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

Wafer level packaging is the key to achieving cost efficiency and high reliability of the packaging of MEMS in high volume production. For many MEMS devices hermetic encapsulation in vacuum is required by their functionality, for example for high dynamic range or high sensitivity. In particular, for uncooled IR microbolometers vacuum packaging is critical for minimizing the heat loss by convection from the IR sensitive pixels to the gas residing in the package. As shown by calculations, at sufficiently low pressures the bolometer performance is not affected anymore by the surrounding pressure (below 1Pa for our device). In this paper a process for wafer-level hermetic vacuum encapsulation based on Cu-Sn SLID (solid-liquid interdiffusion) bonding is presented. It is being developed for Sensonor’s long-wave infrared bolometer, but it can also be used for vacuum encapsulation of other MEMS devices containing fragile or sensitive released structures. In the final product, one of the two wafers to be bonded contains the bolometer read out integrated CMOS circuit on which the IR sensitive pixels are integrated by post processing. These pixels are released by etching of a sacrificial oxide layer using anhydrous vapor HF. At this process step, the metal frames for the final encapsulation are already defined on the wafer. The cap wafer that seals the focal plane arrays contains a patterned thin film getter that will trap the residual gases in the bonded cavities and thereby ensures the required vacuum level for the entire life time of the device. The getter material is activated by a thermal annealing step, typically at 350°C. These particular characteristics of the active devices on the wafers impose certain restrictions to the process used for the final vacuum encapsulation step. The bonding temperature must be low enough to be compatible with the CMOS wafer as well as the pixel materials and structure, while at the same time the joint must resist the temperature of the subsequent getter activation. The bonding frames must withstand the etching of the sacrificial layer for the release of the pixels. Due to the presence of the released pixels, no wet chemical treatment of the wafers is allowed prior to bonding. Cu-Sn SLID bonding has been proven to fulfill the requirements related to the temperature and material compatibility, and we have developed a process that does not require pre-bonding cleaning of the metal surfaces. The bonding process relies on intermetallic compounds that form rapidly by interdiffusion of the two metallic layers, one with high (Cu) and one with low (Sn) melting points, when they are brought into intimate contact and at a temperature above the melting point of the latter. After bonding, the interface layer consists of the intermetallic phases Cu6Sn5 and Cu3Sn, the proportion of which depending on the initial ratio of the available Cu and Sn as well as the process temperature and time. By properly choosing the thicknesses of the initial Cu and Sn layers, and the process parameters, it is possible to raise the melting point of the final joint to a temperature significantly above the temperature of the bonding process. This offers the advantages of both low temperature bonding ( 400°C). Conventional Cu-Sn bonding has relied on a Cu-Sn layer stack on one side and a single Cu layer on the other. However, Cu layers that are exposed to ambient atmosphere oxidize easily, forming a barrier to Cu-Sn interdiffusion that inhibits intermetallic growth at the interface. A pre-bonding surface treatment is therefore required to remove any copper oxide from the top surface of the exposed Cu layer. Several wet chemical treatments are very effective for this purpose, but prohibited from the use on the wafer that contains the released pixels. The novelty of the process that we propose relies on the deposition of a Sn layer on both wafers, protecting the Cu layer underneath from oxidizing. The influence of different parameters on the bonding result has been investigated (process temperature and time, temperature at wafers contact, layout of the frames). Wafer level bonding of Cu/Sn to Cu/Sn layers with very good yield has been demonstrated. Figure 1 presents an IR picture of a 150mm wafer-pair bonded in vacuum. When exposed to atmospheric pressure, the two sides of the bonded cavities deflect towards each other (shown by the concentric interference fringes), confirming that the pressure in the cavities is very low. Further analysis of cross sections through the bonded frames reveals formation of Cu-Sn intermetallic layers as expected, as shown in Figure 2. In this example the bottom frame consists of four metal rails. It can be seen that in the regions where the top and bottom rails overlap, all of the Sn has reacted with Cu and formed Cu3Sn. The intermetallic compound between the rails is Cu6Sn5. The strength of the joints was measured using a shear test, which resulted in an average value of 35MPa. No significant change in shear strength was observed after thermal cycling (1000 cycles, -40°C/150°C). Comparing the caps deflection for vacuum bonded dies before and after annealing at 350°C (simulating the process of the getter activation) have not revealed any significant changes. Nor has any obvious change of the intermetallic structure been observed either.