Deposition of Thin Layers

Construction of electronic devices usually involves deposition of thin layers of semiconducting, metallic, and insulating materials onto a suitable substrate (which might be a wafer cut from a Czochralski silicon boule with a diamond saw). In some cases, it is possible to grow crystalline layers onto a substrate such that the crystallographic order of the atoms in the film is related to that of the surface of the substrate this is known as epitaxial growth. 

Vapor-phase deposition of the sputtered or evaporated layer-forming material avoids the undercooling problems associated with liquid phase epitaxy, but it coats everything in the vaporization chamber unselectively. Sputtering is usually done by forming a plasma (ionized gas) in an electrical discharge in the vapor at low pressure. 

Si02 by CVD of a silane-oxygen or, better, SiH2Cl2 N20 mixture. Layers of silicon nitride, Si3N4, may be similarly made by CVD of silane-ammonia mixtures at 700 °C  

Sol-gel techniques can be used to produce thin layers, as described in Section 19.1. For example, cadmium sulfide layers for photocells are easily made by spraying an ammoniacal solution of cadmium chloride and thiourea (which hydrolyzes to give sulfide ion) onto a substrate surface and baking the resulting CdS film at up to 500 °C  

The solid residue of ammonium chloride left after evaporation of the solvent water decomposes to gaseous HCl and NH3 during the baking. Anti-reflective coatings (e.g., of alumina or titania) may also be applied by sol-gel methodology. 

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In these processes the plastic component is treated in an etching solution to render the surface hydrophilic and to promote adhesion between it and the metallic film the surface then is activated so that later it can catalyse electroless deposition of thin layers of copper or nickel. The thin films so produced are the conductive surfaces on which further metal may be deposited by conventional electroplating. 

Although MBE continues to be the best technique for controlled deposition of thin layers (10-100 A) of materials, the MOCVD process does offer the advantage for rapid deposition over large areas of substrates. The basic principles of the MOCVD technique is the thermal decomposition of volatile molecular precursors to the desired combined form (or even the constituent metal itself) on a selected substrate at not too high a temperature. In addition to the volatility of the precursor and its facile decomposition to the desired combined form for deposition, the whole operation should not, from the practical point of view, involve any toxic/hazardous byproducts, that might entail any environmental problems. 

The decomposition of natural gas by IR radiation of a C02 laser was studied in both the presence and absence of C2H4119. It was found that the rate of decomposition of natural gas was increased by the addition of C2H4. The decomposed gas could be used as medium for deposition of thin layers of conductors, semi-conductors and dielectrics of organic and organometallic substances. 

In thermal evaporation, solid materials are first vaporized by heating at sufficiently high temperature in vacuum, and then a thin film is formed on a cooler substrate by condensation of the vapor. This technique can be used for the deposition of thin layers made of such metals as Ag or Pt that are active in gas separation. 

Physical Deposition of Thin Layers of Non-polar Character... 

Integrated circuits are the most important component in computers as central processing and high-density memory units. They consist of multilayer structures made by repeated deposition of thin layers. Lithography is applied to each layer to form micro- and nanoscale three-dimensional circuits. 

One of the easiest methods to prepare ZnO films is the simple dissolution of zinc acetate in methanol, and the subsequent deposition of thin layers through spin coating [120]. Due to the poor solubility of zinc acetate, only dilute solutions can be obtained and as a consequence the prepared films are thin however, these coatings are ideal as active layers for sensors operating in the SPR mode, which show sensitivity to a variety of VOCs at room temperature. 

Another consideration relative to measures of plasma surface modification with paper-based materials is surface roughness. Surface roughness with papers is inherently influenced within the manufacturing process and will vary dramatically with the side processed. The sides are known as wire side and felt (top) side . The side in contact with the paper machine wire during manufacturing is called the wire side. The opposite side is top side. Prior to the deposition of thin layers of fibers on machine wire, fillers are drained away. As such, the wire side will have less fillers compared to the top side. Process controls predetermine a designated level of surface roughness on each side of papers prior to surface modification by plasma treatment, as well as texture and absorbency. 

Pure Gold Electrolytes Alkaline baths are used exclusively for decorative purposes, for the deposition of thin layers of gold in color finishing of jewelry items and other decorative objects. The composition of typical alkaline cyanide baths used for the electrodeposition of gold is reported in Table 11.4 [2,3,61,62]. 

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