Both the transmittance and reflectance of 2 mm thick MgF2 substrates and of thin films of BaF2, CaF2, LaF3, MgF2, Al2O3, HfO2, and SiO2 deposited on these substrates were measured for the wavelength range 120 nm to 230 nm. Results for BaF2, LaF2 and MgF2 show promise as being good materials from which interference filters can be made. The software and related hardware needed to take large amounts of data automatically in future measurements of the transmittance and reflectance was developed

Peterson, Lennart R.

English

NASA Technical Reports Server

N90-19412
1989
NASA/ASEE SUMMER FACULTY FELLOWSHIP PROGRAM
MARSHALL SPACE FLIGHT CENTER
THE UNIVERSITY OF ALABAMA IN HUNTSVILLE
TRANSMISSION AND REFLECTION STUDIES
OF THIN FILMS IN THE VACUUM ULTRAVIOLET
Prepared by:
Academic Rank:
University and Department:
Lennart R. Peterson
Professor
University of Florida
Department of Physics
NASA/MSFC:
Laboratory:
Division:
Branch:
MSFC Colleague:
Date:
Contract Nnumber:
Space Science Laboratory
Solar-Terrestrial Physics
Atomic Physics
James F. Spann
August 3, 1987
The University of
Alabama in Huntsville
NGT-01-008-021
_. XXII
https://ntrs.nasa.gov/search.jsp?R=19900010096 2020-03-19T22:53:55+00:00Z

TRANSMISSIONAND REFLECTION STUDIES OF
THIN FILMS IN THE VACUUMULTRAVIOLET
by
Lennart R. Peterson
Department of Physics
University of Florida
Gainesville, Florida
ABSTRACT
Both the transmittance and reflectance of 2 _.i thick
MgF2 substrates and of thin films of BaF2, CaF2, LaF3, MgF 2,
A1203, HfO2, and SiO 2 deposited on these substrates have been
measured for the wavelength range 120 nm to 230 nm.
Results for BaF2, LaF 2 and MgF 2 show promise as being good
materials from which interference filters can be made. The
software and related hardware needed to take large amounts
of data automatically in future measurements of the
transmittance and reflectance has been developed.
XXII-i
ACKNOWLEDGEMENTS
The author wishes to express his warm thanks to the
leadership of the Summer Faculty Fellowship Program for
their splendid hospitality and also wishes to acknowledge
the smoothness and quality with which the program was run.
Secondly, the author wishes to thank his MSFC colieague,
Jim Spann, for his helpful, friendly and very professional
support during the i0 week visit.
XXII-ii
LIST OF FIGURES
Figure 1 - The Vacuum Ultraviolet Calibration Facility
Figure 2 - Transmittance and Reflectance Measurements of
2 mm Thick Parallel (for T) and Wedged (for R)
Substrates of MgF 2
Figure 3 - Transmittance and Reflectance Measurements of
a 53 nm Film of BaF 2 on an MgF 2 substrate
Figure 4 - Transmittance and Reflectance Measurements of
a 51 nm Film of LaF 3 on an MgF 2 substrate
Figure 5 - Transmittance and Reflectance Measurements of
a 68 nm Film of MgF 2 on an MgF 2 substrate
XXII-iii

