Coaxial transmission type alignment imaging system

Abstract

Aspects of the present disclosure also provide an imaging system. For example, in the imaging system, a first light source can generate a first light beam having a first wavelength, a second light source can generate a second light beam having a second wavelength, the second light beam having sufficient power to pass through at least a portion of the thickness of the wafer, an alignment module can coaxially align the second light beam with the first light beam, a coaxial module can focus the coaxially aligned first and second light beams onto a first pattern disposed on the front side of the wafer and a second pattern disposed below the first pattern, respectively, and an image acquisition module can acquire a first image of the first pattern and a second image of the second pattern. The second image can be acquired by quantum tunneling imaging or infrared (IR) transmission imaging.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This disclosure claims priority to U.S. Provisional Application No. 63 / 066,779, filed Aug. 17, 2020, entitled "Method for Producing Overlay Results with Absolute Reference for Semiconductor Manufacturing," which is incorporated by reference in its entirety.

[0002] The present disclosure relates generally to methods for manufacturing semiconductor devices, and more particularly to overlay errors. [Background technology]

[0003] The manufacturing of semiconductors involves several different steps and processes. One typical manufacturing process is known as photolithography (also called microlithography). Photolithography is a technique that uses irradiation, such as ultraviolet or visible light, to create fine patterns according to the design of a semiconductor device. Semiconductor manufacturing techniques such as photolithography, etching, film deposition, surface cleaning, and metallization can be used to construct various semiconductor devices, such as diodes, transistors, and integrated circuits. Summary of the Invention [Means for solving the problem]

[0004] Aspects of the present disclosure also provide an imaging system. For example, the imaging system may include a first light source, a second light source, an alignment module, a coaxial module, and an image acquisition module. The first light source may be configured to generate a first light beam having a first wavelength. The second light source may be configured to generate a second light beam having a second wavelength. The alignment module may be configured to coaxially align the second light beam with the first light beam. The coaxial module may be configured to focus the coaxially aligned first and second light beams onto a first pattern disposed on the front side of the wafer and a second pattern disposed below the first pattern, respectively. The image acquisition module may be configured to acquire a first image of the first pattern and a second image of the second pattern. The second light beam may have sufficient power to pass through at least a portion of the thickness of the wafer to reach the second pattern.

[0005] In one embodiment, the second wavelength may be longer than the first wavelength. For example, the first light source may be a UV light source and the second light source may be an IR light source, including, for example, an IR tunable quantum cascade laser. As another example, the first wavelength may be 50-400 nanometers, such as 266 nanometers, and the second wavelength may be 1-10 micrometers, such as 3.6 or 3.7 micrometers.

[0006] In one embodiment, the image acquisition module can acquire a second image of the second pattern via quantum tunneling imaging or IR transmission imaging.

[0007] In one embodiment, the second pattern can be incorporated into a reference plate location on the underside of the wafer, hi another embodiment, the imaging system may further include a substrate holder configured to hold the wafer, and the reference plate is incorporated into the substrate holder.

[0008] As will be appreciated, as fabrication progresses on a given wafer, many different materials and layers may occur depending on the given device being formed. Thus, at each process step, each wafer may have a different shape, which means that different wavelengths may be required to pass through the wafer.

[0009] Of course, the order of description of the different steps described herein is shown for clarity of description. In general, these steps can be performed in any suitable order. Also, although different features, techniques, configurations, etc. of this specification may each be mentioned in different places in this disclosure, it is understood that each concept can be performed independently of each other or in combination with each other. Thus, the present disclosure can be implemented and viewed in many different ways.

[0010] Please note that the summary does not specify every embodiment and / or incrementally novel aspect of the disclosure or the disclosed subject matter of the claims. Rather, the summary merely provides a preliminary description of different embodiments and points out corresponding novel aspects over the prior art. For additional details and / or possible perspectives of the disclosure and embodiments, the reader is directed to the detailed description of the disclosure and corresponding drawings, which are further described below.

[0011] Various embodiments of the present disclosure, presented by way of example only, will now be described in detail with reference to the following figures, in which like reference numbers refer to like elements and in which: [Brief description of the drawings]

