SCANNING VOL. 29, 185–195 (2007) Wiley Periodicals, Inc.

FIB/SEM Characterization of Carbon-based Fibers

S. MAGNI1, M. MILANI1, C. RICCARDI2, F. TATTI3

1 Materials Science Department and Laboratory FIB/SEM ‘Bombay’, University of Milano-Bicocca, Via Cozzi 53,20125 Milano, Italy

2 Physics Department, University of Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy3 FEI Italia S.r.l., Viale Bianca Maria 21, 20122 Milan, Italy

Summary: The aim of this paper is to show howa focused ion beam combined with a scanning elec-tron microscope (FIB/SEM machine) can be adoptedto characterize composite fibers with different electricalbehavior and to gain information about their produc-tion and modification. This comparative morphologyinvestigation is carried out on polyacrylonitrile (PAN)carbon fibers and their chemical precursor (the oxidizedPAN or oxypan) which has different electrical proper-ties. Fibers are imaged by electron and ion beams andsectioned by the focused ion beam (FIB). A sampleof oxypan fibers processed by a radio frequency (RF)plasma is also investigated and the role of the conduc-tive carbon layer around their unmodified, insulatingbulk is discussed. A suitable developed edge detec-tion technique (EDT) on electron, ion images, and afterthe FIB sectioning, provides quantitative informationabout the thickness of the created layer. SCANNING29: 185–195, 2007. 2007 Wiley Periodicals, Inc.

Key words: focused ion beam (FIB), scanning electronmicroscope (SEM), composites (carbon fibers and car-bon precursors), cross-sectional techniques, metrology(spatial dimensions measurement)

PACS: 41.75.fr, 68.37.Hk, 61.82.Pv, 06.20.-f

Introduction

A dual beam system incorporating a focused ionbeam (FIB) with a scanning electron microscope (SEM)adds the microscopic techniques peculiar to the former(micromanipulation, ion microscopy), to the advantages

Address for reprints: Simone Magni, Department of MaterialsScience, University of Milano-Bicocca, Via Cozzi 53, 20125Milan, Italy, E-mail: Simone.magni@mater.unimib.it

Received 26 December 2007; Accepted with revision6 May 2007

of the latter (Phaneuf 1999; Orloff et al ., 2003; Gian-nuzzi and Stevie, 2005). For more than a decade, FIBdevices have been adopted mainly in microelectronicindustry (Cleaver et al ., 1988; Saitoh et al ., 1988;Orloff, 1993; Mallard et al ., 1998).

Significant attention has attracted FIB sectioningwith FIB or SEM imaging, especially for biologicalspecimens (Yang et al ., 1993; Ishitani et al ., 1995;Ballerini et al ., 1997, 2003; Drobne et al ., 2005; Stokeset al ., 2006). As in the case of life sciences, compositeand polymer science potentialities of FIB are stilllargely unexplored, especially when the sample is farfrom being 2-D or spatially homogeneous (Phaneuf,1999). Contributions in this direction can be found inthe works by White et al . (2000); Milani et al . (2004,2005a,b); Stokes et al . (2006).

Carbon and graphite fibers are commonly used asreinforcement materials in advanced structural compos-ites, which are generally lighter, stronger, and moreelastic than many metals. In recent years, carbon fiberswoven in cloth have also been adopted in fuel cell tech-nology, owing to their good electrical conductivity aswell as other features (Pasaogullari and Wang, 2004).

The aim of this paper is to present FIB/SEM imagesand FIB cutting of different polyacrylonitrile (PAN)carbon fibers at different ranges of magnification inorder to assess the quality of the fabric that couldinfluence their electrical conductivity. Scanning ionmicroscopy (SIM) and FIB milling have been per-formed on them for the first time to the best of ourknowledge. As opposed, microscopic characterizationsof carbon fibers have been performed in the past bymeans of an SEM, e.g. to find differences amongPAN-based, pitch-based and rayon-based carbon fibers(Kumar et al ., 1993; Chand, 2000), or the effect ofplasma etching on them (Fukunaga et al ., 1999).

