where s is the emittance of the surface, Tis the absolute temperature, and A is the area of emitting surface. Radiant energy heats all of the surfaces in the deposition chamber, leading to a rise in the substrate temperature, desorption of gases from surfaces, and surface creep of contaminants. Radiant heating of the substrate and interior surfaces can be minimized by:

• Using prewetted evaporant surfaces

• Using radiation shields

• Using shutters over the source until the vaporization rate is established

• Rapid vaporization of the source material onto the substrate

Some materials, such as gold, are good heat reflectors; as soon as a gold film is formed, a high percentage of the incident radiant heat is reflected from the coated surface. Some materials, such as silicon dioxide, do not adsorb infrared radiation very well and are not easily heated by radiation.

Spits and Comets. In the evaporation of materials from heated surfaces, "spits" and "comets" are often encountered. Spits are solidified globules of the source material found on the deposited film. When these poorly bonded globules are disturbed, they fall out, leaving large pinholes in the film. Comets are seen in the system during vaporization as the hot molten globules are ejected from the source.

Molten globules can originate from the molten material by any one of several processes. During heating, particularly rapid heating, gases and vapors in the molten source material agglomerate into bubbles and explode through the surface, giving spits. For example, silver can have a high content of dissolved oxygen and give spitting problems. The source of spits can be continual if new material is continually being added to the melt. Spits can be reduced by using pure vacuum-melted source material, handled and stored in an appropriate way, and by degassing the evaporant charge by premelting with slow heating to melting.

If the molten evaporant is held in a heated crucible, vapor bubbles can form on the crucible surfaces, where they grow and break loose. As the bubbles rise through the molten material, the hydrostatic pressure decreases and the bubbles grow in size. When the bubbles reach the surface they "explode," giving rise to globules of ejected molten material. Materials that have high vapor pressures at their melting points are more likely to give spits than materials that have low vapor pressures at their melting points. Spitting is common in boiling water; in high school chemistry, students are taught to add boiling beads to the water to reduce the violence and splashing. The same approach can be used to prevent spitting from molten material. For example, chunks of tantalum are placed in molten gold to prevent gold spits. The tantalum does not react with the gold and does not vaporize at the gold evaporation temperatures.

Spits from crucibles can be minimized by:

• Using source materials that are free of gases and high-vapor-pressure impurities

• Polishing the crucible surfaces so that bubbles do not stick well and break loose when they are small

• Using "boiling beads" in the molten material to prevent large bubbles from forming

• Using baffle-type sources so that the source material must be vaporized several times before the vapor leaves the source

• Using specially designed crucibles (Ref 58)

• Using electron-beam evaporation where the surface of the evaporant is heated

• Reducing source power if spitting occurs

Spits can also occur during melting and flowing of a material on a hot surface. A solid material placed on a surface has poor thermal contact with the surface, so the tendency is to heat the surface to a very high temperature. When the evaporant melts and spreads over the surface, the very hot surface creates vapor that "explodes" through the spreading molten material. This source of spits can be eliminated by premelting the charge on the surface, to give good thermal contact, and by using shutters in the system, to prevent the substrate from "seeing" the source until the molten charge has wetted the surface and is vaporizing uniformly.

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