Vacuum-Cavity-Insulated Thermal Flow Sensors

US20140069185 (US Patent Application)

Xiang Zheng Tu

This invention relates to a micromachined thermal flow sensor. Particularly, the invention relates to a micromachined vacuum-cavity-insulated flow sensor utilizing a rigid porous silicon membrane as its suspending structure and a micromachined vacuum-cavity as its thermal insulation structure. 

The vacuum-cavity-insulated thermal flow sensor comprising: a single crystal silicon substrate; a porous silicon wall with numerous vacuum-pores therein which is created in said silicon substrate; a porous silicon membrane with numerous vacuum-pores therein which is created in said silicon substrate, surrounded and supported by said porous silicon wall; a cavity with a vacuum-space therein which is created beneath said porous silicon membrane and surrounded by said porous silicon wall; a dielectric layer deposited on the surface of said silicon substrate which includes the surface of said porous silicon wall and said porous silicon membrane; a heating element disposed laterally passing through at a middle of said porous silicon membrane; two temperature sensing elements disposed parallel with said heating element at two opposite sides thereof; and three pairs of metal conducting strips with three pairs of metal bonding pads disposed opposite to two sides of said porous silicon membrane for respectively connecting said heating element and said temperature sensing elements to an external circuit therefore. 

The vacuum-cavity-insulated thermal flow sensor has the following features: 

• One feature of the vacuum-cavity-insulated flow sensor provided by the present invention is to create a vacuum-cavity in a silicon substrate so that there is no vertical thermal conduction from a heating element on a suspended porous silicon membrane over the vacuum-cavity to the silicon substrate.
• Another feature of the vacuum-cavity-insulated flow sensor provided by the present invention is that the suspended porous silicon membrane is supported by a same thick porous silicon wall so that the detachment of the suspended porous silicon membrane from the silicon substrate is prevented.
• Still another feature of the vacuum-cavity-insulated flow sensor provided by the present invention is that the suspended porous silicon membrane is thicker and more rigid so that it is able to withstand at least 7 atm.
• Still another feature of the vacuum-cavity-insulated flow sensor provided by the present invention is that the process for fabricating the sensor is simple and compatible with a CMOS process except anodization in HF solution.
• Still another feature of the vacuum-cavity-insulated flow sensor provided by the present invention is that the vacuum and sealing of a cavity created in the sensor is automatically realized during a dielectric film deposition process without an additional specific processing step.