Pressure Transducer With Reduced Offset Signal

This invention relates to the field of silicon pressure transducers. More specifically, the invention is a method and a device for overcoming packaging and temperature induced offsets in silicon pressure transducers. To make a useful silicon pressure transducer, a silicon die, comprised of a support rim and a diaphragm, was rigidly or semi-rigidly attached to a support housing. This support housing was usually a tube which was used to guide a fluid into contact with the diaphragm. A problem was created, however, in that the structure or tube transmitted stress to the die not related to the pressure applied at the diaphragm. A variable false pressure signal was thus created, the variation occurring with the change in temperature.

A related problem was a transient signal generated by the transducer as the transducer and supporting structure heated up. The supporting structure deformed upon heating. Once again, because the support structure and the transducer were bonded together, stress was transferred to the transducer producing a temperature varying component to the offset signal. This temperature varying component, only stabilized when the temperature of the supporting structure reached a stable level.

Thus, it is an object of the present invention to create a pressure transducer which is more immune to changes in temperature. It is a further object of the present invention to reduce the amount of offset signal generated due to the supporting structure.

The present invention is a method and device directed toward a pressure transducer comprised of a silicon die and a tube wherein the shape of the tube is matched to the shape of the silicon die. The silicon die is comprised of a diaphragm and a support rim surrounding the diaphragm. The tube has an inside shape which is the same as the diaphragm, while having an outside shape which is the same as the shape of the silicon die. By forming the pressure transducer in such a way, the stresses in the die are reduced thus reducing offset signal.

FIG. 1 is a top view of the silicon die of the present pressure transducer

FIG. 1A is a side view of the silicon die depicted in FIG. 1 taken along line 1A--1A.

FIG. 2A is a bottom view of the die of FIG. 1.

FIG. 2B is a bottom view of a tube.

FIG. 3 is a perspective view of a silicon die and tube joined to form a pressure transducer of the present invention.

FIGS. 4A, 4B and 4C show examples of other possible configurations for the tube.

FIG. 5A shows a pressure transducer which can be used to measure differential pressure.

FIG. 5B shows a pressure transducer which can be used to measure absolute pressure.

Referring now to FIG. 1, thereshown is a silicon die 10 as used in the present invention. The silicon die 10 may have any external shape, but is hereshown as having a square shape. Further details of the silicon die 10 may be seen by referring to FIG. 1A which is a side view of the silicon die taken along line 1A--1A.

In FIG. 1A, the silicon die 10 is shown as comprising a substrate 20 and a first layer 50, formed on a first major surface 25 of substrate 20. A portion of substrate 20 has been etched away to define a support rim 40 and a diaphragm 30 having a second major surface 35. The etching of the diaphragm may be done by any of many well known methods. Lastly, piezoresistor 55 is formed in the diaphragm by a, method, for example, ion implantation or diffusion.

Referring now to FIGS. 2A and 2B, thereshown is a silicon die 10 and a tube 60 respectively. Tube 60 is comprised of wall portion 70 and aperture 80. In order to make the pressure transducer of the present invention, aperture 80 must have the same shape, here a rectangle, and cross sectional area as diaphragm 30. Or in other words sides L.sub.5, L.sub.6, L.sub.7, L.sub.8, L.sub.9 and L.sub.10 have at least the following relationship: L.sub.5 .gtoreq.L.sub.7 .gtoreq.L.sub.9, and L.sub.6 .gtoreq.L.sub.8 .gtoreq.L.sub.10, and L.sub.5 XL.sub.6 .gtoreq.L.sub.7 XL.sub.8 .gtoreq.L.sub.9 XL.sub.10. Further, the shape and area of the walls 70 of the tube are substantially the same as the shape and area of the support rim 40 or L.sub.1 through L.sub.10 bear at least the following relation: L.sub.1 .congruent.L.sub.3, L.sub.2 .congruent.L.sub.4.

By joining together a diaphragm and a tube which meet the above description, the pressure transducer 5 of FIG. 3 is thus created. The pressure transducer 5 is comprised of the silicon die 10 and tube 60 having wall portion 70 and aperture 80. By matching the size and shape of the aperture 80 to the diaphragm and the wall size and shape to that of the supporting rim, stress in the x and y directions are equalized. The resistance P=Po+.DELTA.P where P.sub.o =a nominal resistance, .DELTA.P is the change to resistance due to stress. Because of the alignment of the resistors in the silicon die this equation can,be rewritten .DELTA.P.congruent.Po(.pi..sub.11 .tau.x+.pi..sub.12 .tau.y) where .pi..sub.11 .tau..sub.x and .pi..sub.12 .tau..sub.y are the changes in resistivity in the x and y directions due to stress and .tau.x and .tau.y are the stresses, in the x and y direction respectively. .pi..sub.11 and .pi..sub.12 are nearly equal and opposite resulting in .DELTA.P.congruent.Po .pi..sub.11 (.tau.x-.tau.y). Thus the change in resistivity due to stress can be minimized where the stress in the x direction is equal to the stress in the y direction.

Where the support structure (tube) closely matches the silicon die in size and shape, the stresses on the die are equalized. Thus, by closely matching the tube to the silicon die, stress related offsets are reduced. Also as a result of the silicon die-tube matching, temperature transient offsets are also reduced.

Turning now to FIGS. 4A, 4B and 4C, thereshown are other possible tube inside and outside shapes. FIG. 4A depicts a tube 60' having a rectangular external shape and a circular aperture. FIG. 4A shows a tube 60" having a circular external shape and a circular aperture. FIG. 4C shows a tube 60"' having a circular external shape and a square aperture. All three of these tubes are intended for use with a die having the same external shape as the external shape of the tube, and a diaphragm having the same shape as the aperture in the tube.

Referring now to FIG. 5A, thereshown is a pressure transducer 5, comprised of a silicon die and two tubes 60. The tube 60 should be identical in size and shape. This pressure transducer 5' can be used to measure the pressure difference between two isolated regions. The pressure transducer 5" shown in FIG. 5B includes the silicon die 10, a tube 60 and a cap 90. Cap 90 is sized and shaped to cover the diaphragm on the silicon die 5 side away from the tube. By evacuating the volume covered by cap 90, the pressure transducer can be used to measure absolute pressure.

The foregoing has been a description of the applicant's novel and nonobvious pressure transducer. The applicant does not intend to be limited by the foregoing description, but claims the extent of his property in the claims appended hereto.