PATENT DOCUMENT

Publication Number: US-9796578-B2
Application Number: US-201414502835-A
Country: US
Kind Code: B2

Title: Microelectromechanical systems devices with improved reliability

Abstract:
An electronic device may include components that are formed using microelectromechanical systems (MEMS) technology. A MEMS device may include a MEMS structure bonded to a semiconductor substrate. The MEMS structure may be formed from a silicon substrate having a cavity and a moveable member suspended over the cavity and free to oscillate within the cavity. The semiconductor substrate may be a complementary metal-oxide semiconductor substrate having circuitry such as sensing electrodes. The sensing electrodes may be used to gather signals that are produced by movement of the suspended member. One or more of the electrodes on the semiconductor substrate may be covered by a dielectric film to prevent electrical shorts between adjacent electrodes on the semiconductor substrate.

Claims:
What is claimed is: 
     
       1. A microelectromechanical systems device, comprising:
 a first substrate having a cavity and a member that is anchored to the first substrate and suspended over the cavity; 
 a second substrate having circuitry including first and second electrodes that gather signals produced by movement of the member; and 
 a passivation layer that completely covers the first electrode, the second electrode, and a ground electrode that is between the first and second electrodes, wherein the passivation layer prevents electrical shorts between the member and each of the first, second, and ground electrodes. 
 
     
     
       2. The microelectromechanical systems device defined in  claim 1  wherein the first substrate is bonded to the second substrate. 
     
     
       3. The microelectromechanical systems device defined in  claim 1  wherein the passivation layer prevents electrical shorts between the first electrode and the ground electrode. 
     
     
       4. The microelectromechanical systems device defined in  claim 1  wherein the second substrate comprises a complementary metal-oxide-semiconductor substrate. 
     
     
       5. The microelectromechanical systems device defined in  claim 1  wherein the first substrate comprises a silicon substrate having trenches that surround the member and wherein the member is free to vibrate within the cavity. 
     
     
       6. The microelectromechanical systems device defined in  claim 1  wherein the first substrate is bonded to the second substrate using an eutectic bond. 
     
     
       7. The microelectromechanical systems device defined in  claim 1  wherein the passivation layer is interposed between the first electrode and the member and is interposed between the second electrode and the member. 
     
     
       8. The microelectromechanical systems device defined in  claim 7  wherein the ground electrode is held at a higher potential than the first electrode and the second electrode. 
     
     
       9. The microelectromechanical systems device defined in  claim 1  wherein the ground electrode and the member are held at a first voltage, and wherein the first and second electrodes are held at a second voltage that is less than the first voltage. 
     
     
       10. The microelectromechanical systems device defined in  claim 1  wherein the first substrate forms a cap that covers the cavity and the member such that the cavity and the member are interposed between the cap and the second substrate. 
     
     
       11. A microelectromechanical systems device, comprising:
 a first substrate having a cavity and a member suspended over the cavity and free to move within the cavity; 
 a second substrate having circuitry including first and second electrodes, wherein signals produced by movement of the member are capacitively coupled onto the first electrode, and wherein the second substrate has a surface on which the first and second electrodes are formed; and 
 a dielectric film that covers the second electrode, wherein the dielectric film is interposed between the second electrode and the member and prevents electrical shorts between the second electrode and the member, wherein the dielectric film extends across the second substrate and contacts the first electrode without overlapping the first electrode along an axis perpendicular to the surface of the substrate. 
 
     
     
       12. The microelectromechanical systems device defined in  claim 11  wherein the first electrode is a capacitive sensing electrode. 
     
     
       13. The microelectromechanical systems device defined in  claim 11  wherein the first substrate is bonded to the second substrate. 
     
     
       14. The microelectromechanical systems device defined in  claim 11  wherein the first substrate is a silicon substrate. 
     
     
       15. The microelectromechanical systems device defined in  claim 11  wherein the second substrate is a complementary metal-oxide-semiconductor substrate. 
     
     
       16. The microelectromechanical systems device defined in  claim 11  wherein the microelectromechanical systems device comprises an accelerometer.

