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SEMI M53-1103 © SEMI 2003 2 true size of LLSs other than PSL spheres (see Sect ion 3.1). 2.8 This practice suppo rts requirements listed in SEMI M52. NOTICE: This standard does not pu rport to address safety issues, if a…
SEMI M53-1103 © SEMI 2003 1
SEMI M53-1103
PRACTICE FOR CALIBRATING SCANNING SURFACE INSPECTION
SYSTEMS USING DEPOSITIONS OF MONODISPERSE POLYSTYRENE
LATEX SPHERE ON UNPATTERNED SEMICONDUCTOR WAFER
SURFACES
This practice was technically approved by the Global Silicon Wafer Committee and is the direct
responsibility of the North American Silicon Wafer Committee. Current edition approved by the North
American Regional Standards Committee on July 25, 2003. Initially available at www.semi.org October
2003; to be published November 2003. Originally published March 2003.
1 Purpose
1.1 Calibration of a scanning surface inspection system
(SSIS) using a deposition of polystyrene latex (PSL)
spheres as a known source of scatter signal is a
necessary step in matching the responses of SSISs.
However, the measured amplitude of the light scatter
signal depends on both the characteristics of the SSIS
and the characteristics of the scatter source. The use of
a deposition of polystyrene latex (PSL) spheres (see
Section 5.3.4) as a known source of scatter signal
allows meaningful comparisons to be made between
scatter signals from PSL spheres as measured by dark
field detection systems of different designs. On the
other hand, calibration with PSL spheres alone does not
guarantee meaningful performance comparisons to be
made between dark field detection SSISs of the same or
different designs when detecting either real particles of
materials different from PSL spheres or other surface
defects.
1.2 This practice describes calibration of SSIS dark
field channels so that the SSIS accurately sizes PSL
spheres deposited on unpatterned polished, epitaxial, or
filmed semiconductor wafer surfaces (see Related
Information 1).
1.3 This practice defines the use of latex sphere
equivalent (LSE) signals as a means of reporting real
surface defects whose identity and true size are
unknown.
1.4 This practice provides a basis for quantifying SSIS
performance as used in related standards concerned
with parameters such as sensitivity, repeatability and
capture rate.
2 Scope
2.1 This practice covers
• Requirements for the surface and other
characteristics of the semiconductor substrates on
which the PSL spheres are deposited to form
reference wafers (see Section 8.1),
• Selection of appropriate certified depositions of
PSL spheres for SSIS calibration (see Section 8.2),
• Size distribution requirements to be met by the
PSL sphere depositions (but not the deposition
method), and
• Multipoint calibration procedures for dark field
channels.
2.2 Appendix 1 covers a single-point calibration
procedure that may be used in limited production
applications.
2.3 PSL spheres from 10 µm to the smallest size that
can be detected by the SSIS being calibrated can be
used in this practice.
NOTE 1: At the time of development of this edition of the
practice, the smallest practical deposited PSL spheres have
diameters approaching 30 nm, but as IC technology evolves to
smaller and smaller critical dimensions it is expected that
depositions of smaller diameter PSL spheres will become
available.
2.4 Background information to enable an
understanding of the need for the various requirements
imposed on the PSL sphere depositions is provided in
Related Information 1.
2.5 Both the deposition process and calibration
procedures must be carried out in a Class 4 or better
environment as defined in ISO 14644-1.
NOTE 2: ISO class 4 is approximately the same as Class
M2.5 (Class 10) as defined in Federal Standard 209E.
2.6 Although it was developed primarily for use in
evaluation of SSISs to be used for detection of localized
light scatterers (LLSs) on polished silicon wafers with
geometrical characteristics as specified in SEMI M1,
this practice can be applied to SSISs to be used for
detection of LLSs on other unpatterned semiconductor
surfaces provided that suitable reference wafers are
employed.
2.7 This practice does not in any way attempt to define
the manner in which LSE values are used to define the
SEMI M53-1103 © SEMI 2003 2
true size of LLSs other than PSL spheres (see Section
3.1).
2.8 This practice supports requirements listed in SEMI
M52.
