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Sputtering principles

Sputter deposition is a deposition process of ejecting particles from a target through the collision of activated incident particles.
At this step, the impact energy must be about four times higher than the thermal energy—the energy required to evaporate the target material—
to be sufficient enough to strike an atom from the material and escape.
Almost any material can be used as a target since it is a physical momentum exchange process, not a chemical or a thermal process.

Sputtering mechanism

Inert gas (Ar) is put into the vacuum chamber, with the cathode exerted to the target.
(The heavier the ion is, the higher the sputtering yield, but Ar is used most of the time due to its low price.)

Electrons emitted from the cathode collide with Ar, resulting in Ar ionization.
Ar + e-(primary) = Ar+ + E-(primary) + e-(secondary)

As Ar gets excited and emits electrons, energy is released.
At this time, glow discharge occurs, emitting plasma of a specific color depending on the gas where the ions and electrons coexist.

Ar+ ions in the plasma are accelerated toward the target (cathode) by a large potential difference.
When Ar+ ions collide with the target's surface, neutral target atoms are ejected, forming a thin film on the substrate.

Sputtering Advantages and disadvantages

• Advantages

  The deposition rate is stable and similar even on different materials.

  Uniformed deposition is possible, ensuring excellent step or defect coverage.

  Excellent thin film adhesion

  It is possible to perform deposition on different materials such as metals, alloys, compounds, and insulators.

  Target cooling is possible, enabling the use of large targets.

  Pre-cleaning is possible through sputter etching of the substrate.

  It is possible to form oxide and nitride thin films through reactive sputters such as O₂, and N₂.

• Disadvantages

  The deposition rate is slow.(<10Å/sec)
  Can be improved through magnetron sputtering

  High energy deposition causes non-uniformity and damage to thin films.
  Heat treatment after deposition reduces non-uniformity and damage

  Thin film heating is performed with exposure to electrons, UV, ions, etc. (100~150℃)
  The substrate holder may require cooling.

  The deposition conditions are sensitive, and they can affect each other.
  The sputtering parameters should be adjusted.

DC sputtering Advantages and disadvantages

• Advantages

  It is the most standard sputter equipment with a simple structure.

  The deposition rate is almost constant for various types of metals.

  It is easy to control because the current amount and thickness of the thin film are almost in direct proportion.

  The deposition rate is higher compared to RF sputtering.

  High uniformity in thin films.

  Ensures high adhesion since it requires high energy.

• Disadvantages

  Target material is limited to metals.

  High Ar pressure is required. (10 to 15 mTorr)

  The substrate is prone to overheating.

RF sputtering

In DC sputtering, sputtering is not performed if the target is an oxide or an insulator.
This can be solved through RF sputtering. In particular, it can have a high sputtering yield value even at low pressure
since scattering is relatively reduced while the sputtered material reaches the substrate.

During DC discharge, although the secondary electrons are annihilated to the anode before supplying sufficient energy to the ionization process,
RF can be sufficiently used for ionization by reflecting electrons since the two electrodes have a negative potential compared to plasma.

RF sputtering can sputter non-metals, insulators, oxides, and dielectrics in addition to metals.
Although low MHz frequency is required, the United States Federal Communications Commission regulates operation of a high-frequency power source of 13.56 MHz
in order to avoid interference with radio frequencies in the commission's spectrum management.
High sputtering yield value is possible because scattering is relatively reduced when the sputtered material reaches the substrate.

Reactive sputtering

Unlike the roles of bias sputtering, reactive sputtering is used to form an oxide or a nitride thin film (such as dielectric thin film) according to purposes.
This method is the same as general sputtering, but reactive sputtering
can make a thin film of a desired compound by supplying a small amount of oxygen or nitrogen in addition to Ar.
Compound thin deposition through reactive sputtering is more advantageous
in terms of manufacturing, purity and price than directly sputtering an oxide or nitride target.
This is because gaseous atoms from the target are very unstable
and highly reactive to reactive gases, and atoms reaching the substrate also react quickly in thin film state.
If the substrate temperature is raised during reactive sputtering, it is possible to increase the deposition rate by increasing the compound formation rate.
However, reactive sputtering shortens the lifespan of filaments for vacuum pressure gauge or glow discharge,
and an insulating layer such as oxide or nitride is formed on the target surface during EC sputtering; thus reducing the sputter efficiency.
In DC sputtering, this phenomenon can be improved by rotating the permanent magnet
on the back side of the target from the initial stage of sputtering or by changing the magnet's current.

Magnetizing

Magnetizing can be classified into three depending on the method. Magnetization is the alignment of atomic magnets.
In the case of atomic magnets in a magnetic material before magnetization, they are in a state wherein magnetic field lines are not spread outside (non-magnetized) as the north and south magnetic poles are tailed to each other.

If strong north and south magnetic poles are applied from the outside, however, these atomic magnets are aligned parallel to the magnetic poles
until structural changes occur wherein they can make the magnetic field lines flow outside.
Simply put, when a strong magnetic field is applied from the outside of a magnetic material
to align the magnetic poles of atomic magnets in a certain direction, it is called magnetization.

A commonly used method involves making a strong magnetic field
by flowing enough DC current into the coil and inserting a magnetic material into the magnetic field.
In some cases, as a method of weakening small ferrite material, magnetic materials come in direct contact
with a magnet such as Nd-Fe-B, which is a strong rare earth magnet, to make it temporary instead of using DC power.

Magnetron sputtering

Generally, we can identify the characteristics of magnetic materials by using the hysteresis curve; in the case of permanent magnets, however, the B-H curve is used.
A good magnet refers to a magnet with strong magnetizing force proportional to the magnetic flux density.
Since the magnetizing force cannot stay stable when the coercive force is small despite having a large magnetic flux density,
however, a good magnet is a magnet that has high magnetic flux density and large coercive force.

The curve below shows the demagnetization curve. The values of “b” and “H” at any point “A” on the curve are Ba and Ha.
At this point, the energy used becomes Ba x Ha, and this is called energy.
Bd x Hd at point “D” where Ba x Ha on the curve is maximum is called the maximum energy product, and it is indicated as “BHmax” with unit of “X10GOe.”
In order to get the energy product of any point on the demagnetization curve easily, we connect the points with the same BH and draw them in advance.

Here, if the external magnetic field is gradually reduced,
the magnetic flux density decreases from A to b.
If the external magnetic field is set to 0 (zero), it comes to the “b” point,
and the magnetic flux density of 0 (zero)b remains.
This value is called residual Induction, denoted by Br, and the unit is G (Gauss). If an external magnetic field in reverse direction is applied,
the magnetic flux density gradually decreases and comes to point “c”.
The strength of the external magnetic field in reverse direction required to reach “c”
is called coercive force and is indicated as Hc with unit of Oe (Oersted).