Manufacturing technology of integrated microcircuits

The production of integrated circuits consists of a series of operations, performing which the finished product is gradually obtained from the starting materials. The number of technological process operations can reach 200 or more, so we will consider only the basic ones.

Epitaxy is the operation of growing a single-crystal layer on a substrate, which repeats the structure of the substrate and its crystallographic orientation. To obtain epitaxial films with a thickness of 1 to 15 μm, the chloride method is usually used, in which semiconductor wafers, after thoroughly cleaning the surface from various kinds of contaminants, are placed in a quartz tube with high-frequency heating, where the wafers are heated to 1200 ± 3 °C. A stream of hydrogen with a small content of silicon tetrachloride is passed through the pipe. The silicon atoms formed in the course of the reaction occupy places at the sites of the crystal lattice, due to which the growing film continues the crystal structure of the substrate. When gaseous donor compounds are added to the mixture of gases, the layer being grown acquires p-type conductivity.

Doping is the operation of introducing impurities into a substrate. There are two doping methods: impurity diffusion and ion implantation.

Diffusion of impurities is the movement of particles due to thermal motion in the direction of decreasing their concentration. The main mechanism for the penetration of impurity atoms into the crystal lattice consists in their successive movement along lattice vacancies. Diffusion of impurities is carried out in quartz furnaces at a temperature of 1100-1200 °C, maintained with an accuracy of ±0.5 °C. A neutral carrier gas (N2 or Ar) is passed through the furnace, which transports diffusant particles (B2O3 or P2O5) to the surface of the plates, where, as a result of chemical reactions, impurity atoms (B or P) are released, which diffuse deep into the plates.

Ion doping is widely used in the creation of LSI and VLSI. Compared to diffusion, the ion doping process takes less time and allows creating layers with submicron horizontal dimensions, less than 0.1 µm thick, with high parameter reproducibility.

Thermal oxidation is used to obtain thin films of silicon dioxide SiO2, it is based on high-temperature reactions of silicon with oxygen or oxygen-containing substances. Oxidation takes place in quartz furnaces at a temperature of 800-1200 °C with an accuracy of ±1 °C.

Etching is used to clean the surface of semiconductor wafers from various kinds of contaminants, remove the SiO2 layer, and also to create grooves and depressions on the surface of the substrates. Etching can be both liquid and dry.

Liquid etching is carried out using acid or alkali. Acid etching is used in the preparation of silicon wafers for the manufacture of microchip structures in order to obtain a mirror-smooth surface, as well as to remove the SiO2 film and form holes in it. Alkaline etching is used to obtain grooves and depressions.

Lithography is the process of forming holes in masks used for local diffusion, etching, oxidation, and other operations. There are several variations of this process.

Photolithography is based on the use of light-sensitive materials - photoresists, which can be negative and positive. Negative photoresists polymerize under the action of light and become resistant to etchants. In positive photoresists, light, on the contrary, destroys the polymer chains, so the exposed areas of the photoresist are destroyed by the etchant. In the production of FPGA, a layer of photoresist is applied to the SiO2 surface, and in the production of GIS, it is applied to a thin layer of metal deposited on a substrate, or to a thin metal plate that acts as a removable mask.

The required pattern of IC elements is obtained by irradiating the photoresist with light through a photomask, which is a glass plate, on one side of which there is a positive or negative pattern of IC elements on a scale of 1:1. In the production of ICs, several photomasks are used, each of which sets the pattern of certain layers (base and emitter regions, contact leads, etc.).

After irradiation with light, the non-polymerized areas of the photoresist are removed by an etchant, and a photoresistive mask is formed on the surface of SiO2 (or a metal film).

X-ray lithography uses soft X-rays with a wavelength of about 1 nm, which makes it possible to obtain D » 0.1 µm. In this case, the photomask is a membrane (about 5 μm) transparent to X-rays, on which a pattern of IC elements is created by electron-beam lithography.

Ion beam lithography uses irradiation of a resist with an ion beam. The sensitivity of the resist to ion irradiation is many times higher than to electron irradiation, which makes it possible to use beams with low currents and, accordingly, a small diameter (up to 0.01 μm). The ion-beam lithography system is technologically compatible with ion doping units.

3 TECHNOLOGICAL BASIS OF PRODUCTION

SEMICONDUCTOR INTEGRATED MICROCIRCUIT

The technology for the production of semiconductor integrated circuits (SSIMS) has developed on the basis of planar transistor technology. Therefore, in order to understand the technological cycles of manufacturing ICs, it is necessary to familiarize yourself with the typical technological processes that these cycles are made up of.

3.1 Preparatory operations

Single-crystal silicon ingots, like other semiconductors, are usually obtained by crystallization from a melt - Czochralski method. With this method, a seeded rod (in the form of a single crystal of silicon) after contact with the melt is slowly lifted with simultaneous rotation. In this case, after the seed, the growing and solidifying ingot is drawn out.

The crystallographic orientation of the ingot (its cross section) is determined by the crystallographic orientation of the seed. More often than others, ingots with a cross section lying in the plane (111) or (100) are used.

The typical diameter of ingots is currently 80 mm, and the maximum diameter can reach 300 mm or more. The length of ingots can reach 1-1.5 m, but usually it is several times less.

Silicon ingots are cut into many thin plates (0.4-1.0 mm thick), on which integrated circuits are then made. The surface of the plates after cutting is very uneven: the dimensions of scratches, protrusions and pits far exceed the dimensions of future IC elements. Therefore, before the start of the main technological operations, the plates are repeatedly ground and then polished. The purpose of grinding, in addition to removing mechanical defects, is also to ensure the required thickness of the plate (200-500 microns), which is unattainable during cutting, and the parallelism of the planes. At the end of grinding, a mechanically disturbed layer several microns thick still remains on the surface, under which there is an even thinner, so-called physically disturbed layer. The latter is characterized by the presence of "invisible" distortions of the crystal lattice and mechanical stresses that arise during the grinding process.

Polishing consists in removing both damaged layers and reducing surface irregularities to the level characteristic of optical systems - hundredths of a micrometer. In addition to mechanical polishing, chemical polishing (etching) is used, i.e., in essence, the dissolution of the surface layer of the semiconductor in certain reagents. The protrusions and cracks on the surface are etched faster than the base material, and the surface is generally leveled.

An important process in semiconductor technology is also the cleaning of the surface from contamination by organic substances, especially fats. Cleaning and degreasing is carried out in organic solvents (toluene, acetone, ethyl alcohol, etc.) at elevated temperatures.

Etching, cleaning and many other processes are accompanied by washing the plates in deionized water.

3.2 Epitaxy

epitaxy called the process of growing single-crystal layers on a substrate, in which the crystallographic orientation of the layer being grown repeats the crystallographic orientation of the substrate.

Currently, epitaxy is usually used to obtain thin working layers up to 15 µm of a homogeneous semiconductor on a relatively thick substrate, which plays the role of a supporting structure.

Typical - chloride the process of epitaxy in relation to silicon is as follows (Figure 3.1). Monocrystalline silicon wafers are loaded into a "boat" crucible and placed in a quartz tube. A stream of hydrogen is passed through the pipe, containing a small admixture of silicon tetrachloride SiCl4. At a high temperature (about 1200°C), the reaction SiCl4 + 2H2 = Si + 4HC1 occurs on the surface of the plates.

As a result of the reaction, a layer of pure

silicon, and HCl vapor is carried away by a hydrogen flow. The epitaxial layer of deposited silicon is single-crystal and has the same crystallographic orientation as the substrate. The chemical reaction, due to the selection of temperature, occurs only on the surface of the plate, and not in the surrounding space.

Figure 3.1 - Epitaxy process

The process that takes place in a gas stream is called gas transportation reaction and the main gas (in this case, hydrogen), which carries the impurity to the reaction zone, is carrier gas.

If pairs of phosphorus compounds (РН3) or boron compounds (В2Н6) are added to silicon tetrachloride vapors, then the epitaxial layer will no longer have its own, but, accordingly, electronic or hole conductivity (Figure 3.2a), since donor atoms will be introduced into the deposited silicon during the reaction phosphorus or acceptor boron atoms.

Thus, epitaxy makes it possible to grow on a substrate single-crystal layers of any type of conductivity and any specific resistance, having any type and value of conductivity, for example, in Figure 3.2a, a layer n is shown, and a layer n + or p + can be formed.

