Patent Publication Number: US-2022221804-A1

Title: Overlay marks for reducing effect of bottom layer asymmetry

Description:
PRIORITY DATA 
     This application is a divisional application of U.S. patent application Ser. No. 16/295,510, filed Mar. 7, 2019, which claims the benefit of U.S. Provisional Application No. 62/733,125, entitled “Overlay Marks for Reducing Effect of Bottom Layer Asymmetry,” filed Sep. 19, 2018, each of which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. However, these advances have increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC processing and manufacturing are needed. In the course of integrated circuit evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. 
     Overlay marks have been used to measure the overlay or alignment between various layers of an IC. However, conventional overlay marks still have shortcomings. For example, the measurement accuracy of a conventional overlay mark with an upper layer and a lower layer (sometimes referred to as a “bottom layer”) may be affected by asymmetry of the gratings in the bottom layer. The asymmetry in the bottom gratings can induce additional diffraction orders, resulting in reduced overlay accuracy. Therefore, while existing overlay marks and have been generally adequate for their intended purposes, they have not been entirely satisfactory in every aspect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  is a schematic view of a lithography system constructed in accordance with some embodiments of the present disclosure. 
         FIG. 2  is a sectional view of a EUV mask constructed in accordance with some embodiments of the present disclosure. 
         FIG. 3  illustrates a simplified fragmentary cross-sectional view of an overlay mark  100  in accordance with some embodiments of the present disclosure. 
         FIG. 4A  illustrates fragmentary cross-sectional view of an upper layer  1400  and a lower layer  1300  in accordance with some embodiments of the present disclosure. 
         FIG. 4B  illustrates fragmentary cross-sectional view of an upper layer  1600  and a lower layer  1500  in accordance with some embodiments of the present disclosure. 
         FIG. 5  illustrates a top view of an embodiment of an overlay mark on a substrate in accordance with some embodiments of the present disclosure. 
         FIG. 6  illustrates a top view of another embodiment of an overlay mark on a substrate in accordance with some embodiments of the present disclosure. 
         FIG. 7  is a flowchart illustrating the process flow associated with the overlay marks in accordance with some embodiments of the present disclosure 
         FIG. 8  illustrates a fragmentary top view of mandrels for forming a bottom layer of an overlay mark on a substrate in accordance with some embodiments of the present disclosure. 
         FIG. 9A  illustrates a fragmentary cross-sectional view of the mandrel features in  FIG. 8 , according to embodiments of the present disclosure. 
         FIG. 9B  illustrates a fragmentary cross-sectional view of spacer material deposited over the mandrel features in  FIG. 9A , according to embodiments of the present disclosure. 
         FIG. 9C  illustrates a fragmentary cross-sectional view of planarized spacers and mandrel features on a substrate, according to embodiments of the present disclosure. 
         FIG. 9D  illustrates a fragmentary cross-sectional view of spacers on a substrate, according to embodiments of the present disclosure. 
         FIGS. 10 and 11  are flowcharts illustrating methods of semiconductor fabrication associated with the overlay marks in accordance with some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     To ensure accurate alignment (also referred to as overlay) between the various layers in a semiconductor device during a semiconductor fabrication process, overlay marks (or alignment marks) are used to measure the alignment between the layers. However, conventional overlay marks may have shortcomings. For example, lower layers of conventional overlay marks can have asymmetric gratings, resulting in overlay inaccuracy. 
     To overcome the problems discussed above, the present disclosure provides embodiments of overlay marks that can reduce overlay inaccuracy resulting from bottom grating asymmetry. The various aspects of the present disclosure will be discussed below in greater detail with reference to  FIGS. 1-9D . First, a EUV lithography system will be discussed below with reference to  FIGS. 1-3  as an example lithography context in which the overlay mark of the present disclosure may be used, although it is understood that the overlay mark discussed herein may be used for other types of non-EUV lithography contexts too. Next, the details of the overlay mark according to embodiments of the present disclosure are discussed with reference to  FIGS. 4A-9D . 
       FIG. 1  is a schematic view diagram of a EUV lithography system  10 , constructed in accordance with some embodiments. The EUV lithography system  10  may also be generically referred to as a scanner that is configured to perform lithography exposure processes with respective radiation source and exposure mode. The EUV lithography system  10  is designed to expose a photoresist layer by EUV light or EUV radiation. The photoresist layer is a material sensitive to the EUV light. The EUV lithography system  10  employs a radiation source  12  to generate EUV light, such as EUV light having a wavelength ranging between about 1 nm and about 100 nm. In one particular example, the radiation source  12  generates a EUV light with a wavelength centered at about 13.5 nm. Accordingly, the radiation source  12  is also referred to as EUV radiation source  12 . 
     The lithography system  10  also employs an illuminator  14 . In various embodiments, the illuminator  14  includes various refractive optic components, such as a single lens or a lens system having multiple lenses (zone plates) or alternatively reflective optics (for EUV lithography system), such as a single mirror or a mirror system having multiple mirrors in order to direct light from the radiation source  12  onto a mask stage  16 , particularly to a mask  18  secured on the mask stage  16 . In the present embodiment where the radiation source  12  generates light in the EUV wavelength range, the illuminator  14  employs reflective optics. In some embodiments, the illuminator  14  includes a dipole illumination component. 
     In some embodiments, the illuminator  14  is operable to configure the mirrors to provide a proper illumination to the mask  18 . In one example, the mirrors of the illuminator  14  are switchable to reflect EUV light to different illumination positions. In some embodiment, a stage prior to the illuminator  14  may additionally include other switchable mirrors that are controllable to direct the EUV light to different illumination positions with the mirrors of the illuminator  14 . In some embodiments, the illuminator  14  is configured to provide an on-axis illumination (ONI) to the mask  18 . In an example, a disk illuminator  14  with partial coherence σ being at most 0.3 is employed. In some other embodiments, the illuminator  14  is configured to provide an off-axis illumination (OAI) to the mask  18 . In an example, the illuminator  14  is a dipole illuminator. The dipole illuminator has a partial coherence σ of at most 0.3 in some embodiments. 
