Autumn 1998 Yield Management Solutions 11

LithographyF E A T U R E S

The basic concept of the focus moni-tor reticle is simple and elegant. Abar-in-bar overlay target is written ona reticle. Both the inner and outerbars are printed at the same time,with part of the target phase shiftedby 90 degrees and the other partunshifted (figure 1). If the stepper isperfectly focused, the overlay errorwill be exactly zero in both x and y.If, on the other hand, there is a focuserror, the phase shifted half of theoverlay target will move relative tothe unshifted part; and the result isinterpreted as an overlay error whichis a direct measure of the focus error.Even though the method requires aone-time calibration between theactual focus and the overlay misregis-tration (figure 2), the linearity of thebehavior as presented in figure 3 is astrong advantage of the method. Asopposed to alternative solutions thatare quadratic in nature, the PSFmethod can easily detect small focuschanges about the optimal operatingpoint.

With the introduction of new analy-sis software, such as KLA-Tencor’sKLASS PSF, semiconductor manufac-turers have been quick to utilize thespeed and power of this new tech-nique. Unlike other focus techniques,which require qualitative estimates ofimage quality to determine the “best”

Stepper Focus Metrology andAnalysis 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 inthe 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 ShiftFocus Monitor reticle is a tool which uses this effect for stepper diagnostic and calibration.1

focus position, the PSF method provides an operator-independent, quantitative measure of the best focusposition. In addition, the high throughput of overlaymeasurement 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.

This new capability has allowedstepper engineers to apply the samemathematical rigor to focus analysiswhich has long been available forstudying overlay. Focus variationswithin the field and from field-to-field across a wafer can now be math-ematically modeled to determinelens 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), andother focus anomalies.

The speed of the overlay measure-ment tools also allow the engineer tomeasure several wafers to average outwafer flatness effects that would oth-erwise distort the data. The quanti-tative power of this technique is abreakthrough step in the analysis ofstepper focus. Used as a daily focusmonitor, it becomes an invaluablephase for any advanced process con-trol implementation targeting dosecontrol for improved critical dimen-sion performance.

1 The Phase Shift Focus Monitor reticle is exclusive-ly available from Benchmark Technologies, Inc.

Figure 3. Calibration linearity.

Figure 4. Field tilt analysis.

Figure 5. Field curvature after removal of field tilt.

Figure 6. Lens astigmatism.

Figure 7. Wafer and chuck flatness.

Figure 8. Lens heating ef fects.

circle RS#027

F E A T U R E S