Application of glow discharge mass spectrometry and sputtered neutral massspectrometry to materials characterizationPaul K. Chu, John C. Huneke, and Richard J. Blattner Citation: Journal of Vacuum Science & Technology A 5, 295 (1987); doi: 10.1116/1.574148 View online: http://dx.doi.org/10.1116/1.574148 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/5/3?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Erratum: Application of glow discharge mass spectrometry and sputtered neutral mass spectrometry to materialscharacterization [J. Vac. Sci. Technol. A 5, 295 (1987)] J. Vac. Sci. Technol. A 5, 2986 (1987); 10.1116/1.574245 Glow discharge mass spectrometry for sputtering discharge diagnostics J. Vac. Sci. Technol. A 3, 625 (1985); 10.1116/1.572965 Glow discharge mass spectrometry of sputtered tantalum nitride J. Vac. Sci. Technol. 18, 324 (1981); 10.1116/1.570751 A mass spectrometric study of neutral−sputtered species in an rf glow discharge sputtering system J. Vac. Sci. Technol. 12, 151 (1975); 10.1116/1.568745 Mass Spectrometry of Ions in Glow Discharges. V. Oxygen J. Chem. Phys. 38, 1031 (1963); 10.1063/1.1733757

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Application of glow discharge mass spectrometry and sputtered neutral mass spectrometry to materials characterization

Paul K. Chu, John C. Huneke, and Richard J. Blattner Charles Evans & Associates, Redwood City, California 94063

(Received 9 June 1986; accepted 4 January 1987)

Postsputtering ionization using an inert gas plasma to decouple the sputtering and ionization processes minimizes matrix effects commonly associated with conventional secondary ion mass spectrometry. Glow discharge mass spectrometry (GDMS) utilizes ions generated in a dc inert gas plasma to sputter atoms into the ambient plasma from the surface of a cathode composed of the material to be studied. The sputtered atoms, predominantly neutral, are then ionized in the plasma by Penning and electron impact ionization. GDMS provides excellent sensitivity and signal stability, and has found wide uses for characterization of bulk materials such as metals and GaAs. The sensitivity and large sampling volume also make it ideal for determining nonuniform trace contaminants in materials, e.g., U and Th in metals. Sputtered neutral mass spectrometry uses either low-energy plasma ions (direct bombardment mode or DBM) or an independent focused ion beam (separate bombardment mode or SBM) to sputter atoms into a low-pressure high-frequency plasma for electron impact ionization. DBM offers better depth resolution than conventional surface analytical techniques and is ideal for thin-film studies. The plasma electrons compensate for sample charging in SBM and make this method ideal for the analysis of insulating material such as phosphosilicate glass.

I. INTRODUCTION

Since the advent of secondary ion mass spectrometry (SIMS), its application to materials characterization and failure analysis has been hampered by the strong variations in sputtered ion yields of different elements as well as by the large variation in ion yield for a particular element sputtered from different matrices (the SIMS "matrix effects"). Accu­rate quantitation is therefore not straightforward. 1.2 In order to provide quantitative SIMS data, several methods have been developed to compensate for these ion yield variations and matrix effects. The most common approach is to use calibration standards in which the elements of interest are incorporated into a matrix identical or very similar to the matrix of the analytical unknown. These standards can be fabricated both as bulk-doped materials and as ion im­plants. 3 Although excellent quantitative results have been reported using this analytical approach, good calibration standards are often difficult or impossible to fabricate, espe­cially for complex materials or heterogeneous multiple­layered samples.

