Nanoimprint lithography
At this point, nanoimprint lithography has been added to the International Technology Roadmap for Semiconductors (ITRS) for the 32 and 22 nm nodes.
In a standard T-NIL process, a thin layer of imprint resist (thermoplastic polymer) is spin-coated onto the sample substrate.
The cold-welding approach has the advantage of reducing surface contact contamination or defect due to no heating process, which is a main problem in the latest development and fabrication of organic electronic devices and novel solar cells.
[4] In photo nanoimprint lithography (P-NIL), a UV-curable liquid resist is applied to the sample substrate, and the mold is normally made of transparent material like fused silica or PDMS.
Further, a single-step nanoimprint directly molds thin film materials into desired device geometries under pressure at elevated temperatures.
Amorphous semiconductors (for example, chalcogenide glass[5][6]) demonstrating high refractive index and wide transparent window are ideal materials for the imprint of optical/photonic device.
[7] In thermal nanoimprint methods the trade-off between full pattern transfer and deforming the substrate creates limitations in quality of fabrication.
To ensure the pressure and pattern uniformities of full-wafer nanoimprint processes and prolong the mold lifetime, a pressing method utilizing isotropic fluid pressure, named air-cushion press (ACP)[10] by its inventors, is developed and being used by commercial nanoimprint systems.
Alternatively, roll-on technologies (e.g. roll to plate) in combination with flexible stampers (e.g. PDMS) have been demonstrated for full-wafer imprint.
For optics and photonics, intensive study has been conducted in fabrication of subwavelength resonant grating filter, surface-enhanced Raman spectroscopy (SERS) sensor,[12] polarizers, waveplate, anti-reflective structures, integrated photonics circuit and plasmonic devices by NIL.
In the context of opto-electronic devices such as LEDs and solar cells, NIL is being investigated for out- and incoupling structures.
Currently, NIL is used to shrink the size of biomolecular sorting device an order of magnitude smaller and more efficient.
The single greatest cost associated with chip fabrication is the optical lithography tool used to print the circuit patterns.
Optical lithography requires high-power excimer lasers and immense stacks of precision-ground lens elements to achieve nanometer-scale resolution.
The successful implementation of a functional imprint material would result in significant cost reductions and increased throughput by eliminating many difficult chip-fabrication processing steps.
Such an approach has been successfully demonstrated by Greener et al. whereby robust templates were rapidly fabricated by optical patterning of a photoresist-coated metal substrate through a photomask.
A very efficient and precise AFM based method for characterizing the degradation of PDMS stamps enables to optimize materials and processes in order to minimize wear.
It has been proposed to combine photolithography and nanoimprint lithography techniques in one step in order to eliminate the residual layer.
Although the area that can be patterned using Focused Ion Beam is limited, it can be used, for example to imprint structures on the edge of optical fibers.
Using UV-NIL of off-stoichiometric thiol–ene-epoxy polymer it is possible to fabricate robust, large-area, and high-aspect-ratio nanostructures as well as complex hierarchically layered structures with limited collapse and defectivity.
Laser assisted direct imprint (LADI)[33] is a rapid technique for patterning nanostructures in solid substrates and it does not require etching.
The high resolution and speed of LADI, attributed to molten silicon's low viscosity (one-third that of water), could open up a variety of applications and be extended to other materials and processing techniques.
Ultrafast Nanoimprint Lithography[34] or Pulsed-NIL is a technique based on the use of stamps with an heating layer integrated beneath the nanopatterned surface.
Injecting a single, short (<100 μs), intense current pulse into the heating layer causes the surface temperature of the stamp to raise suddenly by several hundreds degrees °C.
[11] For UV-Roller-NIL on opaque substrates, the UV light must flash through the flexible stamper, e.g. by integrating UV-LEDs into a quartz glass drum.
Nanoimprint lithography is a simple pattern transfer process that is neither limited by diffraction nor scattering effects nor secondary electrons, and does not require any sophisticated radiation chemistry.
