1、Designation: E2382 04 (Reapproved 2012)Standard Guide toScanner and Tip Related Artifacts in Scanning TunnelingMicroscopy and Atomic Force Microscopy1This standard is issued under the fixed designation E2382; the number immediately following the designation indicates the year oforiginal adoption or,
2、 in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 All microscopes are subject to artifacts. The purpose ofthis document is to prov
3、ide a description of commonlyobserved artifacts in scanning tunneling microscopy (STM)and atomic force microscopy (AFM) relating to probe motionand geometric considerations of the tip and surface interaction,provide literature references of examples and, where possible,to offer an interpretation as
4、to the source of the artifact.Because the scanned probe microscopy field is a burgeoningone, this document is not meant to be comprehensive but ratherto serve as a guide to practicing microscopists as to possiblepitfalls one may expect. The ability to recognize artifactsshould assist in reliable eva
5、luation of instrument operation andin reporting of data.1.2 A limited set of terms will be defined here. A fulldescription of terminology relating to the description,operation, and calibration of STM and AFM instruments isbeyond the scope of this document.1.3 The values stated in SI units are to be
6、regarded asstandard. No other units of measurement are included in thisstandard.2. Referenced Documents2.1 ASTM Standards:2E1813 Practice for Measuring and Reporting Probe TipShape in Scanning Probe Microscopy3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 artifactany feature
7、of an image generated by anAFM or STM that deviates from the true surface. Artifacts canhave origins in sample preparation, instrument hardware/software, operation, post processing of data, etc.3.1.2 imagesurface topography represented by plottingthe z value for feature height as a function of x and
8、 y position.Typically the z height value is derived from the necessary zvoltage applied to the scanner to allow the feedback value toremain constant during the generation of the image. The“image” is therefore a contour plot of a constant value of thesurface property under study (for example, tunneli
9、ng current inSTM or lever deflection in AFM).3.1.3 tipthe physical probe used in either STM or AFM.For STM the tip is made from a conductive metal wire (forexample, tungsten or Pt/Ir) while for AFM the tip can beconductive (for example, doped silicon) or non-conductive (forexample, silicon nitride).
10、 The important performance param-eters for tips are the aspect ratio, the radius of curvature, theopening angle, the overall geometrical shape, and the materialof which they are made.3.1.4 cantilever or leverthe flexible beam onto which theAFM tip is placed at one end with the other end anchoredrigi
11、dly to the microscope. The important performance param-eters for cantilevers are the force constant (expressed in N/m)and resonance frequency (expressed in kHz typically). Thesevalues will depend on the geometry and material properties ofthe lever.3.1.5 scannerthe device used to position the sample
12、andtip relative to one another. Generally either the tip or sample isscanned in either STM or AFM. The scanners are typicallymade from piezoelectric ceramics. Tripod scanners use threeindependent piezo elements to provide motion in x, y, and z.Tube scanners are single element piezo materials that pr
13、ovidecoupled x,y,z motion. The important performance parametersfor scanners are the distance of movement per applied volt(expressed as nm/V) and the lateral and vertical scan ranges(expressed in microns).3.1.6 scan anglethe angle of rotation of the x scan axisrelative to the x-axis of the sample3.1.
