薄膜結構殘留應力的聲彈效應

Abstract

結構聲波的波速受到殘留應力的影響而改變，稱為聲彈效應。本文根據徹體波之聲彈理論及相關的三階彈性常數，建立數值分析程式，探討薄膜層板結構之藍姆波相速度與殘留應力間的聲彈效應。數值例題分析矽、鍺異質結構晶格不相配所引致的殘留應力。異向性薄膜結構之低模態藍姆波相速度受到殘留應力的影響極輕微，實際應用時，應採用較高模態之藍姆波相速度差值作為殘留應力反算的依據。 實驗結合雷射生成超音波與二維傅立葉轉換方法，以刀鋒技術與共焦式法布里派洛光學干涉兩種非接觸量測方法，量測單軸應力負載下之金屬箔片的藍姆波頻散曲線，點或線聚焦之脈衝雷射生成超音波的頻率仍不夠高，不足以觀測箔片上高模態藍姆波相速度受到聲彈效應的變化。

Phase velocity change of the structural acoustic waves due to residual stresses of the elastic media is known as acoustoelastic (AE) effect. This thesis formulates AE effect in thin-film layered structures with the associated third-order elastic constants by modifying the AE theory for bulk acoustic waves. The single crystalline Ge films deposited on the Z-cut silicon substrate are investigated as numerical examples. Significant biaxial residual stresses induced in the films are caused by misfit between different lattices of the heterostructures. The residual stress has weaker influence on the phase velocities of lower modes of Lamb waves in anisotropic thin-film layered media. On the contrary, the measured phase velocity changes of higher modes of Lamb waves become more significant and are capable of determining the residual stress for practical applications. The experiments used laser-induced ultrasound technique and two dimensional Fourier transform method to measure phase velocities of Lamb waves traveling in metal foils under uniaxial tension. Two kinds of non-contact point-detecting methods, knife-edge technique and confocal Fabry-Perot interferometry, are employed to measure the waveforms. It is of difficulty to generate high-frequency ultrasound using either point-focus or line-focus pulsed laser. Experimental evidence indicates that phase velocity change of higher modes of Lamb waves resulted from the AE effect is not able to be observed in the current set-up.