ASTM F576-00

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: F 576 – 00
Standard Test Method for
Measurement of Insulator Thickness and Refractive Index
on Silicon Substrates by Ellipsometry
This standard is issued under the fixed designation F 576; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
When this test method was developed in the mid-1970’s, manual-null ellipsometers, which are the
basis of this test method, were in routine use. More recently, faster, automated instruments have
replaced manual-null ellipsometers for all common use in the semiconductor industry. There are two
basic types of such automated instruments commonly used: the rotating element null ellipsometer and
the rotating element photometric ellipsometer. For each of these, microprocessors or microcomputers
are used to operate the instrument and to analyze the data. Details of the procedures utilized in these
instruments are usually considered to be proprietary by the instrument manufacturers.
Despite the fact that this test method is not commonly used in its present form, it embodies all the
basic elements of this test method and a simple analysis of data. Thus, it provides useful guidance in
the fundamentals and application of ellipsometry to film thickness measurements. Until a test method,
or test methods, can be developed that cover the newer, automated instruments, this test method
provides the only such information that is available in a standard test procedure. It also contains results
of a test of interlaboratory precision on silicon dioxide films from 20 to 280 nm using manual null
ellipsometers, and of a test of interlaboratory precision of films of 5 to 550 nm using both manual null
ellipsometers as well as automated ellipsometers of both types just mentioned.
Two major changes have occurred since this test method was initially adopted. First, reference
materials certified for the thickness of silicon dioxide layers on silicon are available both from the
National Institute of Standards and Technology and from commercial sources. These can be used to
evaluate the performance of automated ellipsometers. Second, significantly improved materials and
procedures have been developed for storage of reference wafers needed for long term testing of
baseline performance of ellipsometers. It is not uncommon for reference wafers simply to be stored
“clean” with no further wafer-cleaning utilized. If cleaning steps are in fact, utilized, they are not those
described in this test method. The cleaning steps detailed in this test method are retained, however, to
provide background information on procedures used for the first of the interlaboratory tests.
1. Scope 1.4 The precision of this test method is reduced by varia-
tions, over regions smaller than the light-beam spot size, in
1.1 This test method covers the measurement by ellipsom-
substrate flatness, insulator thickness, and index of refraction.
etry of the thickness and refractive index of an insulator grown
1.5 Film thickness measurements determined by ellipsom
or deposited on a silicon substrate.
etry are not unique. When the film thickness is greater than that
1.2 This test method uses monochromatic light.
2 2 1/2
calculated from the expression Nl/[2(n − sin f ) ], where
1.3 This test method is nondestructive and may be used to
N is an integer, l the measurement wavelength, n the index of
measure the thickness and refractive index of any film not
refraction, and f the angle of incidence, the thickness value
absorbing light at the measurement wavelength on any sub-
determined by this expression must be added to the thickness
strate (1) not transparent to light at the measurement wave-
value determined by ellipsometry to obtain the correct film
length, and ( 2) of a material for which both the refractive
thickness. The value of N must be obtained by another
index and the absorption coefficient are known at the measure-
procedure.
ment wavelength.
1.6 Two procedures for computing the results are provided.
If the graphical procedure is used, the measuring wavelength
This test method is under the jurisdiction of ASTM Committee F01 on
shall be either 546.1 or 632.8 nm, and the angle of incidence
Electronics and is the direct responsibility of Subcommittee F01.06 on Electrical
shall be 70 6 0.1°.
and Optical Measurements.
Current edition approved Dec. 10, 2000. Published February 2001. Originally
1.7 This test method may be used for referee measurements
published as F 576 – 78. Last previous edition F 576 – 95.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
F 576
with computer calculations. 3.1.2 fast axis—in optics, of a doubly refracting crystal, that
1.8 This standard does not purport to address all of the direction in which the velocity of light is a maximum.
safety concerns, if any, associated with its use. It is the 3.1.3 optic axis—of a doubly refracting crystal, that direc-
responsibility of the user of this standard to establish appro- tion through the crystal along which no double refraction
priate safety and health practices and determine the applica- occurs.
bility of regulatory limitations prior to use. Specific hazard 3.1.4 polarization—in optics, the term used to describe the
statements are given in Section 9. orientation of the time-varying electric field vector in an
electromagnetic wave.
