Applications of LIGA technology to precision manufacturing
of high-aspect-ratio micro-components and -systems: a review
Chantal Khan Malek
a,*, Volker Saile
b aLaboratoire de Physique et Me´trologie des Oscillateurs (LPMO)-CNRS 32 Av. de l’Observatoire, Besanc¸on 25044, France
b
Institut fu¨r Mikrostrukturtechnik (IMT), Universita¨t Karlsruhe and Forschungszentrum Karlsruhe GmbH, and ANKA, Angstro¨mquelle Karlsruhe GmbH, P.O. Box 3640, D-76021 Karlsruhe, Germany
Received 9 December 2002; revised 22 September 2003; accepted 3 October 2003
Abstract
The by far leading technology for manufacturing MEMS devices is Si-micromachining with its various derivatives. However, many
applications of microsystems have requirements on materials basis, geometry, aspect ratio, dimensions, shape, accuracy of microstructures,
and number of parts that cannot be fulfilled easily by mainstream silicon-based micromachining technologies. LIGA, an alternative
microfabrication process combining deep X-ray lithography, plating-through-mask and molding, enables the highly precise manufacture of
high-aspect-ratio microstructures with large structural height ranging from hundreds to thousands of micrometers thick. These tall
microstructures can be produced in a variety of materials with well-defined geometry and dimensions, very straight and smooth sidewalls,
and tight tolerances. LIGA technology is also well suited for mass fabrication of parts, particularly in polymer.
Many microsystems benefit from unique characteristics and advantages of the LIGA process in terms of product performance. The LIGA
technology is briefly reviewed. The strengths of the manufacturing method and its main fields of application are emphasized with examples
taken from various groups worldwide, especially in micromechanics and microoptics.
q
2003 Elsevier Ltd. All rights reserved.
Keywords: LIGA; 3D-micromachining; High-aspect-ratio; Micromechanics; Microoptics; Microfluidics
1. Introduction
Several microfabrication technologies are available
today and are used to fabricate microcomponents and
systems. The most successful micromachining technologies
have been developed as extensions of standard IC and
microelectronics planar silicon-based processing. Others are
based on advanced precision engineering and laser
structur-ing. 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 devices to be
made,
† The specificity of surroundings in which they will
operate,
† The optimum cost/performance ratio for the targeted
application.
Interest in a number of non-Si based machining methods
stems from major deficiencies of IC-based machining
techniques:
† The need for using application-specific materials to
optimize the functions and performance of various
devices,
† The need to reduce cost by choosing low-cost materials,
† The difficulty to construct truly 3D objects with
planar-based processing continues to be a challenge.
Precision and ultra-precision mechanical,
electro-dis-charge, LIGA-based, and laser-based, micromachining
techniques, to mention the most current ones, are such
alternative techniques, each with their specific application
domains and relative merits. LIGA-based processing, a
sequence of microfabrication steps combining a step of deep
X-ray lithography
[1]
[(DXRL), also called by some authors
0026-2692/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2003.10.003
www.elsevier.com/locate/mejo
* Corresponding author.
E-mail addresses: [email protected] (C. Khan Malek), [email protected] (V. Saile).
deep etch X-ray lithography], and subsequent additive
processing of plating-through-mask and molding
[2]
, has
moved from emerging microfabrication technology to
well-established non-silicon alternative microfabrication
tech-nology for MEMS. The LIGA techtech-nology provides unique
advantages over other manufacturing methods in the
fabrication of microstructures. LIGA-based technologies
are used and 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. Commercial application of the
LIGA process is occurring.
This short review will not go back to the physical and
technological fundamentals of the technique
[3 – 5]
; it is
rather intended to recall the essential steps of the process
sequence and focus on a number of selected examples from
recent work performed in various LIGA groups around the
world and show the usefulness and advantages of this
technology.
The LIGA technology has been developed over a rather
long time span of two decades. During that time other
high-aspect-ratio technologies such as UV photolithography in
thick resist like SU8, often referred to as ‘UV – LIGA’ and
Deep Reactive Ion Etching (DRIE) of silicon have evolved
as well and challenge LIGA successfully in some specific
application areas. For planning LIGA role in future
manufacturing, a review of potential applications may
serve as a basis.
The fabrication of LIGA-parts concerning in particular
the lithographic aspect, materials base expansion through
replication technologies such as electroplating and
mould-ing and some associated challenges, as well as some
materials issues was reviewed in a former article by one of
the authors
[6]
.
1The basic LIGA process and some aspects
of the process are recalled here to illustrate its strengths and
discuss challenges, not in terms of materials properties
[7]
but in terms of applications. The purpose of this article is
thus providing input on the discussion of the LIGA potential
by summarizing proposals and ideas for LIGA applications
found in literature.
2. LIGA process and strengths
2.1. Basic process
The basic LIGA process is described in
Fig. 1
. In the first
step of the LIGA process, an X-ray sensitive polymer (resist)
layer up to several millimeters thick, typically
polymethyl-methacrylate (PMMA) is coated onto a conductive substrate.
A pattern from a mask is therefore transferred into the thick
resist layer via a 1:1 shadow proximity printing scheme using
hard X-rays from a synchrotron radiation source. After
exposure, selective dissolution of the chemically modified
irradiated parts of the resist layer in a chemical developer
results in a polymeric relief replica of the mask pattern. Then,
depending on the material and number of parts selected for
the final product, different fabrication routes can be chosen,
which may include further steps of microreplication through
electroforming and/or a variety of molding techniques
(injection molding, embossing, casting, compression
mold-ing, etc.). The polymeric microstructure can be used:
† Simply as-is;
† As a lost mold for the formation of ceramic microparts;
† As an electroplating template to generate metallic
microparts. The microstructures are often further defined
by precision lapping to control thickness.
† As an electroplating template to produce a metallic
master mold, which can then be used multiple times
to mold cost-effective replicates in other materials,
primarily polymers. When producing large numbers of
electroplated components, the molded polymer parts
are used as lost molds for a second plating process.
The unique processing feature that enables the
manu-facture of thick microstructures characterized by very steep
walls and very tight tolerances is the creation of highly
Fig. 1. (a) Illustration of the basic LIGA process steps. The six panels stand for lithography, polymer components after development, electroplating and overplating the polymer template, a metal mold, and replication. (b) Basic LIGA process sequence.
precise resist template by deep X-ray lithography using
X-ray photons from a synchrotron radiation source. Features
characterizing this process are listed below:
† As a result of their high energy, these X-rays are
capable of deeply penetrating thick (e.g. hundreds of
micrometers or even millimeters) layers of polymeric
resist, allowing uniform deposition of energy in the
depth of the resist and the formation of tall
microstruc-tures in one exposure step.
† The short wavelengths of X-ray photons provide high
resolution for patterning due to low diffraction effects.
† The very small vertical angular divergence of the X-ray
beam achieves high accuracy in pattern transfer from
the mask. Due to their excellent collimation the X-rays
penetrate thick resists with extremely low horizontal
run-out (less than 0.1 mm/100 mm thickness), thereby
producing the substantially vertical walls for which
LIGA structures are well known.
† The almost parallel (well collimated) light of X-ray
beams produced by synchrotron radiation sources also
allows printing with large depth-of-field. A large
working gap between mask and substrate can then be
used in non-traditional pattern transfer as for the
manufacture of slanted structures or for pattern
formation on substrates presenting a large topography.
