The entire system is essentially a dynamic gas balance system.
At this point: the vacuum level is merely a result,
while the pumping speed determines: the system's ability to handle these gas changes.
Those who work in PVD almost always pay special attention to one number: vacuum level.
Can the equipment achieve:
10⁻³ Pa 10⁻⁴ Pa Lower ultimate vacuum? Often, people assume that:
the higher the vacuum level, the more stable the process.
However, after working in the field for a long time, you'll discover a very counterintuitive phenomenon:
Some equipment has excellent ultimate vacuum and stable pressure readings, yet in actual production, issues such as discharge fluctuations, film bleaching, abnormal oxygen content, and a gradual decrease in yield still occur. Conversely, some equipment may not have particularly impressive vacuum specifications, but its processes remain consistently stable.
The problem often isn't:
"Is the vacuum low enough?"
But rather: Can the gas be removed in time?
The real key behind this is actually: The pumping speed.
Many people's understanding of vacuum systems is limited to:
"How much gas is left in the cavity?"
However, PVD production is not a static process.
During coating, the following processes are constantly occurring within the system:
Workpiece venting; cavity venting; target material releasing adsorbed gas; process gas continuously entering.
The entire system is essentially a dynamic gas balance system.
At this point: the vacuum level is merely a result,
while the pumping speed determines: the system's ability to handle these gas changes.
Why do film layers differ so completely under the same pressure?
This is one of the most difficult aspects to understand in many field operations.
For example: both devices showed 0.5 Pa, and the parameters were similar. Result: One device produced a stable film, while the other experienced continuous problems.
The reason is:
A pressure gauge can only tell you:
"What is the total pressure?"
But it won't tell you:
What kind of gas is inside? How much water vapor is there? How much oxygen remains? Is there gas accumulation in certain areas?
That is to say:
Two systems with the same pressure may have completely different internal environments.
The extraction speed actually affects these three things:
1. Whether pollutants can be removed in a timely manner.
Many membrane problems are not because:
"Sudden contamination."
But rather because: Contaminants have been slowly accumulating.
For example: Water vapor, Volatile organic compounds, Residual oxygen
If the pumping speed is insufficient: These gases will remain in the cavity for a long time.
This ultimately leads to: increasingly unstable discharge, increased oxygen content in the film layer, and decreased adhesion. Many people see:
"The parameters haven't changed, but the results have worsened."
In reality: The cavity environment has gradually changed.
2. Will reactive gases accumulate locally?
This problem is particularly noticeable in reactive sputtering.
For example, when creating oxide films:
If the pump speed is insufficient,
you'll find that:
the target is extremely prone to poisoning.
Many people's first reaction is:
The oxygen flow is too high.
But the reality might be:
The oxygen isn't being carried away in time.
It remains in the area in front of the target.
The local oxygen concentration keeps increasing.
Finally:
The target surface becomes completely unbalanced.
3. Is the discharge environment stable?
Many people overlook one point:
Plasma is actually very sensitive to the environment.
if:
The residual gas is constantly changing, the water vapor concentration fluctuates, and the chamber atmosphere is unstable.
The discharge state will slowly drift.
It manifests itself as:
Voltage starts to change, ignition increases, process window gets narrower and narrower
Why are large chambers more prone to problems?
Many large pieces of equipment encounter a situation where: the vacuum figures are good,
but the process is unstable.
The reason is: The biggest challenge with large chambers is not "achieving a low vacuum."
It's: maintaining a consistent vacuum throughout the entire chamber.
Because with a large chamber: Gas turnover time is longer; dead zones are more numerous; and localized stagnation is more pronounced.
Sometimes: The pressure gauge area is very stable, but the atmosphere in some areas is completely different.
A particularly easy pitfall to fall into: Upgrading only the vacuum pump.
Many people, when faced with a problem, will immediately:
Replace with a larger pump
Increase the ultimate vacuum
But ultimately find:
The effect is not significant.
Because what truly determines the effective pumping speed is not just the pump itself.
How do truly experienced engineers view vacuum systems?
Their focus is no longer on:
"The minimum evacuation depth."
Instead, they focus on:
"How fast is the gas exchange rate? Can contaminants be removed promptly? Will the chamber environment continuously drift? Are the discharge conditions stable over the long term?"
Because a PVD vacuum system, in essence, is not:
"Removing all the air."
It's about:
"Continuously maintaining a stable, controllable, and constantly evolving process environment."
