High Resolution Patterning¶
The patterning and transfer of sub 20 nm features ([Wolf2015]) requires that the following conditions are met:
A new cantilever with a sharp tip is used
The Surface Approach can be used to estimate the tip diameter (see Figure 21). An adhesion length below 20 nm is desireable.
Shallow patterning depth (for example 6 nm)
Because of the conical shape of the tip, the width of the written structure grows with the patterned depth. The transfer process sets the lower limit on the patterning depth. Typically depths below 4 nm place unrealistic restrictions on the etch transfer process.
Low reading forces or writing without reading
To minimize the modification of the written surface by the reading (imaging) process read forces should be set to the smallest possible value.
Alternatively to write without reading (reading out of contact), set the backward height equal to the forward height, the read force to 0 V, and make sure that the z drift correction and depth feedback are disabled. Before writing without reading it is recommended to optimise the write parameters by writing with reading in a test area of the sample.
Low cantilever temperature (< 800 C)
The critical challenge for high resolution patterning is correct selection of the writing temperature. If the temperature is too cold then too little resist is removed if it is too hot too much resist is removed and the resolution suffers. The heater (writer) temperature which will yield best resolution varies between cantilevers and should be first optimised using a test pattern. Figure 22 gives an example of an appropriate test pattern.
Figure 22 Lines with different pitches of 4, 6 and 8 pixels. The bar on the left is used to have enough data for the depth feedback. A pixels size of 4 nm is used for patterning.
Figure 23 to Figure 25 shows the result of writing a test pattern at progressively lower temperatures until high resolution is achieved.
Figure 23 Lines written with a writer heater temperature of 1000 °C. The line profile shows that the depth of the 24 nm HP lines is 6 nm but in between the line the PPA is recessed by 1 to 2 nm. Final lines amplitude is 4 nm. The lines with 16 nm HP are merged.
[Wolf2015] | Heiko Wolf et al., Sub-20 nm silicon patterning and metal lift-off using thermal scanning probe lithography, Journal of Vacuum Science & Technology B, 33, 02B102 (2015). |