Coating processes – Direct application of electrical – magnetic – wave – or... – Plasma
Reexamination Certificate
1999-12-17
2003-01-28
Padgett, Marianne (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Plasma
C427S584000, C427S529000, C427S255190, C427S255320, C427S255360, C427S255250, C427S422000, C427S376200, C438S758000, C438S778000, C438S785000
Reexamination Certificate
active
06511718
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to methods for depositing high quality films of complex materials on substrates at high deposition rates, and apparati for effecting such methods. Particularly, the invention relates to enhanced chemical vapor deposition from liquid sources of high quality thin films of a large variety of complex metal-oxide compounds at high deposition rates, and apparati for effecting such methods. More particularly the invention relates to apparati and methods for fabricating high quality thin films of ferroelectric layered superlattice materials.
2. Statement of the Problem
There are known methods for depositing thin films of complex compounds, such as metal oxides, ferroelectrics, super-conductors, materials with high dielectric constants, gems, etc. Such known methods include RF sputtering, chemical vapor deposition (“CVD”), and spin coating. RF sputtering does not provide thin films of suitably high quality for practical integrated circuit uses, and it is hard if not impossible to control the stoichiometry so as to produce materials within the strict requirements of integrated circuit uses. Spin coating avoids the above defects of sputtering, but does not have good step coverage and suitably high fabrication rates for commercial uses. Present methods of chemical vapor deposition, while having good step coverage, are simply not able to form complex materials of suitable quality for integrated circuit use.
It has been recently discovered that certain layered compounds, referred to herein as layered superlattice materials (or layered superlattice compounds), are far better suited for use in ferroelectric and high dielectric constant memories than any prior art materials. These materials are highly complex, and no method is available to reliably fabricate high quality layered superlattice compounds in commercial quantities, at high deposition rates, and with step coverage that is suitable for making state-of-the-art integrated circuits. The application of known CVD methods to complex materials, such as layered superlattice materials, results in premature decomposition of the reagents and, often, a dry dust, rather than a solid material deposited on the substrate, or result in inferior quality materials that are not suitable for use as active components in an integrated circuit.
In conventional CVD methods, one or more liquid precursors are vaporized to the gaseous state using bubblers, in which carrier gas is bubbled through the liquid precursor. This process step requires that the precursor have sufficient volatility at the bubbling temperature to enable sufficient mass transfer rate for a commercially viable process. Even under good mass transfer conditions, however, the mass transfer rate is difficult to control accurately and precisely. When a plurality of liquid precursors are gasified, any uncontrolled variations in mass transfer rate and mass transport in the process streams result in fluctuations in product stoichiometry. Further, in order to vaporize, that is, gasify, sufficient quantities of liquid precursor at a commercially viable rate it is typically necessary to heat the liquid precursor during bubbling. But, the precursors used in the prior art are typically chemically unstable at the higher temperatures necessary to achieve sufficient mass transfer of the precursor from the liquid phase to the gaseous phase. As a result, premature decomposition of the chemical compounds contained in the precursor occurs. Premature decomposition causes undesirable, uncontrolled changes in the chemical stoichiometry of the process streams and the final product, as well as uneven deposition on the substrate in the CVD reactor. Premature decomposition, therefore, results in poor electronic and ferroelectric properties. Premature decomposition also leads to rapid fouling of the CVD-apparatus, necessitating frequent shut-downs for cleaning.
Another typical process for vaporizing liquid precursors in the prior art is to create a mist of small droplets by pushing the liquid through a needle syringe. The mist is usually injected directly into a deposition reactor. The temperature of the deposition reactor must be high enough to rapidly gasify the mist droplets. Such a procedure, however, does not produce a continuous stream, nor does it result in a stream comprising liquid droplets of small mean particle size with a narrow, controllable size distribution. The gasification of larger-sized liquid droplets requires higher temperatures, which inevitably leads to premature decomposition of the precursors. When the precursors decompose in the deposition reactor away from the substrate, they form particles on the substrate instead of a continuous, uniform film of material. Also, fouling of the apparatus occurs.
Still another conventional method of vaporizing the liquid precursor in a CVD process is to use an ultrasonic mist generator to form a mist of liquid droplets, and then transport the droplets into a heated zone of the deposition reactor itself to gasify the droplets at elevated temperature. It has been found that the ultrasonic mist generators add so much energy to the liquid precursors that they become chemically unstable and prematurely decompose. Also, the sizes of liquid droplets reated by the ultrasonic mist generators vary over a wide range and, therefore, are gasified at different rates. Finally, extra high temperatures are needed in the deposition reactor to gasify the liquid droplets, and the high temperatures lead to premature decomposition.
A common feature of CVD processes and apparati in the prior art is the failure to sufficiently mix misted precursors and gas-phase reactants to ensure desired control of reaction conditions in the CVD and control over the stoichiometry and quality of the deposited thin film. Also, the liquid precursors in the prior art have low vapor pressures, and they tend to decompose at the high temperatures necessary for gasifying them.
It would be helpful to have a method, an apparatus, and liquid precursors for fabricating thin films in integrated circuits in a commercially viable manner that would allow good control of stoichiometry in the deposited thin film, avoid the problem of premature decomposition, and provide the advantages usually associated with CVD processes, such as good step coverage and uniform film quality.
3. Solution to the Problem
The invention solves the above problems by providing methods, precursors, and apparati for the chemical vapor deposition (“CVD”) of thin films of metalorganic compounds, particularly precursors of layered superlattice materials, that avoid premature decomposition of the reagents, provide easily controlled composition and flow rate of a gas phase reactant stream to the CVD reactor, and result in a thin film containing small grains having mixed orientation and good electrical properties.
The invention provides at least one liquid precursor containing at least one complex metalorganic compound.
The invention provides a multi-step gasification (or vaporization) process, comprising: production of a mist of each liquid precursor by a venturi mist generator; and rapid, low-temperature gasification of the mist in a separate gasifier. Preferably, the mists of each liquid precursor are combined and mixed in a separate mist mixer before gasification.
The invention provides a method and an apparatus for forming a mist using a venturi mist generator. The mist generator produces a mist of variable, controllable mass flow rate, comprising droplets of narrow, controllable size distribution. The mist droplets have a mean droplet diameter of less than one micron, and preferably in the range of 0.2 to 0.5 micron. Because the mass flow rate and chemical composition of the mist is known, it is possible to deposit a thin film of uniform, desired composition and stoichiometry.
The invention provides for a venturi mist generator comprising a variable gas inlet passage and a variable gas passage throat. The invention also provides for a venturi mist generator compris
Bacon Jeffrey W.
McMillan Larry D.
Paz de Araujo Carlos A.
Solayappan Narayan
Padgett Marianne
Patton & Boggs LLP
Symetrix Corporation
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