Patent Publication Number: US-10308503-B2

Title: Dual cavity pressure structures

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is divisional of prior application Ser. No. 15/071,499, filed Mar. 16, 2016, which claims the benefit of U.S. Provisional Patent Application Ser. No. 62/134,455 filed Mar. 17, 2015, entitled “DUAL CAVITY PRESSURE SCHEMES”. 
    
    
     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. In many cases, but not always, MEMS interact with electrical signals. A MEMS device may refer to a semiconductor device that is implemented as a micro-electro-mechanical system. A MEMS device includes mechanical elements and may optionally include electronics (e.g. electronics for sensing). MEMS devices include but are not limited to, for example, gyroscopes, accelerometers, magnetometers, pressure sensors, etc. During fabrication, it may be desirable to create various different MEMS devices on the same wafer. Furthermore, it may be desirable to form the various different MEMS devices with different internal pressures. 
     SUMMARY 
     An apparatus includes a cavity within a substrate. A MEMS structure is within the cavity, wherein the cavity includes the MEMS structure. A trench is connected to the cavity, wherein the trench is not directly opposite the MEMS structure. An oxide layer lines the trench and the cavity. A seal layer seals the trench and traps a predetermined pressure within the cavity and the trench. 
     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 first substrate including a trench according to one aspect of the present embodiments. 
         FIG. 2  shows the substrate with the addition of cavities according to one aspect of the present embodiments. 
         FIG. 3  shows a thermal oxide growth on the trench and the cavities according to one aspect of the present embodiments. 
         FIG. 4  shows the first substrate fusion bonded to a second substrate including standoffs according to one aspect of the present embodiments. 
         FIG. 5  shows the formation of MEMS features and eutectic bonding of the second substrate to a third substrate according to one aspect of the present embodiments. 
         FIG. 6  shows removal of a portion of the first substrate to reveal the trench and adjust the pressure in one cavity according to one aspect of the present embodiments. 
         FIG. 7  shows the addition of a seal to set the adjusted pressure of the trench cavity according to one aspect of the present embodiments. 
         FIG. 8  shows a first substrate including a trench according to one aspect of the present embodiments. 
         FIG. 9  shows the substrate with the addition of cavities according to one aspect of the present embodiments. 
         FIG. 10  shows a thermal oxide growth on the trench and the cavities according to one aspect of the present embodiments. 
         FIG. 11  shows the first substrate fusion bonded to a second substrate including standoffs according to one aspect of the present embodiments. 
         FIG. 12  shows the formation of MEMS features and eutectic bonding of the second substrate to a third substrate according to one aspect of the present embodiments. 
         FIG. 13  shows removal of a portion of the first substrate to reveal the trench according to one aspect of the present embodiments. 
         FIG. 14  shows removal of the exposed portion of the lining to reveal the trench and adjust the pressure in one cavity according to one aspect of the present embodiments. 
         FIG. 15  shows the addition of seals to set the adjusted pressure of the trench cavity according to one aspect of the present embodiments. 
         FIG. 16  shows an exemplary flow diagram for setting a pressure within a trench and cavity according to one aspect of the present embodiments. 
         FIG. 17  shows an exemplary flow diagram for sealing a first pressure in a second cavity and sealing a second pressure in a first cavity according to one aspect of the present embodiments. 
     
    
    
     DESCRIPTION 
     Before various embodiments are described in greater detail, it should be understood by persons having ordinary skill in the art 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 by persons having ordinary skill in the art that the terminology used herein is for the purpose of describing the certain concepts, and the terminology is not intended to be limiting. 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,” “forward,” “reverse,” “clockwise,” “counter clockwise,” “up,” “down,” or other similar terms such as “upper,” “lower,” “above,” “below,” “vertical,” “horizontal,” “proximal,” “distal,” 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. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by persons of ordinary skill in the art to which the embodiments pertain. 
     MEMS devices may include mechanical elements that work best when sealed in specific predetermined pressures. For example, a gyroscope MEMS device optimally operates at a lower pressure than an accelerometer MEMS device, which optimally operates at a higher pressure. Furthermore, some MEMS devices operate within different environments. For example, some MEMS devices may need a helium environment, while other MEMS devices may need a hydrogen environment. During fabrication, it may be desirable to create various different MEMS devices with different pressures and/or environments on the same wafer. Embodiments disclose methods for fabricating MEMS devices with different pressures and/or environments on the same wafer, as well as the novel characteristics of the MEMS devices fabricated by the disclosed embodiments. 