Figure 3.2 - Substrates with epitaxial and oxide films

The boundary between the epitaxial layer and the substrate does not turn out to be perfectly sharp, since impurities partially diffuse from one layer to another during the epitaxy process. This circumstance makes it difficult to create ultrathin (less than 1 μm) and multilayer epitaxial structures. The main role, at present, is played by single-layer epitaxy. It significantly expanded the arsenal of semiconductor technology; obtaining such thin homogeneous layers as epitaxy provides is impossible by other means.

In Figure 3.2a and following, the vertical scale is not respected.

In the installation shown in Figure 3.1, some additional operations are provided: purging the pipe with nitrogen and shallow etching of the silicon surface in HCl vapor (for cleaning purposes). These operations are carried out before the start of the main ones.

The epitaxial film may differ from the substrate in chemical composition. The method of obtaining such films is called heteroepitaxy, Unlike homoepitaxy, described above. Of course, in heteroepitaxy, both the film and substrate materials must still have the same crystal lattice. For example, a silicon film can be grown on a sapphire substrate.

In conclusion, we note that in addition to the described gas epitaxy, there is liquid epitaxy, in which the growth of a single-crystal layer is carried out from the liquid phase, i.e., from a solution containing the necessary components.

3.3 Thermal oxidation

Silicon oxidation is one of the most characteristic processes in the technology of modern FPIMs. The resulting film of silicon dioxide SiO2 (Figure 3.2b) performs several important functions, including:

protection function - passivation surface and, in particular, the protection of vertical sections p - n transitions coming to the surface;

The mask function, through the windows in which the necessary impurities are introduced by diffusion (Figure 3.4b);

The function of a thin dielectric under the gate of a MOSFET or capacitor (Figures 4.15 and 4.18c);

Dielectric base for connecting the elements of the PCB IC with a metal film (Figure 4.1).

The silicon surface is always covered with its "own" oxide film, resulting from "natural" oxidation at the lowest temperatures. However, this film is too thin (about 5 nm) to perform any of the listed functions. Therefore, in the production of semiconductor ICs, thicker SiO2 films are obtained artificially.

Artificial oxidation of silicon is usually carried out at high temperature (°C). Such thermal oxidation can be carried out in an oxygen atmosphere. (dry oxidation), in a mixture of oxygen and water vapor ( wet oxidation) or simply in water vapor.

In all cases, the process is carried out in oxidizing furnaces. The basis of such furnaces is, as in epitaxy, a quartz tube in which a "boat" with silicon plates is placed, heated either by high-frequency currents or in another way. A stream of oxygen (dry or humidified) or water vapor is passed through the pipe, which reacts with silicon in the high-temperature zone. The SiO2 film thus obtained has an amorphous structure (Figure 3.2b).

Obviously, the growth rate of the oxide must decrease with time, since new oxygen atoms have to diffuse through an increasingly thick oxide layer. The semi-empirical formula relating the thickness of the oxide film to the time of thermal oxidation has the form:

where k - parameter depending on the temperature and humidity of oxygen.

Dry oxidation is ten times slower than wet oxidation. For example, it takes about 5 hours to grow a SiO2 film 0.5 μm thick in dry oxygen at 1000°C, and only 20 minutes in wet oxygen. However, the quality of films obtained in humid oxygen is lower. With a decrease in temperature for every 100 ° C, the oxidation time increases by 2-3 times.

In IC technology, “thick” and “thin” SiO2 oxides are distinguished. Thick oxides ( d = 0.7-1.0 microns) perform the functions of protection and masking, and thin (d = 0.1-0.2 µm) - gate dielectric functions in MOSFETs and capacitors.

One of the important problems in growing a SiO2 film is to ensure its uniformity. Depending on the quality of the wafer surface, the purity of the reagents, and the growth mode, some or other problems arise in the film. defects. A common type of defects are micro- and macropores, up to through holes (especially in thin oxide).

The quality of the oxide film increases with a decrease in its growth temperature, as well as with the use of dry oxygen. Therefore, a thin gate oxide, the quality of which determines the stability of the MOS transistor parameters, is obtained by dry oxidation. When growing a thick oxide, dry and wet oxidation are alternated: the first ensures the absence of defects, and the second allows to reduce the process time.

Other methods for obtaining a SiO2 film are discussed in.

3.4 Lithography

Masks occupy an important place in the technology of semiconductor devices: they ensure the local nature of deposition, doping, etching, and, in some cases, epitaxy. Each mask contains a set of pre-designed holes - windows. The production of such windows is task of lithography(engraving). The leading place in mask manufacturing technology is maintained photolithography and electron lithography.

3.4.1. Photolithography. Photolithography is based on the use of materials called photoresists. This is a type of photographic emulsion known in conventional photography. Photoresists are sensitive to ultraviolet light, so they can be processed in a room that is not very dark.

Photoresists are negative and positive. Negative photoresists polymerize under the action of light and become resistant to etchants (acidic or alkaline). This means that after local exposure, non-exposed areas will be etched (as in a regular photo negative). In positive photoresists, light, on the contrary, destroys polymer chains and, therefore, illuminated areas will be etched.

The drawing of the future mask is made in the form of the so-called pho­ totemplate. The photomask is a thick glass plate, on one side of which a thin opaque film is applied with the necessary drawing in the form of transparent holes. The dimensions of these holes (drawing elements) on a scale of 1: 1 correspond to the dimensions of future IC elements, i.e., they can be 20-50 microns or less (up to 2-3 microns). Since ICs are made by a group method, a lot of the same type of drawings are placed on the photomask along the “rows” and “columns”. The size of each drawing corresponds to the size of the future IC chip.

The photolithography process for obtaining windows in the SiO2 oxide mask covering the surface of a silicon wafer is as follows (Figure 3.3). On the oxidized surface of the plate is applied, for example, a negative photoresist (FR). A FS photomask is applied to a plate coated with a photoresist (with a pattern to the photoresist) and exposed to ultraviolet (UV) rays of a quartz lamp (Figure 3.3a). After that, the photomask is removed, and the photoresist is developed and fixed.

If a positive photoresist is used, then after developing and fixing (which consists in hardening and heat treatment of the photoresist), windows are obtained in it in those places that correspond to transparent areas on the photomask.

As they say, picture moved from photomask to photoresist. Now the photoresist layer is a mask that fits snugly against the oxide layer (Figure 3.3b).

Through a photoresistive mask, the oxide layer is etched up to silicon (this etchant does not affect silicon). Hydrofluoric acid and its salts are used as an etchant. As a result, the pattern from the photoresist is transferred to the oxide. After removal (etching) of the photoresistive mask, the end result of photolithography is a silicon wafer covered with an oxide mask with windows (Figure 3.3c). Diffusion, ion implantation, etching, etc. can be carried out through windows.

Figure 3.3 - The process of photolithography

In the technological cycles of manufacturing IC elements, the photolithography process is used repeatedly (separately for obtaining base layers, emitters, ohmic contacts, etc.). In this case, the so-called photomask alignment problem arises. With repeated use of photolithography (up to 5-7 times in the PPIMS technology), the alignment tolerance reaches fractions of a micron. The combination technique consists in making special “marks” (for example, crosses or squares) on the photomasks, which turn into a pattern on oxide and shine through a thin film of photoresist. Applying the next photomask, in the most accurate way (under a microscope) the marks on the oxide are combined with similar marks on the photomask.

The considered process of photolithography is typical for obtaining oxide masks on silicon wafers for the purpose of subsequent local diffusion. In this case, the photoresistive mask is intermediate, auxiliary, since it cannot withstand the high temperature at which diffusion is carried out. However, in some cases, when the process is running at low temperature, photoresistive masks can be basic - working. An example is the process of creating metal wiring in semiconductor ICs.

When using a photomask, its emulsion layer wears out (erases) after 15-20 applications. The service life of photomasks can be increased by two orders of magnitude or more by metallization: replacing the emulsion film with a film of wear-resistant metal, usually chromium.

Photomasks are made in sets according to the number of photolithography operations in the technological cycle. Within the set, the photomasks are coordinated, i.e., they ensure the alignment of the patterns when the corresponding marks are aligned.

3.4.2 Electrolithography. The described methods have long been one of the foundations of microelectronic technology. They still have not lost their significance. However, as the degree of integration increased and the size of IS elements decreased, a number of problems arose, which have already been partially resolved, and partially are under study.

One of the fundamental limitations concerns resolution, i.e. the minimum dimensions in the generated mask pattern. The fact is that the wavelengths of ultraviolet light are 0.3-0.4 microns. Therefore, no matter how small the hole in the photomask pattern is, the dimensions of the image of this hole in the photoresist cannot reach the specified values ​​(due to diffraction). Therefore, the minimum width of the elements is about 2 microns, and at deep ultraviolet (wavelength 0.2-0.3 microns) - about 1 micron. Meanwhile, sizes of the order of 1–2 μm are already not small enough when creating large and super-large ICs.

