Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a three-axis (XYZ) acceleration sensor and its method of manufacture, more particularly to a simplified method of providing improved impact resistance. 
     2. Description of the Related Art 
     Technology for making an acceleration sensor resistant to destruction by impact is described in, for example, Japanese Patent Application Publication No. 2004-198243. The acceleration sensor described therein has a mass attachment section, a peripheral attachment section surrounding the mass attachment section, beams flexibly linking the mass attachment section to the peripheral attachment section, resistive elements disposed on the surfaces of the beams and producing piezoresistive effects, a mass secured to the mass attachment section, a frame securing the peripheral attachment section to its package so that the rest position of the mass is a prescribed distance above the floor of the package, stoppers attached to the peripheral attachment section to restrict the motion of the mass, which is disposed between the stoppers and the floor of the package, and aluminum reinforcements on the stoppers. Under acceleration in a given direction the mass is displaced in the opposite direction, the beams bend, and the electrical resistance of the resistive elements disposed on the surfaces of the beams changes. The magnitude and direction of the acceleration are calculated from these resistance changes. 
     If the acceleration is directed upward from the floor of the package, the beams sag and the mass moves downward, stopping when it strikes the package floor. If the acceleration is directed downward toward the package floor, the mass moves away from the package floor, stopping when the outer corners of the mass strike the stoppers. The aluminum reinforcements on the stoppers enable the stoppers to withstand the impact of the mass caused by sudden acceleration in the downward direction. A typical requirement is for the acceleration sensor to be able to survive a fall from a height of one and a half meters (1.5 m), which generates an impact force or acceleration equivalent to about six thousand times the acceleration caused by gravity (6000 G). 
     A problem with this acceleration sensor is that in order to obtain an impact resistance rating of at least 6000 G, the conventional stopper structure requires the deposition of an aluminum reinforcing film at least several tens of micrometers thick. If this film is deposited by one of the processes commonly used in semiconductor fabrication, e.g., a sputtering process, deposition takes much time and the acceleration sensor cannot be manufactured efficiently. Additional mask deposition and patterning steps are also required, making the manufacturing process still more time-consuming, complex, and expensive. A simpler way to reinforce the stoppers is needed. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to simplify the manufacture of an acceleration sensor. 
     Another object is to reduce the manufacturing cost of an acceleration sensor. 
     The invention provides an acceleration sensor having a mass movably linked to a peripheral attachment section so that, in response to acceleration, the mass can move in at least one direction relative to the peripheral attachment section. At least one stopper is fixedly attached to the peripheral attachment section to stop the motion of the mass in this one direction. The stopper has a first major surface and a second major surface. The first major surface is disposed between the second major surface and the mass and, absent acceleration, is spaced from the mass in the above direction. A quantity of a curable elastic adhesive is disposed on the second major surface of the stopper to absorb impact of the mass on the first major surface of the stopper. The curable elastic adhesive may adhere to the cover of a package in which the acceleration sensor is enclosed. 
     The curable elastic adhesive can be applied as a drop or swath from a dispenser. This process is considerably simpler and less expensive than the conventional process of depositing an aluminum film to reinforce the stopper. 
     The invention also provides a method of manufacturing the above acceleration sensor. The method starts from a semiconductor wafer having a first semiconductor layer, a second semiconductor layer, and a bonding layer joining the first semiconductor layer to the second semiconductor layer. 
     The manufacturing steps include: 
     forming a strain sensing device for converting mechanical strain in the first semiconductor layer to electrical output; 
     forming a plurality of openings in the first semiconductor layer to define a peripheral attachment section, a mass attachment section, at least one beam flexibly linking the mass attachment section to the peripheral attachment section, and at least one stopper disposed on an inner side of the peripheral attachment section; 
     removing part of the second semiconductor layer, leaving a mass and a frame surrounding the mass, the frame including a peripheral part of the second semiconductor layer; 
     removing the bonding layer between the stopper and the mass, leaving a part of the bonding layer joining the mass attachment section to the mass and a part joining the peripheral attachment section to the frame; and 
     applying silicone rubber from a dispenser to the stopper. The silicone rubber is applied in a liquid state and cures to an elastic solid state. 
     The silicone rubber may also be applied to an adjacent surface of the peripheral attachment section. 
     The silicone rubber may be applied as, for example, one drop per stopper; a plurality of drops per stopper; or a single swath covering at least part of all of the stoppers and part of the adjacent surface of the peripheral attachment section. 
