It cannot be both in the same metal sheet, as it will compensate each other.
It all depends on the chemicals used in the process., so it can be both negative or positive. Still it does not matter, the effect will be the same.
During ion etch positive charge accumulates. During CMP (or more exactly: after CMP, during wafer cleaning and drying) either pos. or neg. charge accumulation can occur, as yashiro noted above.
We usually add a Antenna diode to discharge the charges. If both type of charges gets accumulated then how the diode operation will be?
My assumptions, (diode is placed on p-substrate)
Case1 : Metal is charged with negative ions then,
diode is forward biased and it will discharge the accumulated charges.
Case2 : If metal is charged with positive ions then,
diode is reverse biased, will this current sufficient to discharge the accumulated charges?
The charges accumulated will create a potential of hundreds or maybe thousands of volts so the diode will operate in either forward or reversed biasing and at this voltage level it will be opened either way.
If it will be forward biased it will open at around 0,6V
If it will be reversed biased, it will have a opening voltage of around -60V (it depends on the technology and diode size)
The diode doesn't have a lot of current to deal with, and
most likely the UV / soft X-rays of the plasma generator
will make it conduct plenty in either direction. You only
need a non-insulating shunt path across any gate oxide
(though for tiny metal bits, tunneling current alone can
suffice, which is why there is a length below which you
aren't dinged).
During plasma etch in any case: it's the cathode of the etch system. HV ion acceleration is needed to get the necessary etch energy + perpendicular etching.