FAQ / Education

Our goal is to proactively provide our customers with information that will help them be successful. If you have additional questions, please contact us at SCIService@sputteringcomponents.com.

What is thin film coating?

A thin film is a layer of material that is applied to a substrate. Thin film thickness ranges from fractions of a nanometer to several micrometers.

The controlled synthesis of materials as thin films (a process referred to as "deposition") is a crucial need in many industries. For example, architectural glass, displays, touch panels and solar cells all contain thin films. A familiar example is a mirror, which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface.

The most important coating process to produce thin film coating is a type of physical vapor deposition called sputter deposition, or sputtering.

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What is physical vapor deposition?

In physical vapor deposition (PVD) processes, source material starts from a condensed phase and then transports though a vacuum or low pressure gaseous environment in the form of vapor (plasma). This plasma is sometimes called “the fourth state of matter.” The vapor then condenses on a substrate to produce a thin film coating.

PVD is a physical process, not a chemical process. The most common types of PVD processes are sputtering and evaporation. 

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What is sputtering?

Sputtering is a type of physical vapor deposition. Unlike some other vapor deposition methods, the material does not melt. Instead, atoms from the source material (target) are ejected by momentum transfer from a bombarding particle, typically a gaseous ion.

In the gaseous ion sputtering configuration, the substrate that will be coated is set inside a vacuum chamber. Air is removed from the chamber, and an inert gas (usually argon) is pumped in at low pressure. Inert gas is used because it does not react chemically to the target material.

The sputtering process sequence is as follows:

  1. Voltage is applied to the target (the material that will be deposited onto the substrate), making it a cathode (negatively charged). The positive anode is the chamber body, which acts as the ground.
  2. The voltage causes free electrons to flow from the negatively charged target material and collide with the outer electronic shell of the inert gas atoms. The free electrons drive electrons off the inert gas due to their like charge. The inert gas atoms become positively charged ions.
  3. The inert gas ions attract to the negatively charged target material. During a collision cascade, this attraction ultimately causes inert gas ions to strike the target at an extremely high velocity.
  4. The bombarding ions have sufficient force to dislodge and eject (sputter off) atoms from the face of the target. The atoms from the target cross the vacuum chamber and are precisely deposited in a typical line-of-sight cosine distribution on the substrate surface as a thin film of material.
  5. The glowing “plasma” is created when the inert gas ions recombine with free electrons into a lower energy state. The excess energy is emitted as light.

Sputter deposition is used in the following commercial applications:

  • Architectural and anti-reflective glass coating
  • Solar technology
  • Display web coating
  • Automotive and decorative coating
  • Tool bit coating
  • Computer hard disc production
  • Integrated circuit processing
  • CD and DVD metal coating

Please watch our YouTube video for more information about sputtering:

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What is a rotary (cylindrical) cathode?

A rotary cathode (cylindrical cathode) is a device used for sputtering target material from the surface of a rotating target tube onto a substrate.

A hollow tube of target material rotates around a stationary magnet array suspended inside the tube. This stationary magnet array is directed at the substrate and holds the process plasma in the desired location.

Electric power is transmitted to the rotating target tube. The cathode is the target tube surface since the electrons leave from the target surface and enter the plasma. Cooling water is usually used inside the target tube for sputtering process cooling

Powered by a motor, an end block rotates the target tube. It has rotary seals for both cooling water and vacuum. While the cathode assembly is mounted to the inside lid of a vacuum chamber, the end blocks can be external to the chamber or inside the chamber.

Please watch our YouTube video for more information about rotary cathodes. 

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What is magnetron sputtering?

While conventional cathode sputtering can deposit extremely thin films down to the atomic scale, it tends to be slow and most effective only with small substrates. The bombardment of the substrate can also create overheating or damage to the object to be coated.

With magnetron sputtering, magnetic fields increase plasma density for a higher deposition rate and a more efficient process. 

Magnets (usually permanent magnets) behind the cathode confine electrons over the negatively charged target material. Electrons follow helical (spiral) paths around magnetic field lines. The longer paths mean a higher probability of ionizing collisions with gaseous neutrals near the target surface than would otherwise occur. The result is very dense plasma and a higher deposition rate. The sputtered atoms are neutrally charged and are unaffected by the magnetic trap.

Magnetron sputtering results in very uniform and smooth coatings. Almost any metallic material can be sputtered. Sputtering magnetrons can be oriented in any position to accommodate equipment design. The target is water cooled to minimize radiation heat.

Sputtering magnetrons can be planar magnetrons or rotating (cylindrical) magenetrons

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What are the advantages of rotary (cylindrical) magnetrons over planar magnetrons?

While a planar magnetron uses flat plate or tile targets, a rotary magnetron (cylindrical magnetron) uses target tubes that are rotated by a drive (end block) system. 

Most target materials that are commonly sputtered can be made into tubes, often at lower material costs. The initial capital in investment in a rotary magnetron is lower. Operating costs are also less because coating campaign times are longer and downtime is reduced. In some cases, quality is better: 

Targets last longer because 1) more target material is available on a cylinder than on a planar target of equal width, and 2) target utilization is higher (exceeding 90%) because the target moves through the stationary plasma and is eroded all around its circumference. Planar magnetrons have wide magnetic fields to increase target material utilization.

Deposition rates are higher because higher power densities are achievable due to better target cooling. Cooling is improved because 1) the target surface area is constantly passing though the plasma and is not stationary within the plasma like it is for a planar cathode, so the plasma heats a small portion of the rotary cathode target material at any given time while a planar magnetron heats as much of the target surface area as possible, and 2) the cooling surface area and cooling water flow rate inside a rotary cathode target are much larger than in a planar target.

