RBS AND ERD ANALYSIS IN MATERIALS OF THIN FILMS

Transcription

1 Philips J. Res. 47 (1993) RBS AND ERD ANALYSIS IN MATERIALS OF THIN FILMS RESEARCH by DOEKE J. OOSTRA Philips Research Laboratories, P.O. Box 80000,5600 JA Eindhoven, The Netherlands Abstract Rutherford backscattering speetrometry (RBS) is a well-established technique in thin film research. By detection of energetic ions elastically scattered from nuclei, both the number of atoms of an element present in a sample and the elementary composition are determined. The technique yields the amount of atoms present quantitatively without the need for any calibration. Furthermore, the crystalline perfection of samples can be investigated. Information is obtained from the surface down to a depth of approximately 1.51lm without the need of sputtering. This makes the technique non-destructive and fast and thus very appropriate for application in thin film research. Thin film reactions can easily be followed in situ by cycles of annealing and analyses of samples. Hydrogen contents in samples can be determined byelastic recoil detection (ERD) of hydrogen recoiled out of the sample. The combination of ERD and RBS in one analysis chamber is a powerful tool for analysis of thin films. Examples show that RBS and ERD are indispensable tools in all materials research in which surface or thin film layers are involved. Keywords: composition, defects, elastic recoil detection, epitaxy, impurities, ion beam analysis, Rutherford backscattering spectrometry, thin films. 1. Basic principles. In the technique of Rutherford backscattering speetrometry (RBS) a beam of energetic ions is directed on to a target and the energy and quantity of ions backscattered from the nuclei are determined"), To make the technique quantitative it is required that fully elastic Coulomb scattering events take place with the atomic nuclei. Only then can the (Rutherford) cross-section for scattering into a known geometry be calculated. This condition is fulfilled for high energetic ions; typically 2 MeV He+ ions are used. The energy of the backscattered He+ ions is generally measured with a surface barrier detector, Philips Journal of Research Vol.47 Nos. ~

2 DJ.Oostra A12~ _ 2MeV He ~ --, Det YBa2Cu30x 16.0 o ~ 27.0 AI ~ Ba ~ energy (MeV) - Fig. I. RBS spectrum of YBa 2 Cu 3 0, on A The surface energy positions of the relevant elements are indicated by arrows. The corresponding masses are given in atomic mass units. positioned at a scattering angle of typically (lo to the incoming beam). The pulse generated in the detector is analysed by a multi-channel analyser. By measuring sets of samples with known masses or using radioactive sources a linear conversion from the channel scale to the He energy scale is effected. The energy of the backscattered He particles is directly related to the mass of the target atom at which the scattering event took place by the laws of conservation of momentum and energy, as follows: J(M: - M~sin 2e)+ Mpcos ()J2 E, = KEo = [ Eo (I) M,+Mp where Eo and E,, are the kinetic energy of the incoming and backscattered ion, respectively. MI, and 'M p are the masses of the target and primary particle, respectively, and ()is the scattering angle. The energy ratio E, /Eo or K is known as the kinematic factor. In an RBS spectrum the number of detected He particles is shown as a function of their kinetic energy. In this way a (nonlinear) mass scale appears horizontally. The mass resolution is determined by the kinematic factor and the energy resolution of the detector, which is of the order of 12 key. An example is given in fig. 1, which shows the RBS spectrum ofa thin layer ofyba 2 Cu 3 0 x on A The indicated mass positions on the energy scale clearly demonstrate the non-linear conversion to a mass scale. The absolute number of atoms, Ni' of a specific element present per square centimetre is determined from the total fiuence <p of incoming He+ ions, the 316 Philips Journalof Research Vol:47 Nos. '3-5, 1993

