Modern methods for determination of paper surface topography
1
Book: Analytical methods for structure characterization of printing material
(Editor: Diana Gregor-Svetec)
Modern methods for determination of paper surface topography
Aleš Hladnik, Department of Textiles, Faculty of Natural Sciences and Engineering, University of
Ljubljana, Slovenia
Gary Chinga-Carrasco, Paper and Fibre Research Institute (PFI), Trondheim, Norway
Alojz Suhadolnik, Faculty of Mechanical Engineering, University of Ljubljana, Slovenia
1 Introduction
The quantification of surface topography is of major importance for several industry sectors.
For the paper industry, the surface topography is essentially important for printing paper
grades. The topography affects several paper and print properties like gloss, missing dots, ink
distribution, ink transfer, mottling and picking. It is thus important to have a detailed and
standardised assessment of surface topography for understanding its influence on the paper
and print characteristic details.
The quality of a given surface structure is commonly characterised by the quantification of its
roughness. Although there are several roughness parameters (see Lipshitz et al. 1990;
Peltonen et al., 2004; Chinga et al., 2007b), the root-mean-square is the most used roughness
descriptor. The surface roughness may be divided into several scales, each scale affecting a
given paper or print property. A proper description of a given surface structure requires
reliable image acquisition devices. This is most important as the quality of a given surface
representation will determine or limit the extraction of valuable numerical data, affecting a
given surface property. During the last two decades several methods have been proposed for
assessing the surface structure of paper material. This includes stylus profilometry (see e.g.
Wagberg and Johansson, 1993; Enomae and LePoutre, 1995), laser profilometry (Chinga
2004), photometric stereo method (Hansson and Johansson, 1999), CLSM (Béland XXXX;
Dickson, 2005), SEM (Enomae et al. XXXX; Reme and Kure, 2004), AFM (Niemi et al.,
2002). This chapter will focus on i) laser profilometry, ii) photometric stereo methods, iii)
SEM and iv) AFM. The mentioned characterization methods are complementary and cover a
wide range of roughness scales, from the micron-level (laser profilometry, photometric stereo
method) to the nano-level (SEM and AFM).
2 Laser profilometry
In optical profilometry a sensing head scanns across the surface of a given specimen to create
profiles and 3D surface representations of topography structure. Contrary to mechanical stylus
profilometry, laser profilometry applies no physical contact between the sensor and the
substrate surface, thus avoiding surface damaging and resulting in a faster scanning. Laser
profilometry is also characterized by high vertical and lateral resolution.
Modern methods for determination of paper surface topography
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2.1 Optical triangulation
Laser profilometry is often based on a principle referred to as optical triangulation (Fig. 1). It
comprises three basic elements: a light source, imaging optics, and a photodetector (sensor).
The light source and imaging optics are used to generate a focused beam of light that is
projected onto a target surface. Although any light source can be used for basic optical
triangulation, the most frequently applied are diode lasers, due to their high brightness,
narrow-band wavelength, and phase coherence [1]. An imaging lens captures the scattered
light and focuses it onto a photodetector, which generates a signal that is proportional to the
position of the spot in its image plane. The incident light beam from the laser source has to be
well collimated to produce a uniformly small spot size over the entire measuring range thus
resulting in good spatial resolution of the image. As the distance to the specimen surface
changes, the imaged spot shifts due to parallax. To generate a three-dimensional image of the
surface, the sensor is scanned in two dimensions, thus generating a set of distance data that
represents the local surface topography.
The photodetector may be either a single-element position-sensing device (PSD) or a chargecoupled device (CCD). CCD arrays can be used to accurately determine the shape and
intensity distribution of the light spot on the detector, whereas PSDs only determine the
position of the spot’s centroid and total intensity. On the other hand, CCD arrays have slower
signal response and require more signal processing circuitry than PSDs.
In addition to the above mentioned components, a dedicated software is required to coordinate
probe motion, data acquisition, analysis, post-processing and reporting. Data may also be
exported for further analysis and processing with appropriate scientific software packages.
Fig. 1. Optical triangulation principle [2].
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2.2 Alternative measurement principles
Along with a high-precision computer controlled X-Y movable stage to which the specimen is
attached, commercially available instruments can be equipped with one of various point
sensors depending on the specific application. Laser profilometers by Solarius [3] utilize
autofocus or confocal point sensors (see Table 1 for specifications), among others. In
autofocus measurement (Fig. 2), condensed light is focused from a laser diode onto the
specimen surface. The reflected light is directed to a focus detector, which measures
deviations from the ideal focus to within a few nanometers. The deviation in focus generates
an error, which is used to re-focus the objective. The position of the objective represents an
absolute measurement of the height.
In confocal principle, a point light source and detector pinhole are used by the sensor to
discriminate depth. The point light source emitts the laser beam that is focused on a specimen
through an objective moving rapidly up and down. The maximum light intensity occurs when
the specimen lies within the focal plane of the objective. As the objective moves closer to or
farther from the specimen, however, the reflected light reaching the pinhole is defocused and
does not pass through it. As a result, the quantity of light received by a detector behind the
pinhole decreases rapidly. A detection signal is only generated when the maximum of light
goes through the pinhole. Continuous scanning along the z-axis ensures precise height
measurement of the illuminated point.
