4. Pulsed Sputtering Deposition of AlN Thin Films
4.3 Influence of plasma power on pulsed sputtering deposition of
4.3.5 Al-N cluster
High-energy Al penetrated into the thin film, resulting in defects due to residual strain. Strain due to peening was expected to be induced in Al-N clusters. Al-N clusters have various geometric arrangements as shown in Figure 4.7. Table 4.1 shows the theoretical calculation results. [9]
Table. 4.1 Reported geometries of Al-N clusters used in the theoretical calculation [9]
Cluster Distance Geometry Wavenumber IR Raman Al2N Al-N=1.731
133.3 cm-1 23.67 0 525.4 cm-1 0 11.13 1052.4 cm-1 86.14 0
Al3N2 Al-N=1.705 N-Al=1.713
42.4 cm-1 6.97 0 154.0 cm-1 29.53 0 376.4 cm-1 0 83.17 675.9 cm-1 201.19 0 1166.3 cm-1 1098 0 1167.3 cm-1 0 166.68 AlN2 Al-N = 1.804
157.2 cm-1 53.9 0 163.9 cm-1 81.1 0
646.2 cm-1 0 706
660.1 cm-1 1036 0
AlNNN
Al-N = 1.826 N-N = 1.206 N-N = 1.138
97.1 cm-1 0.85 7.38 496.4 cm-1 141 42.3 627.0 cm-1 21.2 0.89 627.8 cm-1 21.2 0.89 1463.3 cm-1 277.8 0.26 2266.9 cm-1 850 48.7
AlN2Al Al-N = 1.888
N-N = 1.204
73.2 cm-1 4.4 0.2 209.6 cm-1 0.1 19.26 332 cm-1 8.7 676 606 cm-1 341.6 10.67 1744 cm-1 7.9 5878
Al3N Al-N = 1.850
156.3 cm-1 3.8 5.1
223.62 cm-1 0.2 0
427.52 cm-1 0 56.9 749.44 cm-1 326.4 0.10 749.46 cm-1 326.4 0.11
Figure 4.8 shows the FTIR spectra results to confirm the effect of high residual strain. Al-N clusters of Al2N (521cm-1), AlN2Al (591cm-1), AlN (620-670cm-1) and Al2N3 (1339cm-1) were observed in all samples. It expect that residual strain was associated with Al-N clusters.
Therefore, it is necessary to confirm the correlation between plasma power and Al-N cluster. Figure 4.9 shows the variation of Al-N cluster intensity with plasma power. The intensities of Al2N and Al2N3 clusters increased linearly with increasing plasma power. In other words, Al-N cluster interacts with the peening effect.
From these results, it can be seen that the AlN thin film needs to be fabricated at low plasma power in order to reduce the residual strain.
However, low plasma power also reduces migration length. The crystal quality is judged to be lower and further studies on other sputtering conditions are needed.
Figure 4.7 Geometric arrangement of clusters of AlxNy such as (a) AlNNN, (b) Al2N, (c) AlN2, (d) Al2N2, (e) Al3N2 and (f) Al3N
600 800 1000 1200 1400 1600 1800
(e) 700W
Absorbance (a.u)
Wavenumber (cm-1)
521 Al2N
(f) 800W 592 AlN2Al
(d) 600W (c) 500W (b) 400W (a) 300W
620-670 AlN 1339 Al2N3
632 760 890
Substrate peaks : Al2O3
Figure 4.8 FTIR spectra at different plasma power from 300 to 800W
300 400 500 600 700 800
F: 50kHz, toff 2.0μs Ar : N2 ~ 3:2, Pc: 0.1mtorr RT
Al2N (521cm-1)
Al2N3(1339cm-1)
FTIR Relative absorbance (a.u.)
Plasma power (W)
Figure 4.9 FTIR relative absorbance at different plasma power from 300 to 800W
4.4. Influence of gas pressure on the pulsed sputtering deposition of AlN thin films
The gas pressure of sputtering conditions is commonly used as an element to increase the aspect ratio. The lower pressure can be supplied as highly directional sputtered molecules of a high aspect ratio to the thin film. Also the gas pressure is related to a mean free path of the atoms and have been reported as a sputtering parameter that can control the residual strain.
