Acta Polytechnica Acta Polytechnica 53(2):155–159, 2013 © Czech Technical University in Prague, 2013 available online at http://ctn.cvut.cz/ap/ PHYSICAL AND ELECTRICAL PROPERTIES OF YTTRIA-STABILIZED ZIRCONIA THIN FILMS PREPARED BY RADIO FREQUENCY MAGNETRON SPUTTERING Dmitriy A. Golosov∗, Sergey M. Zavatskiy, Sergey N. Melnikov Thin Film Research Laboratory, Belarusian State University of Informatics and Radioelectronics, Minsk, Belarus ∗ corresponding author: dmgolosov@gmail.com Abstract. This paper presents the electrophysical characteristics of a 7 mol.% yttria-stabilized zirconia (YSZ) thin film deposited by radio-frequency magnetron sputtering. In order to form the crystalline structure, the deposited films were annealed in air over a temperature range of 700 ÷ 900 ◦C. By XRD analysis it was established that as the deposited films were amorphous, they crystallized into a pure cubic structure as a result of annealing in air at a temperature above 820 ◦C. The electrophysical properties of YSZ films were investigated on structures such as Ni/YSZ/Pt/Ti/Si and Ni/YSZ/Si. Film features ε > 20 and tg δ < 0.05 were obtained. An estimate of the capacity–voltage characteristic proved that the Ni/YSZ/Si structures possessed a hysteresis. This hysteresis resulted from the drift of the mobile ions in the YSZ film. High-temperature ionic conductivity of the stabilized zirconia was determined by the measurements of the electric resistivity of the YSZ films at 1 kHz over the temperature range from ambient to 800 ◦C. The YSZ film conductivity obtained was 1.96 × 10−2 S/cm under 800 ◦C. Keywords: yttria-stabilized zirconia, RF sputtering, X-ray diffraction, dielectric constant, loss tangent, capacity-voltage characteristic, ionic conductivity. 1. Introduction Yttria-stabilized zirconia (YSZ) is recognized as a very attractive electrical insulator because it is character- ized by high chemical stability, resistivity, and rel- ative dielectric constant. In microelectronics, films of stabilized zirconia are used as buffer layers, prevent- ing the functional layers from interacting chemically with silicon, in particular, in high-temperature super- conductors [2, 8]. Today these films are widely ap- plied as gate dielectrics in field-effect transistors and dynamic random-access memories (DRAM) [15, 11], and also as an insulator in silicon-on-insulator struc- tures [7, 5]. Doped zirconia has ionic transport properties under high temperatures, and thus can be used as a solid electrolyte in micro solid-oxide fuel cells (MSOFC) [3, 13, 10] or as sensitivity elements in integrated gas sensors (GS) [4, 14]. For these devices, the thick- ness of the solid electrolyte layer must not exceed 1 ÷ 2 µm in order to reduce ohmic loss and decrease the operating temperatures down to 500 ÷ 600 ◦C [6]. The requirements imposed on the solid electrolyte are rather tough: it has to be mechanically strong and chemically reliable at high temperatures, mechanically and chemically stable in time, should have maximal ionic and minimal electronic conductivity, and also be gasproof. The aim is therefore to obtain and study the prop- erties of doped zirconia films grown by plasma de- position methods [1]. RF magnetron sputtering, un- like other techniques, enables dense uniform films to be deposited under comparatively low tempera- tures; features stability of the process, the possibility of standalone control of the deposition parameters, and supports deposition over large-area substrates [9]. However, the electrophysical properties of RF sput- tered doped zirconia thin films need further study. In the present paper, the authors report the syn- thesis of YSZ thin films by RF magnetron sputtering and report the results of studying the structural and electrical properties of YSZ films. 2. Experiment Figure 1 illustrates our experimental installation for thin-film YSZ deposition by RF magnetron sputter- ing. The installation is based on the Leybold–Heraeus A550 VZK exhaust cart. The vacuum chamber is equipped with an external flanged Hall current ion source. It is used for substrate pre-cleaning. The orig- inally designed RIF.039.001 magnetron sputtering sys- tem with a � 39 mm target was used to sputter a ce- ramic ZrO2 + 7 mol.