The chemical precursor, i.e. the oxidized PAN fibers(Perepelkin, 2003) is also considered in our presentwork. The oxidized PAN is also employed as a cheapsupporting network in fireproof clothes and shields.Although it is commonly accepted that the morphology

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of the precursors has a strong influence on the finalproduct (Edie, 1998), electron microscopy is hardlyever realized on oxidized PAN (Wangxi et al ., 2003);this could be thought of as a consequence of itsinsulating character, which hinders charged particlemicroscopy.

The oxidized PAN fibers have been exposed to aradio frequency (RF) plasma, obtaining a plasma-drivensuperficial carbonization of the fibers. The microscopictechnique can be coupled to a one-dimensional edgedetection technique (1-D-EDT) to obtain informationabout the carbon layer thickness for analysis on thereliability of plasma-induced carbonization.

The paper is organized as follows. The sectionreferred to as “Experimental” is devoted to a descriptionof the materials, the FIB/SEM system and the tech-niques employed. The section on “Results” features thediscussion about the imaging and the sectioning of car-bon fibers and the microscopic characterization of thetransformation induced by the plasma on oxidized PANfibers. Final conclusions and outlooks are reported inthe section on “Discussion and Conclusion”.

Experimental

Materials

We will briefly summarize the processes of PAN car-bon fibers production (Chand, 2000), before describingthe fibers investigated here. PAN; (–CH2–CH(CN)–)x )

is the starting chemical precursor and it is preliminar-ily annealed in air at 230 ◦C (Edie, 1998). After thisstage, fibers are heated in air or oxygen environment at300 ◦C to get a simultaneous oxidation and cyclizationof PAN. This intermediate product is the oxidized PAN,henceforth “oxypan” (e.g., Perepelkin (2003)), whichwithstands up to temperatures of 800 ◦C without lossin properties. The final step is the carbonization of theoxypan by heating up to 1,200 ◦C–3,000 ◦C, through asettled ramp in an inert atmosphere (nitrogen, argon)(Wangxi et al ., 2003).

PAN carbon fibers are composed of graphite-likelayers that have generally no regular three-dimensional(3-D) arrangement. In experts’ jargon, these fibers aresaid to have a “turbostratic” self-organization (Edie,1998; Johnson, 1987); the graphite planes are thoughtto be essentially parallel with some undulations atthe border region. Going further from the skin to thecentral part, the structure becomes more random. Asfar as the inner region is concerned, many modelsassume that layer planes fold and curve, enclosingsharp-edged voids in a hairpin or a ribbon-like manner;in the current literature, these hypotheses are basedon observations obtained by means of transmissionelectron microscopy (TEM) (Edie, 1998; Johnson,

1987) and X-ray diffraction (Johnson, 1987; Pariset al ., 2001).

Some information can be gained from the obser-vation of the fracture surface of these fibers. In anearly work by Knibbs (1971), PAN carbon fibers wereinvestigated by means of a polarized light microscope.He identified three different structures of PAN-basedcarbon fibers (cf. Figure 7 in Chand (2000)). Morerecently, SEM was utilized to observe fracture surfacesalong the cross section of these fibers (Kumar et al .,1993). In this work, the authors report that most of thePAN carbon fibers have a “particulate” morphology, asopposed to the pitch-based carbon fibers which displaya sheetlike aspect. This “particulate” morphology origi-nates from the longitudinal interlinking organization ofthe graphite layers (cf. Figure 3 in Edie (1998)).

We investigated two prototypes of PAN-based carboncloths that are adopted in fuel cell technology andare produced with different ramps (final temperature∼1,500 ◦C) and the oxypan fibers. A fourth sample offibers is obtained by a RF plasma processing oxypancloth. The discharge parameters are tuned so thatthe plasma induces a local heating of the oxypanreaching the energy threshold sufficient for its partialcarbonization.

Typically the texture of a cloth is characterized by thenumber per length unit of warp yarns, i.e. the threadsrunning lengthwise in the fabric, twisted round eachother and crossed at the right angle to the weft, whichis instead the thread shuttled back and forth across thewarp. In Table I, we report the mean number of warpyarns per length unit and the percentage of free surface(FS), along with some physical features such as cloththickness, free surface density, and surface electricalresistivity.