2. Referenced Documents
NOTE 1—If the electric field vector is confined to a plane containing the
2.1 ASTM Standards:
direction of propagation of the wave, the wave is said to be plane
D 5127 Guide for Ultra Pure Water Used in the Electronics
polarized. If the vector rotates around the direction of propagation as an
axis but remains constant in magnitude, the wave is said to be circularly
and Semiconductor Industry
polarized. If the amplitude does not remain constant, so that the end of the
E 177 Practice for Use of the Terms Precision and Bias in
3 vector traces out an ellipse, the wave is said to be elliptically polarized.
ASTM Test Methods
E 284 Terminology of Appearance 3.1.5 polarized light—in optics, light exhibiting different
F 95 Test Method for Thickness of Lightly-Doped Silicon properties in different directions at right angles to the line of
Epitaxial Layers on Heavily-Doped Silicon Substrates propagation.
Using a Dispersive Infrared Spectrophotometer 3.1.6 relative minimum—in optics, a minimum in the
2.2 SEMI Standard: amount of light transmitted through a polarizer and analyzer
C19 Specification for Acetone combination that results from varying either the polarizing
C31 Specification for Methanol angle or the analyzing angle (with the other angle fixed).
2.3 ASTM Adjuncts: 3.1.7 Other terms used in this method are defined in
Large size figures Terminology E 284, Test Method F 95.
4. Summary of Test Method
3. Terminology
4.1 The apparatus is assembled as shown in Fig. 1. Light
3.1 Definitions:
emitted from the monochromator is plane polarized after
3.1.1 ellipticity—in optics, of elliptically polarized light, the
passing through the polarizer. The compensator is set at − 45°
angle x given by the inverse tangent of the ratio of the minor
(or + 315°) to convert the plane-polarized light to elliptically
to the major axis of the ellipse described by the electric vector
polarized light. The azimuth angle and degree of ellipticity of
of the light.
the light incident on the specimen are determined from the
settings of polarizer and compensator. The incident light
undergoes a change in degree of ellipticity and azimuth when
Annual Book of ASTM Standards, Vol 11.01. reflected from the specimen. The system is adjusted for signal
Annual Book of ASTM Standards, Vol 14.02.
extinction at the detector by alternately changing the polarizer
Annual Book of ASTM Standards, Vol 06.01.
and analyzer settings with the result that the incident light on
Annual Book of ASTM Standards, Vol 10.05.
Available from the Semiconductor Equipment and Materials Institute, 625 Ellis the specimen surface is elliptically polarized and the reflected
St., Suite 212, Mountain View, CA 94043.
light is plane polarized. The film thickness and index of
Also available as large-size figures from ASTM Headquarters, 100 Barr Harbor
refraction are calculated either by a manual graphical method
Drive, West Conshohocken, PA 19428. Order Adjunct ADJF0576.
Reprinted by permission of R. F. Spanier, from Industrial Research, IDRSA, September 1975, p. 75.
FIG. 1 Schematic of Ellipsometer Apparatus
F 576
or by means of a computer program (1). 7.4 Compensator— Doubly refracting plate, with known
constants T and D (see 13.1), used to convert plane-polarized
c c
5. Significance and Use
light to elliptically polarized light, and mounted in a divided
5.1 Thin insulator films are used in semiconductor device
circle that can be accurately positioned to within 60.1°.
fabrication for isolation, passivation, masking in diffusion
NOTE 4—If the constants T and D are not known, they may be
c c
processes, and in some applications as a part of the device.
determined experimentally in accordance with Section 12 provided that
Precise knowledge on the part of the device designer and
the calculations are performed by means of a computer program (1). For
fabricator of actual insulator thickness or index of refraction, or
this purpose, the test specimen is replaced by a metal specimen known to
both, provides information useful for the optimization of
be free of any film. The ellipsometer parameters calculated in 12.8.1,
quantities such as device operating parameters, yield, and
12.10.1, 12.13, and 12.15 are used as input data for the computer program,
reliability. The measurements are also useful for process and the compensator constants are calculated by the program.
control. Since the interlaboratory precision and accuracy of this
7.5 Specimen Table— Specimen mounting table with gradu-
test method have not yet been determined (see 15.3), it is not
ated circle for measuring the angles of incidence on and
recommended that the test method be used for materials
reflection of light from the specimen to within 60.1°. At its
acceptance purposes.
center, the table shall incorporate an X-Y stage suitable for
5.1.1 The threshold voltage for a MOSFET device is related
mounting the specimen and capable of positioning different
to the thickness of the gate insulator.
regions of the specimen in the light beam for the measure-
5.1.2 The capacitance of a capacitor is inversely propor-
ments.
tional to the insulator thickness.