     Referring now to  FIG. 1 , a first substrate including a trench is shown according to one aspect of the present embodiments. A first substrate  102  is provided. The first substrate  102  may be, for example, a silicon wafer, however it is understood that embodiments are not limited to silicon wafers. The first substrate  102  includes a first side  104  and a second side  106 . A trench  108  is formed in the first side  104  of the first substrate  102  by any suitable fabrication method (e.g. etching, cutting, laser ablation, etc.). The trench  108  may also be referred to as a chimney, and in various embodiments the trench  108  is a high aspect ratio trench (e.g. &lt;70:1). In some embodiments the width of the trench  108  is 2 to 4 microns, and the length of the trench  108  is not limited. In various embodiments at this stage of fabrication, the trench  108  does not extend from the first side  104  entirely to the second side  106  (e.g. the trench  108  stops before reaching the second side  106 ). 
     Referring now to  FIG. 2 , the substrate with the addition of cavities is shown according to one aspect of the present embodiments. A first cavity  210  and a second cavity  212  (also referred to as upper cavities) are formed in the first side  104  of the first substrate  102  by any suitable fabrication method (e.g. etching, cutting, laser ablation, etc.). The first cavity  210  is connected to the trench  108 , and the trench  108  is positioned such that it is not over the moveable portion of the MEMS structure in order to minimize process impact on the moveable structure. The second cavity  212  does not include a trench structure. 
     It is understood that for simplicity of illustration only two cavities are shown, but any number of cavities and trenches may be fabricated on a substrate. In addition, the patterns on the substrate are not limited. For example, cavities without trenches may be next to each other, and cavities with trenches may be next to each other. 
     In various embodiments, the trench  108  and the first cavity  210  share a common and fluidly connected open space, such that the trench  108  opens directly down into the first cavity  210 . In the illustrated embodiment, the trench  108  shares a left side wall with the first cavity  210 . However, it is understood that the trench  108  may be on either side of the first cavity  210 . Furthermore in some embodiments, the trench  108  may be positioned in any offset location above the first cavity  210  that is not directly above a MEMS structure (not shown, but see  FIG. 5 ). 
     Referring now to  FIG. 3 , a thermal oxide growth on the trench and the cavities is shown according to one aspect of the present embodiments. A lining  314  is formed over the first side  104  using any suitable fabrication method (e.g. deposition, growth, sputtering, etc.). For example, the lining may be a thermal oxide growth (e.g. silicon dioxide). The lining  314  covers the first side  104 , the trench  108 , the first cavity  210 , and the second cavity  212 . In various embodiments, the lining  314  may be used to set or reduce the width of the trench  108  to a predetermined width. For example, the width of the trench  108  after the formation of the lining may be 0.5 to 2.0 microns. In some embodiments, the narrowing of the width of the trench  108  improves the sealing of the trench (not shown, but see  FIG. 7 ). 
     Referring now to  FIG. 4 , the first substrate fusion bonded to a second substrate including standoffs is shown according to one aspect of the present embodiments. A second substrate  416  is bonded to the first substrate  102 . For example, the second substrate  416  may be fusion bonded to the first substrate  102 , thereby forming a silicon oxide silicon bond. In various embodiments, the second substrate  416  is a structural layer including silicon with MEMS features (e.g. bump stop, damping hole, via, port, plate, proof mass, standoff, spring, seal ring, proof mass, etc.). The MEMS features (not shown, but see  FIG. 5 ) may include one or more moveable elements. 
     In various embodiments, the first substrate  102  is a MEMS substrate. The MEMS substrate (first substrate  102 ) provides mechanical support for the structural layer (second substrate  416 ). The MEMS substrate may also be referred to as a handle substrate or handle wafer. In some embodiments, the handle substrate serves as a cap to the structural layer (e.g. forming the first cavity  210  and the second cavity  212 ), and may be referred to as a cap layer. 
     In some embodiments, standoffs  418  are patterned on the second substrate  416 . The standoffs  418  define the vertical clearance between the structural layer and an IC substrate (not shown, but see  FIG. 5 ). The standoffs  418  may also provide electrical contact between the structural layer and the IC substrate. 
     Referring now to  FIG. 5 , the formation of MEMS features and eutectic bonding of the second substrate to a third substrate is shown according to one aspect of the present embodiments. First MEMS features  520  and second MEMS features  521  have been formed in the second substrate  416  using any suitable fabrication method. Fabrication methods may include, but are not limited to, etching, cutting, laser ablation, deposition, growth, sputtering, etc. 
