Sapphire (Single-Crystal Alumina)

Single-crystal sapphire (~2000 HK) is one of the hardest oxide materials. Since it is a single crystal with no grain boundaries, the preparation objective is revealing crystal defects (dislocations, inclusions, growth features) rather than grain structure. Diamond grinding is mandatory. The crystal orientation affects polishing behavior and must be considered. Final polishing with colloidal alumina (NOT colloidal silica) is preferred to avoid silica contamination on the alumina surface. Nomarski DIC microscopy is the primary imaging technique for revealing surface topography and crystal defects.

Use a precision diamond wafering saw with a thin resin-bonded or electroplated diamond blade. Cutting speed: 100-200 RPM with very low feed rate and continuous coolant. Sapphire is extremely hard and brittle; aggressive cutting causes cracking and chipping. For wafer specimens, scribing and controlled fracture along crystal planes can produce clean sections. Diamond wire saws are also effective for sapphire sectioning. Leave 0.5-1 mm allowance for grinding. The cut surface will have significant subsurface damage that must be removed.

Hot compression mounting or cold mounting with epoxy are both suitable. Sapphire is thermally stable at all mounting temperatures. For thin wafer cross-sections, sandwich between support materials (glass slides or dummy wafers) before mounting to prevent fracture during grinding. Cold mounting with a clear epoxy allows transmitted light examination of transparent sapphire specimens. Edge-retaining compounds are not necessary since sapphire is harder than any mounting resin.

Diamond grinding is MANDATORY. SiC papers are ineffective against sapphire (~2000 HK). Use diamond grinding discs or diamond lapping plates.

Grinding sequence:

• 40 μm diamond disc: Remove sectioning damage (120-300 seconds). Firm pressure (30-45 N). Material removal is very slow.
• 15 μm diamond disc: Remove previous scratches (120-300 seconds). Moderate pressure (25-40 N).
• 6 μm diamond disc: Fine grinding (120-300 seconds). Moderate pressure (20-35 N). Subsurface damage extends deeper in sapphire than in metals; ensure adequate removal at each step.

Disc speed: 200-300 RPM. Use complementary rotation. Grinding times are significantly longer than for metals due to the extreme hardness. Thorough ultrasonic cleaning between steps.

Diamond polishing followed by alumina final polish. Extended polishing times are required.

Diamond polishing sequence:

• 6 μm diamond: 10-20 minutes on a hard composite pad with firm pressure (25-40 N). The material polishes slowly.
• 3 μm diamond: 10-20 minutes on a medium-hard synthetic pad (20-35 N).
• 1 μm diamond: 10-15 minutes on a synthetic pad (15-30 N).

Final polishing:

• 0.05 μm colloidal alumina: 15-30 minutes on a soft chemically-resistant pad, or vibratory polishing for 4-8 hours. Use colloidal alumina, NOT colloidal silica, to avoid silica contamination on the alumina surface. The colloidal alumina provides a slight chemical-mechanical polishing (CMP) action on Al2O3 that improves surface quality beyond what pure mechanical polishing achieves.

For optical-quality surfaces (LED/laser substrates), commercial CMP slurries specifically designed for sapphire are available and produce superior results.

Since sapphire is a single crystal with no grain boundaries, chemical etchants have limited utility. Thermal etching and DIC microscopy are the primary techniques.

Thermal Etching - Primary technique for dislocation analysis:

• Application: Heat polished surface to 1500-1600°C in air or controlled atmosphere for 30-60 minutes.
• Reveals: Dislocation etch pits at points where dislocations intersect the surface. The pit shape depends on crystal orientation (triangular on c-plane, elongated on other planes). Dislocation density is a critical quality metric for LED substrates.
• Note: Requires a high-temperature furnace. Cool slowly to avoid thermal shock cracking.

Hot Phosphoric Acid (Chemical Etching) - Alternative:

• Composition: Concentrated H3PO4 (85%)
• Application: Immerse in boiling H3PO4 (~300°C) for 5-30 minutes.
• Reveals: Dislocation etch pits similar to thermal etching but with different pit morphology. Also reveals subsurface damage from polishing if present.
• Safety: Hot concentrated acid. Extreme caution required. Full PPE, acid-resistant gloves, face shield.

Nomarski DIC Microscopy - Non-destructive primary technique:

• Application: Examine polished surface under Nomarski differential interference contrast optics.
• Reveals: Polishing scratches, growth striations, inclusions, surface waviness, subsurface damage (appears as haze). No etching required. The most commonly used technique for sapphire surface quality assessment.