Motivation

4.4 Anodic dissolution of Cobalt in hydrogen peroxide solutions with and without complexing agent: Kinetic analysis by electrochemical impedance spectroscopy

4.4.1 Motivation

Cobalt (Co) has emerged as one of the most potent barrier metals in the semiconductor devices owing to its lower resistivity (~6.2 Ωm), non-requirement of seed layer deposition, improvement in electro-mitigation of interconnect etc.(Li et al., 2005; Wu et al., 2017) Hence, understanding the chemical and physical changes occurring on cobalt surface upon exposure to various solutions during fabrication of microelectronic chips is vital.(Lu, Zeng, et al., 2012;

Peethala et al., 2012; Li et al., 2016; R Popuri, Sagi, Alety, Peethala, Amanapu, Patlolla, et al., 2017)

Co usually forms a passivating film of Co (II) or Co (III) compounds on its surface when in contact with a reacting environment.(Ismail and Badawy, 2000) At neutral and lower alkaline region a passivating film of

Co OH ( )

₂ {Co(II) compounds} is predominant on the metal surface.(Badawy, F M Al-Kharafi and Al-Ajmi, 2000)

( ) ( )

2

Co OH

⁺

+ OH

⁻

→ Co OH

^[4.4.1]

At higher alkaline region,

Co O

_{3 4 and CoOOH} {Co (III) compounds} are dominant as the passivating film. (Badawy, F M Al-Kharafi and Al-Ajmi, 2000; Lu, Wang, et al., 2012; Lu, Zeng, et al., 2012; Peethala et al., 2012; Jiang, He, Yan Li, et al., 2014b; Fu et al., 2018;

Hazarika and Rajaraman, 2020)

2 3 4 2

3 Co OH ( ) + 2 OH

⁻

 Co O + 4 H O + 2 e

^{− [4.4.2]}

2 2

( )

Co OH +OH⁻ CoOOH +H O+e^{− [4.4.3]}

The formation of these compounds are dependent on pH value, oxidative environment or

the products formed is obligatory to study both the oxidation and the dissolution of the Co metal in the solution of interest.

The studies on reaction mechanism of Co anodic dissolution in different media are modestly low as reported in literature. Most of the suggested models are mainly based on bicarbonate/carbonate solutions(Calderón, Barcia and Mattos, 2008; Real, Ribotta and Arvia, 2008) which are discussed herewith. Davies et.al.(Davies and Burstein, 1980) and Burstein et.al.(Burstein and Davies, 1980) investigated the Co behavior in a carbonate/bicarbonate solution. They suggested the formation of CoO film in the dissolution and also claimed the presence of {

Co CO (

_{3 2}

)

²⁻} intermediate complex ion.

1

3( ) 3 ( )

( ) _aq ^k ( )_{s aq}

Co s +HCO⁻ ⎯⎯→CoCO +H⁺ +e^{− [4.4.4]}

2

2 ^k ( )_s 2 (_aq)

Co H O+ ⎯⎯→CoO + H⁺ +e^{− [4.4.5]}

3

( ) 3( ) 3( )

k

s aq s aq

CoO +HCO⁻ ⎯⎯→CoCO +OH^{− [4.4.6]}

4 4

2

3( )_s 3 ^k ( 3 2() _aq) (_aq)

CoCO HCO k Co CO H

−

−⎯⎯→ − +

+ ⎯⎯ + ^[4.4.7]

Gervasi et.al(Gervasi et al., 1991; Gervasi, Vilche and Alvarez, 1996) suggested a similar dissolution mechanism as Davies and Burstein(Burstein and Davies, 1980; Davies and Burstein, 1980), however he proposed that Co3O4 is formed instead of CoO since CoO film is sensitive to

HCO

₃⁻ion. Real et.al.(Real, Ribotta and Arvia, 2008) suggested a model comprising of formation of Co(I)(

Co OH ( )

_ad) as intermediate species followed by soluble Co(II)(mass transfer contributions)(

Co CO (

_{3 2}

)

²⁻) ions as the final products in an alkaline carbonate–bicarbonate based solution.(Real, Ribotta and Arvia, 2008)

1 1

( ) ( 2 )_i ^k ( )_ad ( )_i

Co s + H O ⎯⎯⎯⎯→k⁻ Co OH + H⁺ +e^{− [4.4.8]}

2 2

( )_ad ^k ( )

Co OH ⎯⎯⎯⎯→k⁻ Co OH ⁺+e^{− [4.4.9]}

3 3

2

3 3

( ) ^k

Co OH ⁺+CO ⁻⎯⎯⎯⎯→k⁻ CoCO +OH^{− [4.4.10]}

4 4

2

3 3 ^k [ ( 3 2) ]

CoCO +HCO⁻⎯⎯⎯⎯→k⁻ Co CO ⁻ +H^{+ [4.4.11]}

A two electro dissolution paths initiating with formation of

( CoHCO

₃

)

_adfollowed by formation of

Co CO (

_{3 2}

)

²⁻ (mass transfer phenomenon) and CoO film respectively was observed by Calderon et.al(Calderón, Barcia and Mattos, 2008) in a carbonate/bicarbonate solution.

