Moisture sensor using reactive sputtered TiO2 thin film with negative substrate bias

Moisture is known as a major factor in degrading the reliability of electronic packages. It is estimated that over 40% of failures in electronic devices are induced by moisture. The use of micro-moisture-sensing-chips in electronic package reliability tests can provide in-situ and real-time monitoring of device failures under controlled environmental conditions. Some previously published fabrication techniques of high performance moisture sensors include a very high temperature powder sintering method (Jain et al, 1999; Katayama et al, 1990; Slunecko et al, 1992), the sol-gel method (Montesperelli et al, 1995) and etching porous silicon (Macko, 1982) under complex and sophisticated process control. All of these methods are incompatible with conventional IC processing. In this paper, simple and low temperature fabricated moisture sensing chips using a reactive sputtering process in thin film technology are characterized. The reactive sputtering process has been widely employed in IC batch production for its high process control and uniformity. The adsorption response, which is well fitted into a Brunauer, Emmett and Teller (BET) type III model, and atomic force microscope (AFM) images illustrate that the sensing films are pore-free. A process window for improving hysteresis performance is obtained by applying a negative substrate bias during sputtering and increasing the annealing time. The sensitivity of four orders of magnitude in DC current change over 11%-97% relative humidity (RH) is achieved