Dynamic in situ diagnostics using high-energy ion beam analysis

Beam Iderections Materials A Atoms

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and Methods in Physics Research B 136-138 (1998) 1203~121I

Dynamic in situ diagnostics using high-energy ion beam analysis W. Miiller *, W. Fukarek, Insritzrtc~ of IonBrum

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Abstract MeV ion beam analysis (IBA) is presented as a powerful tool for in situ, real-time process diagnostics. With minor additional experimental equipment such as differential pumping or the installation of special detectors, dynamic stoicbiometric profiles can be measured during ion beam or plasma exposure. Spectrum acquisition times of the order of a few minutes or less enable the study of fast and transient diffusion effects and transient surface coverages. Examples are given addressing the diffusion and trapping of hydrogen in nickel around room temperature, the ion beam synthesis of carbon-nitrogen films, the ion beam nitriding of stainless steel. and the diagnostics of the layered structure of cubic boron nitride films. The latter example combines stoichiometric in situ analysis using dynamic ERD and structural 0 1998 Elsevier Science B.V. in situ analysis using optical ellipsometry.

1. Introduction Over the last four decades, MeV ion beam analysis (IBA) has been developed as a powerful tool for the analysis of the stoichiometry and structure in near-surface layers [1,2]. It is basically non-destructive and offers a high degree of versatility for different problems, such as Rutherford backscattering (RBS) for typically heavy atoms in a light matrix, elastic recoil detection (ERD) for light elements, and nuclear reaction analysis (NRA) for lighter atoms with a high selectivity even of different isotopes. These methods are capable of providing standard-free depth profiles in a depth range up to approximately 1 urn and with a depth resolution typically between 1 and 100 nm. They are well established in numerous appli-

*Corresponding

author.

0168-583X/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved PIISOl68-583X(97)00813-6

cations such as in electronics and microelectronics, deposition of tribological, anti-corrosive, optical and superconducting thin films, of ion beam and plasma surface modification, and in fusion research. Conventionally, IBA is performed ex situ, i.e. samples are removed from the processing device in which surface modification is accomplished, and are mounted for analysis in a vacuum chamber which is connected to the beam-line system of an MeV accelerator. As the sample is normally carried through air, the transport may cause serious contaminations mainly due to sorption of oxygen and water. In addition, the analysis for certain parameter regimes is excluded, as, e.g., the non-perturbed analysis after a low-temperature treatment. Moreover, the procedure is rather inefficient and time-consuming in case of a timedependent process study. Either a frequent removal and re-installation is required if only one sample

is used. or many samples have to be prepared and analysed. These problems may be overcome by in situ installations which allow a stepwise, alternate processing and analysis, e.g., by a quick sample transfer in-vacua or in the so-called ‘dual-beam’ arrangements. Fast and efficient experimental procedures are achieved, reducing contamination and extending the range of feasible experimental parameters. Still, in many systems the surface composition may change after turning off the process of modification, e.g., due to fast diffusion or desorption. Consequently, a further extension of the capabilities of IBA is possible when the analysis is applied simultaneously with the processing for modification. This “dynamic” or “real-time” analysis opens new diagnostic regimes in particular for processes with fast transients, and thus enables new insight into the mechanisms of an individual process. In 1995, the MRS Bulletin [3] published a volume on “In situ, real-time characterization of thin film growth processes”, exclusively addressing optical methods, reflection high-energy electron diffraction (RHEED), X-ray fluorescence, and low-energy ion scattering (1%). Besides the limited applicability of some of these methods (RHEED, ISS), it is obvious that absolute stoicbiometric profile information is missing. It is the purpose of the present paper to demonstrate that this gap can be filled by dynamic in situ high-energy IBA. Appropriate operating conditions will be discussed and characteristic fields of application will be identified. Some specific studies will be described, one of which will address the possibility of combining with optical in situ diagnostics.

