Patent Publication Number: US-2023161267-A1

Title: Precision multi-axis photolithography alignment correction using stressor film

Description:
INCORPORATION BY REFERENCE 
     This present disclosure claims the benefit of U.S. Provisional Application No. 63/281,431, “Precision Multi-Axis Photolithography Alignment Correction Using Stressor Film” filed on Nov. 19, 2021, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates to semiconductor fabrication, and more particularly, to wafer curvature, bow and overall wafer shape. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Semiconductor fabrication involves multiple varied steps and processes. One typical fabrication process is known as photolithography (also called microlithography). Photolithography uses radiation, such as ultraviolet or visible light, to generate fine patterns in a semiconductor device design. Many types of semiconductor devices, such as diodes, transistors, and integrated circuits, can be constructed using semiconductor fabrication techniques including photolithography, etching, film deposition, surface cleaning, metallization, and so forth. 
     Exposure systems (also called tools) are used to implement photolithographic techniques. An exposure system typically includes an illumination system, a reticle (also called a photomask) or spatial light modulator (SLM) for creating a circuit pattern, a projection system, and a wafer alignment stage for aligning a photosensitive resist-covered semiconductor wafer. The illumination system illuminates a region of the reticle or SLM with a (preferably) rectangular slot illumination field. The projection system projects an image of the illuminated region of the reticle pattern onto the wafer. For accurate projection, it is important to expose a pattern of light on a wafer that is relatively flat or planar, preferably having less than 10 microns of height deviation. 
     SUMMARY 
     Aspects of the present disclosure provide a method for improving overlay alignment of patterning by correcting wafer shape. For example, the method can include receiving a wafer having a working surface with at least partially-fabricated semiconductor devices, and a backside surface opposite to the working surface. The method can also include forming a first stressor film on the backside surface. The first stressor film can modify overlay alignment of the working surface in a first direction across the working surface of the wafer, and include no overlay alignment in a second direction across the working surface of the wafer. The second direction is different than the first direction. The method can also include forming one or more first semiconductor structures on the working surface of the wafer. The first semiconductor structures are aligned in the first direction. 
     In an embodiment, the second direction can be rotated at least 15 degrees relative to the first direction. For example, the second direction can be rotated 45 degrees relative to the first direction. As another example, the second direction can be orthogonal to the first direction. 
     In an embodiment, the method can further include subsequent to forming the first semiconductor structures on the working surface of the wafer, forming a second stressor film on the backside surface, the second stressor film modifying overlay alignment of the working surface in the second direction across the working surface of the wafer, and forming one or more second semiconductor structures on the working surface of the wafer, the second semiconductor structures aligned in the second direction. In another embodiment, the method can further include subsequent to forming the first semiconductor structures on the working surface of the wafer, forming a second stressor film on the backside surface, the second stressor film modifying overlay alignment of the working surface in the second direction across the working surface of the wafer, and modifying the first semiconductor structures on the working surface of the wafer, the modified first semiconductor structures aligned in the second direction. In yet another embodiment, the method can further include prior to forming the first semiconductor structures on the working surface of the wafer, forming a second stressor film on the backside surface, the second stressor film modifying overlay alignment of the working surface in the second direction across the working surface of the wafer; and subsequent to forming the first semiconductor structures on the working surface of the wafer, modifying the first semiconductor structures on the working surface of the wafer, the modified first semiconductor structures aligned in the second direction. For example, the first stressor film and the second stressor film are formed in a litho-etch-litho-etch (LELE) process or a litho-freeze-litho-etch (LEFE) process. 
     In an embodiment, the first stressor film can be patterned based on a bow measurement of the wafer in the first direction. For example, the first stressor film can be patterned using direct write patterning. 
     Aspects of the present disclosure also provide another method for improving overlay alignment of patterning by correcting wafer shape. For example, the method can include receiving a wafer having a working surface with at least partially-fabricated semiconductor devices, and a backside surface opposite to the working surface. The method can also include forming a stressor film on the backside surface. The stressor film can modify overlay alignment of the working surface in first and second directions across the working surface of the wafer. The method can also include forming one or more semiconductor structures on the working surface of the wafer. The semiconductor structures are aligned in the first and second directions. 
