Methods of chemical vapor deposition and powder formation

Details Methods of chemical vapor deposition and powder formation Methods of chemical vapor deposition and powder formation

2001-08-03

2004-09-21

Chen, Bret (Department: 1762)

Coating processes

Spray coating utilizing flame or plasma heat

C427S450000, C427S455000, C427S215000

Reexamination Certificate

active

06793975

ABSTRACT:

BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates to methods of powder formation and thin film deposition from reagents contained in liquid or liquid-like fluid solutions, whereby the fluid solution, near its supercritical point temperature, is released into a region of lower pressure causing a superior, very fine atomization or vaporization of the solution. Gasses are entrained or fed into the dispersed solution and rapidly flow into a flame or plasma torch. The reagents react and form either: 1) powders which are collected; or 2) a coating from the vapor phase onto a substrate positioned in the resulting gases and vapors. Release of the near supercritical point temperature fluid causes dispersion and expansion resulting in a very fine nebulization of the solution, which yields improved powder and film quality, deposition rates and increases the number of possible usable precursors.
II. Background of the Invention
Chemical vapor processing has been used extensively for the production of powders and coatings. Chemical vapor deposition (“CVD”) is the term used when coatings onto a substrate are formed. CVD production of coatings is widespread. Many of these coating are only nanometers thick and smooth to less than 5% percent of coating thickness. Reaction and agglomeration of the reacted vapor material in the gas stream forms powders which can be commercially useful. In fact, nanopowders are required in the formation of nanomaterials which have different properties from those of bulk materials. These materials' properties can be tailored by controlling the cluster size of the nanopowder. Similarly, coatings of less than 50 nm can have properties which are different from thicker films, and the properties change further as the coating thins.
It is desirable to form such powders and coatings at low production and capitalization costs and with simple production processes. However, for many materials there is a very limited selection of available precursors which can be vaporized and used for traditional CVD. Being able to form coatings in the open atmosphere tremendously eases substrate handling and flow through the coating process. In addition to thin films low cost quality thick coatings and bulk materials are also desirable.
Combustion chemical vapor deposition (“CCVD”), a recently invented CVD technique, allows for open atmosphere deposition of thin films. The CCVD process offers several advantages over other thin-film technologies, including traditional CVD. The key advantage of CCVD is its ability to deposit films in the open atmosphere without any costly furnace, vacuum, or reaction chamber. As a result, the initial system capitalization requirement can be reduced up to 90% compared to a vacuum based system. Instead of a specialized environment, which is required by other technologies, a combustion flame provides the necessary environment for the deposition of elemental constituents from solution, vapor, or gas sources. The precursors are generally dissolved in a solvent that also acts as the combustible fuel. Depositions can be performed at atmospheric pressure and temperature within an exhaust hood, outdoors, or within a chamber for control of the surrounding gasses or pressure.
Since CCVD generally uses solutions, a significant advantage of this technology is that it allows rapid and simple changes in dopants and stoichiometries which eases deposition of complex films. In contrast to conventional CVD, where the precursor vapor pressure is a concern which dictates expensive high vapor pressure precursors, the CCVD technique generally uses inexpensive, soluble precursors. In addition, precursor vapor pressures do not play a role in CCVD because the dissolution process provides the energy for the creation of the necessary ionic constituents. In general, the precursor materials used for traditional CVD depositions are between 10 and 1000 times more expensive than those which can be used in CCVD processing. By adjusting solution concentrations and constituents, a wide range of stoichiometries can be deposited quickly and easily. Additionally, the CCVD process allows both chemical composition and physical structure of the deposited film to be tailored to the requirements of the specific application.
Unlike CVD, the CCVD process is not confined to an expensive, inflexible, low-pressure reaction chamber. Therefore, the deposition flame, or bank of flames, can be moved across the substrate to easily coat large and/or complex surface areas. Because the CCVD process is not limited to specialized environments, the user can continuously feed materials into the coating area without disruption, thereby permitting batch processing. Moreover, the user can limit deposition to specific areas of a substrate by simply controlling the dwell time of the flame(s) on those areas. Finally, the CCVD technology generally uses halogen free chemical precursors having significantly reduced negative environmental impact compared to conventional CVD, resulting in more benign by-products.
Numerous materials have been deposited via CCVD technology with the combustion of a premixed precursor solution as the sole heat source. This inexpensive and flexible film deposition technique permits broader use of thin film technology. The CCVD process has much of the same flexibility as thermal spraying, yet creates quality, conformal films like those associated with CCVD. Traditional CVD often requires months of effort to successfully deposit a material. With CCVD processing, a desired phase can be deposited in a few days and at a fraction of the cost of traditional CVD.
By providing these coating capabilities inexpensively, the CCVD process can broaden the commercial opportunity for thin films, including use in tribological, thermal protective, wear, space environment protective, optic, electronic, structural and chemical resistant applications. Thus, government and commercial users can benefit from the advantages of thin films over thick films, including their high adhesion to the substrate, controlled microstructure, greater flexibility, reduced raw material consumption and reduced effect on the operating characteristics and/or dimensions of the coated system.
Ichinose, H., Shiwa, Y., and Nagano, M., Synthesis of BaTiO
3
/LaNiO
3
and PbTiO
3
/LaNiO
3
Thin Films by Spray Combustion Flame Technique, Jpn. J. Appl. Phys., Vol. 33, 1, 10 p. 5903-6 (1994) and Ichinose, H., Shiwa, Y., and Nagano, M., Deposition of LaMO
3
(M=Ni, Co, Cr, Al)—Oriented Films by Spray Combustion Flame Technique, Jpn. J. Appl. Phys., Vol. 33, 1, 10 p. 5907-10 (1994) used CCVD processing, which they termed spray combustion flame technique, by ultrasonically atomizing a precursor containing solution, and then feeding the resulting nebulized solution suspended in argon carrier gas into a propane combustion flame. However, these atomization techniques cannot reach the highly desirable submicron capabilities which are important to obtaining improved coating and powder formation.
U.S. Pat. No. 4,582,731 (the “'731 patent”) discloses the use of a supercritical fluid molecular spray for the deposition of films. However, the '731 patent is for physical vapor deposition (PVD), which differs from the independently recognized field of CVD by having no chemical reagents and normally being operated at high vacuum. Additionally, no flame or plasma torch is used in this method, and only supercritical fluid solutions are considered. Chemical reagents are beneficial because of there physical properties, including higher solubility. The flame and plasma torch enable coatings in the open atmosphere with no additional heat source. The '731 deposition material, however, does not start from a reagent, and thus will not react at supercritical conditions.
U.S. Pat. No. 4,970,093 (the “'093 patent”) discloses the use of supercritical fluids and CVD for the deposition of films. Work related to the '083 patent is described in B. M. Hybertson, B. N. Hansen, R. M. Barkley and R. E. Sievers,. Supercritical

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