100kV Electron Beam Lithography System:JBX-9300FS

Hirofumi Ohki and Hitoshi Takemura

Semiconductor Equipment Division, JEOL Ltd.

Accelerating voltage 100 kV 50 kVMax. field size 500 μm 1000μmMax. sub-deflection size 4 μm 8μmMin. beam size 4 nm 7 nmAddress unit 1 nm 2 nmLaser resolution λ/1024=0.6 nm Same as leftMain deflection DAC 20 bit Same as leftSub-deflection DAC 12 bit Same as leftMax. scanning speed 25 MHz Same as leftWriting area 230 mm×230 mm Same as leftLoadable-wafer size 300 mm Same as leftLoadable-mask size 230 mm Same as leftStitching accuracy ≦20 nm ≦25 nmOverlay accuracy ≦25 nm ≦30 nmPositional accuracy ≦30 nm ≦30 nmwithin material surfaceMin. writing linewidth ≦20nm ≦35 nm

1. IntroductionThe JBX-9300FS is a 100kV spot e-beam

lithography system for fabricating sub-100 nmsize devices and masks of X-ray or e-beamprojection lithography[1].

We describe its system specifications andkey technologies to achieve high accuracywriting.

2. System Specifications ofthe JBX-9300FS

We show the system specifications in Table1. The JBX-9300FS employs a ZrO/W emitterfor forming a spot beam, a vector-scanning sys-tem, and step & repeat stage-movement sys-tem.

Adapting to the increase of writing materialsizes, we have made it possible to load wafersup to 300 mm and mask blanks up to 230 mm.

Accelerating voltage can be switched from50 (100) kV to 100 (50) kV.

Field size is 1000μm at 50kV and 500μmat 100 kV.

We have employed a two-stage electrostaticdeflector system, to increase the writing speedadapting to the high sensitivity e-beam resists.

Critical dimension (CD) uniformity within awriting field has been improved by dynamicfocus and astigmatism correction. And writingaccuracy has been increased by employingsome new technologies for height correction,signal processing, and mark alignment in directwriting.

3. System Implementation of the JBX-9300FS

Figures 1 shows the external view of JBX-9300FS, Figures 2 the electron optics system,and Figures 3 the block diagram of the controlsystem.

3.1. Electron Optics SystemAs shown in Fig.2, an electron source image

at a ZrO/W emitter is optically demagnified

Spot electron-beam(e-beam) lithography systems are promising equipment for fabrication and research of sub-100nm size devices. And, adding to the conventional applications such as fabrication of high speed transistors oroptical devices, new demands for higher voltage lithography for X-ray or e-beam projection mask making have beenincreasing.

Now, sub-50nm size fabrication is aimed at and the required writing accuracy will increase as feature sizesdecrease. To play an important role in next generation device fabrication, we have developed the JBX-9300FS, a100kV spot e-beam lithography system.

In this paper we introduce the JBX-9300FS in terms of system implementation, performance,and writing results.

Table 1. Basic specifications for JBX-9300FS

Fig.1. External view of JBX-9300FS.

(26) JEOL News Vol. 35E No.1 26 (2000)

Aperture

ElectronGun

ZrO/WSuppressor

1st anode2nd anode

AccelerationElectrode

Ground Anode

Aligner

Blanking Plate

Dynamic FocusCorrection Lens

AlignerDynamic Stigmator

Scanning DEF

StigmatorPosition DEF

BE DetectorSubstrate

ObjectiveLens

Aperture

2nd Lens

3rd Lens

Demagnification Lens(Zooming Work)

and projected to the writing plane. Generated electron beam is accelerated to

50 kV or 100 kV through the four-stage accel-erating electrodes. This accelerating field playsthe role of an electrostatic lens, and the firstcrossover is formed at the center position of thesplit blanking electrodes.

Two condenser lenses work as zoom lens,which means that the 3rd condenser lens valueis set up linked to the 2nd lens value, in orderto project a demagnified image onto the fixedpoint as the third crossover.

Finally, the objective lens projects the thirdcrossover onto the writing plane as a spotbeam.

3.2. Deflection SystemWe have employed a two-stage electrostatic

deflector system adapting to a high resolutionand high sensitivity e-beam resists.

One is the deflector that designates writingpositions (positioning deflector), and the otheris the deflector that writes patterns rapidly(writing deflector).

The positioning deflector consists ofoctopole cylindrical electrodes and is locatedin the objective lens. It uses 20-bit DACs.

The writing deflector, on the other hand,consists of quadrupole cylindrical electrodesand is located above the objective lens. It uses12-bit DACs.

Furthermore, we have implemented twoelectrodes to improve the writing uniformitywithin a writing field correcting deflectionaberration and astigmatism.

The former works for dynamic focus cor-rection and the latter works for dynamic astig-matism correction.

These correction values are revolved in awriting field using third-degree polynomial andthey are supplied as a correction voltage in pat-tern writing.

3.3. Signal Detection SystemsThe signal detection systems we have newly

developed are the height correction, backscat-tered electron (BE) signal processing, andabsorbed electron (AE) signal processing.

The purposes of height measurement arecorrections of objective lens focusing, deflec-tion amplitude and rotation of main- and sub-deflectors.

Two pairs of height measurement systemsdetect height difference between the heightstandard plane on the stage and writing plane.And acquired height fluctuation is fed back tothe focusing and deflection control unit.

BE signal is used for mark position determi-nation and correction of deflection amplitudeand rotation of main- and sub-deflectors.Detected signals from alignment marks indirect writing are also processed as BE signals.

The BE signal processing unit consists ofBE signal detectors and the signal processingsystem. Detected digital BE signal is processedby correlation operation and a mark position isdetermined from the top peak signal position.

where f(i) is detected signal, -j ~ j is calcula-tion range of correlation operation, and g(i) iscorrelation operation processed signal.

Practical mark detection signals in directwriting (from top: raw signal, differential sig-nal, correlation operation processed signal) areshown in Figures 4.

AE signal is mainly used for focus adjust-ment at the focus standard plane.

