Adhesive Wafer Bonding Using Ultra-Thin Spray-Coated BCB Layers (2020)

Adhesive wafer bonding based on BCB bonding layers is already a proven industrial technology. Besides its material properties (optical, mechanical) interesting for various applications the use of BCB for wafer bonding is bringing significant benefits such as good adhesion (high bond strength), low thermal budget, high tolerance to substrate surface quality (microroughness, scratches, small size particles, topography) and the ability to join practically any types of materials [1]. Typically the BCB bonding layers are deposited on the surface of one or both substrates using spin-coating method for a final layer thickness ranging typically between 0.5 – 1 µm and up to 50 µm [2]. During the bonding process the BCB material is softening and becomes viscous: this behavior can be both useful and not useful for applications. The softening of the BCB layer during bonding allows for compensation of surface defects (surface scratches), incorporation of small particles in the layer or even embedding of significant topography in the bonding layer (electrical interconnects, patterns) [3]. The main negative effect of BCB softening is revealed when the two wafers have to be precisely aligned: the compression of the soft bonding layers introduces also a shift of the two wafers, resulting in a significant misalignment budget. One way of overcoming this issue is the partial crosslinking of the BCB material prior bonding [4]: in this approach the BCB is baked additionally or UV flood exposed after the post-coating soft bake. The bond quality is similar (voids, bond strength) to the standard process but with the major benefit of totally preventing the viscous flow during bonding. In this work we report the fabrication of ultrathin BCB bonding layers ranging from 25 µm to 125 µm by spray-coating deposition. Our study is addressing some important technical and economical items related to adhesive wafer bonding using BCB layers: Technical: the use of a thin BCB layer just with soft baking will still allow for extremely low compression of the layer but with no major misalignment budget development. This way it can be used simply as a low temperature bonding layer with the advantage of maintaining the alignment accuracy. Economical: thinner layers will obviously reduce the amount of precursor material used for coating, while the use of spray coating instead of spin coating will further reduce the amount. In spray coating the layer uniformity is not reached by spin-drying (when also a loss of material occurs) but by the spray coating itself. A proprietary spray coating technology developed in house was used to deposit BCB layers on Si test wafers. This technique facilitates the precise droplet size control and therefore allows for obtaining optimized and homogenous ultra-thin layers. For BCB layers quality assessment we used Variable Angle Spectroscopic Ellipsometry (VASE) – for layer thickness and surface microroughness, Atomic Force Microscopy (AFM) – surface microroughness, layer thickness (by scratch in the layer) and Scanning Electron Microscopy (SEM) – precise layer thickness. The redundant use of the three methods is important for results confirmation as they are using significantly different dimensions range for the sampling: SEM is using nanometer range, AFM in micrometer range while VASE is working in millimeter range. In order to successfully adopt the Omnispray® technology in industrial processes the process reproducibility is a key factor. In order to study this aspect, the wafer-to-wafer as well as lot-to-lot variations were qualified in this work. Bonding quality was assessed using Scanning Acoustic Microscopy (SAM) for voids inspection. References F. Niklaus, G. Stemme, J.-Q. Lu , and R. J. Gutmann, „Adhesive wafer bonding“, J. Appl. Phys., 99 , 031101 (2006) V. Dragoi, T. Glinsner, G. Mittendorfer, B. Wieder, and P. Lindner, „Adhesive wafer bonding for MEMS applications“, SPIE Proc. Vol. 5116, 160 (2003). V. Dragoi, M. Alexe, M. Hamacher, and H. Heidrich, “Microring resonators fabrication by BCB adhesive wafer bonding”, ECS Trans., 16(8), 105 (2008). 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, “Effects of bonding process parameters on wafer-to-wafer alignment accuracy in benzocyclobutene (BCB) dielectric wafer bonding”, MRS Proc. Vol. 863, B10.8.1 (2005).