Voltage measurements on integrated circuitsusing the scanning electron microscope

A.R. Dinnis

Indexing terms: Integrated circuit testing, Monolithic integrated circuits, Scanning electron microscopeapplications, Voltage measurement

Abstract: The scanning electron microscope (s.e.m.) is widely used for the topographical examination andX-ray elemental analysis of integrated circuits, and is increasingly being used in electrical contrast modes. Thevoltage-contrast (v.c.) mode is particularly important because of the small dimensions of the devices found incurrent l.s.i. circuits which preclude the use of mechanical probes, and because of the minimal circuit loadingwhich the electron beam imposes in high-frequency applications. Voltage contrast can now give a fair degreeof measurement accuracy both in the static and dynamic cases. The dynamic situations often require the useof stroboscopic and sampling techniques, where the primary electron beam is chopped at a frequency lockedto that of the signal being examined.

1 Introduction

The purpose of this paper is to give an introduction to theuse of the scanning electron microscope (s.e.m.) in thevoltage-contrast (v.c.) mode for the engineer who isprimarily concerned with the design and production ofintegrated circuits. Such a person will not necessarily beinterested in the s.e.m. as such, but needs to be able to useit effectively in the same way that he uses other instruments,such as oscilloscopes, to measure the performance ofdevices.

One of the reasons for the versatility of the s.e.m. is thenature of scanning-image-formation systems; the imageon the face of the cathode-ray-tube display is formed froman electrical signal which has a particular value at eachpoint in the image. The parameter which is imaged doesnot have to be focused in the way in which, for example,electrons are focused after passing through the specimenin a conventional transmission electron microscope. Theoriginal, and most commonly used, method of formingthis electrical video signal is to detect the secondary andbackscattered electrons emitted from the specimen, whichproduce the normal topographical image. Another signalmay be generated from the high-energy photons, or X-rays,emitted from the specimen, which can be used to analysewhich elements comprise the specimen.

A purely electrical signal can be generated in semi-conductor devices, where electron-hole pairs generatedby the high-energy primary beam are affected by the strongelectric fields present near p-n junctions in the semicon-ductor. The strong field causes the electrons and the holesto separate, thereby causing a current to flow in an externalcircuit. This electron-beam induced-current (e.b.i.c.) modecan show up defects in the basic material and faults inprocessing, such as inversion layers under the oxide orpinholes in the gates of m.o.s. transistors, as well as thepresence of depletion layers at p.n.junctions.

It is important to realise that if electron beams ofsufficient energy to penetrate several microns beneath thesurface are used to examine working devices, then theirelectrical characteristics can be severely altered, particularly

Paper T292 S, first received 17th July and in revised form 8thNovember 1978Dr. Dinnis is with the Department of Electrical Engineering,University of Edinburgh, King's Buildings, Edinburgh EH9 3JL,Scotland

with m.o.s. devices. It is therefore normal to use primarybeam energies of below lOkV for work on examiningdevices which are to be kept operational. If devices have apassivating insulating film covering regions of interest,energies near to lOkV are used in order that some pen-etration of the film takes place, thereby rendering it slightlyconductive and allowing the surface to approach thepotential of the underlying conductor. In the absence ofa passivating layer, energies of below 5 kV may be used.The use of these low accelerating voltages also has theadvantage that the secondary-electron yield is better thanat the normal 20 or 30 kV e.h.t. used for high-resolutiontopographical work, and it is the secondaries which areimportant in voltage-contrast work. There are two funda-mental approaches to the problem of measuring potentialsof specimen surfaces with an electron beam. They differ inwhether the measurement is done using the effect of thesurface voltage on the primary, incoming, beam or on thesecondary, emitted, electrons. While the second method isthe one most used in scanning microscopy, the other onehas its own particular merits.

