Abstract:
A MEMS transducer comprises a substrate ( 124 ). A membrane layer ( 120 ) is supported by the substrate ( 124 ). A back-plate layer ( 110 ) is supported by the membrane layer ( 120 ), said back-plate layer comprising a respective sidewall portion and a respective raised portion. One or more columns ( 116 ) are provided separate from the sidewall portion of the back-plate layer, said columns ( 116 ) connecting the back-plate layer, the membrane layer and the substrate.

Description:
[0001]    This invention relates to the field of MEMS technology, and relates in particular, but not exclusively, to a MEMS device and a process for manufacturing that device. 
       BACKGROUND 
       [0002]    Consumer electronics devices are continually getting smaller and, with advances in technology, are gaining ever-increasing performance and functionality. This is clearly evident in the technology used in consumer electronic products such as mobile phones, laptop computers, MP3 players and personal digital assistants (PDAs). Requirements of the mobile phone industry for example, are driving the components to become smaller with higher functionality and reduced cost. It is therefore desirable to integrate functions of electronic circuits together and combine them with transducer devices such as microphones and speakers. 
         [0003]    The result of this is the emergence of micro-electrical-mechanical-systems (MEMS) based transducer devices. These may be for example, capacitive transducers for detecting and/or generating pressure/sound waves or transducers for detecting acceleration. There is a continual drive to reduce the size and cost of these devices. 
         [0004]    Microphone devices formed using MEMS fabrication processes typically comprise one or more membranes with electrodes for read-out/drive deposited on the membranes and/or a substrate. In the case of MEMS pressure sensors and microphones, the read out is usually accomplished by measuring the capacitance between the electrodes. In the case of transducers, the device is driven by a potential difference provided across the electrodes. 
         [0005]      FIGS. 1 and 2  show a schematic diagram and a perspective view, respectively, of a known capacitive microphone device. The capacitive microphone device comprises a flexible membrane  1  that is free to move in response to pressure differences generated by sound waves. A first electrode  3  is mechanically coupled to the flexible membrane  1 , and together they form a first capacitive plate of the capacitive microphone device. A second electrode  5  is mechanically coupled to a generally rigid structural layer or back-plate  7 , which together form a second capacitive plate of the capacitive microphone device. 
         [0006]    The capacitive microphone is formed on a substrate  9 , for example a silicon wafer. A back-volume  11  is provided below the membrane  1 , and is formed using a “back-etch” through the substrate  9 . A plurality of openings  13 , referred to hereinafter as acoustic holes, are provided in the back-plate  7  so as to allow free movement of air molecules, such that the sound waves can enter a cavity  15  above the membrane  1 . A plurality of openings  17 , hereinafter referred to as bleed holes, connect the cavity  15  with the back-volume  11 . The cavity  15  and back-volume  11  allow the membrane  1  to move in response to the sound waves entering via the acoustic holes  13  in the back-plate  7 . 
         [0007]    Thus, in response to a sound wave corresponding to a pressure wave incident on the microphone, the membrane  1  is deformed slightly from its equilibrium position. The distance between the lower electrode  3  and the upper electrode  5  is correspondingly altered, giving rise to a change in capacitance between the two electrodes that is subsequently detected by electronic circuitry (not shown). 
         [0008]      FIG. 2  shows a perspective view of the MEMS device illustrated in  FIG. 1 . 
         [0009]      FIG. 3  shows a simplified cross-sectional view of a conventional MEMS device such as that shown in  FIGS. 1 and 2 . As mentioned above, a first electrode (not shown) forms part of the flexible membrane  1 , and a second electrode (not shown) is attached to or embedded in the rigid back-plate  7  above the membrane  1 . Both the back-plate  7  and the membrane  1  may be formed from silicon nitride, for example, and the substrate from silicon. However, the thermal expansion coefficient of silicon is greater than that of silicon nitride and this leads to stresses at the interface between the two dissimilar materials. 
         [0010]    The structure of  FIG. 3  is formed by various processes of depositing layers and then selectively dry or wet etching portions of the layers away again. These processes take place at relatively low temperatures (in the order of 10-400° C.). When the layers are deposited, there are no intrinsic stress concentrations in the structure. When the structure is released by removal of the sacrificial layers the tensile stress of the deposited layer causes a torsional moment in the membrane sidewall. This leads to a tensile stress concentration on the outer sidewall edge and a compressive stress concentration on the inner sidewall edge. A similar stress can be found in the membrane  1 . 
