Characterization of semiconductor materials and devices by surface analysis techniques

Vacuum/volume Printed in Great

34/number Britain

1 O/l 1 /pages

881 to 892/l

0042-207X/84$3.00 Pergamon

984

Characterization of semiconductor materials devices by surface analysis techniques A van Oostrom,

Philips Research Laboratories,

PO Box 80.000,

5600 JA Eindhoven,

Pless

+ .OO Ltd

and

The Netherlands

in this review we consider some major surface analysis techniques: Rutherford backscattering (RBS); Auger electron spectroscopy (AES); X-ray photoelectron spectroscopy (XPS); ion scattering spectrometry (ISS) and secondary ion mass spectrometry (SIMS). Combined with ion bombardment for in-depth profiling some of these techniques provide three-dimensional composition distributions in a thin film. New instrumental developments are smaller electron and ion beam sizes and the increased use of position sensitive detectors. Spatial resolution and quantitative aspects are discussed; examples used are taken from semiconductor materials and device work.

1. Introduction The interest in surface science has grown considerably during the last 25 years. Initially, the interest was concentrated upon the basic ph.enomena connected with the interaction of photons, electrons and ions with solid surfaces. New techniques were developed which were used to investigate the physical properties of solid surfaces. Particularly, the catalytic activity of metal surfaces has been the subject of many investigations and this work still continues. In more recent years semiconductor materials and devices have become increasingly important. Surface analysis techniques are making valuable contributions to understanding such processes as molecular beam epitaxy, metallization, ion implantation, laser and electron beam annealing and plasma etching. In principle, any characterization of a solid surface can be regarded as a surface analysis technique. However, techniques providing information on surface structure e.g. low energy electron diffraction (LEED) or electronic configuration e.g. angle resolved ultra-violet photoelectron spectroscopy (ARUPS) are excluded. Surface analysis techniques are used to determine the elemental composition of the outermost atomic layers of a solid. However, a combination of a surface analysis technique with other techniques is often mandatory in clarifying the complex phenomena occurring at a surface. Surface analysis techniques are also frequently used for thin film analysis. Depth profiling proceeds by removing the thin film by ion bombardment. Depending upon the depth and lateral resolution of the technique a three-dimensional elemental distribution in the thin film can be obtained. This aspect is becoming increasingly important since dimensions of semiconductor devices have now reached the submicron region in all three dimensions. Among the many surface analysis techniques available some have qualified as more useful than others. This is illustrated in Figure 1 which shows the number of papers published per annum

/’

P

I

I

900 -

d

8CO-

s

I

AES

5

700-

3

600-

i a4

soo-

7

I

0-1

P :

/

1971

197L

.h4f

1977

1980

1983

Figure 1. The number of papers published per annum in the period 1971-1983 [or Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), Rutherford backscattering (RBS) and low energy electron diffraction (LEED). Source: INSPEC.

in the period 1971-1983 for Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and Rutherford backscattering (RBS). Low energy electron diffraction (LEED) a technique for the determination of surface structure is also shown. A rapid increase is observed for all these techniques and there is no sign yet of levelling-off. AES and XPS seem to be the most established 881

A van Oosfrom’

Characterization

of semtconductor

materials

and devices

by surface

2. Major surface analysis techniques spectroscopy. As we have seen Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) are widely used techniques. They are based on ionization of a core level of a surface atom by either an electron (AES) or a photon (XPS). Figure 2 illustrates the Auger process for an ionized Klevel. the subsequent filling of the hole by an L-electron and the escape of an Auger electron with a kinetic energy characteristic for the surface atom. The kinetic energy of the Auger electron is measured in an energy analyser. In AES the cylindrical mirror analyser (CMA) has become popular since the natural line widths of the levels involved in the Auger process are such that a resolution in the range 0.3 l.O”,, IS sufficient. Figure 2 also illustrates the shift in position of the SI (KLL) line due to chemical bonding in SiO, and Si,N,. The significant parameters in the Auger process are the cross-section for ionization r~. the backscatter correction I’ and the Inelastic mean free path i. A quantitative technique as .4ES requires precise knoivledge of these parameters in order to relate the measured Auger electron current to theconcentration ofan element in the surface region. Thecroszsection is best described b> Gr\zinski’s” or Drawin’h” formula. the Inelastic mean free path by Seah and Dench3”. The

2.1. Electron

AES

ENIE!

EYIEI

ENIE,

Electron

Table I. Comparlzon of some surface anallbls techniques. Some technique\ pro\ldc Depth resolution (nm). lateral resolution fnml and +enhltl\lr! (ppm) 3rc 31~ g\en quantitatl\e (*I: short (***I \$ long (*I and IOU (***) 15 htgh (*I.

