Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Radiopaque Coating for Biomedical Devices
Invented by
David A. Glocker
Mark M. Romach
Technical Field
The present invention relates to medical devices.
Background
Stents have become extremely important devices in the treatment of
cardiovascular
to disease. A stent is a small mesh "scaffold" that can be positioned in an
artery to hold
it open, thereby maintaining adequate blood flow. Typically a stent is
introduced into
the patient's system through the brachial or femoral arteries and moved into
position
using a catheter and guide wire. This minimally invasive procedure replaces
surgery
and is now used widely because of the significant advantages it offers for
patient care
and cost.
In order to deploy a stent, it must be collapsed to a fraction of its normal
diameter so
that it can be manipulated into the desired location. Therefore, many stents
and guide
wires are made of an alloy of nickel and titanium, known as nitinol, which has
the
unusual properties of superelasticity and shape memory. Both of these
properties
result from the fact that nitinol exists in a martensitic phase below a first
transition
temperature, known as MG and an austenitic phase above a second transition
temperature, known as At. Both Mt and At can be manipulated through the ratio
of
nickel to titanium in the alloy as well as thermal processing of the material.
In the
martensitic phase nitinol is very ductile and easily deformed, while in the
austenitic
phase it has a high elastic modulus. Applied stresses produce some martensitic
material at temperatures above At and when the stresses are removed the
material
returns to its original shape. This results in a very springy behavior for
nitinol,
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35 referred to as superelasticity or pseudoelasticity. Furthermore, if the
temperature is
lowered below Mf and the nitinol is deformed, when the temperature is raised
above
Af it will recover its original shape. This is described as shape memory.
Stents having superelasticity and shape memory can be compressed to small
40 diameters, moved into position, and deployed so that they recover their
full size. By
choosing an alloy composition having an Af below normal body temperature, the
stent
will remain expanded with significant force once in place. Remarkably, during
this
procedure the nitinol must typically withstand strain deformations of as much
as 8%.
45 Stents and similar intraluminal devices can also be made of materials
like stainless
steel and other metal alloys. Although they do not exhibit shape memory or
superelasticity, stents made from these materials also must undergo
significant strain
deformations in use.
50 Figure 1 illustrates one of many stent designs that are used to
facilitate this
compression and expansion. This design uses ring shaped "struts" 12, each one
having corrugations that allow it to be collapsed to a small diameter. Bridges
14,
a.k.a. nodes, that also must flex in use connect the struts. Many other types
of
expandable geometries, such as helical spirals, braided and woven designs and
coils,
55 are known in the field and are used for various purposes.
One disadvantage of stents made from nitinol and many other alloys is that the
metals
used often have low atomic numbers and are, therefore, relatively poor X-ray
absorbers. Consequently, stents of typical dimensions are difficult or
impossible to
60 see with X-rays when they are being manipulated or are in place. Such
devices are
called radio transparent. There are many advantages that would result from
being
able to see a stent in an X-ray. For example, radiopacity, as it is called,
would result
in the ability to precisely position the stent initially and in being able to
identify
changes in shape once it is in place that may reflect important medical
conditions.
Many methods are described in the prior art for rendering stents or portions
of stents
radiopaque. These include filling cavities on the stent with radiopaque
material (US
= 6,635,082; US 6,641,607), radiopaque markers attached to the stent (US
6,293,966;
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US 6,312,456; US 6,334,871; US 6,361,557; US 6,402,777; US 6,497,671; US
70 6,503,271; US 6,554,854), stents comprised of multiple layers of
materials with
different radiopacities (US 6,638,301; US 6,620,192), stents that incorporate
radiopaque structural elements (US 6,464,723; US 6,471,721; US 6,540,774; US
6,585,757; US 6,652,579), coatings of radiopaque particles in binders (US
6,355,058),
and methods for spray coating radiopaque material on stents (US 6,616,765).
