Radical-enhanced Atomic Layer Deposition Research Articles

Characterization of radical-enhanced atomic layer deposition process based on microwave surface wave generated plasma

A plasma-assisted atomic layer deposition system employing a microwave surfatron plasma was developed and characterized by spatially resolved Langmuir probe diagnostics and optical emission spectroscopy. The deposition process was applied on TiO2 thin films prepared on Si wafers. The surfatron is equipped with a small ring electrode serving as a source of weak radio frequency plasma helping with fast and reliable ignition of the discharge in molecular gas. Results evaluated in the pure argon plasma proved that the plasma potential and the plasma density are homogeneous in the radial direction, while a rapid decrease was observed in the axial direction. Adding up to 30% of nitrogen into the gas mixture led to less homogeneous plasma parameters in the radial direction together with the increase of the electron effective temperature. Optical emission spectra revealed many Ar I lines of neutral atoms with only a few Ar II ions’ lines. The gradual addition of nitrogen causes a systematic decrease in the Ar I line intensity. We expect that excited nitrogen molecules are produced by the inelastic collisions with electrons and by the collisional quenching of metastable Ar(4s) states. On the other hand, oxygen atom and ion lines are detected when oxygen was mixed with argon. Deposited TiO2 thin films are characterized by the anatase phase when the substrate temperature is 250 °C. The anatase phase is observed even for the substrate temperature of 200 °C; however, the microwave power delivered into the surfatrons must be lower in comparison with the prior case.

Impact of the Atomic Layer-Deposited Ru Electrode Surface Morphology on Resistive Switching Properties of TaOx-Based Memory Structures.

Resistive switching (RS) device behavior is highly dependent on both insulator and electrode material properties. In particular, the bottom electrode (BE) surface morphology can strongly affect RS characteristics. In this work, Ru films with different thicknesses grown on a TiN layer by radical-enhanced atomic layer deposition (REALD) are used as an inert BE in TaOx-based RS structures. The REALD Ru surface roughness is found to increase by more than 1 order of magnitude with the increase in the reaction cycle number. Simultaneously, a wide range of RS parameters, such as switching voltage, resistance both in low and high resistance states, endurance, and so forth, monotonically change. A simplified model is proposed to explain the linkage between RS properties and roughness of the Ru surface. The field distribution was simulated based on the observed surface morphologies, and the resulting conducting filament formation was anticipated based on the local field enhancement. Conductive atomic force microscopy confirmed the theoretical expectations.

Charge Transport Mechanism in Atomic Layer Deposited Oxygen‐Deficient TaOx Films

TaOx is a promising candidate for random access memory formation. To provide the possibility of 3D integration, it is grown by the atomic layer deposition (ALD). Herein, such properties of the pristine TaOx grown by radical‐enhanced ALD (REALD) as oxygen vacancies concentration and the trap energy are evaluated to reveal whether it can replace more common ion‐sputtered TaOx both in filamentary and nonfilamentary resistive random access memory (ReRAM) stacks. For this purpose, charge transport mechanism in the TiN/TaOx/Pt stacks is analyzed. It is found that charge transport through TaOx is described by the phonon‐assisted tunneling between traps model. The thermal Wt = 0.85 eV and optical Wopt = 1.7 eV trap energies are determined. The trap concentration N = 1 × 1021 cm−3 found from transport corresponds well with the oxygen vacancy concentration, obtained by ab initio simulation of valence band X‐ray photoelectron spectrum. Derived parameters are typical for the ion‐sputtered TaOx. As a result, it can be considered to replace ion‐sputtered TaOx in the nonfilamentary bilayer stacks. Moreover, the observed properties explain the possibility to achieve electroforming‐free resistive switching in a single layer REALD grown TaOx‐based stack in combination with extracting Ta electrode.

