Open Access

The impact of thickness and thermal annealing on refractive index for aluminum oxide thin films deposited by atomic layer deposition

  • Zi-Yi Wang1,
  • Rong-Jun Zhang1Email author,
  • Hong-Liang Lu2,
  • Xin Chen3Email author,
  • Yan Sun3,
  • Yun Zhang3,
  • Yan-Feng Wei4,
  • Ji-Ping Xu1,
  • Song-You Wang1,
  • Yu-Xiang Zheng1 and
  • Liang-Yao Chen1
Nanoscale Research Letters201510:46

https://doi.org/10.1186/s11671-015-0757-y

Received: 19 November 2014

Accepted: 15 January 2015

Published: 6 February 2015

Abstract

The aluminum oxide (Al2O3) thin films with various thicknesses under 50 nm were deposited by atomic layer deposition (ALD) on silicon substrate. The surface topography investigated by atomic force microscopy (AFM) revealed that the samples were smooth and crack-free. The ellipsometric spectra of Al2O3 thin films were measured and analyzed before and after annealing in nitrogen condition in the wavelength range from 250 to 1,000 nm, respectively. The refractive index of Al2O3 thin films was described by Cauchy model and the ellipsometric spectra data were fitted to a five-medium model consisting of Si substrate/SiO2 layer/Al2O3 layer/surface roughness/air ambient structure. It is found that the refractive index of Al2O3 thin films decrease with increasing film thickness and the changing trend revised after annealing. The phenomenon is believed to arise from the mechanical stress in ALD-Al2O3 thin films. A thickness transition is also found by transmission electron microscopy (TEM) and SE after 900°C annealing.

Keywords

ALDAl2O3 thin filmOptical propertiesSpectroscopic ellipsometry

Background

Aluminum oxide (Al2O3) thin films are used as gate dielectric films in electronic devices [1], protective coating layer in magnetic read heads [2], encapsulation layer in light emitting diodes [3], antireflection coating in solar thermal cells [4], and many other areas [5-7]. These applications benefit from the excellent optical and electrical properties of Al2O3 films such as wide bandgap, high conduction, high compatibility with Si substrate, and high dielectric constant [8]. The properties of Al2O3 films have been studied a lot [9-11]. However, most articles focused on electrical and mechanical properties of Al2O3 films. The research on optical properties of Al2O3 films, especially for Al2O3 films thinner than 50 nm, is still lacking. The applications of optical critical dimension, in situ spectroscopic ellipsometry (SE) and phase measurements in inspection are widely used in semiconductor process and solar cells. These applications are dependent on the accuracy of dielectric constants. The inaccurate optical constant of Al2O3 can introduce errors in fabricating procedure and further influence the performance of devices. So the study of optical properties of Al2O3 thin films is needed.

Atomic layer deposition (ALD) is one of the most popular chemical vapor deposition methods used in oxide film fabrication [12]. For its low temperature and monolayer deposition, Al2O3 ultrathin films with a smooth and defect-free surface can be deposited. Therefore, ALD-Al2O3 films are widely used in recent researches [13,14].

SE is routinely used in optical characterization and film thickness determination. In the SE measurement, a linearly polarized light is illuminated on the sample. The polarization state will be changed after the light reflected. Two parameters, the amplitude ratio (Ψ) and phase shift (Δ) between reflected p- and s-polarized light, are obtained from the measurement [15]. The ellipsometric spectra can be fitted to the optical model based on the film structure, then the optical properties and film thickness of the measured material can be revealed [16-18]. Its noncontact, nondestructive characteristics are ideal for many situations when film thickness or dielectric constants are needed [19,20].

In this paper, the thickness dependence of refractive index for ALD-Al2O3 films is investigated by SE. An anomaly change trend of refractive index for Al2O3 films was reported. The changing trend reversed after the Al2O3 films were annealed in nitrogen condition at different temperatures. The thickness transition was observed through transmission electron microscopy (TEM) and SE. The change of dielectric constant was explained by the changing of dielectric polarization after annealing.