     Applying one drop per stopper maximizes the simplicity and minimizes the cost of the silicone rubber application step. 
     Applying a plurality of drops of silicone rubber per stopper improves the impact resistance of the acceleration sensor and enables the impact sensor to be enclosed in a thinner package, since the individual drops can be lower in height than a single drop. 
     Applying the silicone rubber as a single swath also enables the impact sensor to be enclosed in a thinner package. A uniformly thin swath of silicone rubber can be applied by having the dispenser retrace the final part of the swath, moving backward from the final application point, and lifting the dispenser away from the swath as the dispenser moves over the adjacent surface of the peripheral attachment section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the attached drawings: 
         FIG. 1  and  FIG. 2  are simplified plan views of an acceleration sensor in a first embodiment of the invention; 
         FIG. 3  is a sectional view through line A 1 -A 2  in f2b of the acceleration sensor in the first embodiment; 
         FIG. 4  is a simplified perspective view of the acceleration sensor in the first embodiment; 
         FIG. 5  is a bottom plan view of the acceleration sensor in the first embodiment; 
         FIGS. 6 ,  7 ,  8 ,  9 ,  10 ,  11 ,  12 ,  13 ,  14 ,  15 ,  16 ,  17 ,  18 , and  19  are sectional views illustrating a manufacturing process for the acceleration sensor in the first embodiment; 
         FIG. 20  is a plan view of an acceleration sensor in a second embodiment; and 
         FIG. 21  is a plan view of an acceleration sensor in a third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention will now be described with reference to the attached drawings, in which like elements are indicated by like reference characters. 
     First Embodiment 
     A first embodiment will be described with reference to the top plan views in  FIGS. 1 and 2 , the sectional view in  FIG. 3 , the perspective view in  FIG. 4 , and the bottom plan view in  FIG. 5 . 
     The first embodiment is an acceleration sensor formed in a silicon-on-insulator (SOI) wafer by etching and other processes. The SOI wafer includes a first silicon substrate  10  approximately ten micrometers (10 μm) thick, a second silicon substrate  20  approximately 525 μm thick, and an insulating bonding layer  30  by which the first silicon substrate  10  and second silicon substrate  20  are joined. 
     The first silicon substrate  10  of a single acceleration sensor has a substantially square shape measuring about two and a half millimeters (2.5 mm) on a side, in which four openings or trenches  11  are provided to define a peripheral attachment section  12 , a mass attachment section  13 , beams  14 , and stoppers  15 . The peripheral attachment section  12  is an area approximately 500 μm wide, disposed at the periphery of the first silicon substrate  10 . The mass attachment section  13  comprises a central mass attachment section  13   a  and four outer mass attachment sections  13   b . The central mass attachment section  13   a  has a substantially square shape measuring about 700 μm on a side and occupies the center of the first silicon substrate  10 ; the outer mass attachment sections  13   b  have substantially inverted triangular shapes and are attached to the four corners of the central mass attachment section  13   a . The trenches  11  are shaped so that they partially surround each outer mass attachment section  13   b . The peripheral attachment section  12  and central mass attachment section  13   a  are mutually linked by the four beams  14 , which are about 400 μm wide and extend orthogonally from the sides of the peripheral attachment section  12  and central mass attachment section  13   a . On the surfaces of the beams  14 , resistive elements  16  are formed that produce piezoresistive effects by which mechanical strain is converted to a variation in electrical resistance. 
     Stoppers  15  with a substantially right triangular shape are disposed at the inside corners of the peripheral attachment section  12  so as to face respective outer mass attachment sections  13   b  across the trenches  11 . A plurality of small openings  17  are formed in each of the stoppers  15 . 
     The second silicon substrate  20  includes a frame  21  approximately 500 μm wide, formed at the periphery below the peripheral attachment section  12 , and a mass  23  surrounded by the frame  21  and separated therefrom by a space  22 . The mass  23  has a shape corresponding generally to the combined shape of the mass attachment section  13  and stoppers  15  in the first silicon substrate  10 , comprising a prismatic central mass  23   a  corresponding to the central mass attachment section  13   a , and four prismatic outer masses  23   b  corresponding to the outer mass attachment sections  13   b  and stoppers  15 , attached to the four corners of the central mass  23   a . The silicon between each adjacent pair of outer masses  23   b  is removed to form four trenches  24 , disposed generally below the beams  14  in the first silicon substrate  10 . As can be seen in  FIG. 3 , the mass  23  is thinner than the frame  21  by an amount equal to the maximum allowable displacement (for example, 5 μm). 
