Direct wafer bonding of ALD Al2O3

Atomic layer deposition (ALD) is a method to grow conformal thin films. It is a chemical vapor deposition (CVD) technique using self terminating reactions of two or more reactants. In ALD the thickness control and the thickness uniformity of the films exceeds the capabilities of conventional CVD. Large number of materials can be deposited with ALD, including metals, oxides and nitrides [1].

Direct wafer bonding of hydrophilic silicon is well-known process to fabricate silicon-on-insulator wafers. In the process two smooth and clean silicon wafers, one typically thermally oxidized, are brought into a contact in air or in vacuum. Subsequent annealing (typically at 1100°C) is used to improve the bond strength. In our research the goal has been to combine these two techniques to modify the properties of SOI wafers.

To test the process, we bonded thermally oxidized Si wafers covered with Al2O3 to Si wafers having native SiO2, thermally grown SiO2 or Al2O3 surface. The Al2O3 was deposited on thermally oxidized wafers to avoid blistering of the films. Wafers were bonded in a vacuum and subsequently annealed. For some wafers argon plasma activation was used prior contacting [2]. The bond quality was then evaluated by investigating the amount of voids using scanning acoustic microscope (SAM), measuring the bond strength using the crack opening method [3] and taking closer look on the bonded interface with scanning transmission electron microscope (STEM).

Our experiments showed that it is possible to obtain high quality bonding between Al2O3 and SiO2. The bonding is void-free after annealing at 1100°C (Figure 1)
and high bond strengths can be obtained (Figure 2). STEM images show lattice planes on Al2O3 which indicates that the amorphous aluminium oxide had crystallized during the bond annealing at 900°C (Figure 3, top right and bottom left images). Argon plasma activation seemed to have only small or no influence on the bond strength.