U.S. patent application number 15/136809 was filed with the patent office on 2016-08-18 for three-dimensional thin-film nitinol devices.
The applicant listed for this patent is NSVascular, Inc.. Invention is credited to Alfred David Johnson, Colin Kealey.
Application Number | 20160235564 15/136809 |
Document ID | / |
Family ID | 52993521 |
Filed Date | 2016-08-18 |
United States Patent
Application |
20160235564 |
Kind Code |
A1 |
Johnson; Alfred David ; et
al. |
August 18, 2016 |
THREE-DIMENSIONAL THIN-FILM NITINOL DEVICES
Abstract
A method of manufacturing three-dimensional thin-film nitinol
(NiTi) devices includes: depositing multiple layers of nitinol and
sacrificial material on a substrate. A three-dimensional thin-film
nitinol device may include a first layer of nitinol and a second
layer of nitinol bonded to the first layer at an area masked and
not covered by the sacrificial material during deposition of the
second layer.
Inventors: |
Johnson; Alfred David; (San
Leandro, CA) ; Kealey; Colin; (Los Angeles,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NSVascular, Inc. |
Los Angeles |
CA |
US |
|
|
Family ID: |
52993521 |
Appl. No.: |
15/136809 |
Filed: |
April 22, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/US2014/061836 |
Oct 22, 2014 |
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15136809 |
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61894826 |
Oct 23, 2013 |
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61896541 |
Oct 28, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 14/165 20130101;
A61L 31/088 20130101; C23C 14/0005 20130101; C23C 14/042 20130101;
C23C 14/34 20130101; A61F 2230/0069 20130101; A61F 2/844 20130101;
A61F 2002/068 20130101; A61F 2/06 20130101; B81C 1/00476 20130101;
C23C 14/5846 20130101; C23F 1/44 20130101; A61F 2/90 20130101; A61F
2210/0076 20130101; A61F 2240/001 20130101; A61F 2002/823 20130101;
A61F 2/86 20130101; A61F 2230/0017 20130101; A61F 2210/0014
20130101 |
International
Class: |
A61F 2/90 20060101
A61F002/90; C23C 14/58 20060101 C23C014/58; A61F 2/844 20060101
A61F002/844; C23C 14/34 20060101 C23C014/34 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with government support under NIH
grant 1-R41-NS074576-01 awarded by the National Institutes of
Health (NIH). The government has certain rights in the invention.
Claims
1. A method comprising: deep reactive ion etching a pattern of
grooves on a surface of a semiconductor substrate, the grooves
corresponding to fenestrations in a desired three-dimensional
nitinol structure; depositing a lift-off layer on the grooved
semiconductor substrate surface; depositing a first NiTi layer over
the lift-off layer; depositing a sacrificial layer to partially
cover the first NiTi layer, the sacrificial layer corresponding to
a lumen in the desired three-dimensional nitinol structure; and
depositing a second NiTi layer over the sacrificial layer.
2. The method of claim 1, further comprising: removing the lift-off
layer and the sacrificial layer so that the first and second NiTi
layers are separated from the semiconductor substrate and so that
the lumen is formed in the resulting three-dimensional nitinol
structure.
3. The method of claim 1, wherein depositing the lift-off layer
comprises depositing a copper or chromium lift-off layer.
4. The method of claim 1, wherein depositing the sacrificial layer
comprises depositing a chromium sacrificial layer.
5. The method of claim 1, wherein depositing the sacrificial layer
comprises depositing the sacrificial layer through a first mask,
the method further comprising: depositing an aluminum bonding layer
onto the first NiTi layer through a reverse mask prior to
deposition of the second NiTi layer, the reverse mask being
approximately a reverse image of the first mask.
6. The method of claim 5, further comprising heating the aluminum
bonding layer so that the aluminum bonding layer bonds the first
NiTi layer to the second NiTi layer.
7. The method of claim 2, further comprising inserting a mandrel
into the lumen of the three-dimensional nitinol structure, and
heating the three-dimensional nitinol structure while it is on the
mandrel to crystallize the first and second nitinol layers.
8. The method of claim 7, wherein the three-dimensional nitinol
structure is a flow diverter stent cover, the method further
comprising covering a flow diverter stent with the flow diverter
stent cover.
9. A nitinol stent cover, comprising: a first nitinol layer; an
aluminum bonding layer; and a second nitinol layer, wherein the
aluminum bonding layer is configured to bond longitudinal edges of
the first nitinol layer to longitudinal edges of the second nitinol
layer.
10. The nitinol stent cover of claim 9, wherein the first nitinol
layer and the second nitinol layer each includes an array of
fenestrations.