The most obvious way to increase the resolution of lithography is to use shorter wavelength radiation during exposure.

In recent years, methods have been developed electronic lithography . Their essence is that a focused beam of electrons scan(i.e., they are moved “line by line”) over the surface of the plate coated with an electron resist, and the beam intensity is controlled in accordance with a given program. At those points that should be "illuminated", the beam current is maximum, and at those that should be "darkened", it is equal to zero. The diameter of the electron beam is in direct proportion to the current in the beam: the smaller the diameter, the lower the current. However, as the current decreases, the exposure time increases. Therefore, an increase in resolution (a decrease in the beam diameter) is accompanied by an increase in the duration of the process. For example, with a beam diameter of 0.2-0.5 μm, the scanning time of the wafer, depending on the type of electronoresist and the size of the wafer, can range from tens of minutes to several hours.

One of the varieties of electron lithography is based on the rejection of electron-resistive masks and involves the action of an electron beam directly on the oxide layer of SiO2. It turns out that in the places of "flare" this layer is subsequently etched several times faster than in the "darkened" areas.

The minimum dimensions for electron lithography are 0.2 µm, although the maximum achievable is 0.1 µm.

Other methods of lithography are under investigation, for example, soft X-rays (with wavelengths of 1-2 nm) allow obtaining minimum dimensions of 0.1 µm, and ion-beam lithography 0.03 µm.

3.5 Doping

The introduction of impurities into the original wafer (or into the epitaxial layer) by diffusion at high temperature is the initial and still the main method of doping semiconductors in order to create transistor structures and other elements based on them. However, recently another method of doping, ion implantation, has become widespread.

3.5.1 Diffusion methods. Diffusion can be general and local. In the first case, it is carried out over the entire surface of the plate (Figure 3.4a), and in the second - in certain areas of the plate through windows in the mask, for example, in a thick layer of SiO2 (Figure 3.4b) .

General diffusion leads to the formation of a thin diffusion layer in the wafer, which differs from the epitaxial layer by an inhomogeneous (in depth) impurity distribution (see N(x) curves in Figures 3.6a and b).

Figure 3.4 - General and local diffusion

In the case of local diffusion (Figure 3.4b), the impurity spreads not only into the depth of the plate, but also in all perpendicular directions, i.e. under the mask. As a result of this so-called lateral diffusion, the region of the p-n transition that comes to the surface turns out to be “automatically” protected by oxide . The ratio between the depths of the lateral and main -

"vertical" diffusion depends on a number of factors, including the depth of the diffusion layer . A value of 0.8×L can be considered typical for the depth of lateral diffusion .

Diffusion can be carried out once or repeatedly. For example, during the 1st diffusion, an acceptor impurity can be introduced into the original n-type plate and a p-layer can be obtained, and then, during the 2nd diffusion, a donor impurity can be introduced into the resulting p-layer (to a shallower depth) and thereby provide a three-layer structure. Accordingly, a distinction is made between double and triple diffusion (see Section 4.2).

When carrying out multiple diffusion, it should be borne in mind that the concentration of each new impurity introduced must exceed the concentration of the previous one, otherwise the type of conductivity will not change, which means that a p-n junction is not formed. Meanwhile, the impurity concentration in silicon (or other source material) cannot be arbitrarily large: it is limited by a special parameter - limit impurity solubilityNS. The limiting solubility depends on the temperature. At a certain temperature, it reaches a maximum value, and then decreases again. The maximum limiting solubilities, together with the corresponding temperatures, are given in Table 3.1.

Table 3.1

Therefore, if multiple diffusion is carried out, then for the last diffusion it is necessary to choose a material with the maximum limiting solubility. Since the range of impurity materials is limited,

it is not possible to provide more than 3 consecutive diffusions.

Impurities introduced by diffusion are called diffusers(boron, phosphorus, etc.). The sources of diffusants are their chemical compounds. These can be liquids (ВВr3, ROSl), solids (В2О3, P2O5) or gases (В2Н6, РН3).

The incorporation of impurities is usually carried out by means of gas transport reactions, in the same way as in epitaxy and oxidation. For this, either single-zone or two-zone diffusion ovens.

Two-zone ovens are used in the case of solid diffusants. In such furnaces (Figure 3.5) there are two high-temperature zones, one for evaporation of the diffusant source, the second for diffusion itself.

Figure 3.5 - Diffusion process

The diffusant source vapors obtained in the 1st zone are mixed with the flow of a neutral carrier gas (for example, argon) and together with it reach the 2nd zone, where the silicon wafers are located. The temperature in the 2nd zone is higher than in the 1st. Here, the diffusant atoms are introduced into the plates, while other components of the chemical compound are carried away by the carrier gas from the zone.

In the case of liquid and gaseous diffusant sources, there is no need for their high-temperature evaporation. Therefore, single-zone furnaces are used, as in epitaxy, into which the diffusant source enters already in a gaseous state.

When using liquid sources of diffusant, diffusion is carried out in an oxidizing environment by adding oxygen to the carrier gas. Oxygen oxidizes the silicon surface, forming oxide SiO2, i.e., in essence, glass. In the presence of a diffusant (boron or phosphorus), borosilicate or phosphosilicate glass. At temperatures above 1000°C, these glasses are in a liquid state, covering the silicon surface with a thin film. , so that the diffusion of the impurity proceeds, strictly speaking, from the liquid phase. After solidification, the glass protects the silicon surface at diffusion points,

i.e. in oxide mask windows. When using solid sources of diffusant - oxides - the formation of glasses occurs in the process of diffusion without specially introduced oxygen.

There are two cases of impurity distribution in the diffusion layer.

1 The case of an unlimited source of impurity. In this case, the diffusant continuously flows to the plate, so that the impurity concentration in its near-surface layer is maintained constant equal to NS. As the diffusion time increases, the depth of the diffusion layer increases (Figure 3.6a).

2 The case of a limited impurity source. In this case, first a certain amount of diffusant atoms is introduced into the thin near-surface layer of the plate (time t1), and then the diffusant source is turned off and the impurity atoms are redistributed over the depth of the plate with their total number unchanged (Figure 3.6b). In this case, the impurity concentration on the surface decreases, while the depth of the diffusion layer increases (curves t2 and t3). The first stage of the process is called "forcing", the second - "distillation" of the impurity.

Figure 3.6 - Diffuser distribution

3.5.2 Ion implantation.

Ion implantation is a method of doping a wafer (or epitaxial layer) by bombarding impurity ions accelerated to an energy sufficient for their penetration into the depth of a solid.

The ionization of impurity atoms, the acceleration of ions, and the focusing of the ion beam are carried out in special facilities such as particle accelerators in nuclear physics. The same materials are used as impurities as in diffusion.

The depth of ion penetration depends on their energy and mass. The greater the energy, the greater the thickness of the implanted layer. However, as the energy increases, so does the amount radiation defects in the crystal, i.e., its electrical parameters deteriorate. Therefore, the ion energy is limited to 100–150 keV. The lower level is 5-10 keV. With such an energy range, the depth of the layers lies in the range of 0.1 - 0.4 μm, i.e., it is much less than the typical depth of diffusion layers.

The impurity concentration in the implanted layer depends on the current density in the ion beam and the process time, or, as they say, on expo time-positions. Depending on the current density and the desired concentration, the exposure time ranges from a few seconds to 3-5 minutes or more (sometimes up to

1-2 hours). Of course, the longer the exposure time, the greater the number of radiation defects.

A typical impurity distribution during ion implantation is shown in Figure 3.6c as a solid curve. As we can see, this distribution differs significantly from the diffusion distribution by the presence of a maximum at a certain depth.

Since the area of ​​the ion beam (1-2 mm2) is less than the area of ​​the plate (and sometimes the crystal), one has to scan beam, i.e. move it smoothly or “in steps” (with the help of special deflecting systems) one by one along all the “rows” of the plate, on which individual ICs are located.

Upon completion of the alloying process, the plate must be subjected to annealing at a temperature of ° C in order to order the silicon crystal lattice and eliminate (at least partially) the inevitable radiation defects. At the annealing temperature, diffusion processes somewhat change the distribution profile (see the dashed curve in Figure 3.6c).

Ion implantation is carried out through masks, in which the ion path must be much shorter than in silicon. The material for masks can be silicon dioxide or aluminum common in ICs. At the same time, an important advantage of ion implantation is that ions, moving in a straight line, penetrate only into the depth of the plate, and there is practically no analogy with lateral diffusion (under the mask).

