Coating apparatus – Gas or vapor deposition – With treating means
Reexamination Certificate
1998-04-27
2002-02-26
Mills, Gregory (Department: 1763)
Coating apparatus
Gas or vapor deposition
With treating means
C118S728000, C118S7230ER, C118S7230ER
Reexamination Certificate
active
06349668
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus and method to atomize a liquid to form charged particles, which are concentrated into a smaller volume of carrier gas and then deposited onto a substrate. An electric field is used to aid in uniform deposition of the particles.
Technologies for creating thin films on surfaces are basic to the semiconductor industry. Thin films of oxides, nitrides, ceramics, and other materials are used as insulating layers in thin film transistors as part of large-scale integrated circuits (IC) devices. Such devices are used as microprocessors, volatile and non-volatile memories, digital signal processing chips, among others. Conductive metal thin films must also be deposited and be used as conductive pathways, or inter-connects, among thin-films transistors used in an IC or for connection to external circuits. Photo-sensitive materials, which are commonly called photoresists, must also be deposited in thin film form during IC fabrication in order to create the needed geometrical patterns by photo-lithography.
Technologies for thin film deposition are well developed for silicon, and other semiconductor materials, such as gallium arsenide. Commonly used methods for thin film deposition include chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), election beam (E-beam) evaporation and sputtering. In all of these methods, the precursor chemicals used in creating the thin films are produced in vapor form. Atoms or molecules of the vapor are then deposited on the semiconductor wafer surface to create the thin film, the exception being photoresist thin films, which are usually produced by the spin-coating process, in which a solution containing the photoresist and a volatile solvent is first applied to a spinning wafer to form a liquid thin film. The volatile solvent is then allowed to evaporate to produce a photoresist thin film on the wafer.
The technologies described above for fabricating thin film transistors on semiconductor wafers have also been adapted for use in making flat-panel displays. The substrates used for flat-panel displays are usually glass and the areas of the substrate are also much larger than that of the wafer. Currently, the largest wafer used in the commercial production of semiconductors is 8 inches (200 mm) in diameter, although wafers of 12 inches (300 mm) diameter are expected to be in wide-spread commercial use beginning in 1999 or in the year 2000. In comparison, flat-panel displays usually involve substrate areas that are much larger. For use as a television (TV) monitor, a flat-panel may need to be as large as 60″ in diagonal measure to compete with a TV made with a conventional picture tube. Such large area flat panel displays must be made with a process that is economical and capable of being used for large scale commercial production.
There are two major technical difficulties in producing large area flat panels by means of the convention thin film deposition process. One difficulty is that the conventional CVD, PECVD, E-beam evaporation and sputtering process must be carried out in vacuum. The vacuum typically ranges from a few milli Torr to a few Torr in pressure. To carry out the process in vacuum, a vacuum chamber must be large enough to contain the substrates while carrying out the deposition process. The substrates must first be introduced into the vacuum chamber for thin film processing, and then be removed from the vacuum chamber for additional processing. Large vacuum chambers are expensive to build and operate, and the need to introduce and remove substrates from a vacuum chamber slows down the process, making it very expensive to make large flat panels. A second difficulty relates to the operation of the CVD, PECVD, E-beam evaporation, and sputtering processes, all of which involve depositing one molecule or atom at a time. The process is too slow except for the very thin films that can be used in semiconductors.
Other thin film deposition process have also been tried in the past for semiconductor fabrication. Drakitchiev U.S. Pat. No. 4,996,080 describes a photoresist coating process in which the photoresist solution is atomized to form a spray. The spray droplets are then introduced into a chamber and allowed to settle by gravity onto a substrate at the bottom of the chamber. The substrate is then spun to form a uniform coating of photoresist on the substrate. Donovan et al. U.S. Pat. No. 5,229,171 describes a similar approach in which a photoresist spray is first created and then allowed to deposit on the substrate in a vacuum. Electrodes are incorporated in the apparatus to create an electric field to aid in the deposition process.
The need to create thin films of complex chemicals with special electrical properties has led some inventors to develop thin film deposition by droplets. One such invention is that described by McMillan et al. in U.S. Pat. No. 5,316,579, in which a fine mist is formed by a rotating turbine blade within an enclosure. The mist is then withdrawn under vacuum and allowed to deposit onto a substrate in vacuum by gravity. Methods of creating thin films of ferroelectric, super-conducting, and high dielectric thin films by the method are claimed. Other patents such as U.S. Pat. Nos. 5,456,945, 5,540,772, 5,614,252, and 5,688,565 relate to the improvement of the so-called Liquid Source Misted Chemical Deposition (LSMCD) process and application of the process for creating thin films of barium strontium titanate (a high dielectric constant material) or layered superlattice materials, among others. In all of these inventions, the substrate is placed in a vacuum chamber, and the deposition is also carried out in vacuum, again leading to slow processing and expensive equipment.
As discussed just above, a variety of methods have been developed to atomize liquid to form droplets. Centrifugal atomizers, ultrasonic nebulizers, pump sprayers, and compressed air atomizers
ebulizers are some examples of atomization or nebulization devices that are used in spray painting, in personal care products, such as hair sprayers, deodorizers, and drug delivery devices to deliver drugs in aerosol form into the human lung. Most of these devices produce droplets that are either uncharged or carry only a weak natural electrical charge. The most well known exception is electrostatic spray painting in which a high electrostatic charge is placed on the atomized paint droplets by applying a high voltage to the paint during atomization. Because the paint is held at a high voltage, the paint must be electrically non-conductive. This restricts the conventional electrostatic spray guns to organic solvent based paints. When water based paints such as latex paints and other paints consisting of dispersion of paint in water is to be sprayed, the conventional electrostatic spray guns cannot be used.
In integrated circuit applications, there is considerable interest in recent years in applying thin films onto a semiconductor wafer surface using aerosol deposition processes. These processes can also be used to provide thin film deposition in manufacturing flat-panel displays. Typically, the precursor chemical used in the aerosol deposition process is a chemical compound dissolved in a solvent or a complex mixture of several chemical species dissolved in a solvent. To preserve the chemical nature of the material or the composition of the desired chemical elements in the thin film, the liquid solution is atomized to form a droplet aerosol—also referred to as a mist—which in turn is deposited onto the surface. Conventional chemical vapor deposition using a vapor source does not work well with such materials because the vaporization process often alters the chemical nature or the mixture composition which can lead to inferior quality films. An example of the misted deposition process used in fabricating ferroelectric and other high dielectric thin films such as BST (barium, strontium titanate) or SBT (strontium bismuth titanate is that described McMillan et al. U.
Liu Benjamin Y. H.
Marple Virgil A.
Sun James J.
Mills Gregory
MSP Corporation
Westman Champlin & Kelly P.A.
Zervigon Rudy
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