Developments in the field of atomic layer deposition (ALD) have a long history since the 1950s, when for the first time the concept of this method was proposed in the works of prof. V.B. Aleskovsky (USSR). In 1974, the ALD technology was patented in Finland by Dr. Tuomo Suntola, who had been engaged in various R&Ds in this area for a long time. Currently, there are many brands of equipment on the market that implement the principles of ALD, but Picosun (Finland) – the company inspired by T. Suntola, – of course, holds a special position. For a long time, Picosun’s technical director was Sven Lindfors, who for 40 years was engaged in the development of ALD equipment for various applications. The vast experience of the research team in the field of creating various ALD equipment and constant improvements in technology also contribute to the company’s leadership. Picosun is an example of the fact that scientific research can and should lead to success when many industries are interested in the practical application of their results.
TECHNOLOGY ALD is based on the principle of self-saturation, according to which atoms or molecules of type A react on the surfaces of products only with the free chemical bonds of atoms (molecules) of the previous layer (type B) covering the entire surface of products with a uniform monolayer. If there is no free chemical bond, the reaction on the surface of the products does not occur. Since the reaction chamber in which the product is located is constantly flushed with nitrogen or argon, the excess of type A particles and the reaction products are removed, thus preventing possible "parasitic" chemical reactions on the surface of the products. The molecules of the next layer (type B) react only with molecules of type A and are also adsorbed on the surface with one layer, after which the chamber is again purged with nitrogen (argon). Then the cycle is repeated to achieve the desired film thickness. The sequence of pulsed puffing and removal of process gases is the main difference between ALD and the traditional method of chemical vapor deposition (CVD), in which the reaction gases remain in the working chamber at the same time for a considerable period (up to tens of minutes). Due to the self-saturation effect of chemical reactions on the surface of the product, thin film growth by the ALD method can be controlled at the level of atomic layers with the highest reproducibility and uniformity. Another advantage of ALD over other thin film deposition technologies is the unique possibility of synthesizing homogeneous films on steps and micro-grooves with ultra-high aspect ratios. These circumstances are extremely relevant in connection with the trend of miniaturization of microelectronic products and devices based on microelectromechanical systems (MEMS). The temperature of the ALD processes depends on the materials of the deposited film and the product at which it is synthesized. As a rule, it is in the range from 100°C to 400°C. With the use of plasma stimulation, in many cases it is possible to achieve a reduction in operating temperatures to values close to room temperature, which is especially important for products that are critically sensitive to heat. Thus, the ALD provides the following main advantages: • precise control of film thickness and growth; • excellent conformality and uniformity; • absence of micropores and defects; • high reproducibility of film growth both from wafer to wafer and between cassettes with wafers; • relatively low process temperatures. The downside of the indisputable advantages is the low deposition rate. As a result, in applied use, the thickness of the synthesized films rarely exceeds 50–100 nm. Other methods for producing thin films do not have the accuracy of ALD growth control (Table 1). In addition, a common drawback of physical deposition methods (magnetron sputtering, electron beam, thermal evaporation) is the difficulty and, in some cases, the practical impossibility of obtaining a uniform coating on structures of complex shape. The fact is that the flow of particles from the source to the substrate in these technologies has a linear direction, the angle of incidence relative to the surface varies very much, with some areas being shaded. Examples of complex surfaces are MEMS and structures like TSV. Therefore, in their production, ALD has become one of the key technologies, since it allows applying a uniform coating on the walls of micro-grooves with ultra-high aspect ratio (including steps), in through-holes, etc. (Fig.1). The list of materials of films that can be synthesized using ALD is rather wide: dielectrics (oxides, oxonitrides, etc.), A2B6 and A3B5 semiconductors, ternary compounds (SrTiO3, BaTiO3, etc.), metallized compounds (titanium nitride), metals, including aluminum, titanium, ruthenium, copper, silver, gold, palladium, iridium and platinum. The following is a brief list of Picosun equipment applications: • growing different layers in the production of MEMS; • synthesis of film structures for manufacture of semiconductor devices and ICs; • conductive and barrier layers in TSV; • passivation of various materials (solar cells, anti-tarnishing, etc.); • layers in optical devices and decorative layers; • layers that prevent mechanical wear and chemical attack, as well as hydrophobic, hydrophilic and super omniphobic coatings; • synthesis of catalysts and deposition of films on nanoparticles and nanotubes; • deposition of biocompatible and biodegradable coatings; • deposition of films for targeted drug delivery; • deposition of diffusion barriers for moisture and oxygen (in the manufacture