[4] M. Ohtsuka et al. has reported the effect of gas pressure on AlN thin films that high gas pressure at low mean free path can reduce residual strain due to peening effect. [4]
In this experiment, too low (<0.1 mtorr) high (> 1.2 mtorr) pressure was plasma unstable. Therefore, It was investigated at stable plasma forming conditions of 0.1 to 1 mtorr.
Figure 4.10 shows the change in residual strain at gas pressures in the range of 0.1 to 1 mtorr. As the gas pressure increased, the residual strain decreased from 0.002 to 0.008. It has been demonstrated as the Al atoms of energy decreased due to reduced mean free path. [4]
Figure 4.11 shows the variation in FWHM at gas pressures ranging from 0.1 to 1 mtorr. As the gas pressure increased, the FWHM increased from 0.57 to 0.58. The reason for the deterioration of the crystallinity is that the migration length is expected to decrease as the energy of Al atoms decreases due to the reduced mean free path.
0.1 1 0.010
0.005 0.000 -0.005 -0.010
F: 50kHz, toff 2.0μs Ar : N2 ~ 3:2 P.P.: 300W RT
Chamber pressure, P
c(mTorr)
Lattice strain (a.u.)
Figure 4.10 Lattice strain at different chamber pressure from 0.1 to 1 mtorr
0.1 1
0.55 0.56 0.57 0.58 0.59
0.60 F: 50kHz, toff 2.0μs
Ar : N2 ~ 3:2 P.P.: 300W RT
Chamber pressure, P
c(mTorr)
FWHM of theta-2theta peaks (deg.)
Figure 4.11 FWHM of theta-2theta peaks at different chamber pressure from 0.1 to 1 mtorr
4.5 Influence of substrate growth temperature on the pulsed sputtering deposition of AlN thin films
Previously, the gas pressure decreased the residual strain, but decreased the crystallinity. The parameters related to the Al supply energy generated during sputtering were limited in that the crystal quality and the residual strain were inversely proportional to each other. Therefore, the anode parameters such as the substrate growth temperature influenced after the Al supply need to be adjusted. In previous studies, the substrate growth temperature was reported to increase the surface migration length. [10]
Figure 4.12 shows the change in theta-2theta peak FWHM of AlN thin films prepared from room temperature to 500 ℃. As the growth temperature increases, the FWHM of the thin film is decreased from 0.57 to 0.28 deg due to the increase of the surface movement length.
Figure 4.13 shows the lattice strain from room temperature to 500 ℃.
The lattice strain also was decreased with increasing temperature. At 500
℃, thin films with the same lattice constant were observed compared to bulk AlN. Al-N clustering is also expected to decrease.
The substrate growth temperature in the PSD affected to increase both the crystal quality and the lattice strain due to the increase of the surface movement length.
Figure 4.14 shows the XRD patterns of films grown at room temperature, 300 ℃ and 500 ℃. The high FHWM and residual strain were observed at room temperature, and FHWM and residual strain decrease with increasing temperature.
Figure 4.15 shows these phi-scan patterns. In the samples at RT and 300
℃, no peaks were observed due to low crystallinity and high strain results.
It can expect the presence of large amounts of tilt and twist dislocation.
However, in a sample at 500 ℃, a 6-fold symmetric peaks at 60o intervals
were observed, which means that the wurtzite structure of AlN is well aligned. In addition, improvement of crystallinity of in plane can be judged to have a high c-axis direction because defects such as dislocation are reduced. Table 4.2 summarizes the results measured by various methods of XRD.
Table. 4.2 The values of various XRD measurements at different growth temperature
ω rocking curve FWHM
θ-2θpeak
FWHM φ scan HT-AlN (500oC) 1.53 deg. 0.28 deg. 6-fold
LT-AlN (300oC) 3.01 deg. 0.34 deg. U.A.
RT-AlN (30oC) 8.61 deg. 0.58 deg. U.A.
As a result, a thin film approximating to residual strain of 0 was fabricated at a growth temperature of 500 ℃, an Ar / N2 gas flow ratio of 3/2, and a plasma power of 300 W. The crystallinity with 6-fold symmetry was observed (ω rocking curve FWHM: 1.5 deg. Θ-2θ peak FWHM: 0.28 deg.).
