[ ME 539 ]

Surface Science & Engineering

Assoc. Prof. Dr. A. Tolga BOZDANA

Mechanical Engineering Department

© 2020

[ Chapter 2 ]

Optical Surface Metrology

#1

Interferometry: superimposing (interfering) waves to detect differences between them.

It is a non-contact measurement technique in optical metrology, using the interference of light waves to determine surface shape and transmission properties.

Interferometry

Interference of Waves:

Interference occurs when two waves come across at the same time and location.

Constructive interference occurs if the waves are in-phase (i.e. their peaks coincide). They will add together to form a single wave with higher peak (larger amplitude) of the same frequency.

Destructive interference occurs if the waves are out of phase (i.e. the peak of one wave coincides with the valley of other wave). The amount of interference is dependent upon amplitude and frequency of the waves.

#2

Simple interferometry setup uses parallel beam of monochromatic light.

An optical flat (a disc of stress-free glass or quartz with a highly polished surface) is placed on top of the surface to be measured at a very small angle of θ.

The light beam from source (S) is projected onto optical flat. Two reflected components of light wave (partially reflected from a and b) are collected, and combined view is obtained.

Further along the surface at a distance of half-wavelength (λ/2) and due to the angle of θ, the ray (light beam) leaving the source S will again split into two components.

Therefore, the surface will be crossed by a pattern of dark bands (fringes), which are straight for the case of a flat surface.

Principle of Interferometry

t 2sin

interference fringes (red bands)

time

inte

ns

ity

t

#3 Fringe Patterns

Straight Fringe Pattern:

Such pattern indicates a very flat surface.

Suppose that the wavelength (λ) is 0.6328 µm. Then, the flatness is 0.3164 µm. 2Flatness

Flat Surface

Curved Fringe Pattern:

In such case, two red lines tangent to the center of two adjacent fringes are drawn. The blue line indicates the center of a single fringe.

Suppose that distance between red lines (a) is 5.02 µm and between red and blue lines (b) is 1.24 µm. Then, the flatness is ≈ 0.078 µm.

a b

ab 2Flatness

Flatter Spherical

Circular Fringe Pattern:

Such pattern is obtained if the surface is convex or concave. So, the total distance between peak and valley (i.e. the height) can be calculated.

Since there are 5 fringes, the height is found to be 1.582 µm. fringes of #2Height

Optical Flat

Spherical

#4

Special-purpose software/programs are required for accurate analysis of complex fringe patterns.

They are used for analysis of surface topography to determine flatness, waviness, roughness, etc.

Fringe Analysis

#5

Various systems are available for different type of measurements based on the application.

Common Interferometers

Michelson Interferometer

Sagnac Interferometer Fabry-Perot Interferometer

Mach-Zehnder Interferometer

#6

Laser displacement probe (laser triangulator) uses high-speed non-contact laser probe to perform complex profile measurements.

Charge Coupled Device (CCD) sensor in the probe can take range (line) measurements (rather than measuring single point), which dramatically decreases scan time.

Thus, such probes can be used for measurement of distance and/or surface topography within certain measurement ranges.

Laser Triangulation

laser

scanner

laser

pointer

More information on laser displacement probe: www.keyence.com

#7

Such systems are used for 3D fringe projection of complex (particularly free-form) surfaces.

They consist of two digital cameras and a pattern (fringe) projector enclosed at a fixed angle.

Light waves with shifted patterns of parallel sinusoidal stripes are projected onto the surface. If the surface is not plane (flat), then the patterns get distorted. Both cameras record an image of the distorted fringe pattern, and hence the surface topography could be constructed.

3D Scanning Systems

More information on 3D scanning systems: www.gom.com/metrology-systems/atos.html

#8

Confocal Microscopes: They can solve problem of Depth of Field (DoF) in classical microscopes, which prevents higher vertical resolution.

This is accomplished by a modified optical path within the microscope.

Recently, laser has been used as light source. This type of microscope is known as “Laser Scanning Confocal Microscope (LSCM)”.

3D Microscopy

Photo: Olympus LEXT OLS5000 (www.olympus-ims.com/en/metrology/ols5000)

Variable-Depth Focal Microscopes: Focal depth variation is achived by vertical scanning of surface. This is also known as “Through-Focus Scanning Optical Microscopy (TSOM)”.

This is accomplished using a highly sophisticated software that calculates the best focus from continuously recorded data, which enables reconstruction of 3D surface topography.

Photo: Alicona InfiniteFocus (www.alicona.com/en/products/infinitefocus)

#9

Electron microscopy allows to visualize the spots down to 0.1 nm (1Å).

Unlike light microscopy (i.e. illumination with light), the surface is bombarded by electrons.

Scanning Electron Microscope (SEM) uses fine-beam of focused electrons to scan the part surface. The microscope records information about the interaction between the electrons and the surface. Such interaction creates secondary electrons, backscattered electrons, and X-rays. Thereby, they are captured to create a magnified image.

Electron Microscopy 1

cm

1 m

m

100 µ

m

10 µ

m

1 µ

m

100

nm

10 n

m

1 n

m

0.1

nm

(1 Å

)

Electron Microscopy

Light Microscopy

ductile-forced fracture in reinforced plastic stress-corrosion cracking in brass laser-burned hole in steel

#10

Tactile (Contact) Measurement:

Simple and sufficient for measurements requiring only 2D profiles (i.e. roughness, waviness, form)

Never lured by optical properties of part (such as highly reflective, transparent, micro-structured, etc.)

Ignores the surface conditions arising from surface processing (e.g. oil film covering the part surface after machining)

Tactile vs Optical Surface Metrology

Optical (Noncontact) Measurement:

No damage (scratch) on the part surface

High speed measurement (quick detection of large number of points on surface)

Constructing 3D surface topography rather than 2D profiling

Measuring surface through transparent medium (such as glass or plastic film)

Could be the only solution when the part surface is very soft (e.g. pollution deposit) or very hard (e.g. abrasive paper)