Trimethylaluminum Pulse Research Articles

Surface reaction during thermal atomic layer etching of aluminum oxide films using fluorine radicals and trimethylaluminum

We investigated the surface reaction of the thermal atomic layer etching (ALE) of Al2O3 film using fluorine (F) radicals and trimethylaluminum (TMA). As a strong fluorination source to replace HF, we used NF3 remote plasma to generate F radicals without spontaneous or anisotropic etching by ion bombardment. The mass was increased by F radicals and decreased by TMA. X-ray photoelectron spectroscopy (XPS) analysis showed that AlF3 and AlOxFy were formed by F radicals, and all AlF3 and a significant portion of AlOxFy were removed by TMA. At 250 °C, the etch per cycle (EPC) was fully saturated at 1.58 Å/cycle for the NF3 remote plasma of 4 s and the TMA pulse time of 3 s. Based on the saturation behavior, isotropic etching was demonstrated for the Al2O3 film prepared on a trench pattern. The EPC increased progressively with temperature from 0.1 Å/cycle at 198 °C to 4.2 Å/cycle at 320 °C. Since the thickness of the fluorinated layer did not increase significantly with increasing temperature, the removal step determined the EPC.

Atomic layer deposition of α-Al2O3 from trimethylaluminum and H2O: Effect of process parameters and plasma excitation on structure development

Aluminum oxide (Al2O3) thin films were deposited from trimethylaluminum (TMA) and H2O on Si substrates and α-Cr2O3 seed layers using thermal and plasma-assisted atomic layer deposition (ALD). All films deposited on Si were amorphous. On the α-Cr2O3 seed layers, α-Al2O3 was formed at temperatures ≥350 °C in the thermal ALD and at ≥325 °C in the plasma-assisted ALD processes. The Ar plasma used in the experiments most significantly contributed to crystallization when applied during the purge period following the H2O pulse or during the H2O pulse and the following purge. Application of plasma during the TMA pulse, during the purge following the TMA pulse, or only during the H2O pulse suppressed the crystal growth and could potentially be applied as a crystal growth inhibitor.

Investigation of hydrogen impurities in PE-ALD AlN thin films by IBA methods

Hydrogen impregnation is one of the main sources of contamination in AlN thin films obtained by atomic layer deposition method (ALD) as many studies report. Such impurities can be potentially detrimental to mechanical and electrical properties of the material. In this study methods belonging to ion-beam analysis (IBA) group, namely: elastic recoil detection analysis (ERDA), nuclear reaction analysis (NRA) and secondary-ion mass spectrometry (SIMS) were utilized to examine AlN thin films obtained by plasma-enhanced atomic layer deposition (PE-ALD) with use of trimethylaluminium (TMA) and N2+H2 gas mixture as precursors. Several depositions of AlN layers were carried out with different values of N2+H2 flux intensity (20, 100, 150 sccm) and TMA pulse (0.06, 0.12, 0.15 s) in order to investigate whether such alterations would produce any effect on the resulting films. Data obtained within the scope of this study suggest that AlN grown by PE-ALD exhibit rather low amounts of hydrogen impurities, especially when compared to thermal ALD. Furthermore, the effect of high-temperature treatment in N2 atmosphere was studied and was found out to produce a positive effect on the chemical composition of AlN films.

Interface State Reduction by Plasma-Enhanced Atomic Layer Deposition of Homogeneous Ternary Oxides.

Homogeneous ternary oxides of silicon-, niobium-, and molybdenum-aluminate were deposited by plasma-enhanced ALD using sequential metal precursor pulses prior to the oxidation step, to reduce interfacial defects usually observed in nanolaminate growth. The growth kinetics can be understood in terms of competitive adsorption. Trimethyl aluminum (TMA) is strongly chemisorbed to the growth surface and does not permit coadsorption of any of the other precursors; when we lead with a TMA pulse, the resulting film is always Al2O3. When we lead with the Si or Nb precursors, the growth surface is partially saturated, but open sites are available for TMA coadsorption. The Mo precursor is weakly chemisorbed and is largely displaced by a subsequent TMA dose. As compared to nanolaminate films of the constituent binary oxides, the interface state density is reduced by up to a factor of 5.

