Abstract:
A method is presented for fabricating an electrically isolated MEMS device having a conductive outer MEMS element, and an inner movable MEMS element spaced apart from the conductive outer MEMS element. The inner element includes a nonconductive base having a plurality of conductive structures extending therefrom. The conductive components are formed by plating a conductive material into a pre-formed mold which defines the shape of the conductor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to microelectromechanical systems (MEMS) and, in particular, relates to the fabrication of electrically isolated MEMS devices using plating techniques.  
           [0003]    2. Discussion of the Related Art  
           [0004]    Microelectromechanical systems (MEMS) components are being progressively introduced into many electronic circuit applications and a variety of micro-sensor applications. Examples of MEMS components are electromechanical motors, radio frequency (RF) switches, high Q capacitors, pressure transducers and accelerometers. In one application, the MEMS device is an accelerometer having a movable component that, in response to acceleration, is actuated so as to vary the size of a capacitive air gap. Accordingly, the current output of the MEMS device provides an indication of the strength of the external stimulus.  
           [0005]    One current method of fabricating such components, often referred to as surface micro-machining, uses a sacrificial layer, such as silicon dioxide, that is deposited and bonded onto a substrate, such as single crystal silicon which has been covered with a layer of silicon nitride. A MEMS component material, for example polycrystalline silicon, is then deposited onto the sacrificial layer, followed by a suitable conductor, such as aluminum, to form an electrical contact with the ambient environment. The silicon layer is then patterned by standard photolithographic techniques and then etched by a suitable reactive ion etching plasma or by wet chemistry to define the MEMS structure and to expose the sacrificial layer, which may comprise silicon dioxide. The sacrificial layer is then etched to release the MEMS component. This leaves only a single material, the structural material.  
           [0006]    One significant disadvantage associated with current surface fabrication techniques involves the lack of electrical isolation that is achieved. The present inventors have discovered that a MEMS device may be used as a current or voltage sensor, in which the device may receive high voltages at one end of the device, and output an electrical signal at the other end of the device to, for example, a sensor. The output could be a function of the capacitance of the MEMS device, as determined by the position of a movable MEMS element with respect to a stationary element. However, because the entire movable MEMS element achieved using conventional surface fabrication techniques is conductive, the input and output ends of the MEMS device are not sufficiently isolated from one another, thereby jeopardizing those elements disposed downstream of the MEMS output.  
           [0007]    Another significant disadvantage associated with current surface fabrication techniques is that the process is inherently limited to thin structural layers (on the order of 1 to 2 μm) due to stresses which may be introduced during the fabrication. The thinness of the layers limits the amount of capacitance that can be obtained in the sensor portion of the MEMS device, and thus limits the magnitude of any output signal. This in turn limits the overall resolution obtainable  
           [0008]    It is therefore desirable to provide a method for fabricating a MEMS device using surface fabrication techniques having greater thickness than that currently achieved to enhance sensitivity, while providing sufficient electrical isolation for the device.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    The present inventors have recognized that a MEMS device may be fabricated using an insulating material, a sacrificial material, a mold material, and a conducting mechanical structural layer that may be plated onto an insulating substrate.  
           [0010]    In accordance with one aspect of the invention, a method for fabricating a MEMS device, comprising the steps of providing a substrate having an upper surface, and depositing a sacrificial layer onto the upper surface of the substrate. A nonconductive layer is then deposited onto the upper surface of the sacrificial layer. Next, a mold is deposited onto the substrate, wherein the mold has at least one void aligned with the insulating layer. A conductive material is then deposited into the at least one void to form conductive elements extending from the nonconductive layer. Finally, the mold and sacrificial layer are removed to release a movable element including the nonconductive layer and conductive layer from the substrate.  
           [0011]    The conductive material may be electroplated or electrolessplated onto the nonconductive layer.  
