Patent Publication Number: US-11661332-B2

Title: Stiction reduction system and method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit and priority to the U.S. Provisional Patent Application Ser. No. 62/807,865 filed Feb. 20, 2019, entitled “METHOD OF FORMING ROUGH CAVITY;” U.S. Provisional Patent Application Ser. No. 62/807,867 filed Feb. 20, 2019, entitled “METHOD OF FORMING BUMPSTOP ON MEMS DEVICE;” and U.S. Provisional Patent Application Ser. No. 62/825,585 filed Mar. 28, 2019, entitled “UCAV STICTION REDUCTION IN MOTION SENSOR;” which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     MEMS (“micro-electro-mechanical systems”) are a class of devices that are fabricated using semiconductor-like processes and exhibit mechanical characteristics. For example, MEMS devices may include the ability to move or deform. Stiction is a problem in MEMS due to their large surface area-to-volume ratio, as surface force becomes greater than gravity and inertia causing two or more components to stick to one another without releasing. In other words, stiction results in large surface adhesion since the restoring force cannot overcome the attractive interfacial forces caused by capillary and electrostatic forces. Stiction can occur during fabrication (release stiction) and/or in applications (in-use stiction). The surface adhesion and stiction adversely affect the reliability and long-term durability of MEMS devices. 
     SUMMARY 
     Accordingly, a need has arisen to reduce the adhesion and stiction in MEMS devices. Methods and systems for reducing stiction through roughening the surface and reducing the contact area in MEMS devices are disclosed. 
     In some embodiments, a method includes forming an oxide mask on a portion of a surface of a MEMS device substrate. The method further includes depositing a bumpstop material over the oxide mask and further over an exposed portion of the surface of the MEMS device substrate. The method in some embodiments includes etching the bumpstop material to expose the oxide mask. According to some embodiments, the method further includes removing the oxide mask to expose the portion of the surface of the substrate and the bumpstop material. In one illustrative embodiment, the method further includes fusion bonding the MEMS device substrate to a MEMS cap substrate. 
     In some embodiments, the MEMS device includes a MEMS accelerometer or a MEMS gyroscope. According to some embodiments, the bumpstop material includes one or more of polysilicon, dielectric, metal, and single crystal silicon. It is appreciated that the oxide mask may be removed through buffered oxide etch (BOE), concentrated hydrofluoric acid (HF), or Vapor phase HF. 
     In some embodiments, a MEMS device includes a MEMS device substrate and a cap substrate. The MEMS device further includes one or more bumpstops formed on the surface of the MEMS device substrate. According to some embodiments, the forming of the bumpstop includes forming an oxide mask on a portion of a surface of the MEMS device substrate. The forming of the bumpstop further includes depositing a bumpstop material over the oxide mask and further over an exposed portion of the surface of the MEMS device substrate. In one illustrative embodiment, the forming of the bumpstop includes etching the bumpstop material to expose the oxide mask. The forming of the bumpstop in some embodiments includes removing the oxide mask to expose the portion of the surface of the substrate and the bumpstop material. In some embodiments, the MEMS cap substrate is fusion bonded with the MEMS device substrate. 
     In some embodiments, the MEMS device substrate includes a MEMS accelerometer or a MEMS gyroscope. It is appreciated that the bumpstop material may include one or more of polysilicon, dielectric, metal, and single crystal silicon. According to some embodiments, the oxide mask is removed through buffered oxide etch (BOE), concentrated HF, or vapor phase HF. 
     In some embodiments, a method includes forming a hardmask on a first surface of a substrate. The method further includes forming a photoresist layer on at least one exposed portion on the first surface of the substrate. According to some embodiments, the method includes etching a first exposed portion of the first surface of the substrate, in which the first exposed portion is a portion of the first surface not covered by the photoresist or the hardmask. The method in some embodiments includes removing the photoresist layer. In one illustrative embodiment, the method includes etching a second exposed portion on the first surface of the substrate, in which the second exposed portion on the first surface of the substrate is not covered by the hardmask. In some embodiments, the method includes applying roughening etchant to the second exposed portion of the first surface of the substrate. It is appreciated that the hardmask is removed. According to some embodiments, the method includes forming a fusion bond oxide layer on the first surface of the substrate. The method in some embodiments further includes fusion bonding the substrate to a device substrate. 
     In some embodiments, the device substrate includes a MEMS accelerometer or a MEMS gyroscope. It is appreciated that the hardmask may be an oxide layer, silicon nitrite, or metal. According to some embodiments, the roughening etchant includes one or more of xenon difluoride, sulfur hexafluoride (“SF6”), and potassium hydroxide (“KOH”). 
     In some embodiments, a method includes forming a hardmask on a first surface of a substrate. The method further includes forming a photoresist layer on the hardmask and on at least one exposed portion on the first surface of the substrate. According to some embodiments, the method includes etching a first exposed portion of the first surface of the substrate, in which the first exposed portion is a portion of the first surface not covered by the photoresist or the hardmask. It is appreciated that the photoresist layer is removed. The method in some embodiments includes forming another photoresist layer on the hardmask and further selectively on an exposed portion of the substrate. In one illustrative embodiment, the method includes etching a second exposed portion on the first surface of the substrate, in which the second exposed portion on the first surface of the substrate is not covered by the another photoresist layer or the hardmask. In some embodiments, the method includes removing the another photoresist layer to expose the substrate and the hardmask. According to some embodiments, the method includes etching a third exposed portion of the first surface of the substrate, wherein the third exposed portion of the first surface is not covered by the hardmask. It is appreciated that the hardmask may be removed. In one illustrative embodiment, the methods includes forming a fusion bond oxide layer on the first surface of the substrate. The method in some embodiments further includes fusion bonding the substrate to a device substrate. 
