Patent Publication Number: US-2017356929-A1

Title: Z-axis structure of accelerometer and manufacturing method of z-axis structure

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of micro-electromechanical systems (MEMS), and more particularly, relates to a micro-electromechanical accelerometer, in particular to a Z-axis structure of an accelerometer. The present invention further relates to a manufacturing method of the Z-axis structure. 
     BACKGROUND OF THE INVENTION 
     Conventional Z-axis accelerometers are flat-plate-capacitive accelerometers, and the movement mode of mass blocks is similar to that of a seesaw structure. Referring to  FIG. 1 , there are two metallic fixed electrodes  2  on a substrate  1  below a mass block  3 , and the two fixed electrodes  2  are attached onto the surface of the substrate  1 . The capacitor (C 1  or C 2 ) is formed between each fixed electrode  2  and the corresponding mass block  3 , wherein the mass block  3  is supported above the substrate by an anchor  4 . 
     This Z-axis structure is relatively sensitive to deformation caused by changes of external stress and temperature. The deformation caused by the changes of the external stress and the temperature acts on the substrate  1  first, and then transfers to the fixed electrode  2 . As the fixed electrode  2  is attached onto the substrate  1 , deformation of the substrate  1  is directly to be reflected on the fixed electrode  2 . Under normal circumstances, the deformations of the two fixed electrodes  2  are unequal, which results the capacitances of the two fixed electrodes  2  are unequal in the absence of an accelerometer input, and finally outputs deviation signal, which forms the zero point offset of an accelerometer reflected on chip. From the view of the designer, the smaller the zero point offset is, the better the effect of an accelerometer is. However, because of the structure of an accelerometer, the zero point offset caused by the changes of the external stress and the temperature is unavoidable. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a new technical solution of Z-axis structure of an accelerometer. 
     According to the first aspect of the present invention, there provides a Z-axis structure of an accelerometer. The Z-axis structure comprises a substrate, a fixed electrode and a mass block, wherein a first anchor is arranged on a surface of the substrate; the fixed electrode is connected onto the corresponding first anchor at an end thereof; the fixed electrode is suspended above the substrate via the first anchor; an intermediate anchor is also arranged on the surface of the substrate; and the mass block is suspended above the fixed electrode via the intermediate anchor. 
     Preferably, the fixed electrode is integrally formed with the first anchor. 
     Preferably, the first anchor is adjacent to the intermediate anchor. 
     Preferably, the fixed electrode is made of monocrystalline silicon material or polycrystalline silicon material. 
     Preferably, a plurality of through holes is formed on the mass block and the fixed electrode respectively. 
     Preferably, a lower surface of the fixed electrode is further provided with a reinforcing structure. 
     The present invention further provides a manufacturing method of a Z-axis structure, comprising the following steps: (a), etching to form two first anchors and a first intermediate anchor located therebetween on a lower surface of a fixed electrode; (b), press-fitting fixed electrode onto a substrate by the first anchor and the first intermediate anchor; (c), etching an upper surface of the fixed electrode, except for a location of the first intermediate anchor, to make the first intermediate anchor to be higher than other locations on the upper surface of the fixed electrode; (d), etching away locations between the first anchor and the first intermediate anchor on the fixed electrode to separate the first intermediate anchor from the fixed electrode, and etching the fixed electrode into a predetermined size; (e), press-fitting the mass block at an upper end of the first intermediate anchor; and (f), etching on the mass block to form a second intermediate anchor located on the first intermediate anchor as well as a connecting beam for connecting the mass block and the second intermediate anchor. 
