Imprint Goes Flexible as It Goes Big
With a tenfold increase in turnout, the more than 200 lithographers who attended the imprint session at the SPIE Advanced Lithography Conference last month showed that imprint is becoming a manufacturing patterning technology of choice for certain applications.
Michael P.C. Watts, Consultant, Impattern Solutions, Austin, Texas -- Semiconductor International, 3/24/2009
At the SPIE Advanced Lithography Conference last month, more than 200 lithographers turned up for the imprint session — a tenfold increase over two years ago — reflecting a growing realization that for certain applications, imprint is becoming a manufacturing patterning technology of choice.
Imprint is a molding process rather than an imaging process. The mold, which is a 3-D copy of the target pattern, is pushed into a resist-material-coated substrate that is UV or thermally cured, and then peeled apart to leave a copy of the mold on the surface of the substrate. One advantage of imprint is that there appears to be no practical resolution limit — the mold appears on the substrate down to features 2 nm or 10 atoms wide.
Other advantages are the ability to pattern non-flat substrates that would prevent imaging systems from focusing, and the ability to pattern large areas at low cost. To utilize these advantages today, target applications must also have limited overlay and defect-density requirements.
Passive optics and LEDs
The largest volume of products being manufactured today by imprint are passive optical components on either flexible film or rigid transparent wafers. Examples of passive optical components include holograms up to 1.8 m wide, prism films for LCD backlights, diffusers, diffractive optical elements, or sub-wavelength moth’s-eye films. More than $1B in prism films is made every year worldwide. Passive optics have little or no overlay needs.
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| Heptagon (Rueschlikon, Switzerland) uses imprint lithography to create passive optical devices, using a wafer-scale process flow. (Source: www.heptagon.fi) |
LEDs are the second application in manufacture today, patterned with photonic crystals, or as passive optics on an active device to improve light extraction. Luminus Devices Inc. (Billerica, Mass.), for example, uses imprint lithography to create LEDs for projection displays, and has licensed its technology to Japan’s Nichia. Epistar and Luxtalek also are working on this application.
The challenge for LED imprint is that the wafers are notoriously non-flat. A gallium nitride-based semiconductor is grown on a silicon carbide substrate at more than 900ºC. Because the materials have different coefficients of expansion, the combination acts like a bimetallic strip. As a result, a 2-in. wafer can be bowed by 50 µm. This bowing prevents the industry from migrating to larger wafers. To make matters worse, the epitaxy produces nodules or bumps that are several microns high. Finally, a typical LED fab is not very clean, because it does not need to be.
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| Obducat uses a two-step process, creating an inversion of the hard stamp Si-Master into an Intermediate Polymer Stamp (IPS), and performing the simultaneous thermal and UV imprint process on the final substrate (Source: Obducat) |
The imprint solution from Obducat (Malmö, Sweden) uses an intermediate single-use or “throw away” film as a mold. The film is thermally imprinted at a first imprint head and then used as a mold to imprint the wafer at a second imprint head. The flexible film will conform to the bowed wafer and any nodules. Any particles on the substrate are kept away from the first mold by the discarded film. As a result of this flexible approach, Obducat has won multiple orders from Luxtalek and Epistar. The Obducat systems can be configured for either manual or automated wafer loading, and are capable of ~30 wph. EV Group (St. Florian, Austria) and SUSS MicroTec AG (Garching, Germany) have announced flexible mold solutions as add-ons for their contact printers.
HDD patterned media
The hard-disk industry appears to be on the cusp of volume manufacturing for hard-disk media with densities of 1 terabit per in.2 or higher. At these densities, the size of a bit must be smaller than the natural magnetic domain size; therefore the 20 × 20 nm bit must be patterned. The disks are polished to fractions of nanometer tolerances, and are kept exceptionally clean so that the read head can fly within a few nanometers of the surface.
Finally, the HDD system has sophisticated error correction and can tolerate a moderate level of defects. The need for feature sizes in advance of the ITRS roadmap, on flat clean disks, adds up to be a good match for imprint lithography. Based on announced orders, the Molecular Imprints Inc. (MII, Austin, Texas) disk imprint tool appears to be best suited to this hard-substrate application. At the SPIE conference, MII executives said the company has sold 10 systems to HDD media manufacturers, including multiple orders for the high-volume systems that deliver 180 double-sided disks per hour. The MII solution uses a flexible hard-surface glass mold, and drop-on-demand delivery of a very low viscosity fluid. The hard surface works well on clean flat surfaces, and the low viscosity fluid is the key to high throughputs.
Biological applications
Imprint also has been used in a number of biological research applications, including microfluidics, scaffolds for cell growth, analysis, and drug delivery. The challenge with biological applications is the length of time to get revenue. Not only must an imprint vendor find customers, but the company must prove to a third party (i.e., the Food and Drug Administration) that the product provides statistically significant benefits. Liquidia is a venture-funded startup focusing on using imprint to create structured nanoparticles, and has recently announced a partnership with Abbot Labs to develop RNA therapeutics.
Integrated circuits
The use of imprint for silicon integrated circuits is still unclear. The application requires very small overlay errors and low defect densities. Sematech have taken delivery of a MII step and repeat tool, and reported at SPIE that the tool was within 3-4× of meeting production throughput and overlay goals, and within 100× of defect density and process life objectives. Also at the conference, MII announced that they are moving to a mold style where the glass is thinned to 1 mm under the device area, so that they can improve throughput by bowing the mold.
There is a possibility that flexible backplanes for “e-paper” displays will beat silicon IC’s as an application for imprint. The breakout success of the Kindle e-book from Amazon, which uses a rigid E-ink display, will accelerate these developments. Paper Logic has made a SVGA flexible active-matrix backplane for an e-paper display. Seiko Epson and LG Phillips have reported on significant development programs. HP has disclosed a process that uses a single multilayer imprint to produce a self-aligned transistor, which gets around the problem of overlay tolerance on a flexible, stretchable substrate. Plastic electronics have appeared commercially in RFID tags, the transition to large-area devices appears imminent.
Author
Mike Watts is an independent consultant leading Impattern, which helps companies match their product ideas to the most appropriate patterning technology. He spent 28 years working in all aspects of semiconductor patterning and materials, and was a member of the founding team at Molecular Imprints Inc.
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no comments
CUA - 3/25/2009 1:38:00 PM CDT -
no comments
CUA - 3/25/2009 1:25:00 PM CDT -
All you need is a million AFMs...
IBM - 3/25/2009 8:20:00 AM CDT -
Ebeam for imprint mask takes too long. I think some ideas from maskless might project over to imprint though.
other - 3/25/2009 7:21:00 AM CDT -
It can replace 193/248 nm at lower cost, if overlay, throughput and defects were not the big concerns.
guest - 3/25/2009 7:02:00 AM CDT
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