Biomedical Engineering Reference
In-Depth Information
Table 3.2.15-1 Definition of surface parameters
Parameter
Definition
lm
Evaluation length ¼ the horizontal limitation for the assessment of surface parameters
lv
Pre-travel length ¼ the distance traversed by the tracing system over the sample before the tracing (lt)
starts
ln
Over-travel length ¼ the distance traversed or area scanned by the tracing system over the sample
after the tracing (lt)
lt
Tracing length ¼ the distance traversed by the tracing system when taking a measurement. It
comprises the pre- and overtravel, and the evaluation length
le
Sampling length ¼ a standardized number of evaluation lengths/areas as required to obtain a proper
surface characterization
Ra/Sa
Arithmetical mean roughness ¼ the arithmetical average value of all vertical departures of the profile
or surface from the mean line throughout the sampling length/area
Rq/Sq
Root-mean square roughness ¼ the root-mean square value of the profile or surface departures
within the sampling length/area
Rt/St
Maximum roughness depth ¼ the distance between the highest and lowest points of the profile or
surface within the evaluation length/area
Rz/Sz
Mean peak-to-valley height ¼ the average of the single peak-to valley heights of five adjoining
sampling lengths/areas
Rsk/Ssk
Skewness ¼ measure of the symmetry of the amplitude density function (ADF)
ADF
Amplitude density function ¼ the graphical representation of the material distribution within the
evaluation length/area
Rku/Sku
Kurtosis ¼ fourth central moment of the profile or surface amplitude density with the evaluation
length/area. Kurtosis is the measure of the sharpness of the profile or surface
Rcx/Rcy
¼ mean spacing between surface peaks of the surface/area profile along the X or Y direction
Scx/Scy
Sti
¼ surface texture index, i.e. min. (Rq/Sq divided by max. Rq/Sq þ min. Rsk/Ssk divided by max Rsk/
Ssk þ min.(q divided by max.)q þ min (Rc/Sc divided max. Rc/Sc) divided by 4
Dq
¼ the root mean square slope of the rough profile throughout the evaluation length/area
lq
¼ the root mean square of the spacings between local peaks and valleys, taking into account their
relative amplitudes and individual spatial frequencies
surface profiles can be created (
Fig. 3.2.15-2
). Occa-
sionally, techniques are used in which the reflected light
is not directly translated to an electrical signal. In these
so-called interferometers a surface profile is created by
combining light reflecting off the surface with light
reflecting off a reference substrate. When those two light
bundles combine, the light waves interfere to produce
a pattern of fringes, which are used to determine surface
height differences.
The resolution of noncontact methods can be in the
nanometer range. The limiting factor is the spot size.
Several scans have to be taken to obtain a representative
surface area. Occasionally, this is impossible or too la-
borious. In light beam interferometry, an additional
disadvantage is that the substrate surface has to provide
at least some reflectivity.
Atomic force microscop
y
Atomic force microscopy (AFM) is a direct method for
determining high-resolution surface patterns (
Binnig
et al.
, 1986
; van der
Werf
et al.
, 1993
) (also see Section
3.1.4). In AFM the substrate surface is brought close to
a tip on a small cantilever which is attached to a piezo
tube. The deflection of the cantilever, generated by in-
teraction forces between tip and substrate surface, is
detected and used as an input signal for a measuring