The AE signal processing unit consists ofmesh marks with the role of knife edge, e-beamdetectors below the meshes, and the signalprocessing system.

Detected digital AE signal when e-beampasses across a mesh mark is fitted to an errorfunction, and a beam size is defined from coef-ficients of the error function.

Adding to the beam size measurement func-tion, an automatic focus adjustment function tominimize a beam size is attached.

3.4. Chip Mark Alignment MethodSome wafers used in direct writing have

some distortions such as warping, expanding,and shrinking through several thermal pre-pro-cesses.

To compensate these distortion influenceson writing accuracies, four chip marks provid-ed around a writing chip are detected for mea-suring how the chip region is distorted.

Assuming the designed mark coordinates as(xi, yi) (i=1~4) and the actual mark coordi-nates including wafer distortion as (Xi, Yi)(i=1~4), we have the equations shown below.

Xi = A0 + A1・xi + A2・yi + A3・xi ・yi

Yi = B0 + B2・xi + B1・yi + B3・xi ・yi

Solving these equations, we can obtain thedistortion coefficients Aj (j=0~3), Bj (j=0~3).

By using these coefficients, the designedcoordinates of field corners fj (i=1~4) shownin Figures 5 can be converted tof́ i (i=1~4).Here the equations showing the conversion offj→ f́ i are obtained in the same manner as forthe distortion mentioned above. Actual writingpositions are then determined using the con-version coefficients for fj→ f́ i .

The pattern writing program send thesecoefficients to the hardware arithmetic unit.

Fig.2. Electron optics system of JBX-9300FS.

g(i)=Σf(i－j)･f(i+j)n

j=1

JEOL News Vol. 35E No.1 27 (2000) (27)

HEIGHT MEAS

LASER MEAS. SYSTEM

STG DRIVER

WAFER ALIGNER &CASSETTE TRANSFERSYSTEM

Cassettemotor

GUN

LENSES

D.F.

D.S. SCANNINGDEF

POSITION DEF

HEIGHT MEAS.SYSTEM (X2)

SCANNINGDEF. AMP.

POSITIONDEF. AMP.

DATAPARTITION

DATATRANSFER

COMPUTERCONTROL SYSTEM

pump pumppump

GUN CONT.

BLK AMP.

EOS CONT.

CORRECTIONDATA OPERATION

CORRECTIONDATA TABLES

SCAN GENE.

WaferWafer

COAT &DEVELOPMENTSYSTEM

Then this hardware calculates compensatedwriting positions using coefficients and patternwriting goes on.

When one field writing is completed, thenext coefficients are sent and the next fieldwriting starts.

Repeating this procedure over the entirewriting chip, direct writing is achieved withhigh accuracy.

4. Writing ResultsFigures 6 and 7 show results of line and

space pattern writing on ZEP520 (40 nm thick)

on Si. Accelerating voltage is 100 kV andbeam current is 100 pA. Line width of 15 nmin the X direction and 17 nm in Y directionhave been attained.

5. ConclusionIt has been ascertained that high-accuracy

writing results were obtained by employmentof some technologies such as improvement ofaddress unit resolution, newly developed sig-nal processing systems, and dynamic correc-tion within a writing field.

6. AcknowledgmentsA part of this work was performed under

the management of ASET in the MITI’s R&DProgram supported by NEDO.

References1. Y. Ochiai, H.Takemura et al.「PERFORM-

ANCE OF NEW E-BEAM LITHOG-RAPHY SYSTEM JBX-9300FS」Micro-and-nano-engineering 99, pp147-148,1999

Fig.6. X-direction line and space patternwriting (15nm)

Fig.3. Block diagram of the JBX-9300FS’s control system.

Fig.4. Mark detection signals.

Fig.7. Y-direction line and space patternwriting (17nm)

(M1-M4): Region defined by thedesigned four chip marks(M1́ -M4́ ): Region defined by theactual four chip marks

O : Average coordinate of the designed four chipmarks.

f1-f4 : Designed field shape.f1́ -f4́ : Field shape determined from the four chip

marks.

Fig.5. 4-chip marks alignment.

(28) JEOL News Vol. 35E No.1 28 (2000)

In early 1999, a typical SiO2 gate oxide wasabout 14 oxygen atoms thick. In early 2000, theoxide thickness had shrunk to 8 oxygen atoms.Between 2003 and 2008, the projected gateoxide thickness will be 4 oxygen atoms (1 nm).It is now technologically possible to producenano-transistors with 5 oxygen-atom-thick gateoxides and 35 nm channel lengths [1]. Never-theless, there are serious technological chal-lenges that face the semiconductor industry iflarge-scale integrated circuits are to be manu-factured using such devices [2]. The aggressivescaling of the gate dielectric presents chal-lenges in controlling reliability, leakage anddrive currents. The leakage due to direct tun-neling through an SiO2 gate oxide increases byroughly an order of magnitude for everyAngstrom decrease in thickness [1]. The moti-vation for reducing the oxide thickness is toincrease the electric field in the channel, there-by increasing the drive current. However as theelectric field is increased above 1 MV/cm, themobility is reduced due to surface roughnessscattering. This is reflected in a decrease indrive current in transistors with gate oxidesthinner than 1.3-1.5 nm for the smoothestSi/SiO2 interfaces [1,3]. Consequently precisemeasurement and control of the oxide thicknessand roughness is essential. Annular dark field(ADF) imaging offers quantitative measures ofoxide thickness and roughness even when con-ventional high-resolution transmission electronmicroscopy (HRTEM) is beset with imagingartifacts.

A further complication is that the Si-SiO2

interface is not atomically abrupt. For such thindevices even a 1 monolayer-thick suboxide canrepresent 40 % of the gate oxide thickness(since there are 2 Si/SiO2 interfaces). Usingatomic-scale electron energy loss spectroscopy

Gate Oxide Characterization using Annular Dark Field Imaging

D. A. Muller, F. H. Baumann

Bell Labs, Lucent Technologies

Modern integrated circuits can contain transistors smaller than 100 nm and gate oxides as thin as 2 nm. As a 0.1nm decrease in oxide thickness can lead to an order of magnitude increase in leakage current, precise measure-ment of the oxide thickness is critical. We find annular dark field imaging in a scanning transmission electron micro-scope to be a useful method of measuring the thickness and roughness of a gate oxide. Thickness measurementsare still possible in cross-sectioned samples as thick as 6000 Å. Imaging of dopant atom distributions is also possi-ble, sometimes with single atom sensitivity.

