**2.1 Fabrication process of GMR filters by microtransfer molding**

Soft lithography uses soft elastomeric materials to make patterns and structures without the use of complicated and expensive facilities that conventional photolithography uses. Therefore, soft lithography has been shown to be a simple and cost-effective method for pattern transfer. Xia and Whitesides reported several soft lithography methods including replica molding (REM), microtransfer molding (µTM), micromolding in capillaries (MIMIC), and solvent-assisted micromolding (SAMIM) (Xia & Whitesides, 1998). Of the four methods, we choose to conduct preliminary fabrication using the µTM method. REM duplicates the structure with a thick supporting layer, which is not desired for GMR device implementation. MIMIC requires very low-viscosity materials and it cannot get capillary filling with the optical prepolymers in Table 1. SAMIM is a method which applies a solvent that can dissolve or soften the material to form the structure. It is also not proper for our application.

We present guided-mode resonance (GMR) filters fabricated by soft lithography with hybrimer materials. The term GMR refers to a rapid variation in the intensities of the electromagnetic fields in a periodic waveguide, or photonic crystal slab, as the wavelength or the angle of incidence of the excitation light varies around their resonance values. A resonance occurs when incident light is phase-matched to a leaky guided mode allowed by the waveguide-grating structure (Magnusson & Wang, 1992). Numerous potentially useful devices based on resonant waveguide modes have been theoretically predicted and experimentally verified (Magnusson & Wang, 1992) (Avrutsky & Sychugov, 1989) (Peng & Morris, 1996) (Ding & Magnusson, 2004) (K.J. Lee & Magnusson, 2011)(Sharon et al., 1996) (Brundrett et al., 1998) (Priambodo et al., 2003) (Liu et al., 1998) (K.J. Lee et al., 2008). However, these devices were designed to work with conventional materials and processes. Therefore, additionally, we provide example fabrication and characterization of GMR filters made by soft lithography. As these resonant elements are highly sensitive to parametric variations, it is important to develop methods for their reliable fabrication. Thus, we provide a fabrication process that is consistent and simple, employing an elastomeric mold and a UV-curable organic-inorganic hybrid material. A particular fabricated device exhibits measured spectra showing ~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. We conclude that soft lithography combined with hybrimer

media is an advantageous methodology for fabricating resonant photonic devices.

Before hybrimers were considered UV-curable materials for surface-relief structures, commercially available optical prepolymers such as J-91, SK-9 (Summers Optical), and NOA-73 (Norland Products Inc.) were applied in soft lithography. Table 1 shows the main

Product name Material Refractive index Viscosity (cps) J-91 polyurethane 1.55 250 ~ 300 SK-9 polyacrylate 1.49 80 ~ 100 NOA-73 polyurethane 1.56 130

Soft lithography uses soft elastomeric materials to make patterns and structures without the use of complicated and expensive facilities that conventional photolithography uses. Therefore, soft lithography has been shown to be a simple and cost-effective method for pattern transfer. Xia and Whitesides reported several soft lithography methods including replica molding (REM), microtransfer molding (µTM), micromolding in capillaries (MIMIC), and solvent-assisted micromolding (SAMIM) (Xia & Whitesides, 1998). Of the four methods, we choose to conduct preliminary fabrication using the µTM method. REM duplicates the structure with a thick supporting layer, which is not desired for GMR device implementation. MIMIC requires very low-viscosity materials and it cannot get capillary filling with the optical prepolymers in Table 1. SAMIM is a method which applies a solvent that can dissolve or

**2. Fabrication of a surface-relief structure with optical polymers** 

properties of these materials (Summers) (Norland).

Table 1. The properties of three commercial optical prepolymers

**2.1 Fabrication process of GMR filters by microtransfer molding** 

soften the material to form the structure. It is also not proper for our application.

Fig. 1 summarizes the procedure for fabrication of the GMR device using a µTM method. The first step of the fabrication is making a master template, which has the requisite grating structure on the surface of a silicon wafer. The silicon grating structure can be made by photoresist spin-coating, holographic recording, photoresist development, and plasma etching. These conventional fabrication process steps using photolithography are described, for example, in (Priambodo et al., 2003). However, for the results reported here, two types of commercial holographic gratings (Newport Co.) are used as master templates. One has 556 nm grating period (1800 grooves/mm) and ~170 nm grating depth for developing GMR devices operating in the near-infrared region (~850 nm wavelength). Another grating has 1111-nm grating period (900 grooves/mm) and ~340 nm grating depth for the communication band (~1550 nm wavelength). These gratings have sinusoidal profiles.

