2.1.

Fabrication and Structural Characterization

Polymeric 3D microstructures and devices were fabricated by MPL in IP-Dip, a commercially available cross-linkable acrylate photopolymer (NanoScribe), using methods described previously.²⁵ In preparation for SEM imaging, the 3D microstructures were sputter coated (Emitech K675X) with a $\sim 20\text{-}\mathrm{nm}$ thick layer of Au/Pd. A Zeiss Ultra 55 FEG SEM was used to image metal-coated 3D microstructures before and after etching.

2.2.

Preparation of Etching Solutions

Ethaline solutions were prepared²⁴ by mixing choline chloride (Sigma-Aldrich $\ge 98\%$, CAS 67-48-1) and ethylene glycol (Sigma-Aldrich $\ge 98\%$, CAS 107-21-1) in a 1:2 molar ratio and heating to 60°C for 5 min to 10 min to dissolve all solids completely. Ethaline can be prepared in advance and kept in a sealed plastic or glass container under cool and dry conditions for future use. With time, choline chloride may crystallize and settle at the bottom of the container, but it can be redissolved by heating the solution immediately prior to use.

Etchant solutions were prepared immediately prior to use by dissolving ${\mathrm{I}}_2$ (Fisher Scientific $\ge 99.5\%$, CAS 7553-56-2) in ethaline to a concentration of 0.125 M at 60°C. Saturated aqueous potassium iodide (KI) was prepared by dissolving 150 g of KI (Acros Organics, CAS 7681-11-0) in 100 mL of deionized water (18 MΩ) with constant stirring at 20°C.

2.3.

Metal Etching

A metal-coated microstructure was placed in a 10-mL glass beaker. Warm ${\mathrm{I}}_2$-etchant (2 mL, 60°C) was gently added dropwise to avoid dislodging the structure from the substrate. The submerged sample was maintained at 60°C for 15 min. The etchant was then removed slowly using a pipette, and the sample was rinsed by soaking in saturated KI (15 min, two portions), followed by deionized water (10 min). The water was then removed by pipette, and the sample was dried in air.

2.4.

Optical Characterization of Structures

The optical properties of 3D microstructures were measured using the scanned-optical-fiber system shown in Fig. 1(a). An early version of the system is reported elsewhere,²⁶ and additional enhancements are described here. Light injected into the scanned-fiber system had a wavelength in vacuum of ${\lambda }_0=1.55\mu \mathrm{m}$ and was derived from an optical parametric amplifier pumped by an amplified femtosecond laser (coherent). The laser beam was routed through an optical filter to remove residual components having ${\lambda }_0<1\mu \mathrm{m}$ and then split into three “source” beams: (1) a main beam, used for optical alignment and calibration; (2) a zero-order diffracted beam, used for broad-bandwidth optical characterization; and (3) a spectrally dispersed, first-order diffracted beam was used for narrowband optical characterization. The bandwidth of the source beams was measured using a spectrophotometer (OceanOptics NIRQuest512-2.2 NIR). The beams were combined onto a common path, but optical measurements were performed using only one source beam at any given time. An objective lens coupled the source beam into a single-mode “source fiber” (Thorlabs 1550BHP, mode-field $\text{diameter}=9.5\mu \mathrm{m}\pm 0.5\mu \mathrm{m}$) that delivered light onto a microstructure under study. A “detection fiber” of the same type on the opposite side of the sample was coupled to a PbSe detector (ThorLabs PDA20H) and used to record beam profiles, measure optical transmission, and estimate the spot size of an emerging beam. A computer-controlled three-axis stage scanned the output fiber spatially during automated measurements. Signals were normalized against that from a reference detector to remove shot-to-shot fluctuations due to the laser.

Fig. 1

(a) Scanned-optical-fiber system used to characterize optical beams transmitted through the microstructures. (b) Line scans illustrating natural divergence of light exiting the source fiber and propagating distance $z$. (c) Comparison of theoretical and experimentally measured widths of line scans (as FWHM) for light exiting the source fiber. The theoretical scan widths were obtained by propagating a Gaussian beam exiting the source fiber and convoluting its width at position $z$ with the response function of the detection fiber.