     The lithography system  10  also includes a mask stage  16  configured to secure a mask  18 . In some embodiments, the mask stage  16  includes an electrostatic chuck (e-chuck) to secure the mask  18 . This is because gas molecules absorb EUV light, and the lithography system for the EUV lithography patterning is maintained in a vacuum environment to avoid the EUV intensity loss. In the disclosure, the terms of mask, photomask, and reticle are used interchangeably to refer to the same item. 
     In the present embodiment, the lithography system  10  is a EUV lithography system, and the mask  18  is a reflective mask. One exemplary structure of the mask  18  is provided for illustration. The mask  18  includes a substrate with a suitable material, such as a low thermal expansion material (LTEM) or fused quartz. In various examples, the LTEM includes TiO 2  doped SiO 2 , or other suitable materials with low thermal expansion. In some embodiments, the LTEM includes 5%-20% by weight TiO 2  and has a thermal coefficient of expansion lower than about 1.0×10−6/° C. For example, in some embodiments, the TiO 2  doped SiO 2  material of the LTEM has a coefficient thermal expansion such that it varies by less than 60 parts-per-billion for every 1 degree Celsius of temperature change. Of course, other suitable materials having thermal coefficient of expansion that is equal to or less than TiO 2  doped SiO 2  may also be used. 
     The mask  18  also includes a reflective multilayer (ML) deposited on the substrate. The ML includes a plurality of film pairs, such as molybdenum-silicon (Mo/Si) film pairs (e.g., a layer of molybdenum above or below a layer of silicon in each film pair). Alternatively, the ML may include molybdenum-beryllium (Mo/Be) film pairs, or other suitable materials that are configurable to highly reflect the EUV light. 
     The mask  18  may further include a capping layer, such as ruthenium (Ru), disposed on the ML for protection. The mask  18  further includes an absorption layer deposited over the ML. The absorption layer is patterned to define a layer of an integrated circuit (IC). Alternatively, another reflective layer may be deposited over the ML and is patterned to define a layer of an integrated circuit, thereby forming a EUV phase shift mask. 
     The lithography system  10  also includes a projection optics module (or projection optics box (POB)  20  for imaging the pattern of the mask  18  on to a target  26  secured on a substrate stage  28  of the lithography system  10 . The POB  20  has refractive optics (such as for UV lithography system) or alternatively reflective optics (such as for EUV lithography system) in various embodiments. The light directed from the mask  18 , diffracted into various diffraction orders and carrying the image of the pattern defined on the mask, is collected by the POB  20 . The POB  20  may include a magnification of less than one (thereby the size of the “image” on a target (such as target  26  discussed below) is smaller than the size of the corresponding “object” on the mask). The illuminator  14  and the POB  20  are collectively referred to as an optical module of the lithography system  10 . 
     The lithography system  10  also includes a pupil phase modulator  22  to modulate optical phase of the light directed from the mask  18  so that the light has a phase distribution on a projection pupil plane  24 . In the optical module, there is a plane with field distribution corresponding to Fourier Transform of the object (the mask  18  in the present case). This plane is referred to as projection pupil plane. The pupil phase modulator  22  provides a mechanism to modulate the optical phase of the light on the projection pupil plane  24 . In some embodiments, the pupil phase modulator  22  includes a mechanism to tune the reflective mirrors of the POB  20  for phase modulation. For example, the mirrors of the POB  20  are switchable and are controlled to reflect the EUV light, thereby modulating the phase of the light through the POB  20 . 
     In some embodiments, the pupil phase modulator  22  utilizes a pupil filter placed on the projection pupil plane. A pupil filter filters out specific spatial frequency components of the EUV light from the mask  18 . Particularly, the pupil filter is a phase pupil filter that functions to modulate phase distribution of the light directed through the POB  20 . However, utilizing a phase pupil filter is limited in some lithography system (such as an EUV lithography system) since all materials absorb EUV light. 
     As discussed above, the lithography system  10  also includes the substrate stage  28  to secure a target  26  to be patterned, such as a semiconductor substrate. In the present embodiment, the semiconductor substrate is a semiconductor substrate, such as a silicon substrate or other type of substrate. The target  26  is coated with the resist layer sensitive to the radiation beam, such as EUV light in the present embodiment. Various components including those described above are integrated together and are operable to perform lithography exposing processes. The lithography system  10  may further include other modules or be integrated with (or be coupled with) other modules. 
     The mask  18  and the method making the same are further described in accordance with some embodiments. In some embodiments, the mask fabrication process includes two operations: a blank mask fabrication process and a mask patterning process. During the blank mask fabrication process, a blank mask is formed by deposing suitable layers (e.g., reflective multiple layers) on a suitable substrate. The blank mask is then patterned during the mask patterning process to achieve a desired design of a layer of an integrated circuit (IC). The patterned mask is then used to transfer circuit patterns (e.g., the design of a layer of an IC) onto a semiconductor substrate. The patterns can be transferred over and over onto multiple substrates through various lithography processes. A set of masks is used to construct a complete IC. 
