Qin Y. Micromanufacturing Engineering and Technology

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is strained with a defined force that is generated

by a brake motor and a haul-off unit. After use the

wire is cut, chopped and recycled.

Generator Types

Modern EDM machines provide two different

types of generators in order to fit a wide range

of different applications: static pulse generators

for roughing operations and relaxation generators

for micro-structuring and finishing operations.

Static Pulse Generators. Static pulse generators

are charac terized by a controlled current or volt-

age source which generates rectangular single

pulse discharges [12]. The source is directly con-

nected to the discharge path via electronic switch-

ing elements (Fig. 3-22). Pulse shape, pulse dura-

tion t

, and discharge duration t

can be adjusted

and controlled for the realization of a relatively

low wear of the working electrode.

In contrast to relaxation generators static pulse

generators allow the setting of the discharge dura-

tion t

independently of the discharge current i

and are normally used for intensive material

removal processes with long discharge durations

and high discharge currents i

Relaxation Generators. The principle setup of a

relaxation generator is a direct current source in

combination with different storage elements (RC,

RLC or LC circuits) [12] (Fig. 3-23). In case of

charged storage elements the gap between the tool

electrode and the workpiece serves as a switch.

After excee ding the dielectric strength of the

dielectric fluid, an oscillating discharge is

released. Discharge duration t

and discharge cur-

rent i

depend on the value of the capacity and

cannot be controlled independently as this is the

case with a static pulse generator. Additionally,

the pulse frequency depends on the charging volt-

age of the capacity. Due to very small discharge

FIGURE 3-22 Static pulse generator: schematic illustration (a), voltage and current diagram including ignition delay time

), discharge duration (t

), pulse duration (t

), and pulse cycle time (t

) (b).

FIGURE 3-23 Relaxation generator: schematic illustration (a), voltage and current characteristic including discharge

duration (t

), and pulse cycle time (t

) (b).

CHAPTER 3 Micro-EDM 51

energies of down to W

= 0.1 m J at high pulse fre-

quencies of up to f

= 10 MHz, relaxati on genera-

tors are used for micro-machining and precise

surface finishing.

Adaptive Feed Control. In contrast to cutting

technologies, the feed rate in EDM is adaptively

controlled by the control of the EDM machine. A

constant feed rate can lead to short circuits, result-

ing in a destruction of the micro- structures. The

adaptive feed control analyzes the measured para-

meters of the electrical impulse characteristics and

classifies them using a pulse detection unit.

According to that detection, the feed rate is regu-

lated by a control circuit.

The electrical impulses are classified in four

different types (Fig. 3-24) which result from the

conditions inside the gap: no-load discharge, nor-

mal discharge, arc discharge, and short circuit.

No-load discharges occur when the working

gap is oversized. As a consequence the build-up

of a plasma channel is prevented and the discharge

current does not flow. The adaptive feed control

reacts with an increase of the feed rate. At a nor-

mal discharge the width of the working gap has an

optimal size. For the following discharges the

adaptive feed control tries to keep a constant feed

rate. An arc discharge is characterized by a prema-

ture ignition of the spark. The real ignition delay

time t

is far below the set value of the ignition

delay time t

. This type of discharge is caused by a

high concentration of electro conductive particles

inside the gap. This status is normally followed by

a series of normal discharges. The adaptive feed

control reacts with a decrease in feed. Short cir-

cuits can either occur due to contact between the

tool electrode and the workpiece or because of

remaining particles that form an electrical linkage

[13]. To leave that state, the tool electrode is

pulled back along the machining path.

Depending on the application, different con-

trol technologies and strategies based on fuzzy

logic are used in modern m-EDM machines such

as ada ptive control optimization (ACO) or adap-

tive control constraint (ACC) are used in modern

m-EDM machines.

DESIGN CONSIDERATIONS

m-EDM Optimized Design

In the case of manufacturing a punch and die set, it

is beneficial to use identical materials, ideally a

FIGURE 3-24 Illustration of the principle controlling strategies.