0 100 200 300 400 500 0.2
0.3 0.4 0.5 0.6 0.7
0.8 F: 50kHz, toff 2.0μs
Ar : N2 ~ 3:2 P.P.: 300W RT
Growth temperature (
oC)
FWHM of theta-2theta peaks (deg.)
RT
Figure 4.12 FWHM of theta-2theta peaks at different growth temperature from RT to 500oC
0 100 200 300 400 500
0.010 0.005 0.000 -0.005 -0.010
F: 50kHz, toff 2.0μs Ar : N2 ~ 3:2 P.P.: 300W RT
Growth temperature (
oC)
lattice strain (a.u.)
RT
Figure 4.13 Lattice strain at different growth temperature from RT to 500oC
34 36 38 40 42 AlN (0002) Al2O3 (0006)
(c) 500oC, FWHM: 0.28
(b) 300oC, FWHM: 0.34
Intensity (a.u)
2theta (deg.)
(a) RT, FWHM: 0.58
Figure 4.14 XRD theta-2theta patterns of (a) RT, (b) 300 oC and (c) 500
oC
0 60 120 180 240 300
(c) 500oC
(b) 300oC
Intensity (a.u)
(a) RT
Phi (φ)
Figure 4.15 XRD phi patterns of (a) RT, (b) 300 oC and (c) 500 oC
4.6 Conclusion
In this chapter, the PSD condition of the AlN thin film was optimized.
Plasma power affected the amount of Al and the chemical stoichiometry of the film as well as migration length and plasma damage. Plasma damage induced residual strain, which was directly related to Al-N cluster.
The gas pressure was able to reduce the residual strain in relation to the mean free path of Al molecules, but the high gas pressure was induced to poor crystallinity due to low energy Al atoms.
As the growth temperature increased, both crystallinity and residual strain were improved.
Using these a thin film approximating to residual strain of 0 was fabricated at a growth temperature of 500℃, an Ar/N2 gas flow ratio of 3/2, and a plasma power of 300W. The crystallinity with 6-fold symmetry was observed.
Reference
[1] Sato, K. et al., 2009. Room-Temperature Epitaxial Growth of High Quality AlN on SiC by Pulsed Sputtering Deposition. Applied Physics Express, 2(1), 011003.
[2] Thornton, J. A. & Hoffman, D. W., 1977. Internal stresses in titanium, nickel, molybdenum, and tantalum films deposited by cylindrical magnetron sputtering. Journal of Vacuum Science and Technology, 14(1), pp.164-168.
[3] Takeuchi, H., Ohtsuka, M., & Fukuyama H., 2015. Effect of sputtering power on surfacecharacteristics and crystal quality of AlNfilms deposited by pulsed DC reactive sputtering. Phys. Status Solidi B, 252(5), pp.1163-1171.
[4] Ohtsuka, M., Takeuchi, H., & Fukuyama, H., 2016. Effect of sputtering pressure on crystalline quality and residual stress of AlN films deposited at 823 K on nitrided sapphire substrates by pulsed DC reactive sputtering. Japan Society of Applied Physics, 55, 05FD08.
[5] Cherng J. S., & Chang D. S., 2010. Effects of pulse parameters on the pulsed-DC reactive sputtering of AlN thin films. Vacuum, 84, pp.653-656.
[6] Stuart, R. V., Wehner, G. K., & Anderson, G. S., 1969. Energy Distribution of Atoms Sputtered from Polycrystalline Metals. Journal of Applied Physics, 40, pp.803-812.
[7] Soukup, R. J., Kulkarni, A. K., & Mosher, D. M., 1979. Electrical properties of sputtered epitaxial films of GaAs. Journal of Vacuum Science and Technology, 16 pp.208-211.
[8] Pan J.-H. et al., 2011. Epitaxy of an Al-Droplet-Free AlN Layer with Step Flow Features by Molecular Beam Epitaxy, Chinese Physics Letter, 28, 068102
[9] Correa, R. B., Rodríguez-García, M. E., & Mora, Á. P. 2014. Annealing effect on vibration modes of aluminum nitride thin films.
Mometnto-Revista de Física, 48, pp.64-76.
[10] Barshilia, H. C., Deepthi, B., & Rajam K.S., 2008. Growth and characterization of aluminum nitride coatings prepared by pulsed-direct current reactive unbalanced magnetron sputtering. Thin Solid Films, 516, pp. 4168-4174.
