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Abstract
In this paper, enhanced photolithography with Al film insertion for large-scale patterning of chemical-vapor-deposited graphene is proposed and demonstrated. An Al film was evaporated onto the surface of graphene, and then traditional photolithography and wet etching were used to make an Al mask. After Ar plasma treatment and removal of the Al mask, patterned graphene was obtained on the target substrate. Graphene patterns fabricated by conventional photolithography were utilized as reference samples. Compared to conventional patterned graphene, the graphene patterns exhibited obviously less residual photoresist (PR), high resolution, and can be applied to various shapes and sizes. It is worth mentioning that the pitch can reach the limit of photomasking utilized in this study. In addition, the graphene also shows a relatively lower work function (~4.2 eV), better photoelectric properties (sheet resistance ~531 Ω/sq, T550 = 94.2%), and outstanding surface hydrophilicity (water contact angle 49°). It is anticipated that graphene patterns realized by the proposed approach could make the application of graphene more promising in emerging and future optoelectronic devices.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Graphene has attracted intense interest in the fields of optics, electronics, biology and energy-related devices because of its good optical, electrical, and mechanical properties [1–5]. Methods of synthesizing graphene mainly include micromechanical stripping [6,7], chemical reduction of graphene oxides [8–10], and chemical vapor deposition (CVD) [11–13]. CVD-grown graphene is considered as the most promising way for application as transparent conductive electrodes (TCEs) in optoelectronic devices due to scalability and quality requirements [14,15].
For the practical application of CVD-grown graphene in electronics, the challenges in graphene patterning, such as good quality, consistency, and scalability, must be addressed for device integration. Previous attempts to pattern graphene include transfer printing [16,17], laser scribing [18], direct patterning using ion beams [19], direct growth of patterned graphene [20–23], nanoimprint lithography [24], focused electron beam of a transmission electron microscope (TEM) [25], photolithography [26]. However, current approaches still suffer from several disadvantages. The transfer printing method is restrained by the materials, including the graphene source, target substrate, and transfer layer. In addition, the complicated processes and lack of uniformity also restrict its wider application [16,17]. The laser scribing approach faces two problems. One is that it is difficult to define the dimensions accurately, and the other is that the graphene in the laser-exposed regions wraps easily on the substrate to form bundles due to the heat generated by the laser [27]. Direct patterning using ion-beam methods is limited by pattern precision [19]. For direct growth of patterned graphene, a pre-patterned metal catalyst is used for the graphene growth, but the patterned graphene suffers from serious damage during the transfer processes due to the micro-level size of the graphene patterns. The focused electron beam of TEM suffers from complicated operation and high cost even though it can fabricate micro and nanoscale devices such as MEMS, NEMS [25]. Although nanoimprint lithography has the advantages of low cost and low line size to 5 nm, its intricate fabrication and non-uniformatiy for the transfer layer are the limitations of this method. Photolithography, which is widely used in the semiconductor industry, has been attempted in graphene patterning in recent years. Relatively speaking, it has the advantages of high resolution and ease of mass production, and it is compatible with common optoelectronic device processes. However, photolithography suffers from deposits of residual photoresist on the graphene surface, leading to a low-quality patterned graphene, with such deficiencies as loss of transmittance, reduced surface cleanliness and evenness level, reduced conductivity, etc [28]. Furthermore, graphene has advantages in the applications of flexible and wearable electronics, while less compatibility between the intended plastic substrates and the organic solvents used in photolithography can be expected [29].
In this study, we demonstrate a revised photolithography method to eliminate the residual photoresist of patterned graphene by utilizing an Al interlayer as a mask and sacrificial layer. The patterned graphene fabricated by the proposed method displays enhanced performance, and the possible reasons for this enhancement are explored.
2. Experiments and characterization
2.1 Fabrication of graphene patterns
As discussed above, photolithography is the method most likely to be applied in mass production of multiscale patterned graphene if the problem of residual photoresist can be overcome. Considering the physical and chemical differences of atom-thick graphene on various surfaces [30], insertion of a sacrificial layer that introduces a different type or magnitude of interaction force might be a good solution. In this work, we tried to introduce an Al film as a sacrificial layer. Al was chosen as a sacrificial layer, which also acts as a mask during the patterning processes, owing to its low melting point, good etching precision, low cost, and good adhesion with graphene, as opposed to other metals, such as silver and copper.