     In addition, the second substrate  416  has been bonded to an additional substrate (e.g. a third substrate  522 ). For example, in some embodiments the standoffs  418  form a eutectic bond  524  with bond pads  526  on the third substrate  522 , thereby providing an electrical connection between the second and third substrates. In various embodiments, the eutectic bond  524  is an aluminum-germanium (AlGe) bond as described in U.S. Pat. No. 7,442,570, the contents of which are incorporated by reference. In other embodiments, the eutectic bond can be formed by tin-copper, tin-aluminum, gold-germanium, gold-tin, or gold-indium. In some embodiments, the third substrate  522  is an integrated circuit (“IC”) substrate. An IC substrate may refer to a silicon substrate with electrical circuits, for example CMOS (“complementary metal-oxide semiconductor”) electrical circuits. 
     In some embodiments, the standoffs  418  and the third substrate  522  define a first lower cavity  528  and a second lower cavity  529 . The standoffs  418  and the eutectic bond  524  provide a seal that defines a first enclosure  530  and a second enclosure  531 . In various embodiments, the enclosure may refer to a fully enclosed volume typically surrounding the MEMS structure and typically formed by the IC substrate, structural layer, MEMS substrate, and the standoff seal ring. For example in the illustrated embodiment, the first enclosure  530  refers to the fully enclosed volume surrounding the first MEMS features  520 , including the first cavity  210  and the first lower cavity  528 . In addition, the second enclosure  531  refers to the fully enclosed volume surrounding the second MEMS features  521 , including the second cavity  212  and the second lower cavity  529 . In further embodiments, the eutectic bond  524  may be substituted with other forms of bonding (e.g. solder, adhesive, etc.). In some embodiments, the seal may be a hermetic seal. 
     Before bonding the second substrate  416  and the third substrate  522 , thereby sealing the first enclosure  530  and the second enclosure  531 , the pressure and/or environment may be set to a predetermined state. For example, the chamber in which the bonding is being performed may be adjusted to increase or decrease the ambient pressure, thereby causing the pressure in the first enclosure  530  and the second enclosure  531  to match the altered ambient pressure. Likewise, the chamber in which the bonding is being performed may be adjusted to increase or decrease levels of gas (e.g. helium, hydrogen, nitrogen, etc.), thereby causing the gas levels in the first enclosure  530  and the second enclosure  531  to match the altered ambient gas composition. After the gas has been adjusted, the first enclosure  530  and the second enclosure  531  are sealed, thus trapping the gas pressure and composition within. 
     Referring now to  FIG. 6 , removal of a portion of the first substrate to reveal the trench and adjust the pressure in one cavity is shown according to one aspect of the present embodiments. A portion of the second side  106  of the first substrate  102  is removed to open the trench  108  and expose a portion  634  of the lining  314 . Removal of the portion of the second side may be done through any suitable method (e.g. etching, cutting, polishing, grinding, etc.). It is understood that the removal of the portion of the second side  106  is controlled to expose the portion  634  of the lining  314  in the trench  108 , while not exposing the lining  314  in the second enclosure  531 . Thus, the second enclosure  531  remains sealed. 
     In the present embodiment, the portion  634  of the lining  314  is permeable to various gasses. In some embodiments, the portion  634  of the lining  314  is permeable to all gasses. In further embodiments, the portion  634  of the lining  314  is permeable to only one or only a few gasses (e.g. helium and/or hydrogen). 
     As a result of the permeability of the portion  634  of the lining  314 , the pressure and/or environment within the first enclosure  530  may be adjusted to a second predetermined state. For example, the chamber in which the post processing is being performed may be adjusted to increase or decrease the ambient pressure, thereby causing the pressure in the first enclosure  530  to match the altered ambient pressure. Likewise, the chamber in which the post processing is being performed may be adjusted to increase or decrease levels of gas (e.g. helium, hydrogen, etc.), thereby causing the gas levels in the first enclosure  530  to match the altered ambient gas composition. 
     While the environment within the first enclosure  530  is being adjusted, the second enclosure  531  remains sealed. As a result, different MEMS devices with different internal environments may be created on the same MEMS wafer. Some of the advantages include reducing process steps, speeding up fabrication, and allowing different MEMS devices to be more closely integrated. For example, an accelerometer may optimally function in a higher pressure environment. In addition, a gyroscope may optimally function in lower pressure environments. Embodiments allow fabrication of both the accelerometer and the gyroscope using shared process steps. 
