Classifications

• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
  • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
  • G03F7/70—Microphotolithographic exposure; Apparatus therefor
  • G03F7/70425—Imaging strategies, e.g. for increasing throughput or resolution, printing product fields larger than the image field or compensating lithography- or non-lithography errors, e.g. proximity correction, mix-and-match, stitching or double patterning
  • G03F7/70466—Multiple exposures, e.g. combination of fine and coarse exposures, double patterning or multiple exposures for printing a single feature
• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
  • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
  • G03F7/70—Microphotolithographic exposure; Apparatus therefor
  • G03F7/70216—Mask projection systems
  • G03F7/70283—Mask effects on the imaging process
  • G03F7/70291—Addressable masks, e.g. spatial light modulators [SLMs], digital micro-mirror devices [DMDs] or liquid crystal display [LCD] patterning devices
• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
  • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
  • G03F7/70—Microphotolithographic exposure; Apparatus therefor
  • G03F7/70483—Information management; Active and passive control; Testing; Wafer monitoring, e.g. pattern monitoring
  • G03F7/70491—Information management, e.g. software; Active and passive control, e.g. details of controlling exposure processes or exposure tool monitoring processes
  • G03F7/70516—Calibration of components of the microlithographic apparatus, e.g. light sources, addressable masks or detectors
• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
  • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
  • G03F7/70—Microphotolithographic exposure; Apparatus therefor
  • G03F7/70483—Information management; Active and passive control; Testing; Wafer monitoring, e.g. pattern monitoring
  • G03F7/70605—Workpiece metrology
  • G03F7/70616—Monitoring the printed patterns
  • G03F7/70633—Overlay, i.e. relative alignment between patterns printed by separate exposures in different layers, or in the same layer in multiple exposures or stitching
• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
  • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
  • G03F7/70—Microphotolithographic exposure; Apparatus therefor
  • G03F7/70691—Handling of masks or workpieces
  • G03F7/70791—Large workpieces, e.g. glass substrates for flat panel displays or solar panels
• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
  • G03F9/00—Registration or positioning of originals, masks, frames, photographic sheets or textured or patterned surfaces, e.g. automatically
  • G03F9/70—Registration or positioning of originals, masks, frames, photographic sheets or textured or patterned surfaces, e.g. automatically for microlithography
  • G03F9/7003—Alignment type or strategy, e.g. leveling, global alignment
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/027—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34
  • H01L21/0271—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34 comprising organic layers
  • H01L21/0273—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34 comprising organic layers characterised by the treatment of photoresist layers
  • H01L21/0274—Photolithographic processes
  • H01L21/0276—Photolithographic processes using an anti-reflective coating
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/67—Apparatus specially adapted for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus specially adapted for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components ; Apparatus not specifically provided for elsewhere
  • H01L21/68—Apparatus specially adapted for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus specially adapted for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components ; Apparatus not specifically provided for elsewhere for positioning, orientation or alignment

Definitions

• This invention relates in general to printing patterns on a substrate and in particular, to printing multiple patterns on a substrate while dynamically aligning the patterns.
• Lithography was introduced to the electronics industry in the 1950's. The first integrated circuit was produced in 1960 by the firms Fairchild Semiconductor and Texas Instruments. Lithographic processes are used in both back plane and front plane display manufacturing. During the past 10 years, lithographic processes have been applied to new areas of precision patterning such as patterned synthesis of nucleic acid structures required for DNA testing.
• the second challenge involves the spatial relationship of the multiple spatial light modulators or printhead assemblies with one another as well as the spatial relationship between these assemblies and the substrate.
• U.S. Patent No. 6,251,550 (Ishikawa) describes a maskless lithography system, which utilizes an area exposure system. He mentions both LCD displays used as electronic masks and micro-mirror devices. It is advantaged in that the traditional alignment step involving a lateral shift of a mask can occur through electronic means. The traditional shifting of a mask in X and or Y may be seen as a global alignment process. As the display industry moves to fabrication on larger substrates and flexible support there is a need for a means of dynamically detecting and compensating for local changes in the registration of the patterning channels with patterns on the substrate in process.
• a method for dynamically registering multiple patterned layers on a substrate comprises: depositing a first layer on the substrate; printing a first pattern on the first layer; depositing a second layer on the first pattern; and printing a second pattern on the second layer while dynamically detecting the first pattern to align the second pattern with the first pattern.