Details of the synthesis and transfer processes of graphene by CVD can be found in our previous paper [31]. Figure 1 is a schematic of the process to fabricate CVD-grown graphene patterns on a SiO2/Si substrate. Al film was evaporated onto the graphene as a sacrificial layer, which also plays the role of a mask in the Ar plasma treatment process. After ultraviolet (UV) treatment for 30 minutes to clean organic residual on the surface of graphene, the graphene samples were placed into the chamber (SD400M Multi Source Organic Molecular Vapor Deposition System) and heat to above 660°C to fuse Al at a pressure of about 10−3 Pa. Then, Al film was evaporated onto the surface of graphene to an optimized thickness of 200 nm at a rate of 5 Å/s. Finally, the chamber was cooled down to room temperature in vacuum and samples were taken to pattern. Positive photoresist (Merk China, AZ SFP-1400K (10 cp) (19L NOWPAK)) was spin-coated onto the graphene at 1000 rpm for 1 minute and annealed at 125 °C for 90 s on the hotplate. Next, traditional photolithography (OAI, J500 MASK ALIGENR) technology (exposure time for 90 s and development time for 20 s) was performed to achieve photoresist patterns on the surface of Al film. Then, the samples were soaked into Al etching solution (H3PO4 + HNO3 + CH3COOH) for 4 minutes to etch Al film which was not coated with photoresist and rinsed with DI water to remove residual Al etching solution. The samples were soaked into acetone for 5 minutes to remove the photoresist and annealed at 120 °C for 10 minutes to dry graphene film. A precise Al mask on the surface of graphene was obtained after removing the photoresist. Last, argon (Ar) plasma etching (18 W, 6 min) which utilize inert Ar ion to bombard the surface of undesired graphene to achieved graphene patterning, because the Ar plasma etching is different from active oxygen and hydrogen plasma etching which can bring about defects and chemical doping such as substitutional doping, modifying the bulk structure and could be not reversible. Al mask was removed by Al etching solution for 4 minutes, and the samples were rinsed by DI water and annealed at 120°C for 10 minutes. During the Ar plasma treatment, the graphene film was protected from plasma etching by the Al mask, while the graphene film on exposed areas was etched and finally removed from the SiO2/Si substrate surface. With the aid of the Al mask and Ar plasma treatment, precise dry etching realized graphene patterns with high resolution at desired positions.
2.2 Characterization
The sizes of the graphene patterns were recorded using a microscopy (Nikon ECLIPSE L200N) and the surface morphologies were tested by atomic force microscope (AFM, SPA-400 SPM). The photoresist residual was measured with Fourier Transform Infrared Spectroscopy (FTIR, VERTEX70). The surface morphologies of the graphene patterns were recorded using a scanning electron microscope (SEM, HITACHI 4800) and the electrical properties of the graphene patterns were obtained using a Hall Measurement System (ACCENT HL5550LN2). The selected area electron diffraction (SAED) to graphene patterns was characterized using a transmission electron microscope (TEM, FEI Talos). The X-ray photoelectron spectroscopy (XPS, ThermoFischer, ESCALAB 250Xi) was used to quantify the Al residual of graphene patterns. The optical properties were measured with UV-visible spectra (U-3900H), and the work function was tested using a Surface Work Function Equipment (SunMonde). The Raman spectra were evaluated with a Raman microscopy system (LabRAM HR XploRA) at a laser wavelength of 532 nm. The contact angle was measured by a Contact Angle Instrument (Vino, SL200KS).
3. Results and discussions
To confirm the crystallinity and the layers of graphene fabricated by CVD process, SAED was conducted on the graphene transferred to TEM grid and the results are shown in Fig. 2. The clear diffraction pattern (Fig. 2(a)) identifies the good crystallinity of graphene. The line profile of the diffraction patterns (Fig. 2(b)) demonstrates the monolayer nature of the graphene domain. To further characterize the quality of graphene, Raman spectra (eight points at same line with step of 1 µm) and AFM of graphene transferred to SiO2/Si were carried out and the results are shown in Fig. 2(c). The low D peak and root-mean-square (RMS = 1.98 nm, inset graph of Fig. 2(c)) also indicate that the graphene possesses good quality. However, it can also be seen that the graphene quality is not totally same at different areas.
Fig. 2 (a) SAED pattern of graphene, (b) profile plots of the diffraction peak intensities along the arrows in Fig. 2(a), (c) Raman spectra and the inset graph is the AFM of graphene transferred to SiO2/Si substrate.