Other sensor types can be applied as well. The chromatic white light sensor [4] is also based
on the confocal technique. The function of the pin-hole diaphragm is assumed by an objective
lens with high chromatic aberation, while a spectrophotometer is used to measure the heights
from colour differences. Due to its compact design, this type of sensor is especially suited for
measuring in inaccessible places. The holographic sensor can be applied when form and
geometry with large differences in height are of interest.
Fig. 2. Autofocus (left) and confocal (right) measurement principle [3].
Modern methods for determination of paper surface topography
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Table 1. Sensor measurement specifications [3].
3 Photometric stereo method
Photometric stereo method is based on the utilization of stereo or binocular images to derive
height information. Optical and electron microscope images can be used [6]. The range of
resolutions is quite wide, from perhaps several tens of nanometers up to centimeters. Both the
speed and accuracy have improved significantly over the last few years so the technique has
been used successfully in a number of applications.
A method which determines the shape of an object from a single image is known as shape
from shading method [7] and relies on the intensity variation in the image. Unfortunately only
intensity can be determined from a single image point so to be able to describe sample surface
orientation, another variable is required. Another drawback of the shape from shading method
is susceptibility to intensity noise in the image. In general the shading of the surface depends
on the reflectance properties of the surface and on the light source illumination properties. By
contrast, with a photometric stereo method several images at different angles are acquired and
there is no dependency on the reflectance of the surface.
3.1 Irradiance
Let us denote by z  f ( x, y ) the object surface function in Euclidian coordinate system. We
presume orthogonal projection of the image surface onto the image plane over the compact
region  . The surface and the image have the same coordinate system. The surface gradient
(p, q) is defined by equations:
f ( x, y )
,
x
(1)
f ( x, y )
.
y
(2)
p
q
Modern methods for determination of paper surface topography
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In order to recover the object surface, gradients must be integrated for each ( x0 , y0 )   . The
integration is performed along some path and starting at point ( x0 , y0 ) . The image irradiance
I ( x, y ) is then equal to the so called reflectance map R ( p, q ) :
I ( x, y )  R ( p , q )
(3)
which determines image intensity as a function of surface orientation. The reflectance map
combines information about the surface illumination, viewing geometry and surface material
by mapping from the surface orientation to image intensity. If the surface is assumed to be
Lambertian illuminated with diffused light than the irradiance on the camera can be written
as:
I ( x, y)  I 0 RN( x, y)  S
(4)
where R is the reflectance of the surface, I 0 is the irradiance of the light source, S is the
direction to the light source and N( x, y ) is the surface unit length normal at point (x,y)
defined as follows (Fig. 3):
N ( x, y ) 
 p, q,1
(5)
1  p2  q2
Fig.3. Illumination conditions with two light sources S1 and S2 at angle
The unit surface normal is denoted by N,
1
and
 between source and detector.
 2 are angles of incidence for both illuminations and
f ( x, y ) the object surface function.
With photometric stereo method which was introduced by Woodham [8] the estimation of the
local surface orientation is performed by several simultaneous images acquiring the same
surface at different illumination directions.. Number of the light sources depends on the prior
knowledge of the surface reflectance. In most cases the number of light sources is two, three,
four or even more. In case of two light illuminations (Fig. 1) the light source directions are:
S1    sin  ,0,cos  
and
(6)
Modern methods for determination of paper surface topography
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S2  sin  ,0,cos  
(7)
If both light sources have equal I 0 and the incident angles are 1 and 1 , the sum of two
irradiances is:
I1  I 2  I 0 R(N  S1  N  S 2 )  I 0 R(cos ¸1  cos  2 ) ,
(8)
where scalar multiplication of vectors N and S gives the value of cos  . By putting equations
5, 6, 7 into equation 8 the final irradiance sum is equal:
I1  I 2  I 0 R
2 cos 
1  p2  q2
 2 I 0 R cos 
(9)
Last approximation is valid if the gradients p and q are small. From this equation the
reflectance R is then equal:
R
I1  I 2
I 0 2 cos 
(10)
The difference of the light illuminations is:
I 2  I1  I 0 R (N  S1  N  S 2 )  I 0 R
2 p sin 
1 p  q
2
2
2
f ( x, y )
I 0 R sin 
x
(11)
The surface gradient in x direction is finally calculated from the intensity ratio:
f ( x, y ) I 2  I1 1

p
x
I 2  I1 tg 
(12)
If Lambertian irradiation equation (4) is corrected for the experimentally determined
correction factor  , then the irradiation is:
I  I 0 R(cos    )
(13)
and the surface gradient calculated in equation 13 becomes:
f ( x, y ) I 2  I1 cos(    )

x
I 2  I1 sin 
(14)
The correction factor is determined empirically and has value of 0.07 in case of unpolarized
illumination and value of 0.04 in case of p-polarized illumination and s-polarized detection.
