Chemical vapor deposition
The term chemical vapor deposition ( chemical vapor deposition English CVD) refers to a group of coating methods which are used inter alia in the manufacture of microelectronic components and optical waveguides.
History
The term chemical vapor deposition was coined in 1960 by John M. Blocher, Jr.. With this concept, the chemical vapor deposition should be distinguished from physical coating method, the John Blocher, the term PVD (English: physical vapor deposition) summarized.
The history of the process, however, begins much earlier. Already in 1852, reported the German chemist Robert Wilhelm Bunsen on the deposition of Fe2O3 of gaseous iron chloride ( FeCl3) and water vapor. Depending on the definition of terms can be also significantly older reports of CVD processes take place.
Process principle
To the heated surface of a substrate, a solid component is deposited as a result of chemical reaction from the vapor phase.
The only prerequisite is that volatile compounds of the layer components exist which separate the solid layer at a given reaction temperature.
The method of chemical vapor deposition is characterized by at least one reaction in the surface of the workpiece to be coated. In this reaction, at least one gaseous starting material (starting material ) and at least two reaction products - to be involved - at least one of them in the solid phase.
To over competing vapor phase reactions to promote those reactions at the surface and thus to avoid the formation of solid particles, chemical vapor deposition processes are generally operated at reduced pressure (typically from 1 to 1000 Pa).
A special feature of the method is the conformal layer deposition. In contrast to physical methods, the chemical vapor deposition allows the coating of complex, three-dimensionally shaped surfaces. For example, fine recesses in the hollow body, or wafers can be uniformly coated on its inside.
Precise deposition can also be achieved by means of focused electron beams or ion beams. The charged electrons and ions cause precipitate the dissolved substances in the gas at the illuminated points. Such electron beam may be generated, for example, a synchrotron. The ion beams may be generated with an FIB apparatus. These additionally allow for a selective gas-assisted ion beam etching.
Examples
- Synthetic crystalline diamond layers can be deposited from a gas phase consisting % hydrogen and only about 1 % by volume of a carbon source ( methane, acetylene ), generally to about 99 vol. The gases are either thermally activated by a plasma or a laser. The excess of hydrogen is suppressed, inter alia, the simultaneous formation of sp ² - hybridised carbon species ( graphite, amorphous carbon).
- A silicon nitride layer is produced from ammonia and dichlorosilane.
- For silica layers to use silane and oxygen or TEOS ( tetraethylorthosilicate ) and oxygen.
- For the production of metal / silicon hybrids ( silicides ), tungsten hexafluoride is used.
- Titanium nitride coatings for hardening of tools (drills, cutting tools ) are generated from TDMAT and nitrogen.
- Tin oxide layers are deposited of tin chloride or tin- organic compounds and oxygen or water vapor on flat glass and container glass.
- Silicon carbide layers are deposited on hot surfaces (above 800 ° C) of a mixture of hydrogen and methyl -trichloro- silane ( CH3SiCl3 ).
- Arrays of carbon nanotubes can be synthesized on a substrate.
Application
Coatings are used in the electronics industry to, for example, Si3N4, SiO2, poly Si, crystalline Si ( epi- Si), and SiONx deposited on wafer surfaces.
Prior to deposition of the wafers in a dry etching (English: dry etch process) is cleaned, in which either the sulfur hexafluoride or a mixture of tetrafluoromethane and oxygen of high purity are used. Nitrogen and hydrogen serve as carrier gases. The CVD reaction chambers are cleaned with nitrogen trifluoride.
For the structuring of silicon by etching a boron-doped epi- Si layer can be deposited as an etch stop layer by vapor phase epitaxy.
Outside the electronics industry, the processing of glass, and the production of optical fiber cables for the optical communications is one of the largest areas of application of the chemical vapor deposition. Thus, about 10 million m² architectural glass are coated with thermal insulation layers of fluorine -doped tin oxide annually. Another important application of the layers is tin oxide, the protection of container glass. The coating on the outer surfaces of the glass protects against mechanical shock loads, such as bottling plants. Other applications include optical coatings on glass, plastic, and on gas-tight barrier layers.
Boron-doped CVD diamond electrodes are used, inter alia, in industrial water treatment to wastewater oxidation and disinfection of process water.
Method limits
Not desirable for each layer, it is a gaseous compound, from which it could be produced.
Another limitation of the method is the high temperature load of the substrate represents the heat load can, among other arrears on workpieces imply or above the softening temperature of the material to be coated are, so the method can not be applied. In addition, at high temperatures the diffusion processes, thereby doping metals or smeared by a diffuse coating processes. However, there are also variations where the thermal load is low and thereby the adverse effects are reduced.
Variants
By plasma enhanced chemical vapor deposition (English: plasma enhanced CVD, PECVD), the thermal stress of the substrate can be reduced. Here, above the wafer, a plasma is ignited, this can be either inductive (English: inductively -coupled PECVD, ICPECVD ) or capacitive (English: capacitance -coupled PECVD) happen. This CVD method is carried out at temperatures between 200-500 ° C.. At low temperatures because the thermal energy for the pyrolysis is not sufficient, the gas is excited and decomposed by a plasma. Furthermore, the deposition rate is increased by the plasma excitation. However, this has the disadvantage that the crystal structure of the wafer is affected by the radiation of the plasma. In addition to these direct plasma method, there is the RPECVD (English: Remote plasma- enhanced CVD), wherein the plasma is separated spatially from the substrate. This keeps the load of the substrate by ion bombardment and radiation is reduced.
The HFCVD method (English: hot filament CVD, dt "hot wire -activated chemical vapor deposition " ), also called hot -wire CVD or catalytic CVD called, allows the film deposition by strained in the recipient filaments (wires ), which are usually made of tungsten, tantalum or rhenium. By an applied voltage, the filaments are brought to the annealing, wherein temperatures of the wire up to 2600 ° C are reached. The gases used can be cleaved by these high temperatures on the filaments to form radicals and the species formed in this way ensure that the layer structure (for example, the manufacture of the polycrystalline diamond layer ).
A low pressure CVD (English: low pressure chemical vapor deposition, LPCVD), the method commonly used in semiconductor technology for the deposition of silicon oxide, silicon nitride and polycrystalline silicon, and metals. The process takes place in tube furnaces, nowadays usually in vertical furnaces.
APCVD (English: atmospheric pressure chemical vapor deposition, dt " chemical vapor deposition at atmospheric pressure " ) is operated, in contrast to most CVD processes, not under a reduced but at atmospheric pressure.
With metal organic chemical vapor deposition (English: metal organic chemical vapor deposition, MOCVD, also: OMCVD ) the chemical deposition is called from organometallic starting compounds. A subset of the MOCVD is the vapor-phase epitaxy (English: metal organic vapor phase epitaxy MOVPE), are produced in the crystalline layers of very high quality. In the area of compound semiconductor manufacture such as III-V and II-VI semiconductors are these names, depending on the language room, for identical processes.
A specialty that utilizes the particular advantage of the CVD process, be able to coat porous objects evenly, the chemical vapor infiltration (English: chemical vapor infiltration, CVI). This method is used e.g. for the coating of fiber bundles.