Plasma enhanced chemical vapour deposition (PECVD) is a key deposition technique used in the fabrication of silicon solar cells. PECVD reactors are used to deposit thin-film layers of silicon nitride (SiNx), and more recently, aluminium oxide (AlOx) in the fabrication of PERC solar cells. Also, PECVD reactors can be used to deposit intrinsic and doped amorphous silicon (a-Si) for silicon-based heterojunction solar cells, and therefore the technology is widely applicable in photovoltaic solar cell manufacturing.
PECVD SiNx deposition for silicon solar cells has three key advantages, including:
- The alibility to serve as an anti-reflection coating for improved light absorption.
- The passivation of defects at the surface of the silicon wafer.
- The ability to provide bulk passivation via the release of hydrogen from the film into the silicon bulk.
PECVD reactors are typically classified ass either direct or remote systems. In a direct PECVD reactor, silicon wafers are placed between two parallel electrodes, in direct contact with the electrodes and the excited plasma. In a remote PECVD reactor, the samples are not placed in direct contact with the plasma forming electrode and the plasma is introduced onto the surface of the wafer. A main disadvantage of the direct PECVD reactor is that there can be near surface damage due to in ion bombardment which increases the recombination rate in the affected region. However, SiNx:H deposited by direct PECVD are typically found to be denser and have a lower pin-hole density. Both types are currently used in solar cell manufacturing. A schematic of a direct and remote PECVD system are shown in Figure 1.
A typical deposition process occurs on a heated substrate, typically in the 350-450 °C. The most commonly used precursors used for the deposition of SiNx:H are silane (SiH4), ammonia (NH3) typically mixed with inert gasses such as argon (Ar) or nitrogen (N2). The tuning of the gas flows and temperature allows the properties such as the thickness, refractive index and hydrogen content to be tuned. In most cases, the SiNx:H film is optimised to provide optimal surface and bulk passivation after the high-temperature firing step.