High-resolution PMMA one layer resist

Utilizing SX AR-P 640/2 (PMMA 90K) as one layer system, an even higher resolution can be achieved if process parameter are varied, in particular the exposure dose. Generally can be stated that the sensitivity as indicated for each e-beam resist is extremely dependent on the size of the desired structure. To achieve the maximum possible resolution, the required dose may be up to 10 times higher than the dose required for e.g. 1 x 1 µm squares.

Process parameters:
Resist SX AR-P 640/2 was applied in a film thickness of 60 nm and dried for 60 min @ 200°C on a hot plate, followed by exposure using electron beam lithography at an acceleration voltage of 30 kV. Exposed were here so-called single pixel lines, i.e. lines with minimal width.

The development was carried out for 1 min in pure isopropanol. A 6 nm thick layer of gold was vapor deposited, and the lift-off was performed in acetone.

Results:
Very narrow lines with very small periods could be fabricated. The resist is thus well suited for high-resolution electron beam applications. The results of the above cited publication using this resist could be confirmed. At an acceleration voltage of 30 kV, an exposure dose of more than 6000 µC/cm² is required. Thin lines of 8 nm can successfully be generated, and also gold lines with very small spacing are possible. The smallest period obtained was measured to be 45 nm (see figure).

Figures 1 and 2: Line with a width of 8 nm after lift-off. The vapor-deposited gold layer has a thickness of 6 nm. Due to the granular nature of gold, the structure consists of chains of individual dots after the lift-off.

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