Crystalline and Amorphous Electroless Co-W-P Coatings
S. Armyanov,
a,
*
,z
E. Valova,
a
A. Franquet,
b
J. Dille,
c
J.-L. Delplancke,
c
A. Hubin,
b,
*
O. Steenhaut,
b
D. Kovacheva,
d
D. Tatchev,
a
and Ts. Vassilev
a
a
Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
b
Department of Metallurgy, Electrochemistry and Materials Science, Vrije Universiteit Brussel,
B1050 Brussels, Belgium
c
Department of Materials Science and Electrochemistry, Université Libre de Bruxelles, B1050 Brussels,
Belgium
d
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
Electroless deposition onto polycrystalline Cu, Au and amorphous Ni-P substrates was applied to prepare Co-W-P coatings of
two types: crystalline hexagonal close packed, hcp, with low phosphorus content about 2.4–2.7 atom %, and amorphous, with P
concentration within 7.4–8.3 atom %. Tungsten content varied typically in the narrow range of 2.9–3.7 atom % in both types of
coatings. Atomic force microscopy revealed substantial difference in their morphology. Polycrystalline Co-W-P coatings consist of
grains of stacked plates lamellas, confirmed by transmission electron microscopy also. Amorphous films are smoother and
uniform. The crystalline structure promotes the surface oxidation to a higher extent than the amorphous structure, as shown by the
X-ray photoelectron spectroscopy XPS. Auger electron spectroscopy depth profiles display oxidation, smoothly diminishing
toward the inside of the crystalline films. Amorphous coatings are oxidized only at the surface. Inside both types of coatings,
however, all alloy components are in nonoxidized form, according to XPS data. Differential scanning calorimetry DSC studies
of amorphous coatings revealed three transformation peaks, ascribed to a crystallization of hypoeutectic alloy and a transition of
Co-based hcp phase into face-centered cubic. Magnetic properties variation with temperature is in agreement with DSC results.
© 2005 The Electrochemical Society. DOI: 10.1149/1.1990124 All rights reserved.
Manuscript submitted February 9, 2005; revised manuscript received March 25, 2005. Available electronically July 28, 2005.
A comprehensive investigation on the electroless deposition,
structure, and properties of Co-P and Co-W-P alloys was published
30 years ago.
1
Renewed interest in the electroless deposition process
during the last decade is due to its promising application in ul-
tralarge scale integration technology and microelectromechanical
systems fabrication. After replacing aluminum with copper in the
integrated circuits, the barrier layer preventing the diffusion of the
latter became imperative. Barrier properties of thin-film Co-P alloy
2
are significantly improved when tungsten is added to it.
3
It was
emphasized that copper interface diffusion was reduced by Co-W-P
surface coating.
4,5
In addition, similar studies are the focus of re-
search in many parts of the world.
5-8
Extensive investigations of the
structure of electroless deposited Co
0.9
W
0.02
P
0.08
films and their
evolution with thermal annealing have been conducted.
7-9
The re-
quest of alternatives for electrodeposited chromium brings another
aspect of interest in electrodeposited Co-W-P alloy.
10,11
All this is
motivating the interest in the structure and properties of electroless
Co-W-P coatings.
Experimental
The bath formulation was based on an alkaline citrate solution
for electroless plating of Co-Ni-P alloy used before for the magnetic
layer of thin-film memory disks preparation.
12,13
Sodium hypophos-
phite was applied as a reductor, and cobalt sulfate and sodium tung-
state were used as Co and W source, respectively. The main bath
components are listed in Table I and the rest of solution ingredients
are described elsewhere.
12
The bath does not contain free ammonia
ions. Proper choice of complexing agents mainly citrate and care-
ful solution buffering by including some amino-alcohols and boric
acid beside the citrate prevented the precipitation of insoluble metal
species and ensured pH stability in the working alkaline region.
12,13
No stabilizers were applied in the solutions for electroless plating.
Deposition temperature was kept constant at 89 ± 1°C, and solution
pH 8.5 ± 0.5 was adjusted with H
2
SO
4
or NaOH. All chemicals
were pro analysis grade. Deionized water obtained from a Milli-Q
Millipore system was used for solution preparation.
Several types of substrates were used: i sheets of pure 99.99%
polycrystalline Cu 75 m thick, facilitating substrate dissolution
during preparation of free Co-W-P foils; ii ceramic platelets with
glassy surface coated by physical vapor deposition with thin mirror-
like Au film designated as Au; and iii coupons of Al-Mg alloy
substrate for thin-film memory disk, coated with amorphous electro-
less Ni-P layer with polished surface R
a
250 Å, specified as
Al/Ni-P.
Adequate cleaning preceded the specific preplate treatment of
any type of substrates. Copper substrates were activated with Pd
prior to plating with Co-W-P. Au substrates were etched for 30 s in
hot 50% H
2
SO
4
solution. Polished Al/Ni-P substrates were “re-
freshed” by electroless deposition of thin catalytic Ni-P film imme-
diately before immersion in the Co-W-P plating bath.
12,13
Each pre-
treatment step was followed by a thorough rinse with deionized
water.
The deposit thickness was assessed gravimetrically and varied as
a rule within 1 to 1.5 m. For the X-ray diffraction XRD analy-
ses, thicker films about 10 m were prepared. After deposition,
the samples were washed thoroughly with deionized water, dried in
air, and stored in ambient atmosphere, with no precautions prevent-
ing their oxidation. All samples studied in this work were obtained
within several days, meaning their condition at the moment of
analysis is comparable.
The surface morphology of Co-W-P films deposited on Au and
Cu substrates was characterized by the atomic force microscope
AFM incorporated in the TriboScope from Hysitron, Inc. The sys-
tem is dedicated to nanometer-scale tribological and mechanical
tests, allowing imaging by AFM of the sample surface. The em-
ployed probe tip of the nanoindenter was a tree-sided Berkovich
142.3° diamond pyramid with radius of curvature within
100–200 nm. The samples were imaged in the gradient mode, al-
lowing presentation of the slope gradient of the surface, instead of
the more frequently applied topography mode. The obtained images
gave a more exact picture of sample relief with asperities and
grooves of significant amplitudes.
Scanning electron microscopy SEM combined with energy-
dispersive spectroscopy of X-rays EDX was applied using JSM
733 and JSM 820 to determine the elemental composition of the
plated alloys. A high resolution scanning transmission electron mi-
croscope Philips CM20 equipped with an EDX system was used
for transmission electron microscopy TEM analysis. The Co-W-P
TEM foils were prepared in two steps: The thin Cu-sheet substrates
were dissolved using a solution containing NH
4
OH, NH
4
Cl, and
* Electrochemical Society Active Member.
z
E-mail: armyanov@ipchp.ipc.bas.bg
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