Abstract:
A semiconductor pressure sensor having a cylindrical housing with an opening formed at its tip end which is exposed to a pressure atmosphere and a metal diaphragm for receiving pressure, which is formed in a wall of the cylindrical housing defining the opening. A metal oxide layer is formed in a surface of the metal diaphragm by oxidizing the surface of the metal diaphragm, and a glass layer is formed on the metal oxide layer. A semiconductor chip, on which a strain gauge is formed, is firmly and surely bonded to the metal diaphragm through the glass layer by virtue of the metal oxide layer. This semiconductor pressure sensor can measure high pressure with sufficient sensitivity and high accuracy.

Description:
This is a division of application Ser. No. 07/050,031, filed May 15, 1987, now U.S. Pat. No. 4,840,067. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor pressure sensor and a method for bonding a semiconductor chip to a metal diaphragm thereof, and more particulary to an accurate semiconductor pressure sensor suitable for the detection of high pressure and a bonding method therefor. 
     2. Description of the Prior Art 
     In many cases, there have been used pressure sensors, each being provided with such a pressure sensing diaphragm as to deform in response to the measured pressure. These pressure sensors sense the amount of deformation of the diaphragm by means of a strain gauge. In particular, for measuring high pressure, it is desired to use a metal diaphragm having pressure resistance. For example, Japanese examined patent publication No. Sho 58-5372 discloses a pressure sensor provided with a measuring portion composed of a metal diaphragm and a metal strain gauge bonded to the metal diaphragm. However, this metal strain gauge has a defect that the output sensitivity is low. This results in multistage amplification and noise removal being required, and accordingly the measuring circuit becomes complex. 
     Recently, pressure sensors, each being provided with a semiconductor strain gauge having a linear and high sensitive output have been noted. In each of these pressure detectors, a semiconductor chip itself composes a pressure receiving diaphragm (U.S. Pat. No. 3,858,150 for example). Therefore, these pressure sensors cannot be used for sensing high pressure. 
     For obtaining a pressure sensor durable against high pressure and having excellent output sensitivity, it has been thought of to combine the metal diaphragm with the semiconductor chip composing the semiconductor strain gauge. However, such a bonding structure to bond the semiconductor chip to the metal diaphragm with sufficient strenght has not been found as yet. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a semiconductor pressure sensor of good performance, wherein a strain gauge semiconductor chip may be firmly and surely bonded to a metal diaphragm. 
     The semiconductor pressure sensor according to the present invention has a metal diaphragm for receiving pressure, a metal oxide layer formed in a surface of the metal diaphragm by oxidizing the surface of the metal diaphragm, a glass layer formed on the metal oxide layer and a semiconductor chip on which a strain gauge is formed. The semiconductor chip is firmly bonded to the glass layer. 
     In the semiconductor pressure sensor having the above-described structure, the semiconductor chip is firmly and surely bonded to the metal diaphragm. And this semiconductor pressure sensor can measure high pressure with sufficient sensitivity and high accuracy. 
    
