Note, as a further contamination test, we blew the N2 gun across a new 4" witness wafer for the same amount of time/distance as we did during the discharge and inspected.  We blew for 30sec at 20psi 2" from the wafer.  Upon inspection, we found the wafer still looked very clean, as expected.

We have been stating the witness wafers as 3" wafers, but in-fact they are 4" wafers.  There are not 2 sizes on site.