2 Primary-beam voltage contrast

The essence of this method is that the specimen and thecathode of the electron gun are close together in potential,so that small changes in specimen voltage can significantlyaffect the behaviour of primary electrons as they near thespecimen surface. This effect is most commonly used inthe Vidicon television-camera tube, where electrons areaccelerated to 300 V and then are guided perpendicularlythrough a fine mesh grid placed a small distance from,and parallel to, the target surface. A retarding field regionexists between the grid and the specimen, so whether anelectron is collected on the specimen or is reflected andreturned through the grid depends on the potential ofthe target with respect to the cathode; if this potential ismore positive than a certain threshold value, then all theelectrons will be collected, while as the potential becomesmore negative than the threshold, increasing numbers ofelectrons are repelled. Flemming2 has used such a systemfor measurements on semiconductor devices, and morerecently Christou et al.3 have used the same principle tocharacterise certain defects in semiconductor wafers,where a voltage resolution of 25 mV was achieved. Thespatial resolution of such systems has been of the order of

SOLID-STA TE AND ELECTRON DEVICES, JANUAR Y1979, Vol. 3, No. 1 25

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several microns because of the electron optical limitationsrelated to the type of lens and deflecting system used,together with the presence of the grid close to the specimen.However, it is possible that the resolution may be improvedas the technique is, in essence, similar to scanning electronmirror microscopy (s.e.m.m.), where a normal s.e.m.column and accelerating voltages are used, but the specimenis held at a potential within a few volts of the cathode.4

The ultimate spatial and voltage resolutions of this typeof system will be affected by similar factors to those whichlimit the performance of the secondary-electron voltage-contrast technique when applied to integrated circuits.Strong electric fields exist close to the surface of i.e.conductors, and these have a major effect on the trajec-tories of the low-energy primary electrons. By deflectingthe position of the beam they distort and defocus thepicture, and by affecting the collection of electrons theydistort the voltage measurement accuracy.

3 Secondary-electron voltage contrast

The existence of voltage contrast in the s.e.m.5 has beenknown for many years, and its use is now fairly common.In its simplest form, it appears using the microscope in itsstandard configuration when static potentials are appliedto the circuit under examination. A typical example isshown in Fig. 1. To ensure that this contrast appears, it isadvisable to elaborate matters slightly by placing aconducting shield above the specimen, as indicated in Fig. 2.

Fig. 1 Voltage contrast on a c.c.d. device: static positive potentialsare applied

Device provided by the Wolfson Microelectronics Institute, Universityof Edinburgh

This simultaneously prevents the primary beam fromstriking insulating parts of the device package and preventsbackscattered primary electrons from contributing signifi-cantly to the image. The image is therefore formed mainlyfrom secondary electrons, which will normally have anenergy spread of from 0 up to 10 or 15 eV as they leavethe specimen surface. If the point of emission has a negativepotential with respect to the surrounding surface, then theelectrons will be accelerated away from the surface, and theangular spread in the direction of emission will decrease, asindicated in Fig. 3. Consequently,.these electrons are morelikely to reach the collector than those from an unbiased

point, and a negatively biased region will therefore be seenon the display tube to be brighter than its surroundings.Conversely, if the point under examination is positivewith respect to its surroundings, electrons will be retardedand spread out further in angular distribution, many ofthem actually returning to the specimen surface, as shownin Fig. 4. Thus, such a point appears darker than itssurroundings.

back scatteredelectrons

primary beam

secondaryelectrons

shield

Fig. 2 Arrangement of shield above insulating package to preventcharging problems and to reduce number of backscattered electronsreaching collector

s

/ S f / / f

Fig. 3 Electrons accelerated from negative conductor

/1

> / ^ / \ \\

Fig. 4 Electrons retarded from positive conductor

The technique provides a means for ascertaining thepresence or absence of a potential, and such simple in-formation can be very valuable. For example, breaks inconductors can be readily detected even when these are notvisible under the closest topographical examination. Thefact of most significance is that an image is presented ofa large portion of a complex device; this means that anobserver can very rapidly identify the precise area wheretrouble is occurring, without having to probe eachindividual point in the circuit. It is interesting to comparethis method of fault location with the methods used ondiscrete component circuits, where probing with oscillo-scopes and meters is employed at discrete critical points, asthere is no technique in use at present which can directlygive an image of how potential varies over a large area ofsuch a circuit. It would not seem unreasonable to suppose

