Thomas Welzel

The impact of negative oxygen ion bombardment on electronic and structural properties of magnetron sputtered ZnO:Al films

In order to study the impact of negative oxygen ion bombardment on the electronic transport properties of ZnO:Al films, a systematic magnetron sputtering study from ceramic targets with excitation frequencies from DC to 27 MHz, accompanied by strongly varying discharge voltages, has been performed. Higher plasma excitation frequencies significantly improve the transport properties of ZnO:Al films. The effect of the bombardment of the films by energetic particles (negative oxygen ions) can be explained by the dynamic equilibrium between the formation of acceptor-like oxygen interstitials compensating the extrinsic donors and the self-annealing of the interstitial defects at higher deposition temperatures.

Negative oxygen ion formation in reactive magnetron sputtering processes for transparent conductive oxides

Reactive d.c. magnetron sputtering in Ar/O2 gas mixtures has been investigated with energy-resolved mass spectrometry. Different metal targets (Mg, Ti, Zn, In, InSn, and Sn), which are of importance for transparent conductive oxide thin film deposition, have been used to study the formation of negative ions, mainly high-energetic O−, which are supposed to induce radiation damage in thin films. Besides their energy distribution, the ions have been particularly investigated with respect to their intensity in comparison of the different target materials. To realize the comparability, various calibration factors had to be introduced. After their application, major differences in the negative ion production have been observed for the target materials. The intensity, especially of O−, differs by about two orders of magnitude. It is shown that this difference results almost exclusively from ions that gain their energy in the target sheath. Those may gain additional energy from the sputtering process or reflection at the target. Low-energetic negative ions are, however, less affected by changes of the target material. The results concerning O− formation are discussed in term of the sputtering rate from the target and are compared to models for negative ion formation.

The correlation between the radial distribution of high-energetic ions and the structural as well as electrical properties of magnetron sputtered ZnO:Al films

The origin of the pronounced radial distributions of structural and electrical properties of magnetron sputtered ZnO:Al films has been investigated. The film properties were correlated with the radially resolved ion-distribution functions. While the positive ions exhibit low energies and a radial distribution with a maximum intensity opposite the center of the target, the negative ions can have energies up to several hundred eV, depending on the target potential, with a radial distribution with two maxima opposite the erosion tracks. The most prominent positive ion is that of the working gas (Ar+), while the highest flux of the negative ions is measured for negative oxygen O−. The radial distribution of the flux of the high-energetic negative ions can clearly be related to the radial variations of the structural (c-axis lattice parameter, crystallite size) and electronic (resistivity) properties for sputtering from the planar target, which points to the decisive role of the high-energetic negative oxygen ...

Comparison of ion energies and fluxes at the substrate during magnetron sputtering of ZnO : Al for dc and rf discharges

Energy spectra of positive and negative ions have been measured in magnetron sputtering of a ZnO : Al target. The discharges have been operated in dc or rf, the latter with a frequency of 13.56 or 27.12 MHz, at the same geometry, argon pressure, and discharge power. While the average energy of the positive ions against a grounded surface increases with increasing frequency due to increased plasma potential, the average energy of the negative ions strongly decreases due to a target voltage decrease. The flux of negative ions simultaneously decreases, whereas that of the positive ions slightly increases. Combining these quantities to the energy flux onto floating substrates shows a drastic decrease of the energy flux with increasing frequency for high-energetic negative ion bombardment of the films while the low-energetic positive ion impingement is weakly increased. From the point of the energetic bombardment rf discharges are hence favoured to obtain high-quality films. The energy distributions of negative ions have a sharp peak in the dc mode but broad distributions in the rf discharges. Additionally, the latter are characterized by an overlaid peak structure that is explained by the oscillation of target and plasma potential.

Development of a compact combined plasma sensor for plasma surface engineering processes

A combined sensor for the investigation of plasma-based surface engineering processes has been developed, which basically consists of a quartz crystal microbalance that is simultaneously used as a heat flux sensor and a planar Langmuir probe in one active element. The sensor can thus measure deposition flux, heat flux, and charged particle flux laterally resolved at the same time and position. The setup and working principle of the sensor are shown, and the suitability for process investigations is demonstrated exemplarily for a dc magnetron sputtering discharge for Ti thin film deposition.

A multifunctional plasma and deposition sensor for the characterization of plasma sources for film deposition and etching

Our recently reported multifunctional plasma and deposition sensor [Welzel et al., Appl. Phys. Lett. 102, 211605 (2013)] was used for the characterization of two different plasma sources: a magnetron sputtering deposition source and an ion beam source. The multifunctional sensor, based on a conventional quartz crystal monitor (microbalance) for mass increase/decrease measurements, can measure quasi-simultaneously the deposition/etching flux, the energy flux, and the charged particle flux. By moving the sensor or the plasma source stepwise against each other, the lateral (radial) flux profiles of the different sources can be measured with a lateral resolution of about 8 mm, the diameter of aperture in front of the quartz crystal. It is demonstrated that this compact and simple multifunctional sensor is a versatile tool for the characterization of different kinds of plasma sources for deposition and etching purposes. By combining the different measured quantities, the ion-to-neutral ratio and the mean energ...

Cluster ion formation during sputtering processes: a complementary investigation by ToF-SIMS and plasma ion mass spectrometry

Plasma ion mass spectrometry using a plasma process monitor (PPM) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) have been complementarily employed to investigate the sputtering and ion formation processes of Al-doped zinc oxide. By comparing the mass spectra, insights on ion formation and relative cross-sections have been obtained: positive ions as measured during magnetron sputtering by PPM are originating from the plasma while those in SIMS start at the surface leading to large differences in the mass spectra. In contrast, negative ions originating at the surface will be accelerated through the plasma sheath. They arrive at the PPM after traversing the plasma nearly collisionless as seen from the rather similar spectra. Hence, it is possible to combine the high mass resolution of ToF-SIMS to obtain insight for separating cluster ions, e.g. Znx and ZnOy, and the energy resolution of PPM to find fragmentation patterns for negative ions. While the ion formation processes during both experiments can be assumed to be similar, differences may arise due to the lower volume probed by SIMS. In the latter case, there is a chance of small target inhomogeneities being able to be enhanced and lower surface temperatures leading to less outgassing and, thus, retention of volatile compounds.

Observation of the Magnetic Separatrix Between a Magnetron and an Electron-Cyclotron Resonance Discharge

A circular planar magnetron sputtering source has been combined with an electron-cyclotron resonance (ECR) plasma source to reduce the operating voltage of the magnetron in order to avoid high-energetic particle bombardment of sensitive thin films during deposition. Although the ECR source produces a downstream plasma density of several 1010 cm-3, the voltage reduction of the magnetron is limited to only a few percent. Using a geometry with the ECR source facing the magnetron target for maximum coupling, the shape of the discharge clearly shows a magnetic shielding of the magnetron against the ECR discharge limiting the electron injection into the magnetron torus.