450 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 1, NO.

2, NOVEMBER 2011

THz Frequency Selective Surface Filters for Earth

Observation Remote Sensing Instruments
Raymond Dickie, Robert Cahill, Senior Member, IEEE, Vincent Fusco, Fellow, IEEE, Harold S. Gamble, and
Neil Mitchell, Senior Member, IEEE

Abstract—The purpose of this paper is to review recent develop- the FSS exhibit very low signal band insertion loss and simulta-
ments in the design and fabrication of Frequency Selective Sur- neously meet the conflicting requirement for high isolation be-
faces (FSS) which operate above 300 GHz. These structures act tween adjacent frequency bands. This is required to minimize
as free space electromagnetic filters and as such provide passive
remote sensing instruments with multispectral capability by sep- the overall noise performance of the instrument and thereby
arating the scene radiation into separate frequency channels. Sig- achieve high receiver sensitivity which is necessary to detect
nificant advances in computational electromagnetics, precision mi- weak molecular emissions at THz wavelengths. In addition the
cromachining technology and metrology have been employed to FSS must also exhibit high performance at large incident an-
create state of the art FSS which enable high sensitivity receivers gles to reduce the footprint of the feed train and moreover the
to detect weak molecular emissions at THz wavelengths. This new
class of quasi-optical filter exhibits an insertion loss 0.3 dB at structure should be sufficiently robust to withstand the launch
700 GHz and can be designed to operate independently of the po- forces of the space vehicle and operate without failure in the
larization of the incident signals at oblique incidence. The paper harsh thermal environment.
concludes with a brief overview of two major technical advances The purpose of this paper is to acquaint the reader with
which will greatly extend the potential applications of THz FSS. the principle application of this technology and to present an
Index Terms—Frequency selective surfaces (FSS), liquid overview of a multidisciplinary research project at Queen’s
crystals, mesh filters, micromachined structures, polarization University Belfast (QUB) which has exploited state of the
converter, quasi-optical technology, THz filters. art developments in silicon microtechnology to create a new
class of substrateless FSS that satisfies the electromagnetic
requirements for remote sensing instruments that will enter
I. INTRODUCTION
service in the 21st century. These FSS will operate at 45
incidence and exhibit very high mechanical strength and
O VER the past decade major advances have been made in
space borne THz instrument technology, primarily to ad-
dress the need to study the processes driving the climate, and
suitable CTE properties. In addition to the use of new mi-
cromachining technology, innovative electromagnetic design
to monitor the changes and provide a health check on the envi- strategies and measurement techniques have been employed to
ronment in which we live [1]. This requires complex imaging of create quasi-optical filters which can separate either linear or
clouds [2] and spectroscopic characterization of carbon dioxide simultaneously separate vertical and horizontal polarized com-
and other greenhouse gases in the Earth’s atmosphere using re- ponents of naturally occurring thermal emissions with spectral
mote sensing instruments which operate over wide bandwidths efficiencies exceeding 93% at frequencies up to 700 GHz. The
covering the thermal emission lines of the gases being observed electromagnetic performance exhibited by this new class of
[3]. To satisfy satellite payload constraints on cost, mass and en- FSS is presented for the MARSCHALS airborne limbsounder
ergy consumption, passive Earth observation radiometers tradi- (294–380 GHz) [4], [5], European Space Agency (ESA) dual
tionally employ a single mechanically scanned aperture antenna polarization FSS technology demonstrator (316.5–358.5 GHz)
to collect the radiation. Frequency selective surface (FSS) de- [6] and the Microwave Imager (MWI) instrument (113–670.7
multiplexing elements are a key enabling technology for these GHz) which is currently being developed for the European Post
advanced instruments and are used in the quasi-optical receiver EPS mission [2]. This paper concludes with a brief overview
to spectrally separate the signals that are collected by the scan- of two new innovative THz FSS structures which are currently
ning antenna [3]. The key technology challenge is to ensure that being developed at QUB. One FSS variant provides conversion
from linear to circular polarization whereas the other structure
exhibits electronic shutter operation by exploiting the dielectric
Manuscript received January 28, 2011; revised March 04, 2011; accepted
March 04, 2011. Date of publication April 15, 2011; date of current version
anisotropy property of nematic state liquid crystals.
October 28, 2011. This work was supported in part by ESA under Contract
19854/06/NL/JA, EPSRC under Grant EP/E01707X1 and Grant EP/S13828/01, II. FSS DESIGN AND SPECTRAL PERFORMANCE
by EADS Astrium UK, CEOI (www.ceoi.ac.uk), and by the European Re-
gional Development Fund under the European Sustainable Competitiveness A. Evolution of FSS Architecture
Programme for Northern Ireland.
The authors are with The Institute of Electronics, Communications and In- Radiometric remote sensing instruments in service before
formation Technology, The Queen’s University of Belfast, Belfast BT3 9DT, 2000 were generally designed to collect data using a single aper-
Northern Ireland, U.K. (e-mail: r.cahill@ecit.qub.ac.uk). ture antenna and drilled plate waveguide filters were deployed
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org. in the quasi-optical feed train to separate the signals and direct
Digital Object Identifier 10.1109/TTHZ.2011.2129470 these to the spatial location of the individual channel mixer
2156-342X/$26.00 © 2011 IEEE
DICKIE et al.: THz FSS FILTERS FOR EARTH OBSERVATION REMOTE SENSING INSTRUMENTS 451

Fig. 2. MARSCHALS quasi-optical feed train—courtesy of STFC.

