Transparent and conductive ZnO:Al films deposited by large area reactive
magnetron sputtering
Abstract
A new coating technology has been developed for large area deposition of transparent and conductive ZnO:Al films. ReactiveAC magnetron sputtering from metallic Zn:Al has been performed at low substrate temperature below 200 8C using the newCleanMag technology of applied films based on moving magnets for layers with low defect density and with conventional reactiveAC magnetron sputtering using static magnets (Applied Films TwinMag). Process stabilization at non-stable process conditionsin transition mode has been achieved using closed loop control of discharge power according to fast oxygen partial pressuremeasurements taken by a lambda probe. Films with film thickness of 800–1000 nm deposited with dynamic deposition rate ofa )60 nm mymin exhibit low resistivity of r-270 mV cm and small absorption (k-2=10y3) in the visible range. TheDhomogeneity of sheet resistance on float glass substrates with dimension of 600=1000 mm2 is better than "6%. Film propertiessuch as etching characteristics and haze can be controlled due to adjustment of total pressure and substrate temperature for largeare a-Si:H thin film photovoltaic applications w1x._ 2003 Elsevier B.V. All rights reserved.
Keywords: Zinc oxide; Sputtering; Optical coatings; Electrical properties and measurements
1. Introduction
The large area deposition of transparent conductiveoxides is crucial for a variety of applications such asthin film solar cells w2x, flat panel displays w3x andelectrochromic glazings w4x. At the present time, thedevice technology for systems mentioned above is basedon conventional transparent and conductive oxide layerssuch as In2O3:Sn (ITO) deposited by magnetron sputteringand SnO2:F layers deposited by chemical vapordeposition. These established techniques suffer fromcertain drawbacks, i.e. high material costs, high substratetemperature and poor chemical stability in reducingambient for ITO layers and poor electrical and opticalproperties for SnO2:F layers, respectively.At Fraunhofer IST, we have investigated the reactivesputter deposition of alternative TCO films such asSnO2:Sb w5x, ZnO:In w6x and ZnO:Al w7–10x to overcomethe drawbacks of the established materials ITOand SnO2:F. This paper reports on the large area depositionof transparent and conductive ZnO:Al layers using*Corresponding author.an in-line process on flat glass substrates of dimensionsup to 1000=600 mm2.
2. Experimental
ZnO:Al films have been deposited by reactive ACmagnetron sputtering (TwinMag) using a vertical in-linesputter system which has been described elsewhere w10x.Two different cathode types have been used: First, theconventional Leybold PK 750 cathode with stationarymagnetic units (series A films), second, the a newdeveloped cathode type (CleanMag, series Bfilms ).The magnet system of the CleanMag cathode is connectedto a stepper motor via an eccentric tappet. Thus,the magnetic unit performs a rotating plane motion with14-mm radius. The frequency can be adjusted between0 and f100 U miny1.In order to stabilize the reactive sputter process in thenon-stable transition mode, a closed loop control systemhas been established for the adjustment of dischargepower according to reactive gas partial pressure takenby a l-sensor. For series A sputtering, the process gasfor l-sensor measurement has been extracted betweenthe magnetrons via a turbo pump


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