[0012] [Figure 1A] This shows the overlay problem during mass production. [Figure 1B] 1 illustrates the reduction of superposition load using an exemplary reference pattern, according to some embodiments of the present disclosure. [Diagram 2] FIG. 1 illustrates a functional block diagram of an exemplary imaging system according to some embodiments of the present disclosure. [Diagram 3] 3 illustrates an enlarged view showing a portion of a coaxially aligned light beam produced by the exemplary imaging system of FIG. 2. [Figure 4A] 3 illustrates a close-up top view of overlaid images of a portion of a wafer captured by first and second image capture devices of the exemplary imaging system of FIG. 2, according to some embodiments of the present disclosure. [Figure 4B] 1 illustrates an exemplary image analysis for overlay calculation using an absolute independent reference pattern, according to some embodiments of the present disclosure. [Diagram 5] 1 shows a flow diagram illustrating an exemplary imaging method according to some embodiments of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] According to the present disclosure, imaging systems and imaging methods are provided that use an absolute and independent reference pattern as an alignment mark for aligning a feature pattern. The feature pattern may be formed on the front side of the wafer, and the reference pattern is independent of the front side of the wafer. For example, the reference pattern may be formed inside or on the underside of the wafer. A first light beam having a first wavelength (e.g., an ultraviolet (UV) light beam) can be used to image the feature pattern formed on the first side of the wafer, and a second light beam having a second wavelength (e.g., an infrared (IR) light beam) can be used to image the reference pattern formed inside or on the underside of the wafer. In one embodiment, the second light beam can be coaxially aligned with the first light beam. Since the reference pattern is formed inside or on the underside of the wafer, the second light beam needs to "penetrate" some or all of the thickness of the wafer to image the reference pattern. For example, the second light beam may have sufficient power or intensity to quantum tunnel through some or all of the thickness of the wafer depending on whether the reference pattern is formed within or under the wafer to capture an image of the reference pattern using quantum tunneling imaging, IR transmission imaging, or the like. Thus, a UV image of the feature pattern and an IR image of the reference pattern may be captured on the same optical axis and overlaid on one another. Image analysis may then be performed to expose, inspect, align, or otherwise process. The UV and IR images are captured coaxially, but transmission to the image detectors may or may not be coaxial. For example, the coaxially captured images may be optically separated and transmitted to separate image detectors as described below.

[0014] The following disclosure provides many different embodiments or examples that implement different features of the provided subject matter. Specific examples of elements and configurations are described below to simplify the disclosure. Of course, these are merely examples and are not intended to limit the invention. For example, forming a first feature above or directly above a second feature in the following description may include embodiments in which the first and second features are formed in direct contact, as well as embodiments in which an additional feature is formed between the first and second features such that the first and second features are not in direct contact. Also, reference numbers and / or letters may be repeated in various examples of the disclosure. This repetition is for the purposes of brevity and clarity, and does not in itself dictate a relationship between the various embodiments and / or configurations described. Furthermore, spatial relationship terms such as "top," "bottom," "lower," "lower," "lower," "upper," "top," and the like may be used herein to facilitate the description of the relationship of one element or feature to another element(s) or feature(s) as illustrated. The spatial relationship terms are intended to include different orientations of the device in use or operation in addition to the orientation depicted. The device may be in other orientations (rotated 90 degrees or at other orientations) and the spatial relationship descriptors used herein may be interpreted accordingly.

[0015] The order of description of the different steps described herein is shown for the sake of clarity. In general, these steps can be performed in any suitable order. Also, although different features, techniques, configurations, etc. of this specification may each be mentioned in different places in this disclosure, it is understood that each concept can be performed independently of each other or in combination with each other. Thus, the present disclosure can be implemented and perceived in many different ways.

[0016] Microfabrication is the formation and processing of multiple films and layers on a wafer. This may include stacking 10 or more layers of films on the wafer. The patterns provided on the wafer to form the various films and layers must be aligned to previously formed patterns. Traditionally, such alignment is achieved by using a portion of the wafer to form alignment marks and scribe lines. However, the inventors have recognized that various film deposition, etching, and processing techniques sometimes cover the alignment marks or may completely remove the alignment marks. Because the alignment marks are sometimes covered or missing, errors may occur when subsequent patterns are formed on the wafer. The term overlay or overlay error refers to the difference between the placement of a given pattern and a previously placed pattern. If the alignment marks are constantly destroyed, overlay errors may accumulate as layers are added, resulting in performance degradation and device malfunction.

[0017] FIG. 1A illustrates the industrial problem of overlay. Each arrow in the figure has a start point (e.g., 111A, 111B, 121A, and 131A) corresponding to the location of a predecessor pattern, and an end point or arrowhead (e.g., 111A, 111N', 121N') corresponding to the location of a subsequent pattern. As a result, each arrow represents an overlay value or overlay error when a subsequent pattern is formed overlaying or side-by-side with a corresponding predecessor pattern. For example, in step 110A, no grid or reference plate is present when placing an initial pattern. Thus, the start point 111A of the first arrow is likely to be misaligned, i.e., the initial pattern may have a placement error, e.g., relative to the edge of the wafer. The subsequent pattern then attempts to align based on the corresponding last pattern. As shown in FIG. 1A, the start point of the subsequent arrow (e.g., 111B) overlaps with the end or previous arrowhead (e.g., 111A') of the corresponding last or previous arrow. It should be noted that even in a theoretically perfect system, overlays may occur. For example, if a system has a pattern placement tolerance of + / - 4nm and each level references the previous level, then a put-out may occur. Take the reference level as 0 error. The first layer may therefore be off by +4nm. The alignment of the second layer relative to the first layer may be off by +4nm, meaning the second layer is off by +8nm from the reference. Even if the initial alignment marks are visible, there are process factors that create or dissipate stresses throughout the manufacturing process that can cause put-outs / misalignments that can add to the accumulated error.

