First: Prepare the Junction

The first step in measuring the contact resistance of a metal-semiconductor junction is to deposit the metal on the semiconductor and pattern it so that you have lots of identical pads spaced varying distances apart.

• Above is an example of the metal pattern.  Ideally, the pattern includes many rows of various pad sizes.
• All the pads in any one row must be the same size, and the distances between pads must vary. 
• After the metal is patterned, anneal according to your specific recipe.

Second: Take measurements

• The measurements required are simple IV curves.  This is most easily done at a probe station, and using a parameter analyzer like our HP4145.
• Apply a voltage between several pairs of adjacent pads in a row, and measure the current flow.  From this, calculate the resistance between those two pads (R=^V/_I).

Third: Calculate the Contact Resistance

• As illustrated in the picture above, we can think of the total resistance between any two pads as the series combination of 3 resistors: metal to semiconductor, through the semiconductor, and back into metal.
• Since ohmic contacts are the same for both polarities, what we have is 2R_pad + R_semi.

• In the limit where the distance between the pads approaches 0, R_semi goes to 0 and what is left is 2R_pad.
• Now that you have R_pad, multiply that value by the area of the metal pads (in cm²).
• That's all! You now know the contact resistance of your metal-semiconductor junction.

So why is contact resistance so important?

• 10^-5ohm-cm² sounds pretty small to me... Consider that in the modern 90nm processes, the vias that contact the silicon have a contact area of about (.1um)², or 10^-8cm².  If your contact resistance is 10^-5ohm-cm², that amounts to 1 kohm resistor just to get into the silicon. (plus another to get out!)
• A good contact resistance is on the order of 10^-7ohm-cm².