An In Situ Nanoindentation Specimen Holder for a HighVoltage Transmission Electron MicroscopeM. A. WALL AND U. DAHMEN*Chemistry and Materials Science Department, Lawrence Livermore National Laboratory, Livermore, California 94550

KEY WORDS Nanoindentation; cross-section; in situ TEM

ABSTRACT We describe in detail, the design, construction, and testing of a specimen holderthat allows for the nanoindentation of surfaces while viewing in cross-section in a high voltagetransmission electron microscope (TEM). This nanoindentation specimen holder, having three-axisposition control of a diamond indenter in combination with micromachined specimens, allows for thefirst time the dynamic observation of subsurface microstructure evolution under an indenter tip.Additionally, the sample design techniques that have been developed for these procedures mayeliminate the need for TEM specimen preparation for additional ex situ nanoindentation experi-ments. Initial experimental results from in situ indentation of Si samples in the high voltageelectron microscope are reported here to demonstrate the capability of this new specimen holder.Miscrosc. Res. Tech. 42:248–254, 1998. r 1998 Wiley-Liss, Inc.

INTRODUCTIONThere is currently a considerable amount of interest

in the measurement of the mechanical properties (i.e.,hardness, delamination, tribology, etc.) of nanostruc-tured materials and in extremely small volumes; forexample: surfaces, single and multicomponent films,and nano-size particles. Great progress in the measure-ment of the mechanical properties in these materialshas been made possible by the invention and evolutionof the nanoindentation instrument (Doerner and Nix,1986; Oliver and Pharr, 1992).

In many of the mechanical properties studies onnanostructured materials, the as-synthesized micro-structure is typically characterized by microscopy anddiffraction related techniques. Often though, the de-formed ‘‘site-specific’’ internal microstructure is notcharacterized after mechanical testing. As for nanoin-dentation, post-mortem examination of the ‘‘site-specific’’ internal microstructures often is not per-formed because of the difficulty in preparing TEMsamples. TEM replication, scanning electron and atomicforce microscopy are often utilized to measure andobserve the surface indent morphology. With any ofthese characterization approaches one can only attainan indirect conclusion of the microstructure responseduring indentation. Techniques such as micro-Ramanspectroscopy (Kailer et al., 1997) and resistivity (Grid-neva et al., 1972) have been utilized to detect phasetransformations within indents. However, the specificlocation and volume of these new phases are in questionand are often not retained after the indenter is re-moved.

There are few published reports that have character-ized post-mortem nanoindents with TEM. Two of thefirst reports, and related to our work on the indentationof Si, were by Page et al. (1992) and Callahan andMorris (1992). In these reports the importance of TEMcharacterization towards understanding microstruc-tural evolution effects such as elastic recovery, deforma-tion mechanism, local fracture, and phase transforma-

tions were emphasized. Even in these detailed studiesthere were still specific, dynamic events measured inthe typical load-displacement curve that might only belinked to a specific microstructure response by utilizingin situ microscopy techniques.

The inception of the indenter holder was first de-scribed by Wall and Dahmen (1995) with an update byWall and Dahmen (1997). In the initial report, theability to position a primitive Sapphire tip onto thesurface of a cleaved Si specimen, then indent untilfractured while viewing in cross-section, was demon-strated. Since this report, several modifications havebeen made to the specimen holder, and new sampledesigns utilizing lithography techniques have beenemployed. The following is a report on the latestdevelopments and demonstrated ability of our indenta-tion specimen holder and in situ indentation experimen-tation.

DESIGN AND CONSTRUCTIONThe basic design of the indentation specimen holder

employs several engineering features based not onlyupon the goal of being able to perform in situ indenta-tion, but also having a flexible design so that otherexperiments that would require 3-axis positioning of anobject or devise in a specific area on or about a specimenwould be possible. The basic design considerations areas follows: (1) The need for both range of motion andresolution dictated that most of the positioning deviceswould be located outside the vacuum. (2) The indentertip would be attached to a removable mount so that tipsor any other device could be mounted outside of the

*Correspondence to: U. Dahmen, Chemistry and Materials Science Depart-ment, Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, CA94550.