To characterize the practicability and precision of the approach, we fabricated graphene patterns with tunable sizes and shapes with plastic film masks. Figure 3 shows optical microscope images of the patterned graphene of different types, including nets, lines, and triangles. The highly regular patterns are clearly evident and indicate the controllability of line width. Based on these optical microscope patterns, defects such as wrinkles and cracks are not observable, demonstrating high quality of our patterned graphene. The line widths of the plastic masks were 80, 60 and 20 µm, with a machining accuracy of ± 2 µm. The realized sizes of the graphene using the proposed approach are 78.57 ± 0.45, 60.56 ± 1.34 and 20.42 ± 1.11 µm, respectively. Using the designed size as a reference standard, the relative error between patterned graphene and design size are 3.2%, 3.8% to 6.4% for line widths of 80, 60 and 20 µm, respectively. Defining the dimension accurately is a difficult task. We reckon that dimension deviation of graphene pattern which is less than 10% is regarded as high accuracy. All the results show that the size accuracy of pattern turns out to be better than 10%, indicating that graphene pattern of micrometer dimension is generated at a good resolution style. The deviation is acceptable and could be smaller if machining tolerance was considered. The enhanced photolithography method enables a minimum size of patterned graphene of 20 µm, which reaches the patterning limitation of the plastic film mask, showing its wide and potential application in interconnection, transistor or device terminals [32,33]. As the patterning limitation of 20 µm of plastic film mask is well known, it is difficult to investigate fabrication of finer graphene patterns. However, finer and costly glass mask whose line width can be as low as 1um has been fabricated [34], we will investigate finer graphene patterns by using the proposed photolithography method in the future.
Fig. 3 (a)–(c) Optical microscope images of various patterned graphene of different sizes transferred onto SiO2/Si substrates. All scale bars: 50µm.
Photoresist must be used regularly in photolithography, but the photoresist residuals on the surface of graphene have a negative impact on the quality of graphene, especially in terms of photoelectric performance. To compare the effect of the Al insertion layer on the photoresist residuals, images of graphene patterns using the proposed and conventional photolithography processes were captured. Figure 4(a) shows an optical image of a graphene pattern exposed to the Ar-plasma-etched region. The inset of Fig. 4(a) shows the graphene strip realized by conventional photolithography processes. Mass photoresist residuals (green region in Fig. 4(a)) on the surface of graphene can be observed, while no obvious trace of photoresist can be found for the graphene pattern used the proposed approach. In order to detect the residuals quantitatively, we measured the Fourier-transform infrared (FTIR) spectra of graphene with Al film as a sacrificial layer (hereafter designated “graphene-Al”) and graphene that uses conventional photolithography (hereafter designated “graphene-PR); the results are displayed in Fig. 4(b). The inset shows the structural formula of propylene glycol methyl ether acetate (PGMEA), which is the main ingredient of the photoresist utilized in this study. The characteristic peak at 1125 cm−1 could be assigned to –OCH3, which is a functional group of PGMEA. We can observe that the photoresist sample has an highest peak value near 1125 cm−1, and the characteristic peak of the photoresist for graphene-Al is distinctly weaker than that of graphene-PR, demonstrating that the content of the residual photoresist on the surface of graphene-Al is much lower than that of the graphene-PR. Negligible photoresist residual on the surface of graphene-Al is ascribed to the existence of the Al sacrificial layer that prevented the photoresist from directly contacting the graphene surface. The interfacial physics between atom-thick graphene and the target substrate could be a reasonable explanation for the above results.
Fig. 4 (a) Graphene at Ar-plasma-etched region; inset shows edge of graphene strip (green region) realized by conventional photolithography process. (b) FTIR spectra of photoresist residual on surface of graphene transferred onto 300-nm-thick SiO2/Si substrate; inset shows the structural formula of main ingredient of photoresist. (c) Transmittance of graphene with and without thermal annealing. (d)Raman spectrum of Ar-plasma-etched region with respect to distance from edge of graphene pattern as marked in (a).
Thermal post-treatment with e.g. Ar/H2 gas is believed to be able to remove residuals of PR and improve the mobility of graphene. In order to verify the applicability of the proposed approach, thermal annealing of the samples after standard photolithography was carried out at temperature of 350 °C under a mixed gas (H2:Ar = 1:9) flow of 100 sccm (~650 mTorr) for 30 minutes. Higher annealing temperature is not tried in this paper since it has been reported that too high temperature may lead to damage of graphene. Transmittance of graphene after photolithography with and without Ar/H2 gas annealing are compared as shown in Fig. 4(c). Decrease of transmittance is obtained after thermal annealing, possible reason is that the decomposition PR is not one step reaction. Similarly to the polymethylmethacrylate (PMMA) residual, complete removal of PR is not an easy issue and the propose of the Al insertion to avoid the residual PR is very necessary.