† The vertical sidewalls are optically smooth with typical
local roughness of the order of 10 nm and longer-range
waviness such as slope errors or steps determined solely
by the accuracy of mask writing.
Some aspects in utilizing synchrotron radiation for
LIGA are:
† A widely perceived drawback in using LIGA-like
processes is that synchrotron radiation is the optimum
exposure source and thus requires access to large
accelerators. However, deep X-ray lithography is one of
the demanded services, which has led to improved storage
ring access to the point where it may be acquired as a
foundry service and can be provided as part of a
distributed manufacturing process, as for example ion
implantation. It is worth noting in this context that the cost
of lithography at synchrotron radiation sources is not a
limiting factor in the overall cost of a mold. Entirely
different is the case of ‘direct LIGA’
[8]
when each batch
of parts is produced with X-rays. Then access to a
synchrotron facility, intensity of the source, cost per hour
of X-rays, exposure field size, and others become crucial
factors for the economics of LIGA fabrication.
† Specialized equipment for the LIGA process is currently
commercially available (from companies such as, e.g.
Jena-Optics and Technotrans), besides the custom
equip-ment that various groups have developed. Companies and
institutes worldwide provide services for mask making
(e.g. ANKA GmbH), X-ray exposure (e.g. ANKA GmbH
and CAMD), development, electroplating (e.g. TVJ—
Dover Industrial Chrome, Inc., Dynamics Research
Corporation (DRC), etc.), post-processing, molding (e.g.
microParts GmbH, Mezzo Systems Inc.,), etc.
† Exposure cost is being dramatically reduced by the use of
a new very sensitive X-ray resists such as SU8 (which has
been developed as a UV resist for photolithography but
proved very sensitive to X-rays as well
[9]
).
† Little effort has been devoted up to now to optimize the
cost of exposure and access for LIGA applications on
synchrotron facilities that have originally been developed
for totally other applications. Therefore, the margin for
improvement in that field is very large
[10]
.
† Once the master mold is prepared, the replication steps
of microelectroplating and micromolding can be
carried out away from the synchrotron source. The
development and utilization of new application-specific
materials has become one of the key challenges for the
commercial production of microcomponents and
sys-tems. This applies, in particular, to the LIGA process
and is offered through various replication techniques.
Electroforming and various types of molding
tech-niques extend the materials basis. They also allow for
low-cost mass production.
The best strengths of the LIGA technology lie in several
key functional areas:
† Large to very large structural heights, typically from
hundreds to thousands of micrometers thick that can be
formed in one single step,
† Smallest lateral dimension of a few micrometers with
structural details in the sub-micrometer range,
† Access to a large base of functional materials:
electro-plated metals and alloys, molded polymers
[11]
,
ceramics, composites, multilayered materials,
graded-materials, nanograded-materials, etc. The development and
utilization of application-specific materials have become
one of the key challenges for the commercial production
of microcomponents and microsystems. This applies, in
particular, to the LIGA process and is offered through the
various replication techniques that extend the materials
basis.
† Formation of complex shapes,
*
Free lateral shape,
*
Mastering of the third dimension, not only by
achieving deep structures with high aspect ratios,
but also more complex structures involving multilevel
and oblique shape structures (see paragraph 3),
† Structural accuracy of features,
*
Good dimensional control over the entire structure
height,
*
Very precise shape definition of parts, both laterally in
terms of dimensional control and in term of
straight-ness and planarity of sidewalls,
*
Low surface roughness of side-walls (rms roughness
of 20 nm or better), which is an essential factor for
microoptical components
[12]
,
*
Typical feature sizes of several micrometers with
structural height of several 100 mm to several
1000 mm and sub-micrometer details are routinely
available
[13]
.
*
Extreme parallelism/verticality of sidewalls with
slopes of the order of 1 mrad.
† Mixing of scales (small features on large parts), which is
important for several applications that call for a
macro-or meso-scale main part with microscale features, e.g.
patterning of optical waveguides.
† Optical transparency of plastics, mainly PMMA-based,
within a wide range of wavelengths (typically between
200 and 900 nm) and low autofluorescence, which
makes
these
materials
well
suited
for
optical
applications.
† Large process latitude. In particular, X-ray lithography
functions with large depth of field, allowing for pattern
formation on non-planar surfaces
[14]
.
† Combination with CMOS type processes possible as well
as with other microfabrication techniques, e.g. high
precision mechanical engineering, silicon surface and
bulk micromachining, sacrificial layers, membrane
technique, etc.
† Collective manufacturing by parallel processing of parts
(batch fabrication) to reduce cost of fabrication.
† Possibility of high-volume low-cost production. The
original formulation of the LIGA process included
molding, by injection molding or hot embossing, as the
technology for mass fabrication.
† Finally, fab-lines for LIGA components have been
established complying with rigorous quality
manage-ment systems
[15]
.
2.2. Towards building 3D systems with LIGA
Lithography is often considered as a planar process step.
Deep X-ray lithography extends to a third out-of-plane
dimension through forming microstructures with high
vertical or oblique sidewalls. Typically LIGA structures
allow for the free choice of the lateral 2D pattern that is
projected into the third dimension to form prismatic or
cylindrical geometries. This technique has generally been
used to produce structures with straight walls. However, for
all major LIGA process steps, variations have been
developed that increase the fabrication flexibility.
Geo-metrical variations in that third vertical dimension are
possible and can be obtained in different ways by modifying
or combining process steps, in particular for producing
shapes with increased dimensionality. The 3D structural
construction with LIGA technique can be primarily
classified in two categories:
† One based on the sequential planar formation of
individual levels to form 3D structures
[16]
.
Fabrica-ting such 3D structures by multiple level, aligned
X-ray exposures requires additional processes such as
*
An accurate replanarization process such as
pre-cision lapping after each electroplating step
enabling subsequent resist application,
*
A mask-alignment procedure between each
con-ventional DXRL cycle with special alignment gear
and alignment marks on the mask and the substrate.
† The other with spatial variation of the absorbed dose.
Off-axis X-ray exposures and exposures where mask
and/or substrate are moved in non-standard schemes
during irradiation are capable of forming complex
three-dimensional structures. They require:
*
Tilting and/or rotation of the mask and/or substrate
system during exposure
[17 – 20]
or oscillation of the
mask
[21]
; the use of specialized exposure fixtures or
sophisticated stages permit angular positioning and
rotation of the substrate and/or mask during X-ray
exposure. The capability of nearly arbitrary angled
resist patterning allows for the formation of multiple
angled cylindrical surfaces and a wide range of
geometries.
*
Software packages modeling the dose deposited
inside the resist with complex exposure scheme.
The technique can also be extended to the manufacture of
3D, more complex systems by:
† Stacking and assembling various levels of
microfabri-cated structures or by bonding several superimposed
structure levels
*
Multiple materials (e.g. non-silicon materials, e.g. for
magnetic actuation) and multiple levels; free-moving
parts,
*
The release of parts through the use of sacrificial
layers,
*
Requires small part handling.
† Forming stepped structures, thus shaping the third
dimension, for example by combining DXRL with
other process steps or other techniques, e.g. by
machining the substrate into a 3D geometry before
applying the resist layer.
