Lasers and Laser Systems for Micro-machining

Transcription

1 Lasers and Laser Systems for Micro-machining Martyn Knowles Oxford Lasers Ltd Unit 8, Moorbrook Park Didcot, Oxfordshire, OX11 7HP Tel: +44 (0) Lasers and Laser Systems for Micro-machining The Challenges Laser Ablation Processes Lasers Systems Software Making It All Work 1

2 Micro-Machining Features of size microns in size Feature accuracy microns Minimize undesirable thermal effects Wide range of materials Make from bulk material or add features to existing device Advantages of Laser Micromachining Genuinely micro 1um corner radius <1um surface roughness Small Heat Effected Zone um Many & Difficult materials Aerospace alloys Ceramics, glass, Polymers Soft Tooling Fully Programmable Process 2

3 Lasers & Laser Systems Wish- List Easy to use Never breaks down Can machine all materials Very high speed Nano-resolution & accuracy Low cost Laser Micro-Machining The processes are Laser Micromachining is a typical of a new technology Process?*! Unfamiliar Involve several different physical effects Only documented in Research Papers Which Laser? Design & Specification of the System is Complex & Critical!! Which System? 3

4 Lasers - Which One? Nanosecond solid state Picosecond solid state Femtosecond solid state Excimer & F2 Wavelength 1064, 532, 355, 266nm 1064, 532, 355nm 1030, 800nm 157, 193, 248, 308, 351nm Laser Power (longest wavelength) 50W TEM00 1kW M 2 ~25 50W TEM00 25W TEM00 500W Frequency khz khz 1 5 khz or kHz 0 6 khz NOTE : - not all specs achievable simultaneously - solid state harmonic powers significantly less than 1064nm Laser Systems Considerations Size Ease of use Safety Stability Software Motion system Automation Beam delivery method Maintenance 4

5 Laser Micro-machining : Parameters Laser Material Processing Type Wavelength Output Power Pulse Energy Rep.Rate Pulse Length Beam Diameter Beam Polarisation Beam Divergence Beam Intensity Profile OPTICAL Absorptivity Reflectivity Refractive Index Surface Roughness THERMOPHYSICAL Thermal Conductivity Specific Heat Melting Point Boiling Point Evaporation Enthalpy Surface Tension Vapour Pressure MECHANICAL Density Hardness Poisson Ratio Young's Modulus Lens NA Spot Size Shot Overlap Gas Assist Focal Plane Processing Speed Drilling Technique Spatial Temporal! IMPORTANT: Most parameters are interrelated!! Laser Micro-Machining Start by understanding the application and what process will meet it. This defines the laser and system requirements. 5

6 Short-Pulse Laser Ablation Laser Ablation - material removal by a combination of evaporation and melt expulsion. Laser Irradiation Light Absorption 100% 80% Proportion of evaporation vs melt Vaporisation 60% 40% Ablation Material Removal Ejected material (vapour & melt) Recast (melt) material 20% 0% 100ns 1ns 100ps 1ps Melt Vapour At 1J/cm 2 courtesy Dausinger et al. Ablation Model Intense, nanosecond pulse 0ns Pulse strikes surface, W/cm 2 From Industrial Applications of High Power CVLs Warner et al. Pulsed Metal Vapour Laser NATO ASI Series 5ns 8ns 12ns 30ns 50ns Ablating surface temperature reaches 12,000 C Material is evaporating, producing vapour plume Incident light strongly absorbed in vapour plume leading to ionization, i.e. plasma Heat starts to conduct from plasma into material Ablating surface is 6,500 C 0.5 µm removed by evaporation Laser pulse ends, 1.1µm material evaporated 0.5 µs Plasma continues to heat and melt ejection begins 5 µs Temperature now 3000 C and melt ejection ceases 6

7 Processing with Ultrashort Pulses (ps & fs) Laser pulse penetration depth determined by optical and thermal properties L = L op + L th Dielectrics -optical penetration dominates over thermal - for long pulses strongly depends on wavelength. Metals - optical penetration is very short (typically tenth of wavelength) - thermal penetration dominates L th = 2 (K.t) for t >10ps K = thermal diffusivity, t = pulse duration Processing with Ultrashort Pulses - Metals Simple Model - Laser energy is initially absorbed in electrons - Electrons thermalize in about 100fs - Thermal equilibrium between the electrons and lattice occurs after a few electron-phonon relaxation times, ie typically ~10ps. Simple conclusion For laser pulses less than the electron-phonon relaxation time then no heat transferred to lattice, therefore no melt, thermal damage etc. However, this in not always observed in practice evaporation starts and continues for several ns during this time there must be some molten material Process Optimization can reduce thermal effects so melt layer is never zero but does approach a minimum which is sub-micron. 7

