Microsystem Technologies 10 (2004) 699–705 Ó Springer-Verlag 2004
DOI 10.1007/s00542-004-0396-1
Microfabrication of an electromagnetic power relay
using SU-8 based UV-LIGA technology
J. D. Williams, W. Wang
699
The fast developing technology of microelectromechanical systems (MEMS) has opened up new opportunities for developing new types of micro-relays [Peterson,
1978; Sakata, 1989]. Like the revolution started by the
miniaturization of electronic circuits, the miniaturization
of mechanical systems will definitely have a huge impact
on various industries. The principal technology for fabricating micro-mechanical elements has been the siliconbased technology, which is relatively mature due to years
of microelectronics R & D. Most of the commercially
available micro-mechanical components and systems are
made of silicon.
Compared with the solid-state switches, electromechanical micro-relays have the same advantages as normal
mechanical relays: lower on-resistance, higher off-resistance, higher dielectric strength, lower power consump1
tion, and lower cost. Furthermore, by using MEMS
Introduction
technology, the size, cost, and switching time of mechanRelays are widely used in various industries for a wide
variety of applications. Traditional mechanical relays are ical relays are greatly improved, and they can also be
combined with other electronic components. These
large, slow, noisy devices, but are still widely used in
advantages are even greater with multi-contact relays, such
various machines and processes for control purposes.
as matrix switches.
Solid-state switches, which have much longer life times,
There have been several different prototypes of microfaster response, and smaller sizes when compared with
relays reported, most of which were electrostatically
conventional mechanical relays, have been available in
actuated. Gretillat [Gretillat et al., 1995] reported an elecdifferent forms for many years. However, solid-state
trostatic polysilicon micro-relay integrated with MOSswitches have low off-resistance and high on-resistance,
which result in high power consumption, and poor elec- FETs. Drake et al. (1995) reported an electrostatically
trical isolation (typically no better than )30 dB). Design actuated micro-relay for use in automatic test equipment
trade-offs for reducing the ‘‘on-resistance’’ of solid-state (ATE). Guckel et al. (1996) reported an electromagnetic
microactuator with inductive sensing capability. It uses a
relays tend to increase output capacitance, which introLIGA-fabricated (German acronym for lithography, elecduces additional problems in applications requiring the
trodeposition and plastic molding) magnetic core wrapped
switching of high frequency signals.
with a mechanically wrapped coil. The electromagnetic
actuator has a planar design and relatively larger size (in
the millimeter range). Other groups have also reported
Received: 8 August 2003/Accepted: 6 November 2003
work on electromagnetic driven micro-relays. In 1994,
Hosaka et al. (1994) reported some fundamental work on
J. D. Williams, W. Wang (&)
electromagnetically actuated micro-relays. In their reDepartment of Mechanical Engineering,
search, a miniature (inch-size) electromagnetic relay was
Louisiana State University,
constructed using a microfabricated spring and a large
Baton Rouge, LA 70803, USA
external coil assembled together.
e-mail: [email protected]
Rogge et al. (1995) the first LIGA fabrication process for
The research work reported in this paper was made possible by micro-relays. In their study, an integrated magnetic core
support from National Science Foundation under grant ECSwrapped with a microfabricated planar coil is made with
#0104327; Louisiana Board of Regents, National Aeronautics and
the LIGA process. Many planer electromagnetic coils have
Space Administration, and the Louisiana Space Consortium
also been developed for electromagnetic actuation from
under agreement NASA/LEQSF (2001–2005)-LaSPACE, and
silicon-based technologies (Chaing et al., 1993; Yao, 1995;
NASA/LaSPACE under grant NTG5–40115. The authors would
also like to thank the Center for Advanced Microstructures and Ribbas et al., 2000; Jiang et al. 2000;) These devices use
Devices of LSU for the use of the cleanroom facility.
solenoids that yield unconfined magnetic fields perpenAbstract Most of the MEMS relays reported in the field
now are based on silicon fabrication and cannot be used
for power applications. In this paper, we report a research
effort to microfabricate an electromagnetic relay for power
applications using a multilayer UV-LIGA process. A
mechanically wrapped coil was used and very simple design for the magnetic circuit was adopted to increase the
design flexibility and performances. The broad material
selection and the capability of making high aspect ratio
microstructures of the UV-LIGA make the technology best
suited for fabricating microelectromechanical relays for
power applications. The prototype relay has a truly threedimensional structure and very suitable for large power
capacity applications.