The fabrication of micromechanical sensors and
actuators often requires movable parts, which can be
achieved by combining the LIGA process with a sacrificial
layer technique. Many microsystems make use of a
combination of various microfabrication techniques.
Micromechanical and microoptical structures can be
combined with microelectronic devices to form intelligent
microsystems. Several methods have been used to
combine various functions and materials.
Many devices successfully demonstrated interfacing of
bulk or surface silicon micromachining and LIGA through
assembled and co-processed integration in advanced,
integrated systems. However, in most cases a monolithic
approach where the LIGA components such as sensors and
actuators are co-processed on the same chip with the
silicon-based processing circuitry is difficult due to low
compatibility of processes, though some examples exist
such as the fabrication of LIGA microstructures on CMOS
wafers using deep X-ray lithography
[22]
or molding
techniques
[23,24]
. Alternative hybrid concepts for
inte-grating components and circuitry made from the most
appropriate materials made on separate substrates offer the
possibility to build up miniaturized systems with optimum
performances and reduced cost. For example an accelerator
sensor has been integrated with ASICs
[25]
. To date, the
majority LIGA – MEMS has been accomplished by hybrid
integration of individual LIGA components with
elec-tronics. In practice, only components with the highest
precision requirements are fabricated by LIGA, whereas
the other parts are produced by other methods.
Approaches to microassembly of high aspect-ratio metal
mechanisms benefit from conventional and silicon
micro-machining technology. Bonding approaches such as
diffusion bonding or press-fit assembly proved powerful
approaches to achieving multilevel mechanisms. Complex
three-dimensional millimeter-sized structures can also be
constructed using a ‘peg-board’ approach, which offers
increased flexibility for batch assembly of LIGA fabricated
devices
[26]
. A flip-chip assembly technique by means of
electroplating at a low processing temperature has also
been applied to the integration of high-aspect-ratio
microstructures with substrates that have pre-fabricated
microelectronics massively and in parallel. As such, it
provides a powerful way to achieve the integration of
meso- and microscopic electromechanical systems
[27]
. In
many cases, hybrid integration and in particular hybrid
assembly imposes good tolerances and alignment accuracy.
The evolution of microfabrication technologies from
laboratory-scale prototyping to a viable manufacturing
process requires the ability to set dimensional tolerances
and clearance fits at the design stage. To do this requires
evaluation, characterization and understanding of the
magnitude and origin of dimensional variation in
micro-fabricated components
[28]
as well as development of a
consistent set of specifications for clearance and tolerance
in microfabricated and micro assembled parts
[29,30]
.
Specific tools and methods for automated and parallel
assembly of LIGA-fabricated systems are being developed
[31 – 43]
.
3. Applications of LIGA
Applications for deep microstructures exist in many
sectors of R&D activity and as industrial products,
worldwide
[44 – 50]
. The presence of thick, deep and highly
precise microstructures with high aspect ratios is a
requirement for a number of micromechanical, optical or
packaging applications, as well as in other fields.
3.1. MEMS, high-precision parts and tooling
One primary application area for the LIGA technique is
the batch fabrication of precision actuators and mechanisms
[51,52]
.
† The goal is producing mechanically sturdy and stable, yet
highly precise structures for microactuators for a variety
of applications.
† Large structural heights are also advantageous to achieve
microdevices, which are capable of generating sufficient
force and/or torque to be functional. Most difficulties for
producing powerful microactuators arise from the use of
essentially planar technologies and severe materials
restrictions associated with them.
† The material basis is enlarged in particular towards
metallic and magnetic materials. The possibility of
accurately shaping magnetic materials with a large
volume through electroplating or casting and sintering
allows large magnetic forces. Typical examples are
magnet rings, motor parts
[53]
.
† High accuracy and tight tolerance for a variety of moving
elements such as shafts, contacting parts, bearings
[54]
and gears. Miniaturized gear systems
[55]
are a typical
example of micromechanical systems where LIGA
technology demonstrates its superiority by producing
the various gears with sufficient height and excellent
tolerances. The precision of LIGA allows for the
fabrication of gears with minimized backlash and
friction. The free design in two dimensions allows for
the creation of structures with specific optimized
geometries
[56,57]
. The steep and precise profile in
conjunction with the smoothness of side-walls are very
well suited to good fitting and assembly of various
components such as rotor and shaft.
† Many components with those improved performances
have a high potential in application fields such as
microsurgery, robotics and assembly, automotive and
aerospace industry, military safing and arming
devices for munitions
[58,59]
(force, precision and
reliability):
*
Micromotors
[60 – 67]
-rotors and -drivers with
appreciable torques
[68]
, microturbines
[69]
generat-ing power for a variety of applications, various
actuators
[53,70 – 73]
and sensors based on
electro-static or electromagnetic-based principle
[74,75]
, e.
g. mechanical microconnectors using magnetic force
[76]
, high precision micromechanical actuators for
electrical switches and relays, optical switches, fluid
control valves and pumps, etc.
*
Various sensors and actuators where surface area
dependent structures benefit from the
height/mass/-volume for better performances such as improved
sensitivity/resolution, for actuators systems, lower
driving voltage, etc. Examples are found in
micro-gyroscopes
[77,78]
, and capacitive acceleration
sensors
[79]
for automotive and aerospace industry,
trajectory sensing devices, mass spectrometers
[80]
,
switches and relays
[81]
, magnetic sensor
[82]
,
magnetoelectric devices
[83]
, electrostatic actuators,
e.g. for magnetic head tracking system of hard disk
drives
[84]
, new type of transistors
[85]
, various
types of transducers
[86 – 88]
, and detectors
[89 – 96]
.
*
More robust microprobes
[97,98]
, grippers
[99 – 102]
,
and manipulators for interfacing the
micro/nano-world with the macro-micro/nano-world or for precision handling
and manipulating mechanical or biological
micro/-nano-objects;
*
Flexural stiffness of microstructures constraining the
motion to a plane, e.g. springs and coils; the height
provides greater structural strength and rigidity.
† Nozzles for a variety of applications. The attractive
feature is here the high precision combined with the
smooth surface state as well as application-specific
tailored geometries and materials. Examples are:
*
Spinnerets for textile fiber
[103]
of better quality and
lifetime,
*
Nozzles for ink-jet printing type applications,
*Nozzles for fluid injection systems in various
applications: automotive, aerospace, etc.
† Passive high-added-value highly precise parts for
integrated tooling and mold manufacturing. The
precision of fabrication, the accuracy of assembly
process, and the resistance to wear are here attractive
features. The examples include microtools for various
techniques
[104]
:
*
Mold inserts/dies for a number of replication
technologies
[105,106,50]
. The capability to have
straight and smooth sidewalls facilitates the
de-molding process of the molded microparts. In many
cases, complex molds are built up in a modular way
by combining levels made by LIGA with those made
by precision engineering techniques,
*
Electrode arrays for microEDM
[107]
,
*IC leadframe punches
[108]
;
*
3D carriers for microdevices have also been
produced by LIGA
[109]
.
† LIGA is also a powerful tool for manufacturing
positioning structures with the highest of precision.
Structural heights are sufficiently thick to form passive,
robust fixturing guides for other components such as
fibers, lenses, etc. (see next paragraph) and sufficiently
precise to form edge reference structures in a number of
applications.
