U.S. patent number RE34,783 [Application Number 08/111,298] was granted by the patent office on 1994-11-08 for method for determining absolute reflectance of a material in the ultraviolet range.
This patent grant is currently assigned to Nanometrics Incorporated. Invention is credited to Vincent J. Coates.
United States Patent |
RE34,783 |
Coates |
November 8, 1994 |
**Please see images for:
( Reexamination Certificate ) ** |
Method for determining absolute reflectance of a material in the
ultraviolet range
Abstract
A method for determining a value of absolute reflectance of a
material at .[.a predetermined.]. .Iadd.any .Iaddend.wavelength, in
the ultraviolet range from its measured reflectance which includes
system losses contributed by optics, illumination sources,
detectors, etc. The method involves the measurement of reflectance
from a known material such as single crystal silicon whose absolute
reflectance is well known, dividing the measured value by the
absolute value to obtain a system efficiency coefficient at the
known wavelength and then, without changing the illumination or
optics, measuring the reflectance of the unknown material and
applying this coefficient to this measured value to obtain its
absolute value.
Inventors: |
Coates; Vincent J. (Palo Alto,
CA) |
Assignee: |
Nanometrics Incorporated
(Sunnyvale) N/A)
|
Family
ID: |
23880429 |
Appl.
No.: |
08/111,298 |
Filed: |
August 23, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
473649 |
Feb 1, 1990 |
05045704 |
Sep 3, 1991 |
|
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Current U.S.
Class: |
250/372; 356/445;
356/448 |
Current CPC
Class: |
G01N
21/55 (20130101); G01N 2021/1748 (20130101); G01N
21/33 (20130101) |
Current International
Class: |
G01N
21/55 (20060101); G01N 21/31 (20060101); G01N
21/33 (20060101); G01J 001/42 () |
Field of
Search: |
;250/372
;356/445,448,51 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Barkley, John R., "Measurement and Analysis of Reference Spectra of
SiP.sub.2," Applied Optics vol. No. 9, (Sep. 1972) pp. 1928-1935.
.
Book by Dr. Horst Piller, Microscope Photometry, published by
Springer-Verlag Berlin Heidalberg (1977), Chapter 9..
|
Primary Examiner: Dzierzynski; Paul M.
Assistant Examiner: Dunn; Drew A.
Attorney, Agent or Firm: Castle; Linval B. MacDonald; Thomas
S.
Claims
I claim:
1. A method for determining an absolute reflectance of material
from a microscopic measurement of its measured reflectance in the
ultraviolet radiation range, said method comprising the steps
of:
determining a value of absolute reflectance of a known material at
a predetermined wavelength;
measuring the reflectance of said known material to obtain a value
of measured reflectance with a microscope illuminated with
radiation at said predetermined wavelength;
with said values of absolute reflectance and measured reflectance,
calculating an efficiency coefficient representing all absorption
and losses caused by the microscope optical system, its reflectance
detectors and its illumination system at said predetermined
wavelength;
measuring the reflectance of an unknown material to obtain a second
value of measured reflectance with said microscope illuminated with
said radiation at said predetermined wavelength;
applying said efficiency coefficient to said second value of
measured reflectance to obtain a value of absolute reflectance of
said unknown material.
2. The method claimed in claim 1 wherein said step of applying
includes the step of .[.multiplying.]. .Iadd.dividing .Iaddend.said
second value by said efficiency coefficient.
3. The method claimed in claim 2 wherein said microscope is a
reflecting microscope.
4. The method claimed in claim 3 wherein said predetermined
wavelength is in the ultraviolet radiation range.
5. The method claimed in claim 2 wherein the determined values of
absolute reflectance of said known material, said value of measured
reflectance of said known material and said value of measured
reflectance of said unknown material are stored in a memory of a
computer that performs the step of calculating said efficiency
coefficient and said value of absolute reflectance of said unknown
material.
Description
BRIEF SUMMARY OF THE INVENTION
This invention relates to the determination of reflectance from a
material and particularly to the determination of absolute
reflectance independent of losses in an associated optical
system.
Some materials are transparent with very little incident energy
being reflected from the surface and some materials are opaque and
absorb nearly all incident energy and reflect very little. In many
materials the .[.ratio of incident energy to reflected energy,
or.]. reflectance.[.,.]. varies according to the wavelength of the
incident radiation. For example, silicon as used in electronic
microcircuits is transparent to infrared wavelengths, translucent
between about one micron and infrared, and opaque to ultraviolet
radiation.
It is often necessary to determine the energy absorption of some
material with unknown chemical contents. An accurate value for
absorption can easily be computed from a knowledge of the absolute
reflectance from the unknown material since the incident energy can
only be divided into absorption and reflectance. But a value for
absolute reflectance is not readily obtainable since any measured
value of reflectance at some predetermined wavelength is
contaminated by losses contributed by the system optics, such as
absorption of lenses, illumination sources, beamsplitters,
gratings, detectors, etc., all of which also vary with
wavelengths.
