Micromachining with tailored Nanosecond Pulses...Micromachining with tailored Nanosecond Pulses Hans Herfurth a, Rahul Patwa a, Tim Lauterborn a, Stefan Heinemann a, Henrikki Pantsar

Micromachining with tailored Nanosecond Pulses

Hans Herfurtha, Rahul Patwaa, Tim Lauterborna, Stefan Heinemanna, Henrikki Pantsarb a)Fraunhofer USA, Center for Laser Technology (CLT), 46025 Port Street, Plymouth, MI 48170

b) VTT, Technical Research Center of Finland.

ABSTRACT

Fiber lasers in MOPA configuration are a very flexible tool for micromachining applications since they allow to independently adjust the pulse parameters such as pulse duration, repetition rates and pulse energy while maintaining a constant beam quality. The developed fiber laser provides an average power of 11 W and maximum pulse energy of 0.5 mJ for a wide range of pulse parameters at diffraction limited beam quality. Its pulse duration and repetition rate are continuously adjustable from 10 ns to cw and from 10kHz to 1MHz respectively. Ablation experiments were carried out on stainless steel, nickel and silicon with the goal of optimizing removal rates or surface finish using nanosecond pulses of different parameters. Maximum removal rates are achieved on all three materials using relatively similar pulse parameters. For silicon, pulse duration of 320ns at 100kHz and 45mJ resulted in optimum removal. In single shot experiments on silicon a significant influence of the pulse duration was found with a distinct optimum for removal rate and surface finish. The optimum intensity at the work piece is in the range of 35MW/cm2 to 70MW/cm2. Lower values are below the ablation threshold, while the plasma shielding effect limits considerable increases in removal rates for intensities exceeding 70MW/cm2.

Keywords: Fiber laser, flexible pulse parameter, ablation, metal, silicon,

1. INTRODUCTION Pulsed lasers find more and more applications in the industry as the equipment cost continue to decrease and at the same time new products are launched to the market. New fiber lasers offer a cost effective solution to many applications but often the parameter range of these lasers is limited. Fraunhofer CLT has developed a new flexible pulsed fiber laser which can be operated using a very wide parameter window. The purpose of the study was to establish optimal parameters for processing different engineering materials and to establish the effect of pulse duration on the quality and efficiency of different laser processes. The results showed that optimal parameter ranges could be determined for all tested materials. It was found that especially the pulse energy and the pulse width have a great effect on the processing results. Reasons for this behavior are discussed.

2. EXPERIMENTAL SET-UP 2.1 Pulsed fiber laser

The pulsed fiber laser is a joint development of Fraunhofer USA, Center for Laser Technology (CLT) and the University of Michigan (UoM), College of Engineering. While the University of Michigan develops advanced fiber designs and testing, the laser system was engineered by Fraunhofer CLT. The system is based on the Master Oscillator Power Amplifier (MOPA) approach consisting of a master oscillator (seeder) and three amplifier stages. The principle set-up is shown in Figure1.

The pre-amplifier is based on standard single mode fiber components, as they are used in the telecommunications industry. The power amplifier is currently based on standard large mode area fiber (30 µm core, 250 µm cladding). The unique feature of the fiber based MOPA system is the continuously adjustable pulse duration, repetition rate and pulse energy at constant beam quality, even at tens of watts average power. The system has a proven high pulse to pulse stability. A novel fiber concept for obtaining single mode radiation out of a large core fiber even without coiling or taper is currently under development. The performance of this new, patented fiber has been demonstrated for passive fibers and will be extended to active fibers in the near future. This development will enable scaling of the pulse energy in the multi milli-joule range.

Photonics North 2007, edited by John Armitage,Proc. of SPIE Vol. 6796, 67961G, (2007)

0277-786X/07/$18 · doi: 10.1117/12.778742

Proc. of SPIE Vol. 6796 67961G-1

Downloaded from SPIE Digital Library on 23 Feb 2011 to 141.213.32.23. Terms of Use: http://spiedl.org/terms

C

IDI

IDI

400WPeak power0.18WAverage power4.0µJPulse energy0.8WPump power1995Amp. factor33 dBAmplification

50kWPeak power15WAverage power0.5mJPulse energy30WPump power159Amp. factor22 dBAmplification

Isolator,taper

Third stage

14 dB 19 dB 23 dBSignal diode

First stage OutputIsolator Second stage

300mW

Isolator

500mW 30W

LMA30/250SMSCSMSC

PowerAmplifier

Pre-amplifiersSeeder

Pumpdiode

Pumpdiode

Pumpdiode





Isolator,taper

Third stage

14 dB 19 dB 23 dBSignal diode

First stage OutputIsolator Second stage

300mW

Isolator

500mW 30W

LMA30/250SMSCSMSC

PowerAmplifier

Pre-amplifiersSeeder

Pumpdiode

Pumpdiode

Pumpdiode

Fig. 1. Principle MOPA set-up with seeder, two pre-amplifiers and power amplifier.

