Transducer Apparatus Employing Convoluted Semiconductor Diaphragms

This invention relates to pressure sensors and more particularly to a pressure sensor employing a semiconductor convoluted diaphragm.

Typical prior art pressure sensors employing piezoresistive devices operated with a thin "coin-like" diaphragm of a circular, square or rectangular plan configuration. The piezoresistive sensors were then diffused or otherwise deposited upon the diaphragm.

Many examples of such structures exist in the prior art. See for example U.S. Pat. No. 3,654,579 entitled ELECTROMECHANICAL TRANSDUCERS AND HOUSINGS by A. D. Kurtz et al issued on Apr. 14, 1972 and assigned to the assignee herein.

In displacement or pressure transducers the diaphragm is subjected to a force the magnitude of which is designated by the resistance change of the sensors. The desirable mode of operation for such diaphrams is bending. This bending produces surface fiber strains which are sensed by piezoresistive elements. In any event, at low pressure and large forces the diaphragm apart from bending tends to stretch. The stretching of the diaphragm is a nonlinear function of applied force or pressure.

A diaphragm may be looked upon as a spring element which supports the applied laod. Deflection of the center is caused by the application of load. If all the applied load is supported by bending, then this deflection is linear. If on the other hand, a portion of the applied load is resisted by a restoring force caused by stretching, then little deflection is caused by this portion of the load. It is found that the center deflection becomes nonlinear in a regressive and in turn the fiber strains are similarly nonlinear. This stretch effect is very noticeable and objectionable in low pressure transducer structures because such structures employ thin diaphrams which off little resistance to stretching. In general such low pressure piezoresistive transducers must be down rated or reduced in full range sensitivity to maintain linear response at low pressure. The greater the stiffness of the diaphragm, the greater the pressure that the diaphragm can handle and thick diaphragms are usable as high pressure transducers. Thin diaphragms are rather difficult to manufacture and therefore low pressure diaphragms require a relatively large area.

In order to alleviate the nonlinearity due to stretching, the prior art employed convoluted or corrugated diaphragms. These diaphragms exhibit a large range of deflection to applied pressure and therefore the stretching effect was not as pronounced. Typically, such diaphragms are used in conjunction with a push rod and beam to form a relatively complex pressure responsive structure. In any event, such diaphragms were machined from a suitable metal and were and are extremely difficult to manufacture. Furthermore, the strain sensors or gages were separately positioned and mounted on the convoluted structures or associated ancillary flexures resulting in additional problems which affected the transducer performance and linearity. These problems were inherent in the bonding techniques used to secure the sensors to the diaphragm, the dissimilar materials and associated temperature effects and a host of other problems.

It is therefore desirable to provide a convoluted silicon diaphragm for low pressure operation (15 psi or less) wherein semiconductor sensors are directly diffused into the diaphragm by integrated circuit techniques. Semiconductor technology provides a precise, economical and effective means of both fabricating the complex nonplanar diaphragm structure and integrating the piezoresistive elements.

A pressure or displacement transducer employs a semiconductor convoluted diaphragm having a central portion manifesting a plurality of concentric recesses or corrugations on both sides with a thick outer peripheral area with at least one semiconductor sensor deposited on said diaphragm within the peripheral area with a portion of said sensor being located proximate to an edge of an outer convolution.

FIG. 1 is a top plan view of a pressure or displacement transducer according to this invention.

FIG. 2 is a cross-sectional view taken through line 2--2 of FIG. 1.

Referring to FIG. 1, there is shown a top plan view of a pressure sensor 10 employing a convoluted or corrugated diaphragm 11.

Essentially, the diaphragm 11 is shown as relatively square in configuration although rectangular, circular or other configurations could be employed as well. The diaphragm 11 is fabricated from single crystal silicon and is typically square having sides of about 0.08 inches with a thickness of 0.0035 inches. It is understood that the dimensions are given by way of example.

In general, a square or a circular diaphragm is symmetrical about the center point of the diaphragm. The aspect ratio, which is the ratio of the length of the diaphragm to the width is 1:1. In a square or circular diaphragm the radial stress is of a substantial magnitude and of opposite sign at the center of the diaphragm than at its periphery. In a rectangular diaphragm the length to width is greater than 1:1 and the ratio of longitudinal to transverse stress at the center which is one for a diaphragm which has an aspect ratio of one and correspondingly increases for higher aspect ratios. The longitudinal stress is defined as the stress parallel to the short axis of the diaphragm.

In a convoluted structure such as that shown in FIG. 1, the square diaphragm configuration is conveniently fabricated and hence formation of the diaphragm and of the recesses or convolutions is easy to implement. The use of an anisotropic etch is very convenient for the formation of the structure shown. Such an etch produces sharp well-defined contours of precise dimensions. The surface of the diaphragm 11 contains a series of concentric grooves of increasing size which form increasing squares in the plan view. The preferred embodiment uses a square diaphragm but other plan views are possible or desirable. For instance, a circular plan configuration can be fabricated using isotropic etchants. Alternately, a diaphragm of a quasi-rectangular plan view similar to the structure shown in U.S. Pat. No. 4,236,137 entitled SEMICONDUCTOR TRANSDUCERS EMPLOYING FLEXURE FRAMES issued on Nov. 25, 1980 to Anthony D. Kurtz, et al and assigned to the assignee herein.

Referring to FIG. 2, there is shown a cross section of the pressure sensor 10 taken through line 2--2 of FIG. 1.