Fig. 1. Schematic fabrication process of a GMR device by TM

Guided-Mode Resonance Filters Fabricated with Soft Lithography 229

the grating layer for the master template and the PDMS stamp are ~170 nm and ~168 nm, respectively. Odom et al. demonstrated that soft lithography techniques are useful for the patterning on the size scale of 500 nm and larger. However, the application of these methods is limited in the sub-100-nm range because of the low modulus (2.0 N/mm2) of PDMS (Odom et al., 2002). During the process, deformations such as roof collapse and lateral collapse can occur in the PDMS stamp and details of deformations are discussed in (Schmid & Michel, 2000) (Odom et al., 2002) (T.-W. Lee et al., 2005). This deformation is also found in our processes reported here. For example, Fig. 3 shows AFM images of the deformed area. This deformation is more evident if there are higher aspect ratios. Fig. 4 shows AFM images of a PDMS stamp having a 520-nm grating period and 220-nm grating depth, which is prepared by photoresist grating. Pairings of grating lines are easily observed in this image. Delamarche et al. discussed the stability of lines molded in PDMS with different mold thicknesses and showed the limitation of shape formation in PDMS without deformation (Delamarche et al., 1997). They also proposed that surface treatment with 1% sodium dodecylsulfate in water and a heptane rinse could provide the recuperation of the paired lines. Even though their method is applied to this stamp, it

Fig. 3. AFM image of a deformed area on a PDMS stamp. The size of the scanned area is

shows little improvement.

5 m x 5 m

As an elastomeric mold, polydimethylsiloxane (PDMS) is commonly used in soft lithography. Sylgard 184 silicone elastomer from Dow Corning, which is commonly used, is applied. Sylgard 184 silicone elastomer is mixed in a 10:1 ratio of base and curing agent and its mixture is degassed in a vacuum to eliminate bubbles. The prepolymer of Sylgard 184 is poured onto the top of the master template and cured at room temperature for 48 hours. The result is a mold with the negative replica of the grating on its surface. Atomic force microscope (AFM) images of the master template and the replica are shown in Fig. 2. These AFM images are obtained by Asylum MFP-3D AFM (Asylum Research).

A few drops of a UV-curable optical prepolymer are applied on the patterned surface of the elastomeric mold. The excess prepolymer is scraped off using a flat PDMS block, such that the grooves of the PDMS mold are filled with UV-curable prepolymer. Next, the filled PDMS mold is put in contact with a silicon nitride (Si3N4) thin-film on a glass substrate. As a waveguide layer, the Si3N4 thin-film is prepared by sputtering; this layer can also be replaced by a high-refractive index polymer material, such as a titanium dioxide (TiO2)-rich film made by spin-coating (Wang et al., 2005). The prepolymer is cured using a UV lamp (central wavelength λ = 365 nm). Patterned grating structures are obtained on the Si3N4 film after the PDMS mold is peeled off.

Fig. 2. AFM images of (a) the master template and (b) the replica. The size of the scanned area is 5 m x 5 m

#### **2.2 Discussion of the fabrication process**

Fig. 2 shows AFM images of the master template and the PDMS stamp. Both images show approximately sinusoidal profiles and a grating period of 556 nm. The average depths of

As an elastomeric mold, polydimethylsiloxane (PDMS) is commonly used in soft lithography. Sylgard 184 silicone elastomer from Dow Corning, which is commonly used, is applied. Sylgard 184 silicone elastomer is mixed in a 10:1 ratio of base and curing agent and its mixture is degassed in a vacuum to eliminate bubbles. The prepolymer of Sylgard 184 is poured onto the top of the master template and cured at room temperature for 48 hours. The result is a mold with the negative replica of the grating on its surface. Atomic force microscope (AFM) images of the master template and the replica are shown in Fig. 2. These AFM images are obtained by Asylum MFP-3D AFM