           [0012]    All of the aforementioned aspects are not necessary to carry out the invention. Furthermore, these and other aspects of the invention are not intended to define the scope of the invention for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    Reference is hereby made to the following figures in which like reference numerals correspond to like elements throughout, and in which:  
         [0014]    [0014]FIG. 1 is a schematic sectional side elevation view of a MEMS device constructed in accordance with a preferred embodiment of the invention;  
         [0015]    [0015]FIG. 2 is a sectional side elevation view of a structure having a substrate, sacrificial layer, and insulating layer that is used to fabricate the MEMS device illustrated in FIG. 1 in accordance with one embodiment of the invention;  
         [0016]    [0016]FIG. 3 is a sectional side elevation view of the structure illustrated in FIG. 2 having a portion of the insulating and sacrificial layers removed;  
         [0017]    [0017]FIG. 4 is a sectional side elevation view of the structure illustrated in FIG. 3 having a plating mold attached thereto;  
         [0018]    [0018]FIG. 5 is a sectional side elevation view of the structure illustrated in FIG. 4 after patterning the mold using standard photolithographic techniques; and  
         [0019]    [0019]FIG. 6 is a sectional side elevation view of the structure illustrated in FIG. 5 after plating a material onto the insulating layer. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    Referring initially to FIG. 1, a MEMS device  10  includes a stationary MEMS element  12  and a movable MEMS element  14 , both attached to a substrate  16 . The substrate  16  may be either conducting or insulating, depending on the intended application, and may comprise glass, high resistivity silicon, crystalline sapphire, crystalline silicon, polycrystalline silicon, silicon carbide, or ceramic such as alumina, aluminum nitride, and the like, or gallium arsenide. In fact, the substrate may comprise any material whatsoever that is suitable for supporting a MEMS device. In the embodiment shown in FIG. 1, the stationary MEMS element  12  comprises a pair of stationary conductive members  13  which extend outwardly from the substrate. The movable MEMS element  14  includes a base layer  17  which supports separated conductive members  18  that extend outwardly from the base  17 . Movable element  14  is disposed between the stationary members  13 . It should be appreciated by those having ordinary skill in the art that movable MEMS element  14  is a beam that is supported at its distal ends by, for example, the substrate such that the middle portion of element  14  is free and movable relative to the stationary members  13 , as illustrated.  
         [0021]    It should be appreciated by one having ordinary skill in the art that FIG. 1 illustrates a portion of a MEMS structure  10 , and that inner MEMS element  14  is connected to substrate  16  at its two distal ends, as disclosed in patent application Ser. No. 09/805,410 filed on Mar. 13, 2001 and entitled “Microelectricalmechanical System (MEMS) Electrical Isolator with Reduced Sensitivity to Internal Noise” the disclosure of which is hereby incorporated by reference. Accordingly, while the outer portions of movable element  14  are connected to the substrate, an elongated section of element  14  is suspended and free from the substrate, thereby permitting deflection of the free portion of the movable MEMS element with respect to the substrate  16 . The stationary members  13  are separated from the moveable MEMS element  14  by a variable size gap  19 , which could be the gap between the adjacent plates of a detection capacitor, as will become more apparent from the description below. The size of gap  19  changes as the movable element deflects in response to a stimulus.  
         [0022]    In the MEMS device  10  illustrated in FIG. 1, there are two different structural materials that remain after the movable element  14  is released from the substrate  16 . In particular, an insulating material that forms the base layer  17  and a conducting layer that forms the other portions of the device  13  and  18 . As such, fabrication of devices of this type utilizes at least three unique materials, in addition to the substrate: a conducting material, an insulating material, and at least one sacrificial material.  
         [0023]    If base layer  17  is formed utilizing an insulating material, as is the case in accordance with the preferred embodiment, the conductive members  18  become electrically isolated from each other, thereby minimizing the risk that an electrical input will conduct across the device  10 , which would jeopardize those elements disposed downstream of the MEMS output, in a useful circuit application. The insulation layer  17  thus provides sufficient electrical isolation across the movable element  14 , thereby rendering the device  10  usable, for example, as a current or voltage sensor.  
         [0024]    The MEMS device  10  could therefore perform any function suitable for a MEMS application. For example, the device could comprise an accelerometer whose movable MEMS element  14  is a beam that deflects in response to the external stimulus, such as an acceleration or vibration of the device  10 . Accordingly, as the size of the gaps  19  vary, so will the output capacitance, thereby providing a measurement of the amount of deflection of the movable MEMS element  14 . A measurement of the amount of acceleration may thereby be obtained by measuring the capacitance of the device. The device  10  constructed in accordance with the present invention could furthermore incorporate a wafer level cap and electrical traces connected to the stationary members  13 , as is described in U.S. patent application filed on Sep. 26, 2001 and entitled “Method for Constructing an Isolated Microelectromechanical System (MEMS) Device Using Surface Fabrication Techniques” the disclosure of which is hereby incorporated by reference as if set forth in its entirety herein.  