     In an embodiment, the stressor film can be patterned based on bow measurements of the wafer in the first and second directions. In another embodiment, the stressor film can be patterned based on a times of a bow measurement of the wafer in the first direction and (1-α) times of a bow measurement of the wafer in the second direction, where 0&lt;α&lt;1. For example, α is ½. 
     Note that this summary section does not specify every embodiment and/or incrementally novel aspect of the present disclosure or claimed invention. Instead, this summary only provides a preliminary discussion of different embodiments and corresponding points of novelty over conventional techniques. For additional details and/or possible perspectives of the present disclosure and embodiments, the reader is directed to the Detailed Description section and corresponding figures of the present disclosure as further discussed below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of this disclosure that are proposed as examples will be described in detail with reference to the following figures, wherein like numerals reference like elements, and wherein: 
         FIG.  1    is a diagram showing an alignment tree that is prepared before photo masks are produced; 
         FIGS.  2 A to  2 D  show an example of an overlay patterning error; 
         FIG.  3 A  illustrates alignment of two layers without wafer stress correction; 
         FIG.  3 B  illustrates alignment of two layers with wafer stress correction in X direction in accordance with some embodiments of the present disclosure; 
         FIG.  3 C  illustrates further alignment of the layers shown in  FIG.  3 B  with wafer stress correction in Y direction in accordance with some embodiments of the present disclosure; 
         FIG.  4    is a flow chart illustrating an exemplary method for improving overlay alignment of patterning by correcting wafer shape in two directions in accordance with some embodiments of the present disclosure; 
         FIGS.  5 A to  5 D  illustrate modifying overlay alignment of a working surface of a wafer in a first direction in accordance with some embodiments of the present disclosure; 
         FIGS.  6 A to  6 D  illustrate modifying overlay alignment of the working surface of the wafer shown in  FIGS.  5 A to  5 D  in a second direction in accordance with some embodiments of the present disclosure; 
         FIGS.  7 A to  7 D  illustrate a litho-etch-litho-etch (LELE) process used in accordance with some embodiments of the present disclosure; 
         FIGS.  8 A to  8 D  illustrate a litho-freeze-litho-etch (LFLE) process used in accordance with some embodiments of the present disclosure; 
         FIGS.  9 A to  9 C  illustrate correcting wafer bow and curvature in only X direction in accordance with some embodiments of the present disclosure; 
         FIGS.  10 A to  10 C  illustrate correcting wafer bow and curvature in only Y direction in accordance with some embodiments of the present disclosure; 
         FIG.  11    is a flow chart illustrating an exemplary method for improving overlay alignment of patterning by correcting wafer shape in a single axis in accordance with some embodiments of the present disclosure; 
         FIGS.  12 A to  12 D  illustrate an example result of average X and Y correction in accordance with some embodiments of the present disclosure; and 
         FIG.  13    is a flow chart illustrating an exemplary method for improving overlay alignment of patterning by correcting wafer shape in accordance with some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Semiconductor fabrication development now incorporates techniques such as advanced patterning and 3D device construction to reduce feature size and increase device density. The implementation of these techniques, however, has created new challenges for successful micro fabrication. These new fabrication approaches include the creation of multiple layers of film of various materials, on the wafer surface. Each layer, however, adds additional stress to the surface of the wafer. As the layers of film build up, the induced stress distorts the flatness of the wafer. This distortion has been shown to reduce the size uniformity of critical features across the surface of the wafer. 