3.2 Integration
The surface function f(x,y) can be determined from gradients (p,q) by one of the integration
techniques. The local integration technique enables the surface normal vector determination at
two or eight adjacent points to a given point by calculating the average tangent trough the
given point and than interpolating the height and the surface normal. This technique uses the
following curve integral:
f ( x, y )  f ( x0 , y0 )   p( x, y )dx  q ( x, y )dy

(15)
Modern methods for determination of paper surface topography
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Variable  represents arbitrary integration path from point ( x0 , y0 )   to point ( x, y )   .
In this calculation the initial height value must be known. The errors propagate along the
integration paths and the results depend strongly on data accuracy.
The equations 1 and 2 link together the surface height and gradients. The global integration
technique minimizes the following functional which is also called the cost function:
2
2


df ( x, y )
 df ( x, y )

W ( x, y)   
 p( x, y ) 
 q( x, y ) dxdy.
dy
 
 dx


(16)
 df ( x, y ) df ( x, y) 
In this integral 
,
 is gradient field of the surface and  p, q given nondy 
 dx
integrable gradient field. Furthermore the gradient field can be expressed as
 df ( x, y ) df ( x, y ) 
,

   p, q   x ,  y  where expression  x ,  y  denotes the correction of
dx
dy


the gradient field which inverts non-integrable field to integrable one.
Integration in the frequency domain with the Fourier transform is also possible. In case of two
f ( x, y )
light sources the partial derivative
is unknown and we assume that the mean value of
y
the profile function f ( x, y ) with constant y is zero. If the noise n( x, y ) in the acquired image
f ( x, y )
and light spreading in the material is taking into account, the partial derivative
x
becomes:
g ( x, y ) 
f ( x, y )
* PSF( x, y )  n( x, y ) ,
x
(17)
where PSF denotes the point spread function and * convolution. This equation in frequency
domain with the spatial frequencies u and v is expressed as
G (u, v)  2 iuF (u, v)OTF(u, v)  N (u, v),
(18)
The Fourier transform of the PSF is the optical transfer function OTF. The estimated surface
function f R ( x, y) can be calculated by the inverse Fourier transform:
f R ( x, y )  F 1 ( H R G (u, v))
(19)
Before the inversion process G (u, v) is convolved with a Wiener filter H R :
HR 
H I*
H I  SNR(u, v)1
2
,
(20)
where H I  2 iuOTF(u, v) , H I* denotes complex conjugate, H I
STR(u, v)  F (u, v) / N (u, v)
2
2
2
is power spectrum and
is the signal-to-noise ratio in frequency domain. This
calculation is performed if f ( x, y ) is assumed to be a stochastic process with known spectral
density.
Modern methods for determination of paper surface topography
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In case of paper sample the OTF can be approximated by:
OTF 
4000
4000  4 2 (u 2  v 2 )
(21)
where u and v are in mm-1 and signal-to-noise ratio (SNR) by:
SNR 
4
1000  (u  v 2 )x 2
2
(22)
where x is the resolution in millimeters in the x direction.
4 Scanning electron microscopy
Resolution of the conventional optical – light – microscope is limited by the visible light
wavelength (λ = 400-700 nm) to approximately 0.2 m. In addition, out of focus light from
points outside the focal plane reduces image clarity. An important progress in the
investigation of material surfaces came about with the development of an electron microscope
– starting with the transmission electron microscope (TEM) and continuing with the scanning
electron microscope (SEM). Principles of SEM were developed in the early 1950s at the
University of Cambridge, U.K., but the technique did not become commercially available
until 1965. Since then, numerous improvements have been made on the instrument in terms of
lens design, electron sources, detectors, and electronic signal processing. Today, SEM is one
the most widely used analytical techniques providing means to study both the morphology
and composition of various materials. Its main advantages [9] are the high lateral resolution
(1-10 nm), large depth of focus (100 m at 1000 x magnification), wide range of
magnifications (20-300.000 x) and numerous types of electron-specimen interactions that can
be used for further material examination or processing. The technique is being implemented
in a vast range of applications, such as semiconductor research and manufacturing, metalurgy,
biology, geology and also for paper and print investigations.
A detailed discussion on various SEM components, functions and capabilities has been
provided by numerous authors – see, e.g. [10, 11] – so only fundamentals of SEM and its
modes of operation are presented here. More emphasis will be given to the implementation of
this technique for paper- and print-related surface analysis.
4.1 Instrument design
SEM basically contains two main components (Fig. 4) [9]: an electron-optical column and a
vacuum system. An electron gun is placed on top of the column and its cathode filament can
be made of various materials: in conventional SEM, it is produced of tungsten or LaB6 –
usually with an additional electrode (Wehnelt) placed between the cathode and anode – while
in the modern field-emmission gun SEM (FEGSEM) it is made of an extremely thin tungsten
monocrystal needle or a ZrO2 monocrystal attached to a tungsten wire. Upon heating the
cathode, a thermoionic (or field-emission in FEGSEM) emission of electrons takes place.
