Z-AXIS ACCELEROMETER

• US 20030209076A1
• Filed: 04/08/2003
• Published: 11/13/2003
• Est. Priority Date: 10/16/2000
• Status: Active Grant

• Current Assignee: Institute of Microelectronics
• Original Assignee: Institute of Microelectronics
• Inventors: Zhang, Qinxin, Kumar, Rakesh, Sridhar, Uppili, Nagarajan, Ranganathan, Miao, Yubo
• Granted Patent: US 6,662,654 B2
• Time in Patent Office: Days
• US Class Current: 73/514.32
• CPC Class Codes: B81B 2201/0235 Accelerometers
• CPC Class Codes: B81B 2203/053 Translation according to an axis perpendicular to the substrate
• CPC Class Codes: B81B 3/0051 For defining the movement, i.e. structures that guide or limit the movement of an element mechanical arrangements for preventing or damping vibration or shock H01H3/60
• CPC Class Codes: G01P 15/0802 Details
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 2015/0814 for translational movement of the mass, e.g. shuttle type
• CPC Class Codes: Y10T 29/49005 Acoustic transducer

Abstract

An accelerometer design is described. It operates by measuring a change in capacitance when one plate is fixed and one is mobile (free to accelerate). Unlike prior art designs where such changes are caused by variations in the plate separation distance, in the design of the present invention the plate separation distance is fixed, it being the effective plate area that changes with acceleration. A key feature is that the basic unit is a pair of capacitors. The fixed plates in each case are at the same relative height but the mobile plates are offset relative to the fixed plates, one mobile plate somewhat higher than its fixed plate with the other mobile plate being somewhat lower. Then, when the mobile plates move (in the same direction), one capacitor increases in value while the other decreases by the same amount. This differential design renders the device insensitive to sources of systematic error such as temperature changes. A process for manufacturing the design is described.

Claims

1. A Z-axis accelerometer, comprising:

left and right parallel plate capacitors, each plate being parallel to the Z-axis;

each capacitor further comprising a fixed plate and a mobile plate, all plates having upper and lower opposing edges;

said mobile plates being constrained to move only along the Z-axis;

the upper and lower edges of the left mobile plate being an offset distance above the upper and lower edges, respectively, of the left fixed plate; and

the upper and lower edges of the right mobile plate being said offset distance below the upper and lower edges, respectively, of the right fixed plate, whereby motion of the mobile plate causes a change in capacitance of the left capacitor and an opposite change in capacitance of the right capacitor.

2. The accelerometer described in claim 1 wherein the fixed and mobile plates are separated from each other by at least 1 micron.

3. The accelerometer described in claim 1 wherein said accelerometer has a low temperature dependence.

4. The accelerometer described in claim 1 wherein each capacitor has a capacitance of between about 0.1 and 10 pF.

5. A Z-axis accelerometer, comprising:

an X-Y plane that defines positive and negative X and Y directions;

a mobile part and an immobile part;

said mobile part further comprising;

a rectangular prism having upper and lower surfaces in the X-Y plane, a pair of opposing long edges that parallel the X direction, a pair of opposing short edges that parallel the Y direction, and two side surfaces between long edges;

two opposing beams, one attached to each short edge and extending therefrom in both Y directions, each beam having first and second ends;

four springs, two disposed to lie along the X direction and two disposed to lie along the Y direction, each of said springs having a first and a second end;

the first and second ends of one beam being attached to the first ends of springs disposed along the X and Y directions respectively, the first and second ends of the other beam being attached to the first ends of springs disposed along the Y and X directions respectively;

four mobile capacitor plates, each having a thickness in the X direction, attached in pairs to said side surfaces and extending therefrom in the Y direction;

two of the mobile capacitor plates each having an upper corner that is level with said upper surface and a lower corner that is located an offset distance above the lower surface;

two of the mobile plates each having a lower corner that is level with said lower surface and an upper corner that is located the offset distance below the upper surface;

said immobile part further comprising;

a substrate containing a rectangular cavity having four corners;

four pairs of immobile capacitor plates extending inwards from opposing sides of the cavity so that each of the mobile capacitor plates lies between, and parallel to, one of said immobile capacitor plate pairs and is offset therefrom in the Z direction by said offset distance;

said mobile and immobile capacitor plates being electrically insulated from each other; and

each second spring end being attached to one of said four corners.

