Vaporization Sources

Common heating techniques for evaporation/sublimation include resistive heating, high-energy electron beams, low-energy electron beams, and inductive (radio frequency) heating. Figures 7, 8(a), and 8(b) show some vaporization source configurations. Resistive heating is the most common technique for vaporizing materialat temperatures below about 1800 °C (3270 °F), while focused electron beams are most commonly used above 1800 °C (3270 °F).

Fig. 7 Sources used for resistive heating of materials in evaporation processing
Fig. 8(a) Focused electron-beam sources used for evaporation processing. Bent-beam electron gun (top); long-focus gun (bottom)
Fig. 8(b) Unfocused electron-beam sources used for evaporation processing. Work-accelerated gun (top); setup with magnetic confinement of electrons (bottom)

Resistive Heating. Resistively heated sources are the most widely used vaporization sources (Ref 26, 27). Typical conductive source materials are tungsten, tantalum, molybdenum, carbon, and BN/TiB2 composite ceramics. Resistive heating of electrically conductive heater is typically by low voltage (<10 V), very high alternating current (several hundred amperes) transformer supplies. It is generally better to slowly increase the heater current than to suddenly turn on full heater power. Due to the low voltages used in resistive heating, contact resistance is an important factor in source design. As the temperature increases, thermal expansion causes the evaporator parts to move; this movement should be accounted for in the design of the heater fixturing. Because metals expand on heating, the contacting clamps between the fixture and the source may have to be water cooled to provide consistent clamping and contact resistance (Ref 27). Figure 7 shows some typical resistively heated source configurations. The resistively heated vaporization sources are typically operated near ground potential. If the sources are to be operated much above ground, filament isolation transformers must be used.

Electron-beam heating sources can be classified as being either focused or unfocused.

Focused, high-energy electron beams are necessary to provide local high temperatures when required for the evaporation of refractory materials, such as most ceramics, glasses, carbon, and refractory metals. This electron-beam heating is also useful for evaporating large quantities of materials. Graper has tabulated the electron-beam vaporization characteristics of a large number of materials (Ref 28). Figure 8(a) and 8(b) shows two sources that use focused electron-beam heating: the deflected electron gun and the long-focus gun (Fig. 8(a)).

With the deflected or bent-beam electron gun source, the high-energy electron beam is formed using a thermionic-emitting filament to generate the electrons, high voltages (typically 10 to 20 kV) to accelerate the electrons, and electric or magnetic fields to focus and deflect the beam (Ref 29, 30, 31). Electron-beam guns for evaporation typically require 10 to 50 kW of power. Using high power electron-beam sources, deposition rates as high as 50 pm/s (0.002 in./s) have been attained (Ref 32) from sources capable of vaporizing aluminum at rates of up to 10 to 15 kg/h (22 to 33 lb/h). Electron-beam evaporators can be made compatible with ultrahigh vacuum (UHV) processing (Ref 33). Electron-beam evaporators are typically built to deposit material in the vertical direction, but high-rate electron-beam source installations that deposit material in a horizontal direction are being used (Ref 34).

In many applications, the electron beam is magnetically deflected through greater than 180°, to avoid deposition of evaporated material on the filament insulators and to focus the beam onto the source material, which is contained in a water-cooled copper hearth "pocket". The electron beam can be rastered over the surface to produce heating over a large area. The electron bombardment produces secondary electrons that are magnetically deflected to ground. The high-energy electron bombardment also produces soft x-rays that can be detrimental to sensitive semiconductor devices (Ref 35, 36, 37). The electrons ionize a portion of the vaporized materials and these ionscan be used to monitor the evaporation rate. With the electron-beam evaporation of some materials (for example, beryllium), a significant number of ions are produced that can be accelerated to the substrate to modify the film microstructure (Ref 38).

Electron gun sources can have multiple pockets so that several materials can be evaporated. By moving the beam or the crucible, more than one material can be heated with the same electron source.

The long-focus gun uses electron optics to focus the electron beam on a surface that can be an appreciable distance from the electron emitter (Ref 32, 39). The optic axis is often a straight line from the emitter to the evaporant, and therefore the gun must be mounted off-axis from the source-substrate axis.