11. The nitinol stent cover of claim 10, wherein the fenestrations
are diamond-shaped.
12. The nitinol stent cover of claim 11, wherein the first and
second nitinol layers form a wire mesh having a width of 5 to 20
microns.
13. The nitinol stent cover of claim 11, wherein each
diamond-shaped fenestration has a longitudinal extent of
approximately 300 microns and a lateral extent of approximately 150
microns.
14. The nitinol stent cover of claim 11 where the percent metal
coverage of the nitnol film is .ltoreq.15% when covering the
fully-expanded stent backbone.
15. The nitinol stent cover of claim 11 where the density of
fenestrations is between 15 and 25 fenestrations per square
millimeter when covering the fully-expanded stent backbone.
16. The nitinol stent cover of claim 9, further comprising a flow
diverter stent within a lumen of the nitinol stent cover.
17. The flow diverter stent of claim 16 where the percent metal
coverage of the total device (i.e. thin film nitinol cover and
stent backbone is .ltoreq.20%.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of International Patent
Application No. PCT/US2014/061836, filed Oct. 22, 2014, and
entitled "THREE-DIMENSIONAL THIN-FILM NITINOL DEVICES," which
claims the benefit of U.S. Provisional Application No. 61/894,826,
filed Oct. 23, 2013, and entitled "SPUTTERED TiNi THIN FILM," and
U.S. Provisional Application No. 61/896,541, filed Oct. 28, 2013,
and entitled "THREE-DIMENSIONAL THIN-FILM NITINOL DEVICES," which
are hereby incorporated by reference in their entirety.
TECHNICAL FIELD
[0003] The present disclosure generally relates to intravascular
devices and, more particularly, to production, using Nitinol
thin-film techniques, of devices used for treatment of intracranial
aneurysm.
BACKGROUND
[0004] There is a relatively narrow band of temperature in which
nitinol can be mechanically stressed so as to transition at least
partially into the martensite phase from the austenite phase
despite being above the transformation temperature. This property
of nitinol is referred to as superelasticity and is quite
advantageous in that--as the name suggests--superelastic nitinol is
very flexible compared to conventional metal alloys. If the stress
is removed and the nitinol is above the transformation temperature,
the nitinol will revert back to the austenite phase and assume its
unstressed original shape. For example, a cylindrical nitinol stent
may be deformed into the superelastic state so that it can be
packaged and delivered into a blood vessel using a catheter. As the
stent is released from the catheter, the stent reverts to its
original cylindrical shape in the blood vessel. Nitinol is thus
also denoted as a shape memory alloy.
[0005] In one application, nitinol may be used to construct
neurovascular flow diverter nitinol stents that may be placed in
blood vessels in the region of a cerebral aneurysm. The flow
diverter stent essentially takes the shape of the blood vessel
prior to the formation of the aneurysm, which is then cutoff from
the blood flow. The blood within the diverted aneurysm clots, which
neutralizes the aneurysm. Although such flow diverter therapy shows
great promise, its application is extremely challenging. The
affected cerebral vessels may be very small--for example, a vessel
to be stented may have a diameter of just three millimeters such
that they are very delicate and prone to rupture. Balloon expanded
stents are thus too risky for neurovascular applications. In
contrast, a superelastic nitinol stent is far safer and is also
biocompatible.
[0006] To choke off the aneurysm, flow diverter stents are sheathed
in a flow diverter cover. The cover has to satisfy two opposing
goals. On the one hand, the cover should inhibit blood flow into
the aneurysm so that its blood pools and thereby clots. A
completely sealed cover would thus best satisfy such a goal. On the
other hand, the aneurysm may be adjacent to various feeder vessels
that branch off from the area to be stented. If these feeder
vessels are choked off by the flow diverter stent cover, the
patient may suffer an ischemic stroke, a potentially catastrophic
complication. To achieve these conflicting goals, the flow diverter
cover may comprise a fine wire mesh made from a thin film nitinol
(for example, 50 microns or less in thickness) to allow blood to
escape from the flow diverter stent into any feeder vessels that
would otherwise be occluded. Fine-wire-mesh thin-film flow diverter
nitinol stent covers with perforations of 100 to 300 microns in
length offer great promise. The "wire" in the fine wire mesh should
be quite thin (for example, 5 to 20 microns in diameter) because it
is the edges of the wire that assist in the flow diverting effect.
But it is very challenging to form a fine wire mesh thin film
cylindrical nitinol stent cover.
