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

www.nature.com/scientificreports OPEN received: 21 November 2016 accepted: 30 January 2017 Published: 02 March 2017 High-speed maskless nanolithography with visible light based on photothermal localization Jingsong Wei1, Kui Zhang1, Tao Wei1,2, Yang Wang1, Yiqun Wu1 & Mufei Xiao3 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. Recently, there has been growing interest in nanostructure-based optical elements that are able to manipulate visible light for various photonic purposes, such as nanosieve1, metalens2, metamaterial3,4, and metasurface5–8, etc. Among others, a common feature in these photonic tools is the presence of a large amount of nanostructures (ranging from 200 nm to sub-100 nm in size) distributed usually on a large circular disk (ranging from a few centimeters up to a meter in diameter). Therefore, fabrication of these photonic elements especially for practical industrial applications becomes challenged as conventional lithography techniques can hardly meet the requirements simultaneously. Nanolithography techniques based on ion/electron beams and soft x-ray9,10 are widely employed in industrial applications because a shorter wavelength avoids effectively the diffraction limit. However, the need for high-vacuum environment and low speed restricts the techniques to small area patterning. Alternatives, such as stamper-based nanoimprint and mask projection lithography11,12, are also unrealistic due to the difficulty in fabricating large stampers with nanoscale features. Some scanning probe lithographies13,14 are able to fabricate nanoscale arbitrary patterns in atmospheric environment, but the lithography remains of low speed and fits only for small area. In view of above, one concludes tentatively that there seems a lack of effective methods for high-speed fabrication of micro/nanostructure-based optical elements on a large disk (see also, for instance, a recent review on maskless lithography in Ref. [15]). In the present work, we resorted to the widely used laser scanning writing method, where a commercially used GaN diode violet laser at wavelength 405 nm as the light source and an optical lens for focusing the light. On one hand, a direct writing system using a GaN diode laser may solve most of the aforementioned difficulties for high-speed nanolithography as it operates in air, at room temperature, and has virtually no limit on working area and scanning speed; on the other hand, the GaN diode laser is stable and low cost, and the wavelength of 405 nm is close to the limit of visible light. Obviously, the only remaining hurdle is the diffraction limit imposed on the lithography resolution16–18, for example, for a writing system operating at 405 nm wavelength with a focusing lens optics of numerical aperture (NA) 0.90, the resolution is limited to about 1.22λ/NA = 550 nm. In the past years, one also proposed different methods for obtaining below-diffraction-limited pattern structures through using special resists19, such as two-photon lithography20,21, metal hydrazone complex22, and two-color irradiation scheme23; however, the patterning speed is low and can not meet the real application 1 Laboratory of High-Density Optical Storage, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, 201800, China. 2University of Chinese Academy of Sciences, Beijing, 100049, China. 3Centro de Nanociencias y Nanotecnología, Universidad Nacional Autónoma de México, km. 107 Carretera Tijuana-Ensenada, Ensenada, Baja California, CP 22860, México. Correspondence and requests for materials should be addressed to J.W. (email: ) Scientific Reports | 7:43892 | DOI: 10.1038/srep43892 1 www.nature.com/scientificreports/ Figure 1. The experiment. Schematic of the high-speed maskless nanolithography on an AgInSbTe coated sample, where heating spot is far smaller than the light spot due to the photothermal localization response. requirements. The present report shows that the diffraction limit can be overcome by selecting AgInSbTe as the writing material. The reason for the breakthrough stems from a strong photothermal localization response on the surface of the AgInSbTe thin film, so that the heating spot becomes far smaller than the light spot, where the photothermal localization response comes from three aspects, including nonlinear saturation absorption, phase change threshold effect, and the manipulation of thermal diffusion channel. In the rest of the report, we shall first explain the experiment and present experimental results, and then analyze the involved physics with numerical simulations. The experiment and experimental results The nanolithography experiment is illustrated schematically in Fig. 1. The nanolithography is based on a high-speed rotational direct writing system (SpinDWL405)24. A collimated beam from a GaN-based diode laser (λ  = 405 nm) is focused into an optical writing spot. The writing spot irradiates on the surface of a circular disk of 120 mm in diameter mounted on a rotating stage (the detail of writing system is presented in Supplementary Materials). The substrate on the disk is glass. An AgInSbTe thin film of 100 nm thick was deposited on the glass substrate through magnetron-controlled sputtering method at room temperature. The sputtering background pressure was approximately 5 ×  10−4 Pa, the sputtering pressure was about 0.5 Pa in the Ar environment, and the sputtering power was 70 W. AgInSbTe belongs to Te-based chalcogenide phase change material. In general, the crystal structure is changed and maintained in the AgInSbTe thin film when the laser beam spot irradiates. The lithography pattern is formed on the AgInSbTe thin film with laser marked and unmarked areas. Subsequently, the lithography pattern undergoes a wet-etching process in an ammonium sulfide solution, where the laser-marked area is to be etched off. The pattern and scanning speed can be conveniently controlled by external electronics. However, we have discovered that when AgInSbTe is used as (...truncated)