Background Statement for SEMI Draft Document 5433
NEW STANDARD: TEST METHOD FOR IN-LINE CHARACTERIZATION
OF PV SILICON WAFERS REGARDING GRAIN SIZE
Notice: This background statement is not part of the balloted item. It is provided solely to assist the recipient in
reaching an informed decision based on the rationale of the activity that preceded the creation of this Document.
Notice: Recipients of this Document are invited to submit, with their comments, notification of any relevant
patented technology or copyrighted items of which they are aware and to provide supporting documentation. In this
context, “patented technology” is defined as technology for which a patent has issued or has been applied for. In the
latter case, only publicly available information on the contents of the patent application is to be provided.
The grain structure of multicrystalline Si wafers or the newly-developed so-called "mono-like" Si wafers
for PV applications contains important information about the wafer’s position in the original ingot and
about the solidification process. Number and size of grains also may impact the efficiency of resulting
solar cells and the yield of manufacturing lines. For specifying such wafers it is important to know the
size and area distribution of grains or grain boundary length.
The corresponding SNARF was approved by the PV Materials Committee in its meeting in Munich on
June 13, 2012. The draft document was developed since July 2012 and was approved for yellow letter
ballot in cycle 4 or 5 2013 by the PV Materials Committee in its meeting in Munich on June 20, 2013, to
be adjudicated in Dresden in October 2013 in the meetings in conjunction with the SEMICON Europe.
The ballot results will be reviewed and adjudicated at the meetings indicated in the table below. Check
www.semi.org/standards under Calendar of Events for the latest update.
Review and Adjudication Information
Task Force Review
Committee Adjudication
Group:
PV Si Materials Task Force
Europe PV Materials Committee
Date:
October 7, 2013
October 7, 2013
Time & Timezone: 1:00-3:00 PM CEST
3:00-4:30 PM CEST
Location:
Messe Dresden
Messe Dresden
City,
State/Country:
Dresden, Germany
Dresden, Germany
Leader(s):
P. Wagner
H. Aulich, P. Wagner
Standards Staff:
Y. Guillou < yguillou@semi.org >
Y. Guillou < yguillou@semi.org >
This meeting’s details are subject to change, and additional review sessions may be scheduled if
necessary. Contact Standards staff for confirmation. Telephone and web information will be distributed
to interested parties as the meeting date approaches. If you will not be able to attend these meetings in
person but would like to participate by telephone/web, please contact Standards staff.
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DRAFT
SEMI Draft Document 5433
NEW STANDARD: TEST METHOD FOR IN-LINE CHARACTERIZATION
OF PV SILICON WAFERS REGARDING GRAIN SIZE
1 Purpose
1.1 Multicrystalline silicon (mc-Si) wafers consist of a multitude of crystallographically differently oriented grains.
The term multicrystalline includes also so-called “mono-like” Si wafers in the context of this test method.
1.2 The number and size of these grains vary significantly depending on the crystallization method and the wafer’s
position in the ingot.
1.3 Grain boundaries may be regions of high recombination activity in a multicrystalline wafer. They also may
getter unwanted impurities from the interior of the grains and diminish their detrimental impact on solar cell
conversion efficiency.
1.4 An appropriate solar cell manufacturing process can reduce the detrimental effects of grain boundaries and
internal grain defects.
1.5 An optimized grain size distribution for a specific solar cell manufacturing process is preferable.
1.6 A standardized test method for measuring the grain sizes and their distribution is required to establish wafer
specifications regarding grain sizes.
2 Scope
2.1 This test method evaluates dimensional characteristics of cross-sections of grains of mc-Si as they appear on a
wafer surface.
2.2 It employs an in-line, non-contacting and non-destructive method for characterizing clean, dry Si wafers that are
supported by a mechanism that move the test specimen through the measurement equipment.
2.3 The surface condition of the wafers may be as cut or as etched.
2.4 The test method covers square and pseudo-square Si wafers for photovoltaic (PV) applications, with a nominal
edge length ≥ 125 mm and a nominal thickness ≥ 100 µm.
2.5 The test method is intended for in-line high throughput measurements. Therefore it is mandatory to operate the
measurement system under statistical process control (SPC, e.g. ISO 11462) in order to obtain reliable, repeatable
and reproducible measurement data.
2.6 The test method is based on recording and evaluating images of the wafer surface obtained by a digital camera
under directional transmitted or reflected light illumination.
