Magnetron Sputtering

The deposition of ZnO thin films

Sputtering • Introduction of inert gas between two

electrodes

• Target material at the cathode and substrate at the anode

• Plasma formation

Magnetron Sputtering

• The use of magnetic field – Lorentz force: F = q v x B

• More collisions between gas atoms and electrons

– Radius of circular motion: r = mv/qB • Mass dependent

• Bending the electron path

• Ions not affected

– This magnet is a permanent magnet placed inside the target material.

Plasma

• Partially ionized gas

• Creation of secondary electrons

• Shield in front of the cathode

Crooke´s dark space

• A dark space in front of the cathode

• Contains a large concentrations of electrons

– Emitted secondary electrons from the cathode

– Not enough energy to excite Ar atoms

– A large electric field due to the small separation of negative and positive charge

• The critical process in a sputtering system occur in Crookes dark space:

– This electric field accelerates the Ar ions towards the target

RF Sputtering

• When depositing insulating materials

– Otherwise the plasma will be extinguished

• AC signal at radio frequncy

• Alternating voltage

– Target bombarded with electrons and ions alternately

Ion bombardment

Different processes depending on the energy of the ion: - Secondary electrons - Reflection/adsorbtion (E ≤ 10 eV) - Impantation (E ≥ 10 keV) - Sputtering

Deposition rate • The deposition rate depends on:

– Sputter yield (probability)

– Ion flux to the target

• Depends on: Ion mass, voltage difference and the thickness of the cathode dark space (Crookes)

– Transport through the plasma

• Described by computational fluid dynamics

• Deposition rate: approximately 2 nm/min

Sputter yield

• The probability that a target atom will be sputtered

• Sputter yield: S = Ze/Zi

– number of sputtered atoms per incident ion

– Depends on:

• The ion mass, the angle and the target crystallinity.

Thin film properties

• The target atom mobility:

– Kinetic energy, substrate temperature and binding energy

– High kinetic energy High surface mobility

– Island formation which grows

• Step coverage will be improved by heating the substrate due to surface diffusion

• Properties and quality depends on:

– Substrate temperature, chamber pressure and target effect.

– Low temperature and ion energy:

• Low mobility atoms will not settle in the most favorable spots

• Zone 1 - Amorphous film with low mass density

– Raising the temperature or lowering the pressure:

• Higher mobility

• T-zone : small grains and a smooth surface microelectronics

•Higher temperature or ion energy • Zone 2 - Grain size becomes large TCO applications

•Highest temperature and lowest pressure • Zone 3 – The film will be dominated by large 3D grains and a rough surface.

Magnetron Sputtering at UiO

• At MiNaLab UiO

– Semicore Tri-Axis with 3 cathodes (1 DC and 2 RF )

• Allows sputtering of more than one material at once

• Uniformity measurement variyng the angle.

– The target distance was set to 11,7 cm.

– Found that 300 gave the best (possible) result

Pre-sputtering: - Avoid contaminations - Remove native oxides at the target.

Deposition of ZnO: -Initial chamber pressure:

Below 2*10^-6 Torr – to avoid contaminations -Working pressure:

17,0 mTorr - Pressure after Ar gas is inserted -Gass flow:

Argon gass, 70 SCCM - Target power: 50 W

Typical deposition

• Cleaning the substrate; Acetone, 2-propanol and water.

• Pump down the chamber.

• Start the deposition: – Temperature

– Start the turbo pump

– Rotation

– Ar flow

– Power, Igniter, RF ON

– Pre-sputtering

• Vent the chamber and the deposition is done!

References

• Stephen Campbell. Fabrication Engineering at the micro- and nanoscale

• B. Chapmann. Glow discharge processes: Sputtering and plasma etching

• K. Ellmer. Magnetron Sputtering of Transparent Conductive Zinc Oxide