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Understanding Interfacial Deviations in Lithographic Pattern Profiles

Joseph L. Lenhart

A combination of analysis tools (x-ray and neutron reflectivity and scattering, contact angle, fluorescence labeling, and near edge x-ray absorption fine structure, NEXAFS) is being developed and adapted to tackle important technical obstacles facing the lithography community.  One current focus is on identifying the mechanisms that cause lithographic patterns to deviate near an interface. Photolithography is the process used by integrated circuit (IC) chip manufacturers to print circuit tree patterns on wafers and it accounts for about 35% of the manufacturing cost of todays IC chip.  As IC chip feature sizes shrink, the demand on the performance of lithographic films is ever more challenging (e.g. well defined features less than 100 nm in dimension with line edge roughness < 5 nm will be routinely expected). 

Chemically amplified photo-resists are extremely prone to interfacial or surface phenomenon, which causes deviations in the pattern profile near the interface.  Striking examples include T-topping, footing, and undercutting.  One critical challenge facing the industry is to eliminate these surface effects in order to obtain uniform patterns with sub-100 nm dimensions.   In particular, initial focus is placed on the interface between the anti-reflective coating (ARC), and the chemically amplified photo-resist.  An ARC is a thin film, placed between the substrate and photo-resist, which prevents reflection of the exposure radiation from the substrate, which can degrade the pattern quality.  The objective is to characterize the structure and properties of thin ARC layers and the nature of the chemical and physical interactions between the ARC and the photo-imaging layer (the photo-resist).

Since organic based ARCs are typically cross-linked polymer films, an epoxy network was initially chosen as a model ARC system.  Epoxy films were cast onto silicon wafers with a variety of surface treatments.  A transition from bulk to confined expansion occurs in the range of (20 to 40) nm, where thinner films exhibit smaller expansion coefficients.  The thermal expansion behavior of the epoxy films was independent of the substrate surface treatment, which varied both in surface energy, and the strength of bonding interactions with the polymer. The thinnest epoxy films (< 12 nm) exhibited typical glassy expansion values even at temperatures (20 to 40) oC above the bulk polymer glass transition temperature, independent of the surface treatment.  The thermal properties of the epoxy films were dependent on the molecular weight between cross-links.  Loosely cross-linked films exhibited confined thermal properties. Tightly cross-linked films did not. This suggest that the polymer cross-links can dampen or screen the influence of polymer / substrate interactions. Since ARCs are tightly cross-linked, deviations in the thermal properties of thin ARC coatings are unlikely.

The ARC industry has observed that the deviations in the interfacial line profile are dependent on ARC processing.  The ARC surface chemistry is thought to play a critical role in footing / undercutting of resist line profiles, possibly leading to subtle chemical contamination of the photo-resist in the interfacial region.  However, a combination of NEXAFS and contact angle techniques was used to show that the ARC surface chemistry was independent of the ARC processing.