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Patterning micro- and nano-scale optical elements on nonplanar substrates has been technically challenging and prohibitively expensive via conventional processes. A low-cost, high-precision fabrication process is thus highly desired and can have significant impact on manufacturing that leads to wider applications. In this paper, we present a new hot embossing process that enables high-resolution patterning of micro- and nano-structures on non-planar substrates. In this process, a flexible elastomer stamp, i.e., PDMS, was used as a mold to perform hot-embossing on substrates of arbitrary curvatures. The new process was optimized through the development of an automated vacuum thermal imprinting system that allows non-clean room operation as well as precise control of all process parameters, e.g., pressure, temperature and time. Surface profiles and optical properties of the fabricated components, including micro-lens array and optical gratings, were characterized quantitatively, e.g., RMS ~λ/30 for a micro-lens, and proved to be comparable with high cost conventional precision processes such as laser lithographic fabrication.
© 2015 Optical Society of America
1. Introduction
Hot embossing process, first proposed by Chou in 1995, was introduced to pattern high resolution structures on thermoplastic materials [1]. A hard mold, e.g., silicon and Ni, containing nanoscale features was utilized in this process. During the imprint process, the mold and a polymer substrate (e.g., PMMA, PS, PC, and PE) were both heated to above the glass transition temperature (Tg) of the polymer and brought into close contact to generate nanoscale features of 25 nm on the substrate [1]. However, using a hard mold might have the following drawbacks: (1) submicron structures in the mold can be easily polluted and become difficult to clean due to polymer adhesion; (2) pressure distribution during imprinting process is non-uniform, which limits the accuracy and area of fabrication and may even damage the mold; and (3) the hard mold cannot be used to pattern non-planar substrates. To date, patterning micro- and nano-structures on nonplanar substrates has been technically challenging and prohibitively expensive via existing manufacturing processes, e.g. direct laser writing, where a substrate is fixed to a multi-axis precision positioner [2]. Accordingly, new low-cost, parallel precision fabrication technologies can generate significant impact and enable a wide range of applications.
In this paper, we present a new hot embossing process through the adoption of an elastomer mold, where molded structures are generated by the mechanical deformation of the soft mold. In addition, an automated vacuum thermal imprinting system was developed to scale up the new process with precision. Compared with the conventional hot embossing process, the use of a soft elastomer mold has the following advantages: (1) elastomeric molds are cost effective and can be easily replicated from a master; (2) elastomer molds, e.g., polydimethylsiloxane (PDMS) molds, being chemically inert and anti-adhesive to polymers, can be smoothly demolded without cracking as often seen in the hard mold; (3) the flexible mold enables conformal contact and uniform pressure distribution during molding, and can perform hot-embossing on substrates of arbitrary curvatures.
To demonstrate the unique capability and advantages of the soft mold-based hot embossing process, two types of challenging optical components are selected to be fabricated: (1) artificial compound eyes, i.e. micro-lens arrays on convex substrates, and (2) concave gratings, i.e. optical gratings patterned on concave substrates. The artificial compound eyes are first inspired by insect eyes, where thousands of micro-lenses with compact arrangement are uniformly distributed on a convex surface, with each lens pointing to slightly different directions. This device offers unique optical characteristics including ultra-wide field-of-view (FOV) and high sensitivity [3, 4]. So far, the existing fabrication methods, e.g., UV molding, thermal extrusion, and nanoimprint lithography, still cannot produce uniform and high-quality micro-lens arrays over a large area on substrates of arbitrary curvatures [5–7]. A concave grating integrates two optical functions, i.e. a dispersive element and a focusing element, into a single and compact one. As a result, it is extensively used in imaging and spectroscopic applications [8]. Conventionally, there are three ways to fabricate grating structures on a concave substrate, including mechanical ruling [9], interference lithography [10] and polymer replication [11], which are limited by the high fabrication cost and low throughput. Here we show that our soft mold-based hot embossing process is capable of producing micro-lens array and optical gratings with comparable quality as those produced by the high cost conventional precision processes. The design principle of the vacuum thermal imprinting system and the optimization of the fabrication/imprinting processes are presented in the following sections.
