MTE3203 Week 6 Thin films and quantum tech

What is the difference between PVD and CVD?

PVD moves material from a condensed phase (solid or liquid) onto the substrate physically, with no chemical reaction. CVD uses vapour phase precursors that react chemically to form a solid non-volatile film.

What is sputtering and what are its four types?

Ejecting atoms from a target by bombarding it with energetic particles, usually ions. Types: DC, RF, reactive, and magnetron.

How does magnetron sputtering work?

A magnetic field in front of the target traps secondary electrons near it, so more electron and gas molecule collisions occur and ionisation increases.

What is molecular beam epitaxy (MBE) and what are its trade-offs?

Growth on a heated substrate in ultra high vacuum using beams of atoms from heated cells. The vacuum stops the beams scattering off stray gas. Excellent control and very pure sharp layers, but slow. Used for semiconductors.

What is pulsed laser deposition (PLD)?

A strong laser pulse ablates a small spot on the target into a plasma plume of atoms and ions, which travels across the vacuum and builds a uniform ultra-thin film. Gives precise thickness control. The target must be dense and absorb the laser well.

What are the six key steps of CVD?

Gases flow in, react in the gas phase, travel to the surface, adsorb, diffuse along the surface and build the film, then the leftovers desorb and are pumped away.

What are the three types of CVD?

Thermal (heat driven, too hot for sensitive substrates), plasma-enhanced (plasma breaks the gases apart so the substrate stays cool), and photo-assisted (light breaks the gases apart).

What is the difference between homoepitaxy and heteroepitaxy?

In homoepitaxy the film and substrate are the same material with matching lattice parameters. In heteroepitaxy they are different materials and the lattice parameters are likely mismatched.

What three states can a heteroepitaxial film end up in, and how is this characterised?

Matched, strained, or relaxed, where relaxed means misfit dislocations have formed. Lattice parameters are characterised by XRD.

What is lithography and what is it used for?

Turning a plain film into a pattern: coat with light sensitive resist, bake, expose through a mask or write directly with an electron beam, dissolve the exposed parts, then etch. E-beam writing gives smaller features but is slow and costly. Used to make microchips.

What is quantum technology?

Devices that rely on quantum mechanical effects such as superposition and entanglement, covering computing, communications and sensing.

What five things does a working quantum system need?

Set the starting state, hold that state long enough, perform operations, read the answer out, and scale up.

Why do quantum computers have much higher error rates than classical ones?

Quantum states are fragile and are easily disturbed by heat, background radiation and noise.