Mechanics and Tribology of MEMS Materials - prod.sandia.gov ...

7 Impact of Silane Degradation Due to Water Vapor and Radiation
Exposure on Tribological Behavior
7.1 ABSTRACT
Microelectromechanical systems (MEMS) with high out-of-plane stiffness are less
susceptible to adhesion than more compliant structures, but reliable operation of sliding contacts
still requires surfaces that exhibit adequate friction and wear performance after long periods of
storage. Alkylsilane monolayers are popular surface treatments for silicon devices, and there has
been some research to understand the performance of monolayers as a function of environment.
However, there have been limited investigations of the tribological behavior of these surface
treatments after exposure to harsh environments. There is a need to quantitatively determine the
effects of storage environments on the performance of MEMS interfaces, rather than verifying
device functionality alone. To this end, surface micromachined (SMM) structures that contain
isolated tribological contacts have been used to investigate interface performance of alkylsilane
monolayers after storage in inert environments, and after exposure to a variety of thermal and
radiation environments. Results show that both octadecyltrichlorosilane (ODTS) and
perfluorodecyltrichlorosilane (PFTS) exhibit little change in hydrophobicity or friction after Co-
60 radiation exposures at a total dose of up to 500 krad. However, exposure to temperature
cycles consistent with packaging technologies, in the presence of low levels of water vapor,
produces degradation of hydrophobicity and increase in static friction for ODTS films while
producing no significant degradation in PFTS films.
7.2 INTRODUCTION
Alkylsilane coupling agents have been used to create hydrophobic surfaces on silicon
MEMS and thus prevent adhesion of structures due to adsorption of water and the creation of
capillary forces between surfaces in contact [7.1-7.2]. Hydrophobic films are typically
terminated by a methyl group, which creates a low energy surface. These films were originally
applied to silicon surface micromachined MEMS in order to alleviate adhesion after the aqueous
release etch used to remove sacrificial oxide layers and free the movable structures (release
adhesion) [7.2]. They also reduce the tendency of surfaces to adhere during operation (in-use
adhesion). More recently, in devices that rely on regular contact and shear between contacting
surfaces, alkylsilane films have been called upon to reduce static and dynamic friction between
silicon surfaces. Static and dynamic friction coefficients for the methyl-terminated films on
MEMS devices are typically below 0.1 in the as-deposited condition [7.3].
Srinivasan et al. [7.4] demonstrated that exposure of alkylsilane films to water vapor at
elevated temperatures, similar to those that may be present during back-end-of-line processes
such as packaging, can cause treated silicon surfaces to become less hydrophobic. These
experiments were performed on blanket films of polycrystalline silicon, treated with
octadecyltrichlorosilane (chemical formula CH3(CH2)17SiCl3, abbreviated ODTS) or
perfluorodecyltrichlorosilane (chemical formula CF3(CF2)7(CH2)2SiCl3, abbreviated PFTS), and
then exposed to nitrogen or ambient air. The water vapor content of the air was not specified,
but was probably near 50% relative humidity. Samples were exposed to these environments for
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