Plasma characteristics in pulsed direct current reactive magnetron sputtering of aluminum nitride thin films

Abstract

Plasma diagnostics is important to identify plasma parameters and generate reproducible plasma in magnetron sputtering. Langmuir probes have been used to measure local plasma parameters such as electron temperature (Te), charge densities (ne and ni), and plasma potential (Vp). Pulsed direct current (dc) power in the midfrequency range (50–250 kHz) has been used in growing insulating films without charging accumulations at target. Recent investigations showed increased energetic particle bombardment of the substrate in pulsed power. In this work, aluminum nitride thin films were fabricated by pulsed dc power sputter deposition. The argon and nitrogen plasma was characterized by Langmuir probe measurement. The electron temperature in argon and nitrogen plasma was observed to increased from 3.06 to 5.32 eV when the pulsed dc frequency increased from 75 to 250 kHz. The ion density and energy flux were found to increase with frequency. This is believed to be from the stochastic heating generated by the fast oscillation in the target voltage wave form. The measured plasma characteristics were correlated with the crystal orientation of AlN thin films. The crystal structure of AlN thin films changed to (002) preferred orientations as the ion and energy flux increased.

Effects of pulse frequency on the microstructure, composition and optical properties of pulsed dc reactively sputtered vanadium oxide thin films

“…Unlike conventional DC sputtering, many recent works on plasma diagnostics of pulsed DC magnetron sputtering has revealed that pulsing the magnetrons may modify many intrinsic plasma parameters [20][21][22] . Especially, with the increase of pulse frequency, the electron stochastic heating generated by fast switch of the target voltage between positive and negative will leads to a significant elevation in the electron temperature, electron density, ion energy and ion flux etc [24] .…”

“…Unlike conventional DC sputtering, many recent works on plasma diagnostics of pulsed DC magnetron sputtering has revealed that pulsing the magnetrons may modify many intrinsic plasma parameters [20][21][22] . Especially, with the increase of pulse frequency, the electron stochastic heating generated by fast switch of the target voltage between positive and negative will leads to a significant elevation in the electron temperature, electron density, ion energy and ion flux etc [24] . And on the other hand, higher pulse frequency means shorter pulse cycle, the transition period between the negative period and positive period will become a larger part in the pulse cycle, the power density in each pulse cycle and the high voltage overshoots at the beginning of the positive voltage period will be increased evidently.…”

“…Since it is has been demonstrated that as the pulse frequency increased, the "dead time" which represent a period where sputtering of target is negligible will become a greater proportion of the total on-time [31] and meanwhile the electron temperature and density will also be elevated which makes the argon-oxygen plasma become hotter and more chemical activity [20,21] . Therefore, we think the main reasons for the compositions of these VOx films strongly dependent on the pulse frequency, maybe attribute to two aspects: First, due to the effective sputtering time decrease with the increase of pulse frequency that will lead to the vanadium on the target surface has more time to react with the oxygen in the plasma before it is sputtered.…”

“…J.W. Lee et al through increase the pulse frequency during pulsed reactive magnetron sputtering the aluminum nitride changed the crystal structure of the films [24] . J.Y.…”

“…The role of such pulsing parameters in changing the properties of the plasma and film has become a subject of growing number of studies. It has been demonstrated that the pulse frequency is not only the critical parameter to establish an arc-free condition in the Ar-O 2 reactive sputtering process [18,19] , but also an important means to change the plasma properties [20,21] and thermal power density delivered to the substrate [22] . Y.T.…”