Electron Beam Lithography Research Articles

Single‐Component High‐Resolution Dual‐Tone EUV Photoresists Based on Precision Self‐Immolative Polymers

Electron beam (EB) and extreme ultraviolet (EUV) lithography are advanced techniques capable of achieving sub‐10 nm resolutions, critical for fabricating next‐generation nanostructures and semiconductor devices. However, developing EUV photoresists that meet all demands for resolution, line edge roughness (LER), and sensitivity (RLS) remains a significant challenge. Herein, we introduce high‐performance photoresists based on single‐component self‐immolative polymers (SIPs) with inherent signal amplification via cascade degradation. These SIPs function as dual‐tone photoresists under both EB and EUV lithography, with performance primarily determined by the exposure dose. Lithographic evaluations show that discrete SIPs provide significant improvements over disperse counterparts, achieving higher resolution and reduced LER. Specifically, a discrete SIP with a DP of 12 produces a line‐space pattern with a resolution of approximately 18 nm and an LER of 1.8 nm, compared to 21 nm resolution and 2.5 nm LER for disperse SIPs. Additionally, these SIP‐based photoresists, enriched with aromatic structures, exhibit excellent etch resistance. The single‐component nature and potential to address the RLS trade‐off underscore the promise of discrete SIPs for EUV lithography.

Optimizing dose parameters for enhanced maskless lithography in MoS2-based devices

Maskless lithography simplifies the fabrication process and reduces costs compared to electron beam (E-beam) lithography, making it a more efficient choice for patterning nano-devices. Maskless lithography presents a promising avenue for expediting device fabrication by eliminating the need for masks. This technique can streamline the production of basic electronic devices, offering an efficient and low-cost alternative to traditional lithographic methods, like E-beam lithography. This study utilized a 405 nm photodiode to achieve pattern-writing with a minimum linewidth of 1 μm. Exploring optimal parameters includes adjustments in beam intensity, scan speed, and step size. Maskless lithography was applied to 2D transition metal dichalcogenides (TMDCs) material, MoS2, to investigate their electrical transport characteristics. The fabricated device exhibits an ON/OFF ratio of ∼1.7 × 106 and a mobility of ∼0.833 cm2/V·s, indicating a high switching efficiency. The results demonstrate optimized maskless lithography's potential for swift and cost-effective fabrication, offering intermediate-resolution patterning capabilities.

Vortex Domain Wall Thermal Pinning and Depinning in a Constricted Magnetic Nanowire for Storage Memory Nanodevices.

In this study, we investigate the thermal pinning and depinning behaviors of vortex domain walls (VDWs) in constricted magnetic nanowires, with a focus on potential applications in storage memory nanodevices. Using micromagnetic simulations and spin transfer torque, we examine the impacts of device temperature on VDW transformation into a transverse domain wall (TDW), mobility, and thermal strength pinning at the constricted area. We explore how thermal fluctuations influence the stability and mobility of domain walls within stepped nanowires. The thermal structural stability of VDWs and their pinning were investigated considering the effects of the stepped area depth (d) and its length (λ). Our findings indicate that the thermal stability of VDWs in magnetic stepped nanowires increases with decreasing the depth of the stepped area (d) and increasing nanowire thickness (th). For th ≥ 50 nm, the stability is maintained at temperatures ≥ 1200 K. In the stepped area, VDW thermal pinning strength increases with increasing d and decreasing λ. For values of d ≥ 100 nm, VDWs depin from the stepped area at temperatures ≥ 1000 K. Our results reveal that thermal effects significantly influence the pinning strength at constricted sites, impacting the overall performance and reliability of magnetic memory devices. These insights are crucial for optimizing the design and functionality of next-generation nanodevices. The stepped design offers numerous advantages, including simple fabrication using a single electron beam lithography exposure step on the resist. Additionally, adjusting λ and d allows for precise control over the pinning strength by modifying the dimensions of the stepped areas.

Influence of Chemical Structures on E-Beam Lithography Performance of Polysilsesquioxanes.

