● A Novel Liquid Cobalt Precursor for Low
Temperature Deposition
Advanced Materials Research Laboratory, TOSOH Corporation
Sagami Chemical Research Institute

Hiroyuki OIKE
Teppei HAYAKAWA
Yuki YAMAMOTO
Ken-ichi TADA

have some drawbacks. First, since most of them are

1．Introduction

solid at room temperature, it is difficult to vaporize them

The design of semiconductor devices is becoming

at a constant rate by keeping them in a liquid state.

highly miniaturized and three-dimensional year by year.

Second, they are too heat-labile to feed into the CVD/

However, the development of new materials is essential

ALD reactor.

to realize the designed device from the imagination.

In this paper, we report the development of a novel

One of the important keys to manufacturing theseliquid Co-amide complex, [(2-dimethylamino-1,1-

advanced devices is the introduction of innovative

dimethylethyl)(trimethylsilyl)amino][bis(trimethylsilyl)materials to replace conventional materials, i.e. copper

amino]cobalt [Ts-Co9], as a precursor for low(Cu) interconnect and tungsten (W) contact. Cobalt (Co)

temperature deposition of Co thin films.3 In addition,

have attracted much attention as an excellent material

we also show the possibility of Ts-Co9 for area-selectivethat can overcome various problems such as increased

deposition (ASD), which allows a desired thin film

resistance and wiring life caused by the decrease of line

to form in one region of a surface. ASD is currently

width.

regarded as one of the most important technology for

Similar to research on new materials for next-

next-generation semiconductor manufacturing, since

generation semiconductor, technologies for forming

this process eliminates many steps associated withultra-thin films on 3D substrates are also being

conventional patterning.4

developed. Currently, chemical vapor deposition (CVD)

Our development will contribute to improvement inand atomic layer deposition (ALD), are promisingenergy efficiency through semiconductor technology

methods for forming a highly conformal metal thin film

innovation.

even on a deep trench substrate.
Low temperature deposition is quite important forachieving a conformal ultra-thin film with excellentsmoothness. Since CVD/ALD precursor for low

2．Experimental
2.1

Preparations

temperature deposition is required to have highAll manipulations were carried out under argon

reactivity with a reactant gas, our study focused on(Ar) atmosphere with standard Schlenk techniques.

amide complexes with Co-N bonds, which are expected

Anhydrous CoCl 2 was obtained from FUJIFILM

to be extremely reactive.

Wako Pure Chemical Corporation and were used

During the past some decades, several Co-

as received. Tetrahydrofuran (THF) and hexane

amide compounds have been investigated aswere purchased from Kanto Chemical Co. Inc. ast1dehydrated grade. Lithium bis(trimethylsilyl)amide

Co[N(SiMe 3 ) 2 ] 2 (THF), Co[N(SiMe 2 Et) 2 ] 2 (THF),

in THF (1.3 mol/L), butyllithium in hexane (1.6 mol/Co[N(SiMe3)2]2(pyridine), Co[N(SiMe3)2]2(NMe2Et),L), chlorotrimethylsilane, and chloroform-d 1 (CDCl3)

Co[N(SiMe 3 ) 2 ] 2 (NMeEt 2 ), Co[N(SiMe 3 ) 2 ] 2 (NEt 3 ),were purchased from Tokyo Chemical Industry Co.,C V D / A L D p r e c u r s o r s , s u c h a s C o ( N ( B u ) 2) 2,

C o [ N ( S i M e 3) 2] 2( 1 - m e t h y l - p y r r o l i d i n e ) , a n dLtd. and used without further purification. Benzene-d 6

Co[N(SiMe2Et)2]2(NMe2Et).2 However, these complexes(C6D6, purchased from FUJIFILM Wako Pure Chemical
69

TOSOH Research & Technology Review Vol.65（2021）

Corporation) was purified by distillation from Na/

reduced pressure (103 ℃/56 Pa) to afford Ts-Co9 as

1benzophenone ketyl, and then stored under Ar. N ,a dark green liquid (19.7 g, 89%). 1H-NMR (400 MHz,N 1, 2-Trimethylpropane-1,2-diamine (1) was prepared

C6D6, δ/ppm): 149.5 (brs, 6H), 118.7 (brs, 2H), 62.0

5 1

according to a literature procedure. H-NMR spectra(brs, 6H), 32.7 (brs, 9H), -12.7 (brs, 18H).

were recorded on a Bruker Avance HD 400 MHz
spectrometer at 293 K.

