DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] An embodiment of the present invention will be described with reference to FIGS. 1 and 2 , showing a cross-sectional view and a plan view of an semiconductor acceleration sensor 100 , respectively. FIG. 2 shows a plan view viewed in direction A shown in FIG. 1 with a cover plate removed. The acceleration sensor 100 is composed of a semiconductor sensor chip 5 and a substrate 3 both contained in a package case 1 .

[0021] The package case 1 is made of ceramics, and a cavity 1 b for containing the sensor chip 5 and the substrate 3 therein is formed in the package case 1 . The substrate 3 , which is a semiconductor IC-chip having a circuit for processing sensor signals (referred to as a circuit chip), is mounted on a mounting surface 1 c in the cavity 1 b via adhesive 2 . The semiconductor sensor chip 5 is mounted on the circuit chip 3 via an adhesive film 4 . Pads 1 a , 3 a and 5 a made of aluminum or the like for wire-bonding are formed on the package case 1 , the circuit chip 3 and the sensor chip 5 , respectively. The package case 1 and the circuit chip 3 are electrically connected to each other by wires 6 a connecting the pads 1 a and 3 a . The circuit chip 3 and the sensor chip 5 are electrically connected by wires 6 b connecting the pads 3 a and 5 a . The wires 6 a and 6 b made of gold, aluminum or the like are formed in a wire-bonding process.

[0022] The semiconductor sensor chip 5 is made from an SOI wafer (Silicon-On-Insulator) composed of a first silicon layer 51 , a second silicon layer 52 and an oxidized film 53 interposed between both layers 51 , 52 . The sensor chip 5 includes a beam structure 56 consisting of fixed electrodes and movable electrodes and detects acceleration based on static capacitance changes between the fixed electrodes and the movable electrodes. Since the acceleration sensor chip of this kind is known, its structure will be briefly described below with reference to FIG. 2 .

[0023] As shown in FIG. 2 , the beam structure 56 having comb-shaped fixed electrodes 55 and movable electrodes 54 is formed in the second silicon layer 52 . The movable electrodes 54 facing the fixed electrodes 55 displace in direction X shown in FIGS. 1 and 2 when acceleration is imposed on the movable electrodes 54 . Static capacitance between the fixed and movable electrodes 54 , 55 changes according to the displacement of the movable electrodes 54 . Sensor signals representing the capacitance changes are fed from the sensor chip 5 to the circuit chip 3 through the wires 6 b and are converted into voltage signals or the like in the circuit chip 3 . The converted signals are led to the package case 1 through the wires 6 a and are output to outside devices through wires (not shown). Thus, the acceleration is detected as electrical signals.

[0024] A surface 51 a of the first silicon plate 51 is fixedly connected to a first surface 3 b of the circuit chip 3 by means of the adhesive film 4 made of thermosetting or thermoplastic resin such as polyimide or acrylic resin. Preferably, the adhesive film 4 having thickness not exceeding 50 μm (thicker than 15 μm, for example) is used, considering sensor characteristics described later. Further, it is preferable to use the adhesive film 4 having an elasticity coefficient (Young's modulus) equal to or lower than 3,000 MPa (mega pascal). The adhesive film 4 may be either electrically conductive or non-conductive.

[0025] While the sensor chip 5 is connected to a first surface 3 b of the circuit chip 3 via the adhesive film 4 , a second surface 3 c of the circuit chip 3 is connected to the mounting surface 1 c of the package case 1 via adhesive 2 . The circuit chip 3 may be mounted on the package case 1 via the adhesive 2 coated on the entire area of the second surface 3 c . However, it is preferable to place the adhesive 2 only on the four corners of the second surface 3 c , as shown in FIG. 2 .

[0026] The opening of the cavity 1 b of the package case 1 is closed with a cover plate 7 made of iron or the like to protect the sensor chip 5 and the circuit chip 3 contained in the cavity 1 b . The cover plate 7 is fixed to the package case 1 by welding a metal member 8 interposed therebetween.

[0027] Now, a process for connecting the sensor chip 5 to the circuit chip 3 and for mounting the circuit chip 3 on the package case 1 will be described with reference to FIGS. 3 A- 3 H. The components constituting the semiconductor dynamic sensor 100 are all shown in cross-sectional views in FIGS. 3 A- 3 H. The pads 1 a , 3 a and 5 a for wire-bonding are not shown in those drawings. A wafer 20 including plural sensor chips therein is manufactured in a known semiconductor manufacturing process, using the SOI substrate.

