Abstract Electroless cobalt films have been obtained by deposition using a plating bath containing two reducing agents: dimethylamineborane (DMAB) and sodium hypophosphite. This formulation allows spontaneous activation on copper followed by auto catalytic electroless plating. CoWBP and CoBP films are proposed as diffusion barriers and encapsulation layers, for copper lines and via contacts for ULSI interconnect applications. The crystalline structure, chemical composition and oxidation states of the elements were studied, as well as the electrical resistivity, topography and morphology of the films. The film composition was characterized as a function of the solution composition; the barrier properties of the films were tested and an oxidation resistance study was conducted. The films were characterized and the results show that they can be applied as capping layers for ULSI copper metallization

I Goldfarb

R Ofek Almog

Y Shacham-Diamand

Y Sverdlov

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Microelectronic Engineering 84 (2007) 2450–2454CoWBP capping barrier layer for sub 90 nm Cu interconnects
R. Ofek Almog a,*, Y. Sverdlov a, I. Goldfarb b, Y. Shacham-Diamand a
a Department of Physical Electronics, Tel-Aviv University, Tel-Aviv 69978, Israel
b Department of Solid Mechanics, Materials and Structures, Tel-Aviv University, Tel-Aviv 69978, Israel
Received 15 May 2007; accepted 21 May 2007
Available online 26 May 2007Abstract
Electroless cobalt films have been obtained by deposition using a plating bath containing two reducing agents: dimethylamineborane
(DMAB) and sodium hypophosphite. This formulation allows spontaneous activation on copper followed by auto catalytic electroless
plating. CoWBP and CoBP films are proposed as diffusion barriers and encapsulation layers, for copper lines and via contacts for ULSI
interconnect applications. The crystalline structure, chemical composition and oxidation states of the elements were studied, as well as
the electrical resistivity, topography and morphology of the films. The film composition was characterized as a function of the solution
composition; the barrier properties of the films were tested and an oxidation resistance study was conducted. The films were character-
ized and the results show that they can be applied as capping layers for ULSI copper metallization.
 2007 Elsevier B.V. All rights reserved.
Keywords: Cobalt; Tungsten; Phosphorous; Boron; Electroless deposition1. Introduction
ULSI interconnects are made of copper due to its high
resistance to electromigration and stress voiding and its
low resistivity. However, there are a few major problems
with copper metallization. Copper diffusion through the
adjacent insulators into the semiconductor results in device
or inter-level dielectric failure. Additionally, copper oxida-
tion must be prevented during the steps coming after chem-
ical–mechanical polishing (CMP) planarization [1,2] and
cleaning steps that are essential to the damascene process.
Electroless deposition has been recently introduced in
micro and nano technologies for depositing barriers
against Cu diffusion in advanced microelectronic devices
due to its low processing temperature and selectivity [3].
Recent application of ternary cobalt based layers as dif-
fusion barriers and capping layers for Cu metallization
improves interconnection performance and solves some
critical problems such as maintaining low interconnect
resistance while preventing contamination by copper [4,5]0167-9317/$ - see front matter  2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.mee.2007.05.031
* Corresponding author. Tel.: +972 3 6406284; fax: +972 3 642 3508.
E-mail address: ofek.almog@gmail.com (R.O. Almog).and improving the electromigration lifetime [6]. Adding
phosphorous or boron enables deposition of amorphous
or nano-crystalline films with relatively low grain boundary
Cu diffusivity. Adding tungsten, or other refractory metals,
may further improve the thin film barrier properties by
forming either ‘‘stuffed’’ polycrystalline or amorphous dif-
fusion barrier. In some cases electroless cobalt alloys from
an amorphous phase with an embedded nano-crystalline
structure [7,8].
The advantages of electroless CoWP alloys were shown
by Shacham-Diamand et al. [8,9] and its deposition mech-
anisms and properties were discussed. One major problem
in cobalt phosphorous alloy deposition is that Pd seeding
was necessary to start the electroless plating reaction at
the Cu surfaces. It was shown that employment of an elec-
troless plating bath with boron containing reducing agent
can initiate thin film electroless plating [10]. ‘‘Pd free’’ pro-
