Micromechanical component and equalization method

Details Micromechanical component and equalization method Micromechanical component and equalization method

2003-01-09

2004-08-31

Lefkowitz, Edward (Department: 2855)

Measuring and testing

Fluid pressure gauge

Electrical

C073S708000, C073S719000

Reexamination Certificate

active

06782756

ABSTRACT:

FIELD OF THE INVENTION
The present invention concerns a micromechanical component, in particular, a pressure sensor and a corresponding equalization method.
Although applicable to any micromechanical components, the present invention will be explained with reference to example embodiments of a micromechanical pressure sensor.
BACKGROUND INFORMATION
German Patent Application 197 01 055 A1 describes a semiconductor pressure sensor for measuring an externally applied pressure.
FIG. 7
shows a plan view of this conventional pressure sensor. A sectioned drawing of this pressure sensor along section line A-A′ is shown in FIG.
8
. The pressure sensor is manufactured on a substrate 2 made of silicon which has a [100] orientation. Located on the underside of a membrane
10
is a depression in the shape of a truncated pyramid. Its configuration is such that at the location of the truncated pyramid, only a small residual thickness of silicon material (membrane
10
) remains. The delimiting lines of the truncated pyramid are drawn with dashes in FIG.
7
and run parallel to the [110] and [1I 0] directions, whose orientation is indicated in
FIG. 7
with arrows
40
and
41
. The portion of silicon substrate
2
that is not thinned is also called support
11
.
A measuring resistance
4
that extends in the [110] direction is located on membrane
10
close to the membrane edge. Two electrodes
6
, which are made of vacuum-deposited aluminum in the exemplified embodiment selected here, are located on support
11
; one electrode is configured in front of and one behind measuring resistance
4
, as a respective elongated metallization extending perpendicular to measuring resistance
4
. A compensating resistance
5
is located at the left electrode, extending perpendicular to the direction of measuring resistance
4
in the [1I0] direction. Compensating resistance
5
is connected at one end to measuring resistance
4
via a connecting conductor
7
, and at its other end to electrode
6
via a second connecting conductor
7
. Double arrows
30
and
31
indicate mechanical stresses that are relevant in terms of explaining the manner of operation of this conventional pressure sensor with hysteresis balancing.
FIG. 8
shows a cross section through the pressure sensor of FIG.
7
. Substrate
2
has a depression that is trapezoidal in cross section and is delimited by support
11
and membrane
10
. Measuring resistance
4
is located in the surface of membrane
10
. Measuring resistance
4
is implemented by introducing a local doping zone into the silicon material.
The manner of operation of the conventional pressure sensor with hysteresis balancing is as follows. The pressure sensor is mechanically deformed by a pressure acting externally on the pressure sensor. The thickness of support
11
is typically several hundreds of micrometers, whereas the thickness of membrane
10
is typically several micrometers. Because of the resulting difference in stiffness, the mechanical deformation in support
11
is negligible compared to the mechanical deformation in the membrane. The mechanical stress or deformation resulting from the externally applied pressure is illustrated by an arrow
31
, whose length is an indication of the deformation. The mechanical deformation is depicted by way of example at a point, namely at the location of measuring resistance
4
.
Also present in the pressure sensor is a first deformation
30
whose cause is a mechanical interference stress which in the present case is based on the differing coefficients of thermal expansion of the aluminum of the electrodes and the silicon of substrate
2
. A first mechanical stress or deformation
30
of this kind can be associated with each point in the pressure sensor, but only two points in the pressure sensor will be considered. These two points are assumed to be the location of measuring resistance
4
and the location of compensating resistance
5
. In the example embodiment selected here, it is assumed that first deformation
30
is identical everywhere.
Measuring resistance
4
and compensating resistance
5
are dimensioned so that their piezoresistive coefficients are of identical magnitude. The absolute electrical resistance values are also assumed to be identical under identical external conditions. The changes in electrical resistance in measuring resistance
4
and in compensating resistance
5
as a result of first deformation
30
are thus of identical magnitude. Because one resistance is positioned in the direction of the deformation and one perpendicular to the deformation, the two changes in resistance have different signs. The total change in resistance resulting from first deformation
30
in the equivalent resistance for the series circuit made up of measuring resistance
4
and compensating resistance
5
is therefore zero. All that remains, therefore, is the change in the measuring resistance as a result of second deformation
31
, to which compensating resistance
5
(located on support
11
) is not exposed.
The conventional approach to compensating for hysteresis described above has proven to be disadvantageous in that it exhibits only low efficiency and results in a loss of sensitivity.
In the conventional integrated micromechanical pressure sensor, without compensation a hysteresis of the output signal over temperature therefore generally occurs at the converter element. The hysteresis is generally brought about by a plastic deformation of the aluminum conductor paths of the evaluation circuit, which are located in the surrounding region on support
11
. If the sensor element is heated to above &Dgr;T=60 degrees C., the differing coefficients of thermal expansion of aluminum and the silicon substrate cause mechanical stresses of 100 MPa to occur in the aluminum. Above these stress levels, the aluminum begins to flow. Upon cooling, the same happens in the opposite direction.
FIG. 9
illustrates this hysteresis of the mechanical stress as a function of temperature, and
FIG. 10
shows the hysteresis of a further conventional pressure sensor without compensation as a function of membrane edge length mk, for various circuit inner radii SIR. In contrast to the example above, it is assumed here that the circuit conductor paths of the evaluation circuit completely enclose the membrane. The chip size is 4 mm. The global effect predominates in the negative hysteresis region, and the edge effect in the positive hysteresis region.
The hysteretic behavior of the overall aluminum wiring of the evaluation circuit has an integral remote effect on the piezoresistances of the converter element, specifically by way of a “bimetallic deformation” (aluminum layer on silicon) of the overall sensor element (global effect), and by way of a local effect when the distance from the circuit edge to the piezoresistance is less than 100 micrometers (edge effect).
Depending on the geometric layout, either the global effect or the edge effect predominates. The influencing variables are:
a) Distance from membrane edge to edge of evaluation circuit;
b) Membrane size;
c) Chip geometry;
d) Glass thickness, cut width of glass saw;
e) Glass size;
f) Solder thickness, adhesive thickness, and mounting substrate.
In sensors soldered on the back side, the hysteresis of the solder partially counteracts the influences caused by the top side of the chip (i.e., aluminum wiring of the circuit). The hysteresis of the solder should therefore be taken into account for absolute hysteresis calculation.
Four measuring resistances are conventionally provided for the micromechanical pressure sensor, and are located (depending on type) in the region in which the edge effect or global effect is dominant. They are connected into a Wheatstone bridge whose output signal consequently also exhibits a temperature hysteresis. This temperature hysteresis overlies the actual sensor signal.
SUMMARY
The micromechanical component according to the present invention exhibits a high maximum compensation effect and a s

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