2. Experiment
In this section, we present the experimental procedures of the hot embossing process including the methodology to prepare the soft mold and to engage the vacuum imprinting system to enhance the precision of the fabricated components. PDMS is selected as the material for the soft mold for its superior flexibility, thermal stability (up to 200°C) and low surface energy. It is worth mentioning that the conventional hot embossing process utilizes a hard mold, e.g., silicon, to transfer pattern on flat substrates [1].
2.1 Fabrication of soft membrane with micro- and nano-structures
To replicate micro-/nano-structures using PDMS, a master must be first prepared. Typically, masters with micro- or nano-scale features are produced by standard or e-beam lithography processes respectively. As shown in Fig. 1(a), the fabrication of the micro-lens array master consists of a lithographic patterning process, followed by a thermal reflow process. First, a silicon wafer with 20 μm-thick photoresist (AZ4620) is pre-baked at 90°C for 10 minutes. Next, the wafer is exposed at the UV aligner (Karl Süss MA-4) with a photomask of hexagonal arrays for 12 seconds with an irradiance of 20 mW/cm2. For the thermal reflow step, the wafer is heated to 160°C for 80 seconds; at this temperature, the surface tension causes the melted photoresist to form lenses with desired geometries [12].
Fig. 1 (a) Fabrication process of micro-lens array master as well as the PDMS soft mold of negative micro-lens patterns; (b) Cross-section view of a double-layer PDMS mold and an example double layer mold that replicates a grating pattern from a quartz ruled blazed grating master.
To fabricate the soft mold, a liquid prepolymer mixture of PDMS (containing a 10:1 mixture of Sylgard 184A and 184B) is spin-cast on a rigid master whose surface has been patterned in an appropriate relief structure, i.e. a micro-lens array or a diffraction grating. For the micro-lens array mold, the PDMS prepolymer is spin-cast at 1,000 rpm and then cure at 70°C for 10 minutes. This process is repeated 4 times. Subsequently, the PDMS is post-baked in an oven at 80°C for 3 hours to allow the formation of a flexible membrane. Finally, a cross-linked PDMS replica of 280 μm thickness is peeled off from the master to serve as the soft mold. As shown in Fig. 1(a), the membrane is patterned with micro-lens array with bright focal spots in each micro-lens. Note that a metal ring is installed inside the PDMS mold, shown in Fig. 1(b), to ensure the mold remains flat and stress-free after peeling.
To replicate submicron structures with PDMS, a complex stamp of two layers are required. The stamp schematic is shown in Fig. 1(b). The h-PDMS layer for replicating grating pattern at nanoscale is fabricated with 3.4 g of VDT-731 (acting as Sylgard 184A), 18 μL of SIP 6831.1 (adhesion fortifier), and 1 g of HMS-301 (acting as Sylgard 184B) [13]. The mixture is spin-cast on a quartz grating mold at 1,500 rpm to form a thin film with a thickness below 500 nm. This coated master is then baked at 60°C for 20 minutes. Afterwards, the mixture of Sylgard 184A and 184B is cast to form the s-PDMS layer, which provides strength and flexibility to the sold mold. Figure 1(b) shows an image of the PDMS membrane containing a negative replica of the blazed grating with a blaze angle of 10.3°.
2.2 Vacuum imprinter and soft hot embossing process
Figure 2 presents the schematics of the imprinting system, where the PDMS mold is installed in the middle of the chamber, separating the room into two independent chambers (A and B). The chambers are made of Bakelite with a high-temperature tolerance up to 260 °C. The substrate is placed in the bottom chamber. Pressure in chamber A and B are controlled and monitored independently with 4 air valves and 2 pressure sensors respectively. An infrared lamp is installed in chamber A to provide heat for hot embossing process. In chamber B, a load cell is integrated with the substrate holder to monitor the printing force in real time. A thermocouple is installed in chamber B to measure and control the temperature. A heat isolator is mounted between the sample holder and the load cell to protect the load cell from thermal damage. A heat sink is installed underneath the sample holder to promote heat diffusion during the cooling process.
Fig. 2 Schematic and photograph of the vacuum-assisted soft hot embossing apparatus. The vacuum imprinter consists of two chambers, a soft membrane, an IR lamp and a pneumatic system.
