Evaluation of Oxygen Plasma and UV Ozone Methods for Cleaning of Occluded Areas in MEMS Devices

UV ozone and oxygen plasma treatments are two common procedures for cleaning silicon surfaces. The extent to which hidden surfaces of microelectromechanical systems (MEMS) are cleaned by these methods has not been well documented. To probe and compare the effectiveness of the two methods for cleaning occluded regions in MEMS, devices consisting of large movable flaps were fabricated to produce hidden surfaces whose occluded regions exceeded the aspect ratios that typically occur in MEMS devices. The gaps between the flap and the substrate in the custom flap devices were designed to be variable in extent. Their interior regions were initially coated with chemisorbed monolayers and then subjected to cleaning. Both techniques removed monolayers on exposed surfaces and both, to some extent, removed monolayers present on the occluded surfaces. Oxygen plasma was found to be a far more effective method for cleaning the occluded surfaces than the UV ozone method. However, in occlusions with exceptionally large aspect ratios of 1700 : 1, even oxygen plasma could not remove all traces of the chemisorbed monolayers.