Virtues...
|
Consider a typical capacitive plasma. The electron temperature
might be 3 eV; the ion and neutral temperatures are much smaller. As
shown in the table below, the number of electrons with sufficient
energy to dissociate a molecule is 10 to 100 times larger than the
number that can ionize; the plasma will produce a large supply of
excited and dissociated molecules and thus reactive radicals, even
for a modest fractional ionization. |
|
substance |
dissociation energy (eV) |
ionization energy (eV) |
n(diss)/n(ion) @ Te = 3 eV |
|
H2 |
4.5 |
15.4 |
38 |
|
O2 |
5.2 |
12 |
9.6 |
|
CH4 |
4.5 |
12.6 |
15 |
|
F2 |
1.6 |
15.7 |
109 |
|
|
The first reason to employ a plasma in deposition is thus to
crack relatively stable molecules and encourage deposition at much
lower temperatures and pressures than would be required for thermal
CVD. |
|
film |
precursors |
thermal deposition |
plasma-enhanced |
|
silicon nitride |
SiH4 or SiH2Cl2 and NH3 |
750 C |
200-500 C |
|
silicon dioxide |
SiH4 and O2 |
350-550 C |
200-400 C |
|
|
TEOS and O2 |
700-900 C |
300-500 C |
|
amorphous silicon |
SiH4 |
550-650 C |
200-400 C |
TEOS = tetraethyl orthosilicate, Si(OC2H5)4 |
|
The second reason to use plasma deposition is more subtle but of
great importance. Recall that surfaces exposed to a capacitive
plasma are subject to bombardment by energetic ions, whose kinetic
energy can vary from a few eV to 100's of electron volts. Ion
bombardment of this nature has very significant effects on the
properties of the deposited film. Increasing ion bombardment tends
to make films denser and cause the film stress to become more
compressive.
It is often the case than dielectric films which are porous and
under tensile stress cause reliability problems in IC fabrication.
Plasma-enhanced deposition techniques can deposit dense films at
temperatures hundreds of degrees less than required for thermal
densification. However, excessive compressive stress can also lead
to impaired reliability. The ability to adjust stress, through
changes in process conditions, chamber geometry, or excitation (dual
frequency mixtures) is of considerable importance in successful
semiconductor processing. |
 |
|
A final important benefit of plasma deposition is the ability to
easily clean the reactor. For example, by introducing a
fluorine-containing gas (e.g. CF4) and igniting a plasma, one can
clean silicon, silicon nitride, or (with rather more difficulty)
silicon dioxide from the electrodes and chamber walls. Chamber
cleaning is of great practical importance; thick films built up on
the parts of a chamber may "spall" (break off), creating
particles which can fall onto the substrates and cause defects in
circuit patterns, reducing the yield of good circuits from the
process. |
|
...and Vices...
|
The bombardment of surfaces with ions also leads to sputtering:
the displacement of atoms from the surface into the gas phase by
incoming ions. The displaced ions diffuse through the gas and can
land anywhere in the chamber, including on the substrates.
Sputtering can be an important source of trace metallic
contamination in PECVD films.
Sputtering of substrate material can be intentionally exploited
to improve the conformality and gap filling ability of deposited
films: this is typically done using high-density plasma reactors,
and will be discussed in more detailed in the section on reactor
designs. |
 |
|
Ion bombardment is different on horizontal and vertical surfaces;
thus, the composition and density of films can depend on the
topography. |
 |
|
Plasma deposition of metallic or other highly conductive films is
challenging: the deposited film tends to short out the powered
electrode of a capacitive plasma reactor, or coat the dielectric
window of an inductive reactor, shielding the chamber from the
magnetic field. |
|
|
The plasma generation apparatus adds considerably to the
complexity and cost of reactors, and complicates operation. The
plasma requires one or more power supplies, each with an appropriate
matching network, and electrically insulating but mechanically sound
materials for isolating powered electrodes. Substrate heaters must
be electrically isolated from the plasma. Choice of materials is
influenced by the possibility of contamination. |
|
|
