SPUTTERING THEORY

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Sputtering theory

Sputtering principle

Sputtering deposition is the deposition process caused by particle emission of target depending on the collision between activated incident particles.
At this time, impact energy shall be more than 4 times of thermal energy required for evaporating target material to strike atom of material and
enable it to escape. It is not a chemical or thermal process but a physical momentum exchange process. Almost all materials can be used as targets.

Sputtering mechanism

  • Inert gas (Ar) is inserted in vacuum chamber and voltage (cathode) is applied to target. (Although heavy ion shows a high sputtering yield,
    Ar is frequently used because of its low price)
  • The electrons emitted from cathode collide with Ar gas and ionize Ar. Ar + e-(primary) = Ar+ + E-(primary) + e-(secondary)
  • If it is excited and emits electron, energy is emitted. At this time, glow discharge is generated, showing the plasma of specific color depending
    on the gas where ions and electrons coexist.
  • Ar+ ion of plasma is accelerated in the direction of target (cathode) due to big potential difference. If it collides with target surface, neutron
    target atoms are projected to form thin sheet on board.

Sputtering Advantage, Disadvantage

Advantage Disadvantage
  • Film generation speeds are stable and similar in various different materials.
  • Equal films can be generated. Step and defect coverage are good.
  • Adhesion of film is good.
  • Films for various materials such as metal, alloy, compound and insulating
    material are available.
  • Target can be cooled down so big target is available.
  • Pre-cleaning is available through sputter etching of board.
  • Oxide and nitride film can be generated through reactive sputter of O2 and N2
  • Film generation speed is low. (<10Å/sec)
    It can be improved through magnetron sputtering.
  • Film inequality and damage are caused by high energy deposition
    After film generation, inequality and damage can be
    reduced by thermal treatment.
  • Film is heated by the exposure to electron, UV, ion, etc. (100~150℃)
    Depending on need, it is necessary to cool down board holder.
  • Film generation condition is sensitive and gives an effect to each other.
    It is necessary to adjust sputtering parameters

DC sputtering

As the sputtering method to use DC, it has following characteristics.

Advantage Disadvantage
  • Its structure is simple. It is the most standard sputter device.
  • Film generation speeds are almost constant in various metals.
  • It is easy to adjust film thickness because current is in direct proportion
    to film thickness.
  • Its film generation speed is higher than that of RF sputtering.
  • Film is equal.
  • Adhesion strength is high because it is the process of high energy.

  • Target material is restricted to metal.
  • High Ar pressure is required. (10 ~ 15 mTorr)
  • Board is apt to be overheated.

RF sputtering

In DC sputtering, oxide or nitride target is not available for sputtering. The disadvantage can be solved through RF sputtering. Especially, in the
process that sputtered material reaches substrate under the low pressure condition, scattering is relatively reduced, resulting in high sputtering yield value.

Although in DC discharge, the secondary electron is extinguished in anode before it provides Ionization process with sufficient energy, RF can reflect
electron and use it for Ionization because two electrodes have negative potentials when compared to plasma.

In RF sputtering, it is possible to sputter non-metal, insulating material, oxide and dielectric substance as well as metal. Although low ㎒ frequency
shall be used, the electric wave control of Federal Communication Regulation regulates the high frequency power of 13.56MHz in order to avoid the
intervention in radio frequency. Although RF sputtering looks like DC device, it has the advantage that it can achieve accurate organization with
superior adhesion by applying bias voltage to board.

Magnetron sputtering

It is the method to improve sputter yield by installing permanent magnet or electromagnet in the rear of target to improve ionization of target,
collecting the electrons emitted from cathode locally in the magnetic field formed outside target and facilitating the collision with Ar gas atom.

Advantage Disadvantage
  • Sputtering efficiency increases.
  • It is possible to reduce the collision of electron with board and film through
    the vortex motion of electron. Therefore, the temperature increase
    of board is reduced.
  • Film generation speeds are also high in insulating materials such
    as SiO2 and Al2O3
  • Sputtering or reactive sputtering is also available in dielectric substance.
  • Film generation speed is constant in a given input power. (±10%)
  • It is easy to adjust uniformity of film thickness through proper
    arrangement of permanent magnet and use of shield.

