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Publication numberUSRE34783 E
Publication typeGrant
Application numberUS 08/111,298
Publication dateNov 8, 1994
Filing dateAug 23, 1993
Priority dateFeb 1, 1990
Also published asUS5045704
Publication number08111298, 111298, US RE34783 E, US RE34783E, US-E-RE34783, USRE34783 E, USRE34783E
InventorsVincent J. Coates
Original AssigneeNanometrics Incorporated
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Method for determining absolute reflectance of a material in the ultraviolet range
US RE34783 E
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.
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Claims(5)
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, Zλ, at wavelength λ, 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 λ, the system efficiency coefficient, Zλ at that wavelength, is determined by measuring the measured reflectance, mR, and .[.divide.]. .Iadd.dividing .Iaddend.by aR:

Zλ=mR/aR                                            (1)

To determine the absolute reflectance, aRx, of a material, x, measure the .[.measured.]. reflectance, mRx, of that unknown material, x, at the same wavelength, λ, and with the same optical system, and .[.multiply.]. .Iadd.divide .Iaddend.the results by the system efficiency coefficient, Zλ.

.[.aRx=mRx(Zλ).]. .Iadd.aRx=mRx/Zλ.Iaddend.  (2)

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, Zλ 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 Zλ, in memory. The computer performs the .[.multiplication.]. .Iadd.division .Iaddend.of Equation (2) to obtain the absolute reflectance, aRx of the unknown material, x.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US3751643 *May 23, 1972Aug 7, 1973IbmSystem for performing spectral analyses under computer control
US4035085 *Dec 5, 1975Jul 12, 1977Ppg Industries, Inc.Method and apparatus for comparing light reflectance of a sample against a standard
US4455090 *Jul 17, 1980Jun 19, 1984The Wiggins Teape Group LimitedApparatus for measuring surface reflectance characteristics
US4575252 *Oct 14, 1983Mar 11, 1986Shimadzu CorporationApparatus for measuring absolute reflectance
US4899055 *May 12, 1988Feb 6, 1990Tencor InstrumentsThin film thickness measuring method
US5101111 *Jul 6, 1990Mar 31, 1992Dainippon Screen Mfg. Co., Ltd.Method of measuring thickness of film with a reference sample having a known reflectance
US5120966 *Feb 6, 1991Jun 9, 1992Dainippon Screen Mfg. Co., Ltd.Method of and apparatus for measuring film thickness
JPS6413438A * Title not available
Non-Patent Citations
Reference
1Barkley, John R., "Measurement and Analysis of Reference Spectra of SiP2," Applied Optics vol. No. 9, (Sep. 1972) pp. 1928-1935.
2 *Barkley, John R., Measurement and Analysis of Reference Spectra of SiP 2 , Applied Optics vol. No. 9, (Sep. 1972) pp. 1928 1935.
3 *Book by Dr. Horst Piller, Microscope Photometry, published by Springer Verlag Berlin Heidalberg (1977), Chapter 9.
4Book by Dr. Horst Piller, Microscope Photometry, published by Springer-Verlag Berlin Heidalberg (1977), Chapter 9.
