ASTM E2382-04(2012)

Standard Guide to Scanner and Tip Related Artifacts in Scanning Tunneling Microscopy and Atomic Force Microscopy

SIGNIFICANCE AND USE
4.1 This compilation is limited to artifacts observed in scanning tunneling microscopes and contact-mode atomic force microscopes. In particular, this document focuses on artifacts related to probe motion and geometrical considerations of the tip and surface interaction. Many of the artifacts described here extend to other scanned probe microscopies where piezoscanners are used as positioning elements or where tips of similar geometries are used. These are not the only artifacts associated with measurements obtained by STM or AFM. Artifacts can also arise from the following: control electronics (for example, improper feedback gains); noise (mechanical, acoustic, or electronic); drift (thermal or mechanical); problems unique to signal detection methods (for example, laser spillover in optical lever schemes); improper use of image processing (real time or post processed); sample preparation, environment (for example, humidity) and tip-surface interaction (for example, excessive electrostatic, adhesive, shear, and compressive forces). It is suggested that these other types of artifacts form the basis of future ASTM guides.
SCOPE
1.1 All microscopes are subject to artifacts. The purpose of this document is to provide a description of commonly observed artifacts in scanning tunneling microscopy (STM) and atomic force microscopy (AFM) relating to probe motion and geometric considerations of the tip and surface interaction, provide literature references of examples and, where possible, to offer an interpretation as to the source of the artifact. Because the scanned probe microscopy field is a burgeoning one, this document is not meant to be comprehensive but rather to serve as a guide to practicing microscopists as to possible pitfalls one may expect. The ability to recognize artifacts should assist in reliable evaluation of instrument operation and in reporting of data.  
1.2 A limited set of terms will be defined here. A full description of terminology relating to the description, operation, and calibration of STM and AFM instruments is beyond the scope of this document.  
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

General Information

• Status: Historical
• Publication Date: 31-Oct-2012
• ICS: 17.040.20 - Properties of surfaces
• Technical Committee: E42 - Surface Analysis
• Drafting Committee: E42.14 - STM/AFM