A few drops of a UV-curable optical prepolymer are applied on the patterned surface of the elastomeric mold. The excess prepolymer is scraped off using a flat PDMS block, such that the grooves of the PDMS mold are filled with UV-curable prepolymer. Next, the filled PDMS mold is put in contact with a silicon nitride (Si3N4) thin-film on a glass substrate. As a waveguide layer, the Si3N4 thin-film is prepared by sputtering; this layer can also be replaced by a high-refractive index polymer material, such as a titanium dioxide (TiO2)-rich film made by spin-coating (Wang et al., 2005). The prepolymer is cured using a UV lamp (central wavelength λ = 365 nm). Patterned grating structures are obtained on the Si3N4 film

Fig. 2. AFM images of (a) the master template and (b) the replica. The size of the scanned

Fig. 2 shows AFM images of the master template and the PDMS stamp. Both images show approximately sinusoidal profiles and a grating period of 556 nm. The average depths of

(Asylum Research).

area is 5 m x 5 m

**2.2 Discussion of the fabrication process** 

after the PDMS mold is peeled off.

the grating layer for the master template and the PDMS stamp are ~170 nm and ~168 nm, respectively. Odom et al. demonstrated that soft lithography techniques are useful for the patterning on the size scale of 500 nm and larger. However, the application of these methods is limited in the sub-100-nm range because of the low modulus (2.0 N/mm2) of PDMS (Odom et al., 2002). During the process, deformations such as roof collapse and lateral collapse can occur in the PDMS stamp and details of deformations are discussed in (Schmid & Michel, 2000) (Odom et al., 2002) (T.-W. Lee et al., 2005). This deformation is also found in our processes reported here. For example, Fig. 3 shows AFM images of the deformed area. This deformation is more evident if there are higher aspect ratios. Fig. 4 shows AFM images of a PDMS stamp having a 520-nm grating period and 220-nm grating depth, which is prepared by photoresist grating. Pairings of grating lines are easily observed in this image. Delamarche et al. discussed the stability of lines molded in PDMS with different mold thicknesses and showed the limitation of shape formation in PDMS without deformation (Delamarche et al., 1997). They also proposed that surface treatment with 1% sodium dodecylsulfate in water and a heptane rinse could provide the recuperation of the paired lines. Even though their method is applied to this stamp, it shows little improvement.

Fig. 3. AFM image of a deformed area on a PDMS stamp. The size of the scanned area is 5 m x 5 m

Guided-Mode Resonance Filters Fabricated with Soft Lithography 231

In addition, the UV-curable polymers can stick to the PDMS stamp and this causes deformation of the structure during the process. To solve this problem, silane treatment is widely used for the surface of the PDMS stamp and acts as a releasing layer. Tang et al. reported that the modification of PDMS stamps by an adsorbed monolayer of bovine serum albumin (BSA) can provide distortion-free separation between the PDMS stamp and molded gel from the stamp (Tang et al., 2003). In our process, we find that the cured polymer does not attach well to the waveguide but tends to remain on the PDMS stamp after separation. Therefore, we apply the BSA treatment to improve the release. The PDMS stamp is immersed in a 3% solution of aminopropyldimethylethoxysilane (Gelest, SIA0603.0) in methanol for 2 hours. The modified PDMS is next rinsed in phosphate buffer saline (PBS, Fisher Scientific, BP2438-4) solution for 90 seconds. A solution of BSA (100 mg/ml Fisher Scientific, NC9806065) is applied over the patterned surface of the PDMS for 60 seconds. The treated PDMS is then rinsed in a PBS solution for 90 seconds to remove any unbounded BSA. After this BSA treatment, it is possible to successfully get the structure to adhere to the

Fig. 5 shows the experimental spectral response of a fabricated GMR device using the NOA-73 prepolymer. A tunable Ti:Sapphire laser pumped by an Argon-ion laser is used to measure its spectral response. The resonance wavelength (maximum point of reflectance) is at = 863.6 nm and the full-width at half-maximum (FWHM) linewidth is 4.0 nm. The reflectance at resonance is about 33%. This relatively low efficiency is attributed to the loss of the Si3N4 thin-film (measured extinction coefficient, k ~ 10-3) and imperfections on the grating surface. Even though the efficiency of the fabricated device is low, it can be used in resonant sensor devices because high-efficiency is not essential in GMR sensor application and the key quantity is the resonance wavelength shift. Fig. 6 shows AFM images of the GMR device fabricated with NOA-73. The cross-sectional view shows that the thickness of the grating layer is not uniform. This thickness nonuniformity is due to the scraping process applied to remove excess prepolymer by hand.

