Transcript THE ENERGY INFLUX FROM AN RF PLASMA TO A SUBSTRATE

13
n
The energy influx from an rf plasma to
a substrate during plasma processing
W.W. Stoffels, E. Stoffels,
H. Kersten*, M. Otte*, C. Csambal* and H. Deutsch
Department of Physics, Eindhoven University of Technology,
PO Box 513, 5600 MB Eindhoven
* Institute for Physics, University of Greifswald,
Domstr. 10a, D-17487 Greifswald, Germany.
Acknowledgment
The work has been supported by the Royal Dutch Academy of Sciences (KNAW) and the
Deutsche Forschungsgemeinschaft (DFG) under SFB198/A14.
Abstract

Aim: determine the energy flux to a substrate
in an low pressure rf plasma
 Method: calorimetric probe

Results:
– Argon:
• heat flux is few times 10-3 W/cm2
• heating mainly due to ions and electrons
– Oxygen
• 50% higher heat flux than argon
• molecular surface processes are important as well
Substrate heating: csdT/dt = Fin-Fout

Fin = Heat flux Jx times probe surface:
J i  ji Ei  ji e0 (Vpl  V fl )  ne vamb e0 (Vpl  V fl )  ne
kTe
exp 0.5  e0 (Vpl  V fl )
mi
– ions: kinetic
J rec  ji Erec
recombination
 e0 (V pl  V fl ) 
kTe
J

n
exp
– electrons:kinetic

  2kTe
e
e
2me
kTe


– neutrals: kinetic, internal, association, chemical
– photons: blackbody, plasma

Fout:
– thermal conduction of gas and substrate
– radiation
Note
The ion and electron heating depends on surface potential:
=>Separation of neutral component possible by using a bias voltage
Cs: heat capacity substrate; ji,je ion/electron flux; Vpl -Vfl acceleration voltage of ions in sheath
Thermal probe

Principle: heat flux determines the heating
time of the probe
 dT 


 dT  
Qin  H S (heat )  H S (cool )  mc S    S  
 dt heat  dt cool T
substrate
dsh
Fig.4
Photograph of the thermal probe placed
in the glow at substrate position.
The probe is a Cu plate, diameter
3.4 cm, height 0.002 cm. Mounted
to a thermocouple and shielded
from below (see picture). It can be
moved (x,y,z) and rotated.
Thermal probe: raw data
36
2
34
0
-2
IS [mA]
T S [°C]
32
30
-95V
-4
Ar, 1Pa
-6
28
O 2 , 1Pa
-46V
0V
-8
26
0
Fig.5
100
200
time [s]
300
-10
-120
400
Fig.6
TS(t)-curves as measured during the argon
plasma process (p=1Pa, P=15W) for three
substrate voltages (0, -46, -95V).
Rising edge plasma on, decreasing edge
plasma off. The plasma heat flux is
determined from the derivative signal
-100
-80
-60
-40
-20
0
20
V S [V]
Current-voltage characteristic of the
thermal probe for argon and oxygen.
The measured electron and ion flux
is used to separate ion and electron
heating from neutral heating.
Experimental setup

Langmuir-probe
Capacitively coupled
13.56 MHz plasma.
– Al electrode D=130mm
– Spherical reactor D=400mm.
powder
injection
thermal probe

Diagnostics:
– Thermal probe
– Langmuir probe
– CCD camera
CCD
p
argon
oxygen
RF-match
I-V
RF
Fig.1

Typical conditions:
– 1Pa, 15W Ar or O2
– Argon: Te = 3.5 eV
ne = 2 1015 m-3
Results: Argon
0,20
measurement: Ar, 15W , 1Pa
0,20
model: V s *Is
J i+J e +J rec (probe)
model: J i+J e +J rec probe
J i (probe)
0,15
model: J i+J e +J rec substrate
0,15
J e (probe)
Q in [J/s]
Q in [J/s]
J rec (probe)
J e +J rec (substrate)
0,10
0,10
0,05
0,05
0,00
0,00
-100
Fig.10
Left
Right
-80
-60
-40
V S [V]
-20
0
20
-100
-80
Fig.11
-60
-40
-20
0
V S [V]
Calculated contributions by ions (Ji, Jrec) and electrons (Je) to the
thermal balance of the substrate. The calculations are based on ne
measured by the Langmuir probe and a Bohm flux. For the electron
current (right branch) the measured substrate current is used.
Measured data fitted by the model results.
20
Results: Oxygen
0,20
Q in [J/s]
0,15
0,10
0,05
O 2 , 1Pa, 15W
Ar, 1Pa, 15W
0,00
-100
Fig.12
-80
-60
-40
-20
0
20
V S [V]
Measured integral energy influx (Qin) for argon and oxygen,
respectively, for the same macroscopic discharge conditions.
Comparison with argon
– Similar trends for oxygen and argon
– Overall higher heat flux in oxygen due to neutral heating
– ne(oxygen) < ne(argon) so electron branch is smaller
Conclusions

Thermal heat flux to a substrate can be measured by probe

Electron, ion and neutral heating can be separated

Argon 15W, 1Pa:
– heat flux few times 10-3 W/cm2.
– Increases with bias voltage
– mainly ion (and electron) heating

Oxygen 15 W, 1Pa:
– same trends
– significant influence of neutral heating

These data are also valid for heat flux in dusty plasmas