Fabrication and Characterization of Plasmon-Active Infra-Red Enhanced Micromembranes for Gas Diffusion Studies

Surface Enhanced Infra-Red Absorption Spectroscopy (SEIRAS) is an emerging tool for analytical sensing.[1] It has been applied to applications such as catalysis [2], biosensing [3] and studying molecular transport [4]. A typical configuration used in all these systems is the lithographic fabrication of periodic and resonantly tuned nanostructures (ex. nanoslits) supported on dielectric substrates. In this work however, we report an innovative method to fabricate plasmon-active porous micromembranes with an end use in applying them for monitoring gas diffusion. Here, the porosity of the membrane is generated by completely etching of the fabricated nanoslits. To optimize the fabrication process flow, we use and compare two different lithography techniques; electron beam lithography (EBL) and mask-less laser writing (MLW). The membranes are fabricated by etching a 500 nm thick Si-rich nitride (SiRN) layer and are made plasmon-active by sputtering 50 nm gold (Au). The fabrication is carried out by first etching resonantly tuned nanoslits on a SiRN coated Si substrate. A backside KOH etch is then performed to suspend the membrane by removing the excess silicon. A schematic of the fabrication process flow can be seen in Figure 1.
Compared to MLW, EBL offers the advantage of fabricating nanostructures with nanometer precision, with the smallest feature size in the order of 10-100 nm. However, commonly used EBL photoresists are not suitable for fabrication of thick membranes, due to their inability to resist long etching times. MLW however, utilizes standard ultra-voilet photoresists which are durable during long etching steps. MLW also takes only 20 mins to write a pattern design which takes approximately 3 hours with EBL. The only negotiable drawback with MLW is the ability to fabricate nanostructures with the smallest feature size in the range of 500-600 nm. It is visible from the scanning electron microscopy (SEM) image in Figure 2(b) that MLW has more rounding at the edges of the nanoslits as compared to EBL fabricated nanoslits (Figure 2(a)). Figure 2(c) shows the titled SEM view of the micromembrane fabricated with MLW where the complete etching of the SiRN membrane can be seen. A comparative summary of the differences between the two techniques can be found in Table 1.
Keeping in mind the end application, the SEIRAS plasmonic activity of the SiRN membranes is simulated using a finite-difference time-domain (FDTD) software. Figure 3(a) shows the spectral tuning of the membrane that can be achieved by fabricating different slit lengths. The results obtained are also compared to simulations of standard SEIRAS substrates found in literature.[1] Additionally, the simulated enhancement of the porous Au-SiRN membranes can be found in Figure 3(b). It can be clearly seen that the localized sensitivity arises from the pores at the edges of the Au/SiRN interface. With our work, we show the innovative capability to fabricate plasmonic-active SEIRAS micromembranes with the potential to be applied to gas sensing applications. For this we have optimized the SiRN etching process to fabricate porous membranes using MLW lithography.