Plasma-assisted Atomic Layer Deposition Process Research Articles

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.

Growth chemistry of cobalt nitride by plasma enhanced atomic layer deposition

State-of-the-art atomic layer deposition (ALD) and photoemission characterisation are applied to grow and characterise cobalt nitride, a material that has applications in renewable energy and semiconductor technologies. The growth process is characterised using an in situ cycle-by-cycle methodology to identify the main factors which underpin optimal material growth. The role of co-reactant dosing and substrate temperature is analysed in detail to demonstrate the impact these parameters have on the overall composition of the film. The in situ approach, combined with high-energy synchrotron-based photoemission studies of the resulting films, enables understanding of the bulk chemical properties without need for physical removal of material by sputtering. The results provide an insight into optimising plasma assisted ALD processes for deposition of cobalt nitride, and strategies for minimizing carbon incorporation into the film from the precursor ligands.

Atomic layer deposition of cobalt phosphate from cobaltocene, trimethylphosphate, and O2 plasma

Electrodeposited cobalt phosphate has been reported in the literature as a robust alternative to noble metal-based electrocatalysts for the O2 evolution reaction. In parallel, atomic layer deposition (ALD) has been acknowledged as a key technology for the preparation of thin films for energy applications. With the present work, the authors have addressed the preparation of cobalt phosphate thin films by a plasma-assisted ALD process. The process developed consists of cobaltocene (step A) and trimethyl phosphate (step C) exposures alternated by O2 plasma (steps B and D) in an ABCD fashion. The process shows a linear growth with a growth per cycle of 1.12 ± 0.05 Å at 300 °C and no nucleation delay. The ALD saturation behavior has been demonstrated for each dosing step, and the process shows minimal inhomogeneity on 100 mm diameter wafers in terms of film thickness (<1.0%) and refractive index (<0.5%). The chemical characterization of the layers deposited shows that the composition is close to the stoichiometric (Co3.1P2O8.3 for a deposition temperature of 200 °C) and the oxidation states of cobalt and phosphorus and agrees with those reported in the literature. The cobalt-to-phosphorous ratio has been found to correlate with the values of absorption coefficient (k) and refractive index (n) of the layers. Furthermore, the authors have gained insights into the surface reactions occurring during each ALD step by quadrupole mass spectrometry investigation. The results suggest that cobaltocene undergoes associative adsorption and the cyclopentadienyl ligands are removed during subsequent O2 plasma exposure. Moreover, the authors have indirectly identified cyclopentadienone as an intermediate of the oxidation and removal of cyclopentadienyl ligand. On the other hand, the trimethyl phosphate dosing is characterized by chemisorption via the elimination of methanol.

Investigation of interface characteristics of Al2O3/Si under various O2 plasma exposure times during the deposition of Al2O3 by PA-ALD

Plasma-assisted atomic layer deposition (PA-ALD) is more suitable than thermal atomic layer deposition (ALD) for mass production because of its faster growth rate. However, controlling surface damage caused by plasma during the PA-ALD process is a key issue. In this study, the passivation characteristics of Al2O3 layers deposited by PA-ALD were investigated with various O2 plasma exposure times. The growth per cycle (GPC) during Al2O3 deposition was saturated at approximately 1.4 Å/cycle after an O2 plasma exposure time of 1.5 s, and a refractive index of Al2O3 in the range of 1.65–1.67 was obtained. As the O2 plasma exposure time increased in the Al2O3 deposition process, the passivation properties tended to deteriorate, and as the radio frequency (RF) power increased, the passivation uniformity and the thermal stability of the Al2O3 layer deteriorated. To study the Al2O3/Si interface characteristics, the capacitance-voltage (C-V) and the conductance-voltage (G-V) were measured using a mercury probe, and the fixed charge density (Qf) and the interface trap density (Dit) were then extracted. The Qf of the Al2O3 layer deposited on a Si wafer by PA-ALD was almost unaffected, but the Dit increased with O2 plasma exposure time. In conclusion, as the O2 plasma exposure time increased during Al2O3 layer deposition by PA-ALD, the Al2O3/Si interface characteristics deteriorated because of plasma surface damage.