(EELS) [4] and ab-initio electronic structurecalculations [5, 6], it has been shown that theelectrical transition region from Si to SiO2

occurs over a region that is 0.3-0.4 nm wide,even when the structural transition is atomical-ly abrupt. The electrical and optical propertiesof the interfacial region are dramatically differ-ent from the bulk properties, but can be probedusing EELS [4]. Thickness measurements arestill possible using annular dark field (ADF)imaging as will be discussed here.

A second set of challenges facing devicedesigners is the positioning and activation ofdopant atoms. As devices shrink, the dopantconcentrations in the source and drain mustincrease to ensure good contact to the transis-tor. Such concentrations can already exceed thesolid solubility limits and are rapidly approach-ing the 1-% level. Locating individual dopantatoms with atomic resolution is certainly withinthe capabilities of ADF imaging [7, 8].

In this paper we discuss the use of a JEOL2010F, operated as a scanning transmissionelectron microscope (STEM), for measuringdopant distributions and gate oxide thicknesses.Electronic properties of the Si/SiO2 interfacewere measured using EELS. The microscopewas fitted with the analytical (Cs=1 mm) pole-piece, JEOL ADF detector, Gatan imaging fil-ter and BF/ADF detectors. STEM images couldbe recorded with both the Emispec and Gatansystems.

In a scanning-transmission electron micro-scope (STEM), the atomic-resolution imageformed by scanning an atom-wide electronbeam and collecting the signal on an annulardark field detector can also be used to align andlocate the small probe needed for electron-ener-gy-loss spectroscopy. This makes it possible tomeasure the chemical composition and bondingof an interface with sub-nanometer spatial reso-lution [4, 9-11].

Apart from the manner in which the image is

formed (scanning versus parallel illumination),the main difference between ADF and HRTEMimaging is the contrast mechanism. For verythin specimens, HRTEM imaging is primarily acoherent, phase-contrast imaging techniquewhile ADF imaging is an incoherent, ampli-tude-contrast technique[12-16]. Figure 1shows the contrast transfer functions (CTFs)for ADF and HRTEM images for the JEOL-2010F-ARP. The incoherent nature of the ADFimage (which measures the square of the elec-tron wave function) largely removes the con-trast reversals present in phase-contrastHRTEM. ADF imaging also trades off higherresolution for reduced contrast. Consequently,raw ADF images always look more `blurry’than HRTEM images, which in turn are artifi-cially sharpened by the microscope (TheHTREM CTF removes the lower frequencies,while the ADF CTF enhances them). Thesemodels are no longer qualitatively correct forthicker specimens where multiple scattering issignificant, but they still serve as a useful guideto the differences in image formation.

Figure 2 emphasizes the higher point-pointresolution of incoherent imaging (roughly afactor of 40% better), showing the 1.63 Åspacing expected from figure 1. Note that anti-mony dopant atoms show up roughly ninetimes brighter than a silicon atom would - thecross section scales roughly as Z1.7, so ADFimaging is sometimes known as “Z-Contrast”imaging [16], although not all contrast effectsare related to atomic number alone. The effectsof crystal orientation and strain fields can bepronounced [17], so it is always important tocheck the ADF result with an analytical tech-nique such as EELS. This has been done for thecase of antimony in silicon, where the ADFsensitivity is estimated to be +/- 0.5 atoms, andthe EELS detection limit is +/- 2 atoms for a100 kV VG-STEM [7]. We have succeeded inreproducing these experiments on the JEOL-

( 6) JEOL News Vol. 35E No.1 6 (2000)

Murray Hill, NJ 07974. U.S.A

Fig.1. Contrast Transfer Functions (CTF’s) for a JEOL2010F operated in HRTEM mode (with a0.7 mrad beam convergence ) and ADF-STEMmode (with a 1 Å diameter source, and a 10 mradprobe forming aperture). The spherical aberra-tion coefficient of the objective lens is 1.0 mm, atypical value for an analytical polepiece.Theoptimal focus is chosen for each mode. Note thatthere are contrast reversals in HRTEM mode,but not in STEM. The dashed arrow marks theScherzer limit of 2.3 Å beyond which informationin HRTEM mode can only be recovered by imagereconstruction techniques. In ADF-STEM mode,information out to 1.5 Å (the solid arrow atk=0.666 Å–1) can be directly interpreted. At thispoint the ADF-CTF has dropped to 5 % of its ini-tial value, which is comparable to the noise lev-els in the image.

Fig.2. Annular dark field STEM image of an antimony delta-layer in[110] oriented Si, showing an information limit better than0.163 nm, and minimal drift during the 40 second acquisition. Thepeak antimony concentration is 20 atoms or about 0.2 monolayers.(Digiscan and JEOL 2010F-ARP)

Fig.3. Bright Field STEM image of a gate oxide recorded with the Emispec system. The interface roughness was roughly 8 Å peak-peak (or 1.7 Å rms). The collection semi-angle was 1 mrad, and the probe forming aperture was 10 mrad semi-angle. (Emispecand JEOL 2010F-ARP)

Polysilicon Gate

Silicon Substrate

3 nm

JEOL News Vol. 35E No.1 7 (2000) ( 7)

220

311

GrowthDirection

10 Å

2010F and find the sensitivity to be between 1and 2 antimony atoms. Imaging As atoms hasalso proved practical. Light atoms, such asboron, can be imaged by reducing the ADF col-lector angle and imaging the strain field sur-rounding the dopant atoms. Single atom sensi-tivity for light elements in ADF is not yet pos-sible at room temperature, but calculations sug-gest it should be possible with a liquid Nitrogenstage [8].

