Protection Of Chips In The Bulk-Si Etching In The MEMS Post-Processing

The sensors fabricated with CMOS-MEMS process have many advantages such as low cost,easy to achieve batch-fabrication over the traditional sensors. Wet etching is a typical MEMS process in CMOS-MEMS process. The micro-structures such as caves and grooves can be fabricated with the anisotropic property of alkaline solutions and KOH,EDP and TMAH are the most common choices. But the problem is that the Aluminum pad and poly-structures can't resist the erosion of the solutions above. So the chips can't be processed by wet etching without some protection steps.The purpose of this paper is to find ways to protect the pad and the structures on chip during the wet etching. Wet etching has different applications in the bulk-silicon etching, so methods to protect the chips are divided into protection of back-side etching and protection of front-side etching. Acrylonitrile-Butadiene-Styrene (ABS) is a kind of plastic of engineering, it has good adhesion to the substrate and can resist the erosion of alkaline solutions. So a novel process to protect the front side of chips with ABS film is applied in the back-side etching. The ABS clue is mixed by ABS powder and butanone according to some ratio. The process to deposit and bake the film is just like the photolithography process. A lot of experiments test the properties of the film and the effect of protection is researched by varying the parameters of thickness of films and temperatures of baking. With this protection method, the back-side of the flow sensor and the pressure sensor is etched. The function of back-side etching is to minimize the thermal capacity of the sensors, so the sensitivity of the sensors will be improved. The protection of the circuits on chip is also needed to consider in the front-side etching. Generally the alkaline solutions such as KOH and TMAH etch the aluminum very fast, but the Al etching rate will drop with the increase of amounts of doped silicon. So the method of TMAH solutions doped with silicon is adopted to release the structure. The properties of different concentrations of TMAH solution are researched and the effect of the additives is also considered. The front-side micro-structures of the chips are successfully released without etching the pad.All the experiments are completed with the facilities in the key laboratory of MEMS of Ministry of Education. The methods we present have the advantages of simplicity and low cost.