26 SOLID-STATE AND ELECTRON DEVICES, JANUARY 1979, Vol. 3, No. 1

that, if the voltage-contrast technique could be refined togive the same sort of information at a point on the circuitas is available from an oscilloscope, then fault finding inintegrated-circuit devices might even be easier than in thediscrete component case, given similar levels of circuitcomplexity.

The overall aim of providing a method which is accurate,fast and easy to use would seem to be best met byproviding the large-area 'voltage contrast' picture of theconventional kind, together with the facility to instantlyswitch to a method which will precisely measure static anddynamically changing potentials at any chosen point onthe integrated circuit.

4 Quantitative methods

If the voltage-contrast signal can be made to vary mono-tonically with actual potential, then it should be possible toassign certain potentials to every distinguishable grey level,e.g. black = + 5 V to white = — 5 V. In the simple casewith no special electron-detector system this is by nomeans always the case, and the contrast may disappear insome parts of an i.e. or even reverse the polarity. However,provided that contamination darkening induced by electronbombardment does not become too obvious, this can be avery valuable machanism in practice, even if only two orthree different levels can be distinguished.

Methods that have been devised by Tee & Gopinath,6 byHannah7 and by Balk et al.,8 for the reliable measurementof surface potentials in integrated circuits all depend upondetection of the shift in the distribution of the secondary-electron energies due to the potential of the point ofinterest. This energy distribution has the general formshown in Fig. 5a, with a peak in the curve at about 3 eV forthe materials commonly encountered; clearly, if the surfacehas a negative potential of V, then the peak will occur atV + 3 eV. An electron spectrometer and associatedelectronics can be used to detect this shift and so measurethe potential with a fair degree of precision, 1% inreasonably favourable circumstances. There are somedifficulties in employing this technique for conductorswhich are positive with respect to their immediate sur-roundings, but by employing extracting fields above thesurface of the specimen as used by Hannah,7 Balk8 andDinnis and McCarte,9 these problems can usually beovercome. MacDonald10 has overcome the problem ofextracting electrons by employing Auger electrons, whichhave higher energies than secondaries.

5 High-frequency measurements

While the advantages of the s.e.m. for measuring d.c.voltages are clear enough, namely that the probe has verysmall dimensions and does not inflict mechanical damageon the specimen, its use as a voltage probe at highfrequencies has the additional advantage that it imposesvery little loading on the circuit, and so allows the investi-gation of effects which would not otherwise be detectable.A good example is the work of Feuerbaum11 where thedynamically varying voltage on a capacitor of about 10"1

pF was measured.The most straightforward technique for observing a.c.

signals is to stop the beam on the point of interest andexamine the signal coming out of the detector system.If 'simple voltage contrast' is being used, i.e. the con-ventional scintillator is used in conjunction with a simpleshield above the specimen to stop backscattered electrons

reaching the detector, then no severe problems ariseprovided the signal is within the bandwidth of the videosignal path. As systems suitable for t.v. rate picturegeneration must have a bandwidth of about 5 MHz, wave-forms containing frequencies below this limit should bereproduced adequately. However, the introduction ofan electron spectrometer of the types so far discussedwill drastically curtail the frequency response of thesystem. This is a consequence of their modes of operation,which involve either a feedback loop to adjust their energycutoff point, or an energy-scan system to do effectivelythe same thing.

