Microstrip Patch with Adaptive Conductivity
*Joseph Flemish, jflemish@psu.edu
Randy L. Haupt, rlh45@psu.edu
The Pennsylvania State University, Applied Research Laboratory,
State College, PA 16801
Introduction
We are investigating techniques that electrically change the conductivity of microstrip
patch antennas. Altering the conductivity of certain areas of a patch antenna can be useful
for controlling the weighting of elements in an array or for tuning the antenna to a
desirable frequency. Materials that have adjustable conductivity include conductive
electoactive polymers [1] and silicon. Conductive electoactive polymers have a
conductivity that is proportional to an applied electric potential. Polypyrroles and
polyanilines are examples that exhibit changes in conductivity between 10-7 and 103
S/cm. Conducting polymeric materials have controllable conductivity at microwave
frequencies [2]. Applying a small dc potential across a poly(aniline)-silver-polymer
electrolyte composite quickly changes its conductivity. These materials have been
incorporated into a Salisbury screen to dynamically alter the radar cross section of large
surfaces [3]. In this paper, we report on modeling and experimental results aimed at
creating a reconfigurable patch antenna partly made with high-purity silicon which has a
conductivity that can be varied over a wide range using infrared illumination.
Computer Model of a Patch Antenna with Variable Conductivity
A portion of a microstrip patch element was replaced with a material of variable
conductivity with εr=11.9 to simulate silicon which could be varied between values of
high to moderate resistivity. A numerical model was built using CST Microwave Studio.
The patch geometry and variables are shown in Figure 1. Two regions are considered in
this model: the lower portion of the patch and the microstrip transmission line are
assumed to be perfect electrical conductors with a thickness of 0.01 mm, and the upper
portion is silicon 0.1 mm thick with the resistivity value deliberately varied in each
simulation run. The value of length LT was chosen for a 50Ω impedance match when the
resistivity of the variable material is greater than 10,000 Ω -cm. (vs. compromise value
for two states.) Far field parameters of gain and total efficiency were then found at the
resonant condition. Table 1 describes the parameters of Figure 1.
Table 2 shows the resonant frequency, gain, and efficiency for several values of
resistivity. The results show that the antenna efficiency significantly degrades as a
portion of the radiating element is replaced with lossy material such as silicon. Our
preliminary calculations using high-resistivity silicon as a photoconductive material for
the purpose of offering a means of modulating the elements’ conductivity have suggested
that it may be possible to vary the resistivity from over 10,000Ω-cm in the off state to
perhaps less than one Ω-cm in the on state. The simulation results presented in Table 2
suggest that it may be possible to modulate the gain of such an element from a maximum
of 6.3 dB with a total efficiency of 74% when Si is in a non-conductive state, to a gain of
below -3 dB with an efficiency of only 5% is the Si region is modulated to 2 Ω-cm.
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These extremes represent an ability to turn the patch element essentially on or off by
modulating the conductivity of the Si region. It is noteworthy that the greatest changes
occur in the resistivity range of 2 to 100Ω-cm where a decrease in value per runs B
through E causes a reduction in the resonant frequency, gain and total efficiency of the
antenna. As resistivity values are decreased further below 1 Ω-cm, the resonance peak
stops shifting and the gain and efficiency increase again. The latter result suggests that if
a wide range of resistivity modulation is possible, then the element can be switched
between two frequency states in addition to being switched off.
W
L1
L2
LT
Wg
WT
Figure 1. Geometry of reconfigurable patch antenna simulations.
Table 1. Simulation Results for Patch Antenna with Variable Conductivity when
substrate is 3mm SiO2 (loss free, εr= 3.8) with backside ground plane.
Parameter Value (mm)
Description
W
54.0
patch width
L1
L2
Wg
WT
LT
6.0
32.0
6.0
3.2
6.0
length of variable resistivity
Length of PEC
gap width of feed inset
microstrip line width
Inset for Impedance match
Table 2. Simulation Results for Patch Antenna with Variable Conductivity.