It is well known both that the magnitude of the SIMS matrix effect is materials dependent and that usually a large fraction of the sputtered species are neutral. The ionization of these neutral particles in the gas phase would minimize or even eliminate the ion yield dependence on the matrix, while not sacrificing sensitivity to a large extent. Several different methods of achieving this postatomization ionization have been developed. Resonance ionization mass spectrometry (RIMS) employs a tunable dye laser to selectively ionize the element ofinterest.4 The ionization mechanism involves the absorption of several photons by resonant states of the atom. This type of ionization can be extremely selective and can provide high sensitivity, but the technique requires precise selection of the laser wavelength and only one element can

therefore be detected at a time. While it is advantageous to measure one element at a time in some uses, it may present a problem for accurate data quantitation in some applications because an internal reference (typically a matrix ion) cannot be measured under identical analytical conditions to ac­count for instrumental sensitivity fluctuations from sample to sample, although these instrumental variations can be minimized by proper instrument tuning and operation. Mul­tiple elemental analysis can be achieved by utilizing nonre­sonant multi photon ionization (MPI). Becker and Gillen5

have developed analytical methods based on ion beam sput­tering with an inert gas combined with MPI utilizing a high­power excimer laser. Since the efficiency of MPI is quite low (for example, the cross section of an MPI process requiring two photons is on the order of 10-47 cm4 S2), very high pow­er density laser pulses (> lOll W/cm2) are needed to pro­duce useful analytical signals. The sensitivity of the MPI technique is generally quite good because the high power density laser pulses that are available from excimer lasers are usually sufficient to ionize most neutral species. Although both RIMS and MPI are very promising techniques, the lack of commercial instrumentation at the present time hinders the development of and access to routine analysis.

An alternative means to decouple the atomization and ionization processes is to employ an inert gas plasma, and commercial instruments for glow discharge mass spectrom­etry (GDMS) and sputtered neutral mass spectrometry (SNMS) employing ionization in a plasma are now avail­able. In GDMS, a dc inert gas plasma produces ions which are attracted to and continuously sputter the cathode/sam­ple surface and also provides a medium for Penning and electron impact ionization of the sputtered neutrals. GDMS is primarily used for determination of bulk trace impurities. In SNMS, the sample can be sputtered by either low-energy inert gas ions created in a high-frequency (hf) plasma or by

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296 Chu, Huneke, and Blattner: Application of glow discharge mass spectrometry 296

an independent focused ion beam, and ionization occurs in the overlying low-pressure plasma by electron impact. Ex­cellent depth resolution and the capability to analyze insu­lating materials are several of the advantages offered by SNMS. In this paper, besides a brief review on the basic principles and instrumentation of both GDMS and SNMS, some recent applications of these two techniques to metal, semiconductor, and insulating materials are presented.

II. GLOW DISCHARGE MASS SPECTROMETRY

In glow discharge mass spectrometry, the sample consti­tutes the cathode of a dc glow discharge. The sample is main­tained at a lower voltage than the anode to sustain a contin­uous dc discharge between the electrodes in a rare gas (e.g., Ar) ambient of about 1 Torr (Fig. 1). Electrically conduct­ing samples can be analyzed directly by this technique, but insulating samples must be pelletized with a conducting binder such as silver. The sample surface is bombarded con­tinuously by inert gas ions which are created in the plasma and accelerated to the sample surface by the voltage differen­tial (~1000 eV) between the plasma (anode) and the sam­ple (cathode). Positive secondary ions are automatically at­tracted back onto the sample surface. Neutral species that are sputtered from the sample surface diffuse into the dc plasma, and ionization of sputtered neutrals takes place via two mechanisms (Fig. 2):

(1) Penning ionization: Ar* + M --. M + + e - + Ar, where * designates a metastable state, or

(2) electron impact ionization: M + e- --.M+ + 2e-. In a relatively high-pressure dc plasma (about 1 Torr)

such as the one employed in GDMS, the primary mecha­nism is believed to be Penning ionization. 6 In addition, since the pressure is relatively high, multiple collisions encoun­tered during the passage through the plasma lead to dissocia­tion of molecular species and thus reduce the amount of mo­lecular mass spectral interferences. In this pressure range, some sputtered material is redeposited onto the sample sur­face, although this should be of little significance in bulk

ANODE ION EXIT SUT

QUARTZ

GLOW DISCHARGE

FIG. I. A generic glow discharge ion source. The sample is biased electrical­ly negative with respect to the anode to attract Ar+ from the plasma for sputtering.

