Injection seeded F2 laser with line selection and discrimination

[0002] This Application is a Continuation-In-Part of Ser. No. 09/855,310, filed May 14, 2001, Ser. No. 09/848,043 filed May 3, 2001, “Injection Seeded Laser with Precise Timing Control”, Ser. No. 09/829,475 filed Apr. 9, 2001, “Injection Seeded F₂Laser With Pre-Injection Filter”, Ser. No. 09/473,795 filed Dec. 28, 1999 now U.S. Pat. No. 6,381,257, “Very Narrow Band Injection Seeded F₂Lithography Laser”, Ser. No. 09/459,165 filed Dec. 10, 1999 U.S. Pat. No. 6,370,174, “Injection Seeded F₂Lithography Laser”; 09/438,249 filed Nov. 12, 1999 U.S. Pat. No. 6,330,260, “F₂Laser with Visible and IR Control”; Ser. No. 09/421,701, filed Oct. 20, 1999 U.S. Pat. No. 6,359,922, “Single Chamber Gas Discharge Laser with Line Narrowed Seed Beam”, and Ser. No. 09/407,120 filed Sep. 27, 1999, “Line Narrowed Laser with Etalon Output Coupler”, now issued as U.S. Pat. No. 6,240,110. This invention relates to lasers and in particular to injection seeded lasers useful for integrated circuit lithography.

[0003] KrF excimer lasers are the state of the art light source for integrated circuit lithography. Such lasers are described in U.S. Pat. No. 4,959,840, U.S. Pat. No. 5,991,324 and U.S. Pat. No. 6,128,323. 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. 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, increasing the discharge voltage and modifying the controls and instrumentation to accommodate the slightly different wavelength.

[0004] A typical prior-art KrF excimer laser used in the production of integrated circuits is depicted in FIGS. 1, 1A and 1B. A cross section of the laser chamber of this prior art laser is shown in FIG. 1B. As shown in FIG. 1A, 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 to 2000 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 to 2,000 cm 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 (sometimes referred to as a line narrowing package or LNP). 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:

[0005] Laser Chamber Module,

[0006] High voltage power supply module,

[0007] High voltage compression head module,

[0008] Commutator module,

[0009] Output Coupler Module,

[0010] Line Narrowing Module,

[0011] Wavemeter Module,

[0012] Computer Control Module,

[0013] Gas Control Module,

[0014] Cooling Water Module

[0015] 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. 1B. Flow is counter-clockwise in this view. One corner 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 bank 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, one or two preionizers 56 weakly ionize the lasing gas between electrodes 6A and 6B and as the charge on capacitors 19 reaches 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 to 2 cm per millisecond, created by blower 10, is sufficient to provide fresh laser gas between the electrodes in time for the next pulse occurring one half to one millisecond later.

[0016] 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. 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.

[0017] A well-known technique for reducing the band-width of gas discharge laser systems (including excimer laser systems) involves the injection of a narrow band “seed” beam into a gain medium. In one such system, a laser called the “seed laser” or “master oscillator” is designed to provide a very narrow laser band beam and that laser beam is used as a seed beam in a second laser. If the second laser functions as a power amplifier, the system is typically referred to as a master oscillator, power amplifier (MOPA) system. If the second laser itself has a resonance cavity, the system is usually referred to as an injection seeded oscillator (ISO) and the seed laser is usually called the master oscillator and the downstream laser is usually called the power oscillator.

[0018] In gas discharge lasers of the type referred to above, the duration of the electric discharge is very short duration, typically about 20 to 50 ns (20 to 50 billions of a second). Furthermore, the population inversion created by the discharge is very very rapidly depleted so that the population inversion effectively exists only during the discharge. In these two laser systems, the population in the downstream laser must be inverted when the beam from the upstream laser reaches the second laser. Therefore, the discharges of the two lasers must be appropriately synchronized for proper operation of the laser system. This can be a problem because within typical pulse power systems there are several potential causes of variation in the timing of the discharges. Two of the most important sources of timing variations are voltage variations and temperature variations in saturable inductors used in the pulse power circuits. It is known to monitor the pulse power charging voltage and inductor temperatures and to utilize the data from the measurements and a delay circuit to normalize timing of the discharge to desired values. One prior art example is described in U.S. Pat. No. 6,016,325 which is incorporated herein by reference. There in the prior art timing errors can be reduced but they could not be eliminated. These errors that ultimately result are referred to as “jitter”.

[0019] A typical KrF laser has a natural bandwidth of about 300 pm (FWHM) centered at about 248 nm and for lithography use, it is typically line narrowed to about 0.6 pm. ArF lasers have a natural bandwidth of about 500 centered at about 193 nm and is typically line narrowed to about 0.5 pm. These lasers can be relatively easily tuned over a large portion of their natural bandwidth using the line narrowing module 18 shown in FIG. 2. Also for the KrF and ArF lasers, the absolute wavelength of the output beam can be determined accurately by comparing its spectrum to atomic reference lines during laser operation. F₂lasers typically produce laser beams with most of its energy in two narrow lines centered at about 157.63 nm and 157.52 nm.

[0020] Often, the less intense of these two lines (i.e., the 157.52 nm line) is suppressed and the laser is forced to operate at the 157.63 nm line. The natural bandwidth of the 157.63 nm line is pressure dependant and varies from about 0.6 to 1.2 pm. An F₂laser with a bandwidth in this range can be used with lithography devices utilizing a catadiophic lens design utilizing both refractive and reflective optical elements, but for an all-refractive lens design the laser beam should have a bandwidth to produce desired results. It is also known that the centerline wavelength of the output beam will vary somewhat depending on condition in the discharge region.

