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.

Note:
If you are interested in nanoimprint lithography, please visit our nanoimprint lithography service page.

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