Influence of ALD Ru bottom electrode on ferroelectric properties of Hf0.5Zr0.5O2-based capacitors

We report the influence of an ultrathin Ru bottom electrode on ferroelectric properties of fully atomic layer deposition (ALD)-grown Hf0.5Zr0.5O2 (HZO) and La-doped Hf0.5Zr0.5O2 (HZLO)-based ferroelectric capacitors. We show that the Ru bottom electrode deposited by radical enhanced ALD (REALD) improves the remanent polarization of both capacitors considerably. The origin of such a phenomenon is established by grazing-incidence and symmetrical θ–2θ x-ray diffraction measurements. HZO films on Ru exhibit the orthorhombic phase, which is highly (002)-textured in the out-of-plane direction as compared to HZO on TiN. HZLO films demonstrate the rise of (111) intensity of the orthorhombic phase when it is grown on Ru. Both types of capacitors with Ru exhibit a lower wake-up degree as compared to the ones with TiN, which is assumed to be due to the difference in the bottom interface properties. At the same time, both HZO and HZLO on Ru suffer from the relatively early breakdown during electric field cycling, which is presumably due to the high surface roughness of REALD Ru. Taking into account the continuous search for the new precursor's chemicals and ALD processes for Ru, which would be able to provide smother films, ALD Ru might be promising for the hafnium oxide-based ferroelectric random access memory.

Radical-Enhanced Atomic Layer Deposition of a Tungsten Oxide Film with the Tunable Oxygen Vacancy Concentration

This work reports a radical-enhanced atomic layer deposition (REALD) process using WH2(Cp)2–O*–H* reaction cycles (Cp = cyclopentadienyl group) to grow WO3–x films with a wide range of tunable oxyg...

Temperature controlled Ru and RuO2 growth via O* radical-enhanced atomic layer deposition with Ru(EtCp)2.

This work demonstrates by in vacuo X-ray photoelectron spectroscopy and grazing-incidence X-ray diffraction that Ru(EtCp)2 and O* radical-enhanced atomic layer deposition, where EtCp means the ethylcyclopentadienyl group, provides the growth of either RuO2 or Ru thin films depending on the deposition temperature (Tdep), while different mechanisms are responsible for the growth of RuO2 and Ru. The thin films deposited at temperatures ranging from 200 to 260 °C consisted of polycrystalline rutile RuO2 phase revealing, according to atomic force microscopy and the four-point probe method, a low roughness (∼1.7 nm at 15 nm film thickness) and a resistivity of ≈83 µΩ cm. This low-temperature RuO2 growth was based on Ru(EtCp)2 adsorption, subsequent ligand removal, and Ru oxidation by active oxygen. The clear saturative behavior with regard to the precursor and reactant doses and each purge time, as well as the good step coverage of the film growth onto 3D structures, inherent to genuine surface-controlled atomic layer deposition, were confirmed for the lowest Tdep of 200 °C. However, at Tdep = 260 °C, a competition between film growth and etching was found, resulted in not-saturative growth. At higher deposition temperatures (300-340 °C), the growth of metallic Ru thin films with a resistivity down to ≈12 µΩ cm was demonstrated, where the film growth was proved to follow a combustion mechanism known for molecular oxygen-based Ru growth processes. However, this process lacked the truly saturative growth with regard to the precursor and reactant doses due to the etching predominance.

Atomic layer deposition of YMnO3 thin films

YMnO3 (YMO) thin films were synthesized by radical-enhanced atomic layer deposition (RE-ALD) on silicon (Si) and yttria-stabilized zirconia (YSZ)substrates, to investigate the effect of film composition and substrates on their intrinsic magnetic properties. The crystalline phase of these ultra-thin films depends on both the processing conditions and the substrate lattice parameters. The Mn/Y atomic ratio of the YMO thin films could be controlled near unity by adjusting the Mn:Y precursor pulsing ratio during the RE-ALD processes. The ALD YMO thin film on Si (111) was orthorhombic, regardless of the film thickness with a Néel temperature (TN) between 48 ∼ 62 K, as determined through the anomalies observed during DC magnetic susceptibility measurements. However, ultra-thin ALD YMO films (∼6 nm) on YSZ (1 1 1), at a Mn/Y atomic ratio near unity, has both orthorhombic- and hexagonal- phases, yielding two TN anomalies measured at ∼48 K and ∼85 K. The induction of magnetization of ultra-thin YMO film on Si (1 1 1) under an in-situ 20 V electric poling indicates that the magnetoelectric coupling was observed below TN, showing that the ALD synthesis could be a promising technique to deposit ultra-thin magnetoelectric films.

