Kompetenzzentrum für Mikro- und Nanotechnologien

Within this contribution we introduce a method using microfabricated polymer masks as highly homogenous and highly sensitive substrates for surface enhanced Raman spectroscopy (SERS). Pure polymers surfaces are Raman-active and due to there huge background signals not directly suitable for SERS applications. However, low-cost polymer or plastic substrates with reproducible field enhancement properties might be a strategy for the acceptance and the employment of the SERS technique for (bio)analytical applications. Proposals using imprinted plastic hole-gratings covert with a silver layer, which are thick enough to suppress the background signal were already shown in the literature. Our approach combines e-beam lithography for the PMMA mask preparation, atomic layer deposition of aluminum-oxide as protection layer for the PMMA mask and silver evaporation for the SERS-active metal film. The patterns which follow up our former investigations are regular arranged two-dimensional gratings with periods of 200 and 400 nm (Figure 1 and 2), exposed in 70 nm thick PMMA on quartz substrates. After development the resist mask is completely covert with a 20 nm thick Al2O3 film, done by atomic layer deposition. On top, a 40 nm thick silver overlayer serves as the structured plasmonic film for the field enhancement of the light. The alumina-ALD-layer suppresses the PMMA Raman signal, further it is hard and dense enough to protect the polymer against organic solvents and allows the cleaning of the SERS substrate and so the multi-use for further SERS measurements. The well conform-coated plasma enhanced ALD-layer conserve the resist mask geometry (no mask degrading compared to plasma etch processes) and gives the possibility to trim the gap size inside the grating in a very precisely way. Here, SERS measurements of PMMA substrates using crystal violet as model analyte will be presented. A typical SERS fingerprint spectrum of crystal violet is shown in Figure 3. The influence of different geometrical parameters, like the dot diameter to the SERS enhancement was investigated (Figure 4). Compared to our former investigations with silver deposited etched quartz SERS-substrates recently described in [2] the polymer SERS substrates show the same reproducible signal enhancement across the whole grating area In summary, PMMA substrates are easier to fabricate and the presented method provide the potential to use low-cost imprint techniques towards low-cost SERS substrates.