Differential height measurement using an interposed mirror plate.

Abstract

An apparatus and method are provided that includes an imaging device, a stage movable relative to the imaging device, an isolation plate disposed between the imaging device and the stage, the isolation plate having a horizontal glass plate, and a plurality of interferometers in electronic communication with a processor. Three of the plurality of interferometers are disposed on the imaging device and configured to direct a first set of beams toward the horizontal glass plate along a first direction, and three interferometers are disposed on the stage and configured to direct a second set of beams toward the horizontal glass plate along a second direction opposite the first direction. The processor is configured to measure an imaging device-to-isolation plate distance and a stage-to-isolation plate distance based on the first and second sets of beams reflected from the horizontal glass plate.

Description

[Technical Field]

[0001] TECHNICAL FIELD This disclosure relates to metrology systems, and more particularly to metrology systems for inspection systems. [Background technology]

[0002] [CROSS-REFERENCE TO RELATED APPLICATIONS] This disclosure is based on and claims priority to U.S. Provisional Patent Application No. 63 / 240,568, filed September 3, 2021, the entire contents of which are incorporated herein by reference.

[0003] As the semiconductor manufacturing industry evolves, the demands on yield management, particularly for metrology and inspection systems, are increasing. As critical dimensions continue to shrink, the industry is still under pressure to reduce the time to achieve high yields and high-value production. Reducing the total time from detecting a yield problem to correcting it is critical to the return on investment for semiconductor manufacturers.

[0004] In the fabrication of semiconductor devices, such as logic and memory devices, semiconductor wafers are typically processed using a number of manufacturing processes to form the various features and multiple levels of the semiconductor devices. For example, in a semiconductor manufacturing process known as lithography, a pattern is transferred from a reticle onto a photoresist disposed on the semiconductor wafer. Further examples of semiconductor manufacturing processes include, but are not limited to, chemical mechanical polishing (CMP), etching, deposition, and ion implantation. Multiple semiconductor devices can be fabricated in an array on a single semiconductor wafer and then separated into individual semiconductor devices.

[0005] Inspection processes are used at various steps during semiconductor manufacturing, whereby detecting wafer-side defects can help increase yield and, therefore, profits in the manufacturing process. Inspection has always been an important part of the manufacture of semiconductor devices, such as integrated circuits (ICs). However, as semiconductor device dimensions shrink, inspection becomes more important for the successful manufacture of acceptable semiconductor devices because even smaller defects can cause device failures. For example, as semiconductor device dimensions shrink, the detection of smaller-sized defects becomes necessary because even relatively small defects can cause unwanted errors in those semiconductor devices.

[0006] Some inspection processes are performed using an imaging mirror assembly (IMA) aimed at a target on a stage. Because the IMA is highly sensitive to vibrations, it is mounted on a passive vibration isolator assembly, allowing the IMA's position to vary during the inspection process. However, for accurate inspection, it is important that the system be able to determine the IMA's position relative to the stage. Existing systems rely on measurements from six interferometers (corresponding to six degrees of freedom) mounted on the IMA and / or stage or at separate fixed locations within the system. The beams from the interferometers must be bounced off large horizontal mirrors fixed to the IMA and / or stage, which can degrade system performance.

[0007] The IMA must be placed in a vacuum region separated from the vacuum region around the stage. Therefore, an isolation plate is placed between the IMA and the stage. However, to perform interferometric measurements, a 45-degree mirror must be installed to convert the horizontal beam into a vertical beam and pass it through the isolation plate. The performance is impaired by having to pass the vertical beam through a hole or window in the isolation plate. This solution increases the system complexity and introduces some error into the measurement results due to signal loss. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] U.S. Patent Application Publication No. 2013 / 0342827 Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, what is needed is a system for accurate measurement of the IMA-to-stage distance that is simple, provides high accuracy, and requires fewer penetrations through the isolation plate than existing methods. [Means for solving the problem]

[0010] In one embodiment of the present disclosure, an apparatus is provided. The apparatus can include an imaging device configured to receive a light beam from a target reticle. The apparatus can further include a stage movable relative to the imaging device. The stage can be configured to hold the target reticle. The apparatus can further include an isolation plate disposed between the imaging device and the stage. The isolation plate can include a horizontal glass plate. The apparatus can further include a plurality of interferometers in electronic communication with a processor. Three interferometers of the plurality of interferometers can be disposed on the imaging device and configured to direct a first set of beams toward three locations on the horizontal glass plate along a first direction. Three additional interferometers of the plurality of interferometers can be disposed on the stage and configured to direct a second set of beams toward three locations on the horizontal glass plate along a second direction opposite the first direction. The processor can be configured to measure the imaging device-to-isolation plate distance and the stage-to-isolation plate distance at each of the three locations on the horizontal glass plate based on the first set of beams and the second set of beams reflected by the horizontal glass plate.

[0011] According to certain embodiments of the present disclosure, the first direction can be downward and the second direction can be upward.

