Effects of Bonding Process Parameters on Wafer-to-Wafer Alignment Accuracy in Benzocyclobutene (BCB) Dielectric Wafer Bo
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Effects of Bonding Process Parameters on Wafer-to-Wafer Alignment Accuracy in Benzocyclobutene (BCB) Dielectric Wafer Bonding F. Niklaus, R.J. Kumar, J.J. McMahon, J. Yu, T. Matthias*, M. Wimplinger**, P. Lindner*, J.-Q. Lu, T.S. Cale, and R.J. Gutmann Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, NY, USA. * EVGroup, Schaerding, Austria. ** EVGroup Inc., Tempe, AZ ABSTRACT Wafer-level three-dimensional (3D) integration is an emerging technology to increase the performance and functionality of integrated circuits (ICs). Aligned wafer-to-wafer bonding with dielectric polymer layers (e.g., benzocyclobutene (BCB)) is a promising approach for manufacturing of 3D ICs, with minimum bonding impact on the wafer-to-wafer alignment accuracy essential. In this paper we investigate the effects of thermal and mechanical bonding parameters on the achievable post-bonding wafer-to-wafer alignment accuracy for polymer wafer bonding with 200 mm diameter wafers. Our baseline wafer bonding process with softbaked BCB (~35% cross-linked) has been modified to use partially cured (~ 43% crosslinked) BCB. The partially cured BCB layer does not reflow during bonding, minimizing the impact of inhomogeneities in BCB reflow under compression and/or slight shear forces at the bonding interface. As a result, the non-uniformity of the BCB layer thickness after wafer bonding is less than 0.5% of the nominal layer thickness and the wafer shift relative to each other during the wafer bonding process is less than 1 µm (average) for 200 mm diameter wafers. The critical adhesion energy of a bonded wafer pair with the partially cured BCB wafer bonding process is similar to that with soft-baked BCB. INTRODUCTION Adhesive wafer bonding has been used for fabrication of three-dimensional integrated circuits (3D-ICs) [1-3], advanced microelectromechanical systems (MEMS) [4, 5], bioMEMS [5, 6] and packaging applications [7, 8]. Wafer-level 3D integration is an emerging technology to increase the performance and functionality of ICs. In one of the attractive technology platforms for manufacturing of 3D-ICs, adhesive wafer bonding is a key processing step [1-3]. In such an application, the dielectric adhesive layer needs to be very uniform, and an extremely precise wafer-to-wafer alignment accuracy (≤ 1 µm) of the bonded wafers is required [1, 9-11]. Previously reported adhesive wafer bond schemes using softbaked benzocyclobutene (BCB) as the dielectric polymer adhesive do not provide sufficient control of the adhesive layer thickness uniformity and post-bond wafer-to-wafer alignment accuracy [12]. Since soft-baked BCB layers reflow during the wafer bonding process, the liquid BCB redistributes at the bond interface and creates non-uniform BCB layer thickness. During the bonding process, the wafers also can shift relative to each other by several micrometers, significantly decreasing the post-bond alignment accuracy and repeatability. Frictional surface structures have been suggested in order to prevent wafers from shifting r
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