1.  Field of the Invention
The present invention generally relates to sputtering apparatus and methods used in semiconductor device fabrication. 
2.  Description of the Related Art
In a physical vapor deposition ("PVD") process, atoms sputtered from a target are deposited onto a semiconductor substrate.  The target, which could be made of aluminum, for example, is located a distance away from the substrate.  A plasma of a gas suitable for sputtering, such as argon, is maintained between the target and the substrate.  Ions of the sputtering gas are accelerated onto the target by applying a negative potential thereon.  When accelerated ions hit the target, atoms are sputtered from the target and onto the substrate. 
In a conventional PVD process, magnetic fields are employed to cause the electrons to spiral through the plasma, thereby increasing the electrons' residence time and ionization efficiency.  This leads to higher ionization levels which, in turn, increases the deposition rate because there are more ions available to sputter atoms off the target and onto the substrate.  The use of magnetic fields in a PVD process is also known as magnetron sputtering.  In conventional magnetron sputtering, the ionization levels are not high enough throughout the chamber to yield substantially more ionized sputtered metal atoms than neutral metal atoms.  Because neutral atoms are not affected by electric or magnetic fields, their trajectory towards the substrate cannot be influenced by using magnets or electric potential. 
The control of the direction or flow of sputtered atoms onto the substrate is very important in any PVD process.  For example, in applications requiring the deposition of a thin layer of barrier or liner metal in a trench or via, deposition of sufficient material on the bottom and sidewalls (step coverage) depends on the capability of the PVD process to direct the flow of sputtered atoms onto the substrate.  In gap-fill applications, or filling of vias and trenches with primary metals, obtaining good step coverage similarly requires directionality of sputtered atoms.  Conventional magnetron sputtering has proven to be inadequate in the aforementioned applications because it does not yield high levels of ionized atoms whose trajectories can be influenced by using magnetic or electric fields.  This problem is exacerbated in the manufacture of advanced semiconductor devices where narrow and high-aspect ratio structures are used. 
It is also important to obtain good step coverage uniformity across the substrate.  Step coverage uniformity depends on precise control of the flow of sputtered atoms to the substrate. 
From the foregoing, it is highly desirable to be able to control the direction of sputtered atoms in a PVD process in order to achieve excellent step coverage with good deposition uniformity across the substrate.