Formation of Vias in Dielectrics

Formation of vias in polymer dielectric through reactive ion etching. 

To reduce the number of process steps and, ultimately, cost involved in via formation through RIE process, photosensitive dielectrics may be used. This process necessitates just two fabrication steps — exposure and development — as contrasted with up to seven steps in the RIE process. The exposure of dielectric to UV light results in a different solubility for exposed and unexposed areas when placed in a developer solution. Following exposure, developer is sprayed onto the dielectric, and the exposed areas of dielectric dissolve in the solution, leaving vias in the dielectric. Usually, some dielectric residue remains on the metal pads under vias, and a short RIE cleaning process may be added to ensure direct contact of metal in the adjacent layers. 

Because the developer also attacks the exposed dielectric laterally, sloped-wall vias are produced. Consequently, metallization of these vias entail a metal deposition technique that fully covers the sloped wall. In addition, vias in the adjacent layers must be staggered, which decreases the interconnect density. Besides being a simple process, ease of reworking is another advantage of the utilization of a photosensitive polymer dielectric. Being able to easily remove the xmwanted dielectric layer before hard baking avoids the stripping process. 

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