3 ~---~ RBS and ERD analysis in materials research of thinfilms differential Rutherford cross-section for scattering into the specific geometry (da/dn) and the opening angle of the detector (An): Ni = AljJ(dajdQ)AQ (2) where A is the area of the peak in the spectrum corresponding to mass i. The absolute error is determined by the error in the measurement of the total fluence and is therefore usually of the order of 5%. The Rutherford crosssection is given by ( Z pz 2 l e )2 1 a(e) = ~ sin 4(ej2) (3) where Z, and Z, are the atomic number of the primary and target atom respectively, e is the scattering angle and e is the elemental charge. In a specific experimental setting the detection limits are determined by Z~ and by the substrate background. The detection limits in silicon substrates range from 1011 cm? for Sb to cm? for N. 2. Depth scale by electronic stopping The Rutherford cross-section is very small. Most of the incoming He+ ions therefore travel through microns of matter without any high-impact nuclear collision. During this flight path the ions lose energy by electronic stopping, i.e. by interaction with the electron clouds around target atoms. This energy loss introduces a depth scale in the RBS spectrum. An ion that is scattered at some depth has experienced an additional energy loss on the way in and out of the target. Thus it ends with a kinetic energy lower than that of an ion which experiences an elastic collision at the target surface. In a specific geometry the depth resolution is determined by the energy resolution of the detector. In a scattering geometry the depth resolution is typically nm. The resolution can be improved to approximately 1nm for surface layers by a glancing-angle geometry. A layer with an atomic density N and a thickness x produces an energy difference AE in the RBS spectrum, given by AE = [~e in + Ie ouijnx (4) cos in COS out where Ein and EOUI are the electronic stopping cross-sections at the incoming and outgoing paths respectively. e in and e OUI are the angles between the substrate normal and the incoming and outgoing He beam line respectively. Electronic stopping cross-sections of the elements have been measured and are reported by Ziegler"), usually in units of ev cm' per atoms. For a certain element Philips Journal of Research Vol. 47 Nos

4 DJ.Oostra I I Si Co channeled _,,\... _-...,... ~, \ I OL- ~ ~...;'... J ~ ~~~~~ ~ \, channel - Fig. 2. RBS spectrum ofsi(dol) implanted with Co + and annealed for 30 min at IOOODCobtained along a random direction (solid line) and the RBS spectrum obtained along the (DOl) direction ofthe Si substrate (broken line). The surface energy positions ofthe relevant elements are indicated byarrows. the energy loss in ev can be transformed into a depth scale, assuming that the atomic density is known. For compounds and alloys the stopping power is calculated by a linear addition of the elemental values, the so-called Bragg's rule. For a material consisting of elements A and B in a composition A:B I- x:x the total stopping power is given by where EA and EB are the stopping cross-sections of elements A and B respectively. An example of how depth information is obtained from an RBS spectrum is given in fig. 2. In this figure the solid line represents the RBS spectrum of an Si sample implanted with Co+ after an anneal at looo C. The energies of He backscattered from Co and Si at the surface as calculated from eq. (1) are indicated by arrows. The spectrum shows a rectangular Co profile. The high energy edge ofthe Co peak appears at an energy lower than expected from (1). Apparently there is additional electronic stopping in a layer above the Co. The edge ofthe Si signal appears at the energy as calculated from (1). This indicates (5) 318 Philips Journalof Research Vol.47 Nos

5 RBS and ERD analysis in materials research of thin films that an Si layer is present at the surface. In the Si signal a dip is present at a lower energy, i.e. below the surface. This dip indicates that over a range which causes a certain amount of electronic stopping the total amount of Si atoms present is lower than in pure Si. Thus, besides Si another element is present which causes electronic stopping. Apparently a layer exists with a composition different from pure Si. By calculating the stopping by the Si top layer it is deduced that the Co profile comes from the same depth as the dip in the Si signal. Thus Co and Si are present in this buried layer. The composition in this layer is determined as Co:Si = 1:2. This stoichiometry can be given very accurately because the integrated areas of the two peaks or the channel heights in the spectrum are compared. Variations in the parameters in (2), e.g. in the incident ion fluence, therefore do not effect the measurement. Hence, it is concluded that a CoSi 2 layer is present below a Si top layer. It has to be noted that a small non-linearity in the detection system may cause a systematic error. When the amount of Si in the top layer is deduced solely from the energy shift of the Co surface peak, it is extremely important to know exactly the channel corresponding to surface Co atoms. A shift of one channel typically means a shift of 2-4 kev, which may result in an error of 10 nm in the thickness of the Si top layer. In our example this systematic error is avoided by also evaluating the backscatter signal from the top Si layer in the spectrum. X-ray diffraction and cross-section transmission electron microscopy studies confirm that a buried layer of CoSi 2 has formed"). RBS cannot give this type of information about the material; only the average composition can be determined. 3. III situ analyses RBS is a "non-destructive" technique. Information as a function of depth is obtained without the need for sputtering. Most ofthe He+ ions pass through the top 1.5.um without colliding with the target nuclei because of the small scattering cross-sections. Ion fluences are generally of the order of tens of micro-coulombs. Consequently, extremely little damage is done in the layer to be investigated. These points make RBS an ideal technique for in situ analyses. Process steps, for example, can easily be followed in this manner. As an example we discuss the interaction of a ferroelectric material (PbZrTi0 3 (PZT)) with a Pt electrode. New non-volatile memory applications may be realized by using ferroelectrics in a silicon technology"). However, the combination of selected materials has to be compatible with all processing steps in the silicon technology. Figure 3 shows the RBS spectra of a thin film of PZT Philips Journalof Research Vol.47 Nos