In principle, ion implantation, like diffusion, can be carried out repeatedly by "embedding" one layer into another. However, the combination of energies, exposure times, and annealing modes required for multiple implantation turns out to be difficult. Therefore, ion implantation has received the main distribution in the creation of thin single layers.

3.6 Deposition of thin films

Thin films are not only the basis of thin-film hybrid ICs, but are also widely used in semiconductor integrated circuits. Therefore, methods for obtaining thin films are among the general issues of microelectronics technology.

There are three main methods for depositing thin films on a substrate and on top of each other: thermal(vacuum) and ion-plasma spraying, which has two varieties: cathode sputtering and actually ion-plasma.

3.6.1 Thermal (vacuum) spraying.

The principle of this sputtering method is shown in Figure 3.7a. A metal or glass cap 1 is located on the base plate 2. Between them there is a gasket 3, which ensures that the vacuum is maintained after the air has been evacuated from the cap space. The substrate 4, on which the deposition is carried out, is fixed on the holder 5 . Adjacent to the holder is heating (sputtering is carried out on a heated substrate). The evaporator 7 includes a heater and a source of spray material. Rotary damper 8 blocks the vapor flow from the evaporator to the substrate: the deposition lasts for the time when the damper is open.

The heater is usually a filament or spiral made of a refractory metal (tungsten, molybdenum, etc.), through which a sufficiently large current is passed. The source of the sprayed substance is associated with the heater in different ways: in the form of brackets ("hussar"), hung on the filament; in the form of small rods covered by a spiral, in the form of a powder, poured into

Figure 3.7 - Application of films

a crucible heated by a spiral, etc. Instead of filaments, heating with the help of an electron beam or a laser beam has recently been used.

The most favorable conditions for vapor condensation are created on the substrate, although partial condensation also occurs on the walls of the hood. Too low substrate temperature prevents the uniform distribution of adsorbed atoms: they are grouped into "islands" of different thicknesses, often not connected with each other. On the contrary, too high a substrate temperature leads to detachment of newly settled atoms, to their "re-evaporation". Therefore, to obtain a high-quality film, the temperature of the substrate must lie within certain optimal limits (usually 200–400°C). The film growth rate, depending on a number of factors (substrate temperature, distance from the evaporator to the substrate, type of deposited material, etc.), ranges from tenths to tens of nanometers per second.

The bond strength - adhesion of a film to a substrate or other film - is called adhesion. Some common materials (such as gold) have poor adhesion to typical substrates, including silicon. In such cases, the so-called underlayer, which is characterized by good adhesion, and then a base material is sprayed onto it, which also has good adhesion to the sublayer. For example, for gold, the sublayer may be nickel or titanium.

In order for the atoms of the deposited material flying from the evaporator to the substrate to experience the minimum number of collisions with the atoms of the residual gas and, thereby, the minimum scattering, a sufficiently high vacuum must be provided in the space under the cap. The criterion for the required vacuum can be the condition that the mean free path of atoms is several times greater than the distance between the evaporator and the substrate. However, this condition is often insufficient, since any amount of residual gas is fraught with contamination of the deposited film and changes in its properties. Therefore, in principle, the vacuum in thermal spray installations should be as high as possible. The vacuum is currently below 10-6 mmHg. Art. is considered unacceptable, and in a number of first-class sputtering installations it has been brought up to 10-11 mm Hg. Art.

The principle of formation of microcircuit structures. Electronic vacuum hygiene

Basic principles of integrated technology. The principle of locality. The principle of layering. Dusty air environment. Temperature and humidity of the air. Cleanliness of premises and local volumes. Modular clean rooms.

Water, gases and gaseous media used in the production of ICs

The need to use clean water, gas and gas mixtures. Cleanliness of equipment, premises and personal hygiene of workers.

Requirements for technological processes. Requirements for the conditions for the production of microelectronic devices

Reliability. Profitability. Safety. Manufacturability. The need to develop design and technological documentation.

Preparing ingots and cutting them into wafers

Ingot orientation. Formation of the base cut. Cutting ingots into plates.

Machining of plates. Abrasive materials and tools

Necessity and essence of machining of plates. Abrasive materials and tools used in grinding and polishing plates.

Grinding and chamfering, polishing plates

Plate grinding. Plate polishing. Chamfer removal. Methods and technology

9Quality control of wafers and substrates after machining

Measurement of the geometric dimensions of plates after machining. Surface quality control of plates. Measurement of the height of microroughnesses on the plate.

10Cleaning the plates. Methods and means

Classification of contaminants and cleaning methods. Degreasing by immersion, jet, etc. Methods for monitoring the surface cleanliness of plates.

11Chemical treatment and cleaning of the surface of the plates. Intensification of cleaning processes

Degreasing in solvents, degreasing in solvent vapors, degreasing in detergent powders, in alkalis, in peroxide-ammonia solutions. Ultrasonic degreasing, hydromechanical cleaning, jet cleaning, boiling, etc.

Plate etching

Silicon etching kinetics. Selective and polishing etching. Dependence of the etching rate on the properties of the materials used.

13Dry cleaning. Gas discharges at low pressure

Spray coefficient. Distinctive features of etching. Ion-beam etching.

14Plasma etching methods

Physics of the ion etching process. Surface spray efficiency. Etching in diode and triode chambers. Features of their designs, advantages and disadvantages.

15Ion-plasma and ion-beam etching.

Reactive methods of plasma etching: ion-beam and ion-plasma etching. Plasma etching using gas-containing mixtures.

16Plasma chemical etching, reactive ion etching

plasma etching. Radical plasma-chemical etching. Reactive ion-plasma etching and ion-beam etching Etching anisotropy and selectivity.

17Factors determining the rate and selectivity of etching

Energy and angle of incidence of ions. The composition of the working gas. Pressure, power density and frequency. Flow rate. The temperature of the treated surface.

18Quality control of wafers and substrates

Plate surface control. Surface cleaning quality control (glowing point method, drop method, tribometric method, indirect method).

19Photolithography. Photoresists. Photolithography operations

active resists. Photochemical processes occurring in the photoresist upon irradiation of negative and positive photoresists. Features of operations for obtaining a pattern on a photoresistive film.

20Technology of photolithographic operations

Methods and essence of the operation of photolithography. Photoresistive film processing modes and the need for their exact observance.

21Non-contact photolithography. Limitations of contact photolithography. Projection photolithography

Microgap photolithography. Projection photolithography with 1:1 image transmission and image reduction. Physical and technical limitations of contact photolithography.

22Thermal vacuum deposition

The formation of a vapor of a substance. Propagation of vapor from the source to the substrates. Vapor condensation on the substrate surface. Formation of a thin film. Thermal vacuum spraying technique. Advantages and disadvantages of the method.

Variants of methods for obtaining oxide films on silicon wafers

Thermal oxidation at elevated pressure. Thermal oxidation with the addition of hydrogen chloride vapors. Choice of regimes and conditions for growing thermal oxide.

26Properties of silicon dioxide

Structure of silicon dioxide Factors affecting the porosity of silicon dioxide.

Metallization of structures

Requirements for ohmic contacts, current-carrying tracks and pads. Technology and features of metallization of structures.

Preparation of semiconductor structures for assembly

Control of finished structures by electrical parameters. Bonding plates to an adhesive carrier. Requirements for the process of separating wafers into crystals. Diamond and laser scribing of plates and substrates. Scribing plates with a diamond cutter. Features of the process, advantages and disadvantages.

61 Oriented plate separation methods

Separation of plates into crystals with preservation of their orientation. Features of the technological process. Advantages and disadvantages of disc cutting. Breaking plates. Separation of plates without the use of further breaking

Shatalova V.V.

Questions prepared by the teacher

1. Malysheva I.A. Technology for the production of integrated circuits. - M .: Radio and communication, 1991

2. Zee S. VLSI technology. - M.: Mir, 1986

3. Till U., Lakson J. Integrated circuits, materials, devices, manufacturing. – M.: Mir, 1985.

4. Maller R., Keimins T. Elements of integrated circuits. – M.: Mir, 1989.

5. Koledov L.A. Technology and designs of microcircuits, microprocessors and microassemblies - M .: Lan-press LLC, 2008.

6. Onegin E.E. Automatic IC assembly - Minsk: Higher school, 1990.

7. Chernyaev V.N. Technology of production of integrated circuits and microprocessors. - M .: Radio and communication, 1987

8. Parfenov O.D. Microchip technology, - M .: Higher school, 1986.

9. Turtsevich A.S. Films of polycrystalline silicon in the technology of production of integrated circuits and semiconductor devices. - Minsk: Bel science, 2006.

