Paper PS-ThP17
Spatial Evolution of the Electron Energy Distribution Function in a Microwave Surface-Wave Plasma

Thursday, November 12, 2009, 6:00 pm, Room Hall 3

Microwave surface-wave discharges operating within a wide power and pressure window can be used to produce large-area plasmas of densities exceeding 10¹¹/cm³. Due to its inherent diffusion characteristics, away from the discharge source one can expect a relatively high density, quiescent, uniform, and low-temperature single Maxwellian plasma near the wafer region. That is, one would have a unique plasma tool with totally decoupled source of discharge and wafer process region. These merits are highly desired in large-area microelectronic technologies, such as in plasma-enhanced chemical-vapor deposition and etching processes. Because of these promising features, we are trying to understand the mechanisms of the microwave surface-wave plasma such as electron heating and power absorption in the discharge region and the spatial evolution of plasma parameters in the entire plasma volume. Understanding the evolution of plasma parameters in the entire plasma volume would help the development of tools based on microwave surface-wave plasmas and the design of process recipes. The plasma source used in this work consists of a radial line slot antenna (RLSA) which transmits 2.45 GHz microwaves into a large quartz resonator disk which then couples to the plasma. The plasma parameters of a nitrogen plasma, e.g., electron energy distribution functions (EEDF’s) are measured using a cylindrical Langmuir probe. EEDF measurements are carried out from 8 mm below the bottom surface of the resonator disk to the wafer surface, a span of ~140 mm in the vertical direction. A wide pressure-power range has been investigated, i.e., pressures from 20 to 800 mT and powers from 2 to 4 kW. Based on initial global-modal analysis of experimental observations, EEDF’s are analyzed using a curve fitting method developed in-house that assumes electrons in the microwave surface-wave plasma consist of two Maxwellian distributions and a drifting Maxwellian that models a beam component.¹ The relative population and magnitude of these electron components vary as a function of vertical location in the plasma volume. High populations of energetic electrons with energies exceeding 20 eV are typically observed near the resonator disk, i.e., the EEDF is dominated by the beam component. Away from the resonator disk, the EEDF transitions to two Maxwellians then thermalizes to a single cold Maxwellian of Te ~1 eV near the wafer surface. Pressure and power are found to have strong effects on the transition of the beam component to Maxwellian component. Particle-in-cell simulations¹ are conducted to understand the experimental observations.

¹R. V. Bravenec et al., presentation at this conference.