Achieving industrial deposition with constant current control

Paul Boughton

Deposition is a process that is regularly used to coat a thin layer of precious materials (gold, chrome, titanium, etc.) onto a substrate that is ‘technically poor’ or with different mechanical characteristics; the primary reason for using thin layer coatings is to obtain a better surface property, which can range from improved hardness or resistance to corrosion, to a glossy or matt appearance.

One of the several different methods of deposition is ‘vacuum evaporation’. The basic principle consists of the material to be deposited being evaporated from a source, accelerated by an electric field applied appropriately, and then deposited onto the substrate.

Electron Beam Evaporation is a process that takes place in a vacuum chamber. The filament is heated by applying a DC voltage across its terminals and, when it has reached a suitable temperature, electron extraction is possible. Using an applied electric field, these electrons are accelerated from the filament to the crucible that holds the material to evaporate. The beam is focused using the magnetic fields generated by the coils present close to the source and crucible.

Constant current power supplies are used to power the filament, magnets and focus circuitry (see Fig. 1) in order to:

* Ensure the temperature of the filament remains constant; any fluctuations may result in variation in the flow of electrons.
* Focus the beam using Magnetic Flux (programming the current allows the user to vary the focus of the beam).
* Maintain deflection of the magnetic flux (programming the current allows the user to vary the beam’s deflection).

The precise control of all these parameters, together with the use of high performance power supplies with low ripple and high dynamic response, allows the user to obtain a deposition of high quality and uniformity. A better, more reliable process minimises waste and increases product reliability.

TDK-Lambda has developed several ‘Constant Current’ power supply solutions.

The ZUP, Z + and Genesys allow full control of the current and the output voltage, as well as feedback on the same parameters. In addition, the availability of isolated interfaces, analogue, digital RS232/485, IEEE488 and LAN allows a high flexibility of integration and system control. The power ratings range from 200W to 15KW with the possibility of placing up to 4 units in parallel, to increase the power up to 60KW and output currents up to 4000A.
In addition, TDK-Lambda has the capability to optimise a standard product or even develop a custom solution to meet the specific needs of a particular user application.

Giacomo Mazzullo is with TDK-Lambda.

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