Study On Mechanism And Control Method Of Periodic Structure On Polyimide Film Surface Induced By Nanosecond Laser

Laser induced periodic surface structures(LIPSS)is a kind of periodic ripple on the surface induced by laser radiation.It can be produced under a certain laser fluence and pulse number,and appears on almost all materials such as metals,semiconductors,dielectrics and so on.It is a ubiquitous surface structure.It can improve many surface properties,such as friction properties,wetting properties,biocompatibility and optical properties,so it is widely used in microelectronics,biomedicine,aerospace and other fields.This paper focuses on the regulation method and formation mechanism of periodic ripple structure on the surface of polyimide films induced by UV nanosecond laser.LIPSS are created by a 355 nm linearly polarized laser with the pulse duration of 7 ns and the pulse repetition rate of 1 Hz on polyimide films.The influences of different laser parameters on the morphology of the formed LIPSS are investigated.It is found that there is a certain fluence threshold and pulse number threshold for the generation of periodic structure.When the laser fluence is in the range of 54 m J/cm~2-586 m J/cm~2,and the number of pulses is between1-50,the well-ordered LIPSS are formed with the periods between 4 μm-6.5 μm.The number of the pulse and the fluence of the laser are changed in the experiment.With the same laser fluence,increasing the pulse number,or with the same pulse number,increasing the laser fluence,the period of the ripples can be increased.It is also observed that the formation of LIPSS is accompanied by the removal of materials,with the increase of the number of pulses,the depth of the crater increases,and LIPSS can continue to appear at the bottom of the crater.It provides a certain research accumulation for the effectively regulating the surface periodic structure of polyimide films.A new method of controlling the direction of LIPSS by the shape of laser spot is proposed.The direction of LIPSS can be changed by introducing a slit into the optical path to shape the laser spot slightly.The effect of spot shape on the surface periodic structure regularity of polyimide film was found through experiments.When the laser spot is shaped under the same energy density,the regularity is improved by 12.00%-52.17%,and the greater the energy density,the greater the influence of the spot shape on the regularity of LIPSS,which provides experimental accumulation for the large-scale production and processing of the periodic structure on the surface of laser-induced polyimide film.A mechanism model for the formation of micron-scale periodic ripple on the surface of polyimide films induced by 355 nm UV nanosecond laser is proposed.The temperature field distribution of the material after laser irradiation is simulated by the thermal conductivity model.The mechanical state of polymer changes with the change of temperature.Using the soft material growth model based on elasticity for reference,it is calculated that the period of LIPSS is 0.96 μm-9 μm,which is consistent with the experimental results.According to the characteristics of periodic structure on the surface of polyimide film induced by laser in this work,the LIPSS on the surface of polyimide film is equivalent to the grating structure.For terahertz frequency band,the sub wavelength structure can be seen on the LIPSS on the surface of polyimide film.Based on the equivalent dielectric theory,the refractive index of the grating structure is calculated for terahertz frequency band,The reflectivity of the periodic structure on the surface of polyimide film in 0.5 THz-3 THz band is obtained.Compared with the surface without laser treatment,the reflectivity in 0.5 THz-3 THz band is reduced by 7.64%-15.70%.It has practical significance for the application of terahertz wave in transmission imaging and other fields.