High Throughput Structuring: Basics, Limitations and Needs

Transcription

1 High Throughput Structuring: Basics, Limitations and Needs B. Neuenschwander Bern University of Applied Sciences Engineering and Information Technology Laser Surface Engineering 1

2 Outline Introduction Theoretical considerations Volume ablation rates Optimum repetition rates Maximum volume ablation rates Beam Shaping Optics Limiting factors Needs Conclusions

3 Surface Microstructures Goal: Generation of surface microstructures in e range of 1 μm. High roughput means ablation volumes of a few mm 3 /min wi adequate surface roughness no oxidation of e surface geometrical precise Picosecond laser systems a promising candidate to fulfill is requirements. 3

4 High Power ps systems tbwp FUEGO Lumera Hyper Rapid TrueMicro 55 P av > 45 W f rep khz 8 MHz Δτ 1 ps M < 1.3 P av > 45 W f rep khz 1 MHz Δτ < 15 ps M < 1.5 P av 5 W f rep khz Δτ < 1 ps M < 1.3 4

5 Hammer and Nail Bern University of Applied Sciences 5

6 One high energy puls <-> several low energy pulses?? ΔV 1 ΔV ΔV 3 Is it better to distribute e energy among several pulses? If yes, what is e optimum pulse number? 6

7 Ablation of metals wi short and ultrashort pulses zabl / μm The logarimic law: z abl φ δ ln φ δ: Penetration dep Fluence / J/cm The logarimic law has often been reported and confirmed in e literature. Threshold Fluence φ We concentrate on metals and assume at a laser pulse does not influence e physical parameters of e surface for e next pulse. 7

8 Distribute e same energy among several equal pulses zabl / μm pulses 7 pulses 8 pulses 6 pulses 5 pulses 4 pulses 3 pulses pulses 1 pulse z abl φ δ ln φ δ: Penetration dep 1 1 Threshold Fluence φ Fluence / J/cm There exist an optimum pulse number to reach e maximum ablation dep. 8

9 Distribute e same energy among several equal pulses z tot,max 1.5 z tot φtot n δ ln n φ ztot / μm 1 dz tot dn n max.5 n max 1 φtot e φ n max n z tot,max 1 φtot δ e φ Remark: The above result for n max is not automatically a nonnegative integer. For an exact calculation is has to be taken into account. 9

10 Which volume can be ablated by one Pulse Pulse energy: E p Spot radius: w Top Hat: Top Hat 6 5 Gaussian beam: Top Hat 6 5 φ / J/cm 4 3 φ / J/cm r / μm r / μm E p r w π w r > w φ () r φ() r Introducing into: z tot () r φ e E φ π w () r φ nδ ln nφ p r w 1

11 Bern University of Applied Sciences 11 Which volume can be ablated by one Pulse Pulse energy: E p Spot radius: w Top Hat: Gaussian beam: () > ln w r w r r z abl φ φ δ () > R r R r w r r z abl ln φ φ δ w R φ φ wi :

12 Which volume can be ablated by one Pulse Pulse energy: E p Spot radius: w Top Hat: Gaussian beam: ΔV π w zabl φ π w δ ln φ ΔV z () r π R abl 1 π δ w 4 r dr dπ ln φ φ 1

13 Bern University of Applied Sciences 13 Which volume can be ablated at a constant power Top Hat: Gaussian beam: Δ w V φ φ δ π ln Δ w V φ φ δ π ln 4 1 Average Power: P av Spot radius: w Repetition rate: f av w f P f w V φ π δ π ln 4 1 & av w f P f w V φ π δ π ln & f P E av p leads to: w f P av π φ w f P av π φ and Wi follows: V f V Δ &

14 Ablation rate as a function of e repetition rate Ablation rate /mm 3 /min dv/dt max dv/dt max Top Hat Gauss P av 1W, J φ 1 cm δ.61 μm w 15 μm f opt f opt Repetition rate / khz There exist an optimum repetition rate f opt going wi a maximum ablation rate. The Top Hat beam shape shows an about 35% higher ablation rate. For a Gaussian beam shape working at a too high repetition rate is preferred compared to a too low repetition rate. 14

15 Ablation rate as a function of e repetition rate Ablation rate /mm 3 /min dv/dt max dv/dt max Top Hat Gauss Pav 1W, w J φ 1 cm δ.61 μm Top Hat: 1 P fopt π φ V& av e w δ P av max e φ 15 μm f opt f opt Repetition rate / khz Optimum repetition rate and maximum ablation rate directly scale wi e average power. Optimum repetition rate directly scales wi 1/w, i.e. smaller spots lead to higher optimum repetition rates. Gauss: 1 f opt P π φ av e w P av V& δ max e φ 15

16 Ultrashort laser pulses Ablation of stainless steel wi Δτ 15 fs. Two regimes: J φ, 1.16 ; δ1. 4 μm cm J φ, 5. ; δ. 167 μm cm From NCLA Galway, Ireland Does is change e obtained results? zabl / μm r / μm 16

17 Ultrashort laser pulses Ablation of stainless steel wi Δτ 15 fs. 3 Ablation rate / mm 3 /min Exact Regime 1 Regime Repetition rate / khz The exact curve equals e curve of e first regime. Differences are only observed for very low repetition rates. 17

