Structural and Electrical Properties of Metal Silicide Precipitates in Silicon

M.Seibt et al.:Structural and Electrical Properties of Precipitates in Si301phys.stat.sol.(a)171,301(1999)Subject classification:61.72.Nn;66.30.Hs;71.55.±±i;S1.61;S5.11Structural and Electrical Propertiesof Metal Silicide Precipitates in SiliconM.Seibt,H.Hedemann,A.A.Istratov1 F.Riedel,A.Sattler,and W.SchroÈterIV.Physikalisches Institut der UniversitaÈt GoÈttingen,Bunsenstr.13±±15,D-37073GoÈttingen,Germany(Received October19,1998)This paper summarizes current understanding of structural and electronic properties of metal sili-cide precipitates in silicon and their bined studies of high-resolution transmission electron microscopy and deep level transient spectroscopy together with numerical simulations show that the bounding dislocation of nickel silicide platelets is the key to understand their rapid growth and electrical properties.Different misfit relaxation phenomena govern the structural evo-lution of copper silicide precipitates from their early stages to the well-known colony growth.This evolution involves different types of secondary defects indicating that the deep band-like states observed throughout this process are associated with the silicide precipitates themselves.1.IntroductionThe3d transition metals in silicon combine unusual physical properties with detrimental effects in device manufacturing.Especially,the late3d elements cobalt,nickel and cop-per exhibit solid solubilities which strongly decrease with decreasing temperature and high interstitial diffusivities with small migration barriers[1to4].As a result,these impurities are mobile even under conditions of large supersaturation leading to favour-able conditions for precipitation.The high quality of present day silicon materials in combination with the realisation of well-defined annealing and cooling conditions allowed to establish structural proper-ties of silicide precipitates on the atomic scale by means of high-resolution electron microscopy(HRTEM).Deep level transient spectroscopy(DLTS)combined with ap-propriate modeling and quantitative simulations provided insight into the electronic structure of such defects.This paper summarizes current knowledge about the interrela-tion of formation,atomic structure and electrical properties of silicide precipitates in silicon obtained from the combination of the above methods.For nickel,plate-shaped precipitates consisting of two f111g planes of NiSi2with diameters between7and100nm form upon quenching.High-resolution and conven-tional TEM show these platelets to be bounded by a dislocation with a Burgers vector of b a a4h111i which is a geometrically necessary dislocation resulting from the atom-ic structure of the precipitate/matrix interfaces.This dislocation is the key to understand the rapid precipitate growth as well as the deep band-like states revealed by DLTS, which rapidly transform into localized states during internal ripening which is a phe-nomenon related to the metastability of the platelets.1 Present address:Department of Materials Science,UC Berkeley,Berkeley,CA94720-1760, USA.302M.Seibt et al. Precursor defects to the well-known copper silicide colonies are also associated with band-like states.These defects consist of copper silicide platelets which frequently give rise to the formation of a bounding extrinsic stacking fault.During internal as well as Ostwald ripening2 band-like states are observed which are tentatively attributed to precipitate/matrix interfaces.2.Experimental Techniques2.1Preparation of silicide precipitatesInterstitially dissolved cobalt[5],nickel[6]and copper[4]in silicon are highly mobile even at temperatures below300 C where typical impurity concentrations lead to highly supersaturated solutions with large chemical driving forces for precipitation.As a con-sequence,these impurities tend to precipitate during or immediately after quenching from high temperatures and the resulting structure,density and spatial distribution of silicide precipitates critically depend on experimental conditions.Among others,the most important are the impurity concentration(which determines the temperature be-low which the interstitial solution is supersaturated),the density and spatial distribution of pre-existing crystal defects as e.g.dislocations,grain boundaries or microdefects,and the cooling conditions subsequent to high temperature annealing(for a detailed discus-sion,cf.[2,7]).For nominally defect-free FZ-silicon,cooling rates below about10K/s usually lead to predominant precipitation at wafer surfaces which is related to the phe-nomenon of haze formation[8].For the investigations summarized in this paper,impur-ity in-diffusion at high temperatures was terminated by quenching the samples into different liquids(10%NaOH,ethylene glycole or silicone oil).This leads to estimated cooling rates between2000and250K/s[9]if a vertical furnace is used which to our experience is mandatory to obtain reproducible results.2.2Deep level transient spectroscopyDLTS is a well-established technique to investigate deep levels associated with point defects in semiconductors.For spatially extended defects,however,asymmetrically[12] or symmetrically[13]broadened DLTS lines are frequently observed together with a logarithmic capture law in a certain range of filling pulse lengths,t p,i.e.D C m G ln t p [14to17],where