700
dicular to the plane of the wafer. These electromagnets can
then be placed in close proximity, above or below a microactuator to induce a mechanical motion. Typical coils
produced by this technology are comprised of very thin
wires that cannot carry high currents. They typically have
less than 20 turns yielding a low magnetic flux.
Recently, Kim and Allen (1998) produced a multilayer
UV LIGA fabricated electromagnetic solenoid (Park and
Allen, 1999). The solenoid was made using multiple resist
patterning and electrodeposition steps to create a copper
coil around a permalloy material. Other similar devices
have been developed (Chen et al., 2001; Siedemann, 2003).
using the same fabrication techniques. They all use thick
UV resist processing to produce a coil pattern that is
electroplated to produce a solenoid parallel to the surface
of the wafer. This process requires long distances to wrap a
high number of turns and provides no magnetic confinement.
The practical utilization of electromagnetic actuators in
MEMS systems is still very limited. The most common
actuation principle is still the electrostatic one. This is
mainly due to the difficulties in fabrication of micro-sized
electromagnetic actuators. Another reason is that the
electromagnetic actuation would typically require a sizable
current and therefore contribute to power consumption
and heat generation.
control devices and isolation wiring are required to handle
these voltages, resulting in a higher overall cost.
Advantages in using UV-LIGA to fabricate power microrelays
To achieve higher power capacity, a micro-relay based on
MEMS technology should have following characteristics:
(1) Materials of high electric conductivity, for example,
metals or alloys such as copper should be used as electrodes; (2) relative thick wire pattern to carry larger current, other than thin films as in silicon based surface
fabrication technology; (3) high aspect-ratio structures
should be used in the design of the electromagnetic actuator to boast the electromagnetic force and the maximum
moving range, therefore permit a smaller driving voltage;
(4) and finally, to compete with the solid state devices, the
MEMS relays should also have a simple design so that the
final product can be commercially competitive.
Flexibility in materials selection and the capability of
making high aspect ratio microstructures with the LIGA
process make it the best technology for microfabricating
high power micro-relays and relay arrays. Compared with
the more conventional MEMS processes based on silicon
technology, the LIGA process has some unique advantages: (1) it allows the fabrication of high aspect ratio
microstructures with structural heights up to hundreds or
one thousand micrometers; (2) different materials such as
plastics, metals, alloys, ceramics, or a combination of
these materials can be used with the ‘‘LIGA’’ process.
Advantages of electromagnetic actuation
This second advantage makes it very suitable for
The practical utilization of electromagnetic actuators in
MEMS systems is still very limited. This is partially due to Microfabrication of the power micro relay. For example:
beryllium-copper can be used for making a micro-spring,
the difficulties in fabrication of micro-sized electromagnetic actuators with silicon-based MEMS technology, and thick metal structures can be used as electrodes to carry
partially because of some misconceptions about electro- larger current, good electrically conducting materials
such as beryllium-copper and gold can be electroplated
magnetic actuation. Busch-Vishniac (1992) provided a
as electrodes for better contacting condition and reduce
detailed discussion and strong arguments for electrospark corrosion.
magnetic micro-actuators by direct comparison of a
magnetic and an electrostatic microfabricated actuator. It
was concluded that the case for magnetic actuation is
2
compelling at most of the scales of interest.