52 CHAPTER 3 Micro-EDM

fine grained steel alloy. Especially in the case of

m-WEDM with deionized water as dielectric fluid,

it is recommended to use stainless steel. It is nec-

essary to consider clear reference sides and to

prevent machining operations from different

directions due to multiple clamping and con-

nected inaccuracies. At least one outside surface

is needed for electrical touching. If possible, sub-

sequent heat treatm ent should also be prevented

due to risk of shape deviations. If heat treatment is

still necessary it should be applied before EDM

processing.

In m-WEDM the workpiece height is limited by

the thermal and mechanical stability of the wire

electrode which depends on the diameter of the

wire. Inside contours generally require a start

hole for the wire. For proper insertion of the

wire the start hole should be larger than the wire

diameter. Modern WEDM machines are able to

insert a wire in a star t hole with 30 mm oversize

automatically.

The main fact to consider during the design of a

part to produce with m-die-sinking EDM is the

shape of the tool electrode. The applied shaping

process, e.g. micro-milling, laser ablation or

m-WEDM, must be able to produce the negative

shape of the workpiece contour. A preferred shap-

ing process for thin contours is m-WEDM instead

of micro-milling due to the smaller structure

dimensions and higher accuracy that it allows.

In m-EDM the application of the same clamping

system for the shaping process is needed due to a

defined clamping accuracy if a reworking of the

electrode is needed.

Material Selection

Every material with an electrical conductivity

above k = 0.01 S/cm can be machined by EDM

(Fig. 3-25). Mechanical properties like Young’s

modulus, hardness or strength do not have an

influence on the process behavior. Consequently,

EDM is especially used for the machining of hard

and difficult-to-cut materi als such as cemented

carbides for punching tools, PCD for cutting

inserts or high alloy steels for injection molding

tools.

The amount of material being removed is influ-

enced by its thermal and electrical properties.

Contrary to a high electrical conductivity

(Fig. 3-26(a)) that promotes material removal,

high thermal conductivity and high melting tem-

perature reduce the material removal (Fig. 3-26(b)

and (c)). This is of special importance when select-

ing wear resistant electrode materials.

Tool Electrodes for m-EDM

Up-to-date tool electrode materials for m-EDM

are electrolyte copper, tungsten and tungsten

composites such as tungsten copper or tungsten

carbide.

FIGURE 3-25 Machinable and non-machinable materials.

CHAPTER 3 Micro-EDM 53

The attractive attributes of electrolyte copper

are its high electrical and thermal conductivity.

Electrolyte copper is easy to obtain, very consis-

tent in quality, and low in cost. A disadvantage of

the material is its limited grindability [12]. Due to

mechanical instability, the material’s ductility

and softness can cause problems when milling

small structures. Another problem is the high

wear of electrodes made from electrolyte copper.

Tungsten is a metal of very high strength, den-

sity and hardness. With a melting point near

3400



C, it resists thermal damaging effects of

the EDM process very well. Disadvantages of

tungsten are a very high material price. It is also

difficult to machine because of its high hardness.

To make tungsten more attractive for certain

applications it is combined with ductile materials

such as copper.

The resulting material, tungsten copper, is eas-

ier to machine, more conductive than normal cop-

per and extremely wear resistant. Especially in the

field of micro-structuring and surface finishing,

tungsten copper is used as the tool electrode

material.

APPLICATIONS

A final summary of the chapter and the wide

application field of m-EDM are shown in

Table 3-2.

TECHNOLOGICAL

COMPETITIVENESS

AND OPERATION ECONOMICS

In various industrial sectors a number of micro-

technical components with a volume of less than

1mm

and structure dim ensions in the range of

micrometers come into application. The produc-

tion of these components is usually done by tech-

nologies that originated in the semiconductor

industry. To avoid the technological limitations

of these processes as well as the very high invest-

ments, the mass production of micro- or minia-

ture parts is increasingly based on replication

technologies such as hot embossing, micro-injec-

tion molding or bulk forming. To achieve a suffi-

cient tool life, functional materials like hardened

steel or cemented carbide are used due to the high

thermal and mechanical loads in the forming pro-

cess. The mechanical properties of these materials

limit the variety of structuring processes which

can be applied. Next to micro-cutting, laser

machining, and LIGA technology, electrical

discharge machining (EDM) is a manufacturing

technology which can be applied. Due to the non-

contact thermal material removal mechanism,

EDM technologies offer a suitable alternative in

terms of obtainable structure dimensions and

accuracy. Since EDM can be economically

applied in single and small batch production, in

addition to its use for the primary structuring of

micro-parts, it is predetermined for the produc-

tion of micro-dies and molds [33–35].

