Depositing at high rates and on ever larger areas are important objectives for the reduction of thin film silicon modules costs. High deposition rates have been achieved so far using different plasma sources but uniformity over large areas is still problematic. Matrix distributed electron cyclotron resonance (MDECR) systems consist of individual ECR plasma sources, which can be arranged in arrays with virtually no size limitations. Deposition rate of silicon alloys exceeding 10 nm/s, has been demonstrated, but little is known so far about the precursors’ dissociation and species fluxes onto the surface.

 

We study here the dissociation of Silicon film precursors, using optical emission spectroscopy and quadrupole mass spectrometry, both in the plasma phase and in the fore line. We correlate the dissociation on the one hand to the electron density and temperature measured using Langmuir probes and microwave interferometry, and to film growth rate and structure on the other.

It is found that more than 90% of SiH4 is used across a wide range of microwave powers, with a small dependence on gas residence time and pressure. Deposition rate depends primarily on the total flow of SiH4, and values above 3 nm/s are easily obtained. Transition from amorphous to microcrystalline silicon growth, on the other hand, occurs at high powers, and correlates to the electronic temperature and the high H/Si ratio observed in the gas phase. Direct dissociation of SiH4 by electron impact, followed by deposition from atomic Si and subsequent crystallization by hydrogen is therefore a satisfactory explanation.

 

On the contrary, for deposition from SiF4 , consumption is found to be lower than 60%, and independent of the gas residence time and microwave power. The deposition rate is also much lower, on the order of 0.3 nm/s. SiF4 consumption increases with the total hydrogen flow rate added to the mixture. The total concentration of atomic Si in the gas phase varies linearly with the product of SiF and H species, estimated by actinometry, suggesting fluorine abstraction by hydrogen as a possible mechanism. Deposition rate is also proportional to the Si content in the plasma phase and increases with the hydrogen flow rate added to the plasma. We propose that film growth from MDECR plasmas of SiF4/H2 mixtures occurs through the deposition of atomic silicon, obtained from hydrogen abstraction of fluorine in the gas phase, direct electron impact dissociation of SiF4 playing a smaller role.

 

This illustrates the importance of gas phase reaction for achieving high deposition rates, even in the case of high density low pressure plasma sources.