         [0011]    These stress concentrations tend to cause cracking originating at the points labelled A and B in  FIG. 3 , and can lead to failure of the MEMS device. This stress can also render the MEMS device more susceptible to failure during fabrication. For example, when multiple MEMS devices are fabricated on a single wafer and subsequently separated using a technique known as singulation or dicing, the stress at points A and B can cause the device to crack and fail. After failure at these points, the transducer is rendered useless. 
         [0012]    It is therefore an aim of the present invention to provide a MEMS device that does not suffer from the disadvantages mentioned above. 
       SUMMARY OF INVENTION 
       [0013]    According to a first aspect of the invention, there is provided a MEMS device comprising: a substrate; a membrane layer, supported by the substrate; a back-plate layer, supported by the membrane layer, said back-plate layer comprising a respective sidewall portion and a respective raised portion; and one or more columns separate from the sidewall portion of the back-plate layer, said columns connecting the back-plate layer, the membrane layer and the substrate. 
         [0014]    According to a second aspect of the invention, there is provided a method for manufacturing a MEMS device, said MEMS device comprising a substrate, a membrane layer supported by the substrate, and a back-plate layer supported by the membrane layer. The method comprises the steps of: depositing a sacrificial layer on the substrate; removing the sacrificial layer from one or more selected positions, said selected positions defining the positions of one or more columns; and depositing the back-plate layer over the substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the following drawings, in which: 
           [0016]      FIG. 1  shows a cross-sectional view of a prior art MEMS microphone; 
           [0017]      FIG. 2  is a cross-sectional perspective view of the MEMS microphone shown in  FIG. 1 ; 
           [0018]      FIG. 3  is a simplified diagram showing a cross-sectional view of a basic MEMS structure; 
           [0019]      FIG. 4  is a perspective view of a MEMS device according to an embodiment of the present invention; 
           [0020]      FIG. 5  is a cross-sectional view of the MEMS device of  FIG. 4 ; 
           [0021]      FIG. 6  is a perspective view of a portion of the MEMS device shown in  FIGS. 4 and 5 ; 
           [0022]      FIG. 7  is a cross-sectional view of the portion shown in  FIG. 6 ; and 
           [0023]      FIGS. 8   a  to  8   g  show an example of a process for fabricating the MEMS device of  FIGS. 4 to 7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    It is noted that the description of the invention will be made in connection with a MEMS transducer, and in particular a MEMS transducer in the form of a capacitor microphone. However, it is noted that the invention is not limited to MEMS transducers, and is applicable to any MEMS device. 
         [0025]      FIG. 4  shows a perspective view of a MEMS transducer  100  according to the present invention. 
         [0026]    As is conventional, the transducer  100  comprises a back-plate  110  in which an electrode  112  is embedded. Not shown is a flexible membrane underneath the back-plate  110  which reacts to pressure waves. A second electrode is embedded in the membrane, and the changes in capacitance between the two electrodes used to detect the incoming pressure waves. The transducer  100  further comprises a plurality of acoustic holes  114  formed in the back-plate  110 , in order that pressure waves can reach the membrane underneath the back-plate  110 . 
         [0027]    However, as mentioned above, the back-plate  110  is highly stressed, and this can cause cracking in the sidewalls. Therefore, according to the present invention, the transducer further comprises one or more columns  116  formed around the periphery of the back-plate  110 . The columns  116  act to reduce the stress in the structure of the back-plate  110  by reducing the torsional moment in the sidewalls. The effect of the columns  116  is to create a “bridge structure” that is supported at two points, i.e. the columns and the base of the sidewalls. 
         [0028]    In the embodiment shown in  FIG. 4 , the columns are circular, such that any stress built up around the columns  116  is evenly distributed. However, alternative shapes may be thought of by those skilled in the art without departing from the scope of the invention. In particular it is envisaged that any shape column will have some stress-reducing properties in the back-plate  110 . 
         [0029]      FIG. 5  is a distorted perspective view of a cross-section of the transducer  100 . 
