利用共振法量測薄膜卜松比之新型微結構

Abstract

本論文提出一創新十字形（Crisscross-shaped）微結構，用於量測薄膜材料之卜松比。微機電系統（Micro-Electro-Mechanical System, MEMS）製造技術裡，面型微細加工（Surface micromachining）指在基材上進行薄膜的沉積、成長與蝕刻，而殘留應力（Residual stress）是影響薄膜行為的主要因素之一。利用薄膜材料設計並製造微元件，與分析其機械效能時，此影響必須謹慎加以考量；而實際分析殘留應力時，薄膜材料的基本機械性質，如楊氏係數和卜松比是不可或缺的重要參數。由於薄膜與塊材的機械性質通常有很大的不同，對製作微結構與微元件而言，能清楚了解並精確掌握材料的機械性質參數，有助於預期其效能，並且提高產品的可靠度和良率。相較於楊氏係數，研究量測薄膜材料卜松比的文獻相當少。本研究中從基本材料力學與振動學出發，設計一創新十字形微結構，作為量測薄膜材料卜松比的測試元（Test key），利用激發微結構產生共振，經由量測其彎曲與扭轉共振頻率，可直接得到材料的卜松比，並結合測試元件與光學量測系統－電子斑點干涉儀（Electronic speckle pattern interferometry, ESPI），建構一套量測流程，期能提供在微機電製程中，量測薄膜材料卜松比的一種可行方法。 此創新微結構設計的最大優點在於將兩種共振模態去耦合（Decoupling），並且不需精準量測薄膜的厚度，易於快速量測共振頻率，達到非破壞量測的目的，因此適用於大量生產時線上即時偵測。本論文將詳細介紹此方法的理論架構、模擬結果，並以絕緣層上覆矽（Silicon on Insulator, SOI）晶圓為基材，說明其設計與製程所需注意事項，及實驗量測與結果。製程中包含面型微細加工與體型微細加工（Bulk micromachining）技術，並大量使用乾、濕蝕刻技巧，當中所累積的蝕刻經驗，可作為將來懸浮微機電元件時之參考。

In this study, a novel crisscross-shaped micro test structure to characterize the Poisson's ratio of thin film materials is proposed. In micro-electro-mechanical-system （MEMS）fabrication technology, the surface micromachining means that the deposition, growth, and etching of thin films are processed on the surface of a substrate. The residual stress is one of the important issues to influence the behavior of thin films. This issue must be considered seriously when designing, fabricating, and analyzing the micro devices used film materials. The basic material properties, such as Young’s modulus and Poisson's ratio, are almost used in the analysis of residual stresses in practice. The material properties of thin films are usually different from those of bulk materials, and it is crucial to determine the mechanical properties of thin films to not only predict the performance but also enhance the reliability and yield of MEMS devices. There are very few investigations regarding the characterization of Poisson's ratio of thin films compared with Young’s modulus. This thesis starts with basic mechanics of materials and vibration analysis, and presents a novel crisscross-shaped test key to facilitate the measurement of the Poisson's ratio of thin films. Through exciting the microstructure and measuring the bending and torsional vibration modes, the Poisson's ratio can be determined directly. With the combination of test keys and optical measurement system which is electronic speckle pattern interferometry（ESPI）, they construct a measurement process, and are expected to provide a feasible way to extract the Poisson's ratio of thin film materials. The primary advantage of this design is that it decouples the bending and torsional vibration modes without measuring the thickness of the thin films precisely, so it is easy and fast to measure the resonant frequencies, and to achieve the purpose of nondestructive measurement. This method is feasible for on-line inspection of mass production in MEMS industry. The detail theory and simulation will be explained clearly and the issues of design, fabrication, and experiment results are also proposed by using silicon on insulator（SOI）wafers as the substrates. The bulk and surface micromachining are included in fabrication process, and the rich experiences of using dry and wet etching techniques could be the dependable references for releasing MEMS devices afterward.