Philips Journalof Research Vol.47 Nos.3-5 1993 303

Philips J. Res. 47 (1993) 303-314

LASER SCAN MASS SPECTROMETRY-A NOVELMETHOD FOR IMPURITY SURVEY ANALYSIS

by F. GRAINGERPhilips Research Laboratories, Cross Oak Lane, Redhill, UK

AbstractThin layer and bulk semiconductor materials are analysed, by raster scanerosion of the sample surface under a focused Q-switched Nd- YAG laserbeam, in the source chamber of a high resolution MS702 massspectrometer. Interpretation of the spectra produced by the laser plasmagives a complete impurity survey of the material down to detection limitsof approximately I part in 109 (mid 1013 cm="). Results have shown thatsurface impurities are effectively removed in the first scan and subsequentscans over the same area have given true measurements of impurities intypical materials. The method gives automatic successive erosion of samplesurface areas from 0.1-130mm2 with ionisation and mass analysis of thesample material removed. The depth of penetration per scan is dependenton the material being analysed and the laser beam power at the samplesurface. In general it is variable between 0.3 pm and 4 pm for each scan.Most materials, including insulators, can be analysed providing they arenot completely transparent to the laser light. Quantitative measurements ofimportant dopants such as iodine and phosphorus in cadmium mercurytelluride, difficult to make by other assessment methods, can be simplyperformed by laser scan mass spectrometry.Keywords: analysis, impurity, laser scan, mass spectrometry, survey.

1. Introduetion

For many years spark source mass speetrometry (SSMS) was unrivalled forthe survey impurity analysis of semiconductor crystals and related materials.Developed in response to the need for measuring' a broad range of dopant andimpurity elements in these materials, the r.f. spark excitation source gavemulti-element capability with low detection limits. Recently, however, therehas been an increasing demand to provide high sensitivity element surveyanalysis of semiconductor layers for which the r.f. spark is not suited. Assess-ment of thin film semiconductor material requires complete impurity analysis

F. Grainger

to 1 part in 109 (mid 1013 cm") with a depth resolution of a few microns orless. Laser scan mass spectra metry (LSMS) has been developed for thispurpose in these laboratories, based on the original work by lansen andWitmerl) on bulk materials, and has been used successfully as a routinemethod for thin layers as well as bulk materials"), The high power Nd- YAGlaser makes it possible to obtain the ion yield required for trace analysisindependent of sample conductivity. Semi-quantitative impurity surveyanalyses can be made without individual element calibration, since relativesensitivities of the elements are approximately equal for all elements, even ineasily volatilised materials such as cadmium mercury telluride (CMT). LSMShas already proved invaluable in assessing the role of impurities, for examplethe investigation of electrical compensation in heavily silicon doped GaAsgrown by MBE, and the characterisation of CMT grown by LPE and MOVPE.

High frequency energetic light pulses, from the laser, are brought to a fixedfocal point on the ion optical axis ofthe mass spectrometer, to give a reproduc-ible ion yield from a sample eroded at the focal point. Focusing produces anincrease of 104 in the laser power level, ensuring that erosion and ionisation areonly possible from the sample surface and there are no direct backgroundcontributions arising from the internal surfaces of the ion source. The laserenergy is released at the sample surface, providing it is not completely trans-parent at the laser wavelength. The sample area is scanned precisely under thefocused laser beam by a computer-controlled motor-driven manipulatorsystem. Complete coverage of the surface is achieved by successive equallyspaced linear tracks of overlapping craters.

Areas from 0.1 up to 130mm! can be scanned and single scans are shownto effectively eliminate surface impurity errors in layer and bulk analysis.Erosion depth, which can vary from 0.3 to 4,um, is dependent on the materialanalysed and can also be controlled by the laser power and crater overlap.Ionisation efficiency has been shown to be a function of laser beam power,levels of around 5 . 109W cm? being necessary for uniform ionisation. Theion yield at the detector, per unit weight of eroded sample, is up to two ordersof magnitude higher than conventional r.f. excitation for the materialsexamined. The relatively high energy spread of ions, from a few hundreds upto around 1000 eV, makes it essential to use the double focusing mass spec-trometer to obtain high resolution mass spectra on the photoplate detector.

