Sputter Deposition Flashcards
What is sputtering
Removal of material by energetic ion impingement
What is ion beam deposition
Simple from of sputter deposition (low rate).
Ion beam incident on target which sputters and condenses onto a substrate
Explain the properties of Plasma
T+ = 500 K
T- = 23, 000K
Electrons more mobile
Plasma is a good conductor
More electrons impinge on a surface embedded in a plasma than ions
Explain Plasma Potential
A plate that is isolated from earth and is immersed in plasma charges negatively until Ar+ and e- impingement rates balance.
In the cathode sheath ions scatter and swap electrons with neutrals - energy is divided between ions and neutrals
What is diode sputter deposition?
Higher deposition using D.C. plasma
Dark space forms at target
Ar+ accelerated across dark-space by electric field and sputters material of target (cathode)
Sputtered atoms make their way through the plasma and condense on the substrate (anode)
Process also liberates electrons to sustain the plasma by impacting on Ar neutrals to from Ar+ and e-
Discharge is controllable so deposition rate is controllable
Gives uniformity
Operating pressure of diode sputter deposition?
0.2-1mbar
Mean free path is 0.001 - 0.005 m
Negatives to diode sputter deposition
Depositing atoms undergo many collisions on their way to the substrate and thermalise:
Arrive with little kinetic energy
- Poor adhesion and film structure
- Energy is needed to make a good film
Reducing pressure to increase mean free path does not allow the plasma to sustain
How to improve diode sputter deposition
Use a magnetic field to confine and sustain the plasma at lower pressures
Electrons emitted by target are confined by the magnetic field giving them a longer track through the Ar plasma, increasing chance of ionising with Ar atom
Increases the mean free path and enables higher energy deposition giving better film structure
Pressure now 0.001 - 0.050 mbar
Target erodes in circular track
What is the modified zone diagram
Additional zone T is a denser transition zone between 1 and 2.
Sputter deposition provides extra energy to the growing film filling in voids by forward sputtering, enhancing diffusion, improving adhesion
Consists of densely packed grains
RF sputtering benefits
Enables sputtering of insulators
Negative surface charge due to highly mobile electrons attracts argon ions
What is Reactive Sputtering
Dc magnetron sputtering with Ar gives metallic films
To get oxide or nitride films, add 10-30% O2 or N2 to Ar, which reacts with the growing film to make oxide
- Frequently use a metallic target but deposit in the presence of a reactive gas
- Newly deposited metallic atoms will then react with impinging reactive gas molecules to form a compound
What are the complications with reactive sputtering?
Reactive impinges on the surface of the growing film but also on the fresh metal deposited on chamber wall and onto target surface
Fresh metal on chamber pumps the reactive gas by gettering so partial pressure of reactive gas always lower than expected
When r4eactive gas introduced the p.pressure stays near zero for a wide range of incoming flow. Then rises linearly until target poisoning.
When reactive gas impinges on target it reacts with it to reduce area of metal available to sputter and also forms a compound with lower sputtering rate
Target poisoning leads to hysteresis
What is a Quartz crystal monitor
Monitors thin film deposition rate and thickness
Crystal is piezoelectric, so when oscillation potential difference is applied it will vibrate
Crystals are machines to be a whole number of wavelengths wide
Standing wave bounces back and forth causing resonance
When material is deposited on one face of the monitor, the standing wave changes and the resonant frequency decreases
Shift in frequency can be used to measure thickness
Look at table of comparison scribble image and tick or cross
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