[0012] According to certain embodiments of the present disclosure, a reflective coating layer can be provided on a first side of the horizontal glass plate such that the first set of beams and the second set of beams are reflected by the reflective coating layer, and an anti-reflective coating layer can be provided on a second side of the horizontal glass plate opposite the first side such that one of the first set of beams and the second set of beams passes through the anti-reflective coating layer.

[0013] According to certain embodiments of the present disclosure, the imaging device and the stage may be provided in separate vacuum regions separated by the isolation plate.

[0014] According to certain embodiments of the present disclosure, the imaging device may be disposed on a passive vibration isolator.

[0015] According to certain embodiments of the present disclosure, the first and second sets of beams can be prevented from being blocked between the interferometer horizontal glass plates.

[0016] According to an embodiment of the present disclosure, the relative motion between the imaging device and the stage between t1 and t2, where t2 is the current time and t1 is a time prior to t2, can be determined by comparing the sum of the imaging device-to-isolation plate distance and the stage-to-isolation plate distance at each of the three locations with a preset sum. The relative motion can be associated with motion in one of six degrees of freedom, including surge, heave, sway, roll, pitch, and yaw. The preset sum can be associated with the sum of the imaging device-to-isolation plate distance and the stage-to-isolation plate distance at each of the three locations measured at t1.

[0017] Another embodiment of the present disclosure provides a method. The method can include an imaging device configured to receive a light beam from a target reticle. The method can further include a stage movable relative to the imaging device. The stage can be configured to hold the target reticle. The method can further include an isolation plate between the imaging device and the stage. The isolation plate can include a horizontal glass plate. The method can further include a plurality of interferometers in electronic communication with a processor. Three interferometers of the plurality of interferometers can be disposed on the imaging device and configured to direct a first set of beams toward three locations on the horizontal glass plate along a first direction. Three more interferometers of the plurality of interferometers can be disposed on the stage and configured to direct a second set of beams toward the three locations on the horizontal glass plate along a second direction opposite the first direction. The method can further include directing the first set of beams and the second set of beams toward the horizontal glass plate. The method further includes using the processor to measure the imaging device-to-isolation plate distance and the stage-to-isolation plate distance at each of the three locations on the horizontal glass plate based on the beam reflected by the horizontal glass plate.

[0018] According to certain embodiments of the present disclosure, the method may further include using the processor to compare a sum of the imaging device-to-isolation plate distance and the stage-to-isolation plate distance at each of the three locations with a preset sum. The method may further include using the processor to determine relative motion between the imaging device and the stage between t1 and t2, where t2 is the current time and t1 is a time prior to t2. The relative motion may be associated with motion in one of six degrees of freedom, including surge, heave, sway, roll, pitch, and yaw. The preset sum may be associated with the sum of the imaging device-to-isolation plate distance and the stage-to-isolation plate distance at each of the three locations measured at t1.

[0019] According to certain embodiments of the present disclosure, a non-transitory computer-readable storage medium can be configured by which one or more steps of the method can be performed.

[0020] For a more complete understanding of the nature and purpose of the present disclosure, reference should be made to the accompanying drawings in conjunction with the detailed description below. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a block diagram of an apparatus according to an embodiment of the present disclosure. [Figure 2A] FIG. 2 illustrates a first side view of a horizontal glass plate according to an embodiment of the present disclosure. [Figure 2B] FIG. 10 depicts a second side of a horizontal glass plate according to an embodiment of the present disclosure. [Figure 2C] 1A and 1B depict a downward view of a stage and corresponding metrology in an apparatus according to an embodiment of the present disclosure. [Figure 3] FIG. 1 is a block diagram illustrating measurements made by an apparatus according to an embodiment of the present disclosure. [Figure 4] FIG. 10 is a block diagram illustrating measurements in an apparatus according to another embodiment of the present disclosure. [Figure 5] 1 is a flowchart of a method according to an embodiment of the present disclosure. [Figure 6] 4 is a flowchart of a method according to another embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0022] Although the claimed subject matter is described in terms of certain embodiments, other embodiments are also within the scope of this disclosure, including embodiments that do not provide all of the benefits and features described herein. Various structural, logical, process step, and electronic changes may be made without departing from the scope of this disclosure. Thus, the scope of this disclosure is defined solely by reference to the appended claims.

[0023] In one embodiment of the present disclosure, an apparatus 100 is provided, as shown in FIG. 1 . The apparatus 100 may include an imaging device 110. The imaging device 110 may be configured to receive an EUV light beam 111 (e.g., 13 nm wavelength) from a target reticle 112. The light beam 111 may be configured to perform inspection of the target reticle 112. The target reticle 112 may be a wafer, a workpiece, or any other object that may be inspected by the imaging device 110. The imaging device 110 may be disposed on a passive vibration isolator 113. The passive vibration isolator 113 may include a spring / damper system configured to provide horizontal and vertical absorption. For example, the passive vibration isolator 113 may include an elastomer enclosed in a vacuum bellows.

