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Tool/Fab Support Strategies

Pushing dry 193-nm lithography technology to the limit

Holly H. Magoon, Nikon Precision

In the last 25 years, there has been dramatic progress in the pursuit of enhanced lithographic resolution, enabling continuous improvements in IC production. These advancements have been driven by reducing the exposure wavelength and increasing the projection lens numerical aperture (NA). The current generation of photo engineers has extended optical lithography far beyond what previous generations considered feasible, and many in the industry believe that the next step forward will be the implementation of immersion lithography. While the use of immersion tools is rapidly approaching the production stage, continuing efforts to extend the capabilities of affordable dry 193-nm lithography will be critical to maintaining cost-effective semiconductor manufacturing for some time.

As defined in Rayleigh's resolution equation, R = k1 X (λ/NA), the primary method for lowering the minimum resolution has been through reduction of the exposure source wavelength (λ) in conjunction with increasing the lithography system's projection lens numerical aperture. A 193-nm exposure wavelength has been the industry standard for several years, and today's critical scanners feature lens NAs >0.90, very close to the dry numerical aperture limit of 1.0. Thus, the remaining means to extend dry technology is through optimization of k1, where k1 represents the process-dependent factors affecting resolution. Previous process-related resolution enhancement efforts have focused on reticle design, using methodologies such as phase shifting or pattern splitting on dual masks. While these techniques improve imaging, they also have significant drawbacks, including throughput loss and the high cost of reticle generation. When investigating k1 optimization, it is vital to take into account all of the factors that influence resolution (including many aspects of the lithography scanner itself).1

This article discusses scanner factors and characteristics vital to the extension of dry lithography. It also examines ways to manipulate scanner capabilities to meet the aggressive imaging challenges of the future. In addition, it focuses on advanced polarization control, which contributes significantly to enhancing image contrast and providing the process latitude required for sub-90-nm applications. Also considered are scanner lens design and manufacturing methodologies, scanner functions useful in optimizing imaging performance beyond initially specified levels, and ways to minimize exposure to environmental contamination through scanner design.

Polarization Control

While modern ultrahigh-NA lenses provide imaging capabilities far exceeding those of previous-generation lithography systems, industry experts agree that advanced polarization control will be required to successfully achieve sub-90-nm critical dimensions (CDs) with sufficient process latitude. Polarization control enhances image contrast, resulting in improved resolution and translating into k1 benefits for the user. Heightened contrast also reduces the lithography process's sensitivity to deviations in dose or focus errors, extending process latitude.

Standard scanner illumination systems are designed to provide a random mixture of s-polarized light, which enhances image contrast at higher NAs, and p-polarized light, which degrades image contrast at higher NAs. This random light was sufficient for yesterday's lower-NA systems with far less stringent imaging requirements, but it is no longer adequate. To achieve the critical imaging objectives of sub-90-nm applications, it will be necessary to control the type of polarized light used; that is, the s-polarized light must be maximized and the p-polarized light suppressed.

There are two main methods of polarization control. One technique is to incorporate a polarization filter that separates the s- and p-polarized light components. The use of filters can cause at least 50% of the light to be lost and thus result in lower throughput. If laser power is increased in an effort to compensate for lost light, the usable lifetime of both the laser and the optics is reduced, which has a negative impact on the tool's cost of ownership.2

Figure 1: Effect of a traditional polarization filter, which leads to reduced light intensity (top), and advanced polarization control without loss of intensity (bottom).

An alternative technology, developed by Nikon (Tokyo) and known as Polano, has recently been made available to lithographers.3 This technology enhances image contrast, without loss of illumination power, as seen in Figure 1. It is designed to utilize the polarized laser beam efficiently, controlling the polarization direction while maintaining the desired degree of polarization in the illumination homogenizer.4 In addition, the control provides significant flexibility, allowing user selection of the delivered polarization distribution via independent settings for each different mask exposure file.

Figure 2: Effects of advanced polarization control on depth of focus for a 60-nm line/space pattern: (a) without polarization control, (b) with polarization control.

