Nanotechnology 24 (2013) 115306
M Stach et al
Figure 1. Schematic plots of the operations of the WGPs. Strains applied in the x-direction prolong the length of the metal nanowire in the
x-direction but shrink its width in the y-direction for all cases. In the elastic region, the WGP structure returns to its original dimensions
after release. In the plastic region, the WGP structure does not return to its original dimensions after release, producing a different pitch.
Further straining of the WGP structure generates defects such as metal nanowire cracks, delaminations and fall-ons. In the breakdown
region, the polymer substrate breaks with a seriously damaged WGP structure above it. The WGPs in this study operated in small and large
plastic strains in the x-direction to compare their physical and optical characteristics with the zero-strain WGP. The subscript x denotes the
strain direction. The dashed lines indicate possible locations of the three patterns.
substrates were generally fabricated simply as test patterns in
the commercial software Lumerical FDTD (ver. 8) under high
studies for their easy observation of delamination [21, 22],
accuracy settings, with a minimum mesh step of 1 nm, to
buckling [23] and bending [23, 24], which show no electrical
identify optical characteristics. The duty cycle of the structure
or optical functionalities.
was defined as the metal linewidth divided by the pitch for the
Within the applications provided by inorganic nanowires,
following studies.
wire grid polarizers (WGPs) are being widely used for visual
Figure 2(a) shows the simulated transmittance for both
and telecommunication systems. Thus, a WGP operating
TE and TM modes for different pitches with the same duty
in the ultraviolet (UV), visible (VIS) or near infrared
cycle of 75%. This figure implies that the transmittance
(NIR) region is a good device for evaluating optical
changes under different strains provided that the strain
behaviors between the transverse magnetic (TM) mode and
modified the structure dimensions properly. In this figure, the
the transverse electric (TE) mode [25]. From a manufacturing
strained TM mode transmittance could achieve higher average
viewpoint, achieving good WGP performance for a short
values than the unstrained one and its extinction ratio (ER)
wavelength requires a subwavelength structure period [26]
increased accordingly (figure 2(b)). Here, the ER in decibels
that usually exceeds the exposure resolution limitation of a
(dB) was defined as the logarithmic ratio of the TM mode
conventional (e.g. i-line) system.
transmittance (TTM) to the TE mode transmittance (TTE)
To improve the resolution as the first step, the proposed
before being multiplied by 10 [33] as
method uses laser interference lithography (LIL) [27, 28]
TTM
to achieve a faster process and finer resolution. This LIL
ER = 10log
.
10 TTE
process is also potentially suitable for roll-to-roll (R2R)
production, and has attracted research attention in flexible
electronic studies [29­31]. Based on the LIL manufactured
3. Laser interference lithography
WGP, this study presents a novel method of achieving
pattern size reduction by applying external strains. This
A homemade two-beam LIL consisting of an i-line light
study investigates post-lithography pattern modification and
source (wavelength of 365 nm), shutters, lenses, spatial filters
presents a proof-of-concept of a tunable WGP device.
and a Lloyd's mirror was used to form the designed WGP
pattern [34]. A PEN thickness of 125 µm was chosen to
2. Design
show better thermal-stability properties during lithography.
The PEN substrate was spin-coated with a 200 nm bottom
Figure 1 represents the elastic and plastic behaviors of
anti-reflection coating (BARC; Brewer Science, XHRiC-16)
the metal WGP on polyethylene naphthalate (PEN; Teijin
and baked at 130 C for 90 s for solidification before
DuPont, Q65). The elastic and the breakdown regions for
spin-coating with 200 nm photoresist (PR; Tokyo Ohka
the PEN have a strain in the x-direction (x) of less
Kogyo, AR80) plus a second solidification process at 90 C
than 4.5% and greater than 59%, respectively. To change
for 90 s. In the UV­VIS­NIR region, the transmittance
polarization characteristics before and after straining, the
differences between the bare PEN and the WGP on PEN
WGP was designed with titanium (Ti) and aluminum (Al)
fabricated by LIL were less than 2%. This difference
for a linewidth, pitch and height of 180, 210 and 30 nm,
corresponded to the measurement tolerance, proving that
respectively. Ti was chosen for its superior adhesion to PEN
the LIL process did not affect the final performance of the
with compatible Poisson ratio [32] and Al was chosen for its
WGP. After developing the sample, e-beam physical vapor
proper optical transmittance. The WGP was simulated with
deposition was used to deposit a metallic multilayer of 5 nm
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