electrical processing has recently been given by Lange et al. [lo] and Kreissig et al. [l 11.Addressing the mechanisms of anodic bonding, that is the bonding of glass onto silicon under the influence of an electric field, they employed a model system consisting of a 1 mm glass plate covered with thin Al electrodes. Using ERD with 35 MeV incident Cl ions. they studied the dynamic evolution of the drift profiles of Nat and Li’ ions at the anodic side at different temperatures and different electric fields across the glass. The remaining fraction of the present paper will concentrate on dynamic investigations of surfaces during ion and plasma processing (Fig. 1). With respect to ion beam processing, Allen et al. [ 121 and Farley et al. [13] recently reported on the dynamic contamination of Si during 0 implantation for the formation of SIMOX (Separation by IMplanted Oxygen) layers, using the backscattered 0 ions for analysis. Abel et al. [14] studied the degradation of thin polystyrene films during the bombardment with different ions, employing both NRA and RBS. A number of early applications were reported from nuclear fusion research: Morita et al. [ 151 and Scherzer [16] investigated the possible formation of a dynamic inventory during hydrogen implantation into graphite by means of ‘He+ induced ERD. Interestingly. conflicting results were obtained probably due to an analysing beam effect in Ref. [15]. The transient trapping and diffusion of deuterium in Ni was studied by Petitpierre et al. [17.18]. This will be described in detail below.

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2. Surface processes for dynamic IBA Dynamic in situ studies using high-energy IBA can be applied to a number of different processes of surface modification. An example of thermal treatment after ion implantation is given by Yu et al. [4], who applied in situ RBS during isothermal annealing. Several papers have addressed surface dynamics and the adsorption of gases and other atoms on surfaces [5-71, as, e.g., for the study of catalytic processes [8,9]. An example of

Fig. I, Dynamic in situ analysis using high-energy IBA: (a) Ion beam processing; (b) low-pressure plasma processing.

W. Miilier et al. I Nucl. Instr. and Meth. in Phys. Rex B 136-138 (1998) 1203-1211

A first application of high-energy ion beam surface analysis during the exposure to a low-pressure plasma has been reported by Langhoff and Scherzer [19] using a device which had been initiated by the first author of the present paper. The authors studied the hydrogen inventory during the deposition of thin hydrocarbon films from an RF plasma in methane, again using 2.6 MeV 4Hef ERD.

3. Experimental considerations Under appropriate conditions, the environment of the above processes will not conflict with the principles of standard IBA. At pressures well below 1 Pa, where most ion beam treatment and many low-pressure plasma processes are applied, the transversed distances of the analysing ingoing and outgoing ion beams correspond to area1 densities in the order of monolayers or less in a solid, which is well below the depth resolution of standard IBA. However, some additional experimental provisions are usually necessary. As the required vacuum in the beamlines for analysis has to be at considerably lower pressures than the typical process vacua, differential pumping has to be applied. However, it is straightforward to install diaphragms with low-gas conductance, as the diameter of the analysing beam is normally rather small. For ion beam processing, it is desirable to make use of a broad beam source (see Fig. l(a)) with a beam diameter of 1 cm or more at the substrate position. Otherwise, the mutual alignment of the processing and the analysing beams might become difficult. This is particularly critical for analysis at glancing angles such as ERD. The ion flux from the processing source or from the plasma will normally be several orders of magnitude larger than the flux of the analysing ions. Therefore, a direct electrical dose measurement at the sample cannot be used for normalisation of the measured energy spectra. Therefore, other techniques have to be employed such as a rotating vane wheel well in front of the sample or an additional detector monitoring the incident ion backscattering, in cases where an internal calibration of the spectra is impossible.

1205

Special considerations are necessary for the choice of the analysing detector. The ambient of ion or plasma processing may be incompatible with standard surface barrier detectors. High-rate backscattering of the low-energy process ions may be detrimental in terms of energy resolution or may even destroy the detector, as well as an operating pressure around 0.1 Pa or the light being emitted from a plasma. Taking the detector to a remote position would conflict with the major requirement of high sensitivity, as a main aim of dynamic in situ IBA is the fast acquisition of data allowing the characterization of fast and transient surface processes. As the analysing beam dose should always be kept as low as possible (see below), it is therefore desirable to maximize the solid angle subtended by the detector. A large solid angle, however, usually results in a deterioration of the energy and depth resolution due to kinematic spread 220,211. For the special case of heavy-ion ERD, which is particularly suited for dynamic in situ analysis due to its high sensitivity, the above problems can be solved by the use of a specific ionization chamber [22]. In some of the experiments to be described below, such a chamber was mounted outside the process chamber and separated from its vacuum by a thin mylar foil of 1.2 urn thickness, through which the recoil ions emitted from the sample enter the gas volume (typically lo-100 mbar isobutane, active length 28 cm). The ions are slowed down and stopped between a cathode and an anode, which are mounted parallel to the mean direction of flight. The anode is split into a shallow energy loss (AE) section and a residual energy (E,,,,) section, from which particle identification is obtained. For this, one has to maintain the independence of the anode signals, AE and Erest, from the position of the ion trajectory by means of to the so-called Frisch grid in front of the anode, which separates the anode from the electrical field inside the chamber. In order to determine the lateral position of any specific ion trajectory, the cathode is split into two halves with a “backgammon” structure, from which the lateral position in the direction parallel to the electrode surfaces can be determined. The position normal to this direction can be calculated from the height of the total cathode signal, which

