1035
Anal. Chem. 1993, 85, 1035-1037
Reduction of Secondary Ion Mass Spectrometry Matrix Effect for High Dose Chromium and Cobalt Implanted Silicon Chunsheng Tian' and Gerhard Stingeder Institut fur Analytische Chemie, Technische Universitat Wien, Getreidemarkt 91151, A-1060 Wien, Austria
Henning Bubert Institut fur Spektrochemie und angewandte Spektroskopie, Bunsen-Kirchhoff-Strasse 11,D-4600 Dortmund 1, F.R.G.
Thb paper reports a SIMS matrix effect on ion yields caused by a high level (up to 15 atom %) of Cr and Co implantedinto the rillcon matrix. The dependence of %r+ ion yield on Cr concentratkn In the Si matrlx is investigated by recording energy spectra of sputtereds2Cr+ions. The results show that the matrix effect influences mainly iow-energy ions. It can be eliminated wlthln the analytical accuracy if the secondary ion energy b hlghor than 60-100 eV. Although tendivlty is lost by - 2 orders of magnitude, a detection limn of 1 ppm atomlc ( lo4 atomic fractlon) and detailed depth profiles are obtained for wch samples. The rewits are very rhnilar to RBS but provide more detailed information. They show that an energy-fliterlngtechnique can be usedto eff ectiveiy reduce or eiknlnate this matrix effect and quantitatlve depth profiles of metal rliicide specimens wHh a high dynamic range can be obtained by SIMS.
INTRODUCTION In recent progress in VLSI technology, an ion beam synthesis (IBS)technique has been extended to the formation of buried silicide layers.'-3 Such layers, by virtue of their low resistivities ( 10p a cm),have obviousapplications as buried or surface interconnects and have the potential of being used as buried collectors or bases in bipolar techn010gy.4~5 This technique includes a high-dose implantation followed by annealing at a high temperature. In order to understand the formation mechanism of silicide layers, detailed depth distributions are required for as-implanted profiles and the resulting layers. The concentration distributions of the silicides and implanted metals cover several orders of magnitude (percenttonanograma per gram). Conventionally, depth profiling for such high-concentration specimens (up to 10-20 atom % in our case) is preferably done by RBS or AES. Because of their relatively high detection limits, only a small part of the profile can be recorded with detailed information. In the analysis of Cr., Co-, and Ni-implanted profiles in Si, we obtained detection limits of approximately 5 X 1W and 5 X 103 (atomic fraction) for RBS and AES,respectively. In addition, the concentrations at the peaks of the profiles determined by AES using the cW(E)/dEmodel and elemental sensitivity factors show an error of up to 55% relative, compared with RBS results. It is suggested that this is due
-
(l)Stephens, K. G.; Reeson, K. J.; Sedy, B. J.; Gwilliam, R. M.; Hemment, P. L. F. Nucl. Instrum. Methods Phys. Res. 1990, B50,368. (2)Hull, R.; White, A. E.; Short, K. T.; Bonar, J. M. J . Appl. Phys. 1990,68, 1629. (3)Celler, G. K.;White, A. E. MRS Bull. 1992, 17 (6),40. (4)Bourret, A. Microscopy of Semiconducting Materials; Gullis, A. G., August-, P. A,, Eds. Inst. Phys. Conj. Ser. 1987, No. 87, 39. (5)Hensel, J. C.;Levi, A. F. J.; Tung, R. T.; Gibson, J. M. Appl. Phys. Lett. 1985, 41, 151. 0003-2700/93/0365-1035$04.00/0
to a matrix effect caused by the change of the chemical state of the specimensas a result of the formation of metal silicides. More details concerning the conversion of Auger peak-topeak height (APPH) into mole fractions are presented in ref 6. It is well-known that SIMS is characterized by high sensitivity and excellent depth resolution. However, the quantification of SIMS signals is complicated by the chemical matrix effect? Thus, usually the matrix composition should be constant and the concentration level of doped element should be lower than 1atom % At higher concentrations, the matrix composition is modified by the doped element; ion and/or sputter yields may become dependent on the concentration of the dopant and have to be corrected. Depth profiling for silicide layers by SIMS has been reported (e.g., in ref 81,but the matrix effect was not taken into account. The matrix effect, in general, is reported to be caused by the presence of reactive speciesgJ0 such as 0, C1, F, and Cs on the surface or by the variation of matrix composition.11-13 This matrix dependence of ion yield cannot be accurately described by existing models of ion emission by ion bombardment of solid surfaces. However, energy spectra of secondary ions can provide some useful information about the ion formation process and for practical chemical analysis. It has been found by Slodzian and Hennequin14 that the oxygen-induced increase in secondary ion yield is most pronounced at low-energy ions. In the study of secondary ion emission from binary alloy systems, Yu and Reuter15 noted that the ion yield enhancement of Ni+ by 02 adsorption is only observable on ions with energies less than 40 eV, whereas the high-energyions are not affected by the oxidation process. In quantitative analysis of geological and glass specimens, Ray and Hart16 and EngstrBm et al.17 have reported that at
-
.