     The present invention further provides a manufacturing method of a Z-axis structure, comprising the following steps: (a), depositing a first sacrificial layer on a substrate, and etching to form regions for a first anchor and a first intermediate anchor on the first sacrificial layer; (b), depositing fixed electrode layer on the first sacrificial layer and the regions for the first anchor and first intermediate anchor; (c), etching on the fixed electrode layer to form a pattern of fixed electrode connected with the first anchor and a pattern of the first intermediate anchor, and etching to form a plurality of through holes; (d), depositing a second sacrificial layer on the fixed electrode and a region for the first intermediate anchor; (e), etching away the second sacrificial layer located right on the first intermediate anchor; (f), depositing a mass block layer on the second sacrificial layer and etching on the mass block layer to form patterns of the mass block and the second intermediate anchor, wherein the second intermediate anchor is located right on the first intermediate anchor; and etching on the mass block to form a plurality of through holes; and (g), removing the first sacrificial layer and the second sacrificial layer to form a Z-axis structure. 
     Preferably, the manufacturing method comprises, between steps (b) and (c), a step of flattening the fixed electrode layer to a predetermined thickness. 
     Preferably, the step (f) further comprises flattening the mass block layer to a predetermined thickness before etching on the mass block layer to form the patterns of the mass block and the second intermediate anchor. 
     According to a Z-axis structure of the present invention, a fixed electrode is connected to a substrate by a first anchor, in order to form certain gap between the fixed electrode and the substrate. Because of the gap, the path for deformation to transmit from the substrate to the fixed electrode is cut off, such that the contact area between the fixed electrode and the substrate is reduced, effectively preventing the deformation of the substrate caused by changes of external stress and temperature from transmitting to the fixed electrode, and greatly reducing zero point offset of a Z-axis structure. 
     The inventor of the present invention has found that in the prior art, the deformation of a substrate caused by the changes of the external stress and the temperature can be transmitted to a fixed electrode, thus leading to a fixed electrode to deform, and resulting in the difference values between the two capacitors. Therefore, the technical mission to be achieved or the technical problem to be solved in the present invention is unintentional or unanticipated to those skilled in the art, and thus the present invention refers to a novel technical solution. 
     Further features of the present invention and advantages thereof will become apparent from the following detailed description of exemplary embodiments according to the present invention with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description thereof, serve to explain the principles of the present invention. 
         FIG. 1  is a schematic diagram of a conventional Z-axis structure. 
         FIG. 2  is a schematic diagram of the Z-axis structure in the present invention. 
         FIGS. 3-9  show a schematic flow chart of the manufacturing method of the Z-axis structure shown in  FIG. 2 . 
         FIG. 10  is a schematically structural view of the Z-axis structure according to another embodiment of the present invention. 
         FIGS. 11-18  show a schematic flow chart of the manufacturing method of the Z-axis structure shown in  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Various exemplary embodiments of the present invention will now be described in detail with reference to the drawings. It should be noted that the relative arrangement of the components and steps, the numerical expressions, and numerical values set forth in these embodiments do not limit the scope of the present invention unless it is specifically stated otherwise. 
     The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses. 
     Techniques, methods and apparatus as known by one of ordinary skill in the relevant art may not be discussed in detail but are intended to be part of the specification where appropriate. 
     In all of the examples illustrated and discussed herein, any specific values should be interpreted to be illustrative only and non-limiting. Thus, other examples of the exemplary embodiments could have different values. 
     Notice that similar reference numerals and letters refer to similar items in the following figures, and thus once an item is defined in one figure, it is possible that it need not be further discussed in the accompanying drawings. 
     In an accelerometer adopting a conventional structure, its X-axis and Y-axis directions utilize the mode of translation, while its Z-axis direction utilizes the mode of seesaw-type deflection. Compared with the structure of conventional Z-axis accelerometer, the present invention provides a Z-axis structure in an accelerometer, which can be configured to detect Z-axis acceleration signal in the vertical direction. 