     The mask  18  includes a suitable structure, such as a binary intensity mask (BIM) and phase-shifting mask (PSM) in various embodiments. An example BIM includes absorptive regions (also referred to as opaque regions) and reflective regions, patterned to define an IC pattern to be transferred to the target. In the opaque regions, an absorber is present, and an incident light is almost fully absorbed by the absorber. In the reflective regions, the absorber is removed and the incident light is diffracted by a multilayer (ML). The PSM can be an attenuated PSM (AttPSM) or an alternating PSM (AltPSM). An exemplary PSM includes a first reflective layer (such as a reflective ML) and a second reflective layer patterned according to an IC pattern. In some examples, an AttPSM usually has a reflectivity of 2%-15% from its absorber, while an AltPSM usually has a reflectivity of larger than 50% from its absorber. 
     One example of the mask  18  is shown in  FIG. 2 . The mask  18  in the illustrated embodiment is a EUV mask, and includes a substrate  30  made of a LTEM. The LTEM material may include TiO 2  doped SiO 2 , and/or other low thermal expansion materials known in the art. In some embodiments, a conductive layer  32  is additionally disposed under on the backside of the LTEM substrate  30  for the electrostatic chucking purpose. In one example, the conductive layer  32  includes chromium nitride (CrN). In other embodiments, other suitable compositions are possible, such as a tantalum-containing material. 
     The EUV mask  18  includes a reflective multilayer (ML) structure  34  disposed over the LTEM substrate  30 . The ML structure  34  may be selected such that it provides a high reflectivity to a selected radiation type/wavelength. The ML structure  34  includes a plurality of film pairs, such as Mo/Si film pairs (e.g., a layer of molybdenum above or below a layer of silicon in each film pair). Alternatively, the ML structure  34  may include Mo/Be film pairs, or any materials with refractive index difference being highly reflective at EUV wavelengths. 
     Still referring to  FIG. 2 , the EUV mask  18  also includes a capping layer  36  disposed over the ML structure  34  to prevent oxidation of the ML. In one embodiment, the capping layer  36  includes silicon with a thickness ranging from about 4 nm to about 7 nm. The EUV mask  18  may further include a buffer layer  38  disposed above the capping layer  36  to serve as an etching-stop layer in a patterning or repairing process of an absorption layer, which will be described later. The buffer layer  38  has different etching characteristics from the absorption layer disposed thereabove. The buffer layer  38  includes ruthenium (Ru), Ru compounds such as RuB, RuSi, chromium (Cr), chromium oxide, and chromium nitride in various examples. 
     The EUV mask  18  also includes an absorber layer  40  (also referred to as an absorption layer) formed over the buffer layer  38 . In some embodiments, the absorber layer  40  absorbs the EUV radiation directed onto the mask. In various embodiments, the absorber layer may be made of tantalum boron nitride (TaBN), tantalum boron oxide (TaBO), or chromium (Cr), Radium (Ra), or a suitable oxide or nitride (or alloy) of one or more of the following materials: Actium, Radium, Tellurium, Zinc, Copper, and Aluminum. 
     The EUV lithography system discussed above in  FIGS. 1-2  is merely an example lithography system for which overlay marks can be used. However, the overlay marks of the present disclosure may be used for other types of lithography systems having different light sources. The overlay marks of the present disclosure will now be discussed below in more detail. 
       FIG. 3  illustrates a simplified fragmentary cross-sectional side view of an overlay mark  100 . The overlay mark  100  includes an upper layer  100 A and a lower layer  100 B. In some embodiments, the upper layer  100 A includes a patterned photoresist layer, and the lower layer  100 B includes a patterned spacer layer on a substrate. In other embodiments, the upper layer  100 A and the lower layer  100 B may include different patterned layers on a substrate. 
     The upper layer  100 A and the lower layer  100 B each include a plurality of patterned components, also referred to as gratings. For example, the upper layer  100 A includes a plurality of gratings  110 A, and the lower layer  100 B includes a plurality of gratings  110 B. The gratings  110 A and  110 B are elongated features that extend in a certain direction, for example in a direction orthogonal to the cross-section in which the cross-sectional view of  FIG. 3  is taken. In some embodiments, the gratings  110 A are periodically distributed, and/or the gratings  110 B are periodically distributed. In other words, the gratings  110 A are separated from one another by a constant spacing, and the gratings  110 B are separated from one another by a constant spacing. 
     An overlay between the upper layer  100 A and the lower layer  100 B may be measured by light diffraction. For example, in response to incident light projected onto the overlay mark  100 , different orders of diffracted light may be produced as a result. In  FIG. 3 , a 0th order diffracted light is shown as I 0 , a +1 order diffracted light is shown as I +1 , and a −1 order diffracted light is shown as I −1 . The intensities of the various diffracted orders of light may be measured by an optical measurement tool. In some embodiments, the optical measurement tool includes a scatterometry machine. In some other embodiments, the optical measurement tool includes a diffractometry machine. It is understood that the optical measurement tool may also be configured to generate the incident light in some embodiments. Based on the measured I +1  and I −1  data, asymmetry information (As) associated with the overlay mark  100  can be defined as =I +1 −I −1 . The asymmetry information is used to determine overlay, as discussed in more detail below. 
