Electronic copper foil is an important material for circuit boards and plays an essential role in interconnecting circuits and electronic components. Ultrasmooth electronic copper foils are a crucial for manufacturing high-end circuit boards. However, traditional polishing methods have some limitations in the high-efficiency polishing of large-area ultrathin metal foils. Laser shock peening and forming is a manufacturing technology that uses ultrahigh pressure and high-speed shock force generated by a short-pulse laser to achieve a high strain rate plastic deformation of materials. The laser shock peening can improve the material's comprehensive mechanical properties by inducing grain refinement and residual compressive stress on its surface. Through shock wave pressure, the laser shock imprinting causes plastic deformation to form a metal foil, allowing for fabricating micro and nanostructures on the metal foil surface, which can be used in electronic and plasma sensing. Because the laser shock technology can cause plastic deformation on the metal foil surface, it can enable the surface polishing of electronic copper foils. It has been reported that a shock wave generated by a high-energy pulsed laser strikes the metal foil with the bottom plate to duplicate the smooth surface of the bottom plate and obtain less surface roughness of the metal foil. However, because of the large pulse energy and spot size, large plastic deformation occurs, causing profile fluctuation of the spot transition zone. Therefore, studying the microscale laser shock flattening (MLSF) of electronic copper foils using a low-energy pulsed laser is important.
The MLSF experimental device comprised a pulsed laser, beam expander and collimator, galvanometer system, and target. The pulse width, wavelength, and repetition frequency of the laser were 10 ns, 355 nm, and 1 kHz, respectively. The collimated laser beam was scanned using a galvanometer and was focused on the surface of an electronic copper foil, with a focusing diameter of 20 μm. The morphology and roughness of the electronic copper foil were measured before and after the MLSF using an atomic force microscope (CSPM5500). The samples were scanned in the contact mode. The scanning frequency and area were 0.3 Hz and 30 μm×30 μm, respectively. After performing the roughness test, the data was processed using the CSPM imager 4.60 software. The roughness value of the electronic copper foil is the average value of three measurements. The samples with a diameter of 3 mm were cut and subsequently prepared using double jet polishing. A transmission electron microscope (TEM) JEM2100 with a working voltage of 200 kV was used to perform the surface transmission analysis of the copper foil. The Gatan 1.2.44 software was used to perform the inverse fast Fourier transform (IFFT) and filtering of high-resolution TEM images.
The surface roughness of the annealed copper foil is approximately 22.7 nm, and the height distribution of microprotrusions is concentrated in the range of 39.2-227.5 nm. When the laser pulse energy is 100 μJ, the surface roughness of the flattened copper foil is approximately 8.1 nm. After performing MLSF three times, the surface roughness of the copper foil is 5.0 nm. The surface roughness of the electronic copper foil gradually decreases with increasing MLSF. When the laser-induced stress wave passes through the Al/Cu interface, it amplifies and acts on the copper foil. Thus, the copper foil collides with the base plate (K9 glass) under the high pressure of the stress wave. Finally, a surface roughness closing to that of the K9 glass is obtained in the shock area of the copper foil surface (Fig. 7). There are no obvious fluctuations at the spot overlap zone on the surface of the electronic copper foil according to the surface morphology and cross-sectional profile of the foil after the MLSF (Figs. 4 and 5). According to the microstructure analysis of the electronic copper foil before and after the MLSF, laser shock only induces numerous dislocation entanglements, dislocation cells, and lattice distortions in the electronic copper foil. However, it does not lead to the formation of deformation twins and grain refinement. This phenomenon primarily occurs due to the use of small laser pulse energy and the propagation form of the two-dimensional ellipsoidal shock wave in MLSF. Thus, the peak pressure of the shock wave is small, which cannot cause large plastic deformation of the copper foil and is insufficient to induce deformation twins and dynamic recrystallization of the copper foil that has a medium stacking fault energy. The MLSF of the electronic copper foil is primarily caused by the plastic deformation of microprotrusions on its surface.
The MLSF of electronic copper foils is studied. The effects of laser pulse energy and shock times on the MLSF as well as the surface deformation mechanism of the MLSF are studied. The MLSF can effectively reduce the surface roughness of the electronic copper foil, reaching a minimum of 5.0 nm. The MLSF leads to the formation of numerous dislocation structures with different configurations, such as dislocation entanglements and small dislocation cells. The small plastic deformation inside the metal foil and the large plastic deformation near the bottom surface are the surface deformation mechanisms of the MLSF.
