Topic 7 In micro-and nano-electronics (4:00)

Plan of the lecture

1. Overview of semiconductor materials

2. Single-crystal growth

3. Melt extraction method with a continuous replenishment of polycrystalline silicon

Semiconductor materials rightfully occupy a leading position in a number of important materials that determine the level of development of world civilization. They form the basis of elements of modern electronic technology, which today is unthinkable without the scientific and technical progress. With the development of solid-state electronics (and, above all, microelectronics) involves the successful solution of the problems of large-scale computerization and information, the creation of modern communications and television transmission and efficient power conversion, diverse household, medical and special electronic equipment. Important role played by these materials in addressing development of clean energy and refrigeration, a modern monitoring systems of environmental pollution, as well as a highly sensitive sensor technology wide functionality.

Development of solid-state electronics hardware components received much attention in all the advanced countries. Only in 1996, the world production of semiconductor devices in terms of money exceeding $ 160 billion, and in 2000 it amounted to about 300 billion dollars. Each year in the development of this field of science and technology to invest billions of dollars. Achievements in physics, physical chemistry and technology of semiconductor materials and semiconductor materials are essential to progress in the development of solid-state electronics. Our country has traditionally taken (and is now) a leading position in Materials Science and has highly qualified scientists and engineers who are able to date to solve the most complex scientific and technical problems of technology of semiconductor materials.

A characteristic feature of the present stage of development of electronic technology is the scope of its involvement in the direct interests of a wide range of semiconductor materials. The most important are: silicon, gallium arsenide, and a large group of other binary compounds and multicomponent solid solutions, germanium, silicon carbide, binary compounds based on narrow-band and wide-band connections, and touch a variety of thermoelectric materials.

The main material of modern solid-state electronics is silicon. World production of silicon single crystal is 8 ... 9 tons / year Today, single-crystalline silicon - the most perfect crystalline material from a huge variety of materials ever created by man or nature. Characterized by a high level of quality and a number of other well established semiconductor materials. Modern semiconductor industry - a prime example of the outstanding achievements of the human mind to the development of world civilization, and its further progress related to the solution continuously increasing complexity of scientific and engineering problems.

Next, we consider some of the most pressing problems of the present stage of development of semiconductor technology and materials science.

Single-crystal growth

Single-crystal growth - one of the most critical steps towards the creation of device structures. Carved out of single crystal plates are used either for direct formation based on these integrated circuits and discrete devices, or as substrates in the process of obtaining thin film epitaxial structures. In both cases, the quality of single crystal wafers very high demands.

The main trend in the development of technologies for a wide range of single-crystal semiconductor is grown larger diameter ingots, while the continuous strengthening of requirements for perfection of the crystal structure and the uniformity of distribution of electrical characteristics defined in the bulk material. In the case of silicon, it is about getting dislocation-free single crystals with a diameter up to 450 mm in the case of GaAs, InP, GaSb, InSb, CdTe, etc. - malodislokatsionnyh single crystals with a diameter of up to 100 ... 150 mm.

The most universal method of growing single crystals of large diameter is the Czochralski method. Increasing the diameter of grown single crystals and the need to ensure high economic efficiency of the process chain in the crystal - a plate requires a precision of heavy, high-performance and fully automated plant growth. If, for example, in the production of silicon single crystals with a diameter of 200 mm are used to load the oven to 150 kg and a diameter of 300 mm - 250 ... 300 kg, moving to build single-crystal 450 mm requires a growth equipment to load 550 ... 600 kg. The diameter of the quartz crucibles used increases to 0.9 ... 1.0 m.

At present, the main products in the world market are single crystals of semiconductor silicon wafers and 150 and 200 mm. In 2001, the leading silicon manufacturers began to produce dislocation-free single crystals and wafers 300 mm in diameter compared with those for 200 mm wafers, and further significantly exceed them. Also, keep in mind that is already developed and pilot-testing of industrial technology for growing dislocation-free single crystals (and manufacture of these plates) with a diameter of 400 ... 450 mm.

Tackling the growing single crystals of large diameter by successive increase in the mass of initial load and size of the quartz crucibles at each new stage of increasing the diameter of the ingot becomes less cost-effective due to the significant increase in energy consumption, cost, crucibles and increased costs to provide safe working conditions. From this point of view, special attention should melt extraction method with a continuous replenishment of granular or particulate polycrystalline silicon. The main advantage of this method is the ability to grow crystals of the large mass of relatively small and constant over the volume of the molten bath in the crucible of smaller size. There are other fundamental advantages: to provide uniform distribution of impurities increase the length and cross section of the grown crystal, solved the problem of maintaining a constant shape of the crystallization front and constant thermal conditions at the interface between the crystal - melt for almost the entire process. Currently this method is reduced to the level of industrial use.

