Introduction

In this paper we describe UV light induced deformation phenomena of photosensitive hybrid sol-gel materials. UV radiation (365 nm) is used to pattern periodic structures by proximity-printing with a binary transmission photomask. The hybrid sol-gel material undergoes an expansion in the exposed areas resulting in a surface-relief pattern.

Hybrid sol-gel materials have been previously used to fabricate diffractive optical elements with binary and grayscale photomasks [1,2]. More recently, hybrid sol-gel materials have been patterned simultaneously with a diffractive optical element and opto-mechanical features intended to allow accurate, zero-alignment assembly of complex, multi-component micro-optical systems [3]. The associated fabrication processes require a structure development step using organic solvents that may potentially decrease the surface quality of the optical elements. By applying UV light induced deformation of optical materials, which has been observed in a variety of materials, the development and/or etching steps can be eliminated.

The deformation effect in terms of volume can be either positive or negative and depends on the material composition. In the cases of azo-dyes-containing polymers, liquid crystals, and sol-gel matrices, a negative volume change in regions exposed to light has been observed [4–6]. A similar effect has also been detected with an acrylate-rich hybrid glass material [7]. The opposite reaction, i.e., a volume expansion of the material, has been observed under bandgap illumination in fluorozirconate or chalcogenide glasses [8]. The understanding of the mechanisms for deformation reactions is still evolving and several models have been introduced [4,9]. With azo-dye chromophores, trans-cis isomerization upon photoexcitation is found to play a major role in the deformation reaction [4]. However, the surface-relief formation can also be explained by a chromophore migration from high to low light intensity regions [9]. Similar types of reactions can also occur within acrylate-rich hybrid glasses together with material shrinkage [7]. The expansion phenomenon of glass materials is explained in terms of an increase of structural randomness or structural reorganization [8]. In the case of photopolymers, monomer diffusion to illuminated regions of a film has been used to fabricate optical components [10,11].

This paper presents results on UV-light induced expansion of polyethylene-oxide-acrylate modified hybrid glass films. This phenomenon offers the potential for lithographic printing of micro-optical elements that exhibit low surface roughness because of eliminating the development and etching steps associated with more conventional fabrication techniques [11]. Low surface roughness translates into reduced scattering of light at printed optical surfaces, thus making micro-optical elements fabricated by printing suitable for applications such as optical imaging or a micro-optical resonator.

Experimental Description

The siloxane prepolymer solution was synthesized by using 3-(glysidoxypropyl)-trimethoxysilane (GPTMS) : dilute hydrochloric acid aqueous solution (0.01 M HCl) : methanol (MeOH) : 3-(methacryloxypropyl)-trimethoxysilane (MAMPS) : silicon tetrachloride (SiCl₄) in molar ratios of 8 : 26.4 : 0.21 : 8 : 1, respectively. Among the ingredients, GPTMS and MAMPS were used as precursor monomers to form the polymer network. SiCl₄ was used as a Lewis acid to catalyze a ring opening reaction of epoxy rings of glysidoxypropyl side groups of the GPTMS resulting in formation of polymeric ethylene oxide network. The SiCl₄ is the ideal catalyst since it can potentially take a part in silicon oxide network formation and therefore minimize the amount of material non-uniformities. The ethylene oxide network can be thought of as a support matrix containing the other film-forming components. The MAMPS monomers generate photo-crosslinking properties in the hybrid glass. The acid aqueous solution was used to catalyze the hydrolysis of the methoxy groups of the GPTMS and MAMPS. MeOH was used as reaction activating solvent. To increase the viscosity and lower the shrinkage of the material upon baking, the volatile solvent MeOH and byproducts including HCl and water were removed by using vacuum distillation. The resulting clear, homogenous and transparent solution was relatively viscous and remained stable under normal laboratory conditions. Before the film deposition, the silica prepolymer solution was sensitized by using 2 w-% of phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide as a photoinitiator.

