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Multi-Beam High Resolution UV Wavelength Reticle Inspection

by C.C. Hung, C.S. Yoo, C.H. Lin, Taiwan Semiconductor Manufacturing Company W. Volk, J. Wiley, S. Khanna, S. Biellak, D. Wang, KLA-Tencor Corporation

A new reticle inspection system with three parallel scanning laser beams for UV imaging for both contamination and pattern inspection has been developed to detect defects on advanced reticles for DUV steppers and low k1 lithography for 0.13 µm and extensions to 0.10 µm design rules. The development of the new three-beam architecture at UV wavelength has significantly increased system throughput while improving the resolution of the imaging optics for inspecting advanced reticles including Halftone, Tri-Tone, and Alternating PSM’s and reticles with aggressive OPC. The system is capable of running multiple inspection algorithms simultaneously in transmitted and reflected light to achieve concurrent pattern and STARlight™ inspection, thus improving both sensitivity and inspection thoroughness with a single inspection. These improvements enable fast inspections of reticles for 4X lithography design rules at 0.18 µm, 0.15 µm, and 0.13 µm.

Initial simulations were performed to optimize performance of optical components and a new defect detection algorithm. The simulations identified that with the optics changes to achieve three beam scans and with new algorithms, the inspection was more sensitive to all defect types including on edge contamination defects, which can be particularly difficult to detect. Using both PSL and programmed defect test masks and real production reticles, initial observations of the nature and the frequency of defects detected with this 100 nm sensitivity instrument will be presented.

With more defects to review, the system software provides concurrent or remote defect review so time to disposition defects does not effect system inspection capacity.

With smaller defects to review, the quality of defect review images has a direct impact on the effectiveness and ease-of-use of reticle inspections systems. The smaller review pixel with the system, combined with a suite of review imaging tools, yields high quality images for defect dispositioning.

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Introduction

Reticle inspection has become a key aspect of integrated circuit (IC) manufacturing. As IC design rules shrink below 180 nm, reticle linewidths fall below 700 nm and Optical Proximity Correction (OPC) features present an even greater inspection challenge, as they can be as small as 100 nm.

The implementation of low k1 lithography into wafer production is becoming more common as industry design rule roadmaps are accelerating. Reticle inspection systems with smaller linewidth capability, higher sensitivity, and extended capability for OPC and PSM inspection are a critical component to obtaining high yields in low k 1 lithography.

Through the use of OPC and phase shifting techniques on reticles, DUV lithography has been extended to support 0.18 µm, 0.13 µm, and 0.10 µm design rules allowing low k 1 lithography to be used as a standard practice. As this practice has become more widely used, it has resulted in reticle specification change — the requirement of a post pellicle pattern inspection in the mask manufacturing operations as a final and comprehensive final check for pattern and contamination defects

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on critical layer reticles. Thus, more inspections are required in the mask manufacturing operations because pattern inspections must be run after pelliclization.

New inspection system designs now must not only meet the needs of the next generation design rules for sensitivity and OPC and PSM inspectability, but must also have significant throughput increases to keep the cost per inspection at a production worthy level.

The work discussed was a result of the Joint Development Partnership between TSMC and KLA-Tencor to develop a highly productive reticle inspection system for next generation design rules.

Inspection is a specific problem created by the new applications of aggressive OPC. Next generation, most advanced reticles using OPC assist features challenge today’s inspection systems. Current inspection systems such as the 305UV system were originally designed years ago when OPC was not widely used in new device designs. As we move into 0.13 µm design rules, OPC assist features will be used as standard practice and inspections must tolerate and inspect these features without false or nuisance defects. In Figure 1 a defect map is shown from the current 305UV reticle inspection system with the 150 nm pixel using the AOP algorithm

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from the inspection of a 0.15 µm / 0.13 µm design rule reticle with aggressive OPC assist features. This map shows an unmanageable number of false and nuisance defects totaling 1400, caused by sub-resolution assist features. Less than 1% of the defects are real defects. If the inspection were to be de-sensed we would not only reduce the false defect count, but also reduce the real defect count, which is unacceptable for the inspection of such advanced reticles with high mask error enhancement factors. This inspection issue must be solved for aggressive OPC inspection for 0.13 µm design rules, otherwise reticle inspection will be a roadblock for advanced reticle designs.

