size of the diamond. For comparison,the resonant frequency for standardTappingMode AFM cantilevers isusually about 300kHz.

A typical indentation cantilever has a spring constant of 150N/m; length, width, and thickness of about350, 100, and 13µm, respectively (Figure 1); and a resonant frequencyof 50kHz. Veeco Probes measures andsupplies the customer with the springconstant of each cantilever purchased.The typical indentation force rangeavailable with our instruments is

Figure 3. (a) Indentations on a 15nm thick diamond-like carbon (dlc) thin film using three forces of about 15, 20, and 25µN respectively. All indentations are less than 10nm deep. The smallestindentations are approximately 3nm deep as seen in the section view (b). 800nm scan.

curve is recorded utilizing the DCdisplacement of the cantilever versusthe extension of the scanner. Manyindentations may be made usingvarious forces, rates, etc. Upon exitingindentation mode, TappingMode AFMis turned back on and the instrumentreturns to imaging mode; theindentations made on the surface maythen be imaged.

Scratching is done in a similar manner.First, engage on the surface with anindentation cantilever and image thesurface to locate the desired positionfor the scratch. Next, select ScratchMode where, once again, the lateralscanning halts and the tip is liftedslightly off the surface. Then, set thescratch parameters, such as force,rate, scratch length and direction, andthe scratch is executed. During thescratching, the cantilever oscillation isturned off, the tip is forced into thesurface until the specified cantileverdeflection is reached, and the tip isthen moved laterally according to thepreset length, rate, and direction.Multiple scratches may be made usingvarious forces, lengths, rates, anddirections. Upon exiting ScratchMode, TappingMode AFM is turnedback on, and the instrument is ready to image the scratches made on thesurface of the sample.

Indentation Cantilevers

Indentation cantilevers are thicker,wider, and longer than standard AFMcantilevers, and are composed ofstainless steel. The typical range forspring constants of contact mode,TappingMode, and indentationcantilevers are 0.01-1.0N/m, 20-100N/m, and 100-300N/mrespectively. The fundamentalresonance frequency for indentationcantilevers is in the range of 35-60kHz, depending on thedimensions of the cantilever and the

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Figure 2. Indentations on a 1µm goldruling at various forces used for testing thesharpness and orientation of the diamondtips mounted on the indentation cantilevers.3µm scan.

1-100µN with resolution better than0.5µN. Larger forces up to about300µN can be accommodated using custom cantilevers with higherspring constants.

The diamond tip mounted to the end of the cantilever has a tip radius that is less than 25nm to assure goodimaging resolution and nanometerscale indents and scratches. Thediamond tip is the apex of three sidesof a four-sided pyramid with an apexangle of about 90 degrees. Toprovide symmetric indents, thediamond is mounted such that thevertical axis of the pyramid isapproximately normal to the samplesurface when mounted on the microscope.

Unlike contact mode, our patentedTappingMode technique allows use of the high spring-constant cantileversrequired for nanoindentation, while still imaging the surface with minimaldamage. This is illustrated by theindentations made in the 1µm goldruling shown in Figure 2. Theindentation cantilever that was used toindent and image the surface has aparticularly large spring constant ofabout 400N/m. If this cantilever wereused to image the gold in contact

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mode, it would have damaged thesurface sufficiently to preventmeaningful imaging of either theindentations or the rulings. VeecoProbes also provides an image of agold ruling, similar to Figure 2, foreach indentation cantilever sold. Theindents and the image are capturedwith the same cantilever that is sold tothe customer.

Examples

Nanoindentation with an AFMcantilever is useful for investigating the hardness of films as thin as 5nm.Since the macroscopic properties ofthin films of a material may be differentfrom the microscopic properties, theability to perform tests at thenanometer level is of great importancein many processes. The 15nm thick diamond-like carbon (dlc) film in Figure 3a was indented five times with three different forces: about 15,20, and 25µN, respectively. Theindentations were then imaged withthe same tip, which did the indenting.In the cross-section image (Figure 3b),the depth of the 15µN indentations(measured after any elastic recovery of the film has occurred) is only about3nm. The projected area of theseindentations is approximately1,600nm2. The 20 and 25µNindentations have depths of about 5nm and 7nm and projected areas of about 2,000 and 2,500nm2,respectively. Given the small size ofthese indentations, the ability to imageimmediately with the same instrumentand tip is a major benefit.

Using indentation cantilevers, it ispossible to indent various samples withthe same force in order to comparehardness. Figure 4 shows two differentdlc thin films that were indented fourtimes with three different forces of 23,34, and 45µN. The same forces wereused for both films by using the same

Figure 5. Array of scratches performed on four different 10nm thick diamond-like carbonthin films, all using the same force and cantilever in order to compare film adhesion anddurability. The scratches are 1µm long and less than 10nm deep. 2µm scans.

indentation cantilever under the sameconditions. The indentations on the leftfilm are smaller than the ones on theright film. For the film on the left, theaverage indentation depth for each ofthe forces used is about 1.5, 3, and6nm, respectively. For the film on theright, the indentations are about twiceas deep: 3, 7, and 13nm. Theimages of the two films also show adifference in the way that the materialhas piled up around the indentations.

