We present a high-precision hybrid imprinting method to fabricate multi-layer micro-optical structures on nonplanar substrates using a custom-built vacuum imprinting system; with the application of kinematic couplings that align the flexible stamps in all six degrees of freedom, a cross-layer pattern registration precision of 400 nm has been achieved on nonplanar substrates. To demonstrate the precision and feasibility of the new process and instrument, we have designed and fabricated a multi-layer artificial compound eye (ACE) for multispectral imaging. The shapes and sizes of all 12 micro-lenses on the ACE are optimized and integrated with different color filers (red, green and blue) so that the light from different channels and of different spectral contents will focus to the same plane, where the photodetector is located. Next, the multi-layer ACE is installed in a portable optical system for simultaneous multispectral imaging, i.e., to perform pattern detection by looking at specific frequency windows. Imaging experiments are devised and performed on (1) color blindness test cards, (2) space image, and (3) breast and gastric tumor samples. The results confirm the system’s capabilities of frequency separation, extraction of hidden information, and tumor identification.
© 2017 Optical Society of America
Fabrication of multi-layer micro-/nano-structures on nonplanar surfaces has been a challenging, yet largely unsolved problem. Such structures can lead to many new devices of important applications. For example, the fabrication and integration of micro-lenses and photoreceptors on a spherical substrate may result in a nonplanar charge-coupled device (CCD), leading to new optical systems, e.g., spectrometer, with greater efficiency or more compact envelope. Another example would be the integration of color filter arrays with an artificial compound eye (ACE) which forms a new optical component for developing a compact multispectral imaging system. To realize multi-layer fabrication on nonplanar substrates, two issues need to be addressed: (1) fabrication of structures of selected materials with precision and uniformity, and (2) precise registration between each layer. Early attempts to fabricate multi-layer structures on nonplanar surfaces include, for example, the application of laser-induced self-focusing processes for forming micro-lenses and waveguides on a convex substrate ; and the adoption of a multi-step manual alignment process to fabricate multi-layer structures [2–4]. However, these methods are either greatly limited by the material choices, or provide low registration accuracy, i.e., 5 – 10 microns. Other related methods are limited to fabricating single-layer structures on nonplanar substrates [5–9]. A new method that provides multi-layer fabrication capability with submicron registration precision and wide material choices has yet to be developed.
In this paper, we present a hybrid imprinting process and the related instrumentation development that enables the precision fabrication of multi-layer micro-optical structures on nonplanar substrates. Based on a custom-built vacuum imprinting system with integrated kinematic couplings, we demonstrate a cross-layer pattern registration precision of 400 nm, where the pattern transfer process is performed via a flexible elastomeric stamp. This new method presents a low-cost, high-throughput solution for fabricating complex structures on curved substrates that cannot be done in the past. Section 2 presents the detailed design and error analysis of the ACE; Section 3 presents the new hybrid imprinting process as well as the precision vacuum imprinting system; Section 4 presents a compact multispectral imaging system enabled by the unique ACE; imaging experiments on color blindness test card, space image, and various tumor tissues have been performed to characterize and demonstrate the frequency separation and material identification capability of the system.
2. Design of multi-channel ACEs
Figure 1 presents the schematic of a multi-channel ACE that consists of three structural layers: (1) micro-lens arrays on a convex dome for image formation; (2) a thin gold layer that fills the interstitial spaces among the micro-lenses to reject diffused light; and (3) multi-channel color filter arrays, installed at the bottom of the dome, aligning with the micro-lenses, for frequency separation. Specifically, a plano-convex lens of 20 mm diameter, a focal length of 50 mm and a refractive index of 1.47 is used as the base substrate. During experiments, a photodetector array is placed beneath the ACE for image acquisition. Since the photodetector array is flat, each micro-lens on the ACE must have different focal lengths to simultaneously focus onto a plane where the photodetector is located. To achieve this goal, ray-tracing analysis is performed to determine the required curvature and diameter for each micro-lens. Figure 2 presents the top view as well as the ray-tracing analysis results of the ACE with associated micro-lens groups. Based on the results, a 3 × 4 micro-lens array is designed and divided into four groups, where the diameter of each micro-lens group is 900, 896, 891, 887 microns respectively; the sagittal height from the lens vertex are 24 microns for all micro-lenses; the center distance between neighboring micro-lenses is 2.54 mm. The refractive index n of the lens (i.e., SU8 2010) is ~1.65. The design ensures all micro-lenses have proper dimensions to be fabricated via the vacuum-imprinting processes [10, 11] and the geometric aberration induced root mean square (RMS) radius is less than the full width at half maximum (FWHM) of the micro-lenses’ point spread function.