Designs for the electromagnetic power micro-relay
If the efficiencies of electrostatic and electromagnetic
Figure 1.(e) shows a schematic design of the design for
actuators are compared, the smaller the actuator the more
electromagnetic micro-relay, and Figs. 1. (a), (b), (c) and (d)
advantageous the electrostatic type, with the transition
show the operation principle of the relay. The relay is
point around 1 mm [Hosaka et al., 1994]. From a practical
comprised of the following components: a microfabricated
point of view, this size, which is two to four orders of
spring, two electrodes – one for the input and the other for
magnitude smaller than conventional relays, is sufficiently
output, an electromagnetic actuator, and the magnetic core
small because, even if it were further reduced, total system
for coil and magnetic flux circuit. The magnetic core (the
size would not change much since the peripheral circuit
circular post in the center) and magnetic flux circuit will be
would be the dominant feature.
used as one of the electrodes, and the spring, which is
Two key factors in selecting an actuation mechanism
electrically isolated from the remaining part of the relay, will
are reliability and cost. First, since closing and opening of
be used as the other electrode. An electromagnetic coil,
the mechanical contacts are dust-making processes, the
which is not shown in Fig. 1(e), is inserted on the central
actuator for the micro-relays must be unaffected by parpost in the diagram. This arrangement helps to reduce the
ticulate. Electromagnetic actuators are superior to elecnumber of components and simplifying the overall design.
trostatic actuators in this aspect, since the latter are
The design presented here is a truly three-dimensional deaffected by contaminant adhesion. Secondly, when consign, unlike previous prototypes reported in the literature.
sidering the overall cost of micro-relays, both the relays
A typical operation sequence for the device can be
and the peripheral circuit costs must be taken into acshown schematically in the Fig. 1(a), (b), (c) and (d):
count. Electromagnetic actuators can be controlled with
low-cost electronics, but electrostatic actuators typically
(1) If there is no control current supplied to the coil, the
need high voltage (>30 v) to operate. Specially designed
spring remains in the neutral position and the relay is
operation of the micro relay
701
Fig. 1. Schematic diagrams for the design
and operation of the micro-relay
netic circuit is designed in such a way so that the magnetic
flux is well confined.
The flat spring in the design is mounted on separate
supporting structures to have it electrically isolated from
the magnetic core and flux circuit. This arrangement is
necessary because both the magnetic core and the spring
are used as electrodes and the signal wires will be connected to them for design simplicity. Another design option is to use the flux circuit as the supporting structure
and directly bond the spring to it. In that case, an insulating layer will be required between them. To lower the
Most previous micro-relay designs have attempted to micontact resistance between the electrodes, a thin film of
crofabricate coils, therefore have significantly increased
gold may also be deposited on the electrode surfaces.
the complexity of the fabrication process. The LIGA
technology, and silicon-based MEMS technology alike,
Electromagnetic and dynamic analysis of the micro-relay
should only be used for the work best suited for it. A
Since detailed modeling and analysis of the prototype
hybrid approach may be taken for fabrication of the
electromagnetic micro-relay is not the purpose of this
electromagnetic components of the micro-relay. Using the
presentation, only a brief of discussion of the related
UV-LIGA process for fabrication of a magnetic core and
mathematical model will be provided here. The main
magnetic flux circuit has overwhelming advantages over
purpose of this discussion is to provide a realistic assesssilicon-based MEMS technology because of the unique
ment of the performance and a guide for the basic design
advantages of LIGA in making microstructures with high
of the relay. For the purposes of brevity, specific details are
aspect ratios and the flexibility in materials selection.
not presented.
However, making micro-coils with either silicon-based
A lumped-parameter approach can be adopted to derive a
MEMS technology or the LIGA process is not a job that
simplified, closed form mathematical model for the case of a
can be easily and efficiently done. Insulated copper wires
permalloy spring (no permanent magnet required). The
with diameters as small as 25 lm are commercially availmagnetic flux density in the air gap between the magnetic
able and coils can be custom-ordered. A micro-sized
core and the spring is assumed to be uniform. Then the
electromagnetic actuator with the magnetic core, flux cirsystem in Fig. 1 may be modeled as a mass-spring-damper
cuit, and springs made with the UV-LIGA process,
system, where the equivalent spring constant of the flat
assembled with a mechanically-wrapped coil would have
spring is K, the damping effect is b and the equivalent mass is
much lower overall cost than a monolithic microfabricated
M. The equivalent resistance of the coil is R.
system.
