METHOD OF ADJUSTING BRIDGE CIRCUIT OF SEMICONDUCTOR PRESSURE SENSOR

Abstract: A method of adjusting a bridge circuit of a semiconductor pressure sensor comprises the steps of providing conductive lines connected to at least one of diffused resistors at predetermined intervals during the wiring process, said diffused resistors being provided to connect strain gauge resistors constituting said bridge circuit; providing means for adjusting the resistance value of at least said one of said diffused resistors; making vacuum suction to the back surface of a diaphragm of said semiconductor pressure sensor in the testing process to make a state wherein pressure is virtually applied to said diaphragm from the surface side thereof; measuring the pressure sensitivity of said semiconductor pressure sensor from the surface side of the diaphragm by making use of said bridge circuit; and adjusting the resistance value of at least said one of said diffused resistors by said adjusting means based on the pressure sensitivity thus measured to adjust the balance of said bridge circuit.

Description: BACKGROUND OF THE INVENTION This invention relates to a method of adjusting a bridge circuit of a semiconductor pressure sensor under the wafer process and more specifically to a method of adjusting a bridge circuit of a semiconductor pressure sensor typical of a semiconductor pressure sensor fitted to the tip of a catheter for medical use. When a mechanical stress is applied to a semiconductor crystal of silicon or the like, its resistance changes greatly because of the piezoelectric resistance effect and the perception of this fact has led to the development of a semiconductor pressure sensor. The process of making such a semiconductor pressure sensor comprises the steps of forming a strain gauge resistor by diffusion of impurity ions on the surface layer of a silicon single crystal, assembling four of the strain gauge resistors into a Wheatstone bridge, forming a recess in the back surface of the silicon monocrystal by etching, and disposing electrodes in suitable places on the surface thereof with the thin portion as a diaphragm. When pressure is applied to the semiconductor pressure sensor, the diaphragm is deformed and the resistance value of the strain gauge resistor changes to a greater extent because of the piezoelectric resistance effect, so that a bridge output proportional to the pressure is obtainable. Such a semiconductor pressure sensor is known, for example, from prior art US-A-3697918. The aforesaid semiconductor pressure sensor is extremely small in size and, particularly in the case of a semiconductor pressure sensor for medical use, a plurality of semiconductor pressure sensors are fitted to the tip of a catheter and inserted into a body. Accordingly, even in a semiconductor pressure sensor incorporating peripheral circuits such as a temperature compensating circuit, a pressure sensitivity compensating circuit and the like, a side of a chip should be about 1 mm or smaller in length. Variations in resistance value of each of the strain guage resistors and diffused resistors at their formation and uniformity in the thickness of the diaphragm produced by the etching process result in variations in the characteristics of the semiconductor pressure sensor. However, the semiconductor pressure sensor is extremely small in size and it is very difficult to measure the electrical characteristics while pressure is being applied actually and adjust the resistance balance of the bridge circuit. Actually, the measurement of such electrical characteristics while the pressure is being applied is omitted, whereas only the electrical characteristics of the bridge circuit are measured to find and discard what exceed a predetermined tolerance as inferior goods. Accordingly, yields of the semiconductor pressure sensor are poor and, in addition to this problem, the measurement of electrical characteristics has posed lack of reliability because the degree of deformation of the diaphragm resulting from the pressure actually applied thereto is not measured. SUMMARY OF THE INVENTION In view of the aforesaid problems, an object of the present invention as defined in claim 1 is to provide a method of adjusting a bridge circuit of a semiconductor pressure sensor to improve yields by adjusting the output balance of the bridge circuit based on the output of the semicondutor pressure sensor in a state closely approximated to that wherein the semiconductor pressure sensor is actually in use. In order to accomplish the aforesaid object, the method of adjusting the bridge circuit of a semiconductor pressure sensor having a plurality of diffused resistors for connecting strain guage resistors constituting the bridge circuit, comprises the steps of providing a plurality of conductors connected to at least one of the plurality of diffused resistors at predetermined spaces during the wiring process, the plurality of conductors serving as a means for adjusting the resistance value of the at least one of