机械电子工程专论.doc

机械电子工程专论The characteristics and applications of femtosecond laser machiningAbstract: Femtosecond laser processing technology ，involving mechanics, physics, optics, chemistry , material and other interdisciplinary fields, is an advanced technology and provides a new revolutionary technology for precision micro machining. It can be used in the manufacture of micro and nanoscale electromechanical system, optoelectronic devices, energy devices, sensors, actuators, fluid system, optical fiber communication system, biological, medical / diagnosis instrument, the future soldier system, nano satellite and lab on a chip etc. It has play a decisive role in the core electronic components, high-end general chips and great scale integrated circuit manufacturing equipment and complete sets of technology in the Twelveth five major national science and technology projects, which is currently one of the research hotspots in the international. This article, mainly focused on the characteristics and advantages of femtosecond laser machining mechanism and application research, analyses and discusses the femtosecond laser processing technology.Keywords: femtosecond laser; ultrafast; manufacturer;application.1. IntroductionFemtosecond (fs, 10-15s) laer, in the form of pulse laser, is a laser with an extremely high peak power and short pulse width. In 1981, R. L. Fork and his partners in Baer laboratory in American obtained laser pulse with width less than 100fs using passive mode-locked principle in Rhodamine6G collision as the gain medium mode-locked dye laser. It marks the beginning of femtosecond laser. However, there are many disadvantages in dye laser; the media is required dissolved in organic solvent; the jet mode with complex structure is difficult to debug and not easy to use and carry; the laser is difficult to miniaturized and applied; most of the dyes are toxic. Therefore, dye laser is not conducive to the practical application and commercialization. In 1991, the British D. E. Spence first successfully developed 60fs laser pulse output in Ti: sapphire femtosecond laser using the 20mm long Ti: sapphire crystal as the gain medium. They used argon ion laser fully pumping to insert SF14 glass prism into the laser cavity of Ti: sapphire on the material dispersion compensation with a self-mode locked Ti: sapphire laser pulse. With the advantages of the simple structure, wide tunable range, high output power, stable performance, long service life, no pollution and etc, the Solid state lasers, represented by Ti: sapphire laser, have gradually replaced dye laser and obtained rapid development.Femtosecond laser with high spatial and temporal resolution and strong electric field, magnetic field, high pressure and high temperature and extreme physical conditions, which were never reached in laboratory condition, leads to a widespread and profound revolution in science and technology. It has a very wide range of applications in femtosecond chemistry, femtosecond biology and other field. Femtosecond laser, which can be used to study the dynamics of physical, chemical and biological processes, has become an important means of observing and recording the explosive and chemical process of ultrafast process. In October 2002, Germany and Austrian scientists using femtosecond laser successfully observed electron motion, thus revealed the microscopic material movement in atomic level.High peak power of femtosecond laser makes its broad application in high-order harmonic generation, laser plasma, laser processing, laser fusion and fission, simulated cosmology and etc. Femtosecond laser with its unique advantages, overcomes the disadvantages of long pulse for processing material selection and precisely process different materials. Gaseous, liquid, solid matter in radiation of high intensity femtosecond laser turn into plasma instantaneously. The plasma can radiate lasers with different wavelengths. 2 Characteristics of femtosecond laser processing 2.1 small heat affected zoneThe interaction time duration with material while processing is short because of the short pulse of fs laser. During the time of femtosecond laser processing, the electron temperature rises to high heat in a very short period of time so that heat has not transferred to surrounding material when laser action is ended. Thus the thermal impact area is very small and material melting in heat affected zone caused by long pulse laser can be almost completely ignored here, which can greatly reduce the recasting, thermal damage (crack) and the heat affected zone. The machining area, controlled accurately in the laser focus with precise processed neat edge, can greatly improve the processing precision.In order to study theoretical model of laser ablation and demonstration the advantages of high machining precision by femtosecond laser micro-machining, B.N.Chichkov and C.Momma et al. set up a series of experiments of laser and solid effect. Figure 1 shows the morphology of ablation holes (experiment result of of using different pulse width laser to ablate 100 m thin steel plate in vacuum with femtosecond laser, nanosecond and picosecond laser of laser energy density slightly higher than the ablation threshold).