2. System description

The modified Masse MS702 double focusing mass spectrometer (fig. la) isa high resolution system capable of resolving mass lines separated by about 20

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Phihps Journalof Research Vol.47 Nos.3-5 1993 305

Laser scan mass speetrometry for impurity analysis

Spectrometerentrance slit

P'"l 'To ion optics

Lasershutterclosed

LoserShutteropen

Fig. I. a) Complete LSMS system; b) source area; c) raster scan procedure.

F. Grainger

millimass units. The Nd- YAG Q-switched laser (wavelength 1064 nm) is fittedat the front of the MS702 as illustrated and produces up to 10 m.I per pulsewith a duration of 15ns and a repetition rate from 1 to 50Hz. A beam steeringmirror system introduces the laser beam to the source unit where it is focusedat a fixed point on the mass spectrometer ion axis, 1cm in front of thespectrometer slit (fig. lb). A tantalum shield encloses the ion source, to providea field-free ionisation zone and it also reduces any secondary ion productionfrom the internal surfaces of the source chamber. Only a fraction of thepositively ionised material produced passes through the mass spectrometerand the laser lens is protected from evaporated deposits by a removable glassslide.The mass spectrometer gives a seriesof complete element spectra, recordedwith decreasing ion charge exposures on a photographic plate.

The scanning system, attached to the source housing, consists of a sampleplaten angled to the laser beam and spectrometer entrance slit, driven bymicro-manipulators in each ofits three directions oftravel by stepping motorscontrolled by a microcomputer (Hewlett Packard 9816). Initially, the scanningparameters are set and a rectangular area is defined. The sample is then movedunder the stationary laser beam to maintain a focused spot on the surfaceduring the scan motion (fig. le). The selected area is covered by successivelinear erosion tracks as shown. The location of the laser spot on the sample isdisplayed graphically on the computer monitor. The program allows succes-sive scans to be inset thus avoiding the edges of the previously eroded area.

3. Advantages of the laser system

The enhanced efficiency of laser excitation is apparent from a comparisonof the ion yields obtained from CMT and GaAs with LSMS and r.f. sparkionisation"), where the ion charge collected per unit weight of samplevaporised was between one and two orders of magnitude greater with LSMS.This is important since in layer analysis, the sample weight available is in-herently limited by the slicedimensions and the layer thickness. The steady ionyield from the laser source leads to reduced fogging of the photographic plate,normally observed in long exposures generated by the more erratic r.f, spark.The impressive analytical performance of this system may be judged frommeasurements made on 10pm thick layers of MBE-grown GaAs where adetection limit of 1 ppb atomic (4.1013 cm") was obtained for 2J1m depthpenetration over the maximum sampling area of 1.3cm' .The reproducibility of the laser process gives a steady ion beam, typically

better than ± 10% at optimum laser power. This leads to a constant scanningerosion rate and good depth resolution. For overlapping craters (normally

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Laser scan mass speetrometry for impurity analysis

30-50 pm in diameter) the erosion depth, with constant laser power and pulsefrequency, is inversely related to scanning speed along the "y" axis. Erosiondepth is measured by the weight loss of a scanned area and represents a meanvalue; typically this is 1 .urn in GaAs, 2.um in silicon and 4 .urn in CMT per scan.

The relative sensitivity factor (RSF) is defined as the ratio of the knownconcentration of an element to the concentration given by isotope line inten-sities on a calibrated mass photoplate; for most matrices this can be assumedto be near unity for r.f, spark excitation. However, CMT is an exception,giving sensitivity factors which can vary by up to 10 times for some elements').Standard multi-element doped CMT, well characterised by atomic absorptionspectrophotometry (AAS), has been used to obtain RSF values by the lasertechnique with this material and are much nearer unity than those obtained bythe r.f. spark source. This allows estimates of impurity concentrations withina factor of two without individual element calibration and is consistent withresults obtained by previous workers on other materials.