[0021] Lasers for lithography equipment are very complicated and expensive. Further reduction in bandwidth could greatly simplify the lens design for lithography equipment and/or lead to improved quality of integrated circuits produced by the equipment. Thus, a need exists for lithography lasers (including KrF, ArF and F₂lasers) with substantially reduced bandwidth.

[0022] The wavelength of KrF and ArF lasers is relatively easily controlled over ranges of a few hundred picometers corresponding to their natural bandwidths. The F₂laser or the other hand has in part been considered untunable since the a large portion of its output is concentrated in two narrow lines. Several techniques have been prepared for selecting one of the lines and eliminating energy in the other line.

[0023] In many line selected laser systems, such as the KrF and ArF systems, a buildup in a narrow frequency band suppresses buildings in other spectral regions. However, weak interaction between the various laser frequencies may reduce the effect of this mechanism. F₂laser systems provide very large gains allowing substantial intensity build-up or merely a single pass through the gain medium. Initial light levels make originals as so-called spontaneous emissions at any potential laser line. When these spontaneous emissions are amplified, the light is referred to as amplified spontaneous emissions (ASE) and it may become a part of the output laser beam reducing the quality of the beam. In the case of the F₂laser many efforts have been made to suppress the 157.52 nm line while maintaining efficient production of the desired 157.63 nm line. For use as a lithography light source, there is a desire to reduce the intensity of the 157.52 nm line to less than 01.% of the 157.63 nm line.

[0024] What is needed is a better F2 laser system in which the 157.52 nm line is suppressed to insignificance.

[0025] The present invention provides a narrow band F₂laser system having two laser subsystems. The first laser subsystem is configured to provide a very narrow band pulsed beam at a first narrow wavelength range corresponding to a first natural emission line of the F₂laser system. This beam is injected into the gain medium of the second laser subsystem in a first direction where the beam is amplified to produce a narrow band pulsed output beam. The seed laser subsystem also produces a second pulsed beam at a second wavelength range corresponding to a second natural emission line of the F₂laser. This line is injected into the gain medium of the second laser subsystem in a second direction opposite said first direction. The second beam is amplified in the gain medium of the second laser subsystem depleting the gain medium of gain potential at the second wavelength range. (This amplified second beam is preferably wasted.) With the gain potential at the second undesired wavelength the range thus reduced the portion of light at the second wavelength range in the output beam is greatly reduced. In a preferred embodiment a pulse power supply is provided which is specially configured to precisely time the discharges in the two laser subsystem so that the discharges are properly synchronized, and laser gas comprises F₂at a partial pressure less than about 1% with a buffer gas comprised of helium or neon or a combination of helium and neon. Control of center wavelength of the output beam may be provided by adjusting one or more of the following parameters in the first laser: the total laser gas pressure, the relative concentration of helium or neon, F₂partial pressure, laser gas temperature, discharge voltage and pulse energy.

[0026] For precise jitter control in preferred embodiments include a pulse power system with a pulse transformer unit having two sets of transformer cores. A single upstream pulse compression circuit provides high voltage pulses in parallel to the primary windings of all of the cores in both sets. Separate secondary conductors (one passing through one set of cores and the other passing through the other set of cores) provide very high voltage pulses respectively to separate downstream circuits supplying discharge pulses to the electrodes in each of two separate laser chambers. In preferred embodiments line narrowing is accomplished within the resonant cavity of the seed laser and/or the output of the seed laser could be line narrowed using a pre-gain filter.

[0027] FIG. 1 is a drawing of a prior art commercial excimer lithography laser.

[0028] FIG. 1A is a block diagram showing some of the principal elements of a prior art commercial excimer lasers used for integrated circuit lithography.

[0029] FIG. 1B is a drawing of the laser chamber of the FIG. 1 laser.

[0030] FIG. 2 is a block diagram showing features of the present invention.

[0031] FIG. 3 is an electrical circuit drawing showing features of a preferred embodiment of the present invention.

[0032] FIG. 3A is a drawing of a pulse transformer.

[0033] FIG. 3B is a drawing showing features of FIG. 3A.

[0034] FIG. 3C is a modification of the FIG. 3 circuit providing an adjustable delay.

[0035] FIG. 3C1 is a B-H curve.

[0036] FIGS. 3D and 3E show alternate delay adjustment technique.

[0037] FIG. 3F shows pulse shapes.

[0038] FIGS. 4 and 5 are black diagrams of F₂laser systems.

[0039] FIGS. 6 and 6A show features of a first grating monochromator.

[0040] FIG. 7 shows features of a second grating monochromator.

[0041] FIG. 8 shows features of an etalon filter.

[0042] FIGS. 9 and 10 are diagrams of power gain stages.

[0043] FIGS. 11, 11A, 11B, 11C and 11D show features of a pulse power system.

[0044] FIG. 12 shows a pulse energy detector in a feedback control system.

[0045] FIGS. 13 through 17 show preferred embodiments of the present invention.

[0046] In order to understand and characterize the effect of variations in various F₂laser operating parameters on the centerline wavelength and bandwidth of F₂laser systems, Applicants have conducted careful experiments and have determined precisely absolute values of the two major lines of the F₂laser beam spectrum. Applicants have also determined that the centerline wavelength of the F₂laser can be controlled to some extent by controlling laser operating parameter, especially laser gas pressure. Bandwidth can also be controlled to an extent by controlling these parameters.

[0047] A summary of all this test data is set forth below in Table I.

[0048] Applicants' summary conclusion from these tests are:

[0049] 1) The determination of a “zero pressure” wavelength and pressure shift coefficients allows for a universal specification of the central wavelength.

[0050] 2) The central wavelength can be inferred to sufficient accuracy that a wavemeter is not needed.

