Experimental, Numerical, and Analytical Studies of a Staggered Double Vane Structure for THz Application 10 th International Vacuum Electronics Conference (IVEC2009) April th th International Vacuum Electronics Conference (IVEC2009) April th 2009 (Presentation #: ) 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 / 2 2 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 / GHz Sheet Beam TWT - Vacuum Electronic Circuit Elements A.High current density, long-life cathode (UCD and Teledyne Scientific) Tungsten-Scandate Nanopowder cathodes capable of 80 A/cm 2 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-stick C.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 / 4 4 Nano-Composite Cathode 1150 o C for 800 hrs (33 days) UCD Cathode: 80A/cm 2 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 (Sc 2 O 3 -W) Cathode Sol-gel Method EDX Analysis on the SpectraMat 311X Surface See Zhao et al., Session 23 - Cathodes II

TIME / 31 March 2008 / 5 5 Cathode Testing at UC Davis High Current Density Operation of Spectra-Mat: 311X At 1272 °C b the emission at 50 A/cm 2 is still only about 70% SCL Even at 30 A/cm 2 it takes at least 1250 °C b to reach ~90% SCL UC Davis Cathode

TIME / 31 March 2008 / 6 6 Cathode Testing at UC Davis Cathode Testing Rapid button test Multiple rapid cathode life test facility New High Perveance Cathode 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 / 7 7 Overview of Interaction Structure Machined WSBK Circuit Circuit Dimensions: Beam Tunnel (b x h)770 x 150 µm Vane Period (d)460 µm Vane Height (L)270 µm Vane Thickness (d-a)115 µm Vane 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/cm 2

TIME / 31 March 2008 / 8 8 Overview of Interaction Structure Comparison of double vane structures

TIME / 31 March 2008 / 9 9 Circuit Design and Analysis - Operating Conditions  Beam Voltage ( V e )= 20 kV  Beam current ( I e ) = 0.25 A  Center frequency ( f c ) = 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 , 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 , Oct. 12 (2007) 2) Young-Min Shin and Larry R. Barnett, Appl. Phys. Lett. 92, (2008). 3) Young-Min Shin, Larry R. Barnett, and Neville C. Luhmann Jr., Appl. Phys. Lett. 93, (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 / Broadband Coupler Tapered Transition (Vane Width) Ka-band circuits

TIME / 31 March 2008 / MAGIC3D: Gain and Stability Growth Rate ~ 14dB/cm 30dB

TIME / 31 March 2008 / Sensitivity Studies Misalignment Effect dy dz Off-Centered Beam Effect x y z x y z

TIME / 31 March 2008 / KMPR UV LIGA Process MEMS Facility UV-LIGA Northern California Nanotechnology Center UCD

TIME / 31 March 2008 / 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) :  ~  m (3)Lapped and polished to within 0.25 µm (4)Less than 1 µm undercut after mold removal

TIME / 31 March 2008 / Surface Roughness Analysis of Resistivity versus S 21 and S mm long BS circuit model Transmission (S 21 ) Graph Reflection (S 11 ) Graph Resistivity versus S 21 /S 11 Only  1 dB in S 21 and S 11 is changed by up to 5 × Resistivity (OHFC)  Interaction circuit is insensitive to surface conditions (roughness)

TIME / 31 March 2008 / Sensitivity Analysis of Fabrication Errors 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 / Process Optimization - Before Softbake - Right After Heating - After 4.5 hr Softbake * 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 / 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 ~  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 / 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 / Summary and Future Plans Accomplishments –Sc 2 O 3 -W Nano-Powder Cathode A. Maximum Emission Current Density: ~ 80 A/cm 2 at 1150 o C (Full Space Charge Limited) B. Life-Time: 768 hrs (950 o C), 800 hrs (1150 o C) C. ~ 50 A/cm 2 at 1050 o C –Sheet Beam Gun Design (CPI and Teledyne-MEC) –Circuit Design and Analysis A. Operational Bandwidth : 70 GHz 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 (~ nm), Aspect Ratio (~ 10 : 1) –CNC Fabrication Research Schedule –750 A/cm 2 and 25 : 1 Sheet Beam Transport Test –220 GHz Circuit Cold-Test