BACUS symposium goes strong, despite downturn
12/01/2001
Meeting in Monterey, CA, more than 400 maskmakers discussed new technology, cost factors, and potential future problems at the 21st annual BACUS Symposium on Photomask Technology. A record number of papers were submitted, while the number of exhibitors stayed steady with last year.
New reticle pattern generators excited the most interest, with the ETEC (an Applied Materials company) MEBES eXara roll-out as well as first reports from Micronic Laser Systems of SLM imaging performance. The long awaited MEBES eXara 50 keV electron-beam pattern generator was presented as capable of 100nm photomask production and 70nm reticle development. Equipped with a vacuum-compatible air bearing stage with 13-in. movement, the MEBES eXara can pattern substrates of any likely size. Since it uses ETEC's familiar 320MHz raster scanning system — rather than the shaped-beam vector scan system of current 50keV tools — the writing time is supposedly independent of the mask complexity, recently increased by pervasive OPC.
 RAVE's nm1300 system combines AFM, cantilevers, nanotechnology, and 3D software. |
Alan Stivers of Intel presented one surprising paper predicting increased printability for small reticle defects after the transition from DUV to EUV lithography. Essentially, the increased k1 factor of the EUV system results in better printing fidelity for defects that previously had been too tiny to image. The result would be decreased control of edge geometries, especially for the gates with exponentially critical effects on circuit behavior.
Cecilia Smolinski of IBM gave a paper from the IBM/Photronics EUV mask consortium that revealed a prohibitive number of defects — many thousands per cm2 — on multilayer EUV mask blanks. Simulated reticles made on 200mm silicon wafers with only the top layers of an EUV mask displayed fewer than 100 defects each in die-to-die inspection, suggesting potential improvement as the technology matures.
Advances in reticle enhancement techniques (RETs) are leading some lithographers to consider using 248nm tools for sub-100nm gates, and even high mask costs have not discouraged current worldwide use of alternating aperture phase shift masks (AAPSM) in late pilot or early production, noted Jim Reynolds of Reynolds Consulting, Sunnyvale, CA.
A key process step in manufacturing "strong shifter" AAPSM masks is a deep trench etched into the quartz mask substrate. Focused ion beam (FIB) and laser repairs have not been successful in repairing quartz bump defects that can result from this trench etch process. Left behind are edge roughness and surface imperfections that can affect the transmission characteristics of the mask.
RAVE LLC, Delray Beach, FL, has developed a nanomachining technology that combines atomic force microscopy (AFM), cantilevers, nanobit technology, and 3D software. RAVE's nm 1300 advanced mask repair system is essentially a computer-controlled milling and shaping machine that operates at the nanometer level. This technology can repair MoSi and carbon patch defects on AAPSMs, carbon patch defects on chrome masks, and quartz defects on AAPSMs within sub-500nm line/spaces (see illustration). The nm 1300 can repair defects in deep trenches with a depth-to-width ratio exceeding 0.75:1, quartz trench depths down to 450nm.
This new tool can detect defects smaller than 100nm in size and has sub-10nm trench depth and sub-25nm edge controls. In a comparison of nanomachining and FIB repairs of bump defects on AAPSMs, AMD, Sunnyvale, CA, and DuPont Photomasks, Round Rock, TX, reported FIB had 45% successful repairs on 30 sites while nanomachining demonstrated 100% successful repairs on 10 sites.
In the realm of exposure tools, 193nm is ramping up. This year 40-50 193nm scanners have been delivered, according to Gartner Dataquest's Klaus-Dieter Rinnen. The shorter wavelength brings up issues not critical in 248nm systems, such as the protection of optics.
Current filtration systems used in 248nm tools can adequately protect DUV resists and the optical system. However, at some cutoff, shorter wavelength, higher energy photons may interact with refractories commonly found in the fab environment and cause permanent damage to the lens systems.
While organic solvents such as isopropyl alcohol and acetone can be turned into volatile species such as CO2, effectively cleaning the lens surface, this is not so for refractories. Because of an attached inorganic atom, when refractories are absorbed on the surface of optics and interact with high-energy photons, sticky free radicals form on the lens, attenuating or occluding light. A transition point for refractories occurs between 248 and 193nm light, thus the need for filtration systems, such as the Extraction 1000, which provides both resist and optics protection.
Jim Wiley of KLA-Tencor reported the highlights of a panel discussion at Photomask Japan 2001 on 100nm reticle technology. According to Toshiba, masks are consuming the majority of the CD error budget for 110nm devices. Additionally, mask costs have risen by a factor of 10 for every factor of 4 reduction in critical chip dimension. That still represents a reduction of 19% in cost/unit pixel area for each factor of 2 in CD shrink, but individual masks do seem fated to reach six-figure costs soon.