INTRODUCTION
The theory and understanding of thln-film optics has
a long and honored history I°2. In addition, the use of
thin-film interference effects to design high reflectivity
mirrors and narrow band-pass filters is a well established
technology, especially at the visible wavelengths. Not so
well established at present, however, is the extension of
this technology down to the vacuum ultraviolet (120 nm -
230 nm). Because of the shorter wavelengths of UV compared
to visible light (by factors of 1/3 or 1/4), the required
film thicknesses must not only be thinner but also must be
more carefully controlled. Furthermore, values of the
refractive indices n and the extinction coefficients k are
poorly known at these wavelengths for materials likely to
be useful as film materials.
In the face of the extensive work that needs to be
done in the vacuum UV, there is also a natural and growing
need to improve optical instrumentation designed for use in
the UV. For example, satellite UV imaging of auroral and
other emissions from Earth has distinct advantages over
visible and IR imaging. Since the earth's atmosphere does
not backscatter solar UV efficiently and is also highly
absorptive of UV, satellite UV images of Earth allow for a
reasonably direct viewing of auroral and airglow emissions,
day or night, uncontaminated by the earth's albedo.
Determination of the wavelength-dependent optical
constants n and k in the range 120 nm to 230 nm for a
variety of materials is crucial. In particular, these
values are necessary to the design of narrow band-pass
filters needed for the UV Imager to be flown on the POLAR
satellite of the ISTP Program. This satellite, scheduled
for launch in 1993, will be in an eccentric polar orbit at
heights ranging from about 2 Re to 9 Re and will provide an
ideal platform for imaging the aurora. The emissions of
interest are the strong H-Lyman alpha llne at 1216
Angstroms, the 1304 and 1356 atomic oxygen lines, the
weaker LBH N 2 bands between 1400-1700 and 1700-2300, and
the atomic nitrogen lines at 1493 and 1744.
XXII-I
OBJECTIVES
The focus of this research study then is to determine
the transmittance and reflectance of films having known
thicknesses and composition. The materials are 2mm thick
high quality commercially made MgF 2 substrates and thin
films of BaF 2, CaF 2, LaF 3, MgF2, A1203, HfO2, and SiO 2
deposited on these substrates by the Optical Aeronomy
Laboratory at the University of Alabama in Huntsville. The
measured values of transmittance (T) and reflectance (R)
are sufficient for calculation of the more fundamental
quantities n and k as functions of wavelength. The actual
process of obtaining n and k from the reported T and R data
is to be carried out by Zukic, et. al. 3"4. These results
are part of the program to design and construct five narrow
band-pass filters for the UV Imager on satellite POLAR,
including band-pass windows centered on the wavelengths
1216, 1304, 1356, 1500 and 1800 Angstroms.
Because of the tlme-consuming nature of taking large
quantities of data under hlgh-vacuum conditions, there is
also a need for as much automation of data acquisition as
is practical. Thus, a useful task is to interface the
existing hardware and software with upgraded versions. An
existing computer program will be modified and enhanced to
accomplish these tasks.
DESCRIPTION OF FILM SAMPLES
The film depositions are carried out by the Optical
Aeronomy Laboratory at the University of Alabama in
Huntsville. The film materials, BaF 2, CaF 2, LaF 3, MgF2,
A1203, HfO2, and SiO 2 are deposited in vacuum on MgF 2
substrates 12.7 mm in diameter by 2 mm thick. When needed,
a 3 degree wedged substrate is used to eliminate background
contributions to reflectance coming from reflection off the
backside surface of the substrate. Film thicknesses for
samples measured vary from about 50 nm to i00 nm. The
substrates have a surface roughness of about 2.5 nm,
marginally smooth for the film thickness deposited. All
handling, deposition and transportation of substrates is
done under standard clean-room procedures to ensure a
minimum of contamination.
XXII-2 ___
DESCRIPTION OF THE VACUUMULTRAVIOLET CALIBRATION FACILITY
The vacuum ultraviolet calibration facility is a i000
liter hydrocarbon-free system currently being used to
measure reflectance and transmittance. The configuratlon at
present is shown in Figure i. Briefly stated, the facility
consists of a main chamber containing an instrument
platform on which filter samples can be mounted. Mated to
the main chamber is the smaller source chamber containing
a .2 meter monochromator and an optical system capable of
delivering a collimated UV beam having a 1 nm bandwldth and
dimensions of approximately 2 cm square. The main chamber
is a cylinder having diameter 1 m and length 1.25 m; the
source chamber is also a cylinder but having a diameter .3
m an0 length 1 m. The light source is a sealed deuterium
