S. H. Paek

Control of the leakage current in SrTiO3 films by acceptor doping

Stoichiometric SrTiO3 (STO) films doped with Fe or Cr were prepared by r.f. magnetron sputtering technique. The effects of Fe or Cr doping in the SrTiO3 films were studied on the leakage current property which was discussed by defect chemistry. The experimental results can be explained by a model in which oxygen vacancies are the key defects responsible for the leakage current. Acceptor doping, with a small concentration of Fe or Cr, has led to a substantial improvement to 10−9 order in the leakage current density. Above the concentration of 0.01∼0.02 mol% Fe2O3, Cr2O3, however, as the concentration increased, the leakage current increased. These acceptors in Ti4+ site are expected to electrically compensate for donor species such as oxygen vacancies, thereby reducing the concentration of mobile carriers that contribute to electrical conduction. Consequently, acceptor doped STO films have been shown to be superior to undoped films for applications requiring high leakage resistance, such as dynamic random access memory capacitors.

The leakage current mechanism of PZT thin films deposited by in-situ sputtering

PZT thin film has been expected to replace conventional dielectric as a charge storage element in DRAM without use of a sophisticated structure such as a trench, because PZT has a dielectric constant much higher than silicon dioxide [1]. However, PZT thin film has relatively high leakage current [2]. To improve the high leakage current, the mechanism must be studied. Some studies have reported that the leakage current mechanism of PZT thin films on platinum showed Schottky emission [3, 4]. Their interpretation depends on the linearity of l n J versus E 1/2 (J: current density, E: electric field). This method is ,not able to distinguish Schottky emission from Poole-Frenkel emission, which may be one of the main mechanisms in PZT thin films. The equation for Poole-Frenkel emission predicts a straight line graph In J against E 1/2. That is, under most circumstances, experimental results can be plotted as a good straight line on either type of graph. Therefore, the mechanism cannot be separated by this means. To tell them apart, it must be determined that the leakage current results from a bulk-limited process or a barrier limited process, Poole-Frenkel emission being bulk limited and Schottky emission being barrier limited [5]. In this study, we measure J V curves for two different polarities using different electrodes, for example, upper electrode aluminium and bottom electrode platinum. In this case, if the leakage current depends on the barrier limited process, the J V curve will be asymmetrical when electrode polarity is reversed because the work functions of aluminium and platinum are different [6]. If leakage current depends on the bulk limited process, the J V curve shows symmetrical characteristics. Platinum film (250nm) with an intervening titanium (300 nm) layer as an adhesion layer were deposited as the bottom electrode by DC sputtering on thermally surface oxidized Si substrate, and then PZT films were deposited by RF magnetron sputtering, the films having a compositional Zr/Ti ratio of 50/50 and a thickness of 200 nm. The deposition conditions are shown in Table I. The aluminium top electrode of radius 100 ~m was deposited by thermal evaporation. X-ray diffractometry revealed that the film was of (1 1 1)-oriented perovskite phase. Leakage current at various voltages was measured using a HP 4145B semiconductor parameter measurement equipment.

Characteristics of SrTiO3 thin films deposited under various oxygen partial pressures

SrTiO3 thin films were deposited by rf-magnetron sputtering under various sputtering conditions followed by conventional furnace annealing at 600 and 700 °C. The amorphous SrTiO3 thin films crystallized into polycrystals at 600 °C. The leakage current of the SrTiO3 thin films decreased with increasing oxygen partial pressure in the sputtering gas. On the contrary, the dielectric constant increased with increasing the oxygen content in the sputtering gas. The leakage current and dielectric constant increased with increasing substrate temperature and post-annealing temperature. The ratio of Sr∶Ti approached 1∶1 with increasing oxygen content in the sputtering gas and substrate temperature during deposition. The oxygen content in the film decreased with increasing the substrate temperature. The capacitance-voltage (C-V) curves showed that the capacitance was nearly independent of the applied voltage. Scanning electron microscopy (SEM) micrographs showed that interdiffusion between the bottom electrode (Pt) and the buffer layer (Ti) occurred during post-annealing, but that the interface between SrTiO3 and Pt was stable.

X‐ray photoelectron spectroscopy and cross‐sectional transmission electron microscopy studies of titanium nitride/titanium/silicon structures after thermal annealing

Oxygen diffusion/incorporation behaviors of TiN/Ti/Si structures after thermal annealing in nitrogen ambients have been studied by x‐ray photoelectron spectroscopy and cross‐sectional transmission electron microscopy measurements. At the interface between Ti and TiN, titanium dioxides, thermodynamically most stable, are formed as a consequence of grain boundary diffusion, while inside TiN layer the contents of TiO and Ti2O3 compounds increases as the annealing temperature increases. At the interface between Ti and Si, titanium silicide formation is observed in the samples annealed above 450 °C consuming a part of pure Ti layer. One thing to note is that a severe blistering is observed in a sample annealed at 600 °C, probably caused by the difference of thermal expansion coefficients between TiSi2 and TiO2.

