AVS2013 Session TF+AS+EM+NS+SS-ThM: Thin Film: Growth and Characterization I

Gold was likely the first metal discovered by man, more than 11,000 years ago. However, unlike copper (~9000 BC), bronze (~3500 BC), and wrought iron (~2500-3000 BC), it was too soft for use as tools and weapons. Instead, gold was used for decoration, religious artifacts, and commerce. The earliest documented inorganic thin films were gold layers, < 3000 Å thick, produced chemi-mechanically by Egyptians more than 5000 years ago. Examples, gilded on statues and artifacts (requiring interfacial adhesion layers), were found in early stone pyramids dating to ~2650 BC in Saqqara, Egypt. Spectacular samples of embossed Au sheets date to at least 2600 BC. Fatty-acid-based monolayers were deposited in King Hammurabi's time (~1800 BC, Mesopotamia); modern experiments were carried out much later by Ben Franklin, Lord Rayleigh, and Irving Langmuir. Although there is forensic archeological evidence for electroplating as early as the first few centuries BC, no written evidence survived. The earliest published electroplating experiments were ~1800 AD following the invention of the dc electrochemical battery by Volta. Chemical vapor deposition (CVD) of metal films was reported in 1649. Sols were produced in the mid-1850s (Faraday) and sol-gel films synthesized in 1885.

Vapor phase film growth including sputter deposition (Grove, 1852), arc deposition ("deflagration," Faraday, 1857), atmospheric plasmas (Siemens, 1857), plasma-enhanced CVD (1869, Berthelot), and evaporation (Stefan, Hertz, and Knudsen, 1873-1915) all had to wait for the invention of vacuum pumps whose history ranges from ~1650 for mechanical pumps through ~1865 for mercury pumps that could produce ballistic pressures in small systems. The development of the science of crystallography, leading to Miller indices (1839) for describing orientation and epitaxial relationships in modern thin film technology, was already well advanced by the 1780s. The starting point for the development of heterogeneous thin film nucleation theory was provided by Thomas Young (contact angle equation) in 1805.

While an historical road map tracing the progress of thin film technology is interesting in itself, the stories behind these developments are even more fascinating and provide insight into the evolution of scientific reasoning.

Metal-insulator transition (MIT) in strongly correlated oxides has been an intriguing topic of condensed matter physics over many decades. SrVO₃ (SVO) with a 3d¹ electronic configuration for vanadium is a typical strongly correlated system for studying MIT. High quality epitaxial SVO thin films of various thicknesses were grown on (001)-oriented LSAT single crystal substrates by using a novel pulsed electron-beam deposition (PED) technique. Thick SVO films (~25 nm) exhibited metallic behavior with the electrical resistivity following the T² law that corresponds to a Fermi liquid system, the resistance ratio R(300K)/R(2K) was ~1.6. We observed a temperature driven MIT in SVO films with thicknesses below 6 nm. The emergence of this MIT can be attributed to the reduction in the effective bandwidth due to a crossover from a three-dimensional metal to a two-dimensional insulator. We also synthesized SrTi_1-xV_xO₃ (0 ≤ x ≤ 1) thin films with thicknesses of ~15 nm to study the chemical doping of Ti⁴⁺ ions in the SVO system. The films with high vanadium content (x > 0.7) were metallic following the T² law, and the films with low vanadium content (x < 0.7) were semiconducting following the variable range hopping mechanism. The x = 0.7 film showed a temperature driven MIT at ~100K. The observed MIT induced by the substitution of Ti⁴⁺ ions for V⁴⁺ ions could be interpreted by the induced Anderson localization that trapped the electrons below a mobility edge.

Transparent semiconducting amorphous indium gallium zinc oxide (IGZO) films have attracted great attention due to their excellent electrical properties and possible utilization in thin film transistors or in photovoltaic applications. It is known that the properties of IGZO films prepared by magnetron sputtering are highly sensitive to process parameters, especially to oxygen partial pressure. In this study we have focused on the comparison of various types of power supplies with precise control of process parameters in order to optimize electrical and optical properties of the IGZO thin films. We employed dc and pulsed dc power supplies for single magnetron sputtering and bipolar and mid frequency sine wave power supplies for dual magnetron sputtering. All power supplies were equipped with advanced process stabilization solutions, ultra-fast arc management characterized by very low stored energy, and a digital control platform enabling highly flexible software algorithms design. Magnetrons were equipped either with ceramic InGaZnO targets or with metallic InGaZn targets. A novel fast process control system was used in order to precisely control deposition conditions during reactive magnetron sputtering of IGZO films from metallic targets. It is shown that the electrical resistivity can be effectively controlled in the wide range from 10^-3 to 10⁵ Ω.cm and the field effect mobility can reach values up to 40 cm²/V.s at a film thickness of 270 nm. In parallel to the difference between individual power supplies (sputtering techniques), the effect of the discharge power and the oxygen partial pressure on deposition rate, optical and electrical properties and film structure will be discussed in detail.

