Objective The materials system of Tedlar composite-aluminum film, which has the advantages of high specific strength and modulus, light weight, good stability, and high reflectivity, can be used for frequency choice (Frequency Selecting Surfaces, FSS) preparation. FSS are widely used in the field of spatial filters, radar antenna masks, antenna array radar scattering cross sections, reflectors for transmitted and reflected electromagnetic waves in different spectra, and so on. With the development of satellite antennas to high-frequency bands, the graphics size of the FSS unit is required to be smaller and smaller, and the dimensional accuracy is required to be higher. At present, the market has an urgent processing precision demand of less than 10 μm, the traditional microprocessing technology being unable to meet the technical requirements. The technology of a femtosecond pulse laser has the characteristics of short pulse width and high peak power. It has little thermal effect on the etching process of composite-aluminum film, and the overall effect of etching the boundary of graphic unit is better than with a nanosecond laser. Therefore, there are important theoretical significance and application value to carrying out relevant studies on femtosecond laser etching of aluminum film on a Tedlar composite substrate.
Methods In this paper, the effects of laser power, laser spot diameter, and scanning rate on the properties of Tedlar composite substrate -2 μm aluminum film using femtosecond laser etching technology are studied by the method of combining theoretical simulations and experiments. The optimal technological parameters are obtained by theoretical optimization using ANSYS 14.0 software. The femtosecond laser etching composite technology under different process parameters is verified using Pharos-type equipment, and the surface morphology and sample size of the etched-processed samples are tested using noncontact three-dimensional surface graphic analysis equipment.
Results and Discussions The simulation results show that the femtosecond laser etching pulse laser beam has enough energy to make the crystal lattice temperature exceed the melting point of aluminum in a very short time so that the aluminum film material on the composite substrate surface expands and is etched away rapidly (Fig.2). The interface temperature increased as the power of the femtosecond laser etching increased. When the laser power increased from 3.0 W to 5.5 W, the interface temperature between the Tedlar composite and aluminum film increased from 417.68 K to 513.19 K, causing damage to the Tedlar substrate and affecting the performance of the intrinsic material (Figs.3 and 4). When the laser scanning speed increased from 350 mm/s to 600 mm/s, the discontinuous point size increased from 1.2 μm to 2.7 μm in the femtosecond laser etching process, resulting in the size error of the laser etching process becoming greater than 10 μm (Fig.5). The experimental result shows that the larger the laser spot diameter and the smaller the laser spot overlap ratio are, the higher the scanning etching error is, while a smaller laser spot size and higher laser overlap ratio are conducive to the improvement of laser etching accuracy. When the spot diameter is 40 μm, the overlap ratio is 25%, and the etching error is 6.77 μm so that the technical accuracy requirements of less than 10 μm are fully met (Fig.6). When the laser power is less than 4.0 W, there is a large amount of residual aluminum. When the laser power is greater than 4.0 W, the substrate is damaged around the etched area. When the laser power is 4.0 W, the aluminum film on the substrate is completely removed and Tedlar composite substrate is not damaged (Fig.7). When the laser scanning rate is 500 mm/s, the spot overlap rate is 37.5% and metal-aluminum film is completely etched and removed. However, when the laser scanning rate is 550 mm/s, the spot overlap rate is 25%. At this time, the metal-aluminum film is etched clean and only a very small amount of metal aluminum remained. When the scanning rate continued to increase to 600 mm/s and 650 mm/s, the spot overlap rate is 0, but there is a large amount of etched metal-aluminum residue. With the increase of scanning rate, the residual aluminum content increased (Fig.8). The simulation results are consistent with the experimental results.
Conclusions Under the conditions of laser output power is 4.0 W, spot diameter is 40 μm, and scanning rate is 500 mm/s, the surface of the material is clean, the residual aluminum content is very small, the etching diagram is arranged in a neat array, and the size accuracy and relative position accuracy of aluminum film graphics after laser etching are better than 10 μm. Femtosecond laser etching technology can meet the high precision requirement of aluminum film micromachining on a Tedlar composite surface.