Primary electrons are accelerated using anode voltage typically ranging from 500 to 30.000 V.
Modern methods for determination of paper surface topography
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A system of magnetic lenses – sometimes combined with electrostatic ones – and apertures
directs and focuses electron beam onto the specimen surface. The condenser lenses and spray
apertures are responsible for changing the beam divergence angle and therefore the probe
current, thus affecting the probe diameter. Purpose of the objective lens is to focus the
electron beam into an extremely fine spot on the surface of the sample: in modern
conventional SEM using tungsten cathode its diameter is as low as 10 nm. Scanning
(rastering) of the focused electron beam across the specimen surface is accomplished by
special coils located in the bottom part of the electron-optical column. The magnification and
scan velocity are varied by changing scan coil excitation. Rastering together with the signals
generated in the sample by the incident electron beam (see below) are monitored
simultaneously. The signals are collected by special detectors, amplified, displayed and stored
in a computer memory (or on a photographic film) for further image processing.
Vacuum in the SEM specimen chamber is 10-3 – 10-4 Pa. Some modifications of this
instrument operate at a much lower vacuum – low vacuum SEM (LVSEM) – or at a controlled
low pressure of certain gases – environmental SEM (ESEM) and are especially suitable for
investigation of nonconductive samples containing water, oils, organic solvents, etc.
Fig. 4. SEM schematic cross section [12].
4.2 Electron beam-specimen interaction
When primary electrons hit the solid sample in a vacuum, numerous signals are generated as a
result of electrostatic interactions with the nuclei and electrons of the target atoms (Fig. 5).
Modern methods for determination of paper surface topography
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These interactions, i.e. scattering, can be either elastic or inelastic. Elastic scattering is a result
of primary electron interactions with the specimen atomic nuclei. After multiple scattering
events and changes of direction, a portion – about 30% – of the incident electrons orient
themselves towards the surface eventually leaving the sample. These electrons are called
back-scattered electrons (BSEs); all electrons having energy E > 50 eV belong to this
category. The number of BSEs increases with an increase in target's atomic number, Z.
In an inelastic scattering, electrons are losing their kinetic energy due to their contact with the
core and valence electrons of the sample. This results in the emission of secondary electrons,
Auger electrons, characteristic X-rays, continuum X-rays (Bremsstrahlung), etc. The excited
valence electrons, called secondary electrons (SEs), have low energy (E < 50 eV) and are
quickly absorbed by the sample, so only those generated close to the surface – typical depth
50 nm – can be detected.
Conventional SEMs are normally equipped with BSE and SE detectors, which provide
topographical information about the specimen surface, while generation of characteristic Xrays and Auger electrons can, for example, be used in qualitative and quantitative chemical
analysis of sample composition (EDX, WDX, AES).
Primary electron beam
Brehmsstrahlung
Back-scattered electrons (BSE)
Cathodoluminescence
Secondary electrons (SE)
Characteristic X-rays
Auger electrons
Sample
Absorbed current
Fig. 5. Signals generated upon interaction of primary electron beam with a specimen in SEM.
4.3 Preparation of paper samples
Samples to be analyzed by the conventional SEM need to be electrically conductive, stable
under vacuum and insensitive to the local heat generated during electron beam-specimen
interaction. Pretreatment of nonconductive materials involves coating the specimen surface
with a thin metal or carbon layer of 5-40 nm thickness. For the preparation of paper samples
the following procedure has been recommended [13]:
1. Cut a paper sample the size of the microscope stub with a razor blade while taking care not
to touch the surface with either fingers or the instrument to avoid deforming and compressing
the paper.
Modern methods for determination of paper surface topography
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2. Stick the paper sample onto the stub with double-sided tape, preferably a conductive carbon
tape. For thick or rough samples a conductive pathway of silver paste or colloidal silver paint
in an ethanol base should be made between the paper surface and the metal stub.
3. Leave the sample overnight in a desiccator to allow the silver paint solvent to evaporate and
to remove any possibly present residual moisture.
4. Use a sputter coater or an evaporation coater to coat the sample with a thin film (not
exceeding 30 nm in thickness) of a heavy metal, e.g. gold, palladium or a mixture of both.
The film thickness should be such that the surface details are not obscured; for smooth
samples, such as coated papers, 10 nm should be appropriate. Papers with a rougher surface
need thicker metal layers to bridge small gaps and create a continuous conducting layer.
By preparing cross-sections of the paper sheets, information about paper structure in zdirection can be obtained using SEM (Williams and Drummond, 1995?; Allem, 1998; Chinga
and Helle, 2002). For such purposes, the paper sample should first be embedded under
vacuum in a polymer followed by cutting and polishing the resulting stub. To improve the zdirection contrast, the sample should be coated with a thin film of carbon to prevent charging.
