Corrosion Failure Analysis: Accelerated Testing for Electrochemical Corrosion

That Batch Was Mysteriously Scrapped, Only to Discover a Ghost in the Etching Machine

Seriously, do you remember that last batch where 20% failed directly during pre-packaging testing? The entire fab was stunned. The yield plummeted from a stable Cpk of 1.3 directly to 0.8, and the production line manager's face turned as green as a ghost. We spent half a day investigating, checking every single aspect from process to materials. And what did we find? Ultimately, we discovered it was the pipelines inside the etching machine, where several spots exhibited strange corrosion. What's more outrageous, it was almost invisible to the naked eye; those tiny holes only became apparent after 1000x magnification.

What Was the Problem? Why Did It Corrode?

Frankly, in a chemical-rich environment like a semiconductor fab, corrosion is simply commonplace. Many of the chemicals we use are strong acids and bases, such as hydrofluoric acid, sulfuric acid, and hydrogen peroxide. These substances have a certain corrosivity towards metals, plastics, and even ceramics. Do you think equipment made of stainless steel is immune? Heh heh, that's too naive. Even stainless steel, under specific conditions—for instance, in the presence of chloride ions (Cl-) or localized stress concentration—will still develop "holes." Our problem this time was electrochemical corrosion, which, simply put, is a "rusting" phenomenon that occurs when metal is in an electrolyte solution due to a potential difference.

So the key question is: will the lifespan of equipment or materials shorten after exposure to chemicals? By how much? Can we predict this problem before it truly fails?

How Is It Actually Done? Accelerated Testing Helps You Catch the Ghost

Alright, we know the problem, so what do we do? We can't just wait until the equipment rusts and leaks, can we? This is where "accelerated testing" comes into play. The most common and practical method is to conduct an "electrochemical corrosion accelerated test."

1. Select the Right Environment: We immerse the suspect material (e.g., etching machine pipeline material) in a solution "similar" to the actual chemicals used in the process. Note that "similar" here does not mean directly wasting expensive production chemicals; we adjust the concentration, temperature, and even add "catalysts"—for instance, increasing concentration by 10 times or raising the temperature by 20 degrees.
2. Apply Potential: To accelerate the process, we apply an "external potential" to the material. This is like giving the corrosion reaction a stimulant. For example, if the pipeline typically has a 0.5V potential difference across its ends during normal operation, we might directly raise it to 1V or 1.5V during testing.
3. Monitor Current: Here's the most crucial part: we continuously monitor the material's "corrosion current density." The larger the corrosion current, the faster the material corrodes. If the current starts at 100 nA/cm² but spikes to 500 nA/cm² after 100 hours, then you should start worrying.
4. Calculate Lifespan: Based on this change in corrosion current, combined with empirical formulas (such as Arrhenius Law), we "extrapolate" the expected lifespan of the equipment under normal operating conditions. For example, if after 100 hours of accelerated testing, we find the material's corrosion depth reaches 10 micrometers, and our acceleration factor is 100x, then this means the material would require 10,000 hours under normal use to reach the same corrosion depth.

In other words, using this method, we can simulate several months or even years of equipment "wear and tear" within just a few days. This is far more cost-effective and hassle-free than waiting for it to actually break down before repairing it.

The Most Common Pitfalls: Don't Blindly Jump In

The most common mistake I've seen is "over-accelerating." Some rookie engineers, in their haste, push test conditions to the extreme—for example, raising the temperature close to the material's melting point, or applying such high potential that the material starts smoking. And what's the result? The corrosion mechanism of the material changes completely; it's no longer the corrosion that would occur under actual conditions. If you present those test results to your boss, you'll just be in for a scolding.

Also, don't forget the "control group." You can't just test the problematic material; you also need a material "known to be good" to serve as a control group. This way, you can determine whether the corrosion you measured is due to the material's inherent properties or if an actual "abnormality" has occurred. I once saw someone who tested a bunch of data without a control group, had no idea if the data was good or bad, and ultimately had to redo everything, wasting several months.

One Thing You Can Do Today

Go back and check your fab's chemical storage area for any signs of "discoloration" or "leakage" in equipment pipelines. If there are, investigate immediately!

Article Category: Reliability