Calibration standard for critical dimension verification of sub-tenth micron integrated circuit technology

[0001] (1) Field of the Invention

[0002] This invention relates to testing and diagnostics of line processes used for the manufacture of integrated circuit devices and more particularly to critical dimensional measurement of sub-tenth micron pattern features during wafer processing.

[0003] (2) Description of Prior Art

[0004] The manufacture of large scale integrated circuits in a mass production facility involves hundreds of discrete processing steps beginning with the introduction of blank semiconductor wafers at one end and recovering the completed chips at the other. The manufacturing process is usually conceived as consisting of the segment wherein the semiconductor devices are formed within the silicon surface (front-end-of-line) and the portion which includes the formation of the various layers of interconnection metallurgy above the silicon surface (back-end-of-line). Most of these processing steps involve depositing layers of material, patterning them by photolithographic techniques, and etching away the unwanted portions. The materials consist primarily of insulators and metal alloys. In some instances the patterned layers serve as temporary protective masks. In others they are the functional components of the integrated circuit chip.

[0005] In order to monitor the integrated circuit manufacturing process, test structures, representative of the circuit elements are typically incorporated in regions of the wafer outside the integrated circuit chips. Examples of these in-line test devices are a dumb-bell structure testable with a four point probe to establish proper resistivity of a deposited layer, or long serpentine metal lines which can be tested to establish the presence of particulate defects by testing for electrical opens and shorts. These devices are typically designed with critical areas much larger than their corresponding elements in the integrated circuit so they are more sensitive to defects and can be tested at various stages during processing. In addition to such devices which characterize the cleanliness and integrity of the process line, tests sites must also be provided which can characterize the integrity of the pattern alignment and its planar dimensions. For this it is desirable to have a means of providing features which are not produced by the same process which produces the pattern. In other words, it would be desirable to have structures by which the can provide a dimensional and alignment reference for evaluating the patterning process. Of particular interest in this application is the ability to accurately measure and characterize the planar integrity of polysilicon lines which are patterned in a deposited polysilicon layer by plasma etching. For this purpose CD (critical dimension) control wafers are used. In present technology, the control wafers are prepared with a polysilicon layer into which lines have been patterned using conventional photolithography and etching. The lines are then calibrated by measurement with a laboratory standard scanning electron microscope (SEM). The calibrated CD control wafer is then used to calibrate and monitor the line SEMs which are routinely used for quality control by the integrated circuit manufacturing line. The line SEMs are used to monitor and verify the critical dimensions of features on and product wafers.

[0006] Optical photolithography has been the preferred method for patterning features on integrated circuits for many decades. While its limitations have often been wrongly anticipated for years as integrated circuit technology advanced to smaller and smaller dimensions, optical photolithography has nevertheless managed to keep in step and remain the most cost effective and reliable patterning process. At present, device dimensions are at the sub tenth micron level and are expected to continue to shrink in the future. The desired dimensional and alignment reference structures mentioned supra must therefore be formed by a non-photolithographic process.

[0007] Unfortunately, polysilicon lines patterned by these conventional photolithographic and etching processes, which are the essentially the same as those used to manufacture the IC product, suffer from a number of problems which compromise their desirability as calibration standards. These problems include edge roughness and poor dimensional uniformity. In addition, because the lines are of polysilicon, they tend to become charged during the SEM measurement resulting in serious image distortion. It would therefore be desirable to have a critical dimension calibration standard for manufacturing SEMs which includes SEM measurable features which have dimensional parameters comparable to the IC product but without the shortcomings of those formed by the manufacturing processes of the IC product. For example the problem of charging during SEM measurement would be greatly reduced if not entirely eliminated if the calibration feature (lines) on the SEM calibration wafer were made of a material more conductive than polysilicon, such as aluminum or gold. Similarly, the problems of edge roughness and uniformity could be overcome on the calibration wafer by forming the line by a process other than photolithography and etching. These problems are addressed and overcome by the teaching of the present invention.

