Micro-machined accelerometer with composite material springs

• US 5,134,881 A
• Filed: 06/01/1990
• Issued: 08/04/1992
• Est. Priority Date: 06/22/1986
• Status: Expired due to Fees

• Current Assignee: Tritton Technologies
• Original Assignee: Tritton Technologies
• Inventors: Ip, Matthew W., Henrion, W. S.
• Primary Examiner(s): CHAPMAN JR, JOHN E
• Application Number:
  US07/532,207
• Time in Patent Office: 795 Days
• Field of Search: 73/514, 73/517 R, 73/517 B, 73/497, 361/280, 357/26
• US Class Current: 73/514.35
• CPC Class Codes: F16F 15/073 using only leaf springs
• CPC Class Codes: G01P 15/0802 Details
• CPC Class Codes: G01P 15/131 with electrostatic counterbalancing means
• CPC Class Codes: G01P 2015/084 the mass being suspended at more than one of its sides, e.g. membrane-type suspension, so as to permit multi-axis movement of the mass

Abstract

A transducer fabricated by micro-machining includes E-shaped leaf springs suspending a mass from a support. The transducer is formed by chemical etching through openings of opposite faces of a silicon wafer on which etch stop layer patterns are diffused. Sense and force conductive patterns are diffused onto opposite faces of the suspended mass. The legs of the E-shaped leaf springs are formed of a composite of a polysilicon base layer, a silicon dioxide insulating layer and a gold conducting layer. The thickness of such layers are selected to minimize bowing effects of the spring legs due to materials of different coefficients of thermal expansion. The spring-mass-support structure is sandwiched between opposite plates having corresponding force and conductive patterns which face such patterns on the suspended mass.

Claims

1. An improved spring structure in a micro-machined transducer having a support frame and a sensing mass, where said sensing mass is at least partially supported from said support frame via at least one E-shaped leaf spring structure, said spring structure defined by a base and three legs including two outer legs and an inner leg, each of said three legs being connected to said base at one end, said two legs connected to said frame at their other ends, said inner leg connected to said mass at its other end, and wherein said transducer is fabricated by etching of a semi-conductor wafer which has been doped with a dopant of one type, the improvement comprising,

said base characterized by a coefficient of stiffnes substantially greater than that of said legs, and

said base of said spring structure being fabricated of semi-conductor material and having an impurity diffused therein of an opposite type from that of which said semi-conductor wafer was doped and wherein said legs of said spring structure are each a composite structure of a base layer of semi-conducting material, a conducting layer and an insulating layer placed between said conducting layer and said semi-conducting material, said legs each characterized by a thickness which is small compared to the thickness of said base.

2. The improved spring structure of claim 1 wherein said improvement further includes

said legs each having substantially the same length.

3. The improved spring structure of claim 1 wherein said semi-conducting material in polysilicon, said conducting layer is gold and said insulating material is silicon dioxide.

4. The improved spring structure of claim 1 wherein said semi-conductor material is silicon and said impurity diffused therein is of an N type impurity.

5. The improved spring structure of claim 1 wherein said semi-conductor material is silicon and said impurity diffused therein is of a P type impurity.

6. An improved spring structure in a micro-machined transducer having a support frame and a sensing mass, where said sensing mass is at least partially supported from said support frame via at least one E-shaped leaf spring structure, said spring structure defined by a base and three legs including two outer legs and an inner leg, each of said three legs being connected to said base at one end, said two outer legs connected to said frame at their other ends, said inner leg connected to said mass at its other end, the improvement comprising,

said legs including a base layer of semi-conductor material, a conducting layer and an insulating layer placed between said conducting layer and said semi-conductor material, and

wherein said semi-conductor material is polysilicon, said conducting layer is gold, and said insulating material is silicon dioxide.

7. The improved spring structure of claim 6 further comprising a primer layer of chromium placed between said gold conducting layer and said silicon dioxide insulating layer.

8. The improved spring structure of claim 7 wherein the thickness of each of said material layers is selected to produce an approximately flat composite structure at room temperature.

9. The improved spring structure of claim 8 wherein

the thickness of said polysilicon layer is approximately 7320 Å

,

the thickness of said silicon dioxide layer is approximately 2000 Å

,

the thickness of said chromium layer is approximately 250 Å

, and

the thickness of said gold layer is approximately 2250 Å

.

10. The improved structure of claim 9 wherein said layers of silicon dioxide, chromium and gold are narrower in width than is said polysilicon layer.

11. A spring-mass transducer comprising

a support frame structure,

a sensing mass structure, and

an E-shaped leaf spring structure, said spring structure defined by a base and three legs including two outer legs and an inner leg, each of said three legs being connected to said frame structure at their other ends, said inner leg connected to said mass structure at its other end,

said support frame structure and said sensing mass structure including top and bottom layers of etch stop material which sandwich single semi-conductor crystal wafer which has been undercut from said top and bottom layers and severed by semi-conductor etchant,

said base of each said E-shaped spring structures having a coefficient of stiffness, substantially greater than that of said legs, and

said legs of each of said E-shaped spring structure including a composite of a base layer of polysilicon, a layer of conducting material and a layer of insulating material between said base layer and said conducting material.

