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Low-temperature deposition of nanocrystalline Al2O3 films by ion source-assisted magnetron sputtering
Vacuum 149 (2018) 284e290
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Low-temperature deposition of nanocrystalline Al2O3 films by ion source-assisted magnetron sputtering Ji Cheng Ding a, Teng Fei Zhang a, Rajaram S. Mane a, Kwang-Ho Kim a, Myung Chang Kang a, Chang Wei Zou b, *, Qi Min Wang b, ** a b
Global Frontier R&D Center for Hybrid Interface Materials, Pusan National University, Busan, 609-735, South Korea School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou, 510006, China
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a b s t r a c t
Article history: Received 2 September 2017 Received in revised form 5 January 2018 Accepted 5 January 2018 Available online 6 January 2018
In this paper, bipolar pulse reactive magnetron sputtering with ion source assisted deposition method has been utilized for the deposition of nanocrystalline alumina (Al2O3) films at the temperature of 300 C onto silicon (111) wafers, and cemented carbide substrates. The influence of ion source power, i.e. 0, 1.0, 1.5 and 2.0 kW on the structure, morphology, compressive stress and mechanical properties of the Al2O3 films were investigated. In absence of ion source power assistance, an amorphous Al2O3 film was dominant at deposition temperature of 300 C. With ion source assisted deposition, the crystalline gAl2O3 films were obtained at the same conditions, suggesting an importance of ion source power in crystallinity development of metal oxide films obtained from magnetron sputtering deposition method. Images of surface morphology clearly demonstrated the difference in granular sizes of film surfaces prepared with and without ion source powers. With increasing ion source power from 1.0 to 2.0 kW, the micro-hardness and compressive stress of the films were increased from 7 GPa to 13 GPa and 0.3 GPa to 1.1 GPa, respectively. Results revealed that the reactive magnetron sputtering with the ion source assisted deposition was a simple and effective way to prepare nanocrystalline g-Al2O3 films at low temperature. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Low temperature deposition Nanocrystalline g-Al2O3 film Microstructure Hardness
1. Introduction Alumina (Al2O3) thin films have aroused increasing attention of the scientists in last decades because of their applications in surface passivation on crystalline silicon wafers [1], as high quality waveguiding material [2] or to be an important catalyst in the petroleum industries [3,4] etc. Also, due to the considerable hardness and thermal stability at high temperatures (1000 C), it provides an excellent high-temperature abrasion protection for coated tool during the process of high speed machining [5,6]. The high resistance to radiation of the Al2O3 thin film has made it an ideal choice for application in harsh environment, and also its application in catalysis due to their large surface area [3]. The Al2O3 thin films have been previously synthesized by various methods like pulse DC/RF reactive magnetron sputtering, pulsed dc magnetron sputtering, multi-arc ion plating, electron beam evaporation, pulsed
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C.W. Zou), [email protected] (Q.M. Wang). https://doi.org/10.1016/j.vacuum.2018.01.009 0042-207X/© 2018 Elsevier Ltd. All rights reserved.
plasma-assisted magnetron sputtering, and ion beam sputtering etc. [2,7e14] which commonly are known as physical vapor deposition (PVD) technology. However, depositing crystalline Al2O3 thin films by using PVD method at high deposition rate are usually difficult, especially, the as-deposited films are often presented an amorphous structure at a low deposition temperature [15]. It is quite general that the crystalline Al2O3 film offers higher density, hardness and also chemical and thermal resistance than its corresponding amorphous counterpart [16]. To obtain a-alumina films, chemical vapor deposition (CVD) technology is commonly used in industrial fabrications [16,17]. The substrate temperatures are normally above 1000 C, which prohibits the use of many temperature sensitive substrate materials, e.g., glass materials and tool steel. Due to it has limited potential for commercial scale, applying PVD deposition to grow crystalline alumina at low temperature is desired to be studied [18,19]. As on today, PVD technology with low-temperature and high deposition rate is considered to be the most important practical method to deposit crystalline Al2O3 films. Many methods used for obtaining crystalline Al2O3 film by virtue of the PVD technology at low deposition temperatures have been reported. Due to the a-
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Cr2O3 is isostructural with a-Al2O3 and relatively easy to deposit at low temperature [20], so the growth of a-Al2O3 films using PVD technology through a crystallographic template a-Cr2O3 is an effective approach, which has been widely reported in Refs. [21e24], the Cr2O3 template was strongly conducive to the formation of the a-Al2O3 phase through acting as a nucleation layer and a well crystallized Al2O3 film was achieved at a deposition temperature as low as 400 C. This could be a promising candidate for deposition crystalline alumina compared with CVD method. Resputtering technique with high bias voltage was also used to improve the crystallization of alumina film during preparation at low temperature. Zhang et al. [12] deposited the g-Al2O3 films using 800 V bias voltage at 300 C and found that good optical properties with higher refractive index compared with the amorphous films. However, the ion energy of high bias voltage was hard to control and would be easy to destroy the film except the low deposition rate influenced by resputtering effect. The basic work using ion assisted deposition in crystalline alumina film were studied in detail by Schneider et al. and Helmersson et al. [25,26]. An inductively coupled RF discharge was used to effectively ionize not only the Ar but also more important Al and O2. It demonstrated that the ion energy and ion flux had a significant impact on the structure and properties of deposited k-Al2O3 films. Also, the crystalline g-Al2O3 could be obtained by using ion beam sputtering deposition solely with oxygen ion beam assistance device at more than 500 C [27]. Their deposition systems with complex equipment and these methods are also not suitable for large-scale industrial application. Otherwise, annealing is another approach for crystallizing amorphous Al2O3 films. However, this approach has limitations as; a) while annealing several cracks or voids are generated which destroy its practical application potential, b) it is time consuming process, and c) there are considerable changes in phases. In this work, crystalline Al2O3 thin films were deposited by an anode layer ion source assisted bipolar pulse reactive magnetron sputtering method at 300 C. Due to a low temperature and low bias voltage (for this method), this method of deposition eliminates the requirement of heat sensitivity of the substrate. In order to find the appropriate synthesis parameters for synthesizing crystalline Al2O3 films, ion source power were varied and the change of surface morphology were initially confirmed. The crystallinity of the Al2O3 films were identified by grazing incidence X-ray diffraction (XRD) patterns and High-resolution transmission electron microscopy (HRTEM). The hardness and compressive stress properties of the Al2O3 films were also examined and reported. 2. Experiment details Al2O3 thin films were deposited by an anode layer ion sourceassisted (530 102 mm2) bipolar pulse reactive magnetron sputtering method using a laboratory-scale PVD system (F760 700 mm2) (Fig. 1) in this work. The sputtering power supply is Bi-polar pulsed DC (Bipolar 4020, Huttinger, Germany). The anode of the ion source is driven by a pulsed DC power (PDC mode) supply (Pinnacle Plus, AE, USA) and the cathode is grounded. It can ionize the reaction/sputtering gases and enhance the activity of these gases. As to this experiment, the argon ions were obtained from the ion source with different powers. This system is equipped with four arc evaporation sources, a high power pulse magnetron sputtering source, four cylindrical heaters, a planetary table for the substrate, double, unbalanced, magnetron sputtering columnar/ plane targets and a feedback system for plasma emission monitoring. The system is also equipped with a separated anode layer ion source device so that the setup has the capability to deposit insulation films and to improve the ionization rate of the metal. The
285
Fig. 1. Schematic diagram of the deposition chamber.
