Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
2000-05-26
2001-11-13
McDonald, Rodney G. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192160, C204S192220, C204S192260, C204S192110
Reexamination Certificate
active
06315873
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns the formation of thin or thick films containing fluorinated compounds and, more particularly, a sputter vapor deposition process that produces such films to possess highly desirable properties. The process of the present invention is seen to have application in various fields of industry, such as in the semiconductor and optical fields where fluorinated compounds are utilized; for example, in anti-reflection coatings, UV-transmitting optics, corrosion-resistant coatings, electro-optics, and lithography masks.
DESCRIPTION OF THE RELATED ART
The vacuum deposition of fluoride films in research and industry is predominantly achieved via vacuum evaporation techniques; namely through the use of boat and e-beam systems which thermally transfer material to the substrate from a source material of roughly the same composition. This method of depositing fluorides, while adequate for many applications, has many of the same drawbacks associated with evaporative vacuum deposition in general. Such drawbacks including high heat transfer to the substrate, poor adhesion without additional processing, low-density films without unduly high substrate temperatures, and difficulty in achieving the desired stoichiometry.
Such drawbacks might be addressed through application of the more energetic sputtering deposition techniques. The term “sputtering” refers to a group of mechanisms by which material is ejected from a solid, or sometimes a liquid, target surface into a vapor form. This latter effect is due, at least in part, in either physical or chemical sputtering, to the kinetic energy transferred to the target atoms or molecules by bombarding particles. Because the particles generated in sputtering processes tend to be more activated—kinetically and chemically—than in the case of evaporation processes, the resultant condensed films tend to be relatively dense, well-adhering, and stoichiometric. These mechanisms are utilized in sputter deposition processes categorized generally as laser sputtering, ion beam sputtering, and plasma (or glow discharge) sputtering. Plasma sputtering, which is utilized in the preferred embodiment of the present invention, refers to a wide variety of sources including the earlier diode sputtering, magnetron sputtering, helical plasma sputtering, electron cyclotron resonance (ECR) sputtering, and so forth.
A number of attempts have been made to develop a successful sputtering process for depositing fluoride films. In most of these previous studies, a sputtering target comprising the desired fluoride material is sputtered with a radio frequency (r.f.) discharge in an inert process gas, such as argon. Earlier attempts to deposit metal fluoride (namely MgF
2
) films experienced many difficulties. The more persistent of such difficulties include fluorine-deficiency in the deposited films, lack of a specular, optical-quality microstructure in the deposited film, and the inability to supply a reasonable power density to the sputtering plasma without fracturing the fluoride target, due to thermal shock. Various attempts have been made to address these difficulties. Martinu
1
, et al, used a fluorinated process gas, consisting of CF
4
+Ar+O
2
, to increase fluorine adsorption; in this case, oxygen was introduced to prevent incorporation of carbon in the deposited film. The films deposited in this latter experiment were nonetheless found to be fluorine-deficient, as well as to contain oxygen impurities. In subsequent years, researchers experimented with other fluorinated gas combinations with an aim to simultaneously achieve optical quality microstructures and the desired fluorine content
2,3
. These later efforts have included use of such fluorinated process gases as CF
4
, SF
6
, and NF
3
. However, in all such cases, the problem of fluorine deficiency has persisted, along with the—perhaps, related—problem of poor film microstructure. In recent years, instances of greater success have been reported in sputter depositing fluoride films. In these recent cases, fluorinated gases were not used. Instead, more radical equipment designs have been applied. For example, Dudney
4
utilized facing-target and off-axis sputtering, with Ar and Ne process gas, to achieve significant improvements in film stoichiometry. Kawamata
5
, et al, utilized an oxygen background and a heated target, purportedly to maintain the sputtered vapor in a molecular state. While these latest efforts are deemed the most successful to date, it is not yet evident whether such fluoride sputtering processes are ready for commercial applications in optical-quality coatings.
SUMMARY OF THE INVENTION
The present invention addresses the previously cited problems in forming films of fluoride compounds, in part, through the utilization of the novel compound, xenon difluoride (XeF
2
). While XeF
2
has been used successfully in etching systems, its highly oxidizing properties require careful manipulation for reliable thin film deposition. The present invention provides, in its first preferred embodiment, a highly safe and repeatable means of depositing optical-quality fluoride thin films by reactive magnetron sputtering. This latter condition is best found through the use of “metallic mode” sputtering and careful control of separate gas flows, so that the unique chemistry of XeF
2
may be selectively applied.
One object of the present invention is to provide a method of sputter depositing thin films of stoichiometric fluorinated compounds of sufficiently low porosity, low optical absorption, and featureless surface morphology, as to be compatible with the requirements of optical applications.
Another object of the present invention is to provide a means of depositing dense fluoride compounds at low process temperatures compatible with plastics and other heat sensitive substrates.
Yet another object of the present invention is to sputter deposit fluorinated compounds while substantially avoiding contamination due to inclusions of process gas precursors.
Another object of the present invention is to provide a relatively inexpensive process for forming films of fluorinated compounds in a high deposition rate sputtering process.
Another object of the invention is to develop a method of forming a fluoride film that allows minimization of the thermal shock and mechanical stress induced in the film or substrate.
Other objects, advantages and novel features of the invention will become apparent from the following description thereof.
REFERENCES:
patent: 4992839 (1991-02-01), Shirai
patent: 2-289495 (1990-11-01), None
patent: 56-98475 (1981-08-01), None
L. Martinu, et al. Thin Films Prepared by Sputtering MgF2in an rF Planar Magnetron. Vacuum. Dec. 1985; p 531-5. vol. 35 n12, Pergamon Press. Great Britain.
J.G. Cook, et al. RF Magnetron Deposition of Calcium Fluoride. Thin Solid Films. Sep. 1992; p 87-90 vol. 217 n1-2.
N. Marechal, et al. Radio Frequency Sputter Deposition and Properties of Calcium Fluoride Films. J. Appl. Phys. Oct. 15, 1993; p 5203-11. vol. 74 n8 APS Press. USA.
N.J. Dudney. Radio Frequency Magnetron Sputter Deposition of CaF2Films. JVST. Mar.-Apr. 1998; p 615-23 vol. 16 n2 APS Press. USA.
K. Kawamata. KMS. (Keep Molecule Sputtering) Deposition of Optical MgF2Thin Films. Vacuum. Dec. 1998; p 559-64 vol. 51 n4. Pergamon Press, Great Britain.
Hilliard Donald Bennett
Lowe Gregory Kent
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