13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- 50 100 150 200 250 300 350 400 450 500 550 0 2 4 6 8 10 12 14 Voltage (V) Tensile strength (MPa) 300 325 350 375 400 0 2 4 6 8 10 Temperature(oC) Tensile strength (MPa) 500 1000 1500 2000 2500 3000 0 2 4 6 8 10 12 Tensilestrength (MPa) Aluminum-film Thickness( )A o (a) Voltage (b) Temperature (c) Thickness Figure 9. Bonding tensile strength Figure 9(a) shows the tensile strength increases with the bonding voltage ranged from 150 to 450 V. The specimens in Figure 9(a) were bonded with 700 µm thick Pyrex 7740 glass wafer at 300°C, and their aluminum film is 1300 Å thick. In the process of anodic bonding, the higher bonding voltage means the larger force produced by the electrostatic field across the bonding interface, and brings about much larger and more intimate contact area between the bonding pairs, that is, more chemical bonds generated at the bonding interface and an enhancement of the bonding strength. On the other hand, the higher the bonding voltage is, the more amounts of movable ions will be produced in the glass and then the more anions, mainly O2-, will accumulate in the bonding glass adjacent to the interface of glass/Al, which provides more opportunities to form the chemical bonds between oxygen and aluminum. Unfortunately, a high bonding voltage will introduce a risk of electric breakdown, and the maximum bonding voltage is limited mainly by the breakdown voltage over the depleted layer. The tensile test results tell that the maximum failure loads also increases with the bonding temperature from 325 to 375°C, as shown in Figure 9(b). The specimens used in Figure 9(b) were bonded with 500 µm thick Pyrex 7740 glass wafer, and the thickness of aluminum film is 500 Å. Pyrex 7740 glass has a complicated chemical composition and contains some important alkali elements, which are responsible for the ionic current in the glass during the anodic bonding process. The increasing of bonding temperature gives rise to more amounts of the cations, and correspondingly induces a large number of anions to accumulate near the bonding interface, which also provides more opportunities to produce chemical bonds with aluminum in anode and then improves the bonding strength. But a high bonding temperature not only will lead to the degradation of metal leads and integrated circuits in MEMS device, but also will induce large thermal stress. In the high temperature bonding tests, cracks were found in the glass of some specimens due to thermal stress, as shown in Figure 5. When comparing the tensile failure loads from the specimens which were bonded under the same temperature, voltage, and duration time but with the different thicknesses of aluminum film, we find that their bonding tensile strength increases with the decrease of aluminum film thickness in our tested range, as shown in Figure 9(c). All the tested samples in Figure 9(c) were anodically bonded under 300°C and 400 V. The mean tensile strength increases from 6.6 to 9.9 MPa, while the Al film thickness decreases from 2300 to 950 Å, which indicate a fact that the experimental results show the remarkable size effect. The bonding strength is affected by the thickness of Al film within the tested range of Al thickness.
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