- Email: [email protected]
Preparation and tunable optical properties of ion beam sputtered SiAlON thin films
Vacuum 101 (2014) 1e5
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Preparation and tunable optical properties of ion beam sputtered SiAlON thin films Guanghui Liu a, *, Zhenzhen Zhou a, Qinhua Wei a, b, Fan Fei a, Hua Yang a, b, Qian Liu a a State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, China b Graduate University of the Chinese Academy of Science, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 April 2013 Received in revised form 7 July 2013 Accepted 9 July 2013
One kind of quaternary thin films of SiAlON were deposited by ion beam sputtering method at ambient temperature, through a sintered SiAlON ceramic target. Ultraviolet, visible and near infrared (UVeVIS eNIR) Spectrophotometer, Fourier transformed infrared spectroscopy (FTIR), glancing incidence X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS) were employed to characterize the optical, crystalline and chemical composition properties. The optical properties of the deposited amorphous films could be tailored by changing the relative content of O or N in the films, through introducing different working gas and gas flux adjusting (oxygen, nitrogen or their mixtures). By this way, the refractive index at 850 nm can be tailored from 1.53 to 1.83, and the chemical composition data from XPS analysis supplied strong evidences. Reasonably, the SiAlON films were thought to be one kind of candidate films either as a buffer layer benefit from their high temperature stability, or a mid infrared antireflective layer for silicon and other infrared window materials. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: SiAlON films Optical properties Antireflective layer Thermal stability
1. Introduction From the day it was discovered in 1970s, constant research work has been employed on silicon aluminum oxynitride (SiAlON) ceramics [1e4], for its excellent mechanical properties, thermal stability, and so on. The SiAlON ceramics are in nature a solid solution of Si3N4 by partially substitution of Al and O for Si and N, so, as a multi-element compound (quaternary), the properties of SiAlON ceramics can be adjusted and controlled following by the composition design and tuning. It could be found that the huge bygone research work about SiAlON ceramics was mainly on its improvement of mechanical and thermal shock resistivity, as well as the transmittance upgrading, served as translucent ceramics for mid-infrared (3e5 mm) windows [3e6]. Besides, when rare earth elements, such as Ce and Eu, etc, are incorporated, the doped SiAlON powder shows excellent photo luminescence properties for its multi color exhibition (blue, red, green and orange), which is very suitable for a white light-emitting diodes (LED) and as a display materials [7e9].
* Corresponding author. Tel./fax: þ86 2152412404. E-mail address: [email protected] (G. Liu). 0042-207X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2013.07.015
Although the SiAlON bulk materials exhibit outstanding mechanical, thermal and optical properties, less research work has been employed on SiAlON thin film materials by now [10e12], and even more rarely on the optical properties of the SiAlON thin films. Furthermore, compound films always exhibit adjustable properties especially when it is a quaternary one. So it is necessary to carry out some basic research work, aiming to prepare SiAlON thin films and get to know and optimize their properties, and finally find their application potentials. In this work, the authors successfully prepared SiAlON thin films through an ion beam sputtering process. The optical properties (in UVeVISeNIR), mainly the refractive index of the films, were carefully tuned by changing compositions of the samples, through adjusting working gas and gas flux (oxygen, nitrogen or their mixture) during deposition process. The relative content of Si, Al, O and N elements can be verified by the chemical composition data from XPS analysis results. The high transmittance value of the SiAlON/Si stack, in MideFar infrared range, confirmed SiAlON an unambiguous candidate of antireflective films for middle infrared window materials such as silicon or zinc sulfide, etc. Additionally, the as deposited and high temperature (600 C, 700 C, and 900 C, in air) annealed samples, were found to be amorphous based on the GIXRD pattern analysis. The high temperature stability of the film may help it become a potential buffer layer in optoelectronic fields.
2
G. Liu et al. / Vacuum 101 (2014) 1e5
2. Experiments SiAlON ceramic target was prepared following a mature process described in our previous work [1]. The SiAlON ceramics were then sputtered by an ion beam sputtering (IBS) system named IM100IBS, which was pumped to a base pressure around 5 108 Torr before deposition. During the deposition, argon (Ar) was introduced to act as working gas, after which the working pressure of the deposition chamber was increased to 2 104 Torr, then oxygen (O2) or nitrogen (N2) or their mixture were introduced as reactive gas, aiming to adjust the properties of SiAlON films, detailed parameters can be found in Table 1. The SiAlON films were formed on the unheated fused silica and silicon substrates, with a deposition rate of 0.5 A/s for all the samples, the thickness and deposition rate were both monitored by a quartz oscillator. After the deposition, a half of the samples were annealed in air, with temperature of 600 C, 700 C, and 900 C respectively, for the purpose to review their crystalline structure at high temperatures. A series of characterization techniques were employed in sequence to assess the properties of the SiAlON films. Firstly, an UVeVISeNIR Spectrophotometer and FTIR spectrum (Bruker, Vertex 70) were used to measure the optical properties of the SiAlON films with a normal incidence, from UVeVISeNIR to MideFar infrared wavelength. The refractive index and extinction coefficient of the films were deduced by a spectroscopic ellipsometry (SE), as a function of wavelength of 200e1100 nm. A detailed measurement and data fitting process can be found in many references such as Ref. [13], etc. Then the crystalline structure of the as deposited and annealed samples were inspected by a glancing incidence X-ray diffraction (GIXRD) method with a fixed glancing angle of 0.5 , equipped by a Cu-Ka radiation of 0.15418 nm (X’Pert). Finally, the chemical compositions were determined by means of an XPS system, equipped with a Mg-Ka radiation with energy of 1253.6 eV, and the power was 500 W, the detection process were carried out at an ambient temperature and a base pressure of about 5 109 Torr. Before detection, a pre-sputtering process of 60 s was preceded for all the samples, with the purpose of cleaning the surface of the SiAlON films, and during detection, a neutralization gun was opened to minimize the sample charging problem, because SiAlON is one kind of dielectric materials. 3. Results and discussion 3.1. Tunable transmittance and refractive index of SiAlON films in UVeVISeNIR region The normal incidence transmittance spectra of the SiAlON ceramic films are shown in Fig. 1, one can find the working gas and its flux affect the optical transmittance obviously. Sample 1 # without any working gas incorporation shows a bad transmittance performance in 200e900 nm wavelength range, especially in the UVeVIS region, because of the great absorption induced by the lacking of oxygen and nitrogen elements, which work as
Fig. 1. Transmittance spectra of SiAlON films grown at different gas input and flux.