[0018] Additionally, alignment marks may be destroyed in step S120 of the manufacturing process and repositioned without the reference marks. Degradation of the alignment marks may result in accumulation of alignment errors in post-processing. Similar to the starting point 111A, the starting point 121A of the new arrow may also be misaligned. In the example of FIG. 1A, the starting point 121A is misaligned from the arrowhead 111N'. The process proceeds by aligning subsequent patterns based on the corresponding last pattern until the alignment marks are destroyed again in step S130. Similarly, placement is performed without the reference marks and the starting point 131A is misaligned from the arrowhead 121N'. As can be seen from FIG. 1A, as layers are added, overlay errors may accumulate leading to reduced manufacturing yields, device malfunctions, etc. It should be noted that process 110A is a non-limiting example. Other processes (e.g., 110B and 110C) may have different overlay values ​​(different arrows) and / or different steps.

[0019] The techniques herein do not require the creation and re-creation of alignment marks. Instead, an absolute reference pattern is required, i.e., a reference pattern that is independent of the wafer or patterns stacked on the wafer. Each pattern applied to the wafer can be aligned to that independent reference pattern without relying on previous patterns.

[0020] FIG. 1B illustrates the reduction of overlay load using an exemplary reference pattern according to some embodiments of the present disclosure. With the techniques herein, all patterns placed on the front side 191 of the wafer 190 (e.g., the pattern having the origin 141A) are based on the same reference pattern 102. In one embodiment, the reference pattern 102 may be placed on the underside of the front side 191 of the wafer 190. For example, the reference pattern 102 may be formed on the back side 192 of the wafer 190 or may be built into the wafer 190. As another example, the reference pattern 102 may be built into a reference plate (not shown in FIG. 1B), which may be placed on the underside of the wafer 190, placed or glued to the back side 192 of the wafer 190, or built into a substrate holder of a photolithography scanner or stepper (not shown in FIG. 1B) used to hold the wafer 190. In other words, the reference pattern 102 is not affected by lithographic processes such as etching, deposition, chemical mechanical polishing, etc., that are performed on the front side 191 of the wafer 190 to form a pattern. Thus, the reference pattern 102 is independent of the front side 191 of the wafer 190 and is not affected during lithographic processing of the wafer 190. Thus, the reference pattern 102 is considered to be available and absolute, or rather, independent of any patterns formed on the front side 191 of the wafer 190 and does not change due to the various deposition and etching steps performed on the wafer 190. In one embodiment, the reference pattern 102 can be compared to the wafer 190 when placing a new pattern. In the case of an initial pattern, this means that the pattern can be fitted to the reference pattern 102. In the case of a subsequent pattern, this means that one or more patterns can still be compared to the reference pattern 102 and overlay corrections can be calculated to bring them back into the same alignment.

[0021] For example, in step 140, the initial pattern can be aligned to the front side 191 of the wafer 190 using the reference pattern 102. In one embodiment, the reference pattern 102 can be provided at a fixed location relative to the wafer surface, such as by embedding the reference pattern 102 in the wafer 190 or by providing the reference pattern 102 fixedly on the back side 192 of the wafer 190. As a result, the start point 141A of the first arrow is aligned to the reference pattern 102 at a location shown as the reference line 150. Subsequent patterns are also aligned using the fixed, absolute and independent reference pattern 102. Although a new layer of photoresist may be formed for each subsequent pattern, there is no need to create and / or destroy alignment marks on the wafer 190 due to the reference pattern 102. As a result, multiple arrows converge around the reference line 150, meaning that the subsequent patterns are aligned to the reference pattern 102. The alignment can occur, for example, by moving a mask of the pattern image or by moving the wafer 190 relative to the mask. Therefore, there is little chance of overlay errors accumulating as additional layers are formed.

[0022] 2 is a functional block diagram of an exemplary imaging system 200 according to some embodiments of the present disclosure. For example, the exemplary imaging system 200 can be implemented in a scanner or stepper of a lithography system. As another example, the exemplary imaging system 200 can be implemented in a resist coating tool, such as a CLEAN TRACK™ ACT™ 12 manufactured by Tokyo Electron Limited, which includes multiple mask-specific modules, such as an advanced soft bake oven unit, an edge bead removal module, and a cleaning system. The exemplary imaging system 200 can coaxially align two light beams having different wavelengths and focus the two coaxially aligned light beams onto a first pattern disposed on a front side of a substrate (e.g., a wafer) and a second pattern disposed below the first pattern, respectively, to obtain images of the first and second patterns. For example, the exemplary imaging system 200 may include a first light source 210, a second light source 220, an alignment module 230, a coaxial module 240, a first image acquisition device 250, and a second image acquisition device 260. The first image acquisition device 250 and the second image acquisition device 260 may be collectively referred to as an image acquisition module.

