Al-Al thermocompression bonding has been studied using test structures relevant for wafer level sealing of MEMS devices. Si wafers with protruding frame structures were bonded to planar Si wafers, both covered with a sputtered Al film of 1 µm thickness. The varied bonding process variables were the bonding temperature (400 °C, 450 °C and 550 °C) and the bonding force (18, 36 and 60 kN). Frame widths 100 µm, 200 µm, with rounded or sharp frame corners were used. After bonding, laminates were diced into single chips and pull tested. The effect of process and design parameters was studied systematically with respect to dicing yield, bond strength and resulting fractured surfaces. The test structures showed an average strength of 20-50 MPa for bonding at or above 450 °C for all three bonding forces or bonding at 400 °C with 60 kN bond force. The current study indicates that strong Al-Al thermocompression bonds can be achieved either at or above 450 °C bonding temperature for low (18 kN) and medium (36 kN) bond force or by high bond force (60 kN) at 400 °C. The results show that an increased bond force is required to compensate for a reduced bonding temperature for Al-Al thermocompression bonding in the studied temperature regime. HIGHLIGHTS Al-Al thermocompression bonding was demonstrated at a temperature as low as 400 °C. High dicing yield was achieved by applying a high bond force of 60 kN.  The possibility of reducing the bond force to 18 kN was demonstrated. The reduction in bond force required an increase in the bonding temperature to 450 °C.  Cohesive fracture in the bulk silicon below the Al layers was observed to a large extent, indicating good adhesion between Si and Al, and strong Al-Al bonded interfaces.

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.

Hermeticity, reliability and strength of four laminates bonded at different temperatures by Au-Au thermocompression bonding have been investigated. Laminates with a diameter of 150 mm were realized by bonding a wafer containing membrane structures to a Si wafer with patterned bond frames. A bond tool pressure of 2266 mbar was applied for 15 minutes at temperatures ranging from 150-300 • C. The hermetic properties were estimated by membrane deflection measurements applying white-light interferometry after bonding. Reliability was tested by exposing the laminates to a steady-state life test, a thermal shock test, and a moisture resistance test. Bond strength was estimated by pull test measurements. A dicing yield above 90% was obtained for all laminates. Laminates bonded at 200 • C and above had significantly higher hermetic yield than the laminate bonded at 150 • C. No degradation in hermeticity was observed after the reliability tests. The maximum leakage rate (MLR) was estimated from two measurements of membrane deflection executed at two different times and was below 10 −11 mbar • l • s −1 . The average bond strength ranged from 44 to 175 MPa.

Undoped and aluminium (Al)-doped zinc oxide (ZnO) nanorods have been synthesized by electrochemical route. The synthesized materials have been characterized by X-ray diffraction, UV-visible spectrometer and scanning electron microscope. The Al-doped ZnO nanorods have been coated with polyvinyl alcohol. Currentvoltage characteristics have been investigated in dark and under UV-light illumination. Aluminium doping in ZnO increase its electrical conductivity and further polyvinyl alcohol coating on Al-doped ZnO increase UV sensitivity of the material. Response and recovery time of Al-doped ZnO and PVA-coated Al-doped ZnO nanorods have been recorded. PVA-coated Al-doped ZnO nanorods shows very fast response and recovery time of 10 s in comparison to uncoated ZnO (20 min) nanorods.

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Au–Au thermocompression bonding is a versatile technique of high interest for a variety of applications. We have investigated Au–Au bonding using sputter deposited Au films under conditions of low temperature (150–250 °C) and low bonding pressure (∼3 MPa) for short times (15 min). The combination of low temperature and short times is important for applications involving both hermetic sealing and packaging scaling. The initial surface roughness of the Au film was in the 3–5 nm range with peak-to-valley heights of 20–30 nm and a lateral correlation length of ∼400 nm. For samples bonded at 150 °C, the void morphology at the bonded interface was related to the initial surface roughness. The void morphology was different when bonding at the higher temperatures: the void length (along the bonded interface) decreased significantly but the void height (perpendicular to the interface) increased. These results can be understood in terms of a combination of increased surface Au diffusivity and decreased yield stress and elastic modulus with increased bonding temperature.

The residual gas pressure (RGP) in sealed cavities was measured by residual gas analysis (RGA). The cavity sealed by fusion bonding had the lowest RGP of 16 µbar. Cavities sealed by Au-Au thermocompression bonding and plasma bonding had RGPs of 0.18 and 0.22 mbar, respectively. The cavity sealed by Al-Al thermocompression bonding had RGP of 1.3 mbar. Cavities sealed by thermocompression bonding contained 0.16 – 0.21 mbar Ar. The leak rates of the four seal types were estimated by three methods. RGA measurements revealed that the maximum leakage rates were between 10-13 and 10-15 mbar·l·s-1.

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