non-stoichiometric defects in the films [14,15]. When N2 is introduced, the optical transmittance of sample 2 # becomes a little better with a blue shift of the optical band edge from 385 nm to 300 nm, benefitting from a nitridation process during SiAlON films growth. As a result, nitrogen non-stoichiometric defects are reduced. As oxygen incorporated, the SiAlON ceramic films present a huge transmittance enhancement (sample 3 # and 4 #), which are highly transparent in the whole wavelength region of UVeVISeNIR. The oxidation function proceeds much more sufficiently as more oxygen is introduced. And as we all know, it is much easier for an oxidation process to occur than that of a nitridation reaction [16], so, a much more obvious increase of optical transmittance than that of samples 2 #, which seems not difficult to be interpreted. It is easy to deduce that SiAlON films should experience an enough oxidation and nitridation process, when oxygen and nitrogen were both introduced into the deposition chamber at the same time, following by a reduction of oxygen and nitrogen nonstoichiometric defects, and thereby a reduction of optical absorption. In Fig. 1, one can find that the samples 5 # shows the most excellent optical performance among all samples, when the evaluation criterion is confined to the optical absorption. Refractive index tailoring has been attracted many research interests for years, due to its fantastic ability to gain film with specific or a wide range of refractive index needed. One can realize the refractive index tailoring through methods of combining compositions with high and low refractive index, as well as a deposition process adjusting and something else [17e19]. In this work, the refractive index of the SiAlON films was flexibly tailored from 1.53 to 1.83 at 850 nm, though a working gas tuning (flux tuning of O2, N2 or their mixtures), the results are shown in Fig. 2. As can be seen, the as deposited films without any working gas input own the biggest refractive index (1.83 at 850 nm). After oxygen and nitrogen
Table 1 Preparation parameters of SiAlON films. Samples
Rate ( A/s)
Thickness (nm)
Reactive gas
Reactive gas flux (sccm)
Working pressure (Torr)
1 2 3 4 5
0.5 0.5 0.5 0.5 0.5
200 200 320 280 600
None N2 O2 O2 O2:N2 ¼ 4:1
e 1.5 1.5 2.5 2.0
2.0 3.0 3.0 6.0 4.5
# # # # #
104 104 104 104 104
Refractive index (n@850 nm)
Extinction coefficient (K@850 nm)
1.832 1.801 1.710 1.531 1.563
4.54 1.85 3.23 2.81 1.84
102 102 103 103 104
G. Liu et al. / Vacuum 101 (2014) 1e5
3
Fig. 2. Refractive index of the SiAlON films from different reactive gas.
reactive gases are introduced, the refractive index decreases, in which the oxidation process is much more efficient to decrease the refractive index (for example, 1.53 at 850 nm for sample 4 #), as well as to restrain the optical absorption shown in Fig. 1. It can also be found that, in Table 1, the extinction coefficient of the SiAlON ceramic films decrease as the oxygen and nitrogen are introduced, which is consistent to the optical absorption results shown in Fig. 1. The mechanisms of the refractive index tailoring process are seriously restrained by accurately controlling of compositions. In fact an oxidation process is one of important routes for refractive index tailoring and can make the refractive index of films decrease according to the reduction of dielectric constants closely related with oxygen defects, which is determined in nature by the dipoles distance between the metal and oxygen atoms [16,18]. In another word, an oxidation process makes compounds especially metal oxides much more like a dielectric medium, which always owns smaller dielectric constants than those of correlated metals [16,18]. The same mechanism holds true to a nitridation process, with only a difference that nitrogenemetal dipoles have higher polarizability than those of relevant metaleoxygen dipoles, so a nitridation process is inclined to cause a higher refractive index than that of an oxidation process. 3.2. Enhanced transmittance of SiAlON film coated substrates in MideFar infrared region Several SiAlON films were deposited on a double-side polished silicon substrate, the thickness of the films on each side was approximately 600 nm. Then a FTIR inspector was adapted to characterize the MideFar infrared properties of the SiAlON films. Fig. 3 shows that both the single and double side coated films are transparent extending to almost 8 mm wavelength, where a broad absorbing band starts to appear, which should be related to SieN bond (700e1200 cm1, 8.5e14.3 mm) [20] and also to the SieO bond (1040 cm1, 9.6 mm). The transmittance of the coated samples is higher than that of bared silicon substrate, because the refractive index of the SiAlON films is smaller than that of silicon substrate, which makes the SiAlON films work as an antireflective layer for the silicon substrate. As can be seen in Fig. 3, both the single and double sides coated SiAlON films show the maximum transmittance at 3.8 mm, with a transmittance of 67.3% and 88.3%, respectively. A great transmittance enhancement (1.64) is acquired in contrast to bared silicon substrate (53.9% at 3.8 mm).
Fig. 3. Infrared spectra of the single and double side coated silicon substrate with SiAlON films.