(6)Bubert, H.; Palmetahofer, L.; Stingeder, G.; Wielunski, M. Anal. Chem. 1991,63, 1562. (7)Wilson, R. G.; Stevie, F. A.; Magee, C. W. Secondary Ion Mass Spectrometry: A Practical Handbook for Depth Profiling and Bulk Impurity Analysis; John Wiley & Sons: New York, 1989. (8)Van Ommen, A. H.; Bulle-Lieuwma, C. W. T.; Ottenheim, J. J. M.; Theunissen, A. M. T. J. Appl. Phys. 1990, 67, 1767. (9)Yu, M. L. Nucl. Instrum. Methods Phys. Res. 1986, B15,151. (10)Lareau, R. T.;Williams, P. In SIMS V; Bennighoven, A., Colton, R. J., Simons, D. S., Werner, H. W., Eds.; Springer-Verlag: Berlin, 1986, p 149. (11)Richter, C.-E.; Trapp, M.; Gericke, M. Vacuum 1990, 40, 499. (12)Gao, Y.; Godefroy, S.; Mircea, A. In SZMS VI; Bennighoven, A., Huber, A. M., Werner, H. W., Eds.; John Wiley & Sons: New York, 1987, p 761. (13)Gauneau, M.; Chaplain, R.; Salvi, M.; Duhamel, N. In SIMS VI; Bennighoven, A., Huber, A. M., Werner, H. W., Eds.; John Wiley & Sons: New York, 1987;p 295. (14)Slodzian, G.; Hennequin, J. F. C.R. Acad. Sci. Paris 1966,263& 1246. (15)Yu, L. M.; Reuter, W. J. Appl. Phys. 1981,52, 1489. (16)Ray, G.; Hart, R. S. Znt. J . Mass Spectrom. Ion Phys. 1982,44, 231. (17)Engstr6m,E.V.;Lcddmg, A.;Odeliis, H.;SMervall,V.Mikrochim. Acta 1987, 1,387. 0 1993 American Chemical Society
1086
ANALYTICAL CHEMISTRY, VOL. 65, NO. 8, APRIL 15, 1993
the high-energy component the secondary ion yields of both matrix and trace elements show less surface chemical effects. By using only the higher energy ions, the measurement reproducibility is significantly improved. In addition, the variation of ion yields due to the change of matrix composition for the GaAs/Gal,AlxAs matrix has been investigated intensively.11-13 The ion yield of trace element FeI3 varies -1 order of magnitude as the Al component changes from x = 0 to x = 0.6. The energy distributions of Fe+ show that the increase of Fe+ion yield with Al content is only apparent for low-energy ions. At energies larger than 80 eV, Fe+ ion yield becomes independent on the Al concentration. The results mentioned above indicate that ion yield is very sensitive to matrix effects for low-energy ions which are characterized by an energy range of a few tens of electronvolts. Energy filtering (rejecting low-energy ions) can therefore be used to reduce or eliminate the effect of matrix dependence of ion yields. This of course causes a loss of the intensity (by a factor of 50-1001, but in many cases energy filtering has to be applied anyway to reduce molecular ion interferences. This paper reporta a matrix effect on ion yields caused by a high level (up to 15 atom %) of Cr and Co implanted into a silicon matrix. The dependence of Cr ion yield on Cr concentration in the Si matrix is investigated by recording energy spectra of sputtered 52Cr+ions. The results show that the matrix effect influences mainly low-energy ions. It can be eliminated within the analytical accuracy if the secondary ion energy is higher than 60-100 eV. Although sensitivity is lost by -2 orders of magnitude, a detection limit of 1ppm atomic (10-8 atomic fraction) and detailed depth profiles are obtained for such samples. The results are very similar to RBSB but provide more detailed information.
