Abstract:
Backplates for MEMS devices. In one embodiment, the backplate includes an interconnect layer, a first layer, a second layer and a plurality of openings. The interconnect layer includes a first side and a second side that is opposite from the first side. The first layer is coupled to the first side of the interconnect layer. The second layer is coupled to the second side of the interconnect layer. The plurality of openings are located between a first side of the backplate and a second side of the backplate.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 14/894,390, entitled “MULTI-LAYER COMPOSITE BACKPLATE FOR MICROMECHANICAL MICROPHONE” filed Nov. 27, 2015, which is incorporated herein by reference in its entirety. U.S. patent application Ser. No. 14/894,390 is a 371 application of International Application No. PCT/2014/039793, entitled “MULTI-LAYER COMPOSITE BACKPLATE FOR MICROMECHANICAL MICROPHONE” filed May 28, 2014, the entire contents of which is also incorporated herein by reference. International Application No. PCT/2014/039793 claims priority to U.S. Provisional Application No. 61/827,982, entitled “MULTI-LAYER COMPOSITE BACKPLATE FOR MICROMECHANICAL MICROPHONE” filed May 28, 2013, the entire contents of which is also incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to micromechanical systems (“MEMS”), such as, for example, MEMS microphone systems. 
       SUMMARY 
       [0003]    One embodiment provides a backplate for a MEMS device. In one embodiment, the backplate includes an interconnect layer, a first layer, a second layer and a plurality of openings. The interconnect layer includes a first side and a second side that is opposite from the first side. The first layer is coupled to the first side of the interconnect layer. The second layer is coupled to the second side of the interconnect layer. The plurality of openings are located between a first side of the backplate and a second side of the backplate. 
         [0004]    Another embodiment provides a backplate for a MEMS device. In one embodiment, the backplate includes a plurality of openings, a first layer, a second layer, and an interconnect layer. The interconnect layer is positioned between the first layer and the second layer. The interconnect layer couples the first layer to the second layer. 
         [0005]    Yet another embodiment provides a backplate for a MEMS device. In one embodiment, the backplate includes a plurality of openings, a first layer, and an interconnect layer. The interconnect layer is coupled to the first layer. The interconnect layer includes a first portion and a second portion. The second portion is positioned between the first portion and the first layer. A width of the second portion is different than a width of the first portion. 
         [0006]    Still another embodiment provides a MEMS microphone system. The system includes a membrane and a perforated counter electrode opposite the membrane, also referred to as a backplate regardless of position relative to the membrane. The backplate includes multiple thin layers of varying materials which are stacked to produce a rigid, strong, and flat backplate. Thick MEMS layers are prone to stress gradients which cause curvature, but by alternating thin layers of different materials it is possible to minimize backplate curvature. Additionally, through a composite layered backplate, it is possible to combine tensile material layers with compressive material layers to adjust the amount of net tension in the backplate affecting the strength and rigidity of the backplate. Thin layers can also be more easily patterned to tighter tolerances. In particular, one embodiment of the disclosure related to CMOS MEMS provides a backplate including an upper metal layer and a lower metal layer with similar embodiments related to traditional MEMS using other material layers. An interconnect layer connects the two metal layers. The interconnect layer can have a smaller width than the metal layers and can be constructed from a different metal (e.g., tungsten). The interconnect layer can be constructed of multiple materials including using a sacrificial material (e.g. silicon dioxide) encapsulated by another material or combination of materials. In one embodiment, an upper layer is used to encapsulate and protect an unetched but otherwise sacrificial interconnect layer. The upper and lower layers may each also be constructed with different widths, different thicknesses, and different opening sizes with respect to each other. A person skilled in the art would know that the structures described herein are made by known methods such as depositing layers and patterning them. 
         [0007]    Yet another embodiment provides a MEMS device. In one embodiment, the MEMS device includes a membrane, and a reinforced backplate having a plurality of openings. The reinforced backplate include a first layer, and a second layer coupled to the first layer. 
         [0008]    Other aspects of the disclosure will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic cross-sectional view of a portion of a MEMS microphone system. 
