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

• Where

ET ES

sp (1 sp )

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 increase from normal ( =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 CIon energy dependence

Mech et al JNM 255 (1998) 153

10-1

10-2

10-3

10 20 50 100 200 eV

Yield almost independent of energy

Chemical sputtering of CSurface temperature dependence

Mech et al JNM 255 (1998) 153

400 600 800 1000 K

10-2

10-3

10-4

CD4

CH4

C2H4

C2D4

The behaviour is complex and not understood (to my knowledge)

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 conditionsresults 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 sputteringFrom particle balance of confinement times and sputteringyields we can get

Where is a screening coefficientWe can get a measure of the edge Te from energy balance

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

m , pYm ,Yp

nmnp

m p

Yp1 Ym

PH PH PH T a npV / p

Calculation of radiated power and Zeff vs ne : Comparison of Be and W

Calculations based on the global modelAt 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)

• 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

M fi1 Ym Yp

Figure of merit for materialsas 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 tracksBecause 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!