     Referring now to  FIG. 7 , the addition of a seal to set the adjusted pressure of the trench cavity is shown according to one aspect of the present embodiments. A seal layer  736  is formed over the second side  106  of the first substrate  102  using any suitable fabrication method (e.g. deposition, growth, sputtering, etc.). The seal layer  736  covers the portion  634  of the lining  314 , thereby sealing the trench  108  and the first enclosure  530 . For example, a metal layer (e.g. Al or AlCu) may be deposited by physical vapor deposition on the second side  106  of the substrate  102 . The metal layer traps the gas within the trench  108  and the first enclosure  530 , thereby locking in the environment surrounding the corresponding MEMS device. In various embodiments, the seal layer  736  hermetically seals the trench  108  and the first enclosure  530 . 
     In the illustrated embodiment, the seal layer  736  covers the entire second side  106  of the first substrate  102 . However in some embodiments, the seal layer  736  may cover only portions of the second side  106 . For example, the seal layer  736  may only cover the portion  634  of the lining  314  and an area of the second side  106  surrounding the seal layer  736 . Thus the seal layer  736  may be selectively applied in discrete areas to seal the trench  108  and the first enclosure  530 . 
     As previously stated, the arrangement of enclosures with and without trenches is not limited. As a result, a number of combinations of MEMS devices may be fabricated using the described embodiments. For example, MEMS devices requiring a first environment may be fabricated directly next to each other, or they may be separated by one or more MEMS devices requiring a second environment. Thus, many different combinations of MEMS devices may be fabricated, and should not be limited by the embodiments described herein. 
     Referring now to  FIG. 8 , a first substrate including a trench is shown according to one aspect of the present embodiments. A first substrate  802  is provided. The first substrate  802  may be, for example, a silicon wafer, however it is understood that embodiments are not limited to silicon wafers. The first substrate  802  includes a first side  804  and a second side  806 . A trench  808  is formed in the first side  804  of the first substrate  802  by any suitable fabrication method (e.g. etching, cutting, laser ablation, etc.). The trench  808  may also be referred to as a chimney, and in various embodiments the trench  808  is a high aspect ratio trench (e.g. &lt;70:1). In some embodiments the width of the trench  808  is 2 to 4 microns, and the length of the trench  808  is not limited. In various embodiments at this stage of fabrication, the trench  808  does not extend from the first side  804  entirely to the second side  806  (e.g. the trench  808  stops before reaching the second side  806 ). 
     Referring now to  FIG. 9 , the substrate with the addition of cavities is shown according to one aspect of the present embodiments. A first cavity  910  and a second cavity  912  (also referred to as upper cavities) are formed in the first side  804  of the first substrate  802  by any suitable fabrication method (e.g. etching, cutting, laser ablation, etc.). The first cavity  910  is connected to the trench  808  and the first cavity  910 . It is understood that it can also be stated that the trench  808  is located within the first cavity  910 . As such, the trench  808  is positioned such that it is not over the moveable structure of the first cavity  910 . The second cavity  912  does not include a trench structure. 
     It is understood that for simplicity of illustration only two cavities are shown, but any number of cavities and trenches may be fabricated on a substrate. In addition, the patterns on the substrate are not limited. For example, cavities without trenches may be next to each other, and cavities with trenches may be next to each other. 
     In various embodiments, the trench  808  and the first cavity  910  share a common and fluidly connected open space, such that the trench  808  opens directly down into the first cavity  910 . In the illustrated embodiment, the trench  808  shares a left side wall with the first cavity  910 . However, it is understood that the trench  808  may be on either side of the first cavity  910 . Furthermore in some embodiments, the trench  808  may be positioned in any offset location above the first cavity  910  that is not directly above a MEMS structure (not shown, but see  FIG. 5 ). 
     Referring now to  FIG. 10 , a thermal oxide growth on the trench and the cavities is shown according to one aspect of the present embodiments. A lining  1014  is formed over the first side  804  using any suitable fabrication method (e.g. deposition, growth, sputtering, etc.). For example, the lining may be a thermal oxide growth (e.g. silicon dioxide). The lining  1014  covers the first side  804 , the trench  808 , the first cavity  910 , and the second cavity  912 . In various embodiments, the lining  1014  may be used to set or reduce the width of the trench  808  to a predetermined width. For example, the width of the trench  808  after the formation of the lining may be 0.5 to 2.0 microns. In some embodiments, the narrowing of the width of the trench  808  improves the sealing of the trench (not shown, but see  FIG. 15 ). 