2.7 Two procedures for evaluating the grain size characteristics are defined. Both methods are based on obtaining a
digital image of the wafer surface displaying the grain structure that is digitally processed. The first procedure
straightforwardly evaluates the grain sizes; the second procedure follows a statistical approach used in metallurgy as
described in ASTM E112. The constraints of the second procedure must be checked before applying it.
2.8 The test may also be used for off-line characterization of Si wafers provided the requirements of the test method
are met.
2.9 Other measurement techniques may also provide similar information as compared to this test method, but they
are not the subject of this test method.
NOTICE: SEMI Standards and Safety Guidelines do not purport to address all safety issues associated with their
use. It is the responsibility of the users of the documents to establish appropriate safety and health practices, and
determine the applicability of regulatory or other limitations prior to use.
3 Limitations
3.1 This method is only valid for wafers which are cut by a slurry-based technology.
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3.2 The contrast between the wafer image and the background must be ≥ 10 %.
3.3 Weak contrast of the different grains of a wafer may result in erroneous grain size values.
3.4 Variations in illumination (incident angle, irradiance) and orientation of a specimen with respect to light source
and camera may impact the precision of repeated measurements.
3.5 Residue on the wafer surface also may impact the measurement results.
NOTE 1: The contrast of the different grains seen on a wafer surface depends on surface texture, illumination (wavelength,
incident angle, polarization) and orientation of the wafer with respect to light source and camera. Therefore repeated or
reproduced measurements of a specimen have to be performed under identical conditions regarding these parameters.
4 Referenced Standards and Documents
4.1 SEMI Standards and Safety Guidelines
SEMI E89 — Guide for Measurement System Analysis (MSA)
SEMI M59 — Terminology for Silicon Technology
SEMI MF1569 –– Guide for Generation of Consensus Reference Materials for Semiconductor Technology
4.2 ISO Standards1
ISO 11462-1 — Guidelines for implementation of statistical process control (SPC) – Part 1: Elements of SPC
ISO 11462-2 –– Guidelines for implementation of statistical process control (SPC) – Part 2: Catalogue of tools and
techniques
4.3 ASTM Standards2
ASTM E112 –– Standard Test Method for Determining Average Grain Size
ASTM E1382 –– Standard Test method for Determining Average Grain Size Using Semiautomatic and Automatic
Image Analysis
NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions.
5 Terminology
(Refer to the SEMI Standards Compilation of Terms (COTs) for a list of the current Abbreviations, Acronyms,
Definitions, and Symbols.)
5.1 Terms and acronyms relating to silicon and other semiconductor technology are defined in SEMI M59.
5.2 Other Abbreviations and Acronyms
5.2.1 MSA — measurement system analysis
5.3 Definitions
5.3.1 grain — a single-crystalline volume in the bulk of a material. Also used for denoting a cross section of the
grain seen on the surface of a slice through the bulk material.
5.3.2 grain boundary — the perimeter of a 2-dimensional cross-section of a grain.
6 Summary of Test Method
6.1 The wafer resting on a transport mechanism is moved in the x-direction, the direction of travel, past a
directional white light source and a digital matrix camera (see Figure 1).
6.2 For off-line measurement the wafer is positioned on an appropriate support.
1
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, http://www.iso.ch
2
ASTM International, 100 Bar Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, USA; Telephone: 1.877.909.2786 (USA &
Canada), or 1.610.832.9585 (International), http://www.astm.org
This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted Standard or Safety Guideline.
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6.3 A gray scale picture of the wafer is recorded by the digital camera and processed according to § 13 in order to
identify and to outline the grain boundaries.
6.4 Grain area characteristics (maximum, minimum, mean, size distribution) are evaluated according to two
procedures – denoted A and B – in § 14
6.4.1 Procedure A directly counts the pixels within the grains and reports the number of grains as well as their size
statistics.
6.4.2 Procedure B follows a statistical approach as described in ASTM E112 or ASTM 1382 for data analysis of
metallurgical samples resulting in an average grain size 3.
6.4.3 Procedure A and B may be used alternatively or jointly.
6.5 The results of the evaluation are reported according to § 14
7 Apparatus (see Figure 1)
7.1 Light source — the wafer is homogeneously illuminated by light sources Ln (n = 1, 2, …, N) with directional
light. The light sources illuminate the wafer under incident angles (with respect to the wafer surface normal) n and
wavelength n.