     The first silicon substrate  10  and second silicon substrate  20  are mutually joined through the oxide films  31  and  32  constituting the parts of the bonding layer  30  left below the peripheral attachment section  12  and mass attachment section  13 , respectively. As shown in  FIG. 3 , the surface of the first silicon substrate  10  is covered with an insulating protective film  40 . Interconnection pads  41  comprising, for example, an aluminum film are formed on the protective film  40  in the peripheral attachment section  12  and connected to the resistive elements  16 . 
     Referring to  FIGS. 2 and 3 , to reinforce the stoppers  15 , a curable elastic adhesive  50  is applied to the protective film  40  on each stopper  15  as a drop from a dispenser. The drop includes a prescribed quantity of adhesive, sufficient to make the drop at least 250 μm thick. The adhesive  50  should have good mechanical and chemical properties. To facilitate application of the desired quantity, the adhesive preferably takes the form of a highly viscous liquid paste and cures to an elastic solid state after application. In the following description, the curable elastic adhesive will be assumed to be silicone rubber. 
     The acceleration sensor having the structure described above is mounted on a sensor mounting area (for example, the floor  61  of a package  60  as shown in  FIG. 3 ) and secured with an adhesive or the like. 
     Next, fourteen steps in a method of manufacturing the acceleration sensor in  FIG. 1  will be described with reference to  FIGS. 6 to 19 . 
     Referring to  FIG. 6 , in the first step, an SOI wafer is obtained. The wafer includes, for example, an n-type first silicon substrate  10  with a thickness of 10 μm and a volume resistivity of about six to eight ohms per centimeter (6-8 Ω/cm), a second silicon substrate  20  with a thickness of 525 μm and a volume resistivity of about 16 Ω/cm, and a silicon oxide bonding layer  30  with a thickness of about 4 μm, by which the first silicon substrate  10  and second silicon substrate  20  are joined. 
     Referring to  FIG. 7 , in the second step, a protective film  40  of silicon oxide approximately 0.4 μm thick is formed on the surface of the first silicon substrate  10  by thermal oxidation in a wet atmosphere at a temperature of about 1000° C. 
     Referring to  FIG. 8 , in the third step, openings  40   a  are formed in the protective film  40  by photolithography and etching, and boron is diffused through the openings  40   a  to form p-type diffusion layers  18 , which become the resistive elements  16  and other circuit elements. In addition, a protective oxide film  40   b  is formed on the surface of the diffusion layers  18  by chemical vapor deposition (CVD). 
     Referring to  FIG. 9 , in the fourth step, electrode lead openings  40   c  are opened in the protective oxide film  40   b  by photolithography and etching, and aluminum is deposited on the protective film  40  by a metal sputtering method. The aluminum is then patterned by photolithography and etching to form interconnection wiring  41 . 
     Referring to  FIG. 10 , in the fifth step, a protective silicon nitride film  43  is formed on the surfaces of the protective film  40  and the interconnection wiring  41  formed thereon by plasma reactive deposition (PRD). For clarity, the silicon nitride film  43  will be omitted in the subsequent drawings. 
     Referring to  FIG. 11 , in the sixth step, a layer of photoresist (not shown) is applied to the silicon nitride film  43 , and trenches  11  and openings  17  are formed by photolithography and etching. The trenches  11  define the beams  14  and stoppers  15 . The openings  17  will be used for removing the bonding layer  30  disposed between the outer masses  23   b  and stoppers  15  in a later step. 
     Referring to  FIG. 12 , in the seventh step, an oxide film  44  is formed by CVD on the reverse side of the SOI wafer, that is, on the surface of the second silicon substrate  20 . The central portion of the oxide film  44  is removed by photolithography and etching to form an opening  44   a , while the periphery of the oxide film  44  is left intact beneath the frame  21 . A photoresist mask  45  masking what will become the mass  23  is formed in the opening  44   a.    
     Referring to  FIG. 13 , in the eighth step, using the oxide film  44  left in the periphery and the photoresist film  45  as an etching mask, the surface of the second silicon substrate  20  is etched to a depth of about 20 μm by a gas chopping etching technique (GCET), also known as the Bosch method, to form a recessed area  20   a . The photoresist film  45  is then removed. 
