Plasma Induced Chemical Changes at Silica Surfaces During Pre-Bonding Treatments
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Plasma Induced Chemical Changes at Silica Surfaces During Pre-Bonding Treatments Darren M. Hansen1, C.E. Albaugh2, Peter D. Moran1, and T. F. Kuech1 1 Department of Chemical Engineering, University of Wisconsin-Madison, Madison, WI 53706, U.S.A. 2 Department of Chemistry, Xavier University, Cincinnati, OH 45207, U.S.A. ABSTRACT Plasma-treated and DI H2O rinsed oxide layers are commonly used in wafer bonding applications. Borosilicate glass (BSG) layers deposited by low-pressure chemical vapor deposition (LPCVD) treated with an O2 plasma in reactive ion etching (RIE) mode at 0.6 W/cm2 and rinsed with DI H2O readily bond to GaAs and Si. The chemical role of this pre-bonding treatment was investigated using attenuated total reflection Fourier transform infrared (ATRFTIR) spectroscopy. The peak intensities for both the Si-O and B-O absorbance bands decreased in intensity as a result the plasma treatment consistent with the uniform sputter etching of BSG. The effect of changing the total plasma treatment time was investigated in terms of the total amount of material removed. Polarization-dependent ATR-FTIR revealed that the H2O/OH absorbance bands decreased in peak intensity with the OH groups preferentially oriented perpendicular to the sample surface after the plasma treatment. The subsequent DI H2O rinse restores the water to the surface while changing the surface BSG composition. ATR-FTIR studies suggest that for oxide compositions greater than 10 mole % B2O3, the top 4 nm of B2O3 was removed or leached from the oxide layer during the DI H2O rinse.
INTRODUCTION Wafer bonding often employs oxide layers as versatile bonding media.1 SiO2 grown on Si substrates by thermal oxidation has primarily been employed in these applications.1 Doped or multi-component oxides, however, provide greater flexibility in controlling the physical properties of the bonding medium. For example, alloys of B2O3 and SiO2, forming a borosilicate glass (BSG), permit control over the viscosity of the oxide over many orders of magnitude at a given temperature.2 The potentially low, controlled viscosity of BSG has been used in integrated circuit technologies to lower the thermal budget associated with planarization and reflow processes. Independent of the oxide composition employed, strong bonding requires that the surfaces to be bonded be smooth, particle-free, and have surface H2O/OH groups.3 A hydrophilic oxide surface for wafer bonding can be prepared by either aqueous treatments or dry plasma treatments.3-7 A common pre-bonding treatment that is being investigated as a means of forming strong room temperature bonds is exposure of the surface to an O2 plasma followed by a DI H2O rinse.4-7 Previous studies have shown that bonding with GaAs and borosilicate glass (BSG)/GaAs is improved with this pre-bonding treatment.8 The role of the pre-bonding O2 plasma treatment has been suggested to include the introduction of a surface charge4, the removal of hydrocarbons by etching of the surface6, or the creation of a damaged oxide with unsatisfied surfa
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