[0007] In particular, thin film nitinol is conventionally
manufactured by being sputtered onto a suitable substrate such as
silicon. The sputtering is problematic, however, in that the
resulting thin film nitinol is prone to having an undesirable
crystalline structure as opposed to a desired amorphous state. An
amorphous film can be crystallized by heating to approximately
500.degree. C. in a process known as annealing. Such a crystalline
structure is essential is to achieving the austenite-to-martensite
phase change that is the hallmark of a shape memory alloy. But
conventional sputtering techniques will often form a thin film
having a columnar crystalline structure. The columns are only
loosely bound with each other such that the resulting film is quite
brittle and unsuitable. Accordingly, there is a need in the art for
improved thin film nitinol manufacturing techniques that can
reliably form high-quality amorphous thin film that may be
subsequently crystallized through annealing.
[0008] Setting aside the difficulties with regard to forming
amorphous thin film nitinol, it is desirable that the resulting
stent cover formed from suitable thin film nitinol be fenestrated
as discussed earlier. To form openings in the thin film, it is
conventional to etch the film using photolithographic techniques.
The resulting opening can then be expanded by stretching the etched
thin film nitinol to fully open up the desired fenestrations such
that the film forms a wire mesh analogous to a chain-link fence
except that there is no weaving of the resulting wire mesh. The
wire mesh may be relatively thin in comparison to the
fenestrations. For example, the fenestrations may have a length of
approximately 300 microns whereas the wire itself may be just 20
across or even thinner. The resolution of wet etching is relatively
coarse such that if the wire mesh is etched to the desired thinness
(for example, 5 to 20 microns in diameter), the mesh is then prone
to tearing and other flaws. The resolution of wet etching is
relatively coarse such that if the wire mesh is etched to the
desired thinness (for example, 5-20 microns in diameter), the mesh
is then prone to tearing and other flaws.
[0009] The substrate upon which the nitinol is sputtered includes a
release layer so that the etched thin film nitinol can be removed
from the substrate. But the etched thin film nitinol is essentially
two dimensional (if one ignores the third dimension resulting from
its relatively small thickness). This two-dimensional thin film
must be sealed onto itself in some fashion to form in a cylinder or
other type of three-dimensional structure. To seal one edge of the
thin film to another edge, it was known to use glue or stitching.
But nitinol bonds poorly with glue. Similarly, stitching opposing
edges together is also problematic given the relatively tiny
dimensions of the resulting wire mesh.
[0010] Given the difficulties with joining layers of nitinol to
form a three-dimensional structure, it is also known to deposit
nitinol onto a cylindrical mandrel to form a cylindrical nitinol
film. But such deposition is not amenable to mass production as the
mandrel results in just one cylindrical structure. In contrast,
conventional planar techniques can mass produce assorted
cylindrical structures simultaneously across a wafer substrate. In
addition, deposit onto mandrel produces a solid film that must then
be fenestrated upon removal from the mandrel. The resulting
cylindrical structure is not amenable to photolithographic etching
so it is fenestrated using a laser, which results in relatively
coarse features. Accordingly, there is a need in the art for
improved techniques for manufacturing fine wire mesh thin film
nitinol three-dimensional structures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1A is a plan view diagram illustrating a portion of a
silicon wafer substrate and associated structures, in accordance
with an embodiment of the present disclosure.
[0012] FIG. 1B is a more detailed view of a portion of FIG. 1A as
indicated.
[0013] FIG. 1C is a more detailed view of another portion of FIG.
1A as indicated.
[0014] FIG. 1D is a cross-sectional view of the silicon wafer
substrate of FIG. 1B along dashed line D prior to deposition of the
nitinol layer.
[0015] FIG. 2 is a cross sectional view diagram showing a portion
of a silicon wafer substrate and structures, in accordance with one
embodiment.
[0016] FIG. 3 is a perspective view diagram showing an example of a
structure formed on a silicon wafer substrate, in accordance with
an embodiment.
[0017] FIG. 4 is a schematic block diagram illustrating a modified
structure formed on a silicon wafer substrate, in accordance with
an embodiment.
[0018] FIG. 5 is a flow diagram illustrating a method for forming a
structure on a silicon substrate, in accordance with one or more
embodiments.
[0019] FIG. 6 illustrates steps in the formation of a
three-dimensional nitinol structure using an aluminum bonding
layer.
[0020] FIG. 7A illustrates expanded diamond-shaped fenestrations in
a nitinol stent cover.
[0021] FIG. 7B is a close-up view of the longitudinal intersection
between adjacent fenestrations in FIG. 7A.
