# High Vacuum Packaging of MEMS Devices Containing Heterogeneous Discrete Components
## 1 Introduction  
Due to the attributes of MEMS devices, vacuum packaging for MEMS devices becomes more and more challenging than traditional semiconductor devices. Generally, MEMS devices contain fragile parts that need to be protected from high temperature and humidity for a long time [1]. Besides, some MEMS devices such as infrared bolometers and mechanical oscillators have an enhanced performance in a vacuum environment [2]. The increasing requirements of vacuum packaging have driven the research for robust and high vacuum packaging techniques of MEMS devices. Many technologies have been proposed to satisfy this goal, such as anodic bonding [3, 4], eutectic bonding [5, 6], glass drift bonding [7, 8], laser welding [9], thermal compression bonding [10, 11] and polymer bonding [12, 13]. In our case, the physics package of a chip-scale atomic clock (CSAC) is an optoelectronic system that consists of heterogeneous discrete components such as a vertical cavity surface emitting laser (VCSEL), photodiode (PD), attenuator, MEMS \({}^{87}\)Rb cell and 795 nm quarter-wave plate (QWP). The MEMS \({}^{87}\)Rb cell is fabricated by anodic bonding a top Pyrex glass, a middle silicon substrate with a cavity in the center and a bottom Pyrex glass together at 440 \({}^{\circ}\)C, where \({}^{87}\)Rb atoms, N\({}_{2}\) and Ar are sealed in the cavity. Therefore, the hermetic sealing technique should be performed at a temperature lower than 410 \({}^{\circ}\)C. Anodic bonding is known as field-assisted bonding, which is usually performed at a temperature in the range of 300-500 \({}^{\circ}\)C, under compressive stress and ata voltage of several hundred volts [3]. However, the VCSEL is electrostatic sensitive and can be damaged from electrostatic discharge of less than 200 V. Anodic bonding is avoided here for protection of the VCSEL. The preferred pressure for laser welding under a vacuum is in the range of 0.1-10 kPa [14], which is too high for the physics package. Polymer bonding is used for the hermetic package while it only provides a limited vacuum and can suffer from delamination problems [15]. Glass frit bonding has good topography tolerance and good hermeticity, which is widely used as a hermetic bonding technique in the MEMS field. The bonding temperature is usually between 400 and 500 \({}^{\circ}\)C [16]. To reach a lower process temperature of less than 450 \({}^{\circ}\)C, lead or lead silicate glasses have to be used [17]. The high temperature and use of lead make glass frit bonding unsuitable here. Besides, glass frit bonding has the risk of the glass material flowing into the structures, causing contamination and pad corrosion [17]. Au-Au thermal compression bonding at a lower temperature is achieved by smoothing the Au surface at the atomic level [18], which is very complex. Among those vacuum packaging technologies, the liquidus of eutectic solder is the same as the solidus and thus eutectic bonding is chosen as the vacuum packaging technology of the physics package for its relatively low bonding temperature, outstanding fatigue and creep resistance, superior corrosion resistance and low leakage [19; 20; 21; 22]. [PERSON] et al. studied the oxidation and reduction kinetics of different eutectic solders [23]. [PERSON] et al. developed a hermetic vacuum sealing approach based on indium-lead solder [24]. Their research demonstrates the superiority of eutectic bonding for vacuum packaging of MEMS devices. However, the use of lead does not comply with the restriction of hazardous substances (RoHS) compliance. In this paper, a systematic method for high vacuum packaging of MEMS devices containing heterogeneous discrete components is proposed. The physics package of a CSAC is a typical kind of MEMS device that consists of optical and electronic components. In order to get a high vacuum, the general use of adhesive and epoxy is avoided due to the outgassing of organics and solvent.