TF_O3 TF_O4 TF_O5 TF_O6 TF_O7 TF_O8 TF_O9 TF_O10 TF_O11 X-ray absorption spectroscopy of indium nitride, indium oxide, and their alloys Jiraroj T-Thienprasert, Suranaree University of Technology Structural and optical properties of diamond-like carbon films deposited by pulsed laser ablation Kanchaya Honglertkongsakul, Chulalongkorn University Preparation and Characterization of Mg x Zn 1-x O Nanostructure for Dye-Sensitized Solar Cell Kittisak Umma, Chiang Mai University Evolution of ZnO Thin Films Prepared by Reactive DC Sputtering Technique using the Mixed Gases of N 2 O and N 2 Kriangkrai Wantong, Chulalongkorn University The Optical Properties of ZnO/Eosin Y/CuPc Electrode for Dyesensitized Solar Cells application Pikaned Uppachai, Chiang Mai University Effects of Se Flux on 2-Stage Growth of Cu(In,Ga)Se 2 Thin Film Solar Cells Rachasak Sakdanuphab, Chulalongkorn University Development of Surface Magneto-Optical Kerr Effect Measurement System at Beamline 4 of the Siam Photon Laboratory Ritthikrai Chai-ngam, Mahasarakham University and Synchrotron Light Research Institute MeV-ion Beam Analysis of Atomic Layer-deposited Ultra-thin Oxide Films on Single Crystalline Silicon Teerasak Kamwanna, Thailand Center of Excellence in Physics Effect of Abrasion Methods on Nucleation Density for Growing of Diamond Thin Films by Microwave Plasma Enhanced Chemical Vapor Deposition S.Tippawan Khlayboonme, King Mongkut's Institute of Technology Ladkrabang 111 112 113 114 115 116 117 118 119 TF_O12 TF_O13 Temperature Dependent Photoluminescence of Copper Phthalocyanine (CuPc) Organic Semiconductors Thin Films Wichan Techitdheera, King Mongkut's Institute of Technology Ladkrabang Optical Properties of Metal Oxide Films Characterized by Transmission Spectroscopy Wisanu Pecharapa, King Mongkut's Institute of Technology Ladkrabang 120 121 XIX
MeV-ION BEAM ANALYSIS OF ATOMIC LAYER-DEPOSITION ULTRA-THIN OXIDE FILMS ON SINGLE CRYSTALLINE SILICON T. Kamwanna 1*, A. Deachana 2, U. Tippawan 1,2, L.D. Yu 1,2, D. Boonyawan 1,2, P.K. Chu 3 and S. Singkarat 1,2 1 Research Center in Particle Beam and Plasma Physics (PBPP), Thailand Center of Excellence in Physics, P.O. Box 70, Chiang Mai University, Chiang Mai-50202, Thailand 2 Plasma and Beam Physics Research Facility (PBP), Department of Physics, Faculty of Science, Chiang Mai University, Chiang Mai-50200, Thailand 3 Plasma Laboratory, Department of Applied Physics, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China Abstract Ultra-thin aluminum oxide (Al 2 O 3 ) film is currently being explored as a high dielectric constant gate dielectric for the next generation CMOS and related devices. Among many methods to produce such a film, atomic layer deposition (ALD) is a very attractive technique in the sense that it enables deposition of ultra-thin layers on the substrate with monolayer control. In this work, the Al 2 O 3 film was deposited using ALD technique on a single crystalline silicon (100) substrate with a plasma grown silicon dioxide (SiO 2 ) layer sandwiched in between as a buffer interface. The total oxide layered structure was characterized by Rutherford backscattering spectrometry (RBS) in the channeling mode with MeV He 2+ -ions as the analyzing probe. The RBS/channeling analysis detects O, Al and Si atoms in the ultra-thin oxide layers with clear separation from their Si substrate. Our evaluation was compared with results obtained by other standard technique, such as x-ray photoelectron spectroscopy (XPS). Keywords: ALD; Aluminum oxide; RBS; channeling; XPS 1. INTRODUCTION It is widely known that atomic layer deposition (ALD) [also known as atomic layer epitaxy (ALE)] is a technique excellent for uniformity, conformality and thickness control of the deposited thin film down to nanometer range [1,2]. This is based on the selfterminating reactions during each growth cycle [2]. It is an attractive technique used to synthesize an aluminum oxide (Al 2 O 3 ) film which is exceptional in a novel dielectric material for deep submicron processes. Due to its special properties such as thermal and chemical stability, excellent dielectric properties, and good adhesion to many surfaces, Al 2 O 3 has been considered a promising candidate to replace SiO 2 for gate dielectrics [3]. Therefore, Al 2 O 3 is selected as a coating material for the present study. Following to the fact that ion scattering is extremely sensitive to the top most atomic layer, MeV He-ion backscattering was used in this work as a means to characterize thickness and structural features of the ultra-thin oxide films on a single crystal silicon (100) substrate in a non-destructive manner. The channeling phenomenon of He-ions along the <100> axial direction of the Si substrate was used in order to reduce the intensity of Si-bulk RBS signal and to improve detection of oxygen and aluminum atoms present in the ultra-thin film. The principle involved in this analytical method is described. In order to confirm our evaluation, the thickness of Al 2 O 3 films was also investigated by other standard technique, such as x-ray photoelectron spectroscopy (XPS). 