Conventional Atomic Layer Deposition Research Articles

Plasma-enhanced atomic layer deposition of BaTiO3

Among high-k thin films, perovskite BaTiO3 (BTO) is an attractive candidate due to its exceptionally high dielectric constant. In contrast to conventional atomic layer deposition (ALD), plasma-enhanced ALD (PEALD) has several advantages such as lower process temperature, improved film quality and the deposition of a wider spectrum of materials. We report the successful deposition of high-k BTO thin films by PEALD. Compositional, morphological, and crystallographic characterizations of PEALD BTO are presented. The electrical performance of PEALD BTO thin films as a function of Ba-to-Ti-ratio is shown. Slightly Ti-rich BTO has the lowest equivalent oxide thickness while Ba-rich films show the lowest leakage current.

Self-Correcting Process for High Quality Patterning by Atomic Layer Deposition.

Nanoscale patterning of materials is widely used in a variety of device applications. Area selective atomic layer deposition (ALD) has shown promise for deposition of patterned structures with subnanometer thickness control. However, the current process is limited in its ability to achieve good selectivity for thicker films formed at higher number of ALD cycles. In this report, we demonstrate a strategy for achieving selective film deposition via a self-correcting process on patterned Cu/SiO2 substrates. We employ the intrinsically selective adsorption of octadecylphosphonic acid self-assembled monolayers on Cu over SiO2 surfaces to selectively create a resist layer only on Cu. ALD is then performed on the patterns to deposit a dielectric film. A mild etchant is subsequently used to selectively remove any residual dielectric film deposited on the Cu surface while leaving the dielectric film on SiO2 unaffected. The selectivity achieved after this treatment, measured by compositional analysis, is found to be 10 times greater than for conventional area selective ALD.

A Spatial ALD Oxide Passivation Module in an All-Spatial Etch-Passivation Cluster Concept

Conventional Atomic Layer Deposition (ALD) is also a cycled process, with the extra benefit of the half-reactions being self-limiting, thus enabling layer-by-layer growth. A major obstacle holding ALD from large-scale, high-throughput manufacturing has been the very low deposition rate. This was overcome only recently by introducing the spatial atmospheric-pressure version 1]. Today, spatial ALD is used commercially by the solar industry, and soon other application domains will follow. Shallow and deep plasma etching with nanoscale accuracy is also increasingly based on alternating half-cycles of a directional etch and an isotropic deposition of a passivating polymer layer (usually fluorocarbon or oxyfluoride). Also here the process rates are slow, and to speed up atomic precision etching we conceived a spatially-divided concept as well [2,3]. The process is converted from time- into spatially separated by inserting inert gas-bearing ‘curtains’ of heights down to ~100 μm that confine the reactive gases to individual injection slots in a gas injector head. By moving substrates back and forth under the head one accomplishes the alternate etch/ passivation cycles without idle times needed for switching pressure or purging. The pressure settings range from low-pressure for etching to (sub)atmospheric for passivation, and can be optimized for each separate process zone thus maximizing the total etch rate. A further improvement towards an all-spatial etch approach is in the passivation cycle where the conventional polymer passivation layer is replaced by a spatial atmospheric-pressure ALD-grown oxide passivation layer; see Fig. 1). We have conducted both thermal ALD of Al2O3 at 200 oC and plasma-enhanced SiO2 ALD at 50 oC in pre-etched high aspect ratio trenches (65 nm width and 9 μm depth) in a rotary spatial atmospheric-pressure reactor, and showed - with non-optimized processing and equipment - step conformality values of ³ 80 % for the Al2O3 and 70 % for the SiO2 which is sufficient for most passivations. We conducted a preliminary etch/ALD oxide passivation/etch cycle on a Si-sample with a trench-patterned hard SiO2 mask on top. The sample was processed in two separate (non-connected) reactor chambers, each chamber being supplied with a surface Dielectric Barrier Discharge (DBD) micro-plasma source. The following process cycle was carried out: 1) a 1 min. plasma etch step using SF6/O2 on a biased, non-moving Si-substrate in the low-pressure chamber at room temperature, 2) the thermal deposition of an Al2O3 passivation of only 5 monolayers (5 rotations) in the rotary spatial atmospheric pressure ALD reactor described as above, 3) an identical 1 min. plasma etch step in the low-pressure chamber. The cross-sectional etch profile thus obtained (Fig. 2) showed two typical scallops on the trench sidewall, revealing that both directional and lateral etch had been accomplished in a mixed anisotropic/ isotropic regime where as little as 5 monolayers of Al2O3 passivation were sufficient to preserve the shape of the scallop carved out in the first cycle. Another remarkable observation was the flat etch front of the trench bottom. This may be indicative for the robustness of ALD-grown Al2O3 passivation, and desired in atomically smooth precision etching. In continuous etching trench bottoms are often micro-trenched due to ion scattering from the trench sidewalls which results in faster etching of the side corners. We note that these results have been obtained with non-optimized processing and equipment, so we are confident that further optimization will give more improvements and insights. The process variables are manifold: gas concentration, pressure in the etch and passivation chambers, plasma power, substrate voltage biasing, room temperature processing, plasma source-to-substrate distance, passivation with other spatial ALD oxides like SiO2. We conclude by stating that this all-spatial concept, called ALD-enabled RIE, may find industrial use in cost-effective back-end- and front-end-of-line processing, especially in patterning of structures that require minimum interface, line edge and fin sidewall roughness (i.e., atomic-scale fidelity with selective removal of atoms and retention of sharp corners). Besides in CMOS scaling this concept may also become an important option for fast etching of silicon (or III/V) in die dicing, TSV and MEMS processing.