single crystalline Si (111) (30 5 0.6 mm3) wafers (for XRD, XPS, SEM, TEM and stress tests) and cemented carbide (10 10 4 mm3) (for hardness test) were used for substrates. The size of the Al target (purity 99.99 at. %) was 443 69 8 mm3. Prior deposition, all the substrates were cleaned ultrasonically in acetone and alcohol for 15 min each and then were dried in air. The substrates were fixed on the holder facing the sputtering target with a target-substrate distance of about 9 cm and a single fold rotation at a speed of 3 rpm. The ion source faces to the substrate at an angle of 30 . The chamber was firstly evacuated to a base pressure of lower than 9 104 Pa at temperature of 450 C, then cool down to deposition temperature i.e. 300 C and maintain two hours. High purity argon (99.999%) was used as sputtering gas through the ion source to set at different powers and high purity oxygen (99.99%) was used as reactive gas through feedback system in whole deposition process. The deposition pressure was kept at 7 101 Pa. For simplicity, the films synthesized with various ion source powers of 0, 1.0, 1.5 and 2 kW were labeled as A, B, C and D in this work, respectively. The 1000 V bias voltage (PDC mode) was applied for 10 min to the rotating stage to further remove any contamination and oxidation on the substrate surfaces. Then 90 min time was used while depositing all Al2O3 films. The detailed parameters used in deposition process are listed in Table 1. The structure of the as-deposited and power-mediated Al2O3 films were measured by using grazing incidence X-ray diffraction (XRD, D8 Advance Brucker) with Cu Ka radiation (l ¼ 0.154 nm) at 40 kV and 40 mA. The scanning diffraction angle 2q was recorded in the range 30 e70 with a 0.02 step size. The plane-view and the
Table 1 Deposition parameters of alumina thin films. Parameters
Value
Base pressure (Pa) Working pressure (Pa) Substrate temperature ( C) Target to substrate distance (mm) Target power (kW) Duty cycle (%) Ion source power (kW) Substrate bias voltage (V) Working gas (Sccm) Autorotation speed (rpm) Deposition time (min) Thickness (nm)
<9.0 104 0.7 300 90 6 60 0, 1, 1.5, 2 100 Ar (90), O2 (17) 3 90 342, 423, 432, 801
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cross-sectional surface images were recorded by using a scanning electron microscope (SEM, Hitachi, S3400N). High-resolution transmission electron microscopy of the films was performed on FEI TecnaiG2 F20S-Twin microscope with a 200 kV acceleration voltage. The TEM specimen was prepared by mechanical polishing and precision ion polishing system (Gatan PIPS691). The composition and chemical bonds of Al2O3 were analyzed using the X-ray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250Xi, USA) with an Al Ka source (hv ¼ 1486.6 eV). The XPS spectra were obtained after removing the surface contamination layer of the samples by using Arþ ions (3 keV) etching for 200 s (about 10 nm), and were calibrated by carbon peak C1s at 284.6 eV, then the detective analysis depth was about 10 nm. The hardness of the films was measured by using a microhardness tester (HXD-100000TM/ LCD). For obtaining the more precise experiment values, we made ten indentations on each sample at random locations and then the hardness values at the average of the ten measurements were calculated. The residual stress in the films were evaluated by using the Film Stress Analyzer (FST-150, SuPro Instruments, China) according to substrate curvature method based on Stoney's equation [28] at varying ion source powers.
