Written By Matt Hughes - President - Semicore Equipment, Inc.

magnetron sputtering source

RF or Radio Frequency Sputtering is the technique involved in alternating the electrical potential of the current in the vacuum environment at radio frequencies to avoid a charge building up on certain types of sputtering target materials, which over time can result in arcing into the plasma that spews droplets creating quality control issues on the thin films – and can even lead to the complete cessation of the sputtering of atoms terminating the process.

Traditional DC Sputtering is a cost effective way of applying metal target coatings that are electrical conductors like gold. However, DC Sputtering is limited when it comes to dielectric target materials – coatings which are non-conducting insulating materials that can take on a polarized charge. Examples of common dielectric coating materials used in the semiconductor industry include Aluminum Oxide, Silicon Oxide and Tantalum Oxide.

Diagram of the RF Sputtering Process

Diagram of the RF Sputtering Process

As with DC Sputtering, RF Sputtering runs an energetic wave through an inert gas in a vacuum chamber which becomes ionized. The target material or cathode which is to become the thin film coating is bombarded by these high energy ions sputtering off atoms as a fine spray covering the substrate to be coated.  RF Magnetron sputtering uses magnets behind the negative cathode to trap electrons over the negatively charged target material so they are not free to bombard the substrate, allowing for faster deposition rates.

Over time, positive ions are produced which accumulate on the surface of the target face giving it a positive charge. At a certain point this charge can build up and lead to a complete secession of sputtering atoms being discharged for coating.

By alternating the electrical potential with RF Sputtering, the surface of the target material can be “cleaned” of a charge buildup with each cycle. On the positive cycle electrons are attracted to the target material or cathode giving it a negative bias. On the negative portion of the cycle - which is occurring at the radio frequency of 13.56 MHz used internationally for RF power supply equipment - ion bombardment of the target to be sputtered continues.

RF Sputtering offers several advantages depending upon your specific application. RF plasmas tend to defuse throughout the entire chamber rather than concentrating around the cathode or target material as with DC Sputtering.

RF Sputtering can sustain a plasma throughout the chamber at a lower pressure (1-15 mTorr). The result is fewer ionized gas collisions equaling more efficient line-of-site deposition of the coating material.

Because with RF Sputtering the target material is being “cleaned” with each cycle from building up a charge it helps reduce arcing. Arcing is where there is an intensely focused and localized discharge emanates from the target material or cathode into the plasma that creating droplets and problems with non-uniform film deposition. RF Sputtering greatly reduces the buildup of a charge in a specific location on the surface of the target material that leads to the sparks that creates the arc which causes numerous quality control issues.

Racetrack Erosion of Target Coating

Racetrack Erosion of Target Coating

RF Sputtering also reduces the creation of “Race track erosion” on the surface of the target material. With Magnetron Sputtering, a circular pattern becomes etched into the surface of the target material as a result of the circular magnetic field of the magnetron focusing the charged plasma particles close to the surface of the sputter target. The diameter of the circular pattern is the result of the magnetic field.

With RF Sputtering the width and depth of the race track is much less due to the AC nature of the RF discharge with electrons less confined by the magnetic field. The plasma spreads out more producing a larger, wider and shallower racetrack. This makes for better, more uniform and efficient utilization of target coating materials without the deep etching of “Race track erosion”.

Another advantage of RF Sputtering is that there is no disappearing anode effect when the substrate to be coated becomes insulated and acquires a charge as with DC Sputtering. All surfaces develop a charge in a plasma as a result of electrons moving much faster than ions due to their smaller size and kinetic energy.

However, as a result of the AC modulation of the power at radio frequencies, the material to be coated with RF Sputtering does not acquire as great a charge buildup due to it being discharged each half cycle and becoming insulated - which over time can eventually lead to a cessation of the thin film deposition.  With RF Magnetron Sputtering the magnetic field forms a boundary "tunnel" which traps electrons near the surface of the target improving the efficiency of gas ion formation and constraining the discharge of the plasma. In this way, RF Magnetron Sputtering allows for higher current at lower gas pressure that achieves an even higher deposition rate.

While RF Sputtering offers many very attractive benefits depending upon the type of material to be coated, there are several important costs involved that must be considered. Because RF Sputtering uses radio waves instead of DC current, deposition rates are considerably slower with RF Sputtering and require significantly higher voltages. A DC Sputtering system typically requires between –s 2 to -5kV , whereas RF Sputtering needs 1012 volts to sputter dielectric insulators.

Radio waves require much higher voltage to achieve the same deposition results as with direct current, and so overheating becomes an issue. Applying RF power is complicated requiring high voltage power supplies that are expensive. Advanced circuitry is required that can pose additional overheating problems.

Another issue is that the RF currents travel on the “skin” or surface of conductors and not through them. This means that special cabling/connectors are critical.

Another major consideration that needs to be factored with RF Sputtering rates is the decrease in deposition rates due to the lack of secondary electrons being trapped above the target as with conventional Magnetron Sputtering for gas ionization. With all types of sputtering, the plasma is sustained by the breakdown and ionization of an inert gas such as argon - which is most widely used due to its larger mass compared to the other inert gasses, helium and neon.

By alternating the current at a high radio frequency a plasma can be sustained with much lower pressure due to the kinetic energy resulting from accelerating and reversing the electrons for a sufficient distance in the plasma. The difference in the mass between the ionized gas particles and the electrons enables a plasma to be sustained without depending upon the trapping of secondary ions above the target material as with conventional Magnetron Sputtering.

However, this also results in a slower deposition rate compared to DC Sputtering due to the lack of secondary electrons above the target. Because RF Sputtering deposition rates are slower than DC Sputtering rates and have much higher power costs, on a practical level this translates into RF Sputtering usually being used on smaller substrates to be coated.

While RF Sputtering can be used for most types of thin film deposition coatings, it has become the thin film deposition technique of choice for many types of dielectric coatings - insulating coatings which are non-conducting that can take on a polarized charge. RF Sputtering is at the heart of the semiconductor industry producing highly insulating oxide films between the thin film layers of microchip circuitry including Aluminum Oxide, Silicon Oxide and Tantalum Oxide.

 

 

Matt Hughes is President of Semicore Equipment Inc, a leading worldwide supplier of sputtering equipment for the electronics, solar energy, optical, medical, military, automotive, and related high tech industries. Please let our helpful support staff answer any questions you have regarding “What is RF Sputtering?” and how to implement the best techniques and equipment for your specific Thin Film Deposition RF Sputtering Equipment needs by contacting us at sales@semicore.com or by calling 925-373-8201.

 

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Photo:  CC by Inmodus