Formation of CoO film is favored at higher anodic potentials (autocatalytic reactions) whereas formation of

( CoHCO

₃

)

_ad is favored at lower anodic potentials.(Calderón, Barcia and Mattos, 2008)

1

3 3

( )

^k

( )

_ad

Co s + HCO

⁻

⎯⎯→ CoHCO + e

^{− [4.4.12]}

2 2

2

3 3 3 2 2

( )_ad 2 ^k ( )

CoHCO HCO OH k Co CO H O e

−

− −⎯⎯→ − −

+ + ⎯⎯ + + ^[4.4.13]

3 3

3 3 2

( )_ad 2 ^k

CoHCO OH k CoO HCO H O e

−

−⎯⎯→ − −

+ ⎯⎯ + + + ^[4.4.14]

4 2

k 2

Co CoO+ ⎯⎯→Co ⁺+CoO+ e^{− [4.4.15]}

Other than the dissolution studies in carbonate/bicarbonate solution, mechanistic analysis of Co anodic dissolution in glycine solution at alkaline conditions are reported in literature.(Paul and Srinivasan, 2020) Paul et.al(Paul and Srinivasan, 2020) proposed a catalytic mechanism comprising of four adsorbed intermediates {two of each Co(II) and Co(III)} in a glycine system at pH 10. They suggested that the chemical dissolution rate increases with anodic potential while electrochemical dissolution is maximum at intermediary potential. However, the chemical dissolution is found to be saturated at larger anodic potential. The corresponding

2 2

A A

k k ads

Co Co e

−

+ −

⎯⎯→ +

⎯⎯ ^[4.4.16]

2 B 3

B

k

ads k ads

Co Co e

−

+ ⎯⎯⎯⎯→ + + − ^[4.4.17]

3 k_c 3

ads sol

Co

⁺

⎯⎯→ Co

^{+ [4.4.18]}

2 k_D 3

ads sol

Co

⁺

⎯⎯→ Co

⁺

+ e

^{− [4.4.19]}

3 3 3

3

kE

ads ads sol

Co

⁺

+ Co ⎯⎯→ Co

⁺

+ Co

⁺

+ e

^{− [4.4.20]}

In semiconductor industry, a polishing slurry is being used in chemical mechanical polishing (CMP) process to remove the excess Co. The slurry mainly consists of oxidizer along with complexing agent ^13,17,18such as Glycine(Lu, Zeng, et al., 2012; Kwon et al., 2020; Paul and Srinivasan, 2020), arginine(Peethala et al., 2012), acetic acid(Zuo et al., no date), EDTA(Kwon et al., 2020), citric acid(Peethala et al., 2012), oxalic acid(Oxide et al., 2002; Lowalekar, 2006;

Peethala, 2011; Raj et al., 2012) etc.

One such potent component known to form complexes with Co barrier metal is oxalic acid.

17,50 The carboxyl group in an oxalic acid forms complexes with the oxidized Co layer and intensifies the removal rates.(V R K Gorantla et al., 2005; Venkata R K Gorantla et al., 2005;

Ramakrishnan et al., 2007; Janjam et al., 2008, 2009; Miller and Granstrom, 2017) These complexes have an enhanced solubility in the alkaline region.(Meites, 1950; Barney, Argersinger and Reynolds, 1951; McAuley and Nancollas, 1960)

Similarly, hydrogen peroxide is commonly preferred oxidizer for the planarization of various metals such as Cu, Ru, Mo etc.(Amanapu et al., 2013; Jiang, He, Yan Li, et al., 2014b; Hu, Pan, Xu, et al., 2020b; Zhang, Wang and Lu, 2020; Poddar et al., 2021) Although the kinetics and mechanistic reaction pathway of Co in carbonate/bicarbonate, glycine etc. are investigated and reported in literature, the Co anodic dissolution mechanism in H2O2 (oxidizer) solution is yet to be studied. Reaction mechanistic analysis of Co in H2O2 solution in the presence of

complexing agent (oxalic acid) is also not reported in any literature to the best of our knowledge.

Thus, this work presented here focus on comparing the physio-chemical characteristics of Co metal upon exposure to only H2O2 solution and to H2O2 + oxalic acid solution. Various