Fig. 5

electron energy

Electron energy distributions for a surface0 V— 3V+ 3V

Even if these systems are eventually developed to haveimproved frequency response, there will always be re-quirements to work at frequencies higher than either they,or a t.v. bandwidth system, can cope with. The techniqueadopted, originally described by Plows and Nixon,12 is tochop the beam at a high frequency related to the frequencyof the signal on the circuit. The electron stream beingemitted from the specimen will therefore be modulatedboth by this frequency and the frequency of the voltage onthe specimen. The resulting modulated signal can bearranged to have a frequency within the bandwidth of anydetector system, and if the two frequencies are lockedtogether, the output has a d.c. component. The lattersituation is known as the 'stroboscopic' mode and can beused, if a short beam pulse is used, to measure the voltageat a chosen phase point in a repetive waveform. Themethod is indicated in Fig. 6. If the frequencies are notprecisely equal, the phase between them is controlled tovary slowly with the time-base of the display device; theactual waveform of the signal can be observed, as indicatedin Fig. 7. This is a process virtually identical to thatemployed in sampling oscilloscopes; hence the name'sampling' mode. The upper limit on frequency is essentiallythat at which the beam can be chopped, though signal/noiseconsiderations also apply because of the small duty cycleof the primary beam. A high-performance system has beendescribed by Gopinathan and Gopinath.13

6 Future developments

As individual devices in an i.e. become smaller andconductor linewidths grow narrower, inspection by s.e.m.becomes more important both in the topographical modeand in the 'electrical' modes. Emphasis on nondestructiveinspection will grow: at present it is not at all common touse the s.e.m. to examine devices on the slice before it isdivided into individual chips. This is because present day

SOLID-STA TE AND ELECTRON DEVICES, JANUAR Y1979, Vol. 3, No. 1 27

s.e.m.s are unable to form good images at the centre of sucha specimen as electrons cannot escape to a conventionalcollector. However, because suitable detectors will bedeveloped as a result of the work at present being done onvoltage-contrast detectors, this restriction should soon belifted and it will no longer be necessary to break up a sliceto obtain a good topographic image anywhere on itssurface.

choppingplates

signalgenerator

display

Fig. 6 Stroboscopic mode voltage contrast

The beam can be scanned to produce a voltage contrast map on thedisplay

Electrical examination of devices on the slice saves thetime and the labour of mounting and bonding each device,but involves the provision of additional facilities on thespecimen stage. There must be a specimen table capable of±50mm movement to accomodate slices 100 mm indiameter, together with probes to make contact to thebonding pads on the chip, as described by Wolfgang14 andby Fazekas et al.rs In addition of course, the electronextractor/spectrometer assembly must be incorporated. Thismust be able to examine all parts of the chip without inter-fering with the contact probes, so that careful mechanicaldesign is called for.

A circuit-testing system which is capable of making a.c.and d.c. measurements on devices which are on a 10-16 cm(4 in) wafer will have a considerable degree of complexity.Taking into account possible electronic data storage, pro-cessing and display systems, this complexity can approachor exceed that of the s.e.m. itself. It is therefore reasonableto envisage that an s.e.m. should be entirely dedicated tothe function of evaluating i.c.s. and should not be used forany other purpose which would involve removal or dis-turbance of any of the equipment. Such a situation islogical for an i.e. manufacturer, and would encourage fulluse of the s.e.m. facility. In cases where there is a demand

Fig. 7 Sampling mode voltage contrast

The sampling point is continuously shifted in time, so that thewaveform of the repetitive waveform can be built up

that the s.e.m. should be used for other purposes, in a moregenerally oriented research organisation, it is essential thatpressures to keep the instrument a 'general purpose' oneshould be resisted if a useful and reliable set up is to bemaintained.

