Sputtering is a physical vapour deposition (PVD) method use for thin film growth. Like any other physical vapour deposition, sputtering involves the deposition of an atom, ion or molecule corresponding to a target material onto the substrate.
Sputtering employs electrical generation of plasma between the target material, which is the material to be deposited and the substrate. Whereas, other PVD methods follow heating or thermal evaporation of the target material to generate vapour and then this vapour condenses on the substrate to form the coating of a thin layer of the material.
In the sputtering process, atoms are ejected (sputtered) from the target material by the impact of energetic ions accelerated towards the target. Magnetron sputtering is a sputtering method widely used in semiconductor industry, optics and microelectronics.
Magnetron sputter deposition method uses magnets beneath the target material as shown in the title figure to confine the electrons present in the plasma towards the target material. This helps to enhance the deposition rates.
In the magnetron sputtering process, the sputtering gas chosen is often Argon, Krypton or Xenon gas as it possesses high molecular weight and thereby helps to achieve high deposition rate. However, the sputtering gas can also be chosen from particle accelerators, plasma and constructed ion sources. If the deposition requires reactive sputtering, gases like oxygen or nitrogen will also pass to the chamber.
The cathode is located behind the target material and anode is connected as electrical ground to the chamber where the deposition process occurs (Refer the title figure).
Depending on the nature of the power source use, magnetron sputtering is called direct current (DC) magnetron sputtering, radio frequency (RF) magnetron sputtering etc. The power source for DC sputtering is the direct current, on the other hand RF sputtering alternates the charge which in turn helps to prevent accumulation of charge on the target material. The target material has to be conductive for DC sputtering, while RF sputtering is suitable for conductive as well as non-conductive target materials.
The substrate has to be mounted on the substrate holder usually placed in the load lock chamber. The deposition chamber is maintained in a vacuum environment. After making the load lock chamber also to vacuum condition, the substrate can move to the deposition chamber from the load lock chamber by opening the gate in between the chambers.
The deposition chamber contains the sputter gun with the target material placed over it. Magnets are placed beneath the target material to confine the electrons in the sputtering gas. As a next step, the sputtering gas is allowed to pass to the chamber once the base pressure is reached. Power must be supplied with a ramp up from low voltage to the required level.
Pre-sputtering cleans up the surface of the target by knocking out the atoms from the surface for a specified time by keeping the shutter to the substrate closed. After the pre-sputtering, the substrate shutter will be open for the deposition of thin films.
Step by step procedure in magnetron sputtering
- In order to avoid the risk of potential contaminants and to reduce the partial pressure of background gases, the chamber has to evacuate to high vacuum.
- Once the base pressure is reached, high energy ions, called sputtering gas, are passed to the deposition chamber and the pressure is maintained in the range of milli Torr using a pressure control system.
- A high voltage is applied between the cathode and the anode using the external power source to initiate the plasma generation.
- The free electrons in the plasma spiral around the target material due to the influence of magnetic field produced by the magnets placed behind the target material. These electrons collide with the nearby atoms in the sputter gas and ionize the atoms.
- The positively charged atoms in the ionized sputter gas (for instance Ar+) gets attracted towards the negatively charged cathode and thereby it knocks out atoms from the surface of the target material
- As the ejected (sputtered) out atoms are electrically neutral, they are free to travel around in the vacuum chamber without getting affected by the electric charges and the magnetic field.
- The sufficiently high energy acquired by the liberated atoms from the particle collision enables it to move towards the substrate and get adhered to the substrate.
Advantages of Magnetron sputter deposition method
- Magnetron sputter deposition doesn’t require thermal evaporation, heating or melting of the target material. This opens up scope to deposit a wide range of materials using magnetron sputtering irrespective of their melting point.
- Electron confinement in the plasma near the target using the magnets helps to achieve high density deposition
- Trapping of electrons using magnets near the surface of the target prevents damage to the substrate and to the growing film as it avoids direct impact of atoms in plasma on to the substrate and to the growing film.
- Deposit films with uniform distribution over large substrate area
- Deposit films with high purity
Despite the advantages, difficulty in stoichiometry control and undesired outcomes from reactive sputtering are the challenges to tackle while using magnetron sputter deposition method to grow thin films.