by George Marsh

Predictions that Moore’s law – which states that the

number of components fabricated on a chip will

double every 18 months – is about to come up

against a physical ‘red brick wall’ seem perennial. But

optical lithographers have so far confounded the

prophets of doom, managing to etch ever finer

details onto Si. Now, though, the limits are truly in

sight, leaving chip makers wondering what

technology can carry them forward towards the

billion-transistors-on-a-chip mark and whether a

successor will finally be needed for complementary

metal oxide semiconductor (CMOS). The likely

answer to these questions may appear surprising.

Photolithography is still working wonders for the chip

industry. Not so long ago, moving from the 248 nm

laser wavelength standard and the 120 nm features

that it can routinely produce to a 193 nm laser

wavelength and 90 nm features seemed a big

challenge. Yet, even as chip makers adopt this new

standard and consider enhancing it through two or

three generations to produce features of 60 nm and

less, lithographic tool makers are starting to gear up

for 157 nm deep ultraviolet (DUV) technology as the

next step. Predictions are that 10 GHz microprocessor

clock speeds, compared with the present 2.8 GHz

state-of-the-art, should be achieved by 2006.

But, because of fundamental physical limitations, that is

about as far as conventional optical lithography can in

improving resolution by reducing the imaging wavelength.

A wavelength of 157 nm, generated by fluorine lasers, is

already at the UV edge of the visible spectrum and moving

beyond this takes us into the realm of vacuum UV and

X-rays. As current lens materials are opaque at these

wavelengths, existing optical techniques may no longer have

anything to contribute. Conventional wisdom says that

patterning at a scale of only a few tens of nanometers

requires completely new technologies utilizing electron or ion

beams. However, the discovery that there is a band in the

‘soft’ X-ray region (2-50 nm) amenable to techniques still

recognizably optical, suggests that the boundaries of this

type of lithography can be pushed back yet again. Some

argue that this band, better termed extreme UV (EUV) for

chip technology purposes, may not only render 157 nm

Moore’s lawat the extremes

May 200328 ISSN:1369 7021 © Elsevier Science Ltd 2003

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photolithography (and a 126 nm successor proposed in some

quarters as a ‘last gasp’ for optically-based technology)

unnecessary, but could even take us all the way to the

natural limits of CMOS-based computing, where leakage and

other phenomena occurring between transistor elements

might disrupt normal electronic functioning.

The first suggestions that EUV lithography (EUVL) could

viably extend Moore’s law came in 19881,2. Groups from the

Lawrence Livermore National Laboratory (LLNL) and Bell Labs

succeeded in applying reflected EUV rays in projection

lithography systems at 4.5 nm and 37 nm, respectively. They

relied on large synchrotron sources to create the required

soft X-ray beams and used grazing incidence reflection.

Within a year, researchers at the Nippon Telephone and

Telegraph LSI Laboratories in Japan provided the first

demonstration of EUVL imaging using normal incidence,

multilayer-coated optics. Utilizing 13 nm radiation, this work

yielded 0.5 µm features3. However, it was Bell Labs who first

demonstrated the true potential of the technique in 1990.

With 13 nm radiation, Bell researchers achieved 0.05 µm

features on a PMMA (poly [methyl methacrylate]) resist4 –

the detail resolution being in line with the general rule that

features can be realized to about half of the light wavelength

used. The following year, Sandia National Laboratories took

matters further, demonstrating the first EUVL imaging system

in which a compact laser, rather than a synchrotron, was the

radiation source5. Five years after that, Sandia produced the

first workable device able to achieve EUVL patterning, using a

microstepper developed in conjunction with Bell6.

Up to this point the technique had been known as ‘soft

X-ray projection lithography’, but the name was changed to

EUVL to avoid confusion with proximity X-ray lithography.

Unfortunately, the US Department of Energy, which had been

the main source of funding for this work, withdrew its

support as a result of changes in government priorities during

the mid-1990s. But, so promising were the results, a

consortium of IC manufacturers including Intel, Motorola, and

Advanced Micro Devices decided to sponsor LLNL, Sandia, and

the Lawrence Berkeley National Laboratory (LBNL). The three

labs refer to themselves as the Virtual National Laboratory

(VNL). A year and a half ago, the VNL unveiled an EUVL

prototype machine with a numerical aperture of 0.1 able to

produce 70 nm features – already almost comparable to the

best obtainable with conventional optical lithography7.

