Removing Passivation
MOSIS Technical Notes
Removal of passivation from a chip is a very difficult process. If you have not made mask provision for opening holes to your pads then there is no possible way to open holes to the pads afterward. The only other option is to remove passivation from the entire chip. Unfortunately passivation removal is not very simple because the passivation is composed of two components that are chemically very different.
The top layer of the passivation is usually silicon nitride. Below the silicon nitride there is usually either a combination of silicon dioxide and silicon oxinitride or just silicon dioxide. Both sets of materials have very different etch chemistry. To make matters more difficult, it is very difficult to prevent the silicon dioxide etch chemistry from removing interlevel dielectrics between metal layers below the pad level.
When a normal pad opening mask feature is used, the problem with stopping the etch at the pad is taken care of by the metal itself. This etch-stop feature of the metal prevents removal of interlevel dielectrics. Without some fairly sophisticated etch facilities, the chances of removal of passivation without destroying the chip is very remote.
If you have access to a commercial integrated circuit failure analysis service in your area, then you could have them remove passivaion from the chip without great risk to the underlying circuitry. Even with such facilities, the risk of damage is not zero.