Study of the structure in rf glow discharges in SiH₄/H ₂ by spatiotemporal optical emission spectroscopy: Influence of negative ions

Radio frequency glow discharges in SiH₄(50%)/H₂ at 13.56 MHz and 100 kHz have been investigated by using time- and space-resolved optical emission spectroscopy. The relative net-excitation rate is obtained by deconvolving the spatiotemporal emission profile considering the influence of the radiative lifetime. From the results, we infer the behavior of electrons with energy greater than the threshold energy for the dissociative excitation and we discuss the discharge structure. Lines of SiI(4s ¹¹P ⁰→3p ²¹D), SiH(A²Δ;v'=0→X ²Π;v'=0), Hα, and H₂(d ³Π _u→a ³Σ⁺_g) are selected for observation. It is concluded that there exists a considerable population of negative ions compared with positive ions, which has not been previously proposed in high-frequency discharges in SiH₄ because of the small degree of electron attachment. At 13.56 MHz, the optical emission oscillates at 2ω in the bulk plasma region, in addition to the emissions at the plasma-sheath boundaries. It implies that the field in the bulk is strengthened by the external sinusoidal field to make up for the loss of electrons by electron attachment. The bulk field is estimated at 69 sin(ωt) V cm ^-1 from V-I characteristics. The existence of a double layer by the modulation of electrons is strongly suggested even in a high frequency at 13.56 MHz. At low frequency (100 kHz), a double layer is observed, which is formed by the large modulation of negative and positive ions. Consequently, excitation by electrons accelerated in the field at the double layer is observed in addition to excitation by secondary electrons from ion bombardment. Consideration of the role of negative ions in high-frequency SiH₄ discharges at 13.56 MHz is necessary to understand the discharge structure of the silane plasma and the deposition of the hydrogenated amorphous silicon.