Molecular Glass Resists for High Resolution Lithographic Patterning

The microelectronics industry is increasingly looking towards high resolution lithographic techniques, such as electron beam (E-Beam) and extreme ultraviolet (EUV) lithography, in order to fabricate sub 100 nm features. Complimentary to this field of research is the design and evaluation of new photoresists to enable such processes. Molecular glass resists are possible candidates for this application. These are small, monodisperse, amorphous organic molecules which result in high resolution patterns with excellent low line edge roughness. Here, we describe our work investigating the lithographic characteristics of C-4 Hydroxylphenyl-calix[4]resorcinarene, a new molecular glass that enables the patterning of sub 100 nm high resolution feature sizes. Summary: The molecular glass resists were synthesized and characterized using conventional wet-chemistry techniques. Small organic molecules have a tendency to crystallize, so these potential resists were designed with bulky side groups to ensure an amorphous character, and a rigid core to ensure a high glass transition temperature (Tg) [1]. It was demonstrated using XRD and DSC that the fully and partially t-Boc protected C-4-hydroxyphenyl-calix[4]resorcinarenes compounds under investigation were amorphous materials with high enough Tg’s to withstand the baking steps in the lithographic process. Lithographic evaluation was performed using equipment in the Cornell Nanoscale Science and Technology Facility (CNF). The molecular resists were dissolved in propylene glycol methyl ether acetate (PGMEA) (~10 wt %), and to this was added triphenylsulfonium perfluoro-1-butanesulfonate as a photoacid generator (PAG) in 5 wt % with respect to the molecular resists. The resist solution was filtered through a 0.2 μm membrane filter three times Molecular Glass Resists for High Resolution Lithographic Patterning CNF Project # 386-90 Principal Investigator(s): Christopher K. Ober and spin-coated onto silicon wafer primed with HMDS vapor at 2000 rpm for 30 seconds. After baking for 120 seconds at 100°C, the resists were patterned using a Leica VB6 E-beam tool operating at 100 kV. Using E-beam lithography positive-tone resist formulations, we have mainly focused on feature sizes below 100 nm. After exposure, the wafers were baked at 80°C for 30 seconds to ensure complete deprotection of the resist, and then developed using 0.026N tetramethylammonium hydroxide. The resulting SEM images showed reproducible feature sizes below 100 nm, and resolution as low as 50 nm was observed under certain conditions. These compounds have higher sensitivity (~180 μC/cm2) than previously reported for molecular glass resists [2]. In addition, low line edge roughness was observed at high resolution, calculated using sUMMIT image analysis software. Currently, we are investigating modified structures of these compounds as target materials for commercial EUVL resists based on t-Boc chemistry or analogs thereof in the near future [3, 4]. References: [1] Yashukiro, S., “Photoand Electroactive Amorphous Molecular Materials-Molecular Design, Synthesis, Reactions, Properties and Applications” Journal of Materials Chemistry, 2005, 15, 75-93. [2] Fujita, M., Ohnishi, Y., Ochiai, Y., Nomura, E., Matsui, S., “Nanometre Scale Resolution of Calixerenes Resists in Electron Beam Lithography”, Journal of Vacuum Science and Technology B, 1996, 14, 6, 4772-4276. [3] Chang, S.W., Felix, N., Yang, D., Ayothi, R., Ober, C.K., “Lithography Based on calix[4]resorcinarenes and Related Molecular Glasses” PMSE Preprints 2005, 92, 131-132. [4] Chang, S.W., Ayothi, R., Bratton, D., Yang, D., Felix, N., Cao H.B., Deng, H., and Ober, C.K, “Sub 50 nm Feature Sizes using Positive Tone Molecular Glass Resists for EUV Lithography” Manuscript in preparation.