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New trends in SIMOX
Nuclear Instruments and Methods in Physics Research B39 (1989) 194-202 North-Holland, Amsterdam
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Section III. SIMOX NEW TRENDS
and silicides
IN SIMOX
A.H. VAN OMMEN
Recent developments of the SIMOX technology, in which a silicon-on-insulator structure is synthesized by high-dose implantation of oxygen into Si, have been reviewed. A problem with these structures is that the Si layer generally contains a high density of dislocations (lo”-lo9 dislocations/cm2). However, recently three different methods have been reported in which the density of these defects was reduced to less than 10’ dislocations/cm* by optimization of the implantation conditions. These methods have been discussed and relations have been established between the processing conditions, the steady-state point defect concentrations and the microstructure of the material. The relations may be used as guidelines for further improvement of these structures. The success of the SIMOX technology has also stimulated the research on the synthesis of other compounds in Si. In general the structures formed by the ion beam synthesis technique are not only of good quality, but they frequently exhibit unique features, illustrating the potential of the SIMOX and other ion beam synthesis techniques,
1. Introduction The feasibility of the technology of forming a silicon-on-insulator structure by implanting a high dose of oxygen into Si at elevated temperatures was first demonstrated by Izumi et al. [l] in 1978. He named the technique SIMOX, an acronym for Separation by IMplanted Oxygen. In the last decade the quality of the material, that is formed by this technique, has been steadily improved. Moreover, high current (100 mA) oxygen implanters have become commercially available [2]. Small geometry CMOS devices fabricated in this material exhibit characteristics that are superior to those of similar devices formed in bulk Si [3]. Additionally, some circuits of moderate complexity like a 16K SRAM [4] and a microprocessor [5] have been formed successfully in this material. Consequently, it appears that all boundary conditions are fulfilled for SIMOX to become a mainstream technology. In the SIMOX process oxygen is impl~ted into Si at a high energy so that the maximum in the oxygen distribution, where the oxide layer will start to form, lies well be low the surface. The implantations are performed at elevated temperatures to continuously anneal the damage created by the collision cascades, accompanying the penetration of energetic ions. The second step comprises a high-temperature annealing treatment which is required to remove the precipitated oxygen from the silicon film above the dielectric. Jaussaud et al. [6] demonstrated that temperatures above 1200 o C are necessary to remove the oxide precipitates. Celler et al. [7] developed a special technique in which the implanted wafers can be annealed just below the melting point of silicon. These high-temperature annealing treatments render silicon-on-insulator (SOI) struc0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
tures with atomically sharp SiO,/Si interfaces and a precipitate free Si film. A problem with these structures is that they generally contain a very high density of dislocations. After implantation a dislocation density of 109-10’o dislocations/cm’ is commonly reported, Annealing can reduce this density by about one order of magnitude, so that the remaining defect density is still rather high. However, recently there have been several reports on SIMOX material with a dislocation density of less than lo5 dislocations/cm’. The improvement was achieved in all cases by optimization of the implantation conditions, since these defects are a remnant of the implantation damage. The three different methods of improvement that have been established thusfar are: Implantation at low dose rates into a channeling direction of the Si crystal [S] giving rise to ordering of the precipitates [9], sequential low-dose implantation and annealing [lo-121 and implantation at high dose rates Ieading to the formation of cavities in the silicon overlayer 113,141. We will discuss these new developments and relate the observations to phenomena that occur during the implantation. The success of the SIMOX technology has also been a stimulating factor in the research on the formation of other compounds in Si. The synthesis of buried Si,N, layers by implantation of nitrogen has been studied parallel to the work on oxygen (see e.g. ref. [1.5]). In addition to these insulating layers, buried semiconducting SIC films have been formed by implantation of carbon into Si [16,17]. A very spectacular development is the formation of buried single crystaltine silicide layers by implantation of transition metals. This technique was introduced by White et al. [IS], who named it “mesotaxy” to indicate that in this process the growth takes place in the bulk of the crystal. In the
A.H. Van Ommen / New trends WISIMOX
meantime buried layers of CoSi, [18], Nisi,, TiSi,, C&i,, YSi, and even Co,Ni,_,Si, [19] have been synthesized in Si. We will also discuss some of the exciting results in this area. The developments discussed above indicate that SIMOX is just one example of the vast potential of ion beam synthesis of buried compounds.