According to Don Sweeney, LLNL-based chief technology

officer of the VNL, a commercialized version of the

technology should be able to support IC fabrication at 45 nm

initially, scaling through successive generations down to

22 nm without significant loss of production throughput.

Some researchers believe that the method could eventually

realize features as small as 10 nm, about the same scale as

molecular electronic devices.

All done by mirrorsThe basic geometry of an EUVL system is shown in Fig. 1.

Light from a 13.5 nm source is reflected from a patterned

mask and imaged with a four-times reduction onto a

photoresist-coated wafer via a series of mirrors.

Central to the EUVL ‘magic’ is the use of mirrors in place

of transmissive lenses within the beam focusing optics. This is

because all known lens materials absorb these soft X-ray

frequencies. The mirrors must be slightly off-axis so as not to

obscure the light path and require carefully computed

nonspherical shapes to maintain sharp focusing. Although

functionally similar to optical mirrors, those used in EUVL are

physically very different and construction is a major

challenge. To achieve useful levels of reflectance, they rely on

multiple reflections occurring at different levels within a

multilayer stack made up of alternate layers of Mo and Si.

These are deposited by magnetron sputtering, electron beam,

or evaporation techniques. More than 80 layers have been

used in experimental systems to yield some 70% reflectance

overall. So that the individual reflections are additive, layers

need to be of a precise thickness – 2.8 nm for Mo and 4.1 nm

for Si. Because of reflectance losses at each mirror, designers

May 2003 29

Reflectivemask

Absorberpattern

Multilayermirror

Wafer torecord 0.1 µmfeatures orsmaller, overcm2 dimensions

4:1reductionoptics,aspheric,multilayercoated

λ = 13 nm

Fig. 1 Schematic of the EUVL system. The light source, along with the associatedcondenser, is not shown. The system requires a four-times reduction camera (for clarityonly two mirrors are shown), a patterned reflective mask, and an EUV-sensitive resist.

must work with as few optical surfaces as possible and

cannot rely on further mirrors to correct aberrations. Instead,

corrections are accommodated within the very specific

shapes of the aspheric mirror surfaces. The VNL’s prototype

camera system has just four mirrors with a numerical

aperture of 0.1, but commercial systems will probably have

six mirrors and a numerical aperture of 0.25. (Resolution

scales directly with numerical aperture.)

Producers of commercially viable demagnifying EUV optics

will have to meet stringent requirements for these mirrors

and the multilayer structure. Standards for the IC industry’s

45 nm node dictate that etched lines do not vary by more

than ~5 nm and are placed on the photoresist mask to within

~15 nm. To meet these requirements, multilayer thickness

over the entire optics must be held to within one part in

5000 and the pass bands of the multilayers should match to

within 0.01 nm. Mirror substrates must be polished, typically

by ion bombardment, with extremely low error levels. The

mirror surfaces in the VNL’s prototype system are finished to

within 0.1-0.2 nm rms over clear aperture dimensions of

50-150 nm. Geometric shape errors for the assembled system

should not exceed 0.4 nm rms. As well as complying with

these tolerances, the multilayer material must be stable and

durable so that commercial EUV optical systems can last at

least five to ten years in service.

Achieving these parameters reliably is difficult, particularly

given the limitations of existing polishing and metrology

tools. Only visible wavelength interferometry methods are

powerful enough to support fabrication of the mirror shapes.

Once fabricated, the mirrors must be mounted and aligned

relative to each other in a stable manner. The mounts

themselves and the distortion effects of gravity must all be

taken into account. Changing the orientation of the mirrors

relative to gravity will deform the mirrors many times

beyond the required specifications. Only the most

sophisticated computer-based designs and control methods

allow the system to be assembled and maintained.

