PATENT DOCUMENT

Publication Number: US-9510110-B2
Application Number: US-201514611948-A
Country: US
Kind Code: B2

Title: Open top back plate optical microphone

Abstract:
A micro-electro-mechanical system (MEMS) optical sensor and method of manufacturing a MEMS optical sensor. The MEMS optical sensor may be a MEMS optical microphone including a compliant membrane configured to vibrate in response to an acoustic wave, the compliant membrane having a grating suspended therein. The optical sensor further including a back plate positioned above the compliant membrane, the back plate having a reflector suspended within a center portion of the back plate and aligned with the grating. The optical sensor further including a light emitter positioned below the compliant membrane and configured to transmit a laser light toward the grating and the reflector. The optical sensor also including a light detector configured to detect an interference pattern of the laser light after reflection from the reflector, wherein the interference pattern is indicative of an acoustic vibration of the compliant membrane. Other embodiments are also described and claimed.

Claims:
What is claimed is: 
     
       1. A micro-electro-mechanical system (MEMS) optical microphone comprising:
 a substrate; 
 a compliant bottom plate positioned above the substrate, the bottom plate configured to vibrate in response to an acoustic wave and having a grating suspended therein; 
 a rigid top plate positioned above the bottom plate, the top plate having a reflector suspended therein; 
 a light emitter positioned on the substrate, the light emitter configured to transmit a laser light toward the grating and the reflector; and 
 a light detector positioned on the substrate, the light detector configured to detect an interference pattern of the laser light after reflection from the reflector, wherein the interference pattern is indicative of an acoustic vibration of the bottom plate. 
 
     
     
       2. The MEMS optical microphone of  claim 1  wherein the grating is suspended within the bottom plate by a spring. 
     
     
       3. The MEMS optical microphone of  claim 2  wherein the spring is configured to reduce a tension on the grating. 
     
     
       4. The MEMS optical microphone of  claim 1  wherein the grating is suspended within a center portion of the bottom plate. 
     
     
       5. The MEMS optical microphone of  claim 1  wherein the bottom plate comprises an opening formed around the grating. 
     
     
       6. The MEMS optical microphone of  claim 1  wherein an area around the reflector comprises a plurality openings, and wherein each opening is defined by a space between two spokes that extend from the reflector to a periphery or boundary portion of the top plate that is affixed to a support member. 
     
     
       7. The MEMS optical microphone of  claim 1  wherein the reflector is within the same plane as the top plate. 
     
     
       8. The MEMS optical microphone of  claim 1  wherein the reflector is suspended within a frame of the top plate by a plurality of spokes. 
     
     
       9. A micro-electro-mechanical system (MEMS) optical microphone comprising:
 a substrate; 
 a diaphragm positioned above the substrate, the diaphragm having a spring suspended grating formed therein; 
 a back plate positioned above the diaphragm, the back plate having an opening, and a reflector is suspended within the opening by a plurality of spokes; 
 a light emitter positioned below the diaphragm, the light emitter configured to transmit a laser light through the grating and toward the reflector; and 
 a light detector positioned below the diaphragm, the light detector configured to detect an interference pattern of the laser light after reflection from the reflector. 
 
     
     
       10. The MEMS optical microphone of  claim 9  wherein the grating is larger than the reflector. 
     
     
       11. The MEMS optical microphone of  claim 9  wherein an area of the back plate around the reflector is substantially open such that the reflector and the grating can be visually aligned from a top side of, or above, the back plate. 
     
     
       12. The MEMS optical microphone of  claim 9  wherein the back plate comprises a frame from which the reflector is suspended within a plane of the back plate by the plurality of spokes. 
     
     
       13. The MEMS optical microphone of  claim 9  wherein the back plate and the reflector are substantially rigid structures. 
     
     
       14. A method of manufacturing a micro-electro-mechanical system (MEMS) optical microphone comprising:
 providing a substrate; 
 forming a compliant membrane over the substrate, the compliant membrane having a grating; 
 forming a rigid back plate over the compliant membrane, the back plate having an inner plate suspended from an outer portion of the back plate; and 
 applying a reflective coating to the grating and the inner plate by introducing a reflective coating material from a top side of the back plate. 
 
     
     
       15. The method of  claim 14  wherein an opening is formed around the inner plate such that the reflective coating material passes through the back plate to the compliant membrane. 
     
     
       16. The method of  claim 14  wherein forming the compliant membrane comprises forming a suspension member around the grating, wherein the suspension member is configured to reduce a tension on the grating. 
     
     
       17. The method of  claim 16  wherein the suspension member is a spring. 
     
     
       18. The method of  claim 14  wherein forming the back plate comprises forming a spoke within the back plate for suspension of the inner plate within a center portion of the back plate. 
     