     Embodiment I 
     Referring to  FIG. 2 , the present invention provides a Z-axis structure of an accelerometer. The Z-axis structure comprises a substrate  1 , a mass block  3  and two fixed electrodes  2 , wherein two first anchors  20  are arranged on a surface of the substrate  1  and configured to connect the two fixed electrodes  2 , respectively; the fixed electrode  2  is connected onto the corresponding first anchor  20  at an end thereof; the fixed electrode  2  and the corresponding first anchor  20  can be integrally formed and are L-shaped; and the fixed electrode  2  is located in the horizontal direction, while the first anchor  20  is located in the vertical direction. The fixed electrode  2  is approximately parallel to the substrate  1 . Due to the arrangement of the first anchor  20 , there is a certain gap between each fixed electrode  2  and the substrate  1 . That is, the fixed electrode  2  is suspended above the substrate  1  via the first anchor  20 . The fixed electrode  2  can be fixed by the corresponding first anchor  20 , and of course, can be fixed by a plurality of anchor. 
     An intermediate anchor  4  is arranged between the two first anchors  20 , and is fixed onto the substrate  1 . The mass block  3  is suspended above the fixed electrode  2  via the intermediate anchor  4 . For example, the mass block  3  is connected with the intermediate anchor  4  by an elastic beam, so that the mass block  3  is elastically supported above the substrate  1  and the fixed electrode  2 . Of course, there is certain gap between the mass block  3  and the fixed electrode  2 , so that the detection capacitor is formed between each fixed electrode  2  and the mass block  3  respectively. This is well known to those skilled in the art, and the description is omitted herein. 
     In the Z-axis structure of the present invention, the fixed electrode  2  is connected to the substrate  1  by the first anchor  20 , in order to form certain gap between the fixed electrode  2  and the substrate  1 . Because of the gap, the path for deformation to transmit from the substrate  1  to the fixed electrode  2  is cut off, such that contact area between the fixed electrode  2  and the substrate  1  is reduced, effectively preventing the deformation of the substrate caused by changes of external stress and temperature from transmitting to the fixed electrode, and greatly reducing zero point offset of the Z-axis structure. 
     In the Z-axis structure of the present invention, the first anchor  20  is adjacent to the intermediate anchor  4 . The two first anchors  20  are symmetrically distributed at two sides of the intermediate anchor  4 . Without affecting the acceleration performance, in order to greatly reduce capacitance output deviation caused by changes of external stress and temperature, the first anchor  20  is as close as possible to the intermediate anchor  4 . 
     Further, the fixed electrode  2  is made of monocrystalline silicon material, in order to improve the resistance to deformation of the fixed electrode  2 . Preferably, the thickness of the fixed electrode  2  is 10 microns or above. Furthermore, it may be 20-30 microns. In order to further ensure the strength of the fixed electrode  2 , a reinforcing structure such as a mesh reinforcing rib structure can be arranged on a lower surface of each fixed electrode  2 . 
     Referring to  FIGS. 3-9 , the present invention further provides a manufacturing method of a Z-axis structure, and the method comprises the steps as follows. 
     (a) Two first anchors  20  and a first intermediate anchor  21  located therebetween are etched to form on a lower surface of fixed electrode  2 , wherein the two first anchors  20  are symmetrically distributed at the two sides of the first intermediate anchor  21 , and can be as close as possible to the intermediate anchor  21 . Referring to  FIG. 3 , there is gap among the three anchors. 
     (b) The fixed electrode  2  is press-fitted onto the substrate  1  by the first anchor  20  and the first intermediate anchor  21 . That is, free ends of the first anchor  20  and the first intermediate anchor  21  are press-fitted on the substrate  1 . Referring to  FIG. 4 , the press-fitting manner therebetween can be bonding manner, such as silicon-silicon bonding, silicon-silica bonding or alloy bonding. 
     (c) An upper surface of the fixed electrode  2 , except for the location of the first intermediate anchor  21 , is etched, so that the first intermediate anchor  21  is higher than other locations on the upper surface of the fixed electrode  2 . Referring to  FIG. 5 , it can be also understood that an upper end of the first intermediate anchor  21  is etched out from the upper surface of the fixed electrode  2 , so that the upper end of the first intermediate anchor  21  is higher than the upper surface of the fixed electrode  2 . 