[0023] In one embodiment, the first light source 210 may be configured to generate a first incident light beam having a first wavelength. For example, the first light source 210 may generate a first incident light beam of 50 to 400 nanometers, such as 266 nanometers (UV in FIG. 2). incident As another example, the first light source 210 may be an UV light source generating a 1000 nm wavelength (shown as 1000 nm) of 1000 nm light (shown as 1000 nm) in the form of a 1000 nm wavelength. As another example, the first light source 210 may be an Optowaves (Optowares Inc., Massachusetts, USA) solid-state laser, such as a pumped nanosecond laser for surface imaging.

[0024] In one embodiment, the second light source 220 may be configured to generate a second incident light beam having a second wavelength. According to some aspects of the present disclosure, an absolute and independent reference pattern is placed below the pattern to be formed on the front side of the wafer and the second incident light beam is used to image the reference pattern, which must penetrate at least a portion or all of the thickness of a wafer, such as wafer 290.

[0025] For example, the second incident light beam has sufficient power or intensity to penetrate the entire thickness (e.g., 750 micrometers) of the wafer 290 to acquire an image of the reference pattern using quantum tunneling imaging, IR transmission imaging, etc. As another example, the second light source 220 may emit a 1-10 micrometer, e.g., 3.6 or 3.7 micrometer (IR in FIG. 2 ) irradiated light. incident The second light source 220 may be an IR light source generating a second incident light beam (denoted as IR 1 / e). In one embodiment, the second light source 220 may be an IR tunable quantum cascade laser available from Pranalytica, Inc. (California, USA). According to the evanescent wave theory, the intensity of a light beam impinging on a surface (e.g., the front side 391 of the wafer 290 as shown in FIG. 3) between two different media (e.g., the wafer 290 and the air or liquid in an immersion lithography where the coaxial module 240 is located) decays exponentially perpendicular to the surface. The penetration depth at which the intensity drops to 1 / e (approximately 37%) depends, inter alia, on the wavelength of the light beam. A typical penetration depth may be a fraction of the wavelength of the light beam, e.g., one-fifth of the wavelength, depending on the angle of incidence of the light beam with respect to the surface. The second incident light beam IR incident The second wavelength is the first incident light beam UV incident Since the wavelength of the second incident light beam IR is much longer than the first wavelength, if the output is properly controlled, incident can pass through the entire thickness of the wafer 290.

[0026] In one embodiment, the relative positions of the first (UV) light source 210 and the second (IR) light source 220, also referred to as the relative positions of the red and blue calibrations, can be periodically calibrated. For example, the relative positions of the first light source 210 and the second light source 220 can be maintained within a few decades, thus within a fairly wide sensor dynamic range. However, normalization can be performed by imaging a stage artifact of known relative transmittance as needed, for example once a day, so that any relative intensity normalization can be easily performed. Relative position or TIS tool-derived shift calibration is common to metrology instruments. The relative positions are recalibrated to the grid plate in real time as measurements are made. Thus, the exemplary imaging system 200 can always have a real-time absolute reference. Digital image acquisition and regression may be used.

[0027] In one embodiment, the alignment module 230 aligns the second incident light beam IR incident The first incident light beam UV incident For example, the alignment module 230 may be configured to align the first incident light beam UV incident The optical fiber optics may include a first optical beam splitter capable of splitting the first incoming optical beam UV into two portions, transmitting one portion and reflecting the other portion. In one embodiment, the first optical beam splitter may be a prism. In another embodiment, the first optical beam splitter may be a transparent plate, such as a glass or plastic sheet, coated on one side with a partially transparent thin film of metal, such as aluminum, and configured to split the first incoming optical beam UV. incident In the exemplary imaging system 200, a first light source 210 and a first optical beam splitter can transmit a first incident light beam UV incident can be positioned to be incident at a 45 degree angle on the first optical beam splitter.

[0028] For example, the alignment module 230 may further include a second incident light beam IR incidentThe second optical beam splitter may include a second optical beam splitter capable of splitting the second incoming optical beam IR into two portions, reflecting one portion and transmitting the other. For example, the second optical beam splitter may be a prism. As another example, the second optical beam splitter may be a glass or plastic sheet coated on one side with a thin film of aluminum, and may be configured to split the second incoming optical beam IR into two portions, reflecting one portion and transmitting the other portion. incident In the exemplary imaging system 200, the second light source 220 and the second optical beam splitter can reflect a portion of the second incident light beam IR and transmit another portion. incident can be positioned to be incident at a 45 degree angle on the second optical beam splitter.

[0029] For example, the alignment module 230 may further include a third beam splitter that can reflect or transmit light beams of different wavelengths. For example, the third beam splitter may reflect or transmit a first incident light beam UV having a first wavelength transmitted from the first optical beam splitter. incident a second incident light beam IR having a second wavelength that is reflected and transmitted from the second optical beam splitter. incident In one embodiment, the third beam splitter may be a transparent plate coated on one side with a dichroic material that transmits the transmitted second incident light beam IR. incident However, the first incident light beam UV incident A second incident light beam IR is coaxially aligned with and transmitted through incident and the reflected first incident light beam UV incident are designed and positioned to move along the same optical path to the wafer 290 .

