by Dr. Joachim Schulz
An Attempt to Describe the Unique Features of LIGA
LIGA Over the Years in View of Market Needs
LIGA and Molding
Access to Synchrotrons
Over the past decades, many applications have been proposed as commercially
viable for LIGA (Lithographie, Galvanoformung, Abformung (Lithography, Electroplating,
and Molding)). However, most of them were pushed into the market via silicon
MEMS or even classic microdrilling and micromilling. To date, not a single commercial
x-ray LIGA effort has prospered and support for academic LIGA centers is fading.
However, the authors believe that the current demand for LIGA applications,
including x-ray optical elements such as LIGA-lenses, precision apertures etc.,
is the advent of a second dawn for LIGA.
This paper will give a more strategic than technical overview of the past
and future of LIGA.
In the 1990's, LIGA was a pacemaker technology for MEMS, used for many
smart systems such as acceleration sensors or optical devices integrated with
electronic read-out circuits. This first LIGA hype resulted in many different
showpieces that proved in an impressive way that the technology is very versatile.
Even micropumps were made using LIGA, though this kind of application does not
really utilize any of the unique features of LIGA. When judged from their dates
of academic publication, several of these LIGA systems preceded similar systems
from competing technologies. Yet LIGA's breakthrough on the market still
hasn't happened. Why not?
The most apparent reasons are that other technologies such as silicon micromechanics
were pushed by big companies, and that classic technologies such as EDM, milling,
drilling and the like were pushed by many small and medium-sized companies to
achieve ever more challenging fine features. So at a first glance it looks like
not enough engineering effort was put into the advancement of LIGA's capabilities.
Public institutions might also share the blame: None of the LIGA centers was
ever forced to earn its money on the free market. Yet another reason might be
that LIGA is simply too complicated, time consuming and expensive. And finally,
while other technologies have advanced quite a bit and have reached sufficient
complexity to accommodate more functionality in micropatterned structures, LIGA
is still severely restricted to essentially two dimensions on a substrate with
limited options for several mask levels and only a handful of materials.
All of the above arguments bear some truth. Nevertheless, microworks is convinced
that LIGA's technological advancements in recent years and the needs of
the market are merging so much that there will be niches for LIGA. One longstanding
niche is microgears. Another one is currently being stimulated by demand from
synchrotrons for x-ray optical elements. Other examples could be listed.
LIGA structure in nickel, 1600µm
high, aspect ratio of 160.
LIGA structure in SU8 used to focus
x-rays. The narrowest parts are 2 µm wide and extend several hundred
LIGA structure in gold as an absorption
grating for x-rays. The aspect ratio of these structures is near 50
Figure 1. Various
structures that are typical for LIGA: high-aspect ratios, high-precision,
An Attempt to Describe the Unique Features of LIGA
The four strong points that were used to promote LIGA in the 1990's
are quickly listed:
- High-aspect ratio
- High sidewall quality / low roughness
- Extreme precision
- Complete 2-D freedom
In the beginning, these four points were sufficient to convince relevant players
in technology screening, from governments to larger companies like IBM and Bosch,
to look into LIGA. The drive for more miniaturization was apparent; in silicon,
however. only bulk micromachining with its severe restrictions was available.
And nearly as quickly, one could list how these strong points tumbled over
the years. High-aspect ratios of, say, 20 at completely free 2D geometry can
now be easily fabricated by DRIE in silicon; this technology can be tuned to
high sidewall quality as well, and, if need be, cryoetching can be applied.
Extreme precision has always been claimed for LIGA, but a methodology to prove
it has only recently been developed. The results so far are not conclusive.
Then there is UV-LIGA, which exploits the potential of SU8 resists to fabricate
nearly anything that fits the following requirements: +/-2 µm precision,
less than 800 µm in height, and made of nickel or nickel-phosphorous as
a material. This technology was excellently placed on the market by mimotec
in Switzerland, apparently leaving little room for x-ray LIGA. On the other
hand, it's success clearly shows that LIGA is a competitive technology.
One positive aspect for x-ray proponents: exposure to x-rays makes the process
chain more expensive, but only by 30 to 50%.
