Qin Y. Micromanufacturing Engineering and Technology

Подождите немного. Документ загружается.

threshold intensity for ablat ion. Threshold inten-

sities are given for homogene ous material by the

threshold intensity for melting I

and for evapo-

ration I

(Fig. 4-2(a)). Solid materials feature a

melt enthalpy that is always smaller than the

evaporation enthalpy, resulting in larger proces-

sing diameters for melting than for evaporation

(Fig. 4-2(b) for I

= I

/2).

The processing diameter of focused Gaussian

radiation can be reduced steadily by decreasing

the intensi ty close to the threshold intensity for

ablation I

. Processing diameters much smaller

than the focal diameter 2w

are achievable

(Fig. 4-2(c)).

Thermal and/or photo-physical processes gov-

ern ablation. Thermal processes are represented

by heating of matter without change in the chem-

ical constitution, whereas photo-physical pro-

cesses imply changes in the chemical constitution

or irradiated matter.

Thermal Effects on Ablation Precision

and Ablation Geometry

Today, for precise laser ablation, single nanosecond

pulses with time spacings between the pulses in the

range of 10 ms are used. Ablation rates of between

0.1 and 1 mm

/min are achieved with depth accu-

racies of 1 to 2 mm. In the nanosecond range the

thermal influence of the laser irradiation results in

typical melting depths of several micrometers and a

surface quality which is controlled and defined by

the melt resolidification and the surface tension of

the melt. With this technology, ablation accuracies

of typically 1 mm can be achieved and a surface

roughness of 0.6 mm can be obtained. Recent inves-

tigations using ps-lasers showed even higher

accuracy, which is due to a totally different laser

interaction regime with significant differing ther-

mal influence and resulting quality. In nanosecond

beam interaction the energy deposition and heat

transfer in the material is taking place within the

pulse duration. In ultra-short pulse processing the

energy transfer to the lattice occurs after the entire

pulse duration. The temperature increase and ther-

mal behavior are then described by a two-temper-

ature model which takes into account the laser/

material interaction that takes place generally

between photons and electrons with an overheated

electron gas; and an energy transfer to the lattice by

a material-specific coupling coefficient.

The effect of the absorbed optical energy on

thermal ablation of matter is driven by the pulse

duration and is subdivided into four regimes for

linearly absorbing matter:

Absorption of the optical energy by quasi-free

electrons (t

< 10 fs);

Thermalization of the electrons, called the elec-

tron system (t

< 100 fs);

Interaction between the electron and the pho-

non system (t

< 10 ps);

Thermalization of the phonon system

< 100 ps).

The durations of the single processes within the

brackets are exemplary if given for copper. Two

limiting cases can be distinguished:

1. Pulse duration larger than the thermalization

of phonons t > t

. For t > t

the times for

FIGURE 4-2 Extension of the melted (faint) and ablated (dark) region for intensities smaller (a), larger (b), than I

. For I  I

and I > I

demonstrates differences in the extension of the ablated region (c).

CHAPTER 4 Laser Micro-Structuring 61

absorption of the photons, and the thermaliza-

tion of the electron and phonon system, are

much smaller than the pulse duration, resulting

in the description of heating by one tempera-

ture for the electron and the phonon system.

Ablation affects the subsystems as an instanta-

neous process. The threshold fluence for

ablation (fluence = energy per area) scales with

the square root of the pulse duration and a

thermal penetration depth can be defined as:

therm

¼ 2

ﬃﬃﬃﬃﬃﬃﬃﬃ

ð3Þ

depending on the thermal property of matter,

with k the thermal conductivity, c

the heat

capacity,

r the density and t

the pulse dura-

tion of the applied radiation. The thermal

penetration depth

d defines the region beyond

the focus diameter (tool diameter), which can

be thermally modified, such as by the amor-

phization of crystalline substrate. This regime

is called the heat affected zone (HAZ). The

processing diameter is given by 2(w

+ d).

