Micromachining arbitrary 3D silicon structures for micro-electromechanical systems can be accomplished using gray-scale lithography along with dry anisotropic etching. In this study we have investigated two important design limitations for gray-scale lithography: the minimum usable pixel size and maximum usable pitch size. Together with the resolution of the projection lithography system and the spot size used to write the optical mask, the maximum range of usable gray levels can be determined for developing 3D large area silicon structures. An approximation of the minimum pixel size is shown and experimentally confirmed. Below this minimum, gray levels will be developed away due to an excessive amount of intensity passing through the optical mask. Additionally, oscillations in the intensity are investigated by the use of large pitch sizes on the optical mask. It was found that these oscillations cause holes in the photoresist spaced corresponding to the pitch used on the gray-scale mask and penetrate the thickness of the photoresist for thin gray levels. From the holes in the photoresist, significant surface roughness results when used as a nested mask in reactive ion etching, and the very thin gray levels are lost.

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ISR develops, applies and teaches advanced methodologies of design and analysis toAbstract-Rolling element bearing is a well-known concept in macroscale machinery applications. They are prospective candidates for friction reduction in microelectromechanical system (MEMS), as well as for providing stable, robust support for moving micromechanisms. The characteristics of rolling element bearings need to be investigated to facilitate their applications in MEMS. It is well understood that the measured data on the macroscale cannot be directly applied to the microscale. This paper presents an in-situ noncontact experimental system to characterize the friction behavior of microball bearings on the microscale. The methodology presented in this paper provides a useful template to study the dynamical behavior of linear microball bearings with a variety of materials, geometries, and surface qualities. The system, actuated by a motor, affords wide ranges of motion for measuring the dynamic friction using a vision system. It allows the determination of the coefficient of friction (COF) without any interference due to the measurement system. With careful optimization, the error in measurement has been reduced to 2%. Different designs of microball bearings are proposed to achieve lower friction. The studied microball bearings demonstrated an average static COF of 0.01 and an average dynamic COF of 0.007 between stainless-steel and silicon-micromachined contacting surfaces at 27 C and 40% relative humidity.Index Terms-Microelectromechanical systems (MEMS), microball bearings, rolling friction, silicon micromachining, V-grooves.