(Invited) Process Control of Advanced Cobalt Interconnect Plating Bath

Abstract

Cobalt made an inevitable move into more and more applications as device features shrink. It has largely been replacing tungsten in prior forays: for encapsulating and for high-aspect-ratio fill. Cobalt now has a new target in its sights: copper interconnect [1]. Copper replaced aluminum about 20 years ago and had been successfully utilized in numerous semiconductor manufacturing nodes. However, when the IC industry moved beyond the 10 nm node, copper interconnects became more compact, causing an increase in the resistance-capacitance delay. One way to solve the problem is to utilize a cobalt metallization alternative that provides resistance benefits over conventional technology without compromising reliability and yield [2]. In this electroplating process, a super-filling relies on depletion of protons inside feature, leading to increased current efficiency for Cobalt. Depletion of protons in this case leads to reduction of suppression efficiency and further enhancement of cobalt deposition as compared to field [3, 4]. These publications agree that a successful super-filling requires tight pH control. This presentation reveals results of advanced method that allow to reach 0.005 pH repeatability required for electroplating process. While this cobalt plating process is maturing, tighter process control is required to provide more accurate and precise data as well as faster analysis data generation. Previously developed analytical methods [5, 6] had been refined and improved to address challenges of the modern industrial cobalt plating processes. New, efficient analytical techniques were also developed. These methods can also report additional properties of a plating solution (conductivity and pH) critical for bath performance. Fig 1. shows the electrochemical responses of different proprietary organic additives used in commercial cobalt plating baths. Optimized electroanalytical and hydrodynamic parameters enable the determination of a wide range of concentrations of organic additives without interference from other bath components, as well as from bath pH and temperature. Analysis of organic additives can be performed with relatively simple fluidics that include a single mini electrochemical cell with accurate temperature control. Typical analysis steps include fast electrode conditioning in support electrolyte, several or single injection of plating solution to determine changes in plating rate, and then subsequent calculation of the concentrations. One of the newly developed methods is non-reagent spectroscopic analysis of boric acid. This method can also be combined with non-reagent analysis of cobalt providing very fast analysis of both components. Fig. 2 shows correlation of obtained and expected values for wide range of boric acid concentrations. This presentation will provide more detailed information related to the electrochemical behavior of organic additives and inorganic components. The results of Electrochemical Impedance Spectroscopy investigations and Raman spectroscopy will also be presented. The presentation will be concluded with a discussion of analytical results for all bath components during electroplating process.