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Background Statement for SEMI Draft Document 5532
NEW STANDARD: TEST METHOD FOR MEASUREMENT OF CRACKS IN PV SILICON WAFERS IN PV MODULES BY LASER SCANNING
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.
General Background
Cracks in silicon PV cells in PV modules are key parameters affecting module performance and reliability. Laser scanning (LS) is particularly advantageous in detection of cracks in all kinds of crystalline silicon cells. The standardized test method for cracks in PV Si wafers by LS is required for improving PV module performance and reliability. The corresponding SNARF was approved by Japan PV Materials Committee on November 14, 2012. Since then, the draft document has been developed.
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 Materials Task Force meeting / PV Materials Japan TC Chapter meeting
Date: / Monday, June 30, 2014 / Friday, July 4, 2014
Time & Timezone: / 13:30-17:00, Japan Time / 13:30-17:00, Japan Time
Location: / SEMI Japan Office / SEMI Japan Office
City, State/Country: / Tokyo, Japan / Tokyo, Japan
Leader(s): / Tetsuo Fukuda
(National Institute of Advanced Industrial Science and Technology) / Masaaki Yamamichi (Advanced Industrial Science and Technology),
Takashi Ishihara (Mitsubishi Electric),
Kazuhiko Kashima (GlobalWafers Japan)
Tetsuo Fukuda (National Institute of Advanced Industrial Science and Technology)
Standards Staff: / Chie Yanagisawa (SEMI Japan)
+81.3.3222.5863 / / Chie Yanagisawa (SEMI Japan)
+81.3.3222.5863 /
Task Force Review 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.
If you need a copy of the documents in order to cast a vote, please contact the following person within SEMI.
Chie Yanagisawa
SEMI Standards, SEMI Japan
Tel: 81.3.3222.5863
Email:
SEMI Draft Document 5532
NEW STANDARD: TEST METHOD FOR MEASUREMENT OF CRACKS IN PV SILICON WAFERS IN PV MODULES BY LASER SCANNING
1 Purpose
1.1 Multi-crystalline and mono-crystalline silicon wafers for photovoltaic (PV) cells are manufactured by casting, Czochralski technique or other controlled solidification methods.
1.2 In PV cell fabrication process using these silicon wafers, macroscopic defects such as chips, scratches, and cracks are often generated due to mechanical and thermal stresses. Chips and scratches can be origins for crack generation.
1.3 Cracked wafers are sometimes broken in module fabrication process causing lower productivity and cost increases. Also broken wafers in a module may result in degradation of the module reliability.
1.4 Therefore, in-line characterization methods are required that allow discriminately inspecting cracks with high through put and repeatability, and that measure the number and the length of those.
1.5 This method is based upon the measurement of Laser Beam Induced Current (LBIC) and can detect only cracks in wafers of a module by controlling bias voltage applied to the module.
2 Scope
2.1 This test method identifies and measures cracks in crystalline silicon wafers of a module.
2.2 This test method employs an in-line, non-contacting and non-destructive method for characterizing multi- and mono-crystalline silicon wafers of a module supported by a mechanism that moves the test specimen through the measurement equipment.
2.3 This test method covers square and pseudo-square silicon wafers of PV cells with nominal edge length≧125mm and a nominal thickness≧100mm.
2.4 It is more effective to operate this test method under the statistical process control (SPC, e.g. ISO 11462) to obtain the data with high reliability and repeatability.
2.5 This test method detects cracks from the distribution of leakage current density on the PV cell obtained by scanning its front side by laser.
2.6 Other measurement method may also provide similar information about the number and the length of cracks of wafers with this test method. Their subjects are, however, different from that of this test method.
2.7 This test method is also applicable to off-line measurement provided that the requirements of the test method are satisfied.
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 Minimum detectable size/dimension of a crack is affected by a resolution and frequency character of the detecting system.
3.2 Wafer temperature must be within 25±5℃.
3.3 Electrode pattern of PV cells and cycle of laser scanning may interfere with each other, which may cause interference patterns. In such a case the automatic recognition may be difficult.
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
4.2 ISO Standards
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
NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions.
5 Terminology
5.1 General terms, acronyms, abbreviations and symbols associated with silicon technology and used in this Standard are listed and defined in SEMI M59.
5.2 Other Abbreviations and Acronyms used in this document are defined as follows:
5.2.1 LS ― laser scanning
5.2.2 VOC ― open circuit voltage of cluster
5.2.3 ISC ― short circuit current of PV devices
5.3 Definitions
5.3.1 Cluster ― Series-connected photovoltaic cells protected by a bypass diode.
5.3.2 Cracked reference PV module (a reference module) ― PV modules in which positions and length of cracks are known beforehand.
6 Summary of Test Method
6.1 Inspection is done by controlling light sources in a cluster unit and laser-scanning by two rows.
6.2 Light sources should be a main light, laser beam, and sub lights with high directionality.
6.3 Wavelength of lights should range from visible radiation (red) to near infrared (up to about 1,000 nm).
6.4 Laser power should be arbitrarily adjusted to conform to types of PV cells.
6.5 Illumination intensity of sub lights should be set at the value with which ISC of PV cells is larger than that at laser-beam irradiation.
