Rapid Selective Etching of PMMA Residues from Transferred Graphene by Carbon Dioxide

Abstract

During chemical-vapor-deposited graphene transfer onto target substrates, a polymer film coating is necessary to provide a mechanical support. However, the remaining polymer residues after organic solvent rinsing cannot be effectively removed by the empirical thermal annealing in vacuum or forming gas. Little progress has been achieved in the past years, for little is known about the chemical evolution of the polymer macromolecules and their interaction with the environment. Through in situ Raman and infrared spectroscopy studies of PMMA transferred graphene annealed in nitrogen, two main processes are uncovered involving the polymer dehydrogenation below 200 °C and a subsequent depolymerization above 200 °C. Polymeric carbons over the monolayer graphitic carbon are found to constitute a fundamental bottleneck for a thorough etching of PMMA residues. The dehydrogenated polymeric chains consist of active CC bonding sites that are readily attacked by oxidative gases. The combination of Raman spectroscopy, X-ray photoemission spectroscopy, and transmission electron microscopy reveals the largely improved carbon removal by annealing in oxidative atmospheres. CO2 outperforms other oxidative gases (e.g., NO2, O2) because of its moderate oxidative strength to remove polymeric carbons efficiently at 500 °C in a few minutes while preserving the underlying graphene lattice. The strategy and mechanism described here open the way for a significantly improved oxidative cleaning of transferred graphene sheets, which may require optimization tailored to specific applications.

Graphene‐SiO2 Interaction from Composites to Doping

“…As shown in Figure , the first treatment induced no relevant change to the G, 2D Raman bands apart from a small decrease of their intensity ratio from 2.8 down to 2.0. The successive vacuum treatment up to 18.5 h gives a blue shift of the two bands, some further reduction of their intensity ratio and the appearance of a large unresolved signal below the G band that can be associated to some amorphous carbon contribution arising from residual PMMA carbonization . The treatment of a sample in CO 2 atmosphere gives again minor changes to the positions of the Raman bands and a decrease of their intensity ratio down to 1.7.…”

Cleaning Transferred Graphene for Optimization of Device Performance

“…Therefore, there have been many efforts to remove these PMMA residues by various techniques, among which thermal annealing in vacuum or inert gases are most widely applied. Nonetheless it has been reported that some PMMA residues still remain on the surface after thermal annealing, resulting in scattering and defects .…”

The role of contact resistance in graphene field-effect devices

“…Despite the good results discussed above, the most common procedure to remove resist residue (introduced during the fabrication process) from the contact areas is the thermal annealing [110,121,161,162]. Above 200°C the decomposition of the resist residues takes place [152,161], but if annealing is performed after the metal deposition of the contact electrodes, it is difficult that residues may be removed. Indeed, for Ni-contacted graphene it has been reported that there is no relevant improvement of contact resistance after thermal annealing performed on the already contacted device [162].…”