     In some embodiments, before removing the hardmask, the method further includes forming a rough surface layer on the hardmask and the first surface of the substrate. The method may further include etching back the rough surface layer to transfer a roughness of the rough surface layer to the substrate. According to some embodiments, the rough surface layer comprises polysilicon. In some embodiments, the etching back is more than a sum of a thickness of the rough surface layer and a thickness of the hardmask. 
     According to some embodiments, before removing the hardmask, the method further includes applying roughening etchant to the third exposed portion of the first surface of the substrate. In some embodiments, the device substrate comprises a MEMS accelerometer or a MEMS gyroscope. In one illustrative embodiment, the method further includes forming a standoff on a surface of the device substrate opposite to a surface of the fusion bond. 
     In some embodiments, a method includes forming a hardmask on a first surface of a substrate. The method further includes forming a photoresist layer on at least one exposed portion on the first surface of the substrate. According to some embodiments, the method includes etching a first exposed portion of the first surface of the substrate, in which the first exposed portion is a portion of the first surface not covered by the photoresist or the hardmask. It is appreciated that the photoresist layer may be removed. The method in some embodiments includes etching a second exposed portion on the first surface of the substrate, wherein the second exposed portion on the first surface of the substrate is not covered by the hardmask. In one illustrative embodiment, the method includes forming a rough surface layer on the hardmask and the first surface of the substrate. In some embodiments, the method includes etching back the rough surface layer to transfer a roughness of the rough surface layer to the substrate. It is appreciated that the hardmask is removed. In one illustrative embodiment, the method includes forming a fusion bond oxide layer on the first surface of the substrate. The method in some embodiments further includes fusion bonding the substrate to a device substrate. 
     In some embodiments, the rough surface layer comprises polysilicon. According to some embodiments, the etching back is more than a sum of a thickness of the rough surface layer and a thickness of the hardmask. It is appreciated that the device substrate may include a MEMS accelerometer or a MEMS gyroscope. 
     These and other features and aspects of the concepts described herein may be better understood with reference to the following drawings, description, and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    shows a MEMS device substrate at an early stage of manufacture according to one aspect of the present embodiments. 
         FIG.  2    shows formation of a hardmask on a portion of a surface of the MEMS device substrate according to one aspect of the present embodiments. 
         FIG.  3    shows deposition of bumpstop material over the hardmask and further over an exposed portion of the surface of the MEMS device substrate according to one aspect of the present embodiments. 
         FIG.  4    shows etching back or chemical-mechanical polishing (CMP) of the bumpstop material to expose the hardmask according to one aspect of the present embodiments. 
         FIG.  5    shows the hardmask being removed from the surface of the MEMS device substrate according to one aspect of the present embodiments. 
         FIG.  6    shows fusion bonding a MEMS cap substrate to the MEMS device substrate according to one aspect of the present embodiments. 
         FIG.  7    shows a substrate at an early stage of manufacture according to one aspect of the present embodiments. 
         FIG.  8    shows formation of a hardmask and a photoresist layer on a first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  9    shows etching of a first exposed portion of the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  10    shows removal of the photoresist layer from the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  11    shows etching of a second exposed portion of the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  12    shows application of roughening etchant to the first surface of the substrate to enhance surface roughness according to one aspect of the present embodiments. 
         FIG.  13    shows removal of the hardmask from the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  14    shows formation of a fusion bond oxide layer on the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  15    shows fusion bonding the substrate to a device substrate according to one aspect of the present embodiments. 
         FIG.  16    shows a substrate at an early stage of manufacture according to one aspect of the present embodiments. 
         FIG.  17    shows formation of a hardmask on a first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  18    shows formation of a first photoresist layer on the hardmask and on some portions of the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  19    shows etching of a first exposed portion of the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  20    shows removal of the first photoresist layer from the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  21    shows formation of a second photoresist layer on the hardmask and on some portions of the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  22    shows etching of a second exposed portion of the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  23    shows removal of the second photoresist layer from the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  24    shows etching of a third exposed portion of the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  25    shows formation of a rough surface layer on the hardmask and on the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  26    shows etching back of the rough surface layer according to one aspect of the present embodiments. 
         FIG.  27    shows removal of the hardmask from the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  28    shows formation of a fusion bond oxide layer on the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  29    shows fusion bonding the substrate to a device substrate according to one aspect of the present embodiments. 
         FIG.  30    shows formation of a standoff on a surface of the device substrate opposite to a surface of the fusion bond according to one aspect of the present embodiments. 
         FIG.  31    shows formation of a rough polysilicon layer on a surface of the device substrate that is opposite to the surface of the fusion bond and on the standoff according to one aspect of the present embodiments. 
         FIG.  32    shows formation of a germanium (Ge) layer on the rough polysilicon layer according to one aspect of the present embodiments. 
         FIG.  33    shows formation of an ACT layer patterning on the device substrate according to one aspect of the present embodiments. 
