Development Of Flexible Microwave Integrated Circuits

Flexible microwave integrated circuits（fMICs）have the advantages of lightweight,bendability and confomability,which is a hot spots research direction in the wireless electronics.It is widely used in the fields of wearable devices,telemedicine and biotechnology.Although the microwave hybrid integrated circuits（ICs）based on silicon（Si）,gallium arsenide（GaAs）,gallium nitride（GaN）and other traditional compound semiconductor devices and hard substrate has been very mature,the mechanical performance of the traditional semiconductor devices is poor,which limits the further development of microwave flexible electronic.In recent years,the development of flexible microwave substrates and atomic-layer-thick low dimensional carbon materials has brought new opportunities for the f MICs.This dissertation aims to improve the mechanical flexibility and operation frequency of f MICs.Based on the theory of semiconductor devices and micro-machining technology,active transistors using low dimensional carbon materials（graphene and carbon nanotubes）and microwave passive devices that can withstand large tensile strains are studied.The main studying contents include:1.High oscillating frequency graphene based flexible microwave transistor.At present,the transfer method of graphene films mainly relies on the organics polymethylmethacrylate（PMMA）films.However,the graphene will be easily contaminated by the PMMA films during the transfer process.In this dissertation,a transfer method of graphene using Au nanofilms is proposed.First of all,a 30 nm thick Au nanofilm is first sputtered on the surface of graphene,then transfer the graphene films to the flexible substrate surface to avoid the direct contact between the graphene surface and PMMA.Then the parasitic resistance of the graphene-based transistors is improved,and the working frequency of the graphene-based transistors is increased.The measured results show that the maximum oscillation frequency(f_max)of the transistor is up to 30GHz,which is more than 50% higher than the traditional PMMA transfer preparation method.The test results show that the f_max of the transistor is still better than 15 GHz under the tensile strain of 2%.The graphene based flexible transistors have been used in the design of X-band flexible graphene microwave low noise amplifier,which effectively improves its operating frequency.2.High oscillating frequency carbon nanotube based flexible microwave transistor.Flexible substrates are usually unable to withstand high-temperature annealing.In this dissertation,a high-temperature annealing technology suitable for flexible carbon nanotube field effect transistors（CNT-FET）is proposed.First of all,high purity（99.9%）semiconducting CNT films are deposited on silicon（Si）substrates,then annealed at 900℃on Si substrates.Finally,the annealed CNT films are transferred to flexible substrates and finish the fabrication of transistors.The results show that the effective carrier mobility(μ_eff)of the transistor is up to 60 cm²/（V.s）,which is about 33%higher than that of the traditional unannealed preparation method.The f_max of the transistor exceeds 1 GHz for the first time,and the maximum f_max even reaches 2.1 GHz.Furthermore,bending test results show that the f_max of the transistor is still better than 1.1 GHz under the tensile strain of 2%,which provides a research direction for the subsequent CNT-based f MICs.3.Highly flexible microwave filter.Considering that the traditional glass fiber and other microwave composite substrates show thick thickness and poor tensile strength,flexible microwave filters based on 50μm ultra-thin liquid crystal polymer（LCP）are developed in this dissertation.Due to the narrow transmission line of ultra-thin LCP substrate,it is difficult to realize the coupling element with high coupling coefficient with typical PCB process.Therefore,an electromagnetic and mechanical coupling design method based on the stepped impedance resonator element and bilateral coupling structure is proposed to reduce the influence of deformation on the electromagnetic characteristics and increase the maximum tensile strain of the device.The experimental results show that the insertion loss of the 0-8 GHz low-pass filter is better than 1.2 dB and return loss is better better than 15 dB;the insertion loss of the 9.3-9.6GHz flexible bandpass filter is better than 1.8 dB,and the return loss is better than 12 dB;the insertion loss of 2-18 GHz UWB filter is better than 2.8 dB,and the in-band return loss is better than 12 dB.Furthermore,bending tests are carried out.The results show that the maximum tensile strain of the LCP filter in this dissertation can be up to 4.2%under the condition of constant RF performance,which is more than 4 times higher than that of the traditional 100μm thick LCP filter,and provides a solution for the highly flexible microwave circuits.4.Stretchable microwave integrated inductor and filter.To solve the problem that fMICs can only be bended but not stretched,a novel method of fabricating stretchable microwave integrated circuitss is proposed.In this method,a polymide layer（PI）layer is firstly spin-coated on the surface of Si substrate,and then flexible microwave devices are fabricated on the PI layer.Finally,the devices are transferred to stretchable substrates.Based on this method,a double-helix stretchable microwave inductor is proposed.The Q value of the stretchable microwave inductor is increased,and the influence of deformation on the high frequency electromagnetic characteristics of the inductor is reduced.The tested results show that the maximum tensile yield of the inductor is up to 20% at Q value～12.6 and self-resonant frequency f _SRF～11.6 GHz.Finally,a 1.5-2.6 GHz stretchable bandpass filter is designed based on the double-helix inductor.The filter’s return loss is better than 12 dB,the lowest insertion loss is about 2.3 dB,and it can work normally at 20%elongation and on pork skin,which provides an important foundation for stretchable wireless technology in the future.