Deposition by evaporation
The two main methods to evaporate a material are:
- Evaporation by Joule Effect (resistance heated evaporation)
- Evaporation by electrons beam (e-beam evaporation).
This method consists in evaporating a material (Grains, pieces) placed in a resistive boat heated up by the passage of an electrical current. The boat is generate made of tungsten, tantalum, molybdenum or carbon.
- Potential contamination coming from the boat used.
- Impossible to evaporate materials with a high melting point.
- Limitation on the thickness of the layer obtained due to the small quantity of material which can be placed in the boat.
This method consists in evaporating a material (Grains, pieces, tablet) placed in a crucible heated up by a highly energetic electron beam.
The electron source is a white-hot tungsten filament. The emitted electrons are accelerated to bombard the surface of the material contained in the crucible. In details, a negative voltage ( – 4 kV to -10 kV) is applied to the filament causing a acceleration of electrons along the field lines of the electric field, towards the lower edge of the plate constituting the anode. The geometry of the cathode placed in front of the filament allows the impact of the electrons on the lower edge of the anode. Then, the electron beam is 270° deflected by a magnetic field. The kinetic energy of the electrons is converted into heat at the impact point (The energy flow is in the range of 100 W.cm-2). Since the beam is concentrated on the surface of the material, the material itself can be contained in a cooled crucible. Actually, mainly the surface of the material is melted. However, the material touching the walls of the crucible is at solid state avoiding the contamination problems 1.
- Purity of the layer deposited (High vacuum < 10-6 mbar, see note 1)
- X-Ray emission which can damage certain layers of the substrate to coat.
- Potential droplets ejection which can happen if a to high energy is used.
Deposition by sputtering
The deposition by sputtering is carried out in several steps:
- A plasma is initiated. Ions generated are accelerated toward the surface of the target.
- Some atoms are ejected from the surface due to the bombardment of the surface by ions.
- Sputtered atoms redeposit onto the surface of samples to obtain a thin film.
The working principle is similar to a DC diode system. This system is composed of a chamber under vacuum partially filled with a gas at low pressure (Inert gas such as Ar for non-reactive sputtering ). In the chamber, there are two electrodes : The anode (the substrate) which is positively charged and the cathode (the target) which is negatively charged. Between these two electrodes, a potential difference is applied to obtain a electric field.
Radio Frequency (RF) sputtering is used for insulating materials in which DC power cannot flow current through and cannot initiates a plasma. Capacitively coupling the cathode to an RF generator induces a negative DC bias with respect to the anode and allows to initiate the plasma. The RF frequency of 13.56 MHz is typically used.
In magnetron sputtering, a magnetic field is used to increase the plasma density. The magnetic field is perpendicular to the electric field and therefore forces the electrons to follow a gyrating motion inside a torus. This forced motion of electrons extends their path through the plasma and increases the probability of ionization collisions, ultimately leading to an increased plasma density.
Evaporation vs. Sputtering
The avantages of depositing a material by evaporation:
- A limited contamination considering the vacuum level in the chamber during the deposition (generally lower than 10-6 mbar).
- “Silicon Processing for the VLSI era” , Volume 1 – Process Technology, S. Wolf and R.N. Tauber, Lattice Press, 1986.
- “VLSI Technology”, S. M. Sze, Mc Graw-Hill International Editions, 1988.
- “Thin film Deposition – Principles & Practice”, D. L. Smith, Mc Graw-Hill International Editions, 1995.