利用原子力顯微鏡對材料剛性與阻尼性質之成像

Abstract

為了研究材料表面的奈米尺度特性，原子力顯微鏡是一個常用的工具。文獻已經發表數種方法藉其懸臂之探針觀測材料的剛性。為了研究材料表面的剛性和阻尼特性，本研究模擬懸臂樑在拍打模式時的運動行為，發現探針如果碰撞高阻尼係數的材料表面，會激發懸臂樑的高階振動模態。本研究利用小波分析高頻振動發生的時間，得知懸臂樑在剛撞擊到表面時引發的高階振動模態特別強烈。在實驗中，觀察到當探針在與二氧化矽或玻璃表面保持較近距離振盪時，會間歇地激發高階振動模態。本研究利用兩次掃描的方法消除表面高低起伏的影響，並獲得高階諧波響應之表面成像，對於鍍鉻和鍍鋁薄膜結果顯示本方法可區別不同材質。當第一次掃描使用接觸模式時，在不同材質上所得的諧波響應變化較為明顯且諧波訊號較為穩定。在二氧化矽表面上造成的相位差較在鋁表面上大，而且經過探針與表面的一次碰撞所引起的振盪時間在鋁表面較二氧化矽上長，兩個結果都顯示二氧化矽的阻尼特性較鋁大。

To investigate material surface properties in nanoscale, atomic force microscopy is a useful tool. Several methods using the atomic force microscopy have been developed in the literature for exploiting surface stiffness properties. To obtain the stiffness and damping properties, this study simulated the cantilever vibration in a tapping mode, and discovered that high-order resonances of the cantilever are excited in collision with material of high damping coefficient. Analyzing the occurrence time of the high frequency vibration by using wavelet transforms, high-order resonances are the strongest at the instant of the first collision with samples. In experiments, it is observed that high-order resonances are excited intermittently when the tip vibrates and keeps a closer distance with the SiO2 or glass sample. A two-pass method is used to deduct the influence of topography, so as to obtain images at harmonics. Results show that the proposed method can distinguish different materials. The difference of harmonics signals among difference materials is more evident and harmonics signals are more stable when the first pass uses the contact mode. The phase shift on SiO2 is larger than Al, and the vibration duration which is excited by one tip-sample collision is longer on Al than SiO2. Both results show that the damping is larger on SiO2 than Al.