cesses may also require a preliminary surface treatment of
the Cu surface prior to the cobalt electroless plating [10].
The advantages of combining two reducing agents in the
electroless plating bath: hypophosphite as a source for
phosphorus and dimethylamineborane as a source for
boron, are: (a) Pd-free deposition and (b) adding another
R.O. Almog et al. / Microelectronic Engineering 84 (2007) 2450–2454 2451parameter for the thin film properties control. Similar to
the previously discussed CoWP [5,6] and CoWB [10],
CoWBP alloys also demonstrated high stability under ther-
mal treatment in vacuum.
2. Experimental
Sputtered Cu seed (5 nm) on SiO2/Si system was used
as a substrate. A 15 nm thick titanium film was sputtered
on the silicon oxide prior to the Cu seed in order to
improve adhesion between the Cu layer and the oxide.
Electroless deposition was carried out without Pd-activa-
tion. The treatment of the surface in 2–10% DMAB solu-
tion serves as an activating step.
Electroless deposition of CoWBP and CoBP was per-
formed on the copper surface from sulfate-citrate electro-
less baths with dimethylamineborane (DMAB) as the
primary reducing agent and source for boron. Sodium
hypophosphite was used as second reducing agent and a
source for phosphorous. Tri-sodium citrate was used as a
complexing agent to form strong cobalt complexes. The
working pH value of 9.5–9.7 was maintained by potassium
hydroxide. The concentration of NaH2PO2 was either 1 or
7.3 g/l in order to study its effect on the thin film composi-
tion and properties. Sodium tungstate (Na2WO4) was
employed as a source for tungsten for CoWBP (Table 1).
The tungstate concentration in the CoWBP deposition
bath was kept constant at 1 g/l (0.003 M). All chemicals
were analytical purity grade.
Electroless deposition was performed at 80–85 C in a
temperature controlled cell. The CoWBP and CoBP layers
thickness was controlled by deposition time and were in the
range of 6–97 nm. Post-deposition vacuum annealing was
performed in vacuum under residual pressure less than
2 · 107 Torr at 350 C, 450 C and 550 C for 1 h.
The thin film thickness, composition, and electrical
properties were characterized. The layer thickness was
measured using Tencor ‘‘Alpha-step 500’’ profilometer.
Thin films composition after deposition and after anneal-
ing, and oxidation states were measured using XPSTable 1
Solution composition for electroless deposition of CoWBP and CoBP films
Material
Name Form
Role
Co source Cobalt sulphate heptahydrate CoSO
DMAB (CH
Reducing agent Sodium hypophosphite NaH
W source Sodium tungstate Na2W
Complexing agent Sodium citrate Na3C
Additive Hydrogen chloride HCl
Additive Surfactant RE-610
Variable
pH Adjusted by a
Temp. [C](X-ray photoelectron spectroscopy) 5600 Multi Technique
PHI.
The sheet resistance and resistivity of the films were
measured by the in-line four point probe made by Lucas/
SignatoneTM. TEM images and electron diffraction were
carried out using a high resolution TEM model Philips
Tecnai F20 and HRSEM images were taken by a cold field
emitter high resolution SEM model JEOL 6700F.
3. Results and discussion
Electroless deposited CoWBP and CoBP thin films dem-
onstrated high adhesion to the substrate, uniformity over
the sample surface and good coverage. The deposition
reaction has an autocatalytic nature and the layer thickness
is controlled by deposition time and reagent concentration
in solution.
3.1. Deposition rate
The deposition rate was40 nm/min for the solution with
DMAB only and it dropped to 16 nm/min when the
hypophosphite ion was added (Fig. 1). The deposition rate
of CoBP is 10 nm/min, which is slightly lower than that
of CoWBP.
3.2. Copper diffusion into CoBP and CoWBP
The barrier properties of the films were tested by XPS
profiling after annealing in vacuum at 350 C, 450 C and
550 C for 1 h. For CoBP films with the thicknesses of
128.5 Å and 55.7 Å, a significant amount of copper diffu-
sion occurred into the film after annealing at 450 C. In
CoBP films with a thickness of 321.2 Å, only minor
amount of diffusion occurred at this temperature. How-
ever, in CoWBP films that were deposited from solutions
with [P] = 1 g/l and with [P] = 7.3 g/l, there was no signif-
icant diffusion at this temperature. Copper diffusion into
the CoWBP thin film could be seen after annealing at
550 C. (Fig. 2).Mw [g] Concentration g/l (Molar)
ula
4 Æ 7H2O 281 0.1 (28.1)
3)2NHBH3 58 0.069 (4)
2PO2 111 0.0094 (1) or 0.069 (7.3)
O4 330 0 (0) or 0.003 (1)
6H5O7 Æ 2H2O 294.1 0.3
36.5 (0.083)
1.5 · 102 cc/l
Value
dding NaOH 9.5–9.7
80–85
0
50
100
150
200
250
300
350
400
0 5 10 15
Time (min)
T
hi
ck
ne
