Characterization and analysis on chemical and mechanical interactions during chemical mechanical planarization (CMP) of copper

Bum Soo Kim, Purdue University

Abstract

Chemical mechanical planarization (CMP) is one of the most important processes in semiconductor processing. Each layer on the wafer needs to be planarized before the next layer is deposited. This planarization is often accomplished using CMP. Better understanding of the mechanical properties of consumables and the roles of chemical reactions during polishing is required for the development of future CMP innovations. The mechanical properties of the CMP pad are very important to the polishing outcome and were investigated. The elastic moduli of commercial and noncommercial CMP pads were measured under various CMP-relevant conditions. The CMP pad was modeled using stiff and flexible springs, and interpretation was made based on a spring model. The remainder of this dissertation focuses on the chemical aspect of copper CMP. Specifically, surface reactions on copper in phosphoric acid- and nitric acid-based solutions with and without benzotriazole (a corrosion inhibitor) and hydrogen peroxide were investigated using electrochemical tools, including potentiodynamic (PD) scans, electrochemical impedance spectroscopy (EIS), and studies of the time-evolution of impedance at different DC potentials. During EIS, impedance at high frequencies represents the ohmic resistance which is the sum of the resistances of the electrolyte, the connections between the electrodes and the potentiostat, and precipitated salt or resistive viscous liquid film on the electrode of interest. Since the first two resistances are regarded as constant for the conditions studied here, observed changes in impedance at high frequencies are interpreted to be the result of the formation of a porous salt/viscous liquid layer at the electrode surface. The dynamics of the formation of this layer are observed by measuring the changes in the impedance of a bare copper substrate at 100 kHz in different applied DC potentials. The substrate is created when a Teflon-coated copper sample is cut in an electrolyte solution using a guillotine electrode apparatus. PD scans were performed in each solution in order to characterize the anodic behavior of copper. Different DC potentials from the active and passive region were applied while the impedance was monitored. The open circuit potential was also studied in this manner. Interpretations were made for each outcome in terms of surface layer formation (porous salt film and/or viscous liquid film).

Degree

Ph.D.

Advisors

Beaudoin, Purdue University.

Subject Area

Chemical engineering

COinS