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Solvent-Free Method for Intense Vaporization of Solid Molecular and Inorganic Compounds
By Constantin Vahlas, Herve Guillon, François Senocq, Brigitte Caussat and Samuel Bonnafous
New tools have been developed for vaporization of solid precursors
to meet the demands of high feed rate for CVD, ALD and other deposition processes.
Chemical vapor deposition (CVD) processes have been extensively used in microelectronics
for more than thirty years. Atomic layer deposition (ALD) has been more recently
developed in the same field. The potential implementation of CVD and ALD techniques
in other domains of industrial interest (cutting tools, aeronautics, optics,
energy, etc.) has now been acknowledged; indeed, CVD coatings and films can
be used in numerous applications targeting, for instance, thermal insulation,
thermoabsorption and thermoelectricity, catalysis, magnetic, optical and electronic
devices and high-density data storage, or efficient commutation related to the
switching speed of electronic components such as CMOS transistors.
In order for CVD to successfully meet the high electrical potential of the
above mentioned applications, a number of difficulties have to be resolved.
Because numerous CVD reagents, commonly called precursors, are low vapor-pressure
liquids or solids, one of these difficulties is the production of vapors from
the reactive gases that are decomposed in the deposition chamber to provide
the desired film. Such a production has to be supplied at high rate and must
be reproducible and stable during the whole process. The development of new
evaporation technologies called DLI (direct liquid injection) has allowed difficulties
to be overcome in numerous cases . Nevertheless, as it is necessary to dissolve
solid precursors in an organic solvent when one wants to vaporize them with
a DLI source, this process may yield low quality films (carbon contaminated)
and may raise environmental concerns (organic vapors and/or carbon dioxide release).
Furthermore, several low cost and potentially interesting solid CVD precursors
cannot be vaporized by DLI sources because it is not easy, and sometimes even
impossible to dissolve them in an organic solvent. These precursors are thus
used as powders that are sublimated.
Three primary problems occur with sublimation of solid CVD precursors. The
main problem encountered is the low saturating vapor pressure of most solids
of interest (typically below 100 Pa at room temperature) compared to liquids.
This problem is further complicated by the fact that it is often difficult to
saturate the carrier gas with the precursor vapors. Raising the temperature
of the precursor vessel/sublimator (and all lines and valves in contact with
the carrier gas flow) increases the gas phase precursor concentration; this
can lead to precursor degradation and limits the selection of gas line components.
The second, often-encountered problem is maintaining a reproducible precursor
vapor pressure. During a single run, especially if it is lengthy, the gas phase
precursor concentration may decrease with time. From run-to-run similar changes
are often observed. The third problem is particle contamination of the film.
It occurs if suitable safeguards are not built into the precursor delivery line.
Moreover, the low molar fraction of precursor in the gas phase often implies
longer CVD runs and increases the risk of precursor thermal degradation in the
Inefficient mass and heat transport contribute to the first two of the above
listed problems. In a typical solid precursor delivery system, the inability
to saturate the carrier gas flow with the vapors of the solid is related in
part to the inefficient mass transport away from the gas/ solid surface interface.
This would be a potential problem even if the surface transport of a precursor
was completely surrounded by the gas flow. 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). 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. For instance, 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 and patented  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
fluidized 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
at an appropriate temperature in a bed fluidized by an inert gas like nitrogen
(N2), the powder of the precursor is sublimated, providing high, stable and
fully reproducible vapor concentration. With vapors of a solid precursor carrier
gas, it is possible to saturate flows as high as a few tens of slm.
Figure 2 presents the schematic of the fluidized bed sublimator (FBS). It has
been designed to follow the classical concepts of fluidization engineering.
It is composed of a vertical stainless steel tube, connected at its lower part
with an N2 line. The top part corresponds to an expansion zone, to allow particles
entrained by the gas flow to drop back into the bed. Its dimensions have been
calculated using the concept of transport disengagement height (TDH) for a wide
range of particle diameters. Indeed, since sublimation occurs into the fluidized
bed, the diameter of particle always decreases with time. This mainly impacts
the risk of particle elutriation and presents the possibility of distributor
clogging. As a consequence, an appropriate gas distributor has been designed,
positioned at the bottom of the reactor to keep the whole bed of particles at
rest and to distribute the fluidization gas. A micronic filter has been placed
near the exit in order to collect the finest particles and to avoid their transport
to the CVD reactor. The FBS is fed with pure nitrogen. Mass flow controllers
regulate its flow rate. Temperature is one of the most influential operating
parameters for this sublimation process. It is precisely regulated for the whole
set up, from the exit of the flow meters to the entrance of the upward CVD reactor.
Another extremely important parameter is the fluidization quality of particles.
Only optimum fluidization conditions will provide high and stable vapor concentration.