electrical design parameters are precisely known [11]. Struc-

tures of this type are employed in the Millimetre-wave Airborne
Receiver for Spectroscopic CHaracterisation of Atmospheric
Fig. 1. 10 mm diameter 600 GHz waveguide FSS—TM 15 [8], [9]. Limb-Sounding (MARSCHALS) radiometer which is currently
being developed by a European consortium led by Rutherford
Appleton Laboratories in the U.K. [4]. The main objective of
detectors. For example the ’Advanced Microwave Sounding the instrument is to measure vertical profiles of ozone, water
Units (AMSU-B) which were launched on NOAA-15, 16, and vapor, carbon dioxide and other gaseous components in the
17 satellites between 1998–2002, used two waveguide array upper troposphere. Three printed FSS are deployed in the
FSS to separate three signals bands centered at 89, 150, and quasi-optical feed train, two function as image band rejection
183 GHz [7]. The structures were fabricated using a computer filters which separate the bands 316–326 GHz/350–359 GHz,
controlled mill to precision drill an aluminum disk to the 342–349 GHz/373–380 GHz, and the third FSS is used to
nominal dimensions of the two close packed arrays. Although provide demultiplexing of the channels 342–380 GHz/294–306
this type of FSS is mechanically robust the spectral response is GHz. A photograph of the instrument and the location of the
very sensitive to angle of incidence, the insertion loss can be three filters are depicted in Fig. 2. The FSS were constructed
high and the structures lacks design flexibility because the filter from ring element arrays printed on opposite sides of quartz
only provides high pass mode operation. Nevertheless until the wafers of thickness in the range 100–130 m. The structures
beginning of the millennium this was the only space qualified allow transmission of radiation with a maximum insertion
method available to construct compact devices for spatially loss 1.0–1.5 dB in the TM 45 plane and for all three FSS the
separating signals at millimeter wavelengths. To address the image/channel band rejection is better than 10 dB. The filter ar-
need to move to THz wavelengths in which there are resonances chitecture and spectral performance are similar to a FSS which
of ClO, BrO, HCl, and NO, an FSS demonstrator was designed was designed to separate the signal band 297–304.5 GHz from
to operate at 15 incidence and allow transmission of 540–660 the image band 275–282 GHz in the Advanced Microwave
GHz radiation and reject signals below 500 GHz. Fig. 1 depicts Atmospheric Sounder (AMAS) [12]. This instrument was
a photograph of the structure and the predicted and measured developed by Rutherford Appleton Laboratories under contract
spectral response where it is shown that the transmission inser- to Dornier GmbH. An SEM and a plot comparing the mea-
tion loss is in the range 0.5–2.0 dB [8], [9]. sured and simulated spectral response are depicted in Fig. 3.
Post AMSU-B a more versatile architecture consisting of The MARSCHALS and AMAS radiometers are designed to
printed FSS became available. These had the advantage that separate signal channels which are very closely spaced, with
they could easily be designed to operate at large incident angles, edge of band frequency ratios in the range 1.053:1–1.12:1.
in high pass, low pass and bandpass modes [10]. Furthermore For this reason the structures require two periodic screens to
conventional photolithographic techniques similar to those achieve the fast transmission roll off which is required below
used in the semiconductor wafer industry were employed to resonance [9]. However the insertion loss of multilayer printed
print the periodic structures which generally consist of strongly FSS is inversely proportional to the signal channel spacing
shaped metal elements. The substrate choice for space science [11] and therefore this can compromise the performance of the
instruments is SiO since this material is space qualified and the instrument. Moreover, another major practical consideration is
452 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 1, NO. 2, NOVEMBER 2011

Fig. 4. Comparison between transmission response of 2 layer ring FSS with

SiO spacer and a 2 layer air spaced FSS—TM 45 incidence (solid line and
open circles measured and predicted SiO backed FSS) [14].

The efficiency enhancement was quantified by comparing the

spectral transmission of a two layer freestanding FSS with a
classical printed FSS in the frequency range 300–500 GHz. The
simulated results depicted in Fig. 4 highlight the large increase
in the signal transmission (1 dB) in the 350 GHz channel and
the increase in the depth and bandwidth of the rejection band
Fig. 3. Scanning electron micrograph (SEM) and spectral transmission of the centered at 410 GHz.
AMAS FSS—TM 15 incidence [12].
Two or more self supporting screens each consisting of an
array of slot elements is required to design and construct the
FSS. The importance of removing all of the substrate material
the structural integrity of dielectric backed FSS. For example in the resonant elements is essential because as shown in [15] the
to achieve 1.5 dB insertion loss and 18 dB isolation between insertion loss measured for dipole and dogbone slot FSS screens
two channels centered at 501 and 550 GHz [8], a two-layer ring backed by high resistivity silicon is typically 1.6 dB at 300 GHz.
FSS was designed using a quartz wafer of thickness 70 m. The manufacturing route must be capable of creating high con-
Although the specified electromagnetic performance was sat- ductivity perforated metal structures that are substrateless, rigid
isfied, the structure was too fragile for deployment in airborne and optically flat. In [16] two different fabrication technologies
and space science instruments, therefore, this observation sug- were employed to construct 300 GHz high- multilayer FSS.
gests that the maximum operating frequency of high- printed One manufacturing route used electroplating to create a low
FSS is far below 500 GHz. stress copper layer around patterned photoresist pillars with the
same dimensions as the slot elements. The individual 30 mm di-
B. Freestanding Mesh FSS
ameter, 10 m thick solid metal perforated screens were formed
A new class of substrateless FSS has been developed to on the center portion of 2 inch quartz wafers by removing the
overcome the main drawbacks of printed structures, namely pillars in an ultrasonic bath and chemically etching the back sur-
the higher than desirable insertion loss which is unavoidable face of the handle wafer. This approach employs several newly
because of the unavailability of alternative space qualified developed processing steps to improve on previously reported
materials with a lower loss tangent at THz wavelengths, and the techniques for constructing an electrically thick dichroic plate
fragility of thin quartz wafers. In the 1960’s an intensive study which was designed to have a cut-off frequency of 950 GHz
started into slot type periodic surfaces in an attempt to address [17], and freestanding crossed dipole FSS operating in the range
the need to reduce the radar crossection of missile radomes. 585 GHz to 2.1 THz [18]. The second fabrication method em-
A patent [13] was granted to Professor Munk in 1978 for a ploys an 8 m thick layer of polymer which is etched to form
two-layer slot FSS which exhibits a bandpass filter response the slot array and then totally encapsulated in 1 m of alu-
with sharp skirts above and below resonance. This arrangement minum. As in the previous case the structure is fabricated on
provided a narrow transmission window for the on board a 2 inch quartz wafer which is etched away from one side to
missile guidance radar however at out of band frequencies the produce the 30 mm diameter FSS. Prior to this slot arrays had
FSS scatters the signals incident on the conical radome thereby been printed on 3.8 m thick polyester films and these exhibited
reducing the radar crossection of the target. In 2001 [14] the an insertion loss of 1.9 dB at 2.2 THz [19]. From these reported
deployment of substrateless slot FSS in radiometers operating developments it became clear that the fabrication technologies
at THz wavelengths was proposed as a solution to overcome the were suitable for constructing thin periodic screens consisting of
spectral and mechanical limitations of classical printed filters. simple dipole slots, however because of cantilever droop, they
DICKIE et al.: THz FSS FILTERS FOR EARTH OBSERVATION REMOTE SENSING INSTRUMENTS 453