         FIG.  34    shows eutectic bonding a MEMS substrate to a complementary metal-oxide (CMOS) layer according to one aspect of the present embodiments. 
         FIG.  35    shows a substrate at an early stage of manufacture according to one aspect of the present embodiments. 
         FIG.  36    shows formation of a hardmask and a photoresist layer on a first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  37    shows etching of a first exposed portion of the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  38    shows removal of the photoresist layer from the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  39    shows etching of a second exposed portion of the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  40    shows formation of a rough surface layer on the hardmask and on the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  41    shows etching back of the rough surface layer according to one aspect of the present embodiments. 
         FIG.  42    shows removal of the hardmask from the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  43    shows formation of a fusion bond oxide layer on the first surface of the substrate according to one aspect of the present embodiments. 
         FIG.  44    shows eutectic bonding a MEMS device substrate to a CMOS substrate according to one aspect of the present embodiments. 
         FIG.  45    shows a method of reducing stiction by fabricating bumpstops on a surface of a MEMS device substrate according to one aspect of the present embodiments. 
         FIG.  46    shows a method of reducing stiction by applying roughening etchant to a first surface of a substrate according to one aspect of the present embodiments. 
         FIGS.  47 A- 47 B  show a method of reducing stiction of a MEMS device by reducing contact area between proof mass and an upper cavity surface according to one aspect of the present embodiments. 
         FIG.  47 C , together with  FIG.  47 A , show an alternative method of reducing stiction of a MEMS device by reducing contact area between proof mass and upper cavity surface and increasing roughness of the upper cavity surface according to one aspect of the present embodiments. 
         FIG.  48    shows a method of reducing stiction by depositing a rough layer to a surface of a substrate according to one aspect of the present embodiments. 
     
    
    
     DESCRIPTION 
     Before various embodiments are described in greater detail, it should be understood that the embodiments are not limiting, as elements in such embodiments may vary. It should likewise be understood that a particular embodiment described and/or illustrated herein has elements which may be readily separated from the particular embodiment and optionally combined with any of several other embodiments or substituted for elements in any of several other embodiments described herein. 
     It should also be understood that the terminology used herein is for the purpose of describing the certain concepts, and the terminology is not intended to be limiting. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood in the art to which the embodiments pertain. 
     Unless indicated otherwise, ordinal numbers (e.g., first, second, third, etc.) are used to distinguish or identify different elements or steps in a group of elements or steps, and do not supply a serial or numerical limitation on the elements or steps of the embodiments thereof. For example, “first,” “second,” and “third” elements or steps need not necessarily appear in that order, and the embodiments thereof need not necessarily be limited to three elements or steps. It should also be understood that, unless indicated otherwise, any labels such as “left,” “right,” “front,” “back,” “top,” “middle,” “bottom,” “beside,” “forward,” “reverse,” “overlying,” “underlying,” “up,” “down,” or other similar terms such as “upper,” “lower,” “above,” “below,” “under,” “between,” “over,” “vertical,” “horizontal,” “proximal,” “distal,” “forming,” “formation,” “depositing,” “removing,” “removal,” “reducing,” “etching,” “etching back,” “fusion bonding,” and the like are used for convenience and are not intended to imply, for example, any particular fixed location, orientation, or direction. Instead, such labels are used to reflect, for example, relative location, orientation, or directions. It should also be understood that the singular forms of “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. 
     Terms such as “over,” “overlying,” “above,” “under,” etc. are understood to refer to elements that may be in direct contact or may have other elements in-between. For example, two layers may be in overlying contact, wherein one layer is over another layer and the two layers physically contact. In another example, two layers may be separated by one or more layers, wherein a first layer is over a second layer and one or more intermediate layers are between the first and second layers, such that the first and second layers do not physically contact. 
     Stiction is an issue in MEMS devices, and there exists a need to reduce MEMS failure due to stiction. One embodiment described herein is directed to reducing the contact surface area of MEMS proof mass by engineering a MEMS device substrate prior to fusion bonding. Prior to fusion bonding, the MEMS device substrate has silicon oxide deposited and patterned. The oxide mask protects the silicon surface for fusion bonding. The bumpstop material is then deposited over the patterned oxide mask. The bumpstop material is then etched back, exposing the oxide mask underneath. The oxide mask is removed exposing the silicon for fusion bonding. It is appreciated that the silicon surface was protected, using the patterned oxide mask, from being degraded during bumpstop material deposition. Fabricating bumpstops on top of the MEMS device substrate reduces the occurrence of stiction to the cap without degrading the quality of silicon for fusion bonding. 
     Another embodiment described herein enhances the surface roughness of the MEMS encapsulation surface during engineering of the capping substrate. Engineered cavity substrates are prepared per motion process. After two steps of cavity etching, before removal of hardmask, a roughening etchant is used to enhance the silicon surface roughness. The enhanced roughness improves the MEMS performance by reducing stiction. Applying a surface roughening etchant independently and selectively controls the surface roughness of the MEMS cavity substrate. 
