How to Select Polishing Slurry Additives? Dispersion, Wetting and Corrosion Protection Solutions
1. Why do polishing slurries impose demanding requirements on additives?
Polishing slurry (lapping fluid) is a core consumable in precision processing—semiconductor CMP, metal workpiece polishing, stone/glass/gem lapping are essentially abrasive particles acting on the workpiece surface at high speed within a liquid carrier to achieve material removal and surface planarization. Regardless of the process, a polishing slurry must simultaneously satisfy the following:
- Abrasives must “disperse well and settle slowly”: The finer the particle size and the larger the specific surface area of abrasives such as alumina, silica, ceria and diamond micropowder, the more prone they are to agglomeration and settling. Once abrasives agglomerate into large particles, efficiency drops at best, and scratches on the workpiece surface occur at worst—the most dreaded defect in the polishing industry;
- The fluid must “spread quickly and wet evenly”: The contact between polishing slurry and the high-speed rotating polishing pad/workpiece occurs on a millisecond timescale. The fluid must spread rapidly into a continuous liquid film in order to uniformly deliver abrasives and carry away swarf and heat;
- Bubbles must “form less and break faster”: High-speed polishing pad agitation and recirculating pump delivery entrain air. Foam interrupts the liquid film, causing localized dry polishing and scratching, as well as unstable liquid levels and difficult make-up addition;
- Metal surfaces must “polish bright and stay bright”: Freshly polished metal surfaces (aluminum alloy, stainless steel, copper alloy, etc.) are highly active and prone to oxidation, tarnishing, rusting and water spotting during storage.
In short: Polishing slurry additive selection is not a single-function issue; it is a four-in-one formulation project of “dispersing + wetting + foam control + corrosion protection.” Failure of any one link will show up in the workpiece surface quality.
2. Four additive functions and mechanisms of action
| Function | Problem addressed | Mechanism of action | Corresponding additive direction |
|---|---|---|---|
| Abrasive dispersion | Agglomeration, settling, scratching, efficiency loss | Adsorbs onto abrasive particle surfaces, providing steric hindrance/electrostatic repulsion to maintain the dispersed state | Polymeric dispersants (Toynol® DS/ADS/HD series) |
| Rapid wetting | Discontinuous liquid film, uneven wetting, localized dry polishing | Lowers dynamic surface tension, accelerating fluid spreading over workpiece/abrasive surfaces | Acetylenic diol wetting agents (FS-204, Superwet series) |
| Foam control | Foam interrupting the liquid film, cavitation, unstable liquid level | Low-foam by design at the source plus foam breaking; silicone-free systems avoid residual contamination | Low-foam surfactants/defoamers (DF series, etc.) |
| Metal corrosion protection | Oxidation discoloration, rusting and water spotting after polishing | Corrosion inhibitor molecules adsorb onto the metal surface to form a film, isolating oxygen and water | Vanconol® corrosion inhibitors (matched by metal type) |
2.1 Abrasive dispersion: “source-level treatment” of scratches
Most polishing scratches originate from abrasive agglomeration. The role of a dispersant is to anchor-adsorb onto abrasive particle surfaces and separate the particles through the steric hindrance (or electrostatic repulsion) of polymer chains, keeping abrasives at their original particle size and stably suspended. In waterborne systems, the Toynol® DS series polymeric dispersants can be evaluated; for inorganic abrasive systems (alumina, silica, ceria), the ADS series can also be referenced. Specific grades should be matched by abrasive type and particle size and are subject to the TDS.
Note: More dispersant is not necessarily better—excess free dispersant can increase viscosity and cause foaming. A dosage screening test is needed to find the balance point.
2.2 Rapid wetting: dynamic surface tension is the key
Polishing wetting is a high-speed dynamic process, and the decisive indicator is dynamic surface tension, not the static value. Water has a static surface tension of approximately 72 mN/m; after adding a wetting agent, it should drop significantly and do so quickly. Acetylenic diol surfactants (TMDD family) have a compact molecular structure that combines fast wetting with low foaming, making them well suited to such high-speed processes. Toynol® FS-204 is an acetylenic diol, nonionic wetting/foam-controlling agent with a declared active matter content of ≥98% on the official website; its 0.1% aqueous solution shows a static surface tension of 32.8 dyn/cm and a dynamic value of 33.3 dyn/cm (subject to the official website/TDS). The small difference between static and dynamic values indicates outstanding dynamic wetting performance. When even lower foaming is required, the Superwet series can be evaluated.
2.3 Foam control: source-level foam reduction with defoaming backup
High-speed polishing conditions create strong foaming pressure. The preferred foam-control approach is “source reduction”—selecting a low-foam wetting agent (acetylenic diol types are inherently low-foaming)—backed up by a defoamer. Silicone-free defoamers (DF series) are a safer option for polishing slurries: silicone-based defoamers may remain on workpiece/polishing pad surfaces and affect downstream processes (e.g., plating or coating adhesion).
2.4 Metal corrosion protection: polish bright and keep it bright
Metal polishing slurries/lapping fluids are typically weakly alkaline or weakly acidic. After polishing, the fresh metal surface is highly active and readily oxidizes and tarnishes if cleaning is delayed. The approach is to incorporate a corrosion inhibitor into the formulation: for aluminum/magnesium alloy workpieces, Vanconol® ALI-910 (aluminum-magnesium alloy corrosion inhibition direction) can be evaluated; for acidic polishing/pickling conditions, IR-902 pickling corrosion inhibitor can be referenced; for waterborne rust prevention, the ET series can also be evaluated (e.g., ET-103, water treatment/waterborne rust prevention direction). Specific grades should be matched by workpiece material and system pH, and are subject to the TDS and actual coupon testing.