         [0030]    The cross-section shows the presence of the flexible membrane  120  located underneath the back-plate  110 . As is conventional, the membrane  120  is located over a back-volume  122  so that it is free to react to the changes in pressure caused by incident acoustic waves. The back-volume  122  is etched in a substrate  124  that supports the membrane  120  and the back-plate  110 . 
         [0031]      FIG. 5  also shows the columns  116  in more detail. As can be seen, in this embodiment, the columns are comprised of a first part  116 a which is in contact with the substrate. In this embodiment, as will be explained in greater detail below with reference to  FIGS. 8   a  to  8   g , the first part  116   a  is formed by selectively depositing and patterning a sacrificial layer, and then depositing the back-plate  110  on top of the patterned sacrificial layer. The back-plate layer itself then forms the first part  116   a  of the columns. Thus, in this embodiment, the columns  116  also comprise a second part  116   b  above the first part  116   a  which is the absence of the back-plate  110 . It will be appreciated that the reference made above to the first part  116   a  being in contact with the substrate does not necessarily imply that the two features are in direct contact. For example, one or more other layers may be interposed between the substrate and the first part  116   a  of the column. 
         [0032]      FIG. 6  is a perspective cross-section showing the columns  116  in yet more detail. As a consequence of the manufacturing process, the walls of the columns  116  have a stepped profile. Further, the steps of the walls are not vertical, but rather sloped slightly. Both the steps and the sloping nature of the walls of the columns  116  give additional support to the back-plate  110 . 
         [0033]    The columns  116  are advantageous over a large range of dimensions. However, certain dimensions lead to the most effective amount of stress relief in the sidewalls of the back-plate  110 . For example, for a transducer diameter of 1 mm, column widths of about 12 μm and spacing from the sidewall of about 10 μm give the most effective relief. An optimal stress relief is also found when the column widths are approximately equal to column separation. Again for the example of a transducer diameter of 1 mm, 144 columns equally spaced around the periphery of the transducer results in a centre-to-centre column separation of approximately 22 μm (i.e. for columns widths of 12 μm, the edge-to-edge column separation is about 10 μm). For alternative transducer diameters, the optimal column widths and spacings will in general vary. 
         [0034]      FIG. 7  is a horizontal cross-section of a column  116 . 
         [0035]    The column  116  is formed on a substrate  124 . A membrane layer  120  is formed on the substrate  124 . The membrane layer  120  generally comprises a raised portion formed over a gap  126 . However, at the site of the column  116   a , the membrane layer  120  forms part of the column  116   a , and is in contact with the substrate  124 . The gap  126  may be formed by depositing one of the layers of the MEMS transducer, for example an etch stop layer, such as an etch stop polyimide, and then etching that layer away to release the membrane layer  120  from the substrate  124 . In other words, when depositing the etch stop layer the etch stop layer is patterned to leave spaces around the periphery of the device at the sites of the columns  116  (described in greater detail in  FIGS. 8   a  to  8   g  below), such that the subsequent depositing of the membrane layer results in corresponding dips in order to form the columns  116 . A back-plate layer  110  is formed above the membrane layer  120 . The back-plate layer  110  also comprises a raised portion such that a gap  128  is formed between the back-plate layer  110  and the membrane layer  120 . 
         [0036]    Thus, in the above-described embodiment, the column  116  is formed by the back-plate layer  110  in conjunction with the membrane layer  120 . It will be appreciated by one skilled in the art that  FIG. 7  shows only one cross-section through a column  116  near the periphery of the back-plate  110 . As seen in  FIG. 6 , a majority of cross-sections through a MEMS transducer would not pass through a column  116 . That is,  FIG. 7  shows two portions labelled  126 , and two portions labelled  128 . As  FIG. 7  is a cross-section, the two gaps labelled  126  are in fact part of the same gap underneath the membrane layer  120 —they are linked to one another into and out of the plane of the diagram around the column  116 ; similarly for the two portions labelled  128 . The invention leads to the edge of the “active” area of the membrane being defined by the columns, rather than the edge of an etch stop as defined by the prior art. 
         [0037]      FIGS. 8   a  to  8   g  show steps in a process of manufacturing a column  116  of a transducer according to the present invention. 
         [0038]      FIG. 8   a  shows a substrate  124 . The substrate  124  may be silicon, or any other suitable material. 
