Masaharu Shiratani

SURFACE REACTION PROBABILITIES AND KINETICS OF H, SIH3, SI2H5, CH3, AND C2H5 DURING DEPOSITION OF A-SI:H AND A-C:H FROM H2, SIH4, AND CH4 DISCHARGES

The relations between the surface reaction probability β of an atom or a radical in a reactive gas discharge, its diffusive flux to the wall, spatial density profile and temporal density decay during the postdischarge, are examined. Then, the values of β for H, SiH3, and Si2H5 on a growing a-Si:H film, and CH3 and C2H5 on an a-C:H film are derived from the temporal decay of radical densities during the discharge afterglow by using time-resolved threshold ionization mass spectrometry. For SiH3 on a-Si:H, β=0.28±0.03 in excellent agreement with previous determinations using other experimental approaches, and for Si2H5, 0.1<β<0.3. For H on a-Si:H, 0.4<β<1 and mostly consists of surface recombination as H2, while the etching probability of Si as SiH4 is only e≈0.03 at 350 K in good agreement with other studies of H reaction kinetics on crystalline silicon. At high dilution of SiH4 in H2 the sticking probabilities of Si hydride radicals are affected by the flux of H atoms of hydrogen ions which enhances surfac...

Effects of low‐frequency modulation on rf discharge chemical vapor deposition

Low‐frequency square‐wave modulation of a rf discharge in silane diluted with a rare gas brought about an improvement in the deposition rate of amorphous hydrogenated silicon films and in the film quality as well as a drastic suppression of powder concentration in the discharge space. These results can be explained by a SiH3 density in the modulated discharge that is high compared to that without modulation, because of the electron density enhancement resulting from the modulation and also because the lifetime of SiH3 radicals is much longer than those of SiHn radicals (n=0–2).

Simultaneous in situ measurements of properties of particulates in rf silane plasmas using a polarization-sensitive laser-light-scattering method

A polarization‐sensitive laser‐light‐scattering method is developed for simultaneous in situ measurements of properties (size, size dispersion, density, and refractive index) of particulates formed in processing plasmas. The developed system is applied to observe the growth processes of particulates in a range of their size larger than about 10 nm in rf silane plasmas. A size, a size dispersion (logarithm of a standard deviation of size), a density, and a refractive index of particulates in the plasmas are found to be 10–200 nm, about 0.1, 107–109 cm−3 and about 3–5i, respectively. The former three of such values agree fairly well with ones deduced from scanning electron microscopic (SEM) observation. These particulates grow through three phases of nucleation and initial growth, rapid growth, and growth saturation. Coexistence of two size groups of particulates with narrow size dispersions during and after the rapid growth phase verified by the SEM observation may be explained by a model taking into account coagulation between oppositely charged particulates.

Effects of Gas Temperature Gradient, Pulse Discharge Modulation, and Hydrogen Dilution on Particle Growth in Silane RF Discharges

The effects of gas temperature gradient, pulse discharge modulation, and hydrogen dilution on the growth of particles below about 10 nm in size in silane parallel-plate RF discharges are studied using a high-sensitivity photon-counting laser-light-scattering (PCLLS) method. Thermophoretic force due to the gas temperature gradient between the electrodes drives neutral particles above a few nm in size toward the cool RF electrode which is at room temperature. Pulse discharge modulation is much more effective in reducing the particle density when it is combined with the gas temperature gradient, and particles above a few nm in size cannot be detected by the PCLLS method even after 2 h. Hydrogen dilution of a high H2/SiH4 concentration ratio above about 5 is also useful in suppressing particle growth in the radical production region around the plasma/sheath boundary near the RF electrode.

Powder‐free plasma chemical vapor deposition of hydrogenated amorphous silicon with high rf power density using modulated rf discharge

Deposition of hydrogenated amorphous silicon films from SiH4/He gas mixtures was performed by using a square wave amplitude modulated rf discharge. The modulation was used for controlling radical densities in plasmas which led to a high rate deposition of good quality films. The fairly high deposition rate of 6 A/s was obtained for a low concentration of 5% SiH4 and a high rf peak power 200 W (0.8 W/cm3) without any appreciable amount of powder particles in the reaction chamber. The optical gap of the films was 1.8–1.95 eV. Emission intensities of HeI 388.9 nm and SiH 413.5 nm linearly increased with rf peak power and were well correlated with the deposition rate.

