Experimental, Numerical, and Analytical Studies of a Staggered Double Vane Structure for THz

Application

10th International Vacuum Electronics Conference

(IVEC2009)April 28 - 30th 2009

(Presentation #: 1558344)

Young-Min Shin, Larry R. Barnett, Jinfeng Zhao, Diana Gamzina, and Neville C. Luhmann Jr.

Department of Applied Science, University of California-Davis (UCD), CA 95616, USA

17:00 Tuesday, Session 8 - THz

Supported by the DARPA HiFIVE program through a subcontract from Teledyne Scientific.

TIME / 31 March 2008 / 22

Overview

• Motivation and Goal

• Cathode and Electron Gun

• Circuit Design and Analysis

• Interaction Circuit Fabrication

- KMPR UV LIGA

- High Precision CNC Machining

• Cold-Test Setup

• Summary and Future Plans

TIME / 31 March 2008 / 33

220 GHz Sheet Beam TWT

- Vacuum Electronic Circuit ElementsA. High current density, long-life cathode (UCD and Teledyne Scientific)

Tungsten-Scandate Nanopowder cathodes capable of 80 A/cm2

B. High aspect-ratio electron beam (CPI and Teledyne-MEC)Elliptical cathode with beam compression produces 7:1 aspect

ratio beam for SBTWT, 25:1 aspect ratio beam for beam-stickC. High efficiency interaction structure (UCD and Teledyne Scientific)

Novel staggered vane interaction structure at 220 GHz shows 50 GHz bandwidth, 13 dB/cm gain, and 100 W output

D. High power MMIC driver (Teledyne Scientific and UCSB)InP based solid state amplifier 50 mW output at 220 GHz

E. High efficiency thermal management (Teledyne Scientific)Integrated cooling channels around MEMS interaction structure

• MEMS-Integrated High Power Vacuum Amplifier for THz Communication and Sensing Systems

TIME / 31 March 2008 / 44

Nano-Composite Cathode

1150oC for 800 hrs (33 days)UCD Cathode: 80A/cm2 fully space charge limited

• Current Density versus Cathode Button Voltage

After Furnace Sinter: Grain size in matrix is 272 nm and very uniform

• Scandate Nano-Composite (Sc2O3-W) Cathode

Nanograin(100-200 nm)

tungsten

+

Osmium / Rhenium and Scandium /

Yttrium Oxide Cryogenic Milling: incorporation of emission enhancing compounds

Spark Plasma Sintering

Current:-3 kA-Pressure:--3 kN- 50 kNAtmosphere:--Vacuum

Current:-3 kA-Pressure:--3 kN- 50 kNAtmosphere:--Vacuum

Current:-3 kA-Pressure:--3 kN- 50 kNAtmosphere:--Vacuum

Current:-3 kA-Pressure:--3 kN- 50 kNAtmosphere:--Vacuum

Current:-3 kA-Pressure:--3 kN- 50 kNAtmosphere:--Vacuum

Current:-3 kA-Pressure:--3 kN- 50 kNAtmosphere:--Vacuum

Current:-3 kA-Pressure:--3 kN- 50 kNAtmosphere:--Vacuum

Current:-3 kA-Pressure:--3 kN- 50 kNAtmosphere:--Vacuum

Sol-gel Method

1000 100001

10

100

7543

80

1150o

C-Cathode #3

1150o

C-Cathode #2-311X

400

Cur

rent

den

sity

(A/c

m2 )

Voltage (V)

604030

20

2000 4000

2

Cathode #2---Spectra-Mat 311X

Cathode #3---UCD powder+Spectra-Mat recipe

1000 100001

10

100

7543

80

1150o

C-Cathode #3

1150o

C-Cathode #3

1100o

C-Cathode #3

1100o

C-Cathode #3

1150o

C-Cathode #2-311X

1150o

C-Cathode #2-311X

1100o

C-Cathode #2-311X

1100o

C-Cathode #2-311X

400

Cur

rent

den

sity

(A/c

m2 )

Voltage (V)