     Referring to  FIG. 14 , in the ninth step, using the oxide film  44  as an etching mask, the surface of the second silicon substrate  20  is etched to an additional depth of about 5 μm by GCET, thereby obtaining a mass  23  having a thickness approximately 5 μm thinner than the thickness of the frame  21 . 
     Referring to  FIG. 15 , in the tenth step, an etching mask  46 , which will be used to create the space  22  between the frame  21  and mass  23  and the trench  24  in the second silicon substrate  20 , is formed by photolithography. 
     Referring to  FIG. 16 , in the eleventh step, the space  22  and trenches  24  are formed on the second silicon substrate  20  by GCET. 
     Referring to  FIG. 17 , in the twelfth step, the SOI wafer, which has completed the processes in steps 1 to 11, is dipped in buffered hydrofluoric acid to etch the bonding layer  30  between the first silicon substrate  10  and second silicon substrate  20 . During the dip, the acid penetrates from the openings  17  in the first silicon substrate  10  and the space  22  and trenches  24  in the second silicon substrate  20 , and removes the part of the bonding layer  30  disposed between the mass  23  and stoppers  15 . 
     In the thirteenth step silicone rubber  50  is applied by using, for example, a dispenser  70  as shown in  FIG. 18 . The dispenser  70  is a generally tubular device containing silicone rubber in a highly viscous liquid state, having at one end a nozzle  71  from which a certain quantity of the silicone rubber  50  is dispensed when a predetermined pressure is applied at the other end. Exemplary application conditions (dispensing conditions) for the silicone rubber  50  are as follows: 
                                                                   Pressure   400   kPa           Application speed   400   ms           Nozzle diameter   150   μm           Gap (g)   0.10   mm                Silicone rubber   viscous paste                        
The viscosity of the silicone rubber  50  should be such that when applied to the stoppers  15 , the silicone rubber  50  will not flow through the openings  17  therein.
 
     Under dispensing conditions such as those described above, the dispenser  70  is moved successively to a position above each stopper  15 , with a prescribed gap (g) between the tip of the nozzle  71  and the protective film  40  covering the stopper  15 ; the prescribed amount of silicone rubber  50  is dispensed from the nozzle  71  in the liquid paste state; then the dispenser  70  is pulled back. This procedure applies one drop of silicone rubber  50  with a height of 250 μm or more to the stopper  15 . The silicone rubber  50  is left for about thirty minutes to one hour at a temperature of 100° C. to 120° C. to cure from the liquid state to an elastic solid state. The stoppers  15  are thereby reinforced by an elastic material. 
     In the fourteenth step, as in ordinary semiconductor fabrication, an acceleration sensor chip  80  is diced from the SOI wafer and mounted in, for example, a package  60  like the one shown in  FIG. 19 . The package  60  has a hollow structure with a solid floor  61  on which the chip is mounted and an open top that is sealed by a lid  62  after the chip has been mounted. In this example, an integrated circuit chip  81  (referred to below as an ‘IC chip’) for controlling the acceleration sensor is bonded on the floor  61  and the acceleration sensor chip  80  is stacked on the IC chip  81 . The acceleration sensor chip  80  and IC chip  81  are electrically interconnected and are connected to package terminals or leads (not shown) by wires  82 ; then the package  60  is sealed by the lid  62 , completing the mounting process. 
     In a variation of the thirteenth and fourteenth steps, the silicone rubber  50  is applied after the acceleration sensor chip  80  is diced from the SOI wafer and mounted in the package  60 . Further, although a stacked structure is shown in  FIG. 19 , the acceleration sensor chip  80  and IC chip  81  may be bonded side by side to the package floor  61 , or the IC chip  81  may be located outside the package  60 . 
     The acceleration sensor chip  80  manufactured by the above method operates as follows: 
     If an upward acceleration is applied to the acceleration sensor chip  80  housed in the package  60 , the beams  14  bend down and the mass  23  moves downward. The downward movement of the mass  23  stops when its bottom surface strikes the floor  61  of the package  60  (in the stacked structure, the surface of the IC chip  81 ), which blocks further downward movement. If a downward acceleration is applied to the acceleration sensor chip  80 , the beams  14  bend up and the mass  23  moves upward, stopping when the outer masses  23   b  strike the stoppers  15 , which block further upward movement. Since the stoppers  15  are reinforced by the drops of silicone rubber  50 , even when a sudden strong acceleration causes the outer masses  23   b  to impact forcefully on the stoppers  15 , the impact force is absorbed by the silicone rubber  50 , which significantly improves the ability of the acceleration sensor chip  80  to survive impact. 
