Wafer polishing fails on two things: metallic contamination and oversized particles. Everything else is secondary. Choose the grade by purity and PSD tail first, then by crystal type, then by price.
Why Wafers Are Different
A wafer is a device substrate, not a mechanical part. Anything left on the surface or driven into it becomes a yield problem later in the fab. That changes the specification in ways that do not appear on a normal abrasive datasheet.
Three properties move to the front of the list:
- Metallic impurity level. CMP slurry work commonly specifies metal content below 10 ppm, because iron, copper and nickel create electrical defects.
- PSD tail control. A single particle above the target size produces a scratch that can kill a die. D97 and D99 matter more than D50.
- Slurry stability. The powder must stay dispersed for the life of the batch. Settling and re-agglomeration change the removal rate mid-run.
The Size Ladder
Wafer work spans roughly 0.5 to 60 micrometres depending on the step and the material. The ladder below reflects how the band is usually split in practice.
| Step | Typical size | Grade direction | Notes |
|---|---|---|---|
| Coarse grind / backside | 20 – 60 um | HDY, HDG | Stock removal, damage layer management |
| Lapping | 8 – 16 um | HDY, HDYP | Flattening before polish |
| Pre-polish | 3 – 6 um | HDY, HDYP | Removes lap damage |
| Polish | 1 – 3 um | HDYP, HDP | Surface quality step |
| Final / CMP | 0.5 – 1 um and finer | HDP, HDPN, HDYM | Defect-limited step |
SiC And Sapphire Behave Differently
Silicon carbide is harder and more chemically inert than sapphire, and it removes more slowly. Sapphire is anisotropic: removal rate and scratch behaviour change with crystal orientation, so a grade that works on the c-plane may behave differently on the a-plane.
The practical consequence is that SiC usually needs a more aggressive starting point and a longer ladder, while sapphire rewards tighter PSD control at the finish. Neither responds well to a single universal grade.
Crystal Choice At The Finish
Monocrystalline powder cuts faster and is the economical choice for lapping and pre-polish. At the final step, polycrystalline material is normally preferred because the particle fractures into smaller cutting points instead of ploughing.
- HDYP covers 0.7 to 40 micrometres and is the general poly route for wafer pre-polish and polish.
- HDP is detonation-synthesised poly, roughly 0 to 1 micrometre down to 1/8 micrometre.
- HDPN continues from 5 to 500 nanometres for the finest steps.
- HDYM at W0.5 to W0.1 is the mono nano option when a customer prefers monocrystalline chemistry.
The Purity Question
Ask for the analytical method, not just the number. “Low metal content” means nothing without knowing whether it was measured by ICP-OES, by XRF, or estimated from ash. Ash content itself varies widely by grade; measured ash on high-strength grit runs from 0.0002% to 0.0093%, and the low end is what wafer work needs.
Coating, Carrier And Cleaning
Three process details decide whether a correct grade performs correctly on a wafer line.
- Coating. Nickel and titanium coatings exist to anchor grit in a bond or protect it from heat. In a slurry they add metal to a process that is trying to avoid metal. Specify uncoated unless the process explicitly calls for coated powder.
- Carrier. The carrier has to keep the powder suspended and keep the surface wetted without attacking the workpiece. For sapphire and SiC, mildly alkaline water-based systems are common; the exact choice depends on the pad and the post-clean step.
- Cleaning. A particle that is removed from the slurry but stays on the wafer becomes a defect at the next step. Rinse and clean sequences matter as much as the polish itself, and a change in powder can change how easily residue releases.
This is also why slurry suppliers ask about the downstream clean: a formulation that polishes well and cleans badly is not a working formulation.
What A Wafer Customer Usually Sends
A useful first enquiry names the substrate and orientation, the step in the process, the current slurry and its solids loading, the pad type, the target removal rate and the defect specification, plus the cleaning process that follows. With those six items a supplier can recommend a grade and a starting ladder in one round.
Common Mistakes
- Buying on D50 only. Two powders with identical D50 can differ by an order of magnitude in the number of particles above 5 micrometres.
- Ignoring slurry ageing. A slurry that passes on day one and scratches on day three is an agglomeration problem, not a grade problem.
- Using the same grade across materials. SiC and sapphire need separate ladders, and often separate carriers.
How To Verify
Request a full PSD curve rather than summary numbers, plus the impurity report with the method stated. Run a coupon test on the actual material and orientation, count scratches at the same magnification, and check the slurry after 24 and 72 hours for settling and re-dispersion behaviour.
Next Step
Products: HDYP, HDPN nano, HDYM. Applications: SiC and sapphire wafer polishing and CMP pad conditioners and slurries.
Related reading: ash and magnetic content.