7.2 Camera –– a digital camera capable for taking an image with at least 2048 pixels resolution above the wafer
surface. The line of sight of the camera is perpendicular to the wafer surface and the distance camera-wafer surface
is ≥ 3 times wafer diagonal. The exposure time of the camera has to be set so that the image sharpness is better than
3 pixels. The noise level (3 ) of the camera shall be ≤ 1 % of the peak signal and the dynamic range shall be ≥ 255
gray levels.
7.3 Computer –– for controlling the measurement system and equipped with software for recording and processing
the camera images according to § 13
7.4 Wafer Transport — consisting of a mechanism that transports the wafer continuously through the measurement
apparatus without obstructing the line of sight of the camera and the illumination. The mechanism shall not leave
traces or residue on the wafer surface.
8 Safety Precautions
8.1 The entire equipment must be placed in a closed housing and secured with a safety lock that stops the belts and
switches the tool off when the housing is opened if lasers are used for illuminating the wafer.
9 Test Specimens
9.1 Clean, dry Si wafers with as-cut or etched surfaces.
10 Preparation of Apparatus
10.1 The suitability of the equipment is determined by performing a statistically based MSA to ascertain whether
the equipment is operating within the manufacturer’s stated specification, e.g. according to SEMI E89.
10.2 Verify that the camera, illumination and wafer are aligned and adjusted according to the manufacturer’s
specifications.
10.3 Define the SPC control limits for the measurement equipment with a set of selected wafers.
NOTE 2: As this test method is intended for a high throughput, high volume measurement the equipment cannot be calibrated
for measuring each individual wafer. Therefore careful SPC has to be performed.
11 Calibration and Standardization
11.1 Verify that the camera, illumination and wafer are aligned and adjusted according to the manufacturer’s
specifications.
3
M. Schumann, T. Haas, T. Orellana Pérez, S. Riepe, Proc. of the 26th European Photovoltaic Solar energy Conference, Hamburg, 2011, H.
Ossenbrink, A. Jäger-Waldau and P. Helm, Eds., DOI: 10.4229/26thEUPVSEC2011-2BV.4.22
This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted Standard or Safety Guideline.
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11.2 The equipment is calibrated by using a reference wafer with known dimensions.
11.3 Measure the reference wafer according to § 12
11.4 Compare the measured side lengths with the known dimensions of the reference wafer and determine
correction factors fx and fy for the x- and y directions, respectively.
11.5 Calculate the effective pixel area of the camera.
12 Procedure
12.1 Adjust the equipment and calibrate it according to the supplier's instructions.
12.2 Determine the calibration factors fx and fy.
12.3 Verify the equipment is within SPC limits.
12.4 Measure the wafer.
12.4.1 Place a wafer on the transport mechanism so that its surface normal is tilted ≤ 3 deg with respect to the line
of sight of the camera or put it on a support for off-line measurement
12.4.2 Align the wafer so that its leading edge is perpendicular to the transport direction.
12.4.3 Move the wafer through the measurement station.
12.4.4 Take a raw image RI and process it according to § 13
12.4.5 Report the grain size distribution according to § 14
12.5 Repeat with the next wafer.
13 Calculations and Image Processing
13.1 Process the raw wafer image RI according to the five main steps (Figure 2):
13.2 Step 1: wafer edge detection
13.3 Step 2: filtering
13.4 Step 3: identification of grain boundaries
13.5 Step 4: determination of grain sizes
13.6 Step 5: counting of grains and determination of grain sizes.
13.7 The following calculations are performed automatically within the instrument. An outline of the calculation
structure is provided here to indicate the nature of the procedure.
13.8 Assign a rectangular coordinate system to the image so that the x-axis is parallel to the wafer transport
direction and the y-direction perpendicular to it. The origin of the coordinate system is at the lower left corner of the
camera image.
13.9 Identify each pixel by a pair (x, y) of integer index numbers, e.g. (215, 30), corresponding to the indices of the
pixel in line 30 of column 215 of the pixel array of the image.
13.10 Identify the gray-scale image brightness of the pixel at (x, y) by I(x, y).
13.11 The binary operation of thinning is used in the image processing steps. Use the structure elements E1 to E8
displayed in Fig. 2 for thinning.
NOTE 3: For thinning, the structure elements are applied repeatedly on the binary image in the order they are displayed in Fig. 2
until no changes in the image occur.
13.12 Other structure elements as the ones displayed in Fig. 2 may be used, but then their size and pattern shall be
reported.
13.13 Step 1, wafer edge detection:
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13.13.1 Apply an appropriate filter to the raw image RI for detecting the wafer edges and define the x-y-coordinates
representing the wafer edges. Report the type of filter used.
