Why RF Sputtering Process?
Direct current (DC) sputtering is a cost-effective method for thin layer deposition of electrically conductive metallic targets. But this method is not applicable for non-conductive dielectric target materials, since bombarding such targets with positive ions causes charging the surface of the target, which repels further positive ion bombarding the surface, resulting in the cessation of sputtering process and arcing into the plasma. In order to overcome DC sputtering shortcomings, RF sputtering is widely used for electrically non-conductive target materials deposition.
What is the difference between RF and DC sputtering?
In RF sputtering, DC power source is replaced with an AC one in the vacuum chamber, in which the polarity of the power supply changes alternatively. Thus, the electrons reach the target when it possesses the positive pole in the half-cycle and neutralize the positive ions collected on the target surface; while in the other half-cycle, target atoms sputtered by positive ions bombarding the target are deposited on the substrate and form a layer (Figure 1 and 2).
To electrically discharge the target during sputtering a frequency of 1MHz or higher is needed. Application of an alternative current to an insulating target in this frequency range is equivalent to current flow through dielectric media of capacitors in series.
Since the frequency normally used in this method is in the range of 5-30 MHz, it is commonly known as Radio Frequency (RF) Sputtering.
Why frequency of 13.56 MHz is used?
In order to prevent the interference between the frequencies used in telecommunication services, the standard radio frequency recommended by the ITU Radio Regulations (2012) for operating industrial (I), scientific (S), and medical (M) instruments, which is called ISM, is centered at 13.56 MHz with a bandwidth of 14 kHz.
Also this frequency is low enough to provide sufficient time for the momentum transfer of argon ions to the target. At higher frequencies, Ar ions are practically immobilized and electrons play effective role in the sputtering process (more like e-beam evaporation method).
RF Sputtering Advantages over DC Sputtering
Now, we will examine DC vs RF sputtering and explain the advantages of RF magnetron sputtering.
- The plasma formation is not limited to the cathode or target surface and can extend in the vacuum chamber.
- Higher plasma currents in lower working pressure: Plasma can be maintained in less working gas pressure (1-15 mTorr), which results in less collision between sputtered atoms and chamber molecules and larger mean free path for target atoms. Also, the magnetic field of the magnetron creates a boundary tunnel which traps the electrons near target surface and increases sputtering yield in lower pressures.
- By eliminating charge build up on the cathode surface, plasma arcing and layer quality control issues will be eradicated, so more uniform layer deposition is possible.
- In RF sputtering larger surface of the target is involved in the sputtering process, resulting in decreasing the so called ‘Race Track Erosion’ on its surface, so the lifetime of the target is enhanced.
Disadvantages of RF Sputtering
- Compared to DC Sputtering, higher voltages should be applied in order to increase the sputtering rate, leading to more heating effect on the substrate.
- This method is more complicated and expensive compared to traditional DC sputtering.
- RF current is transported on the skin or surface of the conductors and not through them, so special connectors and cables is needed for RF sputtering.
- With decrease in secondary electrons over cathode, deposition rate is lower than DC method and higher power level is needed to increase deposition rate.
- As a consequence of lower sputtering yields of electrically insulating targets, resulting in lower deposition rates, RF sources with higher powers should be employed, in contrast to DC sputtering.
RF Sputtering Systems
VacCoat RF Sputtering Systems
VacCoat Ltd. offers variety of RF sputtering systems. The single-target sputter coater (DST1-300) and triple-target sputter coater (DST3) with thermal evaporator (DST3-T) are equipped with 600W DC power supply, 300 W RF power supply (optional) with auto matching box. Also, plasma cleaning option is provided in these models to clean substrate surface through plasma treatment.
These high vacuum sputtering systems are able to deposit wide ranges of materials, including metals, metal-oxides, semiconductors, and ceramics on different surfaces for thin film applications such as micro/nano-electronics and FESEM sample preparation.
The Other Sputter Coaters
- “The Low Pressure Plasma Processing Environment” Donald M. Mattox, in Handbook of Physical Vapor Deposition (PVD) Processing (Second Edition), 2010
- “Improved electrochromic performance of a radio frequency magnetron sputtered NiO thin film with high optical switching speed” RSC Adv., 2016, 6, 79668-79680, https://doi.org/10.1039/C5RA27099E