Abstract:
An integrally micromachined acceleration sensor has a mass with a surface facing a stopper. At least one protrusion projects from this surface toward the stopper. In the absence of acceleration, the protrusion is spaced apart from the stopper, but by limiting motion of the mass toward the stopper, the protrusion improves the shock resistance of the acceleration sensor. The protrusion also prevents the mass from sticking to the stopper during the fabrication process. The stopper may have a pattern of holes surrounding the protrusion, so that the protrusion is produced naturally during the wet etching process that separates the mass from the stopper. The holes also shorten the wet etching time.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a micromachined acceleration sensor, more particularly to an acceleration sensor with features that aid the micromachining process and improve the robustness of the sensor. 
         [0003]    2. Description of the Related Art 
         [0004]    Known micromachined acceleration sensors include three-axis acceleration sensors having a mass flexibly linked to a frame by beams with microelectronic strain detectors. Acceleration sensors of this type can be classified into a bonded type, which is formed by micromachining different layers of the sensor on separate substrates and then bonding the layers together, and an integral type, which is formed by micromachining a substrate that already has a layered structure. The present invention relates to a three-axis acceleration sensor of the integral type, such as the one described in Japanese Patent Application Publication (JP) No. 2004-198243. 
         [0005]    The frame of this type of acceleration sensor includes stoppers that limit the motion of the mass. Because the sensor is of the integral type, the micromachining process includes a wet etching step that separates the stoppers from the mass, followed by a cleaning step that rinses the etching solution out from the space between the mass and the stoppers. The dimensions of the acceleration sensors now being produced have become so small that after the cleaning process, the mass and stoppers may still be joined by drops of rinsing solution. This leads to a fabrication problem, because as the remaining rinsing solution dries, its surface tension draws the mass toward the stoppers and may cause the mass and stoppers to stick together. 
         [0006]    JP 2004-294401 (U.S. Patent Application Publication No. 20040187592) discloses a single-axis capacitive acceleration sensor in which the bottom surfaces of the mass and moving electrodes are etched laterally in such a way as to leave protrusions to prevent the bottom surfaces from sticking to the base layer of the substrate, but the formation of these protrusions requires laterally convex extensions of the mass and electrodes. Similar protrusions between the mass and stoppers of a three-axis acceleration sensor could be considered, but in a three-axis sensor the necessary laterally convex extensions would undesirably limit the freedom of motion of the mass. If the lateral dimensions of the mass were to be reduced to regain the necessary freedom of motion, the resulting loss of inertial mass would reduce the sensitivity of the sensor, which would also be undesirable. 
       SUMMARY OF THE INVENTION 
       [0007]    An object of the present invention is to prevent the mass of an acceleration sensor from sticking to the stoppers during the fabrication process. 
         [0008]    Another object of the invention is to shorten the fabrication process. 
         [0009]    Still another object is to increase the robustness of the acceleration sensor. 
         [0010]    Yet another object is to increase the sensitivity of the acceleration sensor. 
         [0011]    The invented acceleration sensor has a patterned layer including a mass attachment section, a peripheral attachment section, at least one beam flexibly linking the mass attachment section to the peripheral attachment section, and at least one stopper contiguously joined to the peripheral attachment section. A mass having a surface facing the stopper is joined to the mass attachment section by a first joining layer. A frame surrounding the mass is joined to the peripheral attachment section by a second joining layer. 
         [0012]    The surface of the mass that faces the stopper has at least one protrusion that protrudes toward the stopper. Absent acceleration, the protrusion is spaced apart from the stopper. Preferably, there are a plurality of such protrusions, which may be arranged in a two-dimensional array extending over substantially the entire surface of the mass that faces the stopper. The protrusions are preferably made of the same material as the first and second joining layers. 
         [0013]    The stopper preferably has a plurality of holes positioned such that each protrusion is disposed between geometric projections of at least two of the holes onto the surface of the mass. 
         [0014]    The invented acceleration sensor may be fabricated by a method including the steps of: 
         [0015]    preparing a substrate having a first layer, a second layer, and a joining layer through which the first layer is joined to the second layer; 
         [0016]    patterning the first layer to form a mass attachment section, a peripheral attachment section surrounding and spaced apart from the mass attachment section, at least one beam flexibly linking the mass attachment section to the peripheral attachment section, and at least one stopper contiguously joined to the peripheral attachment section and spaced apart from the mass attachment section and the beam; 
         [0017]    patterning the second layer to form a mass spaced apart from the stopper, having a surface facing the stopper, and a frame surrounding and spaced apart from the mass; and 
         [0018]    selectively removing the joining layer to leave a first joining layer joining the mass to the mass attachment section, a second joining layer joining the frame to the peripheral attachment section, and at least one protrusion protruding from said surface of the mass toward the stopper, the protrusion being spaced away from the stopper. 
