Materials Science & Engineering
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Item PLASMA INTERACTIONS WITH MASKING MATERIALS FOR NANOFABRICATION(2011) Weilnboeck, Florian; Oehrlein, Gottlieb; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Plasma-based transfer of patterns into other materials is a key process for production of nano-scale devices used in micro-electronic technology. With the continuously decreasing feature-size of integrated circuits, manufacturing tolerances are becoming increasingly smaller and complex interactions of plasmas and patterned mask materials require an atomistic understanding to meet future processing tolerances. In this work, we investigated how plasma-material interactions in typical low-k pattern transfer processes depend on individual plasma components and properties of polymeric and metallic masks. First, we studied modifications of 193nm and 248nm photoresist (PR) by plasma ultraviolet/vacuum ultraviolet (UV) radiation, quantifying contributions of plasma radiation to the overall material modifications for direct interaction with plasma. Energetic ions (~125 eV) led to rapid (~3-5 s) formation of a graphitic ion-crust (~1.8 nm) and introduced together with simultaneous UV modifications of the material bulk significantly higher roughness for 193nm PR (~6 nm) than for 248nm PR (~1 nm). During ion-crust formation, 193nm PR softened by chain-scissioning and pendant group detachment in a depth of ~60 nm by UV radiation, while 248nm PR was radiation stable showing surface-close cross-linking (~4 nm). Pretreating 193nm PR with a radiationdominated He plasma and introducing UV modifications before ion-crust formation in the subsequent plasma etch reduces synergistic roughness formation as explained by wrinkling theory. Second, we studied interactions of fluorocarbon (FC) plasmas with Ti and TiN and compared these with organosilicate glass (OSG). Metal hardmasks are expected to provide improved etching selectivity (ES) and low-k damage compared to PR during pattern transfer. Erosion stages and dependencies of etch rates (ER) on FC layer thickness and energy deposition by ions were identified. ES were low (~4-8) in the diffusion-limited regime (thick FC layers) where OSG experienced strong reduction in ER, but high (up to 15) in the chemical sputtering regime (thin FC layers) at low ion energies where removal of Ti etch products was limited. TiN exhibited higher ER and lower ES than Ti due to increased surface reactivity after rapid removal of N. Overall, findings give directions for rational design of masking materials and plasma discharges for future nanofabrication pattern transfer processes.Item INFLUENCE OF POLYMER STRUCTURE ON PLASMA-POLYMER INTERACTIONS IN RESIST MATERIALS(2010) Bruce, Robert Lawson; Oehrlein, Gottlieb S; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The controlled patterning of polymer resists by plasma plays an essential role in the fabrication of integrated circuits and nanostructures. As the dimensions of patterned structures continue to decrease, we require an atomistic understanding underlying the morphological changes that occur during plasma-polymer interactions. In this work, we investigated how plasma surface modifications and the initial polymer structure influenced plasma etch behavior and morphological changes in polymer resists. Using a prototypical argon discharge, we observed polymer modification by ions and vacuum ultraviolet (VUV) radiation from the plasma. A thin, highly dense modified layer was formed at the polymer surface due to ion bombardment. The thickness and physical properties of this ion-damaged layer was independent of polymer structure for the systems examined here. A relationship was observed that strongly suggests that buckling caused by ion-damaged layer formation on a polymer is the origin of roughness that develops during plasma etching. Our results indicate that with knowledge of the mechanical properties of the ion-damaged layer and the polymer being processed, plasma-induced surface roughness can be predicted and the surface morphology calculated. Examining a wide variety of polymer structures, the polymer poly(4-vinylpyridine) (P4VP) was observed to produce extremely smooth surfaces during high-ion energy plasma etching. Our data suggest that VUV crosslinking of P4VP below the ion-damaged layer may prevent wrinkling. We also studied another form of resists, silicon-containing polymers that form a SiO2 etch barrier layer during O2 plasma processing. In this study, we examined whether assisting SiO2 layer formation by adding Si-O bonds to the polymer structure would improve O2 etch behavior and reduce polymer surface roughness. Our results showed that while adding Si-O bonds decreased etch rates and silicon volatilization during O2 plasma exposure, the surface roughness became worse. Enhanced roughening was linked to the decrease in glass transition temperature and elastic modulus as Si-O bonds were added to the polymer structure. For polymers used as resists it is required that the mechanical properties of the ion-damaged layer and the polymer be taken into account to understand their roughening behavior.