The FIB/SEM or Dual System

In our work, a FEI Strata “DualBeam” 235 device(FEI Company, Hillsboro, Oregon, U.S.A.) is used.This system combines a high-resolution FIB column(Ga+ ions from a liquid metal ion source) with aSchottky Field Emission Gun (SFEG) electron source(SEM column). A FEI DualBeam Quanta 200 3-D isalso employed to obtain imaging in low vacuum watervapor environment before and after FIB operations. Areview on the physical insight of FIB systems can befound in Orloff et al . (2003).

Digital images are reconstructed from signals pro-vided by different kinds of secondary particle detectors:a standard secondary electron Everhart-Thornley detec-tor (SED), a Continuous Electron Dynode Multiplier(CDM), able to detect secondary electrons or ions, anda through-the-lens detector (TLD). A large field detec-tor (LFD) is employed in low vacuum modality, whichreduces sample charging during electron imaging.

S. Magni et al.: FIB/SEM Characterization of Carbon-based Fibers 187

TABLE I. The main parameters of the considered cloths: mean warp yarns/mm, percentage of free surface (FS), mean cloththickness (Th), mean surface density (d), and mean surface electrical resistivity ρs (approximate values). One can link theweight reduction of the oxypan converted to carbon fibers to the consequent shrinking of the cloths (lower thickness and higheryarns/mm). The surface resistivity of type 2 is nearly double that of type 1 carbon cloth because type 2 is about half as thickas type 1.

Cloth Yarns/mm Free surface (FS) (%) Th [µm] Density (d ) (g/m2) ρs (�/mm2)

Oxypan 1.10 30 220 82 >1 × 106

Carbon type 1 1.39 26 203 56 0.3Carbon type 2 1.45 51 124 33 0.5Plasma treated oxypan n.a. n.a. n.a. 73 2.5 × 104

Both systems are equipped with a gas injector systemto perform an ion beam-assisted platinum deposition.FIB imaging can achieve a lateral resolution of 5 nm.The limit in spatial resolution of FIB micromachiningis 20 nm. In the best of the systems we have employed,electron imaging can reach a resolution down to 1 nm.

Methods

The main difference between SEM and SIM lies inthe different interactions occurring between the speci-men and the particles of the beam. A peculiar contrastarises from SIM as ions are different from electronsin charge, mass, and momentum transfer and conse-quently both in penetration depth and surface modifica-tion (Orloff, 1993; Orloff et al ., 2003; Goldstein et al .,2003; Giannuzzi and Stevie, 2005). An objective com-parison between SEM and SIM is further discussed inthe works by Ishitani and Tsuboi (1997) and Ohya andIshitani (2003).

In low vacuum one can achieve electron imaging ofnonconducting specimen. One can refer to the workby Thiel (2004) for a discussion about the mechanismof charge reduction, of SE signal amplification andcollection.

FIB micromanipulation refers to a set of machine-assisted operations oriented to modify the sample for asubsequent analysis and/or to fabricate features on it, ata submicrometer scale (Orloff et al ., 2003; Giannuzziand Stevie, 2005).

FIB sectioning could be considered as a branch ofmicromanipulation. A rough FIB milling in the currentrange of 1–20 nA, at a beam energy of 30 keV is usedto obtain a (nearly) full cut of the considered fibers.The cross-section face is polished with a lower beamcurrent in the range of 300–1000 pA. Currents in therange of 10–150 pA are used for ion imaging.

The Edge Detection Technique (EDT)

In image processing, the EDT refers to a set ofoperations carried out on the gray level of a 2-D imagein order to localize its significant variations and then

identify the physical phenomena at their origin (Ziouand Tabbone, 1998).