EXPERIMENTAL SECTION A Cameca IMS-3f ion microscope was used in this study. To measure theenergy spectraof s2Cr+ions at various concentrations of Cr, 52Cr+ ions were implanted into a (100)n-type Si wafer with an energy of 300 keV and a fluence of 1.44 X lo1' ions cm-2. 02+primary ions with an impact energy of 5.5 keVand an incident angle of 4 2 O with respect to surface normal were used. A constant primary ion current of 250 nA was rastered over an area of 400 X 400 pm2. Secondary ions of 5zCr+were detected from an area of 25 pm in diameter. The sputtering position was aligned at the center of the hole (6 mm in diameter) of the specimen holder mask to avoid the field distortion near the hole edge.ls The ion energy spectra were recorded with an energy window of 2-3 eV. The focusing conditions of the secondary ion column were kept optimized and constant throughout all energy spectrum measurements. The energy spectra were recorded sequentially through the Cr implantation profile, which is shown in Figure 1. As the sputtering was in progress, the energy spectra were recorded sequentially with a time interval of 5 min. The time consumed for a spectrum was 1 min. Each energy spectrum correspondsto a given intervalof Cr concentration and toa depth of 4 nm. The totaldepth of sputtered crater was determined by means of surface profilometer (Sloan-Dektak I1 A). The concentrationprofiles were obtained by SIMS(usingenergy filtering) and were confirmed by RBS as reported elsewherea6 The Gaussian-likedistributionof the Cr profile resulted in a distorted shape for the energy spectra. The change of Cr concentration during each measurement was approximatelylinear (in the depth intervalof 4 nm). The resulting concentrationgradient was used to correct the raw energy distributions. The useful yields for Cr and Co, defined as the integrated secondary ions detected divided by the implanted dose, were determined by using implants with doses of 4.5 X 1015, 1.44 X 10I6, 4.5 X and 1.44 X lo1' cm-2. Here, the width of the energy window was 100 eV. It was set from 0 to 100 eV and 60 to 160 eV by tuning the sample voltage. (18)Lau, W.M. Nucl. Instrum. Methods Phys. Res. 1986, B16, 41.
0
50
100
150
0
Cycle Figuro 1. SIMS depth proflle of Cr In Si Implanted wlth a Ruence of 1.44 X 1 0 1 7 c m 2 a n d a n e ~ o 300keVatdmsnnrtemrgywindows. f The energy values followlng the element symbols denote the lowest energy limit. The width of the window is -100 eV. 1E+1
1-
1.55E20
& .
.-v)
1E+O
C
a
-c U .--wa 4-
1E-1
1E-2
E
z"
1E-3
1E-4
I 0
20
40
60
80
100
120
Energy (ev) Flguro 2. Energy dlstrlbutlons of 62Cr+ recorded at different bcal Cr concentrations (atoms cm3) on (100) SI Implanted wlth a flwnce of 1.44X 1017cm-2andanenergyof300keV.TheIntensHyisnonnaked to the high-energy tail.
RESULTS AND DISCUSSION Normalized energy distributions of Cr ions, corresponding to different concentrations, are shown in Figure 2. It is evident that, with increase of the Cr Concentration, the peak of the energy spectrum becomes broader. The parameter full-width a t halfmaximum (fwhm)describing this behavior is presented in Figure 3. It is important to point out that the curves shown in Figure 2 do not represent the true distributions, because they are distorted by ion optic effects.19 Each distribution is actually the product of the ionizationprobability, the sputter yield, and the transmiasion of the ion analyzer, kept constant in this experiment. The dependence of the sputter yield on Cr concentration does not show a visible effect within the error of the depth measurement.6 Consequently, it can be considered that the variation of the energy spectra with Cr concentration is due to the change of ionization probability. With increasing Cr concentration the ion yields of 62Cr+ions are suppressed, while in contrast, the ion yields of mSi+ are enhanced, which is demonstrated by the increase in the %it signal with Cr concentration, as shown in Figure 1. The ion yield of 3oSi+with an energy of >lo0 eV is nearly constant. (19)Blaise, G.; Slodzian, G . Reu. Phys. Appl. 1973,8, 105.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 8, APRIL 15, 1993 1097 1E+04
2 ,
I20
8E+03
1,5
-2
6E+03
w-
3 -0 .-It]a
4E+03
E
2E+03
0
D
a,
I
........ .... ....
