Interface conference shows optical lithography's milestones
01/01/2001
Exposure tools and resists for 157nm lithography are still very much in the laboratory; 193nm lithography is making some progress in broad development, perhaps slightly behind its anticipated learning curve, but workhorse 248nm lithography continues to make somewhat remarkable progress. These conclusions were outlined by the three lead papers (listed below) and others presented at Arch Chemicals's Microlithography Symposium Interface 2000, held late last year. Together, these papers clearly ranked the status of optical lithography research and production:
- "Status of 157nm lithography" from MIT Lincoln Labs;
- "Determining the current limits of 193nm lithography" from International Sematech; and
- "Techniques for printing sub-0.2—m contact holes in logic devices at 248nm," from TSMC (Taiwan Semiconductor Manufacturing Co.)
In his keynote talk on the status of 157nm, MIT's Rod Kunz noted critical items included timing, cost of ownership, contamination, a suitable resist, and pellicles. Kunz said all the issues seem susceptible to engineering solution, but organic pellicle materials presently have lifetimes 100 times too short. Surface contamination of exposure tool lenses can be removed using ppm concentrations of oxygen in the nominally oxygen-free environment. Tiny amounts of contamination originating in gas plumbing can prove disastrous, but electro-polished stainless steel tubing keeps the gas clean. One unresolved question is whether the large fluorine content of anticipated 157nm resists will contribute so much atomic fluorine to etch plasmas that silicon dioxide layers will no longer act as etch-stops.
International Sematech, as reported by James Beach, used an installed 193nm exposure tool and an alternating phase shift mask (PSM) to achieve 1:1 100nm lines, 130nm contacts with 9% attenuated PSM, and 52nm lines by overexposing the 100nm 1:1.5 features (see SEMs). "The 100nm features were obtained with a 0.4—m DOF without process optimization. These early results suggest that this 0.6 NA tool is capable of supporting work at the 100nm node," said Beach. However, details, such as the correct anti-reflection coating (ARC) material, softbake temperature, and the suppression of airborne chemical contamination, matter.
TSMC's advanced production level work, described by J.H. Chen, described how to print sub-0.2—m contact holes and contact hole arrays using established 248nm exposure and attenuating PSMs. This method relied on a variety of transparent assist features to suppress unwanted side-lobe exposures and extend depth of focus (DOF), but today's software is not able to handle the resulting complexity on a full logic chip.
193nm feature slimming
According to Beach (and three Interface 2000 papers), researchers working with 193nm resists have found an unexpected effect — resist slimming caused by SEM metrology — that calls into question the precision of SEM linewidth measurements on ArF resist. The linewidths of ArF resists shrink when exposed to scanning electron beams, sometimes by as much as 8% per measurement, reported Tom Sarubbi and his co-authors from Arch Chemicals, Honeywell, KLA-Tencor, LETI-CEA, STMicrolectronics, Applied Materials, and Clariant. Acrylate-based materials seemed somewhat worse than maleic anhydride co-polymers and the SEM system and voltages also matter. Nevertheless, this electron-beam induced linewidth slimming was dramatically larger for ArF materials than for previous DUV formulations. The conditions that seemed to minimize shrinkage also reduced image quality. If two seconds or more were spent trying to acquire a decent image, the linewidths could shrink and then begin increasing linearly.
Copper lithography
Deeper into Interface, two papers reported improvements in dual damascene copper patterning using Arch's TIS-2000 bilayer resist system. According to Ivan Pollentier and his co-authors from IMEC, Belgium, conventional DUV resist materials can show severe footing when coated on low-k dielectrics and also fail to pattern properly at the edges of regions with the high topography characteristic of via-first damascene. The thick planarizing underlayer of the TIS-2000 system permits patterning of 160nm dense and isolated vias with 193nm radiation as well as 250nm vias over severe topography using 248nm exposure.
Bill Gadson of Tactical Fabs, Fremont, CA, said the development of the TIS underlayer (in an experimental dry developer supplied by his co-authors from Lam Research) controllably recessed the top of the via plug below the nitride hard mask defining the bottom of the trench. This prevents the formation of ring-shaped fences or veils around via openings in the bottom of the trenches caused by excess bottom antireflection coating (BARC). While the bilayer process showed the expected improvement in process window compared to conventional thick DUV resist over BARC, it also showed an unexpectedly large dense-isolated bias, necessitating OPC correction. — M.D.L., P.B.
ASML launches new 193nm exposure tool
While the annual Interface Microlithography Symposium is devoid of equipment and materials exhibits, ASM Lithography used the gathering of key industry lithographers to announce its second generation 193nm (ArF) exposure tool — the PAS 5500/1100.
 ASML's projection for the market transition from 248nm (KrF) to 193nm (ArF) optical lithography. |
Equipped with a 600kg Zeiss Starlith 1100 projection lens, AERIAL II illuminator (with the QUASAR off axis illumination option) and an ATHENA wafer alignment system, the 0.75NA system is intended for production through the 100nm node. The first use, however, is expected to be by low volume ASIC producers who cannot afford expensive reticle-level enhancement technology for 130nm (see graph), said Ron Kool, ASML product marketing manager. "As the industry migrates towards 100nm over the next three years, more manufacturers will adopt ArF, including DRAM makers, ultimately," he said. First shipments of the Model 1100 are anticipated in April 2001.
Accommodating the new heavier lens required redesigning the dynamics of the mechanical system. Other innovations include a 193nm wavelength reticle-wafer stage alignment system, which allows the elimination of long-wavelength alignment apparatus inside the lens. However, the alignment system reduces the field size to 26mm x 32mm. A sensor used for reticle alignment on the wafer stage also facilitates aberration mapping and correction, according to ASML Fellow Donis Flagello.
"With controls for magnification, field curvature, third-order distortion, coma and spherical aberration, the Model 1100 is an engineer's dream," reports Flagello. — M.D.L.