Rutherford scattering analysis:a tool for semiconductor - device technology

D. V. MorganIndexing terms: Semiconductor devices, Surface structure, Passivation, Metallisation, Semiconductor-metal

boundaries, Ion scattering

Abstract: In the paper we consider how the Rutherford scattering of high-energy (MeV) light ions can beused as a nondestructive technique for studying thin films on semiconductor surfaces. A number of examplesare discussed which illustrate the potential and limitations of the technique for studying semiconductor-device structures.

1 Introduction

During the past 25 years there has been rapid progress insemiconductor-device technology, and developments whichhave moved from the simple transistor structure fabricatedby Bardeen, Brattain and Shockley to the very complexmicrotechnology of integrated circuits. This swift advance insemiconductor technology has depended on two inter-related areas of work: basic device physics/technology andmaterials and device characterisation techniques. Anadvancement, for example, in the techniques available formeasuring residual impurities in solids would enable thematerials to be better characterised, would result in im-proved purity and, eventually, device performance. In thispaper one such technique is explored — Rutherford scat-tering of MeV ions — and in particular, its contribution tothe technology of semiconductor device interfaces.

Ion beams have been widely used for solid-state studiesfor well over 10 years. Ion implantation, for example, hasbeen successfully used to alter and control the near-surfaceproperties of solids, with many notable successes in thefield of semiconductor technology.1'2 In the role of ananalytical technique, ion beams have been used for lattice-site location studies,1 surface analysis,3 impurity profiling3

and thin-film analysis.4 The physical phenomena used foranalysis can vary widely, depending on the particularapplication — many of these have been reviewed in detail inReferences 4, 5 and 6. In this paper discussion will belimited to one particular analysis technique — Rutherfordscattering of high-energy light particles (e.g. 1—4MeVHe+

ions). Furthermore, no attempt will be made to reviewwork in the field of lattice location or radiation damagestudies, the primary concern will be to show how thetechnique has been used to study the following problems:semiconductor passivation and metallisation systems, ohmicand Schottky contacts (including silicide formation) inter-diffusion of thin films and contact degradation. It is notintended that an exhaustive review of all the work conductedin this area should be presented since it has been recordedin the proceedings of a series of three international confer-ences.4"6 Some examples will be carefully selected whichillustrate both the potential and the limitations of thetechnique.

2 Rutherford scattering analysis

In order to understand the physical principles on whichthe analysis is based, consider a beam of mono-energetic

Paper T21, first received 3rd May and in revised form 24th June1976Dr. Morgan is with the Department of Electrical & ElectronicEngineering, The University of Leeds, Leeds LS2 9JT, England

(light) ions falling onto a solid surface, as depicted schema-tically in Fig. 1A. This example shows a silicon crystalcovered with a thin passivation layer of silicon dioxide. Ifwe consider only the elastic interactions between thebombarding ions and the target atoms, then from a consider-ation of conservation of momentum and energy we candeduce that particles scattered through an angle 0 from thesurface region of the solid will have an energy Eo, given by

£"0(0) = (1)

Ei is the incident energy and k2 the kinematical factorgiven by

k2 =x cosd

My +M2

SiO,

(2)

solid statedetector

acceptanceangle SO

Fig. 1A Schematic of an experimental arrangement suitable forRutherford-scattering analysis

3x10?

,2x10" Oin SiOr

VSi substrate

Si inSi02

04 0 6 08 10 12 14energy, MeV

Fig. 1B Spectrum obtained when 2 MeV He ions are scatteredfrom a silicon crystal (0 -- 160°) covered with a 400 nm film ofSiO2

The signal from the Si substrate is at a lower energy to that from theSi in SiO2 owing to the energy loss when the particles pass into andout of the oxide layer

SOLID-STATE AND ELECTRON DEVICES, JANUARY 1977, Vol. l,No. 2

Mi is the mass of the incident ion and M2 the scatteringatom. From this equation we see that each species of surfaceatoms (M2) has its own unique value of k2 and hence willyield its own unique value of Eo(0). Since the particlesscattered from a solid surface will be those with the greatestenergy, we can see with reference to Fig. IB that a compari-son of E0(0)si/Ei and E0(0)o/Ei for the leading edges ofthe silicon and oxygen spectra with the calculated k2 valuesfor both these species enables their identity to be confirmed.This is particularly valuable in impurity analysis and will bediscussed in some detail later in this paper. In Fig. 2A a plotof k2 versus atomic mass for4 He+ ions is shown. The relation-ship is highly nonlinear, the mass resolution [8(k2)/8M2]decreases as the mass M2 increases. Consequently, theheavier mass elements all pile up at the high-energy (k2)end of the curve. This situation can be improved by increas-ing the mass of the probing beam — which helps to separatethe k2 values for the different species. However, increasingthe probing ion mass does not always result in a net im-provement, since it changes other features of the spectra.This effect will be discussed later in this paper.