12. The transducer of claim 11 wherein said etch stop layers are applied to major (100) faces of a single semi-conductor crystal wafer, and wherein said E-shaped leaf spring structures are oriented substantially at forty-five degree angles with respect to intersection lines of (111) planes of said wafer.

13. The transducer of claim 11 wherein said E-shaped leaf spring structure includes diagonally opposed E-shaped undercut spring structures on opposite sides of said single semi-conductor crystal material.

14. The transducer of claim 13 wherein two pair diagonally opposed E-shaped undercut spring structures are formed on opposite sides of said single semi-conductor crystal material for supporting said sensing mass structure from said support frame structure, each of said two pair of spring structures being orthogonal to the other.

15. The transducer of claim 14 wherein said support frame structure and said sensing mass structure are separated by open areas where no etch stop layer has been diffused into said wafer including etch-time and undercut control (ETUC) areas between each of said E-shaped undercut spring structures, each of said ETUC areas having an outer edge coinciding with an intersection line of a (111) plane of the wafer and a major face of said wafer, each of said ETUC areas having an inner edge coinciding with an intersection line of a (111) plane of the wafer and major face of said wafer which defines an outer edge of said sensing mass structure,

each of said outer edges of said ETUC areas coinciding with (111) planes which extend outwardly beyond said E-shpaed undercut spring structure on either side of said opening, and

wherein said E-shaped spring structures are undercut to said outer edge coinciding (111) planes by etchant introduced into said ETUC areas.

17. The transducer of claim 15,

wherein adjacent outer edges coinciding with a (111) plane of said wafer and a major face of said wafer which define outer edges of said sensing mass structure are connected by a cat head and ears shaped areas of etch stop material, whereby,

said inner leg of an E-shaped leaf spring structure is connected to the head of said cat head and ears shaped area, and where each of said ears of said sensing mass structures extend into a respective space between an outer leg and said inner leg of said E-shaped leaf spring structure.

18. The transducer of claim 11 futher comprising,

force conductive areas on said opposite sides of said sensing mass structure;

sense conductive areas on said opposite sides of said sensing mass structure; and

means for conductively isolating said force areas from said sense areas.

19. The transducer of claim 18 further comprising,

a force conductor from a location exterior of said spring structures and said sensing mass structures to both of said force conductive areas on opposite sides of said sensing mass structure via respective top and bottom layers of conducting material of one set of oppositely faced E-springs, and

a sense conductor from a location exterior of said spring structures and said sensing mass structures to both of said sense conductive areas on opposite sides of said sensing mass structures via respective top and bottom layers of conducting material of a different set of oppositely faced E-springs.

20. The transducer of claim 19 further comprising,

first and second plates of semi-conductive material, each of said plates having a peripheral outside area disposed about force and sense conductive areas which are electrically isolated from each other and which are mirror images of said force and sense conductive areas disposed on said opposite sides of said sensing mass structure,

first and second force conductors respectively connected to said force conductive areas of said first and second plates,

first and second sense conductors respectively conneted to said sense conductive areas of said first and second plates,

said first and second plates placed so as to form a sandwich structure with said sensing mass structure disposed between said first and second plates with said force and sense conductive areas of said first and second plates facing said force and sense conductive areas of said opposite sides of said sensing mass structure, said sensing mass structure being supported via said E-spring structure from said support frame structure, said support frame structure secured to peripheral outside areas of said first and second plates.

21. The transducer of claim 11 wherein said conducting material is gold and said insulating material is silicon dioxide.

22. The transducer of claim 21 further comprising a primer layer of chromium placed between said gold conducting layer and said silicon dioxide insulating layer.

23. The transducer of claim 22 wherein the thickness of each of said material layers is selected to produce an approximately flat composite structure at room temperture.

24. The transducer of claim 23 wherein

the thickness of said polysilicon layer is approximately 7320 Å

,

the thickness of said silicon dioxide layer is approximately 2000 Å

,

the thickness of said chromium layer is approximately 250 Å

, and

the thickness of said gold layer is approximately 2250 Å

.

25. The transducer of claim 24 wherein said layers of silicon dioxide, chromium and gold are narrower in width than is said polysilicon layer.

16. The transducer of calim 15 wherein said etch stop layer pattern defining said support frame structure includes a hockey stick shaped area between one of said outer legs of said E-shaped spring structure and an adjacent etch time and undercut control area,

said hockey stick shaped area defines a foot and a leg, where said outer leg of said E-shaped spring structure is connected to the top of the foot of said hockey stick shaped area, and

said foot of said hockey stick shaped area provides a strength region for connecting said outer leg to said support frame structure.