A PDMS-Based Pressure-Tunable Nanograting

A PDMS-Based Pressure-Tunable Nanograting

Steven Foland1?, Ke Liu1, Kyung-Hak Choi1, Duncan MacFarlane1, and Jeong-Bong Lee1

1Department of Electrical Engineering, The University of Texas at Dallas, Richardson, TX, USA

(? E-mail: stevenfoland@https://www.360docs.net/doc/716349283.html,; Tel: (972) 883-6549; Fax: (972) 883-2893) Abstract — We report the design, fabrication, and

characterization of a pressure-tunable guided-mode resonance

grating. A thin, flexible polydimethylsiloxane (PDMS)

membrane was fabricated with an embedded titanium dioxide

(TiO2) nanograting. When pressure is applied to the device, the

membrane expands, and the pitch of the grating increases; this

causes a shift in its resonant wavelength. By varying the

pressure by only 3500 Pa, a tunable range of over 35 nm can be

observed.

Index Terms – Flexible, Grating, Guided-mode Resonance,

Membrane, Tunable .

I.I NTRODUCTION

Guided-mode resonance (GMR) devices are a type of

dielectric diffraction grating consisting of periodically

varying regions of high and low refractive index (RI)

materials [1-3]. At specific wavelengths, these gratings

exhibit a strong resonance effect, changing the response to

incident light from mostly-transmissive to mostly-reflective.

Due to this resonance’s sensitivity to the refractive index of

surrounding materials, GMR devices have garnered a great

deal of attention in recent years for their applications in

label-free biosensing [4-8].

The majority of GMR gratings are produced on rigid substrates, and thus have a fixed period. It is, however, possible to fabricate a grating structure on a flexible substrate, thus allowing modulation of the grating period [9]. Such devices could not only act as new types of optical sensors, but could also be used to actively tune the grating period, allowing for novel methods of characterization; one such method has been theoretically demonstrated in [10].

A GMR grating is presented here which may be tuned over a wide range of pitches by varying the air pressure acting on a thin, flexible membrane which serves as its substrate. This tunability allows for fine control over the wavelength of the grating’s resonance response. To the best of our knowledge, this is the first time such a device is reported.

II.D ESIGN AND S IMULATION

The device consists of a one-dimensional array of titanium dioxide (TiO2) bars embedded in one side of a 3mm wide, 120 μm thick polydimethylsiloxane (PDMS) membrane. Fig. 1(a) shows a schematic diagram of the device’s mechanical structure. Pressure is applied to the side of the air-suspended membrane opposite the grating, inflating the

membrane and straining the grating in its direction of periodicity.

A schematic diagram of the unstrained grating (no applied pressure) is pictured in Fig. 1(b). The 160 nm thick periodic region consists of alternating regions of high refractive index TiO2 (n = 2.3) and relatively low refractive index PDMS (n = 1.4). The grating’s period is 560 nm, 260 nm of which is occupied by the TiO2. The grating is surrounded by PDMS on one side and a medium of either air (n = 1.0) or water (n = 1.33) on the other.

Mechanical simulation of the membrane and grating was accomplished using COMSOL Multiphysics, a finite element method software package. A three-dimensional model of the membrane and grating structure was created, and a distributed pressure load was applied to the surface of the membrane. Since TiO2 has such a high Young’s modulus (2.8 GPa) compared to that of PDMS (750 kPa), the embedded TiO2 bars were negligibly deformed under applied pressure; as pressure was increased, the distance between the TiO2bars (i.e. the grating pitch) was the only grating parameter significantly affected.

Rigorous coupled wave analysis (RCWA) was used to generate the optical simulation data using the grating pitch values obtained by mechanical simulation [11-13]. Fig. 2 shows the transverse electric (TE) reflectivity spectrum of Fig. 1 (a) Mechanical model schematic. Channel structure walls are significantly larger than membrane thickness to avoid channel deformation. (b) Optical model schematic. Material parameters are as follows: n PDMS = 1.4, n TiO

2

= 2.3, and n Medium = 1.00 for air, 1.33

for H2O.

