Ultramicroscopy 111 (2011) 1093–1100

Contents lists available at ScienceDirect

Ultramicroscopy

0304-39

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/ultramic

A high signal-to-noise ratio toroidal electron spectrometer for the SEM

H.Q. Hoang, M. Osterberg, A. Khursheed n

Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, Singapore

a r t i c l e i n f o

Article history:

Received 16 December 2010

Received in revised form

13 June 2011

Accepted 21 June 2011Available online 28 June 2011

Keywords:

Toroidal electron energy spectrometer

Scanning electron microscope

Secondary electron spectrum

Backscattered electron spectrum

Voltage contrast

91/$ - see front matter & 2011 Elsevier B.V. A

016/j.ultramic.2011.06.003

esponding author. Tel.: þ6565162295; fax: þ

ail address: eleka@nus.edu.sg (A. Khursheed).

a b s t r a c t

This paper presents a high signal-to-noise ratio electron energy spectrometer attachment for the

scanning electron microscope (SEM), designed to measure changes in specimen surface potential from

secondary electrons and extract specimen atomic number information from backscattered electrons.

Experimental results are presented, which demonstrate that the spectrometer can in principle detect

specimen voltage changes well into the sub-mV range, and distinguish close atomic numbers by a

signal-to-noise ratio of better than 20. The spectrometer has applications for quantitatively mapping

specimen surface voltage and atomic number variations on the nano-scale.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

The secondary electron (SE) energy spectrum inside a ScanningElectron Microscope (SEM) has been widely used for manyapplications such as Electron Beam Testing (EBT) of integratedcircuits [1], monitoring specimen charging [2], and semiconductordopant mapping [3,4]. Most previous SE spectrometers used forvoltage contrast applications have been of the closed-loop retard-ing field type, whose operational principle is illustrated in Fig. 1.There is usually an extraction grid biased to a positive potential of1–2 kV (VE) that first accelerates secondary electrons away fromthe surface of the specimen, followed by a retarding field region,which filters out lower energy secondary electrons by making theretarding grid voltage VR more negative than the specimenvoltage VS. In the open-loop mode of operation, the potential VR

is varied, typically from 0 V to a certain negative voltage, creatingan output signal that is closely related to the integrated form ofthe SE energy spectrum. This signal is normally referred to as an‘‘S-curve’’. Changes in surface specimen voltage, indicated by VS1

and VS2 in Fig. 1, alter the energy barrier, e(VS–VR), where e is theelectron charge, and therefore cause shifts in the output S-curve.By monitoring S-curve shifts, specimen surface potential varia-tions can be quantitatively detected by the SEM, either in theform of a 2D map over the specimen surface or in time as signalwaveforms at specific points of interest on the specimen surface(such as exposed tracks on an integrated circuit). Open-loopsignals of this type have been used to measure the potentialdifference across a Si p–n junction specimen [3]. In order to

ll rights reserved.

6567791103.

simplify the measurement procedure, the retarding grid voltageVR can be biased 1–2 V more negative than the specimen voltageVS, and subsequently adjusted by a feedback loop to keep theoutput current constant when changes in specimen voltage occur.In this way, adjustments in the retarding grid, DVR, are able todirectly track specimen surface potential changes DVS. The closed-loop feedback retarding field spectrometer method of using theSEM to measure surface voltage changes was widely used in theElectron Beam Testing of integrated circuit specimens [1].

Another way of obtaining specimen voltage variations via the SEMis to detect the SE energy distribution more directly via the use ofeither a band-pass energy analyzer or a parallel multi-channel energyspectrometer. Fig. 2 illustrates this method by the use of an electricsector band-pass energy spectrometer. The analyzer pass energy isscanned over the SE energy range by varying the deflector voltage VD,and specimen voltage changes are detected in the form of linear shiftsin the output energy spectrum signal. Although the band-pass/multi-channel spectrometer method is a more direct way of obtaining theSE spectrum, it is more complicated than the closed-loop feedbackmethod for making voltage measurements, since it relies on detectingthe shifts of a non-linear output signal. Unlike retarding field spectro-meters, where shifts in the linear portion of the output S-curve aremonitored, the SE spectrum signal of band-pass/multi-channel doesnot have a clearly identifiable linear part to it; some method, such aspeak position detection, is required. There is also the added compli-cation of how to apply an extraction field close to the specimensurface, which is required in some applications; for these reasons, themajority of voltage contrast spectrometers designed for the SEM wereof the closed-loop retarding field spectrometer type [1].