(Invited) Vacuum Ultraviolet Photochemical Atomic Layer Deposition of Alumina and Titania Films

Conventional atomic layer deposition (ALD) is a thermo-chemical process where co-reagents are sequentially pulsed in cycles onto a heated substrate. As an alternative to substrate heating, various forms of other “energy – enhanced” ALD processes are being investigated. Herein, the photochemical atomic layer deposition of Al2O3 and TiO2thin films at 60°C is reported using a shuttered vacuum ultraviolet (VUV) light source to excite molecular oxygen as a co-reagent with the metal precursors. The growth mechanisms using trimethyl aluminium (TMA) and titanium isopropoxide precursors, are investigated using in-situ quartz crystal microbalance (QCM) and post-deposition ellipsometric measurements. The figure below illustrates the mass gain / loss processes during a series of ALD cycles of TMA with O2/ UV excitation from a VUV lamp with peak emissions at 125 nm (9.9 eV) and 160 nm (7.7 eV). Further, the figure shows the growth per cycle (GPC) derived from the QCM data and also separately from single wavelength (633 nm) ellipsometry measurements made on silicon (100) samples exposed to the same growth conditions. Both data sets indicate that as the VUV illumination time is extended beyond 5 s to 10 s the overall mass gain per cycle saturates and becomes approximately constant, which is consistent with other self limiting ALD processes. The photochemical process is exploited to achieve area-selective film deposition. Selective area deposition was achieved by projecting the VUV light through a metalized magnesium fluoride photolithographic mask. The selectivity of deposition is a function of the UV exposure time and the mask – substrate separation. The photochemical atomic layer deposition of TiO2 thin films from titanium isopropoxide exhibits similar growth characteristics to the Al2O3 photochemical process, however subtle differences in the growth mechanisms are attributed to the different reactions of the excited oxygen species (O( 3P)) with the Al-CH3 and Ti-OCH(CH3)2 bonds. Figure 1

Room Temperature Photoluminescence Characterization of Interface Quality of SiN/SiO2/Si Prepared under Various Deposition Techniques and Conditions

