Mechanical Polishing

Mechanical polishing in sample preparation is the progressive grinding and polishing of a specimen to create a smooth, flat cross-section surface suitable for microscopic examination — the traditional and cost-effective method for preparing large-area cross-sections of semiconductor devices, packages, and materials when site-specific FIB precision is not required.

What Is Mechanical Polishing?

• Definition: A multi-step process that removes material from a specimen by abrading it against rotating platens or polishing cloths loaded with progressively finer abrasive particles — transitioning from coarse grinding (~30 µm grit) through fine polishing (0.05 µm colloidal silica) to produce a mirror-finish surface.
• Principle: Each polishing step removes the damage layer created by the previous coarser step — the final step produces a surface smooth enough for microscopic examination with minimal preparation artifacts.
• Cost: The most economical cross-section method — polishing equipment and consumables cost a fraction of FIB systems.

Why Mechanical Polishing Matters

• Large Area: Produces cross-sections spanning millimeters to centimeters — far larger than FIB cross-sections (typically 20-50 µm). Essential for examining large-scale features and overall package structure.
• Package Analysis: The standard method for cross-sectioning IC packages, PCBs, and solder joints — FIB is too slow for these large structures.
• Economic: Polishing equipment costs $10K-$50K versus $1M-$5M for FIB systems — accessible to any failure analysis lab.
• Parallel Processing: Multiple specimens can be prepared simultaneously in mounting fixtures — higher throughput than serial FIB processing.

Mechanical Polishing Process

• Step 1 — Mounting: Embed specimen in epoxy or acrylic resin — protects edges and provides stable geometry for grinding.
• Step 2 — Sectioning: Cut specimen close to the target area using a diamond saw — reduces grinding time.
• Step 3 — Coarse Grinding: SiC paper (120-600 grit) removes material quickly to approach the target plane.
• Step 4 — Fine Grinding: Diamond lapping films (9 µm → 3 µm → 1 µm) refine the surface with decreasing scratch depth.
• Step 5 — Final Polish: Colloidal silica (0.05 µm) or alumina (0.3 µm) on polishing cloth — produces mirror finish suitable for microscopy.
• Step 6 — Cleaning: Ultrasonic cleaning to remove all polishing residue before examination.

Polishing Artifacts to Avoid

Mechanical polishing is the workhorse cross-section preparation method for semiconductor packaging and failure analysis — providing large-area, cost-effective specimen preparation that remains indispensable even as FIB technology has advanced, particularly for the package-level and board-level analysis that FIB cannot practically address.