F2 laser with visible red and IR control

[0002] This Application is a Continuation-In-Part of Ser. No. 09/273,446, Reliable, Modular, Production Quality Narrow-Band High Rep Rate F₂Excimer Laser filed Mar. 19, 1999. This invention relates to lasers and in particular to narrow-band F₂excimer lasers.

[0003] KrF excimer lasers are the state of the art light source for integrated circuit lithography. One such laser is described in U.S. Pat. No. 4,959,340 issued Sep. 25, 1990. The lasers operate at wavelengths of about 248 nm. With the KrF laser integrated circuits with dimensions as small as 180 nm can be produced. Finer dimensions can be provided with ArF lasers which operate at about 193 nm or F₂lasers which operate at about 157 nm.

[0004] These lasers, the KrF laser, the ArF laser and the F₂lasers, are very similar, in fact the same basic equipment used to make a KrF laser can be used to produce an ArF laser or an F₂laser merely by changing the gas concentration and modifying the controls and instrumentation to accommodate the slightly different wavelength.

[0005] Control of lithography lasers and other lithography equipment require laser pulse energy monitors sensitive to the UV light produced by these lasers. The standard prior art detectors used for monitoring pulse energy in state of the art integrated circuit lithography equipment are silicon photo diodes.

[0006] A typical prior-art KrF excimer laser used in the production of integrated circuits is depicted in FIG. 1 and FIG. 2. A cross section of the laser chamber of this prior art laser is shown in FIG. 3. A pulse power system 2 powered by high voltage power supply 3 provides electrical pulses to electrodes 6 located in a discharge chamber 8. Typical state-of-the art lithography lasers are operated at a pulse rate of about 1000 Hz with pulse energies of about 10 mJ per pulse. The laser gas (for a KrF laser, about 0.1% fluorine, 1.3% krypton and the rest neon which functions as a buffer gas) at about 3 atmospheres is circulated through the space between the electrodes at velocities of about 1,000 inches per second. This is done with tangential blower 10 located in the laser discharge chamber. The laser gases are cooled with a heat exchanger 11 also located in the chamber and a cold plate (not shown) mounted on the outside of the chamber. The natural bandwidth of the excimer lasers is narrowed by line narrowing module 18. Commercial excimer laser systems are typically comprised of several modules that may be replaced quickly without disturbing the rest of the system. Principal modules include:

[0007] Laser Chamber Module,

[0008] Pulse Power System with: high voltage power supply module,

[0009] commutator module and high voltage compression head module,

[0010] Output Coupler Module,

[0011] Line Narrowing Module,

[0012] Wavemeter Module,

[0013] Computer Control Module,

[0014] Gas Control Module,

[0015] Cooling Water Module

[0016] Electrodes 6 consist of cathode 6A and anode 6B. Anode 6B is supported in this prior art embodiment by anode support bar 44 which is shown in cross section in FIG. 3. Flow is clockwise in this view. One comer and one edge of anode support bar 44 serves as a guide vane to force air from blower 10 to flow between electrodes 6A and 6B. Other guide vanes in this prior art laser are shown at 46, 48 and 50. Perforated current return plate 52 helps ground anode 6B to the metal structure of chamber 8. The plate is perforated with large holes (not shown in FIG. 3) located in the laser gas flow path so that the current return plate does not substantially affect the gas flow. A peaking capacitor comprised of an array of individual capacitors 19 is charged prior to each pulse by pulse power system 2. During the voltage buildup on the peaking capacitor, two preionizers 56 weakly ionize the lasing gas between electrodes 6A and 6B and as the charge on capacitors 19 reach about 16,000 volts, a discharge across the electrode is generated producing the excimer laser pulse. Following each pulse, the gas flow between the electrodes of about 1 inch per millisecond, created by blower 10, is sufficient to provide fresh laser gas between the electrodes in time for the next pulse occurring one millisecond later.

[0017] In a typical lithography excimer laser, a feedback control system measures the output laser energy of each pulse, determines the degree of deviation from a desired pulse energy, and then sends a signal to a controller to adjust the power supply voltage so that energy of the subsequent pulse is close to the desired energy.

[0018] These excimer lasers are typically required to operate continuously 24 hours per day, 7 days per week for several months, with only short outages for scheduled maintenance. One problem experienced with these prior-art lasers has been excessive wear and occasional failure of blower bearings. A need exists in the integrated circuit industry for a modular, reliable, production line quality F₂laser in order to permit integrated circuit resolution not available with KrF and ArF lasers.

[0019] The present invention provides a reliable, modular, production quality F₂excimer laser capable of producing, at repetition rates in the range of 1,000 to 2,000 Hz or greater, laser pulses with pulse energies in the range of a few mJ with a full width half, maximum bandwidth of about 1 pm or less at wavelengths in the range of 157 nm. Laser gas mixtures are disclosed for maximizing laser efficiency while reducing unwanted infrared and visible emissions from the laser. Also disclosed are UV energy detectors which are substantially insensitive to infrared and visible light. Preferred embodiments of the present invention can be operated in the range of 1000 to 4000 Hz with pulse energies in the range of 1.0 to 10 mJ with average power outputs in the range of about 10 to 40 watts. Using this laser as an illumination source, stepper or scanner equipment can produce integrated circuit resolution of 0.1 μm or less. Replaceable modules include a laser chamber and a modular pulse power system.

[0020] In a preferred embodiment the laser was tuned to the F₂157.6 nm line using a set of two external prisms. In a second preferred embodiment the laser is operated broad band and the 157.6 nm line is selected external to the resonance cavity. In a third preferred embodiment a line width of 0.2 pm is provided using injection seeding. In a fourth embodiment one of the two F₂lines is selected with an etalon output coupler. Another embodiment utilizes a grating for line selection and increases the tuning range by operating the laser at a pressure in excess of 4 atmospheres.

[0021] FIG. 1 is a drawing of a prior art commercial excimer lithography laser.

[0022] FIG. 2 is a block diagram showing some of the principal elements of a prior art commercial excimer lasers used for integrated circuit lithography.

[0023] FIG. 3 is a drawing of the laser chamber of the FIG. 2 laser.

[0024] FIG. 4 is a drawing of a preferred embodiment of the present invention.

[0025] FIG. 5 is a drawing showing a blower drive unit including magnetic bearings.

[0026] FIGS. 6 and 6A are cross section drawings of laser chambers of preferred embodiments of the present invention.

[0027] FIG. 7 is a drawing showing features of a preferred preionizer tube.

[0028] FIG. 8A is a block diagram of a pulse power system of the preferred embodiment of the present invention.

[0029] FIG. 8B is a simplified circuit diagram of the above preferred embodiment.

[0030] FIG. 8C is a combination block diagram, circuit diagram of a high voltage power supply which is part of the above preferred embodiment.

[0031] FIG. 8D is a prospective assembly drawing of a pulse transformer used in the above preferred embodiment.

[0032] FIG. 8E is a drawing of a primary winding of a pulse transformer used in the above preferred embodiment.

[0033] FIGS. 8F1, 8F2 and 8F3 are time line charts showing pulse compression using the above preferred embodiment.

[0034] FIGS. 8G1 and 8G2 are drawing showing two views of a saturable inductor.

[0035] FIGS. 8H1 and 8H2 shows the mounting of a compression head in a preferred embodiment.

[0036] FIGS. 9A and 9B are drawings describing a preferred heat exchanger design.

[0037] FIGS. 10A through 10F are graphs of test data taken during experiments with a prototype F₂laser.

[0038] FIGS. 11A and 11B show two preferred F₂system configurations.

[0039] FIGS. 12A through 12E show various anode support bar designs.

[0040] FIG. 13 describes a preferred enclosure cooling system.

[0041] FIGS. 14A, 14B and 14C show preferred blower blade structure designs.

[0042] FIG. 15A shows graphs of maximum pulse repetition rate and pulse energy as a function of fluorine concentration.

[0043] FIG. 16 shows a big manifold gas supply system.

[0044] FIG. 17 shows an optical purge system.

[0045] FIG. 18 shows a preferred pulse energy detector system.

[0046] FIG. 19 shows a technique for line narrowing an F₂laser.

[0047] FIG. 20 shows qualitatively the broadening of the F₂laser beam with increased pressure.

[0048] FIG. 21 shows the variation of pulse energy and light spectrum with increasing neon concentration where the other buffer gas is helium.

[0049] FIGS. 22A, 22B, 22C and 22D show features of a preferred pulse transformer for producing high voltage pulses.

[0050] A preferred embodiment of the present invention can be described by reference to the drawings.

[0051] A front view of a preferred embodiment of the present invention is shown in FIG. 4 respectively. This drawing emphasizes the modular nature of their particular invention which allows very quick replacement of modules for repair, replacement and maintenance. The principal features of this embodiment are listed below corresponding to the reference numbers shown on FIG. 4.

[0052] 201 Laser enclosure

[0053] 202 Gas module

[0054] 203 Cooling water supply module

[0055] 204 AC/DC distribution module

[0056] 205 Control module

[0057] 206 Line narrowing module

[0058] 207 Compression head

[0059] 208 High voltage pulse power supply module

[0060] 209 Commutator module for pulse power supply

[0061] 210 Metal fluoride trap

[0062] 211 Laser chamber

[0063] 213 Wavemeter module

[0064] 214 Automatic shutter

[0065] 216 Output coupler

[0066] 217 Blower motor

[0067] 218 Metal fluoride trap power supply

[0068] 219 Status lamp

[0069] 220 24 volt power supply

[0070] 221 Chamber window

[0071] 222 Gas control flexible connection

[0072] 224 Vent box

[0073] A preferred embodiment of the present invention is an improved version of the laser described in FIGS. 1, 2 and 3. This preferred embodiment includes the following improvements:

[0074] 1) A single tube larger preionizer replaces the prior-art combination of a two-tube preionizer to provide improved efficiency, better preionization and improved laser gas flow between the electrodes;

[0075] 2) A silicon-free fan blade which may be a one-piece machined blade;

[0076] 3) The solid-state pulse power system has been modified to produce faster rise time, providing more consistent pulses, and improved laser efficiency at higher voltages;

[0077] 4) More precise control of the charging voltage of the pulse power system;

[0078] 5) A computer controller programmed with a new algorithm providing a much improved control of pulse energy and burst energy; and

[0079] 6) Electrode spacing has been reduced to 10 mm.

