Qin Y. Micromanufacturing Engineering and Technology

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due to their achievable focus diameter opens up

new fields of use in micro-mechanics and micro-

electronics, such a s the replacement of thermo-

sonic ribbon bonding for high power electronics.

REFERENCES

[1] L. Bosse, N. G

€

obbels, A. Olowinsky, A. Gillner, R.

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systems with modular Pick-&-Join-tools, Proc. Third

Int. WLT-Conf. on Lasers in Manufacturing LIM

(2005) 785–789.

[2] G. Humpston, D.M. Jacobson, Principles of Solder-

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[3] C.F. Bohmann, The laser and microsoldering, SME

Technical + Paper No. 10: AD74-810 (1974).

[4] F. Burns, C. Zyetz, Laser microsoldering, Electronic

Packaging & Production(1981).

[5] D. Chang, Experimental investigation of laser beam

soldering, Welding J. 65 (10) (1986) 33–41.

[6] F.G. Meyer, Laserl

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oten unter besonderer Ber

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tigung der SM-Technologie und des L

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[7] R. Vanzetti, A.S. Dustoomian, Intelligence comes to

laser soldering, Electronics(July 1986) 75–77.

[8] F.G. Meyer, B.H. Klimt, Laser soldering of surface

mounted devices, Proc. SPIE 744 (1987) 8–90.

[9] R. Keeler, Lasers for high-reliability soldering – for

soldering military, computer, and other high-rel pro-

ducts lasers surpass mass-soldering methods, Elec-

tronic Packaging & Production 27 (10) (1987).

[10] M. Haag, M. Brandner, Diodenlaser – innovatives

Werkzeug f

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ur die Produktion, Proc. LaserOpto 3

(2000) .

[11] L. Bosse, A. Koglin, A. Olowinsky, V. Kolauch, M.

Nover, Laser beam soldering – an attractive alterna-

tive to conventional soldering technologies, Proc.

SPIE 4977 (2003) 473–480.

[12] L. Bosse, A. Gillner, R. Poprawe, Temperature con-

trolled selective soldering with laser radiation, Proc.

of the 6th International Symp. on Laser Precision

Microfabrication (2005).

[13] L. Bosse, A. Schildecker, A. Gillner, R. Poprawe,

High quality laser beam soldering, Microsystem

Technologies 7 (2002) 215–219.

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power profile for laser beam soldering with solder

paste, Proc. SPIE 4406 (2001) 76–81.

CHAPTER 12 Laser Beam Micro-Joining 201

Deep X-Ray Lithography

Pascal Meyer, Joachim Schulz and Volker Saile

INTRODUCTION

By far the leading technology for the manufactur-

ing of MEMS devices is silicon micro-machining

with its various derivatives. However, many

applications of micro-systems have requirements

in respect of materials, geometry, aspect ratio,

dimensions, shape, accuracy of micro-structures,

and number of parts that cannot be fulfilled easily

by mainstream silicon-based micro-machining

technologies. LIGA, a German acronym for Lithog-

raphy (Lithographie), Electroplating (Galvanofor-

mung ), and Mol ding (Abformung) enables the

highly precise manufacture of high aspect ratio

micro-structures with large structural thickness

ranging from hundreds to thousands of micr ons.

Thes e tall micro-structu res can be produced in a

variety of materials with well-defined geometry

and dimensions, very straight and smooth side

walls, and tight tolerances. LIGA technology is

also well suited for the mass fabric ation of parts,

particularly in polymers. Many micro-syst ems

benefit from the unique char acteristics and

advantages of the LIGA process in terms of prod-

uct performance. In this chapter the strengths of

the manufacturing method and its mai n fields of

application are emphasized, with examples taken

from various groups worldwide, especially in

micro-mechanics and micro-optics.

Several micro-fabrication technologies are

available at the present time and are used to fab-

ricate micro-components and -systems. The most

successful micro-machining technologies have

been developed as extensions of standard IC and

micro-electronics planar silicon-based processing.

Others are based on advanced precision engineer-

ing and laser structuring. However, individual

technologies including Si micro-machining or laser

structuring are far from being sufficient to fulfill

the needs of the variety of problems posed by:

the great variety of functions of most of the

devices to be made;

the conditions of the surroundings in which

they will operate;

the optimum cost/performance ratio for the

targeted application.