of printed circuit boards, for collectible coins, etc.). EQUIPMENT Picosun manufactures ALD equipment for both scientific research and small pilot production (R-series), and for industrial large-scale production (P-series). It should be noted that the equipment has a modular design and makes it easy to move from R&D to industrial production. The R-series (Fig.3) allows deposition of ALD films of the highest quality with excellent uniformity even on the surface of the most complex structures with deaf and through pores, ultra-high aspect ratio, as well as on nanoparticles and on flexible materials with roll feed. Highly functional and easily replaceable sources for liquid, gaseous and solid precursors (reagents) provide deposition of films of different composition without post-process particles on various materials, three-dimensional objects and wafers having nanoscale features of surface geometry. The equipment allows you to connect up to 12 sources of reagents, is equipped with generators of ozone and plasma, allows you to handle both single wafers with a size from 76 mm to 200 mm, and cassettes with vertically arranged wafers up to 150 mm in size. ALD systems can be equipped with analytical instruments, including a quadrupole mass spectrometer and piezo quartz microbalance, an ellipsometer for in situ measurements of film thickness, as well as instruments for in-vacuo measurements, for example, for X-ray spectrometry analysis (Fig.4). Integration with vacuum loading lock-chambers with manual (Fig.5) or automated loading of wafers into the reaction chamber, as well as with Brooks Automation MX400 (Fig.3) and MX700 robotic vacuum modules with loading via SMIF ports is possible. Such solutions are used for atmosphereless loading of wafers into the reaction chamber, for example, for growing films of nitrides or metals. In addition, it is possible to combine the ALD module with the glove box, one of the variants of which is shown in Fig.6. The equipment is also integrated into other vacuum technological lines and processes, for example, with plasma etching modules, molecular beam epitaxy equipment, etc. Table 2 presents a brief list of standard film materials and their characteristics obtained using Picosun R-200 Advanced. For industrial production, Picosun P-series systems are offered for single and cassette processing of wafers of up to 450 mm in size, growing films on large three-dimensional products, working with large volumes of fine powders, as well as processing wide flexible roll materials. Some characteristics of the films obtained on Picosun P-300S, are presented in Table 3. Industrial ALD systems Picosun P-300S (Fig.7) and Picosun P-300B (Fig.8) are designed for single and cassette processing of wafers with a size of up to 300 mm, as well as for processing a large number of three-dimensional objects and fine powders. It is possible to connect from eight (P-300B) to twelve (P-300S) reagent sources. P-300B can be equipped with a mechanical or automated device for loading and unloading heavy products into the reaction chamber. It is possible to integrate P-300S with a loading chamber with a manual manipulator for airless feeding of single wafers, as well as equipping with a Brooks Automation Marathon 2 vacuum robotic module (Fig.7) allowing airless loading of single 300 mm wafers into the reaction chamber via FOUP ports. Industrial ALD system Picosun R-300BV (Fig.9), designed for large-scale industrial production, allows you to connect up to eight sources of reagents and can be equipped with two automated vacuum loading chambers for atmosphereless loading of the cassette with wafers up to 200 mm in size into the ALD reaction chamber. Loading chambers with the heating option allow loading/unloading of sensitive wafers and high-performance deposition of materials such as metal nitrides. The results obtained by Picosun P-300B users for Al2O3 films when loading two cassettes with 27 200 mm wafers each are shown in Table 4. The flagship of the Picosun industrial series of equipment is the Picosun P-300F (Fig.10). The main feature of the system is automated loading using a Brooks Automation MX400 or MX700 vacuum cluster system with the function of flipping a cassette with wafers up to 200 mm in size in the reaction chamber. Up to twelve reagent sources can be connected to the equipment. P-300F is certified according to SEMI S2/S8 standards and can be integrated into automated industrial production thanks to the SECS/GEM option. Productivity reaches 1 thousand wafers per day at a thickness of the deposited film of 15 nm (Al2O3). The P-300F is characterized by the lowest operating cost compared with competing equipment of similar performance. For processing large three-dimensional objects, glass sheets of up to 400Ч600 mm in size or cassettes with silicon wafers up to 450 mm in diameter, the Picosun P-1000 ALD module is available in the Picosun equipment range (Fig.11). Up to ten reagent sources can be connected via six separate vacuum inputs. The main purpose of this system is the application of passivating and barrier layers to increase productivity and increase the service life of products. In addition to manufacturing equipment, Picosun also offers development services based on customer specifications, foundry services and test depositions, assists in the selection of necessary reagents, and also supplies them to users. Picosun and its customers have created an extensive database of ALD processes that are available to all of the company’s customers. In addition, Picosun offers various levels of technical support for users and various ALD equipment training programs. ■