Contract grant sponsor: U.S. Department of Energy; Contract grant number:W-7405-Eng-48; Contract grant sponsor: Office of Energy Research, Office ofBasic Energy Sciences, Materials Sciences Division of the U.S. Department ofEnergy; Contract grant number: DE-ACO3–76SF00098.

Received 3 March 1998; accepted in revised form 17 March 1998

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r 1998 WILEY-LISS, INC.

specimen holder. (3) There would also be a removablespecimen mount having 2-axis positioning capabilityfor pre-alignment with the indenter tip under a stereoscope.

Figure 1 shows the general design and constructionemployed in order to achieve our initial requirements.In this design, a long stiff rod runs the length of thespecimen holder. It pivots (and slides) about a universalfour-point bearing surface, yielding a lever arm reduc-tion in lateral motion of approximately 10 to 1. One endof the rod passes through a bellows assembly and isattached to an ultra-low reduction gearmotor drive forin and out (Z axis) control. In series with this drive is apiezo stack located on the vacuum side of the bellows.The rear portion of the bellow assembly and Z axispositioning motor slides laterally across and is heldagainst (by vacuum and the natural spring of thebellows) three adjustable nylon points. Screw piezodrives are utilized for their resolution and range topush on this bellows assembly for the X and Y axispositioning. Return springs push back on the piezodrives for reverse motion. The diamond indenter is a3-sided, 30° pyramid, and is Boron doped to eliminatecharging under the electron beam.

A number of test experiments were performed todetermine the repeatable specifications of the specimenholder. The current specifications for the specimenholder are as follows. The Z direction gearmotor drivehas a range of 6 2.5 mm and a minimum step of 5 10nm. The Z direction gear motor can be used for indent-ing at a minimum constant displacement rate of < 20nm/sec. The Z direction piezo has a range of < 0.5 µmand a resolution of , 0.2 nm. The lateral positioningscrew piezo drives have a range and resolution of 6 0.2mm and , 5.0 nm, respectively. There is currently noforce measurement capability during an indenting ex-periment. Work is ongoing to develop force measure-ment capability. The specimen holder is shown inFigure 2 in its current configuration.

Sample DesignSeveral methods were considered for preparing speci-

mens for in situ indenting experiments. For test experi-

ments, cleaving of a Si wafer was fast and easy butthese specimens proved mechanically unstable duringindentation. For controlled experiments, well-definedsurface area and volumes are needed. To fabricatesamples, standard lithography techniques were em-ployed. A series of masks, resists, and etching proce-dures were used to produce a row of small mesa on alarger single mesa (Fig. 3a). When turned on edge,these mesa are effectively a cross-section sample of theoriginal Si wafer surface. As etched, the mesa dimen-sions typically range from 2 to 6 µm long by 1 to 4 µmwide and 2 µm deep. When turned on edge, the widthbecomes the viewing direction thickness in the TEM.The wafer is then diced into 2 by 4 mm slices, eachcontaining a single row of many mesa. The Si wafer is[110] oriented with the shape of the mesa being definedby [110], [111], and [111] planes (Fig. 3b).

EXPERIMENTAL PROCEDURESThe basic procedural steps are as follows. A Si

specimen is attached to the specimen mount withepoxy. The specimen mount is then positioned in the tipof the specimen holder. The indenter tip is moved alongthe Z axis with the gear motor drive until it is within <0.5 mm of the Si sample. The Si sample can then berepositioned vertically (Y axis) and laterally (X axis) sothat the mesa are within 6 0.2 mm of alignment withthe indenter tip. The holder is now ready for insertinginto the TEM. Once a mesa near the center of the Sisample is located and oriented for TEM observation,the indenter tip is translated on the X and Z axis nearthe mesa to be indented. The final alignment in thedirection of the electron beam (y axis), is the mosttedious. While moving the indenter tip in the 1 or - Ydirection the amount of focus change between the mesaand the tip is measured (i.e., focus steps). If the amountof focus change becomes less while moving the tip, thenthe tip is moving closer to the mesa, or else furtheraway. This defocus procedure will get the tip to within< 1µm of the mesa. The final y axis alignment proce-dure is to move the indenter tip towards the side of themesa and just past the indenting surface (1Z direc-tion). With continued slow translation of the tip to-

Fig. 1. A schematic of the indentation specimen holder showing the general design and construction.

