THz Absorbers Fabricated from an SU-8 Photoresist Dielectric Layer
Rolando Perez and George Omictin
Department of Physics, Naval Postgraduate School, Monterey, CA 93943
Dr. Dragoslav Grbovic, Microsystems Fabrication Laboratory
Abstract
This project reports on continued research into micro
electro-mechanical systems (MEMS) based terahertz
(THz) thermal sensors used in THz imaging devices. THz
radiation is non-ionizing and will penetrate most nonmetallic materials (Fig. 1). These properties make it very
attractive for imaging applications in the medical and
security industries, as it is not harmful to the human body
yet will still detect metallic objects. In fabricating these
metamaterial THz absorbers the need arose for a more
streamlined and accessible process. The purpose of this
project is to characterize the effectiveness of the
photoresist material SU-8 as a dielectric. This photoresist
is a low cost material whose fabrication methods are less
hazardous than silicon dioxide, making it an appropriate
alternative for the average clean room facility. SU-8 was
incorporated as a dielectric layer in THz absorbers by
utilizing existing fabrication techniques such as
photolithography. Building on existing experimentation,
testing, and modeling software, the use of SU-8 as a
dielectric layer proved to be feasible and showed THz
absorption comparable to silicon dioxide.
Figure 1. The electromagnetic spectrum showing the
THz band gap in red text.
Figure 6. From left to right: 3D rendition of metamaterial layers,
optical micrograph of metamaterial showing the array of Al
squares.
Figure. 3 From left to right: Resistive thermal evaporator, and
ultrasonic Agitator.
Previous research into THz detectors has focused on
bi-material based imagery utilizing a SiO2 dielectric layer
and Al ground and top planes. Impedance matching
between the Al planes and the SiO2 layer causes THz
absorption, with samples using 1000-1400 um SiO2 layers
absorbing as much as 95% of the incident radiation at a
specific frequency .
COMSOL multiphysics and Labview simulated
absorption spectra were used to predict and confirm
experimental results and showed a good correlation with
the measured values. Measurements and characterization
of reflection spectra were carried out using a Fourier
Transform Infrared Spectrometer (FTIR) at a 15 degree
angle of incidence (Fig. 4). Absorption calculated using
the relation
A = 100 - R - T
(1)
Figure 2. From left to right: spin coating machine used to
deposit SU-8 photoresist, contact aligner used to expose
the SU-8 photoresist to UV light.
The COMSOL multiphysics software integrates the
power flow along the boundary of the layer with an
incident power of 1 W, thereby obtaining the reflection
spectrum of the metamaterial (Fig. 8). Since the skin
depth of Al in the THz regime is less than the thickness
of the metamaterial’s Al ground plane, the transmission
can be taken to be zero. Thus the absorbance can be
calculated using the equation
A = 100 - R
(2)
where R is the reflection obtained by either direct FTIR
measurement or via simulation. (1.)
where A is the percent absorption, R is the reflection
coefficient given by the FTIR, and T is the transmission
coefficient (zero here because of the Al ground plane that
acts as a mirror). (1.)
Materials and methods
To create the SU-8 metamaterial, micro electro-mechanical
systems (MEMS) fabrication processes were utilized.
Beginning with an un-doped silicon wafer as the substrate,
a resistive thermal evaporator was used to deposit an
Aluminum ground plane. The Al ground plane was
followed by a 400 nanometer thick layer of SU-8 negative
photoresist, acting as the dielectric, deposited using a spin
coater (Fig. 2). To employ the lift-off method a 7
micrometer thick layer of SPR-220.7 photoresist was spun
onto the SU-8. The SPR-220.7 was then exposed to UV
radiation in a contact mask aligner, followed by various
baking phases, and developed to reveal the exposed
pattern on the SPR-220.7 (Fig. 2). The pattern
development was followed by a hard-bake and subsequent
deposition of Al on the SPR-220.7 using the resistive
thermal evaporator (Fig. 3). The final lift-off of SPR-220.7
was accomplished using an ultrasonic agitator (Fig. 3),
leaving the Al square arrays on the SU-8 dielectric.
