Films with the best electrical characteristics present an average dielectric constant of 7.2 and 8.7 standing electric fields up to 4.5 and 2 MV/cm without observing ...
J O U R N A L O F M A T E R I A L S S C I E N C E : M A T E R I A L S I N E L E C T R O N I C S 1 6 (2 0 0 5 ) 657 – 661
Optical, structural and electrical characteristics of aluminum oxynitride thin films deposited in an Ar-N gas mixture RF-sputtering system J. J. ARAIZA Unidad Academica ´ de F´ısica, Universidad Autonoma ´ de Zacatecas 98060, Zacatecas, Mexico ´ M. AGUILAR-FRUTIS CICATA-IPN, Miguel Hidalgo 11500, Mexico, ´ DF Mexico ´ C. FALCONY, MA. JERGEL CINVESTAV-IPN, Apdo. Postal 14-740, 07000, Mexico, ´ DF Mexico The optical, structural and electrical characteristics of aluminum oxynitride thin films deposited on silicon by rf-sputtering under a fixed oxygen flow and two different Ar and N gas flows are reported. The stoichiometry of the films was studied by EDS as a function of the deposition parameters. In general, the relative oxygen content within the films was higher for a high N/Ar (5/1) gas flow ratio, these films presented refractive indexes in the range of 1.5–2.0, with deposition rates close to 4.0 nm/min, and surface roughness of ˚ Films deposited with a low N/Ar (1/5) flow ratio presented refractive approximately 13 A. indexes in the range of 1.7 to 2.0, deposition rates of 7 nm/min and surface roughness of ˚ IR spectroscopy measurements on these films presented an absorption band 26 A. spreading from 500 to 900 cm−1 . The width and peak of this band depends on the rf power and correlates with the oxygen content in the films. Films with the best electrical characteristics present an average dielectric constant of 7.2 and 8.7 standing electric fields up to 4.5 and 2 MV/cm without observing destructive dielectric breakdown for high and low C 2005 Springer Science + Business Media, Inc. N/Ar gas ratios respectively.
1. Introduction Aluminum oxynitride thin films (Alx N y Oz ) have found several applications in different fields of technology; they are used as optical and protective coatings against wear, diffusion and corrosion and in research areas such as microelectronics and optoelectronics [1, 2]. Aluminum oxynitride thin films have as specific advantage that their physical properties can be tailored between those of aluminum oxide (Al2 O3 ) and aluminum nitride (AlN). In this way, the characteristics of Al2 O3 , such as; high electrical resistivity, high chemical stability, high hardness, etc., and the characteristics of AlN, such as; high refractive index, low thermal expansion coefficient, high surface acoustic wave velocity and high mechanical properties, could be combined in a single Alx N y Oz film, depending on the amount of aluminum, nitrogen and oxygen within the film [1–4]. Aluminum oxynitride thin films have been deposited using dc and ac-magnetron sputtering [1–3], magnetron sputtering ion plating (MSIP) [4], and by chemical vapor deposition techniques [5–7]. Homogeneous and metastable Alx Ny Oz layers were produced by reactive MSIP using an Al target and an Ar-O2 -N2 gas mixture at a substrate temperature of 190 ◦ C with different oxygen and nitrogen content [4]. Wang and Guo
[3] found that both the refractive index and the breakdown electric field depended non-linearly on the oxygen content in their films. (Al2 O3 )1−c -(AlN)c composite films have also been deposited using a laser assisted chemical vapor deposition technique with good optical and electrical characteristics. These films were obtained using trimethylaluminum and ammonia as reactant gases [5]. Pure AlN thin films have been prepared from gaseous AlCl3 and NH3 ; pure Al2 O3 films were prepared from the reaction of AlCl3 , CO2 and H2 , and pseudobinary AlN-Al2 O3 films were obtained by combining the above gases and varying the gas phase NH3 /CO2 ratio. These films were deposited in the range of 770 to 900 ◦ C by chemical vapor deposition [6–7]. In this work, we report the characteristics of Alx Ny Oz thin films deposited on silicon substrates by rf-sputtering using two different N/Ar gas mixtures and a fixed amount of oxygen. The oxygen content in the films was found to be dependent on the deposition parameters and on the nitrogen to argon ratio.