8 Processing with Ultrashort Pulses - Dielectrics Optical penetration dominates over thermal penetration. High intensity rips electrons out of the lattice. Resulting ions repel each other and cause a Coulomb explosion. Coulomb explosion is a non-thermal ablation mechanism. Tests have shown that in dielectrics material is partly removed by Coulomb explosion and partly thermally. In metals there is no evidence for Coulomb explosion. Ultra-high intensity is desirable to enhance the Coulomb explosion in dielectrics crystals or glass so fs is preferable to ps for processing crystals and glass. Which Laser to use? Important Parameters feature size aspect ratio feature quality processing speed cost Quality Laser Wavelength Laser Pulse Width Laser Beam Profile Proc.Speed Laser Power Laser Rep.Rate Laser Focussability Exact shape of curve is open to debate From P.R. Herman et al.rapplied Surface Science (2000)

9 Short Pulse Lasers for Micro-Machining ns ps fs Sapphire Glass Polymers Silicon Ceramics Metals Deep UV Near UV Visible Near IR Other factors (speed, quality, cost) further refine choice Process requirements for different materials Drilling or Cutting with Nanosecond Laser Polymers Ceramics Metals Wavelength Laser Power Frequency Excimer or 355nm or 266nm 355nm or 532nm Low Medium - High 0 5 khz 5-25 khz 1064nm or 532nm Medium - High 5-25 khz 9

10 Laser Micro-machining Systems Stability Design Principles - stable frame < 1um pointing accuracy - anti-vibration mounts Frame - Metal or Granite options Protection Contamination Accidental abuse Temperature variation Safety - steel enclosure - interlocked doors - Class 1 safe Laser Micro-machining Systems Design Principles Laser Motion System - diode pumped solid state - nanosecond or picosecond or femtosecond , 532, 355 or 266nm - excimer - linear, rotary, lift, goniometer axes - up to 32 axes Options - galvo systems (2 axis and 3 axis) - optical trepanning heads for µdrilling - auto-focus - auto-align - attenuators 10

11 Laser Micro-machining Systems System Options Automation Laser Motion System Vision System Diagnostics - Manual load / Pick & Place / Reel to Reel systems etc - Auto-focus - multi-wavelength systems for advanced processes - multi-laser solutions - custom software solutions - CADCAM interfaces - custom 2D calibration - on or off-axis viewing - automated alignment - measurement - in-chuck beam profiler - datalogging of system parameters Multi-Axis Capability XYZ Theta & Phi Rotary Axes Dual Scan Heads Granite base and gantry High stability Anti-vibration mounting Laser and optics on upper gantry Motion system on base Vision Systems 11

12 Industrial Micro-machining Systems Nano Second Systems Alpha Series - Desk Top, R&D A Series - Entry level, R&D / pilot production C Series - Shop floor Production system E Series - Advanced R&D system G Series - Dual Beam systems Ultrafast Systems J Series - Picosecond & Femtosecond systems Custom Systems Excimer, Femto, CVL, etc E Series System Advanced R&D 1.5 ton granite frame Class 1 enclosure mounted on floor High Power Solid State Laser Automated optical attenuator Integral laser power meter Vision system for alignment / process monitoring Cimita software - common across range 12

13 Laser Micro-machining Software Requirements Laser - Control - Fault warning - Data Logging Motion Control - XYZ - Galvos - Rotary axes - CADCAM Integrated into a single software package Process Control - Attenuators - Auto Focus - Special Processes eg Optical Trepanning Vision System - Visual monitoring - Automatic or manual alignment - Measurement Automation Cimita Software Suite - Basic Version Motion Process Control Laser Programming & Pre-programmed Routines Vision 13

14 Making It All Work Most new users of laser micro-machining processes do not have this experience. Who can help? AILU Universities & institutes like TWI, TNO, Fraunhofer etc Experience system integrators Micro-Drilling 150 µm dia thro 1mm steel 50 µm dia thro 0.1mm steel 150 µm dia thro 0.5mm silicon 30 degree hole in steel 14

15 Nanosecond Pulse Ablation : Micro-Milling Examples of optimized processes with nanosecond laser sources Alumina 511nm Tungsten 511nm Diamond 511nm Polyimide 355nm Micro-Cutting Steel Diamond Silicon Polyimide Expert Process Technology 20 years experience in laser micromachining 15

16 Removal of ITO from Glass Patterning - 10 micron wide tracks on 70 micron pitch Expert Process Technology 20 years experience in laser micromachining Patterning Thin Films on Flexible Substrates Laser scribed ITO on Flexible Substrate using 532nm (a) 4 micron spots with depth of 100nm and (b) scanning five times slower. Bar denotes 9 microns Expert Process Technology 20 years experience in laser micromachining 16

17 Lasers and Laser Systems for Micro-machining Conclusions Laser Micro-machining.. Powerful processing tool Undergoing Rapid Development Wide range of lasers and systems available Constant advances in lasers and systems are enabling new process capability Advanced Laser Micro-machining Solutions 17