Unfocused, high-energy electron-beam heating can be accomplished with an electron source by applying a voltage between the electron emitter and the source material or source container, which is usually at ground potential (Fig. 8(b)). Such a source is referred to as a work-accelerated gun (Ref 40, 41). Magnetic confinement of the electrons along the emitter-source axis (Fig. 8(b)) can also be used to increase the electron path length and so increase the ionization probability (Ref 42, 43).

Unfocused, Low-Energy Electron-Beam Heating. High-current, low-energy electron beams can be produced by thermionic-emitting surfaces, plasma arcs (Ref 44), or hollow cathodes (Ref 45, 46, 47, 48, 49). They can be accelerated to several hundred volts and magnetically deflected onto the source, which is at ground potential. Low-energy electron beams are typically not very well focused, but they can have high current densities. The vaporization of a surface by the low-energy electron beam can provide appreciable ionization of the vaporized material because the vaporized atoms pass through a high-density, low-energy electron cloud as they leave the surface.

Electron-Beam Guns in a Plasma Environment. Electron-beam guns are not generally used in a plasma environment because of sputter erosion of the filament by positive ions. There are also problems with the reaction of the hot emitting filaments in reactive gases. In order to use an electron-beam evaporator in a plasma or reactive gas environment, the electron-emitter region can be differentially pumped by being isolated from the deposition environment. This is accomplished by having a septum between the differentially pumped electron-emitter chamber and the deposition chamber. This septum incorporates a small orifice for the electron beam to pass from one chamber to the other (Ref 50).

Inductive Heating. Inductive heating couples radio frequency (rf) energy directly into electrical conductors such as metals or carbon (Ref 51). The rf source can be used either to heat the source material directly or to heat the container ("susceptor") that holds the source material. This technique has been particularly useful in evaporating aluminum from boron nitride and BN/TiB2 crucibles (Ref 52). When the source material is heated directly, the containing crucible can be cooled.

Evaporation sources must contain molten liquid without extensive reaction and prevent the molten liquid from falling from the heated surface. This is accomplished by having a wetted surface or by using a container (Ref 53). Commercial evaporation source manufacturers provide lists of recommended sources for various materials.

Wetted Sources. Wetting is desirable to obtain good thermal contact between the hot surface and the material being vaporized. Wetted sources are also useful for depositing downward, sideways, or from nonplanar surfaces. Metallic stranded wire, coils, and baskets are relatively cheap and can be used in many applications. Wires for evaporation are typically tungsten (Ref 2, 54, 55) but can be molybdenum or tantalum. Wire meshes and porous metals, through which the molten metals wet and wick by capillary action, can be used for large-area vaporization sources.

Wire and coil filaments have the often-frustrating property that the molten material runs to the low spots, where it can drop off. To help alleviate this problem, stranded wire is used to hold the molten material by capillary action. Bends or kinks can be put in the wire at selected points to collect the molten material, or coils of tantalum wire can be wrapped on the filament to hold the molten material at specific points.

Solid evaporants have poor thermal contact with the heater surface until they melt and wet the surface. To obtain wetting of the evaporant on the heater surface, it is often necessary to have a temperature in excess of that needed for a reasonable evaporation rate. When the material becomes molten and wets the surface, the vaporization rate is very high and can cause "spitting" as the molten evaporant spreads over the superheated surface. Refractory metals used for vaporization are covered with oxides, which volatilize at temperatures lower than the vaporization temperatures of many source materials. If film contamination by these oxides is to be avoided, the heater material should be cleaned before installation, shutters should be used, or the surface should be prewetted by the source material.

Premelting and wetting of the evaporant on the heater surface prior to the beginning of the deposition has several benefits:

• Good thermal contact can be established.

• There is volatilization of volatile impurities and contaminants from the evaporant and from the surface of the heater.

• Overheating of the heater surface is avoided, thereby minimizing spitting and radiant heating from the source.

Premelting can be done external to the deposition system if care is used in handling the source afterwards, to prevent surface contamination. Premelting can be done in the evaporator system by using shutters to prevent the deposition of undesirable material on the substrate before film deposition begins.

