Atsuo Uchiyama

Silicon Wafer Bonding Mechanism for Silicon-on-Insulator Structures

X-ray diffraction topography and tensile testing are used to study the perfection of bonded interfaces in the sandwich structure where one of the two silicon wafers used had an SiO2 layer applied to it first. The tensile strength and the formation of unbonded areas (voids) were compared to the cases where two bare silicon wafers were used and where both wafers were coated with oxide. There are two mechanisms for wafer bonding: one is for a lower temperature and another is for a higher temperature range. It is concluded that a strong affinity between the two wafers at low temperatures is essential to obtaining tight bonding after a high-temperature anneal. A proper amount of H, OH and H2O on the wafers plays an important role in good chemical bonding below 800°C. Above 1000°C an interaction between adjacent atoms to create covalent bonding and deformation of the SiO2 layer are effective in establishing good bonding.

Fabrication and Bonding Strength of Bonded Silicon-Quartz Wafers

A multiply repeated process of thinning of silicon layer and annealing of the bonded silicon-quartz interface is proposed for tight bonding between a silicon wafer and a quartz wafer which have different thermal expansion coefficients. Silicon layers on quartz with a thickness of 2 µm±0.5 µm were debonded by a high temperature annealing over 650 C, whereas in the case of thinner silicon layers with a thickness under 0.5 µm tensile strengths over 80 MPa (800 kgf/cm2) were obtained in the temperature range from 700 C to 1100 C.

Silicon Wafer-Bonding Process Technology for SOI Structures

The perfections of bonded interfaces with the sandwich structure of a SiO2 layer are studied by the x-ray diffraction topogrcphy and the tensile testing. The tensile strengths of this structure were compared with that for two bare silicon wafers and two wafers with oxide coatings. It is discussed that a proper amount of H, OH and HzO on wafers play an important role on the chemical bonding under 800C and an interaction between adjacent atoms and the deformation of SiO2layer-are effective for tight bonding over 1000C.

Dislocation-Free Silicon on Sapphire by Wafer Bonding

A 300 µm thick silicon wafer with oxide of 200 A in thickness which prevents void formation is sticked with a sapphire wafer at room temperature. To avoid the crack generation of a silicon wafer, a sticked wafer is heated up to 270C for 2 h and then a silicon layer is removed by grinding from 525 µm to 10 µm. To remove grinding damage and to further thin the silicon layer to 3 µm, KOH etching at 80C is used. Finally, to obtain the silicon layer having the thickness range of 0~3 µm, polishing is employed. Although the high density of dislocations is observed in the 2.2 µm thick specimen annealed at 900C for 2 h, however, when the specimen is thinned down to 0.2 µm, silicon layer becomes dislocation-free, as confirmed by double crystal X-ray topography and transmission electron microscope (TEM). The thin oxide between silicon and sapphire plays an important role in the prevention of diffusion of boron as a contaminant at the bonded interface.

Surface Impurities Encapsulated by Silicon Wafer Bonding

Wafer bonding techniques are shown to provide an important addition to methods used for the detection of residual impurities on the surfaces of polished and cleaned silicon wafers. Impurities were encapsulated in the interface made by wafer bonding, and analyzed by SIMS depth profiling. Significant concentrations of H, C, N, O, F and Cl were detected. The concentration of these elements did not change after two hour wafer bonding anneals in the range of 200°C to 800°C. For anneals at 1000°C and above both the diffusion of H and C from the bonded interface, and the aggregation of N and O were observed. It was confirmed by IR absorption and HR-TEM that oxygen in CZ crystals outdiffuses into the bonded interface and produces an SiO2 layer. Low-oxygen FZ wafers were used as a reference comparison. Elements such as F and Cl contained in the chemicals used to clean the wafers remained fixed at the bonded interface for the entire temperature range tested.

Encapsulation of Surface Impurities by Silicon Wafer-Bonding

Residual impurities on cleaned silicon wafers arc encapsulated in the interface made by wafer-bon{ing and analysed by SIMS depth profiling. H. C. N. O. F and Cl with high concentrations are detected and their diffusions with increasing temperature are alio observed. It is confirmed by IR absorption that oxygen in CZ crystals out-diffuses into the bonded interface and produces a SiOz layer. The elements such as F and Cl contained in ttre final cleaning chemicals are detected in the bonded interfaces.