13.14 Step 2, filtering:
13.14.1 Optionally: Smooth the entire RI first with a Gaussian filter and subsequently with a Median filter to obtain
the filtered image FI. Report the filter characteristics.
NOTE 4: Smoothing RI levels contrast variations within grains, if necessary.
13.15 Step 3, identification of grain boundaries:
13.15.1 Enhance the contours in FI by applying a Sobel4 filter in x- as well as in y-direction. Report the filter
characteristics.
13.15.2 Define two thresholds th1 and th2 so that th2 < th1.
13.15.3 Color all pixels in FI with I(x,y) above th1 e.g. blue and all pixels with I(x,y) above th2 e.g. red.
13.15.4 Perform a thinning operation on all marked pixels so that only 1 pixel wide lines are obtained.
13.15.5 Discard all blue pixels that are not immediately adjacent to red pixels.
13.15.6 Identify open ends of lines and color them e.g. green.
13.15.7 Connect close-by green pixels, which are separated from each other by a gap ≤ g1 pixels, so that regions Gi
with closed loop boundaries are obtained that represent the grains. The index i denotes a specific grain.
13.15.8 Connect the still remaining open ends with the nearest grain boundary that is ≤ g2 pixels in x- or y-direction
away from the open end.
NOTE 5: Steps ¶¶ 13.15.6 – 13.15.8 may be skipped if method B (see ¶ 13.16.2) is used for further evaluation.
13.15.9 The connected regions with closed loop boundaries represent the grains Gi, where the index i denotes the
individual grains.
13.15.10 Report th1, th2, g1, g2 .
13.16 Step 5, counting grains and determination of grain sizes. Two methods for characterizing the wafer may be
applied:
13.16.1 Method A:
13.16.1.1 Count the number NG of grains Gi.
13.16.1.2 Determine the area Ai of each Gi in units of mm2.
13.16.1.3 Determine Amax as the maximum Ai in units of mm2.
13.16.1.4 Determine Amin as the minimum Ai in units of mm2.
13.16.1.5 Calculate the mean grain area Amean in units of mm2.
13.16.1.6 Define a threshold area Ath in units of mm2.
13.16.1.7 Count the number NGth of grains with an area ≤ Ath.
13.16.1.8 Report NG, NGth, Amax, Amin and Ath.
13.16.2 Method B (according to ASTM E112):
13.16.2.1 Inscribe concentric squares Qm (m = 1, 2, …, M) on the images of the wafer surface with side lengths sm
in mm , so that the squares are centered and their sides are nominally parallel to the wafer edges (see Figure 3). M
shall be ≥ 3.
13.16.2.2 Choose the side length s1 of the smallest square.
13.16.2.3 Choose a distance s between the squares.
4
See http://en.wikipedia.org/wiki/Sobel_operator
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13.16.2.4 The side length sq (q = 2, …, M ) of the remaining squares are determined by
sq  sq1  2s
(1)
as long as sq ≤ w –2d, where w is the nominal wafer side length, d is a specified distance from the wafer edge (edge
exclusion) and M is the index of the largest square meeting the requirement.
13.16.2.5 Count the number Im of intersections of the squares Qm with the boundaries of the grains Gi.
13.16.2.6 Determine the length lmj in mm of the individual lineal intercept (m = 1, 2, ..., M, j = 1, 2, ... , Im) of the
squares with the grains.
NOTE 6: For stable statistics the sum of all Im should be ≥ 100.
13.16.2.7 Calculate the average intercept length lmean as follows
13.16.2.7.1 Calculate the mean lineal intercept lmean,m by
l mean,m 
sm
Im
(2)
13.16.2.7.2 Calculate the total mean lineal intercept lmean by
lmean 
1
M
M
l
m 1
(3)
mean,m
13.16.2.7.3 Calculate the standard deviation sl for lmean by
Im
M
sl 

(lmj  lmean ) 2
m1 j 1
(4)
MI m  1
13.16.2.7.4 Determine the distribution of the intercept lengths lmj by generating a histogram with B equally sized
bins (B ≥ 10) of width b and report the counts Cn per bin (n = 1, 2, ..., B). Report both B and b.
NOTE 7: The bin limits may be determined by measuring a set of wafers selected by the user of the document before measuring
unknown specimens.
13.16.2.7.5 Define a length threshold lth , count the number S of grains where lmj < lth and report S.
13.16.2.7.6 Calculate the corresponding total approximate average grain size Amean by
Amean 
4

lmean
2
(5)
NOTE 8: Equation 3 is an approximation and fully correct only for circular grains.