Various types of stacked dielectric films are used in complimentary metal-oxide-semiconductor (CMOS) devices in gates, capacitors and device isolation structures. They are also used as insulating materials between conductors, as inter-metal dielectrics (IMD) and inter-layer dielectrics (ILD). The preparation technique, structure and properties of dielectrics strongly influence the quality of the dielectrics/Si interface. Nominally, similar dielectrics/Si structures, even prepared by the same technique, often show different electrical properties depending on subtle differences in process conditions (hidden variables). Monitoring of physical dimensions and chemical compositions of dielectrics often miss unexpected variations in electrical properties. Non-contact electrical property characterization techniques lack lateral resolution and often results in impairment of the dielectrics/Si interface due to strong external electric fields from the measurement process. Alternative dielectrics/Si interface characterization techniques, which can provide insights into electrical properties without external electric field, would be very useful as a complimentary process and material quality monitoring technique. In this study, we have investigated the effect of SiN film deposition techniques on the interface quality of the resulting SiN/SiO2/Si using spectroscopic room temperature photoluminescence (RTPL) under multiwavelength excitation. The sample preparation procedures are as follows. First, ultra thin (~2.2 nm thick), gate oxide quality, thermal oxide films were grown on 300 mm Si(100) wafers. Nominally 5.0 nm thick SiN films were grown on ~2.0 nm thick SiO2/Si wafers using conventional low pressure chemical vapor deposition (LPCVD) and atomic layer deposition (ALD) under various nitrogen source flow conditions. The film thickness was measured (after SiO2 film deposition) before and after SiN deposition using ellipsometry. Figure 1 (a) and (b) show RTPL spectra measured at the center of seven different SiN/SiO2/Si wafers under 650 nm excitation and showing maximum RTPL intensity under 650 nm and 827 nm excitation. Significantly large RTPL intensity variations were measured among the seven SiN/SiO2/Si wafers. The SiN film deposition conditions have modified the underlying SiO2/Si interface characteristics. Non-contact RTPL measurements are very sensitive to the thin film deposition techniques and conditions. It is effective as a complimentary dielectrics/Si process and interface characterization and monitoring technique. Figure 1

Few-layered titanium trisulfide (TiS3) field-effect transistors.

Titanium trisulfide (TiS3) is a promising layered semiconductor material. Several-mm-long TiS3 whiskers can be conveniently grown by the direct reaction of titanium and sulfur. In this study, we exfoliated these whiskers using the adhesive tape approach and fabricated few-layered TiS3 field-effect transistors (FETs). The TiS3 FETs showed an n-type electronic transport with room-temperature field-effect mobilities of 18-24 cm(2) V(-1) s(-1) and ON/OFF ratios up to 300. We demonstrate that TiS3 is compatible with the conventional atomic layer deposition (ALD) procedure for Al2O3. ALD of alumina on TiS3 FETs resulted in mobility increase up to 43 cm(2) V(-1) s(-1), ON/OFF ratios up to 7000, and much improved subthreshold swing characteristics. This study shows that TiS3 is a competitive electronic material in the family of two-dimensional (2D) transition metal chalcogenides and can be considered for emerging device applications.

Resistive switching phenomena in TiOx nanoparticle layers for memory applications

Electrical characteristics of a Co/ TiOx/Co resistive memory device, fabricated by two different methods, are reported. In addition to crystalline TiO2 layers fabricated via conventional atomic layer deposition (ALD), an alternative method has been examined, where TiOx nanoparticle layers were fabricated via sol-gel. The different devices have shown different hysteresis loops with a unique crossing point for the sol-gel devices. A simple qualitative model is introduced to describe the different current-voltage behaviours by suggesting only one active metal-oxide interface for the ALD devices and two active metal-oxide interfaces for the sol-gel devices. Furthermore, we show that the resistive switching behaviour could be easily tuned by proper interface engineering and that despite having a similar active material, different fabrication methods can lead to dissimilar resistive switching properties.

