Speaking about the development of nanotechnology, we usually see them at the forefront of research and development. However, in some cases, they have already outgrown the stage of R&D and firmly established position in the industrial production. As an example, consider the technology of Atomic Layer Deposition (ALD), a ultra-thin film manufacturing technique by the sequential deposition of the reactants from the gas phase in a cyclic process.
ALD was patented in 1974 in Finland by Dr. Tuomo Suntola and since then has come a long way. Today many companies manufacture equipment that implements the principles of ALD, but the leadership belongs to Picosun, in which Dr. T.Suntola is the mastermind and member of the Board of Directors. Great experience of the ALD equipment development and continuous efforts to improve the technology allow leadership. Picosun is an example of what research can and must lead to success, when their application will benefit many industries and businesses around the world. In Russia the Picosun’s solutions represents the Evrointech LLC (www.eurointech.ru).
Basics of the technology
ALD is based on the elegant idea that atoms or molecules of a certain type (type A) react on the surface of objects with atoms (molecules) of the previous layer (type B), evenly covering the whole surface and forming a homogeneous layer, whose thickness is of the order of angstrom. After that, an excess of particles of type A and the reaction products are removed from the chamber. The next layer of molecules (type B) react with molecules of type A and also are adsorbed on the surface in one layer. Then the cycle is repeated many times to achieve the desired thickness.
Other methods of thin film deposition (PVD and CVD) do not have such precise control of growth. In addition, a common drawback of the PVD technologies, whether magnetron sputtering, electron-beam or thermal evaporation, is the difficulty, and in some cases the practical impossibility of obtaining a uniform coating on the structures of complex shape. The flow of particles from the source to the substrate in these methods has a linear orientation, and the angle of incidence in the objects of complex shape is changing greatly, so some areas are "shaded". An example of such complex surfaces are microelectromechanical systems (MEMS). ALD has emerged as a key technology for their production because it allows to apply a uniform coating on the walls of ultra-high aspect ratio micro grooves, holes, etc.
The temperature of ALD depends on the material to be used, and for the most frequently used processes lies in the range from 200 to 400°C. However, using plasma stimulation in many cases it is possible to reduce the temperature to values close to the room, which is especially important for products that are critically sensitive to heat.
Thus, the ALD method has the following advantages:
•precision thickness control; •excellent conformability and uniformity; •the absence of micropores and defects; •excellent reproducibility of results both from the wafer to the wafer, and from the cassette to the cassette; •relatively low temperature of process. The reverse of advantages is the low deposition rate. As a consequence, in practical applications the thickness of the resulting films rarely exceeds 50 nm. Because of this limitation we refer the ALD method to nanotechnology.
The list of materials that can be obtained by ALD, is quite wide: dielectrics (oxides, nitrides, etc.), semiconductors (types A2B6, A3B5) and metals, including copper, palladium and ruthenium.
Today there are many applications of ALD in a variety of industries. Since the format of this article does not allow to consider each in detail, we will briefly note the most typical.
Micro- and nanoelectronics are traditionally the areas of introduction of the most advanced technologies, including ALD. The continuously increasing demands for reduction in thickness of the insulating, conducting and other functional layers, on the one hand, and for the improvement of their characteristics on the other, has opened up prospects for the application of ALD in the production of semiconductor devices and integrated circuits. A typical example are modern MOS devices with high requirements for the electrodes of the gates and the gate insulators. ALD is also widely used in the manufacture of LEDs, transistors with high electron mobility, DRAM, etc.
It should pay special attention to the production of three-dimensional integrated circuit and the metallization of holes in a silicon wafer: the ALD method with unrivalled capabilities in the application of homogeneous films on 3D structures, was in this case indispensable.
It should pay special attention to the production of three-dimensional integrated circuits that requires metallization of through holes in the silicon wafer. ALD, which has unmatched capabilities for applying uniform films on 3D-structures, is in this case indispensable.
In MEMS among various functional layers (insulating, conductive, optical, seeding, passivating etc.) it should be noted coatings that reduce friction of moving parts and, consequently, reduce frictional wear. The quality of the ALD is especially important, as even a slight wear of parts critically affects the characteristics and performance of MEMS.
One of the actively developing areas in electronics is the creation of organic light-emitting diodes (OLED) based on polymeric materials, which are most promising for the next generation of displays, including fabricated on a flexible basis. The need to protect the OLEDs, which are extremely sensitive to oxidation from moisture and oxygen, makes ALD popular in this area, because this method does not form the micropores and microcracks.
Creation of protective layers is also important for printed electronics, which is actively developed recently, where patterns are created on a flexible film, in contrast to the conventional solid-state silicon electronics. Therefore, companies producing equipment for the ALD, in particular Picosun, pay special attention to the development of Roll-to-Roll (R2R) machines.
Biocompatible ALD coatings find applications in medicine, for example, in the manufacture of implants, and in the future their role will grow.
One of the most promising areas of development of the passive electronic elements are ultra-capacitors, which has increased capacity with minimum external dimensions. These devices with an internal porous structure and the dielectric layer between plates are manufactured using ALD.
ALD is widely used in the creation of anti-corrosion, anti-friction, as well as decorative coatings for various mechanical parts. The protective films that prevents tarnishing are demanded in the jewelry industry and in the manufacture of coins. Deposition of nanoscale films on cutting edges helps protect the tool from wear.
Actual scientific task is the deposition of thin films on nanoparticles and nanotubes, the production of graphene and other promising areas.
The General global trend is: after ALD successfully passes the trials in research centers, begins her introduction in industry. For example, over the past two years clustered equipment was purchased by the largest manufacturers in Taiwan and USA. Russian researchers have already accumulated rich experience in use of ALD equipment, and there has been a steady growth of interest in this technology by large industrial enterprises, primarily leaders of electronic industry of Russia.
The world’s leading manufacturers of equipment for ALD, including Picosun, strive to ensure that the result obtained in laboratories, can be reproduced in a production environment. The number of new applications of ALD is constantly growing, and in most cases they belong to the most promising areas of microelectronics, IT, bio- and nanotechnology. Changing our lives, ALD constantly being improved in accordance with the new requirements. ■
Special thanks to Alexei Veselov (Picosun Oy) for scientific editing of the article.