HIGH-RATE LASER MICRO PROCESSING USING A POLYGON SCANNER SYSTEM

#M1208

Udo Loeschner

a), Joerg Schille

a), Andre Streek

a), Tommy Knebel

a), Lars Hartwig

a), Robert Hillmann

b),

Christian Endischb)

a) Laserinstitut Hochschule Mittweida, Technikumplatz 17, 09648 Mittweida, Germany

b) SITEC Industrietechnologie GmbH, Bornaer Straße 192, 09114 Chemnitz, Germany

Abstract

This paper discusses results obtained in high-rate laser

micro processing by using a high average power high-

PRF (pulse repetition frequency) ultrashort pulse laser

source in combination with an in-house developed

polygon scanner system.

With the recent development of ultrashort pulse laser

systems supplying high average power of hundreds

watts and megahertz pulse repetition rates, a signifi-

cant increase of the productivity can potentially be

achieved in micro machining. This permits upscaling

of the ablation rates and large-area processing, gaining

increased interest of the ultrashort pulse laser technol-

ogy for a large variety of industrial processes. Howev-

er, effective implementation of high average power

lasers in micro processing requires fast deflection of

the laser beam. For this, high-rate laser processing by

using polygon scanner systems provide a sustainable

technological solution.

In this study, a picosecond laser system with a maxi-

mum average power of 100 W and a repetition rate up

to 20 MHz was used. In raster scanning using the pol-

ygon scanner, the laser beam with a focus spot diame-

ter of 44 µm was deflected with scan speeds of several

hundred meters per second. The two-dimensional

scanning capability of the polygon scanner supplied a

scan field of 325 mm x 325 mm. The investigations

were focused on high-rate large-area laser ablation of

technical grade stainless steel as well as selective thin

film ablation from bulk substrates. By variation of the

processing parameters laser fluence, as well as tem-

poral and spatial pulse-to-pulse distance, their impact

onto the ablation process was evaluated with respect to

the ablation rate, processing rate, surface quality, and

ablation efficiency.

Introduction

High-PRF ultrashort laser sources supplying high av-

erage laser power of more than 100 W have recently

become available, allowing excellent machining quali-

ty and high processing throughput at the same time.

Initial studies in this field are focused on micro vias

generation [1], micro hole processing in steel and cop-

per foils [2], refractive index changes in transparent

materials [3], and laser ablation of micro cavities [4,5].

In addition, the potential of the technology has been

already demonstrated by means of specific machining

examples, such as micro pyramids and embossing tools

[6-9].

The influence of laser fluence and pulse repetition rate

on material removal rate and process efficiency was

studied in [6,9] by using a high-PRF picosecond laser

source. By applying an average laser power of 3 W, a

maximum removal rate of 0.16 mm³/min was achieved

with pulse repetition rates between 50 kHz and

100 kHz. Due to constant average laser power, the

pulse energy was gradually reduced while the pulse

repetition rate increased up to 300 kHz, causing the

decrease of the ablation rates. Furthermore, higher

ablation rates can be achieved performing multi pass

irradiation in the “low-fluence” regime while the laser

fluence is above the threshold limit. Therefore, it has

been shown that the material removal rate can be in-

creased up to 7 mm³/min at a pulse repetition rate of

20 MHz [10].

However, by using high-PRF laser sources fast beam

deflection systems are required prospectively permit-

ting a processing speed of several 100 m/s. A first

approach of a fast laser beam deflection system was

realized by means of an acousto-optic deflector [7].

Therewith a laser engraving process was performed on

the outer mantle of a fast rotating cylinder with a pro-

cessing speed of 40 m/s while the material removal

rate was 3 mm³/min.

Laser micro machining with considerably higher scan

speeds was conducted by the use of a polygon scanner

in combination with a single mode cw fibre laser [11].

