Bonding and contacting of MEMS-structures on wafer level

Effective and cost favorable procedures for hermetical encapsulation of MEMS-structures on wafer level can be fabricated by wafer bonding technologies like the seal glass bonding and by suitable connection technologies routing the electrical potential through the chip structure. Within the paper the parameters of the print and bonding process will be presented and the print process limits will be demonstrated by means of print and bonding results. Screen printing and bonding process development Screen-printing is a method for the selective layer deposition of a specific material on flat surfaces. During the print process the material will be pressed through a textured screen (see Figure 1). For encapsulation of micro mechanical components by seal glass bonding it is necessary to create small structures of seal glass like bond frames at one of two substrates. To achieve this aim two different material types of screen fabric layers were evaluated. The two materials were polyester and stainless steel. The experiments showed that screens made of stainless steel are more suitable for printing smaller (< 200 μm) structures than polyester screens. A second important value to deposit enough glass material on the substrate is the mesh opening of the screen-printing fabric. After evaluating different sizes, a mesh opening of 50 μm proved to be the best solution. Two different seal glasses were used for our experiments. At first the seal glass paste FX11-036 from Ferro Corp. was tested. The greatest thickness, which could be achieved with this glass, was 35 μm after printing. That means this structure is approx. 25 μm high after sealing. Such a printed line shows a typical profile. After printing an edge bulge of approximately 5μm height is visible on the feature edges. This bulge can prevent the direct contact between both substrates during the bonding process. The seal glass needs three temperature-stepped treatments after deposition on the substrate. This procedure is necessary to evaporate all solvents, to drive out organic substances and to melt the glass. The parameters of these thermal treatments are given in figure 2. The annealing steps led to a reduction of surface roughness of glass paste but not to a decrease of the bulge height. To bond with seal glass a temperature is needed, which enables the glass to decrease its viscosity. For FX11036 this is at 450 °C. At this temperature a bond between two wafers can be established. For a good bond it is necessary to apply a pressure. The amount of pressure depends on the contact area between the wafers. Typical values are between 0.5 bar and 5 bar [5]. Fabrication of gas-proof, electric feed throughs A very simple version to fabricate electrical feed-through is the direct print of glass paste over prefabricated conducting paths. Our investigation concerning the electrical resistance and material interactions have shown that there is no influence from the paste material on the aluminums lines used as conductor material. It is also possible to embed the aluminum lines in the paste to create tight bonds. Silicon Direct Bonding (SDB) is one of the most frequently used wafer level integration methods. In further experiments an improved SDB process with embedded access lines is investigated to fabricate a hermetical encapsulation with electrical connection from inside to outside of sensor components. This process may be utilized for the packaging of high quality factor micro sensors such as accelerometers or micro gyros. Figure 1: Structure of a textured stainless steel screen Figure 2: Thermal treatment of FX11-036 and G018-173 after printing 0 50 100 150 200 250 300 350 400 450 500 0 100 200 300 400 time in min te m p er at u re in ° C G018-173 FX11-036 Seal ing 430°C, 10 min Glaz ing 335°C, 15 min Drying 120°C, 7-15 min te m p er at u re in ° C