2. Microstructure
of SIMOX
structures
The general microstructure of an as-implanted SIMOX structure (2.5 x 10” O/cm*; 250 keV; 7’, = 600°C) is shown in the cross-sectional transmission electron microscope (XTEM) micrograph of fig. 1, together with some high resolution electron microscopy (HREM) images of specific microstructural features. The micrograph shows that a 490 nm thick buried oxide layer has been formed during the implantation below a 350 nm thick layer of Si. The oxygen concentration in the top Si film greatly exceeds the solid-solubility limit and precipitation occurs in the form of oxide precipitates. The left-hand
195
HREM micrograph of the upper Si-SiO, interface (a) shows these spherical precipitates. The size of these precipitates increases with increasing oxygen concentration and is also dependent on the implantation temperature [20,21]. Fig. 1 shows that the upper interface is much sharper than the lower interface. The HREM image of the interface region (A) shows that it is built up from rows of large precipitates embedded in Si. At a somewhat greater depth there is a region (B) in which we find the same spherical precipitates as above the oxide. The size of the precipitates below the oxide decreases rapidly with depth. The roughness of the lower interface is related to the fact that the oxidation stops at this point once a continuous oxide layer forms during implantation. From that moment on, Si interstitials that are generated to compensate the volume effect of the oxidation reaction can no longer reach the surface to annihilate, and the saturation of interstitials prevents further oxidation [15]. Fig. 1 reveals a very high density of dislocations (= 10” dislocations/cm*) in the top Si film. Other defects that have been found in this region after implantation are dislocation loops [22], stacking faults [21],
Fig. 1. Cross-sectional TEM micrograph and high resolution images of the microstructure commonly obtained after high-dose implantatio n of oxygen into Si. Implantation conditions were 2.5 X 10” O/cm*, 200 keV, 600 o C. III. SIMOX AND SILICIDES
A. H. Van Ommen
196
microtwins [21] and large interstitial Frank dipoles on (ill)-Si planes [22]. Below the oxide, only a high density of (113) defects is found [23] in region C of fig. 1. initially, we [23] interpreted these defects as the highpressure silica phase coesite [24], but Bourret’s more recent explanation [25] in terms of precipitated excess Si interstitials appears more likely.
3. Relation between conditions
microstructure
and implantation
The two predominant processes that occur during the implantation are [26]: (i) Frenkel defect generation by the collision cascades of the oxygen ions: Si” 8 si’ + v. (ii) Reaction
(I)
of the implanted
2Si”+ 20’ * (SiO,)‘+ 40 Aa
oxygen ions to SiO,:
Si’
(2)
46 A3
in which the superscripts indicate substitutional (s) and interstitial (i) lattice positions. Reaction (1) is the dominant source of point defects, since each oxygen ion typically generates lO’-lo3
New trends in SIMOX
Frenkel pairs. The distribution of these defects is determined by the damage profile, which has its maximum closer to the surface than the oxygen concentration profile. Additionally, the oxidation reaction (2) gives rise to the generation of silicon interstitials, since volume has to be conserved in this internal oxidation process. Although the number of defects generated by this reaction is much smaller, it shows that the reaction rate depends on the point defect concentrations. A supersaturation of Si interstitials will stop the reaction, as is observed at the lower oxide interface. Another important effect of reaction (2) is that it gives rise to internal strain in the Si. It is the combination of the supersaturation of point defects and the presence of this strain that will give rise to the nucleation of extended defects. The preceeding shows that the microstructure will greatly depends on the steady-state point defect concentrations. Point defects are generated by reactions (1) and (2) at a rate proportional to the ion flux. The generated defects will migrate through the crystal with a diffusion coefficient which depends on the implantation temperature, and thus controls the rate at which the defects are annihilated. It should however be noted that, due to the different migration enthalpies of vacancies and interstitials, changing the implantation temperature
f
510
520 Stokes
530
shift (cm-‘)
Fig. 2. Micro-Raman measurements, recorded from a low-angle lapped bevel of the as-implanted structure. Bevel shown in the optical micrograph on the right. Implantation conditions were: 2 X 10” O/cm’, 200 keV, 600 * C. The wavelength of the laser light was 410 nm, limiting the information depth to 125 nm. The locations of the spectra correspond to the positions at which they were recorded. The various depths can be read on the right-hand scale.