5 Atomic force microscopy
Together with scanning tunneling microscopy (STM), atomic force microscopy (AFM)
belongs to a family of scanning probe microscopy techniques, which offer the highest
resolution available for studying surfaces of various materials. AFM was invented in 1986 by
Binning, Quate and Gerber [14] and only three years later the first commercial instrument was
produced by Digital Instruments (USA). Resolution of the surface features is typically on the
nanometers scale laterally and on the angstrom scale vertically, although atomic resolution
can be achieved under certain conditions [15]. Sample preparation is quick and relatively
simple, requiring no stains, contrast agents, or conductive coatings (unlike e.g. in SEM, see
above). Other advantages of the technique include its nondestructiveness, potential to use a
broad range of environmental conditions – ambient air, various gases, humidity levels,
temperatures – and ability to study short- as well as long-range molecular and atomic forces.
AFM was implemented to investigate a large variety of materials (thin and thick film
coatings, ceramics, composites, glasses, synthetic and biological membranes, metals,
polymers, and semiconductors) and phenomena (abrasion, adhesion, cleaning, corrosion,
etching, friction, lubrication, plating, and polishing) [16].
5.1 Operating principle
The probe used in AFM is an extremely sharp tip – typically less than 5 μm tall and less than
10 nm in diameter at the apex [19] – integrated at the end of a 100-500 m long cantilever.
The cantilever bends or deflects due to the forces between the tip and the sample surface
while either the tip is scanned over the sample, or the sample is scanned under the tip.
Changes in the angle of the cantilever caused by changes in sample topography result in
different voltage levels out of the detector. These voltages are sent to a computer for
processing and display of the topographic image.
Modern methods for determination of paper surface topography
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Most commercial instruments use optical techniques to detect the position of the cantilever
(Fig. 6). Here a light beam from a laser bounces off the back of the cantilever and onto a
position-sensitive photo-detector (PSPD) or a photodiode. Bending of the cantilever causes a
change in the laser beam position on the detector.
Fig. 6. Basic operational principle of AFM with an optical detection system [20].
5.2 Probe, scanner and detector
The probe, i.e. cantilever with the tip, is made of a solid and inert material, such as silicon
nitride (or silicon for intermittent mode, see below) to prevent mechanical deformations of the
tip and tip/substrate chemical reactions. To get an atomic-scale resolution the tip has to be
extremely sharp with its radius of curvature ranging from 5 nm to 60 nm. Tips for some AFM
applications may be coated with metals (e.g. gold) or other materials (colloid particles,
monolayers, magnetic materials) or can have globular shape where nanometric resolution is
not crucial. Important cantilever parameters include its geometry (triangular or rectangular),
flexibility (spring constant) and resonance characteristics (frequency).
AFM scanner is made of piezoelectric (PZT) ceramics that expands and contracts in response
to the applied electric field. It allows for an extremely precise sample movement in a defined
– raster – pattern consisting of a series of rows in a zigzag pattern. As mentioned above, the
sample can be mounted directly onto the scanner and rastered underneath the cantilever tip, or
the cantilever can be mounted to a scanner tube and rastered over a sample fixed below it. The
former case is preferential when imaging larger samples, it also increases the imaging speed.
In modern AFMs the cantilever deflection is usually detected with a photodiode consisting of
four quadrants, i.e. separate diodes. Interferometry can also be used as a detection system. Its
phase change is compared with that of a standard to determine the shift in cantilever position.
Another option is integration of the detector into the cantilever using the piezoresistive
properties of silica [15].
5.3 Operational modes
Modern methods for determination of paper surface topography
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In order to monitor topographical images with AFM, investigate different surface sample
properties, or modify its surface, several modes of operation are available (Fig. 7). In contact
mode the tip is in constant contact with the substrate surface so short-range ionic repulsive
forces dominate. The tip follows the surface topography pattern. Here two possibilities for
generating topography data exist: Z feedback (loop) can be either turned on or off. Most
applications are based on Z feedback being turned on – so called constant-force mode. Here
vertical (Z) position of the sample changes so that the force between the tip and the sample
(and therefore cantilever deflection) remains constant. By contrast, in the constant-height
mode Z feedback is turned off so the sample vertical position during the measurement
remains unchanged while force between the tip and the sample changes. The constant-height
mode is often used to obtain atomic-scale images of very flat surfaces and for recording realtime images of changing surfaces where high scan speed is necessary.
In noncontact mode the tip oscillates up to approx. 10 nm above the surface with a low
amplitude and resonance frequency by a piezoactuator, longer-range attractive – mainly van
der Waals – forces prevail. The cantilever is stiffer than that that used in the contact mode. As
the tip does not touch the substrate, it can not be contaminated. This mode is applicable for
measuring very delicate samples, but due to a higher distance to the sample it is difficult to
achieve an atomic-scale resolution.