When the powder is fluidized, the gas pressure drop across the bed (versus gas
velocity), stabilizes on a plateau, corresponding to the particles’ weight-per-unit
surface area. The gas flow rate is then fixed using this criterion, by monitoring
the pressure drop using a differential pressure transducer.
An initial prototype implementing the above-mentioned process has been developed
by CIRIMAT and LGC. It has allowed for the validation of the new concept. Figure
3 shows some results obtained with this prototype. It can be seen that a fixed-bed
sublimator can saturate up to approximately 1 slm of carrier gas with the vapors
of Al(acac)3 while the prototype fluidized bed sublimator can do it up to around
From the initial prototype, KEMSTREAM has developed the first industrial fluidized
bed sublimator Sublibox 50, shown in Figure 4. It features six independent heating
zones and is able to handle up to 50 slm of carrier gas and to work up to 300°C.
It has been designed so that it can saturate using the vapors of a solid precursor
carrier gas, flows as high as a few tens of slm and to operate several hours
without precursor refilling. Its design allows easy precursor powder filling
and easy maintenance. For air sensitive precursors, precursor filling under
inert atmosphere conditions can be done. Sublibox 50 is equipped with a programmable
logic controller (PLC) and a gas panel. An industrial carrier gas heater heats
the carrier gas before it enters the sublimation zone. All the electrical and
pneumatical components of Sublibox 50 are connected to a PLC. It can be fully
controlled from a host computer communicating with the PLC. PC control software
is also provided.
(Click Image For A Larger Version)
- H. Guillon, S. Bonnafous. “Vaporization of Solid or Liquid Organic, Organometallic
or Inorganic Compounds,” Gases & Instrumentation, (May/June 2008) pp. 17-19.
- C. Vahlas, B. Caussat, F. Senocq, W.L. Gladfelter, “Device For Providing
Vapors Of A Solid Precursor To A Processing Device”, US Patent Application,
Publication number: US2008268143 (A1), Application number: US20050792020 20051130,
Priority number(s): FR20040052817 20041130; WO2005EP56358 20051130
Constantin Vahlas is a National Center for Scientific Research (CNRS) research
director at the Inter- University Materials Engineering Center (CIRIMAT ) in
Toulouse, where he is head of the chemical vapor deposition group. He received
a Chemical Engineering degree from the National Technical University of Athens
and a Masters Degree and a Doctorate in Metallurgy from the National Polytechnic
Institute of Grenoble. Dr Vahlas has held visiting fellowships in Metals and
Ceramics Division at Oak Ridge National Laboratory, at the un iversity of Pau
and at the University of Delaware. His actual research interests are focu sed
on metalorganic CVD of metals and oxides for metallurgical and func tional applications.
He can be reached at email@example.com
Herve Guillon is President of Kemstream. Mr Guillon was the R&D Manager
of Jipelec (now a division of Qualiflow Therm) and then of Qualiflow (now called
Qualiflow Therm). Guillon is a graduate of the University of Nice Sophia-Antipolis
in France, where he received his PhD in molecu lar chemistry wi th a specialization
in MOCVD precu rsors for YBa2Cu3O7-x superconduc ting materials and associated
buffer layers deposition. He can be reached at firstname.lastname@example.org.
François Senocq is a graduate of the University of Toulouse, where he received
his Ph.D. in molecu lar chemistry, wi th a specialization in homogeneous catalysis
precu rsors and homogeneous catalysis processes. He is presently a National
Center for Scientific Research (CNRS) Senior Scientist at the Inter-University
Materials Engineering Center (CIRIMAT ) in Toulouse. His actual research topics
are OMCVD of metallic films and particles on flat surfaces and on powders for
metallurgical and func tional applications, and on CVD of polymers.
Brigitte Caussat is an associate professor, teaching at the National Engineering
School in Chemical Arts (ENSIACET) of the National Polytechnic Institute of
Toulouse (INPT ), and researching in the Chemical Engineering Lab. (LG C-CNRS),
where she is head of the CVD (Chemical Vapor Deposition) group. She is an engineer
in Chemical Engineering and received a Ph.D. in Chemical Engineering from INPT
in Feb. 1994. Her research interests are focu sed on (1) CVD of siliconbased
films for nanoelectronics, (2) metal oxides and silicon nanostruc ture synthesis
on powders in fluidized bed by CVD and (3) gas-solid fluidization processes
of micro and nano powders. She has co-authored over 120 publications in international
journals and conferenc e proceedings among whi ch 2 review articles, 2 book
chapters, and 3 patents. She can be reached at Brigitte.Caussat@ensiacet.fr
Samuel Bonnafous is Engineering Manager at Kemstream. He has worked at
Qualiflow (now called Qualiflow Therm) for 4 years, where he develops valves,
MFCs and vaporization systems. He is a graduate of Louisiana State University,
where he received his master’s degree in Mechanical Engineering. He can be reached
at samu el.bonn email@example.com.
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