Instrument for measuring accelerations and process of making the same

• US 5,065,628 A
• Filed: 06/04/1990
• Issued: 11/19/1991
• Est. Priority Date: 12/03/1987
• Status: Expired due to Term

• Current Assignee: Fraunhofer Gesellschaft Zur Foerderung Der Angewandten Forsching E.V.
• Original Assignee: Fraunhofer Gesellschaft Zur Foerderung Der Angewandten Forsching E.V.
• Inventors: Benecke, Wolfgang
• Primary Examiner(s): Powell, William A.
• Application Number:
  US07/477,964
• Time in Patent Office: 533 Days
• Field of Search: 156/647, 156/648, 156/651, 156/652, 156/653, 156/656, 156/657, 156/659.1, 156/661.1, 156/662, 29/610.1, 29/620, 29/621.1, 29/25.35, 73/510, 73/514, 73/516 R, 73/517 R, 73/517 B
• US Class Current: 73/514.33
• CPC Class Codes: G01P 15/0802 Details
• CPC Class Codes: G01P 15/123 by piezo-resistive elements, e.g. semiconductor strain gauges
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/18 in two or more dimensions
• CPC Class Codes: G01P 2015/0828 the mass being of the paddle type being suspended at one of its longitudinal ends
• CPC Class Codes: G01P 2015/0845 using a plurality of spring-mass systems being arranged on one common planar substrate, the systems not being mechanically coupled and the sensitive direction of each system being different
• CPC Class Codes: G01P 2015/086 using a torsional suspension spring
• CPC Class Codes: Y10T 29/42 Piezoelectric device making

Abstract

Disclosed is an instrument for measuring accelerations, with the aid of which changes in motion in the three axes of space can be measured with selective sensitivity.

In order to detect multi-dimensional changes in motion, three micromechanical sensors, each sensitive to the acceleration in a selected direction, are integrated in a crystal. The sensors are composed of torsion bars having masses eccentrically attached thereto, which, in the event of changes in motion, exercise torques about the axes of the torsion bars. The torques are measured with the aid of integrated piezo-resistances. The planar integration of the acceleration meter permits integrating the evaluation circuit on the same semiconductor surface.

Acceleration meters of this kind are fabricated with the prescribed process in planar technology with the aid of epitaxy, lithography and anisotropic etching methods.

High precision and high miniaturization make this acceleration meter suited for application in overland and aviation navigation and for component positioning in robotry.

Claims

1. An instrument for measuring accelerations, comprising a reset element, a mechano-electric converter, and three micromechanical sensors, each consisting of an inert mass and sensitive to a motion in one direction to define multiple directions of sensitivity, integrated in a crystal surface layer, wherein the multiple directions of sensitivity of the sensors point in different directions and do not lie in one plane.

2. An instrument for measuring accelerations according to claim 1, wherein said three directions of sensitivity are approximately perpendicular to one another.

3. An instrument for measuring accelerations according to claim 2, wherein said reset element of each sensor is as torsion bar onto which the inert mass is placed, the center of gravity of which is non-coincident with the axis of rotation of the torsion bar.

4. An instrument for measuring accelerations according to claim 3, wherein said torsion bars are made of silicon.

5. An instrument for measuring accelerations according to claim 4, wherein the base material of said instrument is a silicon mono crystal.

6. An instrument for measuring accelerations according to claim 5, wherein said mechano-electric converters are designed as piezo-resistive resistances, which are integrated utilizing the piezo-resistive effect in silicon for measuring the mechanical tensions in the torsion bars.

7. An instrument for measuring accelerations according to claim 6, wherein said sensor elements are integrated together with an evaluation circuit on the same crystal surface.

8. An instrument according to claim 3, wherein said measuring of the mechanical tensions in the torsion bars occurs indirectly by a capacitive method.

9. A process for making an instrument for measuring accelerations having a reset element, a mechano-electric converter and three micromechanical sensors, each consisting of an inert mass and sensitive to a motion in one direction to define multiple directions of sensitivity, integrated in a crystal surface layer, wherein the multiple directions of sensitivity of the sensors point in different directions and do not lie in one plane, comprising the process steps of:

precipitating a semiconducting epitaxy layer having impurities of one kind onto a silicon wafer doped with impurities of a second kind to form a pn-transition,

precipitating a passivation layer of silicon nitride on the silicon wafer,

integrating piezo-resistances on a side of the wafer,

photolithographic structuring on the front and rear sides of the wafer,

etching the passivation layer,

metallizing said front side of said wafer,

Anisotropic electro-chemical etching of said rear side of said wafer in a KOH-H₂ O etching solution, with the etching process stopping automatically at the pn-transition,

passivating said rear side of said wafer,

removing said metallizing layer from said front side,

anisotropic etching said epitaxy layer on said front side of said wafer, and

removing said passivation layer on said rear side.

10. A process for making an instrument according to claim 9, wherein, in step A of said process silicon oxide is precipitated as said passivation layer.

11. A process for making an instrument according to claim 9, wherein said semiconducting epitaxy layer is replaced in said step A of said process by a succession of layers composed of high dropped p- and a low doped layer.

12. A process for making a micromechanical, integrated acceleration meter according to claim 9, wherein the structure of said sensors is built up on the substrate with the aid of X-ray depth lithography and galvanic casting.