Methods

The Al2O3 films were deposited by a thermal ALD reactor (Picosun R-series, Espoo, Finland) on Si substrate. Trimethylaluminum (TMA; Al(CH3)3) and water (H2O) were used as metal and oxidation precursors, respectively. The reacting temperature is 100°C. The characteristic analysis of surface morphology was performed by atomic force microscopy (AFM; Bruker Dimension Icon VT-1000, Santa Barbara, CA, USA) in tapping mode. The ellipsometric spectra were measured by a SE system (J.A. Woollam Co. M2000X-FB-300XTF, Lincoln, NE, USA) over the wavelength range of 250 to 1,000 nm at incident angle of 65°. The thickness of SiO2 and Al2O3 layers were identified by TEM (FEI Tecnai G2 F20, Hillsboro, OR, USA), Then the ALD-Al2O3 samples were cut into three pieces and annealed at 400°C, 600°C, and 900°C in nitrogen atmosphere. A rapid thermal process system (RTP; AS-ONE, Montpellier, France) was used. The annealed samples were researched by AFM, TEM, and SE again to perform further analysis.

Considering the Si substrate always have a native oxide layer [21], the ellipsometric spectra were collected for the Si substrate with oxide layer and ALD-Al2O3 thin film, respectively. The RMS roughness obtained from AFM helps determining the thickness of roughness layer. So the optical model used in SE fitting is Si substrate/SiO2 layer/Al2O3 layer/surface roughness/air ambient, as shown in Figure 1. The dispersion model of Al2O3 used in SE fitting is Cauchy model [22].
Figure 1

The schematic of optical model used in SE fitting for Al 2 O 3 thin films.

Results and discussion

The numbers of ALD cycles in the deposition were 50, 100, 300, and 500. Figure 2 shows the AFM images of selected ALD-Al2O3 thin film. The surface of the samples is smooth and crack-free, which indicates that Al2O3 films were well fabricated. The root mean square roughness (RMS roughness) information of all samples is listed in Table 1. The thickness of surface roughness layer used in SE fitting is fixed as the RMS value. And the roughness layer is described by a Bruggeman effective medium approximation mixed by 50% Al2O3 and 50% void [23].
Figure 2

AFM images of 500 cycles Al 2 O 3 film. (a) As deposited and annealed at (b) 400°C, (c) 600°C, and (d) 900°C.

Table 1

RMS roughness of ALD-Al 2 O 3 thin films

 

Sample (cycles)

50

100

300

500

Annealing temperature (°C)

RMS roughness (nm)

As deposited

0.65

0.62

0.49

0.60

400

0.54

0.55

0.58

0.42

600

0.49

0.51

0.53

0.52

900

0.54

0.46

0.40

0.54

Considering Al2O3 is transparent in visible region, the optical model of Al2O3 used in SE fitting is Cauchy model, which is defined as follows [22]:
$$ n\left(\lambda \right)=A+\frac{B}{\lambda^2}+\frac{C}{\lambda^4} $$
(1)
$$ k=0 $$
(2)
where A, B, and C are the material coeffients that define the real part of the refractive index n(λ). Figure 3a illustrates the thickness dependence of refractive index for as deposited ALD-Al2O3 films revealed by SE fitting. It can be found that with increasing thickness, the refractive index of Al2O3 is decreasing, which is contrary to Al2O3 films thicker than 50 nm [24-26]. From the inset of Figure 3a, we can know that the Al2O3 films were grown at a speed of 0.88 Å/cycle. The growth rate becomes stable when ALD cycle is higher than 100.
Figure 3

Thickness dependence of refractive index for ALD-Al 2 O 3 films. (a) As deposited and annealed at (b) 400°C, (c) 600°C, and (d) 900°C in nitrogen. The inset is the growth rate of as deposited Al2O3 films.

The Al2O3 thin films were then annealed at 400°C, 600°C, and 900°C, respectively. The changing trend of refractive index at each annealing temperature is illustrated in Figure 3b,c,d. It is indicated that the thickness dependence of refractive index for Al2O3 films reversed and shows regular evolution rule after annealing. Furthermore, the thicknesses of Al2O3 films show a significant decrease after 900°C annealing. TEM pictures in Figure 4 also support the SE results. The thickness of SiO2, 300 cycles of Al2O3, and RMS layer at different annealing temperatures in SE fitting and TEM measurements are compared in Table 2. The thickness of SiO2 layer slightly increased after annealing. And the Al2O3 film went through a densification process after annealing.
Figure 4

TEM pictures of 300 cycles of Al 2 O 3 film. (a) As deposited and annealed at (b) 600°C and (c) 900°C.