  • Entrance direction of magnetic field shall be perpendicular to target surface.
  • Many targets are consumed because of optional sputter
    around magnetic field.
    (Efficiency: about 25%)
  • When magnetic materials (Co, Ni, etc.) are sputtered,
    it is difficult for magnetic field to go out to the outside of target.
    Therefore, thin target of 1/8” ~ 1/32” shall be used.

Bias sputtering

Bias sputtering can be divided into sputter etch that applies (-)bias to board and sputters it before sputtering and bias sputter that applies (-)bias,
which is significantly smaller than target, to board and prevent the collision between ion and electron. Sputter etch (Back sputtering) removes
impurity existing on board surface by applying RF or DC(-) bias of -100V ~ -200V to board and making Ar+ion collide with board If about (-)bias of -1kV is applied to target and (-)bias of -60 ~ -100V is applied to board in bias sputtering, Ar+ ion collides with and sputters target. In addition,
because O-2 ions and electrons existing in chamber get away from board due to (-) bias applied to board, it is possible to reduce the amount of oxygen mixed in film. Generally, the infiltration of oxygen gives bad effects including oxidation of film so bias sputtering is essential in the sputtering of easily oxidizable atom (Ta, Mo, Nb, Cr and Rare earth atom).

Reactive sputtering

Reactive sputtering, which plays the opposite role of bias sputtering, is used to generate oxide or nitride film(dielectric substance film, etc)
depending on purpose. Although its method is same as that of normal sputtering, it is possible to generate desired compound film by providing
small amount of oxygen or nitrogen in addition to Ar gas. Compound film generation through reactive sputtering has advantages from the aspect
of manufacture, purity and price, when compared to the direct sputtering of oxide or nitride target. Its reason is that gaseous atom of target is very unstable and easily responds to reactive gas so the atom reaching board responds rapidly in the status of film. Increase of board temperature during reactive sputtering increases compound generation speed, resulting in increase of film generation speed.However, reactive sputtering makes shorten the life span of filament for vacuum pressure gauge or glow discharge and forms insulating layer of oxide or nitride on target surface so it decreases efficiency of sputter. The phenomenon can be improved by turning permanent magnet located in the rear of target or changing current of magnet
from the beginning of sputtering in DC sputtering process.

Magnetic substance

Magnetic substance is a general term for the substance magnetized in magnetic field. The wide meaning of magnetic substance also includes air.
Briefly speaking, although there is a difference in degree, all substances existing on earth can be called magnetic substances. However, the
magnetism (property absorbed into magnet) level is significantly different by substance. The substance showing strong magnetism like iron is called ferromagnetic substance and the substance showing almost no magnetism like aluminum is called paramagnetic substance for classification. Here,
paramagnetic substance is also called nonmagnetic substance. Among paramagnetic substance, there is the substance showing diamagnetism like
gold or copper. It is called diamagnetic substance. Diamagnetic substance has wholly different property from that of ferromagnetic substance.
Because its effect is very timid, however, it is included in paramagnetic substance in classification.

Ferromagnetic substance Fe, Ni, Co Paramagnetic substance Al, Cr, Pt Nonmagnetic substance Cu, Au, Ag

Relationship between spontaneous magnetism and temperature

In the temperature lower than Curie point, the electronic spin of ferromagnetic substance like iron is arranged in a certain direction. If many spins
are arranged, the substance becomes magnet easily and is magnetized in small external magnetic field. In this case, because it is considered that
mini magnet is formed in the direction of spin, it can be understood as a kind of atomic magnet. Ferromagnetic substance possesses magnetism
naturally so it is called spontaneous magnetism. Spontaneous magnetism is dependent on temperature and it becomes maximum at -273℃
(absolute temperature). As temperature increases, magnetism decreases and it disappears in Curie point. In other words, the substance becomes
nonmagnetic substance (paramagnetic substance).

