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Abstract
Graphene interacts with electromagnetic waves strongly in a wide range from ultra-violet to far-infrared, making the graphene coating suitable for a variety of applications. In this study, a novel localized rapid heating technique utilizing micro-patterned silicon stampers with carbide-bonded graphene coating, which directly heats up by absorbing mid-infrared light radiation, is implemented in rapid precision optical molding. The graphene network, as a functional coating to obtain thermal energy and improve the anti-adhesion of the mold surface, can heat up the mold surface rapidly (up to 18.16 K/s) and evenly above glass transition temperature over a large area within several seconds. Since the graphene coating was around tens of nanometers (∼45 nm) thick, the rapid precision surface molding process can be shortened into tens of seconds. Furthermore, the thermal response and repeatability of the graphene coated silicon wafer is investigated by repeated thermal cycling. This novel rapid precision surface molding technique is successfully tested to replicate grating structures and periodic patterns from silicon molds to thermoplastic substrates with high accuracy. Compared with conventional methods, this new approach can achieve much higher replication fidelity with a shorter cycle time and lower energy consumption.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Graphene, a novel 2D material, monatomic layer of carbon atoms composing of sp2-bonded with the honeycomb lattice structure, has fascinated researchers for decades [1]. It has been widely adopted in many applications because of its unique electric and optical properties [2,3]. The effective mass of carriers and high carrier mobility make graphene a possible candidate for ultrafast opto-electronic devices, such as transistors [4] and photodetectors in the visible and near-infrared windows [5,6]. In the long wavelength range (i.e., mid- to far-infrared), graphene, like other materials consisting of carbon, presents strong resonance absorption [7]. On the other hand, the building of a strong graphene coating as a protective layer on silicon substrates provides the surface with a combination of many unique properties, such as low-adhesion, great thermal & electrical conductivity, high hardness and low surface friction [8]. The combination of the aforementioned advantages makes graphene a potential alternative coating material that can be used with micro/nano patterned silicon wafers for high-volume precision glass optics fabrication, thus resulting in a rapid localized molding process with low cost.
In conventional precision glass molding (PGM), the cycle time of such process is time-consuming, which cuts down the high throughput replication. One inherent issue is that the whole large thermal mass of precision glass molding tools, which need to be heated above the glass transition temperature Tg of glass materials before molding and cooled down after molding in a common molding process. Even with the aid of high-power heating units, the thermal cycling is still in very slow rate and spends long cycle time in order of 10 mins or even longer [9]. Furthermore, for the replication of surface functional microstructures, the conventional precision glass molding process leads to some properties’ transition of molded materials in such thermal variation, resulting in non-uniform material properties and influencing the performances of replicated optical elements [10,11]. Therefore, new techniques, which can achieve short thermal cycling with high heating/cooling rates, are critical to overcome these drawbacks.
During the last decade, many techniques have been attempted to achieve rapid thermal cycling in precision glass molding process [9,12–15]. Among the existing rapid heating methods, the coating is the most interesting and popular approach. On one hand, coating materials with different physical properties from substrate materials, such as electrical, thermal and optics, provides more possibilities on applications. On the other hand, nanoscale coating materials are generally ultra-smoothness, remaining the fidelity and reducing surface shear stresses in micro-fraction [16]. The ideal coating materials for precision glass molding process should meet the requirement on good durability for industrial production, great thermal conductivity to enhance heat transfer, and ultra-smoothness to facilitate filling & demolding. Graphene and its derivatives are the perfect potential mold coating materials.
