1. Introduction

A nano-hole array, that is an array of periodic sub-wavelength apertures fabricated in an optically thick metal film, can exhibit extraordinary optical transmission (EOT). This unique property has enabled researchers to design and miniaturize optical elements in a way that surpasses the diffraction limit from standard aperture theory [1]. The EOT or optical resonance transmission property of nano-hole arrays is caused by the interaction of light with the Surface Plasmon (SP) that exists between the metal and dielectric surfaces and results in the resonant transmission of light through nano-holes by exploiting the tunneling effect [2]. That is, light incident on one side of the metallic film of the nano-hole array is coupled to SPs that enter the nano-holes and decouple on the other side of the film. The EOT properties of nano-hole arrays have many potential applications in sub-wavelength photolithography, near-field scanning optical microscopy, wavelength-tunable filters, surface enhanced fluorescence spectroscopy, and molecular sensing [3-7].

It is well-known that the optical resonance transmission properties of nano-hole arrays depend greatly on the lattice arrangement of holes, the distance between adjacent holes (periodicity), and the dielectric constants of the metal and surrounding dielectrics. For normal incidence of light, the spectral position of the optical transmission maximum of a nano-hole array is well-described by${\lambda }_{\max }=\frac{a_0}{\sqrt{i^2+j^2}}\sqrt{\frac{{\epsilon }_m{\epsilon }_d}{{\epsilon }_m+{\epsilon }_d}}$, and its corresponding optical transmission minimum by${\lambda }_{\max }=\frac{a_0}{\sqrt{i^2+j^2}}\sqrt{{\epsilon }_d}$. These relations hold for holes in a square lattice arrangement, where a₀ is the periodicity of holes, ε_d and ε_m are the dielectric constants of the incident medium (at the top or bottom surface of the nano-hole) and the metal film, and i and j are integers expressing the scattering mode indices [1,2]. Many studies have investigated the effect of various metals and dielectrics on the optical transmission properties of nano-hole arrays [8-12]. For instance, nano-hole arrays fabricated in metallic films such as Ag, Au, and Cu have larger optical resonance transmission peaks than nano-hole arrays with the same geometrical parameters in a perfect metal conductor [8]. However, Cr and Ni have high absorption properties in the optical region, which greatly influences the optical transmission properties of nano-hole arrays incorporating these materials [9]. Matching the dielectric constant surrounding the back and front of nano-hole array in a metal film enhances the optical resonance transmission efficiency due to the coincidence of SP energy on both sides [10]. In addition to the material effects on the optical resonance transmission properties, various geometrical effects such as hole shape, hole size, and hole periodicity have been explored [11-13]. For instance, the optical resonance transmission peaks of nano-hole arrays with elliptical hole shape depend highly on the angle of polarization of the incident light although no polarization dependency was observed for circular and square hole shapes [11]. Also, the hole size has a significant effect on the optical resonance transmission efficiency and the resonance bandwidth of nano-hole arrays [12,13].

2. Motivation and objective

A nano-hole array fabricated in a noble metal such as gold has a higher optical resonance transmission compared to nano-hole arrays in other metals [8]. Unlike other noble metals such as Ag and Cu, gold does not suffer from oxidation and is chemically unreactive. As a result, gold has been the material of choice for fabrication of nano-hole arrays for applications such as bio-sensing and chemical sensing based on the Surface Plasmon Resonance (SPR) [14]. Nevertheless, there is a need for a thin adhesion layer between the glass substrate and the gold film during fabrication. Several studies have explored the effect of the adhesion layer on the optical properties of various plasmonic structures and its impact on performance in various applications [15-20]. For example, a thin titanium adhesion layer between gold and a semiconductor substrate caused a 20 nm red-shift on the resonance position of slit-ring resonators (SRR) [15]. Also, the effect of the adhesion layer on the Short Range Surface Plasmon Polariton (SR-SPP) of gold bowtie antennae was numerically calculated and it was observed that the SR-SPP quenched when the adhesion layer was titanium or chromium. The suppression of the SR-SPP was stronger for chromium due to its higher extinction coefficient [17]. Motivated by the reported effects of the adhesion layer on the optical properties of slit-ring resonators and bowtie antennae, we performed a systematic study on the effect of the adhesion layer on the EOT properties of nano-hole arrays in a gold film, which could potentially impact spectral filtering and bio-sensing applications.

