Polymers Main Page > Research Highlights > Detail     Technical Highlights   Profiling the Photoresist-Liquid Interface: Fundamentals for Immersion Lithography and Polymer Dissolution   Recent advancements in the semiconductor industry have resulted in new problems involving the photoresist-liquid interface. For immersion lithography, the water profile within a resist film impacts pattern quality from changes in photoacid generator diffusion or optical transparency. For the development step, where a latent image is realized into the final structure, an improved understanding of photoresist swelling and dissolution mechanisms is needed to address stringent line-edge roughness requirements. Data from neutron reflectivity measurements provide critical insight needed to understand and optimize next-generation photoresists and process strategies.   Bryan D. Vogt and Vivek M. Prabhu   Introduction Polymer thin film photoresists comprise the materials foundation for the production of semiconductor devices with nanoscale dimensions. The extension of optical methods has been problematic due to challenges arising from the implementation of shorter exposure wavelengths. The past focus of the semiconductor industry has been the development of sufficiently transparent photoresist materials for future exposure sources. However, future progress requires depth profile information at the photoresist-liquid interface, due to the emergence of immersion lithography and the increased influence of the photoresist development process on lithographic performance.   Using neutron reflectivity (NR), NIST successfully quantified the profile of water and aqueous base counterions in model photoresist films. NR provides structural information regarding the composition profile normal to the thin film surface with isotopic selectivity between protons and deuterium. Selective deuteration of components in the system allows for the quantification of water or counterion distribution within photoresist films despite the negligible differences in physical density. This data provide critical insight needed to refine models for immersion lithography and polymer dissolution.   Immersion Lithography: Water Profile Recently, immersion lithography has emerged as the key strategy to extend existing optical tools. A liquid, such as water, is placed between the lens and photoresis thin film to enhance resolution. The industry anticipates using immersion lithography for production in 2007 at the 65 nm node. The role of liquids in contact with photoresist films is important; not only for component leaching and contamination, but also due to the detrimental influence of trace levels of water on the reaction and diffusion of photoacid generators. Additionally, a non-uniform water profile within thin films leads to incorrect assumptions regarding the transmission and reflection at the photoresist-anti-reflective coating interface.   Figure 1: Volume fraction of water distributed near the PBOCSt/HMDS treated substrate during immersion, as determined by neutron reflectivity. The excess of water at the substrate reaches a maximum concentration of 17 % by volume. Inset. Dependence of initial film thickness on the total film swelling for two model photoresists in thin and ultrathin films.   Figure 1 shows the measured volume fraction profile of water versus the distance from a trimethylsilane-primed silicon oxide interface. Far from the substrate, the 248 nm photoresist, poly(4-tert-butoxycarbonlyoxystyrene) (PBOCSt), shows bulk water absorption, near 2.5 %. However, significant deviations occur near the interface. An excess of water, up to 17 %, extends 40 Å from the substrate. This enriched interfacial water was previously unknown and unaccounted for.   In contrast, a depletion of water was observed for poly(4-hydroxystyrene) (PHOSt), the developer soluble resist, even though the bulk of the film absorbs 25 %. This depletion indicates the interface cannot accommodate excess water. It appears that the relative hydrophobicity between polymer and substrate controls the amount of interfacial water.   The total film swelling is also observed to be a function of the initial thickness and interfacial water. For thinner resist films the interfacial water dominates the swelling as shown in the inset to Fig. 1 for PBOCSt and PHOSt. For these different resists, the swelling becomes similar for ultrathin films with thickness less than 200 Å. The data are consistent with an interfacial water thickness and content for each film thickness. The interfacial concentration is strongly dependent upon the surface chemistry, but relatively independent of the photoresist material or film thickness.   Dissolution Effects: Aqueous Base Profile The development step of a latent image in an aqueous base contributes significantly to undesirable line-edge roughness (LER). There is a strong need for improved dissolution models incorporating photoresist-developer interactions. A key component is the depth profile of the aqueous base counterion through a photoresist film because it controls the dissolution mechanisms that lead to unacceptable LER. We provided the first direct measurement of the aqueous base distribution (tetramethylammonium hydroxide) (TMAH) within the film. These measurements quantify the extent of developer penetration and the influence of ionization on the response of the photoresist to the developer solution.   We used a zero-average contrast (ZAC) experiment where the neutron scattering length density of a thin film of poly(norbornene hexafluoroisopropanol) (PNBHFA), a model 157 nm photoresist, is matched to the developer solution with a D2O/H2O mixture. As shown in the schematic of Fig. 2, the contrast matched film and solvent (equal color) eliminates neutron contrast at this interface. However, when base (d12-TMAH) enters the film, the reflectivity contrast is enhanced. These changes allow quantification of both film swelling and the base profile through the film.   Figure 2: Schematic of the zero-average neutron reflectivity experiment. The contrast between the film and solvent are equal, only the enrichment of deuterated aqueous base within the film provided the reflectivity enhancement.   The volume fraction of d12-TMA+ is plotted versus distance from the substrate in Fig. 3 for four different equilibrating concentrations of base. The change in the counterion profile illustrates the advancement of the swollen solid front. A key finding was that the film expansion proceeds via base transport throughout the entire film rather than gradually through the film. In addition, a depletion of base was observed at the substrate. The film-solution interfacial width increases with higher base concentration. These measurements show that ionization induced swelling occurs at the dissolution front. At even higher base concentrations, the photoresist film dissolves quickly. Understanding the transition from swelling to rapid dissolution with base concentration will provide guidance into developer-induced roughness.   Figure 3: Volume fraction profiles of d12-TMA+ within the PNBHFA film. The aqueous base profile illustrates the enhanced swelling and content within the thin film with higher equilibrating base concentration.   These base profilometry experiments were followed by a deswelling step by rinsing with pure water. The rinse kinetically traps base within the film where it could contaminate other processes during device fabrication.   NR measurements provide new information about the liquid-photoresist interface needed to improve current models of dissolution. In future work, we will examine the effects of liquid-photoresist interactions on surface roughness with atomic force microscopy measurements of surface features.   For More Information on this Topic   C. Soles, M. Wang, C. Wang, R. Jones, W. Wu, E. Lin (Polymers Division, NIST); S. Satija (NCNR, NIST); D. Goldfarb, M. Angelopoulos (IBM T.J. Watson Research Center); H. Ito (IBM Almaden Research Center).   "Water immersion of model photoresists: Interfacial influences on water concentration and surface morphology" B.D. Vogt, et al., J. Microlithography, Microfabrication, and Microsystems, in press.                       NIST Material Science & Engineering Laboratory - Polymers Division