UV VIS Studies of Ce(III)/Ce(IV) Redox Reactions to Understand Abrasive and Work Surface Interactions during STI CMP

In the current fabrication processes for integrated chips (IC), shallow trench isolation (STI) scheme is used to isolate and insulate transistors. Chemical mechanical planarization (CMP) is a key step in STI integration scheme. High selectivity slurries (HSS) which yield high silicon dioxide to silicon nitride polish rate are often used, and this is achieved by controlling the solid loading and the chemical concentration of additives [1]. Ceria based slurries with suitable additives are known to provide high selectivity [2, 3, 5].  Ceria is known to interact chemically with the silicon dioxide surface during CMP [2,3] and it is proposed that Ce(III) might play a role in the chemical interaction [4], but the exact nature of the species interacting with oxide are not known.

Addition of hydrogen peroxide (H₂O₂) to ceria based slurries is reported to suppress the oxide and nitride removal rates [4]. However, when other abrasives are employed, hydrogen peroxide does not modify the removal rate significantly [4]. Ce (III) and Ce (IV) ions are known to exhibit UV VIS absorption peak at different wavelength regions.

In this work a study on the kinetics of Ce (III) and Ce (IV) ions in the presence of H₂O₂is undertaken using UV-Visible Spectroscopy. Ce (III) chloride and Ce (IV) sulfate are employed for the initial study and the effect of hydrogen peroxide on changing the equilibrium composition is analyzed. The absorption is also monitored as a function of pH and time, to understand the dynamics of the system. Further, ceria abrasives are employed in the study and the nature of the species dissolving into the aqueous medium in presence of and absence of hydrogen peroxide, at different pH values are analyzed. The results show that the equilibrium between Ce (III) and Ce (IV) is shifted by the hydrogen peroxide which may help explain the suppression of oxide and nitride CMP rate by the addition of peroxide to ceria based slurries.

References:

1. J.M. Steigerwald, S.P. Murarka, R.J. Gutmann, Chemical Mechanical Planarization of Microelectronic Materials, John Wiley and Sons, New York, 1997.

2. L.M. Cook, Chemical process in glass polishing, J. Non-Cryst. Solids 120 (1990) 152–171.

3. L.Y. Wang, K.L. Zhang, Z.T. Song, et al., Ceria concentration effect on chemical mechanical polishing of optical glass, Appl. Surf. Sci. 253 (2007) 4951–4954.

4. R.Manivannan, S.Ramanathan, Appl Surf Sci.,Volume 255 (2009), 3764–3768

5. Naresh K. Penta, B.C. Peethala, S.V.Babu et al, Role of Hydrogen Bonding on the adsorption of several amino acids on SiO2 and Si3N4 and selective polishing of these materials using ceria dispersions, Colloids and Surfaces A : Physiochem. Eng. Apects 429(2013)67-73