882

techniques

ments. the importance of electromc and lonlc sputtering for surface analysis, some aspects of spatial resolution and quantitative analysis.

techniques. but RBS and SIMS are growing since 1978. The latter two techniques are often used in the characterization of semiconductor materials. Although the numbers given in Figure 1 refer to published papers and much work carried out in an industrial environment remains unpublished, the trend shown seems clear enough. Surface analysis techniques differ m the experimental methods used. but also in the results obtained. Some techniques are more quantitative than others, the sensiti\,it) may differ and the spatial resolution. For solving a particular problem one technique ma\ be more suitable than others. Often a number of techniques IS needed to analyse and solve the problem. IU .sir~r analysis combined with other techniques ma!’ also be required. In all cases the necessity for close collaboration between the ‘customer‘ and the operator of the analytical instrument should be stressed. Surface analysis techniques require a considerable capital investment. analysing a particular problem may be time consuming and so the cost may be high. Yet. a lot of money and time can be saved bg tackling a problem utth surface analysis techniques. However. in many cases the high cost of anal!& raises the question of strategy: which technique to use first and ho\\ much effort should be put into a particular aspect of a problem. These questions get different ansivers depending upon the use of surface analysis techntques in rebearch. development or qualit) control In manufacturing. Some surface analysis techniques not onl! pro\ Ide elemental concentration. but also chemical Information. This is mdlcated in Table 1 Mhich compares some of the techniques. Apart from .4ES. XPS. RBS and SIMS two other techniques. the atom-probe field ion microscop! (AP) and low energy ion scattering spectrometr! (ISS) hale been included. The comparison S~OMs large differences in lateral resolution and sensltlvit!. .4ES. RBS and XPS are quantitative techniques. RBS 15 faioured for it\ small tlmc consumption per analysis. Mhile ISS onlh nerds a Iok\ capital investment. For AES the capital In\cstment ranges from lo\+ for- a simple LEED Auger system to high fol- a sophisticated computer controlled scanning Auger mlcroprobe (SAM ). In the literature man) excellent re\‘le\+ papers can bc found describing one technique in particular’ ‘) or re\ie\\mg recent result> coLering the complete field of surface analysis’” ‘1 Proceedings of conferences deiotrd to surface analysis or related subjects also provide useful information” I h. b:r hale restricted ourselves to references published in the last t\\o bears. Last but not least an increasing number of books ha5 appeared’” ‘^. After a brief description of some of the malor surface anal!xi\ techniques we shall concentrate upon neu Instrumental develop-

Elemental: E Chemical. C Depth resolution Inm I Lateral resolution (nm) Sensitl\lt! (ppm I Quantification Time conwmp,lon pel- analy~15 Caplral m\eslment

analysis

Energy

)

Wi

-‘:

III addltlon IO elemenlal IEI alw chemical I(‘) ~nform,+tlnn. 71~ \tar\ Indlcatc rrspwrl\el!. mo\t quan,lrarl\e (***I L\ Irasr

AES (SAM)

.AP

ISS

RBS

SIMS

E+ <‘ 0.3 3

FI 03

E O.?

50

(I 2 SIngI ** * **

f 0 3 31 IO”

F v-c (I.3 20 0.3 3 10’ IO 10 ’ 10~ IO” I(1

IO’ IO *** ** 1;

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.

IO” IO’ IO’ ** * ***

IO? IO’ *** *** **

* ** *

XPS

*** ** **

A van Oosrrom: Characterization

of semiconductor

materials and devices by surface analysis techniques

backscattering of primary electrons which subsequently produce Auger electrons in the surface region complicates matters. In electronic devices with multilayer structures an accurate determination of the backscattering factor can be difficult. Corrections proposed by Reuter31 and Shimizu32.33 are satisfactory for relatively simple systems only. For thin films of SiO, or Si,N, on silicon aQuantitative analysis with an absolute accuracy of about 10” 0 can be achieved34. It has been shown that in some cases the accuracy can be improved lo a few per cent35. The effect of backscattering is now often exploited in scanning electron microscopy (SEM). Figure 3 shows SEM-images36 taken at different primary beam energies of a silicon substrate covered partly with platinum silicide (50 nm), a TiW (lo/SO) diffusion barrier (100 nm) and an Al (500 nm) top layer. Since higher Zmaterials as W and Pt produce more backscattering than light elements as Si or Al, the penetration depth of the primary beam can be used to detect thin films below the surface. The problem of backscattering primary electrons is not encountered in XPS, where core level ionization usually proceeds by the excitation with MgK, or AlK, radiation. The kinetic energy of the emitted photoelectron has to be determined with greater precision (0.1 eV) than in AES. Therefore, hemispherical analysers with pre-retardation and a high resolution are often preferred. Table 2 summarizes the line position we measured for Si, SiO, and Si,N, with the aid of a double pass CMA. The Auger lines and the Auger parameter are also given. The results are in good agreement with other data published elsewhere3’. As AES,

Table 2. Measured AES and XPS line positions (eV) for Si (100). SiO, and Si,N,. The Auger parameter x is defined as the sum of the Auger kinetic energy and the photoelectron binding energy

AES and XPS hne positlons (eV)

Si(L\‘VI 90.0 73.0

SI(KLLl 1616.5 16060

Si(2pl

Sl(lOiI) Si02

103.8

r 1716.5 1709.8

SiANA

76.5

1609.0

102.3

1711.3

100.0

O(lS)

N(ls)

533.2

39x. 1

XPS is a quantitative surface analysis technique. The crosssection for core level ionization has been calculated by Scofield3’, while the inelastic mean free path is given by Seah and Dench3’. XPS provides chemical information of the specimen analysed by shifts in the photo-electron binding energy measured. Since depth profiling by ion bombardment will destroy such information, angular dependent detection of photoelectrons has been used. This is illustrated in Figure 4 for a slightly oxidized silicon surface. The Si 2p-peak at 103.8 eV shows up much better at a take-off angle of (I= 15 as compared to the perpendicular case39. Although XPS has been used extensively in research on catalysis, corrosion and polymers, its application in the semiconductor field has been limited. However. interesting results have been obtained in investigations on MNOS structures4’ and plasma-etching4’. Since XPS is a technique providing information on the chemical bond of surface atoms, it would be valuable to obtain such information with a high lateral resolution. Scanning XPS-systems have been proposed with limited resolution”‘.” I10 100 itm). but have not found wide application. SimultaneousI). progress has been made in the development of X-ray microscopes using soft (5 nm) X-rays. It is believed that multilayer coatings on spherical

