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Focus on the Edge—for a Competitive Edge Characterizing and Monitoring Focus and Exposure Edge Effects Venky Subramony, Dawn Goh, Pei Chin Lim, TECH Semiconductor Brad Eichelberger, Augustine Chew, Berta Dinu, Kevin Monahan, KLA-Tencor
To improve yield and chip performance, it has become important for fabs to characterize and correct changes in the effective focus and exposure at the edge. Monitoring focus and exposure on product wafers is the most effective means for correction, since product wafers provide the most realistic view of exposure tool interactions with the process. In this work, on-product monitoring and correction is based on optical measurement using a compact line-end shortening (LES) target that provides a unique separation of exposure and focus on product wafers. Our ultimate objective is indirect critical dimension (CD) control, with maximum yield and little or no impact on productivity.
Introduction
The past few years have witnessed an increasing interest in the monitoring of focus on production wafers. This is driven by the ever shrinking process windows required to print smaller and smaller device geometries. As an example, focus windows are expected to shrink from about 400 nm at the 130 nm technology node to less than 100 nm at the 65 nm node. Focus error alone may contribute to more than 50 percent of the CD variation.1 Currently, there is no reliable technology with sufficient sensitivity to monitor focus in real time on standard production wafers. Focus is currently monitored primarily through the use of test wafers which adequately monitor the parameter on a periodic basis, but do not take into account the dynamic factors affecting focus on a wafer-by-wafer basis. Many techniques exist for focus monitoring on test wafers, but none of them consider the effects of exposure so that accurate focus values can be determined. This barrier limits these
techniques to test wafer applications only. Exposure is currently monitored and controlled through metrology performed on CD structures. Traditional metrology techniques can measure CD, but cannot separate the focus contribution from the exposure, leading to some degree of uncertainty in the correctable. Cross wafer yield shows that there are typically areas of lower yield across the wafer. These drops in yield are invariably the result of uneven processing or non-uniformities of CD across the wafer. These variations can be monitored through increased sampling on traditional technologies, but generally remain unsampled as a tradeoff for throughput. These unseen focus and exposure variations degrade first-pass yield, cause unnecessary rework, and reduce scanner productivity. This article will present an application using a dual tone LES target to characterize and monitor focus-exposure on edge die. The metrology used to control focus and exposure at the lithography level is KLA-Tencor’s MPX (Monitor Photo eXcursions) option, performed on the Archer AIM overlay metrology platform. Winter 2005
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cost-effective method to monitor focus and exposure on production wafers. The use of two target dimensions allows simultaneous determination of focus-exposure conditions on standard production wafers.2 This technique requires a single calibration step before production monitoring can begin. The calibration step uses focus exposure matrices (FEMs), and the measured TD1 and TD2 data is fed into an empirical model of the form: TD = k0 + k1E + (k2 + k3E)(F – F0)2. This model is solved for the model coefficients K0 – K3 and ‘Best Focus’ position Z0. Z0 of the target is typically within < 30 nm of the standard process ‘Best Focus,’ as determined by the SEM results. The model coefficients are then saved to a reference table and used during production monitoring to calculate focus-exposure values for each measured location. This methodology is very quick and simple to implement in a high-volume production line. Figure 1. Dual tone MPX target based on line end shortening.
Metrology methodology
The dual tone LES technology target is shown in Figure 1. It uses grating targets consisting of arrays of lines and spaces at or near the CD of the device layer. The technique takes advantage of LES of array targets in brightfield “islands” or darkfield “windows,” respectively. Relative to CDs, LES will be several times (10x) more sensitive to focus-exposure changes. The behavior of both target dimensions (TD1 and TD2) is approximately parabolic through focus, and approximately linear through small ranges of exposure dose as shown in Figure 2. The combination of these two properties enables the technique to separate focus from exposure. The target, though compact enough to fit in the most restricted scribe lines, contains metrology dimensions large enough to be measured using a standard optical overlay tool. The coupling of focus-exposure metrology with overlay metrology maximizes the value of the optical overlay tool. As a combination, it provides a
Figure 3. Off-line calibration, inline monitoring.
