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Film
stress was measured by a FSM128 stress measurement system.
This instrument utilizes
a laser optical lever to measure the curvature change of a film
stack on a thick substrate.
The curvature of a blank silicon wafer was initially
measured and recorded as a
reference. Then a second measurement was conducted after the e-beam
deposition on the same
wafer to determine the curvature change induced by the film
stress. Stress of a thin film is calculated based on the
Stoney’s equation:
where E/(1-n) is the biaxial elastic
constants for the substrate, which
is 180.5GPa for the (100)
Si5.
ts and tf are the substrate and thin film thick nesses. R and R0 are the substrate curvature measured before
and after deposition.
E = Young's modulus of
the substrate
v = Poisson's ratio of
the substrate
D = thickness of the
substrate
R = net radius of
curvature
t = thickness of the
film