     
       19. The method of  claim 14  wherein the compliant membrane and the back plate are formed such that the grating and the inner plate are vertically aligned. 
     
     
       20. The method of  claim 14  wherein the substrate is a single substrate upon which the compliant membrane and the back plate are both formed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the earlier filing date of U.S. Provisional Patent Application No. 62/021,624, filed Jul. 7, 2014 and incorporated herein by reference. 
    
    
     FIELD 
     An embodiment of the invention is directed to a micro-electro-mechanical system (MEMS) device, more specifically, a MEMS optical microphone having a substantially open back plate with a reflective surface and a grating formed in a diaphragm. Other embodiments are also described and claimed. 
     BACKGROUND 
     MEMS devices generally range in size from about 20 micrometers to about 1 millimeter and are made up of a number of even smaller components which can be formed in layers on a substrate using various MEMS processing techniques (e.g. deposition processes, patterning, lithography, etching, etc.). MEMS devices can be processed for many different applications, for example, they may be sensors or actuators. One example of a MEMS sensor is a laser microphone. A MEMS laser, or optical, microphone refers to a microphone which uses a laser beam to detect sound vibrations of an associated diaphragm. The microphone may include two essentially flat, horizontally arranged, surfaces. One of the surfaces may be a diaphragm, which can vibrate in response to sound waves, and the other surface may be a substantially stiff structure having a grating. A light emitter and a light detector may be associated with a substrate positioned below the flat surfaces. The light emitter may be a laser (e.g. a vertical cavity surface emitting laser (VCSEL)) configured to direct a light beam toward a reflective portion of the diaphragm. Typically, the substantially stiff structure having the grating is positioned between the diaphragm and the light emitter such that the light beam first passes through the grating. The light beam is diffracted by the grating and then reflected off of the reflective portion of the diaphragm back to the light detector. The light detector detects the interference pattern created by the diffracted light rays and converts the light into an electrical signal, which corresponds to an acoustic vibration of the diaphragm, which in turn provides an indication of sound. 
     SUMMARY 
     An embodiment of the invention is directed to a MEMS sensor which can be formed by MEMS processing techniques and includes one or more plates. Representatively, in one embodiment, the MEMS sensor is a very high signal-to-noise ratio (SNR) laser (or optical) microphone having a grating suspended in one plate and a reflector suspended in another plate. The plate having the grating may be a compliant membrane that serves as a microphone diaphragm. The plate having the reflector may be a substantially rigid back plate, which is positioned above or over the compliant membrane. The grating may be suspended within the compliant membrane by suspension members. The suspension members may help to reduce stress on the compliant membrane, and in turn, reduce, minimize, or perhaps eliminate, bowing of the grating. The back plate may include spokes which suspend the reflector within a center portion of the back plate. Both the compliant membrane and the back plate may include openings around the grating and the reflector, respectively, which allow for a coating (e.g. a gold coating) to be applied to the grating and the reflector, from above or the top of the back plate. Since the coating can be applied to both the grating and the reflector from the top structure, the compliant membrane and back plate can be formed from a single wafer (e.g. substrate), as opposed to separate wafers (one being a back plate and the other being a diaphragm) which are patterned into the desired plate or membrane or layer, and then attached together. The optical microphone may further include a light emitter and a light detector mounted to, or formed within, a substrate. The light emitter may be positioned such that it directs a light ray or beam toward the grating and reflector. The light detector may be positioned such that it detects an interference pattern of the laser light after reflection from the reflector. 
     A process for manufacturing a MEMS optical microphone may include providing a substrate and forming a compliant membrane over the substrate. A grating may further be formed in the compliant membrane. The process may further include forming a back plate over the compliant membrane. A center plate may be formed in the back plate. A reflective coating may be applied to the grating and the center plate by introducing a reflective coating material from a top side of, or above, the back plate. 
     The above summary does not include an exhaustive list of all aspects of the present invention. It is contemplated that the invention includes all systems and methods that can be practiced from all suitable combinations of the various aspects summarized above, as well as those disclosed in the Detailed Description below and particularly pointed out in the claims filed with the application. Such combinations have particular advantages not specifically recited in the above summary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and they mean at least one. 
         FIG. 1  illustrates a cross-sectional side view of one embodiment of a MEMS optical microphone. 
         FIG. 2  illustrates a top plan view of a back plate of the MEMS optical microphone of  FIG. 1 . 
         FIG. 3  illustrates a top plan view of a compliant membrane of the MEMS optical microphone of  FIG. 1 . 
         FIG. 4  illustrates a top plan view of the MEMS optical microphone of  FIG. 1 . 
         FIG. 5A  illustrates one embodiment of a processing step for fabricating the optical microphone of  FIG. 1 . 
