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Description  |
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BACKGROUND OF THE INVENTION
The invention relates generally to reducing microstructural defects in
line-of-sight deposited films and more particularly to the elimination of
columnar growth defects in physical vapor deposited films by improved
deposition methods.
Line-of-sight deposition and coating techniques are quite useful to
industry. On a macroscopic scale such techniques provide the capability of
uniformly applying a thin film or coating to a surface. Moreover, physical
vapor deposition methods, such as sputtering, evaporation and similar
line-of-sight deposition techniques, make possible the deposition of solid
coatings directly from a vapor state. The resultant coatings have
microscopic characteristics that are unobtainable by any other means.
Physical vapor deposition methods, and especially sputtering, also enable
deposition of films and coatings of a virtually infinite variety of
materials. Such coatings can be amorphous or crystalline, metallic or
nonmetallic, and can be uniformly composed of non-equilibrium combinations
of elements in proportions which ordinarily form, in equilibrium, a
non-uniform composition or structure when deposited by other techniques.
In general, physical vapor deposition employs some mechanism to eject atoms
of coating material from a source or target with sufficient energy to
travel along a line-of-sight to the surface of substrate to be deposited
thereon. Physical vapor deposition includes sputtering, evaporative
deposition, ion plating, and neutralized ion beam coating. It does not
ordinarily include chemical vapor deposition, electroplating, or rapid
solidification coating techniques. Ion plating is a variation of both
sputtering and evaporative deposition which involves the ionization of
atoms in the vapor followed by attraction of some portion of the ionized
atoms to the substrate with an electric field. The principal
characteristic of these techniques is that they utilize a line-of-sight
access of some portion of the material source to the surface to be coated.
The concept of line-of-sight access is broadened slightly in ion plating.
That method modifies slightly the trajectory of ionized atoms of coating
material to enable application of some material to portions of a substrate
that are not on a true line-of-sight from the source. However, all of
these techniques are essentially line-of-sight deposition methods whose
coatings are influenced in generally the same way by line-of-sight
defect-producing mechanisms. One such mechanism is geometrical shadowing,
which produces columnar growth defects as hereafter described and shown.
Since sputtering is the most important of the presently known physical
vapor deposition methods, and is representative of the other methods, the
remainder of this discussion will concentrate on sputter deposition.
However, the problems and principles discussed hereinafter are to be
considered as equally applicable to all physical vapor deposition
techniques and to other line-of-sight deposition methods as well.
Sputtered atoms which are emitted generally in the direction of the
substrate are deposited as a film or coating on the surface of the
substrate. If the substrate and target are aligned parallel plates, and
the minimum angle of adatom incidence is large, then the entire coating
will have a uniformly high quality. However, if the substrate is angled
with respect to the target, is large, is wider than the target, or has a
three-dimensional surface with portions angled from target, then at least
a portion of the coating will be of poor quality. This problem is
illustrated in greater detail in FIGS. 2, 5a to 5c, and 14a to 14f.
It has been determined experimentally that the angle of incidence of the
net flux on the substrate surface strongly influences the quality of the
resultant coating. Geometric shadowing was found to be a principal
mechanism by which columnar growth defect structures separated by open
boundaries are formed. These structures are generally associated with
reduced corrosion resistance and other localized degradation of coating
properties. The results of these studies are reported in the article "The
Influence of Surface Topography and Angle of Adatom Incidence on Growth
Structure in Sputtered Chromium," by J. W. Patten, presented in April,
1979 at the American Vacuum Society's International conference on
Metallurgical Coatings, San Diego, Calif., and published in Thin Solid
Films, Vol. 63, 1979, pages 121-129. Pertinent aspects of these results
are discussed hereinafter with reference to FIGS. 5a to 5c and 14a to 14f.
