Evaluation of Thin Dielectric-Glue Wafer-Bonding for Three Dimensional Integrated Circuit-Applications
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Evaluation of Thin Dielectric-Glue Wafer-Bonding for Three Dimensional Integrated Circuit-Applications Y. Kwon, J. Yu, J.J. McMahon, J.-Q. Lu, T.S. Cale, and R.J. Gutmann Focus Center - New York, Rensselaer: Interconnections for Hyperintegration Rensselaer Polytechnic Institute, Troy, New York 12180-3590, [email protected] ABSTRACT The critical adhesion energy of benzocyclobutene (BCB)-bonded wafers is quantitatively investigated with focus on BCB thickness, material stack and thermal cycling. The critical adhesion energy depends linearly on BCB thickness, increasing from 19 J/m2 to 31 J/m2 as the BCB thickness increases from 0.4 µm to 2.6 µm, when bonding silicon wafers coated with plasma enhanced chemical vapor deposited (PECVD) silicon dioxide (SiO2). In thermal cycling performed with 350 and 400 oC peak temperatures, the significant increase in critical adhesion energy at the interface between BCB and PECVD SiO2 during the first thermal cycle is attributed to relaxation of residual stress in the PECVD SiO2 layer. On the other hand, the critical adhesion energy at the interface between BCB and PECVD silicon nitride (SiNx) decreases due to the increase of residual stress in the PECVD SiNx layer during the first thermal cycle. INTRODUCTION Monolithic wafer level three-dimensional (3D) integration is an emerging technology to increase interconnect performance and functionality of integrated circuits (ICs) [1,2]. In our approach, fully processed wafers are aligned and bonded with a dielectric adhesive glue, followed by top-wafer thinning and inter-wafer interconnection [1,2]. We have developed a baseline wafer bonding process using benzocyclobutene (BCB) glue [1-3]. In this paper, emphasis is placed on thin BCB wafer bonding, which is desirable for reducing via aspect ratio of inter-wafer interconnect for 3D ICs. The critical adhesion energies of wafers bonded with various BCB thicknesses are determined by four-point bending. The effects of high temperature thermal cycling on critical adhesion energy at the interfaces between BCB and plasma enhanced chemical vapor deposition (PECVD) silicon dioxide (SiO2), and between BCB and PECVD silicon nitride (SiNx) are evaluated, since both SiO2 and SiNx are commonly used on top of interconnects. EXPERIMENTAL PROCEDURES Material preparation Five sets of 200 mm wafers were used in this work: (a) silicon wafer, (b) glass wafer whose coefficient of thermal expansion (CTE, 3.78 ppm/oC) is matched with that of silicon (CTE, 2.6 ppm/oC), (c) silicon wafer with 2 µm thermal SiO2, (d) silicon wafer with 1 µm PECVD SiO2, and (e) silicon wafer with 170 nm PECVD SiNx. Silicon wafers, glass wafers, and thermally oxidized silicon wafers are obtained from commercial vendors. PECVD SiO2 layers are deposited using SiH4 and N2O at 300 oC in a Plasmatherm 73 (Plasma-Therm Inc., Voorhees, NJ) with a chamber pressure of 0.9 Torr and rf power of 25 W. PECVD SiNx layers are deposited using SiH4/N2/He gases at 300 oC in the Plasmatherm 73 with a chamber pressure of
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0.89 Torr and power o
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