=
Fig. 5. Free standing resonant slot FSS filters [5] (a) Three-layer TE FSS L
460 m, W = 15 m, D = 490 m, D = 500 = 357 5
m, S : m
(b) Two-layer TM FSS L = 470 m, W = 20 = 540
m, D = m, D
452 m, S = 143 m.

were not suitable to construct arrays of more highly shaped ele-

ments, such as those shown in Figs. 13 and 16. Building on this
observation, we developed advanced silicon micromachining
processes to manufacture ultra low loss multilayer mesh filters
Fig. 6. Computed and measured frequency response of freestanding FSS at 45
for space science instruments operating at THz wavelengths [6]. incidence. (a) Three-layer TE. (b) Two-layer TM [5].
These are shown to be robust and give a better spectral perfor-
mance than drilled plate and printed periodic arrays. The im-
proved transmission efficiency afforded by the freestanding FSS
architecture was demonstrated experimentally by designing a
structure using the specified frequency separation plan of the
Band C single sideband filter of the MARSCHALS radiometer.
The spectral response [5] was then compared to a printed FSS
previously supplied by QUB and deployed in the instrument
which has since undergone flight trials. The latter filter yielded
approximately 1.5 dB insertion loss in the signal (lower side)
band (LSB: 316.5–325.5 GHz) and 20 dB isolation of the image
(upper side) band (USB: 349.5–358.5 GHz) when operated at
45 incidence in the TM plane. Given that the instrument is re-
quired to detect either TE or TM polarized signals, freestanding
FSS designs were optimized to give the lowest signal transmis-
sion loss and simultaneously exhibit a minimum channel isola-
tion of 20 dB for both orientations of the incident electric field. Fig. 7. Quasi-optical transmission test setup.
The geometry of the two arrangements and the dimensions used
to define the computational unit cell within HFSS [20] are de-
picted in Fig. 5. The optimum performance in the TE plane is ob- this reduces to 0.5 dB. The transmission response of the two
tained with three perforated screens, however for TM incidence FSS was measured using an AB vector network analyzer [21]
a simpler structure consisting of just two periodic arrays was de- in conjunction with a quasi-optical test bench. The feed train
signed. Computed transmission coefficients between 250–400 employs two wideband corrugated feed horns at the waveguide
GHz are plotted in Fig. 6. These show that the insertion loss ports of the source and detector and ellipsoidal mirrors which
of the TM polarized FSS at resonance (322 GHz) is approxi- focus the Gaussian beam to produce a beam waist radius of ap-
mately 1.0 dB lower than the printed FSS. For a perfect elec- proximately 4 mm at the position of the FSS in the feed train.
trical conductor (PEC) the computed transmission at resonance The edge illumination on the 30 mm diameter filter orientated
is almost 100% therefore this result suggests that the efficiency at 45 is lower than 35 dB, and so beam truncation effects can
factor is limited by Ohmic loss and not by reflections from the be neglected [22]. The measurements were performed by ra-
screens. The insertion loss of the three-layer TE polarized FSS tioing the spectra with and without the FSS in the sample holder,
is in the range 1.8–2.2 dB, however when modeled as a PEC QO bench and holder as shown in Fig. 7. Excellent agreement
454 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 1, NO. 2, NOVEMBER 2011

Fig. 9. SEM images of 448 GHz MWI FSS [23].

Fig. 8. MWI radiometer quasi-optical feed train layout. (DP) is dual polariza-
tion; (SP) is single linear polarization.

between the simulated and measured spectral responses is ob-

served in Fig. 6. The two-layer FSS shown in Fig. 5(b) was ret-
rofitted to the MARSCHALS radiometer and subsequently the
remaining two printed quartz FSS were also replaced by high
performance freestanding structures. Measured system level re-
sults obtained from recently completed airborne flight trails con-
firmed the expected improvement in the instrument sensitivity.
The Microwave Imager Instrument (MWI) [2] is an advanced
passive radiometer that is currently being developed by the Eu-
ropean Space Agency to provide complex imaging of clouds.
Retrieved data obtained from the space science instrument will
be employed in numerical prediction models to improve the ac-
curacy of medium and long term weather forecasting [23]. This
mission is part of the Post EPS mission to replace the MetOp
satellites in the 2018–2020 time frame. FSS are required to pro-
vide spatial separation of the scene radiation into five narrow Fig. 10. CST predictions and measured transmission coefficients of the MWI
448 GHz FSS in discrete frequency channels—TM 45 incidence.
channels centered at 664 GHz (2%), 448 GHz (3.6%), 325 GHz
(6.8%), 243 GHz (3%), and 183 GHz (10%). All but the 664
GHz FSS are required to transmit (and reflect) linearly polar-
ized signals, with the electric field vector orientated vertically the reflection responses (not shown for brevity) and the max-
or horizontally. One proposed layout of the quasi-optical feed imum measured loss was found to be 0.76 dB (336 GHz) which
train is depicted in Fig. 8. Spatial separation of the 448 GHz exceeds the performance specified for the MWI optical filter
channel from the three lower frequency bands requires an FSS elements.
which exhibits a maximum insertion loss of 0.5 dB in the trans-
C. Dual Polarization Freestanding Mesh FSS
mission band and 1 dB loss in the four reflection channels.
The optimum design consists of a single screen FSS perforated Passive remote sensing of the atmosphere from space in the
with 326 m long dipole slots as shown by the SEM images frequency range 100–500 GHz is currently performed using
in Fig. 9, and arranged in a rectangular lattice. The computed linear polarized molecular emission spectroscopy. Therefore to
insertion loss of the FSS is 0.2 dB at the passband centre (448 address this technology need, research over the past decade has
GHz) and 0.3 dB at the upper and lower passband edges. The focused on creating FSS structures which separate signals with
numerical simulations were performed using the frequency do- electric vectors that are either orientated parallel (TM) or per-
main solver of CST MICROWAVE STUDIO [24]. The screen pendicular (TE) to the plane of incidence. In 2007 a major exten-
thickness, 12.5 m, and the bulk conductivity value of silver, sion to this work was initiated to provide a technical solution to
6.17 10 S/m, were included in the computer model. At the facilitate the detection and analysis of dual polarized radiation.
upper edge of the three lower frequency channels the computed This additional capability will be incorporated in future space
reflection loss is 0.85 dB (336 GHz), 0.22 dB (247 GHz), and science missions that are planned by ESA. The data retrieved
0.11 dB (192 GHz). In Fig. 10 the experimental spectral trans- will provide information on the size and shape of water ice par-
mission coefficients are shown to be in very close agreement ticles in cirrus clouds and these observations will ultimately be
with the numerical results. Similar agreement was observed for used to quantify their effect on the Earth’s radiation budget.
DICKIE et al.: THz FSS FILTERS FOR EARTH OBSERVATION REMOTE SENSING INSTRUMENTS 455

Fig. 11. Geometry and dimensions (m) of unit cell, and scanning electron
micrographs of MWI 664 GHz FSS [23] L = 158, L = 179, W =
15, W = 30, END = 64, END = 65, Dx = 230, Dy = 250.