2. Pulse duration smaller than the electron–pho-

non relaxation time

t < t

. For t < t

the

processes within the electron and the phonon

system are decoupled. The temperature devel-

opment for these systems are described by a

two-temperature model representing two cou-

pled differential equations:



þ S  m T

 T



¼ m T

 T



ð4Þ

and C

represent the heat capacities of the

electron and the phonon system, k the heat

conductivity of the electron system, S the

applied optical energy, m the electron–phonon

coupling constant and T

and T

the tempera-

tures of the two systems. Ablation of materials

is characterized by negligible melt and small

mechanical load of the irradiated region. The

processing diameter is comparable to the

focus diameter.

In nanosecond laser processing this energy

transfer occurs during the duration of the

laser pulse, whereas in picosecond laser pro-

cessing the energy transfer occurs after a

certain interaction time. For metals, this

transfer time is generally in the range of some

picoseconds. In this case, the material is

heated up after the end of the laser pulse, so

that there is no interaction of the photons

with melted and evaporated material. The

res u l t is a much more accurate ablation

because the ablation is mainly due to vapori-

zation of the material and not by melt expulsion.

The second reason for the use of picosecond

pulse durations in laser ablation is the very

high intensity of the pulses. With peak intensi-

ties of more than 10

W/cm

all materials are

vaporized rather than merely melted. As a con-

sequence of these high intensities, the ablation

rate per pulse is quite low, because for vapori-

zation an excessive amount of energy is needed.

In Fig. 4-3(a) direct comparison of laser ablated

steel with nanosecond pulses and picosecond

pulses is shown. It can be seen c learly that in

nanosecond ablation the residual melt after

ablation is much stronger and the geometry

is affected by melt resolidification. In pico-

second ablation almost no melt can be found

in the ablation area and clean surfaces with a

surface r oughness of <0.5 mm can be produced

by laser ablation.

PROCESS PRINCIPLES FOR LASER

ABLATION

Micro-structuring with laser radiation adopts

one of the follow ing techniques:

the mask technique

the scribing techni que.

The mask technique is appropriate for 2

dimensional structuring, for example appli-

cation in lithography and the generation of

micro-fluidic systems and nano-scaled optical

devices such as gratings. The scribing technique

is ad opted for full 3D struct uring and is com-

parable to mechanical milling.

62 CHAPTER 4 Laser Micro-Structuring

Mask Technique

For mask-based laser ablation the laser radia-

tion is formed and homogenized by a telescope

and projected onto a mask representing the

desired intensity distribution. The mask is

imaged by an objective to the final intensity

distribution. The transversal miniaturization m

is given by:

jmj¼

g  f

; ð5Þ

This relation is known from the optical geom-

etry, with the focal length of the lens f and the

distance of the mask to the lens g (Fig. 4-4).

The precision of the working tool is des-

cribed by Abbe’slaw,whichgivesthetheoretical

resolution limit Dx of an objective for non-

coherent radiation:

Dx ¼ 1:22

¼ 0:61

ð6Þ

and depends on the objective diameter D

and on

the numerical aperture of the objective NA.

Scribing Technique

Scribing is a common technique for structuring

with laser radiation. Similar to milling, the work-

ing tool ‘laser radiati on’ is moved relatively to the

substrate and remo ves matter in the focal regime

by melting, vaporization or photo-ablation.

The spatial intensity distribution of the laser

radiation depends, among other things, on the

applied laser resonator. Of special importance

for micro-structuring concerning focusability is

FIGURE 4-4 Principle of mask projection imaging. Laser radiation is collimated and homogenized by a telescope and a

homogenizer to illuminate the mask. The mask itself is imaged on the target.

FIGURE 4-3 Comparision of nanosecond laser ablation (left) and picosecond laser ablation (right) of steel.

CHAPTER 4 Laser Micro-Structuring 63

the radiation, which should exhibit a spatial

Gaussian intensity distribution. Micro-structur-

ing, applying this radiation, benefits the property

of Gaussian radiation to be focused to the smallest

possible value, called the diffraction-limited

focus, resulting in a beam radius of:



2lf



ð7Þ

with l the wavelength of the applied radiation, f the

focal length of the objective and D the beam diam-

eter of the radiation close to the objective. The

working tool diameter is given by 2w

and defines

the working tool precision. The numerical aperture

is given by NA = D/f for objectives with large focal

length (f >> 15 mm) and by NA = n sin u for

microscope objectives (f < 15 mm), with the refrac-

tive index of the medium between objective and the

substrate and u the aperture angle of the objective.