MPX is currently in the early phases of adoption but is already providing very useful information about the effects of focus-exposure across the wafer. The optical technology enables quick characterization to detail the effects. Typical variations occur as hot spots, edge die excursions, radial effects, and leveling effects—all of which are not typically seen on most test wafer monitors used today. Examples of these effects can be seen in Figure 4 where the color contours represent the magnitudes in focus or exposure. Red and blue represent the higher magnitudes, while green and yellow represent more acceptable parameter levels. Data and results
Metrology verification Figure 2. Dual tone MPX target based on line end shortening.
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The first step in the investigation is to gain confidence in the metrology used to deter-
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Edge Focus Excursion
Radial Focus Effect
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Edge Dose Excursions
Figure 4.
mine the level of exposure and defocus variation. Therefore, we performed correlations between the MPX focus-exposure output to exposure tool inputs settings using FEM wafers.
estate consumed by the FEM. Printed on a 300 mm wafer, the FEM consumes an area of approximately 170 mm by 85 mm. The results of our qualifying experiment are shown in Figure 5.
Two FEM wafers were created to cover the process window. One wafer would be used to create an FE model, for the LES method, and the second wafer would be used to correlate the MPX calculated exposure versus exposure tools programmed offsets. An FEM provides programmed exposure and focus offsets, enabling the evaluation of MPX’s ability to separate the effects of the parameters. The FEMs that were used were created with a focus range of ± 300 nm with 50 nm focus increments. The exposure was changed by .5 mJ though a range of ± 1 mJ or ± 5% of nominal for the given layer is acceptable. In an effort to gather the most accurate results, we tried to minimize the effect of process and wafer influences by minimizing the real
The data shows excellent correlation between the MPX exposure and focus output versus the scanner exposure and focus input values. Despite the excellent results, we must consider that some of the small uncertainties can be attributed to cross wafer variations in process and the wafers themselves. These results are key to understanding the performance levels of MPX technology, given the fact that it will be used in a production environment.
Production assessment For the next step in our investigation, we were required to characterize a few wafers to understand the effects of focus and exposure across the wafer, the edges. From
FEM Dose Linearity
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Figure 5. Focus and exposure linearity
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MPX Exposure Output Summary 1.0% _ 0.5% _ 0.0% _
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Figure 6. Production focus and exposure distribution summar y.
these results we could outline the next steps in the investigation. For this characterization, we measured 25 standard production wafers from multiple lots at the 0.13 µm gate level process. A single point within each field was measured for all fields available on the wafer. The results of this analysis are summarized in Figure 6. These charts show that there is a high ‘effective’ exposure at the edge of the wafer. By ‘effective’ exposure we refer to effects—including wafer, process, and exposure—that appear to be exposure-related. The types of effects have the same impact on CD as they do the line-ends of the MPX target. However the MPX target’s effect is more clearly seen due to the significantly higher response to the effects of exposure and focus. 4
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As a verification step in our process we evaluated the effect of the rouge fields at etch by measuring the final CD after etch with a CD SEM. As expected, the data indicated the edge of the wafer produced a smaller yield than normal CDs. This result can have a devastating effect on yield and chip quality given today’s tight processing requirements.
Compensation experiment For the next step in our investigation we designed a controlled experiment which allowed us to evaluate cross wafer improvements realized from custom exposure and focus settings. This experiment started by taking wafers from standard production lots and characterizing them to baseline the edge effects. The baselining was performed on an Archer 10 MPX tool and a CD SEM.
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Figure 7. Pre-compensation, MPX exposure results at litho.