3.2. MOEMS, optics and communication
Another application, for which LIGA components
provide superior performance, is in the field of microoptical
components and systems
[110,111]
. The technical viability
of the LIGA-approach for fabricating microoptics has been
demonstrated for a variety of cases such as:
† Miniature optically-based sensors such as
microspect-rometers for different wavelength ranges
[112 – 114]
,
interferometers for various applications
[115]
,distance
sensor
[116]
, polarization sensor
[117]
, microchopper
[118]
, wavefront sensor
[119]
, etc.
*
The use of microoptical systems in optical telecom
and datacom
[120]
. Applications in this field require a
variety of optical microcomponents, both active and
passive, many of which can be economically
pro-duced through LIGA. In particular, through LIGA,
polymers offer a broad range of materials that can also
be tailored to specific requirements, providing
flexi-bility and cost advantage over other materials.
Additionnally, the LIGA-based replication processes
enable high-volume production of parts and systems
of low-cost and high performance. They include:
– Batch fabrication of optical components
[121]
such
as microlenses
[122,123]
, gratings
[124]
, mirrors
[125]
, and filters. The sidewalls of LIGA
com-ponents are inherently optical flats with surface
roughness (of 30 nm or under) low enough to make
those structures suitable for optical applications as
well as compatible with industry specifications on
elimination of scattering. Some possible
appli-cations of multiple oblique X-ray exposure
tech-niques are the fabrication of optical devices such as
microprisms
[126]
, cat-eye reflectors, two and 3D
photonic band gap microstructures
[127 – 129]
…
2– The high aspect ratio capabilities of LIGA
combined with its excellent spatial resolution and
patterning accuracy over large field sizes are
utilized for manufacturing a variety of passive
optical components based on light-guiding
elements, optical benches, base plates and high
precision mechanical holders and fixtures, as well
active components. They include:
– Complex planar optical waveguiding
struc-tures
[130,131]
are achieved through stacking
a combination of transparent optical polymer
layers of adequate refractive index on top of
each other. This layer stack can be structured
2
A special paragraph further down in the text deals on X-ray components, including crossed lenses for X-ray focusing produced by multiple oblique X-ray exposures (Ref.[162]).
to perform optical functions both horizontally
and vertically, for example with mirrors,
lenses, 3D gratings, Hollow waveguides and
waveguide cavities have also been made for
visible light, longer wavelength in the IR, and
for the terahertz range
[132]
.
– Various high precision mechanical positioning
structures such as mounts, supports and
connectors for accurate and robust aligning,
mounting,
coupling
and
interconnecting
[133 – 141]
, assembling, and packaging of
components
[142,143]
, devices, and systems.
– Splices, directional couplers, holders for
optical fibers or optical stops can be produced
by deep X-ray lithography, where features
such as grooves or pins allow for reproducible
positioning of ball lenses, detectors, light
sources, fibers, etc. without time consuming
manual alignment. Waveguides and fiber
positioning grooves (including V-grooves
with arbitrary slope, which is an advantage
compared to what can be produced with
silicon-based grooves) can also be achieved
using aligned and stepped exposures and/or
embossing elements. The extremely high
precision achieved garantees the exact
pos-ition of the optical components, minimizing
optical losses.
– For hybrid optical systems and free space
microoptical set-ups, an optical bench shows
advantages upon actively aligned counterparts
[144,145]
. The reduction of the number of
degrees of freedom in alignment leads to
distinct savings in cost and time. In addition
the use of LIGA allows extending the
materials base for microoptical benches from
polymers to ceramics for better thermal
management and lower thermal expansion.
Micrometer to sub-micrometer positioning
accuracy of the microoptical elements is
guaranteed by the LIGA technique. LIGA is
also a scalable approach well suited for
monolithic integration in one substrate of
optical and mechanical functionalities,
allow-ing the reduction of the degrees of freedom for
critical alignment. For example, passive
fiber-to-chip coupling with fiber alignment to
optical chips based on mechanical alignment
can be realized by structures for guiding pins.
The accuracy of patterning such systems, even
in multiple levels with accuracies in the range
of several micrometers, is ideally suited for
assembly and packaging of hybrid optical
systems at micron or sub-micron precision
without the need for active alignment.
– Optical processing elements to perform functions
such as filtering, beam splitting, attenuating, and
redirecting optical signals: 3D diffraction gratings;
a variety of switches
[146 – 149,47,150]
, which are
key components for optical networks: by-pass
switch, cross-connect matrix switch for
reconfi-guration of systems in high-speed optical datacom
networks;
– Coupling and interconnecting components for
optical backplane for computer applications such
as 1 £ N splitters and N £ N star couplers
[151]
.
† 3D high-power microwave, millimeter-wave photonics
and RF components
[152]
and devices
[153]
,
trans-mission lines, antennas, couplers, filters and resonators
[154]
, etc. Since the sidewalls of these structures can be
made accurate, smooth, and highly vertical, closely
spaced, tall microstructures in thick metal able to
conduct heat in highly coupled circuit topologies can
be fabricated.
† X-ray optical components
[155 – 162]
† Various components for accelerators: mm-wave linac
[163]
, W-band klystron, and accelerator structures
[164]
.
3.3. Microfluidics and bio-MEMS
The development of miniaturized devices for
micro-fluidics holds great promise for high throughput screening
and drug discovery as well as therapeutic drug delivery: in
particular multifunctional analytical systems in the rapidly
growing field of lab-on-a-chip devices or
microTotal-Analysis Systems (m-TAS). Besides the medical/biological
fields, microfluidics systems also find applications in many
other fiels, such as chemical and biotech systems
[165]
,
optical systems, ink jet printers, heat exchange, etc. They
consist of a number of components and systems that can be
fabricated in part or in total by LIGA and exploit a number
of characteristics linked to the high aspect ratio nature of the
microfabricated structures. For many applications such as
reactions of fluids in crossing channels well-defined
interaction volumes provided by LIGA are of crucial
importance. Furthermore, plastic microstructures are
desir-able as low-cost material for microfluidic components for a
number of portable analytical and single-use disposal
medical or biological applications.
LIGA was used in diverse microrofluidic building blocks
[166,167]
such as microchannels, reaction chambers,
integrated heat exchangers
[168]
, mixing or separation
[1]
capabilities, micropumps
[169 – 172]
, microvalves
[173 – 175]
, microsieves, filters and membranes
[176 – 178]
,
microinjectors
[179]
and-extractors, catalyst carriers,
micro-plates
[180]
, various sensors and actuators
[181 – 184]
, as
well as various integrated systems such as inhalers
[185]
,
electrophoretic devices
[186,187]
, gas chromatographs
[188]
, microthrust and heat generator
[189]
, as well as
complete systems such as miniature modular microfluidic
chips with processing units, monitoring and diagnosing
systems
[190]
. Cells combining electrochemistry and
spectroscopic applications have also been produced, for the
control of electrochemical microreactions
[191]
.