CNJ 1 0

EobQ-05ouOQjC

9=160

2 0 °Ore- p rJ°°A^ AC Si Cr Ge Ag Au

atomic mass, M2Fig. 2A Plot of the k2 values for 4He ions (0 = 160°) against'ltomic massAppropriate values for a range of ions are shown on the mass scale

The analysis can be put on a quantitative basis by usingthe Rutherford scattering cross-section to relate the back-scattering yield of particles to atomic concentration density

P100

crD ' ' ' 'RD 100

C Si CrGe Ag Auatomic number 2.^

co

10u

ou

crt_o

Fig. 2B Plot of the Rutherford-scattering cross-section (arbitraryunits) for He ions against atomic numberThe full line is a linear plot of a and the broken line shows log aNote: the increased sensitivity for high-atomic-number elements

N, and atomic numbers of the incident and scattering ionsZx and Z2, respectively, and the incident charge Q

a = cosec — (3)

and

Yield (Y) = NoASlQAz

We see from this equation that a decreases rapidly as thescattering angle 6 increases from 0 to 180°. For a violentscattering process to take place (i.e. 6 > few degrees) theions must interact with the atoms at separations of typically10~s to 10~6 nm, which of course is very small comparedwith the mean spacing of atoms in the target, typically ofthe order of 0-2 nm. We can gain some idea of the smallmagnitude of the yield of Rutherford scattering by notingthat in the example of a gold film which is 1 jum thick, thetotal fraction of a 4 MeV beam of helium ions scatteredthrough a solid angle between 6 = 50 to 52° is — 5 x 10~5.The dependence of the cross-section on Z2 is shown inFig. 2B. If we need to relate absolute yield to concentrationwe have to calculate the geometry of the system accuratelyto obtain 0 and Afi. However, if we keep the geometryconstant, the use of a suitable standard target enablesthe absolute concentration of a species to be calculated bycomparison of its yield with that of the standard. Since thescattering yield is given by

ZxZ2e2 Jd

— cosec I -

scattering angle6 = 160° M2—

N SiS1^=1401 *(nitrogen)

M1=1-0(

Cr Ga AsU

An AuA i _

1 _(hydrogen) Q~ 1002 0 4 0 6 08

relative energy [E0(0)/E.]-Fig. 2C Plot ofk2 for a number of elements (d = 160° )The lower plot is for protons and the upper curve for nitrogen ions

1500

ari 000

5 0 0

thin film ofAuAg.and Cu

HeE0=2-5MeV

light substrate

Au

He1 . .A.

;l2O

mass h H 1

63 65 107 109 197

2 5 energy, MeVSo00

Fig. 3 Spectrum obtained by backscattering a 25MeVHe ionbeam from a thin sample composed of equal numbers of gold,silver and copper atoms (6 = 164° )The broken line shows the (Z2)2 dependence of the scatteringcross-section (after Nicolet et al.1)

38 SOLID-STATE AND ELECTRON DEVICES, JANUARY 1977, Vol. 1, No. 2

a comparison with the yield from a standard specimenYstand gives US

= Y,standZx Vstand I ™stand

(5)

This is illustrated by showing in Fig. 3 the backscatteredspectrum for a 2-5 MeV beam of 4He+ ions incident on avery thin film composed of an equal mixture of Au, Ag andCu deposited on a very light substrate. The use of the lightsubstrate ensures that its spectrum does not interfere withthe impurity peaks. The energies Eo for each of the threepeaks enables the three constituents to be identified througheqn. l.The broken line shows the Rutherford scatteringyield normalised to the Au peak.

The Agand Cu peaks fall below this line, because boththese elements have two isotopes, resulting in two separatepeaks for each of the two elements. Consequently, thedetailed structure of these double peaks can yield isotopicinformation, provided, of course, that the two separatepeaks can be resolved. This example illustrates the importantpoint, that a correct measure of atomic concentration is thearea under the peak, and not the peak height. This spectrumillustrates another very important feature of Rutherfordscattering. The finite energy width of the Au peaks in Fig. 3is a result of the finite energy resolution of the particledetector and its associated electronics. Thus for two massesclose to each other, as for example in the two Cu isotopes,the peaks can only be resolved if the energy separation ofthe two peaks x and y is greater than the detector resolu-tion; i.e.

Et{k% •— > detector energy resolution (AE) (6)

From this equation we see that good resolution may beachieved by

(i) improving the detector resolution (i.e. decreasingAE)

