Exhaust Aerosol of a Plasma Enhanced CVD System: II. Electrical Charging and Transport

The microelectronics industry has been concerned about the loss of product yield in its semiconductor wafer processing steps from killer defects caused by the presence and deposition of contaminant particles. Reactant gases used in plasma enhanced chemical vapor deposition (PECVD) can form nanometer sized particles from homogeneous nucleation. Once the particles grow to a few nanometers, they become negatively charged due to the collected ion and electron currents on the particle surface. A gradient of the electron and ion concentration between the sheath and bulk plasma causes an electric field to develop directed to the walls. Contaminant particles can eventually become trapped in electrostatic potential wells due to the higher charge density of positive ions near the powered electrode. The traps fill in the plasma sheath region until some particles are "leaked out" by gas drag forces. Therefore particles formed solely in the plasma volume are theorized to possess a distinct charge level from condensation particles originating within the exhaust line alone. Consequently, plasma properties determine the size and charge distribution of contaminants that exit the reactor and enter the exhaust line. The intended contribution of this research was to (1) develop the capability to monitor charged and uncharged contaminant particle fractions during a thin film deposition cycle and (2) to understand the coupled effect of particle transport and charging in an radio frequency (RF) plasma. Therefore the size distribution and absolute electrical charge were measured in the vacuum exhaust line during a SiO 2 thin film cycle using an integral mobility charge analyzer. Experiments confirm that over 50% of fine particles in the exhaust were neutral. However, a moderately charged coarse fraction was measured between 0.15 and 0.3 w m. The larger particles are hypothesized to be reactor particles gradually released from the potential wells in the plasma sheath. Process variables, including the RF power and the reactor pressure, directly affected the size and charge characteristics measured on particles in the exhaust line. The first paper in the series "Exhaust Aerosol of a Plasma Enhanced CVD System" uses an in-line exhaust sampling system to measure the size distribution of contaminant particles exiting the plasma.