Research Of Key Technologies About The Applications Of CVD Diamond On TWT And MPM

As a new kind of microwave power modules,MPM is a high density compactmicrowave amplifier which includes a mini-traveling wave tube (TWT),a solid-statedriver amplifier (SSA),and a high density electronic power conditioner (EPC).MPM isa very important part in a next-generation electronic military system,and the millimeterwave power module (MMPM) will be well developed.For the development of widerban d width,high efficiency,high power,reliability and long lifetime of microwavetube,it is important to improve the thermal performance of the mireowave tube.Thethermal management under high temperature affects the reliability of tube and poor heatmanapement will decrease the output power and lifetime and limit the development ofhigh power and small size tube.Using an ideal dielectric material with high thermalconductivity is an important means to improve the performance of the microwave tube.The ideal dielectric materials possess low dielectric losses,high mechanical strength,high thermal conductivity,low thermal expansion,and a low dielectric constant.It is recognized diamond is one of the ideal dielectric materials.It should be usedin the wide-band high power TWT and MMCM to improve the thermal managementgreatly.In the thesis,the thermal dissipation of CVD diamond for vacuum electronicdevices (such as TWT helix rod,power windows and MMCM) has been analyzed.The main results are as follows:1.The project supported by government is to develop a CVD diamond thick filmand possible technology which can be taken over the traditional BeO rod and used in thewide-band high power TWT to improve the thermal management greatly.Havingexperienced over three years' efforts,we have successfully found a good way tomanufacture the right rectangular diamond rod by CVD,special Laser cutting andpolishing technology.After material tests and evaluation,parts assembly and tests andfinal TWT assembly and tests,we have got the first good results as follows:Thermalconductivity:κ＞12 W/cm.℃,ten times better than BeO Rod;Dielectrical constant:ε=4.75～5.0;RF loss:tgδ＜1×10^-2;Resistivity:ρ＞10¹³Î©/cm;Slow wave partsassembly thermal test:3.0 times better than BeO.The first TWT with diamond rods has passed all cold tests and has got following hot test results:Δf=4～8GHz,P_out=95W,G=35dB.2.Diamond and diamond-like films are excellent field emission materials.Theyare strong candidates for field emission micro-cathodes applied in microwave tubes,because of their low or even negative electron affinity and chemical inertness.Undopeddiamond film on Cu and Si has been successfully fabricated using microwave plasmachemical vapor deposition (MPCVD) technology.Electron emission testing indicateslow turn-on voltages.Current densities from 0.5mA/cm2 to 100mA/cm2 had beenobtained for applied fields of 2-3.5V/μm.The bright light spot had been observed,while emitted electron bombardment fluorescent screen.The application is immaturenow,but their potential is very big.3.CVD diamond films is one of the ideal sink material of 3D-MCM,which isapplicable to the next generation high power MPM and high speed computers.Thisproject has innovative contribution in the following aspects:(1) Discuss the currentstatus of diamond films applied in microwave tubes.Study the merit,operating,physicaldesign and application possibility for microwave power modules and 3D-MCM.(2)Being based on actual multi-chips module packaging structure a three dimensionalthermal analysis model is built,thermal analysis on multi-chips module is conductedusing diamond substrate and thermal interfacial materials;2D and 3Dchip-adhesive-substrate thermal stress model are built,and interfacial thermal stressdistributions are computed based on different area ratios and thickness ratios ofsubstrate to chip.A stacked MCM model was developed,computational fluiddynamics solution was utilized to proceed the thermal dissipation analysis of thestacked MCM with CVD-diamond substrates,to simulate the heat transfer process andthe temperature distribution of the MCM under forced air-cooling conditions,and theeffects of different factors on the 3D-MCM temperature distribution are investigated,the affecting parameters include thermophysical properties and design structuralproperties.Although CVD diamond is one of the ideal dielectric materials,and high-qualitydiamond films have been used in some microwave tubes,there is still a gap to itscommercial application.