[0051] 3) Variability in the central wavelength of the F₂laser, due to normal changes in laser operating parameters, appears limited to ±0.02 pm

[0052] 4)

[0053] FIGS. 2 through 11D describe various techniques for designing and operating injection seeded F₂laser systems. An F₂injection seed light source may be a conventional F₂laser, using either a plane-parallel optical resonator, or an unstable resonator configuration. The pulse power supply for both laser subsystems is preferably provided utilizing one of the techniques described above. This assures that the timing of the discharge of each laser subsystem is adequately synchronized. In a preferred F₂embodiment the seed laser beam is filtered downstream of the seed laser. The seed laser preferably will generate enough energy such that, after filtering, 10-100 μJ of narrow-band energy is available for seeding the F₂power gain stage. An unstable resonator will produce a lower divergence, more spatially coherent beam than a stable resonator, which may be of some advantage in coupling energy through the injection spectral filter. For instance, if the filter is a simple monochromator, a lower divergence beam will be more easily focused down to the input slit of the monochromator. Another design option is to operate the first F₂light source laser at relatively low pressure (≈100-200 kPa). This produces a substantially reduced spectral width: 0.3-0.6 pm. A lower spectral width means a greater fraction of the energy entering the post gain filter will make it through the filter. The raw output energy from the first F₂light source will be much lower, but this may not be a practical disadvantage because the maximum energy that the injection filter can handle is similarly limited.

[0054] A conventional F₂laser for use as the first F₂light source in a preferred embodiment a standard KrF lithography laser system modified for operation as a F₂laser. These KrF lithography lasers are well known and there are more than 1,000 of these units operating today in integrated circuit fabrication plants as light sources for integrated circuit lithography machines. These lasers produce laser pulses at rates in the range of 1000 to 2000 pulses per second and are available from suppliers such as Cymer, Inc. with offices in San Diego, Calif. These lithography lasers are described in detail in many patents such as U.S. Pat. No. 5,991,324 and U.S. Pat. No. 6,128,323 both of which are incorporated herein by reference. The major modifications needed for operation as an F₂laser are to change the gas mixture to about 0.1 percent fluorine and the remainder helium (although a neon or a combination of helium and neon could be used) and preferably the upper range of the discharge voltage is increased from about 26,000 volts to about 36,000 volts. A basic prototype F₂laser system used for both the first F₂light source and the power gain stage is described below in the section entitled “F₂Laser System Designs”. That section describes the significant improvements to the prior art KrF laser system to produce an F₂laser.

[0055] One technique for producing a narrow band laser beam is to filter the output of the seed laser beam prior to injecting it into the amplifier stage. A conventional grating-monochromator pre-power gain filter is described by reference to FIG. 6. This filter, when used with a free-running F₂laser as the master oscillator, preferably must slice out a 0.1 pm bandwidth portion of the free-running spectrum from the master oscillator, and be capable of producing the 10-100 μJ of narrow-band energy required by the following amplifier stage. Light from the master oscillator is first focussed down onto an input slit 50. Light passing though the input slit is collimated preferably by a curved mirror 54, which may be a simple spherical mirror, or an off-axis paraboloid, and the collimated light is directed to a grating 54. The grating is a high dispersion type (e.g. an echelle grating) chosen to disperse light in the 157 nm wavelength range. The grating is in the Lithrow configuration. Light at a selected very narrow range which is reflected back along the beam path 54 is re-imaged on an exit slit 57 with the aid of beam splitter 56. The various geometric and optical parameters of the arrangement (i.e. slit widths, grating dispersion, curved mirror focal length) determine the bandwidth of the light leaving the exit slit. One design problem that must be overcome is the high peak intensities that are reached at the input and exits slits when one attempts to couple the desired amount of energy through the monochromator. One method to handle these high intensities is to use refractive slits, i.e. knife edge wedges that refract the unwanted light into another direction, without absorbing the energy. Such a slit arrangement is shown in FIG. 6A.

[0056] In addition to its function as a filter, the arrangement of FIG. 6 includes an additional beamsplitter 58 and a linear detector array 60 placed at an exit image plane. This addition solves an important remaining problem: how to maintain the (tunable) injection filter at the desired wavelength. If the grating angle is in error by more than a few 10's of micro-radians, the seed beam will miss the exit slit, and there will be no narrow-band energy to lock the following power amplifier stage. If the angle is very slightly incorrect the amplified spectrum will not be at the desired wavelength. Fortunately, the monochromator can, in essence, monitor itself. The image formed by the grating and curved mirror is the dispersed spectrum of the light entering the monochromator. The second beam splitter produces two identical images (spectra), one at the exit slit, and one at the linear detector array. The linear detector array senses the relatively broad spectrum from the free-running master oscillator and converts it into a video signal representative of the spectral intensity at each point on the array. Since the free-running wavelength is stable and well-known, it serves as a calibration standard for the monochromator. A controller 66 reads out the linear detector array, and adjusts the grating angle so as to place the image of the spectrum centered at a desired point on the array, near the center for example. In this way the monochromator is self-stabilized to the free-running spectrum of the master oscillator. Since the exit slit and linear array are basically duplicate image planes, the exit slit position corresponds to a particular position, and hence wavelength, on the linear array. Therefor, with the free-running spectrum from the master oscillator as a spectral reference, the wavelength of the light leaving the exit slit can be precisely determined.

[0057] One method for calibrating this arrangement is to place a beamsplitter 62 in the path of the beam exiting the monochromator and to monitor the beam energy with energy detector 64. Such a detector is desirable in any event, since the energy of the injection seed needs to be monitored.

[0058] A calibration sequence would proceed as follows: (1) With the grating at a starting angle, the laser is fired and the output energy from the exit slit is monitored, along with the spectral image falling on the linear array. The peak of the spectrum is determined in terms of the position, in pixels, on the array. (2) The grating angle is incremented and the measurements are repeated. (3) After the grating angle is scanned though a range, the resulting data is examined. The position of the spectrum on the array (in pixels) where the output energy maximizes corresponds to the equivalent position of the slit.