A few attempts have been made to use carbide-bonded graphene coated silicon molds in precision glass molding. For example, He et al. [8] conducted a comparative molding test between a silicon wafer mold with carbide-bonded graphene coating and a silicon wafer mold without graphene coating. Their investigation demonstrated that graphene coating allows silicon to be used as a mold material by preventing silicon to glass adhesion at elevated temperature. Additionally, graphene coating was adopted as a conductive heating layer in polymer hot embossing. Xie et al. [9] developed a rapid hot micro-embossing technique utilizing micro-patterned silicon stampers coated with a carbide-bonded graphene network to implement rapid heating and cooling processes. The hot embossing technique was successfully implemented to imprint micro-channel and micro-lens arrays onto thermoplastic polymer substrates with high precision. More recently, Li et al. [13] further extended the rapid heating technique to chalcogenide glass molding with graphene coated fused silica wafer. The feasibility of this process was validated by both experiments and numerical simulation. Zhang et al. [14] employs high-frequency electro-magnetic field as an energy source into the ultrathin conductive graphene coating. The heating and cooling rates were greatly improved, and the entire molding process could be finished within several mins. However, some inherent issues can not be overcome in nature and the process cycle was far from wide applications in industry. Issues contributing to this problem can be summarized as follows,
(I) Contact issues. Graphene is a great 2D conductive material, which presents great conductivity and ultra-low sheet resistance [17], even better than some of the metals. Most of the previous studies are focused on electro-heating process. Contact resistance becomes one of the most critical issues on the joint of graphene coating and power supply. The concentrated resistance on the joint results in mass of energy consumption and heat concentration, leading to structure failure and coating damage. Additionally, due to its ultra-smoothness, the graphene is widely used as a high-performance anti-adhesion coating. Most of the generally employed conducting resins/tapes lost their effectiveness in adhesion on graphene coating. Lastly, as graphene coating is a great thermal conductor, the joints would likely degrade in the molding process, melt or decompose at elevated temperature, which limits its wide applications in industrial conditions.
(II) Failure at elevated temperature. Silicon is a semiconductor, which presents high resistance at ambient temperature. However, the resistance would be largely reduced when the temperature rises up [18]. At that time, the rapid surface heating system, regarded as a bulk heating system, loses its advantage on thermal efficiency at elevated temperature, or even worse than the conventional heating system with cartridge heaters. Again, the thermal applications were restricted within low-temperature range and lose the universal applicative values in industry.
(III) Lack of practicability. Due to structure difference, most of the precision glass molding apparatus need to be modified to meet the requirements of rapid molding system. For example, a high voltage power source is one of the most essential components. Since the resistance of the graphene coating is low in nature, a high-voltage power may be applied to the graphene coating to maximize the performance, which in turn exposes operators to potential safety issues. By customizing a low-voltage power source and reducing thermal performance, the potential hazard can be minimized at the expense of production efficiency and accuracy.
Facing these technical challenges, we propose a novel rapid precision molding system for polymer micro-optics with mid-infrared heater and graphene-coated silicon molds. The energy transformation in this system adopts high-efficient non-contact strategy, which intends to overcome the aforementioned issues. The micro-patterned silicon stampers were coated with a thin layer of newly developed carbide-bonded graphene using atmosphere pressure chemical vapor deposition (APCVD). Using the thin graphene coating as a thin-film thermal absorber, which transfers photon energy into phonon motion and heats up the entire mold surface rapidly, the heating and cooling phases of the molding process can be shortened to less than 10% of the conventional production cycle. A series of experiments were conducted to investigate the stability, reliability and repeatability of the graphene coating by repetitive heating/cooling cycle. Finally, a molding process of a thermoplastic polymer with relatively lower Tg than most oxide glasses was employed in the experiment. Since the similarity on amorphous structures of polymer and glass, polymers were adopted to verify the feasibility of the proposed method. Due to the high absorption of mid-infrared, the ultrathin layer of graphene can be heated above Tg within several seconds. The uniformity, surface quality and optical performance of the molded samples has been validated.
2. Modeling and experiment
2.1 Color dispersion of diffraction gratings
In optics, the diffraction grating was an important optical element with periodic micro/nano-structures. It can diffract white light into several iridescent colors dispersing into different directions, and the directions of the dispersed beams depended on the incident angle and parameters of the gratings, as shown in Fig. 1(a). The rainbow-like colors due to the diffraction of the periodic structures can be described by the grating equation, which states the relationship between the grating spacing and the angles of diffracted light as:
Fig. 1. (a) Schematic diagram of the reflective-type diffraction grating, and (b) dispersion angle for different grating periodic structures for 1st order.
If the incident white light is perpendicular to the surface, the viewing angle of a certain color of wavelength λ, is given by the diffracted angle maxima, which is given by [19]:
Following Eq. (1), an angular dispersion map of grating with spacing distances of 1.0 µm, 1.6 µm, 2.4 µm, 3.2 µm and 4.0 µm was illustrated in Fig. 1(b) at the normal view angle (θm=0). From the results, we can determine the resonant wavelength for certain gratings by calculating the intersection coordinate between the vertical line and the grating curves. For example, if we set the illumination angle at 10°, three intersection coordinates can be determined, which showed blue, yellow and red colors in our eyes.Based on the diffraction equation, a 4×4 square array of diffractive gratings with different spacing distances from 1.2 µm to 2.6 µm with an increment of 0.1 µm was designed. Each element is arranged in a square of 3.0 mm × 3.0 mm. In addition, two complex images from video game characters, a red bird and yellow bird, were also designed using different spacing to present different colors at the view angle of 10°. Figure 2 shows the schematics of red bird with micro surface gratings dispersion incident white light into different angles.