The liquid phase deposition of the thin films was executed by applying a phased spin-coating procedure. First the solution was spread on a substrate at 500 rpm for 10 seconds and then instantly spun on at 3000 rpm for 60 seconds. The solution was delivered slowly onto the substrate through a 2 µm membrane filter to remove residual non-dissolved photoinitiator particles. All films were deposited on cleaned borosilicate glass substrates. After the spin-coating, the samples were prebaked under normal atmosphere conditions at 90 °C for 30 minutes to semi-harden the film structure. The UV light exposure experiments utilized a mercury UV source with a 350 nm to 380 nm bandpass filter. The exposures with the photomask were done by proximity printing with a 2 µm gap between the mask and the photosensitive hybrid glass film. The photomask contained four amplitude gratings. In all gratings, the line width of the bright field line was 6 µm. The distance between the bright field lines was different in each grating. The four grating periods were 24 µm, 18 µm, 12 µm, and 9 µm.

The refractive-index values were measured from slab films by using a fiber-bundle based thin film measurement system (Filmetrics F-20). UV light induced surface relief grating profiles were characterized an optical non-contact surface profiler (WYKO NT-2000). In the diffraction-efficiency measurements, the gratings were illuminated with a HeNe laser (632.8 nm) and the intensities of the diffraction orders were determined with an optical-power meter (ANDO AQ-1135E). The light diffracted into each order was normalized to the total intensity transmitted through the grating.

The transmission values of the solid films were measured from UV-exposed and non-exposed samples using spectrophotometer (Cary 5G UV-VIS-NIR Spectrophotometer) in wavelengths ranging from 250 nm to 3,000 nm. Samples were prepared by spin coating the prepolymer solution on glass slides. The samples were first spun on substrates at 500 rpm for 30 seconds and then instantly spun on at 2000 rpm for 60 seconds. A subset of the samples was then irradiated with UV light. UV-exposed and unexposed samples were baked at 130 °C for 1 hour.

Results and Discussion

The observed surface deformation reaction was found to be of an opposite type compared to previous studies of hybrid glass materials [7]. Instead of trans-cis isomerization or localized shrinkage-related compaction effects, the material undergoes an expansion reaction within the regions exposed to light. The surface corrugation depth was found to be proportional to the period of the gratings in the photomask: 6.2% of the initial film thickness for 24-µm grating period, 4.0% for 18-µm grating period, 2.3% for 12-µm grating period, and 0.3% for 9-µm grating period. The initial film thickness of non-exposed areas was measured to be 10.4 µm.

The surface-relief grating profile formation by material expansion may partially be explained by a diffusion reaction of the hydrolyzed MAMPS monomers (also small molecular weight MAMPS oligomers appear due the partial condensation) from non-exposed regions to the high light-intensity regions, i.e., transparent regions in the photomask. Light interacts with the photoinitiator and generates free radicals that lead to photopolymerization of the MAMPS monomers. As the photoreactive monomer molecules are consumed during the polymerization, concentration and density gradients cause migration of the monomers from the unexposed regions into exposed parts of the film. A similar type of monomer diffusion property has been observed with photopolymers although no expansion phenomenon has been reported [12]. However, the material diffusion from “dark” areas to “bright” areas cannot totally explain the observed material behavior since the film expansion can also take a place with full film area exposure, e.g., when no periodic photomask is applied during the exposure.

Figure 1 shows the transmission curves of an UV-exposed and an unexposed sample. The thickness of the UV-exposed sample was measured to be 19.1 µm. The thickness of the unexposed sample was 17 µm.

 

Fig. 1. Optical transmission of the UV-exposed and the unexposed thin film samples as a function of the wavelength. The discontinuity at 800 nm is an artifact associated with changing the detector in the spectrophotometer. The inset table lists extinction coefficients at eight representative wavelengths.