Description:
This application claims the benefit of provisional patent application No. 61/950,712 filed Mar. 10, 2014, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to electronic devices and, more particularly, to electronic devices that include microelectromechanical systems (MEMS) devices. 
     Electronic devices often include MEMS devices. MEMS devices may, for example, be used to form accelerometers, gyroscopes, microphones, and other types of sensors. MEMS devices are sometimes formed by bonding a MEMS structure having a suspended moveable member to a complementary metal-oxide-semiconductor (CMOS) substrate having circuitry for sensing movement of the suspended MEMS structure. For example, the semiconductor substrate may include capacitive sensing electrodes that are configured to gather signals produced by movement of the suspended MEMS structure. 
     The suspended microstructures in a MEMS device are typically formed using a deep reactive ion etch (DRIE). In many cases, the deep etch process used to form trenches around a suspended MEMS structure will leave behind undesirable surface features such as scalloped sidewalls. The presence of scalloped sidewalls and rough surfaces in a MEMS device can make the device more susceptible to fractures and chipping. If care is not taken, the particles released from impact-induced fractures in the silicon can cause electrical shorts between metal contact pads on the semiconductor substrate. 
     It would therefore be desirable to be able to provide electronic devices with improved MEMS devices. 
     SUMMARY 
     An electronic device may include components that are formed using microelectromechanical systems (MEMS) technology. For example, an electronic device may include one or more sensors such as an accelerometer and/or gyroscope that are formed by a MEMS device. 
     A MEMS device may include a MEMS structure bonded to a semiconductor substrate. The MEMS structure may be formed from a silicon substrate having a cavity and a moveable member suspended over the cavity. The moveable member may be integral with the silicon substrate but may be substantially isolated from the silicon substrate by trenches. A thin beam of silicon may couple the moveable member to the peripheral silicon and may allow the suspended member to oscillate within the cavity. 
     The semiconductor substrate in a MEMS device may be a complementary metal-oxide semiconductor substrate having circuitry such as sensing electrodes. The sensing electrodes may be used to gather signals that are produced by movement of the suspended member. 
     One or more of the electrodes on the semiconductor substrate may be covered by a dielectric film to prevent electrical shorts between adjacent electrodes on the semiconductor substrate. The dielectric film may be etched during two separate etching steps. In the first etching step, raised protrusions may be formed in the dielectric film. The raised protrusions may provide strength to the bonding region where the MEMS structure is bonded to the semiconductor substrate. After the second etching step, some of the metal on the semiconductor substrate may be exposed, while one or more electrodes on the semiconductor substrate may remain covered by the dielectric film. If desired, the dielectric film may cover capacitive sensing electrodes on the semiconductor substrate without affecting the sensing ability of the capacitive sensing electrodes. 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative electronic device of the type that may be provided with a MEMS device in accordance with an embodiment of the present invention. 
         FIG. 2  is a schematic view of an illustrative electronic device of the type that may be provided with a 
       MEMS device in accordance with an embodiment of the present invention. 
         FIG. 3  is a cross-sectional side view of an illustrative MEMS device having a MEMS structure bonded to a semiconductor substrate in accordance with an embodiment of the present invention. 
         FIG. 4  is a perspective view of an illustrative MEMS structure in accordance with an embodiment of the present invention. 
         FIG. 5  is a diagram showing illustrative steps involved in forming a MEMS structure in accordance with an embodiment of the present invention. 
         FIG. 6  is a diagram showing a conventional MEMS device. 
         FIG. 7  is a diagram showing a MEMS device in which an electrode on a semiconductor substrate is covered by a dielectric layer in accordance with an embodiment of the present invention. 
         FIG. 8  is a diagram showing a MEMS device in which electrodes including a sensing electrode are covered by a dielectric layer in accordance with an embodiment of the present invention. 
         FIG. 9  is a diagram showing illustrative steps involved in forming a MEMS device having a MEMS structure bonded to a semiconductor substrate on which electrodes are covered by a dielectric layer in accordance with an embodiment of the present invention. 