Hydrogenated silsesquioxane (HSQ) is a key inorganic electron beam resist, celebrated for its sub-10 nm resolution and etching resistance, but it faces challenges with stability and sensitivity. Our innovative study has comprehensively assessed the lithographic performance of three functionalized polysilsesquioxane (PSQ) resist series─olefins, halogenated alkanes, and alkanes─under electron beam lithography (EBL). We discovered that the addition of olefin groups, such as in the HMP-30 formulation with 30% propyl acrylate, remarkably increased the sensitivity to 0.6 μC/cm2. The inclusion of halogenated aromatic and hydrogen-substituted methyl groups further enhanced sensitivity and contrast, with HClBN-50 achieving a 22.9 nm resolution pattern. At the same time, the storage of PSQ resists was significantly improved compared to commercial HSQ with increasing alkane group content. Crucially, our research has unveiled the lithography reaction mechanism, highlighting how group encapsulation and steric hindrance influence PSQ performance. This insight is groundbreaking, offering a deeper understanding of the molecular structure-performance relationship and laying the groundwork for developing next-generation electron beam resists with superior sensitivity, resolution, and contrast for microelectronics manufacturing.

Imaging of Volatile Organic Compounds Using a Single Nanowire-Based Electronic Nose for Future Biomedical Applications.

This study introduces an array of semiconductor oxide single nanowires fabricated using advanced semiconductor processing techniques, including electron beam lithography and thin-film deposition, which is well-suited for large-scale nanowire integration. A four-channel nanowire array consisting of tin oxide (SnO2), indium oxide (In2O3), ferric oxide (Fe3O4), and titanium oxide (TiO2) was developed. As a proof of concept, we converted the response curves of the sensor array to heat maps, enabling comprehensive feature representation. The fabricated electronic nose (E-nose) was utilized to detect three types of volatile organic compounds (VOCs), with the results visualized in a heat map format. Additionally, the performance of each individual sensor was quantitatively studied, highlighting the array's potential for enhanced gas detection and analysis. To further illustrate the interaction between gas molecules and the nanowires, we visualized the gas response results by mapping the sensor's signal changes. These visualizations provide a clear representation of how different gas molecules interact with specific nanowires. For example, the heat maps reveal distinct response patterns for each type of VOC, allowing for the identification and differentiation of gases based on their unique signatures. This visualization technique not only enhances the understanding of gas-nanowire interactions but also demonstrates the effectiveness of the E-nose in distinguishing between various VOCs. The SnO2 nanowire gas sensor showed enhanced gas response compared to other materials. The SnO2 and TiO2 gas sensors showed enhanced response (62 and 56 s) and recovery times (100 and 37 s).

High density nanofluidic channels by self-sealing for metallic nanoparticles detection

High density nanofluidic channels were successfully fabricated by a novel process, nicknamed as self-sealing process, for the detection of metal nanoparticles dispersed in water using color changes excited by polarized electromagnetic waves. The permittivities of aqueous solutions with various concentrations of metal nanoparticles were calculated by a corrected plasma model. Systematic simulations using finite difference time domain method were carried out in investigating the detection capabilities of the nanofluidic channels for silver, beryllium and copper nanoparticles in water. The pronounced color shifts indicates that the channels possess high sensitivity in the metal nanoparticles detection. The designed nanofluidic channels were then fabricated by a direct flood deposition of a silica film on a pre-replicated hydrogen silsesquioxan (HSQ) grating using electron beam lithography (EBL). The self-sealing technique possesses advantages in simplified processing, encapsulation free and potential of multi-layer nanochannels.

Research on preparation of micron-scale orthogonally aligned HMS patterns based on soft lithography

The soft lithography enables the fabrication of microfluidic honeycombs, which are micron-scale, orthogonally aligned hexagonal mesoporous silica (HMS) patterns with channels embedded within them, on silicon substrates. The process entails meticulous cleaning, precise master mold design (including features for channel alignment) using electron-beam lithography, generation of polydimethylsiloxane (PDMS) mold creation, ink formulation, and pattern transfer. Subsequent steps include the sol-gel transition, hardening, alignment verification, characterization, and optional functionalization. It is of the utmost importance to optimize these steps and the master design in order to achieve success. Therefore, the paper examines the creation of micron-scale HMS patterns with orthogonal alignment through the innovative application of soft lithography. The technique encompasses the strategic design of hexagonal motifs, precision master mold crafting, the PDMS stamps, and a meticulous sol-gel procedure, resulting in HMS structures that exhibit remarkable precision and adaptability. This paper underscores the transformative potential of soft lithography in the realm of advanced materials, which offers a controlled and uniform approach to size and uniformity. The refined soft lithography process not only achieves the precise orthogonal alignment of HMS patterns but also highlights its adaptability and economic viability in nanotechnology ventures.