• the advantages of the invention heretofore described include the ability to coordinate multiple printheads, the ability to track dimensional changes such as that due to thermally induced distortion of the substrate and/or the printhead, dimensional shifts such as that due to the hop and weave of a web during transport, or the misregistration of a printhead to a substrate due to abbe offsets contributed by the structure of the patterning motion system.
• corrections of the location of the written spot are altered in both X and Y dimensions across the substrate image plane, resulting in improvements in the registration of one layer of patterning to a subsequent layer of patterning.
• the system utilizes information about the prior patterning layers to enable precision detection at high speed of the relative location of the printhead and the substrate pattern.
• the system affords the capability of recognizing errors in patterning, which maybe used to prompt corrective actions in an automated manner.
• this system employs a laser source, a means of modulating the light from the laser source, a means of altering the location of the modulated laser light in two dimensions so as to alter the written spot location.
• the system utilizes a high-speed detection system to gather data, which in combination with position information and substrate content data allows the system to determine the alignment error.
• Figure 1 is a schematic of a printing system according to the present invention.
• Figure 2 is a schematic perspective showing components of the alignment detection system.
• Figure 3 shows the overlay of the first pattern image with the mask pattern.
• Figure 4 shows the overlay of the first pattern image and the mask pattern for the nominal case as the first pattern image crosses the vertical portion of the mask pattern.
• Figure 5 shows the overlay of the first pattern image and the mask pattern for the nominal case as the first pattern image crosses the slanted portion of the mask pattern.
• Figure 6 shows the overlay of the first pattern image and the mask pattern for a negative cross-scan error case as the first pattern image crosses the vertical portion of the mask pattern.
• Figure 7 shows the overlay of the first pattern image and the mask pattern for a negative cross-scan error case as the first pattern image crosses the slanted portion of the mask pattern.
• Figure 8 shows the overlay of the first pattern image and the mask pattern for a positive cross-scan error case as the first pattern image crosses the vertical portion of the mask pattern.
• Figure 9 shows the overlay of the first pattern image and the mask pattern for a positive cross-scan error case as the first pattern image crosses the slanted portion of the mask pattern.
• Figure 10 shows examples of the alignment detection output signal with varying cross-scan offsets.
• Figure 11 shows details of a simplified signal processing.
• Figure 12 shows a block diagram of the dual split half aperture auto focus system
• Figure 13 shows a typical auto focus feedback signal
• Figure 14 shows an auto focus feedback signal in response to an edge.
• Figure 15 shows a block diagram or a pictorial representation of an interferometer system.
• Figure 16 shows a block diagram or a pictorial representation of an optical coherence tomography system.
• Figure 17 shows a pictorial representation of a grazing incidence interferometric system.
• the present invention is closed loop control system, which acts to align a second pattern that is being generated with a first pattern.
• the closed loop system operates dynamically, meaning that it operates during and integral with the pattern forming process. As a result, no additional alignment overhead is required.
• the preferred embodiment describes a process whereby a substrate having a patterned first layer is coated with sensitive materials 4, as shown in Figure 1.
• a pattern is formed in this second coated layer.
• One means of forming this pattern is through laser ablation.
• the pattern can be formed through modification of a material coupled with a subsequent step. For example, one could sinter a material and wash off the not sintered components of the second layer. Or, one could polymerize a material and wash off the non-polymerized components of the second layer.
• pattern formation approaches that are additive are also compatible with this invention. Rather than applying a second layer that is to be patterned, one can apply the pattern directly. Suitable additive processes include ink jet, gravure and laser thermal transfer. Finally, when considering the various pattern forming approaches, combinations of the subtractive and additive techniques may be used. Since the ability to locate a patterned feature, for example, by ink jet may not meet the applications requirements, one can pattern by ink jet and then trim by ablation. The operation of the preferred embodiment is consistent with any of the aforementioned pattern forming approaches. The system components are described below and shown in Figure 1.
• Both the second pattern (not shown) and the first pattern 20 are generated via a maskless lithographic process using a multi -channel laser printhead 1 which imparts patterns of light 2 onto a substrate 3.
• Substrate 3 includes a support 21, a first pattern 20 and sensitive materials 4 which are sensitive to the wavelength or range of wavelengths of the patterning light from the multi-channel printhead.
• the patterns of light are emitted from the modulator 10, which receives drive signals 18 from the modulator driver electronics 9.
