Last updated on January 11th,

Why RF Magnetron Sputtering?

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.

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How Triple Target Sputter Coater DST3-A Works?

What is the Difference Between RF and DC Sputtering?

In RF sputtering, an RF power source, usually called RF signal generator replaces the conventional DC power supply, connected to the cathode in the vacuum chamber, so the electric polarity of the cathode 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).

What is RF Sputtering?

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 Sputtering (RF).

RF Sputtering Principles

In RF sputtering method, cathode and anode are serried with a blocking capacitor (C) (Figure 3). This capacitor is part of an impedance matching network for optimizing power transfer from RF source to the plasma. All Vac Coat RF power supplies are provided with an RF matching network to precisely tune the RF power, manually or automatically. RF voltage is an alternative voltage in the form of equation (1):

VRF(t) = VRF Sin(wt)                       Equation (1)

If VRF is set about 500 (V) (Figure 4-a), without blocking capacitor, the target current will be in the form of figure 4-b. Since the plasma potential is close to ground, the current density will change between Jion and &#;qz with changing the sign of VRF. So net current is not zero. When using blocking C and applying a self-bias DC voltage, the cathode voltage is given by Equation (2) (Figure 4-c):

Vcath(t) = VRF(t) &#; VDC                         Equation (2)

The target potential is lower than the plasma mostly. In order to reach zero net current in RF cycle, the electrode potential must be negative in most of the cycle. When the cathode potential is positive, electrons bombard the target and it is negatively charged. Since electrons have higher mobility compared to argon ions and are faster, electron bombarding time should decrease.

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So duration of negatively charged target bombarding with argon ions is increased to neutralize the collected charge on the target surface (Figure 4-d). So the area under the curve of electrode current, that shows average current, is set equal to zero.

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 () 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).

A Common Problem

As mentioned earlier, insulating (Non-conductive) target materials can be deposited through RF sputtering technique. Since these materials are poor thermal and electrical conductors, they should be back-plated with a thin conductive sheet so as to prevent thermal shock and charge accumulation on the target.

The dielectric (Non-conductive) target surface should completely cover the conductive part (Backing plate) beneath, otherwise, the backing-plate is exposed to the electric field and the capacitance will be short-circuited. This is the major problem when the dielectric surface does not cover the metal backing plate in the case of plasma treatment, cleaning or coating, of dielectric surfaces.

RF Sputtering Advantages Over DC Sputtering

1. The plasma formation is not limited to the cathode or target surface and can extend in the vacuum chamber
2. 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
3. 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
4. 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

Sputtering: A Process Animation

Disadvantages of RF Sputtering (DC vs RF Sputtering)

1. Compared to DC Sputtering, higher voltages should be applied in order to increase the sputtering rate, leading to more heating effect on the substrate
2. This method is more complicated and expensive compared to traditional DC sputtering
3. 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
4. With decrease in secondary electrons over cathode, deposition rate is lower than DC method and higher power level is needed to increase deposition rate
5. 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

How Does Our Sputter Coater Work?

RF Sputtering Systems

Vac Coat RF Sputtering Systems

Vac Coat Ltd. offers variety of RF sputtering systems using RF generators with different powers (0.2-5 kW) supplied with precise RF matching network. The single-target sputter coater (DST1-300) and triple-target sputter coater (DST3) with thermal evaporator (DST3-T) are equipped with 600 W DC power supply and 300 W RF power supply (Optional) with auto matching box (You can read more about our sputter coaters here).

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. You can read more about FESEM and difference between SEM and FESEM, and our SEM coaters here.

What is SEM?

FESEM vs SEM

The Other Sputter Coaters

References

1. https://www.sputtertargets.net/sputter-coating-technologies-radio-frequency-rf-sputtering.html
2. https://www.sciencedirect.com/topics/materials-science/radio-frequency-sputtering
3. &#;The Low Pressure Plasma Processing Environment&#; Donald M. Mattox, in Handbook of Physical Vapor Deposition (PVD) Processing (Second Edition),
4. https://baalkikhaal.github.io
5. http://www.semicore.com/news/92-what-is-rf-sputtering
6. &#;Improved electrochromic performance of a radio frequency magnetron sputtered NiO thin film with high optical switching speed&#; RSC Adv., , 6, -, https://doi.org/10./C5RAE
7. https://www.rfwireless-world.com/Terminology/RF-sputtering-vs-DC-sputtering.html
8. https://www.tn.ifn.cnr.it/facilities/rf-sputtering-facility/rf-sputtering-principles
9. Park, Sang Eun, et al. &#;Properties of gallium-doped zinc-oxide films deposited by RF or DC magnetron sputtering with various GZO targets.&#; Journal of the Korean Physical Society 54.3 (): -.

DC Sputtering VS RF Sputtering

views, Updated: -12-16 What is Sputtering:

Sputtering is a vacuum deposition technique used to deposit thin-film of a material onto a surface.

The substrates are placed into the vacuum chamber and pumped down to process pressure. First, creating gaseous plasma accelerates the ions from this plasma into some source material. It is used to knock atoms out of the target. The molecules, atoms, ions, and electrons on the target's surface are sputtered out in the process.

The emitted particles have a certain kinetic energy and are directed toward the substrates' outer surface in a particular direction to deposit on the surface of the substrate. The higher pressure can help to generate better coverage. The excess energy of metal ions can increase surface mobility in the sputtering process.

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DC Sputtering:

&#; The power source is the Direct Current (DC) type. &#; Chamber pressure is usually from 1 to 100 mTorr &#; DC power is usually preferred for electrically conductive target materials as it&#;s effective and economical. Such as pure metal sputtering targets, Iron (Fe), Copper (Cu), Nickel (Ni). &#; It is a simple technique when processing large quantities of large substrates. &#; The deposition rate is high for some pure metal sputtering targets &#; The positively charged sputtering gas is accelerated towards the target in DC sputtering, and the ejection of atoms gets deposited on substrates.

RF Sputtering&#;

&#; The power source is AC (Alternating Current). The power supply is a high voltage RF source often fixed at 13.56 MHz.  &#; RF peak to peak voltage is V, electron densities are 109 to Cm-3, and the chamber pressure is from 0.5 to 10 mTorr. &#; RF sputtering has a wider range of applications and is suitable for all the materials for conductive and non-conductive materials. However, it is most commonly used for depositing dielectric sputtering target materials.  &#; The deposition rate is lower compare with DC sputtering.  &#; It is used for smaller substrate sizes due to the high cost. &#; The RF sputtering involves two processes. In the first cycle, the target material is negatively charged. This results in atoms' polarization, and the sputtering gas atoms are attracted to the source, where they knock outsource atoms.  Due to polarization, the source atoms and ionized gas ions remain on the target surface.  &#; In the second cycle, the target is positively charged. Due to reverse polarization, this causes the ejection of gas ions and source atoms. These ions and atoms accelerated toward the substrate to form deposition.

Following table summarizes difference between RF sputtering and DC sputtering 


Features DC Sputtering RF Sputtering Sputtering Type Magnetron only Magnetron or diode Target Materials Conductive only All the targets (conductive and non-conductive materials) Sputtering Rate 100% of DC 20% of DC, no magnetron Cost and Complexity Best Expensive and complex than DC sputtering Campaign Length (i.e. loss of anode) Good Excellent
AEM Deposition provides various sputtering targets (Pure metal sputtering targets, Oxide sputtering targets, Sulfide sputtering targets) suitable for DC & RF sputtering.


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