Атомдық қабатпен жабындау технологиясы

Thin film deposition technique that deposits one 1 atom thick layer at a time

Atomic layer deposition (ALD) is a thin film deposition technique based on the sequential use of a gas phase chemical process; it is a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals called precursors (also called "reactants"). These precursors react with the surface of a material one at a time in a sequential, self limiting, manner. A thin film is slowly deposited through repeated exposure to separate precursors. ALD is a key process in fabricating semiconductor devices, and part of the set of tools for synthesizing nanomaterials.

During atomic layer deposition, a film is grown on a substrate by exposing its surface to alternate gaseous species (typically referred to as precursors or reactants). In contrast to chemical vapor deposition, the precursors are never present simultaneously in the reactor, but they are inserted as a series of sequential, non overlapping pulses. In each of these pulses the precursor molecules react with the surface in a self limiting way, so that the reaction terminates once all the available sites on the surface are consumed. Consequently, the maximum amount of material deposited on the surface after a single exposure to all of the precursors (a so called ALD cycle) is determined by the nature of the precursor surface interaction. By varying the number of cycles it is possible to grow materials uniformly and with high precision on arbitrarily complex and large substrates. ALD is a deposition method with great potential for producing very thin, conformal films with control of the thickness and composition of the films possible at the atomic level. A major driving force for the recent interest is the prospective seen for ALD in scaling down microelectronic devices according to Moore's law. ALD is an active field of research, with hundreds of different processes published in the scientific literature, though some of them exhibit behaviors that depart from that of an ideal ALD process. Miikkulainen et al., and Mackus & Schneider et al An interactive, community driven database of ALD processes is also available online The purpose was to experimentally build upon the theoretical considerations of the "framework hypothesis" coined by Aleskovsky in his 1952 habilitation thesis. In 1974, when starting the development of thin film electroluminescent displays (TFEL) at Instrumentarium Oy in Finland, Tuomo Suntola devised ALD as an advanced thin film technology. Suntola named it atomic layer epitaxy (ALE) based on the meaning of "epitaxy" in Greek language, "arrangement upon". The technology was first disclosed in 1980 SID conference. The developers of ML and ALE met at the 1st international conference on atomic layer epitaxy, "ALE 1" in Espoo, Finland, 1990. The name "atomic layer deposition" was apparently proposed for the first time in writing as an alternative to ALE in analogy with CVD by Markku Leskelä (professor at the University of Helsinki) at the ALE 1 conference, Espoo, Finland. It took about a decade before the name gained general acceptance with the onset of the international conference series on ALD by American Vacuum Society.

In 2000, Gurtej Singh Sandhu and Trung T. Doan of Micron Technology initiated the development of atomic layer deposition high κ films for DRAM memory devices. This helped drive cost effective implementation of semiconductor memory, starting with 90 nm node DRAM. Intel Corporation has reported using ALD to deposit high κ gate dielectric for its 45 nm CMOS technology. ALD has been developed in two independent discoveries under names atomic layer epitaxy (ALE, Finland) and molecular layering (ML, Soviet Union). it resulted in several publications reviewing the historical development of ALD under the names ALE and ML. In ALD, enough time must be allowed in each reaction step so that a full adsorption density can be achieved. When this happens the process has reached saturation. This time will depend on two key factors: the precursor pressure, and the sticking probability. Therefore, the rate of adsorption per unit of surface area can be expressed as:

Where R is the rate of adsorption, S is the sticking probability, and F is the incident molar flux. However, a key characteristic of ALD is the S will change with time, as more molecules have reacted with the surface this sticking probability will become smaller until reaching a value of zero once saturation is reached. The specific details on the reaction mechanisms are strongly dependent on the particular ALD process. With hundreds of process available to deposit oxide, metals, nitrides, sulfides, chalcogenides, and fluoride materials, Some representative examples are shown below.

ALD is a useful process for the fabrication of microelectronics due to its ability to produce accurate thicknesses and uniform surfaces in addition to high quality film production using various different materials. In microelectronics, ALD is studied as a potential technique to deposit high κ (high permittivity) gate oxides, high κ memory capacitor dielectrics, ferroelectrics, and metals and nitrides for electrodes and interconnects. In high κ gate oxides, where the control of ultra thin films is essential, ALD is only likely to come into wider use at the 45 nm technology. In metallizations, conformal films are required; currently it is expected that ALD will be used in mainstream production at the 65 nm node. In dynamic random access memories (DRAMs), the conformality requirements are even higher and ALD is the only method that can be used when feature sizes become smaller than 100 nm. Several products that use ALD include magnetic recording heads, MOSFET gate stacks, DRAM capacitors, nonvolatile ferroelectric memories, and many others.

Deposition of the high κ oxides Al2O3, ZrO2, and HfO2 has been one of the most widely examined areas of ALD. The motivation for high κ oxides comes from the problem of high tunneling current through the commonly used SiO2 gate dielectric in MOSFETs when it is downscaled to a thickness of 1.0 nm and below. With the high κ oxide, a thicker gate dielectric can be made for the required capacitance density, thus the tunneling current can be reduced through the structure.

Transition metal nitrides, such as TiN and TaN, find potential use both as metal barriers and as gate metals. Metal barriers are used to encase the copper interconnects used in modern integrated circuits to avoid diffusion of Cu into the surrounding materials, such as insulators and the silicon substrate, and also, to prevent Cu contamination by elements diffusing from the insulators by surrounding every Cu interconnect with a layer of metal barriers. The metal barriers have strict demands: they should be pure; dense; conductive; conformal; thin; have good adhesion towards metals and insulators. The requirements concerning process technique can be fulfilled by ALD. The most studied ALD nitride is TiN which is deposited from TiCl4 and NH3.