           [0010]      FIG. 2  is side, cross-sectional view of a composite backplate. 
           [0011]      FIGS. 3 a  and 3 b    are side, cross-sectional views of alternative constructions of a composite backplate. 
           [0012]      FIGS. 4 a  and 4 b    are top, plan views of a composite backplate including an interconnect layer composed of discrete posts. 
           [0013]      FIG. 4 c    is a top, plan view of a composite backplate including an interconnect layer composed of a continuous wall. 
           [0014]      FIG. 4 d    is a top, plan view of a composite backplate including an interconnect layer composed of individual continuous walls surrounding each backplate opening. 
           [0015]      FIGS. 5 a - c    are side, cross-sectional views of alternative configurations of a composite backplate. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Before any embodiments of the disclosure are explained in detail, it is to be understood that the disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The disclosure is capable of other embodiments and of being practiced or of being carried out in various ways. 
         [0017]      FIG. 1  is a side, cross-sectional view of a portion of a MEMS microphone system  100 . As described in further detail below, the system includes a membrane  102  that moves in response to acoustic pressures and a counter electrode opposite the membrane (referred to as a backplate)  104 . An electrical circuit detects movement of the membrane  102  relative to the backplate  104  (e.g., due to varying capacitance) and generates an electrical signal indicative of the acoustic pressure (i.e., sound). CMOS and/or ASIC components (e.g., integrated with the system  100  or external to the system  100 ) process the electrical signal. As illustrated in 
         [0018]      FIG. 1 , the backplate  104  can include holes or vents  106  that allow air to pass between the membrane  102  and the backplate  104 . For optimal performance and durability, the backplate needs a balance between size and strength. For example, a thick backplate provides robust strength but reduces the acoustic noise performance of the system  100 . Similarly, a thin or highly perforated backplate may not provide adequate strength and may not provide adequate particle filtration. By using a composite construction of thin material layers, as show in  FIG. 2 , it is also possible to pattern thinner web sections between vent holes allowing for the placement of more and potentially smaller vent holes  106  in the backplate. Furthermore, the design of the backplate, including both the thickness of the backplate and the size and number of vent hole openings  106 , impacts the microphone capacitance and sensitivity and impacts the acoustic signal-to-noise ratio of the system. Therefore, the design of the backplate impacts the performance of the system  100 . 
         [0019]      FIG. 2  is a cross-sectional view of the backplate  104  in more detail. As illustrated in  FIG. 2 , for one embodiment of this disclosure, the backplate  104  includes an upper metal layer  200   a  and a lower metal layer  200   b.  An interconnect layer  200   c  connects the upper layer  200   a  and the lower layer  200   b.  Each of the upper and lower metal layers can include a composite stack of different metals. For example, in one embodiment related to CMOS MEMS, at least one of the upper and lower layers  200   a  and  200   b  are composed of a stack of the following layers: titanium nitride, titanium, aluminum copper, titanium, and titanium nitride. Also, in some embodiments, at least one of the layers  200   a,    200   b,  and  200   c  is formed out of an insulator rather than metal. Accordingly, in general, the composite backplate  104  can include conductive or insulating upper and lower layers  200   a  and  200   b  and a conductive or insulating interconnect layer  200   c.    
         [0020]    The vertical interconnect layer  200   c  can be constructed of a different material than the upper and lower layers  200   a  and  200   b,  such as a standard CMOS via material layer such as tungsten. In some embodiments, as illustrated in  FIG. 2 , the interconnect layer  200   c  has a smaller width than the upper layer  200   a  and/or the lower layer  200   b.  Also, in some embodiments, each layer has a different width. Furthermore, in an additional embodiment, the interconnect material  200   c  is removed so that the upper layer  200   a  connects directly to the lower layer  200   b.    
         [0021]    In addition, as illustrated in  FIG. 2 , additional layers can be added to the backplate  104 . For example, as illustrated in phantom, another interconnect layer  200   c  and another upper metal layer  200   a  can be added to the backplate. Similarly, as illustrated in  FIGS. 3 a  and 3 b   , in some embodiments, only one of the upper and lower layers  200   a  and  200   b  are used in the backplate  104  where no metal (or insulator) layer is present above or below the interconnect layer  200   c.    