     Another embodiment described herein reduces the adhesion force between proof mass and UCAV (“Upper Cavity”) surface in MEMS devices. In order to reduce stiction, the cavity depth may be increased such that proof mass movement does not make contact with the UCAV surface causing stiction. However, cavity depth controls the pressure in the cavity and increasing the cavity depth reduces pressure which adversely impacts performance of the accelerometer. In some embodiments, contact area between the proof mass and the UCAV surface is reduced with three etching process steps by adding a first step of shallow etching process before two cavity etching steps. Therefore, the contact area between proof mass and the UCAV surface is reduced while the volume of the accelerometer cavity does not increase much. In other words, stiction is reduced without adversely impacting the accelerometer performance by maintaining its cavity pressure. In some embodiments, the adhesion between the proof mass and the UCAV surface is reduced by increasing the surface roughness of the UCAV surface. The surface roughness of the UCAV surface may be increased by depositing a rough layer (e.g. polysilicon) on the hardmask and the surface of the substrate and etching back the rough layer to transfer the roughness to the UCAV cavity surface. 
     Another embodiment described herein enhances the surface roughness of the MEMS encapsulation surface during engineering of the cap substrate. Engineered cavity substrates are prepared per motion process. After two steps of cavity etching, before removal of hardmask, a rough layer (e.g. polysilicon) is deposited on the hardmask and is then etched back to transfer the roughness. The enhanced roughness improves the MEMS performance by reducing stiction. Depositing and then etching back a rough layer independently and selectively controls the surface roughness of the MEMS cavity substrate. 
     Referring now to  FIG.  1   , a MEMS device substrate at an early stage of manufacture according to one aspect of the present embodiments is shown. A MEMS device substrate  110  (Silicon substrate) is provided. 
     Referring now to  FIG.  2   , formation of a hardmask on a portion of a surface of the MEMS device substrate according to one aspect of the present embodiments is shown. The hardmask  101  is formed on a surface of the MEMS device substrate  110 . The hardmask  101  may be an oxide hardmask, a nitride hardmask, a metal hardmask, or a photoresist mask. The hardmask  101  may be patterned. Accordingly, certain portions of the surface of the substrate  110  is exposed while other portions of the surface is covered by the hardmask  101 . 
     Referring now to  FIG.  3   , deposition of bumpstop material over the hardmask and further over an exposed portion of the surface of the MEMS device substrate according to one aspect of the present embodiments is shown. The bumpstop material  102  is deposited on the hardmask  101  as well as on certain areas (portions) of the surface of the substrate  110  that is exposed. In some embodiments, the bumpstop material  102  may be polysilicon, dielectric, metal, or single crystal silicon. 
     Referring now to  FIG.  4   , etching back or chemical-mechanical polishing (CMP) of the bumpstop material to expose the hardmask according to one aspect of the present embodiments is shown. It is appreciated that the bumpstop material  102  forms the bumpstop  103 , as shown in  FIG.  5   . 
     Referring now to  FIG.  5   , the hardmask being removed from the surface of the MEMS device substrate according to one aspect of the present embodiments is shown. It is appreciated that the bumpstop  103  becomes exposed once the hardmask  101  is removed. In some embodiments, the oxide mask is removed through buffered oxide etch (BOE), concentrated HF, or Vapor phase HF. In some embodiments, a portion of the surface of the MEMS device substrate  110  that is not covered by the bumpstop  103  gets exposed. In some embodiments, two bumpstops  103  are fabricated after the hardmask  101  is removed. However, it is appreciated that illustration of two bumpstops  103  is for illustrative purposes and should not be construed as limiting the scope of the embodiments. 
     In some embodiments, the MEMS device substrate  110  may include a MEMS accelerometer  111  or a MEMS gyroscope  112 . In an illustrative embodiment, a bumpstop  103  is located within the cavity for the MEMS accelerometer  111  cavity and another bumpstop  103  is located within the cavity for the MEMS gyroscope  112 . 
     Referring now to  FIG.  6   , fusion bonding a MEMS cap substrate to the MEMS device substrate according to one aspect of the present embodiments is shown. In other words, a fusion bond oxide layer  104  of the MEMS cap substrate  100  is fusion bonded to the MEMS device substrate  110 . 
     Referring now to  FIG.  7   , a substrate at an early stage of manufacture according to one aspect of the present embodiments is shown. In some embodiment, a substrate  200  may be a silicon substrate. In one illustrative embodiment, the silicon substrate  200  is a MEMS cap substrate. A first surface  215  is a surface of the substrate  200 . 
     Referring now to  FIG.  8   , formation of a hardmask and a photoresist layer on a first surface of the substrate according to one aspect of the present embodiments is shown. A hardmask  201  is formed on a first surface  215  of the substrate  200 . The hardmask  201  may be an oxide hardmask, a nitride hardmask, a metal hardmask, or a photoresist mask. The hardmask  201  may be patterned in some embodiments. Accordingly, certain portions of the first surface  215  of the substrate  200  is exposed while other portions of the first surface  215  is covered by the hardmask  201 . In some embodiments, a photoresist layer  202  is formed on certain areas (portions) of the first surface  215  of the substrate  200  that is not covered by the hardmask  201 . The photoresist layer  202  is patterned. In one illustrative embodiment, certain portions of the first surface  215  of the substrate  200  is exposed as it is not covered by either the hardmask  201  or the photoresist layer  202 . 
     Referring now to  FIG.  9   , etching of a first exposed portion of the first surface of the substrate  200  according to one aspect of the present embodiments is shown. It is appreciated that the exposed portion (not covered by either the hardmask  201  or the photoresist layer  202 ) of the first surface  215  of the substrate  200  may be etched. 