% Y2O3 target. The magnetron was mounted in the place of the target unit in the ion source. A 13.56 MHz RF power source having 1 300 W in the maximal power output, was used as the power supply. Monocrystalline superalloyed silicon Si (100), and structures of Si3N4 (1 µm)/Si, Pt (150 nm)/Ti 155 http://ctn.cvut.cz/ap/ Dmitriy A. Golosov, Sergey M. Zavatskiy, Sergey N. Melnikov Acta Polytechnica S S N S S N N To pump Substrate IS Ar O2 MFC MSS RF power suppl 13.56 MHz y Matching network IS PS 5.0 kV 300 mA Figure 1. Experimental installation for deposition of thin-film doped zirconia by RF magnetron sputter- ing: MSS – magnetron sputtering system, IS – ion source, MFC – mass flow controller. (100 nm)/Si were used as substrates. In the series of ex- periments, the substrates were mounted in a rotating substrate holder. The vacuum chamber was pumped down to a residual pressure of 8 × 10−4 Pa. The sub- strates were pre-cleaned with an ion beam. For this purpose, Ar was added into the gas distribution sys- tem of the ion source at the flow rate QAr = 10 sccm. During all the experiments the cleaning time, ion en- ergies and discharge currents were constant, at 3 min, 700 eV, and 40 mA, respectively. Then the targets were cleaned of impurities, for which the substrates were taken away from the de- position zone, and the working gases were supplied di- rectly into the magnetron discharge zone (Ar/O2 mix- ture). The argon flow rate was QAr = 50 sccm, while the oxygen flow rate was QO2 = 10 sccm. The target cleaning modes for all the experiments were constant: forward power PF = 70 W, reflected power PR = 5 W, and sputtering time of 20 min. Then the substrates were placed into the deposi- tion zone. The total gas flow rate was kept constant at a level of 60 sccm. The oxygen content in the gas mixture was varied from 0 to 30 %, whereas the pres- sure in the chamber was maintained at 0.2 Pa. The dis- charge power stabilization mode with 125 W and 80 W in the forward power was used for thin-film deposition, while the level of the reflected power did not exceed 10 % of the forward power. The deposition time in all the experiments was kept constant at 180 min. The substrate was placed 82 mm away from the target surface. The thickness of the deposited films varied within 200 ÷ 400 nm, depending on the oxygen content in the Ar/O2 gas mixture. Then the deposited films were annealed in air, us- ing the Isoprin IR heating system. The annealing temperature varied within the temperature range of 700 ÷ 900 ◦C (annealing time 15 min). The phase composition of the YSZ films was de- termined by means of the X-ray diffraction (XRD) method, using the DRON-3 system in CuKα radiation. Si (100) substrate SiO Ti Pt NiZrO +Y O2 2 3 2 Figure 2. Ni/YSZ/Pt capacitor stucture. The X-ray patterns were obtained at a rate of 60 ◦/h in the angular range of 2θ = 20 ÷ 80 ◦, and room temperature. The thickness of the deposited layers was determined with the POI-O8 optical interfero- metric profilometer. The capacitor structures (Fig. 2) were formed to measure the electrophysical charac- teristics of the YSZ films. The doped zirconia thin film (200 ÷ 400 nm in thickness) was deposited onto superalloyed silicon Si (100) and Pt/Ti/Si structures, followed by film annealing. The upper Ni electrode was deposited by ion-beam sputtering through a mask. The capacitors obtained were 0.8 × 0.8 mm. The capacitance, the dielectric loss tangent, and the C–V characteristics were obtained using the LCR meter E7-20 at frequencies between 25 Hz and 1.0 MHz. The dielectric constant values were calculated on the basis of the thickness and capacitance of the YSZ layer, using the formula ε = Cd/ε0S, where ε0 = 8.85 × 10−12 F/m, and S is the capacitor area S = 6.4 × 10−7 m2. The ionic conductivity of the stabilized zirconia was determined through measure- ments of the YSZ film electric resistance at 1 kHz and over the temperature range from room temperature up to 800 ◦C. 3. Results We aimed to