Nanoimprint lithography on silica sol-gels: a simple route to sequential patterning.

Figure 1. Schematic illustration of the NIL imprinting process of ICSG Since the pioneering work of Chou et al., nanoimprint lithography (NIL) has emerged as a promising technique for surface patterning, applicable to numerous applications ranging from nanophotonics to microfluidics. NIL basically consists in the stamping of deformable surfaces or films. Preferred materials are thermoplastics and UV-curable resists. So far, most papers report on single-imprinting methods, for which the same surface is imprinted only once. However, many applications, such as biomimetic materials and photonics, would benefit from a more versatile imprinting technique, where successive imprinting on the same area with different stamps generates complex structures through the combination of simpler templates. Such sequential patterning strategies are an emerging topic in the field of NIL. An interesting method was proposed by Lee and co-workers and Low and co-worker: they demonstrated that for poly(methyl metacrylate) (PMMA) the strain softening occurring during the first imprint step allows secondary imprints to be performed below the glass transition temperature of the polymer without relaxation of the first imprints. The method, which is based on plastic deformations, requires that high pressure be applied during the imprinting process. Moreover, when using a polymer layer, the resulting structure has limited thermal and mechanical stability. Therefore, for improved stability, the imprinted layer is often used only as an etching mask to transfer the structures into a stable substrate. A major breakthrough for easier and faster processing, especially on brittle ceramic and glass substrates, would be a low-pressure patterning technique on a directly functional thin film or resist, omitting the final pattern-transfer step. In that respect, inorganically cross-linked sol–gel (ICSG) resists appear as very attractive materials, due to their low initial viscosity and outstanding thermal, chemical, and mechanical stability. Very few previous studies have addressed the imprinting of suchmaterials. In the present paper, we report the imprinting of square silica structures from simple line gratings, and demonstrate how the specific thermo-rheological behavior of ICSG resists can be harnessed to form complex structures by sequential imprinting at low pressures. Even though our goal is pure-silica structures, the strategy for thermal imprinting of ICSG resists is to start from hybrid-silica precursors for a better control over condensation. A patterned and flexible stamp is pressed at ambient temperature onto the ICSG resist (Fig. 1A); the system is heated at a temperature T1 for a period of time t1 (Fig. 1B), revealing the imprinted structures after stamp removal (Fig. 1C). The structures formed during this single-step imprinting are organic/inorganic hybrids. To turn them into pure silica, the organic moieties are oxidized by annealing at high temperatures (700 8C). This annealing is possible only if a condensation threshold has been exceeded during the imprinting step; otherwise, the structures relax during annealing, because the material temporarily turns into a fluid state as the temperature increases. By controlling the thermo-rheological properties of the ICSG resist, we have been able to directly imprint features with high aspect ratios (>4) and limited shrinkage even after annealing. The strategy for the sequential thermal imprinting of ICSG resists is as follows. Step 1 is identical to single-step imprinting (Fig. 1A–C), and results in a set of primary structures. During Step 2, another patterned stamp is pressed onto this pre-patterned region (Fig. 1D) – here we use the same stamp after a 90 8 in-plane rotation. The sample is then heated again at a temperature T2 during a time t2 (Fig. 1E), resulting in the formation of secondary structures (Fig. 1F). Annealing is carried out only after Step 2. Effective double imprinting is achieved if the secondary structures can be formed without relaxation of the primary structures. Excessive condensation during Step 1 will clearly prevent the formation of the secondary structures, while poor condensation will result in the relaxation of the primary structures during Step 2. Therefore, successful double imprint-