Atomic Layer Deposited Oxygen‐Deficient TaOx Layers for Electroforming‐Free and Reliable Resistance Switching Memory

Oxygen‐deficient TaOx layers are grown by a radical‐enhanced atomic layer deposition (REALD) process on a chemically active bottom electrode (Ta) to create electroforming‐free resistance switching random access memory (ReRAM) cells. When the top electrode is Pt, the Ta/TaOx/Pt device shows a completely electroforming‐free behavior with a pulse‐switching induced endurance cycle of up to 6 × 106 cycles. This is due to the abundantly present oxygen vacancies within the TaOx layer, which eliminate the need to create further oxygen vacancies during the electroforming cycles in other types of oxide‐based ReRAM cells. The adoption of the Pt top electrode contributes to the suppression of the leakage current, making the switching reliable. When the top electrode is changed to the more production‐compatible Ru, which is also grown by another REALD, the devices show very mild electroforming behavior with an increased leakage current. Due to these slight decreases in the electrical performance, the switching endurance can be confirmed only up to 6 × 104 cycles with the help of an incremental step pulse programming technique.

Magnetic Properties of CoFe2O4 Thin Films Synthesized by Radical-Enhanced Atomic Layer Deposition

A radical-enhanced atomic layer deposition (RE-ALD) process was developed for growing ferrimagnetic CoFe2O4 thin films. By utilizing bis(2,2,6,6-tetramethyl-3,5-heptanedionato) cobalt(II), tris(2,2,6,6-tetramethyl-3,5-heptanedionato) iron(III), and atomic oxygen as the metal and oxidation sources, respectively, amorphous and stoichiometric CoFe2O4 films were deposited onto SrTiO3 (001) substrates at 200 °C. The RE-ALD growth rate obtained for CoFe2O4 is ∼2.4 Å/supercycle, significantly higher than the values reported for thermally activated ALD processes. Microstructural characterization by X-ray diffraction and transmission electron microscopy indicate that the CoFe2O4 films annealed between 450 and 750 °C were textured polycrystalline with an epitaxial interfacial layer, which allows strain-mediated tuning of the magnetic properties given its highly magnetostrictive nature. The magnetic behavior was studied as a function of film thickness and annealing temperature: saturation magnetization (Ms) ranged from 260 to 550 emu/cm3 and magnetic coercivity (Hc) ranged from 0.2 to 2.2 kOe. Enhanced magnetic anisotropy was achieved in the thinner samples, whereas the overall magnetic strength improved after annealing at higher temperatures. The RE-ALD CoFe2O4 thin films exhibit magnetic properties that are comparable to both bulk crystal and films grown by other deposition methods, with thickness as low as ∼7 nm, demonstrating the potential of RE-ALD for the synthesis of high-quality magnetic oxides with large-scale processing compatibility.

Synthesis and Characterization of BiFeO3 Thin Films for Multiferroic Applications by Radical Enhanced Atomic Layer Deposition

A radical-enhanced atomic layer deposition (RE-ALD) process is described for the synthesis of BiFeO3. Metalorganic precursors β-diketonate, tris(2,2,6,6-tetramethyl-3,5-heptanedionato) iron(III) (Fe(TMHD)3), and Bi(TMHD)3 are coreacted with oxygen radicals (produced by a coaxial microwave cavity) as the oxidation source. Stoichiometric BiFeO3 films were deposited on SrTiO3 (001) substrates at 210 °C and crystallized from 450 to 750 °C. The crystallinity and phases in the films were characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The BiFeO3 was determined to be phase-pure and epitaxial when annealed at 650 °C. To study the functional properties, ferroelectric switching was demonstrated using piezoresponse force microscopy (PFM), while weak ferromagnetic behavior was measured using superconducting quantum interference device (SQUID) magnetometry. The films have properties comparable to those of films grown by other methods—growth rates are superior in comparison to ot...

