Surface Roughness Measurement—Terms and Standards

Industrial Microscopes

Various Measurement Instruments Are Capable of Measuring Surface Roughness

Surface roughness measurement instruments can be categorized into contact-based and noncontact-based instruments.
There are pros and cons to both methods, and it is important to select the most suitable instrument based on your application.

Overview

Method	Measurement instrument	Advantages	Limitations
Contact-based measurement	Stylus roughness instrument	Enables reliable measurement as the sample surface is physically traced with a stylus Has been used for a long time	Limited to measuring a single section with a reduced quantity of measurement information Incapable of measuring adhesive surfaces and soft samples Difficult to precisely position the probe Incapable of measuring details smaller than the stylus probe tip diameter
Non contact-based measurement	Coherence scanning Interferometers	Quick measurements Enables sub-nanometer measurement of smooth surfaces at low magnification	Has trouble measuring rough surfaces Has trouble measuring samples with significant differences in brightness Low contrast makes it difficult to locate the areas subject to measurement Low XY resolution
	Laser microscope	High angle detection sensitivity, enabling analysis of steeply inclined slopes High XY resolution, providing for clear, high-contrast images	Incapable of conducting sub-nanometer measurements Inferior height discrimination capabilities at lower magnification rates
	Digital microscope	Enables many kinds of observations and a simple level of measurement	Not suitable for measuring component roughness (suitable for measuring waviness) Incapable of measuring sub-nanometer irregularities Low XY resolution
	Scanning probe microscope (SPM)	Enables measurement of sub-nanometer surfaces Enables measurement of samples with a relatively high aspect ratio	It's difficult to precisely position the probe Slow Not suitable for measuring μm irregularities

Another disadvantage of a stylus is that it requires direct contact between the probe and sample surface. For soft or delicate samples, the stylus can actually cause damage.

Issues2

^{Stylus probes may damage the sample surface}

Because the laser used by the OLS5000 microscope acquires information without touching the sample, you can acquire accurate roughness measurements without causing damage.

Solutions2

^{Adhesive tape 256 × 256 μm}

The OLS5000 microscope, on the other hand, uses a laser to make measurements and has dedicated objectives with a high numerical aperture. These features enable you to obtain accurate measurements regardless of the sample’s surface, even if it’s very steep. The high-quality objectives also enable you to view your sample while capturing measurements and obtain image data while making your measurements.

Issues5

OLS5000 laser microscopes accomplish sub-nanometer-level measurements much more quickly. They also enable you to observe submicron irregularities using a broad field of view. The stitching function can be used to further expand the area of analysis.

Issues5

	Profile method type	Areal method type
Surface texture parameters	ISO 4287:1997	ISO 25178-2:2012
	ISO 13565:1996
	ISO 12085:1996
Measurement conditions	ISO 4288:1996	ISO 25178-3:2012
Measurement conditions	ISO 3274:1996	ISO 25178-3:2012
Filter	ISO 11562:1996	ISO 16610 series
Categorization of measurement Instruments	－	ISO25178-6:2010
Calibration of measurement Instruments	ISO 12179:2000	Under preparation
Standard test pieces for calibration	ISO 5436-1:2000	ISO25178-70:2013
Graphic method	ISO 1302:2002	ISO25178-1:2016

Primary profile curve

The curve obtained by applying a low-pass filter with a cutoff value of λs to the primary profile measured. The surface texture parameter calculated from the primary profile is referred to as the primary profile parameter (P-parameter).

Roughness profile

The profile derived from the primary profile by suppressing the long wave component using the high-pass filter with a cutoff value of λc. The surface texture parameter calculated from the roughness profile is referred to as the roughness profile parameter (R-parameter).

Waviness profile

The profile obtained by sequential application of profile filters with cutoff values of λf and λc to the primary profile. λf cuts off the long wave component while the short wave component is cut off with filter λc. The surface texture parameter calculated from the waviness profile is referred to as the waviness profile parameter (W-parameter).

Profile filter

The filter for the isolation of the long and short wave components contained in the profile. Three types of filters are defined:

λs filter: Filter designating the threshold between the roughness component and shorter wave components
λc filter: Filter designating the threshold between the roughness component and waviness components
λf filter: Filter designating the threshold between the waviness component and longer wave components

Cut-off wavelength

Threshold wavelength for profile filters. Wavelength indicating 50% transmission factor for a given amplitude.

Sampling length

The length in the direction of the X-axis used for the determination of profile characteristics.

Evaluation length

Length in the direction of the X-axis used for assessing the profile under evaluation.

Conceptual drawing of profile method

^{Conceptual drawing of profile method}

Scale limited surface

The surface data are serving as the basis for the calculation of areal surface texture parameters (S-F surface or S-L surface). This is sometimes referred to as 'surface.'

Areal filter

The filter for the separation of the long and short wave components contained in the scale-limited surfaces. Three types of filters are defined according to function:

S filter: Filter eliminates small wavelength components from scale-limited surfaces
L filter: Filter eliminates large wavelength components from scale-limited surfaces
F operation: Association or filter for the elimination of specific forms (spheres, cylinders, etc.)

Note: Gaussian filters are generally applied as S and L filters, and the total least square association is applied for the F operation.