From the point of view of increasing the economic performance of the process, of course, and the method is promising semi pulling single crystals with periodic dozagruzkoy hot quartz crucible through a special bunker without depressurization and cooling growth vessel. Serious and not yet fully solved the problem in this case (as, indeed, in the previous) is the gradual accumulation of unwanted background impurities in the molten bath (and, correspondingly, in the grown single crystal) as the number dozagruzok. Significantly constrain the broad development of the method and insufficient mechanical strength and heat resistance currently used heavy quartz crucibles. However, in this direction in recent years there have been significant improvements.

Very important role in obtaining high-quality single crystals of large diameter design is thermal unit growth vessel. Optimization of thermal units of modern growth setups for growing single crystals is carried out not only with the need precise control of the processes of heat and mass transfer (SST) in the melt a large mass, and thermal fields in the grown single crystal. This approach provides today receive dislocation of large diameter silicon single crystals with controlled nature and density of micro-defects present in them.

Optimization of thermal units require very time-consuming research on communication thermal growing conditions with electrophysical characteristics and structural features of the resulting crystals. As you increase the diameter of grown single crystals, such studies are becoming more labor intensive and require high material costs. Therefore, in recent years to optimize the design of thermal units and growth units of the thermal conditions of growing single crystals are increasingly used methods of mathematical and physical modeling, taking into account not only the thermal characteristics of the simulated growth processes, but specific mechanisms of defect formation in the growth of crystals.

In recent years, process control LUT melts great masses begin extensive use of electromagnetic effects on the melt by means of magnetic devices based on superconducting materials. Most widespread use of static magnetic fields. However, intensive study possible changes in the electromagnetic interference in the first place, to rotate. Electromagnetic effects due to substantial increases in the effective viscosity of melts of semiconducting materials can almost completely suppress the turbulent flow in the molten bath due to thermal convection, and thus drastically reduce temperature fluctuations in the melt podkristalnoy area (and thus, the level of vibration caused by the rate of crystallization) . This leads to a significant increase in the homogeneity of the distribution of impurities and reduction of structural defects in the bulk grown single crystals. However, in different electromagnetic influences significantly expanded the creation of controlled hydrodynamic flow that optimizes the conditions of TMP in the melt. In this regard, special attention should be combined electromagnetic effects.

Thermal units of modern growth setups are made of high-purity isotropic graphite (heaters, pots, stands) for the wide use of carbon composite materials with good thermal insulation properties (screens).

In the design of modern heavy plants growing single crystals of large diameter has to simultaneously solve the problem of creating a reliable system to support very heavy ingot during its extraction and production sites with appropriate accessories for the transportation and installation of the graphite parts thermal unit and quartz crucibles for unloading and transporting grown crystal and its calibration, provide a safe working environment. All this involves increasing the level of automation, robotics and standardize the processes involved, which requires greater technological equipment and accessories by means of modern high-sensitivity sensor technology.

When applied to single crystals, "decaying" compound semiconductor (GaAs, InP, GaP, CdTe, etc.) Czochralski method implemented in the form of liquid melt in the crucible sealing layer of boric anhydride. Growth of single crystals is carried out in a fully automated high pressure, providing growing ingots up to 150 mm in diameter and weighing up to 30 kg. This uses a combined and separate processes for the synthesis of the parent compound and crystal growth. The materials used for the manufacture of crucibles quartz glass and pyrolytic boron nitride.

If the problem of obtaining dislocation-free silicon single crystals of large diameter when grown by the Czochralski method is solved relatively simply, in the way of this method of obtaining large malodislokatsionnyh single crystals of semiconductor compounds occur most fundamental difficulty. They are due in the first place, significantly lower values of the critical stresses of dislocation in these materials, their lower thermal conductivity and difficulty ensuring stoichiometric composition in the growth process.

To reduce the density of dislocations in single crystals grown in this case is widely used doping to relatively high concentrations of the so-called hardening alloy above the critical stress of dislocation in the respective materials. As such strengthening impurities well established isovalent impurities with high solubility in the respective semiconductor materials and has little effect on their physical properties (eg, In a single crystal GaAs; Zn in single-crystal CdTe).

However, the preferred method of obtaining crystals malodislokatsionnyh is to reduce thermal stresses in the grown ingot melt, as it is thermoplastic deformation in this case is the main cause of the generation of dislocations. Since the level of thermal stress is directly related to the magnitude of the axial and radial temperature gradients in the grown crystal, the question naturally arises the task of reducing the latter. It is of fundamental importance is linear or close to the nature of the axial temperature distribution in the crystal in the region adjacent to the crystallization front.

Fulfill these conditions in the traditional drawing process of single crystals from a sealing layer of flux, with no significant degradation of the surface of the ingot in the field of flux, can not. New features in this plan provides recently developed a method of growing single crystals by the Czochralski method with liquid seal melt while maintaining the gas growth atmosphere over a layer of flux controlled vapor pressure of the volatile component of the corresponding compounds, achieved the goal of creating the necessary thermal growing conditions without fear the expansion of the growing crystal surface. Today, this method has been successfully used to obtain single crystals of GaAs and InP large diameter (150 mm ... 75 mm, respectively) with the dislocation density.

