Abstract:
Plural semiconductor chips such as acceleration sensor chips formed on the first surface of a substrate are separated into individual pieces by dicing the substrate from the second surface thereof. A groove surrounding each sensor chip, along which the sensor chip is diced out, is formed at the same time the sensor chip is formed on the first surface. Before dicing, a protecting sheet covering the first surface is pasted along the sidewalls and the bottom wall of the groove. The groove is made sufficiently wide to ensure that the protecting sheet is bent along the walls of the groove without leaving a space between the groove and the protecting sheet. Thus, dicing dusts generated in the dicing process are prevented from being scattered and entering the sensor chip.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    This application is based upon and claims benefit of priority of Japanese Patent Application No. 2000-193150 filed on Jun. 27, 2000, the content of which is incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a sensor device having a semiconductor chip diced out from a semiconductor substrate having plural chips formed thereon and to a manufacturing process of such a sensor device.  
           [0004]    2. Description of Related Art  
           [0005]    Semiconductor sensor chips manufactured by dicing a substrate having plural sensor chips formed thereon are known hitherto. The sensor chip includes stationary electrodes and movable electrodes facing the stationary electrodes. Both electrodes form a variable capacitance which varies according to a dynamic force such as an acceleration force imposed on the sensor chip. The sensor chip detects the dynamic force based on the capacitance of the sensor chip.  
           [0006]    However, there has been a problem caused by dusts generated in the dicing process of the conventional sensor chips. That is, swarfs (dicing dusts) scattered in the dicing process enter into the sensor elements such as the movable electrodes. Such swarfs cause a malfunction in the sensor operation.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved sensor chip which is kept free from the dicing dusts. Another object of the present invention is to provide a manufacturing process in which the dicing dusts are prevented from entering into semiconductor sensor elements.  
           [0008]    A protecting sheet is pasted on a first substrate surface on which plural sensor chips are formed, and then the substrate is diced from a second surface of the substrate thereby to separate the plural sensor chips into individual pieces. The sensor chip includes a beam structure composed of a pair of stationary electrode portions and a movable electrode portion. The beam structure forms a pair of capacitances which vary in accordance with a dynamic force such as an acceleration force imposed on the beam structure.  
           [0009]    To prevent dicing dusts (swarfs) generated in the dicing process from being scattered and entering the sensor chip, a groove surrounding the sensor chip is formed on the first substrate surface. The width of the groove is made sufficiently wide so that the protecting sheet can be bent along the side walls and the bottom wall of the groove. The sensor chips are diced out from the substrate along the groove. Since no space in which the dicing dusts scatter is formed between the protecting sheet and the bottom wall of the groove, the sensor chip is kept free from the dicing dusts. Thus, the sensor malfunction otherwise caused by the dicing dusts is prevented.  
           [0010]    Alternatively, a peripheral bank surrounding the groove may be further formed, and the dicing may be performed along the peripheral bank. In this case, the peripheral bank and the sensor portion inside the grooves are electrically connected. Since no space is formed between the protecting sheet and the peripheral bank, the dicing dusts are prevented from scattering.  
           [0011]    According to the present invention, the dicing dusts do not scatter in the dicing process, and the sensor chip is kept free from the dicing dusts. Thus, the sensor malfunction due to the dicing dusts is avoided.  
