Nanomechanical Analysis of Polydimethylglutarimide Based Lift Off Resist Used for Temporary Bonding and Film Transfers

Temporary bonding has attracted renewed 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. In this quest polydimethylglutarimide (PMGI) based lift-off (LOR) resist has been found suitable to serve as a temporary sacrificial adhesive for clean film exfoliation, layer transfer and temporary bonding applications [1]. Furthermore polydimethylglutarimide (PMGI) lends itself easily for temporary bonding purposes, because subsequent complete de-bonding can be readily achieved using an n-methyl-2-pyrrolidone (NMP)-based solvent. In this study we focus on the nanomechanical analysis of PMGI using the Nanoindenter XP by Agilent with continuous stiffness measurement (CSM) attachment. This investigation is aiming to elucidate the relationship between bonding parameters, bond strength and the nanomechanical properties of Young’s modulus and hardness of polydimethylglutarimide (PMGI) as a function of prebake temperature and PMGI resist film thickness. The influence of the spin-on process and the pre-bake treatment on the structure and properties of the PMGI adhesive layers is discussed and correlated to the resulting hardness, modulus and final bond strength and the corresponding Thermal Gravimetric Analysis (TGA) tracking the mass loss of the resist as a function of temperature. . For this experiment Si wafers were first cleaned with standard clean 1 (H2O:NH4OH:H2O2=5:1:1). Afterwards the wafers were spin coated at 2000 rpm with LOR 5A (from MicroChem), which is based on PMGI, and subsequently pre-baked in order to partially evaporate the solvent in the resist. The spin speeds were optimized to achieve 1.00 μm and 500 nm resist thickness for this investigation. Thicker lift-off resist thickness is not practical for a load-depth analysis with a Berkovich tip, because the nanoindenter is depth limited. The solvent, which is used with PMGI lift-off resist is cyclopentanone with a boiling point of 131ºC. With a glass transition point (Tg) of 180-190 ºC, we have chosen to conduct a series of prebakes at 50 °C, 70 °C, 90 °C, 110 °C and 130 °C, 150 °C & 160 °C each for 5 min and to benchmark the results against the hardness and bond strength achieved after thermal compression bonding of Silicon wafer pairs via adhesive PMGI layers at 250 C. This series of different thermal treatments influenced the resist mechanical properties, hardness and modulus. Our nanoindentation experimental results are indicated in Figure 1. As expected, the 500 nm films exhibited high hardness values as compared to the 1 µm thick films. Both PMGI lift-off resist films hardness approach the Si substrate hardness. 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). Figure 1