Aluminum nitride thin films report - Research Paper Example

The of the art of thin film deposition of the aluminum nitride a Faculty Group, Department, Street Address, Country, Postal Code bCompany Name, Street Address, City, Country, Postal Code Abstract. Thin films of aluminum nitride are usually deposited on a substrate of silica by a direct current that is reactive to the magnetron that is sputtering under a flow of nitrogen gas through a mixture of argon and nitrogen (Dimitrov et al, 2009). The film that is deposited as a result of the reaction is normally characterized by an incidence of x-ray diffraction, an atomic force microscope and a secondary mass of ion spectroscopy. Results from the x-ray diffraction procedure have revealed that the fine grained microstructures have an average roughness that ranges from 6nm and 8nm (Visput et al, 2005). The measurements of the spectroscopic values have been used to show that the band gap and the refractive index of the film normally fall in the range of 5.0eV and 5.48eV and 1.58Ev and 1.84eV respectively. Measurements of the secondary mass of ion spectroscopy indicate that oxygen is present in the thin film of aluminum nitride. Keywords: aluminum nitride, substrate, spectroscopy, deposition. Address all correspondence to: First author, University Name, Faculty Group, Department, Street Address, City, Country, Postal Code; Tel: +1 555-555-5555; Fax: +1 555-555-5556; E-mail: myemail@university.edu Introduction Aluminum nitride is a group 3 and period 5 compounds that exhibits very good properties in terms of their physical characteristics, chemical properties and the mechanical characteristics. Thin aluminum nitride films of high quality have been used in a number of devices and some sensors which include the optical and the optical electronic devices (Schupp et al, 2010). The optical electronic devices have a wide gap of approximately 6.2eV and a refractive index of about 2.0 which make the aluminum nitrides very attractive for use in optics. In addition, the thermal and the chemical stability of aluminum nitride films normally make it suitable for it to be used in environments of difficulties (Takikawa et al, 2001). Today, the aluminum nitride films and coatings have been produced using several methods such as the pulsed laser deposition method, the reactive molecular beam epitaxy method, the vacuum arc deposition method, the DC/RF reactive sputtering technique, the ion beam sputtering method, the metal organic chemical vapor deposition method and a number of other techniques (Cheng et al, 2003). As a result of the simplicity and the ease of up scaling and reproduction of the aluminum nitride films, the sputtering of magnetrons becomes the most common method of growing aluminum nitride films for the various applications that exist. However, it is very difficult to obtain insulating films of high quality using the direct current magnetrons sputtering method but it holds a notable advantage of having a high rate of deposition making it more suitable for the production of high quality and cost effective films that are dielectric in nature (Venkataraj et al, 2006). The properties of the aluminum nitride films are dependent on the structural nature of the materials, the microstructure, orientation and the chemical composition. These factors determine the amount of film deposited the ratio of argon to nitrogen flow and the pressure of the sputtering gas (Venkataraj et al, 2006). The effect of the nitrogen argon ratio on the optical properties of the aluminum nitride film can be noted by in the direct current magnetron sputtering method. In this review, we shall explore the different methods of deposition of aluminum nitride film and determine the state of art technique available in the industry today. Experimental Methods 1. The reactive magnetron sputtering methods In this case, the aluminum nitride films are deposited by sputtering the magnetrons using either aluminum nitride as a target or pure aluminum. The system that undergoes the process of sputtering normally uses the RF power and the plasma of argon ions in order to deposit the films. The most recent reactive technique is the magnetron sputtering method which allows the deposition of aluminum nitrides films using an aluminum target with plasma that consists of ions nitrogen and argon (Venkataraj et al, 2006). The reactive magnetron sputtering method is normally based on energy of formation that produces low energy of formation for the aluminum nitride at 298 K, 5276.1kcal/mol. When the ions of aluminum sputter in the ions of argon they react with the ions of nitrogen that are present in the plasma. The rate of deposition of the AlN is found to be very low in addition to the added high cost of the source of power and the matching circuit of the RF system (Mahmood et al, 2003). These shortcomings can be overcome using direct current sources of power for sputtering of the aluminum target. The process of forming an insulating film through the reaction of the aluminum target surface and the ions that have built up as a result of the bombardment of the ions normally gives rise to the formation of an electric field that easily exceeds the dielectric strength of the aluminum film at the surface of the target and the arc of formation of the film (Chen et al, 2006). The process of arcing is normally avoided in RF sputtering whereby the electrons are attracted to