Low-Temperature Processing of Electronic Materials Using Uniform Microwave Fields

This article presents the recent literature on the unique and surprising capability of microwave energy to effect chemical reactions at lower temperatures. These lower processing temperatures provide improved thermomechanical properties of organic and inorganic material combinations used in microelectronic assemblies. Variable-frequency microwave (VFM) technology eliminates arcing and produces the large and highly uniform fields necessary for use in high-volume manufacturing. Wafer coatings of polyimide thermoplastics can be fully polymerized 100-200 degrees C lower with VFM than with thermal ovens, which lowers stress and warpage. VFM curing of thermoset adhesives for underfill, bonding, and package assembly can be done well below the usually inviolate "ultimate" glass transition temperature without vitrification and in less time. The unique kinetics and thermodynamics of microwave energy produce high chemical reactivity even after the high-viscosity thermoset gelation point. VFM-cured thermosets are highly uniform with less stress and warpage even on large substrates and panels. Field uniformity and penetration depth produce better adhesion and higher fracture strength. The lower process temperatures inherently produce more stable and reliable final products and allow for less costly low-temperature build materials and temperature-sensitive device designs. The anneal and activation process temperatures of implanted ions in silicon can also be reduced by half with VFM, which results in nearly zero dopant diffusion and high-quality SPE regrowth and low numbers of defect sites.