Due to throughput considerations, SEM sampling was not performed across the full wafer. We then applied the MPX correctables for a designated number of fields to wafers in the rest of the lot. In order to apply custom correctables to specific fields, the scanner job requires modification so that specific exposures can be programmed for specific field locations. The results for two of the wafers used as a baseline for the two lots are shown in Figure 7. The results of these lots agreed with the data collected on the standard production lots (Figure 6). From the results, you can see high peaks which represent higher levels of dose, as compared to the other areas on the wafer. The results, shown in cross wafer 3 sigma as a percentage of nominal, can be seen in Table 1. The wafers were also measured after etch, with the results showing consistency between litho and etch. Higher levels at the edge were mainly caused by process-induced effects; therefore, these levels could be considered ‘effective exposure or dose’. Regardless of the cause, the result is the same: a smaller CD. With the results of the pre-compensation wafer known, we identified the field locations representing the higher levels of effective exposure and designated the associated MPX correctable as feedback for the field. These field-specific correctables must be manually inputted into the scanner job for the second round of wafers from each lot. An example of the new scanner wafer map is shown in Figure 8. The colored fields designate which fields have special exposure settings. The color of each field represents the magnitude of the offset to the
exposure. The only change on these wafers will be exposure correction on the targeted fields. The final results are shown in Table 2. This table shows the cross wafer variation, as a percentage of nominal for the wafers pre- and post-exposure compensation. The
Lot 1 Lot 2
Cross Wafer 3σ Before Compensation Before Etch After Etch 2.9% 3.1% 2.9% 3.3%
Table 1. Pre-compensation results.
Scanner Wafer Map
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Figure 8. Compensation map.
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Figure 9. Post-compensation, MPX exposure results at litho.
results are shown for sampling at both litho and etch processing points. The chart to the right of the table shows the delta percent between the pre- and postcompensated wafers. The delta percentage shows that, on average, there was about a 1.2 percent improvement in cross wafer exposure 3 sigma. We also performed the same investigation with focus. Since the sign of defocus was not known from the MPX focus output, the sign of the focus correction was assumed to be positive. Nine fields were offset by small amounts of focus ranging from +20 nm to +40 nm. A
Cross Wafer 3σ Before Compensation Before Etch After Etch Lot 1 Lot 2
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complete compensation map can be seen in Figure 10. We measured the wafers before and after the focus compensation as shown in Figures 11 and 12. These charts have been normalized to the minimum focus on the wafer. From our focus experiment we concluded that, after compensation, the focus levels are within the standard cross wafer focus noise. This is shown in the focus plot in Figure 12. It should be noted that two of the pink peaks in Figure 12 are from points for which we did not intentionally compensate. Lot 2 results are not shown because they exhibited the same results.
Scanner Wafer Map
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Table 2. MPX pre/post compensation results.
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Legend
Figure 10. Focus compensation map.
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Figure 11. Pre-focus compensation.
Figure 12. Post-focus compensation.
Production implementation
both systematic and random. The random variation will typically occur lot-to-lot, whereas the systematic error is consistent like exposure. An example of a typical sample plan can be seen in Figure 13.
Characterization and process setup
Characterization should be performed following a sequence similar to that performed in this experiment. Out of about five lots, we would sample three wafers per lot and perform full wafer sampling using the MPX technology. SEM results should also be collected to validate the MPX results and to provide a SEM baseline so that CD improvement can be determined after compensation. With the combined data, specific fields can be identified as potential areas for improvement. The number of compensated fields is dependent upon the desired level of uniformity required. In our experiments, we compensated for 10 specific fields, with all but one being located at the edge of the wafer.
With a sampling plan strategy ready, MPX tests can be added to the standard overlay recipes. This will maximize the efficiency of the overlay tool by eliminating extra wafer loading and unloading. This methodology takes advantage of the fact that the wafers will already be processed through the optical overlay tool. This means that, for a given wafer, focus and exposure at the edge of the wafer can be monitored efficiently.
Edge Field Monitor
After collecting the average systematic offsets from the five lots, field-by-field offsets can be entered into the scanner recipe for the corresponding device and layer. A few wafers should be exposed to validate the custom offsets. To validate the change, MPX and SEM measurements should be taken to realize the amount of uniformity improvement gained by the offsets. Keeping this in mind, the next section discusses a monitoring strategy that can be used to ensure that the offsets remain consistent on future lots. Monitoring strategies
Establishing an edge field monitoring strategy is rather trivial. Based on the initial characterization that was done, you can determine the appropriate fields that will require monitoring. From our work we have seen that, for exposure, the signature is consistent from wafer to wafer and lot to lot. Relative to focus, the errors can be
Notch Figure 13. Edge metrology sampling.
