石墨烯微机电开关设计与制备

DESIGN AND FABRICATION OF GRAPHENE MEMS SWITCHES

曾维军

11749166

硕士

材料工程领域工程

宋学锋

2020-06-02

2020-07-20

哈尔滨工业大学

深圳

射频开关是无线通信系统的常用器件。快速发展的微电子通信领域对射频开关的射频性能的要求越来越高，固态开关已无法满足需求，微机电技术的发展则有利于应对这样的需求。相比于固态开关，微机电开关虽然具有优异的射频性能，但可靠性却不足，静电驱动的微机电开关还具有吸合电压高的弊端，阻碍着微机电开关的商业化应用。究其原因，是由于Si和金属材料本身的特性，不易进行结构加工以降低吸合电压，并且在反复开关的过程中由于力、电、热的作用容易出现断裂、黏附等失效情况。二维材料石墨烯的出现为解决这一困境提供了一种可能性。一方面，石墨烯在厚度上的优势，容易获得较低的吸合电压。另一方面，石墨烯强度高、表面疏水的特性还利于避免断裂和黏附等失效。石墨烯微机电开关的制备方法仍是当前研究的一个重点，制备关键是实现石墨烯的悬空结构。近年来提出的各种制备方法不可避免地具有工艺复杂、耗时长、精度差等问题。基于以前的研究成果，论文提出一种基于高精度石墨烯转移技术的石墨烯微机电开关的制备方法，对制备过程的关键过程进行研究、成功制备样品并进行性能测试。针对微机电开关的关键参数之一的吸合电压，对桥式和悬臂式两种结构开关的吸合电压进行理论推导，分别讨论载荷分布、作用位置，可动电极等效弹性系数、长度、宽度、厚度、杨氏模量以及电极间隙对吸合电压的影响。根据各种参数对吸合电压的影响规律，从降低微机电开关吸合电压角度出发，提出开关结构设计和材料选择的指导意见。指出选择石墨烯作为开关可动电极材料的合理性。针对制备石墨烯微机电开关的关键问题——如何实现石墨烯悬空结构，研究分析现有的制备方法，比较各种方法的特点，指出这些方法存在的不足。提出石墨烯转移技术的改进方案和石墨烯微机电开关的制备方案。针对石墨烯转移技术的改进方案，设计相应的实验进行可行性验证。分别研究了不同烤胶温度对PVA-PMMA双层膜中PVA溶解性的影响和PVA的引入对PVA-PMMA双层胶中PMMA的电子束曝光的影响。研究结果表明选择PMMA适合的烤胶温度对PVA-PMMA双层膜进行烤胶不会对PVA-PMMA双层膜中PVA的溶解性造成不利影响，由于PVA的引入，在进行电子束曝光时需要适当增加曝光剂量。利用高精度石墨烯转移方法进行石墨烯微机电开关的制备并对制备的样品进行SEM表征，结果表明本方法可以实现石墨烯的自然翘曲，即实现了石墨烯的悬空结构。对制备的开关样品进行初步的性能测试。测试结果表明使用石墨烯作为可动电极可以获得较低的驱动电压，石墨烯电极不容易失效，失效的样品多发生于Au电极。

RF switches is one of commonly used devices in wireless telecommunication systems. With the fast development of micro electronic telecommunication technology, we've seen a surging demand for RF switches with higher RF performances that cannot be met by solid state switches but can be possibly met by fast-developing MEMS technology. However, despite its excellent RF performances compared with its solid state counterpart, MEMS switches is inferior in reliability. Worse is that most electrostatic MEMS switches require relatively higher voltage to be actuated, delaying the commercialization of MEMS switches. By inspection, this problem can be ascribed to properties of materials like Si and metals. These materials cannot be easily shaped to lower pull-in voltages and electrodes made from these materials are commonly suffered from cracking and adhesion due to co-function of force, electricity and heat. Brighter future may be brought from the discovery of two-dimensional material graphene, because thin graphene can keep pull-in voltage low and its high strength and hydrophobic surface can help to avoid cracking and adhesion.One research focus is designing fabrication technique for graphene MEMS switches and the key to fabricate is realizing suspended graphene structure. Fabrication techniques proposed recently feature commonly a complex and time-consuming route resulting poor precision. Here we propose a technique to fabricate graphene MEMS switches based on high precision graphene transfer coming from our previous research work. We study key fabrication process, fabricate samples and test them.In view of pull-in voltage, one of key parameters of MEMS switches, relations are analytically studied for switches featuring fixed-fixed beam and cantilever beam respectively. Impacts of different parameters on pull-in voltage are studied, including load distribution and location, equivalent spring constant, length, width, thickness and Young's modulus of movable electrode and gap between electrodes. Given impacts of these parameters, we provide guiding advice in designing switch structure and choosing materials to realize low pull-in voltage switches. Furthermore, we justify the reason why choose graphene as material for movable electrode.In order to find ways to fabricate suspended graphene, most techniques available so far are perused and analyzed. We compare these techniques and point out their defects and then propose an improved graphene transfer technique and a graphene MEMS switch fabrication procedure.In order to find out feasibility of the improved fabrication procedure, corresponding experiments were designed and conducted. Impacts of baking on solubility of PVA and impacts of introducing PVA on EBL exposure of PMMA in PVA-PMMA double-layered membranes are studied respectively. It is shown that there is no disadvantaged impacts of setting baking temperature according to PMMA on solubility of PVA in PVA-PMMA double-layered membranes and that due to introducing of PVA EBL exposure dose needs to be increased accordingly. Switch samples are fabricated using the high precision transfer technique and characterized via SEM. It is observed that graphene is raised above horizontal plane naturally hence a suspended graphene structure is realized.Preliminary testing is conducted on fabricated switch samples. Testing shows that relatively low pull-in voltages can be realized by using graphene as movable electrode material, graphene electrode features a low failure tendency, and failures are usually observed in Au electrodes.

石墨烯 微机电开关 微加工 转移方法

graphene MEMS switches micro fabrication transfer techniques

中文

联合培养

南方科技大学

曾维军. 石墨烯微机电开关设计与制备[D]. 深圳. 哈尔滨工业大学,2020.