Method of Producing Semiconductor Displacement Transducer

This invention relates to a method of producing a semiconductor displacement transducer in which a semiconductor strain detector is bonded to a metal strain generator by means of a metal junction layer.

The semiconductor displacement transducer usually utilizes the mechano-electrical conversion effect in a semiconductor which is usually the piezo-resistance effect in principle. It is a well-known fact that a semiconductor strain detector using the piezo-resistance effect has an output sensitivity several tens of times higher than that of the conventional strain detector using a metal strain gauge.

In this type of semiconductor strain detectors, a strain-sensitive region is formed by diffusing an impurity into a portion of the main surface of a semiconductor single crystal substrate. Since the semiconductor single crystal substrate is an excellent elastic strain generator, the substrate itself is often used as an elastic strain generator in the structure of a semiconductor displacement transducer. However, this constitution is limited only to the case where strain to be generated is relatively small since the semiconductor single crystal substrate is brittle. Accordingly, in order to realize a semiconductor displacement transducer serviceable in relatively large strain conditions, a semiconductor strain detector must be bonded in metal junction to a metal strain generator by means of an insulating oxide film, as disclosed in, for example, U.S. Pat. No. 4,151,502, issued to Y. Kurihara et al on Apr. 24, 1979, entitled "Semiconductor Transducer".

Such semiconductor displacement transducers are usually used in a wide range of temperatures, i.e. from -40.degree. C. to 120.degree. C., and therefore the metal junction layer undergoes creep due to repeated generation of strain especially at high temperatures. As a result, a zero point drift occurred in the output so that a stable characteristic cannot be obtained.

An object of this invention is to provide a semiconductor displacement transducer which suffers little from zero point drift even at high temperatures and therefore has a stable characteristic.

According to one feature of this invention, strain gauges are formed on one main surface of a semiconductor single crystal substrate while an insulating oxide film is formed on the other main surface of the substrate; first and second metal layers and a eutectic alloy layer are formed in this order on the insulating oxide film; the thus prepared substrate is mounted on a metal strain generator; and this assembly is heated for bonding to a temperature approximating to the eutectic point of the eutectic alloy.

FIG. 1 shows in cross section a semiconductor displacement transducer according to this invention.

FIG. 2 shows in plan a semiconductor strain detector.

FIG. 3 shows in cross section a principal portion of the transducer shown in FIG. 1.

FIG. 4 shows the steps of a procedure for joining metal junction layers together.

FIG. 5 shows in cross section the principal portion of the transducer shown in FIG. 3 as a result of the step of heating.

FIG. 6 shows in graphical representation the relationship between the thickness of the eutectic alloy solder layer and the zero point drift of the output.

In FIG. 1, one end of a metal strain generator 10, made of a Fe-Ni or a Fe-Ni-Co alloy, is rigidly attached to a stationary member 12. Semiconductor strain detectors 16 are attached via metal junction layers 18 onto the upper and lower surfaces 14 of the metal strain generator 10, near the stationary member 12. Accordingly, if external force is applied to the remaining free end of the metal strain generator 10 as indicated by the double-headed arrow in FIG. 1, the metal strain generator 10 undergoes displacements so that strains having the same magnitude but the opposite signs are induced in the semiconductor strain detectors 16.

FIG. 2 shows in plan a semiconductor strain detector. The structure of the semiconductor strain detector 16 is as follows. P-type semiconductor strain gauges 22 and 24 are formed in one of the main surfaces of an N-type silicon single crystal substrate 20, through the diffusion of an impurity such as, for example, boron. Also, on the main surface of the substrate 20 are formed gauge pads 28, 30, 32 and 34 used for connecting the electrodes 26 of the gauges 22 and 24 with leads for conducting electric signals to external circuits. Further, a substrate pad 36 is formed to keep the substrate 20 at a high electric potential for the purpose of improving the signal-to-noise ratio attributable to external electric noise. An insulating oxide film 38 of SiO.sub.2 is so formed as to cover the side surfaces and the other main surface of the semiconductor substrate 20, in order to insulate the semiconductor strain detector 16 from the metal strain generator 10.

