**3. Improved fabrication of GMR filters using hybrimers and MIMIC**

As described in section 1, hybrimers have many advantages, especially low viscosity, which can be lowered to ~8 cps. This low-viscosity material is applied with the MIMIC method. The combination of hybrimers and MIMIC to fabricate GMR filters is described in this section. It is shown to provide much-improved GMR devices relative to the devices reported above.

#### **3.1 Fabrication process and characterization**

Fig. 7 shows a schematic procedure for fabrication of a GMR device using the MIMIC method. As described in section 2.1, MIMIC is one of several soft lithography methods proposed by Whitesides and co-workers, and it is simple to apply (Xia & Whitesides, 1998) (Xia et al., 1999). The first step of the fabrication is preparing a master template, which has the grating structure on the surface of a silicon wafer or a glass substrate. In this section, a commercial holographic grating, which has a 1111-nm grating period and ~340-nm grating depth with a sinusoidal profile is used as a master template.

Fig. 6. AFM images of fabricated GMR devices with NOA-73 at two different locations. The

The structure of GMR filters fabricated by J-91 and SK-9 is not much different from that of the NOA-73 filter as shown in Fig. 6. The improved µTM method with a machine-controlled system was reported by (J.-H. Lee et al., 2005). They used a dragging speed of ~30 μm/sec

As described in section 1, hybrimers have many advantages, especially low viscosity, which can be lowered to ~8 cps. This low-viscosity material is applied with the MIMIC method. The combination of hybrimers and MIMIC to fabricate GMR filters is described in this section. It is shown to provide much-improved GMR devices relative to the devices

Fig. 7 shows a schematic procedure for fabrication of a GMR device using the MIMIC method. As described in section 2.1, MIMIC is one of several soft lithography methods proposed by Whitesides and co-workers, and it is simple to apply (Xia & Whitesides, 1998) (Xia et al., 1999). The first step of the fabrication is preparing a master template, which has the grating structure on the surface of a silicon wafer or a glass substrate. In this section, a commercial holographic grating, which has a 1111-nm grating period and ~340-nm grating

and a metal blade, which was controlled by mechanical actuators for scraping.

**3. Improved fabrication of GMR filters using hybrimers and MIMIC** 

size of the scanned area is 5 μm 5 μm

**3.1 Fabrication process and characterization** 

depth with a sinusoidal profile is used as a master template.

reported above.

Fig. 7. Schematic fabrication process of a GMR device using the MIMIC method

Fig. 8. Configuration of the composite stamp. The h-PDMS is spin-coated on the master template followed by pouring Sylgard 184 PDMS

Guided-Mode Resonance Filters Fabricated with Soft Lithography 235

To realize a GMR filter, the 10mm 10mm composite mold is placed in contact with a Si3N4 thin film on a glass substrate. The Si3N4 film, prepared by plasma-enhanced chemical vapor deposition (PECVD, Surface Technology Systems), serves as the waveguide layer of the GMR device. A diluted hybrimer is prepared to obtain a lower viscosity prepolymer. A few drops of the UV-curable hybrimer are applied at one edge of the patterned surface of the composite mold as indicated in Fig. 7. The applied hybrimer spreads through the channels, which are formed by contact between the patterned mold and the thin-film layer on the substrate. Then the composite mold set is put into a vacuum chamber and is allowed to remain in low vacuum (~450 Torr) for 12 hours so that the channels fill with the hybrimer prepolymer by capillary force. Subsequently, the hybrimer in the h-PDMS channel is cured using a UV lamp (central wavelength λ = 365 nm). The surface-relief type grating structure remains on the Si3N4 film after the composite mold is peeled off. This easy-release process is

A tunable laser source (HP 8168F) is used to measure the spectral response. The angle of incidence (*θ*in) is set at 10º to locate the resonance wavelength of the GMR device within the operating spectral range of the laser source. Next, the reflected and transmitted power are measured in the wavelength range of 1450 nm to 1590 nm, in 0.5 nm steps. A polarizer is

Fig. 10. Experimental spectral response of a fabricated GMR filter for TE polarization

associated with the hybrimers medium.

used to set the polarization state.

Composite polymeric stamps as elastomeric molds are applied to achieve high-quality patterning. The composite stamps consist of two parts as shown in Fig. 8; one part is h-PDMS, which has different mechanical properties than the Sylgard 184 silicone elastomer. The other part is Sylgard 184 PDMS to support h-PDMS and maintain the flexibility of the stamp. The h-PDMS is prepared as follows (Odom et al., 2002): a vinyl PDMS prepolymer (3.4 gram, Gelest Inc., VDT-731), platinum (Pt) catalyst (18 μL, Gelest Inc., SIP6831.1), and modulator (50 μL, Sigma-Aldrich, Product No. 87927) are mixed and degassed in a vacuum for 5 minutes. A hydrosilane prepolymer (1.0 gram, Gelest Inc., HMS-301) is added into the mixture and stirred gently to avoid bubbles. Within 5 minutes after stirring, this h-PDMS prepolymer is spin-coated (1000 rpm, 40 sec.) on a commercial holographic grating and cured in an oven at 60 C for 30 minutes. Then a prepolymer of Sylgard 184 silicon elastomer is poured on the h-PDMS layer and is cured in an oven again at 60 C for 3 hours. Therefore, a pattern with a negative replica of the master template is formed on the h-PDMS surface. AFM images of the surface of the master template and the composite stamp are shown in Fig. 9. We obtain faithful replication and a high-quality, scatter-free surface.

