An OASIS by the sea at BACUS 2002
12/01/2002
The atmosphere of the 22nd BACUS Photomask Symposium in Monterey, CA, was oddly upbeat, with a record 156 papers and high attendance, in spite of depressed industry conditions. The mask industry has been hard hit by the slow shift to 130nm and beyond, since most of industry revenue comes from such advanced reticles.
However, as Naoya Hayashi of Dai Nippon Printing (DNP) reported, the advanced pattern generators needed to print such masks cost four times more than previous equipment but run 10 times slower, resulting in a fortyfold cost increase. Yet the advanced technology supports only a 5¥ reticle price increase from the 500nm node.
The keynote speaker, Buno Pati of Numerical Technologies, hailed the challenge of manufacturing production-quality, subwavelength masks as an opportunity for the maskmaking industry to recover its former prominence in semiconductor manufacturing and capture more of the value it creates (see Fig. 1). Pati made the case for the maskmakers to leverage the investment made and drive the semiconductor industry.
Many changes will be needed to profit from the opportunity, however, and some of them will conflict with the interests of established players. To thrive, the mask industry must take control of its own agenda, noted Pati.
Of special concern is the potential to lose ASIC customers. He urged the industry to accommodate this valuable market segment, particularly by reducing the number of copies of die on critical layer reticles for low-volume production runs, where, he notes, the higher cost of wafer lithography is more than offset by the savings in mask costs. He also called for greater communication between IC designers and mask designers.
"Communication by itself won't solve the problem," stated Dinesh Bettadapur, president and CEO of ASML MaskTools. "We need an infrastructure to promote better understanding of each other's domains. People have become so specialized that we need an integrated way of looking at the problem — local optimization in a global context."
 Figure 1. How the semiconductor industry shares in value creation. |
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Even the old standby used in other industries — systems engineering — won't solve the problem, according to Bettadapur, because the differences between the disciplines are too great. Bettadapur is more optimistic when it comes to a discussion of maskmakers enabling their ASIC customers' success. "ASIC designers won't become FPGA designers overnight," he explains. "There might be a market share loss, but ASICs won't go away anytime soon."
One highlight was the roll-out of the replacement for GDSII as the data format for specifying masks. The new proposed 64-bit stream format, called OASIS, is dramatically more efficient in almost all contexts and has been widely endorsed, but realizing the full benefit of updating the bit-level layout paradigm will require implementation of a "universal data model" also being developed by Semi. According to Tom Grebinski, Semi Data Path Task Force Chairman, $4–6 billion is being lost each year because of inefficiencies and poor communication embedded in the data path. Replacing GDSII is just a start toward more productive practices.
Selete and Semi had been developing competing standards, but during the conference Selete announced that, after collaborating with task force members, it now supports the Semi effort. Grebinski said Selete plans to refocus its efforts further downstream to optimize the native intermediate formats used by electron-beam mask generators.
"Selete will be supporting the development of OASIS and Semi will be offering help to Selete with development of its data link to OASIS and with ongoing concerns of Selete's membership," Grebinski said.
Pattern generators
The first of the new generation of fast stepper-like laser pattern generators has been delivered to DuPont Photomasks, according to Tor Sandstrom of Micronic Laser Systems. The tool benefits from some of the advantages of the partially coherent imaging used in wafer exposure tools, but cannot allow its illumination to be optimized for any particular pattern.
Nevertheless, by tilting the micromirrors that form the images beyond the level that gives "dark," the system can produce destructive interference between pixels comparable to attenuated phase shifting masks, improving contrast and resolution.
Tom Newman of Etec Systems/Applied Materials described an extension of the MEBES eXara to a "raster shaped beam" technology that allows real time proximity correction and should support 50nm photomask development — with 4nm precision and less!
Inspection tools
In a potentially major development, Anja Rosenbusch of Etec Systems/Applied Materials described a new 193nm aerial image inspection system that employs technology similar to the Zeiss MSM-193 review station.