Cost‐effective Atomic Layer Deposition Synthesis of Pt Nanotube Arrays: Application for High Performance Supercapacitor

Due to the unique advantages of Pt, it plays an important role in fuel cells and microelectronics. Considering the fact that Pt is an expensive metal, a major challenging point nowadays is how to realize efficient utilization of Pt. In this paper, a cost-effective atomic layer deposition (ALD) process with a low N2 filling step is introduced for realizing well-defined Pt nanotube arrays in anodic alumina nano-porous templates. Compared to the conventional ALD growth of Pt, much fewer ALD cycles and a shorter precursor pulsing time are required, which originates from the low N2 filling step. To achieve similar Pt nanotubes, about half cycles and 10% Pt precursor pulsing time is needed using our ALD process. Meanwhile, the Pt nanotube array is explored as a current collector for supercapacitors based on core/shell Pt/MnO2 nanotubes. This nanotube-based electrode exhibits high gravimetric and areal specific capacitance (810 Fg(-1) and 75 mF cm(-2) at a scan rate of 5 mV s(-1) ) as well as an excellent rate capability (68% capacitance retention from 2 to 100 Ag(-1) ). Additionally, a negligible capacitance loss is observed after 8000 cycles of random charging-discharging from 2 to 100 Ag(-1) .

Atomic layer deposition of zirconium oxide on copper patterned silicon substrate

Atomic layer deposition (ALD) was performed on copper patterned silicon substrates using zirconium precursor and ethanol as both an oxygen source and reducing agent. Ethanol targeted copper oxide formed on the copper surface, reverting it back to metallic copper. Selective ALD (SALD) of metal oxides on silicon surfaces over copper surfaces has been demonstrated up to 2-3 nm, though the process seems to lose its selectivity afterwards. We strive to maintain selectivity to thicker films by stepping away from conventional ALD processes utilizing oxidants. From previous studies with HfO2 and TiO2 SALD, we speculate that the oxidation of copper to copper oxide spoils selectivity. In this present study, we carried out oxidant-free ALD by using ethanol as a co-reactant solely on the silicon portion of these substrates. This process will occur in-situ every 20-30 ALD cycles for ALD of ZrO2. As expected, reduced ALD growth rate was observed with ethanol compared to that of water or ozone, with a growth rate of about 0.04 nm/cycle on the silicon portion of the substrate.

Rapid vapor deposition SiO2 thin film deposited at a low temperature using tris(tert-pentoxy)silanol and trimethyl-aluminum

Rapid atomic layer deposition (ALD) SiO2 thin film was deposited at various temperatures (below 250 °C) using liquid tris(tert-pentoxy)silanol (TPS) as a SiO2 precursor and trimethyl-aluminum (TMA) as a catalyst. The rapid ALD SiO2 films showed saturated growth behavior similar to conventional ALD, exhibiting a high growth rate (35 nm/cycle) at 120 °C. As the growth temperature increased to 250 °C, the growth rate decreased slightly to 10.5 nm/cycle. This behavior may have originated from two competing growth mechanisms: the Al catalytic reaction vs. a cross-link reaction. There is no carbon and very little hydrogen in rapid ALD SiO2 films according to Rutherford backscattering spectroscopy (RBS) and elastic recoil detection (ERD). As the growth temperature increases, the Si–O stretch bond in the Fourier transmittance infrared spectroscopy (FTIR) results increases but the Si–OH stretch bond decreases simultaneously. The rapid ALD SiO2 films also show an amorphous phase and a smooth surface despite the very high growth rate. In addition, the SiO2 films exhibited excellent electrical properties despite the low growth temperatures, showing a low leakage current (2.5 × 10−11 A at 8 MV cm−1) and a high breakdown field (over 11 MV cm−1 at all growth temperatures). The obtained dielectric constant of the SiO2 films ranged from 6.14 to 4.15 as the growth temperature increased.

