Laser ablation of monocrystalline silicon under pulsed-frequency fiber laser
Subject of research. The paper deals with research of the surface ablation for single-crystal silicon wafers and properties of materials obtained in response to silicon ablation while scanning beam radiation of pulse fiber ytterbium laser with a wavelenght λ = 1062 nm in view of variation of radiation power and scanning modes. Method. Wafers of commercial p-type conductivity silicon doped with boron (111), n-type conductivity silicon doped with phosphorus (100) have been under research with a layer of intrinsical silicon oxide having the thickness equal to several 10s of nanometers and SiO2 layer thickness from 120 to 300 nm grown by thermal oxidation method. The learning system comprises pulse fiber ytterbium laser with a wavelenght λ = 1062 nm. The laser rated-power output is equal to 20 W, pulse length is 100 ns. Pulses frequency is in the range from 20 kHz to 100 kHz. Rated energy in the pulse is equal to 1.0 mJ. Scanning has been carried out by means of two axial scanning device driven by VM2500+ and controlled by personal computer with «SinMarkТМ» software package. Scanning velocity is in the range from 10 mm/s to 4000 mm/s, the covering varies from 100 lines per mm to 3000 lines per mm. Control of samples has been carried out by means of Axio Imager A1m optical microscope Carl Zeiss production with a high definition digital video camera. All experiments have been carried out in the mode of focused laser beam with a radiation spot diameter at the substrate equal to 50 µm. The change of temperature and its distribution along the surface have been evaluated by FLIR IR imager of SC7000 series. Main results. It is shown that ablation occurs without silicon melting and with plasma torch origination. The particles of ejected silicon take part in formation of silicon ions plasma and atmosphere gases supporting the plasmo-chemical growth of SiO2. The range of beam scanning modes is determined where the growth of SiO2 layer is observed. Beginning with scanning velocity equal to 2000 mm/s, the torch existence is not fixed visually; silicon oxide origination is stopped, and silicon particles with nanometer sizes are formed as a result of silicon destruction. The process of silicon destruction is followed by sounds of different frequency depending on scanning velocity. Practical significance. The surface ablation for single-crystal silicon has been shown for the first time ever to be influenced significally by such features of laser radiation as laser beam scanning velocity and the covering of scanning lines. Laser beam scanning modes of pulse fiber ytterbium laser with a wavelenght λ = 1062 nm have been found out providing the synthesis of nanostructured silicon dioxide particles or silicon nanoparticles.