Design And Fabrication Of Passive Devices Working At Terahertz Frequency

With the development of millimeter wave and terahertz wave technology,the requirements of passive devices in the communication system are getting much stronger.Due to the fabrication limitation and the lossy characteristics of electromagnetic wave,passive devices such as antennas,resonators,and filters demonstrate drastically degraded performance beyond 100 GHz.In this paper,various kinds of terahertz structures such as substrate integrated waveguide(SIW),three-dimensional substrate integrated circular waveguide(3D-SICW)and so on are achieved by adopting innovative fabrication technology.The main work of this paper includes the following aspects:1.Design and optimization the fabrication technology of the wide bandgap semiconductor,to make it better meet the requirements of the terahertz and millimeter wave passive devices.Gallium nitride(GaN),silicon carbide(SiC)and other wide bandgap semiconductor materials are very suitable for the millimeter-wave and terahertz applications because of the charactristics such as wide bandgap and low dielectric loss.However,the fabrication technology based on wide bandgap semiconductors is not mature,especially for the terahertz frequency application.Therefore,based on the demanding of terahertz wave passive device,the fabrication technology of the wide bandgap semiconductor is first optimized and improved.The improvements are mainly as follows: 1.Optimization of the process flow,i.e.,the process steps are designed and optimized to be compatible with the existing process for integrated circuits,which reduce the fabrication difficulty and the cost.2.Metal roughness control,i.e.,by optimizing the electroplating process,the metal roughness is significantly reduced,which improve the loss characteristic of passive devices;3.Thinning process optimization,i.e.,the uniformity of the thinned substrate is enhanced by the process optimization,which improve the manufacturing accuracy;4.Etching mask optimization,i.e.,by adjusting the sputtering,electroplating parameters,the adhesion of the etching mask is increased resulting the reduction of the error rate in the subsequent etching process;5.The optimization of the etching process,i.e.,via-holes with different diameter,hole distance,and shapes are achieved by optimizing the etching parameters.The other parameters of the via-hole,such as the morphology,sidewall roughness and the inclination angle are also optimized by adjusting the relevant etching parameters;6.Metallization process optimization,i.e.,by adjusting the sputtering parameters,the metallization effect of the via-holes is well enhanced.Ultimately,the feasibility of the wide bandgap semiconductor based millimeter and terahertz wave passive devices is significantly enhanced by adopting the optimized fabrication technology.2.Based on the optimized fabrication technology of wide bandgap semiconductor,various kinds of passive devices such as SiC base SIW transmission line,grounded coplanar waveguide(GCPW)transmission line,air bridge structural coplanar waveguide transmission line,resonant stub loaded SIW filter and air bridge structural yagi antenna are designed and fabricated.(1)By optimizing the mechanical strength and etching parameters of the substrate,the terahertz SIW transmission line based on silicon carbide material is achieved for the first time.The loss of transmission line at 220 GHz is about 0.45 dB / mm.(2)By adopting the SIW concept,the terahertz GCPW transmission line with metailized via-holes on both sides is achieved.Variety of parasitic modes in the traditional GCPW transmission line is well suppressed in the new structure,and the loss of novel GCPW transmission line at 220 GHz is about 1.5 dB/mm.(3)Design and realize the resonant stub loaded filter based on SIW cavity.The filter adopts the source/load multi-resonator coupling technology and the higher mode coupling technology,which generate two transmission zeros without increasing the insertion loss.The insertion loss of the filter is less than 1 dB at 183 GHz.(4)Design of the air bridged GCPW transmission line with controllable impedance.The impedance of the transmission line can be effectively controlled without changing the structure of the air bridge which reduce the difficulty in fabrication.(5)A planar Yagi antenna based on the air bridge GCPW is designed.The working bandwidth of antenna is more than 100 GHz and the radiation efficiency is more than 80%.3.Based on the low temperature co-fired ceramic(LTCC)technology,3D structural terahertz filtering antenna and filter are achieved.Benefiting both from the reduction of terahertz wave wavelength,and the multi-layer realizability of the LTCC technology,the three-dimensional structure becomes feasible.Different from the traditional planar structure(the energy transmission along the horizontal direction),the 3D structure can make the energy flow in the vertical direction,which means the the effective area of the circuit is increased.The quality factor of the 3D-SICW based resonant cavity is 30% higher than that of SIW based cavity.3D-SICW based Filter and filtering antenna operating around 180 GHz are designed.The insertion loss of the filter is about 1.9 dB at 174 GHz and the relative bandwidth is about 13%.The filtering antenna also has a good match around 180 GHz,achieving the filtering and radiation functions at the same time.The measurement is in good agreement with simulation.