Oxygen assists vacuum-UV micropatterning of organic surfaces

Although photolithography is a powerful means of fabricating microstructures on material surfaces, it depends on photochemical reactions of particular organic thin films: photoresists. These require complicated processing with many steps: these may include resist coating, curing, exposure, development, pattern transfer, resist removal, etc. However, light in the so-called vacuum-ultraviolet (VUV) range (1–200nm) has a high photon energy. As a result, it induces photochemical reactions on a wide variety of material surfaces that have no sensitivity to the conventional UV used in photolithography. VUV can therefore be used to directly micropattern surfaces without the use of photoresists.^1–3 However, light in this wavelength band is heavily attenuated in air. Accordingly, VUV-based patterning generally has to be conducted in a vacuum (as its name suggests). In order to overcome this shortcoming, we have developed a VUV-lithographic system that is operable in air.

Figure 1 shows a schematic illustration of our VUV-exposure system, which employs a Xe-excimer lamp emitting light at a wavelength of 172nm. The light propagates only a very short distance through air due to attenuation by oxygen molecules: these strongly absorb VUV. At 172nm, for instance, an air layer 10mm thick has a transparency of just 10%. In our new design, the sample is placed in air, while the space between the photomask and lamp is purged with nitrogen to avoid the absorption of VUV.^4,5 The photomask serves as a wall between the nitrogen and oxygen atmospheres. Furthermore, the proximity gap between the photomask and the sample is controlled precisely to an accuracy of 0.1μ m using a mechanical stage.

The performance of the VUV-exposure system was characterized by measuring water contact angles of VUV-irradiated samples: that is, Si substrates covered with a native oxide and alkylsilane monolayer—see Figure 2(A)—in that order. The results are shown in Figure 2(B). The monolayer surface becomes hydrophilic due to VUV irradiation because polar functional groups are formed through the oxidation of the alkyl chains.⁶ Finally, at a certain VUV-dose, the surface wets completely because all the organic molecules decompose and are removed, revealing the underlying SiO₂ surface: here the water contact angle is almost 0°. A VUV-dose of about 8 J/cm² is required in order to decompose the monolayer in vacuum at a pressure of 10Pa. However, using our system, the required dose for micropatterning is just 3 J/cm² or less (as indicated by the closed circles in the figure). The dose is further reduced to almost 1 J/cm² (as indicated by the open circles) with a gap of 1μ m between the photomask and the sample.

An example of a printed pattern on the monolayer is shown in Figure 3. Fine lines of 5–20μ m wide were successfully transferred from a photomask. These patterns were observed by a field-emission scanning electron microscope (FE-SEM) based on the difference in secondary-electron emission efficiencies between the organic molecules and SiO₂.

Organic materials consisting of saturated hydrocarbon molecules are barely etched by conventional VUV-irradiation at 172nm. Etching of the alkyl monolayer proceeds only under the presence of atmospheric oxygen. We considered this VUV-etching mechanism as follows. The VUV light excites the oxygen molecules, resulting in the generation of oxygen atoms in the singlet and triplet states: O(1D) and O(3P), respectively. Since these oxygen atoms, particularly O(1D), have a strong oxidative reactivity, the surface terminating -CH₃ groups on the monolayer are oxidized to -COOH, -CHO, and so forth. The monolayer surface, thus, becomes more hydrophilic. Such oxygen-containing functional groups are decomposed by VUV-excitation. The oxidation, and subsequent decomposition, reactions of the alkyl chains proceed simultaneously: this results in the etching of the monolayer.

To conclude, we have used VUV light at 172nm to micropattern organic materials. In this process, oxygen molecules present above the sample surface play the crucial role. Despite the simplicity of this system, line patterns of 1μ m-wide or less were successfully printed on organic monolayer surfaces.⁷ The system will be useful as a research tool for both microbiology and microchemistry.