Shapes of agglomerates in plasma etching reactors
Dust particle contamination of wafers in reactive ion etching (RIE) plasma tools is a continuing concern in the microelectronics industry. It is common to find that particles collected on surfaces or downstream of the etch chamber are agglomerates of smaller monodisperse spherical particles. The shapes of the agglomerates vary from compact, high fractal dimension structures to filamentary, low fractal dimension structures. These shapes are important with respect to the transport of particles in RIE tools under the influence electrostatic and ion drag forces, and the possible generation of polarization forces. A molecular dynamics simulation has been developed to investigate the shapes of agglomerates in plasma etching reactors. We find that filamentary, low fractal dimension structures are generally produced by smaller (<100s nm) particles in low powered plasmas where the kinetic energy of primary particles is insufficient to overcome the larger Coulomb repulsion of a compact agglomerate. This is analogou...
Plasma transport around dust agglomerates having complex shapes
Dust particles generated in low temperature plasmas as used for microelectronics fabrication are often agglomerates of smaller monodisperse particles. The transport of these agglomerates, and the subsequent contamination of surfaces, depends on the details of ion-momentum transfer (ion drag) to, and electrostatic forces on, the agglomerate. Given that the charge distribution on the surface of the agglomerate and local electric fields in the vicinity of the agglomerate depend on its shape, the subsequent forces on the agglomerate will also be a function of shape. In this article, we describe results from a simulation in which plasma transport around, and the charging of, agglomerates are investigated. We find that the charge distribution on the agglomerates is generally nonuniform, a consequence of both shadowing and charge depletion. The ion-momentum transfer cross section, calculated using a Monte Carlo simulation, also depends on the shape of the agglomerate.
Electrostatic trapping of contamination particles in a process plasma environment
A number of authors have observed, by using light scattering from a laser beam, contamination particles suspended in an rf process plasma. The region of space occupied by the particles appears finite, e.g., a ring; there is experimental evidence that the particles are negatively charged. We show, by using a tuned Langmuir probe, that the trap is electrostatic in nature. The volume of a trap is as much as 5 V larger in electrostatic potential than the surrounding plasma. This means that the volume of the trap is positively charged with the electric field being directed outward from the trap. Thus, negatively charged particles will flow into it. The electrostatic potential rises so rapidly at a trap boundary that a double layer may exist there. Finally, the plasma‐sheath interface is found to follow the topographic contour of the rf electrode surface.