Research On Silicon Integrated Ferroelectric Thin Film Phase Shifters At Tera-hertz

Phased Array Radar（PAR）has the advantages of fast scan speed,small volume and light mass.The PAR in Tera Hertz（THz）have great potential in detecting through obstacle,high-resolution imaging,etc.Many groups around the world are developing various types of single chip radar system（SCRS）.Modern semiconductor technology,especially silicon based CMOS,is providing the opportunity to develop low cost SCRS.Last decade,THz generator and sensor have taken much attention,but few researches about THz preprocessor were reported.In PAR system,phase shifter（PS）is the largest demand preprocessor.However,existing phase shift technologies are difficult to meet the needs of THz SCRS development.Perovskite（Ba,Sr）TiO₃（BST）films,whose dielectric constant will decline when applying an electrostatic field,have great potential in the fabrication of PS.The BST films have the advantages of low leakage currents,fast response,easy integration,etc,so they are suitable for the THz PS.To fabricate THz PS on BST films,this research is done from three aspects: the BST films deposition,the PS design and fabrication,and graphene electrode research.The major contribution and innovations can be summarized as follows.（1）The Ba_0.5Sr_0.5TiO₃ films are deposited on the high resistivity silicon（hrSi）substrate.A postdeposition anneal（～750?C in O2）was used to improve the microwave characteristic of BST films,especially for the withstand voltage.In order to extract the high frequency permittivity of BST films,A conformal mapping（CM）accelerated threedimensional（3D）finite element method（FEM）model is proposed.The method extracts permittivity from the propagation-constant of the coplanar waveguides（CPWs）fabricated on the BST films.With this method,the BST films permittivity in the frequency range from 1 GHz to 110 GHz was extracted.Compared to existed method,the 3D FEM method would help to improve the accuracy of the results.（2）A left-hand（LH）transmissionline PS is designed and fabricated,which works in the frequency of 100 GHz,and has a bandwidth of 10 GHz.To reduce the insert loss,two type of low-loss improvement design are proposed and applied to the left-hand PS.The one fabricates the direct-current（DC）bias voltage electrodes with Cr,the other photoetches the BST films into small pieces to reduce the dielectric loss.The existed LH PSs work in the frequency range bellow 20 GHz,and their bandwidths are within 2 GHz.Compared with them,our design works in much higher frequency and wider bandwidth.Furthermore,the proposed low-loss designs have an insert loss of 5 dB and 7 dB,which are 3 dB and 1 dB lower than existed work respectively.（3）A wide bandwidth distributed capacitance PS is designed and fabricated.The measurement result shows that,the insert loss bellow 5.7 dB in the frequency from 75 GHz to 100 GHz.The existed distributed capacitance PSs work in the frequency bellow30 GHz,compared with which,our work have a great improvement in frequency and bandwidth.The two type of low-loss design methods,Cr DC electrodes and photoetched BST films,are applied in distributed capacitance PSs design,which would decline the insert loss to 2.5 dB and 1.9 dB respectively.（4）Because of the skin effect of metal conductor,the inset loss of PSs will rise up with the frequency,which would be a great challenge in improving the frequency of devices.Graphene has great potential in high frequency devices,especially up to THz,for its high electron transfer rate and not obvious skin effect.In order to reduce the conductor loss of PS,a Graphene/Au electrode structure is proposed in this paper.Multi-Layer Graphene films are grown with chemical vapor deposition,then successfully transferred onto the Au/Cr/hrSi substrate.After that,CPW transmissionlines made of Graphene/Au electrodes are fabricated to investigate the characterization of the proposed graphene electrodes.It is a meaningful exploration of transmission line with new material.