Microstructures, like a cavity, are generated by

ablation with laser radiation displacing the laser

radiation relative to the workpiece and carrying

out a meander trajectory, removing the material

in layers (Fig. 4-5). In general, the laser displace-

ment of the laser beam is realized by high speed

galvanometer mirrors. Focusing of the laser beam

is made by either adjustable lenses set before the

scanning mirrors or by telecentric lenses.

The ablation rate per length depends on the

ratio between the applied intensity to the thresh-

old intensity for ablation and on the overlap, o:

o ¼ 1 

2 f

ð8Þ

where w

represents the processing diameter, v

the relative velocity, and f

the pulse repetition

rate. The processing diameter for mechanical

ablation (e.g. milling) is given by the tool diameter

whereas the processing diameter for laser ablation

is given by the focal diameter (tool diameter) and

the process precision, described below. Applying

laser radiation for micro- and nano-structuring,

the approp riate tool diameter is obtained by

reducing the wavelength and increasing the

numerical aperture. The last one is obt ained by

decreasing the focal length of the objective or by

increasing the beam diamete r close to the objec-

tive (example: l = 355 nm; NA = 0.5 ) w



0.7 mm).

Microscope objectives are often used for focus-

ing laser radiation to small focal diameters

<10 mm. The focal diam eter is varied during pro-

cessing by changing the objective, or changing the

beam diameter in front of the objective, or posi-

tioning the laser radiation extra focally. Compared

to mechanical milling, laser radia tion is contact

FIGURE 4-5 Principle of 3D laser machining by laser scribing technology.

64 CHAPTER 4 Laser Micro-Structuring

free and mass-less, resulting in reduced delay

times for positioning of the working tool.

Mechanical micro-milling with a tool diameter

<100 mm exhibits positioning times in the range

of minutes whereas laser radiation is positioned

within fractions of seconds.

EXAMPLES

In the production of micro-scaled products and

products with micro- and nano-scaled surface

functionalities, laser ablation becomes an even

more important tool which is able to generate

structure sizes in the range of 10–100 mmand

with new machining strategies, even within a

range smaller than one micrometer. 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 energies, the sur-

face quality of metal micro-ablation has been

increased significantly. With a combination of

ultra-short pulse s and high resolution interference

methods, structures with dimensions of less than

300 nm can be generated, either in polymer parts

directly, or in steel replication tools. For mass

replication of those structures to achieve improve-

ments in wetting capabilities or optical properties

of surfa ces, a new laser supported embossing tech-

nology has been developed.

The availability of these new process variants

and improved beam sou rces qualifies the laser

as a universal tool, especially in the area of

FIGURE 4-6 Laser manufactured micro-injection molding tool insert.

FIGURE 4-7 Micro-structured replication tool for self-cleaning surfaces (left). Super lotus effect on micro-structured

polymer surface (right).

CHAPTER 4 Laser Micro-Structuring 65

micro-mold processing (Fig. 4-6). The wide vari-

ety of materials processed by lasers range from

hard materials such as tungsten carbide for tool

technology to ceramics, glass and diamond, with

accuracies of bett er than 10 mm, which make the

technology useful for the processing of innovative

tools, and also for high temperature mass repli-

cation processes.

With the newly developed laser ablation tech-

nology using ultra-short pulsed lasers in the pico-

second range, accura cies of better than 5 mm and

surface roughnesses of less than 0.5 mm can be

provided for the manufacturing of micro-molding

tools.

Using this new laser ablation technology opens

a new field in the micro-processing of tools, which

allows the manufacture of micro- and even nano-

scaled functional structures on polymer, metal

and glass components. With laser ablation using

picosecond lasers, a machining technology with

accuracies <1 mm is available which can be

applied to all kinds of materials. Figure 4-7 shows

the surface of a replication tool, whereby laser

ablation in tungsten carbide micro-pits with sizes

of between 1 and 5 mm have been produced. Due

to the pro cess characterist ics of laser ablation,

sub-micrometer scaled substructures are pro-

duced, which further increase the surface area.

