Hot-filament chemical vapor deposition (HFCVD) has become the commercially most widely used technique for producing diamond thin-film coatings due to its scalability and relatively low cost. Maintaining the geometrical stability of the hot filament, however, is challenging since filament deformation varies the distance between the filament and the substrate. This study presents a dual optimization strategy for a newly developed HFCVD reactor, combining mechanical stabilization
with computational modeling.
Multiple novel hot filament clamp concepts were explored. From this, a novel hot filament clamp using tungsten support loops suspended from alumina (Al2O3) crossbeams was designed. Thermal simulations confirmed its feasibility, with alumina components reaching a maximum of 1250 °C, safely below the working temperature of this ceramic. 3D-printed prototypes allowed for the use of tab-and-slot connections with slight tolerances. The clamp was ultimately manufactured and assembled, although reactor maintenance prevented experimental evaluation of its influence on diamond deposition.
Hereafter, a COMSOL Multiphysics model of the reactor was developed. The substrate temperature distribution is modeled as a function of deposition parameters. Achieving more even substrate temperature distributions is beneficial for diamond quality. Parametric sweeps of key deposition parameters revealed that filament-filament spacing (D) had the strongest influence on substrate temperature uniformity. When D < 8.25 mm, the temperature range across the substrate increases.
In contrast, when D > 8.25 mm, the average substrate temperature decreases, and the amplitude of periodic temperature fluctuations grows. Additionally, simulations of sagged filaments showed a significant increase in standard deviation of substrate temperature, from μ = 3.67 °C to μ = 5.33 °C, highlighting the importance of maintaining filament geometry.
This work provides a mechanical solution to filament sagging and a framework for optimizing reactor parameters through simulation. The findings offer a pathway towards scalable, high-quality diamond film deposition in this setup.