It should be noted that STEM is not exclu-sively restricted to ADF imaging. If a smallcollector (collector angle much smaller than theprobe-forming aperture) is placed on axis, thenthe resulting bright field STEM image is for-mally equivalent to a conventional phase con-trast HRTEM image. The collector angle inSTEM plays an analogous role to the illumina-tion angle in HRTEM. Figure 3 shows abright-field STEM image of a gate oxide,which would be hard to distinguish from aHRTEM image.

A serious limitation to all TEM imagingmethods is that the sample is viewed in projec-tion. All information along the path of the

beam is collapsed into a single intensity mea-surement. Roughness is imaged differently inphase and amplitude contrast images, especial-ly in thick samples. Figure 4 shows the “blackband” that is typically present in HRTEMimages of Si/SiO2 interfaces. (It is only inextremely thin samples that this band can beavoided). The EELS spectra on the left of thefigure show that there is a substantial fractionof oxygen in the black band, as would beexpected for a 6 Å peak-peak interface rough-ness. (This would correspond to a 1 Å rmsroughness, which is typical). The “black band”is probably a more general case of dynamicaldiffraction artifacts seen in the roughness mod-els of Akatsu and Odomhari [18]. Their workdemonstrates that HRTEM images can make arough interface appear quite smooth when thefilm thickness exceeds the roughness correla-tion length (typically 10-100 Å).

In contrast, ADF images degrade quitegracefully with thickness, displaying nodynamical diffraction artifacts (provided alarge detector inner angle is used). Figure 5shows that the gate oxide thickness can be mea-

sured in specimens as thick as 6000 Å. Aver-aging of the roughness over the sample thick-ness still occurs in ADF imaging, so a roughinterface will have a smooth edge, however theintensity of the bulk lattice will gradually fadeaway. The result is that the edge of the oxidedoes not appear to be well defined (since inprojection it is not!). Fortunately, the incoher-ent nature of ADF imaging means that a lineprofile through the oxide will provide both theoxide thickness (from the full width at halfmaximum) and the roughness (convolved withthe probe size). Figure 6 shows the fit of agaussian roughness profile to each interface. Ingeneral we find a good correspondence (within10%) between x-ray and ADF measurements ofinterface roughness [4].

Some caveats are in order. As a scannedprobe forms the ADF images, time-varyinginstabilities can lead to distortions in the image(which in HRTEM would only reduce the con-trast). Spatial distortions over the field of vieware almost always larger than 0.2 Å. Neverthe-less, the almost-incoherent nature of ADF-image formation should make the method far

Fig.4. The HRTEM image recorded in the JEOL 2010F showing the 2 nmthick gate oxide and used to position the probe for spectroscopy. AGatan imaging filter was used as the spectrometer, as the photodi-ode array of a conventional PEELS is not sensitive enough to detectthe energy losses from a 20 pA, 0.2 nm sized probe. The interfacestates are visible as a pre-peak 3 eV below the O-K edge. Thisimplies a reduced energy gap at the Si/SiO2 interface [4].

Fig.5. ADF images of a 2 nm gate oxide recorded at different sample thick-nesses. The inner semi-angle on the ADF detector is 40 mrad and theprobe forming aperture was 10 mrad semi-angle. The main effect ofsample thickness is a reduction in signal/noise. The sample thick-ness was determined by EELS. (Digiscan and JEOL 2010F-ARP)

(a)

Gate Oxide

Polysilicon Gate

( 8) JEOL News Vol. 35E No.1 8 (2000)

6000 Å

5500 Å

3000 Å

Poly-Si SiO2 C-Si

1 nm

(b)(b)(b)(b)Silicon SubstrateSilicon SubstrateSilicon Substrate

less sensitive to many of the problems experi-enced with phase-contrast HRTEM imaging.ADF images can only be formed over a muchnarrower range of specimen tilts than HRTEMimages, reducing artifacts from misalignmentof the crystal [15]. Contrast reversals withdefocus are very much less likely to occur inADF imaging, unless very large defocii or largeobjective apertures are used [15]. ADF imagesof crystals do not appear to reverse contrastwith thickness as do HRTEM images [19], butthey can do so at defects such as dislocations,in very thick films [17]. As in HRTEM, strainfields can complicate the image interpretation[17], but this can be minimized by using thinsamples and large ADF collection angles.

In summary, ADF imaging has been demon-strated to provide quantitative information onthe thickness and roughness of gate oxides. Theimages remain interpretable, even in thick sam-ples. In the search for replacement gatedielectrics, the chemical sensitivity of ADFimaging is proving useful for quickly measur-ing reactions between silicon and the SiO2

replacement.

References1. G. Timp et al. (26 others), IEDM Techni-

cal Digest (1998) 615.2. Semiconductor Industry Association, The

National Technology Roadmap for Semi-conductors (Sematech, Austin, 1997), 71-81.

3. G. Timp et al. (19 others), IEDM Techni-cal Digest (1999) 55.

4. D. A. Muller, T. Sorsch, S. Moccio, F. H.Baumann and G. Timp, Nature, 399, 758-761 (1999).

5. C. Kaneta, T. Yamasaki, T. Uchiyama, T.Uda, K. Terakura, Microelectronic Engi-neering 48 (1999) 117-120.

6. J. B. Neaton, N. W. Ashcroft, D. A. Muller,Phys. Rev. Lett . (in press)

7. P. H. Citrin, D. A. Muller, H.-J. Gossmann,R. Vanfleet and P. A. Northrup, Phys. Rev.Lett. 83 3234 (1999).

8. S. Hillyard and J. Silcox, Ultramicroscopy,58, 6 (1995).

9. N. D. Browning and M. M. Chisholm andS. J. Pennycook, Nature 366, 143 (1993).

10. D. A. Muller and Y. Tzou and R. Raj and J.Silcox, Nature 366, 725 (1993).

11. P. E. Batson, Nature 366, 728 (1993).12. O. Scherzer, J. Appl. Phys 20, 20 (1949).13. E. Zeitler and M. G. R. Thomson, Optik

31, 258 (1970).14. G. Black and E. H. Linfoot, Proc. Roy.

