Equipment for thin film growth using atomic layer deposition
The basis of the ALD is the principle of saturation, according to which the atoms or molecules of a certain type (type A) react on processed surface with atoms (molecules) of the previous layer (type B), evenly covering the entire surface with a uniform layer thickness of the order of angstrom. After this, the excess particles of type A and the reaction products are removed by purging with nitrogen or argon from the chamber to prevent parasitic chemical reactions. Molecules of the next layer (type B) react only with molecules of type A, and also are adsorbed on the surface after which the camera is purged again. Then the cycle is repeated many times to achieve the required film thickness. A cyclical gas puffing and freeing is the main difference between ALD and the chemical vapor deposition (CVD). In contrast to ALD, in CVD reactive gases are located in the working chamber simultaneously for a considerable time (up to tens of minutes). The limitation of the surface reactions in ALD makes it possible to growth thin films with controlled deposition at the level of atomic layers.
The ALD temperature depends on the material of the deposited film, and for the most frequently used in practice processes is in the range from 200 to 400°C. With plasma enhancing in many cases it is possible to reduce operating temperatures up to values close to the room, which is especially important for products that are critically sensitive to heat.
Thus, ALD has the following main advantages:
precise control of thickness and of film growth;
excellent conformability and uniformity;
the absence of micropores and defects;
excellent reproducibility of film growth from plate to plate, and between the cassettes with the wafers;
relatively low temperature of processes.
The reverse side of indisputable advantages is the low deposition rate. As a consequence, in practical applications the thickness of the film rarely exceeds 50–100 nm.
Other methods of thin film deposition, in particular magnetron sputtering and CVD, do not allow such precise control of growth (see tab.1). In addition, a common drawback of the physical deposition, whether magnetron sputtering, electron-beam or thermal evaporation, is the complexity, and in some cases practical impossibility of obtaining a uniform coating on structures of complex shape. The particles flow from the source to the substrate in these methods is linear, so, if the angle of incidence against surface normal changes very much, some areas are shaded. Examples of such complex surfaces are MEMS, some microelectronic devices and TSV-structures (through-silicon via). For their production ALD became one of the key technologies because it allows to grow a uniform film on the walls of micro-grooves with ultra-high aspect ratio (including steps), cutting holes, etc. (fig.1).
The list of materials that can be deposited using ALD is quite wide: dielectrics (oxides, nitrides, etc.); semiconductors (A2B6, A3B5); ternary compounds (SrTiO3, BaTiO3, etc.); metals, including ruthenium, copper, silver, gold, palladium and iridium.
Main applications of ALD-films (fig.2) are the following:
different layers for MEMS;
structures used in manufacturing of semiconductor devices and IC;
the conductive layers in the TSV structures;
layers for optical devices and decorative coatings;
layers, preventing mechanical wear and chemical influences, hydrophobic and hydrophilic coatings;
synthesis of catalysts and deposition of films on the nanoparticles;
passivation of various materials (solar cells, protection against tarnishing, etc.).
Picosun manufactures ALD equipment for research and small pilot production (R-series) and for industrial applications (R-Pro and P-series).
PICOSUN R-series ALD tools (fig.3) enable the deposition of the highest quality ALD films with excellent uniformity even on the most challenging through-porous, ultra-high aspect ratio, nanoparticle samples and flexible materials for the roll feed. Highly functional and easily exchangeable precursor sources for liquid, gaseous, and solid chemicals enable particle-free processing of a wide range of materials on wafers, 3D objects, and all nanoscale features. The equipment has up to 12 sources of reagents, is equipped with ozone and plasma generators, and allows processing of from 76 to 200 mm single wafers, and of the cassette with up to 150 mm wafers. The main options are analytical instruments, e.g. quadrupole mass spectrometer, QCM and ellipsometer for in situ measurement of film growth, and also the glove box (fig.4), vacuum load lock (fig.5) and analytical equipment for in vacuo measurements, for example, for x-ray spectrometric analysis (fig.6).
Table 2 presents a brief list of films and their characteristics received by the users of PICOSUN R-200 Advanced.
PICOSUN R-series-Pro (fig.7) can be used for both R&D and production of various products. Depending on customer requirements, it can be equipped with 12 sources, including a plasma generator, and the same set of options, as the R-series tools. In addition, the integration with the other production lines using robotic vacuum manipulators and SMIF-loader is possible.
PICOSUN P-series is developed for industrial production with the use of ALD. P-series allows handling wafer batches up to 450 mm size, to grow films on large three-dimensional objects, to work with large amounts of fine powders, to handle a wide and flexible product in rolls. It is possible to integrate the equipment into the vacuum clusters PICOPLATFORM. Some characteristics of the films obtained using PICOPLATFORM 300, are presented in table.3.
Industrial ALD system PICOSUN P-300 (fig.8) allows to connect up to 12 reagents and is intended for both single and batch processing of wafers up to 300 mm size. It is possible to equip it with manual or automated manipulator for loading/unloading of heavy parts, semi-automatic cassette loader through the vacuum locks (fig.9), robotic manipulator for automatic loading of wafer in the cassette in the vacuum, with subsequent rotation and loading into a vacuum chamber (fig.10), and SMIF- and FOUP-interfaces. This equipment can be integrated with other vacuum technological lines and processes.
The data obtained by users of PICOSUN P-300B and R-200 tools for Al2O3 ALD films with single loading of two 27 wafers cassettes, of 200 mm size are shown in table.4.
In addition, the range of equipment includes PICOSUN P-1000 tool (fig.11) for ALD on up to 450 mm wafers in the cassette and large three-dimensional objects, as well as on the glass sheets of 400Ч600 mm size. It is possible to connect up to 12 reagents through 8 separate vacuum inlets.