All of the prior art methods for imparting radiopacity to stents significantly
increase
the manufacturing cost and complexity and/or render only a small part of the
stents
radiopaque. The most efficient method would be to simply apply a conformal
coating
of a fully dense radiopaque material to all surfaces of the stent. The coating
would
have to be thick enough to provide good X-ray contrast, biomedically
compatible and
corrosion resistant. More challenging, however, it would have to be able to
withstand
the extreme strains in use without cracking or flaking and would have to be
ductile
enough that the important thermomechanical properties of the stent are
preserved. In
addition, the coatings must withstand the constant flexing of the stent that
takes place
because of the expansion and contraction of blood vessels as the heart pumps.
Physical vapor deposition techniques, such as sputtering, thermal evaporation
and
cathodic arc deposition, can produce dense and conformal coatings of
radiopaque
materials like gold, platinum, tantalum, tungsten and others. Physical vapor
deposition is widely used and reliable. However, coatings produced by these
methods
do not typically adhere well to substrates that undergo strains of up to 8% as
required
in this application. This problem is recognized in US 6,174,329, which
describes the
need for protective coatings over radiopaque coatings to prevent the
radiopaque
coatings from flaking off when the stent is being used.
Another important limitation of radiopaque coatings deposited by physical
vapor
deposition is the temperature sensitivity of nitinol and other stent
materials. As
mentioned, shape memory biomedical devices are made with values of Af close to
but
somewhat below normal body temperature. If nitinol is raised to too high a
100 temperature for too long its Af value will rise and sustained
temperatures above 300-
400 C will adversely affect typical Af values used in stents. Likewise, if
stainless
steel is raised to too high a temperature, it can lose its temper. Other stent
materials
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would also be adversely affected. Therefore, the time-temperature history of a
stent
during the coating operation is critical. In the prior art it is customary to
directly
105 control the temperature of a substrate in such a situation,
particularly one with a very
low thermal mass such as a stent. This is usually accomplished by placing the
substrate
in thermal contact with a large mass, or heat sink, whose temperature is
controlled.
This process is known as controlling the temperature directly or direct
control.
Because of its shape and structure, controlling the temperature of a stent
directly during
110 coating would be a challenging task. Moreover, the portion of the stent
in contact with
the heat sink would receive no coating and the resulting radiographic image
could be
difficult to interpret.
Accordingly, there is a need in the art for biomedical devices having
radiopaque
coatings thick enough to provide good x-ray contrast, biomedical compatibility
and
corrosion resistance. Further, the coating needs to withstand the extreme
strains in use
without cracking or flaking and be sufficiently ductile so that the thermo-
mechanical
properties of the device are preserved.
Summary
120 The present invention is directed towards a medical device having a
radiopaque outer
coating that is able to withstand the strains produced in the use of the
device without
delamination.
Accordingly, in one aspect of the present invention there is provided an
intraluminal
125 medical device for placement within a vascular structure comprising:
a. a flexible body; and
b. a porous columnar metallic coating comprising a plurality of independent
columnar structures separated from one another by an opening and disposed on
at least
a portion of the flexible body, wherein
130 (i) the porous columnar metallic coating extends in a substantially
normal
direction from a surface of at least a portion of the flexible body;
(ii) flaking of the porous columnar metallic coating being less than 1% of the
porous columnar metallic coated surface when the flexible body and porous
columnar
metallic coating are strained up to 8%; and
135 (iii) the combination of the size of the columnar structures of the
porous
columnar metallic coating and the size of the openings between the columnar
structures
of the porous columnar metallic coating results in an emissivity in the
visible spectrum
range of at least 80%.
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According to another aspect of the present invention there is provided an
intraluminal
140 medical device for placement within a vascular structure comprising:
a. a flexible body fabricated from a shape memory alloy having an Af value
below body temperature; and
b. a porous columnar metallic coating disposed on at least a portion of the
flexible body comprising a plurality of independent columnar structures
separated
145 from one another by an opening, wherein
(i) the independent columnar structures extend in a substantially normal
direction from a suiface of at least a portion of the flexible body;
(ii) the flexible body and porous columnar metallic coating are operable to
withstand strains up to 8%, wherein flaking of the porous columnar metallic
coating
150 is less than 1%; and
(iii) the combination of the sizes of the independent columnar structures of
the porous columnar metallic coating and the sizes of the openings between the
independent columnar structures results in an emissivity in the visible
spectrum range
of at least 80%.