is obtained by adding the individual signals from the two halves. A workstation performs these calculations during the measurements and in particular corrects for the kinematic deviations of the individual ion tracks. Using 3.5 MeV Cl as primary ions and a scattering angle of 35”, a position resolution of 2.5 mm (FWHM) is obtained for the corresponding detection of, e.g., 18 MeV nitrogen. A rather large solid angle of 10 msr can be tolerated. With an angle of incidence of 17.5”, the set-up achieves an energy resolution of 350 keV for nitrogen, corresponding to a depth resolution of 11 nm at the surface for an iron sample. Without the kinematic correction, these resolutions would be worse by about one order of magnitude. The count rate of the detector is limited to about 1 kHz due to the ion drift time to the anode of about 1 ms and the pile-up due to the relatively large background of backscattered primary ions (about 95% for an Fe sample), and recoils from the sample. As usual for IBA, its non-destructive nature has to be carefully checked for each system under investigation. In addition to depth profile distortions, the analysing beam might change the dynamics of the surface processes during real-time analysis. Therefore, its fluence should always be kept as small as possible. Obviously, checks can be performed at varying beam dose or frequency of the analysis.

carbon foil to suppress the scattered low-energy D+ ions. Absolute calibration was achieved in an independent measurement with a fixed amount of implanted D at low-temperature, relating the ERD signal to the RBS signal and the yield of the D(jHe,“He)H nuclear reaction. Fig. 2(a) shows the dynamic depth profiles at different temperatures and a flux of 10” D/(cm’s) in the limit of large fluences. Each collection of a depth profile took about 1 min only. At about 140 nm. a peak is visible due to the presence of hydrogen at the surface. (Due to the lower energy transfer to hydrogen its surface signal appears at lower energies than for deuterium, and therefore seemingly at this depth when the energy spectrum

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About a decade ago. an example of dynamic IBA of ion-surface interaction has been given by Petitpierre et al. [17,18]. 10 keV Di ions were implanted into Ni from a remote and differentially pumped ion source through an analysing magnet. Consequently, for ERD analysis using 2.6 MeV surface barrier detector “He+ ions a standard could be employed, which was equipped with a metal foil to retard the elastically scattered ‘He ions. The spectra were normalized to the RBS signal measured by means of an additional surface barrier detector, which was placed behind a thin

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Fig. 2. (a) Stationary diffusion profiles of IO keV D’ implanted into Ni at various temperatures. (b) Mean concentration of IO keV D. implanted into Ni for a beam-onlbeam-off cycle. at different temperatures. The lines denote the results of model calculations (see Ref. [17]).

W. Mdier

et ul. I Nucl. Instr. und Mrth. in Phys. Res. B 136-138 (1998)

is evaluated for deuterium.) Apart from this, the profiles clearly exhibit a diffusional nature. The transient character of the deuterium inventory is demonstrated in Fig. 2(b) for a beam on/off procedure at each temperature. Time constants in the order of I min can be resolved. The evaluation showed that both the depth profiles and the time-dependent inventories can be reasonably well explained by diffusion under the influence of traps. The strong increase of the stationary inventory was found to be due to the decoration of very shallow traps with an activation energy of about 0.1 eV only, Obviously, dynamic depth profiling proved to be mandatory for this characterization of fast diffusion and shallow trapping. 4.2. Suturation

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For a number of years, several experimental groups have tried to synthesize carbon nitride (C3N4). the existence of which has been theoretically predicted by Liu and Cohen [23], without any convincing success so far. Among other processes, ion implantation of nitrogen into carbon represents a possible route, since high concentrations of the implant might be built up, being buried deeply below the surface to avoid fast outdiffusion. Although several attempts [24-281 to achieve the stoichiometric composition by keV N+ ions had failed, we decided to perform a dynamic experiment [29,30] in order to obtain more information about the mechanisms of nitrogen saturation and reemission. In addition, this study would be relevant to the plasma-wall interaction in nuclear fusion devices, where plasma-facing components from carbon may be exposed to a plasma containing significant amounts of nitrogen for plasma-boundary cooling [3 11. Again from a remote ion source with magnetic analysis, 20 keV N+ ions were implanted into a highly tetrahedral amorphous carbon (ta-C) film [32] deposited on a silicon substrate. Simultaneous ERD analysis of nitrogen and carbon was performed using 35 MeV “Cl ions. For the detection of the recoils, a Bragg ionisation chamber [33] was employed, being equipped with a thin Mylar entrance foil and operated at 120 mbar of isobutane. Quantitative depth profiles were obtained by using