Surface cleaning is an important step prior to analysis. With the r.f. spark,extensive etching and pre-sparking of bulk electrodes is used to clean a surfacefor analysis, in the vacuum. With many thin layers surface cleaning is notpossible and the amount of sample is limited for excessive pre-rastering of thesurface. In a typical LSMS analysis of a CMT layer high concentrations foundin the first surface scan are reduced by factors of between 40 and 100 in thesecond for most elements. By the third scan most impurities are reduced belowthe detection limit of the analysis, leaving those which persist into the slice asreal impurities. Providing the first scan contains 100 ppb atomic or less, thesecond scan provides true layer impurity information down to ppb atomiclevels, particularly important when layer thickness is insufficient for multiplescans. The laser beam can be defocused at the sample surface, by moving thesample away from the focal point by a controlled amount, to give a veryeffective and quick method of surface cleaning and reducing the eroded depth.After defocusing by 0.5 mm around l um of CMT and 0.3.um of GaAs areremoved for each scan, although defocusing does not reduce the ion yield.

4. Some typical survey analyses

4.1. CMT layers

Epitaxiallayers of CMT, grown on CdTe substrates, are routinely analysedby LSMS. A typical recent analysis of a 15.um thick layer is given in Table Iand shows the concentrations of elements above the detection limit of 5 ppbatomic") (2'1014 cm "). Analyses are shown for four regions in the structure,i.e. the top surface-where contamination effects arise primarily from

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TABLE ILSMS analysis of undoped CMT layer

Element Concentration (.1015 atomscm ")

Surface Bulk Interface Substrate

C 3000 3000 3000 600 3000 30 600 0.6F 9 0.9 1.5 <0.2Na 9 0.9 1.5 6*Al 6 3 0.6 0.3Si 60 <0.9 1.5 < 1.5p 0.9 <0.2 0.3 <0.2S 150 0.6 15 0.6Cl 90 0.3 30 <0.3K 6 0.9 0.6 6*As 0.3 <0.2 0.6* <0.2Se 60 60 <0.3 <0.6* Heterogeneous distribution.

atmospheric contamination and handling after growth, the bulk or centreregion of the layer, the interface and the CdTe substrate. The surfacecontaminants were removed during the analysis and away from the surfaceregion C and 0 are the major impurities, decreasing significantlyon reachingthe substrate. Experience indicates that the concentrations of these elementsmay show quite large variations without exerting a significant effect on theHall measurements. There is evidence of high impurity levels for someelements in the region ofthe interface but the substrate has impurities at levelsbelow the detection limits in most cases. An interesting feature of LSMSanalysis of other CMT samples is the oxygen detection limit of 5 ppb atomic(2.1014 cm -3), which is extremely low for a large area analysis.A further technique development, arising from CMT studies, has been made

for the analysis of copper. Normally the two isotopic Cu+ lines are masked byTe++ lines originatingfrom the matrix element. Experimentally it has beenshown that the interference is reduced by a factor of about 104 when the Cu ++species are monitored. and the laser excitation process is carried out using alower power density, thus reducing the relative contribution of Te4+. Usingsuch conditions, the detection limit for copper is 50 ppb atomic (2.1015 cm").

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TABLE 11Comparative analyses of spiked CdTe

Element Concentration (ppb atomic)

LSMS GDMS

Li 30 5000B <1 <20Al <30 100Si <30 400S 1000 3000Cl 40 <700K 100 30Cr 5 < 10Fe 3000 8100Cu 3000 23000Ga 3 <10In 1000

I ppb atomic = 3·1013cm-3•

4.2. CdTe substrate material

GFAAS

<30

40005100

2500

The quantitative nature as well as the survey capability of LSMS is clearlydemonstrated by the results of a comparative analytical exercise using a bulkingot of CdTe, widely used as a substrate for the growth of infrared sensitiveCMT layers"). There is a range of electrically active elements which are likelyto diffuse into the active CMT layer; it is therefore important that these areabsent from the substrate material. Consequently, there is a clear need forquantitative methods which directly identify and measure impurities down to20 ppb atomic (6.1014 cm") or lower. The only real contenders for this roleare LSMS, secondary ion mass speetrometry (SIMS) and glowdischarge massspeetrometry (GDMS). Of these only LSMS and GDMS are truly surveytechniques and a comparison of these was made, with other techniques, on aspecial doped ingot of CdTe containing spike elements of Fe, Cu and In at the5000 ppb atomic level (1.1017 cm").The concentrations of the spike elements were determined initially by

graphite furnace atomic absorption (GF AAS), a method known to give excel-lent quantitative values on individual elements"), Table 11shows the valuesobtained by all three techniques on a selection of the elements determined.LSMS shows very good agreement with the known spike concentrations,

