SU-8 negative resists designed to produce uniform thick films in a single spin-coating step are widely used in the fabrication of MEMS devices. A potential limitation in using resists of this type in large-scale production is the relatively long processing cycle time, which is determined by the choice of solvent and bake conditions. A wellknown disadvantage of SU-8 is its inability to wet low surface energy substrates which can result in non-uniform coating. In this paper we present the results of the first part of a study aimed at improving SU-8 process capability in which the focus is on solvent replacement. Some of the important trade-offs that should be considered in the selection of a practical solvent are discussed. The results of an investigation that led to the selection of cyclopentanone to replace gamma-butyrolactone are presented, the relative drying rates are compared and the drying mechanism is discussed. This study led to the commercialization of a new family of cyclopentanone based resists, SU-8 2000 (referred to as SU-8 CP in this paper) which shows significantly improved wetting, faster drying for film thicknesses up to about 50 microns and clean edge bead removal without the need for an intermediate bake step.

SU-8 Resists are used widely in the development and fabrication of MEMS devices [1-4]. The resist's images are the product of photochemical and thermal cationic processes and result in vertical sidewalls and high aspect ratio features. These are among SU-8's most desirable attributes. The process, which creates the desirable images, however, generates considerable internal stress, which produces cracked features. It would be beneficial, therefore, to investigate modifications of the SU-8 resist formulation which incorporate materials that will attenuate stress cracking without significant image degradation. Work reported previously by IBM focused on compositions containing SU-8 targeted for applications in which lithographic performance could be sacrificed for high flexibility [5]. In this paper we present our results of studies directed toward the selection of materials compatible with SU-8, which will significantly reduce cracking and also retain high aspect ratios images and vertical sidewalls. To this end, we examined mono and polyfunctional epoxides from the following chemical classes; siloxanes, urethanes, cycloaliphatics, a bicycloaliphatic copolymer with phenol, bisphenol-A derivatives, a long chain aliphatic polyol and some long chain aliphatic esters. This work allowed us to identify compositions having improved crack resistance and adhesion over standard SU-8 resist, without sacrificing lithographic performance, which may provide benefits for applications such as a dielectric barrier.

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As Wafer Level Fan-Out Packaging (WLFO) designs continue to evolve, higher pattern densities for Cu lines and interconnects continue to increase while thickness continues to decrease. As Cu densities increase, the patterning resolution of the RDL dielectric needs to shrink to allow increased bump density. The higher Cu density in turn requires enhanced dielectric performance to minimize Cu migration, whilst utilizing lower temperature and shorter cure times which result in lower levels of wafer warpage. Minimizing mechanical stress, continues to be a critical function of the RDL dielectric. Warpage leads to poor yields, distorting the planarity of the package and ultimately leading to stress induced failure from cracking and delamination. Reduction of the thermal budget is the primary means of reducing mechanical stress in WLFO designs. The amount of Cu in the package continues to increase. Differences in thermal expansions of such and the dielectric increase with temperature. Further, conventional solder reflow processes and set points will continue to be used, therefore the RDL dielectric must continue to be thermally and mechanically stable but capable of being cured at lower temperatures, 180–200 °C to minimize overall mechanical stresses from thermal expansion. We present a novel polyamide based RDL dielectric, KMRD, designed to achieve current and future WLFO design requirements. KMRD is a low temperature curable, aqueous (2.38%TMAH) developable dielectric capable of meeting industry standards for mechanical and electrical requirements, with a high level of reliability while utilizing a single stage cure at 185 °C. KMRD provides clear advantages over incumbent materials, with low temperature cure, improved pattern resolution, wide process latitudes, superior adhesion, chemical compatibility, and cost benefits using standard processing equipment and chemistry.

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