4. LIGA products
Though there is active R&D in LIGA in several groups
around the world and prototyping with commercial
companies is active, commercialization of LIGA products
has been rather slow but is developing. Products
manufac-tured by using LIGA and associated processes have
emerged worldwide in companies
[192 – 196]
. In addition,
LIGA foundry capabilities have been established, providing
easy access to synchrotron radiation
[197,13]
. Several
groups are also offering process know-how that can be
licensed by commercial entities interested in LIGA
production. Applications in the defense sector are also
expanding, especially in the US
[198]
. Exactly what market
share will be for LIGA still remains to be determined. A few
products with LIGA-parts are presently commercially
available, such as:
† MicroSpectrometer (see
Fig. 2
) for visible and IR ranges
[192]
,
† Mechanical gear systems
[193,194]
,
† Optical fiber connectors
[141]
,
† Microalignment flexures for photonics platform
[196]
.
This success was made possible in part by a consistent
and consequent focus on making the technology reliable for
manufacturing purposes
[199]
. Nevertheless, LIGA has
been limited to niche markets, predominantly due to the
complex production infrastructure required. Unlike for Si
microfabrication, where a vast array of processes, originally
developed with billions of US$ for the electronics industry,
is available, LIGA processes have been developed at
institutes with comparatively modest resources. However,
the superior qualities, in particular of LIGA molds, for mass
production of polymer parts are increasingly recognized by
industries and will push fabrication of such molds forward.
Much effort still has to be devoted to the commercialization
of LIGA-based products, in particular, time-to-market,
process stability, quality, and cost are key issues for a
broad industrial implementation of LIGA.
5. Conclusion
LIGA is a flexible technology that offers several
advantages over other microfabrication techniques.
The fabrication of components, systems or molds using
Deep-X-Ray-Lithography remains the most precise batch
technique available for the manufacture of microobjects
with large structural height and high aspect ratio. Most of
the interest in LIGA is associated with the ability to provide
these microstructures in a large selection of materials, in
particular in metal or polymer, through replication
techniques. The usefulness and flexibility of the technique
is further extended through advanced processing,
sacrificial layers, multilevel and oblique exposures, and
combination with other microstructuring processes through
multiprocessing or assembly.
LIGA technology is particularly well suited for
fabricat-ing polymer components where high aspect ratios, smooth
surfaces and submicron accuracies are required. The main
fields of current applications are in mechanical devices and
microoptics, in particular, for sensing, optical telecom and
datacom networks. Future high-impact fields may include
large-area high-precision patterning and, in general, the
fabrication of moulding tools for various applications
including polymer fluidics. In the latter example the
extremely smooth surfaces are the most attractive feature
of LIGA when compared to more traditional fabrication
techniques such as milling or microelectro-discharge
machining. A second route, producing components directly
with Deep-X-Ray-Lithography is currently being explored.
Here cost has been prohibitively high due to the long
exposure times of the PMMA resist. However, recent
progress with new resists, in particular EPON SU-8 with
dramatically increased sensitivity as compared to PMMA
has been encouraging. After establishing a robust process
with such a resist ‘Direct-LIGA’ could become feasible, at
least for small and medium size volume production of
specific components.
References
[1] E.W. Becker, et al., Production of separation nozzle systems for uranium enrichment by a combination of X-ray lithography and galvanoplastics, Naturwissenschaften 69 (1982) 520 – 523. [2] W. Becker, et al., Fabrication of microstructures with high aspect
ratios and great structural heights by synchrotron radiation lithography, galvanoforming, and plastic moulding (LIGA process), J. Microelectron. Engng. 4 (1986) 35 – 56.
[3] M. Madou, Fundamentals of Microfabrication, CRC Press, Washing-ton, DC, 1997.
[4] C.R. Friedrich, High aspect ratio processing, in: P. Rai-Choudhury (Ed.), Handbook of Microlithography, Micromachining and Micro-fabrication, vol. 2: Micromachining and MicroMicro-fabrication, SPIE Press Monograph, PM 39, 1997, pp. 299 – 377, Chapter 6 [5] W. Menz, J. Mohr, O. Paul, Microsystem Technology, Wiley VCH,
Weinheim, 2001.
[6] C. Khan Malek, Fabrication of high-aspect-ratio precision MEMS with LIGA using synchrotron radiation, SPIE Proc. 4592 (2001) 119 – 130.
[7] J. Hormes, et al., Materials for LiGA and LiGA-based microsystems, Nucl. Instrum. Meth., B 1999 (2003) 332 – 341.
[8] H. Guckel, et al., Direct high throughput LIGA for commercial applications: a progress report, J. Microsyst. Technol. 6 (2000) 103 – 105.
[9] R. Lawes, et al., LIGA: a fabrication technology for industry ?, Proc. SPIE 4593 (2001) 145 – 155.
[10] V. Saile, Strategies for LIGA Implementation, in: H. Reichl, E. Obermeier (Eds.), Microsystem Technologies 98, Sixth International Conference on Micro Electro, Opto, Mechanical Systems and Components; Potsdam, VDE-Verlag, 1998, pp. 25 – 30.
[11] P. Bley, Polymers—an excellent and increasingly used material for microsystems, SPIE Proc. 3876 (1999) 172 – 184.
[12] D. Munchmeyer, W. Ehrfeld, Accuracy limits and potential applications of the LIGA technique in integrated optics, SPIE Proc. 803 (1987) 72 – 79.
[13] ANKA GmbH:http://www.anka-online.de
[14] F.T. Hartley, C. Khan Malek, Nanometer X-ray lithography, SPIE Proc. 3892 (1999) 69 – 79.
[15] D. Maas, B. Karl, V. Saile, J. Schulz, Manufacturing of microcomponents in a research institute under DIN EN ISO 9001, Proc. SPIE 4174 (2000) 416 – 426.
[16] S. Massoud-Ansari, P.S. Mangat, J. Klein, H. Guckel, A multi-level, LIGA-like process for three dimensional actuators, IEEE Proc. MEMS’96 (1996) 285 – 289.
[17] O. Tabata, et al., 3D fabrication by moving mask Deep X-ray Lithography (M2-DXL), with multiple stages, IEEE Proc. MEMS’2002 (2002) 180 – 183.
[18] G. Feiertag, W. Ehrfeld, H. Lehr, M. Schmidt, Sloped irradiation techniques in deep X-ray lithography for 3D shaping of microstruc-tures, SPIE Proc. 3048 (1997) 136 – 143.
[19] D.Y. Oh, et al., A tetrahedral three-facet micro mirror with the inclined deep X-ray process, Sens. Actuators A93 (2001) 157 – 161. [20] S. Sugimaya, H. Ueno, Novel shaped microstructures processed by deep x-ray lithography, Transducers ’01-Eurosensors XV, 11thInt
Cont on Solid-Stake Sensors and Actuators, Munich, Germany, June (2001) 10 – 14.
[21] K.-C. Lee, S.S. Lee, 3D fabrication using deep X-ray mask with integrated micro-actuators, IEEE Proc. MEMS’03 (2003) 558 – 561.
[22] S. Stadler, P.K. Ajmera, Integration of LIGA structures with CMOS circuitry, SPIE Proc. 3046 (1997) 230 – 241.
[23] A. Both, et al., Fabrication of LIGA acceleration sensors by aligned molding, Microsyst. Technol. 2 (3) (1996) 104 – 108.
[24] K.-D. Mu¨ller, et al., Flexible integration of nonsilicon microstruc-tures on microelectronic circuits, IEEE Proc. MEMS’98 (1998) 263 – 267.
