Japanese Journal of Applied Physics 49 (2010) 071103
REGULAR PAPER
Effects of Energetic Ion Bombardment on Structural and Electrical Properties of Al-Doped ZnO Films Deposited by RF-Superimposed DC Magnetron Sputtering Norihiro Ito1;2 , Nobuto Oka1 , Yasusi Sato1 , and Yuzo Shigesato1 1 2
Graduate School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara 229-8558, Japan Microfabrication Process Development Center, Panasonic Electric Works, Co., Ltd., 1048 Kadoma, Kadoma, Osaka 571-8686, Japan
Received January 20, 2010; accepted April 6, 2010; published online July 20, 2010 The bombardment of various types of energetic ions during rf-superimposed dc magnetron sputter deposition was investigated in detail and their effects on the structural and electrical properties of Al-doped ZnO (AZO) films were analyzed. Aside from the expected energetic negative oxygen ions (i.e., O and O2 ), various other negative ions (i.e., AlO , AlO2 , AlO3 , ZnO , and ZnO2 ) with a high energy were clearly observed. Such negative ions were found to be generated on the target surface and accelerated towards the substrate by the full cathode voltage. Furthermore, we found that the energy of these negative ions decreased with decreasing plasma impedance by superimposing rf power on dc sputtering. The resistivity of the AZO films deposited using the rf-superimposed dc sputtering was much lower than that of the films deposited using conventional dc sputtering. Such a decrease in resistivity should be attributed to reducing the damage of AZO films by suppressing the bombardment energies of various types of energetic negative ions impinging on a growing film surface. # 2010 The Japan Society of Applied Physics DOI: 10.1143/JJAP.49.071103
1.
Introduction
Low-pass filter
ZnO-based transparent conductive oxide (TCO) films such as Al-doped ZnO (AZO)1) or Ga-doped ZnO (GZO)2,3) should be promising candidate alternatives to indium-based TCO films, such as Sn-doped In2 O3 (ITO) or amorphous indium–zinc oxide (IZO),4,5) owing to their advantages of low cost, resource availability, and nontoxicity. The most practical deposition method for TCO films should be magnetron sputtering using a ceramic oxide target because of its high potential for uniform coatings in large areas with high reproducibility. TCO films deposited by rf magnetron sputtering have a relatively lower resistivity than those deposited by dc magnetron sputtering because the low cathode voltage of rf discharge substantially suppresses high-energy bombardment.6) However, rf sputter deposition shows a smaller deposition rate as a result of the lower cathode voltage and limited power impress.7,8) Hence, an interesting alternative should be to add a certain amount of rf power to the impressed dc power, i.e., a technique known as rf-superimposed dc sputtering.7,9) In sputter deposition, the structural, electrical, and optical properties of deposited AZO films are strongly affected by the bombardment of various energetic particles on growing film surfaces.2,10–12) In this study, we carried out an in-situ analysis of energetic ions and their energy distributions during sputter deposition using dc–rf coupled magnetron sputtering with an AZO ceramic target. The effects of energetic ion bombardment on the structural and electrical properties of AZO films are discussed in detail. 2.
Experimental
AZO films were deposited on alkali-free glass (Corning 1737) and silicon substrates by rf-superimposed dc magnetron sputtering using an AZO ceramic target (Al2 O3 : 2.0 wt %, Tosoh) and Ar gas at the total gas pressure (Ptot ) of 0.35 Pa. Figure 1 shows a schematic diagram of the dc–rf superimposed magnetron sputtering system. The energetic ion bombardment on a growing film surface was analyzed using a quadrupole mass spectrometer combined with an
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Matching box Sputter Chamber Magnetron
Target
Ar gas
Pump
Substrate
Analyzer
Fig. 1. Schematic diagram of rf-superimposed dc magnetron sputtering system and quadrupole mass spectrometer combined with electrostatic energy analyzer.
electrostatic energy analyzer (Pfeiffer Vacuum PPM 422), which was mounted at the substrate location on the surface of the AZO target with a diameter of 8 in., as shown in Fig. 1. In-situ measurement has been performed with the extraction orifice on floating potential, the same as for the substrate. Ion energy should be determined relative to the floating potential of the orifice. Magnetron discharge was generated by an rf source (13.56 MHz) and dc power was simultaneously impressed on the target. The dc power supply was protected from rf waves by a low-pass filter. All the deposition conditions for the sputtering are listed in Table I. Film thickness was measured using a surface profiler (Veeco Dektak8 ). The crystallinity of the films was analyzed by X-ray diffraction (XRD; Shimadzu XRD-6000) analysis using 40 kV, 20 mA Cu K radiation. Precise XRD measurement was carried out in the step-scan mode at step intervals of 0.008 to obtain lattice spacing (d-value). Internal stress () was estimated using Stoney’s formula13) from the deformation in the curvature of the silicon substrate measured using an ex situ surface profiler:
071103-1
# 2010 The Japan Society of Applied Physics
Jpn. J. Appl. Phys. 49 (2010) 071103
N. Ito et al.
Table I. Deposition conditions for conventional dc magnetron sputtering and rf-superimposed dc magnetron sputtering.
10
AZO[Al2 O3 (2.0 wt %), 8 in.]
Substrate
Alkali-free glass, Si wafer
Substrate temperature ( C) Target-substrate distance (mm)
Unheated (
