Microfabrication and tailoring of permselective properties of microporous polymers by Deep X-ray Lithography

Thermally rearranged (TR) polymers are engineered micro-porous polymers which have interconnected microcavities with very high free volume (up to 30%) and extraordinary properties in terms of permeability and selectivity (permselective functionality). They are prepared by thermally rearranging organic precursors at high temperature (typically 450 ˚C). The dimensional pore distribution is bimodal (0.3–0.4 nm and 0.7–0.9 nm), and this provides exceptionally fast and selective diffusion characteristics for small gas and ion molecules. Furthermore, TR polymers present a unique advantage: the internal micropore size can be tuned by thermal treatment (temperature and time). Despite their extraordinary permselective properties, device fabrication that takes advantage of their superior properties was not yet investigated.  In the present study, Deep X-ray lithography (DXRL) was proposed as patterning technique for TR-polymers. The irradiation with X-rays, performed at the DXRL beamline, allowed for control of the micropattern while simultaneously enhancing the permselective properties of TR-polymer.
The proposed lithography technique does not require any chemical modification of the molecular structure or photoinitiator to induce the micropattern formation. The one-step irradiation process performed using hard X-rays makes the process simple and convenient to fabricate extremely reliable high-throughput microdevices.
Membranes and films prepared from the polymer precursor were exposed to X-rays through a mask containing the desired micropatterns. The radical formation induced by the X-rays in the unmasked regions made these irradiated regions insoluble to any organic solvent, while the masked areas (unirradiated) could be removed using ethanol. This process allowed for the fabrication of multileveled structures by simply repeating the deposition and lithography steps, as illustrated in Figure 1. The obtained micropatterns underwent thermal treatment.
 

Figure 1:    Fabrication of a two level array of microchannels 50 µm wide each : (a) silicon substrate, (b) coating of the silicon with the TR-polymer, (c) X-ray irradiation of the polymer through a slit-shaped gold mask, (d) development of the exposed structure to obtain an one-directional series of microchannels, (e) optical microscope image of microchannels, (f) coating of the series of channels with a new layer of polymer, (g) X-ray exposure through the slit-shaped gold mask at 90 ° with respect to the slit mask of the first irradiation,  (h) development of the exposed structures to obtain a two layer array of microchannels (i) optical microscope image of the final structure. The scale bars in (e) and (i) are 200 µm.

Different irradiation doses were tested (performing the so called dose matrix) in order to evaluate the best conditions to obtain high quality, high resolution patterns. The structures were investigated by many techniques: SEM and AFM microscopy, FTIR, ellipsometry, WAXD. AFM evidenced that higher doses form sharper edges.Both exposed and unexposed regions remained transparent. However, the radiation effect that generates radicals induced a visible colour change due to the formation of charge transfer complexes (CTCs). The higher the irradiation dose, the more yellow the exposed regions became. FTIR measurements highlighted the chemical transformation induced by X-rays; the intermolecular linked network progressively induced by the X-ray irradiation resulted in a solubility change.  By conducting ellipsometry measurements, it was observed an increase of the refraction index and a decrease of the film thickness with increasing exposure dose. WAXD experiments on the exposed polymers allowed for the identification of the average intermolecular distances.
The variation of microporosity was examined through positron annihilation lifetime spectroscopy (PALS). The measurements showed that after the thermal rearrangement of the irradiated samples, there was a decreasing cavity size with increasing irradiation dose. After determining the gas permeability for various gases, it was found that the permeability of small molecules through TR-polymers increased with the X-ray dose. This was explained considering that even if the pores are smaller, they are better interconnected.
Combining such porous polymers with patterning techniques will enable the fabrication of a new generation of devices, like microsensors or microvascular circuits with superior molecular size-selective feature.