(ii) increasing the beam energy E{

(iii) increasing (kx — k2,).Consider the first of these possibilities. A solid-state par-ticle detector will have a typical energy resolution ~10—16 keV for 2 MeV alpha particles (these good values arethe result of the substantial efforts which have been devotedto the problem).8 The resolution can be improved greatlyby using an electostatic or magnetic spectrometer. However,these instruments are more cumbersome and difficult touse and consequently have only been used extensively forsurface studies9'10 and radiation-damage studies.11 Thesecond possibility is to increase the beam energy. In prac-tice two problems may arise. First, there is the energylimit available on most accelerators — the majority ofmachines in use are limited to 2 MeV for singly chargedions. If one is prepared to sacrifice some beam current,4 MeV can readily be achieved for doubly charged ions.A more important limitation imposed on the use of highenergy is the onset of nuclear reactions and resonances inthe Rutherford scattering cross-section. The onset on thissort of phenomenon makes quantitative measurementsvery difficult. This problem will be discussed later in thepaper. The third possibility is to increase the (k2

x — k2)term. This can be achieved by increasing the mass of theprobing beam, which has the effect of separating thevarious impurity peaks. This is illustrated in Fig. 2C bythe k2 values being shown for a range of ions, first for ahydrogen ion beam and then for a nitrogen ion beam. In

the case of the proton beam the peaks for Ga, As, Ag andAu are all located close together at around k2 ~ 0-95 andwould be difficult to resolve. For the case of a nitrogenbeam these peaks spread out over the range of k2 from 0-45to 0-75. Unfortunately, increasing the mass of the probingbeam introduces new problems: the detector resolutionwill decrease and its performance will degrade more rapidlywith ion dose, owing to the increased radiation damageresulting from heavier ions. These two effects combine tospread out the impurity peaks and will counteract thedesirable improvement in mass resolution. Consequently,we cannot make hard and fast general rules for the selectionof an incident ion and its energy, since the choice can onlybe made after a detailed preliminary study of the parametersdiscussed above.

2.1 Energy-to-depth conversion

Thus far discussion has been confined to surface impuritiesor thin films whose thickness is equal to or less than the

V. cU O

Eo(z)Eo(0)energy

E0(z*Az) scatteredbeam

incidention beam

surface

Fig. 4 Illustrating how the energy scale of a Rutherford-back-scattering spectrum can be converted into a depth scaleIn this simple example the incident beam is assumed to be perpen-dicular to the surface

depth resolution of the detector. Consider now the effectsdue to a specimen of finite thickness, as shown in Fig. 4,where for geometric simplicity the case of normal incidencehas been taken. The particles scattered from some depthz in the solid chosen will return with some energy E0(z)which will be less than E0(Q) -since the particle will loseenergy traversing the solid. If the rate of loss of energySX(E) in the material (x) is known, then the energy scalecan be converted into a depth scale. For the simple geo-metry shown in Fig. 4 we can write

E0(z) = k2xEt - z [Sx(Ei)kl - SX(EO)/cos0] (7)

the energy loss parameter

Sx = [Sx(Et)k2x-SX(EO)Icosd] (8)

then provides the conversion from energy to depth. In thecase of a compound solid the stopping power for thatcompound must be used together with the appropriatevalue of k2 for the particular constituent that causes the

SOLID-STATE AND ELECTRON DEVICES, JANUARY 1977, Vol. l,No. 2 39

Rutherford scattering. From eqn. 7 the energy-to-depthconversion relationship can be rewritten as

AEAz = AE/[klSM)-Sx(E0)lcose] = — (9)

This equation allows the depth resolution Az of the systemto be determined from the detector resolution AE. Typi-cally, for 2 MeV He ions scattered through 150° in GaAs,Sx ^600eVnm~1, and for a solid-state detector with asystem resolution AE =* \5kcV,Az, the depth resolution,is approximately equal to 25 nm. One way of improvingthe depth resolution is to increase Sx by making 6 tendtowards 90°, thus increasing the path length in the target ofthe emerging ions. This procedure has been used withnotable success by Williams.13 In practice, however, it isbest to make the angle of incidence as well as the angle ofemergence into glancing angles, hence maximising the pathlength for each element Az of the solid perpendicular tothe surface. Using this technique Williams13 has reducedthe depth resolution of He in Si to 3 nm. This technique isunfortunately very restrictive since it requires targets whichcan be prepared with highly flat surfaces.

From this discussion we see that the energy scale can beconverted into both a mass scale (eqns. 1 and 2) and adepth scale through eqn. 7. This dual nature can in certaincircumstances give rise to problems of interpretation: thescattered energy by itself cannot be used to identify aspecies since, for example, the backscattered energy of aburied impurity could be misinterpreted as a lighter massimpurity at the surface. For simple structures this ambiguitycan be partly removed by tilting the crystal and taking asecond spectrum.4 For complex structures, however, someprior knowledge of the system is an advantage. Frequently,this means the use of alternative techniques to supplementRutherford scattering.

2.2 Spectrum heights for thick targets

For targets thicker than the depth resolution of the detectorsystem (Z 25 nm) the Gaussian peak gives way to a welldefined flat plateau region, as for example in the Si and0 peaks in Fig. IB. The height Y of the yield gives thenumber of scattering events in a distance Az. This thicknessis related to the channel width AEby eqn.9;AZT = [Sx] Az.(AE is fixed by the gain of the electronic system and istypically ~ 2—5 keV). Thus we may write

= QAQNaAz

AE (10)Yx = QAQ.No

[Sx]

Hence, the plateau height increases as o/[Sx] (i.e. increasesat greater depths in the target partly due to the l/E2 depen-dence of a and partly because [Sx] varies with energy).