3. Results and discussion 3.1. Hysteresis and deposition rate Fig. 2a shows the variation of the target voltage with increase of the oxygen flow rate into the deposition chamber. Typical hysteresis behavior can be understood when sputtering from a plane metal aluminum-target occurred. During deposition measurement period, the argon flow rate was kept constantly at 90 sccm (standard cubic centimeter per minute) through ion source ionized into the chamber. As the reactive gas was introduced, such as under the 15 sccm of the oxygen flow rate, the deposition rate was high and the obtained film was sub-stoichiometric composition with lack of oxygen. That's ascribe to the oxygen content was less and not sufficient to form a stable oxide layer on the target surface, which causing the target surface etching area of Al2O3 layer growth rate was less than the target reaction sputtering rate and then the metal aluminum was the major material from sputtering target. Otherwise, the target voltage was relatively high and kept around at 500 V. All of these phenomena were belong to the characteristic of metallic deposition stage, which were consistent with reports of Kubart et al. and Wallin et al. [29,30]. At the range of 15e20 sccm, due to the lower sputtering yield of Al2O3 as compared to metallic Al and the fact that compounds have higher secondary electron emission yield than pure metals, the rate of Al2O3 formation was
slightly exceeded the deposition rate as the oxygen flow was increased and the dropped target voltage, thereby, the whole transition process was identified as an oxidic deposition [31]. When the oxygen flow rate was further increased over 20 sccm, a certain thickness insulating layer of Al2O3 was formed on target surface, leading to a steeper decrease of the target voltage (from 520 V to 345 V) and then nearly constant, which imply the target poisoning effect was occurring [7,32]. We were not found any arcing effect because of the deposition of insulating layer of Al2O3 during the entire deposition run process which lasted for 90 min. Deposition rate was nearly 4 nm/min (Fig. 2b) without ion source assisted deposition. With added ion source power, the deposition rate was initially increased slightly. When the ion source power increasing to 2 kW, the deposition rate of Al2O3 film was remarkably increased compare with A to C series of samples. There are two possible reasons could be explaining the deposition rate. One is more argon gas were ionized to Arþ carrying high energy through ion source. Due to the high sputtering capability of argon ions compared with oxygen ions, the emitting atoms and ions from the Al target can be greatly increased by sputtering effect with increasing ion source power; the other one is not only improving the ionization rate of metal atoms in the plasma with increasing power but also the energetic Arþ ions arriving to the surface of substrate, increasing the mobility of adatoms, which are also contributed to the film growth [33].
3.2. Chemical composition and phase structure In order to determine Al2O3 film electronic structure and elemental composition ratio, the typical XPS spectra were obtained for A and B samples. The XPS spectra of O1s and Al2p peak are shown in Fig. 3 (a, b). It was evident from these figures that the each spectrum was demonstrating a single peak, ascribed to AleO bonds of Al2O3 phase, centered around 74.9 eV and 531.5 eV, respectively [34,35]. The absence of shoulder region around 72.5 eV, and 532.5 eV, for AleAl bonds of Al2O3 and chemisorbed oxygen, were not seen, confirming the aluminum in the film was completely oxidized into Al2O3 [34,36]. The 1.5 atomic ratio of oxygen to aluminum in the deposited Al2O3 film were estimated from the total amounts of oxygen and aluminum presenting in Al2O3 and were summarized in Fig. 3c. When the ion source power increased to 2.0 kW, the O/Al ratio value was slightly increased compared to other powers of the ion source. It was attributed to the redundant oxygen ions were adsorbed on the surface of growing film by subsequent ion bombardment injecting into the film, thus, the oxygen content was slight increased. Fig. 4(aed) shows the XRD spectra of Al2O3 films obtained for
Fig. 2. (a) Variation of Al-target voltages with increasing oxygen flow rate, and (b) the relation between deposition rate and ion source power (A, B, C, and D are 0, 1.0, 1.5, and 2.0 ion source powers).
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Fig. 3. (a, b) O1s and Al2p XPS spectra of the Al2O3 films deposited at 0 and 1.0 kW ion source power i.e. A and B. (c) O/Al ratio values of Al2O3 films obtained for 0, 1.0, 1.5 and 2.0 ion source powers.
Fig. 4. (a) The XRD spectra of Al2O3 films obtained for 0, 1.0, 1.5, and 2.0 ion source powers. (b, c, and d) TEM, HR-TEM and SAED pattern images of Al2O3 film deposited at 1.0 kW of ion source power i.e. B part of (a).
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three ion sources powers viz. 1, 1.5 and 2 kW in addition to the pristine i.e. free from the ion source power. The XRD results revealed that the Al2O3 film (sample A) was amorphous phase structure at 300 C without ion source assisted deposition. A seen in the XRD patterns of B, C and D samples i.e. Al2O3 films were deposited at 1.0, 1.5 and 2.0 kW and at 300 C demonstrated that diffraction peaks were at 39.5 , 45.8 and 66.3 , which were assigned to (222), (400) and (440) plane phases of g-Al2O3, respectively [7,37]. Actually the diffraction angles in this work were all negatively deviated compare with the standard Al2O3 phase according to the phase card (JCPDF 29-0063). This implies that compress stress exists in the films which is also evidenced in section 3.4. Interestingly, the loss of intensities and line broadening of signals for films meaning that the plane phase preferred orientation weakening and grain refinement were simultaneously found at higher ion source power (2 kW) compared with 1.5 and 1 kW conditions. Otherwise, the higher ion source power might destroy the structure of Al2O3 film by diffusing ions from lattice host to guest places through introducing stacking faults/defects which eventually create cracks/voids in films, consistent to surface morphology results. Fig. 4b,c presents the TEM images of Al2O3 film synthesized in 1.0 kW ion source power at two different magnifications which verified that was crystalline structure in agreement with former XRD result. Al2O3 film containing agglomerated-type surface which was polycrystalline and the grains of irregular dimensions were also clearly seen in high resolution image. The lattice fringes were also clearly seen in Fig. 4b,c. The lattice spacing value was 0.228 nm, which was belong to (222) plane of g-Al2O3 [38]. The selected area diffraction patterns (SADP) (Fig. 4d) showed almost a pattern of continuous diffraction rings instead of diffuse halo, indicating an involvement of nanocrystalline microstructure with the small grain size of film. Reflection peaks such as (222) and (440) (JCPDF 29-0063) were confirmed through measuring the
diameters of circular rings obtained in Fig. 4d. The typical TEM conclusion combined with the XRD result of sample revealed that the nanocrystalline g-Al2O3 films were obtained by altering the ion source power. Khanna et al. [7] obtained g-Al2O3 through inverted cylindrical magnetron sputtering method without external heating but the target surface ion current density used in deposition process system was 16 mA/cm2, while in this paper the parameter was only about 6.3 mA/cm2. Engelhart et al. [38] reported the deposition of g-Al2O3 coatings on cemented carbide substrate by dual magnetron sputtering method at 550 C deposition temperature. We were successful in synthesizing crystalline g-Al2O3 films at 300 C temperature (which was lower than other literature reports) just by varying ion source power. This was mainly due to the ion bombardment during the unique ion source-assisted step which supplied energy to the film surface, compensating for the energy lost by lowering the temperature. The ion bombardment during the ion source-assisted step removed the loosely bounded atoms on the surface by knocking few atoms nearby to the surface into the regular atomic sites of Al2O3 crystal, resulting in the formation of polycrystalline Al2O3 film. 3.3. Surface morphology The plane-view SEM images were used to confirm surface morphologies of with and without ion source power-mediated Al2O3 films i.e. 1.0, 1.5 and 2 kW (Fig. 5 aed). Interestingly, surface of Al2O3 sample without ion source power was smooth and featureless, which was the typical characteristic of amorphous phase structure, indicating in spite of 300 C temperature, there was no crystal growth on the surface of an amorphous alumina substrate (Fig. 5a). The granular sizes of Al2O3 were varied from 0.1 to 0.8 mm with the ion power source power were increased from 1.0 to 2.0 kW. The result revealed that the small grains could
Fig. 5. (aed) The plane-view surface SEM images of Al2O3 films obtained for 0, 1.0, 1.5 and 2.0 kW ion source powers.