show how the RF magnetron sputtering conditions of ZrO2 + 7 % Y2O3 target affect the depo- sition rate. The deposition rate vs. discharge power and vs. Ar/O2 producer gas mixture curves were plotted in Fig. 3. The deposition rate decreased prac- tically linearly as the oxygen content in the Ar/O2 gas mixture was raised, and the discharge power dropped. When the oxygen content in the Ar/O2 gas mixture was increased to 5 ÷ 7 %, the deposition rate was observed to drop by 20 ÷ 25 %. The estimated sput- tering coefficient for the ZrO2 + 7 %Y2O3 target was YYSZ = 0.17. The dependence of the phase composition in the YSZ films on the annealing temperature was studied, with Si3N4/Si (100) structures used as sub- strates. A silicon nitride layer 1 µm in thickness was formed on the surface of the monocrystalline sili- con by the CVD method. Yttria-stabilized zirconia thin films were deposited up to a thickness of more than 500 nm under the following conditions: forward power 125 W, reflected power 10 W, QAr = 35 sccm, QO2 = 25 sccm. Then the samples were annealed in air over the temperature range of 700 ÷ 900 ◦C, for 15 min. XRD was used to analyze the positions and the in- tensity of the peaks in the respective zirconia phases 156 vol. 53 no. 2/2013 Physical and Electrical Properties of Yttria-Stabilized Zirconia Thin Films D e p o s i t i o n r a t e , n m / s 0 0.01 0.02 0.03 0.04 0.05 5 10 15 20 25 O ,%2 а b Figure 3. YSZ film deposition rate as a function of oxygen percentage in the Ar/O2 gas mixture at var- ious forward power values: a – 80 W, b – 125 W. (Fig. 4). It was determined that regardless of the depo- sition conditions, wide implicit peaks with a high noise level were present in the X-ray diagrams of the as- deposited films, implying that originally the films were almost non-crystalline and/or amorphous (Fig. 4a). The positions of the peaks corresponded to the mon- oclinic modification of the crystalline structure. An- nealing at temperatures up to 800 ◦C resulted in an increase (−111) and (022) in the peak intensity of the monoclinic modification of the crystal lattice, and evoked cubic lattice peaks (Fig. 4b, c). The peak positions in the films annealed at temperatures above 820 ◦C corresponded to the pure cubic modification of the crystal lattice. (111), (200), (220), (311), and (400) cubic peaks with preferable orientation (200) were observed (Fig. 4d). The frequency curves for the dielectric constant and the dielectric loss tangent in YSZ films were obtained. Figures 5 and 6 plot the dielectric con- stant and the loss tangent curves of the deposited YSZ films in Ni/YSZ/Pt and Ni/YSZ/Si structures. The following thin-film YSZ deposition conditions were applied: forward power 125 W, reflected power 11 W, QAr = 50 sccm, QO2 = 10 sccm. The dielectric constant ε = 6.0 and the loss tangent tg δ = 0.11 were observed at 1 MHz of frequency for the deposited films in Ni/YSZ/Pt structures, whereas ε = 13.2, tg δ = 0.4 were observed at 1 kHz of frequency, re- spectively. Ni/YSZ/Si structures exhibited dielectric constant ε = 6.0 and the loss tangent tg δ = 0.06 at 1 MHz, and ε = 6.8 and tg δ = 0.34 at 1 kHz, re- spectively. At frequencies below 500 Hz, the capacitor structures were characterized by high values of dielec- tric loss and electric conductivity. As a result of annealing in air under a tempera- ture exceeding 820 ◦C as the YSZ cubic structure was formed, the dielectric constant of the films increased, whereas the loss tangent dropped. Figures 7 and 8 plot the dielectric constant and loss tangent curves for Ni/YSZ/Pt and Ni/YSZ/Si structures annealed in air at a temperature of 850 ◦C (annealing time 15 min). The YSZ films deposited on the Pt electrode demonstrate a dielectric constant such as ε > 20 and loss tangent tg δ < 0.05 in 25 Hz ÷ 1.0 MHz frequency Figure 4. XRD patterns of the YSZ films annealed at different temperatures: a – as-deposited, b – 700 ◦C, c – 800 ◦C, d – 900 ◦C (annealing time 15 min). L o s s t a n g e n t , t g δ 10 10 10101010 5 0.2 0 0 10 0.4 0.6 15 0.8 20 1.0 32 4 5 6 Frequency, Hz D i e l e c t r i c c o n s t a n t , ε Figure 5. Dielectric constant and loss tangent vs. frequency curves for as-deposited YSZ films in Ni/YSZ/Pt structures. 