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US5717216 *Oct 16, 1996Feb 10, 1998Reynolds Metals CompanyThickness gauging using ultraviolet light absorption
US5747813 *Apr 14, 1994May 5, 1998Kla-Tencop. CorporationBroadband microspectro-reflectometer
US5805285 *Mar 17, 1997Sep 8, 1998J.A. Woollam Co. Inc.Multiple order dispersive optics system and method of use
US6181427Jul 6, 1999Jan 30, 2001Nanometrics IncorporatedCompact optical reflectometer system
US6381009Jun 29, 1999Apr 30, 2002Nanometrics IncorporatedElemental concentration measuring methods and instruments
US6713753Jul 3, 2001Mar 30, 2004Nanometrics IncorporatedCombination of normal and oblique incidence polarimetry for the characterization of gratings
US6795184Sep 26, 2001Sep 21, 2004J.A. Woollam Co., IncOdd bounce image rotation system in ellipsometer systems
US6898537Apr 27, 2001May 24, 2005Nanometrics IncorporatedMeasurement of diffracting structures using one-half of the non-zero diffracted orders
US6949462Apr 4, 2002Sep 27, 2005Nanometrics IncorporatedMeasuring an alignment target with multiple polarization states
US6982793Apr 4, 2002Jan 3, 2006Nanometrics IncorporatedMethod and apparatus for using an alignment target with designed in offset
US6987566Nov 27, 2002Jan 17, 2006Itt Manufacturing Enterprises, Inc.Methods and apparatus for analyzing mirror reflectance
US6992764Sep 30, 2002Jan 31, 2006Nanometrics Incorporatedcomprises measuring two periodic patterns (diffraction gratings) with an incident beam and detecting the intensity of the resulting polarized light
US7026626Sep 23, 2003Apr 11, 2006Metrosol, Inc.Semiconductor processing techniques utilizing vacuum ultraviolet reflectometer
US7061615Sep 20, 2001Jun 13, 2006Nanometrics IncorporatedSpectroscopically measured overlay target
US7067818Sep 23, 2003Jun 27, 2006Metrosol, Inc.Vacuum ultraviolet reflectometer system and method
US7075649Sep 4, 2001Jul 11, 2006J.A. Woollam Co.Discrete polarization state rotatable compensator spectroscopic ellipsometer system, and method of calibration
US7084978Jul 8, 2005Aug 1, 2006J.A. Woollam Co., Inc.Sample orientation system and method
US7099006Jul 7, 2003Aug 29, 2006J.A. Woollam Co., IncEllipsometer or polarimeter and the like system with beam chromatic shifting and directing means
US7115858Sep 25, 2000Oct 3, 2006Nanometrics IncorporatedApparatus and method for the measurement of diffracting structures
US7126131Jul 30, 2004Oct 24, 2006Metrosol, Inc.Broad band referencing reflectometer
US7136162Oct 12, 2003Nov 14, 2006J.A. Woollam Co., Inc.Alignment of ellipsometer beam to sample surface
US7189973Apr 27, 2006Mar 13, 2007Metrosol, Inc.Vacuum ultraviolet reflectometer integrated with processing system
US7193710Apr 22, 2004Mar 20, 2007J.A. Woollam Co., Inc.Rotating or rotatable compensator spectroscopic ellipsometer system including multiple element lenses
US7230699Sep 2, 2003Jun 12, 2007J.A. Woollam Co., Inc.Sample orientation system and method
US7230705Mar 7, 2005Jun 12, 2007Nanometrics IncorporatedAlignment target with designed in offset
US7236244Mar 9, 2005Jun 26, 2007Nanometrics IncorporatedAlignment target to be measured with multiple polarization states
US7271394Apr 27, 2006Sep 18, 2007Metrosol, Inc.Vacuum ultraviolet reflectometer having collimated beam
US7282703May 5, 2006Oct 16, 2007Metrosol, Inc.Method and apparatus for accurate calibration of a reflectometer by using a relative reflectance measurement
US7304737Aug 3, 2006Dec 4, 2007J.A. Woollam Co., IncRotating or rotatable compensator system providing aberation corrected electromagnetic raadiation to a spot on a sample at multiple angles of a incidence
US7304792Apr 5, 2006Dec 4, 2007J.A. Woollam Co., Inc.System for sequentially providing aberation corrected electromagnetic radiation to a spot on a sample at multiple angles of incidence