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Fig. 6. Variation of; (a) micro-hardness value, and (b) compressive stress as a function of ion source power in A, B, C and D Al2O3 films.
agglomerate to form relatively bigger cauliflower-like morphology. The surface of Al2O3 film deposited at 1.0 kW (Fig. 5b) was smooth, uniform, crack/void-free and compact with an average granular size of 0.2 mm, which was bigger than those films obtained at 1.5 and 2.0 kW. The surface of Al2O3 film obtained at 1.5 kW (Fig. 5c) was equally compact but with higher average granular size i.e. 0.4 mm than previous one. When the ion source power was increased to 2.0 kW (Fig. 5d), in addition to cracks and voids, the surface of Al2O3 film was rough and average granular size was increased to 0.8 mm. This variation could be attributed to two factors: one is the bombardment of the ionized Arþ onto substrate from ion source is known to impart extra energy to the growing film and prompt small grains agglomerated into big particles and the other one is the increase of ion source power leads to increase the temperature in growing film, which eventually results in the inhomogeneous distribution of the stress, leading to develop cracks/voids on the surface of the film [9,39]. This study revealed that the ion sourceassisted deposition technique has a significant impact on the surface morphology of the Al2O3 film, showing this method depositing metal oxide film is a promising for protective film in engineering application. 3.4. Micro-hardness and stress measurements Micro-hardness testing was calculated to examine the hardness of the Al2O3 films deposited by pulse reactive magnetron sputtering with different ion source powers. Fig. 6a shows the hardness values of all Al2O3 films i.e. A, B, C and D. Sample A, an amorphous Al2O3 film, showed the hardness of 7.5 GPa, which was lower than the hardness values of other crystalline Al2O3 films, suggesting that the ion source-assisted deposition could favor of the formation of crystalline Al2O3, so the hardness values of B, C and D samples were higher than that of the sample A. However, all crystalline Al2O3 films exhibited nearly the same hardness in the range of 12.3e13.4 GPa and the highest value of 13.4 GPa was obtained for sample prepared at 1.5 kW of ion source power. The increase of the film hardness was mainly caused by the variation of phase structure, which changed from amorphous to crystalline structure. With applying the ion source power, the crystalline structure appeared and maintained stabilization, therefore, the films hardness of crystalline structure were not dramatically changed. This tendency is in good agreement with the reference [40]. Fig. 6b shows the variation of residual compressive stress vs. ion source power for Al2O3 films obtained for 0, 1.0, 1.5 and 2.0 kW. The stress value was slight increased, that's ascribed to the energetic ion bombardment and the changed of the film microstructure with the increasing ion source power [41]. Generally, the compressive stress is conducive to the hardness enhancement, nevertheless, due to the stress value is
small and does not significant impact on the hardness improvement in this work. The phase structure changing is the main reason for higher hardness compare with the amorphous phase film. 4. Conclusions The ion source-assisted pulse reactive magnetron sputtering deposition with various powers i.e. 0, 1.0, 1.5 and 2.0 kW has been used to fabricate the crystalline g-Al2O3 films at low temperature and low bias voltage conditions. The various deposition rates were obtained by controlling the ion source power under the oxidic mode and the maximum value is nearly 9 nm/min at 2 kW ion source power. The surface morphology of samples was changed from smooth, featureless to compact, agglomerate in cauliflowerlike shape with increasing ion source power. The XRD result and typical sample through TEM analysis revealed that the nanocrystalline g-Al2O3 films were obtained by altering the ion source power. The hardness values of crystalline Al2O3 films prepared with various ion source powers were higher and nearly kept constant than that prepared without ion source assistance, implying that the phase structure changing was the main reason for hardness enhancement even though the compressive stress were also slightly increased in the deposition process. The synthesis approach of crystalline Al2O3 films in this work is promising for large-scale industrial applications. Acknowledgments The work was supported by the NSFC project (51522502) and Guangdong Natural Science Fund (2016A050502056) and the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6B1078874). References [1] J. Benick, B. Hoex, M.C.M. van de Sanden, et al., Appl. Math. Lett. 92 (2008) 2535041e2535043. [2] M. Serenyi, T. Lohner, G. Safran, J. Szivos, Vacuum 128 (2016) 213e218. n, M.L. Danielle, et al., J. Solid State [3] L. Samain, Aleksander Jaworski, Mattias Ede Chem. 217 (2014) 1e8. [4] M. Trueba, S.P. Trasatti, Eur. J. Inorg. Chem. 17 (2005) 3393e3403. [5] A. Senthil Kumar, A. Raja Durai, T. Sornakumar, J. Mater. Process. Technol. 173 (2006) 151e156. [6] A. Senthil Kumar, A. Raja Durai, T. Sornakumar, Tribol. Int. 39 (2006) 191e197. [7] A. Khanna, D.G. Bhat, Surf. Coating. Technol. 201 (2006) 168e173. [8] R. Snyders, K. Jiang, D. Music, S. Konstantinidis, et al., Surf. Coating. Technol. 204 (2009) 215e221. [9] V. Edlmayr, M. Moser, C. Walter, C. Mitterer, Surf. Coating. Technol. 204 (2010) 1576e1581.