System changes

KLA-Tencor’s STARlight inspection tool is today’s industry standard for reticle contamination inspection.

The STARlight architecture has been extended to the new TeraStar system, which is designed to meet pattern and contamination inspection requirements for 0.13 µm design rules for production, and 0.10 µm design rules for research and development. The term Tera represents the system’s ability to inspect one tera-pixel (one million x one million pixels). The term Star is an acronym for Simultaneous Transmitted and Reflected Light, meaning that both the transmitted light response and the reflected light response are used on the system for defect detection.

To meet these requirements, the new TeraStar inspection architecture includes1: • an increased numerical aperture as a standard feature, improving the inherent resolution of the system and thus improving its defect detection sensitivity • replication of the UV beam into three parallel scanning beams, significantly increasing the system throughput • a new optics bench with improved alignment features • reduced system vibration to increase the signal-tonoise ratio to improve defect detection capability • a new die:die algorithm, XPA, operating on a new image computer for multi-beam inspection of pattern defects for better OPC inspectability • a new STARlight algorithm, TR412, for multi-beam contamination inspection

F i g u re 1. 305UV Defect Map p15 0 pixel AOP315.

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Using this new architecture, the TeraStar inspection system was developed. The optical characteristics of inspection were identified to be different with multiple inspection beams, and therefore it was critical to system design to perform the following: • investigate the differences of multi-beam and single beam UV inspection through computer simulations.

• based on these findings, create new algorithms for optimizing defect sensitivity with the new multibeam optics.

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• evaluate each of the algorithm options developed and determine which has optimum performance.

The new optics architecture is shown in Figure 2.

Like the 300UV system, this system also uses active beam steering for better reliability and performance.

Unlike the 300UV system, all subsystems of the TeraStar are included within the inspection station, with the exception of the user interface. This is a table top monitor and keyboard placed nearby the system.

All facility components and image computing components are within the inspection station

F i g u re 2. Multi-beam optics over view an d concurrent a lgorithm s fo r pattern and cont amination defect detection.

System Changes and Associated Benefits Challenges Detect smaller contamination Inspect smaller design rules Inspect OPC assist bars/slots Inspect half tone PSMs Inspect Tri-tone PSMs

Software New multi-beam UV algorithms Methods to tolerate assist features Improved overall signal:noise ratio On edge defect detection optimized Multi-edge calibration for Tri-tone PSM

This development project was different from others because it focused on increasing defect sensitivity through reducing the K d of the inspection system by improving algorithms and signal:noise ratio (see Figure 3). Most 70

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Results Inspects 0.13 µm, 0.10 µm designs Contamination sensitivity of 0.10 µm Hard defect sensitivity of 0.10 µm Zero false defects on OPC assists Tri-tone PSM inspection

other projects have focused on moving to either a smaller inspection wavelength or to a higher numerical aperture for increasing system sensitivity.

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F i g u r e 3. The K d factor a nd the relationshi p b etween λ, NA and defect size.

Observations

Simulations were previously reported for UV inspection1, where the behavior of the single beam UV inspection system was studied with respect to image quality, sensitivity, and advanced OPC and PSM capabilities.

Figure 4 illustrates the results of optical simulations of the single beam UV inspection on a binary reticle across a chrome/quartz edge. The vertical axis is the reflected light signal and the horizontal axis is the response position offset in reference to the chrome edge at 0.5 µm.

The simulations modeled the optical path of the two inspection systems to the finest detail. Figure 1 shows a clear difference in near-edge signal modulation between the 488 nm STARlight system and the 364 nm UV STARlight system. This difference is called the UV on Edge Effect (UEE). Because of this difference, new algorithms were investigated for the implementation and optimization of UV inspection.

