Chemistry Reference
In-Depth Information
but a U-shaped curve additional extreme values occur in the second derivative of the
probe characteristic indicating bad rf-compensation [39,48].
6.1.3.5 Probe Measurements in Magnetron Discharges
Magnetron discharges are widely used in plasma-aided etching and deposition tech-
nologies. An overview about probe measurements in such discharges is given in [50].
The presence of a magnetic field generally reduces the current drawn from a plasma
by a probe and causes an anisotropy of the EEDF. For a cylindrical probe the first
effect is minimized if the probe axis is positioned perpendicular to the magnetic field
lines and if the probe diameter is chosen as small as possible. The second effect
appears if the ratio of magnetic field strength and pressure
B
p
is several of T/Pa
[51]. In magnetron discharges this ratio is lower by about two orders of magnitude
[50]. An anisotropy of the EEDF is not detectable there [52]. Based on measurements
in a cylindrical magnetron discharge performed in argon at a pressure of 1.5 Pa in
[50] was shown that in a weak magnetic field the error in the measured electron
density obtained from the probe electron current at plasma potential according to
Equation 6.3 caused by the magnetic field is not bigger than 20%.
/
6.1.3.6 Probe Measurements in Depositing Discharges
Surface contamination is generally a serious problem in probe diagnostics. Every new
probe tip should be conditioned in the plasma bringing it to white heat (
T
p
>
1500
◦
C)
by drawing a high electron saturation current [21]. In depositing plasmas probe
contamination is the limiting factor of probe usefulness. Here a layer at the probe
surface is formed which may disturb the probe characteristic. Generally, probe con-
taminations change the probe surface work function, causing a shift of the probe
characteristic and/or a hysteresis [53,54]. Dielectric layers, such as polymer films
or Al
2
O
3
, additionally flatten the characteristic [54,55] causing an overestimation of
electron temperature, whereas a conductive or semiconductive film, such as α-Si:H
or SnO
2
, weakly disturbs the probe characteristic [54,56-58]. But also an influence
of metallic layers was reported [59,60]. Probe contamination may lead to additional
inflection points in the probe characteristic and thus to additional extreme values
in its second derivative [55,61]. If insufficient rf-compensation as a reason may be
excluded, such additional extreme values are a strong hint to probe contamination.
At first the following measures may be used to avoid probe contamination:
Moving probe into plasma only during measurement, short plasma operating times,
holding the probe at cleaning potential [62] or retracting the probe tip into its insu-
lating capillary during nondata acquisition times [63]. Essential in the construction
of cylindrical probes is the centering of the probe tip wire inside the insulating
capillary [4,64].
6.1.3.6.1 Pre-Cleaning
The simplest possibility to carry out probe measurements in depositing plasmas is
to clean the probe before each measurement by applying a voltage
V
V
s
(ion
bombardment) or/and
V
V
s
(heating by a strong electron saturation current to
red or white heat) and to perform the measurement in a time short compared
with deposition time. Examples are given in [13] (plasma: N
2
, CCl
4
, CCl
4
/N
2
,
>
