Hydrogel-Mediated Semiconductor Bonding for Photovoltaic Applications
Kodai Kishibe& Katsuaki Tanabe
Kyoto University, Kyoto, Japan

Hydrogel-mediated semiconductor wafer bonding has been demonstrated for low-cost fabrication of high-efficiency lattice-mismatched multijunction solar cells. Wafer direct bonding can be hardly applied in the presence of wafer surface roughness or particles. Therefore, the wafer bonding process has so far needed polished wafer surfaces and been commonly conducted in cleanrooms (particle densities < 10,000/m3) with high operation costs. In this study, we have introduced a thin hydrogel film between wafers. Our hydrogel bonding scheme benefits from high surface roughness and particulate tolerances, optical transmittance, and electrical conductivity by utilizing the hydrogels' versatile properties suitable particularly for photovoltaic applications. We have investigated three types of hydrogels and all of them exhibit sufficient transmittance (higher than 95%), bonding strength (higher than 100 kPa), and conductivity (1.3 ohm cm2) for the fabrication of high-efficiency multijunction solar cells. The thickness of the polyacrylamide film at the bonded interface was about 5 um which is larger than typical diameters of particles in a regular, non-cleanroom environment (< 3 um). Therefore, even in the presence of such particles on wafers, hydrogel films change their own morphology to enclose the particles and result in particulate tolerances. Moreover, the roughness of epitaxially grown semiconductor top surface is generally much smaller than the thickness of such hydrogel films. Therefore, our hydrogel-mediated bonding scheme will also mitigate the smoothness requirements on the surface roughness of epitaxial compound semiconductors to be bonded to fabricate multijunction solar cells.