U.S. patent application number 10/014310 was filed with the patent office on 2002-05-30 for methods of forming metal articles.
Invention is credited to Segal, Vladimir, Shah, Ritesh P..
Application Number | 20020063056 10/014310 |
Document ID | / |
Family ID | 22270767 |
Filed Date | 2002-05-30 |
United States Patent
Application |
20020063056 |
Kind Code |
A1 |
Shah, Ritesh P. ; et
al. |
May 30, 2002 |
Methods of forming metal articles
Abstract
Described is the production of a metal article with fine
metallurgical structure and texture by a process that includes
forging and rolling and control of the forging and rolling
conditions. Also described is a metal article with a minimum of
statically crystallized grain size and a uniform (100) cubic
texture.
Inventors: |
Shah, Ritesh P.; (Liberty
Lake, WA) ; Segal, Vladimir; (Veradale, WA) |
Correspondence
Address: |
WELLS ST. JOHN P.S.
601 W. FIRST
SUITE 1300
SPOKANE
WA
99201-3828
US
|
Family ID: |
22270767 |
Appl. No.: |
10/014310 |
Filed: |
December 11, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10014310 |
Dec 11, 2001 |
|
|
|
09098760 |
Jun 17, 1998 |
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Current U.S.
Class: |
204/298.13 ;
148/407; 148/422; 420/427 |
Current CPC
Class: |
C22C 27/00 20130101;
C23C 14/3414 20130101; B21J 5/00 20130101 |
Class at
Publication: |
204/298.13 ;
420/427; 148/422; 148/407 |
International
Class: |
C22C 027/02; C23C
014/34 |
Claims
What is claimed is:
1. A metal article, such as a sputtering target, comprising at
least about 99.95 wt. % tantalum and a substantially uniform (100)
cubic texture.
2. A high purity tantalum sputtering target comprising at least
about 99.95 wt. % tantalum and a substantially uniform (100) cubic
texture at the target surface.
3. A high purity tantalum sputtering target according to claim 2
produced from a frictionless forged billet.
4. A high purity tantalum sputtering target according to claim 2
having substantially uniform (100) cubic texture at the center
location, mid-radial location and edge locations of the target
surface.
Description
[0001] The invention relates to metal articles with fine uniform
structures and textures and methods of making such articles. In
particular, metal articles of type described are especially useful
as sputtering targets.
[0002] Sputtering targets of high purity metals and alloys are
widely used in electronics and semiconductor industries for
sputtering thin films. It is desirable to obtain large size
targets.
SUMMARY OF THE INVENTION
[0003] In accordance with the invention there is provided a high
purity tantalum article, such as a sputtering target having
substantially uniform texture. In particular, the invention
comprises a tantalum sputtering target of at least about 99.95%
tantalum and a substantially uniform (100) cubic texture.
[0004] A process to provide the tantalum sputtering target is
disclosed in Application No. ______ filed on even date herewith,
the disclosure of said application is expressly incorporated herein
by reference. The process comprises:
[0005] 1) providing a metal billet;
[0006] 2) heating the billet to a forging temperature below the
recrystallization temperature of the metal;
[0007] 3) applying a solid lubricant between the ends of the billet
to be forged and press plates of a forging machine in which the
billet is to be forged to reduce the friction during forging;
[0008] 4) forging the billet to a desired billet thickness with
about 70% to 95% reduction;
[0009] 5) bringing the forged billet to about room temperature;
[0010] 6) rolling the billet to plate with a reduction in thickness
per rolling pass sufficient to provide near uniform strain
distribution; and
[0011] 7) recrystalization annealing the plate.
[0012] It is also advantageous to machine shallow pockets in both
ends of the billet ends prior to applying the solid lubricant of
sufficient thickness. Preferably, the billet is forged at a
temperature below the minimum temperature of static
recrystallization and then rolled and annealed at a time and
temperature to provide the beginning stage of static
recrystallization.
[0013] The rolling reduction per pass is desirably in accordance
with a relationship of the minimum reduction per pass, the roll
diameter and the desire billet thickness after forging. Generally,
the reduction per pass during rolling is about 10% to 20% per
pass.
[0014] Another embodiment the invention comprises a metal article,
such as a sputtering target, having a near-to-minimum of statically
crystallized grain size, and uniform texture.
[0015] The present process can be applied to different metals and
alloys that display good ductility and workability at temperatures
below corresponding temperatures of static recrystallization. Among
metals with which the invention can be applied are Al, Ti, Ta, Cu,
Nb, Ni, Mo, Au, Ag, Re, Pt and other metals, as well as their
alloys. One embodiment of the method comprises the steps of
processing an ingot to a semi-finished billet, including, for
example, melting, ingot casting, homogenizing/solutionizing heat
treatment, hot working to break down the cast structure, and billet
preparation followed by billet shaping and thermomechanical
treatment to fabricate a product, for example a sputtering target,
and refine the metallurgical structure and produce a desired
texture. By one embodiment of the process of the invention,
cold/warm working and annealing are used to develop extremely fine,
uniform structures and strong, uniform textures that result in
improvement in performance of sputtering targets.
DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a photomicrograph showing grain structure of
tantalum target; center location on target, 100.times. 25
microns;
[0017] FIG. 2 is a photomicrograph showing grain structure of
tantalum target; mid-radial location on target, 100.times. 25
microns;
[0018] FIG. 3 is a photomicrograph showing grain structure of
tantalum target; edge location on target, 100.times. 25
microns;
[0019] FIG. 4 is an inverse pole figure showing {100} cubic
texture; center location;
[0020] FIG. 5 is an inverse pole figure showing {100} cubic
texture; mid-radial location; and
[0021] FIG. 6 is an inverse pole figure showing {100} cubic
texture; edge location.
DETAILED DESCRIPTION
[0022] To optimize thermomechanical treatment, it is desirable to
attain intensive and uniform strains before recrystallization
annealing. Typically, targets are thin discs fabricated from a
single billet processed by rolling or upsetting-forging operations.
In both cases, an original billet length (Ho) is reduced to a final
thickness (h) and an average strain may be calculated by the
formula:
.epsilon.=(1-h/Ho) 100%=[1-(M/Mo) 2/3] 100% (1)
[0023] where Mo=Ho/Do and M=h/d are height-to-diameter ratios of
the original billet and the worked product, correspondingly. The
final ratio (M) is prescribed by the desired target shape and is
usually in the range of from M=0.07 to M=0.5, while the original
billet ratio Mo may be in the range of from about 1.86 to 0.5 and
yields limits of strain shown in previously described equation (1)
as follows:
73%<.epsilon.<95% (2)
[0024] Strain in equation (2) is high enough to optimize static
recrystallization only for thin targets. But even for these targets
non-uniformity in strain distribution through a billet volume may
significantly reduce the amount strain in some areas. Also, demands
on capacity of a forging press or rolling mill necessary to provide
strains of equation (2) above for large target billets may be too
high for some applications. Therefore, there may be restrictions on
attainable strains by rolling or forging operations.
[0025] Rolling is most suitable for processing to produce thin and
large targets. But the original billet ratio (Mo) advantageously
should be less than 1, otherwise the end effect during rolling of
long cylindrical billets develops very strong non-uniformity in
strain distribution. In addition, to provide near uniform strains
even for thin billets, the roll diameter advantageously should be
significantly larger than the billet thickness and the number of
reductions per pass can influence the result. Because of the
foregoing, rolled billets may have concave-like shapes with maximum
strain at contact surfaces and minimum strains at the middle billet
section that are not sufficient to optimize recrystallization and
develop most useful structures. Recently published Japan Patent No
08-269701 describes a titanium target manufactured by intensive
cold rolling of sheet from stock and low temperature annealing.
However, this technology cannot be applied to plates and although
fine grain size is described for some target parts, the Japanese
patent data shows large deviation in grain diameters.
[0026] Strain non-uniformity from forging is much stronger than for
rolling. Because of contact friction, extensive "dead metal" zones
are present at the central billet area. This results in low strains
inside these zones and high pressure and load for thin billets.
Upsetting bulk targets from a large billet with a large
thickness-to-diameter ratio requires very powerful presses and
expensive tools but cannot produce products with uniform grain
diameters. That is why the forging operation is mostly used for hot
breakdown of cast ingots only.
[0027] One attempt to overcome these problems is described in
Japanese Patent No 08-232061. The patent describes a combination of
forging and rolling for titanium targets at temperatures below the
temperature of phase transformation. The process uses a temperature
below the phase transformation temperature but well above the
temperature of static recrystallization for heavy worked materials.
As a result, the process cannot optimize recrystallization and
develop very fine and uniform structures/textures.
[0028] In contrast to the foregoing, the present invention
includes:
[0029] 1) performing the forging step as frictionless upsetting to
provide stress-strain uniformity and intensive working without
material cracking and press over-loading; and
[0030] 2) performing the forging step at temperatures below the
minimum temperature of static recrystallization for corresponding
conditions to provide the finest and most uniform
structures/textures. The steps of forging, rolling and annealing
can be optimized to provide cost-effective processing and target
performance.