     Referring now to  FIG. 11 , the first substrate fusion bonded to a second substrate including standoffs is shown according to one aspect of the present embodiments. A second substrate  1116  is bonded to the first substrate  802 . For example, the second substrate  1116  may be fusion bonded to the first substrate  802 , thereby forming a silicon oxide silicon bond. In various embodiments, the second substrate  1116  is a structural layer including silicon with MEMS features (e.g. bump stop, damping hole, via, port, plate, proof mass, standoff, spring, seal ring, proof mass, etc.). The MEMS features (not shown, but see  FIG. 12 ) may include one or more moveable elements. 
     In various embodiments, the first substrate  802  is a MEMS substrate. The MEMS substrate (first substrate  802 ) provides mechanical support for the structural layer (second substrate  1116 ). The MEMS substrate may also be referred to as a handle substrate or handle wafer. In some embodiments, the handle substrate serves as a cap to the structural layer (e.g. forming the first cavity  910  and the second cavity  912 ), and may be referred to as a cap layer. 
     In some embodiments, standoffs  1118  are patterned on the second substrate  1116 . The standoffs  1118  define the vertical clearance between the structural layer and an IC substrate (not shown, but see  FIG. 12 ). The standoffs  1118  may also provide electrical contact between the structural layer and the IC substrate. 
     Referring now to  FIG. 12 , the formation of MEMS features and eutectic bonding of the second substrate to a third substrate is shown according to one aspect of the present embodiments. First MEMS features  1220  and second MEMS features  1221  have been formed in the second substrate  1116  using any suitable fabrication method. Fabrication methods may include, but are not limited to, etching, cutting, laser ablation, deposition, growth, sputtering, etc. 
     In addition, the second substrate  1116  has been bonded to a third substrate  1222  (e.g. an additional substrate). For example, in some embodiments the standoffs  1118  form a eutectic bond  1224  with bond pads  1226  on the third substrate  1222 , thereby providing an electrical connection between the second and third substrates. In various embodiments, the eutectic bond  1224  is an aluminum-germanium (AlGe) bond. In some embodiments, the third substrate  1222  is an integrated circuit (“IC”) substrate. An IC substrate may refer to a silicon substrate with electrical circuits, for example CMOS (“complementary metal-oxide semiconductor”) electrical circuits. 
     In some embodiments, the standoffs  1118  and the third substrate  1222  define a first lower cavity  1228  and a second lower cavity  1229 . The standoffs  1118  and the eutectic bond  1224  provide a seal that defines a first enclosure  1230  and a second enclosure  1231 . In various embodiments, the enclosure may refer to a fully enclosed volume typically surrounding the MEMS structure and typically formed by the IC substrate, structural layer, MEMS substrate, and the standoff seal ring. For example in the illustrated embodiment, the first enclosure  1230  refers to the fully enclosed volume surrounding the first MEMS features  1220 , including the first cavity  910  and the first lower cavity  1228 . In addition, the second enclosure  1231  refers to the fully enclosed volume surrounding the second MEMS features  1221 , including the second cavity  912  and the second lower cavity  1229 . In further embodiments, the eutectic bond  1224  may be substituted with other forms of bonding (e.g. solder, adhesive, etc.). In some embodiments, the seal may be a hermetic seal. 
     Before bonding the second substrate  1116  and the third substrate  1222 , thereby sealing the first enclosure  1230  and the second enclosure  1231 , the pressure and/or environment may be set to a predetermined state. For example, the chamber in which the bonding is being performed may be adjusted to increase or decrease the ambient pressure, thereby causing the pressure in the first enclosure  1230  and the second enclosure  1231  to match the altered ambient pressure. Likewise, the chamber in which the bonding is being performed may be adjusted to increase or decrease levels of gas (e.g. helium, hydrogen, nitrogen, etc.), thereby causing the gas levels in the first enclosure  1230  and the second enclosure  1231  to match the altered ambient gas composition. After the gas has been adjusted, the first enclosure  1230  and the second enclosure  1231  are sealed, thus trapping the gas pressure and composition within. 
     Referring now to  FIG. 13 , removal of a portion of the first substrate to reveal the trench is shown according to one aspect of the present embodiments. A portion of the second side  806  of the first substrate  802  is removed to open the trench  808  and expose a portion  1334  of the lining  1014 . Removal of the portion of the second side may be done through any suitable method (e.g. etching, cutting, polishing, grinding, etc.). It is understood that the removal of the portion of the second side  806  is controlled to expose the portion  1334  of the lining  1014  in the trench  808 , while not exposing the lining  1014  in the second enclosure  1231 . Thus, the second enclosure  1231  remains sealed. 
