Investigation On Transfer Induced Residues Of CVD Graphene

Graphene has been widely used in many fields because of its excellent properties.At present,the chemical vapor deposition?CVD?using metal substrate as catalyst has basically achieved high quality and large area controllable preparation,but the precondition for the subsequent processing of CVD graphene is to separate from the metal substrates and be transferred to the target substrates.As a key step in the application of graphene,transfer greatly affects the quality of graphene and the performance of devices.In order to solve the problem of contamination of CVD graphene by traditional wet transfer,the control variable method was used with optical microscopy,Raman spectrometer,Hall measurement system and x-ray photoelectron spectroscopy analysis?XPS?to study the types and components of the residues of traditional wet transfer,and effective measures were taken to reduce the residues.The main achievements are as follows:1.It is clear that the traditional wet transfer residues are mainly back graphene residues,metal substrate residues,etchant residues,PMMA residues and silicon wafer impurities.Further characterization by XPS shows that PMMA residues can be classified into intrinsic PMMA residues and denatured PMMA residues.2.Optimize the methods of inhibiting and removing PMMA residues in the transfer process.On the one hand,inserting rosin layer between PMMA and graphene weakens the binding force between PMMA and graphene;using low molecular weight and low concentration PMMA solution to reduce the folding of long carbon chains;spinning at higher rotating speed to obtain smooth and thin PMMA films;and increasing the solubility of PMMA in organic solvents by uniformly heating the film can effectively inhibit the residual PMMA.The intrinsic PMMA residues can be effectively removed by further promoting the decomposition of PMMA into small molecule MMA by deep ultraviolet exposure,dissolving it with hot acetone and adding subsequent isopropanol cleaning steps.On the other hand,the denatured PMMA can be effectively avoided by covering the PMMA film with PET as a protective layer to isolate the FeCl₃ etching solution,so as to avoid the reaction of the two to produce insoluble organic solvents,and using the mobile phone film?silicone+PET?instead of PMMA as an intermediary layer.3.Optimize the methods of inhibiting or removing other residues in the transfer process.The back graphene residue can be effectively avoided by edge sealing of graphene.By adding dilute HCl into the etching solution,the residual oxide of metal substrate can be effectively reduced.By using?NH₄?₂S₂O₈ or low concentration Na₂S₂O₈instead of FeCl₃ as etching solution,metal particle residues can be effectively avoided;after FeCl₃ etching,high temperature and low concentration“modified RCA clean”steps are added to remove metal particle residues.The organic contaminations on silicon wafer can be removed by acetone ultrasonic cleaning,and the metal contaminants can be removed by KOH solution instead of HF solution by thinning SiO₂ layer.In this paper,the traditional wet transfer residues are distinguished and targeted removal methods are proposed,which greatly improves the cleanliness and integrity of the transfer.This study provides a basis for the preparation and accurate characterization of graphene devices,and is conducive to the research and development of graphene non-destructive transfer technology.