Fabrication of Circular Defects in 2-Dimensional Photonic Crystal Lasers with Convex Edge Structure

We have developed circular defects in 2-dimensional photonic crystal lasers that allow current injection for interconnected optical communications. However, when cleaving the sample to measure the output light, the output light intensity changes due to the cleaving position. In a previous study, we proposed a new end face structure called a convex edge structure. In this paper, we design the electron beam lithography patterns to fabricate this structure. With this design, it is possible to eliminate the effect of different cleaving positions and ensure that the cleavage tolerance is larger than the cleavage position error. We also develop the fabrication technology for this structure, fabricate samples, and measure the output light experimentally. The optical properties of the fabricated sample are similar to well-fabricated samples with normal cleavage edge faces. We are assured that these results contribute to future work such as accurate manufacturing and improving the end face configuration to obtain higher outputs.

Repair Engineering of Crystal Structure in van der Waals Materials by Probe Electron Beam.

The advancement of electronic technology has led to increasing research on performance and stability. Continuous electrical pulse stimulation can cause crystal structure changes, affecting performance and accelerating aging. Controlled repair of these defects is crucial. In this study, we investigated crystal structure changes in van der Waals (vdW) InSe crystals under continuous electric pulses by using electron beam lithography (EBL) and spherical aberration corrected transmission electron microscopy (Cs-TEM). Results show that electrical pulses induce amorphous regions in the InSe lattice, increasing the device resistance. We used Cs-STEM probe scanning for precise repair, abbreviated SPRT, to optimize device performance. SPRT is related to electric fields induced by the electron beam and can be applied to other 2D materials like α-In2Se3 and CrSe2, offering a potential approach to extend device lifespan.

Dry etch performance of Novolak-based negative e-beam resist

Electron beam lithography (EBL) is pivotal for micro- and nanoscale fabrication, offering sub-micron precision. This study explores the utilization of the Novolac-based negative resist AR-N 7520 for EBL and its potential as an etch mask for reactive ion etching (RIE) of silicon. Recent comparisons of negative EBL resists have revealed promising results for AR-N 7520 in terms of resolution and adaptability with other lithography techniques. In this article, we conduct an exploration of patterning of AR-N 7520 (new) for EBL, addressing key parameters in achieving optimal patterning fidelity. Furthermore, we investigate its compatibility with RIE processes, aiming to provide insights into its effectiveness as an etch mask for creating sub-micron silicon structures. Experimental results show that optimal e-beam dose with 100 kV exposure is 300–350 μC/cm2. Selectivity of around 9:1 can be achieved by optimizing etching parameters for a continuous etch and higher than 14:1 for a cyclic etch process.

Wafer-scale nanofabrication of sub-5 nm gaps in plasmonic metasurfaces

Abstract In the rapidly evolving field of plasmonic metasurfaces, achieving homogeneous, reliable, and reproducible fabrication of sub-5 nm dielectric nanogaps is a significant challenge. This article presents an advanced fabrication technology that addresses this issue, capable of realizing uniform and reliable vertical nanogap metasurfaces on a whole wafer of 100 mm diameter. By leveraging fast patterning techniques, such as variable-shaped and character projection electron beam lithography (EBL), along with atomic layer deposition (ALD) for defining a few nanometer gaps with sub-nanometer precision, we have developed a flexible nanofabrication technology to achieve gaps as narrow as 2 nm in plasmonic nanoantennas. The quality of our structures is experimentally demonstrated by the observation of resonant localized and collective modes corresponding to the lattice, with Q-factors reaching up to 165. Our technological process opens up new and exciting opportunities to fabricate macroscopic devices harnessing the strong enhancement of light–matter interaction at the single nanometer scale.

Novel Etch-Resistant Molecular Glass Photoresist Based on Pyrene Derivatives for Electron Beam Lithography.

