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Nano-Scale Protective Oxide Films for Semiconductor Applications & Beyond

Final Report Summary - NANO-PROX (Nano-Scale Protective Oxide Films for Semiconductor Applications & Beyond)

The main objective of the NANO-PROX (Nano-Scale Protective Oxide Films for Semiconductor Applications & Beyond) project was to establish a fundamental understanding on growth of nano-scale protective metal-oxide thin films through experimentation and simulation. One of the main applications the project aimed to contribute was the Chemical Mechanical Planarization (CMP) process. In CMP, the formation and uniform removal of the metal oxide thin films are very critical to be able to control the removal rates, defectivity and topographic selectivity. Hence the project focused to meet the current CMP requirements of the microelectronics manufacturing in addition to the forthcoming needs and demands since more challenging microelectronics circuitries are continuously being designed.

The fundamentals of the NANO-PROX project targeted to create a basis for the new generation semiconductor industry that deals with atomic scale devices and also expected to be utilized in many other fields such as advanced coatings, interfacial adhesion, biological systems i.e. corrosion prevention and enhanced biocompatibility of bio-implants and nano/bio interfaces. Initially, NANO-PROX project resources were concentrated on the establishment of the CMP and surface chemistry laboratory at the Ozyegin University as it was outlined in the research tasks. A materials and surface chemistry laboratory has been established and equipped at OzU including the purchases of basic chemicals, glassware, fundamental laboratory equipment (stirrers, pH meters, hot plates, ultrasonic baths, fume-hoods, DI water systems, autoclave etc’), in addition to the specific surface chemistry equipment such as surface tensiometer and contact angle goniometer. Another critical equipment purchase has been the table-top polisher for the CMP tests, which was purchased earlier than planned to speed up the progress. The laboratory facilities and equipment list can be reviewed at https://faculty.ozyegin.edu.tr/nano-prox/lab-facilities/ which are further being improved through the new projects originated from the resources of the Nano-PROX project.

The technical accomplishments of the project can be grouped in two aspects. The first part focused on the accomplishments on the microelectronics applications of the CMP process as it is used for the current and future semiconductor applications, such as metal CMP applications of the tungsten T-gate transistors, high speed shallow trench isolation transistors with germanium and furthermore isolating thin films for advanced microelectronics applications such encapsulation of the PZT arrays of the ferroelectric memory applications. The second part focused on the applications of the CMP induced metal oxide thin films on biomaterials, conventional metals such as steel that is used for heating elements and aluminum used for airplane bodies to improve the corrosion resistance.

First a fundamental understanding on the growth of nano-scale protective oxide thin films was studied to determine the effect of oxidizer type and proper oxidation time on the changes in surface properties of the thin films. The preliminary model was developed on the very well established tungsten, which is being utilized 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. 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 how the chemically modified thin films were also expanded 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. While the self-assembled surfactant structures help improve slurry stability, they may retard the material removal rates by inhibiting the particle surface interactions. 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. However, surface quality and the selectivity of the Ge/SiO2 systems were tuned through adjusting the concentration of the oxidizer and surfactant type or chain length and tuning the ionic strength of the system to optimize the planarization performance.

In the second part of the project, we have implemented CMP as a technique to induce self-protective metal oxide nano films on metals for applications other than semiconductors. Initially, a similar study to W and Ge evaluations was conducted on titanium metal to investigate the use of CMP for enhancing the surface properties of bio-materials applications. In this part of the project investigation we quantified the effect of controlled surface roughness induced by CMP on the infection resistance and cell attachment capability of the titanium-based bio-materials which are commonly used for orthopedic devices. It was observed that the formation of self protective oxide layers help limit the growth of bacteria and hence can help control infection on the bioimplants. Furthermore, the cell attachment was also found to be related to the topographic nature of the surface. In addition, the impact of CMP induced self protective oxide films of alumina was studied as a part of a senior thesis through the Nano-PrOX project funding to improve the corrosion resistance of the aluminum surfaces used for plane bodies and landing gears as requested by Turkish Airlines Technique. Another PhD project was started on implementation of the CMP process to stainless steel surfaces of the heating elements to control corrosion and lime-scale prevention with Wiessman. Throughout all these applications, it was also clear that a 3-D CMP application was needed to be developed. This new initiative has led to five new IP applications and is believed to drive new research projects in the near future.

In summary, the Nano-PrOX project have resulted in five patent applications, total 26 publications including a book, a book chapter, journal papers published and to be published (held up due to IP applications) and proceedings, 25 presentations as well as the establishment of OzU Nano Team and the Materials and Nanotechnology laboratories. The project have led to 8 confidentiality agreements with the private sector and more than five new project applications among which 2 received Eureka support and one is funded by the company and one more is being submitted to Eurostars program. The research group of Dr. Basim have participated in the Euro-Nano Forums for past three years and have presented OzU Nano activities at the exhibition in 2013 (Dublin) and in 2014 (Athens) meetings. The project has reached its goals of reintegrating Dr. Basim to academia in Turkey as she has been promoted to Associate Professor level, awarded new grants and established many industrial and research partnerships.