More on particulate contamination:
I suspected that the flow in the EY cartridge cleanroom area was not working as we planned.  John helped us look closer at the flow in the cleanroom "system" we have for the cartridge work.  The ETMy/SUS/ISI cartridge is sitting in a large cleanroom (CR), with a smaller CR placed on either side.  We determined that the smaller CRs on opposing sides of the main CR are forcing air into the main one.  This is because the smaller rooms have lower ceilings (thus lower fans closer to the ETM "action"), a smaller gap between the curtains and the floors, and have a higher number of fans/volume.  As well, in the main room the ISI is blocking most of the downward flow from CR fans.  We also have been pinning the curtains open between between the small and large CR, increasing the work real estate.  One of these now open walls makes a direction flow line-of sight to ETMy-HR.
Long story short, we'll need to re-think this next time we do an end station cartridge install.  Small things will help, such as purging more flow from the smaller rooms (raising them to add more gap between floor/curtain).