Electrical characteristics of TaSi2p+Si and TiSi2p+Si contacts formed by rapid thermal annealing

Abstract Silicidation reactions of sputtered tantalum and titanium thin films on p + Si are performed by rapid thermal annealing with a halogen lamp. Ohmic contacts of TaSi 2 and TiSi 2 to p + Si are electrically analysed for various contact sizes and annealing temperatures. A four-terminal Kelvin structure has been used to measure the contact resistance accurately. The sheet resistance of the silicide layers was measured with a four-point probe. All the results are discussed in comparison with TaSi 2 p + Si and TiSi 2 p + Si contacts. The TiSi 2 contact resistance to p + Si is higher than that of TaSi 2 to p + Si within the contact sizes studied. This result can be explained by titanium diboride (TiB 2 ) formation at the interface during the heat treatment. However, the measured sheet resistance of the TiSi 2 layer is lower than that of the TaSi 2 layer.

Effects of BF2+ implants on titanium silicide formation by rapid thermal annealing

The formation of titanium silicides on Si implanted with different BF2+ dosages has been studied by secondary ion mass spectrometry and transmission electron microscopy measurements. The thickness of the silicide layer formed in the temperature ranging from 600 to 800 °C has been investigated as a function of the implanted BF2+ dosage up to 1×1016 cm−2. Annealing at 700 °C results in conversion of the titanium film into predominantly C49 TiSi2, and most of it is transformed into the C54 phase at 800 °C or higher, resulting in a lower sheet resistance (16 μΩ cm). The titanium silicide thickness formed after the rapid thermal annealing (RTA) treatment depends on the implanted BF2+ dosage, caused by the native oxide enhanced by increased damage. Boron is redistributed into the silicide layer up to the solid solubility limit during annealing, leading to an accumulation at the silicide/silicon interface. The lowest contact resistance (with a size of 0.7 μm×0.7 μm) of 35 Ω is obtained at the annealing temperatu...

Formation and characteristics of lead lanthanum zirconium titanate thin films formed by using the r.f. sputtering method

PLZT (lead lanthanum zirconium titanate) thin films were prepared by using the r.f. magnetron sputtering method and post-annealing for crystallization at 650 ‡C. The films which were annealed at 650 ‡C for 10 min consisted of a metastable phase and a stable phase. However, another film which was annealed at 650 ‡C for 20 min had only stable perfect perovskite phase. The stability of the post-annealed thin film and substrate interfaces was observed by using scanning electron microscopy. The longer the annealing time, the more unstable were the interfaces. By analysing the EDX data, the composition difference between the sputtering target and thin films, and the composition variation between as-deposited and post-annealed PLZT were studied. The films annealed at 650 ‡C for 20 min showed good ferroelectric and electrical properties with a remanent polarization (Pr) of 11.5 ΜC cm−2, and a coercive field (Ec) of 164 kV cm−1.

Phase formation and characteristics of r.f.-sputtered barium-strontium titanate thin films on various bottom layers

Abstract(Ba0.7Sr0.3)TiO3 thin films were deposited by r.f.-magnetron sputtering on Pt/Ti/Si, Pt/TiSi2/Si and Pt/Ti/SiO2/Si substrates, respectively, and annealed at 650 ‡C for 30s by Rapid Thermal Annealing (RTA). XRD (X-ray diffraction) patterns revealed that the BST films had perovskite structure without preferred orientation. Auger depth profiles of barium-strontium titanate (BST) films on various substrates were performed. In the Pt/Ti/Si and Pt/TiSi2/Si structures, Si diffused into the BST film, but in the Pt/Ti/SiO2/Si structure, the diffusion of Si into the BST film was prevented and the interface between the BST film and the electrode was stable. The dielectric constants were about 310–260 (100 kHz–1 MHz).

Characteristics of TiSi2 contact to BF2-doped single-silicon

The reaction mechanism of titanium silicide was investigated for varying amounts, of BF2 dopant on a Si-substrate. Titanium thin films were prepared by direct current sputtering on non-doped and BF2-implanted silicon wafers. The heat treatment temperatures, by rapid thermal annealing (RTA), were varied in the range 600–800 °C for 20 s. C49 TiSi2 forms at 700 °C and almost all of its phase is transformed into C54 TiSi2 with a very low resistivity value (16 μΩ cm) at 800 °C. When the amount of impurities is increased, the sheet resistance of Ti-silicides also increased while its thickness decreased. The main cause of the thickness reduction of Ti-silicide is the growth of enhanced native oxide. Dopants are chiefly redistributed in the interface between the Ti-silicide and the Si-substrate. It is believed that the formation of titanium boride increases the contact resistance during the Ti-silicide formation for samples annealed at 750 °C and 800 °C.

Thermally stable ternary titanium‐tantalum silicide formation on polycrystalline silicon

A material for thermally stable self‐aligned silicide technologies has been developed using sequentially deposited Ti/Ta on polycrystalline silicon. At lower annealing temperatures below 1000 °C two separate phases were found by cross‐sectional transmission electron microscopy to exist in the form of bilayer TiSi2/TaSi2. The formation of a ternary phase (TiTa)Si2 has been observed at a higher temperature of 1000 °C. Consequently, the ternary (TiTa)Si2 layer could be kept extremely flat, with a sheet resistance of 5 Ω/⧠, even after 1000 °C, 30 min annealing. Cross‐sectional transmission electron micrographs of the structure clearly reveal that no agglomeration occurs during the heat treatment.