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Title:Investigating surface behavior of copper electrodeposition electrolytes and additives
Author(s):Rooney, Ryan T.
Director of Research:Gewirth, Andrew A
Doctoral Committee Chair(s):Gewirth, Andrew A
Doctoral Committee Member(s):Girolami, Greg S; Kenis, Paul J; Nuzzo, Ralph G
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
electrolyte additive
Raman spectroscopy
surface enhanced Raman spectroscopy
quartz crystal microbalance, QCM
Abstract:Fabrication of microelectronics relies on copper (Cu) electrodeposition to form metal interconnects between the miniaturized circuit components within semiconductor devices. Cu electrodeposition for these applications is performed in the presence of organic additives to achieve super-conformally filled features without detrimental voids, seams or bumps. Fundamental understanding of Cu electrodeposition in this context is important for the advancement and innovation of the microelectronic fabrication industry. The first chapter introduces motivation for the microelectronics industry and fundamental concepts understood about Cu electrodeposition with additives. The second chapter discusses the role hydrophobicity plays in suppressing agents such as poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG). The third chapter describes the unique suppression behavior of PEG in methanesulfonic acid (MSA) electrolyte, an environmentally friendly alternative to sulfuric acid. The fourth chapter discusses the difference in interaction between bis-(sodium sulfopropyl)-disulfide (SPS) accelerator and PEG suppressor and between SPS and amine-based levelers. Chapter 1 briefly explains the microelectronics industry and the role Cu technology played in its evolution. Cu replaced aluminum as the interconnect material of choice in the late 1990s due to its superior physical properties. The Damascene process was developed to efficiently incorporate Cu into patterned low-k dielectric material to create precise interconnects joining the logic capable chip components with a larger scale of connections. Damascene processing relies upon Cu electrodeposition to fill nano and micro trenches/vias. To achieve proper filling without seams or voids, additives are included in standard sulfuric acid and copper sulfate electrolyte to influence the Cu reduction mechanism at the surface. The primary additives are chloride (Cl-), suppressors, anti-suppressors (accelerators), and levelers. Cl- adsorbs to Cu and facilitates all additives’ interaction to the surface. Suppressors inhibit deposition by preventing Cu2+ diffusion to the surface and heavily rely on adsorbed Cl- to function properly. Anti-suppressors, also known as accelerators enhance the deposition rate primarily through displacement of suppressor. SPS is ubiquitously used as accelerator. Levelers inhibit deposition, but operate to smooth out rough deposition evolution. Levelers are chemically distinguished from suppressors by a nitrogen-containing moiety imparting cationic quality. Synergistic interactions between all additives at the surface during deposition yield proper super-conformal filling of Damascene features and are explained thoroughly by the Curvature Enhancement Accelerator Coverage Model. Cu electrodeposition has also been recently been applied to achieving three dimensional packaging of microelectronics. Continual advancements in Damascene processing and new applications in advanced packaging of semiconductor devices keep the pursuit of fundamental Cu electrodeposition insight at the forefront of scientific inquiry. Chapter 2 details investigation into the fundamental surface behavior of Cu plating suppressor additives poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) using normal Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS), and electrochemical quartz crystal microbalance (QCM) measurements. Raman and SERS show a clear spectroscopic trend of increased intensity in higher wavenumber modes in the CH stretching region as the environment is changed from pure material to solution to surface for both PEG and PPG. The spectral changes associated with PEG are larger than those associated with PPG, suggesting that the relatively more hydrophilic PEG undergoes more conformational changes upon surface association relative to the more hydrophobic PPG. Calculated Raman spectra show the observed spectroscopic trend is associated with increased gauche character in the polymer backbone. QCM measurements show PEG adsorbs to the surface only in the presence of Cl-, while PPG adsorbs to the surface both with and without Cl- present. In the presence of Cl-, PPG forms a denser surface layer compared to PEG on a Cu underpotential deposition (UPD) layer on Au. These differences are attributed to differences in relative hydrophobicity between PPG and PEG, highlighting the property’s importance in dictating surface behavior of suppressor additives. Chapter 3 describes a fundamental investigation of the unique suppression behavior of PEG in methanesulfonic acid (MSA) Cu plating baths using electrochemical methods, normal Raman spectroscopy, SERS, and electrochemical QCM measurements. MSA is an alternative electrolyte to sulfuric acid, gaining recent interest for possessing superior Cu salt solubility and being more environmentally friendly. Furthermore, suppression of Cu electrodeposition by PEG in H2SO4 electrolytes only occurs in the presence of Cl-, whereas Cl- is not required in MSA electrolytes. SERS measurements of MSA electrolytes without PEG at a Cu surface show MSA molecules undergo re-orientation at negative potential, as evidenced by potential-dependent symmetry changes. The re-orientation of MSA in MSA and PEG electrolyte is delayed due to interaction between the two species. Eventually, MSA re-orients and PEG leaves the surface, in coordination with onset of Cu reduction current, suggesting the suppression interaction of PEG at a Cu surface is facilitated by MSA. QCM measurements demonstrate a similar departure of PEG mass at potentials negative of the MSA re-orientation. Chapter 4 describes an investigation into intermolecular interactions between Cu electrodeposition accelerator, suppressor and leveler at a Cu surface during deposition using electrochemical injection experiments and SERS. Due to cationic, nitrogen-containing functionalities, levelers interact with the SPS sulfonate moiety through ion-pairing and deactivate SPS’s acceleration ability, whereas non-cationic suppressors cannot. A shift in energy of the carbon-sulfonate bond observed with SERS provides direct evidence of an ion-pairing interaction between amine leveler molecules ethylenediamine (EDA) and diethylenetriamine (DTA) and SPS. Such a shift does not occur when PEG is included with SPS. Furthermore, the ratio between the gauche and trans conformations of SPS’s alkyl chain backbone has been previously correlated to acceleration performance. Analysis of the SPS gauche:trans in the presence of additives reveals that additives affect SPS structure and therefore acceleration ability in the following order: PEG < EDA < DTA. Despite its large molecular weight, PEG influences SPS structure significantly less than EDA or DTA. The difference in SPS structural behavior between interaction with PEG and interaction with amines indicates the importance of cationic functionality for proper leveler interaction with SPS. Continually, DTA’s ability to affect SPS structure more drastically than EDA suggests molecular weight of properly functionalized molecules also plays a role in leveler interaction.
Issue Date:2019-06-05
Rights Information:Copyright 2019 Ryan Rooney
Date Available in IDEALS:2019-11-26
Date Deposited:2019-08

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