退火溫度對原子層沉積氧化鋅薄膜的摩擦特性之影響

Abstract

微機電(MEMS)系統典型的由多層薄膜與複雜結構堆疊成，在元件內部結構間可能相對運動接觸，其中表面薄膜與結構黏結強度或電路短路會直接影響到元件壽命。氧化鋅薄膜可作為介電材料來改善結構層之間發生電路短路，並提供耐磨性與防止薄膜黏附，然而這部分在文獻中極少有研究著墨。為了達到良好的表面成形性與覆蓋性，本研究利用原子層沉積氧化鋅薄膜在結構表面，再透過奈米壓痕與奈米刮痕技術量測分析，建立完整之薄膜機械特性與摩潤性能資料。 研究中首先透過原子沈積技術成長氧化鋅薄膜，發現沉積溫度200度時，沉積速率約2 Å，薄膜厚度30奈米其表面粗糙度為0.5奈米，經由X光繞射分析才有形成良好結晶特性。成長方向為(002)面的六方晶系結構，其奈米壓痕量測硬度為7.1 GPa、模數為147.7 GPa。後續使用不同的退火熱處理，探討薄膜內部造成熱影響與薄膜內聚力失效現象。結果顯示，硬度由7.1 GPa提高至13.2 GPa、模數由147.7提高至198.9 GPa。而在奈米刮痕量測力量與深度曲線的觀察中，可以釐清臨界附載與薄膜黏附力，並顯示氧化鋅之未退火臨界負載約240 μN，黏附力約685.3 μN；在300 度退火處理臨界負載約 256 μN，黏附力約836.5 μN；在400 度退火處理臨界負載約347 μN，黏附力約857.8 μN。 本研究為了進一步強化氧化鋅薄膜高溫機械特性，在單層氧化鋅上再度上一層二氧化鈦來改善薄膜表面與介面之間的粘附或內聚失效的塑性變形。結果顯示其奈米刮痕量測臨界負載值大幅改善到21.8 μN，300度為22.4 μN，和400度為36 μN。透過穿透式電子顯微鏡與X光繞射分析顯示出此薄層二氧化鈦為非晶態，且二氧化鈦/氧化鋅薄膜在400度退火的條件，可表現出更高的耐磨擦性和較低的磨耗率。 本研究在薄膜表面與內層界面區域，經由探針的連續位移與側向力負載過程來分析氧化鋅(及二氧化鈦/氧化鋅)的特性，結果顯示降低臨界負載與增加退火溫度可以有效降低薄膜的磨耗率。本研究的結果將有助於探討如何以氧化鋅薄膜來提升微機電元件壽命與使用周期，進而評估其可靠度特性。

Micro-electromechanical systems (MEMS) are typically constructed by multilayer films and complex structures, where the internal structure of the device might contact with relative slide motion. The thin film surface , the strength of structural bond, or the short circuit will directly affect the device life. Zinc oxide thin films have been coated on the surface of these devices as a dielectric material to reduce short circuit occurrences, but few studies have focused on the abrasion resistance and adhesion prevention of zinc oxide films. Therefore, this study used atomic layer deposition to form zinc oxide film on the surface of the structure to achieve good surface properties. The nanoindentation and nanoscratch measurements were used to identify the mechanical properties and tribological properties of deposited films. During the atomic layer deposition to grow zinc oxide thin film, we found that with process temperature of 200 °C, the deposition rate was about 2 Å, with a film thickness of 30 nm and surface roughness of 0.5 nm. Through x-ray diffraction analysis, the film showed good crystallinity characteristics, i.e., a growth orientation of (002) plane of the hexagonal crystal structure, nanoindentation hardness of 7.1 GPa, and modulus of 147.7 GPa. In addition, the heat-affected and cohesion failure phenomenon of zinc oxide films annealing was also analyzed at different temperatures ranging from room temperature to 400 °C annealing. The results show that hardness increase from 7.1 GPa (as-deposited) to 13.2 GPa and modulus from 147.7 to 198.9 GPa. Moreover, the force and depth curve from nanoscratch measurement clearly revealed the critical load and film adhesion strength, which demonstrated that the as-deposited zinc oxide has a critical load of about 240 μN and adhesion of about 685.3 μN. At 300 °C annealing, it has a critical load about 256 μN and adhesion force of about 836.5 μN. At 400 °C annealing, it has a critical load of about 347 μN and adhesion of about 857.8 μN. In order to reduce film damage at high temperature, a titanium dioxide layer was applied on zinc oxide to ameliorate plastic deformation of the interface to solve the film surface adhesion or cohesive failure. Results show that the anti-scratch property significantly improved, as critical load increased from 21.8 μN as-deposited, to 22.4 μN at 300 °C annealing, and to 36 μN at 400 °C annealing. Transmission electron microscope and X-ray diffraction analyses revealed that the titanium dioxide was amorphous and that titanium dioxide / zinc oxide thin film annealed at 400 °C exhibited higher abrasion resistance and low wear rate. In summary, through the continuous displacement and lateral force loading process provided by probes of nanoindentation and nanoscratch tests on the interface region of the deposited film and silicon substrate, the reduced critical load and the increase of annealing temperature of zinc oxide( and titanium dioxide / zinc oxide ) thin film can effectively reduce the wear rate and to increase life and reliability of MEMS devices.