     This distortion also results in overlay errors and challenges. Various fabrication process steps can cause expansion and/or contraction of the substrate, resulting is a warped or bowed substrate. For example, during exposure a substrate is heated locally due to the energy transferred to the substrate from an exposure beam. Substrates are also heated during anneal processes. This heating causes the substrate to expand. If the substrate expansion is unchecked, the expansion exceeds overlay error requirements. Moreover, if the clamping force between the substrate and the substrate chuck is insufficient to prevent substrate expansion, then the substrate can slip on the substrate chuck and larger substrate expansion will occur, resulting in larger overlay errors. Slipping can be more pronounced in some processes, such as in extreme ultraviolet (EUV) systems, because the environment surrounding the substrate during exposure is a vacuum. Thus, vacuum clamping is not always possible, and the weaker electrostatic clamping must be used in lieu of a vacuum clamp. 
     Other fabrication steps can also cause substrate expansion and contraction. For example, deposited films can cause substrate contraction. Also, various annealing and doping steps can create substantial amounts of bow in a given substrate. Annealing steps can especially create overlay challenges. The result of these various fabrication steps is a substrate that is uneven or non-planar. For example, a backside of the substrate can have z-height variations (i.e., variations in vertical heights) that have both high spots and low spots. Z-height variations due to such bowing can be on the order of about a micron to approximately 100 microns. This fluctuation is significant because semiconductor devices or structures being exposed by various exposure tools are being exposed on scales of tens of nanometers to hundreds of nanometers. Thus having deflection variations of thousands of nanometers to 10,000 nanometers can dramatically reduce yield. 
       FIG.  1    is a diagram showing an alignment tree  100  that is prepared before photo masks are produced. In the alignment tree  100 , a layer L 7  is aligned, using an alignment mark of a layer L 1 , and layers L 11  and L 12  are aligned, using an alignment mark of the layer L 7 . The layer L 7  is aligned to the layer L 1  directly, and the layers L 11  and L 12  are aligned to the layer L 1  indirectly. The layers L 1 , L 7 , L 11  and L 12  can be transistors, fins, metal layer, masks, or any other sacrificial or final structure on a wafer formed during microfabrication. Walkout of successive layers may occur. With unavoidable stresses induced on layers during microfabrication processes, successive layers can become progressively misaligned to each other and to starting layers. 
     Microfabrication causes stresses on a wafer due to various materials deposition, removal, annealing, etching, and so forth. Because of the process stress on the wafer some of the possible wafer bows are the Zernike Polynomials, including vertical tile, horizontal tilt, oblique astigmatism, defocus, etc.  FIGS.  2 A to  2 D  show an example of an overlay patterning error. There are many different patterns of wafer bow and curvature. This particular example shows oblique astigmatism, with the result that a layer L 11  has misalignment in both X and Y direction and to various degrees (the dashed line shows where the layer L 11  is desired to be placed, and the solid line shows where the layer L 11  is actually placed due to the wafer bow). 
     Conventional techniques used to address substrate bow and uneven curvature on partially-processed substrates focus on chucking techniques to chuck (or clamp/suck) a substrate to a substrate holder to flatten curvature. With relatively significant bowing, however, it can be very difficult or impossible to accurately flatten a substrate by chucking alone. Thus, it is desirable to have a substrate bow correction technique to correct substrate bow and improve overlay prior to being sent or returned to a scanner for additional exposures. 
     Techniques herein include methods of improving overlay alignment of patterning by correcting wafer shape or correcting wafer bow and curvature, in order to more accurately print patterns on a wafer. Techniques herein include single axis or single direction alignment or correction followed by secondary alignment or correction in a different axis or direction. Such incremental multi-axis correction can provide greater accuracy compared to correcting bow for all directions at once. Accordingly, techniques herein correct alignment in separate directions incrementally or successively. Techniques also include correction in one direction without correction in another direction, typically orthogonal. 