Figure 3. Backscattered electron images (BSE) in a Philips SEM505 for a low (4.9 kV) and high (30.1 kV) voltage. The specimen consists of a silicon substrate uith squares of platinum silicide lines (thickness 50 nm): the central round region is covered with an aluminium layer (thickness 500 nm) as seen in the Al-map. Between the aluminium and the substrate is a TI’W (10,‘90) dUTusion barrier (IO0 nm). the outer ring only has in addition a 50 nm film of platinum siliclde. At the IOU voltage the aluminium shields the silicide and Ti;W layers, at the high voltage these layers become visible. Note the disappearance of the thin platinum silicide

lines.

could provide low magnification images with a resolution of 50 nrn’j. Focusing and scanning an electron beam is considerabl! simpler. so scanning Auger microscopy (SAM) has become an important tool for investigating electronic materials and decices. The high lateral (- 100 nm) and depth (- 1 nm) resolution permits the analysis ofsmall volumes ofmaterial corresponding to mirrors

883

A van Oostrom:

Characterization

of semiconductor

N(E)

sllzpl

Anglo

materials and devices by surface analysis techniques

where I’ is the interatomic separation, c the electronic charge, Z,.Z, the atomic numbers of the elements involved and Q,(r/a) the screening function. The screening length a is a function of Z, and Z15’. Kalbitzer er a15’ noted some years ago that the Thomas-Fermi potential used by Lindhard” provided large deviations from experimental results of projected range measurements. Wilson, Haggmark and Biersack calculated interatomic potentials for a number of diatomic interactions using a Molieretype potentia15”. The agreement between theory and experiment considerably improved by using the free electron screening function for a representative case: KrC. The reduced nuclear stopping power they derived fits experimental data on ion implantation ranges and sputtering reasonably well. Recently, Biersack and Ziegler showed that the accuracy can be further improved by extending Firsov’s treatment to include additional terms for exchange and correlation energy54. Figure 5 shows calculated potential curves from their work which give even better agreement with experiments particularly in the low energy range.

XPS

Integrated

NIE) I

e=900

~ 103.8

100.0

NIEI

8.150

L!LL lob

x)00

Binding Energy

WI

Figure 4. X-ray photoelectron spectroscopy (XPS). Angle integrated and resolved Si (2~) spectrum for an oxidized silicon surface39. The clean silicon peak at 100.0 eV is depressed, the silicon oxide peak at 103.8 eV emphasized for 6’= 15‘. 0 is the angle between the detector axis and the

substrate. about lo6 atoms. In addition to Auger-images and elemental mapping, elastic backscattering and secondary electron images can be obtained from the same sample surface. The shift in the onset of the secondary electrons in the energy spectrum can be used to make a work function map45. This method proposed by Venables has found application in mapping the work function distribution of impregnated cathodes46. It has also been used to determine variations in the Schottky barrier height for platinum and titanium on silicon4’. A similar principle has been applied by Pantel to determine the position of a shallow p-n junction in silicon from the shift ofthe Si (LVV) peak4*. Since shifts of 20 meV were measured, low concentrations (- 10” cmm3) of electrically active As could be indirectly detected. In many cases AES and SAM are used as an analytical tool with the spectrum taken in the derivative mode to remove the background. However, valuable information is often lost, as the peak shape in the N(E)-mode contains information on the valence band density of states for transitions involving valence electronsz3. High resolution Auger spectroscopy combined with a suitable data handling system can provide information on the chemical environment of carbon, nitrogen and oxygen. X-ray excited Auger spectra (XAES) from small condensed molecules containing oxygen and nitrogen have been investigated”. 2.2. RBS and KS. Rutherford backscattering (RBS) and ion scattering spectrometry (1%) are both techniques based on the scattering oflight ions from a solid target. The energy distribution of the backscattered particles contains information on the elemental compositon of the target material. Only a fraction of the particles is reflected, many of them are implanted in the substrate. The interaction of the incoming ion beam and the solid is largely determined by the interaction potential of two colliding atoms. In the past various simple universal potentials have been proposed of the type

Z, Z,e2

V(r) = ___

I

884

4 I

a

(1)

0

5

10

104

15

0

R=t”rlA)

5

10

u

15

~=z’“rlA)

Figure 5. Sum of Coulomb and kmettc energy depicted as RI’,(R) \s R (left) and the sum of exchange and correlation energies for tv,o overlapping atoms depicted as RL’2(R) is R (right). c1,(~)=0.09 expt-0.19.y)+O.61 exp(-0.57.~)+0.? exp(-2\): Y=I’ u: (bz(.~)=0.07 exp[-(1’7R))‘-R 4-(R 7)‘]:R=Z’“r.CalculattonsforpairsofAu.St. Ag. N and He atoms after Biersack and Ztegler5”.