Fig. 5. Experimental spectral response of a fabricated GMR filter for transverse-electric (TE)

waveguide layer with no remaining polymer in the PDMS stamp.

**2.3 Fabrication results** 

polarization

Fig. 4. AFM images of a PDMS stamp having a 520-nm grating period and 220-nm grating depth showing grating-line pairing. The size of the scanned areas are (a) 5 μm 5 μm and (b) 20 μm 20 μm

Schmid et al. formulated an alternative siloxane polymer, called hard-PDMS (h-PDMS), which has a higher modulus (~9 N/mm2) than that of Sylgard 184 silicone elastomer (Schmid & Michel, 2000) (Odom et al., 2002). Odom et al. and Lee et al. demonstrated improved results of surface structure formation by using h-PDMS compared with Sylgard 184 PDMS (Odom et al., 2002) (T.-W. Lee et al., 2005). In our research, we apply h-PDMS as described in Section 3.

In addition, the UV-curable polymers can stick to the PDMS stamp and this causes deformation of the structure during the process. To solve this problem, silane treatment is widely used for the surface of the PDMS stamp and acts as a releasing layer. Tang et al. reported that the modification of PDMS stamps by an adsorbed monolayer of bovine serum albumin (BSA) can provide distortion-free separation between the PDMS stamp and molded gel from the stamp (Tang et al., 2003). In our process, we find that the cured polymer does not attach well to the waveguide but tends to remain on the PDMS stamp after separation. Therefore, we apply the BSA treatment to improve the release. The PDMS stamp is immersed in a 3% solution of aminopropyldimethylethoxysilane (Gelest, SIA0603.0) in methanol for 2 hours. The modified PDMS is next rinsed in phosphate buffer saline (PBS, Fisher Scientific, BP2438-4) solution for 90 seconds. A solution of BSA (100 mg/ml Fisher Scientific, NC9806065) is applied over the patterned surface of the PDMS for 60 seconds. The treated PDMS is then rinsed in a PBS solution for 90 seconds to remove any unbounded BSA. After this BSA treatment, it is possible to successfully get the structure to adhere to the waveguide layer with no remaining polymer in the PDMS stamp.

#### **2.3 Fabrication results**

230 Recent Advances in Nanofabrication Techniques and Applications

Fig. 4. AFM images of a PDMS stamp having a 520-nm grating period and 220-nm grating depth showing grating-line pairing. The size of the scanned areas are (a) 5 μm 5 μm and

Schmid et al. formulated an alternative siloxane polymer, called hard-PDMS (h-PDMS), which has a higher modulus (~9 N/mm2) than that of Sylgard 184 silicone elastomer (Schmid & Michel, 2000) (Odom et al., 2002). Odom et al. and Lee et al. demonstrated improved results of surface structure formation by using h-PDMS compared with Sylgard 184 PDMS (Odom et al., 2002) (T.-W. Lee et al., 2005). In our research, we apply h-PDMS as described in Section 3.

(b) 20 μm 20 μm

Fig. 5 shows the experimental spectral response of a fabricated GMR device using the NOA-73 prepolymer. A tunable Ti:Sapphire laser pumped by an Argon-ion laser is used to measure its spectral response. The resonance wavelength (maximum point of reflectance) is at = 863.6 nm and the full-width at half-maximum (FWHM) linewidth is 4.0 nm. The reflectance at resonance is about 33%. This relatively low efficiency is attributed to the loss of the Si3N4 thin-film (measured extinction coefficient, k ~ 10-3) and imperfections on the grating surface. Even though the efficiency of the fabricated device is low, it can be used in resonant sensor devices because high-efficiency is not essential in GMR sensor application and the key quantity is the resonance wavelength shift. Fig. 6 shows AFM images of the GMR device fabricated with NOA-73. The cross-sectional view shows that the thickness of the grating layer is not uniform. This thickness nonuniformity is due to the scraping process applied to remove excess prepolymer by hand.

Fig. 5. Experimental spectral response of a fabricated GMR filter for transverse-electric (TE) polarization

Guided-Mode Resonance Filters Fabricated with Soft Lithography 233

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

Fig. 6. AFM images of fabricated GMR devices with NOA-73 at two different locations. The size of the scanned area is 5 μm 5 μm

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