Characterization of Lift Off Resist during Thermal Compression Wafer Bonding Based on Polydimethylglutarimide (PMGI)

With the emergence of various 3-D integration techniques, wafer bonding technology has experienced renewed interest.  Decades of relentless device scaling leading to today’s state-of-the-art nanotechnology have ultimately focused on the third dimension for stacking of device layers. Temporary bonding is also of interest for the integration of dissimilar materials in device structures, which cannot be achieved by epitaxial growth techniques alone when the lattice mismatch between the dissimilar materials is too large. Recently lift-off resists (LOR) based on polydimethylglutarimide (PMGI) have been found to be especially suitable for film exfoliation and layer transfer of 2D materials by temporary bonding [1]. For the experimental preparation the 2-inch diameter Si wafers were cleaned with Standard Clean 1 (H₂O:NH₄OH:H₂O₂=5:1:1) before spin coating the Si wafers with polydimethylglutarimide LOR resist at an optimized spin speed of 2000 rpm. This was followed by a pre-bake process at 160ºC for 5 min in order to evaporate the cyclopentanone solvent with a boiling point of 131ºC. In the last process step two LOR coated Si wafers were bonded face to face via the LOR resist in an EVG®501 Wafer Bonding System in a vacuum of 10^-3 Pa and at a temperature of 250ºC. The EVG®501 Wafer Bonding System is capable of thermo-compression bonding with up to 10 kN force.

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 (T_g) 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.

References:

[1]  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).