Research On Some Basic Issues Of MEMS Packaging Based On Process Mechanics

Micro-Electro-Mechanical-System (MEMS) is a new technology field which has potential applications such as aviation, aerospace, automobile, microelectronics, telecommunications etc. Packaging is one of enabling technologies for volume production of MEMS devices and systems, and the packaging cost generally covers 50-90% of the whole cost of a MEMS device and system. With the development of MEMS, packaging has been the technology bottleneck to hinder the commercialization of MEMS. Reliability of MEMS devices and systems depends on the reliability of MEMS packaging, and the reliability of MEMS packaging need fundamentally understand the packaging materials, packaging precesses and structures within the framework of the process mechanics. Based on the process mechanics framework, this dissertation placed emphasis on some basic issue of packaging materials, key packaging processes, typical device packaging structures, and the primary contents and innovative work presented in the dissertation were summarized as follows:Process mechanics framework was applied for MEMS packaging and the thesis implemented the concept of MEMS packaging process mechanics. Material mechanical properties, thermal-mechanical properties and analysis methods for MEMS packaging process mechanics have been reviewed. It was stressed that reliability of MEMS packaging must be understood within the framework of MEMS packaging process mechanics.In MEMS/Microelectronics packaging processing, the trend of lead-free about the interconnecting materials will be inevitable due to Pb and Pb-containing commpound's toxicity. Temperature property, strain rate property and creep property of three lead-free solder alloys (Sn96.5Ag3.5,Sn96.5Ag3Cu0.5,Sn99.3Cu0.7(Ni)) were obtained which were critical for the process mechanics based reliability analysis and prediction. Compared with the typical Pb-containing solder SnPb, three lead-free solder alloys showed excellent mechanical properties which could substitute the Pb-containing solder in the packaging of some MEMS devices. In terms of the effect of reflow soldering, it was found that the shear strength of Sn96Ag3.5Cu0.5/Ni solder joint was lower with the increase of reflowing time. The thickness of interfacial intermetallic compound (IMC) of SnAgCu/Ni increases significantly after several reflow times and a phenomenon of IMCs spalling from IMC/Ni interface were responsible for the decrease of shear strength of soldering joint.As the typical process of MEMS packaging, die attach and bonding process steps were investigated. The effect of voiding and attach materials on the reliability of die attach was studied by experimental study and building packaging process mechanics model. It was found that the thermal contact resistance increased with increasing void sizes and the chip temperature will also increase rapidly. It was concluded that the choice of attach materials should consider the process mechanics in die attach material and process. In vacuum packaging circumstance, the process of anodic bonding and Au-Si bonding was established and successfully achieved. Using the principle of selection of attach materials and the process of vacuum bonding, the compatible attach material was selected and the glass stacking was successfully achieved in bonding on the back of pressure sensor chip which was based the packaging mechanics simulation, zero point drift and temperature cycle hysteresis of pressure sensor was reduced sharply.The packaging process of vacuum reflowing soldering using in MEMS micro-gyro packaging was built and realized. The process parameters of vacuum reflowing soldering were optimized by analyzing the factors which affacted hermeticity maintenance. It was found that vacuum maintenance was improved after desorption of the gas absorbed by the drying packaging materials and packaging components. After vacuum reflowing soldering, the interface of vacuum sealing of micro-gyro was void-free and delamination-free, having a high welding strength which improved the reliability of micro-gyros.The effect of die attach materials, interface thichness on the sensitivity of SOI pressure sensor was defined with the FEM method based packaging mechanics. The process of die attach used AuSn eutectic solder was realized which had the excellent consistency. In the packaging process of SOI pressure sensor, die attach of AuSn eutecnic avoid the low sensitivity, high hysteresis which was found by the use of epoxy. The die attach of AuSn eutectic enabled the volume production of SOI pressure sensor.