Role of low-energy ion irradiation in the formation of an aluminum germanate layer on a germanium substrate by radical-enhanced atomic layer deposition

Radical-enhanced atomic layer deposition uses oxygen radicals generated by a remote microwave-induced plasma as an oxidant to change the surface reactions of the alternately supplied trimethylaluminum precursor and oxygen radicals on a Ge substrate, which leads to the spontaneous formation of an aluminum germanate layer. In this paper, the effects that low-energy ions, supplied from a remote microwave plasma to the substrate along with the oxygen radicals, have on the surface reactions were studied. From a comparative study of aluminum oxide deposition under controlled ion flux irradiation on the deposition surface, it was found that the ions enhance the formation of the aluminum germanate layer. The plasma potential measured at the substrate position by the Langmuir probe method was 5.4 V. Assuming that the kinetic energy of ions arriving at the substrate surface is comparable to that gained by this plasma potential, such ions have sufficient energy to induce exchange reactions of surface-adsorbed Al atoms with the underlying Ge atoms without causing significant damage to the substrate. This ion-induced exchange reaction between Al and Ge atoms is inferred to be the background kinetics of the aluminum germanate formation by radical-enhanced atomic layer deposition.

Radical‐Enhanced Atomic Layer Deposition of Silver Thin Films Using Phosphine‐Adducted Silver Carboxylates

AbstractMetallic silver films are deposited by radical‐enhanced atomic layer deposition (REALD) using (2,2‐dimethylpropionato)silver(I)triethylphosphine and hydrogen radicals. The silver precursor used is synthesized in‐house, and characterized using CHN elemental analysis, infrared (IR) spectroscopy, nuclear magnetic resonance (NMR), mass spectrometry (MS), and thermogravimetric analysis/single differential thermal analysis (TGA/SDTA). The crystal structure of Ag(O2CtBu)(PEt3) is also solved. Trimeric units are revealed as the building blocks. The hydrogen radicals are produced by dissociating molecular hydrogen with a microwave plasma discharge. The evaporation temperature of the silver precursor is 125 °C, and the film deposition temperature is 140 °C. The deposition is successful on glass and silicon, and the films are conformal. The saturated growth rate is 0.12 nm per cycle, with a 3 s silver precursor pulse and 5 s hydrogen radical pulse time. The overall cycle time is 14 s. The films are polycrystalline and are visually mirror‐like. The films contain 10 at.‐% oxygen, 4.0 at.‐% phosphorous, 1.0 at.‐% carbon, and 5.0 at.‐% hydrogen as impurities. Nevertheless, the films exhibit low resistivity, only 6 μΩ cm for a 40 nm thick film.

Radical Enhanced Atomic Layer Deposition of Titanium Dioxide

AbstractTitanium dioxide films are grown with radical‐enhanced atomic layer deposition (RE‐ALD) from titanium isopropoxide and oxygen radicals at between 50 and 300 °C on silicon, glass, platinum, and RuO2 surfaces. Additionally, the films are grown on polymers and natural fibers at 50 °C. The oxygen radicals are produced by dissociating molecular oxygen with a remote microwave plasma discharge. Purified argon is used as the carrier and purge gas. Growth rate saturation and conformal growth are observed, and thus the film growth in the ALD mode was proven. The saturated growth rate is 0.19 nm per cycle at 50 °C, with an overall cycle time of 11 s. The film grown at 50 °C has a dielectric constant of 20, and undergoes catastrophic breakdown at 0.6 MV cm–1 electric field. It contains 13 at.‐% hydrogen and 4 at.‐% carbon. As the deposition temperature is increased to 250 °C, the impurity contents decrease to 0.5 at.‐% for hydrogen and 0.4 at.‐% for carbon. The film density and refractive index are 3.2 g cm–3 and 2.2, respectively, for a film deposited at 50 °C, and increases to 3.8 g cm–3 and 2.4, respectively, for a film deposited at 250 °C. All the films grown below 250 °C are amorphous according to X‐ray diffraction (XRD).

Nanostructure and temperature-dependent photoluminescence of Er-doped Y2O3 thin films for micro-optoelectronic integrated circuits