In this study processing speeds of 300 m/s have been

demonstrated by irradiating a continuous wave laser

beam of 2 kW laser power and a spot size of 21 µm on

metal sheets.

First results obtained in large-area micro processing

demonstrated once more the potential of the innovative

high-PRF ultrashort pulse laser technology. A ripple

pattern was generated on stainless steel with a dimen-

sion of 80 mm x 80 mm in width and length while the

processing rate was 25 cm²/min [10].

Research activities with high-PRF ultrashort pulse

laser radiation yielded novel phenomena in laser beam

material interaction, those are in contrast to low repeti-

tion rate ultrashort pulse laser processing [2,5,12,13].

On the one hand, a decrease of the ablation rate was

observed with pulse repetition rates of several

100 kHz. For this, plasma shielding induced by inter-

action of the incident laser pulse with the plasma

plume, which originated from the previous laser puls-

es, was suggested. On the other hand, the ablation rate

rises when laser pulses with a frequency in the mega-

hertz range impinge onto the surface of a poor heat-

conducting material like stainless steel, potentially due

heat accumulation effects. Moreover, a further phe-

nomenon called micro cone formation on the scale of

some 10 µm was observed in high-PRF ultrashort

pulse processing with femtosecond laser pulses. Shape,

size, and density of the structures could be influenced

by the laser parameters. Even a selective elimination of

the structures was realized that is in contrast to pico-

second laser pulses [14]. Micro cone formation can

modify the properties of technical grade surfaces with

respect to wettability, roughness, and reflection charac-

teristics.

In conclusion, high average power high-PRF laser

sources combined with ultrafast beam deflection sys-

tems seems to be a promising technology in order to

scale the productivity of high-PRF ultrashort pulse

micromachining with respect to processing throughput

and large-area laser processing. In this work, a novel

machining setup consisting of a high-PRF high average

power picosecond laser source and an ultrafast polygon

scanner was investigated. The study was concentrated

on high-rate ablation of bulk material as well as selec-

tive large-area thin film removal from bulk substrates.

Finally, the potential of this technology was demon-

strated on large-area processed samples.

Experimental

In this study, a high-PRF picosecond laser source of

the PX series from Edgewave GmbH emitting a linear-

ly polarized Gaussian beam at the fundamental wave-

length of 1064 nm was used. Including all losses in the

optical beam path the laser supplied an average power

of 75.9 W onto the work piece surface. The maximum

repetition rate was 20 MHz while the pulse width was

10 ps. Further, the maximum available single pulse

energy decreased with rising repetition rate as a conse-

quence of limited average laser power.

For ultrafast beam deflection across the sample surface

an in-house developed polygon scanner was utilized.

As a special feature the polygon scanner system is

equipped with an additional deflection unit permitting

two-dimensional raster scanning. The laser beam was

focused onto the material surface with a spot size of

44 µm by using an f-theta objective which allows a

maximum scan field of 325 mm x 325 mm. With this

setup scan speeds vsc up to 800 m/s were achieved.

In general, the maximum utilization rate ηf,max of a

single polygon facet was limited by the technical fac-

tors of the geometrical arrangement of the optical

components of the polygon scanner. For the polygon

scanner used in this study, the maximum utilization

rate ηf,max was determined to be 49 %. With respect to

the used f-theta objective, this corresponds to a maxi-

mum processable length lmax of 325 mm. In conclusion,

for laser processing with the maximum possible length

lmax of 325 mm performed with more than one facet a

duty cycle with a ratio of 49:51 has to be considered.

As a result, the maximum achievable effective pro-

cessing speed veff,max corresponds to the duty cycle and

is approximately half of the scan speed vsc, see equa-

tion 1:

Furthermore, a shorter processing length results in a

less effective speed veff given by the ratio ηf of the

current processed length lc and the maximum length

lmax. Hence, the effective processing speed veff can be

calculated with equation 2:

In this study the ablation characteristics of two differ-

ent types of materials were investigated: 1.5 mm thick

technical grade stainless steel (AISI 304) for high-rate

volume ablation and a 75 nm thick silicon nitride thin

film deposited on a multi crystalline silicon wafer in

order to demonstrate high-rate large-area ablation.

