Alameda Applied Sciences Corporation

Transcription

1 Alameda Applied Sciences Corporation Coaxial Energetic Deposition (CED TM ) of superconducting thin films of Nb for RF cavities* Mahadevan Krishnan, Andrew Gerhan, Kristi Wilson, Jason Wright, Brian Bures and Don Parks Alameda Applied Sciences Corporation, San Leandro, CA, USA Anne-Marie Valente-Feliciano, Genfa Wu * and Xin Zhao Thomas Jefferson National Accelerator Facility, VA, USA (* Now at Fermilab) 3rd International Workshop on "Thin films applied to Superconducting RF and new ideas for pushing the limits of RF Superconductivity - Jefferson Lab, VA - USA * This research was supported by DOE Grants #DE-FG02-04ER83896 and #DE-FG02-07ER84741

2 Description of CED, a cathodic arc deposition process CED applied to RF cavities Macroparticle filters Future work Outline

3 Coaxial Energetic Deposition TM Cathode: 60 cm conducting rod (1 cm dia). Anode: 45 cm Mo mesh tube (4.5 cm ID) Substrate: 5 cm ID minimum for this configuration Solenoid: B ranges from 0-10 mt in z + or z - direction. Note: Anode does not collect all the arc current. Mesh spacing is much larger than Debye length. Current is measured between the power supply and cathode through known resistor.

4 Magnetic field profile along z B z (mt) Solenoid 1.0 I solenoid =50A Position from Center of Solenoid (cm) Center of the anode is at the peak in B z

5 Coaxial Energetic DepositionTM

6 Rotating arc moves down the axis of a 4 φ tube

7 Cathodic Arc deposition is very different from sputtering Comparison of low energy deposition, cathodic arc deposition, and cathodic arc deposition with high voltage bias Film stress vs. incident ion energy

8 Early results revealed importance of better vacuum and higher substrate temperature on RRR Sample Effect of better vacuum on RRR Base vacuum (Torr) Ambient resistivity ( -cm) Thickness () Tc (K) RRR AASC-126-Nb AASC-126-Nb AASC-126-Nb Effect of substrate temperature on RRR

9 Nb film thickness scales linearly with total charge: erosion rate is constant y = x R 2 = Number of 200A pulses

10 RRR increases with film thickness until ~2µm

11 Latest T c & RRR results Today, vacuum is better, substrates are heated and thicker films are grown Superconducting Nb thin films with high RRR have been deposited with CED T c =9.2K and RRR=88 were measured at JLab for unfiltered film deposited on sapphire coupon T c & T c of a 0.7µm actively filtered film were measured to be 8.79K/0.46K JLab has obtained SEM, XRD and EBSD data on selected films (see talk on Thursday by Xin Zhao et al)

12 AASC-126-nb-022 analyzed by Xin Zhao (JLab) Confidence Index: 0.43

13 AASC coated Nb on Black Labs. Cu (analyzed by Xin Zhao) UNS C10100 Nb/Cu Confidence Index: 0.72 Confidence Index: 0.48

14 CED demands better Macroparticle filter CED produces micron-sized droplets of molten niobium called macro-particles along with deposition ions Macroparticles increase film roughness, limiting RF power capabilities of Nb-coated copper cavities We have demonstrated a 10x reduction in film macroparticle density using prototype coaxial filters The passive filter is inefficient: deposition rate is greatly reduced; requiring more pulses and thus producing more macroparticles that must be filtered Copper Substrate Macroparticle filter Nb Coating Nb Cathode Grounded passive filter blocks line of sight macroparticles Filter collects electrons and ions are repelled from and travel around vanes

15 Design of an active multi-particle filter Three types of active filters were tested straight vanes angled vane oriented with rotation direction angled vane oriented against rotation

16 Active Flat Macroparticle filter as designed QuickTime TIFF (LZW) de are needed to see The filter is supplied with 200A of current to create a B-field calculated to be several 100s of gauss

17 Schematic drawing of vane filter and B-field simulation

18 Throughputs of active multi-particle filters Nb film thickness was measured for the three filters Sample Description of test Thickness [nm] ratio: unfiltered/filtered # pulses thickness/pulse [nm] measurement method 7A unfiltered control profilometer B flat filter passive SEM 9A unfiltered control profilometer B flat filter active SEM CED chamber VCED-II VCED-II 12A unfiltered control profilometer VCED-I 12B angled filter orientation B passive SEM 11A unfiltered control profilometer VCED-I 11B angled filter orientation A passive SEM 14A unfiltered control profilometer VCED-I 14B angled filter orientation A active SEM 18A unfiltered control profilometer NA VCED-I 18B angled filter orientation A active profilometer 13A unfiltered control profilometer VCED-I 13B angled filter orientation B active SEM Throughput of: Flat filter passive: ~1% (poor) Flat filter active: 2.4% (better, but not acceptable) Angled filter orientation A active; 5.3% Angled filter orientation B active; 5.3% Similarity of angled filters A and B proves that ion flight time through filter is «plasma dwell time in vane