The intuitive definition of an edge is the set of points(pixel location) with an abrupt change of gray level.Since we approach a discrete value problem, numeri-cally one has to detect the point in which the profilederivative overruns a fixed threshold. We can esti-mate in a very fast way the number of pixels betweentwo edges, and rescale it in units of length, once weknow the format of the image and its Horizontal FieldWidth (HFW), obtaining a length measurement, accu-rate enough for an estimation of the plasma processinvolving the oxypan fibers. In SEM, the origin of theedge is due to the electron-specimen interactions andits identification can be subject to errors such as, e.g.discussed in Cazaux (2004).

In this work, we used a quick, simplified EDT inone dimension (1D-EDT), facing only the discrete grayprofile along a definite direction in the 2-D image. Agray profile is firstly exported from the digital imageusing the program Scion Image (Scion Corporation,Frederick, Maryland, U.S.A.). Then, we have adopteda fast fourier transform (FFT) to smooth this profileon bins of 4–10 points. Then, the numerical derivativeis estimated by a finite difference method. For theseoperations, we used the software OriginPro 7.0 (OriginLab Corporation, Northampton, Massachusset, U.S.A.).This technique differs from the one described in Milaniet al . (2005b) or Ciardi and Milani (2006) and it allowscontrol of independently smoothing and differentiationoperations. In the results section we will show acomparison between the two methods.

Results

Carbon and Oxypan Fibers

Firstly, we focus our attention on the characters atlow magnification of the considered cloths (left panelsof Figure 1). In this way, one can investigate the fabricin the precursor, checking how it is conserved afterthe carbonization, or after the exposure to the plasma.This characterization is important in technological

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Fig 1. (a) Top view with a HFW of 2.03 mm; type 1 carbon fibers, (b) oxypan fibers, (c) oxypan fibers undergone plasma etching,(d) cut ends of a single fiber of carbon, (e) oxypan, and (f) plasma-treated oxypan. The HFW is 25.3 µm for (d), and 30.4 µm for(e) and (f). These images are taken by means of an electron beam with an energy of 5 keV. In the oxypan fibers one can appreciatethe brighter zones because of charge accumulation.

applications, as unwanted modifications in the fabriccould influence the overall electrical conductivity ofthe cloth. Some small ruptures of the carbon fibers canbe seen.

Oxypan fibers have an insulating behavior. Westarted with an acceleration of 5 keV to performtheir electron imaging. Lower accelerating values havebeen checked down to 1 keV without appreciably

S. Magni et al.: FIB/SEM Characterization of Carbon-based Fibers 189

decreasing charging, but only losing much of thetopography. At low magnifications, the brighter zonesof Figure 1(b) show that charge accumulation has takenplace on the oxypan cloth, as the local intensity ofthe image is somewhat proportional to the chargeaccumulated on the surface. The charging effect ispartially mitigated by the plasma treatment, as can beseen in Figure 1(c).

The different morphology of the three fibers canbe appreciated in the SEM images of Figure 1—rightpanels. They were mechanically cut with scissors.Figure 1(d) refers to a type 1 carbon fiber (cf. Table I).Figure 1(e) (oxypan) is characterized by brighter anddarker zones. Here, brighter regions again point outthe charge accumulation, that has been minimized byreducing dwell time, probe current, and also by tiltingthe fiber with respect to the primary beam. All theseparameters can reduce charging phenomena during

SEM imaging of uncoated insulating materials (Joy andJoy, 1996).

In Figure 1(f) one can observe how a part of thelayer created by the plasma etching on the oxypan hasbeen broken during cutting operations. We found somesimilar cracks both at the end of the fibers and in theirmiddle, almost all probably arising from sample prepa-ration and handling. Furthermore Figure 1(f) shows thepresence of an amorphous carbon film produced bythe plasma poorly grafted to the rest of the oxypanfiber.