     Referring now to  FIG. 14 , removal of the exposed portion of the lining to reveal the trench and adjust the pressure in one cavity is shown according to one aspect of the present embodiments. The exposed portion  1334  of the lining  1014  (e.g oxide layer) is removed by any suitable method (e.g. etching, cutting, polishing, grinding, etc.), thereby creating an opening  1435  within the trench  808 . In some embodiments, only a portion of the exposed portion  1334  of the lining  1014  is removed. For example, an oxide layer may line the trench, thereby forming a trench lining. An portion of the oxide layer may be exposed, thereby forming an exposed oxide portion of the trench lining. The entire exposed oxide portion of the trench lining or only a portion of the exposed oxide portion of the trench lining may then be removed. 
     As a result of the opening  1435 , the pressure and/or environment within the first enclosure  1230  may be adjusted to a second predetermined state. For example, the chamber in which the post processing is being performed may be adjusted to increase or decrease the ambient pressure, thereby causing the pressure in the first enclosure  1230  to match the altered ambient pressure. Likewise, the chamber in which the post processing is being performed may be adjusted to increase or decrease levels of gas (e.g. helium, hydrogen, etc.), thereby causing the gas levels in the first enclosure  1230  to match the altered ambient gas composition. 
     While the environment within the first enclosure  1230  is being adjusted, the second enclosure  1231  remains sealed. As a result, different MEMS devices with different internal environments may be created on the same MEMS wafer. Some of the advantages include reducing process steps, speeding up fabrication, and allowing different MEMS devices to be more closely integrated. For example, an accelerometer may optimally function in a higher pressure environment (e.g. greater than 50 millibar). In addition, a gyroscope may optimally function in lower pressure environments (e.g. less than 8 millibar). Embodiments allow fabrication of both the accelerometer and the gyroscope using shared process steps. 
     Referring now to  FIG. 15 , the addition of seals to set the adjusted pressure of the trench cavity is shown according to one aspect of the present embodiments. A first seal layer  1536  (i.e. oxide layer) and a second seal layer  1538  are formed over the second side  806  of the first substrate  802  using any suitable fabrication method (e.g. deposition, growth, sputtering, etc.). The first seal layer  1536  and the second seal layer  1538  cover the opening  1435 , thereby sealing the trench  808  and the first enclosure  1230 . In various embodiments, the first seal layer  1536  and the second seal layer  1538  hermetically seal the trench  808  and the first enclosure  1230 . 
     For example, an oxide seal may be deposited by chemical vapor deposition on the second side  806  of the substrate  802 . In addition, a metal layer (e.g. AlCu) may be deposited by physical vapor deposition on the oxide seal. The metal and oxide layers trap the gas within the trench  808  and the first enclosure  1230 , thereby locking in the environment surrounding the corresponding MEMS device. It is understood that the metal layer and oxide layer are exemplary and not limiting. In other embodiments, the oxide layer may overlay the metal layer. In further embodiments, seals other than metal and oxides may be used. In some embodiments, one layer or more than two layers may be used as seals. 
     In the illustrated embodiment, the first seal layer  1536  and the second seal layer  1538  cover the entire second side  806  of the first substrate  802 . However in some embodiments, the first seal layer  1536  and the second seal layer  1538  may cover only portions of the second side  806 . For example, the first seal layer  1536  and the second seal layer  1538  may only cover the opening  1435  and an area of the second side  806  surrounding the opening  1435 . Thus the first seal layer  1536  and the second seal layer  1538  may be selectively applied in discrete areas to seal the trench  808  and the first enclosure  1230 . 
     As previously stated, the arrangement of enclosures with and without trenches is not limited. As a result, a number of combinations of MEMS devices may be fabricated using the described embodiments. For example, MEMS devices requiring a first environment may be fabricated directly next to each other, or they may be separated by one or more MEMS devices requiring a second environment. Thus, many different combinations of MEMS devices may be fabricated, and should not be limited by the embodiments described herein. 
     In embodiments where the metal layer is in direct contact with the second side  806 , the metal layer advantageously provides an EMC shield. In addition, in embodiments including the metal layer, the seal quality is advantageously improved. 