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Fig. 2. Photograph of the indentation specimen holder in its current configuration.

Fig. 3. a: Scanning electron micrograph of a row of various size Si mesa on a larger single Si mesa. b: Higher magnification of a 2 µm x 4 µm mesa.

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wards the mesa, the side of the indenter tip willeventually just touch the mesa. Because the (111) planesides of the mesa are tapered, the indenter tip is now inalignment at the center of the width of the mesa. Theindenter tip (-Z) is retracted and translated over to thelateral center (X axis), indentation can now begin.

RESULTSSeveral indentation experiments have been per-

formed to date. These test experiments have deter-mined the best experimental procedure, assessed thecapability of the specimen holder, and observed at afundamental level the microstructural evolution of Siunder the indenter tip. Figures 4a–f and 5 are micro-graphs and excerpts from video recordings from two insitu experiments. In Figure 4a the diamond indentertip has been aligned with a 1.5 µm thick by 4.0 µm widemesa and is positioned within 1–2 nm of the surface. InFigure 4b the diamond tip has been indented to a depthof < 10 nm. We clearly observe the elastic response ofthe Si by the creation and motion of bend contours. Alarge bend contour with a radius of approximately 0.15µm is seen in the Si directly below and relativelycentered about the indenter tip. At this point there is noindication that dislocations or cracks have nucleated inthe bulk Si. With continued indentation to a depth of <40 nm, a larger crack has nucleated and propagateddirectly under the indenter as seen in Figure 4c. Thereis considerably more elastic strain as seen by therapidly changing diffracting conditions around the in-dent. Again, there are no obvious signs of dislocations.With continued indenting to a final depth of < 80 nm(Fig. 4d), we observe that the crack under the indentertip continued to propagate deeper into the Si. The bendcontour has moved out to a radius of approximately 0.3µm, which is the edge of the viewing area of the video.Because there are no strong diffracting conditions nearthe indenter tip/Si interface, it is difficult to see if anyother cracks exist, or even the possibility that a fewdislocations exist. Upon removing the indenter tip, aconsiderable amount of elastic recovery was observed,as indicated by the reversal of motion of the bendcontours. A minor amount of plastic response was seenalso. Figure 4e shows the microstructure of indent afterremoval of the indenter. There is a strong, permanentbend contour, indicating residual elastic strain. A pla-nar crack (or fault) is seen along with a few dislocationsnear the indent and the crack. The indent was thenreindented to a depth of 100 nm, then unloaded (Fig.4f).Dislocation nucleation and motion under the indentertip were observed as well as signs of small cracking.Upon indentation to these depths, plastic deformationis now observed on the lateral portions of the indenta-tion as well as directly underneath. It should be notedthat during indentation several bursts of plastic defor-mation were observed. During these bursts, the in-denter tip was still observed to move smoothly into theSi, and there were associated relaxations of the diffrac-tion bend contours with these bursts. Upon removal ofthe indenter tip, a considerable amount of ‘‘reverse’’deformation was observed in the form of dislocationactivity (apparent nucleation in addition to motion) andpossibly minor cracking. Residual strain fields stillexist as seen by bend contours, and dislocations areprimarily localized around the Si/indent interface.

Figure 5 is another image of a completed, 0.1-µm-deep indentation. Again, crack nucleation and propaga-tion are seen below the indenter. Dislocation activitybecame more obvious toward the greater depths ofindentation. In this particular indent, the asymmetri-cal shape of the tip interacting with the Si has resultedin an asymmetrical density of dislocations around theindent.