The nanostructure and photoluminescence of polycrystalline Er-doped Y2O3 thin films, deposited by radical-enhanced atomic layer deposition (ALD), were investigated in this study. The controlled distribution of erbium separated by layers of Y2O3, with erbium concentrations varied from 6to14at.%, was confirmed by elemental electron energy loss spectroscopy (EELS) mapping of Er M4 and M5. This unique feature is characteristic of the alternating radical-enhanced ALD of Y2O3 and Er2O3. The results are also consistent with the extended x-ray absorption fine structure (EXAFS) modeling of the Er distribution in the Y2O3 thin films, where the EXAFS data were best fitted to a layer-like structure. X-ray diffraction (XRD) and selected-area electron diffraction (SAED) patterns revealed a preferential film growth in the [111] direction, showing a lattice contraction with increasing Er doping concentration, likely due to Er3+ of a smaller ionic radius replacing the slightly larger Y3+. Room-temperature photoluminescence characteristic of the Er3+ intra-4f transition at 1.54μm was observed for the 500Å, 8at.% Er-doped Y2O3 thin film, showing various well-resolved Stark features due to different spectroscopic transitions from the I13∕24→I15∕24 energy manifold. The result indicates the proper substitution of Y3+ by Er3+ in the Y2O3 lattice, consistent with the EXAFS and XRD analyses. Thus, by using radical-enhanced ALD, a high concentration of optically active Er3+ ions can be incorporated in Y2O3 with controlled distribution at a low temperature, 350°C, making it possible to observe room-temperature photoluminescence for fairly thin films (∼500–900Å) without a high temperature annealing.

Er coordination in Y2O3 thin films studied by extended x-ray absorption fine structure

Extended x-ray absorption fine structure (EXAFS) spectroscopy was employed to study the Er coordination in polycrystalline Y2O3 thin films, which was found to dictate their photoluminescence (PL) properties. Incorporation of Er with concentrations varying from 6to14at.% was achieved by radical-enhanced atomic layer deposition at 350°C. In all samples, Er was found to be in the optically active trivalent state, confirmed by their x-ray absorption near-edge spectroscopy spectra. Modeling of the EXAFS data revealed that the local structure of Er3+ is similar to that of Er3+ in Er2O3. Specifically, Er3+ is coordinated with six O at 2.24 and 2.32Å. Excellent fits to the EXAFS for samples with Er3+ concentration less than 8at.% were achieved when the second coordination shell was modeled as a mixture of Y3+ and Er3+, indicating a complete miscibility of Er3+ in the Y2O3 matrix under these experimental conditions. This behavior is attributed to the almost perfect ionic size match between Y3+ and Er3+, having identical valence state and coordination characteristics. For thin films with higher Er concentrations, the EXAFS analysis revealed an exsolution with Er2O3 domain. Since there is no indication of Er clustering, it is concluded that the PL quenching observed in samples with the Er doping level exceeding 8at.% is likely due to Er ion-ion interaction but not Er immiscibility in Y2O3. Specifically, an increase in the Er3+ concentration implied an increase in the average number of Er3+ in the second coordination shell, thus making ion-ion interaction possible. The critical interionic distance between two Er3+ was determined to be ∼4Å, thus setting an upper limit on the Er3+ concentration in Y2O3 at ∼6×1021cm−3, at least three orders of magnitude higher than the Er3+ solubility limit in the conventional SiO2 host (<1018cm−3).

Erbium Incorporation in Yttrium Oxide Thin Films by Radical-enhanced Atomic Layer Deposition

not Available.

Radical-enhanced atomic layer deposition of Y 2O 3 via a β-diketonate precursor and O radicals

Thin films of Y 2O 3 were deposited on Si(1 0 0) by radical-enhanced atomic layer deposition (ALD), using Y(TMHD) 3 and O radicals. First, the self-limiting mechanism of this process was investigated in situ from 473 to 573 K, using a quartz crystal microbalance. Specifically, adsorption of Y(TMHD) 3 can only be initiated after reactive sites are created on the surface by the exposure to O radicals. However, due to the bulky β-diketonate ligands, only a sub-monolayer of metal oxide was achieved after these ligands were removed by a subsequent O radical exposure. These alternating reactions were found to be self-limiting and temperature insensitive from 473 to 573 K. A reactant pulse time of 7 min was required for both Y(TMHD) 3 and O radicals to achieve saturation, yielding a maximum deposition rate of 0.5 Å/cycle in the ALD temperature window from 473 to 573 K. A lower deposition rate of 0.3 Å/cycle was obtained when both reactant pulse times were reduced to 30 s. X-ray photoelectron spectroscopy analysis suggests the formation of a thin yttrium silicate interfacial layer between Y 2O 3 and Si. The compositional analysis indicates stoichiometric Y 2O 3 films were deposited, with the carbon content decreased from 24 at.% at 473 K to 4 at.% at 623 K. The atomic force microscopy images revealed fairly smooth surfaces of films deposited at 573 K, showing a small root mean square roughness of 5 Å for a 115-Å Y 2O 3 film. Excellent conformal deposition of an 800-Å Y 2O 3 film over 0.5 μm features with an aspect ratio of 2 was accomplished. These results demonstrate that radical-enhanced ALD is a viable technique for conformal low-temperature deposition of stoichiometric and smooth metal oxide thin films with minimal carbon contamination.