         [0019]    The step of selectively removing the joining layer is preferably carried out by wet etching. 
         [0020]    The step of patterning the first layer preferably also forms a plurality of holes facing respective areas on said surface of the mass, each protrusion being disposed between at least two of these areas. 
         [0021]    The protrusions prevent the mass from sticking to the stopper during the fabrication process. 
         [0022]    The holes formed in the stopper shorten the fabrication process by facilitating the etching of joining-layer material between the mass and stopper and naturally leading to the formation of the protrusions. 
         [0023]    The protrusions increase the robustness of the acceleration sensor by shortening the distance through which the mass can travel toward the stopper, thereby reducing the risk of beam or stopper damage caused by shock. 
         [0024]    By slightly increasing the amount of mass, the protrusions increase the sensitivity of the sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    In the attached drawings: 
           [0026]      FIG. 1  is a perspective view of an acceleration sensor embodying the present invention; 
           [0027]      FIG. 2  is an upper plan view of the acceleration sensor in  FIG. 1 ; 
           [0028]      FIG. 3  is a sectional view through line AA′ in  FIG. 1 ; 
           [0029]      FIG. 4  is a sectional view through line BB′ in  FIG. 1 ; 
           [0030]      FIG. 5  is a partial perspective view of the mass in  FIG. 1 ; 
           [0031]      FIG. 6  is an upper plan view of the first layer in  FIG. 1 ; 
           [0032]      FIG. 7  is an upper plan view of the joining layer in  FIG. 1 ; 
           [0033]      FIG. 8  is an upper plan view of the second layer in  FIG. 1 ; 
           [0034]      FIGS. 9 ,  10 ,  11 ,  12 , and  13  are sectional views illustrating steps in the fabrication of the acceleration sensor in  FIG. 1 ; 
           [0035]      FIGS. 14 ,  15 ,  16 , and  17  are plan views illustrating possible layouts of the holes and protrusions in  FIG. 1 ; 
           [0036]      FIG. 18  is a perspective view of a conventional acceleration sensor; 
           [0037]      FIG. 19  is an upper plan view of the conventional acceleration sensor; 
           [0038]      FIG. 20  is a sectional view illustrating a starting state in the fabrication of the conventional acceleration sensor; 
           [0039]      FIGS. 21A ,  22 A, and  23 A are sectional views through line AA′ in  FIG. 18 , illustrating successive steps in the conventional fabrication process; 
           [0040]      FIGS. 21B ,  22 B, and  23 B are corresponding sectional views through line BB′ in  FIG. 18 ; and 
           [0041]      FIGS. 21C ,  22 C, and  23 C are corresponding upper plan views of various layers in  FIG. 18 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0042]    Embodiments of the invention will now be described with reference to the attached drawings, in which like elements are indicated by like reference characters. 
         [0043]    A three-axis acceleration sensor embodying the present invention is shown in perspective view in  FIG. 1 . The acceleration sensor is fabricated in a substantially square substrate having a first layer or patterned layer  101  joined by a joining layer  102  to a second layer  103 . The peripheral section  110  of the acceleration sensor includes a peripheral attachment section  111  formed in the first layer  101 , joined through the joining layer  102  to a frame  113  formed in the second layer  103 . Four beams  120  extend in the first layer  101  from the peripheral attachment section  111  toward the central section  130  of the acceleration sensor. The central section  130  includes a mass attachment section  131  formed in the first layer  101 , joined through the joining layer  102  to a mass  133  formed in the second layer  103 . Each beam  120  is integrally attached at a first end  121  to the peripheral attachment section  111  and a second end  122  to the mass attachment section  131 , and includes piezoresistive elements (not shown) for sensing strain when the beam  120  bends. 
         [0044]    The part of the joining layer  102  that joins the mass attachment section  131  to the mass  133  will be referred to as the first joining layer  132 ; the part of the joining layer  102  that joins the peripheral attachment section  111  to the frame  113  will be referred to as the second joining layer  112 . 
