Exploring the needs and tradeoffs for immersion resist topcoating
07/01/2004
Immersion lithography has quickly become the leading candidate to extending 193nm scanners to the 45nm process node. While it appears there are no major showstoppers facing immersion lithography, key questions remain about the interplay between 193nm photoresists and the fluid filling the space between exposure lenses and wafers. One potential measure to protect resist and fluid from contaminating each other is a topcoat over the photoresist. However, several shortcomings must be addressed with topcoat materials, including defects and extended process time. This article examines the practical issues associated with using a topcoat and the options available.
 Figure 1. SEM photos show 193nm exposure results using JSR resist and Nikon Corp.'s interferometric immersion tool set; a) and b) 55nm half-pitch; c) 45nm half-pitch. |
Tremendous efforts to extend optical lithography beyond the 45nm node are ongoing, and while 193nm resists continue to be the workhorse for advanced 90nm node production and 65nm node development, tight pitch requirements and small critical dimensions for features such as contact holes and isolated trenches are forcing lithographers to explore alternatives. Immersion lithography has recently been proposed as the bridge to extend 193nm lithography to the 45nm node, given that it provides a potential means for improving resolution and depth of focus (DOF) while using the existing optical infrastructure such as light source, illumination optics, masks, and pellicle materials. This technology involves filling the space between the lens and the resist with a material with a high index of refraction.
For first-generation 193nm immersion systems, water has become the most obvious choice. Initial tests and studies have demonstrated the feasibility of immersion lithography using existing 193nm technologies, including resist materials (Fig. 1). But questions remain:
- Does fluid affect photoresist performance?
- Do we need a barrier between the resist and the high-index fluid?
- Does the fluid contaminate the photoresist, or the photoresist contaminate the fluid?
From a resist standpoint, a significant challenge for immersion implementation at 193nm is immersion fluid contamination by resist components leaching into the water lens. Studies from several universities and International Sematech have confirmed this leaching of components, such as photo-acid generators (PAGs), quenchers, and other low molecular compounds, from the resist surface into the immersion liquid.
This contamination has the potential to create defects and to change the resist's PAG/quencher distribution, which can result in a resist pattern profile change. A U. of Texas study showed extraction of PAG (both exposed and unexposed) by water took place as quickly as 30 sec after initial contact between resist and water. Potential issues associated with this extraction of components from the resist may be emphasized since the intrinsic transmission of water at 193nm is so high, even a small amount of dissolved contaminant can dominate absorbance.
 Figure 2. Demonstration of the process flow for initial immersion studies. |
Preliminary lithographic data has demonstrated that the impact of resist component leaching on the resist profile and process window can be controlled dependent on resist platforms (Figs. 2 and 3). However, the potential for chemical contamination from resist to final optical lens worries both tool and device makers. While fused silica is the primary lens material at 193nm, most lens designs incorporate a final element made of CaF2 [1]. The deployment of an immersion topcoat material is proposed to stop contamination from both sides — chemical contamination from resist to final optical elements and water contamination to resist film — by forming a firm hydrophobic film over the resist surface. However, the issues associated with CaF2 solubility in water are being addressed by researchers with thin layers of SiO2 acting as a protective coating.
 Figure 3. a) Resist A shows little to no change in CD with respect to various soaks, while b) Resist B shows a large profile and CD changes over various soaking conditions. |
Other issues of immersion liquid contamination have been discussed with respect to refractive index change during exposure. Localized temperature change caused by exposure can change the fluid's refractive index, which in turn could result in process latitude changes and CD control. However, specific simulations have showed that this is relatively small. According to work from Nikon Corp., a temperature change as small as 0.011K can change wavefront aberrations by 1.7mλ, distortion by up to 3nm max, and focus shift by up to 7nm max [2].
Bubble trouble?
Bubble formation has proven to be another potential challenge for 193nm immersion. Water flow issues and photoresist outgassing can cause microbubbles. Numerous experiments have been conducted to test the formation of microbubbles as a function of water injection and dispense angle, flow rate, gap height between the lens and the resists, and wafer topography. This testing has shown that bubbles can be avoided by fine-tuning water dispense method and flow rate, and using de-gassed water.
Another concern is that microbubble formation from resist outgassing can take place during exposure. Microbubbles may form from volatile resist components, which adhere to the surface of the immersion fluid. The microbubbles created can cause light scattering and may lead to killer defects in the circuit pattern. One study has shown that careful selection of the protecting group in the resist resin can minimize or completely eliminate microbubble formation [3].
The application of a dense topcoat material has been thought to prevent volatile components from leaving the resist surface and forming microbubbles in the immersion fluid. However, the effectiveness of stopping volatile components has been questioned when the topcoat material is only coated at ~300Å thickness, which is the ideal film thickness based on theoretical calculations (λ/4ntarc).
Swing curve suppression
From an optical standpoint, we need to think about the transparency as well as the refractive index of the topcoat at 193nm. By choosing the proper refractive index, we can ensure that the material can function not only as a protective coating, but also as an antireflective coating (ARC), which can reduce the swing curve. This is critical when designers are dealing with large amounts of topography and reflective substrates where reflective notching and CD variation across the chip can be affected.