Additional indents were also performed. Some in-dents were to depths as great as 0.3 µm. At thesedepths, the Si mesa typically were destroyed by crackspropagating to the outer sides of the Si mesa. Otherindents were quite shallow. Even 10-nm-deep indentsleft residual strain fields, presumably from permanentplastic deformation, which could not be resolved indetail in these experiments.

DISCUSSIONThere are a number of engineering design ap-

proaches that could have been taken to construct thisspecimen holder. The results of the experiments de-tailed here demonstrate that the current design doesmeet the criteria of having 3-axis motion with both goodresolution and range. The specimen holder can betranslated and tilted with little or no disturbance to theindenter tip and/or its position with respect to thespecimen. There did not appear to be any concerns withdrift of the indenter tip with respect to the specimeneither. There is a small amount of vibration thoughduring lateral positioning due to the rapid mechanicalmotion of the screw piezos. The specimen holder hasalso proven to be flexible as demonstrated in experi-ments that deformed carbon nanotubes in situ (Cheng,1996, personal communication). Basically, one shouldbe able to attach (accounting for size requirements) anymicro-mechanical/electrical/magnetic device to the3-axis positioning rod and position it on or near aspecimen.

As for the results reported here, it is believed that this isthe first direct observation of internal microstructure evo-lution and response during an indentation experiment.It should be noted that a complimentary experiment onthe in situ observation of the bending of micro-machined Siwedges was reported by Langer and Katzer (1994).

The following is a general discussion of the compari-son of our observations with the reported post-mortemTEM work of Page et al. (1992). At the onset ofindentation in conventional indentation experiments,there is some concern over possible ‘‘surface flexure’’that would lead to an inaccurate indenter depth mea-surement. Sample motion on the scale on 1–2 nm wasnot observed in our experiments, which would be thesmallest detail that could be measured at the magnifica-tions at which our experiments were performed. How-ever, it was observed that the diffracting conditions inthe crystalline diamond tip did change during indenta-tion, an indication of the elastic response of the dia-mond. It would be entirely possible to perform theseindents at a much higher magnification to look forsample compliances other than the elastic response ofthe indenter and the region under the indenter.

After the initial indentation, through the elasticresponse zone and into the plastic deformation mode, itappears that cracking and not dislocation nucleationand motion, is the first mode of deformation. As noted in

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the observations above, we did see signs of intermittentbursts of plastic deformation around the indenter,which was independent of larger cracking. The exactnature of this plastic deformation remains unclear.There are some signs of dislocations after unloading

from deeper indents but because of the highly strainedmaterial around the indenter during indentation, thediffraction imaging conditions for imaging of disloca-tions varied rapidly and/or were low in contrast. Asdescribed in the observations the imaging conditions

Fig. 4. a: Showing alignment of a diamond indenter near thesurface of a 1.5-µm-thick Si mesa prior to indentation. b: Theindentation to a depth of <10 nm. Elastic response of the Si is seen bythe bend contour under the indenter tip. c: Indentation to a depth of<50 nm has precipitated a crack underneath the indenter as well asincreased the amount of elastic strain under the indenter. d: Contin-ued indentation to a depth of 80 nm increases the amount of elastic

distortion and propagates the crack under the indenter tip. e: Uponremoval of the indenter, another planar fault or crack is visible as wellas dislocations. f: Re-indentation to a depth of <100 nm results in thecreation of numerous small cracks, dislocations and minor extrusion ofSi above the surface. (a–d are from a video and e and f are micro-graphs.)

