Abstract:
The present invention discloses a monolithic z-axis torsional CMOS MEMS accelerometer, it includes a curl matching frame, two anchors, a first comb structure, a second comb structure and a proof mass. With the implementation of the present invention, the capacitance sensitivity of Z+ direction and Z− direction sensing signals by the accelerometer can be improved. On the other hand, due to the feasibility of applying micromachining etch processes from the top side, the ease and the yield of production are both promoted.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to MEMS accelerometers, and more particularly, to a monolithic z-axis torsional CMOS MEMS accelerometer. 
         [0003]    2. Description of Related Art 
         [0004]    Due to rapid development of semiconductor processes and increasingly sophisticated MEMS technology, sensor structures have a trend toward miniaturization, thereby broadening their application. In this regard, accelerometers are widely used in portable devices and mobile application devices nowadays. 
         [0005]    However, the manufacturing processes of most accelerometers are intricate. As a result, their production yield and detection accuracy are greatly limited. Process innovations are put forth at times, but they are focused largely on the field of application and are seldom conducive to the enhancement of the precision and production yield of accelerometers. 
         [0006]    Accordingly, it is imperative to provide an accelerometer structure which is easy to manufacture, faces little difficulty in the manufacturing process, exhibits high production yield, has a widened range of operation of the accelerometer, and enhances the sensitivity of the accelerometer. The accelerometer structure thus provided is not only important to the semiconductor industry and MEMS industry but also crucial to the research and application of handheld, mobile, and miniaturized portable devices. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The present invention provides a monolithic z-axis torsional CMOS MEMS accelerometer made of a complementary metal-oxide semiconductor (CMOS) and a MEMS semiconductor. The monolithic z-axis torsional CMOS MEMS accelerometer comprises a curl matching frame, two anchors, a first comb structure, a second comb structure, and a proof mass. According to the present invention, the monolithic z-axis torsional CMOS MEMS accelerometer manifests structural asymmetry and features an impedance structure composed of the first comb structure and the second comb structure which oppose each other, and therefore the monolithic z-axis torsional CMOS MEMS accelerometer is conducive to the enhancement of the sensitivity of an accelerometer and the symmetry of sensing signals. In addition, an etching process can be performed on the front of a complementary metal-oxide semiconductor (CMOS) layers, it reduces the difficulty in the manufacturing process but increases the production yield. 
         [0008]    The present invention provides a monolithic z-axis torsional CMOS MEMS accelerometer made of a complementary metal-oxide semiconductor (CMOS) layers with a micromachining process. The monolithic z-axis torsional CMOS MEMS accelerometer comprises: a curl matching frame comprising a first side zone, a second side zone, a third side zone, and a fourth side zone which are successively connected in an enclosing manner, wherein the first side zone and the third side zone are opposite each other, whereas the second side zone and the fourth side zone are opposite each other; two anchors disposed at the first side zone and the third side zone, respectively; a first comb structure adjoining the second side zone, a second side zone-adjoining portion of the first side zone, and a second side zone-adjoining portion of the third side zone; a second comb structure adjoining the fourth side zone, a fourth side zone-adjoining portion of the first side zone, and a fourth side zone-adjoining portion of the third side zone; and a proof mass disposed between the first comb structure and the second comb structure inside the curl matching frame to adjoin the first comb structure and the second comb structure, wherein an axis of the proof mass is defined by a virtual line which connects the two anchors, and two disconnected regions which are symmetrical to each other but do not adjoin the proof mass are disposed between the axis and the first comb structure. 
         [0009]    Implementation of the present invention at least involves the following inventive steps: 
         [0010]    1. the manufacturing process is simple and incurs low costs; 
         [0011]    2. the accelerometer exhibits high sensitivity and satisfactory symmetry; and 
         [0012]    3. an etching process can be performed on the front of a complementary metal-oxide semiconductor and the front of a MEMS structure (wherein the fronts oppose the semiconductor substrate) twice to thereby reduce the difficulty in the manufacturing process but effectively increase the production yield. 