[0024] The apparatus 100 may further include a stage 120. The stage 120 may be movable relative to the imaging device 110. For example, the stage 120 may be movable relative to the imaging device 110 to perform various inspection processes, such as swathing, step-and-settle, hot spot, etc., on the target reticle 112. The stage 120 may be configured to hold the target reticle 112.

[0025] The apparatus 100 may further include an isolation plate 130. The isolation plate 130 may be disposed between the imaging device 110 and the stage 120. For example, the imaging device 110 and the stage 120 may be disposed in separate vacuum regions 101, 102. Because the stage 120 must move over long distances and carry many loads, the stage vacuum region 102 may be hundreds of times dirtier (both chemically and particulately) than the imaging vacuum region 101. Therefore, it may be desirable to prevent contamination of the sensitive optics in the imaging vacuum region 101. Separating the vacuum regions 101, 102 with the isolation plate 130 separates the imaging device 110 from the stage 120. The isolation plate 130 may comprise a metal sheet. The isolation plate 130 may comprise a horizontal glass plate 131. The horizontal glass plate 131 may be constructed of fused silica or any other glass that is transparent to the light beam and mechanically stable. Other types of glass, such as sapphire or ultra-low expansion (ULE) silicates, may also be used. The horizontal glass plate 131 may be approximately 6 mm thick. The horizontal glass plate 131 may have an opening 134 through which the light beam passes. The diameter of the opening 134 may be approximately 6 mm.

[0026] The apparatus 100 may further include a plurality of interferometers 140, which may be fiber-optic-based interferometers. The plurality of interferometers may be in electronic communication with a processor 150, which may be coupled to the plurality of interferometers 140 via the imaging device 110 and the stage 120. The processor 150 may be a programmable processor programmed in software and / or firmware to perform the functions described herein, as well as digital and / or analog interfaces for connection to other elements of the apparatus 100. Alternatively or additionally, the processor 150 may include hardwired and / or programmable hardware logic circuitry that performs at least some of the functions of the processor 150. 1 shows processor 150 as a single functional block for simplicity, in practice processor 150 may be combined with multiple interconnected control units with appropriate interfaces for receiving and outputting signals as depicted in the drawings and described in the text. Program code or instructions for causing processor 150 to perform and implement the various methods and functions disclosed herein may be stored in a readable storage medium, such as memory within processor 150 or other memory. Typically, this electronic circuitry would be mounted outside the vacuum chamber (for greater ruggedness, lower cost, and easier operation).

[0027] The plurality of interferometers 140 may include three interferometers 141 a, 142 a, and 143 a disposed on the imaging device 110. The interferometers 141 a, 142 a, and 143 a may be configured to direct a first set of beams 151 a, 152 a, and 153 a along a first direction D1 toward three locations X1, Y1, and Z1 on the first side 131 a of the horizontal glass plate 131. For example, the first interferometer 141a can be configured to direct a first beam 151a of the first beam set toward a first location X1 on the horizontal glass plate 131, the second interferometer 142a can be configured to direct a second beam 152a of the first beam set toward a second location Y1 on the horizontal glass plate 131, and the third interferometer 143a can be configured to direct a third beam 153a of the first beam set toward a third location Z1 on the horizontal glass plate 131.

[0028] The plurality of interferometers 140 may further include three interferometers 141b, 142b, and 143b disposed on the stage 120. The interferometers 141b, 142b, and 143b may be configured to direct a second set of beams 151b, 152b, and 153b along a second direction D2 toward three locations X2, Y2, and Z2 on the second side 131b of the horizontal glass plate 131. For example, first interferometer 141b can be configured to direct first beam 151b of the second set of beams toward a first location X2 on horizontal glass plate 131, second interferometer 142b can be configured to direct second beam 152b of the second set of beams toward a second location Y2 on horizontal glass plate 131, and third interferometer 143b can be configured to direct third beam 153b of the second set of beams toward a third location Z2 on horizontal glass plate 131. The second direction D2 can be opposite to the first direction D1. For example, the first direction D1 can be downward and the second direction D2 can be upward.

[0029] Three locations X1, Y1, and Z1 (shown in FIG. 2A ) on a first side 131 a of the horizontal glass plate 131 can be aligned with three locations X2, Y2, and Z2 (shown in FIG. 2B ) on a second side 131 b of the horizontal glass plate 131. In this way, the first set of beams 151 a, 152 a, and 153 a can be aligned with the second set of beams 151 b, 152 b, and 153 b on opposite sides of the horizontal glass plate 131 and based on the three shared locations X, Y, and Z. The three shared locations X, Y, and Z can be any three non-collinear points on the horizontal glass plate 131. For example, the three shared locations X, Y, and Z can be arranged in a triangular configuration. The three shared locations X, Y, and Z can surround the light beam 111 or can be arranged on one side of the light beam 111. The three shared locations X, Y, and Z may surround the aperture 134. According to certain embodiments of the present disclosure, the three shared locations X, Y, and Z may be located farther from the light beam 111 than the edges of the target reticle 112. For example, the target reticle may be approximately 6 inches square, and each of the three shared locations X, Y, and Z may be located approximately 3.5 inches (1 inch = approximately 2.54 cm) from the light beam 111. Arranging the three shared locations X, Y, and Z as close as possible to the light beam 111 may improve measurement accuracy.

