Microsyst Technol (2006) 12: 545–553
DOI 10.1007/s00542-005-0073-z
T E C H N I C A L P A P E R
Ren Yang Æ J. Jiang Æ W. J. Meng Æ Wanjun Wang
Received: 19 May 2005 / Accepted: 30 September 2005 / Published online: 14 January 2006
Ó Springer-Verlag 2006
Abstract Techniques for economic fabrication of highaspect-ratio microscale structures (HARMS) are being investigated intensely. Microdevices employing metalbased HARMS are of particular interest for mechanical, electro-mechanical, and chemical applications. In many applications, HARMS with two or more heights are needed. Fabrication of these multi-level HARMS by compression molding requires two-level or multi-level mold inserts. In addition, tapered mold inserts would help achieving easy insert-part separation. This paper reports a process for fabricating two-level tapered mold inserts by combining UV-lithography of SU-8 resist, one-step metal electrodeposition, polish and level, followed by SU-8 resist removal. Without tilting and rotation during the lithography step, tapered plating molds are obtained by employing characteristics of UVlithography and resist development. The SU-8 removal method used does not reduce the strength of the electrodeposited mold insert. Efficacy of our approach is demonstrated with a two-level mold insert prototype.
Keywords SU-8 resist
1 Introduction
Æ Multi-level
One-step electrodeposition Æ
Æ Tapered insert
SU-8 stripping
Æ
The majority of current microelectromechanical systems
(MEMS) are fabricated with either surface micromachining or bulk micromachining of silicon based materials. In applications requiring metal-based or polymerbased microscale structures, the Lithographie, Galvanoformung, Abformung (LIGA) process is the preferred fabrication technology. The LIGA process, first developed using X-ray lithography of PMMA (Menz et al.
R. Yang Æ J. Jiang Æ W. J. Meng Æ W. Wang (
&
)
Mechanical Engineering Department, Louisiana State University,
Baton Rouge, LA 70803, USA
E-mail: wmeng@me.lsu.edu
E-mail: wang@lsu.edu
Tel.: +1-225-5785807
1991
; Mohr et al.
1992 ; El-Kholi et al.
1993
; Kondo et al.
1998 ; Ehrfeld and Schmidt
1998 ; Henry et al.
1999 ;
Guckel et al.
2000 ; Singleton et al.
2002 ; Schulz and
Bade et al.
2004 ; Heckele and Schomburg
2004
), has been used to fabricate high-aspect-ratio microscale structures (HARMS) for applications in mechanical, electrical, chemical, and biomedical devices and systems
(Menz et al.
1991 ; Mohr et al.
1992 ; El-Kholi et al.
1993 ;
Kondo et al.
1998 ; Ehrfeld and Schmidt
1998 ; Henry
et al.
1999
; Guckel et al.
2000 ; Singleton et al.
2002
;
Schulz and Bade et al.
2004 ; Heckele and Schomburg
2004
), including micro accelerator (Qu et al.
1999
), RF inductor (Sadler et al.
2001
), micro mixer/reactor (Yang et al.
2004 ), polymerase chain reaction (PCR) system
(Hupert and Witek
2003
), and others. The molding replication step of the three-step LIGA process of lithography, electrodeposition, and molding is the most important for low-cost HARMS production. Efforts of using LIGA or LIGA-like technologies for producing polymer-based, metal-based, and ceramic-based
HARMS have been reported (Guckel et al.
2000 ;
Heckele and Schomburg
2004
; Hupert and Witek
2003 ;
Cao et al.
2003
,
2004
; Cao and Meng
2004 ; Cao 2004 ;
Dinakar
2003 ). X-ray lithography of PMMA produces
high-quality plating molds, with attributes such as high aspect-ratios and vertical sidewalls. PMMA can also be easily removed after electrodeposition (Menz et al.
1991 ;
Mohr et al.
1992 ; El-Kholi et al.
1993 ; Kondo et al.
1998
; Ehrfeld and Schmidt
1998 ; Henry et al.
1999
;
Guckel et al.
2000 ; Singleton et al.
2002 ; Schulz and
Bade et al.