D C m denotes the amplitude of the DLTS line.Recently,it has been recognized that such line shapes and the non-exponential capture law,can be consis-tently interpreted if in addition to the capture barrier internal transitions between the states±±although not directly contributing to the measured signal±±are taken into account[18].The internal equilibration time G i may be used to distinguish the two limiting cases of band-like(G i(RÀ1e Y RÀ1c)and localized(G i)RÀ1e Y RÀ1c)states,where R e and R c denote emission and capture rate of the defect,respectively(compare Fig.1).In addition,Fig.1shows the capture barrier d E c a FÀF N which results from the Coulomb interaction of free charge carriers with the defect if its occupation F dif-2 The termªOstwald ripeningºdescribes precipitate coarsening where large precipitates grow at the expense of small ones.For more detailed descriptions,cf.[10,11].fers from the occupation F N of the neutral defect (`neutral occupation').3 For band-like states,the total occupation F can be described in terms of a Fermi distribution with a time-dependent quasi-Fermi energy,whereas localized states independently exchange charge carriers with the conduction and valence band and are only coupled via d E c .Experimentally,the two limiting cases may be distinguished by the qualitatively differ-ent dependence of their DLTS spectra on the filling pulse length,t p :±±For band-like states,the high temperature side of the DLTS lines are mainly inde-pendent of t p as has been shown by simulations [18]and also analytical calculations [19].±±For localized states,the high temperature sides of the DLTS lines coincide after normalization with respect to their maxima [18to 20].Numerical simulations show that the defect parameters cannot be established by con-ventional DLTS analysis [20]so that the current strategy consists of fitting theoretical to experimental spectra on the basis of numerical solutions of the differential equation describing capture and emission of charge carriers from the deep states.3.Nickel Silicide PrecipitatesFor nickel,the phase in equilibrium with silicon is NiSi 2which has the cubic CaF 2structure consisting of a face-centred Bravais lattice with a three-atomic basis,i.e.one nickel atom at the origin and two silicon atoms at Æa a 4 111 .Hence,the formation of a unit of NiSi 2in silicon may be viewed as the incorporation of an interstitial nickel atom on a substitutional site of the silicon lattice and the incorporation of the remaining silicon atom into one of the nearest tetrahedral interstitial sites.Since the lattice param-eter of NiSi 2differs by less than 0.4%from that of silicon,the volume change asso-ciated with nickel precipitation is small.After rapid quenching from high temperatures,plate-shaped precipitates are ob-served by TEM [21].These platelets have diameters which vary between 7and 100nm depending on the initial nickel concentration and quenching rate.From HRTEM imag-ing it has been shown that the platelets consist of two (111)layers NiSi 2[21](Fig.2a).As indicated by the rigid shift shown in Fig.2a,the platelets introduce a stacking faultStructural and Electrical Properties of Metal Silicide Precipitates in Silicon303Fig.1.Band diagram of electronic states at an extended defect showing the capture barrier d E c a ÀF ÀF N Ádue to occupations F larger than the neutral occupation F N .The internal equilibration time G i may be used to classify deep states as band-like (G i (R À1e Y R À1c )and lo-calized (G i )R À1e Y R À1c ),where R e and R c denote the emission and capture rate,respectively3Please,note that the equation d E c a F ÀF N defines the a which enters into numerical calculations as a free parameter.into the silicon lattice due to the atomic structure of the Si(111)/precipitate interfaces which are both built up by Si±Si bonds.All nickel atoms within platelets consisting of two (111)layers NiSi 2belong to these interfaces where nickel atoms are sevenfold coordinated compared to their eightfold coordination in bulk NiSi 2which reveals the metastable nature of such defects.For geometrical reasons,the platelets are bounded by a dislocation (Fig.2b)with a Burgers vector b a a 4h 111i inclined with respect to the platelet normal which compensates the stacking fault.Owing to the strain energy of the bounding dislocation as well as the energy asso-ciated with the large area of the NiSi 2/(111)Si interfaces,the observed particle morphol-ogy must be viewed as energetically unfavourable.Rather its occurrence shows that the incorporation of nickel atoms via the bounding dislocation is much faster compared to 304M.Seibt etal.Fig.2.NiSi 2platelets after quenching from high temperatures:a)HRTEM micrograph of the NiSi 2a 111 Si interface which is built up by Si±Si bonds [21],b)weak-beam dark field image show-ing the bounding dislocation which has a Burgers vector of b =a a 4h 111i ,and c)experimental DLTS spectra and simulations based on a two-dimensional density-of-states function (compare [18]for details of the simulations)and a neutral occupation F N 044Recent fits show that the neutral occupation of states associated with NiSi 2platelets is F N 0X 3[23].that across the (111)interfaces indicating a kinetically selected precipitate morphology.This point of view is further corroborated by the phenomenon of internal ripening which will be described in more