The principles of RBS are illustrated in Figure 6. A beam of 2.0 MeV He’ ions strikes a solid target. is partly backscattered and collected by a solid state detector. The scattering cross-section is given by the familiar Rutherford formula which makes RBS a technique suitable for quantitative analysis. For He+ ions scattered by surface atoms the energy E, is given by

,I,-[

cos 8+JA2-sin2B

EO

A+1

1 ’

(2)

in vvhich H is the scattering angle. A = Mi.!Mo and E,, MO energy’ and mass of the incoming He’ ion. Ifthe ion penetrates the target. extra energy is lost and a broadened peak is observed, as indicated in the figure. For the case drawn M, > M2 and so the peak from the thin film with mass M, is clearly separated from the contribution from the bulk with mass M2. The energy spectrum is a superposition of a mass analysis and a depth analysis of the target. For M, tM2 the peaks will not be separated and the accuracy is reduced. However. in the case of a crystalline substrate alignment of the substrate orientation with the incoming beam will reduce backscattering from the bulk and will show more clearly the surface atoms peak. In ISS the energy of the incoming noble gas ion is reduced to a few keV. This is shown in Figure 7. At these low energies mainly the first monolayer of the sample contributes to the signal of the backscattered ions. The energy loss is again given by formula (2).

A van Oosfrom: Characterization of semiconductor materials and devices by surface analysis techniques RBS NON -ALIGNED

I

ALIGNED

I

THIN

E2

FILM

1

1

SURFACE

El

E-WYE

increasingly important. The scattered ion yield detected may only be l?; of the initial ion beam striking the target. A comparison of the scattering process of He+ and Li+ ions shows that the Li+ ions are less likely to be neutralized6’. Backscattered He’ ions almost exclusively come from the first atomic layer of the sample after a binary collision. Multiple collisions and deeper layers neutralize He’ ions almost completely. Alkali ions can be used to study structural effects and multiple scattering in more detail 61. These new developments in ISS explain the interest in time-of-flight measurements capable of detecting both neutrals and ions. Moreover, Aono has shown that a scattering angle of 180’ enables the positions of surface atoms to be determined more accurately. By varying the angle of incidence detailed structural information can be obtained62.63.

El Energy

E

Figure 6. Rutherford backscattering

(RBS). The non-aligned (left) and aligned (right) case are shown for 2.0 MeV He+ particles. The detector is drawn for glancing take-off angle and high depth resolution. A thin film with thickness I, atomic mass M. and atomic number Z,. is assumed to be on a substrate with atomic mass M, and atomic number Z,.

2.3. SIMS. The interaction of an energetic ion with a solid target will lead to either reflection or implantation of the primary ion. Secondary processes will cause sputter removal of target atoms, mostly as neutrals, partly as ions. The principles of secondary ion mass spectrometry (SIMS) are indicated in Figure 8. A primary SIMS

1%

CMA

l

i j

SOLID

VACUUM

Lens+Deflecton

Plates

I ti

0

OL

l-&_-L Depth

Si

E 5

0.2

0;ions.

0.6 Ok

I

1.0

bwrwsl

12keV-15pA

B

700

am

900 Energy

(&‘I

0

1000

10

20

MASS

-

30

LO

SCALE

El/E0

Figure 7. Ion scattering spectrometry (ISS). The experimental set-up is shown, the energy loss for He’ and Ne’ ions as a function of atomic mass of the solid and an example of surface segregation in a Cu.‘Ni-aIloy.55.

Figure 8. Secondary ion mass spectrometry (SIMS). The experimental setup is shown, a mass spectrum and a result for 70 keV i’B* ions implanted in siliconb4. Primary 0; ion energy: 12 keV.

The scattering angle drawn is for use of a CMA as energy analyser. In order to reduce the effect of sputtering He+ ions are preferably used. However, although all elements with Z > 2 can be detected the resolution for heavy elements is poor and Ne’ ions should be used. Since the technique is extremely surface sensitive, surface segregation in binary alloys has been studied by 1%. An example for a Cu/Ni alloy is shown in the figure55. Copper segregates to the surface upon heating to 51O’C. Medium ion energies (a 100 keV) can also be applied in backscattering experiments. As in the case of RBS at higher energies, the method is still quantitative. Neutralization effects important in ISS are still negligibly small. Saris and his FOMgroup have used such ion energies for ion beam crystallography of surface?. They studied the reconstruction and oxidation of nickel surfaces and the initial stages of silicide formation. Nea trends in RBS are furthermore the introduction of uhv-target microbeam attachments with a chambers and manipulators5’, lateral resolution down to a few microns?* and the use of protoninduced X-ray emission (PIXE)5g. For low ion energies neutralization of ions is becoming

ion beam of Ar+, Cs+ or 0: ions produces ionized particles coming from the target which are detected by a mass spectrometer. The use of reactive cesium or oxygen results in a significant increase of the negative or positive ion yield. It is this high sensitivity which makes SIMS an excellent technique for determining profiles of impurities implanted in semiconductor materials. Obviously, the use of cesium and oxygen ions also complicates analysis and a better understanding of the sputtering phenomena of an altered layer is needed. Yet, careful calibration methods and the use of standards have provided good results. The example given in Figure 8 stresses the large dynamic range of most SIMS-work. The depth resolution of the technique is strongly related to the sputter-process which will be discussed later on. Sometimes the distinction is made between static and dynamic SIMS. In the first case detection is based upon only a small fraction of the top monolayer being removed and analysed. The surface remains essentially unchanged in that case. In the second case the sputter erosion rate is fast to increase sensitivity. Other aspects of SIMS include the feasibility to detect isotopes and to make a three-dimensional analysis”. For the latter application 885

A ~/an Oostrom.

