Design And Realization Of Terahertz Passive Devices Based On Silicon Carbide Substrate

Based on the enormous potential of the GaN-on-SiC active devices under the application scenarios of high frequency and high power,facing the realization of the terahertz monolithic integrated circuit,this paper designed and manufactured the passive devices in terahertz band on the SiC substrate.The size of the devices is shrunk because of the short wavelength in this band,and passive devices created by traditional methods are faced with the problem of big volume,difficult integration,low efficiency,and huge loss.On the other hand,the relevance of wireless communication in daily life is increasing,particularly the higher importance of portable,wearable and miniaturized wireless communication devices,which necessitates the high performance of the passive devices,as well as the size and integrability.Based on the foregoing,this study has conducted extensive research on the following topics:1.Developed and optimized the semiconductor passive devices manufacturing process,which is suitable for the terahertz band.Wide-bandgap semiconductors,such as silicon carbide(SiC)and gallium nitride(GaN),are particularly well suited for processing terahertz passive devices due to their low dielectric loss and high dielectric constant;on the other hand,in recent years,researches into semiconductor active devices have made significant progress,providing a technical basis for the promising realizing of terahertz monolithic integrated circuits(TMICs).However,wide-bandgap semiconductor processing technology is still in its infancy.As a result,the first part of this paper examines,analyzes,and optimizes the relevant processing technology in light of the characteristics of passive devices in the terahertz band and the needs of TMIC circuits.It contains the following aspects in particular:(1)Metal fabrication on the front side.For distinct application scenarios,two sets of deposition schemes are discussed,with comparative analysis and accompanying experimental optimization to reduce the roughness of the metal surface as much as possible,further reducing the metal loss of the passive devices.(2)Fabrication process of the largescale air bridges.Because the air bridge structure has significant promise in the implementation of passive devices in the terahertz frequency band,experimental explorations on the fabrication of large-scale air bridges in the terahertz frequency band have been carried out.(3)The reduction process of the semiconductor substrates.The terahertz passive devices have severe constraints on substrate thickness,necessitating high accuracy and low roughness reduction process on different kinds of substrates.Experiments on different semiconductor substrate materials,abrasives,and substrate bonding methods have proceeded,and a mature process scheme for the reduction of SiC substrates has been proposed,taking into account efficiency,accuracy,and flatness.(4)The etching of the deep via-holes.For terahertz passive devices,the shape and surface roughness of the via-holes are important factors affecting the devices’ performance.In processing the passive devices on the SiC substrates,the employment of bonding chemicals in the deep via-holes etching method causes the substrate temperature to rise during long-term etching,affecting the etching process.To address this issue,the cyclic etching method was first proposed to stabilize the substrate temperature,the parameters affecting the etching rate and surface roughness as well as the detailed etching mechanism were thoroughly investigated experimentally,and finally,an effective etching process was proposed.2.A variety of substrate integrated waveguide(SIW)cavity filters and a filter antenna are designed and fabricated,and a terahertz planar filter is creatively designed and compared with the cavity filters in the aspect of process and performance.(1)A terahertz integrated cavity filter was designed and fabricated based on cascade triplet topology and over-moded cavity,which achieved an insertion loss of 1.55 d B and a relative bandwidth of 9.7% at185 GHz,and introduced two transmission zeros near 201 GHz and 214 GHz,enhancing the out-of-band suppression characteristics of the filter.(2)A new structure named integrated coupling cavity is innovatively designed.The introduction of this structure can control the coupling between the cavities and the position of the transmission zeros at the same time.Based on this structure,a terahertz narrow-band filter is designed,by cascading with an overmoded cavity,a terahertz narrow-band band-pass filter is realized with three transmission zeros at both sides of the passband,and with an insertion loss of 5.6d B and a relative bandwidth of 1.5% at 195.6GHz.Besides,a terahertz filtering antenna is designed based on the above filter and air-bridge structure,which ensures the filter’s passband properties and achieves a maximum gain of roughly 6.4d Bi in the passband.(3)To simplify the fabrication process and reduce the cost,a terahertz coplanar waveguide(CPW)planar integrated filter on SiC substrate is realized.Although the performance is inferior to that of the cavity structure,it does not require via-hole etching or a reduction procedure,considerably simplifying the process flow and lowering the processing cost.The filter exhibits a center frequency of 279 GHz,with a relative bandwidth of 5.7%.Due to the increase of the radiation loss of a planar structure in the terahertz band,revealing the insertion loss of 5.1d B.These filters are all based on SiC substrate,compared with the widely used structures like CNC and MEMS,the volumes of these filters are greatly reduced.3.To complement semiconductor-based passive devices to some extent and to make a contract,this study designs and produces a low-temperature co-fired ceramic(LTCC)based band-pass filter and a filtering antenna.The filter antenna reaches a maximum gain of4.06 d Bi in the passband,while the band-pass filter achieves an insertion loss of 1.58 d B and a relative bandwidth of 2.8% at 97.6GHz.A dual-passband filter is also created and processed.The two passbands are at 93.9GHz and 105.8GHz,respectively,and the two passbands are separated by two transmission zeros,resulting in good passband isolation.The final measurement results are in good agreement with the simulation.Compared with the SiC based passive devices mentioned above,LTCC passive devices have a similar structure,which has the value of mutual reference in design.Meanwhile,due to the characteristics of multi-layer structure,LTCC passive devices have higher potential in the realization of complex structure devices such as filtering antennas.By analysis and comparison,the SiC and LTCC based passive devices have their respective advantages in different aspects,which provides more sufficient technical support for the selection of terahertz passive devices in different application scenarios.