Evaluation of a liquid metal ion source for secondary ion mass spectrometry

Nuclear Instruments and Methods in Physics Research 218 (1983) 303-306 North-Holland, Amsterdam

EVALUATION OF A LIQUID METAL ION SOURCE SPECTROMETRY H. G N A S E R

FOR SECONDARY

303

ION MASS

* and F.G. RUDENAUER

Austrian Research Center Seibersdorf, A - 1082 Vienna, Austria

Positive and negative secondary ion emission of 23 pure elements have been studied for 10 keV In + and 10 keV 02- bombardment. In + ions were produced in a liquid metal ion source. For most of the elements investigated positive and negative secondary ion yields under In + impact are comparable to those obtained with 02+ primary ions. Admission of oxygen into the sample chamber enhances positive and negative ion intensities in a strongly element-specific manner. Depth profiles of a Ni/Cr multilayer (100 .~ single-layer thickness) using 5 keV In + primary ions show that these ions may also be applied successfully for secondary ion mass spectrometric depth profiling.

1. Introduction Secondary ion mass spectrometry (SIMS) has proven to be an extremely sensitive method for the 2- and 3-dimensional microanalysis of solids [1,2]. Basically two types of instruments for the mapping of elemental distributions have been developed: direct imaging " i o n microscopes" [3] and scanning " i o n microprobes" [4]. At the present stage of development, lateral resolution of both types of instruments is roughly of the order of 1 micron or slightly better [5]. Significant improvement of lateral resolution, without excessive loss of sensitivity, appears feasible only with the scanning ion probe principle [6]. Primary ion beams, focused to extremely small diameters, are required in this case. The ultrahigh brightness [7] and the extremely small beam focus ( < 1000 A) [8] of newly developed liquid metal (LM) ion sources make them prime candidates for submicron primary ion sources in SIMS. Experiments with a LM ion source coupled to a secondary ion mass spectrometer were successfully carried out by Prewett and Jefferies [9], who obtained a 0.5 micron diameter primary beam of Ga + and were able to record mass separated secondary ion distribution maps from microelectronic circuits. Higatsberger et al. [10] developed an indium LM ion source of the capillary type; the applicability of this source in the primary gun of a scanning ion microprobe has been demonstrated by Ri~denauer et al. [11]. This source has several attractive features: (a) reliable and simple startup and shutoff procedure; (b) no poisoning by exposure of the cold source to atmosphere; (c) high sputtering yields (useful in depth profil* Present address: lnstitut far Grenzfl~tchenforschung und Vakuumphysik, Kernforschungsanlage Jtilich, D-5170 Jtilich, Fed. Rep. Germany. 0167-5087/83/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

ing); (d) high atomic mass of the bombarding ion minimizes cascade mixing and increases depth resolution in depth profiling studies; (e) reliable operation over a large energy range (2 keV to > 10 keV) with little loss of intensity at low energies. High secondary ion yields of elements under metal ion bombardment are obviously an essential condition for the microanalytical usefulness of LM ion sources in SIMS. The aim of the present study is to provide secondary ion yield data under In + bombardment for a great number of elements and to compare these data to those obtained under commonly used O~ bombardment conditions. Also an example of a SIMS depth profile using In + primary ions is given. While the In LM ion source used in this work is not of the extreme microfocus type (5 /~m spot diameter at 10 keV and 20 nA), the data presented should not depend on primary beam spot size and therefore should allow predictions of the capabilities of a submicron source.

2. Experimental Experiments were performed on a quadrupole scanning ion microprobe, described in detail elsewhere [2]. In addition to a conventional mass separated, differentially pumped 10 keV O~- ion gun with a duoplasmatron source the instrument is fitted with a second primary gun equipped with a LM ion source (not mass separated) using In + as primary species [10,11]. Both beams can be focused to a diameter of 5 # m and raster scanned across the target. The angle of incidence in both cases is 45 ° . The pressure in the target chamber was in the lower 10 8 Torr range when either gun was in operation. A second series of experiments was per-

304

H. Gnaser, F.G. Riidenauer / Evaluation of a liquid metal ion source

oxygen jet in operation. The general trend in Sp-values is more or less similar to that found by other authors [12,13] for rare gas- and oxygen-ion bombardment. Practical sensitivities for all elements and for both primary species are higher when using the oxygen jet, with the possible exception of A1 under O~ impact. This confirms that, similar to the case of rare gas-ion bombardment, introduction of oxygen into the sample chamber strongly enhances positive secondary ion emission also under In + bombardment. It is not surprising that the yield enhancement is stronger for In + primary ions since in the case of O~ considerable yield enhancement is already effected by the surface concentration of oxygen, implanted as the primary species. In general, practical sensitivities under In + impact are higher than under 02 ~ impact when the oxygen jet is on (the only exception is Zn). This might be due to the higher sputtering yields under In bombardment. Without the oxygen jet no general trend can be established for which the primary ion species gives the higher Sp-value. However, only in a very few cases does the difference exceed a factor of 3. For negative secondary ions practical sensitivities are

formed at elevated oxygen partial pressure (5 × 10 -6 Torr). This was achieved by leaking oxygen into the sample chamber via a " j e t " close to the sample surface. The samples were nominally pure elements in the form of thin foils; a bulk purity of better than 99.9% was specified with the exception of Ti, which had a purity of 99.6%. Actual measurements were performed at a primary ion current density of about 0.3 m A / c m 2. At high oxygen partial pressure in some cases a significant increase of the secondary ion signal was observed when the current density was reduced. For these elements the current density was lowered until saturation of the ion signal was obtained.