The width of a beam emerging from the source fiber, and/or transmitted through a microstructure, was obtained by recording signal $S(x,y,z)$ as the detection-fiber scanned across the beam in the $xy$ planes, and at varying distance $z$ between the source and detection fibers. Figure 1(b) shows a series of line scans obtained by measuring signal for the beam directly exiting the source fiber. All line scans have a Gaussian shape which increases with $z$. The signal $S$ is a convolution of the irradiance profile of the beam with the input response function of the detection fiber. A single-mode, step-index fiber outputs a beam that can be approximated as having a Gaussian profile of the form in Eq. (1).^27,28 The input response function of the detection fiber can also be assumed to be Gaussian, and their convolution should then yield a Gaussian signal profile that widens with $z$, as seen in Fig. 1(b).²⁹

Line scans like those in Fig. 1(b) were fit to a Gaussian function to obtain the $1/e^2\text{-width}=\mathrm{FWHM}/{(2\ln 2)}^{1/2}$, where FWHM denotes the full-width at half-maximum. Sputter-coated devices only partly transmitted incident light and generated distorted beams that could not be meaningfully fit to a Gaussian function, so to facilitate comparison, all line scans presented in Sec. 3 are compared in terms of FWHM. But here the performance of the scanned-fiber system and the ability to detect changes in beam size can be demonstrated by comparing experimentally measured scan widths to those predicted for a Gaussian beam exiting the source fiber and diverging in air.³⁰ When the source and detection fibers are coupled end-to-end ($z=0$), the measured signal $S$ is a convolution of the input response function of the fiber with itself, so scan $\text{width}(0)=w_0\surd 2$, where $w_0$ is the $1/e^2\text{-width}$ of the beam at $z=0$. Figures 1(b) and 1(c) show that the line scan $S(z=0)$ has $\mathrm{FWHM}=9.0\mu \mathrm{m}$, which gives $w_0=5.4\mu \mathrm{m}$. The corresponding mode-field-diameter is $2w_0=10.4\mu \mathrm{m}$, which agrees with the value provided by the manufacturer of the fiber. Upon propagating distance $z$, the beam diverges to $w(z)$ calculated with Eqs. (2) and (3).³¹ Convolution of the diverged beam with the input response function of the fiber gives theoretical scan $\text{width}(z)={[w{(z)}^2+w_0^2]}^{1/2}$. The theoretical and experimentally measured scan widths for the beam exiting the source fiber are plotted together in Fig. 1(c) and seen to agree well. This example illustrates that changes in the beam width of a few microns can readily be observed using the scanned-fiber system:

Microstructures were optically characterized by inserting them between the source and detection fibers and performing line scans across the beam transmitted through the sample. Changes in the width of the scans were used to observe how the structure altered propagation of the beam, and how that varied between pristine, metal-coated, and etched samples. Optical transmission through the sample was obtained as the ratio of the peak signal before and after the structure. Ratios of the integrated incident and transmitted signals confirmed that the peak signals provide a suitable measure of optical transmission. Uncertainties were obtained by comparing the variation between repeat measurements with the standard error obtained from fits and reporting the larger of the two.

3.1.

Processing Samples with Etchant

The etchant solution is dark purple due to dissolved ${\mathrm{I}}_2$, so the removal of metal from the sample is not easily observed during etching. Because the etching process is mild, no bubbling, foaming, or other obvious signs of chemical reaction are visible. After the etch time has elapsed, the sample is removed, rinsed with saturated aqueous KI, then rinsed with water. Rinsing the sample first with pure water would precipitate microcrystals of ${\mathrm{I}}_2$ onto the surface and within internal voids of the microstructure. Therefore, the sample is rinsed first with saturated KI because ${\mathrm{I}}^{-}$ reacts with ${\mathrm{I}}_2$ forming soluble ${{\mathrm{I}}_3}^{-}$, which is soluble in water.

After rinsing, the effect of the etching is immediately obvious from a change in reflectivity of the sample, as seen in Fig. 2. Au/Pd sputter coated onto the sample for SEM imaging makes the supporting substrate reflective, to a degree that depends upon the thickness of the metal layer, but the camera used to record the images is clearly visible [Fig. 2(a)]. After etching, the substrate is again highly transmissive [Fig. 2(b)], indicating that metal was removed.

