Nanoimprint technology – Canon to acquire Molecular Imprints Inc.

Nanoimprint technology

Canon Inc. of Japan has finalized a deal to acquire Austin-based Molecular Imprints Inc., according to press release issued on Feb. 13. Molecular Imprints, one of the market and technology leaders for nanopatterning systems solutions, has agreed to sell its semiconductor nanoimprint lithography equipment business to Canon as part of Canon`s bid to bolster its chipmaking equipment business. The price for the acquisition of Molecular Imprints was not disclosed, but the Nikkei business daily estimated it at more than $98 million.

In 2004, Canon began its own research into nanoimprint technology to enter the market for lithography equipment for leading-edge high-resolution patterning. A business alliance was established with Molecular Imprints in 2009, when Canon carried out joint development with the US company for mass production of semiconductor components using the Jet and Flash Imprint Lithography (J-FIL) technology of Molecular Imprints. This partnership aimed to enable the industry to continue on Moore’s Law bypassing the cost burden and resolution limits of traditional optical and EUV lithography. J-FIL is planned to be adopted for advanced FLASH memory within the next two years.

Canon, a stepper manufacturer, is facing increasing difficulties to compete in the stepper market due to the increasing costs of R&D associated with immersion and EUV technologies. Since the stepper market is becoming increasingly consolidated and in the hands of the market leader ASML, Canon is now looking to diversify its micro and nano-patterning device offer by exploring technologies alternative to the traditional optical patterning

Molecular Imprints was founded by S.V. Sreenivasan and Grant Wilson, both professors at the University of Texas at Austin. It is an industry leader for high-resolution, low-cost-of-ownership nanopatterning systems and solutions. Mark Melliar-Smith, CEO of Molecular Imprints, stated that the business alliance with Canon had yielded great progress in the pursuit of low-cost nanolithography solutions for the semiconductor industry.

Dr. Toshiaki Ikoma, CTO of Canon, notes that the acquisition of Molecular Imprints will strengthen the “Industry and Others” business unit of Canon. The acquisition is part of Phase IV of Canon`s Excellent Global Corporation Plan, where one of the strategies is business development via globalized diversification.

Canon decided to make Molecular Imprints a wholly owned unit after considering the outlook for volume production using the technology in the mid-to-long term future. The terms of the agreement will allow Molecular Imprints to retain the same company name and with most of the personnel. It was also reported that Canon plans to set up an office at the same place where Molecular Imprints is currently established.

The merger has the advantage of having a new company with the same name, the same key personnel, intellectual IP rights jointly owned with Canon, and multiple system platforms that are designed to support the growing need for nanoscale patterning in consumer electronics and biomedical applications. David Gino, COO of Molecular Imprints, stated that the company is looking forward to provide low-cost nanoscale manufacturing solutions to the hard disk drive, display, biotechnology, and other emerging markets. Gino is set to become the CEO of the new Molecular Imprints once the merger is complete. The deal is expected to be completed by April this year after government and shareholder approvals.

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Variants of nano imprint lithography

Variants of nano imprint lithography

In addition to the above mentioned thermal and UV-nano imprint lithography, there a big number of variety of processes about nanoimprint that have flourished in recent years, such as laser assisted direct imprint, Sub-10 nm NIL, combined thermal and UV nanoimprint, electrical field-assisted NIL, soft UV-NIL, reverse NIL, Jet and Flash imprint lithography process

Reverse imprint lithography process Using the reverse nanoimprint process, instead of having the polymer spin coated on the substrate, we have polymer coated on the mold and then transferred into the substrate due to the different surface energy between the mold and the substrate that allows an easy transfer of polymer from the mold to the substrate.

Using this process, it is possible to create 3d structures relatively easier than with other methods, but the downside is that the mold gets easily dirty and the process can not therefore be used for large scale production.

Applications of the reverse imprint lithography process are: metallic nanowires, optical devices and microchannels.

Roll-to-roll imprint lithography process

In order to overcome one of the major limitations of traditional nano imprint lithography, that is the inability to imprint large areas at low cost, a new technology called roll-to-roll imprint has been developed.

Roll-to-roll imprint is a continuous and simple process that can allow patterning of large areas at a relatively low cost.

The process is relatively simple and involves a roller mold that imprints the flat substrate by rolling over it in a continuous and uniform way: with roll-to-roll imprint lithography features down to 50nm and lower have been demonstrated for large areas.

Nanoimprint on large areas

Large area imprint lithography can be done in other ways other than roll-to-roll imprint and step-and-repeat imprint.

In detail, preparing a large area mold that completely covers the substrate to be imprinted in a one-shot imprint is something not easy but somehow possible in some cases. In order to be able to perform a uniform imprint over the whole wafer surface and to overcome issues of flatness and uniformity several techniques have been pioneered, such as the Air Cushion Press that is used to ensure the pressure and pattern uniformities.

