THIN
FILM ANALYSIS USING SPECTROMETRY AND ELLIPSOMETRY
ABSTRACT
An
ellipsometer is currently used for analyzing thin film on top of metal
substrates. However, an ellipsometer can only perform this type of
analysis. On the other hand, spectrophotometers are much more versatile and widely used. This
paper develops techniques for using the spectrophotometers to perform the
analyses performed by ellipsometers. Thus, labs that already possess a spectrophotometer
would not need to purchase an ellipsometer nor outside ellipsometry services. Along
with the cost savings, there are added advantages to using spectrophotometer for these analyses, which are discussed along with the
disadvantages.
The four main objectives of this project are as follows:
1. Develop methods and
formulas for using a spectrophotometer to accurately measure the thickness
(d), the index of refraction (n), and the dispersive formula of a thin
dielectric film on a metal substrate.
2. Compare results and
advantages of three methods of measuring the d and n (described in Objective
1).
The three methods include: spectrophotometry
(developed in Objective 1), ellipsometry, and spectrophotometry by use of the
NanoSpec.
3. Use all three
methods (of Objective 2) to determine the n and d for a polymer film layered
on top of a silicon oxide film, which is, itself, on top of a silicon
substrate.
4. Develop much less expensive instrument that performs the key analyses
of the ellipsometer.
I.
INTRODUCTION: MAIN OBJECTIVES:
This introductory section will expound upon the four Objectives stated
in the Abstract:
1. Develop methods and
formulas for using a spectrophotometer to measure the thickness (d), the index
of refraction (n), and the dispersive formula of a thin dielectric film on a
metal substrate.
Determination of the physical properties of oxides (grown or native)
and other dielectric thin films on metal surfaces is important to many
industries and research fields. This
is especially true of the semiconductor industry, where oxides and nitrides
are grown on silicon or gallium arsenide to serve as precise insulators or
masks. The main properties are
the thickness (d) and the index of refraction (n) of the dielectric film.
The thickness is useful for determining the insulation and isolation
values of the film and the index (n) reveals the chemical and physical quality
of the film.
This first objective seeks to adapt a Cary 5 spectrophotometer to
evaluate these properties using interference theory.
The Cary 5 was used, prior to the start of this project exclusively for
transmission studies, in which the entire sample is transparent at appropriate
wavelengths. The studies in this
work use reflection. With the
previous transmission application the spectrophotometer could only determine
the product d*n (thickness times index) of a film.
Thus, requiring prior knowledge of either d or n for the
spectrophotometer tests to be of any use.
A method will be developed here for using the reflection method to
accurately determine both d and n independently, as well as the dispersive
equation for the index of the film.
To accomplish this first objective, samples where prepared in the
Cleanroom, an adaptive fixture was designed and built for the Cary 5
spectrophotometer, measurements where made, and formulas where derived for use
with this set-up. The principles,
errors, and concerns in using this system will be discussed in this report.
Finally, an accurate method for improving the system will be suggested,
but not tested.
2. Compare results and
advantages of three methods of measuring the d and n (described in Objective
1).
The three methods include: spectrophotometry
by use of The Cary 5 (developed in Objective 1), ellipsometry, and
spectrophotometry by use of The NanoSpec.
The three methods mentioned above will be compared.
Their advantages and disadvantages will be discussed.
As well, data from prepared samples will be taken with each and the
results compared.
3. Use all three
methods (of Objective 2) to determine the n and d for a polymer film layered
on top of a silicon oxide film, which is on top of a silicon substrate.
In the semiconductor/integrated circuits industry, multiple layers of
films covering a semiconductor substrate is common.
It is therefore, useful to determine d and n for such multi-layers.
However this is a more complicated and less accurate procedure.
For this objective, an vitreous silica layer was grown on a silicon
wafer, the thickness of the oxide layer was determined by ellipsometry, then a
thin polymer film was laid on top of the silica, and the three methods (of
Objective 2) were used to determine the d and n of the polymer film.
This work required extensive formula derivation and computer
programming. A comparison of
results from all three methods will be discussed.
4. Develop much less expensive instrument that performs the key analyses
of the ellipsometer.
Go to Objective 1
Go to Objective 2
Go to Objective 3
Go to Objective 4
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