         FIG. 5B  illustrates one embodiment of another processing step for fabricating the optical microphone of  FIG. 1 . 
         FIG. 5C  illustrates one embodiment of another processing step for fabricating the optical microphone of  FIG. 1 . 
         FIG. 5D  illustrates one embodiment of another processing step for fabricating the optical microphone of  FIG. 1 . 
         FIG. 5E  illustrates one embodiment of another processing step for fabricating the optical microphone of  FIG. 1 . 
         FIG. 5F  illustrates one embodiment of another processing step for fabricating the optical microphone of  FIG. 1 . 
         FIG. 5G  illustrates one embodiment of another processing step for fabricating the optical microphone of  FIG. 1 . 
         FIG. 6  illustrates one embodiment of a simplified schematic view of one embodiment of an electronic device in which the optical microphone may be implemented. 
         FIG. 7  illustrates a block diagram of some of the constituent components of an embodiment of an electronic device in which an embodiment of the invention may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     In this section we shall explain several preferred embodiments of this invention with reference to the appended drawings. Whenever the shapes, relative positions and other aspects of the parts described in the embodiments are not clearly defined, the scope of the invention is not limited only to the parts shown, which are meant merely for the purpose of illustration. Also, while numerous details are set forth, it is understood that some embodiments of the invention may be practiced without these details. In other instances, well-known structures and techniques have not been shown in detail so as not to obscure the understanding of this description. 
       FIG. 1  illustrates a cross-sectional side view of one embodiment of a MEMS optical microphone. Microphone  100  may include back plate  102  (also referred to herein as an upper or top plate), a compliant membrane  104  (also referred to herein as a lower or bottom plate), a light emitter  124 , a light detector  126  and circuitry  128  formed on substrate  110 . It should be understood that although back plate  102  may be referred to as a top plate, it may not be at the highest end of the microphone structure, rather, just higher than, for example, compliant membrane  104 . Similarly, although compliant membrane  104  may be referred to as a bottom plate, it may not be at the lowest end of microphone structure, rather, just lower than, for example, back plate  102 . Each of back plate  102  and compliant membrane  104  are parallel to one another and extend horizontally between vertically extending support members  112 A or  112 B of substrate  110 . In one embodiment, vertically extending support members  112 A and  112 B may be sidewalls of a cavity  114  which is pre-formed within substrate  110  before each of back plate  102  and compliant membrane  104  are formed using MEMS processing techniques (e.g. deposition processes, patterning, lithography, etching, etc.). Back plate  102  and compliant membrane  104  may be fixedly attached to support members  112 A and  112 B at their ends such they maintain a fixed vertical position. In one embodiment, back plate  102  may be positioned over, or above, compliant membrane  104  and compliant membrane  104  may be positioned over, or above, substrate  110 . In other words, compliant membrane  104  is positioned between back plate  102  and base portion  140  of substrate  110 . 
     Back plate  102  may be a substantially rigid plate which provides a reflective surface for light emitted from light emitter  124 . For example, back plate  102  may be made of a thick and stiff silicon plate. Back plate  102  is considered “rigid” relative to, for example, compliant membrane  104 , which is not considered rigid, but rather compliant in that it can vibrate to achieve acoustic pick up as will be described in more detail below. Back plate  102  may be considered an upper plate or top plate because it is above compliant membrane  104 . Back plate  102  may include an outer frame portion  118  and a center portion  116  which is suspended within frame portion  118 . Center portion  116  and frame  118  may be within the same plane, in other words, within a plane of back plate  102 . Center portion  116  may include a reflective surface  134  formed along a side facing light emitter  124 . In this aspect, center portion  116  serves as a reflector for light emitted by light emitter  124  and may be referred to herein as a reflector. In some embodiments, the center portion  116  is made of a reflective material (e.g. metallic foil) while in other embodiments, reflective surface  134  is formed by application of a coating (e.g. metal coating such as gold) to center portion  116 . Although reflective surface  134  is shown positioned only within center portion  116 , it is contemplated that the reflective surface may extend beyond center portion. Back plate  102 , including center portion  116  and reflective surface  134 , may be built upon substrate  110  using MEMS processing techniques (e.g. deposition processes, patterning, lithography, etching, etc.). 
     Compliant membrane  104  may be positioned below back plate  102  (i.e. between back plate  102  and base portion  140  of substrate  110 ) and may therefore be considered a lower or bottom plate. Compliant membrane  104  may be configured to vibrate in response to sound (S) (acoustic waves) entering enclosure  120  through acoustic port  122 . In this aspect, compliant membrane  104  may also be referred to as a diaphragm or sound pick up surface. Compliant membrane  104  may be made of any material and have any dimensions suitable to provide a semi-rigid or compliant membrane that vibrates in response to sound waves, for example, polysilicon. 
