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Stepper Focus Metrology and Analysis Using a Phase Shift Mask by Patrick J. Lord, Senior Product Marketing Manager and Michelle Zimmerman, Product Marketing Manager
Since the introduction of phase shifting masks, engineers studying this imaging technique have been aware that errors in the phase of the mask (non-180 degree shifters) would cause asymmetries of the printed image as the image was defocused. These asymmetries would create a translational offset in the printed image as a function of the focus offset. The Phase Shift Focus Monitor reticle is a tool which uses this effect for stepper diagnostic and calibration.1 The basic concept of the focus monitor reticle is simple and elegant. A bar-in-bar overlay target is written on a reticle. Both the inner and outer bars are printed at the same time, with part of the target phase shifted by 90 degrees and the other part unshifted (figure 1). If the stepper is perfectly focused, the overlay error will be exactly zero in both x and y. If, on the other hand, there is a focus error, the phase shifted half of the overlay target will move relative to the unshifted part; and the result is interpreted as an overlay error which is a direct measure of the focus error. Even though the method requires a one-time calibration between the actual focus and the overlay misregistration (figure 2), the linearity of the behavior as presented in figure 3 is a strong advantage of the method. As opposed to alternative solutions that are quadratic in nature, the PSF method can easily detect small focus changes about the optimal operating point. With the introduction of new analysis software, such as KLA-Tencor’s KLASS PSF, semiconductor manufacturers have been quick to utilize the speed and power of this new technique. Unlike other focus techniques, which require qualitative estimates of image quality to determine the “best”
focus position, the PSF method provides an operatorindependent, quantitative measure of the best focus position. In addition, the high throughput of overlay measurement tools compared to CD measurement systems such as SEMs makes it possible to measure the best focus at many positions within a lens field in a very short amount of time.
Figure 1. Typical phase shift mask focus target (courtesy of Benchmark Technologies, Inc.).
Figure 2. Meander focus setup.
Autumn 1998
Yield Management Solutions
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Figure 3. Calibration linearity. Figure 5. Field cur vature after removal of field tilt.
This new capability has allowed stepper engineers to apply the same mathematical rigor to focus analysis which has long been available for studying overlay. Focus variations within the field and from field-tofield across a wafer can now be mathematically modeled to determine lens tilt (figure 4), field curvature (figure 5), astigmatism (figure 6), wafer and chuck flatness (figure 7), the impact of lens heating and barometric pressure (figure 8), and other focus anomalies.
Figure 4. Field tilt analysis.
The speed of the overlay measurement tools also allow the engineer to measure several wafers to average out wafer flatness effects that would otherwise distort the data. The quantitative power of this technique is a breakthrough step in the analysis of stepper focus. Used as a daily focus monitor, it becomes an invaluable phase for any advanced process control implementation targeting dose control for improved critical dimension performance. 1 The Phase Shift Focus Monitor reticle is exclusive-
ly available from Benchmark Technologies, Inc.
circle RS#027
Figure 6. Lens astigmatism.
Figure 7. Wafer and chuck flatness.
Figure 8. Lens heating effects.