Abstract:
The present invention discloses a method for making a MEMS device, comprising: providing a zero-layer substrate; forming a MEMS device region on the substrate, wherein the MEMS device region is provided with a first sacrificial region to separate a suspension structure of the MEMS device from another part of the MEMS device; removing the first sacrificial region by etching; and micromachining the zero-layer substrate.

Description:
BACKGROUND 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to a method for making a micro-electro-mechanical system (MEMS) device, in particular to a method for making a MEMS device which is compatible to a standard CMOS process and can be easily integrated with post treatment processes such as surface micromachining or bulk micromachining. 
         [0003]    2. Description of Related Art 
         [0004]    MEMS devices are used in a wide variety of products such as micro-acoustical sensor, gyro-sensor, accelerometer, etc. Most prior art MEMS manufacturing processes are not fully compatible with a standard CMOS process. For example, U.S. Pat. Nos. 5,490,220; 7,190,038; 7,202,101 disclose MEMS manufacturing processes which require special material or equipment that are not provided in a standard CMOS process, and thus it costs more to manufacture MEMS devices thereby. Moreover, it is often required to perform a post treatment process, such as surface micromachining or bulk micromachining, on the manufactured MEMS device to enhance the device performance. There are only few MEMS manufacturing processes that are compatible with a standard CMOS process, such as U.S. Pat. No. 5,970,315. However, this conventional process does not take into consideration how a post treatment process can be integrated therewith. Thus, it is desired to provide a method for making a MEMS device, which is fully compatible with the current CMOS process, and can be easily integrated with MEMS post treatment processes. 
       SUMMARY 
       [0005]    It is an objective of the present invention to provide a method for making a MEMS device, which is fully compatible with the current CMOS process, and can be easily integrated with post treatment processes such as surface micromachining or bulk micromachining. 
         [0006]    In accordance with the foregoing and other objectives of the present invention, and from one aspect of the present invention, the present invention discloses method for making a MEMS device, comprising: providing a zero-layer substrate; forming a MEMS device region on the substrate, wherein the MEMS device region is provided with a first sacrificial region to separate a suspension structure of the MEMS device from another part of the MEMS device; removing the first sacrificial region by etching; and micromachining the zero-layer substrate. 
         [0007]    In the above method, the step of removing the first sacrificial region and the step of micromachining the zero-layer substrate can be performed in any order. 
         [0008]    The micromachining step in the method may be surface micromachining, or bulk micromachining, or both. 
         [0009]    In one embodiment, the zero-layer substrate is a silicon substrate, and the surface micromachining step includes isotropically etching the zero-layer substrate by XeF2. 
         [0010]    In another embodiment, the zero-layer substrate is a silicon substrate, and the bulk micromachining step includes: anisotropically etching the zero-layer substrate from its back side to reduce its thickness in a selected region, followed by anisotropically etching the zero-layer substrate from its top side. The anisotropic etch may be ICP (Inductively Coupled Plasma) etch. 
         [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-1D  show an embodiment according to the present invention. 
           [0014]      FIGS. 2A-2D  show another embodiment according to the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    The drawings as referred to throughout the description of the present invention are for illustration only, but not drawn according to actual scale. 
         [0016]    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 , a first metal layer  13   a , a first via layer  12   b , a second metal layer  13   b , and a second via layer  12   c . In one embodiment, the contact layer and the via layers can be made of tungsten; the metal layers can be made of aluminum; and the dielectric layer 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. The pattern of each of the layers  12   a - 12   c  and  13   a - 13   b  is thus that a sacrificial region  14   a  and an isolation region  14   b  are formed therein. The detailed pattern of each layer is not shown for simplicity of the drawings. The regions  14   a  and  14   b  may be made of the same or different materials; in this embodiment they are both made of an oxide, such as silicon dioxide. The sacrificial region  14   a  is provided so that the suspension structure of the MEMS device to be manufactured is separated from the silicon substrate (also referring to  FIG. 1D ). The isolation region  14   b  is provided to isolate the MEMS device region  100  from the other circuit region  200 ; its cross sectional pattern need not be exactly as shown but can be of any other shape. 
         [0017]    Referring to  FIG. 1B , next via layers  12   d - 12   f  and metal layers  13   c - 13   f  are deposited to form the MEMS structure. The number of the via layers and metal layers can be modified; what is shown is only one example. A sacrificial region  14   c  is formed to separate the suspension structure of the MEMS device from another part of the MEMS device. According to the design of the MEMS device structure, in this embodiment, a sacrificial region  14   d  is also formed to separate an upper structure of the MEMS device from a lower structure thereof (also referring to  FIG. 1C ). This sacrificial region  14   d  can be omitted if the suspension structure of the MEMS device is one integrated body. The sacrificial regions  14   c  and  14   d  can be made of oxides such as silicon dioxide. The sacrificial regions  14   a ,  14   c  and  14   d  and the isolation region  14   b  can be formed concurrently with the patterns of the via layers and the metal layers. 
