In many PVD projects, a very typical problem is frequently encountered:
On the same workpiece, some areas are very thick, while others are very thin, sometimes with very obvious differences.
Many people's first reaction is: Is there a problem with the rotating frame? Is the rotation uneven?
So they start: Checking the motor, adjusting the speed, optimizing the fixture.
But the result is often: Limited improvement, or even no change at all.
This illustrates a key point:
Uneven thickness is not just a "rotation problem," but also a "plasma distribution problem."
First, the conclusion: This is a "spatial distribution problem."
The film thickness essentially depends on: the atomic flux reaching the surface per unit time.
And this flux is often: spatially non-uniform.
In other words: the "material flow" received by the workpiece varies at different locations.
Second. What exactly does rotation solve?
Many people's understanding of rotation is:
"Make the film layer even"
But a more accurate understanding is:
Rotation just "averages unevenness" rather than eliminating it.
For a simple example: If the plasma distribution is: strong on one side, weak on one side, after rotation: The workpiece will periodically pass through the strong and weak regions. The result: An "average value" is obtained.
But the premise is:
The difference cannot be too big
If the difference is too big:
No matter how much you turn it, it will never be completely even.
Third. When is the "rotation problem"?
Feature 1: Thickness changes periodically
For example:
The thickness changes regularly with each revolution
Feature 2: Poor consistency among different workpieces; some are good, some are bad.
Feature 3: Significant improvement in speed adjustment.
Common causes: Unstable rotation speed; Rotational eccentricity; Asymmetrical workpiece placement.
Conclusion: The core issue in rotation problems is "uneven time distribution".
Forth. When is it a "plasma problem"? (More common)
Characteristic 1: Always thick in a certain direction
No matter how it rotates, this region is always thicker.
Feature 2: Overall "spatial offset"
For example: The side closer to the target is thicker, while the side farther away is thinner.
Feature 3: All workpieces exhibit similar patterns.
Common causes:
1. Uneven magnetic field distribution; Uneven target erosion; Uneven plasma density
2. Influence of cavity structure: Obstruction, Reflection
3. Multi-target interference: Uneven superposition between different targets.
4. Unreasonable workpiece position design
Conclusion:
The core of the plasma problem is "uneven spatial source"
Fifth. A Key Misconception
Many people, when faced with unevenness,
their first reaction is to adjust the rotation. But in reality: Rotation can only alleviate the problem, not cure it.
If the root cause is severe plasma inhomogeneity, then increasing the rotation speed is merely a temporary fix.
Sixth. How to quickly determine the type of problem?
Here's a practical method:
Method 1: Fix the workpiece (without rotating)
Observe: Film thickness distribution
If at this point: The distribution is already severely uneven; Conclusion: Plasma problem.
Method 2: Change the rotational speed
If:
Change in rotational speed → Significant change in uniformity
Conclusion: Rotational problem
Method 3: Change the workpiece position
If: Change position → Change in distribution
Conclusion: Spatial distribution problem
Seventh. Truly Effective Optimization Strategies
1. Prioritize Addressing "Spatial Distribution"
Optimize target position; adjust magnetic field; improve cavity structure
2. Further Optimize the Rotation Strategy
Rotation Speed
Rotation Trajectory
Oscillating Mode
3. Optimize workpiece placement: Avoid obstruction and improve symmetry.
4. Multi-target collaborative design: Enabling different targets to form a "uniform superposition".
Eighth. A Key Cognitive Upgrade
Many people understand the problem as:
"Uneven membrane structure is a mechanical problem."
But a more fundamental understanding is:
This is a problem of "uneven mass transport."
In other words, the path of the material from the target to the workpiece is spatially non-uniform.
Ninth. Final Summary
Is a large thickness difference on the same workpiece a rotational or plasma-related issue?
The answer is:
It could be a rotational issue, but more often it's a plasma distribution issue. The key difference is:
Rotational issues → temporal unevenness; Plasma-related issues → spatial unevenness.
