Study of the sputtering process with Rutherford backscattering

NUCLEAR INSTRUMENTS AND METHODS 137 (1976) 553-557; © NORTH-HOLLAND PUBLISHING CO.

STUDY OF T H E S P U T T E R I N G P R O C E S S W I T H R U T H E R F O R D BACKSCATTERING H. KR,~UTLE* Max-Planck-lnstitut fiir Kernphysik, Heidelberg, W. Germany

Received 28 June 1976 The amount of sputtered materials of Au, AI and Si targets bombarded with keV AI, Au and Ge ions has been measured. The backscattering technique allows to determine the various numbers of atoms of the target and of the implanted atoms. Together with the information of the concentration in depth of the different atomic species, measured with the backscattering method or with the secondary ion microscope, the influence of the different ions, the target substrates and the change in composition of the target on the sputtered material is shown and compared with calculations.

1. Introduction The backscattering technique allows to measure the mass and the number of atoms of thin layers quantitativelyl, 2). In addition, this method gives information about the depth distribution of atoms with different mass. To avoid the overlap of the spectra of the different atomic species, the mass of the target atoms and implanted ions have to be quite different. With the depth information about the different atoms, a detailed picture of the implantation, or from another point of view, sputtering process can be obtained. Backscattering data compared with calculations, which simulate the sputtering process3), allow to study the dependence of the sputtering coefficients on the target composition, depending on dose, range and straggling of the incident ions. A1, Au and Ge are used as bombarding ions and A1, Au and Si as targets. They have the advantage of a very low respectively high sputtering behaviour. The backing for the films is low-Z material, such as Be in the present experiments. It allows even to check the amount of oxygen and carbon in the evaporated layers. The thickness of the Au films is chosen so thin (2000 ,~) and the energy of the analysing He beam so high (2 MeV), that the He ions backscattered from A1 atoms at the surface have lower energy than He ions backscattered from the rear of the Au film. That means that the number of A1/cm 2 and Au/cm 2 can be calculated from the integral of the completely separated signals4). For thick targets like Si, markers can be used, which locate a well defined layer inside the target. The shift of the marker indicates the change of the number of atoms/cm 2 on top of this layer. It allows, after sub* Present address: Max-Planck-Institut for Radioastronomie, 53 Bonn, W. Germany.

tracting the contribution of the implanted ions, the calculation of the loss of the original target atoms.

2. Experiments Layers of few thousand A are evaporated on Be targets with a standard hv evaporation system. The thickness of the layers is continuously controlled with a quartz oscillator microbalance. The Rutherford backscattering technique is used for thickness and purity measurements. The Si wafers are chemically polished and cleaned ultrasonically in detergents and rinsed with deionized water. Au, AI and Ge ions of an energy between 15 and 50 keV, and currents of few flA/cm 2 are implanted into the Au and A1 films and into the Si wafers, using the implantation system of Thomson CSF. The procedure of homogeneous implantations, which allows to implant various targets at the same time, is described elsewhereS). The targets are investigated with the 3 MeV Van de Graaff at the CNRS Strasbourg before and after implantation. The current of the 2 MeV He ions is lower than 30 nA. The energy of the backscattered He ions is measured with a solid state surface barrier detector with a resolution of 15 keV for 5 MeV a-particles. With an electron suppressor mounted before the target, the dose can be measured very accurately. The height of the spectrum of the backing material and the signal from the evaporated film is used to check the dose of the He ions. Measuring different arrays of the target the homogeneity of the layers is checked. It is found that each target was very uniform. The variation in thickness of different evaporations is eliminated by measuring the thickness of each sample before and after implantation. To eliminate systematic errors of different measurements, half

554

H. KRAUTLE

of the target is shielded during implantation, so that the evaporated layer before and after implantation can be measured under identical experimental conditions. As mentioned before, the loss of atoms from thick targets can be measured with the backscattering technique by marking a layer inside the target, which cannot be influenced by the implantation process. For this purpose A u ions o f 60 keV (Emax o f the implantation system) are implanted. Rutherford backscattering is very sensitive for atoms with high Z. Therefore the dose can be selected so low (10~4-10 ~5 Au/cm z) that the implanted material does not change the composition o f the target significantly. To see what happens to a layer near the surface, a second marker (20 keV Au) is implanted. The depth distribution o f high dose A u implants is measured with 250 keV He ions backscattered under 127 ° and analysed with a 90 ° electrostatic analyser I

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(ESA)O, 7). The accuracy o f the measurements depends on the energy straggling of the backscattered He ions 8) and on the overlap of the spectra of He ions backscattered from atoms with different mass in different depths. The depth distribution of complicated systems is measured with the secondary ion microscope ( C A M E C A ) at the Institut fiir Angewandte Physik at Tfibingen (see re(. 9). The sputtering gas is oxygen, and the ion beam is scanned to get a better depth resolution. The depth scale is obtained by measuring the thickness of the sputtered layer interferometrically. 3. Results and discussion