Efforts should be directed towards making theinstrument and its associated equipment easy to use by thei.e. engineer. Specimen manipulation should be largelyautomatic so that the risk of damaging the specimen eithermechanically or electrically is minimised. Electronirradiation damage can be reduced by image-storagetechniques; these will have to be developed, in any case, inorder that appropriate image processing and presentationcan be carried out. The presentation of voltage-contrastmeasurements as a colour-coded image superimposed on atopographical image is one obvious example of the latter.9

The s.e.m. is bound to be used more and more as aneveryday tool by the i.e. designer and quality controlengineer. While it is clear that the i.e. engineer should havea certain basic knowledge of the workings of the s.e.m. it isalso of the utmost importance that efforts should beintensified to provide adaptations to s.e.m.s that will permitthe extraction of the maximum possible amount of usefulinformation about the i.e.

7 References

1 THORNTON, P.R.: 'Scanning electron microscopy'(Chapman &Hall, London, 1968), pp. 217-218

2 FLEMMING, J.P.: 'A low energy scanned electron beampotential probe', J. Phys. E., 1968, 1, pp. 1179-1182

3 CHRISTOU, A., NELSON, G., JENKINS, W., DAVEY, J.E., andWILKINS, W.: Characterization of semiconductor wafer defectsby ASLEEP, SEM, and scanning AES'. Proceedings of the SEMsymposium, SEM Inc., Chicago, 1978, Vol. 1, pp. 273-281

4 COX, S.B.: 'Applications of scanning electron mirror microscopyto electronics'. Proceedings of the SEM symposium, IITResearch Institute, Chicago, 1977, pp. 433-440

5 OATLEY, C.W., and EVERHART, T.E.: 'Examination of p-njunctions with the scanning electron microscope', J. Electr. &Control, 1957, 2, pp. 568-570

6 TEE, W.J., and GOPINATH, A.: 'A voltage measurement schemefor the scanning electron microscope'. Proceedings of the SEMsymposium, IIT Research Institute, Chicago, 1976, Vol. 1, pp.595-601

7 HANNAH, J. M.: 'SEM applications to integrated circuit testing'.Ph.D. thesis, University of Edinburgh, 1974

8 BALK, L.J., FEUERBAUM, H.P., KUBALEK, E., and MENZEL,E.: 'Quantitative voltage contrast at high frequencies in theSEM'. Proceedings of the SEM symposium, IIT Research Insti-tute, Chicago, 1977, Vol. 1, pp. 615-624

9 DINNIS, A.R., and MCCARTE, J.T.: 'Improving SEM voltagecontrast measurements'. Proceedings of microcircuit engineering(Cambridge Press, 1978)

10 MACDONALD, N.C.: 'Potential mapping using Auger electronspectroscopy'. Proceedings of the SEM symposium, IIT ResearchInstitute, Chicago, 1970, pp. 481-487

11 FEUERBAUM, H.P., and HERNAUT, K.: 'Applications ofelectron beam measuring techniques for verification of computersimulations for large scale integrated circuits'. Proceedings of theSEM symposium, SEM Inc., Chicago, 1978, Vol. 1, p. 788 andpp. 795-800

12 PLOWS, G.S., and NIXON, W.C.: 'Stroboscopic scanningelectron microscopy',7. Phys. E. 1968, 1, pp. 595-600

13 GOP1NATHAN, K.G. and GOPINATH, A.: 'A sampling scanningelectron microscope', ibid., 1978, 11, pp. 229-233

14 WOLFGANG, E., OTTO, J., KANTZ, D., and LINDNER, R.:'Stroboscopic voltage contrast of dynamic 4096 bit MOS RAMs:failure analysis and function testing'. Proceedings of the SEMsymposium, IIT Research Institute, Chicago, 1976, Vol. 1,pp. 625-632

15 FAZEKAS, P., LINDNER, H., LINDNER. R., OTTO, J., andWOLFGANG, E.: 'On-wafer defect classification of LSI-circuitsusing a modified SEM'. Proceedings of the SEM symposium(SEM Inc., Chicago, 1978), Vol. 1, pp. 801-806

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