Under the stated assumptions, the flux linkage k can be
A mechanically wrapped micro-coil was used in the
derived as
electromagnetic actuator. A permalloy magnetic core and
magnetic flux circuit was fabricated with UV-LIGA prol0 AN 2 i
k
¼
:
ð1Þ
cess. The micro-coil was then inserted vertically onto the
gþx
magnetic core. The spring was made using the LIGA
The instantaneous electromagnetic force working on the
process with either copper alloy, or permalloy. If the
permalloy flat spring Fe can be expressed as:
spring is made of permalloy, then a closed circuit for
magnetic flux can be made and a permanent magnet for
1
L0 i2
e
ð2Þ
self-latching may not be necessary. Both approaches will F ¼ 2 ;
2
gð1
þ
x=gÞ
be evaluated and their performance compared. The magoff (electrodes do not make contact) as shown in
Fig. 1(a).
(2) If a current is supplied to the coil, the electromagnetic force generated by the electromagnet pulls
the spring downward until contact is made between
the two electrodes (the spring and the magnetic
core).
(3) If the control current is turned off, the elastic force
restores the flat spring to the horizontal position,
contact is broken and the relay is off.
where L0=self-inductance from the gap flux when the
center of the flat spring touches the magnetic core:
N ¼ number of turns on the coil, l0 ¼ permeability of the
air gap, A ¼ cross-sectional area of the air gap, x ¼
displacement of the center of the flat spring, g ¼
displacement of the outer rim of the flat spring.
Then the electrical and mechanical dynamic equations
describing the system are:
702
3
The multi-layer UV-LIGA process
From the design of the relay, it can be seen that the electrical wire patterns, the magnetic core, and the supporting
pads for the suspension spring are all at different heights.
The thickness of the wire patterns and the suspension
spring set the limit for the power capacity. Therefore
several layers of metal/alloy structures at different thickness need to be fabricated. A multi-layer UV-LIGA process
L0 dt
L0 i
dx
was developed. The detailed fabrication process is pre
þ
Ri
¼
v
;
ð3Þ
1 þ x=g dt gð1 þ x=gÞ2 dt
sented in Table 1. Figures 2 through 10 show shows a
group of photographs of the prototype electromagnetic
and
relay at different process stages.
d2 x
dx
To reduce the probability of electrostatic breakdown,
e
M 2 þ b þ Kx ¼ F ;
ð4Þ alumina was used as the substrate for this process. UV
dt
d
lithography processing of SU-8 on alumina substrates has
From these two equations, expressions for both the elecpreviously demonstrated aspect ratios over 40:1 [17].
trical and mechanical time constants can be derived:
Fabrication of thick metal layers on a porous ceramic
substrate proved difficult. To prevent under-etching of the
l0 AN 2
se ¼
:
ð5Þ metal layer, a liftoff process was used. AZ 4580 was used to
Rgð1 þ x0 =gÞ
create the pattern for the plating base, and a 1 lm thick
and
copper layer was deposited into the pattern by physical
vapor deposition. The resist was then removed to reveal
2M
:
ð6Þ the pattern shown in Fig. 2. Copper leads extend from the
sm ¼
b
pattern to larger electrical contacts used for electrodeThe damped natural frequency of the mechanical system is position. A subsequent layer of SU-8 between 50 and
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
250 lm thick was then patterned over the plating base to
2ffi
reveal the initial layer of the relay. Nickel was uniformly
K
b
xnd ¼
:
ð7Þ electrodeposited into the pattern, as shown in Fig. 3, using
M
2M
a modified nickel sulfamate bath. Chemical mechanical
The system parameters can be set based on an analysis of polishing was perfomed if needed to provide a uniform
the performance using the model.