Table 2

Thickness comparison between SE and TEM on 300 cycles Al 2 O 3 film

 

Thickness (nm)

SE

TEM

Annealing temperature (°C)

SiO 2

Al 2 O 3

RMS

Al 2 O 3

SiO 2

RMS

As deposited

1.0

25.9

0.49

0.7

25.6

-

600

1.0

24.8

0.53

1.3

24.5

-

900

1.0

20.6

0.40

1.5

20.2

-

Generally, the ALD-Al2O3 film will be under a stress state after it is deposited. And for thin films under 50 nm, the effect of internal stress is strongly related to thickness [27,28]. The anomaly changing trend of refractive index for Al2O3 thin films is only observed in the as deposited samples. The reverse of changing trend may be contributed to two reasons caused by annealing process: stress release and phase transition.

To further understand the effect of annealing process, the refractive index depending on annealing temperature for each sample are researched and given in Figure 5. A significant increase in refractive index after 900°C annealing can be noted. This variation can also be observed from the thickness decreasing illustrated in Figures 3 and 4, which means the films are more compact or may become crystallized. But the crystal grain is not observed from TEM pictures in Figure 4b,c. The Al2O3 thin films are not crystallized after annealing. The transition of the films is due to stress release and densification caused by annealing.
Figure 5

Annealing temperature dependence of refractive index for ALD-Al 2 O 3 with different thicknesses. (a) 50 cycles, (b) 100 cycles, (c) 300 cycles, and (d) 500 cycles.

The refractive index of Al2O3 films increased after thickness transition. This phenomenon can be explained by classical dielectric theory. For a transparent material, the dielectric constant ε is given by [22].
$$ \varepsilon =1+\frac{P}{\varepsilon_0E}={n}^2 $$
(3)
$$ P={\displaystyle \sum_i}{q}_i{l}_i $$
(4)
where P is the dielectric polarization, n is the refractive index, ε 0 is the free-space permittivity, and E is the electric field. The q i and l i are electric charge of electric dipole and distance between the charge pair, respectively. So the dielectric polarization is related to the electric charge and distances of dipoles in the material. The dielectric constant will become larger if the dielectric polarization is larger.

In the process of annealing, the vacancies are filled during annealing and the thickness of Al2O3 films will decrease. The charge of electric dipole is then increased and leads to a higher dielectric polarization. This is often accompanied with a decreasing of total binding energy [29], which agrees with previous reports on Al2O3 films [10,27].

The annealing process, which is believed as an efficient method to release the stress or leads to a thickness transition, turned the changing trend of refractive index back to normal. The stress in ALD-Al2O3 thin films caused the anomaly trend. And the stress has been released after a 400°C annealing. A higher annealing temperature further led to a thickness transition of Al2O3 films. Both stress release and thickness transition will have a significant influence on the refractive index of Al2O3 films.

Conclusions

In summary, the ALD-Al2O3 thin films with various thicknesses were fabricated and annealed at different temperatures. The AFM measurement indicated that the surface roughness of Al2O3 thin films was less than 1 nm. The SE analysis revealed that the refractive index of as deposited Al2O3 thin films decreases with increasing film thickness. And this anomaly phenomenon disappeared after annealing. Further analysis on SE and TEM data shows that the stress of as deposited Al2O3 caused the anomaly changing trend of refractive index. And the refractive index becomes higher after 900°C annealing, which is contributed by vacancy filling induced higher dielectric polarization. The revolution of optical constant will affect other properties of Al2O3 thin films and leads to new features. The results given in this work will be helpful in further fabrication and application of Al2O3 thin films.