Renewed interest in capturing the full SE spectrum to makevoltage contrast measurements came from the realization that ithas significant signal-to-noise advantages over the retarding field

VR

VS1 VS2

Output Current

Offsetlevel

SEM objective lens Primary beam

Offset

Detector

RetardingGrid VR (t)

EE2

Collectedelectrons

SpecimenVS

E1ExtractionGrid VE

ΔVR

Fig. 1. Principle of a retarding field spectrometer for voltage contrast: (a) spectro-

meter layout and (b) output S-curve signals (integrated SE spectra).

VS1 VS2

⏐VD⏐

Output Current

SEM objective lens

Primary beam

DetectorSpecimen

VS

+VD

-VD E1

E3

E2

E1 < E2 < E3

Fig. 2. Principle of a band-pass spectrometer for voltage contrast: (a) spectro-

meter layout and (b) output energy spectrum.

H.Q. Hoang et al. / Ultramicroscopy 111 (2011) 1093–11001094

spectrometer method. With this advantage in mind, some multi-channel voltage contrast spectrometers were proposed [5,6]. Shotnoise, generated within the SEM primary beam, and its interac-tion with the specimen usually set the limit on the minimumdetectable shift in the spectrometer output signal, and it deter-mines, in most situations, the accuracy (voltage resolution) to

which specimen voltage changes can be measured. In the contextof Electron Beam Testers, a widely used formula to characterize aspectrometer’s voltage resolution DVr involves the primary beamcurrent IPE, the data-acquisition time, taq, and a spectrometerconstant C, and is given by [1]

DVr ¼ C1

2taqIPE

� �1=2

ð1Þ

Experimental values for C typically lie in the 6�10–9–5�10–8 V(As)1/2 range. As more and more electrons contribute to theoutput signal, the shot noise limit will naturally decrease, andbeyond a certain limit, parameters like power supply instabilitiesand background noise in the detector set the ultimate limit on thevoltage resolution of the spectrometer. Signal-to-noise analysisshows that multi-channel voltage contrast spectrometers arepredicted to have over 30 times shorter data-acquisition timesthan retarding field spectrometers (around 5.5 smaller values ofthe spectrometer constant C) for the same voltage resolution[7,8]. This advantage comes from the fact that the sensitivity ofthe SE spectrum to specimen voltage changes varies considerablyas a function of energy; lower energy electrons have much greatersensitivity than higher energy ones, mainly due to the shape ofthe SE spectrum, which initially rises steeply and then falls offgradually. If the full SE spectrum is captured, the more sensitiveparts of the SE spectrum (to specimen voltage changes) can inprinciple be separately monitored and utilized. On the other hand,only a small part of the retarding field spectrometer output signalis made up from electrons that are sensitive to surface voltagevariations (those that have kinetic energies just above thespectrometer’s potential barrier), the majority of the output signalis made up of electrons that are insensitive to specimen voltagechanges. These higher energy secondary electrons form a back-ground noise signal, which degrades the output signal-to-noiseratio. In practice, the situation is somewhat worse than thisbecause this background signal also contains backscattered elec-trons (BSE). Dubbledam [7] proposed a double-channel retardingfield spectrometer, consisting of a lower energy channel and ahigher energy channel, and it was able to achieve an experimen-tally measured spectrometer constant C of 4.2�10–9 V(As)1/2 inits lower energy channel, a factor of 2–3 times better thanconventional retarding field spectrometers. However, one disad-vantage of this voltage contrast spectrometer was that its perfor-mance could only be optimized for a given secondary electronenergy, and this sets an additional limit on the voltage resolutionthat can be achieved in practice.

