Direct calorimetric measurements in a PBII and deposition (PBII&D) experiment with a HiPIMS plasma source
Introduction
The utilization of HiPIMS as a source in plasma based ion implantation and deposition (PBII&D) effectively combines the advantages of magnetron sputtering and ion implantation [1]. By utilizing very low duty cycles, HiPIMS, as an enhancement of the common DC magnetron sputtering, allows to use very high peak powers ranging up to several kW/cm2 [2] while keeping the thermal load on the sputtering target and the substrate low. In this way, during the pulse, very high plasma densities (∼ 1019/m3) and a high degree of ionization (10%–80 %) of the sputtered particles are achieved [3]. The resulting flux of energetic ions towards the substrate can be effectively exploited by application of high voltage pulses to the substrate such as used in plasma based ion implantation (PBII).
Due to the complex temporal evolution of the particle density during the HiPIMS period [4], critical deposition parameters such as the energy of the depositing particles (PBII voltage), the ratio between highly energetic and less energetic particles (delay), or even the ratio between the different contributions in reactive sputtering (delay and PBII voltage) can be adjusted in a straight forward manner [5]. In 2010, first studies on such a combined system were presented by Wu et al. who demonstrated its efficacy by deposition of CrN with excellent adhesion properties [6]. In the following years the method has been further refined and applied in the deposition of various material systems [[7], [8], [9], [10], [11]].
As in most deposition systems, the energy influx towards the substrate during plasma operation plays a critical role as it is directly affecting the surface temperature of the substrate and the film properties [12,13]. In [14], Anders presents a refined structural zone model based on previous models by Thornton [12] and Movchan [15], which points out the substrate temperature and the energy flux from impinging particles as an important parameter for tuning the film's microstructure. Recently, we emphasized the difficulties of energy flux measurements on high voltage pulsed substrates and introduced an indirect method which allows to obtain qualitative results without major modifications to the setup of the classical passive thermal probe [16]. However, this method also showed some drawbacks like the complexity associated with the use of a grid in the plasma and the lack of absolute values. To eliminate these disadvantages we modified the thermal probe to allow direct application of high voltages onto the dummy substrate of the probe. With this setup, the grid can be removed and absolute values for the energy flux in a PBII&D process can be obtained, allowing for direct comparison with the measured electrical PBII power.
Section snippets
Experimental setup and plasma diagnostics
The probe was placed in a rectangular vacuum vessel with a volume of approximately 300 l which was equipped with a 4 in. copper magnetron. The magnetron had a balanced magnetic field configuration and was powered by a Melec HiPIMS power supply with rectangular voltage pulses. To realize the calorimetric measurements by passive thermal probe (PTP), the magnetron was equipped with a pneumatic shutter mounted directly in front of the magnetron. The substrate dummy of the PTP was pulsed with a RUP6
Variation of PBII voltage
The PBII voltage is a crucial parameter in the combined PBII&D system. By altering the voltage it is possible to switch the process between a deposition dominated system to a system with equal deposition and implantation to a system dominated by etching and implantation processes [14]. As the differences between these modes are characterized by substantially different energy fluxes, an investigation of this parameter is of particular interest. The combined diagnostics of energy flux and
Conclusion
Using a modified passive thermal probe, time resolved information about the energy flux to the substrate in a PBII&D system were obtained for different PBII and HiPIMS parameters. The combination of VI- and energy flux measurements allowed us to obtain information about the secondary electron yield of our substrate which could be reasonably matched with comparable values from literature. All results showed a peak-like shape for the delay dependence which reflects the evolution of the ion
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