         FIG. 10  is a diagram showing illustrative steps involved in forming a MEMS device having a MEMS structure bonded to a semiconductor substrate on which electrodes are covered by a dielectric layer in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     An illustrative electronic device that may be provided with one or more MEMS devices is shown in  FIG. 1 . Electronic devices such as device  10  of  FIG. 1  may be cellular telephones, media players, other handheld portable devices, somewhat smaller portable devices such as wrist-watch devices, pendant devices, or other wearable or miniature devices, gaming equipment, tablet computers, notebook computers, desktop computers, televisions, computer monitors, computers integrated into computer displays, or other electronic equipment. 
     In the example of  FIG. 1 , device  10  includes a display such as display  14 . Display  14  has been mounted in a housing such as housing  12 . Housing  12 , which may sometimes be referred to as an enclosure or case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of any two or more of these materials. Housing  12  may be formed using a unibody configuration in which some or all of housing  12  is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.). 
     Display  14  may be a touch screen display that incorporates a layer of conductive capacitive touch sensor electrodes or other touch sensor components (e.g., resistive touch sensor components, acoustic touch sensor components, force-based touch sensor components, light-based touch sensor components, etc.) or may be a display that is not touch-sensitive. Capacitive touch screen electrodes may be formed from an array of indium tin oxide pads or other transparent conductive structures. 
     Display  14  may include an array of display pixels formed from liquid crystal display (LCD) components, an array of electrophoretic display pixels, an array of plasma display pixels, an array of organic light-emitting diode display pixels, an array of electrowetting display pixels, or display pixels based on other display technologies. The brightness of display  14  may be adjustable. For example, display  14  may include a backlight unit formed from a light source such as a lamp or light-emitting diodes that can be used to increase or decrease display backlight levels and thereby adjust display brightness. Display  14  may also include organic light-emitting diode pixels or other pixels with adjustable intensities. In this type of display, display brightness can be adjusted by adjusting the intensities of drive signals used to control individual display pixels. 
     Display  14  may be protected using a display cover layer such as a layer of transparent glass or clear plastic. Openings may be formed in the display cover layer. For example, an opening may be formed in the display cover layer to accommodate a button such as button  16 . An opening may also be formed in the display cover layer to accommodate ports such as speaker port  18 . 
     In the center of display  14 , display  14  may contain an array of active display pixels. This region is sometimes referred to as the active area of the display. A rectangular ring-shaped region surrounding the periphery of the active display region may not contain any active display pixels and may therefore sometimes be referred to as the inactive area of the display. The display cover layer or other display layers in display  14  may be provided with an opaque masking layer in the inactive region to hide internal components from view by a user. 
     A schematic diagram of device  10  is shown in  FIG. 2 . As shown in  FIG. 2 , electronic device  10  may include control circuitry such as storage and processing circuitry  40 . Storage and processing circuitry  40  may include one or more different types of storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory), volatile memory (e.g., static or dynamic random-access-memory), etc. 
     Processing circuitry in storage and processing circuitry  40  may be used in controlling the operation of device  10 . The processing circuitry may be based on a processor such as a microprocessor and other suitable integrated circuits. With one suitable arrangement, storage and processing circuitry  40  may be used to run software on device  10  such as internet browsing applications, email applications, media playback applications, operating system functions, software for capturing and processing images, software implementing functions associated with gathering and processing sensor data, software that makes adjustments to display brightness and touch sensor functionality, etc. 
     Input-output circuitry  32  may be used to allow input to be supplied to device  10  from a user or external devices and to allow output to be provided from device  10  to the user or external devices. 
     Input-output circuitry  32  may include wired and wireless communications circuitry  34 . Communications circuitry  34  may include radio-frequency (RF) transceiver circuitry formed from one or more integrated circuits, power amplifier circuitry, low-noise input amplifiers, passive RF components, one or more antennas, and other circuitry for handling RF wireless signals. Wireless signals can also be sent using light (e.g., using infrared communications). 