Magnetic recording heads utilize electric fields to polarize particles and leave a magnetized pattern on a hard disk. AlO ALD is used to create uniform, thin layers of insulation. By using ALD, it is possible to control the insulation thickness to a high level of accuracy. This allows for more accurate patterns of magnetized particles and thus higher quality recordings.

DRAM capacitors are yet another application of ALD. An individual DRAM cell can store a single bit of data and consists of a single MOS transistor and a capacitor. Major efforts are being put into reducing the size of the capacitor which will effectively allow for greater memory density. In order to change the capacitor size without affecting the capacitance, different cell orientations are being used. Some of these include stacked or trench capacitors. With the emergence of trench capacitors, the problem of fabricating these capacitors comes into play, especially as the size of semiconductors decreases. ALD allows trench features to be scaled to beyond 100 nm. The ability to deposit single layers of material allows for a great deal of control over the material. Except for some issues of incomplete film growth (largely due to insufficient amount or low temperature substrates), ALD provides an effective means of depositing thin films like dielectrics or barriers.

The use of ALD technique in solar cells is becoming more prominent with time. In the past, it has been used to deposit surface passivation layers in crystalline silicon (c Si) solar cells, buffer layers in copper indium gallium selenide (CIGS) solar cells and barrier layers in dye sensitized solar cells (DSSCs). For e. g., the use of ALD grown Al2O3 for solar cell applications was demonstrated by Schmidt et al. It was used as a surface passivation layer for the development of PERC (passivated emitter and rear cell) solar cells. The use of ALD technique to deposit charge transport layers (CTLs) is also being explored widely for perovskite solar cells. The ability of ALD to deposit high quality and conformal films with precise control on thickness can provide great advantage in finely tailoring the interfaces between CTL and perovskite layer. Moreover, it can be useful in obtaining uniform and pin hole free films over large areas. These aspects make ALD a promising technique in further improving and stabilizing the performance of perovskite solar cells.

As photonic integrated circuits (PICs) emerge, often in a manner similar to electronic integrated circuits, a wide variety of on chip optical device structures are needed. One example is the nanophotonic coupler that behaves as a micrometer size beamsplitter at the intersection of optical waveguides in which high aspect ratio trenches (~100 nm width x 4 micrometer depth) are first defined by etching then back filled with aluminum oxide by ALD to form optical quality interfaces.

Understanding and being able to specify the surface properties on biomedical devices is critical in the biomedical industry, especially regarding devices that are implanted in the body. A material interacts with the environment at its surface, so the surface properties largely direct the interactions of the material with its environment. Surface chemistry and surface topography affect protein adsorption, cellular interactions, and the immune response. Some current uses in biomedical applications include creating flexible sensors, modifying nanoporous membranes, polymer ALD, and creating thin biocompatible coatings. ALD has been used to deposit TiO films to create optical waveguide sensors as diagnostic tools. Also, ALD is beneficial in creating flexible sensing devices that can be used, for example, in the clothing of athletes to detect movement or heart rate. ALD is one possible manufacturing process for flexible organic field effect transistors (OFETs) because it is a low temperature deposition method. Nanoporous materials are emerging throughout the biomedical industry in drug delivery, implants, and tissue engineering. The benefit of using ALD to modify the surfaces of nanoporous materials is that, unlike many other methods, the saturation and self limiting nature of the reactions means that even deeply embedded surfaces and interfaces are coated with a uniform film.

ALD can be used as a permeation barrier for plastics. For example, it is well established as a method for encapsulation of OLEDs on plastic. ALD can also be used to inoculate 3 D printed plastic parts for use in vacuum environments by mitigating outgassing, which allows for custom low cost tools for both semiconductor processing and space applications. ALD can be used to form a barrier on plastics in roll to roll processes.

The quality of an ALD process can be monitored using several different imaging techniques to make sure that the ALD process is occurring smoothly and producing a conformal layer over a surface. One option is the use of cross sectional scanning electron microscopy (SEM) or transmission electron microscopy (TEM). High magnification of images is pertinent for assessing the quality of an ALD layer. X ray reflectivity (XRR) is a technique that measures thin film properties including thickness, density, and surface roughness. Another optical quality evaluation tool is spectroscopic ellipsometry. Its application between the depositions of each layer by ALD provides information on the growth rate and material characteristics of the film. Applying this analysis tool during the ALD process, sometimes referred to as in situ spectroscopic ellipsometry, allows for greater control over the growth rate of the films during the ALD process. This type of quality control occurs during the ALD process rather than assessing the films afterwards as in TEM imaging, or XRR. Additionally, Rutherford backscattering spectroscopy (RBS), X ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and four terminal sensing can be used to provide quality control information with regards to thin films deposited by ALD.

Atomic layer deposition instruments can range anywhere from $200,000 to $800,000 based on the quality and efficiency of the instrument. There is no set cost for running a cycle of these instruments; the cost varies depending on the quality and purity of the substrates used, as well as the temperature and time of machine operation. Some substrates are less available than others and require special conditions, as some are very sensitive to oxygen and may then increase the rate of decomposition. Multicomponent oxides and certain metals traditionally needed in the microelectronics industry are generally not cost efficient.

The process of ALD is very slow and this is known to be its major limitation. For example, Al2O3 is deposited at a rate of 0.11 nm per cycle,