RF Power and Thermal Annealing Effect on the Properties of Zinc Oxide Films Prepared by Radio FrequencyMagnetron Sputtering

Polycrystalline zinc oxide (ZnO) films were prepared by radio frequency (RF) magnetron sputtering under different powers. The XRD results showed that ZnO crystallite size along c-axis decreased by 43% with deposition power increased from 60 W to 300 W, increased 36% with annealing temperature rising to 400◦C. TDS measurement revealed that the desorption peaks of both atomic Zn (60 W-deposited) and oxygen molecule (180 W and 300 W-deposited) obtained from ZnO films were originated from 300◦C. When annealing temperature was higher than 300◦C, the sheet resistance dramatically decreased, and compressive stress in the (002) plane changed to tensile stress as well. The comparison measurements of ZnO films crystallinity strongly suggested that both lower deposition power and certain thermal annealing temperature over 300◦C would contribute to the formation of high quality ZnO films.


INTRODUCTION
In recent years, applications of ZnO as switching devices and majority carrier devices in thin film transistors (TFTs) have attracted many researches [1][2][3].Numerous techniques [4][5][6][7][8][9] have successfully been used to synthesize ZnO films.Effects of sputtering variables on ZnO film properties have been some of the major research topics.In our previous work, we already found that the average crystallite size of film was strongly dependent on the gas flow ratio of O 2 /Ar and deposition pressure [10,11].Despite considerable efforts have been done by different groups, the performances of ZnO films are still unsatisfactory.In order to extend industrial application, further improvement is needed to optimize the deposition conditions.Usually, a gate insulator is deposited on the ZnO layer at about 250 • C for top-gate fabrication in ZnO thin-film transistors (TFTs) [10].Therefore, either deposition parameters or thermal-annealing effect is necessary for investigation of future commercialization of TFT fabrication processes.In the present work, properties comparison of ZnO film deposited with different powers as well as thermal annealing effects were investigated in detail, which will give a significant help on the low-temperature TFTs fabrication processes.

EXPERIMENTAL PROCEDURES
ZnO films were deposited onto alkali-free glasses (OA-10, Nippon Electric Glass Co., Ltd) using an RF (13.56 MHz) magnetron sputtering system .A ZnO target with 99.999% in purity and 4 inches in diameter was located on the cathode which was 88 mm away the substrate stage.Prior to deposition, the chamber was evacuated to 10 −5 Pa with a turbo molecular pump and the substrate on anode was preheated to 150 • C. A gas mixture of Ar/O 2 (10/30 sccm) was used as working gases.The composition was regulated by mass flow controllers (MFC's).The total pressure was kept constant as 1 Pa.The RF power was varied from 60 W to 300 W with an interval of 60 W in order to examine power dependence of quality of ZnO films.The thicknesses of all ZnO films   The crystal structure of the ZnO film was inspected using an X-ray diffraction (Rigaku ATX-G diffractometer), performed in 2θ/ω sweeping from 27 to 40 degree in 2θ with a step width and a scan speed at 0.01 degree and 0.5 degree/min, respectively, employing a Cu Kα tube (λ = 0.154178 nm) radiation (50 kV, 300 mA).The thermal stability of ZnO films was investigated by a thermal desorption spectroscopy (TDS) (ESCO.Ltd., EMD-WA 1000SW) with a 60 • C heating rate.Optical emission spectroscopy (OES) measurement using a photonic multichannel spectra analyzer (PMA U6039-01) was set up in situ monitoring plasma variation under different powers.The surface morphology and root-mean-square (RMS) surface roughness of the films were characterized on an area of 1250 × 1250 nm 2 by using an atomic force microscope (AFM) (Nanoscope II produced by Digital Instruments).

