Stefan Bienholz

On the electrical asymmetry effect in large area multiple frequency capacitively coupled plasmas

Recently, many publications have dealt with the electrical asymmetry effect in capacitively coupled radio frequency-discharges. The idea of this concept is the possibility of controlling the self-bias voltage by tuning the relative phase of harmonics in relation to the fundamental wave. In this work, we apply the electrical asymmetry effect on a large-area multiple frequency capacitively coupled plasma used for reactive sputtering by varying the relative phase of the 13.56 and 27.12 MHz excitation. The resulting voltage waveforms at the electrode are recorded using a high-voltage probe. The shape of the waveform is then analysed by Fourier analysis to study the influence of higher harmonics excited at the non-linearity of the plasma boundary sheath. To investigate the influence of the relative phase on the plasma itself, radially resolved multipole resonance probe measurements are performed.

Characterization of low-pressure microwave and radio frequency discharges in oxygen applying optical emission spectroscopy and multipole resonance probe

Optical emission spectroscopy (OES) and multipole resonance probe (MRP) are adopted to characterize low-pressure microwave (MW) and radio frequency (RF) discharges in oxygen. In this context, both discharges are usually applied for the deposition of permeation barrier SiOx films on plastic foils or the inner surface of plastic bottles. For technological reasons the MW excitation is modulated and a continuous wave (cw) RF bias is used. The RF voltage produces a stationary low-density plasma, whereas the high-density MW discharge is pulsed. For the optimization of deposition process and the quality of the deposited barrier films, plasma conditions are characterized using OES and MRP. To simplify the comparison of applied diagnostics, both MW and RF discharges are studied separately in cw mode. The OES and MRP diagnostic methods complement each other and provide reliable information about electron density and electron temperature. In the MW case, electron density amounts to ne = (1.25 ± 0.26) × 1017 m−3, and kTe to 1.93 ± 0.20 eV, in the RF case ne = (6.8 ± 1.8)×1015 m−3 and kTe = 2.6 ± 0.35 eV. The corresponding gas temperatures are 760±40 K and 440±20 K.

Multiple frequency capacitively coupled plasmas as a new technology for sputter processes

A novel large area multiple frequency coupled plasma is introduced for sputter deposition purposes. The discharge is driven by three different excitation frequencies (13.56, 27.12 and 60 MHz) simultaneously for advanced control of Ar ion flux and energy at the target by applying the electrical asymmetry effect during sputter processes.Optical emission spectroscopy is performed to characterize the sputter plasma with respect to plasma parameters as well as the Al transport through the plasma. The spectroscopic data are compared with TRIDYN calculation in combination with a simulation of the transport of atoms through the plasma volume.

Study on electrostatic and electromagnetic probes operated in ceramic and metallic depositing plasmas

This paper discusses plasma probe diagnostics, namely the multipole resonance probe (MRP) and Langmuir probe (LP), operated in depositing plasmas. The aim of this work is to show that the combination of both probes provides stable and robust measurements and clear determination of plasma parameters for metallic and ceramic coating processes. The probes use different approaches to determine plasma parameters, e.g. electron density ne and electron temperature Te. The LP is a well-established plasma diagnostic, and its applicability in technological plasmas is well documented. The LP is a dc probe that performs a voltage sweep and analyses the measured current, which makes it insensitive against conductive metallic coating. However, once the LP is dielectrically coated with a ceramic film, its functionality is constricted. In contrast, the MRP was recently presented as a monitoring tool, which is insensitive to coating with dielectric ceramics. It is a new plasma diagnostic based on the concept of active plasma resonance spectroscopy, which uses the universal characteristic of all plasmas to resonate on or near the electron plasma frequency. The MRP emits a frequency sweep and the absorption of the signal, the |S11| parameter, is analysed. Since the MRP concept is based on electromagnetic waves, which are able to transmit dielectrics, it is insensitive to dielectric coatings. But once the MRP is metallized with a thin conductive film, no undisturbed RF-signal can be emitted into the plasma, which leads to falsified plasma parameter.In order to compare both systems, during metallic or dielectric coating, the probes are operated in a magnetron CCP, which is equipped with a titanium target. We present measurements in metallic and dielectric coating processes with both probes and elaborate advantages and problems of each probe operated in each coating environment.

