| Si-based microelectromechanical system(MEMS) devices cannot run reliably due to their dissatisfying tribological performance such as failures caused by large adhesion or excessive wear between contacting components. Hydrophobic and anti-wear films that can be coated uniformly and conformally by atomic layer deposition(ALD) on the surface of MEMS devices present great potential and broad prospects in solving the tribological problems. In this study, the tribological performance of the films prepared by ALD was studied and verified in real MEMS devices.ZnO films were deposited on a Si(100) substrate using ALD, and the influence of their nanocrystalline characteristics, crystal size and crystal structure on macro-tribological performance of the ZnO films was studied. The results reveal that, the nanocrystalline characteristics can significantly increase the ductility of the films, which explains why the Zn O films exhibit low friction coefficient and long wear life at the ambient air. Besides, the ZnO films vary in crystal size and crystal structure, and it is revealed that crystal structure, rather than crystal size, is the main factor that influences the friction coefficient of the ZnO films. The ZnO films with(002)-orientated crystals have larger friction coefficient than those with other orientated crystals.Hydrophobic ZnO films with various thicknesses were prepared by ALD on the Si(100) substrate at temperature of 150 °C, and the influence of adhesion force and mechanical property on nanotribological behavior of the ZnO films was investigated by an atomic force microscope(AFM). The results reveal that the root-mean-square roughness of the films increases with the increase of film thickness, which decreases the real contact area between the AFM tip and films and decreases the adhesion force correspondingly. Thus, in adhesion force dominated condition, thick ZnO films possess smaller friction force due to their smaller adhesion force. With the decrease of film thickness, the hardness of film/substrate system increases. For thin Zn O films, most normal load is borne by the film/substrate system. However, the friction force of the ZnO films is mainly caused by the ploughing of the films themselves. Due to the small hardness of the ZnO films, the shear strength of the films is low, which results in the low friction force.Ultra-thin Al2O3 films were deposited on the Si(100) substrate using ALD, and the tribological performance of the Al2O3 films was studied. The results of macroscale friction test reveal that the Al2O3 films possess small friction coefficients. However, due to their poor ductility, Al2O3 films wear out quickly. The Al2O3 films deposited at 150 °C are hydrophobic. Due to the hydrophobic nature, the Al2O3 films possess small adhesion and friction forces. In addition, the results of nanoscale scratching experiment show that ultra-thin Al2O3 films have high mechanical strength. Films with high mechanical strength cannot be plowed easily.ZnS and TiS2 films were deposited on the Si(100) substrate using ALD. The composition of the films was adjusted by changing the pulse time of precursors. The results of macroscale friction test reveal that ZnS and TiS2 films possess high friction coefficients and poor wear resistance. After annealing, the tribological performance of ZnS films increases significantly due to their enhanced adhesion to the Si(100) substrate, while the tribological performance of TiS2 films shows little change.In order to vefify the tribological performance of the films prepared by ALD, ZnO and Al2O3 films were deposited on the surface of the MEMS devices fabricated by bulk silicon technology. It is revealed that ZnO and Al2O3 films can decrease the static friction force of the MEMS devices. The friction coefficient of the bare MEMS devices is 0.47-0.65, while after film deposition, the friction coefficient decreases to 0.4-0.56. |
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