Keren Deng

Performance Impact of Monolayer Coating of Polysilicon Micromotors

This paper reports the impact on performance of flange-bearing polysilicon micromotors for different self-assembled monolayer coatings on the surface of released motors. Octadecyltrichlorosilane (OTS) and (3,3,3-trifluoropropyl) trichlorosilane (TFP) are found to be promising as they significantly improve micromotor performance. Micromotors coated with OTS show a stable rotor speed and minimum operating voltage during a nine-month testing period. The experiments on gear ratio as a function of wobble cycles indicate that wear in a bearing without OTS coating is significant and results in changes in the gear ratio from the start of micromotor operation by as much as 40%, while the change of gear ratio is within 4% for near 80 million wobble cycles over a nine-month testing period for motors with OTS coating. For motors coated with TFP, no stiction and no significant change of the gear ratio are observed for the testing duration. However, the study of gear ratio as a function of wobble cycles shows that the rotor speed fluctuates in the beginning and then stabilizes for wobble micromotors coated with TFP. OTS coating is found to decrease the flange frictional force/torque by a factor of about 1.5. This net reduction of the flange friction force/torque comes about from the combined action of increasing the frictional coefficient from 0.36 to 0.55 and decreasing the normal contact force associated with the rotor/flange contact friction from near 0.8 μN to near 0.3 μN

A study of static friction between silicon and silicon compounds

Presents results from measurements of static coefficient of friction between materials of interest to microelectromechanical systems (MEMS). The materials studied include silicon, silicon dioxide, and silicon nitride. Two measurement techniques have been used to this study. In the first technique, static friction between two millimeter-sized flat components was measured in a 10-6 Torr vacuum chamber. In the second technique, static friction between a three-millimeter radius aluminum bullet coated with a material of interest and a flat substrate was measured in a approximately 5*10-10 Torr ultra-high vacuum (UHV) chamber. The results show that the coefficient of friction, mu , between silicon and silicon compound contacts in vacuum is in the range 0.2 to 0.7. The coefficients of friction between silicon dioxide/silicon dioxide and silicon dioxide/silicon contacts increase by 55% to 157% with increased exposure to humidity. Additionally, friction between similar materials behaves differently than that between dissimilar materials.

Modeling of the electromechanical performance of piezoelectric laminated microactuators

Piezoelectric microactuators may be considered as multilayer laminated composites consisting of alternating layers of piezoelectric and non-piezoelectric materials. A generalized analytical formulation of mechanical deformations as a function of applied electric field is derived for an arbitrary lay-up of such laminates. Based on the generalized theory developed, specific formulae for the electromechanical performance of two cantilever microactuator lay-up geometries. including a single piezo/elastic laminate and a bimorph, are derived. In the modeling of the electromechanical performance of piezoelectric microactuators, the present model incorporates both d211 and d222. Furthermore, the elastic properties of both the piezoelectric and the elastic materials have been considered. The developed model is evaluated based on a comparison with results from experiment.

Outer-rotor polysilicon wobble micromotors

This paper reports the development of a new class of polysilicon surface micromachined wobble micromotors in which the rotor outer radius is exposed for mechanical coupling. These motors are fabricated in a three-mask process which results in a flange bearing. The rotor and seater are fabricated from phosphorus-doped polysilicon, while the bearing is made of an insulating material such as polyimide or silicon-rich silicon nitride. Rotor radii of 75 to 125 /spl mu/m and rotor/stator thicknesses of 3 to 5 /spl mu/m are typical. The 5 /spl mu/m-thick micromotors with silicon nitride bearings operate smoothly and reproducibly in room air for at least several months after release. Minimum operating voltages can be as low as 14 V, while maximum rotor speeds have been limited by the power supply to 35 rpm. These micromotors have gear ratios over 300 and motive torques over two orders of magnitude larger than previously reported polysilicon micromotors.

A simple fabrication process for polysilicon side-drive micromotors

A simple fabrication process for rapid prototyping of side-drive polysilicon micromotors has been developed. This process uses three low-pressure chemical vapor depositions and three photolithography steps, and it enables fabrication of new micromotors and flange bearing designs. An important feature of this process is that the rotor, stator, and rotor/stator gap pattern definition is the first photolithography step and is performed over a flat surface. As a result, excellent linewidth resolution is possible for defining the rotor/stator gaps. Conventional wobble and salient-pole micromotors fabricated with this process have been operated for months after release, For wobble micromotors with 3-/spl mu/m-thick rotor/stator polysilicon films, minimum operating voltages have been 25 V across 1.5-/spl mu/m rotor/stator gaps; maximum operating speeds have been 1000 rpm. limited by the power supply. Corresponding salient-pole micromotors have had minimum operating voltages of 50 V; their maximum operating speeds have been near 5000 rpm. >

Micromotor dynamics in lubricating fluids

This paper presents results from step-transient measurements on salient-pole, side-drive, polysilicon micromotors operated in different viscosity silicone lubricating oils under different temperatures and excitation voltages. The step transient has been found to be overdamped, with the rise time decreasing with decreasing viscosity of the oil. The rise time, which is in the range of 1 to 20 msec, is around one to two orders of magnitude larger in oil than that in gaseous environments because of the larger viscous drag contribution. In oil, voltages as low as 12 V across 2.5 mu m rotor/stator gaps are sufficient to operate the micromotors. Micromotor speeds up to 12500 rpm are achieved with 110 V excitations across 1.5 mu m rotor/stator gaps. Based on the experimental findings, a model for micromotor dynamics in lubricating oils is examined in which the Coulomb frictional torque is neglected, since micromotor operation in oil is dominated by viscous drag.

Measurement of micromotor dynamics in lubricating fluids

Results from step transient measurements of salient-pole, side-drive, polysilicon micromotors operated in different viscosity silicon lubricating oils under different temperatures and excitation voltages are presented. A preliminary dynamic model has been used to estimate the electrostatic motive torque, the velocity-dependent viscous drag torque, and friction torque. The results indicate that the total frictional torque may be an order of magnitude smaller in silicon oil than in gaseous environments in which no liquid lubricant is used. In oil, voltages as low as 12 V across 2.5- mu m roto/stator gaps have been sufficient to operate the micromotors. Micromotor speeds up to 12500 rpm have been achieved with 110-V excitations across 1.5- mu m rotor/stator gaps.<<ETX>>

Fabrication of polygon mirror microscanner by surface micromachining

This paper describes the fabrication process and preliminary experimental results of a polygon mirror microscanner. The fabrication process is established by the integration of polysilicon surface micromachining and electroless plating of nickel. The scanner structures studied are similar to conventional mechanical polygon mirror scanners but utilize micromotors with diameters up to one millimeter. The micromotors are fabricated by polysilicon surface micromachining. The optical elements are hexagonal nickel reflectors up to 20 micrometers tall fabricated by selective electroless plating of nickel on the rotor of the micromotor. Successful fabrication of the micromotors with nickel reflectors on the motor has been demonstrated. Optical measurements have been performed at visible wavelength (633 nm) using multi-mode optical fibers for the source illumination positioned roughly in the plane of the substrate. The scanned, reflected radiation was detected via scattering from the substrate and by a television camera in the plane of the substrate.