         [0022]    The interconnect layer  200   c  can be provided as discrete connections (e.g. as intermittent linear segments or circular posts) or as a continuous wall. For example,  FIG. 4 a    is a top, plan view of the backplate  104  where the interconnect layer  200   c  is formed to create posts  300  positioned between the holes  106 . As illustrated in  FIG. 4 a   , the posts  300  can be solid. In other embodiments, as illustrated in  FIG. 4 b   , the posts  300  can be constructed as a ring wall that surrounds a core  302  (e.g., an oxide core). The core  302  can contain one or more materials or can be hollow. Although the posts  300  are illustrated in  FIGS. 4 a  and 4 b    as being circular, it should be understood that the posts  300  can have any desired shape, such as triangular, square, polygonal, etc., and any size. 
         [0023]    In some embodiments, rather than being constructed as discrete posts  300 , the interconnect layer  200   c  can be constructed as walls. For example,  FIG. 4 c    is a top, plan view of the backplate  104  where the interconnect layer  200   c  consists of a single continuous wall  304  between the holes  106 . Although the walls  304  are illustrated in  FIG. 4 c    as continuous straight walls, the walls  304  can be continuous or intermittent and straight or curved and can include one or more multiple walls (e.g., two parallel walls encapsulating a region between the walls that can be hollow or filled with one or more materials).  FIG. 4 d    is a top, plan view of the backplate  104  where the interconnect layer  200   c  is constructed of multiple materials including one material used to form a plurality of continuous walls surrounding the backplate perimeter and each backplate opening in order to encapsulate and protect a second interconnect layer material in a region interior to the walls. 
         [0024]      FIGS. 5 a - c    illustrate additional alternative constructions for the composite backplate  104 . In particular,  FIG. 5 a    illustrates a composite backplate  104  including an interconnect layer  200   c  having a double wall  304  (see  FIG. 4 c   ) or discrete ring posts  300  (see  FIGS. 4 a  and 4 b   ). The wall  304  or ring posts  300  protect a core  310  that can be filled with a different material than the interconnect layer  200   c,  such as silicon dioxide. In other embodiments, the core  310  can be hollow. An optional release hole  312  can be formed in the upper layer  200   a.  The release hole  312  allows removal of material to create a core  310  that is hollow. It should be understood that the release hole  312  can alternatively or in addition be included in the lower layer  200   b.    
         [0025]      FIG. 5 b    illustrates an alternative backplate  104  that includes an interconnect layer  200   c  (e.g., formed as either walls  304  or posts) where an upper portion of layer  200   c  is wider or has a different shape than the lower portion. For example, in some embodiments, as illustrated in  FIG. 5   b,  the upper portion of the layer  200   c  matches a shape or size of the upper layer  200   a.  Forming the interconnect layer  200   c  in this fashion stiffens and provide more support to the upper layer  200   a.  It should be understood that the constructions of the interconnect layer  200   c  illustrated in  FIG. 5 b    can alternatively or in addition be used with the lower layer  200   b.    
         [0026]    A similar construction of the interconnect layer  200   c  can also be used with no upper layer  200   a  to form the backplate  104  (see  FIG. 5 c   ). As previously noted, forming the interconnect layer  200   c  as illustrated in  FIG. 5 c    stiffens and provides support to the backplate  104  (e.g., the lower layer  200   b ). It should be understood that the constructions of the interconnect layer  200   c  illustrated in  FIG. 5 c    can alternatively or in addition be used when the backplate  104  does not include a lower layer  200   b.    
         [0027]    Thus, embodiments of the disclosure provide, among other things, a composite backplate that is thin and highly perforated yet strong and flat with adequate tensile properties. The composite backplate can also provide better particle filtration with less reduction of acoustic signal-to-noise ratio than existing backplates. It should be understood that the same patterns can be used as a front plate in a MEMS system. Also, the backplate (or frontplate) of this construction can be formed using CMOS MEMS material layers or traditional MEMS material layers and processing steps.