DESIGN AND FABRICATION OF A THz NANOKLYSTRON Harish M. Manohara, - - PowerPoint PPT Presentation

DESIGN AND FABRICATION OF A THz NANOKLYSTRON

Harish M. Manohara, Peter H. Siegel, Colleen Marrese

Jet Propulsion Laboratory California institute of Technology 4800 Oak Grove Drive Pasadena, CA 91109

Jimmy Xu, Baohe Chang

Brown University Division of Engineering Providence, RI

OVERALL OBJECTIVE Develop a milliwatt level, fixed frequency, CW THz source for space borne Earth and planetary remote sensing instruments IMPLEMENTATION Extend vacuum tube reflex klystron oscillator to THz frequencies.

TECHNICAL APPROACH

Analyze millimeter-wave klystron performance limitations Design THz monolithic circuit based on silicon DRIE process Propose compatible cavity, bunching grid, repeller, output structure Realize ultra-high current density field-emission cathode Incorporate built-in low-voltage emitter/focusing grid with cathode Combine drop-in cathode/grid with cavity/output coupler Develop high vacuum sealing technique compatible with RF output Increase power output or frequency agility through array integration

SCHEMATIC OF A SIMPLE REFLEX KLYSTRON

Heater Bunching Grids Re-entrant Cavity Electrostatic Focus Repeller Cathode Output Coupler Beam Coupling hole AC Field B Accelerator

MODIFICATIONS NEEDED TO REALIZE THZ MONOLITHIC DESIGN

Physical layout must be made compatible with standard MEMS processing Including emitter, re-entrant cavity, focusing electrodes, repeller, output coupler, beam forming antenna Split block construction required to allow sculpting of cavities and insertion

• f wires, focusing electrodes, emitter, repeller

Tuning & output Q controllable via simply varied geometric parameters Current densities of existing hot cathodes must be increased dramatically

MODIFICATIONS NEEDED TO REALIZE THZ MONOLITHIC DESIGN

Cold cathode operation preferred for space operation and reduced thermal load Cold cathode operation implies integrated emitter grids and extra beam focus Vacuum sealing techniques/window compatible with low RF output loss Early design flexibility needed to allow some trial and error testing Detailed analysis of full circuit and RF beam interactions essential

SCHEMATIC CONSTRUCTION WITH REALIZED STRUCTURES

Vacuum sealed split block Emitter & Focusing grids 10 µm Reflector V Silicon host wafer Cathode Silicon host wafer Cathode Silicon wafer - bottom Resonator cavity Cathode Shaped Repeller Output waveguide and transformer Cold Cathode Bunching grids Dielectric seal Beam V Reflector V Integral grid Grid V Beam Focus Silicon host wafer - top Nanotube or Spindt cathode

0.05 µm

Brown Univ. highly ordered carbon nanotube array (cathode) Li. et.al. APL 75, no.3, Jul 19, 1999

Silicon micromachined cavity (JPL) Micromachined emitter grid (JPL)

SIMPLIFIED BEAM ANALYSIS FROM J.J. HAMILTON (1958)

dgap Reduced height

• utput guide

Iris-coupled Cylindrical Re-entrant cavity h ra rb a x b rhole RR CP LP n:1 Z0 Resonator equiv. circuit Coupling iris cavity waveguide

1200 GHz Example With 500V beam, 3mA current: 52 mW produced by beam, 49 mW lost in cavity, 3 mW delivered to output load

1200 GHZ RIDGED-WAVEGUIDE RE-ENTRANT CAVITY ANALYSIS FOR NANOKLYSTRON USING QUICKWAVE FDTD

Ez excitation at gap 1200 GHz

S11 with Coaxial gap excitation

FIELD DISTRIBUTIONS

Ez in Grid Gap and transformer Ez along center line

FABRICATION OF 640 GHZ CIRCUIT USING PRECISION METAL MACHINING

35 µm

640 GHz Nanoklystron fabricated using precision machining in metal split block. The smallest feature is the 0.0015” diameter bunching grid

• hole. The assembled unit with an output waveguide horn is shown on

the right.