[0030] In one embodiment, the coaxial module 240 splits the first incident light beam UV reflected from the third beam splitter. incident 3. The second incident light beam IR is focused onto a first pattern 301 (shown in FIG. 3) disposed on the front side 391 of the wafer 290 and transmitted from a third beam splitter. incidentmay be configured to focus the coaxially aligned first incident light beam UV onto a second pattern 302 (or reference pattern) disposed below the first pattern 301. For example, the coaxial module 240 may be designed and configured to adjust for the tolerance of the placement of the first pattern 301 and the second pattern 302 (i.e., depth of focus (DOF)). For example, a level sensor may be used to track the top of the first pattern 301 and subtract the height of the wafer 290 from the height of the first pattern 301 to focus the coaxially aligned first incident light beam UV. incident and a second incident light beam IR incidient The DOF can be automatically adjusted simultaneously. Damage to the photoresist by deep UV (DUV) light is negligible. The field of view (FOV) of 250 micrometers in this specification corresponds to about 60 nanometers per pixel for 4K resolution. This is sufficient resolution to measure registration errors of 0.1 nanometers. By having a light source of sufficient power or intensity, shadowing of the metal layer can be mitigated. Although FIG. 3 shows imaging of a physical pattern formed on the wafer 290, the image of the pattern to be formed (i.e., before exposure to the activating light) can be achieved, for example, with light having a wavelength that does not activate the photoresist in the wafer.

[0031] In one embodiment, the coaxial module 240 may include 2-12 individual optical elements, for example, 6 optical elements. Each optical element may include sapphire, AlN, MgF, CaF, BaF, LiF, Ge, Si, or the like.

[0032] First incident light beam UV incident is reflected by the first pattern 301 to form a first reflected light beam UV refelection The first reflected light beam UV reflection The second incident light beam IR can be reflected by the third beam splitter and the first optical beam splitter and captured by the first image capture device 250, which can form a corresponding first image of the first pattern 301. For example, the first image capture device 250 can be a DataRay camera. incidentis reflected by the second pattern 302 to form a second reflected light beam IR reflection A second reflected light beam IR can be formed. reflection may be sequentially transmitted by the third beam splitter and the second optical beam splitter and captured by the second image capture device 260, which may form a corresponding second image of the second pattern 302. For example, the second image capture device 260 may be a high speed, high definition mid-wavelength IR (MWIR) camera, such as a FLIRX8500MWIR. In one embodiment, image analysis may be performed on the first image and the second image to calculate a registration value to determine the placement of the first pattern 301. For example, the image analysis may be accomplished by overlaying a first image of the first pattern 301 and a second image of the second pattern 302 on one another to identify a coordinate location of the first pattern 301 relative to the second pattern 302. In some embodiments, the image analysis may be performed in real time such that the placement of the first pattern 301 may be adjusted in real time.

[0033] In one embodiment, the alignment module 230 may further include a first lens set and a second lens set. For example, the first lens set may be configured to focus the first incident light beam UV generated by the first light source 210. incident , and collimated the first incident light beam UV incident As another example, the second lens set may also include reflective and / or refractive optics to direct the second incoming light beam IR generated by the second light source 220 to the first optical beam splitter. incident and collimating a second collimated incident light beam IR incident The optical fiber may include reflective and / or refractive optics to direct the first optical beam to the second optical beam splitter.

[0034] In one embodiment, the exemplary imaging system 200 may further include a third lens set 270 and a fourth lens set 280. For example, the third lens set 270 may focus the first reflected light beam UV reflectionThe fourth lens set 280 may also include reflective and / or refractive optics to focus the second reflected light beam IR onto the first image capture device 250. As another example, the fourth lens set 280 may also focus the second reflected light beam IR onto the first image capture device 250. reflection The second image capture device 260 may also include reflective and / or refractive optics to focus the reflected light onto the second image capture device 260.

[0035] In one embodiment, the exemplary imaging system 200 may further include optics capable of acquiring and directing the diffracted light beams outside the coaxial module 240 to the first image acquisition device 250 and the second image acquisition device 260 .

[0036] 3, the first pattern 301 may be included on a photomask (not shown) disposed on the front side 391 of the wafer 290. In one embodiment, the photomask may be disposed in direct contact with the wafer 290 in a contact printing system. In another embodiment, the photomask may be disposed at a location remote from the wafer 290 in a proximity printing system or a projection printing system.