Fig. 12. Computed and measured spectral responses of the MWI 664 GHz dual
polarization FSS [23]—the plot also shows the measured response of a second
For these instruments innovative freestanding FSS filters are re- technology demonstrator ‘Final FSS’.
quired to simultaneously operate in dual polarized mode when
the filter is orientated at 45 to the incident beam.
The first European Earth observation radiometer which will
require a polarization independent THz FSS is the MWI instru-
ment. Fig. 8 shows that the first FSS is required to separate the
664 GHz channel from the four lower frequency bands (63%
bandwidth) when the filter operates in both the TE and TM
planes and is orientated at oblique incidence in the radiometer.
The specified maximum signal loss in the transmission and
reflection bands is 0.5 dB and for this application the frequency
separation ratio of the FSS is 1.44:1 (657.3/456.7 GHz), there-
fore a single freestanding screen can satisfy the requirements.
The geometry and dimensions of a unit cell of the Jerusalem
Cross FSS is depicted in Fig. 11. Coincident spectral responses
were obtained by adjusting the individual lengths of the vertical
and horizontal main arms and increasing the physical width of
the latter to remove passband narrowing which is observed when Fig. 13. Geometry and SEM of prototype dual polarized annular slot FSS [25]
Dx = 475 m, Dy = 540 m, A = 102 m, A = 27 m, Short =
the structure is excited by a TE polarized wave at 45 incidence. 27 m, Short = 57 m, Dia = 263 m, Dia = 357 m, Depth =
Moreover the capacitive loading introduced by the end caps of 12:5 m.
the Jerusalem Cross elements reduces the area occupied by the
slot in each unit cell and this increases the structural integrity
of the filter. The predicted and measured results are depicted in the orthogonally orientated signals transmit through the FSS.
Fig. 12 where it is shown that the maximum loss in the trans- Optimization of the design was made by increasing the width
mission and reflection band is below 0.5 dB in both planes of of the inner slot to reduce the spectral roll-off rate above the
polarization. The numerical simulations, which included the passband in the TM plane and by employing two single screens
finite conductivity of the metal, were performed using the fre- separated by a distance 475 m. One of the perforated screens
quency domain solver of CST. was rotated by 180 to make full use of the interlayer coupling.
The design of a polarization independent FSS is signifi- The simulated copolar and cross-polar spectral responses are
cantly more complicated when the transmission and reflection plotted in Fig. 14, where it is shown that the lower sideband
bands are very closely spaced. Suitable topologies that can be and upper sideband spectral responses in the orthogonal polar-
employed to create a FSS which separates the Band C signal ization planes are coincident [25]. The numerical results were
(316.5–325.5 GHz) and image (349.5–358.5 GHz) channels obtained from CST using a screen thickness of 12.5 m and the
of the MARSCHALS radiometer have recently been studied. bulk resistivity value of copper, 1.72 10 m. These are
Fig. 13 shows a periodic array of nested short circuited annular in close agreement with the experimental results which show
slots which can be designed to provide spatial demultiplexing of that the maximum passband loss is 0.9 dB, the crosspolar
the two channels (frequency separation ratio of 1.07:1). At the levels are 21 dB and the minimum image band rejection is
specified centre operating frequency (321 GHz) the length of 20 dB. Although dual polarization operation is demonstrated
the inner slot is and the outer slot length is . There- using the nested annular slot architecture, the electromagnetic
fore when these are excited by TM and TE polarized waves, performance falls short of the most demanding specifications
respectively, the structure resonates at the same frequency and imposed on FSS structures which are to be deployed in the
456 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 1, NO. 2, NOVEMBER 2011

Fig. 14. Predicted copolar and measured copolar and cross-polar spectral re-
sponse of two-layer annular slot FSS at 45 incidence in TE and TM plane [25].

Fig. 16. Computed electric field distribution, SEM and photograph of the two-
layer dual polar FSS mounted in an invar holder [6].

Fig. 15. Geometry and dimensions (m) of the nested two-layer dual polar-
ization FSS [6].

next generation passive radiometers. To achieve the desired

receiver sensitivity the filter should ideally exhibit a maximum
loss of 0.5 dB and simultaneously provide 30 dB channel
isolation. Strategies for improving the spectral performance
of the FSS plotted in Fig. 14 were investigated using detailed
parametric studies. In common with the prototype FSS previ-
Fig. 17. Measured and computed spectral response of polarization independent
ously described, the design specification was also based on the FSS [6].
MARSCHALS Band C single sideband filter. CST predictions
were employed to show that a significant reduction in the
insertion loss and higher channel isolation are obtained from a An additional resonance is generated at 450 GHz by the
more advanced periodic array design. The unit cell of the two horizontally orientated linear slot. This design feature removes
layer arrangement depicted in Fig. 15 consists of two nested the undesirable effect of the weaker transmission roll-off
short circuited rectangular loop slots and a rectangular dipole which is observed above the passband in the TE plane thereby
slot. Reference [6] describes the systematic design approach creating coincident reflection channels. The computed trans-
which was employed to achieve the desired filter performance. mission coefficients using the bulk conductivity value of silver
The electric field distribution at resonance (321 GHz) in the 6.17 10 S/m and the experimental results plotted in Fig. 17
TM plane (inner slot) and the TE plane (outer slot) and a show that a significant performance improvement is obtained.
scanning electron micrograph image of three unit cells of the The structure exhibits a maximum insertion loss of 0.6 dB
assembled and bonded two layer FSS are depicted in Fig. 16. (316.5–325.5 GHz), 30 dB rejection (349.5–358.5 GHz) and
DICKIE et al.: THz FSS FILTERS FOR EARTH OBSERVATION REMOTE SENSING INSTRUMENTS 457