           [0012]    Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a plan view showing a semiconductor acceleration sensor chip made as a prototype sample;  
         [0014]    [0014]FIG. 2 is a cross-sectional view showing the acceleration sensor chip shown in FIG. 1, taken along line II-II in FIG. 1;  
         [0015]    FIGS.  3 A- 3 C are cross-sectional views showing a manufacturing process of the acceleration sensor chip shown in FIG. 1, FIG. 3C showing a partly enlarged cross-section;  
         [0016]    [0016]FIG. 4 is a plan view showing a semiconductor acceleration sensor chip as a first embodiment of the present invention;  
         [0017]    [0017]FIG. 5 is a cross-sectional view showing the acceleration sensor chip shown in FIG. 4, taken along line V-V in FIG. 4;  
         [0018]    FIGS.  6 A- 6 C are cross-sectional views showing a manufacturing process of the acceleration sensor chip shown in FIG. 4, FIG. 6C showing a partly enlarged cross-section;  
         [0019]    [0019]FIG. 7 is a plan view showing a semiconductor acceleration sensor chip as a second embodiment of the present invention;  
         [0020]    [0020]FIG. 8 is a cross-sectional view showing the acceleration sensor chip shown in FIG. 7, taken along line VIII-VIII in FIG. 7; and  
         [0021]    FIGS.  9 A- 9 C are cross-sectional views showing a manufacturing process of the acceleration sensor chip shown in FIG. 7, FIG. 9C showing a partly enlarged cross-section. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    (Prototype)  
         [0023]    A prototype sample of a sensor device which was made before devising the preferred embodiments of the present invention will be described with reference to FIGS.  1 - 3 C. The sensor chip  10  shown in FIG. 1 is a semiconductor sensor chip for measuring acceleration by means of capacitance changes. The sensor chip  10  is used as an acceleration sensor for controlling devices such as an air-bag, an ABS (an anti-block braking system), a VSC (a vehicle stability controller) mounted on an automobile vehicle. FIG. 1 shows a plan view of the sensor chip, FIG. 2 a cross-sectional view thereof taken along line II-II shown in FIG. 1, and FIGS.  3 A- 3 C a process of manufacturing the sensor chip.  
         [0024]    Plural sensor chips  10  formed on a semiconductor substrate are separated into individual chips by dicing the substrate. As shown in FIG. 2, the semiconductor substrate  1  is an SOI substrate composed of a first silicon layer  11 , a second silicon layer  12  and an oxidized film  13  interposed between the first and second silicon layers  11 ,  12 . The plural sensor chips  10  are formed on the second silicon layer  12  by a known process. The top surface of the second silicon layer  12  is referred to as a first surface  10   a  of the sensor chip, and the bottom surface of the first silicon layer  11  is referred to as a second surface of the sensor chip. In FIG. 1, portions where the top surface of the oxidized film  13  is exposed are marked with dots to differentiate those portions from other portions.  
         [0025]    A beam structure having a movable portion  20  and a pair of stationary portions  30 ,  40  is formed on the second silicon layer  12 . The beam structure is referred to as a semiconductor element. A portion of the first silicon layer  11  and the oxidized film  13  corresponding to the beam structure is etched out to form an opening  13   a.  The movable portion  20  is composed of a pair of anchor portions  23   a,    23   b,  a pair of beams  22 , a weight  21  and movable electrodes  24  connected to the weight  21 . The pair of anchor portions  23   a,    23   b  are anchored on the oxidized film  13  supported on the first silicon layer  11  at the fringes of the opening  13   a,  so that the movable portion  20  is bridged across the opening  13   a.  In this manner, the beams  22  and the weight  21  having the movable electrodes  24  are positioned above and across the opening  13   a.    
         [0026]    Each beam  22  is shaped in an elongate frame and has a spring function resiliently movable in direction X shown in FIG. 1. More particularly, the beams  22  are displaced in the direction X when the weight  21  is accelerated in the direction X, and the beams  22  return to the original position when the acceleration in the direction X disappears. Thus, the movable electrodes  24  swing in the direction X according to the acceleration imposed on the weight  21  and the movable electrodes  24 . The movable electrodes  24  connected to the weight  21  extend therefrom in a direction perpendicular to the direction X. In the embodiment shown in FIG. 1, three movable electrodes  24  extend to each side of the weight  21 .  