     The electrical resistance of the resistive elements  16  formed in the four beams  14  varies depending on the amount by which the beams  14  bend and the direction of the bend. The magnitude and direction of the acceleration can be calculated from the resistance variations of the resistive elements  16 . 
     The first embodiment provides at least the following four effects: 
     (i) Excessive movement of the mass  23  is blocked in the upward direction by the stoppers  15 , which are reinforced by the silicone rubber  50 , and in the downward direction by the floor  61  of the package  60  (in the stacked structure, the front surface of the IC chip  81 ). Destruction of the acceleration sensor due to excessive bending of the beams  14  is thereby prevented. 
     (ii) Compared with the conventional structure in which the stoppers are reinforced by thick aluminum films, the structure according to the present embodiment, in which the stoppers  15  are reinforced by drops of silicone rubber  50 , is easier to manufacture, requires fewer manufacturing process steps, and has a lower manufacturing cost. This is because the prescribed quantity of silicone rubber  50  can be simply and accurately applied from a dispenser. 
     (iii) When the stoppers  15  are not reinforced, as shown in  FIG. 1 , the impact resistance is, for example, 2000 G or less; reinforcement of the stoppers  15  by application of silicone rubber  50  as shown in  FIG. 2  improves the impact resistance to, for example, 6000 G or more. 
     (iv) If a structure is employed in which the top of the silicone rubber  50  adheres to the back surface of the lid  62  of the package  60 , it is possible not only to strengthen the reinforcement of the stoppers  15  but also to make the package  60  thinner. 
     Second Embodiment 
     In a second embodiment, the silicone rubber reinforcement is applied as a plurality of drops (for example, three drops  50 - 1 ,  50 - 2 ,  50 - 3  as shown in  FIG. 20 ) to each of the stoppers  15  from the dispenser  70 . Otherwise, the structure and manufacturing method are the same as in the first embodiment. 
     In the first embodiment, since the silicone rubber  50  is applied as one drop, a height of, for example, 250 μm or more is required for adequate reinforcement, and the package  60  must be thick enough to accommodate this height. In the second embodiment, since the silicone rubber applied to each stopper  15  is dispensed from the dispenser  70  as a plurality of smaller drops  50 - 1  to  50 - 3 , the heights of the drops can be reduced to as little as about 150 μm while still providing adequate reinforcement, so the thickness of the package  60  can be reduced. Impact resistance is also improved in that the reinforcement is spread over a wider area of the stoppers  15 . 
     Third Embodiment 
     In a third embodiment, silicone rubber  50 A is applied by the dispenser  70  as a swath covering at least part of all of the stoppers  15  and part of the adjacent surface of the peripheral attachment section  12  to reinforce the stoppers  15  and peripheral attachment section  12 . As shown in  FIG. 18 , for example, if the dispenser  70  moves in a line along the peripheral attachment section  12 , starting from one stopper  15  and ending at another stopper  15 , the silicone rubber  50 A can be applied as a single continuous swath. In this case, a uniform swath of silicone rubber  50 A can be applied by lifting the dispenser  70  away from the peripheral attachment section  12  as the dispenser  70  retraces the final part of the swath, moving backward from the final application point. 
     In  FIG. 21 , since the silicone rubber  50 A is not applied in a complete loop, there is a part of the peripheral attachment section  12  to which no silicone rubber is applied. Electrical interconnection terminals can be located in this area, so that the silicone rubber  50 A does not interfere with electrical interconnections. 
     According to the third embodiment, since the silicone rubber  50 A is applied as a swath by the dispenser  70  to reinforce the stoppers  15  and peripheral attachment section  12 , the height of the silicone rubber  50 A can be reduced to a level even lower than in the second embodiment. 
     The present invention is not limited to the above embodiments; various modifications are possible. For example: 
     (a) The acceleration sensor need not be square in shape; it may be rectangular or circular. The dimensions of the first and second silicon substrates and other dimensions are not limited to the exemplary values given above. 
     (b) The silicone rubber used as a curable elastic adhesive to reinforce the stoppers in the embodiments above can be replaced with any other material that can be dispensed as a viscous liquid, cures to an elastic adhesive form, and has good mechanical and chemical properties. 
     (c) The methods of manufacturing and mounting the acceleration sensor are not limited to the methods described above. 
     Those skilled in the art will recognize that further variations are possible within the scope of the invention, which is defined in the appended claims.

Technology Category: 3