Next, we investigate the required fabrication/registration precision for the micro-lenses. Figure 3 presents the results of ray-tracing analyses of Group 1 micro-lenses of increasing misalignments between the lens and the light-blocking gold layer; misalignment errors of 0, 5, 10, 20, 30, 40, and 50 microns are analyzed via Zemax. Figure 3(a) and 3(b) illustrate fabrication misalignments and the associated negative effects of wavefront errors and decrease in intensity, i.e., decreasing Strehl ratio. Figure 3(c) and 3(d) present the point spread function (PSF) and the FWHM under increasing fabrication errors. From the results, we learn the ideal resolution of micro-lenses is 4.5 μm with a Strehl ratio of 0.987. In the next section, we will describe how these results can be achieved via our new hybrid imprinting process.
3. Hybrid imprinting process
3.1 Fabrication process of ACE
Figure 4 presents the hybrid imprinting process for fabricating the multi-layer ACE. In step 1 - 2, microcontact printing (MCP)  is used to fabricate the light blocking layer via a custom-designed vacuum nanoimprinting system [10, 11]. In this process, a metal ring that holds the polydimethylsiloxane (PDMS) stamp carrying distortion corrected micro-lens patterns is installed to separate the printing chamber into two regions; the pressure difference between the top and bottom chambers drives the PDMS stamp (inked with thiols) to make conformal contact with the gold coated dome. After etching and cleansing, the light blocking layer is completed. (The preparation of micro-lens array master is reported in .)
In step 3 - 4, the dome is first spin-coated with SU8-2010 and placed in the printing chamber again. Following the same procedure, the pressure difference drives the PDMS stamp to transfer the micro-lens patterns to the dome. After UV light exposure and demolding, the micro-lens array is completed. As the same PDMS stamp is used to pattern both the gold layer and the micro-lens array, good alignment, i.e., submicron level alignment, between the micro-lenses and the light blocking layer can be achieved when the substrate and the stamp are precisely positioned to the original place and driven the same way for printing. Section 3.2 presents how this can be achieved via a modified precision vacuum imprinting system.
Figure 5 presents the fabrication of multi-channel color filters. The color filter array is fabricated via photolithography on a thin glass substrate sequentially. The completed filter array is bonded to the bottom of the ACE, aligned with the micro-lenses.
3.2 Precision vacuum imprinting system based on kinematic couplings
To achieve the required registration precision between the micro-lens array and the light blocking layer, kinematic couplings have been designed and installed on the PDMS stamp holder, as shown in Fig. 6, where three V-grooves, each spaced 120° apart, are paired with three truncated balls threaded to the stamp holder to achieve exact constraint. The required preload is automatically generated by the atmospheric pressure when the vacuum chambers pump down, resulting in an estimated preload force of ~870 N in normal printing conditions. To avoid yielding, Hertzian contact stresses are calculated to determine the ball radius; a 5 micron titanium nitride (TiN) coating is applied to both the balls and the V-grooves to minimize wear. The detailed design parameters of the kinematic couplings are summarized in Table 1. As in the hybrid imprinting process, the substrate also needs to be removed for etching and spin-coating, it is critical it can be placed to the original position in the printing chamber; accordingly, the substrate mount is integrated with a magnetically preloaded kinematic base (Newport M-BK-3A).
To verify the alignment precision, repeatability experiments have been devised and performed. In the experiments, the substrate and stamp are lifted and repositioned for 30 times; the relative displacements between the center of the stamp and the substrate are recorded. The results are presented in Fig. 7, showing the repeatability between the substrate and the stamp holder is ~400 nm.