In Situ Observation of Two-Dimensional Electron Gas Creation at the Interface of an Atomic Layer-Deposited Al2O3/TiO2 Thin-Film Heterostructure

A two-dimensional electron gas (2DEG) was formed at the interface of an ultrathin Al2O3/TiO2 heterostructure that was fabricated using atomic layer deposition (ALD) at a low temperature (<300 °C) on a thermally oxidized SiO2/Si substrate. A high electron density (∼1014 cm–2) and mobility (∼4 cm2 V–1 s–1) were achieved, which are comparable to those of the epitaxial LaAlO3/SrTiO3 heterostructure. An in situ resistance measurement directly demonstrated that the resistance of the heterostructure interface dropped significantly with the injection of trimethylaluminum (TMA), indicating that oxygen vacancies were formed on the TiO2 surface during the TMA pulse in the ALD of Al2O3 films, such that they provide electron donor states to generate free electrons at the interface of the ultrathin Al2O3/TiO2 heterostructure. The activation energy of the electron donor states to move to the Ti 3d conduction band plays an essential role in the electrical characteristics of the 2DEG. Interestingly, the donor state level can be tailored by the control of TiO2 crystallinity, which eventually adjusts the electron density. The activation energy was decreased to less than 20 meV to generate ultrashallow donor states while improving the TiO2 crystallinity, such that the 2D electrons become readily delocalized, even at room temperature, to create a 2DEG.

Reduction of Carbon Impurities in Aluminum Nitride from Time-Resolved Chemical Vapor Deposition Using Trimethylaluminum

Aluminum nitride (AlN) is a semiconductor with a wide range of applications from light-emitting diodes to high-frequency transistors. Electronic grade AlN is routinely deposited at 1000 °C by chemical vapor deposition (CVD) using trimethylaluminum (TMA) and NH3, while low-temperature CVD routes to high-quality AlN are scarce and suffer from high levels of carbon impurities in the film. We report on an atomic layer deposition-like CVD approach with time-resolved precursor supply where readsorption of methyl groups from the AlN surface is suppressed by the addition of an extra pulse, H2, N2, or Ar, between the TMA and NH3 pulses. The suppressed readsorption allowed deposition of AlN films with a carbon content of 1 at. % at 480 °C. Kinetic and quantum-chemical modeling suggests that the extra pulse between TMA and NH3 prevents readsorption of desorbing methyl groups terminating the AlN surface after the TMA pulse.

First principles study of reactions in alucone growth: the role of the organic precursor.

Organic-inorganic hybrid materials are a unique class of materials with properties driven by the organic and inorganic components, making them useful for flexible devices. Molecular layer deposition (MLD) offers novel pathways for the fabrication of such hybrids by using inorganic metal precursors and the vast range of organic molecules with tunable properties. To investigate and understand the mechanism of growth a combination of theoretical and experimental data is needed. In this contribution, we present a first principles investigation of the molecular mechanism of the growth of hybrid organic-inorganic thin films of aluminium alkoxides, known as "alucones" grown by MLD. We explore the interactions between precursors by analyzing the MLD reaction products of the alumina surface terminated with Al(CH3) groups after the trimethyl aluminium pulse; this yields monomethyl-Al2O3 (Al-CH3-Al2O3) and dimethyl-Al2O3 (Al(CH3)2-Al2O3) terminated surfaces. The organic precursors are ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG) and tetraethylene glycol (FEG). A detailed comparison with alucones grown with ethylene glycol (EG) and glycerol (GL) precursors is presented to assist the interpretation of experimental findings regarding the differences in the hybrid films grown by EG and GL. The results show that Al-O formation with release of methane is favorable for all precursors. EG and GL can lie flat and create so-called double reactions through the reaction of the two terminal hydroxyl groups with the surface fragments. This phenomenon removes active hydroxyl sites for EG. However, for GL the third hydroxyl group is available and growth can proceed. This analysis shows the origin of differences in thickness of alucones found for EG and GL.

Impact of the sequence of precursor introduction on the growth and properties of atomic layer deposited Al-doped ZnO films

Atomic layer deposition relies on surface chemical reactions which implies that the order of the precursor pulses (so-called “sequence”) impacts the growth, especially for multinary compounds. In the case of Al-doped zinc oxide (AZO) thin films, the sequence of introduction of precursors tri-methyl aluminum (TMA)/diethylzinc (DEZ)/H2O has been reported to impact their growth and some of their properties. Here, five different Al sequences for doping the AZO films in Al have been tested at a constant deposition temperature of 160 °C and a TMA:DEZ ratio of 1:10, and the film growth and properties are investigated by in situ quartz crystal microbalance (QCM) measurements and ex situ characterizations. This paper provides evidence of the impact of the Al sequence on AZO material and proposes an explanation of the macroscopic properties based on the nature of chemical surface reactions evidenced by QCM. The growth rate, composition, electrical and optical properties, and, to a lesser extent, structural properties are affected by the TMA/DEZ/H2O pulse sequence. In particular, better electrical properties are obtained by reducing the Al content incorporated per cycle when the TMA pulse follows a DEZ pulse and, second, the optical band gap size is increased when the TMA pulse is between two DEZ pulses. Mass variations during surface reactions of TMA on hydroxyl and zinc-alkyl surface groups have also been proven to be temperature dependent when comparing growth mechanisms at 160 and 200 °C. It is also observed that the increase in temperature has a similar impact on the mass variations as the increase in the TMA:DEZ ratio. This is probably because of an improved intermixing between Al species and ZnO layers.