Hafnium oxide (HfO_2­) is a unique material characterized by excellent chemical and physical properties. It is a wide band gap ( 5.5 eV) material, which makes it attractive for optoelectronics since it transparent from the ultraviolet to the mid-infrared region. Being a promising high-k dielectric, HfO₂ has a strong potential to replace silicon oxide as the insulator in CMOS devices. HfO₂ exhibits various polymorphs; the thermodynamic stability and phase existence depends on the temperature and pressure conditions. Controlled growth and manipulation of specific crystal structures of HfO₂ at the nanoscale dimensions has important technological implications. The present work entails a detailed analysis of growth behavior, microstructure, and optical properties of monoclinic HfO₂ films as a function of growth temperature. In addition, the effect of post-deposition annealing temperature was also studied. HfO₂ thin films were grown by RF magnetron sputtering onto silicon (100) and quartz substrates by keeping power, pressure, and flow of Ar and O₂ and their ratio (70:30) constant, but varying growth temperature from room temperature to 600 C. A thorough characterization was performed through scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and grazing incidence x-ray diffraction (GIXRD). Optical properties were evaluated using spectrophotometric and ellipsometry measurements. GIXRD data revealed a well oriented structure along (-111) as temperature increases, and an evident crystallization temperature at 300 C. The grain sizes measured were in the range of 15 to 20 nm; grain sizes increased with temperature. While SEM and AFM analyses also indicate the grain size with increasing temperature, roughness of the films exhibits a decreasing tendency with increasing temperature. Optical data revealed a double band gap for temperatures higher than RT as well as an increase in band gap with increasing growth temperature. The band gap for at HfO₂ thin films grown at RT was found to be 5.7 eV. The band gap increased to values of ~6.2 eV with increasing growth temperature. A correlation between growth conditions, microstructure and optical properties of nanocrystalline HfO₂ thin films is discussed.

In order to meet the increasing demand for high frequency electronic devices, the physical dimensions of MOSFETs have been continuously scaled down to nanoscale. However, one of the bottlenecks we encountered during the down-scaling process is the tunneling current leakage at gates. SiO₂, the most commonly used traditional gate dielectric experiences an appreciable amount of tunneling current when gate thickness is below 1-1.2 nm. The leakage greatly degrades the performance of nanoelectronics. Therefore, we propose using high-k dielectrics to replace SiO₂, which can effectively limit the tunneling leakage without losing the current control at gates. Our research has mainly focus on Al₂O₃, HfO₂ and Al_xHf_yO_z deposit on silicon via Atomic Layer Deposition. The Aluminum to Hafnium ratio in the oxide is tuned to maximize the electrical and physical properties of the film. We expect Al_xHf_yO_z to have a better interface than HfO₂ and an intermediate band-gap between Al₂O₃ and HfO₂, as well as better thermal stability. The electrical properties of each oxide will be characterized by fabricating transistors with gate oxide thicknesses of 5, 10, and 15 nm. In addition to the C-V and I-V measurements for capacitors and transistors, the films will be characterized by XPS, AFM, and spectroscopic ellipsometry. To improve the electrical property of the film, we will incorporate N into the high-dielectric films using Plasma ALD. Finally, the effects of various annealing and deposition temperatures at the silicon-oxide interface will be studied using TEM.

Optical and electrical properties of TiO₂ thin films are largely effected by film morphology. TiO₂ usually crystallizes into anatase or rutile phase. The rutile phase is generally stable while the anatase phase is difficult to produce in pure form through annealing. TiO₂ thin films were grown by atomic layer deposition (ALD) on Si (100) surfaces using tetrakis(dimethylamino) titanium (TDMAT) and H₂O. Films were grown at every 50°C increment from 100 to 300°C approximately 100 nm thick, determined by nominal growth rates derived from spectroscopic ellipsometry (SE) measurements. The growth rate varied from 0.4Å/cycle at the ALD optimal temperature of 200°C to 1.2Å/cycle at 300°C. Films were annealed post-deposition at temperatures of 500°C, 700°C and 900°C for 3, 5 and 10 minutes in an argon environment using a rapid thermal annealer. Characterization was completed using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). FTIR measurements indicate that the as-deposited films have an amorphous structure. Annealed films exhibit various crystallization to rutile phase, anatase phase, or a mixture of the two phases, depending on the deposition and post-deposition annealing conditions, as confirmed by XRD. Specifically, TiO₂ films grown at 100°C and annealed for 10 minutes at 900°C exhibit only amorphous and anatase phase TiO₂. It is shown that TiO₂ crystallinity can be tuned between amorphous, anatase and rutile phases by controlling the deposition temperature and post-deposition annealing parameters.