     Input-output circuitry  32  may include input-output devices  36  such as button  16  of  FIG. 1 , joysticks, click wheels, scrolling wheels, a touch screen such as display  14  of  FIG. 1 , other touch sensors such as track pads or touch-sensor-based buttons, vibrators, audio components such as microphones and speakers, image capture devices such as a camera module having an image sensor and a corresponding lens system, keyboards, status-indicator lights, tone generators, key pads, and other equipment for gathering input from a user or other external source and/or generating output for a user. 
     Sensor circuitry such as sensors  38  of  FIG. 2  may include an ambient light sensor for gathering information on ambient light levels, proximity sensor components (e.g., light-based proximity sensors and/or proximity sensors based on other structures), accelerometers, gyroscopes, magnetic sensors, and other sensor structures. Sensors  38  of  FIG. 2  may, for example, include one or more MEMS sensors (e.g., accelerometers, gyroscopes, microphones, force sensors, pressure sensors, capacitive sensors, or any other suitable type of sensor formed using a MEMS device). If desired, other suitable components in device  10  may be formed using MEMS technology. 
     A cross-sectional side view of an illustrative MEMS device that may be used in an electronic device such as electronic device  10  of  FIGS. 1 and 2  is shown in  FIG. 3 . As shown in  FIG. 3 , MEMS device  42  may include a MEMS structure such as MEMS structure  44  bonded to a semiconductor substrate such as semiconductor substrate  62 . 
     MEMS structure  44  may, for example, be formed from silicon and may include a moveable member such as moveable member  46  suspended over a cavity such as cavity  54 . Member  46  may be integral with MEMS structure  44  but may be substantially isolated from the surrounding portions of structure  44  so that member  46  is free to move or vibrate within cavity  54 . 
     Semiconductor substrate  62  may include circuitry such as circuitry  82 , conductive pads  64  (e.g., electrodes), and traces  80 . Conductive pads  64  may be used as capacitive sensing electrodes that gather signals produced by movement of suspended member  46 . Signals such as gathered sensor signals may be conveyed between electrodes  64  and circuitry  82  via conductive traces  80 . Circuitry  82  may, for example, include inductors, capacitors, resistors, switches, amplifiers, and/or other circuitry. Semiconductor substrate  62  may be formed using CMOS technology, NMOS technology, PMOS technology, or other suitable semiconductor technology. Arrangements in which semiconductor substrate  62  is formed using a CMOS die are sometimes described herein as an example. 
     A perspective view of a an illustrative MEMS structure that may be used in a MEMS device of the type shown in  FIG. 3  is shown in  FIG. 4 . As shown in  FIG. 4 , MEMS structure  44  may be formed from a silicon substrate and may include a cavity such as cavity  54  over which a moveable member such as moveable MEMS member  46  is suspended. Trenches such as trenches  58  may partially surround member  46  and may separate member  46  from the surrounding silicon bulk (e.g., peripheral portions  44 P of MEMS structure  44 ). For example, trenches  58  may surround suspended member  46  on one, two, three, or all four sides of member  46 . 
     Suspended member  46  (sometimes referred to as a suspended membrane or a diving board) may be coupled to peripheral portions  44 P of structure  44  by a bridge structure such as bridge structure  52 . Bridge structure  52  may, for example, be formed form a narrow beam of silicon that bridges the trench between suspended structure  46  and peripheral portions  44 P of structure  44 . Bridge structure  52  (sometimes referred to as a beam member or spring member), may be used to anchor suspended structure  46  to peripheral portions  44 P while allowing movement of suspended member  46  within cavity  54  (e.g., end portion  46 A of member  46  may be free to move in the x, y, and/or z directions as indicated in  FIG. 4 ). 
     The fixed-free arrangement of  FIG. 4  in which suspended member  46  is anchored on one end (e.g., end  46 B) while the opposing end (e.g., end  46 A) is free to move is merely illustrative. If desired, a fixed-fixed arrangement may be used in which opposing ends of suspended MEMS member  46  are anchored to peripheral portions  44 P of structure  44  using first and second beams  52 . In general, member  46  may be suspended using any suitable number of beams  52  (e.g., one, two, three, or more than three beams). Suspended members of the type shown in  FIG. 4  are sometimes referred to as diving boards. This is, however, merely illustrative. If desired, other types of micromechanical structures may be used. 
     If desired, MEMS structure  44  may include first and second substrates (e.g., first and second silicon substrates) such as first substrate  50  and second substrate  48 . First substrate  50  may sometimes be referred to as a silicon cap structure. Second substrate  48 , from which one or more suspended MEMS structures  46  are formed, may sometimes be referred to as a MEMS layer. Cavity  54  may be formed by etching a recess into first substrate  50 , and trenches  58  may be formed by etching openings into second substrate  48 . First substrate  50  and second substrate  48  may be bonded together to form MEMS structure  44 . 