Novel t-butyloxycarbonyl-protected molecular glass photoresists with pyrene as the core (Pyr-8Boc and Pyr-4Boc) were designed and synthesized. The thermal stability and film-forming ability were measured to assess their applicability for lithography. Pyr-Boc (Pyr-8Boc and Pyr-4Boc) photoresists were evaluated by high-resolution electron beam lithography (EBL), acting as chemically amplified resists. Pyr-4Boc showed a better lithography performance, achieving 25 nm line/space patterns at the dose of 50 μC/cm2. Under SF6/O2 plasma, the etch selectivity of the Pyr-4Boc photoresist to silicon was 12.3, which is twice that of the commercially available poly(methyl methacrylate) photoresist (950 k). The lithography mechanism of EBL was further investigated. Theoretical calculations of HOMO/LUMO orbital energies, cyclic voltammetry, and fluorescence quenching experiments were conducted to confirm the electron-transfer reactions between the Pyr-Boc and photoacid generator. The study provides an option of high sensitivity and etch-resistant photoresist for EBL.

Cryogenic Thermo-Optical Coefficient of SU-8 Measured Using a Racetrack Resonator at 850 nm

SU-8 is an emerging polymer material for integrated optical circuits that has demonstrated good structural properties in a cryogenic environment. In this article, we investigate the thermo-optical properties of SU-8 for a wavelength λ = 850 nm, from room temperature to cryogenic temperature down to 14 K. To measure the material properties, we designed and fabricated SU-8 racetrack resonators via electron beam lithography. While cooling the device in a closed-cycle cryostat, we measured the resonance spectrum as a function of the temperature from which we determined the temperature-induced variations of the group and effective indices of the waveguide. With the aid of waveguide eigenmode simulations, we used these data to derive the temperature dependence of the SU-8 refractive index nSU−8 (T). At room temperature (T~295 K), the thermo-optic coefficient dnSU−8/dT = −5.3 ± 0.2 × 10−5 K−1. At low temperature (T~14 K), dnSU−8/dT = −1.27 ± 0.05 × 10−4 K−1. Our research shows the potential of SU-8 photonics in a cryogenic environment, suitable for the integration with quantum light sources emitting in the near infrared (NIR).

Contact Resistance Engineering in WS2-Based FET with MoS2 Under-Contact Interlayer: A Statistical Approach.

One of the primary factors hindering the development of 2D material-based devices is the difficulty of overcoming fabrication processes, which pose a challenge in achieving low-resistance contacts. Widely used metal deposition methods lead to unfavorable Fermi level pinning effect (FLP), which prevents control over the Schottky barrier height at the metal/2D material junction. We propose to harness the FLP effect to lower contact resistance in field-effect transistors (FETs) by using an additional 2D interlayer at the conducting channel and metallic contact interface (under-contact interlayer). To do so, we developed a new approach using the gold-assisted transfer method, which enables the fabrication of heterostructures consisting of TMDs monolayers with complex shapes, prepatterned using e-beam lithography, with lateral dimensions even down to 100 nm. We designed and demonstrated tungsten disulfide (WS2) monolayer-based devices in which the molybdenum disulfide (MoS2) monolayer is placed only in the contact area of the FET, creating an Au/MoS2/WS2 junction, which effectively reduces contact resistance by over 60% and improves the Ion/Ioff ratio 10 times in comparison to WS2-based devices without MoS2 under-contact interlayer. The enhancement in the device operation arises from the FLP effect occurring only at the interface between the metal and the first layer of the MoS2/WS2 heterostructure. This results in favorable band alignment, which enhances the current flow through the junction. To ensure the reproducibility of our devices, we systematically analyzed 160 FET devices fabricated with under-contact interlayer and without it. Statistical analysis shows a consistent improvement in the operation of the device and reveals the impact of contact resistance on key FET performance indicators.

Approaching Angstrom-Scale Resolution in Lithography Using Low-Molecular-Mass Resists (

Resists that enable high-throughput and high-resolution patterning are essential in driving the semiconductor technology forward. The ultimate patterning performance of a resist in lithography is limited because of the trade-off between resolution, line-width roughness, and sensitivity; improving one or two of these parameters typically leads to a loss in the third. As the patterned feature sizes approach angstrom scale, the trade-off between these three metrics becomes increasingly hard to resolve and calls for a fundamental rethinking of the resist chemistry. Low-molecular-mass monodispersed metal-containing resists of high atom economy can provide not only very high resolution but also very low line-width roughness without sacrificing sensitivity. Here we describe a modular metal-containing resist platform (molecular mass <500 Da) where a molecular resist consists of just two components: a metal and a radical initiator bonded to it. This simple system not only is amenable to high-resolution electron beam lithography (EBL) and extreme ultraviolet lithography (EUVL) but also unites them mechanistically, giving a consolidated perspective of molecular and chemical processes happening during exposure. Irradiation of the resist leads to the production of secondary electrons that generate radicals in the initiator bonded to metal. This brings about an intramolecular rearrangement and causes solubility switch in the exposed resist. We demonstrate record 1.9-2.0 nm isolated patterns and 7 nm half-pitch dense line-space features over a large area using EBL. With EUVL, 12 nm half-pitch line-space features are shown at a dose of 68 mJ/cm2. In both of these patterning techniques, the line-width roughness was found to be ≤2 nm, a record low value for any resist platform, also leading to a low-performance trade-off metric, Z factor, of 0.6 × 10-8 mJ·nm3. With the ultimate resolution limited by instrumental factors, potential patterning at the level of a unit cell can be envisaged, making low-molecular-mass resists best poised for angstrom-scale lithography.

