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Nano-Stamp Lithography from LIGA to LADI

By demi in Technology
Mon Jun 24, 2002 at 07:29:03 AM EST
Tags: Science (all tags)

Nanotechnology is founded upon the coarse union of an intuitive plan to realize nanometer-scale analogues of macro-scale machines using atoms and the counterintuitive quantum mechanical principles of condensed matter. Its current sophistication owes much to the reservoir of engineering tricks that have been tapped by integrated circuit (IC) manufacturers in their efforts to further refine photolithographic fabrication and extend the exponential technological growth described by Moore's Law. Photolithography uses as its scalpel the sharpest tool that was available at the time Jack Kilby sketched his first ideas in 1958: the wavelength of visible light (about 400-800 nanometers).

Computer circuitry will soon have densified to the point where even ultraviolet laser light is insufficient to resolve the tiniest features (~65 nm by 2007). Shorter wavelength light than that has very limited capability for manipulation by lenses of known materials, so there is an impending technological uncertainty that threatens the reliable, sustained growth of the IC industry. As the industry searches for solutions, scientists are eagerly exploring (and patenting) possible alternative fabrication and computation technologies that may become critical once the industry decides on a new direction. Among the more conventional ideas are plans to replace light-based patterning with the exquisitely detailed stamps, ones with features far smaller than lightwaves and closer in dimension to large molecules. Despite the advanced nature of their application, the theory behind these techniques is surprisingly elementary. LADI, a recent discovery in silicon nanofabrication, has technological antecedents in lithographic printmaking, potato stamps, and inkwell pens.

LIGA: early 1980's

LIGA is a process that was developed in the early 1980's by W. Ehrfeld at the West German IMT in Karlsruhe. It is an acronym standing for the stages [informative image] in the overall process: LIthogafie Galvanoformung Abformung. LIGA was one of the first major techniques to allow high aspect ratio structures (in other words, very skinny and tall) with lateral dimensions below one micron in size (~100's of nm) to be manufactured on-demand in a research environment (you still need a synchrotron radiation source, however).

But in a loose sense, LIGA is a deep X-ray + electrochemistry version of the late 18th century process of lithography (writing in stone), in which a thin coating of wax, grease, or ink on stone or metal is scribed with a pick, and the unprotected areas are etched with acid. When the wax is removed by heating, you are left with a patterned piece of material, suitable as a work of art, or as a replica master stamp that can be used to print almost unlimited negative duplicates of the original hand-sketched pattern. While photolithography has been used extensively in integrated circuit manufacture for almost 50 years, nanostructured stamp lithography techniques to pattern silicon and other device-relevant materials have recently been improved enough to offer a realistic alternative to purely radiation-based patterning and formation methods.

Whereas LIGA greatly assisted research in micromachining and MEMS, leading elements of nanotechnology will likely take advantage of the various new forms of nano-contact printing.

Soft Lithography: mid-1990's

Soft Lithography is a process developed at Harvard by researchers in the laboratories of chemist George M. Whitesides. Some of the most immediately useful applications of the micro-contact printing process [informative images therein] have been in microfluidics and NEMS (nano version of MEMS). Instead of having a resist layer that is exposed by radiation, physical abrasion, or molecular impact, a few-nanometer thick self-assembled monolayer is the "ink" that is stamped onto the patterned surface before etching. The height dimension of the molecular resist layer allows for moderately high aspect ratio structures to be made down to the 20-25 nm range (and lower in certain circumstances), limited to restrictions in patterning accuracy by chemical etching effects and the magnetic optical elements used in electron-beam lithography (which must be used at least once to make the nano stamps in the first place).

The stamps are used the same way a potato stamp is used if you ever made one in kindergarten (just press them onto something), and the molecular impressions they leave on surfaces can be used to seed crystal growth, bind strands of DNA for bio-analysis, or protect part of a surface from an etchant.

Another way of patterning the surface with monolayer ink that doesn't require the nano stamps is to use a tiny stylus to write it onto a surface serially. Dip-pen nanolithography is an example of this motif [many informative images within], where the tip of an atomic force microscope on gold acts somewhat like an inkwell pen on paper. The areas that have not been inked can be etched to leave terraces, mesas, and embossed structures that can later be used as stamps for printing.

Good beginners' references for Soft Lithography processing are: Xia Y. and Whitesides G. M., "Soft Lithography," Angew. Chem. Int. Ed. 1998, 37, 550-575 and Jackman R. J. and Whitesides, G. M., Chemtech 1999, 5, 18.