Tools like this have been successfully tested for

the replication of polymers to achieve functional

surfaces. In Fig. 4-7 the result of a wetting test

shows that by micro-structuring, a super hydro-

phobic effect can be produced.

By mask technique ablation with UV-excimer

lasers, polymer parts especially can be produced

in a very flexible way. Among others, micro-flu-

idic systems (Fig. 4-8) requ ire very small chan nels

and reservoirs in the geometry range of several

10 mm. With mask-based excimer ablation a tool

for rapid prototyping is available for the manu-

facture of single parts in small lot sizes [5].

CONCLUSIONS

Laser micro-a blation has been shown to be a

versatile tool in the machining and production

of micro-parts and micro-replication tools. Using

ultra-short pulsed lasers with material-adapted

wavelengths the quality of the ablation process

can be increased significantly compared to long

pulses. With an optimized energy deposition, a

significant reduction of the surface roughness

down to R

< 0.7 mm could be achieved over a

wide parameter range, the best achieved rough-

ness being R

= 0.5 m m. Using this optimized

parameter, laser ablation can be used for the gen-

eration of micro-replication tools as well as for the

direct manufacturing of small parts in different

materials such as ceramics, steel and sintered

metals showing the potential for high precision

tool manufacturing for high accuracy parts.

REFERENCES

[1] J.-C. Diels, Ultrashort Laser Pulse Phenomena, Aca-

demic Press, Boston (1996).

FIGURE 4-8 Micro-fluidic channels ablated with excimer laser radiation.

66 CHAPTER 4 Laser Micro-Structuring

[2] J. Jandeleit, A. Horn, R. Weichenhain, E.W. Kreutz,

R. Poprawe, Fundamental investigations of micro-

machining by nano- and picosecond laser radiation,

Applied Surface Science 127 (1998) 885–891.

[3] F. Korte, et al., Sub-diffraction limited structuring of

solid targets with femtosecond laser pulses, Optics

Express 7(2) (2000) 41–49.

[4] H.-G. Treusch, Geometrie und Reproduzierbarkeit

einer plasmaunterst

€

utzten Materialabtragung durch

Laserstrahlung, Dissertation, TH Darmstadt, (1985).

[5] M. Wehner, Excimer Laser Technology, Edt.: D.

Basting, Lambda Physik AG G

€

ottingen (2001).

[6] R. Poprawe, A. Gillner, D. Hoffmann, J. Gottmann,

W. Wawers, W. Schulz, High speed high precision

ablation from ms to fs, Proc. SPIE 7005, 12 S, 200.

[7] A. Gillner, J. Holfkamp, C. Hartmann, A. Olo-

winsky, J. Gedicke, K. Klages, L. Bosse, A. Bayer,

Laser applications in microtechnology, J. of Materi-

als Processing Technology 167 (2005) 494–498.

[8] G.J. Schmitz, C. Br

€

ucker, P. Jacobs, Manufacture of

high-aspectration micro-hair sensor arrays, J. of

Micromechanics and Microengineering 15 (2005)

1904–1910.

[9] M. Wehner, S. Beckemper, P. Jacobs, S. Schillinger,

D. Schibur, A. Gillner, Processing of polycarbonate

by high-repetition rate ArF excimer laser radiation,

Proceedings of Lasers in Manufacturing 3 (2005)

557–561.

[10] E. Bremus-K

€

obberling, U. Meier-Mahlo, O. Henken-

johann, S. Beckemper, A. Gillner, Laser structuring

and modification of polymer surfaces for chemical

and medical micro components, Proceedings of SPIE

5662 (2004) 274–279.

[11] E. Bremus-K

€

obberling, A. Gillner, Laser structuring

and modification of surfaces for chemical and medi-

cal micro components, Proceedings of the 4th Inter-

national Symposium on Laser Microfabrication LMP

4 (2003) 1–5.

