A Practical Guide to Surface Metrology

By Michael Quinten

This book offers a genuinely practical introduction to the most commonly encountered optical and non-optical systems used for the metrology and characterization of surfaces, including guidance on best practice, calibration, advantages and disadvantages, and interpretation of results. It enables the user to select the best approach in a given context.

Most methods in surface metrology are based upon the interaction of light or electromagnetic radiation (UV, NIR, IR), and different optical effects are utilized to get a certain optical response from the surface; some of them record only the intensity reflected or scattered by the surface, others use interference of EM waves to obtain a characteristic response from the surface. The book covers techniques ranging from microscopy (including confocal, SNOM and digital holographic microscopy) through interferometry (including white light, multi-wavelength, grazing incidence and shearing) to spectral reflectometry andellipsometry. The non-optical methods comprise tactile methods (stylus tip, AFM) as well as capacitive and inductive methods (capacitive sensors, eddy current sensors).

The book provides:

• Overview of the working principles
• Description of advantages and disadvantages
• Currently achievable numbers for resolutions, repeatability, and reproducibility
• Examples of real-world applications

A final chapter discusses examples where the combination of different surface metrology techniques in a multi-sensor system can   reasonably contribute to a better understanding of surface properties as well as a faster characterization of surfaces in industrial     applications. The book is aimed at scientists and engineers who use such methods for the measurement and characterization of
surfaces across a wide range of fields and industries, including electronics, energy, automotive and medical engineering.

 

• Language: English
• Publisher: Springer
• Release date: Jan 1, 2020
• ISBN: 9783030294540

Book preview

© Springer Nature Switzerland AG 2019

M. QuintenA Practical Guide to Surface MetrologySpringer Series in Measurement Science and Technologyhttps://doi.org/10.1007/978-3-030-29454-0_1

1. Introduction to Surfaces and Surface Metrology

Michael Quinten¹ 

(1)

Aldenhoven, Germany

I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of science, whatever the matter may be.

Lord Kelvin (1824–1907)

Abstract

Components and systems used in and manufactured for technical applications consist of two distinct areas, a bulk and a surface. The surface of a workpiece forms the interface to the surrounding. Its status is not invariant. Rather, water (vapor) and air and all in water or air solved or dispersed substances as well as temperature effects alter the surface gradually even without human influence. Changes that even affect the whole workpiece, like melting, corrosion, or abrasion, start at the surface. A technical surface can also carry signatures of processing techniques like grinding, lapping, or pitch polishing. Moreover, the workpiece can be given new properties by structuring, deposition, or coating that affect primarily the surface. In summary, the surface often determines the functionality and the appearance of the component and is of importance for the proper function and performance of the manufactured component. Therefore, it is essential for product development, process control, quality control, and failure analysis to examine the surface of a workpiece.

Already in the first third of the twentieth century many examinations on technical surfaces were carried out and published. In 1936 Gustav Schmaltz published the book Technische Oberflächenkunde [1] where he carefully reviewed all known insights and findings up to this date. With this book he made a big contribution to the development of surface metrology as an autonomous scholarship.

For examination of the surface several measuring techniques had been developed. Some of them mainly analyze the chemical composition of the surface. This group of methods includes

Scanning Electron Microscopy with Energy Dispersive X-ray Analysis (EDX)

Secondary Ion Mass Spectroscopy (SIMS),

Glow Discharge Spectroscopy (GDS),

Auger Electron Spectroscopy (AES),

Photo Electron Spectroscopy (UPS, XPS, ESCA),

Small Angle X-ray or Neutron Scattering (SAXS, SANS), and some more.

Another group takes an image of the surface and evaluates the 2D information with means of digital image processing for pattern recognition and defect inspection. For the main part this is done in industrial image processing. A second well-established imaging method for pattern recognition and defect inspection on surfaces is scanning electron microscopy. Further imaging methods well-established in other branches are hyperspectral imaging, optical coherence tomography, and terahertz spectroscopy that become more and more relevant also for surface analysis.

The biggest group of methods is the group of methods for examination of form deviations, topography, roughness, and waviness. It comprises

Tactile Profiling,

Scanning Atomic Force Microscopy (AFM),

Capacitive and Inductive Surface Profiling,

Confocal Optical Profiling,

Light Sectional Methods,

Various Microscopy Methods,

Various Interferometric Methods,

Wave Front Sensing,

Deflectometry, and

Elastic Light Scattering.

Complementary to these methods, spectral reflectometry and ellipsometry for determination of layer thickness and optical constants are used.

The intention of this book is to introduce in most of these surface metrology techniques except of those for the chemical composition and methods for hardness and friction. But before going into detail of the measuring techniques in the following chapters, this chapter provides first a closer look on the surface itself in a microscopic view and a macroscopic view and on the measurement of surface properties and its validation.