[0080] As shown in FIG. 6, a single larger preionizer tube 56A has replaced the two-preionizer tubes 56 shown in FIG. 3. The single tube preionizer is fabricated in accordance with the description in U.S. Pat. No. 5,719,896, issued Feb. 17, 1998, which is incorporated herein by reference. Applicants have discovered that one preionizer tube is not only sufficient, but very surprisingly provides improved performance over the two-preionizer design. In this embodiment the preionizer is located upstream of the electrodes. Applicants have determined that the one tube preionizer improves in the pulse-to-pulse stability by providing improved spatial stability of the discharge.

[0081] Referring now to FIG. 7, this preionizer utilize an integrated tube design, having bushing element 180 with anti-tracking grooves 170 incorporated therein as an integral component of the tube. The diameter of the rod portion 145 and the OD of the bushing portion 180 of the preionizer is ½ inch. The inside conductor rod 146 has a diameter of {fraction (7/37)} inch and the connecting wire extending through the bushing section to make a ground connection is about {fraction (1/16)} inch diameter. Prior preionizer tube designs utilized a two-diameter design, with the rod portion at about ¼ inch diameter and the bushings at about 1 inch diameter. This necessitated, for manufacturing purposes, a bonding process to join the bushing component with the tube component. The constant diameter, thicker tube design is contrary to conventional design rules, which would predict a reduction in ionization due to lower capacitances. In most designs, the tube thickness is dependent upon the dielectric strength of the material selected. Those skilled in the art will recognize that the prior art conventional preionizer tube design technique is to select a material with the highest dielectric strength and determining a wall thickness to match this capacity. For example, a sapphire material is known to have a dielectric strength ranging from 1200 volts/mil to 1700 volts/mil. Therefore, a dielectric thickness of 0.035 inches thick, provides a safety factor of 2 if the laser operates at 25 kV. This design yields a lower capacitance; however, the actual effect of this reduced capacitance on laser operation was discovered to be negligible, with a surprising increase in the measured geometric irradiation of the electrode gap. Because of the constant diameter, thicker tube wall, integral bushing design, a single piece of material can be machined to provide anti-tracking grooves 170. Because of the single piece construction, there is no need to use ultra-pure (i.e., 99.9%) polycrystalline translucent aluminum oxide ceramic, although Applicants continue to use the ultra-pure material. There is no requirement to perform the difficult surface polishing of tube geometries in preparation for diffusion bonding to artificially create the integral relationship between bushing 180 and tube 145. In fact, it has been determined that high purity is not as important a property as porosity of the material. It has been found that the greater the porosity, the more the dielectric strength is reduced. As a result, a commercial grade ceramic, preferably with purity of at least 99.8% and low porosity, such as that manufactured by Coors Ceramics Company under the material No. AD-998E and having a dielectric strength of 300 volts/mil may be used. Bushings 180, having anti-tracking grooves 170 disposed therein, as previously described, act to prevent high voltage tracking axially along the surface of the tube from the cathode to the ground plane 160.

[0082] As explained above, Applicants have discovered that a single preionizer works dramatically better than two preionizers, and as explained above the first preferred embodiment places the single preionizer system upstream of the electrodes. Applicants have also experimented with the single preionizer located downstream and has discovered that at certain blower speeds this arrangement produces substantially better pulse energy stability than the upstream arrangement on the two tube arrangement.

[0083] Improvements have been made to the chamber to improve the efficiency of the laser. A single piece cathode insulator 55A comprised of alumina, Al₂O₃insulates the cathode from the upper chamber structure as shown in FIG. 6A. In a prior art design, eight separate insulators were needed to avoid insulator cracking due to thermal expansion stress in the insulator. This important improvement permitted the head portion of the chamber to be made shorter which significantly reduced the distance between cathode 83 the peaking capacitor 82. The individual capacitors 54A forming the peaking capacitor array 82 were moved horizontally in closer to the cathode as compared to the prior art. Prior art cathodes for commercial lithography lasers were typically supported by a cathode support bar 53 as shown in FIG. 3. In this preferred embodiment, the cathode support bar was eliminated and the cathode 83 was made slightly thicker and mounted directly on the single piece insulator 55A. The cathode 83 is connected to the high voltage side 82A of peaking capacitor 82 by 15 feed through rods 83A and connecting nuts 83B. In the preferred embodiment, a new anode support bar 84A is substantially more massive than prior art anode support bars and comprises fins 84B located in the gas flow region. Both of these features minimize temperature variations of the anode.

[0084] Applicants have discovered that prior art elastomer seals reacted with fluorine gas to produce contaminants in the laser gas which degraded laser performance. A preferred embodiment of the present invention uses all metal seals to seal the laser chamber. The preferred metal seals are tin plated inconel 1718 seals.

[0085] Applicants have also discovered that elements of stainless steel also react with fluorine to produce contaminants in the laser gas. Therefore, in this preferred embodiment, prior art stainless steel current return structures and gas flow vanes have been replaced with monel current returns 250 and monel flow vanes 252 and 254.

[0086] This preferred embodiment of the present invention includes a braze free blower blade structure. The blower blade includes a non-symmetrical blade arrangement which greatly decreases resonance effects and improves bearing life.

[0087] Silicon Free Fan Blade Structure

[0088] Applicants have discovered that a brazing material commonly used in blower blade construction was the primary source of SiF₆in the laser chamber. This gas significantly degraded laser performance for KrF lasers but was a total disaster for ArF lasers and F₂lasers. Applicants have identified four solutions to this problem. First the blade structure was machined in segments from a solid block of material (in this case aluminum). Another solution was to cast the blade structure in segments. The segments then are welded together using electron beam welding in which no new material is added. It is also feasible to fabricate the blade structure by joining blades to a frame structure but in this case the joining is by electron beam welding instead of the prior art brazing process. The fourth method is to join the blade to a frame structure using a soldering process using a silicon free solder. Aluminum 6061 is used as the base material for all of the component pieces. These parts are then copper-plated in prelude to the soldering process. With all of the parts assembled, the fan is then soldered together using a low temperature solder, typically 91% tin (Sn) and 9% Zinc (Zn) in a vacuum furnace. This solder is chosen due to its lack of silicon and its ability to work with copper plated aluminum. The assembled and soldered fan is then nickel-plated. This method of construction yields a non-silicon fan that is inexpensive to manufacture.

[0089] Reducing Resonance Effects

[0090] Prior art blower blade structures consisted of a tangential blower with 23 longitudinal blades. These blades were mounted symmetrically at the circumference of the structure. Substantial resonance effects were measured both with respect to fan parameters and actual laser performance. Perturbations in the laser beam were shown to correspond to acoustic waves at 23 times the rotating frequency of the fan. Adverse affects on bearing performance were also measured corresponding to 23 times the fan's rotating frequency.

[0091] Improvements in fan structure design call for a non-symmetrical blade arrangement such as that shown in FIG. 14A. An alternative as shown in FIG. 14B where the fan blade structure is formed of 16 separate machined or cart segments with each segment having 23 blades is to rotate each segment by 360°/(15×23) or about 1° relative to the adjacent segment. Another improvement which is made relatively easy in the machine or cast approach to fan blade structure fabrication is to form the blades into air foils as shown at 320 in FIG. 14C. Prior art blades were stamped and a cross section of the two of the stamped blades are shown for comparison at 314. The direction of rotation is shown at 318 and 330 represents the circumference of the blade structure. Whereas conventional blades are uniform in thickness, airfoil blades have a tear shape profile including a rounded leading edge, a thickened midsection and a tapered trailing edge.

[0092] Embodiments of the present invention will be made available with one of two alternative bearing improvements over the prior art.

[0093] Ceramic Bearings

[0094] A preferred embodiment of the present invention includes ceramic bearings. The preferred ceramic bearings are silicon nitride lubricated with a synthetic lubricant, preferably perfluoropolyalkylether (PFPE). These bearings provide substantially greater life as compared to prior art excimer laser fan bearings. In addition, neither the bearings nor the lubricant are significantly affected by the highly reactive fluorine gas.

[0095] Magnetic Bearings

[0096] Another preferred embodiment of the present invention comes with magnetic bearings supporting the fan structure as shown in FIG. 5. In this embodiment, the shaft 130 supporting the fan blade structure 146 is in turn supported by an active magnetic bearing system and driven by a brushless DC motor 130 in which the rotor 129 of the motor and the rotors 128 of at least two bearings are sealed within the gas environment of the laser cavity and the motor stator 140 and the coils 126 of the magnetic bearing magnets are located outside the gas environment. This preferred bearing design also includes an active magnetic thrust bearing 124 which also has the coils located outside the gas environment.