Target changes are easier and quicker. Most SCI rotary cathode systems have only one end block where all utilities are introduced. The other end is a simple support.

Particle contamination and arcing are reduced, and process stability over the lifetime of the target material is better. This is because arcing and debris showers are reduced. By eliminating the re-deposition areas around the sputter region found on planar cathodes, coating buildup in these regions is eliminated. On a planar cathode this area can generate debris that falls on the substrate in the sputter down situation and can also cause charge buildup arcing, which tends to ‘blast’ debris from the target to the substrate even in a vertical or sputter up situation. So with a rotary cathode, longer runs can be achieved before getting ‘dirty’.

Rotary targets typically do not need to be burned in more than once. Although rotary targets are sometimes ‘burned in’ at the start of a run, planar cathode ‘burn in’ often interrupts production.

With some target materials, there is less process drift and better films due to lower impedance. For some processes, deposition rate is also increased because lower impedance in rotary magnetrons will allow the target to be run at higher power. The lower impedance is because the magnetic field designs are much narrower and stronger than the wide, weaker planar magnetron magnetic fields.

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What is a large area substrate?

Large area substrates are usually wide flat rigid panels (such as glass or plastic) or wide flexible webs (typically plastic or metal) that are coated as they pass by the stationary magnetrons. Some examples of products made from these substrates include Low-e window glass, solar panels, solar control window tint and flat panel displays.

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How do I connect to my SCI rotary magnetron?

You will need to provide four types of connection to your SCI rotary (cylindrical) magnetron:

Cooling water for SCI rotary magnetrons: Provided to the cathode using ¾” or 1” flexible hose, connecting to a hose barb or other customer-specified water fitting. Nominal inlet pressure should be 40 psi, with a maximum of 100 psi. Water flow rates should be 1 liter per minute per kW of applied power, or greater. Water quality should be 100-300 micro Siemens, with 75 micron filtration. Inlet temperature should be less than 30°C and maintained above the ambient dew point to prevent condensation.

Power supply connection for the SCI rotary magnetrons: Various connection styles are provided,using M8 or M10 screws. SCI can provide custom length, low impedance cable assemblies, which are optimized to your cathode power rating and mounting pattern.

Drive motors: SCI provides AC inverter duty motors with its end blocks.The inputs to this motor are 230 V, three-phase,10-90 Hz, 480 V is available. Speed control is provided by the use of a customer supplied variable frequency drive (VFD.) The VFD can be selected to accept your local input voltage and number of phases and convert to the motor inputs. SCI can also supply these drives upon request.

Drive encoders: SCI provides encoders with every rotary magnetron.These encoders are used to ensure cathode rotation and, if desired, measure rotation speed. The coder is connected depending upon how it will be used:

  • To confirm rotation only: The encoder can be connected to a signal conditioner (such as the Red Lion IFMR0036) that will convert the encoder pulses to a digital signal, which can be used to interlock rotation. SCI can provide these signal conditioners upon request.
  • To measure rotation speed: The encoder is a simple 100 pulses per revolution quadrature encoder. The pulses from channel A (and/or B) can be used for speed measurements. The SC/SM cathodes are set to 294 pulses per target revolution while the MC/MM is 235 PPR.

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How long should my target be?

The length of the target tube is determined by the substrate length and the coating uniformity requirements. SCI refers to the overall length of the backing tube as the basis for calculation as it will define the system geometry. The actual length of the material to be sputtered will be shorter than the backing tube length. 

The standard calculation that we use is the following:

Backing Tube Length = Substrate Length + 4 * Target-to-Substrate (TTS) distance

This equation is typically used for TTS distances between 75 to 150 mm. If the application has uniformity requirements higher than +/-2%, we suggest to add additional length to the target in order to reduce the initial uniformity tuning required for rotary-type sputtering cathodes.

For an illustration of how to apply this calculation, click here

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How many magnetrons do I need for my application?

SCI will calculate the deposition rates and the number of rotary magnetrons required for your application. Please contact us at SCIService@sputteringcomponents.com.

If you wish to do the calculation, continue reading:

Each target material and coating process has limitations as to how fast the material can be deposited from each magnetron.

Thus each different process has a limited dynamic deposition rate (DDR).These limits for most materials have been experimentally obtained and are correlated with the power density that was applied to the target at the time of the deposition.

Using this information we create a unit for calculation the normalized dynamic deposition rate (nDDR) that has the units of (nm-m/min)/(kw/m).This unit can be used along with the maximum power density that target vendors recommend using with their targets to figure out approximately what the dynamic deposition rate of each rotary magnetron will be for a particular process.

Desired (or maximum) power density = Power applied from power supply in kW divided by the target tube length in meters -> (kW/m)

nDDR(nm-m/min)/(kW/m) * Desired power density(kW/m) = DDR

DDR / substrate velocity = coating Thickness per magnetron

Number of cathodes required = Target coating thickness / Coating thickness per magnetron

Here is a list of typical nDDR values.

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How do I calculate my dynamic deposition rate?

To determine your dynamic deposition rate, multiply the thickness of the coating you want to make in nanometers by the velocity of your substrate in meters per minute.

Example: With SiO2 coating that is 20 nm thick and a substrate speed of 1.5 meters per minute, the dynamic deposition rate is 30 nm * m/min:

20 nm * 1.5 m/min = 30 nm * m/min

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