[0059] Once this calibrated position is determined, the known dispersion of the monochromator can be used to retune the grating to other wavelengths. For instance, suppose the monochromator dispersion is 0.1 pm/pixel, and further suppose the calibrated position of the exit slit is pixel 300. If the desired wavelength of the output is 157.6299 nm (157,629.9 pm), the center of the free-running spectrum, then the grating angle is adjusted so that the center of the image falls at pixel 300. If the desired wavelength is ±0.2 pm away from the center (157.6301 nm), then the grating would be moved so that the center of the image would fall at pixel 302. A further refinement is to include the pressure of the master oscillator in the calibration and subsequent use of the monochromator, since the center of the free-running spectrum is pressure dependent. This pressure dependence must be included in the calibration, especially if the pressure of the master oscillator is allowed to vary significantly. We have determined that the center wavelength of the free-running laser has a pressure shift coefficient in the range of about 1 to 2 fm/kPa, when helium is used as a buffer. For any given pressure, a good estimate of the wavelength is therefor 157.6309+0.00000192* P nm, where P=pressure in kPa. Other pressure shift coefficients can be used if other buffer gasses are used (neon for instance, or mixtures of helium and neon).

[0060] An alternate method for producing the narrow-band light is with a modified grating monochromator as shown in FIG. 7. This filter filters the output beam of a master oscillator which produces a well-collimated, nearly diffraction limited coherent beam, and in this case we eliminate the entrance slit of the monochromator.

[0061] A beam expander 70 is used to reduce the divergence from the master oscillator and to physically match the size of the beam from the master oscillator to grating 54. The dispersed light from the grating is focussed via a curved mirror (or lens) to an exit slit 72 where the desired wavelength is selected. The operation of the linear detector array 60 and controller 66 is the same as previously described. The advantage of this arrangement is that it eliminates the need for an entrance slit and the associated problems with high peak intensities. This arrangement has a disadvantage in that the pointing stability of the master oscillator is now a factor in the position of the image on the array, and hence the wavelength selection process. For slowly varying changes in the input angle from the master oscillator, the controller can retune the grating and keep the wavelength constant.

[0062] An etalon 78 can also be used as a bandpass filter as shown in FIG. 8. As with the monochromator filters, it is desirable that the etalon be self-referenced to the free-running spectrum of the master oscillator, which is used as an atomic standard. The beam from the master oscillator is first expanded with beam expander 70, both to lower its divergence and to reduce the power density on the etalon. After expansion, the beam passes through a special “partial diffuser” 74, an optical element which transmits most of the light unaltered, but scatters a small faction into a range of angles. Examples of this are diffractive optics with low diffraction strength, or very lightly and finely ground optical flats. The light then passes through the etalon at near normal incidence. The etalon's bandpass characteristics are determined by its free spectral range (FSR) and finesse. For instance, an etalon with an FSR of 2 pm and a finesse of 20 will have a bandpass of 0.1 pm FWHM. The etalon will then transmit a 0.1 pm slice of the spectrum from the free-running master oscillator. As with the monochromator, it may be advantageous to operate the master oscillator at low pressure, and hence reduced bandwidth, in order to limit the power loading on the etalon. In addition, a narrower starting spectrum will reduce the amount of energy in the adjacent transmission orders of the etalon (±1 FSR from the central wavelength). After passing through the etalon, a lens 76 (or curved mirror) focuses the light to a point, where an aperture is placed. A portion of the beam is split off by beam splitter 80 and monitored by a photodiode array 82 which provides center wavelength and bandwidth signals to controller 66 which uses this information to control the etalon 78. The purpose of the aperture is to block all light except the on-axis, non-diffused component of the beam. This light, which is narrow-band, is then sent on to the power amplifier stage. A beam splitter and optical detector follows the aperture, to monitor the energy leaving the injection filter.

[0063] In general, the central wavelength of the etalon's bandpass will not align with the center of the spectrum of the free-running master oscillator. The etalon needs to be tuned. Four methods are possible, depending on whether the etalon is a solid plate type or an air-spaced type. For solid etalons, the etalon can be tilted with respect to the incoming beam, or the temperature of the etalon can be varied, which effectively changes the optical thickness of the plate. For air-spaced etalons, the angle can be varied, the plate spacing can be varied (by PZT actuators, for instance), or the index of refraction of the gas between the plates can be varied by changing the density of the gas.

[0064] In a preferred embodiment, the etalon is an air-spaced type which is enclosed in a pressure-tight housing 84 as indicated in FIG. 8. A pressure controller is used to vary the pressure of the gas within the housing (at constant temperature), thereby pressure tuning the etalon. Since the amount of tuning required is very small (about ±0.2 to ±0.5 pm), the pressure change that is needed is also very small, about ±3 to ±8 torr with nitrogen as the gas. This can be achieved either by changing the volume of the (sealed) housing, or by actively introducing or withdrawing gas from a suitable supply. As the etalon is pressure tuned, the output intensity will alternately increase and extinguish as the bandpass wavelength sweeps through the spectrum from the free-running master oscillator.

[0065] Control over the tuning of the etalon is attained by including additional optical elements that turn the etalon into its own etalon spectrometer. An additional beamsplitter is located prior to the exit aperture, and forms a second focal plane of the lens. In this focal plane (as in the first), the intensity distribution consists of an intense spot at the center of focus plus a much weaker conventional etalon ring pattern. The intense spot is formed by the non-diffused portion of the beam passing through the etalon, whereas the etalon ring pattern is formed by the diffused portion of the beam. The intense central spot is not used here and is blocked with a beam stop 81. A linear detector array 82 is then placed in the focal plane to read out the etalon ring pattern. This arrangement is very similar to the design of current wavemeters used in lithographic lasers. For a given optical arrangement, there is a direct relationship between the diameters of the etalon rings and the central wavelength of the etalon bandpass.