ss
 (
nm
)
CoWBP
[P/B]=0.1388
CoWB
CoBP
[P/B]=0.1388
Fig. 1. Thickness of electroless CoWBP films as a function of deposition
time.
Fig. 3. Electron diffraction of CoBP (made from solution where
½H2PO2  ¼ 1g=l, see Table 1).
2452 R.O. Almog et al. / Microelectronic Engineering 84 (2007) 2450–24543.3. Resistivity
The sheet resistance was measured after deposition and
after thermal treatments at 350 C, 450 C, 550 C and
650 C for 1 h, in vacuum. The sheet resistance of CoWBP
films is in the range of 2.3 X/sq. After thermal treatment at
350 C the sheet resistance decreased to 1.8 X/sq. Further
treatment at at 450 C did not change the sheet resistance
while the sheet resistance increased after thermal treatment
at 550 C and 650 C to 2.7 X/sq and 2.8 X/sq, respectively.
The decrease at 350 C is due to the annealing of the film,
and the increase at 550 C and 650 C is due to interdiffu-
sion between the Co and the Cu, creating a Co–Cu alloy.
The resistivity of CoWBP films is in the range of 20–
32 lX cm ±DR where DR/R was estimated to be in the
range of 10–20%. The resistivity value was extracted by
taking into consideration the thin copper film on which
the CoWBP films were deposited; the measurement
includes the parallel resistance of the copper field, therefore
since both the Cu and the CoWBP are thin there is a signif-
icant error in the range of ±20% for CoWBP films thinner
than 300 Å.
Even when assuming the worst case value for the resis-
tivity it can be concluded that the sheet resistance andO
Si
Ti
Co
Cu
0
20
40
60
80
100
0 20 40 60 80 100
Sputter Time (min)
A
to
m
ic
 c
on
ce
tr
at
io
n 
[%
]
Fig. 2. Copper diffusion into CoWBP, ½H2PO2 =½DMAB ¼ 1, XPSthe resistivity of CoWBP is low in comparison to other
common barrier layers such as TaN or WN.
3.4. Structure characterization
Electron diffraction showed a pattern of concentric
sharp circles of high intensity (Fig. 3). They were identified
as diffraction patterns from copper and a cobalt planes, as
follows: diffractions from the hexagonal cobalt phase were
from (100) plane, d = 2.165Å and from (102) plane,
d = 1.48 Å. Diffractions from copper phase from the
(11 1), (200), (220), (222) and (311) planes. One diffrac-
tion at d = 2.44 Å might be from CoB (10 1) plane.O
Si
Ti
Co
Cu
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80
Sputter Time (min)
A
to
m
ic
 c
on
ce
tr
at
io
n 
[%
]
profile (a) as deposited and (b) after annealing at 550 C for 1 h.
0
1
2
3
4
5
6
0 0.2 0.4 0.6 0.8 1 1.2
[H2PO2 -]/ [DMAB ] molar ratio in the liquid
1/
[B
] 
%
at
 in
 th
e 
so
lid
 f
ilm
Fig. 5. Boron concentration as a function of the concentration of the
reducing agents in a CoWBP film.
R.O. Almog et al. / Microelectronic Engineering 84 (2007) 2450–2454 2453The concentric sharp circles indicate a randomly ori-
ented poly crystals, and the diffractions from the cobalt
phase indicates a hexagonal microstructure.
3.5. Morphology
In order to determine the crystalline structure of the
films, high resolution TEM analysis was performed. The
structure of the films after deposition was mostly amor-
phous with an embedded nano-crystalline structure. The
size of most of the crystallites is in the range of 4–7 nm.
However, there are some larger grains with dimension in
the range of 10–12 nm (Fig. 4).
3.6. Concentration model
Electroless deposition of CoWPB alloy thin layers was
performed on copper surface from sulfate-citrate electro-
less bath with DMAB and sodium hypophosphite as reduc-
ing agents. Concentration of NaH2PO2 was varied in range
of 0–8.05 g/l in order to study its effect on the thin film con-
tent and properties. The sodium tungstate was employed as
a source for tungsten. Its concentration was constant at
1 g/l (0.003 M). The film composition was characterized
as a function of the solution composition using XPS and
SIMS analysis. The results indicated that the film’s compo-
sition is most likely determined by the reducing agents’ co-
adsorption rates.
A complete adsorption limited deposition model that
formulates the dependence between the concentration of
the reducing agents and the concentration of the cobalt,
boron and phosphorous in the solid film was presented
elsewhere [11]. A complete model was devised and com-
pared successfully to the results.
For example, the dependence of the boron concentra-
tion in the film on the ratio of the two reducing agents is
given byFig. 4. Bright field TEM plan view images of CoBP½BFilm;0
½BFilm
¼ 1þ kB1:
½H2PO2 
½DMAB ð1Þ
Where [B]Film is the concentration of boron in the film,
[B]Film,0 is the concentration of boron in the film at the
beginning, kBl is a constant (as explained elsewhere [11]),
[H2PO

