Bias is the negative voltage which applied to the substrate during the vacuum coating process. The positive terminal of the bias power supply is connected to the vacuum chamber, while the vacuum chamber is connected to the ground. The negative terminal of the bias power supply is connected to the workpiece. As the earth's voltage is generally considered to be zero potential, so the voltage on the workpiece is considered as negative bias voltage, bias for short.

The function of negative bias
Provide particle energy;
Heating the substrate;
Removal the adsorbed gas and oil on the substrate, improve the film adhesion;
Activate the substrate surface;
Purification of large particles in Arc Ion Plating.

Bias classification
According to the waveform can be divided into:
DC bias;
DC pulse bias;
DC superimposed pulse bias;
Bipolar pulse bias.


Comparison of DC bias and pulse bias
The traditional arc ion plating applied DC negative bias to the substrate to control the ion bombardment energy, this deposition process has the following disadvantages:
The substrate temperature is increased, not good for deposition hard film on the substrate with low tempering temperature. 
High-energy ion bombardment caused severe sputtering, can not simply improve the ion bombardment energy to synthesize high reactive threshold energy hard film.

In the process of DC bias arc ion plating, in order to suppress the substrate surface high temperature caused by ion bombardment, mainly adopts the measures of reducing the deposition power, shorten the deposition time, use the intermittent deposition and others to reduce the deposition temperature. These measures can generally called the energy control method. Although this way can reduce the deposition temperature, but it will reduce some properties of the film. At the same time, it also reduce the production efficiency and the stability of the film quality. Therefore, it is difficult to promote the application.

In the process of pulse bias arc ion plating, as the ions bombard the surface of the substrate by a non-continuous pulse manner, the temperature gradient between the inside of the substrate and the surface can be changed by adjusting the duty ratio of the pulse bias, thereby changing the thermal isostatic compensation effect between the inside of the substrate and the surface, to achieve control the deposition temperature purposes. In this way, the height of the bias and the temperature of workpiece can be separately from  (with little or no influence with each other). 

Bias impact on the film
The influence mechanism of the bias on the film is very complicated. Below we list some influences. 
         Film structure, crystal structure orientation, structure
         Deposition rate
         Large particle purification
         Film hardness
         Film density
         Surface topography
         Internal stress

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