604030

20

2000 4000

2

Cathode #2---Spectra-Mat 311XCathode #3---UCD powder+Spectra-Mat recipe

EDX Analysis on the SpectraMat 311X Surface

See Zhao et al., Session 23 - Cathodes II

TIME / 31 March 2008 / 55

Cathode Testing at UC Davis

High Current Density Operation of Spectra-Mat: 311X

• At 1272 °Cb the emission at 50 A/cm2 is

still only about 70% SCL • Even at 30 A/cm2 it takes at least  1250 °Cb to reach ~90% SCL

2000 3000 4000 5000 6000 7000

10

100

80

605040

30

Cur

rent

den

sity

(A/c

m2 )

Voltage (V)

1100oC 1150oC 1180oC 1203oC 1252oC 1273oC

20

1500 3000 4500 6000

10

20

30

40

506070

Cathode #1 (UCD pellet and Procedure+Impregnated by Spectra-Mat)

Cur

rent

den

sity

(A/c

m2 )

Voltage (V)

1050oC 1000oC 950oC

UC Davis Cathode

TIME / 31 March 2008 / 66

Cathode Testing at UC Davis

Cathode TestingRapid button test

Multiple rapid cathode life test facility

New High PerveanceCathode Life Test Vehicle

System operational

Three 3.0 P CLTVs completed

Cathode testing and life testing underway at UC Davis: eight vehicles operational with another four nearing completion

G. Scheitrum and A. Hasse

TIME / 31 March 2008 / 77

Overview of Interaction Structure

Machined WSBK Circuit

Circuit Dimensions:Beam Tunnel (b x h) 770 x 150 µmVane Period (d) 460 µmVane Height (L) 270 µmVane Thickness (d-a) 115 µmVane Width (h) 770 µm

Approach: Staggered double vane structure provides large bandwidth and good coupling to sheet electron beam

Beam Size: 700 μm × 100 μm (7 : 1), 400 A/cm2

TIME / 31 March 2008 / 88

Overview of Interaction Structure

Comparison of double vane structures

TIME / 31 March 2008 / 99

Circuit Design and Analysis

- Operating Conditions Beam Voltage (Ve)= 20 kV Beam current (Ie) = 0.25 A Center frequency (fc) = 220 GHz Opt. phase-shift () = 2.5 (n = 1)

- Dimensional Parameters L = 270 m h = 770 m b = 150 m d = 460 m

- Bandwidth (Cold) ~ 70 GHz (30%)

* Larry R. Barnett and Young-Min Shin, US Patent Application No. 60979392, Oct. 12 (2007)

• 3D Model

Transmission Loss :~ - 0.6dB (avg.) Attenuation: ~ 0.15dB/cm (avg.)

• Dispersion and Transmission Graphs

• MAGIC3D PIC Simulation Analysis

150 ~ 275W

1) Larry R. Barnett and Young-Min Shin, US Patent Application No. 60979392, Oct. 12 (2007) 2) Young-Min Shin and Larry R. Barnett, Appl. Phys. Lett. 92, 091501 (2008). 3) Young-Min Shin, Larry R. Barnett, and Neville C. Luhmann Jr., Appl. Phys. Lett. 93, 221503 (2008)4) Young-Min Shin, Larry R. Barnett, and Neville C. Luhmann Jr., IEEE Trans. Elec. Dev. (in press), (May. 2009)

TIME / 31 March 2008 / 1010

Broadband Coupler

• Tapered Transition (Vane Width)

• Ka-band circuits

TIME / 31 March 2008 / 1111

MAGIC3D: Gain and Stability

Growth Rate ~ 14dB/cm

30dB

TIME / 31 March 2008 / 1212

Sensitivity Studies• Misalignment Effect

dy dz

• Off-Centered Beam Effect

x

yz

x

yz

TIME / 31 March 2008 / 1313

KMPR UV LIGA Process

• MEMS Facility• UV-LIGANorthern California Nanotechnology Center (NCNC) @ UCD

TIME / 31 March 2008 / 1414

UV LIGA Technical Issues

• Underexposure

• Overexposure

• Non-Uniform Resist Thickness

wrinkles

• Heavy table that can absorb vibrations and can be leveled

• Stone hotplate with low thermal expansion coefficient that can also be leveled accurately

