Composition sensor with minimal non-linear thermal gradients

• US 4,706,061 A
• Filed: 08/28/1986
• Issued: 11/10/1987
• Est. Priority Date: 08/28/1986
• Status: Expired due to Fees

• Current Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Original Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Inventors: Johnson, Robert G.
• Primary Examiner(s): NOT, DEFINED
• Assistant Examiner(s): Monroe, James B.
• Application Number:
  US06/901,215
• Time in Patent Office: 439 Days
• Field of Search: 428/698, 428/699, 428/614-617, 428/620, 428/621, 428/627, 428/136, 428/137, 428/209, 428/210, 73/25, 73/26, 73/27 R, 73/204, 73/719-721, 73/725-727, 156/647, 338/25, 338/292, 338/293, 338/319, 338/34, 338/318, 338/2-5, 357/55, 422/98
• US Class Current: 338/34
• CPC Class Codes: G01N 27/18 caused by changes in the thermal conductivity of a surrounding material to be tested G01N27/20 takes precedence
• CPC Class Codes: Y10T 428/12493 Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
• CPC Class Codes: Y10T 428/12528 Semiconductor component
• CPC Class Codes: Y10T 428/12535 with additional, spatially distinct nonmetal component
• CPC Class Codes: Y10T 428/12576 Boride, carbide or nitride component
• CPC Class Codes: Y10T 428/24314 Slit or elongated
• CPC Class Codes: Y10T 428/24322 Composite web or sheet
• CPC Class Codes: Y10T 428/24926 including ceramic, glass, porcelain or quartz layer

Abstract

A gaseous composition sensor which is a microstructure device comprising a heated planar thin film diaphragm sensor member suspended over a shallow flat bottomed etched pit in a single crystal silicon substrate.

Claims

1. A gaseous composition sensor comprising:

a semiconductor body comprising (100) silicon and having a (100) plane and a <

100>

direction, a first surface of the body being substantially parallel to the (100) plane;

a thin film layer of Si₃ N₄ covering at least a portion of said first surface;

square shaped diaphragm means comprising the thin film Si₃ N₄, said square shaped diaphragm means having successive corners A,B,C and D, where corners B and D are diagonally opposite, said diaphragm means being oriented so that a line joining diaphragm means opposite corners B and D is at an angle of substantially 45°

to the <

110>

direction, said diaphragm means further comprising resistive heater means embedded in the Si₃ N₄ layer generally centered along the diagonal diaphragm area from corners B to D, said diaphragm means having lines of lateral heat flow from the heater means over most of the diaphragm means, said diaphragm means having perimeter lines AB, BC, CD and DA;

a series of closely spaced approximately parallel first slots through said diaphragm, the length of said slots being aligned approximately perpendicular to the line joining corners B and D and approximately parallel to the lines of lateral heat flow from the heater over the diaphragm so that said slots are thus at an angle of substantially 45°

to the <

110>

direction, said slots extending at the outer end to one of perimeter lines AB, BC, CD and DA, said slots providing openings through the Si₃ N₄ to the silicon for purposes of etching the silicon;

two additional slots called a second and a third slot aligned with each other along the line joining B and D and oriented orthogonally to said first slots, said second slot having its outer terminus at said corner B, said third slot having its outer terminus at said corner D;

and a square shaped shallow flat bottomed anisotropically etched pit in said silicon beneath said square shaped diaphragm means and conforming to said diaphragm means in orientation and size.

2. The invention according to claim 1 in which said Si₃ N₄ layer is about 1 micron thick.

3. The invention according to claim 1 in which said series of parallel slots are each about 2 microns in width.

4. The invention according to claim 1 in which said square shaped diaphragm means and said square shaped pit are about 240 microns on a side.

5. The invention according to claim 4 in which said shallow pit is about 0.001 inch in depth.

6. The invention according to claim 4 in which said series of parallel slots are spaced about 20 microns apart.

7. The invention according to claim 1 in which the resistive heater means is a nickel-iron alloy.

8. A method of fabricating a microbridge gaseous composition sensor comprising a silicon nitride thin film diaphragm covering a shallow depression etched into a silicon substrate, the steps comprising:

providing a single crystal silicon substrate comprising (100) silicon and having a (100) plane and a <

110>

direction, with a first surface of the substrate being substantially parallel to the (100) plane;

depositing a thin-film layer of silicon nitride onto the first surface to become a diaphragm;

providing an electrical heater on a portion of said silicon nitride layer by forming a thin film strip of resistive metal thereon;

planning a square depression in said silicon surface beneath said Si₃ N₄ layer, where one side of said square is parallel with the <

110>

direction of said silicon, said square having first and second adjacent sides and third and fourth adjacent sides;

preparing a first series of closely spaced parallel slots through the Si₃ N₄ and oriented at substantially 45°

to the <

110>

direction and to the first and second adjacent sides;

preparing a second series of closely spaced parallel slots through the Si₃ N₄, the slots lying in a direction parallel to the first series slots and oriented at substantially 45°

to the <

110>

direction and to the third and fourth adjacent sides;

said slots being of differing lengths and each slot having an exterior end terminating at the first, second, third or fourth side;

preparing another pair of slots axially aligned with each other and oriented orthogonally to the previous defined slots and located between the first and second series of slots; and

,

applying an anisotropic etch through said slots to the silicon first surface, the slots being oriented at substantially 45°

to the <

110>

direction, to undercut the diaphragm and create a shallow square depression.

9. A gaseous composition sensor comprising:

a semiconductor substrate comprising (100) silicon having a (100) plane and a <

110>

direction;

a thin-film layer means of Si₃ N₄ on the semiconductor substrate surface;

a resistive metallic heater embedded in the Si₃ N₄ layer;

a first series of closely spaced parallel slot means extending through the Si₃ N₄ surrounding the location of said resistive heater, said slots being oriented at an angle of substantially 45°

to the <

110>

direction for the purpose of providing etchant access for anisotropically etching a shallow pit in the silicon under a portion of the Si₃ N₄ ;

a square shaped outline for the shallow pit, the outline having successive corners A, B, C and D, corners B and D being at diagonally opposite corners, a line connecting corners B and D being at an angle of substantially 45°

to the <

110>

direction, the square shaped outline having perimeter sides AB, BC, CD and DA, said slot means extending at the outer end to one of the perimeter sides;

and further slot means orthogonal to said first slot means located on said BD line and having the slot means terminate at B and D.