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.
- 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 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.