155
According to yet another aspect of the present invention there is provided an
intraluminal stent, comprising:
a. a substrate body fabricated from a shape memory alloy, wherein the shape
memory alloy comprises first Af value below body temperature before a porous
160 columnar metallic coating is applied to a surface of at least a portion
of the substrate
body; and
b. the porous columnar metallic coating comprising a plurality of independent
columnar structures separated from one another by a space, wherein each of the
independent columnar structures extend in a substantially normal direction
from the
165 surface of at least a portion of the substrate body and the porous
columnar metallic
coating has an emissivity in the visible spectrum range of at least 80%.
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Brief Description of the Drawings
These and other features, aspects and advantages of the present invention will
become
better understood with regard to the following description, appended claims,
and
170 accompanying drawings where:
Figure 1 illustrates a stent found in the prior art;
Figure 2 is a top view of a Ta target surrounding stents;
Figure 3 is a side cross-sectional view of the target surrounding stents of
Fig.
2;
175 Figure 4 illustrates a cross section of a conformal coating of Ta on
a strut 12
of a stent;
Figure 5 is a graph showing the reflectance of a Ta coating made according to
the present invention with respect to wavelength;
Figure 6 is a graph showing the x-ray diffraction pattern of a Ta coating made
180 according to the present invention;
Figure 7 is a side cross-sectional view of the target surrounding stents in
position C of Figure 3 with a plate above the stents;
Figure 8 is a top view of a Ta target surrounding stents;
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170 Figure 9 is a side cross-sectional view of the target surrounding
stents of Fig.
8;
Figure 10 is a side elevation view of stents positioned beside a planar target
at
a high angle of incidence; and
Figure 11 shows s scanning electron micrograph of the surface of a Ta coating
175 applied to a polished stainless steel surface.
Description
Tantalum has a high atomic number and is also biomedically inert and corrosion
180 resistant, making it an attractive material for radiopaque coatings in
this application.
It is known that 3 to 10 microns of Ta is sufficiently thick to produce good X-
ray
contrast. However, because Ta has a melting point of almost 3000 C, any
coating
process must take place at a low homologous temperature (the ratio of the
deposition
temperature to the melting temperature of the coating material in degrees
Kelvin) to
185 preserve the Af values of the stents as described previously. It is
well known in the art
of physical vapor deposition that low homologous coating temperatures often
result in
poor coating properties. Nevertheless, we have unexpectedly found that
radiopaque
Ta coatings deposited under the correct conditions are able to withstand the
strains
inherent in stent use without unacceptable flaking.
190
Still more remarkable is the fact that we can deposit these adherent coatings
at high
rates with no direct control of the stent temperature without substantially
affecting Af.
Since normal body temperature is 37 C, the Af value after coating should be
less than
this temperature to avoid harming the thermomechanical properties of the
nitinol.
195 The lower Af is after coating the more desirable the process is.
For a thermally isolated substrate, the equilibrium temperature will be
determined by
factors such as the heat of condensation of the coating material, the energy
of the
atoms impinging on the substrate, the coating rate, the radiative cooling to
the
200 surrounding chamber and the thermal mass of the substrate. It is
surprising that this
energy balance permits high-rate coating of a temperature sensitive low mass
object
such as a stent without raising the temperature beyond acceptable limits.
Eliminating
the need to directly control the temperature of the stents significantly
simplifies the
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coating operation and is a particularly important consideration for a
manufacturing
205 process.
This patent relates to coatings that render biomedical devices including
intraluminal
biomedical devices radiopaque and that withstand the extremely high strains
inherent
in the use of such devices without unacceptable delamination. Specifically, it
relates
210 to coatings of Ta having these properties and methods for applying them
that do not
adversely affect the thermomechanical properties of stents.
An unbalanced cylindrical magnetron sputtering system described in US
6,497,803
B2, which is incorporated herein by reference, was used to deposit the
coatings.
