Abstract:
A method of forming a device with a controlled electrode gap width includes providing a substrate, forming a functional layer on top of a surface of the substrate, forming a sacrificial layer above the functional layer, exposing a first portion of the functional layer through the sacrificial layer, forming a first spacer layer on the exposed first portion of the functional layer, forming an encapsulation layer above the first spacer layer, and vapor etching the encapsulated first spacer layer to form a first gap between the functional layer and the encapsulation layer.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to fabrication processes for semiconductor devices. 
       BACKGROUND 
       [0002]    Microelectromechanical systems (MEMS), for example, gyroscopes, resonators and accelerometers, utilize micromachining techniques (i.e., lithographic and other precision fabrication techniques) to reduce mechanical components to a scale that is generally comparable to microelectronics. MEMS typically include a mechanical structure fabricated from or on, for example, a silicon substrate using micromachining techniques. 
         [0003]    The mechanical structures in MEMS devices are typically sealed in a chamber. The delicate mechanical structure may be sealed in, for example, a hermetically sealed metal container (for example, a TO-8 “can” as described in U.S. Pat. No. 6,307,815) or bonded to a semiconductor or glass-like substrate having a chamber to house, accommodate or cover the mechanical structure (see, for example, U.S. Pat. Nos. 6,146,917; 6,352,935; 6,477,901; and 6,507,082). In the context of the hermetically sealed metal container, the substrate on, or in which, the mechanical structure resides may be disposed in and affixed to the metal container. The hermetically sealed metal container also serves as a primary package as well. 
         [0004]    In the context of the semiconductor or glass-like substrate packaging technique, the substrate of the mechanical structure may be bonded to another substrate whereby the bonded substrates form a chamber within which the mechanical structure resides. In this way, the operating environment of the mechanical structure may be controlled and the structure itself protected from, for example, inadvertent contact. The two bonded substrates may or may not be the primary package for the MEMS as well. 
         [0005]    The sensitivity of a particular device is a function of the spacing between the electrodes in a device and the device element. A typical gap between the electrode and the device element may be on the order of 1 micron to 10 microns. Provision of a small gap is desired to increase the performance capability of the device. By way of example, the sensitivity of a particular device is proportional to 1/d 2  wherein d is the width of the gap. Additionally, the power and voltage requirements for electrostatic actuation of the device are proportional to d 2 . 
         [0006]    What is needed is a method of forming wafers such that the electrode spacing can be accurately determined. A further need exists for such a method which does not significantly increase the cost of producing the wafer. Yet another need exists for such a method which improves the antistiction performance of the device. 
       SUMMARY 
       [0007]    In accordance with one embodiment of the present invention, there is provided a method of forming a device with a controlled electrode gap width including providing a substrate, forming a functional layer on top of a surface of the substrate, forming a sacrificial layer above the functional layer, exposing a first portion of the functional layer through the sacrificial layer, forming a first spacer layer on the exposed first portion of the functional layer, forming an encapsulation layer above the first spacer layer, and vapor etching the encapsulated first spacer layer to form a first gap between the functional layer and the encapsulation layer. 
         [0008]    In accordance with a further embodiment, a method of forming a device with a z-axis electrode includes providing a substrate, forming a functional layer on top of a surface of the substrate, forming a sacrificial layer above the functional layer, etching a first electrode hole in the sacrificial layer, forming a first spacer layer within the first electrode hole, forming a first encapsulation layer portion above the sacrificial layer and above the first spacer layer, and removing the encapsulated first spacer layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  depicts a side cross-sectional view of a wafer device with a gap between a Z-axis electrode and a resonator in accordance with principles of the present invention; 
           [0010]      FIG. 2  depicts a flow chart of a process for manufacturing a device with a gap between a Z-axis electrode and a resonator in accordance with principles of the present invention; 
           [0011]      FIG. 3  depicts a cross-sectional view of a substrate, which in this embodiment is a silicon on insulator (SOI) substrate, with a photomask, which may be used in a device in accordance with principles of the present invention; 
           [0012]      FIG. 4  depicts a cross-sectional view of the substrate of  FIG. 3  with trenches formed in the functional layer of the substrate; 
           [0013]      FIG. 5  depicts a cross-sectional view of the substrate of  FIG. 4  with the trenches sealed with a sacrificial layer and holes for defining an electrical contact and an electrode formed in the sacrificial layer; 
           [0014]      FIG. 6  depicts a cross-sectional view of the substrate of  FIG. 5  with a spacer layer formed on a portion of the functional layer which was exposed through the sacrificial layer; 
           [0015]      FIG. 7  depicts a cross-sectional view of the substrate of  FIG. 6  with a thin portion of an encapsulating layer formed over the sacrificial layer and the spacer layer; 
           [0016]      FIG. 8  depicts a cross-sectional view of the substrate of  FIG. 7  with vent holes formed in the thin portion of the encapsulation layer; 
           [0017]      FIG. 9  depicts a cross-sectional view of the substrate of  FIG. 8  after vapor etching has been used to define the electrical contact, to provide a gap between the electrode and the resonator structure, and to release the resonator structure; 
           [0018]      FIG. 10  depicts a cross-sectional view of the substrate of  FIG. 9  after the remaining portion of the encapsulation layer has been formed and vent holes have been etched through the encapsulation layer; 
           [0019]      FIG. 11  depicts a cross-sectional view of the substrate of  FIG. 10  with an oxide layer defining an electrical contact hole formed above the encapsulating layer; and 
           [0020]      FIG. 12  depicts a cross-sectional view of the substrate of  FIG. 11  with an electrical contact formed in the electrical contact hole of the oxide layer. 