13.16.2.7.7 Calculate the standard deviation sA for Amean by
M
sA 
Im

m 1 j 1
4 2
( lmj  Amean )2

(6)
MI m  1
13.16.2.7.8 Optionally, calculate the grain size number GN according to ASTM E112 by
GN  6.643856  log 10 lmean  3.288
(7)
NOTE 9: For large grains GN may become negative.
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Document Number: 5433
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14 Report
14.1 The report shall contain the following elements.
14.1.1 General information
14.1.1.1 Date and time of test.
14.1.1.2 Identification of operator.
14.1.1.3 Identification number of measurement equipment.
14.1.1.4 Software revision.
14.1.1.5 Calibration and SPC status of measurement equipment.
14.1.1.6 Ambient temperature.
14.1.1.7 Lot identification, including each wafer ID.
14.1.2 Equipment
14.1.2.1 Illumination mode used, transmitted or reflected light.
14.1.2.2 Number N of light sources.
14.1.2.3 Incident angles n of light sources.
14.1.2.4 Wavelength n of light sources.
14.1.3 Grain parameters
14.1.3.1 Method used, A or B
14.1.3.1.1 Method A:
14.1.3.1.1.1 Number of grains NG.
14.1.3.1.1.2 Number of grains below threshold NGth.
14.1.3.1.1.3 Maximum grain area Amax.
14.1.3.1.1.4 Minimum grain area Amin.
14.1.3.1.1.5 Mean grain area Amean.
14.1.3.1.1.6 Threshold area Ath.
14.1.3.1.2 Method B:
14.1.3.1.2.1 Total mean intercept length lmean.
14.1.3.1.2.2 Standard deviation sl of lmean.
14.1.3.1.2.3 Mean grain area Amean.
14.1.3.1.2.4 Standard deviation sA.
14.1.3.1.2.5 Number of histogram bins B and bins width b.
14.1.3.1.2.6 Counts Cn per bin for all B bins.
14.1.3.1.2.7 Threshold thl.
14.1.3.1.2.8 Number S of grains with lmj < thl.
14.1.3.1.2.9 Grain size number GN, if required.
14.1.4 Image processing parameters
14.1.4.1 Filter characteristics of Gaussian and Median filter, if applicable.
14.1.4.2 Filter characteristics of Sobel filter.
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14.1.4.3 Thresholds th1 and th2.
14.1.4.4 Gap parameters g1 and g2.
14.1.4.5 Parameters of thinning operation, if other structure elements are used than the ones displayed in Fig. 2.
15 Precision and Bias
15.1 No certified reference materials currently exist for establishing the accuracy of measurement equipment that
can be used with equipment as described in this test method. Therefore, bias of equipment performing these
measurements has not been developed.
15.2 Provisional reference materials may be qualified by using appropriate equipment that is capable measuring
certified reference materials as well as silicon wafers (SEMI MF1569).
15.3 In the absence of inter-laboratory test data to establish its reproducibility this test method should be used for
materials specification and acceptance only after the parties to the test have established reproducibility and
correlation.
digital camera
directional
light source
directional
light source
trailing edge of
wafer
surface normal and line
of sight of camera
wafer
leading edge of wafer
transport belts
wafer transport
direction, x-direction
NOTE: The light-gray dashed lines depict the field of view of the camera and the illumination of the wafer by the light sources.
The dashed dotted lines are supplementary lines for defining the center of the wafer. More than two light sources may be used.
The belts represent generically the transport mechanism.
Figure 1
Schematic Drawing of the Set-up for Recording the Wafer Image in Reflected Light Mode
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0
1
1
1
0
0
1
1
1
0
1
1
1
0
1
E1
1
1
1
0
0
1
E2
1
0
E5
1
0
1
1
0
1
1
0
0
0
0
E3
1
1
0
0
1
1
0
0
0
1
0
E6
E4
1
0
0
1
0
1
1
1
1
1
E7
E8
NOTE: The origin of the structure elements E1 to E8 is at their centers.
Figure 2
The Structure Elements to Be Used for the Thinning Operation
w
d
wafer
s
squares for evaluating
grain size
s1
s2
s3
NOTE: Dash-dotted lines are supplementary lines for identifying the wafer center.
Figure 3
Schematic Drawing Outlining the Position of Three Squares for Evaluating the Average Grain Size
According to ASTM E112
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