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Low-temperature deposition of nanocrystalline Al2O3 films by ion source-assisted magnetron sputtering Ji Cheng Ding a, Teng Fei Zhang a, Rajaram S. Mane a, Kwang-Ho Kim a, Myung Chang Kang a, Chang Wei Zou b, *, Qi Min Wang b, ** a b
Global Frontier R&D Center for Hybrid Interface Materials, Pusan National University, Busan, 609-735, South Korea School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou, 510006, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2017 Received in revised form 5 January 2018 Accepted 5 January 2018 Available online 6 January 2018
In this paper, bipolar pulse reactive magnetron sputtering with ion source assisted deposition method has been utilized for the deposition of nanocrystalline alumina (Al2O3) films at the temperature of 300 C onto silicon (111) wafers, and cemented carbide substrates. The influence of ion source power, i.e. 0, 1.0, 1.5 and 2.0 kW on the structure, morphology, compressive stress and mechanical properties of the Al2O3 films were investigated. In absence of ion source power assistance, an amorphous Al2O3 film was dominant at deposition temperature of 300 C. With ion source assisted deposition, the crystalline gAl2O3 films were obtained at the same conditions, suggesting an importance of ion source power in crystallinity development of metal oxide films obtained from magnetron sputtering deposition method. Images of surface morphology clearly demonstrated the difference in granular sizes of film surfaces prepared with and without ion source powers. With increasing ion source power from 1.0 to 2.0 kW, the micro-hardness and compressive stress of the films were increased from 7 GPa to 13 GPa and 0.3 GPa to 1.1 GPa, respectively. Results revealed that the reactive magnetron sputtering with the ion source assisted deposition was a simple and effective way to prepare nanocrystalline g-Al2O3 films at low temperature. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Low temperature deposition Nanocrystalline g-Al2O3 film Microstructure Hardness
1. Introduction Alumina (Al2O3) thin films have aroused increasing attention of the scientists in last decades because of their applications in surface passivation on crystalline silicon wafers [1], as high quality waveguiding material [2] or to be an important catalyst in the petroleum industries [3,4] etc. Also, due to the considerable hardness and thermal stability at high temperatures (1000 C), it provides an excellent high-temperature abrasion protection for coated tool during the process of high speed machining [5,6]. The high resistance to radiation of the Al2O3 thin film has made it an ideal choice for application in harsh environment, and also its application in catalysis due to their large surface area [3]. The Al2O3 thin films have been previously synthesized by various methods like pulse DC/RF reactive magnetron sputtering, pulsed dc magnetron sputtering, multi-arc ion plating, electron beam evaporation, pulsed
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C.W. Zou), [email protected] (Q.M. Wang). https://doi.org/10.1016/j.vacuum.2018.01.009 0042-207X/© 2018 Elsevier Ltd. All rights reserved.
plasma-assisted magnetron sputtering, and ion beam sputtering etc. [2,7e14] which commonly are known as physical vapor deposition (PVD) technology. However, depositing crystalline Al2O3 thin films by using PVD method at high deposition rate are usually difficult, especially, the as-deposited films are often presented an amorphous structure at a low deposition temperature [15]. It is quite general that the crystalline Al2O3 film offers higher density, hardness and also chemical and thermal resistance than its corresponding amorphous counterpart [16]. To obtain a-alumina films, chemical vapor deposition (CVD) technology is commonly used in industrial fabrications [16,17]. The substrate temperatures are normally above 1000 C, which prohibits the use of many temperature sensitive substrate materials, e.g., glass materials and tool steel. Due to it has limited potential for commercial scale, applying PVD deposition to grow crystalline alumina at low temperature is desired to be studied [18,19]. As on today, PVD technology with low-temperature and high deposition rate is considered to be the most important practical method to deposit crystalline Al2O3 films. Many methods used for obtaining crystalline Al2O3 film by virtue of the PVD technology at low deposition temperatures have been reported. Due to the a-
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Cr2O3 is isostructural with a-Al2O3 and relatively easy to deposit at low temperature [20], so the growth of a-Al2O3 films using PVD technology through a crystallographic template a-Cr2O3 is an effective approach, which has been widely reported in Refs. [21e24], the Cr2O3 template was strongly conducive to the formation of the a-Al2O3 phase through acting as a nucleation layer and a well crystallized Al2O3 film was achieved at a deposition temperature as low as 400 C. This could be a promising candidate for deposition crystalline alumina compared with CVD method. Resputtering technique with high bias voltage was also used to improve the crystallization of alumina film during preparation at low temperature. Zhang et al. [12] deposited the g-Al2O3 films using 800 V bias voltage at 300 C and found that good optical properties with higher refractive index compared with the amorphous films. However, the ion energy of high bias voltage was hard to control and would be easy to destroy the film except the low deposition rate influenced by resputtering effect. The basic work using ion assisted deposition in crystalline alumina film were studied in detail by Schneider et al. and Helmersson et al. [25,26]. An inductively coupled RF discharge was used to effectively ionize not only the Ar but also more important Al and O2. It demonstrated that the ion energy and ion flux had a significant impact on the structure and properties of deposited k-Al2O3 films. Also, the crystalline g-Al2O3 could be obtained by using ion beam sputtering deposition solely with oxygen ion beam assistance device at more than 500 C [27]. Their deposition systems with complex equipment and these methods are also not suitable for large-scale industrial application. Otherwise, annealing is another approach for crystallizing amorphous Al2O3 films. However, this approach has limitations as; a) while annealing several cracks or voids are generated which destroy its practical application potential, b) it is time consuming process, and c) there are considerable changes in phases. In this work, crystalline Al2O3 thin films were deposited by an anode layer ion source assisted bipolar pulse reactive magnetron sputtering method at 300 C. Due to a low temperature and low bias voltage (for this method), this method of deposition eliminates the requirement of heat sensitivity of the substrate. In order to find the appropriate synthesis parameters for synthesizing crystalline Al2O3 films, ion source power were varied and the change of surface morphology were initially confirmed. The crystallinity of the Al2O3 films were identified by grazing incidence X-ray diffraction (XRD) patterns and High-resolution transmission electron microscopy (HRTEM). The hardness and compressive stress properties of the Al2O3 films were also examined and reported. 2. Experiment details Al2O3 thin films were deposited by an anode layer ion sourceassisted (530 102 mm2) bipolar pulse reactive magnetron sputtering method using a laboratory-scale PVD system (F760 700 mm2) (Fig. 1) in this work. The sputtering power supply is Bi-polar pulsed DC (Bipolar 4020, Huttinger, Germany). The anode of the ion source is driven by a pulsed DC power (PDC mode) supply (Pinnacle Plus, AE, USA) and the cathode is grounded. It can ionize the reaction/sputtering gases and enhance the activity of these gases. As to this experiment, the argon ions were obtained from the ion source with different powers. This system is equipped with four arc evaporation sources, a high power pulse magnetron sputtering source, four cylindrical heaters, a planetary table for the substrate, double, unbalanced, magnetron sputtering columnar/ plane targets and a feedback system for plasma emission monitoring. The system is also equipped with a separated anode layer ion source device so that the setup has the capability to deposit insulation films and to improve the ionization rate of the metal. The
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Fig. 1. Schematic diagram of the deposition chamber.
