CVD: Chemical Vapor Deposition
Chemical evaporation is used for the preparation of very pure layers and are widely used in semiconductor technology. A wide range of materials, including dielectrics and silicon single crystal materials, polymeric materials and metal materials, can be coating with this method. The oldest applications, it can be noted to make the pigments. Powders: TiO2, SiO2, Al2O3, Si3N4 and black carbon are made routinely by CVD method.
Because of the nature of CVD, this method is used for the accumulation of very pure metals. In chemical coating methods such as physical methods, several parameters affect on the quality of layer. The most important of them, it can be pointed out to cases such as substrate temperature, concentration of the reaction materials, the gas pressure and gas flow rate.
According to pressure, chemical reactions may be performed in Atmospheric Pressure CVD that called APCVD or in a Low Pressure CVD that called LPCVD (about 0.2 to 20 torr) and even in a ultra-high vacuum HVCVD: High Pressure CVD.
The most important part of this process are chemical reactions that occur between the gases source and as a result of this reaction, the desired solid material create on the substrate surface. Chemical reactions may be caused by heat (CVD), energy RF: PECVD, or by light (PHCVD).
In the process, one or more gas went usually into the reactor at low pressure and therefore, under controlled conditions, on the surface or near the substrate, reaction takes place between them. However, due to the nature of some gases in creating products toxic or corrosive during the deposition process should be done high attention on the selection of targeted gases.
The main performed reaction in a chemical vapor include:
1. Analysis
2. Composition
3. Hydrolysis
4. Oxidation
5. Rehabilitation
In Table 1 some examples of these reactions is given.
CVD system structure
A modern system of CVD is including a measurement and control system for the specific mixture of carrier and reaction gases, one heated reaction chamber, and one system for interaction and disposal of exhaust gases. Figure 1 shows a schematic view of an industrial CVD system. Gas mixture (which is included, resuscitation gases such as hydrogen, inert gas like Ar, reactive gases such as metal halides and hydrocarbons as well as gases such as.......) carried into the reaction chamber and heated under suitable heat.
Figure 1: Various parts of CVD, which works at atmospheric pressure: 1- reactor, 2- heaters, 3- the reaction chamber , 4- cooled flange by water, 5- power controller, 6- pressure gauge, 7- temperature sensor and controller, 8, 10 and 11- The source of gases (required sources), 9- the metal vapor source (in liquid form), 12 and 13- The gas purification, 14- The gauge of rate gas, 15-rate control valve 15 , 16- The regulator of gas capsules, 17- substrate holder, 18- the substrate.
Pressure and temperature conditions of the system is shown for some gases in Table 2.
Table 2: usual parameters in one CVD process.
All CVD systems need a mechanism to control the reactions products. These products include various reaction components and hazardous substances, which must be trapped in a neutral area, before they go out toward the outside. In addition, many of the CVD processes carried in lower pressure than atmospherics and in this case, it must be protected the pump equipment from corrosive gases as well as overheating.
Addition to main reactions, subsidiary reactions may also occur in the reactor, and thus unwanted products are achieved, which sometimes cause problems in the coating process.
For example, as shown in Figure 2, to form a SiO2 thin layer is used from reaction between the gas of SiH4 and oxygen that exist inside the reactor. It can be seen, in addition to SiO2 thin layer, hydrogen gas is produced also, which is as a unwanted product and finally driven out to the out of reactor.
Figure 2: forming a layer on the substrate in chemical deposition method.
One of the substantial cases in chemical vapor deposition method is the nature of gases or the vapor of used material in the reactor. For example, it can be pointed out toxicity, flammability and corrosivity of some of this gases. This can cause various problems in coating and also inside the reactor. Table 3 is the most common used gas in the system are.
In general, it can be summarized one CVD system, in three main sections:
1. Gas Distribution System
2 reactor
3. Discharge System
The initial chemical evaporation deposition systems, as shown in Figure 3, were made in the form of a horizontal tube furnace. As shown in Figure, In this type of furnace is used of three heating element in three different areas of the chamber, for heating the furnaces. On the other hand, one control unit controls the temperature of the heated three areas until set the furnace temperature and therefore, have more uniform temperature in the whole furnace.
In horizontal furnace, the uniformity of temperature is achieved in 0 0.5 ± 1200 ° C as well. Although the reactions are desired usually at lower temperatures.
