Compensation of the Internal Stress Gradient in Ultrananocrystalline Diamond for the Fabrication of Microactuators and T
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COMPENSATION OF THE INTERNAL STRESS GRADIENT IN ULTRANANOCRYSTALLINE DIAMOND FOR THE FABRICATION OF MICROACTUATORS AND TRIBOMETERS ON A CHIP Federico Buja1, Ralu Divan2, Anirudha V. Sumant2, David Czaplewski2, W. Merlijn van Spengen1 1 2
TU Delft, 3ME, MNE, Delft, Nederlands. Center for Nanoscale Material, Argonne National Laboratory, Argonne, IL 60439, U.S.A.
ABSTRACT The excellent tribological performance of ultrananocrystalline diamond (UNCD) makes this material a potential candidate for the fabrication of long endurance micro/nano-electromechanical systems (MEMS/NEMS) that could involve contacting surfaces. In this work, UNCD and nitrogen incorporated UNCD (N-UNCD) microstructures have been produced and investigated, in order to analyze their intrinsic stress component. A solution for stress reduction is proposed: the application of a titanium stress-compensation coating seems to be an optimum route to obtain flat, free-standing N-UNCD films. INTRODUCTION Silicon based micro-machines encounter high friction and wear during sliding motion due to the poor mechanical and tribological properties of silicon. The idea of fabricating tribometers on a chip is useful to investigate the behavior of materials under different sliding conditions, and decide whether or not a given material is a good candidate for the production of successful MEMS with sliding or contacting surfaces. In this paper, the fabrication of micro-devices, using UNCD as a structural material is reported. The main attention at this stage is mostly directed to the fabrication of diagnostic structures that can reveal the stress components that affect the material. With the development of micromachining, the determination of residual stress is made on the same dimensional scale as the film of interest, and can be measured with high precision. It has been seen that there is a noticeable difference between the intrinsic stress in UNCD and that of N-UNCD. In fact, the stress gradient that arises from the mismatch in substrate/layer temperature and grain sizes distribution, shows different values for the two different films, being much larger for N-UNCD. The incorporation of nitrogen, that preferentially ends up at the grain boundaries of the UNCD, demonstrated to have electrical conductivity of ~260 Ωcm-1 [1], and it is essential to fabricate moving micromechanical devices with electrostatic and thermo-electro actuation. The exact cause of the higher stress in this material is unknown, but probably the change of grain boundary width, along with increased sp2 phase, plays a major role [2]. The main step is to evaluate the magnitude of the stress gradient, and find a solution to overcome or balance it, in order to develop devices that can maintain an acceptable degree of flatness after release. The out of plane displacement upon release should be small enough that it doesn’t compromise the interaction between the two sliding test surfaces in a friction experiment. The stress components of the film have been evaluated in situ, monitoring the behavior of the r
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