2004 ; Heckele and Schomburg
2004
). X-ray lithography of SU-8, a negative resist, has also been used in LIGA processing (Ryan
2002 ; Jian 2002
; Turner and
Desta et al.
2003
; Yang and Wang
2005
) because of the much reduced exposure time.
UV-lithography of SU-8 offers excellent quality for thick HARMS at much lower cost, and therefore an alternative to X-ray lithography. Recent progress in understanding the UV-lithography process and improving the lithography quality has enabled the fabrication of SU-8 microscale structures with aspect-ratios

546 exceeding 100 (Yang et al.
2003
). Careful control of the lithography conditions has also led to reasonably good quality vertical sidewalls (Yang et al.
2003
). UV-LIGA technologies based on UV-lithography of SU-8 resist have also been used in fabricating several device prototypes (Williams et al.
2005
; Dentinger et al.
2002 ). Sev-
eral efforts on removal of cross-linked SU-8 resist have been reported in the literature (Muralidhar et al.
2001 ;
Rees
2001 ; Rosato et al.
2000
).
The process for fabricating mold inserts of a single height is well understood. Fabrication of multi-leveled
HARMS dictates more complex processing. The most common approach is to use aligned multiple lithography and electroplating steps (Ehrfeld and Schmidt
1998 ;
Williams et al.
2005
). Mechanical polishing and alignment are needed between steps. As shown schematically in Fig.
1 , within the electroplated structure, contact
areas due to multiple electroplating steps will likely possess reduced strengths. These strength reductions are further exacerbated by fluctuations of plating conditions between multiple electroplating steps. The molding performance of multi-level mold inserts fabricated by such multi-step processes may therefore be compromised.
An alternative approach to fabricating multi-leveled
HARMS is to use pre-structured substrates (Qu et al.
1999 ). Carefully designed microscale structures, which
serve as a sacrificial layer, are made on the substrate based on desired heights of the final HARMS. The prestructured substrate is then used in a lithography and plating process. After the plating pattern is filled with desired metal and overplated, the remaining resist is then stripped. The pre-structured features (sacrificial layer) are then chemically etched to release the mold insert from the substrate. The major difficulty with this approach is non-uniform electroplating rates resulting from uneven substrate surfaces. Patterns at a higher initial height tend to have much higher plating rates. In general, plating into deep features with small openings tends to be difficult. The raised regions with higher plating rates may be overplated quickly and grow into
‘‘mushroom’’ structures as shown schematically in
Fig.
2 . These ‘‘overflowing’’ metal then cover the adja-
cent deeper micro patterns and prevent further plating into them. This leads to no-filling or partial-filling of smaller/deeper holes/channels.
Kim et al. ( 2002 ) reported an interesting approach to
fabrication of multilevel high aspect ratio microstructures and mold insert. They introduced an intermediate step in the aligned multiple lithography and plating process by using a PDMS mold. In their method, a SU-8 pattern was fabricated using a combination of UV and
X-ray lithography. A PDMS mold was then made from the SU-8 pattern. The PDMS mold was then sputtered with seed layer and plated with nickel. The over-plated nickel structural was then released from the plating mold by peeling off the PDMS mold. The released nickel microstructure can then be used as a mold insert. The approach solved the structural problem in traditional aligned multiple lithography and plating process. It can be very useful for some applications requiring hollow structures or without significant mechanical load, such as micro needle array (Kim et al.
2002
). One potential drawback in using the PDMS intermediate step is reduced accuracy of the final mold insert. Another limitation is that some voids may form in the plated structures because the seed layer is sputtered on the whole surface of the PDMS mold.
Separation between the mold insert and the molded material during the demolding process may be difficult.
The volume of the microfeatures on the insert decreases faster than its surface area as the characteristic linear dimension decreases. This scaling suggests that frictional forces experienced by the microfeatures may become significant and exceed their load bearing capacities at sufficiently small characteristic feature sizes. To avoid plastic deformation of the insert or tearing of the molded part, a taper angle may be helpful (Rosato et al.
2000
;
Janczyk et al.
1997
). Stereolithography can make mold inserts with a taper angle (Janczyk et al.