[25] M. Strohrmann, et al. Smart acceleration sensor systems based on LIGA micromechanics, in: H. Reichl (Ed.), Microsystem Technol-ogies 94, 1994, pp. 753 – 762.
[26] T.R. Christenson, Advances in LIGA-post-mold fabrication, SPIE Proc. 3511 (1998) 192 – 203.
T.R. Christenson, D.T. Schmale, A batch wafer scale LIGA assembly and packaging technique via diffusion bonding, IEEE Proc. MEMS’99 (1999) 476 – 481.
[27] L.-W. Pan, L. Lin, J. Ni, A flip-chip LIGA assembly technique via electroplating, J. Microsyst. Technol. 7 (1) (2001) 40 – 43. [28] C.G. Khan Malek, R. Wood, B. Dudley, P. Genova, Metrology study
of structural transfer accuracy for high-aspect-ratio microelectro-mechanical systems (MEMS): from optical mask to polished electroplated parts, J. Vac. Sci. Technol. B 16 (6) (1998) 3552 – 3557.
[29] B.L. Dearth, R.G. Steinhoff, LiGA measurement and acceptance evaluation, Microsyst. Technol. 9 (3) (2003) 197 – 203.
[30] H. Last, et al., MEMS reliability, process monitoring, and quality assurance, SPIE Proc. 3880 (1999) 140 – 147.
[31] W.K. Schomburg, et al., Assembly for micromechanics and LIGA, J. Micromech. Microengng. 5 (1995) 57 – 63.
[32] U. Gegenbach, Automatic assembly of micro-optical components, Proc. SPIE 2906 (1996) 141 – 150.
[33] C.M. Egert, K.W. Hylton, Automated assembly: a high throughput, low cost assembly process for LIGa-fabricated micro-components, Microsyst. Technol. 4 (1) (1997) 25 – 27.
[34] T.R. Christenson, Advances in LIGA-based post-mold fabrication, SPIE Proc. 3511 (1998) 192 – 203.
[35] J. Feddema, et al., Experiments in micromanipulation and CAD-driven microassembly, Proc. SPIE 3202 (1998) 98 – 107.
[36] J. Feddema, T. Christenson, Parallel assembly of high-aspect-ratio microstructures, Proc. SPIE 3834 (1999) 153 – 164.
[37] T.R. Christenson, D.T. Schmale, A batch wafer scale LIGA assembly and packaging technique via diffusion bonding, IEEE Proc. MEMS’99 (1999) 476 – 481.
[38] Y. Ansel, et al., Development of tools for handling and assembling microcomponents, J. Micromech. Microengng. 12 (2002) 430 – 437. [39] M. Niehaus, et al., Tools and methods for automated assembly of
miniaturized gear systems, Proc. SPIE 4194 (2000) 33 – 43. [40] W. Ehrfeld, et al., Highly parallel mass fabrication and assembly of
microdevices, Microsyst. Technol. 7 (4) (2001) 145 – 150. [41] A. Gerlach, et al., Assembly of hybrid integrated micro-optical
modules using passive alignment with LIGA mounting elements and adhesive bonding techniques, Microsyst. Technol. 7 (1) (2001) 27 – 31.
[42] U. Wallrabe, et al., Characterization of a micro optical distance sensor, Proceedings of IEEE/LEOS, International Conference on Optical MEMs, Lugano, August, 2002, pp. 20 – 23.
[43] P. Li-Wei, L. Lin, Batch transfer of LIGA microstructures by selective electroplating and bonding, IEEE Proc. MEMS’2000 (2000) 259 – 264.
[44] A. Rogner, et al., The LIGA technique—what are the new opportunities?, J. Micromech. Microengng. 2 (3) (1992) 133 – 140. [45] W. Ehrfeld, A. Schmidt, Recent developments in deep X-ray
lithography, J. Vac. Sci. Technol. B16 (6) (1998) 3526 – 3534. M. Abraham, et al., Microsystem: between research and industrial application, Engineering 41 – 42 (1998) 47 – 52.
[46] W. Menz, LIGA and related technologies for industrial applications, Sens. Actuators A 54 (1 – 3) (1996) 785 – 789.
[47] J. Mohr, LIGA—a technology for fabricating microstructures and microsystems, Sens. Mater. 10 (6) (1998) 363 – 373.
[48] J. Skardon, M. Vandenberg, Developing new markets for high aspect ratio micro-machined devices, Microsyst. Technol. 5 (1998) 65 – 68. [49] S.-J. Chung, et al., LIGA technology today and its industrial
applications, SPIE Proc. 4194 (2000) 44 – 55.
[50] J. Hruby, The state of LIGA development, Semicon 2000, San Francisco, CA, July 2000.
J. Hruby, LIGA technologies and applications, MRS Bull. April 2001 337 – 340.
[51] T.R. Christenson, et al., Micromechanics for actuators, SPIE Proc. 2220 (1994) 39 – 47.
H. Guckel, Progress in magnetic microactuators, Microsyst. Technol. 5 (2) (1998) 59 – 61.
[52] J. Schulz, A comparison of micro-actuators concepts in view of fabrication technologies and applications, Electrochem. Soc. Proc. 95-18 (1995) 436 – 450.
[53] H. Guckel, et al., Laterally driven electromagnetic actuators, Tech. Digest 1994 Solid State Sensor and Actuator Workshop, Hilton Head Island, SC, pp. 49-52,1994.
[54] L.S. Stephens, K.W. Kelly, K.W., Bearings and mechanical seals enhanced with microstructures, US patent 6617556, 2000. [55] C. Thueringen, et al., Construction and manufacturing of a
microgearhead with 1.9 mm outer diameter for universal appli-cations, SPIE Proc. 360 (1999) 526 – 533.
[56] S.J. Chung, et al., A micro-cycloid gear system fabricated by multi-exposure LIGA, J. Microsyst. Technol. 6 (4) (2000) 149 – 153. [57] R. Degen, et al., Hollow shaft micro-servo actuators realized with the
Micro Harmonic Drivew
, Proc. Actuators 2002, Bremen June (2002). [58] L. Fan, et al., SLIGA based underwater weapon safety and arming
system, J. Microsyst. Technol. 4 (4) (1998) 168 – 171.
[59] C.H. Robinson, Miniature, planar, inertially-actuated delay slider actuator, US patent 5,705,767, 1998.
C.H. Robinson, Microelectromechanical systems (MEMS)-type high-capacity inertial-switching device, US patent 6,314,887, 2001. C.H. Robinson, Microelectromechanical systems (MEMS)-type devices having latch release and output mechanisms, US patent 6, 321,654, 2001.
[60] H. Guckel, et al., Fabrication and testing of the planar magnetic micromotor, J. Micromech. Microengng. 4 (1991) 40 – 45. [61] H. Lehr, et al., Microactuators as driving units for microrobotic
systems, SPIE Proc. 2906 (1996) 202 – 210.
[62] T.R. Ohnstein, et al., Micromechanical stepper motor, US patent 5, 929,542, 1999.
[63] M. Nienhaus, et al., Design and realization of a penny-shaped micromotor, SPIE Proc. 3680 (1999) 592 – 600.
[64] V.D. Samper, et al., Multistator LIGA-fabricated electrostatic wobble motors with integrated synchronous control, IEEE J. Microelectromech. Syst. 7 (2) (1998) 214 – 223.
