Chalcogenide thin films are versatile phase-change materials that are currently used as optical information storage and solid-state memory media; these applications respectively make use of the different optical and electrical characteristics of the amorphous and crystalline phases of chalcogenide materials. Now, by exploiting four phases of chalcogenide thin films — the liquid and gaseous phases, in addition to the amorphous and crystalline solid phases — Rui Wang, Jinsong Wei and Yongtao Fan in China have demonstrated the use of these materials as greyscale photolithography materials (Opt. Express 22, 4973–4984; 2014).
Greyscale photolithography has been used to produce three-dimensional microstructures, microfluidic devices and microlens arrays, but the two main methods that are currently employed (one which involves film deposition, lithography, etching and resist removal of chrome on glass and the other electron-beam writing of high-energy-beam-sensitive glass) are complex and costly.
In contrast, Wang et al.'s technique is simple and cost effective, as it merely involves inscribing greyscale patterns on chalcogenide thin films by direct laser writing. This processing forms bumps in chalcogenide thin films on glass substrates through the heat generated by pulsed laser irradiation. By controlling the pulse energy of the laser irradiation, bumps with different heights and volumes can be formed. As these bumps exhibit different optical reflection and transmission characteristics, high-resolution continuous-tone greyscale patterns can be readily produced on chalcogenide thin films.
Laser irradiation produces four kinds of structures depending on the laser pulse energy used. Low pulse energies result in the formation of small concave dimples in the film due to the higher density of the crystalline phase relative to the amorphous phase. Irradiation at high pulse energies produces protruding solid bumps through melting of small volumes of the film. Still higher pulse energies generate bumps containing cavities as the result of vaporization. Finally, these bumps rupture when even higher pulse energies are used.
The researchers demonstrated their technique using Sb2Te3 films on glass and a homebuilt direct laser writing system. They found that the reflectivity of processed films in the wavelength range 450–600 nm increased with increasing laser power density as the result of the formation of larger microstructures in the film. They then used the technique to produce high-resolution greyscale images. The team anticipates that the technique could be used to produce high-resolution images for micro- and nanoimage storage, microartwork and greyscale photomasks.