In Situ Observation of Growing Process of Particles in Silane Plasmas and Their Effects on Amorphous Silicon Deposition
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IN SITU OBSERVATION OF GROWING PROCESS OF PARTICLES IN SILANE PLASMAS AND THEIR EFFECTS ON AMORPHOUS SILICON DEPOSITION MASAHARU SHIRATANI AND YUKIO WATANABE Kyushu University. Department of Electrical Engineering, Hakozaki, Fukuoka 812, Japan ABSTRACT In rf silane plasmas, growing process and behavior of dust particles and presence of negative ions are studied by square-wave-amplitude-modulating an rf discharge voltage. By combining the rf modulation method with Mie scattering methods, particle behavior together with temporal evolutions of particle size and density is revealed. While the rf modulation is effective for the suppression of particle growth, small particles are found to exist still in the discharge space for a larger duty cycle. Further, particles seem to be suspended around an rf electrode by counter balance between an electrostatic force due to time-averaged sheath electric field and a force increasing with particle size. The development of particle growth also affects the rf discharge itself, resulting in inhomogeneity of film properties. Negative ion detection is carried out as a first step for studying whether they cause the particle formation. It is found that negative ions are formed in rf silane discharges even for a low concentration of 0.5%SIH4 and their formation is due to attachment to dissociated radicals rather than dissociative attachment of SiHl4. Introduction It has been pointed out that particles formed in silane (Sill4) plasmas deteriorate quality of hydrogenated amorphous silicon (a-Si:H) films, leading to declining a yield of a-Si devices such as thin film transistors. Authors have proposed a reaction-control method by modulating the rf voltage with a square-wave, the frequency of which is selected by taking into account lifetimes of SiHn (n=0--3) radicals. [1-5] This rf modulation method has been found to be effective for suppressing particle growth and also for improving film quality and deposition rate. [1] Recently, much attention has been paid to this method. [6-14] The method is also useful for observing the 'growing process and behavior of particles and further for getting information on which radical and/or ionic species contribute to the particle growth. Some features of particle behavior have been revealed by using the method together with a Mie scattering method: for square-wave-amplitude-modulated rf discharges, the growth of particles is drastically suppressed over the whole discharge space, while for cw rf discharges, particles are mainly observed around plasma-sheath interfaces; their shapes are almost spherical and their sizes below about 400 nm; particles are driven out of a discharge space by a gas flow in rf-power-off periods; they are negatively charged and electrostatically trapped in the discharging space; a growth time of particles is much longer than their extinction time which is closely related with the gas flow. The last three features have been found to be one of mechanisms bringing about the suppression of particle growth for modulated rf discharges.
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