       FIG. 5  is a diagram of illustrative steps involved in forming a MEMS structure such as MEMS structure  44  of  FIG. 4 . At step  200 , a first substrate such as silicon substrate  50  having opposing planar surfaces  50 A and  50 B may be provided. 
     At step  202 , etching equipment may be used to etch a recess or cavity such as cavity  54  in surface  50 A of substrate  50 . Any suitable etching process may be used to etch cavity  54  (e.g., plasma etching such as deep reactive ion etching, wet etching, dry etching, etc.). 
     At step  204 , upper surface  50 A′ of substrate  50  may be oxidized to form a thin surface layer of oxide (e.g., silicon dioxide) such as oxide layer  60  on upper surface  50 A′. 
     At step  206 , a second substrate such as silicon substrate  48  may be bonded to substrate  50 . Oxide layer  60  may be interposed between first substrate  50  and second substrate  48 . If desired, step  206  may include thinning substrate  48  to remove a portion of substrate  48  (e.g., portion  48 ′) and thereby reduce the thickness of substrate  48  as desired. 
     At step  208 , etching equipment may be used to etch trenches  58  in substrate  48  and to thereby form suspended MEMS member  46 . Trenches  58  may surround or partially surround MEMS member  46 . Any suitable etching process may be used to etch trenches  58  (e.g., plasma etching such as deep reactive ion etching, wet etching, dry etching, etc.). 
     The deep etch process used to form trenches  58  in substrate  48  may sometimes lead to scalloped or roughened surfaces on substrate  48  that can in turn lead to fractures or chips in the silicon. If care is not taken, particles that result from stress-induced cracks in the silicon can cause electrical shorts in MEMS devices.  FIG. 6  is a diagram showing how fractures in a MEMS structure can result in electrical shorts in a conventional MEMS device. 
     As shown in  FIG. 6 , conventional MEMS device  420  includes member  460  suspended over CMOS substrate  620 . Member  460  is anchored via spring member  520  but is allowed to move in the z-direction. CMOS substrate  620  includes a high voltage (HV) metal pad  640 H interposed between low voltage (LV) metal pads  640 H. LV pads  640 L are used to read out capacitance changes as member  460  moves and the distances D 1  and D 2  between member  460  and LV pads  640  change. HV pad  640 H is not typically used for signal sensing and may be used as a ground contact. 
     As shown in  FIG. 6 , metal pads  640 H and  640 L are exposed. The exposure of metal pads such as HV metal pad  640 H can lead to electrical shorts  610  between LV metal pad  640 L and HV metal pad  640 H. For example, in a drop event, fractures in the silicon (e.g., fracture  612  in member  460 ) can result in the release of particles that in turn cause electrical shorts  610  between LV pad  640 L and HV pad  640 H. Electrical shorts of this type can lead to inaccurate sensor readings or may otherwise decrease the reliability of the MEMS device. 
     Electrical shorts in a MEMS device may be prevented by providing a layer of dielectric over some or all of the metal pads on the semiconductor substrate of a MEMS device. An illustrative arrangement in which metal pads of a semiconductor substrate in a MEMS device are covered with dielectric material is shown in  FIG. 7 . As shown in  FIG. 7 , MEMS device  42  may include a moveable MEMS structure such as moveable member  46  suspended over a semiconductor substrate such as semiconductor substrate  62 . Member  46  may be anchored at one or more locations by a spring member such as spring member  52  (e.g., a thin beam of silicon that couples suspended member  46  to the peripheral portions of the MEMS structure). Member  46  may be suspended over a cavity such as cavity  54  and may therefore be allowed to move or oscillate within cavity  54  (e.g., in the x, y, and/or z directions as indicated in  FIG. 7 ). Semiconductor substrate  62  may include circuitry such as electrode  64 A, electrode  64 B, and electrode  64 C (sometimes referred to as conductive pads). Electrodes  64 A,  64 B, and  64 C may be formed from a metal such as silver, nickel, zinc, aluminum, or copper (as examples) or may be formed from a conductive material such as indium tin oxide or other conductive material. Electrodes  64 A,  64 B, and  64 C may, for example, be located directly under suspended member  46 . 
     Some of the electrodes on semiconductor substrate  62  may be signal-sensing electrodes (sometimes referred to as sensing electrodes, capacitive sensing electrodes, or signal electrodes), while others may be non-signal-sensing electrodes. For example, electrodes such as electrode  64 A and electrode  64 C may be signal sensing electrodes that are configured to gather signals produced by oscillations of suspended member  46 . Conductive electrode  64 A may be configured to detect capacitance changes as distance D 1  between electrode  64 A and member  46  changes, while electrode  64 C may be configured to detect capacitance changes as distance D 2  between electrode  64 C and member  46  changes. If desired, signal-sensing electrodes such as electrodes  64 A and  64 C may be low voltage electrodes (as an example). 