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The data can be monitored in an SPC fashion by tracking the wafer mean and 3 sigma statistics for exposure and focus. A shift in the mean may indicate a change in the signature, which may require further characterization. A higher than normal 3 sigma would indicate a potential excursion in the focus or exposure.
Benefits and return on investment Due to the interaction of more complex designs with rapidly shrinking process windows, pattern-related yield loss is dominating early production. For design rules smaller than 130 nm, CD-limited yield is by far the most challenging problem, and associated production delays can cost tens of millions of dollars per product. Failures in CD control originate from many root causes, most of which change the effective focus and exposure dose in the lithography cell. Consequently, we are seeing a trend toward “indirect CD control” using focus-exposure monitors. Specifically, semiconductor manufacturers are seeking to monitor focus-exposure directly on production wafers in order to control CDs without impacting productivity. Responding to this need, optical overlay platforms have been adapted to provide simultaneous inline monitoring of focus, exposure, and overlay. Unique dual-tone, LES targets have been designed to decouple focus and exposure measurements (Figure 1). Small LES grating targets (about 13 x 22 mm) can be placed at multiple locations in scribe lines to monitor focus-exposure variation across the field and across the wafer. Relative to device CDs, these targets have much higher sensitivity to focus-exposure variation (e.g., < 0.5% in exposure and 30 nm in focus). Since about 50 percent of CD variation can be traced to focus deviations, improved focus control inevitably leads to more stable, dose-based APC performance. In addition, when LES targets are used in focus-exposure matrices, “best focus” repeatability can approach 1 nm. Within the wafer, these targets enable monitoring of systematic focus excursions due to reticle scan direction, wafer stage direction, and wafer topography. In most cases, focus excursions are binned with better accuracy and purity than the CD variations they cause (Figures 2 and 3). Across the lithographic field, LES targets enable monitoring and correction of focus tilt, a common cause of yield-affecting bimodal CD distributions. In particular, since the Archer MPX monitor is often implemented on an existing overlay tool, the incremental ROI may be much greater than 10x per year, when based on improvements in scanner OEE and pattern-related yield loss. 8
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Conclusions
As a result of this work, we conclude that exposure variations for a given process are relatively stable from lot-to-lot and wafer-to-wafer. Therefore, the next opportunity for improvement lies in field-to-field exposure correction. Using this method, we can achieve improved cross-wafer uniformity by offsetting the effects of a non-uniform process. Improved uniformity results in improved CD distributions, correlating to improved chip performance and yield. In this case, Archer MPX provides a cost-effective optical method for characterizing and monitoring edge effects in mass production. In addition, MPX effectively monitors focus excursions on both test and product wafers. Focus feedback for APC systems, however, will require a monotonic response to defocus. To support these new APC requirements, we are evaluating a low-cost directional target and plan to introduce this technology later in the year. Beyond the 90 nm technology node, indirect CD control is likely to become even more critical as the industry struggles to extend 193 nm lithography and to maintain pattern-limited yield. Current projections indicate that controlling simple focus and exposure errors could recover several percent of the pattern-related yield loss and tens of millions of dollars in lost revenue per factory per year3. Acknowledgements
Thanks go out to all of the authors for their input and insights into this topic. We would also like to thank the litho team at TECH Semiconductor in Singapore for permitting the use of their MPX data, as well as the local Singapore applications team for their contributions to making this article a reality. This article was previously published in the SPIE Proceedings, vol. 5375. References 1. S. Hannon, H. Kennemer, K. Monahan, B. Eichelberger, B. Dinu, C. Nelson, H. Pedut “Product-Wafer Focus-Exposure Monitor for Advanced Flash Memory”, Proc. of ISSM, presented at ISSM 2003. 2. B. Eichelberger, B. Dinu, and H. Pedut, “Simultaneous Dose and Focus Monitoring on Product Wafers,” Proc. of SPIE, Vol. 5038, pp. 247-254, February 2003. 3. K. Monahan, B. Eichelberger, M. Hankinson, J. Robinson, and M. Slessor, “Yield Loss in Lithographic Patterning at the 65 nm Node and Beyond”, Proc. SPIE, Vol. 5378, February, 2004.