Photo-Polymer Wafer Bonding for Double Layer Transfer
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Photo-Polymer Wafer Bonding for Double Layer Transfer Vorrada Loryuenyong1, Tim Sands1, 2, and Nathan W. Cheung3 1 Department of Materials Science & Engineering, University of California-Berkeley, Berkeley, CA 94720 2 School of Materials Engineering and School of Electrical & Computer Engineering, Purdue University, W. Lafayette, IN 47907-2036 3 Department of Electrical Engineering and Computer Sciences, University of CaliforniaBerkeley, Berkeley, CA 94720 ABSTRACT Temporary bonding is necessary for the two-stage transfer of thin films from one substrate to another while preserving the original orientation of the film. The process developed in this study uses an SU-8 photo-polymer both as a bonding material and as a delamination material for layer transfer. SU-8 was spun onto an optically transparent handle wafer (e.g., quartz or glass) and then bonded to the layer to be transferred. The processing parameters such as pre-baking temperature and UV exposure dose affect the bonding strength. UV curing of the SU-8 through the backside of the handle wafer could be performed to obtain the ultimate bond strength at low temperatures. A laser pulse from a KrF excimer laser (λ = 248 nm and τ = 38 ns) at a fluence of 50-150 mJ/cm2 was used to delaminate the SU-8 bonding layer to complete the layer transfer. INTRODUCTION The development of layer transfer for integration of dissimilar materials relies greatly on the “paste-and-cut” approach. This approach consists of a film grown on one substrate (the donor substrate) to a receptor substrate, followed by the delamination of the film from the handle substrate. In many applications, the original orientation of the transferred layer must be preserved on the receptor substrate after transfer. A two-stage process known as “double transfer” is, therefore, necessary (Figure 1). In double transfer, the film is first transferred to a temporary receptor or “handle” substrate using a temporary bond, and then the film is bonded to the final substrate (receptor) with a permanent bond. Delamination of the handle substrate completes the transfer process. For successful transfer, the bond strength at the interface between the temporary bond and the handle substrate must be stronger than the bond strength at the interface between the film and the donor substrate during the “cutting” process (γbond > γsubstrate). In the second stage of double transfer, however, the bond strength of the interface between the film and the handle wafer must be lower than the bond strength at the interface between the film and the receptor substrate (γbond < γreceptor). Based on these inequalities, it is clear that choice of the temporary bonding material and engineering of the bonding strength are essential to the success of the double-transfer process. Polymer adhesives are typically used as a temporary bond for double transfer technology. The separation of the temporary bond is often achieved by chemical under-etching, which demands high etching selectivity and can be time-consuming for large pattern sizes
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