the method of internal calibration. Fig. 3 shows the dynamic nitrogen depth profiles at different fluences, obtained during implantation at room temperature. Only at the lowest fluence, the ballistic range profile of the implanted nitrogen is reflected. At increasing fluence, the profiles show an asymmetric broadening and saturation at 0.30.4 nitrogen atoms per carbon atom, far below the stoichiometric value of 1.33. After switching off the beam, no alteration of the nitrogen profile has been observed within the error of the experiment, indicating that no additional dynamic inventory is formed during implantation. The result clearly indicates that nitrogen implantation at room temperature and in the present range of energy is incapable of forming C-N compounds with a composition close to C3N4. Using cross-sectional TEM and Raman spectroscopy, it was demonstrated [34] that the saturation of the N contents at large fluence is accompanied by the formation of bubbles in the bulk of the material, which contain NZ gas. 4.3. Ion nitriding

of stainless steel

The nitriding of steels is widely employed to improve the tribological and corrosive properties of steels, using salt baths, high-pressure gas treatment, or ion nitriding depending on the individual material, the availability of modern vacuum equipment, or environmental preferences. Mostly, nit-

riding is required to a depth of up to some 100 pm, which is accomplished by diffusion at elevated temperature (typically 500.-600°C) [35]. Thus. the efficiency of a selected process in combination with a given material depends on the boundary conditions at the surface which determine the retention and release of implanted nitrogen and thereby the nitrogen fux which is available for diffusion. A detailed understanding of these mechanisms is largely missing, in particular for vacuum processing such as plasma or ion beam nitriding. Obviously, this understanding can be improved by dynamic surface diagnostics. The nitriding of stainless steel has proven to be difficult basically for two reasons: The substrate temperature has to be kept at moderate temperature (typically about 400°C) in order to avoid phase formations that are detrimental to the anti-corrosion properties. Therefore. excessively long processing times of up to many hours are required [36]. The chromium oxide layer at the surF&ce has been suggested to act as a diffusion barrier for nitrogen [37]. Consequently, the process can be improved by employing ion nitriding using higher ion energies, as was recently demonstrated by means of plasma immersion implantation at energies of some tens of keV [38]. However. due to the required cost-efficiency of any technical process, the ion energy should be kept as low as possible. Thus, detailed investigations will not only improve the basic understanding, but also help optimization for industrial applications. In order to study the nitrogen retention and release which will be affected by the oxide diffusion barrier and therefore by sputtering and re-oxidation from the residual gas, an in situ experiment was set up with a Kaufman [39]-type broad-beam ion source delivering mainly N,’ ions (with an admixture of N’ ions) at energies up to 2 keV, a residual gas control combined with a variable oxygen inlet, and a differentially pumped beamline to a tandem accelerator delivering 35 MeV Cl ions for dynamic ERD analysis. An ionization chamber with a backgammon cathode, as described in Section 3, was used as detector. Fig. 4 shows a typical, time-integrated AE-E spectrum obtained with the ionization chamber during ion nitriding of stainless steel. The signals

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of the different light species (C. N. 0) are clearly resolved, corresponding to the individual absolute depth profiles. They show an oxygen peak due to the surface oxide layer, a diffusional concentration of nitrogen, and the low-concentration carbon constituent of the material. From the parameters of the ERD analysis, the depth of detection is limited to about 170 nm. Fig. 5(a) shows a selection of time-resolved nitrogen profiles extracted from the two-dimensional spectra as shown in Fig. 4. Fig. 5(b) correlates the integrated amounts of 0 and N within the maximum depth of detection as function affluence. Each collection of a depth profile took 30 s only. It is seen that the surface oxide layer is quickly reduced, with a correlated increase of the nitrogen retention. Further experiments are in progress, but these preliminary results clearly demonstrate the new experimental possibilities made available by dynamic in situ IBA.

