The development of a vacuum ultraviolet spectrophotometric system for measuring transmittance and reflectance at variable angles is presented. Using various detectors and sources, the spectrophotometric system has been used for wavelengths from 80 nm to 300 nm with optical components up to 80 mm in diameter. The capability exists to make measurements through the visible range

Keffer, Charles E.

Spann, James F.

Zukic, Muamer

English

NASA Technical Reports Server

(
t
NASA-CR-199510
A Vacuum Ultraviolet Spectrophotometric System / /J
' /
James F. Spann1, Charles E. Keffer2, MuamerZukic2
1
 NASA/Marshall Space Flight Center
Space Sciences Laboratory
Huntsville, AL 35812
Department of Physics
The University of Alabama in Huntsville
Optics Building, Suite 300
Huntsville, AL 35899
Abstract
The development of a vacuum ultraviolet Spectrophotometric system for
measuring transmittance and reflectance at variable angles is presented. Using
various detectors and sources, the Spectrophotometric system has been used for
wavelengths from 80 nm to 300 nm with optical components up to 80 mm in
diameter. The capability exists to make measurements through the visible
range.
Key Words: Spectrophotometer, ultraviolet filters, transmittance, reflectance
(NASA-CR-199510) A VACUUM N96-11509
ULTRAVIOLET SPECTROPHOTOMETRIC
SYSTEM (NASA. Marshall Space
Flight Center) 17 p Unclas
G3/25 0068430
Jt
https://ntrs.nasa.gov/search.jsp?R=19960001502 2020-06-16T06:41:32+00:00Z
A. Introduction
The need for spectrophotometric measurements of reflectance and transmittance
in the vacuum ultraviolet (VUV) region of the spectrum, from 0.2 nm to 200 nm ,1
has become more important in recent years with the development of all-dielectric
narrowband filters in the far ultraviolet (FUV) from 200 nm to 100 nm 2 and the
extreme ultraviolet (EUV) from 100 nm to 30 nm ,3 and multilayer normal
incidence mirrors in the x-ray ultraviolet (XUV) from 30 nm to 0.2 nm .4 The
characterization of optical materials and measurements of spectral performance
of optical components has been an essential ingredient in the recent
developments.
Spectrophotometric systems that have been described in the literature are
typically for in-air measurements 5-8. However, the use of many optical
components render these techniques impractical for the VUV region . There
have been some references to techniques which can be employed in the VUV
region,9 and a commercial system is available for small samples up to 50 mm
square.10 Some work has been done related to developing a method of
calibrating VUV radiation sources.11 However, the accuracy of
spectrophotometric measurements is derived from system design and
characterization of all sources of error and not from a fundamental physical
standard.
In this paper, we describe a spectrophotometric system we have developed
which is useful in the VUV region of the spectrum. The system has been used
for various activities including the characterization of optical properties of thin
films 12'13 and space flight hardware for the Ultraviolet Imager (UVI) of the Global
Geospace Science (GGS) POLAR spacecraft.14
B. Measurement System
VUV optics are extremely sensitive to contamination, especially hydrocarbon
deposition. In addition, multilayer dielectric optics cannot be easily cleaned
without damage to the coating. Scattered light due to particulate contamination
is also a serious problem in the VUV range. In order to minimize exposure of
test optics to these types of contamination, the spectrophotometric system is
maintained in a class 1000 clean area in a stainless steel vacuum chamber.
Carbon vane blowers or venturj.pumps are used in the rough vacuum range and
cryogenic pumps are used in the high vacuum range in order to avoid
contamination from hydrocarbon-based pump fluids. The vacuum system
operates with a base pressure in the 10-7torr range.
For FUV measurements, a high pressure arc discharge deuterium lamp is used
as the source. A 0.2 m vacuum monochromator, with a concave holographic
grating, coupled to a 1.5 m focal length collimating UV enhanced mirror system
produces a monochromatic collimated incident beam. The beam can be directed
via a two axis gimbal system to which the collimating mirror is mounted. The
beam is apertured to ensure that the entire beam is incident on the detector.
Figure 1 schematically illustrates the system. The detector is a photomultiplier
