Johannes Strümpfel

Optical investigations in a PEM controlled reactive magnetron sputter process for aluminium doped zinc oxide layers using metallic alloy targets

The use of aluminium doped zinc oxide (ZAO) as transparent conductive layer (TCO) has growing importance especially for production of solar modules. A significant cost reduction of TCO thin film production is expected by use of much less expensive metallic alloy targets. ZAO films with good TCO properties can be achieved only in the transition mode of the reactive sputter process. To stabilize the discharge for long stretched magnetron sputter sources on the working point needed a process control system is required. The intensity of zinc emission lines is used as control parameter. The control is performed by adding varying amounts of oxygen gas to the discharge region using the Plasma Emission Monitor PEM®05. This allows a combination of reproducible TCO film properties with stable high deposition rates. By use of an optical emission spectrometer AOS-4 it was investigated in situ, which emission lines are best suited for the process control. Herefore the chronogram mode of the spectrometer is used, which allows spectral lines to be monitored with a time resolution of a few milliseconds. Furthermore, other spectral line intensities are investigated for use as regulation parameter. A concept for regulating a reactive sputter process by use of several emission lines is discussed. The concept was applied for DC discharge and MF discharge as well. A time-resolved investigation of the interesting spectral lines in MF discharge is carried out with microsecond resolution.

Deposition of TCO films by reactive magnetron sputtering from metallic Zn:Al alloy targets

Abstract The use of aluminum doped zinc oxide (ZAO) as transparent conductive oxide(TCO) has gained importance with the beginning of Cu(In,Ga)Se2 (CIGS)-based thin-film module production. Commonly these layers are deposited from ceramic ZAO targets. However, a high cost reduction is expected if it is possible to use much less expensive metallic alloy targets. A comparison of the results of applying different reactive sputtering techniques is presented. Plasma emission monitoring (PEM) is needed to control the process. A multichannel PEM system is used to obtain homogeneity over long target lengths. A comparison between sputtering from Zn:Al targets and pure Zn targets aids to understand specifics of the sputtering process. Samples deposited by the different techniques are characterized by resistivity and transmittance measurements. It is shown that ZAO films deposited from metallic targets show comparable properties compared with films deposited from ceramic targets. As a first result of applying this reactive sputtering technique with metallic targets, module efficiencies exceeding 10% could be realized on 30×30 cm2 CIGS modules.