A FIB cross sectioning has been performed on type 1carbon fibers (Figure 2(a), (b)). A remaining part of thefiber can be seen on the right. Other full cross sectionshave been carried out. The bulk aspect of the fiber,Figure 2(a), points out that a full carbonization processtakes place. The absence of internal voids larger thanfew tens of nanometers (nm) qualifies these fibers as

Fig 2. Two FIB cross sections of carbon fibers (a) and (b) and an oxypan (c) and (d). A comparison between secondary electronimaging obtained by primary electrons (a) and primary ions (b) of a single carbon fiber. This fiber has been previously sectioned bymeans of a FIB milling (current 3 nA). HFW is 15.2 µm for (a) and 12.2 µm for (b). The energy of the electron beam used for (a)is 5 keV. During imaging of (b) the ion beam current is 47 pA with an energy of 30 keV. A remaining part of the fiber is evidenton the left of both figures. In (c) (ion image 30 kV, 10 pA, HFW = 21.3 µm), the FIB deposited Pt strip with an area of 20 µm2

and with a thickness of 500 nm. On the left, the silver paint can be seen. (d) shows the cross-section face after a 1-nA milling and a300-pA polishing (low vacuum SEM, 10 keV, HFW = 12.8 µm, pressure 80 Pa). The platinum layer is consistently reduced duringFIB milling.

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high-quality carbon fibers. Similar results were foundfor type 2 carbon fibers.

The fiber displays some striations on the border,along the axis of the fiber itself. These striations impartelasticity to the fibers and are explained in the currentmodels of PAN-based carbon fibers reported in section“Materials” (Johnson, 1987; Edie, 1998; Paris et al .,2001).

The ion image, Figure 2(b), reveals a different con-trast in comparison with Figure 2(a), for example thesharper contrast of some fiber striations. In carbonfibers, many crystallites are formed by graphite planeinterlinking. The question whether channeling effecttakes place in these materials in a similar fashion tometals (Phaneuf, 1999) or not remains open.

FIB imaging and milling revealed themselves to betroublesome operations on the oxypan fiber, becauseof its insulating properties. We tested some metal

coatings on these fibers to avoid charging. Howeverthey lacked sufficient uniformity, as sputtercoatingbuilds metallic bridges between the warp yarns withoutcompletely coating a single fiber. The only way toperform a FIB sectioning of the oxypan was thedeposition of a platinum strip by means of a FIB-assisted deposition (Figure 2(c)). This strip was laidto cover the cutting zone up to a point in which somesilver paint established a good path to ground. Indeed,the importance of the grounding is crucial for FIBmilling as we have experienced an incontrollable beamdrift in the case of an ungrounded platinum layer. TheFIB milling was started with a current of 1 nA, avalue lower than usual in order to prevent an excessiveion implantation. The total time for the operationsof FIB deposition, milling, and polishing was about45 min. The imaging of the cross-section face was thentaken in low vacuum at a chamber pressure of 80 Pa

Fig 3. A close comparison between different kinds of fibers after mechanical cut. Those figures show: (a) a zoomed zone of Figure 1(d)(type 1 carbon fiber) revealing the “particulate” morphology typical of PAN carbon fibers (HFW = 6.08 µm); (b) a type 2 carbon fiberthat displays a more amorphous aspect than type 1 (HFW = 6.08 µm); (c) the oxypan fiber which is the precursor of fibers (a) and (b)(low vacuum SEM, HFW = 18.3 µm, energy 20 keV, pressure 100 Pa); and (d) a SEM induced by the ion beam (HFW = 17.4 µm,energy 30 keV, current 24 pA) of plasma-treated oxidized PAN. The thick carbon layer reduces sample charging, thereby disclosingthe inner structure of the precursor. (a) and (b) are conventional secondary electron imaging images with a beam energy of 5 keV. Theblack arrows in (d) indicate the two directions in which the 1D-EDT has been carried out.

S. Magni et al.: FIB/SEM Characterization of Carbon-based Fibers 191

with an electron beam energy of 10 kV (Figure 2(d)),neutralizing sample charging that in this case gets worsethan usual after FIB milling.

The particulate morphology we referred to in section“Materials” is evident, observing Figure 3(a) whichdisplays a fracture of type 1 carbon fibers obtained witha scissors cut. Furthermore, the type 1 seems to stick toKnibbs’ structure of the second kind (cf. e.g., figure 7Chand (2000)). It is worth noting that this scheme waselaborated for low-quality carbon fibers and it has stilla wide range of validity for high-quality fibers, such asthose under consideration.