Such as temperature, the inside pressure of furnace also control in some parts. The pressure control system allows that pressure level inside the vacuum chamber controlled independently from the gas flow. The amount and rate of gases are controlled in these systems using multi-tap MFC: Mass Flow Control. Todays, all quantities, such as temperature, pressure and gas flow are controlled by computer-assisting.
Figure 3: chemical deposition system with horizontal furnace.
Table 3: The nature of the conventional gases in chemical evaporation coating.
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Neutral
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N2
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Nitrogen
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Neutral
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Ar
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Argon
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Oxidizers
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N2O
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Nitrogen Oxide
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Toxic
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AsH3
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Arsine
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Oxidizers
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O2
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Oxygen
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Toxic, flammable
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B2H6
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Diborane
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Toxic, corrosive
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PH3
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Phosphine
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flammable
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H2
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Hydrogen
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Toxic, corrosive
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SiH4
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Silane
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Toxic, corrosive
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HCL
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Hydrogen chloride
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hjhj
For example, we look at to coating a hypothetical sample in one single-wafer chamber (substrate), which is used for coatings in the micrometer dimensions:
The wafer is placed into the system and the chamber is evacuated by a vacuum pumps. Meanwhile, thermal lamps that are located under the wafer turned on and heat is driven to up and below of the wafer. Then, the gases are entered from the top of the chamber and flows towards bottom, and are distributed towards substrate through the holes. The gases reacts on the wafer, which is warming, and the desired layer is formed. The chamber walls are kept cool for prevent the formation of layers on them, and as a result, the attracted particles reach to lowest amount.
Such methods are usually simple. In chemical coating the obtained layers are of great uniformity. The most important issue In this process is a high temperature (over 600 ° C) and the low deposition rate.
In recent decades, different ways, such as system with an intermediate temperature, Moderate Temperature CVD that called MTCVD, using plasma, Plasma Enhanced CVD that called PECVD, and recently using laser (LASER CVD) are regarded by scientists.
In MTCVD, the temperature decreases to 850 ° C, with the use of organic- metal compounds as a precursor that in this condition known as Metal Organic CVD technique or MOCVD.
Chemical evaporation coating has also different advantages and disadvantages, like other methods. Here give briefly the advantages and disadvantages of different methods of CVD:
a) The advantages of chemical evaporation method.
- There is the possibility of creating the very complete and very pure epitaxial layers in this method.
- Due to the nature of reaction, coating can be done well on porous substrates.
- Thickness is uniform and independent from the shape of substrate.
- The rate of deposition is relatively high (100-10 nm /s).
- Its adherence is better than physical methods.
- This method is very suitable for multilayer coating.
- The process can be controlled in the usual pressure.
b) The disadvantages of chemical evaporation methods:
- With regard to the fact that the acceptable reactions can be done rarely in low temperature, so in this method the reactions for doing have need to high temperatures, which will be caused thermal stresses that is a negative factor for the layer and the substrate.
- In some coating reactions there is the possibility of the substrate destruction because of using corrosive vapors.
- The control of reactions and in result control of uniformity is very difficult.
- The thermodynamic details in this method is very complex and their control can be very hard sometimes.
- There is the possibility of unwanted reactions in this method, which can sometimes lead to serious problems in coating or inside the reactor.
- There is the possibility of hydrogen explosion hazard in the reactor.
- The most of reactive materials are expensive.
The mentioned items are as general items of chemical coating. But as we have seen, the chemical vapor deposition method is performed himself in different forms, which are selected, proportion to the required precision, and the type of target material, and the desired limits.
Table 3: general specifications of the most common chemical evaporation methods.
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PECVD
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APCVD
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LPCVD
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MOCVD
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*
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Low temperature for deposition
High adhesion
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Simple, high deposition rate, cheap.
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Excellent uniformity, high purity
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Usable for metals, semiconductors and dielectrics .
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Advantages
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Plasma destroys layers and even sample sometimes.
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Less uniformity, Less purity
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Low accumulation rate
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Very toxic, expensive
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Disadvantages
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Dielectric coating
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Thick oxide layer
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The coating of dielectrics and polysilicon layers
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Making LED, diode lasers, semiconductors
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Common use
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