     Referring now to  FIG. 4A , shown therein is fragmentary cross-sectional view of an upper layer  1400  and a lower layer  1300  of an overlay mark  202  according to some embodiments of the present disclosure. In some embodiments, the overlay mark  202  includes region  1  and region  2 . In some implementations, the upper layer  1400  and the lower layer  1300  may be two different layers of the overlay mark  202  on a substrate, such as a photomask. The lower layer  1300  includes a plurality of compound gratings  130  in region  1  and a plurality of compound gratings  132  in region  2 . The plurality of compound gratings  130  extend along the y direction (into and out of the cross-sectional plane). In some implementations, the plurality of compound gratings  130  in the lower layer  1300  includes one elongated element  130 A having a width W 1 , a plurality of elongated elements  130 B each having a width W 2 , one elongated element  130 C having a width W 3 , and one elongated element  130 D with a width W 4 , where the widths are measured in an X-direction perpendicular to the Y-direction. In the implementations represented by  FIG. 4A , the width W 2  is smaller than each of the width W 1 , the width W 3  and the width W 4 . The width W 1 , the width W 3  and the Width W 4  can be different from one another due to intentional loading effect differential introduced by different widths of the mandrels used to form the elongated elements. In one non-limiting example, the width W 4  is greater than the width W 3 . In some embodiments, the plurality of compound gratings  130  in region  1  of the lower layer  1300  includes a gap  130 E between the elongated element  130 C and the elongated element  130 D. In some instances, the gap  130 E includes a width W 5 , and W 5  is represents a width of a removed mandrel that is used to form the elongated element  130 C and the elongated element  130 D. In some embodiments, the plurality of elongated elements  130 B is periodically disposed at a pitch P, and each of the plurality of elongated elements  130 B is separated from one another by a constant spacing. The constant spacing is smaller than the width W 5 . In some instances, the plurality of elongated elements  130 B includes 2 to 15 elongated elements, for example 4 to 12 elongated elements. While the embodiments shown in  FIG. 4A  include pluralities of compound gratings  130  and  132  extending along the Y direction, the pluralities of compound gratings may be arranged to extend along the X direction. It is also understood that the region  1  may have multiple groups of the plurality of compound gratings  130 . In some embodiments, these groups of the plurality of compound gratings  130  are periodically repeated. 
     The plurality of compound gratings  132  in region  2  is a mirror image of the plurality of compound gratings  130  in region  1  with respect to the borderline  210  between region  1  and region  2 . The plurality of compound gratings  132  in the lower layer  1300  includes one elongated element  132 A having the width W 1 , a plurality of elongated elements  132 B each having the width W 1 , one elongated element  132 C having the width W 3 , and one elongated element  132 D with the width W 4 . Similarly, in some instances, the width W 1  is greater than the width W 4 , the width W 4  is greater than the width W 3 , and the width W 3  is greater than the width W 2 . In some embodiments, the plurality of elongated elements  132 B is periodically disposed at the pitch P, and each of the plurality of elongated elements  132 B is separated from one another by the constant spacing. In some embodiments, the plurality of compound gratings  132  in region  1  of the lower layer  1300  includes a gap  132 E between the elongated element  132 C and the elongated element  132 D. In some instances, the gap  132 E includes the width W 5 , and the width W 5  is greater than W 1 . The width W 5  is greater than the constant spacing. In some embodiments, the plurality of elongated elements  132 B includes 2 to 15 elongated elements, for example 4 to 12 elongated elements. It is also understood that the region  2  may have multiple groups of the plurality of compound gratings  132 . In some embodiments, these groups of the plurality of compound gratings  132  are periodically repeated. 
     In the embodiments represented by  FIG. 4A , the upper layer  1400  includes a plurality of gratings  140  in region  1  and a plurality of gratings  142  in region  2 . Both the plurality of gratings  140  and the plurality of gratings  142  extend along the Y direction (into and out of the cross-sectional plane) as well. In some implementations, the plurality of gratings  140  in region  1  includes elongated elements  140 A disposed at the pitch P, and the plurality of gratings  142  in region  2  includes elongated elements  142 A disposed at the same pitch P. The plurality of gratings  140  and the plurality of gratings  142  are identical and equally pitched. Each of the gratings in the plurality of gratings  140  and in the plurality of gratings  142  has the width W 2 . In some embodiments as shown in  FIG. 4A , the plurality of gratings  140  is disposed above and over the plurality of compound gratings  130 , and the plurality of gratings  142  is disposed above and over the plurality of compound gratings  132 . 
     A known bias may be introduced between the upper layer  1400  and the lower layer  1300 . For example, although the plurality of gratings  140  in region  1  of the upper layer  1400  shares the same pitch P and the same width W 2  with the plurality of elongated elements  130 B in the lower layer  1300 , the plurality of gratings  140  is shifted by a distance d with respect to the plurality of elongated elements  130 B along the −X direction (e.g., shifted to the “left” as shown in  FIG. 4A ). This shift in region  1  can be referred to as bias −d. Similarly, the plurality of gratings  142  in region  2  of the upper layer  1400  is shifted by a distance d with respect to the plurality of elongated elements  132 B along the +X direction (e.g., shifted to the “right” as shown in  FIG. 4A ). This shift in region  2  can be referred to as bias +d. The bias −d in region  1  and the bias +d in region  2  may be intentionally configured or implemented as part of the design of the photomask. 
     As shown in the embodiments represented by  FIG. 4A , out of the elongated elements of each of the plurality of compound gratings  130 , the elongated element  130 A with the width W 1  is the closest to the borderline  210 , and the elongated element  130 D with the width W 4  is the farthest away from the borderline  210 . The plurality of compound gratings  132  in region  2 , being the mirror image of the plurality of compound gratings  130  in region  1 , includes a symmetric arrangement. The elongated element  132 A with the width W 1  is the closest to the borderline  210 , and the elongated element  130 D with the width W 4  is the farthest away from the borderline  210 . In some instances, the plurality of compound gratings  130  in region  1  can be referred to as “normal” gratings, and the plurality of compound gratings  132  in region  2  can be referred to as “inverse” gratings. Taking into consideration of the known bias introduced into the overlay mark  202 , region  1  can be referred to “−d normal” and region  2  can be referred to as “+d normal.” 