In general, carbon nanomaterials have been well studied by Raman spectra, which can provides useful information about the number of layers and the quality in a material [35,36]. D peak (~1350 cm-1) is related to defects in a patterned graphene structure. G peak (~1580 cm-1) and 2D peak (~2700 cm-1) are related to opposite vibration of each carbon lattice and the second order vibration by phonon scattering respectively [35]. Raman spectra at different distances from the Ar-plasma-etched edge were carried out at a pattern width of 60 µm, and Fig. 4(d) shows the Raman spectra variation with distance from the edge spot of the graphene pattern shown in Fig. 4(a). The neglectable characteristic peak of graphene can be found at the edge, showing that there is only a little bit graphene residual on the region. The characteristics G and 2D peaks of graphene began to be observed obviously when the distance is 1 µm, and the peaks nearly remain unchanged upon further increasing distance. Furthermore, the low D peak appeared once in a while and it has no direct relationship with the distance from the margin, demonstrating that the graphene is protected effectively in the whole patterning processes. The existence of D peak and variation of graphene quality are considered to be due to the nonuniformity of CVD graphene itself. The introduction and removal of Al metal make patterned graphene have a little higher value of D peak than CVD-grown graphene (Fig. 2(c)), but the patterned graphene shows more superior properties, such as less photoresist residual, good conductivity, optical transmittance and surface morphology, which are particularly important for photoelectronic devices.
In order to further investigate the edge quality of the patterned graphene, AFM and SEM images were obtained, and the results are shown in Fig. 5, where the inset graph of Fig. 5(b) shows the edge information via an Ir-laser-patterned multi-layer graphene (MLG) film [26]. Precise graphene pattern can be obtained by the conventional photolithography as shown in Fig. 5(a). However, obvious residual can be found in the graphene pattern area. As shown in Fig. 5(b), clean and precise patterned graphene with a definite periphery, without obvious breakage, was obtained using the proposed process. The worse surface morphology of graphene by conventional photolithography can also confirm the existence of more residual compared with photolithography with Al insertion layer as shown in Fig. 5(c) and Fig. 5(d). From a comparison of the results, it can be concluded that photolithgraphy with an Al insertion layer can obtain a much better delineated periphery of patterned graphene comparing with conventional photolithography and laser scribing method. V. Georgakilas showed that polymers with a π bond and correct geometrical configurations can bind with graphene through π-π interactions [37]. Therefore, the photoresist is expected to have higher absorption energies to graphene compared with Al sacrificial layer since their interaction only determined by van der Waals forces.
Fig. 5 (a) and (b) are SEM images, (c) and (d) are AFM images of patterned graphene by conventional photolithography and photolithography with Al insertion layer respectively. The red dotted area of (a) shows the photoresist residual and inset graph of (d) shows the Ir-laser-patterned MLG film [26].
The work function (WF) of graphene plays a key role in the application of semiconductor devices. A custom-built WF instrument was used to determine the WF, and the results are shown in Fig. 6. The instrument was confirmed to be simple and practical [38], and a schematic of the corresponding mechanism and an image of the device are shown in the insets of Fig. 6. The WF of patterned graphene-Al is approximately 4.2 eV, which is lower than that of pure graphene (~5.1 eV) and Indium Tin Oxide (ITO) (~5.0 eV), indicating that graphene-Al may be more suitable as a cathodic material for application in optoelectronic devices. It is well accepted that the development of cathode alternatives is more urgent than anode alternatives, since commonly used cathodic materials are metal dominated, which hinders the optical performance and structural diversity of devices. The relatively low WF of graphene-Al is likely to be due to the doping effect of residual Al particles deposited after the wet-etching process.
Fig. 6 Work function of three different kinds of samples. Top left and lower right insets are a schematic and an image of WF tester, respectively.
Sheet resistance and optical transmittance are two important parameters by which the performance of patterned graphene can be evaluated, especially in the optoelectronic devices field. Generally, the sheet resistance of graphene transferred onto the SiO2/Si substrates was measured by Hall Measurement System. In order to eliminate random error, we measured the sheet resistance and Hall mobility of 20 samples of each type, and the results distribution is shown in Fig. 7(a). Graphene-Al has a significantly higher number of samples with low sheet resistance than graphene-PR does. In addition, a number of graphene-Al samples were reduced with increasing sheet resistance, demonstrating the good uniformity of the sheet resistance. The average sheet resistance is 500, 565, 531 Ω/sq, and the average Hall mobility is 0.1239, 0.1159, 0.1407 m2/V·s for graphene-Al, graphene-PR, and graphene respectively. During the process of patterning, Al can be easily oxidized to be transferred to Al2O3 on its surface or boundary between Al and graphene, but the etching solution which include dilute HNO3, H3PO4 and CH3COOH can etch Al2O3 easily and completely in the wet etching process. The lower sheet resistance and higher Hall mobility of graphene-Al compared with graphene-PR is believed to partially originate from the metal Al doping. Moreover, a significant amount of photoresist residuals adhered to the graphene-PR surface, which can be regarded as insulator and act as scattering center, is another possible reason for its relatively high sheet resistance and lower mobility. The inset of Fig. 7(a) is a photo of LED lighting with 3 × 8 mm2 patterned graphene as part of the circuit, from which we can directly see that the patterned graphene is still conductive and that the patterning process did not lead to serious breakage or essential changes to the graphene.