215 Figures 2 and 3 illustrate the setup. Two Ta targets 20, each 34 cm in
diameter and 10
cm high, separated by 10 cm, were used. They were driven with either DC power
or
AC power at 40 kHz. Xenon or krypton was used as the sputter gas. The total
power
to both cathodes was either 2 kW or 4 kW and a bias of either ¨50 V or ¨150 V
was
applied to the stents during coating. Other devices well known to those in the
art,
220 such as vacuum pumps, power supplies, gas flow meters, pressure
measuring
equipment and the like, are omitted from Figures 2 and 3 for clarity.
In each coating run, stents 22 were placed at one of three positions, as shown
in
Figures 2 and 3:
225
Position A- The stents were held on a 10 cm diameter fixture 24 that rotated
about a
vertical axis, which was approximately 7 cm from the cathode centerline. The
vertical position of the stents was in the center of the upper cathode.
Finally, each
stent was periodically rotated about its own vertical axis by a small
"kicker", in a
230 manner well known in the art.
Position B- The stents 22 were supported from a rotating axis that was
approximately
7 cm from the chamber centerline. The vertical position of the stents was in
the
center of the upper cathode.
235
Position C- The stents 22 were on a 10 cm diameter fixture or plate 24 that
rotated
about a vertical axis, which was approximately 7 cm from the cathode
centerline. The
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vertical position of the stents was in the center of the chamber, midway
between the
upper and lower cathodes. Finally, each stent was periodically rotated about
its own
240 vertical axis with a "kicker."
Prior to coating, the stents were cleaned with a warm aqueous cleaner in an
ultrasonic
bath. CrestTM 270 Cleaner (Crest Ultrasonics, Inc.) diluted to 0.5 pounds per
gallon of
water was used at a temperature of 55 C. This ultrasonic detergent cleaning
was done
245 for 10 minutes. The stents were then rinsed for 2 minutes in
ultrasonically agitated
tap water and 2 minutes in ultrasonically agitated de-ionized water. The
stents were
then blown dry with nitrogen and further dried with hot air. The manner in
which the
stents were cleaned was found to be very important. When the stents were
cleaned
ultrasonically in acetone and isopropyl alcohol, a residue could be seen on
the stents
250 that resulted in poor adhesion. This residue may be a consequence of
material left
after the electropolishing process, which is often done using aqueous
solutions.
The Ta sputtering targets were preconditioned at the power and pressure to be
used in
that particular coating run for 10 minutes. During this step a shutter
isolated the stents
255 from the targets. This preheating allowed the stents to further degas
and approach the
actual temperature of the coating step. After opening the shutter, the coating
time was
adjusted so that a coating thickness of approximately 10 microns resulted. At
a power
of 4 kW the time was 2 hours and 15 minutes and at a power of 2 kW the time
was 4
hours and 30 minutes. These are very acceptable coating rates for a
manufacturing
260 process. The stents were not heated or cooled directly in any way
during deposition.
Their time-temperature history was determined entirely by the coating process.
Figure 4 illustrates the cross section of a conformal coating of Ta 40 on a
strut 12,
shown approximately to scale for a 10-micron thick coating. Stents coated in
this
265 manner were evaluated in several ways. First, they were pressed into
adhesive tape to
see if there was any flaking or removal when the tape was peeled away. Next,
the
stents were flexed to their maximum extent and examined for flaking. In all
cases this
flexing was done at least three times and in some cases it was done as many as
ten
times. Finally, the Af values for the stents were measured by determining the
270 temperature at which they recovered their original shape using a water
bath.
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Table 1 summarizes the results. The level of flaking and Af temperatures at
positions
A and B were very similar in the experiments and were averaged to produce the
values shown. The level of flaking was ranked using the following procedure:
275
Level 5: Approximately 10% or more of the coated area flaked.
Level 4: Between approximately 5% and 10% of the coated area flaked.
Level 3: Between approximately 1% and 5% of the coated area flaked.
Level 2: Between approximately 0.1% and 1% of the coated area flaked.
280 Level 1: An occasional flake was observed, but less than approximately
0.1% of the
coated area flaked.
Level 0: No flakes were observed.