Soc. (London) A239, 522 (1957).15. J. Silcox and P. Xu and R. L. Loane, Ultra-

microscopy 47, 173 (1992).16. S. J. Pennycook, Ultramicroscopy 30, 58

(1989).17. D. D. Perovic and C. J. Rossow and A.

Howie, Ultramicroscopy 52, 353 (1993).18. H. Akatsu, I. Ohdomari, Applied Surf Sci-

ence, 41 357 (1989).19. S. J. Pennycook and D. E. Jesson, Phys.

Rev. Lett . 64, 938 (1990).

Fig.6. Quantitatively measuring the thickness of a 2.5 nm gate oxide froman ADF image. The gaussian interface roughness model fit is shownas a dotted line. The lower and upper interface roughnesses were1.75 Å and 3 Å rms respectively. The inner semi-angle on the ADFdetector is 40 mrad and the probe forming aperture was 10 mradsemi-angle. (Digiscan and JEOL 2010F-ARP)

Polysilicon Gate

Silicon Substrate

2 nm

JEOL News Vol. 35E No.1 9 (2000) ( 9)

Introduction of New Products

JSM-6500F

EDS

WDS

EBSP

(32) JEOL News Vol. 35E No.1 32 (2000)

Multi Purpose High PerfomanceFE SEM

JSM-6500F multi purpose high performance FE SEM isequipped with the newly developed In-Lens Thermal FEG,unique combination of the thermal FEG (Field Emission Elec-tron Gun) and the condenser lens produces high probe currentsufficient for EBSP, WDS and CL analysis as well as highmagnification image observation and EDS. A high probe cur-rent in a very small electron probe size enables one to analyzesmaller area.

●Small probe diameter attainable at the analytical conditionsrequiring high probe current : 3.0 nm (Probe current : 5 nA,Accelerating voltage: 15 kV, WD:10 mm)●High resolution : 1.5 nm at 15 kV, 5.0 nm at 1kV●Newly developed In-Lens Thermal FEG : Maximum 200 nA

probe current available●Emission noise free, stable probe :

Thermal FEG produces high stability for analytical applications●Multi purpose specimen chamber : Maximum 150 mm diameter

specimen●Motorized specimen stage : X-Y-Rotation motorized (5 axes

motor control: option)●One action specimen exchange : Fail safe simple procedure ●High Resolution, Crisp Live Image Display : 1,280×1,024

pixels digital image●Variety of analytical capability : Integrated EDS, WDS, EBSP,

CL available as option

The JAS-WPA wafer surface particle analyzer is a laser Ramanspectrometer combined with an optical microscope and adefect-review system. Linked with a defect coordinate fileobtained by the wafer inspection system, this instrument iscapable of not only reviewing defects and particles but also per-mitting them to be identified by observing the Raman spectra. Itcan analyze them down to 0.2μm in size and also identify bothorganic and inorganic compounds effectively. The WPA is areliable tool that clarifies the cause of occurrence of defects andparticles and improves the product yield.

●Laser : Ar+ (wavelength 514.5 nm) standard●Wafer size : 125 mm, 150 mm, 200 mm●Stage position accuracy : ± 2.5 mm●Coordinate link : KLA (KRF), SURFSCAN (tff) and others

Wafer Surface Particle Analyzers

JAS-WPA series

JEOL News Vol. 35E No.1 33 (2000) (33)

Analysis ExampleThis is an example of analysis of submicron-level particles on awafer surface. It reveals that a satisfactory spectrum can beobtained by measurement within several tens of seconds. Theinclusion in particles is identified as ammonium sulfate (NH4)2SO4

by the spectrum collation search with the data base. Even a com-pound having a complex structure that cannot be identified by ele-ment analysis alone can be accurately identified by using the WPA.

Introduction of New Products

The JWS-7855S has the capability to investigate reticles, X-ray masks, MR Heads and wafers at high specimen stage tiltangles with high resolution. It has a wide range of electronbeam accelerating voltages (0.5 kV－15 kV) for both mor-phological observation and elemental analysis by EDS. Itenables the user to make excellent reproducible CD (criticaldimension) measurements on flat position of the specimenstage and height CD measurement at 60° as an alternative toolof wafer cross section SEM. When a scheduled program isedited beforehand, the operator is required only to operate atrackball and to control on the operation panel for imageobservation and schedule program execution. Automaticdefect review (ADR) system and automatic defect classifica-tion (ADC) system can be integrated for the yield manage-ment of semiconductor production lines.

●High SEM resolution: 5 nm (1 kV) ●High quality SEM images (1280 3 1024 picls)●Wide range of accelerating voltage (0.5 kV－15 kV) ●Stage tilt : –15° to 60° (wafer)

–15° to 50° (mask) ●High throughput●Convenient job schedule functions●Less particle contamination and lower damage

Mask Observation Scanning Microscope

JWS-7855S

(34) JEOL News Vol. 35E No.1 34 (2000)

Low accelerating voltage and process monitor analysis appli-cation Tilt : 60°, Acc.v, : 500V

Defect review application at 60° tilt

Dual Beam FIB

JFS-9855S

The JFS-9855S is an upgraded model of JFS-9855 designedas a “CUT and SEE” defect review tool. It allows both in-situfabrication of defect portion and high resolution SEM obser-vation of the cut section in real time. The capability of quickmode switching between SIM and SEM modes helps to per-form “CUT and SEE” analysis very easily. The JFS-9855Scan be used as a high-resolution wafer inspection SEM,because it has the column of the JWS-7000 series dedicatedto wafer inspection. Another important aspect is the expend-ability to attach EDS so that it can be used as an analyticalSEM.Automatic defect review (ADR) system and automatic defectclassification (ADC) systems can be integrated for the yieldmanagement of semiconductor production lines. This systemis now applied to the production of many other devices suchas MR Head production lines. Thus, the JFS-9855S allowsthe use as a surface defect analysis tool as well. Such a func-tional integration will contribute greatly to reduce the timeneeded for failure analysis, thus allowing quick control in pro-duction line.