Fig. 7 (a) Sheet resistance distributions; inset is a photo of LED lighting with 3 × 8 mm2 patterned graphene as part of the circuit. (b) Optical transmittance; inset shows the patterned graphene-Al on the glass. (c) Water contact angle of graphene samples.
Figure 7(b) shows the optical transmittances of three types of graphene supported on glass substrates. The transmittances of graphene, graphene-Al, and graphene-PR were 97.2%, 94.2%, and 89.9%, respectively, at a wavelength of 550 nm. The transmittance of 97.2% for graphene without any etching process also indicates its monolayer nature of good quality. The obvious decrease of transmittance for graphene-PR is mainly due to the significant amount of residual photoresist. Optical transmittance of patterned graphene which utilize an Al interlayer is about 4% higher than graphene-PR. The slight shrinkage of graphene-Al compared with the transferred graphene because of the doping effect of Al particles which bring about absorption and scattering of photos. The inset graph of Fig. 7(b) depicts the patterned graphene-Al on the glass. That no visible disparity can be observed from the graph in the area with or without graphene also confirms the good transparency of graphene-Al.
The wetting property of graphene directly influences its application in photoelectric devices. Figure 7(c) shows the results of wetting property measurements using a droplet of deionized water. The water contact angles (WCAs) of graphene, graphene-PR, and graphene-Al are 77°, 76°, and 49°, respectively, demonstrating that graphene-Al is much more hydrophilic than the two other types of samples. The wetting property of graphene-based TCE is an issue of wide concern, and its WCA varies from ~40° to ~90°. It has been widely accepted that the WCAs is related to physico-chemical composition, surface roughness, surface functional groups, and doping level of graphene-based TCE, which eliminate equipment error. The unintentional doping of Al may be one of the possible reasons for the improvement in hydrophilicity. According to first principles and atomistic calculations, they show that doping modulates the binding energy between graphene and water and thus improve wetting property of graphene [39]. The work of Liu et al. [40,41] showed that alkali adatoms adsorption on graphene donates their electron easily and acts as good agents for n-doping of graphene. Hence, the interaction between alkali-metal adatoms and graphene has a large ionic character. For Al/graphene system, there is about 0.8-0.9 electron transfer, and it is consistent with physisorption picture discussed before [42–48]. Compared to undoped graphene on a SiO2/Si substrate, the introduction of Al particles makes n-doped graphene whose binding energy has been changed exhibit a lower water WCAs. On the other hand, HNO3 which is included in Al mask etchant can be generally used as a p-type dopant on graphene. However, HNO3 residual is negligible because it is volatile and is easy to dissolve in water, and it is almost removed by the rinse of DI water after etching. Therefore, we emphasize that the increase in wetting property of graphene-based TCE can be helpful in the application of transparent conductive electrodes in optoelectronic devices.
In order to quantify the residues of Al to graphene pattern, XPS analysis was carried out and the results are shown in Fig. 8. Due to the exposure to atmospheric environment, the main components of graphene are O, C and Al.
In conclusion, a precise and large-scale fabrication method for patterned graphene utilizing an Al film as an interlayer between the photoresist and graphene was presented. The Al layer was used both as a mask and as a sacrificial layer during the etching process. By this method, CVD-grown graphene was fabricated into high-quality patterned graphene with well-defined sizes and shapes and negligible photoresist residuals compared with conventional photolithography. Worth noting is that the pitch can reach the limit of photomasking utilized in this study. Furthermore, the good quality and relatively low work function of the patterned graphene make it more suitable as a transparent conductive film. In addition, no direct contact with the photoresist and Al doping effects result in the fabricated patterned graphene exhibiting better sheet resistance, transmittance, and hydrophilicity, which is applicable in photoelectric devices.
Funding
National Natural Science Foundation of China (51505270); Science and Technology Committee of Shanghai (15590500500).
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