Proceedings Article | 16 April 2008
KEYWORDS: Image quality, Iterated function systems, Scanning electron microscopy, Image processing, Manufacturing, Critical dimension metrology, Etching, Process control, Metrology, Optical alignment
In lithography and etch processing, the control inputs (dose or gas flow, etc) use the critical dimension
measurements from CDSEM as feedback and/or feed forward parameters. Thus the image quality of the
metrology tools is critical for controlling litho and etches processes. With wafer size increasing while CD
and features shrinking, even tighter controls on CD are required. It has been shown in literature that during
24 hour period, the beam alignment can drift severely enough to cause a shift of over 10 nm in the
measured CD. Though auto focus tuning is provided on some CDSEMs, our tests show that, depending on
the focus algorithm used, the insitu autofocus may shift from the best focus. In practice, the tuning of
CDSEM settings largely depends on the operator's "eyeball" judgments, thus the quality of the SEM
images is dependent on the judgment of the operators. In this paper, we propose an objective and
quantitative image quality monitor for focus monitor based on image processing and optimization.
For focus monitor and optimization, a series of through-focus images are taken for a CDSEM tool. By
processing the images using image processing toolbox in Matlab, an IFQ (image focus quality) score, used
to quantify the image focus quality, is assigned to each image. The fitting of this data to a predefined
polynomial can be used to determine best focus. The algorithm is robust and fast, and has been integrated
into the existing manufacturing infrastructure for tool performance tracking and monitoring.
The paper is organized as following: In the introduction, some background information on CDSEM, as well
as existing and alternative image quality monitoring methods are reviewed. In the second part, we introduce
the methodology and steps for the new focus monitor. The third part covers the experiments for CDSEM
parameter optimization, robustness tests and validation. The next part explains the implementation of the
focus monitor in manufacturing environment. In summary, the proposed method for focus monitor is fast,
robust and manufacturability.