Extreme masksA similar challenge exists in producing EUV masks able to

deliver high-definition patterning. The basic EUV mask is

shown in Fig. 2. It has the same shape as today’s 6” x ¼”

photomask, but the similarities end there. The EUV mask is

reflective and requires a very high quality coating. The

multilayer on the mask is effectively imaged onto the wafer

so any defects in the mask will be transferred to the

photoresist, producing defects in the final product. The

surface of the multilayer is overlaid with a patterned,

absorbing material such as 100 nm of Cr to encode the

design to be imaged onto the wafer. Buffer and capping

layers between the multilayer and the absorber protect it

from damage during patterning and pattern-repair processes.

Industrial requirements are extreme. The mask surfaces

must be ten times flatter than present mask substrates, the

substrate coefficient of thermal expansion must be close to

zero, and reflective surfaces must be defect-free. Any defects

that are present must be smaller than 50 nm. The reflecting

properties of the multilayer coating cannot be compromised

by the mask-making process. Surface quality must be

maintained without protective covers (pellicles), which are

used in conventional optical masks, since EUV equivalents of

these do not exist. Finally, achieving all this at an affordable

price will be difficult.

Much research is currently under way around the world

aimed at meeting these challenges, including the EXTUMASK

(Extreme UV Lithography Masks) collaborative project taking

place under the auspices of Europe’s MEDEA+

(Microelectronics for European Applications) program. Due to

finish in October, this three-year program seeks to narrow

the technology choices and provide an infrastructure for

experimental EUV masks. Labs in several countries are

working together to select appropriate materials and coating

techniques, and to specify the laminate structure.

Researchers have focused on the optimization and

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May 200330

Absorberpattern Buffer layer

Capping layer

Substrate:Low thermal expansion material (LTEM)(6" square × 1/4" thick)

Multilayer Coating

Fig. 2 The EUV mask must be reflective. It consists of a Mo/Si multilayer on a flat, lowthermal expansion glass substrate. Two extra capping and buffer layers protect themultilayer during patterning and operation. The patterned absorber consists of about 100 nm of a material such as Cr.

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industrialization of evaporation and electron-beam coating of

surfaces for both optics and masks. Related work involving

Carl Zeiss, Philips Research, and lithography toolmaker ASML,

for instance, has been aimed at optimizing ion beam energies,

incidence angles, and other polishing settings. Angular scan

reflectivity monitoring systems are being developed so that

high coating consistency can be achieved from one run to

another. Hard X-ray, diffractometry, reflectrometry, photo-

electron spectroscopy, and EUV microscopy tools are also

being utilized to secure multilayer quality.

A criticism leveled by supporters of competing

technologies, such as electron-beam lithography, is that such

sandwich structures will be difficult to repair. For example,

techniques will be required to repair opaque defects in the

absorber material at the multilayer surface. Using focused ion

beams for local defect removal is an obvious approach, but

risks damaging the multilayer. One possible solution is to

incorporate a thin C layer as a buffer to prevent ions from

penetrating below the surface. Researchers have verified,

both theoretically and experimentally using secondary ion

mass spectroscopy, that a 25 nm layer effectively bars the

passage of most ions, making ion beam removal of opaque

mask defects feasible.

PhotoresistsEUVL photoresists are basically the same as those currently

used for DUVL. To date, therefore, most research has focused

on surveying existing DUV resists. One difference, though, is

that the resists absorb EUV light fairly readily so that they

have to be thinner than for DUV. A typical layer thickness is

100-130 nm, about a fifth of a DUV equivalent. Modifications

to DUV resists require formulations to obtain coatings of this

thickness.

Resists acceptable for high-volume manufacture must

exhibit good resolution, high contrast for printing, good etch

resistance, a sensitivity of ~5 mJ/cu•cm, and low outgassing.

A resist must also address the line edge roughness that has

become an increasing problem for fine-detail lithographies.

Three basic approaches are under investigation: silylated

single-layer resists, ultra-thin single layer resists on top of a

refractory hard mask, and bilayer resists. Regardless of the

approach, the active imaging layer must be only ~100 nm.

