Silicon-rim-reinforced Silicon Nitride Micro-scanner and Wafer-level Vacuum Packaging

Abstract

This dissertation presents a novel method to achieve large dynamic range as well as large radius-of-coverture (ROC) of silicon nitride (SiN) microscanner. Optically flat mirror plate in conjunction with a large tilt angle and low driving voltage in SiN microscanner was obtained through the use of a silicon rim and wafer-level vacuum packaging. The silicon rim, attached to the edge of the SiN film, increases the flatness of the mirror plate and enables a vertical comb to be assembled. The vertical comb provides the SiN scanner with a large tilt angle. In addition, vacuum packaging offers low driving voltage of proposed SiN scanner. This work proposed and demonstrated the design and the fabrication process for implementing the silicon-rim-reinforced SiN microscanner. Diameter of mirror plate was 1 mm. 20-100 μm-width silicon rim was tested to investigate optimized width of silicon rim. Thickness of silicon rim and torsional spring was 50 μm. Vertical combs were 10 μm in width, 100 μm in length, and 25 μm in thickness, respectively. Conventional silicon mirror plate with 50-μm-thick mirror plate was fabricated to compare the performance with silicon nitride scanner. All of them were designed to be same resonant frequency of 15 kHz by different spring width. Mechanical and optical properties were measured, and ROC of more than 300 mm (minimum value required) and driving voltage of 40 % decrease from silicon mirror plate were shown. Moreover, large dynamic range of 32 ˚ was measured. This was more than 6 times of that of the previous work, and the best result thus far. This paper not only demonstrated SiN microscanner with large dynamic range, but also achieved the modulation of vacuum-packaged SiN microscanner in wafer-level. Vacuum packaging offers additional shrinkage of the driving voltage owing to the low energy loss due to the viscous flow of the gas molecules. Furthermore, it protects the SiN scanner from the environment and secures the reliability of the performance of the device. This paper proposed and successfully demonstrated the wafer-level vacuum packaging of silicon-rim-reinforced SiN microscanner. Two kinds of fabrication techniques were proposed. Firstly, wafer-level vacuum packaging using glass cap with deep cavity and vertical through-via was proposed. High vacuum level of 1.3 Torr, 1/700 of atmosphere, was demonstrated. Dynamic range was 32 ̊ at 85 Vrms at a resonant frequency of 14.82 kHz. In addition, he silicon nitride microscanner packaging with silicon through-via substrate has proposed, and led to better production yields and simplicity of back-end process including the wiring and PCB attachment. The through-wafer interconnection (TWIn) substrate was adapted to offer advanced packaging process. Fabrication method for deep cavity of TWIn substrate was investigated and optimized for optical transparency of cavity in order to prevent from optical signal distortion. Optical verification of cavity suggests its feasibility as optical window for microscanner. SiN scanner packaging using proposed TWIn substrate is demonstrated. Vacuum level of 3.0 Torr and tilt angle of 18˚at 53.5 Vrms have been measured.. The proposed novel SiN scanner has demonstrated superior properties of dynamic range of six times or more of that of reported works and driving voltage of 41% decrease from conventional silicon microscanner. This work contributes to the significant improvement of SiN microscanner, and opens the door to a high resolution microscanner with a low driving voltage.