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Semiconductor Process6 min read

CMP Chemical Mechanical Planarization Process Monitoring Strategies

This article details a past major crisis in wafer production where an alarming CPK report highlighted significant process control issues. It elaborates on how passivation in the Chemical Mechanical Planarization (CMP) process, akin to uneven surface treatment, can critically impact wafer thickness and explains the strategies implemented to identify and resolve these challenges.

That day the CPK report came out, the whole room fell silent for three seconds

I still remember years ago, we had a batch of 12-inch wafers that, after CMP polishing, suddenly showed abnormal thickness—a cliff-like drop. That evening, I was in the Control Room with Ah-Zhe from the night shift, staring at that appalling CPK report. It showed a red value of 1.08, accompanied by a DPMO 6210 alert. Ah-Zhe rubbed his eyes and said to me, "Senior, this batch is going to fail." My heart sank, and only one thought remained in my mind: How could this happen?

Where did the problem lie?

In essence, the biggest fear in CMP process monitoring is "passivation." Think of CMP as "exfoliating" the wafer, removing excess material from the surface to make it flat and smooth. However, if even a slight deviation occurs in the slurry composition, the condition of the polishing pad (Pad), or even the equipment parameters, the wafer surface condition will become uneven, much like a facial mask applied improperly—some areas absorb well, while others get stuck, resulting in non-uniform final polished thickness. The most common issues we encounter are the effective lifespan of the Slurry and the wear of the Pad; these can slow down or destabilize the removal rate, leading to wafer thickness deviations.

Therefore, the key is not to wait until the CPK report comes out to discover problems. Process monitoring means nipping problems in the bud, preventing them from becoming a major bomb.

How is it actually done?

Frankly, CMP's process monitoring strategy revolves around two points: "parameter stability" and "real-time feedback."

  1. Real-time Monitoring of Removal Rate: We install real-time thickness measurement instruments on each machine. After polishing each wafer, the Removal Rate is immediately measured and recorded. If this value continuously exceeds the LSL (Lower Specification Limit) or USL (Upper Specification Limit) for three consecutive wafers—for example, if the LSL is set at 1000 Å/min and the USL at 1100 Å/min, an alarm will sound immediately if the rate is 980 Å/min or 1120 Å/min.
  2. Regular Checks of Slurry Concentration and pH Value: These two are crucial for determining Slurry performance. We perform at least four sample checks daily to ensure the concentration remains within a ±5% range and the pH value within a ±0.5 interval. If either parameter is exceeded, the chemical supplier will be "invited" for questioning.
  3. Pad Wear Monitoring: The polishing pad is like your sandpaper; it dulls with prolonged use. In addition to setting fixed replacement times for the Pad, we also monitor the Surface Roughness after polishing. If the roughness suddenly deteriorates—for example, skyrocketing from an original 0.5 nm to 1.5 nm—the Pad must be replaced prematurely, even if the scheduled time hasn't arrived.

In other words, these measures are building lines of defense for your process.

The Most Common Pitfall

Speaking of the most common pitfall, it's definitely "data drift without awareness." Often, parameters don't instantly jump to the red line but slowly "drift" along the edge of the green line. There was a newcomer who watched the Removal Rate data slowly drop from 1050 Å/min to 1030 Å/min, then 1010 Å/min. Since it was still within specification, he thought nothing was wrong. It wasn't until it dropped to 990 Å/min and the alarm sounded that he realized the previously accumulated wafers were already too thin. At that point, salvaging them was costly.

Therefore, merely checking whether current data exceeds limits is insufficient; you also need to observe trends. We later introduced SPC (Statistical Process Control) charts precisely to catch these "slow but fatal" trends.

One Thing You Can Do Today

Re-examine your process parameter monitoring frequency.

Want to try it yourself?

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