Max-Planck-Institutfür PlasmaphysikEURATOM Assoziation

Interaction of nitrogen plasmas with tungsten

Klaus Schmid,A. Manhard, Ch. Linsmeier, A. Wiltner, T. Schwarz-Selinger, W. Jacob,

Stephan Mändl

K. Schmid 2009

• Introduction

• Summary

Outline

• Experiment & Results

N accumulation & sputtering of W

Nitride phase formation

K. Schmid 2009

Introduction

ASDEX Upgrade (now full W) experiments show performance increase for N2 seeding in the divertor compared to Ne or Ar [1]:

Why N and W ?

[1] A. Kallenbach, et. Al. Nuclear Fusion 49 (2009) 045007

Question: Influence of nitrogen ions on tungsten PFCs ?

Sputtering of tungsten by nitrogen

Nitrogen accumulation in tungsten

Thermodynamics and thermal stability of the tungsten-nitrogen system

Perform experiments on:

+ Improved energy confinement+ Smaller ELMs

N2 gas puffing is used for edge plasma cooling in high-Z PFC fusion experiments

K. Schmid 2009

Experiment & ResultsN-accumulation and Sputtering of W by N

grid

Sample holder

UDC, bias

• ECR plasma ion source with freely expanding plasma beam

• Homogenous irradiation of samples

• Ion energy by DC biasing up to 500V

• Water cooled sample holder

• Ion flux & energy distribution measured by retarding field analyzer: 3 to 4x1018 Ny

+ (m-2 s)

• Molecular ion distribution measured by plasma monitor Dominantly N2

+ ions

PLAQ

SamplesWCu

Si

PVD tungsten layer ~ 500 nmPVD copper interlayer ~100 nm

stress reliefSilicon substrate

well-defined surface for RBS analysis

Implantation setup

K. Schmid 2009

Experiment & ResultsN-accumulation and Sputtering of W by N

N2 plasma flux and composition

0 100 200 300 400 5000

1

2

3

4

Bias -500 VBias -400 V

Bias -300 VBias -200 V

d I

on

/dE

(1

01

7 c

m-2

s-1

eV

-1)

Ekin / ion (eV)

floating

Bias -100 V

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.10

10

20

30

40

50

60

70

80

90

100

flu

x fr

acti

on

(%

)

pressure (Pa)

N2+

N+

N3+

Retarding field analyzer measurements

Flux :3 - 4 x 1014 N2+ / cm2 s in main

peak

Most ions have energy corresponding to bias

Plasma monitor measurements

Plasma dominated by N2+ ions

N3+ ions occur at highest

pressures ~1Pa

N+ ions maintain constant level with pressure

Implantations were performed at 0.25 Pa 90% N2+

K. Schmid 2009

0 5000 10000 150001

10

100

1000

p1+2

6x1016 N/cm2 in W

Cou

nts

Proton energy (keV)

40uC of 3.8MeV 3He

p0

p4 p3

Experiment & ResultsN-accumulation and Sputtering of W by N

Measuring N accumulation in W by nuclear reaction

NRA with 3.8 MeV 3He

Reaction used: 14N ( 3He, p ) 16O

Sensitivity: 1015 N/cm2 0.13 cts/C

Peak integral of p1 + p2 protons is

evaluated to obtain the N areal density

Measuring W sputtering via thickness change of W layer

Rutherford backscattering (2.3 MeV 4He)

Yields tungsten layer thickness

Sensitivity: 1012 W atoms / cm2

0 200 400 6000

2000

4000

6000

8000

10000

12000

Si-Substrate

Cu-Layer

Co

un

ts

Channel

W-Layer

K. Schmid 2009

Experiment & ResultsN-accumulation and Sputtering of W by N

0.0 0.5 1.0 1.5 2.00

2

4

6

8

10

Ret

aine

d N

(10

15 c

m-2)

Implantation fluence (1019 N/cm2)

20 V 50 V 100 V

TRIDYN prediction 20 V 50 V 100 V Unimplanted

Implantation in plasma ion source PlaQ

Low energy ions

Sample temperature room temperature

Quick saturation of retained N amount

TRIDYN calculations predict similar levels

Diffusion of N in W low (similar to C in W)

N accumulation controlled by the implantation range

For our low implantation energies one expects accumulation in the range of:

2cmN1030.5cm10cm106 15

n compositio nitrideW

7322

NN cr

K. Schmid 2009

Experiment & ResultsN-accumulation and Sputtering of W by N

0 100 200 300 400 500 600

1E-3

0.01

0.1

Sp

utt

ere

d W

pe

r N

or

Ne

Acceleration voltage drop (V)

Experimental N W Static TRIM N W Dynamic TRIM N W Experimental Ne W Static TRIM Ne W

Sputter yield of W by N from an N2 plasma

Sputter yield is much lower that expected from static TRIM

Accumulation of N in W surface reduces partial sputter yield

Good agreement between dynamic TRIM and experiment (dyn. Surface evolution)

For Ne no accumulation in W surface Ne Sputter yield matches static TRIM

N accumulation in W surface shields W from erosion by N

Could partly be the reason for the good AUG performance with N-puffing

K. Schmid 2009

Experiment & ResultsNitride phase formation

Bombardment of W by 3keV N ion beam

XPS analysis to identify nitride formation

Literature data on W4f shift due to nitride formation varies strongly

We find a shift of 0.45eV for nitride peaks. (lies within the literature data range)

The intensity of nitride phase decreases with temperature

K. Schmid 2009

Experiment & ResultsNitride phase formation

W-N Phase diagram calculated by ThermoCalcTM

Based on very little available data

N2 gas phase suppressed

At ambient pressures WN instable

above ~600K

XPS measurements confirm thermodynamic

modelingWN decomposes at high temperatures and

N is lost as degassing N2.

K. Schmid 2009

600 650 700 750 8005

6

7

8

9

6.0 0.5

4.6 0.4

3.2 0.3

Ret

aine

d N

(10

16 c

m-2

)

Temperature during implantation (K)

Decrease for high temperatures

1.7 0.3

Fluence

(1018 cm-2)

Plasma immersion ion implantation at IOM Leipzig

10 kV pulses

Sample heated by pulses

Implantation fluence comparable to plasma implantation

Retained amount of N decreases with temperature above 650K

Experiment & ResultsNitride phase formation

Decay above ~650K Nicely fits predictions by thermodynamic modeling

As expected: Total accumulated amount higher than in our plasma implantation due to higher energies

K. Schmid 2009

Summary

N accumulation quickly saturates once implantation range is filled with nitride phase

Total amount of N is determined by N energy via the implantation range

N accumulation in the surface leads to a reduction in the partial W sputter yield

Advantage over noble gas seeding species

XPS measurements of nitride fraction & measurements of total N amount indicate that the nitride decomposes at elevated temperatures

This is line with thermodynamic calculations by ThermoCalcTM