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2. LITERATURE REVIEW
2.5 Study on Chemical Mechanical Polishing of Cobalt (Co)
corrosion between Cu and Ru. Hence to minimize the galvanic corrosion and excessive dissolution of Cu, different additives are proposed.(Patlolla et al., 2018) 2,2'-[[(methyl-1H- benzotriazol-1-yl)methyl]imino]diethanol (TT) added as an inhibitor to ammonium sulfate and H2O2 slurry suppresses the dissolution of Cu by forming adsorbed passivation on its surface.(Patlolla et al., 2018) However, such effect was not seen on Ru surface but a reduced dishing and erosion defects were observed in both the metals. Addition of ascorbic acid and BTA as cathodic and anodic inhibitors to KIO4 based solution reduced the galvanic corrosion of Cu-Ru to 20 mV.(Peethala, Roy and Babu, 2011a) It was seen that potassium molybdate ions modifies the Cu and Ru surface in a BTA+ KIO4 based slurry. The ions adsorbed facilitates the BTA absorbed-passive layer thereby suppressing the galvanic corrosion between them.(Cheng et al., 2014) Insoluble Cu(IO3)2·nH2O compounds on the surface also suppresses the dissolution rate of Cu.(Cheng et al., 2018) Similar inhibition properties as BTA can be observed using 1,2,4 triazole.(Cheng, Wang and Lu, 2020) BTA usually inhibits by forming a 3-dimensional adsorbed structure on the Cu surface. However, the adsorption of the 1,2,4 triazole is 2 dimensional, and an excess of it can destabilize the passivation film, and reduce its inhibition efficiency on Cu. Tian et.al (Tian et al., 2022) suggested that potassium Tolyltriazole (TTAK) as inhibitor in a H2O2-glycine based slurry effectively reduces the corrosion rates of both Cu and Ru by forming a passive film on its surface. A reduced erosion and dishing are also observed in its presence.
drawbacks such as corrosion and defectivity post CMP are mainly observed for Co (CVD/PVD) films.(Hu, Pan, Li, et al., 2019) The corrosion could be general corrosion or pitting or galvanic corrosion. It is to be noted that, Co can only be commercialized for the next generation semiconductors if the mentioned issues are eliminated. The dissolution of Co via Co2+ ions is illustrated below.(Peethala et al., 2012)
Co + 2H+ → Co2++ H2 [2.14]
To eliminate such corrosion via dissolution, formation of passive layer is the utmost priority.
The passive layer could either be in the form of Co (II) or/and Co (III) oxide/hydroxides. Lower potential Co (II) oxide/hydroxide are usually formed at acidic pH and at lower potentials. At higher potentials and at neutral and alkaline pH, Co (II) oxide/hydroxide oxidizes to form Co(III) oxides/hydroxides.(Park, Paluvai and Venkatesh, 2018) The passive layer formed at highly alkaline region is more stable in nature.(S. Yang et al., 2019)
Co + H2O → Co(H2O)(ads) [2.15]
Co(H2O)(ads) → Co(OH)++ H++ 2e− [2.16]
Co(OH)++ H2O → Co(OH)2+ H+ [2.17]
Co(OH)2 → CoO + H2O [2.18]
Co(OH)2+ OH− → CoOOH + H2O + 2e− [2.19]
3CoO + 2OH−→ Co3O4+ H2O + 2e− [2.20]
The issues can be controlled by proper tuning of the slurry chemistry. Different oxidizers such as ammonium per sulphate,(Ranaweera et al., 2019), H2O2(Hu, Pan, Xu, et al., 2020a),
potassium per sulphate(Zhang, Wang and Lu, 2020) ; complexing agents such as:, cystine(Zhi Wang et al., 2019), citric acid(R Popuri, Sagi, Alety, Peethala, Amanapu and Patlolla, 2017), glycine(Paul and Srinivasan, 2020), hydroxyethylidene diphosphonic acid (HEDP)(Hu, Pan,
Xu, et al., 2020a), potassium tartrate(Hu, Pan, Li, et al., 2019), ethylenediaminetetraacetic acid (EDTA)(Kwon et al., 2020) and inhibitors such as: 1,2,4 triazole(Zhong et al., 2014), BTA(Ryu et al., 2020), diethanolamine (DEA)(Xu et al., 2021), potassium oleate(Ranaweera et al., 2019) etc. are suggested in literature to attain a planar Co surface with reduced defects and corrosion. Alkaline solutions are usually preferred over acidic solution due to controlled occurrence of dissolution at this region. Although BTA was the most preferable inhibitor in most of the studies done till date, it was seen that it forms easily soluble Co-BTA passive layer.(Ryu et al., 2020) This gives rise to unwanted residue generation post CMP. Ji et.al (Ji et al., 2017) reported that presence of ethylene diamine tetra acetic acid (EDTA) derivative, in an oxidizer absent slurry can also give and enhanced removal rate for Co.
2.5.1 Galvanic Corrosion of Cobalt and Copper
Co, being more fragile chemically as compared to Cu is highly inclined towards corrosion and oxidation.(S. Yang et al., 2019) Also, the difference in reduction potential makes them more prone to galvanic corrosion.(Peethala, Roy and Babu, 2011a) In addition to that that Co (Mohs hardness 5) is mechanically harder than Cu (Mohs hardness 3).(Peethala et al., 2012) Hence, maintaining desired removal selectivity along with reduced galvanic corrosion is a big challenge. Difference in open circuit potential (OCP) between Cu and Co acts as the main driving force behind galvanic corrosion. Hence the slurry formulated should be capable enough to reduce the difference between them.(Peethala et al., 2012)Slurry solutions containing BTA and glycolic acid at pH 2 were investigated and it was observed that Co corrosion can be minimized in the presence of BTA and glycolic acid.(Bilouk et al., 2009a) Literature proved that pH of a slurry has a major role in attaining selectivity between Co/Cu.(Nishizawa, Nojo and Isobe, 2010b) Nishizawa et.al(Nishizawa, Nojo and Isobe, 2010b) also proposed a slurry comprising of H2O2, citric acid and BTA, however the slurry lead to high current densities with increase corrosion defects. For devices less than 14 nm, an abrasives slurry comprising of
mainly potassium acetate, hydrogen peroxide, and benzotriazole gave a controlled corrosion for Co/Cu system.(Johnson, Wei and Roy, 2018) Hu et.al reported that, novel inhibitor, TT- Lyk adsorbs on the Cu surface forming Cu-TT-Lyk passivating film, thereby reducing the potential difference between the two metal.(Hu, Pan, Zhang, et al., 2019) A higher galvanic corrosion at static condition is observed as compared to dynamic condition. Zhang et.al proposed a green inhibitor, sarcosine that inhibited the corrosion in two different ways: forming passive layer and hindering further oxidation, reducing the glycine-metal complexes on the metal surface.(Zhang et al., 2022) It was reported that potassium hydrogen phthalate (KHP) and 1, 2, 4-triazole (TAZ) as chelating agent and inhibitor in a H2O2 based slurry gave a reduced Cu/Co potential gap better surface quality.(Yang, Zhang and Yang, 2022)