FIG. 3 shows the semiconductor strain detector 16 as joined to the metal strain generator 10 by means of a metal junction layer 18, which includes a binder layer 180, a stopper layer 182 and a eutectic alloy solder layer 184. As the binder layer 180 is used such metal as Cr that can be firmly bonded to the insulating oxide film 38 and can easily form intermetallic compound with the eutectic alloy solder layer 184 for alloy junction. As the stopper layer 182 is used such metal as Cu or Ni that can suppress the reaction of the binder layer 180 with the eutectic alloy solder layer 184. A eutectic alloy such as of Au-Ge or Au-Si system is used as the eutectic alloy solder layer 184.

FIG. 4 shows the steps of a process for forming the above-mentioned metal junction layer. An oxide film 38 is formed on the surface of a silicon substrate 20 and chromium is vacuum-deposited, as a binder layer 180, on the surface of the oxide film 38. A Cr-Cu alloy layer 181 is formed on the Cr layer 180 through vacuum deposition and then a Cu layer to serve as a stopper layer 182 is vacuum-deposited on the Cr-Cu layer 181, which serves to improve adhesion between the layers 180 and 182. A Au layer 183 is vacuum-deposited on the stopper layer 182 and finally a Au-Ge eutectic alloy layer to serve as a eutectic alloy solder layer 184 is vacuum-deposited on the Au layer 183, which serves to improve adhesion between the layers 182 and 184. On the other hand, the surface of a metal strain generator 10 is plated with gold layer 186 by ion-plating method and the semiconductor strain detector 16 is mounted on the metal strain generator 10 in such a manner that the eutectic alloy solder layer 184 is in contact with the metal strain generator 10 (in this case, two detectors 16 are placed on both the surfaces of the generator 10). And this assembled condition is maintained by means of jigs.

Next, this assembly held by the jigs is heated to a temperature nearly equal to the eutectic point (about 400.degree. C.) of the Au-Ge eutectic alloy. The Au layer 186 serves to ensure bonding of the detectors 16 to the generator 10. In this treatment, as shown in FIG. 5, the binder layer 180 and the eutectic alloy solder layer 184 form a first intermetallic compound 187, i.e. Cr-Ge system, and at the same time the constituent element Ni of the metal strain generator 10 (Fe-Ni or Fe-Ni-Co alloy) diffuses in solid phase into the eutectic alloy solder layer 184 to form a second intermetallic compound (Ni-Ge system) 188. As a result, the thickness of the eutectic solder layer 184 is reduced to 1.5 .mu.m. Moreover, the first and the second intermetallic compounds 187 and 188 are stable substances free from such an undesirable phenomenon as creep and since the Au-Ge eutectic alloy melts at about 400.degree. C., it can firmly join the semiconductor strain detector 16 and the metal strain generator 10 without adversely affecting their desirable properties. The constituent element Cu of the stopper layer 182 diffuses into the eutectic alloy solder layer 184 so that the stopper layer 182 cannot exist as a single, independent layer. Likewise, the layers 181, 183 and 186 are no longer independent ones in macroscopic observation.

FIG. 6 shows the relationship between the thickness of the eutectic alloy solder layer and the zero point drift of the output, this relationship being discovered as a result of the present inventor's experiments. In FIG. 6, solid curve indicates the zero point drift at a high temperature (120.degree. C.) and dashed curve indicates the zero point drift at a room temperature (20.degree. C.). According to this invention, the semiconductor strain detector and the metal strain generator are joined together by means of the metal junction layer and the first and the second intermetallic compounds are formed by the heat treatment at a temperature nearly equal to the eutectic point of the eutectic alloy solder layer so that, as apparent also from FIG. 6, the thickness of the eutectic alloy solder layer can be reduced to substantially not greater than 1.5 .mu.m and that the zero point drift at a high temperature can be reduced to less than 0.1% (with respect to the full scale of an indicating meter).