Wan-Chih Wang

Band alignment and electron traps in Y2O3 layers on (100)Si

Y2O3 films deposited by atomic vapor deposition on (100)Si with a 2 or 5 nm thick pregrown thermal SiO2 are investigated as possible charge trapping layers. Analysis of these structures using spectroscopic ellipsometry, photoconductivity, and internal photoemission reveals that Y2O3 has a 5.6 eV wide optical bandgap and a 2.0 eV conduction band offset with silicon. Photo(dis)charging experiments show that the optical energy depth of most of the traps exceeds 1.5 eV with respect to the Y2O3 conduction band, explaining the observed charge retention time of ∼108 s at room temperature, even in the absence of a blocking insulator.

Impact of bottom electrode and SrxTiyOz film formation on physical and electrical properties of metal-insulator-metal capacitors

Metal-insulator-metal capacitors with SrxTiyOz (STO) dielectric films on TiN, Ru, and RuOx bottom electrodes with TiN top electrodes were studied. Metastable perovskite STO films with compositions in the Sr/(Sr+Ti)∼54–64 at. % range were obtained by crystallization at 600 °C in N2 of dielectric stacks grown by atomic layer deposition consisting of Sr-rich STO films [Sr/(Sr+Ti)∼64 at. %] on thin interfacial TiOx layers. The significant decrease in equivalent oxide thickness (EOT) and STO lattice parameter observed with increasing TiOx thickness indicates full intermixing of the TiOx and STO layers during the crystallization anneal, which results in the formation of an STO layer with higher Ti content and higher dielectric constant. The Sr-rich STO on TiOx stacks crystallize with small grain size, favorable for low leakage (JG). A significant improvement in JG for e-injection from the bottom electrode is obtained when using RuOx, as compared to TiN or Ru. A milder JG improvement with RuOx bottom electrode i...

Applying Complementary Trap Characterization Technique to Crystalline

The operation and reliability of nonvolatile memory concepts based on charge storage in nitride layers, such as TANOS (TaN/Al₂O₃/Si₃N₄/ SiO₂/Si), require detailed information on the energy and spatial distribution of the charge defects in both the nitride and the Al₂O₃ blocking dielectric. This paper focuses on the characterization of Al₂O₃. We have successfully applied complementary trap characterization techniques to crystalline γ-phase- Al₂O₃ in order to obtain a complete picture of the spatial and energetic distribution of the defect density. As a result, two defect types at energy levels 1.8 and 3.5 eV below the conduction band edge are found.

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Metastable perovskite SrxTiyOz (STO) films were formed over a wide composition range by crystallization of layers grown by atomic layer deposition. An expansion of the lattice, decrease in permittivity and mild increase in band gap are observed with increasing Sr content. Sr-rich films [Sr/(Sr+Ti)∼62 at. %] show significant improvement in leakage current at low equivalent oxide thicknesses (EOT) as compared to stoichiometric films (Sr/(Sr+Ti) ∼50 at. %). TiN/STO/TiN capacitors with leakage ∼10−6 A/cm2 at 1 V were obtained at 0.6 nm EOT for crystalline Sr-rich STO. The difference in leakage behavior was found to correlate with different microstructures developed during crystallization.

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We present direct physical evidence supporting the mechanisms we proposed to explain low leakage (Jg) and low equivalent oxide thickness (EOT) in RuOx/TiOx/Sr-rich SrxTiyOz(STO)/TiN metal-insulator-metal capacitors which achieve Jg = 10−7 A/cm2 (1 V) at 0.45 nm EOT: (1) healing of STO traps by O incorporation from RuOx during STO crystallization anneal for low leakage and (2) TiOx/Sr-rich STO intermixing during STO crystallization anneal resulting in a higher Ti-content, higher k-value STO layer for low EOT. Proof of oxygen incorporation into the STO layer from the RuOx electrode as well as of TiOx/STO intermixing were obtained by secondary ion mass spectrometry performed on samples fabricated using O18 for Ru oxidation. We also show excellent match between measured leakage vs. voltage and simulations of trap-assisted leakage with asymmetric trap profiles due to STO trap healing next to RuOx, further supporting our model.