[0066] Calibration of the arrangement is done in a manner similar to the procedure previously described for the monochromator filter. A calibration sequence would proceed as follows: (1) With the etalon pressure-tuned to a starting wavelength, the laser is fired and the output energy from the exit aperture is measured, along with the etalon ring pattern falling on the linear array. The diameter of the innermost, fully formed ring is determined. (2) The pressure controller increments the pressure in the etalon, and the measurements are repeated. (3) After the etalon is pressure-tuned through one free-spectral range, the resulting data is examined. The diameter of the innermost ring where the output energy maximizes corresponds to the condition where the bandpass of the etalon is exactly tuned to the peak of the free-running spectrum from the master oscillator which is pressure dependent.

[0067] Once this calibrated diameter is determined, the wavelength of the etalon bandpass filter can be stabilized by varying the pressure in the etalon so as to maintain this diameter. As a further refinement, the etalon ring pattern can be converted directly into a “wavelength” by employing the same non-linear etalon equations used in the lithographic wavemeters. This allows the bandpass function to be detuned from the peak of the master oscillator spectrum by a known amount. A further refinement is to include the pressure of the master oscillator in the calibration and subsequent use of the etalon, as has been previously described.

[0068] Normally the narrow spectral band (of about 0.1 pm or less) chosen for operation of the laser system will be the narrow band meeting desired bandwidth specifications with the maximum output pulse energy. However, a limited amount of wavelength tuning is possible with the pregain filters described above. Applicants expect that a tuning range of at least about 1.2 pm can fairly easily be achieved. Additional tuning is feasible with some significant compromise of output pulse energy. The tuning range is a function of laser gas pressure as indicated above. Therefore a longer overall range can be achieved by adjusting the pressure in the laser.

[0069] Two preferred power gain stages can be described by reference to FIGS. 9 and 10.

[0070] The power gain stage can be configured as a power oscillator as shown in FIG. 9. Many different resonators can be used for the PO design, depending on the desired output. In a preferred embodiment, the resonator is an off-axis hybrid unstable resonator, formed by two split-coated mirrors, all as shown in FIG. 9.

[0071] The injected seed beam 90 is aligned to a central axis along the top of an unstable resonator 92, and first travels through a 50% partial reflector 93A. The rear resonator optic 94 is a zero power meniscus type, which does not disturb the collimation of the injected beam. The injected beam fills the Fresnel core of the resonator, establishing control over the intercavity field (the Fresnel core in this case is the volume formed between 50% R mirror 93A and 100% R mirror 94A). After propagating with amplification to the front optic, the beam is reflected from the 100% reflective, convex surface. The beam expands and is amplified as it travels to the rear optic, where a portion reflects off of the 100% reflective, concave surface 93B and a portion reflects off surface 93A. This re-collimates the beam, which is further amplified by a third pass through the gain. The lower portion 94B of the output coupler is anti-reflective coated, allowing the beam to exit with minimal loss. As with the rear optic, the front optic is a zero-power meniscus type, to preserve the collimation of the output beam. This type of a resonator forms a power oscillator because the 50% and 100% reflective surfaces provide feedback into the Fresnel core of the resonator. The advantage of this type of resonator is (1) there is no central obscuration or hole in the beam, and (2) it requires very little seed energy to lock the power oscillator to the seed.

[0072] A power gain stage in the form of a power amplifier is shown in FIG. 10. In this case the resonator is similar to the one shown in FIG. 9 except the feedback has been eliminated by changing the 50% reflective surface 93A to an anti-reflective surface. This configuration produces an off-axis, multipass power amplifier.

[0073] Several prototype F₂laser systems have been built and tested by Applicants and their fellow workers to serve as gain media for both as the first F₂light source, and as the power gain stage.

[0074] These systems are largely 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.

[0075] In these prototype units, for both the first F₂light source and the power gain stage, a modified pulse power system is used and a circuit diagram for the system is shown in FIG. 11 and in FIGS. 2 and 3. The major difference between the pulse power system for these embodiments and corresponding systems for prior art KrF lasers is the pulse transformer 56 which for the F₂laser provides higher output voltage and, as described above, the portion of the circuit upstream of the transformer is a single circuit and the portion downstream of the transformer is a divided circuit. In this pulse transformer the single four-section stainless steel rod (described in U.S. Pat. No. 6,128,323 referred to above) 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. 11A, 11B and 11C. The secondary conductor consists of two coaxial assemblies (the cross-section of which are depicted in FIGS. 11B and 11C) connected with bus bar as shown at 302 and HV cable as shown at 304. FIG. 11D shows the same cross-sections as 11B and 11C 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. 11D 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. 11A, 11B and 11C. 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 36,000 volts as indicated at 318 in FIG. 11A. Thus, each of the two pulse transformer sections feeding the two lasers, is comprised of 12 induction units (instead of the 23 shown in FIG. 3A-B). However, the three secondary conductors passing through the 12 induction units produce voltage amplification of 36.

[0076] 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:

[0077] The modified pulse power system in this prototype embodiment produces an output rise time of about 80 ns into the peaking capacitor bank. The step-up ratio of the pulse transformer is 36× (as compared to the 24× in the embodiment described in detail above). This permits the laser to operate at substantially higher voltages with correspondingly lower F₂concentrations as compared to the unmodified pulse transforms. Applicant has determined that the higher voltage operation improves discharge stability, and permits higher repetition rates. As explained above, in this embodiment two separate transformers are provided each of which are supplied by a primary current from a common source as shown in FIG. 3B but each of the transformers are configured a shown in FIGS. 11A, B, C and D to provide a 36× step up instead of the 24× as shown in FIG. 3B.

