Toward High Carrier Mobility and Low Contact Resistance: Laser Cleaning of PMMA Residues on Graphene Surfaces
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Poly(methyl methacrylate) (PMMA) is widely used for graphene transfer and device fabrication. However, it inevitably leaves a thin layer of polymer residues after acetone rinsing and leads to dramatic degradation of device performance. How to eliminate contamination and restore clean surfaces of graphene is still highly demanded. In this paper, we present a reliable and position-controllable method to remove the polymer residues on graphene films by laser exposure. Under proper laser conditions, PMMA residues can be substantially reduced without introducing defects to the underlying graphene. Furthermore, by applying this laser cleaning technique to the channel and contacts of graphene field-effect transistors (GFETs), higher carrier mobility as well as lower contact resistance can be realized. This work opens a way for probing intrinsic properties of contaminant-free graphene and fabricating high-performance GFETs with both clean channel and intimate graphene/metal contact.
KeywordsGraphene PMMA residues Laser exposure Carrier mobility Contact resistance
Graphene, a single layer of sp 2 bonded carbon atoms, has attracted considerable interests for its intriguing physical properties such as high carrier mobility and thermal conductivity and held great promise for future integrated electronics [1, 2, 3]. Being a truly two-dimensional (2D) material, however, graphene is extremely sensitive to adsorbates and molecules in contact with its surface. The intrinsic properties of graphene are thus severely degraded because any surrounding medium may act as a dominant source of doping or scattering [4, 5, 6]. Unfortunately, contamination of graphene films with external molecules is inevitable in successive fabrication processes of devices, especially polymer residues.
To fabricate graphene field-effect transistors (GFETs), graphene grown by chemical vapor deposition (CVD) need to be transferred from a metal foil to an insulating substrate using a polymer such as poly(methyl methacrylate) (PMMA) as a support layer. PMMA is also commonly used as a mask material for electron beam lithography (EBL). Yet, a thin layer of PMMA residues (1–2 nm) after organic solvent (e.g., acetone) cleaning cannot be completely removed due to strong physical (van der Waals interactions) or chemical (covalent bonds formed between functional groups of PMMA and defect sites of graphene) adsorption effects .
Previous studies show that polymer residues left on graphene surfaces result in shift of the Fermi level and decrease of carrier mobility [4, 5]. Likewise, the polymer residues trapped at the interface of graphene/metal contact for GFETs fabricated in standard process considerably reduce graphene/metal interactions and lead to a broken ambipolar Fermi energy modulation and an increased contact resistance [8, 9]. To obtain a clean surface, graphene samples are empirically heated at 150–300 °C under Ar/H2 atmosphere or vacuum [7, 10, 11].
However, previous studies of transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and electrical measurements reveal that thermal annealing still cannot remove the polymer residues thoroughly. Furthermore, high-temperature heating process may intensify graphene/substrate and graphene/atmosphere interactions, causing graphene to be highly doped with severe mobility degradation [10, 11, 12, 13]. In addition to thermal annealing, electric current-induced annealing [14, 15], wet chemical treatment [12, 16], plasma treatment [17, 18], and ultraviolet ozone treatment  have also been developed to address the problem of polymer residues. However, current-induced annealing is limited to GFETs with ready-made electrodes [14, 15]; wet chemical treatment by chloroform or formamide is often toxic and may bring in new species of contaminants [12, 16]; Ar or O2 plasma treatment is aggressive and needs to be operated with extremely low plasma density and delicate time control [17, 18]; ultraviolet ozone treatment has poor reproducibility and may induce serious oxidation of graphene under the same condition [19, 20].
Here we propose a new technique using a laser beam to eliminate polymer residues and recover clean graphene surfaces. Our laser cleaning technique, unlike previous methods, can be specially applied to targeted positions without introducing additional contaminants and defects. In the following contexts, detailed descriptions on laser cleaning process and optimization conditions are given. Then the laser cleaning technique is applied to GFETs, which shows that higher carrier mobility as well as lower contact resistance can be realized. Finally, mechanisms of laser cleaning are discussed in three ways: agglomeration, decomposition, and expulsion.
2 Experimental Details
2.1 Graphene Preparation and Measurements
Graphene was prepared using both mechanical exfoliation and CVD methods. The exfoliated graphene films were peeled off from natural flake graphite using an adhesive tape (3 M) at ambient conditions and transferred onto a heavily doped Si wafer coated with a 300-nm-thick thermally grown SiO2 layer. The CVD graphene films were grown on polycrystalline copper foil (25 μm thick, 99.8 %, Alfa Aesar) in a gas mixture of methane, hydrogen, and argon at 1000 °C. Then graphene films were transferred to a Si/SiO2 substrate. To describe the process of laser cleaning, both the exfoliated and CVD graphene films on Si/SiO2 substrate were intentionally spin-coated with a 270-nm-thick PMMA layer (Allresist AR-P 679.04), baked at 170 °C for 2 min, cooled to room temperature, and then placed in an acetone bath for 2 h to dissolve PMMA. The number of layers was first characterized by optical microscopy (Olympus BX51) and then confirmed by Raman spectroscopy (532 nm laser wavelength, 50× objective) and atomic force microscope (AFM, Bruker Dimension Icon) in air.
2.2 GFET Fabrication and Electrical Measurements
Back-gated GFETs were fabricated in a top-down process. The graphene channels were patterned using e-beam lithography (EBL) followed by inductively coupled plasma reactive ion etching (ICP-RIE). The source (S) and drain (D) electrodes were fabricated by EBL, e-beam metal evaporation, and subsequent lift-off process. The exfoliated and CVD graphene films as the channel were contacted with Ti/Au (10/70 nm) and Pd/Au (20/60 nm), respectively. The structure of GFETs was inspected by optical microscope and AFM at tapping mode. All electrical measurements of GFETs were carried out on a probe station (Signatone WL-210E) using an Agilent B1500A semiconductor device analyzer under ambient conditions.
