ZnO films have been prepared by rf sputtering a Zn target in a planar magnetron system with controlled Ar/O2 gas mixtures. The films were deposited on unheated glass substrates which were either stationary in front of the target or in constant motion. Both the system pressure and plasma impedance changed when an oxide layer formed on the target surface. This occurred at an oxygen flow rate which increased almost linearly with rf power; at 500 W, the required flow rate was 9 ml/min and the pressure increased from 0.1 to 1.2 Pa due to the reduced oxygen gettering. High resistance ZnO films were deposited at oxygen flow rates above this threshold value. The target self-bias voltage increased by 30 V at this value; it is affected by both the system pressure and the power. The deposition rate increased linearly with power at approximately 0.03 (μm/min)/(W/cm2) which appears to be typical of sputtering from a ZnO layer or target. For continuous substrate motion, the average rate was approximately 7% of this value. All the films were polycrystalline ZnO with a preferred orientation, the c axis of the hexagonal structure being within a small angle of the substrate normal; this orientation was improved by motion of the substrate past the target. Films deposited at pressures of approximately 0.4 Pa had a large internal stress, as revealed both by substrate bending and x-ray measurements. Increasing the pressure to 4.7 Pa decreased the stress by an order of magnitude. SEM analysis showed that this was associated with the development of a columnar structure. The refractive indices obtained from guided wave measurements were 1.940±0.006 and 1.962±0.003, which correspond to 97% of the single crystal values. The resistivity measured normal to the film plane was greater than 107Ω cm. The changes in film stress and structure are similar to effects in metal films. The electromechanical coupling coefficients obtained from SAW measurements are approximately half the best reported value.
An Al target has been sputtered in a planar magnetron system using argon and argon–oxygen mixtures at pressures from 0.1 to 2 Pa. The characteristics of both dc and rf discharges have been studied. In the dc case, the current is given by a relation of the form KVn where V is the applied voltage and K and n are pressure-dependent parameters. Values of n up to 9 were obtained at the higher pressures but n decreased at lower pressures. For the rf discharge, the target self-bias voltage, Vsb, at a given power decreased with decreasing pressure and was related to the average rf power by a relationship of the form CVsb/(Vsb−V) where V was approximately 1000 V and C dereased from 900 to 500 W with increasing argon pressure. The deposition rate of Al increased linearly with average dc and rf power and there was good agreement between the values for the same power; the rate was 0.93 μm/min at the maximum power of 2.5 kW. When sufficient oxygen was added, the target surface was oxidized and the deposition rate of Al2O3 was only 5% of the Al rate. This is about half the ration predicted for sputtering yields and can be accounted for by a secondary electron emission coefficient, due to ion bombardment, of between 0.4 and 1.2. This is much higher than the value for Al and accounts for a sharp decrease in Vsb when oxide forms on the target surface. At an average power of 400 W at a pressure of 1 Pa, Vsb is 380 V for the Al case and 190 V for the oxidized target.
Detailed analysis by reflection electron diffraction (RED), x-ray diffraction (XRD), and scanning electron microscopy (SEM) was performed on thin ZnO layers which were formed under reactive and nonreactive rf-sputtering conditions. A variety of textures and morphologies were observed. 100% reproducible piezoelectric layers, preferred oriented with [002] perpendicular to the layer within 7°, could be obtained by reactive sputtering from a zinc target at rf power of 150 W, oxygen-argon atmosphere of 8×10−3 Torr with 35% O2, and with the glass substrate being kept at room temperature by a cooling device. No differences in the surface-acoustic-wave properties were found between reactively and nonreactively sputtered ZnO layers which had similar texture and morphology. SEM techniques proved to be extremely misleading in the study of this type of layer; there is no relationship between an observed columnar structure and the texture of the layer which is determined by RED, and also between the column thickness (∼1 μm) and the actual crystallite size (∼200 Å) which is determined by XRD. It is shown that the observed columns must be polycrystalline.
The crystal structure of Cs2HfC16 was determined to be cubic, Fm3m, similar to K2PtC16, with parameters: a0 = 10.42+0.01 A, and u=0.247_+0.003. Powder diffraction data on which the determination was based are given.
The I-V characteristic of dc and rf magnetron sputtering systems has been shown to fit an expression of the form I=β(V−V0)2, where V0 is the minimum voltage necessary to maintain a discharge. New experimental data is presented for a dc planar magnetron with Al, Cd2Sn, Cr, and CdSe targets in argon discharges and for a dc Research S-gun with an Al target in argon and argon/oxygen discharges. Literature data for planar, S-gun, and cylindrical magnetrons has also been shown to fit the above expression; for rf magnetrons, I is the rms current and V is the target self-bias voltage. V0 decreases from about 400 V at 0.1 Pa to 250 V at 1 Pa and then decreases at higher pressures. β increases from about 50 to ≤300 A/kV2 as the pressure increases from 0.1 to 10 Pa. The actual values of V0 and β depend on the system, target, and sputtering gas. It is shown that the I∝V2 dependence is due to a space-charge-limited electron current in the magnetron geometry which reduces the electron mobility by several orders of magnitude from the zero magnetic field value; the mobility ratio depends on the electron-atom collision frequency.
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