2. EXPERIMENTAL 2.1 Sample preparation The ALD of Al 2 O 3 films was carried out in an ALD-PBP reactor, which was a self-developed system at the Plasma and Beam Physics Research Facility (PBP), Chiang Mai University. Standard p-type Si(100) wafers were used as substrates. Prior to deposition, the substrates were cleaned by following the standard RCA procedure [4]. A silicon dioxide (SiO 2 ) layer was grown on the Si surface by oxygen plasma as a buffer interface between Al 2 O 3 film and Si substrate. Trimethyl Aluminum [TMA, Al(CH 3 ) 3 ] and O 2 were used as depositing precursors for aluminum and oxidation, respectively. Argon was used as a carrier gas for transporting the precursors from bubble vapors to the substrate surface and purging reaction gases from the chamber during each reaction cycle. Details of the coating process are shown in Fig. 1. TMA and O 2 flow rates were 1.0 and 7.5 sccm, respectively. While the purge Ar flow rate was 1.5 sccm. Total cycle length was 6.5 s. In this work, * Corresponding author. E-mail: teerasak@fnrf.science.cmu.ac.th
Figure 1: Schematic of ALD coating process of Al 2 O 3 film used in this work. several ultra-thin Al 2 O 3 films were grown with different number of coating cycles, i.e., 100, 200, 300 and 400 cycles (abbreviated as ALD100, ALD200, ALD300 and ALD400 hereafter, respectively). During the deposition, the Si substrates were kept at a temperature of 150 C. The process pressure was maintained at 2.3 Torr during the introduction of vaporized precursors. 2.2 Backscattering measurements The composition and thickness of the ALDdeposited ultra-thin Al 2 O 3 films on the single crystalline silicon substrates were investigated by means of Rutherford backscattering spectrometry in the channeling mode (abbreviated as RBS/C hereafter) [5,6]. RBS/C analysis was performed using He 2+ -ions produced by a standard 1.7-MV tandem Tandetron accelerator at Chiang Mai University. The He beam energy was typically 2.1-MeV for a standard RBS/C analysis, but 2.7-MeV energy was also used which will be seen later. The RBS/C measurement system and the analysis procedure have been described in detail elsewhere [7]. 3. RESULTS AND DISCUSSIONS Fig. 2 shows a typical 2.1-MeV RBS/C spectrum of one of the Al 2 O 3 films. The peaks from oxygen, aluminum and silicon atoms at the Si wafer surface are clearly seen. The Al and Si peaks are almost overlapping due to their close in mass, which needs more effort for a quantitative analysis. In principle, the separation between the Al and Si peaks will increase with higher energy of the incident ion beam. However, the effects of non-rutherford cross-section will complicate the quantitative analysis, especially for low-mass elements. Deviations of the scattering cross section from the Rutherford formula are observed for He-ion scattered from oxygen atoms at energy above 2.25-MeV [8]. There are resonances at 2.5-MeV and 3.05-MeV. In order to avoid this effect, the He-ion energy of 2.7-MeV was chosen for this measurement, since the cross-section for 16 O(α,α) 16 O does not Figure 2: RBS/C spectrum of 2.1-MeV He 2+ -ions backscattered from thin ALD100-Al 2 O 3 film on a Si(100) substrate. change rapidly in this region, and quantitative analysis is therefore reliable. In Fig. 3, the 2.7-MeV He 2+ RBS/C spectra obtained from Al 2 O 3 ultra-thin films deposited on Si(100) wafers are compared for different number of coating cycles. As shown in the figure, the Si surface peaks are slightly shifted from the surface peak of the virgin Si(100) surface toward lower backscattered ion energy due to the energy loss of He 2+ -ions in traversing the Al 2 O 3 and SiO 2 top layers. As the number of deposition cycles increased, the thickness of the Al 2 O 3 film naturally increased and thus the width of the Al peak in the spectrum increased so that the more number of the cycles, the more the Al and Si peaks overlapped. Figure 3: Measured RBS/C spectra of a 2.7-MeV He 2+ beam aligned along the <100> axial direction of Si wafers deposited with Al 2 O 3 film at four different number of coating cycles. As a reference, the backscattering yield spectrum along the <100> axis of a virgin Si wafer is also included.