         [0027]    As shown in FIG. 1, a first stationary portion  30 , and a second stationary portion  40  are supported on the fringes of the opening  13   a,  respectively, where the movable portion  20  are not anchored. That is, the first stationary portion  30  having three stationary electrodes  32  connected to a wiring portion  31  is positioned at the left side of the movable portion  20 . The second stationary portion  40  having three stationary electrodes  42  connected to a wiring portion  41  is positioned at the right side of the movable portion  20 . Both stationary portions  30 ,  40  are electrically insulated from each other. Both wiring portions  31 ,  41  are fixed on the oxidized film  13  supported on the first silicon layer  11 , and each stationary electrode  32 ,  42  is positioned between the movable electrodes  24  with a certain space therebetween, thereby forming a comb-shaped electrode structure.  
         [0028]    Each stationary electrode  32 ,  42  extending from the respective wiring portions  31 ,  41  has a rectangular cross-section. The stationary electrodes  32  positioned at the left side and the movable electrodes  24  form a first variable capacitor, and the stationary electrodes  42  positioned at the right side and the movable electrodes  24  form a second variable capacitor. A stationary electrode pad  31   a  is connected to the wiring portion  31 , and another stationary electrode pad  41   a  is connected to the wiring portion  41 . A movable electrode pad  25   a  is connected to the anchor portion  23   b  of the movable portion  20 . Those electrode pads are made of aluminum or the like.  
         [0029]    Plural through-holes  50  are formed in the weight  21 , the movable electrodes  24  and the stationary electrodes  32 ,  42 , respectively, as shown in FIGS. 1 and 2. The through-holes  50  serve to reduce the weight of the movable and stationary electrodes  24 ,  32 ,  42  and to enhance a mechanical strength against a torsional force imposed thereon. A ditch that reaches the top surface of the oxidized film  13  is formed surrounding the beam structure composed of the movable portion  20  and the pair of stationary portions  30 ,  40 . A filed portion  60  formed outside the ditch is electrically insulated from the beam structure by the ditch. The beam structure is electrically shielded from outside by the field portion  60 . A peripheral groove  17  that reaches the top surface of the oxidized film  13  is formed at the outermost periphery of the sensor chip  10 .  
         [0030]    As shown in FIG. 2, the bottom surface of the sensor chip  10  is mounted on a sensor package  81  via adhesive  80 . The electrode pads  25   a,    31   a,    41   a  of the sensor chip  10  are electrically connected to a circuitry (not shown) contained in the package  81  by wire-bonding or the like.  
         [0031]    The first variable capacitor CS 1  formed by the first stationary electrodes  32  and the movable electrodes  24  and the second variable capacitor CS 2  formed by the second stationary electrodes  42  and the movable electrodes  24  are used as capacitors for detecting acceleration. That is, when an acceleration force is imposed on the movable portion  20 , the movable electrodes  24  are displaced in the direction X under the spring function of the beams  22 , and thereby the capacitances of both capacitors CS 1 , CS 2  change according to the displacement of the movable electrodes  24 . The circuitry contained in the sensor package detects a difference between CS 1  and CS 2  (CS 1 −CS 2 ) and outputs an electrical signal representing the acceleration imposed on the sensor chip  10 .  
         [0032]    Now, a manufacturing process of the sensor chip  10  will be described with reference to FIGS.  3 A- 3 C. The cross-sectional views shown therein correspond to the cross-sectional view shown in FIG. 2. FIG. 3C shows a partly enlarged cross-sectional view of a dicing portion. Plural sensor chips  10  are formed on a first surface  1   a  (on the second silicon layer  12 ) of the SOI substrate  1  through known processes such as photolithography and dry or wet etching. The sensor chips  10  formed on the substrate, each having the beam structure  20 ,  30 ,  40 , the field portion  60 , the peripheral groove  17  and so on, are separated into individual sensor chips  10  by dicing.  
         [0033]    More particularly, a conductor film of aluminum or the like is formed on the first surface  1   a  of the SOI substrate  1 . The conductor film is patterned to form the electrode pads  25   a,    31   a,    41   a  under photolithography and etching processes. Then, a masking film (a plasma-SiN film or the like) for etching the opening  13   a  is formed on the second surface  1   b  of the SOI substrate  1 . Then, a PIQ (polyimide) film is coated on the second surface  1   b  of the substrate  1 , and the beam structure  20 ,  30 ,  40 , the field portion  60  and the peripheral groove  17  are patterned by etching the PIQ film. Then, a resist film as a protective film is coated on the PIQ film, and the second surface  1   b  side is deep-etched by KOH aqueous solution or the like. In this deep-etching process, the oxidized film  13  acts as a stopper because an etching speed of the oxidized film  13  is slow compared with that of silicon.  