[0030] The plurality of interferometers 140 may include more than three pairs of interferometers. Increasing the number of interferometer pairs can improve measurement accuracy and simplify calculations. For example, the plurality of interferometers may include four interferometers disposed on the imaging device 110 and four interferometers disposed on the stage 120. The beams from the four pairs of interferometers may then be aligned at four common locations on opposite sides of the horizontal glass plate 131. The four common locations may be any four non-collinear points on the horizontal glass plate. For example, the four common locations may be arranged in a rectangular configuration. The four common locations may surround the light beam 111 or may be arranged on one side of the light beam 111. The four common locations may surround the aperture 134. According to certain embodiments of the present disclosure, the four shared locations can be located farther from the light beam 111 than the edges of the target reticle 112. For example, the target reticle can be approximately 6 inches square, and each of the four shared locations can be approximately 3.5 inches from the light beam 111.

[0031] The processor 150 can be configured to measure distances S1a, S2a, and S3a between the imaging device 110 and the isolation plate 130. For example, as shown in FIG. 3 , the processor 150 can be configured to measure a first distance S1a between the first interferometer 141a and a first location X1 based on a first beam 151a of the first set of beams reflected by the first side 131a of the horizontal glass plate 131, measure a second distance S2a between the second interferometer 142a and a second location Y1 based on a second beam 152a of the first set of beams reflected by the first side 131a of the horizontal glass plate 131, and measure a third distance S3a between the third interferometer 143a and a third location Z1 based on a third beam 153a of the first set of beams reflected by the first side 131a of the horizontal glass plate 131. The first set of beams 151 a , 152 a , 153 a can be unobstructed between the respective interferometers 141 a , 142 a , 143 a and the first side 131 a of the horizontal glass plate 131 .

[0032] Processor 150 may be further configured to measure distances S1b, S2b, and S3b between stage 120 and isolation plate 130. For example, as shown in FIG. 3 , processor 150 may be configured to measure a first distance S1b between first interferometer 141b and first location X2 based on a first beam 151b of the second set of beams reflected by second side 131b of horizontal glass plate 131, measure a second distance S2b between second interferometer 142b and second location Y2 based on a second beam 152b of the second set of beams reflected by second side 131b of horizontal glass plate 131, and measure a third distance S3b between third interferometer 143b and third location Z2 based on a third beam 153b of the second set of beams reflected by second side 131b of horizontal glass plate 131. The second set of beams 151 b , 152 b , 153 b can be unobstructed between the respective interferometers 141 b , 142 b , 143 b and the second side 131 b of the horizontal glass plate 131 .

[0033] According to an embodiment of the present disclosure, the processor 150 may be further configured to determine sums S1, S2, and S3 of the distances S1a, S2a, and S3a between the imaging device 110 and the isolation plate 130 and the distances S1b, S2b, and S3b between the stage 120 and the isolation plate 130 at three locations on the horizontal glass plate 131. For example, a first sum S1 may be the sum of the first distances S1a and S1b, a second sum S2 may be the sum of the second distances S2a and S2b, and a third sum S3 may be the sum of the third distances S3a and S3b. By comparing these sums S1, S2, and S3 with preset sums P1, P2, and P3 in processor 150, the relative motion between imaging device 110 and stage 120 between t1 and t2, where t2 is the current time and t1 is a time prior to t2, can be determined. The relative motion can be associated with motion in one of six degrees of freedom, including surge (forward / backward motion along the Y direction), heave (up / down motion along the Z direction), sway (left / right motion along the X direction), roll (rotation about the Y axis), pitch (rotation about the X axis), and yaw (rotation about the Z axis). For example, the sums S1, S2, and S3 can be used to determine heave, pitch, and roll.

[0034] According to certain embodiments of the present disclosure, these preset sums P1, P2, and P3 can be mapped to sums S1, S2, and S3 of the distances S1a, S2a, and S3a between the imaging device 110 and the isolation plate 130 and the distances S1b, S2b, and S3b between the stage 120 and the isolation plate 130 measured at time t1. This allows the relative motion determined by the processor 150 to be based on the measured changes in the sums S1, S2, and S3 between time t1 and time t2. The measurement frequency can be in the MHz range, which allows for separation of mechanical frequency issues (typically less than 200 Hz) from measurement issues. This also allows for averaging of the measurement results to reduce noise.

