Understanding photoresist - electroplating bath interactions using HPLC methodology

With the rapidly expanding range of design and integration flows required for advanced packaging, both foundries and OSATs alike are evaluating new types of materials for additive manufacturing - this is especially pertinent for both resist materials and galvanic plating baths. This problem is far from new (1), it is a part of the regularly required re-evaluation of process-material interactions (2) for the new packaging integration schemes, driven by changes in design, process flow, and technology requirements. With increasing demands for bath longevity and robustness of plating performance, material vendors are motivated to minimize interactions between the plating bath chemistries and photoresists, defining the plating pattern. In this study, we have evaluated the effect of several factors including - resist type (i.e. positive, negative, general-purpose vs “plating/packaging” type) and process parameters for both resist and bath material (by comparing several resist materials from different manufacturers with varying processing conditions on silicon wafers). We used reverse phase HPLC with UV-detection as a method of choice in this study to compare resist extractables into the galvanic baths. We utilized a copper plating packaging type galvanic bath as a representative bath material. We evaluated semi-quantitatively the extractable via their HPLC signature into the bath to compare resist properties and further attempt to extrapolate resist stability and breakdown magnitude with possible effects on bath life. The utility of the method developed here allows for comparing and quantifying (note - evaluation is semi-quantitative since no standard solutions of leached components exist) extractables via HPLC (high-performance liquid chromatography) allows for a better understanding of resist bath stability “factors”, and can be further expanded into an online method for detecting early signs of bath contamination by a photoresist. It can also be used for aiding lithographers to better pre-assess photoresist capabilities and aid in the material selection for future plating applications.