Atomic Layer Deposition for SRF Cavities

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Atomic Layer Deposition for SRF Cavities
Jeff Elam , Michael PellinJerry Moore, Jim
NoremArgonne National LaboratoryClaire
AntoineCEA Saclay/Fermialso MassThink

• SRF Materials Workshop
• FNAL, May 23-24, 2007

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Outline

• What is Atomic Layer Deposition (ALD)?
• Why should ALD be considered for use in SRF
  cavities?
• Near term ALD effort.
• Replace the Niobia (NbxOy) layer inevitably
  present with an well defined, thin barrier layer.
• Barrier layer should be
• H diffusion barrier
• O, N (H20, O2, etc.) diffusion barrier
• Long term ALD effort.
• Superconducting layer growth (Nb, NbN, Nb3Sn,
  MgB2, etc.)
• Increase radius of curvature
• Conclusions

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Atomic Layer Deposition-What is it?

• Atomic Layer by Layer Synthesis Method similar to
  MOCVD
• Used Industrially
• Semiconductor Manufacture for high K gate
  dielectrics
• Abrupt oxide layer interfaces
• Pinhole free at 1 nm film thicknesses
• Conformal, flat films with precise thickness
  control
• Electroluminescent displays
• No line of sight requirement
• Large area parallel deposition
• Parallel film growth technique Inside of large
  tubes can be done at once.

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ALD Reaction Scheme

• ALD involves the use of a pair of reagents.
• each reacts with the surface completely
• each will not react with itself
• This setup eliminates line of site requirments
• Application of this AB Scheme
• Reforms the surface
• Adds precisely 1 monolayer
• Pulsed Valves allow atomic layer precision in
  growth
• Viscous flow (1 torr) allows rapid growth
• 1 mm / 1-4 hours

• No uniform line of sight requirement!
• Errors do not accumulate with film thickness.
• Fast! ( mms in 1-3 hrs ) and parallel
• Pinholes seem to be removed.
• Bulk

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Apparatus

• Hot wall reactor (RT-400 C)
• Always coat the wall now it will be useful

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Demonstrated ALD A/B Reactions
Element
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Atomic Layer Deposition of Tungsten ( analogous
to Nb synthesis using NbCl5 )
Overall Reaction WF6 Si2H6 ? W products
ALD W Binary Reaction Sequence

• A) WSiHSiH3 2 WF6? WWWF4 2 SiF4 3/2 H2
  HF
• B1) WF4 Si2H6 ? WSiH2F SiHF3 3/2 H2
• B2) WSiH2F ½ Si2H6 ? WSiHFSiH3 1/2H2

• ALD Growth Rate 2 ML W per cycle
• Small CVD component from Si2H6 thermal
  decomposition
• Difficult to nucleate W ALD on many surfaces
• W ALD nucleates well on Al2O3 ? Deposit ALD
  Al2O3 seed layer

F.H. Fabreguette et al, Thin Solid Films 488
(2005) 103 110. Grubbs et al, J. Vac. Sci.
Technol. B 22(4) 1811.
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ALD for SRF Cavities

• ALD is an ideal tool for coating SRF cavities
• Precision synthesis of ultra-thin films on
  complex surfaces
• Films are adherent (growth mechanism requires it)
• Films are pinhole free even at 1-2 nm film
  thicknesses (hence semiconductor industrys
  choice of ALD for gate dielectric films)
• Conformal coating -gt reduced point radius of
  curvature
• Alumina by ALD forms an extremely effective
  diffusion barrier to H, O, N etc.
• TiN ALD reduces secondary electron surface yield
  and is a diffusion barrier.
• With development ALD can grow the thin
  superconducting films necessary to enhance
  superconductor lower critical fields.

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Near Term ALD Research Replace poorly
constrained Niobia with a more optimum dielectric
Al2O3 capping layers for Nb surfaces

• Surface behavior of Nb
• In air, Nb oxide layer
• Annealed in vacuum
• Al2O3 Capping Layer
• 10 nm
• Al(CH3)3 alternating H2O cycles
• 20 C -gt200 C deposition temperature
• Anneal at 200 C to dissolve Niobia
• Other caps?
• TiN (Reduce Multipactering)

Ciovati, APL, 89 (2006) 022507/1-3 Delheusy,
Antoine et al, thesis work
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Conformal Coating Reduces Particulate Radius of
Curvature

• TiN coating or Alumina coating - residual
  particle effect might be reduced

• Progressive growth of Ag nanoparticles with
  increasing number of Al2O3 cycles (c)
• Ag nanoparticles still visible under 60 nm Al2O3
  coating
• AFM linescans show no net change in height ?
  Al2O3 grows equally on Ag and SiO2

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Where are we?

• Nb coupons have been coated with alumina and TiN
• Films are thin, pinhole free, adherent,
• Coating reduces Nb Oxidation State
• Initial TiN studies are encouraging.
• We are working to coat a single cell cavity.

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Long Term Goal of ALD SCRF

• Build nanolaminates of superconducting
  materials
• 10- 30 nm layer thicknesses

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What has been done.

• Nanolaminates of Al2O3 and W
• Growth conditions can set RMS roughness to lt 0.4
  nm
• Higher temperature -gt greater crystallinity

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Proposal for bringing ALD to SCRF fabrication

• Investigate the properties of Al2O3 capping
  layers for Nb surfaces
• Al2O3 is the ALD poster child
• Stabilize Nb surface oxides
• Anneal to reduce oxide thickness
• We are already using it to cap FePt particles
  (resist oxidation to 1000 C _at_ 10 nm thick film)
• Synthesize Nb bulk layer on Al or Cu cavities
• Develop the chemistries for clean deposition
  and crystallite control
• In situ capping
• Grow NbN, Nb3Sn, MgB2 layers on the Al2O3 capped
  Nb
• Develop the chemistries for clean deposition
  and crystallite control
• Test for enhanced field behavior
• Scale-up
• ALD was made for growing films on the inside of
  tubes precision layer by layer growth at aspect
  ratios exceeding 10000
• Coating is of all surfaces, no field penetration
  at the corners or edges

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Synthesize Nb bulk layer on Al or Cu cavities

• Why?
• Limit amounts of Nb needed
• Single apparatus for Nb and cap layer oxygen
  control
• Reactions not demonstrated
• NbCl5 Si2H6 ? Nb SiCl4 HCl
• Analogous to W reactions with similar enthalpies
• Precursors are volatile, stable, and purchasable
  at high purity
• Challenges
• Control of impurities (Cl)
• Control of crystallite size (may require high
  temperature flash)

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Atomic Layer Deposition of NbN
Overall Reaction (T 400 C) 3NbCl5 5NH3 ?
3NbN N2 15 HCl

• ALD Growth Rate 0.03 nm/cycle NbN per cycle
• NbN ALD nucleates well on Al2O3 or SiO2
• Need to find synthesis conditions for
  cleandeposition
• Need to control crystallinity

Higher-TcSC NbN
Nb, Pb
Van Hoornick et al, Journal of the
Electrochemical Society 153 (2006) G437. Alén et
al, Thin Solid Films 491(1-2) (2005) 235.
Insulating layers
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Conclusions ALD to SCRF fabrication

• ALD is an intriguing synthesis technique with
  many advantages for scrf cavities
• Conformal coating means reduced radius of
  curvature
• Parallel (non-line of sight) method
• Flat samples directly map to complex shape
  samples even with high aspect ratios
• Layer by layer growth on complex shapes
• Clean path from caps to layered structures with
  much higher field gradients