The maximum structural depth for the first etching step is determined by the height of the imprint template and the etching rate ratio between the imprint resist and the ITO layer. To perform an etching rate analysis at the chosen etching parameters, we prepared several planar samples with and without the imprint resist. We etched partially masked samples for different times before measuring the resulting height difference with a profilometer. The measurements shown in Fig. 2 indicate constant etching rates over the entire process duration corresponding to an etching rate ratio of 1.2. Despite the low selectivity of the etching process (which is expected for a purely physical etching), it is suitable to transfer the grating pattern into the underlying ITO layer. However, as the ITO etching rate is lower than the etching rate of the imprint resist the maximum structural depth possible is only 116 nm (83% of the height of the imprint template).
The actual etching time for each sample was chosen according to the desired etching depth assuming the etching rates presented before. In order to account for potential thickness variations of the imprint resist during deposition, the etching process was additionally tracked in-situ by time-of-flight secondary ion mass spectrometry (TOF-SIMS), as shown in Fig. 3. Since the imprint resist contains zirconium (Zr), one can qualitatively track the presence of resist on the sample surface by monitoring the Zr content in the secondary ion gas mixture. While etching a planar sample, the Zr count decreases sharply after approximately 7 minutes indicating complete removal of the imprint resist and exposure of the ITO layer underneath. At the same time the indium (In) count rises abruptly corresponding to the beginning of the ITO etching. After a total etching time of 15 minutes the ITO count declines again indicating that the ITO layer is entirely removed. The sudden increase of the Si count at this point can be attributed to the etching of a 20 nm thick SiO2 passivation layer underneath the ITO coating. Considering the etching durations for planar layers of the imprint resist (approx. 200 nm) and the ITO coating (140 nm ± 20 nm) indicated in Fig. 3b), an etching rate ratio of 1.45 ± 0.2 is estimated. The discrepancy between this value and the ratio calculated from the etching rate analysis may be mainly attributed to uncertainties in total layer thickness. This emphasizes the importance of the in-situ tracking as it allows precise detection of the starting point of the ITO etching and calculation of the required etching time irrespective of the actual imprint resist thickness.
By repeating the imprint and etching process rotating the imprint template by an angle for each imprint step, we obtain two-dimensional greyscale patterns in the ITO. In order to determine the etching time for subsequent patterning steps, the existing nanograting has to be taken into account. During coating, the imprint resist fills the voids of the underlying pattern while partly preserving the structure through the layer. However, application of the imprint template forces a new pattern onto the surface, causing differences in resist thickness, which result from both the existing pattern underneath the resist and the newly imprinted grating. As the etching rate of the imprint resist is higher than the etching rate of the ITO, this effect finally leads to flattening of the existing pattern during successive etching steps. While the flattening may be utilized to easily obtain greyscale features, one can also diminish it by choosing the etching time accordingly. The final shape of the resulting nanopattern is determined by the number of the performed imprint and etching steps as well as the angles in which the imprint templates are applied. One can simply obtain square pillar patterns by using the same imprint template holding a 1D grating structure and rotating it by 90° for the second imprint step. Other designs (such as rectangular or rhomboid shaped pillars) can be fabricated by using multiple 1D templates with different period lengths or applying the PDMS templates at different angles. Examples for various different greyscale pattern designs fabricated with our method are presented in Fig. 4. The generation of more complex patterns such as isosceles triangles requires more than two imprint steps and therefore exact alignment of the imprint templates. For this work, all alignment was done manually, hence preventing the fabrication of those patterns. However, as alignment precision below 100 nm has been shown to be feasible for NIL34, we expect our method to be suitable for the generation of arbitrary periodic and aperiodic patterns from appropriate master templates.
Since the generation of complex multi-dimensional patterns from one-dimensional master templates requires multiple imprint and etching steps, exact reproducibility between different samples may be difficult to achieve. We therefore envision the main application of our method to be the fabrication of new master template patterns. Subsequently, one may utilize these master templates for pattern duplication via highly reproducible standard nanoimprint lithography techniques. Generating new variable master templates by nanoimprint lithography and ion beam etching using only one or few existing templates is especially beneficial in cases where greyscale EBL may not be available or not feasible due to size constraints. In order to show the applicability of our method to this workflow, we covered the etched ITO gratings with an anti-sticking layer (BGL-GZ-83) and used them as a mold for PDMS imprint templates. With these PDMS stamps we performed a standard UV-NIL process yielding a nanopatterned layer of imprint resist on a glass substrate, which was coated with a thin silver layer for SEM imaging. A comparison of the employed ITO templates and the resulting imprinted nanopatterns is presented in Fig. 5. Both the one-dimensional grating pattern (a) and the two-dimensional pillar pattern (c) show very good conformity to the period length and grating depth of the templates (b) and (d) confirming that the etched ITO samples can indeed be used as master templates for pattern replication. The imprinted samples exhibit higher surface roughness and less distinct pattern features than the templates. We attribute this mainly to the silver coating on top of the resist and not to be a result of the imprint process itself. This is due to the fact that silver may form rough films especially when deposited at the low rates necessary for the fabrication of very thin layers35, whereas we did not observe roughening of the pattern during any other imprints using the same template material and imprint resist.