     Referring now to  FIG. 4B , shown therein is another overlay mark  204 . Similar to the embodiment represented by  FIG. 4A , along the Z direction, the overlay mark  204  includes a lower layer  1500  and an upper layer  1600 ; and along the X direction, the overlay mark  204  includes a region  1  and a region  2 . In some embodiments, the upper layer  1600  of the overlay mark  204  is substantially identical to the upper layer  1400  of the overlay mark  202 . In region  1  of the lower layer  1500  is a plurality of compound gratings  150 . In region  2  of the lower layer  1500  is a plurality of compound gratings  152 . In some implementations, the plurality of compound gratings  152  in region  2  is substantially identical to the plurality of compound gratings  130  of the overlay mark  202 , and the plurality of compound gratings  150  in region  1  is substantially identical to the plurality of compound gratings  132  of the overlay mark  202 . That is, the plurality of compound gratings  152  in region  2  may be referred to as “normal” gratings and the plurality of compound gratings  150  in region  1  may be referred to as “inverse” gratings. In some implementations, each of the plurality of compound grating  150  includes one elongated element  150 A, a plurality of elongated elements  150 B, one elongated element  150 C, and one elongated element  150 D. In some embodiments, each of the plurality of compound gratings  152  includes one elongated element  152 A, a plurality of elongated elements  152 B, one elongated element  152 C, and one elongated element  152 D. Out of the elongated elements  150 A- 150 D, the elongated element  150 D is the closest to the borderline  220  between region  1  and region  2 , and the elongated element  150 A is the farthest away from the borderline  220 . Out of the elongated elements  152 A- 152 D, the elongated element  152 D is the closest to the borderline  220  between region  1  and region  2 , and elongated element  152 A is the farthest away from the borderline  220 . 
     In some embodiments, similar to the overlay mark  202 , the plurality of gratings  160  and the plurality of gratings  162  in the upper layer  1600  of overlay mark  204  each include the pitch P. In a similar fashion, the pluralities of elongated elements  150 B and  152 B include the pitch P as well. The known bias d can be introduced between the upper layer  1600  and the lower layer  1500  in regions  1  and  2 . In region  1 , the plurality of gratings  160  is disposed above the plurality of compound gratings  150  and is shifted in the −X direction by a distance d. In region  2 , The plurality of gratings  162  is above the plurality of compound gratings  152  and is shifted in the +X direction by a distance d. Viewing the exemplary overlay mark  202  in  FIG. 4A  and overlay mark  204  in  FIG. 4B  as a whole, region  1  of  FIG. 4A  can be referred to as “−d normal,” region  2  of  FIG. 4A  can be referred to as “+d inverse,” region  1  of  FIG. 4B  can be referred to as “−d inverse” and region  2  of  FIG. 4B  can be referred to as “+d normal.” The plurality of elongated elements  150 B includes 2 to 15 elongated elements, for example 4 to 12 elongated segments. The plurality of elongated elements  152 B includes 2 to 15 elongated elements, for example 4 to 12 elongated elements. 
     Embodiments of the present disclosure provide advantages. Taking the overlay mark  202  in  FIG. 4A  as an example, the overlay information of region  1  can be described as (OVL −OVL BGA ). In the expression, the plurality of elongated elements  130 B in region  1  of the lower layer  1300  and the compound gratings  140  in region  1  of the upper layer  1400  contribute to the overlay term OVL and provide alignment information. The elongated element  130 A, the elongated element  130 C and the elongated element  130 D contribute to the additional bottom grating asymmetry (BGA) error term −OVL BGA . The overlay information of region  2  can be described as (OVL+OVL BGA ). In the expression, the plurality of elongated elements  132 B in region  2  of the lower layer  1300  and the compound gratings  142  in region  2  of the upper layer  1400  contribute to the overlay term OVL and provide alignment information. The elongated element  132 A, the elongated element  132 C and the elongated element  132 D contribute to the additional BGA error term +OVL BGA . Viewing the overlay mark  202  as a whole, the overlay information of the overlay mark  202  can be expressed as (OVL−OVL BGA +OVL+OVL BGA )/2. Because the BGA error term from region  1  and the BGA error term from region  2  are substantially equal in magnitude and opposite in polarity, the BGA error terms can be canceled out, and the above expression (OVL−OVL BGA +OVL+OVL BGA )/2 can be simplified as OVL. The designed-in BGA error terms for region  1  and region  2  reduce the asymmetry in the lower layer (or referred to as the bottom layer), improving the overlay accuracy. 
     In some instances, the design of the certain compound gratings in the lower layer (or bottom layer) can have varying densities therein. Taking the compound gratings  132  as an example, due to the presence of the gap  132 E, the gratings on the left-hand side of the compound gratings  132  are denser than the right-hand side thereof. This design of compound gratings  132  includes a wider mandrel to form the gap and a plurality of narrower mandrels to form the denser side of the compound gratings  132 . The difference in mandrel density can introduce different loading and create unevenness or imperfection in the compound gratings  132 . The compound gratings  130  are a mirror image of the compound gratings  132 . Because formation of the compound gratings  130  includes a mirror image of the mandrels used to form the compound gratings  132 , the unevenness or imperfection in the compound grating  130  is likely a mirror image of the unevenness or imperfection in the compound grating  132 . This mirror imaging allows the error terms in region  1  and region  2  to cancel each other out, yielding better alignment accuracy. In other words, although imperfections may be caused by the different densities of the compound gratings  132 , these imperfections may be obviated by the fact that the compound gratings  130  are designed as a mirror image of the compound gratings  132 . 