Fig. 2

Photographs of a sample sputter coated with Au/Pd for SEM imaging (a) before and (b) after etching to remove the metal. After sputter coating with Au/Pd, the sample is highly reflective. The circular feature visible at the top of the image is the lens of the camera used to record the image, visible as a reflection from the metal-coated substrate. After etching, the sample is highly transmissive, and the reflected image of the camera is no longer visible.

Generally, strong oxidizing agents are needed to etch Au, Pd, and other noble metals. Yet strong oxidizers can destroy soft materials like polymers, fragile microstructures, and adhesion layers that bind a structure to its substrate. ${\mathrm{I}}_2$ is a mild oxidizing agent that does not attack saturated polymers, and ${\mathrm{I}}_2$ by itself is not strong enough to oxidize noble metals. The species ${\mathrm{Au}}^{3+}$, ${\mathrm{Pd}}^{2+}$, and oxidized forms of related noble metals form stable complexes with chloride anions, so the base metals are easier to oxidize in the presence of ${\mathrm{Cl}}^{-}$.^32,33 The etching agent used in this work is based on ethaline, a eutectic mixture of choline chloride, and ethylene glycol, which readily dissolves ${\mathrm{I}}_2$ and provides a high concentration of ${\mathrm{Cl}}^{-}$ ($\sim 5\mathrm{M}$). In ethaline, ${\mathrm{I}}_2$ is strong enough to oxidize Au and Pd, so the etchant can dissolve the noble metals without harming the polymeric microstructures.

3.2.

Structural and Optical Characterization of Etched Photonic Crystals

Uniform-lattice photonic crystals (ULPCs) that self-collimate light³⁴ were fabricated and characterized to study how the etching process affects the optical properties of a nanophotonic device. The self-collimation effect suppresses beam divergence, so a light beam coupled into a UPLC should emerge with the same spot size as that of a beam propagating and diverging in free space over an equivalent physical distance. Simple-cubic photonic crystals can be engineered to self-collimate light.²⁵ The ULPCs discussed here are a $20\times 20\times 20$ lattice of simple-cubic unit cells having a lattice spacing of $a=1.05\mu \mathrm{m}$ and a fill factor of $\sim 45\%$, for operation at ${\lambda }_0=1.55\mu \mathrm{m}$. The UPLCs were fabricated by MPL in IP-Dip photoresist, structurally characterized by sputter coating and SEM imaging, and optical characterized by scanned-optical-fiber measurements.

The difference between metal-coated and etched UPLCs was first examined via visual inspection and optical microscopy. Freshly fabricated microstructures are optically transmissive, but after sputter coating with Au/Pd, they are opaque and reflective. After etching, the microstructures were restored to optically transmissive, indicating that metal was removed without compromising the structures.

SEM imaging was used to study how etching affects the integrity of the microstructures. SEM images as top and side views of the ULPCs before and after etching are shown in Figs. 3(a) and (b). To obtain the first set of images, samples were fabricated, sputter coated with metal, and imaged by SEM. To obtain the second set of SEM images, the same samples were subjected to the etching process, then sputter coated again for a second round of SEM imaging, to reveal changes potentially caused by etching. The SEM images in Fig. 3 show that structural integrity was maintained throughout the etching process. Individual lines comprising the lattice are rigid and connected to the sample and the form is virtually unchanged. The sample is also free of ${\mathrm{I}}_2$ crystals or other deposits, indicating that the rinsing procedure is sufficient to remove the etchant and leave samples free of contaminants. Intriguingly, even the finest defects originating from fabrication are preserved through the etching process. Red ellipses added to the high-magnification images in Figs. 3(b) and 3(d) encircle a defect formed during fabrication of the structure. The defect is clearly visible and unchanged before and after etching. These data confirm that the etching process does not compromise the form or integrity of the microstructures.

Fig. 3

SEM images of a UPLC (a) before and (b) after etching. Components A and C are top views, looking down onto the structure and its supporting substrate. Components B and D are side views, looking parallel to the substrate.