       FIG. 16  an exemplary flow diagram for setting a pressure within a trench and cavity is shown according to one aspect of the present embodiments. At a block  1650 , a trench is formed in a first side of a first silicon wafer. For example, in  FIG. 1  a trench is formed in the first side of the first substrate by any suitable fabrication method (e.g. etching, cutting, laser ablation, etc.). 
     At a block  1652 , a cavity connected to the trench is formed in the first side of the first silicon wafer. For example, in  FIG. 2  a first cavity is formed in the first side of the first substrate by any suitable fabrication method (e.g. etching, cutting, laser ablation, etc.). The first cavity is connected to the trench. 
     In some embodiments, an additional cavity is formed in the first side of the silicon wafer. For example, in  FIG. 2  a second cavity  212  is formed in the first side of the first substrate by any suitable fabrication method (e.g. etching, cutting, laser ablation, etc.). In some embodiments, the additional cavity in the first side of the first silicon wafer includes a different pressure from the pressure within the trench and the cavity. For example, in  FIG. 6  while the environment within the first enclosure is being adjusted, the second enclosure remains sealed. As a result, different MEMS devices with different internal environments may be created on the same MEMS wafer. 
     At a block  1654 , an oxide layer is formed on the first side and in the trench. For example, in  FIG. 3  a lining is formed over the first side using any suitable fabrication method (e.g. deposition, growth, sputtering, etc.). In some embodiments, the lining may be a thermal oxide growth (e.g. silicon dioxide) that reduces the width of the trench. For example, in  FIG. 3  the lining may be used to set or reduce the width of the trench to 0.5 to 2.0 microns. 
     At a block  1656 , the first side of the first silicon wafer is bonded to a second silicon wafer. For example, in  FIG. 4  a second substrate is bonded to the first substrate. 
     At a block  1658 , a MEMS structure is formed in the second silicon wafer. For example, in  FIG. 5  first MEMS features are formed in the second substrate using any suitable fabrication method. Fabrication methods may include, but are not limited to, etching, cutting, laser ablation, deposition, growth, sputtering, etc. In some embodiments, an additional MEMS structure is formed in the second silicon wafer. For example, in  FIG. 5  second MEMS features have been formed in the second substrate using any suitable fabrication method. 
     In some embodiments, a standoff is formed on the second silicon wafer. For example, in  FIG. 4  standoffs are patterned on the second substrate. The standoffs define the vertical clearance between the structural layer and an IC substrate, as well as providing electrical contact between the structural layer and the IC substrate. 
     At a block  1660 , the second silicon wafer is bonded to a third silicon wafer, wherein the bonding seals the MEMS structure between the third silicon wafer and the cavity. For example, in  FIG. 5  the second substrate has been bonded to a third substrate with a eutectic bond, thereby sealing a first enclosure. The first enclosure is the fully enclosed volume surrounding the first MEMS features, including the first cavity and the first lower cavity. 
     In some embodiments, the bonding the second silicon wafer to the third silicon wafer provides a hermetic seal and an electrical connection. For example, in  FIG. 5  the eutectic bond is a seal that may be a hermetic seal. In addition, the eutectic bond provides an electrical connection between the second substrate and the third substrate. In further embodiments, the bonding the second silicon wafer to the third silicon wafer includes forming an aluminum-germanium bond. For example, in  FIG. 5  the eutectic bond may be an aluminum-germanium (AlGe) bond. 
     At a block  1662 , a portion of a second side of the first silicon wafer is removed, wherein the removing exposes the trench. For example, in  FIG. 6  a portion of the second side of the first substrate is removed to open the trench. 
     In some embodiments, a lining is formed within the trench, which reduces the width of the trench. For example, in  FIG. 3  a lining is formed over the first side using any suitable fabrication method (e.g. deposition, growth, sputtering, etc.), and the formation of the lining reduces the width of the trench. In further embodiments, the removal of the portion of the second side of the first silicon wafer exposes a portion of the lining, and the exposed portion of the lining is removed. For example, in  FIG. 14  the exposed portion of the lining is removed by any suitable method (e.g. etching, cutting, polishing, grinding, etc.), thereby creating an opening within the trench. 
     At a block  1664 , a pressure is set within the trench and the cavity. For example, in  FIG. 6  the pressure and/or environment within the first enclosure is adjusted to a second predetermined state. In some embodiments, the oxide layer is permeable to a gas, and setting the pressure within the trench and the cavity includes passing the gas through the oxide layer. For example, in  FIG. 6  the portion of the lining is permeable to various gasses. As a result of the permeability of the portion of the lining, the pressure and/or environment within the first enclosure may be adjusted to a second predetermined state. 
