Green Removal of DUV-Polarity-Modified PMMA for Wet Transfer of CVD Graphene

Abstract

To fabricate graphene-based high-frequency electronic and optoelectronic devices, there is a high demand for scalable low-contaminated graphene with high mobility. Graphene synthesized via chemical vapor deposition (CVD) on copper foil appears promising for this purpose, but residues from the polymethyl methacrylate (PMMA) layer, used for the wet transfer of CVD graphene, drastically affect the electrical properties of graphene. Here, we demonstrate a scalable and green PMMA removal technique that yields high-mobility graphene on the most common technologically relevant silicon (Si) substrate. As the first step, the polarity of the PMMA was modified under deep-UV irradiation at λ = 254 nm, due to the formation of ketones and aldehydes of higher polarity, which simplifies hydrogen bonding in the step of its dissolution. Modification of PMMA polarity was confirmed by UV and FTIR spectrometry and contact angle measurements. Consecutive dissolution of DUV-exposed PMMA in an environmentally friendly, binary, high-polarity mixture of isopropyl alcohol/water (more commonly alcohol/water) resulted in the rapid and complete removal of DUV-exposed polymers without the degradation of graphene properties, as low-energy exposure does not form free radicals, and thus the released graphene remained intact. The high quality of graphene after PMMA removal was confirmed by SEM, AFM, Raman spectrometry, and by contact and non-contact electrical conductivity measurements. The removal of PMMA from graphene was also performed via other common methods for comparison. The charge carrier mobility in graphene films was found to be up to 6900 cm²/(V·s), demonstrating a high potential of the proposed PMMA removal method in the scalable fabrication of high-performance electronic devices based on CVD graphene.

Keywords: DUV; PMMA; THz-TDS; graphene.

Figure 1

Optical absorption spectra of pristine PMMA/quartz (black line), DUV-exposed PMMA/quartz irradiated in air (red line), DUV-exposed PMMA irradiated in argon (blue line). PMMA film thickness, 0.25 µm.

Figure 2

Transmission FTIR spectra of (a) DUV-irradiated PMMA/CaF₂ sample and (b) pristine PMMA/CaF₂ sample. The difference of the two spectra is shown as (c). Spectra are shifted vertically for clarity.

Figure 3

(a) Water contact angle on PMMA before (left) and after (right) DUV exposure. Increased hydrophilicity observation corresponds to increasing polarity of new chemical products received after DUV exposure of PMMA. (b) Schematic representation of reconstruction of water: water and IPA: IPA clusters into non-clustered water: IPA mixture to act as an effective solvent, where under specific water: IPA mixture due to formation of hydrogen bonds connect effectively to the polymer.

Figure 4

(a) Schematic of graphene wet transfer and PMMA removal processing steps. (b) Optical microscope image of graphene edge area.

Figure 5

SEM images of the graphene films after different PMMA removal methods: (a) DUV exposure and dissolution in IPA/water, (b) dissolution in chloroform, (c) dissolution in acetone.

Figure 6

Topography and conductivity images (10 × 5.4 μm²) and topography profiles obtained via AFM microscopy of the graphene samples after different PMMA removal methods: (a,d,g) DUV exposure and IPA/water dissolution; (b,e,h) dissolution in chloroform; (c,f,i) dissolution in acetone. Topography profiles (g–i): A—graphene surface roughness, B—height of contaminants on the graphene, C—hole in graphene showing distance to substrate.

Figure 7

Raman measurements of wet-transferred CVD graphene after PMMA removal: (a) Raman spectra of the selected sample DUV3 demonstrating shape and position of G and 2D lines (spectra are shifted vertically for visual clarity). Spectra were measured every 10 µm steps along a line on the graphene. (b) Correlation analysis of the G and 2D peak positions for all samples under study, i.e., 3 DUV method samples (black squares), chloroform sample (green squares), acetone sample (yellow squares), and commercially transferred sample (orange squares). The black solid line demonstrates the dependence correlation between the frequencies of the G and 2D lines in the Raman spectrum of undoped graphene under biaxial strain. Blue dashed lines show correlation dependence for doped graphene under biaxial strain. Black cross marks G and 2D line positions in undoped and unstrained graphene.

Figure 8

The THz transmission spectrum of graphene on HR Si wafer (DUV3 sample). The experimental data (red circles) was described by Drude conductivity model (black line), the usage of which allows one to estimate the electrical conductivity of the graphene.