Electron holographic characterization of electrostatic potential distributions in atransistor sample fabricated by focused ion beamZhouguang Wang, Tsukasa Hirayama, Katsuhiro Sasaki, Hiroyasu Saka, and Naoko Kato Citation: Applied Physics Letters 80, 246 (2002); doi: 10.1063/1.1432746 View online: http://dx.doi.org/10.1063/1.1432746 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/80/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Fabrication of p-type silicon nanowires for 3D FETs using focused ion beam J. Vac. Sci. Technol. B 31, 06FA01 (2013); 10.1116/1.4823763 Extending the detection limit of dopants for focused ion beam prepared semiconductor specimens examined byoff-axis electron holography J. Appl. Phys. 106, 064506 (2009); 10.1063/1.3195088 Focused ion beam specimen preparation for off-axis electron holography using Si, Ga, and Au ions Appl. Phys. Lett. 93, 043510 (2008); 10.1063/1.2960351 Improvement in electron holographic phase images of focused-ion-beam-milled GaAs and Si p - n junctions by insitu annealing Appl. Phys. Lett. 88, 063510 (2006); 10.1063/1.2172068 Characterizing an implanted Si/Si p–n junction with lower doping level by combined electron holography andfocused-ion-beam milling Appl. Phys. Lett. 81, 478 (2002); 10.1063/1.1491606

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Electron holographic characterization of electrostatic potential distributionsin a transistor sample fabricated by focused ion beam

Zhouguang Wanga) and Tsukasa HirayamaJapan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan

Katsuhiro Sasaki and Hiroyasu SakaDepartment of Quantum Engineering, Nagoya University, Nagoya 464-8603, Japan

Naoko KatoSemiconductor Failure Analysis Laboratory, International Test and Engineering Services (ITES) Co. Ltd,800, Ichimiyake, Yasu-cho, Yasu-gun, Shiga-ken 520-2392, Japan

~Received 24 July 2001; accepted for publication 31 October 2001!

A cross-sectional sample of a silicon-metal-oxide semiconductor field-effect transistor, which wasdirectly cut from an integrated circuit wafer, has been prepared carefully using a focused-ion-beamtechnique and examined by means of off-axis electron holography. In the reconstructed phase image,heavily doped source, and drain regions are revealed clearly as bright contrast, from which ann-channel transistor is identified. In addition, two-dimensional phase distributions around bothsource and drain regions show a core area with relatively high phase in the heavily doped region,which may be attributed to the effect of doping atoms and residual defects and strains remainingafter implantation. The electrostatic potentials across the core area and depletion layer are estimatedand discussed. This work demonstrates the feasibility of using a focused-ion-beam technique toprepare electron holographic sections of a wide range of semiconductor devices. ©2002 AmericanInstitute of Physics.@DOI: 10.1063/1.1432746#

Electron holography, based on a transmission electronmicroscope~TEM! equipped with a field emission sourceand an electron biprism, can record two-dimensional phaseinformation about an object wave relative to a referencewave, thus making it possible to directly image and analyzelocal electric and magnetic field distributions with high spa-tial resolution.1 Owing to their importance in the semicon-ductor industry,p–n junctions, the basic building blocks ofsemiconductor devices, have been investigated by electronholography.2–4 Recently two-dimensional electrostatic poten-tials in semiconductor transistor structures have also beensuccessfully mapped by Rauet al.5 These studies demon-strate the validity of electron holography as a practical toolfor imaging the potential distribution in a semiconductor de-vice with nanometer spatial resolution and;0.1 V potentialsensitivity.

The sensitivity of the phase to thickness change acrossthe sample means that a uniform thickness in the observedarea is necessary in order to obtain accurate informationabout the potential distribution. This, to some extent, in-creases the difficulty of preparing the TEM sample. In Rau’sstudy,5 all observed transistor cross-sectional samples weresuccessfully prepared in predetermined microscopic regionsby means of argon ion milling but for a practical semicon-ductor device with complicated structures and compositions,it is difficult to position the region of interest and control thethinning thickness by the ion milling method. Due to someunique advantages, such as high accurate position selectivity,high process uniformity, high removal rate, etc., the focused-ion-beam ~FIB! technique is now widely used to prepare

TEM samples in the semiconductor industry. To investigatethe application of electron holography to practical devicesamples prepared by FIB, in this work a silicon-metal-oxidesemiconductor field-effect transistor~MOSFET! sample sec-tioned directly from a silicon integrated circuit wafer wasfabricated using the FIB technique to produce a cross sec-tion, and electrostatic potential distributions in the transistorwere examined by means of off-axis electron holography.