From eqn. 10 we can determine the ratio of two con-stituents in a thin film thus

Yy Ny oy Sx

or

*y °x

A more detailed discussion of the physical basis of interpret-ation is given in Reference 12.

3 Applications to semiconductor device technology

In this Section a selection of problems related to semi-conductor-device technology that have been studied usingthe Rutherford scattering technique are examined. Thediscussion will be selective and the examples chosen toillustrate the current achievements and future potential ofthe technique.

3.1 Compositional and impurity analysis of thin films

A technique with the combined facility of identifyingimpurity masses and measuring their atomic density findsan obvious application in the field of compositional andimpurity analysis of thin films. Fig. IB for example showsthe spectrum obtained from a 400 nm film of SiO2 depositedon a silicon surface by sputtering. Both the silicon (in SiO2)and oxygen peaks are visible, together with the backgroundsilicon. Since the shape of the background Si spectrum canbe obtained from a separate experiment involving a cleansilicon surface, the total spectrum can be separated to yieldthe Si (in SiO2) and O spectra. The comparative yields fromthe Si and O peaks confirm that the stoichiometry is SiO2

(approximately equal to ± 5%). We note further that thefinite thickness of the SiO2 spectra enables information ofstoichiometry with depth to be obtained. In this case, thecomposition remains constant throughout the film. Incontrast, vacuum deposited films frequently exhibit com-positional variation of as much as 20% as the surface isapproached14 (for example see Fig. 6, Reference 14). In theexample of Fig. IB, although the oxygen peak is super-imposed on the substrate spectrum (Si), the large concen-tration of O together with the closeness of the atomicnumbers of Si and O (i.e. the Rutherford scattering yieldswill be comparable) makes it relatively easy to extract theoxygen spectrum. If the substrate atoms are considerablyheavier than the lighter species (e.g. copper oxide on copper),the small yield of oxygen relative to copper would makeit impossible to detect the oxygen signal with any accuracy.One solution to this problem is to use resonances in theion-atom interaction processes. For example, Dearnaleyand Suffield15 have studied the oxidation of titanium byusing the broad resonance (i.e. increased yield) in theRutherford-scattering yield of protons on oxygen. Similarly,Amsel and coworkers16 have used nuclear reactions forlooking selectively at light impurities in heavy solids.Cachard et al.11 have successfully used the l6O(d, p)17 O and the 28Si(d, p) 29Si reactions to study the stoichi-

Si

0-6 08 10 1-2energy, MeV

Fig. 5 Energy spectrum from a SiOx film (6 =160°,MeV, M, = 4-0) vacuum deposited onto a carbon substrate

40 SOLID-STATE AND ELECTRON DEVICES, JANUARY 1977, Vol. l,No. 2

ometry of evaporated silicon oxide films. The nuclearreaction technique complements Rutherford scattering,enabling the combination of techniques to provide us witha powerful analytical tool.

When interest is confined to a thin film, or a combinationof thin films, the background problem can be removed byusing a light substrate such as carbon. In. Fig. 5 the spectrumobtained from an evaporated silicon-oxide (SiOx) filmdeposited on a carbon substrate is shown. The low mass ofcarbon ensures that it does not interfere with the mainspectrum. The substantial improvement gained as ananalytical tool is clear from this spectrum. The removal ofthe background spectrum has enabled some 12 impuritypeaks to be identified and measured. The oxide in thisexample is SiOi.os and the aluminium peak is a result ofthe evaporation of boat contaminants. (The molybdenumboat had been contaminated by a previous evaporation ofAl, some of which had condensed on the outside of the boatcontaining the silicon monoxide.) The source of the remain-ing peaks was not identified. The presence of these impuritiesdrastically reduces the electrical strength of this supposedlyinsulating layer making them unsuitable as insulating layersin, for example, MOS technology.

6x10'

54x10'o

CdS CdAl

A\\\YGlass

\

0-6 10energy, MeV

14 18

Fig. 6 Energy spectrum obtained by scattering 2 MeV He ions(d = 150°) from a CdS/Al/glass structure

Studies of this kind are not confined to single layers, butcan readily be extended to multiple-layer structures. Fig. 6shows the spectrum obtained from a Glass/Al/CdS structure;we see from this figure that the background glass spectrumcan be removed to yield Cd, S and Al peaks. (Note that theglass spectrum can be identified by conducting a secondmeasurement on the glass alone.) The improved sensitivityfor high atomic numbers is clearly seen — in this particularcase NCdlNs = (1 -0 ± 0-05) indicating good stoichiometricproportions in the film which had been grown by co-vacuumdeposition of the two constituents. The Al film was used asa counter-electrode for electrical studies in the semiconduc-tor films. We note also the appearance of a small peak,

possibly Zn, just below the cadmium peak — most probablya result of contamination from the vacuum deposition sys-tem which had also been used to deposit ZnS. However,without any extra information it is only possible to say thatthis mass corresponds closely to Zn.