6 DJ. Oostra 0; 's.. Q).!::! ëö E 0 c: energy (MeV) _ PZT as inserted '@500 Ti '@700 loet 2MeVHe! /.."::.., : / i\ i\ \ \ \ \ ; I \''1''-'''-''\ I i 50 I \ I /\ \ I/ \ i \i \\ channel Fig. 3. RBS spectrum of a Ti/PbZrTi0 3 /Pt/Ti/Si0 2 layer as deposited (solid line), after annealing for 15 min at 500 C (dotted line) and after annealing for 15 min at 700 C (broken line). The arrows indicate the surface energy positions of the elements of interest. For explanation of the spectrum, see text. sandwiched between a Ti top electrode and a Ti/Pt bottom electrode"). The arrows indicate the surface energy positions of the elements of interest in the as-deposited state. The signal near channel 370 is caused by He+ ions backscattering from Pb in the PZT layer below the top Ti electrode. The peak at channel330 is caused by backscattering from the underlying Pt layer. The two small peaks near channel 290 and 240 are caused by backscattering from the Ti top layer and the bottom Ti layer respectively. The He+ backscatter yield between these two peaks is caused by Ti in the PZT layer. Upon annealing in vacuum the backscatter yield near channel 370 of Pb in the PZT layer decreases. The backscatter yield from the deeper-lying Pt layer increases. Apparently Pb disappears out of the PZT layer and moves into the underlying layers. This in situ RBS analysis clearly demonstrates that this selection of processed materials is unstable at certain processing conditions. 4. Crystallinity and ion channelling Crystallinity of samples is investigated with RBS by aligning a crystal axis of a crystalline material with the incoming ion beam. When the He+ ions enter the material along a crystal axis or plane, the probability of a Rutherford collision with a target atom is dramatically reduced because channels exist Pt l Pb 320 Philips Journni of Research Vol. 47 Nos

7 RBS and ERD analysis in materials research of thinfilms "C "Q; 's:.. "C Ol.~ (ij E 0.5 o c: O~~~L-L-~~~~~ rotation (degrees) Fig. 4. Backscattering yield of Sr from a crystalline SrTi0 3 substrate as a function of rotation angle between the [OOI]substrate direction and the He + ion beam line. between the atomic rows in this orientation"). This is reflected in a reduced yield of backscattered He particles. An example is given in fig. 4, where the backscatter yield of He+ ions colliding with Sr in a crystalline SrTi0 3 target is shown as a function of the angle between the [OOI] axis and the ion beam. In the channel minimum the yield is decreased to approximately 5% of the yield in a random direction. The full width at half-maximum is typically 1.5. Channelling is used to investigate, for example, the crystallinity of epitaxial layers, ion implantation damage or the lattice positions of impurities. An example is given in fig. 2. The RBS spectrum obtained in a random direction (solid line) is discussed above. The broken curve indicates the RBS spectrum obtained with the incoming He+ ion beam aligned along the [OOI] direction. In the channelled orientation the yield of backscattered He particles has decreased more than an order of magnitude in comparison with "the random orientation. The spectrum demonstrates that both the top Si layer and the buried CoSi 2 are present mainly epitaxial on the Si substrate, Channelling can also be obtained along various other crystallographic axes or planes. Figure 5 shows an example in which channelling minima are obtained at the (lil) direction ofepitaxial CoSi 2 on an Si(OOI) substrate"), The cubic CoSi 2 lattice has a -1.2% lattice mismatch with the Si substrate at room temperature. For pseudomorphic growth a two-dimensionaliateral extension of the lattice in the interface plane is needed which causes a vertical contraction. The channelling yield' minimum of the [I ii] Si substrate is at 54.74, as expected for a cubic lattice. The minimum ofthe backscatteririg yield from Co is obtained at an angle 0.30 larger than that ofthe Si substrate. This demonstrates that the CoSi 2 crystal has a tetragonal distortion of the cubic lattice. With XRD the strain in the perpendicular direction can be measured. Philips Journal of Research Vol.47 Nos