     Referring now to  FIG.  10   , removal of the photoresist layer  202  from the first surface  215  of the substrate  200  according to one aspect of the present embodiments is shown. As such, certain portions of the first surface  215  of the substrate  200  that was previously covered by the photoresist layer  202  gets exposed. 
     Referring now to  FIG.  11   , etching of a second exposed portion of the first surface of the substrate according to one aspect of the present embodiments is shown. In other words, a second exposed portion of the first surface  215  of the substrate  200  which is not covered by the hardmask  201  is etched. In one illustrative embodiment, the etched exposed portions of the first surface  215  of the substrate  200  creates two separate cavities on the first surface  215  of the substrate  200 . It is appreciated that in some embodiments, one cavity may correspond to a MEMS accelerometer and the other cavity may correspond to a MEMS gyroscope. 
     Referring now to  FIG.  12   , application of roughening etchant to the first surface of the substrate to enhance surface roughness according to one aspect of the present embodiments is shown. In some embodiments, roughening etchant  204  is applied to enhance the surface roughness of the exposed surfaces. As such, the roughness of the exposed portion of the first surface  215  is enhanced to create the enhanced surface roughness  203 . It is appreciated that any appropriate roughening etchants may be used. For example, the roughening etchant may include xenon difluoride, sulfur hexafluoride (“SF6”), and potassium hydroxide (“KOH”). 
     Referring now to  FIG.  13   , removal of the hardmask from the first surface of the substrate according to one aspect of the present embodiments is shown. It is appreciated that the entire first surface  215  of the substrate  200  becomes exposed after the hardmask  201  is removed. 
     Referring now to  FIG.  14   , formation of a fusion bond oxide layer  205  on the first surface  215  of the substrate  200  according to one aspect of the present embodiments is shown. It is appreciated that the fusion bond layer  205  may be an oxide layer, silicon nitrite, or metal. 
     Referring now to  FIG.  15   , fusion bonding the substrate to a MEMS device substrate according to one aspect of the present embodiments is shown. In some embodiments, the substrate  200  may be a MEMS cap substrate that is fusion bonded to the MEMS device substrate  210 . In some embodiments, the fusion bond layer  205  may be used to fusion bond the MEMS cap substrate  200  to the MEMS device substrate  210 . In some embodiments, the MEMS device substrate  210  includes a MEMS accelerometer  211  or a MEMS gyroscope  212 . In one illustrative embodiment, the two cavities on the first surface of the MEMS cap substrate  200  correspond to the MEMS accelerometer  211  or the MEMS gyroscope  212  of the MEMS device substrate  210  respectively. 
     Referring now to  FIG.  16   , a substrate at an early stage of manufacture according to one aspect of the present embodiments is shown. In some embodiment, a substrate  300  may be a silicon substrate. In one illustrative embodiment, the silicon substrate  300  is a MEMS cap substrate. A first surface  315  is a surface of the substrate  300 . 
     Referring now to  FIG.  17   , formation of a hardmask on a first surface of the substrate according to one aspect of the present embodiments is shown. A hardmask  301  is formed on a first surface  315  of the substrate  300 . The hardmask  301  may be an oxide hardmask, a nitride hardmask, a metal hardmask, or a photoresist mask. In this illustrative embodiment, the hardmask  301  is an oxide hardmask. The hardmask  301  is patterned. Accordingly, certain portions of the first surface  315  of the substrate  300  is exposed while other portions of the first surface is covered by the hardmask  301 . 
     Referring now to  FIG.  18   , formation of a first photoresist layer on the hardmask and on some portions of the first surface of the substrate according to one aspect of the present embodiments is shown. In some embodiments, a first photoresist layer  302  is formed on the hardmask  301  and certain areas (portions) of the first surface  315  of the substrate  300  that is not covered by the hardmask  301 . The first photoresist layer  302  is patterned. In one illustrative embodiment, certain portions of the first surface  315  of the substrate  300  is exposed as it is not covered by either the hardmask  301  or the first photoresist layer  302 . 
     Referring now to  FIG.  19   , etching of a first exposed portion of the first surface of the substrate according to one aspect of the present embodiments is shown. It is appreciated that the exposed portion (not covered by either the hardmask  301  or the first photoresist layer  302 ) of the first surface  315  of the substrate  300  is etched. 
     Referring now to  FIG.  20   , removal of the first photoresist layer  302  from the first surface  315  of the substrate  300  according to one aspect of the present embodiments is shown. As such, certain portions of the first surface  315  of the substrate  300  that was covered by the first photoresist layer  302  is exposed. 
     Referring now to  FIG.  21   , formation of a second photoresist layer on the hardmask and on some portions of the first surface of the substrate according to one aspect of the present embodiments is shown. In some embodiments, a second photoresist layer  303  is formed on the hardmask  301  and certain areas (portions) of the first surface  315  of the substrate  300  that is not covered by the hardmask  301 . The second photoresist layer  303  is patterned. In one illustrative embodiment, certain portions of the first surface  315  of the substrate  300  is exposed as it is not covered by either the hardmask  301  or the second photoresist layer  303 . 
