Patent Publication Number: US-11665897-B2

Title: Improving surface topography by forming spacer-like components

Description:
PRIORITY DATA 
     This is a divisional of U.S. patent application Ser. No. 16/721,565, filed Dec. 19, 2019, which is a continuation application of U.S. patent application Ser. No. 15/796,992 filed Oct. 30, 2017, now U.S. Pat. No. 10,522,557, issued on Dec. 31, 2019, the entire disclosure of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     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 IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component that can be created using a fabrication process) has decreased. However, conventional semiconductor device fabrication may still have certain drawbacks. For example, semiconductor device fabrication may involve topography variations. For example, some regions of the wafer may be “taller” or “shorter” than other regions of the wafer. The topography variations may lead to degraded device performance or even device failures. Conventional semiconductor fabrication methods have not provided a satisfactory solution to the topography variation problem. Therefore, while existing semiconductor devices and the fabrication thereof have been generally adequate for their intended purposes, they have not been entirely satisfactory in every aspect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is 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 and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIGS.  1  and  3 - 10    are cross-sectional side views of a semiconductor device at different stages of fabrication according to various embodiments of the present disclosure. 
         FIG.  2    is a three-dimensional perspective view of the semiconductor device at a stage of fabrication according to an embodiment of the present disclosure. 
         FIG.  11    is a top view of an overlay mark according to an embodiment of the present disclosure. 
         FIG.  12    is a flowchart of a method for fabricating a semiconductor device in accordance with embodiments of the present disclosure. 
     
    
    
     It is understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. 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 sake of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Moreover, various features may be arbitrarily drawn in different scales for the sake of simplicity and clarity. 
     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. For example, if the device in the figures is turned over, elements described as being “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. 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. 
     As semiconductor device sizes keep on shrinking with each technology generation, new fabrication challenges continue to arise. One of the challenges pertains to topography variations in semiconductor fabrication. Topography variations may occur when some regions of a wafer may be “taller” (e.g., have a greater vertical height) than some other regions of the wafer, or vice versa. The “taller” or “shorter” regions may include different layers, such as semiconductor layers, dielectric layers, or conductive (e.g., metal) layers. Topography variations may not be an important concern in older technology generations, since the device sizes for the older technology generations either exceed the topography variations, or at least are big enough in comparison to the topography variations, such that the topography variations are unlikely to cause any problems. As such, conventional semiconductor fabrication methods have not devised a solution to address any potential problems that could be caused by the topography variations. 
     Unfortunately, as the semiconductor fabrication technology node has progressed to the nanometer (nm) nodes (e.g., 22-nm or below), topography variations may cause problems. For example, if a wafer&#39;s surface topography variation is not flat or smooth but rather contains a first region that is taller than a second region, then a layer formed on both the first region and the second region may “break” or experience a discontinuity. In other words, the layer may become damaged as a result of being formed over an uneven surface or terrain. The damaged layer may also lead to problems in subsequent fabrication processes, for example over-etching of layers that should not be etched, etc. Consequently, device performance may be degraded, and device failures may even occur in some instances. 
     The inventors of the present disclosure have discovered the potential problems caused by topography variations as a part of semiconductor device fabrication. Accordingly, the present disclosure pertains to a method of overcoming these potential problems, for example by “smoothing over” the topography variations. Certain various aspects of the present disclosure will be discussed in more detail below using a flash memory device as an example device in which the topography variations may occur, and a solution according to the various aspects of the present disclosure is implemented. However, it is understood that the flash memory device is merely an example, and various aspects of the present disclosure may apply to other types of devices as well. 
     Referring now to  FIG.  1   , a simplified diagrammatic fragmentary cross-sectional side view of a portion of a memory device  100  is illustrated. In some embodiments, the memory device  100  includes a flash memory device, which is a form of electronically erasable programmable read-only memory (EEPROM). A flash memory device may include a plurality of blocks or arrays or individual cells that contain NAND logic gates or NOR logic gates. In addition to being capable of storing electronically erasable data, some other advantages of flash memory devices include their relatively fast access times, mechanical shock resistance, and non-volatility. Due at least in part to these advantages, flash memory devices have been widely used in many consumer and professional products, for example memory cards, USB (Universal Serial Bus) flash drives, solid-state drives, personal computers (including desktops and/or laptops), digital cameras, mobile telephones, tablet computers, video games, robotics, and medical devices, etc. 