It would be preferable if line-of-sight deposited coatings, and especially
physical vapor deposited coatings could be formed without defects due to
geometrical shadowing, and particularly without columnar growth structures
and open leaders or boundaries between such structures. The open
boundaries degrade the mechanical, electronic, and other physical
properties of coatings and thus detract from their usefulness in
engineering applications. For example, such coatings fail to protect the
surface of the substrate from penetration of foreign substances,
particularly corrosive fluids. They are also more susceptible to
mechanical failure than coatings lacking such defects. The surface of such
coatings also are often rough. These features are all highly
disadvantageous for protective coatings applied to such substrates as
marine gas turbine vanes and blades.
Several techniques have been tried to eliminate columnar growth defects
from such coatings. One approach involves rotating the substrate as
material is being deposited thereon. This technique results in a uniformly
mediocre coating which still contains columnar growth defects. Another
approach has been to try to manipulate the static geometrics of the target
or the substrate or both so as to deposit uniformly at a right angle
everywhere on the substrate, as disclosed in FIGS. 5 and 5a of U.S. Pat.
No. 4,038,171 to Moss, et al. However, this method also does not
satisfactorily eliminate defects due to geometrical shadowing.
Another technique involves heating the substrate after coating to increase
the lateral thermal diffusion of material deposited thereon to "heal" the
defects and thereby reduce the porosity of the coating. However, heating
sufficiently to diffuse the materials laterally, for example, to a
temperature of about 80% of the Kelvin melting point for a material such
as sputtered copper alloy, allows the deposited materials to segregate
into equilibrium crystallites of different phases. The hotter or the
longer the heat treatment, the greater this tendency toward equilibrium.
Phase segregation reduces both structural and compositional homogeneity of
the entire coating, not only in those areas containing columnar growth
defects, but in those regions having a high quality closed microstructure.
Consequently, one of the principal purposes of physical vapor deposition,
namely obtaining a non-equilibrium homogenous coating structure and
composition, is defeated. Another problem with heat treating is that it
tends to increase grain size in the coating. The disadvantages of
inhomogeneous structure or composition, or large grain size, are readily
apparent to persons skilled in the coating art.
Another difficulty arising from heat treatment is the degradation of
coating-to-substrate adherence. If the thermal expansion coefficient of
the substrate and the coating are quite different, fracturing at their
interface can occur. In addition, vertical diffusion of material away from
the interface is likely to occur producing voids at the interface or, in
some instances, brittle phases. Both of these mechanisms weaken the
coating-to-substrate adherence.
A related technique involves coating at an elevated temperature so that
sufficient lateral diffusion occurs as the coating is deposited to produce
a dense coating. The same disadvantages as those described above apply.
Mechanical treatment of the coating, such as shot-peening, in combination
with heat treatment allows somewhat lower temperatures to be used.
However, shot-peening can also degrade coating-to-substrate adherence,
particularly if the Young's modulus of the substrate differs substantially
from that of the coating, by causing fracturing at the interface. In the
case of very brittle coatings shot-peening without fracturing the coating
is impossible.
Even combining deposition of a first material onto highly cleaned pin
surfaces while rotating, followed by deposition of an overlayer of a
different material and subsequent heat treatment, fails to eliminate
defects due to geometrical shadowing, including columnar growth defects.
Referring to FIG. 18, many voids or leaders remain, and some extend
vertically through more than half of the thickness of the coating. After a
portion of the coating wears away during use, such voids will be exposed.
Accordingly, there remains a need for a physical vapor deposition method
which eliminates columnar growth defects without requiring mechanical or
thermal treatments. For many purposes, it would, at least, be desirable to
obtain a coating in which voids or leaders do not extend completely
through the coating; that is, are limited to a fraction of the thickness
of the coating. It would be even better if such voids or leaders were
limited to about the height of the asperities which cause them. However,
it would be most preferable to have a deposition method which would
provide extremely high quality coatings which are essentially unaffected
by geometrical shadowing.
A variety of sputtering methods have been proposed whose objectives are to
obtain specific coating characteristics. For example, in U.S. Pat. No.