US7333198Jul 29, 2006Feb 19, 2008J.A. Woollam Co., Inc.Sample orientation system and method
US7345762Aug 15, 2005Mar 18, 2008J.A. Woollam Co., Inc.Control of beam spot size in ellipsometer and the like systems
US7372565Sep 28, 2006May 13, 2008Nanometrics IncorporatedSpectrometer measurement of diffracting structures
US7391030May 3, 2007Jun 24, 2008Metrosol, Inc.Broad band referencing reflectometer
US7394551Sep 23, 2003Jul 1, 2008Metrosol, Inc.Vacuum ultraviolet referencing reflectometer
US7399975 *Aug 31, 2004Jul 15, 2008Metrosol, Inc.Method and apparatus for performing highly accurate thin film measurements
US7446876Sep 8, 2006Nov 4, 2008Metrosol Inc.Vacuum ultra-violet reflectometer with stray light correction
US7511265May 5, 2006Mar 31, 2009Metrosol, Inc.Method and apparatus for accurate calibration of a reflectometer by using a relative reflectance measurement
US7518725Oct 23, 2006Apr 14, 2009J.A. Woollam Co., Inc.Temperature controlled lens
US7535566Jun 15, 2006May 19, 2009J.A. Woollam Co., Inc.Beam chromatic shifting and directing means
US7619752Oct 30, 2007Nov 17, 2009J. A. Woollam Co., Inc.Sample orientation system and method
US7663097Apr 25, 2007Feb 16, 2010Metrosol, Inc.Method and apparatus for accurate calibration of a reflectometer by using a relative reflectance measurement
US7804059Aug 31, 2004Sep 28, 2010Jordan Valley Semiconductors Ltd.Method and apparatus for accurate calibration of VUV reflectometer
US7948631Nov 30, 2009May 24, 2011Jordan Valley Semiconductors Ltd.Method and apparatus for using multiple relative reflectance measurements to determine properties of a sample using vacuum ultra violet wavelengths
US8014000Nov 3, 2009Sep 6, 2011Jordan Valley Semiconductors Ltd.Broad band referencing reflectometer
US8054453Sep 7, 2010Nov 8, 2011Jordan Valley Semiconductors Ltd.Broad band referencing reflectometer
US8059276Dec 8, 2008Nov 15, 2011J.A. Woollam Co., IncEllipsometric investigation and analysis of textured samples
US8119991Aug 12, 2010Feb 21, 2012Jordan Valley Semiconductors Ltd.Method and apparatus for accurate calibration of VUV reflectometer
US8153987May 22, 2009Apr 10, 2012Jordan Valley Semiconductors Ltd.Automated calibration methodology for VUV metrology system
US8169611Jun 23, 2009May 1, 2012University Of Nebraska Board Of RegentsTerahertz-infrared ellipsometer system, and method of use
US8248607 *Aug 3, 2010Aug 21, 2012J.A. Woollam Co., Inc.Empirical correction for spectroscopic ellipsometric measurements of rough or textured surfaces
US8416408Jun 11, 2010Apr 9, 2013J.A. Woollam Co., Inc.Terahertz-infrared ellipsometer system, and method of use
US8488119Jun 14, 2010Jul 16, 2013J.A. Woollam Co., Inc.Terahertz-infrared ellipsometer system, and method of use
US8564777Aug 13, 2011Oct 22, 2013J.A. Woollam Co., Inc.System and method for compensating detector non-idealities
US8564780Jul 28, 2010Oct 22, 2013Jordan Valley Semiconductors Ltd.Method and system for using reflectometry below deep ultra-violet (DUV) wavelengths for measuring properties of diffracting or scattering structures on substrate work pieces
US8565379Mar 14, 2012Oct 22, 2013Jordan Valley Semiconductors Ltd.Combining X-ray and VUV analysis of thin film layers
US8570513Nov 3, 2011Oct 29, 2013J.A. Woollam Co., Inc.Ellipsometric investigation and analysis of textured samples
US8705032Mar 7, 2013Apr 22, 2014J.A. Woollam Co., IncTerahertz-infrared ellipsometer system, and method of use
Classifications
U.S. Classification250/372, 356/448, 356/445
International ClassificationG01N21/55, G01N21/33
Cooperative ClassificationG01N21/33, G01N21/55, G01N2021/1748
European ClassificationG01N21/55
Legal Events
DateCodeEventDescription
Jan 16, 2007RRRequest for reexamination filed
Effective date: 20061102
Jan 9, 2007RRRequest for reexamination filed
Effective date: 20061102