NOTICE: This standard does not purport to address
safety issues, if any, associated with its use. It is the
responsibility of the users of this standard to establish
appropriate safety and health practices and determine
the applicability of regulatory or other limitations prior
to use.
3 Limitations
3.1 LLSs are normally assigned only LSE sizes, not
physical diameters, because the response of an SSIS to
an LLS depends on the SSIS optical system
characteristics as well as the size, shape, and
composition of the LLS. The LSE size assigned to a
particular LLS by an SSIS calibrated against PSL
spheres may be different from that assigned to the same
LLS by another similarly calibrated SSIS, because
different SSISs have different optical system
characteristics.
3.2 PSL spheres may have specified characteristics
(mean diameter uncertainty, diameter distribution,
spread between mean and modal diameter) that differ
significantly from the characteristics of the resulting
deposition due to the transfer function of the deposition
system. For this reason the practice is limited to the use
of PSL sphere depositions that are appropriately
characterized. See Related Information 1.
3.3 If calibration points occur at, or near, dips in the
response curve (see Section 5.3.17), the calibration
curve will not be single valued; therefore dips must be
avoided as calibration points if a monotonic calibration
curve is to be obtained. However, if a complete
calibration is desired, the locations and magnitudes of
the dips must be determined. In this case there may not
be a one-to-one correlation between SSIS response and
LSE size; i.e., a particular instrument response may
correspond to more than one LSE size.
1
(See Section
5.3.13.1.)
3.4 Background Contamination
3.4.1 The presence of localized light scatterers with
LSE sizes near that of the nominal PSL diameter on the
reference wafer may skew the results. This condition
1 See, for example, Locke, B. R., and Donovan, R. P., “Particle
Sizing Uncertainties in Laser Scanning of Silicon Wafers,” Journal of
The Electrochemical Society, Vol 134, No. 7, 1987, pp. 1763–1771;
or Liu, B. Y. H., Chae, S.-K., and Bae, G.-N., “Sizing Accuracy,
Counting Efficiency, Lower Detection Limit and Repeatability of a
Wafer Surface Scanner for Ideal and Real-World Particles,” ibid., Vol
140, No. 5, 1993, pp. 1403–1409.
may result in a large error or poor equivalent sizing
accuracy.
3.4.2 High levels of localized light scatterers on the
reference wafer or wafers may overload the SSIS or
obscure the peak of the deposited PSL sphere
distribution. This condition may also result in a large
error or poor equivalent sizing accuracy.
3.4.3 For these reasons, both the deposition process
and calibration procedures must be carried out in a
clean environment, and the reference wafers must be
handled in such a way as to avoid contamination
between deposition process and calibration.
3.5 If the surface roughness of the reference wafer or
wafers is excessive, the peak of the PSL sphere
distribution may be obscured or distorted.
3.6 If the SSIS being calibrated is not operating in a
stable condition, the calibration may not be appropriate
for subsequent use of the system. System stability can
be evaluated by making repeated calibrations, in
accordance with this practice, over suitable time
periods.
4 Referenced Standards
4.1 SEMI Standards
SEMI M1 — Specification for Polished Monocrystal-
line Silicon Wafers
SEMI M20 — Specification for Establishing a Wafer
Coordinate System
SEMI M50 — Test Method for Determining Capture
Rate and False Count Rate for Surface Scanning
Inspection Systems by the Overlay Method
SEMI M52 — Guide for Specifying Surface Scanning
Inspection Systems for Silicon Wafers for the 130-nm
Technology Generation
SEMI MF1241 — Standard Terminology of Silicon
Technology
4.2 Federal Standard
2
Fed Std 209E — Airborne Particulate Cleanliness
Classes in Cleanrooms and Clean Zones
2 Standardization Documents Order Desk, Bldg. 4 Section D, 700
Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS (This
standard has been superseded by ISO 14644-1 and may no longer be
available.)
SEMI M53-1103 © SEMI 2003 3
4.3 ISO Standards
3
ISO 14644-1 Cleanrooms and associated controlled
environments — Part 1: Classification of airborne
particulates
ISO Guide 30: 1992 Terms and definitions used in
connection with reference materials
NOTICE: Unless otherwise indicated, all documents
cited shall be the latest published versions.