     Techniques disclosed herein use stressor films on a backside surface of a wafer. Certain embodiments use only direct write laser treatment for modification of a stress value to create optimum wafer shape using a wafer shape correction tool. Such stress correction tools are configured to modify a wafer shape (wafer bow, wafer curvature) to improve overlay. One technique is to deposit one or more stressor films on a backside surface of a wafer. The one or more stressor films can have different initial internal stresses. The stressor films can be patterned and then etched to selectively increase/decrease stresses at given point locations across the backside surface, thereby modifying wafer shape, which can improve overlay values. Patterning can be executed using direct-write laser exposure. Direct write patterning is beneficial because patterns can be created by software and laser control intensity at each coordinate location on a wafer based on a wafer shape signature or a wafer bow measurement. Thus, exposure patterns can easily change wafer to wafer. Of course, mask-based exposure is also contemplated. A given stressor film herein can be removed in selective regions of the backside surface as an option. Stressor films herein can be reset and be kept in place or removed. Stressor films herein can be a single layer or N layers. Stressor films herein can be modified between X and Y directions, or multi-axis photolithography alignment corrections. 
       FIG.  3 A  illustrates alignment of a layer L 2  to a layer L 1  without wafer stress correction. Note that there is misalignment in both X and Y directions. This is the problem to be solved.  FIG.  3 B  illustrates alignment of the layer L 2  to the layer L 1  with wafer stress correction first in X direction in accordance with the present disclosure. Note that the layer L 2  is aligned to the layer L 1  in X direction correctly, but is still misaligned to the layer L 1  in Y direction.  FIG.  3 C  illustrates further alignment of the layer L 2  to the layer L 1  with wafer stress correction in Y direction in accordance with the present disclosure. Note that the layer L 2  is also aligned to the layer L 1  in Y direction. X and Y directions extend across a working surface of a wafer and are orthogonal to each other. Accordingly, the wafer stress is corrected and the layer L 2  is aligned to the layer L 1  in two steps. 
     Techniques disclosed herein use backside films stress modification and novel lithography integration. 
       FIG.  4    is a flow chart illustrating an exemplary method  400  for improving overlay alignment of patterning by correcting wafer shape in two different axes in accordance with some embodiments of the present disclosure. For example, the method  400  can correct wafer bow and curvature in two or more independent directions to more accurately print patterns on a wafer. Accordingly, two masks may be used. Nevertheless, additional masking layers can be integrated with backside stressor films with wafer-shape correction tools. In an embodiment, the two or more independent directions can include X and Y directions. In another embodiment, the two or more independent directions can include a first direction and a second direction being rotated at least 15 degrees relative to the first direction, e.g., 45 degrees. 
     In general, one or more stressor films can be deposited on a backside surface of a wafer, which is opposite to a working surface where active devices are disposed. Then, a photoresist film can be deposited on the backside surface and patterned. Patterning can be done via a photomask and scanner, but preferably patterning is executed using direct-write laser exposure. Direct write is preferred because exposure patterns can be modified for or tailored to each individual wafer. For example, the shape of each wafer can be measured to create a wafer bow measurement. The wafer bow measurement can be, for example, a map of relative z-height variations across a wafer or other overlay misalignment measurement. After the photoresist formed on the backside surface is patterned and developed, the one or more stressor films can be etched. This modifies an internal stress of the wafer to result in a modified wafer shape, that, for example, improves overlay or flattens the wafer. The stressor films can have an initial internal stress which can be tensile or compressive. Selectively removing material from coordinate locations accordingly modifies the internal stress of the wafer. 
     Additional backside film correction techniques are introduced to induce/modify stresses, such as an epoxy film that has its internal stress changed in response to a pattern of light corresponding to particular locations of the epoxy film. Thus, stress modification can occur without etching needed. Accordingly, stressor films herein refer to any film formed on a backside surface of a wafer and treated to cause a stress modification to the wafer. This can include depositing one (or more) stressor film, such as a silicon oxide or silicon nitride film, coating the stressor film with a photoresist layer, exposing (e.g., direct-write) and developing the photoresist layer, etching the stressor film, and removing the photoresist layer. At this point, patterning on a working surface of the wafer can continue. Techniques herein have discovered that there are distinct advantages to incremental overlay correction. One discovery is that correcting alignment in one direction at a time can provide greater accuracy compared to correcting bow for all directions at once. Accordingly, techniques herein correct alignment in separate directions incrementally or successively. Techniques also include correction in one direction without correction in another direction, typically orthogonal. 
