Fundamentals of thin film preparation

This article briefly introduces the relevant knowledge of semiconductor coating, and the basic thin film preparation methods include thermal evaporation and sputtering.

Evaporation

Thermal evaporation is a mature and widely used method for preparing thin films. At high temperatures, when the film material is heated to a higher temperature, the atoms or molecules of the film material will evaporate from the surface of the film and adhere to the surface of the substrate to form a thin film. Depending on the evaporation source, thermal evaporation can be divided into the following two categories.

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(1)Resistance heating method

At a vacuum of 10-6 Torr or more, the material is heated to escape from the evaporation source, transform into a vapor phase, and then deposit into the matrix and its surroundings to form a thin film. This process is based on resistance heating, through a continuous supply of electricity to produce the Joule heating effect, high energy so that atoms or molecules gain a certain kinetic energy, forming a thin film on the surface of the substrate.

(2)Thermal evaporation of the electron beam

The electron beam evaporation method mainly uses the electron gun emitter to emit electrons to the surface of the membrane, and the membrane material is bombarded by electrons to generate internal energy, and the particles in the membrane convert the internal energy into kinetic energy and evaporate to the surface of the substrate.

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Magnetron sputtering

Sputtering technology includes direct current sputtering, AC sputtering, reaction sputtering and magnetron sputtering, which is a preparation method in which the atoms or molecules on the surface of a solid target are ejected by the bombardment of the surface of a solid target by charged particles in a vacuum environment.

The RF magnetron sputtering process is to fill an appropriate amount of argon under the condition of high vacuum, apply a radio frequency (13.56 MHz) power supply between the cathode (cylindrical target or planar target) and the anode (coating chamber wall), and produce a magnetron abnormal glow discharge in the coating chamber, and the electrons collide with the argon atoms in the process of flying to the substrate under the action of the electric field E, so that the argon gas ionizes (the Ar atoms are ionized into Ar+ and electrons under the action of high voltage), and the incident ions (Ar+) bombard the target under the action of the electric field. The neutral atoms or molecules on the surface of the target can obtain enough kinetic energy to leave the surface of the target and be deposited on the surface of the substrate to form a thin film.

The secondary electrons produced will be affected by the electric and magnetic fields, resulting in a drift in the direction of E (electric field) × B (magnetic field), referred to as E×B drift, whose trajectory is similar to a cycloid. In the case of a toroidal magnetic field, the electrons move in a circular motion on the target surface in the form of an approximate cycloid, and their path is not only long, but also bound to the plasma region close to the target surface, where a large amount of Ar+ is ionized to bombard the target, thus achieving a high deposition rate.

As the number of collisions increases, the energy of the secondary electrons is depleted, gradually moving away from the target surface, and finally deposited on the substrate under the action of the electric field E. Since the energy of this electron is very low, the energy transferred to the substrate is small, resulting in a low temperature rise of the substrate.

Compared with the film made by thermal evaporation technology, the optical film prepared by sputtering technology is of better quality. The reason is that the energy of sputtered particles is an order of magnitude greater than that of thermally evaporated particles, which ensures that the film has a stronger binding force to the substrate, a higher aggregation density, and a refractive index closer to that of the bulk material.