Wafer-scale transfer of graphene grown on thin Cu film

Graphene has sparked a great deal of interest in the microelectronics field since the discovery of its excellent electronic transport properties. The main steps toward the development of high-performance graphene based devices are the synthesis of graphene and its transfer from the synthesis substrate onto a specific substrate, typically a dielectric material. Currently, chemical vapor deposition of graphene on Cu foil is considered as the most promising technique to produce large-scale high-quality single-layer graphene films. The challenge for device fabrication lies in the development of a reliable technique allow- ing the transfer of graphene without generating cracks, folds and ripples which are known to compromise graphene properties. It also emerged that Cu surface morphology on which graphene grows is of first importance to guaranty a successful defect-free transfer of graphene. This work investigates the use of thin Cu film with varying thicknesses (from 1 micron down to 350 nm) as catalyst instead of Cu foil in order to improve graphene transfer. Wafer-scale high-quality single-layer graphene is produced by low pressure chemical vapor deposition (LPCVD) on the Cu film evaporated on a 300 nm-SiO2/Si wafer. Graphene grown on a thin Cu film does not present the rough morphology mimicking the deep Cu grain boundary grooves and ripples induced by the rolling process of Cu foils. The experimental results demonstrate that using a thin Cu film instead of a Cu foil effectively mitigates graphene morphology and results in graphene films with reduced defects after a conventional transfer procedure. Starting from a Cu film instead of a Cu foil, we also established alternative transfer approaches. The flat and rigid Cu substrate can be exploited to directly process graphene by conventional thin film technologies. The Cu film can be used as a sacrificial layer after graphene growth to directly deposit graphene on the underlying SiO2/Si substrate. Alternatively, lift-off process can also be used to pattern the Cu thin film prior to CVD graphene growth in order to prevent residues induced by direct contact of resist on the top of graphene during lithography process. The patterned Cu film can be sublimated afterward to circumvent the use of wet etching which is known to cause an adverse impact on graphene quality. Scanning electron microscopy (SEM) and Raman spectroscopy are used at every single step of the transfer processes in order to systematically study their impact on physical properties of graphene such as doping and structural quality. The developed proximity transfer techniques based on the removal (wet etching or sublimation) of the thin Cu film demonstrated their efficiency to produce high-quality and uniform single-layer graphene on any final rigid substrate of interest.