[0037] In the exemplary embodiment shown in FIG. 3, the second pattern 302 is disposed on the backside 392 of the wafer 290 and is irradiated with a second incident light beam IR. incidenthas sufficient power to penetrate the entire thickness of the wafer 290 to capture a second image of the second pattern 302 using quantum tunneling imaging, IR transmission imaging, or the like. In one embodiment, the second pattern 302 can be formed on a reference plate 310. For example, the reference plate 310 can be a grid plate including a plurality of nearly perfectly aligned 20 micrometer by 20 micrometer squares, and the second pattern 302 can be at least one corner point of the plurality of squares. As another example, the reference plate 310 can include at least one of dots, lines, corners, boxes, numbers, marks, or any other pattern suitable for alignment purposes, and the second pattern 302 can be one of these. In one embodiment, the reference plate 310 can be bonded to the backside 392 of the wafer 290. Thus, the reference plate 310 and the wafer 290 can function as one module. In another embodiment, the reference plate 310 can be integrated into a substrate holder 320 of a photolithography scanner or stepper. Each time a given wafer may be placed on the substrate holder 320 in a different position or orientation compared to previous placements, but this is not critical. When placing or exposing a given new pattern, the wafer can be imaged by a reference plate 310, e.g., a grid plate. The reference plate 310 thus provides a relative reference point that identifies vectors representing two or more points from which overlay correction adjustments in the next exposure can be calculated using vector analysis. For example, when a wafer 290 that does not yet have a pattern is placed on the reference plate 310, the wafer 290 is roughly pre-aligned with respect to the reference plate 310. As another example, when a wafer 290 that already has an existing pattern is placed on the reference plate 310, the existing pattern and the reference plate 310 can be coaxially aligned. In conventional lithography processes, measurement errors caused by wafer backside scratches, backside dust, and / or thermally induced substrate distortion can affect overlay, but conventional overlay systems often fail to recognize these issues. The technique of the present invention includes an independent reference plate and high spatial resolution that overcomes these issues.

[0038] In one embodiment, the second pattern 302 may be formed on the backside 392 of the wafer 290 and the second incident light beam IR incident is also sufficiently strong to penetrate the entire thickness of the wafer 290 to obtain a second image of the second pattern 302 using quantum tunneling imaging, IR transmission imaging, etc. Other techniques may include embedding the second pattern 302 (e.g., grid lines) in the wafer 290, such as with an emissive or fluorescent material.

[0039] In one embodiment, the second pattern 302 can be formed on the front side 291 of the wafer 290, and then a layer of silicon and / or silicon oxide is deposited thereon. For example, the layer of silicon and / or silicon oxide can have a thickness of 1-5 micrometers such that the second pattern 302 is effectively "embedded" in the wafer 290, forming a pattern on the layer of silicon and / or silicon oxide. Thus, the second incident light beam IR incident should be strong enough to pass through the silicon and / or silicon oxide layer to obtain a second image of the second pattern 302 using quantum tunneling imaging, IR transmission imaging, or the like. As another example, the second pattern 302 can be formed on the backside 292 of the wafer 290 before a protective layer, such as silicon or silicon oxide, is formed on the backside 292 of the wafer 290. As a result, the second pattern 302 can be embedded in the wafer 290. Thus, the second incident light beam IR incident must be strong enough to penetrate the entire thickness of the wafer 290 to capture a second image of the second pattern 302 using quantum tunneling imaging, IR transmission imaging, or the like. In one embodiment, the second pattern 302 can be formed on the front side of the carrier wafer before the front side of the carrier wafer is bonded to the back side of the target wafer (e.g., back side 392 of wafer 290). As a result, the second pattern 302 can be sandwiched between the carrier wafer and the target wafer, which together function as a single wafer. Thus, the second incident light beam IR incidentmust be strong enough to penetrate the entire thickness of the target wafer to obtain a second image of the second pattern 302 using quantum tunneling imaging, IR transmission imaging, etc. In some embodiments, optical projection may be used. For example, the second pattern 302 may be a projection grid that is not physically present in the wafer 290 placed on the substrate holder, or a grid plate below the substrate holder. In some embodiments, the second pattern 302 may be a combination of physical marks and optical projection. For example, physical reference marks may be provided in peripheral areas of the substrate holder that are not covered by the wafer placed on the substrate holder, and the optical projection may complete the reference pattern in areas of the wafer such that tunneling effects are not required.

[0040] FIG. 4A shows a close-up top view of an overlay image of a portion of a wafer 290 captured by a first image capture device 250 and a second image capture device 260, the portion having a first pattern 301 and a second pattern 302, according to some embodiments of the present disclosure. FIG. 4B shows an exemplary image analysis for overlay calculation using a first pattern 301, which serves as a reference pattern in the alignment process, according to some embodiments of the present disclosure. FIGS. 4A and 4B show how the absolute and independent first pattern 301 can be used to calculate the overlay value of two patterns. This can be done by knowing each reference pattern common to a coordinate system and using the reference pattern to know "where" each pattern is in the coordinate system. Once the reference patterns, e.g., the distance between each layer, are known, the vector calculations required to extract the overlay value can be done with simple vector algebra. This can be considered mix-match overlay (MMO), for which powerful tools are always at hand.