Highly Conducting and Very Thin ZnO:Al Films with ZnO Buffer Layer Fabricated by Solid Phase Crystallization from Amorphous Phase

We propose a novel method of oxide crystal growth via atomic-additive mediated amorphization. By utilizing this method, solid-phase crystallization (SPC) of ZnO from amorphous phase has been successfully demonstrated via nitrogen atom mediation. The resultant SPC-ZnO films are highly orientated and the crystallinity is higher than that of the films prepared by conventional sputtering. By using the SPC-ZnO as a buffer layer, the resistivity of ZnO:Al (AZO) films is drastically decreased. 20-nm-thick AZO films with a resistivity of 5×10-4 Ω cm and an optical transmittance higher than 80% in a wide wavelength range of 400–2500 nm have been obtained.

Particle Growth Kinetics in Silane RF Discharges

Growth kinetics of particles above 10 nm in size in silane RF discharges has been extensively studied and fairly well clarified. Moreover, recent developments of new measurement methods for particles below 10 nm in size have led to a rapid advance in understanding the growth processes of small particles. Such previous studies are reviewed with the accent on their initial growth phase corresponding to a particle size range below 10 nm. The notable effect of pulse modulation of the RF discharges of suppressing particle growth is also described. Both the growth of particles and their suppression by the modulation can be explained by a model taking into account the production of key radicals (highly reactive neutral radicals of SiH2 having a high production rate), particle growth reactions initiated by the key radicals and particle diffusion in the radical production region.

In situ observation of nucleation and subsequent growth of clusters in silane radio frequency discharges

Growth processes of clusters in low-pressure and low-power silane radio frequency discharges are studied by using the newly developed double-pulse-discharge method which realizes in situ measurement of their size and density in a size range of 0.5–4 nm. The clusters begin to be composed of two size groups at about 10 ms after the discharge initiation: clusters in the small size group have an almost constant average size of about 0.5 nm through the discharge period, while those in the large one grow at about 4 nm/s in a monodisperse way. Time evolution of the measured average sizes and densities in the groups is transformed into that of size distributions assuming that the density of SinHx clusters for the small group decreases exponentially with the increase in the number of Si atoms, n, of them, and the size distribution for the large group is the lognormal one. The results show that a critical cluster size for nucleation is SinHx (n∼4).

Growth processes of particles in high frequency silane plasmas

Growth processes of particles in high frequency silane plasmas are studied as a parameter of discharge frequency (3.5–28 MHz) or by modulating the amplitude of discharge voltage (125–275 V). Except for the 28 MHz case, particles tend to grow through three phases of nucleation and subsequent initial growth, rapid growth, and growth saturation. A detailed study for 6.5 MHz explains the following features: morphology of particles shows that coagulation of particles plays a crucial role in the rapid growth phase; a coagulation rate of 200 s−1 observed in the rapid growth phase is extremely high compared to a thermal collision rate of 5 s−1 between particles; coagulation almost stops when decreasing the discharge power by about one‐fourth at the middle of the rapid growth phase; two size groups of particles with narrow size dispersions coexist during and after the rapid growth phase. For 28 MHz, while, as compared to 6.5 MHz, particles appear early after the initiation of discharge and their density is high by about two orders, their growth rate in the subsequent phase is quite low. To properly explain most rapid growth features, a model, taking into account coagulation between oppositely charged particles, is proposed.

Recent progress in understanding the behavior of dust in fusion devices

It has been known for a long time that microscopic dust appears in plasmas in fusion devices. Recently it was shown that dust can be responsible for the termination of long- discharges. Also, in ITER-scale experiments dust can pose safety problems related to its chemical activity, tritium retention and radioactive content. In particular, the presence of dust in the vacuum chamber of ITER is one of the main concerns of the ITER licensing process. Here we review recent progress in the understanding of different experimental and theoretical aspects of the physics of dust dynamics and transport in fusion plasmas and discuss the remaining issues.