High-speed maskless nanolithography with visible light based on photothermal localization

High-speed maskless nanolithography is experimentally achieved on AgInSbTe thin films. The lithography was carried out in air at room temperature, with a GaN diode laser (λ = 405 nm), and on a large sample disk of diameter 120 mm. The normal width of the written features measures 46 ± 5 nm, about 1/12 of the diffraction allowed smallest light spot, and the lithography speed reaches 6 ~ 8 m/s, tens of times faster than traditional laser writing methods. The writing resolution is instantaneously tunable by adjusting the laser power. The reason behind the significant breakthrough in terms of writing resolution and speed is found as the concentration of light induced heat. Therefore, the heat spot is far smaller than the light spot, so does the size of the written features. Such a sharp focus of heat occurs only on the selected writing material, and the phenomenon is referred as the photothermal localization response. The physics behind the effect is explained and supported with numerical simulations.

is the beam waist radius, which can be calculated as ~0.61 / . The light passes through the 1 layer and decays into the 2 layer due to the absorption effect. If is large enough, the exiting intensity for every layer can be calculated using the Beer-Lambert formula.
The 1 layer can be considered as a sample surface, for which the absorption coefficient ( , ) and exiting light intensity ( , ) can be obtained as, where the and are linear and nonlinear absorption coefficients of writing material, respectively.
At the layer, At the last layer, that is, the layer, The absorbed energy profile inside the writing material can be calculated by the formulas (1)-(4).
The absorbed laser energy (photons) excites the electrons and holes in the writing material.
The photoexcited electrons and holes usually recombine non-radiatively, and the absorbed photon energy is transferred into temperature rise above ambient due to strong electron-phonon coupling. According to formulas (1)-(4), the absorbed laser power unit volume can be rewritten as ∆ ( , , ) = ( , ) ( , , ).
Assuming that the heat loss from the sample surface is proportional to temperature rise of ( , = 0, ), the temperature rise profile ( , , ) is determined by the non-steady state heat-conduction eqauation 1 : The laser spot is easy to defocus in the high-speed writing process due to the influence of sample surface flatness, external vibration, the fluctuation of movement stage, etc. To overcome these obstacles, the autotracking module based on astigmatic method is installed in rotation-type direct laser writing system 2 . The autotracking module can find the focus displacement of the sample by detecting the focus error signal and make the sample always be located in the focal plane. Figure s1 is the real picture of established system in this work.

Some other results on high-speed maskless lithpgraphy
In the process of photothermal localization lithography, the line patterns with different linewidth were directly obtained through rapidly changing the laser power. Figure s2 gives the experimental results. In order to enhance the pattern contrast between the written and original areas, the written patterns were further wet-etched by ammonium sulfide solution. Figure s2(a) presents the two-dimensional AFM imaging and cross-section analysis, and the inset is the height of the line patterns. The three-dimensional AFM image is presented in Figure s2(b).
One can see that the line patterns are very clear, and the different linewidth patterns are written on the AgInSbTe thin films, and the FWHM depth changes from 250 nm to 50 nm through quickly changing the laser intensity, accordingly.  Generally speaking, it is difficult to fabricate complex (arbitrary) patterns for high-speed rotation polar coordinate laser writing system. However, in our system, one can transfer the patterns into bitmap format. Then the laser writing system automatically transfers bitmap to polar coordinates. Therefore the complex (arbitrary) patterns can be written in the format of bitmap. Figure s4 presents all kinds of complex patterns obtained through using the high-speed rotation-type laser writing system, where the complex patterns were etched by ammonium sulfide solution. Figure s4a is the lion image, one can see that the appearance and expression of the lion have been fully written on the AgInSbTe thin films. Figure s4b is the gear pattern, and Fig. s4c-s4f are all kinds of snowflake patterns with fine structures.