The depth of focus (DOF) enhancement that was achieved using advanced polarization control is demonstrated in Figure 2. These results were obtained at Nikon using the company's NSR-S308F deep-ultraviolet (DUV) lithography tool with an NA of 0.92, a 6% attenuated phase-shift mask, and standard dipole illumination. A separate evaluation by Infineon Technologies (Dresden, Germany) using an NSR-S307E DUV lithography tool with an NA of 0.85 and an alternating phase-shift mask for a 62-nm line/space pattern also confirmed the benefits of polarization control. Exposure latitude, depth of focus, and the mask error enhancement factor all improved using polarization control, as shown in Figure 3.6

Figure 3: Improvements in exposure latitude, depth of focus, and mask error factor achieved using advanced polarization control for a 62-nm line/space pattern.

Lens Design and Manufacturing

When considering a scanner's effect on k1, it is necessary to look at the earliest phases of the lens design and manufacturing process, as both have a significant impact on the tool's imaging capabilities. The first crucial element is the lens design. The use of kinematic lens mountings, for example, eliminates stress-induced birefringence. Another vital aspect is the quality of the materials used. Impurities in calcium fluoride (CaF2), which is used in some lens elements, are known to increase short-range flare, which can degrade image contrast and, if varied across the field, can contribute to across-field CD variation. Therefore, only materials with the highest purity and lowest possible birefringence should be used.6

In addition, because surface roughness also affects short-range flare, lens polishing, annealing, and assembly techniques have a significant influence on a scanner's imaging performance. Effective lens-manufacturing methodologies incorporate sophisticated polishing techniques, including the repolishing of aspherics, which can be extremely effective in compensating for higher-order aberrations. In addition, a power spectral density analysis should be performed for each lens element surface to ensure that superior quality control is achieved.

Another important aspect of successful lens-manufacturing processes is the ability to measure the wave-front aberration content accurately. This capability is achieved by using a phase-measurement interferometry (PMI) system, which determines Zernike coefficients up to F81. For optimal results, PMI data measurements should be performed at several stages throughout the lens-manufacturing process. As indicated in Figure 4, progress in these technologies has enabled a continued decrease in projection lens aberration content over several generations of scanners.

Figure 4: Residual lens aberration versus NA for four generations of Nikon scanners.

Application-Specific Lens Tuning

A scanner's imaging performance can also be enhanced by fine-tuning the lens aberration content for user-specific applications and imaging configurations (i.e., NA and illuminator settings). Such tuning can have a significant impact on the tool's full-field resolution capabilities. As discussed elsewhere, it is understood that aberrations in areas of a lens not being used to image a particular pattern (or patterns) do not influence ultimate imaging capabilities.7 This combination can therefore provide a pathway for application-specific lens tuning.

Before a scanner can be optimized for specific pattern and imaging conditions, however, a variety of information is required. It is necessary to understand the present aberration state of the lens, which is obtained via in situ PMI tools such as the ISI aberration measurement instrument from Litel Instruments (San Diego).8 In addition, the present adjustment state of the lens, which is read from the scanner itself, must be understood. Knowledge of the impact of various adjustments on the aberration state, which is inherent to the lens design, is also required. Beyond these prerequisites, a vehicle to convert the scanner's present aberration state, specific pattern details, and imaging goals into a usable lens prescription—paired with a lens capable of field tuning—is necessary.

Nikon's technology for aberration optimization (TAO) is one such vehicle. After the user defines the specific pattern requirements and imaging performance criteria (e.g., allowable limits and assigned priorities), software determines the lens prescription for that scanner that best satisfies the user's objectives.

To evaluate the system's capabilities, tests were run using five-bar patterns of 150-nm lines on a 300-nm pitch at four different orientations (0°, 90°, 45°, and 135°). The scanner used in the tests had not been adjusted previously and had a considerable amount of linewidth abnormality (LWA), as seen in Figures 5a and 5c. However, its spherical aberration (SA) and total focus deviation (TFD) were considered acceptable. Therefore, the goal was to reduce LWA without negatively affecting TFD or SA. As shown in Figures 5b and 5d, results improved markedly following implementation of the TAO-recommended prescription, with LWA well below the 0.03 target. In addition, SA was reduced and TFD was successfully maintained. After the implementation, measured results mirrored the software's prediction, confirming its simulation capabilities.

