7.3.4 Physical vapour deposition (PVD), sputtering
An ion hitting a metal surface after acceleration through more than 100 V will not stick or bounce off but will burrow into the surface, splashing atoms outwards. This is known as sputtering and provides a versatile alternative to thermal evaporation for metal-vapour deposition: more controllable, with adjustable uniformity, able to cope with alloys and high-melting-point metals and suitable for production-line automation. Given these advantages, it is also worth the effort to heat the wafer to several hundred degrees centigrade to aid crystallisation.
There are several variants of this physical vapour deposition (PVD) technique. In the simplest, suitable for metal deposition, the chamber ceiling is replaced with a target of the metal to be deposited and a DC current is passed from an anode electrode across the plasma and into the face of the target. As with electroplating, the deposition rate on the wafer depends on the current, as this is proportional to the flux of ions, and the yield of sputtered material per ion is approximately fixed. Typical industrial systems draw about 50 A, with a voltage of −200 V on the target, and hence a 10 kW DC power supply is required.
The efficiency can be improved if the plasma is prevented from leaking to the chamber walls using shaped magnetic fields, with techniques similar to those in nuclear fusion reactors (albeit on a very different scale). This magnetron is designed to confine the plasma at the target face, where it is needed, increasing the sputtering rate and allowing the plasma to be sustained at lower gas pressure, improving the cleanliness of the film. Adjustments to the field geometry can also be used to optimise thickness uniformity.
Since the wafer is also bathed in the fringes of the plasma, it is common to apply a second voltage to the platen on which it sits. This draws ions down onto the growing film, and the bombardment densifies and compresses it. The wafer may not conduct DC current, so an alternating voltage is used for this platen bias, usually at a radio frequency of 13.56 MHz that is set aside for industrial use. The edge of the plasma rectifies this voltage because it is much easier for light, mobile, negatively charged electrons to flow onto the wafer than heavy, relatively slow, positive ions. The wafer surface quickly accumulates a negative static charge, slowing the electron leakage to equal the ion current. In this subtle way, the RF bias generates a DC voltage that controls the ion bombardment energy. Figure 33 shows a tantalum barrier layer deposited by PVD with ~60% step coverage.