Results and Discussions Two kinds of micron-level double-layer composite structures fabricated by one-step photolithographic preparation technology are presented (Fig. 3 and Fig. 4). The experimental results show that the depth of the concave notch can be effectively controlled by adjusting the exposure energy. The basic rule is that as the exposure energy becomes larger, the depth of the concave notch will be greater. When the exposure energy is large enough, there is no residual photoresist at the central concave notch after development. At this time, the micron-level double-layer composite structure disappears, and two micron-size discrete structures are obtained on the substrate (Fig. 3). The influence of exposure energy on the depth of the concave notch is studied with identical development time (Fig. 4). For 8 μm thick AZ9260 photoresist film, the exposure energy not higher than 160 mJ/cm² is the key to preparing the micron-level double-layer composite structures (Fig. 4). Schematic diagram of yz plane of the simulation model is demonstrated (Fig. 5). The light field distribution behind the mask during lithography exposure is analyzed by the finite-difference time-domain method (Fig. 6). The simulation results show that the exposure efficiency of the photoresist under 4 μm narrow slit is lower than that under 40 μm wide slit, which is the fundamental reason why the micron-level double-layer composite structures can be prepared by one-step lithography technology.

In bionics research, micron-level double-layer composite structures can usually show better mechanical, optical, and chemical properties than single-layer structures. Designing and constructing these unique biomimetic microstructure surfaces for human use is a hot research topic in recent years. The traditional photolithography technology is very convenient and has the advantage of simple process when it is applied to prepare the micron-level single-layer structures. However, when the traditional photolithography technology is adopted to prepare the micron-level double-layer composite structures, it needs to use the overlay lithography process for many times, which will greatly increase the manufacturing difficulty and processing cost of the microstructure. To overcome the above difficulties, researchers have developed a variety of microstructure processing methods, such as dry/wet etching, nanoimprinting, 3D printing, self-assembly, laser processing, photolithography, replication molding, and electrospinning. A variety of single-layer microstructure surfaces can be prepared by using these technology combinations, and even multi-scale micron-level composite structure surfaces can be prepared. However, the combinations often lead to more cumbersome processing procedures and higher costs of micron-level composite structures. To solve these problems, a method to adjust the exposure efficiency of photoresist by changing the width of the light transmitting part on the mask is proposed. By this method, the micron-level double-layer composite structures can be obtained on the positive photoresist with only one exposure and one development, which greatly reduces the processing difficulty and manufacturing costs of such structures and provides a new strategy for fabricating multi-scale micron-level composite structures.

Two lithographic masks with different parameters are designed and purchased from the 55th Research Institute of China Electronics Technology Group Corporation. The pattern of the mask and the schematic diagram of the photolithography process are shown in Fig. 1 and Fig. 2, respectively. The specific experimental process is detailed as follows. The K₉ glass substrate is cleaned by an ultrasonic cleaner in acetone, ethanol, and deionized water for 5 min each and then dried with nitrogen flow. The AZ9260 photoresist film with a thickness of 8 μm is spin-coated on a 5.08-cm K₉ substrate at 2500 r/min for 40 s using Laurell WS-650Mz spin coater. After standing at room temperature for 10 min, the substrate is placed on a 65 ℃ hot plate for 5 min, a 95 °C hot plate for 10 min and a 110 °C hot plate for 5 min, and finally cooled to room temperature. The ultraviolet (UV) lithographic exposure process is performed on Midas MDA-400M. The exposure energy of different samples is set between 125 mJ/cm² and 240 mJ/cm². The photoresist development is carried out with AZ400K developer (the volume ratio of AZ400K developer to deionised water is 1∶3) after UV exposure. Leica Microsystems DM8000M optical microscope is used to characterize the obtained samples. In addition, the light field distribution during mask exposure is analyzed by the finite-difference time-domain method to find out the formation mechanism of the micron-level double-layer composite structures.

The application of the one-step photolithographic preparation technique proposed in this paper can effectively reduce the difficulty in fabricating micron-level double-layer composite structures. The experimental results show that the fabrication process of the micron-level double-layer composite structures using the proposed method is very simple compared with that using overlay lithography technology, and only one exposure and one development process are needed. The maximum exposure energy of 8 μm thick AZ9260 photoresist should not be higher than 160 mJ/cm² to obtain micro-level double-layer composite structures. The light field distribution behind the mask during lithography exposure is analyzed by the finite-difference time-domain method. The simulation results show that the exposure efficiency of the photoresist under 4 μm narrow slit is lower than that under 40 μm wide slit, which is also the fundamental reason why the micron-level double-layer composite structures can be prepared by one-step lithography technology. The developer renewal speed in the 4 μm narrow slit is less than that in the 40 μm wide slit, further promoting the formation of the micron-level double-layer composite structures. During the mask lithography, the photoresist under the wide light transmitting area on the mask will be developed to the substrate faster than that under the narrow light transmitting area. According to this rule, various masks can be designed, and it is expected to prepare a variety of micron-level double-layer composite structures or even micron-level multi-layer composite structures by this method. The masks shown in this paper have reference significance for preparing various micron-level multi-layer composite structures.