         [0045]    Four stoppers  140  are disposed in the first layer  101  at the four inner corners of the peripheral attachment section  111 , to which they are connected. Each stopper  140  has the shape of a right isosceles triangle. A plurality of holes  141  are formed in each stopper  140 , extending from its top surface to its bottom surface. 
         [0046]    The mass  133  has for square lobes, each with a surface that extends partly beneath one of the stoppers  140 . A plurality of protrusions  150  extend from this surface toward the facing undersurface of the stopper  140 . As shown by the top plan view in  FIG. 2 , the protrusions  150  project toward points disposed between the holes  141  in the stopper  140 . 
         [0047]    The mass attachment section  131  is spaced apart from the sides of the beams  120 , and from the stoppers  140 . The four lobes of the mass  133  are spaced apart from the frame  113 , and absent acceleration, the protrusions  150  are spaced apart from the stoppers  140 , as shown in  FIG. 3 . The central part of the mass  133  is widely spaced apart from the frame  113  by cavities below the beams  120 , as shown in  FIG. 4 . 
         [0048]    The protrusions  150  have a square pyramidal shape, as best seen in  FIG. 5 . This drawing shows part of one lobe of the mass  133 . The facing stopper  140  is omitted from  FIG. 5  for clarity, but the part of the surface of the mass  133  that faces the stopper  140  is bounded by the dotted line  151 . Circular dotted lines in  FIG. 5  define areas  152  facing the holes  141  in the stopper  140 . The protrusions  150  are disposed between these areas  152 , which are geometric projections of the holes, and the protrusions  150  are oriented so that their sides face toward these areas  152 . 
         [0049]    The greater the height of the protrusions  150 , the less the mass  133  can move toward the stoppers  140 . The height of the protrusions  150  should be chosen to allow enough motion for acceleration to be sensed but not so much motion that the beams  120  might break under strong acceleration. 
         [0050]    Most of the part of the square lobe of the mass  133  that does not face the stopper  140  is joined by the first joining layer  132  to the mass attachment section  131 , as shown at the back of  FIG. 5 . The space between the dotted line in  FIG. 5  and the first joining layer  132  corresponds to the space between the mass attachment section  131  and stopper  140  in  FIGS. 1 and 2 . 
         [0051]    Although the substrate layers  101 ,  102 , and  103  are unitarily contiguous and cannot be separated from one another, strictly for explanatory purposes,  FIGS. 6 ,  7 , and  8  show top plan views of the three layers separately. 
         [0052]    The first layer  101 , shown in  FIG. 6 , is a silicon layer with a preferred thickness in the range from three to eight micrometers (3-8 μm). The mass attachment section  131  is separated from the beams  120  and stoppers  140  by trenches  401  with a preferred width of 10-25 μm. 
         [0053]    The joining layer  102 , shown in  FIG. 7 , is a silicon oxide layer with a preferred thickness of 1-3 μm. The joining layer  102  includes not only the second joining layer  112  that joins the peripheral attachment section  111  to the frame  113  and the first joining layer  132  that joins the mass attachment section  131  to the mass  133 , but also the protrusions  150 . A plurality of protrusions  150  are formed below each stopper  140  to ensure that, if acceleration drives the mass  133  toward the stoppers  140  at an angle such that the protrusions  150  strike the stopper  140  in only one corner of the sensor, the impact force will not be concentrated on just one protrusion  150 , which might damage the sensor. 
         [0054]    The first joining layer  132  in  FIG. 7  has the same plan geometry as the mass attachment section  131  in  FIG. 6 , and the second joining layer  112  has the same plan geometry as the peripheral attachment section  111 . Below the beams  120  and stoppers  140 , the joining layer  102  is removed during the fabrication process, except for the protrusions  150 . 
         [0055]    The second layer  103 , shown in  FIG. 8 , which includes the peripheral frame  113  and mass  133 , is a silicon layer with a preferred thickness of 200-400 μm. The shape of the mass  133 , with large outer lobes and a smaller central part, is designed to maximize its total size and hence its total inertial mass, while also maximizing the length of the beams; both of these factors enhance the sensitivity of the acceleration sensor. The thickness of the mass  133  is preferably 8-15 μm less than the thickness of the frame  113 . This thickness difference, best seen in  FIG. 3 , corresponds to the maximum distance through which the mass  133  can move from its rest position in the direction away from the stoppers  140 . 
