Using UHV Nude Bayard-Alpert Ionization Gauge as a Langmuir Probe to Characterize Plasma Properties Generated in a Molecular Beam Epitaxy System

In molecular beam epitaxy (MBE), wide range of functional oxide thin film materials are important to quantum phenomena studies in high-electron-mobility semiconductor (SrTiO3) and to applications relevant to field of waveguiding devices as electro-optic modulator and surface acoustic waves (SAW) transducers (products made of LiNbO3) [1-2]. It is crucial to synthesize these thin films with lowest number of defects, caused by energetic ionic particle impingements from surrounding plasma, during their fabrication process to achieve optimal performances; therefore, a Langmuir probe, modified from a Bayard-Alpert ionization gauge, should be designed to characterize the oxygen radio frequency (RF) plasma properties inside an MBE growth system. The ionization gauge is converted to a Langmuir probe by inserting the collector and a grounded filament of the gauge into the MBE growth chamber enriched with oxygen RF plasma and disconnecting the gauge from its vacuum gauge controller. The probe is connected in series with a Keithley 6485 picoammeter and a Keithley 2400 voltage source meter. The probe current-voltage (I-V) characteristics are measured under constant oxygen plasma power and constant gas pressure conditions to estimate plasma’s ion saturation and floating potentials. The first derivatives of the probe I-V measurements under these conditions are investigated to determine kinetic energy of the oxygen RF plasma. For comparison, the Langmuir probe I-V measurement is also carried out in an argon RF plasma under constant gas pressure condition.

In conclusion, the ion saturation current from the I-V measurement in oxygen plasma increases in magnitude with decreasing pressure (7~4 Torr) due to elevated charge carrier mobility, but with increasing plasma power (150~400 W) because of increased number of charge carriers. The kinetic energy of the oxygen plasma, analyzed from dI/dV,increases with decreasing pressure due to reduced particle collisions, but with increasing plasma power because of elevated electric potential energy arisen from massive charge carrier accumulation on biased probe tips. In argon plasma, kinetic energy and ion saturation current is only investigated under constant pressure condition, and they follow the similar variation trend observed in the oxygen plasma analysis.