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Polishing slurries used in chemical-mechanical planarization ͑CMP͒ of copper typically include a complexing agent and an oxidizer. Our present work investigates the effectiveness of citric acid as a complexing agent for Cu with H 2 O 2 employed as an oxidizer. We show that the rate of copper removal from Cu increases when both citric acid and H 2 O 2 are used in acidic solutions ͑pH 4.0͒ and decreases drastically in alkaline solutions ͑pH 8.0͒, as well as in citric acid without any H 2 O 2 in the slurry. We investigate the underlying surface reactions of these effects by using Fourier transform electrochemical impedance spectroscopy ͑FTEIS͒ in combination with potentiodynamic measurements. We analyze the relative roles of citric acid, H 2 O 2 , and solution pH in Cu removal, and develop a reaction scheme describing the surface chemistry of Cu in this system. The results presented here also demonstrate how FTEIS can be used for quantitative investigation of surface reactions in complex CMP systems.Copper damascene structures are now widely used in integrated circuits, and chemical-mechanical planarization ͑CMP͒ is the preferred planarization technique for fabricating these structures. 1-4 The polishing slurry used in CMP contains a number of additives that have different chemical functions. 2 For instance, typical copper polishing slurries contain an oxidizer to form porous unstable surface oxides, and a complexing agent to dissolve material from the sample surface in the form of water-soluble complexes. 1 Understanding the relative roles of these different chemical additives is crucial to developing efficient polishing slurries. Our present work is centered on this specific issue and probes the chemical effects of citric acid as a complexing agent in the presence of a commonly used oxidizer, H 2 O 2 .Though citric acid has been evaluated as a complexing agent for Cu-CMP slurries, 5-9 a fundamental understanding of the surface reactions of citric acid with Cu at different pH values and/or oxidizer concentrations is still lacking. Our goal here is to probe these reactions in detail. As we have recently demonstrated, 10,11 Fourier transform electrochemical impedance spectroscopy ͑FTEIS͒ is an ideally suited technique for quantitative investigation of multistep reactions in CMP systems. FTEIS allows for fast ͑time-resolved͒ detection of impedance spectra 12-16 and hence, is particularly useful for studying transient corrosion effects. In this approach, circuit models of the reactive interfaces are obtained through rigorous complex nonlinear least square ͑CNLS͒ analysis of voltage and/or solution-dependent impedance spectra. 10,11 Subsequently, detailed reaction schemes are derived by comparing the CNLS analyzed results with those of potentiodynamic polarization measurements. 10,11,17,18 In this work, we utilize these capabilities of FTEIS in combination with standard CMP and potentiodynamic measurements to study pH-dependent surface chemistries of citric acid and H 2 O 2 on Cu. We demonstrate here that citric acid ac...

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Infrared spectroscopic ellipsometry (IRSE) of organic self-assembled monolayers (SAMs) commonly uses the external reflection geometry and a three-phase system (bulk substrate, SAM, and ambient medium). In the present work, we study a four-phase system for IRSE, where a gold substrate film is sandwiched between a CaF2 prism and a SAM of octadecylmercaptan (ODM). This sample configuration can be employed for internal reflection IRSE (using a continuous Au film), as well as to boost the detection sensitivity of IRSE through surface-enhanced infrared absorption (using discontinuous film of Au nanoislands). We study how the thickness and morphology of the Au layer in the four-phase structure affect the IRSE results for ODM by using three Au substrates:  an optically thick ∼1000 Å continuous film, a continuous ∼230 Å thick film, and a discontinuous ∼220 Å thick film of Au nanoislands. The spectral features of ODM in the last case are different form those of the first two, and are associated with surface-enhanced IRSE. Surface morphologies of the Au substrates are characterized by scanning electron microscopy. The IRSE results are discussed with use of currently known theoretical considerations for differential spectroscopy.

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By combining the polarization modulation (PM) technique with Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS), one can substantially improve the detection sensitivity and data collection efficiency. Using PM, the step of "background subtraction" can be eliminated for quantifying adsorbed species or covalently attached monolayers, and the IR spectra can be taken in situ in aqueous solutions. We demonstrate here that such a combination of PM and FT-IRRAS is possible, not only for external reflection measurements, but also for the attenuated total internal reflection (ATR) arrangement. The unique advantage of this second combination is that it couples the surface sensitivity of ATR with the experimental convenience and rapid data collection capability of polarization modulated FT-IRRAS. In the present work, we combine numerical calculations with experiments to study octadecylmercaptan (ODM) monolayers, self-assembled onto a gold-coated calcium fluoride prism. Results, including IR spectra (2800-3000 cm -1 ) of ODM on Au are presented for PM as well as for parallel (p) and perpendicular (s) polarization selective detection schemes and for both external reflection and ATR experimental geometries.