The multi-channel voltage contrast spectrometers proposed so farfor the SEM cannot be easily incorporated into conventional SEMs asattachments. The double-channel retarding field spectrometer pro-posed by Dubbledam [7] was constructed on top of an immersionobjective lens column that was specially built to incorporate it. TheKienle and Plies [6] multi-channel proposal was specifically designedfor a low voltage column SEM, the one that uses a single-polemagnetic immersion lens in combination with a retarding fieldobjective lens. The scattered electrons from the sample travel upthrough the objective lens, are deflected by a Wien filter from theprimary beam optical axis, then dispersed through an electricspherical deflector energy analyzer, and subsequently focused onto a scintillator detector by an electrostatic zoom lens unit. This kindof through-the-lens voltage contrast spectrometer cannot be imple-mented as an easy to use attachment for most SEMs. Its considerablecomplexity also brings in other parameters that limit the finalvoltage resolution. For a beam current of 50 nA, a voltage resolutionof 50 mV was measured, and instability of the high voltage electro-des in the spherical deflector energy analyzer was cited for degradingthe voltage resolution beyond what was expected based upon shot

PMT + Scintillator

SEM objective lens

PE

Aperture

+VSC-VD

0V

0Vshielding

VS

SE detector

Secondaryelectrontrajectories

Rotationalaxis

Fig. 3. Experimental layout of the high-resolution toroidal secondary electron

spectrometer inside the SEM. Sixteen electron trajectory paths with an input

angular spread of 781 around the central angle of 451 are simulated.

Specimen

Hemispherical caps

Spectrometerdeflectorelectrodes

Aperture

52.5 mm

Fig. 4. Prototype of the toroidal spectrometer attachment (a half). Azimuthal

deflection angle is 1001.

H.Q. Hoang et al. / Ultramicroscopy 111 (2011) 1093–1100 1095

noise considerations alone. The spectrometer constant C was mea-sured to be 5.6�10–8 V(As)1/2. The time of flight multi-channelvoltage contrast proposal made by Khursheed [5], based upon usingthe time dispersion of secondary electrons that travel through amagnetic immersion objective lens field, has the disadvantage ofrequiring that the primary beam be blanked to nano-second pulsewidths, and is also not straightforward to implement into mostconventional SEMs as an add-on attachment.

Band-pass voltage contrast spectrometers are a much simplerway of obtaining the full SE spectrum than multi-channel spectro-meters, and will in comparison to open-loop retarding fieldspectrometers have much better signal-to-shot noise character-istics. However, a simple way of using parts of the output signalthat are most sensitive to specimen potential variations isrequired, the one that is comparable in simplicity to makingmeasurements with closed-loop retarding field spectrometers.

The most recent incorporation of a SE voltage contrast spectro-meter into a SEM was for the mapping of dopant concentration insemiconductor specimens [3,4]. An open-loop retarding field spectro-meter was designed and integrated into a magnetic immersionobjective lens, where shifts in output signal (integrated form of theSE spectrum) were monitored from point to point on a pn junctionspecimen. A relatively low beam current of 5 pA was used in order tominimize the production of electron-hole pairs. The surface potentialchange across the p–n junction (0.72 V) was measured to anaccuracy of 70.15 V. The output signals were differentiated in orderto obtain SE spectra, and an average voltage shift over the lowerenergy portion was used [3]. The resolution to which dopantconcentrations can be measured by this technique is criticallydependent on the degree to which voltage shifts in the output signalcan be distinguished, mV resolution is required.