the surface of the target on the peak of the cycle that is positive and this insulates all the regions that have been formed. In regard to the dielectric constant of the reacting products and the density of the gases, the layers that are now charged are discharged at frequencies that have a relatively low magnitude (Zetterling et al, 2007). The source of power that is used in the reactive magnetron sputtering of the aluminum nitride film. This is achieved at a frequency of 75 kHz and the pulses of direct current are modulated at a frequency of 2.5 KHz (Chen et al, 2006). In the pulsed direct current magnetron sputtering, a trap of electrons is formed at the cathode surface and this leads to the generation of intense plasma at this surface. The ions that emanate from the plasma are usually drawn onto the target surface and this changes the potential causing the electrons to leak from the plasma (Thapa et al, 2009). The electrons that are produced are then attracted to the anode surface of the system and a return current to the source of power supply s generated. The chamber surface is coated with the insulating aluminum nitride film blocking any return film that may be available for the electrons to flow to the power supply and this raises the impedance of the load and the voltage of performance of the system. This causes the formation of diffusive plasma which can be avoided by frequently cleaning the chamber after the aluminum nitride has been deposited (Thapa et al, 2009). Studies have shown that the rate of deposition of the aluminum nitride film normally reduces as the frequencies over the 30 kHz mark. The rate of deposition however is approximately 80% greater than at the RF frequency. The film quality has been found to be independent of the magnitude of the frequency and the available stoichiometric films that have been generated using the pulsed direct current source. The main advantage of the magnetron sputtering technique is that it allows for deposition of the film in large areas up to a diameter of 8 inches (Kelly and Arnell, 2009). The quality of the deposited films of aluminum nitride is dependent on the purity of the argon and the argon gases. The substrate can be cleaned by heating the surface of the oxide using a HF solution that is saturated up to % concentration. Figure 1: The pinciple of the process of sputtering 2. The pulsed laser deposition technique This technique is very important for the deposition of films of compounds such as aluminum nitride. Studies have shown that the pulse laser detection (PLD) technique normally lends itself to processing at low levels of temperature since the average energy of the materials in the plume that is evaporated with the laser is very high as compared to the thermal energy of evaporation (Sellers, 2008). Therefore, the ratio of the species of ions that are present in the plasma generated from the pulse laser detection (PLD) technique is much higher as compared to any other technique. The atomic and molecular energy that is added during the ablation of the laser normally improves the mobility of the film and this results in the commencement of the process of crystallization at temperatures that are very low in comparison to the films that are deposited using the equilibrium evaporation techniques (Cho, 2011). Therefore, the temperatures of the substrate remain very important in achieving crystalline films of high quality. The supersaturated vapor in the plasma generated by pulse laser detection (PLD) technique has a very high energy which is achieved in a very short period of time and this satisfies the substrate thus classifying the pulse laser detection (PLD) technique as being highly non-equilibrium in nature of formation of the compound (Moreira et al, 2011). A notable feature of the pulse laser detection (PLD) technique is that is able to preserve the stoichiometry of the nitrogen and oxygen at the surface of the target of the films that are deposited. The process of ablation of the laser on the aluminum target in the presence of nitrogen plasma may fail to be most suitable for the formation of a film of nitride of high quality in the form of aluminum and it is known to result into the ejection of aluminum particulates in high quantity in the pulse laser detection (PLD) and this cannot undergo reaction to form an aluminum nitride (Wang et al, 2006). Employing a compound aluminum nitride film target in order to grow the pulse laser detection (PLD), the aluminum nitride compound that is used as the target in the pulse laser detection (PLD) technique will often contain smaller quantities of materials to be used in binding and this is incorporated into the system in the form of impurities. In some cases, this situation that is not favorable to the formation of the thin film may be transformed into strength of the system through incorporation of some specific species of the atoms to modify the nature of the films that is deposited. The polycrystalline aluminum nitride target that is sintered normally produces oxygen as a result of desorption due to the process of heating of the target surfaces. The surface of the target must be cleaned before embarking on the process of deposition of the aluminum nitride film while the substrate is kept covered using the shutter (Arwin et al, 2004). A notable efficient source of either atomic or ionic nitrogen is important to the process of formation of the epitaxial