[0031] The original billet has a cylindrical shape and a volume and
length-to-diameter ratio Mo. Cold upsetting is preferable, but in
some cases preheating of the billet and tool to a temperature below
the temperature of static recrystallization may be used to reduce
working pressure and load. Two thin sheets of solid lubricant (3)
are placed between the billet end and forging plate (4) mounted in
a press. It has been found that best results are obtained with
lubricant polymers that exhibit visco-elastic behavior at working
conditions, such as polyethylene, polytetrafluroethylene or
polyurethane.
[0032] In accordance with the present invention, visco-elastic
polymer film is used to entirely separate the billet and tool.
During upsetting, the polymer flows into contact with the billet.
It has been found that with the invention the original billet ratio
(Mo) may be as large as Mo=1.86, and the polymer lubricant film
enables partial reductions of up to 75%. Because of increase of the
original billet ratio Mo=1.86, the limits for attainable strain
(see equation (1) are much better than (2)
87%<.epsilon.<95% (3)
[0033] that in conjunction with uniform strain distribution allows
one to optimize recrystallization in most cases. Also, thin billet
after forging (up to M=0.16) provides the best conditions for
following rolling.
[0034] The preliminary forged billet is rolled for further
reduction of thickness. Cold or warm rolling may be used. Rolling
may be performed in two or four mutually perpendicular directions
to produce a product with a circle-like shape. It is important to
provide the most uniform strain distribution during rolling by
controlling roll diameter-to-billet thickness ratios (/H), billet
thickness-to-diameter ratio (M) and reductions per pass. An
important aspect is to prevent buckling along the free surface of a
cylindrical billet at the beginning of rolling. It has been found
that buckling area (T) is approximately equal to a billet-roll
contact length (L), and buckling is eliminated if contact length
exceeds a billet thickness h1 after the first pass. In other words,
if L>H, then 1 / H 4 ( 1 - ) 2 + 2 2 ( 4 )
[0035] where .phi. is the roll diameter, .epsilon.=(1-h/H) 100% is
rolling reduction per pass. Calculations with formula (4) for
different reductions are shown in Table 1.
1 TABLE 1 .epsilon. 5% 10% 15% 20% 25% .phi./H 36 16 9.7 6.5
4.6
[0036] As can be seen, at an average reduction of 15% or less, the
roll diameter should be at least about 10 times (9.7 in Table 1) as
large as the cylindrical billet thickness. On the other hand, use
of thin billets for rolling without upsetting reduces possible
reductions (1). Conventional target rolling suffer from both
disadvantages, that is, non-uniform and low reductions are equally
unacceptable to optimize structure. In the present invention high
ratios of roll diameter-to-billet thickness (/H) are provided by
preliminary billet upsetting to the necessary thickness (H).
Simultaneously the upsetting operation provides a pre-rolling
billet ratio (m) of less than about 0.5 that is useful to attain
uniform rolling reductions along a billet. Partial rolling
reductions from about 10% to 20% per pass are also useful for near
uniform strain distribution in the final product. Rolling
reductions lower than about 10% develop higher strains at billet
surfaces while reduction more than about 18% develop higher strains
at billet middle section. All these parameters define the best
embodiments for performing upsetting and rolling for targets for
optimum results.
[0037] The last step in target processing is recrystallization
annealing. For many metals and alloys, strains from equation (3)
are enough to optimize static recrystallization. To attain this
goal, first the lowest temperature necessary to start static
recrystallization, and then the shortest time necessary to complete
that at all billet volume should be determined. Corresponding
structures have the minimum grain sizes and the lowest dispersions
of grain diameters inside each local area. As the present method
also provides uniform strains at any part of the billet, the
minimum temperature of static recrystallization may be realized as
the optimal temperature for the whole billet at the shortest time.
This results in very fine and uniform structures and strong,
uniform texture for the target produced.
[0038] Another embodiment of the invention is preforming forging in
a few steps with successive decrease a billet thickness and
resumption of film lubricant at each step. That way forging may be
prolonged to low billet thickness without distortion of
frictionless conditions and strain uniformity under relative low
pressure and load. If forging is continued to the final target
thickness without rolling, corresponding forging textures are
provided for targets. Similarly, in the special cases rolling may
be performed without forging with near uniform strain distribution
in accordance with the invention.
[0039] The following example illustrates one embodiment of the
invention.
[0040] High purity tantalum (99.95% and higher) in the form of
billets of about 178 mm length and about 100 mm were used.
[0041] The composition of the resulting tantalum target is shown in
Table 2, the target comprising 99.95% tantalum and balance as shown
in the table.
2 TABLE 2 ELEMENT TYPICAL ELEMENT TYPICAL C 10 Ca <5 O 15 Fe 15
N 15 Mg <5 H <5 Mn 40 K 0.001 Mo 40 Li 0.001 Nb 150 Na 0.001
Ni <5 Al <5 Si 15 B 2 Sn <5 Cu <5 Ti 5 Co <5 W 25 Cr
<5 Zr <5
[0042] Reported in ppm.