Graphene as plasma-compatible blocking layer material for area-selective atomic layer deposition: A feasibility study for III-nitrides

Plasma-assisted atomic layer deposition (PA-ALD) is a promising method for low-temperature growth of III-nitride materials. However, selective film deposition using PA-ALD is challenging mainly due to the plasma-incompatibility of conventional deactivation/blocking layers including self-assembled monolayers and polymers. The main motivation behind this work is to explore alternative plasma-resistant blocking layer materials. Toward this goal, single/multilayered graphene (SLG/MLG) sheets were investigated as potential growth-blocking layers for III-nitride grown via PA-ALD. Prior to PA-ALD growth experiments, partially graphene-covered Si(100) samples were exposed to N2/H2 and N2-only plasma cycles to evaluate the plasma resistance of SLG and MLG. While SLG degraded fairly quickly showing signs of completely etched areas and rough surface morphology, MLG surface displayed certain degree of plasma-resistance. Based on this result, III-nitride PA-ALD experiments were carried out on MLG-patterned Si(100) samples. Crystalline III-nitride film deposition was observed on both Si(100) and graphene surfaces, confirming the rather ineffective nucleation blocking property of graphene surface against PA-ALD process. However, as graphene layers feature relatively weak van der Waals bonds at the substrate/graphene interface as well as between the multilayer graphene interfaces, conventional lift-off process was sufficient to remove the deposited excessive nitride films. InN and AlN-coated samples were ultrasonicated, and blocked/unblocked surfaces were characterized using scanning electron microscopy, x-ray photoelectron spectroscopy, and spectroscopic ellipsometer. While ∼50 nm thick films were measured in the open Si(100) areas, graphene-coated sample portions exhibited limited material growth in the range of 5–15 nm. Although not completely, the MLG surface has considerably blocked the PA-ALD growth process resulting in a usable thickness difference, enabling growth selectivity with postgrowth etch process. An Ar-based physical dry etching recipe was utilized to completely etch the unwanted nitride films from graphene coated area, while about 30 and 40 nm thick InN and AlN films remained on the nonblocked parts of the samples, respectively. As a result, selective deposition of PA-ALD grown AlN and InN has been achieved via graphene-assisted lift-off technique along with subsequent dry-etch process, achieving a maximum growth selectivity of ∼40 nm. With further process recipe optimization and integrating with a suitable patterning technique, the demonstrated graphene-assisted lift-off technique might offer an alternative feasible pathway toward area-selective deposition of III-nitrides and other plasma-necessitating materials.

Plasma-Assisted Atomic Layer Deposition of Thin Film Li2CO3 for All-Solid-State Li-Ion Batteries

All-solid-state Li-ion batteries offer the advantage of a higher power density, a wider temperature window and an improved safety with respect to batteries containing liquid electrolytes. However, the lack of solid electrolytes with sufficient ion conductivity and wide electrochemical window is currently inhibiting the implementation of solid state batteries. Although the limited ionic conductivity can be overcome by employing very thin solid-state electrolyte films which shorten the ionic diffusion distance, the capacity will still be insufficient. This makes the use of non-planar devices of particular high interest as the application of 3D-(nano)structured materials will increase the total effective battery area. Moreover, the area enhancement will result in an ever higher energy density, faster (dis)charging, better stability, etc. Conventional deposition techniques, such as PVD and PE-CVD, do not generally result in conformal thin layers on 3D structures. Therefore, novel deposition techniques leading to enhanced control in thin film properties and conformality are being introduced in the field of battery research. Atomic layer deposition (ALD), which is based on sequential and self-limiting reactions, has emerged as a powerful tool since it offers exceptional conformality on high-aspect ratio structures, thickness control at sub-nm level, and tunable film properties. In this work, thin film lithium carbonate (Li₂CO₃) is being deposited by ALD. Li₂CO₃ is widely used as a building block for the fabrication of Li-ion battery active materials, such as lithium cobalt oxide (LiCoO₂), which is used for most lithium-ion battery cathodes [1]. It has also been reported that Li₂CO₃, without combining it with other materials, has a pure ion conductive behavior and a good electrochemical stability [2]. The high electrochemical stability of Li₂CO₃ makes it useful as an artificial solid-electrolyte interface (SEI) or buffer layer [3]. The SEI and buffer layer need to be conformal and very thin, for which the use of ALD would be beneficial. In addition, ultra-thin Li₂CO₃ can also improve the stability of known solid electrolyte layers, such as lithium lanthanum titanate or lithium phosphate. More specifically, we focus on the optimization of the plasma-assisted Li₂CO₃ ALD process by (in situ) growth studies and we investigate the influence of the process parameters on the electrical and electrochemical properties. Using plasmas in combination with ALD opens up the possibility to tune the properties of the layer by varying the plasma exposure time and also to deposit at lower temperatures. Although the plasma-assisted ALD process for Li₂CO₃ has been reported in literature before [4], to our knowledge, the optimization of the process was not reported, and also an electrochemical analysis of Li₂CO₃ layers fabricated with this method is still lacking. The ALD process employed, consists of a combination of LiOtBu as lithium precursor and an O₂ plasma as oxidant source, alternated by Ar purges. A growth rate of ± 0.8 Å/cycle was obtained at a process table temperature of 150 °C on TiN substrates in the linear growth regime (figure). TiN substrates are used as an electrical contact for electrical and electrochemical testing of the layers. The film stoichiometry was investigated with Elastic Recoil Detection (ERD) and X-ray Photoelectron Spectroscopy (XPS). The results are in good agreement with the stoichiometry and density of Li₂CO₃. Excellent electronic insulation properties were confirmed by current-voltage and DC-polarization measurements. This observation is in agreement with impedance measurements at open circuit potential (figure), which showed no apparent electronic conductivity of the film. The dependence of ALD process parameters such as temperature and O₂ plasma exposure time on the electrochemical properties is investigated. Moreover, half cells employing Li₂CO₃ with TiO₂, LiMn₂O₄ and Si electrodes were studied for cyclability and rate performance. These and other results will be presented in this contribution. Acknowledgements: This project is financially supported by the Dutch program “A green Deal in Energy Materials” ADEM Innovation Lab. M. E. Donders et al., Journal of The Electrochemical Society, 160 (5), 2013J. Mizusaki et al., Solid State Ionics, 53-56, 1992P. Verma et al., Electrochimica Acta, 55, 2010A. C. Kozen et al., The Journal of Physical Chemistry C, 118, 2014 Figure caption: On the left: growth curve for the plasma-assited ALD process of Li₂CO₃ measured by in-situ spectroscopic ellipsometry on TiN at 150 °C and the corresponding process steps (insert). On the right: Electrochemical impedance spectroscopy measured at room temperature showing the presence of ionic conductivity. Figure 1

