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Ellipsometry is a powerful tool, which allows the investigation of material properties over a broad spectral range. Over the course of several years, the ellipsometry lab at BAM has become an accredited testing lab according to ISO/IEC 17025 laying bare the need of better methods for accuracy and traceability. Despite its wide range of application in both research and development as well as industry, there have been no generally accepted standards dealing with model validation and measurement uncertainties.
Based on the first German standard DIN 50989 – 1: 2018 Ellipsometry - Part 1: Principles (currently international standard ISO 23131: 2021) and under consideration of GUM [1] a series of standards for ellipsometry was developed. The entire 6-part series covers several model-based application cases. This standards series avoids having narrow and material specific application cases but instead classifies applications of ellipsometry according to the sample complexity. The concept of ellipsometric transfer quantities (Ψ and Δ or alternatively the elements of transfer matrices) is implemented in the series. For each application case a model-based validation strategy was developed. Thus, the standards are applicable to all materials, instruments and measuring principles.
The uniform structure concept of the series facilitates its practical applicability for users. The standards include the model-based GUM-compliant determination/estimation of the measurement uncertainties. In addition, the appendices of the documents contain numerous measurement and simulation examples as well as recommendations for measuring practice.
In this contribution we present the application cases and basic structure of the standards developed in collaboration with Accurion GmbH and SENTECH Instruments GmbH in the project SNELLIUS.
Ellipsometry has become a powerful measurement tool in semiconductor industry since the sixties of the last century.
Early standardization activities focused exclusively on SiO2/Si (ASTM F 576-01, SEMI 3624). The first generic standard dealing with ellipsometry is DIN 50989-1:2018 Ellipsometry – Part 1: Principles. Standardization is a prerequisite for accreditation according to DIN EN ISO/IEC 17025 and the evaluation of uncertainty budgets.
The paper addresses “WHITE LIGHT INTERFERENCE MICROSCOPY, ELLIPSOMETRY, AND INSTRUMENTED INDENTATION TESTING AS REFERENCE PROCEDURES“ and is subdivided in the following sections: 1. Measurement vs. testing vs. reference procedures, 2. White light interference microscopy (WLIM) for determination of step height h, 3. Spectroscopic ellipsometry (SE) for determination of layer thickness d and 4. Instrumented indentation testing (IIT) for determination of indentation hardness HIT. The following points are discussed in more detail: methodology of measurement and testing, uncertainty budgets for direct (WLIM), model-based (SE) and formula-based (IIT) reference procedures by means of specific examples. It is shown that standardization efforts are connected to reference procedures.
The comprehensive knowledge of material properties as engineered and the material behavior in use is of huge importance for the functionality and reliability of processes and products. Hence, the accurate determination of material properties is a prerequisite for the applicability of a given material. Nowadays, macroscopic product features are originated by material properties on the microscopic and nanoscopic scale. This is a general challenge for uncertainty budgets in measurement and testing based on a conformity assesment.
Testing labs accreditated according to DIN EN ISO/IEC 17025 have to meet these demands. This paper focuses on the measurement of physical quantities (measurand with value and unit) and on the testing of material properties (qualitative, semi-quantitative or quantitative characteristics) of solid state materials. For both measurement and testing, the expression of uncertainty according to GUM is essential. Regarding the object/sample one has to subdivide in surface and bulk features, from the measurement/testing point of view destructive vs. nondestructive procedures have to be distinguished. Moreover, direct mesurement/testing and model-based determination of quantities/properties of interest require different approaches regarding the expression of uncertainty. For surface quantities and properties as discussed here, further considerations have to be made with respect to localized (lateral extension, native or artificial) or stratified (homogeneity and isotropy) material features vs. mapping (integration area) and imaging (lateral resolution and field of analysis) specifications of the measurement/testing procedure.
The concept of uncertainty budgets and the expression of uncertainty is introduced and discussed in more detail for the nondestructive model-based determination of thickness by spectroscopic ellipsometry (SE), the destructive determination of adhesion by means of centrifugal adhesion testing (CAT). It is shown that the expression of uncertainty for these examples and real materials requires different approaches resulting in either quantitity values with unit or qualitative/quantititative attributes with calculated/estimated uncertainties in agreement with GUM.