With the increasing functional demands of micro- and nano-devices, there is a growing need for complex micro/nano-architectures featuring high spatial resolution and tunable depth profiles. Conventional binary lithography techniques can only produce two-dimensional structures with uniform depth in photoresists, making it difficult to fabricate more sophisticated three-dimensional architectures. In contrast, grayscale lithography enables precise control of the exposure dose of light, electrons, or ions, thereby allowing the formation of three-dimensional grayscale structures with continuously varying heights in photoresists. Such techniques have been widely applied in various fields, including blazed gratings, microfluidic channels, microelectromechanical systems (MEMS), and biomimetic structures.
The vertical resolution of grayscale lithography is typically defined by the minimum height difference between adjacent steps achievable during the fabrication of multilevel staircase structures, which reflects the precision of depth control in the process. However, the vertical resolution of existing grayscale lithography techniques generally ranges from several hundred micrometers down to tens of nanometers. Even the most advanced grayscale electron-beam lithography reported to date has achieved a maximum vertical resolution of only 6 nm. At present, no grayscale lithography process with sub-nanometer vertical resolution has been demonstrated, which significantly limits the in-depth investigation of the unique properties of three-dimensional structures at the sub-nanometer scale.
In this work, researchers develop a grayscale lithography process termed probe-guided laser direct writing (PG-LDW) for grayscale patterning of MoS2 thin films with nanometer-scale thickness. Prior to the formal writing step, a probe similar to that used in atomic force microscopy (AFM) is employed to assist in surface localization, ensuring that the laser operates under optimal focusing conditions and enabling precise energy delivery to the sample. This unique probe-assisted focusing strategy significantly enhances the accuracy of depth control in the PG-LDW process.
AFM characterization of simple three-level staircase structures written at different laser powers confirms that the achievable vertical resolution of this technique is approximately 2 Å (0.2 nm). Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and other characterization methods are further employed to analyze the writing effects, and the underlying mechanisms of laser direct writing on MoS2 thin films under ambient conditions are discussed. To demonstrate the customizable patterning capability of PG-LDW, complex grayscale structures such as the emblem of Beijing Institute of Technology and a portrait of the Mona Lisa are fabricated. Moreover, the probe used for laser focusing can also be utilized to relocate and re-register previously written grayscale structures, endowing the PG-LDW process with a unique capability for multiple modification and refinement steps.
The emergence of PG-LDW establishes a foundation for constructing grayscale structures with unprecedented sub-nanometer precision and opens new opportunities for the development of next-generation micro/nano-integrated devices, effectively breaking through the resolution limits of conventional grayscale lithography.