10 2 5 0 10 15 20 D i e l e c t r i c c o n s t a n t , ε L o s s t a n g e n t , t g δ 10 10 101010 0.2 0 0.4 0.6 0.8 1.0 3 4 5 6 Frequency, Hz Figure 6. Dielectric constant and loss tangent vs. fre- quency curves for as-deposited YSZ films in Ni/YSZ/Si structures. range. YSZ films deposited on an Si substrate in that frequency range showed the dielectric constant as low as ε = 13 ÷ 15 and loss tangent tg δ = 0.04 ÷ 0.1 . The C–V relationships for Ni/YSZ/Pt and Ni/YSZ/Si capacitor structures annealed at 850 ◦C were received at DC bias up to ±10 V (Fig. 9). It was established that the capacitance of Ni/YSZ/Pt structures did not depend on the bias volt- age, which was typical for conventional dielectrics. As for the Ni/YSZ/Si structures, estimates of the capacitance–voltage relationships proved that these structures had hysteresis (Fig. 9b). Capacitance varia- tion at DC bias is typical of metal–insulator–semicon- 157 Dmitriy A. Golosov, Sergey M. Zavatskiy, Sergey N. Melnikov Acta Polytechnica 10 10 10101010 14 0.02 12 0 16 0.04 18 0.06 20 0.08 22 0.1 3 4 5 6 Frequency, Hz D i e l e c t r i c c o n s t a n t , ε 2 L o s s t a n g e n t , t g δ Figure 7. Dielectric constant and loss tangent vs. frequency curves for Ni/YSZ/Pt structures annealed in air at 850 ◦C (annealing time 15 min). 10 10 10101010 14 0.02 12 0 16 0.04 18 0.06 20 0.08 22 0.1 3 4 5 6 Frequency, Hz D i e l e c t r i c c o n s t a n t , ε 2 L o s s t a n g e n t , t g δ Figure 8. Dielectric constant and loss tangent vs. frequency curves of Ni/YSZ/Si structures annealed in air at 850 ◦C (annealing time 15 min). 0 5 10 15-5-10-15 437 439 441 443 445 Bias voltage, V C a p a c i t a n c e , p F 0 5 10 15-5-10-15 490 480 500 510 520 530 540 Bias voltage, V C a p a c i t a n c e , p F a b Figure 9. C-V characteristics of structures such as Ni/YSZ/Pt (a) and Ni/YSZ/Si (b) annealed in air at 850 ◦C (annealing time 15 min). ductor structures and can account for the charge car- rier drift at the YSZ/Si boundary affected by the elec- tric field. The measured P–E characteristics of the YSZ layers showed the absence of dielectric polariza- tion, proving that the deposited YSZ films can be referred to as linear dielectrics. The high-temperature ionic conductivity of the sta- 600 800 10004002000 10 10 10 10 10 10 Temperature, С C o n d u c t i v i t y , S / c m -2 -3 -5 -6 -4 -1 Figure 10. Ionic conductivity the YSZ films in Ni/YSZ/Pt structures annealed in air at 850 ◦C (annealing time 15 min) as a function of temperature (f = 1.0 kHz). bilized zirconia was determined by measuring the elec- tric resistance of the Ni/YSZ/Pt capacitor structures annealed at 850 ◦C at 1 kHz of frequency with the tem- perature varied from ambient up to 800 ◦C. It was established that as the temperature was increased, the film conductivity was also grown in proportion to the temperature. When the substrate tempera- ture was 800 ◦C, the ionic conductivity of the YSZ films reached 0.0196 S/cm (Fig. 10). For comparison: the ion conductivity of the bulk YSZ samples is about 0.025 S/cm at 800 ◦C [12]. 4. Conclusion The RF magnetron sputtering method was used to deposit thin-film 7 mol.%-yttria-stabilized zirconia. By means of XRD analysis, it was established that the deposited films were amorphous, they could crys- tallize into pure cubic structures after being annealed in air under temperatures above 820 ◦C. The an- nealing treatment of YSZ films at temperatures above 700 ◦C causes the dielectric constant to rise, and the di- electric loss tangent to go down. The films that are obtained are characterized by ε > 20 and tg δ < 0.05. Estimates of the capacitance–voltage relationships proved that the Ni/YSZ/Si structures have hysteresis, resulting from the drift of the mobile ions in the YSZ film. It was discovered that the high-temperature ionic conductivity of thin-film YSZ to the temper- ature and reached 0.0196 S/cm with the substrate temperature of 800 ◦C. 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