Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1999-12-23
2004-01-27
Ryan, Patrick (Department: 1745)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S298040, C204S298070, C204S298120
Reexamination Certificate
active
06682634
ABSTRACT:
FIELD OF INVENTION
This invention relates generally to the deposition of thin films, and more particularly to the deposition of thin films with low contamination by extraneous materials and low damage due to bombardment by energetic ions or neutrals.
This invention can find application in a variety of thin film applications such as the deposition of decorative or protective films, the deposition of conducting films for solid state electronics, or the deposition of magnetic films for recording heads or recording media.
BACKGROUND ART
Sputter deposition is widely used for the deposition of thin films. The most closely related prior art is the sputter deposition from a target that results from an energetic beam of ions being directed against that target. The most widely used ion source for such an application is the gridded ion source described in an article by Kaufman, et al., in the
AIAA Journal
, Vol. 20 (1982), beginning on page 745, incorporated herein by reference. Although it would be less likely, a gridless ion source could also be used. The end-Hall type of gridless ion source is described in U.S. Pat. No. 4,862,032-Kaufman, et al., while the closed-drift type of gridless ion source is described in U.S. Pat. No. 5,359,258-Arkhipov, et al., both of which are incorporated herein by reference. The primary advantage of such sputter deposition apparatus is the low pressure that is possible at the deposition substrate. The high pressure required for the generation of ions is confined to the inside of the ion source. The total gas flow is thereby reduced, compared to having the entire volume within the vacuum enclosure at high pressure, and moderate pumping permits the deposition substrate to be maintained at a low background pressure.
There are other means of depositing thin films using sputter deposition. One is radiofrequency sputtering as described by Maissel in Chapter 4 of
Handbook of Thin Film Technology
(Maissel and Glang, eds.), McGraw-Hill Book Company, New York, 1970. Another is the magnetron as described in the article by Thornton in
the Journal of Vacuum Science and Technology
, Vol. 15 (1978), beginning on page 171, or an improved magnetron as described in U.S. Pat. No. 4,588,490-Cuomo, et al. These other means of sputter deposition operate at higher background pressures and are therefore subject to contamination of the thin film that is deposited with background gases.
There are problems with the prior art of sputter deposition from a target using an energetic beam of ions against that target.
One problem is that the ion beam generated by the ion source must be directed only at the sputter target. Even with carefully machined and expensive ion optics grids, it is common for some energetic ions to strike other hardware besides the target and thereby introduce contamination into the deposited film.
Another problem is the reflection of energetic neutrals from the sputter target. Energetic ions become neutralized in striking the target, and are reflected diffusely to strike the thin film being deposited on the substrate. These collisions with the substrate introduce damage sites in the deposited film.
Yet another problem is the reduced ion current capacity of ion optics for gridded ion sources at energies low enough to minimize the above problem of energetic neutral reflection. As described in the above article by Kaufman, et al., in the
AIAA Journal
, the ion current capacity of these ion optics varies approximately as the three-halves power of the voltages. Operation at low ion voltages—and energies—therefore severely restricts the ion beam current and thus the process rate.
A related problem is the large gas flow required to operate a gridded ion source when the source must be large to offset the reduction in ion current capacity due to operating the source at low voltages.
In summary, complicated and expensive apparatus is required for sputter deposition with energetic ion beams. Attempts to reduce the damage due to energetic neutrals by reducing the ion energy can result in an increase in the size of the ion source used which, in turn, can result in the increase of the gas flow and a need for larger, more expensive vacuum pumps.
SUMMARY OF INVENTION
In light of the foregoing, it is an overall general object of the invention to provide an improved apparatus that confines the high pressure required for ion generation to an ion source and deposits thin films in a low background pressure.
Another object of the present invention is to provide an apparatus that minimizes the overall gas flow required and thereby reduces the vacuum pumping requirement.
A further object of the present invention is to provide an apparatus that minimizes the sputtering that results from ions striking components of the apparatus other than the sputter target.
Yet another object of the present invention is to provide an apparatus in which the energies of the ions striking the target are low enough to minimize the damage by reflected energetic neutrals, without the ion current restrictions of gridded ion optics at low ion energies.
A more specific object of the present invention is to avoid any need for expensive ion optics grids.
Another more specific objective of the present invention is to carry out the deposition at a background pressure that is substantially (a factor of two or more) less than the pressure within the ion source.
In accordance with one specific embodiment of the present invention, the apparatus for sputter deposition within an evacuated volume comprises a compact gridless ion source to generate a beam of ions into which an ionizable gas is introduced and from which ions leave with directed energies at or near the sputtering threshold, a sputter target near that source and located within the beam of ions leaving that source, a grounded shield surrounding the sputter target and defining that portion of the target which is to be exposed to sputtering, and a power supply to bias the target negative relative to ground so that ions are attracted to and sputter only the target. Ground is defined as the potential of the surrounding vacuum enclosure, which is typically at earth ground. Particles sputtered from the target are deposited on a deposition substrate separate from both the ion source and the sputter target.
In the case of an insulating target, the target is biased with a radiofrequency power supply and the bias has a mean negative value rather than a direct-current negative value relative to ground. The rate with which ions arrive at the sputter target is controlled by the rate with which the ions are generated by the ion source, while the energy with which these ions strike the target is controlled by the target bias. The rate and energy with which the ions arrive at the target together determine the rate with which that target is sputtered and the rate at which the thin film is deposited on the substrate.
In using a compact gridless ion source, the high pressure required for generating ions is confined to the ion source and the gas load for pumping is reduced. For ion energies at or near the sputtering threshold, the sputtering from extraneous hardware is reduced or eliminated. In addition, the target biases can be low enough to minimize the damage due to energetic neutrals that result from the reflection of energetic ions striking the target.
REFERENCES:
patent: 3156090 (1964-11-01), Kaufman
patent: 4108751 (1978-08-01), King
patent: 4376688 (1983-03-01), Ceasar et al.
patent: 4693805 (1987-09-01), Quazi
patent: 4747922 (1988-05-01), Sharp
patent: 4911809 (1990-03-01), Wort et al.
patent: 5423971 (1995-06-01), Arnold et al.
Ion Beam Neutralization, CSC Technical Note, pp. 4, 5 and 11.
Baldwin David A.
Hylton Todd L.
Kahn James R.
Kaufman Harold R.
Zhurin Viacheslav V.
Edmundson Dean P.
Kaufman & Robinson, Inc.
Mercado Julian
Ryan Patrick
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