Precursor-Derived Sensing Interdigitated Electrode Microstructures Based on Platinum and Nano Porous Carbon.

Interdigital electrodes were prepared using nanoimprint lithography and piezoelectric inkjet printing. These processes are simpler and more cost-effective than the industrially used electron beam lithography because of their purely mechanical process step. For the investigation of material dependence, platinum as well as carbon electrodes were fabricated. Afterwards electrodes with various line widths and spacings were tested for the application as a chemiresistive sensor for ferrocenyl-methanol and the influence of the gap-width and conductor cross-section on the sensitivity was investigated. The general suitability of the systems for the production of such structures could be confirmed. Structures with a limit of detection (LOD) down to 1.2 μM and 35.9 μM could be produced for carbon and platinum, respectively, as well as a response time of 3.6 s was achieved.

Convolutional Neural Network-Assisted Photoresist Formulation Discriminator Design of a Contact Layer for Electron Beam Lithography.

The photoresist formulation is closely related to the material properties, and its composition content determines the lithography imaging quality. To satisfy the process requirements, imaging verification of extensive formulations is required through lithography experiments. Identifying photoresist formulations with a high imaging performance has become a challenge. Herein, we develop a formulation discriminator of a metal oxide nanoparticle photoresist for a contact layer applied to electron beam lithography. This discriminator consists of convolutional neural network photoresist imaging and formulation classification models. A photoresist imaging model is adopted to predict the contact width of variable formulations, while a formulation classification model is used to classify formulations according to relative local critical dimension uniformity (RLCDU). The verification results indicate that the discriminator can accurately recognize photoresist formulations that simultaneously meet the conditions of contact width and RLCDU, and its feasibility has been demonstrated, providing a valuable reference for the preparation of photoresist materials.

Diffractive optical elements on non-planar surfaces for the formation of color 3D images

This paper is dedicated to the development of synthesis methods for diffractive optical elements (DOEs) on non-planar surfaces. To form 3D images, a set of color 2D frames is used, visible to an observer from different observation points. The developed DOEs can be called a stereogram with a 360-deg view when the DOE is illuminated with white light. Methods are developed for computing the phase function of the DOE and are applicable to a wide range of convex surfaces. However, the manufacturing technology used limits this range to conical and cylindrical surfaces. The microrelief of the flat DOE is produced using electron-beam lithography. A video recorded using optical elements produced demonstrates the capabilities of the technologies developed. These DOEs can be used to protect brands against counterfeit.

