Transcript Document

Introduction to Plasma-Surface
Interactions
Lecture 5
Sputtering
Topics
• Physical sputtering
– Sputter yields
– Energy distribution of sputtered atoms
• Chemical sputtering
– Yields
– Flux dependence of yields
• Global Model
– Comparing effect of different materials
Physical sputtering
• This is the most common mechanism for bulk
impurity atoms getting into the plasma.
• Sputtering occurs as a result of momentum
transfer from an energetic incident ion to solid
surfaces
• It is a well understood physical process and results
agree well with calculations
• Calculations are normally made with the TRIM
Monte Carlo code. Tabulation of data for a wide
range of ions and targets & energies are available
Energy threshold
• Because an atom leaving the surface has to
overcome the surface binding energy Es
there is a threshold energy ET for sputtering.
This is given by
ES
ET 
 sp (1   sp )
• Where
 sp  4m1m2 / m1  m2 
2
Sputter yields for Be, C and W by
D and self ions
Note the
increasing
threshold energy
with target mass.
Using D+ ions
yield is ~ same
for Be, C and W
W Eckstein PMI, Garching, Report PP9/8 (1993)
High energy sputtering
• The maximum in the yield and the decreas
at high energies is due to the collision
cascade occurring deeper and deeper in the
solid.
• The surface atoms have less chance of
receiving sufficient energy to be sputtered
Uncertainty in yields
• There is an variation in yields measured
experimentally ~ 2.
• This is not experimental error but genuine
variations which depend on surface
conditions which can affect the binding
energy
• Examples are variation in the structure,
surface roughness or impurity levels
Effect of incident angle
• The sputter yield increases as the angle q
increase from normal (q =0)
• This is due to the increased probability of the
incident ion being backscattered
• At energies <300 eV the variation of yield with
angle is small.
• This is the region of most interest in plasma
physics (the sheath potential tends to make ions
arrive at normal incidence)
Energy distribution of sputtered atoms
• The energy of the sputtered atoms is important
because it determines how far they penetrate into
the plasma
• It too has been well studied and is understood
theoretically
• The most probable energy is 0.5 Es (2 to 5 eV)
• At higher energies the energy distribution has a
tail going as E-2 with a cut-off at the incident ion
energy
Sputtered atom energy distribution for C
Measured
spectroscopically
using doppler shift
and compared with
Thomson model
using B.E.= 9.3 eV
Bogen and
Ruesbueldt JNM
179 (1992) 196
Chemical sputtering
• This only applies to C: but because C is widely
used it has received a lot of attention
• A typical reaction is
•
4 H+ C = CH4
• Methane is the most common product but higher
hydrocarbons are also produced eg C2H4, C3H6
• The details of the reactions are not well
understood and there is no reliable theory
Chemical Sputtering of C
Ion energy dependence
10-1
Yield almost
independent
of energy
10-2
10-3
10
20
50
Mech et al JNM 255 (1998) 153
100
200 eV
Chemical sputtering of C
Surface temperature dependence
10-2
The
behaviour is
complex and
not
understood
(to my
knowledge)
CD4
CH4
C2H4
10-3
C2D4
10-4
400
600
800
Mech et al JNM 255 (1998) 153
1000 K
Extrapolating to high flux conditions
• Over the last few years there has been much
discussion about how the chem. sputt. yield
varies with incident ion flux.
• Results from 7 different devices have been
correlated and analyzed to obtain a
consensus view (Roth et NF 44 (2004) L21)
• Results from this study are presented in the
next slide
Extrapolating to high flux conditions
results from many experiments
Roth Nuclear Fusion 44 (2004) L21
Modelling global behaviout
• The operation of a plasma physics device is
complicated because there are so many interacting
processes.
• These are generally studied using large computer
programs, often using fluid codes. It is difficult to
see the importance of different processes.
• An attempt has been made to present a simpler
analytical model. It is not expected to give
accurate description of the systems but to try and
see the relative importance of different processes
Carbon as a target material
• The reduction of the sputter yield at high
flux compensates for the higher flux
• The concern over chem. sputt. is not as
serious as originally thought.
• However the major concern in using carbon
in a DT machine is th ehigh inventory built
up in the deposited layers
Global model of sputtering
From particle balance of confinement times  m ,  p and sputtering
yields Ym ,Y p we can get
Yp
nm
m

np
 p 1  Ym 
Where is a  screening coefficient
We can get a measure of the edge Te from energy balance
PH  PH  PH   T a n pV /  p
Where PH,PR, and PC are the input, radiated and conducted power.
Although crude this model allows us to see the difference in
behaviour of low and high Z materials
Calculation of radiated power and
Zeff vs ne : Comparison of Be and W
Calculations based on the global model
At low ne : Te is high and W is sputtered
fast, resulting in High Pr and Zeff
Be has reached or is over the maximum
in the sputter yield. Becaause of low Z
and low density Pr and Zeff are low.
At high ne: Te is low and W sputter rate
is low or zero, resulting in low Pr and
Zeff Be sputter rate is still high and ne: is
high so Pr is high
G McCracken and G Matthews JNM
176-177 (1990) 312
Choice of materials
• A figure of merit M was proposed by Lazlo
and Eckstein (1991)
1  Ym 

M f
i
Yp
• Where fi is the maximum impurity
concentration allowed in the plasma.
• The larger M the less power will be radiated
• Both the sputter yields and fi are functions
of edge Te . M can be plotted against edge
Te
Figure of merit for materials
as a function of edge Te
High M is good: low M is bad!
At low enough
Te all materials
are good
At high Te
Mo and W are
useless
For high edge
Te only very
low Z
materials are
tolerable
Health warning!
• Don’t take these models too seriously, but
they are worth thinking about, particularly
in terms of comparing high and low Z
materials
• The thresholds even for hig Z materials like
W are not very high, especially when you
take into account multiply charged ions
Summary - 1
• Physical sputtering is a real threat. Only by
keeping the edge Te low can it be avoided
• At low density i.e. higher Te, only low Z
materials stand any chance
Summary - 2
• Chemical sputtering is only a problem with
carbon.
• Unlike phys. sputt. there is no good
theoretical model and so it is difficult to
include it general plasma codes
• Recent data of lower yields at high fluxes
look helpful
Schematic of arc tracks
Because the arc tracks are driven by an
JxB force, for a fixed field, on a curved
surface the current changes direction
and henc the force changes
This results in curved tracks
Typical patterns seen in tokamaks are
shown
Thhe tracks go in the opposite direction
to the JxB force. There are at least 20
explanations for this effect but none are
very convincing!