LADI: 2002

The recent report in Nature of silicon-quartz nanoimprinting was so important that Slashdot reported it twice. Chou and co-workers at Princeton [somewhat misleading image therein] are using a process called LADI (Laser-Assisted Direct Imprint) where a quartz "stamp" is made first by e-beam lithography, photolithography or soft lithography. That quartz structure is pressed directly against a flat silicon wafer, and then, due to the optical transparency of quartz, a high energy light pulse (20 ns, 308 nm ultraviolet XeCl laser, 1.6 mJ cm-2) melts the first few nanometers of the silicon. The molten silicon flows into voids in the quartz stamp and then cools, all in less than 250 nanoseconds. The simplicity and utility of the LADI scheme [informative image therein] is extremely impressive. There are no "inks", resists, or etchants to speak of, for this step (this would only be one step in an overall process), and the fidelity of the pattern transfer over large areas is very encouraging. The quartz stamp can also be used many times without apparent degradation.

If you have online access to Nature, the article is here, otherwise the reference is: Chou, S. Y., Keimel, C. and Gu, J. Nature 2002, 417, 835 - 837.

The principal investigator is reported by the BBC as saying:

But now Professor Stephen Chou of Princeton University says he has a way of stamping out chips with a die which could keep Moore's Law in operation for decades and maybe even beat it.

This statement is somewhat misleading in that LADI as a front-end processing technology for silicon is a divergent technology with respect to the ITRS roadmap, so the viability of its integration into production is unproven. The ability to grow a high-quality field oxide layer over the LADI structures, so that they may be insulated from the many layers of interconnect wiring directly above them, must be demonstrated. And while this technique has shown excellent structure-forming capabilities, the demands of solid-state electronics at the nanometer scale are sensitive to small perturbations in crystalline and even atomic order, so it remains to be seen what effects the rapid melting-cooling process will have on a doped transistor channel. Finally, the semiconductor industry has many other technological challenges facing it besides the ability to make small structures reliably and economically, so this advance alone cannot make up for the shortcomings in our current knowledge.

The future

Much of the speculation that follows big discoveries in science, especially in this day and age of quasi-realistic corporate forecasting, is colored by a degree of optimism that is commonly encountered in the parents of newborns (she'll grow up to be Chief Justice of the Supreme Court!). But there is a good reason to expect that there will be technological advances in the near future that would seem totally alien to us now. The possible end of the ITRS roadmap brings with it the exciting prospect of uncertainty, and interest in new and unorthodox ways of approaching existing engineering problems has never been greater.


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Related Links
o Slashdot
o integrated circuit (IC) manufacturers
o photolitho graphic fabrication
o Moore's Law
o Jack Kilby
o stages
o LIGA [2]
o lithograph y
o photolitho graphy
o Soft Lithography
o Harvard
o micro-cont act printing
o process
o self-assem bled monolayer
o motif
o Nature
o Slashdot [2]
o reported
o twice
o Princeton
o e-beam lithography
o scheme
o here
o reported [2]
o ITRS roadmap
o many other
o Also by demi

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Nano-Stamp Lithography from LIGA to LADI | 25 comments (23 topical, 2 editorial, 0 hidden)
What about SCALPEL? (3.00 / 1) (#3)
by Ranieri on Mon Jun 24, 2002 at 07:59:37 AM EST

Last time i looked into lithography techniques, the current hype was "SCATTERING WITH ANGULAR LIMITATION PROJECTION ELECTRON BEAM LITHOGRAPHY", or SCALPEL, though i seem to remember there were quite some problem with the mask itself building up a charge and distorting the electron beam. The paper claimed feature sizes of up to 80 nm, putting it close to the target range you mentioned for 2007.

The obvious question if, of course, whatever happened to this technique? Is it still impractical? Is it already obsolete?
Taste cold steel, feeble cannon restraint rope!

some problems with that scheme (5.00 / 1) (#4)
by demi on Mon Jun 24, 2002 at 10:11:21 AM EST

That's a good example of trying to use a diffuse electron beam the same way photolithography is employed (patterning a large area at simultaneously by projection). Usually the electron beam is a single laser-like spot that must be slowly scanned or pulsed across a huge area, which limits the likelihood of its use in mass production. There are still some problems with it wrt to Moore's Law though.

The first is the mask charging that you alluded to. Also, since it's an e-beam system, the process must be done in high vacuum, which limits the number of samples that can be loaded at once. Then there is the problem of optics, since you are talking about a stream of electrons you have to use electromagnetic lenses to focus it (instead of say, glass), but those lenses typically have a huge degree of spherical aberration and are unstable.