[0097] As shown in FIG. 3, prior art gas flow from blower 10 was forced to flow between electrodes 6A and 6B by anode support bar 44. However, Applicants have discovered that the prior art designs of support bar 44 such as that shown in FIG. 3 produced substantial aerodynamic reaction forces on the blower which were transferred to the blower beaings resulting in chamber vibration. Applicants suspect that these vibrational forces are responsible for blower bearing wear and possibly occasional bearing failures. Applicant has tested other designs, several of which are shown in FIGS. 12A-12E, all of which reduced the aerodynamic reaction forces by distributing over a longer time period, the reaction force resulting each time a blade passes close to the edge of support bar 44. One of Applicants preferred anode support bar design is shown in FIG. 6A at 84A. This design has substantially greater mass which minimizes anode temperature savings. The total mass of the anode and the anode support bar is about 3.4 Kg. Also, this design comprises fins 84B which provides added cooling for the anode. Applicants tests have indicated that both the acoustic baffles and the aerodynamic anode support bar tend to reduce slightly the gas flow so that is gas flow is limited, the utilization of these two improvements should involve a trade-off analysis. For these reasons two improvements are shown on FIG. 6A and not FIG. 6.

[0098] A preferred pulse power system is manufactured in four separate modules as indicated in FIGS. 8A and 8B, each of which becomes an important part of the excimer laser system and each of which can be quickly replaced in the event of a parts failure or in the course of a regular preventative maintenance program These modules are designated by Applicants: high voltage power supply module 20, commutator module 40, compression head module 60 and laser chamber module 80.

[0099] High Voltage Power Supply Module

[0100] High voltage power supply module 20 comprises a 300 volt rectifier 22 for converting 208 volt three phase plant power from source 10 to 300 volt DC. Inverter 24 converts the output of rectifier 22 to high frequency 300 volt pulses in the range 100 kHz to 200 kHz. The frequency and the on period of inverter 24 are controlled by the HV power supply control board 21 in order to provide course regulation of the ultimate output pulse energy of the system. The output of inverter 24 is stepped up to about 1200 volts in step-up transformer 26. The output of transformer 26 is converted to 1200 volts DC by rectifier 28 which includes a standard bridge rectifier circuit 30 and a filter capacitor 32. DC electrical energy from circuit 30 charges 8.1 μF C_ocharging capacitor 42 in commutator module 40 as directed by HV power supply control board 21 which controls the operation of inverter 24 as shown in FIG. 8A. Set points within HV power supply control board 21 are set by laser system control board 100.

[0101] The reader should note that in this embodiment as shown in FIG. 8A that pulse energy control for the laser system is provided by power supply module 20. The electrical circuits in commutator 40 and compression head 60 merely serve to utilize the electrical energy stored on charging capacitor 42 by power supply module 20 to form at the rate of 1,000 to 2,000 times per second an electrical pulse, to amplify the pulse voltage and to compress in time the duration of the pulse. As an example of this control, FIG. 8A indicates that processor 102 in control board 100 has controlled the power supply to provide precisely 700 volts to charging capacitor 42 which during the charging cycle is isolated from the down stream circuits by solid state switch 46. The electrical circuits in commutator 40 and compression head 60 will upon the closure of switch 46 very quickly and automatically convert the electrical energy stored on capacitor 42 into the precise electrical discharge pulse across electrodes 83 and 84 needed to provide the next laser pulse at the precise energy needed as determined by processor 102 in control board 100.

[0102] Commutator module 40 comprises C_ocharging capacitor 42, which in this embodiment is a bank of capacitors connected in parallel to provide a total capacitance of 8.1 μF. Voltage divider 44 provides a feedback voltage signal to the HV power supply control board 21 which is used by control board 21 to limit the charging of capacitor 42 to the voltage (called the “control voltage”) which when formed into an electrical pulse and compressed and amplified in commutator 40 and compression head 60 will produce the desired discharge voltage on peaking capacitor 82 and across electrodes 83 and 84.

[0103] In this embodiment (designed to provide electrical pulses in the range of about 3 Joules and 16,000 volts at a pulse rate of 2000 Hz pulses per second), about 250 microseconds (as indicated in FIG. 8F1) are required for power supply 20 to charge the charging capacitor 42 to 800 volts. Therefore, charging capacitor 42 is fully charged and stable at the desired voltage when a signal from commutator control board 41 closes solid state switch 44 which initiates the very fast step of converting the 3 Joules of electrical energy stored on charging capacitor C_ointo a 16,000 volt discharge across electrodes 83 and 84. For this embodiment, solid state switch 46 is a IGBT switch, although other switch technologies such as SCRS, GTOs, MCTs, etc. could also be used. A 600 nH charging inductor 48 is in series with solid state switch 46 to temporarily limit the current through switch 46 while it closes to discharge the C_ocharging capacitor 42.

[0104] Pulse Generation Stage

[0105] The first stage of high voltage pulse power production is the pulse generation stage 50. To generate the pulse the charge on charging capacitor 42 is switched onto C₁8.5 μF capacitor 52 in about 5 μs as shown on FIG. 8F2 by closing IGBT switch 46.

[0106] First Stage of Compression

[0107] A saturable inductor 54 initially holds off the voltage stored on capacitor 52 and then becomes saturated allowing the transfer of charge from capacitor 52 through 1:23 step up pulse transformer 56 to C_p−1capacitor 62 in a transfer time period of about 550 ns, as shown on FIG. 8F3, for a first stage of compression 61.

[0108] The design of pulse transformer 56 is described below. The pulse transformer is extremely efficient transforming a 700 volt 17,500 ampere 550 ns pulse rate into a 16,100 volt, 760 ampere 550 ns pulse which is stored very temporarily on C_p−1capacitor bank 62 in compression head module 60.

[0109] Compression head module 60 further compresses the pulse.

[0110] Second Stage of Compression

[0111] An L_p−1saturable inductor 64 (with about 125 nH saturated inductance) holds off the voltage on 16.5 nF Cpl capacitor bank 62 for approximately 550 ns then allows the charge on C_p−1to flow (in about 100 ns) onto 16.5 nF Cp peaking capacitor 82 located on the top of laser chamber 80 and which is electrically connected in parallel with electrodes 83 and 84 and preionizer 56A. This transformation of a 550 ns long pulse into a 100 ns long pulse to charge Cp peaking capacitor 82 makes up the second and last stage of compression as indicated at 65 on FIG. 8A.

[0112] About 100 ns after the charge begins flowing onto peaking capacitor 82 mounted on top of and as a part of the laser chamber module 80, the voltage on peaking capacitor 82 has reached about 14,000 volts and discharge between the electrodes begins. The discharge lasts about 50 ns during which time lasing occurs within the optical resonance chamber of the excimer laser. The optical resonance chamber is defined by a line selection package 86 comprised in this example by a 2 prism wavelength selector and a R-max mirror together, indicated as 86 in FIG. 8A and an output coupler 88. The laser pulse for this laser is a narrow band, 20 to 50 ns, 157 nm pulse of about 10 mJ and the repetition rate up to 2000 pulses per second. The pulses define a laser beam 90 and the pulses of the beam are monitored by photodiode 92, all as shown in FIG. 8A.

[0113] The signal from photo detector 92 is transmitted to processor 102 in control board 100 and the processor uses this energy signal and preferably other historical pulse energy data (as discussed below in the section entitled Pulse Energy Control Algorithm) to set the command voltage for the next and/or future pulses. In a preferred embodiment in which the laser operates in a series of short bursts (such as 100 pulse 0.5 second bursts at 2000 Hz separated by a dead time of about 0.1 second) processor 102 in control board 100 is programmed with a special algorithm which uses the most recent pulse energy signal along with the energy signal of all previous pulses in the burst along with other historical pulse profile data to select a control voltage for the subsequent pulse so as to minimize pulse-to-pulse energy variations and also to minimize burst-to-burst energy variations. This calculation is performed by processor 102 in control board 100 using this algorithm during a period of about 35 μs. The laser pulses occurs about 5 μs following the T_ofiring of IGBT switch 46 shown on FIG. 8F3 and about 20 μs are required to collect the laser pulse energy data. (The start of the firing of switch 46 is called T_o.) Thus, a new control voltage value is thus ready (as shown on FIG. 8F1) about 70 microseconds after the firing of IGBT switch 46 for the previous pulse (at 2,000 Hz the firing period is 500 μs). The features of the energy control algorithm are described below and are described in greater detail in U.S. patent application Ser. No. 09/034,870 which is incorporated herein by reference.

[0114] This preferred embodiment is provided with electronic circuitry which recovers excess energy onto charging capacitor 42 from the previous pulse. This circuitry substantially reduces waste energy and virtually eliminates after ringing in the laser chamber 80.

[0115] The energy recovery circuit 57 is comprised of energy recovery inductor 58 and energy recovery diode 59, connected in series across Co charging capacitor 42 as shown in FIG. 8B. Because the impedance of the pulse power system is not exactly matched to that of the chamber and due to the fact that the chamber impedance varies several orders of magnitude during the pulse discharge, a negative going “reflection” is generated from the main pulse which propagates back from the chamber towards the front end of the pulse generating system. After the excess energy has propagated back through the compression head 60 and the commutator 40, switch 46 opens up due to the removal of the trigger signal by the controller. The energy recovery circuit 57 reverses the polarity of the reflection which has generated a negative voltage on the charging capacitor 42 through resonant free wheeling (a half cycle of ringing of the L-C circuit made up of the charging capacitor 42 and the energy recovery inductor 58) as clamped against reversal of current in inductor 58 by diode 59. The net result is that substantially all of the reflected energy from the chamber 80 is recovered from each pulse and stored on charging capacitor 42 as a positive charge ready to be utilized for the next pulse. FIG. 8F1, 2 and 3 are time line charts showing the charges on capacitor Co, C₁, C_p−1and Cp. The charts show the process of energy recovery on Co.

[0116] In order to completely utilize the full B-H curve swing of the magnetic materials used in the saturable inductors, a DC bias current is provided such that each inductor is reverse saturated at the time a pulse is initiated by the closing of switch 46.