Compared to silicon substrate with a refractive index of 3.43 at 3.8 mm, zinc sulfide (ZnS) has a smaller refractive index of 2.25 at 3.8 mm, which means that a coated film material with a refractive index nf z 1.5 at 3.8 mm would be just right to realize a perfect antireflection effect on ZnS, according to the following formula [21]:
nf ¼
pffiffiffiffiffiffiffiffiffiffi nZnS
(1)
As mentioned in Section 3.1, the refractive index of SiAlON films can be tailored from 1.53 to 1.83 at 850 nm, there should be no doubt that one can get a refractive index about 1.5 at 3.8 mm by adjusting the working gas flux or deposition parameters during the growth process of SiAlON film, which may make the SiAlON ceramic film an unambiguous candidate antireflective layer for zinc sulfide substrate. 3.3. High temperature stability of SiAlON films The XRD data for the as deposited and annealed SiAlON films (on silicon substrate) are shown in Fig. 4, where the amorphous textures are found for all samples. As we all know, the substrate temperature during films deposition is one of the most important
Fig. 4. XRD spectra of the as deposited and annealed SiAlON films.
4
G. Liu et al. / Vacuum 101 (2014) 1e5
Fig. 5. XPS results of the SiAlON films.
parameters which effects crystallinity of the deposited films, and the cold substrate always supplies an environment lacking of enough driving energy for the deposited atoms or molecules to nucleate, further to form crystallized structures. In this work, an ambient temperature was used during films growth process, and an amorphous structure formed as expected for the as deposited films. As we all know, since SiAlON ceramics have a very high molten temperature zone of 1600e2000 C, it needs a high temperature for the SiAlON ceramics or films to form crystalline structure [5,6]. Fig. 4 shows the GAXRD (0.5 ) results of the samples. After being annealed at 600 C, 700 C, and 900 C in air, SiAlON films all remain amorphous. The high temperature stability of the SiAlON films may make it a candidate as a buffer layer for silicon, and for applications in microelectronics, etc. 3.4. Chemical state and composition of SiAlON films Fig. 5 shows the XPS results of the SiAlON films, characteristic peaks of Si, Al, O, N are all found, and also a carbon peak with a binding energy located at 284.7 eV (not shown in Fig. 5), which was used as a comparison element. For Si 2p spectra in Fig. 5(a), the binding energy of samples 1 #, 3 # and 4 #, are located at around 103.3e103.4 eV, while those of samples 2 # and 5 # shift to lower
binding energy following by a nidridation process during films growth [18,22], The broad binding energy spectra shown in Fig. 5(b), in a range of 73.0e78.0 eV, is corresponding to Al 2p. As we all know, AleN bond always falls in 73.0e75.0 eV and AleO bond is in 74.0e76.0 eV [23], so it can be concluded that AleO bond in samples 1 #, 3 # and 4 # occupy a quite high proportion. Similar to Si 2p, the Al 2p spectrum shift towards a lower energy level (Ale N bond) after N2 (sample 2 #) or the mixture of O2/N2 (sample 5 #) is introduced. O 1s binding energy spectra is shown in Fig. 5(c), the strong peak at around 532.7 eV (for samples 1 #, 3 # and 4 #) should be ascribed to a comprehensive contribution of SieO bonds, and a little of AleO and NeO bonds [22e24]. As N2 is incorporated, O 1s binding energy of samples 2 # and 5 # also shifts to a lower energy level, because some of the SieO bonds transformed to SieN and OeN bonds, accompanied by an increase of electron density of O atoms, then a decrease of binding energy of O 1s. Fig. 5(d) shows the binding energy results of N 1s, the main peak located at between 398.7 eV and 397.2 eV, is ascribed to NeSi bonds originating from SiAlON ceramics target, while the higher energy band (appears in samples 3 # and 4 #) at 403.6 eV and 404.0 eV are ascribed to NeO bonds [24], majorly caused by a further oxidation process of the SiAlON films in an O2 rich environment.
Table 2 Chemical state and composition of SiAlON films from XPS results. Samples Si
Al
O
N
Binding energy (eV) Atomic ratio (%) Binding energy (eV) Atomic ratio (%) Binding energy (eV) Atomic ratio (%) Binding energy (eV) Atomic ratio (%) 1 2 3 4 5
# # # # #
103.3 101.5 103.4 103.4 102.0
35.9 35.6 27.8 27.2 37.8
75.4 74.3 75.6 75.6 75.0
20.5 12.4 10.8 11.3 13.6
532.7 531.8 532.8 532.7 532.6
16.4 15.2 46.4 52.6 17.0
398.0 397.2 398.7 398.6 398.1
27.2 36.8 15.0 9.0 31.6
G. Liu et al. / Vacuum 101 (2014) 1e5
For multi-element dielectric films, the chemical compositions are critical to their optical properties, for instance the refractive index. The XPS results also supplied us with the composition information of the final SiAlON films, listed in Table 2. Higher oxygen contents are found in 3 # and 4 # SiAlON films that experienced an oxygen introducing during films growth process, which should be a good explanation for their resultant lower refractive indexes, illustrated in Fig. 2. The film samples (2 # and 1 #) experienced a nitrogen environment or nothing input during growth possess higher nitrogen contents, and reveal much higher refractive indexes, indicated in Fig. 2, too. Sample 5 # shows an intermediate content of oxygen and nitrogen, and exhibits an intermediate refractive index value as expected. 4. Conclusion In summary, one kind of quaternary SiAlON films were deposited by an ion beam sputtering system, the optical absorption of the final films in UVeVIS region can be improved by adjusting chemical compositions of films through inputting different reactive gas and controlling gas flux during films growth process. The refractive index can also be tuned from 1.53 to 1.83 around 850 nm by adjusting the chemical compositions of the films, through varying the reactive gases. XPS results gave some positive supporting information for the chemical composition change of the final samples. Combining the tunable refractive index and high infrared transmittance results of the SiAlON films, it can be deduced that SiAlON films could serve as an antireflective layer for silicon substrate, especially for ZnS mid infrared window materials, in the infrared wavelength region of 3e5 mm. The as-deposited and annealed (at 600 C, 700 C, and 900 C, in air) SiAlON films all show an amorphous state verified by GIXRD results. The stability at high temperature may make the SiAlON film a candidate material as buffer layers for microelectronics applications. Acknowledgment This work is supported by Science Foundation for Youth Scholar of State Key Laboratory of High Performance Ceramics and Superfine Microstructures (No. SKL201201). References [1] Liu Q, He W, Zhong HM, Zhang K, Gui LH. Transmittance improvement of Dya-SiAlON in infrared range by post hot-isostatic-pressing. J Eur Ceram Soc 2012;32:1377e81.
