Page 272 - Chalcogenide Glasses for Infrared Optics

Early Work at Texas Instruments 247

Boris Kolomiets. After a reading of their papers, it seemed obvious that
a glass could be formed out of Si Te if we only added arsenic as a third
2 3
element. While the author was working with Maurice Brau, the ternary
glass forming composition region for Si-As-Te glasses was established. 2
As mentioned earlier in this book, these results were the basis for the
proposal and resulted in the many years of support by DARPA through
the Office of Naval Research for the chalcogenide infrared glass efforts
at TI. The discussion need not be repeated again here.

10.3 Optical Interference and Film Thickness
Many times the success or failure of a technical program depends upon
the ability to measure accurately the value of a particular system param-
eter. The measurement technique may be totally new to the activity of
the company or may require development for the particular application.
This new capability or ability may become very valuable to the com-
pany’s technical effort in a short time. Such was the case at TI in the early
1960s as, similar to other semiconductor device producers, they began
growing pure silicon layers on highly doped silicon substrates and then
grew dielectric films to protect the electronic devices. The two new tech-
niques were the measurement of the thickness of epitaxial layers by the
infrared scan of reflected light and the thin dielectric film thickness
measurement using ellipsometry. In this chapter, the general principles
behind both techniques are discussed along with how they were modi-
fied for their applications to two semiconductor problems.
Optical interference occurs when light is reflected from a film-
covered surface. As shown in Fig. 10.1a, light intensity I strikes the
0
surface of a film and a fraction of the light is reflected I at the incident
1
angle. The reflected ray I has a phase angle θ . The remaining part of
1 1
I is refracted into the film, partially reflected at the film-surface inter-
0
face and partially transmitted at the film-air interface as the refracted
beam I . The phase angle of I (θ ) is different from that of I (θ ) because
2 2 2 1 1
of the phase lag that occurs as the light travels the extra distance
through the film and back to the same point as I In Fig. 10.1b,
1.
the path difference between light rays I and I is the optical
1 2
distance X = 2d cos θ , where d is the film thickness and θ is the angle
1 1
of refractioncalculated from Snell’s law:
n sin θ = n sin θ
0 0 1 1
The refractive indexes n and n are measurements of the velocity of light
1 2
in medium 1 and medium 2 relative to the velocity of light c in a vacuum:
n = c = λ
1 ν λ
1 1
where ν is velocity of light in medium 1, λ is the wavelength in
1 1
medium 1, and λ is wavelength in air.