• the modulator driver electronics 9 receives image data 16 from electronics called the image data path 5.
• Image data 16 is representative of the second pattern that is to be produced in register to that of the first pattern 20.
• the image data path 5 has several functions. It is responsible for gating image data 16 to the modulator driver electronics 9. It is also responsible for determining when to gate this data to the modulator driver electronics 9. Finally, it computes and sends a cross-scan control signal 28 to the cross-scan correction controller 12.
• the image data path 5 monitors the current region 14 of the substrate 3 as detected by the alignment detection system 15.
• the current region 14 is defined as the area of the substrate 3, which is just about to be patterned.
• the alignment detection system 15 captures an electrical signal, which indicates the changes in reflectivity of the surface as the multi-channel laser printhead 1 and substrate 3 move relative to one another.
• the image data path 5 also processes the current motion control system position 26 provided by the motion control system 7 and the pending image data 16.
• the current position acts as a region of interest indicator and can be used as an enable signal to filter out spurious noise.
• the detection of features on the substrate 3 triggers a synchronization signal.
• the image data path 5 adjusts the gating of the pending image data 16 to the modulator driver electronics 9.
• the adjustment of the gating of the data is referred to as in- scan control.
• Cross-scan control is adjusted in this system by the cross-scan controller 12.
• the image data path 5 computes the misalignment between the pending image data 16 for the current motion control system position 26 and the current region 14 to generate an error signal that represents the correction that needs to be applied in order to counter the misalignment in the cross-scan direction.
• the image data path 5 sends the cross-scan control signal 28 to the cross-scan controller 12.
• the image data path 5 has information that exists within the image file which details process information 8. This process information 8 is critically important in interpreting the output from the alignment detection system. Further, the image data path can send process information 8 to the alignment detection system, prompting adjustment of illumination wavelengths, magnitude, gain of electronic circuitry and the like, in order to optimize the detection process for a specific layer or combination of layers on a substrate 3.
• Process information 8 includes information such as the characteristics of the coated materials, which have been placed on the substrate 3, as well as information concerning the first pattern 20 on the substrate 3, which is currently expected to be within the view of the alignment detection system 15. Further, the process information 8 can include information concerning the complex topology formed by multiple first patterns that are superimposed upon one another.
• the modulator 10 is responsible for altering the pattern of light 2 that is emitted from the multi-channel laser printhead 1.
• a wide array of modulation modalities is known in the art. In general one may divide these into reflective/diflractive and transmissive and would include TIR and DMD devices for example. In all cases control of the individual channels is possible through appropriate modulator driver electronics.
• the comparative process described above is a spatially intermittent process. There will typically be specific areas within a pattern that will best lend themselves to processing. These are referred to as capture windows. These areas may be parts of the functional pattern of the device that is being manufactured, or they may be additional alignment marks. The location of these areas may be predefined. When they are predefined, data identifying their location is embedded in the process information 8. As the image data path 5 transfers the image data 16 to the modulator driver board 9, the image data path 5 recognizes the process information 8 that indicates that the system is approaching or is in a capture window. In this embodiment, the image data path 5 uses this information to trigger the comparative process.
• Alignment correction refers to shifting of alignment in the cross-scan and in-scan directions.
• In-scan correction is accomplished through timing correction.
• the cross-scan correction is accomplished by sending a cross-scan control signal 28 to the cross-scan correction controller 12 to rotate an optical element 13 such that due to refraction the pattern emitted by the printhead is offset by a known amount in the cross-scan direction.
• the alignment detection system 15 transmits the sensing and detecting beam 58 through the optical element 13 such that the movement of the optical element 13 will impact the alignment detection system 15 sensing.
• the in- scan correction is accomplished by the image data path 5 through changes in head load timing.
• Figure 2 shows the components of the alignment detection system 15. These components include the substrate 3 with first pattern 20, the mask 22, mask pattern 23, photodetector 24. The illumination source and detector electronics are not shown. As either the substrate 3 is moved past the multichannel laser printhead 1 or the multi-channel laser printhead 1 is moved past the substrate 3, the pattern on the substrate 3 will create a modulation of signal at the detector based upon how the first pattern 20 on the substrate 3 matches with the mask pattern 23. The use of a vertical slit and angled slits in the mask pattern 23 provides the ability to detect both in-scan and cross-scan misalignment.