single crystalline Si (111) (30 5 0.6 mm3) wafers (for XRD, XPS, SEM, TEM and stress tests) and cemented carbide (10 10 4 mm3) (for hardness test) were used for substrates. The size of the Al target (purity 99.99 at. %) was 443 69 8 mm3. Prior deposition, all the substrates were cleaned ultrasonically in acetone and alcohol for 15 min each and then were dried in air. The substrates were fixed on the holder facing the sputtering target with a target-substrate distance of about 9 cm and a single fold rotation at a speed of 3 rpm. The ion source faces to the substrate at an angle of 30 . The chamber was firstly evacuated to a base pressure of lower than 9 104 Pa at temperature of 450 C, then cool down to deposition temperature i.e. 300 C and maintain two hours. High purity argon (99.999%) was used as sputtering gas through the ion source to set at different powers and high purity oxygen (99.99%) was used as reactive gas through feedback system in whole deposition process. The deposition pressure was kept at 7 101 Pa. For simplicity, the films synthesized with various ion source powers of 0, 1.0, 1.5 and 2 kW were labeled as A, B, C and D in this work, respectively. The 1000 V bias voltage (PDC mode) was applied for 10 min to the rotating stage to further remove any contamination and oxidation on the substrate surfaces. Then 90 min time was used while depositing all Al2O3 films. The detailed parameters used in deposition process are listed in Table 1. The structure of the as-deposited and power-mediated Al2O3 films were measured by using grazing incidence X-ray diffraction (XRD, D8 Advance Brucker) with Cu Ka radiation (l ¼ 0.154 nm) at 40 kV and 40 mA. The scanning diffraction angle 2q was recorded in the range 30 e70 with a 0.02 step size. The plane-view and the
Table 1 Deposition parameters of alumina thin films. Parameters
Value
Base pressure (Pa) Working pressure (Pa) Substrate temperature ( C) Target to substrate distance (mm) Target power (kW) Duty cycle (%) Ion source power (kW) Substrate bias voltage (V) Working gas (Sccm) Autorotation speed (rpm) Deposition time (min) Thickness (nm)
<9.0 104 0.7 300 90 6 60 0, 1, 1.5, 2 100 Ar (90), O2 (17) 3 90 342, 423, 432, 801
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cross-sectional surface images were recorded by using a scanning electron microscope (SEM, Hitachi, S3400N). High-resolution transmission electron microscopy of the films was performed on FEI TecnaiG2 F20S-Twin microscope with a 200 kV acceleration voltage. The TEM specimen was prepared by mechanical polishing and precision ion polishing system (Gatan PIPS691). The composition and chemical bonds of Al2O3 were analyzed using the X-ray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250Xi, USA) with an Al Ka source (hv ¼ 1486.6 eV). The XPS spectra were obtained after removing the surface contamination layer of the samples by using Arþ ions (3 keV) etching for 200 s (about 10 nm), and were calibrated by carbon peak C1s at 284.6 eV, then the detective analysis depth was about 10 nm. The hardness of the films was measured by using a microhardness tester (HXD-100000TM/ LCD). For obtaining the more precise experiment values, we made ten indentations on each sample at random locations and then the hardness values at the average of the ten measurements were calculated. The residual stress in the films were evaluated by using the Film Stress Analyzer (FST-150, SuPro Instruments, China) according to substrate curvature method based on Stoney's equation [28] at varying ion source powers.
3. Results and discussion 3.1. Hysteresis and deposition rate Fig. 2a shows the variation of the target voltage with increase of the oxygen flow rate into the deposition chamber. Typical hysteresis behavior can be understood when sputtering from a plane metal aluminum-target occurred. During deposition measurement period, the argon flow rate was kept constantly at 90 sccm (standard cubic centimeter per minute) through ion source ionized into the chamber. As the reactive gas was introduced, such as under the 15 sccm of the oxygen flow rate, the deposition rate was high and the obtained film was sub-stoichiometric composition with lack of oxygen. That's ascribe to the oxygen content was less and not sufficient to form a stable oxide layer on the target surface, which causing the target surface etching area of Al2O3 layer growth rate was less than the target reaction sputtering rate and then the metal aluminum was the major material from sputtering target. Otherwise, the target voltage was relatively high and kept around at 500 V. All of these phenomena were belong to the characteristic of metallic deposition stage, which were consistent with reports of Kubart et al. and Wallin et al. [29,30]. At the range of 15e20 sccm, due to the lower sputtering yield of Al2O3 as compared to metallic Al and the fact that compounds have higher secondary electron emission yield than pure metals, the rate of Al2O3 formation was
slightly exceeded the deposition rate as the oxygen flow was increased and the dropped target voltage, thereby, the whole transition process was identified as an oxidic deposition [31]. When the oxygen flow rate was further increased over 20 sccm, a certain thickness insulating layer of Al2O3 was formed on target surface, leading to a steeper decrease of the target voltage (from 520 V to 345 V) and then nearly constant, which imply the target poisoning effect was occurring [7,32]. We were not found any arcing effect because of the deposition of insulating layer of Al2O3 during the entire deposition run process which lasted for 90 min. Deposition rate was nearly 4 nm/min (Fig. 2b) without ion source assisted deposition. With added ion source power, the deposition rate was initially increased slightly. When the ion source power increasing to 2 kW, the deposition rate of Al2O3 film was remarkably increased compare with A to C series of samples. There are two possible reasons could be explaining the deposition rate. One is more argon gas were ionized to Arþ carrying high energy through ion source. Due to the high sputtering capability of argon ions compared with oxygen ions, the emitting atoms and ions from the Al target can be greatly increased by sputtering effect with increasing ion source power; the other one is not only improving the ionization rate of metal atoms in the plasma with increasing power but also the energetic Arþ ions arriving to the surface of substrate, increasing the mobility of adatoms, which are also contributed to the film growth [33].