5
[2] Mitomo M, Moriyoshi T, Sakai T, Ohsaka Y, Kobayasgi M. Translucent b-SiAlON ceramics. J Matter Sci Lett 1981;1:25e6. [3] Mandal H. New developments in a-SiAlON ceramics. J Eur Ceram Soc 1999;19: 2349e57. [4] Ye F, Liu LM, Liu CF, Zhang HJ, Zhou Y, Yu J. High infrared transmission of Y3þe Yb3þ-doped a-SiAlON. Mater Lett 2008;62:4535e8. [5] Kurama S, Hermann M, Mandal H. The effect of processing conditions, amount of additives and composition on the microstructures and mechanical properties of a-SiAlON ceramics. J Eur Ceram Soc 2002;22:109e19. [6] Cao GZ, Metselaar R. a-SiAlON ceramics: a review. Chem Mater 1991;3: 242e52. [7] Sakuma K, Omichi K, Kimura N, Ohashi M, Tanaka D. Warm-white lightemitting diode with yellowish orange SiAlON ceramic phosphor. Opt Lett 2004;29:2001e3. [8] Liu LH, Xie RJ, Hirosaki N, Takeda T, Zhang CN, Li JG, et al. Photoluminescence properties of b-SiAlON: Yb2þ, a novel green-emitting phosphor for white light-emitting diodes. Sci Technol Adv Mater 2011;2:034404e9. [9] Xie RJ, Hirosaki N, Mitomo M, Takahashi K, Sakuma K. Highly efficient whitelight emitting diodes fabricated with short-wavelength yellow oxynitride phosphors. Appl Phys Lett 2006;88:101104e6. [10] Lad RJ. Multifunctional silicon aluminum oxynitride (SiAlON) ceramic coatings for high temperature applications. Final performance report, Grant #F4962002-1-0323, P1e23. [11] Jacobs M, Terwagne G, Schryversb D, Rodquiny Ph, Bodart F. Unbalanced magnetron sputtered SieAl coatings: plasma conditions and film properties versus sample bias voltage. Surf Coating Technol 1999;116e119:735e41. [12] Knotem O, Loffler F, Beele W. PVD coatings in the system SiAlON. Key Eng Mater 2002;275:89e91. [13] Dinh NN, Trung TQ, Binh LK, Khoa ND, Thuan VTM. Investigation of zinc oxide thin film by spectroscopic ellipsometry. VNU J Sci Math Phys 2008;24:16e23. [14] Li Q, Wang SJ, Lim PC, Chai JW, Huan ACH, Ong CK. The decomposition mechanism of SiO2 with the deposition of oxygen-deficient M(Hf or Zr)Ox films. Thin Solid Films 2004;462e463:106e9. [15] Sundaram KB, Alizadeh Z, Chow L. The effects of oxidation on the optical properties of amorphous SiC films. Mater Sci Eng B 2002;B90:47e9. [16] Wang XJ, Zhang LD, Zhang JP, Liu M, He G. Effects of nitrogen atom doping on optical properties and dielectric constant of HfO2 gate oxides. Appl Phys Lett 2008;92:202906e8. [17] Rebib F, Tomasella E, Gaston JP, Eypert C, Cellier J, Jacquet M. Determination of optical properties of a-SiOxNy thin films by ellipsometric and UVevisible spectroscopies. J Phys Conf Ser 2008;100:082033e6. [18] Koike M, Ino T, Kamimuta Y, Koyama M, Kamata Y, Suzuki M, et al. Dielectric properties of noncrystalline HfSiON. Phys Rev B 2006;73:125123e8. [19] Vishwasa W, Raob KN, Gowdac KVA, Chakradhard RPS. Optical, electrical and dielectric properties of TiO2eSiO2 films prepared by a cost effective solegel process. Spectrochimica Acta Part A Mol Biomol Spectrosc 2011;83:614e7. [20] Vila M, Prieto C, Lopez JG, Respaldiza MA. Influence of the target and working gas on the composition of silicon nitride thin films prepared by reactive RFsputtering. Nucl Instr Meth 2003;211:199e205. [21] Tang JF, Gu PF, Liu X, Li HY. Modern optical thin film technology. China: Zhejiang University Press; 2006. p. 62. [22] Chen CW, Huang CC, Lin YY, Chen LC, Chen KH. The affinity of SieN and SieC bonding in amorphous silicon carbon nitride (a-SiCN) thin film. Diamond Relat Mater 2005;14:1126e30. [23] Tsai MH, Wang HY, Lu HT, Tseng IH, Lu HH, Huang SL, et al. Properties of polyimide/Al2O3 and Si3N4 deposited thin films. Thin Solid Films 2011;519: 4969e73. [24] Wagner CD, Riggs WM, Davis LE, Moulder JF, Muilenberg GE. Handbook of Xray photoelectron spectroscopy. USA: Perkin-Elmer Inc, Physical Electronic Division; 1979. p. 40e2.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Preparation and tunable optical properties of ion beam sputtered SiAlON thin films Guanghui Liu a, *, Zhenzhen Zhou a, Qinhua Wei a, b, Fan Fei a, Hua Yang a, b, Qian Liu a a State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, China b Graduate University of the Chinese Academy of Science, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 April 2013 Received in revised form 7 July 2013 Accepted 9 July 2013
One kind of quaternary thin films of SiAlON were deposited by ion beam sputtering method at ambient temperature, through a sintered SiAlON ceramic target. Ultraviolet, visible and near infrared (UVeVIS eNIR) Spectrophotometer, Fourier transformed infrared spectroscopy (FTIR), glancing incidence X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS) were employed to characterize the optical, crystalline and chemical composition properties. The optical properties of the deposited amorphous films could be tailored by changing the relative content of O or N in the films, through introducing different working gas and gas flux adjusting (oxygen, nitrogen or their mixtures). By this way, the refractive index at 850 nm can be tailored from 1.53 to 1.83, and the chemical composition data from XPS analysis supplied strong evidences. Reasonably, the SiAlON films were thought to be one kind of candidate films either as a buffer layer benefit from their high temperature stability, or a mid infrared antireflective layer for silicon and other infrared window materials. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: SiAlON films Optical properties Antireflective layer Thermal stability
