Atomic layer deposition(ALD), a chemical vapor deposition technique based on sequential self-terminating gas–solid reactions, has for about four decades been applied for manufacturing conformal inorganic material layers with thickness down to the nanometer range. Despite the numerous successful applications of material growth by ALD, many physicochemical processes that control ALD growth are not yet sufficiently understood. To increase understanding of ALD processes, overviews are needed not only of the existing ALD processes and their applications, but also of the knowledge of the surface chemistry of specific ALD processes. This work aims to start the overviews on specific ALD processes by reviewing the experimental information available on the surface chemistry of the trimethylaluminum/water process. This process is generally known as a rather ideal ALD process, and plenty of information is available on its surface chemistry. This in-depth summary of the surface chemistry of one representative ALD process aims also to provide a view on the current status of understanding the surface chemistry of ALD, in general. The review starts by describing the basic characteristics of ALD, discussing the history of ALD—including the question who made the first ALD experiments—and giving an overview of the two-reactant ALD processes investigated to date. Second, the basic concepts related to the surface chemistry of ALD are described from a generic viewpoint applicable to all ALD processes based on compound reactants. This description includes physicochemical requirements for self-terminating reactions,reaction kinetics, typical chemisorption mechanisms, factors causing saturation, reasons for growth of less than a monolayer per cycle, effect of the temperature and number of cycles on the growth per cycle (GPC), and the growth mode. A comparison is made of three models available for estimating the sterically allowed value of GPC in ALD. Third, the experimental information on the surface chemistry in the trimethylaluminum/water ALD process are reviewed using the concepts developed in the second part of this review. The results are reviewed critically, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials. Although the surface chemistry of the trimethylaluminum/water ALD process is rather well understood, systematic investigations of the reaction kinetics and the growth mode on different substrates are still missing. The last part of the review is devoted to discussing issues which may hamper surface chemistry investigations of ALD, such as problematic historical assumptions, nonstandard terminology, and the effect of experimental conditions on the surface chemistry of ALD. I hope that this review can help the newcomer get acquainted with the exciting and challenging field of surface chemistry of ALD and can serve as a useful guide for the specialist towards the fifth decade of ALD research.

Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process

Atomic layer deposition (ALD) is gaining attention as a thin film deposition method, uniquely suitable for depositing uniform and conformal films on complex three-dimensional topographies. The deposition of a film of a given material by ALD relies on the successive, separated, and self-terminating gas–solid reactions of typically two gaseous reactants. Hundreds of ALD chemistries have been found for depositing a variety of materials during the past decades, mostly for inorganic materials but lately also for organic and inorganic–organic hybrid compounds. One factor that often dictates the properties of ALD films in actual applications is the crystallinity of the grown film: Is the material amorphous or, if it is crystalline, which phase(s) is (are) present. In this thematic review, we first describe the basics of ALD, summarize the two-reactant ALD processes to grow inorganic materials developed to-date, updating the information of an earlier review on ALD [R. L. Puurunen, J. Appl. Phys. 97, 121301 (2005)], and give an overview of the status of processing ternary compounds by ALD. We then proceed to analyze the published experimental data for information on the crystallinity and phase of inorganic materials deposited by ALD from different reactants at different temperatures. The data are collected for films in their as-deposited state and tabulated for easy reference. Case studies are presented to illustrate the effect of different process parameters on crystallinity for representative materials: aluminium oxide, zirconium oxide, zinc oxide, titanium nitride, zinc zulfide, and ruthenium. Finally, we discuss the general trends in the development of film crystallinity as function of ALD process parameters. The authors hope that this review will help newcomers to ALD to familiarize themselves with the complex world of crystalline ALD films and, at the same time, serve for the expert as a handbook-type reference source on ALD processes and film crystallinity.

Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends

Silicon nanowire field-effect transistor-based biosensors for biomedical diagnosis and cellular recording investigation

Thin films are formed by formed by atomic layer deposition, whereby the composition of the film can be varied from monolayer to monolayer during cycles (301) or (450, 455, 460, 470) including alternating pulses of self-limiting chemistries. In the illustrated embodiments, varying amounts of impurity sources (306 or 460) are introduced during the cyclical process. A graded gate dielectric (72) is thereby provided, even for extremely thin layers. The gate dielectric (72) as thin as 2 nm can be varied from pure silicon oxide to oxynitride to silicon nitride. Similarly, the gate dielectric (72) can be varied from aluminum oxide to mixtures of aluminum oxide and a higher dielectric material (e.g., ZrO2) to pure high k material and back to aluminum oxide. In another embodiment, metal nitride (432) (e.g., WN) is first formed as a barrier for lining dual damascene trenches and vias. During the alternating deposition process, copper can be introduced, e.g., in separate pulses, and the copper source pulses (460) can gradually increase in frequency, forming a graded transition region (434), until pure copper (436) is formed at the upper surface. Advantageously, graded compositions in these and a variety of other contexts help to avoid such problems as etch rate control, electromigration and non-ohmic electrical contact that can occur at sharp material interfaces.