1997 ; Cedorge
and Colton
2000 ; Pham and Colton
2002
). However, stereolithography molds are made layer by layer, which may generate stepping effect when a taper angle is set, and lead to increased friction forces (Janczyk et al.
1997 ;
Cedorge and Colton
2000
; Pham and Colton
2002
).
Tapered patterns have been fabricated by X-ray lithography on SU-8 resists by multiple exposures with tilts and rotations (Jian
2002 ; Turner and Desta et al.
2003
;
Yang and Wang
2005 ), at much increased exposure
times and consequently much higher costs.
In this paper, we present a new process for fabricating two-level mold inserts. Diffraction of UV light during the lithography step is employed to obtain tapered patterns in SU-8 resist. Electroplating of two-level mold inserts can be accomplished in a single step, yielding no extraneous interfaces. Removal of the SU-8 resist does not cause obvious changes in the strength of the resulting HARMS.
2 Numerical simulations and fabrication of SU-8 structures with tapered sidewalls
Fig. 1 Structural weakness at electroplating interfaces due to multiple exposures and electroplating steps may compromise performance of such multi-level HARMS as mold inserts
Diffraction effects around edges or slits are more pronounced at wavelengths in the near-UV range, e.g., the

547
Fig. 2 Schematic diagram showing effects of uneven electroplating rates at different regions of pre-structured substrates h -line at 405 nm, as compared to those in the soft X-ray range. This phenomenon is utilized to obtain tapered patterns in SU-8 resist. In order to achieve a useful taper angle, the SU-8 plating mold should have a bigger opening at the top. Figure
3
shows three possible shapes for SU-8 plating molds, at respectively, negative, zero, and positive taper angles. An insert with a positive taper angle is believed to be the best for molding purposes.
Based on the Huygens’ principle, diffraction through an aperture with arbitrary shape in an otherwise opaque partition can be described by the Fresnel–Kirchhoff integral formula:
U p
¼ ikU
0 e
4 p i x t e ik ð r þ r
0
Þ
½ cos ð n ; r Þ cos ð n ; r
0
Þ d s ; rr
0
ð 1 Þ where k =2 p / k and k is the incident light wavelength, U
0 represents incident monochromatic spherical waves, r and r
0 stand for positions of a point on the aperture relative to the screen and the source, respectively, ( n, r ) and ( n, r
0
) denote the angles between the vectors and the normal to the surface of integration, the integration is over the aperture area.
To better understand UV-lithography of SU-8 and its effects on sidewall profiles, numerical simulations were conducted with ZEMAX EE (ZEMAX Development
Corporation, San Diego, CA, USA). This software, based on the principle of diffraction as stated in Eq. 1, can be used to perform Fresnel diffraction simulations in relation to UV-lithography of SU-8 resists.
Refractive indices of SU-8 are n =1.668 at k =365 nm
( i -line) and n =1.649 at k =405 nm ( h -line), respectively.
Its transmission versus thickness curve is shown in Fig.
4
at these two selected wavelengths. For a given mask pattern, the entire SU-8 resist is divided into different numerical layers. Light intensity distribution within different numerical layers were calculated with ZEMAX, and imported to SigmaPlot (Systat Software, Inc. Point
Richmond, CA, USA) to generate the contour of light energy distribution in the entire resist layer. It was assumed that the SU-8’s absorbance of the UV light is directly proportional to the local energy intensity. The dosage contour is therefore congruent with that of the energy distribution. The contour of the total exposure dosage and the development condition combine to determine the final sidewall profile of the SU-8 pattern.
Numerical simulations were conducted for two different mask patterns, shown schematically in Fig.
5
. One is an open slot aperture and another is an obscuration.
These produce, in negative resists such as SU-8, a protrusion and an open channel, respectively. When displaying the simulation results, light intensities were normalized by its value at the resist surface: the light intensity at the same level or higher than the incident lithography light is denoted as 1 and shown as a bright gray; lower light intensity areas in resist are denoted with smaller numbers and shown as darker gray.
