Scalable method for the fabrication and testing of glass filled,
three-dimensionally sculpted extraordinary transmission
apertures
Sameer S. Walavalkar, Pawel Latawiec, Andrew P. Homyk, Axel Scherer
November 27, 2013
1 Detailed Fabrication
The fabrication was done on single-side polished (100) silicon wafers. PMMA A4 950mw
was spun onto the wafers at 4000 RPM for 1 minute, to a height of about 125nm. The
e-beam resist was baked at 180
C
o
for 5 minutes. The patterns were written with a Leica
EBPG 5000+ 100kV electron beam writer with a dose of 1350
μC/cm
2
and developed
for 45 seconds in a 1:3 solution of MIBK:IPA. Aluminum Oxide was sputtered into the
regions where the PMMA was removed. The sputtering was done with a DC magnetron
sputtering gun at 10 mTorr with a 5:1
Ar
:
O
2
gas mixture at 400 W. Approximately 30nm
was deposited. The pattern was lifted off with sonication in dichloromethane for 1 minute.
Etching was performed in an Oxford Plasmalab 100 ICP-RIE 380. Etching was con-
ducted with simultaneous etching (with
SF
6
) and passivation (with
C
4
F
8
). Tuning of the
relative flow rates of the etching and passivation gasses allowed for sidewall sculpting as
the etch progressed into the silicon. This step is shown schematically in figure S1(a) and
an SEM of an array of circular pillars is shown to the right. The samples were then ox-
idized at 1000
C
o
for 8 hours in a dry oxygen ambient (shown in figure S1(b)) to ensure
that no silicon was left within the structures. Note that in this step an equally thick field
of silicon dioxide is grown on the backside of the sample. A 2 nm titanium wetting layer
was sputtered on the structures followed by 250nm of gold. Both were performed with DC
magnetron sputtering in an Ar atmosphere. The conformal metal coating in this step is
shown schematically in figure S1(c) and is also seen in the middle SEM.
The samples were then reflowed using a rapid thermal annealer (RTA). The sample
temperature was ramped from room temperature to 650
C
o
in 90s in a 5%
/
95%
H
2
:
N
2
atmoshere. The temperature was held for 7 minutes and lowered to room temperature in
2 minutes. The rapid heating and cooling caused the gold on the side-walls to wick and
bead atop the structures. This is shown schematically in figure S1(d) and an SEM of the
resulting beading is shown at the bottom.
1
A thin layer of PMMA (A2 950 mw) was spun on to protect the surface and the glass
structures were snapped off using a clean-room compatible q-tip for mechanical cleavage.
The samples were then sonicated in dichloromethane to remove the PMMA protection and
any fragments of the structures left on the surface. These steps are shown schematically in
figure S2(a-c). The resulting glass structures protrude from the gold surface at the height
of the PMMA protection (shown in the top SEM image). The protruding tips are flattened
with a
CF
4
:
CHF
3
:
Ar
(45:15:60 sccm at 20mTorr, 80W fwd) RIE etch. The use of
fluorocarbons preferentially etched the glass while leaving the gold chemically untouched.
A thick layer of PMMA (A8 950mw) is spun on to protect the front side and photoresist
is sput on the backside of the sample (AZ 5214). A Karl-Suss MA6/BA6 backside mask-
aligner is used to align a window in the photoresist to the apertures on the front-side. The
photoresist is developed and buffered hydrofluoric acid is used to etch a window on the
backside of the sample. The photoresist is stripped in IPA. The silicon dioxide window is
used to mask an isotropic
XeF
2
and etch terminates on the silicon dioxide surface on the
front-side of the sample. Finally the protective PMMA layer is removed with an oxygen
plasma. These steps are shown schematically in figure S3(a-d). Images are also shown of
the front and backside of a sample with a dime to show the scale.
2 Polarization rotation animation
An animation demonstrating the effect of rotating the polarization of the input light on
the color transmitted through an array of apertures is available on the web. The figure
shows, centered, the Caltech logo in which the flame ‘turns-on’ as the polarization filter on
the input light is aligned along the short axis of the rectangular apertures that constitute
the flame. For comparison the logo to the left of the frame has only circular apertures
(with the same spacing, area and distribution) and thus the colors do not change during
the polarization rotation.
2
Figure S1: Sample fabrication flow. (a) Silicon nanostructures are etched using an ICP-
RIE using electron beam lithography and an alumina hard mask. (b) The structures are
oxidized to make then transparent. (c) Ti/Au 2/250nm is conformally sputtered on to
the sructures. (d) The gold is reflowed off of the side-walls to the top of the structures to
make mechanical cleavage easier. SEMs show (top) an array of silicon pillars (middle) The
conformal gold coating (bottom) gold nano-beads atop the pillars as a result of the reflow
step.
3