     The method  400  starts with step  410 , at which a wafer is received that has a backside surface and a working surface opposite to the backside surface. The working surface may be with at least partially-fabricated devices, one or more microfabrication steps of which may result in a measure of wafer bow of the wafer. For example, layer L 1  and layer L 7  are formed on a working surface of a wafer, as shown in  FIGS.  5 A and  5 B . It is desired to add a layer L 11  at the indicated location (shown in dashed line), which is spaced from the layers L 1  and L 7  or aligned independent of layers L 1  and L 7  alignment. 
     At step S 420 , a first stressor film  510 , shown in  FIGS.  5 A and  5 B , is deposited and formed on the backside surface of the wafer. In an embodiment, the first stressor film  510  is used to modify overlay alignment of the working surface of the wafer in a first direction, e.g., X direction, across the working surface of the wafer, without any stress correction in Y direction. Accordingly, the first stressor film  510  targets the overlay alignment modification for one direction, e.g., X direction, without targeting for another direction e.g., Y direction, For example, the wafer, with the layers L 1  and L 7  formed on the working surface thereof, can be measured to identify an X-direction bow measurement of the wafer, and the first stressor film  510  can have its X-direction internal stress modified based on the X-direction bow measurement. In an embodiment, the first stressor film  510  is coated with a photoresist layer, the photoresist layer is patterned corresponding to the X-direction bow measurement and developed, the first stressor film  510  is etched using dry or wet etching, and the photoresist layer is then removed, so that the internal stress of the first stressor film  510  is modified to correspond to the X-direction bow measurement of the wafer, and, as a result, the wafer is close to being flat or considered flat in X direction. In another embodiment, the first stressor film  510  can be reactive to heat such that applied heat changes its internal, and a pattern of heat can be applied onto the first stressor film  510 , the pattern of heat corresponding to the X-direction bow measurement. In some other embodiments, the first stressor film  510  can be reactive to a first wavelength of light in that exposure to the first wavelength of light changes its internal stress, and a pattern of the first wavelength of light can be generated to expose the first stressor film  510 , the pattern of the first wavelength of light corresponding to the X-direction bow measurement. The first stressor film  510  thus formed is used to modify overlay alignment in the first direction, i.e., X direction, and includes no substantial or significant overlay alignment in a second direction, e.g., Y direction, that is different from or is orthogonal to the first direction. 
     Then, a photoresist layer L 11   x  can be formed on the working surface of the wafer and aligned to the layers L 1  in X direction. The photoresist layer L 11   x  can be patterned, exposed and developed in a lithographic process to form a hardmask L 11   x , as shown in  FIGS.  5 C and  5 D , and the wafer can be etched accordingly. Note that the hardmask L 11   x  is formed and placed between the layers L 1 , with no alignment correction in Y direction performed. Accordingly, the hardmask L 11   x  covers a portion of the layers L 7 . This may be acceptable depending on etch selectivities and patterning objectives. The hardmask L 11   x  has been aligned in X direction, that is, without overlapping the L 1  layers. 