As can be seen from FIG. 2, the convolutions appear trapezoidal in cross section. Thus, as seen in FIG. 2, the central area of the diaphragm appears convoluted and is of a given thickness (i.e. 0.0005 inches) with the concentric square patterns forming the trapezoidal cross-sectional crests and valleys in a repeating sequence on both sides with crests aligning with valleys as shown. The outer peripheral area 21 of the diaphragm is thicker for support purposes and as will be explained is the area which contains the diffused piezoresistive sensors. Each recess which is a square pattern is formed by an anisotropic etch which results in the sloping sidewalls. In regard to the dimensions given above, the trapezoidal area A possesses the following dimensions: the thickness T of the convoluted area is 0.0005 inches. The dimensions X which determine the slope of the trapezoidal area are 0.0022 inches respectively, with the dimension B being 0.0035 inches. The diaphragm is mounted on a base support member 20 which is bonded about the periphery of the diaphragm by means of a glass or other type of bond. The base member 20 may be fabricated from silicon, metal or glass and has a central aperture 19 which may serve as a pressure port. This operation can be eliminated to form an absolute or sealed transducers as required.

As can be seen from FIG. 2, the sides of the convoluted diaphragm 10 as extending into the large peripheral bonding area 20 slope at the same angle as the sides of the trapezoidal area. This is depicted by numeral 22 on FIG. 2.

Referring to FIG. 1, there is shown four resistor configurations 30, 31, 32, and 33 diffused into the surface of the diaphragm 10 and symmetrically located about the center lines. Resistors 30 and 31 are larger in magnitude than resistors 31 and 33. The resistors are formed on the diaphragm by conventional semiconductor deposition techniques and are thus integrally formed within the diaphragm structure. The techniques of fabricating such resistor structures are well-known in the art and may comprise solid state diffusion or epitaxial growth. For examples of suitable processes for forming such resistors see U.S. Pat. No. 3,654,579 entitled ELECTROMECHANICAL TRANSDUCERS AND HOUSINGS issued on Apr. 4, 1972 to A. D. Kurtz et al and assigned to the assignee herein. See also U.S. Pat. No. 3,873,956 entitled INTEGRATED TRANSDUCER ASSEMBLIES issued on Mar. 25, 1975 to A. D. Kurtz et al and assigned to the assignee herein.

The resistors 30 and 31 respond to transverse stress and as seen in FIG. 1 are located near the edge of recess 35 and overlying the sloped wall 22. This position enables resistors 30 and 32 to respond to transverse stress in an optimum manner. Resistors 31 and 33 are located between the sloping wall section 22 and the outer edge of recess 35 and, based on the configuration and orientation, respond to mainly longitudinal stress. The terminals of the resistors are directed through suitable contact lead areas as 40 and 41 to aluminum terminal pads 45 and 46 located about the periphery of the diaphragm as shown. The formation of the terminal pads as 45 and 46 and the contact lead areas 40 and 41 are also well-known in the art and are formed by conventional diffusion and metallization techniques, examples of which are also given in the above noted patents.

Essentially, as can be ascertained, the resistor configurations are connected on the substrate as a Wheatstone bridge array with the bias for the bridge designated as + in and - in and the output designated as + out and - out. The bridge may have an open bridge to allow resistor or other compensating components to be added between the + out terminals 50 and 51.

As indicated, the square convoluted areas are formed directly on the silicon diaphragm by an anisotropic etch.

As is known, anisotropic etching of silicon varies with the crystallographic orientation of the silicon substrate and is an etch whose rate depends upon the orientation of the crystal and is only effective for etching crystals of silicon of special orientation.

As indicated, the etch rate in anisotropic etching varies with the crystallographic orientation of the wafer or diaphragm 11 and decreases in the following order: (100)>(110)>(111). In order to provide the anisotropic etch, etchants such as pyrocatechol, (C.sub.6 H.sub.4 (OH).sub.2)P, ethylene diamine (NH.sub.2) CH.sub.2 NH.sub.2 (ED) and water are employed.

The use of such etchants to provide anisotropic etches are well known in the art and for example, see an article entitled "Anisotropic Etching of Silicon" by K. E. Bean published in the IEEE Transactions on Electron Devices, October, 1978, Volume ED-25, No. 1 on Page 1185.

The wafer of silicon used to form the diaphragm 11 may be single crystal silicon of (100) orientation. The rate of etch can be accurately controlled and the wafer of silicon may be etched prior to the diffusion of the sensors or piezoresistances 30 to 33.

As indicated, various etching solutions are well known which provide etch rates of 50 micrometers per hour. This low rate of etching assures that one can accurately control the dimensions of the convolutions to an extremely precise value.

Based on the above, we can see that the silicon diaphragm and sensor assembly as above described, due to the convolutions, is anisotropic to stress. The convolutions afford substantial resistance to transverse stretching and, therefore, allows larger forces to be applied with little nonlinear response. On the other hand, the convoluted diaphragm is radially very flexible and full bending response to applied load is allowed. The convoluted diaphragm is substantially more linear in its response than similarly fabricated planar diaphragms.

The diaphragm, due to the convolutions, does not exhibit any appreciable effects due to stretching to enable the sensors as positioned to provide a linear output determinative of applied force or pressure strictly due to the bending of the structure. In this manner the structure can be used to optimumly fabricate pressure or displcement transducers for low pressure operation (15 psi or less). If one compares structures of similar fabrication, then it is seen that a convoluted diaphragm as above described provides 2 to 5 more output for a given linearity than a flat or bossed diaphragm.