Figure
6
shows simulation results for dosage distributions corresponding to a 20 l m slot aperture with i -line
(365 nm) and h -line (405 nm) exposures, respectively. In all simulations, a resist thickness of 500 l m and a zero air-gap were assumed. Brighter and darker regions represent, respectively, higher and lower exposure dosages.
Dashed lines indicate the boundary between regions with or without sufficient dosage to cross-link the SU-8 resin, and therefore potential sidewalls of the final SU-8 structure after development. Figure
6
a shows the dosage
Fig. 3 Different SU-8 plating mold and mold insert geometries

548
Fig. 4 Optical transmission curves of SU-8 distribution produced by light exposure through the 20l m slot aperture considering only diffraction effects.
Figure
6 b shows the dosage distribution caused by
substrate reflection, assuming a reflectivity of 0.85 for the gold-coated plating seed-layer surface. Figure
6
c shows the total exposure dosage distribution, combining diffraction and reflection effects. Figure
6
d shows the total dosage distribution for h -line (405 nm) exposure. It can be seen that the dosage distribution shown in Fig.
6 d is
more uniform, with the top-to-bottom difference significantly reduced. This implies that the width difference between the top and bottom of the final SU-8 structure will be much smaller with h -line exposure as compared to that with i -line exposure.
Simulation results for a 20l m slot obscuration with h -line and i -line exposures are shown in Fig.
7
a, b, respectively. Because of the negative tone of the
SU-8 resist, patterns obtained with a slot obscuration exposure will be channels. As shown in Fig.
7
a, b, the channel obtained with i -line exposure will be narrower at the top and wider at the bottom. Using h -line exposure, the channel will be wider at the top and narrower at the bottom. To obtain the desired taper in the SU-8 plating mold for mold insert fabrication, an exposure wavelength of 405 nm should be selected. Figure
7
b shows that a narrow strip at the middle of the obscured region is also significantly exposed. This localized high dosage strip within the obscured region is responsible for the fact that, when the desired channel pattern becomes too narrow, full development becomes very difficult.
Figure
8
shows a SU-8 protrusion cross with a designed width of 20 l m and a height of 1,150 l m. This structure was obtained with h -line exposure. The scanning electron microscopy (SEM) images show that the width of the SU-8 cross varies from 13.5
l m at the top to
19.7
l m at about 75 l m from the top, finally to 32.8
l m at the bottom. This represents a taper angle of 1 o
.
3 Fabrication of two-level mold inserts
Fig. 5 Two different cases studied in the numerical simulations of
UV-lithography of SU-8.
a Open aperture to generate a post; b obscuration used to produce groves
To demonstrate our UV-LIGA process for multi-level mold insert fabrication, a two-level electroplated Ni mold insert with 500 l m-deep microholes and 250 l mdeep microchannels was built. To form a small positive taper angle on all protruding microscale features on the insert, h -line exposure at 405 nm was chosen.
Numerical simulations were performed for the SU-8 plating mold geometry. The plating mold included holes
200 l m in diameter and 500 l m in depth, together with rectangular channels 200 l m in width and 250 l m in

549
Fig. 6 Simulation results for a 20l m slot aperture.
a Dosage distribution due to diffraction effects with i -line exposure (365 nm); b dosage distribution due to substrate reflection with i -line exposure (365 nm); c total dosage distribution for a 20 l m slot aperture with i -line exposure (365 nm). The resulting structure shows a wider top and a narrower bottom ; d total dosage distribution for a 20l m slot aperture with h -line exposure
(405 nm). The resulting structure shows less taper as compared to the i -line exposure case depth. Using h -line exposure, the simulations showed hole diameter reduction from 200 l m at the top to
193 l m at the bottom, corresponding to a positive taper angle of 0.8
° . Simulations also showed channel width reduction from 200 l m at the top to 197 l m at the bottom, corresponding to a positive taper angle of
0.69
° . Without agitation during resist development after exposure, resist at smaller depths receives longer developing time and exposure to fresher developer liquid. This difference in development tends to further increase the positive taper angle.
The processing flow chart for a two-level mold insert is shown schematically in Fig.
