Aluminum Nitride Substrate

Aluminum nitride is a hard (1100-1200 HV), brittle non-oxide ceramic with exceptional thermal conductivity (170-230 W/mK) and a density of 3.26 g/cm³. The high thermal conductivity is governed by grain boundary phase composition; yttrium aluminate grain boundary phases scatter phonons less than oxygen-containing secondary phases. AlN reacts with water to form aluminum hydroxide and release ammonia, so non-aqueous preparation methods are preferred. Grain size, grain boundary phase distribution, and porosity are the features of interest.

Use a precision wafering saw with a diamond blade (0.15-0.3 mm). Cutting speed: 100-200 RPM with low feed rate. Use oil-based or non-aqueous coolant if available to prevent hydrolysis of the AlN surface. If water-based coolant is used, dry the sample immediately after cutting. The hard, brittle ceramic requires gentle cutting to prevent fracture and chipping. Leave 1-2 mm allowance for grinding.

Cold mounting with castable epoxy is required. Use a low-viscosity, low-shrinkage epoxy resin. Vacuum impregnation is recommended for substrates with any porosity. Compression mounting is not recommended; the brittleness and relatively low fracture toughness (~3 MPa√m) create a risk of cracking under pressure. Ensure the epoxy is fully cured before grinding.

The high hardness (1100-1200 HV) requires bonded diamond grinding discs. SiC paper wears too rapidly and produces poor surface quality on hard non-oxide ceramics. Use non-aqueous or oil-based lubricant to prevent hydrolysis. Disc speed: 200-300 RPM. Apply moderate pressure (20-30 N per 30 mm sample).

Grinding sequence:

• 30μm diamond disc: Remove sectioning damage (30-90 seconds). Moderate pressure.
• 15μm diamond disc: Refinement (30-60 seconds).
• 9μm diamond disc: Final grinding (30-60 seconds). Reduce pressure to minimize subsurface cracking.

If water-based lubrication is used, minimize grinding time and dry the surface immediately after each step. Rotate specimen 90° between steps.

Use firm, napless cloths for all polishing steps. Use oil-based diamond suspensions rather than water-based to prevent hydrolysis. The hard ceramic requires longer polishing times than metals.

Diamond polishing sequence:

• 6μm diamond: 3-5 minutes on a hard napless pad with moderate pressure (20-30 N per 30 mm sample). Oil-based suspension strongly preferred.
• 3μm diamond: 3-5 minutes on a napless pad (20-25 N). Oil-based suspension.
• 1μm diamond: 2-4 minutes on a napless pad (15-20 N).

Final polishing:

• 0.05μm colloidal silica: 3-5 minutes on a soft pad with light pressure (12-18 N). Colloidal silica is water-based; minimize exposure time and dry immediately afterward. Alternatively, use 0.05μm diamond suspension on a soft pad for a fully non-aqueous final polishing step. Vibratory polishing (4-8 hours) produces the best results for grain boundary analysis.

AlN grain boundaries are revealed by thermal etching or aggressive chemical etchants. Thermal etching is preferred when the equipment is available.

Thermal Etching - Primary method:

• Conditions: Heat polished sample to 1700-1900°C in nitrogen atmosphere for 15-30 minutes. An inert or nitrogen atmosphere is essential to prevent oxidation of AlN to Al₂O₃.
• Reveals: Grain boundaries by thermal grooving. Also reveals the grain boundary phase (yttrium aluminate) distribution.
• Note: The very high temperature requires a high-temperature furnace with controlled atmosphere.

Molten NaOH (Chemical Etching) - Alternative:

• Composition: Molten NaOH at 300-400°C
• Application: Immerse polished sample in molten NaOH for 30-120 seconds. AlN dissolves selectively at grain boundaries.
• Reveals: Grain boundaries and grain boundary phase distribution.
• Safety: Molten NaOH is extremely corrosive. Use nickel or stainless steel crucible. Full face shield, heat-resistant gloves, and protective clothing required. Work in fume hood.