     For reasons of simplicity, merely a small portion of the memory device  100  is shown in  FIG.  1   . As shown in  FIG.  1   , the memory device  100  includes a plurality of active regions  110  (also referred to as OD). The active region  110  may be formed over a substrate, for example a silicon substrate. The active region  110  may include one or more doped wells, which may be formed using one or more ion implantation processes that implant dopant ions into the substrate. The dopant ions may include an n-type material in some embodiments, for example arsenic (As) or phosphorous (P), or they may include a p-type material in some other embodiments, for example boron (B), depending on whether an NFET or a PFET is needed. In some embodiments, the active regions  110  may each have a lateral dimension that is in a range between about 50 nm and about 200 nm. It is understood that portions of the active regions  110  may serve as the source/drain regions for a transistor, and other portions of the active regions  110  may serve as the channel regions for a transistor. 
     The memory device  100  also includes an isolation structure  120 . The isolation structure  120  may also be formed over the substrate, for example by etching recesses in the substrate and then filling the etched recesses with a dielectric material. The isolation structure  120  is adjacent to and surrounds the active regions  110 . In some embodiments, the isolation structure  120  includes shallow trench isolation (STI). The isolation structure  120  provides electrical isolation between various components of the memory device  100 , for example including, but not limited to, the active regions  110 . 
     As is shown in  FIG.  1   , the memory device  100  includes a low voltage region (LV), a high voltage region (HV), and a non-volatile memory region (NVM). In some embodiments, the NVM region includes a non-volatile memory device such as flash memory, while neither the LV region nor the HV region contains any non-volatile memory devices. 
     In the LV region, a gate dielectric  130  is disposed over each of the active regions  110 , respectively. The gate dielectric  130  may serve as the gate dielectric of transistors for the LV region. In some embodiments, the gate dielectric  150  may include silicon oxide (SiO2). In other embodiments, the gate dielectric  150  may include a high-k dielectric material, which is a material having a dielectric constant that is greater than a dielectric constant of SiO2 (approximately 4). In an embodiment, the high-k gate dielectric includes hafnium oxide (HfO2), which has a dielectric constant that is in a range from approximately 18 to approximately 40. In alternative embodiments, the high-k gate dielectric may include ZrO2, Y2O3, La2O5, Gd2O5, TiO2, Ta2O5, HfErO, HfLaO, HfYO, HfGdO, HfAlO, HfZrO, HfTiO, HfTaO, or SrTiO. 
     A gate electrode  140  is formed over the gate dielectric  130  in the LV region. The gate electrode  140  may serve as the gate electrode of the transistors for the LV region. In some embodiments, the gate electrode  140  may include polysilicon. In some other embodiments, the gate electrode  140  may include a metal gate electrode. The metal gate electrode may include a work function metal component and a fill metal component. The work functional metal component is configured to tune a work function of its corresponding transistor to achieve a desired threshold voltage Vt. In various embodiments, the work function metal component may contain: titanium (Ti), titanium aluminum (TiAl), titanium aluminum nitride (TiAlN), tantalum (Ta), tantalum carbide (TaC), tantalum carbon nitride (TaCN), tantalum silicon nitride (TaSiN), tantalum nitride (TaN), titanium nitride (TiN), tungsten nitride (WN), or tungsten (W), cobalt (Co), nickel (Ni), manganese (Mn), zirconium (Zr), or combinations thereof. The fill metal component is configured to serve as the main conductive portion of the gate structure. In various embodiments, the fill metal component may contain aluminum (Al), tungsten, copper (Cu), aluminum copper (AlCu), or combinations thereof. 
     For the HV region, a gate dielectric  150  is disposed over each of the active regions  110 , respectively. The gate dielectric  150  may serve as the gate dielectric of the transistors for the HV region. Similar to the gate dielectric  130 , the gate dielectric  150  may include silicon oxide or a high-k dielectric material. 
     A gate electrode  160  is formed over the gate dielectric  150  in the HV region. The gate electrode  160  may serve as the gate electrode of the transistors for the HV region. Similar to the gate electrode  140 , the gate electrode  160  may include a polysilicon material or metal gate material. 