         [0039]    A first step in the process is to deposit a layer  130  on to the substrate  124  (see  FIG. 8   b ). For example, the layer  130  may be an etch stop layer, for example an etch stop polyimide. 
         [0040]    A next step in the process is to pattern the polyimide layer  130  appropriately (see  FIG. 8   c ). The polyimide layer  130  is patterned such that one or more spaces are left on the substrate  124  at selected points around the device, these points defining the positions of the columns  116 .  FIG. 8   c  shows two portions, both labelled as the polyimide layer  130 . However, in reality these portions are connected out of the plane of the cross-section. In other words, the gap shown between the portions  130  in  FIG. 8   c  is effectively a circle in plan view (or other shape for a differently shaped column). 
         [0041]    The patterning may take place through dry or wet etching, or any other process that appropriately removes the polyimide layer  130  without damaging the device. 
         [0042]    A next step in the process is to deposit the membrane layer  120  (see  FIG. 8   d ). The membrane layer  120  may be silicon nitride or any other suitable material. As afore-mentioned, the membrane layer  120  must be flexible enough to respond to acoustic waves, but strong enough not to sag in the middle of the device. 
         [0043]    Optionally, the membrane layer  120  may be patterned to form small “release” holes above the area of polyimide layer  130  between the column  116  and the prospective position of the sidewall of the back-plate layer  110 . The release holes would allow etchant to flow more easily to the area of the polyimide layer between the column and the sidewall, such that the polyimide layer is removed more effectively. 
         [0044]    A next step in the process is to deposit a sacrificial layer  132 , for example a polyimide layer  132  on top of the membrane layer  120  (see  FIG. 8   e ). The sacrificial layer  132  conforms to the shape of the membrane layer  120  underneath. 
         [0045]    In  FIG. 8   f , the sacrificial polyimide layer  132  is patterned, such that the sacrificial polyimide layer  132  forms a stepped structure, the stepped structure forming the subsequent stepped structure in the back-plate  110 . The patterning involves removing the sacrificial polyimide layer  132  from the locations that have been marked for columns, i.e. substantially the positions in which the first etch stop polyimide layer  130  was etched away. 
         [0046]    A next step of the process is to deposit the back-plate layer  110  (see  FIG. 8   g ). The back-plate  110  acts as a fixed reference against which the displacement of the membrane layer  120  varies. Therefore the back-plate layer  110  must be as rigid as possible, and so is thicker than the membrane layer  120 . As can be seen, the back-plate layer  110  fills the recess left by the patterning of the two polyimide layers  130 ,  132 , and forms the column in conjunction with the membrane layer  120 . The back-plate layer  110  may be silicon nitride or any other suitable material familiar to those skilled in the art. 
         [0047]    Optionally, the back-plate layer  110  may be patterned to form small “release” holes above the area of polyimide layer  132  between the column  116  and the position of the sidewall of the back-plate layer  110 . The release holes would allow etchant to flow more easily to the area of the polyimide layer between the column and the sidewall, such that the polyimide layer is removed more effectively. 
         [0048]    As a final step (not shown), the remains of the polyimide layers  130 ,  132  are etched away, so that the membrane  120  is free to move relative to the back-plate  110 . The etchant (possibly a gas or a liquid) flows though the acoustic holes  114  and/or the release holes mentioned above to etch away the remaining polyimide layers  130 ,  132  and create the gaps  126 ,  128  respectively. 
         [0049]    An alternative to the above-described embodiments is a transducer with a substantially flat membrane layer. That is, the membrane layer does not comprise a raised portion, and instead lies substantially flat over the substrate. In this embodiment, the only “active” part of the membrane is that which directly overlies the back-volume and is therefore free to move in response to oncoming pressure waves, etc. In this embodiment, the columns may be formed solely by the back-plate layer. 
         [0050]    It is noted that the invention may be used in a number of applications. These include, but are not limited to, consumer applications, medical applications, industrial applications and automotive applications. For example, typical consumer applications include laptops, mobile phones, PDAs and personal computers. Typical medical applications include hearing aids. Typical industrial applications include active noise cancellation. Typical automotive applications include hands-free sets, acoustic crash sensors and active noise cancellation. 
         [0051]    It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single feature or other unit may fulfill the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.