3.2 Thin Film Deposition Methods
3.2.2 Sputtering technique
Sputtering is a thin film deposition technique where atoms are dislodged from solid target (source) surface and moves to the substrate through impact of gaseous ion mostly argon. In this method atoms are dislodged from the surface of a target by high-energy particle bombardment so that ejected atoms condense on a substrate as thin film. The atoms are released from the source at a much lower temperature than in evaporation technique. The source, also called the target, is put in a vacuum chamber and an inert gas is introduced at low pressures. Gas plasma is then struck with an RF power source, ionizing the gas. The ions are accelerated towards the target making atoms of the source material to break off from the target in vapor form and condense on the substrate hence forming thin film.
The basic sputtering principle is similar for all sputtering technologies with the difference being the manner in which the target is bombarded. Film thickness is controlled by fixing the operating parameters and adjusting the deposition time. The advantage of sputtering over evaporation is that it enables control of alloy composition and other film properties such as step coverage and grain structure. However, this process suffers from high capital expenses and low rate of deposition of some materials such as silicon dioxide (SiO2). Organic solids are easily degraded by ionic bombardment and therefore not used in this deposition process. The high capital expense is due to the fact that sputtering process is energy intensive as it takes much longer time to deposit thin film. Moreover the most targets are very expensive hence raising the sputtering capital.
Sputtering yield (number of atoms ejected per incident ion) is an important factor in sputter deposition process since it affects the deposition rate. Three major factors that affect the sputtering yield are target material, mass of bombarding particles and energy of bombarding particles.
Figure 3.2 shows a schematic diagram of sputtering phenomenon. Energetic argon ions from the plasma of gaseous discharge are attracted to the target (cathode) which they bombard. Due to the bombardment of the target the atoms of the target together with some electrons are ejected from the target surface. The target atoms are accelerated towards the substrate (anode) where they are impinged to form a thin film coating. The emitted electrons contribute in the production of more ions which sustains the discharge.
Sputter deposition may employ multiple targets making it possible to work with different composition. Thin films processed using sputter deposition do not have good step coverage (Burr et al., 2010). A minimum projectile kinetic energy needed to induce sputtering is called threshold energy (Eth) and is given by (Bohdansky,1984) ;
1 o th U E for 0.3 2 1 M M (3.2)where M1 is projectile mass, M2 is mean molecular mass per atom of a target and β is
maximum fractional energy transfer possible in a head-on collision given by:
2 2 1 2 1 4 M M M M (3.3) and for 0.3 2 1 M M , 3 1 2 1 0 8 M M U Eth (3.4)Sputtering processes are wide and varied. There are four spattering arrangements; DC, RF, magnetron and reactive. The arrangements employ almost similar mechanism of atom ejection from the target. The arrangements are briefly discussed below.
3.2.2.1 DC sputtering/RF sputtering
In DC sputtering the target is a plate material to be deposited or the material from which thin film is synthesized. The target is placed at the cathode connected to the negative terminal of the DC/RF supply. Several kilovolts are applied to the target. The substrate that usually faces the target is either grounded, electrically floating, biased positively or negatively. The substrate may also be heated, cooled maintained at room temperature. A
gas, typically argon is introduced into the chamber after evacuation. This serves as a medium in which the discharge is initially sustained. The pressure of the gas usually ranges from a few to 100 mtorr. After a visible glow, discharge is maintained between the electrodes, it is observed that a current flows and a film condenses on the substrate (anode). Microscopically positive ions in the discharge strike the cathode plate and eject a neutral target atom through momentum transfer. These atoms enter and pass through the discharge region to eventually deposit on the substrate. Regardless of the sputtering, however, roughly similar discharge, electrode configurations and gas-solid interaction are involved.
It is also worth noting that the DC and RF sputtering techniques follow similar mechanisms except that RF sputtering has an additional matching network circuitry.
Figure 3.3 (b): Schematic diagram of RF sputtering set up (Ohring M, 1992)
3.2.2.2 Magnetron sputtering
In magnetron sputtering the superposition of electric and magnetic fields between the substrate and the target yields a force on electron within the dual field environment. The force is given by Lorentz equation described in equation 3.5;
F =md𝑣dt = −q(ε + 𝑣𝑋𝐵) (3.5)
Where q, m, 𝑣 are the electron charge, mass and velocity respectively.
By suitable orientation of the target magnets, a “race track” may be defined where the electrons hope around at high speed. Target erosion by sputtering occurs within the track because ionization of the working gas is most intense above it.
Magnetron sputtering takes the lead as the most widely used commercially practiced sputtering method. The main reason for its success is the high deposition rate achieved
(up to 1 𝜇𝑚/𝑚𝑖𝑛) which is higher than rates attained by conventional sputtering techniques.
3.2.2.3 Reactive sputtering
Here thin films of compounds are deposited on the substrate by sputtering from metallic targets in the presence of a reactive gas always mixed with the inert working gas such as argon. Below are the most commonly reactively sputtered compounds;
a) Oxides (oxygen),
b) Nitrides (nitrogen and ammonium), c) Carbides (methane, acetylene , propane), d) Sulphides (H2S),
e) Oxycarbides and oxynitrides of Ti, Ta, Al and Si.
Irrespective of the material during reactive sputtering, the resulting films are either a solid solution alloy of the target metal doped with element, compound or a mixture of the two.