Intermittent contact (tappingTM) mode works by lightly tapping the surface with an
oscillating probe tip. Surface topography causes the cantilever oscillation amplitude to
change, the topography image is obtained by monitoring these changes and closing the Z
feedback loop to minimize them [19]. This mode is useful for measuring soft samples or
molecules that are not strongly attached to the surface, but due to the danger of tip
contamination and change in resonance frequency of the cantilever the latter has to be
recalibrated. Intermittent contact mode also overcomes problems that can occur when using
AFM in an ambient environment – i.e. in air or some other gas – where a liquid layer often
covers the substrate surface.
Fig. 7. Regions of operations for AFM contact, non-contact and tapping mode [21].
Apart from the above described primary AFM imaging modes, there is a large variety of
secondary imaging modes available that are derived from the primary ones, such as lateral
Modern methods for determination of paper surface topography
14
force microscopy, phase imaging, magnetic force microscopy, conductive AFM, tunneling
AFM, electric force microscopy, and others [19].
6 Paper and print applications
The techniques described in this chapter are complementary, thus enabling assessment of
several surface characteristics and scale of roughnesses. During the last years laser
profilometry (LP) has been used extensively for assessing the surface topography of several
paper grades, including newsprints, SC and LWC paper. The method is fast, fully automated,
non-contact, non-destructive and capable of assessing large areas (see Chinga-Carrasco et al.,
2008). However, LP has some limitations with respect to the detection and description of
steep gradients. Such limitation causes noise on the digital images, especially along coarse
surface fibres. Recently Chinga et al. (2007), proposed to cover the surface with a layer of
gold before the image acquisition. The gold layer seems to reduce internal reflections and
reduces the amount of error height values.
Suonstausta (2002) applied LP for assessing the effect of coating and calendering on the
surface structure of paper. The study demonstrated one of the major advantages of
profilometer devices, i.e. a topographical map may be decomposed into several scale of
roughness. This gives the opportunity of assessing the effect of a given scale of roughness on
a specific respons. Wavelength analysis was thus used for exploring the development of the
surface structure depending on a given coating composition and calendering configuration
(Suontausta 2002). Using LP, Holmstad et al. (2004) studied the effect of temperature
gradient calendering on the surface structure of pilot calendered paper. According to the
authors, the calendering conditions used in the study had a major effect on the uppermost
surface structure, being the calendering temperature the variable having the major impact on
the reduction of the surface roughness and thus the increasing of the paper gloss.
LP has been applied for studying the effect of the surface structure of SC and LWC paper on
gloss (Suontausta 2002; Chinga 2004; Holmstad et al., 2004; Chinga-Carrasco et al., 2008).
Gloss is one of the most important paper and print quality parameters of printing paper. Gloss
may be affected by the surface roughness and the mineral pigment particles, used as fillers or
in a coating layer. For the same roughness, clay yields usually higher gloss compared to
ground calcium carbonate (GCC) (see e.g. Stanislawska and LePoutre, 1996). In addition, it
has been demonstrated that the amount of fillers in the surface layers explains most of the
gloss development of commercial SC papers (Chinga et al., 2007a). Compared to SC paper,
LWC paper has commonly higher gloss levels due to the smoothing effect of the coating
layer. In addition, when SC paper is in contact with water a roughening phenomenon occurs
due to the swelling of the fibre material. The roughening is less in coated paper (see e.g.
Chinga et al., 2004). Contrary to water application that causes roughening, a layer of printing
ink usually smoothens a given surface structure (Fig. XX).
Modern methods for determination of paper surface topography
15
Fig. XX. Upper row) Laser profilometry surface representations of an unprinted (left) and
printed (right) LWC sample from the same local area. The calibration bars are given in
micrometers. Lower row) The corresponding 3D surface plot. The 3D plots are generated
with the Interactive 3D surface plot v.2.3 ImageJ plugin, by Kai Uwe Barthel, Internationale
Medieninformatik, Berlin, Germany.
A surface structure can be decomposed into gradients. A gradient is a small facet having a
given angle relative to the paper plane (Fig. XX). For comparison purposes, such analysis has
been performed on the images from Fig. XX. Note that the gradient image of the printed
sample has a lower amount of high angles compared to the gradient image of the unprinted
sample. A lower amount of high angles indicates that the surface has been smoothed due to
the printing ink (see also Chinga et al., 2004). Consequently, the printed surface (Figs. XX
and XX, right) will thus have a higher gloss than the unprinted surface. This exemplifies one
the benefits of using a non-destructive surface assessment device in combination with the
appropriate image analysis for quantifying a given response upon a finishing variable.
Modern methods for determination of paper surface topography
16
Fig. XX Gradient analysis of LWC samples. Upper row) Fig. XX converted into gradients of
the unprinted (left) and printed (right) samples.Each gradient corresponds to an angle relative
to the paper plane. The calibration bar is in degrees. Lower row) the corresponding
histograms. Images generated by G. Chinga Carrasco, PFI, Norway.