●High SIM resolution : 7 nm (30 kV)●High SEM resolution: 5 nm (1 kV)●High quality SEM images (1,280 3 1,024 picls)●Stage tilt : 0° to 60° (wafer)●Quick switching between FIB milling and SEI observation

modes●Equipped with a C to C auto-loader●Coordinate linkage with optical defect inspection system and

laser microscope●Recipe capability for wafer information and observation/milling

condition

JEOL News Vol. 35E No.1 35 (2000) (35)

Cross section of contact holes

Cross section of a defect

IntroductionEarly geophysical observations allowed the

distinction of three layers in our Earth, namelythe crust with 8 to 50 km thickness, the mantledown to 2900 km and the metallic core (Fig.1).Continental and oceanic crust may be studieddirectly by sampling in deep erosional cuts,dredging from ships and by drilling. Upliftedblocks in the Alps even allow insight into thetransition of lower crust to upper mantle. Xeno-liths entrained by fast uprising magmas anderupted to the surface by volcanoes allow thestudy of Earth’s mantle material stemmingfrom up to 200 km depth. A quite detailed pic-ture on the chemistry, mineralogy and tempera-ture distribution begins to emerge from thesestudies. Greater depths and the Earth’s corewere accessible exclusively to geophysical andhigh pressure experimental methods. Theirresults allow the explanation of the mostimportant seismic discontinuities within theEarth’s mantle, namely at 410 km and at 660 km(Fig. 1). The 410 km discontinuity marks thetransition from the upper mantle (garnet peri-dotite) with mostly olivine (a Si-poor Mg-sili-cate Mg2SiO4), orthopyroxene (a Si-saturatedMg-silicate MgSiO3) and garnet (a Mg-Al-sili-cate Mg3Al2Si3O12) to the transition zone,where olivine isochemically transforms to amuch denser spinel structure and pyroxeneincreasingly dissolves into garnet (majorite). At670 km the spinel structured olivine dispropor-tionates into the still denser phase assemblageferropericlase (MgO) + perovskite structuredMgSiO3. Basaltic oceanic crust gets subductedat continental margins and transforms at highpressures first to pyroxene + garnet (eclogite),then, on dissolution of pyroxene into garnet,the entire rock transforms into majorite plussome leftover SiO2 (stishovite). Majoriteexsolves Ca-perovskite (CaSiO3) from about550 km onwards.

Diamonds grow at depths from about140 km onwards and inclusions in these dia-monds enable us to sample deep regions of our

Kankan diamonds (Guinea): probing thelower mantle

Thomas Stachel†, Gerhard P. Brey † and Jeff W. Harris ††

†Institut für Mineralogie, Universität Frankfurt ††Division of Earth Sciences, University of Glasgow

†Senckenberganlage 28, 60054 Frankfurt,GermanyE-mail:stachel@em-uni-frankfurt.de

††Glasgow G12 8QQ, Scotland, UK

Fig.1. Structure of the Earth showing the major phase transitions at 410 and 660km, the beginning of the dissolution of pyroxene into garnet to form majoriteat 300 km and the resulting division into upper mantle, transition zone andlower mantle and the Earth’s core. The lid of the convecting astenosphere isthe lithosphere consisting of the continental (yellow) and oceanic (black) crustand the uppermost part of the upper mantle which may be subducted into themantle below the continents. Diamonds may be brought to the surface fromdepths exceeding 700 km first by convection and from about 200 km onwardsby the explosive force of volcanism.

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Earth’s mantle. Unequivocal information maybe obtained from the inclusions (Fig. 2)because they are protected and isolated by achemically inert container which prevents re-equilibration and retrograde reactions with thesurrounding matrix during exhumation.

We have studied diamonds from the Kankandistrict in Guinea and examined the major andtrace element chemistry of “normal” lithospher-ic inclusion parageneses, diamond populationsderived from the asthenosphere and the transi-tion zone (410-660 km) and further lower man-tle inclusion parageneses from below 660 kmdominated by the mineral ferropericlase. Thelatter are extremely rare and were describedbefore from only one locality in Brazil.

SamplesIn Guinea, diamonds are mined from alluvial

deposits in the eastern part of the country. Pri-mary diamond sources are provided by Creta-

ceous kimberlite intrusions that range from bar-ren to high diamond grades. In the presentstudy, several thousand Kankan diamonds wereexamined optically from which 90 diamondswere selected for study of the mineral inclusioncontent. After breakage, 70 of these diamondsyielded syngenetic mineral inclusions.

Analytical MethodsThe small size (between 30 and 100 µm) and

the high scientific value of the diamond inclu-sions require non-destructive microbeam tech-niques for chemical analysis. For major andtrace elements (down to about 100 ppm) theelectron microprobe is the obvious choice, as itcombines (i) high spatial resolution (at 20 kVgun potentialthe excited sample volume will beabout 2 µm (ii) in diameter), high precision andaccuracy down to low concentrations and (iii)the ability to check homogeneity of the sampleby electron and X-ray images.

Diamond inclusions may be studied in situby X-ray diffraction for phase identificationand structural state. For chemical analysis theyneed to be released by breaking the diamond.Once released from the host diamond, theinclusions were embedded in araldite® in 5mmbrass rings and subsequently polished on a Pb-Sb plate using 1/4 µm diamond powder. EPMAanalyses of major elements were performed ona Jeol JXA-8900RL at 20 kV gun potential and20 nA beam current using silicate, oxide andmetal standards. Count times range between 30and 90 seconds to ensure detection limits of100 ppm or better for all oxides except SrO andNa2O (200 ppm). Accuracy of major elementanalyses was checked against secondary stan-dards and is better than 1 % (relative). Analyti-cal precision is mainly controlled by countingstatistics (which, for a given oxide species,depend on the concentration) and thereforestrongly declines for minor and trace elements.

Fig.2. Inclusions of orange coloured eclogitic garnet (about 250µm in length) in diamond. Both garnets show a cubo-octa-hedral shape which is imposed by the diamond host andtherefore indicative for a syngenetic formation.