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TABLE IIILSMS analysis of MBE grown layers

Element Concentration (ppb atomic)

Layer 1 Layer 2 Substrate(Si Doped) (Undoped)

B 20 30 200F 20 30 10Na 20 30 5Al 40 50 20Si 30 7 2P 30 50 200K 30 30 3Ca 2 3 <1Fe 4 5 <1

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particularly as the levels measured were arrived at by reference to the knownmatrix composition and not calibrated standards. In particular, the measure-ment of Cu and Fe has interference problems at the singly charged mass lineand the value is derived from the response ofthe doubly charged species usinga known calibration factor. For analysing the unintentionally added back-ground impurities in the crystal the results show that LSMS can achieve therequired detection limit of 20 ppb atomic (6·1 014cm -3) or lower.

4.3. Epitaxial GaAs layers

The samples analysed were IO.um thick GaAs layers, produced by theVarian Gen Il molecular beam epitaxy equipment at PRL, on a GaAs sub-strate grown from a boron oxide encapsulated melt"). An area of around1.3 ern? of each sample was scanned successively through the layer and into thesubstrate ( erosion depth for each scan was typically 1 .urn ). The basic sensitiv-ity ofthe analysis, by combining two scans, gave an overall sensitivity of 1 ppbatomic (4.1013 cm "), The elements detected are listed in Table III and theconcentrations in the layer were measured at least 5.um below the surface.Surface impurities were effectively removed before the analysis.

Layer 1 was doped with silicon at 1.1015 cm ? (30 ppb atomic) while layer2 was undoped (carrier concentration was low 1014 cm "). Most of the im-purities found are electrically inactive and the electrical measurements agree

Laser scan mass speetrometry for impurity analysis

very well with the silicon concentrations detected by LSMS, i.e. 30 ppb atomicin layer 1 and 7 ppb atomic in layer 2.

5. Some typical quantitative analyses

5.1. Arsenic concentration profile in CMT

Experience gained from the analysis of semiconductor layers with thicknessesin excess of 1 f1m has shown that the raster scanning LSMS technique iscomplementary to SIMS. While the profile depth resolution (~1 f1m) is notcomparable with that obtained by SIMS (~ 10nm), the detection sensitivitycan often be superior for some elements. This point is illustrated by the dataobtained from comparative analyses of an arsenic-doped CMT layer, 12f1mthick. Defocused laser conditions were used for surface cleaning during theinitial scan and the remaining scans were carried out with focused beamconditions. The mean eroded depth of 5 f1m enabled two scans to be carried outat maximum analytical sensitivity before the CdTe was eroded. ComparativeLSMS and SIMS depth profiles are given in fig. 2, both techniques werecalibrated with an arsenic-doped bulk crystal. It can be seen that the LSMSdata closely follow the SIMS profile in the layer region of the sample. TheLSMS technique, however, has the advantage of showing a lower detectionlimit (5'1014 cm -3) as compared with SIMS (5'1015 cm -3).

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5.2. Other dopant elements in CMT

Important dopant elements in CMT, such as the non-metallic species iodineand phosphorus, are very difficult to measure with known chemical methodsat the dopant levels required. In this area LSMS has proved very successful,as demonstrated by the analysis of a series of iodine-doped layers with increas-ing concentration levels from 2'1015 cm? to 1'1016 cm? (ref. 4). The singlycharged mass ion, normally used for iodine determinations is masked by a linedue to a hydride ofTe. However, the doubly charged species, recorded at halfthe effective mass/charge ratio has about 10 times less sensitivity but is notobscured by any interfering species. A detection limit of 50 ppb atomic (2' 1015cm") is obtained for this analysis, a sensitivity which is not attainable by othermethods. The quantitative agreement with the electrical characteristics is good,i.e. within a factor of2, considering that the LSMS measurements rely on platecalibration factors and not calibrated standards. To quantify the concentra-tions of the dopants, densitometer measurements of the photographic platewere made on a further iodine-doped layer. The density of the minor isotopelines ofTe were used as an internal standard to compare with the mass ion lines