Based on a LP concept, Sung and Keller (2008) reported a new method defined as a twin laser
profilometer (TLP). The method consists on two laser sensors, placed to each side of a sample
holder. This configuration makes it possible to acquire surface profiles from each side of a
paper sample. The profiles are thus combined to generate a thickness map with a resolution of
1 m. According to the authors, the TLP method yields the intrinsic thickness of a given
sample, thus eliminating the overestimation of thickness that is characteristic of standard
caliper methods (Sung and Keller, 2008).
A photometric stereo method is a relatively simple, yet powerful technique for assessing
surface structures (Hansson and Johansson, 1999). The method is fast and is capable of
assessing large areas. However the maximum resolution of 5 m may be considered low for
some purposes such as the assessment of coated paper sub-micron roughness and how this is
affected by mineral pigment particles. On the other side, an optical stereo method may be
Modern methods for determination of paper surface topography
17
suitable for assessing missing dots in rotogravure printing as the surface cavities inducing
missing dots seem in the range of 80 m in diameter, thus reducing resolution requirements.
A clear example of the applicability of a photometric stereo method is presented in Fig. XX.
Surface depressions below -1 mm can be quantified and related to the occurrence of missing
dots. The method seems to give reasonable results, detecting the surface depressions causing a
poor contact between the paper surface and the gravure printing cylinder. A similar approach
has been applied in flexographic printing, where surface depressions may induce the
occurrence of unprinted areas (see Hansson and Johansson, 1999; Barros and Johansson,
2006). In addition, Barros and Johansson, 2008 described the applicability of this
instrumentation in combination with bandpass filtering for finding the relationship between
surface topography and the reflectance of flexography printed samples. The authors
concluded that small differences in surface roughness (wavelengths 0.8-1.6 mm) influenced
ink distribution and consequently print mottle in full-tone flexography printing.
Fig. XX Photometric stereo method about missing dots.
Fig. XX presents a clear example of the capabilities of the scanning electron microscope
(SEM) for exploring the surface structure of printing paper. SEM is a versatile device for
acquiring structural information. Images can be acquired at several magnifications and with a
resolution unattainable by other techniques. Images can be acquired in secondary electron
(SE) (Fig. XX) or backscatter electron mode (BSE) (Fig. XX). Due to its extensive
capabilities, SEM has been a most used device for assessing the structure of paper and prints
(see e.g. Helle and Johnsen, 1994?; Enomae et al., 1995?; Forseth and Helle, 1997; Allem,
1998; Reme and Kure, 2003?; Chinga and Helle 2003; Zou et al. XXXX; Eriksen and
Gregersen, XXXX).
Fig. XX SEM-SE and SEM-BEI images from the same area.
The SEM is a powerful tool for assessing different characteristics of the surface structure of
paper. In SE-mode the SEM gives a clear 3D impression of the topography of a given surface.
This capability has been used extensively for exploring e.g. the surface development due to
calendering (Holmstad et al., 2004), the consolidation of coating layers (Enomae and
LePoutre XXXX) and the smoothening effect of printing inks (Chinga et al., 2004), to name a
few. Common for the mentioned studies is that the SEM was used for exemplifying a given
phenomenon, however no quantification was performed. The SEM can also be used for
reconstructing surface structures (Fig. XX). The method is based on stereo imaging and
parallax (see e.g. Hein, 2001). Such surface reconstruction makes it possible to perform a
quantitative assessment of the surface topography (see Helle and Johnsen, 1994; Gregersen et
al., 1995; Reme and Kure, 2004). Another approach has been presented by Enomae et al.
(1995??). The authors used a SEM having two SE-detectors. Images were acquired from each
side of the vacuum chamber and a topographical height map was reconstructed.
Modern methods for determination of paper surface topography
18
Fig. XX SEM stereo imaging of a newsprint sample. Note the coarse surface fibre on the left
side of the image. Use red/green stereo glasses for better visualization. The SEM stereo image
has been acquired by G. Chinga Carrasco, PFI, Norway.
Surface coverage by a layer of fillers is an important characteristic of printing paper.
Coverage may determine some paper and print quality properties. SEM in backscatter mode
yields contrast depending on the average atomic number of a given local area. Mineral fillers
such as clay and CaCO3 carbonate may appear lighter that the matrix of darker fibres (Fig.
XX). SEM-BEI mode images with appropriate image segmentation and analysis procedures
make this technique suitable for quantification of coverage and related characteristics. Such
capability has been used for quantifying the coverage of the coating layer on coated papers
(see e.g. Kaartovara 1989; Dickson et al., 2002; Forsström et al., 2003;) and the coverage of
fillers on SC paper surfaces (Chinga et al., 2007). Coverage may be given in percentage and
may be defined as the ratio of areas covered with a coating layer to the whole imaged area.
Kaartovara (1989) quantified several statistics of uncovered areas, such as percentage,
average size and number of uncoated areas. The author found a reduction of uncoated areas as
the coat weight was increased from approximately 8 to 20 g/m2. However, care must be taken
when using the SEM-BEI mode for quantification of coverage, as the amount of uncovered
areas will depend on the accelerating voltage used during image acquisition in the SEM–BEI
mode. It is recommended to use low accelerating voltage in order to assess only the
uppermost layers of the paper surface. The effect of accelerating voltage on the quantification
of coverage is depicted in Fig. XX.