     At a block  1666 , a layer is formed on the second side of the first silicon wafer, wherein the layer seals the trench, the cavity, and the MEMS structure. For example, in  FIG. 7  a seal layer is formed over the second side of the first substrate using any suitable fabrication method (e.g. deposition, growth, sputtering, etc.). 
       FIG. 17  an exemplary flow diagram for sealing a first pressure in a second cavity and sealing a second pressure in a first cavity is shown according to one aspect of the present embodiments. At a block  1770 , a trench is formed in a handle substrate. For example, in  FIG. 8  a trench is formed in the first side of the first substrate by any suitable fabrication method (e.g. etching, cutting, laser ablation, etc.). The first substrate provides mechanical support for the structural layer and may be referred to as a handle substrate or handle wafer. In some embodiments, the handle substrate serves as a cap to the structural layer. 
     At a block  1772 , a trench lining is formed in the trench. For example, in  FIG. 10  a lining is formed over the first side and may be a thermal oxide growth (e.g. silicon dioxide). The lining covers the first side, the trench, the first cavity, and the second cavity. In some embodiments, the width of the trench is 2 to 4 microns before the forming the trench lining, and the width of the trench is 0.5 to 2.0 microns after the forming the trench lining. For example, in  FIG. 8  the width of the trench may be 2 to 4 microns, and in  FIG. 10  the width of the trench after the formation of the lining may be 0.5 to 2.0 microns. 
     At a block  1774 , a first cavity and a second cavity are formed in the handle substrate, wherein the first cavity is connected to the trench. For example, in  FIG. 9  a first cavity and a second cavity (also referred to as upper cavities) are formed in the first side of the first substrate (also referred to as a handle substrate) by any suitable fabrication method (e.g. etching, cutting, laser ablation, etc.). The first cavity is connected to the trench. 
     At a block  1776 , a first MEMS structure and the handle substrate are sealed for maintaining a first pressure within the trench and the first cavity. For example, in  FIG. 12  MEMS features have been formed in the second substrate using any suitable fabrication method. In addition, the bonding of the second substrate to the third substrate seals a predetermined pressure and/or environment within the trench and first cavity. The MEMS structure is within the first cavity, and the first cavity includes a side opposite the MEMS structure. The trench is connected to the side of the cavity opposite the MEMS structure, and the trench is not directly opposite the MEMS structure. 
     At a block  1778 , a second MEMS structure and the handle substrate are sealed for maintaining the first pressure within the second cavity. For example, in  FIG. 12  the bonding of the second substrate to the third substrate seals a predetermined pressure and/or environment within the second cavity containing the MEMS features. 
     At a block  1780 , a portion of the trench lining is exposed. For example, in  FIG. 13  a portion of the second side of the first substrate is removed to open the trench and expose a portion of the lining. In some embodiments, the exposed portion of the trench lining or a portion of the exposed portion of the trench lining is removed. For example, in  FIG. 14  the exposed portion of the lining is removed by any suitable method (e.g. etching, cutting, polishing, grinding, etc.), thereby creating an opening within the trench. 
     In other embodiments, the exposed portion of the lining is not removed. Instead the trench lining is permeable to a gas, and changing the first pressure to a second pressure within the first cavity includes passing the gas through the trench lining. For example, in  FIG. 6  the portion of the lining is permeable to one or more gasses (e.g. helium and/or hydrogen). As a result of the permeability of the portion of the lining, the pressure and/or environment within the first enclosure may be adjusted to a second predetermined state. 
     At a block  1782 , the first pressure is changed to a second pressure within the first cavity. For example, in  FIG. 14  the pressure and/or environment within the first enclosure may be adjusted to a second predetermined state. 
     At a block  1784 , the first cavity and the trench are sealed to maintain the second pressure within the trench and the first cavity. For example, in  FIG. 15  a first seal layer and a second seal layer are formed over the second side of the first substrate using any suitable fabrication method (e.g. deposition, growth, sputtering, etc.). In some embodiments, the sealing includes depositing a metal layer over the trench lining. For example, in  FIG. 15 , a metal layer (e.g. AL or AlCu) may be deposited by physical vapor deposition on the oxide seal. The metal and oxide layers trap the gas within the trench and the first enclosure, thereby locking in the environment surrounding the corresponding MEMS device. 
     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 to persons having ordinary skill in the art to which the embodiments pertain, 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.