Reduction of swing ratio is a well-known advantage of using a top antireflective coating (TARC) and results from destructive interference of the phase difference between the incident and reflected light from the resist to TARC interface. Using an optimum film thickness of a quarter-wave ensures destructive interference between the incident and reflected phases of light (Fig. 4).
What's needed in an immersion topcoat
Assuming that the industry can successfully navigate the issues associated with immersion lithography and assuming that the positives outweigh the negatives for using an immersion topcoat, it then becomes critical to look at what properties are important in a topcoat material.
Resist surface properties must be carefully matched to the topcoat to achieve a successful immersion lithographic process. Dry 193nm resists were originally designed to be hydrophilic for both good wettability with developer and excellent adhesion to the underlying substrate. It was thought that the surface of current dry 193nm resist was too hydrophilic for the immersion liquid, and thus a hydrophobic surface was needed in order to create the optimal meniscus for stage movement and avoid leaving behind water droplets, introducing microbubbles or absorbing water from the immersion fluid.
It is now proposed that through careful engineering with respect to wettability and surface properties, topcoat materials can be tuned to optimally fit the immersion process and dry 193nm resist materials can be used without any modification.
As previously mentioned, a topcoat material must also be highly transparent at 193nm and have antireflective properties so that it serves as not only a protective coating but also as a TARC. Early TARCs incorporated fluorinated polymers and created systems that were able to attain an ideal refractive index to reduce the swing curve of the photoresist. These early TARCs required unique casting solvents and needed to be removed with solvent prior to development of the resist. Therefore, the benefit of swing curve suppression came at an expense to process complexity.
Additionally, environmental concerns also existed with earlier TARC systems due to the exotic casting solvents. To avoid this additional stripping step, TARCs began to use an aqueous media, which allowed for more flexibility from a manufacturing perspective. They could be applied in conjunction with the resist application process using the same equipment already being used for resist coating and exposures.
Today, standard i-line and 248nm TARCs are water-based. This characteristic does not lend itself well to immersion lithography, where water-based TARCs would be completely soluble with the immersion fluid, leading to lens contamination and many of the previously mentioned issues.
On the other hand, immersion topcoats cannot use solvents similar to those used in photoresists since any intermixing between the topcoat and the resist needs to be avoided. Therefore, when choosing a casting solvent for the immersion topcoat, look at a material that is soluble in developer to avoid an added stripping step, but is insoluble to the immersion fluid — water. This last requirement is somewhat challenging due to the fact that developer is mostly water and our first requirement was that the topcoat was water-insoluble. These three characteristics — water insoluble, no resist intermixing, and developer-soluble — will play a large role in choosing the final casting solvent and chemical aspects of designing a topcoat.
Immersion topcoats and/or TARCs could also potentially lead to defect control as seen with earlier KrF and ArF standard TARCs [4]. Because high contrast resist systems are more prone to surface defects, a TARC could prove beneficial.
By designing a specific amount of acid into the topcoat/TARC, acid diffusion from the material could migrate into the upper surface of the resist film. This can in turn make the upper surface region of the resist more susceptible to developer and effectively increase the darkloss in this part of the resist. Finally, profile compatibility to standard 193nm photoresist and compatibility to standard track configurations is also essential when selecting an immersion topcoat.
Summary
While ArF 193nm immersion technology promises great enhancement of resolution and larger DOF, there are also some potential problems facing the implementation of this technology. One potential solution to many of the issues associated with immersion lithography — such as fluid and/or resist contamination, lens degradation, and microbubbles — is an immersion-based topcoat. This topcoat material should be insoluble to water but soluble to standard photoresist developer so that it may be stripped away during the photoresist development step without the addition of any special solvent strip process.
Lithographers and resist companies are working hard to accomplish these tasks and should have development-grade materials available toward the end of this year. However, several shortcomings must be addressed with topcoat materials, such as defects, additional costs, and extra processing steps. The key will be that the benefits outweigh the negatives, helping us to reach our goal of implementing 193nm immersion lithography. Overall, the use of current 193nm resists for immersion lithography has been successful and we are moving forward with cautious optimism. It appears that the water-resist interactions will not pose major roadblocks to the implementation of 193nm immersion lithography. However, access to immersion topcoats could help us move forward with additional confidence.
Acknowledgments
The authors of the paper would like to thank Hiroaki Nemoto, Tsutomu Shimokawa, Hidetoshi Miyamoto, and Kazuo Taira for their guidance and support.
References
- M. Switkes, The Prospects for Liquid Immersion Lithography Are Becoming More Solid, Future Fab Intl.
- S. Owa, H. Nagasaka, et al., "Update on 193nm Immersion Exposure Tool," Nikon Corp. from LA Immersion Workshop, 2004.
- M. Switkes, V. Liberman, et al., "Water for Immersion Lithography," Lincoln Laboratory, MIT, Lexington, MA 02420, from LA Immersion workshop, 2004.
- R. Subramanian, G. Bains, C. Lyons, B. Singh, E. Gallardo, Deep UV Reflection Control for Patterning Dielectric layers, SPIE, 2001.
For more information, contact Mark Slezak at JSR Micro Inc.'s Lithography Group, 1280 N. Mathilda Ave., Sunnyvale, CA 94089; ph 408/543-8800, fax 408/543-8996, e-mail [email protected].