[0035] According to an embodiment of the present disclosure, as shown in FIG. 4 , a reflective coating layer 132 may be provided on one side of the horizontal glass plate 131. For example, the reflective coating layer 132 may be provided on the second side 131b of the horizontal glass plate 131. The reflective coating layer may be a metal (e.g., aluminum or silver) coated with a protective layer (e.g., magnesium fluoride or the like). The first set of beams 151a, 152a, 153a and the second set of beams 151b, 152b, 153b may be reflected by the reflective coating layer 132. An anti-reflective coating layer 133 may also be provided on one side of the horizontal glass plate. For example, the anti-reflective coating layer 133 may be provided on the first side 131a of the horizontal glass plate 131. The anti-reflective coating 133 may be composed of a chemical substance with a refractive index between that of a vacuum (1.0) and that of the type of glass used in the horizontal glass plate 131 (e.g., 1.46 for fused silica). For example, the anti-reflective coating 133 may be composed of magnesium fluoride, which has a refractive index of 1.38 and reduces the reflectivity of non-reflective surfaces from approximately 4% one-way to less than 2% one-way. One of the first set of beams 151a, 152a, 153a and the second set of beams 151b, 152b, 153b may pass through the anti-reflective coating 133. This minimizes any effect of thickness variations in the horizontal glass plate 131 when determining the sums S1, S2, and S3, since only the thickness of the reflective coating 132 is interposed between the first set of beams 151a, 152a, 153a and the second set of beams 151b, 152b, 153b.

[0036] The apparatus 100 provides several benefits to the inspection process. For example, the imaging device 110 and stage 120 do not need to support large mirrors, reducing the weight of these moving parts and allowing the interferometer to be located in a more convenient location on the imaging device 110 and stage 120. Instead, a single horizontal glass plate 131 is mounted on a stationary isolation plate 130. Any drift or movement of the horizontal glass plate 131 due to system vibrations is common between measurements obtained from the imaging device 110 and the stage 120, so no errors are introduced into the system. The horizontal mirrors provide an improved signal-to-noise ratio for the interferometer compared to misalignment-prone 45-degree mirrors. Additionally, the beam path lengths of the first set of beams 151a, 152a, and 153a and the second set of beams 151b, 152b, and 153b are relatively short compared to conventional systems, reducing susceptibility to wavelength instability.

[0037] 1 and 2C , the plurality of interferometers 140 can further include an X interferometer 144, a Y interferometer 145, and an Rz interferometer 146. The X interferometer 144, the Y interferometer 145, and the Rz interferometer 146 can be located within the vacuum region 102 adjacent to the stage 120 or outside the isolation plate 130. The X interferometer 144, the Y interferometer 145, and the Rz interferometer 146 are supported by a support structure 114 coupled to the imaging device 110. The X interferometer 144 can be configured to direct an X beam 154 along the X direction toward a first side 121 of the stage 120. The Y interferometer 145 can be configured to direct a Y beam 155 along the Y direction toward a second side 122 of the stage 120. The Rz interferometer 146 can be configured to direct the Rz beam 156 along the Y direction toward the second side 122 of the stage 120. The Rz interferometer 146 can be positioned a fixed distance, for example 50 mm, from the Y interferometer 145. The first side 121 and the second side 122 of the stage 120 can be mirrors such that the X beam 154, the Y beam 155, and the Rz beam 156 are reflected from the stage 120.

[0038] The processor 150 can be further configured to measure a fourth distance S4 between the X interferometer 144 and the stage 120 based on the X beam 154 reflected from the first side 121 of the stage 120, a fifth distance S5 between the Y interferometer 145 and the stage 120 based on the Y beam 155 reflected from the second side 122 of the stage 120, and a sixth distance S6 between the Rz interferometer 146 and the stage 120 based on the Rz beam reflected from the second side 122 of the stage 120. The processor 150 can compare the distances S4, S5, and S6 with preset distances P4, P5, and P6 to determine the relative motion of the stage 120. For example, the fourth distance S4 can be used to determine sway (left-right movement along the X direction), the fifth distance S5 can be used to determine surge (front-to-back movement along the Y direction), and the fifth distance S5 and the sixth distance S6 can be used to determine yaw (rotation about the Z axis). Thus, the apparatus 100 can be configured to use multiple interferometers 140 to determine relative motion in all six degrees of freedom.

[0039] According to certain embodiments of the present disclosure, the preset distances P4, P5, and P6 can be mapped to the distances S4, S5, and S6 measured at time t1, allowing the relative motion determined by processor 150 to account for changes in the measurement of distances S4, S5, and S6 between time t1 and time t2. The measurement frequency can be in the MHz range.

[0040] Apparatus 100 eliminates the need for a metrology frame extending horizontally near the stage movement area. Such a frame would be difficult to design due to (a) its natural frequency (greater than 300 Hz), (b) its large wingspan (0.8 m), (c) its tight thermal expansion requirements (much less than 1 nm over 15 min), and (d) its very stringent contamination requirements in the space above / near stage 120. A less stringent (on the order of 100 times) vacuum regime can be applied in the space where the fiber is routed to stage 120, and (e) the metrology frame and optics frame can be operated independently while still providing a rigid connection between them through isolation plate 130 in some way.