9 . Two exposure masks
are needed. The finished two-level mold insert has cylindrical microposts with a diameter of 200 l m and a height of 500 l m, together with rectangular protrusions with a length of 18,000 l m, a width of 200 l m, and a height of 250 l m. The fabrication process is as follows:
(1) clean Si wafer with acetone, isopropyl alcohol
(IPA), and de-ionized water, dry; (2) electron-beam evaporate a Cr/Au plating seed layer; (3) pattern alignment marks on the plating seed layer; (4) spin coat
250 l m SU-8 50 at 900 rpm; (5) pre-bake spin coated
SU-8; (6) expose SU-8 with the no. 1 exposure mask;
(7) post-bake exposed SU-8; (8) electron-beam evaporate a second Cr/Au seed layer on the exposed SU-8;
(9) develop SU-8 with agitation, with simultaneous liftoff of the second Cr/Au seed layer attached to the unexposed and developed SU-8; (10) spin coat a second layer of SU-8 50 at 870 rpm; (11) pre-bake spin coated
SU-8; (12) expose SU-8 with the no. 2 exposure mask;
(13) post-bake and develop SU-8; (14) electroplate Ni until complete overplating is achieved; (15) polish backside of the Ni overplate; (16) KOH etch to remove the Si substrate; (17) etch Cr/Au seed layer, then remove SU-8.
In our experiments, a 4.538 mm thick PMMA piece is used as a filter for removing the i -line radiation from the
UV source. The spectral output of the UV source before and after the PMMA filtering are shown in Fig.
10 . The
spectral output after PMMA filtering consists of a combination of the h -line and the g -line (434 nm). The longer wavelength g -line accentuates diffraction effects and consequently the positive draft angle on the result-

550
Fig. 7 Dosage distributions for generating SU-8 channels: a a 20l m slot obscuration with i -line exposure (365 nm); b a 20l m slot obscuration with h -line exposure (405 nm)
Fig. 8 SU-8 protrusions with tapered sidewalls ( narrower top and wider bottom ) ing features. Because the absorbance of the g -line radiation in the SU-8 resist is lower (about 1/3 as compared to that of h -line), the h -line dominates the exposure. The effects of the g -line radiation are therefore neglected in this study. Figure
11
shows an SEM micrograph of a prototype SU-8 two-level plating mold.
Using a two-level SU-8 plating mold, consisting of
500 l m-deep microholes and 250 l m-deep microchannels, a Ni mold insert was fabricated by Ni electroplating into the SU-8 plating mold followed by overplating.
As shown in Fig.
9 e, there are two separate Cr/Au
electroplating seed-layers connected, respectively, to the microholes and the microchannels. During electroplating, the seed layer for the microholes was connected into plating circuits first. After the Ni deposit in the microholes were plated up to the same height as or slightly higher than that of the seed layer for the rectangular protrusions, the two seed layers were connected together electrically while the electroplating continued.
This process led to the microholes and microchannels in the SU-8 plating mold being ‘‘filled up’’ simultaneously.
Continued Ni overplating after complete SU-8 plating mold fill-up produces the backing plate of the Ni mold insert. Electroplating of the Ni insert was therefore completed in a single continuous process, in contrast to the multiple steps of patterning, plating, and polishing as used in other approaches, and results in no weak interfaces in the Ni insert. The plating current used to electroplate posts was 2.74 mA (current density of 10 mA/ cm
2
). The current used to plate the channels was 6.4 mA
(also equivalent to a current density of 10 mA/cm
2
). The total plating time for the posts and channels was about
166 h, about 83 h for each step. The average height for the posts was measured to be 415 l m, and the average height for the channels was about 335 l m as shown in
Fig.
12
a.
After completion of the electroplating process, the backside of the over plated Ni mold insert was polished with a lapping machine, using the Si wafer substrate as the polishing reference. A flat mold insert backside was obtained after polishing. A KOH solution was then used to remove the Si substrate. After etching of the Cr/Au seed layer, the Ni insert together with the SU-8 plating

551
Fig. 9 Flow chart for the fourteen fabrication steps of a 2-level mold insert: a steps 1–7; b step 8; c step 9; d step 10–11; e step 12–13; f step
14–15; g step 16; h step 17
Fig. 10 The spectrum of the lithography light mold was dipped into a Dynasolve solution at 90 ° C, the
SU-8 plating mold was removed in 30 min. According to Bacher et al. (
1998
), the hardness of electrodeposited
Ni shows no obvious change when the annealing temperature is below 200 ° C. Significant softening of the electrodeposited Ni insert is therefore not expected due to the SU-8 removal process. After SU-8 removal, the Ni insert was cleaned with acetone and IPA to remove any residue on the Ni surface. A finished two-level Ni mold insert is shown in the SEM images of Fig.
