Moon Yong Lee

Deposition of extremely thin (Ba,Sr)TiO3 thin films for ultra‐large‐scale integrated dynamic random access memory application

(Ba,Sr)TiO3 (BST) thin films with thicknesses ranging from 15 to 50 nm are prepared by a rf magnetron sputtering on Pt/SiO2/Si substrates. The dielectric constants of BST thin films increase with increasing deposition temperature and thicknesses. The leakage current increases with increasing deposition temperature and this prevents the deposition temperature of the 20 nm thick BST thin film from being increased to a value more than 640 °C. The leakage current is also critically dependent upon the postannealing temperature and atmosphere after the top electrode fabrication. The dielectric constant increases with increasing postannealing temperature which further reduces the SiO2 equivalent thicknesses of the BST thin films. A 20 nm thick BST thin film deposited at 640 °C and postannealed at 750 °C under N2 atmosphere for 30 min, shows a SiO2 equivalent thickness of 0.24 nm, dielectric dissipation factor less than 1%, and leakage current of about 40 nA/cm2 at ∓1.5 V.

A comparative study on the electrical conduction mechanisms of (Ba0.5Sr0.5)TiO3 thin films on Pt and IrO2 electrodes

Electrical conduction mechanisms for Pt/(Ba0.5Sr0.5)TiO3 (BST)/Pt, IrO2/BST/IrO2, and Pt/BST/IrO2 capacitors were studied. The Pt/BST/Pt capacitor shows a Schottky emission behavior with interface potential barrier heights of about 1.5–1.6 eV. The barrier height is largely determined by the surface electron trap states of the BST. The IrO2/BST interface shows an ohmic contact nature due to the elimination of the surface trap states as the result of the formation of strong chemical bonds between the IrO2 and BST which results in the Poole–Frenkel emission conduction mechanism. Pt/BST/IrO2 capacitor shows Schottky emission behavior and a positive temperature coefficient of resistivity (PTCR) effect depending on the bias polarity. The electron trap states at the Pt/BST interface and the positive space charges within the carrier depletion layer result in the PTCR effect.

Depletion layer thickness and Schottky type carrier injection at the interface between Pt electrodes and (Ba, Sr)TiO3 thin films

The electrical conduction properties of rf sputter-deposited (Ba, Sr)TiO3 (BST) films on Pt and IrO2 electrodes and metalorganic chemical vapor deposited (MOCVD) BST films on a Pt electrode were investigated and a new energy band model that satisfactorily explains the observed leakage current characteristics and film thickness dependent dielectric properties is proposed. The BST and Pt junction constituted a blocking contact with interface potential barrier heights of 1.6–1.7 eV and 1.2 eV for the sputtered and MOCVD films, respectively. Schottky emission behavior was observed at measurement temperatures higher than 120 °C and tunneling related conduction behavior appeared below that temperature for a film thickness of 40 nm. A partial depletion model with a very thin (about 1 nm) layer devoid of space charge at the interface with the Pt electrode is proposed to explain the V1/2 dependent variation of ln(Jo) as well as the decreasing dielectric constant with decreasing film thickness.

Deposition and Electrical Characterization of Very Thin SrTiO3 Films for Ultra Large Scale Integrated Dynamic Random Access Memory Application

SrTiO3 thin films are deposited on Pt/SiO2/Si substrates using RF magnetron sputtering in a temperature range from 200° C to 600° C. The film deposited at 600° C shows the best dielectric property and leakage current characteristics due to its good crystallinity and stoichiometric composition. Dielectric constant of the film deposited at 600° C decreases with decreasing thickness from 235 at 120 nm to 145 at 30 nm. Leakage current shows a constant value of about 30 nA/cm2 at 1.6 V in a thickness range from 50 nm to 120 nm but increases rapidly to 5 µ A/cm2 at 30 nm. The electrical properties of the films are explained by a model of the Pt/SrTiO3/Pt capacitor based on the band structure.

Control of Etch Slope during Etching of Pt in Ar/Cl2/O2 Plasmas

Pt patterns of the 0.25 μm design rule were etched at 20°C using a magnetically enhanced reactive ion etcher. The main problem of this device integration process is the redeposition of the etch products onto the pattern sidewall, making it difficult to reduce the pattern size. In both cases using a photoresist mask and an oxide mask, the redeposits of the etch products onto the sidewall were reduced by the addition of Cl 2 to Ar, although the etch slope was lowered to 45°. Using the oxide mask, by adding O 2 to the Cl-containing gas, the etch slope was increased up to 70°, and the redeposits were removed by an HCl cleaning process.

