Research On MEMS Device Design,Packaging Process And Application

Packaging of MEMS (Micro-Electrical-Mechanical-Systems) is an enabling technology for volume production of MEMS devices and systems and the packaging cost generally covers 50-80% of the whole cost of a functional device or system. This dissertation conduct fundamental research into several key issues of hermetic MEMS and optoelectronic devices by integrating theory, numerical simulation, and experiments. Based on the fundamental work conducted, volume production packaging processes have been developed successfully for silicon based micro pressure sensors and gyroscopes. The major innovation points are summarized below. (1) Systematic research has been conducted in this work on the MEMS packaging process for the first time in China and packaging process standards have been established and have been posted on the internet according to the requirement by MOST (Ministry of Science and Technology) of PRC. The research work has been focused on the individual steps of processing, such as cleaning, die attach, wire bonding, and hermetic sealing (capping), and come up with the key issues to ensure the stable and reliable packaging processes. Four die attach models have been established. Effect of voiding in the solder in die attach has been studied for the thermal conducting properties and results show that voiding has significant effect on the thermal conduction of solder joints. When the voids are too large, the thermal contact resistance will increase with increasing void sizes and the chip temperature will also increase rapidly. For instance, using ridge-type wave guide semiconductor laser chip as an example, the effect of the temperature rise due to the voids has been calculated. Effect of the thermal expansion coefficient and the Young's moduli has also been studied for the die attach glue and therefore the polymer based die attach process has been developed. Shear testing has also been conducted and the die attach process has been found to be reliable and consistent. The plasma based cleaning process has been studied by experimental method and the cleaning process by plasma has been established, which successfully enhance the wire bonding strength by using optimal cleaning process parameters. All the above results have laid a solid foundation for the MEMS packaging standard processes for China. (2) According to the hermetic requirements for a high speed Laser diode, complete packaging method and flow chart has been developed. The process steps have been modeled by a novel method called Sequential Process Mechanics Modeling, which was first modeled by Dr. Sheng Liu, and now further developed in this work for the LD hermetic packaging. In this method, model changes, electrical loading histry, die attach materials for many material interfaces, traisient electrical-thermal-deformation -stress coupling has been modeled and nonlinear rate dependent material constitutive models have been implemented into nonlinear finite element code. The results show that the LD chip drifting is slightly bigger than one micron and the temperature rise of LD due to the paralle seam welding is 37℃. This shows the selected process of packaging is reasonable and the model based packaging process and its method is useful for the packaging of other optoelectronic devices. Pressure sensor based on the beam-membrane structure packaging which can resist high temperature of 250℃has been developed and small volume has been proven. However, the yield and the high volume die attach production has been a problem. In this research, four different die attach materials have been chosen. FEM based model has been developed again to model the die attach process and the results have justified the appropriate die attach material and optimal thickness of the solder are the keys in enhancing the yield. (3) This work applies the fundamental principle of piezo-resisitivity and deformation mechanisms of thin membrance and have studied various parameters affecting the performance of pressure sensor. This work has successfully illustrated the design method of silicon based pressure sensor. All the obtained results are important in optimal design of the piezo-resistive based pressure sensors. (4) The work has systematically studied the MEMS pressure sensor packaging processes by both TO-can and isolated media packaging for oil pressure sensor by robust mathematical models, by using visco-plastic constitutive material models. Challenging issues such as zero point drifting, temperature drifting, time drifting, and hystersis have been solved, which have been solved only by a few multinational companies such as Delphi and Motorola. Two standard processes have been developed and two pressure sensors have been developed successfully for both the engine oil pressure and manifold absolute pressure (MAP) sensors. These two products have been tested in the range of -40 to 125℃and the accuracy has been found to be within 0.5% and they have been used bya well known DongFeng Automobile Company in small volume. This technology has been regarded to be the first class in the world and it is critically important to promote the development of MEMS technology in China. (5) By working with colleagues in the Institute of Microsystems of HUST, we developed the first multi-functional vacuum bonder and its specifications are leading in the field in China. By using theory of the gas molecular dynamics, the law governing the motion of the moleculars within the micro vacuum media has been established. The work systematically studies the vacuum reflow process for silicon gyroscope. Optimal process of welding was obtained and realize the vacuum packaging of gyroscope, which is leading in technology in China and only a few leading companies have achieved this. The technology developed will accelerate the commercialization of silicon micro gyroscope into the market. (6) A novel electrical static bonding has been established under our multi-functional vacuum bonder and the results obtained have been compared to the regular electrical static bonding. The results show that the obtained bonding has more uniform bonding, smaller deformation and smaller stress. The shear strength has achieved 14MPa, which can provide good enough quality for a lot of cases. We also combine both the theory and the a three-dimensional (3D) dynamic moirémeasurement for a tiny vacuum cavity for its surface profile, the result of which is believed to be optimal design of micro vacuum cavity.