Interest in a number of non-Si-based machin-

ing methods stems from major deficiencies of IC-

based machining techniques:

the need for using application-specific materi-

als to optimize the functions and performance

of various devices;

the need to reduce cost by choosing low cost

materials;

the difficulty in constructing truly 3D objects

with planar-based processing, which con tinues

to be a challenge.

Precision and ultra-precisi on mechanical,

electro-discharge, LIGA-based, and laser-based,

micro-machining techniques, to mention the most

current, are such alternative techniques, each with

its specific application domains and relative mer-

its. LIGA-based processing, a sequence of micro-

fabrication steps combining a step of deep X-ray

lithography (DXRL), also called by some authors

‘deep etch X-ray lithography’, and subsequent

additive processing of plating-through-mask and

molding, has moved from the position of emerging

from micro-fabrication technology to become

CHAPTER

202

a well-established non-silicon alternative micro-

fabrication technology for MEMS.

LIGA technology provides unique advantages

over other manufacturing methods in the fabrica-

tion of micro-structures. LIGA-based technolo-

gies are used and are being further developed in

a number of R&D institutes around the world.

Spin-off companies and commercial companies

have also evolved around large-scale synchrotron

facilities. Also, commercial applications of the

LIGA process are on the market. LIGA technol-

ogy has been developed over the rather long time

span of two decades [1–4]. During this time other

high aspect ratio technologies such as UV photo-

lithography in thick resist, for example as SU-8,

often referred to as UV -LIGA, and Deep Reactive

Ion Etching (DRIE) of silicon have also evolv ed

and challenged LIGA successfully in some specific

application areas. For planning the role of LIGA

in future manufacturing, a review of potential

applications may serve as a useful basis. The basic

LIGA process and some aspects of the process are

recalled here to illustrate its strengths and discuss

still existing challenges.

THE LIGA PROCESS AND ITS

STRENGTHS

The basic LIGA process is described in Fig. 13-1 .

In the first step of the process, an X-ray-sensitive

polymer (resist) layer of up to several mi llimeters

thickness is coated onto a conductive or non-

conductive substrate. Typically polymethylmetha-

crylate (PMMA) is used as positive resist and an

epoxy-based resist SU-8 [5–9] as negative resist. A

pattern from a mask is then transferred into the

thick resist layer via a 1:1 shadow-proximity print-

ing scheme using hard X-rays from a synchrotron

radiation source. After exposure, selective disso-

lution of the chemically modified irradiated parts

of the positive resist (or dissolution of the non-

irradiated parts of the negative resist) in a chemical

developer results in a polymeric relief replica of the

mask pattern.

Then, depending on the material, number of

parts selected for the final product, accuracy,

quality and price, different fabrication routes

can be chosen, which may include further steps

of mi cro-replication through electroforming and/

or a variety of molding techniques (injection

molding, embossing, casting, compression mold-

ing, etc.). The polymeric micro-structure can be

used:

simply as it is;

as a lost mold for the formation of ceramic

micro-parts;

as an electroplating template to generate metal-

lic micro-parts;

as an ele ctroplating template to prod uce a

metallic master-mold, which can then be used

many times to mold cost-effective replicas in

other materials, primarily polymers. When

producing large numbers of electroplated com-

ponents, the molded polymer parts are used as

lost molds for a second plating pro cess.

The unique processing feature that enables the

manufacture of thick micro-structures character-

ized by very steep walls and very tight tolerances is

the creation of a high precision resist template by

deep X-ray lithography using X-ray photons from

a synchrotron radiation source.

LIGA MANUFACTURING STEPS

Resist Technology

The resist technology consists of applyin g a resist

onto a substrate, the substrates that can be used

depend on the product to be made (for example, a

mold insert), but they should meet the following

criteria:

a high planarity;

the substrate and resist should have good adhe-

sion;

the substrate surface should be conductive if an

electroplating step is needed;

the resist developer should not etch the sub-

strate.