252 M.A. WALL AND U. DAHMEN

(i.e., bend contours) would aperiodically move, thenrelax as the indenter moved further into the Si, indicat-ing a relaxation of stress by plastic deformation. Numer-ous reports have suggested a phase transformation to amore dense phase takes place in Si and may account forthe low dislocation density. Although the observationsreported here cannot rule out a phase transformation,there is some suggestion of another mechanism thatmight explain the observation of the indenter movinginto the Si with little dislocation activity. The tip of theindenter is not very smooth. As the tip region is movinginto initial contact with the Si, the Si is trying toconform to the local (atomic scale) irregularities of thetip. As the tip continues to move past these local contactsites, the Si relieves the local stress though micro-cracking and cleaving. This action then allows for thepossible ejection of the Si particles out of the indentedregion. Certainly, more experiments performed in thediffraction mode and at higher magnification in imagemode are required to elucidate the exact nature of themicrostructure under the indenter tip at this initialstage.

No ‘‘pop-in’’ like behavior was observed in Si duringthese experiments. Page et al. (1992) did not observethis behavior in Si, but in Sapphire they did. This insitu TEM indentation technique may be a way ofelucidating this type of phenomena.

In all of our deeper indents (greater that the tipradius, 50 nm), there was noticeable elastic recoveryand plastic deformation in the form of dislocationnucleation during unloading. This may be the micro-structural response that was noted as ‘‘pushing theindenter upwards’’ or ‘‘reverse thrust.’’ This microstruc-ture response could be explained by a phase transforma-tion. Again, it was not obvious, in our experiments thusfar, that any phase transformation occurred duringindentation or upon removing the indenter. The in situindentation experiments are stable enough that these

could be repeated in the diffraction mode to look for anynew phases.

After indentation we did not see any sign of anamorphous region in the center of the indent. For thesmall indents in these experiments, it would be difficultto observe a low-contrast phase amorphous phase,primarily because the samples are up to 2 µm thick inthe viewing direction. It should be noted that for theexperiments reported here, the indents were not asdeep as in the case of Page et al. (1992) and the tipswere of a different geometry.

Finally, the design of the Si mesa samples appears tobe very complimentary to these experiments. Usinglithography techniques, the samples can be produced inlarge numbers, have a well-defined volume and crystal-lography (which would be necessary for potential mod-eling purposes), and are directly observable in cross-section in the electron microscope without any additionalpreparation procedures. Another possibility that isbeing explored is to perform ex situ nanoindentationexperiments on these Si samples and then performTEM observation. Thus, no site-specific sample prepara-tion of the indents would be needed. These indentationexperiments would allow for the understanding of themechanical response of these mesa as compared to thelarger Si wafer surfaces and possibly a calibration forthe loading during the in situ experiments. Finally, wehave been successful in evaporating thin films ontothese Si mesa. In situ experiments will be performed onthese films in the future.

CONCLUSIONSFrom the results of these experiments and opera-

tional testing of the indentation specimen holder, wefeel that we now have an experimental procedure andinstrument in place that are reproducible, flexible, andhave the potential for yielding a multitude of unique insitu observations in the high voltage electron micro-scope. This was demonstrated through the first everobservations of sub-surface microstructural evolutionin Si under the indenter tip during indentation. Oncewe are able to develop a force-measuring capability, webelieve that we will have created a new area of quantifi-able, in situ, nanoscale mechanical deformation study.Lastly, this holder has already been modified andsuccessful in plastically deforming carbon nanotubes insitu. Again, there is no reason (barring size require-ments) that many types of electro/mechanical/magneticdevices cannot be mounted on the 3-axis positioning rodand positioned on or near a specimen in order toperform many new types of in situ experiments.

ACKNOWLEDGMENTSThe authors thank Richard Gross of Lawrence Liver-

more National Laboratory and Doug Owen of LawrenceBerkeley National Laboratory for their technical sup-port. We also thank Mehdi Balooch of LLNL and TimWeihs of Johns Hopkins University for their manydiscussions. This work was performed under the aus-pices of the U.S. Department of Energy by the LawrenceLivermore National Laboratory under contract W-7405-Eng-48 and by the Director, Office of Energy Research,Office of Basic Energy Sciences, Materials SciencesDivision of the U.S. Department of Energy undercontract DE-AC03–76SF00098.

Fig. 5. Another indent to a depth of > 100 nm. Again cracks anddislocations in the area around the indent are visible.

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