Surface metrology
Encyclopedia
Surface metrology is the measurement
of small-scale features on surfaces, and is a branch of metrology
. Surface primary form
, surface waviness
and surface roughness are the parameters most commonly associated with the field. It is important to many disciplines and is mostly known for the machining of precision parts and assemblies which contain mating surfaces or which must operate with high internal pressures.
Surface metrology is the study of surface geometry, also called surface texture or surface roughness. The approach is to measure and analyze the surface texture in order to be able to understand how the texture is influenced by its history, (e.g., manufacture, wear, fracture) and how it influences its behavior (e.g., adhesion, gloss, friction).
.
Vertical scanning:
Horizonal scanning:
For 3D measurements, the probe is commanded to scan over a 2D area on the surface. The spacing between data points may not be the same in both directions.
In some cases, the physics of the measuring instrument may have a large effect on the data. This is especially true when measuring very smooth surfaces. For contact measurements, most obvious problem is that the stylus may scratch the measured surface. Another problem is that the stylus may be too blunt to reach the bottom of deep valleys and it may round the tips of sharp peaks. In this case the probe is a physical filter that limits the accuracy of the instrument.
indicators obtained from many samples of the surface height. Some examples include:
This is a small subset of available parameters described in standards like ASME B46.1 and ISO 4287.
Most of these parameters originated from the capabilities of profilometers and other mechanical probe systems.
In addition, new measures of surface dimensions have been developed which are more directly related to the measurements made possible by high-definition optical gauging technologies.
Most of these parameters can be estimated using the SurfCharJ plugin http://www.gcsca.net/IJ/SurfCharJ.html for the ImageJ
.
series describes all these roughness values in detail. The advantage over the profile parameters are:
Surfaces have fractal
properties, multi-scale measurements can also be made such as Length-scale Fractal Analysis or Area-scale Fractal Analysis.
Measurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...
of small-scale features on surfaces, and is a branch of metrology
Metrology
Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek μέτρον , "measure" + "λόγος" , amongst others meaning "speech, oration, discourse, quote, study, calculation, reason"...
. Surface primary form
Primary form
Primary form is used in surface metrology to refer to the over-all shape of a surface which can be measured quantitatively, as contrasted with more local or higher-spatial frequency variations in the surface dimensions such as surface waviness or surface roughness or surface finish. Examples of...
, surface waviness
Waviness
Waviness is the measure of the more widely spaced component of surface texture. It is a broader view of roughness because it is more strictly defined as "the irregularities whose spacing is greater than the roughness sampling length"...
and surface roughness are the parameters most commonly associated with the field. It is important to many disciplines and is mostly known for the machining of precision parts and assemblies which contain mating surfaces or which must operate with high internal pressures.
Surface metrology is the study of surface geometry, also called surface texture or surface roughness. The approach is to measure and analyze the surface texture in order to be able to understand how the texture is influenced by its history, (e.g., manufacture, wear, fracture) and how it influences its behavior (e.g., adhesion, gloss, friction).
Equipment
A full list of standardized instruments can also be found in the part 6 document of the ISO series ISO 25178ISO 25178
ISO 25178: Geometric Product Specifications – Surface texture: areal is an International Organisation for Standardisation collection of international standards relating to the analysis of 3D areal surface texture.-New features:...
.
Contact (tactile measurement)
The following instruments are well established technologies. There are many manufacturers implementing these technologies into products:- ProfilometerProfilometerProfilometer is a measuring instrument used to measure a surface's profile, in order to quantify its roughness.While the historical notion of a profilometer was a device similar to a phonograph that measures a surface as the surface is moved relative to the contact profilometer's stylus, this...
- traditionally called a stylusStylusA stylus is a writing utensil, or a small tool for some other form of marking or shaping, for example in pottery. The word is also used for a computer accessory . It usually refers to a narrow elongated staff, similar to a modern ballpoint pen. Many styli are heavily curved to be held more easily...
and works like a phonographPhonographThe phonograph record player, or gramophone is a device introduced in 1877 that has had continued common use for reproducing sound recordings, although when first developed, the phonograph was used to both record and reproduce sounds... - Atomic force microscopeAtomic force microscopeAtomic force microscopy or scanning force microscopy is a very high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit...
Non-Contact (optical microscopes)
Optical measurement instruments have some advantages over the tactile ones. The main advantages are:- no touching of the surface (the sample can not be damaged)
- the measurement speed is much higher (up to a million 3d points can be measured in a second)
- they are designed primarily for 3D surface topography rather than single traces of data
- they can measure surfaces through transparent medium such as glass or plastic film
- they have lower vertical noise
Vertical scanning:
- InterferometryInterferometryInterferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...
- Confocal microscopyConfocal microscopyConfocal microscopy is an optical imaging technique used to increase optical resolution and contrast of a micrograph by using point illumination and a spatial pinhole to eliminate out-of-focus light in specimens that are thicker than the focal plane. It enables the reconstruction of...
- Focus variationFocus variationFocus variation is a method to calculate a sharp image and to measure the depth with an optics with limited depth of field.-Algorithm:The algorithm work as follows:# at first images with difference focus are captured...
- Confocal chromatic aberration
Horizonal scanning:
- Scanning laser microscope (SLM)
- Structured light scanningStructured Light 3D ScannerA structured-light 3D scanner is a device for measuring the three-dimensional shape of an object using projected light patterns and a camera system.- Principle :...
