# 2D Dielectric Nanoimprinted PMMA Pillars on Metallo-Dielectric Films
## 2 Results and Discussion  
Figure 1 is a sketch of the proposed 2D array of PMMA pillars deposited on different configurations of Cu/AlN slabs supported by a SiO\({}_{2}\) substrate. In particular, Figure 1a,b correspond to the bilayer and multilayer, respectively. The arrays show a square lattice with a periodicity \(p\) equal to 450 and 600 nm. The square lattice makes the device polarization-insensitive at normal incidence. Without loss of generality, hereinafter, we will consider three pairs of metallo-dielectric slabs for the multilayer configuration. The Cu/AlN bilayer and multi-layer structures were deposited by a DC magnetron sputtering technique (K.J. Lesker Lab 18 system) without a preheating process of the substrate and performed in a single run of deposition in order to minimize contaminations between the layers. A SiO\({}_{2}\) test sample was used to optimize the thickness of the sputtered materials. The growth conditions were optimized to provide good adhesion properties, high crystallographic quality, low residual stress, and well-controlled deposition rates. In more detail, starting from a base pressure of about \(10^{-7}\) mbar, the Cu layers were sputtered from a high-purity Cu target (99.95%), in an Ar atmosphere at a working pressure of \(5\times 10^{-3}\) mbar and a power of 200 W, with a deposition rate of 0.28 nm/s. The AlN layers were deposited from a high-purity Al target (99.9995%) with a gas mixture of N\({}_{2}\)/Ar (1:1 gas ratio) at a working pressure of \(2.8\times 10^{-3}\) mbar; the sputtering plasma was generated in pulsed mode with a frequency of 100 kHz and a power of 1000 W, showing a deposition rate of 0.55 nm/s. Thicknesses were verified by scanning electron microscope (SEM) inspection. The cross-section of the sputtered materials was investigated by means of a focused ion beam (FIB) system that calculated the slab thicknesses (Figure 2). The grey area on the top of the AlN film refers to the platinum (Pt) deposition used to protect the sample during the milling with FIB (Figure 2a) while Figure 2b shows the measured thicknesses of the Cu (about 100 nm) and AlN (about 150 nm) layers deposited on the test SiO\({}_{2}\) substrate. Based on these results, a stack composed of three alternating couples of Cu (17 nm-thick) and AlN (33 nm-thick) deposited on a SiO\({}_{2}\) substrate was achieved. Subsequently, sub-micron PMMA pillars were realized using the nanoimprint lithography technique (EITRE(r)3 system - Obducat Technologies AB, Malmo, Sweden). A silicon mold was used to investigate the effect of the nanoimprint lithography on the SiO\({}_{2}\) substrate. The nanoimprint lithography technique was used to investigate the effect of the nanoimprint lithography on the SiO\({}_{2}\) substrate. Figure 1: (**a**,**b**) Sketch of fabricated samples with an array of cylindrical pillars on top of (**a**) bilayer and (**b**) multilayer configurations (three pairs), respectively. (**c**,**d**) Sketch of the (**c**) bilayer and (**d**) multilayer configurations, respectively, covered by a thin gold layer. Figure 2: (**a**) Scanning electron microscope (SEM)scan showing the cross-section of bilayer design. Grey area on the top of aluminum nitride (AlN) film is a Pt deposition used to protect layers while milling with focused ion beam (FIB). (**b**) Cu (about 100 nm) and AlN (about 150 nm) layer thicknesses deposited on a test SiO\({}_{2}\) substrate. The measured thicknesses are corrected for the tilt. in the NIL process consisting of two square arrays of holes \(\simeq\) 900 nm depth, with a total footprint of 2.5 mm \(\times\) 2.5 mm. After the molds were fabricated, they were coated with a commercial anti-sticking layer that facilitates the subsequent de-molding process. In order to optimize the NIL process, several tests were performed by varying the temperature (in the range 170-180 \({}^{\circ}\)C) and pressure (in the range 30-40 bar), allowing us to achieve uniform and high-resolution pillars starting with a PMMA layer thickness of 220 nm.