2.2

Measurement of physical properties

Thermogravimetric analysis (TG) was carried out
112

Synthesis of N , N ,2-trimethyl-N -(trimethylsilyl)

under an Ar flow (400 mL/min) at a heating rate of 10

propane-1,2-diamine (2)℃/min with a SII Technology TG/DTA7200 that was

A solution of butyllithium in hexane (2.65 mol/L, 16

contained in a high-purity nitrogen-filled glovebox tomL, 42.4 mmol) was added dropwise to a solution of

prevent decomposition arising from exposure to air.

1 (4.95 g, 42.6 mmol) in THF (20 mL) at -78 ℃. The

Differential scanning calorimetry analysis (DSC) was

reaction mixture was warmed to room temperature andperformed using a SII Technology DSC7020 at a heating

additionally stirred for 15 min. To this stirred solution

rate of 10 ℃/min with a sealed container in a high-chlorotrimethylsilane (4.61 g, 42.4 mmol) was slowly

purity Ar gas atmosphere. The sample weights used for

added at 0 ℃, and then stirred at room temperature for

TG and DSC were 18.9 and 7.5 mg, respectively.

4 h. After the solvents were removed under vacuum, thecrude product was distilled under reduced pressure (80-2.3

82 ℃/40 Torr) to afford 2 as a colorless liquid (7.05 g,

improvement187%). H-NMR (400 MHz, CDCl3, δ /ppm): 2.32 (s, 6H),
2.11 (s, 2H), 1.09 (s, 6H), 1.05 (br, 1H), 0.06 (s, 9H).

Deposition of Co thin films and the film quality

Co thin films were deposited on SiO2 and ruthenium(Ru) substrates at the deposition temperature range
from 100 to 300 ℃ by CVD of Ts-Co9 using CVD

Synthesis of bis[bis(trimethylsilyl)amino]cobalt (3)apparatus we assembled (Fig. 1). The Co precursor was

A solution of lithium bis(trimethylsilyl)amide in THF

supplied by a bubbling method with the vapor pressure(1.3mol/L, 154 mL, 200 mmol) was added dropwise toof 6.67 Pa at 70 ℃. This vapor was transferred to a colda solution of CoCl2 (13.0 g, 100 mmol) in THF (50 mL)

wall type CVD reactor chamber using Ar as a carrier

at -78 ℃. The reaction mixture was warmed to roomgas. Ammonia (NH 3) or a mixed gas [NH 3, H 2, andtemperature and additionally stirred for 17 h. After the

dipivaloylmethane (DPM)] was used as a reactant gas.

solvent was removed under vacuum, the residue was

In the case with DPM, it was supplied by a bubbling

extracted with hexane (90 mL) and filtered throughmethod with the vapor pressure of 800 Pa at 73 ℃. The®

a Celite -padded G4 glass filter. The filtrate wasdetailed deposition conditions for Co thin films by CVD

concentrated, and the residual solid was purified by

are summarized in Table 1.

sublimation under reduced pressure (120 ℃ , 51 Pa) to
1give 3 as a deep green solid (40.1 g, 89%). H-NMR (400

H2 annealing treatment conditions to improve the film
quality are shown in Table2.

MHz, C6D6, δ/ppm): 168.2 (br, 4H), 100.2 (br, 4H),
-15.9 (br, 36H).

2.4

Evaluation of Co thin films

Film thickness was estimated by the X-ray
Synthesis of [(2-dimethylamino-1,1-dimethylethyl)

fluorescence (XRF) calibrated by standard samples.