[0041] While apparatus 100 eliminates the need for a metrology frame, it is contemplated that a metrology frame may be incorporated in certain embodiments of the present disclosure. In such embodiments, an additional interferometer may be provided, pointed toward imaging device 110 and stage 120 and configured similarly to interferometer 140 described above, to measure distance using processor 150 to determine relative motion of the metrology frame.

[0042] In an embodiment of the present disclosure, a method 200 is provided. As shown in Figure 5, the method 200 may include the following steps.

[0043] Step 210 may provide an imaging device. The imaging device may be configured to receive a light beam from a target reticle. The light beam may be configured to perform inspection of the target reticle. The target reticle may be a wafer, a workpiece, or any other object that may be inspected by the imaging device. The imaging device may be disposed on a passive vibration isolator. The passive vibration isolator 113 may include a spring / damper system configured to absorb motion along horizontal and vertical directions. For example, the passive vibration isolator 113 may include an elastomer enclosed in a vacuum bellows.

[0044] In step 220, a stage can be provided. The stage can be movable relative to the imaging device. For example, the stage can be movable relative to the imaging device to perform various inspection processes on the target reticle, such as swathing, step and settle, hot spot, etc. The stage can be configured to hold the target reticle.

[0045] In step 230, an isolation plate can be provided between the imaging device and the stage. The isolation plate separates the imaging device and the stage into separate vacuum regions. The isolation plate can comprise a horizontal glass plate.

[0046] A plurality of interferometers may be provided in step 240. The plurality of interferometers may be fiber optic-based interferometers. The plurality of interferometers may be in electronic communication with a processor.

[0047] The plurality of interferometers can include three interferometers disposed on the imaging device. The three interferometers can be configured to direct a first set of beams along a first direction D1 toward three locations X1, Y1, and Z1 on a first side of a horizontal glass plate. For example, the first interferometer can be configured to direct a first beam of the first set of beams toward a first location X1 on the horizontal glass plate, the second interferometer can be configured to direct a second beam of the first set of beams toward a second location Y1 on the horizontal glass plate, and the third interferometer can be configured to direct a third beam of the first set of beams toward a third location Z1 on the horizontal glass plate.

[0048] The plurality of interferometers may further include three interferometers disposed on the stage. The three interferometers may be configured to direct a second set of beams along a second direction D2 toward three locations X2, Y2, and Z2 on a second side of the horizontal glass plate. For example, a first interferometer may be configured to direct a first beam of the second set of beams toward a first location X2 on the horizontal glass plate, a second interferometer may be configured to direct a second beam of the second set of beams toward a second location Y2 on the horizontal glass plate, and a third interferometer may be configured to direct a third beam of the second set of beams toward a third location Z2 on the horizontal glass plate. The second direction D2 may be opposite to the first direction D1. For example, the first direction D1 may be downward and the second direction D2 may be upward.

[0049] Three locations X1, Y1, and Z1 on a first side of the horizontal glass plate can be aligned with three locations X2, Y2, and Z2 on a second side of the horizontal glass plate. This allows the first set of beams to be aligned with the second set of beams on opposite sides of the horizontal glass plate and with reference to the three shared locations X, Y, and Z. The three shared locations X, Y, and Z can be any three non-collinear points on the horizontal glass plate. For example, the three shared locations X, Y, and Z can be arranged in a triangular configuration. The three shared locations X, Y, and Z can surround the light beam 111 or can be arranged on one side of the light beam 111. The three shared locations X, Y, and Z can surround the aperture 134. According to certain embodiments of the present disclosure, the three shared locations X, Y, and Z can be spaced farther from the light beam 111 than the edges of the target reticle 112. For example, the target reticle may be approximately 6 inches square, with each of the three shared locations X, Y, and Z located approximately 3.5 inches from the light beam 111. By arranging the three shared locations X, Y, and Z as close as possible to the light beam 111, measurement accuracy can be improved.

[0050] In step 250, a first set of beams and a second set of beams can be directed toward the horizontal glass plate. For example, the first set of beams can be directed toward a first side of the horizontal glass plate to locations X1, Y1, and Z1, and the second set of beams can be directed toward a second side of the horizontal glass plate to locations X2, Y2, and Z2.

[0051] In step 260, a processor can measure imaging device-to-isolation plate distances S1a, S2a, and S3a and stage-to-isolation plate distances S1b, S2b, and S3b based on the beams reflected from the horizontal glass plate. For example, the processor can be configured to measure a first distance S1a between a first interferometer and a first location X1 based on a first beam of the first set of beams reflected from a first side of the horizontal glass plate, measure a second distance S2a between a second interferometer and a second location Y1 based on a second beam of the first set of beams reflected from the first side of the horizontal glass plate, and measure a third distance S3a between a third interferometer and a third location Z1 based on a third beam of the first set of beams reflected from the first side of the horizontal glass plate. The processor 150 can also be configured to measure a first distance S1b between the first interferometer and the first location X2 based on a first beam of the second set of beams reflected from the second side of the horizontal glass plate, measure a second distance S2b between the second interferometer and the second location Y2 based on a second beam of the second set of beams reflected from the second side of the horizontal glass plate, and measure a third distance S3b between the third interferometer and the third location Z2 based on a third beam of the second set of beams reflected from the second side of the horizontal glass plate. The first and second sets of beams can be unobstructed between the respective interferometers and the horizontal glass plate.