detail below.The electronic structure of such NiSi 2platelets has been studied by means of DLTS especially on n-type silicon [18,22].Fig.2c shows experimental DLTS spectra obtained for different t p .The coincidence of the high temperature sides of the spectra shows that band-like states are associated with the NiSi 2platelets.From their structure,two possi-ble origins of band-like states can be considered,i.e.1.the NiSi 2/(111)Si interfaces,and 2.the bounding dislocation.For the former,a density of states r E const of two-dimensional systems can be assumed,whereas for the latter r E of one-dimensional systems with 1a E p singularities at the band edges is appropriate.Numerical calcula-tions ±±as exemplified in Fig.2c for a two-dimensional density-of-states function ±±show that both types of density of states are consistent with experimental data,i.e.a distinction between the two models on the basis of DLTS data alone is not possible.As mentioned above,platelets consisting of only two (111)layers NiSi 2are energeti-cally unfavourable mainly due to the large strain energy of the bounding dislocation Structural and Electrical Properties of Metal Silicide Precipitates in Silicon305Fig.3.NiSi 2platelets after internal ripening at 320 C:a)HRTEM micrograph showing a type-B precipitate,b)DLTS spectra for different filling pulse lengths normalized to the line maximum;the coincidence of the high temperature sides after normalization is a fingerprint of localized statesand the large (111)interfaces.Hence,energy will be gained if the platelets transform into more compact morphologies.Such transformations are referred to as internal ripen-ing since they occur without long range diffusion,i.e.at constant precipitate density.Fig.3a shows a HRTEM micrograph of a cylindrical NiSi 2precipitate after additional annealing at 320 C for 20min which completes internal ripening.Structural investiga-tions at various stages of internal ripening show that the morphological transformations proceed by island formation at the border of the platelets and their subsequent growth [24,25].These structural transformations considerably affect the deep states associated with the NiSi 2platelets as is revealed by DLTS.The originally band-like states transform into localized states as is shown by the common high temperature side of DLTS lines after normalization with respect to the line maximum (Fig.3b)[22].This transformation occurs on a much shorter time scale than the structural transformations measured by TEM.This observation has been taken as strong evidence for the bounding dislocation to be the origin of the deep states associated with NiSi 2platelets [25].The authors argue that structural transformations start with the climb of the bounding dislocation out of its initial (111)plane,which immediately destroys the translational symmetry within the core of the dislocation and thus the band-like nature of states.4.Copper Silicide PrecipitatesThe precipitation of copper in silicon as Cu 3Si is associated with a large volume expan-sion of 150%.As a consequence,strain relaxation phenomena like elastic deformation of matrix and precipitates,production of intrinsic point defects and formation of sec-ondary defects govern copper precipitation.The latter process is most important during the late stages of precipitation where copper particles form colonies consisting of planar arrangements of (spherical)silicide precipitates bounded by an extrinisic dislocation 306M.Seibt etal.Fig.4.a)HRTEM micrograph of a copper silicide precipitate after quenching from 900 C,b)en-larged detail (upper right box in a))showing the lattice image of the copper silicide,c)enlarged detail (lower left box in a))showing the rigid lattice shift introduced by the surrounding stacking faultloop.These defects have been investigated in detail in the 1970s (see [26]for an exten-sive list of references)and their formation is well understood within the model of Nes [27]which is based on a growth mode previously described for C precipitation in Nb containing steels [28].Within this model,colony growth is described as repeated (het-erogeneous)nucleation and growth of copper silicide precipitates on pre-existing dislo-cations which climb due to the incorporation of silicon self-interstitials emitted by the precipitates.Colony growth is observed in the presence of extended defects like dislocations [29,30]or stacking faults [31]but also in initially dislocation-free materials.There,dis-locations form as a result of copper precipitation [32].Fig.4a shows a defect configura-tion typical for the precursor stages of colony growth:the defect consists of a copper silicide platelet in the center (see detail in Fig.4b)which is surrounded by an extrinsic stacking fault (Fig.4c).Such stacking faults may serve as heterogeneous nucleation sites for additional copper silicide precipitates and hence may initiate colony growth [26,31].The thickness and diameter of the central platelets critically depend on the copper con-tamination level and cooling rate,and are in the range of 0.6to 5nm and 50to 500nm,respectively.DLTS measurements on n-type Si containing copper silicide precipitates as described above,typically show a single broad line [9](Fig.5).The coincidence of the high tem-perature sides of DLTS lines measured for different filling pulse lengths (Fig.5c)re-Structural and Electrical Properties