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2. Experimental details The aluminum oxynitride films were deposited on n-type silicon (111) wafers with a resistivity of
250–350 -cm for the structural and optical characterization and on Si (100) 0.1–10 -cm substrates for the electrical measurements. The substrates were cleaned by the RCA method [8] previous to the introduction to the deposition system. The sputtering system was a 450 Alcatel system. Two different sets of films were prepared (denoted as R1 and R2-films). R1 and R2 films were deposited with 1, 5, and 0.03 sccm, and 5, 1, and 0.03 sccm of high purity N2 , Ar, and O2 , respectively. The background pressure in the sputtering chamber was less than 10−6 Torr. Upon the introduction of the gases, the total pressure was increased to 1.16 × 10−2 Torr by partially closing the throttle valve of the sputtering system, at this pressure, a 10 min. time of presputtering was given to the target (an aluminum nitride target, 99.99% purity, 3 inch diameter). Then, the films were deposited at a pressure of 1.8 × 10−3 Torr. The target-substrate distance was kept constant at 7 cm. The radiofrequency (RF) power during the deposition process was 50, 100 or 150 watts (the corresponding rf power densities are 1.1, 2.2 and 3.3 W/cm2 , respectively). The films were deposited at three different substrate temperatures: room temperature (25 ◦ C), 150 ◦ C and 300 ◦ C. The thickness and refractive index of the films were measured with a single wavelength ellipsometer (at 633 nm). A 750 Magna-IR NICOLET spectrometer was used for the infrared (IR) measurements. A Si substrate without deposited film was used as reference for these measurements. A scanning electron microscope (Jeol, JSM-6300) was used to plan view the surface of the films and also to carry out the chemical analysis by energy dispersion spectroscopy (EDS) with an ultra thin window and a beam accelerating voltage of 3 kV. An Al2 O3 crystal was used as a reference for the analysis with a Voyager microanalysis system from Noran instruments. The surface roughness measurements were carried out on a Park Scientific Instruments atomic force microscope. The films with the best characteristics were incorporated into Metal-Oxide-Semiconductor (MOS) structures for their electrical characterization by depositing a circular aluminum contact of .011 cm2 on top of the film. The electrical measurements were performed with commercial equipment by Keithley Instruments.
3. Experimental results and discussion The refractive index of the films helps in estimating their relative density and stoichiometry [9]. Fig. 1 shows the refractive index of the aluminum oxynitride films as a function of the sputtering power for the two gas ratios and the different substrate temperatures. The refractive index increased with increasing the sputtering power from 1.5 to 2. Aluminum nitride has a refractive index close to 2.08 [5] and aluminum oxide is in the range of 1.55 to 1.68 [10, 11]. R1-films have in general, for similar deposition conditions, a larger refractive index (indicating larger density and/or a stoichiometry closer to AlN) than R2-films. The observed increment of the refractive index with the applied rf power is in agreement with previous reports on the same type of films[1]. Fig. 2 shows the deposition rate for both types of sam658
Figure 1 Refractive index as a function of applied power of the films of aluminum oxynitride.