[0022] Embodiments of the present disclosure and their advantages
are best understood by referring to the detailed description that
follows. It should be appreciated that like reference numerals are
used to identify like elements illustrated in one or more of the
figures, in which the showings therein are for purposes of
illustrating the embodiments and not for purposes of limiting
them.
DETAILED DESCRIPTION
[0023] The techniques and structures disclosed herein achieve
economical, large-scale production of cylindrical nitinol film
structures at reasonable cost. To provide low-cost mass production,
nitinol film is sputtered deposited onto a semiconductor wafer
substrate. In the prior art, the resulting film was etched using
photolithographic techniques to form the desired fenestrations. In
contrast, the film disclosed herein is sputtered deposited onto a
substrate having dry-etched trenches formed using deep reactive ion
etching (DRIB) techniques. The substrate trenches correspond to the
desired fenestrations in the resulting thin film nitinol deposited
onto the etched substrate. Deep reactive ion etching of the
substrate is quite advantageous as compared to conventional wet
etching techniques to form the fenestrations. For example, deep
reactive ion etching is considerably more precise and thus enables
the formation of features with as little as one micron accuracy. In
addition, the wet etching techniques left residue on the nitinol
film that interfered with joining to another film so as to
construct a three-dimensional structure such as a cylindrical stent
cover. In contrast, deep reactive ion etching of the substrate is
entirely separate from the subsequent deposition of the nitinol
film and thus causes no contamination of the film.
[0024] After the film has been sputtered onto patterned silicon
wafers, it may be removed using a lift-off process by etching away
a sacrificial layer such as a chromium layer. Combining this
lift-off process with multiple-layer depositions of nitinol
separated by layers of sacrificial material enables fabrication of
cylindrical stent covers, which are three-dimensional in the sense
that two layers are joined together along their longitudinal edges
such that the resulting joined layers may be opened up to form a
cylinder.
[0025] The patterned substrate is prepared by deposition of a
chrome lift-off layer. Upon deposition of a first nitinol film onto
the chrome lift-off layer, a chrome sacrificial layer may be
deposited through a mask onto the first nitinol film. The mask
covers substantially all of the patterned area of the substrate
except for the longitudinal edges along which the first nitinol
film is to be joined to a second film. The subsequent deposition of
a second nitinol film then covers both the chrome sacrificial layer
and the unmasked longitudinal edges of the first nitinol layer. The
completed mesh may be removed from the substrate by etching of the
chrome lift-off layer and the sacrificial chrome layer. A mandrel
may be used to shape-set the mesh into the desired cylindrical form
by heating to annealing temperature.
[0026] A major problem is solved herein with respect to the
deposition of the second nitinol layer. In particular, note that
nitinol will promptly form an oxidized surface layer upon exposure
to the atmosphere. This oxidized layer is quite resistant to
bonding to additional nitinol layers. To prevent formation of the
oxidized layer, one could thus mask the first nitinol layer and
deposit the sacrificial chrome layer, and remove the mask and
deposit the second nitinol layer while maintaining a vacuum during
the entire process. But such a procedure is of course very
cumbersome with regard to aligning the mask and then removing it
while maintaining a vacuum during the procedure. A particularly
advantageous aluminum bonding layer is disclosed herein that
obviates the need for maintaining a vacuum over all the
manufacturing steps. In that regard, the first nitinol layer may be
deposited (which of course is done in a vacuum chamber) but the
vacuum may be released while the mask is applied. The subsequent
deposition of the sacrificial chrome layer is performed in the
vacuum chamber. The mask may then be removed without maintaining
the vacuum and a reverse mask applied. As implied by the name, the
reverse mask would be the complement of the mask used to deposit
the chrome sacrificial layer. The reverse mask thus exposes the
longitudinal edges of the first nitinol layer along which it is to
bond to the yet-to-be deposited second nitinol layer so that these
edges may be covered with an aluminum layer.
[0027] Upon deposition of the aluminum layer, the second nitinol
layer may be sputtered deposited in the vacuum chamber. The two
nitinol layers are thus separated by the aluminum layer along the
longitudinal edges where the two nitinol layers are to be joined.
This aluminum layer is quite advantageous as the resulting
structure may be heated to approximately 500 to 600 degrees Celsius
so that the aluminum partially melts. As opposed to the oxidized
aluminum surfaces, the molten aluminum is very chemically reactive
and actively bonds to both nitinol layers. In this fashion, the two
nitinol layers are bonded together despite the formation of an
oxidized layer on the first nitinol layer. The ability to break the
vacuum so as to assist in the mask alignment and other steps
greatly lowers manufacturing costs. In addition, the chemical
bonding of the aluminum layer to the two nitinol layers provides a
very secure bond. As discussed earlier, the conventional
alternative was to glue or stitch the two layers together, which is
quite unsatisfactory from both a production viewpoint as well as
with regard to biocompatibility issues of the glue or problems
caused by the stitching.
