OPTIMIZING OF THERMAL EVAPORATION PROCESS COMPARED TO MAGNETRON SPUTTERING FOR FABRICATION OF TITANIA QUANTUM DOTS Vojtěch SVATOŠ 1, Jana DRBOHLAVOVÁ 1, Marian MÁRIK 1, Jan PEKÁREK 1, Jana CHOMOCKÁ 1, Jaromír HUBÁLEK 1 1 Brno University of Technology, Brno, Czech Republic, EU, drbohla@feec.vutbr.cz Abstract The titania quantum dots (QDs) are fabricated using template based non-lithographic technique. This technique is comparing to the other known methods (such as e-beam or droplet epitaxy, and lithography), cheaper, faster, and well reproducible. Nanoporous template employed in this method is created from aluminum layer. The material in the following step for creating QDs is titanium film located under the aluminum film. These films are prepared using physical vapor deposition (PVD) processes. Electrochemical properties and behavioral of these layers are key parameters for fabrication of QDs. During PVD deposition, it is very important to optimize every parameter of deposition process due to following creation of QDs. In the presented paper the comparison of two deposition methods is proposed and the deposition parameters influence on the nanofabrication of QDs is shown. In the beginning, there is the optimization of the thermal evaporation. The PVD thermal evaporation process is used due the ability of stress reducing, purity of materials, and grain structure growing. The optimizing of the evaporation is very important because of a few techniques and improvements (e.g. the substrate heating) are not allowed to be employed in order to reach the optimal titanium and aluminum layers. The PVD magnetron sputtering is one of the most applied methods for thin film deposition. In this paper, there is a discussion of nanofabrication results, namely characterization of final QDs fabricated in the process of anodization using Ti/Al bilayer deposited via the optimized thermal evaporation and the magnetron sputtering. Keywords: Titania quantum dots, anodization, magnetron sputtering, thermal evaporation. 1. INTRODUCTION The titania QDs have increasingly attacking attention as possible optical detectors of biomolecules, such as proteins, DNA, etc. [1]. Also the nanostructured surface may be a possibility as platform for electrochemical detection e.g. monitoring of adhesion, proliferation and apoptosis of mammalian cells on the electrode surface. Considering previously mentioned potential used there is the wide concern to focus on the QDs fabrication. The fabrication of QDs has been in detail described in previously published papers, e.g. [2]. Employing the non-lithographic technique, the first and fundamental fabrication step is the deposition of metal layers. This paper presents two fundamental physical vapor deposition(pvd) methods, magnetron sputtering and thermal evaporation, as the first step for nanofabrication of titania QDs. In this work there is comparison of these two deposition technique, and the presenting of the optimizing of thermal evaporation to gain the optimal film properties for creating QDs. Deposition of thin films by PVD techniques has found widespread use in many industrial sectors. The PVD has been known and performed for very long time. There have been numerous publishing activities about these techniques since discovering of this methods[3]. Even though there is still need of researching in this field due to different application, different intended film properties, different behavior during the exact fabrication processes [4]. The magnetron sputtering is, without any questions, the most used deposition technique [5]. From different point of view there are still some field thin layers where the only option for creating of optimal film is performing the thermal evaporation.
Evaporated atoms have Maxwell energy distribution which is lower than energy of the particles deposited during sputtering process. The physical vapor deposition of metal films is used for preparation of nanostructures. It is important to prepare the thin layers with absolutely known properties. In this paper it is studied and compared the magnetron sputtering and thermal evaporation. The deposited films have to have certain properties for particularly application in this case nanofabrication of titanium quantum dots. The process starts with silicon substrate on the silicon substrate there is deposited the titanium layer with thickness 50 nm and then aluminum layer with thickness 100 nm. They are many parameters which need to be considered. The adhesion of layers, grain size, micro-structure of film, and the interface between titanium and aluminum, should be controlled and deposited with absolutely correct setup of each deposition. 2. EXPERIMENTAL The fabrication and experimental process was as follows. Both experiments started with a silicon wafer (100) 500µm thickness. Silicon dioxide was created on wafer by thermal oxidation with thickness 1 µm. The substrate was cleaned in isopropanol, rinsed out with deionized water. The following step was the deposition of titanium (50 nm), and deposition of aluminum layer (100 nm). The first deposition was performed using the magnetron sputtering and the second deposition by the thermal evaporation. The anodization process was then performed on each sample to compare these two depositions. 2.1 Magnetron sputtering The magnetron sputtering deposition was performed with the parameters listed in the Table 1. The parameters for the deposition were chosen by generally used setup for the titanium and aluminum deposition. Table 1 Operation condition of magnetron sputtering deposition Parameter Titanium layer Aluminium layer Pressure [Pa] 0.55 1.2 Ambient Flow [sccm] 20 50 Power[kW] 0.5 DC 1 DC Deposition Rate [nm*s -1 ] 0.08 0.10 10 Thickness[nm] 50 100 2.2 Thermal evaporation For the evaporation of aluminum wire is used the tungsten spiral with bounded condition 160 W and the current 40 A. The evaporation has a few steps. First, the aluminum wire is given into the center of the spiral. As the current is slowly increasing the aluminum wire starts to melt and the molten ball is appeared in the center of spiral. With other increasing of current the molten aluminum covers the whole spiral then the evaporation could be started with very slow rate which is needed. Therefore, it is not possible to heat the substrate and improve adhesion in this way. Tungsten boat is used for evaporation of titanium and this deposition must be done very carefully because of impurities, with boundary conditions the power 590 W and 70 A. The titanium has the melting point 1668 C. The temperature of evaporation is a little lower but with the amount of titanium, the deposited film could have some contaminations. Considering the electro-chemical properties, the analysis is processing to be sure about the film purity. The rate of deposition is very important for microstructure and the grain size of titanium layer. The process conditions are listed in the Table 2.