• Figure 3 shows the overlay of the first pattern image 62 with a mask pattern 23 onto a sensor area 60 in a system with no cross-scan errors.
• Figures 4 and 5 show the creation of the alignment detection output signal 11 for this system with no cross-scan error, also known as a nominal case system.
• the first pattern image 62 has just fully intersected with the mask pattern 64, and the waveform shown below the sensor area shows that the alignment detection signal 11 has increased to a peak value and is marked as time A.
• a second peak in the alignment detection output signal 15 is generated and marked as time B.
• the amount of time between the occurrences of A and B is indicative of the cross-scan error. It should be clear from these figures that cross-scan error can be characterized in terms of the distance traveled between the two events, A and B.
• Figures 6 and 7 illustrate the generation of the alignment detection signal 11 for a system with a negative cross-scan error.
• Figures 8 and 9 illustrate the generation of the alignment detection signal 11 for a system with a positive cross-scan error.
• Figure 10 shows representative timing differences for a system with negative, nominal, and positive cross-scan errors. Details of simplified signal processing are shown in Figure 11.
• This figure shows the in-scan and cross-scan synch inputs sent by the image data path 5.
• the third input is the alignment detection signal 11.
• the location of the first maximum A of the alignment detection signal 15 is measured with respect to the location of the in-scan synch pulse generated by the image data path 5.
• the positional difference between these two points is the in-scan error.
• the positional difference from the in-scan synch pulse and the cross-scan synch pulse indicates the predicted time or distance needed to traverse from the first pulse to the second pulse on the alignment detection signal 11.
• the synch delta As cross-scan error increases, the time or distance required to reach the actual second maximum on the alignment detector output is increased, as shown in Figure 10. Capturing the elapsed time or distance from the first maximum A to the second maximum B is the actual delta.
• the cross-scan error signal is the difference between the synch delta minus the actual delta.
• the laser light is 808 nm and the modulator is a TIR transmissive device.
• the material coated onto the substrate 3, meant to act as a resist is laser ablative resist that is sensitive to the IR.
• the channel size is 5 microns and the exposed line widths and gaps that can be produced are as small as 2 microns.
• s Systems that operate in the UV would utilize a UV source and a diffractive modulator and would be capable of smaller channel sizes and feature sizes.
• the detection system would utilize a sensing beam generated by a laser source at a wavelength different from that of the writing beam. It should be noted that this embodiment has been described as part of a maskless lithographic system. However, the ability to detect and dynamically compensate for misalignments between a current and first pattern may be applied to many other areas, which require precision alignment of patterns.
• the alignment detection system 15 can be a camera and high-speed image processing which performs image matching with the expected image region, hi such a system the sensing illumination would be diffuse lighting at a range of wavelengths that differ substantially from that of the writing beam.
• the camera and high-speed image processing is advantaged in that it would enable the determination of rotation.
• One means of working around the optical issues associated with designing an optical system, which transmits writing light at 808 nm and sensing light at a different wavelength, is to place the sensing subsystem outside of the writing path.
• the drawback associated with this alternative is that the sensing optical path is physically displaced from the writing path.
• One means of linking the two systems is to include use of one channel or an additional channel associated with the multi-channel laser printhead as a pointer, which instructs the sensing system where the writing beam has been directed. This pointer would be used by the sensing system to establish the physical relationship between the writing and sensing systems. Use of an additional channel at the same wavelength as the writing beam is possible as long as the energy in this beam is below patterning threshold.
• the process of connecting the sensing system with that of the writing system will need to occur in a non-imaging area of the substrate. If, however, an additional channel is provided that is of a non-patterning wavelength, then the linkage between the camera and the multi channel laser printhead 1 can occur continuously. Use of a different wavelength avoids the risk of patterning artifacts, but requires a more complex optical design.
• a pure edge detection scheme may be employed, utilizing coherent illumination that is focused to a small spot.
• Such an arrangement is similar to that of typical the auto-focus systems.
• Typical auto focus detection systems utilizing the dual half aperture approach will provide an analog signal proportional to the height of the reflecting surface.
• a laser source 16 sends light to the surface of the substrate 3 through a beam splitter 78.
• the light reflected from the surface of the substrate 3, passes through the beam splitter 78 and continues through the dual wedges 74 and optics 72, to impinge upon a quad photodetector 70.
• the dual half aperture focus error detection method creates a differential signal by utilizing both halves of the optical aperture for focus detection.