Fig. 1 shows the loss o f the target atoms and the accumulation of the implanted atoms. Targets bombarded with the same atomic species do not change their composition, and the loss is linear with dose. Au targets show a high sputtering coefficient, S = 50, by A u bombardment, and the AI self-sputtering coefficient is practically zero (see also re(. 10). The energy of the bombarding ions is 50 keV. The influence of different target material is investigated by bombarding Au films with A1 ions and A1 films and Si wafers with A u ions. These systems are interesting, because the range o f the 50 keV ions are of the same order but the self-sputtering coefficients for Au and A1 (or Si) are very different. Therefore, the change in composition of the target caused by the implanted ions results in a nonlinearity of the sput-

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tering rate. Most of the theories and experimental data give averaged sputtering coefficients disregarding the influence and the change in concentration of the implanted material. Calculations, described in a previous paper3), which take into consideration the different ,,;puttering rates S1 for the pure target and $2 for the implanted material (inserted in fig. 1 as S = S1; $2), describe the nonlinearity of the sputtering rate reasonably well. This can be understood by following the idea of the multiple collision process11). The sputlering coefficient S depends on the nucleon-nucleon ('ross section, the different mass influencing the moment Iransfer and the sublimation energy of the surface atoms. The difference of S for Au ions sputtering AI atoms and A1 ions sputtering Au atoms is due to the differential cross section in the laboratory frame, which allows a direct backward scattering for A1 ions. The change of concentration of the implanted ions influences S via the changing averaged cross section and energy transfer. Backscattering spectra of a thick Si crystal implanted with two Au markers (20 and 60 keV) before and after the implantation of 2 x 1017 A1/cm 2 of 15 keV are shown in fig. 2a. Unfortunately the straggling is greater than expected by the LSS theory 12), so that the markers cannot be separated. But the shift of the shoulder of the Au signal to lower energies and the flattening on the high energy side indicate an increase o r A l atoms in the layer in front of the Au marker. The stopping powers of Si and A1 are practically the same, so that from the shift of the low energy shoulder of the Au spectra the increase of the implanted layer can be calculated easily. The sputtering coefficient S = 0 . 5 , derived from this experiment is in agreement with previous measurements~3). The amount of A1 in Si cannot be measured directly with the backscattering technique, because the spectra of A1 and Si overlap, depending on the similar mass. Only a small shift backward and a small maximum between 0.55 MeV and 0.6 MeV in the energy spectrum, indicate the presence of a high concentration of A1 in Si. The spectrum of Ge implanted into a Si target with the Au marker can be seen in fig. 2b. The higher mass of Ge relative to Si separates the backscattering spectra of Si, Ge and Au completely. Also in this case the Au marker changes its shape. The high energy part has flattened, the total number of Au decreases and the rear shoulder shifts slightly to lower energies. This is not due to the increasing number of atoms in the top layer, but on the higher energy loss of the He ions in Ge than in Si 2). The number of the implanted Ge atoms is derived from the area of the Ge spectrum

PROCESS

555

compared with the height of the Si spectrum4). The information about the concentration profile of the implanted Ge ions is as poor as in the case of the Au signal; it is limited by the energy resolution of the detector. Low energy He backscattering spectra which show a high depth resolution of the 50 keV Au implants in A1 films with doses of 1016 and 4 x 1016 ions/cm z, can be seen in fig. 3. Knowing the energy loss of low energy He ions in A1 and Au 2), a depth scale can easily be given. The energy of the analysing He beam is 250 keV. Unfortunately the targets could not be measured immediately after implantation and are stored for several weeks, so that they are oxidized, especially the high dose implant, which causes a small shift of the depth profile of about 50 ~, being, however, within the accuracy of these measurements. The dashed and drawn curves show the results of the calculations with the parameters for SI and $2 given in fig. 1. For the calculations the range and straggling are assumed to be 50% greater than theoretical values ~4). With these modified parameters good agreement with the experimental profiles is obtained. In low energy d + or He + backscattering experiments, the high resolution electrostatic analyser cannot separate the Si, Ge and Au signal for the system shown in fig. 2. Therefore the depth distribution of the

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556

H. KRAUTLE

implanted and target material is obtained from measurements with the secondary ion microscope. This method is insensitive for Au but sensitive for B, which, therefore, is used as a marker. The concentration of the marker and the implanted ions is measured under identical experimental conditions on the same target. Fig. 4a shows the depth distribution of B implanted with an energy of 5 keV before and after A1 implantation of 2 × ]017 ions/cm 2 of 15 keV. The finite distribution of the B atoms has the advantage to mark a well defined Si layer inside the infinite target. Now by weighing each layer of the Si target with the measured B concentration before AI implantation (dash-dotted line), the B distribution after AI implantation can be I