Controlled erbium incorporation and photoluminescence of Er-doped Y2O3

A high concentration of erbium doping was achieved in Y2O3 thin films on Si (100) by depositing Y2O3 alternatively with Er2O3 using radical-enhanced atomic layer deposition (ALD). Specifically, the erbium doping level was controlled by varying the ratio of Y2O3:Er2O3 cycles during deposition, and a 10:5 ratio yielded ∼9at.% erbium incorporation in Y2O3, confirmed by the compositional analysis using x-ray photoelectron spectroscopy. Room-temperature photoluminescence was observed in a 320-Å Er-doped (9 at. %) Y2O3 film deposited at 350 °C. This result is very promising, since the film was fairly thin and no annealing at high temperature was needed to activate the erbium ions. This suggests that radical-enhanced ALD was able to preserve the optically active trivalent state of the erbium ion from its precursor state. The effective absorption cross section for Er3+ ions incorporated in Y2O3 was estimated to be on the order of 10−18cm2, about three orders of magnitude larger than the direct optical absorption cross section reported for Er3+ ions in a stoichiometric SiO2 host. These results validate Y2O3 as a promising Er3+ host material and demonstrate that radical-enhanced ALD is a viable technique for synthesizing these materials.

Surface reaction kinetics of metal β-diketonate precursors with O radicals in radical-enhanced atomic layer deposition of metal oxides

The surface reaction kinetics of Er(TMHD) 3 and Y(TMHD) 3 with O radicals in radical-enhanced atomic layer deposition (ALD) of Er 2O 3 and Y 2O 3 was investigated in situ using a quartz crystal microbalance (QCM). The adsorption isotherms were fitted with the Langmuir–Hinshelwood adsorption model and the extracted adsorption rate coefficient was found to decrease with increasing temperature, exhibiting a negative apparent activation energy of −0.24 ± 0.09 eV for Er(TMHD) 3 and −0.14 ± 0.05 eV for Y(TMHD) 3. The corresponding activation energies for desorption were determined to be 0.29 ± 0.03 and 0.16 ± 0.03 eV. Exposing the adsorbed Er(TMHD) 3 precursors to O radicals at 533 K resulted in a rapid mass decrease followed by saturation, indicating that the reactions proceeded in a self-limiting manner. The critical O radical exposure needed to reach this saturation increased with increasing adsorbed mass and approached approximately 2 minutes as the adsorbed mass increased towards the saturation level. The net mass change ratio per cycle decreased with increasing temperature and reached 0.27 at 603 K for deposition of pure Er 2O 3. In addition to effectively removing the β-diketonate ligands, the O radicals were found to create reactive sites for precursor adsorption. Specifically, when the O radical pulse time was shorter than the critical oxygen radical exposure, the removal of β-diketonate ligands by the O radicals was incomplete, and consequently, less reactive sites were created. This ultimately led to a decrease in adsorption during the subsequent precursor pulse. Finally, radical-enhanced ALD of Er 2O 3 thin films was achieved at temperatures ranging from 473 to 573 K, using alternating pulses of metal β-diketonate precursors and O radicals.

Radical-Enhanced Atomic Layer Deposition of Metallic Copper Thin Films

Radical-enhanced atomic layer deposition (REALD) of metallic copper films from copper(II)acetylacetonate and hydrogen radicals was studied. For this work, a new kind of REALD reactor was developed by adding a surface-wave launcher type of microwave plasma source to an inert gas-valved flow-type ALD reactor. The copper films, grown at 140°C, were polycrystalline, exhibited low resistivity, 15 μΩ cm for a 25 nm thick film, and had relatively low impurity levels. The films had excellent adhesion, and they grew conformally. The successful incorporation of the radical source to the ALD reactor encourages the study of other challenging ALD processes. © 2004 The Electrochemical Society. All rights reserved.