Coating techniques for 3D-packaging applications
Spin-on is the method of choice for the deposition of thin film polymers and photo-resists for standard WLP-processes like wafer bumping and redistribution. Topography and deep via holes are the challenges for the 3-D packaging technologies. For examples the depth and the diameter of TSV (through silicon via) and Si-trenches can be in a range of multiple 10-mu ms being far away for spin-on processes which are needed for lithography of wiring systems and side wall passivation processes. A comparison of different coating techniques has been compared in this paper: For spray-coating the polymer precursors have to be modified by dilution with appropriate solvents. Guidance for the selection of solvents for different material combinations will be discussed. A new technology called diffusion coating by the authors has been introduced in this paper. The process is based on a modified spin-on technique. Highly conformal coatings can be achieved for a wide range of materials. Patents are pending. Electrophoretic coating is a special deposition process using electroplating. Dry-film photo-resist by lamination is the process of choice if only the surface of the wafer has to be structured. The main advantage is that no liquid-based resists may flow into TSV or other cavities.