BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
FIG. 1 is a schematic diagram of a fab;
FIG. 2 is a schematic view of a horizontal transmission robot acquiring a wafer lot;
FIG. 3 is a schematic view of a vertical transmission robot acquiring a wafer lot;
FIG. 4 is a schematic diagram of a wafer dropping from the vertical transmission robot;
FIG. 5 is a schematic diagram of a displaced wafer detection system;
FIG. 6 is a schematic diagram of the arrangement of transmitters and receivers of a sensor;
FIG. 7 is a schematic diagram of the arrangement of transmitters and receivers of another sensor;
FIG. 8 is a side view of a sensor in a first direction;
FIG. 9 is a side view of a sensor in a second direction; and
FIG. 10 is a side view of an exemplary sensor.