[0028] The deposition of the nitinol layers themselves is
problematic. It was conventional for the nitinol layers to be
undesirably brittle from the formation of a columnar crystalline
structure. Alternatively, the nitinol may be deposited so as to
have an undesirable tensile strain that can actually crack or break
the substrate surface from the resulting tensile forces. Applicant
has discovered that a very narrow range of manufacturing parameters
results in high-quality film. In contrast, manufacture outside of
these parameters results in excessively brittle material or
undesirable tensile strain. With regard to these parameters, the
sputtering power, the distance between the sputtering target and
the substrate, and Ar pressure are critical as will be discussed
further herein.
[0029] FIGS. 1A, 1B, 1C, and 1D illustrate a portion of a silicon
wafer substrate 100 in accordance with one or more embodiments. As
shown in FIG. 1A, a thin film layer 101 may be deposited on, for
example, a silicon substrate 100 using sputtering. Since the
surface of the substrate is planar, the resulting thin film layer
101 is also planar. As seen in the cross-sectional view of FIG. 2,
layer 101 comprises a lift-off layer 115 that is initially
deposited onto the surface of substrate 100. A first NiTi layer 121
covers lift-off layer 115. This first NiTi layer 121 forms one-half
of a resulting stent cover (not illustrated). The remaining half of
the stent cover is formed by a second NiTi layer 122 that is
partially separated from first NiTi layer 121 by a sacrificial
layer 116. Sacrificial layer 116 forms what will eventually become
the lumen of the resulting stent cover. NiTi layers 121 and 122 are
not joined together along the longitudinal edges of the resulting
stent cover and thus along the longitudinal edges of sacrificial
layer 116. These longitudinal edges 112 are shown in FIG. 3 after
removal of the sacrificial layer 116 to form a stent cover 114. A
lumen 125 for stent cover 114 exists in the place of the removed
sacrificial layer 116.
[0030] To function as stent cover for neurological applications,
stent cover 114 should have fenestrations 106. Referring again to
FIG. 1A, substrate 100 may be configured so that thin film layer
101 includes the patterns of fenestrations 104 for each
subsequently formed stent cover. These patterns of fenestrations
104 may also be denoted as a fiche 104 in that the fenestrations
104 are in collapsed form on substrate 100. Just like a microfiche,
each fiche 104 or pattern of fenestrations effectively codes for
the resulting fenestrations when the stent cover is expanded to
fully open up the fenestrations. The number of fiches 104 on
substrate 100 thus determines the resulting number of stent covers
114 that will be produced in one given production batch.
[0031] A close-up view of a fiche 104 is shown in FIG. 1B.
Fenestrations 104 at this stage are not expanded and thus are in
the form of narrow columnar apertures. One column of apertures is
staggered with regard to an adjacent column so that when the
fenestrations 104 are later expanded, the resulting stent cover has
a "chain link fence" mesh pattern. As will be explained further
herein, such a mesh pattern is quite advantageous for a flow
diverter stent cover.
[0032] FIG. 1D shows a cross-sectional view of the fiche 104 of
FIG. 1B. But in FIG. 1D, thin film layer 101 has not yet been
formed. To form the desired fenestrations that make up a fiche or
pattern of fenestrations, substrate 100 includes corresponding
grooves 160 formed using a deep reactive ion etching process. Lands
170 support the subsequent thin film layer 101 that will form a
wire mesh between adjacent fenestrations. Referring again to FIG.
1B, each fenestration 104 (prior to being expanded) may be
approximately 5 to 20 microns across and approximately 300 microns
in length. Each land 170 may also be approximately 5 to 20 microns
across. Such a land width means that the resulting wire mesh will
also have a width of approximately 5 to 20 microns across. The wire
depth depends upon the film layer 101 depth, which may be, for
example, from 5 to 20 microns in depth. It will be appreciated,
however, that these dimensions are just examples and may be varied
in alternate embodiments.
[0033] Trenches or grooves 160 may be 50 microns deep in one
embodiment. Following removal of lift-off layer 115 and sacrificial
layer 116, NiTi layers 121 and 122 may be crystallized at
500.degree. deg. C. for about 120 minutes in a vacuum less than
1.times.10.sup.-7 Torr, which may produce, for example, a 6 micron
thick micropatterned Nitinol thin film sheet (e.g., device
component 114) that can be lifted off the silicon substrate (e.g.,
silicon wafer substrate 100).
