Temperature coefficient compensated pressure transducer

A pressure transducer, which includes a silicon diaphragm with ion-implanted resistors in Wheatstone Bridge configuration and in insulating layer covering the diaphragm, is provided with temperature compensation for differences in thermal expansion coefficients of the layers by depositing a layer of aluminum onto the central portion of the diaphragm.

BACKGROUND OF THE INVENTION 
This invention relates generally to pressure transducers of the strain gage 
type and more particularly to pressure transducers having zero temperature 
coefficient (zero TC) compensation. A strain-gage type pressure transducer 
converts a physical displacement into an electrical signal. Strain-gage 
type pressure transducers have been manufactured using integrated circuit 
technology for numerous applications. A typical miniature pressure 
transducer includes a thin silicon diaphragm into which resistors are 
diffused or implanted and then connected to form a Wheatstone Bridge 
circuit. While these pressure transducers offer many advantages, such as 
their use as disposable blood pressure transducers, they do not give 
consistent results in environments with varying temperatures. Inconsistent 
results due to temperature variation is caused by the different thermal 
expansion coefficients of the two or more layers of the composite forming 
the diaphragm, slight variations in the temperature coefficients of 
resistance (TCR) of individual bridge elements, thermal gradients between 
elements, or any combination of these causes. 
Prior art schemes for providing zero TC compensation include on-chip and 
off-chip resistive compensation. Since the silicon diaphragm transducer is 
miniaturized, many manufacturers use circuitry on additional chip(s) which 
is electrically connected to the diaphragm chip to provide zero TC 
compensation. While this scheme can provide acceptable zero TC 
compensation in many cases, the use of off-chip circuitry is not always 
convenient for applications requiring greater miniaturization and may not 
adequately compensate a unit in which a temperature differential exists 
between the diaphragm and compensation chips. 
A prior art method of providing on-chip zero TC compensation is shown in 
FIG. 2. In this pressure transducer, thin-line aluminum resistors are 
deposited on the edge of the diaphragm chip, outside the diaphragm and 
electrically connected in the bridge circuit. Aluminum has a temperature 
coefficient of resistance (TCR) of about 3900 ppm/degree C., while the 
diffused bridge resistors would typically have a design value between 1000 
and 2000 ppm/degree C. The effective TCR of the Wheatstone Bridge arms 
that include the aluminum resistors would then be greater and as a result 
a zero TC would be induced to compensate the transducers. While the 
structure of FIG. 2 is useful for providing zero TC, it is expensive to 
manufacture and requires tight processing control. For example, the amount 
of zero TC compensation provided by the aluminum resistors of FIG. 2 
depends on the following parameters: aluminum resistor TCR and ohmic value 
(i.e. line width, thickness, and resistivity); TCR of the diffused bridge 
resistor; and bridge impedance. Of these factors, variations in line width 
or thickness provide the major shifts in compensation variables. In 
addition, since the compensation resistors are in series with the bridge 
resistors, bridge balance is coupled with zero TC compensation and thus 
will also shift with variations in the value of the aluminum resistors. 
Therefore, it is an object of the present invention to provide a pressure 
transducer having zero TC compensation which is independent of bridge 
balance. 
It is another object of the present invention to provide a pressure 
transducer having zero TC compensation which is simple to manufacture and 
requires a minimum of processing control. 
It is yet another object of the present invention to provide a pressure 
transducer with improved on-chip zero TC compensation. 
Additional objects, advantages, and novel features of the invention will be 
set forth in part in the description which follows, and in part will 
become apparent to those skilled in the art upon examination of the 
following or may be learned by practice of the invention. 
SUMMARY OF THE INVENTION 
To acheive the foregoing and other objects, a pressure transducer may 
comprise a diaphragm, the diaphragm being formed of a material having a 
first thermal expansion coefficient; measuring means for measuring the 
displacement of the diaphragm in response to an applied pressure including 
four piezoresistive resistors disposed on the diaphragm and connected as 
the elements of a Wheatstone Bridge circuit, the resistors being 
positioned near the edges of the diaphragm; an insulation layer covering 
the diaphragm and insulating the diaphragm from the circuit connections of 
the measuring means, the insulation layer being formed of a second 
material having a second thermal expansion coefficient; and a compensation 
layer on the insulation layer, the compensation layer being formed of a 
third material having a third thermal expansion coefficient, wherein the 
size of the compensation layer is chosen such that the pressure transducer 
has a particular value of temperature compensation. 