Our approach was to perform numerical and experimental analyses on the optical resonance transmission properties of a series of nano-hole arrays in a gold film, where various adhesion layers were used (thin Ti, thin Cr, thicker Ti or etched thicker Ti adhesion layer). Nine different nano-hole arrays with various geometrical parameters (hole size and periodicity) were fabricated in a square lattice arrangement in 100 nm gold film for each adhesion layer case. The geometrical design of each nano-hole array was selected so that the optical resonance transmission properties were within the visible and near infra-red regime. Nano-hole arrays were fabricated using Electron Beam Lithography (EBL). Each nano-hole array was characterized with optical transmission spectroscopy and the experimental results were compared with numerical calculations by Finite Difference Time Domain (FDTD) with respect to the (1,0) optical resonance transmission properties related to (1,0) SP excitation from Pyrex-gold side.

3. Methods

3.1 Electron beam lithography (EBL)

We used electron beam lithography (EBL) to fabricate nano-hole arrays in a 100 nm optically thick gold film. The fabrication process is shown in Fig. 1 . In order to fabricate gold nano-hole arrays on a Pyrex substrate using EBL, we first deposited a conductive layer to enable the electron beam to be focused on the Pyrex substrate when writing patterns. We deposited a 3 nm thin conductive layer of chromium or titanium on the Pyrex substrate using electron beam physical vapor deposition (EB_PVD) (see Fig. 1(a)). Then, a 500 nm photo-resist (Negative Tone photo-resist ma-N 2403) was spin-coated and soft-baked on the conductive layer (see Fig. 1(b)). The pattern of nano-hole arrays was written with the EBL machine (LEO, 1530 e-beam lithography) and followed by development of the sample to leave behind photo-resist pillars (see Fig. 1(c)). The adhesion layer (similar metal to the conductive layer with thickness of 5 nm for a thin chromium adhesion layer, 5 nm for a thin titanium adhesion layer and 10 nm for a thicker titanium adhesion layer) was deposited (see Table 1 for a summary of the fabricated nano-hole arrays with various conductive and adhesion layers) (see Fig. 1(d)) and followed by deposition of a 100 nm gold layer (see Fig. 1 (e)). Finally, the sacrificial mask layer (photo-resist pillars) was lifted off to leave behind the nano-hole array patterns in the gold film (see Fig. 1(f)). The SEM images of a fabricated nano-hole array are shown in Fig. 2 . Previous AFM work by another group employing the same EB_PVD system determined that the surface roughness of the gold layer was 1.9 nm [21].

 

Fig. 1 Electron beam lithography (EBL) for fabrication of nano-hole arrays

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Table 1. Summary of the Conductive and Adhesion Layer Composition and Thickness

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Fig. 2 SEM images of a nano-hole array in a 100 nm gold film: a) angle view, and b) top view.

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For one device with a 10 nm titanium adhesion layer, we used a titanium etchant (TFT, Transene company, Inc.) to isotropically etch away the titanium adhesion layer. The etching rate of TFT for titanium was 2.5 nm/s at 20°C. Also, since the etchant contained 30% Hydrofluoric (HF) acid, it also etched the Pyrex substrate at approximate 4.3 nm/s. As a result, after the titanium conductive and adhesion layers within the holes were removed, the etchant had the opportunity to etch the Pyrex substrate and the titanium adhesion layer between the holes and beneath the gold film. For example, a nano-hole array with a 3 nm titanium conductive layer and a 10 nm titanium adhesion layer placed in the titanium etchant for 20 s resulted in removal of the 3 nm conductive layer, the 10 nm adhesion layer, and approximately 64 nm of Pyrex.

We fabricated nine different nano-hole arrays with various hole sizes and periodicities for each adhesion layer case (see Table 1). The ratio of the hole area (area of circle) to the background area (square area of periodicity) was 0.21 and the same for all arrays. A summary of the geometrical parameters (hole size and periodicity) of the fabricated nano-hole arrays is provided in Table 2 . The hole periodicities were chosen to observe optical resonance transmission of each array in the visible and near-infrared regime. The dimension of each array was 50 µm by 50 µm.