The cross-sectional silicon transistor sample for this ob-servation was prepared using a Hitachi FB-2000 FIB thin-ning system. First, the slices containing complementarymetal-oxide-semiconductor~CMOS! transistors were sec-tioned using a dicing saw from a silicon device wafer. Sincewe did not know the positions of the transistors, to protectthem from damage the thickness of the slice was typicallyreduced to;120mm by mechanical polishing. The slice wasthen mounted on a partially cut grid so that a gallium ionbeam could easily be adjusted to impinge normal to theoriginal surface. The surface was imaged to locate the pre-cise position of a CMOS. In this region an;30 mm widetrench was etched little by little from one side along thedirection of the slice width until a MOSFET was found, fol-lowed by thinning from the other side. When a film thicknessof ;2 mm was reached, the surface passivating layer, themetal interconnect lines, and part of the SiO2 layer wereremoved. A protective W layer was then deposited, and thefinal sample suitable for TEM observation was thinned to;200 nm at lower currents. All observations were performedon a Hitachi HF-2000 TEM~200 kV acceleration voltage!equipped with a field-emission source and electron biprisms.Digital 102431024 TEM images and holograms were col-lected using a Gatan 694 slow scan charge coupled devicea!Electronic mail: wangzg@jfcc.or.jp

APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 2 14 JANUARY 2002

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~CCD! system. The hologram was reconstructed using Avi-sion software~Image Sense Co. Ltd.!.

Figure 1~a! shows a cross-section TEM image of theMOSFET. We can see a uniform contrast in the doped re-gions of source and drain parts. This indicates that a rela-tively uniform thickness has been obtained in these twoparts. In the region below the gate, because the WSi conduc-tor of the gate remains during FIB etching, there is a projec-tion in the sample area. This will induce an additional phasein this area, but does not affect our observations in the sourceand drain parts. An electron hologram of the region shown inFig. 1~a! was taken and the corresponding reconstructedphase image is shown in Fig. 1~b!. In the phase image, thephase distributes so the potential distributions in the sourceand drain regions are clearly indicated by the brighter con-trast. The observed difference in contrast reflects the changesin electrostatic potential. In fact, on the two sides of ap–njunction, the potential in then-type region is higher than inthep-type region, so in the phase image the brighter contrastcorresponds to then-type region, and the darker one to theptype. According to this criterion we can identify our deviceimmediately as ann-channel MOSFET, where the sourceand drain aren-type doped.

In Fig. 2, phase images in the~a! source and~c! drainparts are given. To see the potential distributions in these twoparts more clearly, corresponding equiphase contours aredrawn in Figs. 2~b! and 2~d!, for the source and drain, re-spectively. We see a small difference in the potential distri-bution ~junction distribution! between the source and drain.In addition, as noted from the equiphase contours, a core areawith relatively high phase appears in the central part of theheavily doped region@indicated by D in Fig. 2~b! or D8 inFig. 2~d!#. This may correspond to the center of dopant in-jection to form the junction by ion implantation during de-vice fabrication. To quantitatively analyze the detailed phasedistributions across the depletion layers in the source anddrain parts shown in Figs. 2~a! and 2~c!, line profiles AB andA8B8, averaging over 0.4mm are plotted through a distance1.0 mm, respectively, from the positions A and A8 near sur-

faces at the core areas, and the phase profiles plotted in Fig.3. In the line profiles we see apparent phase drops across thejunctions. However instead of the expected flat distributions,it can be seen that continually increasing phase distributionsappear in the highly doped regions of the source and drain.This may be because the interactions between doping atomsand residual defects and strains remaining in the injectioncore areas increase the electrostatic potential slightly.

