Biomedical Engineering Reference
In-Depth Information
particles within agglomerates in the bed. Even though this breakage is
partial and temporary during a single collision event, due to recirculation
and frequent impacts, the entire surface area of each of the primary particles
will be eventually exposed and the individual nanoparticles can be coated
(Hakim et al., 2005b).
8.2.2
In situ
mass spectrometry
Since all precursors and products are gaseous at operating conditions,
residual gas analysis (RGA) is an efficient technique to observe the
termination of each half-reaction. In situ mass spectrometry has been
successfully used for real-time monitoring of gaseous products and reactants
throughout the ALD reaction in an FBR (King et al., 2007). Here, alumina
ALD is used as an example to demonstrate this powerful technique. Al
2
O
3
ALD films have been deposited using repeated exposures of trimethylalu-
minum (TMA) and H
2
O in an ABAB . . . sequence (Dillon et al., 1995; Ott
et al., 1996). Al
2
O
3
ALD is derived from the following binary CVD reaction:
ð
Þ
3
þ
3H
2
O
!
Al
2
O
3
þ
6CH
4
½
2Al CH
3
8
:
1
This binary reaction can be divided into two half-reactions:
þ
2CH
4
A : 2AlOH
þ
2Al CH
3
ð
Þ
3
!
2 AlOAl CH
3
ð
Þ
2
½
8
2
:
þ
3H
2
O
!
Al
2
O
3
þ
2AlOH
þ
4CH
4
B : 2 AlOAl CH
3
ð
Þ
2
½
8
3
:
where the asterisks indicate the surface species (Dillon et al., 1995; Ott et al.,
1996). The expected reaction product is CH
4
, which, however, is also formed
directly from the fragmentation of TMA in the mass spectrometer (Juppo
et al., 2000). To distinguish the amount of CH
4
formed as a reaction product
formed directly from TMA, D
2
O was used instead of H
2
O. The reaction
product from TMA and D
2
O was expected to be CH
3
D, instead of CH
4
.
The m/z peaks of interest (i.e. primary and fragmentation peaks) were 28 for
N
2
,42(
AlCH
3
), 57 (
Al(CH
3
)
2
) and 72 (Al(CH
3
)
3
) for TMA, 18 (
OD)
and 20 (D
2
O) for D
2
O, and 17 for CH
3
D.
The in situ mass spectrometry results of Al
2
O
3
ALD on 16
μ
m high-
density polyethylene (HDPE) particles at 77
C, shown in Fig. 8.3 (King
et al., 2009), indicate that the chemistry of TMA and D
2
O ALD is self-
limiting. When TMA was dosed into the reactor (half reaction A), an
instantaneous increase of the signal of CH
3
D byproduct was not seen. This
is expected, since there was no deuterium element in the system before the
very first dose of D
2
O into the reaction system. TMA is an extremely
reactive precursor and, as such, very high surface conversions (in some
8