     (d) The locations between the first anchor  20  and the first intermediate anchor  21  on the fixed electrode  2  are etched away, so as to separate the first intermediate anchor  21  from the fixed electrode  2  and the first anchor  20 ; and referring to  FIG. 6 , the fixed electrode  2  is etched into a predetermined size. 
     (e) The upper end of the first intermediate anchor  21  is press-fitted with the mass block  3 . That is, the mass block  3  is press-fitted at the upper end of the first intermediate point  21 . Based on the above-mentioned principle, the press-fitting manner therebetween can be bonding manner, such as silicon-silicon bonding, silicon-silica bonding or alloy bonding. Referring to  FIG. 7 , as the first intermediate anchor  21  is higher than the surface of the fixed electrode  2 , there is certain gap between the mass block  3  press-fitted on the first intermediate anchor  21  and fixed electrode  2 . 
     (f) A second intermediate anchor  31  located on the first intermediate anchor  21  as well as a connecting beam (not shown in the drawing) for connecting the mass block  3  and the second intermediate anchor  31  are etched to form on the mass block  3 , and the mass block  3  is etched into a predetermined size. That is, the above-mentioned intermediate anchor  4  of the present invention includes the first intermediate anchor  21  and the second intermediate anchor  31  which are press-fitted together, wherein the first intermediate anchor  21  is etched out from the fixed electrode  2 , and the second intermediate anchor  31  is etched out from the mass block  3 . Referring to  FIG. 8 , the first intermediate anchor  21  and the second intermediate anchor  31  that are press-fitted together to constitute the intermediate anchor  4 , which is configured to support the mass block  3  above the substrate  1  and the fixed electrode  2 . The second intermediate anchor  31  and the connecting beam are etched out from the whole mass block  3 ; in order to support the mass block  3  elastically above the substrate  1 . 
     Preferably, a step of thinning the fixed electrode  2  by etching is further included between step (b) and step (c), and a step of thinning the mass block  3  by etching is further included between step (e) and step (f), so that the damage caused by etching the fixed electrode  2  and the mass block  3  without support structure is avoided. 
     Referring to  FIG. 9 , of course, the manufacturing method provided in the present invention further includes the step of press-fitting a housing  5  on the substrate  1 , so as to encapsulate all components in the housing  5 . 
     Embodiment II 
     Referring to  FIG. 10 , the present invention provides a Z-axis structure of an accelerometer. The Z-axis structure comprises a substrate  1   a , a mass block  3   a  and two fixed electrodes  2   a , wherein two first anchors  20   a  are arranged on a surface of the substrate  1   a  and configured to connect the corresponding fixed electrode  2   a , respectively; the fixed electrode  2   a  is connected onto the corresponding first anchor  20   a  at an end thereof; the fixed electrode  2   a  and the corresponding first anchor  20   a  can be integrally formed and are L-shaped; the fixed electrode  2   a  is located in the horizontal direction, while the first anchor  20   a  is located in the vertical direction. The fixed electrode  2   a  is approximately parallel to the substrate  1   a . Due to the arrangement of the first anchor  20   a , there is certain gap between each fixed electrode  2   a  and the substrate  1   a . That is, the fixed electrode  2   a  is suspended above the substrate  1   a  via the first anchor  20   a . The fixed electrode  2   a  can be fixed by the corresponding first anchor  20   a , and of course, can be fixed by a plurality of anchor. 
     An intermediate anchor  4   a  is arranged between the two first anchors  20   a , and is fixed onto a surface of the substrate  1   a . The mass block  3   a  is electrically suspended above the fixed electrode  2   a  via the intermediate anchor  4   a . For example, the mass block  3   a  is connected with the intermediate anchor  4   a  by an elastic beam, so that the mass block  3   a  is elastically supported above the substrate  1   a  and the fixed electrode  2   a . Of course, there is certain gap between the mass block  3   a  and the fixed electrode  2   a , so that the detection capacitor is formed between each fixed electrode  2   a  and the mass block  3   a  respectively. This is well known to those skilled in the art, and the description is omitted herein. 