     One difference between the LV region and the HV region is that the transistors in the LV region and in the HV region are configured to handle different voltages. For example, the transistor in the LV region may be configured to handle a voltage in a range between about 0.5 volts and about 5 volts, whereas the transistors in the HV regions may be configured to handle a voltage in a range between about 5 volts and about several tens of volts, for example 20 volts. In order for the transistors in the HV region to properly handle the higher voltage, they may have a greater thickness for the gate dielectric  150  than the gate dielectric  130  in the LV region. 
     Note also that a portion  140 A of the gate electrode  140  is formed over a portion of the gate electrode  160 . As such, a step height  170  exists between the LV region and the HV region. The step height  170  is determined by the thickness of the portion  140 A of the gate electrode  140  located over the gate electrode  160 . In some embodiments, the step height  170  is in a range between about 20 nm and about 400 nm. It may be said that a topography variation  180  exists between the LV region and the HV region for the semiconductor device  100  due to the step height  170 . As will be discussed in more detail below, the topography variation  180  between the LV region and the HV region may lead to problems in subsequent fabrication processes. As such, the present disclosure provides a solution to overcome the problems caused by the topography variation, as will be discussed below in more detail. 
     Still referring to  FIG.  1   , a tunnel oxide layer  190  is formed over the active regions  110  and the isolation structure  120  in the NVM region. Charge carriers may be moved through the tunnel oxide  190  via mechanisms such as Polyoxide Conduction, CHEI (Channel hot electron injection), and the high electric field Fowler-Nordheim tunneling. The tunnel oxide layer  190  has a thickness  195 . In some embodiments, the thickness  195  is in a range between about 5 nm and about 50 nm. 
     For the NVM region, a floating gate component  200  is formed over the tunnel oxide layer  190 . In some embodiments, the floating gate component  200  includes a semiconductor material such as polysilicon. The floating gate component  200  is formed to have a thickness  205 . In some embodiments, the thickness  205  is in a range between about 20 nm and about 400 nm. 
     A dielectric structure  210  is formed over the floating gate component  200 . In some embodiments, the dielectric structure  210  includes an “ONO” structure with an oxide component, a nitride component, and another oxide component. For example, the ONO structure may include a first silicon oxide layer formed over the floating gate component  200 , and a silicon nitride layer formed over the silicon nitride layer, and a second silicon oxide layer formed over the silicon nitride layer. In other embodiments, the dielectric structure  210  may include a silicon nitride layer and a silicon oxide layer, but not two silicon oxide layers. The dielectric structure  210  is formed to have a thickness  215 . In some embodiments, the thickness  215  is in a range between about 10 nm and about 60 nm. 
     A control gate component  220  is formed over the dielectric structure  210 . In some embodiments, the control gate component  220  includes a semiconductor material such as polysilicon. In some embodiments, the control gate component  220  includes the same materials as the gate electrode  160  formed in the HV region, and they may be formed simultaneously using the same deposition processes. The control gate component  220  is formed to have a thickness  225 . In some embodiments, the thickness  225  is in a range between about 20 nm and about 400 nm. 
     It is understood that the ranges of the thicknesses  195 ,  205 ,  215 , and  225  may all contribute to a step height  230  at a boundary between the HV region and the NVM region. In some embodiments, the step height  230  may be measured as a distance from an upper surface  250  of the gate electrode  160  in the HV region to an upper surface  260  of the control gate component  220  in the NVM region. In some embodiments, the step height  230  is greater than the step height  170 . For example, the step height  230  may be in a range between about 35 nm and about 510 nm. 
     Again, it may be said that a topography variation  181  exists at the boundary between the HV region and the NVM region for the semiconductor device  100  due to the step height  230 . Since the step height  230  is greater than the step height  170 , the topography variation between the HV region and the NVM region is even greater than the topography variation between the LV region and the HV region. As such, the semiconductor device  100  may experience even more problems caused by the greater topography variation between the HV region and the NVM region. Again, the present disclosure provides a solution to overcome the problems caused by such topography variation, as will be discussed below in more detail. 
     The topography variation between the HV region and the NVM is also illustrated in  FIG.  2   , which is a 3-dimensional perspective view of a portion of the memory device  100 . A part of the HV region is illustrated in  FIG.  2   , as is a part of the NVM region. As shown in  FIG.  2   , the gate electrode  160  is formed over the active region  110  and the isolation structure  120  in the HV region, and the control gate component  220  is formed on the dielectric structure  210  (which may include an ONO structure) in the NVM region. In some embodiments, the gate electrode  160  and the control gate component  220  are connected and are formed as the same layer (or layers). Due to the presence of the floating gate component  200 , the dielectric structure  210 , (and the tunnel oxide layer  190 , which is not specifically illustrated in  FIG.  2    for reasons of simplicity) in the NVM region, the step height  230  exists between the boundary of the HV region and the NVM region. Again, the negative effects caused by this step height  230  will be minimized by various aspects of the present disclosure. 