Figure 5: Measured LWA for five-bar patterns of 150-nm lines: (a) 0° and 90° before lens prescription, (b) 0° and 90° after prescription, (c) 45° and 135° before prescription, and (d) 45° and 135° after prescription. The target LWA was 0.030. LWA values are shown as vector components. (→ 0.020.)

It should be noted that although the level of improvement for a particular scanner depends on such factors as the starting lens adjustment condition and illumination distribution, technologies such as TAO enable lithography engineers to significantly enhance imaging capabilities and provide a means to extend the usefulness of existing systems to next-generation applications. Such technologies also allow a single lens to be optimized and used for multiple layers, reducing the need for dedicated scanners and increasing flexibility in the fab. With the increasing price of each generation of lithography systems, such benefits are valuable to semiconductor manufacturers seeking to control costs.

Other scanner features can also be employed to achieve imaging objectives, such as expanding the depth of focus. Methods such as the continuous DOF expansion procedure (CDP) have been shown to be effective in increasing the DOF for contact holes without affecting throughput. With CDP, the wafer is tilted along the scanning direction, while the wafer stage continuously moves upward or downward during exposure. Recent evaluations of the technology using an S307E scanner with an NA of 0.85 indicated that DOF for a 90-nm contact hole improved from 0.2 to 0.3 Ám. This finding supported earlier data showing a 35% improvement in DOF for 250-nm contact holes (Figure 6). Such technologies provide an enhanced process window, eliminating the need for double exposures and thereby enabling throughputs comparable to those for a standard wafer exposure.

Figure 6: Depth of focus for 250-nm contact holes using a scanner with and without the continuous DOF expansion procedure.

Flexible Dose Adjustment

IC manufacturers are well aware that both their processes and processing equipment can have a significant influence on final imaging. Such process factors as resist coating nonuniformities, varied topography across the field or wafer, and edge effects can all cause across-wafer and shot-to-shot CD variation. Root-cause investigations of variations are frequently time-consuming, and corrections may involve costly changes. Therefore, scanner features able to mitigate process-induced variability will deliver k1 benefits.

After resist is dispensed onto the wafer surface, wafers are spun rapidly to distribute the liquid evenly. Any irregularities in spin-process rotation control can lead to uneven resist coatings. Deviations in temperature control during prebake can also contribute to uneven coatings. Fortunately, scanners that permit flexible dose adjustment across the wafer can mitigate the impact of processing inconsistencies on incoming wafers. There may also be cases where certain exposure shots experience repeatable shifts in optimum dose or abnormal best focus, which cannot be eliminated by standard engineering methods. Irregular shots may be induced by resist-thickness variation or extreme topography, which result in abnormal behavior at the wafer edge. However, regardless of the source of irregularities, the ability to define individual exposure conditions for each shot can compensate for them.

Contamination Prevention

Advanced DUV lithography systems are far more susceptible to the effects of environmental molecular contamination than were earlier systems, which patterned significantly larger features.9 Environmental contaminants can cause film formation involving the scanner's projection optical components, degrading lens coatings and negatively affecting transmittance of the optical elements. The accompanying decrease in illumination power and uniformity contributes to throughput loss and leads to a need to replace costly optics. Such contamination-related films also have detrimental imaging effects, including reduced image contrast and elevated CD nonuniformity.

Some level of environmental contamination is present in all cleanrooms. However, it is possible to minimize tool components' exposure to such contaminants and their effects through meticulous protection of the scanner's main chamber and projection lens. Various protective measures are implemented during system design and manufacture:

Material selection. Only lens-element mountings completely free of contamination and contamination-causing materials should be used.

Engineering solutions. Comprehensive filtration systems and optimized chamber airflow should be coupled with nitrogen purging of projection optics.

