1、Designation: E 2382 04Guide toScanner and Tip Related Artifacts in Scanning TunnelingMicroscopy and Atomic Force Microscopy1This standard is issued under the fixed designation E 2382; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision,
2、 the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) 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 provide a description of com
3、monlyobserved 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 to the source of the art
4、ifact.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 evaluation of instrument op
5、eration andin reporting of data.1.2 A limited set of terms will be defined here. A fulldescription of terminology relating to the description, opera-tion, and calibration of STM and AFM instruments is beyondthe scope of this document.2. Referenced Documents2.1 ASTM Standards:2E 1813 Standard Practic
6、e for Measuring and ReportingProbe Tip Shape in Scanning Probe Microscopy3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 artifactany feature of an image generated by anAFM or STM that deviates from the true surface. Artifacts canhave origins in sample preparation, instrument h
7、ardware/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 y position.Typically the z height value is derived from the necessary zvoltage applied to the scanner to allow the feedback value torem
8、ain constant during the generation of the image. The“image” is therefore a contour plot of a constant value of thesurface property under study (e.g. tunneling current in STM orlever deflection in AFM).3.1.3 tipthe physical probe used in either STM or AFM.For STM the tip is made from a conductive met
9、al wire (e.g.tungsten or Pt/Ir) while forAFM the tip can be conductive (e.g.doped silicon) or non-conductive (e.g. silicon nitride). Theimportant performance parameters for tips are the aspect ratio,the radius of curvature, the opening angle, the overall geo-metrical shape, and the material of which
10、 they are made.3.1.4 cantilever or leverthe flexible beam onto which theAFM tip is placed at one end with the other end anchoredrigidly to the microscope. The important performance param-eters for cantilevers are the force constant (expressed in N/m)and resonance frequency (expressed in kHz typicall
11、y). Thesevalues will depend on the geometry and material properties ofthe lever.3.1.5 scannerthe device used to position the sample 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 sc
12、anners use threeindependent piezo elements to provide motion in x, y, and z.Tube scanners are single element piezo materials that providecoupled 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 vertica
13、l scan ranges(expressed in microns).3.1.6 scan anglethe angle of rotation of the x scan axisrelative to the x-axis of the sample3.1.7 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
14、an algorithmfor determining the tip geometry.3.2 Abbreviations Used in this Document3.2.1 AFMatomic force microscopy (microscope). Werefer here to contact mode AFM as opposed to non-contacttechniques.3.2.2 STMscanning tunneling microscopy (microscope).1This Guide is under the jurisdiction of ASTM Co
15、mmittee E42 on SurfaceAnalysis and is the direct responsibility of Subcommittee E42.14 on STM/AFM.Current edition approved August 1, 2004. Published September 2004.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Boo
16、k of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4. Significance and Use4.1 This compilation is limited to artifacts observed insca
17、nning tunneling microscopes and contact-mode atomicforce 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 piezo
18、scanners are used as positioning elements or wheretips 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 (e.g., improper feedback gains); noise (mechanical,acoustic, or e
19、lectronic); drift (thermal or mechanical); prob-lems unique to signal detection methods (e.g., laser spillover inoptical lever schemes); improper use of image processing (realtime or post processed); sample preparation, environment (e.g.humidity) and tip-surface interaction (e.g. excessive electro-s
20、tatic, adhesive, shear, and compressive forces). It is suggestedthat these other types of artifacts form the basis of futureASTM guides.5. Artifacts in STM and AFM5.1 Artifacts arising from Scanner MotionScanners are made from piezoelectric ceramic materials usedto accurately position the tip relati
21、ve to the surface on thenanometer scale. They exhibit an inverse piezoelectric effectwhere the material will undergo dimensional change in anapplied electric field. Ideal behavior is often assumed whenusing these devices in STM or AFM microscopes. Idealbehavior implies: 1) linear response in dimensi
22、onal change perapplied volt; 2) no dependence of the dimensional response onthe direction of the voltage change, the magnitude of thevoltage change, or the rate of the voltage change (Fig. 1). Themotions of these devices are subject to deviations that includenon-linearity, hysteresis, and creep 1-5.
23、 In addition to thesenon-ideal motions which are characteristic of independentscanner axes, artifacts may arise as a consequence of couplingbetween the axes.5.1.1 Non-linearityNon-linearity means that the response of the scanner innm/V changes as a function of applied voltage. Typically theresponse
24、deviates more at larger positive or negative voltagesthan near zero applied volts 2 (Fig. 2). Non-linear effects inthe lateral direction (x,y) can be observed most clearly whenscanning a periodic structure with known spatial frequenciessuch as a diffraction grating . Since the scanner does not movel
25、inearly with applied voltage, the measurement points will notbe equally spaced. The observed spacings will vary over theimage and some linear features will appear curved. Whileobvious for test structures, this effect could go unnoticed onother samples that do not have evenly spaced surface features.