     A plurality of through holes  6   a  is formed on the mass block  3   a  and the fixed electrode  2   a , so as to release the structure of the fixed electrode and the movable mass block easily. 
     In the Z-axis structure of the present invention, the fixed electrode  2   a  is connected with the substrate  1   a  by the first anchor  20   a , in order to form certain gap between the fixed electrode  2   a  and the substrate  1   a . Because of the gap, the path for deformation to transmit from the substrate  1   a  to the fixed electrode  2   a  is cut off, such that contact area between the fixed electrode  2   a  and the substrate  1   a  is reduced, effectively preventing the deformation of the substrate caused by changes of external stress and temperature from transmitting to the fixed electrode, and greatly reducing zero point offset of the Z-axis structure. 
     In the Z-axis structure of the present invention, the first anchor  20   a  is adjacent to the intermediate anchor  4   a . The two first anchors  20   a  are symmetrically distributed at two sides of the intermediate anchor  4   a . Without affecting the acceleration performance, in order to greatly reduce capacitance difference caused by changes of external stress and temperature, the first anchor  20   a  is as close as possible to the intermediate anchor  4   a.    
     Further, the fixed electrode  2   a  is made of monocrystalline silicon material, in order to improve the resistance to deformation of the fixed electrode  2   a . Preferably, the thickness of the fixed electrode  2   a  is 5 microns or above. Of course, if the capacity for processing can be achieved, the thickness of the fixed electrode  2   a  can be less than 5 microns. In order to further ensure the strength of the fixed electrode  2   a , a reinforcing structure such as a mesh reinforcing rib structure can be arranged on a lower surface of each fixed electrode  2   a.    
     Referring to  FIGS. 11-18 , the present invention further provides a manufacturing method of a Z-axis structure, and the method comprises the steps as follows. 
     (a) A first sacrificial layer  7   a  is deposited on a substrate  1   a , and may be made of silicon oxide material, and the regions for a first anchor and a first intermediate anchor are etched to form on the first sacrificial layer  7   a , in particular, which is determined based on shapes of the first anchor and the first intermediate anchor. Referring to  FIG. 11 , for example, if it is required that the two first anchors are symmetrically distributed at two sides of the first intermediate anchor, a corresponding etching region should be formed on the first sacrificial layer  7   a.    
     (b) A fixed electrode layer a is deposited on the first sacrificial layer  7   a  and the first anchor and first intermediate anchor. Referring to  FIG. 12 , the fixed electrode layer a includes fixed electrode located right on the first sacrificial layer  7   a , as well as a first anchor  20   a  and a first intermediate anchor  21   a  located in the regions for first anchor and first intermediate anchor. The first sacrificial layer  7   a  in this region is etched away, so that the first anchor  20   a  and the first intermediate anchor  21   a  are directly deposited on the substrate  1   a , realizing to connect the first anchor  20   a  and the first intermediate anchor  21   a  to the substrate  1   a . The fixed electrode layer a may be made of polycrystalline silicon material, improving the strength of the fixed electrode layer. 
     Considering the influence of the region for the first anchor and first intermediate anchor, in order to finally obtain the fixed electrode layer with a predetermined thickness, the deposited thickness of the fixed electrode layer is larger than the predetermined thickness, and then the flattening treatment is performed. That is, the deposited fixed electrode layer is thinned by etching, and then step (c) is carried out. 
     (c) Patterns of the fixed electrode  2   a  and the first intermediate anchor  21   a  are formed by etching the fixed electrode layer a, and a plurality of through holes  6   a  is etched on the fixed electrode  2   a . That is, referring to  FIG. 13 , the fixed electrode  2   a  is separated from the first intermediate anchor  21   a , while the fixed electrode  2   a  is connected onto the substrate by the first anchor  20   a.    