Crucible containers can hold large amounts of molten evaporant, but the vapor flux distribution changes as the level of the molten material changes. Electrically conductive crucibles can be heated resistively and are available in the form of boats, canoes, dimpled surfaces, crucibles, and so on (Ref 56). Typical refractory metals used for containers are tungsten, molybdenum, and tantalum, as well as refractory metal alloys such as TZM (titanium and zirconium added to molybdenum for improved high-temperature strength) and tungsten with 5 to 20% rhenium added for improved ductility. Metallic containers are often wetted by the molten material, and the material can spread to areas where it is not desired. This spreading can be prevented by having nonwetting areas on the surface. Such nonwetting areas can be formed by plasma spraying Al2O3 on the surface.

Commonly used crucible construction includes water-cooled copper, ceramics (both conductive and insulating), and glasses:

• Water-cooled copper is used as a crucible material when the evaporant materials are heated directly, as with electron-beam heating. The design of the coolant flow is important in high-rate evaporation from a copper crucible, because a great deal of heat must be dissipated (Ref 57). The water-cooled copper solidifies the molten material near the interface, forming a "skull" of the evaporant material so that the molten material is actually contained in a like material. This prevents reaction of the evaporant with the crucible material. On cooling, the evaporant "slug" shrinks and can be easily removed from the pocket of the electron-beam evaporator. In some cases, a liner can be used with a water-cooled crucible. Typical liner materials include pyrolytic graphite, pyrolytic boron nitride, BN/TiB2, BeO, Al2O3, and other such materials. In general, the liner materials have poor thermal conductivity. This property, along with the poor thermal contact that the liner makes with the copper, allows the liner to heat significantly. Liners can be fabricated in special shapes to obtain specific characteristics (Ref 58).

• Electrically conductive ceramics can be used as containers. Carbon and glassy carbon are commonly used container materials, and when a carbon-reactive material is evaporated from such a container, a carbide skull forms that limits the reaction with the container. For example, titanium in a carbon crucible forms a titanium carbide skull. An electrically conductive composite ceramic that is used for evaporating aluminum is BN-50TiB2 composite ceramic (known by the tradename UCAR) (Ref 59). This composite ceramic is stable in contact with molten aluminum, whereas most metals react rapidly with the molten aluminum at vaporization temperature.

• Glasses and electrically insulating ceramics can be used as crucibles and are often desirable because of their chemical inertness with many molten materials. Typical crucible ceramics are ThO2, BeO, stabilized ZrO2 (additions of HfO2 and CaO to ZrO2), Al2O3, MgO, BN, and fused silica. Kohl has written an extensive review of the oxide and nitride materials that may be of interest as crucible materials (Ref 60). The ceramics can be heated by conduction or radiation from a hot surface though these are very inefficient methods of heating. For more efficient heating, the material contained in the electrically insulating crucible can be heated directly by electron bombardment of the surface or by rf inductive heating from a surrounding coil. Isotopic boron nitride is a good crucible material for containing molten aluminum for rf heating because most other ceramics are attacked by molten aluminum. Metal sources, such as boats, can be coated with a ceramic (for example, plasma-sprayed Al2O3) in order to form a ceramic surface in contact with the molten material.

Feeding sources are sources where additional evaporant material is added to the molten pool without opening the processing chamber. Feeding sources can use pellets (Ref 61), powder, wires, tapes, or rods of the evaporant material. Pellet and powder feeding is often done with vibratory feeders, while wires and tapes are fed by friction and gear drives. Multiple wire-fed electron-beam evaporators are often aligned to give a line source for deposition in a web coater (Ref 62, 63). Rod feeds are often used with electron-beam evaporators where the end of the rod, whose side is cooled by radiation to a cold surface, acts as the crucible to hold the molten material (Ref 18). Feeding sources are used to deposit large amounts of material.

Baffle Sources. Some elements vaporize as clusters of atoms, and some compounds vaporize as clusters of molecules. Baffle sources are designed so that the vaporized material must undergo several evaporations from heated surfaces before it leaves the source, to ensure that the clusters are decomposed. Baffle sources can also be used to allow deposition downward from a molten material. Baffle sources are desirable when evaporating silicon monoxide or magnesium fluoride for optical coatings, to ensure the vaporization of monomolecular SiO or MgF2. Drumheller made one of the first baffle sources, called a "chimney source," for the vaporization of SiO (Ref 64).

