Paul Graeupner

Benefits and limitations of immersion lithography

Liquid immersion has been used for more than 100 years to increase the numeric aperture (NA) and resolution in optical microscopy. We explore the benefits and limitations of immersion technology in lithography. Immersion optical lithography has the potential to extend the resolution below 40 nm. The theory of immersion is decribed. Simulations show that a 193-nm immersion system at NA = 0.95 can double the depth of focus as compared to a dry system. Also, an immersion 193-nm system at NA = 1.05 has slightly more depth of focus than a 157-nm dry system at NA = 0.85. However, the exposure latitude at 193 nm is decreased due to the impact of polarization in imaging. Design schemes are presented to realize an immersion step and scan system. Two configuration approaches are proposed and explored. A localized shower type solution may be preferred over a bath type solution, because the impact on the step and scan platform design is significantly less. However, scanning over the wafer edge becomes the main design challenge with a shower solution. Studies are presented that look at the interaction of immersion fluids with the lens and the photoresist. Water seems to be a likely candidate, as it does not impact productivity of the step and scan system; however, focus and aberration levels need to be carefully controlled. For 157 nm, per-fluor-polyether (PFPE) materials are currently being studied, but their characteristics may limit the productivity of the exposure system. Further research on fluid candidates for 157-nm immersion is required.

Impact of wavefront errors on low k1 processes at extremely high NA

This paper presents a comprehensive study of the impact of wavefront errors on low-k1-imaging performance using high numerical aperture NA lithographic systems. In particular, we introduce a linear model that correctly describes the aberration induced imaging effects. This model allows us to quantify the aberration requirements for future lithographic nodes. Moreover, we derive scaling laws characterizing the imaging performance in dependence on the key parameters exposure wavelength λ, NA, and k1. Our investigations demonstrate, first, that an accurate control of coma is and will be crucial, and, second, that spherical requirements will be very tight for k1<0.3 due to isolated contact printing. Finally, we summarize the results of this paper in a roadmap covering the aberration requirements in optical lithography down to the 45nm node. We conclude that the improvement of wavefront quality is necessary to enable imaging enhancement techniques, but is not sufficient to replace these techniques.

Optimizing and Enhancing Optical Systems to Meet the Low k 1 Challenge

Current roadmaps show that the semiconductor industry continues to drive the usable Rayleigh resolution towards the fundamental limit (for 50% duty cycle lines) at k1=0.25. This is being accomplished through use of various resolution enhancement technologies (RETs), extremely low aberration optics with stable platforms, and resists processes that have ever-increasing dissolution contrast and smaller diffusion lengths. This talk will give an overview of the latest optical mechanisms that can be used to improve the imaging system for low k1 resolutions. We show 3 non-photoresist techniques to measure the optical parameters of a scanner: 1) a new fast phase measurement interferometer to measure aberrations is presented with an accuracy and repeatability of <3mλ, 2) we introduce a method to measure the illumination profile of the exposing source, and 3) a measurement system to monitor scattered light is presented with correlation to other techniques using a salted pellicle experiment to create controlled scattered light. The optimization of illumination and exposure dose is presented. We show the mechanism for customizing illumination based on specific mask layers. We show how this is done and compare process windows to other more conventional modes such as annular illumination or QUASAR. The optimum design is then implemented into hardware that can give extremely high optical efficiency. We also show how system level control mechanisms can be used to field-to-field and across-field exposure to compensate for lithography errors. Examples of these errors can include reticle CD deviations, wavefront aberrations, and across-field illumination uniformity errors. CD maps, facilitated by SEM and ELM, can give the prescribed changes necessary. We present a system that interfaces to new hardware to compensate these effects by active scanner corrections.

