AFM probe characterization
The condition of an AFM's tip can be viewed using an electron microscope, but this is time consuming and often damaging due to the necessary conductive coatings. Alternatively, the AFM can be used to assess the tip being used by the quality of the image of a specific sample. Any material of known size and geometry can be used to visualize the AFM tip, but some material is better suited depending on the user's needs.
The following are some suggestions and sources of things which can be used for the determination of the AFM tip structure.
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Geometry
It is well known that the geometry of the AFM probe tip itself directly affects image resolution, types of imaging artifacts, mechanical stresses imposed on the imaged surface, and the density of bio/chemical functional groups intentionally deposited on the probe surface. Probes come in two basic geometries: as-deposited conospheres and attached spheres. Conospheres are typically Si or Si3N4 pyramids fabricated via etching via standard lithography, terminating in imperfect apexes that are approximated as spherical with radii on the order of 20 nm (approximately the limit of this fabrication technique. Attached spheres are typically polystyrene or borosilicate spheres of micron-scale radius that are glued to a tipless cantilever free-end. There are several approaches by which the user can quantify the geometry of the AFM probe, and also several approaches by which the AFM probe can be used to quantify the geometry of larger probes used in nanoindentation-enabled imaging and characterization of material surfaces. Scanning electron microscopy is commonly used to estimate probe radii from 2D images of the probe, but this approach is tedious, must be done for each probe (risking cantilever damage and probe surface contamination), and subjective in that circles of best fit are imposed on these 2D projections of the probe. Alternatively, scanning of the AFM probe with another, modified AFM probe (eg, one to which a carbon nanotube of sub-20 nm radius has been affixed) can be used, and the resulting 3D height scan analyzed with recourse to best-fit algorithms approximating spheres, cones, or conospheres [cite].
Any material of known size and geometry can be used to visualize the AFM tip, but some material is better suited depending on the user's needs.
The sphere is probably the most extensively used geometry for tip characterization since this class is not only restricted to "true" spheres but also describes the rounded portion of most structures. Calculation of the AFM tip radius (R) is easily obtained using the equation shown provided that only the apex of the tip is used in the image, otherwise the body of the tip will be erroneously measured.
The following are some suggestions and sources of things which can be used for the determination of the AFM tip structure. Elsewhere are some examples of what to look for when using these standards.
- also see SPM Standards and Reference Samples
NOTE
The ratings given for the material presented here are highly subjective - persons interested in calibrating their AFM tips are advised to use this information as an introduction to the subject.
Deposited Material
Colloidal Gold
- Available from most any microscope supplier.
- Random distribution on surface - locating them is hit and miss.
- Come in a range of small sizes (5 to 50 nanometers).
- Can be used to determine AFM tip radius.
- Might contaminate tip.
- Ideally, can codeposit with material being studied.
- Ted Pella supplies a kit for the AFM (#16200).
- Rating: 4 out of 5.
Sources
Structure Probe Inc.
Ted Pella
Nanoprobes
References
- S. Xu and M.F. Arnsdorf, "Calibration of the scanning (atomic) force ficroscope with gold particles" J. Microscopy 173 (1994) pg 199
Polystyrene and Glass Spheres
- As with colloidal gold, spheres are available from many microscope suppliers.
- Range of sizes (50 nanometers to several microns).
- Too big to be used for accurate apex determination using above formula.
- Rating: 3 out of 5.
Sources
Structure Probe Inc.
Ted Pella Image shown
Polysciences, maker of polystyrene microspheres
Banglabs, distributor of microspheres
Duke Scientific, maker of glass and polymer microspheres
Interfacial Dynamics Corporation microspheres
Spherotech, Inc. microspheres
References
- Y. Li and S.M. Lindsay, "Polystyrene Latex particles as a calibration for the atomic force microscope", Rev. Sci. Instrum. 62(11), 1991, pg2630.
Array of Spheres
- Instead of individual spheres, construct an array.
- Can cover large area, visible to naked eye.
- Stable, unlikely to contaminate tip.
- Periodicity = diameter of spheres.
- Any range of sizes.
- Limited area of tip is used in imaging.
- Not to be used for apex determination using above formula.
- Rating: 2 out of 5.
Sources
See above.
References
- Y. Li and S.M. Lindsay, "Polystyrene Latex particles as a calibration for the atomic force microscope", Rev. Sci. Instrum. 62(11), 1991, pg2630.
Biologicals
- Assorted structures.
- Might contaminate tip.
- Examples: DNA - 2 nm, TMV - 15 nm, etc...
- Widely available from most microscope suppliers.
- Rating: 1 out of 5.
Sources
None found.
References
None found.
Fabricated Structures
Random Thin Films
- Sharp, randomly oriented film.
- Covers very large area.
- Useful for quick assessment of tip.
- Ideal for characterizing AFM tip when used with blind reconstruction software.
- "Nioprobe" - niobium film for characterizing last 5-20 nm of tip.
