Line-edge roughness in 193-nm resists: lithographic aspects and etch transfer

Abstract

We describe methods to determine transfer functions for line edge roughness (LER) from the photoresist pattern through the etch process into the underlying substrate. Both image fading techniques and more conventional focus-exposure matrix methods may be employed to determine the dependence of photoresist LER on the image-log-slope (ILS) or resist-edge-log-slope (RELS) of the aerial image. Post-etch LER measurements in polysilicon are similarly correlated to the ILS used to pattern the resist. From these two relationships, a transfer function may be derived to quantify the magnitude of LER that transfers into the polysilicon underlayer from the photoresist.1 A second transfer function may be derived from power spectral density (PSD) analysis of LER. This approach is desirable based on observations of pronounced etch smoothing of roughness in specific spatial frequency ranges. Smoothing functions and signal averaging of large numbers of line edges are required to partially compensate for large uncertainties in fast-Fourier transform derived PSDs of single line edges. An alternative and promising approach is to derive transfer functions from PSDs estimated using autoregressive algorithms.