1. Introduction From the day it was discovered in 1970s, constant research work has been employed on silicon aluminum oxynitride (SiAlON) ceramics [1e4], for its excellent mechanical properties, thermal stability, and so on. The SiAlON ceramics are in nature a solid solution of Si3N4 by partially substitution of Al and O for Si and N, so, as a multi-element compound (quaternary), the properties of SiAlON ceramics can be adjusted and controlled following by the composition design and tuning. It could be found that the huge bygone research work about SiAlON ceramics was mainly on its improvement of mechanical and thermal shock resistivity, as well as the transmittance upgrading, served as translucent ceramics for mid-infrared (3e5 mm) windows [3e6]. Besides, when rare earth elements, such as Ce and Eu, etc, are incorporated, the doped SiAlON powder shows excellent photo luminescence properties for its multi color exhibition (blue, red, green and orange), which is very suitable for a white light-emitting diodes (LED) and as a display materials [7e9].
* Corresponding author. Tel./fax: þ86 2152412404. E-mail address: [email protected] (G. Liu). 0042-207X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2013.07.015
Although the SiAlON bulk materials exhibit outstanding mechanical, thermal and optical properties, less research work has been employed on SiAlON thin film materials by now [10e12], and even more rarely on the optical properties of the SiAlON thin films. Furthermore, compound films always exhibit adjustable properties especially when it is a quaternary one. So it is necessary to carry out some basic research work, aiming to prepare SiAlON thin films and get to know and optimize their properties, and finally find their application potentials. In this work, the authors successfully prepared SiAlON thin films through an ion beam sputtering process. The optical properties (in UVeVISeNIR), mainly the refractive index of the films, were carefully tuned by changing compositions of the samples, through adjusting working gas and gas flux (oxygen, nitrogen or their mixture) during deposition process. The relative content of Si, Al, O and N elements can be verified by the chemical composition data from XPS analysis results. The high transmittance value of the SiAlON/Si stack, in MideFar infrared range, confirmed SiAlON an unambiguous candidate of antireflective films for middle infrared window materials such as silicon or zinc sulfide, etc. Additionally, the as deposited and high temperature (600 C, 700 C, and 900 C, in air) annealed samples, were found to be amorphous based on the GIXRD pattern analysis. The high temperature stability of the film may help it become a potential buffer layer in optoelectronic fields.
2
G. Liu et al. / Vacuum 101 (2014) 1e5
2. Experiments SiAlON ceramic target was prepared following a mature process described in our previous work [1]. The SiAlON ceramics were then sputtered by an ion beam sputtering (IBS) system named IM100IBS, which was pumped to a base pressure around 5 108 Torr before deposition. During the deposition, argon (Ar) was introduced to act as working gas, after which the working pressure of the deposition chamber was increased to 2 104 Torr, then oxygen (O2) or nitrogen (N2) or their mixture were introduced as reactive gas, aiming to adjust the properties of SiAlON films, detailed parameters can be found in Table 1. The SiAlON films were formed on the unheated fused silica and silicon substrates, with a deposition rate of 0.5 A/s for all the samples, the thickness and deposition rate were both monitored by a quartz oscillator. After the deposition, a half of the samples were annealed in air, with temperature of 600 C, 700 C, and 900 C respectively, for the purpose to review their crystalline structure at high temperatures. A series of characterization techniques were employed in sequence to assess the properties of the SiAlON films. Firstly, an UVeVISeNIR Spectrophotometer and FTIR spectrum (Bruker, Vertex 70) were used to measure the optical properties of the SiAlON films with a normal incidence, from UVeVISeNIR to MideFar infrared wavelength. The refractive index and extinction coefficient of the films were deduced by a spectroscopic ellipsometry (SE), as a function of wavelength of 200e1100 nm. A detailed measurement and data fitting process can be found in many references such as Ref. [13], etc. Then the crystalline structure of the as deposited and annealed samples were inspected by a glancing incidence X-ray diffraction (GIXRD) method with a fixed glancing angle of 0.5 , equipped by a Cu-Ka radiation of 0.15418 nm (X’Pert). Finally, the chemical compositions were determined by means of an XPS system, equipped with a Mg-Ka radiation with energy of 1253.6 eV, and the power was 500 W, the detection process were carried out at an ambient temperature and a base pressure of about 5 109 Torr. Before detection, a pre-sputtering process of 60 s was preceded for all the samples, with the purpose of cleaning the surface of the SiAlON films, and during detection, a neutralization gun was opened to minimize the sample charging problem, because SiAlON is one kind of dielectric materials. 3. Results and discussion 3.1. Tunable transmittance and refractive index of SiAlON films in UVeVISeNIR region The normal incidence transmittance spectra of the SiAlON ceramic films are shown in Fig. 1, one can find the working gas and its flux affect the optical transmittance obviously. Sample 1 # without any working gas incorporation shows a bad transmittance performance in 200e900 nm wavelength range, especially in the UVeVIS region, because of the great absorption induced by the lacking of oxygen and nitrogen elements, which work as
Fig. 1. Transmittance spectra of SiAlON films grown at different gas input and flux.