3. Results and discussion

For positive secondary ion detection fig. 1 shows practical sensitivities Sp [defined as detected secondary ions (cps) per primary ion current (nA) and fractional concentration] of various elements as a function of their atomic number for In + and 0~- bombardment; data for both primary species are presented with and without the

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305

H. Gnaser, F.G. Riidenauer / Evaluation of a liquid metal ion source

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given in fig. 2. Some elements could not be detected as atomic ions but only as oxides and are therefore not included in fig. 2. Both with and without the oxygen jet practical sensitivities under In + bombardment are higher, which again might reflect the higher sputtering yield. But this might also be interpreted as showing that the presence of oxygen plays a less important role in the emission process of negative ions. However, as can be seen from the data under I n " impact (open and closed circles in fig. 2), even negative ion emission is enhanced in the presence of oxygen. This effect, which has already been observed for some elements [14,15], seems to be less pronounced than for positive secondary ions (cf. fig. 1). Nevertheless, the enhancement of negative ion yields is found for almost all elements (amounting to an order of magnitude for certain elements) and obviously has to be accounted for when discussing possible ionization mechanisms of sputtered particles [15,16]. Negative ion intensities as a function of the oxygen partial pressure in the sample chamber are presented in fig. 3 for some elements. These curves show features similar to those observed for positive secondary ions [14]: intensities increase first with increasing oxygen

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306

H. Gnaser, F.G. Riidenauer / Evaluation of a liquid metal ion source

eroded depth. These data together with recent results on oxygen i m p l a n t e d silicon [18] show that indium primary ions can also be used - at least in certain cases - for SIMS d e p t h profiling studies.

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pressure, while above a certain limit they start to decrease again when the pressure is further increased. In this context it should be n o t e d that Cs primary ions are k n o w n to produce very high negative ion yields [12]. C o m p a r i n g our data to those of Storms et al. [12] seems to indicate that negative ion yields u n d e r In + impact are approximately two orders of magnitude lower t h a n those u n d e r Cs + b o m b a r d m e n t . Even in the latter case, however, certain elements (e.g. Mn, Z n ) are barely detectable (cf. fig. 2 of ref. 12) as atomic ions. To provide in addition an example of the d e p t h profiling capability of the liquid metal ion source we analyzed a multilayer structure consisting of 100 A, Nia n d Cr-layers evaporated o n a silicon substrate. Fig. 4 shows the S8Ni+ intensity from such a d e p t h profile, using a 5 keV In + primary b e a m for sputter erosion. T h e m e a s u r e m e n t was done at a pressure of 10 8 Torr with a primary ion current density of 5 p , A / c m 2 in a s t a n d a r d m a n n e r with a raster scanned b e a m and electronic gating (25% linear) of the secondary ion signal. The time between actual Ni + data points is 40 s. Despite some irregularities in the last layers (especially a greater a p p a r e n t thickness) which seem to stem from the evaporation procedure, the signal could be recorded t h r o u g h o u t all 24 layers. N o primary b e a m induced artefacts are found as have been observed [17] w h e n using oxygen primary ions for sputter erosion of a similar sample. Closer inspection of the measured profile reveals a d e p t h resolution almost c o n s t a n t with

It has been shown that indium ions produced in a liquid metal ion source can be applied as primary ions in SIMS. Practical sensitivities u n d e r In + impact for all elements investigated are c o m p a r a b l e to those obtainable with the routinely used oxygen primary ions. W h e n the sample c h a m b e r is flooded with oxygen, positive a n d negative secondary ion yields are e n h a n c e d for b o t h p r i m a r y species in an e l e m e n t - d e p e n d e n t manner. The possibility of doing d e p t h profiling with In + primary ions is shown by means of a N i / C r - m u l t i l a y e r structure. T h e results presented make the In L M ion source together with its potentially very small b e a m focus a promising candidate for submicron primary ion guns of the next generation of ion microprobes.

References [1] A.J. Patkin and G.H. Morrison, Anal. Chem. 54 (1982) 2. [2] F.G. R/.idenauer and W. Steiger, Mikrochim. Acta II (1981) 375. [3] R. Castaing and G. Slodzian, J. Microsc. 1 (1962) 395. [4] H. Liebl, J. Appl. Phys. 38 (1967) 5277. [5] F.G. Rtidenauer, in: Secondary Ion Mass Spectrometry SIMS III, eds., A. Benninghoven et al. (Springer, Berlin, 1982) p. 2. [6] H. Liebl, Nucl. Instr. and Meth. 187 (1981) 143. [7] V.E. Krohn and G.R. Ringo, Appl. Phys. Lett. 27 (1975) 479. [8] R.L. Seliger, J.W. Ward, V. Wang and R.L. Kubena, Appl. Phys. Lett. 34 (1979) 310. [9] P.D. Prewett and D.K. Jefferies, Inst. Phys. Conf. Ser. 54 (1980) 316. [10] M.J. Higatsberger, P. Pollinger, H. Studnicka and F.G. Ri~denauer, in ref. 5, p. 38. [11] F.G. Rtidenauer, P. Pollinger, H. Studnicka, H. Gnaser, W. Steiger and M.J. Higatsberger, in ref. 5, p. 43. [12] H. Storms, K.F. Brown and J.D. Stein, Anal. Chem. 49 (1977) 2023. [13] K. Wittmaack, Surf. Sci. 53 (1975) 626. [14] P. Williams and C.A. Evans Jr., Surf. Sci. 78 (1978) 324. [15] K. Wittmaack, Surf. Sci. 112 (1981) 168. [16] P. Williams, Appl. Surf. Sci. 13 (1982) 241. [17] W. Reuter, presented at the 9th Int. Mass Spectr. Conf., Vienna 1982. [18] H. Gnaser, Nucl. Instr. and Meth., in press.