Chapter 5. Characterizations of AlN Thin Films
5.1 Introduction
AlN has characterizations that can be used in various fields such as DUV-LED, RF filter, SAW sensor, heat sink, transparent device and memory device. However, AlN thin films fabricated by sputtering have advantages such as a higher possibility of a low temperature growth, a low roughness of thin film and a low manufacturing cost, but it is not satisfying in terms of crystallinity for certain applications. A study on the characterization of sputtered AlN thin films is needed for wide application of AlN. Therefore, It is important to evaluate feasibility of AlN thin film sputtered.
In this chapter, the characterizations of AlN thin films fabricated with PSD were studied and evaluated to determine the feasibility of various fields.
5.2 Experiment details
The surface, refractive index, luminescence characteristics, permittivity and resistance of the samples of LT-AlN (700 nm)/Al2O3 (XRC FWHM: 3.02 deg.) And HT-AlN (700 nm)/Al2O3 (XRC FWHM: 1.53 deg.) were evaluated.
On the surface, the AFM was measured in the range of 10 × 10 μm2. FE-SEM measurements were performed under accelerating voltage of 5 kV.
and carbon coating was performed on the films before measurement. The refractive index (n) and extinction coefficient (k) were measured at wavelengths ranging from 300 to 1000 nm using an ellipsometer. The luminescence property was measured from 1.5 to 7 eV using CL at 80K.
The dielectric constant was measured in the region of 1 to 4000 MHz using VNA. I-V characterization was measured by fabricating Au/AlN/Al2O3 samples. Figure 5.1 shows a side view of the structure (a), and (b) OM top view image. Two upper Au metals were deposited on AlN/Al2O3 using a thermal evaporator. The thickness of Au is 100 nm and the distance between Au metals is 0.6 mm.
Figure 5.1 (a) The schematic and (b) OM top view image of fabricated MIM structure
5.3 Surface morphology
Figure 5.2 is the surface observed by various methods such as (a) photomicrograph and (b) FE-SEM (c) AFM 3D image of HT-AlN/Al2O3. In figure. 5.2 (a), the transparent characterization was observed in the visible light region and shows the value as a transparent device. The AlN material has a wide band gap and high transmittance in the visible region. In the case of sputtered AlN/glass, transmittance of over 83% was reported in the visible region. [1] In addition, a research has been reported about a transparent device, which is an ITO/AlN/ITO/glass structure. [2]
In figure 5.2 (b) and (c), a very smooth surface with a roughness RMS
value of less than 1 nm is observed. Sputtered AlN films were similar to previous results reported to have low roughness. [3,4] Roughness affects SAW characteristics. The smooth surfaces are required to minimize frequency variations due to local acoustic velocity changes. [5,6] Low roughness shows the research value as a SAW device.
Figure 5.2 (a) Photo image, (b) FE-SEM and (c) AFM 3D image of sputtered HT-AlN/Al2O3
5.4 Refraction index
Figure 5.3 shows a changes in refractive index (n) and extinction coefficient (k) of HT-AlN/Al2O3 depending on the wavelength range from 300 nm to 1000 nm. The value of n is 1.8, which is similar to the reported values of AlN/Glass (1.6) [3] and AlN/Si (1.9) [4].
Also, the value of k was approximated to 0 in the measured wavelength range. This is related to the absorption coefficient (β) as shown in equation 5.1, which means that light of a specific wavelength can be transmitted before it is absorbed.
(5.1)
In the case of semiconductors, absorption coefficient curves have sharp edges in light with energy below the band gap. It can be seen that k ≈ 0 in all wavelength regions and has a low absorption coefficient.
Figure 5.3 n and k value of sputtered HT-AlN/Al2O3
5.5 Luminescence property
Figure 5.6 shows CL spectra ranging from 1.5 to 7 eV at a low temperature of 80 K. Defects such as 3.00 eV nitrogen vacancy (VN) and 4.67 eV interstitial (ON) were observed in all samples of HT-AlN/Al2O3 and HT-AlN/Al2O3. [7] Near-band edge emission is also observed in LT-AlN/Al2O3.