The current Yb-based system is shown in Figure 2. It comprises the pump module for the power amplifier (bottom), the MOPA based fiber laser (middle) including all associated electronics and controls, as well as all optical components of the power amplifier with free space beam delivery and the pulse generator (top).

Specifications:

Wavelength: 1064 +/-4nm

Beam quality: M2<1.5

Average power: 15W

Pulse energy: 0.5mJ

Pulse Length: 10ns to 1µscontinuously adjustable

Repetition Rate: 10kHz-1MHzcontinuously adjustable

Specifications:

Wavelength: 1064 +/-4nm

Beam quality: M2<1.5

Average power: 15W

Pulse energy: 0.5mJ

Pulse Length: 10ns to 1µscontinuously adjustable

Repetition Rate: 10kHz-1MHzcontinuously adjustable

Fig. 2. Pulsed fiber laser system and specifications Pulse generator, MOPA system, and pump module for power amplifier (top to bottom)

The main controller controls and monitors all pre-amplification components. It can be connected to a computer via Ethernet, RS-232 or RS-485. Basic control commands are used to change parameters and read the status of the connected devices. An analog interface is integrated to change parameter settings on a microsecond timescale, such as gating and pulse parameter variation. A fully integrated external frequency/pulse generator (Avtech) is used for the



seeder to allow continuous adjustability of pulse duration, duty cycle and amplitude. It is controlled via a RS-232 interface and provides an analog interface for gating and modulation of the amplitude. The performance of the system is shown in Figure 3 for pulse duration of 10ns and various repetition rates.

0.1

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10

100

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10,000

100,000

Seed First stage Second stage Third stage

Amplifier Stage

Peak

pow

er [W

]

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Ener

gy [µ

J]

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CW

Pulse length: 10ns

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Seed First stage Second stage Third stage

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pow

er [W

]

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0.01

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Ener

gy [µ

J]

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100kHz

1MHz

CW

Pulse length: 10ns

Fig. 3. Peak power and pulse energy of pulsed fiber laser for individual stages

2.2 Processing optics and sample manipulation

The collimated laser beam has a diameter of approximately 1mm at the laser output. A 5x telescope is used to increase the beam size before it enters the galvanometric scanner (Scanlab). Two different lenses with a focal length of 100mm and 160mm are used to focus and direct the beam onto the work piece. They result in spot minimum spot size of xx and xx respectively. While the beam is manipulated by the scanner in x- and y-direction during processing, the material samples are positioned in x-, y-, and z-direction with a high precision motion system (Aerotech). The motion system also repositions the samples in z-direction after the processing for each layer is completed.

3. RESULTS 3.1 Processing of Stainless Steel

The objective for the experiments performed on stainless steel SS304 was to establish the relationship between processing parameters and the material removal rate. This was carried out by engraving 1.8mm x 1.8mm squares into the surface of a stainless steel plate of 2mm thickness. The engraving was done by scanning multiple layers, each of which ablated a thin layer of material from the surface. The total energy applied per square was kept constant at 121.5J. Therefore, for higher scanning velocity, the number of layers was increased respectively and for higher power, the number of layers was decreased to maintain the constant input energy per square. The scanning speed was varied from 100 to 800mm/s, the frequency from 20 to 100kHz and pulse width from 20 to 640ns. Two power levels were tested. With 163mm focal length, which creates an approximately 45µm focal spot diameter on the surface, 4.5W laser power was used. With the shorter focal length of 80mm, delivering an approximate focal spot diameter of 25µm, 2.25W of laser power on work piece surface was used. The displacement between hatched lines was in both cases 12µm and the area was scanned in the x- and y-direction.

The material removal rate was determined by measuring the depth of the squares, and consequently the volume of the removed material. The depth was measured using an optical microscope and measuring the surface of the material, the distance to the highest peaks of the square and the lowest valleys. This did not in detail provide the correct amount of removed material, but gave a good estimate. The accuracy of measurement was the poorest with samples processed with a high scanning velocity and a low pulse repetition rate. For such conditions there was very little overlap of pulses and the engraved surface was very coarse. In these cases, the roughness (Rz) value of the surface is in the range of tens of micrometers. As a result, the volume measurement is not accurate. Figure 4 describes the measurement procedure.



