Inductively-Coupled-Plasma Assisted Sputter-Deposition

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					PLD2009, December 02-04, 2009, IIT Madras, Chennai


                     Inductively-Coupled-Plasma Assisted Sputter-Deposition of
                              Aluminum-doped Zinc Oxide Thin Films

                     Y. Matsuda, R. Kan, T. Iwata, T. Kitagawa, R. Shindo and M. Shinohara
Department of Electrical and Electronic Engineering, Nagasaki University, Bunkyo 1-14, Nagasaki 852-8521, Japan

1. Introduction
          Transparent conducting oxide films have been widely used as transparent conducting electrodes of various
optoelectronic devices such as solar cells, flat panel displays, etc. Tin-doped Indium oxide (ITO) has been mainly
used so far due to high transmittance in the visible region, high chemical stability and low resistivity. For the last ten
years, however, metal-doped zinc oxide (ZnO) has been a focus of attention as one of the alternatives to the ITO.
Metal-doped ZnO has lower inherent electric conductivity than ITO, but has advantages over ITO in environment
resistance and resource cost.1-2 To actually replace ITO with metal-doped ZnO, however, a reproducible and highly-
reliable fabrication process of good quality thin films has to be developed. Thus, for the last few years we have been
investigating the aluminum-doped ZnO (AZO) film deposition process by using inductively coupled plasma (ICP)
assisted sputtering.3,4 . In this paper, we report our experimental results.

2. ICP assisted sputter-deposition of AZO thin films
         A disk target of ZnO: Al2 O3 (2wt%) of 60 mm diameter and 6mm thick was used as target and glass
substrates were set on a earthed substrate holder with a gap length of 80mm. In between these diode electrodes, a
single turn coil antenna of 100 mm diameter was installed and used for the production of 13.56MHz inductively
coupled plasma. The antenna was covered with insulator and water-cooled. The distance from the target to the RF
coil and the distance from the RF coil to the substrate were both 40 mm. Owing to the presence of high density ICP
between the target and substrate, the ICP-assisted sputter-deposition enables the low-voltage and high-current
sputtering and the sputtered particles are efficiently ionized compared to the conventional planar magnetron
sputtering. Thus, enhanced ion fluxes onto the substrate with moderate ion energy would contribute to the decrease
in surface roughness and promote the crystallinity of thin films without intentional substrate heating. In addition,
both the target usability and the spatial uniformity of thin film properties are improved. As a result of optimization
of target-coil-substrate distances and operating pressures, we have succeeded in depositing high quality AZO thin
films with resistivity of about 10-3 ? ?cm by using this technique so far.

3. Plasma diagnostics
          To understand the fundamental plasma surface interactions, atom densities in gas phase and heat fluxes
onto the substrate have been investigated. First, by using hollow cathode lamp absorption, Al and Zn atom densities
were measured during ICP-assisted sputter-deposition of AZO thin films with Ar at 30 mTorr. As a result, Zn and Al
atom densities were evaluated 1011 -1013 cm-3 and 108 -1010 cm-3 respectively depending on the discharge (target input)
power up to 50W. The ratio of Zn density to Al density was about one order of magnitude larger than the element
ratio of Zn to Al in the AZO target (100:4). Such a large spatial density ratio of Zn to Al density may be caused by
re-vaporization of Zn atom Second, heat flux onto the substrate was measured with a thermal probe. As a result,
we found that the heat flux to the substrate was predominantly determined by ICP power and the contribution of
planar magnetron discharge to the total heat flux is less than 10%.

4. Application to ZnO LED
 We plan to apply this technique to the deposition of n-type transparent conducting layer of ZnO base white color

[1] T. Minami, H. Nanto, S. Takata, Jpn. J. Appl. Phys. 23 (1984) L280.
[2] R. Das, K. Adhikary, and S. Ray, Jpn. J. Appl. Phys. 47 (2008) 1501.
[3] Y. Matsuda, M. Iwaya, Y. Koyama, M. Shinohara, and H. Fujiyama, Thin Solid Films, 457 (2004) 64.
[4] S. Iwai, Y. Matsuda, M. Shinohara, and H. Fujiyama, Abstracts and Full-Paper CD of the 18th Int. Symp. on
    Plasma Chemistry, August 26-31, 2007, Kyoto, Japan (2007) 396.