Environmental Engineering Reference
In-Depth Information
the twentieth century. It was later replaced by a cheaper method that produced
ammonia from nitrogen and hydrogen in a high-temperature, high-pressure reac-
tor with a platinum catalyst. Another plasmachemical process involves production
of ozone in a barrier discharge. This method has been used for several decades.
Large-scale development of plasma chemistry was long retarded by the required
high power intensity. When other criteria became the limiting factors, new plas-
machemical processes were mastered. Presently, the array of chemical compounds
produced industrially by plasmachemical methods includes C
2
H
2
,HCN,TiO
2
,
Al
2
O
3
,SiC,XeF
6
,KrF
2
,O
2
F
2
, and many others. Although the number of such
compounds constitutes a rather small fraction of the products of the chemical in-
dustry, this fraction is steadily increasing. The products of a plasmachemical pro-
cess may exist either in the form of a gas or in the form of condensed particles.
Plasmachemical industrial production of ceramic-compound powders such as SiC,
Si
3
N
4
, or powders of metals and metal oxides leads to a yield product of high quali-
ty. Plasmachemical processes with participation of organic compounds are used as
well as those with involvement of inorganic materials. Organic-compound appli-
cations include the production of polymers and polymeric membranes, processes
of fine organic synthesis in a cold plasma [11, 15, 16], and so on. In a qualitative
assessment of the technological applications of plasmas, we conclude that plasma
technologies have a sound basis, and have promising prospects for important fur-
ther improvements.
Etching [15, 16, 23, 24] is a typical example of plasma processing. It is one stage
in the fabrication of microelectronics. The process consists in the replication of
the desired pattern on an element of an integrated microelectronic scheme. The
replicator is a sheet of glass or quartz patterned with a thin film of a metal or
metal oxide that absorbs UV radiation. This sheet, or wafer, is a substrate on which
several layers of different materials are deposited. The upper layer, with thickness
of the order of 1
m, is a photoresist - an organic substance which absorbs UV
radiation that causes volatilization at low temperatures. Other deposited materials
are exemplified by Si, SiO
2
,S
3
N
4
, and Al, which can play the role of a dielectric, a
semiconductor, or a metal in an integrated manufacturing scheme.
AsequenceinthefabricationofpatternsisillustratedinFigure7.3.Thefirststep
of the replication (lithography) process is to establish the replicator pattern on the
sample. Then UV radiation is directed to the sample through the replicator. The
transmitted radiation evaporates a photoresist and transfers the pattern to the sam-
ple in this way (Figure 7.3b). The following stage is the etching process, in which
an underlying film is removed at points where a photoresist has been removed.
This may be accomplished with electron beams, ion beams, X-rays, or by chemi-
cal or plasma methods. The etching process must be anisotropic in that material
must be removed in a vertical direction only. At the same time, the process must
be selective, so that it acts only on the removed material. These requirements seem
to be incompatible. For example, the ion beam method is anisotropic because ions
are directed perpendicular to the surface. But it is not selective because ions de-
stroy the etching layer and the photoresist to an almost equal degree. Discharge
methods that use chemically active particles (O, F, Cl) are selective, as chemical
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