The technique is interesting but still far from commercial utilization due to the issues above: so far results have been achieved for patterns down to 200nm and with wafers up to 200mm

Laser-assisted direct nanoimprint

This technique is used to perform the nanoimprint directly on the surface without the use of a polymer film to transfer the patter from the mold to the substrate.

A laser melts the surface of the substrate so that the mold can directly pattern the substrate.

Usually the mold is made by hard and resistant metals such as nickel and the substrate needs to be made of a material which can be melt quite easily.

The advantage of the technique is that does not require a step of etching of the substrate and that flatness requirements become less stringent due to the fact that the surface of the substrate is melt.

The speed of the process is also a plus and patterns down to 10nm have been produced with this technique

One of the main drawbacks of the technique is that the process is hardly appropriate for full production due to the dirt that inevitably gets stuck into the mold after several imprints

Thermal and UV-NIL combined nanoimprint

This technique has been pioneered by Obducat and allows to combine UV-nanoimprint patterning with the use of heat to achieve better results

Substrate conformal imprint lithography

Substrate conformal imprint lithography bridges the gap between large area lithography using a soft stamp and small area lithography using a small stamp with high resolution: the technique aims at achieving the advantages of both techniques (large area and high resolution) minimizing the drawbacks of the soft stamp, which is the inability to reach high resolutions due to problems of thermal expansion.

Using this technique, patterns down to 50nm have been achieved on substrate that did not meet stringent planarity requirements.

Nanoelectrode lithography

This technique combines the use of an electrode with nanoimprint

The mold has to be conductive and the patterning process is combined with an electrochemical reaction that allows the oxide pattern to get transferred directly into the substrate surface

While still called as “nanoimprint”, the technique is quite different from traditional nanoimprint where the pattern is mechanically transferred to the surface while the process here happens by electrochemical reaction

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The two main categories of nanoimprinting

The two main categories of nanoimprinting

While there are many different flavours of nanoimprinting processes, two main ones are:

      • Thermal nanoimprinting (also called as hot embossing)
      • UV-nanoimprinting

Thermal nanoimprint technology

Thermal nanoimprint technology has been the first kind of nanoimprinting process ever used and the original one adopted by Prof. Chuo back in 1996.

The thermal nanoimprint process is very easy to understand and straightforward: a layer of thermoplastic polymer is deposited on the surface of the substrate by spin coating or alternative method; then the substrate is placed inside the imprint machine along with the mold and the mold is pressed against the substrate at a set pressure.

The substrate received a precise amount of heat from the machine and the temperature of the polymer rises over the glass-transition temperature and becoming soft.

The mold is kept pressed against the substrate for an amount of time, after that the substrate is cooled down and the mold is released

After the pattern is transferred to the mold polymer layer, it is then transferred to the beneath silicon substrate surface by etching process

UV-nanoimprint technology

UV-nanoimprint technology process is different from thermal nanoimprint in two aspects: it does not use heat to soften the polymer layer but UV light and requires a UV-transparent mold.

With UV-nanoimprint technology, the polymer used on the substrate surface is a UV-curable photopolymer and not a thermoplastic one as in the case of thermal nanoimprint; this allows the process to be effective at considerably lower pressures and helps to avoid the issue of thermal expansion

As discussed, the main advantage of thermal nanoimprint over UV-nanoimprint is that molds of any material can be used while, in case of UV-nanoimprint technology, only UV-transparent materials can be used

However, since UV-nanoimprint uses mainly UV light to deform the polymer, considerably lower pressure levels on the substrate are needed to be able to transfer the pattern to the polymer

Moreover, the advantage of not needing much heat to deform the polymer brings other advantages such as much lower special deformation of the pattern due to applied heat and therefore a better alignment accuracy and the ability to imprint larger areas.

As for UV-nanoimprint, there are two sub-categories: hard-mold UV-nanoimprint and soft-mold UV-nanoimprint: as the name implies, hard-mold UV-nanoimprint is a process that uses a quartz or other rigid mold, while soft-mold nanoimprinting uses a soft mold usually made in polymer.

Hard-mold UV-nanoimprint technology has two main shortcomings if compared to soft-mold UV-nanoimprint: the mold is rigid and therefore there are sometimes issues while releasing the mold from the substrate; moreover, there are other issues when trying to imprint large areas due to the inherent issues of flatness of the mold and the substrate.

Lack of perfect flatness in either the mold or the substrate can lead to defectivity in the pattern or, in worst cases, to a breakage or the substrate to the mold.

