Ultra-sensitive magnetoresistive displacement sensing device

• US 6,507,187 B1
• Filed: 08/24/1999
• Issued: 01/14/2003
• Est. Priority Date: 08/24/1999
• Status: Expired due to Fees

• Current Assignee: US Government As Represented By The Director National Security Agency
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Lairson, Bruce M., Ramesham, Rajeshuni, Olivas, John D.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US09/384,363
• Time in Patent Office: 1,239 Days
• Field of Search: 324/207.2, 324/207.21, 324/207.22, 324/207.26, 324/252, 324/259, 338/32.R, 735/142--51, 735/143.1, 737/17, 737/22, 737/23, 737/28, 738/626.25, 738/626.29, 738/626.34, 738/626.37, 738/626.39, 738/626.9
• US Class Current: 324/207.21
• CPC Class Codes: G01B 7/24 using change in magnetic properties
• CPC Class Codes: G01L 9/0042 Constructional details associated with semiconductive diaphragm sensors, e.g. etching, or constructional details of non-semiconductive diaphragms details about the integration or bonding of piezoresistor in or on the diaphragm G01L9/0052 and G01L9/0057 respectively
• CPC Class Codes: G01L 9/007 using variations in inductance
• CPC Class Codes: G01P 15/105 by magnetically sensitive devices

Abstract

An ultrasensitive displacement sensing device for use in accelerometers, pressure gauges, temperature transducers, and the like, comprises a sputter deposited, multilayer, magnetoresistive field sensor with a variable electrical resistance based on an imposed magnetic field. The device detects displacement by sensing changes in the local magnetic field about the magnetoresistive field sensor caused by the displacement of a hard magnetic film on a movable microstructure. The microstructure, which may be a cantilever, membrane, bridge, or other microelement, moves under the influence of an acceleration a known displacement predicted by the configuration and materials selected, and the resulting change in the electrical resistance of the MR sensor can be used to calculate the displacement. Using a micromachining approach, very thin silicon and silicon nitride membranes are fabricated in one preferred embodiment by means of anisotropic etching of silicon wafers. Other approaches include reactive ion etching of silicon on insulator (SOI), or Low Pressure Chemical Vapor Deposition of silicon nitride films over silicon substrates. The device is found to be improved with the use of giant magnetoresistive elements to detect changes in the local magnetic field.

Claims

1. A microelectromechanical system (MEMS) for sensing displacement comprising:

a substrate carrying a magnetoresistive element including a copper layer sandwiched by a pair of cobalt layers, said magnetoresistive element having a variable electrical characteristic determined by a local magnetic field;

a movable silicon nitride microstructure carrying a hard magnetic film, wherein said hard magnetic film is sputtered onto said movable microstructure, said hard magnetic film has a thickness of between 1000 Å and

3000 Å

, and includes a composition to cobalt, chromium, tantalum, and platinum, said movable microstructure traveling along a predetermined direction towards, and away from, said magnetoresistive element, and a displacement of said movable microstructure and said hard magnetic film in the predetermined direction is correlated with a force in the predetermined direction, and

means for measuring a change in the variable electrical characteristic of said magnetoresistive element whereby said change in the variable electrical characteristic is due to a change in the local magnetic field attributable to said displacement of said hard magnetic film on said movable microstructure, and whereby the change in the magnetic field is used to determine the displacement of said movable microstructure in the predetermined direction.

2. The microelectromechanical system of claim 1 wherein said movable microstructure comprises a bridge.

3. The microelectromechanical system of claim 1 wherein said movable microstructure comprises a diaphragm.

4. The microelectromechanical system of claim 1 wherein said movable microstructure comprises a cantilever beam.

5. The microelectromechanical system of claim 1 wherein said movable microstructure comprises a membrane.

6. The microelectromechanical system of claim 5 wherein said movable microstructure comprises a silicon nitride membrane fabricated over a silicon substrate using low-pressure chemical vapor deposition and plasma etching.

7. The microelectromechanical system of claim 5 wherein said movable microstructure comprises a silicon nitride membrane having a maximum thickness of 0.5 microns.

8. The microelectromechanical system of claim 1 wherein said hard magnetic film comprises a composition of cobalt, chromium, tantalum, and platinum in the proportion of 80:

14;

5;

1 respectively, by weight.

9. The microelectromechanical system of claim 1 wherein the magnetoresistive element is a giant magnetoresistive element comprising:

a sensing layer;

a conducting spacer layer;

a magnetically pinned layer; and

an exchange layer;

wherein the exchange layer permits conduction of electrons between the sensing layer and the magnetically pinned layer via the conducting layer.

10. The microelectromechanical system of claim 9 wherein the pinned layer is an antiferromagnetic layer comprising iridium, platinum, and manganese.

11. The microelectromechanical system of claim 9 wherein the giant magnetoresistive element comprises:

an oxidized silicon substrate;

a first layer of tantalum on the oxidized silicon substrate;

a layer of Permalloy on the first layer of tantalum;

a first layer of cobalt on the layer of Permalloy;

a layer of copper on the first layer of cobalt;

a second layer of cobalt on the layer of copper;

a layer of iron-manganese on the second layer of cobalt; and

a second layer of tantalum on the layer of iron-manganese;

where each layer is deposited in a common vacuum chamber.

12. The microelectromechanical system of claim 11 further including a second layer of permalloy between the second layer of cobalt and the layer of iron-manganese.

13. The microelectromechanical system of claim 11 wherein the total thickness of the deposited layers of the giant magnetoresistive element is between 324 Å

and 354 Å

.

14. The microelectromechanical system of claim 9 wherein the giant magnetoresistive element has a characteristic resistance change of 0. 1 ohms per oersted.

15. The microelectromechanical system of claim 9 wherein the giant magnetoresistive element has a percentage change of resistance of less than 5 percent.

16. An accelerometer sensor comprising:

a movable microstructure having a direction of travel along a direction for which acceleration is to be measured, said movable microstructure including a hard magnetic film deposited thereon such that said hard magnetic film travels with said movable microstructure;

a giant magnetoresistive element mounted adjacent said movable microstructure and positioned normal to a line defining the direction for which acceleration is to be measured, said giant magnetoresistive element consisting of;

at least two layers of tantalum;

at least two layers of cobalt;

at least one layer of copper;

at least one layer of iron-manganese; and

wherein the electrical resistance of the giant magnetoresistive element varies according to an imposed magnetic field;

wherein the displacement of the movable microstructure and the hard magnetic film towards and away from the giant magnetoresistive element affect the imposed magnetic field and consequently the electrical resistance in said giant magnetoresistive element.

17. A microelectromechanical system (MEMS) for sensing displacement comprising:

a substrate carrying a hard magnetic film;

a movable microstructure carrying a magnetoresistive element including a copper layer sandwiched by a pair of cobalt layers, said movable microstructure traveling along a predetermined direction towards, and away from, said hard magnetic film, said magnetoresistive element having a variable electrical characteristic determined by a local magnetic field where said local field is affected by the position of said magnetoresistive element relative to said hard magnetic film, and a displacement of said movable microstructure and said magnetoresistive element in the predetermined direction is correlated with a force in the predetermined direction; and

means for measuring a change in the variable electrical characteristic of said magnetoresistive element whereby said change in the variable electrical characteristic is due to a change in the local magnetic field attributable to said displacement of said movable microstructure, and whereby the change in the magnetic field is used to determine the displacement of said movable microstructure in the predetermined direction.

18. The microelectromechanical system of claim 17 wherein the copper layer has a thickness less than half the combined thickness of the pair of cobalt layers.