Pulsed DC reactive magnetron sputtering of aluminum nitride thin films

Abstract

Aluminum nitride thin films have been deposited by pulsed DC reactive magnetron sputtering. The pulsed DC power provides arc-free deposition of insulating films. Two types of pulsed DC (unipolar and asymmetric bipolar) were studied with respect to characteristics and properties of resultant films. The unipolar power supply generates a series of 75 kHz DC pulses modulated with 2.5 kHz frequency. The frequency of asymmetric power supply can be varied from 50 kHz to 250 kHz. The duty cycle, which is a ratio of negative pulse time to total time, can be varied from 60% to 98%. Very fast oscillation and overshoot were observed when the polarity of the target voltage was changed.

The control of crystal orientation of deposited film is important since the properties of AlN film is related with the orientation. For example, the acoustic velocity is high along the c-axis. The electromechanical coupling coefficient is large in a-axis direction. The crystal orientation and microstructure of the AlN films were strongly affected by the deposition conditions such as sputtering power, growth temperature, sputtering gas pressure and frequency/duty cycle. The crystal orientation of AlN films was closely related with the energy of sputtered atoms and mobility of adatoms on substrate. The c-axis oriented films were obtained when the target power and growth temperature were high. This provided higher energy of sputtered atoms and mobility of adatoms. The deposited AlN films have a columnar structure.

The crystal orientation of the AlN films was changed from (101) to (002) by applying an RF bias was applied to the substrate in unipolar pulsed DC sputtering. The columnar structure disappeared when the RF bias was applied to the substrate. Applying bias was thought to increase mobility of adatoms by ion bombardment.

MIM (aluminum-AlN-aluminum or molybdenum) structure was fabricated to measure electric properties of AlN films. Dielectric constants of 8.5 to 11.5 were obtained at 100 kHz. The relation of the current density and electric field of the AlN film followed ohmic conduction. The broad photoluminescence spectrum in the range of 300 nm and 650 nm was observed for the aluminum nitride thin films regardless of the deposition conditions. This spectrum might be due to the defects related with oxygen. The thermal conductivity of AlN films was evaluated by the 3omega method. The values in the range of 12 and 30 W/mK was obtained. Those values are very low compared to those of bulk. The reason might be attributed to the oxygen incorporation as well as the unique microstructure of the sputtered films.

The plasma parameters such as electron temperature and charge density were obtained by the Lagmuir probe. The electron temperature in argon and nitrogen plasma increased from 2.7 to 5.3 eV when the frequency increased from 75 kHz to 250 kHz in asymmetric bipolar pulsed DC sputtering. The electron and ion densities were found to increase with frequency. The stochastic heating due to the fast oscillation in the target voltage waveform may be attributed to the increased electron temperature and electron/ion densities. The measured plasma characteristics were correlated with properties of the AlN film. The ion and energy flux were believed to increased as the frequency increased. The intensity of the (002) peaks was found to increase with the increase in the ion flux and energy flux.