RESULTS AND DISCUSSION
Figure 1 shows θ/2θ XRD spectra of the ZnO films deposited at different RF powers: 60 W, 120 W, 180 W, 240 W, and 300 W, respectively.It was noticed that only (002) reflection peak (34.4 • is obtained from unstressed ZnO powder as a standard data) was observed for all films, which means that all of the as-deposited ZnO films were polycrystalline structured and having highly (002) preferred orientations with a c-axis perpendicular to the substrate.However, comparing to the other XRD pattern, the ZnO film deposited at 60 W showed much sharp peak (position shifted to a higher angel about 34.32 • ) with the strongest peak intensity and the smallest FWHM.It revealed that the ZnO film deposited at 60 W would have much better crystal structure, which was in good agreement with the results of the Ellipso (WASE by J. A. Woollam.Co., Inc) measurement for the same samples, in which the reflective indexes of the ZnO films were slightly increased with decreasing RF power, indicating that the films densities were decreased with increasing RF power.The film deposited at 60 W had higher density than others.
The AFM measurement was used to further confirm the above supposition.Figure 2 shows topographies of the ZnO films obtained at 60 W and 180 W, respectively.The root-mean-square (RMS) average roughness of the ZnO surfaces was calculated for a 1250 nm square scan area.The roughness of the as-deposited film at 60 W (RMS: 0.37 nm) was less than that at 180 W (RMS: 0.42 nm), which revealed that 60 W-deposited ZnO film had higher density and much smoother surface.However, crystallite sizes varied in a reversed tendency of the case of roughness.The crystalline size was estimated according to the Scherrer equation [12].The calculated results showed that crystallite sizes in the c-axis were increased from 10.2 nm to 18 nm as the power decreased from 300 W to 60 W, as will be shown later in Figure 5(a), which also suggested that lower deposition power would contribute to the improvement of the crystallinity of the ZnO films.
The variation of species as a function of the RF power during the plasma deposition was in situ monitored by means of OES.The emission from plasma was collected by an optic fiber externally mounted on the chamber window and pointed to the center position between the cathode and the substrate.During the sputtering process, O 2 served as a reactive gas and Ar acts as a sputtering enhancing gas.The results, as shown in Figure 3(a), indicated that intensities of the excited atomic oxygen O * (777 nm), molecular oxygen ions O + 2 (602 nm), and metal atomic Zn (481 nm) lines versus RF power.It was obviously that O * intensity emission increased roughly linearly and about 10 times larger than O + 2 intensity which slightly increased with RF power.It was reported that O * including both excited atomic oxygen and ionic oxygen served as negative ion candidate, which was generated on the ZnO target surface, then accelerated from cathode to substrate (anode), neutralizing in RF plasma on the way [13].With power increasing, working gas O 2 was further ionized companying with higher kinetic energy contributing to the increasing deposition rate: from 1.6 nm/min at 60 W increased to 17.73 nm/min at 300 W. However, in Figure 3  it was found that emission intensity ratio of O * /Zn decreased with RF power increasing, implying oxidation of metal Zn was suppressed due to the excess metal zinc or incomplete oxidized zinc oxide in the deposited film attribute to increased power.Meanwhile, with the power increasing, ion bombardment effect consequently enhanced leading to increased compressive stress because of atomic peening, an energy-dependent process, furthermore, resulting in crystallite size decreased, as shown in Figure 5(a) and (b) later.TDS measurement was carried out to investigate thermal stability of ZnO films, results shown in Figure 4.The desorption peaks obtained from the molecular oxygen were orig-inated from 300 • C as the deposition power at 180 W and 300 W, which was much larger than the desorption from 60 W-deposited ZnO film.Because much more excess oxygen radicals in the plasma began to be absorbed with weakly bond existing in the deposited films with increased RF power, which will be preferentially desorbed as temperature increasing from the films afterwards.However, atomic Zn desorption only could be observed at temperature of 300 • C for the 60 W-deposited film.With the power increased to 180 W and 300 W, Zn desorption was strongly suppressed, no any desorption peak was observed due to the higher residual compressive stress.Therefore, the TDS results were in good agreement with the OES analysis.The broad peaks appeared for all samples at temperature around 150 • C were due to desorption of water absorbed on the film surface.
After deposition, all samples were annealed in a vacuum (10 −3 Pa) chamber for 2 hours with different temperatures from 200 • C to 400 • C, respectively.XRD measurement was applied to investigate thermal annealing effects on the crystal structure.It was found that there were multiple appearances of sharp XRD peaks with the annealing temperature increased above 300 • C, contrasting to the as-deposited film, indicating a dramatic increase in the crystallinity of the ZnO film, revealed by the FWHM decreasing [14].The most intensive peak and the smallest FWHM (0.33 • ) were exhibited at the 400 • C-annealed ZnO film deposited at 60 W.During the annealing process, ZnO crystalline became recrystallized, where new grains nucleated and grew to replace those deformed by internal stresses.Well-oriented columnar structures were obtained by thermal annealing temperature up to 300 • C. According to the calculation from the FWHM values, the average crystallite sizes deposited with different powers in the c-axis increased by approximately 30%, as shown in Figure 5 to 24.42 nm until the annealing temperature reached 400 • C. Therefore, the quality of ZnO film was significantly improved by thermal annealing process.
During the annealing process, the ZnO atoms gain energy to rearrange in the lattice.As well known, thermal annealing was used to relieve the film stress and improve the crystal structure.Stresses in the sputtered and annealed ZnO films were investigated as a function of the RF power in the range from 60 W to 300 W, as shown in Figure 5(b).The residual stresses of all ZnO films were compressively kept until the annealing temperature increased to 300 • C. It was clearly observed that the compressive residual stresses of ZnO films were reduced towards zero then changed to tensile stress with further increase of the annealing temperature to 300 • C.However, sheet resistances of the films were observed dramatically decreased from 1 × 10 14 Ω/ to 1 × 10 6 Ω/ when the annealing temperature greater than 300 • C.