2011 11th IEEE International Conference on Nanotechnology Portland Marriott

August 15-18, 2011, Portland, Oregon, USA

the grating at various applied pressures for both air and water mediums. A clear shift in peak reflectivity is seen as applied

pressure

is increased, corresponding

to the associated increase in the grating pitch.

III.

D EVICE

F ABRICATION

During device testing, it was found that the vast majority of defects in the grating structures arose due to mishandling of the delicate membranes in which they were embedded. In order to minimize the occurrence of such grating damage, the device was first fully fabricated on a rigid substrate, and then released from the substrate in its completed form. To allow for this release of the device, a 100 nm thin film of molybdenum (Mo) was deposited on a silicon wafer

to serve as a sacrificial layer. A 160 nm thick layer of TiO 2 was then deposited via sputtering, followed by a 30 nm aluminum conduction layer to reduce dielectric charging during electron-beam lithography (EBL) . The grating itself was patterned in poly(methyl methacrylate) (PMMA) using EBL over an area of 750 by 750 μm. A chromium hard mask was created through the lift-off process, and the pattern was then transferred into the TiO 2 film by reactive ion etching. An aluminum etchant was used to remove the remaining metals from the tops of the TiO 2 grating lines, preparing the grating to be embedded in the membrane. PDMS was spin-coated and cured on the wafer surface to form the membrane, and then bonded to a PDMS channel structure. The entire structure was then immersed in hydrogen peroxide (H 2O 2) to etch away the Mo sacrificial layer, releasing the device from the substrate. Fabrication diagrams of the major steps in this process are given in Fig. 3. Scanning electron microscope (SEM) images of the grating structure prior to release from the substrate are shown in Fig. 4(a,b), and the final device is pictured in Fig. 4(c,d).

IV. E XPERIMENTAL S ETUP

In order to properly characterize the device, a means of making fine adjustments to the pressure acting upon the membrane is necessary. To accomplish this, one end of a length of tubing was connected to the air inlet of the device (see Fig. 4(d)), and the other end of the tubing was connected to a closed system consisting of an airtight container and a 30 mL syringe. With the syringe plunger fully withdrawn, the total volume of the closed system was 900 mL. Since the pressure of the system was known to be equal to atmospheric pressure (roughly 101 kPa) initially, the

Fig. 2 Simulation results for (a) air medium and (b) water medium. Optical and mechanical simulation results were obtained using RCWA

and COMSOL Multiphysics, respectively.

Fig. 3 (a) Mo sacrificial layer, TiO 2 layer, and Al conduction layer are deposited on Si wafer. (b) Grating structure is patterned in PMMA using EBL, and 50 nm Cr is deposited. (c) Excess Cr is removed via lift-off process to form grating hard mask. (d) Dry etch is used to transfer pattern into the TiO 2. (e) Metals are removed from the tops of the TiO 2 structures. (f) PDMS is spin-coated on the wafer surface to form membrane. (g) PDMS Channel structure is bonded to the membrane. (h) Mo is etched away in H 2O 2, releasing the membrane

from the substrate.

Fig. 4 (a) SEM micrograph of TiO 2 lines on Mo sacrificial layer. (b) Close-up of TiO 2 lines showing vertical sidewalls produced by dry-etch. (c) Optical microscope image of grating suspended in PDMS, exhibiting only minor damage from release process. The thick vertical lines on the sides of the grating are remnants of EBL alignment marks. (d) Photograph of final device. 750x750 μm grating is clearly visible.

pressure as the syringe was depressed may be calculated by

Boyle’s law,

.2211V

p V p

= (1)