2 3 4 5 6 7
LT- AlN/Al2O3
HT- AlN/Al2O3 Sapphire
(1.71eV)
@80k
6.0-6.5eV Sapphire
3.82eV VN
3.00eV
Relative Intensity (a.u)
Photon energy (eV)
ON 4.67eV
Figure 5.4 CL spectra of HT-AlN/Al2O3 and LT-AlN/Al2O3 at 80K
5.6 Relative permittivity
The dielectric properties can be determined by the memory device. The nitride material is characterized by low voltage/current capability. [8-10] Due to the thermal, dielectric and optical properties of AlN thin films, H. D. Kim et al. have been reported that AlN thin films have excellent value as transparent resistive random access memory (T-ReRAM) devices. [2]
Figure 5.4 shows the variation of dielectric constant according to the frequency of Al2O3 substrate, LT-AlN/Al2O3 and HT-AlN/Al2O3. Regardless of frequency, all samples is observed as parallel permittivity values. The relative dielectric constant of the AlN thin film at 1 MHz was
Al2O3 (4.6) <LT-AlN/Al2O3 (6.27) <HT-AlN/Al2O3 (6.5)
These values are similar to sputtered AlN films reported as values between 6.0 and 7.0. [4]
1000 2000 3000 4000
4 6 8
HT- AlN/Al2O3
Al2O3 substrate LT- AlN/Al2O3
Relative permittivity(ε r)
Frequency (MHz)
Figure 5.5 Relative permittivity of substrate, LT-AlN/Al2O3 and HT-AlN/Al2O3 as a function of test signal frequency
5.7 Electrical resistivity
Figure 5.8 shows the I-V curve from -5V to 5V for Au/LT-AlN/Al2O3 and Au/HT-AlN/Al2O3. Electrical resistivities (ρ) of Au/LT-AlN/Al2O3 (2.83 × 1011 Ωm) and HT-AlN/Al2O3. (4.24 X 1010 Ωm) were observed. These values are similar to the values for the resistive bulk AlN ρ = 109 - 1011 Ωm. [11].
-4 -2 0 2 4
-20 -15 -10 -5 0 5 10 15 20
LT-AlN/Al2O3
HT-AlN/Al2O3
C u rr en t (p A )
Voltage (V)
Figure 5.6 I-V curves of LT-AlN/Al2O3 and HT-AlN/Al2O3
5.8 Conclusion
In this chapter 5, evaluation of AlN thin films using PSD was investigated for feasibility. Table 5.1 shows properties of AlN thin films compared with AlN thin films on different substrates.
Table 5.1 The properties of reported AlN compared with this study
Property AlN/Si AlN/Glass In this study, AlN/Al2O3
Surface 1nm 1.7nm <1nm
Refraction index 1.9 1.6 1.8
Relative
permittivity - 6.0 ~ 7.0 5.7 ~ 6.5
Luminescence
property - - 6.3 eV
(Near band edge) Electrical
resistivity - 1010Ωm 1010 ~ 1011Ωm
The surface of AlN thin films by PSD has low roughness, transparency at visible region. The value of refraction index and the relative permittivity were similar compared with previous research.
As a result, It has similar characteristics compared with the AlN thin films studied in the past, which indicate that research and development of devices using AlN thin films by PSD is possible.
Reference
[1] Dimitrova, V., Manova, D., & Valcheva, E., 1999. Optical and dielectric properties of dc magnetron sputtered AlN thin films correlated with deposition conditions. Materials Science and Engineering, B68, pp.1–4 [2] Kim, H.-D., An, H.-M., Seo, Y., & Kim, T. G., 2011. Transparent
Resistive Switching Memory Using ITO/AlN/ITO Capacitors. IEEE Electron Device Letters, 32(8), pp.1125-1127.
[3] Kumari, N., Singh A. K., Barhai, & P. K., 2014. Study of Properties of AlN Thin Films Deposited by Reactive Magnetron Sputtering.
International Journal of Thin Films Science and Technology, 3 pp.43-49 [4] Andrzej Stafiniak et al., 2009. Properties of AlNx thin films prepared by
DC reactive magnetron sputtering. Optica Applicata 39(4) pp.717-722 [5] Wang, D. Y., Nagahata, Y., Masuda, M., & Hayashi, Y., 1996. Effect of
nonstoichiometry upon optical properties of radio frequency sputtered Al-N thin films formed at various sputtering pressures. Journal of Vacuum Science and Technology A, 14(6), pp.3092-3099
[6] Hsiosaki, T., Harada, K., & Kawabata, A., 1982. Low-Temperature Growth of Piezoelectric AlN Film and its Optical and Acoustical Properties. Proceedings of Japan Journal of Applied Physics, 21, pp.
