Recently we received an enquiry as to the reason behind the changes in deposition rate during the life of a Sputtering Target.
Negatively charged electrons in the plasma are repelled by the negatively charged current applied to the sputtering target surface (like charges repel) while the positively charged ions are drawn toward it (opposite charges attract). Around each negatively charged target there is thus a sheath of definite thickness containing only positive ions and neutral atoms. Electrons are reflected from the outside surface of the sheath while all positive ions which reach the sheath are attracted toward the target surface. It follows directly that no change occurs in the positive ion current reaching the target surface. The target surface is in fact perfectly screened from the discharge by the positive ion sheath, and its potential cannot influence the phenomenon occurring in the plasma nor the current flowing to the target.
What is occurring, however, is the change in the magnetic field potential as a function of distance. This distance (t) being measured from the top of the permanent magnetic array in the cathode assembly to the surface of the target. The magnetic field (M), in conjunction with the fixed negative current supplied by the system power supply (B) makes up the potential at the target surface by the mathematical function MxB. However, the strength of the magnetic field potential varies as a function of distance in the relationship of the reciprocal of the distance squared or 1/t2. This means that as the distance between the top of the magnetic array, which is in a fixed position, and the target surface, which is varying as the target erosion profile becomes deeper, has a major role in the overall potential at the target surface. Thus the overall potential at the target surface increases significantly (by the square of the distance as t gets smaller or decreases as the target erodes) even though the electrical current from the power supply remains constant. Simply put, as the target erodes the deposition rate increases appreciably. This is why a new target (without an erosion profile), which is thicker than a used target deposits at a much slower rate than does the worn out target with a deep erosion profile.