-_-_---
".e-...
0,5
.
-_-_
.....................................
p
-
;,---..-.-y.c..o
-
z0 2E+21
0
4E+21
6E+21
8E+21
1E+22
Cr Concentration (atoms cm-3) Flgurr 3. Secondary ion intensity (solid clrcles) detected at an ion energy of 100 eV vs h i concentrations (atoms ~ m - and ~ ) fwhm (open circles) of the energy distrlbutlons vs local concentrations. The concentrations are determined by RBS and SIMS.
Furthermore, Figure 2 shows that the ion yield dependence on Cr concentration is only pronounced for low-energy ions, specifically,up to -60 eV. All the high-energy tails have the same shape, no matter how high the concentrations. These features can be qualitatively understood by Gries' model20 based on cascade collision. According to this model, lowenergy ions are more sensitiveto surface properties, especially to the surface binding energy. High-energy ions leave the surface quickly and have a low interaction probability. This implies that one can also use energy filtering to reject lowenergy ions and to perform quantitative analysis for highlevel doped samples by SIMS. This is strongly confirmed by the relation of the ion intensity at 100 eV vs concentration, presented in Figure 3. The linear relation corresponds to an ion yield being independent of doping concentration. The conclusions suggested above are supported by Figure 4, which exhibits the relation of the normalized useful ion yield for different energy windows with respect to implanted doses for both Cr and Co. All results are normalized to the lowest implant dose of 4.5 X 1015 cm-2. The ions recorded with energies from 60 to 160 eV have a constant normalized useful ion yield, indicating that the ion yield for the highenergy ions does not vary with elemental concentration. The border of 60 eV for the matrix effect is consistent with the result from energy distribution measurements and similar to the results in refs 11 and 13. From the figure, one can see that the useful yield without energy filtering decreases for a Cr dose of >4.5 X 1016 cm-2 and for a Co dose of >2 X 1016 om-2. At the highest dose, 1.44 X 1017 cm-2,both of them are -30% lower than those at low doses. This shows that the concentration level which cawes a matrix effect varies from element to element. ~~
~
(20) Griee, W.H.Int.
J. Mass Spectrorn. Ion Phys. 1975, 17, 77.
0
'
~
'
1
'
~
'
'
'
'
'
1
CONCLUSION The matrix effect on ion yield due to a high level of implanted Cr or Co in silicon has been studied experimentally. The results show that the ion yield due to this matrix effect is dependent on the impurity concentration and on the elements as well. This effect only acts on secondary ions with low emission energies. At high energies (>60 eV), the ion yield of the secondary atomic species is constant. The intensity from the high-energy portion is linear with respect to the doped element concentration. It proves that an energyfiltering technique can be used to effectively reduce or eliminate this matrix effect caused by a high level of dopant concentration and quantitative depth profiles with a high dynamic range of metal silicide specimens can be obtained by SIMS. Thenatureofthismatrixeffect on theionformation mechanism is still under investigation.
ACKNOWLEDGMENT The authors express their thanks to Mr. K. Piplits for his contribution to this experiment. This work has partly been supported financially by the Austrian National Bank, by the Fonds zur Fijrderung der wissenschaftlichen Forschung (Project P7495),by the Gesellschaft filr Mikroelektronik,and by the Ministerium fiir Wissenschaft und Forschung des Landes Nordrhein-Westfalen and the Bundesministeriumfilr Forschungund Technologieder Bundesrepublik Deutechland.
RECEIVED for review August 18, 1992. Accepted January 11, 1993.
'
~
'
