Hydrogenated microcrystalline silicon thin films deposited by RF-PECVD under low ion bombardment energy using voltage waveform tailoring

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Abstract

We present experimental results for hydrogenated amorphous and microcrystalline silicon (a-Si:H and μc-Si:H) thin films deposited by PECVD while using a voltage waveform tailoring (VWT) technique to create an electrical asymmetry in the reactor. VWT dramatically modifies the mean ion bombardment energy (IBE) during growth, and we show that for a constant peak-to-peak excitation voltage (VPP), waveforms resembling “peaks” or “valleys” result in very different material properties. Using Raman scattering spectroscopy, we show that the crystallinity of the material depends strongly on the IBE, as controlled by VWT. A detailed examination of the Raman scattering spectra reveals that the narrow peak at 520 cm 1 is disproportionately enhanced by lowering the IBE through the VWT technique. We examine this effect for a range of process parameters, varying the pressure, hydrogen–silane dilution ratio, and total flow of H2. In addition, the Sisingle bondHX bonding in silicon thin films deposited using VWT is characterised for the first time, showing that the hydrogen bonding character is changed by the IBE. These results demonstrate the potential for VWT in controlling the IBE during thin film growth, thus ensuring that application-appropriate film densities and crystallinities are achieved, independent of the injected RF power.

Highlights

► We apply non-sinusoidal RF voltage waveforms to PECVD of microcrystalline silicon. ► This technique decouples ion bombardment energy from absorbed RF power. ► Waveform shape determines film microcrystallinity for pressures from 0.25 to 1 Torr. ► Sisingle bondHX bonding configuration also strongly controlled by voltage waveform shape.

Introduction

The industrial use of hydrogenated microcrystalline silicon (μc-Si:H) for photovoltaics requires high-quality thin films deposited at high deposition rates. For radio-frequency plasma enhanced chemical vapour deposition (RF-PECVD) techniques, this introduces the competing requirements of achieving a high injected RF power while maintaining a low mean ion-bombardment energy (IBE). To break the link between these two parameters, we have applied the voltage waveform tailoring (VWT) technique to produce an Electrical Asymmetry Effect (EAE) in the plasma reactor. The use of the EAE to control the IBE in a capacitively coupled plasma (CCP) has been previously explored by the Bochum group, first using two frequencies, the fundamental and its first harmonic, both using PIC simulation [1], as well as demonstrated experimentally [2]. The principle of using a tailored voltage waveform to control ion energy at the substrate has equally been demonstrated in remote plasma configurations for etching [3] and deposition [4] applications. Our group has recently demonstrated that producing the EAE in a CCP deposition system with SiF4 [5] or SiH4 [6] as the precursor gas changes the crystallinity of the deposited silicon thin films by decoupling the IBE from the injected power. In this work, we further examine both the changes in the Sisingle bondSi bonding character induced by a modified IBE controlled through the EAE using more in-depth characterization of the Raman scattering spectra for a broad range of deposition parameters, as well as changes in the Sisingle bondHX bonding configuration controlled by this same effect as measured through infrared absorption spectroscopy.

Section snippets

Experimental

The μc-Si:H thin films were deposited in the laboratory scale “PHILIX” reactor, which consists of a grounded confinement chamber surrounding two 10 cm diameter electrodes, separated by 2 cm, leading to an asymmetry in the electrode areas of Agrounded/Apowered = 2.25. This geometry leads to a DC self-bias voltage (VDC) and thus a decreased mean IBE on the grounded sections (including the substrate holder) and an increase on the powered electrode. The substrates (Corning Glass “Eagle”, 2.5 cm × 2.5 cm

Results

The current and voltage waveforms measured at the RF feedthrough are shown in Fig. 1. The real-time measurements are obtained using the calibrated derivative probe [8], and VDC is measured using the high-voltage probe. It is seen that for the four harmonics controlled by the feedback system, the desired waveforms are observed at the feedthrough to the powered electrode and that the “peaks” and “valleys” voltage signals are mirror images of each other. This generates a large change in the

Discussion

The detailed examination of the Raman scattering spectra presented in Fig. 3(a) and (b) shows that the difference in IBE caused by the use of TVWs primarily impacts the relative strength of the narrow peak at ~ 520 cm 1. Considering this peak as the “large-grain” peak (as-opposed to the “grain-boundary” peak at 500 cm 1, Ref. [9]) suggests that the lower IBE favours the growth of large grains rather than many small grains, although this conjecture should be verified through other means (XRD). In

Conclusion

We have demonstrated that producing an EAE using tailored voltage waveforms is an effective way to control the mean IBE during a μc-Si:H deposition process. Reducing the IBE increases the Raman crystalline volume fraction obtained using otherwise identical process conditions, as well as resulting in material with a lower microstructure factor. Notably, the Raman peak at ~ 520 cm 1 is disproportionately enhanced through the reduction of IBE using VWT, suggesting the growth of larger grains under

Acknowledgements

The work presented herein was funded by the CNRS (Project PIE STEP-UP), by the ANR (CANASTA Project no. ANR-10-HABISOL-002), as well as through a “Bourse Thèse Energie Renouvelable” doctoral bursary from Ecole Polytechnique.

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