[0117] In the case of the commutator saturable inductors 48 and 54, this is accomplished by providing a bias current flow of approximately 15A backwards (compared to the directional normal pulse current flow) through the inductors. This bias current is provided by bias current source 120 through isolation inductor LB1. Actual current flow travels from the power supply through the ground connection of the commutator, through the primary winding of the pulse transformer, through saturable inductor 54, through saturable inductor 48, and through isolation inductor LB1 back to the bias current source 120 as indicated by arrows B1.

[0118] In the case of compression head saturable inductor, a bias current B2 of approximate 5A is provided from the second bias current source 126 through isolation inductor LB2. At the compression head, the current splits and the majority B2-1 goes through saturable inductor Lp−164 and back through isolation inductor LB3 back to the second bias current source 126. A smaller fraction of the current B2-2 travels back through the HV cable connecting the compression head 60 and the commutator 40, through the pulse transformer secondary winding to ground, and through a biasing resistor back to the second bias current source 126. This second smaller current is used to bias the pulse transformer so that it is also reset for the pulsed operation. The mount of current which splits into each of the two legs is determined by the resistance in each path and is intentionally adjusted such that each path receives the correct amount of bias current.

[0119] In this embodiment, we refer to the flow of pulse energy through the system from a standard three-phase power source 10 to the electrodes and to ground beyond electrode 84 as “forward flow” and this direction as the forward direction. When we refer to an electrical component such as a saturable inductor as being forward conducting we mean that it is biased into saturation to conduct “pulse energy” in a direction toward the electrodes. When it is reverse conducting it is biased into saturation to conduct energy in a direction away from the electrodes toward the charging capacitor. The actual direction of current flow (or electron flow) through the system depends on where you are within the system. The direction of current flow is now explained to eliminate this as a possible source of confusion.

[0120] By reference to FIGS. 8A and 8B, in this preferred embodiment Co capacitor 42 is charged to (for example) a positive 700 volts such that when switch 46 is closed current flows from capacitor 42 rough inductor 48 in a direction toward C₁capacitor 52 (which means that electrons are actually flowing in the reverse direction). Similarly, the current flow is from C₁capacitor 52 through the primary side of pulse transformer 56 toward ground. Thus, the direction of current and pulse energy is the same from charging capacitor 42 to pulse transformer 56. As explained below under the section entitled “Pulse Transformer” current flow in both the primary loops and the secondary loop of pulse transformer 56 is toward ground. The result is that current flow between pulse transformer 56 and the electrodes during the initial portion of the discharge (which represents the main portion [typically about 80 percent] of the discharge) is in the direction away from the electrodes toward transformer 56. Therefore, the direction of electron flow during the main discharge is from ground through the secondary of pulse transformer 56 temporarily onto C_p−1capacitor 62 through inductor 64, temporarily onto Cp capacitor 82, through inductor 81, through electrode 84 (which is referred to as the discharge cathode) through the discharge plasma, through electrode 83 and back to ground. Thus, between pulse transformer 56 and the electrodes 84 and 83 during the main discharge electrons flow in the same direction as the pulse energy. Immediately following the main portion of the discharge, currents and electron flow are reversed and the reverse electron flow is from ground up through the grounded electrode 84, though the discharge space between the electrodes to electrode 83 and back through the circuit through transformer 56 to ground. The passage of reverse electron flow through transformer 56 produces a current in the “primary” loops of transformer 56 with electron flow from ground through the “primary” side of pulse transformer 56 (the same direction as the current flow of the main pulse) to ultimately charge Co negative as indicated qualitatively in FIG. 8F2. The negative charge on Co is reversed as shown in FIG. 8F2 and explained above in the section entitled Energy Recovery.

[0121] A more detailed circuit diagram of the power supply portion of the preferred embodiment is shown in FIG. 8C. As indicated in FIG. 8C, rectifier 22 is a 6 pulse phase controlled rectifier with a plus 150 v to −150 V DC output. Inverter 24 is actually three invertors 24A, 24B and 24C. Invertors 24B and 24C are turned off when the voltage on 8.1 μF Co charging capacitor 42 is 50 volts less than the command voltage and inverter 24A is turned off when the voltage on Co 42 slightly exceeds the command voltage. This procedure reduces the charge rate near the end of the charge. Step up transformers 26A, 26B and 26C are each rated at 7 kw and transform the voltage to 1200 volt AC.

[0122] Three bridge rectifier circuits 30A, 30B and 30C are shown. The HV power supply control board 21 converts a 12 bit digital command to an analog signal and compares it with a feedback signal 45 from Co voltage monitor 44. When the feedback voltage exceeds the command voltage, inverter 24A is turned off as discussed above, Q2 switch 34 closes to dissipate stored energy within the supply, Q3 isolation switch 36 opens to prevent any additional energy leaving the supply and Q1 bleed switch 38 closes to bleed down the voltage on Co 42 until the voltage on Co equals the command voltage. At that time Q1 opens.

[0123] The principal components of commutator 40 and compression head 60 are shown on FIGS. 8A and 8B and are discussed above with regard to the operation of the system. In this section, we describe details of fabrication of the commutator.

[0124] In this preferred embodiment solid state switch 46 is an P/N CM 1000 HA-28H IGBT switch provided by Powerex, Inc. with offices in Youngwood, Pa.

[0125] Inductors 48, 54 and 64 comprise saturable inductors similar to those described in U.S. Pat. Nos. 5,448,580 and 5,315,611. A top and section view of a preferred saturable inductor design is shown respectively in FIGS. 8G1 and 8G2. In the inductors of this embodiment, flux excluding metal pieces such as 301, 302, 303 and 304 are added as shown in FIG. 8G2 in order to reduce the leakage flux in the inductors. The current input to this inductor is a screw connection at 305 to a bus also connected to capacitor 62. The current makes four and one half loops through vertical conductors. From location 305 the current travels down a large diameter conductor in the center labeled 1A, up six smaller conductors on the circumference labeled 1B, down 2A, up 2B, down all of the flux excluder elements, up 3B, down 3A, up 4B and down 4A, and the current exits at location 306. Where a pot like housing 64A serves as a high voltage current lead. The “lid” 64B of the saturable inductor is comprised of an electrical insulator material such as teflon. In prior art pulse power systems, oil leakage from oil insulated electrical components has been a problem. In this preferred embodiment, oil insulated components are limited to the saturable inductors and the oil is contained in the pot-like oil containing metal housing 64A which is, as stated above, the high voltage connection output lead. All seal connections are located above the oil level to substantially eliminate the possibility of oil leakage. For example, the lowest seal in inductor 64 is shown at 308 in FIG. 8G2. Since the flux excluding metal components are in the middle of the current path through the inductor, the voltage allowing a reduction in the safe hold-off spacing between the flux exclusion metal parts and the metal rods of the other turns. Fins 307 are provided to increase heat removal.

[0126] Capacitor banks 42, 52 and 62 are all comprised of banks of commercially available off-the-shelf capacitors connected in parallel. These capacitors are available from suppliers such as Murata with offices in Smyrna, Ga. Applicants preferred method of connecting the capacitors and inductors is to solder or bolt them to positive and negative terminals on special printed circuit board having heavy nickel coated copper leads in a manner similar to that described in U.S. Pat. No. 5,448,580.

[0127] Pulse transformer 56 is also similar to the pulse transformer described in U.S. Pat. Nos. 5,448,580 and 5,313,481; however, the pulse transformers of the present embodiment has only a single turn in the secondary winding and 23 separate primary windings. A drawing of pulse transformer 56 is shown in FIG. 8D. Each of the 23 primary windings comprise an aluminum spool 56A having two flanges (each with a flat edge with threaded bolt holes) which are bolted to positive and negative terminals on printed circuit board 56B as shown along the bottom edge of FIG. 8D. Insulators 56C separates the positive terminal of each spool from the negative terminal of the adjacent spool. Between the flanges of the spool is a hollow cylinder 1{fraction (1/16)} inches long with a 0.875 OD with a wall thickness of about {fraction (1/32)} inch. The spool is wrapped with one inch wide, 0.7 mil thick Metglas™ 2605 S3A and a 0.1 mil thick mylar film until the OD of the insulated Metglas™ wrapping is 2.24 inches. A prospective view of a single wrapped spool forming one primary winding is shown in FIG. 8E.

[0128] The secondary of the transformer is a single stainless steel rod mounted within a tight fitting insulating tube of electrical glass. The winding is in four sections as shown in FIG. 8D. The stainless steel secondary shown as 56D in FIG. 8D is grounded to a ground lead on printed circuit board 56B at 56E and the high voltage terminal is shown at 56F. As indicated above, a 700 volt pulse between the + and − terminals of the primary windings will produce a minus 16,100 volt pulse at terminal 56F on the secondary side for a 1 to 23 voltage transformation. This design provides very low leakage inductance permitting extremely fast output rise time.

[0129] The Cp capacitor 82 is comprised of a bank of twenty-eight 0.59 nf capacitors mounted on top of the laser chamber pressure vessel. The electrodes 83 and 84 are each solid brass bars about 28 inches long which are separated by about 0.5 to 1.0 inch. In this embodiment, the top electrode 83 is the cathode and the bottom electrode 84 is connected to ground as indicated in FIG. 8A.

[0130] This preferred embodiment of the present invention includes a compression head mounting technique shown in FIGS. 8H1 and 8H2. FIG. 8H1 is a side section view of the laser system showing the location of the compressor head module 60 in relation to electrodes 83 and 84. This technique was designed to minimize the impedance associated with the compression lead chamber connection and at the same time facilitates quick replacement of the compression head. As shown in FIGS. 8H1 and 8H2 the ground connection is made with an approximately 28 inch long slot tab connection along the back side of the compression head as shown at 81A in FIG. 8H1 and 81B in FIG. 8H2. The bottom of the slot tab is fitted with flexible finger stock 81C. A preferred finger stock material is sold under the trade name Multilam®.

