A particle size distribution has two ends, and they do different jobs. The middle sets how fast the surface comes down; the top end sets how often it comes down damaged. Buyers compare D50 because it is easy to quote. Yield is decided by D99 and by the count of particles above 15 μm.
Read both ends of the distribution
| Parameter | What it controls | Why it shows up on the line |
|---|---|---|
| D50 | Removal rate and the Ra level you can reach | One step finer buys finish and costs cycle time |
| D99 / oversize count | Scratch rate | A single 20 μm particle scores a cover glass |
| Dmax | Pump, valve and seal safety | Sets the filtration you need in the loop |
| BET and morphology | How the powder disperses and stays suspended | Same D50, visibly different removal rate |
| CeO₂ / TREO | Chemical driving force behind the cut | Sets the price band and the over-etch risk |
| Fluorine content | Activity and etch aggressiveness | High F cuts faster and attacks the glass sooner |
| pH | Suspension stability and cleaning load | Drifts as the bath is used, taking the rate with it |
Laser and sedimentation do not agree
The same powder measured by laser diffraction and by sedimentation can differ by up to a factor of two in D50, because the methods weight particles differently. This is the most common reason two powders both sold as “1.5 μm” do not compare. Ask which method, and whether the figure is D50 by volume or by weight. Our tables state the method per grade, and the COA repeats it.
Where scratches actually come from
- Oversize in the powder — the tail you did not specify.
- Dried slurry on the pad or in the pipe — flakes behave like 50 μm particles.
- Contaminated water or a loop that was never flushed after a grade change.
- Pad debris and broken conditioner diamonds.
- Handling: a contaminated brush, a dirty tray, an open drum in a dusty bay.
Diagnose in that order. Filter a sample and look at the residue; SEM the scratch to see whether it is a gouge or an embedded particle; then run a blank pad with fresh DI water. If the scratch disappears, the powder was not the source and no amount of grade changing will fix it.
Powder or slurry
Slurry is powder plus water plus dispersant. You pay freight on the water, and in exchange you get a fixed dilution, no make-up equipment on site, and a concentration that only drifts if circulation stops. High-volume lines with their own slurry plant normally stay on powder. Automated lines with frequent changeovers normally prefer slurry, and the one-to-one numbering between our powder and slurry grades means no new purchasing code and no new approval file.