The impact of optical non-idealities in litho-litho-etch processing

Experimental work reveals that a thermal cure freeze process can alter the refractive index of a 1st pass LLE resist designed for that purpose. Although negligible change in the real index (n) is observed at the actinic wavelength, a 20% increase in the imaginary index (k) occurred. It is also experimentally determined that a second pass resist coated over a frozen first layer, may have a planar or non-planar surface, depending upon its' formulation. Simulation studies show that a non-planarizing 2nd resist will exhibit lensing effects which result in the 2nd pass resist feature showing sensitivity to the CD and profile of the embedded resist features. Other simulations suggest that both non-planar 2nd resist surfaces and mismatching resist n & k values can have a negative impact on the alignment sensitivity of a LLE double patterning process.