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Optical properties of photoresists for femtosecond 3D printing: refractive index, extinction, luminescence-dose dependence, aging, heat treatment and comparison between 1-photon and 2-photon exposure

Michael Schmid, Dominik Ludescher, and Harald Giessen

Open Access Open Access

Abstract

Femtosecond 3D printing has emerged as an important technology for manufacturing nano- and microscopic optical devices and elements. Detailed knowledge of the dispersion in the visible and near-infrared spectral range is crucial for the design of these optical elements. Here we provide refractive index measurements for different UV-doses, aging times, heat treatment and 2-photon exposed structures for the photoresists IP-S, IP-Dip, IP-L, OrmoComp, IP-Visio, and PO4. We use a modified and automized Pulfrich refractometer setup, utilizing critical angles of total internal reflection with an accuracy of 5·10−4 in the visible and near-infrared spectral range. We compare Cauchy and Sellmeier fits to the dispersion curves. We also give Abbe numbers and Schott Catalog numbers of the almost entirely polymerized resists. Additionally, we provide quantitative extinction and luminescence measurements for all photoresists.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Figures (11)
Fig. 1.
Fig. 1. Modified and automized Pulfrich refractometer setup. The light source is a white light laser and an additional laser diode; the angle of incidence is controlled using a mirror on a rotation mount. Measurements are taken at room temperature.

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Fig. 2.
Fig. 2. Refractive index of the photoresists IP-S, IP-Dip, IP-L, and OrmoComp depending on the UV dose. For all photoresists the refractive index rises with longer UV curing times over the entire wavelength range (a). Closer investigation shows a saturation behavior of the refractive index depending on the UV dose (b). However, the saturation times vary; OrmoComp reaches saturation within several seconds, whereas the refractive indices of the other photoresists saturate after more than 30 minutes.

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Fig. 3.
Fig. 3. Refractive index plotted for different aging times. Due to post polymerization the refractive index rises up to several days after the initial illumination. However, the change in the refractive index finally becomes very small.

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Fig. 4.
Fig. 4. Refractive indices after 5 minutes of UV curing before and after 1 hour heat treatment at 60 °C. The refractive indices of all photoresists significantly rise during the heat treatment and reach the same level as they have shown in Fig. 3 after 15 minutes UV treatment and additional post polymerization.

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Fig. 5.
Fig. 5. The refractive index of 2-photon polymerized samples and 15 min UV 1 hour at 60 °C post baked samples in comparison with Sellmeier fits. The 2-photon polymerized samples of IP-S (Laser power: 70% Scan speed: 50000 µm/s Slicing: 2 µm Hatching: 0.8 µm) and OrmoComp (Laser power: 100% Scan speed: 20000 µm/s Slicing: 2 µm Hatching: 0.8 µm) show the same higher refractive index as the post baked samples except for the high-resolution resist IP-Dip (Laser power: 100% Scan speed: 40000 µm/s Slicing: 2 µm Hatching: 0.8 µm). In this case the used slicing and hatching did not provide enough intensity to almost entirely cure the photoresist which results in a much lower refractive index.

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Fig. 6.
Fig. 6. Dispersion of the photoresists IP-S, IP-Dip, IP-L, OrmoComp, IP-Visio, and PO4 in the almost entirely polymerized state. The samples have been UV cured for 15 minutes and afterwards post baked at 60 °C for 1 hour. Further UV illumination, aging or heat treatment did only have minor effect on the refractive indices.

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Fig. 7.
Fig. 7. Abbe diagram of the almost entirely polymerized photoresists. IP-S, IP-L, OrmoComp, and IP-Visio all have similar refractive indices nd between about 1.51 and 1.52 with high Abbe numbers between 47 and 51. IP-Dip has a higher refractive index nd above 1.55 and an Abbe number of 36 and PO4 has the highest refractive index nd above 1.62 and an Abbe number of 25 indicating much stronger dispersion.

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Fig. 8.
Fig. 8. Extinction of the photoresists IP-S, IP-Dip, IP-L, OrmoComp, IP-Visio, and PO4 in the liquid, UV treated and almost entirely polymerized state after heat treatment. The samples have been UV cured for 15 minutes and afterwards post baked at 60 °C for 1 hour.

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Fig. 9.
Fig. 9. Extinction of the photoresists IP-S, IP-Dip, IP-L, OrmoComp, IP-Visio, and PO4 in the liquid state and after UV curing and heat treatment. The samples have been UV cured for 15 minutes and afterwards post baked at 60 °C for 1 hour. The extinction consistently increases from the liquid to the polymerized state for all photoresists.

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Fig. 10.
Fig. 10. Luminescence of the photoresists IP-S, IP-Dip, IP-L, OrmoComp, IP-Visio, and PO4 in the liquid, UV treated and almost entirely polymerized state after heat treatment. The samples have been UV cured for 15 minutes and afterwards post baked at 60 °C for 1 hour.

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Fig. 11.
Fig. 11. Luminescence of the photoresists IP-S, IP-Dip, IP-L, OrmoComp, IP-Visio, and PO4 after UV curing and heat treatment upon comparable excitation and data acquisition conditions. The samples have been UV cured for 15 minutes and afterwards post baked at 60 °C for 1 hour. OrmoComp shows by far the lowest luminescence, followed by IP-Visio, PO4 and IP-S. IP-L and IP-Dip show very similar results with the highest luminescence among the photoresists at around 500 nm.

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Tables (4)

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Table 1. Cauchy parameters for the photoresists IP-S, IP-Dip, IP-L, OrmoComp, IP-Visio, and PO4. We obtain the values by fitting the corresponding equation to the measured refractive index data.

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Table 2. Sellmeier parameters for the photoresists IP-S, IP-Dip, IP-L, OrmoComp, IP-Visio, and PO4. We obtain the values by fitting the corresponding equation to the measured refractive index data.

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Table 3. Measured refractive indices of almost entirely polymerized photoresists (15 min UV, 1 hour at 60 °C post baking).

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Table 4. Measured refractive indices of almost entirely polymerized photoresists (15 min UV, 1 hour at 60 °C post baking). The corresponding Abbe numbers and Schott catalog numbers are given in the last two columns.

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Equations (4)

Equations on this page are rendered with MathJax. Learn more.

θ ( α ) = 60 ∘ + arcsin [ n n 2 sin ⁡ ( arctan ( f 1 f 2 tan ⁡ ( 2 α ) ) ) ] ≈ 60 ∘ + n n 2 f 1 f 2 2 α for small rotation angles α ,
R s = | n 1 cos ⁡ θ − n 2 2 − n 1 2 sin 2 θ n 1 cos ⁡ θ + n 2 2 − n 1 2 sin 2 θ | 2
n ( λ ) = A + B λ 2 + C λ 4 ,
n ( λ ) 2 = 1 + B 1 λ 2 λ 2 − C 1 + B 2 λ 2 λ 2 − C 2 + B 3 λ 2 λ 2 − C 3 .
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