[0131] The high voltage connection is made between a six-inch diameter smooth bottom of saturable inductor 64 and a mating array of flexible finger stock at 89 in FIG. 8H1. As above, a preferred finger stock material is Multilam®. This arrangement permits the replacement of the compression head module for repair or preventative maintenance in about five minutes.

[0132] This preferred embodiment comprises a fluorine control system which permits operation within a chosen sweet spot without the use of a fluorine monitor. This embodiment can be described by reference to FIG. 16.

[0133] Laser chamber 1 comprises about 20.3 liters of laser gas. Nominally as described above, the constituents are 0.1 percent fluorine and the remainder being a buffer gas (preferably, helium, neon or a combination of helium and neon) at a pressure of about 4 atmospheres. The 0.1 percent fluorine represents a volume of about 0.0023 liters or 2.3 ml of fluorine at 4 atm. In mass terms the normal amount offluorine in the laser chamber is about 110 mg . The partial pressure of the pure fluorine is about 411 Pa, pure fluorine (corresponding to about 41 kPa of the 1% fluorine mixture). During normal operations with the laser operating at a duty factor of about 40 percent (which is typical for a lithography laser) fluorine is depleted at a rate of about 4.5 mg per hour (this corresponds to about 4% of the fluorine in the chamber per hour). In terms of partial pressure of pure fluorine, this normal depletion rate of fluorine is about 16 Pa per hour. To make up for this depletion using the 1% fluorine gas mixture, a volume of the mixture equivalent to about 1.6 kPa per hour is added to the chamber.

[0134] The fluorine depletion rate for the laser is far from constant. If the laser fan is operating but no lasing is taking place the fluorine depletion rate is cut approximately in half. If the fan is shutdown the fluorine depletion rate is cut to about ¼ the 40% duty factor depletion rate. At 100% duty factor the depletion rate is about double the 40% duty factor depletion rate.

[0135] The process described above basically replaces depleted fluorine on an almost continuous basis. Since the fluorine gas source is only 1% fluorine it also replaces a portion of the buffer gas or gases in the chamber on an almost continuous basis. Nevertheless, even though a portion of the laser gas is being substantially continuously replaced, operation in this mode results in a build up of contaminants in the laser gas which reduces the efficiency of the laser. This reduction in efficiency requires an increase in the voltage and/or an increase in the fluorine concentration to maintain the desired pulse energy. For this reason, normal practice with prior art systems suggest that periodically the laser be shutdown for a substantially complete gas exchange. This substantially complete gas exchange is referred to as a refill. These periods may be determined based on number of laser pulses such as 100,000,000 pulses between refills, or refill times may be determined based on calendar time since the last refill or a combination of pulses and calendar time. Also the refill times may be determined by the magnitude of the charging voltage needed for a desired output at a particular fluorine concentration. Preferably after a refill a new test for the “sweet spot” should be run. Also, periodically in between fills the sweet spot test should be performed so that if the sweet spot changes the operator will know where the new sweet spot is.

[0136] A refill may be accomplished using the system shown in FIG. 16 as follows. This discussion presumes a laser gas normally consisting of 0.1% Fl and the rest buffer gas. With valves 510, 506, 515, 512, 517, and 504 closed, valves 506 and 512 are opened, vacuum pump 513 is operated and the laser chamber is pumped down to an absolute pressure of less than 13 kPa. (A direct pump down line may be provided between the chamber 1 and vacuum pump 513 to permit a quick pump down.) Valve 512 is closed. Valve 516 is opened and 100% He buffer gas from buffer gas bottle 516 is added to the chamber to fill it to a pressure equivalent to 262 kPa at 50° C. (Note that for this 20.3 liter laser chamber, temperature correction can be approximated using a AP/AT correction of 1 kPa/° C. for a chamber temperature deviation from 50° C. So if the chamber temperature is 23° C. it would be filled to 247 kPa) Valve 517 is closed and valve 515 is opened and a quantity of the 1% Fl, 99% buffer gas mixture from halogen rich gas bottle 514 is added to chamber 1 to fill it to a pressure equivalent to 290 kPa at 50° C. (Note a temperature correction should be used.) This will produce a gas mixture in the chamber of approximately 0.1% Fl and 99.9% buffergas. When the chamber is heated to about 50° C. the pressure will be about 4 atm.

[0137] Because O₂strongly absorbs 157 mn light, O₂must be excluded from the beam path. Applicants have developed an N₂purge system that is greatly improved over prior art systems. All optical components associated with the laser that are outside the chamber are purged with nitrogen. This nitrogen system is operated at a pressure that is during operation of the laser only about 10 pascals in excess of atmospheric pressure. This small pressure differential is preferred to avoid a pressure distortion effect on the optical components. Components purged include the line narrowing module, the output coupler, the wavemeter and the shutter assembly.

[0138] Seals are provided at all potential leakage sites. Output ports consisting of {fraction (1/16)} inch id. tubes about 6 feet long are provided. The flow through the output ports is monitored to assure proper functioning of the purge system. Preferred flow rates of about 4 liters/minute through the {fraction (1/16)}-inch id. 6-foot long tube is a preferred flow rate to correspond to the desired N₂pressure differential. Preferably, the 6-foot output port tube 304 coiled up. Flow may be monitored with a simple flap switch 300 which is held open by the N₂flow and energizes a warning light 302 if the flow is lost as shown in FIG. 17. A similar purge system preferably is applied for the output coupler and the wavemeter.

[0139] Helium can also be used for the purge gas. Advantages of helium include better heat transfer properties and better optical properties. Helium however is much more expensive than nitrogen and if it is used in a production laser, a collection system for recycling the helium should be considered.

[0140] Preferred embodiments of the present invention which is especially useful for operation at repetition rates in excess of 1000 to 2000 Hz, includes a unique cooling technique shown in FIG. 13 for cooling an excimer laser.

[0141] Components of the laser are contained in enclosure 240 which is maintained on the inside at a slight vacuum produced by a blower mounted in a vent as shown at 224 in FIGS. 13 and 4A. The cabinet comprises filtered intake port 241 near the top of the cabinet and a few small leakage sources, such as around gasketed doors, so that the flow of room air through the laser enclosure is about 200 ft³/min which is not nearly sufficient to remove the heat produced by heat producing components of the laser.

[0142] The very great majority (roughly 90 percent) of the waste heat produced by the laser (roughly 12 kw at 100% duty factor) is removed by a chilled water system as shown in FIG. 13.

[0143] In this embodiment the major heat sources in the laser are the high voltage supply 20, the commutator 40, the compression head 60 and the laser chamber 80. For the chamber a water cooled heat exchanger is located inside the chamber and heat is transferred from circulating laser gas to the heat exchanger to the cooling water. Another heat exchanger (not shown) is mounted on an outside surface of the chamber. For the rest of the major heat producing components cooling water is piped to the location of the component and one or more fans force air through a water-to-air heat exchanger onto the component as shown in FIG. 13. For the compression head the circulation is contained as shown, but for the HVPS and the commutator the circulation is onto the component then through other portions of the enclosure to also cool other components before being recirculated back to the heat exchangers.

[0144] Dividing pans 242 and 243 guide the general ventilation air from filter 241 through a path shown by open headed arrows 244 to vent 224.

[0145] This cooling system contains no ducts and except for a water line feeding the heat exchangers inside of and attached to the laser chamber there is no water line connection to any laser component. Since all components (other than the laser chamber) are cooled by air blown about inside the enclosure, there are no cooling connections to make a break when installing and replacing components. Also, the lack of need for ducting greatly increases useable components and working space inside the enclosure.

[0146] The embodiment of the present invention includes a computer controller program with a new algorithm, which substantially reduces prior art variations in pulse energy and total integrated burst energy. The improved equipment and software and a preferred process for reducing energy sigma and burst dose variation is described below.

[0147] As stated in the background section of this specification, the burst mode is a typical mode of operation of an excimer laser used for the light source of a stepper machine in the lithographic production integrated circuits. In this mode the laser is operated to produce “a burst” of pulse at the rate of 1000 Hz for about 110 milliseconds to produce 110 pulses to illuminate a section of a wafer. After the burst the stepper moves the wafer and the mask and once the move is complete which takes typically a fraction of a second the laser produces another 110 pulse burst Thus, normal operation is bursts of about 110 milliseconds followed by dead times of a fraction of a second. At various times, longer dead time periods will be provided so that other operations can be performed. This basic process continues 24 hours a day, 7 days per week, for several months with the laser typically producing several millions of bursts per day. In the above burst mode, it is usually important that each section of the wafer received the same illumination energy on each burst. Also, chip makers want the pulse to pulse variation to be minimized. This preferred embodiment of the present invention accomplishes these objectives with equipment and software which monitors the energy of each pulse (pulse N−1) then controls the energy of the next pulse (pulse N) based on the results of a:

[0148] 1) a comparison of the measured energy of pulse N−1 with a target pulse energy and

[0149] 2) a comparison of the accumulated dose of the burst through pulse N−1 to a target pulse dose through pulse N−1.

[0150] In the typical F₂excimer laser we have been discussing the energy of the first 30-40 ms of a burst is typically less stable than the rest of the burst due to transient effects in the laser gas. After about 40 ms following the first pulse, the pulse energy at constant voltage is relatively constant. In dealing with these early perturbations, Applicants have separated the burst into two time-wise regions, the first region (consisting of a number of the earlier pulses, for example, 40 pulses) called the “K” region and a second region (consisting of the pulses which follow the K region) which Applicants, in this specification, refer to as the “L” region.