Too surface

Highestmeasur,

point

measurementDo'

4.0

E 3.0 —100 mrrVsE

150 mm's2.0

E —200mm's300 mms

1.0w —400mm/s—800mm/s

0.0

0 200 400 600 800 1000

Pulse width Ens]

4.0-)

E 3.0E

o 2.0E

• 1.0a,'5

0.00 200 400 600

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100 mms

150 mm1s

200 mms

300 mms

—400 mrrVs600 mms

800 mm1s

800 1000

Fig. 4. Procedure to determine ablated material volume

Using 2.25W of power and a spot size of 25µm (80mm focal length), the highest material removal efficiency [mm3/kJ] is obtained at low repetition rates and pulse widths ranging form 80 to 320ns. In this parameter range, high peak power is achieved that leads to high material removal rates. The optimal material removal efficiency depends mainly on correlation between pulse width and frequency. At 100kHz highest material removal is obtained using an 80ns pulse width, which resulted in a 280W peak power and an average power density in the laser spot of 57.3MW/cm2. The corresponding pulse energy is 22.5µJ. The dependency between pulse width, scanning velocity and material removal efficiency is presented in Figure 5. The maximum of the material removal rate shifts to longer pulses for lower repetition rates. At 20kHz the highest material removal rate is achieved with a pulse width of 320ns. For these settings the pulse energy is 113µJ, the peak power is 350W and the average power density in the laser spot is 71.6MW/cm2.

Repetition rate: 100kHz Repetition rate: 20kHz

Fig. 5. Material removal as a function of pulse width and scanning speed

The material removal rate is also affected by the scanning velocity. This effect is observed especially for high repetition rates. Figure 6 shows the dependency between the scanning speed and the material removal rate [mm3/min]. For samples that were processed at 100 kHz frequency the material removal rate starts to decrease when the scanning velocity drops below approximately 300mm/s. At this speed, the individual pulses overlap by 88 to 92%, with the pulse-to-pulse distance being only 2 to 3µm. For values higher than 300mm/s the removal rate is rather constant and independent of the scanning velocity. At a repetition rate of 60kHz a similar decrease in the ablation rate occurs at velocities lower than 150 to 200mm/s. This setting results in a pulse-to-pulse distance of 2.5 to 3.3µm and the pulses overlap by 10 to 13%. No distinct correlation between the scanning velocity and the removal rate is observed at a repetition rate of 20kHz. However, the surfaces created at low repetition rates and low scanning velocity are also very rough and therefore, the measurement of the ablated material volume is less accurate.



1.00

0.80E —640nsE 0.60 —320ns>o —l6Ons

0.40

0.20__________________

80 ns

40 ns

20 ns0.00

0 200 400 600 800 1000 1200

Scanning velocity [mm s1]

•'':••jil

Fig. 6. Material removal rate as a function of scanning velocity and pulse width

The ablation process was optimized for highest material removal rate or surface quality. The highest ablation rate as a function of input energy was achieved at 20kHz repetition rate, 320ns pulse width and 150mm/s scanning velocity. At these settings the removed material volume per laser energy was 3.4mm3/kJ and the ablation rate was 0.46mm3/min. However, the surfaces obtained at low repetition rates or low scanning velocities are very rough with peak-to-valley values exceeding 15µm. In general, operating at maximum material removal rates leads to unacceptable surface quality for engraving applications. Figure 7 (left) shows the surface appearance of a sample processing at highest removal rate. In addition to the rough surface there are also small cavities that occur randomly. For comparison, Figure 7 (right) presents the result after optimizing the parameter settings towards high surface quality.

Pulse width: 320ns Repetition rate: 20kHz Scanning speed: 150mm/s

Pulse width: 160ns Repetition rate: 100kHz Scanning speed: 800mm/s

Post-treated surface

Fig. 7 Surface appearance for different processing conditions



40

35

30 —320ns25

—l6Ons20

a)15 —8Ons

010 —4Ons

—2Ons0

0 500 1000 1500 2000 2500

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400 rrmfs500 rrm/s

600 rrmfs

800 rrrfs1000 rin*1500 rrn*2000 rrn*

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Pulse width Ens]

Shorter pulses, increased scanning velocity and repetition rates lead to the significantly improved surface appearance. Even further enhancements are possible if the sample is post processed using parameter settings that do not ablate additional material but remelt a thin layer and smooth out the surface profile.

The power was set to 4.5W on work piece surface for all investigation performed with the 163mm focusing optic. Overall, highest material removal [mm3/kJ] is achieved at a low repetition rate and a pulse width ranging from 160 to 320ns. The optimal material removal rate depends primarily on the dependency between pulse width and frequency. At 100kHz the highest material removal rate is obtained using a pulse width of 80ns, which corresponds to a peak power of 560W and an average power density in the laser spot of 35.4 MW/cm2. The pulse energy for this setting is 45µJ.