There is still another niche for LIGA technology in our opinion: the parallel
mass fabrication of tiny structures with high-aspect ratios. These are typically
arrayed structures which work together over reasonably large lengths or areas.
A good example is the x-ray lenses which were recently developed at KIT (central
picture above), where the focusing range of each lens is very small. However,
when hundreds of them are placed next to each other, the effect can be well
What is needed to exploit the niche for potential markets? We will try to
answer this in the next section.
LIGA Over the Years in View of Market Needs
An excellent summary of LIGA has been given in a fairly recent book, LIGA
and its Applications (Saile et al. 2009)1. The book
clearly identifies LIGA as a powerful tool for making excellent microstructures.
In this section, we will try to evaluate these findings from the point of view
of a small company, microworks. The main criteria are: the time involved before
the first parts are produced; proof of the value proposition through standards;
as well as cost factors.
Many aspects of LIGA technology have been addressed over the past decade, predominantly
as part of government-funded R&D projects. Their goals have typically been
to identify a new use, adapt the LIGA process chain and make first parts. Few
projects have focused on addressing technological aspects of the LIGA process.
In this respect, microworks is thankful for the German government's support
of both FELIG2 and INNOLIGA3,
whereby FELIG focused on automating the process and INNOLIGA on ensuring the
reliability of a resist based on the same basic chemistry as SU8.
LIGA and Molding
In the very first publication on LIGA, Becker et al.4 suggested molding as
the way to go for mass fabrication. From today's point of view, the technology
used to make mold inserts is still unreliable, which makes it hard to guarantee
delivery dates to customers. More important, however, is that the outstanding
features which keep LIGA attractive are essentially impossible to retain in
molding: high-aspect ratios, many features on the substrate and high precision.
All of this is particularly true for injection molding, while hot embossing
does not produce the expected drastic cost advantage over direct LIGA. On the
other hand, the process to use molded structures as a form to electroplate metal
has never been successfully demonstrated.
There is another indication that LIGA molding tools are not commercially viable:
mimotec SA in Switzerland started out in the late 1990s primarily making mold
inserts. Today, these make up less than 10% of their business, whereas 90% is
the direct manufacturing of LIGA metal parts.
Access to Synchrotrons
A main bottleneck for x-ray LIGA has virtually disappeared over the years:
the availability of synchrotron radiation. Increased availability was a major
step towards being able to reliably meet delivery dates.
Some synchrotrons now offer automated sample handling and measurements in protein
crystallography at a high degree of standardization, which should be possible
for LIGA exposures as well. Since the LIGA technique involves more than just
exposures, it unfortunately is still not that easy to transfer a process from
one synchrotron to another. The second source issue has therefore still not
been completely solved, but sufficient arguments are at hand that this will
not pose a problem with respect to potential large orders.
Ron Lawes and others have argued at several HARMST conferences over many years
that the various LIGA groups need to agree on one design, one process and one
set of properties to be measured, possibly by using different methods, making
structures and comparing the results of all the groups. Unfortunately, this
has never occurred. Perhaps the typical researcher saw no R&D reward in
doing the same thing over and over; also, all of the LIGA groups have been in
competition with each other.
As a result, if asked today what specifications LIGA can actually achieve,
the community has no answer regarding precision, maximum aspect ratios, sidewall
roughness, verticality etc. What we do have, however, is a situation in which
all of the major effects that produce non-ideal results have been identified,
and these effects can usually be minimized by typical engineering approaches.
In our opinion, here lie the most severe deficiencies of the last decades:
While other batch technologies have moved from 4-inch to 6- and 8- or even 12-inch
wafers, LIGA still works with 20x60 mm2 titanium membranes (KIT),
30x30 mm2 silicon nitride membranes (CAMD and HTmicro), and 83 mm round
graphite membranes (CAMD and BESSY). Also, several failed projects attempted
to use beryllium for its obvious advantages in x-ray applications5.
The main reason for larger areas has been the cost factor for DRIE, UV LIGA
and the like. This point never had to be addressed by any of the LIGA centers.
However, we argue that the main motivations for developing large-area LIGA will
be its compatibility with other technologies and the option to make large devices.