Atomic Layer Deposition: Overview and Applications

Atomic layer deposition(ALD) is a promising deposition method and has been studied and used in many different areas, such as displays, semiconductors, batteries, and solar cells. This method, which is based on a self-limiting growth mechanism, facilitates precise control of film thickness at an atomic level and enables deposition on large and three dimensionally complex surfaces. For instance, ALD technology is very useful for 3D and high aspect ratio structures such as dynamic random access memory(DRAM) and other non-volatile memories(NVMs). In addition, a variety of materials can be deposited using ALD, oxides, nitrides, sulfides, metals, and so on. In conventional ALD, the source and reactant are pulsed into the reaction chamber alternately, one at a time, separated by purging or evacuation periods. Thermal ALD and metal organic ALD are also used, but these have their own advantages and disadvantages. Furthermore, plasma-enhanced ALD has come into the spotlight because it has more freedom in processing conditions; it uses highly reactive radicals and ions and for a wider range of material properties than the conventional thermal ALD, which uses H2O and O3 as an oxygen reactant. However, the throughput is still a challenge for a current time divided ALD system. Therefore, a new concept of ALD, fast ALD or spatial ALD, which separate half-reactions spatially, has been extensively under development. In this paper, we reviewed these various kinds of ALD equipment, possible materials using ALD, and recent ALD research applications mainly focused on materials required in microelectronics.

Spontaneous formation of aluminum germanate on Ge(100) by atomic layer deposition with trimethylaluminum and microwave-generated atomic oxygen

The application of microwave-generated atomic oxygen as an oxidant is found to change the manner of atomic layer deposition (ALD) of an Al2O3 layer on a Ge substrate, leading to the spontaneous formation of aluminum germanate with a deposition rate higher than that of conventional ALD with water oxidant. Electrical characterization of the Al/aluminum germanate (11 nm)/p-Ge(100) structure indicates that both the bulk and the interface properties of the aluminum germanate are promising with small capacitance-voltage hysteresis of less than 20 mV and interface trap densities ranging from 2×1011 to 6×1011 cm−2 eV−1 in the upper half of the Ge band gap.

Rapid, conformal gas-phase formation of silica (SiO2) nanotubes from water condensates

An innovative atomic layer deposition (ALD) concept, with which nanostructures of water condensates with high aspect ratio at equilibrium in cylindrical nanopores can be transformed uniformly into silica (SiO2) at near room temperature and ambient pressure, has been demonstrated for the first time. As a challenging model system, we first prove the conversion of cylindrical water condensates in porous alumina membranes to silica nanotubes (NTs) by introducing SiCl4 as a metal reactant without involving any catalytic reaction. Surprisingly, the water NTs reproducibly transformed into silica NTs, where the wall thickness of the silica NTs deposited per cycle was found to be limited by the amount of condensed water, and it was on the orders of ten nanometers per cycle (i.e., over 50 times faster than that of conventional ALD). More remarkably, the reactions only took place for 10-20 minutes or less without vacuum-related equipment. The thickness of initially adsorbed water layers in cylindrical nanopores was indirectly estimated from the thickness of formed SiO2 layers. With systematic experimental designs, we tackle the classical Kelvin equation in the nanosized pores, and the role of van der Waals forces in the nanoscale wetting phenomena, which is a long-standing issue lacking experimental insight. Moreover, we show that the present strategy is likely generalized to other oxide systems such as TiO2. Our approach opens up a new avenue for ultra-simple preparation of porous oxides and allows for the room temperature formation of dielectric layers toward organic electronic and photovoltaic applications.