         [0013]    The features and advantages of the present invention are detailed hereinafter with reference to the preferred embodiments. The detailed description is intended to enable a person skilled in the art to gain insight into the technical contents disclosed herein and implement the present invention accordingly. In particular, a person skilled in the art can easily understand the objects and advantages of the present invention by referring to the disclosure of the specification, the claims, and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0014]    The invention as well as a preferred mode of use, further objectives and advantages thereof will be best understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein: 
           [0015]      FIG. 1  is a top view of a monolithic z-axis torsional CMOS MEMS accelerometer according to the embodiment of the present invention; 
           [0016]      FIG. 2  is a schematic view of a first comb structure and a second comb structure, which are formed from sensing units arranged side by side, according to the embodiment of the present invention; 
           [0017]      FIG. 3  is a perspective view of a sensing unit according to the embodiment of the present invention; 
           [0018]      FIG. 4A  is a schematic view of a monolithic z-axis torsional CMOS MEMS accelerometer and the sensing unit according to the embodiment of the present invention; 
           [0019]      FIG. 4B  is a schematic view of the sensing units adapted to form the first comb structure and the second comb structure and electrically connected according to the embodiment of the present invention; 
           [0020]      FIG. 5  is a schematic view of a mobile single unit of a sensing unit, which undergoes upward movement in a direction perpendicular to a plane, according to the embodiment of the present invention; 
           [0021]      FIG. 6  is a schematic view of a mobile single unit of a sensing unit, which undergoes downward movement in a direction perpendicular to a plane, according to the embodiment of the present invention; and 
           [0022]      FIG. 7  is a schematic view of the operation of a monolithic z-axis torsional CMOS MEMS accelerometer according to the embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Referring to  FIG. 1 , in an embodiment of the present invention, a monolithic z-axis torsional CMOS MEMS accelerometer  100 , which is made of a complementary metal-oxide semiconductor (CMOS) layers with a micromachining process, comprises a curl matching frame  10 , two anchors  20 , a first comb structure  30 , a second comb structure  40 , and a proof mass  50 . 
         [0024]    Referring to  FIG. 1 , the curl matching frame  10  comprises first side zone  11 , second side zone  12 , third side zone  13 , and fourth side zone  14  which adjoin successively in an enclosing manner. The first side zone  11  and the third side zone  13  are opposite each other. The second side zone  12  and the fourth side zone  14  are opposite each other. The fourth side zone  14  and the first side zone  11  adjoin. The curl matching frame  10  is made of a complementary metal-oxide semiconductor (CMOS) layers or made of a combination of a complementary metal-oxide semiconductor (CMOS) layers with a micromachining process. 
         [0025]    Referring to  FIG. 1 , the two anchors  20  are disposed at the first side zone  11  and the third side zone  13 , respectively. The anchors  20  are located at the midpoint of the first side zone  11  or the midpoint of the third side zone  13 . 
         [0026]    Referring to  FIG. 1 , the first comb structure  30  adjoins second side zone  12 , a second side zone-adjoining portion of first side zone  11 , and a second side zone-adjoining portion of third side zone  13 . 
         [0027]    Referring to  FIG. 1 , the second comb structure  40  adjoins fourth side zone  14 , a fourth side zone-adjoining portion of first side zone  11 , and a fourth side zone-adjoining portion of third side zone  13 . The second comb structure  40  and the first comb structure  30  are equal in size and shape. 
         [0028]    Both the first comb structure  30  and the second comb structure  40  are made of a complementary metal-oxide semiconductor (CMOS) layers or a combination of a complementary metal-oxide semiconductor (CMOS) layers with a micromachining process. 
