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.
Depth of focus (DOF) has become a victim of its mathematical relationship with Numerical Aperture (NA). While NA is being increased towards one to maximize scanner resolution capabilities, DOF is being minimized because of its inverse relationship with NA. Moore's law continues to drive the semiconductor industry towards smaller and smaller devices the need for high NA to resolve these shrinking devices will continue to consume the usable depth of focus (UDOF). Due to the shrinking UDOF a demand has been created for a feature or technology that will give engineers the capability to monitor scanner focus. Developing and implementation of various focus monitoring techniques have been used to prevent undetected tool focus excursions. Two overlay techniques to monitor ArF Scanner focus have been evaluated; our evaluation results will be presented here.
Device Design criteria and product complexity have reduced the Focus Budget on today's technologies to near zero. Recent years have seen the introduction of a number of focus monitor methods involving new designs and processes that attempt more accurately or more easily to define the focus performance of our imaging systems. We have evaluated several focus monitoring techniques and compared their relative strengths and speed. The objective of this study is to demonstrate each technology's ability to evaluate exposure tool lens performance and quantify those factors that directly degrade depth-of-focus in the process. Baseline focus for process exposure and lens aerial image aberration analysis is evaluated using focus matrices. The remaining contributors to depth-of-focus (DOF) degradation are derived from the opto-mechanical interactions of the tool during full-wafer exposures. Full-wafer exposures, biased to -100 nm focus, were used in the determination of these error sources. Exposing all test sequences on the same 193 nm scanner provided consistency of the comparison. A valid analytical comparison of the technologies was further guaranteed by using a single software tool, Weir PSFM software from Benchmark Technologies, to calibrate, analyze and model all metrology. Two of the four techniques we evaluated were found to require focus matrices for analysis. This prohibited them from being able to analyze the fixed-focus exposure detractors to the DOF. One technique was found to be ineffective at the 193 nm because of the high-contrast response of the photoresists used. An analysis of the aerial image was validated by comparison of each technique to the Z5 Zernike as measured by ASML's ARTEMIS analysis. The ASML FOCAL and Benchmark PGM targets, both replicating dense- packed feature response, best tracked ARTEMIS signature. A whole-wafer, fixed exposure tool focus analysis is used to evaluate wafer, photoresist and dynamic scan contributions to the focus budget. Of the four techniques considered only the PSFM and PGM patterns could be used for this evaluation. Performance response is reported for detractors involving the wafer as well as the mechanical scan direction of the reticle stage.