Cu-Sn Wafer Level Bonding for Vacuum Encapsulation of Microbolometers Focal Plane Arrays

摘要

Wafer level packaging represents the key to achieving cost efficiency and high reliability of packaging of a large variety of MEMS in high volume production, while at the same time allowing the reduction of the final size of the device. Accelerometers, gyroscopes, RF switches, RF resonators, microbolometers, micro-mirrors, all require a package to protect the fragile and sensitive parts and are dependent on a well controlled atmosphere in the package in order to function at the required performance. For many of these devices hermetic encapsulation in vacuum is required, which imposes tough challenges on cost reduction and ensuring long term reliability.rnSeveral wafer-scale packaging technologies have proven their ability to secure vacuum and high reliability in demanding applications, such as in the automotive industry for example. Glass-based bonding methods, i.e. anodic bonding and glass frit, have traditionally been used for most of the wafer level packaging of high volume MEMS, such as pressure sensors, accelerometer, gyroscopes. More recently, metal seals have gained more and more attention, due to the lower permeability rate of metals [1] and therefore to their potential for improved hermeticity achievable with smaller seal rings.rnIn this paper we present a process for wafer-level hermetic vacuum encapsulation for fragile MEMS devices based on Cu-Sn solid-liquid interdiffusion bonding (SLID) [2, 3], also known as isothermal solidification (IS) or transient liquid phase bonding (TLP) [4, 5]. The process is an alternative for encapsulation of Sensonor's long wave infraredThe influence of annealing and thermal cycling on the bonding strength, on the microstructure of the interface layer and on the deflection of the two sides of the bonded cavities has been investigated. No significant changes have been found for the dies that withstood the thermal treatment. However, some of the chips delaminated after annealing at 375°C and 400°C, most probably due to interface voids. More investigations are required to confirm the cause and to fUrther optimize the bonding process.