All significant results presented to date (including those

shown in Fig. 5) have used the ultra-thin single layer

structure. A photoresist of 100 nm is coated on top of a hard

mask of a material such as SiO2. The ultra-thin resist is

exposed to EUV radiation and wet-developed. The pattern in

the developed photoresist is transferred to the hard mask

using a dry etchant. Finally, the hard mask provides a robust

etch mask for transfer to the IC device.

Resists produced to date have a resolution of about

50 nm. This will have to be improved as EUVL moves from

the 45 nm node down to the 22 nm node. Research into

improved resists meeting all the required specifications is

gaining momentum. The necessary experimental facilities

with high resolution (i.e. numerical aperture) imaging test-

beds will be coming on-line within a year.

EUVL sourcesA powerful, compact, and reliable source of 13.5 nm light is

felt to be the most challenging requirement of EUVL.

Synchrotron sources used in the early 1990s generate X-ray

light by emission from electrons. They are, however, large,

expensive, and produce radiation that is too coherent.

Moderate-temperature plasmas (20-100 eV) also emit X-ray

radiation and are regarded as the most promising option.

Plasmas can be produced electrically – by z-pinch, capillary

discharge, hollow cathode discharge, or plasma focus – or by

pulsed laser irradiation of a target material. Schematics of

two options are shown in Fig. 3. In a typical laser-produced

plasma (LPP) system for EUVL, a pulsed Nd:YAG laser is

focused onto a jet of Xe gas to produce a gas plasma. Light at

13.5 nm, emitted as part of the blackbody radiation from this

plasma, can then be collected optically (by a condenser), and

passed onto the reflective mask.

The engineering of sources suitable for commercial

application have been well elucidated by, among others,

researchers from the Royal Institute of Technology in

Stockholm, Sweden8. Probably the greatest difficulty is

attaining 13.5 nm emission of sufficient power at the resist

without damaging the condenser optical system. Lithographic

tool manufacturers such as Canon, Nikon, and ASML estimate

that 50-150 W within a 2% bandwidth of 13.5 nm would

have to be collected by the condenser for a viable production

tool. These power levels are sufficient for a lithographic

throughput of about a hundred 300 mm wafers per hour.

Achieving this with an LPP source requires lasers of high

power and pulse repetition rate, with significantly improved

efficiency in converting drive power to in-band EUV radiation.

To date, efficiency levels have been low – typically ~1%.

May 2003 31

Xe is currently the plasma source of choice for both

discharge and LPP sources because its generation results in

little debris to contaminate the condenser optical mirrors. For

LPP, contamination can be reduced by ensuring that the

plasma is formed away from the nozzle through which the Xe

jet is ejected. Experiments have been carried out with various

forms of debris mitigation, but none have been completely

successful to date. Conversion efficiencies are being tackled

through attention to Xe jet parameters and stability,

optimization of pulse/plasma interaction, trials of different

pulsing techniques, and improving condenser design.

Of the ten or so research efforts in progress worldwide to

develop EUV sources, the current status is about 10 W of in-

band EUV light collected by the condenser – a factor of about

ten less than required.

Working prototypeAn engineering test stand (ETS), effectively a prototype EUVL

system, developed by the VNL offers the best indication yet that

this ‘extreme lithography’ technique can be made to work at the

industrial level (Fig. 4). The system comprises an LPP source with

a 1500 W laser, a four-element condenser, a reflective mask, and

a four-mirror 0.1 numerical aperture imaging system. This entire

system must be operated in a vacuum because EUV light is

absorbed by air. The ETS, with its vacuum system, occupies a

3 m x 3 m x 4 m high enclosure, which is itself kept in a

cleanroom to exclude possible contaminants. A collage of

images produced with the ETS is shown in Fig. 5.

As described earlier, the mask is produced by patterning an

absorber material deposited onto a multilayer-coated, low

thermal expansion mask blank. The required pattern image is

reproduced on the wafer at a quarter of the original scale by

step-and-scan technology similar to that used in today’s

DUVL tools. A high-precision control system governs the

initial die alignment, mask and wafer position control at each

stage, and exposure and focus setting during stage scanning.

