影响微电子机械系统成品率和可靠性的粘合力和摩擦力

Stiction and Friction in Micro Electro Mechanical Systems

文章评述了影响微电子机械系统 (MEMS)成品率和可靠性的粘合力和摩擦力问题。在用氢氟酸(HF)腐蚀牺牲层、释放多晶Si微结构、干燥时 ,由于Si片表面薄层水的表面张力使两片亲水、间隙在微米量级的Si片粘合起来 ,称为“释放有关粘合”。粘合也发生在封装后器件中 ,当输入信号过冲时 ,由于Si片表面的化学状态将Si片粘合起来 ,称为“使用中粘合”。解决粘合的最好办法是 :在MEMS微结构的表面涂以抗粘合薄膜 ,将成品器件在干燥气氛下封装。介绍了抗粘合薄膜的制备工艺和目前存在的问题。相比之下 ,具有高速运动的MEMS ,其摩擦力问题更为复杂。应用抗粘合薄膜 ,解决了粘合 ,也降低了摩擦力 ,但摩擦依然存在。摩擦带来磨损 ,降低器件可靠性和寿命。寻找既抗粘合、又耐磨的薄膜 。

The stiction and friction influencing the yield and reliability of MEMS are reviewed in this paper.The stiction,called release related stiction,can occur within micron gaps of Si microstuctures during the etching of sacrifacial layer by HF and drying.It can also occur after packaging of the device due to over range of input signals and is called in use stiction.It is concluded that the capillary force of water between two Si hydrophilic surfaces at separation gap of micron is the major contributor to the release related stiction.The origin of in use adhesion is from the chemical state of Si surface.Coating of anti stiction films on the surface of Si microstructures and packaging of devices under dry atmosphere or vacuum are the most important methods to prevent the in MEMS devices from stiction.The preparation technology of anti stiction films and its problems are described.Comparing with stiction,the friction is more complex.The commercially produced accelerometers and digital mirror devices are the devices with no direct contact friction.After using anti stiction films,the stiction is fully prevented,and even the friction is apparently decreased.However,friction still exists in MEMS of direct contact moving parts.The wear resoluted from friction will decrease the reliability and lifetime of the devices.To fabricate the films with anti stiction and wear resitsting ability are the key research projects in high speed moving MEMS devices. [