     Some electrodes such as electrode  64 B may be non-signal-sensing electrodes (sometimes referred to as non-sensing electrodes, ground voltage electrodes, fixed voltage electrodes, or reference voltage electrodes) that are not used for gathering signals. Non-signal-sensing electrodes such as electrode  64 B may, for example, be used as reference voltage electrodes, ground voltage electrodes, high voltage electrodes, fixed voltage electrodes, etc. If desired, non-signal-sensing electrode  64 B may be interposed between signal-sensing electrodes  64 A and  64 C and may be used to provide shielding between signal-sensing electrodes  64 A and  64 C. This is, however, merely illustrative. If desired, other arrangements of signal-sensing electrodes and non-signal-sensing electrodes may be used. For example, electrode  64 B may be a signal-sensing electrode and electrodes  64 A and  64 C may be non-signal-sensing electrodes. If desired, all three of electrodes  64 A,  64 B, and  64 C may be signal-sensing electrodes. 
     The arrangement of  FIG. 7  in which electrode  64 B is larger than electrodes  64 A and  64 C is merely illustrative. In general, electrodes  64 A,  64 B, and  64 C may have any suitable size. Similarly, the example in which signal-sensing electrodes  64 A and  64 C are low voltage electrodes and non-signal-sensing electrode  64 B is a high voltage electrode is merely illustrative. If desired, electrodes  64 A and  64 C may be high voltage electrodes and electrode  64 B may be a low voltage electrode. There may be one, two, three, four, or more than four electrodes on semiconductor substrate  62 . Arrangements in which semiconductor substrate  62  includes three electrodes and in which electrodes  64 A and  64 C are signal-sensing electrodes while electrode  64 B is a non-signal-signal sensing electrode are sometimes described herein as an example. 
     To minimize electrical shorts between adjacent electrodes on semiconductor substrate  62 , semiconductor substrate  62  may include a thin layer of dielectric over one or more of electrodes  64 A,  64 B, and  64 C. For example, as shown in  FIG. 7 , a dielectric material such as dielectric material  66  may be formed over electrodes on semiconductor substrate  62  such as non-signal sensing electrode  64 B. Dielectric material  66  (sometimes referred to as a dielectric thin film, a dielectric layer, a passivation layer, or a protection layer), may be formed from silicon nitride, silicon oxinitride, silicon oxide, or any other suitable dielectric material that may be used to passivate the surface of semiconductor substrate  62 . 
     In the example of  FIG. 7 , some of the electrodes on semiconductor substrate  62  are exposed (e.g., are not covered by dielectric material  66 ), while other electrodes on semiconductor substrate  62  are insulated by dielectric material  66 . For example, non-signal-sensing electrode  64 B may be covered by dielectric material  66 , while signal-sensing electrodes  64 A and  64 C may be exposed. Because dielectric material  66  insulates electrode  64 B from electrode  64 A, particles that are released from silicon member  46  in a drop event will not cause an electrical short between electrode  64 A and electrode  64 B. Similarly, the dielectric material between electrode  64 B and electrode  64 C may prevent electrical shorts between electrode  64 B and electrode  64 C. 
     If desired, suspended member  46  may be held at the same potential as insulated electrode  64 B in order to minimize charge build-up in dielectric layer  66  above electrode  64 B. For example, in arrangements where electrode  64 B is a high voltage electrode, suspended member  46  may also be held at a high voltage (HV) as shown in  FIG. 7 . This is, however, merely illustrative. If desired, MEMS structure  46  may be held at a different voltage than electrode  64 B. 
     If desired, dielectric layer  66  may cover both sensing electrodes and non-sensing electrodes on semiconductor substrate  62 .  FIG. 8  is a diagram showing how dielectric layer  66  may cover electrode  64 A, electrode  64 B, and electrode  64 C on semiconductor substrate  62 . Even though signal-sensing electrodes  64 A and  64 C are covered by dielectric thin film  66 , signal-sensing electrode  64 A may still be configured to detect capacitance changes as the distance D 1  between electrode  64 A and suspended member  46  changes, and signal-sensing electrode  64 C may still be configured to detect capacitance changes as the distance D 2  between electrode  64 C and suspended member  46  changes. 
       FIG. 9  is a diagram of illustrative steps involved in forming a MEMS device (e.g., a CMOS-MEMS device of the type shown in  FIGS. 3, 4, 5, 7, and 8 ). 
     At step  300 , semiconductor substrate  62  may be provided having circuitry such as electrodes  64  and conductive pads  84 . Conductive pads  84  may be used in electrically connecting substrate  62  to MEMS structure  44 . A passivation layer such as passivation layer  66  may cover electrodes  64  and conductive pads  84 . Semiconductor substrate  62  may be fabricated using a sequence of material deposition steps, doping steps, lithography steps, and etching steps. At step  300 , passivation layer  66  may have a thickness T 1  over electrodes  64 . 