26、This effect can be compensated for in software by applying anon-linear voltage ramp during scanning based on prior cali-bration (open loop method) or by independently measuring theposition of the scanner using an additional position sensor suchas a capacitor plate (closed loop method) 5. An example
27、ofthe open loop correction method is given in Fig. 3. Non-lineareffects in z or height measurements are less obvious but can bedetected using vertical height standards 4. They are mostnoticeable when trying to measure small features (smallchanges in V) and large features (large changes in V) within
28、thesame scan. They are also more difficult to correct for due to thecomplex coupling of motion of x and y to z, in say, a tubescanner.5.1.2 HysteresisFIG. 1 Ideal behavior of a piezoelectric scanner in one dimension(either x,y, or z).FIG. 2 Non-ideal behavior in a piezoelectric scanner. Non-linearex
29、tension in response to linear applied voltage and hysteresiswhere the sensitivity varies depending on direction of appliedvoltage.E2382042Hysteresis occurs in piezoelectric materials when the re-sponse traces a different path depending on the direction of thevoltage change (Fig. 2). The magnitude of
30、 the effect willdepend on the DC starting voltage, the size of the voltagechange, the rate of the voltage change, and the scan angle. Theeffects of hysteresis can be compensated for by means of asoftware correction. However, the accuracy of the correction islimited by the need to create a model with
31、 a large number ofvariables. In the case where voltage ramps are applied to thescanners, such as in rastering in x,y for STM or AFM imagingor for ramping in z for generating a force vs. distance curve inAFM, the tip or sample will move non-uniformly. Hysteresiscould explain why the distance between
32、the same features in animage might differ depending on the direction of scan (trace vs.retrace), the size of the scan, or the rate at which the tip isscanned. It would also explain inaccuracies in step heights oflarge features where large voltage sweeps are necessary in thez direction 5.5.1.3 CreepC
33、reep describes the continued motion of the scanner after arapid change in voltage, such as might occur when the scannerencounters a large step during scanning. The tube will continueto move even if the voltage remains fixed or changes sign. Thisis a time dependent effect and its magnitude will depen
34、d on theFIG. 3 AFM of a two-dimensional grating (top) without software linearity correction and (bottom) with the open-loop correction. (Imagescourtesy of G. Meyers. Used with permission of The Dow Chemical Company.)E2382043size of the voltage change and the rate of voltage change (Fig.4). Creep acc
35、ounts for the initial lateral drift apparent afterzooming or moving to a new area which will settle out afterseveral scan lines have been recorded (Fig. 5a). Creep accountsfor the overshoot and slopes at both the plateaus and bases inline profiles of periodic, tall features that have been recorded a
36、ta fast scan rate. It is also very noticeable in generating AFMforce vs. distance curves where the x and y scans are disabledand the z element voltage is ramped. Both hysteresis and creepaccount for the higher force seen in the unloading vs. loadingportion of the curves for the same sample displacem
37、ent (socalled “reverse-path” effect 3) seen in Fig. 5b.5.1.4 Dynamic RangeThe maximum extension of a piezoceramic scanner in x, y,or z will depend on the response of the piezo material, the sizeand shape of the scanner, and the maximum voltages that canbe applied to the piezo electrodes. Each scanne
38、r has a statedrange of x, y and z motion. Features in an image can appearclipped if the vertical height exceeds the available range of zmotion prescribed for the scanner in use. If the sample plane issubstantially tilted relative to the scanner, portions of the imagemay appear to go flat as the scan
39、ner is contracted or elongatedto its dynamic range limit. This is most often a concern withlong range scanners that may have lateral to vertical rangeratios in excess of 10:1.5.1.5 Coupled Motion5.1.5.1 BowingIn either tube or tripod scanners the z motion is coupled tox and y motion. For a tube scan
40、ner this results in the tubemoving in an arc as the tube bends in x or y directions duringscanning. If uncorrected this can give the appearance ofbowing (a central dip) in an otherwise flat sample. Somesystems correct for this in real time by using a line by lineplanefit of the data. Alternatively a
41、 polynomial plane can be fitto and subtracted from the data set after image capture.As withdynamic range effects the bowing artifact is more common forlong range scanners.5.1.5.2 Abbe Offset ErrorAnother artifact related to coupled motion is the Abbe offseterror. When the point of interest on the sa
42、mple surface isdisplaced from the true measuring system (that is, the unde-flected scanner tube z-axis), an angular error exists in thepositioning system and, therefore, the measured displacement.The magnitude of this error is directly proportional to thelength of the lever arm times the angular off
43、set in radians. Ina scanned sample configuration the lever length is estimated bythe sum of the tube length plus the distance to the samplesurface. This sum is typically tens of millimeters while thescanning displacement is only a few microns so the angularoffsets are typically 10:1 (2-3) = 5 nmelec
44、trochemicallyetched 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 AsperityF= 50 nm(variable)AThe aspect ratio is defined as the ratio of Ltip:Wtipas shown in Fig. 6.BA cusp is introduced
45、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 side and asymmetric from front to back on the shank. At the tip the cross-section istriangular.DProduced by focused ion-beam
46、 (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 or FEGSEM.FDue to the nature of the cutting, a nanoscale asperity is formed which is responsible for the imagingFIG. 7 FE
47、GSEM image of a silicon field emitter array (sample tilted 85). In the array the emitters are located on a square grid with a 10micron pitch. A FEGSEM image of an AFM cantilever/tip is inset. The cantilever is tilted 10 to simulate its position in the microscope.The emitter tips are longer and sharp
48、er than the AFM tip. (FEGSEM images courtesy of D. Millbrant. Used with permission of The DowChemical Company. The Si emitter sample was provided by H. Busta of Amoco.)E2382047rigorously correct. If this were true it should then be possibleto “deconvolve” the original surface from the image if, fore
49、xample, the tip shape was known exactly. In the example ofFig. 16, any attempt to regain the original surface profile fromthe image profile and actual tip profile would not regenerate thesecond, smaller feature. Consideration of these regions ofinaccessibility of surface features is of utmost importance inattempting to describe the information content of an AFM orSTM image.5.2.1.5 Angular EffectsThe schematics to this point have assumed that the scanningtip z-axis is normal to the surface plane under study. Forpractical reasons, cantilevers are mounted in