Considering the operation economics of

m-EDM, it is necessary to know the workpiece

accuracies that are needed. Although most

m-EDM machines include a cooling system for

the dielectric fluid and therewith offer adequate

thermal long-term stability, an air conditioned

installation is needed to achieve the best geomet-

rical accuracy. Despite an air conditioned instal-

lation the temperature can change around T =1K

which can influence the machine accuracy.

FIGURE 3-26 Influence of thermal and electrical properties on material removal rate.

54 CHAPTER 3 Micro-EDM

Therefore, most m-EDM machines consist of a

machine frame made from granite and have

ultra-precise linear axes. This drives the price to

levels of more than e300,000 for a high end

m-WEDM machine. Considering the tooling costs

of EDM wire electrodes, there are differences

regarding the wire diamete r. Whereas the price

for conventional wire electr odes with dia-

meters from d =300mm down to d =100mmis

around e8/kg to e15/kg, the price for micro-wires

TABLE 3-2

Table of m-EDM Process Variants According to Performance and Applications

Process Variant Geometry

Complexity

Structure

Dimension

Surface Quality Application

Examples

Micro-wire EDM

(m-EDM)

– 2{1/2}D

– Tapered surfaces

with maximal angles

of 15



– Min. inner radii,

cutting width, and

max. workpiece

height depends on

wire diameter, e.g.

at d

wmin

= 0.03 mm,

min

= 0.005 mm,

max

=5mm

 0.07

 0.35

with multiple cutting

strategy

– Forming tools for

opto-electronic

components

– Lead frame

stamping tools

– Spinning nozzles

– Micro gears

Micro-die sinking

(m-die sinking)

–3D

– Freeform surfaces

– Undercuts possible

by planetary

erosion

– Limited by electrode

manufacturing

– Min. structure width

0.02 mm

– Aspect ratio 20

 0.1 mm

 0.5 mm

in all directions

– Micro-

injection molds

– Embossing molds

for micro-optics

Micro-

electrical discharge

drilling (m -ED drilling)

–2D

– Boreholes

– Bore hole diameters

down to

min

= 0.04 mm

– Aspect ratio from

25–50

 0.3 mm

 1.5 mm

lateral

– Injection nozzles

– Micro-fluid systems

– Starting holes

for m-WEDM

Micro-electrical

discharge grinding

(m-EDG)

– 2{1/2}D

– Straight partly

vertically curved

channel structures

– Profile of structure

depends on cross-

section of disc

– Min. structure widths

of 0.07 mm

– Aspect ratio 15

– Possible length

depends on

electrode wear

(>50 mm)

 0.2 mm

 0.8 mm

in all directions

– Embossing and

coining tools

– Micro-

fluid channels

Micro-electrical

milling (m-ED milling)

–3D

– Free-form surfaces

– Min. inner radii

depend on pin

diameter

– Applicable electrode

diameter

min

=0.1mm

 0.2 mm

 0.8 mm

in all directions

– Cavities for micro-

injection molding

– Embossing or

coining tools

Micro-wire electrical

discharge grinding

(m-wire EDG)

Rotational symmetric

structures (discs, pins)

– Min. pin electrode

diameter

min

 0.02 mm

– Aspect ratio 30

 0.15 mm

 0.75 mm

– Pin electrodes

– Rolling tools

CHAPTER 3 Micro-EDM 55

with diameters d < 50 mm can easily be 3 t o 5

timesthisamount.Theoperatingcostsfora

m-EDM machine depend on the diele ctric fluid.