J. Vac. Sci. Technol. A, Vol. 5, No.3, May/Jun 1987

ELEClRON M'ACT IQIIIZA TKlN

SAMPLE

FIG. 2. Schematic of the atom­ization and ionization pro­cesses in a glow discharge.

analyses. However, deposition of sputtered materials on the interior surface of the plasma chamber can contribute signif­icantly to instrument backgrounds due to memory-type mass interferences, and the entire plasma source must there­fore be easily replaceable. Ions in the vicinity of the dis­charge chamber exit orifice are extracted into a mass spec­trometer which is usually either a quadrupole or magnetic sector instrument. While quadrupole based instruments are less expensive in general, the mass resolution attainable is not sufficient for many applications. Magnetic sector instru­ments with mass resolving power over 5000 are required in such situations.

Two of the advantages of GDMS are the absence of sub­stantial matrix effects and relatively uniform ion yields across the periodic table. Consequently, semiquantitative re­sults can be obtained without calibration standards. Table I depicts the relative ion yield factors (normalized to Fe) cal­culated from the results acquired from a pin sample of the National Bureau of Standards (NBS) 661 stainless-steel standard. Also tabulated are the relative ion yield factors calculated from SIMS results obtained by both oxygen and cesium bombardment. The variation of ion yields in GDMS is in general less than an order of magnitude for most ele­ments, much smaller than that observed in SIMS. GDMS thus provides semiquantitative results even without stan­dards.

One of the recent applications ofGDMS is the characteri­zation of semi-insulating undoped GaAs materials. GaAs crystals prepared by the modern liquid-encapsulated Czoch­ralski (LEC) method have impurities with concentration < 10 parts per 109 (ppb) by weight, and high sensitivities are required to quantitatively characterize undoped GaAs crys­tals. The conventional solid-state methods have been sec­ondary ion mass spectrometry and spark source mass spec­trometry (SSMS). In Table II, the detection limits of some of the important trace elements in GaAs using GDMS (with an uncooled discharge chamber), SSMS, and SIMS (em­ploying positive oxygen ion bombardment for electroposi­tive impurities or positive cesium ion bombardment for elec­tronegative impurities) are tabulated for comparison. The GDMS and SSMS results are calculated assuming uniform ion yields for all elements and should therefore be accurate to within ± a factor of 3, whereas the SIMS data are quanti-

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297 Chu, Huneke, and Blattner: Application of glow discharge mass spectrometry 297

TABLE I. Relative ion yields by G D MS measured on NBS661 stainless steel with 2-mA discharge current at I k V in an Ar plasma. SIMS results ob­tained using both oxygen and cesium ion bombardment and 80-eV energy discrimination are provided for comparison. Relative yields are normalized to Fe.

Element

B C Al Si P S Ti V Cr Mn Fe Co Ni Cu As Se Zr Nb Mo Ag Sn Sb Te La Ce Nd W Pb Bi

GDMS

0.64 0.16 0.62 0.96 0.19 0.19 2.43 1.90 0.52 1.76

0.84 0.43 0.18 0.12 0.17 1.55 1.41 0.97 0.22 0.39 0.19 0.20 1.17 1.43 1.16 0.75 0.28 0.17

O/SIMS

0.056 0.0004 3.3 0.18 0.011 0.0064 6.1 2.5 1.9 1.7

0.28 0.15 0.085

0.018 4.3 2.6 1.0

0.041

0.16

0.028

Cs/SIMS

14.1 0.7 0.95

111 46

810 10

1.5 0.95

1 3.7 6.6 5.7

910 17 87 0.95

12 8.9

760

2.7

tated based on ion implant standard analyses and are accu­rate to within a factor of2. It can be observed that the sensi­tivity of GDMS is in general excellent, and in some cases GDMS offers better detection limits than SSMS or SIMS.