10. Shchuka A.A. Nanoelectronics. – M.: Fizmatkniga, 2007.

General characteristics of microcircuit production technology

Basic concepts. Classification and characteristics of integrated circuits (ICs). The main stages of IC manufacturing technology, their purpose and role. Principles of integrated technology, methods for manufacturing microcircuit structures, features of IC production technology.

The main technological processes used in the manufacture of semiconductor integrated circuits are oxidation, photolithography, diffusion, epitaxy, and ion doping.

Silicon oxidation. This process is of great importance in the technology of manufacturing semiconductor integrated circuits. Silicon dioxide Si0 2 is a glassy oxide having the same chemical composition as quartz glass. These oxides are good insulators for individual circuit elements, serve as a mask that prevents the penetration of impurities during diffusion, are used to protect the surface and create active dielectric elements (for example, in MOSFETs). They form a uniform continuous coating on the silicon surface, which is easily etched and removed from local areas. Re-oxidation provides protection P-N-transition from environmental influences. The thermal expansion coefficients of silicon and silicon dioxide are close. Silicon dioxide has good adhesion and is relatively easy to create on the wafer surface.

Depending on the method of preparation, thermal and anodic oxides are distinguished.

Thermal oxides are obtained by heating-accelerated reactions of silicon with oxygen and other substances containing oxygen. Such oxides are ~1 µm thick and have a high density.

The thermal oxidation method has two varieties:

1) high-temperature oxidation in a stream of dry oxygen and humidified gases;

2) oxidation in water vapor at high pressure (up to 50 MPa), at relatively low temperatures (5OO...900°C).

Oxidation in a stream of humidified gases performed according to Fig. 1.8. Silicon wafers are placed in a quartz tube, where the temperature is set to 1100°C. One end of the pipe is connected to a humidifier (deionized water), through which gas (argon, nitrogen, etc.) is passed. When the humidifier is turned off, dry oxygen enters directly into the quartz tube. Oxidation is carried out in the following sequence: preliminary holding in dry oxygen (~15 min); long-term oxidation in moist oxygen (2 h) and final oxidation in dry oxygen. The first operation gives a strong film of small thickness. Thermal treatment in humid oxygen provides rapid film growth (up to 1 μm), but its density is insufficient. The subsequent treatment in dry oxygen leads to a densification of the film and an improvement in its structure.

The most commonly used oxide thickness is tenths of a micrometer, and the upper limit in thickness is 1 µm. The addition of chlorine-containing components to the oxidized medium increases the rate of oxidation and increases the breakdown intensity. The main role of chlorine is the transformation of impurity atoms (potassium, sodium, etc.) that accidentally got into silicon dioxide into electrically inactive ones.

Oxidation of silicon in water vapor at high pressure is carried out in a chamber, the inner surface of which is coated with gold or other inert metal to avoid unwanted reactions. Silicon wafers and a certain amount of high purity water are placed in the chamber, which is heated to the oxidation temperature (500...800°C). The film thickness depends on the duration of oxidation, pressure and water vapor concentration.

The quality of the oxide film is affected by the purity of the working volume in which the process is performed. The ingress of even an insignificant amount of impurity atoms can significantly change the properties of the material of the original workpiece. The most harmful effect is exerted by copper impurities, whose diffusion coefficient in silicon is very high.

Of great importance is the preoxidative purification of silicon from contaminants leading to discontinuities in films. The advantage of high pressure oxidation is the possibility of lowering the process temperature without increasing the duration.

Anode oxidation silicon has two modifications: oxidation in a liquid electrolyte and in a gas plasma. The anodic oxidation process makes it possible to obtain oxide films at lower temperatures, which limits the redistribution of impurities in preformed diffusion regions.

To create interlayer insulation, the oxidation process is not used, and the dielectric layers are obtained by deposition.

Silicon dioxide films as protective layers have the following disadvantages: 1) structure porosity, which leads to the possibility of water vapor and some impurities penetrating to the original silicon surface; 2) the ability of atoms of a number of elements to migrate through a silicon dioxide film, which leads to instability in the characteristics of semiconductor devices.

Photolithography. Photolithography is the process of forming a photoresist image of the circuit topology on the substrate dioxide surface and then transferring it to the substrate. In structure, it coincides with the methods used in the formation of printed circuit board conductors. However, this process has its own specifics, due to the requirements of high resolution and increased requirements for the quality of the materials used and the cleanliness of the environment.

Photoresists are thin films of organic solutions, which should have the properties, after exposure to ultraviolet light, to polymerize and become insoluble. The main requirements for photoresists are high resolution, light sensitivity, resistance to etchants and various chemical solutions, good adhesion to the surface of the product.

The resolution of a photoresist is the number of lines that can be applied to one millimeter of the board surface with a distance between them equal to their width. The resolution depends on the type of photoresist and the layer thickness. With thin layers, it is greater than with thick ones.

According to the way the pattern is formed, photoresists are divided into negative and positive (Fig. 1.9).

Areas of negative photoresist, which are under the transparent areas of the photomask, under the action of ultraviolet light, get the property not to dissolve during development. The areas of the photoresist located under the opaque areas of the photomask are easily removed when developing in a solvent. Thus is created; relief, which is an image of the light elements of the photomask (Fig. 1.9, a).

Negative photoresists are made from polyvinyl alcohol. They are widely used due to the absence of toxic components, acceptable resolution (up to 50 lines/mm), ease of development and low cost. The disadvantage is the impossibility of storing more than 3 ... 5 hours of blanks with a deposited layer, since the latter is hardened even in the dark. In addition, with a decrease in humidity and ambient temperature, the mechanical strength of the photosensitive layer and its adhesion to the surface decrease.

A positive photoresist under the action of irradiation changes its properties in such a way that during processing, its irradiated areas dissolve in developers, and non-irradiated areas (located under the opaque areas of the photomask) remain on the board surface (Fig. 1.9, b).

For positive photoresists, materials based on diazo compounds are used, which consist of a photosensitive polymer base (novolac resin), a solvent, and some other components. In terms of adhesion and resolution, they are superior to negative photoresists, but they are more expensive and contain toxic solvents. The resolution of positive photoresists is up to 350 lines/mm. The advantage of a positive photoresist is the absence of tanning during storage of blanks with a photosensitive layer applied.

In the technological process of IC production, liquid and dry photoresists are used.

Liquid photoresists are applied by dipping (dipping), pouring with centrifugation, rolling with a ribbed roller, and other methods.

Dry photoresists, which have become more widespread due to their greater manufacturability and ease of use, are a thin structure of three layers: an optically transparent film (usually polyethylene terephthalate), a photosensitive polymer, and a protective lavsan film. They are applied at an elevated temperature with the preliminary removal of the protective layer and gluing of the photoresist. After the pattern is exposed, the optical film is removed and the image is developed in water. In this case, the unexposed areas of the picture are removed.

The high resolution of the circuit pattern is provided by positive photoresists. However, their advantages do not exclude the possibility of using negative photoresists, which are more acid-resistant and easier to develop.

The main stages of the photolithography process in the implementation of contact printing are shown in Fig. 1.10.

The preparation of the substrate surface (Fig. 1.10, a) significantly affects the adhesion of the photoresist. The latter should be applied immediately after the plate is oxidized without any additional surface treatments. If the substrates are stored for more than an hour, then heat treatment is performed in dry oxygen or nitrogen at t=1000°C for several minutes. It eliminates the hydrophilicity of the substrate surface.

The photo-resist is applied by centrifugation (Fig. 1.10.6). The optimal thickness of the photoresist layer is in the range of 0.3...0.8 µm. When the layer thickness is less than 0.2 μm, the probability of punctures increases sharply, and at thicknesses greater than 1 μm, the resolution of the process decreases, which makes it impossible to obtain elements with small dimensions.

When applying a photoresist, it is necessary to ensure the uniformity of the layer (the absence of pores, foreign particles, etc.) and its uniformity in thickness. The homogeneity of the layer depends on the purity of the initial photoresist, the purity of the environment, modes and method of drying. The uniformity of the layer thickness depends on the viscosity of the photoresist and the modes of its deposition. The unevenness of the layer in thickness is the reason for the deterioration of the contrast due to the incomplete fit of the photomask to the photolayer during exposure.

Removal of the solvent from the photoresist layer to form a strong and homogeneous film is carried out by drying at t =18...20°C for 15...30 min, and then at t=90...100°C for 30 min.