The most promising method for growing single crystals malodislokatsionnyh decomposing compound semiconductor large geometric dimensions are the methods of horizontal (STC) and vertical (BHK) directional solidification in a container, is placed in a sealed quartz ampoule. Both methods allow you to grow single crystals at relatively low temperature gradients, under strict control of stoichiometry. In recent years, a growing preference for the method of the INC, which provides preparation of crystals in a cylindrical axisymmetric thermal field and maintain a flat solidification front, and the lack of heat to melt the convention. Special preparation of containers of quartz or boron nitride eliminates their negative impact on the quality of grown crystals. Particularly promising embodiment of the method is crystallization BHK in a "driving temperature gradient." Currently using OWC in industrial environments successfully grown GaAs single crystals with a diameter of 150 mm and a mass of 15 ... 30 kg, with a dislocation density and high homogeneity in the distribution of electrical properties of the bulk.

Dramatically in recent years, the interest in such a wide-semiconductor materials such as silicon carbide and nitride elements of the third group of the periodic system. These materials have very high melting points and high vapor pressure is extremely volatile on their own melts. Enough for growing large single crystals of these materials have to be used crystallization from solution and the different methods of crystallization from the gas phase, including a high pressure apparatus. Obtaining sufficiently large and perfect single crystals of wide band gap semiconductors involves overcoming many difficulties and principal, except for silicon carbide, has not gone beyond the laboratory.

The sharp increase in density and decrease in the size of work items in modern integrated circuits dictates the need to reduce the current and voltage. In these conditions, the role of background noise, resulting in the first place, the presence in the active instrument composition of impurities and structural defects, capable of forming a semiconductor material electrically and rekombinatsionnoaktivnye centers. In this regard, much stricter requirements for widely used in solid state electronics single crystals. Suffice it to say that in the dislocation-free silicon single crystals of large diameter, used for the manufacture of integrated circuits ultrasverhbolshih, the total content of heavy metals rapidly diffusing impurities should not exceed 1011 atoms/cm3 and carbon - (1 ... 2) 1015 atoms/cm3. Much attention is paid to ensuring a given concentration and uniform distribution of oxygen in the crystal.

Strict requirements on the content of the electrically active impurities are put forward in the solution of the problem of obtaining high-quality single crystals of undoped semi-insulating GaAs and InP, used in the manufacture of discrete devices and integrated circuits microwave. In this case, besides the heavy metal impurities should strictly limit the content of impurities in single elements of the second and sixth groups of the periodic system - Zn, Cd, S, Se, Te, etc. (atoms/cm3), and silicon (atoms/cm3) and carbon (atoms/cm3).

A very significant role in addressing the purity is given materials (process gases, materials, containers, heating elements and heat shields, chemicals, etc.) materials, the content of limiting residual impurities that do not exceed 10.7 ... 10.9%. However, much depends on ensuring sterility of the growth process. Especially likely additional contamination of the material at the stage of preparation for the loading and in the implementation of most of the operation (all the operations related to the final preparations of both initial load and container, and were placed in the growth chamber, must be conducted in a particularly clean conditions ). Transportation prepared to load growth vessel is sealed in a special clean container. Serious attention is paid to the preparation of the most growth-setting, including prior to annealing of graphite parts thermal unit (and storage) and the exclusion of severe overheating melt during melting download.

Serious new tasks arise in increasingly complex equipment manufacturing methods of quality control, especially as applied to the plates. As the degree of integration of solid-state electronic devices are feeling the need for new high-resolution, rapidity, highly informative and automated non-contact control methods, objectively characterize the suitability of single crystals and wafers for the new challenges. Requirements for the number and size of those present in single crystals and on the surface of the plates defects tougher every year, because the capabilities of traditional optical and electrical control methods are almost exhausted. A shift to a new level of metrology, using the capabilities of scanning tunneling and atomic force microscopy, and other modern methods of control of structure and properties with submicron and nanometer resolution. With new controls should fit well into the ideology of flexible, continuous, high-automated production lines. Becomes very relevant issue and express pollution control wafer surface metallic impurities with a sensitivity at the level of - 108 atoms/cm2.

To make the grown single crystals of various electrical parameters required for successful use in specific areas of semiconductor devices, the processes of doping with certain impurities. Currently, the scope of technology used in the important semiconductor materials dopants is limited. As a rule, the doping is carried impurities forming shallow donor and acceptor levels in the band gap, respectively, at the bottom of the conduction band or from the valence band. Thus able to influence the controllable conductivity type and concentration of charge carriers in a semiconductor. Sometimes used for doping impurities that form deep levels in the band gap, which can influence the diffusion length of the charge carriers and adjust the degree of compensation of electrically active centers in the doped material.

At the same time, it is now well known that the introduction of various dopants can effectively influence the state of the ensemble of intrinsic point defects (STD) in the crystals' behavior of dislocations, which can eventually lead to a significant expansion of the ability to manage effect on the properties of semiconductor material. Of great interest are isovalent impurities and rare earth impurities. We can suggest that in the near future, during crystal with the desired set of properties, greater importance will become a complex process of doping, using both traditional and non-traditional dopants.