     At step S 430 , a second stressor film  610 , shown in  FIGS.  6 A and  6 B , is deposited and formed on the backside surface of the wafer. In an embodiment, the second stressor film  610  is used to modify overlay alignment of the working surface of the wafer in a second direction, e.g., Y direction, across the working surface of the wafer, without any stress correction in X direction. Accordingly, the second stressor film  610  also targets the overlay alignment modification for one direction, e.g., Y direction, without targeting for another direction e.g., X direction, For example, the wafer, with the layers L 1  and L 7  formed on the working surface thereof and the first stressor film  510  formed on the backside surface thereof, can be measured to identify a Y-direction bow measurement of the wafer, and the second stressor film  610  can have its Y-direction internal stress modified based on the Y-direction bow measurement. In an embodiment, the second stressor film  610  is coated with a photoresist layer, the photoresist layer is patterned corresponding to the Y-direction bow measurement and developed, the second stressor film  610  is etched using dry or wet etching, and the photoresist layer is then removed, so that the internal stress of the second stressor film  610  is modified to correspond to the Y-direction bow measurement of the wafer, and the wafer is close to being flat or considered flat in Y direction. In another embodiment, the second stressor film  610  can be reactive to heat such that applied heat changes its internal or be reactive to a second wavelength of light in that exposure to the second wavelength of light changes its internal stress, and a pattern of heat is applied onto the second stressor film  610  or a pattern of the second wavelength of light is generated to expose the second stressor film  610 , the pattern of heat and the pattern of the second wavelength of light corresponding to the Y-direction bow measurement. The second stressor film  610  thus formed is used to modify overlay alignment in the second direction, i.e., Y direction, and includes no substantial or significant overlay alignment in the first direction, e.g., X direction, which is different from or is orthogonal to the second direction. 
     Then, a photoresist layer Lily can be formed on the working surface of the wafer and aligned to the layers L 7  in Y direction. The photoresist layer Lily can be patterned, exposed and developed in a lithographic process to form the hardmask L 11 , as shown in  FIGS.  6 C and  6 D , and the wafer can be etched accordingly. Note that the hardmask L 11  is formed and placed between the layers L 1  and between the layers L 7 , and is aligned in X and Y directions, without overlapping the layers L 1  and L 7 . 
     In an embodiment, the first stressor film  510  can be removed in selective regions of the backside surface of the wafer before the second stressor film  610  is formed. Stress memorization can also be used herein. In some embodiments, even though the first stressor film  510  is removed, the semiconductor lattice still has a memory effect because the stress of the first stressor film  510  has been transferred to the silicon lattice of the wafer. 
     In some embodiments, the second stressor film  610  and the first stressor film  510  can be formed in a double patterning lithography process. For example, in a litho-etch-litho-etch (LELE) process the second stressor film  610  and the first stressor film  510  are formed on a backside surface of a wafer  710 , and a first resist layer  720  is coated on the first stressor film  510  and patterned corresponding to the X-direction bow measurement, as shown in  FIG.  7 A . The first stressor film  510  is etched, as shown in  FIG.  7 B . The first resist layer  720  is stripped, and a second resist layer  730  is coated on the second stressor film  610  and patterned corresponding to the Y-direction bow measurement, as shown in  FIG.  7 C . The second stressor film  610  is etched, and the second resist layer  730  is stripper, as shown in  FIG.  7 D . In the LELE process, two lithography steps and two etch steps are performed. As another example, in a litho-freeze-litho-etch (LFLE) process the first resist layer  720  is coated on the second stressor film  610  and patterned corresponding to the X-direction bow measurement, as shown in  FIG.  8 A , and the patterned first resist layer  720  is frozen using a chemical treatment, as shown in  FIG.  8 B . The second resist layer  730  is coated on the second stressor film  610  and patterned corresponding to the Y-direction bow measurement, as shown in  FIG.  8 C . The second stressor film  610  is etched that has its internal stress modified and corresponding to the X-direction and Y-direction bow measurements, and the first resist layer  720  and the second resist layer  730  are stripped, as shown in  FIG.  8 D . In the LFLE process, two lithography steps and only one etch steps are performed. The first stressor film  510  and the second stressor film  610  can also formed in a litho-cure-litho-etch (LCLE) process, in which the patterned first resist layer  730  is baked, instead of being frozen using the chemical treatment. 
     At step S 440 , one or more semiconductor devices, e.g., the layer L 11 , can be formed on the working surface of the wafer. As the wafer bow of the wafer in the first and second directions is modified and the wafer is close to being flat or considered flat in X and Y directions, the semiconductor devices can be aligned in X and Y directions. 
