Ceramics, semiconductors, and thin films

Why it matters

Ceramics, semiconductors and thin films are where processing control is most extreme. A ceramic part's strength depends on the largest pore left after sintering; a transistor depends on films a few nanometres thick being uniform across a 300 mm wafer. Materials scientists in these fields design and tune processes, then characterize the result with the tools from module 1.

Ceramic processing

Most technical ceramics (alumina, zirconia, silicon nitride, silicon carbide) cannot be melted and cast like metals, so they are made from powders:

1. Powder preparation: particle size, purity and agglomeration control everything downstream.
2. Forming a "green" body: pressing, slip casting, tape casting (thin sheets for capacitors and substrates), injection molding with a binder, or additive manufacturing.
3. Binder burnout at low temperature.
4. Sintering below the melting point: atoms diffuse, necks grow between particles, pores shrink and the part densifies.

Densification means shrinkage. If a green body is 55% dense and sinters to 98%, mass is conserved, so the volume shrinks by the density ratio and each dimension by its cube root:

linear shrinkage = 1 - (rho_green / rho_final)^(1/3) = 1 - (0.55 / 0.98)^(1/3) = 0.175

About 17.5% in every direction, which is why ceramic tooling is oversized and why uneven green density causes warping. Higher temperatures and longer times densify more but also grow grains, and large grains or residual pores lower strength. Because ceramics are brittle, strength is governed by flaws and is statistical (Weibull analysis), so a few large pores matter more than average porosity.

Semiconductor fabrication in one page

Chips are built on single-crystal silicon wafers (grown by the Czochralski method and sliced). A process flow repeats a handful of steps hundreds of times:

• Oxidation grows SiO2 on silicon.
• Deposition adds films: dielectrics, polysilicon, metals.
• Lithography coats photoresist, exposes a pattern through a mask (deep-UV or EUV light), and develops it.
• Etching removes material where the resist is open; plasma (dry) etching gives vertical walls.
• Doping by ion implantation, then annealing to activate dopants and repair damage.
• Chemical mechanical polishing (CMP) flattens each layer.
• Metrology at every step: film thickness by ellipsometry, critical dimensions by SEM, defects by optical inspection.

Everything happens in cleanrooms, because a single particle can kill a die.

Thin-film deposition

Deposition processes fall into a few families. Being able to explain one end to end is a common interview question.

PVD: physical vapour deposition

Material is turned into vapour physically and condenses on the substrate, under vacuum.

• Thermal or e-beam evaporation: heat a source until it evaporates. Simple, high purity, but line-of-sight, so it coats steps poorly.
• Sputtering, step by step: pump the chamber to high vacuum; admit argon at low pressure; apply a voltage to the target to strike a plasma; argon ions accelerate into the target and knock atoms off; those atoms travel to the substrate and build a film. Magnets behind the target (magnetron sputtering) trap electrons near it and raise the rate. Adding nitrogen or oxygen (reactive sputtering) makes nitrides and oxides such as TiN. Power, pressure, substrate temperature and bias control density, stress and grain structure.

CVD: chemical vapour deposition

Precursor gases react on a heated surface to form the film, for example silane (SiH4) decomposing to polysilicon. CVD coats complex shapes more evenly than PVD. Plasma-enhanced CVD (PECVD) runs at lower temperature.

ALD: atomic layer deposition

ALD splits a CVD reaction into two self-limiting half-reactions separated by purges. For alumina from trimethylaluminium and water, each cycle adds roughly 0.1 nm, regardless of small changes in dose. Illustrative calculation: a 10 nm gate-dielectric layer at about 0.1 nm per cycle needs about 100 cycles. ALD gives superb thickness control and conformality inside deep trenches, at the cost of speed.

Epitaxy

Epitaxial growth (CVD epitaxy, molecular beam epitaxy) continues the substrate's crystal lattice in the film. It is used for compound semiconductors such as GaAs and GaN in LEDs and lasers, where lattice mismatch between film and substrate creates strain and defects.

Characterizing films

Thickness by ellipsometry, profilometry or X-ray reflectivity; phase and texture by XRD; morphology by SEM; composition by XPS or EDS; stress from wafer curvature. Film stress is a frequent cause of cracking and delamination.

Common mistakes

• Treating sintering shrinkage as small and machining to final size before firing.
• Choosing evaporation for a stepped surface and getting gaps on sidewalls.
• Ignoring film stress until the coating peels.
• Reporting average porosity when the largest flaw controls ceramic strength.

Safety and training

Furnaces, vacuum systems, plasmas, high voltages and toxic or pyrophoric precursor gases (silane, arsine, trimethylaluminium) are serious hazards. Cleanroom and deposition work requires your facility's training, gowning and supervision before you touch a tool.

Recap

• Ceramics: powder, forming, burnout, sintering; densification shrinks every dimension and flaws control strength.
• Chips: oxidation, deposition, lithography, etching, doping and polishing, repeated with metrology at each step.
• PVD (evaporation, sputtering), CVD and ALD trade rate, purity, coverage and thickness control.