[0028] As shown in FIG. 3 A, the adhesive film 4 is stuck to the surface 51 a of the first silicon layer 51 to cover an entire surface of the wafer 20 . The sensor chip surface opposite to the surface 51 a is covered with a protective sheet 21 made of resin. Plural depressions corresponding to beam structures 56 of each sensor chip 5 are formed on the protective sheet 21 , so that the protective sheet 21 does not contact the beam structures 56 .

[0029] The adhesive film 4 made of polyimide resin is used in this embodiment. To stick the polyimide adhesive film 4 on the surface 51 a of the first silicon layer 51 , the wafer 20 is placed on a plate (not shown) heated to about 180° C., placing the surface 51 a upward. The adhesive film 4 wound on a roll bobbin (not shown) is spread over the surface 51 a , and the adhesive film 4 is stuck to the surface 51 a under heat and pressure. Portions of adhesive film 4 hanging over from the surface 51 a are cutout. Then, the adhesive film 4 stuck to the wafer 20 is provisionally cured on the heated plate, e.g., for 2 minutes at 180° C. Then the protective sheet 21 is stuck on the surface opposite to the surface 51 a . Thus, both surfaces of the wafer 20 are covered with the adhesive film 4 , and the protective sheet 21 , respectively, as shown in FIG. 3A .

[0030] Then, as shown in FIG. 3 B, the wafer 20 is cutout into individual sensor chips 5 together with the adhesive film 4 and the protective sheet 21 by a dicing cutter 22 . Then, the protective sheet 21 is removed, while keeping the adhesive film 4 on the surface 51 a . Thus, the sensor chip 5 having the adhesive film 4 attached thereto is completed, as shown in FIG. 3C .

[0031] On the other hand, as shown in FIGS. 3 D- 3 F, the circuit chip 3 is mounted on the package case 1 by means of adhesive 2 . The adhesive 2 is placed at desired positions, e.g., at four corners, on the mounting surface 1 c of the package case 1 , as shown in FIG. 3E . Then, the circuit chip 3 is bonded to the package case 1 with the adhesive 2 , as shown in FIG. 3F .

[0032] Then, as shown in FIG. 3 G, the sensor chip 5 is bonded to the circuit chip 3 with the adhesive film 4 interposed therebetween. This process will be described in detail, assuming the adhesive film 4 is made of polyimide resin. The package case 1 on which the circuit chip 3 is mounted is placed on an assembling plate (not shown) heated to about 230° C. The sensor chip 5 is placed on the circuit chip 3 , so that the adhesive film 4 contacts the circuit chip 3 . The sensor chip 5 is bonded to the circuit chip 3 via the adhesive film 4 under heat and pressure, e.g., at a temperature of 230° C. and under a pressure of 3 N (newton) Then, the adhesive film 4 is finally cured in an oven for about one hour at about 190° C.

[0033] Then, as shown in FIG. 3 H, the pads 1 a , 3 a , 5 a are electrically connected through wires 6 a , 6 b by wire-bonding. Finally, the cover plate 7 is connected to the package case 1 to close the opening by welding, e.g., by seam-welding. The metal member 8 for welding is bonded or soldered to the package case 1 before the process of mounting the components thereon. Thus, the semiconductor acceleration sensor 100 shown in FIG. 1 is completed. The acceleration sensor 100 is mounted, for example, on an electronic control unit for use in an automobile to detect acceleration.

[0034] Since the adhesive film 4 that has no flowablity is used for fixing the sensor chip 5 to the circuit chip 3 in the embodiment described above, the sensor chip 5 is correctly mounted at a predetermined position on the circuit chip 3 . If a liquid adhesive having flowability is used as in the conventional device, the sensor chip 5 cannot be fixed to the circuit chip 5 at a correct position, because the sensor chip 5 will move relative to the circuit chip 3 in a sticking process or during a process of transporting the device into an oven to cure the adhesive. In other words, misalignment of the sensor chip 5 is avoided in the embodiment of the present invention, and thereby the sensor sensitivity in directions other than in a predetermined direction (referred to as the other axis sensitivity) can be made almost zero.

[0035] The other axis sensitivity mentioned above will be further explained with reference to FIGS. 4A and 4B showing a comparative example 200 in which a liquid adhesive S 1 is used for mounting the sensor chip 5 on the circuit chip 3 . The reference numbers shown in FIGS. 4A and 4B are the same as those of the embodiment of the present invention. As mentioned above, acceleration is detected based on a displacement of the movable electrodes 54 of the beam structure 56 in the X-axis direction (X, Y and Z axes are shown in FIGS. 4A and 4B ). The sensor chip 5 has to be positioned, so that the movable electrodes 54 moves in the X-axis direction and the beam structure plane becomes in parallel to the mounting surface 1 c of the package case 1 .

