In a joint study, scientists fr om the MIPT (Moscow Institute of Physics and Technology), ICP (Institute of Chemical Physics) named after Semenov, MSU (Moscow State University) and IPCP (Institute of Problems of Chemical Physics) have developed a mechanism of laser deposition of patterns on glass with a resolution of 1000 times lower than the width of a human hair. Focusing the laser was conducted with the help of small glass spheres, playing the role of the lens. This mechanism allows inexpensively and relatively easy to apply complex patterns to a glass surface, whereby obtaining a spatial resolution of less than 100 nanometers.
The proposed method will allow sufficiently quick and cheap creation of sensors and microchips of a nanoscale. According to the scientists, it is much cheaper and technologically easier than any of previously used methods, in addition to the fact that the new application allows deposition of the computer drawings on the glass surface with an acceptable resolution. To demonstrate this method, the abbreviation of the Institute of Chemical Physics (ICP) was deposited on glass with a high resolution (about 100 nanometers). In particular, nanoengraving is used to create ultra-precise circuits in microfluidics. The operating fluid can flow through the engraved channels connecting various parts of the circuit; and the smaller the size of such a circuit, the higher the engraving resolution.
A femtosecond laser used by scientists allows deposition of complex two- and three-dimensional patterns on the surface of transparent materials. Resolution - the minimum size of the pattern’s image detail - is always a problem in these kinds of tasks, since it is limited (for physical reasons) by the laser wavelength. The higher the resolution, the smaller the size of patterns applied and the more interesting and sought-after these patterns in technology.
To improve the resolution, the near-field effect is often used. This method involves focusing a laser beam, using metal nanoparticles or a layer of dielectric microspheres as "lens". However, these methods complicate the pattern’s deposition process, as the "lens" are fixed in space.
In their work, the authors propose a different approach: using a beam of light in fluid, they create a kind of a trap wh ere they place glass microspheres. The advantage of this focusing method consists in that, that the "trap" can be moved, thereby moving the "lens" in space and focusing the laser to the desired area of the glass.
Fig. 1. The experimental setup. // ACS Publications/Applied Materials & Interfaces
However, simply moving a laser beam along the surface is not enough. Exposure to laser leads to the formation of hillocks but not craters. These hillocks are quite rough and wide, but the effect of alkali at a temperature of 90° C turns the hillocks into smooth craters with a smaller width. Whereas such a two-step structuring allows achievement of a resolution below 100 nanometers (nm), one-step structuring, when the surface is treated only with a laser, does not allow obtaining precision below 150-200 nm (depending on the complexity of the structure).
Fig. 2. The laser-treated surface of the glass before and after the application of alkali. (from the article)
As a result, the whole process of nanoengraving appears as follows. Initially, the glass surface is irradiated with a femtosecond laser. The laser pulse is focused by means of a glass bead, which is guided by an optical "trap" to a predetermined area of the glass. As a result, wide hillocks are formed on the surface of glass; but then, after surface treatment with an alkaline solution, these hillocks are converted to smaller craters of more streamlined shapes.
In addition to direct patterning process, the scientists investigated dependence of the resolution, i.e. the crater size, from the laser power. The results showed that for obtaining greater precision, it will be more effective to use small spheres, which allow obtaining a resolution below 100 nm.
Fig. 3. The dependence of the width of the craters from the laser radiation power. Red - for microspheres of 2.25 microns in size, blue - 1.15 microns, black - without microspheres. // ACS Publications/Applied Materials & Interfaces
The minimum width of the crater obtained was 70 nm. The figure below shows this particular crater, and the chart shows the shape of the crater on two axes.
Fig. 4. The Image of the smallest crater (70 nm). // ACS Publications/Applied Materials & Interfaces
For practical use of this method - in addition to high resolution, you must also take into account the complexity of applying intricate structures. The above publication shows that if you use the engraving technique described, you will be able to apply relatively complex structures. To prove this, the glass surface was engraved with the abbreviation for the Institute of Chemical Physics (ICP). The average width of each letter is 100 nm, depth - 20 nm (see Fig. below with a scale - 500 nm).
Fig. 5. ICP abbreviation applied to the glass. // ACS Publications/Applied Materials & Interfaces
“Creating thin grooves and channels can be used in chemistry and biology fields - in the production of ‘microfluidics’ and at various ‘nano-plants’," - says Aleksander Shakhov, the article’s co-author, post-graduate of the Faculty of General and Applied Physics at MIPT.
"Channels" for liquids engraved by methods described above are used for the development of small precise sensors working with liquids. The article in question also suggests a sufficiently fast and cheap mechanism for nanostructuring. Such an approach, in particular, can allow rapid and technologically uncomplicated creation of inexpensive devices and sensors by applying complex structures of thin grooves and channels through which operating fluid will flow.
// A. Shakhov, A. Astafiev, A. Gulin and V. Nadtochenko, Femtosecond Nanostructuring of Glass with OpticallyTrapped Microspheres and Chemical Etching, ACS Appl. Mater. Interfaces, 2015, 7 (49), pp 27467–27472