Glow discharge mass spectrometry is well suited for the determination of impurities in metals. In silicon very large scale integrated circuit (VLSI) technology, radioactive al­pha particle emitters like 238U and 232Th existing in packag­ing materials and metallic interconnects can induce soft er­rors in dynamic random access memory (DRAM) cells. As these memory cells shrink in size to submicron dimensions, they are more prone to small amounts of alpha particles. Low U and Th contents in metals used for interconnects and gate silicides are therefore very critical because a few ppb weight of these elements may be detrimental to the operation of the device. The determination of sub-ppb U and Th con­centrations in metals is now routinely done by secondary ion mass spectrometry.7 However, since the distribution of these elements is usually nonuniform, especially at higher concen­trations, care must be taken to assure that the analyzed area is large enough to yield results that are representative of the average bulk concentrations. Figure 3 depicts the SIMS measurements of the U and Th depth profiles in an alumi­num sample. The heterogeneous distribution is confirmed by the U+ ion micrograph shown in Fig. 4. The ability to sam-

J. Vac. Sci. Technol. A, Vol. 5, No.3, May/Jun 1987

TABLE II. Typical detection limits in GaAs by GDMS, SSMS, and SIMS by both oxygen and cesium bombardment (concentrations in ppm atomic).

B C o Na Mg Al Si P S CI Ti V Cr Mn Fe Ni Co Cu Zn Ge Se Mo Cd In Sn Sb Te I Ba

0. 0.

10 5

c: 104 o

c: Q)

u c: o

(..)

10 3

GDMS

0.001

0.005 0.002 0.004 0.014 0.006 0.023 0.113 0.005 0.004 0.006 0.003 0.011 0.007 0.005 0.002 0.003 0.005 0.07 0.025 0.017 0.45 0.006 0.009 0.070 0.005 0.047

5 10

SSMS

0.003

0.003 0.003 0.003 0.1 0.003 0.5 0.004 0.006 0.003 0.003 0.003 0.003 0.006 0.003 0.004 0.01 0.01 0.006 0.01 0.01 0.005 0.01 0.006 0.006 0.003 0.03

15 20

SIMS(O)

0.0003

0.001 0.001 0.01

0.050

0.001 0.001 0.02

0.1

0.020 0.2 0.001

25

Depth (microns)

Th

U

30

SIMS(Cs)

0.05 0.1

0.002 0.05 0.025 0.03

0.005 0.03 0.01 0.0002

0.005 0.050 0.005

35 40

FIG. 3. U and Th depth profiles of an Al metal sample by SIMS employing energetic oxygen ion bombardment. The concentration scale is in units of

parts per trillion weight.

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298 Chu, Huneke, and Blattner: Application of glow discharge mass spectrometry 298

FIG. 4. U+ ion micrograph of an Al metal sample taken by a Cameca IMS-3F ion microscope employing energetic oxygen ion sputtering.

pIe a large area, as is always accomplished in GDMS, is thus a big advantage in this situation. Table III displays the SIMS and GDMS results obtained from the U and Th analyses in three Al alloy samples. The GDMS results are quantitated assuming uniform ion yields, whereas the SIMS data are quantitated based on the assumption that the relative sec­ondary ion yield is proportional to exp[ - IP] where IP is the atomic ionization potential of the element of interest. The discrepancy between the SIMS and GDMS results is mostly explained by the inaccuracy of the GDMS and SIMS data quantitation processes. By way of calibration, an Al sample analyzed for U by thermal ionization mass spectrom­etry (TIMS) has also been measured by SIMS, and the U content measured by TIMS is comparable to that calculated from the SIMS data. Although fairly good approximation can be made by assuming uniform ion yields for GDMS, empirical standards are still necessary for accurate quantita­tion.

In conclusion, GDMS offers excellent sensitivity and rela­tively uniform ionization efficiency, both of which enable semiquantitative standardless bulk trace impurity analysis. More accurate results can be obtained with the use of empiri­cal standards. Sample precleaning is usually necessary prior to data acquisition to reduce the time required for sputter etching the sample surface to achieve equilibrium analytical conditions, particularly at the ppb concentration level. The mass resolving power attainable on magnetic sector GDMS machines permits the separation of isobaric mass spectral interferences. In addition, the appropriate choice of the plas-

TABLE III. U and Th concentrations (ppm weight) measured for three aluminum alloy samples by GDMS and SIMS.