Figure 2 Deposition rate as a function of the applied power of the films of aluminum oxynitride.
ples studied as a function of the rf power for different deposition temperatures. R1-films presented higher deposition rates than R2-films in all cases, with values as large as 7 nm/min. R2-films show deposition rates up to 4.0 nm/min. There is an almost linear behavior with rf power with little dependence with deposition temperatures. The difference observed on the deposition rates for R1 and R2 films may be due to a lower sputtering yield expected from nitrogen ions with respect to argon
Figure 3 Surface morphology determined by atomic force microscopy of a film deposited with a high N/Ar gas ratio a), and low N/Ar gas ratio b), at 150 watts and at room temperature.
ions [12]. The average thickness of R1 and R2 films as ˚ determined by single wave ellipsometry was 1200 A, ˚ and 700 A, respectively. The surface morphology of the aluminum oxynitride films as determined by atomic force microscopy (AFM) at a scan area of 1 × 1 µm is shown in Fig. 3. The average of the root mean square (rms) surface roughness ˚ for all R2-films and 26 ± 3 A ˚ for all was of 13 ± 2 A R1-films with no obvious dependence on rf power or deposition temperature. Thin film with this low values of surface roughness are, in general, required for microelectronic applications [13]. The roughness of these films is comparable to the best roughness values reported for films deposited with similar deposition techniques (A.G. Erlat et al. prepared aluminum oxynitride thin films and obtained a rms surface roughness of 14 ˚ [2]). Table I summarizes the relative chemical comA position in atomic percent of the films studied as measured by EDS considering only the contributions of Al, O and N. The Si signal coming from the substrate was not considered for the atomic percent composition listed in this table. These data has an uncertainty of 2 to 3 percent for the average of the data measured
on all samples deposited with the sputtering conditions quoted. The largest difference between R1 and R2 samples deposited under similar conditions is in the relative content of N and O in all cases. The oxygen content is higher for R2-films than for R1-films. Considering the results shown in Fig. 2 for the deposition rate, it appears that low deposition rates favor the incorporation of oxygen into the film. Aluminum nitride has an IR absorption band centered around 680 cm−1 associated to the overlap of the phonon modes LO at 737 cm−1 , TO1 at 665 cm−1 and TO2 at 630 cm−1 [5, 14]. On the other hand, aluminum oxide presents a similar band centered at 700 cm−1 , approximately, associated to the overlap of the bending mode of O Al O at 700–650 cm−1 and stretching mode of Al O Al at 850–750 cm−1 [15, 16]. Figs. 4 and 5 show the IR spectra for R1 and R2 films deposited
T A B L E I Chemical composition (in atomic%) of the films of AlNx O y determined by EDS and deposited with the different conditions. R1-films T (◦ C)
N
O
R.T. 150 300
42 41 51
16 15 8
R.T. 150 300
43 45 42
16 11 14
R.T. 150 300
34 34 12
28 24 43
R2-films Al 150 watts 42 44 41 100 watts 41 44 44 50 watts 38 42 45
N
O
Al
31 30 26
27 23 26
42 47 48
32 29 29
36 29 29
32 42 42
21 13 18
40 53 49
39 34 33
Figure 4 Infrared spectra of R1-films deposited at room temperature and sputtering-powers of 50, 100 and 150 watts. The inset shows an amplification of
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Figure 5 Infrared spectra of R2-films deposited at room temperature and sputtering-powers of 50, 100 and 150 watts.
