Raman spectroscopy in-situ on diamond HF-CVD
The fabrication of boron-doped diamond (BDD) in this project is done in hot-filament chemical vapour deposition (HF-CVD) processes.
The interaction of the coating parameters such as substrate temperature, pressure, gas mixture and dopant concentration is highly complex but decisive for the later characteristics of the produced BDD. Therefore, an understanding of the formation mechanisms through an in situ monitoring of the coating’s properties helps to optimise the production process.
The figure below represents the Raman spectrometer set up at the CVD-machine for in situ analysis: a) Spectrometer controller, b) Laser and Raman equipment, c) window into CVD machine with Raman probe head and d) CVD coating control screen.
This experiment has just been undertaken last week, where we mounted an external Raman probe head from our Witec alpha 300R spectrometer onto our newly build CVD machine. The general setup is shown in figure 1, where the Raman equipment is surrounding the CVD reactor, placing the probe head in front of a quartz window to look inside the chamber. There its long working distance objective focuses on a 5 mm titanium sample, gathering Raman spectra from the forming BDD layer.
The challenges here are to collect a Raman signal strong enough to shine through the high thermal background radiation of the heating filaments (2100 °C) and sample surface (800 °C). Furthermore, several interference effects mangle the spectra and make the post processing quite demanding (comp. screen a) in fig. 1), but now the diamond quality and the doping concentration can be successfully determined in situ! This is part of our endeavour to improve the BDD performance for the production of hydrogen peroxide in our project.
The figures below represent the Raman probe head in front of the CVD-chamber, focusing a laser spot (green dot) on the 5 mm Ø titanium sample. The sample is at standard coating temperature of 800 °C, heated by the glowing filaments.
