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.

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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.

Abstract:Metal thermocompression bonding is a hermetic wafer-level packaging technology that facilitates vertical integration and shrinks the area used for device sealing. In this paper, Au-Au bonding at 350, 400 and 450 °C has been investigated, bonding wafers with 1 µm Au on top of 200 nm TiW. Test Si laminates with device sealing frames of width 100, 200, and 400 µm were realized. Bond strengths measured by pull tests ranged from 8-102 MPa and showed that the bond strength increased with higher bonding temperatures and decreased with increasing frame width. Effects of eutectic reactions, grain growth in the Au film and stress relaxation causing buckles in the TiW film were most pronounced at 450 °C and negligible at 350 °C. Bond temperature below the Au-Si eutectic temperature 363 °C is recommended. CONFIDENTIAL -FOR REVIEW ONLY JMM-100282.R2Wafer-level Au-Au bonding in the 350-450 °C temperature range Abstract. Metal thermocompression bonding is a hermetic wafer-level packaging technology that facilitates vertical integration and shrinks the area used for device sealing. In this paper, Au-Au bonding at 350, 400 and 450 °C has been investigated, bonding wafers with 1 µm Au on top of 200 nm TiW. Test Si laminates with device sealing frames of width 100, 200, and 400 µm were realized. Bond strengths measured by pull tests ranged from 8-102 MPa and showed that the bond strength increased with higher bonding temperatures and decreased with increasing frame width. Effects of eutectic reactions, grain growth in the Au film and stress relaxation causing buckles in the TiW film were most pronounced at 450 °C and negligible at 350 °C. Bond temperature below the Au-Si eutectic temperature 363 °C is recommended. Submitted to: Journal of Micromechanics and Microengineering IntroductionMicroelectromechanical system (MEMS) technology enables sensitive and reliable devices to be produced at low cost due to the advantages of batch processing. However, packaging of the individual devices can account for more than 70% of the device cost [1]. Wafer-level bonding lowers these costs substantially. Several bonding technologies are used in packaging of commercial MEMS devices. Glass-frit bonding [2-4], anodic bonding [5,6] and fusion bonding [7,8] are well known and widely used techniques for wafer-level packaging and sealing of MEMS devices.Recently, metal thermocompression bonding has found its application as a hermetic waferlevel packaging technology that facilitates vertical integration. Thermocompression bonding, also referred to as diffusion bonding, is a form of solid-state welding. Pressure and heat are applied simultaneously to bring two metal surfaces into close contact. The atoms can then migrate from lattice site to lattice site joining the interface together [9,10]. To enable metal-to-metal contact, the bonding mechanism must deform the two surfaces in contact in order to disrupt any intervening surface films [11].Cu, Al, and Au are the three most commonly applied metals for thermocompression bonding. The lowest proces...

Anodic bonding of glass to aluminium may provide a higher degree of freedom in device design. In this paper, a systematic variation of the bonding parameters for the aluminium-glass bond is presented. Hermetic seals with strengths of 18.0 MPa can be achieved using a 50-100-nm-thick bonding aluminium layer, and bonding at 300-400°C applying a voltage of 1,000-1,500 V for 20 min. With these parameters, bond yields above 95.1% were obtained on 17 wafers. The bonds survived extensive thermal ageing without significant degradation. The possibility of bonding glass to an aluminium layer with buried, electrically isolated conductors underneath is also demonstrated.