Characterlzatlon

of semiconductor

materials

and devices

small diameter ion beams are needed of high current density. Liquid metal ion sources (LMIS) have been proposed for this type of analysis. Submicron lateral resolution has been achieved with such sources.

3. Instrumentation The introduction of surface analysis techniques on a large scale in many branches of research and development has led IO a greater need for calibration and standardization. The American Society for Testing and Materials (ASTM) has established a Commtttee The committee sponsors symposia, E-42 on Surface Analysi?‘. recommends standards, prepares reference materials and organizes round robins. The results ofsuch round robins for XPSh7 and AEShs can be found in the literature. They clearly illustrate the need for improved calibration and operating procedures. Relatively large deviations in both the energy scale and the intensity ratios were observed. A similar round robin was organized for ISS by a small group of people working in the the National Physical Laborator) in fieldh“. Recently, Teddington has also started a ser\,ice for reference materials-‘“. Improvements in instrumentation may he related to better photon. electron and ion sources, beam formation and optics. nen types of energy analysers or mass spectrometers. detectors and data acquisition and handling. We have selected a few example\ of the many aspects and new developments In instrumentation. 3.1. Sources. Synchrotron radiation. MgK, and AIK, radiation arc the main sources for photoelectron spectroscop! : field eml\smn or W and LaB, thermionic emitters are common as electron sources for AES,ISAM; plasmas. surface iontratton and liquidmetal or gas-ion sources are used for RBS. ISS and SIMS. Fat both electrons and ions the optics wj111provide an image of the virtual source on the specimen surface. The brightness /i in a focused spot is the current densit) per unit solid angle (A cm ’ sterad). The minimum spot sire IS determined b> the gaushian heam diameter tl,,. the chromatic aberration tl,. the spherical aberration rl, and the energ) spread in the electron or ion beam (Figure 9). The energy spread AE is usually not determined by the electron emitter. but by the Boersch effect in the crossover of the gun. It is seen that in SAM spherical aberration may be the important parameter as a minimum current of i,,= IO ’ A is required, wjhile in a SEM the beam size is limited bj chromatic

semi-angle

alradl

Figure 9. Spot size vs the semi-angle I illustrating the efkct of chromatic and spherical aherrarlon on the minimum \po~ v~1: Len\ pal-:~nlctcr\ C,=7 mm. C,=2cn~.Tht:dra~n Ilnr\alefor AE= I c\‘. thcdortcd llnc ~OI LIE=? eV. 886

by surface

analysis

techmques

aberration for the example given. At present LaB, sources can provide at 1600 K a brightness fl: 5 x IO” combined with a lo\ rate of vaporization for atoms. An effective work function 4, =2.91 eV has been reported’l. Recent efforts to find ternar! than systems” or other hexaboridesT3 better in performance LaB, failed. In contrast to an electron gun. an ion gun ma! contain undesired ionic or neutral particles. Ion guns often incorporate mass separators, e.g. a Wien filter. Ion optics fol surface analysis instruments has recently been reviev,ed’J. The minimum spot size for ion beams has now reached the submicron region. For XPS a spot size of I50 llrn is still regarded as small”. 3.2. Analysers. As we hake seen. man! surface analysis techniques are based upon an accurate determination of the energy spectrum of charged particles emitted b\ or scattered from the target surface. Depending upon the information required, the emphasis may fall upon the precision of the energy measurement or high sensitivity. corresponding to either a high resolving power E,AE or large ktendue ,: of the spectrometer. In surface analysis techniques as AES.ISAM and ISS a high sensitivity is required and the cylindrical mirror anajyser (CMA) has become the standard instrumentT’.” Thektenduec isdefined asi:=R,.4. uhereR is the solid angle and A IS the area accepted bq the spectrometer. For a CM.4 the &endue is large and the resolving power constant o\er a wide energy range. For higher resolution experiment5 as in XPS OI- ARUPS. it IS advantageous to lower the kinetic energ! b! preretarding the emitted electrons. At low energies the gain in resolving power is proportional to the loss in Ctendue. at higher energies the loss in &endue is more than proporttonal for most spectrometers. In XPS the preretarded electrons pass a hemispherical analyser or a double pazs CMA (DCMAI. In photoemGon experiments the design criteria usualI> Include angular dependent measurements o\er a wide range of polar and azimuthal angles. Small mobile spectrometers have been used for this purpose. By in\crting a rotatuble drum in a DCMA angular resolved XPS spectra can be obtained-‘. The spectra shown above for oxldired silicon Mere taken with such a dc\ice. Several neh designs of htgh-resolution angle-resolving electron energy analysers have recently been proposed T9-H2. It is expected that for most routine work in surface analysis the CM.4 ~ill remain the most common spectrometer. 3.3. Detectors. For the detection of electrons or ion\. Mhlch ha\e passed the analyser or mass separation stage, multistage electron multiplier5 or channeltrons are used. Howecer. a relatively ne\\ development is the introduction of position sensitive detectors combined with electron spectrometers (Figure IO). The detector is a micro channel plate (MCP) assembly coupled with a resistive anode. Channel plates were first introduced in surface science in field ion microscop> H3 and the atom-probe time-of-flight instrumentsHJ. In more recent years the) found applications in LEED”. XPSxh and mass spectrometry”. In Figure 10 the energ\ spectrum of a concentric hemispherical analyser is displayed linearl) on a position sensitive detector. The channel plates can bc clamped in pan-s or even trtplets to achieve a maximum gain of about 5 x IO”. Ion feedback sets a limit. but this problem is overcome by the use of a bias angle. High current channel plates with matched resistance are used to avoid gain variations due to changing count rates. The spatial resolution is limited by the tranaierse electron energ) on lea\ing the MCP. For a channel width of 15 /cm dia. the spatial resolution ~111 be between 50 100 pm