Investigation of Pt/Ti Bottom Electrodes for Pb(Zr,Ti)O 3 Films

Pt films, as bottom electrodes for PZT capacitors, were deposited on Ti/SiO2/Si substrates using DC magnetron sputtering under various deposition conditions. The effects of post-annealing on surface morphology, element diffusion, microstructure and structural phase of the Pt films were investigated. The structure and electrical properties of PZT films deposited on Pt/Ti/SiO2/Si electrodes were also studied. As the deposition temperature of the Pt film increases and the deposition rate decreases, the film becomes dense so that Ti out-diffusion and film deformation are suppressed. The out-diffused Ti faciliates the formation of nucleation sites for perovskite PZT films. However, excess Ti out-diffusion not only decreases the total capacitance of the PZT films due to formation of an interfacial layer having a low dielectric constant but also degrades the leakage current characteristics of the PZT films due to deformation of the Pt electrode.

Remote Plasma-Assisted Metal Organic Chemical Vapor Deposition of Tantalum Nitride Thin Films with Different Radicals

Thin films of tantalum nitride have been deposited from remote plasma-assisted metal organic chemical vapor deposition (RP-MOCVD) using the reaction of pentakis-dimethyl-amino-tantalum (PDMATa) with different activated radicals. Microstructures of deposited films measured by X-ray diffraction (XRD) and transmission electron microscopy (TEM) depend on the deposition temperature and the type of radicals. At temperatures below 300°C, amorphous films are obtained which are independent of the reacting species. On the other hand, at higher deposition temperatures, (111)-preferred cubic TaN films are obtained when they react with ammonia plasma, while the reaction with hydrogen plasma produces amorphous films. All amorphous films obtained are recrystallized at an annealing temperature of 1000°C in an oxygen-containing (10%) ambient, showing (111) TaN, bcc Ta, and signals of orthorhombic Ta2O5. From detailed studies of film composition and chemical bonding in the obtained films, the impurity incorporation, especially carbon, is responsible for the dependence of film microstructures on different deposition conditions.

Barrier Metal Properties of Amorphous Tantalum Nitride Thin Films between Platinum and Silicon deposited using Remote Plasma Metal Organic Chemical Vapor Method.

Amorphous TaN thin films have been prepared by remote plasma-assisted metal organic chemical vapor deposition using pentakis-dimethyl-amino-tantalum (PDMATa) in hydrogen plasma. The dependence of film properties such as resistivity, impurity contents, and microstructures on deposition conditions is reported. All obtained films have been tested as diffusion barriers between platinum and silicon in a stacked-capacitor type memory cell for future, high-density ferroelectric memories. X-ray photoelectron spectroscopy (XPS) has been used to determine the nature of carbon incorporation into the film, which is responsible for the observed microstructure of the deposited film. Recrystallization occurs at an annealing temperatures of 1000°C in an oxygen-containing (10%) ambient, showing (111) TaN, [bcc] Ta, and orthorhombic Ta2O5. It was determined that a TaN barrier layer can be successfully applied as a barrier layer between platinum and silicon (700°C for 30 min in an oxygen-containing ambient), preventing the silicidation reaction of silicon with a Pt electrode as well as the oxidation of the underlying capacitor electrode during the capacitor formation process.

An optimized densification of the filled oxide for quarter micron shallow trench isolation (STI)

Densification methods using H/sub 2/O and N/sub 2/ ambient annealing of the filled CVD oxide for quarter micron STI are compared. Although the H/sub 2/O ambient oxidation is more effective in terms of the resistance against the HF etching, volume expansion by the trench sidewall oxidation generates a large amount of stress in the narrow isolation region. However, an N/sub 2/ gas ambient annealing at high temperature shows a low stress and a low HF etch rate which enable us to fabricate the stable quarter micron STI.

IMPROVEMENT OF LEAKAGE CURRENT CHARACTERISTICS OF BA0.5SR0.5TIO3 FILMS BY N2O PLASMA SURFACE TREATMENT

The effects of plasma surface treatment, using N2O gas, of Ba0.5Sr0.5TiO3 (BST) film on the leakage current characteristic of a Pt/BST/Pt capacitor were investigated. As a result of exposure of BST film to the plasma, the leakage current density of the BST capacitor decreased by two orders of magnitude in the high voltage region, and higher onset voltage of an abrupt increase in leakage current was observed. The improvement of leakage properties of BST films can be attributed to the elimination of the bulged curve in the leakage current characteristics. Thermal desorption spectroscopy showed that the elimination was closely related to the reduction of carbon content in the BST film.