Background Statement for SEMI Draft Document 5905 REAPPROVAL OF SEMI PV15-0211 GUIDE FOR DEFINING CONDITIONS FOR ANGLE RESOLVED LIGHT SCATTER MEASUREMENTS TO MONITOR THE SURFACE ROUGHNESS AND TEXTURE OF PV MATERIALS 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. Background Per SEMI Regulations 8.9.1, the Originating TC Chapter shall review its Standards and decide whether to ballot the Standards for reapproval, revision, replacement, or withdrawal by the end of the fifth year after their latest publication or reapproval dates. The NA PV Materials TC Chapter reviewed and recommended to issue for reapproval ballot. Per SEMI Procedure Manual (NOTE 19), a reapproval Letter Ballot should include the Purpose, Scope, Limitations, and Terminology sections, along with the full text of any paragraph in which editorial updates are being made. Voter requests for access to the full Standard or Safety Guideline must be made at least three business days before the voting deadline. Late requests may not be honored. Review and Adjudication Information Task Force Review Committee Adjudication Group: International PV Analytical Test Methods, PV Materials NA TC Chapter Metrology, and Inspection TF Date: November 4, 2015 November 4, 2015 Time & Timezone: 8:30-10:00 AM PDT 10:30 AM -12:00 PM PDT Location: SEMI HQ SEMI HQ City, State/Country: San Jose, CA/USA San Jose, CA/USA Leader(s)/Authors: Hugh Gotts (Air Liquide) Hugh Gotts (Air Liquide) Lori Nye (Brewer Science) Standards Staff: Kevin Nguyen (knguyen@semi.org ) Kevin Nguyen (knguyen@semi.org ) This meeting s details are subject to change, and additional review sessions may be scheduled if necessary. Contact the task force leaders or 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. Check www.semi.org/standards on calendar of event for the latest meeting schedule.
SEMI Draft Document 5905 REAPPROVAL OF SEMI PV15-0211 GUIDE FOR DEFINING CONDITIONS FOR ANGLE RESOLVED LIGHT SCATTER MEASUREMENTS TO MONITOR THE SURFACE ROUGHNESS AND TEXTURE OF PV MATERIALS 1 Purpose 1.1 Many surfaces used in the photovoltaic industry are textured in order to optimize light absorption and maximize cell efficiency. The required texture usually is not well defined by a single roughness, gloss, or haze specification because the relative amounts of low and high frequency roughness on the surface can both be important. 1.2 Light scatter measured over a range of collection angles (or several individual angles) provides a fast, economical means to monitor, but not to quantify, texture over a range of roughness frequencies. Such measurements allow several degrees of freedom, such as incidence angle, scatter angle, light wavelength, aperture, spot size, etc., which need to be agreed upon between involved parties in order to be able to obtain comparable measurement results. Therefore means are required to uniquely define the measurement parameters, and to allow them to be easily exchanged. 1.3 This guide covers the language necessary to define scatter measurement conditions to enable measurements that describe changes in the scatter pattern that are present with differences in surface texture. It is not intended to distinguish good from bad or desirable from undesirable, to set scatter levels, or to determine the measurements to be taken. 2 Scope 2.1 This guide covers angle resolved light scatter (ARLS) measurements performed on monocrystalline (also known as single crystal) and multicrystalline (also known in some regions as polycrystalline) semiconductor wafer surfaces, transparent substrate surfaces, and coated transparent surfaces that may have been purposely modified (textured) to achieve desired reflective and/or transmissive properties. 2.1.1 Measurement of scatter from both reflective and transparent surfaces is well developed. The basic concepts for angle resolved light scatter (ARLS) measurements are published 1, 2 and the use of bidirectional scatter distribution function (BSDF) units to quantify scattered light has been standardized in SEMI ME1392 and ASTM E2837. Making use of these standard units allows scatter specifications to be written and consistent replication of monitoring systems between different locations. If the scattered light is reflected from the illuminated surface, the term is taken as the bidirectional reflective distribution function (BRDF) and if the scattered light is transmitted through the specimen, the term is taken as bidirectional transmittance distribution function (BTDF). 2.1.2 Many surfaces, textured to increase absorption for use in the photovoltaic industry, are in a roughness range that is too rough to give a strong specular reflection, but too smooth to completely diffuse the reflected light. ARLS measurements take advantage of the fact that the scatter from surfaces of this type depends strongly on texture characteristics. If the texture is changed, the scatter pattern changes. 2.2 This guide provides a framework for defining the conditions of light scatter measurement conditions, under a variety of measurement geometries, as a means to monitor roughness (or texture) on a variety of surfaces manufactured in the photovoltaic industry so that the surface texture can be kept within independently determined acceptable bounds. 2.3 Using scatter measurements to monitor texture does not depend upon knowledge of the detailed surface profile or roughness statistics. 2.4 Angle resolved scatter measurements are known to be more sensitive to texture changes than single value measurements such as haze, RMS roughness, and gloss, which tend to be dominated by low spatial frequency roughness. 1 Stover; J. C. Optical Scattering: Measurement and Analysis. SPIE Press, 1995. 2 Stover, J. C., and Hegstrom, E. L. Scatter metrology of photovoltaic textured surfaces. SPIE Proceedings, August 2010. Page 1 Doc. 5905 SEMI
