Chemistry Reference
In-Depth Information
Θ
A
Θ
Θ
AB
B
Substrate (B)
Adsorbate (A)
Product (AB)
FIGURE 5.4
A supposed situation for modeling of reactive surface etching involving three
components: adsorbed plasma species (A), solid surface atoms (B), and products (AB).
Under the assumption that at each B only either one A or one AB can be adsorbed,
one obtains for the coverage
A
+
B
+
AB
=
1,
(5.13)
where
A
=
n
A
/
n
0
,
B
=
n
B
/
n
0
, and
AB
=
n
AB
/
n
0
. Combining and solving (5.12) and
(5.13), one obtains the normalized etch rate
R
/
n
0
:
τ
AB
des
−
1
+
(
τ
des
γ
A
j
A
)
−
1
+
n
0
k
SFR
(
γ
A
j
A
)
−
1
+
n
−
1
0
R
n
0
=
.
(5.14)
σ
CS
γ
−
1
k
SFR
+
(
τ
des
+
n
0
k
SFR
)
−
1
A
This expression for such a simple example becomes much more difficult in real
systems with more involved components and processes as considered here. Therefore,
it is of importance to study some limiting cases of (5.14). For instance, at a very low
adsorption of A (e.g.,
A
1), as is the case for high substrate temperatures where
τ
des
≈
0, or for low sticking coefficients γ
A
≈
0, the etch rate becomes
τ
AB
des
−
1
R
n
0
=
1
j
A
σ
CS
+
.
(5.15)
In this case, the etch rate
R
is mainly determined by the chemical sputtering of
B and the desorption of the products. If the formed product molecule AB desorbs
