Electron beam lithography tri-layer lift-off to create ultracompact metal/metal oxide 2D patterns on CaF2 substrate for surface-enhanced infrared spectroscopy
Graphical abstract
Introduction
The large free carrier density and relatively small ohmic losses of the noble metals Au, Ag and Cu have enabled many of the breakthroughs in field-enhanced spectroscopy [1], [2], [3], [4], [5]. However, substrates coated with such metals are restricted in many applications such as CMOS or silicon waveguide integrations. Therefore the use of alternative (semiconductor) plasmonic materials, such as Si and ITO, has attracted significant attention in this field [6], [7], [8]. ITO antennas show a strongly reduced plasmon wavelength compared to metals like Au, which holds promise for ultracompact antenna arrays and extreme subwavelength. We have recently shown that ITO antennas can be integrated at extremely high densities with no loss in performance due to suppression of long-range transverse interactions [7]. These and many other actual device measurements require relatively thick films (at least 100 nm) with small edge roughness to enable operation. Here we report the positive tone tri-layer electron beam lithography (EBL) lift-off process required to create such high density patterns for 100 nm thick Au and ITO layers.
Nano fabrication procedures such as electron beam lithography patterning together with electroplating or lift-off pattern transfer have been widely used to fabricate nano-array structures [9], [10], [11], [12]. It has been shown recently by Calafiore et al. [13] that by using a nano-imprint lithography and a tri-layer process, it is possible to achieve 1 dimensional metal lines of 15 (35) nm thickness, 15 (50) nm width and 20 (50) nm half pitch [13]. A tri-layer lift-off process is not required for e-beam lithography, but we show in this work that it is nevertheless extremely beneficial as it gives better resolution than a bilayer process and better line edge roughness than a single layer process, such as applied in Ref. [16], in both 1D and 2D nanostructures.
In our previous work we discussed the limitation of suspended polymethylmethacrylate (PMMA) on copolymer (MMA) bi-layer resist EBL process [14], [15]. For 1 dimensional arrays with a resist layer of around 250 nm, the limit of resist width w is about 100 nm as shown in Fig. 1. For two dimensional 2D arrays the minimum width is about 200 nm as shown in Fig. 2. In order to push the resist width limit down to below 100 nm without reducing the ability to lift off, we have developed a tri-layer lift-off process which results in higher pattern density. A SiO2 layer sandwiched between top and bottom layer of the bi-layer resist breaks the link of the two limitations. The top layer resist which influences the lift-off resolution can be much thinner without affecting the lift-off ability which is now determined by the thickness of the bottom layer.
Section snippets
Fabrication procedure
Demonstrator structures with Au patterns are fabricated on a Si substrate while a CaF2 substrate with ITO features is used for the infrared spectrum measurement. The process is schematically shown in Fig 3. A 250 nm thick MMA layer is spin coated on the substrate. Then a 20 nm thick PECVD SiO2 is deposited at 200 °C. The PECVD is performed in an OIPT SYS100 system. The depositing duration was 24 s following a 5 min stabilizing dummy run. A 150 nm thick PMMA layer is spin coated on the SiO2 layer. On
Tri-layer lift-off
The results of the tri-layer lift-off process are shown in Fig. 4. The comparison with Fig. 1 shows that in the tri-layer process the undercut in the MMA layer is slightly smaller, and, more importantly, that the shape of the undercut is altered. The positive angle of the MMA layer in the tri-layer process is significantly more stable against toppling over than the overhang structure created in the bilayer process. The different undercut comes from the difference between the tri-layer dry etch
Infra-red spectroscopy of ITO structures
The ITO and Au structures on CaF2 using the tri-layer lift-off process are shown in Fig 6. The spacing between the ITO rods (resist width w) is 100 nm and gives well resolved features even for a pitch of 200 nm whereas the limits of the bi-layer process were around a pitch of 400 nm (Fig. 2). Fig. 7 shows the transmission at the resonant wavelength in the infrared spectrum of ITO rods as measured on 100 nm thick ITO and Au arrays as function of rod size L and rod spacing w.
Even when the ITO rods
Conclusion
The fabrication of ITO arrays for surface-enhanced infrared spectroscopy requires relatively thick layers of ITO at extremely high pattern density. The tri-layer e-beam lithography process combines both the ability for lift-off of thick films and high resolution for high density. The dry etching of the MMA layer results in both higher aspect ratio and better undercut structure resulting in the ability to transfer high resolution high density resist patterns.
Acknowledgements
This work was financially supported by EPSRC – United Kingdom through the research Grant EP/J011797/1 and EP/J016918/1.
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