a plurality of diffused resistors; making vacuum suction to the back surface of a diaphragm of said semiconductor pressure sensor in the testing process to make a state wherein pressure is virtually applied to the diaphragm from the surface side thereof; measuring the pressure sensitivity of the semiconductor pressure sensor from the surface side of the diaphragm by making use of the bridge circuit; and adjusting the resistance value of the at least one of a plurality of diffused resistors by said adjusting means based on the pressure sensitivity thus measured to adjust the balance of the bridge circuit. On the other hand, during the wiring process, pads may be installed at the ends of respective conductors and one of the pads may be selected and bonded based on the pressure sensitivity of the semiconductor pressure sensor to adjust the output balance of the bridge circuit. In the method of adjusting the bridge circuit of a semiconductor pressure sensor, the diaphragm is subjected to vacuum suction from its back surface side to subject the diaphragm to negative pressure during the testing process and the resistance value of the strain guage resistor forming the principal part of the bridge circuit formed on the diaphragm is caused to change through the deformation of the diaphragm and the piezoelectric resistance effect. The changes in the output of the bridge circuit can be measured by making use of the electrodes formed on the surface of the semiconductor pressure sensor. The resistance value can be set stepwise by providing the plurality of conductors connected to the diffused resistor at the predetermined spaces and the diffused resistor is distributed by selectively cutting the conductors or bonding one pad whereby the resistance value can simply be adjusted. Accordingly, before the wafer is diced, it is possible to adjust the bridge circuit of the semiconductor pressure sensor readily by adjusting the resistance balance of the bridge circuit while pressure is being applied thereto. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic tip view showing a method of adjusting a bridge circuit of a semiconductor pressure sensor according to a first embodiment of the present invention. Fig. 2 is a cross sectional view of the semiconductor pressure sensor of Fig. 1. Fig. 3 is a schematic sectional view showing an embodiment for measuring the pressure sensitivity of the semiconductor pressure sensor. Fig. 4 is a diagram showing a state wherein pressure is applied to the semiconductor pressure sensor. Fig. 5 is an electric circuit diagram showing an electrical arrangement of the semiconductor pressure sensor of Fig. 1. Fig. 6 is a schematic top view showing a method of adjusting a bridge circuit of a semiconductor pressure sensor according to a second embodiment of the present invention. Fig. 7 is a cross sectional view of the semiconductor pressure sensor of Fig. 6. Fig. 8 is an electric circuit diagram showing an electrical arrangement of the semiconductor pressure sensor of Fig. 6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the accompanying drawings, embodiments of the present invention will be described in detail. Fig. 1 is a schematic top view showing a method of adjusting a bridge circuit of a semiconductor pressure sensor according to a first embodiment of the present invention. Fig. 2 is a schematic cross sectional view of the semiconductor pressure sensor. In Figs. 1 and 2, a semiconductor pressure sensor 1 is extremely small with a thickness of about 400 um and p⁻ type strain gauge resistors 3a, 3b, 3c, 3d of several kΩ are formed on the surface of a substrate 2 composed of an n⁻ type silicon single srystal. Four of the strain gauge resistors are wired in series using p⁺ diffused resistors 4 which are extended around the surface of the substrate 2 and Aℓ pads 5a, 5b, 5c, 5d, 5e are formed on the diffused resistors 4 thus extended. Moreover, a recess 6 is formed in the back surface of the silicon single crystal 2 with a thin portion (with a thickness of less than 30 um) used as a diaphragm 7. In addition to the construction of the aforesaid known semiconductor pressure sensor 1, an Aℓ pad 8 is installed and lines 9 of an electric conductor (for example, aluminum, gold, silver, copper, etc.) are provided to be connected to the diffused resistor 4 wired between the Aℓ pad 5e and the strain gauge resistor 3d at predetermined intervals. The ends of the all lines 9 are connected to the Aℓ pad 8. The Aℓ pads 8 and 5e are connected and further the Aℓ pads 5e and 5a are connected. Voltage is applied across the Aℓ pads 8 (5e) and Aℓ pad 5c to obtain an output between the Aℓ pads 5b and 5d. That is, a bridge circuit is arranged (see Fig. 5). In Fig. 5, r₁ ∼ r₈ represent the resistance values of the diffused resistors 4. The adjustment of the output balance of the bridge circuit is made by selectively cutting the lines 9 by a laser beam and adjusting the resistance value (r₈) between the Aℓ pad 5e and the resistor 3d. The intervals at which the lines 9 are