• Lapped and polished copper substrate with thickness deviation of less than 1 µm over the circuit area

Results: Film Thickness Uniformity Improvement(1) Over the 4” wafer : 5 m(2) Over the circuit area (25 × 25 mm) : ~ 1 - 2 m(3) Lapped and polished to within 0.25 µm(4) Less than 1 µm undercut after mold removal

TIME / 31 March 2008 / 1515

Surface Roughness Analysis of Resistivity versus S21 and S11

• 10 mm long BS circuit model

• Transmission (S21) Graph

• Reflection (S11) Graph • Resistivity versus S21/S11

Only 1 dB in S21 and S11 is changed by up to 5 × Resistivity (OHFC) Interaction circuit is insensitive to surface conditions (roughness)

TIME / 31 March 2008 / 1616

Sensitivity Analysis of Fabrication Errors

• Sidewall Slope versus Frequency Deviation

• Undercut versus Frequency Deviation

θ

Undercut ( 40 m) Frequency Deviation ( 2 %) Undercut size of UV LIGA mold is much smaller : ~ 10 – 20 m

Sidewall Slope (> 2) Frequency Deviation (> 2 %) Vertical sidewall of less than 2 can be controlled by common photo-lithography process.

TIME / 31 March 2008 / 1717

Process Optimization

- Before Softbake - Right After Heating - After 4.5 hr Soft-bake

* 1 fringe = 0.25 m

• System Setup for Process Optimization

- Autocollimator

(Leveling)

- Large Hotplate(Rapid Production)

- Copper Substrate Flatness Analysis

• UV Lithography • Mold Removal

6 wafers/cycle Thickness Uniformity : 1 m over the circuit (25 mm)

Dimensional accuracy : 3 m, sidewall slope: 90 ± 2

Mold removal efficiency: 90 %

TIME / 31 March 2008 / 1818

Completed Circuit and Analysis

• UV LIGA Fabricated BS Circuit

- AFM-Measured Surface Roughness

• Line Resolution and Surface Roughness

- Line Resolution Pattern Images

Aspect ratio ~ 10 : 1

Surface Roughness ~ 50 - 100 m

Young-Min Shin, Diana Gamzina, Larry R. Barnett, and Neville C. Luhmann Jr. “UV Lithography and Molding Fabrication of Ultra-Thick Micrometallic Structure using a KMPR Photoresist ”, IEEE J. Microelec. Micro. Sys. (MEMS) (submitted, 2009)

TIME / 31 March 2008 / 1919

Required Surface Roughness

• Frequency versus Required Skin Depth for Device Operation

220GHz (~0.14 µm)

(2)

(3)

(1) LIGA-(Cu) : 70 nm(2) DRIE-(plated Si) : 100 nm(3) LIGA-(PMMA Mold) : 20 nm

TIME / 31 March 2008 / 2020

Summary and Future Plans

• Accomplishments– Sc2O3-W Nano-Powder Cathode A. Maximum Emission Current Density: ~ 80 A/cm2 at 1150 oC (Full Space Charge

Limited) B. Life-Time: 768 hrs (950 oC), 800 hrs (1150 oC) C. ~ 50 A/cm2 at 1050 oC – Sheet Beam Gun Design (CPI and Teledyne-MEC)– Circuit Design and Analysis A. Operational Bandwidth : 70 GHz (30%) @ 220 GHz B. Output Coupling Bandwidth: 66 GHz (88%) C. MAGIC3D Simulation: Power Growth Rate (14 dB/cm), Max. Efficiency (5%)– MEMS Fabrication A. UV LIGA : Critical Dimensions ( 5 m), Surface Roughness (~ 50-100 nm),

Aspect Ratio (~ 10 : 1) – CNC Fabrication

• Research Schedule– 750 A/cm2 and 25 : 1 Sheet Beam Transport Test – 220 GHz Circuit Cold-Test