3,021,271, to G. Wehner it was proposed to use ion bombardment of the
substrate to effect controlled resputtering of deposited material to
maintain the overall rate of deposition below a predetermined critical
value. The purpose was to grow monocrystalline coatings rather than the
polycrystalline coatings having small crystallites which are formed by
high rates of deposition. In U.S. Pat. No. 3,736,242, to N. Schwartz et
al., resputtering was a means of controlling the crystalline phase
structure and, thus, the resistivity and temperature coefficients of
deposited films. In U.S. Pat. No. 4,036,723, to B. Schwartz et al.,
resputtering at different rates during deposition to avoid initial
preferential etching of crystal grain boundaries in polycrystalline
substrates and to thereby form a smooth insulative layer on a substrate.
U.S. Pat. No. 4,038,171, to Moss et al., discloses a high deposition rate
sputtering apparatus in which the substrate can be negatively biased
during operation. Resputtering can thus be obtained in such apparatus if
desired.
In each of the foregoing patents, the surfaces of the substrate and the
source of sputtered material were parallel and of approximately the same
lateral dimensions. In such patents, substantially all of the material is
deposited nearly perpendicularly to the substrate surface. Hence, the
problem of geometric shadowing was not addressed in these patents.
U.S. Pat. No. 4,006,070 to King et al. discloses apparatus for sputtering
metal oxide films on substrate surfaces of large lateral dimensions, such
as a windscreen for a vehicle. The apparatus includes multiple, laterally
spaced-apart sources of material which are reciprocated laterally along
the substrate during deposition. The amplitude of reciprocation is
sufficient to cause material to be deposited substantially uniformly over
the entirety to the surface. However, it does not appear that King et al.
addressed the problems of geometrical shadowing. Although the windscreens
are curved, the sources are positioned along a parallel curve so that
shadowing in the curved portions of the surface may be reduced. However,
it appears that some geometrical shadowing is likely to occur on portions
of the substrate below the spaces between the sources. Nevertheless, King
et al. make no attempt to minimize the effect of geometrical shadowing on
the resultant film. Defective regions of the coating are simply covered up
as a result of reciprocation of the source assembly during deposition.
In our own prior work, described in ASME Gas Turbine Division Paper
74-GT-100, "Initial Work on the Application of Protective Coatings to
Marine Gas Turbine Components by High Rate Sputtering", authored by E. D.
McClanahan et al., Mar. 30-Apr. 4, 1974, and in 1977 Tokyo Joint Gas
Turbine Congress Paper No. 64, "Recent Developments in the Application of
High-Rate Sputtering Technology to the Formation of Hot Corrosion
Resistant Metallic Coatings", authored by J. W. Patten et al., May 22-27,
1977, we experimented with several approaches to solving the problems of
obtaining high quality coatings. The former paper discloses coatings on
small planar surfaces, both in an as-sputtered condition and after heat
treating. In some of the experiments, biasing of the substrate to -30 and
-50 volts DC was tried and was found to have some effect on the coarseness
of columnar grain structure. However, changes in deposition temperature
had similar effects and the relative contributions of each parameter were
not determined. The latter paper discloses both high- and low-integrity
sputtered coatings on three-dimensional turbine components, both before
and after heat treating, and includes coatings obtained by rotation of the
substrate. However, no reference is made to biasing of the substrate. Nor
does this paper teach the advantages of eliminating columnar growth
defects due to geometrical shadowing by manipulating the method of
deposition rather than resorting to post-deposition heat treatments.
Finally, neither paper discloses the mechanism by which columnar growth
defects are formed or a method for inhibiting their formation or
propagation.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to improve the microstructure
of line-of-sight deposited films, particularly those formed on three
dimensional surfaces.
Another object of the invention is to provide a line-of-sight method of
deposition for reducing coating defects due to geometrical shadowing.
An additional object of the invention is to form physical vapor deposited
coatings without microstructural defects due to geometric shadowing.