CONCLUSIONS
ZnO films deposited with different RF powers appeared to have a preferential orientation with c-axis perpendicular to the substrate.60 W-deposited ZnO film had the highest intensity and the smallest FWHM.The cystallite size of the ZnO films in the c-axis decreased with power increasing.The increase of both reactions between atomic zinc and oxygen and kinetic energy with power increasing resulted in the deposition rate increase.The thermal desorption spectroscopy confirmed that the oxygen molecules desorbed from 300 • C, so did the atomic zinc from the 60 W-deposited film.The investigation of thermal annealing effects showed that crystallite size in the c-axis remarkably increased by 30% as the annealing temperature rose to 400 • C. The film compressive residual stress was relieved and crystal structure was significantly improved with the annealing temperature reached to 300 • C which is also a critical point for sheet resistance variation.The improved crystallinity of ZnO films strongly suggests that both lower power and thermal annealing process will contribute to the formation of high-quality ZnO films for top-gate structure thin film transistors fabrication.

Figure 2 :
Figure 2: Atomic force spectroscopy topography of ZnO films deposited with (a) 60 W and (b) 180 W.
Figure1shows θ/2θ XRD spectra of the ZnO films deposited at different RF powers: 60 W, 120 W, 180 W, 240 W, and 300 W, respectively.It was noticed that only (002) reflection peak (34.4 • is obtained from unstressed ZnO powder as a standard data) was observed for all films, which means that all of the as-deposited ZnO films were polycrystalline structured and having highly (002) preferred orientations with a c-axis perpendicular to the substrate.However, comparing to the other XRD pattern, the ZnO film deposited at 60 W showed much sharp peak (position shifted to a higher angel about 34.32 • ) with the strongest peak intensity and the smallest FWHM.It revealed that the ZnO film deposited at 60 W would have much better crystal structure, which was in good agreement with the results of the Ellipso (WASE by J. A. Woollam.Co., Inc) measurement for the same samples, in which the reflective indexes of the ZnO films were slightly increased with decreasing RF power, indicating that the films densities were decreased with increasing RF power.The film deposited at 60 W had higher density than others.The AFM measurement was used to further confirm the above supposition.Figure2shows topographies of the ZnO films obtained at 60 W and 180 W, respectively.The root-mean-square (RMS) average roughness of the ZnO surfaces was calculated for a 1250 nm square scan area.The roughness of the as-deposited film at 60 W (RMS: 0.37 nm) was less than that at 180 W (RMS: 0.42 nm), which revealed that 60 W-deposited ZnO film had higher density and much smoother surface.However, crystallite sizes varied in a reversed tendency of the case of roughness.The crystalline size was estimated according to the Scherrer equation[12].The calculated results showed that crystallite sizes in the c-axis were increased from 10.2 nm to 18 nm as the power decreased from 300 W to 60 W, as will be shown later in Figure5(a), which also suggested that lower deposition power would contribute to the improvement of the crystallinity of the ZnO films.The variation of species as a function of the RF power during the plasma deposition was in situ monitored by means of OES.The emission from plasma was collected by an optic fiber externally mounted on the chamber window and pointed to the center position between the cathode and the substrate.During the sputtering process, O 2 served as a reactive gas and Ar acts as a sputtering enhancing gas.The results, as shown in Figure3(a), indicated that intensities of the excited atomic oxygen O * (777 nm), molecular oxygen ions O + 2 (602 nm), and metal atomic Zn (481 nm) lines versus RF power.It was obviously that O * intensity emission increased roughly linearly and about 10 times larger than O + 2 intensity which slightly increased with RF power.It was reported that O * including both excited atomic oxygen and ionic oxygen served as negative ion candidate, which was generated on the ZnO target surface, then accelerated from cathode to substrate (anode), neutralizing in RF plasma on the way[13].With power increasing, working gas O 2 was further ionized companying with higher kinetic energy contributing to the increasing deposition rate: from 1.6 nm/min at 60 W increased to 17.73 nm/min at 300 W. However, in Figure3(b),

Figure 3 :Figure 4 :
Figure 3: Optical emission spectroscopy measurement: (a) emission intensities from Zn at 481 nm, O * at 777 nm, O + 2 at 602 nm, and deposition rate, (b) dependence of the emission intensity ratio of O * to Zn on deposition power.

Figure 5 :
Figure 5: (a) Dependence of the average crystallite size on deposition power and annealing temperature and (b) biaxial stress of ZnO films varied with the annealing temperature.