Fig. 9. AFM images of (a) the master template and (b) the composite stamp replica. The size of the scanned area is 10 μm 10 μm

Composite polymeric stamps as elastomeric molds are applied to achieve high-quality patterning. The composite stamps consist of two parts as shown in Fig. 8; one part is h-PDMS, which has different mechanical properties than the Sylgard 184 silicone elastomer. The other part is Sylgard 184 PDMS to support h-PDMS and maintain the flexibility of the stamp. The h-PDMS is prepared as follows (Odom et al., 2002): a vinyl PDMS prepolymer (3.4 gram, Gelest Inc., VDT-731), platinum (Pt) catalyst (18 μL, Gelest Inc., SIP6831.1), and modulator (50 μL, Sigma-Aldrich, Product No. 87927) are mixed and degassed in a vacuum for 5 minutes. A hydrosilane prepolymer (1.0 gram, Gelest Inc., HMS-301) is added into the mixture and stirred gently to avoid bubbles. Within 5 minutes after stirring, this h-PDMS prepolymer is spin-coated (1000 rpm, 40 sec.) on a commercial holographic grating and cured in an oven at 60 C for 30 minutes. Then a prepolymer of Sylgard 184 silicon elastomer is poured on the h-PDMS layer and is cured in an oven again at 60 C for 3 hours. Therefore, a pattern with a negative replica of the master template is formed on the h-PDMS surface. AFM images of the surface of the master template and the composite stamp are shown in Fig. 9. We obtain faithful replication and

Fig. 9. AFM images of (a) the master template and (b) the composite stamp replica. The size

a high-quality, scatter-free surface.

of the scanned area is 10 μm 10 μm

To realize a GMR filter, the 10mm 10mm composite mold is placed in contact with a Si3N4 thin film on a glass substrate. The Si3N4 film, prepared by plasma-enhanced chemical vapor deposition (PECVD, Surface Technology Systems), serves as the waveguide layer of the GMR device. A diluted hybrimer is prepared to obtain a lower viscosity prepolymer. A few drops of the UV-curable hybrimer are applied at one edge of the patterned surface of the composite mold as indicated in Fig. 7. The applied hybrimer spreads through the channels, which are formed by contact between the patterned mold and the thin-film layer on the substrate. Then the composite mold set is put into a vacuum chamber and is allowed to remain in low vacuum (~450 Torr) for 12 hours so that the channels fill with the hybrimer prepolymer by capillary force. Subsequently, the hybrimer in the h-PDMS channel is cured using a UV lamp (central wavelength λ = 365 nm). The surface-relief type grating structure remains on the Si3N4 film after the composite mold is peeled off. This easy-release process is associated with the hybrimers medium.

A tunable laser source (HP 8168F) is used to measure the spectral response. The angle of incidence (*θ*in) is set at 10º to locate the resonance wavelength of the GMR device within the operating spectral range of the laser source. Next, the reflected and transmitted power are measured in the wavelength range of 1450 nm to 1590 nm, in 0.5 nm steps. A polarizer is used to set the polarization state.

Fig. 10. Experimental spectral response of a fabricated GMR filter for TE polarization

Guided-Mode Resonance Filters Fabricated with Soft Lithography 237

Generally, for this device class, there is good agreement between theory and experiment as shown, for example, in (Priambodo et al., 2003) and by the results presented here. As quantified in (Kemme et al., 2003), small variations in the device parameters can shift the location of the resonance peak significantly. A residual layer, shown in the inset in Fig. 11 with thickness dR, can have significant effects on the spectrum and central filter wavelength. By numerical modeling, the thickness of the residual layer can be estimated. This layer, in this work, is an unwanted layer arising during the soft lithography process. Theoretical calculation shows that the resonance wavelength shifts and the FWHM linewidth decreases as the thickness of the residual layer increases as shown in Fig. 12. By comparing the resonance wavelength and linewidth of the calculated data with the measured data, the

Fig. 12. Calculated spectral response of the fabricated GMR filter with varying thickness of

Fig. 13 shows a cross-sectional view of the fabricated GMR filter obtained with a scanning electron microscope (SEM, FEI Company, Quanta 3D FEG) indicating no apparent residual layer for the fabricated filter. Moreover, the SEM confirms the parameters used in the

the residual layer (dR)

theoretical calculations in Fig. 11.

thickness of the residual layer is estimated to be ~11 nm.

#### **3.2 Results and discussion**

Fig. 10 shows the experimental spectral response of the fabricated device. The resonance wavelength (maximum point of reflectance or minimum point of transmittance) is λ = 1538 nm, and the reflectance at resonance is ~81%. The FWHM linewidth is ~4.5 nm, and the sideband reflectance is ~5%. The transmittance at resonance is ~8%. These results pertain to the TE polarization state of the input light.