6.6 Irradiate a PV module with sub lights and scan cells with a laser light one by one.
6.7 A current induced in PV cells should be converted to voltage signals by a current-voltage converter.
6.8 Voltage signals are collected and converted to digital data.
6.9 For each scanning, data are recorded and stored as a line array.
6.10 Image processing software creates module images from line array data.
6.11 The system extracts liner dark areas appeared in images and reports existence or non-existence of cracks and the number, if any.
7 Apparatus
7.1 Schema of device compositions is shown in figure 1, optical unit in figure 2 and measurement system in figure 3 respectively.
7.2 Laser head ― A laser head, configured with a laser, a scanner mirror and an optical lens, irradiates a laser beam and scans the light receiving surface of a PV cell.
7.3 Sub lights ― Highly directional sub lights irradiate all cells except the cell under test.
7.4 Start sensor ― A start sensor detects a laser beam and generates a trigger signal for retrieving data.
7.5 Light unit ― A light unit is a unit comprising of items described in 7.2 to 7.4.
7.6 Bipolar power supply ― A bipolar power supply applies a voltage to PV modules.
7.7 Current-voltage converter ― A current-voltage converter amplifies a current of PV cells to a prescribed voltage value.
7.8 Equalizer ― An equalizer improves a current response of PV cells generated by laser- scanning.
7.9 AD converter ― An AD converter converts a current of PV cells to digital data and transfers them to a computer. Resolution should be 8 bit or more.
7.10 Computer ― Software described below is installed in a computer
7.10.1 Software for controlling the hardware ― Software for controlling the hardware controls a light unit, a current-voltage converter, an equalizer amplifier and a bipolar power supply.
7.10.2 Signal processing software ― Signal processing software filters data retrieved by an AD converter.
7.10.3 Image processing software ― Image processing software visualizes data processed by signal processing software beginning at detection of a start sensor signal, and determines cracks.
8 Safety Precautions
8.1 The entire machine must be contained in a housing and must be equipped with a security lock to stop the machine and to turn off power supply of all lights including a laser light when the housing is open.
8.2 If a laser light and other lights are infrared, which is invisible to human eyes, additional security measures must be taken to protect operators’ eyes during maintenance.
8.3 Operators and maintenance personnel must wear protective goggles in compliance with the IEC safety standard.
9 Preparation of Apparatus
9.1 Move a target board, an optical adjustment device, to a test position and check alignment.
9.2 Misalignment must be adjusted.
9.3 Move the laser head to a position of central coordinate of the target board, and turn on the laser light.
9.4 Confirm that a position of laser beam irradiation is aligned on the center of target board.
9.5 If not, collimate an optical axis of laser beam and align on the center of the target board.
9.6 Confirm that an output irradiation of laser power is at a prescribed value using a laser power meter.
9.7 If not, adjust the power to a prescribed value.
9.8 Run the scanner and laser-scan cells.
9.9 If a track of scanning is not on a printed line of the target board, adjust an angle and modify a course.
10 Calibration and Standardization
10.1 Calibrate the machine by use of one or more reference modules.
10.2 Reference modules should include multiple cracks.
10.3 Test one or more reference modules.
10.4 Adjust image processing parameters in order that the number of cracks detected should be within a prescribed deviation or accuracy range.
NOTE 1: The number of cracks of reference modules should be determined in agreement between a customer and a machine supplier.
11 Procedure
11.1 Insert (load) a PV module into the machine.
11.2 Align the PV module.
11.3 Connect contact probes with electrodes of the PV module.
11.4 Irradiate all clusters with sub lights.
11.5 Using the formula below calculate bias voltage VBIAS where a voltage on both ends of PV cell under test 1 is 0V.
VBIAS＝VOC（1-1/N)
N：Number of serial clusters
NOTE 2: 1 PV cell under test means PV cell under laser scanning.
11.6 When testing multi-crystalline PV cells with multiple grain boundaries that interfere with test images, bias voltage VBIAS should be set at or under Voc.
11.7 Apply bias voltage VBIAS with the bipolar power supply.
11.8 Start scanning by irradiating a light receiving surface of each PV cell with a laser beam.
11.9 Move the laser head along with the serial direction of a PV module at a constant speed to raster-scan PV cells.
11.10 The start sensor outputs a trigger signal when it detects a laser-scan beam.
11.11 When a trigger signal is received, the AD converter starts collecting induced current and stops after a prescribed collection period.
11.12 Switch off the sub lights whose location correspond to the position of PV cells under test.
11.13 Switch on/off sub lights according to a test object switches from one cell to the other.
11.14 Current induced in PV cells are retrieved through contact probes and converted into voltage signals by the current-voltage converter.
11.15 Waveforms of voltage signal are shaped by the equalizer.
11.16 Signals after waveform shaping are converted into digital data by the AD converter and transmitted to the computer.
11.17 For each scanning, data are recorded and stored as a line array.
11.18 Move the light unit to correspond to the next row in the PV module transferring direction after the first row of clusters is tested.
11.19 Test the second row according to the same procedures as the first row.
11.20 Test all the clusters according to procedures § 11.4 to § 11.19.
11.21 Line array data are stacked to produce a digital image by image processing software.
11.22 The digital images are processed and cracks or other defects are determined by image processing software.
11.23 The number and maximum length of cracks and defect dimensions are reported by image processing software.
11.24 Remove contact probes from PV module electrodes.
11.25 Remove (Unload) the PV module from the machine.
Figure 1
Device Composition
Figure 2
Light Unit