     Embodiments herein also include single axis or single direction correction at a time. Some embodiments include alignment correction in only one direction when sufficient for desired microfabrication applications. For example, in some microfabrication steps there may be certain features that only have an X-direction alignment dependence or Y-direction alignment dependence. These features would not need a cut mask approach and could be segregated on X mask and Y mask, respectively. An example with vias  900  is shown in  FIG.  9 A , which illustrates that a layer L 2  that has vias disposed in Y direction is formed on a layer L 1  that has metal signal wires. The X-direction critical vias of the layer L 2  could be on a hardmask L 2   x , as shown in  FIGS.  9 B and  9 C . Another example with vias  1000  is shown in  FIG.  10 A , which illustrates that a layer L 2  that has vias disposed in X direction is formed on a layer L 1  that has metal signal wires. The Y-direction critical vias of the layer L 2  could be on a hardmask L 2   y , as shown in  FIGS.  10 B and  10 C . 
       FIG.  11    is a flow chart illustrating an exemplary method  1100  for improving overlay alignment of patterning by correcting wafer shape in a single axis in accordance with some embodiments of the present disclosure. For example, the method  1100  can correct wafer bow and curvature in a single axis, as described in accordance with  FIGS.  9 A to  9 C  and  FIGS.  10 A to  10 C . The method  1100  starts with step  410 , followed by step S 420 , in which the first stressor film  510  is formed and patterned on the backside surface of the wafer to modify overlay alignment of the working surface of the wafer in a first direction, e.g., X direction. At step S 1130 , one or more first semiconductor devices, e.g., the vias  900  or the layer L 2   x , can be formed on the working surface of the wafer. As the wafer bow of the wafer in the first direction is modified and the wafer is close to being flat or considered flat in X direction, the first semiconductor devices can be aligned in X direction. The method  1100  can then execute step S 430 , in which the second stressor film  610  is formed and patterned on the backside surface of the wafer to modify overlay alignment of the working surface of the wafer in a second direction, e.g., Y direction. At step S 1150 , one or more second semiconductor devices, e.g., the vias  1000  or the layer L 2   y , can be formed on the working surface of the wafer. As the wafer bow of the wafer in the second direction is modified and the wafer is close to being flat or considered flat in Y direction, the second semiconductor devices can be aligned in Y direction. 
     Some embodiments can include average X and Y correction. Layers can be L 1  to Ln (n is 1 to ˜300). For example, an alignment tree is prepared: L 1 --&gt;L 7 -- 22  L 11 , and layers L 1  and L 7  have already been patterned. Ideally, wafer shape for a hardmask L 11   x  is L 1 , while wafer shape for a hardmask L 11   y  is L 7 . In an embodiment, wafer shape of L 11 =αL 1 +(1-α)L 7 , where 0≤α≤1. For example, α=½ and L 11 =(L 1 +L 7 )/2. The advantage is that this process is less costly with less processing (one stressor film).  FIGS.  12 A to  12 D  illustrate an example result. Note that the layer L 11  is aligned within the layers L 1  and L 7 , but with less precision compared to incremental correction and placement. This alignment correction is still better than using no stressor film. 
       FIG.  13    is a flow chart illustrating an exemplary method  1300  for improving overlay alignment of patterning by correcting wafer shape, in accordance with some embodiments of the present disclosure. For example, the method  1300  can include average X and Y correction in wafer bow and curvature, as described in accordance with  FIGS.  12 A to  12 D . The method  1300  starts with step  410 . At step S 1320 , a stressor film is deposited and formed on the backside surface of the wafer. In an embodiment, the stressor film is used to modify overlay alignment of the working surface of the wafer in an average of first and second directions, e.g., X and Y directions, across the working surface of the wafer. For example, the wafer, with the layers L 1  and L 7  formed on the working surface thereof, can be measured to identify X-direction and Y-direction bow measurements of the wafer, and the stressor film can have its X-direction and Y-direction internal stresses modified based on the X-direction and Y-direction bow measurements. For example, the stressor film can have its X-direction and Y-direction internal stresses modified based on a times of the X-direction bow measurement and (1-α) times of the Y-direction bow measurement, where 0≤α≤1, e.g., α=½. The method  1300  can also include step  5440 , in which one or more semiconductor devices, e.g., the layer L 11 , can be formed on the working surface of the wafer. As the wafer bow of the wafer in the first and second directions is modified and the wafer becomes flatter in X and Y directions, as compared with the wafer without the stressor film formed on the backside surface thereof, the semiconductor devices can be aligned in X and Y directions. 