Concerning the resist, a distinction should be

made between positive and negative resist. In the

first case, the radiation will da mage the polymer

by reducing its molecular weight; the most com-

monly used being polymethylmetacrylate

(PMMA); the irradiates parts becom ing soluble

CHAPTER 13 Deep X-Ray Lithography 203

in a developer. In the case of a negative resist, the

radiation will ‘damage’ the polymer by increasing

its molecular weight (curing/cross-linking); the

irradiated volume being insoluble in a developer;

the most commonly used of which is an epoxy-

based resin: the SU-8. In the first case, it is neces-

sary to know the dose to apply to the resist so that

it becomes soluble in the developer; while the

developer must have a negligible influence on

the non-irradiated parts; in the second case, it is

necessary to know the dose to apply to the resist so

that the exposed part becomes insoluble in the

developer. PMMA is still the resist of choice for

many applications; this resist offering very high

resolution and stable processing parameters. Its

sensitivity is low, which means high exposure

costs. SU-8 is a very attractive resist; its sensitivity

is high, reducing exposure times by a factor of 100

FIGURE 13-1 Illustration of basic LIGA process steps.

204 CHAPTER 13 Deep X-Ray Lithography

compared to PMMA. However, SU-8 actually

offers an unreliable performance due to the high

sensitivity of processing parameters’ fluctuation.

Resist Deposition

Concerning PMMA, two possibilities exist:

1. Gluing a commercially available PMMA sheet

with a glue consisting of PMMA dissolved in

methylmetacrylate (MMA). Prior to gluing,

the PMMA sheet is cut and milled to the

desired dimensions, the stress so induced

being removed by annealing under controlled

ramping conditions (the max temperature is

slightly above the glass transition tempera-

ture of the PMMA used). The positioning of

the sheet and the dispersing of the glue can

be made using a robotic dispenser combined

with a pick-and-place machine. This method

is generally used for PMMA thicknesses greater

than 100 mm.

2. Casting. A resin consisting of PMMA dis-

solved in MMA is mixed with di-benzoylper-

oxide and dimethylaniline, and is applied to

the substrate and hardened under a pressure

of 4 bar at room temperature for 4 h; the

dispensing can be effected using a robot dis-

penser. Subsequently, the resist-coated sub-

strate is annealed under ramping conditions.

This method is generally used for PMMA

thicknesses of less than 100 mm.

The SU-8 is spin coated. Expe rimental results

indicate that the coating qualities of SU8 are

affected by several factors, including the spinning

speed, the photoresist viscosity, the initial accel-

eration and the duration. After the resist has been

applied to the substrate, it must be soft baked to

evaporate the so lvent and densify the film.

Irradiation Technology

LIGA technology needs a synchrotron beam-

line to perform the resist exposure, a scanner

to move the sample through the line shaped

beam, and a c omputer program to c alculate the

dose.

Synchrotron Source – Scanner. At the heart

of a synchrotron (see Figs. 13-2 and 13-3) is a

storage ring: a huge donut -shaped vacuum cham-

ber. Electrons are accelerated and confined to

travel around the storage ring at nearly the

speed of light [10–12]. Because the electrons are

constantly forced to travel in a closed loop they

are accelerated, and accelerated electrons lose

energy in the form of synchrotron light. A

synchrotron produces light of exceptional quality

and brightness: a million times more intense

than that of a hospital X-ray machine. A

FIGURE 13-2 Schematic view of a synchrotron (source: Soleil).

CHAPTER 13 Deep X-Ray Lithography 205

charged particle that is constrained to move in a

curved path experie nces centripetal accelera-

tion. Due to this acceleration, the particle radi-

ates energy according to the Maxwell equations.

A non-relativistic partic le emits radiation pri-

marily at its frequency of revolution. However,

as the speed of the particle approaches the speed

of light, the radiation pattern is distorted by

relativistic effects a nd changes to a narrow cone

of radiation with angular spread. The total

energy E for a particle of mass at rest m

moving

at velocity n is:

E ¼ gm

c

ð1Þ

with

g ¼ 1



ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

1 



ð2Þ

The opening angle Dw of the radiation cone can

be expressed, for high values of g, as:

Dw ﬃ g

1

ð3Þ

Synchrotron radiation sources produce

photons with a continuum of energies, from the

infrared to the X-ray region. The spectral range of

photons produced by electrons (energy E

(GeV))

in a bending magnet (radius of curvature: R

magnet field: B (T)) can be characterized with

the critical energy E

, where:

cg

2R

ð4Þ

In practical units, the critical photon energy is

also given by:

ðKeVÞ¼0:6650E

ðGeVÞBðTÞð5Þ

is defined in terms of the spectral power radi-

ated by a relativistic particle: half of the power

spectra is radiated at energies below E

and the

other half at energies above it. A wiggler (a series

of magne ts designed to periodically horizontally

deflect the charged particle) and an undulator (a

series of magnets designed to periodically verti-

cally deflect the charged particle) are also used to

produce synchrotron radiation, the differences of

which are indicate d in Fig. 13-4.