Cubic boron nitride films are of considerable interest as hard coatings and also for possible optoelectronic applications. Their deposition, however, is still in the stage of scientific development. Films

W. Miilkr

et al. I Nucl. Instr. and Meth. in Phys. Rrs. B 136-13X (199X) 1203-121 I

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Fig. 5. (a) Dynamic depth profiles during the implantation of I keV Nj IN + ions into stainless steel at a substrate temperature of 370°C. a flux of 1.2 x lOI cm -? se’, and an oxygen partial pressure of 6 x 10mh mbar. (b) Integrated amount of retained nitrogen and surface oxygen as function of fluence. At Ruences above 6 x IO” cm-‘. the apparent retention of nitrogen is influenced by the maximum depth of detection of the analysis.

with a high fraction of cubic BN can be obtained by a number of different deposition processes such as ion beam assisted deposition, pulsed laser deposition, magnetron sputtering, plasma-enhanced chemical vapour deposition and hollow-cathode assisted PVD. Generally, the initial growth phase results in a layered structure the formation of which is far from being understood. As shown first by Kester et al. [40] from TEM cross-sectional analysis, an amorphous layer with a thickness of about 10 nm is formed on top of the substrate, with has been denoted as amorphous boron nitride. Subsequently, a hexagonal BN layer with a thickness of the order of 50 nm is observed, with is base planes “standing” perpendicular to the sur-

I209

face of the substrate. On top of this layer. the desired film containing close to 100% cubic BN grows under appropriate conditions as a polycrystalline material with a mean grain size of the order of 10 nm. Finally, internal compressive stress limits the maximum attainable thickness of usually less than 300 nm. The physical mechanisms which determine the formation of this layered structure, such as the transformation from amorphous to hexagonal and then from hexagonal to cubic growth, are essentially unidentified. C-BN films were prepared on Si substrates by means of ion beam assisted deposition at a substrate temperature of 400°C by electron beam evaporation of boron and simultaneous irradiation with At+ and Nr ions at a flow ratio of Ar/ N2 = 2:l. an arrival ratio of N/B = 1:1, and an ion energy between 500 and 800 eV [41]. For characterization. the films were transported to a UHV chamber which allows simultaneous plasma treatment, IBA of the surface stoichiometry and structural analysis by means of optical characterization. Using an ECR plasma source running with a mixture of SF6 and O2 (2:l) at a total pressure of 0.3 Pa, the films were slowly etched layer by layer with an etching rate of about 2 nm/min. During the etching, dynamic in situ analysis was performed both using ERD (at identical conditions as described in Section 4.3) and single-wavelength ellipsometry using 632.8 nm laser light. The result is displayed in Fig. 6, which shows the total amount of remaining B and N during the etching process, together with the variation of the ellipsometric angles, Y and d [42]. In connection with ex-situ generalized ellipsometry [43], the in situ ellipsometry allows the identification of the sequence of the cubic. hexagonal, amorphous and substrate layers and the corresponding thicknesses. In connection with ERD, a number of new and useful detailed pieces of information can be extracted: (i) The BN layer, as deposited under the present conditions, exhibits a slight excess of B of about 7%. (This finding, however, does not depend on the availability of dynamic in situ ERD.) (ii) The etching rate of c-BN is lower than that of h-BN by a factor of about 2. This result might be helpful for the understanding of the deposition mechanisms. The formation of the cubic phase

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proved understanding of processes of thin-film formation and surface treatment. with some of these processes being of high technical relevance. In particular for the characterization of plasmasurface interaction during plasma etching and deposition, the technique of dynamic in situ IBA appears to exhibit an extremely large potential for the future. Corresponding experiments are in progress.

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References

Fig. 6. Remaining amount of B and N in a c-BN film structure (full triangles. lines to guide the eye. left-hand ordinate scale) during SF6 + O2 plasma etching. correlated with the ellipsometric angles Y and 3 (open symbols. right-hand scale). as function of the etching time.

[II J.R. Bird. J.S. Williams

rather than the hexagonal one cannot be explained by differences in physical sputtering, as proposed by Reinke et al. [44]. However, effects of surface chemistry cannot be excluded. (iii) For the first time, it is clearly demonstrated that the relatively thick amorphous interface towards the silicon substrate does not contain significant amounts of B nor N. Thus, the generally accepted assignment of amorphous boron nitride has to be revised: The present in situ study with its correlation of structural and compositional diagnostics proves the transformation of the substrate surface into amorphous silicon which also contains a small percentage of oxygen and metallic impurities such as W and Fe (not shown here). These impurities result from contaminations from the Kaufman ion source which had been operated with argon for pre-sputtering of the sample.

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5. Conclusions It has been demonstrated in the present paper that MeV IBA can successfully be employed for dynamic in situ studies during surface processing. Using heavy-ion ERD, compositional depth profiles can be measured at repetition rates larger than one per minute. New and unique experiments become available. which are indispensable for an im-

[I41 [I51 [IhI

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