Fig.3. Phase boundaries and reaction curves relevant for theascent of lower mantle diamonds from Guinea.

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At an oxide concentration of 0.10 wt% errors(2 sigma) range between 4-8 % (relative),except for P2O5, NaO and SrO (12-18 %). Atthe detection limit (0.01-0.02 wt%) errorsincrease to about 20 % for the majority ofoxides, but range between 27-45 % for the lat-ter group.

Composition of Inclusions -14 diamonds yielded inclusions belonging to

the peridotitic suite with garnets, 16 olivines,three clinopyroxenes and four chromites. Rep-resentative microprobe analyses are given inTable 1.

19 Kankan diamonds are eclogitic fromwhich eleven yielded 12 eclogitic garnets.These are mainly high-Ca garnets and six con-tain a significant majorite component (seeTable 1). Majorite component means the solidsolution of pyroxene in garnet to formA2+

4Si[6]Si[4]3O12. The molar proportion of the

majorite component increases with pressureand may be used to estimate depths of origin(see below). In part, majoritic garnets alsoshow very high sodium contents with Na2Oexceeding 1 wt.%. Thirteen clinopyroxeneswere found in nine eclogitic diamonds. Twocoexist with majoritic garnets and show high tovery high potassium contents, which are corre-lated to the Si-excess of their coexisting gar-nets.

Inclusions of likely lower mantle originhave been released from 34 diamonds. By farthe most common inclusion is ferropericlase; atotal of 39 inclusions were recovered from 32diamonds. Kankan ferropericlase is very mag-nesian, At Kankan, significant impurities in fer-ropericlase are formed by NiO (0.08-1.46wt%, av. 1.27 wt%), Cr2O3 (0.01-1.04 wt%,av. 0.55 wt%), Na2O (0.07-0.79 wt%, av. 0.32wt%) and MnO (0.13-0.26 wt%, av. 0.19 wt%).

Seven MgSiO3 inclusions were found in

sample KK-26a KK-26b KK-28b KK-86 KK-81a KK-81b KK-108c KK-108e KK-66amineral olivine garnet cpx garnet garnet cpx Fe-periclase MgSiO3 CaSiO3

paragenesis peridotitic peridotitic peridotitic eclogitic deep-ecl. deep-ecl. lower mantle lower mantle lower mantleP2O5 £0.01 £0.01 £0.01 0.04 0.05 0.02 £0.01 £0.01 0.02SiO2 40.57 42.14 55.03 39.49 45.05 55.21 0.02 57.04 51.08TiO2 £0.01 0.27 0.04 0.72 1.16 0.65 £0.01 0.05 0.07Al2O3 0.02 18.64 0.64 21.31 16.64 6.31 0.06 1.66 0.39Cr2O3 0.04 5.50 0.74 0.04 0.08 0.05 0.57 0.26 0.02FeO 8.18 7.10 3.13 20.86 10.32 4.42 22.47 4.87 0.07MnO 0.12 0.31 0.13 0.42 0.26 0.07 0.20 0.11 0.04NiO 0.38 0.00 0.08 0.00 0.02 0.04 1.38 0.02 £0.01MgO 50.40 21.23 20.78 8.17 15.18 12.35 75.69 35.00 0.08CaO 0.04 4.98 17.93 9.15 9.89 14.79 £0.01 0.10 47.07SrO 0.73Na2O 0.02 0.04 0.59 0.27 1.37 4.05 0.31 0.04 0.07K2O £0.01 £0.01 0.18 £0.01 £0.01 1.44 £0.01 £0.01 0.43Total 99.78 100.21 99.28 100.47 100.02 99.40 100.70 99.16 100.08

P 0.005 0.006 0.001 0.001Si 0.992 6.042 1.991 6.002 6.558 2.001 0.001 1.968 1.985Ti 0.029 0.001 0.083 0.127 0.018 0.001 0.002Al 0.001 3.150 0.027 3.816 2.854 0.270 0.006 0.067 0.018Cr 0.001 0.623 0.021 0.005 0.009 0.001 0.033 0.007 0.001Fe2+ 0.167 0.851 0.095 2.651 1.257 0.134 1.402 0.141 0.002Mn 0.003 0.038 0.004 0.054 0.032 0.002 0.013 0.003 0.001Ni 0.008 0.002 0.003 0.001 0.083 0.000Mg 1.836 4.537 1.121 1.851 3.293 0.667 8.418 1.800 0.005Ca 0.001 0.765 0.695 1.490 1.543 0.574 0.004 1.959Sr 0.016Na 0.001 0.012 0.042 0.079 0.386 0.285 0.045 0.002 0.005K 0.008 0.066 0.021Total 3.008 16.047 4.008 16.036 16.067 4.020 10.001 3.994 4.016[O] 4.000 24.000 6.000 24.000 24.000 6.000 10.000 6.000 6.000

Table 1. Representative electron microprobe analyses (in wt%) and calculated atom-ic proportions of peridotitic, eclogitic and lower mantle inclusions inKankan diamonds.

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four ferropericlase bearing diamonds. Due tothe very strong partitioning of Ni into ferroper-iclase, MgSiO3 inclusions formed in the lowermantle may be readily distinguished fromupper mantle orthopyroxenes by very low Ni(£0.03 wt% NiO) contents.

Five CaSiO3 inclusions have been releasedfrom four diamonds, all but one coexisting withferropericlase. XRD showed two of these tohave reversed into the walstromite structure(the stable modification of CaSiO3 between 3-10 Gpa (Fig 3). In three diamonds bimineralicinclusions of Ca2SiO4-larnite and CaSi2O5-titanite have been identified by in situ XRD. Inone case, the inclusion was subsequentlyreleased for EPMA and found to show a thinband of CaSiO3 (< 10 µm) separating the othertwo phases (Fig. 4). The phase relationships forCaSiO3 (Gasparik et al. 1994) are consistentwith the mineralogical relationships just noted,as CaSiO3-perovskite decomposes to larnite +CaSi2O5-titanite at pressures below 12 GPabefore recombining to form CaSiO3-walstro-mite at pressures below 10 GPa. The threepolymineralic Ca-silicate inclusions, therefore,are pressure-inversion reaction products of pri-mary CaSiO3-perovskite.