Fig. 18. Key processing steps used to construct one single FSS layer.
Fig. 19. FEM structural analysis showing the natural frequency and stress con-
centrations of the FSS [6].
cross-polar levels below 25 dB simultaneously for TE and
TM polarizations at 45 incidence.
icon. Detailed mechanical analysis was carried out by EADS
III. FABRICATION AND SPACE QUALIFICATION Astrium UK Ltd to quantify the dynamic behavior and peak
stress levels of the structure, as shown in Fig. 19. The predicted
The preferred manufacturing method is to form the individual natural frequency obtained from a finite element model of the
perforated screens by high conductivity coatings on silicon 30 mm diameter structure is 148 Hz and therefore meets the
wafers. Single crystal silicon was chosen as the base material of minimum requirement with 48% margin. Fig. 19 also shows a
the structure because it has very high theoretical yield strength, single unit cell containing 50 000 mesh elements. This was used
typically 7000 MPa, and therefore provides a very rigid core to model the stress contours during random vibration and the
with desirable structural properties. The FSS were constructed stress concentrations, which are shown above. The highest stress
from 100 mm diameter silicon-on-insulator (SOI) material levels predicted in the silicon was 487 MPa which is more than
which consists of a handle silicon wafer (typically 400 m ten times lower than the allowable yield (7000 MPa). Random
thick) with a 3 m buried oxide insulating layer on top of which and sine vibration testing in three axes was performed to space
is a 10 m silicon surface. The SOI wafers are coated with qualification levels at the EADS Astrium Portsmouth environ-
photo resist and patterned to form a mask for the deep reactive mental test facility. In addition thermal cycling was used to
ion etching (DRIE) of the 10 m silicon layer which was etched demonstrate that the FSS can survive in—orbit temperatures.
at a rate of 3.5 . DRIE was used to remove the Five cycles between 20 C and C with a dwell time of
exposed silicon under the array and the release rings to create 1 hour was used to test the filter. Visual inspection before and
a 50 mm diameter freestanding structure containing the 30 mm after testing confirmed the robustness of the FSS and pre and
diameter perforated FSS and a 10 mm wide silicon annulus post test spectral measurements showed no degradation in the
with the same thickness as the handle wafer. The FSS was then spectral performance of the filter.
sputter coated with a titanium adhesion layer followed by a
0.25 m thick copper seed layer. The construction was com-
pleted by growing a 1 m thick electrodeposited silver coating IV. FUTURE DEVELOPMENTS
on the seed layer and applying a 25 nm thick layer of gold
A. FSS Polarization Convertor
to prevent oxidation. Fig. 18 summarizes the main processes
steps for the single screen. When another layer was added A new concept for converting an incident linear polarized
separation was controlled by placing epoxy binder containing (LP) wave into circular polarization (CP) upon transmission
precisely dimensioned glass spheres around the screen annulus. through a split slot ring FSS has recently been demonstrated
The measured dimensional tolerances of the slot elements was at 320 GHz. The unit cell geometry depicted in Fig. 20 con-
found to be within m and the separation distance was sists of a nested arrangement of two annular slots suitably ori-
within m of the nominal design value for the multilayer entated [26] so that an incident slant 45 LP signal when re-
arrangements. A more detailed description of the fabrication solved into equal components, aligned along the vertical and
and plating processes which were developed to construct the horizontal directions, exits the FSS with outputs that are equal
freestanding FSS arrays is given in [6]. in amplitude and have a phase difference of 90 . The length of
The construction technique was selected to satisfy the struc- the outer slot is slightly larger than one wavelength at the oper-
tural and thermal demands of the space environment. This ap- ating frequency of the polarizer thus the impedance presented
proach exploits the high mechanical strength and rigidity of sil- by the FSS to the TE wave component at 320 GHz is inductive.
458 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 1, NO. 2, NOVEMBER 2011

Fig. 20. SEM and geometry of single layer ring slot polarization converter de-
= 475
sign: dx = 540
m, dy m, R 1 = 133 6 2 = 157 9
: m, R : m,
R3 = 189 5 4 = 213 5
: m, R : m,  1 = 16 8 2 = 53 6
: , : [26].

Fig. 22. Unit Cell geometry of the reconfigurable FSS device with dimensions;
periodicity dx= 422 = 522
and dy = 184
, slot width w = 262
, slot length l ,
= 300
quartz thickness q lc = 300 ( )
, liquid crystal thickness m [29].

and the insertion loss is 3.38 dB. Previously the authors re-
ported that a single layer polarization converter designed to op-
erate at X-band yielded a similar loss, however computed and
measured results also showed that this can be reduced to 0.5
dB by designing a double screen FSS [28].

B. Electronically Tunable FSS

A new class of adaptive FSS is currently being developed
which allows a small voltage to switch ‘on’ and ‘off’ an incident
signal thus producing an electronically controllable shutter [29].
Fig. 21. Simulated and measured results: (a) amplitude; (b) phase; and (c) AR The tunable bandpass filter response is obtained by exploiting
of transmitted CP signal [26]. the dielectric anisotropy property of a thin layer of nematic state
liquid crystal (LC) molecules which is sandwiched between two
electrodes as shown in Fig. 22. A technology demonstrator has
Conversely for the TM signal the smaller inner slots create a ca- been constructed using a 30 mm diameter printed FSS which
pacitive reactance and thus the two criteria which are required to consists of an array of 259 m long rectangular slots. The peri-
launch a CP wave [27] are satisfied by the single layer FSS. The odic structure is formed on a 300 m thick quartz wafer with a
screen dimensions shown in Fig. 20 were obtained from the fre- boron diffused polysilicon biasing layer formed on the bottom
quency domain solver of CST using the conductivity of copper surface of the wafer. Another 300 m quartz wafer is used to
and the device was micromachined using the process steps out- provide a second biasing layer and to encapsulate the liquid
lined in Section III. The 30 mm diameter, 10 m thick silicon crystals. In the unbiased state the director of the liquid
reinforced metal structure contains 2700 resonator cells. Close crystal molecules are orientated parallel to the surface of the
agreement between the measured and predicted transmission, quartz wafer due to the static action of a polyimide coating on
phase and axial ratio [27] is shown in Fig. 21. At 320.8 GHz, the bias layer. To reconfigure the molecules a voltage is applied
the axial ratio minimum, the measured phase difference is 88 across the biasing layers producing a torque which rotates the
DICKIE et al.: THz FSS FILTERS FOR EARTH OBSERVATION REMOTE SENSING INSTRUMENTS 459