3.2. Chemical composition and phase structure In order to determine Al2O3 film electronic structure and elemental composition ratio, the typical XPS spectra were obtained for A and B samples. The XPS spectra of O1s and Al2p peak are shown in Fig. 3 (a, b). It was evident from these figures that the each spectrum was demonstrating a single peak, ascribed to AleO bonds of Al2O3 phase, centered around 74.9 eV and 531.5 eV, respectively [34,35]. The absence of shoulder region around 72.5 eV, and 532.5 eV, for AleAl bonds of Al2O3 and chemisorbed oxygen, were not seen, confirming the aluminum in the film was completely oxidized into Al2O3 [34,36]. The 1.5 atomic ratio of oxygen to aluminum in the deposited Al2O3 film were estimated from the total amounts of oxygen and aluminum presenting in Al2O3 and were summarized in Fig. 3c. When the ion source power increased to 2.0 kW, the O/Al ratio value was slightly increased compared to other powers of the ion source. It was attributed to the redundant oxygen ions were adsorbed on the surface of growing film by subsequent ion bombardment injecting into the film, thus, the oxygen content was slight increased. Fig. 4(aed) shows the XRD spectra of Al2O3 films obtained for
Fig. 2. (a) Variation of Al-target voltages with increasing oxygen flow rate, and (b) the relation between deposition rate and ion source power (A, B, C, and D are 0, 1.0, 1.5, and 2.0 ion source powers).
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Fig. 3. (a, b) O1s and Al2p XPS spectra of the Al2O3 films deposited at 0 and 1.0 kW ion source power i.e. A and B. (c) O/Al ratio values of Al2O3 films obtained for 0, 1.0, 1.5 and 2.0 ion source powers.
Fig. 4. (a) The XRD spectra of Al2O3 films obtained for 0, 1.0, 1.5, and 2.0 ion source powers. (b, c, and d) TEM, HR-TEM and SAED pattern images of Al2O3 film deposited at 1.0 kW of ion source power i.e. B part of (a).
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three ion sources powers viz. 1, 1.5 and 2 kW in addition to the pristine i.e. free from the ion source power. The XRD results revealed that the Al2O3 film (sample A) was amorphous phase structure at 300 C without ion source assisted deposition. A seen in the XRD patterns of B, C and D samples i.e. Al2O3 films were deposited at 1.0, 1.5 and 2.0 kW and at 300 C demonstrated that diffraction peaks were at 39.5 , 45.8 and 66.3 , which were assigned to (222), (400) and (440) plane phases of g-Al2O3, respectively [7,37]. Actually the diffraction angles in this work were all negatively deviated compare with the standard Al2O3 phase according to the phase card (JCPDF 29-0063). This implies that compress stress exists in the films which is also evidenced in section 3.4. Interestingly, the loss of intensities and line broadening of signals for films meaning that the plane phase preferred orientation weakening and grain refinement were simultaneously found at higher ion source power (2 kW) compared with 1.5 and 1 kW conditions. Otherwise, the higher ion source power might destroy the structure of Al2O3 film by diffusing ions from lattice host to guest places through introducing stacking faults/defects which eventually create cracks/voids in films, consistent to surface morphology results. Fig. 4b,c presents the TEM images of Al2O3 film synthesized in 1.0 kW ion source power at two different magnifications which verified that was crystalline structure in agreement with former XRD result. Al2O3 film containing agglomerated-type surface which was polycrystalline and the grains of irregular dimensions were also clearly seen in high resolution image. The lattice fringes were also clearly seen in Fig. 4b,c. The lattice spacing value was 0.228 nm, which was belong to (222) plane of g-Al2O3 [38]. The selected area diffraction patterns (SADP) (Fig. 4d) showed almost a pattern of continuous diffraction rings instead of diffuse halo, indicating an involvement of nanocrystalline microstructure with the small grain size of film. Reflection peaks such as (222) and (440) (JCPDF 29-0063) were confirmed through measuring the
diameters of circular rings obtained in Fig. 4d. The typical TEM conclusion combined with the XRD result of sample revealed that the nanocrystalline g-Al2O3 films were obtained by altering the ion source power. Khanna et al. [7] obtained g-Al2O3 through inverted cylindrical magnetron sputtering method without external heating but the target surface ion current density used in deposition process system was 16 mA/cm2, while in this paper the parameter was only about 6.3 mA/cm2. Engelhart et al. [38] reported the deposition of g-Al2O3 coatings on cemented carbide substrate by dual magnetron sputtering method at 550 C deposition temperature. We were successful in synthesizing crystalline g-Al2O3 films at 300 C temperature (which was lower than other literature reports) just by varying ion source power. This was mainly due to the ion bombardment during the unique ion source-assisted step which supplied energy to the film surface, compensating for the energy lost by lowering the temperature. The ion bombardment during the ion source-assisted step removed the loosely bounded atoms on the surface by knocking few atoms nearby to the surface into the regular atomic sites of Al2O3 crystal, resulting in the formation of polycrystalline Al2O3 film. 3.3. Surface morphology The plane-view SEM images were used to confirm surface morphologies of with and without ion source power-mediated Al2O3 films i.e. 1.0, 1.5 and 2 kW (Fig. 5 aed). Interestingly, surface of Al2O3 sample without ion source power was smooth and featureless, which was the typical characteristic of amorphous phase structure, indicating in spite of 300 C temperature, there was no crystal growth on the surface of an amorphous alumina substrate (Fig. 5a). The granular sizes of Al2O3 were varied from 0.1 to 0.8 mm with the ion power source power were increased from 1.0 to 2.0 kW. The result revealed that the small grains could
Fig. 5. (aed) The plane-view surface SEM images of Al2O3 films obtained for 0, 1.0, 1.5 and 2.0 kW ion source powers.