         [0018]    Referring to  FIG. 1C , in one embodiment, a mask  16  is formed and patterned; and an etch step is performed to remove the oxides in the sacrificial regions  14   a ,  14   c  and  14   d  (for better identification of these sacrificial regions, the sacrificial region  14   c  is also referred to as the first sacrificial region, the sacrificial region  14   d  is also referred to as the second sacrificial region, and the sacrificial region  14   a  is also referred to as the third sacrificial region). The mask  16  shields the regions other than the sacrificial regions to protect those other regions from etching. The etch for example can be HF (hydrogen fluoride) vapor etch, or BOE (buffered oxide etch) by immersing the whole wafer in an acid tank. The mask  16  for example can be a photoresist layer patterned by lithography, or made of other single or composite materials, such as metal or amorphous silicon. Since a purpose of the mask  16  is to protect the other circuit region  200 , a passivation layer which is often used in the circuit region may be used as the mask  16 . The passivation layer may be a nitride layer or a bi-layer structure including an oxide layer and a nitride layer. 
         [0019]    Referring to  FIG. 1D , after the sacrificial regions are removed, a surface micromachining step is taken to etch the zero-layer silicon substrate from its top side. The etch step for example is an isotropic etch by XeF2 gas. If the mask  16  is a photoresist layer which has not yet been totally consumed, it can be removed after the step of  FIG. 1D . If the mask  16  is a layer with some other function, such as the passivation layer described above, then the process stops at the step of  FIG. 1D , and a desired MEMS device is obtained. 
         [0020]    Now referring to  FIG. 2A  for the second 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 a contact layer  12   a , via layers  12   b - 12   f , and metal layers  13   a - 13   f . Similar to the previous embodiment, the contact layer and the via layers can be made of tungsten; the metal layers can be made of aluminum; and the dielectric layer can be made of oxides such as silicon dioxide. The suspension structure of the MEMS device in this embodiment is different from that of the previous embodiment, so it is not necessary to form a sacrificial region  14   a  (but can be formed if desired); however an isolation region  14   b  is preferably formed to isolate the MEMS device region  100  from the other circuit region  200 . A sacrificial region  14   c  is also formed depending on the design of the MEMS device. The isolation region  14   b  and the sacrificial region  14   c  can be made of oxides such as silicon dioxide. The sacrificial region  14   d  in the previous embodiment is not shown in the figure, but it can be provided if required. 
         [0021]    Next referring to  FIG. 2B , to form the desired MEMS structure, an anisotropic etch step is taken from the back side of the substrate, forming a recess region  15  at the back side of the substrate to reduce the thickness of the substrate in this region. The etch step for example can be an ICP (Inductively Coupled Plasma) etch. This step is part of the bulk micromachining process. Alternatively, this step can be taken after the step of  FIG. 2C . 
         [0022]    Referring to  FIG. 2C , in one embodiment, a mask  16  is formed and patterned; and an etch step is performed to remove the oxide in the sacrificial region  14   c . The mask  16  shields the regions other than the first sacrificial region to protect those other regions from etching. The etch for example can be HF (hydrogen fluoride) vapor etch, or BOE (buffered oxide etch) by immersing the whole wafer in an acid tank. The mask  16  for example can be a photoresist layer, or other single or composite material layers, such as metal layer, amorphous silicon layer, or passivation layer which is often used in the circuit region. The passivation layer may be a nitride layer or a bi-layer structure including an oxide layer and a nitride layer. 
         [0023]    Referring to  FIG. 2D , after the sacrificial region  14   c  is removed, an etch step is taken to etch the zero-layer silicon substrate, by ICP etch for example, to complete the MEMS device. Similar to the previous embodiment, if the mask  16  is a photoresist layer which has not yet been totally consumed, it can be removed. If the mask  16  is a layer with some other function, it can be kept. This embodiment is different from the previous embodiment in that the suspension structure  101  of the MEMS device includes the silicon substrate. 
         [0024]    It can be readily understood by those skilled in this art from the foregoing description of the embodiments that the front end process of the first and second embodiments are identical ( FIGS. 1A-1B  and  FIG. 2A , and even  FIG. 1C  and  FIG. 2B ), except the layout may be different. Thus, according to the present invention, a wafer may be processed by the same steps to form a basic structure, and thereafter subject to surface and bulk micromachining processes at different locations on the same wafer. Such flexibility does not exist in prior art, and therefore the present invention is better. 
         [0025]    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. For example, the materials, number of metal layers, etch details, and so on, can be modified without departing from the spirit of the present invention. One skilled in this art can readily think of other modifications and variations in light of the teaching by the present invention. In view of the foregoing, it is intended that the present invention cover all such modifications and variations, which should interpreted to fall within the scope of the following claims and their equivalents.