         [0056]    A fabrication process for this acceleration sensor will now be described with reference to  FIGS. 9 to 13 , which correspond to sections through line AA′ in  FIG. 1 . 
         [0057]    The fabrication process starts from a silicon-on-insulator (SOI) wafer substrate having a first layer  101 , a joining layer  102 , and a second layer  103  as shown in  FIG. 9 . The joining layer  102  may be a so-called buried oxide layer. Although only one acceleration sensor is shown in the drawings, normally many acceleration sensors are fabricated simultaneously in the same wafer. 
         [0058]    First, standard microelectronic semiconductor fabrication methods are used to form piezoresistive elements (not shown) in the part of the first layer  101  that will become the beams  120 . In addition, the first layer  101  is anisotropically etched to form the trenches  401  shown in  FIG. 6  that define the peripheral attachment section  111 , beams  120 , mass attachment section  131 , and stoppers  140 , and to form a plurality of holes  141  in each stopper  140 . The result is illustrated in  FIG. 10 . 
         [0059]    Next, the underside of the second layer  103  of the wafer is etched to a depth of 8-15 μm in the region that will become the mass  133 , as shown in  FIG. 11 . 
         [0060]    The underside of the second layer  103  is then further etched by an anisotropic etching process to form trenches  502  as shown in  FIG. 12  that separate the mass  133  from the frame  113  and that separate the lobes of the mass  133  from each other. This etching process removes all parts of the second layer  103  from beneath the beams  120  and from a square annular ring just inside the frame  113 ; the etching process ends at the joining layer  102 , which is not etched. 
         [0061]    Finally, a wet etching process is performed by immersing the wafer in an etching fluid that etches the silicon oxide of the joining layer  102  but does not etch the silicon of the first and second layers  101  and  103  (more precisely, the etching fluid etches silicon oxide much more rapidly than silicon). The etching fluid easily reaches the part of the joining layer  102  exposed by the trenches  401  and  502  formed in the preceding steps and removes all of the joining layer  102  from the area beneath the beams  120  and the area between the frame  113  and mass  133 . As wet etching is isotropic, the etching process also proceeds laterally from these trenches  140 ,  152  into the spaces between the stoppers  140  and mass  133 . Additional etching fluid reaches this space through the holes  141  in the stoppers  140 , and by etching isotropically from the ends of the holes  141 , excavates a cavity beneath each hole. The cavity is wider at the top (near the hole) than at the bottom (on the surface of the mass  133 ). As these cavities grow, they shape the protrusions  150 . If the etching conditions are properly selected, protrusions  150  of the desired height will be left on the surfaces of the mass  133  beneath the stoppers  140 , as shown in  FIG. 13 . In experiments by the inventor, appropriate protrusions  150  were formed with a total wet etching time of about seventy minutes. 
         [0062]    After wet etching, the completed acceleration sensor is cleaned to rinse away the etching fluid, and then dried. The protrusions  150  prevent the mass  133  from sticking to the stoppers  140  during the drying process, so the dried acceleration sensor can immediately be diced from the wafer and mounted in an appropriate package. 
         [0063]    The wet etching step may be performed as a single continuous process, or as a series of short etch-rinse cycles. The latter strategy promotes etching by removing the etched silicon oxide material at the end of each cycle and replacing the spent etching fluid, which has already reacted with the silicon oxide, with fresh etching fluid. Etching may be further promoted by immersing the wafer in a surfactant solution before each etching cycle, to reduce the surface tension of the etching fluid and rinsing fluid and enable etching to proceed efficiently even in the narrow space between the mass  133  and stoppers  140 . 
         [0064]    As the wet etching process forms protrusions  150  not in the areas  152  directly beneath the holes  141  but at locations between these areas, if acceleration moves the mass  133  toward the stoppers  140  during operation of the acceleration sensor, the protrusions  150  will strike the surface of the stoppers  140 , as desired, instead of entering the holes  141 . 
         [0065]    The number of holes  141  and protrusions  150  per stopper  140  is not limited to the numbers shown in  FIGS. 1 ,  2 ,  5 , and  6 ; a larger number may be formed, as illustrated in  FIG. 14 , for example. The preferred diameter of the holes  141  is 3-4 μm, and the preferred spacing between the edges of adjacent holes  141  is 4.5-5.5 μm. The center-to-center spacing of the holes  141  is then approximately 8.5 μm. 
