Design of a surface conductance based fully integrated standard CMOS humidity sensor without post-processing

Abstract

Humidity sensors have a wide range of application for commercial and industrial use. Recently, smartphones began to include humidity sensors for ambient weather monitoring and thus the market size of humidity sensors will only grow in the future. Unfortunately, it is difficult to implement a humidity sensor in a standard CMOS technology, as humidity is not an electric signal nor does it change the property of CMOS like temperature. Thus, past humidity sensors require either MEMS or a special humidity-sensitive layer, usually made of polyimide, which is added on to a traditional CMOS IC. Although there are reports that polyimide layer could be obtained as part of passivation layer in a CMOS process, it is not a material that is available in any CMOS process. In this paper, we present a humidity sensor that exploits the surface conductivity of SiO2 that is sensitive to relative humidity. This is in contrast to previous works that rely on humidity-induced change in capacitance or thermal conductivity. Since SiO2 is used in all CMOS processes, the proposed humidity sensor can be manufactured in any CMOS process without requiring post-processing, MEMS or any humidity-sensitive materials. Surface conductance of SiO2 between two metals is exponentially related to relative humidity, due to the hopping process of electrons between places where adsorption of water molecules occurs on SiO2. The chip has been fabricated in a 5-metal 0.25um standard CMOS mixed signal process on an MPW shuttle. To expose the SiO2 surface into air, a pad-open layer is used in the layout process of the design. No post-processing of any sort was used and the chips were not specially treated by any means. The area occupied by the electrodes is 730um x 700um. The circuits are placed underneath the electrodes. 9 chips were tested and characterized from 30 %RH to 100 %RH in a regulated temperature of 23.5+_1oC. The chip-to-chip variation is between +_3.5% for full operating range of humidity.