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.

Note:
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