    
     BRIEF EXPLANATION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a main part of a semiconductor pressure sensor according to the present invention, which is an enlarged sectional view of the portion A of FIG. 2; 
     FIG. 2 is a sectional view of an overall semiconductor pressure detector; 
     FIG. 3 is a plan view of a semiconductor chip; 
     FIG. 4 is a wiring diagram of a strain gauge elements; 
     FIG. 5 is a graph showing the relation between the surface roughness of a sensing body and the tensile strength; 
     FIG. 6 is a graph showing the relation between the thickness of an oxide layer and the tensile strenght; and 
     FIG. 7 is a schematic block diagram of the bonding process. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In FIG. 2, a housing 501 is composed of a cylindrical metal body. The upper half portion of the housing 501 has a hexagonal section and the lower half portion thereof is provided with a screw portion 501a in its outer periphery. 
     A sensing body 1 is inserted into the housing 501. The sensing body 1 is composed of a cylindrical body having a closed upper end and an open lower end. The peripheral edge of the open lower end is welded to the inner edge of an opening of the housing 501. The sensing body 1 is made of Fe-Ni-Co alloy having a low coefficient of thermal expansion. The central portion of the closed upper end surface of the sensing body 1 is formed into a thin-walled diaphragm 11 for receiving pressure. 
     A semiconductor chip 4 is bonded to the upper surface of the diaphragm 11 by a bonding structure as described later. The semiconductor chip 4 is connected to electrodes formed on a ring-shaped ceramic substrate 502 provided on the upper surface of the housing 501 so as to surround the semiconductor chip 4, by way of lead wires 503. 
     A ceramic substrate 504 on which an IC circuit for processing signals is formed, is disposed above the semiconductor chip 4. The IC circuit is connected to the electrodes formed on the substrate 502 through pins 505 which support the substrate 504. 
     A cylindrical cover 506 is bonded to the outer peripheral edge of the upper surface of the housing 501, and a lead holder 507 is closely fit in an upper opening of the cover 506. A lead wire 508 extends from the substrate 504 to the outside of the sensor, penetrating the lead holder 507. In FIG. 2, reference numeral 509 denotes a seal ring, 510, 511 denote seal resins, and 512 denotes a push ring for the seal ring 509. 
     The semiconductor chip 4 is provided with P-type semiconductor strain gauge elements 41, 42, 43, and 44 which are formed at four points on a silicon (Si) substrate surface by doping boron (B) as shown in FIG. 3. The strain gauge elements 41 and 43 are positioned just above the center of the diaphragm 11 while strain gauge elements 42 and 44 are placed just above the peripheral portion of the diaphragm 11. These elements 41 to 44 are connected to each other through electrode leads (not shown) formed on the silicon substrate to form a full bridge as shown in FIG. 4. 
     The bonding structure of the semiconductor chip 4 and the sensing body 1 is shown in FIG. 1. In FIG. 1, the portion encircled by a circle A shown in FIG. 2 is enlarged. 
     The whole upper surface of the sensing body 1 is oxidized to form an oxide layer 2. Then, the semiconductor chip 4 is bonded on the oxide layer 2 through a low melting point glass layer 3. 
     The bonding strength in bonding portions of the bonding structure shown in FIG. 1 is measured by a tensile test, the results being shown in FIGS. 5 and 6. As shown in FIG. 5, when the surface-roughness of the sensing body 1 is 0.5 μm to 3 μm, good tensile strength higher than 150 kg/cm 2  can be obtained. When the depth of the oxide layer 2 is about 1.5 μm to 5.5 μm, on the other hand, sufficient bonding strength higher than 150 kg/cm 2  can be obtained as shown in FIG. 6. 
     Non-crystal low melting point glass containing lead oxide as a main ingredient is used as the glass layer 3. It is preferable to form the thickness of the glass layer 3 to 40 to 60 μm (Table 1) and to form the particle diameter of a filler thereof to about 2.0 μm as an average value and about 20 μm as the maximum (Table 2). However, when the thickness of the glass layer 3 is larger than 60 μm, both the sensitivity and strength are drastically decreased. 
     A thermal shock test of cooling at -40° C. and heating at 120° C. shown in Tables 1 and 2 is repeated 200 times per hour and a high pressure operation test of applying a presssure load of 200 kg/cm 2  four times per minute is repeated one million times. In Tables 1 and 2, &#34;0&#34; denotes that no drift occurs in the output signals from the semiconductor chip after these tests while &#34;X&#34; denotes that some drift occurs in output signals from the semiconductor chip 4 after these tests. 
     The method for forming the above-described bonding structure is roughly shown in FIG. 7. 
     At first, the surface of the sensing body 1 is subjected to the acid-pickling step and the decarburization step. The acid-pickling step is performed by removing oil components from the surface of the sensing body 1, using nitrohydrochloric acid. The decarburization step is performed by supplying steam in a deoxidizing and weak deoxidizing atmosphere containing H 2  and N 2  at 800° to 1100° C. for about 1 hour (Table 3, D, F, J, K, L). 
     Next, the surface of the sensing body 1 is subjected to oxidization process in an O 2  (oxygen) atmosphere at 800° to 850° for 5 to 10 minutes (Table 3, D, F, J, K, L), whereby an oxide layer 2 of which the depth is 1.5 μm to 5.5 μm is formed. 
     The glass layer 3 is formed by printing a glass paste on the oxide layer 2 and calcining the printed glass paste at about 400° C. The thickness of the formed glass layer 3 is preferably 40 μm to 60 μm. The glass paste is obtained by mixing glass powder with an organic solvent. 
     The semiconductor chip 4 is placed on the calcined glass layer 3 and then is sintered at about 500° C., whereby the semiconductor chip 4 is firmly bonded to the sensing body 1. 
     In the semiconductor pressure sensor having the above-described structure, when the diaphragm 11 is deformed due to the introduced pressure, tensile stress is generated in the semiconductor chip 4 bonded to the diaphragm 11. This results in the resistance value of each of the strain gauge elements 41 and 43 on the chip 4 being drastically changed to obtain a linear output signal of high sensitivity, which corresponds to the measured pressure. Furthermore, when the inner peripheral surface of the sensing body 1 composing a pressure inlet port is subjected to the oxidization process, excellent corrosion resistance can be obtained. 
     As described above, according to the present invention, the diaphragm for directly receiving a high pressure is made of a metal of a high strength. Therefore, the diaphragm is durable against high pressure. Furthermore, the semiconductor chip provided with a semiconductor strain gauge is firmly bonded to the metallic diaphrgm through glass under most proper conditions. This results in the highly sensitive pressure sensor being obtained. According to the present invention, the semiconductor chip is previously formed and then jointed on the metal diaphragm. Therefore, mass-producible pressure sensors can be provided. 
     