[0008] Focused ion beams have been used for years to thin or remove layers using a hardmask or a photoresist mask. An early example of this form of micro machining or milling using a focused ion beam is cited as early as 1976 by Garvin, et. al., U.S. Pat. No. 3,988,564. Progress was relatively slow with regard to patterning with a focused ion beam (FIB) slow because, like electron beam patterning, it took a relatively long time to pattern photoresist on a wafer by scanning a single beam. This became even more impractical as wafer size increased from about 75 mm. to the present day 150 mm. wafer. Furthermore, FIB systems could not bring about high ion currents at small spot size. More recently Leung, et. al., U.S. Pat. No. 5,945,667 cited an improved ion beam system which overcome these problems. Nevertheless, full wafer patterning of today's large wafers with a single FIB is still not practical. However, FIB technology, particularly, now with greatly improved small spot size and high ion currents, is found by the present inventors, to be useful as well as practical for providing independent reliable test structures for critical dimension measurements and SEM calibration in the sub tenth micron process technology.

[0009] Russell, et. al., U.S. Pat. No. 6,514,866 B2 shows a method for micro machining a copper film with a focused beam of gallium Ions while the film is in an ambient of organic chloride or organic hydroxide vapor. The gallium ions selectively sputter the copper, producing a rough surface thereon, while neighboring material, such as a dielectric is not removed. In another similar application, Takigawa, U.S. Pat. No. 4,457,803 uses a focused beam of argon ions to selectively sputter an oxide film.

[0010] Azuma, et. al., U.S. Pat. No. 5,683,547 shows a method for using a focused energy beam such as an ion beam to assist the local etching of a material with an etchant gas. Satya, et. al., U.S. Pat. No. 6,528,818 shows a method for scanning a region on a wafer for defects using a charged particle beam. Takano, et. al., U.S. Pat. No. 6,538,844 B2 shows a method for fabricating a magnetic head by focused ion beam etching while Talbot, et. al., U.S. Pat. No. 5,616,921 shows a method for controlling preferential etching during focused ion milling by using a mask image.

[0011] It is an object of this invention to disclose a test structure having multiple metal lines separated from each other by precision micro-machined trenches formed on a control wafer which can be used as a high precision dimensional reference, in particular as applied to the calibration and monitoring the stability of SEMs used for monitoring sub tenth-micron integrated circuit process technology.

[0012] It is a another object of this invention to provide a method for forming precision trenches in a metal layer with low width roughness, low edge roughness, and with reduced charging.

[0013] It is yet another object of this invention to provide a method for forming a reference test structure having multiple precision trenches which can be used as dimensional references for the in-line characterization of polysilicon lines after said lines are patterned.

[0014] It is still another object of this invention to provide a method for forming precision trenches in a metal layer on a substrate, said trenches having a width of between about 30 and 90 nm. and a width uniformity 3-sigma of between about 3.0 and 3.5 nm.

[0015] It is yet another object of this invention to provide a method for forming precision trenches in a metal layer on a substrate, said trenches having a width of and a mean value of width roughness of between about 3.0 and 3.7 nm. and a mean value of edge roughness of between about 1.8 and 2.2 nm.

[0016] These objects are accomplished by depositing a metal film comprising an aluminum copper (AlCu) alloy onto a substrate and then forming trenches in the AlCu alloy with a focused ion beam.

[0017] It is another object of this invention to provide a method for forming a CD (critical dimension) control wafer and calibration standard, having test structures consisting of precision trenches in a metal layer having a width of about 45 nm. and a width uniformity 3 sigma of between about 3.0 and 3.5 nm.

[0018] It is still another object of this invention to provide a method for forming a CD control wafer, having structures consisting of precision trenches in a metal layer having mean values of width roughness of between about 3.0 and 3.7 nm. and mean values of edge roughness of between about 1.8 and 2.2 nm.

[0019] These objects are accomplished by depositing a metal film comprising an aluminum and copper (AlCu) alloy onto a wafer and micro-machining trenches in the AlCu alloy with a focused ion beam.

[0022] In an embodiment of this invention an SEM calibration pattern will be formed on a silicon wafer. The calibration pattern consists of a set of parallel trenches dug into a metal layer using a focused ion beam. The widths of the trenches will be of the order of 50 nm. The widths of the metal lines which separate the trenches is arbitrary and may typically exceed 500 nm. The widths of the trenches and metal lines are verified by measurement in a laboratory calibrated SEM. After calibration, the wafer is used in a manufacturing environment as reference for calibration of SEMs, used to monitor critical dimensions of integrated circuit product flowing through a process line. The wafer may also used to verify and monitor the stability of the SEMs.

