2.4. Cryogenic Deep Reactive Ion Etching

In order to transfer the pattern from the mr-NIL210-200 nm resist to the silicon wafer, it was first required to remove the residual layer (RL—the resist formed between imprinted pillars). This process was performed by oxygen plasma, using an RIE system (Etchlab SI 220-Sentech Instruments, Berlin, Germany). The values of this recipe’s parameters included a pressure of 150 mTorr, an ICP power setting of 200 W and 50 sccm O₂ flow. Due to the significant non-uniformity thickness of the residual layer between wafers, oxygen etching was performed in subsequent steps with optical investigations in between (Figure 4), which allowed us to evaluate the total required etching time.

Optical microscopy image of the metalens after 30 s. of oxygen plasma (during the RL removing process).

For most samples, we allowed for two steps of 15 s each for the complete removal of the residual layer, though in some cases additional plasma exposure was required. The subsequent etching of the silicon wafers was achieved by using the cryogenic process, performed in an Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) system—PlasmaLab 100 (Oxford Instruments Ltd, Yatton, UK). The process was carried out at −115 °C, using the ICP power of 1200 W, RF power of 3 W, a pressure of 7.5 mTorr and 60 sccm SF₆ flow and 8 sccm O₂ flow. Figure 5a shows that high pattern fidelity of the silicon pillars was achieved. Vertical sidewalls and heights of around 800 nm were observed Figure 5b. At the end of the technological flow, the mr-NIL210-200 nm resist was completely removed in oxygen plasma (5 min process).

Cross-sectional view of SEM images of the nanopatterned silicon after cryogenic etching process (tilt 45°)—(a) the fidelity of the nanopillars (b) with corresponding measured heights.