There are research-grade e-beam systems that can do 15 nm fabrication now, but they use cold cathode field emission guns as their beam source, which requires a vacuum of 10exp-10 torr or so. That's way beyond what can be expected for a factory to get acceptable product throughput from, it would take forever to run a single sample, and in order to process many large wafers you would need multiple identical systems operating at once ($$$). It's just too bad, because e-beam is a wonderful technology but in terms of mass production it is too expensive for anything other than photomask making at this point.

[ Parent ]

High vacuum (none / 0) (#5)
by maozo on Mon Jun 24, 2002 at 12:19:03 PM EST

So maybe we'll be seeing a space-based IC fabrication plant sometime in the future?

After we finish that "Space Elevator" project, of course :)

[ Parent ]

It's definitely been considered (none / 0) (#6)
by demi on Mon Jun 24, 2002 at 12:46:01 PM EST

although I don't know exactly what the vacuum of low earth orbit is, or if the radiation at that altitude would be a problem for exposing the resist layers.

[ Parent ]
Space vacuum (none / 0) (#7)
by sigwinch on Mon Jun 24, 2002 at 04:32:56 PM EST

The high speed of an orbiting spacecraft forms a good vacuum in its wake. This article claims 10-14 torr calculated, 10-10 torr measured in an actual experiment.

I have my doubts about the commercial usefulness for conventional wafers, though, since you'd need a separate wake for each process station to keep them from contaminating each other. I think it would be more useful for simple, continuous processes. E.g., get a big spool of stainless steel foil, deposit one layer of semiconductor at one wake shield station, another layer of semiconductor at the next, sputter on the top electrodes through apertures at a third station, and deposit a passivating layer at a fourth station. Viola, acres of solar cells. Co-wind a plastic film at the collection spool to prevent abrasion. The spools are compact and durable for easy shipping. I think space vacuum will be useful, but you have to keep the process simple; conventional complex fab equipment isn't going to fly. (Get it? Fly. Har har.)

BTW, nice article. I appreciate technical stuff like this, even if it doesn't draw a lot of comments.

I don't want the world, I just want your half.
[ Parent ]

thanks (none / 0) (#8)
by demi on Mon Jun 24, 2002 at 09:23:09 PM EST

That link was very interesting. What kind of stuff do you do (since it appears that this article has saturated itself at 7 comments)?

[ Parent ]
What I do (none / 0) (#9)
by sigwinch on Wed Jun 26, 2002 at 02:59:41 AM EST

My day job is electrical engineering, mostly design of high-resolution measurement instruments. At the moment I'm working on a light sensor for a fluorescence based instrument; we hope to achieve a system signal-to-noise ratio of >>>100 dB. On my own time I do software design, writing, physics, mathematics, cryptology, philosophy, you name it. Jack of many trades, master of a few. I try to keep up with nanotech (which I think semiconductors are a subset of) because there's so much promise, and because the field is young enough that there are plenty of easy, good ideas left. I also try to keep up with the basic developments in molecular biology: living things are awesome nanotech + information processing systems. Oh, and I also like photonics, which is closely related to nanotech. E.g., when conductive metal structures get down to a couple of nanometers in size, the metallic lattice resonates at optical frequencies. (Do a web search on surface plasmon resonance. Cool, my employer comes up in the first several hits. (I'd say who except I sometimes use this account for rather extreme political positions and don't want to take any heat. I really ought to make a new account.))

I don't want the world, I just want your half.
[ Parent ]

IBM-GE consortium to develop millipede storage (none / 0) (#10)
by demi on Wed Jun 26, 2002 at 12:27:09 PM EST



AIP virtual journal of Nanotech (none / 0) (#11)
by demi on Wed Jun 26, 2002 at 12:28:15 PM EST


Israel-based Nanolayers company (none / 0) (#12)
by demi on Wed Jun 26, 2002 at 12:29:24 PM EST



IBM-Zurich millipede storage (none / 0) (#13)
by demi on Wed Jun 26, 2002 at 12:33:04 PM EST



resonance raman spectroscopy (none / 0) (#14)
by demi on Sat Jun 29, 2002 at 01:34:36 AM EST


Raman is a technique for identification and analysis of molecular species, it is similar to FT-IR spectroscopy, but it has a few advantages, for example Raman can be used to study, solids, liquids, powders, gels, slurries and aqueous solutions. Raman does not require any special sample preparation meaning that many studies may be performed in situ. An example of this is the analysis of mixed pharmaceutical tablets in a sealed blister pack. Tablets can be identified through a sealed blister pack, without destroying the sample and the relative concentration of the substances in the tablet can even be determined

Raman spectroscopy is based on detection of scattered light i.e. the Raman effect. In general when light interacts with a substance it can do so in three main ways:-

The light may be absorbed
The light may be transmitted
The light may be scattered.