[0043] C, O, N and H by LECO analysis.
[0044] Na, Li and K by SIMS analysis.
[0045] Metallic elements by ICP (inductively Coupled Plasma).
[0046] or GDMS (Glow Discharge Mass Spectroscopy) analysis. Billets
were upset-forged at room temperature to a thickness of 75 mm.
Teflon films of 150.times.150 mm2 and thickness of 1.2 mm were used
as lubricants for frictionless upsetting (alternatively
frictionless upset-forging can also be performed at 300 deg. C).
Thereafter cold rolling with a roll diameter of 915 mm was
performed in sixteen passes with partial reductions of 12% per pass
along four directions under an angle of 45.
[0047] Coupons across the thickness of the rolled billet were cut
from central, mid-radius and external areas and annealed at
different temperatures during 1 hours (h and investigated for
structure and texture and photomicrographs thereof are shown in
FIGS. 1-6. FIGS. 1-3 are photomicrographs of the center, mid-radial
and edge, respectively, showing the fine grain structure of a
tantalum target. FIGS. 4-6 are graphs showing (100) cubic texture
at the center, mid-radial and edge.
[0048] An important advantage of the invention is the production of
very fine and uniform structures and strong uniform textures at any
point of a target which formerly could not be attainable. The
following are various billet dimensions and processing routes which
can be applied to manufacture sputtering targets with uniform
microstructures and crystallographic texture. The method provides
targets with significant improvement in sputtering target
performance.
[0049] The following examples are illustrative for various possible
starting billet dimensions:
3 Billet Height, H.sub.o 7" 6" 4.5 Billet Diameter, D.sub.o 3.75"
3.75 4.5 M.sub.o 1.86 1.6 1
[0050] Process Flow Steps for different billet dimensions.
[0051] Mo=1.86
[0052] Step 1: Anneal the billet in vacuum
[0053] Step 2: Upset-forge billet using teflon as a solid lubricant
at room temperature or at 572F to specific height required for
rolling
[0054] Step 3: Fly-cut surfaces of the forged billet
[0055] Step 4: Roll the billet at room temperature to required
final thickness.
[0056] Step 5: Anneal in vacuum to obtain a fine grain size and
uniform texture
[0057] Alternate route for Mo=1.86
[0058] Step 1: Upset-forge using teflon to a height such that
Mo=1.0
[0059] Step 2: Vacuum anneal the forged billet.
[0060] Step 3: Upset-forge billet using teflon to a final height as
required for rolling operation
[0061] Step 4: Fly-cut the surfaces of the forged billet
[0062] Step 5: Roll the billet at room temperature to the required
final thickness.
[0063] Step 6: Vacuum anneal the rolled target blank in vacuum to
obtain fine grain size and uniform texture.
[0064] Mo=1.6
[0065] Step 1: Anneal the billet in vacuum
[0066] Step 2: Upset-forge billet uisng teflon as a solid lubricant
at room temperature or at 572F to a required final height suitable
for rolling.
[0067] Step 3: Fly-cut surfaces of the forged billet
[0068] Step 4: Roll the billet at room temperature to required
final thickness.
[0069] Step 5: in vacuum to obtain a fine grain size and uniform
texture
[0070] Mo=1.0
[0071] Step 1: Anneal the billet in vacuum
[0072] Step 2: Upset-forge billet uisng teflon as a solid lubricant
at room temperature or at 572F
[0073] Step 3: Fly-cut surfaces of the forged billet
[0074] Step 4: Roll the billet at room temperature to required
final thickness.
[0075] Step 5: Anneal in vacuum to obtain a fine grain size and
uniform texture.
[0076] The following illustrates one embodiment of the process to
obtain tantalum (a 99.95 or higher purity) target blank with a
maximum grain size less than 50 microns and a uniform
crystallographic texture of {100} across the face and through the
thickness of the target.
[0077] 1) working a billet during thermomechanical processing by
combining the frictionless upset forging and rolling;
[0078] 2) frictionless forging during upsetting operation that
develops positive friction along contact surfaces and increases
process stability;
[0079] 3) predetermine parameters of upsetting operation to
increase accumulated strains, reduce press capacity and enable
effective rolling;
[0080] 4) predetermine parameters of rolling conditions to enable
near uniform strain distribution and cylindrical shape (for
sputtering targets) of the product;
[0081] 5) using as the annealing temperature the lowest temperature
of static recrystallization; and
[0082] 6) producing a sputtering target with very fine and uniform
structures and uniform strong textures not previously
attainable.
[0083] It is apparent that various changes and modifications can be
made without departing from the invention. Accordingly, the scope
of the invention should be limited only by the appended claims,
wherein:
* * * * *