NOTE 1: Haze is defined in SEMI M59 descriptively and without numeric values. This term is now also used in another context (for example, solar glass properties) and Haze Instrumentation is now sold. Fortunately, the numerical value for Haze in this new context is equal to the measured total integrated scatter (TIS), which is the subject of SEMI MF1048. Numerically, the measured reflective TIS (or Reflective Haze ) is: Scattered Power Integrated over a Specified Portion of the Sample Reflective Hemisphere Reflective TIS = (1) Total Reflected Power A similar definition is used for the measured transmissive TIS (or Transmissive Haze ): Scattered Power Integrated over a Specified Portion of the Sample Transmissive Hemisphere Transmissi ve TIS = (2) Total Transmitted Power Measured Haze (or TIS) values change with measurement parameters such as the source wavelength, incident angle, polarization, and scatter collection regions. As a result comparing Haze values found for samples where these parameters were different is meaningless. Thus, source wavelength, polarization, incident angle and the scatter collection regions need to be reported directly, or implied by identifying the measurement system used for the tests. 2.5 The specifications for surfaces regarding roughness or texture are expressed as the BSDF obtained with a defined parameter setting, and its specific subsets BRDF and BTDF for measurements performed in reflective and transmissive configurations, respectively. 2.6 If acceptable and unacceptable surface textures can be determined (e.g., by cell performance or some other parameter), then the related scatter patterns can be measured and scatter specifications can be written in BSDF units by identifying sections of the BSDF that have changed significantly. 2.7 Specific values for BRDF or BTDF as well as the measurement conditions are to be agreed upon between the parties to the test. The resulting specifications are flexible enough to accommodate variations in measurement related to different laboratory and production line geometries and situations. 2.8 Profilometer measurements of the surface texture/roughness are not covered in this guide. 2.9 This guide is an extension of SEMI ME1392. Understanding and using this guide requires the use of SEMI ME1392 or a similar document, such as ASTM E2387. 2.10 The guide does not address or limit how measurements are obtained. 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 The amount and nature of required texture/roughness differs widely for surfaces of materials and objects used in PV manufacturing. In general these surfaces are not optically smooth that is, they are not mirrors. Although the measured BSDF can be used to detect changes in surface texture under these conditions, it cannot be used to accurately calculate surface statistics such as rms microroughness (R q), arithmetic average microroughness (R a), and peak-to-valley ratio (R t). 3.2 ARLS measurements may be made over all or a portion of the surface area. Under the latter condition, care has to be taken to select measurement points that are representative of the entire surface. Note that surface nonuniformities affect the results. 3.3 The BSDF of a lambertian surface is independent of scatter direction. If a surface scatters nonuniformly from one position to another then a series of measurements over the sample surface must be averaged to obtain suitable statistical uncertainty. Nonuniformity may be caused by irregularity of the surface microughness or film, optical property inhomogeneity, or subsurface defects. Thus, the orientation of rough surfaces may result in different results being obtained for different angles. This is particularly true in the case of wafer surfaces that have a pattern of saw marks. Although most PV wafers do not have a fiducial, the 15-point measurement plan for square and pseudosquare silicon wafers in EN 50513 (and also cited in this guide) is aligned with the wire saw direction. 4 Referenced Standards and Documents 4.1 SEMI Standards Page 2 Doc. 5905 SEMI
SEMI M59 Terminology for Silicon Technology SEMI ME1392 Guide for Angle Resolved Optical Scatter Measurement on Specular or Diffuse Surfaces SEMI MF1048 Test Method for Measuring Reflective Total Integrated Scatter SEMI MF1811 Guide for Estimating the Power Spectral Density Function and Related Finish Parameters from Surface Profile Data 4.2 ASTM Standard 3 ASTM E2387 Standard Practice for Goniometric Optical Scatter Measurements 4.3 DIN Standard 4 DIN EN 50513 Solar Wafers Data Sheet and Product Information for Crystalline Wafers for Solar Cell Manufacturing NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions. 5 Terminology 5.1 Acronyms, terms, and symbols related to silicon technology, including many of those in this guide, are listed and defined in SEMI M59. 5.2 Additional terminology related to surface finish parameters is discussed in SEMI MF1811. 5.3 Other Acronyms Used in this Guide NOTE 2: Of these, BRDF is included in SEMI M59. 