Depending on the application, some level of flaking may be tolerated and we
consider
285 Level 2, Level 1 or Level 0 flaking acceptable.
Table 1
Run No Power Gas Bias AC/DC Flaking Af
Appearance
1 2 kW Xe 50 AC 5 29 Dull mottled
appearance
2 2 kW Kr 150 AC 0 59
Shiny metallic appearance
3 4 kW Kr 50 - AC 4 57 Dull
mottled appearance
4 4 kW Xe 150 AC 0 60 Shiny
metallic appearance
2 kW Kr 50 DC 0 23 B lack appearance
6 2 kW Xe 150 DC 0 27 Dull mottled
appearance
7 4 kW Xe 50 DC 4 32 Shiny
metallic appearance
8 4 kW Kr 150 DC 1 38 Shiny
metallic appearance
290
It can be seen from the results with respect to positions A and B that a major
factor in
determining adhesion is the bias voltage. A bias of ¨150 V produces much
better
adhesion overall than a bias of¨ 50 V. This is consistent with many reports in
the
295 literature that higher substrate bias produces better adhesion in many
applications.
However, it also produces greater heating at a given power, as determined by
the Af
values.
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An obvious and important exception to the need for high bias to produce good
300 adhesion is Run Number 5, which has both excellent adhesion and the
lowest value
for Af among the coatings. Moreover, the coating appearance of Run Number 5
was
black, which could be appealing visually. This is indicative of a very high
emissivity
in the visible spectrum, characteristic of a so-called black body. As charted
in Figure
5, the reflectance was measured to be about 0.5% at a wavelength of 400nm and
rises
305 to about 1.10% at 700nm. This is an emissivity of approximately 99% or
greater
across the visible spectrum.
The combination of a very low Af and excellent adhesion is very surprising.
Without
being bound to this explanation, one possibility consistent with the observed
results is
310 that the coating is very porous. Low homologous temperatures (the ratio
of the
substrate temperature during coating to the melting point of the coating
material, in
degrees Kelvin) are known to produce open, columnar coating structures. The
observed black appearance may be the result of an extremely porous coating. It
is
also known in the art that such morphology is also associated with very low
coating
315 stress, since the coating has less than full density. However, even if
this explanation
is correct, the excellent adhesion is very surprising. Typically such porous
coatings
have very poor adhesion and we were able to aggressively flex the coating with
no
indication of flaking.
320 Another possible consequence of the high emissivity of the coating is
the fact that the
radiative cooling of the stent during coating is more effective, thereby
helping to
maintain a low coating temperature.
Furthermore, as described in Utility Patent Application Number 11/040,433,
which is
325 incorporated herein by reference, sputtered Ta typically exists in one
of two
crystalline phases, either tetragonal (known as the beta phase) or body
centered cubic
(known as the alpha phase). The alpha phase of Ta is much more ductile than
the beta
phase and can withstand greater strains. Therefore, the alpha phase of Ta is
more
desirable in this application. Figure 6 is an X-ray diffraction pattern of a
coating
330 made under the conditions of Run No. 5 described above, showing that
the coating is
alpha tantalum. It is known in the art that sputtering Ta in Kr or Xe with
substrate
bias can result in the alpha phase being deposited. See, for example, Surface
and
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Coatings Technology 146-147 (2001) pages 344-350. However, there is nothing in
the prior art or in our experience to suggest that alpha Ta coatings of 10
microns
335 thickness can withstand the very high strains inherent in the use of
stents without
delamination and coating failure. There is also nothing in the prior art to
suggest that
alpha Ta can be deposited in such an open, porous structure.
An open, porous structure may have other advantages as well. For example, the
340 microvoids in the coating would permit the incorporation of drugs or
other materials
that diffuse out over time. In the art, drug-eluting coatings on stents are
presently
made using polymeric materials. A porous inorganic coating would allow drug-
eluting stents to be made without polymeric overcoats.
345 Surprisingly, the stents at position C all had adhesion equal to or
better than the stents
at positions A and B, regardless of conditions. Table 2 illustrates the
surprising
results. (NA indicates coating runs for which no data was taken at those
positions.)