The transfer of an image from a photomask to a plate covered with a layer of photoresist is realized by exposure (Fig. 1.10, c). If the process of photolithography is repeated, then it is necessary to combine the previously obtained pattern with the pattern on the photomask. The alignment accuracy is 0.25 ... 0.5 µm. Xenon and mercury-quartz lamps are used as a light source.

The transfer quality is significantly affected by diffraction phenomena that occur when there are gaps between the template and the plate. The gaps arise due to the non-flatness of the substrate, reaching 20 μm. The quality of image transfer from the photomask to the photoresist layer can only be assessed after development.

The development of a latent image (Fig. 1.10, d) in a negative photoresist consists in the removal of areas that were under the dark places of the photomask. In the case of a positive photoresist, the irradiated areas are removed. Negative photoresists are shown in organic solvents (trichlorethylene, etc.), and positive - in alkaline solutions. To improve the protective properties, the resulting layer is dried at t=100...120°C, and then tanned at t=200...250°C for 30...40 min.

The required pattern of the circuit is obtained by etching the areas of the substrate not protected by the photoresist in a mixture of nitric and hydrofluoric acid (Fig. 1.10, e).

Etching should ensure complete etching of oxide films. In this case, there are cases when it is necessary to simultaneously etch oxide films of different thicknesses. The accuracy of etching operations depends on the accuracy of the negative and the quality of the photoresist. In the case of poor adhesion of the layer to the surface of the workpiece, hydrofluoric acid can penetrate under the tanned layer and etch out the areas of the oxide film protected by it. The photoresist layer remaining on the surface is removed in a solvent, which is used as organic liquids and sulfuric acid. After swelling, the photoresist films are removed with a swab.

Photolithography is one of the main technological processes in the production of semiconductor microcircuits. Its widespread use is explained by its high reproducibility and resolution, which makes it possible to obtain a pattern of small sizes, the versatility and flexibility of the method, and high productivity. The disadvantage of contact photolithography is the rapid wear of the photomask and the occurrence of defects on the contact surfaces. Upon contact, the photomask presses any particles (such as dust particles) into the photoresist layer, which lead to defects in the protective layer of the photoresist.

A grain of dust on the surface of the photoresist can prevent its hardening and lead to the formation of a hole (“puncture”) in the oxide. A speck of dust or some dark dots on the transparent part of the photomask can give the same effect. A hole in the darkened part of the photomask may lead to incomplete removal of the oxide film. The sizes of dust particles are commensurate with the sizes of areas of contact elements. Their presence leads to the marriage of the microcircuit.

The probability of defects appearing as a result of insoluble dust particles and other point contaminants entering the silicon surface is proportional to the area of ​​the wafer. The presence of such defects limits the maximum area of ​​microcircuits.

Non-contact (projection) photolithography eliminates contact between the photomask and the photoresist layer, which makes it possible to avoid a number of disadvantages inherent in contact photolithography.

The method of projection printing consists in projecting an image from a photomask onto a plate covered with a layer of photoresist, placed at a considerable distance from each other. The dimensions of the picture on the photomask can be made on an enlarged scale. With this method, the requirements for the flatness of the substrates and the uniformity of the thickness of the photoresist layer are increased. High demands are placed on the lens, which must provide the required resolution over the entire working field of the substrate. At present, the best resolution (0.4 µm) can be obtained on an area of ​​2x2 mm. Difficulties in creating lenses that provide high resolution over a large area hinder the widespread introduction of the method of projection photolithography.

Microgap photolithography combines the advantages of contact and projection methods of photolithography. With this method, a gap of 10 ... 20 microns is established between the plate and the photomask. Such a gap is large enough to minimize the phenomenon of diffraction, and at the same time small enough to neglect non-linear distortions in the gap during image transmission. Industrial micro-gap exposure equipment is much more complex than contact exposure equipment.

Diffusion. This is the process of transferring dopants from areas of higher concentration to areas of lower concentration. If there is a concentration gradient of atoms of any element in a solid, then a directed diffusion motion is created, which tends to equalize the concentration of these atoms throughout the volume. The processes of concentration equalization occur at sufficiently high temperatures, when the particle velocity increases sharply. They are characterized by the diffusion coefficient D, which is determined by the mass of a substance penetrating through a single area per unit of time with a concentration gradient equal to one.

The diffusion coefficient for a certain material and diffusible impurity in the first approximation depends only on temperature (exponential dependence).

The diffusion coefficient of group III elements (B, A1, Ip) into silicon is 1 ... 1.5 orders of magnitude higher than that of group V elements (As; P; Sb). For example, the diffusion coefficient of boron into silicon at t == 1473 K is 10.5 cm 2 /s, arsenic - 0.3 cm 2 /s.

The diffusion process is carried out in two stages. At the first stage, an impurity-saturated layer is created on the crystal from an infinite source (gas phase). This stage is called impurity driving. It is carried out in the presence of oxygen, which contributes to the formation of a layer of borosilicate glass (for B 2 0 3 impurities) or phosphorus-silicate glass (for P 2 O 5 impurities) on the surface. The parameters of the driving process are the concentration of the diffusant and oxygen in the carrier gas, the velocity of the gas mixture and the process time. At the second stage, the admixture undergoes redistribution. This stage is called impurity dispersal. It is performed at t = 800...1000°C in the absence of an external source of impurity. The working atmosphere is a mixture of an inert gas and oxygen. The dispersal of the impurity into the depth of the wafer is accompanied by the growth of a protective silicon oxide film.

Diffusion is carried out in the temperature range of 1100...1300°C, and taking into account the driving process in a two-stage process -1000...1300°. Below 1000 °C, the diffusion coefficients are very small and the diffusion depth is negligible. Above 1300°C, violations of the surface of the plates occur under the action of high temperature.

Solid, liquid and gaseous compounds are used as impurity sources. Boron and phosphorus are most often used in the form of chemical compounds B 2 0 5, P 2 O 5, etc.

Diffusion in a carrier gas flow from a solid source is performed in two-zone installations (Fig. 1.11). The source of impurities is placed in the low-temperature zone, and silicon wafers are placed in the high-temperature zone (1100 ... 1200 ° C). The pipe is purged with a mixture of an inert gas with oxygen, and after establishing the temperature regime, the plates are placed in the working area. The evaporating impurity molecules are carried by the carrier gas to the plates and through the layer of liquid glass fall on their surfaces. Liquid glass protects the surfaces of the plates from evaporation and ingress of foreign particles. Disadvantages of the process of diffusion from a solid source - the complexity of the installation and the difficulty of controlling the vapor pressure.

Diffusion in a carrier gas flow from a liquid source is carried out on a simpler single-zone setup, where it is possible to obtain a wider range of surface concentrations. The disadvantage of such a process is the high toxicity of concentrations.

Diffusion in a closed volume. Such diffusion provides good reproducibility of the parameters of the diffusion layers. In this case, the silicon wafer and the source of impurities are placed in a quartz ampoule, which is pumped out to a pressure of 10 -3 Pa or filled with an inert gas. Then the ampoule is sealed and placed in a heating furnace. Impurity vapor molecules are adsorbed by the surfaces of the semiconductor wafer and diffuse into its depth. This method is used for the diffusion of boron, antimony, arsenic, phosphorus. These impurities are highly toxic, and diffusion in the ampoule eliminates the possibility of poisoning.

The advantage of the method is the possibility of using one oven for diffusion of several impurities without their mutual contamination, the disadvantage is low productivity and the need for careful loading process, since any substance that enters the ampoule diffuses together with the main impurity.

For all diffusion methods, it is necessary to ensure a uniform temperature distribution along the axis of the hot zone. If the tolerance on the depth of the diffusion layer is 100%, then it is sufficient to maintain the temperature with an accuracy of ±5°C. With a tolerance of 20%, the temperature must be maintained with an accuracy of ± 0.5 ° C.

The diffusion depth varies from a few micrometers (for circuit elements) to 10 ... 100 microns for their isolation. A large diffusion depth requires a significant time (up to 60 h).

Impurities diffusing into silicon through a hole in the oxide propagate laterally by almost the same amount as in depth.

The most common diffusion defects are deviations in the depth of the diffusion layer. The reasons for such deviations are dust and other particles on the surface of the plate, as well as residual photoresist. Surface defects and disturbances in the crystal lattice contribute to a deeper penetration of the diffusant into the material. To reduce the number of such defects, it is necessary to carefully observe the cleanliness of the environment, materials and equipment during the preparatory operations and during the diffusion process.