Fig. 11 shows the calculated spectral response of the fabricated GMR device whose model is shown as an inset. The calculations are performed with a computer code that we wrote based on rigorous coupled-wave theory (Gaylord & Moharam, 1985). The device parameters used in Fig. 11 correspond to the experimental values used in the fabrication. The calculated resonance wavelength is λ = 1536 nm at *θ*in = 10.0º, and the FWHM linewidth is 4.4 nm. The calculated reflectance at resonance is ~82%. This non-100% reflection is due to the presence of a higher-order transmitted wave at resonance as shown in Fig. 11 and noted as T+1.

Fig. 11. Calculated spectral filter response with parameters corresponding to those of the fabricated filter for TE polarization. The parameters are as follows: thicknesses, d1 = 333 nm, dR = 0 nm (No residue), d2 = 250 nm; refractive indices nH = 1.51, nL = 1.00, n2 = 1.87, nc = 1.00, ns = 1.50; grating period Λ = 1111 nm; incident angle *θ*in = 10º

Fig. 10 shows the experimental spectral response of the fabricated device. The resonance wavelength (maximum point of reflectance or minimum point of transmittance) is λ = 1538 nm, and the reflectance at resonance is ~81%. The FWHM linewidth is ~4.5 nm, and the sideband reflectance is ~5%. The transmittance at resonance is ~8%. These results pertain to

Fig. 11 shows the calculated spectral response of the fabricated GMR device whose model is shown as an inset. The calculations are performed with a computer code that we wrote based on rigorous coupled-wave theory (Gaylord & Moharam, 1985). The device parameters used in Fig. 11 correspond to the experimental values used in the fabrication. The calculated resonance wavelength is λ = 1536 nm at *θ*in = 10.0º, and the FWHM linewidth is 4.4 nm. The calculated reflectance at resonance is ~82%. This non-100% reflection is due to the presence of a higher-order transmitted wave at resonance as shown in Fig. 11 and noted as T+1.

Fig. 11. Calculated spectral filter response with parameters corresponding to those of the fabricated filter for TE polarization. The parameters are as follows: thicknesses, d1 = 333 nm, dR = 0 nm (No residue), d2 = 250 nm; refractive indices nH = 1.51, nL = 1.00, n2 = 1.87, nc = 1.00,

ns = 1.50; grating period Λ = 1111 nm; incident angle *θ*in = 10º

**3.2 Results and discussion** 

the TE polarization state of the input light.

Generally, for this device class, there is good agreement between theory and experiment as shown, for example, in (Priambodo et al., 2003) and by the results presented here. As quantified in (Kemme et al., 2003), small variations in the device parameters can shift the location of the resonance peak significantly. A residual layer, shown in the inset in Fig. 11 with thickness dR, can have significant effects on the spectrum and central filter wavelength. By numerical modeling, the thickness of the residual layer can be estimated. This layer, in this work, is an unwanted layer arising during the soft lithography process. Theoretical calculation shows that the resonance wavelength shifts and the FWHM linewidth decreases as the thickness of the residual layer increases as shown in Fig. 12. By comparing the resonance wavelength and linewidth of the calculated data with the measured data, the thickness of the residual layer is estimated to be ~11 nm.

Fig. 12. Calculated spectral response of the fabricated GMR filter with varying thickness of the residual layer (dR)

Fig. 13 shows a cross-sectional view of the fabricated GMR filter obtained with a scanning electron microscope (SEM, FEI Company, Quanta 3D FEG) indicating no apparent residual layer for the fabricated filter. Moreover, the SEM confirms the parameters used in the theoretical calculations in Fig. 11.

Guided-Mode Resonance Filters Fabricated with Soft Lithography 239

curable organic-inorganic hybrid material is provided. By combining MIMIC, a hybrimer, and an h-PDMS mold, a photopolymer grating structure is readily fabricated. Measured spectra show ~81% reflectance and ~8% transmittance at a resonance wavelength of 1538 nm. The filter's linewidth is ~4.5 nm, and the sideband reflectance is ~5%. Experimental and theoretical results are in good agreement. The use of hybrimer media yields improved

This work was supported in part by the National Science Foundation (NSF) under grant ECCS-0925774 and by the UT System Texas Nanoelectronics Research Superiority Award funded by the State of Texas Emerging Technology Fund. The authors gratefully acknowledge the NanoPort Applications Team at FEI Company for providing the results shown in Fig. 13. The authors thank Prof. B. Huey from The Institute of Materials Science at the University of Connecticut for providing access to AFM facilities. This work was performed in part at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the NSF under award ECS-0335765. CNS is part of the Faculty of Arts and Sciences at Harvard University. Additional support was provided by the

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**5. Acknowledgments** 

**6. References** 

Fig. 13. SEM images of the fabricated GMR device. (a) Top down view, Magnification = 10,000. The size of the image is ~15 μm. (b) Cross-sectional view, Magnification = 25,000. Note that the white part on top is a platinum (Pt) layer for protection during ion-beam sectioning
