Low temperature germanium to silicon direct wafer bonding using free radical exposure

Abstract

A low temperature germanium (Ge) to silicon (Si) wafer bonding method was demonstrated by in situ radical activation bonding in vacuum. In order to gain further insight into the bonding mechanism, the Ge surface chemistry after either oxygen or nitrogen radical activation was analyzed by means of angle-resolved x-ray photoelectron spectroscopy. After low temperature direct bonding of Ge to Si followed by annealing at 200 and 300 degrees C, advanced imaging techniques were used to characterize the bonded interface. (C) 2010 American Institute of Physics. (doi: 10.1063/1.3360201

Comprehensive investigation of Ge–Si bonded interfaces using oxygen radical activation

“…11 Prior to bonding, radical exposed Ge surface showed higher values of O/Ge ratio, which was due to a thicker GeO 2 film compared to the reference Ge sample. 7 In Fig. 1(a), it is shown that the passive film, formed on hydrophilic cleaning, appears to prevent bulk oxidation after the bonded wafers are annealed at 200 and 300 C for 24 h. On the contrary, an oxygen radical exposed passive film allows for bulk oxidation via diffusion of a reactive H 2 O species by means of hydrophilic reaction (Eq.…”

“…In Ref. 7 it was clear that both cleaned and oxygen radical exposed Ge surface are largely hydrolyzed and the spectra are dominated by the hydroxide component prior to bonding. This is consistent with a surface preparation to ensure hydrophilic bonding.…”

“…This XPS data after bonding and anneal corresponds to our previous result that radical activated samples generate a relatively thicker oxide interfaces which results in minimal intrinsic voids due to enhancement of by-product diffusion. 7 Among non-destructive methods to judge the quality of bonding, x-ray topography is probably the most sensitive technique. 14 Similar to x-ray diffraction, synchrotron x-ray topography is a non-destructive method that is ideal for studying high quality crystalline materials.…”

“…7 In that study, the hydrophilic bonding reactions at the Ge-Si interface have been described using Eqs.…”

“…5,6 In our previous report, a low temperature radical activated Ge-Si wafer bonding method was developed, to achieve good bond strength with a low thermal budget and as a result mitigates defect generation common in epitaxial growth. 7 In this paper, we have investigated the transition of the chemical states of Ge as a function of the hydrophilic reaction at the Ge-Si interface formed by wafer bonding and thermal annealing at low temperature (200 and 300 C) using x-ray photoelectron spectroscopy (XPS). Further, the deformation induced by strain at the bonded hetero-interface was detected by synchrotron x-ray topography.…”