[65] R. Ledworuski, et al., A new ultrasonic catheter system with micromotor and LIGA gear-boxes, Microsyst. Technol. 9 (1 – 2) (2002) 133 – 136.
[66] F. Yi, et al., A new process to fabricate the electromagnetic stepping micromotor using LIGA process and surface sacrificial layer technology, J. Microsyst. Technol. 7 (3) (2001) 103 – 106. [67] E.J. O’Sullivan, et al., Future directions in electroplated materials for
thin-film recording heads, IBM J. Res. Dev. 42 (5) (1998) 1 – 12. [68] A. Cox, E. Garcia, Development of a LIGA-based elastodynamic
flying mechanism, SPIE Proc. 3329 (1998) 2 – 8.
[69] U. Wallrabe, et al., Power characteristics of 3D operated micro-turbines for minimally invasive therapy, Proc. IEEE Transducers (1996) 462 – 466.
[70] R. Kondo, et al., High aspect ratio electrostatic micro-actuators using LIGA process, J. Microsyst. Technol. 6 (6) (2000) 210 – 213. R. Kondo, et al., Fabrication of thick film magnetic core with high aspect ratio and long structure using LIGA Book of Abstract of Fourth International Workshop on High Aspect Ratio Microstructure
Technology, (HARMST’01), Baden-Baden, Germany, June 2001, pp.207 – 208.
[71] H. Guckel, et al., Electromagnetic linear actuators with inductive position sensing for micro relay, micro valve and precision positioning applications, Sens. Actuators A 53 (1– 3) (1996) 386– 391. [72] E.J. Garcia, et al., Design and fabrication of a LIGA
millien-gine, Proceedings of IEEE Transducers’97, Chicago, June 1997, pp. 765 – 768.
[73] H. Guckel, et al., Electromagnetic, spring constrained linear actuator with large throw, Actuator’94, Bremen, Germany, June 15 – 17, 1994.
H. Guckel, et al. Closed loop controlled, large throw, magnetic linear microactuator with 1000 micron structural height Proc. MEMS’98, 1998, pp. 414 – 418.
[74] J. Mohr, et al., Fabrication of microsensor and microactuator elements by the LIGA process, Proceedings of Transducers’91, San Francisco, CA, June 24 – 27, 1991, pp. 607 – 609.
[75] H. Lehr, et al., Application of the LIGA technique for the development of microactuators based on electromagnetic principles, J. Micromech. Microengng. 2 (1992) 229 – 233.
[76] T. Haga, et al., Development of microconnector with an automatic connecting/disconnecting mechanism, Microsyst. Technol. 6 (4) (2000) 157 – 160.
[77] K. Schumacher, et al., Micromechanical LIGA gyroscope, Transdu-cer’99, Sendai, Proceedings, vol. 1, June 1999, pp. 1574-1577. [78] Y.M. Desta, et al., Fabrication of Micromachined Resonating
Cylinder Gyroscopes, Book of Abstract HARMST’01, Baden-Baden, Germany, June 2001, pp. 259-260.
[79] M. Strohrmann, et al., Acceleration sensor with integrated compensation temperature effects fabricated by the LIGA process, Sens. Actuators, A 41 – 42 (1994) 426 – 429.
A. Both, et al., Fabrication of LIGA acceleration sensors by aligned molding, Microsyst. Technol. 2 (3) (1996) 104 – 108.
[80] A. Chutjian, et al., Miniature micromachined quadrupole mass spectrometer array and method of making the same, US patent 6,188, 067, 2001.
[81] E. Tabat, H. Guckel Single coil bistable, bidirectional micromecha-nical actuator, US patent 5,803,385, 1998.
[82] H. Zhang, et al., Microfabrication of a Magnetic Field Sensor Based on Electrodeposited Giant Magnetoresistant Material, Proceedings of IEEE International Symposium on Test and Measurement (ISTM/ 97), Beijing, China, June 1997, pp. 661 – 665.
[83] T. Wang, High-aspect-ratio microstructures for magnetoelectronic applications, SPIE Proc. 4979 (2003) 464 – 471.
[84] S. Nakamura, et al., An electrostatic microactuator using LIGA process for a magnetic head tracking system of hard disk drives, J. Microsyst. Technol. 5 (2) (1998) 69 – 71.
[85] P.K. Ajmera, I.-H. Song, Laterally movable gate (LMGFET) for on-chip integration of MEMS with electronics, SPIE Proc. 4334 (2001) 30 – 37.
[86] B. Choi, et al., Development of pressure transducers utilizing deep X-ray lithography, International Conference on Solid-State Sensors and Actuators (1991) 393 – 396.
[87] Y. Hirata, et al., Piezoelectric composites for micro-ultrasonic transducers realized with deep-etch X-ray lithography, Proc. MEMS’98 (1995) 191 – 196.
[88] S. Takimoto, et al., Transformersa, inductors, power and magnetic device control—fabrication of thick film magnetic cores for high frequency using LIGA process, IEEE Trans. Magnetics 37 (4-1) (2001) 2888 – 2890.
[89] M. Lemonnier, et al., First experimental results on new microstrip with three dimensional geometry, Nucl. Instrum. Meth., A 349 (1994) 274 – 276.
[90] J. Kawarabayashi, et al., Development of a micro-array type electron multiplier, Proc. IEEE MEMS’98 (1998) 791 – 794.
[91] H. Zhang, Microfabrication of a magnetic field sensor based on electrodeposited giant magnetoresistant material, Proceedings of
IEEE International Symposium on Test and Measurements (ISTM’97), Beijing, China, June 1997, pp. 661 – 665.
[92] R.H. Liu, et al., Investigation of the LIGA process to fabricate microchannel plates, Proc. IEEE Transducers’97 (1997) 645 – 648. [93] K.H. Jackson, et al., Fabrication of miniaturized electrostatic
deflectors with LIGA, SPIE Proc. 2640 (1995) 204 – 213.
[94] V.I. Kondratyev, et al., Preliminary testing of microstructured imaging plates with improved resolution, Nucl. Instrum. Meth., A 448 (1 – 2) (2000) 207 – 210.
[95] T.R. Christenson, et al., A variable reluctance stepping micro-dynanometer, Microsyst. Technol. 2 (3) (1996) 139 – 143. [96] M. Inoue, et al., Development of new position sensitive electron
multiplication device fabricated by LIGA process, Microsyst. Technol. 6 (3) (2000) 90 – 93.
[97] S. Akkaraju, et al., LIGA-based family of tips for scanning probe applications, SPIE Proc. 2880 (1996) 191 – 198.
[98] L. Huang, et al., Microfabrication of high aspect ratio Bi – Te alloy microposts and applications in micro-sized cooling probes, Micro-syst. Technol. 6 (1999) 1 – 5.
[99] S. Ballandras, et al., Microgrippers fabricated by the LIGA technique, Sens. Actuators A 58 (1997) 265 – 272.
[100] F. Yi, et al., A new sacrificial layer method of LIGA technology to fabricate movable part of a gripper, J. Microsyst. Technol. 6 (4) (2000) 154 – 156.
