Method of Fabricating Transducer Structure Employing Vertically Walled Diaphragms With Quasi Rectangular Active Areas

This invention relates to transducer structures and more particularly to a transducer structure employing a relatively rectangular diaphragm having a relatively rectangular active area.

The prior art is replete with a number of transducer configurations. In general, such transducers consist of a bridge arrangement of piezoresistors mounted or integrally formed on a diaphragm. The diaphragm in many structures is fabricated from a thin wafer of silicon and possesses an active area which is conventionally manifested by an aperture in the diaphragm over which the piezoresistive elements are mounted. In this manner, a force applied to the diaphragm causes a deflection of the same; which deflection is attendant with a change in stress on the diaphragm and a consequent change in resistance of the piezoresistive elements which change is indicative of the magnitude of the pressure or force applied to the diaphragm.

Most diaphragms depicted in the prior art are circular in configuration. For examples of transducers employing circular diaphragms, reference is made to 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. Other patents such as U.S. Pat. No. 3,748,571, U.S. Pat. No. 3,753,196, U.S. Pat. No. 3,800,264, U.S. Pat. No. 3,739,315 and various other patents too numerous to mention also depict the use of circular diaphragms in conjunction with transducer structures containing various arrangements of piezoresistive devices.

The prior art was also cognizant of the use of other geometric shapes for diaphragms incorporated in transducer structures. As such, certain prior art patents depict rectangular diaphragms employing various grooves and other structures to obtain maximum sensitivity during the force responsive mode. A particular type of diaphragm is depicted in U. S. Pat. No. 4,236,137 entitled Semiconductor Transducers Employing Flexure Frames issued on Nov. 25, 1980 to A. D. Kurtz, et al. and assigned to the Assignee herein. In such a diaphragm structure, a groove frames a central area of the diaphragm with the groove operating as a stress concentrating area to enable sensors mounted and positioned about the groove to provide relatively large output forces upon application of a stress to the diaphragm.

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 of the diaphragm, is 1:1. In such diaphragms, the radial stress is of substantial magnitude and of opposite sign at the center of the diaphragm and at the edge of the diaphragm. In a rectangle, the length to width ratio is greater than 1:1 and for length to width ratios which are greater than 3:1, the ratio of longitudinal to transverse stress approaches theoretical minimum. For the purposes of this invention the longitudinal stress is defined as the stress parallel to the short axis of the diaphragm. In order to utilize a rectangular diaphragm in the most optimum manner, one would desire to take advantage of the maximum longitudinal piezoresistive effect and further, one would desire to substantially reduce or eliminate all transverse stress effects. This is because, for instance in P type silicon the maximum longitudinal coefficient which is in the <111> crystallographic direction is accompanied by a transverse piezoresistive coefficient of 1/2 the mangitude of the longitudinal piezoresistive coefficient and of opposite sign. Thus if such a gage is placed in the center of a circular diaphragm or a square diaphragm a substantial loss in sensitivity occurs. For a rectangular diaphragm however, maximum sensitivity is maintained since there is little transverse stress.

As one can ascertain from the above noted patent literature, the active area of a diaphragm is normally formed by a suitable semiconductor processing technique such as etching an aperture in the diaphragm to define an active area. According to prior art teachings, one incorporates and uses isotropic etching. An isotropic etch is an etchant which etches silicon in all the crystallographic directions at the same rate. The etching materials consist of a mixture of certain acids such as hydroflouric, nitirc and acetic acid (HF, HNO.sub.3, CH.sub.3 OOH).

As one can ascertain, by the use of isotropic etching, one forms an active area which possesses sloping sidewalls. This particular configuration can be shown in U.S. Pat. No. 4,204,185 entitled Integral Transducer Assemblies Employing Thin Homogenous Diaphragms issued on May 20, 1980 to A. D. Kurtz, et al. as well as others of the above cited patents. This shape of the sidewalls of the active area is not ideal, one does not provide the most effective edge clamping and furthermore, one does not utilize the material to its optimum degree. The sloping sidewalls serve to reduce the effective active area of the diaphragm and hence, result in material loss, while reducingg the effectiveness of the edge clamping and resulting in insufficient area for bonding of the rim or disphragm support.

It is therefore an object of the present invention to provide an approximately rectangular diaphragm having a rectangular aperture with essentially vertical sidewalls defining an active area. It is a further object to provide such a diaphragm having a longitudinal stress, while exhibiting a transverse stress.

A diaphragm for a pressure transducer comprising a rectangular planar semiconductor member having a predetermined crystallographic orientation, having located on a major surface thereof a relatively rectangular aperture defining an active diaphragm area with said rectangular aperture characterized in having steep sloped vertical walls.

FIG. 1 is a perspective top plan view of a rectangular diaphragm according to this invention;

FIG. 2A is a perspective bottom plan view; and

FIG. 2B is a side elevational view depicting a method for fabricating the diaphragm structure.