12
.
Electroplated Ni HARMS have been used directly as inserts for molding replication of metal-based structures

552 refractory metals and alloys (Cao et al.
2005
). The use of multi-level, electrodeposited Ni HARMS for direct transfer of multi-level micropatterns to high temperature metals and alloys remains to be explored.
4 Summary and conclusion
Fig. 11 A two-level SU-8 plating mold after engineering its surfaces by depositing a conformal ceramic coating (Cao et al.
2003
; Cao and Meng
2004 ).
More recently, single-level, high-temperature compatible mold inserts were fabricated by transferring microscale patterns as defined by electroplated Ni HARMS to
A UV-LIGA process for fabricating two-level electroplated Ni mold inserts by combining UV-lithography on
SU-8 resist, one-step Ni electroplating, followed by complete SU-8 resist removal was described. Multiple
UV-lithography steps performed on SU-8 resists were used to produce the two-level SU-8 plating mold. A single Ni electroplating step into the SU-8 plating mold was used to fabricate the two-level Ni insert. This process eliminates the presence of extraneous interfaces within the electroplated Ni insert. Numerical diffraction simulations showed that UV-lithography of SU-8 with an h -line-dominant light source helps to generate a desirable positive taper angle for the microfeatures on the insert.
Acknowledgement Partial project support from the National Science Foundation through grants DMI-0400061, EPS-0346411, and the Louisiana Board of Regents through contract LEQSF (2004-
07)-RD-B-06 is gratefully acknowledged.
References
Fig. 12 SEM images of an electroplated prototype Ni two-level molding insert
Bacher W, Bade K, Matthis B, Saumer M, Schwarz R (1998)
Fabrication of LIGA mold inserts. Microsyst Techol 4:117–119
Cao DM (2004) Replication of metal-based high-aspect-ratio microscale structures by high temperature micromolding. PhD
Dissertation, Louisiana State University
Cao DM, Meng WJ (2004) Microscale compression molding of Al with surface engineered LiGA inserts. Microsyst Technol
10:662–670
Cao DM, Guidry D, Meng WJ, Kelly KW (2003) Molding of Pb and
Zn with microscale mold inserts. Microsyst Technol 9(8):559
Cao DM, Meng WJ, Kelly KW (2004) High-temperature instrumented microscale compression molding of Pb. Microsyst
Technol 10:323
Cao DM, Jiang J, Yang R, Meng WJ (2005) Fabrication of highaspect-ratio microscale mold inserts by parallel EDM. Microsyst Technol (submitted)
Cedorge T, Colton J (2000) Draft angle and surface roughness effects on stereolithography molds. Polym Eng Sci 40(7):1581–1588
Dentinger PM, Clift MW, Goods SH (2002) Removal of SU-8 photoresist for thick film applications. Microelectr Eng 61–
62:993–1000
Dinakar P (2003) Micro molding of 3D ceramic parts. MS Thesis,
Louisiana State University, August
Ehrfeld W, Schmidt A (1998) Recent developments in deep X-ray lithography. J Vac Sci Technol B 16(6):3526–3534
El-Kholi A, Bley P, Gottert J, Mohr J (1993) Examination of the solubility and the molecular weight distribution of PMMA in view of an optimised resist system in deep etch X-ray lithography. Microelectr Eng 21(1–4):271–274
Guckel H, Fischer K, Stiers E, Chaudhuri B, McNamara S, Ramotowski M, Johnson ED, Kirk C (2000) Direct, high throughput LIGA for commercial applications. Microsyst
Technol 6(3):103–105
Heckele M, Schomburg WK (2004) Review on micro molding of thermoplastic polymers. J Micromech Microeng 14:R1–R14

553
Henry AC, McCarley RL, Das SS, Khan-Malek CG (1999)
Characteristics of commercial PMMA sheets used in the fabrication of extreme high-aspect-ratio microstructures. J Electrochem Soc 146(7):2631–2636
Hupert M, Witek M (2003) Polymer-based microfluidic devices for biomedical applications. In: Proceedings of SPIE—the International Society for Optical Engineering 4982:52–64. Conference: Microfluidics, BioMEMS, and Medical Microsystems,
Jan 27–29 2003, San Jose
Janczyk M, Mclaughlin R, McCarthy SP (1997) Rapid stereolithography tooling for injection molding: the effect of cooling channel geometry. J Inject Molding Technol 1(1):72–78
Jian Z (2002) LIGA mold insert fabrication using SU-8 photoresist.