Gaussian filter

A type of areal filter normally used in areal measurement. Filtration is applied by convolution based on weighting functions derived from a Gaussian function. The value of the nesting index is the wavelength of a sinusoidal profile for which 50% of the amplitude is transmitted.

Spline filter

A type of areal filter with smaller distortion in the peripheral edge when compared to the Gaussian filter.

Nesting index

The index representing the threshold wavelength for areal filters. The nesting index for the application of areal Gaussian filters are designated in terms of units of length and equivalent to the cutoff value in the profile method.

S-F surface

The surface obtained by eliminating small wavelength components using the S filter and then processed by removing certain form components using the F operation.

S-L surface

The surface obtained by eliminating small wavelength components using the S filter and then eliminating large wavelength components using L filtration.

Evaluation area

A rectangular portion of the surface designated for characteristic evaluation. The evaluation area shall be a square (if not otherwise specified).

Conceptual drawing of the areal method

^{Conceptual drawing of the areal method}

1. From the items listed below, select the appropriate objective lenses based on the item to be measured (roughness, waviness, or unevenness). Be sure that the working distance (W.D.) value exceeds the clearance between the sample and the lens.

2. If there are multiple objective lens candidates, make a final selection. The size of measurement field should be five times the scale of the coarsest structure of interest.

If there are multiple candidates, select the objective lens with the largest possible numerical aperture (N.A.).
If no suitable lens is available, either select again (this time including objectives labeled 'acceptable depending on usage') or consider expanding the measurement area using the stitching function.

Objectives	Specification				Measurement item
Objectives	Numerical aperture (N.A.)	Working distance (W.D.) (unit: mm)	Focusing spot diameter* (unit: μm)	Field of measurement** (unit: μm)	Roughness	Waviness	Unevenness (Z)
MPLFLN2.5X	0,08	10,7	6,2	5120 x 5120	X	X	X
MPLFLN5X	0,15	20	3,3	2560 x 2560	X	X	X
MPLFLN10XLEXT	0,3	10,4	1,6	1280 x 1280	X	○	△
MPLAPON20XLEXT	0,6	1	0,82	640 x 640	△	○	○
MPLAPON50XLEXT	0,95	0,35	0,52	256 x 256	◎	○	◎
MPLAPON100XLEXT	0,95	0,35	0,52	128 x 128	◎	○	◎
LMPLFLN20XLEXT	0,45	6,5	1,1	640 x 640	△	○	○
LMPLFLN50XLEXT	0,6	5	0,82	256 x 256	△	○	○
LMPLFLN100XLEXT	0,8	3,4	0,62	128 x 128	○	○	◎
SLMPLN20X	0,25	25	2	640 x 640	X	○	△
SLMPLN50X	0,35	18	1,4	256 x 256	X	○	△
SLMPLN100X	0,6	7,6	0,82	128 x 128	△	○	○
LCPLFLN20XLCD	0,45	7,4-8,3	1,1	640 x 640	△	○	○
LCPLFLN50XLCD	0,7	3,0-2,2	0,71	256 x 256	○	○	○
LCPLFLN100XLCD	0,85	1,0-0,9	0,58	128 x 128	○	○	◎

--- ---

* Theoretical value.
** Standard value when using the OLS5000.

◎ : Most suitable
○ : Suitable
△ : Acceptable depending on usage
X : Not suitable

The functionality of the respective filters, the combination of filters, and the size of the filters used in surface feature analysis are as described below:

The filtering conditions are determined in accordance with the objectives of the analysis.

Filter functionality

In conducting surface feature parametric analysis, the application of three types of filters (F operation, S filter, and L filter) should be considered for the surface texture data acquired in accordance with the objectives of the measurement.

F operation	S filter (Short-cut filter)	L filter (Long-pass filter)
Nominal form components of samples (spheres, cylinders, curves, etc.) are eliminated	Measurement noise and small feature components are eliminated	Waviness components are eliminated

Method of filter application

Filter combinations

Eight combinations are available for the three filters (F operation, S filter, and L filter). Select the combination of filters to be applied referencing the list of measurement objectives indicated in the following table.

Intended purpose

When analyzing raw acquired data

When eliminating waviness component

When eliminating spheres, curves, and other form components

When eliminating spheres, curves, and other form components in addition to the waviness component

When eliminating small roughness components and noises

When eliminating small roughness components, noise, and waviness components

When eliminating spheres, curves, and other form components along with small roughness components and noise

When eliminating small roughness components and noise, spheres, curves and other feature components in addition to the waviness component

F-operation

－

〇

－

〇

S filter

－

〇

L filter

－

〇

－

〇

－

〇

－

〇

－ : Not applicable
○ : Applicable

Filter size (nesting indices)

Filtering strength (separating capabilities) is referred to as nesting indices (L filters are alternately called cutoffs).

The S filter eliminates increasingly more detailed feature components the larger the nesting index value is
The L filter eliminates increasingly more waviness feature components the smaller the nesting index value is

Although the use of numerical values (0.5, 0.8, 1, 2, 2.5, 5, 8, 10, 20) are recommended when defining nesting index values, the following restrictions apply:

The nesting index value for S filters needs to be specified to exceed the optical resolution (≒ focusing spot diameter) and at least three times the value of the data sampling interval
The nesting index for the L filter needs to be set to a value smaller than the area of measurement (length of the narrow side of the rectangular area)