Modern methods for determination of paper surface topography
19
Fig. XX Coverage and accelerating voltage. A) Secondary electron image showing surface variations in
a LWC sample. B)-F) Images showing the same area, taken in BEI-mode with raising accelerating voltage: 5, 10,
15, 20 and 25 kV resp. Note the gradually emerging fibres when raising the accelerating voltage. Bar: 200 m.
The SEM images have been acquired by G. Chinga Carrasco, PFI, Norway.
In addition to surface assessment, the SEM is a powerful tool for quantification of crosssectional details of paper structure (Allem, 1998; Chinga and Helle, 2002; Holmstad et al.,
2004; Zou et al., XXXX). Such quantification may be used for describing the porosity (Allem
1998; Zhou XXXX?), fibre and pore cross-sectional dimensions (Chinga et al., 2007), filler
distribution (Holmstad et al., 2004), fines distribution (Eriksen et al., 2006) and the structural
details of mineral pigment layers on paper (Chinga and Helle, 2002). However, SEM has the
limitation of yielding 2D images of 3D structures. As an attempt to circumvent this limitation,
serial sectioning and serial grinding has been applied for making 3D reconstructions of paper
structure (Aronsson et al., 2002?; Chinga et al., 2004). This methods yield detailed
information about a given paper structure, though the method is time-consuming and difficult
to use as a standard analytical method for paper structure assessment.
Atomic force microscope (AFM) is a most suitable scientific device for quantifying the micro
(1-100 m), sub-micron (0.1 – 1 m) and nano-structure (1-100 nm) of paper and print
Modern methods for determination of paper surface topography
20
surfaces. According to Niemi et al. (2002) AFM have tree major advantages compared to
other microscopy techniques, i.e. i) no or little preparation, ii) high resolution and threedimensional surface information and iii) the microscope can be used in environments
inaccessible with other techniques.
Fig. XX AFM image of a LWC paper sample and the corresponding 3D surface
representation. The calibration bar is given in micrometers. The 3D plots are generated with
the Interactive 3D surface plot v.2.3 by Kai Uwe Barthel, Internationale Medieninformatik,
Berlin, Germany. The AFM image has been acquired by B. Wang. Dept. of Physics, NTNU.
Norway.
In addition to extracting 3D topographic information (see Peltonen et al., 2004; ChingaCarrasco et al., 2008), AFM can be used in phase mode to distinguish different components in
a coating structure, such as pigment particles and latex (see Niemi et al., 2002). The AFM is
thus a suitable method for assessing the structure of calendered coated paper. Rougher
surfaces have to be analysed with care, as the maximum movement in the z-direction (height)
is only a few micrometers. Local areas of fibre surfaces may also be assessed thus giving a
detailed description of pulp fibres, including the different layers of the fibre walls with their
characteristic arrangements of microfibrils (see e.g. Niemi et al., 2002).
Using AFM the structure of coating structures has been studied in detail (Ström et al., 2003;
Larsson et al., 2007; Järnström et al., 2007; Chinga-Carrasco et al., 2008). Ström et al. (2003)
applied an AFM analysis for assessing the structure of XXXX. The study proved the
suitability of AFM for assessing the structure of coated paper and prints. The topography was
related to the print gloss. A similar approach was applied by Järström et al. (2007) for relating
the surface topography of model calcium carbonate –based coating layers to the
corresponding gloss levels. The authors described a set of roughness descriptors that can be
applied for exploring a surface structure in detail. The applicability of a surface skewness
parameter for exploring the topography development at different scales was discussed. A
coating layer structure is also affected by the coating formulation. Calcium carbonate and clay
Modern methods for determination of paper surface topography
21
pigment particles have different shapes, i.e. ground calcium carbonates particles are blocky,
while clays are platey. They affect the surface development and pore structure in coating
layers in different ways. Larson et al. (2007) performed a study to reveal the effect of several
blends on the surface and bulk structure of coating layers. The behaviour of the coatings upon
calendering was also assessed. The results showed that increasing the amount of clay caused a
smoother surface, a more compact coating bulk structure and consequently higher gloss
levels. Most recently, Chinga-Carrasco et al. (2008) showed the relative relationship between
several scales of roughness and gloss. Roughness below a wavelength of approximately 1 m
did not affect the gloss of LWC paper significantly. The complementary capabilities of LP,
AFM and X-ray microtomography for assessing surface structures was also demonstrated,
thus giving valuable insight into the structure of coating layers.
6 Conclusions
7 References
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Modern methods for determination of paper surface topography
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16 Atomic Force Microscopy; http://www.asmicro.com/Applications/afmpage.htm
19 Practical guide to SPM; http://www.veeco.com/library/resources.php
20 Wikimedia Commons: AFM block diagram.svg; http://commons.wikimedia.org
21 The Opensource Handbook of Nanoscience and Nanotechnology: Nanotechnology/AFM;
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