The stents at position C always had very little or no flaking, even under
coating
conditions where stents in positions A or B had significant flaking. As can be
seen
350 from Table 2, this is true over a wide range of coating conditions. The
Af values of
the stents in position C were comparable to those in the other positions, and
in the
case of the AC coatings they were sometimes significantly lower. Stents in the
C
position that were sputter coated in Kr at a pressure of 3.4 mTorr, an AC
power of 2
kW with ¨150 V bias (Run Nos. 2 and 3) had a metallic appearance and an Af
355 between 38 and 42 C. Those coated in the C position using Kr at a
pressure of 3.4
mTorr, a DC power of 2 kW and ¨50 V bias (Run No. 8) were black in appearance
with an Af of only 24 C. An Af of 24 C is virtually unchanged from the Af
values
before coating. Both the metallic and the black samples had excellent
adhesion. The
fact that position C is preferable for adhesion and Af in virtually every case
is
360 unexpected.
Table 2
Total Gas Bias AC / DC Position A Position B
Position C
Power
Af = 29 C Af = 28 C
Af = 30 C
2 kW Xe 50 AC 5 5 0
At = 59 C
Af = 42 C
2 kW Kr 150 AC 0 NA 0
At = 52 C Af = 45 C
Af = 38 C
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2 kW Kr 150 AC 0 0 0
Af= 56 C Af = 58 C
4 kW Kr 50 AC 4 4 NA
Af> 55 C Af> 55 C
4 kW Kr 150 AC 0 0 NA
Af > 55 C
4 kW Kr 150 AC NA 0 NA
Af > 55 C
4 kW Xe 150 AC NA 0 NA
Af=25 C Af = 22 C Af = 24
C
2 kW Kr 50 DC 0 0 0
Af= 37 C Af= 37 C Af = 38
C
4 kW Xe 150 DC 1 5 0
Af = 32 C Af = 33 C Af = 31
C
4 kW Xe 50 DC 3 5 1
Af= 38 C Af= 38 C Af = 49
C
4 kW Kr 150 DC 1 0 0
Af= 25 C Af= 29 C Af=25 C
2 kW Xe 150 DC 0 0 1
365
Stents in position C receive a generally more oblique and lower energy coating
flux
than stents in positions A or B. By an oblique coating flux we mean that the
majority
of the depositing atoms arrive in directions that are not generally
perpendicular to the
surface being coated. Some of the atoms arriving at the surfaces of the stents
in
370 position C from the upper and lower targets will have done so without
losing
significant energy or directionality because of collisions with the background
sputter
gas. Those atoms, most of which will come from portions of the targets close
to the
stents as seen in Figures 2 and 3, will create an oblique coating flux. Other
atoms will
undergo several collisions with the background gas and lose energy and
directionality
375 before arriving at the substrate surfaces. Those atoms, which will
generally come
from portions of the targets at greater distances, will form a low energy
coating flux.
Westwood has calculated ("Calculation of deposition rates in diode sputtering
systems," W. D. Westwood, Journal of Vacuum Science and Technology, Vol. 15
page 1 (1978)) that the average distance a Ta atom goes in Ar at 3.4 mTorr
before its
380 energy is reduced to that of the background gas is between about 15 and
30 cm. (The
distance would be somewhat less in Kr and the exact value depends on the
initial
energy of the Ta atom.) Because our cylindrical targets have an inside
diameter of
approximately 34 cm, substrates placed in the planes of the targets (positions
A and
B) receive a greater number of high energy, normal incidence atoms than those
placed
385 between the targets (position C).
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The geometry of the cylindrical magnetron arrangement shown in Figures 2 and 3
assures that atoms arriving at the surface of the stents in position C will do
so either at
relatively oblique angles or with relatively low energy. Referring to Figures
2 and 3,
390 when the stents are close to the targets, where the arriving Ta atoms
have lost little
energy, the atoms arrive at oblique angles. And when the stents move closer to
the
center of the chamber where the arrival angles are less oblique, they are
farther from
the target surface so that the arriving Ta atoms have lost energy through gas
collisions.
