Anti-reflective coatings are an essential part of photolithography with the continual shrinking of pattern geometries. Anti-reflective coatings reduce reflectivity at resist interfaces, thus providing better line width control with minimal loss of resist performance. The reflectivity is reduced by either attenuating light that passes through the anti-reflective coatings or by matching the index of refraction of the anti-reflective coatings to the resist system at the exposure wavelength employed.
There are two major types of anti-reflective coatings on the market: top anti-reflective coatings (TARCs) & bottom anti-reflective coatings (BARCs).
TARCs use the refractive index and thickness of two films (resist and TARC) to control reflectivity through destructive interference only. It does not change the reflectivity of the substrate.
| Simplified process |
| Require separate waste drains |
| Environmental hazard (may contain perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) or other perfluorocarbons) |
| No effect on reflective notching and topography control |
| Difficult to control thickness |
| Intermixing can affect resist |
| Only marginal process improvements as TARC polymers don’t have the optimum n value to completely reduce thin film interference |
BARCs use the refractive index, thickness and absorption of light to control reflectivity. They effectively make the substrate non-reflective.
| Better reflection control and process stability |
| Larger process windows |
| Compatible with common resist waste drains |
| Reflective notching and topography control |
| No etch open for wet developable BARCs |
| Requires etch open step for dry BARCs |
Both TARCs and BARCs can be used for the same process, with BARCs giving better CD control and wider process windows. BARCs are the preferred choice when you want to get the most out of your expensive lithography equipment.
TOP ANTI-REFLECTIVE COATINGS VS BOTTOM ANTI-REFLECTIVE COATINGS