When applying deionized water additional costs

such as for deionization resin need to be consid-

ered. In the case of di electric oil as the dielectric

fluid, the costs for renewing the oil have to be

cons idered as well a s the costs for wear str ained

parts in the wire guiding system, deb ris filter, and

electricity (Table 3-3).

As an example, the following listing shows the

average costs of a WEDM machi ne, AGIE Evolu-

tion, in a dual-mode operation (roughing , finish-

ing process for cemented carbide/steel up to

= 0.2 with wires CCA 0.2/0.25) with an aver-

age erodin g performance of 4000 machining

hours per year and the use of the wearing part

kit 8000 for 8000 machining hours. Additional

peripheral units such as air conditioning, cooling

or lighting are not included [36].

Due to the complex machine structure,

machine control, and process technologies, it is

recommended to train technicians with courses

offered from machine suppliers. These courses

last one to two weeks and cost between e5000

and 1 e10,000.

THE OUTLOOK

During to the latest development in end mill

geometry, micro-milling is able to machine mate-

rials with a hardness of up to 60 HRC. Cutting

technologies originated for non-conventional

machining processes are pushing into the market.

Limiting factors for the application of micro-mill-

ing and grinding for structuring micro-parts are

the process forces. Therefore, at a comparable

cost listing for micro-milling, m-EDM, as a pro-

cess that is almost force free and independent of

material properties, is predestined for micro-

structuring. Even though the process is slower

compared to micro-milling, the independence of

the material ’s hardness is a big advantage which

will gain in further importance in the future.

Latest machine developments and research

activities show a trend toward hybrid processes

to combine the possibilities and advantages of

different technologies and therewith reach best

machining results more efficiently. One example

is the ‘Hybrid Wire



’ machine from Sodick



which combines water jet cutting and WEDM.

Further research activities head toward the com-

bination of laser ablation and EDM technologies,

e.g. using laser ablation for roughing operations

and ED contouring for finishing. In the field of

tool electrodes, research concentrates on develop-

ing more wear resistant materials, e.g. electrodes

with special diamond coatings. Also extending the

spectrum towards new workpi ece materials plays

an important role. The develop ment of EDM pro-

cess variants concerning the machining of low

conductive high performance materials related

to even higher wear resistance and new appli-

cation fields especially comes into focus. An

essential demand for new EDM systems is the

fabrication of workpieces with shape accuracies

smaller than 1 mm at shorter process times. There-

fore, new, more precise positioning systems and

generators with higher pulse frequencies than the

present ones are needed that will turn m-EDM into

nano-EDM.

REFERENCES

[1] E. Uhlmann, Trennende Fertigungsverfahren f

€

neue Perspektiven in der Mikrotechnik, Forschung-

spolitische Dialoge in Berlin: MST – Schl

€

usseltechno-

logie des 21, Jahrhunderts, Berlin (1998).

[2] E. Uhlmann, U. Doll, Mikrofunkenerosion zur Ferti-

gung von Mikrosystemen, Jahrbuch Innovative Pro-

duktion 2000, Trade & Contact Verlag, Berlin

€

arz 2000).

TABLE 3-3

Cost Listing AGIE Evolution/

Classic 2/3 S

Wearing parts e0.30

Filter e0.61

Deionization resin e0.51

Water e0.06

Electricity e0.58

Wire costs e2.52

Total e4.58

CHAPTER 3 Micro-EDM

[3] A. Wolf, W. Ehrfeld, H.P. Gruber, Mikrofunkenero-

sion f€ur den Pr

€

azisionsformenbau, wt Werkstatt-

stechnik (1999) H. 11/12.

[4] S. Thiel, S. Lehnicke, O. Zimmer, D. Grimme, U. Doll

and S. Piltz, Abformung von Mikrostrukturen in Glas,

VDI-Z Special Werkzeug- und Formenbau VII/2000.

[5] W. Menz, J. Mohr, O. Paul, Mikrosystemtechnik f

€

Ingenieure, Wiley-VCH Verlag GmbH Co, KGaA,

Weinheim (2005).

[6] B.R. Lazarenko, N.I. Lazarenko, Elektrische Erosion

von Metallen, Cosenergoidat, Moskau (1944).

