PART 2 • BUILDING THE LAYERS

Grow, deposit, etch, polish

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Grow

React the silicon surface with oxygen to make its own insulator, SiO₂

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Deposit

Add a new film on top from reacting gases, sputtered atoms, or plating

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Etch

Remove material wherever a mask leaves it exposed

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Polish

Grind the surface flat again with chemical mechanical polishing (CMP)

Patterning and doping get their own units later. Here you meet the four workhorses that shape every layer.

UNIT 7 • GROWING OXIDE

Heat silicon in oxygen and it grows its own glass

Si + O₂ → SiO₂

At roughly 800 to 1,200 °C, oxygen or steam reacts with the silicon surface and turns it into silicon dioxide, a glass. Because the oxide is made from the wafer itself, the interface between them is clean and nearly free of defects. That is a big part of why silicon won (Unit 1).

Thermal oxide is the best insulator on silicon because it is made of silicon.

WHAT OXIDE DOES

Three jobs for one glass

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Gate insulator

A thin film between the gate and the channel of a transistor

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Isolation

Thick oxide separates neighboring transistors so they do not short together

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Mask and shield

Blocks dopants and etch chemicals from parts of the wafer

The same material works as an insulator, a fence, and a shield.

IT GROWS IN AND OUT

Oxide eats silicon

Growing oxide consumes silicon. SiO₂ takes up about 2.3 times the volume of the silicon it replaces, so about 44% of the final thickness lies below the original surface and 56% above it. Grow 100 nm of oxide and about 44 nm of silicon disappears.

Oxidation moves the silicon surface, and later steps have to allow for that.

DRY VS WET

Pick dry for quality, wet for thickness

Dry oxidation Uses O₂. Slow, but the oxide is dense with the best electrical quality, so it is used for gate oxides

Wet oxidation Uses steam (H₂O). Much faster but slightly less dense, so it suits thick isolation oxide

Temperature Typically 800 to 1,200 °C in a furnace tube. Hotter means faster growth

Native oxide Bare silicon grows about 1 to 2 nm of oxide just sitting in air, so fabs strip it right before critical steps

Speed and quality trade off: the fast oxide is not the best oxide.

HOW THICK?

Growth slows as the oxide thickens

x ≈ √(B · t)

Oxygen has to diffuse through the oxide that already exists to reach fresh silicon, so growth keeps slowing. Once the film is thick, its thickness x grows roughly as the square root of time t. Twice as thick takes about four times as long.

Doubling an oxide's thickness costs about four times the furnace time.

WHEN OXIDE GETS TOO THIN

The gate oxide hit a wall

Around 1 to 2 nm Electrons tunnel straight through a film this thin, so current leaks

Why it matters Leakage wastes power and heats the chip even when it is idle

The fix A high-k dielectric such as hafnium oxide (HfO₂) can be physically thicker for the same electrical effect

When High-k metal gates entered volume production around the 45 nm generation, in 2007

Thermal oxide still rules isolation, but atomic layer deposition took over the gate.

UNIT 7 STUDY COMPLETE

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Ready for the Fab Challenge?

You can explain how oxide grows and why the gate oxide had to change.