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.
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.
TiO 2 thin films were grown by an atomic-layer-deposition process at growth temperatures ranging from 200 to 300°C on Ru and Si substrates using Ti͓OCH͑CH 3 ͒ 2 ͔ 4 and H 2 O as metal precursor and oxygen source, respectively, for metal-insulator-metal capacitor application in dynamic random access memories. The saturated film growth rate on Ru and Si substrates was 0.034 and 0.046 nm/cycle, respectively. The TiO 2 film growth on a Ru substrate showed a rather long incubation period and the incubation period decreased with increasing Ti͓OCH͑CH 3 ͒ 2 ͔ 4 pulse time, whereas the H 2 O pulse time had almost no influence on the incubation period. A growth rate transition, from low to high values, ͑thickness 7-8 nm͒ was observed when the films were grown at temperatures Ͼ250°C, whereas the films grown at lower temperatures did not show the transition. The transition was due to the structural change of the film from an amorphous/nanocrystalline to the well-crystallized polycrystalline anatase phase. The TiO 2 films grown at temperatures Ͼ250°C showed a dielectric constant of ϳ35. A 14-nm-thick TiO 2 film showed an equivalent oxide thickness of 1.7 nm and a leakage current density of 5 ϫ 10 −6 A/cm 2 at 1 V.As the density of dynamic random access memories ͑DRAM͒ increases over 1 GB, the fabrication of capacitors having a cell capacitance high enough to meet the refresh requirement becomes very difficult because of the extremely small cell sizes. If the required capacitance of a DRAM cell is assumed to remain at 25 fF for a DRAM with a 50 nm design rule, a dielectric film with an equivalent oxide thickness ͑t oxeq ͒ of ϳ0.5 nm is necessary for a cylindrical capacitor structure. 1 Obtaining t oxeq of ϳ0.5 nm from a capacitor dielectric is very difficult even with very high dielectric constant ͑k͒ thin films, such as ͑Ba,Sr͒TiO 3 ͑BST͒ when the top or bottom electrode consists of conventional polycrystalline Si. This is due to the inherent formation of an interfacial low dielectric layer by Si oxidation and a rather large charge screening length of the Si even when the impurity doping concentration of the polycrystalline Si increases, reaching the solubility limit ͑Ͼ10 20 cm −3 ͒. Therefore, metal-insulator-metal ͑MIM͒-type capacitors with higher-k dielectric films become crucial for the production as the DRAM feature size gets close to 50 nm. The most probable metal for the capacitor electrode is Ru. The reason for this is the rather matured chemical vapor deposition ͑CVD͒ process on Ru thin films, which results in a reasonable conformality over the severe three-dimensional geometry of the DRAM capacitor, easy etching, and the property of hindering oxidation or forming a conducting oxide. The conformal deposition and fine patterning of a noble metal electrode have been realized only for the case of Ru. 2,3 Recently, the atomic layer deposition ͑ALD͒ of Ru films has been increasingly focused because the ALD process offers an even better conformality than the CVD process.There have been many reports on the growth and charac...
Bi 2 Ti 2 O 7 films were deposited on sputtered Ru/SiO 2 /Si substrates by atomic layer deposition using tris͑1-methoxy-2-methyl-2-propoxy͒bismuth ͓Bi͑mmp͒ 3 ͔, titanium tetraisopropoxide as Bi and Ti precursors, and H 2 O as oxidant, respectively. The wafer temperature was varied from 225 to 300°C. The growth rates decreased from 0.075 nm/cycle at 225°C to 0.055 nm/cycle at 300°C. The Bi concentration in the film decreased with increasing growth temperature and it could be controlled within a certain range by changing the Bi feeding at a given temperature. The as-grown Bi 2 Ti 2 O 7 films were amorphous but contained metallic Bi at high growth temperatures and high Bi concentrations. Bi and Ti piled-up on the film surface and the Ru interface, respectively. The electrical leakage property of the Bi 2 Ti 2 O 7 thin film became worse with increasing deposition temperature and Bi concentration due to the metallic Bi formation. The dielectric constant at the optimum deposition condition ͑225°C͒ was ϳ40, and the leakage current level was Ͻ10 −7 A/cm 2 at 1.0 MV/cm at a film thickness of ϳ15 nm. Reasonable conformal film thickness and cation composition over contact holes with a diameter of 0.15 m and a depth of 1.1 m were obtained.As the density of dynamic random access memory ͑DRAM͒ quadruples every three years, the fabrication of storage capacitors having a cell capacitance high enough to satisfy the refresh requirement becomes difficult. It is expected that the required capacitance of a storage capacitor will be still ϳ25 fF even for DRAMs with a design rule of ϳ50 nm. 1 In order to achieve this capacitance, a dielectric film with an equivalent oxide thickness ͑t oxeq. ͒ of ϳ0.5 nm is necessary for a simple stack-type capacitor. In order to achieve a t oxeq. of ϳ0.5 nm, noble metal top and bottom electrodes have to be used even for high-dielectric-constant thin films like ͑Ba,Sr͒TiO 3 ͑BST͒ to avoid interfacial low-dielectric-layer formation.The most probable metal electrode is Ru for the metal-insulatormetal ͑MIM͒ capacitor fabrication because the chemical vapor deposition ͑CVD͒ process is already matured for thin-film Ru deposition. Conformal deposition and easy etching for fine patterning of Ru are possible. In addition, the oxidation potential of Ru is quite low compared to most of the high-dielectric materials and, if formed, Ru-oxide is a conducting oxide which minimizes the adverse interface effects. 2,3 Recently, increasing focus lay on the atomic layer deposition ͑ALD͒ of Ru films due to a better conformity than obtained by CVD.There have been numerous reports on high-k dielectric films for DRAM capacitors. 4-8 The most promising material for high-k thin films is BST because of its superior dielectric constant ͑Ͼ300 at a thickness of ϳ30 nm͒ compared to other binary metal oxide films. 9 However, the adoption of BST capacitors to DRAMs has been hindered by many issues. 10 One of the most serious difficulties was the limited success of the metallorganic CVD of the complex BST thin films. The difficulty prim...
(BTSO) thin films were grown on eight inch diameter Ru/SiO 2 /Si or bare Si wafers by atomic layer deposition (ALD) method using Bi(mmp) 3 , Ti(mmp) 4 , and Si(OEt) 4 [mmp = 1-methoxy-2-methyl-2-propoxide (OCMe 2 CH 2 OMe, Me = methyl); Et = ethyl] as metal alkoxide precursors and ozone (O 3 ) as the oxidant gas. Transmission electron microscopy (TEM) and Rutherford backscattering spectrometry (RBS) analysis showed that the as-deposited films are amorphous and the cation compositional ratio of Bi:Ti:Si is 0.38:0.37:0.25 of films deposited at 325 C on Ru/SiO 2 /Si Ru substrates. X-ray photoelectron spectroscopy (XPS) shows that the BTSO films are homogeneously dispersed without any phase-separation of the components. The BTSO films show excellent step coverage on a high aspect ratio (4) contact hole structure with a diameter of 300 nm. The leakage current density of BTSO films is on the order of 10 ±8 A cm ±2 at 1 V, meeting present dielectric requirements for a dynamic random access memory storage capacitor with an equivalent oxide thickness of 2.1 nm. From the capacitance± voltage characteristics at 10 kHz of the Pt/BTSO/Ru capacitor, the estimated dielectric constant of amorphous BTSO films is~24.
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