     Referring now to  FIG. 5 , shown therein is a combination overlay mark  300 . The overlay mark  300  includes a region I and a region II. In some embodiments, region I and region II are adjacent to (e.g., contiguous to each another) or near one another. In some embodiments, region I and region II are spaced apart. In some embodiments represented by  FIG. 5 , region I of the overlay mark  300  includes an area A with a +d normal overlay mark  301 , an area A′ with a −d normal overlay mark  302 , an area B with a +d inverse overlay mark  311 , and an area B′ with a −d inverse overlay mark  312 . Region II of the overlay mark  300  includes an area C with a +d normal overlay mark  321 , an area C&#39;s with a −d normal overlay mark  322 , an area D with a +d inverse overlay mark  331 , and an area D′ with a −d inverse overlay mark  332 . In some embodiments, the elongated elements and gratings in areas A, A′, B, and B′ extend along the X direction, and the elongated elements and gratings in areas C, C′, D, and D′ extend along the Y direction. In alternative embodiments, the elongated elements and gratings in areas A, A′, B, and B′ extend along the Y direction, and the elongated elements and gratings in areas C, C′, D, and D′ extend along the X direction. 
       FIG. 6  is another embodiment of a combination overlay mark  400 . The overlay mark  400  includes an area A with a +d normal overlay mark  401 , an area A′ with a −d normal overlay mark  402 , an area B with a +d inverse overlay mark  411 , an area B′ with a −d inverse overlay mark  412 , an area C with a +d normal overlay mark  421 , an area C′ with a −d normal overlay mark  422 , an area D with a +d inverse overlay mark  431 , and an area D′ with a −d inverse overlay mark  432 . In some embodiments represented by  FIG. 6 , the elongated elements and gratings in areas A, A′, B, and B′ extend along the X direction, and the elongated elements and gratings in areas C, C′, D, and D′ extend along the Y direction. In alternative embodiments, the elongated elements and gratings in areas A, A′, B, and B′ extend along the Y direction, and the elongated elements and gratings in areas C, C′, D, and D′ extend along the X direction. 
     It is noted that the mirror image compound gratings pairs do not have to be aligned with and adjacent to one another. In the embodiments represented by  FIGS. 4A and 4B , the plurality of compound gratings  130  in region  1  is the mirror image of the plurality of compound gratings  132  in region  2 . Region  1  of the overlay  202  is aligned with and adjacent to region  2 . In the embodiments represented by  FIGS. 5 and 6 , area A is a mirror image of area B′, area B is a mirror image of area A′, area C is a mirror image of area D′, area D is a mirror image of area C′. 
     The overlay marks disclosed herein, including the upper layers and lower layers of overlay marks  100 ,  202 ,  204 ,  300  and  400 , can be fabricated in any areas of an IC devices. In some embodiments, these overlay marks can be fabricated in scribe lines or scribe areas, which are subject to cutting in singulation processes. In these embodiments, at least a portion of the overlay marks in a singulated die is damaged, leaving behind some remnant overlay marks. In some alternative embodiments, these overlay marks can be fabricated in device areas (i.e. outside of the scribe lines or scribe areas), which are not subject to cutting in singulation processes. In these alternative embodiments, these overlay marks can survive the singulation process and remain intact in a final IC device. Both the intact overlay marks and remnant overlay marks according to the present disclosure can demonstrate a portion of overlay marks being a mirror image or another portion of the overlay marks. 
     Referring now to  FIG. 7 , illustrated therein is a flowchart of a method  500  of fabricating an overlay mark on a substrate. The method  500  is merely an example, and is not intended to limit the present disclosure beyond what is explicitly recited in the claims. Additional operations can be provided before, during, and after the method  500 , and some operations described can be replaced, eliminated, or moved around for additional embodiments of the method. Exemplary operations of the method  500  will be described below with reference to  FIGS. 8 and 9A-9D . 
     Referring now to  FIGS. 7-8 , at operation  510  of the method  500 , a plurality of mandrel features  601 A of an overlay mark  600  is formed in region  1 , and a plurality of mandrel features  601 B of the overlay mark  600  is formed in region  2  on a substrate  608 . The mandrel features can be fabricated with conventional mandrel forming processes. Note that region  1  has multiple sets or groups of the plurality of mandrel features  601 A, and region  2  has multiple sets or groups of the plurality of mandrel features  601 B. The pluralities of mandrel features  601 A and  601 B extend along the Y direction shown in  FIG. 8 . In one embodiment, each of the plurality of mandrel features  601 A includes one elongated element  604 A and a plurality of elongated elements  602 A. The plurality of mandrel features  601 A is disposed at a pitch  605 . For example, the pitch  605  (or a distance measured in the X-direction) separates one group of the mandrel features  601 A from its nearest group of mandrel features  601 A, as shown in  FIG. 8 . The elongated element  604 A includes a width W 6  along the X direction, and each of the plurality of elongated elements  602 A is equally spaced and includes a width W 7 . In some embodiments represented by  FIG. 8 , the width W 6  is greater than the width W 7 . In some embodiments, the width W 6  is at least twice as the width W 7  to ensure meaningful loading effect brought about by the larger width W 6 . In some implementations, the plurality of elongated elements  602 A includes 2 to 15 elongated elements with the width W 7 . The plurality of mandrel features  601 B in region  2  is a mirror image of the plurality of mandrel features  601 A in region  1  with respect to the borderline  610  between regions  1  and  2 . Consequently, the plurality of mandrel features  601 B is also disposed at the same pitch  605 . Each of the plurality of mandrel features  601 B includes one elongated element  604 B and a plurality of elongated elements  602 B. In the embodiments where the elongated element  604 A in region  1  has the width W 6  and each of the plurality of elongated elements  602 A in region  1  has the width W 7 , the elongated element  604 B includes the width W 6 , and each of the plurality of elongated elements  602 B is equally spaced and includes the width W 7 .  FIG. 9A  illustrates the Y-direction cross-sectional view of the mandrel features in  FIG. 8 . The greater width (at least twice of W 7 ) of W 6  can give rise to loading effects and result in intentionally introduced unevenness and imperfection in the overlay mark. Because the plurality of mandrel features  601 B in region  2  is a mirror image of the plurality of mandrel features  601 A in region  1 , the unevenness and imperfection in regions  1  and  2  can cancel out and improve the overlay accuracy. 