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E2382 − 04 (Reapproved 2012)
Standard Guide to
Scanner and Tip Related Artifacts in Scanning Tunneling
Microscopy and Atomic Force Microscopy
This standard is issued under the fixed designation E2382; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope have origins in sample preparation, instrument hardware/
software, operation, post processing of data, etc.
1.1 All microscopes are subject to artifacts. The purpose of
3.1.2 image—surface topography represented by plotting
this document is to provide a description of commonly
the z value for feature height as a function of x and y position.
observed artifacts in scanning tunneling microscopy (STM)
Typically the z height value is derived from the necessary z
and atomic force microscopy (AFM) relating to probe motion
voltage applied to the scanner to allow the feedback value to
andgeometricconsiderationsofthetipandsurfaceinteraction,
remain constant during the generation of the image. The
provide literature references of examples and, where possible,
“image” is therefore a contour plot of a constant value of the
to offer an interpretation as to the source of the artifact.
surfacepropertyunderstudy(forexample,tunnelingcurrentin
Because the scanned probe microscopy field is a burgeoning
STM or lever deflection in AFM).
one,thisdocumentisnotmeanttobecomprehensivebutrather
to serve as a guide to practicing microscopists as to possible 3.1.3 tip—the physical probe used in either STM or AFM.
pitfalls one may expect. The ability to recognize artifacts
For STM the tip is made from a conductive metal wire (for
should assist in reliable evaluation of instrument operation and example, tungsten or Pt/Ir) while for AFM the tip can be
in reporting of data.
conductive(forexample,dopedsilicon)ornon-conductive(for
example, silicon nitride). The important performance param-
1.2 A limited set of terms will be defined here. A full
eters for tips are the aspect ratio, the radius of curvature, the
description of terminology relating to the description,
opening angle, the overall geometrical shape, and the material
operation, and calibration of STM and AFM instruments is
of which they are made.
beyond the scope of this document.
3.1.4 cantilever or lever—the flexible beam onto which the
1.3 The values stated in SI units are to be regarded as
AFM tip is placed at one end with the other end anchored
standard. No other units of measurement are included in this
rigidly to the microscope. The important performance param-
standard.
eters for cantilevers are the force constant (expressed in N/m)
and resonance frequency (expressed in kHz typically). These
2. Referenced Documents
values will depend on the geometry and material properties of
2.1 ASTM Standards:
the lever.
E1813Practice for Measuring and Reporting Probe Tip
3.1.5 scanner—the device used to position the sample and
Shape in Scanning Probe Microscopy
tip relative to one another. Generally either the tip or sample is
scanned in either STM or AFM. The scanners are typically
3. Terminology
made from piezoelectric ceramics. Tripod scanners use three
3.1 Definitions of Terms Specific to This Standard:
independent piezo elements to provide motion in x, y, and z.
3.1.1 artifact—any feature of an image generated by an
Tube scanners are single element piezo materials that provide
AFM or STM that deviates from the true surface.Artifacts can
coupled x,y,z motion. The important performance parameters
for scanners are the distance of movement per applied volt
(expressed as nm/V) and the lateral and vertical scan ranges
(expressed in microns).
This guide is under the jurisdiction of ASTM Committee E42 on Surface
Analysis and is the direct responsibility of Subcommittee E42.14 on STM/AFM.
3.1.6 scan angle—the angle of rotation of the x scan axis
Current edition approved Nov. 1, 2012. Published December 2012. Originally
relative to the x-axis of the sample
approved in 2004. Last previous edition approved in 2004 as E2382 – 04. DOI:
10.1520/E2382-04R12.
3.1.7 tip characterizer—a special sample used to determine
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
thegeometryofthetip.Thetipinquestionisusedtoimagethe
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
characterizer.Theimagethenbecomesaninputtoanalgorithm
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. for determining the tip geometry.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2382 − 04 (2012)
3.2 Abbreviations:
3.2.1 AFM—atomic force microscopy (microscope). We
refer here to contact mode AFM as opposed to non-contact
techniques.
3.2.2 STM—scanning tunneling microscopy (microscope).
4. Significance and Use
4.1 This compilation is limited to artifacts observed in
scanning tunneling microscopes and contact-mode atomic
force microscopes. In particular, this document focuses on
artifacts related to probe motion and geometrical consider-
ations of the tip and surface interaction. Many of the artifacts
described here extend to other scanned probe microscopies
wherepiezoscannersareusedaspositioningelementsorwhere
tips of similar geometries are used. These are not the only
artifacts associated with measurements obtained by STM or
AFM. Artifacts can also arise from the following: control
electronics (for example, improper feedback gains); noise
(mechanical, acoustic, or electronic); drift (thermal or me-
chanical); problems unique to signal detection methods (for
example, laser spillover in optical lever schemes); improper
FIG. 1 Ideal Behavior of a Piezoelectric Scanner in One Dimen-
sion (Either x, y, or z)
use of image processing (real time or post processed); sample
preparation, environment (for example, humidity) and tip-
surface interaction (for example, excessive electrostatic,
adhesive, shear, and compressive forces). It is suggested that
these other types of artifacts form the basis of future ASTM
guides.
5. Artifacts in STM and AFM
5.1 Artifacts arising from Scanner Motion—Scanners are
made from piezoelectric ceramic materials used to accurately
position the tip relative to the surface on the nanometer scale.
They exhibit an inverse piezoelectric effect where the material
will undergo dimensional change in an applied electric field.
Ideal behavior is often assumed when using these devices in
STM or AFM microscopes. Ideal behavior implies: (1) linear
response in dimensional change per applied volt; (2)no
dependenceofthedimensionalresponseonthedirectionofthe
voltage change, the magnitude of the voltage change, or the
rate of the voltage change (Fig. 1). The motions of these
devices are subject to deviations that include non-linearity,
hysteresis, and creep (1-5). In addition to these non-ideal
motions which are characteristic of independent scanner axes,
artifacts may arise as a consequence of coupling between the
NOTE1—Non-linearextensioninresponsetolinearappliedvoltageand
axes.
hysteresis where the sensitivity varies depending on direction of applied
5.1.1 Non-Linearity—Non-linearitymeansthattheresponse
voltage.
of the scanner in nm/V changes as a function of applied FIG. 2 Non-Ideal Behavior in a Piezoelectric Scanner