Receipt P-N-transitions using diffusion methods allows you to accurately control the depth and location of the transition, the concentration of impurities, etc. The disadvantage of the diffusion process is the impossibility of obtaining clear transitions between regions with different types of conductivity.

Epitaxy. This is the process of growing layers with an ordered crystal structure by implementing the orienting action of the substrate. In the production of integrated circuits, two types of epitaxy are used: homoepitaxy and heteroepitaxy.

Homoepitaxy (autoepitaxy) is a process of oriented growth of a crystalline substance that does not differ in chemical composition from the substrate substance. Heteroepitaxy is a process of oriented growth of a substance that differs in chemical composition from the substrate material.

In the process of growing an epitaxial film, dopants can be introduced into it, creating semiconductor films with the desired concentration distribution and a given type of conductivity. This makes it possible to obtain clear boundaries between regions with different types of conductivity.

The most widespread at present is the so-called chloride method for obtaining epitaxial silicon layers, based on the reduction of silicon tetrachloride. The process is carried out in a reactor, which is a quartz tube placed in the inductor of an RF generator. Reactors can be of horizontal and vertical type.

In a horizontal reactor (Fig. 1.12), silicon wafers are placed on graphite supports. Heating is carried out by a high-frequency generator. Before starting the process, the system is filled with nitrogen or helium to remove air and purged with pure hydrogen, which at a temperature of 1200°C reacts with the remains of oxide films on the surface of the substrates and almost completely removes them. The chamber is then filled

mixture HC1 and H 2 for etching a layer several micrometers thick from a silicon wafer. The gas etching operation removes the damaged layer and residues Si0 2. Epitaxial films are obtained without structural defects. After cleaning, the system is purged with hydrogen for several minutes, then SiCl4 and dopant. As a result of the reaction

5iС1 4(gas) + 2H 2(gas) ↔ Si(HARD) ↓ + 4HC1(GAS)

Silicon tetrachloride decomposes and silicon is deposited on the silicon substrate, which takes on the structure of the underlying layer. After the end of the process, the substrate is cooled with a stream of pure hydrogen.

Certain ratios of hydrogen, silicon chloride and impurities are achieved by controlling the feed rate and temperature. The typical flow rate of the carrier gas (hydrogen) is 10 L/min, and the ratio between the amount H 2 and SiCl4 is 1000: 1. A gaseous diffusant is introduced into this mixture in an amount of approximately 300 parts per 1,000,000 parts of the gas mixture.

Phosphine is used as a donor impurity. (RN 3), and to get the layer P-type - diborane (B 2 H 6).

The growth rate of the epitaxial film depends on the consumption SiCl4 and H 2 substrate temperature, the amount of impurity introduced, etc. These variables, which can be controlled quite accurately, determine the duration of the process.

The smallest thickness of the epitaxial film is determined by the presence of crystallization centers. The upper limit of the film thickness free from defects is 250 µm. Most often, the thickness of the epitaxial film is from 1 to 25 µm.

The quality of the epitaxial layer is greatly influenced by the purity of the substrate surface and the gases used. Silicon wafers 150...200 µm thick, free from structural defects, are used as the substrate. The permissible content of impurities in gases is equal to several parts of impurities per million parts of gas.

The control of semiconductor wafers is carried out after finishing polishing, epitaxy, oxidation and diffusion. It is based on visual observation and analysis of the plate image formed on the screen by a homocentric beam of visible light reflected from the plate surface.

Parts of the wafer with a broken structure introduce perturbations into the light beam, due to which wafer defects are visible on the screen as changes in the light intensity in the wafer image, making it possible to evaluate its quality.

Sputtering of thin films. The main methods for obtaining thin films are thermal spraying (evaporation) in vacuum and ion sputtering.

Thermal spraying in vacuum. Such deposition is based on the property of atoms (molecules) of metals and some other materials during evaporation in high vacuum conditions to move in a straight line (beam-like) and deposit on a surface placed in the path of their movement.

Vacuum sputtering installation (Fig. 1.13) consists of a flat plate 6, on which a glass or metal cap is installed 9. In the latter case, it is supplied with a viewing glass. The plate has two insulated vacuum-tight outlets. 4 to power the evaporator 3. A substrate is placed at some distance from the evaporator 10, onto which a thin film is applied. The substrate heats up and is closed by a damper until the set mode is reached. 1.

In accordance with the physical processes that occur during evaporation in a vacuum, the following stages of film formation can be distinguished: 1) transfer of the deposited material into a vapor state; 2) vapor transfer from the source of evaporation to the substrate; 3) vapor condensation on the substrate and film formation.

Transfer of the sprayed material into a vapor state. In the area of ​​vapor production, the material evaporates and heats up until its vapor pressure exceeds the pressure of the residual gases. In this case, the most heated molecules with high kinetic energy overcome the forces of molecular attraction and break away from the surface of the melt. Due to the sharply reduced heat transfer under high vacuum conditions, overheating of the substrates does not occur.

For some materials, the nominal evaporation temperature is lower than the melting point. For example, chromium has a melting point of 1800°C, and evaporates when heated in vacuum at a temperature of 1205°C. The transition of a substance from a solid state to a vapor state without going through a liquid state is called sublimation.

Transfer of vapor from the source of evaporation to the substrate. The area of ​​vapor transfer is 10...20 cm. In order for the trajectories of the molecules of the evaporated substance to be rectilinear, the mean free path of the molecules of the residual gas must be 5...10 times greater than the linear dimensions of the area of ​​vapor transfer.

Free path l- the distance traveled by a vapor molecule of a substance without colliding with molecules of residual gases. In high vacuum, when l ³ d(d is the distance from the source of evaporation to the substrate), the molecules of the evaporated substance fly the distance practically without collisions. This flow of vaporized matter is called molecular and to create it, a vacuum of the order of 10-5 ... 10-6Pa is required.

Vapor condensation on the substrate and film formation. Vapor condensation depends on the substrate temperature and atomic flux density. Atoms of the evaporated substance are adsorbed on the substrate after random migration over its surface.

In terms of mechanical and physical properties, thin films differ significantly from bulk materials. For example, the specific strength of some films is approximately 200 times higher than the strength of well-annealed bulk samples and several times higher than the strength of materials subjected to cold work. This is due to the fine crystalline structure and low plasticity. The evaporation temperature of metals ranges from several hundred degrees (for example, 430 ° C for cesium) to several thousand (for example, 3500 ° C for tungsten). In this regard, evaporators of various designs are used in vacuum evaporation. According to the method of heating the substance, evaporators are divided into resistive, electronic and induction.

In resistive evaporators, thermal energy is obtained due to the release of heat when current passes through the heater or directly through the material to be evaporated. The most commonly used evaporators with indirect heating. In this case, special heaters are provided, with the help of which the evaporated substance is heated to the required temperature. The evaporator material is usually tungsten, tantalum, molybdenum, etc.

The choice of heater material is determined by the following requirements: the evaporated material in the molten state must wet the heater well, forming good thermal contact, and must not enter into a chemical reaction with the heater material. Basically, heaters made of tungsten, molybdenum, tantalum are used.

Resistive evaporators do not provide the required composition of films during the evaporation of alloys. Due to the difference in the vapor pressure of the various components, the composition of the film differs significantly from that of the starting material. For example, a sputtered nichrome alloy (80% Ni and 20% Cr) forms a film on the substrate with a composition of 60% Ni and 40% Cr. To obtain films of the required composition from multicomponent alloys (for example, MLT, etc.), the method of microdosing or explosive evaporation is used. With this method, a tape evaporator heated to a temperature exceeding the evaporation temperature of the most refractory component by 200 ... 300 ° C is fed with a microdose of evaporated alloy powder with a particle size of 100 ... 200 microns. Evaporation of a microdose occurs almost instantly.

In electronic evaporators, the kinetic energy of electrons is converted into thermal energy. The evaporated material is used in the form of a solid wire, the free end of which is exposed to an electron beam. Due to the short duration of heating (10 -8 ... 10 -9 s), various components of the complex compound evaporate and deposit on the substrate almost simultaneously. Electron beam heating makes it possible to evaporate refractory metals and their alloys.

To increase the stability of the parameters, thin metal films are subjected to heat treatment by heating to t=300 ... 400 ° C. In this case, the crystals become coarser, the bond between them increases, the film becomes denser and more compact, and the electrical resistivity decreases.

Vacuum deposition is widely used to obtain resistive films, conductors made of copper, aluminum and some other alloys, silicon oxide dielectric coatings, etc. The main advantages of the process are the high purity of the resulting film, the convenience of controlling its thickness during the deposition process, and ease of implementation. The most significant drawbacks of the process are the change in the percentage ratio of components during the evaporation of substances of complex composition; low film thickness uniformity during deposition over a large area from point sources; difficulty in evaporating refractory materials; high inertia of the process when using resistive evaporators; relatively low adhesive strength of the film with the substrate.