     At operations  520 ,  530  and  540  of the method  500 , spacers are formed over sidewalls of the plurality of mandrel features  601 A and the plurality of mandrel features  601 B. Referring now to  FIG. 9B , at operation  520  of the method  500 , a spacer layer  700  is deposited over the plurality of mandrel features  601 A and the plurality of mandrel features  601 B, including over the space between elongated elements. 
     Reference is now made to  FIG. 9C . At operation  530 , the deposited spacer layer  700  is planarized to expose the plurality of mandrel features  601 A and the plurality of mandrel features  601 B. 
     Referring to  FIG. 9D , at operation  540 , the plurality of mandrel features  601 A and the plurality of mandrel features  601 B are removed, leaving behind a plurality of spacers  800  in region  1  and a plurality of spacers  900  in region  2 . The plurality of spacers  800  may be referred to as a plurality of compound gratings  800  (e.g., as an embodiment of the compound gratings  130  or  150  discussed above with reference to  FIGS. 4A-4B ). The plurality of spacers  900  may be referred to as a plurality of compound gratings  900  (e.g., as an embodiment of the compound gratings  132  or  152  discussed above with reference to  FIGS. 4A-4B ). In some implementations, each of the plurality of compound grating  800  includes one elongated element  800 A, one elongated element  800 B, a plurality of elongated elements  800 C, and one elongated element  800 D. In some embodiments, each of the plurality of compound gratings  900  includes one elongated element  900 A, one elongated element  900 B, a plurality of elongated elements  900 C, and one elongated element  900 D. In the implementations represented by  FIG. 9D , as the plurality of mandrel features  601 B is a mirror image of the plurality of mandrel features  601 A with respect to the borderline  610 , the plurality of compound gratings  900  is a mirror image of the plurality of compound gratings  800  with respect to the borderline  610  as well. In some implementations, each of the elongated elements  800 A and  900 A has the width W 4 , each of the elongated elements  800 B and  900 B has the width W 3 , each of the elongated elements  800 C and  900 C has the width W 2 , and each of the elongated elements  800 D and  900 D has the width W 1 . In the implementations represented by  FIG. 4A , the width W 2  is smaller than each of the width W 1 , the width W 3  and the width W 4 . The width W 1 , the width W 3  and the Width W 4  can be different from one another due to intentional loading effect differential introduced by different widths of the mandrels used to form the elongated elements. In one non-limiting example, the width W 4  is greater than the width W 3 . 
       FIG. 10  is a flowchart illustrating a method  1000  of semiconductor fabrication according to aspects of the present disclosure. The method  1000  includes a step  1002  of patterning an overlay mark on a substrate. The overlay mark includes an upper layer; and a lower layer disposed below the upper layer. The lower layer can include a first plurality of compound gratings and a second plurality of compound gratings. The first plurality of compound gratings extends in a first direction and is disposed in a first region of the overlay mark. Each of the first plurality of compound gratings can include one first element and at least two second elements disposed on one side of the first element. The second plurality of compound gratings extends in the first direction and is disposed in a second region of the overlay mark. Each of the second plurality of compound gratings can include one third element and at least two fourth elements disposed on one side of the third element. Each of the first element and the third element has a first width along a second direction perpendicular to the first direction. Each of the second elements and the fourth elements has a second width along the second direction. The second width is smaller than the first width. The first plurality of compound gratings is a mirror image of the second plurality of compound gratings. The method  1000  further includes a step  1004  of performing one or more semiconductor manufacturing processes using the overlay mark. 
     In some embodiments, each of the first plurality of compound gratings may further include one fifth element and the at least two second elements are disposed between the first element and the fifth element. In some embodiments, each the first plurality of compound gratings may further include one sixth element disposed between the at least two fourth elements and the fifth element. In some implementations, each of the first plurality of compound gratings may further include one gap disposed between the sixth element and the fifth element. In some implementations, the fifth element has a third width and the sixth element has a fourth width. Each of the third width and the fourth width is greater than the second width. In some embodiments, the at least two second elements comprise 4 to 12 second elements. In some embodiments, the upper layer may include a third plurality of gratings and the third plurality of gratings may be shifted with respect to the at least two second elements in the second direction. In some instances, the upper layer may further include a fourth plurality of gratings and the fourth plurality of gratings is shifted with respect to the at least two fourth elements in the second direction. In some embodiments, the first region of the overlay mark is adjacent to and aligned with the second region of the overlay mark along the second direction. 
     It is understood that additional processes may be performed before, during, or after the steps  1002 - 1004  of the method  1000 . For reasons of simplicity, additional steps are not discussed herein in detail. 
       FIG. 11  is a flowchart illustrating a method  1100  of semiconductor fabrication according to aspects of the present disclosure. The method  1100  includes a step  1102  of patterning an overlay mark on a substrate. The overlay mark includes an upper layer that includes a plurality of gratings extending in a first direction, and a lower layer disposed below the upper layer. The lower layer may include a first plurality of compound gratings extending in the first direction and disposed in a first region of the overlay mark and a second plurality of compound gratings extending the first direction and disposed in a second region of the overlay mark. Each of the first plurality of compound gratings may include one first element, one second element, and at least two third elements disposed between the first element and the second element. The first element has a first width along a second direction perpendicular to the first direction and each of the at least two third elements has a second width along the second direction. The second width is smaller than the first width. The second plurality of compound gratings is a mirror image of the first plurality of compound gratings. The plurality of gratings is shifted with respect to the at least two third elements in the second direction. The method  1100  further includes a step  1104  of performing one or more semiconductor manufacturing processes using the overlay mark. 
