Low-Pressure Chemical Vapor Deposition of Borosilicate Glasses and their Application to Wafer Bonding
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Low-Pressure Chemical Vapor Deposition of Borosilicate Glasses and their Application to Wafer Bonding Darren M. Hansen, Peter D. Moran, and T. F. Kuech Department of Chemical Engineering, University of Wisconsin-Madison, 1415 Engineering Drive, Madison, WI 53706, U.S.A. ABSTRACT The essential features of the deposition process and the film properties of borosilicate glasses are presented here as an alternative to pure SiO2 in wafer bonding and compliant substrates. While the deposition of SiO2 is a well-studied system, the deposition of boron-doped films is less understood. The deposition rate of the SiO2 mole fraction in the films was accelerated by the presence of trimethylborate and oxygen and this is associated with an increased adsorption of the tetraethylorthosilicate related precursor in the presence of boranols. Typical deposition conditions result in borosilicate glass films with an r.m.s. roughness of ~0.5 nm as measured by atomic force microscopy. Annealing the films at temperatures above 550°C reduces the film roughness via glass reflow. Room temperature bonding of these films was achieved after a 250 W O2 plasma surface treatment. Fourier-transform infrared investigations of the bonded interface revealed the importance of the role of surface OH and H2O groups in the bonding of these films. INTRODUCTION Oxide layers have been investigated as a versatile bonding medium for the development of wafer bonding and compliant substrate technologies [1]. To a large degree, these investigations have focused on un-doped SiO2 layers [1]. However, the use of doped oxides such as xB2O3·SiO2 (BSG) or xB2O3·yP2O5·SiO2 (BPSG) provides even greater flexibility in determining the properties of the bonding medium through the ability to control the physical properties such as viscosity or thermal expansion coefficient (TEC) [2]. For very low B2O3 composition BSG, the lowered viscosity has been exploited in integrated circuit technologies to lower the thermal budget associated with planarization and reflow processes. High boron content films (x>0.09) are not stable in a room ambient over an extended time due to the reaction of B2O3 with water vapor to form boric acid [3,4]. This has placed a practical limit on the boron content in films that are exposed to the ambient. When these high boron content films are used in wafer bonding applications, this practical limit is removed since the layer of BSG is now effectively “sandwiched” between two semiconductor layers. The ability to engineer the BSG film properties over a wide range has motivated the study of these layers for wafer bonding and compliant substrates [1,5,6]. We report on the low-pressure chemical vapor deposition of BSG from tetratehylorthosilicate (TEOS) and trimethylborate (TMB). These investigations focus on the reaction mechanism for BSG deposition. The deposition of SiO2 from TEOS is catalyzed by the simultaneous deposition of B2O3 from TMB and O2. We demonstrate that this effect is due to an enhanced adsorption of the TEOS-related precursor [7].
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