U

Sample GDMS

<1.3 2 56 3 10

SIMS

0.12 66 9

GDMS

<1.3 143

6

J. Vac. Sci. Technol. A, Vol. 5, No.3, May/Jun 1987

Th

SIMS

0.04 88 6

ELECTRON IMPACT IONIZATION

e=----..MO_M+

(RF PLASMA)

" ELECTRON IMPACT IONIZATION ~ .. ~ e~M°--+M+ ~ <;.~ (RF PLASMA)

~.,.-, (SIMS IONS) e-

~ ~ I M~ 1 ATTRACTED ~ TOWARD SAMPLE

S~~:~ ~

FIG. 5. The direct bombardment and separate bombardment operation modes of SNMS.

ma inert gas can eliminate potential mass interferences, for instance, using Xe instead of Ar to enable the determination of Ca in a specimen.

III. SPUTTERED NEUTRAL MASS SPECTROMETRY

Sputtered neutral mass spectrometry is similar to GDMS in that the technique also utilizes an inert gas plasma to ion­ize the sputtered neutral species. However, in SNMS, the rf plasma is sustained at a low pressure of 10-4 Torr. 8 As with GDMS, ion yields are relatively uniform and matrix inde­pendent, but the ionization mechanism in SNMS is believed to be mainly electron impact due to the low concentration of excited state atoms in the plasma.

There are two modes of operation for SNMS: the direct bombardment mode (DBM), which is the normal method of operation for analyzing electrically conducting or semi-

1.40,----------------------,

~ ~ l--105 A ;:; 0.56

0.28

0L--~-~-__ - ____________ ~_~_~

o 50 100 150 200 250

Sputter time (s.c)

FIG. 6. SNMS profile of W in alternating Wand Si layered structures. The bombarding Ar+ energy is 280 eV.

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299 Chu, Huneke, and Blattner: Application of glow discharge mass spectrometry 299

-1/1

C :::J

>-.. Si (92 eV) III .. -.c .. III

>--1/1 C CII - 1736 eV) x 5 C

0 20 40 60 80 100 120 140 160

Sputter time (min)

conducting samples, and the separate bombardment mode (SBM), which is used to analyze insulating samples (see Fig. 5). In bulk and thin-film analyses of conducting materi­als, the same rf plasma is used both to sputter the sample and ionize the sputtered atoms. The direct bombardment mode requires that the sample be at a more negative potential with respect to the plasma in order to accelerate inert gas ions to the sample surface. Most of the positive secondary ions emit­ted from the surface are attracted back onto the sample, and so the contribution of these ions, and consequently the SIMS matrix effects, are minimal. Certain combinations of sam­ple-plasma spacings and voltages yield a current density of 1 mAl cm2 over a 5-mm-diam spot at an energy of 300 e V. This minimizes ion beam induced mixing effects. Additionally, very little redeposition of sputtered particles onto the sample surface takes place. Hence, the depth resolution attainable by the direct bombardment mode is excellent. Figure 6 shows a tungsten depth profile obtained from a sample con­sisting of alternating layers of Wand Si (each layer pair is 105 A thick) employing 280-eV primary Ar+ ions for sput­tering. The depth profile acquired by Auger electron spec­trometry (AES) from the same sample using an argon sput­tering primary beam of 1 keY and a 5-keV electron beam is shown in Fig. 7. The depth resolution obtained by SNMS in

OJ -I: :::I ,.. ~ -:e to

,.. -

o

L 50 100 150

Sputter time (sec)

FIG. 8. SNMS profile of W in alternating Wand Si layered structure. The sputtering energy is 209 eY.

J. Vac. Sci. Technol. A, Vol. 5, No.3, May/Jun 1987

160 200

Silicon Substrate

220 240 260

FIG. 7. Auger depth profile of W and Si in alternating Wand Si layered structures. The argon bombardment energy is 1 keY.

this case is superior to that achieved by AES. Improved depth resolution can conceivably be attained with AES by reducing the ion beam energy, but the current density gener­ally decreases, thus resulting in an increased total sputtering time. In addition, focusing of a low-energy ion beam may present some technical difficulties. SNMS is therefore more attractive than AES for depth profiling over large areas when depth resolution is important.

