detector pitch smaller than 1mm?

Hi, 
thanks for your great work!
I was trying to define a 2D sensor array with the 'Design Detector Locations' GUI, but I could not get the sensor pitch to be smaller than 1mm.
In my application, I would actually need it to be as small as 20um.
Could you tell me how to do it, if this is possible?
Thank you very much in advance.