This paper presents a band-pass voltage contrast spectrometerattachment for conventional SEMs based upon the recentlydesigned second-order focusing electric toroidal energy spectro-meter [8,9]. For a 10 pA primary beam current and a dwell time of0.1 s for each point in the output signal, shifts larger than 12 mVin the output SE signal can be distinguished. This can be furtherimproved to be shifts larger than 4 mV by biasing the specimenand a neighboring hemispherical electrode negatively (typicallybetween �10 and �20 V), which has the effect of greatly sharpen-ing the output signal peak shape and improving its signal-to-noiseproperties. For these conditions, a typical value of the spectrometerconstant C was measured to be 5.65�10–9 V(As)1/2. The position ofthe peak was found to be very sensitive to specimen voltagevariations, and by calculating the expectation value (signal mean)over 500 points around the peak position (70.5 V), we predict thatthe shot noise limited minimum measurable voltage shift should bearound 35.2 mV, which would be equivalent to a spectrometerconstant of 0.05�10–9 V(As)1/2.

The toroidal spectrometer is also used to examine the signal-to-noise properties of the backscattered energy spectrum, andquantifies the output signal BSE signal’s sensitivity to changes inspecimen atomic number being probed.

In order to avoid charging and transverse surface field effects,known to cause errors for quantitative voltage contrast measure-ments [1], all experimental spectral measurements are made on bulkmetal specimens. This is done in order to establish the signal-to-noiseproperties of the toroidal spectrometer on close to ideal specimenconditions; this information can then be used to better understandhow the spectrometer performs for the more complicated cases.

2. Toroidal electron spectrometer attachment design

Fig. 3 depicts the experimental setup of the spectrometer,fitted as an attachment inside a conventional SEM. The inner

sector is grounded and the outer sector is biased with a negativepotential �VD that is ramped to capture the scattered electronspectrum. The spectrometer is designed to capture an angularspread of 781 with respect to the central entrance angle of 451 inthe polar direction. The input angular spread in the azimuthaldirection is 1001. In this design, the specimen is surrounded bytwo hemispherical caps, where the inner cap is electricallyconnected to the specimen and the outer cap is grounded. Thisarrangement keeps a field-free region around the specimen sur-face while setting up a radial field in the space in between the twocaps, allowing the specimen to be biased to a certain negative orpositive potential. This arrangement can be used for extractionand collimation of SEs into the spectrometer. An aperture isplaced vertically at the spectrometer exit to select electronenergy. A scintillator is used to convert the output electrons intolight, which is then detected by a photomultiplier tube (PMT). Thewhole setup is placed right below the final pole-piece of the SEMobjective lens in order to minimize the working distance, asshown in Fig. 3. The minimum working distance for this setup is15 mm. For the acquisition of SE spectra, the scintillator voltage,VSC, was biased to 5 kV. A 3D drawing of one half of the spectro-meter attachment prototype is illustrated in Fig. 4.

80

90

100 VS = -24V

-20V

H.Q. Hoang et al. / Ultramicroscopy 111 (2011) 1093–11001096

Fig. 3 also depicts 16 simulated secondary electron trajectorypaths traced from the specimen through the spectrometer on tothe scintillator, demonstrating how the spectrometer functions.The electric field distribution solution and electron trajectorieswere obtained by running the program, LORENTZ-2EM [11].In this simulation, the specimen and the inner cap are grounded.The scattered electrons are emitted from the specimen in alldirections and a wide range of energies, from the low energysecondary electrons to the elastic backscattered electrons, entersthe spectrometer. However, only electrons with an initial energyaround the spectrometer passing energy with polar emissionangles between 371 and 531 will travel through the spectrometerand strike the scintillator. When the specimen and the inner capare biased to a certain negative voltage, the arrangement acts as apre-focusing lens in front of the spectrometer, which allows moreelectrons to be collected. More details on the biasing effect aregiven in the following section.

An experimental relative energy resolution measurement of0.38% from this spectrometer attachment has been reportedelsewhere [12]; this value is slightly larger than the correspond-ing simulated resolution of 0.32% (for an angular spread of 781)by direct ray tracing. One reason for the slightly larger experi-mental value may come from misplacement of the exit slit.Ideally, an electronic means of varying the position of the exitfocal point needs to be found in order to optimize the analyzer’sperformance. In the recent experiments for monitoring SE spec-trum inside a SEM, where high energy resolution is not required,the output aperture of the spectrometer was fixed at around100 mm, corresponding to an energy resolution of about 0.6%.