Figure 8 depicts the W depth profile of another sputter deposited W lSi alternating structure with much thinner lay-

71

As 75

x 512 Ga

69

As 75

Ga

69

As 75

FIG. 9. Ratios of ion intensity of As/Ga obtained by SIMS and SNMS (DBM and SBM).

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300 Chu, Huneke, and Blattner: Application of glow discharge mass spectrometry 300

SNMS-SBM Analysis of NBS Glass K 1080 5r--------------------------------------,

COMPOSITION (at %l

L, 8 o M9 AI S Ga T, s, Z,

8.86 127

579 078 649

147 590 055 319 018

1~------------------------------------~ a 24 48 72 96 120 MI.

FIG. 10. Mass spectrum of NBS glass standard K 1080 by SNMS.

ers. Results by Rutherford backscattering spectrometry (RBS) indicate the total thickness of these alternating struc­tures to be 2400 A, and that the atomic ratio ofW to Si is 1 to 2.9. The thickness of each W lSi layer pair is calculated to be 32 A. Since the W to Si ratio is 1 to 2.9, the thickness of each W layer is therefore about 8 A or approximately 3 at. layers. This depth resolution has not been demonstrated employing conventional depth profile techniques such as SIMS and AES.

Like GDMS, SNMS is almost free of matrix effects, but it should be mentioned that in oxygenated or cesiated surfaces, the secondary ion yields of certain elements can approach 100%, and matrix effects can be an issue due to large reduc­tions in the number of neutrals. 9,10 However, the number of sputtered neutrals far exceeds that of secondary ions in most applications. The relatively uniform ion yields achieved in SNMS are indicated in Fig. 9, which depicts the signal inten­sity ratios of Ga to As from a GaAs specimen measured by SIMS and by SNMS (both DBM and SBM).

When the sample potential is biased slightly positive rela­tive to the plasma, Ar+ from the plasma will not be attracted to the sample, and a separate collimated ion beam of several keY energy must be employed to sputter the materiaL While the sputtered neutrals that are released are ionized in the

u

~ ~

c ~ 2 u 0; o

..J

SNMS-SBM Analysis of CdTe

1~------------------------------------~ 100 108 116 124 132 140 MI.

FIG. II. Mass spectrum of insulating CdTe by SNMS.

J. Vac. Sci. Technol. A, Vol. 5, No.3, May/Jun 1987

?fl I

c o

c Q)

" C o II

• 10

3 10

__ 1 a

'" C ::J

o II

'" C Q)

cIa

c o

1 a

c. • A I. TI Mg •• • .·sr LI

.91

zr· .B

10

Ionization Potential(eV)

1 2

o

•

,.

FIG. 12. Relative ion yields of some of the certified elements in NBS glass standard KI080 as measured by SNMS/SBM plotted vs atomic ionization potential.

adjacent rfplasma region, any sample charging created dur­ing ion beam bombardment is neutralized by electrons auto­matically attracted to the sample surface due to the positive bias. Therefore, the separate bombardment operating mode is suitable for the analysis of insulating materials. Figures 10 and 11 depict the mass spectra obtained from an NBS glass

4.0

3.0

"0

~I~ 2.0

~It

''''"Ii 1.0

1.0 2.0 3.0 4.0 5.0

Known P Concentration (at. %)

FIG. 13. P/Si signal intensity ratios by SNMS vs P/Si concentration ratios by wavelength dispersive x-ray spectrometry (WDS). where 1111' and 1 28s;

are the SNMS ion intensities for 31p and 28Si, respectively; A 2RS, is the natu­ral isotopic fraction o[28Si; andAF s, is the atomic fraction ofSi in the PSG.

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301 Chu, Huneke, and Blattner: Application of glow discharge mass spectrometry 301

standard (K1080) and an insulating CdTe specimen, re­spectively. With the exception of oxygen, the SNMS relative ion yields for the various elements in the NBS K 1080 glass standard shown in Fig. 12 are within an order of magnitude of the relative contents. The mass spectra have a relatively high background due to the linear design of the instrument which allows the detection of neutrals, photons, etc. The second-generation SNMS instrument incorporates a 90· electrostatic analyzer which should eliminate most of this background.