18 And in reality? Bern University of Applied Sciences Marking lines in a polished copper surface: v P av 3 W w μm pitch.5 μm M 1.3 (Gaussian Beam) Δτ 1 ps λ 164 nm z / μm Line Cross-Section V& A Av Generate a set of lines at constant average power wi different repetition rates and adapted speed to guarantee a constant pitch y / μm 18

19 And in reality? Bern University of Applied Sciences Marking lines in a polished copper surface: v P av 3 W w μm pitch.5 μm M 1.3 (Gaussian Beam) Δτ 1 ps λ 164 nm.16.1 Copper polished, 164nm, P av 3W Measured Model Very good agreement wi e developed model: φ 1.7 J/cm ΔV/Δt / mm 3 /min.8.4 δ.33 μm f rep / khz 19

20 What does is mean for an average Power of 5 W 15 Top Hat: V& max δ P e φ av mm min fopt / khz 1 Top Hat Gauss Gauss: V& max δ P e φ av 3 mm.5 min w / μm For small spot sizes repetition rates of several MHz are required to obtain maximum ablation rates. For lower repetition rates, e spot size has to be increased. A Top Hat distribution will be more efficient an a Gaussian Beam.

21 Top Hat: Pi-Shaper (Moltech) The Top Hat distribution is only achieved over a small distance in front of e focal plane. Therefore only suited for scanners whit a significantly reduced scan field. A spot diameter of about 5 μm can be achieved wi a f 5 mm lens. This spot corresponds approximately to e spot limit. 1

22 Top Hat: Pi-Shaper (Moltech) When a round top hat beam moves over a surface more energy per area is deposited in e center of e line an at e borders. The result will be a groove wi a round shaped cross section. Quite hard to adjust.

23 Top Hat: Gauss to Top Hat Converter gt (Limo) 3

24 Top Hat: Gauss to Top Hat Converter gt (Limo) Would produce grooves wi rectangular cross section. Achievable spot sizes are limited to 6 μm. The dep of focus is quite small. Not suited for operation wi scanners, i.e. moving work piece. 4

25 Influencing and limiting factors Plasma generation: Vacuum 1-6 mbar Low pressure: 6 mbar Typical lifetime of e plasma plume in air is in e range of μs corresponding to a repetition in e range of 1 MHz. To avoid plasma shielding, e next pulse at e same location should appear after e lifetime of e plasma plume -> reduction of e repetition rate or fast moving of e laser beam. 5

26 Influencing and limiting factors Surface heating: Even in e case of ultrashort pulses, always a part of e energy is converted to heat in e material. For high average powers is can lead to surface melting effects, when e laser energy is concentrated to too low lateral dimension. -> The laser energy has to be spread over a certain area of e surface, i.e. e laser spot has to be moved over e surface. From a repetition rate of about 1 MHz on, e beam spot should be moved by about one spot diameter from pulse to pulse. 6

27 What does is mean for an average Power of 5 W fopt / khz Top Hat Gauss Top Hat: V& V& max Gauss: max δ P e φ av δ P e φ 3 mm 3.4 min av 3 mm.5 min w / μm w 5 μm f opt 87 khz v Beam m s w 6 μm f opt 48 khz v 8 Beam m s Today scanners can not reach marking speeds in e range of m/s wi an f 5 mm Optic. Speed in e range of 3 m/s of e work piece for structures in e range of 1 μm?? 7

28 Structuring wi single Gaussian spots Structuring wi single layers Good results concerning precision and roughness are achieved. Resolution of at least ¼ of e beam diameter is required. For a 1 μm structure e beam diameter should not be bigger an 15 μm. 8

29 Galvo scanner systems The scanner axes should be driven closed loop controlled and should be triggered wi e laser repetition rate. 9

30 What does is mean for an average Power of 5 W fopt / khz 15 Top Hat Gauss 1 5 w / μm Top Hat: V& V& max Gauss: max δ P e φ av δ P e φ FAST AND ACCURATE BEAM GUIDING SYSTEMS ARE REALLY NEEDED!!! 3 mm 3.4 min av 3 mm.5 min φ / J/cm Top Hat r / μm w 7.5 μm f opt 71 khz vbeam 11 m s To achieve a spot diameter of 15 μm, a scanner optics of f 5 μm has to be used. There is absolutely no scanning system on e market which can achieve is speed wi a resolution of 3 μm. 3

31 Conclusions Bern University of Applied Sciences Based on e logarimic ablation law we can find expressions for e ablated volume as a function of e average power and e repetition rate. These expressions show at ere exist optimum repetition rates (at a given average power) to achieve maximum volume ablation rates. In e case of ps-laserpulses ese optimum frequencies lies in e range of a few MHz for an average power of 5 W and a spot radius of about 1 μm. The maximum ablation rates amount several mm 3 /min. There is a great lack of beam guiding systems to really take benefit of ese high average powers. Wi beam conversion optics e optimum repetition rate can be reduced to several 1 khz. Beam guiding is still a problem. Not well suited for 3d micro structuring. Today, we can not really take benefit of e currently available high average power system, for 3d micro structuring. 31

32 ? Thank you for your attention Special anks to: Dr. Guido Bucher, Dr. Valerio Romano, Christian Nussbaum, Benjamin Joss, Martin Muralt, Michael Affolter, Peter Schütz and Urs Hunziker 3