14、7 tip characterizera special sample used to determinethe geometry of the tip. The tip in question is used to image thecharacterizer.The image then becomes an input to an algorithmfor determining the tip geometry.1This guide is under the jurisdiction of ASTM Committee E42 on SurfaceAnalysis and is th
15、e direct responsibility of Subcommittee E42.14 on STM/AFM.Current edition approved Nov. 1, 2012. Published December 2012. Originallyapproved in 2004. Last previous edition approved in 2004 as E2382 04. DOI:10.1520/E2382-04R12.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orco
16、ntact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.2 Abbreviations:3.2
17、.1 AFMatomic force microscopy (microscope). Werefer here to contact mode AFM as opposed to non-contacttechniques.3.2.2 STMscanning tunneling microscopy (microscope).4. Significance and Use4.1 This compilation is limited to artifacts observed inscanning tunneling microscopes and contact-mode atomicfo
18、rce microscopes. In particular, this document focuses onartifacts related to probe motion and geometrical consider-ations of the tip and surface interaction. Many of the artifactsdescribed here extend to other scanned probe microscopieswhere piezoscanners are used as positioning elements or wheretip
19、s of similar geometries are used. These are not the onlyartifacts associated with measurements obtained by STM orAFM. Artifacts can also arise from the following: controlelectronics (for example, improper feedback gains); noise(mechanical, acoustic, or electronic); drift (thermal or me-chanical); pr
20、oblems unique to signal detection methods (forexample, laser spillover in optical lever schemes); improperuse of image processing (real time or post processed); samplepreparation, environment (for example, humidity) and tip-surface interaction (for example, excessive electrostatic,adhesive, shear, a
21、nd compressive forces). It is suggested thatthese other types of artifacts form the basis of future ASTMguides.5. Artifacts in STM and AFM5.1 Artifacts arising from Scanner MotionScanners aremade from piezoelectric ceramic materials used to accuratelyposition the tip relative to the surface on the n
22、anometer scale.They exhibit an inverse piezoelectric effect where the materialwill undergo dimensional change in an applied electric field.Ideal behavior is often assumed when using these devices inSTM or AFM microscopes. Ideal behavior implies: (1) linearresponse in dimensional change per applied v
23、olt; (2)nodependence of the dimensional response on the direction of thevoltage change, the magnitude of the voltage change, or therate of the voltage change (Fig. 1). The motions of thesedevices are subject to deviations that include non-linearity,hysteresis, and creep (1-5).3In addition to these n
24、on-idealmotions which are characteristic of independent scanner axes,artifacts may arise as a consequence of coupling between theaxes.5.1.1 Non-LinearityNon-linearity means that the responseof the scanner in nm/V changes as a function of appliedvoltage. Typically the response deviates more at larger
25、 positiveor negative voltages than near zero applied volts (2) (Fig. 2).Non-linear effects in the lateral direction (x,y) can be observedmost clearly when scanning a periodic structure with knownspatial frequencies such as a diffraction grating . Since thescanner does not move linearly with applied
26、voltage, themeasurement points will not be equally spaced. The observedspacings will vary over the image and some linear features willappear curved. While obvious for test structures, this effectcould go unnoticed on other samples that do not have evenlyspaced surface features. This effect can be co
27、mpensated for insoftware by applying a non-linear voltage ramp during scan-ning based on prior calibration (open loop method) or byindependently measuring the position of the scanner using anadditional position sensor such as a capacitor plate (closed loop3The boldface numbers in parentheses refer t
28、o a list of references at the end ofthis standard.FIG. 1 Ideal Behavior of a Piezoelectric Scanner in One Dimen-sion (Either x, y, or z)NOTE 1Non-linear extension in response to linear applied voltage andhysteresis where the sensitivity varies depending on direction of appliedvoltage.FIG. 2 Non-Idea
29、l Behavior in a Piezoelectric ScannerE2382 04 (2012)2method) (5).An example of the open loop correction method isgiven in Fig. 3. Non-linear effects in z or height measurementsare less obvious but can be detected using vertical heightstandards (4).They are most noticeable when trying to measuresmall
30、 features (small changes in V) and large features (largechanges in V) within the same scan. They are also moredifficult to correct for due to the complex coupling of motion ofx and y to z, in say, a tube scanner.5.1.2 HysteresisHysteresis occurs in piezoelectric materi-als when the response traces a
31、 different path depending on thedirection of the voltage change (Fig. 2). The magnitude of theeffect will depend on the DC starting voltage, the size of thevoltage change, the rate of the voltage change, and the scanangle. The effects of hysteresis can be compensated for bymeans of a software correc
32、tion. However, the accuracy of thecorrection is limited by the need to create a model with a largenumber of variables. In the case where voltage ramps areapplied to the scanners, such as in rastering in x,y for STM orAFM imaging or for ramping in z for generating a force versusdistance curve in AFM,
