Research On MEMS Packaging By Induction Heating And Its Applications

With the development of Micro-Electro-Mechanical-System (MEMS), packaging has been the technology bottleneck to hinder the commercialization of MEMS. In some sense, packaging is not only a manufacturing technology, but also a science, it is necessary to understand the physical essence of materials, process mechanics and thermodynamics and apply them in order to achieve a good packaging.Based on the theory of induction heating, this dissertation places emphasis upon the design, simulation, fabrication technologies and the applications of induction heating for MEMS packaging and wafer bonding. The primary content of the dissertation includes:1)The principle, methods, quality evaluation, design and simulation of wafer bonding have been reviewed. In materials selection and process, some common bonding techniques such as wafer direct bonding (WDB), anodic bonding and eutectic bonding have been analyzed and compared. The effect of post-bonding processes on anodic bonding strength was studied with an in-house anodic bonding system. The results showed that fast cooling rate after bonding would introduce high inner thermal stress in glass, which deteriorated the strength of anodic bonding. Thermal cycle and thermal shock can help to increase the strength. As an application packaging, the packaging design of every kind of MEMS device should be independently, and packaging simulation can help to analyze the design feasibility, optimize and speed up the progress and decrease the cost.2)Owing to its great advantages such as high heating speed, non-contact heating, and selective heating with materials and geometry, induction heating is specially suit to MEMS packaging and bonding. The selection of induction power supply (frequency and power), design of inductor (structure and geometry) and bonding layer (material and size), impedance matching, et al, were carried out so as to optimize the induction heating for packaging. Because electromagnetic field do harm to body, electromagnetic shield is necessary for high frequency induction heating. Infrared thermoscope and temperature indicating paint can be used to measure the temperature of induction heating. The simulation of induction heating is a coupling calculation of electromagnetic field and thermal field.3)Based on the theory of induction heating and above design, global heating and local heating by electromagnetic induction were experimentally discussed. Because electromagnetic field can boost the ion mobility, fast and low temperature anodic bonding can be promised, and high voltage does not exhibit significant influence as resistive heating bonding. At the same time, the principle and process of gold-silicon eutectic bonding were discussed by induction heating. High heating rate can decrease the pollution of gold diffusion into silicon in elevated process, and improve the performances of bonded devices. Moreover, the principles, methods of local heating for packaging were introduced; the design method, bonding process and results of gold-silicon eutectic bonding by induction local heating were discussed in details. The experimental results agreed well with the simulation result of induction local heating by finite element modeling software (ANSYS). In addition, selective induction heating for gold-tin eutectic bonding and anodic bonding were experimented, respectively.4)The results of micro-gyro packaging by local induction heating showed that, due to thermal concentration only in bonding solder layer, thermal effect on the chip in the bottom of the ceramic package was decreased. The temperature test and simulation result agreed with each other. To satisfy with the thermal diffusion of light-emitting diode (LED), copper-tin alloy was chosen to bonding the chip and the heat-sink by induction heating, which improved the performances of LED.5)Three sets of bonding systems such as anodic bonder, high-frequency and radio-frequency(RF) induction heating bonder were self-developed and set up. The process criterions of hermetic MEMS wafer bonding (trial edition) were compiled.In sum, the innovations included in the dissertation are as follow:1)The key technical points in induction heating for MEMS packaging were first and systemically investigated. Based on the theory of the induction heating, the selection of induction power supply, design of inductor and bonding layer, impedance matching, et al, were carried out to optimize the induction heating for packaging. Selective induction heating for micro-gyro and LED packaging has been highlighted, which does not only increase the packaging efficiency and reliability, but also improve the performances of devices.2)Induction heating anodic bonding was first carried out. Because electromagnetic field can boost the ion mobility, low temperature and fast anodic bonding can be promised. Moreover, high voltage does not exhibit significant influence as resistive heating bonding. The principle and process of gold-silicon eutectic bonding were explored by induction heating. High heating rate can decrease the pollution of gold diffusion into silicon in elevated process, which can improve the performances of bonded devices.3)The effect of post-bonding processes on anodic bonding strength was investigated. The results showed that fast cooling rate after bonding will introduce high inner thermal stress in glass, which deteriorated the strength of anodic bonding. Thermal cycle and thermal shock can help to increase the strength.