     Compliant membrane  104  may include a grating  106 . Grating  106  may be vertically aligned with center portion  116  including reflective surface  134 . In other words, grating  106  is aligned with the reflector formed within back plate  102 . Grating  106  is also aligned with light emitter  124  and light detector  126  such that light emitted by light emitter  124  toward, and reflected from, reflective surface  134  of back plate  102  passes through grating  106 . Grating  106  is dimensioned to form an interference pattern that can be detected by light detector  126  and used as an indicator of a movement of compliant membrane  104 . Since the pattern represents a displacement of the compliant membrane  104 , it can be used to provide an indication of sound using a diffraction based optical interferometer method or any other optical interferometric method. Representatively, in some embodiments, grating  106  may also include a reflective coating  136  to facilitate formation of the interference pattern. 
     Grating  106  may be suspended within compliant membrane  104  by suspension members  108 A and  108 B. Representatively, compliant membrane  104  may include a frame portion  138  having an open center. Grating  106  may be suspended within the open center by suspension members  108 A and  108 B. Suspension members  108 A and  108 B may be any type of suspension structure having some degree of elasticity such that a tension (e.g. outward pull) on grating  106  may be reduced, as compared to a membrane or plate having a grating that is not connected to the membrane or plate by a suspension member. Representatively, suspension members  108 A and  108 B may be spring type structures which can expand and contract in response to an outward tension on grating  106  which could be caused by compliant membrane  104 . In this aspect, a bowing of grating  106 , which can be caused by an outward tension, can be reduced, minimized or eliminated. 
     Compliant membrane  104 , including grating  106  and suspension members  108 A- 108 B, may be built upon substrate  110  using MEMS processing techniques (e.g. deposition processes, patterning, lithography, etching, etc.). 
     Microphone  100  may further include a light emitter  124  and a light detector  126 . In some embodiments, light emitter  124  may be a light source such as a VCSEL that is electrically connected to substrate  110 . Light emitter  124  may be configured to emit a laser light (or beam) in the direction of grating  106  and reflective surface  134 , for detection by detector  126 . Detector  126  may, in some embodiments, be a photo detector configured to detect a reflected light (or beam) generated by emitter  124 . Emitter  124  and/or detector  126  may be mounted to, or formed from, substrate  110  using MEMS processing techniques. 
     Representatively, during operation, detector  126  detects light reflected off of grating  106  and reflective surface  134  to provide an indication of sound. In particular, compliant membrane  104  vibrates in response to sound (S). The vibration of compliant membrane  104  modulates an intensity of light  160  reflected off of the reflective surface  134  and grating  106  of compliant membrane  104 . In addition, movement of compliant membrane  104  with respect to back plate  102  (which is rigid) causes an interference pattern formed by grating  106  to change in size. This modulation in intensity (i.e. change in size of the interference pattern) is detected by detector  126  and used as an indication of the movement of compliant membrane  104  and in turn, provides an indication of sound. It is further to be understood that in order to determine sound from the interference pattern, a distance between compliant membrane  104  and back plate  102  is set such that it is an integer multiple of ¼ λ of the light  160 . 
     Microphone  100  may further include a circuit  128  (e.g. an application specific integrated circuit (ASIC)) electrically connected to light emitter  124  and light detector  126  by wiring  130 ,  132 , respectively. In addition, circuit  128  may include wiring  142 ,  144  connected to back plate  102  and compliant membrane  104 , respectively. Wiring  130 ,  132 ,  142 ,  144  may run through substrate  110  to the respective light emitter  124 , light detector  126 , back plate  102  and compliant membrane  104 . In one embodiment, circuit  128  may be configured to receive power from an external source and apply a voltage to one or more of light emitter  124 , light detector  126 , back plate  102  and compliant membrane  104 . For example, in one embodiment, wiring  142 ,  144  may be used to apply a voltage to one or more of back plate  102  and compliant membrane  104  to tune a distance (e.g. change the distance) between the back plate  102  and compliant membrane  104  so as to improve a resonance of an interference pattern used to provide an indication of sound, as will be discussed in more detail below. 
     Each of back plate  102  and compliant membrane  104 , and in some cases emitter  124  and detector  126 , may be built on substrate  110  using MEMS processing techniques. Substrate  110  may be mounted within a frame or enclosure  120 . Enclosure  120  may include an acoustic port  122  through which sound (S) (also referred to as acoustic waves) can travel into microphone  100 . Although acoustic port  122  is illustrated along a top side of enclosure  120 , it could also be along a bottom side or side wall of enclosure  120  and therefore is not limited to the illustrated location. 
