Are EPD and OES enough?

Finding endpoint and keeping final quality consistent are different problems.

In CMP and Etch, knowing when the process reaches a transition point is critical. CMP uses endpoint detection to identify when the target film begins to clear, while Etch can use OES and other sensor signals to detect the transition between the etched layer and the underlying material.

This is more precise than stopping only by fixed time because the transition can follow actual process state.

But accurate endpoint detection does not make final quality identical. In production, a quality-critical region remains after endpoint.

CMP EPD uses changes in friction, acoustic or optical signals, while Etch OES uses changes in the emission spectrum. Both processes still have an over-polish or over-etch region after endpoint, and some locations can retain material at that moment.

CMP continues after endpoint detection

CMP endpoint detection generally captures the point where a target film begins to clear or the underlying film starts to appear. Not every location on a wafer reaches that point at exactly the same time.

• Film-thickness and pattern-density differences across the wafer
• Position-dependent pressure
• Slurry-supply differences
• Pad-contact differences

Some regions may already be clear while others still retain film. CMP therefore often continues for an over-polish interval after endpoint.

• If over-polish is too short: residual film and uncleared regions remain.
• If over-polish is too long: dishing, erosion and underlayer loss increase.

Final quality therefore depends not only on how precisely endpoint is detected, but also on how the post-endpoint polishing condition is managed.

Etch also depends on what happens after the detected endpoint

In plasma Etch, OES measures wavelength and intensity changes in light emitted by the plasma. Changes in the etched material or reaction by-products can indicate when the underlying layer begins to appear.

But the detected endpoint can represent the moment when only part of the wafer starts to clear. Etch rate is affected by several factors.

• Wafer position
• Pattern size and density
• Loading
• Chamber state

Many Etch recipes therefore include an over-etch or finish-etch step after main etch to clear remaining material while limiting underlayer loss, CD change and profile damage.

• If over-etch is insufficient: under-etch and residue remain.
• If over-etch is excessive: the underlayer and structures can be damaged.

Endpoint can therefore be the start of the next control region rather than the final result.

Fixed post-endpoint conditions have limits

The simplest strategy is to run the same over-polish or over-etch time after every endpoint. That can work when the process is stable.

Production conditions, however, continue to change.

• Incoming-wafer film thickness and pattern composition
• CMP pad and slurry aging
• Etch chamber and consumable state such as focus rings
• Changes after cleaning, maintenance and consumable replacement

If time-to-endpoint changes, it may indicate a change in process rate or incoming state. Using the same post-endpoint condition regardless can leave that change in the final-quality distribution.

Better detection alone cannot solve this. Detection identifies when the process changed; it does not determine how the next wafer should be adjusted.

APC manages the quality after endpoint

APC uses process data and quality results to adjust conditions for the next run. CMP can adjust polishing time or other process conditions, while Etch can adjust over-etch time or recipe setpoints.

• If the previous wafer shows residual film → strengthen the next-run condition, such as extending polish or etch time.
• If underlayer loss or excessive removal appears → reduce over-polish or over-etch.

Equipment state, consumable life, recent endpoint time and incoming-wafer information can also be incorporated.

Calculated changes should remain inside explicit safeguards.

• Allowed correction range
• Maximum process time
• Fallback recipe
• Approval conditions

EPD and OES do not replace APC. EPD and OES detect process-state transitions quickly, while APC updates conditions across runs to reduce final-result variation.

Endpoint is not the end of process control

CMP EPD and Etch OES are already valuable automation technologies. They reflect process state better than fixed-time operation and can reduce under-processing and unnecessary over-processing.

Production requires more than finding endpoint consistently. Final quality must remain stable wafer after wafer, including the over-polish and over-etch region after endpoint.