19 Macro-particle distributions measured with ImageJ software Unfiltered CED film on Si substrate at 200x (Nb-005) Start with unfiltered microscope image of CED deposited coupon Do some digital filtering to improve contrast and develop histogram using ImageJ

20 Particle distributions for active and passive flat filters Macro-particle distributions for passive and active flat filter Debris is reduced, but throughput is unacceptably low

21 Passive vane filter A has no effect on debris! Macro-particle distributions for passive vane filter vs. no filter Could vanes themselves be a source of debris? (is the treatment worse than the disease?)

22 Active vane filter B is better, but.. Macro-particle distributions for active vane filters Need RF tests to establish acceptable macro-particle distribution thresholds

23 Cross section SEM image of film 14B This local SEM shows a rather smooth surface after active filtering

24 Hall plasma model for active multi-particle filter Ion momentum eq. r Electron fluid with drift speed ν Which leads to: deflection: Importance of Hall term: Alfven speed scaling: NM d r V dt p e + Ne( r E + = r Jx r B / c ( p e + p i ) r vx r B c ) + Nm τ (r v r V ) = 0 J y = ωτ J r e ( yx ) b ) ) r Jx r d B δv dt 2JBd ρc ρcv 0 θ 1 5 d jbd ρv 0 2 j y = ωτ j n θ 1 5 I arc B NvLρV 0 2 ωτ 1 2π V A 2 = B arc B /4πρ B arc B NvρV ωτ 2 0 θ 2 Nv V A 2 V 0 2 ωτ

25 Measured film thickness enables an estimate of plasma density measured film thickness/pulse= nm average substrate= 6.95E-08 g/cm 2 bulk Nb density anode screen transparency= 0.56 average cathode surface= 1.21E-06 g/cm 2 conserve flux cathode conduction area= 182 cm 2 eroded mass/pulse= 2.20E-04 g flux*area average current= 150 A pulse width= s average erosion rate= 2.0E-05 g/c eroded mass/charge "dwell" time in vane= 4.03E-03 s instantaneous 2.95E-06 g/cm 2 fluence@vane= 7.32E-04 g/cm 2 -s ion speed= 1.00E+06 cm/s electron density in vane= 1.92E+13 cm -3 "spoke" flux -50eV ions

26 Design of an active multi-particle filter d = Lcosα, so that the line of sight along z is completely blocked assume that every particle undergoes an impulsive deflection through a unique angle θ = π/2 - φ on the midline connecting the vanes. Observe that for α = 30 degrees half the incident particles, (those incident with x < d/2), will not reach the midline. Thus the efficiency for transmission can be no greater than fifty percent Only particles entering the filter at x > x 1 will pass through the filter Define the transmission efficiency E by: E = 1 x 1 /d leads to: E = 0.5(tanα)cotφ =0.5(tanα)tanθ θ = B arc Bωτ/(2πN v ρv 02 ) (using V θ ) Nv V ωτ A = B arc B /4πρ 0 Observe that vane filter efficiency (throughput) scales as B 3 (the third factor comes from the Hall parameter): In PhII we will use auxiliary current to vanes V A 2

27 Advanced vane filter concepts QuickTime and a TIFF (LZW) decompressor are needed to see this picture. Two-stage deflection design Three-stage deflection design Plasma physics model provides guidance to higher efficiency vane filter designs that should also reduce filter generated debris

28 Increase process repetition rate through hardware/software modification Reducing deposition chamber base pressure below current 5x10-9 Torr is impractical 1.0E E+04 Operating regime 0.25 Hz operation 2.0 Hz operation Increasing repetition rate allows increased flux of deposition ions at film surface relative to background contaminants: thus film purity is increased 1.0E E E+01 Superconducting properties are greatly affected by film purity 1.0E+00 High RRR 1.0E E E E E E-07 Oxygen Partial Pressure (Torr) To increase the repetition rate, 400A IGBT switches are being installed to control the arc s turn on and turn off Initial switch testing is promising but more experiments are required to optimize switch controls. Higher repetition rate to ~2 Hz should reduce impurities by 10x

29 Future plans Make further improvements to active filters using design parameters developed in Phase I Obtain RF data on coupons to improve specifications on acceptable macro-particle distributions Increase discharge duty factor to further reduce O 2 contamination and increase RRR for smoother, actively filtered films Coat Cu RF cavities and have them tested at JLab