     The operation of the memory device  100  is now briefly described. Referring to  FIGS.  1 - 2   , the floating gate component  200  is configured to store a bit by the presence or absence of an electrical charge. When the floating gate component  200  is not electrically charged (e.g., neutral), then the transistor in the NVM region function similar to a regular metal oxide field effect transistor (MOSFET). For example, a positive charge in the control gate component  220  would create a channel between a source and a drain (e.g., formed in the active regions). On the other hand, when the floating gate component  200  is negatively charged, the negative charge may shield what would have been the channel from the control gate component  220 . As a result, the channel may not be formed between the source and the drain. The presence or the absence of a charge in the floating gate component  200  results in a more positive threshold voltage Vt or more a more negative threshold voltage Vt (where the threshold voltage Vt is voltage applied to the control gate component  220  at which the transistor becomes conductive). In this manner, the memory device may be programmed by injecting electrical charge into the floating gate component  200  or removing electrical charge from the floating gate component  200 . For example, injecting electrical charge into the floating gate component  200  results in writing a logic 0 to a flash memory cell, and removing electrical charge from the floating gate component  200  results in resetting the flash memory cell to a logic 1. 
     Referring now to  FIG.  3   , using a deposition process  300 , a layer  310  is formed over the upper surface of the gate electrode  140  in the LV region, over the gate electrode  160  in the HV region, and over the control gate component  220  in the NVM region. In some embodiments, the deposition process  300  includes chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), or combinations thereof. In some embodiments, the deposition process  300  may include a conformal deposition process, such that the layer  310  is formed conformally over the upper surfaces of the various components in the LV region, the HV region, and the NVM region. In other embodiments, the deposition process  300  may include a non-conformal deposition process as well. 
     In some embodiments, the layer  310  contains a dielectric material, for example a silicon oxide material. The silicon oxide material may include tetraethyl orthosilicate (also referred to as tetraethoxysilane, or abbreviated as TEOS). The layer  310  is also formed to have a thickness  320 . In some embodiments, the thickness  320  is in a range from about 600 angstroms and about 1000 angstroms. As will be discussed in greater detail below, the layer  310  is used as a buffer layer or a transitional layer to “smooth over” the topography variations of the semiconductor device  100 . In some embodiments, the range of values of the thickness  320  is specifically configured to facilitate the function served by the layer  310 —which is to minimize the topography variation effects. In other words, if the value of the thickness  320  is too high or too low, then the layer  310  may not achieve a desired efficacy in terms of reducing the topography variations between the HV region and the NVM region. 
     Referring now to  FIG.  4   , a patterning process  340  is performed to the layer  310  to pattern the layer  310  into components  310 A that are formed on the sidewalls of the portion  140 A of the gate electrode  140 , as well as a component  310 B formed on the sidewalls of the control gate component  220 . In some embodiments, the patterning process  340  is similar to a spacer patterning process, which may include an etching process or a blanket etch-back process. 
     As a result of the patterning process  340  being performed, the components  310 A and  310 B are shaped similar to spacers. For example, the components  310 A each have a somewhat rounded or curved surface  360 , and the component  310 B has a somewhat rounded or curved surface  370 . The curved surfaces  360  and  370  effectively make the upper surface of the semiconductor device  100  more smooth or “flatter.” For example, rather than having a relatively steep and abrupt drop off between the LV region and the HV region due to the step height  170  (see  FIG.  1   ), the curved surface  360  of the spacer-like components  310 A allow for a more gradual transition between the LV region and the HV region. In other words, the previous topography variation  180  (shown in  FIG.  1   ) between the LV region and the HV region is reduced into a smoother topography variation  380 . 
     Similarly, rather than having a relatively steep and abrupt drop off between the HV region and the NVM region due to the step height  230  (see  FIG.  1   ), the curved surface  370  of the spacer-like component  310 B allow for a more gradual transition between the HV region and the NVM region. In other words, the previous topography variation  181  (shown in  FIG.  1   ) between the HV region and the NVM region is reduced into a smoother topography variation  381 . 