Table 2. Summary of the Geometrical Parameters of the Simulated and Fabricated Nano-Hole Arrays

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3.2 Optical characterization setup

To optically characterize each nano-hole array, we employed an inverted microscope (Nikon, TE300) attached to a photometer (PTI, D104), monochromator (PTI, 101), and photo-multiplier tube detector (PTI, 710). Unpolarized white light from a 100 W halogen lamp was focused on to the sample from the air-gold side using the bright-field condenser lens (NA = 0.3) of the microscope. The transmitted light was collected by a 20X objective (NA = 0.45; Nikon, 93150). Utilizing the aperture adjustment on the photometer, light from a desired region was captured for spectroscopic analysis by the monochromator and detector. For each device, optical transmission spectra were collected from the region containing the nano-hole array (sample) and a hole-free region (background). The background and lamp properties were accounted for by subtracting the background spectrum from the sample spectrum and then dividing the result by the measured white light spectrum (collected with the Pyrex substrate in front of the objective).

3.3 FDTD simulation of nano-hole arrays

We used the three-dimensional (3D) FDTD method to simulate the interaction between light and a nano-hole array in an optically thick metal film with the purpose of calculating the optical transmission properties [22,23]. We used the FDTD package from Lumerical Inc. (Vancouver, Canada) with dielectric constants for metallic and dielectric materials provided by Palik [24]. The minimum grid size was 2 nm in the FDTD simulation analysis. The details of the numerical simulation model are described in more detail elsewhere [12].

3.4 Analysis of the optical transmission spectra

In order to analyze the effect of various adhesion layers on the EOT properties of nano-hole arrays, the optical resonance peaks related to SP excitation from the Pyrex-gold side were examined. For each optical transmission spectra, we computed the optical resonance position (resonance wavelength), spectral transmission modulation ratio (STMR), and optical resonance bandwidth. The STMR of the optical resonance peak was defined as$STMR=\frac{T_{\max }-T_{\min }}{T_{\max }+T_{\min }}$, where T_max was the transmission of the (1,0) resonance peak and T_min was the transmission of the (1,0) minimum. The optical resonance bandwidth was computed as the full width of the EOT, where the optical transmission was $\frac{1}{\sqrt{2}}$ of the maximum.

4. Results

All fabricated nano-hole arrays were optically characterized experimentally and with simulation. The optical transmission spectra for nano-hole arrays employing the same composition and thickness adhesion layer obtained by simulation and experiment were in good agreement. The optical transmission spectra for nano-hole arrays simulated and fabricated with various conductive and adhesion layers (5 nm chromium (Cr5), 5 nm titanium (Ti5), 10 nm titanium (Ti10), etched 10 nm titanium (Ti10-etched), and no adhesion layer) between gold and Pyrex substrate are shown in Fig. 3 . The (1,0) optical resonance peaks related to SP excitation from Pyrex-gold side were clearly observed in both simulation and experiment for all nano-hole arrays (brown dashed line in the Fig. 3), except the Ti10 case. The (1,0) transmission minimum before each (1,0) optical resonance peak was clearly observed for each nano-hole array (yellow dotted line in Fig. 3), except for the Ti10 case. As shown in Fig. 3 for both simulation and experimental results, the position, transmission and bandwidth of the (1,0) optical resonance peak was dependant on the type of adhesion layer.

 

Fig. 3 Optical transmission spectra of nano-hole arrays with circular holes, 223 nm in diameter, and 425 nm periodicity in a 100 nm thick gold film for various adhesion layers between gold and Pyrex substrate [5 nm chromium (red curve), 5 nm titanium (blue curve), 10 nm titanium (black curve), etched 10 nm titanium (green curve) and no adhesion layer (purple curve in simulation results)]: a) simulation results and b) experimental results. (Pg: Pyrex-gold side).