       FIG. 2  illustrates a top plan view of the back plate of the optical microphone of  FIG. 1 . From this view, it can be seen that back plate  102  may have a center portion  116  (such as an inner plate or center plate) suspended within an opening  206  of outer frame  118  by arms or spokes  204 A,  204 B,  204 C and  204 D. In this aspect, the area around center portion  116 , and between spokes  204 A- 204 D, is considered open. For example, openings  206 A,  206 B,  206 C and  206 D are formed between spokes  204 A- 204 D and around center portion  116  such that back plate  102  is considered a substantially open structure. Each of frame  118 , center portion  116  and spokes  204 A- 204 D may be substantially rigid structures formed from a single back plate material layer (e.g. a silicon material layer). In this aspect, back plate  102  having frame  118 , center portion  116  and spokes  204 A- 204 D is considered a single, integrally formed structure. In addition, each of the frame  118 , center portion  116  and spokes  204 A- 204 D are all within the same plane. In this aspect, it should be understood that by referring to center portion  116  as being suspended within frame  118 , center portion  116  is considered level with frame  118 . Alternatively, center portion  116  could be suspended above or below frame  118  by spokes  204 A- 204 D. 
     In one embodiment, center portion  116  is a substantially square shaped plate upon which the reflective surface  134  is applied. In this aspect, although center portion  116  is shown as a square shaped structure, center portion  116  may have any dimensions sufficient to reflect light generated by the light emitter. Representatively, in other embodiments, center portion  116  may have any type of quadrilateral shape, or other shapes, for example, a circle, ellipse, oval or the like. In the case of a square shaped center portion  116 , each of spokes  204 A- 204 D may extend from a respective side of center portion  116  to frame  118 . Frame  118 , may in turn, be a square shaped structure. Each of the sides of frame  118  may run parallel to a respective side of center portion  116 . In other embodiments, spokes  204 A- 204 D and frame  118  may be oriented in any manner with respect to center portion  116  that is sufficient to suspend center portion  116  above compliant membrane  104  and emitter  124 /detector  126  as previously discussed. Representatively, spokes  204 A- 204 D may extend from corners of center portion  116  to corners of frame  118 . 
       FIG. 3  illustrates a top plan view of a compliant membrane of the MEMS optical microphone of  FIG. 1 . From this view, it can be seen that compliant membrane  104  may have a similar size and shape as back plate  102 , for example, a square shape. Alternatively, compliant membrane  104  may have any type of quadrilateral shape, or other shapes, for example, a circle, ellipse, oval or the like. 
     Grating  106  may be formed within a center portion of compliant membrane  104 . Grating  106  may have a periodic structure sufficient to split and diffract light emitted from an emitter (e.g. emitter  124 ) into different beams for detection by a detector (e.g. detector  126 ). In some embodiments, the grating  106  causes the formation of an interference pattern which can be used to indicate a movement of compliant membrane  104  in response to sound waves, and in turn, as an indicator of sound. Grating  106  may be formed in a portion of compliant membrane  104  that is aligned with center portion  116  of back plate  102 . For example, complaint membrane  104  may have an outer frame  138  with a center opening  302 . Grating  106  may be coated with a reflective coating  136  and suspended within center opening  302  by suspension members  108 A,  108 B,  108 C and  108 D. 
     Suspension members  108 A- 108 D may, in one embodiment, be spring type structures having an elasticity that helps to reduce a tension on grating  106 . Representatively, in some cases, a grating within a plate or membrane can be subjected to an outward tension or pull that causes the grating to bow. Since suspension members  108 A- 108 D have an elasticity, they can absorb this pull thereby reducing a tension on grating  106  and, in turn, possible bowing. 
     Suspension members  108 A- 108 D can be made from the same compliant material layer used to form compliant membrane  104  and grating  106  such that the entire compliant membrane structure  104  is one integrally formed membrane. The material and/or dimensions of suspension members  108 A- 108 D may be selected to provide elasticity to the members. For example, in one embodiment, suspension members  108 A- 108 D may be corrugated structures which can expand and contract. In some embodiments, suspension members  108 A- 108 D may be relatively narrow structures such that the area  304  between grating  106  and frame  138  remains substantially open to fluid flow, for example, a gas such as air or a liquid. It is noted that since air is free to flow through compliant membrane  104  (e.g. through grating  106  and the open area  304  around grating  106 ) and back plate  102  (e.g. through openings  206 A- 206 D), there is less of a “squeeze film” effect. The squeeze film effect refers to a phenomenon that occurs when air passes between two plates in close proximity. As a result, the noise penalty due to the squeeze film effect is reduced. 
       FIG. 4  illustrates a top plan view of the MEMS optical microphone of  FIG. 1 . From this view, it can be seen that visual alignment of the grating  106  of compliant membrane  104  and center portion  116  (which forms the reflective surface  134 ) of back plate  102 , and in some cases light emitter  124 , are possible through the top side of microphone  100 . In particular, because a substantial portion of back plate  102  remains open around center portion  116 , the underlying grating  106  can be viewed and aligned with center portion  116  from a top side of microphone  100 . Back plate  102  can therefore be considered an “open top” back plate because it is on top of compliant membrane  104  and substantially open. In addition, in some embodiments, grating  106  is larger than center portion  116 , and in turn the reflector formed by reflective surface  134 , such that the location of grating  106  with respect to center portion  116  (i.e. the reflector) can be clearly seen from above. Said another way, when viewed from a top side as shown in  FIG. 4 , grating  106  has a larger overall profile or footprint than the reflector portion (i.e. center portion  116 ) such that the edges defining grating  106  can be viewed from above. In other words, the reflector has a smaller overall footprint than grating  106 . 
