1999's

04/01/1999

In a flurry of next-generation lithography (NGL) activity, three major stepper vendors, including an ASM Lithography-Applied Materials-Bell Labs alliance, have announced plans for development of post-optical technologies.

• The Applied-ASML-Bell Labs deal unites two of the largest equipment companies behind the SCALPEL projection e-beam lithography technology. The agreement calls for the three firms to collaborate on commercializing SCALPEL, a Bell Labs technology under development since 1989. SCALPEL and extreme ultraviolet technology were recently identified by International Sematech as the two most promising next-generation lithography candidates, and will receive near-term funding from the consortium of chipmakers.
  
• Nikon said it would collaborate with IBM on development and commercialization of a reduction-projection
  e-beam system with roots in IBM's in-house e-beam program. The system would have several aspects in common with SCALPEL, including SiN membrane masks used to scatter electrons during the imaging process.
  
• SVG Lithography said it would have a 157-nm small-field miniscanner available in the second half of 2000, with production tools to follow in 2H01, said officials (see "Full speed ahead on 157-nm lithography," p. 26). For geometries below 65 nm, SVGL is pursuing extreme UV technology and will accept orders by year-end. Delivery times are vague; shipments (initially in the form of small-field units) will be no earlier than 2003, but the tools are expected to be ready for production by 2007.

The exact roles of the three SCALPEL partners are still being defined, said Lloyd Harriott, head of advanced lithography development at Lucent Technology's Bell Labs. While Holland-based ASML has extensive experience building steppers, "Applied is a huge equipment manufacturer, and SCALPEL involves vacuum systems and other things that ASML doesn't have experience with."

Bell Labs has a proof-of-concept system running now. The next step is a high-throughput tool capable of 100-nm imaging. This design rule range is a possible insertion point, although advocates of optical extension feel NGL will not be needed for several more generations. Throughput goals are currently about 17, 300-mm wafers/hr, but Harriott called this "very preliminary." Higher beam current and a faster stage are key elements in improving throughput.

IBM also has a prototype version of its system in operation at East Fishkill, NY, and anticipates that production versions could be used in fabs around 2006 at sub-100-nm design rules. There will be no restrictions on Nikon's marketing of any products that may result. The Nikon-IBM team intends to cooperate with the Bell Labs group on mask development; the two systems may be compatible with a common mask. Nikon will continue to explore the feasibility of 157-nm technology, and continue evaluating EUV with the Japanese ASET consortium. - P.N.D.

New technology picks up at CD SEMs' limit

Significant advances in probe tip technology for nanoscale stylus profilometry, together with its deployment in a new metrology tool for CDs, sidewall slopes, and trench depths, may provide a partner to CD SEM technology. CD SEM is entrenched at the leading edge for line-width and aspect ratio measurements. But, "at some point, [with] 180-, 160-, or 130-nm features, it's going to hit the wall," notes Bob Jaynes, VP of sales and marketing, at Surface/Interface Inc., Sunnyvale, CA. Working with technology licensed from Lucent Technologies, Bell Laboratories, and field trials directed by Jeff Bindell at CIRENT Technologies, Orlando, FL, Surface/Interface is set to commercialize a $1.1 million durable optical fiber probe tip with a system that controls sample and probe tip interactions to minimize probe tip erosion.

"The heart of these kinds of tools and their application to CD measurements is in the probe tip. If you cannot clearly define it and be certain it is not changing, you cannot take its contribution out of the linewidth measurement, which means you don't have an accurate measurement," says Jaynes. The response of a stylus microscope to a sample surface is in general a complicated nonlinear function of the shape of both the sample and stylus. If the stylus shape changes during scanning, caused by erosion or contamination, it introduces a corresponding error in the measurement. The durability of the stylus depends on its shape, its composition, the material being measured, and its interactive force with the sample surface.

The new Surface/Interface technology uses a cylindrical, silica glass probe tip made from a specially chosen optical fiber that is drawn from a preform across a pure silica rod. This process has produced tips 85-500 nm in diameter and 250-1500 nm long. Tips are matched to measurement requirements.

The glass probes are integrated with a balance beam force sensor with sensitivity as low as 35 nN. This is a one-dimensional sensor sensitive to vertical forces. "For each data point," says Jaynes, "the probe touches the surface only once, which is a key to probe longevity and stability, with a preset repulsive force. Each Force-Controlled Interaction (a trademark of Surface/Interface) represents a pixel, with resolution determined by the spacing of the pixels." All lateral motion of the probe occurs when it is retracted from the surface being measured.