Graded thin films

Publisher Summary This chapter deals with atomic layer deposition (ALD), which is a chemical gas phase thin film deposition method based on alternate saturative surface reactions. As distinct from the other chemical-vapor-deposition techniques, in ALD the source vapors are pulsed into the reactor alternately—one at a time—separated by purging or evacuation periods. Each precursor exposure step saturates the surface with a monomolecular layer of that precursor. This results in a unique self-limiting film-growth mechanism with a number of advantageous features—such as excellent conformality and uniformity and simple and accurate film-thickness control. As the current interest in ALD is largely centered on non-epitaxial films, the chapter focuses on these materials and the related chemistry. ALD is a unique process based on alternate surface reactions, which are accomplished by dosing the gaseous precursors on the substrate alternately. Under ideal conditions, these reactions are saturative, ensuring many advantageous features, such as excellent conformity, large-area uniformity, accurate and simple film-thickness control, repeatability, and large-batch processing capability.

Atomic layer deposition

Extremely thin (equivalent oxide thickness, Teq=1.2 nm) silicon-nitride high-k (er=7.2) gate dielectrics have been formed at low temperatures (⩽550 °C) by an atomic-layer-deposition (ALD) technique with subsequent NH3 annealing at 550 °C. A remarkable reduction in leakage current, especially in the low dielectric voltage region, which will be the operating voltage for future technologies, has made it a highly potential gate dielectric for future ultralarge-scale integrated devices. Suppressed soft breakdown events are observed in ramped voltage stressing. This suppression is thought to be due to a strengthened structure of Si–N bonds and the smoothness and uniformity at the poly-Si/ALD-silicon-nitride interface.

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NH3-annealed atomic-layer-deposited silicon nitride as a high-k gate dielectric with high reliability

Thin (equivalent oxide thickness Teq of 2.4 nm) silicon nitride layers were deposited on Si substrates by an atomic-layer-deposition (ALD) technique at low temperatures (<550 °C). The interface state density at the ALD silicon nitride/Si-substrate interface was almost the same as that of the gate SiO2. No hysteresis was observed in the gate capacitance–gate voltage characteristics. The gate leakage current was the level comparable with that through SiO2 of the same Teq. The conduction mechanism of the leakage current was investigated and was found to be the direct tunneling. The ALD technique allows us to fabricate an extremely thin, very uniform silicon nitride layer with atomic-scale control for the near-future gate dielectrics.

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Low-temperature formation of silicon nitride gate dielectrics by atomic-layer deposition

ZrO2 thin films for gate dielectrics have been formed at low temperatures (200 °C) by an atomic-layer deposition (ALD) technique using Zr(t-OC4H9)4 and H2O source gases An ultrathin (physical thickness Tphy of ∼05 nm) Si nitride layer was deposited on a Si substrate by ALD before the deposition of ZrO2 Transmission electron microscopy showed that the Si nitride barrier layer successfully suppressed the formation of a SiO2 interfacial layer Because of the extremely uniform thickness control capability in the ultrathin region and the low thermal budget of the ALD process, the ALD process for the ZrO2/Si nitride stack structure is a promising candidate for fabricating the ultrathin gate dielectrics for sub-01-μm complementary metal–oxide–semiconductor transistors

Atomic-layer deposition of ZrO2 with a Si nitride barrier layer

An extremely thin (∼0.4 nm) silicon-nitride layer has been deposited on thermally grown SiO2 by an atomic-layer-deposition (ALD) technique. The boron penetration through the stacked gate dielectrics has dramatically been suppressed, and the reliability has been significantly improved, as confirmed by capacitance–voltage, gate-current–gate-voltage, and time-dependent dielectricbreakdown characteristics. The ALD technique allows us to fabricate an extremely thin, very uniform silicon-nitride layer with atomic-scale control.

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Atomic-layer-deposited silicon-nitride/SiO2 stacked gate dielectrics for highly reliable p-metal–oxide–semiconductor field-effect transistors

We deposited ZrO2 thin films by atomic-layer deposition (ALD) using zirconium tertiary–butoxide [Zr(t-OC4H9)4, (ZTB)] and H2O source gases on Si substrates at low temperatures. We grew ZrO2 films layer by layer in a temperature range of 175–250 °C to minimize surface roughness. The deposited ZrO2 film thickness had self-limiting properties with the exposure time of ZTB and vapor pressures of ZTB and H2O. The deposition rate per cycle was independent of the vapor pressure of ZTB from 0.01 kPa to 0.04 kPa. Transmission electron microscopy revealed that the formation of an SiOx interfacial layer could be suppressed by using an ALD ZrO2/ALD Si-nitride (∼0.5 nm) stack structure. We found the fixed charge, interface trap density, and leakage current density in the ALD ZrO2/ALD Si-nitride stack dielectrics to be less than those in ALD ZrO2 dielectrics. In spite of the same equivalent oxide thickness of 1.6 nm, the relative dielectric constant er (11.5) of the ALD ZrO2/ALD Si-nitride stack capacitor was higher th...

https://ir.lib.hiroshima-u.ac.jp/files/public/1/18594/20141016132519731160/JApplPhys_95_536.pdf

Anri Nakajima

Papers

NH3-annealed atomic-layer-deposited silicon nitride as a high-k gate dielectric with high reliability

Low-temperature formation of silicon nitride gate dielectrics by atomic-layer deposition

Atomic-layer deposition of ZrO2 with a Si nitride barrier layer

Atomic-layer-deposited silicon-nitride/SiO2 stacked gate dielectrics for highly reliable p-metal–oxide–semiconductor field-effect transistors

Growth and electrical properties of atomic-layer deposited ZrO2 /Si-nitride stack gate dielectrics