The next phase for immersion lithography

Immersion Lithography is now the most important technique for extending optical lithographys capabilities and meeting the requirements of the Semiconductor Industry Association (SIA) roadmap. The introduction of water as an immersion fluid will allow optical lithography to progress as far as the 45nm (half pitch) node using ArF scanning systems such as the XT1700i. Developments are under way to explore the use of immersion lithography beyond this performance level and toward the 32nm (half pitch) node. This paper examines the progress that has been made, particularly with the use of 2nd-generation immersion fluids. The requirements of the exposure system are defined. Issues associated with achieving the requirements are reviewed and discussed. Special attention is given to clarifying the optical materials and the issues associated with extending optical designs to hyper-numerical aperture (NA) levels. A number of threshold levels for the numerical apertures are set by the refractive index of the available materials in the lithographic film stack. These are defined. The requirements of high refractive index fluids are detailed. The performance of experimental samples is compared to system requirements. Fluid interaction with photoresists and topcoats are examined. The results of stain tests and soak tests for fluid samples on resist are reported. Data is supplied on resist imaging for 32nm line and space L/S.

Deep Ultraviolet Out-of-Band Contribution in Extreme Ultraviolet Lithography: Predictions and Experiments

Extreme ultraviolet lithography (EUVL) sources emit a broad spectrum of wavelengths ranging from EUV to DUV and beyond. If the deep ultraviolet (DUV) reaches the wafer it will affect imaging performance by exposing the photoresist. Hence it is critical to determine the amount of DUV out of band (OoB) present in a EUVL tool, as well as its effect on the printed features on the wafer. In this study we investigate the effect of DUV OoB in EUVL. A model is developed in order to be able to quantify the DUV/EUV ratio at wafer level and all the required input parameters are estimated in the range from 140 to 400nm, as well as for the EUV at 13.5nm. The transmission of the optical system was estimated based on the optical design and reflectivity measurements of the mirrors. The mask reflectivity for multilayer (ML) and absorber was measured at wavelengths down to 140 nm and for EUV. The sensitivity to EUV and DUV for a variety of resist platforms was measured at 13.5 nm, 157 nm, 193 nm, 248 nm and 365 nm. The source spectra were also measured. By using these inputs, it was possible to estimate the DUV/EUV ratio for two different ASML tool configurations, the EUV Alpha Demo Tool and the NXE:3100. Both NXE:3100 with LPP (laser produced plasma) source and Alpha Demo Tool with DPP (discharge produced plasma) source show less than 1% DUV/EUV ratio in resist. The modeling predictions were compared to experimental results. A methodology is introduced to measure the DUV/EUV ratio at wafer level in situ. With this aim, an aluminum coated mask was fabricated and its reflectivity was qualified in both EUV and DUV wavelength range. By comparing the dose to clear exposures of a reflective blank and of the aluminum mask, it is possible to quantify the DUV/EUV ratio. The experimental results are in order of magnitude agreement with modeling predictions. The proposed experimental approach can be used to benchmark the DUV sensitivity of different resist platforms and may be used to monitor DUV OoB.

Extending immersion lithography with high-index materials: results of a feasibility study

In this paper we report the status of our feasibility work on high index immersion. The development of high index fluids (n>1.64) and high index glass materials (n>1.9) is reported. Questions answered are related to the design of a high NA optics immersion system for fluid containment and fluid handling, and to the compatibility of the fluid with ArF resist processes. Optical design and manufacturing challenges are related to the use of high index glass materials such as crystalline LuAG or ceramic Spinel. Progress on the material development will be reviewed. Progress on immersion fluids development has been sustained. Second-generation fluids are available from many suppliers. For the practical use of second-generation fluids in immersion scanners, we have evaluated and tested fluid recycling concepts in combination with ArF radiation of the fluids. Results on the stability of the fluid and the fluid glass interface will be reported. Fluid containment with immersion hood structures under the lens has been evaluated and tested for several scan speeds and various fluids. Experimental results on scan speed limitations will be presented. The application part of the feasibility study includes the imaging of 29nm L/S structures on a 2-beam interference printer, fluid/resist interaction testing with pre- and post-soak testing. Immersion defect testing using a fluid misting setup was also carried out. Results of these application-related experiments will be presented and discussed.

