PVD

PVD stands for "physical vapor deposition". The word "physical" in this context is used to distinguish processes from other "chemical" processes such as chemical vapor deposition and plasma etch. As one might imagine, there is a considerable amount of "overlap" in such definitions. Most "physical" processes have important "chemical" aspects.

The most common PVD processes are evaporation and sputtering, and all their many variants. The most useful way to distinguish these processes is to pose the question, "what makes the process go"?

Evaporation

For evaporation processes, the driving force is heat. The material to be evaporated is simply heated by some suitable means until the desired evaporation rate is achieved. As a practical matter, evaporation for the deposition of thin solid films always takes place in a high vacuum chamber. The evaporated material travels through the (nearly empty) space within the vacuum chamber until striking a surface that is cold enough for it to condense. Condensation simply means that the atoms or molecules "stick".

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Sputtering

For sputtering, the driving force is the kinetic energy of energetic ions. These ions "bombard" the source of material, called the sputtering target. Atoms are literally knocked off the target surface by these ions in what looks a great deal like a game of "atomic billiards". Like evaporation, sputtering is performed in a vacuum chamber, and the atoms or molecules travel across the chamber until they condense to form a thin film. Unlike evaporation, however, a process gas is generally required to produce the ions. This can be accomplished using an ion source or by means of a plasma.

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More about Evaporation

There are many possible ways to heat material for evaporation; the most common contemporary ways are the following:

Filament Evaporation:

For filament evaporation, we begin with a very robust filament, usually fabricated from tungsten. A suitable AC low voltage high current power supply must be provided to heat the filament.

Advantages:

Low cost, simple, relatively safe.

Disadvantages:

The material to be evaporated may react with the hot filament; relatively small amounts of material can be evaporated at one time. Many highly refractory materials may be impossible, or at least, impractical to evaporate.

"Boat" Evaporation:

An evaporation boat is simply a container for the material to be evaporated and which is heated in the same way a filament is heated. The boats are usually fabricated from tantalum and may be coated with a material to help inhibit any reaction between the boat and the material to be evaporated.

Advantages:

Higher cost than the filament, but still relatively low cost,simple, relatively safe.

Disadvantages:

The material to be evaporated may react with the hot filament; relatively small amounts of material can be evaporated at one time. The power supply required may be quite a bit larger than the power supply required for simple filament evaporation. Water cooled feedthroughs may be required. As before, many highly refractory materials are difficult or even impossible to evaporate.

Electron Beam Evaporation:

The material to be evaporated is placed in a water cooled crucible and bombarded by a high energy beam of electrons. Almost anything can be evaporated in this manner since the power density can be extremely high.

Advantages:

Very capable technique... almost anything that is vacuum compatible can be evaporated, unless it decomposes.

Disadvantages:

The equipment required is much more expensive than equipment required for filament or boat evaporation, and there is a significant high voltage safety hazard. Very high deposition rates may be achieved.

More about Sputtering

As mentioned before, sputtering processes all resemble a game of "atomic billiards" taking place in a low-pressure gas. Any of these processes can be primarily "physical" or may include a significant "chemical" component. The most common examples of the latter are called "reactive sputtering". Reactive sputtering can make some very important thin solid films, such as titanium nitride barrier layer films.

One of the more useful ways to categorize sputtering processes is in terms of the manner in which the ions are generated. There are many possible ways to provide the necessary ions for sputtering, but the most common contemporary ways are the following:

Planar Diode RF Sputtering:

RF planar diode sputtering is an outgrowth of DC planar diode sputtering, which is not used much these days since relatively high pressures, hundreds of mTorr, are usually required. By comparison, RF diode sputtering can be performed at pressures as low as a few mTorr.

Advantages:

Simple inexpensive disk targets; able to sputter dielectric materials, operates well at low pressure, typically a few mTorr to a few tens of mTorr, a suitable range for commercially available cryopumps and turbopumps.

Disadvantages:

The RF power supply is typically expensive, and a rather expensive impedance matching network is required. There is a potential problem with RFI (radio frequency interference) if the system isn't designed and/or operated properly.

DC "Magnetron" Sputtering:

DC "magnetron" sputtering involves the well-known principles of "crossed-field" electrical gas discharges to give very high deposition rates as well as other highly desirable parameters. The high deposition rates simply come from the fact that such magnetically enhanced discharge plasmas allow very high power density under otherwise available conditions.

Advantages:

High deposition rates at low pressures are typical; good uniformity and step coverage is possible. The sources are usually quite rugged. RF operation is possible, but not common.

Disadvantages:

Good deposition uniformity comes at the expense of very non-uniform erosion of the target. This means that target life suffers.

Rotating Magnet DC Magnetron Sputtering:

This technique is relatively recent. It is an attempt to address the question of material utilization efficiency by moving the magnet structure over the surface of the target. "Moving magnet"structures have been around for quite a number of years, of course. The "trick" is to get good material utilization (remember: the erosion profile of a magnetron sputter source is highly non-uniform!) AND good uniformity and step coverage at the same time.

Advantages:

Properly executed, these sources perform very well.

Disadvantages:

Properly executed, these sources perform very well.

Ion Beam Sputtering:

Ion beam sputtering uses a separate ion source, as opposed to the previous two examples where the target electrode is the cathode in the discharge. The discharge, of course, is the source of the ions in such configurations.

Advantages:

Very capable technique... The separation of the ion source from the target allows a great deal of control. It is often possible to operate at much lower pressure than other forms of sputtering. This may significantly reduce contamination of the growing film by residual gas.

Disadvantages:

The equipment required is somewhat more expensive when measured in terms of rate and coated area per unit cost. The ion sources may also be rather fragile.