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calculated (thick dashed line). This means that the concentration of the B atoms decreases simultaneously with the number of Si atoms. Assuming a sputtering coefficient of S = 0.5 good agreement with the measured B distribution (thick drawn line) can be obtained. Also the experimental AI distribution (thin drawn line) agrees well with the calculated profile (thin dashed line). Fig. 4b shows a similar system. The marker is an implanted B profile of 8 and 30 keV ions (dashdotted line). The shift of the 30 keV B peak as a function of the Ge implantation (thick drawn line) indicates the ablation of atoms, similar to the backscattering measurements with Au markers (see fig. 2b). The absence of a shift of the B distribution at a depth of 2000 A indicates a sputtering coefficient of S = 1 + 0.4 for Ge bombarding Si within this dose range. Similar to the AI implant, the Ge implant changes the distribution of the B atoms near the surface which indicates the decrease of the Si concentration. The drawn and dashed thick lines show the experimental and calculated distributions after Ge implantation. The sputtering coefficients for the calculation are S I = 0.8 (Si sputtered by 35 keV Ge) and $ 2 = 4 (for Ge sputtered by Ge ions).

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With the backscattering technique small changes of solid films and thick targets during ion bombardment are measured. It is a nondestructive method to determine quantitatively the number of target and implanted atoms with high accuracy, depending on the mass of the atoms. The same measurements show also the concentration profiles of the target compounds. Especially for systems with atoms of very different mass, the depth information is obtained with low energy backscattering combined with a high resolution detector (ESA). For other systems the concentration profiles are measured with the secondary ion microscope. Thick targets can be examined like thin films by marking a buried layer of the target as a reference layer with implanted ions. The shift and change in concentration of the marker atoms in the surface layer of thick targets show directly the loss of target atoms and the accumulation of implanted ions. Generally it can be seen that the sputtering coefficient S for materials with large S, e.g. Au, decreases rapidly with the concentration of material with a lower sputtering coefficient and vica versaf°). Also the sputtering coeffÉcient of the implanted material is different from the self-sputtering coefficient, depending on the material in which it is implanted. The sputtering

THE SPUTTERING PROCESS coefficient S1 for the pure target is determined by c o m p a r i s o n of calculations with accurate data of the sputtered n u m b e r of each atomic species of the target as a f u n c t i o n of dose c o m b i n e d with c o m p o s i t i o n determination. So the influence of the implanted material to the sputtering coefficient can be eliminated. The a u t h o r wants to t h a n k Dr. Siffert's group at the C N R S at Strasbourg for assistance in p e r f o r m i n g the backscattering experiments. The interest of Dr. Kalbitzer in this work a n d the help of the backscattering group in taking the ESA spectra is greatly acknowledged. The a u t h o r is t h a n k f u l to Dr. Prager for measuring the SIMS spectra at the Institut ffir Angewandte Physik in TiJbingen. The a u t h o r is also grateful for the technical assistance of Mrs. M. Stumpfi and E. Dohm. References ~) M.-A. Nicolet, J. W. Mayer and I. V. Mitchell, Science 177, (1972) 841.

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2) j. F. Ziegler and W. K. Chu, IBM Research RC4288, Yorktown Heights, N.Y. 10598, U.S.A. 3) H. Kr/~utle, Nucl. Instr. and Meth. 134 (1976) 167. 4) H. Kr~.utle, Rad. Effects 24 (1975) 255. 2) H. Kr/iutle, A. Feuerstein, H. Grahmann, S. Kalbitzer, F. Hasselbach and M. Prager, Ion implantation in semiconductors (ed. S. Namba; Plenum Press, New York, 1974) p. 585. 6) A. van Wijngaarden, B. Miremadi and W. E. Baylis, Can. J. Phys. 49 (1971) 2440. 7) A. Feuerstein, Dissertation (Heidelberg, 1975). 8) W. K. Chu and J. W. Mayer, Catania Working Data, private communication. 0) K. H. Gaukler, Quantitative analysis with electron microprobes and secondary ion mass spectrometry (ed. E. Preuss; Jill. Conf. vol. 8, 1973) 19. 279. 1o) O. Alm6n and G. Bruce, Nucl. Instr. and Meth. 11 (1961) 279. 11) p. Sigmund, Phys. Rev. 184 (1969) 383. 12) L. Lindhardt, M. Scharff and H. E. Schiott, Kgl. Dan. Vid. Selsk. Mat. Fys. Medd. 33, no. 14 (1963). ~3) H. Krautle and S. Kalbitzer, Ion implantation in semiconductors and other materials (ed. B. L. Crowder; Plenum Press, New York, 1972) 19. 585. 14) W. S. Johnson and Y. F. Gibbons, Projected range statistics in semiconductors, distr, by Stanford Univ. Bookstore (1964).