nickel layer thickenss. The substrate was then cleaned
The mass M in Eq. (4) can be estimated using the
using a strong base and plasma ashed prior coating a 2nd
conventional method of one third of the weight of the
spring. Given the geometric design and the material of the
spring, it can be easily calculated. The spring constant k Table 1. Fabrication Procedures
can be estimated by treating the flat spring as cascaded
Step#1
Copper electroplating base patterned on 4’’ alumina
cantilever beams. Assume small deflection, the total
substrate using a liftoff process. The result is
deflection of the spring at its center of can be roughly
shown in Fig. 2.
estimated using the fundamental relations in macro scale Step#2
Nickel electroplated into patterned SU-8 layer to
and linear superposition method. Each section of the
produce relay base capable of carrying large
cantilever beam is treated as a beam in bending and torcurrents. The result is shown in Fig. 3.
Step#3
Substrate is coated with 50 lm of SU-8 and exposed
sion.
to reveal interconnect patterns shown in Fig. 4.
The prototype design for a relay with a current
Spin-coat another layer of 1000–1500 lm of SU-8
capacity of 100 mA was modeled and simulated to and Step#4
resist. Expose, and develop to form the negative
the results are provided here to offer some general ideas
pattern of the bottom part of the relay. Figs. 5 and
about the possible design parameters and the relay
6 shows a photograph of the exposed photo pattern
performances. Assume copper is used as the main
obtained.
material for its good electrical and mechanical proper- Step#5
Nickel is then electroplated selectively into the
pattern to produce the magnetic cup, core post and
ties. Assume each piece of flat springs are designed to
electrical leads for the relay. Figs. 7 and 8 show the
have four cascaded beams with a width of c ¼ 50 lm
photographs of the bottom parts of the relay that
and a thickness of h ¼ 15 lm, and length a ¼ 150 lm,
has been electroplated.
the length b ¼ 450 lm. Another spring design called for
Step#6
Solenoid was wrapped using 50 lm diameter copper
a curved beam spring configuration. Beam width and
wire and inserted onto the relay base. Figure 9
thickness remained the same, however the geometry and
shows a micro-coil wrapped mechanically.
therefore the spring constant change. Also assume the Step#7
Magnetic spring made of electroplated NiFe was
made on a second substrate.
diameter of the magnetic core is 200 lm, the height of
Plunger is then released from its substrate and
the magnetic core is 1000 lm, and a 20 lm air gap exist Step#8
bonded on top of the base to yield a device.
between the relay base and the spring-mass. A control
Figure 10 shows a photograph of the relay that has
current of 10 mA is to be supplied to a coil of 30 turns
been assembled.
to drive the relay.
703
Fig. 4. 50 lm thick SU-8 intermediate insulation layer on nickel
Fig. 2. Patterned copper plating base
SU-8 layer. The Intermediate SU-8 layer was then coated
and patterned to reveal only the nickel areas that were to
be plated to the top of the relay base. Figure 4 shows a
photograph of the patterned intermediate layer. The white
rings around the pattern are areas of debonded SU-8 due
to the poor adhesive properties of nickel and SU-8 resist.
The debonded areas shown did not however, did not lead
to underplating and did not affect the final product. After
development, the substrate was dried, cleaned in O2 plasma and coated with the final SU-8 layer.
The third resist layer was required a spin coat between
1000 and 1500 lm thick. Two spin coats were required to
obtain layers thicker than 1300 lm in height [17]. The
pattern presented in Fig. 5 is 1200 lm tall with a core
cylinder wall that is 75 lm thick. The insulation layer
between the magnetic cup and the solenoid is 50 lm wide,
yielding an aspect ratio of 24:1.
Cured SU-8 has been well known to be extremely difficult to strip. At the same time, it also has very good
mechanical properties. It has quite high dielectric constant
and is very good electric insulator. In our design, effort has
been made to avoid the unnecessary stripping procedures
of cured SU-8. While the geometric constraints permitted,
the cured SU-8 is always kept as part of the permanent
structure for the relay. For example, the magnetic core also
serves as the conductor pad for the current to be switched
on, the cured SU-8 was used to serve as the insulator between the coil to be inserted later and at the same time, it
serves as the electroplating mold for fabricate the magnetic
core.