0

10

20

30

40

50

60

70

0

PMT

sign

al (a

.u)

Deflection voltage (V)

-16V

-12V

-8V

-4V

0V

5 10 15 20 25

Fig. 6. Experimental secondary electron output signals at different specimen

biasing voltages.

3. Experimental secondary electron voltage contrast signals

A chromium specimen was used to investigate the SE spec-trum inside the SEM. The specimen was cleaned using an ultra-sonic cleaner with acetone solvent. The specimen was polished tohave a smooth surface, in order to minimize the influence ofsurface topography. All experiments were carried out inside aTungsten JEOL JSM-5600 SEM, where a 5 kV, 10 pA primary beamwas focused on to a 3 nm diameter spot on the specimen. Thedeflection voltage VD in Fig. 3 is software controlled through aPersonal Computer (PC), and the output signal on the PMT is fedinto an electrometer, converted into a digital signal and mon-itored by the PC as VD is scanned. Around 150–200 points are usedin the generation of a full SE spectrum, with each point having a

3.1

3.15

3.2

3.25

3.3

3.35

3.4

1

PMT

sign

al (a

.u)

ΔV=12 mV Curve 1

Curve 2

Deflection voltage (V1.05

Fig. 5. Experimental SE spectrum: (a) full range and (b) selected range in which curve 2

noise limit.

primary beam dwell time of 0.1 s on the specimen. The outputsignal is typically captured at a rate of around 3 points per second.

Fig. 5a shows an experimentally acquired SE spectrum, whileFig. 5b displays a selected part of it in order to study the noise onthe signal. In this experiment, the specimen and the inner cap aregrounded. Fig. 5b shows that the noise is relatively small,allowing for SE spectral shifts of 12 mV or larger to be monitored.

For the next experiment, the specimen is biased with anegative voltage together with the inner cap, while keeping theouter cap grounded. Fig. 6 shows the output signals of thecollected SEs as a function of deflection voltage at differentnegative specimen biasing voltages. Compared to 0 V bias, theoutput signal not only shifts to the right (as expected) but alsogrows significantly in intensity as the biasing voltage increases,forming a sharper peak, which no longer directly represents theSE spectrum. The increase in signal height comes from the factthat negative specimen voltages repel secondary electronstowards the spectrometer entrance, giving them a higher passenergy, enabling a wider energy range to travel through thespectrometer exit slit. This negative biasing also creates anacceleration leakage field penetrating into the inner cap nearthe specimen surface, creating a weak lens focusing action,effectively widening the angular spread into the spectrometer.Since the output aperture size is unchanged, a correspondingly

1

2

3

4

5

0

)1.1 1.15

1 2 3 4 5 6 7 8 9 10

(dotted line) is obtained by shifting curve 1 by 12 mV in order to demonstrate the

H.Q. Hoang et al. / Ultramicroscopy 111 (2011) 1093–1100 1097

higher proportion of secondary electrons pass through the outputslit. The electric field generated between the inner and the outercaps pulls through secondary electrons that would otherwisestrike the inner surface of the inner cap.

Although the SE spectrometer output signals no longer repre-sent the SE energy spectrum for more negative specimen voltages,they are in a more useful form for the purposes of quantifyingspecimen potential changes, due to the presence of a sharp signalpeak. These results illustrate an important point about theacquisition of the SE spectra in general: high energy resolutionis not required, and accurate recording of the SE energy spectrumis unnecessary. By biasing the specimen/inner cap negatively, theSE spectrum is transformed into a much more convenient formfor open-loop specimen voltage measurement than the original SEspectrum. If the surface potential does not change by largeamounts over the area scanned, it is possible to store severalimages, each at a different analyzer deflector voltage, therebyobtaining an estimate of the peak position for each pixel in theimage. In this way, a quantitative voltage map can be super-imposed upon the normal SE image, rapidly acquired in open-loop mode.