films of nitrides of high quality. The molecule of the nitrogen is known to be stable with an approximate energy of dissociation of 9.7eV. This is likely to generate very small fractions of the species of the atoms that have been ionized (Arwin et al, 2004). It is therefore important to note that there are other sources of ionic nitrogen that are suitable in the formation of the aluminum nitrides films. The process of formation of the aluminum nitrides films by PLD technique from the target in cases where the nitrogen ions are present and generated from the Kauffman sources of ions have been seen to be incomplete therefore leaving behind large quantities of aluminum (Hilfiker et al, 2008). In instances where there is the lack of either ionic or atomic nitrogen, the situation can be overcome in the pulse laser detection (PLD) technique by the process of ablation of the aluminum nitride together with the optimized density of the laser energy so that the aluminum nitride film in the molecular form can be generated close to the surface of the substrate. The main setback that is associated with the process of growth of the aluminum films using any method is the incorporation of the oxygen element in the aluminum film due to its high affinity to oxygen (Blanckenhage et al, 2002). The deposition of aluminum nitride films in conditions of ultra-high vacuum is very essential to the reduction of the levels of oxygen in the films that is obtained by pulse laser detection (PLD). The deposition of the aluminum nitride can be conducted using the pulsed exciters (Blanckenhage et al, 2002). The laser beam is focused on the aluminum nitride film target that is placed in the chamber and then evacuated to the pre-deposition vacuum. The AlN film thickness is measured using the profilameter and normally, the films range from 300nm to 400nm in a period of deposition of about 30 minutes. The films that are used in the wear test are deposited at the temperatures of the substrate at approximately 600 degrees. Figure 2: The schematic of the pulse laser detection (PLD) technique 3. Chemical vapor display technique The process of Chemical vapor display (CVD) of the films and coatings normally involves chemical reactions of the reactants in gaseous forms on the surface or close to the surface of the substrate. This atomic based deposition of the film usually provides materials with high levels of purity which have structural control at both the atomic and the nanometer levels on the scale (Méndez et al, 2008). In addition, this method is capable of producing single layers, multiple layers, composite layers, nanostructured layers and coatings that are functionally graded and produced at low temperatures. More to that, the uniqueness of the CVD process in relation to the other techniques allows for the fabrication of the Nano-devices and the free standing shapes of the aluminum film (Dumitru et al, 2000). The practice of the CVD process has resulted into a rapid increase in the methods of processing for the purpose of deposition of the thin films of aluminum nitrides. The key aspects of the Chemical vapor display (CVD) includes the principle of processing, the mechanism of deposition, the chemistry of reaction, the thermodynamic of the reaction and the phenomena of transportation (Drüsedau and Koppenhagen, 2002). Additionally, the aspects and apparatus of the Chemical vapor display (CVD) include the parameter of processing, the control techniques, the range of films that have been synthesized, and the co relationships of the structures. The Chemical vapor display (CVD) technologies are based on heating, the type of precursor, plasma enhanced, photo assisted and metal organic assisted Chemical vapor display (CVD). Chemical vapor display (CVD) techniques including the electrostatic-aerosol techniques have proved to be more cost effective (Dumitru et al, 2000). Figure 3: The schematic of the Chemical vapor display (CVD) techniques 4. Plasma processing method The plasma processing method for the deposition of thin film nitrides of aluminum is achieved through the CVD plasma onto an area that has large substrates of glass. The rate of deposition of the nitride film is high and it can be monitored by adjusting the space that is existent between the inlet of the gas manifold and the substrate of the system (Mohamed et al, 2004). The thin films that are deposited in the different chemical chambers of the set up form part of the large vacuum system. The aluminum nitride in this process can be produced by operation of the system at high pressure thus optimizing the process of deposition and the parameters involved such as the flow of the gases, the RF power of the system and the spacing of the electrodes (Borges et al, 2013). The spacing existent between the inlet of the gas manifold and the substrate of the system is maintained at small sizes and this allows for the process of deposition of the thin film of aluminum nitride. This way, the process of deposition can easily be controlled. The gate silicon nitrides films that are dielectric in nature should be of high quality so that it is bale to form thin films of the nitride (Mohamed et al, 2004). The quality of the film is measured using the wet etch rating using a HF solution that has a concentration of 6:1. The glass plates that are used in the plasma processing method must have a high temperature of about 4500C. The temperatures for deposition