The relay base cross section spliced and imaged in
Fig. 6 shows a 1350 lm tall relay base with a maximum
aspect ratio of 27:1. It can be seen that the micro-patterns
for both the cylindrical magnetic flux path (to hold the
coil) and the magnetic core were designed in such as way
that the exposed SU-8 resist do not need to be stripped and
are kept as part of the overall structure after electroplating
process is completed. The 50 lm SU-8 intermediate
insulation layer patterned around the base of the relay’s
core post is reveled in Fig. 6. The flat pad with a circular
tip in the lower left corner of the SEM image is the first
electroplated nickel layer. This pattern was used to plate
one of the four nickel cylinders surrounding the magnetic
cup that were used to support the Ni-Fe spring mass.
Figs. 7 and 8 are images of a plated relay base after the
cylinders were polished down to a height of 1000 lm. The
insulation layer prevented plating inside the magnetic up
and allowed for easy insertion of a wrapped solenoid for
electromagnetic actuation. After plating, a flash of copper
or gold was plated onto the nickel patterns to enhance the
bonding process.
Fig. 5. Relay base after patterning the 3rd SU-8 layer. The thick
Fig. 3. 200 lm of nickel uniformly plated into the first SU-8 layer layer is 1200 lm tall on top of 200 lm of electrodeposited nickel
and a 2nd 50 lm layer of SU-8
patterned over the copper
704
Fig. 8. SEM image showing electroplated nickel structures in
relay base. A solenoid is inserted into the cup and leads passed
through the slots on either side
Fig. 6. SEM image of relay base cross section. Both the initial
nickel base and the insulation layer are revealed under 1350 lm
The spring mass was then bonded to the top of the
thick SU-8 layer
completed relay base. The bonding process can either be
Figure 9 is a photograph of a 30 turns solenoid wrapped
using 46 gauge insulated copper wire. A stickpin was
placed next to the coil to demonstrate size. After wrapping,
the solenoid was bound using adhesive and placed into the
magnetic cup on the relay base. The wire leads of the coil
were placed into the patterned slots on either side of the
magnetic cup. They were then soldered to square nickel
post shown in Fig. 7. This allows for easy interconnects
between the relay and an external driving source. Placement of the solenoid represents the completion of the relay
base. It can then be used to create local magnetic fields for
electromagnetic actuation of any magnetic material.
The power relay design required the actuation of a thin
magnetic spring mass. To make the spring mass, a flat NiFe spring was also successfully fabricated using a single
layer resist on a silicon substrate and then under-etching a
thick plating base or stripping an exposed negative resist
base. Currently, Fe content in the Ni-Fe alloy plated has
been tested with XES to contain approximately 20% wt Fe.
Fig. 7. Photograph of the relay base
done by soldering or glued using silver paste. Figure 10
shows a photograph of the relay that has been assembled.
The prototype relay is currently being tested in our lab for
response time and current capacity. Different spring
geometries are being evaluated to produce a relay with the
optimum desired response. Results will be presented in a
future work.
Conclusion
Most micro-sized switches (micro-relays) currently on the
market or reported in the literature are solid-state devices
made using semiconductor technology, with silicon the
predominant material. Such devices typically have low
current capacity, low off-resistance, significant on-resistance, significant power consumption, and low dielectric
strength. In recent years, the fast-evolving technology of
(MEMS) has opened up new opportunities for microfabricating microelectromechanical switches. However, most
of the MEMS relays based on silicon fabrication cannot be
used for power applications.
In this paper, the microfabrication of a prototype
electromagnetic actuated power relay was presented. The
device was fabricated using UV-LIGA technology using
Fig. 9. Micro-coil is wound around pin
Fig. 10. Photograph of the assembled prototype electromagnetic
relay
multiple layers of SU-8 and electroplated metal. A
mechanically wrapped solenoid insert was used to provide
the driving mechanism for the actuator. The solenoid can
be wrapped up to 4 layers thick and as tall as 1500 mm.