Silicon Surface Passivation by Mixed Aluminum Precursors in Al2O3 Atomic Layer Deposition

Dimethylaluminum chloride (DMACl) is a cost-effective aluminium precursor alternative to conventional trimethylaluminium (TMA) for Atomic Layer Deposited (ALD) Al2O3. The DMACl water process shows better passivation after high temperature firing when compared with conventional TMA water process. However, after low-temperature post-anneal its passivation quality is slightly worse than with TMA. Here we show that a mixed use of TMA and DMACl precursors in the ALD process results in better surface passivation both after 400°C post-anneal and after an 800°C firing step. The high-quality passivation results from the low interface defect density and high negative charge at the surface. Specifically, we investigate the role of chlorine in the ALD Al2O3 passivation by varying the TMA and DMACl pulse proportions.

PEALD AlN: Controlling growth and film crystallinity

AbstractWe report on the growth kinetics and material properties of aluminium nitride (AlN) films deposited on Si(111), with plasma enhanced atomic layer deposition (PEALD). Tri‐methyl aluminium (TMA) and NH3‐plasma were used as the precursors. The ALD window was identified in terms of the process parameters, and it showed that a 0.1 s of TMA pulse, 8 s of NH3‐plasma pulse, and 350 °C were the optimal conditions for ALD to occur. In‐situ spectroscopic ellipsometry (SE), using a Cauchy optical model, was used to monitor the film growth in real time. The composition of the as‐grown AlN films were determined by X‐ray photoelectron spectroscopy (XPS), which revealed Al/N compositions of approximately 46% and 53%, also with low impurity (O, C) levels. The film crystallinity, measured with grazing incidence X‐ray diffraction (GIXRD), showed polycrystalline hexagonal (wurtzitic) crystalline planes. Finally, several techniques were employed to influence the film crystallinity. These included: ALD at different plasma powers and temperatures, in‐situ treatment of the Si(111) wafer with different plasma parameters (composition, power, duration) prior to deposition, and ALD on Si(100) and SiO2 substrates. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A Single-Event MicroKinetic assessment of n-alkane hydroconversion on ultrastable Y zeolites after Atomic Layer Deposition of alumina

The acid strength of a commercial Y zeolite subjected to Atomic Layer Deposition (ALD) of alumina using alternating pulses of trimethylaluminium (TMA) and water was assessed by means of Single-Event MicroKinetic modelling of n-decane hydroconversion. Catalytic activity changes were rationalized in terms of changes in three catalyst descriptors: the physisorption saturation concentration, the total Brønsted acid site concentration, and the average acid strength of these sites. The former two descriptors were measured independently from N2 adsorption, and pyridine TPD and FT-IR, while the latter was determined as the reference alkene standard protonation enthalpy by model regression. Changes in aluminium content were characterized by means of 27Al MAS NMR. Physico-chemically reasonable differences in sorbate stabilization by Van der Waals interactions with the zeolite framework could not explain the observed differences in n-decane hydroconversion performance between the ALD-treated USY samples. Instead, an increase in catalytic activity by the ALD treatment could be assigned to the creation of new and stronger Brønsted acid sites in the zeolite micropores exhibiting alkene standard protonation enthalpies that were up to 7kJmol−1 more negative on the ALD-treated samples compared to the parent zeolite. The newly formed acid sites were associated with extraframework aluminium species in a distorted tetrahedral configuration and were predominantly obtained using long TMA pulse times and a large number of ALD cycles provided that water was effectively removed from the ALD reaction chamber prior to ALD.