Magnetizing

Magnetizing can be classified into 3 types depending on its method. Magnetizing is to arrange atomic magnets. The atomic magnets of magnetic
substance exist in the status where line of magnetic force is not emitted externally (no magnetizing) because their N pole and S pole are connected
in rapid succession before they are magnetized. If powerful external magnetic field is applied to N pole and S pole, the atomic magnets are arranged
in parallel to external magnetic field and they are changed into the structures that can emit line of magnetic force externally.

Like that, the activity to arrange atomic magnets in a certain direction by applying strong magnetic field outside magnetic substance is called
magnetizing. The method to apply sufficient amount of DC current to coil for creating strong magnetic field (the space where line of magnetic force
forms its power) and magnetize magnetic substance by putting it in the magnetic field is frequently used, In some cases, as the method to
magnetize small ferrite material weakly, magnetic substance is magnetized temporarily by making it contact powerful permanent magnet of rare
earth element, Nd-Fe-B, without using DC power directly.

Magnetic permeability

As the ratio of magnetic flux density to responding magnetizing force (magnetic flux density / responding magnetizing force), magnetic
permeability shows how easily line of magnetic force can pass through substance.As the similar term to magnetic permeability, there is the term,
specific permeability. It means the value that magnetic permeability of magnetic material is divided by magnetic permeability of vacuum.
The material with the highest specific permeability is Fe. Its value is 120~20,000. As other ferromagnetic substances, cobalt is 270 and nickel is 180.
The specific permeability of paramagnetic substance (nonmagnetic substance) is closed to 1 so it is almost same in air or vacuum Because specific
permeability of aluminum or copper is very small, line of magnetic force passes through it although it is a metal. In other words, it has no power to
block line of magnetic force, It is hard for line of magnetic force to pass through the ferromagnetic substance like iron. Therefore, if line of magnetic
force enters the substance, it can’t pass through the substance unless there is a magnetic saturation in the ferromagnetic substance.

Hysteresis loop

If ferromagnetic substance like iron is gradually magnetized, the relationship between magnetic flux density and magnetizing force shows the
character istic of 0 A B C If magnetizing force is gradually decreased from saturation status (saturation point C), magnetic flux density
responds to the decrease and changes from C to D. If more magnetizing force is applied to minus(-) direction, it reaches F via E. If flux magnetizing is applied again, it moves toward I via G H. If magnetizing force increases more and more, it reaches at the initial saturation point C. Its repeated loop is called hysteresis loop. The work performed during the process (equal to loop area) is thermal energy, whose whole amount is consumed in magnetic substance.

BH curve

Generally, it is possible to know the characteristic of magnetic substance through hysteresis loop. However, BH curve is used for permanent magnet.
Generally, good magnet means the magnet with strong magnetic force. It is in proportion to magnetic flux density. In spite of high magnetic flux
density, however, it is hard to maintain the magnetic force if coercive force is weak. Therefore, good magnet means the magnet with high magnetic
flux density and strong coercive force.

The curve below shows BH curve. In the point “A” on curve, the values of “b” and “H” become Ba and Ha respectively. For the point, used energy
becomes Ba X Ha and it is called energy product. In the point “D” on curve where Ba X Ha becomes maximum, Bd X Hd is called maximum energy
product. It is indicated as “BHmax” and “X10GOe” is used for its unit. In order to know the energy product of optional point on BH curve, the points
with same BH are connected by line in advance.

In the status, if external magnetic field gradually decreases, magnetic flux density decreases from A to b. If external magnetic field becomes 0(zero),
magnetic flux density becomes “b” and that of 0(zero)b is remained. The value is called residual induction and is indicated as Br. Its unit is G(Gauss).
If additional external magnetic field is applied to the opposite direction, magnetic flux density gradually decreases to the point “c”. The strength of
external magnetic field of opposite direction required for decreasing magnetic flux density to the point “c” is called coercive force and is indicated
as Hc. Its unit is Oe(Oersted).