33、 the tip or sample will move non-uniformly. Hysteresis could explain why the distance betweenthe same features in an image might differ depending on thedirection of scan (trace versus retrace), the size of the scan, orthe rate at which the tip is scanned. It would also explainNOTE 1(Images courtesy
34、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 CorrectionE2382 04 (2012)3inaccuracies in step heights of large features where largevoltage sweeps are necessary in the
35、z direction (5).5.1.3 CreepCreep describes the continued motion of thescanner after a rapid change in voltage, such as might occurwhen the scanner encounters a large step during scanning. Thetube will continue to move even if the voltage remains fixed orchanges sign. This is a time dependent effect
36、and its magnitudewill depend on the size of the voltage change and the rate ofvoltage change (Fig. 4). Creep accounts for the initial lateraldrift apparent after zooming or moving to a new area whichwill settle out after several scan lines have been recorded (Fig.5a). Creep accounts for the overshoo
37、t and slopes at both theplateaus and bases in line profiles of periodic, tall features thathave been recorded at a fast scan rate. It is also very noticeablein generating AFM force versus distance curves where the xand y scans are disabled and the z element voltage is ramped.Both hysteresis and cree
38、p account for the higher force seen inthe unloading versus loading portion of the curves for the samesample displacement (so called “reverse-path” effect (3) seenin Fig. 5b.5.1.4 Dynamic RangeThe maximum extension of a piezo-ceramic scanner in x, y, or z will depend on the response of thepiezo mater
39、ial, the size and shape of the scanner, and themaximum voltages that can be applied to the piezo electrodes.Each scanner has a stated range of x, y, and z motion. Featuresin an image can appear clipped if the vertical height exceedsthe available range of z motion prescribed for the scanner inuse. If
40、 the sample plane is substantially tilted relative to thescanner, portions of the image may appear to go flat as thescanner is contracted or elongated to its dynamic range limit.This is most often a concern with long range scanners that mayhave lateral to vertical range ratios in excess of 10:1.5.1.
41、5 Coupled Motion:5.1.5.1 BowingIn either tube or tripod scanners the zmotion is coupled to x and y motion. For a tube scanner thisresults in the tube moving in an arc as the tube bends in x ory directions during scanning. If uncorrected this can give theappearance of bowing (a central dip) in an oth
42、erwise flatsample. Some systems correct for this in real time by using aline by line planefit of the data. Alternatively a polynomialplane can be fit to and subtracted from the data set after imagecapture. As with dynamic range effects the bowing artifact ismore common for long range scanners.5.1.5.
43、2 Abbe Offset ErrorAnother artifact related tocoupled motion is the Abbe offset error. When the point ofinterest on the sample surface is displaced from the truemeasuring system (that is, the undeflected scanner tube z-axis),an angular error exists in the positioning system and, therefore,the measur
44、ed displacement. The magnitude of this error isdirectly proportional to the length of the lever arm times theangular offset in radians. In a scanned sample configuration thelever length is estimated by the sum of the tube length plus thedistance to the sample surface. This sum is typically tens ofmi
45、llimeters while the scanning displacement is only a fewmicrons so the angular offsets are typically 10:1 (2-3) = 5 nmelectrochemicallyetched wireSTM W, Au, or PtPt/Ir alloyConical 5:1 (8-10) = 50 nmion-milled wire STM Pt/Ir alloy Conical 5:1 (5) = 5 nmmechanically cutwireSTM Pt/Ir alloy Ill-defined
46、AsperityF= 50 nm(variable)AThe aspect ratio is defined as the ratio of Ltip:Wtipas shown in Fig. 6.BA cusp is introduced at the outer 0.1 micron that results in a sharper point and correspondingly smaller half angle.CDue to the “kite” shaped cross-section the half angle is symmetric from side to sid
47、e and asymmetric from front to back on the shank. At the tip the cross-section istriangular.DProduced by focused ion-beam (FIB) milling of conventional pyramidal silicon nitride tip.EProduced by e-beam deposition of contamination on the apex of a conventional pyramidal silicon nitride tip in an SEM
48、or FEGSEM.FDue to the nature of the cutting, a nanoscale asperity is formed which is responsible for the imagingNOTE 1In the array the emitters are located on a square grid with a 10 micron pitch. A FEGSEM image of an AFM cantilever/tip is inset. Thecantilever is tilted 10 to simulate its position i
49、n the microscope. The emitter tips are longer and sharper than the AFM tip. (FEGSEM images courtesyof D. Millbrant. Used with permission of The Dow Chemical Company. The Si emitter sample was provided by H. Busta of Amoco.)FIG. 7 FEGSEM Image of a Silicon Field Emitter Array (Sample Tilted 85)E2382 04 (2012)7image may contain areas where structural information aboutthe original surface is missing (7, 9, 10, 17, 18). Consider theundercut surface feature mentioned previously. If there is asecond feature in the vicinity of the undercut the tip may notse
