inefficient mass transport away from
the gas/solid surface interface. The
instability of the precursor delivery
rate over time is caused by changes
in the particle size (solids are usually
used as powders) that affect the total
solid surface area. Local variations in
temperature can also alter the precursor vapor pressure.
The most commonly used delivery
system for solid precursor vapors is
the down stream fixed-bed saturator (Figure 1), which doesn’t solve
the above-mentioned problems. It
is not able to saturate with vapors of
a solid precursors carrier gas flows
higher then a few slm (standard liter
per minute). Furthermore, during the
course of a single run, the gas phase
precursor concentration decreases
with time and from run-to-run similar
changes are often observed. Such
non-optimum operation of the process yields films and coatings with
scattered, non reproducible characteristics and is therefore detrimental
to the targeted properties of the
material. A number of alternative
solid precursor delivery methods
have been published or patented.
The simplest approach is to raise the
precursor vessel temperature above
the melting point of the solid, if it is
thermally stable enough. The precursor, (MeCp)Ir(1-5 COD), heated above
its melting point (≈ 40°C) is used
as a liquid precursor in Ir MOCVD
processes. However, that approach
can be used for only a very limited
number of precursors because most
solid precursors start to decompose
at a temperature that is very close to
their melting point.
Alternative delivery systems that
transport solid precursor directly to
a hot zone (flash evaporator) before
the reaction chamber have been proposed, but have encountered little success. This may be due, in some cases,
to the complexity of the device and in
other cases to the physical transport of
powdery precursor to the vaporization
zone. It appears that despite a number
of solutions proposed in the literature
to solve the problem of the controlled
sublimation of solid precursors, none
of them is satisfactory enough to leave
the laboratory environment and to be
integrated into a production line. There
is thus a need for a simple solid precursor delivery device, which could deliver
high, stable and reproductive molar
fractions of precursor in the gas phase
even with solids of low saturating vapor
pressure. Such a device would allow
the use of simple and cheap molecules,
which are not yet commonly used in
CVD and ALD processes.
A new sublimation process and
system has been developed for solid
precursors in the form of powders. It
is based on gas-solid fluidization technology. A fluidized bed is formed in a
vertical cylindrical tube with a perforated plate at its bottom, called a gas
distributor. The powder is placed on
this plate and gas is flowed upwards.
At the appropriate gas flow rate, a fluid-
Figure 1. Down stream fixed-bed saturator
ized bed is formed, i.e. powder behaves
as a fluid due to the intense mixing of
particles generated by the gas flow.
In particular, high thermal and mass
transfer rates exist inside the fluidized
bed, ensuring quasi-perfect isothermal
conditions for the particles. These high
transfer rates ensure that when heated
Figure 2. Schematic description of the fluidized bed sublimator.
