Chemistry Reference
In-Depth Information
Practical application of the EAMS is used for the investigation of larger neutral
moleculesbydirectmeasurementoftheconcerningnegativeionsandforinvestigation
of smaller neutral species which are identified by measurement of the dissociative
products, as F
−
, in dependence on the electron energy. Each compound is charac-
terized due to the resonance process by a narrow peak shaped according to their
attachment cross sections for the specific electron energy.
The experimental equipment for the EAMS consists usually of a quadrupole mass
spectrometerwithelectronimpactionsource,energyanalyzer,quadrupolemassfilter,
and ion detector, evacuated by a turbo molecular pump. The device is separated from
the plasma reactor by an orifice with a diameter of 100-200 μm, which is situated in
the reactor wall. The neutral species flow out of the plasma vessel through the orifice
into the ion source with the ionizing electrons. These electrons are generated by a hot
filament and accelerated by a potential between the filament and a grid surrounding
the attachment region in the source. The electron energy can be scanned from 0.5
to 10 eV with an energy resolution of 0.1 eV. A calibration of the energy scale is
possible by using the dissociative attachment of O
2
with the resonance peak at 6.7 eV
for O
−
-generation [281].
The EAMS is applied preferentially for the investigation of fluorocarbon gases
usedforplasmaprocessinginsemiconductorindustry[333,338-341].Ina13.56 MHz
capacitivelycoupledCF
4
discharge(15 mTorr,2.5 sccm,
30 W)C
2
F
6
andC
3
F
8
could
be identified; C
2
F
6
and C
4
F
8
plasmas were also investigated and series of anions
C
n
F
2
n
−
k
were found, which may originate from C
n
F
2
n
−
k
+
1
[333]. A C
5
F
8
plasma
is investigated by [340]. Figure 6.30 presents the F
−
-signal which shows several
resonance peaks.
≤
1000
2.1 eV
3.0 eV
6.2 eV
800
7.3 eV
4.0 eV
5.2 eV
8.6 eV
600
400
200
0
0
2
4
Electron energy [eV]
6
8
10
FIGURE 6.30
F
−
-signal of neutral gas from a C
5
F
8
plasma in dependence on the electron
energy. The peaks at 2.1, 6.2, and 8.6 eV can be attributed to C
5
F
8
, the 7.3, 4.0, and 3.0 eV
peaks to CF
4
,C
2
F
6
,andC
3
F
8
. (From Imai, S. and Tachibana, K.,
Jpn, J. Appl. Phys.
, 38,
L888, 1999.)