[0052] According to an embodiment of the present disclosure, as shown in FIG. 6, the method 200 may further include the following steps.

[0053] In step 270, the processor can compare the sums S1, S2, and S3 of the imaging device-to-isolation plate distances S1a, S2a, and S3a and the stage-to-isolation plate distances S1b, S2b, and S3b with preset sums P1, P2, and P3. For example, the first sum S1 can be the sum of the first distances S1a and S1b, the second sum S2 can be the sum of the second distances S2a and S2b, and the third sum S3 can be the sum of the third distances S3a and S3b.

[0054] In step 280, the processor can determine the relative motion between the imaging device and the stage between t1 and t2, where t2 is the current time and t1 is a time prior to t2. The relative motion can be associated with motion in one of six degrees of freedom, including surge (forward / backward motion along the Y direction), heave (up / down motion along the Z direction), sway (side / side motion along the X direction), roll (rotation about the Y axis), pitch (rotation about the X axis), and yaw (rotation about the Z axis). For example, the sums S1, S2, and S3 can be used to determine heave, pitch, and roll.

[0055] According to an embodiment of the present disclosure, the plurality of interferometers may further include an X interferometer, a Y interferometer, and an Rz interferometer. The X interferometer, the Y interferometer, and the Rz interferometer may be disposed within a vacuum region adjacent to the stage or outside the isolation plate. The X interferometer may be configured to direct an X beam along the X direction toward a first side of the stage. The Y interferometer may be configured to direct a Y beam along the Y direction toward a second side of the stage. The Rz interferometer may be configured to direct an Rz beam along the Y direction toward the second side of the stage. The Rz interferometer may be disposed at a fixed distance, e.g., 50 mm, from the Y interferometer. The first and second sides of the stage may be mirrors such that the X, Y, and Rz beams are reflected by the stage. Therefore, step 250 may also direct the X, Y and Rz beams towards the stage.

[0056] The processor can be further configured to measure a fourth distance S4 between the X interferometer and the stage based on the X beam reflected from the first side of the stage, a fifth distance S5 between the Y interferometer and the stage based on the Y beam reflected from the second side of the stage, and a sixth distance S6 between the Rz interferometer and the stage based on the Rz beam reflected from the second side of the stage. The processor can compare distances S4, S5, and S6 with preset distances P4, P5, and P6 to determine the relative motion of stage 120. Thus, step 260 can further measure distances S4, S5, and S6.

[0057] The fourth distance S4 can be used to determine sway (side-to-side movement along the X direction), the fifth distance S5 can be used to determine surge (front-to-back movement along the Y direction), and the fifth distance S5 and the sixth distance S6 can be used to determine yaw (rotation about the Z axis). Thus, in step 280, the multiple interferometers can also be used to determine relative motion in all six degrees of freedom.

[0058] According to certain embodiments of the present disclosure, preset sums P1, P2, and P3 can be associated with sums S1, S2, and S3 of the imaging device-to-isolation plate distances S1a, S2a, and S3a and the stage-to-isolation plate distances S1b, S2b, and S3b measured at time t1. This allows the relative motion determined by the processor to take into account changes in the measured sums S1, S2, and S3 between time t1 and time t2. The measurement frequency can be in the MHz range.

[0059] According to certain embodiments of the present disclosure, a reflective coating layer can be provided on one side of the horizontal glass plate. The first set of beams and the second set of beams can be reflected by the reflective coating layer. An anti-reflective coating layer can also be provided on the other side of the horizontal glass plate. One of the first set of beams and one of the second set of beams can pass through the anti-reflective coating layer. In this way, any effect of variations in thickness of the horizontal glass plate can be minimized when determining the sum values ​​S1, S2, and S3.

[0060] Method 200 can provide benefits to the inspection process. For example, because the imaging device and stage do not need to support large mirrors, the weight of these moving parts is reduced and the interferometer can be located more conveniently on the imaging device and stage. Instead, a single horizontal glass plate is mounted on a stationary isolation plate. Any drift or movement of the horizontal glass plate due to system vibration is common between measurements obtained from the imaging device and those obtained from the stage, so no errors are introduced into the system. The horizontal mirror can provide an improved signal-to-noise ratio for the interferometer compared to a 45-degree mirror, which is prone to misalignment. Additionally, the beam path lengths of the first and second sets of beams are relatively short compared to conventional systems, reducing susceptibility to wavelength instability.

[0061] In some embodiments of the present disclosure, a non-transitory computer-readable storage medium configured to perform steps 260-280 of method 200 may be provided.

[0062] Although disclosed in the context of optical systems, aspects of the disclosed embodiments may also be used with electron beam systems.