[0041] Each wafer may have deep scratch effects that affect overlay, thermal effects, chucking issues, etc. The wafer may also have patterning defects, which may be present if lines are not connected as designed, critical dimensions are too small / large, or there are gaps that may cause shorts. Since the second light beam can pass through the wafer 290, it may also encounter defects, and the acquired second image may further include defect information. In one embodiment, image analysis may be performed on the first image of the first pattern 301 and the second image of the second pattern 302 to inspect the wafer 290 for defects.

[0042] In one embodiment, a corner of a first pattern 301 (denoted as point M), for example one of the squares of a grid plate having 20 micrometer by 20 micrometer squares, can be considered absolute or wafer independent and can be used to calculate an overlay value between a second pattern 302 (denoted as point N) and a third pattern 401 (denoted as point P) formed after the formation of the second pattern 302. By overlaying the second pattern 302 on the first pattern 301, a coordinate difference, i.e., a vector from point M of the first pattern 301 to point N of the second pattern 302, can be calculated.

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[0043] Furthermore, the coordinate position of a point from the second pattern 302 (e.g., N(Wx,Wy)) and the coordinate position of a point from the third pattern 401 (e.g., P(Bx,By)) can be used to determine an overlay value or shift from the second pattern 302 to the third pattern 401. This overlay value can then be used to position the third and subsequent patterns to correct the overlay to an independent reference pattern, e.g., the first pattern 301. In some embodiments, having a uniform reference image in all image comparisons allows for maintaining overlay correction based on an initial line or absolute reference, as well as correction of adjacent patterns. Considering concerns about the critical dimension (CD) variation effect on the resist layer, the coordinates of the pattern are extracted without the pattern CD variation effect on the resist layer and the underlying layer (e.g., a metal resist pattern covers most of the via hole pattern). The CD variation effect on the resist layer can be problematic for alignment, but can be ignored by the overlay metrologist. The significant improvement of the present technique is that the reference pattern itself provides a much better indication of pattern placement than the alignment marks, which suffer from CD astigmatism and Zernike induced deviations from the pattern.

[0044] It should be noted that in some embodiments, image registration is not required: coordinate position data can be collected from the reference plate and the working surface of the wafer, and vector analysis can then be used to determine the total misalignment or registration value.

[0045] 5 is a flow diagram illustrating an example imaging method 500 for processing a wafer (e.g., wafer 290) according to some embodiments of the present disclosure. The example imaging method 500 may be applied to the example imaging system 200. In various embodiments, some steps of the example imaging method 500 shown may be performed simultaneously or in a different order than as shown, and may be substituted or omitted by other method steps. Additional method steps may also be performed as desired.

[0046] In step S510, a first light beam having a first wavelength and a second light beam having a second wavelength can be generated. In one embodiment, the first light source 210 can be used to generate the first light beam and the second light source 220 can be used to generate the second light beam. For example, the second wavelength can be longer than the first wavelength. As another example, the first light source 210 can be a UV light source and the second light source 220 can be an IR light source. In one embodiment, the first wavelength is 50-400 nanometers, for example 266 nanometers, and the second wavelength is 1-10 micrometers, for example 3.6 or 3.7 micrometers.

[0047] In step S520, the first light beam can be coaxially aligned with the second light beam. In one embodiment, the alignment module 230 can coaxially align the first light beam with the second light beam. For example, a first light beam splitter can be used to transmit the first light beam, a second light beam splitter can be used to reflect the second light beam, and a third beam splitter can be used to reflect the first light beam transmitted from the first light beam splitter and transmit the second light beam reflected from the second light beam splitter, and align the reflected first light beam with the transmitted second light beam.

[0048] In step S530, the coaxially aligned first and second light beams can be focused on the first pattern disposed on the front side of the wafer and the second pattern disposed below the first pattern, respectively. In one embodiment, the coaxial module 240 can be used to focus the first light beam on the first pattern and the second light beam on the second pattern. The second light beam can have sufficient intensity to pass through at least a portion of the thickness of the wafer. In one embodiment, the second pattern can be incorporated into a reference plate, for example, a grid plate having a plurality of 20 micrometer by 20 micrometer squares with sub-nanometer positional accuracy disposed on the underside of the wafer. For example, the reference plate can be disposed or glued on the backside of the wafer. Thus, the second light beam can have sufficient intensity to pass through the entire thickness of the wafer to reach the second pattern. In another embodiment, the reference plate can be incorporated into a substrate holder or chuck of a photolithography scanner or stepper, and the exemplary imaging method 500 can further include aligning the reference plate with the wafer before focusing the coaxially aligned first and second light beams. In yet another embodiment, the second pattern can be formed on the backside of the wafer. Thus, the second light beam can have sufficient intensity to pass through the entire thickness of the wafer to reach the second pattern. In yet another embodiment, the second pattern can be embedded in the wafer and accessible through one or more layers. Thus, the second light beam can have sufficient intensity to pass through a portion of the thickness of the wafer. For example, the second pattern can include a radioactive or fluorescent material. As another example, the second pattern can include at least one of a dot, a line, a corner, a box, a triangle, a number, or a mark.