(trimethylsilyl)amino][bis(trimethylsilyl) amino]cobalt

The resistivity of the films was measured by a four-(Ts-Co9)

point probe method at room temperature. The atomic

2 (10.2 g, 54.3 mmol) was added to a solution of 3 (24.5

composition of the films was analyzed by means ofg, 54.3 mmol) in hexane (50 mL) at room temperature.

X-ray photoelectron spectroscopy (XPS) after Ar +After stirring for 1 h at this temperature, the mixture

sputtering. Atomic force microscope (AFM) was used

was refluxed for 18 h. The solvent was removed underto characterize surface roughness.

vacuum, and then the crude product was distilled under70

東ソー研究・技術報告

Diluent gas (Ar)

第 65 巻（2021）

Mass flow controller

Reactant/Annealing gas (4% H2/Ar)

Mass flow controller

Reactant gas (NH3)Mass flow controller

Carrier gas (Ar)

Mass flow controller

Carrier gas (Ar)

Mass flow controller

DPM
Chamber
Substrate
Heater

Ts-Co9ExhaustRotary Pump

Fig.１

Table１
Depo. temp.
Depo. press.
Depo. time.
Prec. feed
Reactant gas
Total gas flow
Substrate

Table２

CVD apparatus

Experimental conditions of CVD using Ts-Co9

100－300℃
5 or 10 Torr
15－120 min
0.06 sccm
NH3 (0－80 vol％）A mixture of NH3 (50 vol％), H2 (1.31 vol％), and DPM (0.03 vol％)200 sccm
Ru (20 nm)/Ti (10 nm)/SiO2(100 nm)/Si
SiO2(100 nm)/Si

H2 annealing conditions of the deposited films

Annealing temp.
Pressure
Annealing time
Annealing gas
Total gas flow

200℃
10 Torr
120 min
4％ H2/Ar
200 sccm (H2 : 8 sccm, Ar : 192 sccm)

to the metal center, but in the case of precursors for
low temperature deposition, high reactivity with thereactant gas is also required. Controlling the balance of
these requirements is the decisive factor in determining
the performance of the precursor. In the developmentof Ts-Co9, the requirements were successfully met
by the placement of trimethylsilyl and methyl groups

3．Results and discussion
3.1

Moleculae Design, Synthesis, Physical andChemical properties of Ts-Co9

at specific positions and the effective introduction of
specially designed chelating ligands.
The new cobalt compound Ts-Co9 can be obtainedby treatment of Co(hmds)2(thf) (3) with N-silyldiamine

Essential properties of an excellent CVD/ALD(2) (Fig. 2). The Co precursor, which is a dark green

precursor generally include sufficient volatility andliquid at room temperature (Fig. 3), can be purified

adequate thermal stability. In order to obtain a highby distillation. The vaporization property and the self-

vapor pressure, it is important to reduce intermoleculardecomposition temperature of this complex wereinteractions and, especially in the case of metal

measured with the help of TG and DSC, respectively

complexes, to prevent dinuclearization due to bridging(Fig. 4 and 5). Ts-Co9 represented single-step weight

coordination of the ligand. Improvement of theloss on the TG curve, and approximately 10% residue

thermal stability of metal complexes can be achieved

remained. The DSC measurement confirmed that theby selecting appropriate ligands that can bind tightly

thermal decomposition temperature was 183 ℃ or
71

TOSOH Research & Technology Review Vol.65（2021）

1) BuLi
2) Me3SiCl
N

H2N

THF
0 ℃ → rt

1

Si

2

Si
CoCl2

N Li

N

Si

THF
0 ℃ → rt

Si

Si NH

Si

Si
N Co
O

N

Si

2

Si

hexanert → 50 ℃

Si

NN

Ts-Co93

Synthetic route of Ts-Co9

Heat flux [a.u.]