The object of this invention is to determine the absolute
reflectance value of a test material at a desired wavelength, from
a measured value of reflectance.
Briefly described, the method involves the steps of measuring the
reflectance of a known material, such as single crystal silicon or
aluminum specimen, at the desired wavelength, computing the value
of absolute reflectance from .Iadd.the index of refraction and
.Iaddend.absorption data available in .[.myriads of.]. handbooks,
and then dividing the absolute value .[.by.]. .Iadd.into
.Iaddend.the measured value to derive a .[.product of all optical
system coefficients.]. .Iadd.system efficiency
coefficient.Iaddend.. This value of the .[.coefficients.].
.Iadd.coefficient .Iaddend.is stored. An unknown material is then
tested with the same unchanged optical system and the same
wavelength to obtain a measured reflectance value which, when
.[.multiplied.]. .Iadd.divided .Iaddend.by the stored coefficient,
yields the absolute reflectance value of the unknown material.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings which illustrate the preferred embodiment of the
invention:
FIG. 1 is a schematic drawing illustrating a reflective microscope
and detector processing apparatus for ultraviolet examinations;
and
FIG. 2 illustrate the steps for determining absolute reflectance of
an unknown material.
DETAILED DESCRIPTION
FIG. 1 illustrates a typical microscope for measuring reflectance
in the ultraviolet range. Since the material used in the
construction of typical refractive lenses .[.chromatic or.]. is
opaque to UV radiation, .[.only.]. reflective optical devices
.[.may.]. .Iadd.should .Iaddend.be used. Hence, a UV source 10 with
known output energy, such as a deuterium discharge tube, with
output beam condensed by a small iris 12 is reflected from a planar
quartz beam splitter 14 into a reflective objective lens 16 which
focuses the UV beam onto a specimen 18. The image from the specimen
18 is magnified by the reflective objective and, after passing
through the planar quartz beam splitter 14, is focused into a
monochromator 20 where the image beam is passed through a narrow
slit to isolate a narrow band of wavelengths before detection.
The detected image beam, after being converted by an
analog-to-digital converter 22, is applied to a computer 24 having
a memory 26 and an input-output device 28, such as a keyboard
terminal.
When the microscope system of FIG. 1 is used for measuring
reflectances, all optical elements such as the beam splitter 14,
the several reflective surfaces of the objective lens 16 and the
internal optical elements in the monochromator 20, absorb energy
from the incident radiating beam of the source 10. Further, the
absorbed energy varies with variations in the incident
wavelength.
A value of reflectance can easily be measured with the microscope
system, but that is a measured reflectance, mR, which has little
value since it includes the unknown system losses resulting from
the energy absorption of the optical elements. The type of
reflectance of value is the absolute reflectance, aR. the ratio of
reflected energy to incident energy, independent of system
losses.
Absolute reflectance of an unknown material can be determined if
one knows the values of both absolute and measured reflectance of a
known material at the desired wavelength. With this data, the
system losses, or system efficiency coefficient, Z.lambda., at
wavelength .lambda., is computed by merely dividing the measured
value by the absolute value of reflectance. Many reference books
list tables of refractive index and absorption of various materials
at various .[.frequencies.]. .Iadd.wavelengths .Iaddend.and many
also list the values of absolute reflectance at various
wavelengths. Thus, absolute reflectance values, aR, are available
or calculable for several pure materials, such as single crystal
silicon.
With knowledge of an absolute reflectance value, aR, of a
particular pure material at some known wavelength .lambda., the
system efficiency coefficient, Z.lambda. at that wavelength, is
determined by measuring the measured reflectance, mR, and
.[.divide.]. .Iadd.dividing .Iaddend.by aR:
To determine the absolute reflectance, aRx, of a material, x,
measure the .[.measured.]. reflectance, mRx, of that unknown
material, x, at the same wavelength, .lambda., and with the same
optical system, and .[.multiply.]. .Iadd.divide .Iaddend.the
results by the system efficiency coefficient, Z.lambda..
The determination of absolute reflectance can readily be performed
by the computer system illustrated in FIG. 1. The value aR of the
known material at the predetermined wavelength is entered through
the keyboard 28 into the computer 24 which is programmed to perform
the simple division .[.and multiplication.]. shown in Equations (1)
and (2) above. The value, aR is stored in the memory 26. The
reflectance, mR is then measured of the known material 18 on the
microscopy stage and the detected value is stored into the memory
26. The computer 24 then performs Equation (1) and stores the
efficiency coefficient, Z.lambda. in memory. Without making any
changes in the energy source or the optical system, the known
material 18 is replaced with the unknown material, x, and the
reflectivity is measured to obtain the value, mRx, which is applied
to the computer 24 along with the efficiency coefficient Z.lambda.,
in memory. The computer performs the .[.multiplication.].
.Iadd.division .Iaddend.of Equation (2) to obtain the absolute
reflectance, aRx of the unknown material, x.
* * * * *