395
It is widely known in the art that when the atoms in a PVD process arrive with
low
energies or at oblique angles to the substrate surface, the result is a
coating that is less
dense than a coating made up of atoms arriving at generally normal incidence
or with
higher energies. The black appearance of the low power DC coatings deposited
with
400 low substrate bias (Run 5 in Table 1 and Run 8 in Table 2) may be the
result of
considerable coating porosity. Normally low-density PVD coatings are not
desirable,
but we have found that conditions that result in relatively low density or
porous
coatings produce very desirable results in this application.
405 Further evidence of the importance of the coating geometry is seen in
the following
experiment. A number of coatings were done in Kr at a pressure of 3.4 mTorr, a
DC
power of 2 kW and a bias of-50 V using the fixture shown in Figure 2 and 3 in
position C. As before, the stents were rotating about the vertical rod as well
as about
their own vertical axes. The coated stents made this way were matte black at
the
410 bottom but had a slightly shinier appearance at the top. In contrast,
when coatings
were done on stents 22 under identical conditions, except that a second plate
24 was
placed above the stents as shown in Figure 7, the stents were a uniform black
from
bottom to top.
415 The non-uniformity in appearance that resulted with the fixturing shown
in Figures 2
and 3 in position C indicates that the coating structure depends on the
details of how
the stents and sputter targets are positioned relative to one another. As
discussed
earlier, when the stents are in position Cl in Figure 3, they receive very
oblique
incidence material from portions of the targets that are close, while the
coating
420 material that arrives from other portions of the target has to travel
farther. Therefore,
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all of the coating flux has arrived at high angles or has traveled a
considerable
distance and has lost energy and directionality through collisions with the
sputtering
gas. When the stents are in position C2 in Figure 3, however, they receive a
somewhat less oblique coating from all directions. In the configuration shown
in
425 Figure 3, position C the bottoms of the stents are shielded from the
more direct flux
from the bottom target by the plate that holds them, but the tops of the
stents are not
similarly shielded from the more direct flux coming from the top target. By
adding
the plate above the stents shown in Figure 7, the more direct coating flux is
shielded
at all points on the stents and the coating material either arrives at
relatively oblique
430 incidence or after scattering from the background gas and losing energy
and
directionality. The plate above the stents restores the symmetry of the
situation and
the coatings on the stents become uniformly black overall.
Other methods of positioning and moving the substrates within the chamber can
also
435 produce results similar to those described above and are within the
scope of the
invention. In another experiment three stents were located as shown in Figures
8 and
9. All three stents 22 were held fixed at their positions within the chamber
and were
rotated about their individual vertical axes during the coating run. The
innermost
stent was 3 cm from the cathode centerline, the middle stent was 7 cm from the
440 cathode centerline and the outermost stent was 11 cm from the cathode
centerline.
The deposition was done at a DC power of 2 kW, a Kr pressure of 3.4 mTorr and
with
the stents biased at ¨50 V. These are the same conditions used in Run No. 8 in
Table
2. All three stents had a matte black appearance and exhibited excellent
adhesion
when tested. Therefore, stents placed at virtually any radial position within
the
445 cathodes and rotating about their individual vertical axes will receive
a satisfactory
coating, provided they are located between the targets in the axial direction.
An alternative, although less desirable, approach to oblique incidence
coatings or
large target to substrate distances in order to reduce the energy of the
arriving atoms
450 through collisions is to raise the pressure of the sputtering gas.
Sputtering takes place under conditions of continuous gas flow. That is, the
sputtering gas is brought into the chamber at a constant rate and is removed
from the
chamber at the same rate, resulting in a fixed pressure and continuous purging
of the
14
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PCT/US2005/009651
455 gas in the chamber. This flow is needed to remove unwanted gases, such
as water
vapor, that evolve from the system during coating. These unwanted gases can
become incorporated in the growing coating and affect its properties.
The high vacuum pumps used in sputtering, such as diffusion pumps,
turbomolecular
460 pumps and cryogenic pumps, are limited with respect to the pressure
that they can
tolerate at their openings. Therefore, it is well known that in order to
achieve high
sputtering pressures it is necessary to "throttle" such pumps, or place a
restriction in
the pump opening that permits the chamber pressure to be significantly higher
than
the pressure at the pump. Such "throttling" necessarily reduces the flow of
gas
465 through the chamber, or gas throughput. Surprisingly, we have found
that the
adherence of the coatings is improved at high gas throughputs.