2.2 Micro-feature fabrication on silicon stampers
Silicon (Si) wafers were employed to fabricate the micro-structures as precision molds for their high mechanical strength and excellent surface quality among nonmetallic materials [20]. In this work, we implemented a simple yet well-controllable process to fabricate silicon stampers in cleanroom, as shown in Fig. 3(a). For the visible diffraction patterns, the expected nano-channel size was in the range of hundreds of nanometers to several micrometers. The channels with size larger than this range would drastically reduce the effect of diffraction performance, including diffraction efficiency and dispersion angle. Since the features were within several hundreds of nanometers, the project photolithography process was implemented to fabricate nano-/micro-structures on the silicon wafer. Unlike contact photolithography, in which the features on the mask were copied to the photoresist 1:1, the pattern size can be reduced by 5× through a projection system in photolithography apparatus (GCA 6100C Stepper (I-line)). The substrate was 500 µm thick double-side polished 4 inches diameter silicon wafer. SPR 955 photoresist was deposited for nano- and micro-pattern fabrication on silicon surfaces. After UV exposure, the silicon wafer was dry etched by reactive gas plasma on PlasmaTherm SLR-770 using CF4 at a flow rate of 20 sccm. Finally, the remaining photoresist was stripped and cleaned by acetone/IPA bath to obtain the nano-/micro-channels.
Fig. 3. Schematic diagram of the mold fabrication process, 1. Spin-coating photoresist; 2. UV lithography; 3. inductively coupled plasma reactive ion etching; 4. ultrasonic vibration cleaning; (b) apparatus for synthesis carbide-bonded graphene coating on Si stamper surface in a tube furnace equipped with liquid and gas lines.
2.3 Coating of graphene on the silicon insert
The patterned silicon wafers were coated with a thin layer of carbide-bonded graphene coating using a novel atmosphere pressure chemical vapor deposition (APCVD) process with silicon oxycarbide to accelerate the growth of the graphene network, which was developed from Lee et al. method [21]. As illustrated in Fig. 3(b), a liquid catalyzer, tetraethyl orthosilicatemethane (TEOS) (purchased from Aldrich, USA) with a purity of 99.8% in glass bubbler and a gas carbon source, methane (CH4), were used in graphene coating. The silicon wafer was placed inside a quartz tube furnace. A shield gas, argon, was applied to purge air inside the quartz tube with a flow rate of 50 sccm. Temperature increased from room temperature to 1,100 °C at 10 °C /min heating rate. After the pre-set temperature was reached, the TEOS was introduced into the tube with flow of argon as a carry gas at 15 sccm. At the same time, the methane was brought into the tube for of 15 mins. After deposition, the entire coating system was cooled down with 10 °C /min cooling rate to room temperature and the coated silicon wafer was taken out of the tube furnace.
2.4 Precision glass molding apparatus
As shown in Fig. 4(a), the layout of the rapid surface molding apparatus based on mid-infrared heating was integrated in a vacuum chamber. It is similar to the conventional precision glass molding machine except a ceramic support as an insulator and a mirror-like reflector under and above the graphene coated silicon mold, respectively. The cartridge heaters used in conventional precision glass molding process are no longer used in the graphene coating based molding process. The home-made molding apparatus consists of an infrared heating system, a high-precision driving system, a vacuum chamber and a control system. The mid-infrared light with maximum power of 1,500 W at 110 V was installed above the silicon mold, which is powered by a high current power source (Thermalcraft 35-230-Y07SK). The maximum voltage and current can be up to 230 V and 35A. The infrared light was reflected downward to the graphene coated mold by the mirror surface on the cone by a 45° aluminum reflector. The vertical movement of the upper mold was driven by a high-precision linear actuator with a ball screw drive system. With the aid of linear encoder of servo motor, the displacement during the molding process could achieve position control within 0.6 μm and up to 1000 N load. A modularized LabVIEW controlled system was employed to control and monitor system with National Instrument (NI) DAQ devices. An OMEGA multiple channel USB data acquisition module (OMEGA, OM-DAQ-USB-2401) was adopted to collect temperature profiles of the silicon stamper. Several K-type thermocouples, attached to the sample surface with a thermal conductive insulating tape, were used to provide fast measurement of temperature. The LabVIEW modular program is able to achieve precise temperature control by PID, obtain temperature feedback from sensors, monitor and store all the processing parameters.