Fig.２

N Co

183℃

50

100

150

200

250

300

Temperature [℃]

Fig.３

Fig.５

Appearance of Ts-Co9

higher. This compound has good volatility and favorable0

thermal stability for use as a material for the CVD/ALD
processes.

－40
－60

3.2
－90％

－80
100

200

300

NH 3 concentration dependency of Co thin film
400

deposition rate on Ru and SiO2 substrates at 200 ℃

500

under 10 Torr was examined to determine the optimal

Temperature [℃]concentration in the total reactor chamber pressure

TG curve of Ts-Co9

Depo. rate [nm/h]

Fig.４

Reactant gas concentration dependency(Fig. 6). The concentration of NH 3 were varied 0-8035

1.E＋0730

1.E＋0625

1.E＋0520

1.E＋0415

1.E＋0310

1.E＋025

1.E＋01

0

0

20

40

60

80

Resistivity [μΩcm]

TG [％]－20－100

DSC curve of Ts-Co9

on Ru
on SiO2＊Rs on SiO2

1.E＋00NH3 conc. [vol％]

Fig.６

72

NH3 concentration dependency of deposition rate on Ru and SiO2 substrates
and electric resistivity of Co thin film on SiO2 substrate at 200 ℃

東ソー研究・技術報告第 65 巻（2021）

vol%. The deposition rates on Ru and SiO2 substrates

resistivity of the films on SiO2 substrate was almost

were increased with increasing NH3 concentration andconstant in the temperature range of 175-250 ℃. At

saturated near about 50 vol%, which reveals that Ts-Co9

the deposition temperatures of 200 and 250 ℃, there

has favorable reactivity with NH3. Also, the resistivity

appears to be a slight difference in deposition rates

values of the films deposited on SiO2 substrate in the

between Ru and SiO2 substrates (approximately 4 nm/

presence of 20 vol% or more of NH3 concentration wereh). In the case without NH3, almost no Co thin film was

as low as 123 to 142 μΩcm, while the film prepared

formed at 100-200 ℃, except for film deposition at 300

without NH3 exhibited much higher resistivity.

℃. This suggests that film formation due to thermal
decomposition of the complex is unlikely to occur. From

3.3

these findings, 200-250 ℃ seems to be the optimum

Deposition temperature dependency

temperature for CVD using the Co precursor and NH3.

deposition rate from Ts-Co9 with and without NH 3

The change in Co thin film deposition rate at different(50 vol%) on Ru and SiO2 substrates under 10 Torr isdeposition temperatures using Ts-Co9, NH 3 , H 2 ,presented in Fig. 7. The figures indicate that there

and DPM on Ru and SiO2 substrates under 5 Torr is

is a significant effect of NH3 as a reactant gas on theplotted in Fig. 8. Co thin films were not formed on SiO2

deposition rate at various deposition temperature. Co

substrates at 150-225 ℃, whereas Co thin films werethin films were deposited with NH 3 on Ru and SiO 2grown smoothly on Ru substrate at 200-300 ℃. Since

substrates at 150-300 ℃. At a deposition temperature

the substrate-selective window was observed at 200-

range from 200-275 ℃, the deposition rate was

225 ℃, this compound has a possibility to achieve ASD

saturated on both of Ru and SiO 2 substrates and thearound 200 ℃.

50

1.0E＋0740

1.0E＋06
1.0E＋0530

1.0E＋0420

1.0E＋03100

1.0E＋02

on Ru w/NH3
on Ru w/o NH3
Resistivity [μΩcm]

Depo. rate [nm/h]

The temperature dependency of Co thin filmon SiO2 w/NH3
on SiO2 w/o NH3＊ Rs on SiO2

1.0E＋01
100

150

200

250

300

Depo. temp [℃]

Temperature dependency of deposition rate and electric resistivity of Co thinfilms from Ts-Co9 with/without NH3 on Ru and SiO2 substrates60

on Ru sub.

50
Depo. rate [nm/h]

Fig.７

on SiO2 sub.