Not only does the output signal develop a sharp peak, whichcan be precisely tracked as a function of specimen potential, butthe signal-to-noise ratio is more than an order of magnitudehigher than the case when there is no specimen bias. This isillustrated by the experimental signals shown in Fig. 7, where thebias voltage on the specimen/inner cap changes from �10 to�10.1 V. In Fig. 7a, the spectrometer deflector voltage isrestricted to a small range (70.5 V) around the peak signal valueand a greater number of points in VD are deliberately sampled(5 0 0) in order to make the presence of noise more visible. Fig. 7bshows a small part of the signal, in the range, 7.1oVDo7.16 V, tothe left hand side of the peak value, where the noise is estimatedto limit the voltage shifts in the output signal that can bemonitored to around 4 mV. This is considerably better than the12 mV estimate for the unbiased specimen case, and comes from

Fig. 7. Experimental secondary electron signals for the specimen voltage changing from

and (b) deflection voltage range from 7.1 to 7.16 V.

the fact that parts of the signal develop a greater gradient as thepeak sharpens in shape, and this results in greater sensitivity tospecimen voltage changes. An approximate value for the spectro-meter constant C (given by Eq. (1)), based on data acquisition(dwell time) of 0.1 s, primary beam current of 10 pA and voltageresolution of 4 mV is 5.65�10–9 V(As)1/2, lower than mostexperimental values reported previously [7]. Moreover, this valuecorresponds to around 1001 collection in the azimuthal direction,if a full 3601 is used; this value is expected to drop byapproximately half.

The signal-to-noise performance of the spectrometer can befurther improved using the expectation value in the output signalaround the signal peak for each signal, given by

EðVÞ ¼XN

j ¼ 1

PjVj ð2Þ

where the index j runs from 1 to the number of points sampled in theoutput signal that is examined, N, Vj refers to the deflector voltage, Pj

refers to the probability of each point in the output signal, obtainedby normalizing the output height to the area under the output curve.The change in expectation value due to specimen voltage changerepresents the parameter to be measured, the signal; while noise inthe measurement is the variation in the expectation value caused byshot noise, the difference in expectation value of the signal (withnoise) from the expectation value of a 5th-order polynomial fits to thesignal, denoted by dashed lines in Fig. 7c. Use of the expectationfunction enhances signal-to-noise information for the part of theoutput signal around the peak, since the peak position shiftssignificantly as the specimen voltage is changed, while the effect ofnoise, if it is dominated by shot noise, will be suppressed. Shot noisecauses variations in terms of the detected current at each point in theoutput signal, and any blurring that arises in the rising edge will beapproximately compensated by blurring in the falling edge. If thepeak were perfectly symmetric, there would, on average, be nohorizontal blurring effect on the waveform. These points are

�10 to �10.1 V around the peak value. (a) Deflection voltage range from 7 to 8 V

0

10

20

30

40

50

0

Col

lect

rion

sign

al (a

. u)

Deflection voltage (V)

ExperimentSimulation

VS = -10VVcap = -10V

VS = -14VVcap = -14V

5 10 15 20

Fig. 9. Comparison between experimental and simulated SE output signals for the

specimen/inner cap voltages of �10 and �14 V.

H.Q. Hoang et al. / Ultramicroscopy 111 (2011) 1093–11001098

illustrated by examining the expectation values of the signals shownin Fig. 7a. Here the expectation values are 7.49405 V for VS¼�10 V,and 7.511865 V for VS¼�10.1 V, giving a change of 17.8154 mV forthe specimen/inner cap voltage changing by 100 mV. In comparison,the change in expectation value due to shot noise from a smooth5th-order polynomial fit for the signal at VS¼�10.1 V is calculated tobe 6.272 mV, giving a signal-to-noise ratio of 2840 (17.8154mV/6.272 mV). Assuming that the minimum measurable voltageoccurs at a signal-to-noise ratio of unity, this sets the minimumdetectable shift due to shot noise to be around 35.2 mV. Thecorresponding spectrometer constant C value is 0.05�10–9 V(As)1/2.These kinds of high signal-to-noise measurements of output SE signalshifts are expected to be useful for mapping dopant carrier concen-trations in semiconductor devices, where shifts in SE spectra ofseveral mV need to be monitored [3,4].