Rutherford scattering studies of the kind described abovehave already been used extensively and have resulted inconsiderable improvements in the fabrication of bothpassivation layers and of active semiconductor films.

§1500

_1OOO

5 0 0 \

15 1 75* 025 0-5 0 75 10 125energy, MeV

Fig. 7A Backscattered spectrum of a carbon/silicon oxide/goldstructure after heating for 2 hat 500° C

Note: the gold has almost completely penetrated the oxide

10

?u

i

in

EO 211°

\\

0 100 2OO 300 400depth, nm

Fig. 7B The gold profile calculated from the spectrum of Fig. 7A

3.2 In terdiffusion s tudies

The facility of measuring the depth profile of one or moreconstituents in a multiple-layer structure makes the tech-nique of great value for studying the interdiffusion of thethin films. Rutherford scattering has been used for suchstudies by a number of different groups (see References4—7). To illustrate the process consider the case of a thingold film deposited onto the surface of a silicon oxide filmwhich is deposited on a light carbon substrate by vacuumdeposition. Fig. 7A shows the spectrum obtained from thestructure after annealing for 2 h at 500° C — the goldelectrode has almost completely penetrated the oxide.From the gold peak we derive the depth profile shown in

SOLID-STA TEAND ELECTRON DEVICES, JANUARY 1977, Vol. 1, No. 2 41

Fig. 7B where the energy scale of Fig. 7A has been con-verted to a depth scale using the relationship given in eqn. 7.In this particular example detailed studies have shown thatgold diffuses readily into silicon oxide films even at therelatively low temperature of 200° C. Once the Au-Si eutec-tic point is exceeded (370° C) the inward migration is veryrapid, resulting in the complete destruction of the insulatingproperties of the oxide films. An extension of these studiesto the SiO2 layers normally used for MOS technology shouldprove valuable in understanding the degradation in deviceperformance which occurs at elevated temperatures. Anexcellent example of the potential of the technique forheavy atom diffusion studies in light substrates is the seriesof spectra shown in Fig. 8. These spectra show the progress-ive diffusion of Zn into an Al substrate for progressive timesat 200° C.19 The shape of the Zn spectra indicates thatdeep penetration has occurred and from these data a diffu-sion coefficient of D = 10~14 cm2/s was calculated. It hasbeen estimated by Mayer5 that diffusion coefficients inthe range 10*"11 and 10~lscm2/s can be measured — thisfollows from the relationship D = z2/t where the distance

1-5 MeV He—Zn in Al at 2OO°C

150 200channel number

250

Fig. 8 Backscattering spectra illustrating the diffusion of Zn intoAl after isochromal anneals at 200° CAfter Fontelle? a/.19

z can vary from 25 nm (minimum depth resolution) to afew micrometres, and t from a few minutes to severalhours. These values of diffusion coefficient are below thosethat can be measured conventionally by sectioning tech-niques. This means that scattering spectrometry can be usedto study diffusion at low temperatures. Clearly, in circum-stances where the diffusion ion is lighter than the substrate,the combination of low yield relative to the substrate andthe appearance of the diffused peak on the background willresult in either poor concentration sensitivity or, in theextreme case, the impurity signal will be lost in the back-ground. In cases where the metal layers are thin and thesubstrate is a single crystal it is possible to reduce thebackground spectrum by channelling the ion beam.18

With comparatively thick metal layer structures that are ofinterest in device technology (Z lOOnm) this is of limited

use, but is very important for surface studies or the studyof trace impurities on the surface.

In order to illustrate two important limitations of thetechnique, consider the problem of gold interdiffusion witha GaAs surface. The spectrum of the 'as-deposited' film isshown in Fig. 9A. In the event of gold diffusing into thefilm a change will occur at the lower energy edge of thegold peak. If on the other handGa or As outdiffuse throughthe gold film, this will appear beyond the high energy edgeof the GaAs substrate spectrum. The window region betweenthese two peaks will therefore contain all the interdiffusiondata. Fig. 9B shows two spectra, the closed circles corre-spond to the 'as deposited' film and the open circle afterannealing for 3 hat 350° C. We see from this figure thatthere appears to be a finite count inside the window region

§8000

^6000d

C4000

g2OOO

Au(1845MeV)GaAs(1408MeV)l

I

0 100 "200" """300channel number

4 0 0 5 0 0

Fig. 9A Rutherford scattering spectrum obtained from an unan-nealed specimen consisting of a thin gold film on a clean GaAssurface

104

103

io2

600

300

8

8

S

•

•C

Go t ..