In one experiment, a cylindrical magnetron cathode with an inside diameter of
19 cm
and length of 10 cm was used to coat a stent with Ta at a sputtering pressure
of 30
470 mTorr in Ar. In order to achieve this pressure, it was necessary to
throttle the
turbomolecular high vacuum pump on the vacuum system. The Ar flow during this
coating was 0.63 Ton-liters per second, corresponding to a throttled pumping
speed
of 21 liters per second. The stent was placed in the center of the cathode,
approximately 9 cm from the target surface. The sputtering power to the
cathode was
475 200 W. According to Westwood's calculations, the average distance a Ta
atom
travels in Ar at 30 mTorr before reaching thermal velocities is between 1.7
and 3.4
cm, depending on its initial energy. Therefore, these coating conditions
should result
in a very low-density coating. The black appearance of the coated stent
confirmed
that this was the case. However, the coating had very poor adhesion.
480
In another experiment, coatings were done on stents in the C position using
the 34 cm
diameter dual cathode shown in Figures 2 and 3. The sputtering gas was Kr at a
pressure of 3.4 mTorr. A DC power of 2 kW was used together with a substrate
bias
of¨ 50 V, the conditions of Run No. 8 in Table2. The Kr flow was 28 standard
cubic
485 centimeters per minute, or 0.36 Ton-liters per second. At a pressure of
3.4 mTorr,
this corresponds to a throttled pumping speed of 104 liters per second during
the
process. The resulting black coatings all flaked at levels between level 1 and
level 3
when tested. The position of the pump throttle was then changed and the Kr
flow was
CA 02560232 2012-07-11
increased to 200 standard cubic centimeters per minute or 2.53 Torr-liters per
second.
490 Coatings were done on stents in the C position at the same power,
pressure and bias
levels as before. The only difference was that the throttled pumping speed
during this
process was 744 liters per second. In this case there were no flakes or cracks
in the
coating evident after testing. A scanning electron micrograph of the surface
of a
coating applied to a polished stainless steel surface under these conditions
is shown in
495 Figure 11. The open, porous nature of the coating is clearly visible.
Based on the foregoing results, we conclude that adequate adhesion does not
result at
low gas throughputs, which are usually necessary to achieve high sputtering
500 pressures. The sputtering pressure and system geometry must be chosen
together so
that the coating flux arrives at the substrate surface either at high angles
of incidence
or after the sputtered atoms have traveled a sufficient distance from the
target to
reduce their energies significantly.
505 While the geometry of a cylindrical magnetron makes this possible in an
efficient
way, as we have shown, the same results can be accomplished using planar
targets as
well. In the case of planar targets, the requirement is to place the
substrates far
enough from the target surface(s) that a large target-to-substrate distance is
achieved.
Alternatively, the substrates could be placed to the side of a planar target
so that the
510 material arrives at high incidence angles. This configuration is
illustrated in Figure
10. Of course, the stent positions 22 shown in the case of planar target 50
make
inefficient use of the coating material. Nevertheless, Figure 10 illustrates
how the
inventive method could be used with geometries other than cylindrical
magnetrons.
515
Although the present invention has been described in considerable detail with
_
reference to certain preferred versions thereof, other versions are possible.
For
example, a device other than a stent can be coated with Ta or another
radiopaque
520 material.
16
CA 02560232 2012-07-11
All features disclosed in the specification, including the claims, abstract,
and
drawings, and all the steps in any method or process disclosed, may be
combined in
525 any combination, except combinations where at least some of such
features and / or
steps are mutually exclusive. Each feature disclosed in the specification,
including
the claims, abstract, and drawings, can be replaced by alternative features
serving the
same, equivalent or similar purpose, unless expressly stated otherwise: Thus,
unless
expressly stated otherwise, each feature disclosed is one example only of a
generic
530 series of equivalent or similar features.
The scope of the claims should not be limited by the preferred embodiments
set forth in the examples, but should be given the broadest interpretation
consistent
with the description as a whole.
17
