Investigation On The Ultrashort Pulsed Laser Micro-welding Technique Based On Laser-matter Interaction

The focused ultrashort pulsed laser,with intrinsic feature of ultra-high peak intensity,can selectively excite the nonlinear absorption of materials in threedimensional space,forming a high-density plasma,thereby inducing a local high temperature and pressure,triggering the processes of surface ablation,melting,vaporization,condensation,and so on.When the ultrashort pulsed laser is focused onto the interface of the sample with optical contact,local fusion and mixture of the material can be formed after the laser-matter interaction processes,achieving selective welding between the materials.Therefore,the essence of ultrashort laser micro-welding is that ultrashort laser induces local modification on materals and thereby forming a strong bond within the sample interface.Comparing with the traditional joining methods,including sweating soldering,anodic bonding and adhesive bonding,ultrashort pulsed laser micro-welding has many advantages,such as high applicability,high precision,high spatial selectivity,no inserting interlayer,and “cold welding”,can strongly influence the processing of embedded integrated device in the near future.In this dissertation,we firstly investigate the laser induced modification on material interface.Then we focus on the application of ultrashort pulsed laser in the high-precision selective welding of glass-metal,glass-semiconductor and glass-glass,aiming to explore the mechanism of micro-welding between different kinds of materials,optimize the welding process,and promote the industrial application of ultrashort pulsed laser micro-welding technology.The main work and achievements of this dissertation are as follows:1)To our best knowledge,it is the first time that the transparent glass and metal,without optical contact,were directly welded together by using the femtosecond pulsed laser.The influence of sample gap on the welding quality of glass-metal was explored.This is of great significance for simplifying the welding process.The role of laserinduced interface ablation in bonding process was studied.The welding mechanism of aluminosilicate glass-copper with optical and non-optical contact was illuminated.By optimizing the pulse energy,deposition density of pulse number and interval between adjacent welding lines,a strong bonding strength up to 12 MPa was formed on the sample interface under the non-optical contacting condition.The width of welding line was about 40 ?m,the trajectory shape of welding line was controllable.Further more,the welding technique was successfully applied on the welding of aluminosilicate glassaluminum and aluminosilicate glass-steel,which to some extent demonstrated the high applicability of such ultrashort pulsed laser micro-welding technique.2)The micro-welding between fused silica and silicon carbide with an ultra-high melting point was achieved by using the femtosecond pulsed laser.Through the morphology and chemical composition characterization,combining the theoretical analysis,the welding mechanism of silicon carbide and fused silica was studied.Based on the study of absorption characteristics of materials and the simulation results of the laser-induced thermal relaxation process,the influence of laser focus position along the propagation axis on welding quality was discussed.By optimizing the pulse energy and interval value,a shear joining strength as high as 15 MPa was achieved at interval value of 15 ?m.3)A time-resolved phase-contrast microscopy imaging system was constructed.The transient responsing process of the refractive index of fused silica after irradiated by ultrashort pulsed laser was recorded and investigated.By deriving the relationship between refractive index of fused silica and temperature,the time-resolved thermal relaxation process of the excited fused silica was further obtained.The thermal relaxation process was proved to be finished in the time scale of 10 ?s.In addition,the laser-induced heat-affected zone within the fused silica was demonstrated to be confined in a spatial scale of 3 ?m,indicating great advantages of ultrashort pulsed laser in the high-precision selective welding of transparent materials.4)An ultrashort pulsed laser micro-welding system with on-line monitoring capability was constructed.The characteristics of the structural responses of fused silica under the irradiation of ultrashort pulsed laser were experimentally studied.The influences of pulse duration and pulse energy on the regime and characteristic of the material modification were explored.Based on the study results above,the processing parameters for welding fused silica were optimized.A shear joining strength up to 14.2 MPa was achieved.The welded region was characterized to be in a good transparency.As an application,a microfluidic device was fabricated with fused silica by using the ultrashort laser welding,etching and cuting techniques.5)The Bessel ultrashort pulsed laser micro-welding method was proposed in this work.By tailoring the size of the demagnified Bessel beam,and optimizing the pulse energy and deposition density of pulse number,the high-quality microwelding of fused silica was achieved.The shear joining strength of the sample was characterized to be up to 23 MPa.The width of welding line was about of 10 ?m.The defects within the welded region of the sample were well suppressed comparing with that fabricated by using Gaussian ultrashort pulsed laser.The transmittance of the welded sample at ultraviolet-visible-near infrared band was greatly improved to be above 90%.More importantly,the focal-position tolerant zone for effective welding increased 4-fold compared to the traditional Gaussian beam welding,which is of great significance for promoting the industrial application of ultrashort pulsed laser micro-welding technology.