380channel

X ••• • *• • • • • " • * •

•

400number

0

0 °<o°

CD

0

•

° o o o o •• •

•

•

•

o *

•

t

•

360 380 400channel number

420

1-40 1-50energy, MeV

1-60

Fig. 9B Semilogarithmic plot of two Rutherford scattering spectra,one before and the other after annealing for 3 h at 350° C

42 SOLID-STATE AND ELECTRON DEVICES, JANUARY 1977, Vol. l,No. 2

before annealing — this could correspond to an intake ofGa or As into the gold film during deposition. However, thiscannot be resolved by Rutherford scattering alone, since inthis window region it could also be a result of pulse 'pileup' which extends beyond the GaAs edge of the spectrum(i.e. two separated particles arriving at the detector at thesame time, resulting in a pulse whose magnitude is the sumof the two component pulses). In this example the problemwas resolved by the use of ion-microprobe analysis(CAM EC A)20 where it has been established that the de-posited gold film has taken up Ga from the substrate. Afterheating, we see a substantial increase in the backgroundsignal which, with the assistance of the CAMECA, has beenidentified as Ga outdiffusing, which is also found to accumu-late at the surface. It is fairly clear from this example that,although Rutherford scattering can be used to quantify theimpurity concentration, it is necessary to seek the aid of asecond technique with very good mass sensitivity in orderto identify the impurity uniquely; Auger or ion-microprobeanalysis can be used for this purpose.21 For example, withRutherford scattering alone it would not be possible todetermine whether the outdiffusing species was Ga or Assince the differences in their atomic masses are too small tobe resolved by this technique.

10

10'

10

10

10"

§10'o

10

GaAs

1500A(GaAs)

Au

100 150 200channel number (energy)

250

Ga(T589)As(T615) Au(1844)

backscattered energy, MeV

F ig. 10 Backseattaring spectra for 2 Me V" He* froma 80 nm W-nGaAs annealed for 2 h at 500° C in vacuumb 70 nm Au-nGaAs annealed for 2 h at 2 50° C in air

These results corroborate earlier studies22 on the inter-diffusion of Au-GaAs and W-GaAs systems. In Fig. 10 theback-scattering spectra for the 2-metal systems are shown.We see from this figure that Au-n GaAs shows considerablechanges, corresponding to Ga outdiffusion to the gold sur-

face forming a Ga2O3 surface film, while the Au migratesinto the semiconductor. The identification of surfaceoxygen comes from Auger analysis by Todd etal.21 Incontrast, we see that the W-GaAs interface does not exhibitthis aging at temperatures below 500° C. As is shown inFig. 10fl the Rutherford scattering spectrum shows noobservable change at the interface. These results correlateclosely with the electrical studies on these contacts. In thecase of the W-GaAs Schottky barrier (0^O-66eV) theeffective barrier height does not change with annealing. Incontrast, the effective barrier height of the Au-GaAs sys-tem drops from approximately 0-9 to 0-6eV after a 2 hanneal at 350° C.20'22 The reduction in effective barrierheight is thought to be a result of the formation of an JV*layer under the contact.

Studies of device metallisation systems and their agingcharacteristics are now an important 'semiconductor'application of this technique. Interest is, however, not onlyconfined to the semiconductor-metal interface, the tech-nique has, for example, been used very successfully to lookat the stability of multiple-layer metallisation systems. Onevery good example of this is the effectiveness and stabilityof 'barrier' layers, i.e a thin metal layer such as Pt is intro-duced between the initial ohmic metal (e.g. Chromium onSi) and the final heatsink metal such as Au or Ag. Thebarrier metal is required to stop the Au or Ag diffusingrapidly into the semiconductor and destroying the device.Rutherford scattering is now being used to study theeffectiveness of such barrier layers. In the case of Si IMPATTdiodes23 the rapid interdiffusion of Pt into Ag at relativelylow temperatures (£ 300° C) resulting in the loss of thebarrier layer has been established as the major cause of the

#1000--.Si-Pt structure

unannealed3OO°C 20min35C£C 20min

--40CfC 10min400°C 20min400t 120min450C 20min

JL07 08 09101-1 12 13 14 1516 17 18 19

energy, MeV

1OOOO-?

9000 o8000 p-7000 ™6000 g5000 JD4000 53000-g2000 S1000^0

energy, MeV

Fig. 11 Backseattering spectra showing the changes taking placewhen a thin Pt film on a Si crystal is heated in the temperaturerange 300° C to 450° CInitially the Pt is converted to Pt2Si and at higher temperatures intoPtSi (after Nicolet etal.1)

SOLID-STATE AND ELECTRON DEVICES, JANUARY 1977, Vol. l.No. 2 43

short mean time to failure of these devices when working atjunction temperatures of this magnitude. A basic under-standing of the interdiffusion of these systems is leading toimproved barrier layers and hence longer device lifetimes.

3.3 Kinetics and phase changes

As a final example of Rutherford scattering analysis, aproblem of some considerable technological importancewill be investigated — the kinetics of basic structural trans-formations and phase changes in thin films. There is now awide range of examples in the literature4"7 of the study ofsilicide formation of thin metal layers such as Pt, Th and Hfon silicon single crystals. With this technique it has beenpossible to follow the detailed kinetics of the phase change.In Fig. 11A a series of spectra for a 200 nm platinum filmon a silicon substrate after a range of annealing cycles attemperatures from 300° C to 450° C are shown. Theunannealed spectrum shows the existence of two separatedmaterials Pt and Si. As annealing takes place a Pt2 Si-likecompound begins to form, first at the interface, then itgradually spreads throughout the whole of the Pt film(300°, 350° and 400° C curves). Further annealing resultsin the whole silicide film being transformed into the stablePt-Si compound. The growth changes taking place withinthe compound are shown schematically in Fig. 1 IB. Thesestudies have enabled the stoichiometry and depth depen-dence of the silicide to be determined and have resulted insubstantial new information being evaluated of the dynami-cal processes taking place during growth.