One can notice the higher precision reached by theFIB cross-sectioning in comparison with mechanicalcut by examining Figures 3(a), 2(a), or 2(b). The FIBmilling can achieve a flat surface after the polishingwhile the mechanical cut produces unwanted fractures.The milling, rough, and fine polishing procedures takean average time of 15–20 min. From these imagesone can estimate that the fiber diameter ranges from6 to 8 µm. For the sake of clarity, we note that theFIB milling should be compared to a microtome cutof these fibers. For example, this technique has beenapplied in Prickett et al . (2000) to the study of carbonfiber composites with a time of flight (TOF)-SIMS.Incidentally, both mechanical and microtome cut canprovide only cross sections of warp yarns, in which thefibers are twisted with an unpredictable orientation.

We can establish a comparison between type 1and type 2 carbon fibers, by the electron images inFigure 3(a) and (b). Type 2 appears to be “more amor-phous” than type 1. Type 2 PAN carbon fibers are pro-duced by a less efficient carbonization ramp, reachingthe same final temperature and starting from the sameintermediate precursor (Figure 3(c)). A top view of type2 fibers (not shown here), also displays a more irregu-lar shape with a bottleneck conformation along the fiberaxis. We can guess that an insufficient retention of thesefibers has caused this higher degree of irregularity.

A front view of the precursor fiber is shown inFigure 3(c) (low vacuum SEM, pressure of 100 Pa).This electron image gives evidence of a surface fracturequalitatively different from those characterizing carbonfiber morphology. From Figure 1(d) one can also seethe fragmentation in small clusters. This is due to thefact that oxypan fibers do not have the continuousconjugation system typical of carbon and/or graphitizedfibers that is obtained after the carbonization process(Perepelkin, 2003).

The plasma etching of the oxypan cloth, whichhad been developed for other purposes, can providean alternative solution to the problem of imaging anoxypan fiber. The created carbon layer has a lowconductivity, although sufficient to drain the excess ofcharge to ground. Figure 3(d) displays an ion imagingof such fibers. The inner structure of the precursoris disclosed, as the plasma process involves only few

layers of the fiber skin. The radial fractures produced bythe mechanical cutting are evident in Figure 3(d). Thisfigure, Figures 1(e) and 3(c) provide different images ofthe oxypan intermediate precursor, that one can seldomfind in literature.

The Plasma-Etched Oxypan Cloth

The front of an oxypan fiber after plasma etching isshown in Figure 4(a). It is immediately evident that theamorphous carbon film created by this treatment has ahigher SE yield than the oxypan bulk. One can appre-ciate the formation of many spheroidal carbon clusters.The skin along the fiber has completely changed. Ahigh magnification of the cracks in Figure 1(f) confirmsthat incoming ions of the plasma induced the chemicalmodification up to a limited thickness (cf. Figure 4(b)).This carbon layer appears to be rough and it has anabrupt interface toward the unmodified bulk.

A typical result from many cross sections of thesefibers is shown in Figure 4(c,d). The carbon film has ahalf-moon shape along the fiber and it is thicker in thosezones more exposed to plasma actions, correspondingto the top part in these figures. From geometricalconsiderations about the plasma device, we can saythat the ion dose and kinetic were larger during thetreatment in those zones, with a consequent moreeffective chemical modification.

The action of the plasma can be further charac-terized by SIM at low magnifications. Ion images ofFigure 5(a) and (b) point out many fibers in a bundle,with the skin partially or fully carbonized or with-out any apparent modifications. Fibers with a differentextent of carbonization can also be seen. The bettersurface sensitivity of this technique, indeed, detects thesmallest variations in conductivity and consequently inthe plasma treatment.

We can estimate dimensions of the layer displayedin Figure 4(b) in a quick way. The image format is1024 × 954, and a pixel corresponds to a rectangulararea of 4.6 × 4.3 nm2. The apparent layer thicknessis estimated to be 300 ± 20 nm using the 1D-EDTalong the direction pointed out by the white arrow.The standard deviation refers to fluctuations around themean value of 10 repetitions of the measure, in a certainzone close to the radial direction.

