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SEMI M53-1103 © SEMI 2003 6 approxim ately midway between adjacent pairs of the original set. 8.3 Background Contaminati on 8.3.1 Handle and store reference wafers with great care to avoid conta mination and damage. 8.3.…

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RC depends on the light source used, is in general non-
linear and may contain regions with dips that make the
response/diameter relationship multi-valued. (See
Section 3.3).
5.3.18 scanning surface inspection system (SSIS) — an
instrument for rapid examination of the entire quality
area of a wafer to detect the presence of localized light
scatterers or haze or both; also called particle counter
and laser surface scanner.
5.3.19 threshold — the level set on a scanning surface
inspection system (SSIS) to discriminate between signal
pulses of different size.
5.3.19.1 Discussion — Thresholds may be set to
discriminate between true counts and surface or
electrical noise (nuisance or false counts, respectively)
or between different sizes of light scatterers. Because
of spatial non-uniformity of the intensity of the
scanning beam and the general use of overlapping scans
in an SSIS, a localized light scatterer with equivalent
size near the threshold may generate a signal greater
than or less than the threshold depending on its location
with respect to the path of the scanning beam. The
former is identified as a true count and the latter is
identified as a missing count.
5.3.20 true count — a laser-light scattering event that
arises from the localized light scatterers being
investigated.
5.3.21 unimodal distribution — a distribution
represented by a histogram with constant bin size that
has a single bell-shaped peak.
6 Summary of Practice
6.1 The range of LSE values to be used for calibration
is defined for each dark channel that is to be calibrated.
6.2 The number of calibration points required is
defined for each defined range.
6.3 Suitable reference wafers with certified PSL
distributions are obtained.
6.4 The reference wafers are scanned by the SSIS
being calibrated under machine conditions identical
with those to be used in examining wafers with the
calibrated SSIS.
6.5 The peak diameter of the PSL spheres deposited on
each reference wafer is assigned to the peak value of
the SSIS signal units.
6.6 An RC, the curve of SSIS channel response as a
function of SSIS calibration PSL sphere diameter, is
constructed through the data points obtained. A
separate calibration curve is developed for each
channel.
7 Apparatus
7.1 Scanning Surface Inspection System — designed to
detect, size, and map localized light scatterers (LLSs)
on unpatterned semiconductor wafers, that has the
following capabilities:
7.1.1 Scans the entire fixed quality area of the surface
of a wafer with a laser beam,
7.1.2 Detects localized light scatterers as laser-light
scattering events,
7.1.3 Has a user definable sensitivity threshold,
7.1.4 Can generate a data set file of the distribution of
the detected LLSs as a function of reported size (LSE),
7.1.5 Can generate a histogram from the data set file,
or can output the data set file in a form that can be
imported to a spreadsheet or other application program
that can generate the histogram,
7.1.6 Is sufficiently repeatable for the intended
application, and
7.1.7 Handles wafers in a Class 4 or better clean
environment as defined in ISO 14644-1.
8 Reference Wafers
8.1 Substrates — Use bare semiconductor wafers with
a native oxide (or other filmed) surface of the type
intended to be tested with the SSIS to be calibrated as
substrates for the certified depositions of the PSL
spheres. This is particularly important because SSIS
response is affected by the optical properties of the
substrate. Semiconductor wafer surfaces, such as
unpatterned polished, epitaxial, or film layers, have
different optical properties. The wafers must meet the
dimensional requirements of SEMI M1 for the
appropriate nominal wafer diameter and must be laser
marked in a manner agreed upon between supplier and
user.
8.2 Range of Calibration Diameters — Choose the
diameters of the PSL spheres so that the measurement
range for the intended application is covered. Use
spheres of size ranging from the largest measurable size
down to a size with an estimated capture rate less than
50%. Do not exceed the dynamic range of the SSIS
channel being calibrated. Use sufficient sphere
diameters to achieve the required 5% PSL sphere sizing
accuracy of the response curve between calibration
points using a calibration curve that is produced by
fitting the calibration points. In the absence of other
criteria, choose spheres with diameter ratios of
approximately 1.7.
8.2.1 To accomplish the accuracy check (see Section
10.4.2), choose an additional set of PSL spheres of size
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approximately midway between adjacent pairs of the
original set.