tube (PMT). Depending on the wavelength range of interest, one of four PMTs
referenced in Table 1 is used.
The PMT with a bialkali photocathode can be configured to have an additional
Pyrex window which is coated with a layer of sodium salicylate. The sodium
salicylate layer provides a wavelength shift from the VUV to 420 nm.15 Care
must be exercised when using sodium salicylate due to its fluorescent nature
and propensity to powder when physically disturbed. Figure 2 shows the
configuration for this detector. The layer of sodium salicylate was deposited by
using an atomizer to spray a suspension of ethanol and sodium salicylate onto a
Pyrex substrate. The thickness was determined by weighing the substrate
before deposition and after the layer dried. A layer with an average thickness of
1.2 mg cm-2 was deposited. The bialkali PMT was chosen because its peak
response at 420 nm matched the fluorescent emission peak of sodium salicylate.
The optical samples to be measured are housed in a filter wheel. For 25.4 mm
and 12.7 mm diameter samples, an eight-position filter wheel is used. This
allows up to six samples to be measured with one reference position and one
background position. For larger samples or odd-shaped samples, another filter
wheel is used with four positions. Either filter wheel is mounted on a platform
that can rotated about an axis that is coincident with the front surface of the
sample. The PMT is mounted on an arm which is rotated independently about
the same axis (see Figure 1). Reflectance and transmittance measurements can
be made for multiple angles of incidence. The spectral characteristics of the
incident beam are determined by the wavelength setting of the monochromator
and the width of the variable-width slits. Two different gratings are used in the
monochromator. An iridium-coated grating is used for measurements from 30
nm to 120 nm. An AI/MgF2 coated grating is used for the measurements from
120 nm to 500 nm. Spectral resolution to 0.1 nm is achievable.
The filter wheel position and monochromator wavelength setting are controlled
using stepper motors controlled by a PC. The PMT signal is acquired using an
IEEE 488-controlled picoammeter. Integrated software allows automated
spectrophotometric measurements to be made over the desired wavelength
range.
For measurements that require larger signal and do not require a collimated
incident beam, the system can be moved into another chamber schematically
depicted in Figure 3. The significant difference in this configuration is the lack of
collimating optics. The divergence of the incident beam is on the order of 10
mrad, defined by the f/4.5 monochromator optical system. For the
measurements made to date, this has not been a factor. This configuration is
better suited for EUV measurements because of fewer reflections. The source
used for EUV measurements is a capillary glow dc discharge lamp.16
C. Measurement Process
To measure the transmittance of a sample the filter wheel is oriented to the
desired angle of incidence and the PMT is positioned in line with the incident
beam. The beam is apertured down so that it will fall entirely on the detector
active area even if there is a translation of the transmitted beam due to the
sample substrate thickness. The monochromator is set to the desired spectral
resolution and initial wavelength. The filter wheel is rotated to the reference
position where the incident beam passes unattenuated through to the detector.
In this position, the reference flux is measured. The filter wheel is then rotated to
the sample position and the signal measurement is made. If there are other
samples, then a signal measurement is made for each one as the filter wheel is
rotated to each of those positions. The filter wheel is returned to the reference
position for a second reference measurement. At some point a background
measurement is made by rotating the filter wheel to the position where the
incident beam is blocked. This completes the measurement cycle for that
wavelength. The incident beam's wavelength is changed and the cycle is
repeated until measurements at all wavelengths are completed. The effects of
source intensity drift are minimized using this sequence of measurements;
however, the effects of non-repeatability of the filter wheel positioning are
enhanced. Alternatively, the entire wavelength range is scanned at each filter