Compositionally the inclusions are 99 %pure CaSiO3, with minor Al2O3 (0.03-0.66wt%), SrO (0.06-0.85 wt%) and K2O (0.01-0.73 wt%), which all positively correlate witheach other. SiO2 in the form of coesite was alsoobserved in two diamonds containing Ca-sili-cates, which were examined by XRD only(Joswig et al. 1999). As the stability fields ofCaSiO3-perovskite and coesite do not overlap,these SiO2-phases were most likely also includ-ed as stishovite. Along an adiabatic geothermalgradient, the phase boundary stishovite-coesitecoincides with the reaction curve larnite +CaSi2O5-titanite = CaSiO3-walstromite (Fig.3).

Diamond KK-109 contains (i) two isolatedferropericlase grains with higher Mg/Fe and Crand lower Ni than (ii) the ferropericlase in con-tact with olivine which is heterogeneous inMg/Fe and locally very high in Cr, Al and Ti.These elements correlate positively indicatingsub-microscopic Cr-spinel, which is in accor-dance with the cloudy optical appearance ofthe olivine. High resolution element mappingyields elongated spots (<<1 µm, Fig. 5) withhigh Al and Cr indicating exsolution of spinelfrom olivine. From mixing trends of multiplespot analyses we calculate the composition ofthe spinel exsolution bodies to be aboutFe0.37Mg0.77Cr0.41Al1.46Ti0.03O4. Chemical differ-ences and exsolution features may beexplained if olivine is the final reaction prod-uct of a primary ferropericlase + MgSiO3-per-ovskite inclusion with an excess amount of fer-ropericlase. Upon reaction first to spinel struc-turedγ-olivine the Ni content in the remainingferropericlase increases and becomes higherthan in the isolated ferropericlases. Also, theMg/Fe becomes lower and Al, Cr and Ti dis-solve in the spinel structure which subsequent-ly exsolve as Cr-spinel during conversion toα-olivine. That the reaction of ferropericlase withMgSiO3 did not immediately produce α-olivine but first theγ-phase is indicated furtherby a strong chemical zonation of the olivinetowards the ferropericlase-rim, with the Mg-number increasing from 94.7 to 96.1 and NiO

decreasing from 0.34 to 0.16 wt%. This is dueto diffusion of Fe and Ni into ferropericlaseafter conversion of the spinel phase intoα-olivine, since experimentally determined parti-tioning of Fe/Mg shows significantly lowerDFe/Mg ferropericlase/γ-phase relative to fer-ropericlase/olivine. Our finding here is the firstclear evidence of naturally occurring Mg2SiO4

spinel in terrestrial materials.

Pressure and Temperature Estimates

Using various geothermometers and geo-barometers based on major element composi-tions of coexisting phases pressures and tem-peratures of about 5.5 GPa at 1200 °C and upto 7.8 GPa at 1400 °C were estimated for theperidotitic inclusions. These are typical P-Tconditions for cratonic regions in other dia-mond producing areas with an additional con-tribution of diamonds from unusually greatdepth (approx. 250 km) which may come closeto the base of the lithosphere. The depths oforigin of the majoritic garnets may be estimat-ed from their excess Si-content with shows thatthe deepest majoritic garnet comes from 440km within the transition zone.

No geothermobarometers are available todetermine the PT conditions of formation for

lower mantle inclusions. However, a depth oforigin may be derived from the composition ofthe MgSiO3-perovskites based mainly on theAl-content. Various studies show that at 22.5GPa (at 1630 °C) 1.2-1.9 wt% Al2O3 are dis-solved and at 24 GPa MgSiO3-perovskite hasabout 4 wt% Al2O3. Therefore, the low alu-minium contents of the MgSiO3-perovskitesfrom Kankan (max. 1.7 wt% Al2O3) suggestderivation from the uppermost 10-20 km of thelower mantle.

ConclusionsThe majoritic inclusion paragenesis is

derived from a large depth interval extendingapproximately from 250 to 440 km, i.e. fromthe asthenosphere into the transition zone. Thelower mantle appears to be the dominant sourcefor Guinea diamonds. Apart from frequentinclusions of ferropericlase, we observed sever-al associations that may only exist at depthsexceeding 660 km. Examples are the paragene-ses of ferropericlase with CaSiO3- andMgSiO3-perovskite and stishovite. The low alu-minium contents of the MgSiO3 inclusionsindicate derivation from the uppermost deca-kilometres of the lower mantle.

Several diamonds contain inclusions whichare best explained in terms of re-equilibration

of former lower mantle parageneses at decreas-ing pressure and temperature: (i) CaSiO3-per-ovskites in part decomposed to Ca2SiO4-larniteand CaSi2O5-titanite, which in turn may recom-bine to CaSiO3-walstromite, (ii) CaSiO3- andMgSiO3-perovskite may react to form clinopy-roxene and (iii) MgSiO3-perovskite and fer-ropericlase form olivine with excess ferroperi-clase or enstatite. These observations suggestthat exhumation from the lower mantle wasslow, probably in a rising plume, whichallowed adjustment of external pressure on thehost diamond and internal pressure on theinclusions via plastic deformation.

Fig.4. Backscattered electron image of the polished surface ofthe Ca-silicates in diamond KK-44. Two crystals of larnite(b-Ca2SiO4) form the bright coloured left part of the inclu-sion. The darker coloured upper right consists of CaSi2O5-titanite. The darker veins within the brecciation arechemically not distinct from the host. The thin grey bandseparating larnite and CaSi2O5 consists of CaSiO3.

Fig.5. Wave length dispersive beam scan map showing the distri-bution of Al in the olivine part of KK-109a. Al concentra-tions increase from blue through green and yellow to red.Increased Al corresponds to submicroscopic exsolutions ofspinel. Note that exsolution features less than 0.2 mm indiameter are clearly recognisable without the need to pro-duce ultra-thin sections for transmission electronmicroscopy.

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