[0034] In one embodiment, the DC sputtering process involves the
use of a near equiatomic NiTi alloy target under ultra-high vacuum
(UHV) atmosphere (e.g., base pressure of a sputter chamber may be
set below 5.times.10.sup.-8 Torr and argon (Ar) pressure about
1.5.times.10.sup.-3 Torr). The silicon wafer is rotated adjacent
the heated NiTi target during deposition of the NiTi (for
minimizing compositional variations) so as to fabricate a NiTi film
(e.g., about 6 microns thick or in a range of about 2-12 microns
thick) with a deposition rate of 0.1 microns per minute.
[0035] As seen in FIGS. 1A and 1C, individual web fiches 104 may
spaced apart in a regular pattern (e.g., a web fiche pattern 102 of
FIG. 1A) so that the un-fenestrated spaces in the web fiche pattern
102 between the individual meshes 104 form areas 108 (also referred
to as streets 108) resembling and analogous to streets on a map.
Streets 108 may be formed during the DRIE process of creating
grooves 160 shown in FIG. 1D. There may be a significant difference
in scale between the size of the streets 108 (e.g., 1000 microns)
and the widths for fenestrations 106 (e.g., 10 microns). A mask 110
shown in FIG. 1C may be readily formed that takes advantage of the
difference in scale between the streets 108 and the fenestrations
106 of the individual web meshes 104. In this fashion, mask 110 may
have a spatial alignment resolution of at least 50 microns so that
mask 110 covers streets 108 and the longitudinal edges (112 of FIG.
3) of each individual fiche 104 to a depth of about 10 microns or
more. It is on these areas covered by mask 110 that first and
second NiTi layers 121 and 122 are joined. This joining occurs
because sacrificial layer 116 is deposited through mask 110. When
mask 110 is removed and second NiTi layer 122 deposited over
sacrificial layer 116, NiTi layer 122 will be deposited onto NiTi
layer 121 wherever NiTi layer 121 was masked by mask 110 so that a
bond 112 (shown in FIG. 2) may be formed between the contacting
layers 121 and 122. In alternative embodiments, an additional
bonding layer may be deposited to assist in the joining of layers
121 and 122 as will be explained further herein.
[0036] Sacrificial layer 116 may be sputter deposited onto first
NiTi layer 121 through mask 110. Mask 110 thus prevents the
sacrificial (e.g., Cr) layer 116 from depositing on the streets 108
and on the longitudinal edges of each individual mesh 104 of web
fiche pattern 102. The entire process of forming a
three-dimensional object such as stent cover 114 entails no use of
chemical wet etch except a Cr etch of the finished three
dimensional object to remove the sacrificial Cr layer 116 and
lift-off layer 115. But since layers 121 and 122 are already joined
by that time, the wet etching causes no complications. In contrast,
the wet etching of the prior art to form the fenestrations was
performed prior to the joining of the nitinol layers and thus
interfered with this joining through the resulting chemical
contamination of the first NiTi layer. All of the process
operations up to the final etch of the sacrificial Cr layers, which
release the finished three dimensional object, may be carried out
in a vacuum without exposure to atmosphere so as to ensure a strong
bond 112 between the NiTi layers 121, 122 (e.g., device components
114 of three dimensional device 124). The enhanced quality and
strength of the bond 112 compared to other methods such as
adhesive, laser welding, or suturing may, for example, provide
extra reliability and safety for a stent cover device 124.
[0037] The final etch of the sacrificial Cr layers may produce, as
seen in FIG. 3, a device such as a stent cover 114 having a lumen
125 between two NiTi layers that are joined (e.g., by bonds 112) at
the edges. The device shown in FIG. 3, although appearing
flattened, can be seen to be equivalent topologically to a three
dimensional cylinder. Lumen 125 may be enlarged, as seen in FIG. 4
for example, by insertion of a mandrel, and the two NiTi layers
(e.g., device components 114) may be shape set (e.g., by annealing)
to form a cylindrical stent cover 124 having bonds 112 between the
two NiTi layers (e.g., device components 114). Because the bond
between the two layers is strong (e.g., approaching the strength of
the NiTi material itself) bond 112 can have a width no wider than
the thickness of the individual layers. Hence bond 112 may not
present a significant obstacle to insertion of stent cover 124 in a
catheter for implantation.
[0038] FIG. 5 illustrates a method 500, in accordance with one or
more embodiments, for forming a three dimensional structure on a
silicon substrate without wet etching, other than, for example, to
release the structure from the substrate. Although description of
method 500 refers to production of individual web fiche mesh 104 or
single devices 124, it can be seen from FIG. 1A, for example, that
many devices 124 can be produced simultaneously using the method of
FIG. 5.
