CMOS-sealed membrane capacitors for medical tactile sensors

This paper describes the design, manufacture and properties of membrane structures with capacitive detection of deflection. The purpose of the study is to find a sensing organ to be used in an integrated tactile sensor for medical applications. The designed structures are implemented in a commercial CMOS process with a small number of post-micromachining steps. The process used offers two possibilities to achieve capacitors with a moving membrane, namely sacrificial etching of aluminium or silicon dioxide. Both methods have their strengths, but also limitations. These issues, which are strongly affecting the design flow, are discussed together with measurement results of the fabricated structures. The membranes from sacrificial oxide etching showed reliable dielectric gap formation. However, the large variation in their stress properties together with limited available pitch rendered them inferior to membranes from sacrificial aluminium etching. Moreover, the aluminium release etch can be achieved with a simpler etch-opening formation procedure than the oxide release etch.