‘Hot-wire assisted atomic layer deposition of tungsten films’
This thesis focuses on the application of hot wire atomic layer deposition (HWALD). ‘Ultra-large-scale integration of microelectronics is rapidly evolving,’ Mengdi Yang says. ‘This novel technique is promising for next generation fabrication strategies, to achieve conformal and uniform thin films, with very precise control of thicknesses on structures of increasing complexity.’
Tungsten (W) deposition is realized by providing sequential pulses of atomic hydrogen (at-H) and WF6. Two reactors are used. In the cold-wall reactor, the hot wire is situated much further away from the substrate as compared to the hot-wall reactor.
In both reactors a spectroscopic ellipsometer is installed to in-situ monitor the film growth. Tellurium etching experiments were conducted, to confirm the existence of atomic hydrogen and its delivery to the substrate.
At the beginning of the PhD project the experiments showed no great results because the resulting conductivity on the substrate was too low, Mengdi Yang shares.
‘We then changed and chose another reactor,’ she says. ‘The results were much better. However, we did spend a lot of time tracking down the exact mechanisms causing the low conductivity. One of the main lessons of the PhD project is that lots can be learnt if results are not at first sight satisfactory. On the other hand, at a well-chosen moment, you have to choose a new direction of research, to make progress possible.’
After improving the hot-wire technique further, a patent was obtained. ‘The process as a whole ( placing the entire wafer, monitoring growth, and characterizing the functionalities) is very promising,’ Mengdi says. ‘Together with ASM International - with which we closely collaborated – we now can start thinking to apply this approach for industry. However, this process can be further developed. More efforts must be made to shorten the cycle time.’
The good results were confirmed by using X-Ray Diffractometry and High-resolution Transmission Electromicroscopy. ‘We, as the Integrated Devices and Systems (IDS) Group, collaborated with the Inorganic Materials (IOM) Group, and also with experts from the Technical University Delft,’ Mengdi says. ‘Because of the long-term working relationships that already existed, we were able to benefit from the expertise available at different Groups.’
The HR-TEM analysis of the films showed a uniform and conformal coverage on high aspect ratio structures (up to an aspect ratio of 36), confirming the effective atomic layer deposition process and the sufficient diffusion of both WF6 and at-H into deep trenches. Finally, it was found that W-layers start to become electrically continuous in a thickness range of 2-3 nm.
Mengdi perceives herself as an experimentalist scientist in the first place. ‘Together with process development, that’s where my main interest and drive comes from,’ she says. ‘So, I opt for becoming a real engineer, preferably in industries.