Dependence of Spatial Uniformity of Nanocrystalline Silicon and Silicon Nitride Films on the Thermal Profile of Low Temperature Catalytic CVD Reactor

Application of nanocrystalline silicon (nc-Si) and silicon nitride (SiN_x) thin films on plastic substrates is one of the key technologies for flexible display devices and wearable electronics.  The fabrication of electronic circuitry on flexible substrates often requires a thin film deposition technique that requires a processing temperature substantially lower than the glass transition point of the substrate material. The catalytic chemical vapor deposition (Cat-CVD) technique is one of the best candidates to obtain electronic quality inorganic films at low temperatures: a thin (0.4 – 0.7 mm in diameter) filament is the only component that is heated, saving the substrate from the direct heating, and the source gas is dissociated to energetic radicals upon contact with the filament, boosting the source-to-film conversion efficiency.

The configuration of the filament can be varied in numerous ways to accommodate specific needs for various films.  The distance between the filament and the substrate, the number of segments, the type of connections between segments (serial or parallel), the inter-filament spacing, and the total length of the filament have been changed to grow nc-Si and SiN_xfilms at temperatures below 100ºC.  All these parameters, however, not only affect the process temperature, but also the thermal profile on the substrate surface.  In this study, influence of the thermal profile, resulting from various filament configurations, on the uniformity of the film across the substrate was investigated in terms of thickness and electrical properties.

Figure 1 shows a schematic diagram of the showerhead-filament assembly of a Cat-CVD system.  For a parallel arrangement of the filaments, a pair of molybdenum fixtures is attached to the electrical feed-through where the power is supplied to heat the filaments. The spacing between the filaments can be adjusted at an 8 mm interval.  For a serial arrangement, insulating pillars, as many as eight, can be mounted around the showerhead.  The filament may extend from one feed-through to the other in an arbitrary geometrical pattern, using these pillars as a mechanical support.

The parallel arrangement was more advantageous than the serial arrangement in minimizing the self-heating of the substrate by the radiative heat from the filament.  When only two pieces of filament was used with an inter-filament spacing of 56 mm and a filament-substrate distance of 5 cm, the process temperature was lowered to 100°C and the resulting films showed a good thickness uniformity and adhesion.  As shown in Figure 2, a 2-D cross-sectional simulation with two point heat sources revealed that the temperature profile on the substrate surface had two maxima when the filament-substrate spacing was smaller than 1.7 cm.  Beyond this distance the temperature profile behaved as if there was a single heat source at the center.  When the filament-substrate spacing was larger than 8.5 cm, the temperature across the 100 mm-diameter substrate became uniform.

On the 100 mm-diameter circular substrate, the thickness was measured at a 10 mm interval to investigate the uniformity across the substrate.  The 56 mm inter-filament spacing led to the lowest standard deviation, and the thickness was uniform in the area between the filaments.

Resistivity of nc-Si and breakdown field strength of SiN_x, however, showed little correlation with the filament configuration.  The standard deviation showed the lowest value for the inter-filament spacing of 56 mm, but contour plots for the resistivity and the breakdown field strength appeared to be more affected by the asymmetry of the reactor shape than by the geometry of the filament arrangement.