The periodicity of the Al squares enables the
simulation of the entire metamaterial array using only
the simple unit cell (Fig. 7). This is made possible by
imposing perfect electric and perfect magnetic
conditions on the boundaries of the unit cell, and
calculating the field distribution of a normally incident
plane polarized along one of the edges of the Al square.
Figure 7. From left to right: 2D rendition of unit cell with boundary
conditions, 3D model generated in COMSOL multiphysics.
Figure 8. From left to right: Simulated absorption peak from
COMSOL software, actual data from FTIR measurements.
Conclusions
Through the progression of experimental measurements and
simulations, it was concluded that the absorption of
metamaterials incorporating SU-8 as the dielectric layer in
the 3-10 THz range, is similar to that of a metamaterial
incorporating a silicon oxide dielectric layer. The silicon
oxide dielectric layer shows a higher absorption peak than
that of the SU-8 dielectric layer, however simulations show
that the thickness of SU-8 and absorption are proportional.
This illustrates that, as the SU-8 dielectric layer thickness
increases, the absorption of the metamaterial within the 310 THz range also increases. Further research into
metamaterials containing a thicker layer of SU-8 could
bring the absorption even closer to that of silicon oxide.
Accessibility and low cost make SU-8 an excellent
alternative for the average clean room facility. Moreover,
the metamaterial exhibits very beneficial absorbance that
can be used for the THz thermal sensors.
Figure 9. Photograph of
actual SU-8 metamaterial
wafer.
Results
Figure 4. From left to right: FTIR spectrophotometer bench
schematic, FTIR MappIR setup for wafer analysis.
This experiment used approximately 100 nm Al
ground and top planes, but utilizes a dielectric layer made
of SU-8 photoresist rather than SiO2. As illustrated by
Figure 5, the dielectric layer of SU-8 sits between the 100
nm Al ground plane and an array of Al squares with a
periodicity of 21 um. This metamaterial structure allows
variation of the Al square dimensions to adjust the
resonant frequency of the THz absorption, and the use of
SU-8 as the dielectric layer makes the process more userfriendly and accessible.
Figure 5. A chronologic order of the photolithographic process.
The sample’s absorption in the 3-10 THz range was
measured using an FTIR equipped with an AutoPro
accessory, pyroelectric detector, and a Si beamsplitter. An
aluminum mirror was used to measure the background
signal and the reflectance of the sample was measured
against this background. As shown in Figure 4, the angle
of incidence was 15 degrees. This was shown to produce
only a minor change in the reflectance and thus did not
affect the measurement significantly. The absorbance was
then calculated using equation (2) and compared against
the simulation.
Direct measurement using the FTIR showed a dip
in reflection to nearly 25% (from 95%) at approximately
6.1 THz, corresponding to nearly 70% absorption (Fig. 8).
As seen in Figure 8, the simulation matched the
experimental measurements well. In the process of
matching the simulation to the measurement, the real part
of the index of refraction of SU-8 negative photoresist in
the 3-10 THz region was estimated to be 1.45, and the
imaginary part was taken as 0.005.
Upon further experimentation, absorption of the
metamaterial was found to be proportional to the thickness
of the dielectric layer. Additionally, the frequency of
absorption could be adjusted by varying the dimensions of
the Al squares (Fig. 6).
Literature cited
1. Grbovic, Dragoslav; Alves, Fabio; Kearney, Brian;
Apostolos, Karamitros; Karunasiri, Gamani. 2011.
Optimization of THz Absorption in Thin Films
2. Appl. Phys. Lett. 100, 111104 (2012);
doi:10.10631.3693407
Acknowledgments
Benjamin Waxer of CalTech , Dr. Dragoslav Grbovic of NPS, Dr.
Fabio Alves of NPS, Dr. Gamani Karunasiri of NPS, Brain Kearney of
NPS, Sam Barone of NPS, Pat McNeil, Alison Kerr, Kelly Locke, Andy
Newton.
These internships were funded by the Title V Strengthening Transfer
Pathways Grant and the Title V College to University Success Program
Grant
For further information
Dgrbovic@nops.edu; Rcp.rhcp@gmail.com;
georgecomictin@student.hartnell.edu