CMP Development for New Generation Materials through Metal Oxide Thin Film Characterization
Authors: Basim G. Bahar 
Published: February 2020

Chemical Mechanical Planarization (CMP) process is currently facing the challenge to planarize larger scale wafers at an atomic scale precision. While the horizontal dimension (x-axis and y-axis as the wafer diameter) in the process can extend up to 300, 450 mm, the z-axis is limited to a margin of a couple hundred nanometers. This requirement highlights the need for fundamental understanding on the surface chemistry of the layers being exposed to the planarization process in terms of their interactions with the CMP slurry chemistry. Once the behavior of the wafer surface is understood at an atomic level, the process control metrics can be tuned accordingly.
In our earlier work, we studied the growth of nano-scale protective oxide thin films for metal CMP on tungsten as a model [1]. Tungsten is a well-studied metallic layer in microelectronics and it is used as a gate dielectric for the novel T-gate transistors currently. Thin film analyses were conducted through advanced characterization techniques and also compared to the theoretical calculations for the modeling simulations. Atomic Force Microscope (AFM) was used to measure the surface roughness of the samples conditioned in the oxidizer environment before and after the CMP was conducted. The affect of surface roughness on wettability of the surfaces studied through contact angle measurements on the treated tungsten films. Fourier Transform Infrared Spectroscopy with Attenuated Total Reflectance FTIR/ATR technique in combination with the X-Ray Reflectivity (XRR) was utilized to determine the thicknesses of the oxidized nano films on the tungsten wafers. The results were evaluated through the comparison of the Pilling-Bedworth ratios of the oxidized nano films to determine the ability of the created oxide films as a self-protective oxide. Furthermore, a new modeling approach was introduced to CMP process optimization by means of topographic evaluation of the metal oxide thin films. Cahn Hilliard Equation (CHE) was utilized as an alternative to classical nucleation theory in terms of analyzing the topographic nature of the protective metal oxide nano films and modeling their growth, which was observed to affect the CMP performance [2]. It was concluded that the material removal rate mechanisms and the consequent planarization performance depend on the nature of nucleation of the metal oxide films, which is tailored by the oxidizer concentration.
The basic knowledge defined on the chemically modified thin films has also been extended to the germanium CMP applications. Particularly, formation and selective removal of chemically modified germanium/silica thin films in the presence of cationic and anionic surfactants were evaluated through AFM wear tests as well as CMP and surface wettability responses. It was determined that while the self-assembled surfactant structures help improve slurry stability, they may retard the material removal rates by inhibiting the particle surface interactions [3]. The results of this study have shown that in the presence of hydrogen peroxide in the slurry, removal rates were mainly affected by the oxidizers surface activity resulting in the formation of germanium oxide film. However, surface quality and the selectivity of the Ge/SiO2 systems were tuned through adjusting the concentration of the oxidizer and surfactant type/chain length in the system to optimize the planarization performance [3]. This paper reviews the affects of chemically modified thin films on the CMP performance at an atomic level for tungsten, silica and germanium CMP applications as metal, dielectric and semiconductor materials utilized in the microelectronics industry.
References

A. Karagoz, V. Craciun, G.B. Basim, ECS Journal of Solid State Science and Technology, 4(2), 1 (2015).

A. Karagoz, Y. Sengul, G.B. Basim, ECS Transactions, 61(17), 15 (2014).

A. Karagoz, G.B. Basim, ECS Journal of Solid State Science and Technology, CMP Special Issue, 4(11), 5097 (2015).