     Referring now to  FIG.  22   , etching of a second exposed portion of the first surface of the substrate according to one aspect of the present embodiments is shown. In other words, a second exposed portion (not covered by either the hardmask  301  or the second photoresist layer  303 ) of the first surface  315  of the silicon substrate  300  is etched. 
     Referring now to  FIG.  23   , removal of the second photoresist layer from the first surface of the substrate according to one aspect of the present embodiments is shown. As such, certain portions of the first surface  315  of the substrate  300  that was covered by the second photoresist layer  303  is exposed. In one illustrative embodiment, after the second etch shown in  FIG.  22   , the etched exposed portions of the first surface  315  of the substrate  300  creates a left cavity  313  and a right cavity  314  on the first surface  315  of the substrate  300 . In one illustrative embodiment, the depth of the left cavity  313  is t 1  (i.e., the depth of the first etch); and the depth of the right cavity  314  is t 2  (i.e., the depth of the second etch). 
     Referring now to  FIG.  24   , etching of a third exposed portion of the first surface of the substrate according to one aspect of the present embodiments is shown. In other words, a third exposed portion of the first surface  315  of the substrate  300  which is not covered by the hardmask  301  is etched. In one illustrative embodiment, after the third etch shown in  FIG.  24   , the depth of the left cavity  313  and a right cavity  314  will be incremented by the depth of the third etch (t 3 ). Accordingly, after the third etch, the depth of the left cavity  313  is t 1 +t 3 ; and the depth of the right cavity  314  is t 2 +t 3 . 
     In some embodiments, the left cavity  313  is used as an accelerometer cavity when fusion bonded with a MEMS device substrate  310  (shown in  FIGS.  29 - 34    below). In some embodiments, the right cavity  314  is used as a gyroscope cavity when fusion bonded with a MEMS device substrate  310  (shown in  FIGS.  29 - 34    below). In some embodiments, it is desirable to reduce the contact area between the proof mass and the cavity surface with minimal impact on the cavity volume, e.g., increasing the pressure of the accelerometer cavity (e.g., decreasing the volume of the accelerometer cavity) while reducing (or at least maintaining) the pressure of the gyroscope cavity (e.g., the volume of the gyroscope cavity is increased or at least not decreased). After three etching steps with etching depths of t 1 , t 2 , and t 3  respectively, the depth of the left cavity  313  (accelerometer cavity) becomes t 1 +t 3  and the depth of the right cavity  314  (gyroscope cavity) becomes t 2 +t 3 . With the combination of t 1 , t 2  and t 3 , the low volume of the accelerometer cavity is achieved while a higher volume of the gyroscope cavity is achieved. 
     Referring now to  FIG.  25   , formation of a rough surface layer on the hardmask and on the first surface of the substrate according to one aspect of the present embodiments is shown. In some embodiments, a rough surface layer  304  is formed on the hardmask  301  and the first surface  315  of the substrate  300  that is not covered by the hardmask  301 . In some embodiments, the rough surface layer  304  includes polysilicon. 
     Referring now to  FIG.  26   , etching back of the rough surface layer according to one aspect of the present embodiments is shown. It is appreciated that after the rough surface layer  304  is etched back, the roughness is transferred to the exposed portion of the first surface of the substrate  300 . Accordingly, the exposed surface becomes a rough surface  305 . In an illustrative embodiment, the etching back is more than a sum of a thickness of the rough surface layer and a thickness of the hardmask. In some embodiments, after the etch back, there are still some rough layer remnant  316  left on the first surface of the substrate  300 . 
     Referring now to  FIG.  27   , removal of the hardmask from the first surface of the substrate according to one aspect of the present embodiments is shown. As such, the whole first surface  315  of the substrate  300  is exposed after the hardmask  301  is removed. In some embodiments, the steps of applying and etching back the rough surface layer  304  (shown in  FIGS.  25 - 26    above) are performed before the hardmask  301  is removed. In some other embodiments, the removal of the hardmask  301  occurs after the third etch (shown in  FIG.  24    above) without performing the steps in  FIGS.  25 - 26   . In some further embodiments, before removing the hardmask  301 , a step of applying roughening etchant (such as those shown in  FIG.  12   ) to the third exposed portion of the first surface  315  of the substrate  300  may be performed. 
     Referring now to  FIG.  28   , formation of a fusion bond oxide layer  306  on the first surface  315  of the substrate  300  according to one aspect of the present embodiments is shown. It is appreciated that the fusion bond oxide  306  may be an oxide layer. 
     Referring now to  FIG.  29   , fusion bonding the substrate to a MEMS device substrate according to one aspect of the present embodiments is shown. In some embodiments, the substrate  300  is a MEMS cap substrate. In other words, the fusion bond oxide  306  of the MEMS cap substrate  300  is fusion bonded to a MEMS device substrate  310 . In some embodiments, the MEMS device substrate  310  includes a MEMS accelerometer  311  or a MEMS gyroscope  312 . In one illustrative embodiment, the left cavity  313  and the right cavity  314  on the first surface of the MEMS cap substrate  300  correspond to the MEMS accelerometer  311  or the MEMS gyroscope  312  of the MEMS device substrate  310  respectively. 
     Referring now to  FIG.  30   , formation of a standoff on a surface of the device substrate opposite to a surface of the fusion bond according to one aspect of the present embodiments is shown. In some embodiments, standoff  307  is formed on a surface of the device substrate  310  opposite to a surface of the fusion bond. It is appreciated that standoff  307  may be a vertical structure providing electrical contact. 