In a semiconductor diaphragm with P type silicon sensors, the maximum longitudinal piezoresistive coefficient is defined as .pi..sub.L and is +2/3.pi..sub.44. Such a piezoresistive element will have a transverse piezoresistive coefficient of -.DELTA..pi..sub.44 maximum sensitivity in a diaphragm structure, one desirably should eliminate the transverse stress within the active area.

Referring to FIG. 1, there is shown a rectangular semiconductor wafer 10. The wafer 10 is formed from single crystal silicon having a (110) crystallographic plane. High quality silicon wafers of (110) orientation are readily available in a wide range of thicknesses, surface finishes and conductivity. In such wafers, the orientation is accurately aligned in the (110) orientation within + or -1% of the crystal axis. The wafer 10 has a length to width ratio of greater than 2:1. In this manner, one is assured that the transverse stress approaches its theoretical minimum and the ratio of longitudinal stress to transverse stress is approximately equal to one over Poisson's ratio. This ratio for silicon is approximately 0.3. A recess or aperture 11 (dashed) of a rectangular configuration is shown formed in the surface of the wafer 10. The recess is produced by an anisotropic etching technique to a desired depth. Anisotropic etching of single crystal silicon varies with the crystallographic orientation of the substrate and essentially is an etch whose rate depends upon the orientation of the crystal and is only effective for etching crystals of special orientation.

As indicated, the etch rate in an anisotropic etching varies with the crystallographic orientation of the wafer and decreases in the following order: (100)>(110)>(111). In order to achieve 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).sub.2 NH.sub.2 (ED) and water or others known in the art are used.

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, Oct. 1978, Vol. ED-25, No. 1 on page 1185. A piezoresistive array 15 consisting of four sensors is deposited on the top surface of the wafer 10. The deposition of the array is wellknown as well as the deposition of the terminals 16. The above noted patents shows typical examples.

Referring to FIGS. 2A and 2B, there is shown a wafer 20 of a single crystal silicon of (110) orientation. A thin layer 21 of silicon dioxide or silicon nitride is grown on the top surface of wafer 20. The growth of silicon dioxide or silicon nitride is well known in the art and described in many references. After growth of the layer, one utilizes a rectangular mask definitive of the aperture 22, which mask is impressed upon the layer 21 by a photolithographic technique. The active area is then etched using the above described etchant solution.

Since the aperture is approximately rectangular with edges aligned to the <111> directions in configuration, it exhibits crystal planes in the vertical and horizontal directions as shown in FIG. 2A by arrows 25 at <111> planes. In employing a crystal orientation of (110), the etching stops and does not penetrate at the <111> planes. In this manner, one provides, as shown in FIG. 2A, an aperture 22 having relatively vertical sidewalls.

Due to the crystalline orientation as shown in FIG. 2C, the aperture 22 has extremely steep sidewalls, but essentially possesses the geometrical configuration of a parallelogram. The angle A is about 70.degree. and hence is an acute angle, while the angle B is approximately 110.degree. and hence is an obtuse angle. The depth of penetration of the etch is, of course, a function of the etch rate and strength of the etchant material. In this manner, a uniform and controlled etching rate can be obtained in utilizing etchants of various compositions. A particularly effective composition employs 4 mol percentage of pyrocatechol, 46.4 mol percentage of ethylene diamine and 49.4 mol percentage of water. The solution is employed at its boiling point (118.degree. C.) in a non-oxidizing atmosphere. In using such a solution, the etch rate of the silicon is approximately fifty micrometers per hour. This relatively low etch rate is a distinct advantage in that one can control the depth of the aperture 22 to an extremely precise value.

As indicated, the formation of the above described diaphragm structure results in a rectangular assembly which exhibits a maximum longitudinal coefficient and hence, is extremely responsive to longitudinal stresses. The sensors can be deposited in a bridge array on the top or bottom surface of the diaphragm and within the active area defined by the boundaries of the rectangular aperture 22. Piezoresistive sensors can be implemented in the diaphragm by many well known techniques such as diffusion, epitaxial growth, bonding by means of glass or epoxy bonds and so on. Techniques for forming piezoresistive strain elements on silicon substrates are well known in the art and many examples of suitable methods clearly explained in the above noted patents.

By utilizing the diaphragm depicted in FIGS. 1 and 2 one virtually eliminates transverse stresses. Furthermore, the technique allows one to utilize relatively thick wafer members which can be etched very deeply to form deep apertures and hence, thin diaphragm assemblies. These structures allow relatively rugged transducer assemblies to be provided. Due to the vertical nature of the sidewalls of the aperture forming the active area, one is assured of an optimumly responsive diaphragm and the elimination of the sloping sidewalls further enhance the longitudinal responsiveness of the structure.