Chemical vapor deposition and powder formation using thermal spray

Abstract

A method for chemical vapor deposition using a very fine atomization or vaporization of a reagent containing liquid or liquid-like fluid near its supercritical temperature, where the resulting atomized or vaporized solution is entered into a flame or a plasma torch, and a powder is formed or a coating is deposited onto a substrate. The combustion flame can be stable from 10 torr to multiple atmospheres, and provides the energetic environment in which the reagent contained within the fluid can be reacted to form the desired powder or coating material on a substrate. The plasma torch likewise produces the required energy environment, but, unlike the flame, no oxidizer is needed so materials stable in only very low oxygen partial pressures can be formed. Using either the plasma torch or the combustion plasma, coatings can be deposited and powders formed in the open atmosphere without the necessity of a reaction chamber, but a chamber may be used for various reasons including process separation from the environment and pressure regulation.

Description

I. RELATED CASES[0001] This application is a division of U.S. patent application Ser. No. 09 / 293,867 filed Apr. 16, 1999, which is a divisional of U.S. patent application Ser. No. 08 / 691,853, filed Aug. 2, 1996, now U.S. Pat. No. 5,997,956, which claims the benefit of U.S. Provisional Application Ser. No. 60 / 002,084, filed Aug. 4, 1995, the contents of which are hereby incorporated in their entirety by this reference.II. FIELD OF THE INVENTION[0002] 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 o...

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Benefits of technology

[0019] A reduction in the supercritical temperature of the reagent containing fluid demonstrated superior coatings. Many of these fluids are not stable as liquids at STP, and must be combined in a pressure cylinder or at a low temperature. To ease the formation of a liquid or fluid solution which can only exist at pressures greater than ambient, the chemical precursor(s) are optionally first dissolved in primary solvent that is stable at ambient pressure. This solution is placed in a pressure capable container, and then the secondary (or main) liquid or fluid (into which the primary solution is miscible) is added. The main liquid or fluid has a lower supercritical temperature, and results in a lowering of the maximum temperature needed for the desired degree of nebulization. By forming a high concentration primary solution, much of the resultant lower concentration solution is composed of secondary and possible additional solution compounds. Generally, the higher the ratio of a given compound in a given solution, the more the solution properties behave like that compound. These additional liquids and fluids are chosen to aid in the very fine atomization, vaporization or gasification of the chemical precursor(s) containing solution. Choosing a final solution mixture with low supercritical temperature additionally minimizes the occurrence of chemical precursors reacting inside the atomization apparatus, as well as lowering or eliminating the need to heat the solution at the release area. In some instances the solution may be cooled prior to the release area so that solubility and fluid stability are maintained. One skilled in the art of supercritical fluid solutions could determine various possible solution mixtures without undue experimentation. Optionally, a pressure vessel with a glass window, or with optical fibers and a monitor, allows visual determination of miscibility and solute-solvent compatibility. Conversely, if in-line filters become clogged or precipitant is found remaining in the main container, an incompatibility under those conditions may have occurred.

[0020] The resulting powder size produced by the methods and apparatuses of the present invention can be decreased, and therefore, improved by: 1) decreasing the concentration of the initial solution; 2) decreasing the time in the hot gasses; 3) decreasing the size of the droplets formed; and / or 4) increasing the vapor pressure of the reagent used. Each of the variables has other considerations. For instance, economically, the concentration of the initial solution should be maximized to increase the formation rate, and lower vapor pressure reagents should be used to avoid the higher costs of many higher vapor pressure reagents. Decreasing the time in the hot gasses is countered by the required minimum time of formation of the desired phase. Decreasing the size of the droplets formed can entail increased fluid temperature which is countered by possible fluid reaction and dissolution effects. Similarly, coating formation has parallel effects and relationships.

[0021] Another advantage is that release of fluids near or above their supercritical point results in a rapid expansion forming a high speed gas-vapor stream. High velocity gas streams effectively reduce the gas diffusion boundary layer in front of the deposition surface which, in turn, improves film quality and deposition efficiency. When the stream velocities are above the flame velocity, a pilot light or other ignition means must be used to form a steady state flame. In some instances two or more pilots may be needed to ensure complete combustion. With the plasma torch, no pilot lights are needed, and high velocities can be easily achieved by following operational conditions known by one of ordinary skill in the art.

[0023] Further bulk material can be grown, including single crystals, by extending the deposition time even further. The faster epitaxial deposition rates provided by higher deposition temperatures, due to higher diffusion rates, can be necessary for the deposition of single crystal thick films or bulk material.

Problems solved by technology

However, for many materials there is a very limited selection of available precursors which can be vaporized and used for traditional CVD.

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.

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.

Traditional CVD often requires months of effort to successfully deposit a material.

However, these atomization techniques cannot reach the highly desirable submicron capabilities which are important to obtaining improved coating and powder formation.

Additionally, no flame or plasma torch is used in this method, and only supercritical fluid solutions are considered.

All of the precursors of the '093 patent are carried in the supercritical solution which can limit the usable precursors due to reactivity and solubility in supercritical fluids.

Both Merkle et al. and McHale et al. deposited YBa.sub.2Cu.sub.3O.sub.x from a combusted sprayed solution onto substrates, but the deposition conditions resulted in low quality pyrolysis and particulate type coatings.

al. Even after oxygen annealing, zero resistivity could never be obtained at temperatures above 76.degree. K The solution concentrations used were not reported, but the deposition rates were excessively h

metry. Additionally the resulting droplet size of sprayed solutions was excessively large and the vapor pressure too low for effective vapor depo

The range of desired deposition distances from the plasma source was small due to the rapid temperature drop of the gases.

Many of these fluids are not stable as liquids at STP, and must be combined in a pressure cylinder or at a low temperature.

Conversely, if in-line filters become clogged or precipitant is found remaining in the main container, an incompatibility under those conditions may have occurred.

Thus, materials which are not stable in the presence of oxidants cannot be formed using a flame.

Additionally, the use of hydrocarbon compounds can result in the deposition of extensive carbon.

While some carbon is needed for the formation of carbides, too much carbon could result in high C coatings, and possible elemental carbon.

Keeping the solution below the supercritical temperature until atomization maintains the dissolved amounts of precursor in the region of normal solubility and reduces the potential of developing significant solvent-precursor concentration gradients in the solution.

Small pressure gradients (as they can develop along the precursor-solvent system delivery) can lead to significant changes in solubility as has been observed.

Such solubility changes are potentially detrimental because they may cause the precursor to be driven out of solution and precipitate or react prematurely, clogging lines and filters.

In this case, the sudden pressure drop causes high supersaturation ratios responsible for catastrophic solute nucleation conditions.

This results in a poorly formed flame and, possibly, undesirable liquid contact with the substrate.

This leaves the solutes which can build up and clog the atomizing device.

Precursor molecules then form clusters that adhere to the atomizing device and clog the restrictor.

These spheres are detrimental if dense coatings are desired.

In the no-mist instance, atomization and intermixing is very good but flow stability is reduced, resulting in a flame that can jump from side to side with respect to the direction of the tip.

With such a flame behavior, depositions remain possible, but it can be difficult to deposit films requiring stringent thickness uniformity.

This causes the formation of two separate phases, with the possibility of concentration differences in the two phases due to different solubilities of the solutes.

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