       FIG. 5A  illustrates one embodiment of a processing step for fabricating the optical microphone of  FIG. 1 .  FIG. 5A  illustrates substrate  502  having a cavity  501  formed therein. Substrate  502  may be a silicon substrate, for example, a silicon on insulator (SOI) wafer. Cavity  501  may be defined by vertically extending support member  504 A and vertically extending support member  504 B and a base portion  503  of substrate  502 . In one embodiment, cavity  501  is formed within substrate  502  using a MEMS etching process, for example, reactive ion etching (RIE). Alternatively, cavity  501  may be formed on top of substrate  502  by stacking additional material layers and then patterning the layers to form cavity  501 . MEMS microphone  100  may be formed within cavity  501 . 
     Representatively, in one embodiment, a sacrificial layer  506  may be formed on top of the base portion  503  of substrate  502 . Sacrificial layer  506  may be formed by any MEMS processing technique suitable for forming a sacrificial layer. For example, sacrificial layer  506  may be formed by blanket depositing a sacrificial material over substrate  502  using a chemical vapor deposition (CVD) process and then planarizing the layer to provide a desired layer thickness. Sacrificial layer  506  may be made of any material that can be selectively removed or patterned using MEMS processing steps. Representatively, sacrificial layer  506  may be made of silicon dioxide or a silicate glass. 
     Compliant membrane layer  508  may be formed over sacrificial layer  506 . Compliant membrane layer  508  may be formed by any MEMS processing technique suitable for forming a compliant membrane layer, for example, blanket depositing a compliant membrane layer material using CVD. Compliant membrane layer  508  may be made of any material suitable for forming a compliant membrane that vibrates in response to acoustic waves as previously discussed in reference to  FIG. 1 . Representatively, compliant membrane layer  508  may be made of a material capable of forming a membrane that can function as a microphone diaphragm (e.g. capable of vibrating in response to acoustic waves) or sound pick up membrane, for example, a polysilicon material. 
       FIG. 5B  illustrates one embodiment of another processing step for fabricating the optical microphone of  FIG. 1 .  FIG. 5B  shows compliant membrane layer  508  after a processing step in which portions of compliant membrane layer  508  are removed to form a structure suitable for use as a compliant membrane within microphone  100 . For example, compliant membrane layer  508  may be patterned using different etching steps (e.g. reactive ion etching) to have the shape and dimensions of compliant membrane  104  described in reference to  FIG. 1 . Representatively, where compliant membrane  508  is to be used as both a sound pick up surface (e.g. a microphone diaphragm) and a grating to form an interference pattern that can be detected by a light detector to provide an indication of a movement of compliant membrane  104 , compliant membrane layer  508  is patterned to have grating  512  suspended with a frame portion  510  by suspension members  514 A and  514 B. Grating  512 , frame portion  510  and suspension members  514 A- 514 B may be formed using MEMS processing techniques such that they are substantially similar to grating  106 , frame portion  138  and suspension members  108 A- 108 B, respectively, previously discussed in reference to  FIG. 1 . 
       FIG. 5C  illustrates one embodiment of another processing step for fabricating the optical microphone of  FIG. 1 .  FIG. 5C  illustrates the step of forming a sacrificial layer  516  over compliant membrane layer  508 . Sacrificial layer  516  may be formed using any MEMS processing step suitable for forming a sacrificial layer over another layer. For example, sacrificial layer  516  may be formed by blanket depositing a sacrificial layer material over compliant membrane layer  508  and sacrificial layer  506  using CVD. Sacrificial layer  516  may be substantially similar to sacrificial layer  506 . Sacrificial layer  516  may be of any material that can be selectively removed during a further processing step (e.g. silicon dioxide or silicate glass). 
       FIG. 5D  illustrates one embodiment of another processing step for fabricating the optical microphone of  FIG. 1 .  FIG. 5D  illustrates the step of forming a back plate layer  518  over sacrificial layer  516 . Back plate layer  518  may be formed by any MEMS processing step suitable for forming a back plate layer over sacrificial layer  516 . For example, back plate layer  518  may be formed by blanket depositing a back plate layer material over sacrificial layer  516  using CVD. A suitable back plate layer material may be, for example, a silicon material capable of forming a substantially rigid layer that can function as a substantially rigid reflective surface during operation of the microphone. 
       FIG. 5E  illustrates one embodiment of another processing step for fabricating the optical microphone of  FIG. 1 .  FIG. 5E  shows back plate layer  518  after a processing step in which portions of back plate layer  518  are removed to form a structure suitable for use as a back plate within microphone  100 . For example, back plate layer  518  is processed using MEMS processing techniques to have the shape and dimensions of back plate  102  described in reference to  FIG. 1 . Representatively, an RIE processing technique may be used to pattern back plate layer  518  to include a frame portion  520  and center portion (or center plate)  522 . Frame portion  520  and center portion  522  may be substantially similar to frame portion  118  and center portion  116  previously discussed in reference to  FIG. 1 . For example, the center portion  522  may be a plate suspended within an opening of frame  520  by spokes as previously discussed in more detail in reference to  FIG. 2 . In this aspect, once the sacrificial layers  516  and  506  are removed, a substantially open back plate layer  518  is formed over compliant membrane layer  508 . 