[0078] As indicated above, the output of power gain stage of preferred embodiments of the present invention will have ultraviolet bandwidths of about 0.1 pm or less with a line center within the nominal F₂157.63 nm line which covers a spectral range of about ±0.5 pm around the nominal wavelength. As indicated in the following section, a small amount of light energy in other spectral ranges are produced in F₂lasers especially red and infrared light when helium is used. If this red light is a problem, it can be easily eliminated with well know optical filters designed to transmit 157 nm UV light and absorb or reflect away (not back into the laser) the red light. Also, a post output filter of one of the types described above could be added to further line narrow the output beam in the UV range. However, when used as a post output filter, the components of the filter need to be designed to handle a much higher energy beam.

[0079] Preferred embodiments of the present invention utilize a pulse power system configured to control the timing of the discharge of the two laser systems to produce desired output pulse laser beams. These embodiments make use of a fractional turn pulse transformer design similar to fractional turn pulse transformer described in U.S. Pat. No. 5,142,166. In these embodiments the portion of the pulse power circuits for the two laser systems are separate downstream of the pulse transformer system and portions of the pulse power circuits upstream of the pulse transformer system is common for both laser systems.

[0080] FIG. 3 shows an electrical outline of the principal elements of a preferred pulse power system. The portion of the system upstream of the pulse transformer system is very similar to the circuit shown in FIG. 11 which is described in detail U.S. in U.S. Pat. No. 6,151,346 (hereby incorporated by reference). A single bank of capacitors define a charging C₀capacitor bank 42. Electrical pulses are generated by the closing of switch S₁46 which consists of two IGBT switches mounted in parallel L_oinductor 48 holds off current flow through S₁so it can close without any deterioration to charge up C₁capacitor bank 52. L₁saturable inductor holds off substantial current flow through pulse transformer system 56 until capacitor L₁saturates at which time the primary turns of fractional turn pulse transformer system 56 is pulsed with a short about 0.5 microsecond 1000 volt pulse. In this embodiment, transformer system 56 is comprised of two separate transformer units 56A and 56B which in this case are virtually identical.

[0081] Each of the pulse transformer unit 56 is similar to the pulse transformer described in U.S. Pat. No. 6,151,346. The pulse transformer units of the present embodiment has only a single turn in the secondary winding and 23 induction units. The transformer as configured as an auto transformer as shown in FIG. 3B to provide a 1:24 step-up ratio. Each of the 23 induction units 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. 3A. (The negative terminals are the high voltage terminals of the twenty three primary windings.) 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. 5 of U.S. Pat. No. 6,151,346.

[0082] The secondary of each transformer is a single OD stainless steel rod mounted within a tight fitting insulating tube of Teflon® (PTFE). The transformer units are in four sections as shown in FIG. 3A. The low voltage end of stainless steel secondary shown as 56D in FIG. 3A is tied to the primary HV lead on printed circuit board 56B at 56E, the high voltage terminal is shown at 56F. As a result, the transformer assumes an auto-transformer configuration and the step-up ratio becomes 1:24 instead of 1:23. Thus, an approximately −1400 volt pulse between the + and − terminals of the induction units will produce an approximately −35,000 volt pulse at terminal 56F on the secondary side. A 1000 volt primary pulse produces a pulse on the secondary sides of both transformers of about 24,000 V. This single turn secondary winding design provides very low leakage inductance permitting extremely fast output rise time.

[0083] The general configuration of the pulse transformer system is shown in FIG. 3B. As indicated in this figure, the primary high voltage pulse of about 1000V produced by the upstream portion of the pulse power system arrives at each pulse transformer at exactly the same time and as the corresponding output pulse of each of the transformers will therefore be substantially identical in shape and time. Applicants estimate that the jitter at the output of the two transformer will be less than one nanosecond.

[0084] As indicated in FIG. 3 in this embodiment the portion of pulse power circuits downstream of the pulse transformers are separate but substantially equal so that the jitter at electrodes 83 and 84, A and B, is estimated to be less than 3 ns. Therefore, the gain medium in both lasers is produced at the same time with a variation of less than about 3 ns. The duration of each of the pulses is about 20 to 50 ns so that the laser pulse produced in the first laser is properly amplified in the second laser. Preferably, the circuit is provided with a bias circuit to bias all saturable inductors so that they are reverse conducting prior to each pulse. The bias circuit is designed so that during a short period immediately after the pulse the saturable inductors remain forward conducting so that pulse energy reflected from the electrodes can be recovered as explained in detail in U.S. Pat. No. 5,729,562.

[0085] In preferred embodiments of this invention, the output coupler of the first laser is located about one foot downstream of the input window of the second laser. Therefore, for this reason or for other reasons, it may be desirable to delay the discharge of the second laser as compared to the first laser. Since the electrical pulse travels through a good conductor at a rate of about ins/20 cm, this can easily be accomplished by making a conductor carrying the pulse of the second laser longer (for example, by 20 to 40 cm) than a corresponding conductor for the first laser.