[0151] This embodiment of the present invention utilizes prior art excimer laser equipment for pulse energy control. Pulse energy of each pulse of each burst is measured by photodiode 92 as shown in FIG. 8A. The overall response time of this photodiode and its sample and hold circuit, including time required to reset the circuit, is less than substantially less than 500 microseconds. The accumulated signal resulting from each approximately 15 ns pulse is stored a few microseconds after the pulse is over and this signal is read six times and the average is stored by computer controller 22 approximately 1.0 microsecond after the beginning of the pulse. The accumulated energy of all the previous individual pulses in a burst is referred to as the burst dose value. Computer controller utilizes the signal representing the pulse energy of pulse N along with target pulse energy and the burst dose value in order to specify the high voltage for the pulse N+1. This calculation requires about 200 microseconds. When the value of high voltage for N+1 is determined, computer controller sends a signal to the high voltage command (VCMD) of the high voltage power supply as shown in FIG. 8A establishing the charging voltage for pulse N+1 that takes a few microseconds. Computer controller commands the high voltage power supply to charge up capacitor Co to the specified voltage. (At high repetition rates in excess of 2000 Hz it may be desirable to start the charging before the calculation is complete.) The charging requires about 250 microseconds so that Co is fully charged and ready to go when it receives a trigger signal for pulse N+1 from trigger circuit 13 as shown in FIG. 2 at 0.5 millisecond after the trigger signal from pulse N. On the trigger signal, capacitor C_odischarges its approximately 700 volts into the magnetic compression circuit shown in FIG. 8B over a period of about 5 microseconds and the pulse is compressed and amplified by the magnetic compression circuit to produce a discharge voltage on capacitor Cp of about 16,100 volts which discharges across electrodes 6 in about 100 ns producing a laser pulse of about 10 mJ and about 15 ns in duration.

[0152] A special preferred process for adjusting the charging voltage to achieve substantially desired pulse energies when operating in a burst mode is described below.

[0153] The process utilizes two voltage adjustment algorithms. The first algorithm applies to the first 80 pulses and is called the KPI algorithm. The second algorithm called the PI algorithm applies to pulses after pulse number 40. This time period after the 80^thpulse is herein called the “L region” of the burst. The initials “PI” refer to “proportional integral” and the “K” in “KPI” refers to the “K region” of the burst.

[0154] The K region comprises pulses 1 through k, where k=40 for this preferred embodiment. The algorithm for setting the charging voltage for pulse N is: $(V_N={\left(V_B\right)}_N-{\left(V_C)\right)}_{N-1}N=1.2,\dots k$

[0155] where:

[0156] V_N=charging voltage for N'th pulse

[0157] (V_B)_N=an array of k stored voltages which represent the current best estimate of the voltage required to produce the target energy E_Tfor the N'th pulse in the K region. This array is updated after each burst according to the equation below.

[0158] (V_C)_N−1=a voltage correction based the energy error of the previous pulse and on the energy errors which occurred for the previous pulses in the burst, up to pulse N−1 $\sum _{i=1}^{N-1}\frac{\left(A·{ϵ}_i+B·D_i\right)}{\left(dE/dV\right)}$

[0159] By definition, (V_c)₀=0

[0160] A,B=fractions typically between 0 and 1, which in this preferred embodiment both A and B are 0.5 $\begin{array}{c}{ϵ}_i=\mathrm{the}\mathrm{energy}\mathrm{error}\mathrm{of}\mathrm{the}i\text{'}\mathrm{th}\mathrm{pulse}\\ =E_i-E_T,\mathrm{where}{E}_i\mathrm{is}\mathrm{the}\mathrm{energy}\mathrm{for}\mathrm{the}i\text{'}\mathrm{th}\mathrm{pulse},\mathrm{and}{E}_T\\ \mathrm{is}\mathrm{the}\mathrm{target}\mathrm{energy}\end{array}$

[0161] D_i=the cumulative dose error of the burst, including all pulses from 1 through i $=\sum _{k=1}^i{ϵ}_k$

[0162] dE/dV=a rate of change of pulse energy with charging voltage. (In this embodiment, one or more values of dE/dV is determined experimentally during each burst and a running average of these values is used for the calculation)

[0163] The stored values (VB)N are updated during or after each burst according to the following relation: ${\left(V_B\right)}_N^{M-1}={\left(V_B\right)}_N^M-C·\left(\frac{{ϵ}_N}{\left(dE/dV\right)}-{\left(V_C\right)}_N\right),\mathrm{where}\mathrm{the}\mathrm{index}M\mathrm{refers}\mathrm{to}\mathrm{the}\mathrm{burst}\mathrm{number}$

[0164] C=a fraction typically between 0 and 1, which in this preferred embodiment is 0.3.

[0165] The L region comprises pulses k+1 to the end of the burst (for a preferred $V_N=V_{N-1}-\frac{\left(A·{ϵ}_{N-1}+B·D_{N-1}\right)}{\left(dE/dV\right)}N=k+1,k+2,\dots$

[0166] embodiment, pulse numbers 41 and higher). The algorithm for setting the charging voltage for pulse N is:

[0167] where:

[0168] V_N=charging voltage for N'th pulse

[0169] V_N−1=charging voltage for N−1'st (previous) pulse

[0170] The variables A, B, ε₁, D₁, and dE/dV are defined as before.

[0171] A new value for dE/dV is determined periodically, in order to track the relatively slow changes in the characteristics of the laser. In the preferred embodiment, dE/dV is measured by varying or dithering the voltage in a controlled manner during two successive pulses in the L region. For these two pulses, the normal PI energy control algorithm is temporarily suspended and replaced by the following:

[0172] For pulse j: $V_j=V_{j-1}-\frac{\left(A·{ϵ}_{j-1}+B·D_{j-1}\right)}{\left(dE/dV\right)}+V_{\mathrm{Dither}}$

[0173] where V_Dither=a fixed voltage increment, typically a few volts

[0174] For pulse j+1:

[0175] V_j+1=V_j−2·V_Dither

[0176] After pulse j+1, dE/dV is calculated: $dE/dV=\frac{\left(E_{j+1}-E_j\right)}{2·V_{\mathrm{Dither}}}$

[0177] The calculation of dE/dV can be very noisy, since the expected energy changes due to the dithering voltage can be of the same magnitude as the normal energy variation of the laser. In the preferred embodiment, a running average of the last 50 dE/dV calculations is actually used in the PI and KPI algorithms.

[0178] The preferred method for V_Ditherchoosing is to specify a desired energy dither E_Dither, typically a few percent of the energy target E_T, and then use the current (averaged) value for dE/dV to calculate V_Dither: $V_{\mathrm{Dither}}=\frac{E_{\mathrm{Dither}}}{\left(dE/dV\right)}$

[0179] Pulse j+2 (immediately following the two dithered pulses) is not dithered, but has the special value: $V_{j-2}=V_{j-1}+V_{\mathrm{Dither}}-\frac{\left(A·\left({ϵ}_{j+1}+E_{\mathrm{Dither}}\right)+B·D_{j+1}\right)}{\left(dE/dV\right)}\left(\mathrm{pulse}j+2\right)$

[0180] This special value for V_j+2is corrected for both the applied voltage dither and the expected energy dither from pulse j+1.

[0181] Many variations on the algorithm described above are possible. For example, dE/dV can be determined in the L region as well as the K. The dithering can be performed once per burst, or several times. The dithering sequence may be performed at a fixed pulse number j as described above, or it may be initiated for a randomly chosen pulse number which varies from one burst to the next.

[0182] The reader should recognize that A, B and C are convergence factors, which could have many other values. Higher values than those specified above could provide quicker convergence but could lead to increased instability. In another preferred embodiment, A={square root over (2B)}. This relationship is developed from a recognized technique to produce critical damping. B could be zero in which case there would be no dose correction; however, A should not be zero because it provides a dampening term for the dose conveyance portions of the algorithm.

[0183] If the determined value of dE/dV becomes too small the above algorithm could cause over correction. Therefore a preferred technique is to arbitrarily double dE/dV if the energy sigma value exceeds a threshold. Default values of V and dE/dV are provided for the first pulse of a burst. D is set to zero at the start of each burst. The default dE/dV is set at about three times the expected dE/dV to avoid initial over correction.

[0184] An alternate method of determining ddV without the dither referred to above is to merely measure and store the energy and voltage values during laser operation. (Measured rather than specified voltage values can also be used.) These data can be used to determine dE/dV as a function of V for constant pulse energy. The reader should note that each individual value of dE/dV would contain fairly large uncertainties because the elements of the value are differences of measurements having significant uncertainties. However, averaging large numbers of dE/dV values can reduce these uncertainties.

[0185] The dither exercise to determine dE/dV does not have to be made on each burst but instead could be done periodically such as once every M bursts. Or the measurement of dE/dV could be replaced by a calculation performed by the computer or the value of dE/dV could be inserted manually by the operator of the previous pulse for the calculation of V_N+1. An alternate approach would be to use the actual measured value for V_Nfor this control system. Also the value of V_BINare calculated from specified values, not actual measure values in the above-described embodiment. An obvious alternative would be to use measured voltage values. E_Tis normally a constant value such as 10 mJ but it does not have to be constant. For example, E_Tof the last ten pulses could be smaller than the nominal pulse energy so that percentage deviations from target E_Tfor these pulses would have a smaller effect on the integrated pulse dose. Also, it may be preferable in some situations to program computer controller 22 to provide E_Tvalues that vary from burst to burst.

[0186] A prototype F₂laser system unit was built and tested by Applicants and their fellow workers.

[0187] The prototype laser is largerly based on current production KrF and ArF lasers incorporating several important improvements over prior art excimer laser systems, utilizing a high efficiency chamber and solid-state pulsed power excitation. The discharge is corona pre-ionized to minimize gas contamination. The entire optical beam path is nitrogen purged to avoid light absorption by oxygen and to avoid damage to optical components. All resonator optics were external to the angled chamber window equipped laser chamber. The gas mixture was 0.1% fluorine in 4 atmospheres of helium and the electrode gap was reduced to 10 mm.