For this project, simulations were performed for the new multi-beam UV optics to study the UV on Edge Effect. Figure 5 illustrates the results of optical simulations of the multi-beam UV inspection on a binary reticle across a chrome/quartz edge. The reflected light edge response of the new multi-beam system is better than previous generation systems. With this smoother transition across an edge, the algorithms will perform more reliably and will have better repeatability of finding defects on the quartz side of an edge. This improvement is targeting transmission and contamination defects near edge.

F i g u re 4. UV ins pection optical si mulation results across a c h ro m e / q u a r tz edge.

In addition to optimizing the inspection algorithms for the UEE, the XPA algorithm was optimized for the inspection of aggressive OPC features, including assist features. The design rule of the reticle image shown in Figure 6 is 0.15 µm. At DUV lithography the k1 for this process was less than 0.45, which would cause this programmed OPC defect to print to the wafer. This pin dot defect class may not be new to reticles. But, at the low k 1 lithography and in aggressive OPC these defects will cause yield excursions in wafer fabs. Many OPC programmed defect images were used to help optimize the algorithm performance using the algorithm test bench, a key development to for fast feedback on algorithm changes to allow fast algorithm optimization.

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image and the transmitted reference image. The images of four defects are shown in Figure 7.

Defect A is a very small chrome extension on a primary feature next to neighboring OPC assist features. The new generations of Tera algorithms, including XPA, are not vulnerable to sub-resolution OPC assist features.

Assist features do not degrade the inspection system performance.

Defect B demonstrates the system’s capability of detecting small defects on OPC assist features. This particular defect is a chrome extrusion on the inside of assist feature facing a primary feature.

F i g u re 5. Multi-beam UV inspection optica l s imula tion results acros s a c h ro m e / q u a r tz edge.

Initial characterization

The sensitivity of the TeraStar inspection system has been initially characterized. Using the new XPA algorithm, the inspection issues caused by OPC assist features at the 0.13 µm design rule are no longer a barrier for inspection. Figure 7 shows the defect map of the 150 nm pixel inspection on the same TSMC OPC reticle with 0.15 µm and 0.13 µm design rules with aggressive OPC assist features as was shown earlier in Figure 1 using the current generation 305UV inspection system.

On TeraStar, with the XPA algorithm and the 150 nm pixel, there are no false defects and 26 real defects, which is more than twice the real defects found on the current generation 305UV system.

There are four defect images shown in the review screen image for the die:die pattern defects detected.

The review screen shows both the transmitted test

Pindot between OPC serifs line ends.

F i g u r e 6. Scanned UV In spec tion image of OPC.

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Defect C shows that even short assist features will not impact the inspection’s ability to run without false or nuisance defects. TeraStar tolerates even short OPC assist features without de-sensing the inspection to real defects on the primary geometry of the reticle.

Defect D shows the system is tolerant not only of thin chrome assist features, but also of thin clear assist features with out false or nuisance defects, and without de-sensing the inspection performance. This defect is very small, but if it were located on the other side of the assist feature may impact CD. Immediately following this inspection of the TSMC OPC reticle, a sensitivity verification test used the same sensitivity settings for the XPA algorithm and was run on the Verithoro 5491 test reticle with the 150 nm inspection pixel. System sensitivity was verified at 100 nm using this test reticle. The programmed defects on the Verithoro 5491 test reticle are sized using the 8100XP-R reticle CD SEM system.

The Verithoro 5491 is the standard acceptance test plate which monitors system sensitivity to chrome defects, and monitors system susceptibility to false and nuisance defects. It is currently used for testing the 150 nm pixel inspection on the current generation 305UV systems. It uses geometry as small as 540 nm and has may defect types as illustrated on the right side of figure 8. In Figure 8 the results show that the system fully meets its sensitivity specification of 100 nm for all programmed defects types on the Verithoro 5491 test reticle. The new TeraStar target specification is 100 nm defect sensitivity, and is shown by the red line. In this case, the defect sensitivity using the XPA algorithm meets or exceeds the 100 nm sensitivity requirement for each defect type.

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F i g u re 7. Defect ma p a nd images from Tera Star inspection of the TSMC OPC reticle from Figure 1.