Another object of the invention is to eliminate columnar growth defects in
physical vapor deposited coatings without heat treating subsequent to
deposition of the coatings.
A further object is to provide a method of physical vapor deposition
capable of producing coatings having a closed, non-porous microstructure
uniformly over large planar substrate surfaces or three-dimensional
surfaces.
A further object is to eliminate defects in coatings deposited at
relatively low temperatures, for example, at less than 0.6 times the
absolute melting temperature T.sub.m of the coating material.
Yet another object of the invention is to form sputter-deposited coatings
on three-dimensional and other substrate surfaces having an as-deposited
microstructure free of columnar growth defects substantially uniformly
over such surfaces.
Still further objects of the invention are to form sputter-deposited
coatings having:
1. A closed microstructure characterized by a non-equilibrium composition;
2. A substantially homogeneous microstructure;
3. A substantially homogeneous composition;
4. A very fine grain size; and
5. An as-deposited coating-to-substrate adherence.
The invention takes advantage of the fact that the same line-of-sight
mechanism that produces a high quality deposit in a first region of a
substrate surface and a defective deposit in a second such region can be
used to keep the defective deposit relatively thinner than the deposits of
higher quality. Both the desirable deposition characteristics of sputtered
and other physical vapor deposited coatings and the undesirable columnar
growth defects in such coatings are eliminated when the coated substrate
is subjected to post-deposition heat or mechanical treatments, or when
deposition is carried out at sufficiently high temperatures to effectively
cause heat treatment to occur during deposition. The invention enables
reduction or elimination of columnar growth defects without such
treatments so that the desirable deposition characteristics can be
retained. Heat treatment may still be used following deposition, or
deposition may be carried out at a high temperature, if desired for other
reasons, such as relieving substrate stresses, but these techniques are
not necessary to reduce defects due to geometrical shadowing.
The invention is of a line-of-sight deposition method comprising the steps
of depositing an amount of coating material non-uniformly over a substrate
surface, such that a greater fraction of material is deposited on a first
region at a nearly perpendicular angle of incidence and a lesser fraction
is deposited on a second region at an acute angle, removing a lesser
amount of deposited material uniformly over such surface, and repeating
the foregoing steps in a region adjacent to or overlapping the first
region. Material deposited at a nearly perpendicular angle in a first
region of the surface accumulates more quickly than material deposited at
lesser, or more acute, angles in a second region of the surface. Material
deposited at a nearly perpendicular angle in the first region is
relatively unaffected by geometrical shadowing while material deposited at
increasingly acute angles is increasingly affected by geometric shadowing.
By removing material uniformly from the surface, much of the material
deposited in the second region is removed while a greater portion of the
more desirable material in the first region remains. The more desirable
material preferably has a closed, nonporous microstructure substantially
free of columnar growth defects. After the removal step, the orientation
between the substrate and source can be changed and the nonuniform
deposition and uniform removal steps repeated, so that the more desirable
material is applied to a different region of the surface, still retaining
the more desirable material in the first region. After sufficient
repetition of the steps of depositing from progressively changing
orientations and removing uniformly, the entire surface is coated with
material. Any coating defects due to geometrical shadowing can be held
within specific limits to meet the needs of specific applications of the
coating substrate. Columnar growth defects can be completely eliminated to
obtain coatings of the highest quality. Moreover, the resultant coating
can have all of the native qualities of the material as originally
deposited. These qualities need not be sacrificed by subjecting the
substrate to a post-deposition heat or mechanical treatment in order to
heal defects due to geometrical shadowing. The invention includes a method
for sequentially depositing, removing and changing the substrate to source
orientation. The invention also includes a method for simultaneously
depositing, removing and changing the substrate to source orientation.
The foregoing and other objects, features and advantages of the invention
will become apparent from the following detailed description of preferred
embodiments of the invention, which proceeds with reference to the
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a triode sputtering apparatus which can be used for
performing the present invention, including an enlarged portion of a
cylindrical substrate as viewed in cross-section.