Presented in Ta ble 13-1 is a non-exhaustive

list of synchrotrons with install LIGA beam-

lines.

For example, the synchrotron ANKA in Karls-

ruhe (Germany) is a third-gene ration, medium-

FIGURE 13-3 View of the synchrotron ANKA.

206 CHAPTER 13 Deep X-Ray Lithography

energy source, with an electron beam energy of

2.5 giga electron volts. The X-rays are sent into

the various beam-lines (the straight lines branch-

ing out of the synchrotron). A typical LIGA beam-

line is presented in Fig. 13-5. All of the beam-lines

are equipped with a scanner (see Fig. 13-6), and

optical elements which modify the spectrum of

the source. In fact, the synchrotron spectrum

should be adapted to the needs of the user. The

synchrotron ANKA has three LIGA beam-lines

(LIGA1, LIGA2, LIGA3) sited on a bending mag-

net (1.5 T) with different optics. The characteris-

tics of the beam-lines are given in Table 13-2. The

spectra of the lines after the front end wind ow and

optics are given in Fig. 13-7. The X-rays produced

are highly parallel and to conserve this property

for the lithography aspect, the diffraction (Fres-

nel) effect should be as low as possible and also the

absorption of the photons should take place only

in the designed volume. In X-ray lithography,

typically X-rays in the 0.5–5 10

10

m region are

used, which interact with matter by the photoelec-

tric effect, the Com pton effect, and Rayleigh

scattering. The total effect depends on the cross-

section of the different possibilities of interaction.

The primary dose, which is about 95% of the

deposited dose, is due to the photoelectric effect

in PMMA. Incident photon energy will be dissi-

pated ultimately by secondary electrons generated

by impact ionization; the distance over which the

energy is spread should be as small as possible.

The resolution limit of X-ray lithography is a

TABLE 13-1

Synchrotron Location and Operating Energy (this is a Non-Exhaustive List)

Name BESSY ANKA ELETTRA CAMD AURORA

Country/City Germany/Berlin Germany/Karlsruhe Italy/Trieste USA/Baton Rouge Japan/Kyoto

Operating energy (GeV) 1.7 2.5 1.7 1.3 0.575

Nb. of LIGA beam-lines 2 3 1 4 4

FIGURE 13-4 Different sources of synchrotron radiation with some of their characteristics.

CHAPTER 13 Deep X-Ray Lithography 207

function of diffraction in the mask-to-sample gap,

and the effective range of the photo and auger

electrons related when an X-ray is absorbed. In

Fig. 13-8, it is shown how the spectrum (source

synchrotron ANKA; beam-line LIGA3) is attenu-

ated during the exposure of a 1400 mm PMMA

sample. The carbon and aluminum filters absorb

the low energy radiation to prevent overexposure

of the top surface; the primary dose to be depos-

ited in the resist should be situated in the region

bottom

to D

top

. This region has been determined

by many tests and is the best compromise between

exposure and development time. The mask mem-

brane (beryllium) is quasi-transparent, as it is

required. The absorber thickness should be a

minimum, defined by the threshold dose at which

the resist starts to dissolve.

PMMA Degradation. During X-ray irradiation

of PMMA, synchrotron light is absorbed in the

exposed PMMA area, which results in a chemical

modification; a scission of the polymer chain leads

to a radiation-induced degradat ion of the molec-

ular weight and becomes soluble in an organic

developer. By increasing the dose of radiation,

the average molecular weight decreases from an

initial value M

WDose¼0ðÞ

(about 1.5 10

g/mol) to a

minimum limiting value of between 2500 g/mol

and 3000 g/mol at a very high dose of radiation.

The degradation mechanism of radiation-excited

PMMA depends on the chemical structure of the

FIGURE 13-6 (a) Side and front (scanner open) view of the Jenoptik scanner – beam-line: ANKA-LIGA3, and (b) side

and front (scanner open) view of the Jenoptik scanner – beam-line: ANKA-LIGA3.

FIGURE 13-5 Schematic diagram of an X-ray lithography beam-line (ANKA-LIGA2).