Fig. 23. Measured and predicted spectral response of experimental electroni-

cally reconfigurable liquid crystal based FSS [29]. Fig. 24. Simulated and measured spectral response of optimized prototype
electronically reconfigurable FSS based on BL037 liquid crystals.

molecules perpendicular to surfaces. This changes the permit-

the frequency shifts upwards to 301 GHz and the loss increases
tivity value from to . Once the voltage is switched off the
to 3.5 dB. This produces a dynamic switching range of 16 dB
molecules return to their parallel state due to the action of the
at the upper resonance. In addition to demonstrating electronic
polyimide alignment layer. The dielectric constant of the LC
shutter operation at 301 GHz it is noted that the FSS also pro-
layer varies between these two states and the tunability is de-
vides electronic tuning of the passband peak over a 2% band-
fined as
width. Excellent agreement with predicted results is obtained
using permittivity values of 3.06 (0 V) and 2.67 (20 V) in the
numerical model. It is noted that these are 10% larger than the
The inherent dielectric anisotropy of the liquid crystals can measured design values for BL037 which were obtained from
therefore be exploited in this arrangement to shift the resonant the results plotted in Fig. 23. BL037 is a commercially available
frequency of the bandpass FSS thus creating a structure which liquid crystal material which has been engineered for optical
can both block and be transparent to THz signals on demand. applications, therefore, a significant improvement could be ob-
The experimental device was designed to reconfigure its pass- tained by synthesizing a mixture which yields lower loss at THz
band in the frequency range of 290–310 GHz when oriented wavelengths. The best possible performance was predicted by
at 45 in TE plane. A 300 m thick layer of Merck BL037 setting the loss tangent of the tunable layer to zero in CST. The
liquid crystals was sandwiched between the quartz wafers and computed spectral performance shows that with this arrange-
this was modeled in CST using and ment the insertion loss of the electronically reconfigurable FSS
. These permittivity and loss tangent is less than 0.9 dB and the dynamic switching range is greater
values are given in the literature at 130 GHz [30], however, as than 35 dB.
shown in Fig. 23, the experimental spectral response is better
matched at THz wavelengths using V. CONCLUSION
and in the numerical model. The In this review paper, the authors have briefly described
measured data was obtained with bias voltages of 0 and 15 V. many of the innovative design techniques and manufacturing
This first technology demonstrator therefore confirms the processes that have led to the creation of state of the art FSS
design methodology and provides accurate characterization of filters operating at THz wavelengths. Over the past 20 years
the electrical properties of the liquid crystals in the frequency the technology has mainly been driven by the need to satisfy
range 300–320 GHz. The spectral response of the electronically the stringent electrical and mechanical performance specifi-
tunable FSS has been optimized using these values. cations for separating signals in advanced Earth observation
For operation at 45 incidence in the TE plane the slot length instruments. The manufacturing process developed at QUB is
is increased to 272 m, the periodicity decreased to 512 m and wholly compatible with the structural and thermal requirements
in the numerical model a 350 m thick LC layer was inserted for space hardware. Moreover, the substrateless FSS have very
in the cavity between the two quartz wafers. The periodic array little impact on the receiver sensitivity and their use permits
was modeled using the conductivity of copper. Fig. 24 depicts the deployment of a single reflector antenna to collect energy
the measured transmission response of the FSS in the unbiased over a wide frequency band thus significantly reducing energy
state and with an applied 20 V peak to peak triangular wave- consumption, cost and the size and mass of the payload. New
form at 20 KHz. The measured filter bandpass centre resonant and innovative design strategies for converting from linear to
frequency is 295 GHz in the unbiased state and the insertion loss circular polarization and for dynamically switching signals
is about 2.7 dB. When the FSS is energized by a control voltage incident on the FSS have been demonstrated over the past
460 IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY, VOL. 1, NO. 2, NOVEMBER 2011