Raman spectroscopy is a result of the scattering of light. The radiation may be scattered elastically, that is without any change in its wavelength and this is known as Rayleigh scattering. Conversely the radiation may be scattered inelastically resulting in the Raman effect. There are two types of Raman transitions, upon collision with a molecule a photon may lose some of its energy, this is known as Stokes radiation or the photon may gain some energy and this is known as anti-Stokes radiation. When viewed with a spectrometer it can be seen that both the Stokes and anti-Stokes radiation are composed of lines which correspond to molecular vibrations of the substance under investigation. Each compound has its own unique Raman spectrum which can be used as a finger print for identification.

Raman spectroscopy is similar to I.R. spectroscopy but has several distinct advantages.

Using IR spectroscopy on aqueous samples, results in a large proportion of the vibrational spectrum being masked by the intense water signals. With Raman spectroscopic techniques aqueous samples can be performed with ease as Raman signals from the water molecule are relatively weak.

Using Raman spectroscopy, spectra of samples in transparent containers such as glass or plastic, can be obtained.

Advanced Raman

There are many variations of basic Raman spectroscopy but here we draw attention to just two variants, resonance Raman and surface enhanced Raman spectroscopy (SERS).

Resonance Raman

Resonance Raman techniques require no extra equipment. Resonance Raman scattering occurs when the photon energy of the exciting laser beam matches that of an electronic transition of a chromophoric group within the system under study. Under these conditions bands belonging to the chromophore are selectively enhanced by factors of 103 to 105.

Surface Enhanced Raman

In 1974 it was discovered that pyridine molecules that were absorbed unto a electrochemically roughen surface yield many times more intense Raman Signals than they normally would. This effect was later named Surface Enhanced Raman Spectroscopy (SERS). The theoretical understanding of SERS is not clear but the technique has found application in many areas of physics, chemistry and biology, yielding information on how molecules interact with surfaces whilst allowing detection of very low concentrations of various analytes.

raman (none / 0) (#15)
by demi on Sat Jun 29, 2002 at 01:42:17 AM EST



[ Parent ]

excellent description of raman (none / 0) (#16)
by demi on Sat Jun 29, 2002 at 01:49:19 AM EST


[ Parent ]
Rolltronics (none / 0) (#17)
by demi on Sun Jun 30, 2002 at 10:50:15 PM EST

polysilicon and other dep materials on a roll-tape production line.


ellipsometry and data table (none / 0) (#18)
by demi on Mon Jul 01, 2002 at 01:51:25 PM EST



Metal-Semiconductor Contacts (none / 0) (#19)
by demi on Mon Jul 15, 2002 at 03:57:36 PM EST

Rhoderick, E. H. and Williams, R. H.
"Metal-Semiconductor Contacts"
Monographs in Electrical and Electronic Engineering 19,
eds: P. Hammond and R. L. Grimsdale,
Clarendon Press, Oxford: 1988
ISBN 019859335-X
and ISBN 019859336-8

Introduction to surface chemistry (none / 0) (#20)
by demi on Mon Jul 22, 2002 at 03:22:11 PM EST


electron energy loss spectroscopy (none / 0) (#21)
by demi on Tue Aug 06, 2002 at 11:23:21 PM EST




Solar cell use in BP Amoco gas stations (none / 0) (#22)
by demi on Sat Sep 14, 2002 at 01:46:53 PM EST


Buried-Contact Solar Cell at UNSW (none / 0) (#23)
by demi on Sat Sep 14, 2002 at 01:52:12 PM EST


Solar Cell efficiency tables at UNSW (none / 0) (#24)
by demi on Sat Sep 14, 2002 at 01:53:53 PM EST


[ Parent ]
Key Centre for Photovoltaic Engineering at UNSW (none / 0) (#25)
by demi on Sat Sep 14, 2002 at 01:58:49 PM EST


[ Parent ]
Nano-Stamp Lithography from LIGA to LADI | 25 comments (23 topical, 2 editorial, 0 hidden)
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