208 CHAPTER 13 Deep X-Ray Lithography

resist and the exposure energy. The radiochemis-

try of PMMA is a complex mixture of consecutive

reactions including excitations, fissions, cross-

linking, recombinations, disproportions, rearran-

gements and transfer reactions. The most impor-

tant step of degradation is the scission of the

methyl ester group, which is responsible for the

major amoun t of the gases evolved. The rem ain-

ing polymer chain stabilizes after hydrogen

abstraction by formation of a double bond or by

chain scission. Radio-chemical degradation of

PMMA is categorized into two schemes:

1. About 80% of the main-chain scissions have

been observed after preceding side-chain deg-

radation. The radiation-excited polymer mol-

ecule splits off an ester side-chain. The

remaining chain radical stabilizes after hydro-

gen abstraction by the formation of a double

bond, or reacts by way of a main-chain scis-

sion. In the case of stabilization by hydrogen

abstraction, the molecule has one ester side-

chain less than before irradiation.

2. The remaining 20% of the main-chain scis-

sions are due to the direct decomposition of

the polymer into two macro-molecules.

Recombination of these fragments results in

the primary polymer molecule.

PMMA Development. The dissolution rate is a

function of molecular weight, which is related to

the initial PMMA molecular weight, the dose

and the main-chain scission yield [13–21]. This

reduction of the average molecular weight

causes the solubility of the resi st in the developer

to increase dramatically. A developer suitable

for PMMA in X-ray lithography, commonly

referred to as the GG developer, is composed

of 15 vol% deionized water, 60 vol% 2-(2-

butoxyethoxy)ethanol, 20 vol% tetrahydro-1-4-

oxazine and 5 vol% 2-aminoethanol. For X-ray

lithography process simulation to calculate

TABLE 13-2

Some Characteristics of the Three LIGA Beam-lines of the Synchrotron ANKA

Beam-line LIGA1 LIGA2 LIGA3

Window 175 mm beryllium 225 mm beryllium 350 mm beryllium

Optics single Cr mirror single Ni mirror no optics

Dedicated to X-ray lithography deep X-ray lithography Ultra-deep X-ray lithography

Structure height up to 100 mm 100 to 600 mm 600 to 2500 mm

FIGURE 13-7 Spectrum of the three LIGA beam-lines at ANKA, Germany.

CHAPTER 13 Deep X-Ray Lithography 209

high aspect ratio micro-structures and realistic

MEMS devices of the size order of mm, a mac-

roscopic resist dissolution and an easily m easur-

able approach are required, rather than to

describe the very complicated problem of micro-

scopic resist dissolution. In chemistry, many

models concerning polymer dissolution may be

found. Due to the fact that a dose profile is

deposited during X-ray lithography and the GG

developer consists of four components, these

models cannot be applied easily. The dissolution

rate is described by t he following equation (all

other parameters being constant):

ðÞ

¼ R

þ C M

W Dose¼DðÞ



b

ð6Þ

corresponds to the development r ate of an

unexposed resist M

WDose¼0ðÞ

is negligible

in the case of the PMMA-GG system), and C

and b are characteristic constants of the polymer

and solvent.

In reality, the relationship between the dissolu-

tion rate a nd the dose is influenced by a large

number of parameters related to chemical reac-

tion at the liquid (GG developer) – solid (PMMA)

interface, e.g. the PMMA molecule weight, the

developer temperature and the development

apparatus.

Special Computer Programs for X-ray

Lithography. Different computer programs have

been developed, which meet the requirements of a

LIGA X-ray beam-line [22,23]. The code cur-

rently pe rmits the computation of synchrotron

radiation from bending magnets, the effects of

the optical properties of materi als, and the nec-

essary parameters for the resist expos ure. The

basic calculations ne eded for synchrotron beam-

line design ar e related to the spectral character-

istics and to the mod eling of the optical elements

(mirrors,filters,beam-stop).Forexample,the

following calculations are performed: the dose

rate, th e dose profile from the top to the bottom

of the re sist, the exposure do se (the parameter

which should be given to the scanner which

moves the sample) and the time needed to develop

an irradiated resis- sample. In Table 13-3 are

listed computer programs especia lly dedicated

to deep X-ray l ithography.

FIGURE 13-8 Primary absorbed dose along the X-ray trajectory, including a 1400 mm thick PMMA foil X-ray mask with a

550 mm beryllium membrane and carbon and aluminum filters. The source is ANKA; the beam-line is LIGA3.

210 CHAPTER 13 Deep X-Ray Lithography