[101] M. Nienhaus, et al., Tools and methods for automated assembly of miniaturized gear systems, SPIE Proc. 4194 (2000) 33 – 43. [102] M.C. Carrozza, et al., Towards a force controlled microgripper for
asssembling biomedical microdevices, J. Micromech. Microengng. 10 (2000) 271 – 276.
[103] Y. Cheng, et al., LIGA spinnerets for microfiber, Proceedings of Transducer’99, Sendai, Japan, June 7 – 10, 1999, pp. 1452 – 1455. S.C. Tseng, et al. New pattern on fabrication of fiber spinnerets by LIGA technologySPIE Proc., 3680, 1999, pp. 518 – 525.
[104] H. Yang, et al., Ultra-fine machining tool/molds by LIGA technology, J. Micromech. Microengng. 11 (2001) 94 – 99. [105] J. Fahrenberg, et al., High aspect ratio multi-level mold inserts
fabricated by mechanical micromachining and deep etch x-ray lithography, J. Microsyst. Technol. 2 (1996) 174 – 177.
W. Bacher, et al., Fabrication of LIGA mold inserts, J. Microsyst. Technol. 4 (3) (1998) 117 – 119.
[106] S.M. Ford, et al., Rapid fabrication of embossing tools for the production of polymeric microfluidic devices for bioanalytical applications, SPIE Proc. 4560 (2001) 207 – 216.
[107] K. Takhata, et al., A novel micro-electro-discharge machining method using electrodes fabricated by the LIGA process, Proc. MEMS’99 (1999) 238 – 243.
K. Takhata, et al., High-aspect-ratio WC-Co microstructure produced by the combination of LIGA and micro-EDM, J. Microsyst. Technol. 6 (2000) 175 – 178.
[108] Y. Cheng, et al., Ultra-deep LIGA process and its applications, Nucl. Instrum. Meth., A 467 – 468 (2001) 741 – 744.
[109] A. Cox, E. Garcia, Three-dimensional LIGA structures for use in tagging, SPIE Proc. 3673 (1999) 122 – 126.
[110] D. Munchmeyer, W. Ehrfeld, et al., Accuracy limits and potential applications of the LIGA technique in integrated optics, SPIE Proc. 803 (1987) 72 – 78.
J. Mohr, Micro-optical and opto-mechanical systems fabricated by the LIGA technique, SPIE Proc. 3008 (1997) 273 – 278.
[111] M. Gerner, et al., Micro-optical components for fiber and integrated optics realized by the LIGA technique, Proc. IEEE MEMS’95 (1995) 328 – 333.
[112] J. Mohr, et al., Fabrication of a planar grating spectrograph by deep-etch lithography with synchrotron radiation, Sens. Actuators, A 25 – 27 (1991) 571 – 575.
[113] P. Krippner, et al., Microspectrometer system for the near infrared wavelength range based on the LIGA technology, SPIE Proc. 3912 (2000) 141 – 149.
[114] D. Brennan, et al., Development of a micro-spectrometer system for process control application, Infrared Phys. Technol. 43 (2) (2002) 69 – 76.
[115] A. Ruf, A miniaturized Fabry – Perot AFM sensor, Proceedings of Transducers’95-Eurosensors IX, Stockholm, Sweden, 1995, pp. 660 – 663.
[116] H. Nakajima, et al., Micro-optical distance sensor fabricated by deep x-ray lithography, Opt. Engng. 40 (8) (2001) 1667 – 1673. [117] B. Stenkamp, et al., Miniaturised polarisation sensor as a concept for
new optical microsystems, Proceedings of Transducers’95, Stock-holm, Sweden, June, 1995, pp. 160.
[118] P. Krippner, et al., Electromagnetically driven microchopper for integration in microspectrometers based on LIGA technology, Proc. SPIE 3878 (1999) 144 – 154.
[119] F. Pe´renne`s, et al., Characterisation of adaptive optic pyramid wavefront sensors fabricated by deep X-ray lithography, Microelec-tronic Engng. 67 – 68 (2003) 566 – 573.
[120] T. Paatzsch, et al., Polymer waveguides for telecoms, datacom and sensor applications, SPIE Proc. 3276 (1998) 16 – 27.
H.-D. Bauer, et al., Manufacturing microcomponents for optical information technology using the LIGA technique, Proc. SPIE 3739 (1999) 224 – 229.
[121] W.C. Sweatt, et al., A plethora of micro-optical systems, SPIE Proc. 4437 (2001) 125 – 133.
T.R. Christenson, W.C. Sweatt, Micro-optomechanical uses of deep x-ray lithography (X-optics)Book of Abstract HARMST’01, Baden-Baden, Germany, June (2001) pp. 3 – 4.
[122] A. Picard, et al., Refractive microlens arrays made by contactless embossing, SPIE Proc. 3135 (1997) 96 – 105.
[123] M. Ilie, U. Danut, Miniaturized achromatic hybrid objective containing LIGA-made microlenses, SPIE Proc. 3879 (1999) 196 – 205.
[124] T.R. Ohnstein, et al., Tunable IR filters with integral electromagnetic actuators, Proceedings of Solid-State Sensor and Actuator Work-shop, Hilton Head, South Carolina, June, 1996, pp. 196 – 199. J.A. Cox, et al. Optical performance of high aspect LIGA gratings II, Opt. Engng., 3711, 1998, pp. 2878 – 2884.
[125] D.-Y. Oh, et al., A tetrahedral three-facet micro-mirror with the inclined deep X-ray, Sens. Actuators A93 (2) (2001) 157 – 161. [126] K.-H. Brenner, et al., Micro-optical set-up with microlenses and
microprisms based on refractive optics, Proc. SPIE 1806 (1993) 228 – 233.
[127] G. Feiertag, et al., Fabrication of photonic crystals by deep X-ray lithography, Appl. Phys. Lett. 71 (11) (1997) 1441 – 1443. M. Katsarakis, et al., Two-dimensional metallic photonic band-gap crystals fabricated by LIGA, Microsyst. Technol. 8 (2 – 3) (2002) 74 – 77.
[128] C. Cuisin, et al., Fabrication of three-dimensional photonic structures with submicrometer resolution by x-ray lithography, JVST B 18 (6) (2000) 3505 – 3509.
[129] F. Romanato, et al., Fabrication of metallic photonic crystals by X-ray lithography, J. Microelectron. Engng. 67 – 68 (2003) 479 – 486. [130] A. Rogner, W. Ehrfeld, Fabrication of light-guiding devices and
fiber-coupling structures by the LIGA process, Proc. SPIE 1506 (1991) 80 – 91.
[131] T. Paatzsch, et al., Polymer waveguides for optical backplane interconnects fabricated by the LIGA technique, Proceedings of Microsystem Technology’98, Postdam, VDE-Verlag, Berlin, 1998, ISBN No3-8007-2421-9, pp77 – 82.
H.-D. Bauer, et al. Polymer waveguide devices with passive pigtailing: an application of LIGA technologySynth. Metals, 1151 – 3, 2000, pp. 13 – 20.
[132] I. Turcu, et al., X-ray micromachining of deep 3D terahertz waveguide components using a laser plasma source at 1 nm wavelength, SPIE Proc. 3157 (1997) 291 – 299.
[133] A. Rogner, et al., LIGA-based flexible microstructures for fiber-chip coupling, J. Micromech. Microengng. 1 (1991) 167 – 170.