SILICON DEEP REACTIVE ION ETCH WAFER PROCESSING STEPS

Si wafer w/ SJR 5740 ~ 1.5 mm thick First DRIE step Second DRIE step After several similar DRIE steps… Finished bottom wafer Similarly, finished top wafer

Top view Side view

1. 2. 3. 4. 5. 6.

Top view Side view Top view Side view Top view Side view Top Wafer Side view Bottom Wafer Side view

Backside DRIE to create feed through holes, wafer bonding & finally, dicing to produce finished device (side views shown here)

CUT-VIEW OF A WAFER BONDED NANOKLYSTRON (A MODEL)

Step Waveguide Transformer Resonating Cavity Electron beam coupling Hole Repeller Hole Top Wafer Bottom Wafer

1st ITERATION MONOLITHIC NANOKLYSTRON CAVITY [1200 GHz cavity split into two halves]

Top half micromachined in silicon showing a repeller hole Bottom half of in silicon showing an emitter hole and a 5-step waveguide transformer terminating in a silicon window

BONDED WAFER HALVES WITH CAVITY CUTAWAY

Bonded Region Wafer bonded cavity and a magnified view of the bonded interface showing fused gold layers of the top and the bottom halves

DEVELOPMENT OF COLD EMITTER CATHODES

Electron source for nanoklystrons must be capable of generating current densities of at least 1000 A/cm2 at low operating voltages. Such current densities can be generated by employing cold cathodes, especially carbon nanotube-based field emitters. The small diameter of carbon nanotubes (diameter of a single single-walled- nanotube can be <1 nm) enables efficient emission at low fields, despite their relatively high work function (>4.5eV). At 1-3 V/µm of threshold voltage, carbon nanotubes are the best suited for low-power, high-current density applications.

Efforts are underway to develop flat bed of grid-integrated ordered arrays of carbon nanotubes and tailor their field emission to suit nanoklystron applications.

ORDERED ARRAYS OF CARBON NANOTUBES FOR THE FIRST TIME GROWN ON Al-DEPOSITED Si-WAFER

Nanotubes exposed after ion-milling the anodized pores of alumina Tube diameter is typically 40 nm with a density of ~100 tips/µm2

FIELD EMISSION MEASUREMENTS

A A A Tube anode Grid Deflector Cathode 10 kΩ 10 kΩ UHV Chamber ~ 7 × 10-9 Torr

Electron beam TO 5 Base Nanotubes Conductive glue + ve bias

Micromachined grid with nanotubes for field emission measurement Distance from top of the sample to anode is 2 mm vertically and 5 mm horizontally.

SILICON MICROMACHINED GRID STRUCTURES WITH INSULATING PHOTORESIST SPACER FOR MICRON SEPARATION

Au-Contact PR spacer Au-mesh Emission hole made using DRIE TO 5 Base

Assembly for field emission measurements

ORDERED CNT ARRAY EMISSION MEASUREMENT

50 100 150 200 250 300 1E-9 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3

Data from trials 2, 3 and 4 Data from trial 1

UHV = 7 x 10

• 9 Torr

NASA - JPL

Emission Current VS. Grid Bias (Apr 23, 2001) Data For Trials 1,2,3 & 4 Grid Current, Ig (A) Grid Bias, Vg (V)

Grid area=0.0078 cm2 #tips=100/µm2=1010/cm2

• Equiv. Current

density=.01A/cm2 Typical current/tip=300nA Estimated number emitters=300 for 100µA Number of tips total=7.8*108

NEW NANOKLYSTRON AND EMISSION TEST CHAMBER

Load Lock UHV compatible XYZ-Manipulator Ion Pump Quartz Window Test Chamber Stand

SUMMARY

Design concept, circuit layout & simple analysis of a 1200 GHz

nanoklystron presented

New style ridged waveguide re-entrant cavity designed and analyzed Simple cathode/grid field emission tests performed in existing

chambers.

New assembly/measurement chamber being built. Close-in cold cathode emitter grid developed for carbon nanotube

arrays

Copper 640 GHz nanoklystron cavity completed. First iteration silicon monolithic 300/600/1200 GHz nanoklystron cavities

• completed. Wafer bonding tests successful.