1.1 Microscopic View on a Surface

Considering solid state matter it can be distinguished between a regular arrangement of the atoms which can be described by an elementary cell that is regularly continued within the bulk, and an amorphous structure where elementary cells are not regularly continued but are cross-linked at variable distances. Looking primarily at the regular crystalline structure, the crystal lattice, each atom has a distinct number of next neighbors as long as it is in the bulk. This is dominantly caused by the electronic structure of the single atom and the electronic interaction among the atoms. Each atom is arranged in the crystal so that the resulting electronic states are states of equilibrium of minimal energy.

When approaching the end of the bulk, this is the surface as interface between the solid matter and the surrounding, typical properties of a bulk solid exhibit an abrupt change at a surface. One of the most evident changes occur in the crystalline order of the bulk. It gets disturbed since the atoms in the outmost atom layer do have a lower number of neighbors. Then, the electronic interaction among the atoms leads to a new arrangement of the atoms in the surface with electronic states of minimal energy that are different from that in the bulk. Photoelectron spectroscopy actually reveals that the atoms in the surface exhibit so-called surface states.

In electrochemistry and nanoscience the jellium model [2] is a well-established simple model for separating the conduction electrons of a metal body from the ions. In this model the ions appear as a constant positive charge background with the task to compensate the integrated negative charge of the electrons. While the ionic body has an atomically sharp surface, the electronic body exhibits a smooth transition of the electron density due to the finite length of the electron waves, resulting in negative charge density outside the body. This is called spill-out effect. The electron density varies in an oscillating manner, the so-called Friedel oscillations. This is illustrated in Fig. 1.1 where the gray region is the constant positive charge background. As a consequence from the jellium model the surface of a metal body is soft and the surface region cannot longer be approximated by a sharp two-dimensional plane but forms a three-dimensional extended area. Hence, the question may arise what is in general the extension of the surface? Is it only the outmost atom layer or does it extend into the bulk? How deep does it extend into bulk? The answers to these questions are not clear. The extension of the surface namely depends upon the considered property as the above example of the electron density of metals shows.

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Fig. 1.1

Normalized charge densities of ions and free electrons at a metal surface according to the jellium model

While the different atomar arrangement in the surface only extends over maximum three or four atomic layers, the relevant surface is quite larger when considering interaction with electromagnetic radiation. E.g., the skin depth in metals is in the order of 20–30 nm for radiation from the soft UV to the radio frequency range. After this distance the intensity of the radiation is diminished to 1/e² = 0.135 of the maximum intensity. Metallic nanoparticles with sizes in this order or metallic films with thickness in this order are therefore surface, a surface that extends over 100–150 atomic layers! Another extreme example is the recently discovered graphene. Graphene is a monolayer of linked benzene rings with extraordinary properties. For this atomar carbon layer the difference between surface and bulk has vanished in the third dimension. But also for machined surfaces different boundary layers can be distinguished. The inner boundary layer is the zone where the workpiece is altered due to the machining process. It is covered by the outer boundary layer, a very thin zone where the atoms and molecules are exposed to external forces and chemical reactions. Below the inner boundary layer the structure of the workpiece is that of the crystal lattice.

The extent of the surface is relevant also in manufacturing. E.g., when manufacturing white light LEDs from gallium nitride (GaN) the GaN is deposited on synthetic sapphire or spinel. However, the crystalline structures of GaN and sapphire do not match. The mismatch causes enormous stress and strain at the interface of both materials that can lead to detachment of the GaN. Therefore, GaN is deposited in an epitaxial process where the first GaN atoms brought up form atomar layers that gradually change from the crystalline structure of the sapphire to the crystalline structure of GaN. Stress and strain play also an important role when coating a semiconductor wafer surface with a thin film of even a few 10 nm thickness. The enormous stress of several GPa deforms the wafer which on the other hand affects the further processing of the wafer.

The atoms in the outermost surface area of a common workpiece are permanently in contact with the atoms and molecules in the surrounding medium. In consequence, chemical reactions like oxidation start at the surface. Furthermore, atoms and molecules from the surrounding can condensate on the surface, are adsorbed on the surface, and can build thin contamination films. This is important for example in atomic force microscopy. Adsorption processes are involved in almost all technological processes in which surfaces play a crucial role. The most prominent example is heterogeneous catalysis since usually the reactants have to adsorb on the catalyst before they can react. Traditionally, one-dimensional potential curves are used to describe adsorption. The most prominent one goes back to the semi-empirical formula of John Lennard-Jones [3] that describes the interaction potential between atoms with distance R. It is a combination of the attractive potential of the van-der Waals forces (∝ −1/R⁶) and a repulsive potential (∝ 1/Rm, with m mostly m = 12).