(a) pulse width of 80ps, 900 J, F=3.7J/cm2; (b) pulse width of 3.3ns, 1mJ, F=4.2J/cm2;(c) pulse width of 200fs, 120 J, F=0.5J/ cm2Fig. 1 morphology of ablation holes of different pulse width on thin steel sheet2.2 high precision of machiningThe high energy density of femtosecond laser and material interaction excite some nonlinear effects of laser and material interaction. When laser irradiating material, electrons usually absorbs the energy of one photon, then jump to the excited state from the ground state. Owing to the two photon absorption rate of material is proportional to the square of the excitation light intensity and the Gauss distribution of laser energy, photochemical reaction induced by two photon absorption by will be limited in a extremely small region in the focus of high light intensity. Although the laser itself cannot break through the diffraction limit, the function area is far smaller than the focal spot by means of two-photon excitation. So that the processing region can exceed the laser diffraction limit and processing resolution can be greatly improved far beyond the optical diffraction limit up to nanometer.In 2001, Nature published an article using two-photon polymerization micro machined. In this paper, Kawata from Osaka University in Japan and Sun Hongbo produced 3D bull graphics (as shown in Figure 2), equivalent to red cell with size of 10m in length and 7m in height, breaking the limitation of optical diffraction with resolution of 120 nm.Figure.2 nano bull produced by two-photon polymerizationthe scale bar is 2m2.3 capacity of three dimensional processing Femtosecond laser can generate multi photon nonlinear absorption. In order to make full use of the nonlinear interaction of light and material, near infrared wavelength light source is usually used. In this case, the region of multi-photon absorption must be taken in a very small area in the focus of high power density. In other part of the material laser passes through, material response will not take place. Thus laser beam can pass through large volume with no energy absorption loss. In this way, internal materials cutting can be achieved, enabling the capability of 3D processing. Before processing, 3D pattern can be edited in computer in advance. When processing, laser scans along edited tool path to achieve 3D processing.2.4 wide range of applicationsEach kind of material has the damage threshold of itself when femtosecond laser interacts with matter. After focused femtosecond laser can achieve extremly high strength, exceeding damage thresholds of vast majority of materials. Therefore, femtosecond laser can be applied to various materials including metal, transparent materials, polymer materials and so on. 3. Current status of processing technology of femtosecond laser3.1 capacity of three dimensional processingAccording to the highly focused energy, small heat affected zone, no splash, no slag without special gas environment, no subsequent processes, high processing precision of two-photon polymerization and other advantages, femtosecond laser induced microstructure processing of metal applications and fine processing have made great progress.1) hole machiningIn 1 mm thick stainless steel sheet, femtosecond laser produced nano level sized deep hole with clear edge, deep clean surface and other characteristics (Figure 3a). In metal film, Ti: sapphire femtosecond laser processed micro-nano hole array (Figure 3b). The smallest diameter is 215 m.Figure 3 (a) holes in stainless steel sheet; (b) micro-nano hole array in metal film2) surface modification of metallic materialsThe interaction between femtosecond laser and materials can produce periodic micro structure. By means of femtosecond laser double beam interference, multi beam interference with multiple exposure, multilayer interference and other technologies, 1D, 2D and 3D complex periodic structures can be prepared on the surfaces of various materials, thus achieving the goal of surface modification of materials.3) processing of nano metal particle Since 1993 Henglein A et al first preparaed metal nano particles by laser ablation, many research groups have produced high purity, particle size distribution of metal nano particles. Link H et al. further controlled energy density and irradiation time of femtosecond laser to completely transform metal nano rods into the metal nano particles. Compared with other pulsed laser, metal particle prepared by femtosecond laser is of uniform size and specific shape. So that metal nano particles, especially precious metals (Au, Hg, Pt, Pd), have wide range of applications in catalysis, nonlinear optics, medical science and other fields.3.2 biology and medical fieldFemtosecond laser with advantages of cold processing, low energy consumption, less injury, high accuracy, strictly three-dimensional space location, maximally meet the special requirements of Bio Medical:1) low risk of operation, which can be repeatedly operated on the same site with short wound healing period; 2) compared to traditional operation knife, medicine source of infection decreases; 3)laser operation with no knife and high precision; 4) no painless without complication.3.3 other fieldsIn addition, femtosecond laser has a wide application prospect in some special fields of micro processing: 1) drilling and cutting high thermal conductivity and high melting point metal (such as rhenium, titanium) and high hardness of diamond. 2)The safe cutting of some high explosive dangerous goods such as: LX216, TNT, PETN, PBX and so on, avoiding the disadvantages of long pulsed such as laser linear absorption, energy transfer and diffusion. 3) using femtosecond laser to observe and analyse mechanism of physical and chemical reaction, with expect of to control nuclear fusion in order to obtain controllable nuclear fusion energy with no pollution. 4. ConlusionFemtosecond laser has unique potential applications and becomes the research hotspot in the fields of micro / nano machining, micro / nano processing. Characteristics such as extremly short pulse duration and high instantaneous energy density enable femtosecond laser to reveal the micro machining process of non equilibrium, to break through the bottleneck of coherent limit of processing precision, and finally become it possible for nano machining and many corresponding ideas in micro / nano electronics and micro / nano optics.Reference1 B. N. Chichkov, C. Momma, S. Nolte,et al, Femtosecond picoseconds and nanosecond laser ablation of solids J. Appl. Phys. A, 1996,Vol. 63:109-115.2 Kawata S., Sun H. B. et al. Finer features for functional microdevices- Micromachines can be created with higher resolution using two-photon absorption J. Nature. 2001, 412: 697-698.3 Sun H B, Xu Y, Juo dkazis S, et al. Arbitrary lattice photonic crystal created by multiphoton microfabrication J. Optical Letters, 2001,26(6):3252327.4 Rafael R. Gattass and Eric Mazur. Femtosecond laser micromachining in transparent material