[7] B.R. Lazarenko, Die Elektrofunkenbearbeitung von

Metallen, Vestuik Maschinostroia (1974).

[8] B.N. Zolotych, Physikalische Grundlagen der Elek-

trofunkenbearbeitung von Metallen, SVT 175 VEB-

Verlag Technik, Berlin (1955).

[9] B.N. Zolotych, U

ber die physikalischen Grundlagen

der elektroerosiven Metallbearbeitung, Bd. 1: Elek-

troerosive Bearbeitung von Metallen, Moskau: Aka-

demie der Wissenschaften der UdSSR (1957).

[10] E. Uhlmann, S. Piltz and U. Doll, Machining of

micro/miniature die and moulds by electrical dis-

charge machining – recent development, Journal of

Materials Processing Technology, Elsevier (2005).

[11] R. Br

€

uck, N. Rizvi, A. Schmidt, Angewandte Mikro-

technik – LIGA – Laser- Feinwerktechnik, Carl Han-

ser Verlag M

€

unchen Wien (2001).

[12] W. K

€

onig, Fertigungsverfahren Band 3, VDI-Verlag

GmbH, D

€

usseldorf (1990).

[13] S. Piltz, Grundlagen und Prozessstrategien der Mik-

rofunkenerosion f

€

ur die Bearbeitung von Rotations-

bauteilen, Fraunhofer IRB Verlag, Stuttgart (2007).

[14] F.-J. Siebers, Funkenerosives Senken mit w

€

assrigen

Arbeitsmedien, Grundlagen, Technologie und

Wirtschaftlichkeit, Dissertation, RWTH Aachen

(1994).

[15] J. Sch

€

onbeck, Analyse des Drahterosionsprozesses,

Carl Hanser Verlag M

€

unchen Wien (1993).

[16] H.H. Langen, T. Masuzawa, M. Fujino, Modular

method for microparts machining and assembly with

self-alignment, Annals of the CIRP 44(1) (1995).

[17] J. Fleischer, T. Masuzawa, J. Schmidt and M. Knoll,

New applications for micro EDM, Proc. of the 14th

International Symposium for Electromachining

(ISEM XIV), March 30–April 1, 2004, Edinburgh,

Scotland, UK, published in J. of Materials Processing

Technology 149 (1-3) (2004).

[18] J. Fleischer, J. Schmidt, M. Knoll, S. Haupt, C.

€

uller, R. F

€

orster, A. Gehringer, Mikrobearbeitung

durch Abtragen, wt Werkstattstechnik online, Jahr-

gang (2004) H. 11/12.

[19] R. F

€

orster, Untersuchung des Potenzials elektro-

chemischer Senkbearbeitung mit oszillierender

Werkzeugelektrode f

€

ur Strukturierungsaufgaben

der Mikrosystemtechnik, Albert-Ludwig-Universit

€

Freiburg, Dissertation (2004).

[20] T. Masaki, K. Kawata, T. Masuzawa, Micro electro-

discharge machining and its applications, Proceed-

ings of IEEE MEMS (1990).

[21] T. Masaki, et al. Micro electro-discharge machining,

Proceedings of the 9th International Symposium for

Electromachining-ISEM IX, Nagoya (Japan) (1989).

[22] D. Allen, H. Almond, and P. Logan, A technical com-

parison of micro-electro-discharge machining,

microdrilling and copper vapour laser machining

for the fabrication of ink jet nozzles, Proc. of SPIE

Conference on Design, Test, Integration and Packag-

ing of MEMS/MOEMS, 4019, Paris, France, May 9–

May 11, (2000).

[23] M. Ghoreishi, M. and J. Atkinson, Vibro-rotary elec-

trode, a new technique in EDM drilling – perfor-

mance evaluation by statistical modelling and opti-

misation, Proc. of the 13th International Symposium

for Electromachining (ISEM XIII), Bilbao, Spain,

Vol. II, May 9–May 11, (2001).

