Learn about the Preparation Technology of Atomic Layer Deposition (ALD) Thin Films

Introduction to common film growth techniques

(1)CVD thin film technology
CVD technology is a process of film growth through chemical reaction on the surface of the substrate in a vacuum environment, and the short process time and the high density of the prepared film make CVD technology more and more used in the preparation of inorganic barrier layers in the film encapsulation process.

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0040-09094 CHAMBER 200mm
(2)PECVD thin film technology
Plasma Enhanced Chemical Vapor Deposition (PECVD) uses plasma to compensate for the low reactivity caused by reaction precursors or process temperatures.

(3)Atomic layer deposition technology
Similar to CVD technology, atomic layer deposition (ALD) is also a thin film preparation technology based on the chemical reaction of the substrate surface, and in addition to similar film growth conditions, some precursor materials are also commonly used between the two processes.
The difference is that CVD technology maintains the coexistence of the two precursor materials in a vacuum reaction chamber, and chemisorption occurs on the surface of the substrate to form a thin film. The surface chemical reaction established by ALD technology is that each precursor material occurs independently and alternately, and each precursor material has self-limiting reaction characteristics, and the corresponding self-limiting surface half-reaction grows the substance layer by layer on the substrate surface in the form of a single atomic layer, and the continuous self-limiting surface reaction meets the needs of single atomic layer control and conformal deposition in the process of thin film growth.
The surface reaction process of ALD technology is continuous and self-limiting, as shown in the figure below.

Typical ALD processes often use binary reaction sequences for thin film growth, and the two precursors complete their respective half-reactions sequentially on the substrate surface to achieve a single-layer deposition process of a binary compound film. The active site on the substrate surface is the basis for the growth of ALD films, so the substrate often introduces the active site or increases the active site density through some surface pretreatment before the film growth process begins.For example, the amount of hydroxyl groups (-OH) on the substrate surface can be greatly increased by means of oxygen plasma (O2 plasma) or ultraviolet radiation, as shown in Figure (a).
The binary reaction sequence involved in the ALD process is divided into four steps, as shown in Figure (b).
First, precursor A is introduced into the reaction chamber and the active site on the substrate surface undergoes a self-confined surface reaction to adsorb a single atomic layer and produce the corresponding by-products, and then, the entire cavity and pipeline are purged with the inert gas Ar to empty the residual precursor A and reaction by-products. Next, precursor B enters the reaction chamber and undergoes a self-confined surface reaction with the active site provided by precursor A, adsorbs another layer of monoatomic layers with the production of by-products, and finally, Ar again acts as a cleaning gas to expel the residual precursor B and the corresponding by-products, and the reexposed active site is able to react with precursor A. At this point, a cycle ends and a layer of product completes growth. Repeat the above cycle N times to customize the ALD process parameters according to the usage needs. Because the number of active sites on the substrate surface is limited, the surface material deposited by the semi-reaction is also limited, corresponding to the fact that each surface half-reaction has its own saturation state. If each of the two independent surface half-reactions is self-limiting, then the two reactions can be carried out continuously, alternately, to obtain a layer-by-layer deposition process of thin films that is controllable at the atomic level. The ALD process is controlled by surface chemical reactions, which do not come into contact in the gas phase because the surface reactions are sequential and alternate, and the separation of the two inhibits the possible occurrence of CVD-like gas phase reactions, avoiding the appearance of particle products on the surface of the film. Although the precursor material has self-limiting reaction characteristics, the reaction of the surface active sites also has a sequential order due to the different gas flow rates of the precursor. Precursors may be physically adsorbed in the form of van der Waals forces in the region where the surface reaction has been completed and subsequently desorbed from that region, continuing to react with other unreacted surface regions and produce conformal deposition. Because ALD avoids the randomness of precursor fluxes, the self-limiting nature of surface reactions also results in non-statistical deposition, which causes each surface half-reaction to be driven to near saturation. As a result, the ALD-grown film is very smooth and conformal to the original substrate. Since there are almost no surface-active sites left during film growth, the film tends to be continuous and pinhole-free. This property is very important for the preparation of excellent dielectric films and water vapor barrier films.

Application of ALD thin film technology

At present, ALD technology has great application prospects in the preparation of ultra-thin and ultra-fine films. Typical thin film materials such as Al2O3, SiO2, and ZnO have been used in various electronics industries.
In recent years, thin film deposition and component manipulation have been widely used in micro/nanofabrication techniques such as mechanical structure, galvanic isolation, and connection. The International Semiconductor Technology Development Roadmap (ITRS) applies ALD technology to the fabrication of high-dielectric constant gate oxides in MOSFET structures and copper diffusion barrier layers in back-end interconnects. Due to the miniaturized layout of the semiconductor process and the resulting high aspect ratio structure of the product, the precise control and conformal coating of thin film deposition technology has become a key technical requirement, and the ALD process provides an effective solution to this requirement.In addition, due to the excellent compactness of the thin film grown by ALD technology, it can form a good barrier barrier for gas molecules within 100 nanometers thickness, and the ultra-thin film form provides important technical support for flexible product applications. Therefore, the current ALD technology is widely regarded as one of the effective protection methods for optoelectronic devices in the future, and the thin film packaging technology based on ALD shows a thinner package weight and better flexibility than the existing packaging methods.
Professor S. F. Bent of Stanford University believes that ALD will be an effective solution to the problem of thin film encapsulation because of its precise and controllable growth at the atomic scale. At present, a lot of research work has been carried out on inorganic materials such as Al2O3, ZrO2, SiO2, and HfO2 prepared by ALD technology, and excellent packaging results have been obtained.However, thin film encapsulation materials based on ALD technology are usually dominated by oxides, and the existence of stable binary bonds between metal and oxygen atoms in the molecular structure leads to a high Young's modulus of oxide films, and the films tend to be rigid as the density and thickness of the films increase.

In addition, in order to meet the needs of low-temperature deposition, plasma-assisted ALD (Plasma Enhanced Atomic Layer Deposition) (PEALD) is often used to compensate for the lack of low-temperature reactivity, however, the introduction of O2 plasma brings large residual stress to the inside of the film. The intrinsic properties of inorganic materials attributed to ALD growth, such as low ductility, low fracture toughness, and high brittleness, limit the durability and reliability of inorganic encapsulation materials during mechanical motion.
Similar to ALD technology, molecular layer deposition (MLD) technology enables the deposition of monolayers layer by layer onto the surface of substrates, and is often used for the growth of organic or organic-inorganic hybrid materials. It is worth noting that there are often some organic components introduced in MLD technology, and the organic or organic-inorganic hybrid films prepared by it have excellent mechanical properties. However, MLD often uses organic precursors as the surface growth unit of the monolayer, and the long-chain organic structure contained in it leads to the large molecular volume of the precursor material, which is easy to form steric hindrance on the surface of the substrate during the semi-reaction process, and occludes part of the active sites, so that the saturation degree of the surface reaction is limited, and the residual active sites cause more defect states in the film, which have the opportunity to provide a permeation path for environmental water vapor, which greatly affects the water vapor barrier performance of the film.

Preparation of monolayer and laminated films

During the PEALD and MLD processes, where the pressure of the reaction chamber is maintained at 0.25 Torr and a high-purity Ar (99.999%) with a flow rate of 100 sccm is used as the carrier gas and precursor cleaning gas, both the PEALD and MLD processes occur in the equipment shown in the figure below.