Atomic Layer Deposition of the Quaternary Chalcogenide Cu2ZnSnS4

Atomic layer deposition (ALD) is a layer-by-layer synthesis method capable of depositing conformal thin films with thickness and compositional control on subnanometer length scales. While many materials have been synthesized by ALD, the technologically important metal sulfides are underexplored, and homogeneous quaternary metal sulfides are absent from the literature. We report an ALD process to synthesize Cu2ZnSnS4 (CZTS), a potentially low cost semiconductor being explored for photovoltaic applications. Two strategies are reported: one in which a trilayer stack of binary metal sulfides (i.e., Cu2S, SnS2 and ZnS) is deposited and mixed by thermal annealing, as well as a supercycle strategy that is similar to the conventional ALD procedure for forming nanolaminates. Both routes rely on the facile solid state diffusion of chalcogenides for mixing. For this ALD route to the CZTS system, the challenges are nucleation, ion-exchange between the film and the volatile chemical precursors, and phase-stability of binary SnS2. The thin films were made with no sulfurization step. The X-ray diffraction and Raman spectra were consistent with the formation of CZTS. X-ray fluorescence measurements revealed that the films contained the expected amount of sulfur based on the target oxidation states. Photoelectrochemical measurements under simulated AM1.5 illumination using Eu3+ as an electron acceptor demonstrated that the films were photoactive and had an average internal quantum efficiency (IQE) of 12%.

Investigation on spatially separated atomic layer deposition by gas flow simulation and depositing Al2O3 films

Al2O3 thin films were deposited using tri-methyl aluminum and ozone by spatially separated atomic layer deposition (SALD). A large gap was kept between the reactor and substrates in an attempt to enhance the process gas flow. According to simulation data and deposition results, strong edge pumping for the dominantly lateral flow improved the gas isolation and deposition was very effective, with a resulting gap height of 5 mm. To compare this SALD process with conventional atomic layer deposition (ALD), the authors examined how the amount of source supplied, the deposition temperature, and the number of rotations affected the growth rate. The growth rate per rotation was saturated at ∼0.12 nm/rotation at a deposition temperature of 250 °C, which is comparable to the saturated growth rate of the same film using conventional ALD. The dielectric constant of the films was ∼8 and the film with a capacitance equivalent thickness of 3.2 nm had a leakage level of 9.8 × 10−8 A/cm2 (at −1 V). X-ray photoelectron spectroscopy peak area analysis indicates that the films deposited by SALD consist of Al2O3. This indicates that SALD is a viable option for several mass-production applications that require high throughput.

Effect of hydrogen plasma on growth of Ir thin film by plasma-enhanced hybrid atomic layer deposition

► Growth behaviors of Ir films using hybrid ALD were investigated. ► The 3.2 nm thick Ir film was grown by hybrid ALD under 50 deposition cycles. ► The grown Ir layer exhibited typical self-limited nature of ALD. ► High conformality of 0.88 is achieved on 32 nm wide nano trench. A 3.2-nm-thick Ir layer was grown by a cyclic chemical vapor deposition (CVD)-like hybrid atomic layer deposition (ALD) method on non-patterned Si substrate and a 32-nm-wide TaN-coated nano-trench. Ir metalorganic precursors were mixed with hydrogen reactant and co-fed into the chamber in the same cycle. The Ir metalorganic precursors were effectively decomposed in the gas phase, as in a CVD process, and on the surface of the substrate, as in ALD by hydrogen plasma, which was turned on at feeding time. The nucleation density, which caused a relatively long incubation time, was considerably increased as compared to the conventional ALD process by combining the CVD process with the ALD process. The number of deposition cycles required to obtain a 3.2-nm-thick Ir film on a non-patterned Si substrate was remarkably reduced from 200 to 50 deposition cycles, resulting in a low incubation time. Furthermore, the thickness of the Ir layer increased linearly as the number of deposition cycles increased, which shows the self-limiting nature of typical ALD. The thickness of the Ir layer saturated after 10 s as the precursor injection time was increased, which is further clear evidence of the self-limited nature of ALD. The conformal deposition of Ir was performed on a 32-nm-wide, 3-nm-thick TaN-coated nano trench. The thicknesses of the Ir layer on the top, bottom, left side, and right side were measured to be 3.5, 2.9, 3.3, and 3.4 nm, respectively, as measured by HR-TEM. The calculated conformality was 0.88, which was close to the ideal value without overhang, which often occurs in PVD processes. Our study suggest that this CVD-like hybrid ALD process can be applied to prepare barrier layers with a reduced number of deposition cycles without degradation of the quality of the film, compared to those prepared by the conventional ALD process.