     At step  302 , a first etching step may be performed using a first mask. During the first etching step, the thickness of passivation layer  66  over electrodes  64 A,  64 B, and  64 C may be reduced from thickness T 1  to thickness T 2 . The etch depth E 1  associated with the first etching step  302  may be controlled such that a thin layer of dielectric material  66  remains on all of electrodes  64  and conductive pads  84 . Etch depth may be controlled using a timed etch or using an etch stop (e.g., a dopant etch stop, an electrochemical etch stop, etc.). The etching process of step  302  may be used to form raised protrusions in dielectric material  66  such as raised protrusions  66 S. Raised protrusions  66 S may be used to strengthen the bond region where substrate  62  is bonded to MEMS structure  44 . 
     At step  304 , a second etching step may be performed using a second mask. During the second etching step, dielectric material  66  over one or more of electrodes  64  may be removed. For example, as shown in  FIG. 9 , electrodes  64 A and  64 C may be exposed after the second etching step is performed. Conductive electrode  64 B, on the other hand, maintains a thin film of dielectric  66  over its surface in order to provide insulation between electrode  64 B and adjacent electrodes  64 A and  64 C. Outer conductive pads  84  may also be exposed during step  304 . The exposed portions B of outer conductive pads  84  may be used as bonding regions where substrate  62  is bonded to MEMS structure  44 . 
     The example of  FIG. 9  in which signal-sensing electrodes  64 A and  64 C are exposed during the second etch step  304  (e.g., to form a semiconductor substrate of the type shown in  FIG. 7 ) is merely illustrative. If desired, a thin film of dielectric material  66  may remain on electrodes  64 A and  64 C after the second etch step  304  (e.g., to form a semiconductor substrate of the type shown in  FIG. 8 ). With this type of arrangement, the second etch step  304  may include removing dielectric material  66  in bonding regions B of outer conductive pads  84  to expose portions B of pads  84 . 
     At step  306 , semiconductor substrate  62  may be bonded to a corresponding MEMS structure such as MEMS structure  44  to form MEMS device  42 . As shown in  FIG. 9 , a bonding member such as bonding member  70  may be used to bond MEMS structure  44  to semiconductor substrate  62 . Bonding member  70  may, for example, be an aluminum-germanium eutectic bond that forms a hermetic seal around the MEMS structure while also forming an electrical connection between MEMS structure  44  and semiconductor substrate  62 . As shown in  FIG. 9 , bonding member  70  is interposed between peripheral portion  44 P of MEMS structure  44  and conductive pads  84  of semiconductor substrate  62 . If desired, other bonding techniques may be used to mechanically and electrically bond semiconductor substrate  62  to MEMS structure  44  (e.g., surface bonding, metallic layer bonding, conductive adhesive bonding, etc.). The use of a eutectic bond is merely illustrative. 
     As shown in  FIG. 9 , some of the electrodes on semiconductor substrate  62  are exposed (e.g., are not covered by dielectric material  66 ), while other electrodes on semiconductor substrate  62  are insulated by dielectric material  66 . For example, non-signal-sensing electrode  64 B may be covered by dielectric material  66 , while signal-sensing electrodes  64 A and  64 C may be exposed. Because dielectric material  66  insulates electrode  64 B from electrode  64 A, particles that are released from silicon MEMS member  46  in a drop event will not cause an electrical short between electrode  64 A and electrode  64 B. Similarly, the dielectric material between electrode  64 B and electrode  64 C may prevent electrical shorts between electrode  64 B and electrode  64 C. 
     If desired, a thin layer of dielectric material  66  may cover signal-sensing electrodes  64 A and  64 C in MEMS device  42  (e.g., as shown in  FIG. 8 ). Signal-sensing electrodes  64 A and  64 C may still be used to gather signals that are produced by movement of suspended member  46  even when covered by a thin layer of dielectric material  66 . 
       FIG. 10  is a diagram of illustrative steps involved in forming a MEMS device (e.g., a CMOS-MEMS device of the type shown in  FIGS. 3, 4, 5, 7, and 8 ) according to another suitable method. Similar to the method of  FIG. 9 , the method of  FIG. 10  includes first and second etching steps. In the arrangement of  FIG. 10 , however, a dielectric deposition step takes place between the first and second etching steps. 
     At step  400 , semiconductor substrate  62  may be provided having circuitry such as electrodes  64  and conductive pads  84 . Conductive pads  84  may be used in electrically connecting substrate  62  to MEMS structure  44 . A passivation layer such as passivation layer  66  may cover electrodes  64  and conductive pads  84 . Semiconductor substrate  62  may be fabricated using a sequence of material deposition steps, doping steps, lithography steps, and etching steps. 