         [0029]    Referring to  FIG. 1 , a proof mass  50  disposed between the first comb structure  30  and the second comb structure  40  inside the curl matching frame  10  to adjoin the first comb structure  30  and the second comb structure  40 , wherein an axis  51  of the proof mass  50  is defined by a virtual line which connects the two anchors  20 , and two disconnected regions  52  which are symmetrical to each other but do not adjoin the proof mass  50  are disposed between the axis  51  and the first comb structure  30 . 
         [0030]    Due to the disconnected regions  52 , the portion of the proof mass  50 , which is disposed between the axis  51  and the first comb structure  30 , has a lower weight than the portion of the proof mass  50 , which is disposed between the axis  51  and the second comb structure  40 . Therefore, due to unequal weight of the two ends of the axis  51 , the monolithic z-axis torsional CMOS MEMS accelerometer  100  readily undergoes seesawed movement in the Z-axis direction, with the axis  51  functioning as the fulcrum. 
         [0031]    The Z-axis direction is perpendicular to the top-view plane of the monolithic z-axis torsional CMOS MEMS accelerometer  100  shown in  FIG. 1 . 
         [0032]    Referring to  FIG. 1 ,  FIG. 2  and FIG,  3 , the first comb structure  30  and the second comb structure  40  each comprise a plurality of sensing units  60  arranged side by side. The sensing units  60  are each formed from a first fixing single unit  61 , a mobile single unit  62 , and a second fixing single unit  63  which are arranged successively but not connected. 
         [0033]    Each first fixing single unit  61  and each second fixing single unit  63  of the first comb structure  30  adjoin the second side zone  12 . Each first fixing single unit  61  and each second fixing single unit  63  of the second comb structure  40  adjoin the fourth side zone  14 . Each mobile single unit  62  adjoins the proof mass  50 . Each mobile single unit  62  inside the first comb structure  30  and the second comb structure  40  adjoins the proof mass  50 . 
         [0034]    Referring to  FIG. 3  and  FIG. 4A , the first fixing single units  61  each have a first upper portion  611  and a first lower portion  612  which are separated by a silicon dioxide unit  70 . The first upper portion  611  and the mobile single unit  62  together form a first upper capacitor. The first lower portion  612  and the mobile single unit  62  together form a first lower capacitor. The second fixing single units  63  each have a second upper portion  631  and a second lower portion  632  which are separated by the silicon dioxide unit  70 . The second upper portion  631  and the mobile single unit  62  together form a second upper capacitor. The second lower portion  632  and the mobile single unit  62  together form a second lower capacitor. It is because capacitive coupling occurs between any two conductors or semiconductors which are not in contact with each other, 
         [0035]    Referring to  FIG. 4A  and  FIG. 4B , it is practicable for each first upper portion  611  and each second upper portion  631  of the first comb structure  30  to be electrically connected by a conductor  90 , for each first lower portion  612  and each second lower portion  632  of the first comb structure  30  to be electrically connected by another conductor  90 , for each first upper portion  611  and each second upper portion  631  of the second comb structure  40  to be electrically connected by yet another conductor  90 , and for each first lower portion  612  and each second lower portion  632  of the second comb structure  40  to be electrically connected by a further conductor  90 . 
         [0036]    Therefore, the first comb structure  30  and the second comb structure  40  form an upper capacitor and a lower capacitor, respectively. The upper capacitor of the first comb structure  30  connects with each first upper capacitor by the conductor  90 . The lower capacitor of the first comb structure  30  connects with each first lower capacitor by the conductor  90 . The upper capacitor of the second comb structure  40  connects with each second upper capacitor by the conductor  90 . The lower capacitor of the second comb structure  40  connects with each second lower capacitor by the conductor  90 , 
         [0037]    The first upper portion  611  of the first comb structure  30  is electrically connected to the first lower portion  612  of the second comb structure  40  by a first conductor  91  which penetrates the proof mass  50 . The first lower portion  612  of the first comb structure  30  is electrically connected to the first upper portion  611  of the second comb structure  40  by a second conductor  92  which penetrates the proof mass  50 . Therefore, it is feasible to not only electrically connect the upper capacitor of the first comb structure  30  to the lower capacitor of the second comb structure  40  but also electrically connect the lower capacitor of the first comb structure  30  to the upper capacitor of the second comb structure  40 . 