6. The accelerometer described in claim 5 wherein each spring is U-shaped.

7. The accelerometer described in claim 5 wherein said mobile part has a total mass that is less than about 10⁻

7 Kg.

8. The accelerometer described in claim 5 wherein accelerations between about 1 and 100 g may be measured to an accuracy between about 10⁻

3 and 1 g.

9. The accelerometer described in claim 5 wherein said accelerometer has a total thickness in the Z direction that is less than about 30 microns.

10. The accelerometer described in claim 5 wherein each capacitor plate has an area between about 300 and 3,000 sq. microns.

11. A process for manufacturing a Z-axis accelerometer, having a mobile part and an immobile part, comprising:

providing a substrate having left and right areas;

within the right area, forming a first right pedestal that extends upwards from a first upper surface;

within the left area forming a first left trench that extends downwards from said first upper surface;

coating with a first protective layer and then patterning the protective layer to define the mobile and immobile parts, said parts being connected by a spring structure intended to inhibit movement of the mobile part in a plane parallel to said upper surface;

further patterning said first protective layer to define left and right comb structures, the left comb structure having at least one tooth located over the first left trench and connected to the mobile part, and at least one tooth located outside the first left trench and connected to the immobile part, the right comb structure having at least one tooth located over the first right pedestal and connected to the mobile part, and at least one tooth located outside the first right pedestal and connected to the immobile part;

using the protective layer as a mask, etching the substrate, thereby forming;

a second left trench within which lie the first left trench and at least two pedestals, one pedestal extending upwards from inside the first left trench and connected to the mobile part, and one pedestal extending upwards from outside the first left trench and connected to the immobile part, and

a first right left trench within which lie the first right pedestal and at least two other pedestals, one of the other pedestals extending upwards from within the first right pedestal and connected to the mobile part, and one of the other pedestals extending upwards from outside the first right pedestal and connected to the immobile part;

with the protective layer in place, selectively coating all vertical surfaces with a second protective layer;

isotropically etching the substrate until the mobile part is released from direct contact with the substrate, remaining connected thereto only through said spring structure; and

thereby transforming the left and right comb structures into left and right capacitors. each having mobile arid immobile plates, said mobile plates being limited by the springs to motion normal to the upper surface, whereby motion by both mobile plates, relative to the immobile plates, results in an increase in capacitance by one of the capacitors and a decrease in capacitance by the other capacitor.

12. The process recited in claim 11 wherein the first right pedestal has sloping sidewalls and a height of between about 1 and 5 microns.

13. The process recited in claim 11 wherein the first left trench has sloping sidewalls and a depth of between about 1 and 5 microns.

14. The process recited in claim 11 wherein the second left trench has vertical sidewalls and a depth of between about 3 and 30 microns.

15. The process recited in claim 11 wherein the first right trench has vertical sidewalls and a depth of between about 3 and 30 microns.

16. The process recited in claim 11 wherein the substrate is selected from the group consisting of monosilicon, polysilicon, and metals.

17. The process recited in claim 11 wherein the first protective layer is selected from the group consisting of silicon oxide, silicon nitride, and metals.

18. The process recited in claim 11 wherein the second protective layer is selected from the group consisting of silicon oxide, silicon nitride, and metals.

19. The process recited in claim 11 wherein the step of isotropic etching further comprises using a mix of SF₆ and XeF₂.

20. The process recited in claim 11 further comprising, after isotropic etching, removing all protective layers.