Thin layer

A thin layer is a fine film of a material deposited on another material, called “Substrat”. The goal of the thin layer is to give properties particular to the surface of the part while profiting from the massive properties of the substrate (in general: mechanical resistance), for example:

  • electric conductivity: metallization of surface, for example to observe a sample Insulator with the Electron microscope with optical sweeping
  • : Silvering of the Mirror, anti-reflecting treatment of the objectives of Camera, old Nickel of the Helmet S of firemen to reflect the Heat (Infra-red S), gilding of their visor to avoid the dazzling

Manufacture of a thin layer

The first method to manufacture thin layers was by wet process: a Chemical reaction between a solution and the substrate. A famous example is the Miroir of money: Reduction of Ion S Ag + (for example solution of Silver nitrate AgNO3) by Sugar S.

Now mainly three techniques are used:

  • the vacuum Deposition: the material to be deposited is sublimated, or melted then vaporized, and it will condense on the substrate;
  • reaction in gas phase: method similar to the liquid way, but the reaction is done between a Gaz and the substrate;
  • projection plasma: an high voltage is established in a gas what creates a plasma; the acélérés ions come to tear off the atoms of a target, atoms which settle on the substrate in glance.
  • ablation with the Laser

Characterization of the thin layer

One of the problems is to estimate the thickness of the thin layer. If it lets pass the light, one can use interferometric methods (fringes of Interférence between the rays thought on the surface of the layer and those considered of the interface lay down thin-substrate).

See the detailed article Interference by a thin layer.

When that lends itself to it, one can use the X-rays:

  • by Diffractometry of x-rays:
    • method known as of “reflectometry”, similar to the interference of the light waves; oscillations of the signal are seen when the detector is moved;
    • method by shaving incidence: one makes sweep the detector around a peak characteristic of the substrate (if this one is smooth Cristal), for an incidence of x-rays data; the incidence is increased, and when one sees appearing the peak, the Loi of Beer-Lambert makes it possible to estimate the thickness of the layer;
  • by Spectrometry of x-ray fluorescence: either one measures the absorption of a line emitted by the substrate, or one measures the intensity of a line emitted by the thin layer; this method can also make it possible to determine the chemical composition of the layer

To have information on the texture of the thin layer on the surface, one can use the electronic Microscopie with sweeping. This technique makes it possible to have images of surface and profile. One obtains the thickness thus but also information on the microstructure.

All the other physical properties of the layer can be used: resistance, Mass (one measures the difference in mass between the naked substrate and the part after deposit)…

See too

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