Design And Fabrication Of Transparent Electromagnetic Interference Shielding Multilayer Structures Based On Ultrathin Metal

The innovation of communication technology brings convenience to our lives,but also makes the electromagnetic environment more complicated,which puts forward higher requirements for electromagnetic interference(EMI)shielding materials,especially the materials with visible transparency.The emerging 5G wireless telecommunications feature larger working bandwidth and higher operating frequencies,which challenges the existing EMI shielding materials,in terms of shielding bandwidth,working frequencies,and EMI shielding effectiveness(SE).Though it is highly challenging,it is urgent to enable EMI shielding materials to have the improved EMI SE,visible transmittance,higher operating frequencies,and broader bandwidth.Transparent ultrathin metal films are good candidates for transparent EMI shielding thanks to their excellent optoelectrical properties,flexibility,and scalable fabrication.In this thesis,the ultrathin metal-based transparent EMI shielding materials are explored through studying the thin film preparation process and EMI shielding structures.On the basis of it,the EMI shielding materials with high visible transmittance,SE,and ultra-broadband EMI shielding characteristics are developed.The main contents and results are as follows:1.To simultaneously realize the EMI shielding materials with high SE,visible transmittance,and larger shielding bandwidth,an ultrathin metal multilayer sandwich structure,composed of zinc oxide/ultrathin silver/zinc oxide(Zn O/Ag/Zn O,denoted as SLSO),is introduced.The magnetron sputtering deposition processes and the thicknesses of Ag layer are systematically studied,achieving transparent conducting thin films with low optical loss and low sheet resistances and clarifying the relationship between the Ag thicknesses and the optoelectrical properties in SLSO.Following the guidelines,the fabricated SLSO thin films on flexible PET substrates exhibit excellent optical and electrical properties with a transmittance of 91.9%at 550 nm,higher than that of the bare substrate,and a low sheet resistance of 4.2?/?.2.To alleviate the trade-off between the optical transparency and EMI shielding of transparent EMI shielding materials,the comparative studies are performed on the single-metal layer and the multi-metal layer structures in terms of their SE properties,via a transmission theory of electromagnetic waves in multi-layer media.Eventually,a Fabry-Pe?rot interference-enhanced multi-layer structure is proposed through experimentally comparing the visible transmittance and EMI SE of three different ultrathin metal-based structures,namely single-sided SLSO,laminated SLSO and double-sided SLSO(referred to as SLSO,SLSOs and D-SLSO,respectively).The D-SLSO structure shows the best performance,which is ascribed to the Fabry-Pe?rot interference cavity.The D-SLSO thin films fabricated on 200-?m flexible PET substrates exhibit an SE higher than 40 d B over 4?40 GHz and an absolute transmittance of 88.9%at 550 nm.The SE values can be further enhanced through varying the substrate thicknesses.The D-SLSO samples with 1.1-mm thick glass substrates have an average SE of 62.0 d B at 4?40 GHz(as high as 70 d B at 27.6 GHz).This work clarifies the underlying origin of the high visible transmittance and EMI SE in D-SLSO by the transmission theory of electromagnetic waves in multi-layer media and the equivalent interface theory.In addition,the potential applications of the D-SLSO structure in flexible optoelectronic devices are discussed.3.In order to further improve the EMI shielding performance and broaden the shielding bandwidth,the transparent EMI shielding materials with a multi-cavity Fabry-Pe?rot interference structure are proposed and realized.On the basis of the D-SLSO structure,the metallic mesh,as an interval shielding layer,is introduced,splitting the transparent dielectric layer into multiple spacers.The D-M₁ structure with two-cavity Fabry-Pe?rot interference is achieved by one inserted shielding layer,namely SLSO/substrate/metallic mesh/substrate/SLSO.Experimental results show that the D-M₁ sample with the two-cavity structure have an absolute transmittance of 84.4%at550 nm and an average SE of 84.6 d B(SE>80 d B in K band).Compared with the D-SLSO with a single-cavity,the D-M₁ structure shows an increase of?37%in the average SE.The origin of the enhancement of EMI SE and the broadening of shielding bandwidth is clarified by the finite element numerical calculations and the electromagnetic field transmission analysis.In this thesis,the ultrathin metal-based transparent EMI shielding materials are designed and fabricated,which have either a single or a multi-cavity Fabry-Pe?rot interference structure.The developed materials meet the main requirements of high visible transmittance,strong EMI SE,and ultra-broad shielding bandwidth.These findings provide the design strategy and the experimental approach for studies on novel transparent EMI shielding materials.