We divide the thermal imprinting procedure into 4 steps as illustrated in Fig. 3(a). Step 1: Evacuate both chambers to a low-pressure state, e.g., 2psi, to minimize the trapped air in small pockets and to remove small particles from chamber B. Meanwhile, heat the substrate to the molding temperature with the IR lamp, i.e. 180 °C, to soften the surface of the polymer substrate. Step 2: Initiate the printing process by introducing a pressure difference between both chambers; pressurize chamber A to deform the PDMS stamp until it is in contact with the curved substrate, as shown in Fig. 3(a). The pressure and the capillary filling effect cause the polymer to automatically fill the cavities [14]. Step 3: Cool the substrate to the demolding temperature, i.e. 80 °C, with controlled imprinting force. Step 4: Demold the PDMS by pressurizing chamber B. Note that throughout the printing process, all parameters are controlled in a precise manner, including embossing temperature, imprint pressure and the hold time. Accordingly, precision imprinting is realized by minimizing the defects on the stamp when transferring micro-/nano-structures to curved substrates. In addition, since the vacuum processes remove most dust particles contained in air, all processes are performed in a non-clean room environment. Figure 3(b) plots the temperature, pressure and imprint force versus time in various stages of the hot embossing process.
Fig. 3 (a) Schematic procedures for the soft mold-based hot embossing process: Deforming the soft stamp and patterning micro- or nano-structures on curved substrates (convex or concave surfaces). (b) Temperature control in the hot embossing process and the pressure readings from chamber A and B; imprint force data collected by load cell versus time; middle images show a deformed membrane during the imprint process.
It is worth mentioning that during the vacuuming and demolding process, the soft mold must not make double contact with the substrate. This can be avoided by maintaining the pressure in chamber A slightly lower than chamber B.
3. Results and discussion
3.1 Characterization of the molded micro-lens array
Figure 4(a) shows an optical image of the hot-embossed micro-lens array on a convex PMMA substrate that forms an artificial compound eye with no visible defects. A surface profilometer (Tencor Instruments, Alpha-Step 500) was used to measure the profile of the compound eye. The results are shown in Fig. 4(b), where a smooth cross-section profile of the compound eye (left) was observed. Next, a randomly selected micro-lens near the central region was characterized with a diameter (D) of 187 µm and a sag height (h) of 13.29 µm at the micro-lens vertex. Comparing with the lens-profile on the master (D = 185 µm, h = 15 µm), the diameter of the hot-embossed micro-lens is increased while the sag height is decreased due to expansion of the soft mold.
Fig. 4 (a) Optical images of the micro-lens array imprinted on a convex PMMA substrate; (b) cross-sectional profile of the micro-lens array and a selected individual micro-lens; (c) 3-D surface profile of the selected micro-lens obtained from a Michelson interferometer.
A Michelson interferometer (μPhase 2 from FISBA OPTIK) was used to quantify the 3-D surface morphology of the hot-embossed micro-lens array. Figure 4(c) presents the 3-D profile of an individual micro-lens from the micro-lens array with a surface roughness of 21.37 nm RMS, or approximately λ/30. (The average surface roughness of 10 randomly selected micro-lens is 21.61 nm RMS; the roughness of the master was measured to be 14.45 nm RMS). Overall, the hot-embossed micro-lens array demonstrates exceptional optical quality which may lead to important applications in fields of digital display systems, ultrathin cameras or optical telecommunication [4].
The calculated radius of curvature (R), focal length (f), numerical aperture (NA) and acceptance angle (Δϕ) of the micro-lens are 335.55 µm, 684.79 µm, 0.14 and 31.29° respectively, where the refractive index of PMMA is 1.49. These results were obtained using the equations below based on geometric optics [15]:
3.2 Optimization of process conditions
To attain optimized embossing process, it is essential that uncontrolled flow behavior of the polymer (PMMA) is avoided [12]. As such, we devised a set of experiments to study the influence of various process parameters on the forming of micro-/nano-structures: the embossing temperature ranging from 120 °C to 260 °C, the imprint force from 80N to 220N, and the hold time from 30 seconds to 240 seconds.
