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Startup taps digital holography for wafer inspection

Posted: 26 Dec 2002 ?? ?Print Version ?Bookmark and Share

Keywords:analog holography? wafer inspection? contacts? trench capacitors? semiconductor wafers?

A metrology startup is pitching the use of digital holography to peer into the narrow and deep spaces, such as contacts and trench capacitors, on semiconductor wafers. Alpha versions of nLine Corp.'s Fathom inspection system were installed at International Sematech early this year and at Advanced Micro Devices Inc.'s (AMD) Austin fab in August.

The International Technology Roadmap for Semiconductors lists high-aspect-ratio inspection (HARI) as one of the industry's toughest technical challenges, and nLine claims its digital approach offers a solution. Conventional, analog holography, invented by Dennis Gabor in 1949, requires film, with all of the time delays associated with the chemical development and display of film-based images. What nLine calls direct-to-digital holography records optical holograms, with a CCD camera and computer interface, directly to a hard drive for immediate display on a computer monitor.

Digital holography measures both the amplitude and phase of light. Because the phase is directly proportional to wavelength, and because measurements of phase can be made to one-hundredth or even one-thousandth of the wavelength, digital holography can be used to detect defects that are smaller than the wavelength of deep-ultraviolet light, said nLine founder and CTO, C.E. Thomas.

Other forms of inspection, such as scanning electron microscopes, are relatively limited in their ability to look deep into contacts or DRAM trench capacitors. A contact 220nm wide and 1,500nm high has an aspect ratio of 6.8:1. But nLine's Fathom system can find defects in such high-aspect-ratio structures with a signal-to-noise ratio of 20:1, Thomas said.

This kind of approach has attracted about $35 million in investment funding in the past two years and millions in development funding from the National Institute of Standards and Technology (NIST) for first- and second-generation systems. Illustrating that no development effort is immune to the ravages of the industry's prolonged crunch, nLine laid off 10 of its 34 employees in a cost-cutting move.

Digital's domain

Thomas led the team that developed digital holography at the U.S. Department of Energy's Oak Ridge National Laboratory in Tennessee. Thomas had worked on magnetic fusion from 1975 to 1994 and used holography to measure electron density. When the Department of Energy budget for fusion research was cut back in 1994, a colleague suggested that Thomas' ideas about digital holography were worth exploring for semiconductor inspection.

NIST stepped up with early development funding, and Oak Ridge designed a prototype system. The key patent for digital holography, U.S. Patent No. 6,078,392, was granted to Thomas and others in June 2000. nLine got exclusive rights to use the technology for semiconductor inspection applications.

Over the past two years, the startup has attracted about $35 million in funding, including strategic investments by AMD and Intel Corp. Sematech's yield management program developed benchmark testing wafers (with intentional defects in deep contacts, for example) to develop HARI capabilities within the metrology industry.

NLine CEO Robert Black said sales activity will run in parallel with continuing work to commercialize the Fathom tool, which is based on a 266nm deep-ultraviolet laser and a 2 megapixel CCD camera. The system is targeted at the 90nm process technology node, with an ability to identify 60nm defects at a throughput of one wafer per hour.

Faster throughput

nLine recently won $9.4 million in NIST matching funds to support development of a next-generation system. HARI II is expected to be able to characterize much smaller defects, with a throughput of five to 10 wafers per hour.

That system will use a sub-200nm laser; a higher-resolution CCD camera, with at least 4 million pixels; and a faster stage. Thomas said that this second-generation system will extend high-aspect-ratio inspection to the 25nm node, expected in the 2012 to 2014 time frame.

Black, the former president of Juki Automation America, said nLine has a time-sharing agreement with Sematech for use of the first-generation tool. nLine has a backlog of test wafers coming in from potential customers, some with trench structures with 12:1 aspect ratios; in one case the aspect ratio is 18:1.

Because the Fathom system is adept at distinguishing one material from another, it is particularly suited to use with silicon-on-insulator structures. "Oxide stands out like a beacon," Black said. "Over the next few years we think HARI tools will become a good niche, worth perhaps a few hundred million dollars. We think we are the only automated alternative to high-aspect-ratio inspection, for looking straight down at 180."

Thomas said the semiconductor industry is accustomed to paying $1 million or more for metrology equipment. Digital holography also may be applied to the biotechnology industry at some point, Thomas said, but biotech customers are accustomed to microscopes and other relatively inexpensive gear.

- David Lammers

EE Times

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