Coating processes – Electrical product produced – Integrated circuit – printed circuit – or circuit board
Reexamination Certificate
2000-07-13
2002-05-07
Talbot, Brian K. (Department: 1762)
Coating processes
Electrical product produced
Integrated circuit, printed circuit, or circuit board
C427S124000, C427S126300, C427S255280, C427S255310, C427S255320, C427S383100, C427S901000
Reexamination Certificate
active
06383555
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to materials and methods for fabricating thin film electrical components in integrated circuits and, more particularly, liquid precursor solutions which are used in misted deposition apparatus for depositing thin films.
2. Description of the Related Art
As is well-known in the art, the electrical components in integrated circuits are made up of layers of thin films which are connected by wiring layers and separated by insulating layers. Simple thin film materials and compounds, such as silicon glass, have been formed using a liquid deposition process. Complex compounds, i.e., compounds containing more than two elements, in the prior art have always been formed using processes such as vacuum sputtering (i.e., E-beam, D.C., R.F., ion-beam, etc.), laser ablation, reactive chemical vapor deposition including metalorganic chemical vapor deposition (MOCVD); and liquid application methods using sol-gels (alkoxides) or carboxylates. However, none of these known methods have been able to produce metal oxides with properties that are entirely satisfactory for use in integrated circuits. In all of the prior art processes, except sputtering, the films produced had significant physical defects, such as cracking, peeling, etc.
It was substantially impossible with the conventional processes, particularly sputtering, to reliably and repeatably produce metal oxides with a specific stoichiometry within tolerances required for integrated circuits. Some processes, like MOCVD, could be dangerous or toxic. Most required high temperatures that were destructive to an integrated circuit, and provided poor “step coverage” of a substrate to be covered, e.g., the prior art techniques resulted in a relatively excessive build-up of deposition of the film at the boundary of any discontinuities on the substrate. In prior art liquid deposition processes, it was impossible to control thickness with the degree of accuracy that is required to manufacture integrated circuits. As a result, up to now, metal oxides and other complex materials have not been used in integrated circuits except for one or two specialty, relatively expensive applications, such as the use of sputtered lead zirconium titanate (PZT) in ferroelectric integrated circuits that were expected to have short life times.
U.S. Pat. No. 5,456,945 to McMillan et al. provided a substantial advance in the art by teaching the use of ultrasonic transducers to generate a volume of mist or aerosol into a deposition chamber. Within the deposition chamber, a DC voltage is applied between a substrate holder and a barrier plate for induced polarization in the aerosol particles. A constant flow of carrier gas, e.g., argon, is used to transport the aerosol to an integrated circuit substrate or wafer for the deposition of liquid thin films. These liquid thin films are dried and annealed to yield the thin films of an integrated circuit.
When the precursor is processed in misted deposition apparatus according to the U.S. Pat. No. 5,456,945, the selection or choice of a liquid precursor affects step coverage, liquid film deposition rate, and the final thin film morphology. The use of some liquid precursor solutions results in no material being deposited as desired. Other solutions provide poor step coverage, and yet other solutions have good step coverage with poor film morphology. There is a need for liquid precursor solutions that function in misted deposition apparatus to provide good step coverage and good film morphology. There is also a need for parameters permitting the design and development of precursor solutions that will function appropriately in misted deposition apparatus.
SUMMARY OF THE INVENTION
The claimed invention overcomes the problems outlined above to advance the art by providing a cosolvent system for use in liquid precursor solutions for misted deposition apparatus. The liquid precursors are processed in misted deposition apparatus to yield metal oxide thin films having good step coverage and good film morphology rendering the metal oxide thin films suitable for use in integrated circuits. Additionally, the cosolvent system enhances the film deposition rate.
Method and apparatus according to the invention utilize a liquid precursor solution to form metal compounds. The solution is processed in a misted deposition apparatus to yield films having good step coverage and good morphology. The liquid precursor comprises a metal organic portion including at least one metal organic compound. The metal organic portion has a total metal content in an effective amount for yielding a solid metal compound during an anneal of a thin film of the liquid precursor solution. For example, the metal organic portion may contain barium, strontium, and titanium metals to yield barium strontium titanate (BST) in an oxygen anneal. The resultant BST has a stoichiometry corresponding to that of the liquid precursor solution for these metals less any volatilization losses of metals during the anneal.
The metal organic portion is mixed with a cosolvent system. The cosolvent system includes a first solvent for use in solubilizing said metal organic portion and a thinning agent for use in reducing surface tension in the liquid precursor solution to a value ranging from 10 to 40 dynes per centimeter. This range of surface tension is more preferably from 14 to 34, and is most preferably from 16 to 26. These surface tensions are measured at the ambient temperature in the intended environment of use. This ambient temperature is preferably about 20° C. The metal organic portion, the first solvent, and the thinning agent form a substantially homogenous mixture.
Suitable materials for use as the thinning agent or surface tension reducing agent are typically low molecular weight polar hydrocarbons, e.g., tert-butyl alcohol, n-butylineamine, diethylamine, ethyl ether, and isopentane. Methanol is particularly preferred for use with 2-methoxyethanol solvents. Methyl ethyl ketone is particularly preferred for use with xylenes, octane, and other apolar solvents. Inorganic compounds, such as ammonia may also be used. The thinning agent should be nonreactive in solution, but low revel reactivity is permitted if the solution will not be fundamentally altered before it is consumed. It is preferred that the thinning agent have a surface tension in air of 75% or less than the liquid precursor solution to which the thinning agent is added, This requirement means that the thinning agent will usually have a surface tension less than 20 dynes per centimeter at 20° C. The thinning agent should also have a boiling point greater than 60° C. at atmospheric pressure if it is to be used in misted deposition apparatus. Films of good morphology can be derived from solutions where the thinning agent is present in an amount up to 60% by volume of all of the solvents in the solution.
Mists or aerosols according to the invention are suspensions of liquid particles in a gas. The particles in the mists or aerosols are made of the liquid precursor solution. The particles typically have sizes in the colloidal size range, namely, at least one single dimension in the range from one nanometer to one micron. In this size range, the surface area of the particle is so much greater than its volume that unusual phenomenon occur, i.e., the particles do not settle out of the solution by gravity and are often small enough to pass through filter membranes. Colloidal sizes are not necessarily essential to the formation of an aerosol. It is necessary to deposit these particles on an integrated circuit substrate, e.g., a silicon wafer, to produce a thin film of the liquid precursor solution on the substrate.
A misted deposition apparatus is required for misting of the liquid precursor. Preferred misted deposition apparatus includes ultrasonic or venturi apparatus. The venturi apparatus uses the well-known principles of an automotive carburetor for misting of the liquid. These devices require manual adjustments with empirical measurements of the corresp
Hayashi Shinichiro
McMillan Larry D.
Paz de Araujo Carlos A.
Patton & Boggs LLP
Symetrix Corporation
Talbot Brian K.
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