a prober for the electrical testing of unpackaged integrated circuits on a semiconductor wafer can test these devices between subfreezing to elevated temperatures without generating atmospheric fog or frost on the wafer or associated equipment by directing an enclosed tubular curtain of dry gas , such as heated liquid nitrogen , down from a circular nozzle above the wafer to encircle the device under test . observations can be made and electrical tests performed within the curtain while all moist atmospheric air is purged from the wafer and surrounding areas so that fog and frost cannot form .