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Chemical Mechanical Polishing (CMP) is a process important for semiconductor production, where the wafer surfaces are globally planarised. This technology combines dynamic coupling of chemical etching and mechanical abrasion. It removes excess material and achieves nanoscale flatness through the chemical reaction of the slurry components with the silicon wafer surface material and the physical friction of the polishing pad.
CMP polishing slurry also known as CMP slurry, polishing liquid or abrasive slurry is one of the main steps in CMP process. It's usually ultra-fine solid particles (nanoscale SiO₂, Al₂O₃, CeO₂), abrasives, surfactants, stabilisers, oxidants and chemicals. These components work synergistically to remove material and achieve planarization through a balance of mechanical abrasion and chemical corrosion.
During the CMP process, abrasive particles in the slurry remove material from the workpiece surface under pressure and friction, while the chemical components react with the workpiece to form a thin, weakly bonded reaction layer.
The primary function of CMP slurry is to remove surface materials through micro-cutting, micro-scratching, and rolling, achieving planarization by removing the chemically generated reaction layer. Its performance directly impacts the quality of the polished surface, requiring precise control of its formulation and consistency across batches.
Moreover, the stability of CMP slurry is crucial; it must resist layering or sedimentation over time. To meet the demands of different materials and device structures, CMP slurry compositions can be customized. For instance, in high-end display glass substrate applications, cerium dioxide (CeO₂) is indispensable due to its high reactivity and strong affinity for SiO₂.
| Composition | Description |
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| Abrasive Particles |
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| Chemical Additives |
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| Dispersants |
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| pH Adjusters |
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CMP polishing slurries can be classified by abrasive type, pH, or application field.
By Abrasive Type:
Silicon dioxide slurry: Commonly used for polishing silicon and SiO₂ interlayer dielectrics, offering high activity, easy cleaning, and good dispersion and selectivity, though its hardness may damage the silicon wafer surface.
Cerium oxide slurry: Low hardness, high polishing efficiency, excellent flatness, and cleanliness, but higher cost and severe agglomeration.
Aluminum oxide slurry: High polishing efficiency but high hardness, low selectivity, and severe agglomeration, requiring stabilizers and dispersants.
Nanodiamond slurry: Excellent polishing performance with minimal surface damage.
The particle size distribution (PSD) of the abrasive particles directly influences the important parameters like removal rate, flatness, uniformity and defects in the surface in CMP. Therefore, accurate measurement of PSD is essential.
Currently, several technologies are employed for measuring CMP slurry particle sizes, including:
Dynamic Light Scattering (DLS): Commonly used for particle size measurement, suitable for nanoscale to micron-scale particle size distribution. However, it struggles with large particles, as it may conflate primary and secondary particles.
Laser Diffraction: Provides high-precision particle size distribution information for nano- to micron-scale particles but does not directly measure particle concentration.
Single-Particle Optical Sensing (SPOS): A high-resolution technique that measures one particle at a time, offering precise particle size and concentration results. Particularly suitable for detecting large particles (LPCs) that may cause wafer surface scratches and defects.
Nanoparticle Tracking Analysis (NTA): Captures individual particles/aggregates, avoiding aggregation effects, and enhances PSD analysis accuracy. Particularly effective for analyzing particles larger than 100 nm.
AccuSizer System: Widely used in CMP slurry particle size analysis, covering a dynamic range from 0.15 µm to 400 µm. Provides accurate mean particle size and LPC data. Common models include A7000 AD, A7000 APS, and A7000 FX Nano.
Other Methods: Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) can be used to see particles, but are not resolution-sensitive enough for small particle imaging and are sometimes not suitable.
In summary, the choice of CMP slurry particle size measurement technology depends on specific measurement needs and goals. For large particle detection, SPOS and AccuSizer systems are ideal, while DLS or laser diffraction is suitable for overall particle size distribution analysis. NTA also demonstrates high accuracy and reliability for large particle analysis.
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