Removing of material is done by bombardment of Ar ions (ionised in plasma)
There are no chemical reactions (Ar is a noble gas and nonreactive)
Advantages:
Directionality: - ions are accelerated in one direction only - toward the negative charged substrate where the substrate is placed
the directionality of ions movement insures the fact that almost no etching will take place on the lateral walls - no undercut
Applicability: - etches with the same rate any material or alloys, thus it can be used for any layer, without modifying the gas type
Disadvantages:
Not selective - etches all materials, including the photoresist, thus the resist cannot act as a protecting mask
Low througput - ion sources cannot produce more ions
Ion source for etching - Kaufman source
A current passes through a filament and electrons boil off the filament
Argon is introduced in the chamber and ionised by the accelerated electrons
Low pressure is required so that the ions don't collide (through collisions they can neutralise or loose their kinetic energy)
Ion density is increased by applying a magnetic field to the source - in this way electrons are not lost by collisions with the walls, their trajectory is curved close to the source
The argon ions are extracted from the source toward the chamber by applying a potential
The ion energy depends on the difference between plasma potential and the extraction potential
The ion flux depends on the potential applied to the filament, thus ion energy and ion flux are independent
High density plasma etching
High density plasma can be obtained at low pressure
ECR plasma - another source for plasma assisted etching
High density plasma caused by magnetic field - increases the electron path
Advantages:
less substrate damage
it is remote, thus less damage
high density ions, thus no need of high energy, thus less damage
Low pressure - anisotropy
Reactive species can be used (combination of chemical and physical etching)
Disadvantages:
increased cost and complexity
ICP
RF current through a coil produces an oscillating magnetc field - increases the electron path
Damage in ion milling
due to bombardment of ions, the layer beneath may be damaged
other effects of bombardment
film smoothening due to resputtering of deposited atoms (the corners get higher bombardment, and atoms may be dislodged from the corners and redeposited, causing planarization
if photoresist is sputtered and redeposited, the layer can be contaminated by organic material
faceting (see picture below) is caused by the etching of photoresist at the edges; the film will not be protected at the edges by the thinner photoresist
as solution, photoresist should be thicker to avoid faceting
trenching (see picture below) is caused by reflection of ions from the vertical sides of the photoresist, therefore trenches will be made by the ions at the edges of the photoresist
Ion milling + reactive species
If a reactive gas is introduced besides the noble gas, or in place of it, chemical processes will also appear in the etching
Because the reactive species attack the filament in a Kaufman source, the reactive gas is introduced separately from the noble gas, closer to the wafer - the process is named in this case ion assisted chemical etch
If the wafer is placed in the plasma and gases are introduced together, the process is called reactive ion milling
Substrate preparation