To identify the maximum achievable removal rate

cavities were processed into stainless steel by using the

“line-by-line” and “layer-by-layer” raster scan regime

characterized by the lateral pulse distance dp between

two consecutive incident pulses and the hatch distance

dh between lines. Dc is the cavity depth, and ns is the

number of scans representing the quantity of processed

layers. The averaged ablated volume per single pulse

Vsp can be calculated according to equation 3, taken

from [15]:

(1).

(2).

The cavity depth dc was determined at five individual

positions at the cavity bottom by means of a confocal

point sensor µScan from NanoFocus AG. From equa-

tion 3 the ablation rate dz can be estimated considering

the average number of incident laser pulses per area

associated with the focal spot radius w0:

The material removal rate MRR as an indicator for the

process efficiency results from the averaged ablated

volume per single pulse Vsp and the repetition rate fr

and is given by equation 5:

For large-area laser processing the efficiency is charac-

terised by the processing rate Ap which can be calculat-

ed with equation 6:

It is noteworthy, that the processing rates Ap are realis-

tic rates provided by the setup because the effective

processing speed veff was used.

The surface roughness Sa was measured in accordance

with ISO 25178 utilizing a measurement arrangement

consisting of the Confocam C101 (confovis) and the

LV100D–U microscope (Nikon). The measurement

data were analysed with the Mountains Map® soft-

ware.

Results and discussion

In this work high-rate laser ablation of stainless steel

by applying an average laser power of 75.9 W of a

high-PRF picosecond laser source and an ultrafast

polygon scanner was studied. Even at the highest pulse

repetition rate of 20 MHz the supplied laser fluence at

the material surface of 0.5 J/cm² exceeds the ablation

threshold for stainless steel Hth ~ 0.1-0.15 J/cm² by a

factor of 4.

The ablation experiments were carried out by applying

the maximum average laser power of 75.9 W onto the

stainless steel surface while the lateral pulse distance

and the hatch distance between lines were kept con-

stant at 10 µm. By variation of the repetition rate be-

tween 1 MHz and 20 MHz the laser fluence dropped

continually beginning at 10 J/cm² at 1 MHz down to

0.5 J/cm² at 20 MHz. Figure 1 depicts the ablation rate

as a function of the repetition rate for 100 scans. The

ablation rate was taken from the cavity depth obtained.

In addition, the laser fluence versus the repetition rate

is plotted. The maximum ablation rate of 23 nm/pulse

was achieved at the lowest investigated repetition rate

of 1 MHz corresponding to the maximum applied laser

fluence of 10 J/cm². With rising repetition rate an ex-

ponential decrease of the ablation rate down to

3 nm/pulse was observed mainly caused by lower laser

fluences.

In order to gain information on the process efficiency,

the volume ablation rate and the material removal rate

reveal more detailed facts. Figure 2 plots the volume

ablation rate per laser pulse as well as the material

removal rate versus the repetition rate for a number of

100 scans. As expected, the largest volume ablation

rate was achieved by using the lowest investigated

repetition rate corresponding to the highest laser flu-

ence of 10 J/cm². By applying higher repetition rates,

the volume ablation rate decreases, as shown in fig-

ure 1. However, the material removal rate, which re-

(3).

(4).

(5).

(6).