From the experiments, we have learned that to ensure faithful replication of small features (<5 micron), especially high aspect ratio structures, the optimal embossing temperature is 180 °C, approximately 75 °C above the Tg of PMMA. As evidenced in the experiment, the proper range of imprint force falls between 120N and 180N. To overcome the viscosity between the mold and PMMA, sufficient embossing pressure needs to be applied to avoid incomplete mold-filling. On the other hand, an excessive pressure (> 180 N) can distort or even collapse the mold, resulting in inaccurate pattern dimension. The optimal range of hold time is between 120 and 180 seconds to ensure a complete formation of the polymer in the soft mold. Note that prolonged hold time may increase the internal stress which usually causes cracks in PMMA substrates. The findings in the embossing parameter experiments are summarized in Fig. 5.
3.3 Fabrication and characterization of the concave grating
In the second experiment, we hot-embossed two different types of gratings, i.e. a blazed grating and a holographic grating, on precision plano-concave PMMA substrates (Diameter: 25.4mm; focal length: −50mm; center/edge thickness: 3.5mm/6.8mm). Figure 6(a) shows the optical image and the AFM characterization results of the blazed grating master as well as the molded concave grating respectively. In each characterization, the AFM scanned an area of 10 × 10 µm2, revealing that the pitch and the blazed angle of the molded concave grating were 1900 nm and 8.7° respectively. Comparing with the master, (pitch: 1660 nm; blazed angle: 10.2°), the pitch was increased by 240 nm with a decreased blazed angle due to the expansion of the soft stamp. Nonetheless, such discrepancy is acceptable as long as the expansion is predictable and repeatable (STD = 10.1 nm out of 10 imprinted samples). In general, the experimental results suggested that grating patterns had been successfully transferred to the concave substrates. Figure 6(b) shows the diffraction test result of the molded grating. Before the test, the molded concave grating was first coated with a layer of aluminum (500 nm) by sputtering, and then fixed on a precision holder. A Nd:YAG laser (532 nm) was used to perform the diffraction test. From the results, it can be observed that the −1st order has the highest intensity among the five recorded diffraction peaks, which well preserves the characteristics of a blazed grating.
Fig. 6 (a) Optical image and AFM characterization results of the blazed grating master (left) and the hot-embossed concave grating (right); (b) diffraction test of the hot-embossed blazed grating coated with aluminum
Next, we fabricated an 830 nm-pitch holographic grating on the plano-concave PMMA substrate. To minimize mold expansion effect, the substrate was positioned in close proximity to the PDMS mold (<500 µm) before the imprinting process started. The AFM characterization results of the master and the molded grating are presented in Fig. 7, showing a slightly decreased amplitude (~40 nm) with roughened surface and identical groove frequency. The roughness on the polymer surface is mainly caused by contaminants in air. (The printing was performed in a non-clean room environment). These results have shown the soft mold-based hot embossing process is capable of replicating nanoscale structures on non-planar substrates. The mold expansion effect can be minimized through (1) decreasing the distance between the mold and the substrate, i.e. decreasing required mold expansion, or (2) developing distortion-corrected master pattern.
Fig. 7 Optical image and AFM characterization results of the blazed grating master (left) and the hot-embossed concave holographic grating (right).
4. Conclusion
We have developed a high precision, low-cost hot embossing process based on a soft PDMS mold that enables batch fabrication of high aspect-ratio precision micro- and nano-structures on nonplanar substrates. A vacuum thermal imprinting machine was constructed to control the operating parameters in a precise manner. Experiments were devised to obtain the optimal embossing parameters (temperature: 180 °C, force: 140 N, hold time: 120 seconds). For demonstration, we successfully fabricated and characterized (1) the artificial compound eye by patterning micro-lens arrays on a convex PMMA substrate, and (2) the concave gratings by embossing a blazed grating on 1” PMMA plano-concave substrates.
Acknowledgments
This work was supported by the HKSAR Innovation and Technology Commission under the Innovation and Technology Fund ITS/129/14: Development of a Vacuum Nanoimprinting System for Low-cost Parallel Nanomanufacturing as well as National Natural Science Foundation of China (61006076). The interferometer used in this paper was kindly provided by the CUHK-BIT Joint Research Center for Optomechatronic Design and Engineering.
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