A van Oostrom:

Characterization

of semiconductor

materials

and devices

by surface

analysis

techniques

-

Dose

lC/cm~J

Figure 11. Calculated critical dose for decomposition based upon the published data of Pantano and Madey” and the measured signal-to-noise ratio for silver in a SAM 590. For other elements and Auger transitions correctionsshould bemadefor thesensitivity factors. For aspot size with a diameter of lo3 nm H,O will decompose for Q= 10e9C, but not for Q= lo-“C. The signal-to-noise ratio will be subsequently poorer.

Figure 10. Position

sensitive detector in combination wiith concentric (CHA). Electrons strike the input of a pair of microchannel plates (MCPJ, pass a second stack of MCP’s and are collected by a resistive anode. The signal processor determines the (X, Y) coordinates. With a CHA only one direction on the anode is used. hemispherical

analyser

The collector of the electrons can either be a fluorescent screen with optical readout using a TV-camera, multiple anodes or a resistive anode. So far, the silicon vidicon camera has been mainly used, but solid state arrays as charged-coupled devices (CCD) are now becoming available**. Such arrays consist of a matrix of diodes which are charged by photon or electron induced electronhole pairs. Multiple anodes are an expensive solution, but enable the simultaneous detection of two particles in time and position of arrival with high accuracy”. A simpler solution is the resistive anode. as shown in Figure 109’. In such an anode the electrical charge is measured at its four corners A, B, C and D for each event. The (X, Y) coordinates of each event can be determined from the ratios of the charge delivered at the four corners. The anode can detect single pulses with a maximum count rate of lo5 s-‘. Position sensitive detection with multiple anodes or a resistive anode are also becoming commercially available. The simultaneous detection of a range ofenergies reduces the measuring time or alternatively increases the signal-to-noise ratio.

function of spot size. The critical dose for decomposition of CH,OH, Hz0 and Al,O, are indicated in the figure. The drawn lines are for a total exposure of 10e9 and lo- ’ ‘C respectively. For silver we have measured signal-to-noise ratios of 70 and 7 in these cases. The sensitivity for silver is high compared with most elements and corrections should be made for other elements. The instrumental parameters are also of importance and should be taken into account. Our data were collected in the N(E)-mode with a CMA at an energy resolution of0.6”,, and a primary beam energy of 5 keV. Photon stimulated desorption, although less important than electron stimulated desorption, has been considered by FuggleJ9. The mechanisms involved in both types of desorption are still a matter of controversy’*. However, the revived interest in the subject since the early work of the sixties is indeed timely from a point of view of surface analysis. From Figure 11 it is clear that AES/SAM work on adsorbates can be rather hazardous. Fortunately. the cross-section for decomposition is strongly reduced towards lower electron energies. Electron energy loss spectroscopy (EELS) for hydrogen and water adsorbed on Si (100) can therefore proceed without noticeable radiation damage. Figure 12 is taken from the work of Wagner and Ibach9’. Dissociation of the hydroxyl groups after heating to 550 and 750 K respectively occurs, leaving oxygen bridge-bonded between Si surface atoms. The appearance of the

4. Some other aspects beam of electrons used for ionization of a core level in a surface atom in AES has sufficient energy to cause radiation damage. This damage may result in dissociation of compounds. adsorption of gas atoms or desorption of atoms or molecules. Clearly, a surface analysis technique requires that damage is reduced to a minimum. For normal Auger work radiation damage is often relatively small. since the crosssections for adsorption and desorption are much smaller than for core level ionization. Since the survey written by Pantano and Madey” on the subject. few new data have appeared. For many adsorbate-substrate systems accurate data are missing and yet such data are needed for quantitative analysis by AES. In SAM spot sizes are now well below 100 nm and the electron dose per cm2 required to obtain a meaningful signal-to-noise ratio can be quite high. Figure 11 illustrates the effect of electron dose as a

EELS WATER

4.1. Electron beam effects. The primary

Energy

Loss lcm-‘I

Energy

Loss

km-‘)

Figure 12. Electron energy loss spectroscopy (EELS). Adsorption of atomic hydrogen on Si (100) Z x 1 surface at 370 K: Exposure 4 33 Langmuirs (left) and decomposition of H,O on Si (100) 2 x 1 surface. Oxygen is bridge-bonded between Si surface atoms as indicated by the appearance of the asymmetric stretch frequency at 1050 cm- I, After Wagner and Ibach93. 887

A van Oostrom:

Characterization

of semiconductor

materials

and devtces

So, frequency at 1050 cm _’ is responsible for this description. EELS does not decompose water, can actually detect hydrogen and provides information on surface structure. All three aspects differ from AES/SAM work. However, a small spot size cannot be obtained for EELS. 4.2. Sputtering. In Section 2.2 we have discussed the interatomic potentials calculated for a number of atomic pairss3,‘J. Such interatomic potentials also apply to theoretical models for the description of the sputtering process. Apart from many other applications as in sputter-deposition and plasma-etching, in the present context our interest is connected with SIMS and in-depth profiling in general. We shall restrict ourselves to semiconductor substrates bombarded with inert gas ions in the energy range O-5 keV. In 1969 Sigmund presented a model for sputtering of particles from a solid by incoming ions94. His collision-cascade model has been particularly successful in the explanation of sputtering of elements. For many other, often practical, situations the sputter yields cannot be easily predicted. Zalm was able to show that good agreement can be obtained between theory and experimental results for the low energy inert gas ion bombardment of silicon9’. He inserted in Sigmund’s model the interatomic potential and the reduced nuclear stopping power

of Wilson rr ~11’”with a=0.14 and h=0.42. The values of the constants are the average ones from ref 53. The reduced energ)

by surface

analysis

techniques

compounds and alloys selective sputtering is often observed. leading to an altered layer with changed composition’0’,‘“4. Our lack of a detailed understanding of the sputter removal of a thin film adversely influences the accuracy of in-depth profiling. 4.3. Spatial resolution. Electrical and optical properties of semiconductor devices are influenced by the composition of thin films, lattice defects in the material and sharpness of interfaces. The ultimate goal of surface analysis techniques is to determine accurately the elemental concentration as a function of (X. 1; Z) coordinates in the device. As we have indicated in Table 1 the depth resolution of the techniques discussed falls in the range 0.3 -20 nm. the lateral resolution varies between atomic resolution and 1 mm. Over the years the minimum dimensions in semiconductor devices have decreased in size and are now in the micron and submicron range in both the silicon and GaAstechnology. At present only SAM and to a lesser extent SIMS have sufficient lateral resolution for such devices. Depth profiling by ion bombardment has become a common procedure for thin film analysis with AES and SIMS. However, a depth distribution can also be obtained without sputter profiling by RBS. Changing the detector angle to a glancing position with the substrate provides depth information in XPS and improves depth resolution in RBS. If a sample can be prepared by ball cratering or an ion beam to produce a crater in the thin film, the high lateral resolution of SAM can be ‘translated’ tn a high depth resolution105. This latter method also known as crater-edge profiling has shown to give identical results to normal in-depth profiling. Sputter profiling is often carried out with inert gas ions of a few keV kinetic energy. However. since recoil mixing increases with ion energy, it is an advantage to operate the ion gun at low energies. It has been shown that combining low energy ion bombardment with AES is particularly useful for electronic materials’Oh The ion range for 500 eV argon ions is less than 1 nm in silicon and GaAs, while Al, Si, Ge, Ga and As all have major Auger transitions with an energy larger than 1000 eV. The inelastic mean free path of these electrons is well above 1 nm3’. In Table 3 the sputter rate of some materials is given for 500 eV Ar+ ions. our measurements were made with a SAM for calibrated thin films of known thickness. The depth resolution in sputter profiling is limited by fundamental and instrumental parameters, the properties of the sputtering process and the quality of the sample’O”‘” Among the fundamental and instrumental parameters are the inelastic mean free path i for AES and XPS. misalignment of electron and

Table 3. Sputter

rate (A rnn- ‘,/IA cm ‘) for So0 eV Ar’ Ions at Incidence on some elements and compounds. SI. GaAs and films: SIOz Ga,, fi Al,,., As single crystals: Au and Pt polycr)stalline thermal oxide and LPCVD Si,N,. normal

Element

yield is taken proportional dependmg on the projectile 888

S, (cl is the nuclear stoppmg”. the to S,(i:) wth an adjustable parameter target combmation.

or compound

Sputter rate 500 eV Ar (A min-’ itA cm~‘)

+

Ions (1)

A van Oostrom:

Characterization

of semiconductor

materials

and devices

ion beams and the analyser acceptance area in SIMS. The sputtering process causes atomic and recoil mixing, will make, depending upon dose, a single-or monocrystalline target amorphous, may change the composition by selective sputtering and defect induced migration. The smoothness of the sample surface, impurities and lattice defects present in the substrate may also influence depth resolution. The National Physical Laboratory in Teddington (UK) provides a tantalum pentoxide reference material for calibration purposes’ ’ ’ . For a 100 nm thin film the interfacial resolution, as measured by AES is given as 1.7 _+0.10 nm. This value is probably limited by i and the 3 keV ions used for profiling. Multilayered Ni/Cr structures have also been proposed as reference material by Fine at the National Bureau of Standards in Washington (USA)“‘. A careful analysis can reduce some of the instrumental effect, lower recoil mixing by operating at low ( < 500 eV) ion energies and select samples with smooth surfaces and low impurity levels. Semiconductor materials are often prepared with a considerably lower defect density and impurity level than metals. It is therefore understandable that the best depth resolution performance has been observed with these materials ‘r3,ir4. Figure 14 illustrates this for our work on GaAs/AlGaAs interfaces. After removal of 2.0 pm ofmaterial and passing through several interfaces the interface width was still only 2.5 nm for the LPE-grown layers. The measured interface width was found to degrade by the presence of impurities on the surface or in the film.

by surface

analysis techmques

DEPTH

saoA

LqA

10'

2000 DEPTH

Ga

Depth

,I070eV

(nm)