normally range between 300° and 350° C (Borges et al, 2013). The thickness of the film of aluminum nitride that is deposited normally varies depending on the nature of both the underlying and overlying layers of the deposit. Figure 4: The schematic of the plasma processing method Conclusion From the discussion, there are different methods of deposition of the aluminum nitride. The different methods are their principles of operation have been discussed and the strengths and efficiency of each technique over the other methods have been determined. The magnetron sputtering technique is the most state of start method of deposition as is produces very high quality deposits of aluminum nitride at a fast and efficient rate. This makes magnetron sputtering technique more efficient as compared to the other methods of deposition. References V. Dimitrova, D. Manova, and E. Valcheva, “Optical and dielectric properties of dc magnetron sputtered AlN thin films correlated with deposition conditions,” Materials Science and Engineering B, vol. 68, no. 1, pp. 1–4, (2009). R. D. Vispute, H. Wu, and J. Narayan, “High quality epitaxial aluminum nitride layers on sapphire by pulsed laser deposition,” Applied Physics Letters, vol. 67, pp. 1549–1551, (2005). T. Schupp, K. Lischka, and D. J. As, “MBE growth of atomically smooth non-polar cubic AlN,” Journal of Crystal Growth, vol. 312, no. 9, pp. 1500–1504, (2010). H. Takikawa, K. Kimura, R. Miyano et al., “Effect of substrate bias on AlN thin film preparation in shielded reactive vacuum arc deposition,” Thin Solid Films, vol. 386, no. 2, pp. 276–280, (2001). H. Cheng, Y. Sun, J. X. Zhang, Y. B. Zhang, S. Yuan, and P. Hing, “AlN films deposited under various nitrogen concentrations by RF reactive sputtering,” Journal of Crystal Growth, vol. 254, no. 1-2, pp. 46–54, (2003). S. Venkataraj, D. Severin, R. Drese, F. Koerfer, and M. Wuttig, “Structural, optical and mechanical properties of aluminium nitride films prepared by reactive DC magnetron sputtering,” Thin Solid Films, vol. 502, no. 1-2, pp. 235–239, (2006). A. Mahmood, N. Rakov, and M. Xiao, “Influence of deposition conditions on optical properties of aluminum nitride (AlN) thin films prepared by DC-reactive magnetron sputtering,” Materials Letters, vol. 57, no. 13-14, pp. 1925–1933, (2003). H.-Y. Chen, S. Han, and H. C. Shih, “The characterization of aluminum nitride thin films prepared by dual ion beam sputtering,” Surface and Coatings Technology, vol. 200, no. 10, pp. 3326–3329, (2006). C.-M. Zetterling, M. Östling, K. Wongchotigul et al., “Investigation of aluminum nitride grown by metal-organic chemical-vapor deposition on silicon carbide,” Journal of Applied Physics, vol. 82, no. 6, pp. 2990–2995, (2007). R. Thapa, B. Saha, and K. K. Chattopadhyay, “Synthesis of cubic aluminum nitride by VLS technique using gold chloride as a catalyst and its optical and field emission properties,” Journal of Alloys and Compounds, vol. 475, no. 1-2, pp. 373–377, (2009). P. J. Kelly and R. D. Arnell, “Magnetron sputtering: a review of recent developments and applications,” Vacuum, vol. 56, no. 3, pp. 159–172, (2000.) J. Sellers, “Asymmetric bipolar pulsed DC: the enabling technology for reactive PVD,” Surface and Coatings Technology, vol. 98, no. 1–3, pp. 1245–1250, (2008). S. Cho, “Effect of nitrogen flow ratio on the structural and optical properties of aluminum nitride thin films,” Journal of Crystal Growth, vol. 326, no. 1, pp. 179–182, (2011). M. A. Moreira, I. Doi, J. F. Souza, and J. A. Diniz, “Electrical characterization and morphological properties of AlN films prepared by dc reactive magnetron sputtering,” Microelectronic Engineering, vol. 88, no. 5, pp. 802–806, (2011). D.-Y. Wang, Y. Nagahata, M. Masuda, and Y. Hayashi, “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, vol. 14, no. 6, pp. 3092–3099, (2006). H. Arwin, M. Poksinski, and K. Johansen, “Total internal reflection ellipsometry: principles and applications,” Applied Optics, vol. 43, no. 15, pp. 3028–3036, (2004). J. N. Hilfiker, N. Singh, T. Tiwald et al., “Survey of methods to characterize thin absorbing films with spectroscopic ellipsometry,” Thin Solid Films, vol. 516, no. 22, pp. 7979–7989, (2008). B. Von Blanckenhagen, D. Tonova, and J. Ullmann, “Application of the Tauc-Lorentz formulation to the interband absorption of optical coating materials,” Applied Optics, vol. 41, no. 16, pp. 3137–3141, (2002). M. García-Méndez, S. Morales-Rodríguez, R. Machorro, and W. De La Cruz, “Characterization of ALN thin films deposited by DC reactive magnetron sputtering,” Revista Mexicana de Fisica, vol. 54, no. 4, pp. 271–278, (2008). V. Dumitru, C. Morosanu, V. Sandu, and A. Stoica, “Optical and structural differences between RF and DC AlxNy magnetron sputtered films,” Thin Solid Films, vol. 359, no. 1, pp. 17–20, (2000). T. P. Drüsedau and K. Koppenhagen, “Substrate heating by sputter-deposition of AlN: the effects of dc and rf discharges in nitrogen atmosphere,” Surface and Coatings Technology, vol. 153, no. 2-3, pp. 155–159, (2002). S. H. Mohamed, O. Kappertz, J. M. Ngaruiya et al., “Influence of nitrogen content on properties of direct current sputtered TiOxNy films,” Physica Status Solidi A, vol. 201, no. 1, pp. 90–102, (2004). J. Borges, N. P. Barradas, E. Alves et al., “Influence of stoichiometry and structure on the optical properties of AINxOy films,” Journal of Physics D, vol. 46, pp. 1–11, (2013.) Read More