Sputter coating is a process where the pressure typically ranges from 10^-2 Pa to 10 Pa.
This relatively high pressure plays a significant role in the sputtering process.
It affects various aspects, including the mean free path of the process gas molecules, the angle at which adatoms arrive at the substrate, and the potential for gas absorption into the growing film.
This can lead to microstructural defects.
What is the Pressure of Sputter Coating? (5 Key Factors Explained)
1. Pressure Range and Its Impact on Mean Free Path
In sputter coating, the working pressure is generally between 10^-2 Pa and 10 Pa.
This pressure range is much higher than that in thermal or e-beam evaporation systems, which operate at pressures around 10^-8 Torr (approximately 10^-10 Pa).
At these higher pressures in sputtering, the mean free path (the average distance a particle travels between collisions) is much shorter.
For example, in direct current magnetron sputtering (dcMS) at 10^-3 Torr (approximately 10^-5 Pa), the mean free path is only about 5 centimeters.
This is compared to 100 meters in systems operating at 10^-8 Torr.
2. Effect on Adatom Arrival Angles
Due to the high density of the process gas and the short mean free paths, adatoms in sputtering processes tend to arrive at the substrate at random angles.
This is different from evaporation methods where adatoms typically approach the substrate at a normal angle.
The random angles in sputtering are a result of numerous collisions that occur as the adatoms travel from the target to the substrate.
3. Gas Absorption and Microstructural Defects
The abundance of process gas near the substrate/film interface can lead to some of this gas being absorbed into the growing film.
This absorption can introduce microstructural defects, which can affect the film's properties and performance.
4. Pressure Management in Reactive Sputtering
In reactive sputtering, managing the pressure is crucial to prevent the "poisoning" of the target surface.
This can hinder the growth of the thin film.
At low pressures, the film formation is slow, while at high pressures, the reactive gas can negatively impact the target surface.
This reduces the growth rate of the film and increases the rate of target poisoning.
5. Vacuum System Requirements
The vacuum system for sputtering requires a base pressure in the high-vacuum range (typically 10^-6 mbar or better) to ensure clean surfaces and avoid contamination.
During the sputtering process, the pressure is adjusted to the mTorr range (10^-3 to 10^-2 mbar) by introducing the sputter gas.
This is controlled by a flow controller.
The thickness of the deposited film is also monitored and controlled during this process.
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