The method of calculating the expectation value over a smalldeflection voltage range around the peak position does notrequire a large number of points in the output signal. For 10points evenly spaced in the 7–8 V range of VD (selected from the500 points actually recorded), the change in expectation value is19.75 mV as the specimen voltage goes from �10 to �10.1 V,giving a signal-to-noise ratio of 3149 (19.75 mV/6.272 mV).The total data-acquisition time to achieve this is 1 s (10 points,each with an individual dwell time of 0.1 s).

Simulations were carried out to help understand the specimenbiasing effect. The specimen and the inner cap are biased as thesame potential VS¼Vcap, while the outer cap is grounded.A Chung–Everhart SE distribution is assumed for the simulations[13]. The width and the thickness of the output aperture are 200and 100 mm, respectively. Fig. 8 shows the simulated upper andlower limits of the polar angular range accepted into the toroidalspectrometer for different take-off energies at a specimen/innercap bias of �14 V. It shows that the entrance take-off polarangular range into the spectrometer is sharply energy dependentat low energies. In general, more scattered electrons with take-offenergies below 1 eV are pulled towards the toroidal spectrometerby the electric field set up by the specimen bias. Therefore, morelow energy electrons enter the spectrometer, up to a 501 range for0.1 eV SEs, compared to the 161 acceptance polar angular rangefor a zero specimen/inner cap voltage. The upper polar angularlimit also falls as the take-off energy decreases; this is because avertical focusing effect is also created by the specimen beingbiased negatively (vertical acceleration field); however, loss ofelectrons due to this effect is relatively small, and the net polarangular range of electrons entering the analyzer is considerablywider at lower energies (o1 eV) than it is for higher energies.When the specimen is biased, the kinetic energies of low energy

Fig. 8. Collection efficiency of different SE energies through the two hemispherical

caps for the specimen/inner cap voltage of �14 V.

electrons rise by several orders of magnitude, greatly increasingthe number of secondary electrons that pass through the spectro-meter exit slit, creating a sharp peak in the output signal that hasa much higher amplitude than the case for no biasing.

The simulated SE collection current is compared to theexperimental spectrum as illustrated in Fig. 9 for two specimen/inner cap voltages, �10 and �14 V. It shows that the relativepositions of the simulated and experimentally obtained curvesmatch closely, with the rising edge almost identical. However, thesimulation curve width is larger than the experimental one. Thismay be due to spectrometer manufacturing errors and differencesin the energy distribution of the experimental signal from thatassumed in the simulation model (Chung–Everhart distribution).It may also be due to specimen charging, causing the actualpotential of the specimen to differ slightly from that of the innercap. Two other bias conditions were simulated to further under-stand these effects: first, where the specimen bias is morenegative than the inner cap bias, and, second, where the specimenbias is less negative than the inner cap bias. The results show thatwhen the voltage specimen is more negative than the inner capthe simulated curve fits better to the experimental one than theother two cases where the specimen bias potential is either thesame or less negative than the inner cap voltage. The best fit tothe experimental spectra is obtained when the specimen voltageis set to be one volt more negative than the inner cap voltage.More investigation is required in order to better understand thesedifferences in simulated and experimental SE spectra. In thepresent context, these simulation results demonstrate that thegreater sensitivity of the output signal to specimen voltagevariations is caused both by the acceleration electric fringe fieldand a higher pass energy bandwidth through the spectrometer.

Another point for future work is the situation where there is apotential difference between the specimen and the inner capvoltage. If the difference in potential is small, (51 V), not muchdifference from the signals presented in Figs. 6 and 7 is expected.For larger differences in potential, the height of the output signalwill drop, as electrons either return to the specimen (retardingfield) or are drawn up in the vertical direction (acceleration field),just how this affects the signal-to-noise characteristics of theanalyzer needs further investigation.