non-stoichiometric defects in the films [14,15]. When N2 is introduced, the optical transmittance of sample 2 # becomes a little better with a blue shift of the optical band edge from 385 nm to 300 nm, benefitting from a nitridation process during SiAlON films growth. As a result, nitrogen non-stoichiometric defects are reduced. As oxygen incorporated, the SiAlON ceramic films present a huge transmittance enhancement (sample 3 # and 4 #), which are highly transparent in the whole wavelength region of UVeVISeNIR. The oxidation function proceeds much more sufficiently as more oxygen is introduced. And as we all know, it is much easier for an oxidation process to occur than that of a nitridation reaction [16], so, a much more obvious increase of optical transmittance than that of samples 2 #, which seems not difficult to be interpreted. It is easy to deduce that SiAlON films should experience an enough oxidation and nitridation process, when oxygen and nitrogen were both introduced into the deposition chamber at the same time, following by a reduction of oxygen and nitrogen nonstoichiometric defects, and thereby a reduction of optical absorption. In Fig. 1, one can find that the samples 5 # shows the most excellent optical performance among all samples, when the evaluation criterion is confined to the optical absorption. Refractive index tailoring has been attracted many research interests for years, due to its fantastic ability to gain film with specific or a wide range of refractive index needed. One can realize the refractive index tailoring through methods of combining compositions with high and low refractive index, as well as a deposition process adjusting and something else [17e19]. In this work, the refractive index of the SiAlON films was flexibly tailored from 1.53 to 1.83 at 850 nm, though a working gas tuning (flux tuning of O2, N2 or their mixtures), the results are shown in Fig. 2. As can be seen, the as deposited films without any working gas input own the biggest refractive index (1.83 at 850 nm). After oxygen and nitrogen
Table 1 Preparation parameters of SiAlON films. Samples
Rate ( A/s)
Thickness (nm)
Reactive gas
Reactive gas flux (sccm)
Working pressure (Torr)
1 2 3 4 5
0.5 0.5 0.5 0.5 0.5
200 200 320 280 600
None N2 O2 O2 O2:N2 ¼ 4:1
e 1.5 1.5 2.5 2.0
2.0 3.0 3.0 6.0 4.5
# # # # #
104 104 104 104 104
Refractive index (n@850 nm)
Extinction coefficient (K@850 nm)
1.832 1.801 1.710 1.531 1.563
4.54 1.85 3.23 2.81 1.84
102 102 103 103 104
G. Liu et al. / Vacuum 101 (2014) 1e5
3
Fig. 2. Refractive index of the SiAlON films from different reactive gas.
reactive gases are introduced, the refractive index decreases, in which the oxidation process is much more efficient to decrease the refractive index (for example, 1.53 at 850 nm for sample 4 #), as well as to restrain the optical absorption shown in Fig. 1. It can also be found that, in Table 1, the extinction coefficient of the SiAlON ceramic films decrease as the oxygen and nitrogen are introduced, which is consistent to the optical absorption results shown in Fig. 1. The mechanisms of the refractive index tailoring process are seriously restrained by accurately controlling of compositions. In fact an oxidation process is one of important routes for refractive index tailoring and can make the refractive index of films decrease according to the reduction of dielectric constants closely related with oxygen defects, which is determined in nature by the dipoles distance between the metal and oxygen atoms [16,18]. In another word, an oxidation process makes compounds especially metal oxides much more like a dielectric medium, which always owns smaller dielectric constants than those of correlated metals [16,18]. The same mechanism holds true to a nitridation process, with only a difference that nitrogenemetal dipoles have higher polarizability than those of relevant metaleoxygen dipoles, so a nitridation process is inclined to cause a higher refractive index than that of an oxidation process. 3.2. Enhanced transmittance of SiAlON film coated substrates in MideFar infrared region Several SiAlON films were deposited on a double-side polished silicon substrate, the thickness of the films on each side was approximately 600 nm. Then a FTIR inspector was adapted to characterize the MideFar infrared properties of the SiAlON films. Fig. 3 shows that both the single and double side coated films are transparent extending to almost 8 mm wavelength, where a broad absorbing band starts to appear, which should be related to SieN bond (700e1200 cm1, 8.5e14.3 mm) [20] and also to the SieO bond (1040 cm1, 9.6 mm). The transmittance of the coated samples is higher than that of bared silicon substrate, because the refractive index of the SiAlON films is smaller than that of silicon substrate, which makes the SiAlON films work as an antireflective layer for the silicon substrate. As can be seen in Fig. 3, both the single and double sides coated SiAlON films show the maximum transmittance at 3.8 mm, with a transmittance of 67.3% and 88.3%, respectively. A great transmittance enhancement (1.64) is acquired in contrast to bared silicon substrate (53.9% at 3.8 mm).
Fig. 3. Infrared spectra of the single and double side coated silicon substrate with SiAlON films.