[0188] In this prototype unit, a modified pulse transformer is utilized. In this pulse transformer the single four-section stainless steel rod (described above and shown in FIG. 8D) which functions as a secondary winding is replaced by a transformer secondary conductor consisting of an inner cylindrical rod and two coaxial tubes all connected in series and insulated from each other as shown in FIGS. 22A, 22B and 22C. The secondary conductor consists of two coaxial assemblies (the cross-section of which are depicted in FIGS. 22B and 22C) connected with bus bar as shown at 302 and HV cable as shown at 304. FIG. 22D shows the same cross-sections as 22B and 22C and also the layers 306 of Metglas™ and mylar film which is wrapped around the cylinder portion 308 of the spools forming the primary winding. Also identified in FIG. 22D are the central wire 310 and hollow cylindrical conductors 312 and 314 forming the secondary portion of the pulse transformer. The Metglas™ and mylar layers are not shown in FIGS. 22A, 22B and 22C. A current pulse having a voltage peak of about 1,000 volts (as indicated at 316) will produce a pulse at the secondary HV terminal of about 0-36,000 volts as indicated at 318 in FIG. 22A.

[0189] Coupling between the primary cylinders and the three coaxial secondary conductors is provided by wrappings of Metglas™ and mylar film as described above with reference to FIG. 8E. In this embodiment an extra stage of compression (with one additional capacitor bank C_p−1) is provided. The capacitor banks in this embodiment have the following values:

[0190] C_o=about 12.1 μF

[0191] C₁=about 12.4 μFC

[0192] C_p−2=about 8.82 μF

[0193] C_p−1=about 8.4 μF

[0194] C_p=about 10 nF

[0195] The modified pulse power system in this prototype embodiment produces an output use time of about 80 ns into the peaking capacitor bank. The step-up ratio of the pulse transformer is 36× (as compared to the 23× in the embodiment described in detail above). This permits the laser to operate at substantially higher voltages with correspondingly lower F₂concentrations. Applicant has determined that the higher voltage operation improves discharge stability, and permits higher repetition rates.

[0196] Experimental results from experiments with the prototype unit are described below. The laser power was measured by a standard power meter and cross-correlated with a pyroelectric Joulemeter. Contributions of the red, atomic fluorine laser were subtracted and usually amounted to less than 3% of the total energy. By venting the beam delivery tubes to air, which strongly absorbs 157 nm light, the red radiation could be measured.

[0197] The laser wavelength in this prototype unit was operated in single-line mode at 157.6 nm by tuning with a set of two external prisms. The laser could also be tuned to the 157.5 nm transition line with reduced efficiency. The transition at 156.7 nm was not observed. The laser spectrum as recorded by a 2 meter Jobin Yvon VUV spectrometer indicates a measurement-limited linewidth of 6 pm.

[0198] In broadband (or multi-line) operation a maximum power of 12 W was obtained at a repetition rate of 1000 Hz. The power increased linearly with repetition rate without signs of saturation. The behavior in single line mode was similar, but at one third of the energy. This energy decrease is due to a large cavity length increase in the present prism set-up and can be significantly reduced. In burst mode a 3 sigma stability of 5% was recorded. Only a benign burst transient on the output energy was observed. This compares favorably to ArF lasers which show larger energy instability and a gas flow related burst transient. From this one can conclude that a production fluorine lithography laser will have even better energy stability than present day ArF lasers. The integral square pulse duration was 30 ns which is approaching the performance of ArF lasers.

[0199] Careful selection of laser chamber materials and pre-ionization by a corona discharge enabled laser operation without cryogenic purification and without halogen injection over a period of several hours and 3 M shots with minimal energy degradation.

[0200] The dependence of broadband laser power upon repetition rate is displayed in the top portion of FIG. 10A. The bottom portion of FIG. 10A shows a slight drop off of pulse energy with increasing repetition rate above 1000 Hz. The laser power increases almost linearly with repetition rate up to a power of 15 W at 1 kHz and about 19 W at 2000 Hz. Based on this linear relationship one can assume that the fluorine laser can be scaled further to several kilohertz operation, providing the gas flow is scaled accordingly. Since Applicants were using helium as a buffer gas, only a fraction of the blower power of standard neon based lasers is required and therefore does not present a limitation to higher flow speeds.

[0201] A good measure for energy stability is gained by observing the energy transient in burst mode. For this the laser is repeatedly fired in bursts and the average energy for every pulse position in the burst is recorded. Also, for every pulse number in the burst the average variation in energy from burst to burst is calculated. The resulting energy and stability curves for the fluorine laser and for comparison also for a line-narrowed ArF laser are displayed in FIGS. 10B1 and 2. The fluorine laser exhibits only minor energy variations over a 120 shot burst. The energy stability shows an initial increase in the beginning of the bursts and then stabilizes on a 3 sigma level of about 3%. By contrast the ArF laser exhibits a large transient in the energy and a 3 sigma instability around 7%. The ArF laser obtained a dose stability of 0.5% in a 60 pulse window, therefore the fluorine laser is expected to deliver at least the same dose stability. FIG. 10C comprises pulse energy and 3σ a values at 1000 Hz and 1900 Hz.

[0202] A spectrum of the broadband fluorine laser as recorded by VUV spectrometer is shown in FIGS. 10D1 and 2. Clearly visible are the two transition lines at 157.52 nm and at 157.63 nm. 87% of the laser energy is located in the longer wavelength line at 157.63 nm. The transition at 156.7 nm was not observed. Single-line mode operation at 157.63 nm was achieved by tuning with a set of two external prisms. The laser could also be tuned to the 157.52 nm transition line, but at reduced efficiency. Also shown in FIGS. 10D1 and 2 is an expanded view of the laser line at 157.63 nm. Convolved linewidths of 1.14 pm FWHM and 2.35 pm 95% were measured. These linewidths are much narrower than previously expected. Therefore, a line selected fluorine laser without additional line-narrowing will be sufficient for all but fully refactive imagine systems. The laser power vs. repetition rate behavior of the single line laser exhibits the same linear rise as the broadband laser. However, the maximum power in this initial experiment was limited to 4 W. The reduced output power was caused by reflection losses in the line selection optics and by an overly long cavity length.

[0203] The horizontal and vertical beam profiles were measured at 1 m distance from the laser. (See FIGS. 10E1 and 2). The beam shows smooth profiles with a high degree of symmetry. These kinds of profiles are easily managed by currently used homogenizer technology to produce very uniform illumination.

[0204] As estimation of the gas lifetime is derived by operating the fluorine laser at constant voltage without fluorine injection and recording the evolution of laser power versus the number of shots. No cryogenic purification was used in these measurements. As evident in FIG. 10F, the laser power decreases by less than 20% after 4 million laser shots, which is at least as good as for comparable ArF lasers. From previous experience with ArF lasers one can thus estimate a gas lifetime of about 25 million shots by making use of periodic fluorine injections. This is clearly a result of the choice of compatible materials in the laser chamber and the use of corona pre-ionization. For KrF and ArF lasers a direct correlation between fluorine consumption and chamber lifetime has been established, previously. We can therefore estimate a chamber lifetime of the fluorine laser on the same order as that of an ArF laser.

[0205] FIG. 15A bottom graph shows pulse energy versus F₂concentration and the top graph shows the maximum pulse repetition rate, both at a blower speed of 2500 rpm. FIG. 15B top graph shows pulse shape as function of time indicating a FWHM of 16 ns, and the bottom graph shows that the integrated square pulse width is about 37 ns.

[0206] With the prototype unit described above, prior art UV pulse energy detectors do not provide good results. This is because the pulse energy detectors used in prior art KrF and ArF lasers are very sensitive to light in the red and neon infrared spectral ranges. In fact these standard silicon photodiodes are much more sensitive to red and infared than to laser light at 157 nm. Therefore, even when light is in the range of about 3% as indicated above, the effect on the silicon photodiode is much greater than 3%. For this reason, an energy detector should preferably be provided which is not significantly affected by visible red and infrared light. Detectors are available on the market which are especially sensitive to UV radiation and are relatively or completely sensitive to red and infrared radiation. These detectors generally are referred to as solar blind detectors and are use in above the atmosphere astronomical work but to the best of Applicants knowledge, they have not been applied to measure laser pulse energy. There are a number of methods available to construct a “solar blind” photodetector. Several strategies are discussed below.

[0207] Phototube

[0208] A phototube is a vacuum device consisting of two electrodes, a photocathode and an anode. Light striking the photocathode can cause the ejection of electrons from the cathode material via the photoelectric effect The application of a positive voltage between the anode and cathode will drive the ejected electrons to the anode, generating a current through the device proportional to the number of photons/second striking the photocathode. This effect has a cutoff; irradiation with light of wavelength higher than a certain value does not produce photoelectrons. The maximum wavelength is defined by:

[0209] λ_max=hc/Φ

[0210] where H is Planck's constant, c is the speed of light in vacuum, and Φ is a property of the material called the work function. Choice of a photocathode material with a sufficiently high work function (say, >4 electron volts) will result in the generation of photocurrent only for illumination at wavelengths shorter than 300 nm, i.e., solar-blind response. Examples of photocathode materials with acceptable work functions are CsTe, and CsI diamond films.