Al2O3/InGaAs Metal-Oxide-Semiconductor Interface Properties: Impact of Gd2O3 and Sc2O3 Interfacial Layers by Atomic Layer Deposition

The use of InGaAs as a high carrier mobility CMOS-channel material requires a proper electrical passivation of its interface with the gate dielectric. We investigate InGaAs passivation by Atomic Layer Deposition (ALD) of Al2O3, Gd2O3, and Sc2O3 using tri-methylaluminum (TMA), (iPrCp)3Gd, (MeCp)3Sc, and H2O as precursors. We discuss the impact of the starting precursor and TMA exposure during the initial cycles of Al2O3 on the interface trap density (Dit), frequency dispersion, leakage current, and breakdown field. Increasing the TMA pulse time to five seconds during the first five cycles reduce the Dit to 1.8 × 1012 eV−1 cm−2, while frequency dispersion, leakage current and breakdown field generally improve. Gd2O3 and Sc2O3 interfacial layers between InGaAs and Al2O3 are examined. The initial growth study of Gd2O3 ALD on InGaAs indicates growth inhibition as compared to the hydrophilic SiO2/Si substrate. Gd2O3 and Sc2O3 improve the interface in terms of Dit and border traps. The improvement depends on the initial precursor pulse lengths, and on the Gd- or Sc-content. The lowest Dit values, 2.5 × 1012 eV−1 cm−2 and 1.8 × 1012 eV−1 cm−2, are obtained for four cycles of Sc2O3 and Gd2O3, respectively. Interfacial self-cleaning by TMA, (iPrCp)3Gd, and (MeCp)3Sc is demonstrated by X-ray Photo-electron Spectroscopy and Time-of-flight secondary ion mass Spectroscopy.

Probing the Nucleation of $\hbox{Al}_{2}\hbox{O}_{3}$ in Atomic Layer Deposition on Aluminum for Ultrathin Tunneling Barriers in Josephson Junctions

Ultrathin dielectric tunneling barriers are critical to Josephson junction (JJ) based superconducting quantum bits (qubits). However, the prevailing technique of thermally oxidizing aluminum via oxygen diffusion produces problematic defects, such as oxygen vacancies, which are believed to be a primary source of the two-level fluctuators and contribute to the decoherence of the qubits. Development of alternative approaches for improved tunneling barriers becomes urgent and imperative. Atomic Layer Deposition (ALD) of aluminum oxide (Al2O3) is a promising alternative to resolve the issue of oxygen vacancies in the Al2O3 tunneling barrier, and its self-limiting growth mechanism provides atomic-scale precision in tunneling barrier thickness control. A critical issue in ALD of Al2O3 on metals is the lack of hydroxyl groups on metal surface, which prevents nucleation of the trimethylaluminum (TMA). In this work, we explore modifications of the aluminum surface with water pulse exposures followed by TMA pulse exposures to assess the feasibility of ALD as a viable technique for JJ qubits. ALD Al2O3 films from 40 angstroms to 100 angstoms were grown on 1.4 angstroms to 500 angstroms of Al and were characterized with ellipsometry and atomic force microscopy. A growth rate of 1.2 angstroms/cycle was measured, and an interfacial layer (IL) was observed. Since the IL thickness depends on the availability of Al and saturated at 2 nm, choosing ultrathin Al wetting layers may lead to ultrathin ALD Al2O3 tunneling barriers.

Arsenic decapping and half cycle reactions during atomic layer deposition of Al2O3 on In0.53Ga0.47As(001)

In situ x-ray photoelectron spectroscopy was performed during thermal desorption of a protective As layer and subsequent atomic layer deposition (ALD) of Al2O3 on In0.53Ga0.47As(001). H2O dosing on the As-decapped surface caused formation of As oxides and As hydroxides, which were reduced by a subsequent trimethylaluminum (TMA) pulse. However, when a TMA pulse was performed first, the In0.53Ga0.47As(001) surface did not oxidize during subsequent ALD cycles, suggesting passivation by TMA adsorption at water-reactive sites. Scanning tunneling microscopy performed on a structurally-similar InAs(001) surface after H2O dosing revealed that surface defects are created by displacement of surface As atoms during oxidation. These surface defects act as interface states, consistent with the inferior capacitance-voltage characteristics of H2O-first ALD-Al2O3 capacitors compared to TMA-first samples.

ALD growth of Al2O3 on GaAs: Oxide reduction, interface structure and CV performance

AbstractWe have studied the effect of the trimethylaluminum (TMA) precursor on the reduction of surface “native” oxides from GaAs substrates using medium energy ion scattering spectroscopy (MEIS), X‐ray photoelectron spectroscopy (XPS) and electrical measurements. Our data show that after one single TMA pulse a substantial part of the native oxide is reduced and an oxygen‐rich aluminum oxide layer is formed. Al2O3 films grown with the normal atomic layer deposition cycles of TMA and water show that the growth rate of the Al oxide during this initial reduction of the native oxides is faster than the rate once this reduction is completed. Furthermore, the results of C‐V measurements of Al2O3/GaAs grown under the same conditions along with post deposition annealing indicate a good quality interface. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Reduction of native oxides on GaAs during atomic layer growth of Al2O3

The reduction of surface “native” oxides from GaAs substrates following reactions with trimethylaluminum (TMA) precursor is studied using medium energy ion scattering spectroscopy (MEIS) and x-ray photoelectron spectroscopy (XPS). MEIS measurements after one single TMA pulse show that ∼65% of the native oxide is reduced, confirmed by XPS measurement, and a 5 Å thick oxygen-rich aluminum oxide layer is formed. This reduction occurs upon TMA exposure to as-received GaAs wafers.