Redeposition in plasma-assisted atomic layer deposition: Silicon nitride film quality ruled by the gas residence time

The requirements on the material properties and growth control of silicon nitride (SiNx) spacer films in transistors are becoming ever more stringent as scaling of transistor structures continues. One method to deposit high-quality films with excellent control is atomic layer deposition (ALD). However, depositing SiNx by ALD has turned out to be very challenging. In this work, it is shown that the plasma gas residence time τ is a key parameter for the deposition of SiNx by plasma-assisted ALD and that this parameter can be linked to a so-called “redeposition effect”. This previously ignored effect, which takes place during the plasma step, is the dissociation of reaction products in the plasma and the subsequent redeposition of reaction-product fragments on the surface. For SiNx ALD using SiH2(NHtBu)2 as precursor and N2 plasma as reactant, the gas residence time τ was found to determine both SiNx film quality and the resulting growth per cycle. It is shown that redeposition can be minimized by using a short residence time resulting in high-quality films with a high wet-etch resistance (i.e., a wet-etch rate of 0.5 nm/min in buffered HF solution). Due to the fundamental nature of the redeposition effect, it is expected to play a role in many more plasma-assisted ALD processes.

Use of B2O3 films grown by plasma-assisted atomic layer deposition for shallow boron doping in silicon

Plasma-assisted atomic layer deposition (PALD) was carried for growing thin boron oxide films onto silicon aiming at the formation of dopant sources for shallow boron doping of silicon by rapid thermal annealing (RTA). A remote capacitively coupled plasma source powered by GaN microwave oscillators was used for generating oxygen plasma in the PALD process with tris(dimethylamido)borane as boron containing precursor. ALD type growth was obtained; growth per cycle was highest with 0.13 nm at room temperature and decreased with higher temperature. The as-deposited films were highly unstable in ambient air and could be protected by capping with in-situ PALD grown antimony oxide films. After 16 weeks of storage in air, degradation of the film stack was observed in an electron microscope. The instability of the boron oxide, caused by moisture uptake, suggests the application of this film for testing moisture barrier properties of capping materials particularly for those grown by ALD. Boron doping of silicon was demonstrated using the uncapped PALD B2O3 films for RTA processes without exposing them to air. The boron concentration in the silicon could be varied depending on the source layer thickness for very thin films, which favors the application of ALD for semiconductor doping processes.

In situ diagnostics for studying gas-surface reactions during thermal and plasma-assisted atomic layer deposition

The authors have constructed a novel experimental setup to investigate the surface reaction mechanisms during thermal and plasma-assisted atomic layer deposition (ALD) of thin films by employing in situ surface and gas-phase diagnostic techniques. Specifically, we have used a real-time attenuated total reflection Fourier transform infrared spectroscopy setup to detect the surface adsorbates as well as gas-phase species in each half-reaction cycle during ALD. In this article, we describe the reactor construction and the in situ diagnostics with sufficient details that it can be a useful reference for researchers in the field. To illustrate the use of these in situ diagnostics, we present our results for the ALD of TiO2 from titanium tetraisopropoxide in conjunction with H2O, O3, and a O2 plasma. For O2 plasma-assisted ALD, our measurements are complimented with optical emission spectroscopy to identify the radicals and excited molecular species in the plasma, some of which are produced by the dissociation of the surface reaction products. The authors show that in O3- and O2 plasma-assisted ALD processes, the surface reaction products play an important role in determining the surface chemistry.