F. Grainger

-- LSMS --SIMS

Cd 0.2 Hg0.8 Te -----..*\ ....CdTe-

Depth (urn)

Fig. 2. Comparative analyses of an arsenic-doped layer by LSMS and SIMS.

due to the Te hydride mass lines. The difference between the ratio of the Tehydride mass lines and the known isotopic ratio of Te gives the contributiondue to the singly charged iodine ion. The measured concentration of 2· 1016

cm -3 requires a factor of 2 for agreement with electrical measurements, whichis consistent with the previous calibration for iodine and is entirely consistentwith the known spread of RSF values.

In a similar manner phosphorus doping levels of ,....,1·1017 ern"? weremeasured in a series of 4 J1.m depth scans into a CMT layer, comparing theeffective density of the 31p+ ion mass line against that of the minor isotopelines of Te. The mean concentration level was found to agree well withelectrical figures.

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6. Conclusions

Laser scan mass spectra metry has been shown to be superior to the earlierspark source procedure. In addition to the obvious advantage of being anelectrodeless technique, LSMS shows a more uniform sensitivity for differentelements. It is more reproducible and is capable of analysing conducting andinsulating materials. Variation of the laser operating conditions providesadditional flexibility not available with the spark technique.Results with GaAs and CMT samples have shown ion yields to be essentially

the same although the mean erosion depth is dependent on the materialanalysed. The stable ion yield gives detection limits of 2 ppb atomic (,...,1'1014

cm-3) with each scan for most elements. Surface impurities can be effectivelyremoved by the first scan and successive scans made over the same areaconfirm the layer impurity concentrations. A defocused laser beam provides aquick and effective means of removing surface impurities as well as allowinganalyses with a reduced scan erosion depth, although with a correspondinglyincreased detection limit.Although primarily developed as a survey technique for impurity analysis of

semiconductor layers, LSMS has been shown to give quantitative informationon selected elements ofparticular interest as dopants. In particular, the deter-mined concentrations of iodine and phosphorus dopants in CMT agree wellwith electrical measurements made on the same samples. LSMS has also beenfound to be a useful and sensitive technique for other types of sample, such asquartz tubing and bulk silicon crystals, used as source material in the growthof amorphous silicon.

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Acknowledgements

Thanks are due to David Brown for engineering the system; Alan Mills andIan Gale for computer programming, with the latter also providing the AASmeasurements; Barry Clegg for SIMS measurements. Thanks are above all dueto John Roberts without whose help and support this work would not havebeen possible. This work has been carried out with the support ofProcurementExecutive, Ministry of Defence.

REFERENCES

I) J.A.J. Jansen and A.W. Witmer, Spectrochim. Acta., 37B, 483 (1982).2) F. Grainger and J.A. Roberts, Semicond. Sci. Technol., 3, 802 (1988).3) J.B. Clegg, J.B. Mullin, K.J. Timmins, G.W., Blackmore, G.L. Everett and R.J. Snook, J.

Electron Mater., 12, 879 (1983).4) B.C. Easton, C.D. Maxey, P.A.C. Whiffin, J.A. Roberts, l.G. Gale, F. Grainger, P. Cap per,

U.S. Workshop on the Physics and Chemistry of Mercury Cadmium Telluride and Novel IRDetector Materials (1990). In J. Vac. Sci. Technol. B, 9(3), 1682 (1990).

F. Grainger

5) B.E. Dean, C.J. Johnson and F.J. Kramer, Proc. 3rd Workshop on Purification of Materialsfor Crystal Growth and Glass Processing, Orlando, FL, USA (1989).

6) F. Grainger and l.G. Gale, J. Mater. Sci., 14, 1370 (1979).

AuthorFred Grainger: B.Sc. Hons. (chemistry and physics), Birkbeck College, London University, 1964.After joining PRL in 1967,his early work was related to the analysis ofsemiconductor and othermaterials using atomic absorption, polarography, ion exchange, emission spectrography and solidsource mass spectrography. Since 1983 he has been involved in the development of laser sourcemass speetrometry (LSMS) and its exploitation as an analytical technique.

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