Using MTI’s capacitive thickness gauge Proforma 300i with conductive wafers and thin films

Wafer of thickness between two capacitance probes

G = (a+b+t1+t2 ) Where G is the fixed gap between two probes

When making the thickness measurement t2 can appear as t2 (film thickness) when it is a conductor, or 0 thickness when t2 is just air. So t2 will vary between the actual film thickness t2 if it’s a conductor all the way down to 0 thickness if the film has a dielectric constant of 1 (same as air). Some aero gel films have dielectric constants close to 1.

Apparent thickness of t2 to the system = t2 (1-*1/k) where k is the dielectric constant of the system

Film Medium Dielectric k t2 Apparent Thickness
Air 1 = 0
Dielectric 2 =0.5*t2
Dielectric 5 =0.8*t2
Metal or Si Infinity = t2

Example Use Case Scenarios with various dielectric constants:

Assuming ProForma 300i is calibrated with a 1 mm Si wafer with no film:

  • If we were to insert a 1 mm wafer with a 10 um film that had a dielectric constant of 2 then the wafer would measure as 1.005 mm thick.
  • If we inserted a 1 mm wafer with a 10 um film that had a dielectric constant of 5 then the PF300i would read 1.008 mm thick.

Would you like to contact MTI Instruments directly to learn more about the products used in this application note?

Product Inquiry Short Form