The typical starting material in a semiconductor fabrication process is a silicon wafer. This wafer is called the "substrate".
It is necessary to characterize the substrate in a semiconductor fabrication process because the dopant type and amount must be correct for the process to yield devices with the desired characteristics.
Silicon is grown in ingots, a cylindrical shape resembling a sausage.
The silicon ingots are sliced into many thin pieces to make wafers. Before slicing the ingot into wafers though, a vertical slice is made down one side of the ingot, forming a "flat". This flat side of each ingot indicates the crystalline orientation of the silicon and is also used to position the wafer as it is processed.
Using a diamond scribe or sharp tweezers, it is possible to break a wafer cleanly along a particular crystallographic plane.
One way to determine whether the wafer is doped P or N type is to place a hot probe and a room-temperature probe on the wafer surface, and then measure the direction of current flow between the two probes using a galvanometer. If we are characterizing an N-type wafer, the current flows from the hot probe towards the cold probe. If the wafer is P-type, current flows in the opposite direction.
We are also interested in the dopant concentration. First we use a micrometer to measure the wafer thickness.
Next we use a 4-point probe to measure the sheet resistance in ohms per square.
The wafer is placed under the probe head.
Looking closely, you can see the four probes.
When the lid is closed, the sheet resistance is measured, and the value is displayed.