Surface activation for low temperature wafer fusion bonding by radicals produced in an oxygen discharge

Abstract

A new method of exposing silicon/semiconductor wafers to a mixture of radicals is described, in which these species are generated in an oxygen-rich gas discharge confined between a concentric pair of annular mesh electrodes surrounding the wafers. This approach allows the wafer surfaces to be treated without damage from the energetic ions, strong electric fields, and high UV fluxes associated with direct treatment by exposure to gas discharge plasmas. The process is compared with direct oxygen plasma activation for its latitude with respect to treatment duration, effect on wafer surface roughness and bond strength. Wider process latitude and reduced surface roughening are obtained for treatment by radicals compared with direct plasma exposure. Comparative analysis of treated and untreated silicon surfaces by X-ray photoelectron spectroscopy indicate that traces of fluorine present on the wafer surface before treatment are removed with great efficiency by the process.

A Review of Hydrophilic Silicon Wafer Bonding

“…They show no radial variance in bond strength and this empirical result is supported by a theoretical discussion that indicates that the average rate of diffusion of oxygen radicals in the chamber atmosphere is approximately 1 m min −1 . The 2009 follow up paper by J. Kowal et al 31 repeated much of the background. In order to isolate the effect of radical treatment XPS and AFM studies were carried out.…”

“…The 2009 follow up paper by J. Kowal et al 31 repeated much of the background. In order to isolate the effect of radical treatment XPS and AFM studies were carried out.…”

“…9 However it was desirable to actively probe the chemical states of the treated wafer surfaces and the bond interface, in order to experimentally derive bond densities and the like. Secondary Ion Mass Spectrometry 20,29,30 (SIMS) and X-ray photoelectron spectroscopy 14,31,32 (XPS) can be used in these roles. However both techniques can only be used to probe either a surface or a subsurface and so are not suitable for inspecting the bond interface.…”

The effect of atmospheric moisture on crack propagation in the interface between directly bonded silicon wafers

“…Bonded wafer pairs were produced using the oxygen radical activation technique [8]. These were annealed for 1 hour at 200°C after bonding.…”

Fabrication and Characterization of Capacitive Micromachined Ultrasonic Transducers with Low-Temperature Wafer Direct Bonding

“…Nevertheless, conventional wafer bonding technology requires high temperature (>1000 °C) to ensure good bonding quality, which prohibits monolithic integration of CMUTs [ 15 ]. Additionally, a high temperature could result in the degradation of thermally sensitive devices and residual stress in processed silicon wafers with different thermal expansion coefficients [ 16 ]. To avoid the problems mentioned above, low-temperature wafer bonding which is limited to 400 °C is strongly desirable.…”