[24] M. M. Ghoreishi, J. Atkinson, A comparative exper-

imental study of machining characteristics in vibra-

tory, rotary and vibro-rotary electro-discharge

machining, J. of Materials Processing Technology

120 (2002).

[25] A. Muttamara, Y. Fukuzawa, N. Mohri, T. Tani,

Probability of precision machining of insulating

ceramics by EDM, J. of Materials Processing

Technology 140 (2003).

[26] E. Uhlmann, Funkenerosives Feinbohren kera-

mischer Werkstoffe. Abschlussbericht zum DFG-

Forschungsprojekt UH 100/22-2, Technische Uni-

versit

€

at, Berlin (2002).

[27] N.N., Pressemitteilung der Fa. Sodick Europe Ltd,

Coventry, UK, zur Pr

€

asentation der AE05 Nano

EDM Machine, erschienen in: www.tool-moldmak-

ing.com.

[28] W. Meeusen, D. Reynaerts, J. Peirs, H. Van Brussel

et al., The machining of freeform micro moulds by

micro EDM – work in progress. Proc. of Microme-

chanics Europe Workshop, Cork, Ireland (MME

2001).

[29] A. Wolf, W. Ehrfeld, H. Lehr, F. Michel, T. Richter,

H. Gruber, O. W

€

orz, Mikroreaktorfertigung mittels

Funkenerosion, F&M Fernwerk- und Mikrotechnik

(1997) 6.

[30] H.-M. Chow, B.-H. Yan, F.-Y. Huang, Micro slit

machining using electro-discharge machining with a

modified rotary disk electrode (RDE), J. of Materials

Processing Technology 91 (1999).

[31] C.-L. Kuo, J.-D. Huang, Fabrication of series-pattern

micro-disk electrode and its application in machining

micro-slit of less than 10 mm, Int. J. of Machine Tools

and Manufacture 44 (2004).

[32] S. Appel, Funkenerosive Bearbeitung von ploykristal-

linem Diamant, Technische Universit

€

at Berlin, Dis-

sertation (1998).

CHAPTER 3 Micro-EDM 57

[33] E. Uhlmann, U. Doll, Application of m-EDM in the

machining of micro structured forming tools, Pro-

ceedings of the 3rd Int. Machining & Grinding Conf,

Cincinnati (USA) (1999).

[34] F. Michel, W. Ehrfeld, O. Koch, H.-P. Gruber, EDM

for micro fabrication – technology and applications,

Proceedings International Seminar on Precision Engi-

neering and Micro Technology, Aachen (Germany)

(2000).

[35] T. Masuzawa, State of the art of micromachining,

CIRP Annals – Manufacturing Technology 49(2)

(2000).

[36] AGIE, Cost listing of an AGIE Wire EDM Machine

(2003).

[37] D.M. Allen, Micro electro discharge machining for

MEMS applications, IEE Seminar on Demonstrated

Micromachining Technologies for Industry, Ref. No.

2000/032 (2000).

58 CHAPTER 3 Micro-EDM

Laser Micro-Structuring

Arnold Gillner

INTRODUCTION

In the production of micro-scaled products and

micro-replication tools, laser ablation is becom-

ing an ever more important tool, which is able to

generate structure sizes in the range of 10–

100 mm, not only in metals but also in hard

and ultra-hard materials such as tungsten car-

bide and ceramics. Especially for micro-machin-

ing, laser processes qualify for a wide r ange of

materials, from semiconductors in the field of

micro-electronics, to hard materials such as

tungsten carbide for tool technology, to very

weak and soft materials such as polymers for

medical products. Even ceramics, glass and dia-

mond can be processed with laser technologies

with accuracies better than 10 mm. In comp ari-

son t o the classical technologies, laser processes

are generally used for small and medium lot

sizes but with strongly increased material and

geometric variability.

Using ultra-short pulsed lasers with durations

of 10 ps in bursts of several pulses with a time

spacing of 20 ns each and adapted pulse en ergies,

the surface quality of metal micro-ablation has

been increased significantly and allows the pro-

duction of tools and parts with R

values of less

than 0.5 mm. Compared to conventional EDM

processing, laser manufacturing of parts and tools

can be performed without additional working

tools in reasonable times directly from the CAD-

CAM system.