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The position of the (1,0) optical resonance peak related to SP excitation from the Pyrex-gold side was extracted from the optical transmission spectrum of each nano-hole array for each adhesion layer case. The (1,0) resonance position versus hole periodicity of nano-hole arrays for various adhesion layers are shown in Fig. 4 as well as the theoretical results calculated by the Ebbesen equation. In both simulation and experimental results, the (1,0) resonance position red-shifted as the periodicity of the nano-holes increased. In addition, for a given periodicity, the presence of the Ti and Cr adhesion layers resulted in a red-shift in the resonance peaks compared to the resonance peak of the Ti10-etched. However, the red-shift was greater for Ti5 compared to Cr5 for each periodicity. In the experimental measurements, the average red-shifts of the (1,0) resonance peaks for Cr5 and Ti5 with respect to the resonance position of Ti10-etched were 79 nm and 106 nm for a given periodicity, respectively. The (1,0) resonance position for Ti5 was red-shifted on average 27 nm compared to Cr5. However, the difference in the resonance peak position between Cr5 and Ti5 became smaller as the hole periodicity increased. Both experimental and simulation results for the effect of various adhesion layers with respect to the (1,0) resonance position were similar. Also, the (1,0) resonance positions found by experiment were very close to simulation results except for the nano-hole arrays with lower periodicities fabricated with a Cr adhesion layer, where the simulation results showed a slight shift to shorter wavelengths compared to the experimental data.

 

Fig. 4 The (1,0) optical resonance position versus hole periodicity for nano-hole arrays in a 100 nm gold film with various adhesion layers between the gold and Pyrex substrate [5 nm chromium (red curve), 5 nm titanium (blue curve), etched 10 nm titanium (green curve), and theoretical results (light blue curve in simulation results)]: a) simulation results and b) experimental results.

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Figure 5 shows the STMR versus nano-hole periodicity for nano-hole arrays with various adhesion layers obtained from simulation and experimental results. In simulation, for a given periodicity, the STMR for the Ti10-etched layer was higher compared to the STMR for Cr5 or Ti5. The STMR for Ti5 was higher than the STMR for Cr5 although the difference was not large. As the periodicity of the nano-holes increased, the STMR increased slightly for all nano-hole arrays regardless of the adhesion layer type. From experiment, the STMR for Ti5 had the highest value compared to Cr5 or Ti10-etched for a given periodicity. The STMR for Ti10-etched had a lower STMR compared to Cr5. The STMR remained approximately constant as the periodicity increased for nano-hole arrays with the various adhesion layers. Both simulation and experiments revealed a higher STMR for Ti5 compared to the STMR for Cr5. In contrast to the experimental results, the STMR for Ti10-etched obtained from simulation was higher than the STMR for Cr5 or Ti5.

 

Fig. 5 The spectral transmission modulation ratio for (1,0) resonance peak and its respective minimum versus hole periodicity for nano-hole arrays in a 100 nm gold film with various adhesion layers between gold and Pyrex substrate [5 nm chromium (red curve), 5 nm titanium (blue curve), and etched 10 nm titanium (green curve)]: a) simulation results and b) experimental results.

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The (1,0) optical resonance bandwidth versus periodicity for nano-hole arrays with various adhesion layer materials is shown in Fig. 6 . In both simulation and experimental results, the (1,0) resonance bandwidth was lower for the Ti10-etched layer than Ti5 or Cr5 for a given periodicity. However, the (1,0) resonance bandwidth for Cr5 was higher compared to Ti5. In both the simulation and experimental results, the bandwidth remained nearly constant as periodicity increased for each case.

 

Fig. 6 The (1,0) optical resonance bandwidth versus hole periodicity for nano-hole arrays in a 100 nm gold film with various conductive and adhesion layers between the gold and Pyrex substrate [5 nm chromium (red curve), 5 nm titanium (blue curve), and etched 10 nm titanium (green curve)]: a) simulation results and b) experimental results.