[0211] Photoconductor

[0212] Illumination of some semiconductors and/or insulating materials produces an effect closely related to the photoelectric effect: interaction between photons and electrons in the material excite electrons such that they are no longer bound to a specific site in the material (i.e., in the valence band), but are free to move through the crystal in response to an applied voltage (i.e, the conduction band). Again, a current is generated proportional to the flux of photons striking the material. This effect exhibits a wavelength cutoff identical to that given in the above equation, with the exception that the material work function, Φ, is replaced by a different property usually called the band gap, E_g. Again, a material with sufficiently high band gap will respond only to short wavelength radiation. Diamond, with a 5.48 eV band gap, has a photoconductive response at wavelengths less than 200 nm.

[0213] Photodiode

[0214] Illumination of a diode junction (Schottky, p-n, or p-i-n doped semiconductor) can also promote electrons from the valence band to the conduction band, generating a photocurrent. The diode may be forward biased, in which case its performance is very similar to the photoconductor except that the charge carrier lifetime is longer. When reversed biased, the applied field improves the device response speed. Once more, a junction with sufficiently high band gap will allow response only at UV wavelengths.

[0215] One way to greatly suppress the red response of a low-bandgap photoconductor or diode as compared to its UV response is by appropriate choice of the device thickness. It is characteristic of such materials that penetration of UV photons into the depth of the material is much less than that of visible light. That is to say, complete absorption of the UV light occurs in a much thinner layer near the surface. By choosing device thickness to be comparable to the absorption depth of the UV light (at some cutoff wavelength), one can make the device relatively transparent (and thus unresponsive) to longer wavelengths. While not truly solar-blind, such constructions can diminish the visible/IR response of the device below the threshold of detectability.

[0216] Preferred off-the-shelf solar-blind phototube detectors are available from suppliers such as Hamamatsu, Ltd. with offices in Los Angeles, Calif. and in Japan. Its Model R1187 using CsI has a spectral response in the range of 115 nm to 200 nm. Models R1107 and R763 use CsTe and have a best response range somewhat above 157 nm but could be used.

[0217] FIG. 18 shows a solar-blind detector being used in a laser system to keep the pulse energy and/or dose energy controlled to a desired level. (See prior section for details of controlling pulse and dose energy.)

[0218] The sampled beam (Red and VUV) is reflected off several mirrors with dielectric coatings designed to produce high reflectivity at the (157 nm) VUV wavelength and low reflectivity in the red/infrared (the undesired wavelengths span a range from 635 nm to 755 nm). A typical optic might have 95% reflectivity at 157 nm and 4% reflectivity in the red/IR, giving an extinction ratio of ˜24:1. The desired extinction mtio (typically 500 or 1000:1) may be obtained through successive incidences on several mirrors. The light transmitted through these mirrors should impinge on a red/IR-absorbing color glass filter, or other light trapping element, to prevent scattered light from finding its way to the detector.

[0219] A dispersive element (prism or graining) may be used to separate the VUV from the red/IR wavelengths, and the photodiode is placed to intercept only the VUV beam. With a grating element, the groove spacing must be chosen such that there is no substantial overlap between higher diffraction orders of the red/IR wavelengths and the UV wavelengths. That is, the grating equation must not be simultaneously satisfied at the design angle theta for the ultraviolet wavelengths and the visible/infrared wavelengths by varying the value of m (m is always an integer, e.g. D sin(theta)=m₁*157 nm≠m₂*755 nm (etc., for all visible/IR emission wavelengths).

[0220] A material which, upon exposure to 157 nm light, fluoresces in the visible or infrared is placed in front of a silicon photodetector. By converting the VUV light to visible/IR light, the enhanced red response of the photodiode is counteracted. If the fluorescent emission wavelengths are substantially different from the red/IR laser emission wavelengths, one may employ a red/IR absorbing filter after the fluorescent converter to directly suppress the red/IR emission. The fluorescent converter might also be constructed or doped with such an absorber material.

[0221] Prior art F₂lasers typically utilize helium as the buffer gas. It is known that neon can be used but the efficiency of the laser with the Ne buffer is greatly reduced as compared to the F₂laser using He as a buffer. Generally, the use of Ne as a buffer requires substantially higher concentrations of F₂and also higher voltage to produce equivalent laser pulse energy. Helium is much cheaper than neon, therefore, in the prior art He has been the natural choice for the F₂buffer gas. However, as stated above the F₂laser with the helium buffer produces a significant proportion of visible red and infrared light so that super fluorescence or even lasing occurs at the wavelengths. Normally the contribution is high enough so that lasing actually occurs at these longer wavelengths. These longer wavelengths in the laser beam can cause problems with regard to energy detection as discussed above and also the red wavelengths can cause problems in a downstream lithography system. These problems can be dealt with but a better solution in many cases is to substantially reduce or eliminate the red and infrared light from the beam as it is being created in the laser.

[0222] Applicants have described that the red and near-infrared light in the beam is in fact eliminated when the buffer gas is changed from pure helium to an appropriate mixture of helium and neon. However, in addition to substantially reducing the undesirable red and near-infrared components, the addition of Ne affects the efficiency of the laser at the desired UV wavelength. Therefore the best mixture of He and Ne may be different depending on the importance of maximum pulse energy and the relative importance of eliminating the red and near-infrared light. Normally, the preferred range of Ne as a percent of the total buffer quantity will be from about 40% to 95%. As indicated in FIG. 21, a good buffer gas mixture range providing substantially zero Red-1R but maximum UV pulse energy is between 0.52 and 0.63 neon and the rest helium.

[0223] FIG. 11A shows a preferred single line configuration for a preferred F₂laser system. In this configuration one of two major F2 lines is selected with a simple prism selector as shown in the figure. FIG. 11B shows a preferred line narrowed system in which a power oscillator is seeded by a master oscillator. Both the master oscillator and the power oscillator may utilize the same laser chamber. An etalon output coupler may be used to line narrow the F₂laser using the technique described in patent application, Ser. No. 09/407,120, filed Sep. 27, 1999 and incorporated herein by reference. With this technique, the etalon output coupler is adjusted to reflect about 20% of the light at one of the two principal lines and to transmit light at the other principal F₂line. The laser will then lase at the line which is reflected.

[0224] FIGS.19 and 20 describe a tunable fluorine laser with increased tuning range. The laser resonator consists of an output coupler mirror 32 and a wavelength tuning optical setup. The wavelength tuning optical setup consists of a beam expanding optic subassembly 31 and a diffraction grating 38. The optical gain medium is created within a gas discharge chamber 30 by means of excitation of fluorine gas molecules in a gas discharge. Similar configurations are currently used in narrow band exciplex lasers for lithography operating at longer wavelengths, i.e., the KrF at 248 nm and the ArF laser at 193 nm. The major difference between these lasers is the spectral gain distribution and temporal gain dynamic of the gain medium. Whereas the exciplex molecules typically show gain in a much broader spectral region of at least several 10 pm, the spectral gain distribution of the observed molecular fluorine transitions at approximately 157 nm is, in accordance with recent measurements of multi-line and single-line laser emission spectra, commonly considered to be restricted to about 1.1 pm. However, a closer view reveals that this is not true, since the observed spectrum does not necessarily reflect the true gain distribution of the gain media, i.e., when high specific gain values are involved. Due to the high specific gain of the molecular fluorine laser media and the typical used length of the gain medium of about 0.5 m to 1 m, the observable emitted spectrum gets narrowed and peaked up at the maximum of its natural transition wavelength already within a single path through the gain medium. This physical well-understood behavior is described in many standard laser handbooks, i.e., in A. E. Siegman, “Lasers”, Univ. Science Books, Mill Valley, Calif. Unfortunately, the necessary gain-length product to achieve line-narrowed operation down to a bandwidth which is useful for lithography with dioptic imaging optics using this simple approach requires unreasonable amplification lengths of at least several meters.

[0225] The real available FWHM (Full Width Half Maximum) gain bandwidth of any of the individual UV/VUV molecular fluorine laser transition can be estimated to be 4 pm at 157.6299 nm at typical operating conditions of the gas discharge that generates the gain by exciting fluorine molecules. The optical resonator of the laser and thus the emission wavelength can be tuned over a range of about the FWHM bandwidth of any single transition of the UV/VUV molecular fluorine laser. Since the major contribution to the FWHM gain bandwidth is pressure-induced collision broadening, the tuning range can easily extend by increasing the pressure within the discharge chamber. Almost continuous tuning over an extended range of wavelengths (approximately 50 pm) can be achieved if the adjacent, by approximately 7 pm separated, rotational line transitions of the 157 nm fluorine molecule transition overlap due to pressure broadening of the individual transitions at elevated pressures. FIG. 20 depicts the spectral gain distribution in the vicinity of the strong line at 157.6299 nm at elevated pressures.

[0226] Of course, increasing the laser chamber pressure beyond the typical pressures of about 3 atmospheres used in prior art excimer lasers and can increase the cost of the chamber. However, Applicants propose that pressures in the range of about 7 to 10 atmospheres can be dealt with without major cost increases and would produce substantial broadening of the tuning range.

[0227] The two principal spectral lines of the F2 laser are centered at about 157.630 nm and 157.523 nm. The 157.630 nm line is typically substantially larger than the 157.523 nm line in a free running laser. However, the 157.523 nm is generally large enough to be a problem in many applications. Applicants have discussed the above methods external of the gain medium for discriminating against the smaller line. Applicants have discovered that by reducing the length of the discharge region from about 21.5 inches to about 14 inches and reducing the discharge voltage, the laser could be forced to lase only at 157.630 nm and the 157.523 line does not appear.

[0228] Although this F₂laser system has been described with reference to a particular embodiment, it is to be appreciated that various adaptations and modifications may be made. For example, many alternative embodiments are discussed in the patent applications listed in the first sentence of this specification, all of which have been incorporated herein by reference. An etalon output coupler could be used to provide additional line narrowing. The buffer gas could be neon instead of helium. The invention is to be limited only by the appended claims.