(Invited) Plasmonic Nanofabrication of Chiral Nanodisk Ensembles

Chiral plasmonic nanostructures attract attention because those could be applied to enantioselective chemical sensors, optical isolators, light sources for circularly polarized light (CPL), metasurfaces, and metamaterials. In many cases, chiral plasmonic nanostructures are fabricated by a top-down method such as electron beam lithography (EBL). Since EBL is time-consuming and expensive, we have developed photoelectrochemical methods in which a site-selective reaction is driven by optical near field generated around anisotropic metal nanoparticles under right- or left-CPL. As anisotropic metal precursors for preparation of chiral nanoparticles, we have used gold or silver nanocuboids, nanorods, nanocubes, and triangular or hexagonal nanoplates so far.1-5 In the present work we demonstrate that laterally isotopic, circular nanodisks can also be used as precursors for the preparation of chiral nanostructures.We prepared ensembles of circular gold nanodisks on a glass plate by a nanosphere lithography method. A glass plate was coated with a gold thin film, and an array of polystyrene spheres (500 nm diameter) was prepared on the gold film. The polystyrene spheres were etched by oxygen plasma, and the exposed gold film was etched by argon ion milling. As a result, an ensemble of gold nanodisks (~200 nm diameter) was obtained. Each nanodisk was capped with a polystyrene cone, and the cap was removed by sonication in toluene if necessary. We used both capped and uncapped nanodisk ensembles. The ensemble was irradiated with visible right- or left-CPL in the presence of silver ions and sodium citrate. The obtained Au-Ag nanostructures exhibited circular dichroism (CPL), indicating that the nanostructures have chirality. Initial deposition at arbitrary site may break the geometric symmetry, resulting in the growth into chiral structures. Saito, K.; Tatsuma, T. Nano Lett. 2018, 18, 3209–3212.Morisawa, K.; Ishida, T.; Tatsuma, T. ACS Nano 2020, 14, 3603–3609.Shimomura, K.; Nakane, Y.; Ishida, T.; Tatsuma, T. Appl. Phys. Lett. 2023, 122, 151109.Homma, T.; Sawada, N.; Ishida, T.; Tatsuma, T. ChemNanoMat 2023, 9, e202300096.Ishida, T.; Isawa, A.; Kuroki, S.; Kameoka, Y.; Tatsuma, T. Appl. Phys. Lett. 2023, 123, 061111.

Impact of Silicon Dioxide Periodic Grating Structure on Electrical Properties of Graphene

Our study breaks away from the conventional focus on 3D transistor structures, aiming to explore the potential of two-dimensional materials. Graphene, a key player of two-dimensional materials, draws significant attention due to its exceptional electrical properties and chemical stability. Rather than delving into the intrinsic properties of graphene, our research focuses on examining the impact that other materials in contact with graphene have on graphene’s behavior. With a thickness of just one atomic layer, graphene's band structure is easily influenced by materials in contact with it, giving rise to the Surface Charge Transfer Doping (SCTD) effect. Our study involves the utilization of e-beam lithography to define 140 nm gratings and RIE to etch into SiO2 substrates. Subsequently, graphene is transferred onto these SiO2 gratings. Within these grating regions, in other words, graphene is supported by SiO2, while other part of the graphene is suspended. By strategically forming alternatively supported/suspended type of graphene in period of 140 nm through these gratings, our aim is to unravel the impact on the electrical characteristics of graphene along both the vertical and horizontal axes. In vertical direction, we measured contact resistance between graphene and metal; in horizontal, we measured the difference of Id-Vg curve caused by the grating structure under graphene channel. This exploration emerges from the quest to harness novel properties in the face of evolving fabrication methodologies.To have 140 nm grating, we chose e-beam lithography in defining the nano grating pattern. Initially, a clean wafer was coated with chromium, and e-beam resist Zep-520A was spin-coated and soft-baked. Subsequently, using e-beam lithography, we defined the periodic pattern. Following this, ICP-RIE was employed for dry etching of chromium, and the resist was removed. Then, ICP-RIE, with chromium as the hard mask, was utilized for dry etching of SiO2. Finally, the chromium was immersed in a chromium etchant solution, resulting in a periodic grating structure with a depth of 100nm on SiO2. This marks the completion of the first part of the fabrication process. In the second part of fabrication process, we transferred graphene, prepared through CVD, onto the SiO2 substrate with periodic gratings defined through etching. We employed optical lithography to define patterns for transmission line method (TLM) measurements. These patterns were aligned with positions defined by e-beam, ensuring that the graphene-metal contact positions precisely coincided with the underlying grating structure. Utilizing TLM, we measured the contact resistance, enabling the study of the impact of the grating on the electrical properties of graphene in the vertical direction. The fabrication flow can be schematically shown in Fig. 1.By using ELS-7000 (E-beam writer) and Oxford ICP-RIE, we successfully achieved ~140 nm grating structure on silicon dioxide with depth of 100nm, which is observed on SEM as shown in Fig. 2. Then we measured the I-V curve by 4200A-SCS and calculated contact resistance by TLM. The preliminary results of the contact resistance measurement are shown in the Fig. 3. The I-V curve shows that as the channel length increases, the total resistance increases, which is aligned with the assumptions of TLM. We can further calculate the contact resistance through the Y-axis intercept of channel length-resistance curve. With the same process, we can explore the electrical variations of graphene with graphene-metal contact resistance and current-voltage relationship. Figure 1