three years. These devices exploit the same computational [15] S. Biber, M. Bozzi, O. Gunther, L. Perregrini, and L. P. Schmidt, “De-
analysis techniques and manufacturing processes that have sign and testing of frequency-selective surfaces on silicon substrates
for submillimeter-wave applications,” IEEE Trans. Antennas Propag.,
been developed to produce space qualified demultiplexing vol. 1, no. 9, pp. 2638–2645, Sep. 2006, 54.
filters. The additional functionality afforded by these FSS could [16] R. Dickie, R. Cahill, V. F. Fusco, H. S. Gamble, B. Moyna, P. Huggard,
find applications in imaging devices and other emerging THz N. Grant, and C. Philpot, “300 GHz high Q resonant slot frequency
selective surface filter,” Proc. IET Microw. Antennas and Propag., vol.
technologies. 151, pp. 31–36, Jan. 2004.
[17] P. H. Siegel and J. A. Lichtenberger, “A technique for fabricating free
standing electrically thick metallic mesh and parallel wire grids for use
as millimetre and submillimetre wavelength filters and polarizers,” in
ACKNOWLEDGMENT IEEE MTT-S Int. Microwave Symp. Dig., May 1990, pp. 1311–1314.
[18] D. W. Porterfield, J. L. Hesler, R. Densing, E. R. Mueller, T. W. Crowe,
The studies reported in this paper were carried out at the and R. M. Weikle, “Resonant metal-mesh bandpass filters for the in-
Queen’s University Belfast. The authors would like to acknowl- frared,” Appl. Opt., vol. 33, pp. 6046–6052, Sept. 1994.
edge the contribution to this work which was made by Dr. P. de [19] M. E. MacDonald, A. Alexanian, R. A. York, Z. Popovic, and E. N.
Grossman, “Spectral transmittance of lossy printed resonant-grid tera-
Maagt (ESA); Dr. N. Grant, Ms. Y. Munro, Prof. M. Johnson, hertz bandpass filters,” Proc. IEEE Trans. Microw. Theory Techn., vol.
and P. Howard (EADS Astrium); and Dr. P. Huggard, Dr. M. 48, pp. 712–718, April 2000.
Henry, and B. Moyna (RAL). [20] Ansoft Corp., “HFSS 3D EM simulation software for RF, wireless,
packaging and optoelectronic design,” (Mar. 2011) [Online]. Available:
http://www.ansoft.com
[21] AB Millimetre, Paris, France, “AB Millimetre,” (Mar. 2011) [Online].
REFERENCES Available: http://www.abmillimetre.com
[22] P. F. Goldsmith, “Quasi-optical techniques offer advantages at mil-
[1] C. Mangenot, “Space antennas: ESA’s perspectives on future needs limeter frequencies,” Microw. Syst. News, pp. 65–84, Dec. 1983.
and technologies,” presented at the 13th Int. Symp. on Antennas, Nice, [23] R. Dickie, R. Cahill, V. F. Fusco, H. S. Gamble, Y. Munro, and S. Rea,
France, 2004. “Recent advances in submillimetre wave FSS technology for passive
[2] V. Kangas, C. Lin, and M. Betto, “Microwave instrument requirements remote sensing instruments,” in Proc. 4rd Eur. Conf. on Antennas and
and technology needs for the post-EPS mission,” in Proc 31st ESTEC Propagation (EuCAP), Barcelona, Spain, Apr. 2010.
Antenna Workshop on Millimetre and Sub-Millimetre Waves—From [24] CST-Computer Simulation Technology, Darmstadt, Germany, “CST-
Technologies to Systems, May 2009, pp. 501–506. Comp. Simulation Technology,” 2005 [Online]. Available: http://www.
[3] R. J. Martin and D. H. Martin, “Quasi-optical antennas for radiometric cst.com
remote sensing,” Electron. Commun. Eng. J., vol. 8, pp. 37–48, Feb. [25] R. Dickie, R. Cahill, H. S. Gamble, V. F. Fusco, P. G. Huggard, B.
1996. Moyna, M. Oldfield, N. Grant, and P. de Maagt, “Polarisation inde-
[4] M. Oldfield, B. Moyna, E. Allouis, R. Brunt, U. Cortesi, B. Ellison, pendent bandpass FSS,” Electron. Lett., vol. 43, pp. 1013–1015, Sep.
J. Ellison, J. Eskell, T. Forward, T. Jones, D. Lamarre, J. Langen, 2007.
P. de Maagt, D. Matheson, I. Morgan, J. Reburn, and R. Siddan, [26] M. Euler, V. F. Fusco, R. Cahill, and R. Dickie, “325 GHz single layer
“MARSCHALS: Development of an airborne millimetre-wave limb sub-millimeter wave FSS based split slot ring linear to circular polar-
sounder,” in Proc. SPIE 8th Int. Symp. on Remote Sensing, Sept. 2001, ization convertor,” IEEE Trans. Antennas Propag., vol. 58, no. 7, pp.
vol. 4540, pp. 221–228. 2457–2459, Jul. 2010.
[5] R. Dickie, R. Cahill, H. S. Gamble, V. F. Fusco, A. Schuchinsky, and [27] B. Y. Toh, R. Cahill, and V. F. Fusco, “Understanding and measuring
N. Grant, “Spatial demultiplexing in the sub-mm wave band using mul- circular polarization,” IEEE Trans. Edu., vol. 46, no. 3, pp. 313–319,
tilayer free-standing frequency selective surfaces,” IEEE Trans. An- Aug. 2003.
tennas Propag., vol. 53, no. 6, pp. 1903–1911, Jun. 2005. [28] M. Euler, V. F. Fusco, R. Cahill, and R. Dickie, “Comparison of FSS
[6] R. Dickie, R. Cahill, H. S. Gamble, V. F. Fusco, M. Henry, M. L. Old- based linear to circular polarization converter geometries,” Proc. IET
field, P. G. Huggard, P. Howard, N. Grant, Y. Munro, and P. de Maagt, Microw. Antennas and Propag., vol. 4, pp. 1764–1772, Nov. 2010.
“Submillimeter wave frequency selective surface with polarization in- [29] R. Simms, R. Dickie, R. Cahill, N. Mitchell, H. S. Gamble, and V. F.
dependent spectral responses,” Proc. IEEE Antennas and Propagation, Fusco, “Measurement of electromagnetic properties of liquid crystals
vol. 57, pp. 1985–1994, Jul. 2009. at 300 GHz using a tunable FSS,” presented at the 32nd ESA Workshop
[7] R. Cahill, W. J. Hall, and R. J. Martin, “Technologies for millimeter on Antennas for Space Applications, Noordwijk, the Netherlands, Oct.
remote sensing antennas,” in IEE/SEE Seminar Dig. on Spacecraft An- 2010.
tennas, 1994, pp. 8/1–8/8. [30] W. Hu, R. Dickie, R. Cahill, H. S. Gamble, M. Y. Ismail, V. F. Fusco,
[8] R. Cahill and E. A. Parker, “Frequency selective surface design for D. Linton, S. P. Rea, and N. Grant, “Liquid crystal tunable mm wave
submillimetric demultiplexing,” Microw. Opt. Technol. Lett., vol. 7, pp. frequency selective surface,” IEEE Microw. Wireless Compon. Lett.,
595–597, Sep. 1994. vol. 17, no. 9, pp. 667–700, Sep. 2007.
[9] R. Cahill, I. M. Sturland, J. W. Bowen, E. A. Parker, and A. C. de C
Lima, “Frequency selective surfaces for millimetre and submillimetre Raymond Dickie received the B.Eng. (Hons) and
wave quasi-optical demultiplexing,” Int., J. Infrared and Millimetre Ph.D. degrees in electrical and electronic engineering
Waves, vol. 14, pp. 1769–1788, Sept. 1993. from The Queen’s University of Belfast, U.K., in
[10] R. Cahill and E. A. Parker, “Performance of mm-wave frequency se- 2001 and 2004, respectively.
lective surfaces in large incident angle quasi-optical systems,” Electron. In October 2004 he joined the high frequency elec-
Lett., vol. 28, pp. 788–789, Apr. 1992. tronic circuits and antennas group at The Institute of
[11] R. Cahill, E. A. Parker, and I. M. Sturland, “Influence of substrate loss Electronics, Communications and Information Tech-
tangent on performance of multilayer sub millimetre wave FSS,” Elec- nology (ECIT), Belfast, U.K., where he is now em-
tron. Lett., vol. 31, pp. 1752–1753, Sep. 1995. ployed as a senior engineer working on mm-wave
[12] R. Cahill, H. S. Gamble, V. F. Fusco, J. C. Vardaxoglou, M. Jayawar- components. His work on freestanding frequency se-
dene, B. Moyna, M. Oldfield, G. Cox, and N. Grant, “Low loss FSS lective surfaces has been patented and includes fabri-
for channel demultiplexing and image band rejection filtering,” in cation methods using silicon-on-insulator (SOI), metal and polymer mesh tech-
Proc 24th ESTEC AntennaWorkshop on Innovative Periodic Antennas: nology. He has experience in photolithographic processing including thick posi-
Photonic Bandgap, Fractal and Freq. Sel. Surfaces, May 2001, pp. tive and negative photoresist RIE of polymers and oxides, DRIE of silicon, CVD
103–108. metal deposition, high conductivity stress controlled electroplating, and SEM
[13] B. A. Munk, Frequency Selective Surfaces Theory and De- imaging methods. He is experienced in working in clean room environments
sign. Hoboken, NJ: Wiley, 2000. where he develops MEMS devices. He has co authored over 50 publications,
[14] R. Cahill, J. C. Vardaxaglou, and M. Jayawardene, “Two layer his high frequency research interests include numerical modeling of high fre-
mm-wave FSS of linear slot elements with low insertion loss,” Proc. quency structures and precision quasi-optical measurements in the millimeter
IEE Microw. Antennas and Propag., vol. 148, pp. 410–412, Dec. 2001. and sub-millimeter wave bands.
DICKIE et al.: THz FSS FILTERS FOR EARTH OBSERVATION REMOTE SENSING INSTRUMENTS 461