The beauty of this combination of probe tip and sensor technologies is found in the longevity of the tip and predictable repeatability of the resulting measurements. While wear rate depends on contact force and the material scanned, stylus use is probably most appropriately measured in the number of pixels rather than the conventional total scan distance.

According to Joe Griffith and Eric Houge of Lucent Technologies, tests -which kept materials and the scan algorithm constant and varied probe force by no more than a factor of two - studied a range of materials' CD measurements (see figure, p. 26). This work found that:

• Probe tips were used for extended periods on samples, including photoresist, silicon, and silicon nitride, with little change in their shape from either erosion or contamination.
  
• The cylindrical shape is relatively resistant to changes in width.
  
• The probe's amorphous silica does not suffer plastic deformation typical of metal probes and rarely experienced chipping or other catastrophic failures.
  
• Moderately hard materials, such as silicon and silicon nitride, eroded the glass probes so slowly that it did not
  affect measurements (fastest wear was noted with metal-coated quartz).
  
• No transfer of photoresist to the glass probes was observed.

True line and trench profiles from which measurements are made result from removing the contribution of the fully-characterized probe tips from the raw data. The figure shows such a profile overlaid on the SEM image of the line after cross-sectioning. Nondestructive SNP profiles are obtained significantly faster than SEM images of cross-sectioned features and can be taken at multiple points. This technology can also be used for CD-SEM calibration. - P.B.

0.1-?m silicon from 0.18-?m, double exposure on horizon

Using its leading-edge 248-nm lithography 0.18-?m process, Motorola is fabricating production PowerPC microprocessors with 0.1-?m features. This was accomplished using phase-shifting (PS) and optical proximity correction (OPC) technologies from Numerical Technologies (NumeriTech). The solution lies in integrating OPC and PSM into the process, all the way through design layout, mask manufacturing, process development, and silicon fabrication.

Motorola now has an alliance with NumeriTech, the industry's first for subwavelength design and manufacturing, incorporating PS and OPC to produce additional ICs. NumeriTech also has alliances with KLA-Tencor, Applied Materials, Zygo, DuPont Photomasks, Photronics, and Duet. Fabio Pintchovski, director of Motorola's Advanced Products Research and Development Lab, says, "This enables us to routinely produce logic circuits with high-performance transistors and subwavelength gates. More importantly, it allows current-generation manufacturing tools to provide next-generation performance."

This accomplishment lays bare many arguments against the difficulty in extending optical lithography, and particularly the application of phase-shift technology. "This is a powerful paradigm shift for the industry, one that we have been looking for," says Pintchovski.

NumeriTech CEO Buno Pati said, "Motorola's foresight in integrating our phase-shifting technology will eventually benefit the entire industry. The only currently viable production technology for efficiently producing ICs below 0.18 micron is a judicious combination of PS and OPC." NumeriTech also has a hand in double exposure lithography, which may be an important boost for optical lithography. It has developed a technique for photomask design and inspection that splits a circuit pattern onto two separate masks, one equipped with phase-shift features, the other with a traditional binary pattern.

In addition, stepper vendor Canon says its Innovative Double Exposure by Advanced Lithography (IDEAL) system could allow 193-nm lithography to produce 80-nm features, while next-generation 157-nm machines could print features as small as 50 nm. There are clear downsides to any double-exposure technique, most notably the inherent reduction in throughput and the need for very tight overlay on the second exposure. But these problems might seem attractive to chipmakers when compared to challenges associated with nonoptical technologies. NumeriTech executives say their technology allows two relatively undemanding and less-expensive photomasks to replace a single very costly mask, thus reducing cost of ownership compared to traditional phase-shift methodologies.

To take full advantage of the concept, circuit designs should be tailored for phase-shift technology rather than corrected after the fact, and NumeriTech is working to that end. NumeriTech's alignments with maskmakers stems from the fact that "you need to inspect these masks in a pretty different way, and it's difficult without the right tools, " said Pati. DuPont has done substantial work and fabricated the masks used by Motorola. Likewise, the company's equipment alliances are designed to link inspection tools with a software-based "virtual stepper" that will combine the images from the two masks and estimate the image that will be produced on the wafer. KLA-Tencor is adding the capability to its 300-series inspection tools, and Applied to its 800-series equipment; Zygo is developing an off-line work station.

Canon also said it is optimizing its exposure tools for the IDEAL technology, with the first steps including an extensive lens-tuning program that has reduced wavefront aberration by a factor of four or five.