In the following, we will discuss examples of sys-tematic errors the 1D-EDT can incur. First of all, thefracture of the carbon layer is not completely in thetransverse direction. This should always be taken intoaccount and it might result in an overestimate of theheight of the fracture. Secondly, also the mechanicaltolerance of the stage tilting should be considered inerror estimation. Thirdly, the image and the fracture arenot coplanar. In our case, the first two errors are negli-gible if compared to the last one, which is referred toas the parallax error or as the foreshortening error.

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Fig 4. The effect of the plasma etching on oxypan. The figure shows the following: (a) the treatment at the front of the fiber (HFW =20.3 µm); (b) a closer look at Figure 1(f), revealing that the thickness of the carbon layer is some hundred nm (HFW = 4.67 µm); (c)a fiber is chosen in a bundle, FIB cut and polished in order to estimate the thickness of the carbon layer (HFW = 20.3 µm); (d) a closeview of the carbon layer (HFW = 6.08 µm). Electron beam imaging (5 keV). White arrows indicate directions of 1D-EDT profiles.

In particular, the thickness of the layer in Figure 4(b)might be underestimated by a factor up to 0.707, asthe parallax error is due to an angle α up to π

4 .On the basis of the “real” thickness, this should be350–430 nm with a parallax relative error within therange of 15–40%. The 1D-EDT has also been used inthe study of Figure 3(d). Here, the cross section of thefiber seems to better correspond with the image plane.The profile along a segment indicated by a verticalblack arrow in Figure 3(d) and its numerical derivativesare displayed in Figure 6(a), (b), and (c), respectively.The mean value of its thickness measures 610 ± 35nm, with a parallax relative error that we guess to beless than 10%. For comparison, the thickness alongthe horizontal black arrow in Figure 3(d) is 300 ±15 nm.

Problems arising from the parallax error can beremoved by carrying out measures with the stereopairs method (e.g. cap. 5 in Goldstein et al ., 2003

and references quoted therein). Alternatively, one couldminimize this systematic error by performing the 1D-EDT on a cross section of the fiber after FIB millingand polishing, as in Figure 4(c). The profile along thewhite arrow in Figure 4(d) and its derivative are shownon the right of Figure 6. Here the cross-section planelies exactly on the image plane, as the sample stage istilted by an angle of 52◦ which is the same for the SEMcolumn. In this manner, the only error (∼5%) that wecannot avoid is slicing the fiber at an improper angle.The thickness is 440 ± 35 nm. Finally, a systematicstudy on many fibers allows us to affirm that thethickness of the carbon layer induced by the plasmaprocessing lies in the range (300 ÷ 800) nm.

Discussion and Conclusion

In this work, a FIB/SEM system is used in the studyof different carbon-based fibers and different kinds of

S. Magni et al.: FIB/SEM Characterization of Carbon-based Fibers 193

Fig 5. Two ion-generated electron images at different magni-fications, revealing the inhomogeneity of the plasma treatmentwith a better contrast. In the brighter regions a partial carboniza-tion takes place, making the oxypan fibers slightly conducting.HFW = 1.31 mm for (a) and 203 µm for (b) with ion-beamcurrent 104 pA and energy 30 keV.

behavior are displayed during all the operations. ThePAN carbon fibers are good conductors (cf. Table I),hence, electron and ion imaging as well as FIB millingare successful. The images confirm the previous resultson the subject (Knibbs, 1971; Johnson, 1987; Kumaret al ., 1993; Paris et al ., 2001), i.e. the particulatemorphology of the PAN-based carbon fibers. FIBsectioning with the consequent imaging establishes thata full carbonization takes place for these fibers andcharacterizes them as high-quality fibers. However, areverse engineering operation reveals that one of the

two kinds of carbon fibers that we have considereddisplay a more irregular morphology after the industrialcarbonization process.