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The spatial distributions of the electric field intensity (log scale) are shown in Fig. 7 for nano-hole arrays with 223 nm hole diameter and 425 nm periodicity for various adhesion layer cases at their corresponding (1,0) resonance wavelength. The spatial ditributions were computed for a xy surface near the hole (6 nm away from the hole in air-gold side) and the xz cross section through the hole. For all the adhesion layer cases, the highest electric field intensity was observed at the edges of the holes (hot-spots). The hole with the Ti10-etched layer had a much higher electric field intensity at the edges compared to holes with the Cr5 or Ti5 layers. The enhancement of the electric field intensity in the hot-spot of the Ti10-etched case was 34 and 98 times larger compared to the Ti5 and the Cr5 cases, respectively. The magnitude of electric field intensity was higher for the Ti5 case compared to the hole with the Cr adhesion layer.

 

Fig. 7 The spatial distribution of electric field intensity for a xy surface 6 nm above the hole from air-gold side: a) 5 nm Cr adhesion layer, b) 5 nm Ti adhesion layer, and c) etched 10 nm Ti adhesion layer. Spatial distribution of the electric field intensity in the central xz plane of the structure d) 5 nm Cr adhesion layer, e) 5 nm Ti adhesion layer, and f) etched 10 nm Ti adhesion layer.

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5. Discussion

5.1 Overall findings

The (1,0) optical resonance transmission properties of nano-hole arrays from the Pyrex-gold side depended on composition of the adhesion layer. Both the 5 nm Ti and the 5 nm Cr adhesion layers caused a red-shift in the (1,0) resonance position of nano-hole arrays compared to the etched adhesion layer case. From the experimental results, the (1,0) STMRs for Cr5 or Ti5 were higher than the etched adhesion layer case. Also, the Ti and Cr adhesion layers had a major effect on increasing the optical resonance bandwidth of the nano-hole arrays.

In the simulation results (see Fig. 3(a)), the position of the (1,0) resonance peak was lower for the Ti10-etched case compared to the one without an adhesion layer (no adhesion layer). This difference was caused by the etched region in the Ti10-etched. The etched region decreased the effective refractive index on the Pyrex-gold side and blue-shifted the (1,0) resonance peak compared to the nano-hole array without an adhesion layer. Also, a larger etched region is expected to result in a lower effective refractive index on the Pyrex-gold side of the holes and, in turn, result in a greater blue-shift in the resonance peak.

5.2 Fabrication of nano-hole arrays

In addition to EBL used in this study, Focused Ion Beam (FIB) milling and Nano-Imprint Lithography (NIL) are common methods for fabrication of nano-hole arrays [25-27]. Each method requires a thin adhesion layer to be deposited between the substrate and the gold film; however, the thickness of the adhesion layer can be considerably different for each fabrication methodology. For example, FIB milling requires an adhesion layer with a thickness of 2-3 nm, while due to the aggressive fabrication nature of EBL, a thicker adhesion layer is required. Based on our fabrication experiments, an adhesion layer with a thickness of 4 to 8 nm was sufficient to adhere the gold film to the Pyrex substrate and avoid peel off during the lift-off process.

We employed an EB-PVD system for deposition of the Ti and Cr adhesion layers to achieve uniform deposition of each metal. The deposition rates for both Ti and Cr materials were calibrated in order to achieve the desired thickness. However, due to material property differences between Cr and Ti, the deposition process was expected to result in thickness difference of a few angstroms between the two adhesion layer types for a 5 nm target thickness. This may have contributed to the slight differences in the optical resonance bandwidth and the STMR for nano-hole arrays fabricated with adhesion layers of each material. After the optical characterization of nano-hole arrays with the thicker Ti adhesion layer, we etched the adhesion layer of the samples and very different optical properties were observed in both experiment and simulation (see Fig. 3). According to the measured optical resonance properties of the (1,0) resonance peak from the Pyrex-gold side, it is probable that the Ti within and to the sides of each hole, and the Pyrex beneath and to the sides of each hole were partially etched away. SEM analysis of the etched nano-hole arrays confirmed that the gold film remained intact and did not peel off in the area of nano-hole array. This may be a result of resilient bonding between the gold nano-structure and the gold film beyond the nano-hole array, where the Ti etchant had no effect.

