Abstract:
The present invention discloses an MEMS sensor and a method for making the MEMS sensor. The MEMS sensor according to the present invention comprises: a substrate including an opening; a suspended structure located above the opening; and an upper structure, a portion of which is at least partially separated from a portion of the suspended structure; wherein the suspended structure and the upper structure are separated from each other by a step including metal etch.

Description:
BACKGROUND 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to a micro-electro-mechanical system (MEMS) sensor, in particular to a MEMS pressure sensor; the present invention also relates to a method for making such MEMS sensor, which is compatible to standard CMOS process. 
         [0003]    2. Description of Related Art 
         [0004]    MEMS devices are used in a wide variety of products; examples of such applications are absolute and relative sensors such as blood pressure sensor and micro-acoustical microphone. U.S. Pat. No. 6,012,336; U.S. Pat. No. 6,536,281; U.S. Pat. No. 6,928,879; U.S. Pat. No. 7,121,146; U.S. Pat. No. 6,743,654; and U.S. Pat. No. 7,135,149 are relevant art, but the processes employed in these prior art either are too complicated or require equipment or materials which are not compatible to or not standard in CMOS process. A MEMS sensor having a structure which can be manufactured by a standard CMOS process is desired in this industry. 
       SUMMARY OF THE INVENTION 
       [0005]    It is an objective of the present invention to provide a MEMS sensor which can be manufactured by a process fully compatible to the present standard CMOS process. 
         [0006]    It is another objective of the present invention to provide a method for making such MEMS sensor. 
         [0007]    In accordance with the foregoing and other objectives of the present invention, from one aspect, the present invention discloses a method for making a MEMS sensor, comprising: providing a substrate; forming interconnection on the substrate, part of the interconnection forming an etchable structure separating at least a portion of a suspended structure of the MEMS sensor from the rest of the MEMS sensor; etching the back side of the substrate to expose the etchable structure; and etching the etchable structure. 
         [0008]    From another aspect, the present invention discloses a MEMS sensor comprising: a substrate including an opening; a suspended structure located above the opening; and an upper structure, a portion of which is at least partially separated from a portion of the suspended structure; wherein the suspended structure and the upper structure are separated from each other by a step including metal etch. 
         [0009]    In the above MEMS sensor and the method, the back side of the substrate can be sealed by hermetical package so that the MEMS sensor becomes an absolute sensor. The material for hermetical package for example can be silicon or glass. 
         [0010]    The sensor can further comprise a guard ring encompassing the suspended structure. 
         [0011]    It is to be understood that both the foregoing general description and the following detailed description are provided as examples, for illustration and not for limiting the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings. 
           [0013]      FIGS. 1A-1E  show an embodiment according to the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    The drawings as referred to throughout the description of the present invention are for illustration only, but not drawn according to actual scale. 
         [0015]    Referring to  FIG. 1A  for the first embodiment of the present invention, a zero-layer wafer substrate  11  is provided, which for example can be a silicon wafer so that the process is compatible with a standard CMOS process. Next, transistor devices can be formed by standard CMOS process steps as required (not shown), followed by deposition, lithography and etch steps to form interconnection including a contact layer  12   a,  metal layers  13 , and via layers  12   b.  A dielectric layer  14  is provided to isolate the metal patterns of the layers where there should not be connection. Furthermore, a bond pad pattern  15  is formed on the topmost metal layer, and a passivation layer  16  is formed on top of the overall structure. In one embodiment, the contact layer  12   a  and the via layers  12   b  can be made of tungsten; the metal layers  13  can be made of aluminum; and the dielectric layer  14  can be made of oxides such as silicon dioxide. Other conductive or dielectric materials can be used to replace what are suggested above, and the structure can include more or less number of metal layers. 
         [0016]    In the shown structure, between the innermost first metal layer  13  and the outermost metal structure (item  30  of  FIG. 1C , referred to also as the “upper structure” hereinafter), an etchable structure  20  is formed which includes a portion of the contact layer  12   a,  metal layers  13 , and a via layer  12   b.  The purpose of the etchable structure  20  is to define a suspended structure of the MEMS device, which will become clearer as referring to the following process steps. The outermost metal structure can either provide a guard ring function to protect the circuitry (not shown) from etch damage or moisture, or act as an upper electrode (to be explained later). 
         [0017]    Referring to  FIG. 1B , the back side of the substrate  11  is subject to an etch, such as an ICP (Inductively Coupled Plasma) DRIE (Deep Reactive Ion Etch), to forming an opening exposing the etchable structure  20 . 
         [0018]    Referring to  FIG. 1C , the etchable structure  20  is etched away to leave a space  20  which separates the suspended structure  40  from the rest of the MEMS device. The etch can for example can be wet etch by sulfide acid and hydrogen peroxide solution. Note that the term “separate” in the context of this specification means “at least partially disconnected” but does not have to be “totally disconnected”. In a position not shown in this figure, the suspended structure  40  is still connected with the zero-layer wafer substrate  11 . In one application, the suspended structure  40  functions as a lower electrode and fixed to the substrate; it does not deform during operation of the sensor. On the other hand, the upper structure  30  functions as an upper electrode; its uppermost metal layer  13 , or a composite layer of the uppermost metal layer  13 , part of the dielectric layer  14  and part of the passivation layer  16 , form a thin film structure which elastically deforms during operation of the sensor according to acoustical or air pressure variation. The deformation causes a change of the capacitance. 
         [0019]    After the etchable structure  20  is etched, as shown in  FIG. 1D , part or all of the passivation layer  16  can be removed by lithography and etch, to open the bond pads  15  at the topmost layer of the interconnection. If the upper structure  30  is used as an upper electrode and it is desired to reduce the thickness of the upper electrode thin film, the passivation layer  16  above the upper electrode, or together with the dielectric layer  14  above the upper electrode, can removed in this step. 
         [0020]    If what is intended to make is a relative sensor such as acoustical sensor or accelerometer, the process can stop here. If what is intended to make is an absolute sensor, referring to  FIG. 1E , preferably, a hermetical package is formed on the back side of the zero-layer wafer substrate  11  by a sealing material  17 , which for example can be silicon or glass. 
         [0021]    The foregoing process is superior to the prior art because it is fully compatible with standard CMOS process. By such process, a MEMS device and CMOS devices can be made on the same chip in a CMOS fab conveniently. 
         [0022]    Although the present invention has been described in detail with reference to certain preferred embodiments thereof, the description is for illustrative purpose and not for limiting the scope of the invention. One skilled in this art can readily think of other modifications and variations in light of the teaching by the present invention. For example, the materials, number of metal layers, etch in the shown embodiments are provided as examples; they can be modified in many ways. As another example, the outermost metal structure does not have to form a guard ring. Therefore, all such modifications and variations should be interpreted to fall within the scope of the following claims and their equivalents.