at room temperature (similar behavior is observed for the other substrate temperatures) for the different rf powers studied. These spectra show a dominant absorption band at 500–900 cm−1 and another one at 3500 cm−1 , the latter one is generally associated with –OH bonds. R2-films deposited at 50 watts show a broad band centered approximately between 500 and 1000 cm−1 . As the deposition power is increased, the width of this band is reduced and its center is slightly shifted to lower wave-numbers. On the other hand, R1-films did not show any noticeable shift for this band with the applied rf power. The presence of a single absorption band centered in between the location expected for pure silicon nitride and pure aluminum oxide could be considered as an indication that no phase segregation of either of this two compounds is occurring but rather the formation of an alloy. Figs. 6 and 7 show sequential ramp I-V’s of MOS capacitors formed with R2 and R1 films deposited at room temperature and at 150 watts respectively. The inset shows the capacitance versus voltage graph for the same capacitors. It was observed that the R2-films can stand electric fields up to 4.5 MV/cm, approximately, without observing destructive dielectric breakdown, while R1-films can stand electric fields up to 2.0 MV/cm. The spikes observed in the I-V curves are probably associated local self-healing non-destructive breakdowns. Table II presents the dielectric constant, determined from the capacitance versus voltage measurements in the accumulation region, film thickness as measured by ellipsometry, and the maximum electric field applied to the films without dielectric breakdown, the fixed charge and the threshold voltage determined also from the C-V curves. R1-films show a dielectric constant in the range of 8.3 to 9.3, whereas R2-films present a dielectric constant in the range of 6.1 to 8.3. Previous results found by Wang [17] for the dielectric constant and breakdown field for aluminum nitride are 8.3 and 3 MV/cm, respectively, while for aluminum ox660
Figure 6 Current and capacitance versus voltage measurements of R2films deposited at room temperature. The plot shows four ramp IV measurements performed on the same capacitor. The inset shows the capacitance versus voltage of a capacitor prepared under the same conditions.
Figure 7 Current and capacitance versus voltage measurements of R2films deposited at room temperature. The plot shows four ramp IV measurements performed on the same capacitor. The inset shows the capacitance versus voltage of a capacitor prepared under the same conditions.
ide [13], the values reported are 7.9 and 5 MV/cm. Thus, the electrical characteristics for R1-films are closer to those for aluminum nitride while R2-films characteristics are closer to aluminum oxide. These results
T A B L E I I Dielectric characteristics of films deposited at 150 watts R1-films
R2-films
Substrate temperature (◦ C)
Dielectric constant
Electric field (MV/cm)
Threshold voltage (V)
Effective oxide charge (Cb/cm2 )
Dielectric constant
Electric field (MV/cm)
Threshold voltage (V)
Effective oxide Charge (Cb/cm2 )
Room temp. 150 300 Average
8.3 9.3 8.5 8.7 ± 0.4
≈2 ≈2 ≈2
−0.85 0.87 3.81
−1.2E-7 −1.7E-7 −4.3E-7
6.1 8.3 — 7.2 ± 1.1
≈4.5 ≈4.5
7.6 3.9
−4.5E-7 −3.6E-7
are in agreement with the EDS and IR results above described.
as well as CGPI-IPN (Projs. Nos. 20020280 and 20031364) is acknowledged.
4. Conclusions The characteristics of the aluminum oxynitride thin films deposited on silicon by rf-sputtering were found to be closer to those reported for aluminum nitride or to aluminum oxide films depending on the N/Ar gas ratio supplied to the deposition chamber, on the substrate temperature, and on the sputtering power, even though the amount of oxygen supplied was kept constant during the deposition process. The oxygen content in the films was higher for those films deposited with a higher N/Ar ratio (R2-films). The incorporation of oxygen into the films seems to be favored by the slower deposition rate induced by a richer nitrogen plasma ambient (4.0 nm/min as compared to 7.0 nm/min for films deposited with low N/Ar ratio). The refractive indexes (1.5 to 2.0) and the width and peak position of the IR absorption band for these films are in between the expected values for Al2 O3 and AlN. The surface roughness of ˚ the films was for both types of films lower than 26 A. Films deposited at 150 watts with a high N/Ar gas ratio and at room temperature presented an average dielectric constant of 7.2 and can stand electric fields up to 4.5 MV/cm, without observing destructive dielectric breakdown, while films deposited with low N/Ar gas ratio have a average dielectric constant of 8.7 and can stand electric fields up to 2.0 MV/cm.
References
Acknowledgments The authors would like to acknowledge the technical assistance of A. B. Soto, M. Guerrero, J. Garcia-Coronel and R. Fregoso. The financial support from CONACyTMexico (Projects Nos. J34225-U and G37858-E),
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Received 23 August 2004 and accepted 19 June 2005
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