By judicious choice of the compensation layer material, zero TC is 
accomplished without the use of additional resistors affecting overall 
bridge balance.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 1 and 1A, a typical silicon diaphragm pressure 
transducer is shown. A large substrate of silicon 41 is etched to form the 
thin diaphragm region denoted by numeral 10. Silicon dioxide layer 42 
covers the silicon to provide insulation between the silicon and the 
circuit metallization shown by numeral 20 that connects resistors 11, 12, 
13, 14. In this transducer, four resistors 11 through 14 are formed by 
first etching the silicon dioxide layer at the locations shown, followed 
by diffusion or ion implantation. Resistors 11 through 14 are shown 
connected in a Wheatstone Bridge circuit. The input and output of the 
bridge circuit is shown by metallization areas 4 and 1, and 2 and 3 
respectively. Resistors 11 through 14 are shown implanted near the edges 
of the diaphragm. 
Referring to FIG. 2, the transducers of FIG. 1 has been modified by the 
addition of zero TC resistors 21 and 22. Resistors 21 and 22 are typically 
5-10 ohm aluminum resistors (TCR=3900 ppm/deg C.). Note that resistors 21 
and 22 are part of the bridge circuit and thus affect the bridge balance. 
Aluminum is generally chosen since the metallization is also aluminum and 
thus saves several processing steps. Note also that resistors 21 and 22 
are deposited outside of diaphragm 10. 
Referring now to FIG. 3 is shown a preferred embodiment of the present 
invention. Here, the transducer of FIG. 1 has been modified by the 
addition of compensation layer 31. Layer 31 is shown centrally located on 
diaphragm 10, away from resistors 11 through 14. Layer 31 is not part of 
the bridge circuit and provides independent zero TC compensation. 
A typical miniature diaphragm sensor has an etched silicon diaphragm of the 
order of 15 microns thick and a thermally grown silicon dioxide insulation 
layer of the order of 0.65 microns thick. Since the thermal expansion 
coefficient of silicon is about 2.6 ppm/deg. C. and the thermal expansion 
coefficient of silicon dioxide is about 0.5 ppm/deg. C., the diaphragm 
will become slightly concave as the temperature is increased and a 
non-zero TC will result. The addition of layer 31, formed of aluminum (25 
ppm/deg C. thermal expansion coefficient) on the top surface (i.e. on the 
silicon dioxide) results in a zero TC of opposite sign. The magnitude of 
the resultant zero TC of the composite structure having three layers is 
primarily a function of the relative thicknesses, areas, and values of the 
linear expansion coefficients of the aluminum, silicon, and silicon 
dioxide that form the composite diaphragm. 
The size and thickness of the aluminum layer is determined by using the 
equations for bimetallic plate deformation due to uniform temperature 
(see, for example Roark and Young, Formulas for Stress and Strain, 5th 
Edition, McGraw Hill 1975 pp. 324-413). FIG. 4A is a graph of zero TC 
versus thickness of the metallization layer for both theoretical and 
experimental data. FIG. 4B shows a graph of zero TC versus length of a 
centrally placed aluminum square for both theoretical values and the 
experimental value determined from the following example. 
EXAMPLE 
An etched silicon diaphragm transducer having dimensions of 1000 by 1000 
microns and 15 microns thickness with a silicon dioxide layer with 0.65 
microns thickness was provided with a centrally located square of aluminum 
having dimensions 250 by 250 microns. Tests of several transducers made 
with this configuration indicated an average zero TC of -0.4 mm Hg/deg. C. 
as compared to a value of +0.4 mm Hg/deg C. for uncompensated transducers. 
The transducers were designed to have a negative zero TC since packaging 
of completed transducers introduces a positive zero TC in the range of 0.2 
to 0.5 mm Hg/deg. C.