40
30
20
10
0

150

200

250

300

Depo. temp [℃]

Fig.８

Temperature dependency of Co thin film deposition rateusing Ts-Co9, NH3, H2, and DPM on Ru and SiO2
substrates

73

TOSOH Research & Technology Review Vol.65（2021）

3.4

are unclear at this moment. The use of the mixed gas

Deposition time dependency

Substrate-selective behavior was investigated by CVD
of Ts-Co9 with NH3 or a mixed gas (NH3, H2, and DPM)

as a reactant gas is thought to be a better choice inachieving ASD.

as the reactant gas at 200 ℃ under 10 or 5 Torr on Ru
and SiO2 substrates.

3.5

Film composition

For the deposition test using NH 3 (50 vol%) at 10

Table 3 shows chemical composition (estimated byTorr (Fig. 9), the overall linear Co thin film growth

XPS) of the film prepared by CVD of Ts-Co9 with NH3

without no incubation time was observed on Ru andand (NH3+H2+DPM) mixture at deposition temperatures

SiO 2 substrates. There was almost no difference inof 150-200 ℃ under pressures of 5 and 10 Torr on SiO2

film thickness between the Ru substrate and the SiO2

and Ru substrates.

substrate at each deposition time. At 200 ℃, ASD using
NH3 as a single reactant gas seems difficult.

Co-based film formed using NH 3 at 200 ℃ under
5 and10 Torr had a large resistivity value (159 μΩ

In the case employing the mixed gas at 5 Torr (Fig.

cm on SiO2) and a significant amount of nitrogen (14-10), Co thin film thickness linearly increased on Ru

15 atom%). Some studies reported that cobalt nitride

substrate after an incubation time of about 15 minutes,(Co xN, x = 3-6) films have been deposited by CVD

while no Co thin film was formed on SiO2 substrate. Theof several Co precursors with NH 3 at 200 ℃ andexcellent selectivity between Ru and SiO 2 substrates

are stable at this temperature. 1,6 According to thesewas clearly observed. However, it should be noted

previous studies, the N impurity in the film is thoughtthat an island-like pattern without Co film deposition

to be derived from NH3 rather than the N atoms in Ts-

can be seen on the surface of SiO2. The selectivity on

Co9. Even at 150 ℃ or using the mixed gas (NH3, H2,

both substrates and its pattern on the SiO2 substrate

Film thickness [nm]on Ru sub.
on SiO2 sub.

40
30
20
10
0

Film thickness [nm]

40
50

30
20
10
0

0

30

60

90

0

120

Deposition time dependency of Co thin film thickness
using Ts-Co9 and NH3 on Ru and SiO2 substrates
at 200℃

Table３
Reactant
gas(vol％)

60

90

120

Fig.10

Deposition time dependency of Co thin film thickness
using Ts-Co9, NH3, H2, and DPM on Ru and SiO2
substrates at 200℃

Chemical composition of Co thin films deposited on SiO2 and Ru substrates at 150 and 200℃

Depo. Depo.
H2
Filmpress. temp. Sub. annealing time thickness(Torr) (℃)
(nm)
(min)
23
0
10
21
120
SiO2
18
0
NH3
200
17
120
(20)
5
12
0
Ru
14
0
NH3
SiO2
18
120
10
150
(50)
20
0
Ru
45
NH3/H2/DPM
10
0
200
Ru
31
(50/1.31/0.03)
5

74

30

Depo. time [min]Depo. time [min]Fig.９

on Ru sub.
on SiO2 sub.

