Detection of SiH 3 radicals and cluster formation in a highly H 2 diluted SiH 4 VHF plasma by means of time resolved cav
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0910-A08-07
Detection of SiH3 radicals and cluster formation in a highly H2 diluted SiH4 VHF plasma by means of time resolved cavity ring down spectroscopy Takehiko Nagai, Arno H. M. Smets, and Michio Kondo Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Umezono 1-1-1, Tsukuba, Ibaraki, 305-8567, Japan
ABSTRACT The spatial distribution of the SiH3 radicals between the electrodes of a hydrogen diluted silane VHF plasma under thin film hydrogenated microcrystalline silicon (µc-Si:H) growth conditions has been measured using the time resolved cavity ringdown (τ-CRD) absorption spectroscopy technique. The µc-Si:H growth rate is estimated from the measured spatial SiH3 profiles using a simple model based upon diffusion controlled flux of SiH3 radicals to the electrode surface, where the SiH3 can react with the film surface. The calculated value of µc-Si:H growth rate roughly agrees with the value of the experimentally determined growth rate. This agreement implies that the SiH3 radical is the main growth contributor to the µc-Si:H growth. Furthermore, the τ-CRD reveals the growth kinetics of the clusters in the plasma by light scattering at these clusters on time scales of 1 s after the plasma ignition. INTRODUCTION In the last decade, several plasma diagnostics have been applied to silane (SiH4)/hydrogen (H2) plasmas to study the radicals created and transported to the growth surface [1-8]. These studies show that the SiH3 radical is by far the dominant contributor to the a-Si:H growth, due to its higher density and longer lifetime in the plasma compared to SiHx (x 1 s the cavity loss rapidly increases. The origin of this large additional cavity loss is light scattering at growing clusters in the plasma. Note that typical timescales in which this additional cavity loss shows up is in line with that of cluster formation [10, 11]. Since the clusters are growing in size with time from some nanometers to several tens of nanometers, the light scattering mechanism should be Rayleigh-like in the initial cluster evolution period. The sphere integrated cross section for Rayleigh scattering σs is given by σs = 128/3π5{(ε - 1)/( ε + 2)}2rc6/λ4 with rc
Cavity Loss (10
–3
/pass )
15 Plasma end
10
clusters dusts Plasma start
5 SiH3
～t ～t
0.66 on
5.65
on
0
0
2 4 Delay Time (s)
6
Figure 1. Cavity loss as a function of delay time determined from the τ-CRD transients. The plasma-on and -off period was 4 and 6 s, respectively. the radius of the cluster and ε the dielectric constant of the cluster material. Kujundzic et al. [11] have shown that in hydrogen diluted silane plasma the cluster radius grows linearly with plasmaon-time ton. If we assume that the growth kinetics of the clusters in our VHF plasma is the same, the averaged scatter cross-section will grow with time σs ∼ton6. Furthermore, if the cluster density for (ton>1 s) is only weakly dependent on ton, similar to Ref.[11], it will scale like Nc~ton-χ. Consequently, the cavity loss due to light scatter
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