Inspection
Table 1: Change in film thickness for different wavelengths Table 2: Film thickness at the completion of each cycle
only generated between the electrodes, not in The plasma used to deposit the silicon dioxide
the area of the viewport or the lens, therefore no film contains a high proportion of nitrogen (both
deposition occurs on the optical components as N
2
and N
2
O) and so the spectrum observed
and both remain clear, even after extended use. shows many of the characteristics of a N
2
Silicon dioxide films with a nominal target plasma. (Figure 2). This has strong emission
thickness of 300nm were deposited onto 100mm bands in the region 300 – 400 nm (these are
silicon wafers using a gas mixture of dilute SiH
4
labeled A,B,C,D).
(2% in N
2
) and N
2
O. The total gas flow was When the intensity of a band is measured as
approximately 1.5 litres/min at an operating a film is deposited, it is seen to vary in a cyclical
pressure of 1000mTorr, which gave a nominal manner as expected, each band with a different
26
deposition rate of 100nm/min. cycle time (Figure 3). Knowing the wavelength of
each of these bands, the change in film thickness
www
Theory of Operation for each cycle can be calculated from the above
.eur Interferometry can be used to measure the formula. These values are shown in Table 1.
oasiasemiconductor
thickness of a film by measuring the change in If one of these bands is chosen (e.g. band B
intensity of reflected light that occurs due to at 337nm, since this is the strongest) then, as
interference of light reflected from the top and each cycle is completed, film thickness can be
bottom surfaces of the film. As the film thickness calculated, as shown in Table 2. In practice, film
increases during deposition the intensity of the thickness can be measured at each half cycle
reflected light will vary in a cyclical manner, with since it is possible to detect both minimum and
.com
the thickness d, corresponding to 1 cycle, being maximum points in the intensity values). From
given by: these points in time it is possible to calculate
film thickness versus time as shown in Figure 4.
square4
Issue VI 2009
d = λ/2n Furthermore, by interpolating between points it
is possible to make an accurate estimate of the
where: film thickness at any given time, and to
λ. is the wavelength of the reflected light terminate the process when a target thickness
n is the refractive index of the film at that has been reached.
wavelength.
Outcome
To quantify the improvement in process
performance obtained by using end point
detection, a silicon dioxide film was deposited
on a number of wafers and the wafer to wafer
film thickness variation was measured. In all
cases the process was terminated based on
reaching a target film thickness. As such the
process time was not fixed and varied from run
to run, reflecting the underlying issue that the
deposition rate is varying from run to run. In
Table 3: Summary of wafer deposition run most manufacturing environments where
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