Abbreviations

AFM: 

atomic force microscopy

ALD: 

atomic layer deposition

RMS roughness: 

root mean square roughness

SE: 

spectroscopic ellipsometry

TEM: 

transmission electron microscopy

Declarations

Acknowledgements

This work has been financially supported by the National Natural Science Foundation of China (Nos. 11174058, 61275160, 11374055 and 61427815), the No. 2 National Science and Technology Major Project of China (No. 2011ZX02109-004), the STCSM project of China with Grant No. 12XD1420600, and Key Laboratory of infrared imaging materials and detectors, Shanghai Institute of Technical Physics, Chinese Academy of Sciences.

Authors’ Affiliations

(1)
Shanghai Engineering Research Center of Ultra-Precision Optical Manufacturing, Department of Optical Science and Engineering, Laboratory of Micro and Nano Photonic Structures, Ministry of Education, Fudan University
(2)
State Key Laboratory of ASIC & System, Fudan University
(3)
National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences
(4)
Key Laboratory of Infrared Image Materials and Devices, Shanghai Institute of Technical Physics, Chinese Academy of Sciences

References

  1. Chen S-H, Liao W-S, Yang H-C, Wang S-J, Liaw Y-G, Wang H, et al. High-performance III-V MOSFET with nano-stacked high-k gate dielectric and 3D fin-shaped structure. Nanoscale Res Lett. 2012;7:1–5.View ArticleGoogle Scholar
  2. Groner M, Elam J, Fabreguette F, George SM. Electrical characterization of thin Al2O3 films grown by atomic layer deposition on silicon and various metal substrates. Thin Solid Films. 2002;413:186–97.View ArticleGoogle Scholar
  3. Meyer J, Schneidenbach D, Winkler T, Hamwi S, Weimann T, Hinze P, et al. Reliable thin film encapsulation for organic light emitting diodes grown by low-temperature atomic layer deposition. Appl Phys Lett. 2009;94:233305.View ArticleGoogle Scholar
  4. Nuru ZY, Arendse CJ, Khamlich S, Maaza M. Optimization of AlxOy/Pt/AlxOy multilayer spectrally selective coatings for solar–thermal applications. Vacuum. 2012;86:2129–35.View ArticleGoogle Scholar
  5. Groner MD, George SM, McLean RS, Carcia PF. Gas diffusion barriers on polymers using Al2O3 atomic layer deposition. Appl Phys Lett. 2006;88:051907.View ArticleGoogle Scholar
  6. Dameron AA, Davidson SD, Burton BB, Carcia PF, McLean RS, George SM. Gas diffusion barriers on polymers using multilayers fabricated by Al2O3 and rapid SiO2 atomic layer deposition. J Phys Chem C. 2008;112:4573–80.View ArticleGoogle Scholar
  7. Ding S-J, Chen H-B, Cui X-M, Chen S, Sun Q-Q, Zhou P, et al. Atomic layer deposition of high-density Pt nanodots on Al2O3 film using (MeCp)Pt(Me)3 and O2 precursors for nonvolatile memory applications. Nanoscale Res Lett. 2013;8:1–7.View ArticleGoogle Scholar
  8. Kim SK, Lee SW, Hwang CS, Min Y-S, Won JY, Jeong J. Low temperature (<100 C) deposition of aluminum oxide thin films by ALD with O3 as oxidant. J Electrochem Soc. 2006;153:F69–76.View ArticleGoogle Scholar
  9. Koski K, Hölsä J, Juliet P. Properties of aluminium oxide thin films deposited by reactive magnetron sputtering. Thin Solid Films. 1999;339:240–8.View ArticleGoogle Scholar
  10. Snijders P, Jeurgens L, Sloof W. Structural ordering of ultra-thin, amorphous aluminium-oxide films. Surf Sci. 2005;589:98–105.View ArticleGoogle Scholar
  11. Zhao Y, Zhou C, Zhang X, Zhang P, Dou Y, Wang W, et al. Passivation mechanism of thermal atomic layer-deposited Al2O3 films on silicon at different annealing temperatures. Nanoscale Res Lett. 2013;8:114.View ArticleGoogle Scholar
