TOP ANTI-REFLECTIVE COATINGS VS BOTTOM ANTI-REFLECTIVE COATINGS

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TARC vs BARC

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.

Reasons to Control Reflectivity

There are two major types of anti-reflective coatings on the market: top anti-reflective coatings (TARCs) & bottom anti-reflective coatings (BARCs).

How They Work

Top Anti-reflective Coatings
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.

Benefits
Simplified process
Drawbacks:
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
Bottom Anti-reflective Coatings
BARCs use the refractive index, thickness and absorption of light to control reflectivity. They effectively make the substrate non-reflective.

Benefits:
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
Drawbacks:
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.

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TOP ANTI-REFLECTIVE COATINGS VS BOTTOM ANTI-REFLECTIVE COATINGS

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