         [0034]    Then, the oxidized film  13  and the plasma-SiN film which are exposed are removed by HF aqueous solution or the like, and the resist film protecting the first surface  1   a  is removed. Then, the sensor structure including the beam structure  20 ,  30 ,  40 , the field portion  60 , and the peripheral groove  17  is made by forming cavities in the second silicon layer  12  by dry-etching using the PIQ film as a mask. Finally, the PIQ film is removed by O 2 -ashing or the like. Thus, the plural sensor chips  10 , one of which is shown in FIG. 3A, are formed on the first surface  1   a  of the SOI substrate  1 .  
         [0035]    After the sensor chips  10  are formed on the first surface  1   a  of the SOI substrate  1 , a protecting sheet  110  is pasted on the first surface  1   a  (the first surface  10   a  of the sensor chip), as shown in FIG. 3B. The protecting sheet is a resin dicing tape that is usually used in the dicing process. The plural sensor chips  10  are separated into individual pieces by dicing along dicing lines DL that run through the center of the peripheral groove  17 . As shown in FIG. 3B, a dicing blade  120  is aligned with the dicing line on the second surface  1   b,  and the dicing proceeds from the second surface  1   b  toward the first surface  1   a  on which the protecting sheet  110  is pasted. After the sensor chips  10  are separated into individual pieces by dicing, the protecting sheet  110  is removed from the sensor chip  10 . Thus, the manufacturing process of the sensor chip  10  is completed.  
         [0036]    In the dicing process of the prototype sensor chip described above, the following problem has been found. FIG. 3C shows the dicing portion in an enlarged scale. A hollow space K is formed between the bottom surface of the peripheral groove  17  and the protecting sheet  110 . As the dicing blade  120  cuts through the first silicon layer  11  and the oxidized film  13 , swarfs N (particles produced by dicing) scatter in the hollow space K. Though some swarfs adhere to the protecting sheet  110 , some other swarfs N remain in the sensor chip  10  and adhere to the movable electrodes  24  or other portions. The swarfs N remained in the sensor chip  10  cause malfunction of the sensor chip  10 .  
       First Embodiment  
       [0037]    To eliminate the problem found in the dicing process of the prototype sample, a first embodiment has been devised. It is found out that the swarfs N do not remain in the sensor chip  10  if the hollow space K where the swarfs N scatter is eliminated. If the protecting sheet  110  pasted on the first surface  1   a  is bent along the peripheral groove  17 , the hollow space K will be eliminated. However, it is difficult to bend the protecting sheet  110  along the peripheral groove  17 , because the width of the peripheral groove  17  is too narrow in the prototype. Accordingly, the width of the peripheral groove is widened in the first embodiment.  
         [0038]    Now, the first embodiment will be briefly described with reference to FIGS.  4 - 6 C. Because the first embodiment is almost the same as the prototype described above, only the difference from the prototype will be described. The same reference numerals as those of the prototype denote the same components.  
         [0039]    [0039]FIG. 4 shows a plan view of a sensor device  100  as the first embodiment of the present invention. The width of the peripheral groove  17  of the prototype is widened, and a reference number  70  denotes the widened peripheral groove. Since the surface of the oxidized film  13  is exposed to the bottom of the peripheral groove  70 , the groove carries reference numbers  70 ( 13 ) in FIG. 4. FIG. 5 shows a cross-sectional view of the sensor chip  100 , taken along line V-V shown in FIG. 4. The peripheral groove  70  is widened, compared with that of the prototype. FIGS.  6 A- 6 C show the manufacturing process of the sensor chip  100 , which is similar to that of the prototype sensor chip  10 , except that the peripheral groove  70  is widened.  