ρ (ohm-cm) f0 (GHz) Gain (dB ) Efficiency (%) Return Loss (dB)
10,000
2.243
6.3
74.5
25.8
100
2.243
5.5
60.4
17.4
20
5
2.240
2.175
3.0
-0.6
30.8
9.8
8.8
4.4
2
1
0.2
0.1
2.030
1.951
1.905
1.905
-3.1
-2.3
2.25
3.8
5.2
7.5
28.9
39.1
4.1
6.0
>30
14.6
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Experimental Verification of Materials with Variable Conductivity
Presently, our leading candidate material for varying conductivity over a wide range is
high-purity crystalline silicon employing infrared excitation to render the material
conductive. High-resistivity silicon wafers (>30,000 Ω-cm) with minority carrier lifetime
>1 ms have been obtained for this purpose, as a high lifetime is desireable for
maximizing the photoconductive response. High-emission infrared light emitting diodes
(LEDs) at various wavelengths from 870nm to 950nm have been obtained from several
commercial sources. Silicon switching and resistivity test structures were designed and
fabricated using standard microfabrication techniques. As a starting point, silicon wafers
were thermally oxidized to create a passivating surface layer and then patterned
photolithographically to define aluminum metal contacts. To evaluate the effect of
contact resistance on the measured photoconductivity, one sample was ion-implanted
with 2E15 B+/cm2 at 50kV selectively in the region to be contacted and subsequently
annealed at 1000°C for activation of the boron dopant. For comparison another sample
received no implant or anneal. Prior to evaporation of the metal contacts, the oxide layer
was removed on each sample by CF4/O2 plasma etching in to allow contact to the silicon.
The switch structure had a gap of 10μm between electrodes which were 500μm long.
Two different types of LEDs were used for illumination of the Si in these measurements
with the following manufacturer’s specifications: Diode A had a center wavelength of
950nm, with emission half angle of 15o and output power of 20mW/sr at 100mA. Diode
B had a center wavelength of 870nm, with emission half angle of 22o and output power
of 60mW/sr at 100mA. For each measurement the LED was placed approximately 3mm
above the electrode gap and DC I-V sweeps were taken at various levels of LED forward
current with the results shown in Figure 2. From measurements taken without
illumination value of sheet resistance values of more than 3.9MΩsq corresponds to a dark
resistivity of approximately 220kΩ-cm for the silicon wafers.
Under IR illumination corresponding to a diode current of 250mA the change in
resistivity is most pronounced for the ion-implanted sample illuminated with the 870nm
diode. In this case the the sheet resistance is decreased to approximately 12.6kΩ/sq.
Assuming that an active region of only 0.01cm is participating in the conduction (the
depth corresponding to approximately 95% IR aborption at 870nm) the corresponding
average Si resistivity would be approximately 126Ω-cm. This value is one to two orders
of magnitude greater than what would be useful in the reconfigurable patch geometry and
is several orders of magnitude greater than what should be acheivable if photocarrier
lifetimes approaching one millisecond could be maintained. Therefore, these relatively
high values of resistivity are attributed to short carrier lifetimes due to rapid
recombination of photogenerated electron-hole paris. Efforts are underway to achieve
significant reduction in resistivity through several means including (1) designing
structures which will allow for electric field induced confinement of photogenerated
electrons and holes (2) optimized n-type ohmic contacts to allow efficient electron
injection into the silicon (3) employing an antireflection coating to enhance IR absorption
and (4) selection of LEDs with higher optical power output. If successful, this
achievement will allow for the experimental verification of a photonically reconfigurable
patch antenna.
Conclusions
The results of computer modelling of a patch antenna with a portion of the patch
contructed from silicon and having variable conductivity shows that over a range of
resistivity of 2 to 100Ω-cm the antenna can be effectively switched on or off. At values
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of resistivity higher than this range the antenna characteristics show high gain and
efficiency independent of resistivity. Reduction in resistivity below this range can restore
gain and efficiency with an accompanying shift to lower resonant frequency
demonstrating a reconfigurable nature of the antenna. Experimentally, we have been able
to acheive modulation of Si resistivity from over 200kΩ to approximately 120Ω using IR
LEDs without special optimization of the Si structure or the illumination method.
Therefore, future work will focus on greatly enhancing the range of conductivity
modulation of Si and further optimizing the patch design to increase its reconfigurability
using this technique.
7.0
Non-Implanted (950nm)
Non-implanted (870nm)
Ion-Implanted (950nm)
Ion-Implanted (870nm)
Log Rs (ohm/sq)
6.5
6.0
5.5
5.0
4.5
4.0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
LED current (A)
Figure 2. Measured sheet resistance of Si switch structure under various levels of IR
illumination using LEDs with λ=950nm and λ=870nm.
Acknowledgement
This work was sponsored by Army CECOM under contract N00024-02-D-6604 DO-295.
References:
[1] Wallace, G.G., Spinks, G.M., Kane-Maguire, L.A.P., and Teasdale, P.R., Conductive
electroactive polymers, 2nd ed. (CRC Press, New York, 2003).
[2] P.V. Wright, et.al., "Progress in smart microwave materials and structures," Smart
Mater. Struct. Jun 2000, 9, (3), pp. 273-279.
[3] B. Chambers, "Surfaces with adaptive radar reflection coefficients," Smart Mater.
Struct., Oct 1997, 6, (5), pp. 521-529.
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