Table 2 Operation conditions of thermal evaporation deposition Parameter Titanium layer Aluminium layer Pressure [Pa] 3.2 10-3 2.8 10-3 Current [A] 54 38 Deposition Rate [nm*s -1 ] 0.08 0.10 3 6 Thickness [nm] 50 100 2.3 Anodization process Experimental prove the thermal evaporation could be used very efficiently for nanostructuring is performed by manufacturing of titanium QDs, anodization of aluminum and titanium layers using the utility model equipment for the electrochemical post processing deposition fabricated in our laboratory (detailed description of the tool is reported by Hubalek [6]). Due to the different anodizing behavior of these layers, there is option for optimizing deposition techniques to be more suitable for nanotechnologies. Detailed report about anodization process for Ti QDs is detailed reported by Drbohlavova in [7]. 3. RESULTS AND DISSCUSSION The thermal evaporation has some advantages to sputtering, there is a less stress of coated films and sputter-ejected species have kinetic energies considerably greater than thermal evaporation [8]. Due to previously mentioned fact the interface and the microstructure of layers are not completely suite for manufacturing of QDs. The topography analysis of evaporated film is shown in Figure 1 a), and the sputtered layer is shown in Fig 2 B). The grain size is more homogeneous and partials are smaller. This is necessary for creating the titania QD array. Fig. 1 The SEM images of A-evaporated Al film, B-sputter-deposited Al film. The thermal evaporation sample was use for anodization in the next step. The anodization curve is in shown in the Figure 2. The curve is flat with minimum of oscillation due to homogenous layer. The nanofabrication of titania QDs was achieved with sufficient results. The experiments with these layers has been reproducing very well, and it has been proved for many times in this case.
Fig. 2 The anodization curve corresponding to Al evaporated layer. The sample (shown at the Fig 3) is the thin layer deposited by magnetron sputtering. Previously, it has been shown (see Fig 1) that the microstructure is not optimal and the grain structure is not suitable for the next fabrication steps. Fig. 3 The anodization curve corresponding to Al sputter-deposited layer. At the Fig 3 it is shown the process is irregular and there is no setup to create QDs with these sputtered layers. The current was oscillating due to inhomogeneity mainly in the aluminum layer. There is no possible reproducing of such experiments.
The Fig 4 shows the XPS analysis of thermal evaporation deposited Al layer. During thermal deposition is more likely to detect some impurities. Therefore it is important for nanofabrication to perform the material analysis. The aluminum layer was without impurities of tungsten, even the titanium layer was with such tungsten particles. Only other materials were oxygen and carbon, but when the layer was sputtered due to remove the surface contamination. Fig. 4 XPS analysis of Al layer chemical composition combined with sputtering of surface impurities. 4. CONCLUSION This paper presents comparing of two fundamental PVD deposition methods with respect to nanofabrication of titania QDs. The anodization process serves as kind of research of deposited layer. The optimized thermal evaporation was considered as better type of deposition for this kind of deposition. Magnetron sputtering deposition is the most used kind of deposition, but for several type of nanotechnology is necessary to use different way. There is overview of experiments, deposition parameters, and the anodization processes. This paper can contribute to solve today and future issues with nanofabrication and nanotechnology. ACKNOWLEDGMENT The research was supported by the project GAČR P102/11/1068 NaNoBioTECell. LITERATURE [1] DRBOHLAVOVA, J., et al. Effect of Nucleic Acid and Albumin on Luminescence Properties of Deposited TiO2 Quantum Dots, International Journal of Electrochemical Science, 2012, 7, p. 1424-1432. [2] DRBOHLAVOVA, J., et al. Self-ordered TiO2 quantum dot array prepared via anodic oxidation, Nanoscale Research Letters, 2012, 7, p. [3] KELLY, P.J. and ARNELL, R.D. Magnetron sputtering: a review of recent developments and applications, Vacuum, 2000, 56, p. 159-172. [4] SHARMA, S.K. and MOHAN, S. Influence of annealing on structural, morphological, compositional and surface properties of magnetron sputtered nickel-titanium thin films, Applied Surface Science, 2013, 282, p. 492-498. [5] CONSTANTIN, D.G., et al. Magnetron Sputtering Technique Used for Coatings Deposition; Technologies and Applications RECENT, 2011, 12, p. 29-33.
[6] HUBALEK, J., et al. A new tool for the post-process modification of chips by nanostructures for chemical sensing. In BRUGGER, J.andBRIAND, D., Proceedings of the Eurosensors Xxiii Conference, 2009, vol. 1. Amsterdam: Elsevier Science Bv, pp. 36-39. [7] DRBOHLAVOVA, J., et al. TEMPLATE BASED FABRICATION OF TITANIA QUANTUM DOTS ARRAY, 2011, Tanger Ltd, Slezska. [8] BUNSHAH, R.F. ed. (1994). Handbook of Deposition Technologies for Films and Coatings. Park Ridge New Jersey: Noyes Publicatinos.