5.3.1 ARLS angular resolved light scatter 5.3.2 BSDF bidirectional scatter distribution function 5.3.3 BRDF bidirectional reflectance distribution function 5.3.4 BTDF bidirectional transmittance distribution function 5.3.5 CCBRDF cosine corrected bidirectional reflectance distribution function 5.3.6 CCBTDF cosine corrected bidirectional transmittance distribution function 5.3.7 PLIN plane of incidence 5.3.8 PV photovoltaic 5.3.9 RMS root mean square 5.3.10 TIS total integrated scatter 5.4 Other Terminology Used in this Guide NOTE 3: Many of these terms used to specify BSDF are taken or adapted from SEMI ME1392. Figures 1 and 2 adapted from SEMI ME1392 are useful for understanding these terms. BRDF and BTDF are the reflective and transmissive subsets of the more general BSDF. Other than the difference as to which side of the sample the scatter is measured, the definitions are identical. For convenience in these figures, the source direction is inverted when going from BRDF to BSDF. 5.4.1 angle of incidence, θ i, polar angle between the central ray of the incident flux and the Z B axis, normal to the sample surface. 5.4.2 beam coordinate system, X B Y B Z B a Cartesian coordinate system with the origin on the central ray of the incident flux at the sample surface, the X B axis in the plane of incidence (PLIN) and the Z B axis normal to the surface. 3 American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, Pennsylvania 19428-2959, USA. Telephone: 610.832.9585; Fax: 610.832.9555; http://www.astm.org 4 Deutsches Institut für Normung e. V., Burggrafenstrasse 6, D-10787 Berlin, Germany. Telephone: 49.30.2601.0; Fax: 49.30.2601.1231; http://www.din.de Page 3 Doc. 5905 SEMI
The angle of incidence, scatter angle, and incident and scatter azimuth angles are defined with respect to the beam coordinate system. 5.4.3 bidirectional scatter distribution function (BSDF) a description of the distribution of light scattered by a surface, it is the scattered power per unit projected solid angle divided by the incident power: Ps BSDF = PΩ cosθ i s 1 [ sr ] 5.4.4 cosine-corrected BSDF the scattered power per unit solid angle divided by the incident power: Ps / Ω cosine corrected BSDF = P i 1 [ sr ] 5.4.5 incident azimuth angle, φ i the angle from the X B axis to the projection of the incident direction onto the X B Y B plane. It is convenient to use the beam coordinate system shown in Figure 1, in which φ i = 180, since this makes φ s the correct angle to use directly in the familiar form of the grating equation. Conversion to a sample coordinate system is straight forward, provided the sample location and rotation are known. 5.4.6 incident direction the central ray of the incident flux specified by θ i and φ i in the beam coordinate system. 5.4.7 incident power, P i the radiant flux incident on the sample under test. 5.4.8 plane of incidence, PLIN the plane containing the sample normal (Z-axis) and the central ray of the incident flux. 5.4.9 receiver a system that generally contains apertures, filters, and focusing optics to gather the scatter signal over a known solid angle and transmit it to the scatter detector. 5.4.10 receiver solid angle, Ω the solid angle subtended by the receiver aperture stop from the sample origin. 5.4.11 sample coordinate system a coordinate system fixed to the sample and used to indicate the position on the sample surface for the measurement which is application and sample specific. The Cartesian coordinate system (X Y Z) shown in Figure 2 is recommended for flat samples. The origin is at the geometric center of the sample surface with the Z axis normal to the sample. 5.4.12 scatter the radiant flux that has been redirected over a range of angles by interaction with the sample. (3) (4) 5.4.13 scatter azimuth angle, φ s angle from the X B axis to the projection of the scatter direction onto the X B-Y B plane. Reflection Source P i Z B Specular Direction θ i θ s Scatter Direction P s Sample Surface Face Y B Solid Angle Ω φ s X B Origin O θ i Transmission Source P i Page 4 Doc. 5905 SEMI
NOTE: The plane of incidence is the PI-O-ZB plane. The scatter plane is the PS-O-ZB plane. In the case of reflective scatter (BRDF), the incident beam (Pi) is above the sample surface and in the case of transmissive scatter (BTDF) incident beam (Pi) is below the sample surface. Figure 1 Angle Conventions in the Beam Coordinate System Z Z B Y Y B Sample Surface α X X B Figure 2 Relationship between Sample and Beam Coordinate Systems 5.4.14 scatter direction, P s the central ray of the collection solid angle of the scattered flux specified by θ s and φ s in the beam coordinate system (see Figure 1). 5.4.15 scatter polar angle, θ s polar angle between the central ray of the scattered flux and the Z B axis. 5.4.16 scattering hemisphere a virtual hemispherical surface about which detectors are located. It is defined by the plane of the sample surface and the illumination spot on the sample surface. 5.4.17 specular direction the central ray of the reflected flux that lies in the PLIN with θ s = θ i and φ s = 0 (see Figure 1). NOTICE: SEMI makes no warranties or representations as to the suitability of the standard(s) set forth herein for any particular application. The determination of the suitability of the standard(s) is solely the responsibility of the user. Users are cautioned to refer to manufacturer s instructions, product labels, product data sheets, and other relevant literature respecting any materials or equipment mentioned herein. These standards are subject to change without notice. By publication of this standard, Semiconductor Equipment and Materials International (SEMI) takes no position respecting the validity of any patent rights or copyrights asserted in connection with any item mentioned in this standard. Users of this standard are expressly advised that determination of any such patent rights or copyrights, and the risk of infringement of such rights are entirely their own responsibility. Page 5 Doc. 5905 SEMI