Steven Rabe

Arrays of gated field‐emitter cones having 0.32 μm tip‐to‐tip spacing

We have reduced the gate voltage required to achieve a given emission current density in field‐emitter arrays by scaling down the gate‐to‐tip and tip‐to‐tip spacing to the unprecedented levels of 0.08 and 0.32 μm, respectively. The submicrometer features of our arrays are patterned using interferometric lithography. Electrical tests of arrays we have fabricated have shown a record low turn‐on voltage of 8 V for cesiated molybdenum emitters. Emission current densities of 1600 A/cm2 have been obtained, which is also a record for such structures. These arrays provide large advantages for applications such as flat panel displays and microwave devices.

MEMS microswitches for reconfigurable microwave circuitry

The performance is reported for a new microelectromechanical structure (MEMS) cantilever microswitch. We report on both dc- and capacitively-contacted microswitches. The dc-contacted microswitches have contact resistance of less than 1 /spl Omega/, and the RF loss of the switch up to 40 GHz in the closed position is 0.1-0.2 dB. Capacitively-contacted switches have an impedance ratio of 141:1 from the open to closed state and in the closed position have a series capacitance of 1.2 pF. The capacitively-contacted switches have been measured up to 40 GHz with S/sub 22/ less than -0.7 dB across the 5-40 GHz band.

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 matches 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.

MEMs microswitch arrays for reconfigurable distributed microwave components

A revolutionary device technology and circuit concept is introduced for a new class of reconfigurable microwave circuits and antennas. The underlying mechanism is a compact MEMs cantilever microswitch that is arrayed in two-dimensions. The switches have the ability to be individually actuated. By constructing distributed circuit components from an array, the individual addressability of the microswitch provides the means to reconfigure the circuit trace and, thus, provides the ability to either fine-tune or completely reconfigure the circuit elements behavior. Device performance can be reconfigured over a decade in bandwidth in the nominal frequency range of 1 to 100 GHz. In addition, other circuit-element attributes can be reconfigured such as instantaneous bandwidth, impedance, and polarization (for antennas). This will enable the development of next-generation communication, radar and surveillance systems with agility to reconfigure operation for diverse operating bands, modes, power levels, and waveforms.

Arrays Of Gated Field-emitter Cones Having 0.32-/spl mu/m Tip-to-tip Spacings

For many applications for gated field-emitter arrays it is highly desirable to reduce the gate voltage required to achieve a given amount of current per emitter. In rf devices, the lower gate voltage results in higher transconductance (gm), reduces ionization and breakdown effects, and allows the anode voltage to be set to lower values for lower power dissipation and heating in the anode. For flat-panel displays, it is highly desirable to use field-emitter arrays having a gate voltage below 30 V in order to be compatible with commonly available circuits and power sources.

High-density gated field-emitter arrays

We have developed a process based on laser interferometric lithography that can pattern an array of gated Mo cones having 0.32-/spl mu/m tip-tip spacings, 0.08-/spl mu/m gate-tip spacings, and tip radii of 5 nm. These are 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. Numerical simulations of our cathodes operated in a microtriode configuration have shown that values of unity-current-gain frequency (f/sub T/) above 10 GHz can be obtained using refractory emitter materials such as Mo and ZrC which have work functions between 3.0 and 5.0 V. Lowering the effective work function to 1.0 V or below by using surface treatments of Cs or similar materials is predicted to provide f/sub T/ values above 100 GHz. Experimentally, our gated cone arrays have demonstrated record turn-on voltages as low as 17 V (uncesiated) and 6 V (cesiated). Cesiated devices have shown an emission current density as high as 1600 A/cm/sup 2/ averaged over the array. The gate voltage at this current density was 40 V. In addition, RF modulation of the emission current in these devices has been demonstrated at 1 GHz, an important milestone in transitioning this technology to microwave operation. At present, we are collaborating with a number of other organizations under the Vacuum Microelectronics Initiative to realize an X-band klystrode using gated field-emitter cathodes. Progress in this effort will be described.