Coating apparatus – Gas or vapor deposition – Crucible or evaporator structure
Reexamination Certificate
2002-04-22
2004-12-14
Hassanzadeh, Parviz (Department: 1763)
Coating apparatus
Gas or vapor deposition
Crucible or evaporator structure
C118S724000, C427S008000, C427S249100
Reexamination Certificate
active
06830626
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to material coating and, more particularly, to a method and apparatus for coating a substrate with a deposition material in a vacuum.
2. Brief Description of the Prior Art
Coating a substrate with a deposition material typically involves vaporizing the deposition material in a vacuum such that the vaporized deposition material condenses onto a substrate that is at a lower temperature than the temperature of the vaporized deposition material.
In the production of organic-based devices, a is thin, flat, film-like substrate is coated with a chemical coating, usually organic based, on at least one side of the substrate. The substrate material may be glass or a plastic/polymeric material and though typically planar in configuration, may also consist of a curved or non-planar surface. The size of the substrate being coated is generally limited to a few square inches due to technical capability limitations of current material sources.
During fabrication of most organic-based devices, such as organic-based LED displays, organic-based lasers, organic-based photo-voltaic panels, and organic-based integrated circuits, chemicals or deposition materials are typically applied to the substrate in a vacuum, using a point source crucible A, shown in
FIG. 1
, or a modified point source crucible. When the chemicals are heated, the chemicals vaporize and radiate away from the point source crucible A, through an exit aperture B, in a generally cosine-shaped emission plume C. A substrate D is then typically held in a fixed position or rotated within the emission plume C with a planar side E of the substrate D facing the point source crucible A. A certain amount of vaporized chemicals deposits on the planar side E of the substrate D, forming a film coating.
In some applications, modified point sources are used to produce a gaussian (non-uniform).flux distribution. Examples of modified point sources include R. D. Mathis-type boats, Knudsen cells, or induction furnace sources. A general drawback of point or modified point source crucibles, however, is their design. First, the ability to control evaporation rates of chemicals involves sensitive, precise control over material temperatures and temperature gradients with low heat capacities and poor thermal conductivity. Point sources/gaussian material sources typically use radiant reflectors, insulation, and baffling to create good evaporation rates for metals and salts at higher temperatures of 1,000-2,000° C. However, these material sources are inappropriate for evaporating organic-based chemicals at lower temperatures of 100-600° C. Excessive heat applied to many organic-based chemicals will spit the chemicals out of the material sources, destroying any film being grown on the substrate and requiring the vacuum system to be taken out of service in order to be cleaned and reloaded. Another problem is that the vaporized chemicals frequently condense into the exit apertures of the crucibles of point or modified point sources. The condensation of the vaporized chemicals begins to alter or occlude the exit aperture, causing chemicals to fall back into the crucible's heated interior, and spit onto the substrate. This spitting ruins the homogenous distribution of the chemical film, because films having spit defects exhibit higher surface roughness values and may exhibit pinhole defects entirely through the deposited layers. The source aperture condensation also degrades the uniformity of the deposited film by altering the flux emission distribution.
Another disadvantage of both point and modified point source crucibles is that no axis of flux uniformity can be found. Point source and modified point source crucibles produce relatively uniform films only when flux angles are kept small. As shown in
FIG. 2
, flux angles &agr;, &bgr;, and ⊖ are measured from a normal axis N extending from the exit aperture of the point source crucible to lines L
1
, L
2
, and L
3
representing the edge of the cosine-shaped plume C shown in FIG.
1
. The only way to keep the flux angle small, such as the angle &agr; shown in
FIG. 2
, is to greatly increase the separation distance, or throw distance, between the point source crucible A and the planar side E of a substrate, such as those substrates referred to by reference numerals D
1
, D
2
, and D
3
. For example, substrate D
2
would need to be moved to the position of substrate D
3
to be fully coated, while keeping the flux angle &agr; constant. Such a move would increase the throw distance from TD
2
to TD
3
. Similarly, if substrate D
3
is moved to the position of substrate D
1
, i.e., from TD
3
to TD
1
, then only a small portion of substrate D
3
would be coated, and the deposited coating would be much less uniform. Film uniformity is a very important characteristic of organic layers utilized for photonic and electronic applications as the fabricated devices will not operate properly, if at all, if the organic-based films are not maintained at a 95 percent or higher level of uniformity.
Throw distances can be predicted in order to achieve a uniform film of 95 percent or higher. If this uniformity requirement is applied to a 6-inch square substrate, for example, then a throw distance of approximately 2 ½ feet may be required. By comparison, a 24-inch square substrate would require a throw distance of 9 ½ feet. This increasing throw distance destroys the ability to develop a productive process, because the rate of film growth is inversely proportional to the square of the distance between the crucible and the substrate.
Film growth rates of organic-based materials are typically expressed in single Angstroms per second. For example, a throw distance of one foot or less would be desirable for coating a 12-inch substrate with a 95 percent uniform film coating 1000 Angstroms thick. At the one-foot throw distance, a typical chemical deposition rate would be 18 Angstroms per second, which equates to a coating time of approximately fifty-five seconds conversely, at a throw distance of 9 ½ feet, the typical deposition rate is 2 Angstroms per second, resulting in a 1 ½-hour deposition time.
In addition to increasing film growth rates, increases in throw distance significantly increase production costs. First, vacuum chambers must be large enough to accommodate the increased throw distances, requiring larger vacuum deposition chambers as well as more powerful vacuum pumps. Second, there is a substantial waste of expensive chemicals, since an increase in throw distance decreases deposition efficiency. Third, because the vaporized organic material that does not reach the substrate is deposited on an interior wall of the vacuum chamber, the vacuum chamber must be removed from productive service and cleaned more frequently. Cleaning is expensive because some chemicals, such as those used to produce organic liquid electronic displays, are toxic as well as expensive. Costs are further exaggerated because point or modified point source crucibles only hold between 1 and 10 cubic centimeters of chemicals. Therefore, only a few substrates can be coated before the vacuum chamber must be brought to atmosphere, the vacuum chamber cleaned, the crucibles refilled, and the vacuum chamber re-evacuated.
It is therefore an object of the present invention to produce a method and apparatus for coating a substrate in a vacuum that allows larger substrates to be coated without increasing throw distances as the width of a substrate increases, allowing more deposition material to be deposited on the substrate during coating, reducing loading downtime, and reducing cleaning time.
SUMMARY OF THE INVENTION
In order to help solve the problems associated with the prior art, the present invention generally includes a vacuum deposition system for coating a substrate with a deposition material. The vacuum deposition system includes a vacuum chamber and a material source positioned inside the vacuum chamber. The material source has a body w
Hassanzadeh Parviz
Kurt J. Lesker Company
Moore Karla
Webb Zeisenheim Logsdon Orkin & Hanson, P.C.
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