A.H. Van Ommen
will not only change the absolute defect concentrations but also the relative concentrations of vacancies and interstitials. Annihilation of excess point defects can occur by various mechanisms, but when the diffusion lengths are large enough the surface will be the predominant sink. In that case the chemical state of the surface (i.e. bare, oxide covered, etc.) will also affect the steady-state point defect concentrations. The influence of sinks on microstructure is clearly demonstrated at the lower oxide interface, where the oxidation stops due to the lack of a sink for Si interstitials. Processing variables that will affect the steady-state point defect equilibria and thereby the microstructure of the SIMOX material are: implantation energy, dose, temperature, ion flux, angle of incidence of the ions and chemical state of the surface. Once of the major goals in the present research on SIMOX is to reduce the dislocation density. Excess point defects and strain play a crucial role in the formation of these defects. The oxidation reaction (2) gives rise to compressive stress in the oxide, as is observed by the frequency shift of the infrared absorption bands [27]. The presence of tensile strain in the top Si film of as-implanted SIMOX structures has been shown by Raman scattering measurements [8,27,28]. Fig. 2 illustrates the depth dependence of this strain, as measured after implantation on a low-angle lapped bevel. Taking the perfect Si spectrum as a reference (dashed line), one can see that the Si below the oxide appears to be under compressive strain (shift of the spectrum to higher wave numbers), whereas the Si above the oxide is clearly under tensile strain. Although these are only preliminary results to demonstrate the feasibility of the technique, it does present a good example of the rapid evolution of optical analysis techniques, most of which are nondestructive, that are being developed to characterize SIMOX structures. In addition to infrared absorption [27] and Raman scattering [8,27,28] photoluminescence [29,30], spectroscopic ellipsometry [31], UV reflection [32], elastic light scattering [33] and optical interference methods [34] have been employed.
4. Low-dislocation
density
SIMOX
Now that we have gained some insight into the phenomena that occur during the implantation we will discuss the three methods that have been found to reduce the dislocation density in the top Si film of SIMOX structures.
4.1. Channeling implants at low dose-rates The use of low dose-rates will result in lower steadystate defect concentrations, since there will effectively be more time to anneal the defects. A second way to
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/ New trends in SIMOX
Fig. 3. XTEM micrograph of a SIMOX structure implanted at a low dose rate into a channeling direction. The laminar contrast is due to precipitate ordering.