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Figure 2 shows the measured index of refraction of the hybrid glass material as a function of increased exposure dose when the full film areas were exposed. The decreasing index of refraction suggests a reduction in density of the material as the exposure increases. In addition, due to photochemical reactions in the photosensitive moieties of the material some changes in dipole moment can happen that are observed as a change in the index of refraction. The samples used in the transmission measurements were also made by exposing the whole substrate area. When film thicknesses where measured, expansion of the exposed films relative to non-exposed films was observed. Therefore, it can be speculated that the siloxane chain modified polyethylene oxide partially participates in photochemical reactions while the material is exposed. This may happen, for example, through a reorientation of the ethylene oxide networks or due to the ring opening of non-reacted epoxy groups once the material is exposed. The chemical reaction mechanisms are currently under investigation.

 

Fig. 2. Variation of refractive index with UV exposure dose. See text for measurement-procedure details.

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The maximum measured peak-to-valley modulation depth of a fabricated grating was 0.685 µm and was associated with the 24-µm-period grating (see Figure 3). This modulation-depth value represents an average of measurements taken over four 0.6-mm square areas, i.e., comparable to the grating area illuminated with a 632.8-nm HeNe laser beam used to measure diffraction efficiencies.

The experimentally obtained diffraction efficiencies are listed in Table 1. The surface topography shown in Figure 4 was used to calculate diffraction efficiencies using the material’s index of refraction as a variable. The diffraction efficiencies were computed according to

where 𝓕{.} denotes the Fourier transform, n denotes index of refraction, z(x) denotes the grating-surface topography (see Figure 4), and l denotes wavelength [13]. The best match in a least-squares sense between the measured and computed diffraction efficiencies was found at n=1.48. The computed diffraction efficiencies associated with this index value are listed in Table 1. Based on the exposure dose used to fabricate the grating (2.78 J/cm²), the index of refraction was expected to be n=1.52 (see Figure 2). The mismatch between the computed and the expected refractive indices suggests the presence of a periodic index-of-refraction variation with the same period as the grating and oriented in a direction perpendicular to the grating lines. The index of refraction assumes a higher value under the troughs in Figure 3 and a lower value in regions under the peaks in Figure 3. A possible cause of a variation in index of refraction is due to monomer diffusion not encountered in the full film-area exposure samples.

 

Fig. 3. Surface-topography measurement of a segment of a printed diffraction grating (24 µm period, 0.685 µm peak-to-valley height).

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The rms surface roughness was measured to be 2.2±0.8 nm (based on 28 data points with rms roughness varying from a minimum of 1.1 nm to a maximum of 4.2 nm) for the grating with 24-µm period. The roughness measurements were made within strips measuring 121.5 µm by 1.8 µm located along the peaks and troughs of the surface relief grating [WYKO NT-2000 in phase-shifting interferometry (PSI) mode]. Tilt and cylinder terms were removed from the topography measurements within the strips prior to calculation of the rms surface roughness. Within the unexposed regions of the sample (away from the grating lines), the rms surface roughness was measured to be 0.81±0.27 nm (values ranged from 0.6 nm to 1.1 nm).

Table 1. Diffraction efficiencies of the 24-µm-period grating formed by 2.78 J/cm2 UV dose. The computed diffraction efficiencies correspond to phase grating with a surface topography as shown in Figure 4 and an effective index of refraction of 1.48. See text for details.

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The created micro-structures exhibit chemical, mechanical and thermal stability. For example, the stability against organic solvents such as acetone or alcohols was found to be excellent which is not typically the case with conventional optical polymers. It was also found that a 48-hour-long accelerated aging test at 120 °C had no effect on the optical properties of the material.

 

Fig. 4. Segment of diffraction-grating surface profile (see Figure 3). The surface-profile curve is the result of averaging surface profiles over a width of 25 µm. The black line labeled “Photomask” indicates the opaque and clear areas in the photomask. The clear areas are 6 µm wide. The blue rectangles schematically indicate the illumination pattern on the hybrid glass material.

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Conclusions

In summary, the reported phenomenon was demonstrated in fabrication of near-sinusoidal diffraction gratings [see Figure 4]. The quality of the formed gratings is high in terms of optical as well as stability properties. The obtained effective peak-to-valley modulation depth is sufficient to achieve a phase delay of 1.04π radians. The properties of the hybrid glass material described in this paper indicate that it has a significant potential for one-step lithographic patterning of other micro-optical structures such as lenslets.