Patent Publication Number: US-7585417-B2

Title: Method of fabricating a diaphragm of a capacitive microphone device

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method of fabricating a diaphragm of a capacitive microphone device, and more particularly, to a method of fabricating a diaphragm of a capacitive microphone device that has silicon spacers and corrugate structure. 
   2. Description of the Prior Art 
   Capacitive microphone device has a parallel capacitor composed of a diaphragm and back plate. When the diaphragm senses a sound pressure and vibrates, the capacitance between the diaphragm and the back plate will change. Generally speaking, the capacitive microphone device can be classified into two types: electret type and condenser type. For a capacitive microphone device, the diaphragm is used to sense the sound pressure, and therefore requires good uniformity to accurately reflect the volume and frequency of sound. 
   The diaphragm of a conventional capacitive microphone device is made of plastic, and formed by stamping. The plastic diaphragm is mounted on the back plate by spacers. However, the plastic diaphragm formed by stamping has poor yield and uniformity. In addition, the conventional method, which assembles the diaphragm with spacers after the capacitive microphone device, requires high cost and much cycle time. 
   SUMMARY OF THE INVENTION 
   It is therefore one of the objectives of the present invention to provide a method of fabricating a diaphragm of a capacitive microphone device to improve the uniformity and reliability. 
   According to the present invention, a method of fabricating a diaphragm of a capacitive microphone device is provided. First, a substrate is provided, and a dielectric layer is formed on a first surface of the substrate. Than, a plurality of silicon spacers are formed on a surface of the dielectric layer, and the dielectric layer is patterned to form a plurality of dielectric bumps. Subsequently, a diaphragm layer is formed on a surface of the silicon spacers, a surface of the dielectric bumps, and the first surface of the substrate so that the diaphragm layer has a corrugate structure by virtue of the dielectric bumps. Thereafter, a planarization layer is formed on the diaphragm layer, and a second surface of the substrate is etched to form a plurality of openings corresponding to the corrugate structure. Following that, the dielectric bumps exposed through the openings are removed, and the planarization layer is removed. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  to  FIG. 9  are schematic diagrams illustrating a method of fabricating a diaphragm of a capacitive microphone device according to a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 1  to  FIG. 9 .  FIG. 1  to  FIG. 9  are schematic diagrams illustrating a method of fabricating a diaphragm of a capacitive microphone device according to a preferred embodiment of the present invention. As shown in  FIG. 1 , a substrate  10  e.g. a semiconductor wafer is provided. Subsequently, a dielectric layer  12  is formed on a first surface of the substrate  10 . In this embodiment, a 4-micrometer thick silicon oxide layer is used as the material of the dielectric layer  12 . 
   As shown in  FIG. 2 , a silicon layer  14  is formed on the surface of the dielectric layer  12 . In this embodiment, the silicon layer  14  is a deposited polycrystalline silicon layer, and the thickness of the silicon layer  14  is approximately 10 micrometers. In addition, the stress of the silicon layer  14  is controlled to less than 10 MPa. It is appreciated that the silicon layer  14  can be made of other materials such as amorphous crystalline silicon or single crystalline silicon, and the thickness may be modified if necessary. As shown in  FIG. 3 , a portion of the silicon layer  14  is removed by e.g. lithography and etching techniques to form a plurality of silicon spacers  16 . Please note that each silicon spacer  16  has a vertical sidewall, so as to ensure the diaphragm to be formed having good uniformity. As shown in  FIG. 4 , the dielectric layer  12  is then patterned by such as lithography and etching techniques to form a plurality of dielectric bumps  13 . 
   As shown in  FIG. 5 , a diaphragm layer  18  is formed on the surface of the dielectric bumps  13 , the surface of the silicon spacers  16 , and the first surface of the substrate  10 . The diaphragm layer  18  has a corrugate structure by virtue of the dielectric bumps  13 . In this embodiment, the diaphragm layer  18  is a deposited polycrystalline silicon layer having a thickness of 0.5 micrometer, and the stress is controlled less than 10 MPa. It is appreciated that the diaphragm layer  18  can be made of other materials such as amorphous crystalline silicon or single crystalline silicon, and the thickness may be modified if necessary. Following that, a plurality of vents  20  can be optionally formed by e.g. lithography and etching techniques in the diaphragm layer  18 . The vents  20  can prevent noises resulting from the damping effect while sensing sound signals. It is appreciated that the vents  20  can also be formed in a back plate (not shown), rather than in the diaphragm layer  18 . 
   As shown in  FIG. 6 , a planarization layer  22  such as a photoresist layer is formed on the diaphragm layer  18  for the convenience of successive processes. As shown in  FIG. 7 , the substrate  10  is turned over, and a thinning process can be selectively performed from a second surface of the substrate  10  depending on the initial thickness of the substrate  10 . The thinning process can be implemented by e.g. polishing, grinding, etching, etc. Subsequently, a plurality of openings  24  corresponding to the corrugate structure of the diaphragm layer  18  are formed on the second surface of the substrate  10  by lithography and etching techniques. Then, the dielectric bumps  13  exposed through the openings  24  are etched. Thereafter, a metal layer  26 , which serves as an electrode, is formed on the second surface of the substrate  10  and on the surface of the diaphragm layer  18 . In this embodiment, the metal layer  26  is a titanium/gold layer formed by electroplating, and has a thickness of between 1000 and 2000 angstroms. However, the material of the metal layer  26  is not limited. In addition, the electrode can be incorporated into the diaphragm layer  18  if the diaphragm layer  18  turns conductive. For instance, the diaphragm layer  18  can be doped to turn conductive. 
   As shown in  FIG. 8 , the substrate  10  is turned over again, and the planarization layer  22  disposed on the first surface of the substrate  10  and the surface of the diaphragm layer  18  is removed. As shown in  FIG. 9 , a segment process e.g. a cutting process or an etching process is performed to cut or etch the substrate  10  along scribe lines formed in advance to form a plurality of diaphragm structures  28  having corrugate structure. 
   The diaphragm structure can be combined with a back plate having a stationary electrode, and therefore forms a capacitive microphone device. It is appreciated that the diaphragm structure can be applied to various capacitive microphone devices such as electret type microphone device or condenser type microphone device. In addition, the method of the invention can be modified to be a wafer-level method if the substrate having the diaphragm layer is bonded to another substrate having stationary electrodes prior to performing the segment process. 
   In summary, the method of the invention uses silicon as the material of spacers, and therefore can fabricate diaphragms with high uniformity and high reliability. In addition, the thickness of the diaphragm can be thinner than that of a conventional plastic diaphragm, and thus has broader applications. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.