of Metal Silicide Precipitates in Silicon307Fig.5.DLTS spectra from copper silicide precipitates.a)and b)Measured and simulated correla-tion frequency variations,respectively;c)and d)measured and simulated filling pulse length varia-tions,respectively.Note the common high temperature side for spectra obtained at different filling pulse lengths (parts c)and d))which is a fingerprint for band-like statesveals the band-like character of the deep states associated with the precipitates.Fig.5b and d show simulated DLTS spectra obtained from a two-dimensional density-of-states function with band edges at E c À0X 15eV and E c À0X 45eV ,a 0X 58eV ,and a neutral occupation F N =0.72[33].The spectra are in qualitative agreement with the experi-mental data.Due to the significantly different slopes of the low temperature sides of measured and simulated spectra,however,fits to experimental spectra of copper silicide precipitates are currently not possible.Hence,the above parameter values should be viewed as an estimate.As for NiSi 2platelets,internal ripening of copper silicide precipitates is observed for annealing at temperatures between 260and 400 C [26,33].HRTEM revealed that the main effect is a morphological transformation of the central platelet into a spherical precipitate which has been discussed in terms of misfit accommodation by elastic defor-mation and secondary defect formation [26].DLTS spectra obtained during internal ripening show a different shape (compared to those of Fig.5)but are still due to band-like states [33].308M.Seibt etal.Fig.6.Copper precipitate colony after Ost-wald ripening at 700 C.a)TEM micro-graph showing a central precipitate (P)sur-rounded by spherical particles attached to a dislocation loop,b)DLTS spectra for dif-ferent filling pulse lengths;the common high temperature side is a fingerprint of band-like statesStructural and Electrical Properties of Metal Silicide Precipitates in Silicon309 Conventional Ostwald ripening of copper silicide precipitates has been observed for annealing at700 C leading to the formation of small copper silicide colonies[26]. Fig.6a is a TEM micrograph of such a colony consisting of a central precipitate(P)and spherical precipitates attached to a perfect dislocation loop.The DLTS spectra obtained for different filling pulse lengths(Fig.6b)coincide on their high temperature side. Hence,also precipitate colonies introduce deep band-like states into the bandgap of silicon.Like in the case of NiSi2precipitates,electronic states at precipitate/matrix in-terfaces or at the dislocations may be associated with the DLTS lines,and a distinction between these possibilities on the basis of DLTS alone is not possible.Bearing in mind that the character of the dislocations involved change from partial dislocations bounding the stacking fault of initial defects(Fig.4)to perfect dislocations after Ostwald ripening(Fig.6),the deep band-like states are tentatively attributed to pre-cipitate/matrix interfaces.In this interpretation,morphological changes of the copper silicide precipitates may change the density-of-states function which is reflected in the different shapes of DLTS lines,while the band-like character of the states is conserved.5.Summary and ConclusionsThe interrelation between formation,atomic structure and electrical properties of nickel and copper silicide precipitates in silicon has been described.For nickel,deep band-like states have been attributed to the b a a4h111i dislocation bounding the NiSi2platelets which establishes fast precipitate growth by incorporation of interstitial nickel via its core.During internal ripening,the translational symmetry of the dislocation core is destroyed due to its climb out of the initial(111)plane which results in a transition to localized states.While providing a coherent picture of platelet growth,ripening and electronic structure,the questions how the bounding dislocations nucleate and whether structurally different precursor stages exist,are open.The nucleation problem is related to the more general question of dislocation loop nucleation[34,35]in crack formation or strain relaxation in heteroepitaxial growth:nucleation barriers of such loops for a given mechanical stress or chemical driving force are generally beyond the regime of thermal fluctuations.Defect configurations formed during various stages of copper precipitation in silicon introduce deep band-like states into the bandgap of silicon which are tentatively attrib-uted to precipitate/matrix interfaces.Again a coherent picture of how defects formed during the early stages evolve into the well-known colony growth has emerged from experimental pared to nickel,however,much less is known about the atomic structure of precipitate/matrix interfaces.Furthermore,the discrepancies be-tween experimental and calculated DLTS spectra manifested in different slopes of the low temperature sides,indicate that current understanding of DLTS on extended de-fects is still not complete.Acknowledgements The authors are grateful to M.Griess and U.Gnauert for provid-ing experimental results and V.V.Kveder for critical reading of the manuscript.This work was financially supported by the Sonderforschungsbereich345and the Volkswa-gen Foundation.21physica(a)171/1310M.Seibt et al.:Structural and Electrical Properties of Precipitates in 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