INFICON XTM/2 Thin Film Deposition Monitor - Manual (Page 95)

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XTM/2 Operating Manual
Simple substitutions lead to the equation that was used with the first “frequency
measurement” instruments:
[6]
where the film thickness, T
f
, is proportional (through K) to the frequency
change, DF, and inversely proportional to the density of the film, d
f
. The
constant, K = N
at
d
q
/F
q
2
; where d
q
(= 2.649 gm/cm
3
) is the density of single
crystal quartz and N
at
(=166100 Hz cm) is the frequency constant of AT cut
quartz. A crystal with a starting frequency of 6.0 MHz will display a reduction of
its frequency by 2.27 Hz when 1 angstrom of Aluminum (density of 2.77
gm/cm
3
) is added to its surface. In this manner the thickness of a rigid adlayer
is inferred from the precise measurement of the crystal’s frequency shift. The
quantitative knowledge of this effect provides a means of determining how
much material is being deposited on a substrate in a vacuum system, a
measurement that was not convenient or practical prior to this understanding.
T
f
K F
∆ ( )
d
f
---------------- =