Choice of the right measurement instrument
Because of every instrument has advantages and disadvantages the operator must choose the right instrument depending on the measurement application. In the following some advantages and disadvantages to the main technologies are listed:- Interferometry: This method has the highest vertical resolution of any optical technique and lateral resolution equivalent to most other optical techniques except for confocal which has better lateral resolution. Instruments can measure very smooth surfaces using phase shifting interferometry (PSI) with high vertical repeatability; such systems can be dedicated for measuring large parts (up to 300mm) or microscope-based. They can also use vertical scanning interferomtry (VSI) mode with a white-light source to measure steep or rough surfaces, including machined metal, foam, paper and more. Actually the interaction of light with the sample for this instruments is not fully understood. This means that measurement errors can occure specially for roughness measurement. This is described in 2 articles from national laboratorys (see and )
- Focus variation: This method delivers color information, can measure on steep flanks and can measure on very rough surfaces. The disadvantage is that this method can not measure on surfaces with a very smooth surface roughness like a silicon wafer. The main application is metal (machined parts and tools), plastic or paper samples.
- Confocal microscopy: this method has the advantage of high lateral resolution because of the use of a pin hole but has the disadvantage the it can not measure on steep flanks. Also, it quickly loses vertical resolution when looking at large areas since the veritical sensitivity depends on the microscope objective in use.
- Confocal chromatic aberration: This method has the advantage of measuring certain height ranges without a vertical scan but cannot measure very smooth surfaces.
- ProfilometerProfilometerProfilometer is a measuring instrument used to measure a surface's profile, in order to quantify its roughness.While the historical notion of a profilometer was a device similar to a phonograph that measures a surface as the surface is moved relative to the contact profilometer's stylus, this...
: this method is the most common surface measurement technique. The advantages are that it is a cheap instrument and has higher lateral resolution than optical techniques. New systems can do 3D measurement in addition to 2D traces and can measure form and critical dimensions as well as roughness.
Resolution
The scale of the desired measurement will help decide which type of microscope will be used.For 3D measurements, the probe is commanded to scan over a 2D area on the surface. The spacing between data points may not be the same in both directions.
In some cases, the physics of the measuring instrument may have a large effect on the data. This is especially true when measuring very smooth surfaces. For contact measurements, most obvious problem is that the stylus may scratch the measured surface. Another problem is that the stylus may be too blunt to reach the bottom of deep valleys and it may round the tips of sharp peaks. In this case the probe is a physical filter that limits the accuracy of the instrument.
Roughness parameters
The real surface geometry is so complicated that a finite number of parameters cannot provide a full description. If the number of parameters used is increased, a more accurate description can be obtained. This is one of the reasons for introducing new parameters for surface evaluation. Surface roughness parameters are normally categorised into three groups according to its functionality. These groups are defined as amplitude parameters, spacing parameters, and hybrid parameters.Profile roughness parameters
Parameters used to describe surfaces are largely statisticalStatistics
Statistics is the study of the collection, organization, analysis, and interpretation of data. It deals with all aspects of this, including the planning of data collection in terms of the design of surveys and experiments....
indicators obtained from many samples of the surface height. Some examples include:
| Parameter | Name | Description | Type | Formula |
|---|---|---|---|---|
| Ra, Raa, Ryni | arithmetic average of absolute value Absolute value In mathematics, the absolute value |a| of a real number a is the numerical value of a without regard to its sign. So, for example, the absolute value of 3 is 3, and the absolute value of -3 is also 3... s |
Mean of the absolute values of the profile heights measured from a mean line averaged over the profile | Amplitude | |
| Rq, RRMS | root mean square Root mean square In mathematics, the root mean square , also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids... d |
Amplitude |  |
|
| Rv | maximum valley depth | Maximum depth of the profile below the mean line with the sampling length | Amplitude |  |
| Rp | maximum peak height | Maximum height of the profile above the mean line within the sampling length | Amplitude |  |
| Rt | Maximum Height of the Profile | Maximum peak to valley height of the profile in the assessment length | Amplitude |  |
| Rsk | Skewness Skewness In probability theory and statistics, skewness is a measure of the asymmetry of the probability distribution of a real-valued random variable. The skewness value can be positive or negative, or even undefined... |
Symmetry of the profile about the mean line | Amplitude |  |
| Rku | Kurtosis Kurtosis In probability theory and statistics, kurtosis is any measure of the "peakedness" of the probability distribution of a real-valued random variable... |
Measure of the sharpness of the surface profile | Hybrid |  |
| RSm | Mean Peak Spacing | Mean Spacing between peaks at the mean line | Spatial |  |
This is a small subset of available parameters described in standards like ASME B46.1 and ISO 4287.
Most of these parameters originated from the capabilities of profilometers and other mechanical probe systems.
In addition, new measures of surface dimensions have been developed which are more directly related to the measurements made possible by high-definition optical gauging technologies.
Most of these parameters can be estimated using the SurfCharJ plugin http://www.gcsca.net/IJ/SurfCharJ.html for the ImageJ
ImageJ
ImageJ is a public domain, Java-based image processing program developed at the National Institutes of Health. ImageJ was designed with an open architecture that provides extensibility via Java plugins and recordable macros. Custom acquisition, analysis and processing plugins can be developed using...
.
Areal surface parameters
The surface roughness can also be calculated over an area. This gives Sa instead of Ra values. The ISO 25178ISO 25178
ISO 25178: Geometric Product Specifications – Surface texture: areal is an International Organisation for Standardisation collection of international standards relating to the analysis of 3D areal surface texture.-New features:...
series describes all these roughness values in detail. The advantage over the profile parameters are:
- more significant values
- more relation to the real function possible
- faster measurement with actualiunstruments possible (optical areal based instruments can measure an Sa in higher speed then Ra.
Surfaces have fractal
Fractal
A fractal has been defined as "a rough or fragmented geometric shape that can be split into parts, each of which is a reduced-size copy of the whole," a property called self-similarity...
properties, multi-scale measurements can also be made such as Length-scale Fractal Analysis or Area-scale Fractal Analysis.