Fig. 1: Ablation rate dz and laser fluence H as a func-

tion of the repetition fr rate obtained on stainless steel

Fig. 2: Volume ablation rate Vsp and material removal

rate MRR as a function of the repetition rate fr obtained

on stainless steel

sults from scaling of the volume ablation rate with the

repetition rate increased with rising repetition rate

although the amount of the volume ablation rate

dropped. The maximum material removal rate was

found to be 5.4 mm³/min at a laser fluence of 0.5 J/cm²

and a repetition rate of 20 MHz. The highest ablation

efficiency for ultrashort laser pulses was indicated for

laser fluences about 7.4 times above the ablation

threshold [16]. Within the presented investigations

performed with a picosecond laser system, the supplied

fluence at the highest material removal rate exceeds

the threshold fluence by a factor of 4. So it can be

assumed, that the material removal rate can be maxim-

ized by irradiating laser pulses with a laser fluence in

the range of 0.8 J/cm². For this, a higher average laser

power is needed. It is noteworthy to point out, that the

maximum material removal rate of 5.4 mm³/min is

related to the scan speed of the polygon scanner, which

was 200 m/s. Considering the maximum utilization

rate of the polygon scanner of 49 % used in the inves-

tigations the maximum effective speed was 98 m/s

yielding to a maximum material removal rate of

2.65 mm³/min.

In ultrashort pulse laser processing there is a contro-

versial discussion regarding efficiency analyses be-

tween picosecond laser ablation and femtosecond laser

ablation. The maximum achieved material removal rate

of 5.4 mm³/min in stainless steel, based on the scan

speed, is lower in comparison to the removal rate per-

formed with high-PRF femtosecond laser pulses re-

ported in [10]. There, a scan speed based material

removal rate of 6.8 mm³/min was determined with an

applied laser fluence of 0.85 J/cm² and a repetition rate

of 19.3 MHz by using a galvanometer scanner system.

While the material removal rate achieved with femto-

second laser pulses was obtained at the optimum laser

fluence of 8 times above the threshold level, the rate

for picosecond laser pulses was lower whereas the

applied fluence was below the optimum fluence.

Against this background, similar material removal

rates can be assumed at identical laser fluences. There-

fore, at first glance there is no preference in efficiency

between the picosecond and the femtosecond regime.

But, if the average laser power input is taken into ac-

count, in the picosecond regime an average laser power

of 75.9 W was applied. The investigations with femto-

second laser pulses were performed with an average

laser power of only 31.7 W. To put the rates on a com-

parable basis, the rates were normalized with the laser

power leading to a material removal rate of

0.09 mm³/W/min and 0.21 mm³/W/min for picosecond

laser pulses and femtosecond laser pulses, respectively.

Even though identical material removal rates of

6.8 mm³/min, the laser-power-normalized rates differ

by a factor of more than two indicating a more effi-

cient ablation process of stainless steel for processing

with femtosecond laser pulses.

For a constant total energy input within the ablation

process two different regimes were compared: the

“high-fluence” regime with 10 J/cm² and the “low-

fluence” regime with 0.5 J/cm². Because of the maxi-

mum average laser power of 75.9 W the repetition rate

was 1 MHz for 10 J/cm². Choosing the maximum

repetition rate of 20 MHz the laser fluence of the puls-

es irradiated to the material surface amounts to be

0.5 J/cm². In order to provide a constant total energy

input in the “high-fluence” regime 100 scans and in the

“low-fluence” regime 2000 scans were performed.

Furthermore, the processing time was identically. For

the “high-fluence” regime a volume ablation rate of

2.05 mm³/min was achieved. However, in the “low-

fluence” regime an increase of the ablation rate of

2.58 mm³/min was obtained corresponding to a growth

of 25%.