2 ] and [DMAB] are the hypophosphite and DMAB
concentrations, respectively.
The results of the boron concentration are plotted in
Fig. 5 as the inverse of the boron concentration vs. the hyp-
ophosphite to DMAB concentration ratio. It shows a lin-
ear relation over the entire range starting where the
phosphorous concentration equals zero. This is due to
the competition between the hypophosphite and DMAB
during their initial surface adsorption that is prior to the
actual co reduction. Therefore as the hypophosphite con-
centration in the solution increases, less DMAB is
adsorbed. As a result, the boron concentration in the film
decreases as the concentration of the phosphorous ions in
the solution increases. The theoretical results seem to yield
a similar function to that of what we found experimentally.films, as-deposited, (a) ·38 K and (b) ·400 K.
2454 R.O. Almog et al. / Microelectronic Engineering 84 (2007) 2450–24544. Conclusions
The electroless deposition of cobalt alloys system with
two reducing agents was studied. Using both sodium hyp-
ophosphite and DMAB reducing agents with cobalt metal
ionic solution and tungstate ions at a lightly basic solution
results in a self activated spontaneous deposition of
CoWPB thin-film coatings formation.
The solution composition was studied; the functional
dependence between the solid to the liquid composition
pointed towards an adsorption limited model. The model
predicts a strong dependence of boron composition and a
weak dependence of the phosphorous composition on the
½H2PO2 =½DMAB ratio. That prediction was verified
experimentally.
CoWBP thin films demonstrated high thermal stability
and provided good barrier properties and can be applied
as capping layers for ULSI copper metallization.
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CoWBP capping barrier layer for sub 90 nm Cu interconnects

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CoWBP capping barrier layer for sub 90 nm Cu interconnects

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