Measuring Metal-Semiconductor Junction Contact Resistance Using the Transmission Line Method (TLM)
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.
This 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 2Rpad + Rsemi.
In the limit where the distance between the pads approaches 0, Rsemi goes to 0 and what is left is 2Rpad
Now that you have Rpad, 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.