  12. Puurunen RL. Surface chemistry of atomic layer deposition: a case study for the trimethylaluminum/water process. J Appl Phys. 2005;97:121301.View ArticleGoogle Scholar
  13. Dingemans G, Kessels E. Status and prospects of Al2O3-based surface passivation schemes for silicon solar cells. J Vac Sci Technol A. 2012;30:040802.View ArticleGoogle Scholar
  14. Ghiraldelli E, Pelosi C, Gombia E, Chiavarotti G, Vanzetti L. ALD growth, thermal treatments and characterisation of Al2O3 layers. Thin Solid Films. 2008;517:434–6.View ArticleGoogle Scholar
  15. Tompkins H, Irene EA. A quick guide to ellipsometry. In: Handbook of ellipsometry. Norwich, NY: William Andrew; 2005. p. 4–19.View ArticleGoogle Scholar
  16. Zhang R-J, Chen Y-M, Lu W-J, Cai Q-Y, Zheng YX, Chen LY. Influence of nanocrystal size on dielectric functions of Si nanocrystals embedded in SiO2 matrix. Appl Phys Lett. 2009;95:161109.View ArticleGoogle Scholar
  17. Xu Z-J, Zhang F, Zhang R-J, Yu X, Zhang D-X, Wang Z-Y, et al. Thickness dependent optical properties of titanium oxide thin films. Appl Phys A. 2013;113:557–62.View ArticleGoogle Scholar
  18. Xu J-P, Zhang R-J, Chen Z-H, Wang Z-Y, Zhang F, Yu X, et al. Optical properties of epitaxial BiFeO3 thin film grown on SrRuO3-buffered SrTiO3 substrate. Nanoscale Res Lett. 2014;9:1–6.View ArticleGoogle Scholar
  19. Langereis E, Heil S, Knoops H, Keuning W, Van de Sanden M, Kessels W. In situ spectroscopic ellipsometry as a versatile tool for studying atomic layer deposition. J Phys D Appl Phys. 2009;42:073001.View ArticleGoogle Scholar
  20. Cai Q-Y, Zheng Y-X, Zhang D-X, Lu W-J, Zhang R-J, Lin W, et al. Application of image spectrometer to in situ infrared broadband optical monitoring for thin film deposition. Opt Express. 2011;19:12969–77.View ArticleGoogle Scholar
  21. Ourmazd A, Taylor D, Rentschler J, Bevk J. Si → SiO2 transformation: interfacial structure and mechanism. Phys Rev Lett. 1987;59:213.View ArticleGoogle Scholar
  22. Fujiwara H. Spectroscopic ellipsometry: principles and applications. Chichester: John Wiley & Sons; 2007.View ArticleGoogle Scholar
  23. Bruggeman D. Dielectric constant and conductivity of mixtures of isotropic materials. Ann Phys(Leipzig). 1935;24:636–79.Google Scholar
  24. Chiu R-L, Chang P-H. Thickness dependence of refractive index for anodic aluminium oxide films. J Mater Sci Lett. 1997;16:174–8.View ArticleGoogle Scholar
  25. Hoffman D, Leibowitz D. Al2O3 films prepared by electron-beam evaporation of hot-pressed Al2O3 in oxygen ambient. J Vac Sci Technol. 1971;8:107–11.View ArticleGoogle Scholar
  26. Patil PV, Bendale DM, Puri RK, Puri V. Refractive index and adhesion of Al2O3 thin films obtained from different processes — a comparative study. Thin Solid Films. 1996;288:120–4.View ArticleGoogle Scholar
  27. Krautheim G, Hecht T, Jakschik S, Schröder U, Zahn W. Mechanical stress in ALD-Al2O3 films. Appl Surf Sci. 2005;252:200–4.View ArticleGoogle Scholar
  28. Jen S-H, Bertrand JA, George SM. Critical tensile and compressive strains for cracking of Al2O3 films grown by atomic layer deposition. J Appl Phys. 2011;109:084305.View ArticleGoogle Scholar
  29. Murata M, Wakino K, Ikeda S. X-ray photoelectron spectroscopic study of perovskite titanates and related compounds: an example of the effect of polarization on chemical shifts. J Electron Spectrosc. 1975;6:459–64.View ArticleGoogle Scholar

Copyright

© Wang et al.; licensee Springer. 2015

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.