         [0025]    The MEMS device  10  schematically illustrated in FIG. 1 may be fabricated in accordance with several embodiments of the invention that utilize plating processes, as will now be described.  
         [0026]    In particular, referring now to FIG. 2, the fabrication process begins by providing a substrate  16  that is insulating and comprises either glass or high resistivity silicon in accordance with the preferred embodiment. Other materials, including conducting materials, could be substituted for the substrate material, depending on the intended application of the MEMS device. Several layers are subsequently deposited onto the substrate  16 . The first layer  20  to be deposited will ultimately form a sacrificial release layer and comprises silicon nitride in the preferred embodiment. A skilled artisan will appreciate that any alternative material that is selectively etchable could also be used. The second layer  22  to be deposited will form an insulating base layer and comprises silicon dioxide in the preferred embodiment. The deposition of these materials is well known, and could be achieved by chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), or similar techniques well known to those skilled in the art. The thickness of each layer is selected in anticipation of the desired height of the final MEMS device, and may be on the order of 1-3 microns.  
         [0027]    Referring now to FIG. 3, once the layers  20  and  22  are deposited, they are patterned by standard photolithographic techniques. In particular, photoresist is applied to the top surface of the structure and patterned. The insulating layer  22  is selectively anisotropically etched, followed by selective anisotropic etching of the sacrificial layer  20 , and finally the photoresist is removed which reveals the insulating base  17 , lying on top of patterned sacrificial layer  20 .  
         [0028]    In preparation for an electroplating step, the top surface of the structure may be coated with a shorting layer that is compatible with the metal which will be electroplated. The shorting layer will later facilitate the plating process. For example, when electroplating gold, a tin/gold or a chromium/gold bilayer shorting layer is used. This electrically connects regions where metal deposition is desired. After plating, the gold shorting layer can be removed with a short KI 3  solution and the tin, for example, can be removed using a buffered HF solution.  
         [0029]    In preparation for an electrolessplating step, the top surface of the structure may be coated with a pre-treated catalyst to induce the electroless plating reduction reaction.  
         [0030]    Referring to FIG. 4, the structure is now coated with the mold material  24 , which may comprise a photoresist or other photoactivated polymer material in accordance with the preferred embodiment. Because the plating process is a relatively low temperature process, a high temperature material like that needed for the sacrificial layer is not required for the mold material. In addition, commercial photoresists exist that can be applied to thickness up to and above 10 microns. This increased thickness is beneficial as it will allow the plated conductive layer to achieve a greater thickness. A skilled artisan will appreciate that the mold material could also be an inorganic material, such as the same material employed for the sacrificial material. However, the thickness of such materials is generally limited to 1-3 microns which will limit the overall height of the final conducing layer.  
         [0031]    The photoresist is then patterned with standard photolithographic processes to result in a mold pattern. That is, the photoresist is removed in the areas where the plating is desired, as shown in FIG. 5. Gaps are thereby formed in the mold  24  that will provide the structure for the fabrication of conductive members  18 .  
         [0032]    Referring now to FIG. 6, the conducting material is plated onto the insulating layer  22  using standard plating processes. Conducting material is further plated onto the surface of substrate  16  to form the stationary conductive members  13 . The conducting material could be nickel, gold, copper, or any other suitably conductive metal which can be plated. The metal fills the cavities in the mold and attaches to layers  22  and substrate  16 . Finally the mold material  24  is etched away and the sacrificial layer  20  is etched away using standard techniques, thereby leaving the final released structure depicted in FIG. 1.  
         [0033]    It should be appreciated that the embodiments described herein comprise various layers of conductive and nonconductive materials. While these materials are identified in accordance with the preferred embodiment, it should be appreciated that any alternative materials suitable for use in the intended MEMS application, and that are selectively etchable if necessary, could be substituted for the disclosed materials. For example, sacrificial layer  20  could be silicon dioxide and the insulating layer  22  could be silicon nitride with no change in functionality.  
         [0034]    The above has been described as preferred embodiments of the present invention. It will occur to those that practice the art that many modifications may be made without departing from the spirit and scope of the invention. For example, it is appreciated by one having ordinary skill in the art that the structure of the movable MEMS element  14  may differ so long as it is electrically isolated and includes a conductive member that is operable to create, for example, a capacitance that varies in accordance with the motion. Accordingly, variations of the embodiments described herein will become apparent to those having ordinary skill in the art, and such variations are within the scope of the present invention, as defined by the following claims.