5.3 Analyses of the experimental and simulation results

The notable differences between the optical resonance properties of nano-hole arrays employing various adhesion layers were clearly observed in both simulation and experimental data. The composition and thickness of the adhesion layer had a significant effect on the optical resonance properties of the nano-hole arrays. Due to the high optical absorption properties of Ti and Cr, a greater thickness of these materials between Pyrex substrate and the gold caused a significant change in optical resonance properties related to the SP excitation from the Pyrex-gold side of the nano-hole array. For example, compared to the 5 nm Ti adhesion layer case, the (1,0) optical resonance peak for the 10 nm Ti adhesion layer was suppressed due to the high thickness of Ti and its strong absorption properties. Other major effects of the adhesion layer thickness were the widening of the optical resonance bandwidth and a change in the STMR. The optical resonance peak position was not significantly affected by the thickness of the adhesion layer.

Although there was good agreement between the (1,0) optical resonance position and bandwidth of nano-hole arrays studied in experiment and simulation, there were notable differences in the STMR observed by simulation and experiment. In experimental data, the STMR of nano-hole arrays with a Cr or Ti adhesion layer was higher than the STMR measured from simulation data. This discrepancy could have been due to limitations of the FDTD method when modeling the thin adhesion layer and the material dispersion properties used in the simulations. This is supported by the good agreement between the simulation and experimental optical transmission spectra for the thicker Ti adhesion layer (Ti10).

In both simulation and experimental results, the different absorption properties of Ti and Cr likely resulted in the observed differences in the (1,0) optical resonance positions between nano-hole arrays with Ti and Cr adhesion layers. Based on the dielectric constants of these materials, Cr has a stronger optical absorption compared to Ti although the difference in absorption between Cr and Ti becomes less apparent at higher wavelengths. As a result, the optical resonance positions of Ti5 approached those of Cr5 at higher periodicities.

The STMR measured by experiment for the Ti10-etched cases were lower than the STMR for the Cr5 or the Ti5 cases. This was due to the optical absorption properties of Ti or Cr, which caused the (1,0) transmission minimum to be lower in the Cr and Ti cases resulting in a higher STMR. Also, the large difference between the STMR for Ti5 compared to the STMR for Cr5 may have been due to the higher absorption properties of Cr compared to Ti. Based on the experimental results, a thin adhesion layer of Ti resulted in an increased STMR at the expense of a larger optical resonance bandwidth.

5.3 Implications of the adhesion layer composition to surface Plasmon sensing

Optical resonance peaks of nano-hole arrays can be employed in applications such as spectral filters and Surface Plasmon Resonance (SPR) sensing. In SPR sensing applications such as bio-sensing, a narrow bandwidth and a steep slope in the optical resonance transmission spectrum are desired since they result in enhanced sensitivity of the nano-hole array for detection of bio-molecules. Based on the experimental results, the optical resonance peak for Ti10-etched and Ti5 had narrower bandwidth and sharper slopes compared to the nano-hole arrays with a Cr adhesion layer suggesting nano-hole arrays with a Ti adhesion layer or an etched adhesion layer are more suitable for SPR sensing applications. This is supported by recent SPR sensing work where a Ti adhesion layer was preferred over a Cr adhesion layer in Kretschmann SPR sensing configuration due to the lower absorption properties of Ti compared to Cr [14].

6. Conclusions

In this study, we presented a systematic numerical and experimental analysis on the optical resonance transmission properties of nano-hole arrays in an optically thick gold film with various adhesion layers (5 nm titanium, 5 nm chromium, 10 nm titanium, and etched) between the Pyrex substrate and the gold film. The fabricated nano-hole arrays were optically characterized and compared with corresponding simulation results. Good agreement was observed between the simulation and experimental results for the (1,0) optical resonance bandwidth and the optical resonance position for most nano-hole arrays. However, some differences between the STMR values obtained from simulations and experimental measurements were observed. The composition and thickness of the adhesion layer had a large effect on the optical resonance transmission properties of the tested nano-hole arrays. The Cr and Ti adhesion layers caused an increase in (1,0) optical resonance position and bandwidth compared to the etched case. The measured STMR value was greatest for nano-hole arrays fabricated with a 5 nm adhesion layer of Ti. The results confirmed that the composition of the adhesion layer is an important parameter for optimization of nano-hole arrays for sensing applications.