Binary 193nm photomasks aging phenomenon study
2011
193nm binary photomasks are still used in the semiconductor industry for the lithography of some critical layers for the
nodes 90nm and 65nm, with high volumes and over long period. These 193nm binary masks seem to be well-known but
recent studies have shown surprising degrading effects, like Electric Field induced chromium Migration (EFM) [1] or
chromium migration [2] [3] . Phase shift Masks (PSM) or Opaque MoSi On Glass (OMOG) might not be concerned by
these effects [4] [6] under certain conditions.
In this paper, we will focus our study on two layers gate and metal lines. We will detail the effects of mask aging, with
SEM top view pictures revealing a degraded chromium edge profile and TEM chemical analyses demonstrating the
growth of a chromium oxide on the sidewall. SEMCD measurements after volume production indicated a modified CD
with respect to initial CD data after manufacture. A regression analysis of these CD measurements shows a radial effect,
a die effect and an isolated-dense effect. Mask cleaning effectiveness has also been investigated, with sulphate or ozone
cleans, to recover the mask quality in terms of CD.
In complement, wafer intrafield CD measurements have been performed on the most sensitive structure to monitor the
evolution of the aging effect on mask CD uniformity. Mask CD drift have been correlated with exposure dose drift and
isolated-dense bias CD drift on wafers.
In the end, we will try to propose a physical explanation of this aging phenomenon and a solution to prevent from it
occurring.
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