Optical studies of micron-sized particles immersed in a plasma

Abstract

In the last decade, the formation of dust particles in processing plasmas has received a lot of attention. In those environments, dust formation often is unwanted: it can e.g. reduce production yield or deteriorate film quality. The discovery of the Coulomb crystal, which is an analogy for a solid state crystal, shows that dust formation also has its benefits. Also the perspective to the production of particles with unique and desired qualities for e.g. ceramics, catalysis, and optoelectronics is attractive. In order to gain information about the dust particles in these three fields, it is necessary to use suitable diagnostics, which are capable of monitoring small changes inflicted on the particles. In this thesis optical studies of micrometer-sized dust particles immersed in radiofrequency plasmas are described. The dust particles are injected into the plasma, where they acquire a net negative charge. This charge prevents the particles to escape from the plasma. When the right conditions are chosen, it is possible to confine the particles to a stable position for a long time. In this situation, optical techniques can be applied to measure the influence of the radio-frequency (RF) plasma on the particles. Two of these techniques, rotating compensator ellipsometry (RCE) and Mueller matrix ellipsometry (MME) are based on the same method: the ellipsometric analysis of the change of the polarization state of the scattered light induced by the dust particles. The main difference between the two methods is the number of rotating components; the RCE has one rotating component and analyses a part of the matrix describing the polarization behavior of the dust particles. The MME has two rotating components and measures the full matrix. Both diagnostics measure the scattered light at an angle of 90o in the horizontal plane with respect to the undeflected beam. With the two techniquess the etching of the polymer particles in oxygen RF plasmas has been monitored. In both situations the size of the particles decreases as a result of the plasma operation. From the RCE measurements it can be inferred that the width of the particle size distribution increases. The MME measurements are performed on single particles and point out that the particles remain spherical during their treatment. A third technique is used to measure the angle resolved scattering profile of single particles in the forward direction. One single particle is injected in the plasma and illuminated with a vertically polarized laser beam. The scattering, which is described by the well-known Mie theory, is recorded in the forward direction in the horizontal scattering plane. This method allows for a very accurate determination of particle size and refractive index. It takes time to record the angular profile, and as a consequence the plasma can cause the particle size to change during the measurement. This is beneficial, as it now becomes possible to determine, in addition to the particle size and the refractive index, the etch rate of the particle from one single angular resolved profile. In this thesis also a fourth optical technique is applied to the particles. In contrast to the other three diagnostics, now the particle has been used to study the plasma. In the plasma a cloud of dyed dust particles is injected. The dye, Rhodamine B, has been shown to emit a temperature-dependent fluorescent emission spectrum. In the plasma, the particles are illuminated with an argon ion laser and subsequently the resulting fluorescent emission is recorded. The fluorescent emission depends on the plasma parameters. The particle temperature increases with increasing RF power. These measurements are combined with high-resolution atomic absorption spectroscopy measurements that give information on the gas temperature. Furthermore, Langmuir probe measurements have been performed which yield the electron density and temperature, the ion density, the floating potential, and the plasma potential. All these data have been used to determine the particle thermal balance, i.e. the fluxes arriving at and leaving from the particle. A good agreement has been found between experiment and theory: the dust particles can be used as microprobes.