reduce the densities of these point defects is by implanting into a channeling direction of the Si crystal. This will significantly reduce the number of displacive collisions, specifically in the top Si film [8]. The microstructure that is obtained after channeling implantation of 2.5 x 10’s O/cm2 at an energy of 300 keV a temperature of 570°C and a flux of 1.5 uA/cm’, is shown in the XTEM micrograph of fig. 3. Three regions (I-III) can be distinguished in the top Si film. Region I consists of perfect Si and we have shown [8] that this region is denuded of oxygen by outdiffusion during implantation. Region II exhibits a laminar contrast which is due to ordering of the oxide precipitates [35] into a simple cubic superlattice which has a periodicity of about 5 nm and is aligned with the (100) directions of the Si lattice. The size of the precipitates (2 nm) is fairly constant of this region. A similar microstructure has been observed by a number of other groups working on oxygen implanted Si [9,22,36,37], but also for nitrogen [38] and carbon [17] implanted Si. The superlattice is not very well resolved in the (110) cross-section of fig. 3. Rotation of this structure through 45” to the (010) direction of Si, renders the micrographs in fig. 4 in which the superlattice can be clearly distinguished. The inset shows that it also gives rise to additional spots in the electron diffraction pattern. The thickness of region II (fig. 3) is limited by the maximum concentration of oxygen that can be accomodated in the superlattice structure (1.6 x 102’ O/cm’) [8]. When this concentration is exceeded the microstructure of region III results in which the precipitates are much larger in size (10 nm) and which contains a very high density (= 10’ dislocations/cm2) III. SIMOX
AND
SILICIDES
A.H. Van Ommen / New
trendsin SIMOX
Fig. 5. XTEM micrograph annealing
Fig. 4. XTEM micrograph similar to that of fig. 3 but rotated from the (110) to the (010) direction of Si to show the super-
of the structure in fig. 3 after
at 1300 o C for 1 h.
considerably less strain [El. This is probably related to precipitate ordering, in which the precipitates align with the (100) directions of Si along which strain is most easily accommodated. In general we find that when Raman measurements reveal little strain in the as-implanted state, low-dislocation SO1 is obtained after annealing.
lattice. The inset shows the electron diffraction pattern revealing extra first order diffraction spots in (100) directions, due to the ordered precipitate structure.
of defects. The most important feature in fig. 3 is that these defects are now confined to a region close to the buried oxide interface, whereas in fig. 1 they we present throughout the entire top Si film. The advantage of the confinement of the dislocations immediately becomes clear after annealing at 1300°C for 1 h (fig. 5). At this high temperature all of the small precipitates in the former region II have been dissolved resulting in the growth of the larger precipitates in region III. These growing precipitates pin the dislocations and prevent them from extending to the surface [39]. Prolonged annealing (8 h) at 1300 ’ C leads to the elimination of both the precipitates and the dislocations, resulting in the structure shown in fig. 6. The Si inclusions near the lower SiO,/Si interface are a remnant of the roughness of the interface in the as-implanted state. Plan-view TEM revealed that the dislocation density in the top Si film is less than 10’ disl~ations/cm2 1391. The reduction of the dislocation density is only partially due to the reduction in the steady-state point defect concentrations. Raman measurements have shown that the as-implanted structures also contain
A number of groups [lo-121 have recently reported on low-dislocation SIMOX (< lo5 dislocations/cm’) obtained by sequential implantation and annealing. In the first reports six implants of 3 x lOI O/cm* at 150
micrograph of the structure in fig. 3 after at 1300 o C for 8 h. Part of the Si film was lost by oxidation during the anneal.
Fig. 6. XTEM annealing
A.H.
Az476.4
“m
Van Ommen
5~10170/Ul12 200
fn
510
520 Stokes
keV
530
Shift (cm-‘)
Fig. 7. Stokes Raman spectra recorded form SIMOX structures implanted with a relatively low dose at two different implantation temperatures. The spectrum of virgin Si (dashed) is given as a reference.