Fig. 4. Schematic diagrams of (a) the mid-infrared heating compression molding system, and (b) typical precision glass molding process, 1. heating; 2. pressing and holding, 3. demolding.
A typical surface molding process, consisting with four stages, i.e. heating, pressing, holding and cooling, was conducted on the home-made molding apparatus [22]. Unlike conventional molding process, in the rapid heating molding process, when the heating process started, an initial compression pressure P0 was applied on the substrate to ensure efficient heat transfer with close contact. In addition, the heating time was independent of the substrate thickness because the rapid heating mainly elevated the temperature at the interface between the mold and preform. After the mold surface temperature rose above the Tg of the substrate in several seconds, a holding pressure, either the same as or higher than the initial compression pressure, was applied during holding and cooling phases to drive the viscoelastic material into the mold micro-structures, compensate shrinkage and minimize residual stresses. When the mold surface temperature was lowered to room temperature, the applied load was removed.
2.5 Thermal characteristics by modeling and experiment
Before applied to molding process, the thermal characteristics of the rapid heating system was theoretically and experimentally investigated to ensure the performance of the heating system as what we predicted. To do this, we introduced a simplified heat transfer model of a strip-shaped Si substrate coated with a graphene coating. In this model, it was assumed that no thermal resistance existed between the graphene coating and silicon substrate, and temperature of the graphene/silicon structure was considered to be constant over the thickness [23]. Under these assumptions, the task is reduced, and the temperature field T can be determined by solving the heat transfer equation below.
The first boundary condition for the thermal equation is based on the assuming that the heat transfer from the side surfaces, i.e. from the surfaces perpendicular to the plane, need to be neglected:
Another boundary condition $T = f(t)$ for $x ={\pm} L/2$ can be determined from the law of the conservation of energy in the plane [23]:A series of thermal experiments were conducted. Figure 5(a) shows the experimental (solid line) and calculation (dotted line) results of temperature responses of the graphene coating with different powers during the heating and cooling process. From the result, the model matches the obtained thermal response of the graphene coating. The heating rate increases significantly as the mid-infrared light power goes up. The average heating rates were 4.88 K/s, 6.73 K/s, 8.97 K/s, 11.36 K/s, 13.98 K/s, 17.54 K/s and 18.16 K/s for lamp power of 150 W, 225 W, 300 W, 375 W, 450 W, 525 W and 600 W, respectively. By utilizing this heating setup, the silicon wafer surface could be heated from 25 °C to 220 °C within 10∼40 s with different level of power. When the lamp power was turned down, the mold surface cools down to room temperature within tens of seconds because of the low thermal inertia of the thin layer graphene coating on the silicon wafer. Furthermore, repeated heating/cooling cycles were conducted to evaluate the stability and repeatability of the graphene coating.
Fig. 5. (a) Heating/cooling responses of graphene coating under different infrared heater power, (b) repetitive heating/cooling of graphene coating with 300 W driving power, (c) temperature profiles at three different locations on the Si surface, and (d) temperature variations with/without graphene coating.
As shown in Fig. 5(b), in about 40 mins continuous cyclic thermal testing ranging from 75 °C to 220 °C with 300 W, the temperature profile exhibited excellent repeatability, which implied good thermal stability and repeatability of the graphene coating. The same heating/cooling cycle time indicated constant absorptivity and conductivity after dozens of rapid heating-to-cooling cycles, demonstrating that the graphene bonding strength is sufficient and stable enough to be implemented in repetitive heating/cooling cycles. In addition, total of three thermocouples were attached to the mold surface with insulated tape at the center area, middle area and the edge area to investigate the uniformity of mold temperature, as shown in Fig. 5(c). Temperature variation at these three locations showed no notable difference, which indicated that the graphene coated silicon mold surface remained at the same temperature, which is the optimal conditions for precision molding process. Finally, the temperature profiles of the graphene coated surface were compared with those without graphene coating with same driving powers. The measurements showed large difference in heating/cooling rates, in general, several (2-4) times faster for the graphene coating molds, as illustrated in Fig. 5(d). It indicated that the graphene networks have much better absorption in the mid-infrared range than the silicon wafer.
2.6 Molding material
The molding material employed in our experiments was amorphous polymer poly(methyl methacrylate) (PMMA) with Tg of ∼110 °C. The optical clear PMMA sheets were from McMaster Carr (Cleveland, OH) with dimensions of 25.4 mm diameter and 3 mm thickness. The recommended molding temperature for PMMA is usually 20∼30 °C above its transition temperature Tg. For this study, the molding temperature and load were set as 135°C and 500N, respectively.