     These more gradual transitions between the various regions of the semiconductor device  100  will reduce the likelihood of breakage of a layer to be formed subsequently (formed over the upper surfaces of the LV region, the HV region, and the NVM region). 
     Referring now to  FIG.  5   , using a deposition process  400 , a layer  410  is formed over the upper surface of the semiconductor device  100  in the LV region, the HV region, and the NVM region. The layer  410  is formed to have a thickness  420 . In some embodiments, the thickness  420  is in a range between about 50 nm and about 200 nm. 
     The layer  410  may include a dielectric material, such as an ashing-removable dielectric (ARD) material. In some embodiments, the ARD material may include a material having photoresist-like properties. For example, the ARD material may be capable of being etched and/or patterned, similar to a photoresist material. Thus, whereas an ashing process may be used to remove the photoresist material, it may also remove the ARD material. As an example, the ARD material may be a strippable CVD-deposited hardmask film such as Applied Materials Incorporated&#39;s® Advanced Patterning Film, which is an amorphous carbon material that produces higher definition patterns. Since the ARD material has properties that allow it to be removed by ashing, it may be used as a disposable material, and is used temporarily during the fabrication process, before eventually being completely removed from the final semiconductor package. In the examples herein, the ARD material allows the layer  410  to be patterned as a mask layer, and then the patterned mask layer is then used to pattern the control gate component  220  in a later process. 
     In some embodiments, due to the unique characteristics of the ARD material of the layer  410 , the deposition process  400  used to form the layer  410  may be a non-conformal deposition process. In other embodiments, the deposition process  400  may not be specifically configured to be a non-conformal deposition process, but if the step height  170  or the step height  230  is too great, then the result is that the layer  410  may still be deposited as a non-conformal layer. In any case, the layer  410  is deposited non-conformally over the upper surfaces of the various components in the LV region, the HV region, and the NVM region. If the spacer-like components  310 A and  310 B had not been formed, the non-conformal deposition of the layer  410 , in conjunction with the topography variations of the semiconductor device  100  discussed above (e.g., caused by step heights  170  or  230 ), could cause problems in later fabrication processes. 
     For example, referring to  FIG.  6   , which is a cross-sectional side view showing what the semiconductor device  100  could have looked like if the spacer-like components  310 A and  310 B had not been formed to smooth over the topography variations between the LV, HV, and NVM regions. In that case, the layer  410  deposited by the deposition process  400  may experience a “break”  450 , for example between the HV region and the NVM region. In other words, due to the step height  230  (shown in  FIG.  1   ) between the HV region and the NVM region, the non-conformal deposition process  400  may not be able to form a continuous layer  410 . Instead, the layer  410  may “break”, and portions of the layer  410  may be formed on an upper portion (but not the lower portion) of the sidewall of the control gate component  220 . As such, the layer  410  is now discontinuous and includes a segment  410 A formed over the LV region and the HV region, and a separate segment  410 B formed over the NVM region. 
     The “breaking” of the layer  410  may cause problems later on, when the layer  410  is used to pattern the layers below. For example, the absence of the layer  410  in the “break”  450  means that the layer below may be undesirably etched, when it should have been protected by the layer  410 . As another example, for some type of overlay marks, their boundaries may also correspond to the boundary between the HV region and the NVM region. As such, the breaking of the layer  410  may lead to a damage of the boundary of the overlay mark. As a result, overlay measurements may become inaccurate. 
     It is understood that although  FIG.  6    merely illustrates one “break”  450  (which occurs between the HV region and the NVM region), other “breaks” of the layer  410  may also occur, for example at the boundary or interface between the LV region and the HV region, due to the step height  170  (shown in  FIG.  1   ). When these additional breaks of the layer  410  occur, it may further compound the patterning problems in subsequent processes, as discussed above. 
     Making matters worse, the break  450  (or other similar breaks) may be exacerbated by some subsequent processes, such as during a photoresist rework process. This is shown in  FIGS.  7 - 9   . Referring to  FIG.  7   , in order to pattern the layer  410  into a mask layer, a bottom anti-reflective coating (BARC) layer  500  is formed over the layer  410 , and a photoresist layer  510  is formed over the BARC layer  500 . The photoresist layer  510  and the BARC layer  500  may undergo a photolithography process to define the photoresist layer  510  and the BARC layer  500 , so that they can be used to pattern the layer  410  later. 
