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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 AdjudicationGroup: / 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 / Y. Guillou
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.
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.
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 Standards[1]
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 Standards[2]
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.
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) an and wavelength ln.
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 s) 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.
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:
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.