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2. LITERATURE REVIEW
2.4 Study on Chemical Mechanical Polishing of Ruthenium
Ruthenium (Ru), a noble metal can be used as a barrier metal for lower node devices due its various properties such as low resistivity (~7 μΩ cm), high melting point (2334 °C) and negligible solid solubility with copper.(M. Damayanti et al., 2006; K. V Sagi et al., 2016) However, the planarization of Ru is very challenging because of its high chemical resistance and mechanical hardness. Hence, it was suggested that modification of the chemical composition might enhance the removal rate of Ru. Ru is soluble in an aqueous solution in the
form of RuO4. Hence, the slurry formulated should be such that the Ru surface should be oxidized to RuO4 or RuO2.2H2O on the Ru surface.(Zeng, J.-X. Wang, et al., 2012; Cheng et al., 2016) Addition of only abrasives in the slurry such as silica, alumina and ceria gave negligible polish rate (< 1nm/min).(Peethala, Roy and Babu, 2011a; Victoria et al., 2012;
Yadav, Jitendra C. Bisen, et al., 2017) Therefore, addition of several oxidizing agents and additives were suggested for Ru CMP. Some of the suggested oxidizers are ceric ammonium nitrate (CAN)(Lee and Park, 2004), hydrogen peroxide (H2O2)(Cui, Park and Park, 2013; Duan et al., 2015), percarbonate based slurries(Turk et al., 2013), potassium bromate(Victoria et al., 2010a), Oxone(Victoria et al., 2012), sodium hypochlorite(Yadav, Jitendra C. Bisen, et al., 2017) and sodium periodate (NaIO4)(Cheng et al., 2015) etc.
Ceric ammonium nitrate (CAN) used as oxidizer for Ru polishing in highly acidic (pH =1) region leads to formation of Ru2O3 and RuO2 (Lee and Park, 2004). At acidic medium, Ru oxidizes to form ruthenium dioxide (RuO2) which further converts to ruthenium tetraoxide (RuO4).(Wei et al., 2007a)
RuO4 + 2H2O ↔ H2RuO5 [2.12]
However, use of CAN causes undesirable high etching. The soluble products are unstable in nature hence not preferred. H2O2 based slurries also gave very low removal rates. Addition of guanidine carbonate (GC) along with to H2O2 forms soluble Ru complexes, thereby enhancing the removal rates.(Amanapu et al., 2013). Also, optimized GC concentration along with 1,2,4 triazole in H2O2 based slurry can give a desired Cu-Ru selectivity.(Wang et al., 2018). The RR of Ru can also be enhanced in the presence of potassium ions.(Jiang, He, Yuzhuo Li, et al., 2014) However, the performance of GC ions on enhancing removal rates is comparatively better than that of potassium ions.(Du et al., 2017) However, GC in KMNO4 suppresses the removal rate to unacceptably low values.(K. V. Sagi et al., 2016) It was observed that sodium
dissolution obtained was high (20 nm/min). Also, contamination of sodium is observed. The high dissolution is attributed to formation of soluble RuO4 in abundance. The reaction occurring are Ru surface are as follows.
4RuO4−+ 4H+ ↔ RuO2. 2H2O + 3RuO4 [2.12]
IO4−+ 2(RuO2. 2H2O) ↔ 4H2O + RuO4+ I− [2.13]
RuO4+ H2O ↔ H2RuO5 [2.14]
At acidic pH, Ru dissociates into perruthenate (RuO4−) oxidizes to form ruthenium dioxide (RuO2) and RuO4. At alkaline pH, perruthenate (RuO4−) and RuO2 (insoluble species) were formed and very low etch rates were observed. The RR doesn’t follow Preston equation with respect to pressure and velocity.(Victoria et al., 2012; Yadav, Jitendra C. Bisen, et al., 2017) A slurry comprising of potassium periodate (KIO4) as oxidizer and glycine was studied. Effect of glycine at pH 9 for Ru-Cu selectivity was studied.(Zeng, J.-X. Wang, et al., 2012) It revealed that glycine suppresses etching of Ru while it accelerates dissolution of Cu thereby giving a selectivity of 1:1.
Potentiodynamic polarization studies with potassium bromate as oxidizing agent reveals that current density is increased at the anodic part and the removal is mainly due to mechanical abrasion.(Victoria et al., 2010a) Ammonium ion enhances the surface corrosion rate of Ru by forming water soluble complexes. Hence, with an increase in concentration of the ions, the removal rate increases.(Ziyan Wang et al., 2019) Also, an optimized concentration could give a relatively smoother surface. Wang et.al reported that UV activated potassium persulfate loosen the Ru surface, hence enhancing the RR to 26.36%. (Wang et al., 2021)
2.4.1 Galvanic corrosion of Ruthenium and Copper
In a Ru polishing slurry, Cu being an active metal acts as anode whereas Ru being more noble in nature acts as anode. This difference in current corrosion potential leads to galvanic
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.