                       TABLE 1______________________________________Glass   Thickness of Thermal   High PressureNo.     Glass Layer  Shock Test                          Operation Test______________________________________C       20 μm     X         XC       40 μm     O         OC       60 μm     O         O______________________________________ 
    
     
                       TABLE 2______________________________________  Particle  Diameter         Thermal  High PressureGlass  of Filler        Shock    OperationNo.    Average  Maximum     Test   Test______________________________________A      5.4 μm           110 μm   X      XB      5.4 μm            70 μm   X      XC      2.0 μm            20 μm   O      O______________________________________ 
    
     
                       TABLE 3______________________________________Condition of       Condition of BondingDecarburization       Oxidization  State______________________________________A   --          --           Insufficient                                    X                        Formation of                        Oxide LayerB   --          800° C. × 10 min                        Generation of                                    X                        Bubbles in                        Glass Layer                        (Insufficient                        Bonding Strength)C   1000° C. × 1 hr           --           Insufficient                                    X                        Formation of                        Oxide LayerD   1000° C. × 1 hr           800° C. × 10 min                        Good Bonding                                    OE   1000° C. × 1 hr           800° C. × 1 min                        Insufficient                                    X                        Formation of                        Oxide LayerF   1000° C. × 1 hr           800° C. × 5 min                        Good Bonding                                    OG   1000° C. × 1 hr           800° C. × 30 min                        Too thick Oxide                                    X                        Layer                        (Insufficient                        Bonding Strength)H   1000° C. × 1 hr           700°  C. × 10 min                        Insufficient                                    X                        Formation of                        Oxide LayerI   1000° C. × 1 hr           900° C. × 10 min                        Too thick Oxide                                    X                        LayerJ    800° C. × 1 hr           800° C. × 10 min                        Good Bonding                                    OK   1100° C. × 1 hr           800° C. × 10 min                        Good Bonding                                    OL   1000° C. × 1 hr           850° C. × 5 min                        Good Bonding                                    O______________________________________