These so-called AIMS tools emulate the optical performance of the actual exposure tools and identify defects in the aerial image, rather than the mask itself. Masks incorporating strong phase-shifting technologies require inspection on such a tool. The new Israel-developed Etec system promises to detect 60° and 120° phase defects as well as any other defects capable of causing 10% CD variations at the 120, 90, and 65nm nodes, with 2-hr turnaround time for a 10cm square inspection area.
Initial results on an AIMS tool engineered for 157nm were reported by a team from International Sematech, Zeiss, and Infineon. Because of the need to protect everything from oxygen and moisture, the entire a-tool lives in a glove-box. Targeted for the 65nm and 45nm nodes, this inspection system has already shown adequate laser beam stability and is overcoming the challenges of laser speckle in the image.
 Figure 2. Mask prices are rising quickly. |
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Larry Zurbrick of KLA-Tencor described a phase-contrast enhancement method ("Teraphase") that allowed new KLA mask inspection tools to detect phase defects more efficiently using transmitted light alone.
In another paper, Dan Bald and a team from Intel and KLA-Tencor described the DIVAS system, which saves and analyzes the images of defects seen by mask inspection tools. Subsequently applying simulation technology allows the importance of different anomalies to be judged.
For EUV masks, Alan Stivers of Intel described a multibeam confocal microscope inspection system capable of detecting 93% of programmed surface-bump defects on multilayer substrates. Such defects, due to buried particles or other causes, are expected to be printable even if they are only 1.25nm high and 60nm across. The good news is that the 33 beam optical tool could detect such a large fraction of the programmed defects. The bad news is that it also found many, many more apparently real defects than had been anticipated; these would probably print on wafers. Thus, further study — and possibly actinic inspection — may still be needed.
Repairs
Repairing the anomalies detected on photomasks has long been problematic. Some repair processes do more harm than good. Volker Boegli of NaWoTec GmbH in Rossdorf Germany described a gentle e-beam enhanced chemistry approach to fixing clear and opaque defects.
By locally dissociating an organo-metallic precursor with a tightly focused electron beam, NaWoTec deposits platinum and other metals, even forming bridges on stencil masks. TaN, an absorber used in EUV lithography, can be etched using e-beam activated XeF2, but for now, chrome defects cannot be removed.
Alfred Wagner of IBM described an innovative laser method to remove chrome with femtosecond laser pulses. When such short pulses are focused to high power they directly break chemical bonds, atomizing materials without thermal damage. According to Wagner, IBM uses the second-generation MARS2 tool as its primary means of repairing 248nm and 193nm masks.
The highly automated system can remove material gently in "nibble" mode with 8nm 3s edge placement accuracy, carve 80nm holes and trenches, and keep transmission loss below 2.5%, according to Wagner. What was not presented was any feasible scheme to commercialize a turnkey version of the system.
Robert Muller from LSI Logic described a management system for dealing with photomask defects in a wafer fab. In real life, it sometimes pays to tolerate defects, according to Muller. The challenge is to identify those cases efficiently. LSI developed a web-based detection, classification and dispositioning database tool to help operators of mask inspection stations make the best decisions. Sometimes, when a multichip reticle has a killer defect on one die, it is better to accept the yield loss than to order a new reticle.
"Virtual inking" identifies the known bad die produced by such a defect and saves cost by eliminating further processing. Using this system, LSI entirely eliminated rejections of new photomasks (and related delays and cost increases) for three months.
When good reticles go bad
Kaustuve Bhattachryya of KLA-Tencor described syndromes where DUV exposure caused soft defects to form on the pellicle-protected reticle surface. Most defects caused a ~13% reduction in transmission, but even that could kill yield. Chemical analysis of the material on the surface revealed ammonium sulfate, cyanuric acid, and an unknown organic. The speculation was that the DUV exposure was causing vapor-phase photochemical reactions among the species outgassing into the confined pellicle-reticle space. The less-volatile products deposited on the reticle surface, whereupon crystallites nucleated more rapid deposition.
After an unpredictable number of wafer exposures — perhaps 700, perhaps nine — the reticles became unusable and had to be cleaned and re-pellicled. This phenomenon bodes ill for 157nm lithography schemes involving nonremovable hard pellicles.