High aspect ratio LIGA process
Personnel: Kabseog Kim and Jeong-Bong Lee
The LIGA process, a German acronym that stands for deep-etch x-ray lithography,
electroplating, and molding, is a micromachining technology developed in the early 1980’s at
the Karlsruhe Nuclear Research Center (www.fzk.de). With this technique it is possible to
produce microstructures from a variety of materials with very high aspect ratios (up to 100),
very small structures (in the submicron range) and very smooth sidewalls (surface roughness
less than 50nm) due to reduced diffraction, low resist absorption and minimal proximity effects
[1-3].
The
process
sequence
is
shown
in
the
following
JAVA
applet
(www.utdallas.edu/~jblee/research/LIGA.html). Highly parallel and coherent x-rays from a
synchrotron radiation source are incident on a x-ray mask patterned with high atomic number
absorbers (Figure 1). The absorbers on the mask are thick enough to prevent the penetration of
x-rays. In the open areas of the mask, the radiation passes through and exposes PMMA
(polymethylmethacrylate) resist. The resist is then developed and the resulting PMMA mold is
used to produce metallic microstructures by electroplating. The electroplating is either the final
step in the process or the electroplated microstructure is used as a metallic mold insert for
replication from another material such as plastics or ceramics [4-6] using hot embossing or
injection molding. When producing large numbers of metallic microstructures, the molded
polymer components are used as sacrificial molds for the electrodeposition of metals [7-8].
Figure 1. SEM photomicrographs of X-ray mask of 25 micron gold absorber patterns
on 120 m thick graphite membrane.
Prior to joining UTD, our research group was working at LSU and we have access to the Center
for Advanced Microstructures and Devices (CAMD, www.camd.lsu.edu), South’s only
synchrotron radiation research center.
(a)
(b)
(c)
Figure 2. SEM photomicrographs of (a) PMMA mold after developing and (b-c) electroplated Ni mold
inserts for 500m microstructures with a maximum aspect ratio of 20:1).
Literature
[1] E. W. Becker, W. Ehrfeld, P. Hagmann, A. Maner, and D. Munchmeyer, “Fabrication of
microstructures with extreme structural heights by synchrotron radiation lithography,
galvanoforming and plastic forming(LIGA process),” Microelectron. Eng., vol.4, pp. 35-56,
1986.
[2] A. Rogner, J. Eicher, D. Munchmeyer, R.-P Peters and J. Mohr, “The LIGA technique-what
are the new opportunities,” J.Micromech. Microeng. , 2, pp.133-140, 1992.
[3] M.W. Borner, M. Kohl, F.J. Pantenburg, W. bacher, H. Hein, and W. K. Schomburg,
“Movable microstructures made by a sub-micron LIGA process,” Microsystem technologies,
2, pp. 149-152, 1996.
[4] Homepage of the Institute for Microfabrication Mainz(IMM) at http://www.imm-mainz.de/
[5] Homepage of Jenoptik Gmbh at http://www.jo-mikrotechnik.com/
[6] ] Homepage of Sandia national Lab at http://www.mdl.sandia.gov/Micromachine/
[7] W. Bacher, K. Bade, B. Matthis, M. Saumer, and R. Schwarz, “Fabrication of LIGA mold
inserts,” Microsystem technologies, 4, pp. 117-119, 1998.
[8] W. Ehrfeld, V. Hessel, H. Lowe, Ch. Schulz, and L. Weber, “Materials of LIGA
technology,” Microsystem technologies, 5, pp. 105-112, 1999.