1. The surface of the chamber will be constantly "rewritten"
Every PVD process will leave traces in the chamber.
These traces mainly come from:
Deposition of material sputtered from the target, adhesion and accumulation of reaction products, alternating coverage of multiple materials
Over time, the inner wall of the chamber is no longer the original metal, but a layer of composite films that are constantly superimposed and evolving.
More importantly, this film is not "stable" and will continue to participate in subsequent processes, such as re-sputtering, redeposition, and re-reaction.
In other words, the chamber surface itself has become part of the manufacturing process.
2. The gas environment is also "remembered"
The chamber is not only "growing film", but also "retaining air".
Water vapor, oxygen, and residual process gases will continue to be adsorbed on the chamber surface.
In subsequent processes, these gases may be released and re-enter the plasma.
The characteristics of this process are:
Not completely controllable. Strongly related to temperature, time, and discharge status. Differences will occur in different batches.
The result is:
Even if the gas flow rate is set to be consistent, the atmosphere actually involved in the reaction may be different.
3. The Discharge Path Gradually "Fixes"
The discharge is not stable from the beginning; rather, it gradually forms a structure over long-term operation.
As the process is repeated: Some regions more easily maintain the discharge; others gradually weaken; the plasma distribution tends towards a "habitual path." This process is essentially establishing a "stable structure."
Once formed, it will have a continuous impact on subsequent processes.
4. Why are problems most likely to occur when changing processes?
When a process is switched, the original "steady state" is disrupted: New materials cover the old deposit layer; the gas system changes; the plasma structure redistributes. However, these changes are not immediate but rather a transitional process.
During this process: The system is neither in the "old state" nor has it entered a "new steady state."
Thus, the typical phenomenon occurs: The first few batches show significant fluctuations, followed by gradual stabilization.
5. Why does cleaning equal "resetting memory"?
The function of cleaning is not only to remove dirt, but more importantly, to:
Remove deposits
Disrupt gas adsorption balance
Reset discharge paths
From a system perspective, it's equivalent to:
Completely clearing away long-term accumulated states.
So after cleaning:
The original craft experience is no longer fully applicable and a new balance needs to be re-established.
6. A More Fundamental Understanding
If we were to summarize this phenomenon in a single sentence, it could be understood as follows: The PVD system is not a "memoryless input/output system," but rather a dynamic system continuously influenced by history.
Parameters are only a part of the input, while the "chamber state" itself is a constantly evolving implicit variable.
7. Summary
The formation of the chamber's "state memory" essentially stems from three long-term superimposed processes: the continuous evolution of the inner wall deposit layer, the continuous cycle of gas adsorption and release, and the gradual solidification of the discharge path during repeated processes. These factors work together to make the system exhibit a clear "history dependence".
8. A key conclusion: The discharge state you see each time is not actually the result of the current parameters, but the superposition of the current parameters and the historical accumulated states.
