Chemistry Reference
In-Depth Information
SF
6
), [54] (precursor: aluminum isopropoxid, layer: Al
2
O
3
), [58] (precursor: tetram-
ethyltin (TMT), layer: SnO
2
, [58] (plasma: Ar/C
2
F
3
Cl), [65] (plasma: CH
4
/H
2
,
layer: diamond-like). Additional argon plasma or oxygen plasma cleaning may
be performed after each measurement [58,65]. In a silane plasma stable probe
characteristics may be obtained if the probe ambient temperature is held at about
200
◦
C [56,57].
6.1.3.6.2 Heated Probes
These probes have to be used, if the measuring time is bigger than the deposition
time. In direct heated probes the tip consists of a heated wire which is in direct
contact with the plasma. The wire temperature
T
p
during probe measurement has
to be chosen below the limit of noticeable thermionic emission. If such a probe is
used in an rf-discharge, both connections have to be rf-compensated [66]. Examples
for the use of heated probes are given in [55] (plasma: Ar/benzene,
T
p
=
850
◦
C),
[67] (plasma: Ar/benzene,
T
p
=
1120
◦
C), [68] (plasma: CF
4
,
T
p
≈
900
◦
C), [69]
(plasma: Ar/HMDSO,
T
p
=
1500 K). Sometimes
a heatable probe is heated only during the non-data acquisition times [71] (plasma:
acrylic acid,
T
p
>
900
◦
C), [70] (plasma: Ar/CF
4
,
T
p
≈
2000 K). This eliminates the uncertainty of the probe bias due
to the voltage drop along the probe wire during heating. Indirect heated probes
may be used, if a lower probe temperature is sufficient [59,60] (plasma: potassium,
T
p
600
◦
C).
Heated probes also may be used to determine the plasma potential directly,
especially in depositing discharges. In this case the wire temperature has to be
chosen to allow thermionic emission [8,72]. At probe voltages higher than plasma
potential the emitted electrons are reflected back to the probe and the emissive probe
characteristic is similar to that of the cold probe. At probe voltages lower than plasma
potential the emitted electrons are rejected from the probe and appear as an effective
ion current additionally to the current of plasma ions. This process depends directly
on plasma potential rather than the electron kinetic energy. Thus it is not sensitive
to plasma flow, non-isotropic plasma, and collisions of electrons with heavy neutral
particles. The plasma potential may be determined from the branching point of the
characteristic of the emissive and the cold probe and from the floating potential or the
inflection point of the emitting probe. Unfortunately other plasma parameters than
plasma potential are not detectable by means of emissive probes.
300
/
6.1.3.6.3 Floating Systems of More Than One Probe
Also a layer on the reference electrode can complicate the probe measurement.
In this case double or triple probes may be used. The
double probe
method was
developed originally for electrodeless discharges [73]. From the characteristic of a
probe measured with respect to a second nearby, mostly identical, probe the electron
temperature and the ion density may be derived [4,8,11,74-76].
Because the net current in the double probe circuit is zero, the maximum current is
the ion saturation current. Thus only probe cleaning by ion bombardment is possible
applying a high voltage between the two probes. Inserting a reference electrode allows
probe cleaning by drawing an electron saturation current [75,76]. Furthermore both
probe tips may be designed as heated probes. Often a symmetrical double probe