         [0066]    In the design stage, the holes  141  can be laid out by defining two holes on an imaginary reference line, then translating the line so that one hole occupies the location of the other hole, rotating the line by ninety degrees to define a new hole, and repeating this process until all the necessary holes have been defined. Alternatively, a unit cell A of four holes  141  surrounding one protrusion  150  can be defined; then the unit cell can be stepped horizontally and vertically to define further holes  141 . 
         [0067]    The layout is not limited to the square cell A shown in  FIG. 14 . A triangular cell A with three holes  141  surrounding one protrusion  150  can be used, as shown in  FIG. 15 , or a hexagonal cell with six holes  141  surrounding one protrusion  150  can be used, as shown in  FIG. 16 . The resulting protrusions  150  will then have a triangular pyramidal shape or a hexagonal pyramidal shape, as shown in  FIGS. 15 and 16 . Increasing the number of holes around each protrusion  150  increases the etching speed, so to shorten the etching time, the number of holes  141  may be increased still further.  FIG. 17  shows a unit cell A with eight holes  141 , for example, which produces protrusions  150  with an octagonal pyramidal shape. 
         [0068]    Increasing the number of holes  141  also weakens the stoppers  140 , however, and therefore reduces the ability of the sensor to withstand shock. The number of holes  141  per protrusion  150  and hence the shape of the protrusions  150  should be selected by balancing requirements for quick etching against requirements for a robust acceleration sensor. The square pyramidal shape shown in  FIGS. 5 and 14  is thought to represent an appropriate compromise. 
         [0069]    It not necessary to tile the entire surface of a stopper  140  with unit cells A as in  FIGS. 14 to 17 . A few unit cells may be placed at selected locations in the stopper  140 . This provides another way to achieve an appropriate balance between robustness and short etching time. 
         [0070]    During operation, as noted above, the protrusions  150  reduce the distance through which the mass section  130  can travel in the direction perpendicular to the surfaces of the stoppers  140 . This has the desirable effect of reducing the risk of damage to the acceleration sensor if strong acceleration drives the mass  133  forcefully against the stoppers  140 . 
         [0071]    For comparison,  FIG. 18  shows a conventional acceleration sensor of the type described in JP 2004-198243, comprising a first layer  701 , joining layer  702 , second layer  703 , peripheral section  710 , beams  720 , mass section  730 , and stoppers  740  similar to the corresponding elements in  FIG. 1 , except that the stoppers  740  lack holes.  FIG. 19  shows a plan view of the first layer  701 . 
         [0072]    The fabrication process for this conventional acceleration sensor is virtually identical to the fabrication process for the inventive acceleration sensor described above, except that because of the lack of holes in the stoppers  740 , the wet etching step takes longer and does not leave protrusions. 
         [0073]    The conventional fabrication process begins from an SOI wafer substrate as illustrated in  FIG. 20 . The first layer  701  is anisotropically etched to define the upper parts of the peripheral section  710  and mass section  730 , the beams  720 , and the stoppers  740  as shown in  FIG. 21A  (a sectional view through line AA′ in  FIG. 18 ),  FIG. 21B  (a sectional view through line BB′ in  FIG. 18 ), and  FIG. 21C  (a top plan view of the first layer  701 ). Next the second layer  703  is anisotropically etched to define the lower parts of the peripheral section  710  and mass section  730 , as shown in sectional views in  FIGS. 22A  (another view through line AA′) and  22 B (another view through line BB′) and in a bottom plan view in  FIG. 22C . Finally, a wet etching process is performed to remove the joining layer  702  from the undersides of the beams  720  and stoppers  740 , as shown in sectional views in  FIGS. 23A  (again through line AA′) and  23 B (again through line BB′) and a plan view of the resulting patterned joining layer  702  in  FIG. 23C . 
         [0074]    The total wet etching time in the conventional fabrication process, when performed under the same wet etching conditions as in the above embodiment, is about eighty minutes. The present invention thus reduces the wet etching time by about ten to thirteen percent. Moreover, when the conventional acceleration sensor is dried after wet etching and cleaning, the mass  730  sometimes sticks to the stoppers  740 , as noted above, and further time is required to deal with this problem. The invention thus leads to a quicker manufacturing process, as well as a more robust and more sensitive sensor. 
         [0075]    The foregoing represents one preferred embodiment of the invention. Those skilled in the art will recognize that many other embodiments and variations are possible within the scope of the invention, which is defined in the appended claims.