The sheath of a dual-frequency capacitively coupled rf discharge has been investigated with microparticles as probes based on two different methods: by the first method, intensity profiles of the emitted light of the discharge and the scattered light of the microparticles are used to study the effects of phase variation between the fundamental frequency (13.56 MHz) and its first harmonic (27.12 MHz) or a change in the dc self-bias, respectively, on the expansion of the sheath and the levitation height of microparticles in the sheath. These measurements show that the sheath width as well as the levitation height of the microparticles correlates with the dc self-bias as a function of the phase between the applied voltage harmonics. By a second method, profiles of the force acting on an optically trapped microparticle in an optical tweezer in the sheath were measured. On the basis of these force profiles, the strength of the electric field force as a function of pressure and the dependency on the distance of the probe to the rf electrode as well as the extent of the sheath are determined in both a single- and a dual-frequency discharge. From the results, it can be deduced that the sheath thickness and the electric field force in a dual-frequency discharge are influenced by both excitation frequencies and, thus, differ from a single-frequency discharge.

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