Behavior of Dust Particles in Plasma Etching Apparatus

Abstract

The behavior of dust particles in a plasma etching apparatus was investigated with an in-situ particle monitor. The properties of the particles are classified into three types according to the particle velocity. Slow-velocity particles (less than 1 m/s) are trapped by a plasma-sheath boundary and rarely fall on the wafer during plasma discharge. These particles should be removed from the region above the wafer before turning off the plasma. Increasing gas flow rate and changing plasma density distribution are effective to control the particle transport. Medium-velocity particles (a few meters per second) travel above the wafer surface due to a balance between ion drag and electrostatic forces during plasma discharge. The number of particles that attach to the wafer can be reduced by supplying wafer bias power for the purpose of increasing the electrostatic force. Fast-velocity particles (a few dozen meters per second or more) are generated by the reflection of particles in a turbo molecular pump (TMP), and these may damage fine patterns on the wafer by colliding with it. The fine pattern damage can be reduced by increasing the distance between the wafer and the TMP. The number of particles fall on the wafer is decreased by supplying down-flow gas. It is therefore important to decelerate fast-velocity particles by using gas viscous force.