[0086] Another approach to control the timing of the discharge in one laser relative to the other is to insert a saturable inductor in the circuit shown in FIG. 3 in one of the branches downstream of transformer 56 such as at location 63 shown in FIG. 3C. This saturable inductor is fitted with an adjustable forward bias. The forward bias which is applied is chosen so that the time to complete the forward saturation of the inductor is approximately equal to the desired delay time. The delay time is a function of the number of turns in the saturable inductor, the cross-section of its magnetic core and magnetic flux swing _ΔB of the inductor. Since the required delay is very small the number of turns can be one and the core can be small (such as 2 inch diameter) and the flux swing _ΔB can also be small as indicated in FIG. 3C1. By adjusting the bias the relative delay can be adjusted. The delay control could be incorporated into a feedback loop design to control jitter. Since the delay expected to be required for an oscillator/amplifier configuration is small (on the order of ns or 10's of ns), the delay reactor can be made small. In addition, the core material can be selected to minimize the losses introduced in the circuit, again since the volt-second requirement is likely to be much smaller than that required in the power pulse compression circuit. Another technique for providing for an adjustable delay is shown in FIG. 3D. In this case, a conductor 201 carrying the pulse to one of the lasers is arranged in a single loop coil 202 and a rod 203 having high permeability arranged to be movable into and out of the coil. The rod can be positioned with a fast drive such as a stepper motor or a piezoelectric driver.

[0087] Still another way of providing an adjustable delay is shown in FIG. 3E. In this technique, pulsed current source 86 is used to provide a secondary pulse current for inductor L₄. This current source 86 is triggered by a trigger circuit 84, which also closes the switches. As shown in FIG. 3F pulse I_sstarts at about the same time as switch S closes and lasts a time t_dafter the main compressed current pulse propagates to capacitors C₄and C₄′. t_dcan be about 10 us. The secondary pulsed current serves to provide a saturation for inductor L₄. By changing this current, a saturation point on BH curve of inductor L₄can be changed, causing the delay to change as well. The corresponding inductor in the amplified circuit, L₄is baised with a non-adjustable bias current I_bias. Other embodiments of this technique are also possible, such as providing two pulsed current sources for inductors L₄and L₄′.

[0088] Many variations of the pulse transformers configuration shown in FIG. 3B are possible. The preferred output voltage for the two laser systems may not be the same. If different voltages are needed this can be easily accomplished by providing a smaller or larger number of induction units for one of the transformers relative to the other one. Also, switches could be included in one of the transformers to cut out some of the induction units to reduce the discharge voltage output of that transformer relative to the other one. Taps could be provided between any of the four transformer sections of either transformer to take off the secondary voltage at reduced levels.

[0089] Applicants have done extensive testing to explore techniques for centerline wavelength control of F₂lasers systems as described above. Applicants have determined that in a master oscillator, power amplifier configuration where the seed beam is in the range of 20 μJ to 50 μJ or greater the center wavelength and the band width of the seed beam determines almost exactly the center wavelength and band width of the output beam. Applicants have also measured precisely the effects on center wavelength and band width of laser gas pressure, buffer gas mix, F₂partial pressure, laser gas temperature, discharge voltage and pulse energy. Applicants then use these results to control the center wavelength by varying one or more of the above parameters. The degree of parameter charge necessary to make the centerline wavelength is indicated by the pressure shift coefficients shown in Table I above.

[0090] Applicants believe that the total laser gas pressure will be the more easily used parameter to control centerline wavelength, but the other parameters could be used or a combination of more than one parameter could be used.

[0091] FIGS. 13 through 17 described preferred embodiments of the present invention in which the 157.63 nm line is selected and the 157.52 nm line is actively discriminated.

[0092] FIG. 13 depicts a first embodiment of the present invention. A seed laser 100 is configured as a low-pressure master oscillator for reduced bandwidth, a second laser 102 is configured as an efficient, high-pressure-operated power amplifier. The seed laser 100 employs a resonant cavity comprised of laser chamber 101, partial reflective mirrors 104 and 106, permitting extraction of laser radiation on both ends of the resonant cavity. Prisms 108 and 110 at both ends angular disperse the two dominant laser lines of the F₂laser. The adjustment of these prisms is such as to inject the desired strong laser line at one end of power amplifier 102 via mirror 114 and to inject the undesired laser line at the other end via partial mirror 116. Beam stops 118 prevents the weak line radiation of the seed laser from entering into the entrance of power amplifier 102. Similar, beam stop 120 prevents the strong line radiation of the seed laser from entering into the power amplifier from its output side. With this design, low bandwidth light from the seed laser on the strong laser line will be efficiently amplified in power amplifier 102. In order to reduce the undesired ASE output on the weak line, active gain extraction is provided by means of the corresponding output of the seed laser from the other side. The weak line radiation is amplified within the power amplifier, substantially reducing the potential gain in the spectral region of the weak line, which in turn reduces ASE in this spectral region in the output beam 124. The amplified radiation of the undesired light finally passes through prism 110, where dispersion allows separation from the optical axis of the seed laser. The energy on this wavelength is dissipated in beam stop 122. Obviously, the dispersion of both prisms has to be large enough to ensure sufficient discrimination with the corresponding beam stop. This requires the angular separation to be larger than the intrinsic divergence of the seed laser beam on both ends. Single prisms of CaF₂material, transparent at these VUV wavelengths, can achieve approximately 0.5 mrad angular dispersion between both laser lines, while maintaining reasonable transmission efficiencies. Since the gain in power amplifier 102 is very large, seed laser 100 can be operated with divergence limiting apertures and thus with reduced efficiency. Another method is a master oscillator design with single- or dual axis unstable resonator design, which is known to achieve divergences below 0.5 mrad easily. Multiple prisms could be used instead of single prisms on both ends of the master oscillator to increase dispersion if preferable for a particular implementation.