Half-cycle atomic layer deposition reaction studies of Al2O3 on (NH4)2S passivated GaAs(100) surfaces

“Half-cycle” atomic layer deposition reactions of trimethyl aluminum (TMA) and water on GaAs exposed to wet chemical sulfur treatments are studied for the formation of Al2O3. Trivalent oxides of gallium and arsenic are completely reduced following the first TMA pulse. The same processing step also removes As–S bonding below the level of detection, while the relative concentration of gallium suboxides as well as Ga–S bonds is not affected. A concomitant decrease in the S 2p peak intensity is observed, indicating that sulfur is lost through a volatile reaction product. Further precursor exposures do not measurably affect substrate surface chemistry.

In Situ Mass Spectrometry Study on Surface Reactions in Atomic Layer Deposition of Al2O3 Thin Films from Trimethylaluminum and Water

The surface reactions between trimethylaluminum (TMA) and deuterated water (D2O) in the deposition of Al2O3 were studied by using a mass spectrometer to determine the amount of the reaction product, methane (CH3D), produced in a flow-type atomic layer deposition (ALD) reactor during each deposition step. At low temperatures (<200 °C) more CH3D was produced during the TMA pulse whereas at higher temperatures when the dehydroxylation is more extensive more CH3D was produced during the D2O pulse. In addition to the temperature, the hydroxyl coverage was also noticed to depend on the D2O dose, since D2O reacts with dehydroxylated alumina sites producing adsorbed −OD groups. According to our results, which agree quite well with previous studies carried out by other methods, the dehydroxylation of the alumina surface seems to play a key role in the surface chemistry of the atomic layer deposition of Al2O3.

Dependence of atomic layer-deposited Al2O3 films characteristics on growth temperature and Al precursors of Al(CH3)3 and AlCl3

The materials characteristics of Al2O3 films grown on a Si (100) substrate by traveling wave reactor atomic layer deposition were investigated in the growth temperature ranging from 250 to 500 °C. The Al2O3 films grown using Al(CH3)3 trimethylaluminum (TMA) and H2O as precursors were characterized and also compared with the films grown using AlCl3 and H2O. Both samples grown with different precursors revealed identical chemical binding state of oxidized Al and very flat surface morphology. In the study of impurity incorporation, the films grown using TMA showed the C and H count rates of secondary ion mass spectrometry (SIMS) approximately six and 10 times higher than those of the film grown using AlCl3, respectively. For the Al2O3 films grown using TMA, the results showed that the impurity contents and the growth rate of the films decreased and the refractive index increased with the growth temperature. The content of impurities could be also lowered by increasing the N2 purge time after TMA pulse. However, the effect of purge time increase on the impurity incorporation was very small compared to that of growth temperature. The refractive indices were 1.64 and 1.68 at the growth temperatures of 250 and 400 °C, respectively. The rough estimation of H content using refractive indices and SIMS data indicated that the H content decreased from approximately 8.5 to 3.0 at. % as the substrate temperature increased from 250 to 400 °C. The Al2O3 film grown using AlCl3 and H2O at 500 °C, which contained approximately 0.5 at. % Cl, revealed a refractive index of 1.65.

Low temperature deposition of AIN films by an alternate supply of trimethyl aluminum and ammonia

AbstractAIN thin films were grown by an alternate supply of trimethyl aluminum (TMA) and ammonia, i.e., according to the processing scheme of the atomic layer epitaxy (ALE) technique. In contrast with the ideal ALE process, no saturation of the growth rate was observed with increasing TMA pulse time. In addition, with fixed pulse times the growth rate was strongly temperature dependent. These observations were interpreted in terms of a self‐decomposition of TMA. Despite the absence of self‐limitation, the films were uniform and showed good structural quality. The deposition temperatures were remarkably low, 325–425°C, compared with CVD processes employing the same precursors. Time‐of‐flight elastic recoil detection analysis (TOF‐ERDA) and nuclear resonance broadening (NRB) measurements indicated that the films contained hydrogen, carbon, and oxygen impurities.