Abstract:
A raised microstructure for use in a silicon based device, such as a microphone, is disclosed. The raised microstructure comprises a generally planar film having a ribbed sidewall supporting the film.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not Applicable. FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT:  
           [0002]    Not Applicable.  
         TECHNICAL FIELD  
         [0003]    The present invention relates to raised micro structures such as those found in pressure sensors, accelerometers and silicon based capacitive transducers and microphones. Specifically, the present invention is directed to improving the means of supporting a raised backplate of a silicon based capacitive transducer such as that found in a microphone.  
         BACKGROUND OF THE INVENTION  
         [0004]    The use of silicon based capacitive transducers as microphones is well known in the art. Typically, such microphones consist of four elements: a fixed backplate; a highly compliant, moveable diaphragm (which together form the two plates of a variable air-gap capacitor); a voltage bias source and a buffer.  
           [0005]    The two mechanical elements, the backplate and diaphragm, are typically formed on a single silicon substrate using a combination of surface and bulk micromachining well known in the art. One of these two elements is generally formed to be planar with the surface of the supporting silicon wafer. The other element, while itself generally planar, is supported several microns above the first element by posts or sidewalls, hence the term raised microstructure.  
           [0006]    In general, the positioning of the two elements with respect to each other, affects the performance of the entire device. Intrinsic stresses in the thin films comprising the raised microstructure cause the structure to deflect out of the design position. In a microphone in particular, variations in the gap between the diaphragm and backplate affect the microphone sensitivity, noise, and over pressure response.  
           [0007]    Many other factors also affect the manufacture, structure, composition and overall design of the microphone. Such problems are more fully discussed and addressed in U.S. Pat. No. 5,408,731 to Berggvist; U.S. Pat. No. 5,490,220 to Loeppert, and U.S. Pat. No. 5,870,482 to Loeppert.  
           [0008]    In the specific example of the design of a microphone backplate as a raised microstructure, the goal is to create a stiff element at a precise position relative to the diaphragm. One method to achieve this is to form the backplate using a silicon nitride thin film deposited over a shaped silicon oxide sacrificial layer which serves to establish the desired separation. This sacrificial layer is later removed through well known etch processes, leaving the raised backplate. Intrinsic tensile stress in the silicon nitride backplate will cause it to deflect out of position. Compressive stress is always avoided as it causes the structure to buckle.  
           [0009]    [0009]FIG. 1 depicts one such raised microstructure  10  of the prior art. After the oxide is removed leaving the raised microstructure  10 , an intrinsic tension will be present within the plate  12 . This tension T results from the manufacturing process as well as from the difference between the coefficient of expansion of the material of the raised microstructure  10  and the supporting wafer  16 . As shown, the tension T is directed radially outwards. The tension T intrinsic in the plate  12  will result in a moment as shown by arrow M about the base  18  of sidewall  14 . This moment M results in a tendency of the plate  12  to deflect towards the wafer  16  in the direction of arrow D. This deflection of plate  12  results in a negative effect on the sensitivity and performance of the microphone.  
           [0010]    A number of undesirable means to negate the effects of this intrinsic tension within a thin-film raised microstructure are known in the prior art. Among them are that the composition of the thin film can be adjusted by making it silicon rich to reduce its intrinsic stress levels. However, this technique has its disadvantages. It results in making the thin film less etch resistant to HF acid, increasing the difficulty and expense of manufacture. An additional solution known in the prior art would be to increase the thickness of the sidewall supporting the raised backplate thereby increasing the sidewall&#39;s ability to resist the intrinsic tendency of the thin film to deflect. While this sounds acceptable from a geometry point of view, manufacture of a thick sidewall when the raised microstructure is made using thin film deposition is impractical.  
           [0011]    It is an object of the present invention to overcome the disadvantages of the prior art with respect to the design of raised microstructures in silicon based devices by negating the undesirable effects of the intrinsic thin film tension inherent in said microstructure.  
         SUMMARY OF THE INVENTION  
         [0012]    It is an object of the present invention to provide an improvement to raised microstructures for use in silicon based devices. In accord with one embodiment of the present invention, a raised microstructure for use in a silicon based device is provided comprising a generally planar thin-film and a sidewall supporting the film, wherein the sidewall is ribbed.  
           [0013]    Other features and advantages of the invention will be apparent from the following specification taken in conjunction with the following drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The raised microstructure of the present invention will now be described with reference to the accompanying drawings, in which:  
         [0015]    [0015]FIG. 1 is a cross sectional schematic of a raised microstructure known in the prior art;  
         [0016]    [0016]FIG. 2 is a cross sectional perspective view of a raised microstructure embodying the present invention;  
         [0017]    [0017]FIG. 3 is a cross section of the raised microstructure of FIG. 2; and  
         [0018]    [0018]FIG. 4 is a plan view of FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail a preferred embodiment of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiment illustrated.  
         [0020]    An embodiment of the raised microstructure  10  of the present invention is shown in FIGS. 2 &amp; 3. The raised microstructure  10  comprises a generally circular thin-film plate or backplate  12  supported by a sidewall  14 .  
         [0021]    The raised microstructure  10  is comprised of a thin film plate  12  of silicon nitride deposited on top of a sacrificial silicon oxide layer on a silicon wafer  16  using deposition and etching techniques readily and commonly known to those of ordinary skill in the relevant arts. The sacrificial silicon oxide layer has already been removed from the figure for clarity.  
         [0022]    The sidewall  14  of the raised microstructure  10  is attached at its base  18  to the silicon wafer  16  and attached at its opposite end to the plate  12 . The sidewall  14  is generally perpendicular to plate  12 , but it is noted other angles may be utilized between the sidewall  14  and the plate  12 .  
         [0023]    [0023]FIG. 4 shows a plan view of the assembly of FIG. 2 with a surface of the sidewall  14  of the present invention shown in phantom. It can be seen that the sidewall  14  of the present invention as shown in FIGS.  2 - 4  is ribbed, forming a plurality of periodic ridges  20  and grooves  22 . In the preferred embodiment, the ridges  20  and grooves  22  are parallel and equally spaced, forming a corrugated structure. Furthermore, the preferred embodiment utilizes ridges  20  and grooves  22  of a squared cross section. The effect of corrugating the side wall in this manner is to create segments  24  of the sidewall  14  that are radial, as is the intrinsic tension T of the plate  12 . By making portions of the sidewall  14  radial, as is the tension T, the sidewall  14  is stiffened. It has been found that the sidewall  14  of the prior art, which is tangential to plate  12 , is easily bent as compared to the radial segments  24  of the present invention.  
         [0024]    Other geometries than that shown in FIGS.  2 - 4  of the corrugations or ridges  20  and grooves  22  can be imagined and used effectively to increase the sidewall&#39;s  14  ability to resist moment M and the geometry depicted in the FIGS.  2 - 4  is not intended to limit the scope of the present invention.  
         [0025]    For example, a generally annular geometry, generally triangular geometry or any combination or variation of these geometries or others could be utilized for the ridges  22  and grooves  24 .  
         [0026]    In the preferred embodiment, the corrugations are radial and hence the sidewalls  14  are parallel to the tension in the backplate  12 . Furthermore, the sacrificial material is etched in such a way that the sidewalls  14  are sloped with respect to the substrate to allow good step coverage as the thin film backplate  12  is deposited.  
         [0027]    While the specific embodiments and various details thereof have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention and the scope of protection is only limited by the following claims.