keV (475O C) and six annealing treatments of 16 h at 1275 “C were used. Margail et al. [12] performed a sequence of two implantations of 8 X 10” O/cm2 at 200 keV (600 o C) and two annealing steps of 6 hours at 1300 o C. In addition to a dislocation density of less than lo5 dislocations/cm2 these authors also found that the final structure did not contain the silicon inclusions which are commonly observed near the lower interface (fig. 6). In this method the point defect concentrations are reduced by interrupting the implantation and eliminating them by annealing. However, this procedure still requires optimization of the implantation conditions. In fig. 7 we show Raman spectra in the as-implated state for implantations of 5 X 10” O/cm’ at 200 keV and at temperatures of 550” C implant reveals a significant amount of strain in the Si, whereas for the 6OO’C
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/ New trends WISIMOX
implant hardly any strain is present. Plan-view TEM micrographs (fig. 8) of material annealed for 4 h at 1350” C reveal that the strain introduced by the low-temperature implantation leads to a high dislocation density of 3 x 10’ dislocations/cm2, whereas the specimen implanted at 600” C exhibits a dislocation density of less than lo5 dislocations/cm2. (The dark areas in these figures are due to silicon inclusions and discontinuities in the oxide film.) Evidently, when using optimized implantation conditions, repetition of this implantation and annealing treatments can result in low-dislocation SO1 structures containing thicker buried oxide layers. 4.3. High dose-rate implantations The use of high dose rates (34 pA/cm2) was recently found to lead to the formation of cavities in the upper part of the top Si film. The cavities are believed to be filled with oxygen gas [13]. Maszara [14] applied even higher instantaneous current densities (= 1 mA/cm’) and found that these spherical cavities build up a regular structure consisting of parallel columns of voids. A surprising observation was that the direction of these columns aligned with the ion beam, which was unrelated with any major crystallographic axis of Si. Maszara [14] has tentatively attributed this to overlapping ionization thermal spikes. Voids were only found to occur when the implantation was carried out into bare Si and were absent when the implanation was carried out through a protective layer of SiO,. The XTEM micrograph of Maszara in fig. 9, shows the columns of voids on the left for implantations into bare Si, and the absence of these features on the right, where part of the
Fig. 8. Plan-view TEM micrographs after high-temperature annealing of the specimens in fig. 7. (Conditions: 5 x 10” O/cm’; 200 keV; 4 h at 1350 o C.) The strain after implantation at 550 o C gives rise to a dislocation density of 3 X 10’ cme2 after annealing, whereas for the 600° C implantation it is less than lo5 cmm2. III. SIMOX
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SILICIDES
A.H. Van Ommen / New trends in SIMOX
200
Fig. 9. XTEM micrograph from Maszara [14], illustrating the columnar structure of voids for high dose-rate implantations into bare Si (left) and the absence of these features below a protective oxide (right).
protective oxide can still be seen to be present. There are indications that this material also has potential to render low-dislocation SO1 material after annealing [13]. A low dislocation density has been reported for annealing treatments upto 1250” C. However, at these temperatures large facetted cavities are still found to be present in the top Si layer. After elimination of these cavities at 1300 o C dislocations are found to be present. Hence in this case further optimization of the annealing procedure is required. In these experiments the steady-state point defect concentrations were affected by the high dose rate and by the chemical state of the surface. A bare Si surface is an effective sink for excess interstitials, and the difference in mobility of the defects leads to a supersaturation of vacancies. This again shows that relations exist between the processing conditions, the steady-state point defect concentrations and the microstructure, which can be used as guidelines for further improvement of SIMOX material.
to the formation of buried dielectric layers, Reeson et al. [16,17] have recently also studied the synthesis of the wide band-gap semiconductor SIC by implanting C into Si. They found that after subsequent high temperature annealing a buried /I-Sic film is formed, which has an epitaxial relationship with the (100)~Si substrate. The microstructure of C-implanted Si turns out to be very complex [17] containing interesting microstructural features, like voids and precipitate ordering, that have also been found in oxygen implanted Si.
5. Synthesis of other buried compounds There have been a number of interesting developments in addition to the “traditional” work on buried 50, and Si,N, layers. To combine the strengths of these two materials, Reeson et al. [40] have studied the formation of buried SiO,N, layers by implanting combinations of O+ and N+ ions or NO+ ions. The resulting structure was found to depend strongly on the sequence in which the ions were implanted. In addition
Fig. 10. XTEM micrograph of a buried single-crystalline Co%, layer in (100)-a, formed by implantation of 2 X 10” Co/cm2 and annealing. The insets are HREM images of the interfaces, illustrating the coherence of the two lattices.
