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...
Regimes of particle trapping in inductively coupled plasma processing reactors
Contamination of wafers by particles in plasma processing reactors is a continuing problem affecting yields of microelectronic devices. In this letter, we report on a computational study of particle contamination of wafers in a high plasma density inductively coupled plasma (ICP) reactor. When operating with an unbiased substrate, particles readily contaminate the wafer due to high ion fluxes which produce large ion‐drag forces. Biasing the substrate with a radio frequency (rf) voltage counteracts the ion‐drag forces by increasing the opposing electrostatic forces in the sheath, thereby shielding the wafer from incoming particles. We have found three regimes of particle contamination for different ICP powers and rf biases. At high rf biases and low ICP powers, particles trap at the edge of the sheath. At low rf bias and high ICP power, ion drag forces dominate, particles do not trap, and wafer contamination is problematic. At intermediate powers and biases, particles quasitrap, leading to moderate particl...
Process-induced particle formation in the sputtering and reactive ion etching of silicon and silicon dioxide
Particle formation has been studied in Ar sputtering plasmas and CCl2F2-Ar reactive ion etching plasmas from Si and SiO2 substrates by laser light scattering, scanning and transmission electron microscopy and optical emission plasma diagnostics. Particles nucleate and grow continuously, and are swept out into the exhaust under a wide variety of plasma conditions. Within a more limited range of values for pressure and flow rate, particles grow large enough in the plasma so as to form a particle cloud suspended above the substrate. Stability and position of the cloud depend on the process conditions. When particles become visible by light scattering, at a diameter of about 200 nm, they are essentially spherical and monodisperse in size, but the size distribution becomes much wider as the average particle size increases with increasing discharge time. In sputtering, particles smaller than about 100 nm are quite porous, have a somewhat more irregular shape and exhibit a spherulitic (spherically columnar) mode of growth. Optical emission spectroscopy of the plasma and compositional analysis of the particles indicate that in all cases, Si atoms are responsible for particle nucleation and growth. These observations are discussed in terms of possible mechanisms for generation and transport of plasma species in the discharge, particle nucleation and growth, particle transport and particle cloud formation.