voltage.Typicallytheresponsedeviatesmoreatlargerpositive
or negative voltages than near zero applied volts (2) (Fig. 2).
appear curved. While obvious for test structures, this effect
Non-linear effects in the lateral direction (x,y) can be observed
could go unnoticed on other samples that do not have evenly
most clearly when scanning a periodic structure with known
spaced surface features. This effect can be compensated for in
spatial frequencies such as a diffraction grating. Since the
software by applying a non-linear voltage ramp during scan-
scanner does not move linearly with applied voltage, the
ning based on prior calibration (open loop method) or by
measurement points will not be equally spaced. The observed
independently measuring the position of the scanner using an
spacingswillvaryovertheimageandsomelinearfeatureswill
additionalpositionsensorsuchasacapacitorplate(closedloop
method) (5).Anexampleoftheopenloopcorrectionmethodis
given in Fig. 3. Non-linear effects in z or height measurements
The boldface numbers in parentheses refer to a list of references at the end of
this standard. are less obvious but can be detected using vertical height
E2382 − 04 (2012)
NOTE 1—(Images courtesy of G. Meyers. Used with permission of The Dow Chemical Company.)
FIG. 3 AFM of a Two-Dimensional Grating (Top) without Software Linearity Correction and (Bottom) with the Open-Loop Correction
standards (4).Theyaremostnoticeablewhentryingtomeasure angle. The effects of hysteresis can be compensated for by
small features (small changes in V) and large features (large means of a software correction. However, the accuracy of the
changes in V) within the same scan. They are also more correction is limited by the need to create a model with a large
difficulttocorrectforduetothecomplexcouplingofmotionof number of variables. In the case where voltage ramps are
x and y to z, in say, a tube scanner. applied to the scanners, such as in rastering in x,y for STM or
5.1.2 Hysteresis—Hysteresis occurs in piezoelectric materi- AFMimagingorforrampinginzforgeneratingaforceversus
als when the response traces a different path depending on the distance curve in AFM, the tip or sample will move non-
direction of the voltage change (Fig. 2). The magnitude of the uniformly. Hysteresis could explain why the distance between
effect will depend on the DC starting voltage, the size of the the same features in an image might differ depending on the
voltage change, the rate of the voltage change, and the scan direction of scan (trace versus retrace), the size of the scan, or
E2382 − 04 (2012)
Thisismostoftenaconcernwithlongrangescannersthatmay
have lateral to vertical range ratios in excess of 10:1.
5.1.5 Coupled Motion:
5.1.5.1 Bowing—In either tube or tripod scanners the z
motion is coupled to x and y motion. For a tube scanner this
results in the tube moving in an arc as the tube bends in x or
y directions during scanning. If uncorrected this can give the
appearance of bowing (a central dip) in an otherwise flat
sample. Some systems correct for this in real time by using a
line by line planefit of the data. Alternatively a polynomial
plane can be fit to and subtracted from the data set after image
capture. As with dynamic range effects the bowing artifact is
more common for long range scanners.
5.1.5.2 Abbe Offset Error—Another artifact related to
coupled motion is the Abbe offset error. When the point of
interest on the sample surface is displaced from the true
measuringsystem(thatis,theundeflectedscannertubez-axis),
anangularerrorexistsinthepositioningsystemand,therefore,
the measured displacement. The magnitude of this error is
directly proportional to the length of the ‘lever arm’times the
angularoffsetinradians.Inascannedsampleconfigurationthe
lever length is estimated by the sum of the tube length plus the
NOTE 1—The scanner exhibits a delay in response to sudden voltage
distance to the sample surface. This sum is typically tens of
changes (used with permission from ThermoMicroscopes, now Veeco
Instruments, Inc.).
millimeters while the scanning displacement is only a few
FIG. 4 Creep in a Piezoelectric Scanner is Another Non-Ideal
micronssotheangularoffsetsaretypically<<0.0001(radians).
Behavior
Agood example of this effect is in the measurement of lattice
spacings in cleaved mica using a short tube scanner in contact
mode (6). As the sample height is increased the measured
the rate at which the tip is scanned. It would also explain
lattice spacings decrease for the same xy scan size.
inaccuracies in step heights of large features where large
5.1.6 Ringing—Ringingoccurswhenthefeedbackamplifier
voltage sweeps are necessary in the z direction (5).
gain or filter frequency is too high. This causes the tube to
5.1.3 Creep—Creep describes the continued motion of the
oscillate or ring at high frequency and the image becomes
scanner after a rapid change in voltage, such as might occur
dominated by noise. In extreme cases the ringing is audible.
when the scanner encounters a large step during scanning.The
Sometimes optimum imaging occurs with PID settings set just
tubewillcontinuetomoveevenifthevoltageremainsfixedor
belowtheonsetofringing,however,onceotherparametersare
changessign.Thisisatimedependenteffectanditsmagnitude
changed, for example, scan speed or size, the ringing may
will depend on the size of the voltage change and the rate of
return. Horizontal ringing is responsible for the turnaround
voltage change (Fig. 4). Creep accounts for the initial lateral
effect at image edges where the scanner reverses direction
drift apparent after zooming or moving to a new area which
during scanning.
will settle out after several scan lines have been recorded (Fig.
5.2 Artifacts Caused by the Tip—Artifacts derived from the
5a). Creep accounts for the overshoot and slopes at both the
STM or AFM probe tip is the most common sort of artifact
plateaus and bases in line profiles of periodic, tall features that
observed with scanned probe microscopes. Consideration of
havebeenrecordedatafastscanrate.Itisalsoverynoticeable
the geometry and shape, material of construction, and the
in generating AFM force versus distance curves where the x
possible presence of structural defects and contamination,
and y scans are disabled and the z element voltage is ramped.
assists in recognizing tip artifacts. The heights and depths of
Both hysteresis and creep account for the higher force seen in
major surface features determine what portion of the tip
theunloadingversusloadingportionofthecurvesforthesame
interacts with the surface (and therefore which portion of the
sample displacement (so called “reverse-path” effect (3)) seen
tipneedstobeconsideredasasourceofartifacts).Fig.6shows
in Fig. 5b.
an idealized tip characterized by an opening half-angle, α (α =
5.1.4 Dynamic Range—Themaximumextensionofapiezo-
30°intheexample),anaspectratio(lengthtobasewidth(L/W
ceramicscannerinx,y,orzwilldependontheresponseofthe
= 1 in the example), and a spherical shape at the apex. The
piezo material, the size and shape of the scanner, and the
spherical tip described in Fig. 6 is idealized and one of
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