Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1995-02-14
2002-07-23
McDonald, Rodney G. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192220, C204S192230, C204S192260, C204S298160
Reexamination Certificate
active
06423191
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the coating of substrates by means of physical sputtering effect, in which the transfer of kinetic energy from ions of glow discharge plasma, striking the cathode surface leads to ejection of cathode material from the cathode and to subsequent formation of coating onto the substrate.
More particularly, the invention relates to sputtering process carried out in a diode-type apparatus, where glow discharge is established in the working chamber in an atmosphere of ionizable fluid, maintained at reduced pressure between the cathode, constituting a target, and the anode and where cathode atoms, emitted by the bombardment of plasma ions move towards the substrate, mounted in the same chamber.
The present invention also refers to articles, provided with the coating, deposited by the sputtering method.
BACKGROUND OF THE INVENTION
The occurrence of a metallic coating which sputtered on the glass walls of a discharge tube was observed in the last century; the explanation for this phenomena at the beginning of this century, is the ejection of cathode material by positive ions striking the cathode.
Nowadays the sputtering process, which basically can be attributed to the same phenomena, is used on an industrial scale for the deposition of different kinds of coatings on different substrates. A large variety of devices has been developed for this purpose.
A comprehensive review of the physical methods of discharge sputtering deposition as well of equipment configuration, used for this purpose can be found in the monograph “Thin film processes”, edited by John L. Vossen and W. Kern, Academic Press, 1978.
The simplest system, which is employed for deposition of films by sputtering process utilizes the glow discharge between two electrodes (cathode and anode), established within the evacuated working chamber and is commonly referred to as a diode arrangement.
The cathode in these systems is usually planar and constitutes the target, connected to a negative voltage source, capable of supplying voltage of several kV (dc or ac), while the anode is grounded. The substrate holder is mounted within the chamber and faces the cathode.
A stream of gas (the most common sputtering gas is argon) is continuously introduced into the chamber and is evacuated therefrom so as to provide a medium in which a glow discharge plasma, consisting of a gas ions, can be maintained.
The applied negative voltage urges these ions to strike the cathode, while removing its atoms by momentum transfer. The flux of these atoms moves towards the substrate, usually situated in the vicinity of the glow discharge region and condense onto the substrate surface.
In some cases gases or mixtures of gases other than argon are fed into the chamber so as to cause the deposition of a compound, synthesized on the substrate due to interaction between the atoms, which are dislodged from the cathode plate, and the reactive gas species, which are present in the chamber atmosphere.
TiO
2
, synthesized by sputtering a Ti cathode in a reactive atmosphere of mixture of argon with oxygen, can be mentioned as an example of such a compound.
The main advantage, associated with the planar diode arrangement is its simplicity and possibility of applying rather high voltages to the cathode, resulting in ejection and sputtering of atoms with high energies in the range of 10-100 eV, which reach the-substrate fast and thereon form a coating layer, firmly adhering to the substrate surface.
On the other hand, the high energy of plasma particles reaching the substrate is inevitably associated with exposing of the substrate surface to a heat flux with high density (up to several watt per square centimeter) and heating the substrate up to rather high temperatures of 400-600 deg.C.
Such high temperatures might have number of undesirable consequences, e.g.,
preclude the use of sputtering deposition for coating substrates, made of materials, susceptible to these temperatures e.g. plastics,
cause warping in substrates with a large length-to-thickness ratio,
deteriorate adhesion between the coating and the substrate due to the development of excessive thermal stresses in the interface region, because of the difference in linear thermal expansion coefficients of the coating and the substrate,
promote undesirable chemical reactions at the substrate surface, which is exposed to plasma ions and electrons, and the subsequent formation of undesirable compounds.
The other disadvantage of the planar diode arrangement is associated with relatively high gas pressure, which should be kept within the diode source chamber, so as to maintain condition for self-sustained glow discharge. This pressure might affect dispelling of the stream of the target atoms (so-called collision scattering), moving towards the substrate, which in its turn reduces the sputtering rate and prevents the establishing of conditions for formation of homogeneous coating and might even cause deterioration of some properties of the coating.
Reducing of the pressure can, to some extent, decrease the above-mentioned associated negative effects; however, this pressure can not be kept less than a certain minimum, which is 20-100 millibars; otherwise, the density of ions required for sputtering of target atoms falls too rapidly and sputtering rate becomes too slow.
Typical sputtering conditions, employed in planar high voltage diode sources, e.g., for sputtering of Ni in Ar atmosphere, as described in the above monograph are: cathode-to-substrate separation 4,5 cm, voltage 3 kV, pressure 75 millibar (7,5 KPa), current density 1 mA/cm
2
.
It is known that in order to improve sputtering rates at low pressures the ionization efficiency of available ionizing electrons of the gas should be increased. This effect is provided in diode sources, known as magnetrons, in which a transverse magnetic field, normal to the electric field is applied to the target and is so configured that the ExB electron drift currents close on themselves.
The magnetron mode of operation is defined by magnetic focusing of the glow discharge, which results in the formation of an uniform plasma sheet over the cathode, disposed remotely from the substrate surface and thus preventing its excessive heating, seeing that the substrate is no longer subject to the plasma bombardment.
A typical diode source, operating in the magnetron mode is disclosed, e.g., in U.S. Pat. No. 4,006,073, and comprises a cathode target, made of the sputtering material, a substrate holder mounted opposite said target, at least one anode, the means for supplying an ionizable gas, the means for establishing an electric field between the cathode and anode, sufficient to sustain an electrical discharge between them through said gas and means for establishing a magnetic field to extend through the space surrounding said anode and cathode.
Despite the benefits inherent to the magnetron mode of operation, like relatively low temperatures at the substrate surface (in the range of 50-200 deg.C.) and elimination of collision scattering effect due to reduced pressure, this configuration nevertheless suffers from its intrinsic limitations.
Configuration of magnetic field, formed as a closed-on-itself loop, causes a nonlinear current characteristic of the glow discharge area and does not the allow applyication of voltage as high as in planar diode sources.
The typical magnetron-mode operating conditions for magnetron type diode sources, as listed in the above mentioned monograph are:
voltage 800 V, magnetic field 150 G, pressure 1 millibar (100 Pa), current density 20 mA/cm
2
.
The relatively low voltage, employed in magnetron sources is associated with reduced energy, submitted to atoms being ejected from the target and moving to the substrate with energies of several eV instead of several tenths of eV. Reduced energy of target material flux, reaching the substrate is associated with formation of less dense coating and poor adhesion to the substrate.
A further serious disadvantage of the magnetron configuration is associated with r
Khanukov Ilya
Sorokov Boris
Blank Rome Comisky & McCauley LLP
McDonald Rodney G.
Thin Films, Ltd.
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