The maximum material removal rate shifts to longer pulses when the frequency is decreased. At 40kHz repetition rate, the maximum is at 160ns and at 20kHz, the maximum is observed at a pulse width of 320ns. The peak power is 700W and the average intensity is 44.2MW/cm2 in both settings. The scanning velocity does not significantly effect the material removal rate and only slight changes occur. For higher frequency (100kHz), the material removal rate is rather constant regardless of the velocity. However, at low repetition frequency of 20 kHz, the material removal rate increases with rising scanning velocity up to 800mm/s for pulse width of 160 and 32ns. Other pulse parameters maintained a rather constant material removal rate, regardless of the scanning velocity.

3.2 Processing of Silicon

The objective of processing experiments carried out on silicon wafers was to drill holes and to scribe grooves. The purpose was to determine possible ablation rates and the processing quality and to estimate the optimal parameter settings for silicon processing. Grooves of 40mm length were scribed to the silicon sample using scanning velocities ranging from 200 to 2000mm/s, frequencies between 20 to 200kHz and the pulse widths from 20 to 320ns. All samples were processed using the 80mm focal length focusing optic that produces a spot size of 25µm. The laser power on the work piece surface was 4.5 W.

The depth of each groove was measured; however this applies only to samples in which the relationship between the scanning velocity and frequency was such that a continuous groove is scribed. Some samples exhibit a large amount of recast and debris preventing an accurate measurement of the groove depth. The groove depth is measured from the top level of the sample to the highest and lowest point at the bottom of the groove. The depth is an average value of these measurements.

Generally, the groove depth is maximized at low velocities; a fact that is mainly due to the higher energy per length. The achievable groove depth at 100kHz is shown in Figure 8 (left). The repetition rate has also a significant effect on the ablation rate. The higher ablation rates are observed with decreasing frequency.

Fig. 8. Groove depth as a function of scanning velocity and pulse width at a frequency 100kHz

However, low frequencies limit the applicable scanning velocity, since high velocities result a series of single pulses, but not in a continuous groove. On the other hand, high frequency and low scanning velocity lead to a strong formation of recast and the material ejection from the groove is insufficient.



The depth of the groove is also affected by the pulse width. Short pulses remove only a limited amount of material, while longer pulses result in a considerable increase of the material removal rate (Figure 8 right). However, extending the pulse width also reduces the groove quality and eventually stops the ejection of material. A significant amount of recast is formed, especially at slow scanning velocities. The best quality for scribing grooves in silicon is achieved using a scanning velocity 600mm/s, a repetition rate of 100kHz and a pulse width of 160ns. The edges of the groove are melted and the quality is very consistent. The depth of the groove is 15µm. Figure 9 shows an example for such a groove.

Fig. 9. Laser machined groove in silicon (depth 15µm)

Single pulses were applied to the surface of the silicon wafer to investigate the effect of the pulse duration on the ablation phenomenon. Three different pulse widths were selected: 20, 80, 320, and 1000ns. In each case, the pulse energy was maintained at 45µJ, only the pulse width was been changed. The 1000ns pulse only heated and melted the surface of the material, but hardly removed any material. In this case the resulting peak power was only 45W and the average intensity in the laser spot 9.2MW/cm2. By shortening the pulse to 320ns material is ablated and a clean funnel shaped blind hole with a very small amount of recast is formed. The peak power is 140W and the average intensity is 28.6MW/cm2. Further compression of the pulse width to 80ns clearly induces a more aggressive impact and but the material removal is not very efficient and leaves a blind hole with a depth of only 4µm. The material removal is further reduced at a pulse width of 20ns. The created dimple is less than 2 micrometer deep and a large amount of debris is observed effected area. In this case, the peak power is 2.25kW and average intensity 458MW/cm2. Figure 10 shows processing results for pulse lengths of 20 and 320ns.

Pulse width: 320ns, peak power: 140W, depth: 15µm Pulse width: 20ns, peak power: 2.2kW, depth: >2µm

Fig. 10. Processing results in single pulse mode on silicon



4. SUMMARY AND CONCLUSIONS A new pulsed fiber laser with flexible pulse width, repetition rate, and pulse energy is introduced and results on micromachining of metal and silicon are presented. The experiments show a pronounced effect of the pulse width on the quality and efficiency of the processes. Optimized settings for the pulse width and peak intensity are determined that result in highest removal rates or high surface quality. The material removal rates drop rapidly if the optimum intensity is exceeded. Factors that are assumed to cause this reduction in process efficiency include intra-pulse plasma effects and exothermic reactions during processing. A minimum intensity is required to exceed the ablation threshold of the material and to establish efficient material removal. The high flexibility of the laser source allows exploring a wide parameter range and offers extended capabilities in determining optimum processing conditions for specific materials.



Documents

Micromachining with tailored Nanosecond Pulses...Micromachining with tailored Nanosecond Pulses Hans Herfurth a, Rahul Patwa a, Tim Lauterborn a, Stefan Heinemann a, Henrikki Pantsar