       FIG. 5F  illustrates one embodiment of another processing step for fabricating the optical microphone of  FIG. 1 .  FIG. 5F  illustrates a processing step in which sacrificial layers  506  and  516  have been removed, for example, by a wet or dry etch processing technique. For example, layers  506  and  516  may be removed using a wet etching step with a selective wet etchant including hydrofluoric acid (HF). The wet etchant (HF) etches away sacrificial layers  506  and  516  without etching, or otherwise damaging, the various layers needed to form the microphone, for example, compliant membrane layer  508  and back plate layer  518 . In some embodiments, an opening is formed through substrate  503  and the etchant is introduced through the opening to an underside of complaint membrane layer  508  and back plate layer  518 , while in other embodiments the etchant is applied from above compliant back plate layer  518 . The etchant reaches sacrificial layers  506  and  516  by flowing through the openings in back plate layer  518  and complaint membrane layer  508 . 
       FIG. 5G  further illustrates the step of applying a reflective surface  532  to center portion  522  and a reflective surface  536  to grating  512 . Representatively, in one embodiment, reflective surfaces  532  and  536  are formed by introducing a reflective material  530  (e.g. gold coating) from above back plate layer  518  and through the openings in back plate layer  518  and compliant membrane layer  508  in a manner that allows material  530  to coat center portion  522  and grating  512 . In this aspect, top side metallization may be used to form the reflective surfaces. Since top side metallization can be used, masking steps typically used for bottom side metallization techniques are not necessary. 
     Once each of the layers necessary for operation of microphone  500  are formed, an emitter (e.g. emitter  124 ) and detector (e.g. detector  126 ) can be positioned on substrate  502 , for example on base portion  503 , such that they are aligned with grating  512  and reflective surface  532  formed on center portion  522 . In one embodiment, emitter and detector may be formed monolithically on another substrate using standard MEMS processing techniques, and then positioned within or on substrate base portion  503 . Microphone  500  may then be mounted within an enclosure (e.g. enclosure  120 ) which can in turn be mounted within the desired electronic device. In addition, any circuitry (e.g. wires) connected to the various microphone components, for example, the emitter or the detector may be pre-formed within substrate  502  such that when the components are formed, the circuitry is connected to the components. 
       FIG. 6  illustrates one embodiment of a simplified schematic view of one embodiment of an electronic device in which a MEMS optical microphone, or other MEMS device described herein, may be implemented. As seen in  FIG. 6 , the MEMS device may be integrated within a consumer electronic device  602  such as a smart phone with which a user can conduct a call with a far-end user of a communications device  604  over a wireless communications network; in another example, the MEMS device may be integrated within the housing of a tablet computer. These are just two examples of where the MEMS device described herein may be used, it is contemplated, however, that the MEMS device may be used with any type of electronic device in which a MEMS device, for example, an optical MEMS microphone, is desired, for example, a tablet computer, a desk top computing device or other display device. 
       FIG. 7  illustrates a block diagram of some of the constituent components of an embodiment of an electronic device in which an embodiment of the invention may be implemented. Device  700  may be any one of several different types of consumer electronic devices. For example, the device  700  may be any microphone-equipped mobile device, such as a cellular phone, a smart phone, a media player, or a tablet-like portable computer. 
     In this aspect, electronic device  700  includes a processor  712  that interacts with camera circuitry  706 , motion sensor  704 , storage  708 , memory  714 , display  722 , and user input interface  724 . Main processor  712  may also interact with communications circuitry  702 , primary power source  710 , speaker  718 , and microphone  720 . Microphone  720  may be an optical microphone such as optical microphone  100  such as that described in reference to  FIG. 1 . The various components of the electronic device  700  may be digitally interconnected and used or managed by a software stack being executed by the processor  712 . Many of the components shown or described here may be implemented as one or more dedicated hardware units and/or a programmed processor (software being executed by a processor, e.g., the processor  712 ). 
     The processor  712  controls the overall operation of the device  700  by performing some or all of the operations of one or more applications or operating system programs implemented on the device  700 , by executing instructions for it (software code and data) that may be found in the storage  708 . The processor  712  may, for example, drive the display  722  and receive user inputs through the user input interface  724  (which may be integrated with the display  722  as part of a single, touch sensitive display panel). In addition, processor  712  may send an audio signal to speaker  718  to facilitate operation of speaker  718 . 
