Micro-machining

• US 20020017132A1
• Filed: 07/05/2001
• Published: 02/14/2002
• Est. Priority Date: 09/12/1998
• Status: Active Grant

• Current Assignee: Qinetiq Limited (QinetiQ Group PLC)
• Original Assignee: Secretary of State For Defence
• Inventors: McNie, Mark E., Nayar, Vishal
• Granted Patent: US 6,670,212 B2
• Time in Patent Office: Days
• US Class Current: 73/504.02
• CPC Class Codes: G01C 19/56 Turn-sensitive devices using vibrating masses, e.g. vibratory angular rate sensors based on Coriolis forces

Abstract

A method of fabricating a micro-mechanical sensor (101) comprising the steps for forming an insulating layer (6) onto the surface of a first wafer (4) bonding a second wafer (2) to the insulating layer (6), patterning and subsequently etching either the first (4) or second wafer (6) such that channels (18,20) are created in either the first (2) or second (4) wafer terminating adjacent the insulating layer (6) and etching the insulating layer (6) to remove portions of the insulating layer (6) below the etched wafer such that those portions of the etched wafer below a predetermined cross section, suspended portions (22), become substantially freely suspended above the un-etched wafer. This method uses Silicon on Insulator technology. Also disclosed is a micro-mechanical gyroscope structure (101) allowing an anisotropic silicon to be used to fabricate a sensor functioning as if fabricated from isotropic silicon.

Claims

1. A method of fabricating a micro-mechanical sensor comprising taking a first wafer with an insulating layer formed thereon and with a second wafer bonded to the insulating layer and

(a) patterning and subsequently etching one of either said first or second wafers such that channels are created in said one wafer (the etched wafer) terminating adjacent to said insulating layer;

and

(b) etching said insulating layer to remove portions of said insulating layer adjacent said etched wafer such that those portions of said etched wafer below a predetermined size, suspended portions, become substantially freely suspended above the other wafer.

2. A method according to claim 1 wherein the thickness of said insulating layer is substantially in the range 10 nm to 100 μ

m.

3. A method according to claim 1 wherein said second wafer is a substrate or handle wafer which is provided as a support for the micro-mechanical sensor.

4. A method according to claim 1 wherein the method comprises an additional step before step (a) of polishing said wafer to be etched (which may be either said first or second wafer) to the desired thickness.

5. A method according to claim 1 wherein a mask for the etch of step (a) is optimised so that areas to be etched have substantially equal cross sectional area.

6. A method according to claim 1 wherein suspensory ligaments are provided to join said suspended portions and a remainder of said etched wafer.

7. A method according to claim 1 wherein the method includes the step of cleaning the etch of said insulating layer with a vapour.

8. A method according to claim 7 wherein the structure being fabricated is held above substantially the boiling point of said vapour.

9. A method according to claim 1 wherein some of the channels created in said etched wafer prior to the performing of step (b) are refilled with a refill material.

10. A method according to claim 9 wherein surface layers are laid across said refilled channel.

11. A method according to claim 1 wherein said first and second wafers are provided from a semi-conductor.

12. A method according to claim 1 which comprises providing integrated circuits in association with said sensor such that a sensor is provided on a single wafer with the necessary signal processing electronics.

13. A method according to claim 1 wherein the insulating layer between said first and second wafers comprises more than one material.

14. A method according to claim 13 wherein there is provided at least one layer of undoped oxide and at least one layer of doped oxide, in said insulating layer.

15. A method according to claim 14 wherein there are provided two layers of undoped oxide sandwiching a layer of doped oxide.

16. A method according to claim 1 wherein dopants are included in the insulating layer and include any one from the following list consisting of:

Phosphorous, Boron, Antimony, Arsenic, Germanium.

17. A method according to claim 1 wherein stress between the first and second wafer is reduced by providing said insulating layer as a series of layers.

18. A micro mechanical device fabricated according to the method of claim 1.

19. A device according to claim 18 which is any one of the following:

gyroscope, accelerometer, resonator, actuator, sensor, or other micro part.

20. A method of fabricating a micro-mechanical sensor comprising the following steps:

(a) forming an insulating layer on to a top-most surface of a first wafer;

(b) etching portions of said insulating layer;

(c) bonding a second wafer to said insulating layer; and

(d) etching a bottom most surface of one of said wafers, the etched wafer, in regions adjacent the etched portions of said insulating layer such that portions of said etched wafer become substantially free (suspended portions) from the remainder of said first and second wafers.

21. A method according to claim 20 which comprises a step before step (a) wherein marker channels, having a top end portion near to the surface of the wafer and a bottom end portion distal from the top end portion, are etched into said first wafer.

22. A method according to claim 21 wherein polishing of said wafer removes part of said first wafer such that bottom end portions of said marker channels are revealed.

27. A gyroscope according to claim 26 wherein the gyroscope is fabricated from silicon aligned such the plane of the wafer has the lattice spacing of substantially <

100 >

.

28. A gyroscope according to claim 26 having eight sensing elements wherein four of said sensing elements have a portion of increased thickness when compared with the remaining four of said sensing elements.

29. A method of fabricasting a micro-mechanical ring gyroscope according to claim 26 wherein the method comprises determining the degree of misalignment of the crystal lattice before fabricating the gyroscope.

30. A method according to claim 29 wherein the degree of misalignment is calculated by wet etching alignment markers.

23. A method of forming an alignment marker comprising the following steps:

(a) forming channels in a top-most surface of a wafer substantially perpendicular to the surface of the wafer, said channels having a top-most end portion adjacent the top-most surface of the wafer and a bottom end most portion distal the top-most end portion;

(b) providing a layer onto the surface of the wafer which fills said channels; and

(c) polishing said wafer from a rear-most surface at least until the bottom end portion of said channel is exposed through the rear-most surface of the wafer.

24. A method of separating portions of at least two wafers comprising the following steps:

(a) providing two wafers separated by an insulating layer wherein said insulating layer is fabricated from at least two sub layers of material having different etching rates; and

(b) etching said insulating layer to remove portions wherein the time of the etch is controlled to ensure that at least one sub layer is substantially completely removed within the portion being etched and at least one other sub layer is partially left un-etched in the portion being removed.

25. A method of separating a first portion of material from a second portion of material comprising using a dry etching process to etch between said portions and subsequently using a dry agent to clean any residues remaining from said etch.

26. A micro-mechanical ring gyroscope having a ring element which is fabricated from a material having anisotropic properties, the dimensions of at least a portion of the element being thicker or thinner when compared to the remainder of the element so that said gyroscope functions as if it were manufactured from a material having isotropic properties.

31. A micro mechanical device having at least one element which is fabricated from a material having anisotropic properties, the dimensions of the element having the thickness dimension designed so that said device functions as if it were manufactured from a material having isotropic properties.