7.6 Detector—Photoelectric detector, for determining the
5.1.3 The maximum voltage possible across a MOSFET
minimum of the reflected light signal.
gate is proportional to the insulator thickness.
7.7 Aperture Plates, as required by the apparatus shown in
5.1.4 The effectiveness of a diffusion mask is proportional
Fig. 1, including (1) a variable-aperture plate or several
to insulator thickness.
fixed-aperture plates having apertures ranging in diameter from
1 to 5 mm, inclusive, and used to define the size of the
NOTE 2—MOSFET is an acronym for Metal-Oxide Semiconductor
Field-Effect Transistor. light-beam spot incident on the specimen, and (2) an inter-
changeable aperture-plate assembly.
6. Interferences
7.8 Chemical Laboratory Apparatus, such as plastic beakers
6.1 The presence of fingerprints or other foreign contami-
and plastic-coated tweezers suitable for use with solvents.
nation on the surface may give erroneous results.
7.9 Ventilated Hood— Working space with means for lim-
6.2 If the substrate is not flat, the thickness of the layer is not
iting the concentration of solvent vapors to acceptable levels
uniform, or the index of refraction is not uniform over regions
and for exhausting air containing vapors in a manner consistent
comparable in dimension to the diameter of the light beam, it
with safe practice.
may not be possible to obtain complete extinction (see
7.10 Ultrasonic Cleaner, with operating frequency in the
12.10.1), with the result that the precision of the measurement
nominal range from 18 to 45 kHz and with adequate power to
may be reduced.
clean test specimens.
6.3 If the film is partially absorbing or scattering at the
7.11 Glass Plate, suitable for use in 12.2.
measurement wavelength, a unique solution may not be ob-
7.12 Supports, Mounts, and Other Fixtures, as required.
tainable.
8. Reagents and Materials
6.4 When graphical methods are used in the calculations,
the precision of the method is reduced when the angles D and
8.1 Purity of Reagents—All chemicals for which SEMI
C (see 13.2.1) have a range of values from 140 to 180°,
specifications exist shall adhere to Grade 1 specifications for
inclusive, and from 11.6 to 14.0°, inclusive, respectively.
those chemicals. Reagents for which SEMI specifications have
not been developed shall conform to the specifications of the
7. Apparatus
Committee on Analytical Reagents of the American Chemical
7.1 Light Source, producing a collimated beam of mono-
Society, where such specifications are available. Other grades
chromatic light at the intended measurement wavelength.
may be used provided it is first ascertained that the reagent is
NOTE 3—The source may consist of either (1) a laser, or (2)a of sufficiently high purity to permit its use without lessening
polychromatic lamp with collimator and filters or monochromator for
the accuracy of the determination.
selecting the measurement wavelength.
8.2 Purity of Water— References to water shall be under-
7.2 Polarizer—Doubly refracting crystal used to convert the stood to mean Type I or II water as specified in Guide D 5127.
unpolarized monochromatic radiation from the light source to 8.3 Acetone [(CH ) CO], SEMI C19, grade 1.
3 2
plane-polarized light. The crystal shall be rotatably mounted in 8.4 Methanol (CH OH), SEMI C31, grade 1.
a divided circle that can be read to within 60.1°.
7.3 Analyzer—Doubly refracting crystal of similar con-
Reagent Chemicals, American Chemical Society Specifications, American
struction to that of the polarizer and with the same type of
Chemical Society, Washington, DC. For suggestions on the testing of reagents not
mounting.
listed by the American Chemical Society, see Analar Standards for Laboratory
Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
The boldface numbers in parentheses refer to the list of references at the end of and National Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville,
this test method. MD.
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