Stiction Reduction Agents Studies Using QCM

Abstract

The problem of stiction in microelectromechanical systems (MEMS) is highly limiting their fabrication and functionality. The problem occurs during the fabrication, release stiction, as well as during the use of the devices, in-use stiction. Anti-stiction agents are currently an active area of research in the MEMS field to address this issue. These agents are primarily deposited in the form of self-assembled monolayers (SAM's) on the substrate to change its surface properties; to reduce what is known as the stiction problem. One commonly used SAM is the octadecyltrichlorosilane (OTS). On silicon substrates, the most used material in MEMS fabrication, OTS has shown high effectiveness in reducing devices stiction.

The use of a quartz crystal microbalance (QCM) as an analytical technique for studying the OTS anti-stiction agent is presented in this work. Using the QCM as the primary analytical tool, we were able to extract comprehensive information about the formed SAM. The dependence of SAM deposition on the bulk phase concentration of the deposit solution is shown. A rough estimation of the adsorption kinetics' rate constants were calculated, and the equilibrium constant was determined from their values. The equilibrium constant shows the high favorability of OTS deposition on silicon substrate compared to the reverse desorption process. The complex nature of the OTS SAM and its formation mechanism were also shown. These conclusions were made based upon comparison between the more robust SAM system of thiols on gold and the OTS on silicon.

Finally, the interaction between the OTS and a vapor-phase lubricant (tertiary-butyl phenyl phosphate, TBPP), for friction reduction, was studied. Preliminary QCM results show a change in the adsorption of lubricant on bare silicon compared to OTS coated silicon. In addition, the lubricant film is believed to have higher slippage when OTS was present as an underlayer.