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Fig. 6. Variation of; (a) micro-hardness value, and (b) compressive stress as a function of ion source power in A, B, C and D Al2O3 films.
agglomerate to form relatively bigger cauliflower-like morphology. The surface of Al2O3 film deposited at 1.0 kW (Fig. 5b) was smooth, uniform, crack/void-free and compact with an average granular size of 0.2 mm, which was bigger than those films obtained at 1.5 and 2.0 kW. The surface of Al2O3 film obtained at 1.5 kW (Fig. 5c) was equally compact but with higher average granular size i.e. 0.4 mm than previous one. When the ion source power was increased to 2.0 kW (Fig. 5d), in addition to cracks and voids, the surface of Al2O3 film was rough and average granular size was increased to 0.8 mm. This variation could be attributed to two factors: one is the bombardment of the ionized Arþ onto substrate from ion source is known to impart extra energy to the growing film and prompt small grains agglomerated into big particles and the other one is the increase of ion source power leads to increase the temperature in growing film, which eventually results in the inhomogeneous distribution of the stress, leading to develop cracks/voids on the surface of the film [9,39]. This study revealed that the ion sourceassisted deposition technique has a significant impact on the surface morphology of the Al2O3 film, showing this method depositing metal oxide film is a promising for protective film in engineering application. 3.4. Micro-hardness and stress measurements Micro-hardness testing was calculated to examine the hardness of the Al2O3 films deposited by pulse reactive magnetron sputtering with different ion source powers. Fig. 6a shows the hardness values of all Al2O3 films i.e. A, B, C and D. Sample A, an amorphous Al2O3 film, showed the hardness of 7.5 GPa, which was lower than the hardness values of other crystalline Al2O3 films, suggesting that the ion source-assisted deposition could favor of the formation of crystalline Al2O3, so the hardness values of B, C and D samples were higher than that of the sample A. However, all crystalline Al2O3 films exhibited nearly the same hardness in the range of 12.3e13.4 GPa and the highest value of 13.4 GPa was obtained for sample prepared at 1.5 kW of ion source power. The increase of the film hardness was mainly caused by the variation of phase structure, which changed from amorphous to crystalline structure. With applying the ion source power, the crystalline structure appeared and maintained stabilization, therefore, the films hardness of crystalline structure were not dramatically changed. This tendency is in good agreement with the reference [40]. Fig. 6b shows the variation of residual compressive stress vs. ion source power for Al2O3 films obtained for 0, 1.0, 1.5 and 2.0 kW. The stress value was slight increased, that's ascribed to the energetic ion bombardment and the changed of the film microstructure with the increasing ion source power [41]. Generally, the compressive stress is conducive to the hardness enhancement, nevertheless, due to the stress value is
small and does not significant impact on the hardness improvement in this work. The phase structure changing is the main reason for higher hardness compare with the amorphous phase film. 4. Conclusions The ion source-assisted pulse reactive magnetron sputtering deposition with various powers i.e. 0, 1.0, 1.5 and 2.0 kW has been used to fabricate the crystalline g-Al2O3 films at low temperature and low bias voltage conditions. The various deposition rates were obtained by controlling the ion source power under the oxidic mode and the maximum value is nearly 9 nm/min at 2 kW ion source power. The surface morphology of samples was changed from smooth, featureless to compact, agglomerate in cauliflowerlike shape with increasing ion source power. The XRD result and typical sample through TEM analysis revealed that the nanocrystalline g-Al2O3 films were obtained by altering the ion source power. The hardness values of crystalline Al2O3 films prepared with various ion source powers were higher and nearly kept constant than that prepared without ion source assistance, implying that the phase structure changing was the main reason for hardness enhancement even though the compressive stress were also slightly increased in the deposition process. The synthesis approach of crystalline Al2O3 films in this work is promising for large-scale industrial applications. Acknowledgments The work was supported by the NSFC project (51522502) and Guangdong Natural Science Fund (2016A050502056) and the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6B1078874). References [1] J. Benick, B. Hoex, M.C.M. van de Sanden, et al., Appl. Math. Lett. 92 (2008) 2535041e2535043. [2] M. Serenyi, T. Lohner, G. Safran, J. Szivos, Vacuum 128 (2016) 213e218. n, M.L. Danielle, et al., J. Solid State [3] L. Samain, Aleksander Jaworski, Mattias Ede Chem. 217 (2014) 1e8. [4] M. Trueba, S.P. Trasatti, Eur. J. Inorg. Chem. 17 (2005) 3393e3403. [5] A. Senthil Kumar, A. Raja Durai, T. Sornakumar, J. Mater. Process. Technol. 173 (2006) 151e156. [6] A. Senthil Kumar, A. Raja Durai, T. Sornakumar, Tribol. Int. 39 (2006) 191e197. [7] A. Khanna, D.G. Bhat, Surf. Coating. Technol. 201 (2006) 168e173. [8] R. Snyders, K. Jiang, D. Music, S. Konstantinidis, et al., Surf. Coating. Technol. 204 (2009) 215e221. [9] V. Edlmayr, M. Moser, C. Walter, C. Mitterer, Surf. Coating. Technol. 204 (2010) 1576e1581.
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