Equipment Surface metrology

1 equipment

1.1 contact (tactile measurement)
1.2 non-contact (optical microscopes)
1.3 choice of right measurement instrument
1.4 resolution

equipment


contact (tactile measurement)

stylus-based contact instruments have following advantages:

the system simple , sufficient basic roughness, waviness or form measurement requiring 2d profiles (e.g. calculation of ra value).
the system never lured optical properties of sample (e.g. highly reflective, transparent, micro-structured).
the stylus ignores oil film covering many metal components during industrial process.

technologies:

contact profilometers – traditionally use diamond stylus , work phonograph
atomic force microscope considered contact profilers operating @ atomic scale.

non-contact (optical microscopes)

optical measurement instruments have advantages on tactile ones follows:

no touching of surface (the sample can not damaged)
the measurement speed higher (up million 3d points can measured in second)
some of them genuinely built 3d surface topography rather single traces of data
they can measure surfaces through transparent medium such glass or plastic film
non-contact measurement may solution when component measure soft (e.g. pollution deposit) or hard (e.g. abrasive paper).

vertical scanning:

coherence scanning interferometry
confocal microscopy
focus variation
confocal chromatic aberration

horizonal scanning:

scanning laser microscope (slm)
structured light scanning

non-scanning

digital holography microscopy

choice of right measurement instrument

because of every instrument has advantages , disadvantages operator must choose right instrument depending on measurement application. in following advantages , disadvantages main technologies listed:

interferometry: method has highest vertical resolution of optical technique , lateral resolution equivalent other optical techniques except confocal has better lateral resolution. instruments can measure smooth surfaces using phase shifting interferometry (psi) high vertical repeatability; such systems can dedicated measuring large parts (up 300mm) or microscope-based. can use coherence scanning interferometry (csi) white-light source measure steep or rough surfaces, including machined metal, foam, paper , more. case optical techniques, interaction of light sample instruments not understood. means measurement errors can occur roughness measurement.
digital holography: method provide 3d topography similar resolution interferometry. moreover, non-scanning technique, ideal measurement of moving sample, deformable surfaces, mems dynamics, chemical reaction, effect of magnetic or electrical field on samples, , measurement in presence of vibration, in particular quality control.:
focus variation: method delivers color information, can measure on steep flanks , can measure on rough surfaces. disadvantage method can not measure on surfaces smooth surface roughness silicon wafer. main application metal (machined parts , tools), plastic or paper samples.
confocal microscopy: method has advantage of high lateral resolution because of use of pin hole has disadvantage can not measure on steep flanks. also, loses vertical resolution when looking @ large areas since vertical sensitivity depends on microscope objective in use.
confocal chromatic aberration: method has advantage of measuring height ranges without vertical scan, can measure rough surfaces ease, , smooth surfaces down single nm range. fact these sensors have no moving parts allows high scan speeds , makes them repeatable. configurations high numerical aperture can measure on relatively steep flanks. multiple sensors, same or different measurement ranges, can used simultaneously, allowing differential measurement approaches (ttv) or expanding use case of system.
contact profilometer: method common surface measurement technique. advantages cheap instrument , has higher lateral resolution optical techniques, depending on stylus tip radius chosen. new systems can 3d measurements in addition 2d traces , can measure form , critical dimensions roughness. however, disadvantages stylus tip has in physical contact surface, may alter surface and/or stylus , cause contamination. furthermore, due mechanical interaction, scan speeds slower optical methods. because of stylus shank angle, stylus profilometers cannot measure edge of rising structure, causing shadow or undefined area, larger typical optical systems.

resolution

the scale of desired measurement decide type of microscope used.

for 3d measurements, probe commanded scan on 2d area on surface. spacing between data points may not same in both directions.

in cases, physics of measuring instrument may have large effect on data. true when measuring smooth surfaces. contact measurements, obvious problem stylus may scratch measured surface. problem stylus may blunt reach bottom of deep valleys , may round tips of sharp peaks. in case probe physical filter limits accuracy of instrument.