3. Results and discussion
3.1 Replication fidelity of optical elements
To evaluate the molding process, the surface profiles of the molded samples were measured by an optical profilometer (Wyko NT9100). As shown in Fig. 6, the micro patterns were successfully replicated onto the PMMA substrates in around ∼60 s cycle time using 300 W operation power. Figure 6(a) and (b) presented the three-dimensional (3D) structures of the micro-channel on the silicon mold and molded sample with a magnification of 50 ×, respectively. From the results shown in Fig. 6(b), homogeneous features of both the 3D structures and the profile were obtained, indicating the uniformity of the molded micro-structures. Since distortion of the relative positions was a key factor in the generation of form error of the micro-channel, the corresponding cross-sectional profiles were captured and presented in Fig. 6(c). The average height of the molded micro-channel was about 150 nm, and the distance between two adjacent micro-channels was about 2.4 μm. The observed values were in good agreement with the molds. More importantly, the shapes as well as the sizes of the two profiles along the cross-sectional directions agreed well with each other, showing high consistence and accuracy of the obtained micro structures. The result proved that this novel rapid localized molding process successfully transferred these optical features from mold surface to polymer substrates.
Fig. 6. (a) 3D surface profile of a microchannel silicon mold, (b) 3D surface scan of a replicated PMMA with micro-channels, and (c) comparison of line scans profiles between the silicon molds and the corresponding molded sample.
3.2 Optical characterization of replicated optics
The optical measurement setup for the grating performance was illustrated in Fig. 7(a). As shown in Fig. 7(a), a He-Ne laser beam (632.8 nm) was projected onto the molded PMMA diffractive gratings (2.4 μm and 4.0 μm). When the laser beam strikes the gratings, it was split and scattered into different directions. By using a large screen, the intensity and associated angle of diffraction beams were presented. As shown in Fig. 7(b) and 7 (c), the scattering spots diagram was symmetric, and the central spot had the largest intensity. The second order spots were located next to the central area. The intensity and position of each spot represented the performance of the grating.
Fig. 7. Illustration of (a) setup for testing molded optical gratings, and diffraction patterns on the screen with different gratings (b) 2.4 μm, and (c) 4.0 μm.
Another optical test was conducted with white incident beam. Various grating structures with difference spacing distances presented different diffractive colors and brightness, as shown in Fig. 8(a) and 8(b). The result illustrates the performance of optical gratings strongly depended on grating space and the color for each region varies with different grating profiles. With the same incident light angle, colors in textured regions are different at different viewing angles. In addition, two complex images from the video game characters were also replicated with the aforementioned molding process, as shown in Fig. 8(c) and 8(d). The images with high resolution and brightness are clearly recognized. The success on replication of surface diffractive features to the PMMA molded samples indicated the feasibility of the proposed rapid molding process with high geometrical accuracy.
Fig. 8. 16 sections with different grating spacing viewed at different angles (a) and (b), and two complex images from the video game characters (c) red bird, and (d) yellow bird.
4. Conclusions
In summary, we have presented a novel low-cost and low-energy-consumption rapid heating system for precision optical molding. The apparatus was highly energy efficient since rapid heating in graphene coating under vacuum was employed. Furthermore, the energy transformation is non-contact, which avoids complex wire-joint on graphene coating, making this heating system more flexible in industrial applications. The maximum heating rate of the system can reach 18.16 °C/s at 600 W. The cycle of the entire molding process could be reduced to several minutes (approximately 1-3 minutes). This novel rapid surface molding technique based on mid-infrared heating was successfully implemented to imprint diffractive structures on thermoplastic polymer substrates with high precision. This energy-efficient and unique design of the molding apparatus promoted numerous exciting novel techniques into large-volume industrial production and applications, e.g., surface diffractive structures and moth-eye anti-reflective structures. To implement more industrial applications for the proposed method, some technical challenges still remain to be resolved in our future work, including modeling of the system thermal dynamics, improving energy efficiency by system structure optimization, developing of an in-situ monitoring system, and improving molding stability at elevated temperature (600∼1,000°C).
Funding
Directorate for Engineering (1537212).
Acknowledgments
This work was supported in part by the II-VI Foundation block-gift program. This work was also partially supported by the National Science Foundation (NSF) (Grant No. 1537212). Opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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