Similarly, Rutherford scattering has been used to studythe oxidation of a number of metals. Brown and Mackin-tosh24 have, for example, been involved with detailedstudies of ion migration processes during the anodic oxi-dation of Al. Although the basic process of oxidation isprimarily of importance to corrosion studies, it also has

scattering in areas where supplementary information is ofcrucial importance. The technique is mass sensitive with thelimitation that it has a finite mass resolution defined by thekinematical eqn. 2. We also see from Fig. 2A that the massresolution decreases as the impurity mass increases. Thetechnique gives absolute information on impurity concen-tration to an accuracy of plus or minus 5%. This is in contrastto ion-microprobe and Auger analysis where such quantitat-ive measurements are still not possible. A very importantfeature of this technique is its nondestructive nature. Thismeans that it can be used, for example, to study directlythe composition and changes of passivation layers, metallis-ation films or indeed the active semiconductor layer of realdevice structures. This is already leading to a better under-standing of the so called 'cookery book' fabrication pro-cedures and is giving, for the first time, some direct insightinto aging/degradation phenomena. Such degradation is agreat limitation, when greater power handling is requiredfrom solid-state devices.

Clearly, the facility to study real device structures has itslimitations. For conventional Rutherford scattering theminimum practical beam diameter is in the region of0-25 nm. Consequently, for the majority of devices, studiescan only be conducted on artificially enlarged structures,and it is not always possible to operate these under thesame conditions as the real devices. The relatively largebeam size also means that analysis is averaged over this area.In so many circumstances the real information required ison a comparatively small scale (i.e. a few micrometres).These limitations can be partly overcome by the use of afocused ion beam — this facility has only recently becomeavailable but should result in interesting device informationduring the next few years. The good sensitivity to high massimpurities together with the excellent depth resolution hasenabled diffusion profiles to be measured at low tempera-tures, allowing slow interdiffusion of multiple layers to be

Table 1: Summary of Rutherford-scattering analysis

Probe diameter

Depth resolution

Sensitivity

Mass resolution

0-2-1 mm

Solid-state detectors: ~20—30 nm magnetic orelectrostatic spectrometer ~ 3 nm

Good for heavy elements, very poor for lightelements on heavy substrates. Can be in therange of 10""—0'1 monolayer (Surface impurities)

Good for low masses but poor for high masses. Canbe improved by the use of heavier bombarding ions

Decreases to ~ 10 Mm with focused ion beam

Surface monolayer can be isolated andstudied using channelling

Nuclear reactions can be used for selectiveimpurities (e.g. oxygen). Ion microprobe orAuger can be used to identify impurities

Ambiguity in identification can be removedby the use of auxiliary technique e.g. Auger,or ion microprobe or nuclear reactions

relevance to device technology, since surface oxidation ofSi is crucial to current planar silicon technology. The oxi-dation technology of other important semiconductors, suchas GaAs, is still in its infancy and there is little doubt thatthis technique could have an important role to play in theunderstanding and utilisation of its native oxide for devicefabrication.

4 ConclusionsIn this paper I have sought to outline the power and flexi-bility of the high energy (MeV) Rutherford scatteringtechnique as a quantitative tool for the study of semiconduc-tor-device metallisation and passivation layers. Some of themore important characteristics of Rutherford scattering areoutlined in Table 1. Also included in this Table is a listingof alternative techniques which can supplement Rutherford

studied, and there are now strong reasons to believe thatthis is a major cause of device failure. These studies have,for example, enabled the effectiveness of new diffusionbarrier layers to be evaluated. Clearly, a very severe limi-tation to this technique is its poor sensitivity to low masselements. However, if the technique is supplemented byalternative analytical techniques as is suggested in Table 1,many of the limitations outlined above can be overcome.

Overall, the technique is relatively simple to use, andalthough the time required for a spectrum is small comparedwith Auger analysis for example (i.e. roughly 15mintopump down the chamber and 15min to take a spectrum).Most of the basic studies on depth and mass resolution,sensitivity and associated resonance phenomena have beenreported in the literature enabling the technique to be util-ised fully by the device physicist. Consequently, we will see

44 SOLID-STATE AND ELECTRON DEVICES, JANUARY 1977, Vol. l,No.2

during the next few years new basic'information emergingwhich will result in improved device fabrication, improvedperformance, and very important, a clear understanding ofdevice aging resulting from interdiffusion/chemical reactionand degradation of metallisation and passivation layers.