Beside the ablation rates the surface quality and the

roughness of the ablated areas was evaluated. In fig-

ure 3 SEM photographs are presented, demonstrating

the surface morphology obtained with various laser

fluences.It can be seen, that the surface quality was

strongly influenced by the applied processing parame-

ters. The smoothest surface was obtained by applying

the lowest fluence of 0.5 J/cm² and the highest repeti-

tion rate of 20 MHz. Highly regular ripple formation

with a spatial period of around one micron, which

correlates with the laser wavelength of 1064 nm of the

laser beam [17-21], as well as starting micro crater

development in the grooves between ripples can be

seen. Neither debris nor molten bulges can be observed

on the structure surface indicating a minor thermal

load of the material in spite of high average power

impinging on the sample. This is the result of the low

applied laser fluence of 0.5 J/cm² and very high scan

speed of 200 m/s. Further, with increasing laser flu-

ence the surface appears more roughly. On the bottom

of the cavity micro craters became more pronounced,

growing into deeper regions followed by formation of

micro cones when laser fluence was increased up to the

“high-fluence” regime with gradually lower scan speed

down to 10 m/s at a laser fluence of 10 J/cm². The

surface structures were covered with ripples.

The results of surface roughness measurement, listed

in figure 3f), correlate with the surface morphologies

presented in figure 3a)-e). The smoothest surface,

achieved with the highest repetition rate of 20 MHz

and the lowest laser fluence of 0.5 J/cm² is character-

ised by a surface roughness of 0.6 µm. Surface rough-

ness increases with higher laser fluences and sinking

repetition rates due to pronounced formation of micro

craters and micro cones. The achieved roughness value

of 1.9 µm at a laser fluence of 2 J/cm² is close to the

value reported in [22]. In similar experiments in stain-

less steel, performed with picosecond laser radiation at

a fluence of 2 J/cm², a repetition rate of 150 kHz, and a

comparable ratio between lateral pulse distance and

focal spot diameter, a profile roughness value Ra of

1.6 µm was determined. In conclusion, by applying

low fluences and high repetition rates, maximum mate-

rial removal rates and best surface quality was

achieved at the same time. These results show a similar

tendency observed and discussed in high-PRF laser

ablation with femtosecond laser pulses [10].

figure 3a) 3b) 3c) 3d) 3e)

dp, dc [µm] 10

H [J/cm²] 10 4 2 1 0.5

fr [MHz] 1 2.5 5 10 20

Sa [µm] 3.3 3.1 1.9 1.1 0.6

Fig. 3: a) - e) SEM photographs demonstrate the surface morphology at the cavity bottom obtained with different

processing parameter sets, f) parameters and corresponding roughness values

f)

Up to now, laser micro processing was limited on

comparatively small areas of a few square centimetres.

However, with the investigated laser system a maxi-

mum scan field of 325 mm x 325 mm corresponding to

an area of 0.1 m² can be processed in one pass. The

surface of a stainless steel sheet with a dimension of

265 mm in length and 250 mm in width was laser

processed, presented in figure 4.

With laser pulses supplied at a repetition rate of

20 MHz and a lateral pulse distance of 40 µm, a scan

speed of 800 m/s was realized. If the current processed

length of 265 mm is taken into account, an effective

processing speed of 320 m/s resulted. Further, it yield-

ed to a processing rate of 7680 cm²/min for one scan

which is evident with a processing time of 5.2 s while

the hatch distance was also 40 µm. On closer inspec-

tion, the processed field is characterized by barrel and

pincushion distortion arising from the so far uncorrect-

ed optical system of the polygon scanner. However,

the scanner control provides a feature for scan field

correction which will be tackled in a next step in order

to improve the accuracy of the scanner system.

High-rate large-area laser ablation of a thin film was

conducted on 6” (156 mm x 156 mm) multi crystalline

silicon wafers covered with an approximately 75 nm

thick silicon nitride layer for photovoltaic application.

The challenge of the investigation was to remove the

layer within one pass without any damage of the sili-

con substrate.

First, a grid of lines with a hatch distance of 100 µm

was laser processed by applying a fluence of 1 J/cm²

and a repetition rate of 10 MHz while the lateral pulse

distance was 20 µm corresponding to a scan speed of

200 m/s. Figure 5 shows an optical microscope image

of the processed wafer.