connected to the diffused resistor 4 are set according to the resistance characteristic (Ω - m) of the resistor and, provided the resistance characteristic is linear, an equal interval is preferred. The present invention is not limited to the aforesaid embodiments. The Aℓ pads 5a and 5e are connected, for instance, and voltage is applied across the Aℓ pads 8 and 5d, whereas the resistance value of not only the diffused resistor 4 between the Aℓ pad 5e and the strain gauge resistor 3d but also those of other diffused resistors may be made adjustable, provided the combination and arrangement of parts are modified without departing from the sprit and the scope of the invention as hereinafter claimed. Fig. 6 is a schematic top view showing a method of adjusting a bridge circuit of a semiconductor pressure sensor according to a second embodiment of the present invention. Fig. 7 is a schematic cross sectional view of the semiconductor pressure sensor of Fig. 6. This embodiment is substantially the same as what is shown in Fig. 1 except that Aℓ pads 8′ are connected to the respective conductive lines 9. In this embodiment, the Aℓ pads 5a and 5e are connected and voltage is applied across the Aℓ pads 5a, 5e and the Aℓ pad 8′, whereas an output is taken out from between the Aℓ pads 5b and 5d. That is, a bridge circuit is thus arranged and any one of the Aℓ pads 8′ is properly selected and bonded for the purpose of adjusting the resistance value r₄ (r₅) of the diffused resistor 4 formed between the strain gauge resistors 3c and 3b as the wiring (see Fig. 8). In Fig. 8, r₁ ∼ r₈ represent the resistance values of the diffused resistors 4. A resistance adjusting circuit 110 in this embodiment may be arranged between not only the strain gauge resistors 3b and 3c but also other strain gauge resistors to effect the adjustment of the diffused resistors 4. Moreover, the intervals at which the lines 9 are connected to the diffused resistor 4 are set according to the resistance characteristic (Ω - m) of the resistor and, provided the resistance characteristic is linear, an equal interval is preferred. Fig. 3 is a schematic sectional view showing an example for measuring the pressure sensitivity of the semiconductor pressure sensor, wherein a wafer stage 10 has a vacuum leakage preventing seal material 12 of soft synthetic resin (for example, styrene, butadiene or silicone rubber) with a thickness in the order of 10 um, which is formed on a plate material 11 of stainless or synthetic resin. Further, at least one through-hole 13 is bored in a suitable place of the wafer stage 10 to make vacuum suction to a recess 6 of the semiconductor pressure sensor 1. The recess 6 of the semiconductor pressure sensor 1 is located above the through-hole 13. In the first embodiment shown in Fig. 1, the measuring probes 14 are caused to contact across the Aℓ pads 5a, 5e and the Aℓ pad 5c (between the input terminals of the bridge) and across the Aℓ pads 5b and 5d (between the output terminals thereof) provided on the surface of the semiconductor pressure sensor 1. In the second embodiment shown in Fig. 6, the measuring probes 14 are caused to contact across the Aℓ pads 5a, 5e and the Aℓ pad 8′ (between the input terminals of the bridge) and across the Aℓ pads 5b and 5d (between the output terminals thereof). In order to locate the recess 6 of the semiconductor pressure sensor 1 above the through-hole 13 of the wafer stage 10, the wafer 15 should be moved while the wafer stage 10 and the measuring probes 14 are set still or the wafer stage 10 and the measuring probes 14 should be shifted while the wafer 15 is set still. As set forth above, the wafer 15 is mounted on the wafer stage 10 and is subjected to vacuum suction using the through-hole 13 and the seal material on the wafer stage 10 prevents vacuum leakage through the connection between the silicon crystal 2 and the wafer stage 10. Moreover, the negative pressure corresponding to the pressure applied from the surface of the semiconductor pressure sensor 1 is generated at the recess 6 and the diaphragm 7 is deformed as in the case where the pressure is applied thereto from the surface. Fig. 4 is a schematic view showing the diaphragm 7 thus deformed. The strain gauge resistors 3a and 3c formed by diffusion in the center of the diaphragm 7 out of the four strain gauge resistors 3a, 3b, 3c and 3d constituting the bridge circuit as shown in Fig. 5 or 8, are compressed, whereas the strain gauge resistors 3b and 3d diffused on the periphery of the diaphragm 7 are extended as the diaphragm 7 is deformed. With respect to the strain gauge resistors, use is made of those whose resistance value increases proportionally to the stress. Given the resistance values of the strain gauge resistors 3a, 3b, 3c and 3d respectively at R1, R2, R3 and R4, R2 and R4 increases whereas R1 and R3 decreases as the diaphragm 7 deforms. In other words, the potential V₁ across the terminals of the resistor 3b increases, whereas the potential V₂ across the terminals of the resistor 