5 Acknowledgments

The author wishes to express his sincere thanks to M.J.Howes, C.J. Palmstrom (Leeds), Prof. G. Carter (Salford).Prof. J.W. Mayer and G. Hogsted Phim (Caltech), Prof. J.A.Davies, I.V. Mitchell (Chalk River) and J.M. Poate (Bell)for the many valuable discussions on the topic of this review,to all the authors who have allowed him to reproducetheir work and to the Science Research Council for sup-porting the Rutherford-scattering work at the Universityof Leeds.

6 References

1 MAYER, J.W., ERIKSSON, L., and DAVIES, J.A.: 'Ion implan-tation in semiconductors' (Academic Press, New York, 1970)

2 DEARNLEY.G., FREEMAN, J.H., NELSON, R.S., and STE-PHEN, J.: 'Ion implantation' (North Holland, Amsterdam,1973)

3 MORGAN, D.V.: 'Recent advances in surface studies: ion beamanalysis', Con temp. Phys., 1975, 16, pp. 221-241

4 MAYER, J.W., and ZEIGLER, J.F.: 'Ion beam surface analysis',Thin Solid Films, 1973,15, pp. 1-463

5 ZEIGLER, J.F.: 'New uses of ion accelerators' (Plenum Press,1975)

6 MAYER, O.: 'Ion beam surface layer analysis', Karlsruhe 1975Conference (Plenum Press, 1977)

7 NICOLET, M-A., MAYER, J.W., and MITCHELL, I.V.: 'Micro-analysis of materials by backscattering spectrometry', Science,1972,177,pp. 841-849

8 DEARNLEY,G., NORTHROP, D.C.: 'Semiconductor countersfor nuclear radiations' (Spon, 1966)

9 MORGAN, D.V. (Ed.): 'Channelling: theory observation andapplications' (Wiley Interscience, London, 1973)

10 B0GH, E.: 'Applications to surface studies', ibid., chap. 1511 EISEN,E.H.: 'Applications to radiation damage', ibid., chap. 14

12 CHU, W.K., MAYER, J.W., NICOLET, M-A., BUCK, T.M.,AMSEL, G., and EISEN, F.: 'Principles and applications of ionbeam techniques for the analysis of solids and thin films', ThinSolid Films, 1973,15, pp. 423-463

13 WILLIAMS, J.S.: 'The optimization of a Rutherford backscat-tering geometry for enhanced depth resolution', Nucl. lustrum.& Methods, 1975,126, pp. 205-214

14 MORGAN, D.V.: 'Thin film analysis using Rutherford scattering',/. Phys. D., 1974, 7, pp. 653-662

15 DEARNLEY, G., and SUFFIELD, N.W.: 'Changes in the elec-trical properties of thin anodic TjO2 films induced by ion implan-tation', Proceedings of the International Conference on IonImplantation, Yorktovvn Heights, USA, 1973

16 AMSEL, G., NADAI, J.P., d'ARTEMARE, E., DAVID, D., GIR-ARD, E., and MOULIN, J.: 'Microanalysis by the direct obser-vation of nuclear reactions using a 2 MeV Van de Graff, Nucl.Instrum. & Methods, 1971,92, pp. 481-498

17 CACHARD, A., ROGERS, J.A., PIVOT, J., and DUPAY.C.H.S.:'Analysis of evaporated silicon oxide films by means of (d, p)nuclear reactions and infrared spectrophotometry',/7n'4\ StatusSolidia, 1971, 5, pp. 637-644

18 MAYER, J.W.: 'Applications to semiconductor technology',chap. 16 in Reference 9

19 FONTELL, A., ARMINEN, E., and TURUNEV, M.: 'Applicationof the backscattering method for the measurement of diffusionof zinc in aluminium', Phys. Status Solidia. 1973,15, pp. 113 —119

20 MADAMS, C.J., MORGAN, D.V., and HOWES, M.J.: 'Degra-dation of Au-n GaAs Schottky barriers due to Ga outmigration'(submitted to Solid-State Electron., 1976)

21 TODD.CJ., ASHWELL, G.W.B., SPEIGHT, J.D., and HECH-INGBOTTOM, R.: 'Thermally induced processes at Au GaAsinterfaces', Institute of Physics Conference series 22, London,1975,pp. 171-183

22 SINHA, A.K., and POATE, J.M.: 'The effects of alloying behav-iour on the electrical characterization of N-GaAs Schottkydiodes metallized with W, Au and Pt', Appl. Phys. Lett., 1973,23, pp. 666-668

23 MORGAN, D.V., HOWES, M.J., TAYLOR, D., and BROOK, P.:'Rutherford scattering analysis of Ag/Pt/Cr and Au/Pt/Cr metal-lization on Si impatt diodes' (unpublished work)

24 BROWN, F., and MACKINTOSH, W.D.: 'The use of Rutherfordbackscattering to study the behaviour of ion implanted atomsduring anodic oxidation of aluminium Ar, Kr, Xe, K, Rb, Cs, Cl,Br and I', 7. Electrochem. Soc, 1973,120, pp. 1096-1102

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