As shown in Fig. 3, a small difference appears in thephase distributions near the junction positions at the sourceand drain parts. This may be due to a variety of factors suchas sample preparation~especially beam damage during the

FIG. 1. ~a! TEM image and~b! reconstructed phase image of a Si-MOSFETcross section prepared by FIB. In the phase image,n-type doped regions atthe source and drain are clearly distinguished by brighter contrast from thep-type regions.

FIG. 2. Reconstructed phase images~a! and ~c!, and correspondingequiphase images~b! and ~d! showing two-dimensional phase distributionsaround heavily doped source and drain regions.

FIG. 3. Average phase profiles from Figs. 2~a! and 2~c! showing one-dimensional phase distributions across depletion regions as a function ofdepth for source and drain.

247Appl. Phys. Lett., Vol. 80, No. 2, 14 January 2002 Wang et al.

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FIB process!, thickness fluctuation, and fluctuations duringdevice manufacturing. Despite this small difference, totalphase changes (w tot) across the core areas and depletion re-gions are almost the same and were measured to be;1.44rad. In the equiphase contours shown in Figs. 2~b! and 2~d!,if we think the junctions are situated at positions where thephases begin to decrease abruptly@equiphase lines, C and C8,respectively, in Figs. 2~b! and 2~d!#, the averaging junctiondepth can be extracted as;380 nm. Measuring at this posi-tion, the averaging phase shiftwpn across the junction isabout 1.0460.16 rad; this means the averaging phase incre-mentDw in the core area is about 0.4 rad.

From the above measured phase values, the potentialscan be estimated using the equation5

w5CEV~ t22t0!

wherew is the phase shift induced by potential dropV, CE isa constant~CE57.29531023 rad/~V nm! for a 200 keV elec-tron beam!, t is the sample thickness, andt0 is the thicknessof the dead layer at each sample surface. The sample thick-ness,t, can be derived from the reconstructed amplitude im-age following the method proposed by McCartneyet al.6 Us-ing a theoretical bulk inelastic mean-free-path value of 125nm for Si for 200 keV incident electrons,7 the thickness ofour observed specimen was determined to be;216 nm. Us-ing this value, along with 2t0549 nm, which was measuredby Rauet al. using a FIB fabricated Si/Sip–n junction testsample,5 we estimate the total potential drop,Vtot to be;1.18 V, the potential dropVpn across the depletion regionto be;0.8660.13 V, and the potential increment in the corearea to be;0.32 V.

In conclusion, a Si-MOSFET sample carefully fabricatedby the FIB technique was examined by off-axis electron ho-lography. The success in mapping the electrostatic potentialdistributions of source and drain regions demonstrates thecapability of applying the technique based upon combinedFIB-electron holography to characterize a wide range ofsemiconductor devices.

The authors would like to thank Dr. K. Kitamura of IBMJapan Ltd., Dr. Y. Kohno and Dr. J. M. Won of ITES Co. Ltd.and Dr. N. Shibata and Dr. C. Fisher of JFCC for informativediscussions and comments on the manuscript. This work wassupported in part by the New Energy and Industrial Technol-ogy Development Organization~NEDO!.

1A. Tonomura,Electron Holography. Springer Series in Optical Sciences,Vol. 70, 2nd ed.~Springer, Berlin, 1999!.

2S. Frabboni, G. Matteucci, G. Pozzi, and M. Vanzi, Phys. Rev. Lett.55,2196 ~1985!.

3M. R. McCartney, D. J. Smith, R. Hull, J. C. Bean, E. Vokl, and B. Frost,Appl. Phys. Lett.65, 2603~1994!.

4Z. G. Wang, K. Sasaki, N. Kato, K. Urata, T. Hirayama, and H. Saka, J.Electron Microsc.~to be published!.

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6M. R. McCartney and M. Gajdardziska-Josifovska, Ultramicroscopy53,283 ~1994!.

7This value was calculated from Eq.~10! in Ref. 6 using a collectionsemiangle of 10 mrad.

248 Appl. Phys. Lett., Vol. 80, No. 2, 14 January 2002 Wang et al.

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