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Micro/Nano Lithography

Pushing the Limits

In order to deploy 157-nm lithography in 2005, researchers are rushing to overcome obstacles in lenses, pellicles, resists, and contamination

From oemagazine March 2002
28 March 2002, SPIE Newsroom. DOI: 10.1117/2.5200203.0004

Semiconductor manufacturers have chosen to extend optical lithography technology to the 157-nm wavelength and to provide feature sizes of 70 nm or smaller for high-volume production of a broad range of advanced memory and logic devices. Their roadmaps call for the technology to be available in 2005, but this requires that a number of technical obstacles be overcome (see oemagazine, March 2001, page 20). Industry groups including International Sematech (Austin, TX), Selete (Yokohama, Japan), the Association of Super-advanced Electronics Technologies (ASET; Tokyo, Japan), and MEDEA+ (Paris, France) in Europe are promoting technology reviews with integrated-circuit (IC) manufacturers, suppliers, and universities, as well as awarding development contracts to identify and provide the initial solutions.

Several new materials need to be developed for these next-generation lithography lenses, optical coatings, resists, masks substrates, pellicles, and absorbers. The issues related to the laser source and reticle substrate blank have been solved in principle and are being commercialized. Engineering solutions have been demonstrated for issues related to the optical lens design, optical coatings, effective contamination control and purging of the optical path, and reticle molecular cleaning.

Not all has gone smoothly; reticles defect control and lens materials continue to challenge developers. The supply of calcium fluoride (CaF2) material for lens manufacturing has the highest perceived risk, aggravated by the material's unexpectedly strong intrinsic birefringence. In this article we put these challenges in perspective and describe their current status and the actions that are being taken by equipment suppliers.

materials challenges

Because conventional optical and fused-silica glasses are opaque at 157 nm, CaF2 material is used instead. The intrinsic birefringence of CaF2 was reported by the National Institute of Science and Technology (NIST; Gaithersburg, MD)1 and measured to be an unexpectedly large value of 11 nm/cm to 12 nm/cm.2 Its effect on the image performance has been evaluated and must be accounted for in optical lens design (see page 23). Compensation requires a preselected crystal axis for each lens element. By clocking lens elements of both <111> and <100> crystal orientations, engineers from ASML Wilton (Wilton, CT) and Carl Zeiss (Oberkochen, Germany) produced modeling data demonstrated a residual relative contrast loss of approximately 3%.

As of this writing, the material is still in short supply without a single good solution. This material still needs a factor of three to five improvement in homogeneity and stress birefringence, based on data recently obtained with post-growth annealing of <111> crystals. The new <100> CaF2 material needs to be optimized in quality to obtain the same target as the <111> material. In addition, yield improvements of about 25% per year are required to obtain the necessary quantities in both crystallographic directions.

A pellicle protects photomasks from particulate contaminants out of the focal plane. A soft pellicle is the solution of choice, but development has proved more difficult than originally anticipated. Researchers have readily developed a transmissive polymer, but its useful lifetime, curtailed by photochemical darkening, is still insufficient by almost three orders of magnitude. Each pellicle should be able to handle 5000 wafers for volume production (equivalent to a 1% transmission drop for a total dose of 1.2 kJ/cm2). Failure analysis is necessary to understand the photochemical darkening issue.

Because the industry has lost confidence that soft pellicles will be ready in 2004, research groups are investigating other approaches. Pellicle-less solutions such as thermophoresis, developed for the vacuum systems of extreme ultraviolet (EUV) lithographic units, cannot be applied due to the unacceptable temperature gradient on the lens. Hard pellicles are seen as a secondary alternative. With a typical thickness of 800 µm, however, a hard pellicle would act as an additional optical element and impact the imaging and overlay performances. Developers have achieved good optical homogeneity and surface finish for hard pellicles, but improvements to the mounting frames are required to keep pellicle bending below 4 µm and to avoid significant optical distortion. Hard pellicles, which can cost up to $6000, compared to $800 for a soft pellicle, place additional concern on the system cost of ownership.