• Each half of the optical aperture is sent to either the upper or lower bi-cell of photodetector 70 through the dual wedges 74 that refract one side high and one side low.
• the spot formed on the two bi-cells that form photodetector 70 change in equal and opposite directions.
• Adding the signals from the two opposing sides of each bi-cell produces a signal that is twice that of the half aperture approach.
• any common noise that is riding on the signals from all the cells is removed.
• the result is a cleaner signal that more closely represents the true error signal for focus.
• the difference signal is typically normalized for consistency. The design for such a system has been described in U.S.
• Patent No. 5,406,541 Kay
• Such a detector signal would appear as shown in Figure 13.
• the individual intensity of the sensors in an auto focus system such as that in Figure 12 can also be monitored for changes that would indicate an edge has scattered light.
• Figure 14 shows the signals generated in response to passing over an edge.
• a distinct pulse would be generated when the small spot traversed a change in elevation on the substrate.
• an interferometer can be used to detect changes in topology as the multi-channel laser printhead 1 with interferometer is scanned along the patterned layer surface. This system is depicted in Figure 15. In this system, coherent light is directed to the patterned layer from a laser source 29.
• Part of the illumination passes through a beam splitter 31 and impinges upon a substrate 3.
• the rest of the illumination is deflected to a mirror 30.
• the reflected light from both the mirror 30 and the substrate 3 is directed to the sensor 32.
• the sensor 32 receives this combined light energy which creates an interference pattern.
• This interference pattern is detected by the sensor 32, which is typically a photo-detector or a CCD.
• optical coherence tomography or low coherence interferometry
• OCT optical coherence tomography
• a low coherence light source 34 for the interferometer has a short coherence length.
• Examples of low coherence light sources include super luminescent diodes, lasers with extremely short pulses on the order of femtoseconds and also white light sources.
• the system includes two arms, a sample arm 54 which includes the substrate 3.
• the second arm is a reference arm 56, which includes a reference mirror 40.
• the light from the low coherence source 34 is collimated by a collimation lens 36 and is directed to the two arms by a beam splitter 38.
• the reflected beams are combined at the beam splitter 38, acted upon by beam reducer 44 and impinge upon a photodetector 52.
• Photodetector 52 detects a fluctuating signal that is the result of the interference of the two beams that is directly related to the surface topology of the substrate 3. Interference only occurs when the mirror 42 in the reference arm is in a specific axial position.
• Scanning mirror 46 scans the substrate through the objective lens 48. If the position of the reference arm mirror is closely monitored then a signal can be generated when the axial position of the mirror indicates a specific surface change (within the region of interest).
• Grazing incidence interferometry can also be used for detection of surface topographies.
• Figure 17 shows a laser beam from laser source impinging on holographic diffraction grating 61 and creating two beams of roughly equal intensity.
• the first order beam is separated from the zero order beam by a low angle.
• the first order beam grazes off of substrate 3 at a shallow angle and some of the light reflects and diffracts.
• the light that combines with the reference beam at the second holographic grating 63 will form an interference pattern containing the image of the relief pattern from the substrate 3.
• a sensor/video camera 65 can detect the pattern.
• this invention is suited for adaptive control application. Since it is possible to detect patterns just after they have been created, the system could employ two detectors; one to aid in locating the pattern point and one to inspect the pattern that has been created. Data can be stored from the second detection system that can be used to enhance the next detection process. It can also be used to better calibrate the correction provided by the first sensor and correction system. It can also be used as in process inspection.
• the control algorithms employed by the image data path 5 limit the rate of change of the in-scan control and cross- scan control signal 28. Such a rate limiting control algorithm is often referred to as servo loop. This limitation on the rate of change of the control signals is used to prevent undesirable patterning artifacts.
• the control algorithms employed by the image data path does not limit the rate of change of the in-scan cross-scan control signal 28. This type of control is appropriate where abrupt resynchronization is preferred.
• control algorithms employed may be either rate limiting or non-limiting or a combination thereof.
• the image data path 5 utilizes process information 8 to determine an adaptive control algorithm.
• the substrate can be illuminated with an incoherent source of light and have the image of the substrate projected onto a detector. If the layers on the substrate have sufficient contrast between them then the features in the layers can be discriminated within the image.
• the detector could be an area array, line sensor, or single sensor.
• OCT optical computed tomography

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