Properties such as film adhesion anddurability can also be studied usingindentation cantilevers. Figure 5 showsfour different 10nm thick dlc films(coated on computer hard disks) onwhich a series of scratches were madeat a force of 15µN. The averagedepth of the scratches for each film is3, 6, 6, and 3nm for Figures 5a to5d, respectively. The material deposits,which have piled up near thescratches are likely peels or shavings

Figure 4. Indentations on two different diamond-like carbon thin filmsusing three different forces (23, 34, and 45µN) with four indents made ateach force. Each film was indented using the same forces and cantilever inorder to compare hardness. 500nm scans.

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of the film, which were scratchedaway from the surface. The films mayalso have delaminated from thesubstrates causing taller features(brighter colors) in the images.

The cantilevers used fornanoindentation and scratching canalso be used for wear testing. Figure 6shows four different dlc films (coatedon computer hard disks) on which anarea was worn by scanning thesurface – in this case using contactmode – at a preset force using a rasterscan pattern. The worn areas resultfrom only a single pass (scan) from top to bottom over the square area.Using this type of testing, the durabilityof materials can be compared. In Figure 6a, the film has beencompletely worn away from thesubstrate, whereas the films in Figures 6c and 6d appear to haveworn only slightly with small amountsof debris shown at the edges of thescan. The film in Figure 6b exhibitswear similar to the topmost portion ofFigure 6a; however the film did not failentirely. The image shows that films 6cand 6d are tougher than films 6a and6b. Figure 7 shows a dlc film, whichappears to have delaminated from thesubstrate of the hard disk during thewear test.

The image shown in Figure 8 is agood illustration of the indent size and image resolution possible with the indentation cantilevers. The Digital Instruments logo was indentedon dlc film using the indentingsoftware/hardware and then imagedwith the same tip. The scan size isabout 250nm and the z-range (or fullheight range) is 4nm. The indentationdepth, measured from the image, isabout 1nm for the small indents andabout 2.5nm for the indent used to dotthe “i”. For the small indentations, theprojected area is approximately400nm2, with spacing of 25-30nm

Figure 6. Worn areas on four different 10nm thick diamond-like carbon thin filmsgenerated using the same force and cantilever in order to compare film adhesion anddurability. The worn areas are 5µm square. 9µm scans.

Figure 7. Worn area on adiamond-like carbon thin film inwhich the film delaminated from thesubstrate. The worn area is 2µmsquare. 3µm scan.

Figure 8. Digital Instruments logoindented on a 15nm thick diamond-like carbon thin film. Thesmall indents are 1nm deep and thelarge indent is about 2.5nm deep. Theforces used for the indentations wereabout 5µN and 10µN respectively.250nm scan.

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and indent width of about 20-25nm.The force applied during the indentwas about 5µN for the small indentsand 10µN for the large indent.

Figure 9 shows an array ofindentations, which were performed ontwo polymer films using five differentforces. Figure 9a is PMDA-ODApolyimide and Figure 9b is BPDA-PDApolyimide. The smallest indents areabout 20nm deep and the largestabout 200nm deep for both samples.The average depth of the indentationson the polymer in Figure 9b is about85% of the depth for the polymer inFigure 9a, suggesting that the BPDA-PDA polyimide is harder than thePMDA-ODA. This is consistent with the chemical structures of the twopolyimides, which indicate that theBPDA-PDA has stronger bonding thanthe PMDA-ODA. There is also anobvious difference in the amount ofmaterial, which has piled up at theedges of the indents for each of thepolymers. The PMDA-ODA polyimide,which has deeper indentations, hasless accumulated material than the BPDA-PDA.

The bovine and human sperm nuclei,shown in Figure 10, were indentedusing the same forces in order tocompare the hardness of each. Bothsamples were indented using forces ofabout 8 and 16µN, and then imagedin air using the same indentationcantilever. The hardness of the bovineand human sperm was calculated as0.9 and 0.5GPa, indicating thatbovine sperm is about twice as hardas human sperm. This result isconsistent with what is known aboutthe different proteins, which compactthe DNA in bovine and human sperm. Figure 10c and 10d show full size images of the sperm nuclei,which were imaged using theindentation cantilever.

Figure 9. Indentations on two different polymers using the sameforces to compare hardness. Each sample was indented four timesusing each of five forces. The first sample (a) is a PMDA-ODApolyimide, and the second sample (b) is a BPDA-PDA polyimide. The indentation depths vary from about 20-200nm and are deeperfor the softer PMDA-ODA polyimide. 3µm scans.

Figure 10. Indentations on bovine (a) and human (b) sperm nuclei, imaged in air andindented at forces of about 8 and 16µN. 1µm x 0.25µm scans. The hardness for thebovine and human sperm were calculated at about 0.9 and 0.5GPa, respectively. Fullsize images of bovine (c) and human (d) sperm nuclei were also imaged with theindentation cantilever. 10µm scans.

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