     Storage  708  provides a relatively large amount of “permanent” data storage, using nonvolatile solid state memory (e.g., flash storage) and/or a kinetic nonvolatile storage device (e.g., rotating magnetic disk drive). Storage  708  may include both local storage and storage space on a remote server. Storage  708  may store data as well as software components that control and manage, at a higher level, the different functions of the device  700 . 
     In addition to storage  708 , there may be memory  714 , also referred to as main memory or program memory, which provides relatively fast access to stored code and data that is being executed by the processor  712 . Memory  714  may include solid state random access memory (RAM), e.g., static RAM or dynamic RAM. There may be one or more processors, e.g., processor  712 , that run or execute various software programs, modules, or sets of instructions (e.g., applications) that, while stored permanently in the storage  708 , have been transferred to the memory  714  for execution, to perform the various functions described above. 
     The device  700  may include communications circuitry  702 . Communications circuitry  702  may include components used for wired or wireless communications, such as two-way conversations and data transfers. For example, communications circuitry  702  may include RF communications circuitry that is coupled to an antenna, so that the user of the device  700  can place or receive a call through a wireless communications network. The RF communications circuitry may include a RF transceiver and a cellular baseband processor to enable the call through a cellular network. For example, communications circuitry  702  may include Wi-Fi communications circuitry so that the user of the device  700  may place or initiate a call using voice over Internet Protocol (VOIP) connection, transfer data through a wireless local area network. 
     The device may include a microphone  720 . Microphone  720  may be a MEMS optical microphone such as that described in reference to  FIG. 1 . In this aspect, microphone  720  may be an acoustic-to-electric transducer or sensor that converts sound in air into an electrical signal. The microphone circuitry (e.g. circuit  128 ) may be electrically connected to processor  712  and power source  710  to facilitate the microphone operation (e.g. tilting). 
     The device  700  may include a motion sensor  704 , also referred to as an inertial sensor, that may be used to detect movement of the device  700 . The motion sensor  704  may include a position, orientation, or movement (POM) sensor, such as an accelerometer, a gyroscope, a light sensor, an infrared (IR) sensor, a proximity sensor, a capacitive proximity sensor, an acoustic sensor, a sonic or sonar sensor, a radar sensor, an image sensor, a video sensor, a global positioning (GPS) detector, an RF or acoustic doppler detector, a compass, a magnetometer, or other like sensor. For example, the motion sensor  704  may be a light sensor that detects movement or absence of movement of the device  700 , by detecting the intensity of ambient light or a sudden change in the intensity of ambient light. The motion sensor  704  generates a signal based on at least one of a position, orientation, and movement of the device  700 . The signal may include the character of the motion, such as acceleration, velocity, direction, directional change, duration, amplitude, frequency, or any other characterization of movement. The processor  712  receives the sensor signal and controls one or more operations of the device  700  based in part on the sensor signal. 
     The device  700  also includes camera circuitry  706  that implements the digital camera functionality of the device  700 . One or more solid state image sensors are built into the device  700 , and each may be located at a focal plane of an optical system that includes a respective lens. An optical image of a scene within the camera&#39;s field of view is formed on the image sensor, and the sensor responds by capturing the scene in the form of a digital image or picture consisting of pixels that may then be stored in storage  708 . The camera circuitry  706  may also be used to capture video images of a scene. 
     Device  700  also includes primary power source  710 , such as a built in battery, as a primary power supply. 
     While certain embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that the invention is not limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those of ordinary skill in the art. For example, the devices and processing steps disclosed herein may correspond to any type of optical sensor that could benefit from a substantially open back plate positioned over a compliant membrane having a grating, for example, an inertial sensor, an accelerometer, a gyrometer or the like. The description is thus to be regarded as illustrative instead of limiting.

Metadata:
Filing Date: 20150202
Publication Date: 20161129
Grant Date: 20161129
Priority Date: 20140707
Inventors: LEE JAE H.
AGASHE JANHAVI S.
Assignee: APPLE INC
CPC Classifications: [{"code": "H04R7/18", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R2231/003", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R31/003", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04R2201/003", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R2307/207", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R23/008", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04R31/003", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04R23/008", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04R2201/003", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R7/18", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R2307/207", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R2231/003", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 55017975