     Referring now to  FIG.  31   , formation of a rough polysilicon layer on a surface of the device substrate that is opposite to the surface of the fusion bond and on the standoff according to one aspect of the present embodiments is shown. As such, a rough polysilicon layer  308  is formed on a surface of the device substrate  310  that is opposite to the surface of the fusion bond  306  and on the standoff  307 . 
     Referring now to  FIG.  32   , formation of a germanium (Ge) layer on the rough polysilicon layer according to one aspect of the present embodiments is shown. A germanium (Ge) film  309  is deposited onto the device substrate  310  followed by patterning the germanium to define a standoff  307 . 
     Referring now to  FIG.  33   , formation of an ACT layer patterning on the device substrate according to one aspect of the present embodiments is shown. 
     Referring now to  FIG.  34   , eutectic bonding a MEMS substrate to a CMOS substrate according to one aspect of the present embodiments is shown. In some embodiments, the MEMS cap substrate  300  and the MEMS device substrate  310  are collectively referred to as MEMS substrate. In some embodiments, the MEMS substrate is eutectic bonded to a CMOS substrate  320 . 
     Referring now to  FIG.  35   , a substrate at an early stage of manufacture according to one aspect of the present embodiments is shown. In some embodiment, a substrate  400  may be a silicon substrate. In one illustrative embodiment, the silicon substrate  400  is a MEMS cap substrate. 
     Referring now to  FIG.  36   , formation of a hardmask and a photoresist layer on a first surface of the substrate according to one aspect of the present embodiments is shown. A hardmask  401  is formed on a first surface of the substrate  400 . The hardmask  401  may be an oxide hardmask, a nitride hardmask, a metal hardmask, or a photoresist mask. In this illustrative embodiment, the hardmask  401  is an oxide hardmask. The hardmask  401  is patterned. Accordingly, certain portions of the first surface of the substrate  400  is exposed while other portions of the first surface is covered by the hardmask  401 . In some embodiments, a photoresist layer  402  is formed on certain areas (portions) of the first surface of the substrate  400  that is not covered by the hardmask  401 . The photoresist layer  402  is patterned. In one illustrative embodiment, certain portions of the first surface of the substrate  400  is exposed as it is not covered by either the hardmask  401  or the photoresist layer  402 . 
     Referring now to  FIG.  37   , etching of a first exposed portion of the first surface of the substrate according to one aspect of the present embodiments is shown. It is appreciated that the exposed portion (not covered by either the hardmask  401  or the photoresist layer  402 ) of the substrate  400  is etched. 
     Referring now to  FIG.  38   , removal of the photoresist layer from the first surface of the substrate according to one aspect of the present embodiments is shown. As such, certain portions of the first surface of the substrate  400  that was covered by the photoresist layer  402  is exposed. 
     Referring now to  FIG.  39   , etching of a second exposed portion of the first surface of the substrate according to one aspect of the present embodiments is shown. In other words, a second exposed portion of the first surface of the substrate  400  which is not covered by the hardmask  401  is etched. In one illustrative embodiment, the etched exposed portions of the first surface of the substrate  400  produces two separate cavities on the first surface of the substrate. 
     Referring now to  FIG.  40   , formation of a rough surface layer on the hardmask and on the first surface of the substrate according to one aspect of the present embodiments is shown. In some embodiments, a rough surface layer  403  is formed on the hardmask  401  and the first surface of the substrate  400  that is not covered by the hardmask  401 . In some embodiments, the rough surface layer  403  includes polysilicon. 
     Referring now to  FIG.  41   , etching back of the rough surface layer according to one aspect of the present embodiments is shown. It is appreciated that after the rough surface layer  403  is etched back, the roughness is transferred to the exposed portion of the first surface of the substrate  400 . Accordingly, the exposed surface becomes a rough surface  404 . In an illustrative embodiment, the etching back is more than a sum of a thickness of the rough surface layer and a thickness of the hardmask. In some embodiments, after the etch back, there are still some rough layer remnant  406  left on the first surface of the substrate  400 . 
     Referring now to  FIG.  42   , removal of the hardmask from the first surface of the substrate according to one aspect of the present embodiments is shown. As such, the whole first surface of the substrate  400  is exposed after the hardmask  401  is removed. 
     Referring now to  FIG.  43   , formation of a fusion bond oxide layer on the first surface of the substrate according to one aspect of the present embodiments is shown. It is appreciated that the fusion bond oxide  405  may be an oxide layer. 
     Referring now to  FIG.  44   , fusion bonding of the substrate to a MEMS device substrate according to one aspect of the present embodiments is shown. In some embodiments, the substrate  400  is a MEMS cap substrate. In other words, the fusion bond oxide  405  of the MEMS cap substrate  400  is fusion bonded to a MEMS device substrate  410 . In some embodiments, the MEMS device substrate  410  includes a MEMS accelerometer  411  or a MEMS gyroscope  412 . In one illustrative embodiment, the two cavities on the first surface  415  of the MEMS cap substrate  400  correspond to the MEMS accelerometer  411  or the MEMS gyroscope  412  of the MEMS device substrate  410  respectively. 