[0093] FIG. 14 shows a different, diffraction grating based design. Seed laser 126 is again configured as a master oscillator with a resonant cavity comprising high reflective mirror 128 and partial reflector 130. The seed laser output, containing both wavelengths, is directed towards grating 132, which is used to disperse both wavelengths. The undesired wavelength is cleanly separated with a slit 134 and injected into the front of power amplifier 136 gain generator via turning mirrors 138 and 140, the latter preferably is an non-coated CaF₂substrate at Brewster's angle, so that optical losses from the power amplifier throughput are minimized. Since the seed laser radiation is not or only weakly polarized, coupling via Brewster's angle is possible. The amplified light of the undesired wavelength is propagating backward via mirror 142 to grating 132, where dispersion directs it into beam stop 144. The desired wavelength selected with slit 146 and directed into the entrance of the power amplifier via turning mirror 142. The advantage of this particular design is the increased dispersion of the grating, significantly easing the separation of the two wavelengths.

[0094] FIG. 15 shows an embodiment employing polarization dependent optical retardation/polarization rotation. In this embodiment, polarizers 150 and 152, and partial mirrors 154 and 156 form seed laser 158. One of the polarizers might be omitted under situations of moderate gain, i.e. at lower operating pressures, in generator laser chamber 101. A phase retarder 160 is used to rotate the undesired wavelength polarization by 90 degree while keeping the polarization of the desired wavelength unchanged. A subsequent polarizer 162 rejects transmissions of the undesired wavelength as it has perpendicular polarization. Such retardation-based systems, made of bi-refringent materials, such as MgF₂and polarizers, are called Lyot filters and have been used in wavelength separation applications for many years, i.e. in astronomical investigations, employing relatively large beam divergences. The remaining light of the desired wavelength is passed into the entrance of the power amplifier chamber 103 via turning mirror 164 and polarizing beam splitter 166. On the opposite end of the seed laser a similar arrangement separates the weak line from the output. Phase retarder 168 and polarizer 170 select the weak line to be passed into the front end of chamber 103 of power amplifier 176 via mirror 172 and partial mirror/beam splitter 174. The difference in wavelength selection is achieved by rotation of polarizer 170 by 90 degrees. For this reason, the amplified light of the undesired wavelength propagating backwards in the power amplifier has perpendicular polarization with respect to the desired wavelength propagating forward. This feature is used to separate and dissipate backward propagating energy with polarizing beam splitter 166 and beam stop 178 respectively. Advantageous with this design is the inherent polarization of the power amplifier output, which is a result of the polarized input radiation.

[0095] FIG. 16 shows a simplified scheme, which uses only a one-sided phase retarder/Lyot filter between the seed laser 180 and the power amplifier 182. The seed laser consists of laser chamber 101, mirror 184 polarizer 186 and partial mirror 188. The polarized light is objected to phase retarder 190, which separates the two wavelengths in terms of their polarization. While the polarization of the desired laser line is kept unchanged, the undesired is rotated by 90 degree. Both laser lines are directed into the entrance of laser chamber 103 of power oscillator 182 via turning mirrors 192 and 194 which are not required if the longitudinal extension allows lining-up the two gain generator stages. A polarizing beam splitter 196 at the output discriminates the weak line due its 90 degree rotated polarization and directs it into beam stop 198. The desired wavelength passes polarizing beam splitter 196 practically without loss.

[0096] To further reduce the bandwidth and to spectrally purify the systems output radiation, etalons might replace one or both of the resonator mirrors of the seed laser section. The general idea is very similar to the previously described examples 1, 2 and 3. Since the gain of the power amplifier is not completely homogeneously broadened, part of the spectrum emitted can be suppressed by injection of the according spectra into the front of the power amplifier gain generator.

[0097] FIG. 17 shows a scheme, which is very similar to the FIG. 1 scheme except it employs an etalon 200 at one end of the seed laser 100. The etalon transmission minimum is tuned to the desired wavelength range, such as to reduce the propagation of this spectral content into the front of the power amplifier gain generator 103. At the same time, the transmittance of the undesired spectral content can be maximized by appropriate choice of the free spectral range (etalon spacing). This undesired spectral content is directed into the front of the power amplifier. The particular tuning of the etalon also maximizes the reflection of the desired wavelength within the seed laser resonator towards the power amplifier entrance, while minimizing the amount of the undesired spectral content propagating into this direction.

[0098] Further improvement from the scheme depicted in FIG. 17 can be achieved with scheme where the second seed laser mirror 104 is replaced with an etalon 202 as well. However, this etalon will be tuned complementary to the first etalon by appropriate choice of its free spectral range. This arrangement has improved selectivity/discrimination of the propagation of desired and undesired spectral contents within the system, as now two selective elements are involved.

[0099] Generally, a variety of similar optical schemes are possible, all of them providing the undesired spectral content into the front of the power amplifier thus depleting the optical gain and associated ASE in forward/output direction in the corresponding spectral range. The desired spectrum is generally provided at the power amplifier entrance for efficient amplification.

[0100] Several advantages are achieved with these designs:

[0101] 1. Narrowband output

[0102] 2. High spectral purity

[0103] 3. High efficiency due to low, i.e. no post-filtering required to suppress weak line.

[0104] Ideally the arrangement is made symmetrical, that is, the propagation time of light (equivalent with distance) from the seed laser into the power amplifier is made the same by appropriate optical layout. This way the gain medium of the power amplifier will interacts with both laser lines simultaneously, though from different directions, thereby maximizing wavelength discrimination. However, even quite large deviations from symmetry are allowed as the temporal gain profile is at least several ns wide, which corresponds to distance differences in the range of 1 m.

[0105] In fact, a slight advance of the light injection on the output side of the power amplifier might be used to reduce the red/IR output radiation in the same manner. The gain on these laser transitions is also very high (super-radiant emission) which in turn requires very little amounts, even from stray light levels of input intensity to saturate the gain. Temporal gain and emission on these lines occurs about 10-20 ns earlier than the VUV gain/laser pulse, which suggests to inject the lines earlier from the front.

[0106] Although the present invention has been described in terms of specific embodiments, the reader should understand that the scope of the invention is to be delimited by the appended claims and their legal equivalents.