Fig. 5: Selective ablated lines of a silicon nitride layer

on a silicon wafer (line distance: 100 µm, width of

ablated lines: 30 µm)

The silicon nitride layer was completely removed

within the lines showing a width of 30 µm without

damage of the underlying silicon. Considering the

effective processing speed a processing rate of

2800 cm²/min was obtained yielding to a processing

time of the entire wafer surface of 5.2 s. A more de-

tailed consideration revealed slight variations of the

hatch distance between adjacent lines. This is due to

insufficient manufacturing precision of the polygon

wheel, which is also a task of prospective scan field

correction. By applying a laser fluence of 2 J/cm², a

repetition rate of 5 MHz, a lateral pulse distance and a

hatch distance of 20 µm, and a resulting scan speed of

100 m/s the silicon nitride layer was completely re-

moved from the silicon wafer. For a processed field of

136 mm x 136 mm the processing rate was 245 cm²/s

Fig. 4: Laser processed stainless steel surface, field

size 265 mm x 250 mm

Fig. 6: Selective thin film ablation of a silicon nitride

layer using a special feature of the polygon scanner –

multiple segments regime within one facet passing the

laser beam at a scan speed of 200 m/s, line segment

width 4 mm

according to a processing time of 45.5 s. As a special

feature of the polygon scanner multiple line segments

can be produced while one facet passes the laser beam

realized with fast laser switching as a preliminary stage

of a so called pixel mode. The challenge is an exact

timing of the scanner and the control at high scan

speed in order to process the adjacent line segments

with a high lateral precision. The performance capabil-

ity was demonstrated by selective ablation of the sili-

con nitride layer, shown in figure 6. The pattern con-

sisted of 16 ablated fields with a line segment width of

4 mm processed at a scan speed of 200 m/s. It is re-

markable, that there is only a marginal spatial jitter

regarding the starting points of the line segments, at-

testing a very high degree of temporal synchronization

of the scanner system.

Summary

High-rate laser micro processing by using a high aver-

age power high-PRF picosecond laser source in com-

bination with an in-house developed polygon scanner

system, equipped with a two-dimensional scanning

capability, has been investigated in this study. By ap-

plying a raster scanning regime, high-rate ablation of

bulk material as well as high-rate large-area processing

was analysed in detail.

Stainless steel was irradiated with a maximum average

laser power of 75.9 W. The highest material removal

rate of 5.4 mm³/min was achieved in the “low-fluence”

regime at the highest available repetition rate of

20 MHz. In addition, with these parameters the best

surface quality was obtained, characterized by a mini-

mum surface roughness of 0.6 µm.

With the presented laser system the capability of high-

rate large-area laser processing was demonstrated on a

stainless steel sheet. A field size of 265 mm in length

and 250 mm in width was processed with a processing

rate of 7680 cm²/min for one scan. Further, thin film

ablation of a silicon nitride layer from bulk silicon

substrate was investigated. The layer was completely

removed in one pass in a field of 136 mm x 136 mm

without any damage of the silicon while the processing

rate was 245 cm²/min. Furthermore, selective thin film

removal was performed. For this, a grid of lines with a

hatch distance of 100 µm was laser processed with a

processing rate of 2800 cm²/min.

Acknowledgment

The presented results have been conducted in the

course of the projects ”Innoprofile Transfer – Rapid

Micro/Hochrate-Laserbearbeitung” (03IPT506X),

funded by the Federal Ministry of Education and Re-

search.

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Meet the author

Udo Loeschner is graduated in Physical Engineering

from the University of Applied Sciences Mittweida

(Germany) in 1998. Thereafter he has been a R&D

engineer at the laser institute of the same university.

Since 2006 he joined the Rapid Micro Tooling re-

search group to investigate laser micro processing with

short and ultrashort pulse laser technologies. He com-

pleted his PhD thesis at the Technische Universitaet

Ilmenau in 2007. In 2011 he became an endowed pro-

fessorship and represents the appointment area “High

rate laser processing”.