Characterization of Lift Off Resist during Thermal Compression Wafer Bonding Based on Polydimethylglutarimide (PMGI)
In this study we report on Thermal Gravimetric Analysis (TGA) which is widely applied on thermoplastic polymeric materials in order to elucidate chemical phenomena such as decomposition and mass loss, which also occur during thermal compression bonding of Silicon wafer pairs coated with spin-on LOR resist. From TGA data (Figure 1), the LOR precursor solution lost 93% of its original mass starting at 100˚C because of solvent vaporization. The LOR resist exhibits a glass transition point (Tg) of 180-190 ºC, which aids the wafer bonding process at 250ºC by rendering the resist softer and initiating viscous flow for surface planarization. The final 7% of resist mass was lost started at around 281˚C due to thermal decomposition of PMGI.
The functional groups of polydimethylglutarimide based Lift Off Resist (LOR) layers were studied before wafer bonding and after wafer compression bonding by attenuated total reflection infrared (ATR-IR) spectroscopy (Figure 2). All functional groups before and after bonding via the LOR layers at 250 ºC were identical, except for the disappearance of the two bands at 3188 and 3088 cm-1after thermal compression bonding. The disappearance of these two bands, which are associated with the N–H group, is interpreted as a result of cross linking of polymers. This cross linking of polymer chains is the chemical process that enables the actual wafer bonding across the two joined polydimethylglutarimide resist interfaces.
 T. Matsumae, A. D. Koehler, D. Greenlee, T. J. Anderson, H. Baumgart, G. G. Jernigan, K. D. Hobart and F. J. Kub, ECS Journal of Solid State Science and Technology, 4 (7) P1-P5 (2015).