3 Results and Discussion
To save time, we increase the exposure power of laser. The dependence of R q on exposure time for monolayer graphene with higher exposure powers of 20, 30, and 40 mW is shown in Fig. 3b. With exposure time increasing at the initial stage, R q first increases and then decreases. With exposure time continuing to increase, R q no longer decreases and restores closely to the value of its pristine state (~0.15 nm), which indicates that a nearly complete removal of polymer residues is achieved. The higher the exposure power, the faster the R q decreases and saturates. However, it may induce defects at higher power (e.g., 30 and 40 mW). The dotted red boxes indicate the regions where the disorder-induced Raman D peak occurs. Figure 3c shows the Raman spectra of the monolayer graphene before and after laser cleaning with an exposure power of 30 mW for 180 and 270 s, respectively. The absence of the D peak around 1350 cm−1 indicates that there is no significant damage to the sp 2 hybridized carbon structure under a moderate exposure power of 30 mW for 180 s . However, overexposure (e.g., for 270 s) will induce a few defects as evidenced by the emerging D peak. In the following electrical studies of graphene devices, we set the laser cleaning condition to be 30 mW (180 s)−1 for monolayer GFETs to realize fast, effective, and noninvasive removal of polymer residues.
This laser cleaning technique has also been applied to CVD mono-, bi-, and trilayer graphene, as shown in Fig. S3. The polymer residues left on CVD graphene samples are apparently removed, except at the ripples formed in wet transfer process where few residues may still remain due to increased chemical activity at these sites .
The laser cleaning technique can also be used to remove the polymer residues from the contact regions of GFETs as defined by EBL prior to metal deposition. The previous thermal or current annealing methods, however, are not possible to remove the residual PMMA layer that is already covered by metal. To form intimate graphene/metal contact without polymer residues, generally there exist two kinds of processes in previous reports: the resist-free process and the resist-involved process. However, the resist-free process includes complex steps of non-polymer mask fabrication and alignment [8, 35]. The resist-involved process includes a global treatment by either oxygen plasma or ultraviolet ozone after contact lithography [18, 19]. As it is applied to the whole PMMA mask, resist deformation and thus pattern distortion may be caused. It is easy to remove the polymer residues on contact regions of GFETs using our laser cleaning technique.
It is worth noting that this laser cleaning technique may also be used in contact area with size smaller than 1 μm. The contact region shown in Fig. 5 is 1.8 μm wide. With a manual scanning, the PMMA mask on either side of the contact region was inevitably illuminated by the laser spot (~1.5 μm). However, as we can see from Fig. 5 that after laser cleaning, both sides show no deformation. Furthermore, no degeneration of the PMMA mask occurred because it was easily dissolved during lift-off process.
Besides PMMA residues, our laser cleaning is also effective for other polymer residues, such as novolak-based negative resist (AR-N 7520). After cleaning by 532 nm laser at 30 mW for 180 s, the residual negative-resist residues can also be completely removed, as shown in Fig. S4.
The mechanism of our laser cleaning process can be understood based on laser ablation phenomenon and PMMA behavior under laser exposure. Laser ablation (commonly by ultraviolet or near-infrared pulse lasers) has been widely used and thoroughly investigated for polymer micro-machining since the early 80s . Here we give a qualitative interpretation for our laser cleaning of polymer residues on graphene surfaces in three ways: agglomeration, decomposition, and expulsion. At the initial stage of laser cleaning, the incoming photons penetrate and diffuse into the PMMA-contaminated graphene sample, raising the surface temperature and melting the polymer residues. The melted small PMMA particles, if originally densely packed, may merge into large PMMA droplets . This is why we observe that the RMS surface roughness R q, as shown in Fig. 3a, b, is abnormally increased at the initial stage of laser exposure. With further laser irradiation, decomposition of polymer residues begins when the surface temperature approaches 230 °C . In general, there are two models proposed to explain decomposition of PMMA by laser ablation: in the first model of thermochemical process, laser acts as a heating source and results in a solid–gas phase transition, prevailing in near-infrared lasers [42, 43]; in the second model of photochemical process, high-energy photons directly break the main-chain bonds, dominating in ultraviolet lasers [44, 45]. Previous studies show that decomposition of PMMA includes main depolymerization process into monomers (at least 80 % of the mass loss) and other secondary processes into low-molecular-weight gases (e.g., H2, CO, CO2, CH4, C2H4) in trace amounts [37, 46, 47]. As shown in the AFM images in Figs. 2, 3, 4, and 5 (also in Supporting Information), the PMMA residues are obviously removed from graphene surfaces after laser illumination. We thus speculate that both thermochemical and photochemical processes may potentially be possible to account for decomposition of PMMA in our continuous-wave visible laser cases.
In summary, we have proposed a facile and reliable technique to remove polymer residues on graphene surfaces without generating defects using a visible laser from Raman system. After laser cleaning of the channel in GFETs, carrier mobility has been improved by a factor of 1.5–2.6. Moreover, this technique can be particularly applied to the contact regions as defined by EBL prior to metal deposition to eliminate the polymer residues, which is impossible by previous annealing methods. The contact resistivity of GFETs with cleaning at contacts can be reduced to 1/5–1/3 of those of GFETs without cleaning. This work provides an efficient route to get access to intrinsic properties of polymer residue-free graphene and fabricate high-speed GFETs with high carrier mobility and low contact resistance.
We are grateful to the National Basic Research Program of China (Grant No. 2013CBA01604) and the National Science and Technology Major Project of China (Grant No. 2011ZX02707).
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