     Referring now to  FIG.  45   , a method of reducing stiction by fabricating bumpstops on a surface of a MEMS device substrate according to one aspect of the present embodiments is shown. The method steps described here have been described in  FIGS.  1 - 6   . At step  4510 , an oxide mask is formed on a portion of a surface of a MEMS device substrate. At step  4512 , a bumpstop material is deposited over the oxide mask and further over an exposed portion of the surface of the MEMS device substrate. At step  4514 , the bumpstop material is etched back or chemical-mechanical polished (CMP) to expose the oxide mask. At step  4516 , the oxide mask is removed to expose the portion of the surface of the substrate and the bumpstop material. At step  4518 , the MEMS device substrate is fusion bonded to a MEMS cap substrate. 
     Referring now to  FIG.  46   , a method of reducing stiction by applying roughening etchant to a first surface of a substrate according to one aspect of the present embodiments is shown. The method steps described here have been described in  FIGS.  7 - 15   . At step  4610 , a hardmask is formed on a first surface of a substrate. At step  4612 , a photoresist layer is formed on at least one exposed portion on the first surface of the substrate. At step  4614 , a first exposed portion of the first surface of the substrate is etched, in which the first exposed portion is a portion of the first surface not covered by the photoresist or the hardmask. At step  4616 , the photoresist layer is removed. At step  4618 , a second exposed portion is etched on the first surface of the substrate, in which the second exposed portion on the first surface of the substrate is not covered by the hardmask. At step  4620 , roughening etchant is applied to the second exposed portion of the first surface of the substrate. At step  4622 , the hardmask is removed. At step  4624 , a fusion bond oxide layer is formed on the first surface of the substrate. At step  4626 , the substrate is fusion bonded to a device substrate. 
     Referring now to  FIGS.  47 A- 47 B , a method of reducing stiction of a MEMS device by reducing contact area between proof mass and an upper cavity surface according to one aspect of the present embodiments is shown. The method steps described here have been described in  FIGS.  16 - 24  and  27 - 34   . At step  4710 , a hardmask is formed on a first surface of a substrate. At step  4712 , a photoresist layer is formed on the hardmask and on at least one exposed portion on the first surface of the substrate. At step  4714 , a first exposed portion of the first surface of the substrate is etched, in which the first exposed portion is a portion of the first surface not covered by the photoresist or the hardmask. At step  4716 , the photoresist layer is removed. At step  4718 , another photoresist layer is formed on the hardmask and further selectively on an exposed portion of the substrate. At step  4720 , a second exposed portion on the first surface of the substrate is etched, in which the second exposed portion on the first surface of the substrate is not covered by the another photoresist layer or the hardmask. At step  4722 , the another photoresist layer is removed to expose the substrate and the hardmask. At step  4724 , a third exposed portion of the first surface of the substrate is etched, in which the third exposed portion of the first surface is not covered by the hardmask. At step  4730 , the hardmask is removed. At step  4732 , a fusion bond oxide layer is formed on the first surface of the substrate. At step  4734 , the substrate is fusion bonded to a device substrate. 
     Referring now to  FIG.  47 C , together with  FIG.  47 A , an alternative method of reducing stiction of a MEMS device by reducing contact area between proof mass and upper cavity surface and increasing roughness of the upper cavity surface according to one aspect of the present embodiments is shown. The method steps described here have been described in  FIGS.  16 - 34   . Steps  4710 - 24  and steps  4730 - 32  are substantially similar to that of  FIGS.  47 A- 47 B . In some embodiments, between steps  4724 - 30  there are two extra steps of depositing and etching back rough surface layer to transfer roughness to the surface of the substrate. At step  4726 , a rough surface layer is formed on the hardmask and the first surface of the substrate. At step  4728 , the rough surface layer is etched back to transfers a roughness of the rough surface layer to the substrate. 
     Referring now to  FIG.  48   , a method of reducing stiction by depositing a rough layer to a surface of a substrate according to one aspect of the present embodiments is shown. The method steps described here have been described in  FIGS.  35 - 44   . At step  4810 , a hardmask is formed on a first surface of a substrate. At step  4812 , a photoresist layer is formed on at least one exposed portion on the first surface of the substrate. At step  4814 , a first exposed portion of the first surface of the substrate is etched, in which the first exposed portion is a portion of the first surface not covered by the photoresist or the hardmask. At step  4816 , the photoresist layer is removed. At step  4818 , a second exposed portion is etched on the first surface of the substrate, in which the second exposed portion on the first surface of the substrate is not covered by the hardmask. At step  4820 , a rough surface layer is formed on the hardmask and the first surface of the substrate. At step  4822 , the rough surface layer is etched back to transfers a roughness of the rough surface layer to the substrate. At step  4824 , the hardmask is removed. At step  4826 , a fusion bond oxide layer is formed on the first surface of the substrate. At step  4828 , the substrate is fusion bonded to a device substrate. 
     While the embodiments have been described and/or illustrated by means of particular examples, and while these embodiments and/or examples have been described in considerable detail, it is not the intention of the Applicants to restrict or in any way limit the scope of the embodiments to such detail. Additional adaptations and/or modifications of the embodiments may readily appear, and, in its broader aspects, the embodiments may encompass these adaptations and/or modifications. Accordingly, departures may be made from the foregoing embodiments and/or examples without departing from the scope of the concepts described herein. The implementations described above and other implementations are within the scope of the following claims.