But not only atoms and molecules from the surrounding play a role. A certain number of atoms of the solid matter leaves the solid matter and forms a gas in front of the solid. The maximum number of atoms or molecules that can leave the solid matter is given by the temperature dependent saturated vapor pressure. It increases with temperature. In thermal equilibrium the atoms in the gas phase permanently condensate on the surface but are replaced by other atoms evaporating from the surface region. Condensation and evaporation are driven not only by the temperature but also by the surface tension or better the surface free energy that is proportional to the area and forces evaporation, and the energy for building of bulk matter that is proportional to the volume and forces condensation. The interplay of condensation and evaporation is theoretically described by the nucleation theory of Volmer and Flood [4]. Many effects like the Ostwald ripening in colloidal matter or the latent image in the photographic process can be explained by this way.

Another important quantity of a surface is the work function. It corresponds to the minimum energy needed to bring an electron from the solid to a point close to the surface but outside the solid surface. The work function is a characteristic property of the surface of the material depending on the crystal face and the contamination of the surface.

Finally, the surface diffusion is a general process that involves the motion of adatoms, molecules or atomic clusters at the surface. Similar to the bulk diffusion it is a thermally promoted process with rates increasing with increasing temperature. As the surface diffusion rates and mechanisms are affected by a variety of factors such as the strength of the surface-adparticle bond, the orientation of the surface lattice, attraction and repulsion between surface species, and chemical potential gradients, it is important, e.g. in surface phase formation or epitaxial growth.

1.2 Macroscopic View on a Surface

Unlike for natural surfaces, for technical surfaces manufacturing, processing, and conditioning will leave traces on the surface of a workpiece so that it will deviate from the intended surface. One distinguishes three types of technical surfaces. They are illustrated in Fig. 1.2.

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Fig. 1.2

Illustrations of (a) the nominal surface, (b) the real surface, (c) the measured surface

The nominal surface is the intended target surface when manufacturing the workpiece. The shape and extent of a nominal surface are usually shown and dimensioned on a drawing. The real surface is the actual surface obtained after manufacturing the workpiece. The real surface differs from the nominal surface since it carries signatures of the processing. The measured surface is a representation of the real surface obtained with some measuring instrument. Each measurement method yields an image that is influenced by the measuring method and its resolution. Hence, different measuring methods may result in different images of the same real surface. Are the obtained surface characteristics then actually representative? Of course, a comparison is then possible if the measurements are carried out with the same measurement parameters and the same evaluation methods.

Beyond the deviations caused by the processing the manufactured workpiece may additionally exhibit deviations in its shape from the ideal shape given in the design drawing. These shape deviations can be divided in coarse shape deviations and smooth shape deviations. Both can further be divided into sections as shown in Fig. 1.3.

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Fig. 1.3

Division of shape deviations

Dimensional (Gauge) Deviations are deviations of the dimensions of the workpiece from the tolerances, e.g. the length of a cylinder. Position Deviations are deviations in the position of a single element of the workpiece from the ideal position, e.g. the actual position of drill holes.

Form Deviations are deviations of the real surface from the nominal surface. They include straightness, flatness, roundness, cylindrical form, line shape, and area shape. These deviations result from large scale problems in the manufacturing process such as errors in machine tool ways, guides, or spindles, insecure clamping, inaccurate alignment of a workpiece, or uneven wear in machining equipment. They can affect the performance or the lifetime of a workpiece.

Waviness and Roughness are deviations of the real surface from the nominal surface excluding position and form deviations. Roughness includes the finest (shortest wavelength) irregularities of a surface. Roughness generally results from a particular production process or material condition, e.g. the movement of the cutting tool. Waviness is an unwanted effect coming from the machine tool and is almost always present.

Surface topography measurement is mainly concerned with form deviations, waviness, and roughness. Topography measurement means the exact quantitative determination of the geometry and/or the micro structure of technical surfaces. Roughness and waviness characterize the optical or haptic impression of the surface of the specimen. These quantities are compared with norm values that give information on an average behavior of the surface.

According to the German Industry Norm DIN 4760 [5] all shape deviations are classified into six regimes. The morphological deviations up to the fourth order are summarized in Table 1.1. The fifth order deviation (microporous structure) and the sixth order deviation (lattice structure) are not considered in surface metrology but are subject of materials engineering.

Table 1.1

Morphological deviations according to DIN 4760:1982

Besides the introduced coarse and fine shape deviations a third group of deviations from the nominal surface exist that are not characteristical for the manufacturing process but are important for the effectiveness of the surface: defects. Defects comprises scratches, bumps, and dings as well as contamination