Impact of SiO₂on Al–Al thermocompression wafer bonding

Abstract

Al-Al thermocompression bonding suitable for wafer level sealing of MEMS devices has been investigated. This paper presents a comparison of thermocompression bonding of Al films deposited on Si with and without a thermal oxide (SiO 2 film). Laminates of diameter 150 mm containing device sealing frames of width 200 µm were realized. The wafers were bonded by applying a bond force of 36 or 60 kN at bonding temperatures ranging from 300-550 °C for bonding times of 15, 30 or 60 minutes. The effects of these process variations on the quality of the bonded laminates have been studied. The bond quality was estimated by measurements of dicing yield, tensile strength, amount of cohesive fracture in Si and interfacial characterization. The mean bond strength of the tested structures ranged from 18-61 MPa. The laminates with an SiO 2 film had higher dicing yield and bond strength than the laminates without SiO 2 for a 400 °C bonding temperature. The bond strength increased with increasing bonding temperature and bond force. The laminates bonded for 30 and 60 minutes at 400 °C and 60 kN had similar bond strength and amount of cohesive fracture in the bulk silicon, while the laminates bonded for 15 minutes had significantly lower bond strength and amount of cohesive fracture in the bulk silicon.

Interfacial characterization of Al-Al thermocompression bonds

“…[17][18][19] It was found that Al films sputtered on SiO2 can be bonded at lower temperatures than those sputtered directly on Si wafers. 18 We also found that higher quality bonding in terms of dicing yield and bond strength can be achieved by increasing the bonding temperature and/or bond force.…”

“…[17][18][19] It was found that Al films sputtered on SiO2 can be bonded at lower temperatures than those sputtered directly on Si wafers. 18 We also found that higher quality bonding in terms of dicing yield and bond strength can be achieved by increasing the bonding temperature and/or bond force. 17 In the current work, we use transmission electron microscopy (TEM) to investigate bonded interfaces realized with Al deposited on Si or on SiO2 to explore possible reasons leading to differences in dicing yield.…”

Al-Al wafer-level thermocompression bonding applied for MEMS

“…When a layer of SiO2 was present below the Al, the bonding parameters could be reduced to 350 °C, 102 MPa and 15 min without a significant drop in dicing yield or strength [4]. As expected, higher temperatures, higher bond tool pressures or longer bonding times resulted in further increase in dicing yields and/or bond strengths.…”

Low-Temperature Aluminum-Aluminum Wafer Bonding

“…As a calculation shows (4), the elastic energy is too low to influence the bonding between atoms directly, but the applied stress and the resulting strain breaks up the surface layer. The wafers are bonded at high temperatures, usually in the range of 400°C to 550°C (1)(2)(3)(5)(6)(7). In recent experiments, Malik et al were able to reduce the required bonding temperature to about 300°C by depositing the Al metallization layer onto an intermediate SiO2 layer (3).…”

“…The wafers are bonded at high temperatures, usually in the range of 400°C to 550°C (1)(2)(3)(5)(6)(7). In recent experiments, Malik et al were able to reduce the required bonding temperature to about 300°C by depositing the Al metallization layer onto an intermediate SiO2 layer (3). In the references listed in TABLE I, typically temperature of 400°C-550°C led to bonded Al-Al wafers with Al2O3 precipitates present at the bonded interface and still with reasonably good bonding quality.…”