By exploiting the rapid change in the fluorescence signal intensity at the interface between the photoresist and substrate as well as adopting the appropriate image-processing methods, such as the two-dimensional discrete Fourier transform, ideal band-stop filter, ideal band-stop filter, inverse fast Fourier transform, and Canny edge-detection algorithm, the angle between the objective focal plane and substrate plane is efficiently identified. Subsequently, based on the spatial-position rotation-transformation relationship, the compensation values for each processing point and the relative motion speed of the voxel in three-dimensional space are obtained. A nano-piezo translation stage is utilized to execute the corresponding position and speed to achieve proactive compensation for defocusing.

Two-photon polymerization laser direct writing (TPP-LDW) is a key technique in micro/nanofabrication. It offers many advantages, such as high precision, maskless fabrication, three-dimensional lithography, and diverse customization. Since the lateral and axial linewidths of voxels in TPP-LDW are on the order of hundreds of nanometers and 1?2 μm respectively, a slight inclination between the focal plane of the objective lens and the substrate might cause defocusing, thus resulting in the collapse or deformation of fine structures. Therefore, defocusing correction and compensation is extremely important, particularly in large-scale micro/nanostructure processing. However, most compensation methods used for commercial products and research require additional reference beams and precise compensation for defocusing is based on the accurate calibration of the auxiliary detection optical path with respect to the fabrication beam, which significantly increases the cost, complexity, and difficulty of the TPP-LDW system. In this study, we propose a proactive focus-compensation method based on fluorescence-imaging analysis. This methodology does not require additional reference beams or complex calibration; additionally, it is easy to operate and implement, with high compensation accuracy.

A femtosecond laser with a wavelength of 800 nm is used as the excitation source for the TPP-LDW setup in this study. The photoresist resin adopted in the current experiment contains 7-diethylamino-3-thenoylcoumarin (DETC) with a mass fraction of 0.5% as the initiator, and the monomer is pentaerythritol triacrylate (PETA). The two-photon-excited fluorescence of DETC is obtained using a high numerical aperture objective and imaged using a complementary metal-oxide-semiconductor transistor (CMOS) camera. A six-axis piezoelectric nanotranslation platform is used to support and actuate the samples.

Based on the proposed proactive defocusing compensation method, we successfully fabricated large-area micro/nanowire array structures. In fabricating a nanowire array with an expected height of 300 nm, the average height of the nanowire is measured to be 276.4 nm, the standard deviation is approximately 23 nm, and the inclination correction accuracy is 4.62×10^-4 rad (Fig. 5). Based on precise compensation, the three-dimensional morphology of the nanowires can be controlled by adjusting the focus position in the z-axis direction; subsequently, a micro/nano functional structure with a gradient morphology can be fabricated. The minimum feature sizes of the obtained double-needle structures in the lateral and axial directions are 12 nm and 79 nm, respectively (Fig. 6). In addition, using a self-written automatic calculation and control program, a periodic topological structure with gradient lateral and axial linewidths can be implemented rapidly (Fig. 7).

In this study, we propose a method to achieve stable and automatic compensation for TPP-LDW via fluorescence-image analysis and spatial-coordinate rotation transformation. Without introducing a reference beam, a leveling sensor, a four-quadrant detector, or an optical zoom system, the defocus of the large-scale fabrication is reduced from the micron level to tens of nanometers. In processing large-area micro/nanowire arrays with an expected height of 300 nm, the inclination correction accuracy reaches 4.62×10^-4 rad and the standard deviation of the compensation accuracy is 23 nm. In addition, this methodology is suitable for fabricating large-scale gradient micro/nanostructures. By adjusting the focus position in the z-axis direction, we successfully fabricate a gradient double-needle structure with minimum feature sizes of 12 nm and 79 nm in the axial and lateral directions, respectively. Furthermore, a large-scale periodic topological structure with scale-gradient characteristics can be implemented easily and automatically using a self-written automatic calculation and control program. In summary, the proactive defocusing compensation method, which is simple, easy to use, and does not require additional reference light or complex calibration, offers high precision for defocusing compensation.