MS Thesis, Louisiana State University
Kim K, Park S, Lee JB, Manohara H, Desta Y, Murphy M, Ahn
CH (2002) Rapid replication of polymeric and metallic high aspect ratio microstructures using PDMS and LIGA technology. Microsyst Technol 9:5
Kondo R, Suzuki K, Sugiyama S (1998) Study on fabrication of high aspect ratio electrostatic micro actuators using LIGA process. In: Proceedings of the international symposium on micromechatronics and human science, pp 155–160
Menz W, Bacher W, Harmening M, Michel A (1991) The LIGA technique—a novel concept for microstructures and the combination with Si-technologies by injection molding. In: Proceedings IEEE micro electro mechanical systems, pp 69–73
Mohr J, Bley P, Strohrmann M, Wallrabe U (1992) Microactuators fabricated by the LIGA process. J Micromech Microeng
2(4):234–241
Muralidhar KG, Jason PH, Erol CH, Dinesh KS (2001) Patterning, electroplating, and removal of SU-8 moulds by excimer laser micromachining. J Micromech Microeng 11:133–139
Pham GT, Colton J (2002) Ejection force modeling for stereolithography injection molding tools. Polym Eng Sci 42(4):681–
693
Qu W, Wenzel C, Gerlach G (1999) Fabrication of a 3D differential-capacitive acceleration sensor by UV-LIGA. Sens Actuators A Phys 77(1):14–20
Rees H (2001) Understanding injection mold design. Hanser
Gardner Publications, Cincinnati
Rosato D, Rosato D, Rosato M (2000) Injection molding handbook, 3rd edn. Kluwer, Dordrecht
Ryan T (2002) Tapered mold insert. MS Thesis, Louisiana State
University
Sadler D, Gupta S, Ahn C (2001) Micromachined spiral inductors using UV-LIGA techniques. IEEE Trans Mag 37(41):2897–2899
Schulz J, Bade K et al (2004) Ensuring repeatability in LIGA mold insert fabrication. Microsyst Technol 10:419–422
Singleton L, Schenk R, Haverbeck O, Tunayer A, Himmelsbach A,
Krempel C, Detemple P, Lacher M (2002) Design of experiments optimization of PMMA for LIGA applications. In:
Proceedings of SPIE—the international society for optical engineering, 4690(II):1180–1187
Turner R, Desta Y et al (2003) Tapered LIGA HARMs. J Micromech Microeng 13:367–372
Williams JD, Ren Y, Wang W (2005) Numerical simulation and test of a UV-LIGA fabricated electromagnetic micro-relay for power applications. Sens Actuators A Physical 120(1):154–162.
DOI 10.1016/j.sna.2004.10.034
Yang R, Wang W (2005) A numerical and experimental study on gap compensation and wavelength selection in UV-lithography of ultra-high-aspect-ratio SU-8 microstructures. Sens Actuators
B Chem 110(2):279–288
Yang R, Seok JJ, Wang W (2003) UV-LIGA microfabrication of a power relay based on electrostatic actuation. SPIE Photonics
West, MEMS Components and Application for Industry, San
Jose
Yang R, Williams J, Wang W (2004) A rapid micro-mixer/reactor based on arrays of spatially impinging micro-jets. J Micromech
Microsyst 14(10):1345–1351