[0063] Although the present disclosure has been described in connection with one or more specific embodiments, it will be understood that other embodiments of the present disclosure may be made without departing from the spirit and scope of the present disclosure, which is to be construed as limited only by the appended claims and their reasonable interpretation.

Claims

1. 1. An apparatus comprising: an imaging device configured to receive the light beam from the target reticle; a stage movable relative to the imaging device and configured to hold the target reticle; an isolation plate provided between the imaging device and the stage, the isolation plate including a horizontal glass plate; a plurality of interferometers in electronic communication with a processor; three interferometers disposed on the imaging device and configured to direct a first set of beams along a first direction toward three locations on the horizontal glass plate; and three interferometers disposed on the stage and configured to direct a second set of beams toward the three locations on the horizontal glass plate along a second direction opposite the first direction; a plurality of interferometers including: wherein the processor is configured to measure a distance between the imaging device and the isolation plate and a distance between the stage and the isolation plate at each of the three locations on the horizontal glass plate based on the first set of beams and the second set of beams reflected by the horizontal glass plate.

2. 2. The apparatus of claim 1, wherein the first direction is downward and the second direction is upward.

3. 2. The apparatus of claim 1, wherein a reflective coating is provided on a first side of the horizontal glass plate, and the first set of beams and the second set of beams are reflected by the reflective coating.

4. 4. The apparatus of claim 3, wherein an anti-reflective coating is provided on a second side of the horizontal glass plate opposite the first side, and one of the first set of beams and the second set of beams passes through the anti-reflective coating.

5. 2. The apparatus of claim 1, wherein the imaging device and the stage are provided in separate vacuum regions separated by the isolation plate.

6. 10. The apparatus of claim 1, wherein the imaging device is disposed on a passive vibration isolator.

7. 2. The apparatus of claim 1, wherein the first set of beams and the second set of beams are uninterrupted between each interferometer and the horizontal glass plate.

8. 2. The apparatus according to claim 1, wherein a sum of the distance between the imaging device and the isolation plate and the distance between the stage and the isolation plate at each of the three locations is compared with a preset sum. １ ・t ２ Relative motion between the imaging device and the stage during t ２ is the current time, t １ is ２ A device for determining a time earlier than

9. 9. The apparatus of claim 8, wherein the relative motion corresponds to motion in one of six degrees of freedom including surge, heave, sway, roll, pitch, and yaw.

10. 9. The apparatus of claim 8, wherein the preset sum value is t １ The distance between the imaging device and the isolation plate and the distance between the stage and the isolation plate at each of the three locations measured in the above-mentioned manner are equivalent to the sum of the distances measured in the above-mentioned manner.

11. 1. A method comprising: providing an imaging device configured to receive the light beam from the target reticle; providing a stage movable relative to the imaging device, the stage configured to hold the target reticle; an isolation plate having a horizontal glass plate is provided between the imaging device and the stage; Providing a plurality of interferometers in electronic communication with a processor, the plurality of interferometers comprising: three interferometers disposed on the imaging device and configured to direct a first set of beams along a first direction toward three locations on the horizontal glass plate; and three interferometers disposed on the stage and configured to direct a second set of beams toward the three locations on the horizontal glass plate along a second direction opposite the first direction; and directing the first set of beams and the second set of beams toward the horizontal glass plate; a method for measuring, using the processor, a distance between the imaging device and the isolation plate and a distance between the stage and the isolation plate at each of the three locations on the horizontal glass plate based on the first set of beams and the second set of beams reflected by the horizontal glass plate.

12. 12. The method of claim 11, further comprising providing a reflective coating on a first side of the horizontal glass plate, the first set of beams and the second set of beams being reflected by the reflective coating.

13. 13. The method of claim 12, further comprising providing an anti-reflective coating on a second side of the horizontal glass plate opposite the first side, and causing one of the first set of beams and the second set of beams to pass through the anti-reflective coating.

14. 12. The method of claim 11, wherein the imaging device and the stage are provided in separate vacuum regions separated by the isolation plate.

15. 12. The method of claim 11, wherein the imaging device is disposed on a passive vibration isolator.

16. 12. The method of claim 11, wherein the first set of beams and the second set of beams are not blocked between each interferometer and the horizontal glass plate.

17. 12. The method of claim 11 further comprising: using the processor to compare a sum of the distance between the imaging device and the isolation plate and the distance between the stage and the isolation plate at each of the three locations with a preset sum; Using the processor, t １ ・t ２ Relative motion between the imaging device and the stage during t ２ is the current time, t １ is ２ A method for determining a time earlier than

18. 18. The method of claim 17, wherein the relative motion corresponds to motion in one of six degrees of freedom including surge, heave, sway, roll, pitch, and yaw.

19. 18. The method of claim 17, wherein the preset sum value is t １ a method of associating the measured distance between the imaging device and the isolation plate at each of the three locations with the sum of the measured distance between the stage and the isolation plate at each of the three locations.

20. A non-transitory computer-readable storage medium configured to perform the measurements of claim 11.

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