Method for the fabrication of suspended porous silicon microstructures and application in gas sensors

• US 20040195096A1
• Filed: 01/30/2004
• Published: 10/07/2004
• Est. Priority Date: 07/31/2001
• Status: Abandoned Application

• Current Assignee: Androula G. Nassiopoulou, Angeliki Tserepi, Christos Tsamis
• Original Assignee: Androula G. Nassiopoulou, Angeliki Tserepi, Christos Tsamis
• Inventors: Tserepi, Angeliki, Nassiopoulou, Androula G, Tsamis, Christos
• Application Number:
  US10/485,940
• Publication Number:
  US 20040195096A1
• Time in Patent Office: Days
• US Class Current: 204/426
• CPC Class Codes: B81B 2201/0278 Temperature sensors
• CPC Class Codes: B81B 2203/0109 Bridges
• CPC Class Codes: B81B 2203/0118 Cantilevers
• CPC Class Codes: B81C 1/00682 Treatments for improving mechanical properties, not provided for in B81C1/00658 - B81C1/0065
• CPC Class Codes: B81C 1/0069 Thermal properties, e.g. improve thermal insulation
• CPC Class Codes: B81C 2201/0115 Porous silicon
• CPC Class Codes: G01F 1/6845 Micromachined devices
• CPC Class Codes: G01N 27/12 of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid , for detecting components in the fluid
• CPC Class Codes: G01N 27/16 caused by burning or catalytic oxidation of surrounding material to be tested, e.g. of gas
• 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: H05B 3/265 the insulating base being an inorganic material, e.g. ceramic H05B3/262 takes precedence

Abstract

This invention provides a front side silicon micromachining process for the fabrication of suspended Porous Silicon membranes in the form of bridges or cantilevers and of thermal sensor devices employing these membranes. The fabrication of the suspended Porous Silicon membranes comprises the following steps: (a) formation of a Porous Silicon layer (2) in, at least one, predefined area of a Silicon substrate (1), (b) definition of etch windows (5) around or inside said Porous Silicon layer (2) using standard photolithography and (c) selective etching of the silicon substrate (1), underneath the Porous Silicon layer (2), by using dry etching techniques to provide release of the Porous Silicon membrane and to form a cavity (6) under the said Porous Silicon layer. Furthermore, the present invention provides a method for the fabrication of thermal sensors based on Porous Silicon membranes with minimal thermal losses, since the proposed methodology combines the advantages that result from the low thermal conductivity of Porous Silicon and the use of suspended membranes. Moreover, the front-side micromachining process proposed in the present invention simplifies the fabrication process. Various types of thermal sensor devices, such as calorimetric-type gas sensors, conductometric type gas sensors and thermal conductivity sensors are described utilizing the proposed methodology.

Claims

1. A front-side silicon micromachining process for the fabrication of suspended Porous Silicon membranes in the form of bridges or cantilevers, comprising the following steps:

(a) Formation of a Porous Silicon layer in, at least one, predefined area of a Silicon substrate, (b) Definition of etch windows around or inside said Porous Silicon area using standard photolithography, (c) Selective etching of the Silicon substrate, underneath the Porous Silicon layer, by using dry etching techniques to provide release of the Porous Silicon membrane and to form a cavity under the said Porous Silicon layer.

2. The front side silicon micromachining process of claim 1 for the fabrication of suspended Porous Silicon membranes in the form of bridges or cantilevers, further comprising the formation of a patterned conductive layer on top of said porous silicon membrane. The said patterned conductive layer is used as a heater, for the fabrication of thermal sensor devices.

4. A gas sensor device based on Porous Silicon suspended membranes in the form of bridges or cantilevers fabricated by the front side micromachining process of claim 1 and composed of:

(a) a silicon substrate, (b) a suspended membrane on top of a cavity on the silicon substrate to provide local thermal isolation, (c) a patterned conductive layer on said suspended membrane, (d) an insulating layer on top of said conductive layer, (e) a catalytic material on top of said insulating layer (f) a thermal sensing element to sense the temperature change due to the reaction of the ambient gas with the catalytic material.

5. The gas sensor of claim 4 wherein said sensing element is a Pt resistor.

6. The gas sensor device of claim 4 wherein said thermal sensing element consists of a number of thermocouples, one contact of said thermocouples being on the suspended porous silicon membrane, the other contact being on bulk silicon.

7. A gas sensor device based on Porous Silicon suspended membranes in the form of bridges or cantilevers fabricated by the front side micromachining process of claim 1 and composed of:

(a) a silicon substrate, (b) a suspended membrane on top of a cavity on the silicon substrate to provide local thermal isolation, (c) a patterned conductive layer on said suspended membrane (d) an insulating layer on top of said conductive layer (e) metal electrodes on top of said insulating layer and (f) a catalytic material on top of said electrodes, the resistivity of the said catalytic material being dependent on the ambient gas.

8. A thermal conductivity sensor based on Porous Silicon suspended membranes in the form of bridges or cantilevers fabricated by the front side micromachining process of claim 1 and composed of:

(a) a silicon substrate, (b) a suspended membrane on top of a cavity on the silicon substrate to provide local thermal isolation, (c) a patterned conductive layer on said suspended membrane, where said patterned conductive layer is a Pt resistor. The dimensions of the said porous silicon membrane can be changed in order to minimize thermal losses and modulate the response time of the sensor.