Time Efficient Fabrication of Ultra Large Scale Nano Dot Arrays Using Electron Beam Lithography


Time Efficient Fabrication of Ultra Large Scale Nano Dot Arrays Using Electron Beam Lithography

Grebing, J.; Faßbender, J.; Erbe, A.

Many applications in plasmonics or sensorics require large scale arrays of nano-sized dot structures with rather high surface coverages. Typical dimensions, e.g., 50nm diameter, 200nm center-center distance, 1000µm × 1000µm arrays, quickly lead to extremely high numbers of objects in the design to be handled by both the software and the lithography system. In case of the aforementioned dimensions this would amount to be about 2.5 × 107 objects.

Using regular techniques the dots would be represented by a matrix of circles or dots in the design – or references to those, for that matter. Any changes in the layout of the structures would require a rather large effort since every object or reference would have to be edited. Additionally, with commonly used lithography systems the writing times quickly exceed reasonable values due to the beam settling times normally required for each object in the design. Even with values as low as 1ms the overall sum of the settling times would be roughly 7hrs. This is excluding the actual writing times!

The alternative method for the creation of ultra large scale nano dot arrays presented here is astonishingly simple and does not require any particular efforts: In electron beam lithography, exposure is based on single pixels – or dots – with defined distances (step size). The overlap of the single pixels yields the desired continuous structures. Thus, separating these pixels by the distance required for the lattice of nano dots, i.e., setting the step size to the lattice constant, would result in the desired lattice. This may be achieved by underexposing a regular rectangle by a factor of about 1/10 of the clearing dose, i.e., the result is a lattice of separated dots. The diameter of the dots may be adjusted within certain limits by the dose used for exposure.

As a result not only the writing times are strongly decreased – depending on the parameters many hours of processing time may be saved – but also handling of an array of dots is simplified to the action on rectangles or other shapes in the GDSII editor. No hassle with millions of dots is required.

As a proof of principle, different large lattices have been produced using a Raith 150TWO system in a lift-off process with PMMA or ZEP as resists on silicon. Additionally, a 4000µm × 4000µm lattice with a lattice constant of 500nm has been produced in an etching process with HSQ as the resist and reactive ion beam etching (RIBE) structuring gold on glas as a possible application for plasmonic sensors. Compared to the regular technique the processing times could be greatly reduced. The resulting samples show almost perfect lattices, the shape of the dots is also of good quality. Multiple exposures with shifted write fields also enables fabrication of hexagonal lattices or dipole structures.

Keywords: electron beam lithography; plasmonics; photonic crystal

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Publ.-Id: 15978