Figure 14. Interface between p-GaAs and n-Al,,, Ga,,,. As layer in a laser structure 2.0 pm below the surface. In depth profiling with a SAM and 500 eV argon ion bombardment incident at 32. to the sample normal. The measured interface width (10/90) is 2.5 nm”‘. The importance of recoil mixing in sputter-profiling is also demonstrated in Figure 15 for a multiquantum-well (MQW) structure of alternating thin films of 40 A GaAs and AlAs grown by MO-VPE in our laboratory. Lattice imaging in a TEM showed the abruptness of the GaAs/AlAs interface to be one atomic layer”‘. The results of a SIMS-analysis, obtained by Boudewijn, are shown for bombardment with 0; ions”‘. The “Al’ signal is plotted on a linear scale in Figure 15(a) and on a log scale in Figure 15(b). The depth resolution rapidly deteriorates for an increase in the oxygen ions primary energy. Even at 1.7 keV the Al’ signal in an AlAs-layer remains below the value in the AlAssubstrate. In the 500 A GaAs the Al+ is higher for 12 keV O;-ions than for lower ion energies and the profile does not show a flat bottom. 4.4. Quantitative aspects. The accuracy or reliability of results obtained with surface analysis techniques is limited by the

IAI

2500

IC 100

[AI

Figure 15. SlMS-profile of a MQW-structure of alternating GaAs and AlAs layers. The relative 27 Al’ signal is plotted on a linear scale (top 15(a)) and on a log scale (bottom 15(b))’ I”. The effect of recoil and atomic mixing is clearly observed.

instrument, the experimental procedure and the physical parameters themselves. Interpretation may be difficult and by no means straight forward. Round robins have shown that instruments can provide very different results, but careful calibration and measurement procedures should reduce this problem. Combining several experimental techniques to solve a problem has become a standard procedure in many laboratories. In our laboratory such work has been done e.g. on silicides”’ and silicon nitrides’ I*. RBS is a quantitative method which will supply absolute values of concentration of a particular element A in a host matrix B. As we have seen the spatial resolution of the method is not as high as in some of the other techniques. These other techniques may be less quantitative because of the limited accuracy of escape depth and backscattering in AES and XPS, ion neutralization in ISS, the degree of ionization and matrix effects in SIMS and the sputter rate of a sample which may be complex of structure and composition. We have recently compared RBS, SAM and ISS data obtained from the same silicon wafer implanted with 25 keV As’ ions and subsequently annealed at 700‘C. The RBS data of Figure 16(a) show the separation ofthe As and Si peaks. From the arsenic peak a dose 4.48 x 10” atoms cm-’ was derived. The detector angle with the substrate was reduced to a few degrees for the data of 889

A van Oostrom: Characterizatton

of semiconductor

materials and devices by surface analysis techniques AES

(a)

RES 2MW

SOOeV Ar’ He+

Si

As 25keV ..LB.10’51Cd

DEPTH

tAi -

Figure 17. AES,SAM m-depth profile wrth 500 eV Ar’ Ions for the same wafer as m Figure 16. Alternating sputtering was used under computer control. Prrmary beam energy E,= 5.0 keV

-

JYLY ENERGY WI(bl

I

Eo

120

1Exl

2LO

300

360

DEPTH

DEPTH

‘Figure 16. RBS data for 25 keV As + ions implanted

in silrcon. The 16(b)). The 2 MeV

Figure 16(b). The top of the distribution was found to be at R=215 A. This value was in excellent agreement with range calculations by Littmark. Ziegler and Fiorio”’ based upon the interatomic potential of Wilson, Hagpmark and Biersack”. The maximum concentration of the distribution was 1.6 x IO” As atoms cm ‘. The same wafer was also examined with AES’SAM profilmg with 500 eV At-+-ions. The high energy As peak at 1228 eV was monitored and the Si peak at 1619 eV. The result is shown in Figure 17. Using the sputter rate for silicon giv,en in Table 3 we measured a range R=210 A. The maximum concentration of the distribution was measured to be 1.35 x IO” Asatomscm-” based upon the elemental sensitivjity factors of Si and As. Since As is implanted in a low Z-material, a correction should be made for backscattering. The calculated correction gave a value of 1.6 x IO*’ As atoms cme3 m excellent agreement with the RBSresults. Finally, thewafer was also examined with ISS using 3 keV Ne’ions. The measured distribution is given in Figure IX. The backscattered ions were continuously collected. while the specimen was slowly profiled. The measured range for this dose was 215 8, and good agreement with theory. the RBS- and SAM890

LEO

5LO

6

F4gure 18.ISS in-depth protile wrth 3 heV Ne’ Ions for the same wafer as rn the prebrous figures. Data were collected wrth a CMA. the neon ions s~rrhe the target at normal rncidencc.

(A,--+

scattering angle is 170 (top. 16(a)) and 9.5.3 (bottom. He+ beam strikes the target at normal incidence.

L20

[AI __,

results was obtained. However. the maximum concentration of As atoms could not be determined. since vve do not know the sensitivity of As with respect to Si. If we assume a concentration of 1.6 x IO” atoms cm ’ the sensitivity of As should be two orders of magnitude larger than for Si. A more detailed description of these results will be published elsewhere.

5. Summaq

Surface analysis techniques have developed rapidly during the last decade. This review does not pretend to be exhaustive, but rather aims at describing the present state-of-the-art. We have tried to indicate new instrumental developments and some aspects of radiation damage. spatial resolution and calibration. Acknowledgements 1 am indebted to P Boudewijn. A Kuiper, G van der Ligt. B Volbert, M Willemsen and P Zalm for discussions and the use of material prior to publication. Thanks are also due to K Vrijsen for typing the manuscript.

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