         [0038]    The aforesaid connection technique effectuates compensation and enables Z+ direction and Z− direction (i.e., the two opposite directions of Z-axis direction) to be consistent in capacitance variation. The capacitors of the first comb structure  30  and the second comb structure  40  which flank the axis  51  are connected alternately. Alternatively, electrodes in the semiconductors which flank the axis  51  are equal in their distances from the axis  51 , such that the electrodes are equal in their swings from above and below the Z-axis direction, and the connection effectuated alternately equalizes the sensing capacitance at the left and right ends, thereby attaining equal total sensing capacitance and total capacitance variation. 
         [0039]    Referring to  FIG. 5  and  FIG. 6 , the upper edge of the first fixing single unit  61  and the upper edge of the second fixing single unit  63  together define a XY plane  80 , such that the mobile single unit  62  moves in a direction perpendicular to the XY plane  80 . The XY plane  80  is the top-view plane of the monolithic z-axis torsional CMOS MEMS accelerometer  100  shown in  FIG. 1 . The mobile single unit  62  moves in a direction perpendicular to the XY plane  80 , such that the mobile single unit  62  moves up or down in the Z-axis direction, Upward movement along Z-axis is indicated by the arrow shown in  FIG. 5 , and downward movement along Z-axis is indicated by the arrow shown in  FIG. 6 . 
         [0040]    When the monolithic z-axis torsional CMOS MEMS accelerometer  100  undergoes movement along Z-axis, the proof mass  50  is subjected to forces not uniformly distributed because the weights at the two ends of the axis  51  are unequal. As a result, the proof mass  50  rotates about the axis  51  and drives the mobile single unit  62  on the second comb structure  40  to rotate, and in consequence the capacitance of the first upper capacitor, first lower capacitor, second upper capacitor, and second lower capacitor between the mobile single unit  62  and the first fixing single unit  61  or the second fixing single unit  63  varies when coupled. Given the variations in capacitance, the magnitude of sensed forces applied along Z-axis can be calculated. With the calculated magnitude of the forces applied along Z-axis, it is feasible to estimate or calculate the acceleration along Z-axis, so as for the accelerometer to function well. 
         [0041]    Referring to  FIG. 7 , there is shown a schematic view of the operation of the monolithic z-axis torsional CMOS MEMS accelerometer  100  according to the embodiment of the present invention. As shown in  FIG. 7 , the proof mass  50  rotates about the axis  51 , because the monolithic z-axis torsional CMOS MEMS accelerometer  100  moves in Z-axis direction. Referring to  FIG. 7 , both the proof mass  50  and the second comb structure  40 , which are on the left, move upward in Z-axis direction, whereas both the proof mass  50  and the first comb structure  30 , which are on the right, move downward in Z-axis direction. 
         [0042]    During the manufacturing process, the size of the monolithic z-axis torsional CMOS MEMS accelerometer  100  and the ratio of constituent elements of the monolithic z-axis torsional CMOS MEMS accelerometer  100  are subject to change as needed. For instance, the first comb structure  30  shown in  FIG. 1  extends 80 μm with an error of &lt;10% from the second side zone  12  to the axis  51 , whereas the second comb structure  40  extends 80 μm with an error of &lt;10% from the fourth side zone  14  to the axis  51 . The axis  51  is of a length of 276 μm with an error of &lt;10%, and a width of 6 μm with an error of &lt;10%. 
         [0043]    The embodiments described above are intended only to demonstrate the technical concept and features of the present invention so as to enable a person skilled in the art to understand and implement the contents disclosed herein. It is understood that the disclosed embodiments are not to limit the scope of the present invention. Therefore, all equivalent changes or modifications based on the concept of the present invention should be encompassed by the appended claims.