Description

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.

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