       
    
    
     DESCRIPTION 
       [0021]    For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains. 
         [0022]      FIG. 1  depicts a side cross-sectional view of a wafer device  100 . The wafer device  100  includes a substrate  102 , which, in this embodiment, is a silicon on insulator (SOI) substrate. The substrate  102  includes an SOI handle layer  104 , a buried oxide layer  106  and an SOI functional layer  108 . A sacrificial oxide layer  110  is located above the functional layer  108  followed by an epitaxial encapsulation layer  112  and an oxide layer  114 . 
         [0023]    A chamber  116  extends from the sacrificial oxide layer  110  through the functional layer  108  and into the buried oxide layer  106 . A resonator  118  is located within the chamber  116  and is formed in the functional layer  108 . A Z-axis electrode  120  is located above the resonator  118  and separated from the resonator  118  by a gap  122 . Trenches  124  extend through the encapsulation layer  112  to electrically isolate the Z-axis electrode  120  and trenches  126  extend through the encapsulation layer  112  to electrically isolate an electrical contact  128  which extends through the oxide layer  114 . 
         [0024]      FIG. 2  shows a flow chart  150  of a manufacturing process that may be used to produce the wafer device  100 . The process  150  of  FIG. 2  begins (block  152 ) and a substrate is provided (block  154 ). A photomask defining a resonator structure is then used to form the resonator structure (block  156 ). Once formed, the resonator structures are sealed with a sacrificial oxide layer (block  158 ). Electrical contacts and electrode contacts are then etched into the seal layer (block  160 ). A spacer layer is formed on the electrode contact (block  162 ) and a first portion of an encapsulation layer, which in this embodiment is a thin silicon layer, is formed over the seal layer (block  164 ). Vent holes are etched through the thin silicon layer (block  166 ) and a vapor phase hydrofluoric acid (HF) is used to etch the sacrificial oxide layer to release the resonator structure (block  168 ). The vapor phase etch further etches the spacer layer to provide a gap between the electrode structure and the resonator (block  170 ). 
         [0025]    The second portion of the encapsulation layer is formed (block  172 ) which closes the vents and provides structural stability, and the top surface of the encapsulation layer is planarized using chemical mechanical polishing (CMP) (block  174 ). The planarized surface is etched to provide trenches which define isolated pillars of silicon for electrical throughputs (block  176 ). An oxide layer, deposited on the wafer to close the trenches (block  178 ), is etched to define electrical contacts (block  180 ) and the electrical contact is then formed (block  182 ). The process then ends (block  184 ). 
         [0026]    One example of the process of  FIG. 2  is shown in  FIGS. 3-12 . A substrate  200  is shown in  FIG. 3 . The substrate  200  in this embodiment is a silicon on insulator (SOI) substrate including an SOI handle layer  202 , a buried silicon dioxide layer  204  and a functional SOI layer  206 . A photomask  208  is formed on the exposed upper surface of the SOI active layer  206 . Deep reactive ion etching (DRIE) of the substrate  200  creates trenches  210  which define an unreleased resonator in the functional SOI layer  206 . Next, a sacrificial layer  212  of LPCVD oxide is used to seal the trenches  210  and an electrical contact hole  214  and a Z-axis electrode hole  216  are etched into the sacrificial layer  212  as shown in  FIG. 5 . 
         [0027]    A spacer layer  218  is then formed in the Z-axis electrode hole  216  ( FIG. 6 ) and a first portion  220  of a silicon encapsulation layer is deposited on the sacrificial layer  212 . In one embodiment, the first portion  220  is about 2 microns in depth. Vent holes  222  and vent holes  224  are etched through the first portion  220  as shown in  FIG. 8 . Vapor-phase HF is used to etch the sacrificial layer  212  located adjacent to the vent holes  222  and  224 . Etching of the sacrificial layer  212  adjacent to the vent holes  222  defines an electrical contact  226  in the first portion  220 . Etching of the sacrificial layer  212  adjacent to the vent holes  224  exposes some of the trenches  210  allowing the etch vapor to contact and etch the buried silicon dioxide layer  204 , thereby forming a chamber  228  and to release the resonator structure  230  as shown in  FIG. 9 . The vapor-phase HF further etches the spacer layer  218  creating a gap  232  between the Z-axis electrode  234  and the resonator  230 . 
         [0028]    A second portion  236  of the silicon encapsulation layer  238  is deposited on top of the first portion  220  and vent holes  240  and  242  are etched through the encapsulation layer  238  (see  FIG. 10 ). The vent holes  240  electrically isolate the electrical contact  226  and the vent holes  242  electrically isolate the Z-axis electrode  234 . The vent holes  242  also expose the chamber  228  to the environment above the encapsulation layer  238 . Accordingly, the environment above the encapsulation layer  238  may be modified to result in a desired pressure within the chamber  228 . 
         [0029]    The vent holes  240  and  242  are then closed with an oxide layer  244  and an electrical contact hole  246  is etched through the oxide layer  244  (see  FIG. 11 ). As shown in  FIG. 12 , an electrical contact  248 , which in one embodiment is formed from aluminum, is formed in the electrical contact hole  246 . 
         [0030]    The processes and devices described above may be modified in a number of ways to provide devices for different applications including, but not limited to inertial sensing, shear stress sensing, in-plane force sensing, etc. By way of example, additional chambers may be provided on a single substrate  200 . By selective deposition of one or more spacer layers, gaps of different widths may be realized between electrodes and resonators in the chambers to provide structures of different sensitivity within a wafer. Additionally, the thickness of the encapsulation lay may be selectively increased (decreased) over the entire wafer or over particular electrodes to provide stiffer structures. 
         [0031]    While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.