[0039] At step 501, a first sacrificial layer (e.g., lift-off or
release layer 115 shown in FIG. 2) of Cr (or other sacrificial or
barrier layers) may be deposited on a silicon substrate (e.g.,
silicon wafer substrate 100) in a sputtering chamber while the
substrate is held at high vacuum or under ultra-high vacuum, using
e-beam evaporation or PECVD, for example, as described above. When
subsequently etched away, the lift-off layer may release the
finished product such as device 114 from the substrate (e.g.,
silicon wafer substrate 100) and may thus be referred to as a
release layer. The lift-off layer may be 1700 to 3000 Angstroms of
sputter-deposited chromium.
[0040] Prior to the deposition of the lift-off layer, the substrate
may first (e.g., before deposition) be prepared in step 501, as
described above, by etching (using, for example, dry etching or
DRIE) grooves or trenches that will correspond to fenestrations of
a web fiche pattern 102 or other surface features that may
correspond to structures (e.g., mesh fenestrations) of a finished
product such as device 114. Step 501 and subsequent steps 502
through 506 may all be performed while the substrate continues to
be held under a vacuum in a sputtering chamber and without removing
the vacuum (or removing the substrate wafer or device from the
vacuum chamber) until all depositions are completed, even during
operations of manipulating a shadow mask, such as at steps 503 and
505 of method 500.
[0041] At step 502, a first layer of NiTi (e.g., layer 121 shown in
FIG. 2) may be deposited using one or more sputtering or other
techniques, examples of which are described above. An example
thickness of this first layer (as well as the second layer of NiTi)
is 3 to 5 microns.
[0042] At step 503, a shadow mask (e.g., mask 110) may be placed
over the substrate and the previously deposited layers such as the
release layer 115 and NiTi first layer 121. Manipulation (e.g.,
placing, removing) of the shadow mask may be performed without
interrupting the maintaining under vacuum of the substrate and
previously deposited layers. The shadow mask may protect covered
(or blocked) areas from subsequent deposition of a second Cr
sacrificial layer (or other sacrificial or barrier layers). The
masked (covered) areas may include portions of the first NiTi layer
121 intended to form a bond 112 with the second NiTi layer 122 so
that those same areas (e.g., edges of the individual web fiche mesh
104 to a width of about 10 microns) may be exposed after deposition
of the second sacrificial layer. Thus, a mask 110 may be placed
with a spatial alignment resolution of 50 microns so that mask 110
covers streets 108 and the edges of the individual web fiche mesh
104 to a width in a range of about 5 microns to about 15
microns.
[0043] At step 504, a second sacrificial layer (e.g., layer 116
shown in FIG. 2) of Cr (or other sacrificial or barrier layers) may
be deposited on the silicon substrate (e.g., silicon wafer
substrate 100) in a sputtering (or vacuum) chamber while the
substrate continues to be held at high vacuum or under ultra-high
vacuum, using e-beam evaporation or PECVD, for example, as
described above.
[0044] At step 505, the shadow mask 110 may be removed from the
substrate and the accumulated deposited layers. Removal of the
shadow mask may be accomplished without removing the vacuum or
removing the substrate and accumulated deposited layers from the
vacuum.
[0045] At step 506, a second layer of NiTi (e.g., layer 122 shown
in FIG. 2) may be deposited using one or more sputtering or other
techniques, examples of which are described above. At this step,
deposition of second layer of NiTi 122 may result in second layer
of NiTi 122 bonding to first layer of NiTi 121 at those areas left
exposed by the second sacrificial layer 116, forming, for example,
bonds 112 at the edges of individual web fiche mesh 104.
[0046] At step 507, removal of the sacrificial layers (e.g., first
sacrificial or release layer 115 and second sacrificial layer 116)
may be performed using a wet etch and may be performed after
allowing the vacuum chamber to repressurize or after removing
substrate 100 from the vacuum chamber. Etching the sacrificial
layers may release the device components 114 from the substrate and
may remove interior layers such as second sacrificial layer 116.
The etch may comprise soaking silicon substrate wafer 100 and the
deposited layers in a solution, for example, of Cr etch, and may
create a lumen (e.g., lumen 125 shown in FIG. 3) where sacrificial
layers are removed between the first and second NiTi layers that
are joined at the edges. Further processing may include shaping
device 124 including, for example, shaping device 114 into a more
rounded shape, as shown in FIG. 4, by insertion of a mandrel into
lumen 125 shown in FIG. 3. With device 114 in the desired shape,
the NiTi layers may be crystallized as discussed earlier.
