PVD: Sputtering and Thermal Evaporation

Any wafer fabrication involves deposition of layers of materials, which will serve as a conductor, an insulator, or a buffer. Thin film metal layers like aluminum interconnects are deposited in the wafer substrate through Physical Vapor Deposition (PVD) – a process which uses vacuum deposition technique.

One of the earliest PVD techniques is the thermal evaporation. In thermal evaporation, the metal is heated to its vaporization point. It will then evaporate to the wafer to form the thin film.  However, since evaporated materials tend to be directional, thermal evaporation provides poor coverage.

The poor coverage problem in thermal evaporation was solved when sputtering emerged. Sputtering is one of the most widely used PVD techniques by the semiconductor industry in deployment of thin film metal layers over the wafer substrate. It injects the metal layer in the substrate by ‘knocking-off’ the metal atoms out of the metal material and bombards the ‘knocked-off’ ions on to the wafer. This grants sputtering a better coverage of deposition than thermal evaporation.

Physical Vapor Deposition Techniques

Figure: Physical vapor deposition techniques: (a) thermal evaporation and (b) sputtering

Following are the main differences of thermal evaporation and sputtering:

1. Coverage Area

As mentioned, sputtering has a larger coverage area than thermal evaporation because of the nature of their distribution method. The vapors used in thermal evaporation are directional; while the ion bombardment of the sputtering is ‘rain-like’.

2. Deposition Rate Control Factors

In thermal evaporation, deposition rate can be controlled by the amount of heat supplied on the material (vaporization point). On the other hand, sputtering controls its rate through gas pressure, temperature, the potential difference between the material, which acts as the cathode of system and the wafers placed on the system’s anode.

3. Deposition Rate

When talking about deposition rate, the more controlled the rate is (lower number of layers/second), the better. Sputtering can trim down its deployment of metal layers up to one atomic layer per second. Whereas thermal injection can only control it to hundreds or thousands of atomic layers per second.

4. Choice of Material

Sputtering has a wider range of choices for material than thermal evaporation.

5. Decomposition of Material

The uniformity of decomposition and erosion of the material in sputtering makes it more efficient than thermal evaporation.

6. Equipment Cost

Operating using sputtering will cost more than thermal evaporation because the latter only needs a vacuum chamber with precise thermometers; while the former requires twice or thrice of the energy used in thermal deposition to excite the ion of the material.

7. Surface Damage

Surface damage has higher possibility in sputtering. Its ion particle bombardment can induce damage in the substrate.

THERMAL EVAPORATION

SPUTTERING

Coverage Area poor coverage large coverage
Deposition Rate Control Factors temperature only gas pressure,temperature, andpotential difference between the material and the wafer
Deposition Rate thousand atomic layers per sec one atomic layer per sec
Choice of Material Limited wide range of variety
Decomposition of Material High Low (uniform)
Equipment Cost Cheaper more expensive
Surface Damage very low ion particle bombardment can induce damage

Considering the pros and cons between these two techniques, sputtering will clearly prevail over thermal evaporation for industrial applications. Thermal evaporation, on the other hand, is most of the time preferred in laboratory experiments.

Aside from being the main choice for thin film deposition in semiconductor industry, sputtering is also used in fabrication of compact disks, large area displays, and other deposition process on blades and gears.

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The evolution of physical vapor deposition technology from thermal evaporation to sputtering

PVD (physical vapor deposition) through sputtering

Sputtering deposition is one of the main physical vapor deposition techniques to deposit thin films of metals or other materials over a substrate.

The technique is useful for microelectronics applications and has been widely popular in the industry for several reasons as it can be seen as the natural evolution of thermal evaporation, one of the earliest techniques for thin film physical vapor deposition.

While thermal evaporation requires only a relatively simple vacuum chamber where the material is heated in a controlled way and evaporated, sputtering needs much larger and complicated machinery that allows the substrate to be literally “bombarded” by ions in order to eject particles that get deposited over a substrate placed in front of the target itself
Sputtering is now the deposition method of choice for industrial applications while thermal evaporation is little more than a lab experiment as sputtering allows deposit a large variety of materials in a much tightly controlled way.

While with thermal evaporation the deposition rate can only roughly be controlled by setting the material source at a predetermined temperature, sputtering allows to control more factors such as: the gas pressure inside the chamber, the potential difference between anode and cathode, the target and substrate temperatures, etc.

The main differences between thermal evaporation and sputtering are:

  1. With thermal evaporation only a limited number of materials can be deposited, with sputtering the variety is much larger;
  2. With sputtering it is possible to control the deposition speed down to almost one atomic layer per second while with thermal deposition hundreds if not thousands of atomic layers are deposited in a second. In industrial applications, where the deposition rate must be strictly controlled, thermal evaporation is then not an option
  3. It is possible to deposit larger areas with sputtering than with thermal evaporation
  4. The decomposition of the target material and its erosion during sputtering are uniform and therefore the process makes better use of the target material, which in many cases can be expensive (gold, ruthenium, etc.)

However, sputtering has also some disadvantages if compared with thermal evaporation as:

  1. It needs by far more expensive machinery and devices than thermal evaporation, which basically needs only a simple vacuum chamber coupled with a precise thermometer
  2. The substrate can be damaged due to particle hit during sputtering

From the above considerations, it is clear that the advantages of sputtering over thermal evaporation far outweigh the disadvantages for industrial applications.

Nowadays, sputtering is widely considered the physical vapor deposition method of choice for compact disks, large area displays, and even for applications not related with microelectronics, such as deposition of TiN, CrN, TiC or CrAlN over saw blades and bearing gears.

The sputtering process mechanism works as follows: during the deposition, the disc of materials which needs to be deposited (called as the “target”) has a negative potential and it is bombarded by positive ions of inert gases such as argon or xenon. By purely kinetic energy transfer between the ions colliding on the target and the atoms on the target surface, the latter get ejected from the target surface and collide with the substrate, creating a thin film of material over it.

Keeping all other variables fixed, sputtering deposition rate is proportional to the plasma energy within a certain range and therefore it can be easily controlled.

This is the main reason why sputtering deposition is superior to other thin film physical vapor deposition methods such as thermal evaporation

However, as hinted above, the whole process of sputtering requires a much larger system as it takes at least two or three order of magnitude more energy to liberate one atom from the target during sputtering than during thermal evaporation.

Moreover, the target and the substrate require considerable cooling and therefore a proper refrigerating system needs to be put in place.

The whole sputtering station needs to be into vacuum, and to be kept free from contamination from atmospheric element.

The formation of sustained plasma is also crucial to the reliability of the whole process.
The reason why the conditions of the plasma need to be carefully controlled are several, probably one of the most important parameters is plasma pressure, a factor that is pivotal to the reliability of the whole process.

If the pressure of the plasma is too high, a large number of the sputtered atoms cannot pass through the gas and get reflected back to the cathode, while if the pressure is too low, the number ionizing collisions between secondary electrons released from the cathode and the inert gas is not enough to keep the plasma sustained at the needed levels.

Another point that needs to be carefully considered is the power supply for the whole system as the type of power supply is heavily dependent on the type of material that needs to be sputtered.

For conductors, a typical DC power supply can be used, for insulating materials an RF power supply is needed. Insulating materials are usually harder to be sputtered and therefore sometimes sputtering may not be the best physical vapor deposition method, plasma lased deposition is a another possible option

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