A Study on Electrostatic Actuated RF MEMS Switches with Low Actuation Voltage

伴隨著近幾十年來半導體技術的蓬勃發展，系統單晶化的概念得以實現，低耗能、小體積、低成本的無線通訊電子產品不再是遙不可及的夢想。然而傳統上在射頻前端系統的微波開關大都使用固態電子技術製造的場效電晶體或二極體，其優點為開關狀態切換速度快，但效能會隨著操作頻率的提高而影響其表現。隨著微機電技術的發展，使得微機電微波開關擁有“非常低的插入損失”、“非常高的隔離度”、“近於零的能量損失”及“CMOS製程相容”等特性。相對於傳統電子式開關在無線通訊系統應用上，更能符合未來低耗能、小體積、低成本之需求；然其驅動電壓過高和切換速度過慢等缺點，實為其遲遲未能量產的重要因素。本研究即利用微機電技術與半導體製成相容的特性，希望透過不同於以往的開關機構設計，以降低微機電開關之驅動電壓。基於此目的，提出一新穎靜電式低壓致動器，其為利用雙金屬結構釋放後產生之殘留應力，造成結構彎曲而設計出平行拱形結構，並將之與槓桿樑整合，而成為具有推拉式功能之致動器，兼具低驅動電壓及高隔離度等特性。針對雙層金屬結構因為殘留應力所造成之結構彎曲現象進行理論分析與實驗模擬，並使用輔助模擬軟體設計出相對應的訊號傳輸線，最後將低壓致動器整合至高頻傳輸線，即完成靜電式低壓微機電微波開關之設計。在製程方面，利用表面加工方式研製出平行拱形結構，其頂點翹曲範圍為1.38μm到3.4μm，符合模擬之高隔離度要求。然由於製程上的誤差和不均勻的殘留應力，使得平行拱形結構之頂點翹曲的程度不一；過大的殘留應力，也使得結構在不連續處造成斷裂現象，皆會使得良率大幅下降。最後，本研究針對製程上遇到的問題，提出一些可能的解決方法，做為未來研究的方向。This thesis includes design and fabrication of IC-compatible RF MEMS switch by MEMS technology. Compared to other actuation mechanisms, electrostatic RF MEMS switches have small footprint, extremely low power consumption, high switching speed, and ease of integration with CMOS circuits. However, high actuation voltage of electrostatic driven switches remains an essential issue for commercial applications. To solve this problem, the novel design of low actuation voltage push-pull MEMS switch using residual stress of bi-metallic microstructure is reported in this thesis. Low voltage driving mechanism is realized by taking advantage of residual stress of two bi-metallic parallel arch-like beams, which are mechanically connected with an insulating lever beam. Utilizing the nature of residual stress, a small gap between DC driving pair-electrodes and a large separation between signal line and switching structure can be achieved, resulting in low actuation voltage and high isolation. The primary limitation of this switch is to control the residual stress of microstructure. A non-contact 3D measurement is performed through a WYKO interferometer to evaluate the beam deformation subject to pre-designated residual stresses. It is found that the gap between the apex of each arches and the underneath insulating layer is range from 1.38μm to 3.4μm. The non-uniform stress distribution makes it difficult to fabricate the same apex height of parallel arch-like beams. The stiction problem is observed while releasing the microstructure, which really affects the functionality of the switches. Finally, the suggestions are proposed in the end of this thesis for eliminating these problems.Abstract (English)………………...……………………………………....i Abstract (Chinese)……………………......……………………………...ii Acknowledgment………………………………………………………..iii Table of Content………………………………………………………….v List of Figures……………………………………………………….....viii List of Tables…………………………………………………………….xi Chapter1 Introduction………………………….……………………………1 1.1 Background…………………………………………………….1 1.2 Motivation……………………………………………………...3 1.3 Literature Survey…………………………............…………….6 1.3.1 Electrostatic……………………………………………..6 1.3.2 Thermal………………………………………….……..15 1.3.3 Magnetostatic…………………………………….…….16 1.3.4 Piezoelectric…………………………………………....17 1.4 Organization of this work……………………………………..21 Chapter2 Design of MEMS switches ………………………….………….22 2.1 Preliminary study……………………………………………..22 2.1.1 Principle of electrostatic actuation………………….….22 2.1.2 Spring constant of low-k beams……………………….24 2.1.3 Literature survey of micro-actuators…………………..26 2.2 Topology of low actuation voltage micro-actuator…………...29 2.2.1 Design concept…………………………………….…...29 2.2.2 Theory analysis of bimetallic structure…………….…..32 2.2.3 FEA simulation of arch-like micro-actuator…………...35 2.2.4 Stability analysis of arch-like beam…………….……...39 2.3 Topology of RF transmission line…………………………….42 2.3.1 Comparison of microstrip and CPW……………….…..43 2.3.2 HFSS® simulation of CPW transmission lines………....45 2.4 Equivalent circuit models of DC-contact switches…………...47 2.4.1 Up-state capacitance…………………………………...47 2.4.2 Down-state resistance………………………………….48 2.4.3 Geometry of switches design……………………….….49 Chapter3 Fabrication of MEMS switches……………………….....……...52 3.1 Fabrication of arch-like beams………………………………..52 3.1.1 Layout analysis………………………………………...52 3.1.2 Process flow………………………………….………...53 3.1.3 Fabrication results………………………….…………..55 3.1.3.1 Step coverage……………………………..55 3.1.3.2 Structure release…………………………..59 3.1.3.3 Oxidation of copper………………...…….61 3.2 Fabrication of switches…………...…………………………..62 3.2.1 Layout analysis………………………………………...62 3.2.2 Process flow…………………………….……………...63 3.2.3 Fabrication results………………………….…………..66 3.2.3.1 Different deflection of parallel arch-like beams……………………………….66 3.2.3.2 Residual stress of lever beam…………………..66 3.2.3.3 Release of switches…………………………….67 Chapter4 Testing and measurement…………………………….…………70 4.1 Measurement of micro-actuator………………………………70 4.1.1 Deformation of parallel arch-like beams………………70 4.1.2 Stability of arch-like beams……………………………70 4.1.3 Pull-in voltage measurement…………………………..72 4.1.3.1 Force-deflection relation……………………….73 4.1.4 Structure reliability…………………………………….77 Chapter5 Conclusions and future research……………………….………..81 5.1 Conclusions…………………………………………………...81 5.2 Future research………………………………………………..81 References……………………………………………………………....8