Exposure light passes through resist films and may reflect off the substrate. The reflected light interferes with the incoming exposure light and generates standing waves in the direction normal to the wafer surface. These standing waves have a period of a quarter of the wavelength divided by the index of refraction . Thus, the reflected light causes a corrugated profile in the resist pattern and makes CD control very difficult. If the substrate has patterns, the patterns underneath the resist film may reflect exposure light differently from the way unpatterned areas reflect exposure light, leading to poor CD uniformity across the wafer.
Antireflection coatings (ARCs) have been introduced to control reflection. These ARCs can be located either below the resist film (bottom antireflection coatings or BARCs) or above the resist film (top antireflection coatings or TARCs). TARCs control the reflection at the top resist surface. They are organic solutions that are spin-coated onto the resist surface. BARCs are specifically useful on pattern or topographic substrates and control the reflection from below. BARCs can consist of either organic or inorganic materials. Inorganic BARCs are deposited via chemical vapor deposition (CVD) methods (e.g., SiON films). Organic BARCs are spin-coated onto the substrate and then baked at high temperatures to remove solvent and to cross-link the film.
The disadvantage of inorganic BARCs is that they cannot be removed by solvent and so cannot be reworked. Usually, after pattern transfer, the inorganic film stays on the wafer and becomes part of the film stack. In contrast, the spin-on organic BARCs are more process-flexible and can be easily removed with solvent or plasma etch. Their viscosities can be adjusted, enabling different coating thicknesses from several tens to several hundreds of nanometers.
Due to their fluid nature, organic BARCs may fill the wafer surface topography and planarize surfaces. Some organic BARCs are optimized for this purpose and are called planarizing BARCs. The coating performance of BARCs is strongly related to the molecular weight of their polymers, their viscosity, and their solvent content.