To get a first evaluation of the Al 2 O 3 film thickness, the ALD100-Al 2 O 3 case was considered due to a clear separation between the Al and Si peaks, as shown in Fig. 4; so more reliable quantitative thin film analysis is possible. The total number of counts under the O, Al and Si peaks can be used to determine the thickness of Al 2 O 3 and SiO 2 layers by applying the peak integration method [6]. The integrated areas of the O, Al and Si peaks are evaluated by a straight-line extrapolation of the contribution from the Si substrate. The Al and Si peaks are analyzed by double Gaussian distributions, as shown in the inset of Fig. 4. By referring to the known Rutherford cross-section (σ) and the experimental parameters of the scattering geometry, the measured area (A) in counts can be converted to areal densities (Nt) in atoms/cm 2 for O, Al and Si peaks, from the following equation [5]: Nt = A/σΩQ, (1) where Q is the total number of incident ions and Ω is the solid angle subtended by the SSB detector. Therefore, the resulting total areal densities for O, Al and Si peaks are (8.11±0.47) 10 16, (2.93±0.20) 10 16 and (3.14±0.14) 10 16 atoms/cm 2, respectively. The quoted uncertainties are statistical due to the uncertainty in the integrated areas of the O, Al and Si peaks. The procedures for quantitative analysis of the spectrum shown in Fig. 4 are described as follows. Firstly, all the aluminum atoms are assumed to make compounded with the oxygen atoms in the form of Al 2 O 3. Note that the total areal density of O is a sum of that from Al 2 O 3 and that from SiO 2. Therefore, we can do following calculations. The areal density of oxygen atoms at the zero aluminum atom density is (3.72±0.57) 10 16 atoms/cm 2, calculated from the Al areal density and stoichiometric Al 2 O 3, which corresponds to the areal density of oxygen atoms in the SiO 2 layer. From this value, the density of Si atoms that are contained in the oxide layer can be evaluated using the stoichiometric SiO 2, i.e., equal to (1.86±0.28) 10 16 atoms/cm 2. The Si surface peak in this case has several origins such as the Si atoms contained in the oxide layer, Si atoms in the Si/SiO 2 interface region, and nonshadowed Si atoms in the outermost layers of a perfect Si crystal. Therefore, the areal density of Si atoms at zero oxygen atom density is (1.28±0.31) 10 16 atoms/cm 2, which is greater than the areal density of nonshadowed Si atoms, i.e., (1.08±0.10) 10 16 atoms/cm 2, in the surface region of clean Si(100) crystal. The excess (2.02±0.32) 10 15 atoms/cm 2, approximately three monolayers, are attributed to the displaced Si atoms at the Si/SiO 2 interface. For an evaluation of the physical film thickness, the density of the film is an important input; but it is an unknown in this case. However, it is customary to estimate the film thickness by using the bulk densities. The tabulated values of the bulk densities for Al 2 O 3, SiO 2 and Si are 1.175 10 23, 6.98 10 22 and 4.98 10 22 atoms/cm 3, respectively [8]. Hence, the thicknesses of the Al 2 O 3 film, SiO 2 layer and the interface width of Si/SiO 2 are 6.23±0.43, 8.00±1.20 and 0.41±0.06 nm, respectively. For the other cases (ALD200, ALD300 and ALD 400), the Al and Si peaks are not separated, as shown in Fig. 2. However, the evaluation of the film thickness can be done by assuming equal number of oxygen atoms in the SiO 2 layer in every samples, i.e., (3.72±0.57) 10 16 atoms/cm 2. This assumption is based on the experimental fact that the SiO 2 layers were grown by oxygen plasma at equal time of which the excess oxygen atoms were made compound with aluminum atoms to form Al 2 O 3. The thicknesses of Al 2 O 3 films are summarized in Fig. 5. It shows that the thickness of Al 2 O 3 film is a linear dependence with the number of coating cycles, which indicates a constant growth rate of Al 2 O 3 films Figure 4: RBS/C spectrum of 2.7-MeV He 2+ -ions backscattered from thin ALD100-Al 2 O 3 film on a Si(100) substrate. The inset shows the Al and Si peaks fitted by double Gaussian distribution. Figure 5: The thickness of Al 2 O 3 films are plotted as a function of the number of coating cycles. The results observed from XPS are also included for comparison.