8 DJ.Oostra lo.8 Ol.t:! ïii E 0.6 o c: 0.4 ~. ï JO e qcfi ~,.< de CoSiz..., Si o angle (degree) - Fig. 5. Angular scan around the [Ill] axis on the (110) plane ofa sample of Coêi, on Si(OOI).The yield of Si (8) from the substrate and Co (0) from CoSi 2 has been compared with a random measurement near the [Ill] axis. In the inset the angular scan measurement is indicated. By combining the XRD and RBS results the strain in both the vertical and the lateral direction can be calculated"). A second feature in fig. 5 is the fact that the full width at half-maximum (FWHM) of the Co yield is larger than that of the Si substrate, which indicates that the CoSi 2 layer consists of grains with a certain distribution in their orientation. The minimum backscatter yield from the Si substrate is not as low as is obtained in the [001]direction. The incoming He+ ions experience small-angle forward-scattering events in the top layer, because the channel minimum is not aligned with that of the substrate. This causes some dechannelling. 5. Hydrogen detection Backscattering can only occur from atoms with mass numbers greater than that ofthe incident ions. Therefore hydrogen cannot be detected in RBS. It can be detected, however, by the related technique of elastic recoil detection (ERD)8). In this technique energetic He+ ions scatter with H nuclei, as in RBS. However, rather than measuring the scattered He particles, H atoms recoiling out of the sample are detected. From the laws of conservation of momentum and energy it follows that this can only be done in a scatter geometry with the incoming He+ ion beam under a glancing angle, as given in fig. 6. A foil is inserted in front ofthe ERD detector to discriminate recoiling H particles from other energetic species such as backscattered He particles. The stopping power of particles increases with the atomic number. All species except H can therefore be stopped in the foil by ajudicious choice ofthe thickness ofthe foil. In the specific geometry given in fig. 6 this condition is fulfilled by use of a 9 }lm 322 Philips Journal of Research Vol.47 Nos

9 RBS and ERD analysis in materials research of thin films Fig. 6. Scattering geometry in He-ERD. thick Mylar foil. Because of the scattering geometry and the stopping in the Mylar foil, only H recoiled out of a depth up to approximately 200 nm can be detected. In a glancing angle scattering geometry the depth resolution is optimized according to eq. (4). However, the final depth resolution in ERD is only approximately 10 nm because of energy straggling in the Mylar foil. For 2 MeV He+ IH collisions the scattering process is inelastic. The scattering cross-section can therefore not be calculated a priori. Hence, to determine the amount of H quantitatively, a gauge experiment has to be performed, e.g. by use of a sample implanted with a known amount of hydrogen"). Hydrogen amounts of 0.1 at. % in bulk or atoms cm -2 can be detected. As an example of the use of ERD, fig. 7 shows the RBS and ERD spectra of a hydrogenated diamond-like carbon coating deposited on silicon by a plasmaassisted chemical vapour deposition process"). From the step in the RBS spectrum at the surface energy position of C, the total amount of C is calculated using eq. (2) (fig. 7a). The Si edge is shifted to a lower energy than expected from eq. (1) as a result ofthe stopping ofthe He+ ions in the carbon layer. The theoretical fit reproduces this energy shift so that the amount of C as determined from eq. (1) is in agreement with the energy shift of the Si peak as deduced from eq. (4). Thus the RBS spectrum is internally consistent. The RBS spectrum in fig. 7b) is obtained simultaneously with the ERD spectrum in fig. 7c), hence with the sample rotated such that the He+ ion beam comes in under a glancing angle. The total amount of C being known from the RBS spectrum in fig. 7a), the RBS spectrum in fig. 7b) is then used to check the scattering geometry. The exact angle between the incoming ion beam and the surface normal is deduced from the shift of the surface peak of Si using (4). This check is very important since small deviations in the determination of this angle yield large variations in the total amount of C and H probed by the incoming ion beam. From fig. 7b) also the total ion fluence is derived. From the ERD spectrum in fig. 7c) the amount ofh present in the layer can now be calculated. It was concluded that the composition of the layer is COO.63Ho.37' When the amount ofhydrogen is less than approximately 3%, an additional Philips Journalor Research Vol.47 Nos

10 DJ. Oostra a; ':;' Cl>.!::! m E o c: energy {MeVl - 4or----o~.4~----._----~OT 8~----._-----lT 2~ al Si l 300 channel - a; ':;' Cl>.!::! m E o c: energy {MeVl - 80r---~0~ ~OT 8~ ~1.~2~ bl Si l channel - Fig. 7. RBS spectrum obtained in a standard (5 in, 5 out) scattering geometry of a) a sample of approximately O.4Jlm of diamond-like carbon on Si and b) the corresponding RBS spectrum obtained in a 170 scattering geometry with the ion beam arriving at an angle of 75 from the substrate normal and c) the corresponding ERD spectrum being obtained simultaneously. Broken lines indicate fits to the spectra (see text). 324 Philips Joumnl or Research Vol.47 Nos