The optical behavior of the ULPCs before and after etching was explored using the scanned-optical-fiber system described above, with vertically polarized light having ${\lambda }_0=1.55\mu \mathrm{m}$. Figure 4 shows the FWHM and transmission efficiency for beams exiting etched and unetched ULPCs. For unetched UPLCs, the beam transmits at an efficiency of $75\%\pm 3\%$ with an FWHM of $8\mu \mathrm{m}\pm 0.3\mu \mathrm{m}$. The beam size is comparable to that of the input beam, and it transmits with high efficiency, indicating that the device suppresses scatter and beam divergence, consistent with self-collimation. The FWHM and transmission of beams passing through the etched UPLC match those of the unetched device within one standard deviation. These results confirm that etching does not alter the optical function of the nanophotonic device. Even though metal sputtering makes the devices reflective, causing a radical change to their optical properties, the etching process described here restores optical function.

Fig. 4

Comparison of the width and transmission efficiency of a Gaussian beam passing through the UPLC shown in Fig. 3, before and after metal etching.

3.3.

Structural and Optical Characterization of Monolithic Block

To explore further the effect of sputter coating and etching on the optical behavior of 3D microstructures, a $25\mu \mathrm{m}\times 25\mu \mathrm{m}\times 150\mu \mathrm{m}$ monolithic block was fabricated via MPL in IP-Dip photoresist and then structurally and optically characterized. The monolithic block serves as a complement to the UPLC for assessing the impact of etching. The UPLC is well suited for confirming that the etching process negligibly affects the structure’s form; however, it is not straightforward to intuit how the UPLC’s optical function would be compromised if metal were still present after etching. In contrast, the simple form of the monolithic block does not reveal subtle changes in structure potentially caused by etching, but it does provide a straightforward means for studying how light transmits through the structure before and after etching.

Figures 5(a)—5(d) show SEM images of the as fabricated (unetched) monolithic block and the same structure after etching, sputter coating, and reimaging. The structure was optically characterized at ${\lambda }_0=1.55\mu \mathrm{m}$ (a) immediately after fabrication, (b) after sputter coating with metal for SEM imaging, and (c) again immediately after etching. At all three stages, intensity maps of the transmitted beam were recorded, as shown in Figs. 5(e)–5(g). Line scans through these intensity maps were used to obtain the FWHM of the transmitted beam. The beam widths and optical transmission are presented in Fig. 6.

Fig. 5

(a)–(d) SEM images and (e), (f) scanned-fiber optical characterization of the $25\mu \mathrm{m}\times 25\mu \mathrm{m}\times 150\mu \mathrm{m}$ monolithic block at various stages of processing. Intensity profiles measured for an optical beam at ${\lambda }_0=1.55\mu \mathrm{m}$ exiting from the output face of the block (e) immediately after fabrication, (f) after metal coating, and (g) after etching. (a), (b) Perspective and top views of the block after initial optical characterization shown in (e). (c) A top view of the block after metal sputtering and the second optical characterization shown in (f). (d) A top view of the etched block imaged after the third optical characterization shown in (g).

Fig. 6

Comparison of the width and transmission efficiency of a Gaussian beam passing through the monolithic block shown in Fig. 5 immediately after fabrication, after metal coating, and after etching.

The transmission efficiency of the as-fabricated block was $53\%\pm 3\%$. The material is minimally absorbing at the measurement wavelength,³⁵ so the losses are due primarily to reflection at the higher-refractive-index entrance and exit faces and scattering from the surfaces, which have a nanoscale texture due to digital printing by MPL. When the block is sputter coated, the thin metal coating reduces the transmission significantly, down to $18\%\pm 3\%$. The profile of the weakly transmitted beam is highly misshapen relative to that observed prior to metal sputtering [compare Fig. 5(e) to Fig. 5(f)], and the FWHM drops to $4\mu \mathrm{m}$, with high uncertainty due to the beam distortion. Measurements recorded after removing sputtered metal show that the etching process restores the optical transmission $52\%\pm 3\%$. The profile of the transmitted beam after etching is also similar to that measured prior to metal deposition, and the FWHM of the corresponding beams are identical within uncertainty. These observations provide additional confirmation that gentle etching using the ethaline–iodine solution can be used to remove sputtered metal and return a microstructure to its original form and optical function.