No industry-wide agreement has been reached for removable pellicles. Removable pellicles detach during each exposure, which increases the risk of generating particles. They also require novel reticle handling and reticle stage machine concepts. Thin (less than 10-µm thick) inorganic pellicles are another possibility, although they require much more research.3

Progress in the development of 157-nm resists is illustrated by several new polymer platforms bringing the transmittance of the resist to a practical level. Clariant (Muttenz, Switzerland) recently announced the first commercial 157-nm resist for sub-100-nm resolution.4 Based on university work sponsored by International Sematech, this resist combines low optical density (2.7/µm), straight profiles at moderate resist thickness (1330 Å), and good dry-etch resistance. The industry consensus target for resists includes optical densities below 1/µm and resist film thickness above 2000 Å. To reach this target, all new resists need improved line edge roughness, substrate compatibility, and contrast to achieve sufficient imaging quality for contact holes and dense lines.

systems issues

The main changes in the exposure system platform are related to purging of the optical path. Optical absorption caused by air is four orders of magnitude higher at 157 nm than at 193 nm. The entire exposure system needs to be designed and maintained contamination-free, with the complete optical path (including the wafer and reticle stages) exposed to only part-per-million concentrations of oxygen, water, and organic molecules. Researchers at MIT Lincoln Labs (Lexington, MA) have analyzed materials outgassing. ASML and Zeiss are developing operational functional models for the purification of the purge gases.

Special attention is needed in selecting non-outgassing construction materials, cleaning optics and metals, and the additional ultraviolet ozone (UVO) in-situ lamp cleaning. The increased sensitivity to contamination requires that the reticle handling system be modified. An additional molecular cleaning step is needed before the reticle is exposed. Active pellicle purging is being developed in the reticle handling system to allow the space enclosed by the pellicle to be nitrogen-filled.

An all-refractive optical design would require either a line-narrowed injection-locked laser, which does not appear to be cost effective, or the use of a second refractive material such as barium fluoride (BaF2), which is not available. Therefore, tool suppliers have chosen to use catadioptric projection optics, which are compatible with the larger 1-pm bandwidth line of the natural line-selected high-power F2-molecular excimer laser source.

The timing of introduction of the exposure tools is another critical issue. Sematech has made an Exitech microstepper tool with a 0.6 numerical aperture (NA) lens available for exposure of samples provided by resist vendors. The first full-field exposure tool, ASML's MSVII, will have a 0.75 NA and a usable area of 26 x 34 mm. It will be used for R&D at IMEC (Leuven, Belgium). Such a tool is essential to facilitate early process and device development, to enable a 157-nm learning curve, and to drive the 157-nm infrastructure and vendors. ASML plans to introduce its first 157-nm production tool in 2004. It will be based on the Twinscan platform with a NA of better than 0.8, a 4:1 projection lens reduction ratio, and a 26 x 33 mm field size.

If the history of previous wavelengths is any guide, 157-nm technology is likely to be extended with extremely high NA second-generation tools that enable line features down to about 50 nm. This wavelength is likely to be used until EUV challenges are overcome and inserted for volume production. oe

Acknowledgements

The author would like to thank Bill Arnold, Jan Mulkens, and Paul van Attekum of ASML Veldhoven for reviewing the manuscript, and Reiner Garreis of Zeiss, James McClay of ASML Wilton, Judon Stoeldraijer, and Steve Hansen of ASML TDC for their input.

References

1. J.H. Burnett et al., 2nd International Symposium on 157-nm lithography, May 2001.

2. D. Kraehmer et al., ISMT Workshop on Intrinsic Birefringence, July 2001.

3. M. Switkes et al, 3rd International Sematech Data Review on 157nm Lithography, December 2001.

4. R. Dammel et al., 3rd International Sematech Data Review on 157-nm lithography, December 2001.


Stephane Dana

Stephane Dana is product manager with ASML, Veldhoven, The Netherlands.