Compared to silicon substrate with a refractive index of 3.43 at 3.8 mm, zinc sulfide (ZnS) has a smaller refractive index of 2.25 at 3.8 mm, which means that a coated film material with a refractive index nf z 1.5 at 3.8 mm would be just right to realize a perfect antireflection effect on ZnS, according to the following formula [21]:
nf ¼
pffiffiffiffiffiffiffiffiffiffi nZnS
(1)
As mentioned in Section 3.1, the refractive index of SiAlON films can be tailored from 1.53 to 1.83 at 850 nm, there should be no doubt that one can get a refractive index about 1.5 at 3.8 mm by adjusting the working gas flux or deposition parameters during the growth process of SiAlON film, which may make the SiAlON ceramic film an unambiguous candidate antireflective layer for zinc sulfide substrate. 3.3. High temperature stability of SiAlON films The XRD data for the as deposited and annealed SiAlON films (on silicon substrate) are shown in Fig. 4, where the amorphous textures are found for all samples. As we all know, the substrate temperature during films deposition is one of the most important
Fig. 4. XRD spectra of the as deposited and annealed SiAlON films.
4
G. Liu et al. / Vacuum 101 (2014) 1e5
Fig. 5. XPS results of the SiAlON films.
parameters which effects crystallinity of the deposited films, and the cold substrate always supplies an environment lacking of enough driving energy for the deposited atoms or molecules to nucleate, further to form crystallized structures. In this work, an ambient temperature was used during films growth process, and an amorphous structure formed as expected for the as deposited films. As we all know, since SiAlON ceramics have a very high molten temperature zone of 1600e2000 C, it needs a high temperature for the SiAlON ceramics or films to form crystalline structure [5,6]. Fig. 4 shows the GAXRD (0.5 ) results of the samples. After being annealed at 600 C, 700 C, and 900 C in air, SiAlON films all remain amorphous. The high temperature stability of the SiAlON films may make it a candidate as a buffer layer for silicon, and for applications in microelectronics, etc. 3.4. Chemical state and composition of SiAlON films Fig. 5 shows the XPS results of the SiAlON films, characteristic peaks of Si, Al, O, N are all found, and also a carbon peak with a binding energy located at 284.7 eV (not shown in Fig. 5), which was used as a comparison element. For Si 2p spectra in Fig. 5(a), the binding energy of samples 1 #, 3 # and 4 #, are located at around 103.3e103.4 eV, while those of samples 2 # and 5 # shift to lower
binding energy following by a nidridation process during films growth [18,22], The broad binding energy spectra shown in Fig. 5(b), in a range of 73.0e78.0 eV, is corresponding to Al 2p. As we all know, AleN bond always falls in 73.0e75.0 eV and AleO bond is in 74.0e76.0 eV [23], so it can be concluded that AleO bond in samples 1 #, 3 # and 4 # occupy a quite high proportion. Similar to Si 2p, the Al 2p spectrum shift towards a lower energy level (Ale N bond) after N2 (sample 2 #) or the mixture of O2/N2 (sample 5 #) is introduced. O 1s binding energy spectra is shown in Fig. 5(c), the strong peak at around 532.7 eV (for samples 1 #, 3 # and 4 #) should be ascribed to a comprehensive contribution of SieO bonds, and a little of AleO and NeO bonds [22e24]. As N2 is incorporated, O 1s binding energy of samples 2 # and 5 # also shifts to a lower energy level, because some of the SieO bonds transformed to SieN and OeN bonds, accompanied by an increase of electron density of O atoms, then a decrease of binding energy of O 1s. Fig. 5(d) shows the binding energy results of N 1s, the main peak located at between 398.7 eV and 397.2 eV, is ascribed to NeSi bonds originating from SiAlON ceramics target, while the higher energy band (appears in samples 3 # and 4 #) at 403.6 eV and 404.0 eV are ascribed to NeO bonds [24], majorly caused by a further oxidation process of the SiAlON films in an O2 rich environment.
Table 2 Chemical state and composition of SiAlON films from XPS results. Samples Si
Al
O
N
Binding energy (eV) Atomic ratio (%) Binding energy (eV) Atomic ratio (%) Binding energy (eV) Atomic ratio (%) Binding energy (eV) Atomic ratio (%) 1 2 3 4 5
# # # # #
103.3 101.5 103.4 103.4 102.0
35.9 35.6 27.8 27.2 37.8
75.4 74.3 75.6 75.6 75.0
20.5 12.4 10.8 11.3 13.6
532.7 531.8 532.8 532.7 532.6
16.4 15.2 46.4 52.6 17.0
398.0 397.2 398.7 398.6 398.1
27.2 36.8 15.0 9.0 31.6
G. Liu et al. / Vacuum 101 (2014) 1e5
For multi-element dielectric films, the chemical compositions are critical to their optical properties, for instance the refractive index. The XPS results also supplied us with the composition information of the final SiAlON films, listed in Table 2. Higher oxygen contents are found in 3 # and 4 # SiAlON films that experienced an oxygen introducing during films growth process, which should be a good explanation for their resultant lower refractive indexes, illustrated in Fig. 2. The film samples (2 # and 1 #) experienced a nitrogen environment or nothing input during growth possess higher nitrogen contents, and reveal much higher refractive indexes, indicated in Fig. 2, too. Sample 5 # shows an intermediate content of oxygen and nitrogen, and exhibits an intermediate refractive index value as expected. 4. Conclusion In summary, one kind of quaternary SiAlON films were deposited by an ion beam sputtering system, the optical absorption of the final films in UVeVIS region can be improved by adjusting chemical compositions of films through inputting different reactive gas and controlling gas flux during films growth process. The refractive index can also be tuned from 1.53 to 1.83 around 850 nm by adjusting the chemical compositions of the films, through varying the reactive gases. XPS results gave some positive supporting information for the chemical composition change of the final samples. Combining the tunable refractive index and high infrared transmittance results of the SiAlON films, it can be deduced that SiAlON films could serve as an antireflective layer for silicon substrate, especially for ZnS mid infrared window materials, in the infrared wavelength region of 3e5 mm. The as-deposited and annealed (at 600 C, 700 C, and 900 C, in air) SiAlON films all show an amorphous state verified by GIXRD results. The stability at high temperature may make the SiAlON film a candidate material as buffer layers for microelectronics applications. Acknowledgment This work is supported by Science Foundation for Youth Scholar of State Key Laboratory of High Performance Ceramics and Superfine Microstructures (No. SKL201201). References [1] Liu Q, He W, Zhong HM, Zhang K, Gui LH. Transmittance improvement of Dya-SiAlON in infrared range by post hot-isostatic-pressing. J Eur Ceram Soc 2012;32:1377e81.