High-n immersion lithography

A two-year study on the feasibility of High-n Immersion Lithography shows very promising results. This paper reports the findings of the study. The evaluation shows the tremendous progress made in the development of second-generation immersion fluid technology. Candidate fluids from several suppliers have been evaluated. All the commercial fluids evaluated are viable, so there are a number of options. Life tests have been conducted on bench top fluid-handling systems and the results referenced to full-scale systems. Parameters such as Dose per Laser Pulse, Pulse Rate, Fluid Flow Rate, and Fluid Absorbency at 193nm, and Oxygen/Air Contamination Levels were explored. A detailed evaluation of phenomena such as Last Lens Element (LLE) contamination has been conducted. Lens cleaning has been evaluated. A comparison of High-n fluid-based technology and water-based immersion technology shows interesting advantages of High-n fluid in the areas of Defect and Resist Interaction. Droplet Drying tests, Resist Staining evaluations, and Resist Contrast impact studies have all been run. Defect-generating mechanisms have been identified and are being eliminated. The lower evaporation rate of the High-n fluids compared with water shows the advantages of High-n Immersion. The core issue for the technology, the availability of High-n optical material for use as the final lens element, is updated. Samples of LuAG material have been received from development partners and have been evaluated. The latest status of optical materials and the technology timelines are reported. The potential impact of the availability of the technology is discussed. Synergy with technologies such as Double Patterning is discussed. The prospects for <22nm (hp) are evaluated.

Solutions for printing sub-100-nm contacts with ArF

This study assesses the various approaches to printing contacts in the sub 100nm regime using 193nm. Traditional techniques are analyzed along with the use of tri-tone contacts and pupil filtering. Approaches using attPSM masks looks promising down to pitches of 300nm. Below this, assist features may be needed to prevent residual artifacts due to sidelobes. For pitches > 400nm the use of tri-tone masks show a significant improvement in process latitude and ease of overlapping process windows. The pupil filter solution does not seem provide any significant improvement as compared to other solutions with the exception that it provides the lower MEF. Realization of this solution will increase machine complexity and will possibly impact throughput, especially if using transmission filters. However, pupil filtering can be an option for isolated contact layers that are printed with binary masks. We find that the process and enhancement techniques to print a dense contacts and isolated contacts to be vastly different. This may require a split into two exposures if an extensive pitch range is needed.

Printing 130-nm DRAM isolation pattern: Zernike correlation and tool improvement

To meet lithographic requirements for the 130nm generation, the influence of aberrations on printing of various patterns is investigated. This paper shows a process for patterns that are sensitive to coma and three wave. The aberration sensitivities are calculated and the effect on printing experimentally verified. This analysis leads to slight changes in lens adjustment strategy to accommodate the printing of specific DRAM patterns. Additional improvements in materials and surface figures, as well as reduction in process-induced aberrations and associated RMS wave front error, enable the production of tools that are capable of printing the 130nm device generation. The importance of collaboration between makers of lithography tools and their customers cannot be underestimated in finding tool specific limitations. Because of the length of the design cycle of lithography tools it is necessary to perform analysis of device patterns years in advance. The current work also indicates that patterns historically used to determine lens specifications, such as dense and isolated lines, are insufficient to fully determine lens specifications. This paper also outlines techniques that can be used to reduce aberration sensitivities by use of resolution enhancement techniques. This is another area where close interaction between vendor and customer is needed.

High-NA EUV lithography exposure tool progress (Conference Presentation)

While EUV systems equipped with a 0.33 Numerical Aperture (NA) lens are readying to start high volume manufacturing, ASML and Zeiss are in parallel ramping up their activities on an EUV exposure tool with an NA of 0.55. The purpose of this high-NA scanner, targeting an ultimate resolution of 8nm, is to extend Moore’s law throughout the next decade. A novel lens design, capable of providing the required Numerical Aperture, has been identified; this lens will be paired with new, faster stages and more accurate sensors enabling the tight focus and overlay control needed for future process nodes. In this paper an update will be given on the status of the developments at Carl Zeiss and ASML. Next to this, we will address several topics inherent in the new design and smaller target resolution: M3D effects, polarization, focus control and stitching.