December 20, 2011

Step-and-repeat nanoimprint lithography delivers 7 nanometer nanostructures and promises 4 nanometer features

Step-and-repeat nanoimprint lithography (NIL) is a promising technique to replicate nanoscale patterns at low cost across a large area. Last year, researchers Christophe Peroz and Scott Dhuey and coworkers demonstrated a simplified imprint process to replicate patterns with minimum size down to 14 nm across six inch wafers. To test the ultimate resolution of this process, a novel and robust strategy to fabricate NIL templates with sub-10 nm patterns was explored.

Feature sizes down to 4 nm

This strategy for fabricating very high-resolution molds combines the advantages of electron beam lithography (EBL) and atomic layer deposition (ALD). The first step consists of patterning hydrogen silsesquioxane gratings on a quartz template using EBL. Next, ALD is used to deposit highly conformal alumina films and reduce the final sizes of the trench patterns on the template. Atomic level control over thickness deposited by ALD allows the fabrication of molds with features sizes down to 4 nm. At such a small scale, the deposition rate is highly dependent on the size of the trench and is dramatically reduced for trenches smaller than 8.5 nm.
Scanning electron microscope pictures of gratings (a) on a nanoimprint template and (b) after imprinting and pattern transfer into silicon. High pattern fidelity is reported down to 7 nm.

The step-and-repeat process is performed at room temperature and at low pressure (force <30 N) with an Imprio 55 press (Molecular Imprints). Low viscous resist films are pre-spin coated on six-inch wafers and are imprinted with ultraviolet light. Patterns down to 7 nm can be successfully imprinted with a variation of less than 1 nm between the mold and the final structures. Thanks to a high control over the residual layer underneath the imprinted features, the patterns are easily transferred with high aspect ratio into silicon.

"We believe that our nanoimprint process has not yet reached its limit in terms of resolution, especially for reducing the pitch of gratings," commented Peroz and Dhuey.

Single digit nanofabrication by step-and-repeat nanoimprint lithography

A novel strategy for fabricating nanoimprint templates with sub-10 nm patterns is demonstrated by combining electron beam lithography and atomic layer deposition. Nanostructures are replicated by step-and-repeat nanoimprint lithography and successfully transferred into functional material with high fidelity. The process extends the capacity of step-and-repeat nanoimprint lithography as a single digit nanofabrication method. Using the ALD process for feature shrinkage, we identify a size dependent deposition rate.

Template fabrication scheme by combining electron beam lithography in an HSQ resist (a) and atomic layer deposition of Al2O3 (b). Principle of the imprint process (c), (d) and of the pattern transfer into a functional material (e). Wi defines the initial trench width (a) and Wib the final trench width after ALD deposition (b); hp and ht are the height for the imprinted and transferred patterns respectively; RLT defined the total residual layer thickness underneath the imprinted patterns.

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