Using the same die:die sensitivity data from the TeraStar system, the sensitivity chart, (Figure 9) compares the 305UV system performance to that of TeraStar. As expected, the 305UV system, with the current AOP315 algorithm, performs to the 120 nm defect sensitivity specification with the 150 nm inspection pixel. The new XPA algorithm of TeraStar significantly improves defect sensitivity at the same 150 nm inspection pixel.

By design, the XPA algorithm has a better defect:pixel performance ratio.

available on the older 488 nm wavelength inspection system). By using the VT5491 test mask for this test, the sensitivity is compared. The linewidth capability of the XPA algorithm is also compared to the XPA algorithm. TeraStar outperforms the 300 series in sensitivity, while also running the VT5491 test reticle with no false or nuisance defects. A total of 116 false and nuisance defects appear on the 300 series because the APA algorithm was not designed to run such small geometry when the system was developed years ago.

The next sensitivity test compares the mid-range, die:die sensitivity of the TeraStar’s 250 nm pixel inspection to that of the 300 series 250 nm pixel (the smallest pixel

XPA does not have the same three pixel wide linewidth limitation as the older generation algorithms.

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TeraStar performance specification for defect sensitivity down to 100 nm

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TeraStar performance 300 Series performance

Actual

F i g u re 8. Sens itivity verification of th e TeraStar usi ng VT5491 test

F i g u r e 9. Sensitivity verificat ion of the TeraStar using VT 5491 test

reticl e.

reticle.

Other inspections

During the development of the TeraStar system, many advanced reticles were inspected as part of the standard in-house characterization. Many reticles were of 0.15 um and 0.13 um design rules. Figure 11 shows the capability of the XPA algorithm to detect CD defects. The defect shown is a 80 nm CD defect over a number of SRAM cells probably caused by a large area resist thickness variation. The difference image is a selection on the review menu to help enhance the CD defect area and highlights all the defective pixels found by the inspection system. The XPA algorithm has better sensitivity to CD defects compared to the AOP algorithm of the 300UV series.

TeraStar actual performance 305UV performance TeraStar performance specification

F i g u re 10. Sensitivity co mparison of the TeraStar us ing V T5491 test

On the same reticle shown in Figure 12, the TeraStar system found stitching errors, which are misplaced shots from the pattern generator, using the new XPA algorithm. The stitching error can be seen in the right image and is a vertical offset on five horizontal edges.

Shown in figure 12 is the transmitted light image of the test and reference die.

reticle.

The TeraStar system is also capable of inspection of PSM reticles. Shown in Figure 13 is a defect on an embedded attenuator reticle. This reticle is a tri-tone 74

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Transmitted Image - Reference Die Transmitted Image - Test Die

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Difference Image between Die

F i g u r e 11. CD d e f e c t .

Conclusions

The results of the TeraStar development program: • Freedom from inspection limitations caused by aggressive OPC designs on advanced reticles.

• Improved defect sensitivity to 100 nm through improving Kd — signal:noise and algorithm performance.

F i g u re 12. Stitch ing error c aused by pat ter n genera tor.

• Linewidth capability is better than previous generations reticle inspection systems for die:die inspection, not just by pixel, but the linewidth:pixel ratio is improved significantly. Defect sensitivity is maintained at smaller linewidths.

• Multiple inspection beams and concurrent inspection drives the cost per inspection and cost of ownership lower for the TeraStar reticle inspection system.

These capabilities are a perfect fit for comprehensive final post-pellicle inspections at the mask shop or an incoming inspection in wafer fabs.

F i g u r e13. Chrome extension on top of attenuat or.

reticle, and the defect shown is a chrome extension on top of a high transmission MoSi attenuator material.

Acknowledgment

The authors would like to acknowledge Steve Hentschel and Chris Aquino of KLA-Tencor.

Reference 1 . “High Resolution UV Wavelength Reticle Contamination Inspection”, F. Kalk, W. Volk, J. Wiley, S. Watson, E. Hou., SPIE Vol. 3748, p. 513-519.

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