5 Terminology
5.1 Definitions for general terms for silicon technology
are found in SEMI MF1241.
5.2 Additional terminology is defined in SEMI M1.
5.3 Definitions
5.3.1 capture rate (CR) — the probability that a
scanning surface inspection system (SSIS) detects a
localized light scatterer (LLS) of latex sphere
equivalent (LSE) signal value at some specified SSIS
operational setting.
5.3.2 certified reference material (CRM) — a material
accompanied by a certificate, one or more of whose
property values are certified by a procedure that
establishes its traceability to an accurate realization of
the unit in which the property values are expressed, and
for which each certified value is accompanied by an
uncertainty at a stated level of confidence [ISO Guide
30].
5.3.2.1 Discussion — In the context of this practice, a
CRM is a certified PSL sphere deposition on an
unpatterned wafer with the same surface films and
finish as the wafers to be examined by the calibrated
SSIS. The deposition property values that must be
certified are the peak sphere diameter and the diameter
distribution on the wafer and are determined by both
the PSL sphere source and the deposition process. The
wafer may contain more than one CRM. See Related
Information 1.
5.3.3 coefficient of variation (CV) — one standard
deviation, σ, expressed as a percentage of the mean of a
Gaussian distribution.
5.3.4 deposition — an approximately known number of
PSL spheres of known size distribution placed in a
known location on the surface of a reference wafer.
3 International Organization for Standardization, ISO Central
Secretariat, 1, rue de Varembé, Case postale 56, CH-1211 Geneva 20,
Switzerland. Telephone: 41.22.749.01.11; Fax: 41.22.733.34.30
Website: www.iso.ch; also available in the US from American
National Standards Institute, New York Office: 11 West 42nd Street,
New York, NY 10036, USA. Telephone: 212.642.4900; Fax:
212.398.0023 Website: www.ansi.org, and in other countries from
ISO member organizations.
5.3.5 deposition process — the procedure used to place
the PSL spheres on the reference wafer.
5.3.6 dynamic range — of a scanning surface
inspection system, the signal range covered by an
instrument with one set of measurement conditions.
5.3.6.1 Discussion — The useful dynamic range is
limited on the small signal side by the background
noise or the inherent resolution of the instrument and on
the large signal side by saturation of the detector and/or
the related electronics. The small signal limit is usually
defined as the smallest PSL diameter than can be
measured with a capture rate of at least 95%.
5.3.7 false count — a laser-light scattering event that
arises from instrumental causes rather than from any
feature on or near (in) the wafer surface; also called
false positive; compare nuisance count.
5.3.7.1 Discussion — False counts would not be
expected to occur at the same point on the wafer surface
during multiple inspection scans, and hence they could
be considered as random “noise” that could be
identified by examining the results of repeated scans.
5.3.8 histogram — a representation of a partitioned
(binned) data set as a bar graph in which the widths of
the bars are proportional to the sizes of the bins of the
data set variable, and the height of each bar is
proportional to the frequency of occurrence of values of
the variable within the bin.
5.3.8.1 Discussion — In presenting data for the size
distribution of LLSs, the data set variable is usually the
derived LLS size; in presenting haze data, the data set
variable is usually the haze in ppm. The data set is
usually partitioned into bins of equal size on either a
linear or logarithmic scale, as appropriate. The bins at
the low and high ends of the data set variable range are
customarily plotted with the same width as the
remainder of the histogram even though they may
represent a larger or smaller range of the independent
variable than the rest of the bins.
5.3.9 laser-light scattering event — a signal pulse that
exceeds a preset amplitude threshold, generated by the
interaction of a laser beam with an LLS at a wafer
surface as sensed by a detector.
5.3.9.1 Discussion — The amplitude of the signal into
a single detector, as measured for any combination of
incident beam direction and collection optics, does not
by itself convey topographic information, for example,
whether the LLS is a pit or a particle. It does not allow
the observer to deduce the size or origin of the scatterer
without other detailed knowledge, such as its index of
refraction and shape. In a scanning surface inspection
system, laser-light scattering events and the background