5
[2] Mitomo M, Moriyoshi T, Sakai T, Ohsaka Y, Kobayasgi M. Translucent b-SiAlON ceramics. J Matter Sci Lett 1981;1:25e6. [3] Mandal H. New developments in a-SiAlON ceramics. J Eur Ceram Soc 1999;19: 2349e57. [4] Ye F, Liu LM, Liu CF, Zhang HJ, Zhou Y, Yu J. High infrared transmission of Y3þe Yb3þ-doped a-SiAlON. Mater Lett 2008;62:4535e8. [5] Kurama S, Hermann M, Mandal H. The effect of processing conditions, amount of additives and composition on the microstructures and mechanical properties of a-SiAlON ceramics. J Eur Ceram Soc 2002;22:109e19. [6] Cao GZ, Metselaar R. a-SiAlON ceramics: a review. Chem Mater 1991;3: 242e52. [7] Sakuma K, Omichi K, Kimura N, Ohashi M, Tanaka D. Warm-white lightemitting diode with yellowish orange SiAlON ceramic phosphor. Opt Lett 2004;29:2001e3. [8] Liu LH, Xie RJ, Hirosaki N, Takeda T, Zhang CN, Li JG, et al. Photoluminescence properties of b-SiAlON: Yb2þ, a novel green-emitting phosphor for white light-emitting diodes. Sci Technol Adv Mater 2011;2:034404e9. [9] Xie RJ, Hirosaki N, Mitomo M, Takahashi K, Sakuma K. Highly efficient whitelight emitting diodes fabricated with short-wavelength yellow oxynitride phosphors. Appl Phys Lett 2006;88:101104e6. [10] Lad RJ. Multifunctional silicon aluminum oxynitride (SiAlON) ceramic coatings for high temperature applications. Final performance report, Grant #F4962002-1-0323, P1e23. [11] Jacobs M, Terwagne G, Schryversb D, Rodquiny Ph, Bodart F. Unbalanced magnetron sputtered SieAl coatings: plasma conditions and film properties versus sample bias voltage. Surf Coating Technol 1999;116e119:735e41. [12] Knotem O, Loffler F, Beele W. PVD coatings in the system SiAlON. Key Eng Mater 2002;275:89e91. [13] Dinh NN, Trung TQ, Binh LK, Khoa ND, Thuan VTM. Investigation of zinc oxide thin film by spectroscopic ellipsometry. VNU J Sci Math Phys 2008;24:16e23. [14] Li Q, Wang SJ, Lim PC, Chai JW, Huan ACH, Ong CK. The decomposition mechanism of SiO2 with the deposition of oxygen-deficient M(Hf or Zr)Ox films. Thin Solid Films 2004;462e463:106e9. [15] Sundaram KB, Alizadeh Z, Chow L. The effects of oxidation on the optical properties of amorphous SiC films. Mater Sci Eng B 2002;B90:47e9. [16] Wang XJ, Zhang LD, Zhang JP, Liu M, He G. Effects of nitrogen atom doping on optical properties and dielectric constant of HfO2 gate oxides. Appl Phys Lett 2008;92:202906e8. [17] Rebib F, Tomasella E, Gaston JP, Eypert C, Cellier J, Jacquet M. Determination of optical properties of a-SiOxNy thin films by ellipsometric and UVevisible spectroscopies. J Phys Conf Ser 2008;100:082033e6. [18] Koike M, Ino T, Kamimuta Y, Koyama M, Kamata Y, Suzuki M, et al. Dielectric properties of noncrystalline HfSiON. Phys Rev B 2006;73:125123e8. [19] Vishwasa W, Raob KN, Gowdac KVA, Chakradhard RPS. Optical, electrical and dielectric properties of TiO2eSiO2 films prepared by a cost effective solegel process. Spectrochimica Acta Part A Mol Biomol Spectrosc 2011;83:614e7. [20] Vila M, Prieto C, Lopez JG, Respaldiza MA. Influence of the target and working gas on the composition of silicon nitride thin films prepared by reactive RFsputtering. Nucl Instr Meth 2003;211:199e205. [21] Tang JF, Gu PF, Liu X, Li HY. Modern optical thin film technology. China: Zhejiang University Press; 2006. p. 62. [22] Chen CW, Huang CC, Lin YY, Chen LC, Chen KH. The affinity of SieN and SieC bonding in amorphous silicon carbon nitride (a-SiCN) thin film. Diamond Relat Mater 2005;14:1126e30. [23] Tsai MH, Wang HY, Lu HT, Tseng IH, Lu HH, Huang SL, et al. Properties of polyimide/Al2O3 and Si3N4 deposited thin films. Thin Solid Films 2011;519: 4969e73. [24] Wagner CD, Riggs WM, Davis LE, Moulder JF, Muilenberg GE. Handbook of Xray photoelectron spectroscopy. USA: Perkin-Elmer Inc, Physical Electronic Division; 1979. p. 40e2.