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070330: Extending the lithography-airplane metaphor
Ed’s Threads 070330
Musings by Ed Korczynski on March 30, 2007

Extending the lithography-airplane metaphor
At this year’s SPIE Advanced Lithography conference, Burn Lin, senior director of TSMC's micropatterning division, invoked an extended metaphor of airplanes to explain the evolution of lithography in the semiconductor manufacturing industry. He started with 1:1 masks as propeller planes, moved to reduction steppers as jet aircraft, added double-exposure as the supersonic Concorde, and finally considered EUV as the space shuttle. In contradiction, Larry Thompson declared that EUV is the Concorde in a panel discussion at SEMICON West last year. I resolve this metaphoric contradiction by saying that EUV is neither, but is instead a supersonic sub-orbital plane. For logical consistency, then, 157nm was the Concorde.

This metaphor applies both to the evolution of technology over time, as well as to why specific technologies are used for different applications in any given moment. Propeller planes are 1:1 masks and steppers, and just as propeller planes continue to be used for short trips and small loads, 1:1 lithography tools continue to be used in packaging and non-critical exposures. Likewise, reduction steppers are jets; just as jets developed larger bodies and faster engines, reduction steppers developed faster throughputs and tighter resolutions. While the industry moved to smaller wavelengths of light to print ever smaller features with reduction steppers, the cost to image a given area of silicon continuously decreased all through the 1980s and 1990s.

Then we came upon a fundamental limit of “optical” lithography: silica glass lenses aren’t purely transparent for wavelengths below 193nm. The upper frequency limit on “light” transmission through quartz is to lithography what the speed limit of sound is to air-transport. Both are hard limits on economics, and both can be surpassed but only with great expense for niche markets. Pursuing 157nm lithography required replacing silica glass lenses with calcium-fluoride crystal lenses, and led to rumored >$500 million in calcium-fluoride investments before the economics killed the program.

Consider the economics of state-of-the art sub-sonic air travel. A Boeing 747-400 burns five gallons/mile flown on average, or about 80 miles/gallon/passenger (assuming a fairly full load of 400 passengers). In contrast, a Concorde gave just 17 miles/gallon/passenger, or 4X less fuel-efficiency. Like the Concorde supersonic jet, 157nm litho was always going to cost more than the mainstream could afford and the global semiconductor industry decided to metaphorically ground the fleet before it was even built.

Airbus is now pushing the economic limits of sub-sonic flight with new ultra-light (and expensive) materials to make an ultra-large plane that can skip stop-overs. The Airbus A380F for freight seems to be the limit: an aluminium-lithium skin on parts of the fuselage and wings, carbon-fibre in the fuselage, and high static strength Glare (glassfibre reinforced aluminium) in the fuselage. All of this results in three full cargo levels that can accommodate standard-sized palettes along with US military palettes – a major advantage for customers that have long-haul transportation contracts with the United States Air Force. The A380 with its use of new expensive materials can be compared to 193nm lithography with immersion — both require more capital investment upfront, but promise lower costs in the end. Like an A380, the cost of a 193i stepper makes sense only for big companies doing big things. If new production volumes can justify the investment, then such big new technologies can still be very cost-effective.

Like a 747-400, a 193nm dry stepper is relatively less expensive and thus makes sense for medium to big companies doing lots of different things. Just as some airline companies (whether passenger or freight) with established networks of hubs may use two 747 flights to reach a long haul destination, so too might a fab (whether logic or memory) with established design flows and hardmasks use double-exposure/double-patterning of 193nm dry to make a final nanometer-scale pattern.

Since the visible spectrum of light is roughly 380-720nm, and with 248nm termed “deep UV,” it’s quite extreme to call ~13nm “extreme UV.” It used to be termed “soft x-ray,” but since x-ray lithography (XRL) ended up with a not-so-great reputation for mainstream commercial use, when R&D; began on 13.5nm lithography no one wanted people to think about x-rays. Regardless of terms, in this extended metaphor EUV and XRL are super-sonic sub-orbital x-planes that can fly from New York to Tokyo in a few hours. Seriously cool capabilities, but who can afford them besides the military and Intel?

Most people can afford to fly in planes, jets, and sometimes helicopters. Helicopters are like EBDR in that both provide nearly boundless flexibility and precision, but are inherently too expensive for most work. Helicopters don’t need runways and can hover to drop passengers anywhere in the world. EBDR doesn’t need a mask and can write any pattern on the wafer. Just as you might fly in a jet to an area and a helicopter to reach a specific final destination, fabs may use reduction steppers for most layers and EBDR just for a few.

Just as there’s a place in the market for all of these different aircraft technologies, so too is there a place for all of the different lithography technologies. If you need ultimate precision and can afford to process just fifteen 150mm wafers per hour, then XRL is not “next-generation” but today’s technology for you. EUV may still fly for Intel. Still, reduction steppers will continue to be used for most high-volume applications in the conceivable future.



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070330: Extending the lithography-airplane metaphor

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Ed's Threads is the weekly web-log of SST Sr. Technical Editor Ed Korczynski's musings on the topics of semiconductor manufacturing technology and business. Ed received a degree in materials science and engineering from MIT in 1984, and after process development and integration work in fabs, he held applications, marketing, and business development roles at OEMs. Ed won editorial awards from ASBPE, including interviews with Gordon Moore and Jim Morgan, and is not lacking for opinions.