[12] C. Hartmann, T. Fehr, M. Brajdic, A. Gillner, Inves-

tigation on laser micro ablation of steel using short

and ultrashort IR-multipulses, LAMP 2006, Kyoto,

Japan (2006).

[13] C. Hartmann, A. Gillner, U

. Aydin, et al., Investiga-

tion on laser micro ablation of metals using ns-multi-

pulses, J. Physics 59 (2007) 440–444.

CHAPTER 4 Laser Micro-Structuring 67

Hot Embossing

Matthias Worgull

INTRODUCTION

A further distrib ution of applications with inte-

grated micro-features or micro-systems requires

technologies to replicate the corresponding

micro-structures. In addition to the structuring

of silicon, glass or metals by different structuring

processes, the replication of micro-structures is

based typically on polymers. Polymers, especially

thermoplastic polymers, are commercially avail-

able and are characterized by a large bandwidth of

properties. Here for most applications a suitable

polymer which fulfills the requirements of the

application can be found.

Hot embos sing is one of the established repl i-

cation technologies for the replication of a micro-

structured master, a so-called mold insert. The

available replication processes are listed below

[1–4]:

Micro-reaction injection molding (RIM)

Micro-injection mol ding

Micro-injection compr ession molding

Micro-hot embossing

Micro-thermoforming

Nano-imprint lithography processes (NIL).

None of these processes are in competition

with each other, since they have their own specific

characteristics and merits. Therefore, depending

on the requirements, like stress in molded parts,

flow length, number of replications or cost effec-

tiveness, the replication process can be deter-

mined. Nevertheless, the design and arrangement

of the structure s will define the suitable replica-

tion technology. Hot embossing is a technology

which is suitable for the replication of a large

bandwidth of structures, especially structures

with high aspect ratios. In this chapter such a

process is presented. The technology of hot

embossing machines, hot embossing tools and

examples of typical applications will unde rline

the merits and flexibility of hot embossing.

The replication techniqu e of hot embossing of

micro-structures can be traced back to the second

half of the 20th century. Already in 1970 hot

embossing had been implemented by a group of

researchers from RCA laboratories at Princeton,

NJ, USA [5]. The objective of their work was to

develop a low cost reproduction technique of sur-

face hologram motion pictures for television play-

back. A master tape was made by electr oplating

nickel into photo resist patterns. The master was

run through heated rollers together with a vinyl

tape and, thus, the micro-structure was trans-

ferred into the vinyl. The molded structures were

characterized by a depth of 0.1 mm and a lateral

resolution of 1 mm. In 1978 Gale et al. [6] used a

hot embossing technique for the replication of

surface relief structures for color and black-and-

white reproductions . These relief phase grating

structures refer to a recording of light in zero

order diffraction. These structures, with a height

of 1–2 mm and a spacing of around 1.4 mm, were

replicated from a nickel mold insert into transpar-

ent PVC by hot embossing at a molding tempera-

ture of 150



C and a pressure of 0.3 MPa. The

structures were also replicated in polycarbonate

(PC) and acetate.

The structures replicated by hot embossing

today are characterized by high aspect ratios,

CHAPTER

smaller feature sizes down to the nanometer

range, and vertical sidewalls with a surface rough-

ness below 40 nm (Fig. 5-1).