4. Experimental BSE spectra for elemental identification

Although preliminary experimental BSE energy spectra havebeen reported previously for the second-order focusing toroidal

H.Q. Hoang et al. / Ultramicroscopy 111 (2011) 1093–1100 1099

spectrometer attachment [10], they are not accurate enough togive an indication of the spectrometer’s signal-to-noise capability.The previous results were reported on an initial experimentalsetup, which collected the output signal indirectly, via a metal

0

1

2

3

4

5

6

7

0

PMT

sign

al (a

.u)

Scattered electron energy E (keV)

Au

Cr

Al

C

1 2 3 4 5

Fig. 10. Experimental BSE spectra of different metals.

0

500

1000

1500

2000

2500

3000

3500

4000

0

FWH

M o

f nor

mal

ized

spe

ctra

(eV)

Atomic number N

MC-simulation

Experimental

10 20 30 40 50 60 70 80 90

Fig. 11. Dependence of the BSE spectrum full width at half maximum (FWHM) on

atomic number. Curve fitting was used for both experiment and simulation. Black

square dots indicate actual measured values.

Fig. 12. Experimental BSE spectra for

multiplier and not directly by a PMT as in the present setup. Thisled to signal-to-noise estimates for distinguishing close atomicnumbers to be significantly underestimated. The previous esti-mate of the signal-to-noise ratio for distinguishing materials onthe specimen that differ by one atomic number was estimated tolie between 1 and 2, while the present setup estimates it to beover 20.

Fig. 10 shows the experimentally obtained BSE spectra for speci-mens made from gold (Au), chromium (Cr), aluminum (Al) andcarbon (C). These experimental results illustrate how the BSEspectrum changes with specimen atomic number, which as expectedhas higher BSE yields and narrower widths as the atomic numberrises. The acquisition time for each spectrum is approximately 2 min.Each spectrum plot here is a single recorded data set, where thebackground noise on each signal is limited by shot noise. It is clearthat the shot noise of the setup is relatively small.

If the full width at half maximum (FWHM) of each BSE spectrumis plot as a function of atomic number, an approximate means forquantitative material contrast can be performed with the SEM [10].Fig. 11 shows the plot of FWHM of each BSE spectrum together withcorresponding simulation predictions derived from running MonteCarlo (MC) software [14]. The experimental curve shows moresensitivity to material contrast variations than the MC-simulationcurve. An indication of the SNR from these types of BSE experimentalspectra can be obtained by recording spectra from close atomicnumber specimens. For instance, the spectra for silver (N¼47) andpalladium (N¼46) were captured, as shown in Fig. 12, and theirFWHM difference was compared to variations in FWHM caused bynoise, a SNR of 23.4 was obtained. The noise was estimated by takingan average width over 30 points below and above the 50% level ofthe experimental signal, and comparing it to the FWHM of a signalobtained by performing a 5th-order polynomial fit. Obviously moremeasurements need to be made on a greater variety of specimens;however, these preliminary results indicate that it may be feasible touse the toroidal spectrometer to perform quantitative materialanalysis in the SEM by capturing BSE energy spectra.

5. Conclusion

This paper has presented experimental results from a toroidalenergy spectrometer attachment for SEMs. The signal-to-noisecharacteristics of both the secondary electron and backscatteredelectron spectra have been presented; the SE spectrum in the context

silver and palladium specimens.

H.Q. Hoang et al. / Ultramicroscopy 111 (2011) 1093–11001100

of using the SEM to measure specimen voltage changes, and the BSEspectrum to estimate specimen atomic number. For a 10 pA primarybeam current and a dwell time of 0.1 s for each point in the outputsignal, larger than 12 mV shifts in the output SE signal can bedistinguished. This can be further improved to be larger than 4 mVby biasing the specimen and a neighboring hemispherical electrodenegatively. A simple method of tracking specimen voltage changes tosub-mV precision has been proposed. Experimental BSE spectralresults show that the spectrometer can distinguish materials thatdiffer by one atomic number with a signal-to-noise ratio of greaterthan 20. Future work is required to investigate how the signal-to-noise characteristics of the energy analyzer presented in this paperchange for specific applications, such as semiconductor dopantconcentration mapping, electron beam testing of integrated circuitsand monitoring of specimen charging.

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