Conformal coating and patterning of 3D structures on wafer level with electrophoretic photoresists

Process technology for electronic packaging and MEMS is being confronted with higher topography on the wafer due to higher complexity of the devices. Especially spin coating of photoresists has severe limitations when dealing with larger three-dimensional features leading to excessive thickness in the bottom and inadequate thickness at the top of these features. One method to overcome the limitations of using spin-coated liquid photoresists on wafer surfaces with extreme topography is the electrophoretic deposition of photoresists. The electrophoretic resist coating process is based on the electrodeposition of either a negative tone or a positive tone photoresist from an aqueous solution onto a conductive seed layer. In the aqueous resist emulsion the ionized polymer forms charged micelles comprising solvent, dye, and photoinitiator molecules. When an electric field is applied micelles migrate by electrophoresis towards the corresponding electrode and form on the surface a self-limiting, insulating film. This electrode is the wafer that is supposed to be coated. The electrophoretic deposition is finished very quickly, usually after 10 sec. The coating experiments were mainly performed in a new ED resist coater, developed by MECO Equipment Engineers B.V. This coater is based on semiautomatic single wafer operation. The resulting layer thickness is mainly affected by the applied voltage and the temperature during the deposition. Final resist layer thicknesses between 3-20 µm are obtained in dependence of the applied voltage, the bath temperature, and the used resist type. Cavities, 300-400 µm deep, obtained on Si wafers either by wet chemical etching or by dry plasma etching were conformally coated with electrophoretic resists from Rohm and Haas Electronic Materials (RHEM). It is shown that especially the top corners of the cavities are well covered with photoresist even after the full lithographic process.