lamp with a MgF 2 window and provides continuous useful
spectra from about 115 nm to 230 nm.
Samples are mounted on an 8-slot wheel in the main
chamber, operated by with a small stepping motor controlled
by means of electrical feedthroughs. Behind the filter
wheel is a photometer fixed in a position to measure
transmitted intensity. A second photometer is positioned
to view reflected light under the condition that the filter
wheel is oriented to a 4 degree incident angle with respect
to the beam. The vacuum system provides a hydrocarbon-free
environment at a pressure of order 10 -5 Torr, maintained by
a pair of cryo pumps.
In order to enable a means of monitoring the beam
signal strength as well as of checking background at each
wavelength, three of the eight filter-wheel positions are
occupied. One of the three positions contains a calibrated
UV enhanced aluminum mirror for use in determining the
reference beam intensity for reflectance measurements. The
other two positions are used to monitor the reference beam
and background intensities for transmittance measurements.
Thus, in any single run, 5 or fewer samples can be nleasured
for both T and R at all wavelengths from 120nm to 230nm,
consistent with the range of usefulness of the deuterium
lamp. Measurements are made by setting the monochromator
at a particular wavelength and then successively rotating
the filter wheel to each of the required positions. When
all positions for a given wavelength are complete, the
computer, which has accumulated the photometer current data
directly on-line, calculates the value of T (or R) along
with statistical uncertainties.
XXII-3
Software has been developed which gives the user
several levels of automatic operation. At the lowest level
of automation, the monochromator is set by hand, the filter
wheel is positioned manually by means of a vacuum rated
stepping motor, and on command, the photometer current is
sampled repeatedly by the computer. At the highest level
of automation, the filter wheel and monochromator are set
manually to the starting value, after which the computer
guides the system through a complete set of specified input
wavelengths and filter-wheel positions, both taking and
processing data as obtained. Upon completion of a run,
graphs and tables are produced of T and R as a function of
wavelength.
The monochromator stepping motor (model PH296-01B,
Vextra) has been interfaced with the parallel input/output
port of the computer by means of a motor driver circuit
(model 2D3128, Anaheim Automation). Two blt-posltions of
the eight-bit port are used for motor control. 'fhe in-
vacuum filter wheel stepper motor (model C, Princeton
Research Instruments) has similarly been interfaced using
a motor driver circuit board (model ESH081/R2D23, Cyber-
Research). An optolsolator (model ESH 085, CyberResearch)
protects the computer system from electrical noise and
potentially damaging voltages. Four additional bit-
positions are used for the vacuum stepping motor operation.
RESULTS AND CONCLUSIONS
Examples of transmittance and reflectance measurements
are shown in Figures 2, 3, 4 and 5. As a result of the
measurements, optical constants for these and other film
materials have been determined and will be used in the
design of UV narrowband filters 4. Indications from these
calculations are that high values of the refractive index
make LaF 3 and BaF 2 good for use with MgF 2, having lower
refractive index, in making narrowband transmission
filters. Because the measurement process is now automated,
more accurate and consistent measurements are able to be
obtained than previously, without the stress and tedium
imposed on the operator.
XXII-4
REFERENCES
i. O. S. Heavens, Physics of Thln Films, G. Hass and
R. Thun, Eds. Vol. 2 (Academic Press, New York, 1964)
2. M. Born and E. Wolf, Principles of Optics, (Pergamon
Pre_s, 1980)
3. M. Zukic, Damped Least Squares Technique for the Design
of Optical Multilayer Filters, MS thesis, (unpublished)
Imperial College, London (1984)
4. M. Zuklc, D. G. Torr and J. F. Spann, Optical constants
of BaF2, CaF2, LaF3, MgF2, A1203, HfO2, and SiO 2 thin
films in the VUV, (to be published)
XXII-5
ii
FIGURE 1
.<
5-
t)
E_
I0
8
6
2
I I i I i
0 I I I I I
120 140 160 180 200 220
I00
80
_ 60
_ 40
20
0
240
WAVELENGTH [nm]
<0
z
<
_Z
CO
Z
<
FIGURE 2
._J
XXII-6
[m
Co
25
<
CO
£d
Q_
25
20
OR
¢T
[ ( L I
15
10
5
0
120
I
140
I I ._1
160 180 200
WAVELENGTH [nm]
220
100
80
60
_ 4O
2O
0
240
_J
0
z
<
[-
_E
u_
z
<
E--
(J
Z
<
E-(J
QJ
.J
25
FIGURE 3
20
OR
OT
I I I I
15
10
5
0 i
120 140
I 1 I
180 180 200
WAVELENGTH [.nm]
FIGURE 4
X_XII-7
I
220
I0o
_ 60
40
20
0
240
f"--'-I
O
z
.<
Z
oz
r---i
[.d
¢.0
m
.<
o
[.d
tO
8
4
! I | i I
OT •
2l
0
120
I00
80
60 o
z
<
40 _
20
I I I I l
140 160 180 200 220
0
240
z
<
[-
WAVELENGTH [nm]
J
FIGURE 5
XXII-8


Transmission and reflection studies of thin films in the vacuum ultraviolet

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server