3c decreases. Accordingly, the bridge output, i.e., V₁ - V₂ increases in proportion to the deformation of the diaphragm 7. The pressure sensitivity of the semiconductor pressure sensor 1 can be measured, before the wafer 15 is diced, by measuring the bridge output with the measuring probe 14 using the Aℓ pads of the semiconductor pressure sensor 1. The output of the semiconductor pressure sensor is measured while the pressure is being applied thereto. With respect to the semiconductor pressure sensor whose measured value shows a slight error, the lines connected to the diffused resistor 4 are appropriately cut out by a laser beam (first embodiment), or otherwise one of the Aℓ pads 8′ provided at respective lines 9 connected to the diffused resistor 4 at the predetermined intervals is selected (second embodiment), whereby the resistance value of the diffused resistor can be easily adjusted. Therefore, the output balance of the bridge circuit of the semiconductor pressure sensor can be adjusted before the semiconductor pressure sensor is diced. In a brief summary, the electrical measurement is made from the surface side of the semiconductor pressure sensor 1 and the pressure is applied from the back surface side thereof, whereby the pressure sensitivity of the semiconductor pressure sensor 1 is measured under the testing process. With respect to the semiconductor pressure sensor showing error in the measurement, the output balance of its bridge circuit can be adjusted by adjusting the resistance value of the diffused resistor 4. As set forth above, in the method of adjusting the bridge circuit of the semiconductor pressure sensor according to the present invention, there is created a state in which pressure is virtually applied to the diaphragm from the back surface side of the semiconductor pressure sensor and the electrical output is measured in that state similar to that of actual use. Then the lines connected to the diffused resistor at the predetermined intervals are suitably cut out or otherwise one of the Al pads provided at the respective lines connected to the diffused resistor at the predetermined intervals is properly selected and bonded so that the balance adjustment of the bridge circuit of the semiconductor pressure sensor can be made by adjusting the resistance value of the diffused resistor. Accordingly, yields of the semiconductor pressure sensor are improved. Although, in the above described embodiments, recesses 6 provided in the wafer 15 are subjected to vacuum suction one by one, all recesses may be subjected to vacuum suction simultaneously by providing a porous member, through which a gas can pass, between the wafer 15 and the wafer stage 10 provided the through-hole 13. Also, in the above described embodiments, although one resistance value of a diffused resistor is adjusted, any resistance values of any diffused resistors may be adjusted.

Claims: A method of adjusting a bridge circuit of a semiconductor pressure sensor (1), having a plurality of diffused resistors (4) for connecting strain gauge resistors (3) constituting said bridge circuit, comprising the steps of: providing a plurality of conductors (9) connected to at least one of said plurality of diffused resistors (4) at predetermined spacings during the wiring process, said plurality of conductors serving as a means for adjusting the resistance value of said at least one of a plurality of diffused resistors; applying vacuum suction to the rear surface of a diaphragm (7) of said semiconductor pressure sensor in the testing process to produce a state wherein pressure is virtually applied to said diaphragm from the other surface thereof; measuring the pressure sensitivity of said semiconductor pressure sensor from the surface side of the diaphragm by making use of said bridge circuit; and adjusting the resistance value of said at least one of a plurality of diffused resistors (4) by said adjusting means based on the pressure sensitivity thus measured to adjust the balance of said bridge circuit. A method of adjusting a bridge circuit of a semiconductor pressure sensor as claimed in claim 1, wherein said means for adjusting the resistance value consists of said plurality of conductors (9) and a conductive pad (8) commonly connected to the ends of respective said conductors and wherein the resistance value of said diffused resistor is adjusted by selectively cutting out said condutors based on said pressure sensitivity. A method of adjusting a bridge circuit of a semiconductor pressure sensor as claimed in claim 1, wherein said means for adjusting the resistance value consists of said plurality of conductors (9) and conductive pads (8') connected to respective ends of said conductors and wherein the resistance value of said diffused resistor is adjusted by selecting and bonding one of said pads based on said pressure sensitivity.

Inventors: NISHIGUCHI, MASANORI C/O YOKOHAMA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1987
Main classification: G01L9
IPC codes: G01L9, G01L27