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Analysis of Reticle CD Uniformity with CD SEMs

by Waiman Ng, Ph.D., Senior Product Marketing Engineer

As device dimensions are scaled below 0.18 Âľm, increasing demands are placed on critical dimension (CD) control of the lithography process. One large contributor to this total CD error can be attributed to the reticle. This is pushing the mask industry to improve their capabilities in pattern generation, inspection and metrology in order to meet these stringent CD requirements. Current optical metrology tools are operating at the limit of their resolution. CD SEM technology will be required to help mask makers meet these challenges.

The requirements of semiconductor manufacturers, as evidenced by the SIA roadmap, are driving the reticle development cycle at an ever-increasing rate. With the current trends toward employing optical proximity corrections (OPC) to features to improve the image transfer to the wafer, as well as phase shift masks (PSM), both targeting the extension of the range of optical lithography, even more emphasis is being placed on photomask quality. This is driving the mask industry to improve their capabilities and technology in order to meet these stringent CD requirements.

characterization of both the reticle and printed wafer with the same system. This ability eliminates one possible source of error in reticle to wafer correlation studies. This capability will become increasingly important as CD uniformity and pattern fidelity for OPC features become more critical.

Previous reticle CD metrology is based primarily on optical technologies. As the reticle feature size approaches the wavelength of light used in these metrology instruments, diffraction can cause large proximity effects.

This leads to a highly non-linear optical response for metrology in the sub-micron range. The mask industry must explore new CD measurement technologies in order to meet the current and future CD control requirements.

The KLA-Tencor 8100XP-R makes use of a proprietary advanced charge elimination (ACE™) system, which ensures that local surface changing on the reticle is eliminated. The ACE system allows the 8100XP-R to provide stable imaging and metrology even on isolated chrome features. Figures 1 and 2 illustrate how the 8100XP-R controls surface charging of an isolated chrome feature on a reticle with an average chrome density of 5 percent. This imaging technology allowed us to perform the following experiment to quantify the reticle contribution to CD error.

Low voltage CD SEM-based imaging and measurement systems are the current platform of choice for high throughput automated metrology systems used for lithography process control on silicon wafers.

These systems offer the imaging resolution and precision required for wafer CD control below the 0.18 Âľm level. Recent modification of the wafer CD SEM for reticle use also offers the additional advantage of direct

Adaptation of wafer CD SEMs for reticle CD metrology is not a simple task. SEM imaging of chrome on glass (COG) reticles pose significant challenges, since glass is a highly insulating material. Isolated areas of chrome on glass have no direct path for charge dissipation; the resulting sample charging can be a major cause of image instability in conventional SEMs.

Figure 1. Typical sample charging

Figure 2. The same isolated chrome

effect for isolated chrome feature.

feature imaged with the ACE system.

Spring 1999

Yield Management Solutions

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Reticle CD: Verticle Iso 1130 1120 1110 1100 1090 1080 1070 1060 1050 1040

Figure 3. Reticle CD contour plot, vertical isolated line.

Case study: reticle CD uniformity

Intrafield CD uniformity is becoming an increasing limitation to integrated circuit performance. Tighter restrictions are placed on the across chip linewidth variation (ACLV) to improve yield and device speed on an increasingly larger die. In addition, non-linear transfer of reticle CD errors to the wafer, known as Mask Error Factor (MEF)1, has increased the importance of studying reticle CD variation on printed ACLV. The capability to identify and separate the component sources of variation is essential when working to reduce ACLV. In particular, quantification of the across-reticle contribution of CD variation to the printed CD on the wafer is critical in order to isolate the CD non-uniformity effects of the exposure tool or process2.

Contour maps of the reticle and wafer CD data are plotted to create a “fingerprint” of the reticle contribution to the overall intrafield CD distribution. Figure 3 is the contour plot for the isolated vertical line feature on the reticle. It is clear from this plot that the CD variation on the reticle is not purely random, but it has a strong radial distribution. This CD variation is most likely a signature of the wet process used on the mask. Figure 4 is a contour plot of CD variation of the same feature as printed on the wafers. The CD variations on the wafer also exhibit a radial component, but its distribution is lopsided. To isolate the contribution of the stepper, we removed the reticle CD data from the observed variation of CDs printed on the wafer. Figure 5 is a contour plot of the wafer data with the reticle CD error removed. Figure 5 shows that after the radial component of reticle variation is compensated, a linear component is clearly visible on the plot. This linear CD distribution can be attributed to the stepper and processing of the wafer.

Wafer CD: Reticle CD/4.1: Vertical Iso 25 15 10 5 0 -5 -10 -15 -20 -25

For this experiment, a standard test reticle with CD metrology cells located in a uniform array across a square field was used. First, a set of 0.25 µm features on the reticle was directly measured with a CD SEM. The test reticle was then used to expose a set of test wafers. Next, the corresponding 0.25 µm features on each stepper field on the printed wafer were measured using the CD SEM. These CD measurements were used to examine the impact of systematic reticle CD variations on the estimate of the CD non-uniformity of a stepper projection lens.

Wafer CD: Verticle Iso 285 280

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Figure 5. Compensated CD contour, wafer – reticle/4.

The CD SEM results presented in this article illustrate a method to measure reticle CDs. Separating the reticle CD component from any stepper-induced CD variation allows for a more accurate assessment of the sources of CD non-uniformity. It is possible to reduce the apparent stepper field uniformity by carefully extracting the reticle contribution to the overall measured field distortion. Using the reticle-compensated CD data can yield better correction inputs to the stepper and provide better lithographic performance. circle RS#037

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Acknowledgements: Special thanks to David Witko, Shawn Cassel, James Foster, Geoff Anderson and Richard Elliott. The author would also like to thank Raymond Yip for assistance with data analysis and contour plotting.

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Figure 4. Wafer CD contour plot, vertical isolated line.

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1. J.P. Kujiten, et.al., “Analysis of Reticle Contributions to CD Uniformity for 0.25 µm DUV Lithography”, Presented at SPIE, Feb. 1998.

2. G. Anderson, et.al., “Intrafield CD Uniformity study in 0.25 µm Lithography”, Presented at Olin Interface ’98.