In silicon integrated device technology, phosphosilicate glass (PSG) is widely used as a surface passivant and as an intermetal dielectric. The properties of a PSG film are relat­ed to the concentration and in-depth distribution ofP. At the present time, SIMS employing a separate electron beam for neutralization is one of the best depth profiling techniques for PSG analysis. II However, when the PSG film thickness is larger than 2 fl, sample charging problems are very difficult to circumvent in SIMS. Because of the absence of sample charging, SNMS using SBM is therefore better for the analy­sis of thick PSG films. Figure 13 depicts the P lSi intensity ratios measured by SNMS plotted against the P lSi concen­tration ratios as measured by wavelength dispersive x-ray spectroscopy. The good linearity suggests that SNMS em­ploying SBM is an excellent technique to analyze PSG mate­rials.

In conclusion, SNMS offers relatively uniform ion yields across the periodic table and is therefore suitable for the de­termination of thin-film composition. The direct bombard­ment mode is especially useful for obtaining high depth reso­lution profiles not possible by other conventional depth profiling techniques, and the separate bombardment mode is useful for insulator analysis.

IV. CONCLUSION

In summary, GDMS and SNMS each employ an inert gas plasma for postsputtering ionization, which minimizes ma­trix effects notoriously associated with the SIMS technique and provides relatively uniform ion yields. As a bulk analysis tool, GDMS is better than conventional spark source mass

J. Vac. Sci. Technol. A, Vol. 5, No.3, May/Jun 1987

spectrometry in that the signals are very stable and repro­ducible and the detection limits for some elements are supe­rior. The GDMS technique is therefore useful in determin­ing both major and trace constituents in metals and semiconductor materials, and can yield semiquantitative re­sults even without calibration standards. In addition, be­cause of the large sampling volume, it is suitable to measure trace impurities which are inhomogeneously distributed, such as U and Th in metals. The SNMS technique exhibits excellent depth resolution when operated in the direct bom­bardment mode. Because of the relatively uniform ion yields for different elements, composition determinations on thin films and bulk materials are straightforward. Insulating ma­terials, which are difficult to measure using conventional electron beam and ion beam techniques, can be analyzed more easily using the separate bombardment mode.

ACKNOWLEDGMENTS

The authors wish to express gratitude to I. D. Ward, R. J. Bleiler, and R. Jede for providing some of the experimental results, and C. A. Evans, Jr. and D. A. Reed for valuable comments.

IH. A. Storms, K. F. Brown, andJ. D. Stein, Anal. Chern. 49, 2023 (1977). 2V. R. Deline, C. A. Evans. Jr.. and P. Williams, Appl. Phys. Lett. 33, 578 ( 1978).

ID. P. Leta and G. H. Morrison, Anal. Chern. 52, 277 (1980). "G. S. Hnrst, M. G. Payne, S. D. Kramer, and J. P. Young, Rev. :\.1od. Phys. 51, 767 (1979).

5c. H. Becker and K. T. Gillen, Anal. Chern. 56, 1671 (1984). 6J. W. Coburn and W. W. Harrison, Appl. Spectrosc. Rev. 17,95 (1981). 'J. C. Huneke, C. A. Evans, Jr., C. Wickersham, and P. Turner, Workshop on Refractory Metals and Silicides for VLSI III, San Juan Bautista, CA, 1985.

8H. Oeschner, in Fourth Proceedings of Secondary IOll Mass Spectrometry (Springer. New York, 1984), p. 291.

9},. Sander, U. Kaiser, R. Jcdc, D. Lipinsky, O. Ganschow, and A. Ben­ninghoven, J. Vac. Sci. Technol. A 3, 1946 (1985).

10M. Bernheim and G. Slodzian, J. Phys. Lett. (Paris) 38, 1.325 (1977). "l'. K. Chu and S. L. Grube, Anal. Chern. 57, 1071 (1985).

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