     In another example embodiment, in the A,H, Z example another way to approach the Zx Zy challenge is to pre-emptively ensure the shape of A and H match. Suppose H has a relatively large overlay budget and it would be an easy application of a wafer shape correction tool to manipulate wafer shape to match A. That way when it is time to do layer Z, there is a best Zx--&gt;A and Zy--&gt;H overlay. Even if H doesn&#39;t have a large overlay budget, co-optimization could still take place to match A while staying within budget to make wafer tool correction at Z easier. In the flow described above, A and H are interchangeable. If A has more flexibility its shape can be made to look like H in advance using feedback from a previous lot run. 
     The precision multi-axis nature disclosed herein is in part understood from photolithography exposure practices. When exposing at the limit of resolution of a given photolithography system, shapes are often easier to print when exposed as lines. As such, multiple exposures can be executed to print lines in different directions. Of course, lithographic exposure is distinct from stress correction, but stress modification can be improved herein by separating directional stress modification into components. For example, X direction correction and application is followed by Y direction correction and application. Some transistor designs can have feature shifts of 45 degrees, thus a third alignment modification can be executed as well as a fourth alignment modification, fifth alignment modification, and so on. Accordingly, by separating backside alignment correction into component directions, improved precision and accuracy of aligned patterning is realized. 
     In the preceding description, specific details have been set forth, such as a particular geometry of a processing system and descriptions of various components and processes used therein. It should be understood, however, that techniques herein may be practiced in other embodiments that depart from these specific details, and that such details are for purposes of explanation and not limitation. Embodiments disclosed herein have been described with reference to the accompanying drawings. Similarly, for purposes of explanation, specific numbers, materials, and configurations have been set forth in order to provide a thorough understanding. Nevertheless, embodiments may be practiced without such specific details. Components having substantially the same functional constructions are denoted by like reference characters, and thus any redundant descriptions may be omitted. 
     Of course, the order of discussion of the different steps as described herein has been presented for clarity sake. In general, these steps can be performed in any suitable order. Additionally, although each of the different features, techniques, configurations, etc. herein may be discussed in different places of this disclosure, it is intended that each of the concepts can be executed independently of each other or in combination with each other. Accordingly, the present disclosure can be embodied and viewed in many different ways. 
     Various techniques have been described as multiple discrete operations to assist in understanding the various embodiments. The order of description should not be construed as to imply that these operations are necessarily order dependent. Indeed, these operations need not be performed in the order of presentation. Operations described may be performed in a different order than the described embodiment. Various additional operations may be performed and/or described operations may be omitted in additional embodiments. 
     “Substrate” or “target substrate” as used herein generically refers to an object being processed in accordance with the present disclosure. The substrate may include any material portion or structure of a device, particularly a semiconductor or other electronics device, and may, for example, be a base substrate structure, such as a semiconductor wafer, reticle, or a layer on or overlying a base substrate structure such as a thin film. Thus, substrate is not limited to any particular base structure, underlying layer or overlying layer, patterned or un-patterned, but rather, is contemplated to include any such layer or base structure, and any combination of layers and/or base structures. The description may reference particular types of substrates, but this is for illustrative purposes only. 
     Those skilled in the art will also understand that there can be many variations made to the operations of the techniques explained above while still achieving the same objectives of the present disclosure. Such variations are intended to be covered by the scope of this disclosure. As such, the foregoing descriptions of embodiments of the present disclosure are not intended to be limiting. Rather, any limitations to embodiments of the present disclosure are presented in the following claims.