Robert Cahill (M’10–SM’11) received the B.Sc. Harold S. Gamble graduated from The Queen‘s
(1st class, Hons) degree in physics from the Uni- University of Belfast with a 1st class honours degree
versity of Aston, Birmingham, U.K., in 1979, and in electrical and electronic engineering in 1966, and
the Ph.D. degree in microwave electronics from the received the Ph.D. degree in 1969.
University of Kent, Canterbury, U.K., in 1982. As a research engineer at The Standard Telecom-
He joined The Queen’s University of Belfast munication Laboratories, Harlow, U.K., he estab-
(QUB), U.K., in 1999 after a 17–year career working lished a polysilicon gate process for MOS integrated
in the UK space and defense industry, where he circuits. He was appointed to a lectureship at The
worked on antenna and passive microwave device Queen’s University of Belfast, U.K., in 1973, and
technology projects. During this time he pioneered has lead research there in silicon device design
methods for predicting the performance of antennas and related technology including, CCDs, silicided
on complex scattering surfaces such as satellites and has developed techniques shallow junctions, rapid thermal CVD, GTOs and Static Induction Thyristors.
for analyzing and fabricating mm and sub-mm wave quasi-optical dichroic In 1992 he was promoted to Professor of Microelectronic Engineering, and
filters. Recently, he has established a 100–700 GHz quasi-optical S-parameter until 2010 was the Director of the Northern Ireland Semiconductor Research
measurement facility at QUB. He has exploited the results of numerous Centre. Major activity at present is the use of direct silicon wafer bonding
research projects, sponsored by the European Space Agency, EADS Astrium for producing silicon-on-insulator (SOI) substrates for low power bipolar
Space Ltd., the British National Space Agency, the Centre for Earth Observa- transistor circuits. This includes trench and refill before bond technology and
tion Instrumentation (CEOI), and the UK Meteorological Office, to develop buried metallic layers to eliminate epitaxial layers and to minimize collector
quasi-optical demultiplexers for atmospheric sounding radiometers in the range resistance. Ground plane SOI structures incorporating tungsten silicide layers
89–500 GHz. These include AMSU-B, AMAS, MARSCHALS and the ESA are being investigated for cross talk suppression in mixed signal circuits and for
500 GHz demonstrator. His recent interests also include the characterization of ultra short MOSTs. The silicon wafer bonding combined with the integrated
liquid crystal materials at microwave and mm wavelengths, and strategies for circuit patterning techniques is also being applied to micro-machining appli-
broad banding and creating active reflectarray antennas. He has (co)—authored cations such as sensors, mechanical actuators and 3-D mm wave components.
over 130 publications and holds four international patents. His other projects include multilayer free-standing frequency selective surfaces
for spatial demultiplexing in the sub-mm wave band, thin film transistors in
polysilicon or bonded silicon on glass for displays/imagers, and high density
interconnects produced by sputtering and CVD for IC‘s and MR heads. He has
Vincent Fusco (S’82–M’82–SM’96–F’04) received co-authored over 250 publications in the area of silicon devices and thin film
the Bachelors degree (1st class honors) in electrical technology.
and electronic engineering, the Ph_D. degree in
microwave electronics, and the D.Sc. degree for
his work on advanced front end architectures with
enhanced functionality, from The Queens University Neil Mitchell (M’96–SM’02) received the B.Sc. and
of Belfast (QUB), Belfast, Northern Ireland, in 1979, Ph.D. degrees in electrical and electronic engineering
1982, and 2000, respectively. from The Queen’s University of Belfast, U.K., in
He is the Technical Director of the High Frequency 1982 and 1986, respectively.
Laboratories at The Queens University of Belfast, In 1986 he was appointed as a temporary lecturer
U.K., and is also Director of the International Centre in Queen’s University Belfast, U.K., where he is cur-
for Research for System on Chip and Advanced MicroWireless Integration, rently a senior lecturer in the School of Electronics,
SoCaM. His research interests include nonlinear microwave circuit design, and Electrical Engineering and Computer Science. His
active and passive antenna techniques. He has published over 420 scientific main research interests are in semiconductor and
papers in major journals and international conferences, and is the author of two microelectromechanical systems technology. His
text books. He holds several patents on active and retrodirective antennas and research has encompassed a wide range of device
has contributed invited chapters to books in the fields of active antenna design structures and has included development of technology for fabrication of
and EM field computation. semiconductor devices on substrates including glass and sapphire. His recent
Dr. Fusco is a Fellow of the Royal Academy of Engineering, and a Fellow research has been on technology for fabrication of germanium and germanium
of the Institution of Electrical Engineers (U.K.). In 1986, he was awarded a on sapphire devices. In the micromachining area, he has developed technology
British Telecommunications Fellowship, and in 1997 he was awarded the NI for fabrication of RF MEMS components and sensors for biomedical and
Engineering Federation Trophy for outstanding industrially relevant research. environmental applications. Recent micromachining activity has been on
chemotaxis sensors for biomedical applications, photoacoustic sensors for
greenhouse gas measurement and frequency selective surfaces for RF applica-
tions. He is joint author of over 130 publications.