     At step  402 , a first etching step may be performed using a first mask. During the first etching step, portions of passivation layer  66  over electrodes  64 A,  64 B, and  64 C may be removed to expose electrodes  64 A,  64 B, and  64 C. Portions of passivation layer  66  over pads  84  may also be removed. The etch depth E 1  associated with the first etching step  402  may be controlled such that the etch stops at the interface between passivation layer  66  and metal electrodes  64 . Etch depth may be controlled using a timed etch or using an etch stop (e.g., a dopant etch stop, an electrochemical etch stop, etc.). The etching process of step  402  may be used to form raised protrusions in dielectric material  66  such as raised protrusions  66 S. Raised protrusions  66 S may be used to strengthen the bond region where substrate  62  is bonded to MEMS structure  44 . 
     At step  404 , a second passivation layer such as passivation layer  66 ′ may be deposited over the surface of substrate  62 . Passivation layer  66 ′ may be formed form the same dielectric material as layer  66  or may be formed from a different dielectric material. As shown in  FIG. 10 , passivation layer  66 ′ covers the surfaces of electrodes  64 A,  64 B, and  64 C and the surfaces of pads  84 . 
     At step  406 , a second etching step may be performed using a second mask. During the second etching step, dielectric material  66 ′ over one or more of electrodes  64  may be removed. For example, as shown in  FIG. 10 , electrodes  64 A and  64 C may be exposed after the second etching step is performed. Conductive electrode  64 B, on the other hand, maintains a thin film of dielectric  66 ′ over its surface in order to provide insulation between electrode  64 B and adjacent electrodes  64 A and  64 C. Outer conductive pads  84  may also be exposed during step  406 . The etch depth associated with the second etching step  406  may be controlled such that the etch stops at the interface between passivation layer  66 ′ and metal electrodes  64 . The exposed portions B of outer conductive pads  84  may be used as bonding regions where substrate  62  is bonded to MEMS structure  44 . 
     The example of  FIG. 10  in which signal-sensing electrodes  64 A and  64 C are exposed during the second etch step  406  (e.g., to form a semiconductor substrate of the type shown in  FIG. 7 ) is merely illustrative. If desired, a thin film of dielectric material  66 ′ may remain on electrodes  64 A and  64 C after the second etch step  406  (e.g., to form a semiconductor substrate of the type shown in  FIG. 8 ). With this type of arrangement, the second etch step  406  may include removing dielectric material  66 ′ in bonding regions B of outer conductive pads  84  to expose portions B of pads  84 . 
     At step  408 , semiconductor substrate  62  may be bonded to a corresponding MEMS structure such as MEMS structure  44  to form MEMS device  42 . As shown in  FIG. 10 , a bonding member such as bonding member  70  may be used to bond MEMS structure  44  to semiconductor substrate  62 . Bonding member  70  may, for example, be an aluminum-germanium eutectic bond that forms a hermetic seal around the MEMS structure while also forming an electrical connection between MEMS structure  44  and semiconductor substrate  62 . As shown in  FIG. 10 , bonding member  70  is interposed between peripheral portion  44 P of MEMS structure  44  and conductive pads  84  of semiconductor substrate  62 . If desired, other bonding techniques may be used to mechanically and electrically bond semiconductor substrate  62  to MEMS structure  44  (e.g., surface bonding, metallic layer bonding, conductive adhesive bonding, etc.). The use of a eutectic bond is merely illustrative. 
     As shown in  FIG. 10 , some of the electrodes on semiconductor substrate  62  are exposed (e.g., are not covered by dielectric material  66  or  66 ′), while other electrodes on semiconductor substrate  62  are insulated by dielectric material  66 ′. For example, non-signal-sensing electrode  64 B may be covered by dielectric material  66 ′, while signal-sensing electrodes  64 A and  64 C may be exposed. Because dielectric material  66  insulates electrode  64 B from electrode  64 A, particles that are released from silicon MEMS member  46  in a drop event will not cause an electrical short between electrode  64 A and electrode  64 B. Similarly, the dielectric material between electrode  64 B and electrode  64 C may prevent electrical shorts between electrode  64 B and electrode  64 C. 
     If desired, a thin layer of dielectric material  66 ′ may cover signal-sensing electrodes  64 A and  64 C in MEMS device  42  (e.g., as shown in  FIG. 8 ). Signal-sensing electrodes  64 A and  64 C may still be used to gather signals that are produced by movement of suspended member  46  even when covered by a thin layer of dielectric material  66 ′. 
     The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention.

Metadata:
Filing Date: 20140930
Publication Date: 20171024
Grant Date: 20171024
Priority Date: 20140310
Inventors: CHEN KUAN-LIN
YEH RICHARD
Assignee: APPLE INC
CPC Classifications: [{"code": "B81B3/0086", "inventive": true, "first": true, "tree": "[]"}, {"code": "B81B3/0086", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 56093650