(0.094 nm Al 2 O 3 per cycle). This may implies that the deposition can begin during the first cycle. In Fig. 5, the thicknesses of these Al 2 O 3 films obtained from XPS measurements are also included. The measurements were carried out using an XPS system of model: PHI5802 from Physical Electronics, Inc. (PHI), USA. The analysis was performed using x- ray beam from an Al Kα monochromatic source (1486.6 ev) with an energy step of 0.8 ev. In general, both results show similar growing behavior. However, the growth rate estimated through the XPS is slightly higher than the RBS/C evaluation, especially at high number of coating cycles. This discrepancy may attributed to the fact that the XPS is a surface analysis technique with a sampling volume that extends from the surface to a depth of 20-50 Å [9]; the photoelectrons generated deeper than such depth if can escape will not have sufficient energy to be detected. Alternatively, the sputter depth profiling was utilized by quantifying matrix-level elements as a function of depth. Thus, the accuracy of the evaluation at high numbers of coating cycles is mainly affected by the sputtering rate of the system. On the other hand, for the RBS/C measurement, the 2.7 MeV He 2+ - ion range in Al 2 O 3 film (i.e., 5.7 μm as calculated by the SRIM simulation program [10]) is exceedingly deeper than the film thickness. The He-ions can penetrate both Al 2 O 3 film and SiO 2 layer and be backscattered as well. Therefore, the ions can provide the information in whole range of the films. Further attribution, the film thickness is directly affected by the orientation in sample mounting during analysis. Measurement by the XPS technique in the oblique direction can results in the film thickness thicker than measuring in the normal direction. In RBS/C measurement, the samples are certainly installed normal to the incident ions, since the crystal axis of the sample must be aligned along the incident direction of the ions. But XPS measurements have no alignment procedure of the sample. Thus, we believe that our results are in agreement with the XPS within the experimental errors. 4. CONCLUSIONS The thickness in the nano-scale of aluminum oxide ultra-thin film deposited by ALD was analyzed using backscattering spectrometry of MeV-ion beam. The use of channeling mechanism in the Si substrate and higher ion beam energies helps to separate elements which are close in mass, such as Al and Si. In order to avoid errors related to the absolute value of a scattering cross-section, the probing ion beam energy should be directed away from any sharp resonances in scattering on the elements of interest. ACKNOWLEDGEMENTS The authors would like to express their gratitude to the CoE Programme of Chiang Mai University, the National Research Council of Thailand (NRCT) and the International Atomic Energy Agency (IAEA, Vienna) for financial supports. We thank Mr. C. Thongleurm of Science and Technology Research Institute, Chiang Mai University for technical assistance. A. Deachana wishes to thank the Plasma Laboratory of City University of Hong Kong for collaboration through project no. 8730024. REFERENCES [1] K. Grigoras, S. Franssila, V.-M. Airaksinen, Thin Solid Films 516 (2008) 5551-5556. [2] R.L. Puurunen, J. Appl. Phys. 97 (2005) 121301. [3] J. Kim, K. Chakrabarti, J. Lee, K.-Y. O, C. Lee, Mater. Chem. Phys. 78 (2003) 733-738. [4] W. Kern, D.A. Puotinen, RCA Rev. 31 (1970) 187. [5] W.K. Chu, J.W. Mayer, Marc-A. Nicolet, Backscattering Spectrometry, Academic Press, New York, 1978. [6] L.C. Feldman, J.W. Mayer, S.T. Picraux, Materials Analysis by Ion Channeling, Academic Press, New York, 1982. [7] T. Kamwanna, N. Pasaja, L.D. Yu, T. Vilaithong, A. Anders, S. Singkarat, Nucl. Instr. and Meth. B 266 (2008) 5175-5179. [8] J.R. Tesmer, M.A. Nastasi (Eds.), Handbook of Modern Ion Beam Material Analysis, Materials Research Society, Pittsburgh, PA, 1995. [9] X-ray photoelectron spectroscopy, http://www.mee-inc.com/xray-photo.html. [10] SRIM Program, http://www.srim.org.