In short: Dispersion prevents scratches, wetting ensures uniformity, foam control prevents dry polishing, and corrosion protection preserves appearance—each has its own mechanism, and selection should be verified item by item against the system.
3. Practical route for polishing slurry additive selection
3.1 Define priority of requirements by polishing target
| Polishing target | Typical abrasives | Primary challenge | Additive priority |
|---|---|---|---|
| Semiconductor CMP | Colloidal silica, ceria | Scratching, abrasive agglomeration | Dispersion > wetting > foam control |
| Aluminum/magnesium alloy workpieces | Alumina | Dispersion + corrosion tarnishing | Dispersion = corrosion protection > wetting |
| Stainless steel/copper parts | Alumina, diamond micropowder | Dispersion + surface quality | Dispersion > wetting > foam control |
| Stone/glass/gem | Silicon carbide, ceria | Settling, scratching | Dispersion > suspension stability > foam control |
3.2 Recommended verification route
- Dispersion verification: After preparing the abrasive slurry, measure the particle size distribution (D50/D90), observe settling and stratification during static storage, and confirm the dispersant dosage;
- Wetting verification: Measure dynamic surface tension or observe liquid film spreading to confirm the wetting agent meets high-speed contact requirements;
- Foam control verification: Run a simulated recirculation foaming test (stirring/pump circulation) and observe foam height and break speed;
- Corrosion protection verification: After polishing, clean and store the workpiece, then observe discoloration/rusting over time; conduct coupon tests if necessary;
- Overall verification: Conduct machine polishing trials; check scratch rate, surface roughness (Ra), gloss and batch-to-batch stability.
In short: First define the polishing target and primary challenge, then verify item by item—passing laboratory data does not mean passing on-machine performance; final judgment should be based on actual polishing results.
4. Common pitfalls and points of attention
- Adding only a dispersant and ignoring wetting: The abrasives are dispersed, but the liquid film cannot spread, still causing localized dry polishing and scratching;
- Residual silicone defoamer: Affects downstream coating/plating adhesion; use silicone types with caution in precision polishing;
- Corrosion inhibitors conflicting with the system: Corrosion inhibitors may compete with dispersants for adsorption or alter pH; overall compatibility must be verified;
- Excessive dispersant increasing viscosity: Higher viscosity affects pumping and flow control; start from the TDS-recommended dosage and run a gradient;
- Formulations cannot be copied across different abrasives: The surface properties of alumina and silica differ significantly; dispersant grades must be matched and verified against the actual abrasive.
FAQ
Q1: What causes scratching of workpieces in polishing slurry?
A: The most common cause is abrasive agglomeration—insufficient or ineffective dispersant allows abrasives to re-agglomerate into large particles. The second cause is localized dry polishing caused by a discontinuous liquid film. First measure the slurry particle size distribution to confirm whether agglomeration has occurred, then check dispersant dosage and compatibility.
Q2: The abrasives in our polishing slurry keep settling and caking. What should we do?
A: Settling indicates insufficient dispersion/suspension: check whether the dispersant matches the abrasive system and whether the dosage is sufficient; if necessary, combine with suspension stabilization measures (thickening/spatial network) and verify with a static settling test.
Q3: Metal parts discolor and darken quickly after polishing. How can this be solved?
A: This is caused by oxidation of the fresh metal surface after polishing. Introduce a corrosion inhibitor matched to the material into the polishing slurry formulation (e.g., Vanconol® ALI-910 for aluminum/magnesium alloys) and standardize the cleaning and drying process. For acidic systems, pickling corrosion inhibitor directions can be evaluated (e.g., IR-902).
Q4: Does severe foaming in polishing slurry affect quality?
A: Yes. Foam interrupts the liquid film, causing localized dry polishing and scratching, as well as circulation cavitation and unstable liquid levels. We recommend switching to a low-foam wetting agent (acetylenic diol type) for source-level foam reduction, backed up by a silicone-free defoamer.
Q5: Can a general-purpose dispersant be used in CMP polishing slurry?
A: CMP imposes extremely stringent requirements on particle size, purity and residual metal ions; additives must be specially verified with strict data requirements. A general-purpose dispersant may be used for preliminary evaluation, but full verification against CMP process requirements is mandatory; data are subject to the TDS and actual testing.
Core conclusions
- Polishing slurry/lapping fluid additives form a four-in-one system of “dispersion + wetting + foam control + corrosion protection”: dispersion prevents scratches, wetting ensures uniformity, foam control prevents dry polishing, and corrosion protection preserves appearance.
- Direction of the approach: Toynol® DS/ADS series dispersants + FS-204/Superwet series wetting agents + DF series silicone-free defoamers + Vanconol® corrosion inhibitors (matched by material).
- Selection should start by defining the polishing target and primary challenge, then verify item by item in the order of “dispersion → wetting → foam control → corrosion protection,” with final judgment based on actual polishing performance.
- Use silicone-based defoamers with caution in precision polishing; check corrosion inhibitor compatibility with dispersants; do not copy formulations across different abrasives.