- "TipCheck" - titanium film characterizes the last 100 nm of tip. (image shown)
- Rating: 5 out of 5
Sources
References
- Ken Westra et al., "Tip Artifacts in AFM imaging of thin film surfaces", J. Appl. Phys. 74, 3608 (1993)
- Ken Westra et al., "AFM tip radius needed for accurate imaging of thin film surfaces", J. Vac. Sci. Technol. B 12, 3176 (1994)
- Ken Westra et al., "Effect of tip shape on surface roughness measurements form AFM images of thin films", J. Vac. Sci. Technol. B 13, 344 (1995)
- Ken Westra et al., "The microstructure of thin films observed using AFM", Thin Solid Films 257, 15 (1995)
- Ken Westra et al., "AFM tip radius measurement using the surface of a niobium thin film", J. Vacv. Sci. Technol. A. (accepted)
Sharp Spikes
- Sharp spikes provide direct reflection of AFM tip: closest approximation of a delta function.
- Often, very little deconvolution is required.
- Use sample (NT-MDT) for 1000 nm of tip (image shown).
- Rating: 4 out of 5
Sources
MikroMasch Image shown
References
- V. Bykov, A. Gologanov, and V. Shevyakov, "Test structure for SPM tip shape deconvolution", Applied Physics A: Materials Science & Processing, 66 (1998) 499-502.
Sharp Lines
- These objects provide an image of the tip, but only in one direction.
- "Silicon Nano Edges" - sharp line.
- Silicon-MDT line - Image shown
- Crystal structures - eg. salt (?)
- Rating: 3 out of 5
Sources
Team Nanotec
MikroMasch Image shown
References
S. Hirsekorn, U. Rabe, and W. Arnold, "Ultrasonic Radiation in Dynamic Force Microscopy", Applied Physics A, 72 (2001) 7, S87-S92
Wide Lines
- Can provide an image of the tip, but only in one direction.
- Flat portion provides no information.
- "Silicon Nano Edges" - wide lines (Image shown).
- Also, "Flared Silicon Ridges" - wide lines with undercut.
- Rating: 2 out of 5
Sources
References
None found.
Circular Posts
- The top portion is flat, not rounded. Deconvolution of the images is required.
- Indifferent to scan direction
- Two sizes are available (300 and 700 nm)
- Used primarily for xy linearity calibration
- Rating: 2 out of 5
Sources
Advanced Surface Microscopy, Inc. Image shown
Moxtek
References
None found.
Circular Depressions
- These round holes have been used to characterize tips but have major limitations associated with them (Ref. 1).
- Indifferent to scan direction.
- One size, no longer made (?).
- Intended for xy linearity calibration.
- Rating: 0 out of 5.
Sources
Formerly a free sample from Digital Instruments.
References
- P. Markiewicz and M.C. Goh, "Simulation of Atomic Force Microscope Tip- Sample/Sample-Tip Reconstruction", J. Vac. Sci. Technol. B 13, 1115 (1995).
- P. Markiewicz and M.C. Goh, "Atomic force microscope tip deconvolution using calibration arrays", Rev. Sci. Instrum. 66, pg3186 (1995)
Square Posts
- Flat on the top and with sharp sidewalls, these might give good information on the tip shape.
- The large size of the squares means a lot of data collected is not used in the reconstruction of the tip.
- Dependant on scan direction.
- Use for xy linearity calibration.
- Rating: 2 out of 5
Sources
MikroMasch Image shown
References
R. Schlaf et al., "Using carbon nanotube cantilevers in scanning probe metrology", Proc. SPIE, 4689 (2002) 53-58
Square Holes
- These are extremely poor for tip reconstruction (Ref. 1).
- Dependant on scan direction (eg. square or diamond image).
- Used primarily for xy linearity calibration.
- Require deconvolution procedure (Ref.2)
- Rating: 0 out of 5
Sources
VLSI Standards, Inc.
Nanosensors
References
- P. Markiewicz and M.C. Goh, "Simulation of Atomic Force Microscope Tip- Sample/Sample-Tip Reconstruction", J. Vac. Sci. Technol. B 13, 1115 (1995).
- P. Markiewicz and M.C. Goh, "Atomic force microscope tip deconvolution using calibration arrays", Rev. Sci. Instrum. 66, pg3186 (1995).
Polymer Films
- Good for quick but general assessment of tip.
- Covers large area.
- Rating: 3 out of 5
Sources
Celgard
References
Porous Alumina / Anopore Filter
- Three pore sizes (0.2 µm, 0.1 µm and 0.02 µm).
- If tip penetrates openings it must be sharp.
- Covers very large area.
- Commercially available
- Rating: 3 out of 5
Sources
References
- n/a
"Home-made" structures
- Compression molding (Ref. 1).
- Gold coated Polycarbonate filter (Ref. 2).
- Special polymer blends (Ref. 3).
- Indifferent to scan direction.
- Various sizes possible.
- Random distribution (?).
- Rating: 2 out of 5.
Sources
Not found.
References
- S.Y. Chou, P.R. Krauss, and P.J. Renstrom, "Imprint Lithography with 25-nm Resolution", Science 272, pg 85 (1996)
- J. Tentschert et al., (1.4 Mb postscript file).
- T.O. Glasbey et al., Surface Science 318 (1994) L1219-L1224.