Enhanced hydrophobic bond-strength can be achieved by exposing prime grade silicon wafers to ultraviolet (UV) light and heat prior to bonding. The following independent variables were explored: platen temperature, UV exposure time, oxygen containing versus non-oxygen containing (nitrogen only) bonding atmosphere, and annealing temperature. The results suggest exposure to UV can be used as an activation process which removes the passivation of the silicon surface rendering the silicon highly reactive.Exposure of silicon wafers to UV appears to be a promising low-temperature surface activation method.

This paper reports the successful transfer of a thin single-crystalline silicon film to a flexible, transparent polymer substrate. Thin-film silicon on polymer was realized by bonding a silicon-on-insulator (SOI) wafer to a flexible substrate using a spin-on polymer as an adhesive. The SOI wafer was thinned by a grinding operation followed by chemical mechanical polishing (CMP). The SOI was further thinned to the buried oxide using wet etchants. The residual stress in the transferred substrate was investigated by ultraviolet (UV) microRaman spectroscopy and numerical modeling. Both approaches showed that a low level of stress was created at the bonded interface during the layer transfer.

A 4" silicon-on-insulator wafer was bonded to a flexible plastic substrate using indirect bonding to fabricate thin-film silicon on a flexible substrate. The bonded materials were annealed at 150oC for 48 hours. The bonding process created stress in the silicon (~20 MPa) at the interface due to thermal mismatch. Thin-film silicon was created by removing substrate material. The interfacial stress increased as the silicon was thinned. Relaxation of the stress resulted in residual defects in the silicon.

A comparison of Si surface treatments for hydrophobic wafer bonding was conducted. Surface energies were found for wafers with preactivation treatments consisting of UV and heat exposure. Bonded hydrogen-terminated silicon (100) which was activated simultaneously with short-wavelength (158nm) UV/heat and bonded in an inert (normalN2) environment showed the highest bond strengths. The results are consistent with other studies of the dynamics of hydrogen on silicon (100).

New, cost effective fabrication processes for bonding and thinning silicon wafers for improving device performance and creating 3-D structures were investigated. Wafer bonding and de-bonding are key fabrication steps for many applications such as 3-D devices. Double bond transfer and single bond processes to fabricate silicon thin films on permanent substrates were investigated. Polymer and direct bonding techniques were studied to create both permanent and temporary bonds. De-bonding methods utilizing wet chemistry and laser lift-off are discussed.