The intermediate precursor, i.e. the oxypan, is onthe contrary strongly insulating. Its conventional SEMimaging can be obtained by adjusting beam parame-ters and the tilting of the sample. A more straight-forward technique is to perform their imaging in lowvacuum modality (chamber pressure 80–100 Pa). FIBoperations require some caveats in order to ground themilling zone on these insulating fibers. To this purpose,we have deposited a platinum strip on the top of thefiber with the aid of the ion beam-assisted deposition.

Many studies affirm that PAN-based carbon fibersfollow the original fibril structure of the precursor fibers(Edie, 1998; Wangxi et al ., 2003). A morphologicallink between the former and the latter might beestablished considering a large number of both fibers.

Moreover, with a FIB/SEM system it is possibleto study the morphology not only of the surface butalso of the bulk and this can contribute to resolvethe spatial complexity of the sample. In particular, theFIB can obtain both longitudinal and cross sectionsof carbon fibers suitable for electron microscopy,avoiding artifacts during preparation, which can hinderpossible explanations of the graphite-layer organizationin different kinds of carbon fibers as reported inJohnson (1987). We suggest that, in the near future,such a machine should be adopted to study a widerange of composite or polymeric materials, regardingboth their nature and the industrial process developedon them, for example coating or enhancing adhesiontreatments.

The plasma etching creates a feeble conductive,spatially, inhomogeneous carbon layer on oxypan fibersproviding a further way to disclose their inner structure,while draining the excess of charge to ground. Thisprocess is rather different from the usual plasmaetching sometimes adopted before electron microscopyof polymers (Goldstein et al ., 2003), as it involves awell-defined chemical modification of the sample, i.e.a creation of a carbon layer.

We have shown how FIB imaging enhances imagecontrast because of its better surface sensitivity and howit is useful to discriminate the carbonized zone in theplasma-etched oxypan. We were able to show that theplasma-induced modification is inhomogeneous eitherconsidering a single fiber or a bundle, qualitativelyexplaining the intermediate surface resistivity of thiscloth (cf. Table I).

The thickness of this carbon film, (300 ÷ 800) nm, ismeasured with a 1D-EDT, characterizing the efficiencyof the novel plasma process developed on oxypan. Con-sidering that the radius for this fiber is ∼8 µm, thismodification varies from 4 to 10% of this length. How-ever, if we consider an “ideal” oxypan fiber to be cylin-drical with a radius of 8 µm, the volume modification

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Fig 6. Examples of the thickness estimation by means of the 1D-EDT. Shown here is the study of the gray profile along a verticaldirection of the lower part of Figure 3(d)(left panels) and upper part of Figure 4(d) (right panels). Panels (a) and (d) show the original(continuous line) and the FFT smoothed profiles (dotted lines). Panels (b) and (e), in which the numerical derivatives of FFT smoothedprofiles are estimated. Panels (c) and (f), where the numerical derivatives of original profiles are estimated with the method reportedin Milani et al . (2005b). Results with two different widths of bins are shown: 5 points (crosses) and 10 points (circles).

amounts from 7 to 19% of the volume of the startingfiber. We could conclude that such a modification hasan appreciable effect on the electrical properties of theoxypan cloth. The section on “Results” suggest that theindustrial process (pyrolysis) to produce carbon fibersshould be replaced by this plasma treatment, reducing

time and costs of production, if the plasma process wereefficient enough to fully carbonize the oxypan.

Finally, we expect that we will able to perform aFIB cut of ungrounded oxypan fibers with a FIB/SEMconfiguration in which the electron column can beemployed as a flooding gun to neutralize charging

S. Magni et al.: FIB/SEM Characterization of Carbon-based Fibers 195

occurring during FIB cross-sectioning (Stokes et al .,2006).

Acknowledgements

We would like to thank R. Barni and M. Piselli forsuggestions regarding the experimental setup of plasmadevices, M. Orlandi for fruitful discussion about thechemistry of carbon fibers, L. Ferraro for assistance onEDT software and analysis, D. Drobne for the jointwork in the “Executive Programme of Scientific andTechnological Cooperation between Italy and Slovenia”line SV3 (2006-2009), funded by the Italian Ministryof Foreign Affairs.

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