FIG. 2 is an enlarged diagram of portions of the target and substrate of
FIG. 1.
FIG. 3 is a horizontal cross-sectional view of a triode sputtering
apparatus usable for performing the present invention, multiple turbine
blades being mounted as substrates therein.
FIG. 4 is a vertical cross-sectional view of the apparatus of FIG. 3.
FIGS. 5a, 5b and 5c are further enlarged views of portions of the substrate
of FIG. 2 as they would appear following the non-uniform coating step of
the present invention.
FIGS. 6a, 6b and 6c show the substrate portions of FIGS. 5a, 5b and 5c,
respectively, following the step of uniformly removing an amount A of the
previously deposited material.
FIGS. 7a, 7b and 7c show the step of deposition of material from a
different angle onto the substrate surface regions of FIGS. 6a, 6b and 6c,
respectively.
FIGS. 8a-8f are diagrams of a target and a cylindrical substrate
illustrating the steps of a first embodiment of the invention in which the
steps of the method are performed sequentially.
FIG. 9 shows a coated substrate corresponding to that of FIG. 8f when
somewhat less material is removed in the step of FIG. 8c to produce a high
integrity coating to somewhat less exacting requirements.
FIGS. 10a and 10b show two steps in the method illustrated in FIGS. 8a-8f
when two targets are used to deposit onto the substrate.
FIGS. 11a-11e are diagrams similar to FIGS. 8a-8f illustrating the steps of
a second embodiment of the invention in which the steps of the method are
performed simultaneously.
FIG. 12 is a diagram of a substrate similar to that of FIG. 11a
illustrating the use of two targets to deposit material simultaneously
onto the substrate.
FIGS. 13a-13d illustrate steps of the first embodiment of the invention as
used to coat a substrate of large lateral dimensions.
FIG. 14a is an optical photomicrograph taken at 30.5.times. showing a
cross-section of a cylindrical pin following the non-uniform deposition
step of the method of the present invention, as shown in FIG. 8b.
FIGS. 14b-14e are optical photomicrographs taken at 500.times. showing
regions of the pin and coating of FIG. 14a at 0.degree., 60.degree.,
80.degree. and 100.degree. proceeding clockwise around the surface of the
pin.
FIG. 14f is a scanning electron microscope photomicrograph taken at
2000.times. of the surface of the portion of the coating shown in FIG.
14e.
FIGS. 15a-15e are optical photomicrographs of a cross-section of a
cylindrical pin following the uniform removal step of the method of the
invention, as shown in FIG. 8c, the views corresponding to the views of
FIGS. 14a-14e.
FIG. 16 is an optical photomicrograph taken at 32.times. of a cross-section
of a cylindrical pin after coating in accordance with the invention.
FIGS. 16a and 16b are optical photomicrographs taken at 500.times. and
1050.times., respectively, of a pin like that of FIG. 16 following heat
treatment; FIG. 16b being etched to enhance the microstructure.
FIGS. 17a and 17b are optical photomicrographs of a cross-section of a
cylindrical pin like that of FIG. 16 after coating in accordance with the
invention but without subsequent heat treatments, the views corresponding
to the views of FIGS. 16a and 16b.
FIG. 18 is an optical photomicrograph taken at 850.times. of a
cross-section of a cylindrical pin following deposition while
simultaneously rotating the pin and subsequent heat treatment to heal
columnar growth defects.
FIG. 19 is an optical photomicrograph taken at 1000.times. of the coating
of FIG. 16.
FIGS. 20 and 21 are optical photomicrographs taken at 500.times. of a
cross-section of a portion of a turbine blade coated in accordance with
the invention without any prior surface preparation; FIG. 20 being shown
as-polished, FIG. 21 being etched to enhance the microstructure and blade
surface irregularities.
FIG. 22 is an optical photomicrograph taken at 500.times. of a crosssection
of a pin coated in accordance with the invention so | | |