     In some embodiments, a portion of the second plurality of compound gratings may be shifted with respect to the plurality of gratings in the second direction. In some embodiments, each of the first plurality of compound gratings may further include one fourth element between the second element and the at least two third elements. In some implementations, each of the first plurality of compound gratings may further include one gap disposed between the second element and the fourth element. In some implementations, the fourth element has a fourth width and the fourth width is greater than the second width. In some instances, the at least two third elements may include 4 to 12 second elements. In some instances, the first region of the overlay mark is adjacent to and aligned with the second region of the overlay mark along the second direction. 
     It is understood that additional processes may be performed before, during, or after the steps  1102 - 1104  of the method  1100 . For reasons of simplicity, additional steps are not discussed herein in detail. 
     One embodiment of the present disclosure pertains to an integrated circuit (IC) device. The IC device includes an overlay mark on a substrate. The overlay mark includes an upper layer and a lower layer disposed below the upper layer. The lower layer includes a first plurality of compound gratings extending in a first direction and disposed in a first region of the overlay mark, each of the first plurality of compound gratings including one first element and at least two second elements disposed on one side of the first element, and a second plurality of compound gratings extending in the first direction and disposed in a second region of the overlay mark, each of the second plurality of compound gratings including one third element and at least two fourth elements disposed on one side of the third element. The first element and the third element each have a first width along a second direction perpendicular to the first direction. Each of the second elements and each of the fourth elements has a second width along the second direction, the second width being smaller than the first width. The first plurality of compound gratings is a mirror image of the second plurality of compound gratings. 
     In some embodiments, each of the first plurality of compound gratings of lower layer the further includes one fifth element. The at least two second elements are disposed between the first element and the fifth element. In some implementations, each the first plurality of compound gratings further includes one sixth element disposed between the at least two fourth elements and the fifth element. In some instances, each of the first plurality of compound gratings further includes one gap disposed between the sixth element and the fifth element. In some embodiments, the fifth element has a third width and the sixth element has a fourth width. Each of the third width and the fourth width is greater than the second width. In some implementations, the lower layer further includes a third plurality of compound gratings extending in the second direction and a fourth plurality of compound gratings extending in the second direction. The third plurality of compound gratings is a mirror image of the fourth plurality of compound gratings. In some embodiments, the upper layer includes a third plurality of gratings and the third plurality of gratings is shifted with respect to the at least two second elements in the second direction. In those embodiments, the upper layer further includes a fourth plurality of gratings and the fourth plurality of gratings is shifted with respect to the at least two fourth elements in the second direction. In some instances, the first region is adjacent to and aligned with the second region along the second direction. 
     Another embodiment of the present disclosure pertains to a method of fabricating a semiconductor device. The method includes patterning an overlay mark on a substrate and performing one or more semiconductor fabrication process using the overlay mark. The overlay mark includes an upper layer comprising a plurality of gratings extending in a first direction, and a lower layer disposed below the upper layer. The lower layer includes a first plurality of compound gratings extending in the first direction and disposed in a first region of the overlay mark, each of the first plurality of compound gratings including one first element, one second element, and at least two third elements disposed between the first element and the second element; and a second plurality of compound gratings extending the first direction and disposed in a second region of the overlay mark. The first element has a first width along a second direction perpendicular to the first direction and each of the at least two third elements has a second width along the second direction, the second width smaller than the first width. The second plurality of compound gratings is a mirror image of the first plurality of compound gratings. The plurality of gratings is shifted with respect to the at least two third elements in the second direction. 
     In some embodiments, a portion of the second plurality of compound gratings is shifted with respect to the plurality of gratings in the second direction. In those embodiments, each of the first plurality of compound gratings further includes one fourth element and the fourth element is between the second element and the at least two third elements. Also, in these embodiments, each of the first plurality of compound gratings further includes one gap disposed between the second element and the fourth element. Additionally, the fourth element has a fourth width, and the fourth width is greater than the second width. In some implementations, the lower layer further includes a third plurality of compound gratings extending in the second direction and a fourth plurality of compound gratings extending in the second direction. The third plurality of compound gratings is a mirror image of the fourth plurality of compound gratings. In some instances, the first region is spaced apart from the second region. 
     Another embodiment of the present disclosure pertains to a method of fabricating an overlay mark on a substrate. The method includes forming a first plurality of mandrel features at a pitch in a first region of the substrate, forming a second plurality of mandrel features at the pitch in a second region of the substrate such that the second plurality of mandrel features comprises a mirror image of the first plurality of mandrel features, forming spacers over sidewalls of the first plurality of mandrel features and the second plurality of mandrel features, and removing the first plurality of mandrel features and the second plurality of mandrel features. In this embodiment, the first plurality of mandrel features extends in a first direction. Each of the first plurality of mandrel features includes one first mandrel and at least two second mandrels disposed on a side of the first mandrel. The first mandrel has a first width along a second direction perpendicular to the first direction and each of the second mandrels has a second width along the second direction. The first width greater than the second width. 
     In some embodiments, the first region is spaced apart from the second region. In some implementations, the first width is at least twice of the second width. In some instances, forming of the spacers over sidewalls of the first plurality of mandrel features and the second plurality of mandrel features includes depositing spacer material over the first plurality of mandrel features and the second plurality of mandrel features, and planarizing the spacer material to expose top surfaces of the first plurality of mandrel features and the second plurality of mandrel features. 
     The foregoing has outlined features of several embodiments so that those skilled in the art may better understand the detailed description that follows. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure.