74881m

Experimental Test Results Of Pattern Placement Metrology On Photomasks With Laser Illumination Source Designed To Address Double Patterning

Lithography Challenges

Klaus-Dieter Roeth, Frank Laske, Michael Heiden, Dieter Adam, Lidia Parisoli, Slawomir Czerkas, John Whittey, Karl-Heinrich Schmidt

1 KLA-Tencor MIE Division GmbH, Kubacher Weg 4, D-35781 Weilburg, Germany

ABSTRACT

Double Patterning Lithography techniques place significantly greater demand on the requirements for pattern placement accuracy on photomasks. The influence of the pellicle on plate bending is also a factor especially when the pellicle distortions are not repeatable from substrate to substrate. The combination of increased demand for greater accuracy and the influence of pellicle distortions are key factors in the need for high resolution through-pellicle in-die measurements on actual device features. The above requirements triggered development of a new generation registration metrology tool based on in-depth experience with the LMS IPRO4. This paper reports on the initial experimental results of DUV laser illumination on features of various sizes using unique measurement algorithms developed specifically for pattern placement measurements.

Keywords: Registration Metrology, mask metrology, Double Patterning Lithography, advanced reticles, through-pellicle measurement, in-die registration measurement, LMS IPRO4

1. INTRODUCTION Development of 45nm–node reticles has been completed at most leading edge mask shops, and the development of next generation 32nm–node reticles is currently underway. This will be followed by 22nm node reticle development in the coming months. Double Patterning Lithography (DPL) is considered by many as a backup for EUV in the 2X node and beyond, with or without the aid of advanced illumination techniques such as freeform illumination and Source Mask Optimization. DPL requires that the specifications for pattern placement on reticles tighten by a factor of two or more over previous generation requirements.

Besides so-called spacer technology already in use today, there are two different DPL technologies under consideration. These technologies are based on splitting dense patterns and putting adjacent structures onto two separate reticles. In the cases of LELE (Litho-Etch-Litho-Etch) and LFLE (Litho-Freeze-Litho-Etch) technologies, two masks are used for each critical layer in order to reduce the pitch by a factor of two, thus enabling further extension of 193nm immersion lithography for electronic device manufacturing. The dominant competing technology under consideration, Spacer Pitch Splitting (SPS) does not require a second mask or extremely tight specifications for the critical layers; instead the second mask is a trim mask. In the case of spacer technology the involved process steps may lead to higher manufacturing costs.

Any kind of DPL technology requires significantly improved writing performance of photomask pattern generators. Considering that pattern placement specification tolerances may tighten below 5nm (3 sigma) in 2010, there is a risk that mask production yields may drop considerably. E-beam placement performance in dense pattern arrays is one particular area of concern. In dense pattern arrays, any pattern displacement could lead to fatal CD errors on the wafer. Therefore the actual pattern placement performance of the litho tool should be investigated and tracked within the active array1. In-die photomask pattern placement metrology is critical to ensure stable yields on DPL reticles2,3.

An additional area of growing interest is the impact of pellicle-induced distortions. This seems to be a manageable issue for the current device generation4, but definitely needs R&D attention and improvement in order to meet overall DPL-related reticle registration specifications of 4nm or better since every possible error contributor must be squeezed to the minimum. Leading edge mask shops need to have the flexibility to verify registration error on in-die features as well as characterizing pellicle induced distortions on one single tool. This avoids the need for additional investment for a second registration metrology system. Through-pellicle measurement capability combined with high resolution in-die

Photomask Technology 2009, edited by Larry S. Zurbrick, M. Warren Montgomery, Proc. of SPIE Vol. 7488, 74881M · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.833203

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measurement capability requires a long working distance objective with a high numerical aperture to achieve the needed measurement resolution.

Figure 1 summarizes the technology challenges driving the need for new approaches on the registration metrology tool.

Fig.1: In-die metrology becomes the critical issue with DPL introduction. An additional concern is the error contribution of pellicle-induced distortions, requiring high resolution and through pellicle measurement capability

2. EXPERIMENTAL SETUP The next generation registration metrology tool based on LMS IPRO4 is under development at KLA-Tencor. This tool is designed to meet the tight measurement performance requirements for qualification of DPL reticles for the 22nm node as well as for the qualification and control of future generation e-beam tools.

At the beginning of the development, all major sub-components of the actual LMS IPRO4 were checked, and each sub-component’s contribution to the error budget was investigated. The sub-components were then evaluated for potential improvement to meet target performance of the next generation. Several of the sub-components, like the environmental chamber, could be improved, but other sub-components must be redesigned.

Previous papers have reported that the i-line based LMS IPRO4 system equipped with a high N.A. lens can provide excellent results on dense line/space (L/S) pattern as small as 125nm half pitch as well as on a dense 200nm contact hole arrays2. In the past, some mask manufacturers felt the need to check pellicle induced distortions in order to control and improve the pelliclization process. Therefore, a long working distance (6.9mm) objective is available as an option for the LMS IPRO4. This microscope objective, however, has a numerical aperture of only 0.55, limiting the measurement capability for small dense structures to 250nm HP.

For the next generation metrology system, most mask manufacturers have requested both capabilities in one system: high resolution for in-die measurement on actual device structures and through-pellicle measurement capability. This requires both high resolution and a long working distance greater than 6.3mm (which is the maximum height of the

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pellicle frames). In order to meet these requirements the illumination system for the LMS IPRO5 was completely redesigned. According to the Rayleigh equation (1) optical resolution can be enhanced by either increasing the N.A. or by using shorter wavelength illumination.

Rayleigh Equation: (1)

The IPRO5 development team took both approaches simultaneously: laser illumination at 266nm was implemented and a new objective was developed with a numerical aperture of 0.8. The shorter illumination wavelength provides >25% resolution enhancement and the increased N.A. an additional 30% resolution enhancement.

Using coherent DUV laser illumination instead of a Hg-Xe lamp requires a completely new concept for the illumination path and the imaging objective design. The coherence of a laser light source leads to speckling effects which can be observed as brightness deviations over the illumination field. Although there are commercial solutions available, the actual performance of the readily available commercial units could not meet the required homogeneity for the next generation metrology system. The LMS IPRO4 image analysis algorithms had to be modified and optimized for laser illumination operation. We prepared an LMS IPRO4-based test bench for detailed investigations of the performance of the new illumination components. The measurement capability on small dense structures on state-of-the-art reticles including CoG, PSM, OMOG, and EUV masks has been tested with investigations continuing.

Test bench #1 uses a 266nm CW laser and a proprietary DUV laser de-speckling unit. Since the newly developed high-NA, long-working-distance imaging objective could not be mounted on the focusing device of the bench, a standard high-NA DUV microscope objective was used for the test measurements. The imaging device as well as the related electronics and algorithms were integrated into the test bench. Test bench #1 operates in a standard LMS IPRO4 climate chamber. The automatic LMS IPRO4 handling system was not included in the test bench setup.

A second performance test bench (referred to as test bench #2) was set up in order to verify performance improvements from further development of the laser interferometers and the environmental chamber of the metrology system. The measurement performance of test bench #2 was evaluated on an IPRO4 test mask, following the IPRO4 standard acceptance test procedure for short term repeatability and nominal accuracy. This procedure ensures compatibility with currently available test data and IPRO4 user experience. Twenty dynamic measurement loops were evaluated for short term repeatability using the statistics per point method implemented into the proprietary LMS IPRO4 data evaluation software, DEVA. All data provided here are max 3 sigma values. The mask was rotated on the stage and measurements performed in all four orientations.

3. RESULTS

3.1 Optimum Illumination Performance Achieved

High illumination stability is mandatory to achieve stable measurement results for a registration metrology tool. The stability of the laser source and de-speckling unit were investigated and optimized on test bench #1. For reference, the camera noise was checked first as a function of the number of pixels binned. For this evaluation the illumination source was a mercury-xenon lamp to ensure speckle-free illumination as a base reference. The evaluation was then repeated with the 266nm laser source and the proprietary de-speckling unit in place. Fig.2 shows the final result at optimum de-speckling conditions. The x-axis gives the number of pixels binned in the camera and the y-axis represents the obtained camera noise deviation in digits between lamp setup and laser setup. Under standard measurement conditions, when 3 or more pixels are binned, the impact from the laser illumination on camera noise is less than 0.004 digits. This value far exceeds the development target set at 0.02 digits.

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Figure 2: Illumination noise versus pixels binned on the camera, observed on test bench #1. The impact of the illumination on the camera noise is less than 0.02 digits under standard operation conditions.

3.2 2x Enhanced Measurement Capability on Dense Structures

Several customer test samples were investigated in detail to measure the resolution enhancement expected from the DUV illumination at higher N.A. versus the actual capability improvements on dense arrays of line/space pattern and contact holes. Following the Rayleigh equation (1) resolution, the test setup with 0.55 NA and 6.9mm LWD lens was expected to deliver twice the resolution of the LMS IPRO4 while enabling through-pellicle measurement (fig 3).

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Figure 3: From the Rayleigh equation (1) a 2x resolution enhancement is expected for the next-generation IPRO’s 266nm wavelength and 0.8 N.A. versus the IPRO4’s 365nm and 0.55.

Investigations were performed on Chrome-on-Glass substrates, on phase shift (MoSi) test masks, and on advanced OMOG material. Figures 4-7 compare the images obtained on the standard LMS IPRO4 with LWD lens (top) and the images obtained on the same features on the DUV Test bench #1 (bottom). The green frames indicate which patterns have sufficient contrast to be measured and to achieve at least standard performance specifications. In almost all cases (the only exception is dense contacts on COG mask), a 2x resolution enhancement was observed, as was expected from theoretical values.



.

Figure 4: 200nm dense L/S on MoSi (ARF-HT) PSM can be measured on the IPRO4, while on the DUV test bench, dense L/S features as small as 100nm can be measured.

Figure 5: 200nm dense contacts on MoSi (ARF-HT) PSM can be measured on the IPRO4, while on the DUV test bench, contacts as small as 100nm were measured.

Figure 6: 200nm dense L/S on OMOG reticle can be measured on the IPRO4, while on the DUV test bench, dense L/S features as small as 100nm can be measured.



Figure 7: 200nm dense contacts on OMOG reticle can be measured on the IPRO4, while on the DUV test bench, contacts as small as 100nm can be measured.

3.3 Registration Measurement Performance Results on Test Bench #2

On test bench #2, short term repeatability and nominal accuracy performance were investigated. Twenty dynamic measurement loops were performed for short term repeatability following the standard LMS IPRO4 acceptance test procedure. The mask was then rotated three times (0, 90, 180 degree orientations), and measured in each orientation 10 dynamic loops, and the data evaluated using the proprietary LMS DEVA data evaluation software.

The short term repeatability performance is shown in figure 8 and the accuracy performance in figure 9.

Fig 10 is a comparison of the test bench performance to the average LMS IPRO4 performance. For reference, a red line indicates the LMS IPRO4 specification and a green line the next-generation LMS IPRO specification target. As it can be clearly observed, the measurement performance of test bench #2 is already close to the target specification of the LMS IPRO next generation tool under development.

Figure 8: Dynamic short term repeatability performance of the test bench #2

Figure 9: Measurement accuracy performance of the test bench #2



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Figure 10 Comparison of the performance achieved on test bench #2 versus the average performance of all LMS IPRO4 systems installed. The red line indicates the measurement specifications of the actual IPRO4 and the green line the targets for the next-generation LMS IPRO system.

4. CONCLUSION AND OUTLOOK Registration requirements for DPL reticles will very likely require different sampling, including heavy in-die measurements1,3. In addition, registration requirements are so tight that contributions from pellicle-induced distortions have to be considered and investigated4. The combination of those requirements can be achieved on one tool by utilizing shorter illumination wavelength and a high NA long working distance objective. The DUV laser illumination paired with a 0.8 N.A. long working distance objective for the next-generation LMS IPRO provides a 2x resolution enhancement, as predicted by the Rayleigh equation and supported by measurements in this paper.

The resolution performance of the new optical path of the next generation registration metrology tool was investigated on a test bench. As expected, the optical resolution provides enhanced measurement capability on dense pattern on various types of reticles, including COG, MoSi, halftone PSM, and the latest OMOG mask types. On substrates for future use, e.g. PSM and OMOG, it was demonstrated that dense contacts as small as 100nm and dense L/S patterns with 100nm half pitches provided sufficient contrast for high performance registration measurement (fig. 4-7). This is a two-fold enhancement over the LMS IPRO4 performance when a long working distance objective is used.

In addition, it was verified on the test bench that laser illumination, as proposed for use in the new IPRO tool, can be successfully substituted for the lamp illumination used by the LMS IPRO4. The residual speckling effect contributes less than 0.01% to the camera signal at the actual operation point.

On another test bench, modifications to the optical path and environmental conditions were tested. The actual measurement performance on this test bench was 25-30% better than the average performance of the LMS IPRO4 systems installed, and it is currently very close to the target specifications for the next generation metrology tool (fig10).

Achieving the expected performance data on the test benches inspires confidence that the development project is on schedule and system integration and fine tuning can continue.

ACKNOWLEDGEMENT

The author would like to thank the German BMBF for funding project CDuR32 to support R&D of the next-generation reticle pattern placement metrology system.

In addition, the authors would like to thank Hubert Altendorfer of KLA-Tencor for the fruitful discussions and suggestions.



REFERENCES

[1] G.Hughes: Mask Metrology – Current and Future Challenges, http://www.eeel.nist.gov/812/conference/2009_presentations/Hughes.pdf, 2009

[2] K-D. Roeth, F. Laske, H. Kinoshita, D. Kenmochi, K-H. Schmidt, D. Adam: In-Die Registration Metrology on Future Generation Reticles, Proceedings of SPIE Vol. 7272, 2009,

[3] K-D Roeth, F Laske, M Heiden, D Adam, A Boesser, K Rinn, A Schepp, J Bender, In-Die Mask Registration Metrology for 32nm Node DPT , SPIE Vol. 7379, 2009

[4] R. de Kruif, T. van Rhee, E. v.d. Heijden: Reduced Pellicle Impact on Overlay using High Order Intrafield Grid Corrections, 25th EMLC 2009



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