     Referring now to  FIG.  8   , a photoresist and BARC removal process  530  is applied as a part of the photoresist rework process. In some cases, after the initial patterning of the photoresist layer  510  and the BARC layer  500 , measurements may reveal that the initial patterning of the photoresist layer and the BARC layer  500  may be unsatisfactory. This could be due to inadequate critical dimension (CD) or poor overlay, for example. As such, the photoresist needs to be “reworked”, for example by removing the existing photoresist layer  510  and the BARC layer  500 , and forming a new photoresist layer and a BARC layer. The photoresist and BARC removal process  530  may remove the photoresist layer  510  by photoresist washing, and it may remove the BARC layer  500  by applying an acid to remove the BARC layer  500 . In some embodiments, the acid may include Peroxymonosulfuric acid (H2SO5), also known as Caro&#39;s acid. Unfortunately, the acid may also corrode or otherwise damage the layer  410  if the “broken” layer  410  has a large surface area that may be exposed to the acid. Thus, the photoresist rework process could undesirably enlarge an already-existing “break”  450  into a larger “break”  450 A, as shown in  FIG.  8   . 
     Referring now to  FIG.  9   , as a part of the photoresist rework process, another BARC layer  550  and another photoresist layer  560  are formed over the layer  410 . The photoresist layer  560  and the BARC layer  550  will undergo lithography to be patterned, which in turn can be used to pattern the layer  410  below. However, the “break”  450 A—which was made larger by the acid used to remove the BARC layer  550 —still exists, and it may cause damage to other layers in subsequent fabrication processes. 
     Referring back to  FIG.  5   , since the present disclosure uses the spacer-like components  310 A and  310 B to effectively reduce the step height and thereby make the topography smoother between the LV, HV, and NVM regions, the layer  410  may not be broken even though it is deposited non-conformally. Furthermore, even if the semiconductor device  100  needs a photoresist rework at some point, the layer  410 —which is still formed as a continuous layer due to the presence of the spacer-like components  310 A and  310 B—will not be damaged due to the application of the acid used to remove the BARC layer. For these reasons, the potential problems discussed above with reference to  FIGS.  7 - 9    are unlikely to occur. As such, the method of the present disclosure discussed herein will improve semiconductor fabrication performance. 
     Referring now to  FIG.  10   , the layer  410  is shown as having been patterned by a photo-sensitive material (e.g., the photoresist layer and the BARC layer discussed above). The patterned layer  410  may include one or more openings, such as an opening  600  that exposes a portion of the control gate component  220 . In other words, in the embodiment illustrated in  FIG.  10   , the patterned layer  410  may be used to pattern the gate structure of the transistor in the NVM region. In other embodiments, the layer  410  may be patterned differently to pattern other structures, for example the gate structures of the transistors in the LV region or the HV region. 
       FIG.  11    illustrates a simplified top view of an overlay mark  700 . The overlay mark  700  may be formed on the wafer on which the semiconductor device  100  discussed above is formed. The overlay mark  700  may also be considered to be a part of the semiconductor device  100 . In the illustrated embodiment, the overlay mark  700  has a box-in-box structure, with an inner box  710  and an outer box  720 . The location of the inner box  710  or the outer box  720  may correspond to a location of the semiconductor device  100  where a step height exists. For example, in some embodiments, the inner box  710  or the outer box  720  may correspond to the boundary between the HV region and the NVM region discussed above. If the spacer-like components  310 A- 310 B of the present disclosure had not been formed to “smooth over” the step height, the breaking of the ARD layer may cause damage to the inner box  710 , or the outer box  720 , or both. This could degrade the overlay performance and cause the overlay measurements based on the overlay mark  700  to become unreliable. Fortunately, this is no longer an issue according to the various aspects of the present disclosure, since the spacer-like components lead to a smoother topography for the overlay mark  700  as well, and it can be fabricated without being damaged. 
       FIG.  12    is a flowchart of a method  900  for fabricating a semiconductor device in accordance with various aspects of the present disclosure. The method  900  includes a step  910  of providing a wafer having a first region and a second region. A first topography variation exists between the first region and the second region. In some embodiments, the semiconductor device includes a memory device. In some embodiments, the first region includes a non-volatile memory device. In some embodiments, the second region does not have a non-volatile memory device. In some embodiments, the first region is taller than the second region. In some embodiments, the non-volatile memory device includes a floating gate and a control gate formed over the floating gate. 