wheel position beginning with the first reference scan. A signal scan for all
wavelengths for the first sample is made before going to the next sample; a
background measurement is taken; and another reference scan is completed.
This measurement sequence minimizes the effects of filter wheel position
repeatability, but is more sensitive to nonlinear drifts of the source intensity.
For reflectance measurements, the same steps are taken for either of the
measurement sequences with two exceptions. The detector is positioned so that
the specular reflection of the beam off a calibrated reference mirror falls entirely
on the active area of the PMT. The calibrated reference mirror is used to
calculate the unattenuated reference beam flux by dividing the flux measurement
by the known reflectance of the mirror. The background measurement is made
by rotating the filter wheel to the open position which corresponds to the
reference position for a transmittance measurement.
For each spectrophotometric measurement, a control sample is measured along
with the sample(s). One control sample is used for all transmittance
measurements and one for all reflectance measurements. This enables a quick
evaluation to be done on the validity of a particular measurement, and provides a
measure of repeatability and an indication of system contamination. It also
provides a measure of continuance if the spectrophotometric system is moved
from one vacuum chamber to the other.
D. Error Analysis
The spectrophotometric measurements are made by taking the ratio of the
reflected or transmitted beam flux ls, called signal, and the unattenuated beam
flux I0, called reference. The measured reflectance R or transmittance T is
d)
(2)
Because both of these measurements are made using the same source and
detector, errors resulting from cross calibration of sources and detectors are
eliminated. Errors due to source output and detector response drift are
minimized by measuring the reference before and after the signal. The
reference is linearly extrapolated to the time the signal was measured. In
addition, a background measurement lb is made with the incident beam blocked.
For transmittance measurements
T = v * (3)
/.(',)-'*
where ts denotes the time the signal was measured.
7
A calibrated mirror is used for reflectance measurements. The reference is
determined by the measured signal reflected off the calibrated mirror lm divided
by the known reflectivity of the mirror Rm, or
For reflectance measurements
(4)
R = — " ' ( 5 )
/„(',)-//
The fractional errors associated with the source and detector drift are small
(<0.003) over the time interval between measurements which is typically less
than 1 minute. The background measurement is 2 to 3 orders of magnitude
smaller than the signal. The noise is due to thermal noise of the detector. The
signal and reference measurement uncertainty is a combination of the source
and detector noise. The fractional error in R or T for a given measurement is
typically less than 0.015. The absolute value of R or T is determined by
accessing the repeatability of a measurement. Figure 4 is a plot of the averaged
reflectance measurements of a control sample used for the much of the
reflectance measurements done to date. A total of 150 measurements are
included in this average over a period of 31 months. Measurements in both
vacuum chambers are included. The error bars represent the repeatability that is
achieved. For the peak, the absolute fractional repeatability is less than 0.04.
For values around R=0.05, the absolute fractional repeatability is 0.17.
E. Conclusion
By using various detectors and sources with a single reflection monochromator
in a hydrocarbon free vacuum system, VUV spectrophotometric measurements
have been made to characterize thin films and multilayer optical components.
The absolute fractional repeatability of measurement has been demonstrated to
be better than 0.003. The capability exists to measure the reflectance and
transmittance of different-sized samples, at multiple angles of incidence, and
over the VUV and into the visible spectral range.
With advances of optical components in this wavelength range, requirements for
an improved capability will be forthcoming. Known improvements to the current
system that can be made include the incorporation of photon counting or lock-in
amplifier techniques for weak signals, for example, when attempting to