What has been achieved is that mask-making has become more realiable in combination
with better access to beam time, enabling significantly shorter times for the
development of first products. However, two iterations of a new design are still
necessary in most cases, and that still takes more than six months.
What needs to be achieved for any future perspective for LIGA is an increase
of the mask area with no or virtually no compromise to structural quality.
From the beginning, our mission has been to become a leading LIGA manufacturer.
It has never been a strong "business case", because otherwise we
would not have been the first such endeavor in Europe. We have looked for other
manufacturing opportunities as well, and within a year or so, several projects
were acquired. A portfolio analysis may be rather uncommon for a small company
like microworks, but it helps us to keep our focus. The picture looked very
diverse, in fact way too diverse in 2008. A restructuring of our projects, last
but not least enforced by the financial crisis, makes microworks now look quite
Portfolio of microworks' projects at the end of 2008. The ellipse
size roughly gives the total project value, and the color indicates
the technological risks associated with the technological requirements
(from low to high risk: dark grey, light grey, green, yellow, orange).
Same as fig. 2 but in mid-2010. The differences to 18 months earlier
can be summarized as follows: we agreed to a somewhat higher degree
of technological risks on average but cancelled high-risk projects with
low benefits, added one key R&D project (in cooperation with KIT)
and kept cash cows.
The difference becomes more obvious when we look at another portfolio plot
following Wheelright and Clark6. We shall skip the cloud picture of the status
at the end of 2008 and only show how things look like today:
microworks repositioned itself with respect to the diversity of its
projects. A central R&D project, with the support of the State of
Baden-Württemberg, is set out to increase mask sizes dramatically.
All other projects fit into current platforms or include only minor
The major R&D project focuses on mask-making, with an emphasis on searching
for a way to obtain large-area masks. Unfortunately, we cannot report on these
aspects today since a patent application is still in progress. But we can say
that if we are successful, we will be able to advance to very large areas, say
beyond 10 inches, and still keep the option of patterning the masks in an e-beam.
Over the years, several COMS and HARMST conferences have seen excellent contributions
from LIGA experts on the advancements of this technology. A commercially successful
company has not yet appeared. We have argued in this contribution that the size
constraints of high precision, e-beam-written x-ray masks have been a limiting
factor not only with respect to costs but also with respect to device size and
compatibility with other technologies. Solving these size constraints is the
key issue for the future of LIGA technology. We have also argued that the strategic
positioning of microworks' projects together with a potential solution
to the mask problem may lead to commercially viable x-ray LIGA.
- LIGA and its Applications; V. Saile, U. Wallrabe, O. Tabata,
J. G. Korvink (eds.); Wiley, 2009, ISBN: 978-3-527-31698-4
- FELIG: Modulare Fertigungsstraße für Mikrobauteile
über Röntgentiefenlithographie und Galvanik; Project funded by the
German Ministry of Education and Research, 2005 to 2009; http://www.imt.kit.edu/downloads/FELIG-Broschuere.pdf
- INNOLIGA: Innovative Resist- und Direkt-LIGA-Technologieentwicklung
für eine rentable Fertigungsmethode für LIGA-Präzisionsteile
für KMUs mit einem stabil arbeitenden Röntgen-Negativresist; Project
funded by the German Ministry of Education and Research, 2007 to 2010 under
contract number 16SV3522
- E. W. Becker, W. Ehrfeld, P. Hagmann, A. Maner, and D.
Muenchmeyer, "Fabrication of Microstructures with High-Aspect Ratios
and Great Structural Heights by Synchrotron Radiation Lithography, Galvanoforming,
and Plastic Moulding ( LIGA Process)," Microelectronic Engineering, vol.
4, pp. 35ff, 1986.
- See the article by Jost Göttert in: LIGA and its Applications;
Volker Saile, Ulrike Wallrabe, Osamu Tabata, Jan G. Korvink (eds.); Wiley,
2009, ISBN: 978-3-527-31698-4
- "Creating project plans to focus product development",
S. Wheelright, K. Clark, Harvard Business Review, March-April 1992, pp. 2-14
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