     (d) A second sacrificial layer  8   a  is deposited on the fixed electrodes  2   a  and the first intermediate anchor  21   a . Referring to  FIG. 14 , the second sacrificial layer  8   a  is not only located above the fixed electrodes  2   a  and the first intermediate anchor  21   a , but also deposited in the through holes  6   a  and gap between the first intermediate anchor  21   a  and the fixed electrodes  2   a . Similarly, the deposited thickness of the second sacrificial layer  8   a  is larger than a predetermined thickness, and then the flattening treatment is performed. That is, the second sacrificial layer  8   a  is thinned by etching, and then step (e) is carried out. 
     (e) Referring to  FIG. 15 , the second sacrificial layer  8   a  located right on the first intermediate anchor  21   a  is etched away to form a groove  80   a.    
     (f) A mass block layer is deposited on the second sacrificial layer  8   a . Here, the mass block layer is deposited not only on the second sacrificial layer  8   a , but also in the groove  80   a , and is connected to the first intermediate anchor  21   a . Similarly, considering the influence of the groove  80   a , in order to finally obtain the mass block layer with a predetermined thickness, the deposited thickness of the mass block layer is larger than the predetermined thickness, and then the flattening treatment is performed. That is, the deposited mass block layer is thinned by etching, and then the following etching process is carried out. 
     The patterns of a mass block  3   a  and a second intermediate anchor  31   a  are formed by etching the mass block layer. Referring to  FIG. 16 , the second intermediate anchor  31   a  is located right on the first intermediate anchor  21   a  and a plurality of through holes  6   a  is formed by etching the mass block  3   a . That is, the mass block  3   a  and the second intermediate anchor  31   a  are etched out from the mass block layer, so that the mass block  3   a  and the second intermediate anchor  31   a  are connected together only through an elastic beam finally. That is, the intermediate anchor  4   a  of the present invention includes the first intermediate anchor  21   a  and the second intermediate anchor  31   a  which are deposited together, wherein the first intermediate anchor  21   a  is etched out from the fixed electrode layer, and the second intermediate anchor  31   a  is etched out from the mass block layer. The first intermediate anchor  21   a  and the second intermediate anchor  31   a  that are deposited together to constitute the intermediate anchor  4   a , which is configured to support the mass block  3   a  above the substrate  1   a  and the fixed electrode  2   a.    
     (g) Referring to  FIG. 17 , the first sacrificial layer  7   a  and the second sacrificial layer  8   a  are removed to form the Z-axis structure of the present invention. The first sacrificial layer and the second sacrificial layer  8   a  can be corroded off via an HF solution or gaseous HF. This is well known to those skilled in the art, and the description is omitted herein. Through the through holes in the mass block  3   a  and the fixed electrode  2   a , the corroding speed of the first sacrificial layer  7   a  and the second sacrificial layer  8   a  can be accelerated, so as to quickly release the mass block  3   a  and the fixed electrode  2   a.    
     Referring to  FIG. 18 , of course, the manufacturing method provided by the present invention further includes the step of press-fitting a housing  5   a  on the substrate  1   a , so as to encapsulate all components in the housing  5   a.    
     The first sacrificial layer  7   a  in step (a) and the second sacrificial layer  8   a  in step (d) are not limited to the silicon oxide material, and also can be made of an organic material such as polyimide (PI). 
     In the manufacturing method of the present invention, in the deposition process of the fixed electrode layer, internal stress can be increased by adjusting a process parameter, while in the deposition process of the mass block layer, internal stress of a film can be reduced by adjusting the process parameter. 
     Although some specific embodiments of the present invention have been demonstrated in detail with examples, it should be understood by a person skilled in the art that the above examples are only intended to be illustrative but not to limit the scope of the present invention. It should be understood by those skilled in the art that the above embodiments could be modified without departing from the scope and spirit of the present invention. The scope of the present invention is defined by the appended claims.