The syringe

was

depressed in increments of 0.5 mL, corresponding to a step size in relative pressure of 56~60 Pa over a range of nearly 3500 Pa. The grating’s optical properties were characterized in the following manner. A broad-spectrum halogen light source was connected to one leg of a reflection probe, and the output light was collimated using a series of lenses. Since the grating used produces the strongest resonance response for TE polarized light, a linear polarizer was used to ensure that the electric field of the incident light was aligned parallel to the grating lines. Care was taken to ensure that the resulting illumination spot was aligned with the center of the GMR grating at normal incidence to avoid any unwanted effects associated with off-axis illumination. The light reflected from the grating was analyzed by a spectrometer connected to the other leg of the reflection probe. As the plunger of the syringe was depressed, a full reflectivity spectrum was recorded for each incremental pressure.

V. R ESULTS

Fig. 5 shows the recorded TE reflectance spectra for both air and deionized water mediums at various applied pressures. As was observed in the simulation results, the resonance peaks shift to higher wavelengths as the pressure acting on the membrane is increased. However, it is worth noting that the observed and expected values of peak wavelength differ by around 30 nm. Due to the high sensitivity of the resonant wavelength to changes in grating dimensions and refractive indices, this discrepancy could be caused by nearly any parameter in the fabricated device differing from those used in the optical simulation. More significant than absolute peak wavelength is the relative shift in peak wavelength with applied pressure. Fig. 6 shows that there is good agreement between experiment and simulation, suggesting that the strain acting upon the grating under applied pressure is close to the predicted values.

One other key difference between the experimental and simulation results is that some attenuation of the resonance peaks occurs as the pressure is changed. This is most likely caused by out-of-plane curvature of the grating as the membrane expands, which was neglected in the simulation due to limitations of the frequency-domain approach of RCWA. The amount of attenuation for a given applied pressure (Fig. 7) was consistent over time, however, making

Fig. 7 Attenuation observed in experimental results, most likely due to

curvature of the grating.

Fig. 5 Experimental results for (a) air medium and (b) water medium.

A clear shift in resonant wavelength is observed as pressure is

increased.

Fig. 6 Experimental and simulation values of peak wavelength shift.

Fig. 8 Normalized experimental results for (a) air medium and (b) water medium. These results more closely resemble the simulation data

than the raw reflectivity curves.

it possible to normalize the data by dividing the reflectance

spectrum by the reflectivity at the peak, )

max(R R R norm =

(2)

This results in the curves given in Fig. 8; once attenuation is eliminated, the experimental results much more closely resemble the form of the simulation data, and it becomes much easier to identify the peak resonance shifts associated with changes in pressure. One potentially useful feature of such a widely tunable grating device is the ability to manipulate resonance with respect to wavelength. For example, for a rigid grating, pitch is held constant by design, producing a resonance that occurs at a specific wavelength. Similarly, for a tunable grating, wavelength may be intentionally held constant, resulting in a resonance that occurs at a specific pitch (or pressure). This is seen in Fig. 9 for a fixed wavelength of 825 nm. Just as an increase in RI of the medium causes resonance to shift to higher wavelengths for a fixed-pitch grating, such a change in RI causes resonance of this tunable grating to shift to lower pressures at a constant wavelength.

VI. C ONCLUSIONS

A pressure-tunable nanoscale GMR grating was successfully designed, fabricated, and tested. A tunable range of over 35 nm was demonstrated for both an air and water medium, showing good agreement with expected performance in both cases. Some attenuation of the reflectivity spectra was observed as pressure was varied, but due to the consistency of this attenuation, it is possible to compensate for this. The ability to tune the pitch of the grating via pressure was used to demonstrate a fixed-wavelength resonance response. A pressure-dependant resonance was observed for both air and water media at a wavelength of 825 nm. Such a widely tunable device could find new applications in a number of fields in which rigid-substrate GMR devices are already established.

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Fig. 9 Reflectivity vs. pressure at a fixed wavelength of 825 nm. Peak reflectivity values shift to the left as refractive index of the medium is

increased.