accurately measure small out-of-band filter reflectance or transmittance values.
This work was supported by NASA grant NAG8-834 and NASA contract NAS8-
38145.
References
1. J. C. Boyce, "Spectroscopy in the Vacuum Ultraviolet," Rev. Mod. Phys. 13,
1-57(1941).
2. M. Zukic, D. G. Torr, J. F. Spann, and M. R. Torr, "Vacuum ultraviolet thin
films. 2: Vacuum ultraviolet all-dielectric narrowband filters," Appl. Opt. 29, 4293-
4302(1990).
3. M. ZuKic, D. G. Torr, and J. Kim, "X-ray, extreme and far ultraviolet optical
coatings for space applications," in Trends in Optical Engineering, edited by
Council of Scientific Research Integration, to be published by Research Trends,
in press (1994).
4. T. W. Barbee, Jr., "Multilayers for x-ray optics," Opt. Eng. 25, 898-915 (1986).
5. K. D. Mielenz and K. L. Eckerle, "Spectrophotometer linearity testing using
the double-aperture method," Appl. Opt. 11, 2294-2303 (1972).
6. D. R. Gibson and P.M. Lissberger, "Optical Properties of narrowband spectral
filter coatings related to layer structure and preparation," Appl. Opt. 22, 269-281
(1983).
7. J. Stone and L. W. Stulz, "Reflectance, transmittance, and loss spectra of
multilayer Si/SiO2 thin film mirrors and antireflection coatings for 1.5 urn," Appl.
Opt. 29, 583-588 (1990).
8. J. C. Zwinkels and D. S. Cignac, "Design and testing of a new high-accuracy
ultraviolet-visible-near-infrared spectrophotometer," Appl. Opt. 31, 1557-1567
(1992).
9. D. L. Decker and V. A. Hodgkin, "Techniques of measurement of reflectance
and transmittance of thin film coatings as a function of temperature," in Optical
Thin Films." R. I. Seddon, ed., Proc. SPIE 325, 167-174 (1982).
10
10. Vacuum reflectometer, model VRSC-100, Acton Research Corp., Acton,
Massachusetts.
11. W. R. Ott, J. M. Bridges, and J. Z. Kiose, "Vacuum-ultraviolet spectral-
irradiance calibrations: Method and applications," Opt. Lett. 5, 225-227 (1980).
12. M. Zukic, D. G. Torr, J. F. Spann, and M. R. Torr, 'Vacuum ultraviolet thin
films. 1: Optical constants of BaF2, CaF2, LaF3, MgF2, AI2O3, HfO2, and SiO2
thin films," Appl. Opt. 29, 4284-4292 (1990).
13. C. E. Keffer, M. R. Torr, M. Zukic, J. F. Spann, D. G. Torr, and J. Kim,
"Radiation damage effects in far ultraviolet filters, thin films, and substrates,"
Appl. Opt., in press (1994).
14. M. Zukic, D. G. Torr, J. Kim, J. F. Spann, and M. R. Torr, "Filters for the
International Solar Terrestrial Physics Mission Far-Ultraviolet Imager," Opt. Eng.
32, 3069-3074 (1993)fX
15. J. A. R. Samson, Techniques of Vacuum Ultraviolet Spectroscopy. John
Willey & Sons, Inc., New York, p. 212-216 (1967).
16. ibid., pp. 163-174.
11
Photocathode Window Spectral Response
Range (nm) Peak (nm)
Csl MgF2 115-200 140
CsTe MgF2 115-320 210
Bialkali Quartz 160-650 420
Bialkali Quartz with added 20-350 150
sodium salicylate coated
Pyrex window
Table 1. PMT photocathode and window materials and spectral response
characteristics.
12
Figure Captions
Figure 1. Schematic of collimated spectrophotometric configuration.
Figure 2. Configuration of the bialkali PMT with a sodium salicylate coated
window.
Figure 3. Schematic of uncollimated spectrophotometric configuration.
Figure 4. Reflectance measurement for RF135 control sample. The values are
averages of 150 runs made over a period of 31 months. The error bars
represent one standard deviation.
13
Monochromator Reflectance PMT
Grating Folding Mirror /
Q
Colimating Mirror Aperture
OTransmittance— PMT
Sample
Filter Wheel
Figure 1. Schematic of collimated spectrophotometric configuration.
Figure 1
James F. Spann
Applied Optics
14
Pyrex Substrate
\ Vtton O-ri
Delrin Holder! /
CW
ng
\
Bialkali PMT
Sodium Salicylate Layer
Figure 2. Configuration of the bialkali PMT with a sodium salicylate coated
window.
Figure 2
James F. Spann
Applied Optics
15
Reflectance PMT
Monochromator
Grating Q
£ O
Transmittance
« PMT
Aperture
Sample
Filter Wheel
Figure 3. Schematic of uncollimated spectre-photometric configuration.
Figure 3
James F. Spanri
Applied Optics.
16
0.7
0.6
0.5
o
<U
f^0-,
0)
OS
0.3
0.2
0.1
0.0
120 130 140 150 160 170 160 190 200 210
Wavelength (nm)
Figure 4. Reflectance measurement for RF135 control sample. The values are
averages of 150 runs made over a period of 31 months. The error bars
represent one standard deviation.
Figure 4
James F. Spann
Applied Optics
17


A vacuum ultraviolet spectrophotometric system

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