Composition by XPS
AFMResistivity(atom％)RMS value Image(nm)(Fig.11)CoOCN
Si (μΩ・cm)
2
3.64
3
159
77
5
14
a
1
3.08
2
98
80
16
1
b
1
1.52
ND
159
82
2
15
－
1
－
3
121
85
10
ND
－
－
1.25
－
－
－
－
－
c
1
2.11
ND
229
81
2
16
d
2
－
72
90
8
ND ND
－
－
1.82
－
－
－
－
－
－
－
ND
－
84
ND ND
16
－
ND
2.25
ND
－
84
1
15
e

(a)

Fig.11(b)

東ソー研究・技術報告第 65 巻（2021）

(c)

(d)

(e)

AFM (2μm × 2μm) micrographs for films deposited (a) with NH3 under 10 Torr on SiO2 at
200℃ ; (b) with NH3 under 10 Torr on SiO2 at 200℃, followed by H2 annealing ; (c) with NH3
under 5 Torr on Ru at 200℃ ; (d) with NH3 under 10 Torr on SiO2 at 150℃ ; (e) with NH3, H2,
and DPM undr 5 Torr on Ru at 200℃

and DPM), the film composition was similar to thattreatment is effective in reducing N contamination,using NH3 at 200 ℃. On the other hand, the surface

lowering the resistivity and smoothing the surface. Insmoothness of the film seems to be ameliorated

the case of using the mixed gas, Co thin films were

under low pressure or low temperature conditions,grown on Ru substrate between 200 and 300 ℃, whereas

as indicated by RMS values in Table 3. In addition, it

Co thin films were not formed on SiO2 substrate at 200-

was found that the growth film on Ru substrate had a

225 ℃. Owing to the excellent substrate selectivity,

smaller surface roughness than that on SiO2 substrate,

the use of the mixture as a reactant gas is thought toas shown in RMS values in Table 3 and diagrams in

be a better choice for ASD. It is hoped that Ts-Co9

Fig. 11. Furthermore, 4 vol% H2 annealing treatment at

will contribute to the achievement of low temperature

200 ℃ for 2 h of the film grown on SiO2 at 200 ℃ leaded

deposition.

to an increase in the Co content, a significant decreasein the N content, and improvement in the surface
flatness of the film. However, the origin of the increased

5．References

O content is not well understood at this time. Compared（1）A. N. Cloud, L. M. Davis, G. S. Girolami, and J. R.

to before the annealing, an order of magnitude lower

Abelson, J. Vac. Sci. Technol., A 32, 020606 (2014)

resistivity value (72 μΩcm) approached that for bulk（2）S. Gatineau, M. Kimura, C. Dussarrat, J.-M. Girard,

7

Co (5.6 μΩcm at 0 ℃). These results of experiments

and N. Blasco, Application: WO2017115138; AIR

clearly explain the effectiveness of the H2 annealing for

LIQUIDE: France, 2017.

improving film quality.

（3）We have already reported on Ts-Co10, anothercobalt precursor that can be synthesized from Ts-

4．Conclusions
A novel liquid Co precursor Ts-Co9 was synthesized,
and its thermal properties, deposition and film

Co9.
https://www.tosoh.co.jp/technology/assets/20-4-9.
（4）G. N. Parsons, and R. D. Clark, Chem. Mater . 32,
4920 (2020)

characteristics were evaluated. This precursor has（5）H. Mellstedt, S. Bystroem, J. Vaagberg, and E.

sufficient vapor pressure and thermal stability for

Olsson, Application: WO2016124553; Kancera AB:

CVD/ALD processes. The CVD of Co-based filmusing the Co precursor under non-oxidative conditions
was demonstrated. The differences in Co thin film

Sweden, 2016.
（6）J. Yang, K. Li, J. Feng, and R. G. Gordon, J. Mater.

Chem. C 3, 12098 (2015)

formation and the film properties between NH 3 and（7）P. A. Lane, P. E. Oliver, P. J. Wright, C. L. Reeves, A.

(NH3+H2+DPM) mixture as a reactant gas were also

D. Pitt, and B. Cockayne, Chem. Vap. Deposition 4,

revealed by the deposition test of this complex on SiO2

183 (1998)

and Ru substrates. For CVD with NH 3, Co thin films
were deposited on Ru and SiO2 substrates at 150-300℃. There was no large difference in the deposition
rate between SiO2 and Ru substrates, and the obtained
films deposited under these condition had a significant
amount of nitrogen. It was suggested that H2 annealing75