         [0040]    Referring to FIG. 6C, the widened peripheral groove  70  will be described in detail. To paste the protecting sheet  110  along the bottom surface of the peripheral groove  70 , eliminating the hollow space K otherwise formed between the bottom surface of the peripheral groove  70  and the protecting sheet  110 , the width of the groove  70  has to be sufficiently wide compared with the thickness of the second silicon layer  12 . It is found out that the width of the groove  70  has to be at least 5 times of the thickness of the second silicon layer  12 , preferably, 10 to 13 times, or more. Since the thickness of the second silicon layer  12  is 15 μm, the width of the groove  70  has to be at least 75 μm, preferably 150 μm to 200 μm.  
         [0041]    The dicing dusts (swarfs) problem has been overcome by making the peripheral groove  70  sufficiently wide so that the protecting sheet  110  can be pasted along the bottom surface of the groove  70 .  
       Second Embodiment  
       [0042]    A sensor chip  200  as a second embodiment of the present invention will be described with reference to FIGS.  7 - 9 C. The second embodiment is similar to the first embodiment, except that the peripheral groove  70  of the first embodiment is replaced with a ditch  210  and a peripheral bank  211 . The same components as those of the first embodiment carry the same reference numbers, and only the points of the second embodiment which are different from the first embodiment will be described below.  
         [0043]    As shown in FIG. 7, a peripheral bank  211  surrounding a peripheral ditch  210  that is similar to the groove  70  in the first embodiment is additionally formed. Four electrical bridges  220  formed on the first surface  10   a  of the sensor chip  200  electrically connect each side of the peripheral bank  211  to each side of the field portion  60 . Though at least one electrical bridge  220  is necessary, four bridges are provided in this embodiment. The electrical bridges  220  are formed by leaving a portion of the second silicon layer  12  un-etched.  
         [0044]    Referring to FIGS.  9 A- 9 C which correspond to FIGS.  6 A- 6 C of first embodiment, a manufacturing process of the sensor chip  200  will be briefly described. Components of the sensor chip  200  are formed on the first surface  1   a  of the SOI substrate  1  in the same manner as in the first embodiment. The electrical bridges  220  formed by leaving portions of the second silicon layer  12  un-etched may be replaced with separate wires connecting the field portion  60  to the peripheral bank  211 .  
         [0045]    After the plural sensor chips  200  are formed on the first surface  1   a  of the SOI substrate  1  as shown in FIG. 9A, the protecting sheet  110  is pasted on the first surface  1   a  of the SOI substrate  1  as shown in FIG. 9B. In the second embodiment, the dicing line DL is set through the center of the peripheral bank  211 , not through the center of the peripheral ditch  210 . The sensor chips  200  are separated into individual pieces by dicing along the dicing line DL.  
         [0046]    The dicing portion is shown in FIG. 9C in an enlarged scale. The dicing blade  120  enters the second surface  1   b  and comes out from the first surface  1   a.  Since the protecting sheet  110  is pasted on the first surface  1   a,  there is no hollow space in which the swarfs (dicing dusts) scatter in the dicing process. Therefore, the sensor chip  200  can be kept free from the swarfs, and thereby malfunction of the sensor chip  200  due to the swarfs can be avoided.  
         [0047]    Though the field portion  60  and the peripheral bank  211  are separated by the peripheral ditch  210  in this embodiment, no parasitic capacitance is created between the field portion  60  and the bank  211  because both are electrically connected by the bridges  220 .  
         [0048]    Application of the present invention is not limited to the sensor chips for measuring acceleration, but the present invention may be applied to other semiconductor devices having structures similar to the sensor chips described above. For example, the present invention may be applied to dynamic sensors such as pressure sensors or angular velocity sensors, as long as plural chips are made on the first surface of a substrate and diced out into separate pieces from the second surface after a protecting sheet are pasted on the first surface.  
         [0049]    While the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.