Silicate-Based Packaging Materials for Heterogeneous Integration of Microsystems

Heterogeneous integration (HI) represents the cutting edge of microelectronic design and fabrication. However, the limitations of current, organic polymer-based packaging materials (PMs) prevent implementation of this assembly technology. Building on the previous work, an improved silicate-based PM for 3-D HI of microelectronics with a focus on mmWave RF applications was developed. The sodium-free material was shown to be compatible with common microelectronics fabrication processes, such as positive and negative photolithographic patterning, wet etching, and most chemical cleans. It was polishable to a surface roughness of 197 ± 29 Å, thermally stable up to 400 °C, compatible with high vacuum exposure down to 1.5 <inline-formula> <tex-math notation="LaTeX">$\pm \,\,0.4\times 10^{-5}$ </tex-math></inline-formula> torr, and filled gaps with aspect ratios as high as 18.6:1 using certain techniques. The PM could be deposited at thicknesses ranging from single-digit microns up to several millimeters and exhibited excellent adhesion to silicon at thicknesses below <inline-formula> <tex-math notation="LaTeX">$\sim 30~\mu \text{m}$ </tex-math></inline-formula>. The coefficient of thermal expansion (CTE) was 2.968 ± 0.054 ppm/°C from 50 °C to 400 °C. The dielectric constant and loss tangent were ~4 and ~0.0002 at 110 GHz. Fabrication of coplanar waveguides and reconstructed wafer prototypes demonstrated the potential to make RF devices and enable HI.