69-71
[7] Slack, G. A., Schowalter, L. J., Morelli, D., & Freitas, J. A., 2002. Some effects of oxygen impurities on AlN and GaN. Journal of Crystal Growth, 246(3-4), pp.287-298.
[8] Kim, H.-D., An, H.-M., Kim, K. C., Seo, Y., Nam, K.-H., Chung H.-B., Lee, E. B. & Kim, T. G. 2011. Improved reliability of Au/Si3N4/Ti resistive switching memory cells due to a hydrogen post-annealing treatment. Journal of Applied Physics, 109(1), pp. 016 105-1–016 105-3,
[9] Kim, H.-D., An, H.-M. & Kim, T. G., 2010. Large resistive-switching phenomena observed in Ag/Si3N4/Al memory cells. Semiconductor Science and Technology, 25(6), pp. 065 002–065 005.
[10] Chen, C., Yang, Y. C., Zeng, F., & Pan, F., 2010. Bipolar resistive switching in Cu/AlN/Pt nonvolatile memory device. Applied Physics Letters, 97(8), pp. 083 502-1–083 502-3.
[11] Kumari, N., Singh, A. K., & Barhai, P. K., 2014. Study of Properties of AlN Thin Films Deposited by Reactive Magnetron Sputtering.
International Journal of Thin Films Science and Technology, 3(2), pp.43-49.
Chapter 6. Conclusion
In this study, growth and evaluation of AlN thin films were investigated for widen application of it. The AlN thin films have been deposited by sputter of two types such as DC mode and Pulsed DC mode. The characterization of AlN thin films using by PSD was investigated such as surface morphology, refraction index, relative permittivity, luminescence property and electrical resistivity.
Plasma power and N2/Al gas flow ratio were controled as fundamental experiments using by reactive gas DC magnetron sputter. the sputtering parameters were affected to Al supply amount. It changed the stoichiometric composition of the film. In the N-rich condition, the thin film showed a smooth surface and transparency in the visible region. In the Al-rich condition, the thin film showed dark color and a rough surface because of Al droplet. These conditions affected the crystallinity. It was also confirmed that the proper amount of Al is important for the crystallinity of the thin film.
The sputtering parameters such as plasma power, gas pressure and substrate growth temperature were controled for optimization of AlN growth condition using by PSD. In PSD, also plasma power affected to Al supply amount. However, residual strain, such as tensile strain in the c-axis direction, was observed due to high energy Al supply during sputtering. Also It indicated that Al-N clustering has to be solved. The plasma power and gas pressure were correlated with the kinetic energy of Al supplied, and the relationship between crystal quality and lattice strain was inversely proportional to each other. The substrate growth temperature was controlled, and the crystal quality as well as the lattice strain were also decreased. Using these, a thin film without residual strain was fabricated with 6-fold symmetry.
Feasibility of AlN thin films using by PSD was investigated. It has similar characteristics compared with the AlN thin films studied in the past. It indicates that research and development of devices using sputtered AlN is possible.
Resume
1. Personal Data
Name
(K o r e a n ) 조 성 민
(E n g lis h ) C h o S u n g m in (C h in e s e c h a r a c te r ) 趙 成 珉
Ages/Sex 26 / Man Date of birth 1991. 11. 18 E - m a il [email protected]
Tel. 052)269-1258 H.P. 010-6471-1258 Address #906, 14, Daegongwonipgu-ro, Nam-gu, Ulsan
South Korean
2. Education
Master of Engineering (Graduating in Feb. 2018)
Department of Convergence Study on the Ocean Science and Technology, Graduate School of Korea Maritime and Ocean University (KMOU)
Busan, Republic of korea
(Scholarship by KIOST and KMOU)
2016.03. ~ 2018.02.
Bachelor of Engineering (Graduated)
Major of Electronic Material Engineering, Korea Maritime and Ocean University
Busan, Republic of korea
2010.03. ~ 2016.02.
High school (Graduated)
Natural sciences, Ulsan Jeil Highschool Ulsan, Republic of korea
2007.03. ~ 2010.02.