具有微結構之石英表面聲波感測器之共振頻率數值模擬與分析

Numerical Study of the Resonant Frequency of Micro-structured Quartz SAW Sensors

葉建宏(Chien-hung Yeh)

指導教授 ： 洪銘聰

國立中央大學/工學院/機械工程學系/碩士(2011年)

摘要

表面聲波是由縱波及橫波所組成在固體表面運行的彈性波,利用壓電材料的訊號轉換功能製作指叉電極於基板輸出及輸入端製作指叉電極，以形成表面聲波元件。表面聲波元件由於其高靈敏度、高可靠度、可攜帶性及對溫度的穩定性等，在濾波器、感測器及震盪器皆廣泛被使用。由於表面聲波元件需要使用微製程技術，考慮其製作上的耗時且不易，且大部分的研究是以實驗為主，不易了解細部的物理現象，故本研究以數值模擬分析為主。本研究旨在利用有限元素法(FEM)探討利用微製程蝕刻於石英表面增加微結構物，所造成其表面聲波共振頻率偏移情形。為提升表面聲波元件效益並找出相對較佳的結構增加型態，針對不同表面結構的變化方式，模擬結構物數量、高度變化下的頻率響應情況，並針對結構寬度的變化及不同結構負載前後的變化情況進行分析，以期了解表面聲波的傳輸情形及限制。另外，考慮可能的製程方式，針對蝕刻落差基板表面聲波結構進行模擬，探討蝕刻落差對於靈敏度等的影響；同時，思考不同的增加面積方式，針對濕蝕刻情況和乾蝕刻做比較。最後基於減輕模擬計算量的目的，探討使用等效壓力方式替代負載的可能性。
為了確實擷取表面聲波共振頻率並掌握不同模態下的振動趨勢，本研究採用位移頻率響應及插入損失頻率響應，其中插入損失頻率響應由輸入及輸出端的電場頻率響應比來進一步定義。藉由比較理論分析結果並和模擬結果對照，以期掌握方程式的預測程度並找出預測的合理區域。此外，利用微機電製程實際製作將表面聲波元件，並使用網路分析儀量測其插入損失頻率響應，觀察平面基板在負載前後的頻率偏移情況。
模擬分析結果顯示，不論是增加結構數量或是增加結構物高度，其增加的面積對於分辨負載重量的能力皆能夠線性提升，但其效益的提升皆受微結構物幾何尺寸的限制。由結果判斷增加結構數量在靈敏度的提升上約略優於增加高度，並且在實際製作時有相對不易損毀的優勢。對於在蝕刻落差基板上蝕刻不同數量的微結構，表面聲波特性會因為此高度落差而失去，逐漸變的難以分辨。另外，錐狀結構相較於柱狀結構靈敏度較不顯著。

關鍵字

表面聲波 ； 頻率響應 ； 有限元素法 ； 微質量 ； 感測器 ； 插入損失

並列摘要

Acoustic waves consist of longitudinal and transverse elastic waves that propagate along the surface of solids. Surface acoustic wave (SAW) usually generated on the surface of a piezoelectric material using interdigital electrodes as signal transformers. Due to its high sensitivity, reliability, portability, and capability of the room temperature operation, the surface acoustic wave device has become an important component as filters, sensors, and oscillators. In recent years, SAW devices as high sensitivity gas sensors and biological sensors have been proposed. They usually require microfabrication processes that are time consuming and difficulty to implement. In this study, we perform numerical simulations in order to understand the insight phenomena in the SAW sensors and prevent the cumbersome fabrication processes. The purpose of this study is to investigate the effects of microstructured surfaces on the resonant frequencies of SAW sensors. Different structure geometries of micro-columns are considered in this study to investigate the sensing performance, including the distribution density, the height, and the width of the columns. Besides, considering the feasibility in the SAW structure fabrication, the simulations of stepped substrate are also discussed to accounts for the wet etch and dry etch situations. Finally, a simplified model using equivalent pressure force is compared.
In order to derive the surface acoustic wave frequency in detail and realize the different vibration modes, the displacement frequency response is simulated. To compare with the measurements, the insertion loss frequency response is used, which is the ratio of electric field frequency response at the input IDTs to that at the output IDTs. Meanwhile, the simplified analytical solutions are also applied in comparing with the simulated results to estimate the extent for frequency shift and search for the appropriate sensing regions. To verify the validity of the simulation, experiments that show the frequency shifts between unload and load situations via the Network analyzer are performed. The simulated results agree well with the experimental data.
According to the simulation results, the increased active surface area is linear proportional to the frequency shift; however, this relation can only be applied to certain geometric conditions. The simulations of increasing the number of surface structures or its height are performed. Judging from the results, increasing the number of surface structure is slightly more effective than increasing its height in promoting the sensing performances. The former structure fabrication is much robust as well. Considering the stepped substrate, the features of the surface acoustic wave lose if the step height is too large. Besides, it is justified that the pyramidal structure is less sensitive to the load than columns structure.

並列關鍵字

IDT ； SAW ； resonant frequency ； FEM ； insertion loss ； mass sensor

參考文獻

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