Using a soft mold, however, leads to some shortcomings too: while issues of flatness between the mold and the surface are greatly resolved while using a soft mold and releasing is far easier, problems like thermal expansion and non-uniformities are worsened due to the elasticity of the mold itself

Deformation of the pattern due to even small quantities of heat can become a significant problem at lower resolutions and this may lead to additional issues when the surface to be patterned is large and therefore the thermal expansion rate of the mold polymer varies across the substrate surface

A variant of the UV-nanoimprint process is the so-called Step and Flash ® process.
Using this process, initlally developed at the University of Texas, the polymer is deposited on the substrate not by spin coating or by any other machine, but by a pattern generator which is embedded in the imprint machine. After the polymer has been deposited, a soft mold is gently pressed over the substrate surface and the pattern exposed to a UV-lamp.
After the soft mold is detached, the patterned polymer is etched and the unnecessary features are removed.

Other further categorizations of the process are: single step or multiple step and single layer and multiple layer nanoimprint

Single step nanoimprinting involves the patterning as a one-time process while multiple step process (also called step-and repeat nanoimprint) involves multiple step patterning of usually large areas of substrate with smaller area molds.

Single layer nanoimprinting is the traditional process where one single layer of pattern is transferred into the substrate surface, multiple layer nanoimprinting instead requires thhe patterning of multiple layers of patterning over the same area.

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Introduction to nano imprint lithography

nanoimprint lithography

Nano imprint (or nanoimprint) lithography, abbreviated as NIL, is a relatively old patterning technology which has gained traction in the last few years.

Nano imprint technology was introduced back in 1996 by Prof. Chou as an alternative fabrication method to traditional optical lithography.

Due to increasing costs with traditional optical lithography, the industry has been looking in the past few years for alternative technologies that may help fabricating micro and nano-patterns at a lower cost and high throughput: nano imprint lithography promises to deliver to users the ability to pattern at micro and nano-order without all the burden in terms of costs typically associated with optical technology.

The principle behind NIL is straightforward and simple: instead of using an optical mask to draw patterns on a substrate, the pattern is imprinted directly using a stamp which physical creates the pattern on the substrate by deforming the resist placed over the substrate and subsequently etching the part of substrate which need to be removed to create the pattern

Advantages of the technique are

  • Since NIL relies on deforming the polymer deposited on the substrate by physical means and not by optical exposure + etching, there are no issues about diffraction limits of the incoming light or beam scattering
  • NIL is a potentially very fast technique, the pattern needs to be imprinted using an imprint machine and this process can be repeated relatively fast and seamlessly for a number of times
  • Nano imprint lithography is a much cheaper alternative to optical lithography as it does not involve complex stepper machines but a much simpler imprint machine. The cost of the imprint machine itself is only a small fraction of the cost of a top-of-the-notch stepper
  • With nano imprint lithography, resolutions under 10 nm have been consistently achieved. No other patterning technology can reach this level of resolution at such low costs.
  • Master molds, used to imprint on substrates, can be manufactured with E-beam lithography and then used multiple times to imprint on substrates (up to thousands of times)
  • Possible to pattern large areas, patterning of 4″, 6″ and even 8″ wafers has been demonstrated, using roll-to-roll imprint, even larger areas can be imprinted on film
  • For the above reasons, NIL has been added by the International Technology Roadmap for Semiconductors (ITRS) for the 32 and 22 nm nodes.

Various users, as Toshiba, for example, have however shown the reliability of the technology for patterns lower than 22 mm

For historical reasons, many people associate NIL with hot embossing (or thermal nano imprint) due to the fact that the first iteration of the technology was about imprinting on a substrate heating it at relatively high temperature therefore softening the resist polymer and allowing it to deform according to the pattern drawn on the mold.

More recently, a new technology called UV-nano imprint has also gained strength: instead if using heat to deform the substrate, UV light is used.

The UV-curable polymer is deposited over the substrate and the deformation occurs by exposing it to UV-light

Apart from the two main processes (thermal nano imprint and UV-nano imprint), several other alternatives have been developed in the last few years: laser-assisted nano-imprint, roll-to-roll imprint process, reverse imprint lithography and others

Main applications of nano imprint lithography are: HDD platter patterning, sapphire substrate patterning for LED applications, biological devices such as cell-culture plates, optical elements patterning

However, due to the different technology involved, nano imprint has also several disadvantages and point of attention compared to traditional optical technology.

Disadvantages of the technique are

  • Alignment between the mold and the substrate is a critical aspect of the process. With nano imprint technology it is much more difficult to reach the required alignment between mold and substrate due to the inherent nature of the process
  • Defectivity on the mold is also a serious issue. Due to the direct contact between the mold and the substrate, even the small defect on the mold gets replicated into the substrate
  • Mold release from the substrate may be problematic, mold release may cause new defects on the mold and therefore there is the need of replacement of the mold every 100, 500 or 1000 imprints (depending on various process conditions)
  • Due to the fact that pattern transmission between mold and substrate is direct and not reduced as in stepper lithography, the masks used during nano imprint need to be more accurate than conventional optical lithography masks.

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