Gated field-emitter arrays for microwave-tube applications

As reported in the past, we have developed a process based on laser-interferometric lithography that can pattern an array of gated molybdenum cones having 0.32-/spl mu/m tip-tip and 0.08-/spl mu/m gate-tip spacings. These remain the smallest dimensions and highest density for such cone arrays reported. This geometry offers substantial advantages for achieving lower gate operating voltages, higher transconductance, and higher frequency operation. We are now working as part of a team led by Varian to apply this and related technology in a practical demonstration of a 10-GHz klystrode. Our first major activity in this area has involved the re-engineering of our emitter fabrication process to make it more robust. Gate-film thicknesses have been more than doubled to 700 /spl Aring/ to minimize fragility and lower the RF parasitic gate resistance. It appears this may also reduce the severity of burnout phenomena in these devices. Comprehensive reliability tests have yet to be performed: however, devices have been baked at 400/spl deg/C in a pressure of 10/sup -5/ Torr of H/sub 2/ for 37 hours with no ill effects. To meet the klystrode cathode requirements, we have designed an emitter array comprising four 30-/spl mu/m-long by 240-/spl mu/m-wide annular array segments that are disposed around a 0.024-in.-diameter circle. This structure is intended to deliver 160 mA at a tip loading of 0.57 /spl mu/A/tip. Phase-delay and attenuation effects in the gate and space-charge limitations in the emission current have all been taken into account in the design of these devices and are not expected to present a problem. We have also contributed heavily to the design of the packaging and matching scheme for the field-emitter cathode in the klystrode electron gun. A chip-carrier concept is used to ease the assembly/disassembly, and the microwave matching is done via microstrip circuits. General design of the matching circuits is discussed, and cases for both conjugate and resonant matc- es are shown for the Lincoln array. Provided that the necessary emitter yield, reliability, and emission current can be obtained, klystrode operation having useful output powers and reasonable gains at 10 GHz should be achievable.