Focused vaporization sources can be used to confine the vapor flux to a beam. Focusing can be done using wetted curved surfaces or by using defining apertures. A beam-type evaporation source that uses apertures has been developed to allow the efficient deposition of gold on a small area (Ref 65). This source forms a 2 2° beam of gold and gives a deposition rate of 4 nm/s (40 A/s) at 50 mm (2 in.).

Confined vapor sources confine the vapor in a heated cavity and pass the substrate through the vapor. The vapor that is not deposited stays in the cavity. Such a source uses material very efficiently and can produce very high rates of deposition. For example, a wire can be coated by having a heated cavity source such that the wire is passed through a hole in the bottom and out through a hole in the top. By having a raised stem in the bottom of the crucible, the molten material can be confined in a doughnut-shaped melt away from the moving wire. The wire can be heated by passing a current through the wire as it moves through the crucible.

Porous Evaporation Sources. Porous materials and meshes can be used to evaporate materials that wet and wick through the material. Examples of such materials are porous tungsten, tungsten meshes, and porous ceramics. These types of sources are sometimes called dispenser sources and are similar to the electron-emitted cathodes in electronic devices (Ref 60).

Radiation shields can be used to surround the hot vaporization source to reduce radiant heat loss (Ref 56). Generally radiation shields consist of several layers of refractory metal sheet separated from each other and from the heated surface. These radiation shields:

• Reduce the power requirements of the source

• Reduce radiant heating from the source

• Allow the source to reach a higher temperature

• Have a more uniform temperature over a larger volume

Source Degradation. Vaporization sources can degrade with time due to reaction of the evaporant material with the heated surface. When there is reaction between the molten source material and the heater material, the vaporization should be done rapidly. For example, palladium, platinum, aluminum, iron, and titanium should be evaporated rapidly from tungsten heaters. When tungsten is used as the heater material, crystallization at high temperatures makes the tungsten brittle and causes microcracks, which create local hot spots that result in burnout. On burnout, the tungsten is vaporized and can contaminate the film. In general, it is better to replace tungsten wire heaters after each deposition if such contamination poses a problem. When large masses of material that have wet the surface are allowed to cool in brittle containers (crucibles or boats), the stresses can crack the container material.

Sublimation Sources. Vaporization from solid sources has the advantage that the vaporizing material does not melt and flow. Vaporization from a solid can be sublimation from a chunk or pellet, or sublimation from a solid composed of a subliming phase and a nonvaporizing phase (for example, Ag-50Li and Ta-25Ti alloy wire, known by the tradename KEMET). Heating can be by resistive heating, direct contact with a hot surface, radiant heating from a hot surface, or bombardment by electrons.

A problem with sublimation of a solid material from a heated surface is the poor thermal contact with the surface. This is particularly true if the evaporant can "jump around" due to system vibration during heating. Changing the source setup can often alleviate the problem (for example, changing from a boat to a basket source, eliminating mechanical vibration, using mesh "caps" on open-top sources, etc.). Direct electron-beam heating of the material is generally more desirable than contact heating for heating a subliming material.

Better thermal contact between the subliming material and the heater can be obtained when forming the material in contact with the heater by pressing powders around the heater or by electroplating the material onto the heater surface. Powder pressing generally produces a porous material that has appreciable outgassing. Chromium is often electrodeposited on a tungsten heater. Electroplated chromium has an appreciable amount of trapped hydrogen, and such a source should be heated slowly to allow outgassing of the material before deposition commences.

Exploding Wires. Flash evaporation can be accomplished by "exploding wire" techniques, where very high currents are pulsed through a small wire by the discharge of a capacitor (Ref 66). The majority of the vaporized material is in the form of molten globules. This technique has the interesting feature that the wire can be placed through a small hole and the vaporized material can be used to coat the inside of the hole.

Radiant Heating from the Source. The radiant energy, E, from a hot surface is given by:

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