Manufacturing solutions. Lens components should undergo UV-light and chemical cleaning before being integrated into the scanner.

Storage/shipment considerations. Only containers filled with pure nitrogen should be used to store and ship optical components.

Preventive measures such as these have been effective in maintaining optimal equipment performance.

Conclusion

Dramatic progress has been made in pursuit of pattern resolution enhancements, enabling continuous advances in IC manufacturing. Historically, these technological achievements have been driven by reductions in lithography systems' exposure wavelength paired with increases in lens NAs. A 193-nm exposure wavelength has been used for several years, and critical scanners incorporate ultrahigh NAs. Therefore, future improvements will come from optimizing the process-related factors that can affect resolution.

Many aspects of scanner design and manufacturing have a significant impact on resolution capabilities, including minimization of flare and aberration content, benefits provided by advanced polarization control, and preventive measures to protect sensitive scanner components from environmental contamination. In addition, both dry and immersion systems incorporate many features that can be used to minimize process influences, enable imaging beyond specified levels, and extend the useful lifetime of existing equipment. These factors are critical to the extension of cost-effective dry lithography, a first step toward enabling IC manufacturers to continue to provide technology innovations for years to come.

Acknowledgments

This article is a revised version of a poster presentation at the IEEE/SEMI Advanced Semiconductor Manufacturing Conference, held April 11–12, 2005, in Munich. The author wishes to acknowledge the significant contributions of Steve Slonaker and Gene Fuller. She would also like to thank Bernie Wood and members of the Nikon Precision and Nikon Precision Europe technical departments for their support.

References

1. H Magoon, "Scanner Influences on Resolution Capabilities," in Proceedings of the Advanced Semiconductor Manufacturing Conference (Piscataway, NJ: IEEE, 2005).

2. S Owa et al., "Full-Field Exposure Tools for Immersion Lithography," in Proceedings of SPIE Optical Microlithography XVIII, vol. 5754, (Bellingham, WA: SPIE, 2005), 655–668.

3. P Clarke, "Nikon Supports High-NA Litho with Polarized Illumination," EETimes [online] November 2004 [cited 9 June 2005]; available from Internet: http://www.eetimes.com/news/latest/showArticle.jhtml?articleID=542011431.

4. H Nishinaga et al., "Development of Polarized-Light Illuminator and Its Impact," in Proceedings of SPIE Optical Microlithography XVIII, vol. 5754 (Bellingham, WA: SPIE, 2005), 669–680.

5. R Pforr et al., "Polarized Light for Resolution Enhancement at 70 nm and Beyond," in Proceedings of SPIE Optical Microlithography XVIII, vol. 5754 (Bellingham, WA: SPIE, 2005), 92–106.

6. T Matsuyama et al., "Nikon Projection Lens Update," in Proceedings of SPIE Optical Microlithography XVII, vol. 5377 (Bellingham, WA: SPIE, 2004), 730–741.

7. S Slonaker and H Magoon, "The TAO of Lens Adjustment—Realizing Pattern-Specific Optimization," Microlithography World 13, no. 4 (November/December 2004), 5–25.

8. K Rebitz and A Smith, "Characterizing Exposure Tool Optics in the Fab," Microlithography World 8 (August 1999), 10–26.

9. DA Kinkead et al., "Prevention of Optics and Resist Contamination in 300 mm Lithography—Improvements in Chemical Air Filtration," in Proceedings of SPIE Metrology, Inspection, and Process Control for Microlithography XV, vol. 4344 (Bellingham, WA: SPIE, 2001), 739–752.


Holly H. Magoon joined Nikon Precision in 1995, holding both technical and supervisory positions in the applications department. During that time, she specialized in methods of productivity optimization in the lithography sector and coauthored several publications. In 2003, Magoon joined the company's marketing department as marketing manager, focusing on the development and launch of new products and technologies. She received a BS in chemistry from St. Michael's College in Colchester, VT, in 1995. (Magoon can be reached at 802/879-5027 or hmagoon@nikon.com.)


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