Before the hot embossing process was estab-

lished in micro-structure technology previous

experiments were done by reaction injection

molding. This process enables injection of mixed

liquid monomers and a starter into a micro-

structured mold and the molding occurs by poly-

merization in the mold [7]. Micro-structures of

PMMA and PA (polyamide) were molded success-

fully, but at the beginning of these experiments the

filling and the demolding of filigree micro-struc-

tures with high aspect ratios was a challenging

aspect. Nevertheless, because of the difficulties

in controlling the polymerization and the high

shrinkage of the molded part, the hot embossing

technique based on polymer foils was inves ti-

gated. Beside fundamental tests wi th micro-injec-

tion molding, basic experiments in 1989 with an

embossing technique of PMMA material marked

the start of the use of micro-hot embossing for the

production of LIGA structures [8] (LIGA is a

German acronym for lithography, electroplating

and molding –

Lithographie, Galvanik, Abfor-

mung). With the further development of the LIGA

technique the development of the hot embossing

technique also proceeded. At the beginning

the technique was based on laboratory machines

with a low grade in automation which required

the control of every mol ding step by the user. The

precision of the molding machine regarding the

press force, the molding velocity, and the temper-

ature distribution was also improved. A milestone

in the development was the use of co mputer con-

trolled tensile testing machines for hot embossing

applications. Such machines, already available in

many material testing laboratories, can fulfill the

requirements for micro-hot embossing, including

high stiffness of the machine, precise motion, the

control system for force and velocity combined

with a measurement system, and an interface to

the user. An integration of a molding tool with

a heating and cooling system completed the

required components for hot embossing. Subse-

quently, this concept was further developed into

commercially available machines . Together with

further development of the hot embossing tech-

nique, these kinds of machines are now part of

state-of-the-art hot embossing technology [9].

Apart from the developments based on the use

of tensile testing machines, new concepts based

on the use of the hydraulic drives were also estab-

lished [10]. There are also other types of machines

existing in various research laboratories and

industry, as a result of independent development

and/or customized applications.

On the basis of the hot embossing technology

available at the presen t time, hot embossing is a

well-established process in industry and science

with a large bandwidth of process variations.

The replication of Fresnel lenses for overhead pro-

jectors or concentrating solar systems [11] is also

established, like the replication of CDs by the

FIGURE 5-1 In PMMA replicated structures with high aspect ratios.

CHAPTER 5 Hot Embossing 69

related process of injection compression molding.

The development of nano-imprint lithography

with thermal and UV curable materials opens a

way for hot embossing technology to be extended

to nano-structuring methods. A new level of dif-

ferent applications, such as surface modifications

in the nano-range or the replication of structures

in the wavelength dimensions of light, will pave a

way to a new class of applications. The develop-

ment of the hot embossing technique is closely

linked to the development of structuring techni-

ques for the fabrication of mold inserts. Today,

structures of mold inserts in a range below

10 nm can be fabricated. The hot embossing

process in combination with polymers is well

suited for the replication of these structures

and it can make contributions to the volume

production of new a pplications based on

nano-structuring. One of the key merits is its

flexibility in set-up, which allows for convenient

changing of the mold and molding material.

Therefore, hot embossing is also a popular process

in laboratories for micro-structuring. The process

is characterized by a large bandwidth of possible

process variations, such as double-sided aligned

molding, molding of through-holes, molding of

polymer stacks, thermoforming by polymer melts

or structuring by hot punching. These variants

offer attractive characteristics for micro- and

nano-replications and for the development of

micro- and nano-systems.

HOT EMBOSSING PROCESS

To illustrate the fundamentals of the process, the

case of single-sided hot embossing is described

below. The principle of a one-sided hot embossing

cycle is represented schematically in Fig. 5-2.

Between the micro-structured mold insert and a

metal plate with a rough surface, the so-called

substrate plate, a semi-finished product, i.e. a

polymer foil, is positioned. The thickness of the

foil exceeds the structural height of the tool. The

surface area of the foil covers the structured part

of the tool. The tool and substrate are heated

under vacuum to the polymer molding tempera-

ture. When a constant molding temperature is

reached, the two steps of the molding cycle are

initiated. In a first step, the mold insert and sub-

strate are moved towards each other (in the range

of 1 mm/min) until the preset maximum emboss-

ing force is achieved. In a second step, the emboss-

ing force achieved is held at a constant value for a

defined holding time. To generate a constant

force, the rel ative movement between the tool

and substrate has to be controlled. The force is

now kept constant over an additional time period

(packing time, holding time). During this period,

the plastic material flows in the radial direction

and the residual layer will be reduced under the

acting constant force (packing pressure). At the

same time, the tool and substrate move further

towards each other, while the thickness of the

FIGURE 5-2 Schematic view of the hot embossing process.

70 CHAPTER 5 Hot Embossing