Ionic sputtering. It is based on the phenomenon of the destruction of solid materials when their surface is bombarded by ionized molecules of a rarefied gas. The process is not associated with high temperatures and makes it possible to obtain films of refractory metals and alloys. There are the following types of ion sputtering: cathode, ion-plasma and magnetron.

Cathode sputtering (“diode” system) (Fig. 1.14) is carried out in a vacuum chamber, where two plane-parallel electrodes are located. One electrode (cathode) is made of a spray material and is a target for bombing. The other electrode (anode) serves as a substrate on which the film is deposited. A low pressure is created in the vacuum chamber (10 -3 ... 10 -4 Pa), after which it is filled with an inert gas (usually argon) at a pressure of 1 ... 10 Pa. When a high voltage (1...3 kV) is applied between the electrodes, an independent glowing gas discharge occurs, excited by electron emission. The cathode is the source of electrons needed to maintain the glow discharge. The electrons move towards the anode and, upon collision with neutral gas molecules, knock out new electrons, which leads to a sharp increase in the electron flow. In this case, an inert gas molecule turns from neutral into a positive ion, which has a larger mass compared to an electron. This is how the ionization of a gas occurs, which, with a greater or equal number of electrons and ions, is called plasma. The electrons move to the anode and are neutralized. Positive ions move to another plasma boundary and are accelerated in the dark cathode space, acquiring high energies to sputter the target (cathode). Atoms of the target material with high energy are deposited on the surface of the substrate, which is located close enough to the cathode. Usually this distance is one and a half to two lengths of the dark cathode space.

Cathodic reactive sputtering is carried out in a mixture of inert and active gases. It allows you to get a different composition of the film. Discharge in a mixture of gases "argon - oxygen" is used to obtain oxides. Reactive sputtering of tantalum in an argon atmosphere with the addition of oxygen, nitrogen, and carbon makes it possible to obtain a number of compounds with very different properties.

Ion-plasma sputtering (three-electrode system) is carried out at lower pressures (Fig. 1.15).

A pressure of 10 - 3 Pa is created in the chamber and the cathode glow is turned on. Then it is filled with an inert gas at a pressure of 10-1 Pa. The creation of gas-discharge plasma is provided by an arc discharge that occurs between the anode and cathode at a voltage of 150 ... 250 V. The thermal cathode serves as a source of electrons. The sputtered material (target) is introduced into the gas discharge as an independent electrode that is not associated with maintaining the discharge. The electrons simulated by the thermionic cathode are accelerated towards the anode and ionize the molecules of the residual gas along the way. The density of the resulting plasma is more than an order of magnitude higher than that of the glow discharge plasma. The target cathode and the substrate are placed on opposite boundaries of the active plasma space. Sputtering begins from the moment when a negative potential of 200 ... 1000 V is applied to the target with respect to the anode. This potential repels electrons and attracts ions from the plasma space. The ions bombard the target in the same way as in the considered "diode" version. The sputtered atoms, moving mainly in the direction perpendicular to the surface, are deposited on the substrate. Sputtering at low pressures makes it possible to obtain high adhesion of the film to the substrate due to the greater energy of the sprayed particles. Since at this pressure the mean free path of molecules is several centimeters, the sputtered atoms on their way from the target to the substrate almost do not collide with the molecules and ions of the inert gas and gas impurities, which significantly reduces the degree of contamination of the film with foreign gas inclusions. The possibility of reducing the distance between the target and the substrates is due to the fact that in the triode sputtering system, the formation of electrons and ions occurs autonomously from the target.

Disadvantages of the triode system are the short service life of the wire cathode and the different sputtering rates in individual sections of the flat target.

High-frequency ion sputtering is used for sputtering dielectrics and semiconductor materials. During the usual sputtering of conductive materials that hit the target cathode, the neutral working gas ion receives an electron from the target and discharges, turning into a neutral molecule for some time. If the target material being sputtered is a dielectric, then there will be no neutralization of ions on the target and it is quickly covered with a layer of positive charges that prevent further target sputtering.

The effect of a positive charge can be eliminated by applying an alternating voltage to the metal electrode on which the dielectric being sprayed is fixed. During the period when the voltage on the target is negative, it is sputtered, accompanied by the accumulation of a positive charge. When the polarity is reversed, the positive charge is compensated by electrons drawn from the plasma. Dielectric materials can be sputtered at almost any frequency.

Without what it is difficult to imagine the existence of modern man? Of course, without modern technology. Some things have entered our lives so much, they have become so boring. The Internet, TV, microwave ovens, refrigerators, washing machines - without this it is difficult to imagine the modern world and, of course, oneself in it.

What makes almost all of today's technology truly useful and necessary?

What invention provided the greatest opportunities for progress?

One of the most indispensable discoveries of man is the technology of manufacturing microcircuits.

Thanks to her, modern technology is so small. It is compact and convenient.

We all know that a huge number of things consisting of microcircuits can fit in the house. Many of them fit in a trouser pocket and are light in weight.

thorny path

To achieve a result and get a microcircuit, scientists have worked for many years. The initial circuits were huge by today's standards, they were larger and heavier than a refrigerator, despite the fact that a modern refrigerator does not consist entirely of complex and intricate circuits. Nothing like this! It has one small, but superior in utility to the old and bulky ones. The discovery made a splash, giving impetus to the further development of science and technology, a breakthrough was made. Chip production equipment released.

Equipment

The production of microcircuits is not an easy task, but fortunately, a person has those technologies that simplify the task of production as much as possible. Despite the complexity, a huge number of microcircuits are produced daily around the world. They are constantly being improved, acquiring new features and enhanced performance. How do these small but smart systems appear? This helps equipment for the production of microcircuits, which, in fact, is discussed below.

When creating microcircuits, electrochemical deposition systems, cleaning chambers, laboratory oxidizing chambers, copper electrodeposition systems, photolithographic and other technological equipment are used.

Photolithographic equipment is the most expensive and precise in mechanical engineering. It is responsible for creating images on the silicon substrate to generate the intended chip topology. A photoresist is applied to a thin layer of material, which is subsequently irradiated with a photomask and an optical system. During the operation of the equipment, the size of the elements of the pattern decreases.

In positioning systems, the leading role is played by a linear electric motor and a laser interferometer, which often have feedback. But, for example, in the technology developed by the Moscow laboratory "Amphora", there is no such connection. This domestic equipment has more precise movement and smooth repetition on both sides, which eliminates the possibility of backlash.

Special filters protect the mask from the heat generated by the deep ultraviolet area, enduring temperatures in excess of 1000 degrees for long months of operation.

Low-energy ions are mastered in deposition on multilayer coatings. Previously, this work was carried out exclusively by the magnetron sputtering method.

Chip production technology

The whole process of creation begins with the selection of semiconductor crystals. The most relevant is silicon. A thin semiconductor wafer is polished until a mirror image appears in it. In the future, a mandatory step in the creation will be photolithography using ultraviolet light when drawing a picture. This helps the machine for the production of microcircuits.

What is a microchip? This is such a multilayer pie made of thin silicon wafers. Each of them has a specific design. This same pattern is created at the stage of photolithography. The plates are carefully placed in special equipment with a temperature of over 700 degrees. After roasting, they are washed with water.

The process of creating a multilayer plate takes up to two weeks. Photolithography is carried out numerous times until the desired result is achieved.

Creation of microcircuits in Russia

Domestic scientists in this industry also have their own technology for the production of digital microcircuits. Plants of the corresponding profile operate throughout the country. At the output, the technical characteristics are not much inferior to competitors from other countries. Russian microcircuits are preferred in several states. All thanks to a fixed price, which is less than that of Western manufacturers.

Necessary components of the production of high-quality microcircuits

Microcircuits are created in rooms equipped with systems that control the purity of the air. At the entire stage of creation, special filters collect information and process the air, thereby making it cleaner than in operating rooms. Workers in production wear special protective overalls, which are often equipped with an internal oxygen supply system.

Chip manufacturing is a profitable business. Good specialists in this field are always in demand. Almost all electronics are powered by microcircuits. They are equipped with modern cars. Spacecraft would not be able to function without the presence of microcircuits in them. The production process is regularly improved, the quality is improving, the possibilities are expanding, the shelf life is increasing. Microcircuits will be relevant for long tens or even hundreds of years. Their main task is to benefit on Earth and beyond.