[0047] It will be appreciated that bonding of one NiTi layer onto
another can be problematic in that NiTi readily forms an oxidized
surface layer. This surface layer inhibits the bonding of one NiTi
layer to another. To prevent formation of this surface oxidized
layer requires the first NiTi layer 121 to be maintained in a
vacuum or a non-oxidizing environment before second NiTi layer 122
may be bonded to it, which is cumbersome and increases
manufacturing costs. For example, mask 110 must be applied and
removed while maintaining a high vacuum. The bonding layer
discussed below obviates the need to maintain such a vacuum across
all the manufacturing steps. Referring now to FIG. 6, manufacturing
costs may be lowered as shown in the example manufacturing
flowchart. The first three steps are as described previously. In
that regard, a lift-off layer 115 is applied to substrate 100,
followed by deposition of first NiTi layer 121 and sacrificial
layer 116. But before the second NiTi layer 122 is deposited, an
aluminum bonding layer is applied using a reverse mask (not
illustrated). This reverse mask is (as implied by the name), the
complement of mask 110 used to form sacrificial layer 116. In other
words, the reverse mask covers sacrificial layers 116 and exposes
the uncovered areas of first NiTi layer 121. Aluminum may then be
sputtered through the reverse mask to form bonding layer 600. Since
bonding layer 600 is applied, first NiTi layer 121 may be exposed
to the atmosphere between the masking with mask 110 and the
subsequent masking with the reverse mask. In this fashion,
manufacturing costs are lowered in that the applications of the
masks is greatly aided by performing the mask applications outside
of the vacuum chamber using, for example, conventional
semiconductor pick-and-place equipment. After application of
bonding layer 600, second NiTi layer 122 may be sputter deposited
as discussed earlier. The wafer 100 may then be heated to
approximately 500 to 600 degrees prior to removal of the lift-off
and sacrificial layers. Such heating partially melts the aluminum,
which then becomes very reactive despite the formation of some
aluminum oxides. The molten un-oxidized aluminum is very reactive
and chemically bonds to the NiTi layers, resulting in a very secure
bond, despite the formation of an oxidized NiTi surface on the
first NiTi layer.
[0048] Regardless of whether an aluminum bonding layer is used, the
resulting stent cover is quite advantageous over conventional wire
mesh approaches. For example, a conventional wire mesh to function
as a flow diverter stent cover uses a wire of at least 30 to 40
microns in diameter. Such a relatively thick wire must weave up or
under adjacent strands to form the desired mesh. But the mesh from
the techniques described herein is planar with regard to the wire
intersections. In that regard, the columnar fenestrations may be
expanded into diamond shapes having a length of approximately 300
microns and a width of approximately 150 microns. In contrast, the
resulting wire forming the diamond-shaped fenestrations is only 5
to 20 microns in thickness. Each "corner" of the diamond-shaped
fenestration is thus relatively flat such that a null region with
regard to fluid flow is formed at each corner. This may be better
appreciated with regard to FIG. 7A, which shows the diamond-shaped
fenestrations that result upon expansion of the columnar
fenestrations 104 shown earlier. As shown in the close-up view in
FIG. 7B for the adjacent longitudinal ends of two diamond-shaped
fenestrations, the wire mesh forms regions 700 and 705 in the
interstices of the resulting flat wire mesh that are advantageously
conducive to the desired clotting process so that flow diversion of
aneurysm is safely achieved. Such interstices are absent in a
conventional wire mesh cover because of the weaving of the
relatively coarse wire. In contrast, the width W for the wire mesh
of FIG. 7B may be 10 microns or less.
[0049] As discussed earlier, DC sputtering of NiTi layers 121 and
122 is problematic in that the resulting nitinol may be too brittle
due to an undesirable columnar crystalline structure being formed
upon deposition. Alternatively, the deposition may be amorphous but
possess such tensile strain that it buckles or even cracks the
semiconductor substrate surface. To provide high-quality film and
solve this prior-art issues, DC sputtering may be performed using
the following parameters. In particular, the Ar pressure in the
vacuum chamber should be 3 milli Torr or less, more preferably 2
milli Tarr or less. The sputtering power should be at least 1
kilowatt and more preferably at least 2 kilowatts. Finally, the
distance between the sputtering target and the semiconductor
substrate surface should be between 2 and 3.5 inches.
[0050] Embodiments described herein illustrate but do not limit the
disclosure. It should also be understood that numerous
modifications and variations are possible in accordance with the
principles of the present disclosure. Accordingly, the scope of the
disclosure is best defined only by the following claims.
* * * * *