4. Multispectral imaging system
Multispectral imaging is a technique to collect different image data at different spectrum, which enables extraction of additional wavelength-sensitive information. Multispectral imaging finds important applications in space imaging , examination of arts, e.g., paintings , and biomedical imaging [15, 16]. In conventional multi-spectral imaging systems, different wavelengths are typically separated by filter wheels or a liquid crystal tunable filter [17, 18]. However, such design only captures one spectral image at a time, making the entire operation time-consuming and the system unnecessarily bulky. This issue can be addressed by our compact system. Figure 8 presents the optical configuration of the multispectral imaging system, where the object plane is located between a broadband light source (400 - 1000 nm) and the ACE. The photodetector is a high-resolution CMOS sensor that captures images formed by the micro-lens array. (Note that the system can also be set up in a reflective mode to image opaque objects.)
Figure 9(a) and 9(b) presents the measured transmission spectrum of the four-channel filter array. As the color filter array does not block infrared (IR) light, an IR cut-off filter is installed behind the filter array, as shown in Fig. 8. Accordingly, frequency separation is achieved via the four different color bands, including 400-500 nm (blue), 500-600 nm (green), 600-800 nm (red), and 800-1200 nm (NIR).
5. Imaging Experiments
In this section, we present three multispectral imaging experiments to demonstrate the frequency separation capability and practicality of the system, including imaging results on a (1) color blindness test card, (2) space image, and (3) tumors.
Figure 10 presents the imaging results on a color blindness test card (on a color reversal film), Fig. 10(a). Four groups of images in Fig. 10(b) are obtained simultaneously. From the results, we find the number “86” can be observed from the red band; and the number “9” can be observed from the green and blue bands, indicating good frequency separation.
In the second experiment, we demonstrate the applications of multispectral imaging in space imaging. As every object has its unique frequency spectrum, object identification can be achieved by combining and processing information from different spectral bands. Figure 11 presents the imaging results on a satellite image of earth, Fig. 11(a), containing information of vegetation, water and soil. Figure 11(b) shows the imaging results, where the ocean appears to be black, dark green, and bright blue in the R/G/B channels respectively. Similarly, the land appears to be bright red, bright green and bright blue in the R/G/B channels respectively. These results indicate our compact ACE-based multispectral imaging system may find applications in remote sensing.
In the third experiment, we demonstrate the applications of multispectral imaging in tumor screening, showing healthy and cancerous tissues can be distinguished by studying their spectral information. Figures 12(a) and 12(b) present the imaging results on two common cancerous tumors (prepared on glass slides stained with haematoxylin and eosin), including (a) breast carcinoma; and (b) gastric carcinoma. The yellow lines in the optical images circle the tumor site; the black dashed lines indicate the field of view of the multispectral imaging system. From the imaging result, it can be observed that different tumors appear to have stronger contrasts in specific color bands. For example, breast carcinoma is sensitive to the red band; and gastric carcinoma is sensitive to both the red and green bands. These results confirm the practicality of multispectral imaging in tumor screening.
We have developed a hybrid imprinting process to fabricate a multi-layer ACE with non-homogeneous micro-lenses using a custom-built vacuum nanoimprinting system, where kinematic couplings have been designed and installed to achieve ~400 nm registration precision on a nonplanar substrate. Based on the multi-layer ACE, a compact multispectral imaging system is developed to perform imaging experiments on a (1) color blindness test card, (2) space image, and (3) breast and gastric tumor samples. The results confirm the frequency separation capability and practicality of the system. Better results may be achieved by increasing the number of color bands and reducing the bandwidth. The precision hybrid imprinting process may be used to fabricate other multi-layer devices on nonplanar substrates, e.g., curved screens or photodetectors.
HKSAR ITC, Innovation and Technology Fund, ITS/129/14; HKSAR RGC, General Research Fund (CUHK 14201214). Research projects from the Guangzhou Key Lab (No.201509010015); Guangzhou Pearl River S&T Nova Program (No.201610010009).
We would like to thank Prof. Anthony WH Chan in the Department of Anatomical and Cellular Pathology at The Chinese University of Hong Kong for providing the tumor samples in the imaging experiments.
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