\hbox{Al}_{2} \hbox{O}_{3}

Effective electron tunneling mass mtunnel is extracted from trap-assisted leakage in Sr-rich strontium titanate and rutile titanium oxide films in metal–insulator–metal (MIM) capacitors and compared with theoretical values obtained from first principles calculations of the imaginary band structure. Optimum orientations of films and stoichiometry impacting mtunnel are also discussed. Because future vertical DRAM integration schemes also stipulate maximum thickness of the MIM capacitor, mtunnel is shown to be a critical parameter influencing intrinsic leakage and potentially limiting further scaling.

for Improved Understanding of Nonvolatile Memory Operation and Reliability

The energy distribution of electron trap density in atomic layer deposited Al2O3, LaAl4Ox and GdyAl2-yO3 insulating layers was studied by using the exhaustive photodepopulation spectroscopy. Upon filling the traps by electron tunneling from Si substrate, a broad energy distribution of trap levels in the energy range 2?4 eV is found in all studied insulators with trap densities in the range of 1012 cm?2eV?1. The incorporation of La and Gd cations reduces the trap density in aluminate layers as compared to Al2O3. Crystallization of the insulator by the post-deposition annealing is found to increase the trap density while the energy distribution remains unchanged. The similar trap spectra in the Al2O3 and La or Gd aluminate layers suggest the common nature of the traps, probably originating from imperfections in the AlOx sub-network.

Impact of crystallization behavior of SrxTiyOz films on electrical properties of metal-insulator-metal capacitors with TiN electrodes

We report the lowest leakage achieved to date in sub-0.5 nm EOT MIM capacitors compatible with DRAM flows, showing for the first time a path enabling scalability to the 3X nm node. A novel stack engineering consisting of: 1) novel controlled ultrathin Ru oxidation process, 2) TiO<inf>x</inf> interface layer, is used for the first time to achieve record low Jg-EOT in MIM capacitors using ALD Sr-rich STO high-k dielectric and thin Ru bottom electrode. Record low Jg of 10⁻⁶ A/cm² (10⁻⁸ A/cm²) is achieved for EOT of 0.4 nm (0.5 nm) at 0.8 V. Our data is compared favorably (> 100× Jg reduction at 0.4 nm) to previous best values in literature for MIMcaps with ALD dielectrics.

Direct physical evidence of mechanisms of leakage and equivalent oxide thickness reduction in metal-insulator-metal capacitors based on RuOx/TiOx/SrxTiyOz/TiN stacks

HfTiOx films deposited on TiN, Pt and Al2O3/Si bottom electrodes (BE) by Atomic Layer Deposition (ALD) method were investigated in terms of the crystallization temperature, dielectric constant and leakage current. The as-deposited films show a linear increase of dielectric constant from 20 (HfO2) to 40 (TiO2) as a function of Ti content in the film. On the other hands, the dielectric constant of HfTiOx films with 64% – 30% Hf content increases up to 60 after crystallization annealing, which strongly correlates with the appearance of the orthorhombic HfTiO4 structure. Furthermore, the thermal treatment in O2 ambient is found to have a drastic effect to decrease the leakage current density (Jg). As a result, 10−8 A/cm2 at Vg = +1V with 0.8nm EOT is achieved with 30%-Hf contained film.

Considerations for further scaling of metal–insulator–metal DRAM capacitors

The bandgap width in niobium tantalate (NbxTayOz) films with Ta/(Ta+Nb) ratio of 10.7, 41.9, and 68.4 % are studied in this work. The samples were prepared by atomic layer deposition on (100)Si/TaN substrates, some of them subjected to crystallization annealing. Two photoconductivity spectral thresholds are found in all the studied samples: one at around 4.2 eV and another in range 3.2–3.8 eV, which may be correlated with the bandgaps associated with TaOx and NbOx sub-networks, respectively. The observed preservation of the narrowest bandgap of NbOx despite the addition of the Ta cation indicates that electron states of Nb and Ta have insufficient mutual interaction to affect the gap width of two oxide sub-networks.