Dual frequency capacitive plasmas in Fe and Ni sputter applications: correlation of discharge properties on thin film properties

Dual frequency capacitively coupled plasmas (CCPs) are widely used in (large area) etching and plasma enhanced chemical vapor deposition processes. However, applications in physical vapor deposition (PVD) are still sparse due to the well-established dc magnetron cathode discharges. Nevertheless, there exist critical applications such as ferromagnetic or ceramic thin film deposition which are difficult to handle even for dc magnetron systems. For these materials systems dual frequency CCPs pose a good alternative, because for insulators charging can be avoided and for ferromagnetic materials the target thickness becomes independent of the magnetron configuration at comparable deposition rates.In this work we investigate two separate subjects. First, in dual frequency capacitive discharges a complex coupling of the applied excitation frequencies can be observed, which from a plasma parameter point of view limits the separability of ion flux (usually controlled by frequencies >60?MHz) and ion bombarding energy (usually controlled by frequency <15?MHz) onto the sputter target. By performing deposition experiments it was found that by following simple tuning guidelines a very good degree of separability is achievable. Additionally, the deposition homogeneity is not affected.Second, we correlate the growth conditions with crystalline and magnetic properties as well as the degree of O content for Fe and Ni films. Therefore, we applied different signals as a substrate bias to influence thin film growth. It was found that the crystalline and magnetic properties can be influenced for both Fe and Ni films but is more pronounced for Ni.

On the physics of a large CCP discharge

Demands of the plasma processing industry gradually lead to an increase in electrode areas and driving frequency of the commonly used capacitively coupled reactors. This brings about new phenomena which differ from the well known physics of smaller capacitively coupled plasma (CCP) devices. In this work we compare experimental data and results of numerical modeling for a large CCP discharge having a GEC cell-like geometry currently studied in context of a possible use as a sputtering device. Using an electrostatic implicit particle-in-cell code with Monte-Carlo collisions (PIC/MCC), we have been capable of reproducing all main features of the experimental discharges, which have strong relevance for the processing applications, such as the plasma uniformity and the self-bias. The side chamber proves to play an essential role in defining the physics of the whole device, featuring substantial production of plasma particles and participating in establishing the self-bias due to the telegraph effect observed for higher frequencies.

Kinetic simulations of a large-sized multifrequency CCP-based sputtering source with a PIC/MCC darwin code

Summary form only given. A novel concept of a sputtering source based on a CCP multifrequency large-sized discharge is currently under experimental investigation [1]. The physics of such a discharge is quite complex and includes phenomena taking place on several time and spatial scales. In particular, because of the size and the high frequency harmonics in the driving voltage of such a discharge, the electromagnetic effects may play a significant role. Moreover, use of the electrical asymmetry effect (EAE) to create a self-consistent bias complicates the problem even more. In the present work we report results of our studying such a discharge with a recently developed self-consistent kinetic 2d3c PIC/MCC GPU-parallelized code which uses Darwin approximation [2] for description of the electromagnetic field components. The simulations are made in a geometry close to that of the sputtering source used in the experiments. We discuss interesting features of the discharges arising in the main and the side chambers and compare the simulation results and the experimental data.

Simulations of a CCP-based sputtering source with a PIC/MCC Darwin code

Summary form only given. A novel concept of a sputtering source based on a CCP multi-frequency discharge is studied using a self-consistent kinetic PIC/MCC Darwin code. The cylindrically symmetric 2d3v code allows to follow all the kinetic and all the important electro-magnetic effects taking place in such plasmas. To speedup the calculations, the code is parallelized on a GPU. Our simulations show that the electromagnetic effects can be significantly different from the predictions of simplified analytical models (see, e.g., [1]). To validate the code, we compare the major plasma parameters obtained from numerical simulations of a set of different cases with those provided by the experiment [2]. We give special attention to the plasma properties which are particularly relevant to the use of the discharge as a sputtering source.