The ETS demonstrated full-field sub-100 nm imaging soon

after it first became operational in 2001. This represented an

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May 200332

Fig. 4 Engineering drawing of the ETS prototype tool built by the VNL. The tool includes anEUV source, a condenser illumination system, a four-times reduction projection opticalsystem, a patterned reflective mask, and an EUV-sensitive resist on a wafer. Associatedsubsystems include focus systems, scanning mask and wafer stages, optics housing, waferand reticle handling interfaces, vacuum and thermal management systems, and robotics.

Fig. 3 There are two basic approaches for commercial EUVL sources, both involving emission from a 20-100 eV plasma. (a) Laser-produced plasma source: the plasma is usually producedin a liquid or a gas to avoid debris. (b) Electrical discharge plasma source. While discharge is the more efficient of the two methods, it generates more harmful debris and much excessenergy in a small space, which must be removed.

(a) (b)

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encouraging alpha class capability and supported much early

EUVL system learning. The optical imaging system has a

figure error of 0.7 nm (50 milliwaves) rms and a flare of 20%.

Although commercial systems will have to double both figure

and flare, the ETS values are considered an impressive first

demonstration. Fig. 5 shows dense and isolated lines, along

with contacts down to 70 nm. In a commercial tool with a

higher numerical aperture, a 70 nm feature would scale to

28 nm. The process windows for the ETS show that depth of

focus for 100 nm dense lines is a full micron.

ProspectsClearly, technical challenges remain. Foremost among them

are high power source development and the production of

defect-free masks at commercially acceptable costs of

ownership. Additionally, there are always advocates of

competing technologies. Only time will tell how this will play

out in the marketplace.

The construction and operation of the ETS to demonstrate

EUVL technology has been funded by Extreme Ultraviolet

LLC, an Intel-led consortium that includes five other chip

makers. The next critical phase is for EUVL to leave the

research environment and move into the commercial

marketplace. In 2002, the first beta tool orders were

placed with commercial suppliers, for intended delivery

in 2006. Commercial requirements call for a tool to

support the 45 nm node by the year 2007 and EUVL is

expected to support CMOS IC manufacturing through the

22 nm node.

As a result, an Excellence in Technology Transfer award

has been made to the EUVL team at the VNL. According to

the awarding body, the Federal Laboratory Consortium, the

team has successfully transferred the technology to the

private sector. “Much work remains to be done before

EUVL is ready to be a cost-effective method for IC

manufacturing,” says Sweeney, “but all the pieces are in

place and I am confident that industry can complete the

process. The push for commercialization marks another

phase of what, I believe, offers the best hope, of several

candidate technologies, for extending the potential for

CMOS beyond what anyone could have foreseen a few years

ago. It should lead to microprocessors that are tens of times

faster than today’s most powerful chips, together with

memory chips having storage capacity to match.” If

Sweeney’s hopes, along with those of growing cohorts of

researchers around the world, are fulfilled, EUVL could help

give Moore’s law and CMOS a much needed ten-year life

extension. MT

AcknowledgmentsThe author gratefully acknowledges the help of Don Sweeney in preparing this article andfor supplying illustrative material.

May 2003 33

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3. Kinoshita, H., et al., J. Vac. Sci. Technol. B (1989) 77 (6), 1648

4. Bjorkholm, J. E., et al., J. Vac. Sci. Technol. B (1990) 88 (6), 1509

5. Tichenor, D. A., et al., Opt. Lett. (1991) 1166 (20), 1557

6. Nguyen, K. B., et al., In: OSA TOPS on Extreme Ultraviolet Lithography, Kania,D., and Kubiak, G. D., (eds.), Optical Society of America, Washington, (1996), 44

7. Sweeney, D. W., et al., EUV optical design for a 100 nm CD imaging system. In: Emerging Lithographic Technologies, SPIE, (1998) 33333311, 2

8. Hansson, B. A. M., et al., Liquid-Xenon Jet Laser Plasma Source for EUVLithography. Proc. SPIE, (2001) 44550066

Fig. 5 A collage of images produced by the ETS. The images are top-down scanningelectron micrographs of the developed photoresist.