Micro-ablation by means of laser-based vapor-

ization of materials has been used in a large

variety of applications to produce micro-molds,

functional components in electronics and micro-

drilling applications. Most of these processes were

focused on small parts, where the laser has its

unique position in selectivity and low thermal

input without changing the properties of the

entire part. Recent developments in solar cell

manufacturing, display manufacturing and the

manufacture of products based on electrically

conducting polymers such as OLEDs have opened

a new field in laser micro-manufacturing with

laser radiation, where the specific advantages

and outstanding properties of laser radiation are

highly sought after. Laser micro-ab lation and sur-

face fictionalization with:

micron and sub-micron scale accuracy in abla-

tion depth

nanometer scale in-depth heat input

micron scale lateral accuracy and

sub-micron scale in lateral dimensions with

special process techniques

on almost all types of materials allow new

manufacturing processes, when the production

of components cannot be met by conventional

processes. In this way laser ablation and surface

fictionalization have been already proven as a

non-convertible tool in solar cell and display

manufacturing.

BASIC PROPERTIES OF LASER

ABLATION

The precision of laser ablation is given by a com-

bination of different effects, which is related to

the working tool (geometry of the laser beam)

CHAPTER

the process itself (laser/material interaction)

and

the positioni ng system for the part and the laser

beam.

Laser radiation is used as a working tool to struc-

ture matter by thermal vaporization with well-

defined volumes and high lateral precision. For

this, depending on the type of laser and processing

technology, laser radiation is focused or projected

by objectives onto the surface, and subsequently

formed into a specific spatial intensity distribu-

tion. Depending on the intensity, the absorption

conditions of the materials and the interaction

time, several aspects have to be considered, which

define the ablation geometry and the precision of

the ablation proces s.

The precision of the resulting structures

depends not only on the focus diameter and the

positioning stage, but also on the precision of the

process resulting from the reactions of the matter

during and after irradiation. Thus, the processing

diameter depends not only on the beam radius w

but also on the physical properties of the material,

such as the reflectivity, absorption, melting and

evaporation enthalpy. Depending on the optical

properties of the material and on the applied radi-

ation, the precision of the process diameter

depends on the absorption of the radiation, which

can be linear or non-linear. Additionally, the abla-

tion depends strong ly on the pulse duration and

on the wavelength.

Absorption Effects on Ablation

Geometry

Generally the interaction of photons and m atter

is based on the absorption of the electromagnetic

energy at the free and bound electrons of a

material. The electrons are heated and transfer

their energy after a charact eristic ti me to the

lattice of the material. After transfer of the opti-

cal energy to the latt ice, the material heats up

andstartstomeltandsubsequently vaporizes

with further energy d eposition. The laser-

induced ablation itself can be distinguished

between photo-induced and thermal ablation.

The precision is greater for photo-induced abla-

tion being comparable to t he precision of the

working tool, here resulting in the processing

diameter. In the case of the thermal ablation,

additional processes such as heat diffusion

decrease th e precision and give rise to a larger

processing diameter than the focus diameter.

For most of the materials in laser material pro-

cessing, surface absorption is the dominating pro-

cess. Materials absorb laser radiation linearly,

when an absorption band at the laser wavelength

(Fig. 4-1) or due to free or quasi-free electrons

being present (such as metals or graphite) is given.

This can be described by Lambert–Beer’s law:

IðxÞ¼ð1  RÞI

ax

ð1Þ

where R is the reflectivity, a is the absorption

coefficient (cm

1

), and I

is the threshold intensity

for ablation (W/cm

). Ba sed on this, a logarithmic

dependence of the ablation depth h [cm] on the

intensity can be found:

h ¼



: ð2Þ

The ablation depth defines the precision of the

structures in depth.

Applying Gaussian radiation with a focus

diameter 2w

the processing diameter can be con-

trolled by the intensity of the radiation, depending

on the ratio of the applied intensity to the

FIGURE 4-1 Principle of laser beam interaction with

materials.

60 CHAPTER 4 Laser Micro-Structuring