[0023] Referring to FIG. 1, a silicon wafer 100 is provided. A thin silicon oxide pad layer 102, between about 50 and 200 nm. thick is thermally grown on the wafer 100 by conventional thermal oxidation. The pad oxide layer 102 serves as a stress buffer and adhesion layer for a metal layer 104 which is next deposited onto the wafer 100, preferably by vacuum evaporation. The metal layer 104 preferably comprises an aluminum-copper alloy. The layer 104 is deposited to a thickness of between about 400 and 1000 nm. The aluminum-copper alloy preferably contains between about 0.1 and 1.0 percent copper by weight.

[0024] Referring now to FIG. 2, the CD control wafer 100 is mounted onto a motor driven X-Y stage of a focused ion beam processing tool. A suitable tool is the Model XL 830 manufactured by the FEI Company, 5350 NE Dawson Creek Dr. Hillsboro, Oreg. 97124 USA. After the tool is evacuated to a background pressure of between about 5×10⁻⁷ and 2.3×10⁻⁶ mTorr. A focused beam 106 of germanium ions is selectively directed onto the wafer while the wafer is moved under the beam 106 with the by X-Y stage. The focused ion beam 106 formed of activated germanium ions drawn out into a beam by an electric field while maintaining an operating pressure of between about 1.5×10⁻⁷ and 1.9×10⁻⁷ Torr. The ion beam is directed at the wafer at an ion energy of between about 25 and 35 keV and a current of between about 0.5 and 3 pA. The focused ion beam 106 gouges out linear trenches 108 in the metal layer 102. The trenches 108 are substantially parallel and are between about 50 and 500 nm. wide, and preferably between 30 and 90 nm. wide, at the surface and between about 200 and 500 nm. deep. The pitch 110 of the channels 108 is between about 100 and 600 nm. The trenches typically have mean values of width roughness of between about 3.0 and 3.7 nm. Edge roughness, which is defined as the mean variation of the edge about its linear least square average, typically has values of between about 1.8 and 2.2 nm.

[0025] Wafer 100 is next mounted in the chamber of a calibrated standards laboratory SEM and the widths of the trenches and their pitch are measured and recorded. In addition, the edge roughness is observed and measured to assure standards grade quality of the FIB gouged trenches. The CD wafer 100 is now stored and made available as a reference standard for the calibration of process line quality control SEMs which measure dimensions on product wafers as well as test site wafers passing through in the process line. A typical calibration procedure may demand a daily observation of a CD control wafer by the process SEM. The CD control wafer is mounted in the chamber of the process SEM and the recorded image is measured with reference to dimensions recorded for the CD wafer by the laboratory SEM. After calibration the CD control wafer is removed from the SEM chamber and product wafers from the process line are then measured in a series of routine SEM operations. After a pre-determine time period or after a pre-determined number of product wafer measurements, each involving mounting a product or test site wafer in the SEM chamber, pumping the chamber down, operating the SEM to obtain a measurable image, and then removing the product/test site wafer.

[0026] The frequency of calibration, of course, must be determined empirically, However, a single calibrated CD control wafer may be used over and over again to calibrate the process SEM. Because the pattern on the CD control wafer is formed on a highly conductive metal, the pattern is not subject to charging in the SEM. This greatly extends the life of the control wafer.

[0027] In FIG. 3, there is shown a planar view of a CD control wafer 100. In this illustration a single array of trenches 108 have been formed in a metal layer 102 which covers the entire surface of the wafer. Alternately, the metal layer 102 may itself be patterned, prior to the formation of the trenches. Such a pattern could be a metal region 109 just around the trench array. A smaller pattern such as the region 109 would make the trench pattern easier to find in the SEM. The metal region 109 is preferably rectangular and between about 5 and 50 μm. on a side with the trench array covering a region of about 4×10 μm. It is also advantageous to include several different trench widths in the array in order to test the linearity of the process control SEM.

[0028] While the embodiment utilizes an CD control wafer to measure and verify the dimensions of a polysilicon gate electrode, it should be understood that the CD control wafer, as described and claimed in the present invention could easily and effectively be used to verify critical dimensions of other patterned features, for example metal interconnect lines or additional higher level polysilicon lines.