Lesson 11.9CoatChapter 11 · Coatings and surface finishing

CVD coating

Gases react on a hot surface and leave a film

CVD (chemical vapor deposition) grows a solid film from gases that react on a hot surface. It lays down the ultra-thin layers inside computer chips and the tough coatings on many carbide cutting inserts. Unlike PVD, the gas reaches into holes and around corners, so every surface gets an even coat.

Photo: NASA/GRC, Public domain, via Wikimedia Commons

CVD coating · tube reactor, with a close-up of the surface reactionThe circled numbers match the steps below; the lower box zooms in on the chemistry at the surface.

How it works

Load the parts. Wafers (thin silicon discs for chips) or carbide tool inserts are stacked on racks inside the reactor, a sealed tube or chamber.
Heat and feed the gases. The parts are heated, typically to 500–1,100 °C for thermal CVD. Precursor gases flow in: gases that carry the atoms you want, such as silane for silicon or methane for carbon.
React on the hot surface. Near the hot surface the gas molecules break apart and react. The atoms you want stay behind as a solid, tightly bonded film.
Pump away the by-products. Leftover gases, such as hydrogen or acidic hydrogen chloride, are pumped out and cleaned in a scrubber before release.
Grow, then cool. The film thickens evenly on every surface the gas touches, from a few nanometers on chips to about 5–20 µm on tool inserts. The reactor then cools and the parts come out.

Key facts

Materials: Films: silicon, silicon oxide and nitride, tungsten, TiN, TiC, alumina, diamond, graphene. Parts: silicon wafers, carbide, ceramics, heat-resistant metals
Typical parts: Chip wafers, carbide cutting inserts, solar cells, optical and wear parts
Part size: Wafers up to 300 mm across; inserts of a few centimeters; hundreds to thousands of parts per run
Precision: Films from a few nanometers to about 20 µm, very uniform, even inside holes and trenches
Surface finish: Smooth and dense, copying the surface below; tool coatings look gray, gold or black
Batch size it suits: Large batches: hundreds of wafers or thousands of inserts per run
Tooling and machine cost: High to very high: industrial reactors cost hundreds of thousands to millions of dollars, chip tools at the top
Speed: Slow: a tool-coating run typically lasts many hours; thin chip layers take minutes to hours

Strengths and limits

Strengths

Coats every surface the gas reaches, even inside holes
Very uniform, dense films that bond strongly
Huge range of materials, from silicon to diamond
Exact nanometer layers for chips, or thick wear layers for tools

Limits

High heat, often near 1,000 °C, rules out many steels and plastics
Toxic, flammable or corrosive gases need careful handling
Expensive reactors and long runs
Hard to mask: it also coats places you may not want

Where it's used

ElectronicsInsulating and conducting layers inside chips, solar cells
Cutting toolsThick TiCN and alumina layers on carbide turning inserts
OpticsHard and anti-reflective coatings, synthetic diamond windows
JewelryLab-grown diamonds
AerospaceAluminide coatings that protect turbine blades from hot gases
ResearchGraphene sheets and carbon nanotubes

How to recognize it

An even coating that reaches inside holes, slots and the back side of a part.
Carbide inserts with a thick gold, gray or black coating whose layers show as bands at a chipped edge.
The rainbow colors of a silicon wafer: thin films, often CVD-grown, whose thickness sets the color.
Lab-grown diamond certificates that say CVD.