Vaporization of Materials

Elements. Many elements evaporate, but many (for example, chromium, cadmium, magnesium, arsenic, and carbon) sublime, and many others (for example, antimony, selenium, and titanium) are on the borderline between evaporation and sublimation. Chromium has a vapor pressure of 1.3 Pa (10-2 torr) at 600 °C (1080 °F) below its melting point, and carbon cannot be melted except under high hydrostatic pressure. Materials such as aluminum, tin, gallium, and lead have very low vapor pressures above the just-molten material. Tin, for example, has a vapor pressure of 1.3 Pa (10-2 torr) at 1000 °C (1800 °F) above its melting point. Aluminum and lead have vapor pressures of about 1.3 Pa (10-2 torr) at 500 °C (900 °F) above their melting points.

Most elements vaporize as atoms, but some (for example, antimony, tin, carbon, and selenium) have a significant portion of the vaporized species as clusters of atoms. For materials that evaporate as clusters, special vaporization sources called baffle sources can be used to ensure that the depositing vapor is in the form of atoms. It should be noted that as a material is heated, the first materials that are volatilized are high-vapor-pressure surface contaminants, absorbed gases, and high-vapor-pressure impurities.

Alloys vaporize in a ratio that is the same as their vapor pressures (that is, the high vapor pressure constituent vaporizes more rapidly than the low vapor pressure material). This relationship is called Raoult's Law, and the effect can be used to purify materials by selective vaporization/condensation. When an alloy is evaporated from a molten pool, the higher-vapor-pressure material steadily decreases in proportion to the lower-vapor-pressure material in the melt. For example, when an Al:Mg (6.27 at.%) alloy is evaporated at 1919 K, the magnesium is totally vaporized in about 3% of the total vaporization time (Ref 10). Vaporization of alloys produces a gradation of film composition as the evaporant is selectively vaporized. This can be desirable or undesirable. For example, when copper-gold is deposited on polymers by evaporation of a copper-gold alloy, copper, which has a higher vapor pressure than gold, is deposited at a higher initial rate than the gold. This results in copper enrichment at the interface, which is conducive to better adhesion between the deposited film and the polymer.

In some cases, the nature of vaporization of an element can be changed by alloying it with another material. For example, chromium (melting point of 1863 °C, or 3385 °F), which normally sublimes, can be alloyed with zirconium (melting point of 1855 °C, or 3371 °F) to give a liquid melt from which the chromium evaporates. The eutectic alloy of zirconium-chromium (14 wt%) melts at 1332 °C (2430 °F), at which temperature chromium has a vapor pressure of approximately 1.3 Pa (10-2 torr) and zirconium has a vapor pressure of approximately 0.13 pPa (10-9 torr). Another eutectic alloy of zirconium-chromium (72 wt%) has a melting point of 1592 °C (2898 °F).

Compounds. Many compounds (for example, SiO, MgF2, Si3N4, HfC, SnO2, BN, PbS, and VO2) sublime. Compounds often vaporize with a range of species, from clusters of molecules to dissociated or partially dissociated molecules (Ref 2). The degree of dissociation is strongly dependent on the temperature and composition of the compound (Ref 11). For example, in the thermal vaporization of SiO2, a number of species are formed in addition to SiO2, including (SiO2)x, SiO2-x, SiO, Si, and O.

Evaporation from a very hot surface or by electron beams can give ionization of some of the molecular fragments. These charged species can be detrimental to the electrical properties of the deposited material. The ionized species can be deflected from the vapor flux using electrically charged plates above the source.

0 0

Post a comment