8.3 Background Contamination
8.3.1 Handle and store reference wafers with great care
to avoid contamination and damage.
8.3.2 Establish that the bell-shaped peaks in the SSIS
LLS histogram of each reference wafer to be used,
which are generated from the PSL sphere depositions,
are well defined and well above the background level
over all of the response curve except near the threshold.
Also verify that each unimodal bell-shaped curve
extends to less than 50% of its peak value on both sides
of the peak within a diameter range of ± 15% of the
PSL sphere diameter at the peak of the distribution. If
the deposition on any reference wafer fails either of
these criteria, obtain a new reference wafer.
NOTE 3: In most cases, the LLS histogram peaks are wider
than the actual deposition diameter distributions. See Related
Information 1 for more details.
8.4 Data to Accompany Reference Wafers — A
certificate with the following information must
accompany each reference wafer.
8.4.1 For each CRM on the reference wafer provide:
8.4.1.1 The deposition peak diameter and the
uncertainty in accordance with the requirements of Row
3.3 of Table 3 of SEMI M52.
8.4.1.2 The maximum possible value of the deposition
diameter distribution full width at half max (FWHM)
expressed as a percent of peak diameter. (See Related
Information 1.)
8.4.1.3 The particle count of each CRM and the
associated sample coefficient of variation.
8.4.1.4 The approximate location of each CRM on the
reference wafer by the x- and y-coordinates (as
specified in SEMI M20) of the center of the deposition
area or by a map or drawing of the wafer.
8.4.2 Identification of the deposition system used for
the deposition by model and serial number.
8.4.3 The date of production.
8.4.4 Wafer identification by laser mark on 200 mm
and smaller wafers per SEMI M12 or 300 mm wafers
per SEMI M1.15 including alpha numeric message.
8.4.5 Name and address of the reference wafer
manufacturer.
8.4.6 Identification of the deposited PSL spheres by
manufacturer, lot number and model.
9 Procedure
9.1 Set up the SSIS in accordance with the
manufacturer's instructions for the wafer diameter, sizes
of PSL spheres, and other machine conditions to be
used during the calibration procedure. Ensure that
machine conditions are identical with those to be used
in examining wafers with the calibrated SSIS.
9.2 Ensure that the SSIS is operating properly for the
selected machine conditions.
9.3 Load the first reference wafer into the SSIS.
9.4 Scan the wafer.
9.5 Generate a data set file of the distribution of
localized light scatterers as a function of reported SSIS
signal.
9.6 Repeat Sections 9.3 through 9.5 for each of the
depositions on the remaining reference wafers and for
all of the SSIS channels.
10 Interpretation of Data
10.1 Construct a histogram for the data set from each
of the PSL sphere depositions used for the calibration.
10.2 Determine the standard deviation and peak value
from curve fits to the histograms from each deposition.
10.3 Associate each peak value of reported SSIS
signals determined in Section 10.2 to the certified value
of PSL sphere diameter for that CRM deposited on that
reference wafer.
10.4 Create and check the SSIS response curve.
10.4.1 To obtain the SSIS response curve make a fit to
the set of peak values in such a manner that the
response to PSL sphere deposition CRMs that size
between calibration points meets the uncertainty
requirements of SEMI M52.
10.4.2 Check the accuracy of the resulting SSIS
response curve by either measuring PSL sphere
deposition CRMs with locations mid-way between
calibration points, or by comparison to a modeled
result, or both.
10.4.3 If necessary repeat the curve fitting procedure
until the required accuracy is met.
10.5 Use this fitted response curve to establish the LSE
size of localized light scatterers with any particular
machine response value.
11 Report
11.1 Report the following information:
11.1.1 Operator identification;
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11.1.2 Date and location of measurement;
11.1.3 Manufacturer, model, serial number, and
software version of the SSIS;
11.1.4 Reference wafer characteristics as outlined in
the certificates accompanying the reference wafers (See
Section 8.4);
11.1.5 Histogram for each data set and the assigned
peak value of the distribution of reported diameters
together with the certified peak diameter of the PSL
sphere distributions used to generate the histogram; and
11.1.6 The curve fitted to the peak values and the
associated certified PSL diameters.