Optical emission spectroscopy as a tool for studying, optimizing, and monitoring plasma-assisted atomic layer deposition processes

In this note it is demonstrated that optical emission spectroscopy (OES) is an easy-to-implement and valuable tool to study, optimize, and monitor thin film growth by plasma-assisted atomic layer deposition (ALD). The species in the plasma can be identified through the analysis of the light emitted by the plasma. OES provides therefore information on the reactant species delivered to the surface by the plasma but it also yields unique insight into the surface reaction products and, as a consequence, on the reaction mechanisms of the deposition process. Time-resolved measurements reveal information about the amount of precursor dosing and length of plasma exposure needed to saturate the self-limiting half reactions, which is useful for the optimization of the ALD process. Furthermore, time-resolved OES can also be used as an easy-to-implement process monitoring tool for plasma-assisted ALD processes on production equipment; for example, to monitor reactor wall conditions or to detect process faults in real time.

Reaction mechanisms during plasma-assisted atomic layer deposition of metal oxides: A case study for Al2O3

Plasma-assisted atomic layer deposition (ALD) of metal oxide films is increasingly gaining interest, however, the underlying reaction mechanisms have rarely been addressed. In this work, a case study is presented for the plasma-assisted ALD process of Al2O3 based on Al(CH3)3 dosing and O2 plasma exposure. A complementary set of time-resolved in situ diagnostics was employed, including spectroscopic ellipsometry, quartz crystal microbalance, mass spectrometry, and optical emission spectroscopy. The saturation of the Al(CH3)3 adsorption reactions was investigated, as well as the reaction products created during both the precursor dosing and the plasma exposure step. The generality of the observations was cross-checked on a second commercial ALD reactor. The main observations are as follows: (i) during the precursor dosing, the Al(CH3)3 predominantly binds bifunctionally to the surface at 70°C through a reaction in which H is abstracted from the surface and CH4 is released into the gas phase; (ii) during the plasma exposure, O radicals in the plasma are consumed at the surface by combustionlike reactions with the surface −CH3 ligands, producing mainly H2O, CO2, and CO; (iii) small gas phase densities of CH4 and higher hydrocarbons (C2Hx) are also present during the O2 plasma exposure step indicating complementary surface reactions including a secondary thermal ALD-like reaction by the H2O produced at the surface; (iv) the plasma and its optical emission are strongly affected by the surface reaction products released in the plasma. In the latter respect, optical emission spectroscopy proved to be a valuable tool to study the surface reaction products during the plasma exposure as well as the saturation of the surface reactions. The implications of the experimental observations are addressed and it is discussed that the reaction mechanisms are generic for plasma-assisted ALD processes based on metal organic precursors and O2 plasma as oxidant source.

GeSbTe deposition for the PRAM application

GeSbTe (GST) chalcogenide thin films for the phase-change random access memory (PRAM) were deposited by an atomic layer deposition (ALD) process. New precursors for GST thin films made with an ALD process were synthesized. Among the synthesized precursors, Ge(N(CH 3) 2) 4, Sb(N(CH 3) 2) 4, and Te( i-Pr) 2 ( i-Pr = iso-propyl) were selected. Using the above precursors, GST thin films were deposited using an H 2 plasma-assisted ALD process. Film resistivity abruptly changed after an N 2 annealing process above a temperature of 350 °C. Cross-sectional scanning electron microscope (SEM) photographs of the GST films on the patterned substrate with aspect ratio of 7 shows that the step coverage is about 90%.

Low-Temperature Deposition of TiN by Plasma-Assisted Atomic Layer Deposition

Titanium nitride () films were deposited by a plasma-assisted atomic layer deposition (PA-ALD) process, based on precursor dosing and remote plasma exposure, at temperatures ranging from 100 to . The plasma, the PA-ALD process, and the resulting material properties were extensively investigated. The plasma was studied by optical emission spectroscopy and Langmuir probe, revealing an ion density of and an electron temperature of just above the substrate. Under floating conditions there is thus a considerable ion flux towards the substrate per ALD cycle with a typical ion energy of . film growth was studied by in situ spectroscopic ellipsometry, revealing self-limiting surface reactions for the complete temperature range. At the growth rate of was found to be significantly lower than the growth rate of at . The stoichiometry of the films varied with the plasma exposure time, while the Cl content was mostly affected by the deposition temperature ( at to at ). Resistivities as low as were obtained at a temperature of , while at a fair resistivity of was reached. These results show that PA-ALD with and plasma is well suited for low-temperature deposition of high-quality films.