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Magnetron Sputtering

Magnetron sputtering is used at Teer Coatings Ltd to produce some of the highest quality coatings currently available from any technique. These coatings have excellent adhesion to the workpiece and a range of properties tailored to todays demanding engineering applications. Magnetron sputtering is a powerful and flexible technique which can be used to coat virtually any workpiece with a wide range of materials – any solid metal or alloy and a variety of compounds.

To magnetron sputter coat a component it is first precleaned, then jigged and placed inside the coating system. This is a stainless steel chamber equipped with the pumps necessary to produce a high quality vacuum, sample support and rotation equipment and the sources of coating material known as magnetrons.

Creation of Vacuum conditions

It takes typically 30 minutes of pumping to achieve a vacuum sufficient to begin the coating process – a pressure of less than one ten millionth of an atmosphere. From this point a closely controlled flow of argon, an inert process gas, is introduced, with the pumps still running. This raises the pressure in the system to the levels needed to operate the magnetrons, although it is still only a few ten thousandths of an atmosphere. The combination of pumping down to the high vacuum region and the continual throughput of process gas ensures that coating conditions are both clean and reproducible.

Application of Magnetrons

A magnetron consists of a plate of the material of which the coating is to be made (called the target) with magnets arranged behind it which create a magnetic trap for charged particles, such as argon ions, in front of the target. When the magnetron drive power supplies are turned on, they put the target at a negative voltage of typically -300V or more, which attracts argon ions to the target surface at speed. When they hit it, two important processes take place:

• Atoms are knocked out of the target surface by the ions – this is sputtering. These sputtered atoms are not charged negatively or positively, so they go straight out of the magnetic trap. It is these atoms which hit the workpieces to form the coating.

• The surface also releases electrons, which are negatively charged sub-atomic particles. These are held in the magnetic trap, where their energy is used to produce more argon ions. This means that the ions which are attracted to the surface are replenished all the time, so that the magnetron can operate continually.

Sputtering also leads to the heating of the target, so the magnetron incorporates channels for water cooling during operation.

Ion cleaning

After pumping down, the workpieces are ion cleaned. During this part of the process the magnetrons are run at a low power, which ionises (electrically charges) atoms of the argon process gas, but sputters very little coating material. When the workpieces are biased to -500V or more they attract the argon ions, which bombard the surface with high energies, removing microscopic surface contamination at the molecular level. When the ion clean is complete the bias voltage on the workpieces is reduced and the power to the magnetrons is increased for the deposition of the coating.

Magnetron Plasma

When a magnetron is in operation (viewed through a window in the chamber wall), the space inside the chamber glows, with particularly bright areas in the magnetic traps at the front of each magnetron. This is because of the way the electrons interact with the argon gas, causing it to become excited, then decay back to its original state, giving off its extra energy as light. This glowing mass of charged particles is known as a plasma.

The exact details of the magnetic trap, and the links between the magnetic traps of adjacent magnetrons have to be carefully designed. Adjusting the strength and size of the magnets can effect not only how efficiently the sputtering process takes place, but also how much plasma surrounds the items being coated.

Closed Field System – CFUBMSIP

From years of experience Teer Coatings Ltd have developed and patented the closed field unbalanced magnetron system known as CFUBMSIP. Use of this system produces magnetrons with good operating characteristics while simultaneously generating a dense plasma close to the workpieces. This is important as a few tens of volts applied to the workpiece during coating attract ions out of the plasma with enough energy to densify the coating and improve the adhesion, but not so much energy that the coating is sputtered off. It is this bombardment with very large numbers of low energy ions which gives the coatings their exceptional properties. The very adhesive nature of our coatings, often combined with the high hardness values, low friction co-efficients and exceptional wear resistance which can be produced mean that we are able to assist hundreds of customers world-wide with their coating requirements.

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