Composition of the optical film

The optical film is formed by agglomeration of bulk materials on the substrate after solid-liquid-gas-solid or solid-gas-solid transformation, so its microstructure, crystal structure and film composition will be different from the original bulk material.

Most metals evaporate almost in the form of atoms, but for some semiconductors and semi-metals, they mostly escape as aggregates of two or more atoms. Because the 4 atoms of Sb are almost bonded by similar covalent bonds, the Sb film mainly contains Sb4, while mixing Sb2 and a small amount of Sb. As is mostly As4 and As2, Bi and Te contain considerable amounts of Bi2 and Te2, and similar aggregates are also observed in C, Ce, Se and Si.

The compound situation is more complicated. For chalcogenides, the material decomposes during evaporation, for example, 2ZnS→2Zn+S2. But they are recombined on the substrate, so that a film with a stoichiometry similar to that of the bulk material can still be obtained. CdTe is also decomposed into Cd and Te2. Similarly, fluoride can also cause different degrees of fluorine loss. The following table shows the loss of fluoride in several fluoride films. The moisture absorption capacity of the film layer after fluorine loss is improved. This fluorine loss phenomenon is closely related to the evaporation source, and a large-capacity radiant evaporation source can be used to obtain a film that is approximately the same as the bulk stoichiometry. When evaporating oxides, the decomposed O2 can be observed. Even stable SiO2 also contains SiO and O2. The most serious oxygen loss is TiO2, Ta2O5 and NiO. In contrast, MgO, Al2O3, BeO and CoO2 are almost Close to the bulk material.

Fluoride loss of fluoride film

Sputtering also contains a large number of composite atoms. The higher the ion acceleration voltage, the fewer the single atoms sputtered. Take the sputtered Cu film as an example. For the polycrystalline Cu target, when the acceleration voltage of Ar+ is 100eV, there is only about 5% Cu, and the rest are Cu2. Crystal Cu(100) target, in addition to Cu and Cu2, there is also Cu+n (n=1~11). With Ar+ and Xe+ sputtering Al, Aln (n=1~7) and Aln (n=1~18) can also be observed respectively. With Ar+ sputtering compound GaAs, 99% of the sputtered neutral atoms are Ga and As, and the rest are GaAs molecules.

The analysis of the chemical composition of optical films is mainly by means of surface analysis techniques. The so-called surface analysis technology usually uses photons, electrons or ions as primary particles to bombard the surface of the sample to be analyzed. The particles interact with the solid surface, and the result will cause the emission of excited particles, that is, secondary particles. Can identify the chemical composition of the surface and film.