Self-Aligned Hermetic Packaging Using Eutectic SnPb Solder and Cr/Ni/Cu Metallization Layer

Abstract

This paper reports an easy, low cost, and low temperature hermetic packaging technology utilizing eutectic SnPb solder and Cr/Ni/Cu bonding pad. We investigate the bonding results of silicon-silicon as well as silicon-glass and glass-glass bonding. Most hermetic packaging technologies require a bonding temperature higher than 300°C. Because some devices are sensitive to temperature that decreases their functionalities, two localized heating technology have been proposed. One technology generates heat via built-in-microheaters on the silicon substrate. Another localized heating technology utilizes microwave as a heating source [1]. However, both technologies require high cost and cannot be implemented for mass production. Furthermore, local heating creates a large temperature gradient. The stress causes crack on the substrates, thus limiting the selection of substrate materials. We choose eutectic tin-lead with the melting temperature of 183°C. Metal thin films we choose is also similar to the under bump metallurgy used for flip chip technology. The advantage of solder is its metal property. With a width of a few micrometers, metal can block moisture for over a decade. In addition, solder is known to pertain self-aligning property in flip chip technology. Other kind of solders can also be applied for hermetic packaging as well. Shie et al have tested In-Sn as bonding using the reflow temperature as low as 120 °C [2]. Seong-A Kim et. al have tested Au-Sn solder line at 400°C [3,4]. Due to the difference in melting points, the application of Sn-Pb, Au-Sn and In-Sn can be different. Bonding characteristic of our design is investigated on three different setups: silicon-silicon, silicon-glass, and glass-glass samples. (Fig.1) This experiment consists of three different setups: silicon-to-silicon bonding, silicon-to-glass bonding, and glass-to-glass bonding. These three different setups utilize the same bonding method. The design includes square patterns and circle patterns of 500 μm width as shown in Figure 2. Schematic process flow of sample fabrication is demonstrated in Figure 3. Substrate and cap have identical size with the pattern of square of 1 cm in width or circle of 1 cm in diameter. The bonding pad is composed of three layers of metal from bottom to top: 500 Å of chromium, 2000 Å of nickel and 6000 Å of copper.(Fig.3b) Eutectic SnPb solder is reflowed on square or circle patterns on a hot plate at room ambient.(Fig.3d) Sample pairs are then bonded on a hot plate at 200°C for about 1.5 minutes for silicon-silicon and silicon glass bonding and 3 minutes for glass-glass bonding.(Fig.3e) Before placing the sample pairs on the hot plate, for glass-glass and glass-silicon bonding, we align a pair of chips of matched pattern by visual alignment. For silicon-silicon bonding, we align two chips along the dividing lines. Figure 4a and 4b show a glass and a silicon sample after solder reflow respectively. Sample pairs after bonding process are seen in Figure 5a through 5c. Figures 6a through 6c show the cross-sectional picture of the joint. Figures 6b and 6c are enlarged pictures of left and right side of the joints respectively. The average misalignment is 11.2 μm and 13.6 μm for square and circle samples respectively. Bonding strength of the three setups ranges from 3 MPa to 10 MPa. For leakage rate test, a 3 mm hole in diameter was drilled under the sealed area on the substrate side, followed by connecting a glass pipe to the hole by frit glass. The setup can be pumped down to the order of 10−8 torr.