November 7, 2010 PVD Coating

As this website concentrates on magnetron sputtering the history of PVD coatings concentrates on magnetron sputtering. The history of other deposition methods such as arc evaporation will be added at a later date.

Origin of the term Physical Vapour Deposition, PVD
The term physical vapour deposition, PVD seems to have been originally termed by the authors CF Powell, JH Oxley and JM Blocher Jr. in their 1966 book “Vapor deposition”. However PVD processes were invented much earlier.

The development of vacuum technology, electricity and magnetism and gaseous chemistry
The history of PVD is closely associated with the development of vacuum technology, the discovery of electricity and magnetism and the understanding of gaseous chemistry.

Vacuum technology, glow discharges and sputtering
The first piston type vacuum pump was invented by Otto van Guericke to pump water out of mines as far back as 1640 . However the first person to use a vacuum pump to be able to form a glow discharge (plasma) in a “vacuum tube” was M. Faraday in 1838 who used brass electrodes and a vacuum of approximately 2 Torr. In 1852 W.R. Grove was the first to study what became known as “sputtering” although others had observed the effect while studying glow discharges. Grove used a tip of wire as the coating source and sputtered a deposit onto a highly polished silver surface held close to the wire at a pressure of about 0.5 Torr. He noted a coating on the silver surface when it was made the anode and the wire the cathode of an electrical circuit. In 1858 Prof A.W. Wright of Yale University published a paper in the American Journal of Science and Arts on the use of an “electrical deposition apparatus” that he used to create mirrors. This form of deposition may have been arc evaporation based rather than sputtering as the US Patent Office quoted his work when challenging T. Edison’s patent application for vacuum coating equipment to deposit coatings on his wax cylinder phonographs before subsequent electroplating. Edison successfully argued that his invention was a continuous arc whereas Wright’s process was pulsed arc. Edison could therefore be said to be the first person to make commercial use of sputtering.

Electricity and magnetism
In the late 1930s Penning developed an “electron trap” to confine electrons near a surface using a combination of electric and magnetic fields. This combination of electric and magnetic fields increased the ionization of the plasma near the surface and was named a “Penning Discharge” after it’s inventor. Penning used his invention to sputter from the inside of a cylinder. This was an important development in the history of sputtering and is a basic magnetron.

Lower pressures, lower voltages and higher deposition rates
Such a combination of electric and magnetic fields allowed sputtering to be performed at lower pressures and lower voltages, and at higher deposition rates than were previously possible with DC sputtering without magnets. Variations of the Penning magnetron have subsequently been developed, notably the post cathode magnetron invented by Penfold and Thornton in the 1970s and Mattox, Cuthrell, Peeples and Dreike in the late 1980s.

RF sputtering
The use of radio frequency, RF to sputter material was investigated in the 1960’s. Davidse and Maiseel used RF sputtering to produce dielectric films from a dielectric target in 1966. In 1968 Hohenstein co-sputtered glass using RF and metals (Al, Cu, Ni) with DC, to form cermet resistor films. RF sputter deposition is not used extensively for commercial PVD for several reasons. The major reasons are it is not economic to use large RF power supplies due to their high cost and the fact that you introduce high temperatures, due to the high self-bias voltage associated with RF power, into insulating materials.

Bias sputtering and triode sputtering
In 1962 Wehner patented the process of deliberate concurrent bombardment “before and during” sputter deposition using a “bias sputter deposition” arrangement and mercury ions to improve the epitaxial growth of silicon films on germanium substrates. Later this process became known as bias sputtering. The triode sputtering configuration uses an auxiliary plasma generated near the sputtering cathode by a thermoelectron emitting electrode and a magnetically confined plasma. This configuration was used to increase the level of ionization in the plasma but became obsolete with the development of magnetron sputtering.

“Closed loop” magnetrons
The effects of magnetic fields on the trajectories of electrons had been realized even before Penning’s work and studies continued after Penning published his work. The early Penning discharges used magnetic fields that were parallel to the sputtering target surface. Magnetron sources that use magnetic fields that emerge and reenter a surface in a “closed loop” pattern can be used to confine electrons near the surface in a closed pattern (“racetrack”). These confined electrons generate a high density plasma near the surface and were used in developing the “surface magnetron” sputtering configurations of the 1960s and 1970s.

The “S-gun” and planar magnetrons
In 1968 Clarke developed a sputtering source using a magnetic tunnel on the inside of a cylindrical surface. This source became known as the “sputter gun” or “S-gun”. Various magnetron configurations, including the planar magnetron, were patented by Corbani. Chapin also developed a planar magnetron source in 1974 and is credited with being the inventor of the planar magnetron sputtering source. Major advantages of these magnetron sputtering sources were that they could provide a long-lived, high-rate, large-area, low-temperature vapourization source that was capable of operating at lower gas pressure and offered higher sputtering rates than non-magnetic sputtering sources. With these superior characteristics magnetron sputtering became the most wide-spread PVD coating technique.

Reactive sputtering
The term reactive sputtering was introduced by Veszi in 1953. Reactive sputter deposition of tantalum nitride for thin film resistors was an early application. However it wasn’t until the mid-1970s that reactively sputter-deposited hard coatings on tools began to be developed and they became commercially available in the early 1980s.

Unbalanced magnetron and “closed field” magnetron arrangement
One of the disadvantages of these early magnetron sources was that the plasma was effectively trapped near the surface of the sputtering target. This meant that the reactive gases could not be dissociated effectively near the substrate and the ion bombardment of the substrate was low resulting in poor quality films. The problem was partially solved by adding auxiliary ionization sources or using RF. The invention of the unbalanced magnetron by Windows and Savvides in 1986 offered a better solution. The unbalanced magnetron allows some electrons to escape from the confining E x B field and create plasma in regions away from the target surface. When the escaping magnetic field is linked to other unbalanced magnetron sources (north to south poles), the plasma generation area can be significantly increased.

Coating structure and morphology
With the invention of the scanning electron microscope, SEM in 1965 the growth morphology of the deposited coating could be examined. In 1977 Thornton published a “structure zone model” (SZM) patterned after the Movchin and Demichin diagram for evaporated coatings. This diagram is known as the “Thornton Diagram” and illustrates the relationship between the coating morphology, the deposition temperature and the pressure in the sputtering chamber. Of course the sputtering pressure determines the flux and energy of the reflected high energy neutrals from the sputtering cathode, so the diagram reflects the degree that the depositing material is bombarded by energetic particles during deposition. In 1984 Messier, Giri, and Roy further refined the structure zone model.

Extensive range of applications
The number of applications has increased rapidly from the mid 1970s. To read about the applications of PVD coatings click the link. The information presented here is based on “The Foundations of Vacuum Coating Technology” by D. Mattox, which can be viewed in pdf format here

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PVD Coating

Mark started working in the area of PVD coatings in 1994. He has worked both in research developing coatings and production for Kyocera Unimerco. You can find him on Google + and Twitter