     In some embodiments, the method  900  includes a step  920  of forming a first layer over the first region and over the second region of the wafer. 
     In some embodiments, the method  900  includes a step  930  of patterning the first layer. A patterned first layer causes a second topography variation to exist between the first region and the second region. The second topography variation is smoother than the first topography variation. In some embodiments, the patterning of the first layer includes a spacer patterning process. 
     In some embodiments, the method  900  includes a step  940  of forming a second layer over the first region and the second region. At least a portion of the second layer is formed over the patterned first layer. In some embodiments, the forming of the second layer is performed using a non-conformal deposition process. In some embodiments, the forming of the second layer comprises forming the second layer as a continuous layer. 
     In some embodiments, the method  900  includes a step  950  of patterning the second layer into a patterned mask layer. In some embodiments, the patterning of the second layer comprises a photoresist rework process. In some embodiments, the patterning of the second layer comprises forming an anti-reflective material over the second layer and forming a photo-sensitive material over the anti-reflective material. In some embodiments, the photoresist rework process comprises applying an acid to remove the anti-reflective material. In some embodiments, the patterning of the second layer comprises forming the patterned second layer on a sidewall of the control gate. 
     It is understood that additional process steps may be performed before, during, or after the steps  910 - 950  discussed above to complete the fabrication of the semiconductor device. For example, these additional steps may include contact formation, testing, packaging, etc. For reasons of simplicity, these additional steps are not discussed in detail herein. 
     Based on the above discussions, it can be seen that the present disclosure offers advantages over conventional semiconductor fabrication methods. It is understood, however, that other embodiments may offer additional advantages, and not all advantages are necessarily disclosed herein, and that no particular advantage is required for all embodiments. One advantage is that, due to the spacer-like components formed herein at the boundaries of regions where a step height exists, the device of the present disclosure can achieve a much smoother surface topography. This reduces the likelihood of breakage of subsequent layers (e.g., an ARD layer) formed on the surface. Since the layer is not broken, it will not cause damage to other layers (e.g., the layers to be patterned below) during subsequent fabrication processes. Consequently, semiconductor device performance (e.g., overlay performance) is enhanced, and yield may be improved. This is especially true if a photoresist rework process would need to be performed. During the photoresist rework process, an acid may be applied to remove the BARC layer, and that acid could enlarge a discontinuity of the ARD layer, if that discontinuity already exists as a result of the uneven surface topography. Here, since the ARD layer is formed as a continuous layer (due to the smoother surface topography), the acid applied as a part of the photoresist rework process would still not exacerbate the damage to the ARD layer. In addition, the various aspects of the present disclosure are compatible with current fabrication process flow and are easy to implement, and therefore inexpensive to implement in actual fabrication. 
     One embodiment of the present disclosure involves a method of fabricating a semiconductor device. The method includes: providing a wafer having a first region and a second region, wherein a first topography variation exists between the first region and the second region; forming a first layer over the first region and over the second region of the wafer; patterning the first layer, wherein a patterned first layer causes a second topography variation to exist between the first region and the second region, and wherein the second topography variation is smoother than the first topography variation; and forming a second layer over the first region and the second region, wherein at least a portion of the second layer is formed over the patterned first layer. 
     Another embodiment of the present disclosure involves a method of fabricating a semiconductor device. The method includes: providing a wafer having a first region and a second region, wherein the first region includes a flash memory cell and is taller than the second region; forming a first layer over the first region and over the second region of the wafer; patterning the first layer to form a spacer component at a boundary between the first region and the second region; and forming a continuous second layer over the first region and over the second region, including over the spacer component. 
     Yet another embodiment of the present disclosure involves a method of fabricating a semiconductor device. The method includes: providing a wafer having a first device and a second device located adjacent to the first device, wherein the first device comprises a non-volatile memory device, and wherein a step height exists at a boundary between the non-volatile memory device and the second device; forming a component at the boundary, wherein the component provides a reduction in the step height between the non-volatile memory device and the second device; and forming, via a non-conformal deposition process, a mask layer over the non-volatile memory device and the second device, wherein the mask layer is formed to be continuous at least in part due to the component providing the reduction in the step height. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. 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.