11 RBS and ERD analysis in.materials research of thinfilms t "C ID 's;. "C al ~ CD 200 E c; 100 c: cl energy (MeV) _ channel - Fig. 7 (continued). H peak at the surface position is observed in ERD spectra 11). This surface peak is caused by adsorbed hydrocarbons, probably cracked at the surface by the incoming ion beam. For low H contents ERD therefore requires high-vacuum conditions. 6. Conclusions The basic principles of RBS, channelling-rbs and ERD have been explained. Depth resolutions and detection limits have been indicated. Examples from different studies demonstrate that the techniques discussed are invaluable tools in the analysis of surface layers and thin films. The areas of applications are in fields such as integrated circuits, coatings or ceramics. A drawback of RBS is its relatively low sensitivity to light elements in a matrix of heavy elements. For crystalline samples this drawback can be overcome: channelling is then used to reduce the substrate signal in such a way that the signal-to-noise ratio of the light elements is improved. To measure hydrogen (or deuterium) the ERD technique is used. The complementary nature of the two techniques allows for RBS and ERD spectra to be recorded simultaneously, which is a powerful way of deriving compositions when hydrogen is involved. REFERENCES I) For a detailed treatment of the technique of backscattering spectrometry, see, for example, Philips Journal of Research Vol. 47 Nos

12 DJ.Oostra W.-K. Chu, J.W. Mayer and M.-A. Nicolet, Backscattering Spectrometry, Academic Press, New York, ) J.F. Ziegier (ed.), Helium Stopping Powers and Ranges in All Elemental Matter, Pergamon, New York, 1977; L.R. Doolittle, Nucl. Instrum. Methods, B9, 344 (1985). 3) E.H.A. Dekempeneer, J.J.M. Ottenheim, D.W.E. Vandenhoudt, C.W.T. BulleLieuwma and E.G.C. Lathouwers, App!. Phys. Lett., 59, 467 (1991). 4) P.K. Larsen, R. Cuppens and G.A.C.M. Spierings, Ferroelectrics, 128, 265 (1992). 5) A.E.T. Kuiper, Thin Solid Films, 224 (I), 33 (1992). 6) Channelling is discussed in many review articles. Apart from ref. I, a good introduetion is: L.C. Feldman, J.W. Mayer and S.T. Picraux, Materials Analysis by Ion Channeling. Academic Press, New York, ) F. La Via, A.H. Reader, J.P.W.B. Duchateau, E.P. Naburgh, D.J. Oostra and AJ. Kinneging, J. Vac. Sci. Techno!., BlO, 2284 (1992). 8) C.P.M. Dunselman, W.M. Arnold Bik, F.H.P.M. Habraken and W.F. van der Weg, MRS Bull., 12, 35 (1987). 9) M.F.C. Willemsen, A.E.T. Kuiper, LJ. van Ijzendoorn and B. Faatz, In J.R. Tesner, C.J. Maggiore, M. Nastasi, J.C. Barbour, and J.W. Mayer (eds), Proc. High Energy and Heavy Ion Beams in Materials Analysis, Albuquerque, NM, June 14-16, 1989, Materials Research Society, Pittsburgh PA, USA, 1990, p ) E.H.A. Dekempeneer, R. Jacobs, J. Smeets, J. Meneve, L. Eerseis, B. Blanpain, J. Roos and DJ. Oost ra, Thin Solid Films, 217, 56 (1992). '') A.E.T. Kuiper, Surf. Interface Ana!., 16,29 (1990). Author Doeke J. Oostra: M.Sc. (physics), University of Groningen, 1983; Ph.D., F.O.M. Institute for Atomic and Molecular Physics, Amsterdam 1987; Joint Institute for Laboratory Astrophysics, Boulder, CO, ; Philips Research Laboratories Eindhoven In his doctoral thesis he investigated sputtering of semiconductor materials in a reactive environment. Subsequently, he was concerned with the interaction of thermal beams of group III and group Velements with silicon surfaces. At Philips his work is concerned with the interaction of ion beams with materials, namely both ion beam rnodification and ion beam analysis.," 326 Philip. Journalof Research Vol.47 Nos