[0049] In step S540, a first image of the first pattern and a second image of the second pattern may be acquired. For example, the first image and the second image may be acquired using the first image acquisition device 250 and the second image acquisition device 260, respectively. In one embodiment, the second image of the second pattern may be acquired via quantum tunneling imaging or IR transmission imaging.

[0050] In step S550, image analysis may be performed on the first and second images to calculate a registration value.

[0051] The exemplary imaging system 200 and exemplary imaging method 500 can be implemented as a stand-alone coaxial measurement system and method operable in conjunction with a lithography tool, an integrated track coaxial measurement system and method providing feedforward to a connected lithography cell, or an active coaxial measurement system and method that can be embedded in a lithography tool for real-time correction.

[0052] Aspects of the present disclosure provide imaging systems and imaging methods that provide accurate and precise alignment mechanisms that do not rely on traditional alignment marks formed on the front side of the wafer. Instead, a reliable reference pattern can be repeatedly accessed to perform precise and accurate registration and alignment of subsequent patterns by referencing patterns or grids on the interior / underside of the wafer. The techniques herein eliminate the need for traditional overlay marks. These new paradigms for overlay require no cleaning, no loss of footprint area, and no complex scribe line designs, improving silicon area utilization and eliminating complex integration for alignment marks. The exemplary reference patterns disclosed herein are no longer wiped out by undesirable steps that fabricate elements rather than alignment marks as has been done traditionally. Overlay placement accuracy can be measured from the very first layer where the second pattern is placed, because the reference pattern is not only near perfect every time, but is hidden directly under the stage at all times.

[0053] In the above description, specific details have been disclosed, such as the particular shape of the processing system and a description of the various elements and processes used. However, it should be understood that the technology of the present invention may be practiced in other embodiments that differ from these specific details, and that such details are for the purpose of explanation and are not intended to limit the present invention. The embodiments disclosed herein have been described with reference to the accompanying drawings. Similarly, for the purpose of explanation, specific numbers, materials, and configurations have been disclosed to provide a thorough understanding. Nevertheless, the embodiments can be practiced without such specific details. Elements having substantially the same functional structure are labeled with similar reference numerals, and therefore redundant descriptions may be omitted if any.

[0054] Various techniques have been described as multiple separate operations to facilitate understanding of various embodiments. The order of description should not be construed as to imply that these operations are necessarily order dependent. In fact, these operations need not be performed in the order presented. The described operations may be performed in a different order than in the described embodiments. In additional embodiments, various additional operations may be performed and / or the described operations may be omitted.

[0055] As used herein, "substrate" or "target substrate" generally refers to an object to be processed according to the present disclosure. The substrate may include any material portion or structure of a device, particularly a semiconductor or other electronic device, and may be a base substrate structure, such as a semiconductor wafer, a reticle, or a layer on or overlying a base substrate structure, such as a thin film. Thus, the substrate is not limited to any particular base structure, lower layer, or upper layer, patterned or unpatterned, but is intended to include any such layer or base structure, and any combination of multiple layers and / or base structures. While the above description may refer to a particular type of substrate, this is for illustrative purposes only.

[0056] Those skilled in the art will also understand that many modifications may be made to the operation of the techniques described above while still achieving the same objectives of the present disclosure. Such modifications are also intended to fall within the scope of the present disclosure. Thus, the above description of the embodiments of the present disclosure is not intended to limit the present invention. Rather, any limitations to the embodiments of the present disclosure are set forth in the following claims.

Claims

1. 1. An imaging system comprising: a first light source configured to generate a first light beam at a first wavelength; a second light source separate from the first light source configured to generate a second light beam at a second wavelength; an alignment module configured to coaxially align the second light beam with the first light beam; a coaxial module configured to focus the coaxially aligned first and second light beams onto a first pattern disposed on a front side of a wafer and a second pattern disposed below the first pattern, respectively; an image acquisition module configured to acquire a first image of the first pattern and a second image of the second pattern; having the first light source is an ultraviolet (UV) light source and the second light source is an infrared (IR) light source; The second light beam has sufficient intensity to pass through at least a portion of the thickness of the wafer to reach the second pattern.

2. The first light beam is ultraviolet light, and the second light beam is infrared light.

2. The imaging system of claim 1.

3. 3. The imaging system of claim 2, wherein the first wavelength is 266 nanometers and the second wavelength is 3.6 or 3.7 micrometers.

4. The imaging system of claim 1 , wherein the IR light source comprises an IR tunable quantum cascade laser.

5. The imaging system of claim 1 , wherein an image acquisition module acquires the second image of the second pattern via quantum tunneling imaging or IR transmission imaging.

6. The imaging system of claim 1 , wherein the second pattern is embedded in a reference plate positioned below the wafer.

7. The imaging system of claim 6 , further comprising a substrate holder configured to hold the wafer, the reference plate being integrated into the substrate holder.

Images