Controlled Dopant Distribution and Higher Doping Efficiencies by Surface-Functionalized Atomic Layer Deposition

Reversible surface functionalization allows control of the precursor saturation coverage and growth per cycle in atomic layer deposition. Functionalizing the ZnO surface with alkyl alcohols, we achieve 2× higher doping efficiencies in Al-doped ZnO compared to conventional atomic layer deposition because of the more efficient dopant distribution in the ZnO host

Plasma-Assisted Atomic Layer Deposition: Basics, Opportunities, and Challenges

Plasma-assisted atomic layer deposition (ALD) is an energy-enhanced method for the synthesis of ultra-thin films with Å-level resolution in which a plasma is employed during one step of the cyclic deposition process. The use of plasma species as reactants allows for more freedom in processing conditions and for a wider range of material properties compared with the conventional thermally-driven ALD method. Due to the continuous miniaturization in the microelectronics industry and the increasing relevance of ultra-thin films in many other applications, the deposition method has rapidly gained popularity in recent years, as is apparent from the increased number of articles published on the topic and plasma-assisted ALD reactors installed. To address the main differences between plasma-assisted ALD and thermal ALD, some basic aspects related to processing plasmas are presented in this review article. The plasma species and their role in the surface chemistry are addressed and different equipment configurations, including radical-enhanced ALD, direct plasma ALD, and remote plasma ALD, are described. The benefits and challenges provided by the use of a plasma step are presented and it is shown that the use of a plasma leads to a wider choice in material properties, substrate temperature, choice of precursors, and processing conditions, but that the processing can also be compromised by reduced film conformality and plasma damage. Finally, several reported emerging applications of plasma-assisted ALD are reviewed. It is expected that the merits offered by plasma-assisted ALD will further increase the interest of equipment manufacturers for developing industrial-scale deposition configurations such that the method will find its use in several manufacturing applications.

Subnanometer Palladium Particles Synthesized by Atomic Layer Deposition

Monodispersed palladium nanoparticle catalysts were synthesized by atomic layer deposition (ALD) using alternating exposures of Pd hexafluoroacetylacetonate (Pd(hfac)2) and formalin on an alumina support. The size of the ALD Pd particles could be tuned by adjusting the preparation conditions. Conventional ALD conditions produced Pd particles with an average size of 1.4 nm. Removal of surface hydroxyls from the alumina support by a chemical treatment using trimethyl aluminum (TMA) before performing Pd ALD led to nanoparticles larger than 2 nm. Ultrasmall (subnanometer) Pd particles were synthesized using low-temperature metal precursor exposures, followed by applying protective ALD alumina overcoats. The ALD Pd particles were characterized by transmission electron microscopy, extended X-ray absorption fine structure, and diffuse reflectance infrared Fourier transform spectroscopy techniques. The Pd loadings were measured by X-ray fluorescence. The catalytic performance of ALD Pd particles of different sizes was compared in the methanol decomposition reaction. The specific activity (normalized by Pd loading) of the ultrasmall Pd particles was higher than those of the larger particles. Considering the metal dispersion factor, the turnover frequency (TOF) of the ultrasmall Pd particles is comparable to that of the medium-sized (1.4 nm, on average) Pd particles synthesized under standard ALD conditions. The large Pd particles (>2 nm) are a factor of 2 less active than the smaller Pd particles.

Characteristics and applications of plasma enhanced-atomic layer deposition

Atomic layer deposition (ALD) is expected to play an important role in future device fabrication due to various benefits, such as atomic level thickness control and excellent conformality. Plasma enhanced ALD (PE-ALD) allows deposition at significantly lower temperatures with better film properties compared to that of conventional thermal ALD. In addition, since ALD is a surface-sensitive deposition technique, surface modification through plasma exposure can be used to alter nucleation and adhesion. In this paper, characteristics of PE-ALD for various applications in semiconductor fabrication are presented through comparison to thermal ALD. The results indicate that the PE-ALD processes are versatile methods to enable nanoscale manufacturing in emerging applications.