Because the relays are based on very thick metal pads for
conducting current, they expected to have the high power
capacity, high off-resistance, (completely disconnected)
lower on-resistance, low power consumption and low heat
generation. This provides sufficient flexibility to use the
electromagnetic driver for a wide variety of switching
applications as well as other devices such as sensors and
magnetic positioning devices.
References
Busch-Vishniac IJ (1992) The case for magnetically driven microactuators. Sensors and Actuators A, 33: 207–220
Chen TE; Yoon YK; Laskar J; Allen M (2001) A 2.4 GHz integrated CMOS power amplifier with micromachined inductors.
IEEE MTT-S International Microwave Symposium Digest
June: 67–70
Chaing JYC; Abidi AA; Gaitan M (1993) Large suspended
inductors and their use in 2 lm CMOS RF amplifier. IEEE
Electron Device Let 14: 246–248
Drake J; et al (1995) An electrostatically actuated micro-relay.
Transducer ’95, Eurosensors 1X 2: 380–383
Gretillat MA; Thiebaud P; Linder C; Derooij NF (1995) Integrated-ciruit compatible electrostatic polysilicon microrelays.
J Micromech Microeng 5(2): 156–160
Guckel H; Earles T; Klein J; Zook JD; Ohnstein T (1996) Electromagnetic linear actuators with inductive position sensing.
Sensos and Actuators A 53: 386–391
Hosaka H; Kuwano H; Yanagia K (1994) Electromagnetic microrelays: concepts and fundamental characteristics. Sensors
and Actuators A 40: 41–47
Jiang H; Wang Y; Yeh JLA; Tien NC (2000) On chip siprial
inductors suspended over deep copper-lined cavities. IEEE
Trans. Microwave Theory Tech., 48 (12): 2415–2423
Kim YJ; Allen MG (1998) Surface Micromachined Solenoid
Inductors for High Frequency Applications. IEEE Transactions on Components, Packaging & Manufacturing Technology, Part C (Manufacturing) 21 (1): 26–33
Lakawala H; Zhu X; Santhanam S; Carley LR; Fedder Gk (2002)
Micromachined high –Q inductors in a 0.18 lm copper
interconnect low-k dielectric CMOS process. IEEE J Solid –
State Circuits 37: 394—403
Park JY; Allen MG (1999) High Q spiral-type microinductors on
silicon substrates. IEEE Transactions on Magnetics 35 (5):
3544–3546
Petersen KE (1978) Dynamic micromechanics on silicon techniques and devices. IEEE Trans Electron Devices ED-25 10:
1241–1250
Ribbas R.P; Lescot J; Leclerq JL; Karam J.M; Ndagijimana F
(2000) Micromachined microwave planer spiral inductors and
transformers. IEEE Trans Microwave Theory Tech 48 (8):
1326–1335
Rogge B; Schultz J; Mohr J; Thommes A; Menz W (1995) Fully
batch fabricated magnetic microactuators using a two layer
LIGA process. Transducer ’95, Eurosensors IX: 320–323
Sakata M (1989) An electrostatic microactuator for electromechanical relay. Proc.IEEE MEMS Workshop ’89, Salt Lake
City, UT, USA pp. 149–151
Siedemann V; Kohlmeier T; Gattzen HH; Buttgenback S (2003)
High aspect ratio spiral and helical micro coils for actuator
applications. High Aspect Ratio Micro-Structure Technology
Workshop 2003: Monterey, CA, USA pp. 175–176
Sun Y; Tauritz Jl; Beats RGF (1999)Micromachined RF passive
components and their applications in MMICs. Int.J RF
Microwave CAE 9: 210–325
Williams JD; Wang W (2003) UV lithography process for ultra
thick high aspect ratio SU-8 microstructures. J Microlithography Microfabrication and Microsystems
Yao J; Chang MF (1995) A surface micromachined miniature
switch for telecommunications applications with signal frequencies from DC up to 4 GHZ. Transducer ’95, Eurosensors
IX: 384–387
705