[0036] If the sensor chip 5 is misaligned relative to the circuit chip 3 with an misalignment angle θ 1 in the X-Y plane as shown in FIG. 4 A, the following sensitivity appears in the direction of Y-axis:

(Y-axis sensitivity)=(X-axis sensitivity)×(sin θ 1 )

[0037] If the sensor chip 5 is misaligned with an misalignment angle θ 2 in the Z-axis direction as shown in FIG. 4 B, the following sensitivity appears in the direction of Z-axis:

(Z-axis sensitivity)=(X-axis sensitivity)×(sin θ 2 )

[0038] It is, of course, intended to have the sensor sensitivity only in the X-axis direction. If there appears some sensitivity in other axis directions than X-axis direction, the X-axis sensitivity decreases by the amount of the other axis sensitivity.

[0039] Since the other axis sensitivity is eliminated or is made almost zero by using the adhesive film 4 having no flowability in the embodiment of the present invention, an intended high sensitivity in the X-axis direction can be obtained. Further, the pads 3 a on the circuit chip 3 are prevented from being stained with liquid adhesive flowing over the pads 3 a . Accordingly, the bonding strength is not adversely affected by the adhesive material.

[0040] As mentioned above, the preferable thickness of the adhesive film 4 is less than 50 μm, and its preferable elasticity coefficient (Young's modulus) is lower than 3,000 MPa. The reasons for those will be explained with reference to FIGS. 5 and 6 showing simulation test results.

[0041] In FIG. 5 , the temperature-dependency of sensor offset outputs (in terms of mV) is plotted on the ordinate versus the elasticity coefficient (in terms of MPa) of the adhesive film 4 on the abscissa. The temperature-dependency of sensor offset outputs means a sensor output difference between two ambient temperature levels, 25° C. and 85° C. when no acceleration is imposed on the sensor. According to experiments by the applicant, it is preferable in a practical use that the temperature-dependency does not exceed 50 mV. The temperature-dependency for three adhesive films 4 , each having different thickness, t=10, 50 and 100 μm, is shown in the graph of FIG. 5 . As seen in the graph, the thicker the adhesive film is, the larger the temperature-dependency becomes.

[0042] In FIG. 6, a relation between the temperature-dependency of sensor offset outputs and the thickness of the adhesive film 4 is shown, taking the elasticity coefficient as a parameter. As seen in the graph, the lower the elasticity, the lower the temperature characteristic. Combining both results shown in FIGS. 5 and 6 , it is seen that the condition, under which the temperature-dependency of the offset outputs does not exceed 50 mV, is satisfied if the adhesive film thickness is thinner than 50 μm and its elasticity coefficient is lower than about 3,000 MPa.

[0043] Since the adhesive film 4 is stuck to the wafer 20 that includes plural sensor chips 5 , and the wafer 20 is separated into individual sensor chips 5 by dicing, it is not necessary to stick the adhesive film to individual sensor chips 5 . Therefore, the manufacturing process is considerably simplified. It is preferable to smooth the mounting surfaces, i.e., the first surface 3 b of the circuit chip 3 on which the sensor chip 5 is mounted and the mounting surface 1 c of the package case 1 on which the circuit chip 3 is mounted. It may be necessary to remove the mounting surface inclination relative to the X-Y plane before mounting the sensor chip 5 , if there is such inclination.

[0044] Since the circuit chip 3 is mounted on the mounting surface 1 c by applying the adhesive 2 only to some portions of the mounting surface 1 c , e.g., only to its four corners, the circuit chip 3 can be mounted flat on that surface without being affected by possible small depressions or projections on that surface. Since the adhesive film 4 stuck to the sensor chip 5 is heated to soften the same to a degree not to generate its flowabilty in the process of mounting the sensor chip 5 on the first surface 3 b of the circuit chip 3 , small depressions or projections on that surface can be absorbed by the softened adhesive film 4 . The first surface 3 b may be smoothed by coating resin such as polyimide (PIQ) thereon to enhance a close contact between the sensor chip 5 and the circuit chip 3 .

[0045] The semiconductor sensor chip is not limited to the sensor chip having a beam structure 56 including movable electrodes 54 , but other sensor elements generating electrical signals according to applied acceleration, such as a piezoelectric elements, may be used. Further, elements other than the circuit chip 3 , such as a glass substrate, may be used as the substrate on which the sensor chip 5 is mounted. The present invention is also applicable to other sensor devices such as angular velocity sensors.

[0046] While the present invention has been shown and described with reference to the foregoing preferred embodiment, 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.