Chemistry Reference
In-Depth Information
Minimum
feature size [µm]
Transistors/chip
10
9
100
10
8
10
10
7
10
6
1
10
5
0.1
10
4
10
3
0.01
1960
1971
1978
1985
1993
2000
2005
2011
Ye a r
FIGURE 8.21
Minimum feature size/structure width (
), and number of the transistors
per chip (
) in dependence on time. Moore's law. (From Fahrner, W.,
Nanotechnologie and
Nanoprozesse
, Springer, Berlin, Germany, 2003.)
incidence. Redeposition of sputtered material is avoided operating at low
pressures (
0.01 Pa).
2. Chemical etching: Neutral radicals generated inside the plasma can react
with substrate materials. The volatile reaction products leave the surface. A
product is called volatile if the vapor pressure is larger than 10
−
2
Pa at oper-
ating temperature [80]. The formation of volatile reaction products depends
on the chemical properties of the radical and the substrate. Therefore, this
process is very species selective but isotropic due to the isotropy of the
inflow of the radicals from the plasma Figure 8.22a [76].
Pure chemical etching is used, e.g., for plasma stripping or plasma
ashing in barrel reactors to remove residues of photoresist from wafers
in 13.56 MHz discharges in O
2
or O
2
/N
2
[81]. The plasma does not contact
the wafers and generates O radicals, which oxidize the organic photoresist
to CO, CO
2
, and H
2
O. Also, radicals in downstream microwave plasmas are
used for pure chemical etching [82].
3. Physical-chemical etching: The reactivity of the surface can be enhanced
utilizing the kinetic and potential energy of impinging ions. Heating and
breaking of bonds on the surface facilitates the formation of volatile com-
pounds by reactions with neutral radicals. Ion collisions on the thin polymer
surface enhance the diffusion of radicals, provide the desorption energy for
the etch products, and enhance their diffusion through the polymer film
into the gas phase [83,84]. Also, the activation of adsorbed working gas
molecules is possible. This process is selective, determined by the chemistry
<
