New Insights into Low Temperature Doping TTC Meeting, FRIB - - PowerPoint PPT Presentation

New Insights into Low Temperature Doping

TTC Meeting, FRIB February 21, 2017

• P. N. Koufalis

Motivation

• In high temperature doping (~800–1000 oC) nitrogen diffuses

several microns into the niobium lattice in a matter of minutes

• Reduces electron mea

ean fr free ee pa path th in the RF penetration layer

• Results in anti

anti-Q-slope and hi higher Q0 values

EP + 900 oC N-doped T = 2.0 K 1.3 GHz single-cell cavities

• A. Grassellino et al., Supercond. Sci.
• Tech. 26

26 (2013).

• Requires po

post-tr treatment t che chemistry to remove lossy nitride layer and typically results in lo lower r que quench fie fields in strongly doped cavities

Motivation

• It has been shown that low temperature baking (~160 oC) in a

nitrogen atmosphere also results in anti anti-Q-slope and hi higher Q0

• A. Grassellino and S. Aderhold, New low T nitrogen treatments: cavity results with

record gradients and Q. TTC, Saclay. 2016.

• In this temperature regime, nitrogen should not diffuse more

than a a few nm into the niobium!

• What is going on? Why are the sa

same ef effects ts observed?

Cavit ity Preparation

• 1.3 GHz single cell TESLA-

shaped cavity

• Che

hemistry:

• BCP (in + out)
• EP (16 μm; in)
• Lo

Low tem emperatu ture ba bake:

• 800 oC (10 hr; UHV)
• 160 oC (48 hr; N2)
• 160 oC (168 hr; UHV)

Conti tinuously flo flowing nit itrogen atm atmosphere!

Cavit ity Performance

T = 2.0 K Max ax Qu Quality ty Factor: Q0 = 3.6 .6 × 10 1010

10 @ 16 MV/m

• 160 oC cavity:
• Anti

ti-Q-slope

• Hig

igher Q0

• Similar performance

compared to nitrogen- doped (800 oC) cavities

• Im

Implie ies im impurit ities!

• Quench at Eacc = 25

MV/m (Bpk ~ 107 mT)

Cavit ity Performance

• Reduction of the

BCS su surface resistance with increasing gradients leads to anti anti-Q-slope

• Sa

Same ef effect seen in (800 oC)N-doped cavities

• Should see similar

mean free path

• What is the

impurity?

Sample Analysis

• Sample analyzed with

secondary ion mass spectroscopy (SIMS)

• If nit

nitrogen is responsible for reduction of mean free path, then should

• bserve similar

concentrations compared to the N- do doped sam ample le

• However, the ni

nitr trogen co concentratio ion is much lower than the N-doped sample in the RF layer

C O N N-doped + 10 μm EP RF layer Oxide layer

• N

N concentratio ion @ 5 nm nm: : ~3.6 × 1019 atoms/cm3

• N

N concentratio ion @ 50 nm nm: < 1 × 1019 atoms/cm3

Sample Analysis

• The ca

carbon and oxygen concentration is much higher than that of ni nitr troge gen!

• If mean free path is

responsible for the

• bserved effects, then C

and O must be responsible

• C

C an and O O concentratio ion @ 5 nm nm: : ~1021 atoms/cm3

• C

C and nd O concentratio ion @ 50 nm nm: : ~5 × 1020 atoms/cm3

C O N N-doped + 10 μm EP RF layer Oxide layer

Sample Comparis ison

EP + No Bake 160 oC

• Near identical N concentration
• The difference between the samples is the C and O concentration

in the RF layer!

Sources of C and O?

• The nitrogen supplied during the ‘doping’ step is co

continuously flo flowing and is supplied from li liquid nit itrogen blo blow-off

• Impurity content includes < 5 ppm

ppm O2, < 3 pp ppm H2O, and < 1 pp ppm CO, , CO2, , tot

• tal hydrocarbons
• Possible source of C and O
• Surface carbon/carbides?
• Furnace contamination?
• Backflow from furnace pumps

Dif iffusion Model

• Fick’s second law:

𝜖𝑑 𝜖𝑢 = 𝐸 𝜖2𝑑 𝜖𝑦2 𝑑 = impurity concentration 𝑦 = depth into material 𝑢 = time 𝐸 = diffusion coefficient

• Solution:

𝑑 𝑦, 𝑢 = 𝐷′ + 𝐷′′ − 𝐷′ erfc

𝑦 𝐸𝑢

𝐷′= initial impurity concentration in bulk 𝐷′′= impurity concentration at the surface

Material l Properties

• RF measurements and BCS prediction:
• Very short measured mean free path: l ≈ 7 nm!

l = 7.04 nm

Material l Properties

• At l = 7 nm  penetration depth ~100 nm
• Use depth of 50 nm to calculate mean free path estimate:

∆𝜍 = 𝑏 ∙ 𝑑′

• Mean free path is related to the change in resistivity by:

𝑚 =

𝜏 ∆𝜍

• Even at the RF surface (5 nm) the resulting mean free path

estimate due to only nitrogen is: 110 nm nm!

∆𝝇 = = resis istivit ity 𝒃 = 4.3 .3 × 10 10-8 Ω∙m (C (C, , O) 𝒃 = 5.2 .2 × 10 10-8 Ω∙m m (N (N) 𝒅′ = impurit ity concentratio ion 𝝉 = 0.3 .37 × 10 10-15

15 Ω∙m2

MF MFP @ de depth = 50 50 nm nm Carbon + Oxygen 5 nm Nitrogen Only 712 nm

Concluding Remarks

• For low T baking, C

C and O play the dominant role in the reduction of the mean free path

• At 160 oC, nit

itrogen does not diffuse whereas oxygen and ca carbon diffuse readily

• RF measurements of mean free path consistent with theoretical

calculations based on measured impurity concentrations

• Low temperature treatment results in the same effects:
• anti

anti-Q-slope and im improved Q0

• No post-treatment EP needed

Concluding Remarks

THANK YOU FOR YOUR ATTENTION!