52
LITHOGRAPHY
All nodes to the
challenge
Meeting the
challenges of
advanced
corner rounding in contact layers and on line ends.
Consequently, the elimination of residual mask
process nodes I
t is widely anticipated that upcoming IC
process nodes will be more challenging
than any other technology transition since errors via pattern correction is gaining attention.
with
the first integrated circuit. One of the most difficult New models that correct for both short range
tasks is to extend the lithography process to and long range mask process effects, including
independent 32/22nm while employing essentially the same proximity effects from etching, pattern density
mask process
exposure wave length that the world has been using loading effects, and across mask process non
since 130nm. Until the 45nm node, distortions uniformity, can significantly improve the quality of
correction. involved in making masks had less impact than the masks. Dedicated models that decouple mask
Steffen
wafer lithography process, since mask features are correction from wafer correction can provide added
typically four times larger than the actual features flexibility and improve the overall quality of results
Schulze from on a die. compared to lumping mask errors into the OPC
Mentor
But at 32/22nm, the accuracy requirements for model (Figure 1). In the discussion that follows, we
mask making are much more stringent. The 2007 describe mask correction models for independent
Graphics and ITRS roadmap for optical masks requires that the mask process correction (MPC), and methods to
Ming Jing
‘mean to target’ (MTT) difference between two calibrate the models against measurements from
consecutive layers in a double patterning mask set photo masks. It will be demonstrated that model
Tian from be less than 1.3nm, and mask uniformity must be based mask process correction has the potential to
SMIC explain
within 1.2nm (3σ) for the 32nm node [1]. reduce the residual mask error by a factor of 2x
Traditional optical proximity correction (OPC) through application of a basic mask process model.
the challenges. mainly addresses the wafer image transfer with
some mask making effects included as part of the Requirements for independent
overall OPC model. Existing e-beam proximity MPC correction models
correction on mask writing tools address e-beam Effective mask correction must handle long,
exposure and resist effects, but have known medium and short range effects with a combination
limitations in the areas of narrow features and of models calibrated to physical measurements [9].
corners, non linearity due to etch effects, and Mask errors can be categorised in terms of their
range of influence as proximity, mid range, and
long range effects [6]. Proximity effects have a
range of influence from zero to a few micrometres,
and long range effects have a range of influence
from tens of micrometres to tens of millimetres.
The most significant contributors to long range
mask CD errors are process non uniformity and
pattern density loading effects [3, 4]. Proximity
Figure 1. Dedicated errors are caused by finite patterning resolution,
correction models for resist develop, and etch loading [6]. The
the mask making intermediate mid range effects are assumed to be
step can improve corrected via compensation of the electron beam
overall lithography lithography tools and are not considered here [3,
results at advanced 4, and 6].
technology nodes. Etch can contribute around 40% of the loading
This approach also effects, corresponding to an error larger than
provides added 10nm[2]. In addition to loading, which is
flexibility be correlated to the macro density of the pattern
decoupling mask across the mask, other long range errors include
process models from radial components stemming from chuck
wafer process configuration in bake steps, and the spinning
models motion of the substrate during develop, rinse, and
www.euroasiasemiconductor.com November 2008
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