A.H. Van Ommen
Finally, also buried metallic films can be synthesized within the Si crystal. White et al. [18] have shown that a buried layer of CoSi, can be formed within Si by Co implantation and annealing. CoSi, exhibits the cubic CaF, structure and has a lattice mismatch of only 1.2% with Si. Epitaxial growth of CoSi, has been observed on (111)-a, although in a twinned orientation, but attempts to grow monocrystalline films on (lOO)-Si have been unsuccessful as yet. In fig. 10 we show a XTEM micrograph of a single heteroepitaxial Si/CoSi,/Si structure in (100) oriented Si obtained by implantation of 2 x 10” Co/cm2 at 170 keV (450 o C), followed by an annealing treatment of 30 min at 1000°C. The HREM images of the interfaces show that the buried monocrystalline CoSi, film is coherent with the Si lattice. This example demonstrates that ion beam synthesis can provide unique structures of very high quality.
6. Conclusions Recent developments have shown that the quality of SIMOX material can still be significantly improved by optimization of the implantation conditions the microstructure after implantation has been shown to depend strongly on the steady-state point defect concentrations and on internal strain. Reduction of these two quantities by optimization of the processing parameters will lead to better quality material. The success of SIMOX has also stimulated research into the synthesis of other buried compounds in Si. Semiconducting and metallic buried layers of excellent quality have been synthesized, indicating the great potential of ion beam synthesis techniques like SIMOX. The authors thanks W.P. Maszara for the permission to publish fig. 9 and K.J. Reeson and P.L.F. Hemment for communicating many of their new results prior to publication. He is pleased to acknowledge the valuable contributions from J.J.M. Ottenheim, J. Politiek, H.J. Ligthart, B.H. Koek, M.P.A. Viegers, A.H. Reader, R. Hokke, C.W.T. Bulle-Lieuwma and H. van den Boom.
References PI K. Izumi, M. Doken and H. Ariyoshi, Electron. Lett. 14 (1978) 593. R.E. Kaim and K. PI J.P. Ruffel, D.H. Douglas-Hammilton, Izumi, Nucl. Instr. and Meth. B21 (1987) 229. [31 J.-P. Colinge, IEEE Electra. Dev. Lett. EDL-7 (1986) 244. [41 C.-E. Chen, Mater. Res. Sot. Symp. Proc., vol. 107, Silicon-On-Insulator and Buried Metals, eds. Ck. Chen, P.L.F. Hemment, J.C. Sturm and L. Pfeiffer (The Materials Research Society, Pittsburg, 1988).
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M. Bruel, C. Jaussaud, J. Margail, [51 A.J. Auberton-Herve, W. D’Hespel, J.F. Pere, A. Vitez and A. Tissot, Proc. European SOI Workshop, Meylan, France (March 1988). J. Stoemenos, J. Margail, D. Dupuy, B. [cl C. Jaussaud, Blanchard and M. Bruel, Appl. Phys. Lett. 46 (1985) 1046. K.W. West and J.M. Gib[71 G.K. Celler, P.L.F. Hemment son, Appl. Phys. lett. 48 (1986) 532. PI A.H. van Ommen, Mater. Res. Sot. Symp. Proc., vol. 107, Silicon-On-Insulator and Buried Metals, eds. C.K. Chen, P.L.F. Hemment, J.C. Sturm and L. Pfeiffer (The Materials Research Society, Pittsburg, 1988). [91 A.H. van Ommen, B.H. Koek and M.P.A. Viegers, Appl. Phys. Lett. 49 (1986) 1062. and G. Fraundorf, IEEE SOS/SO1 DOI D. Hill, P. Fraundorf Technology Workshop, Durango, Colorado (1987). P11 T. Cheek and C.-E. Chen, Mater. Res. Sot. Symp. Proc., vol. 107, Silicon-On-Insulator and Buried Metals, eds. C.K. Chen, P.L.F. Hemment, J.C. Sturm and L. Pfeiffer (The Materials Research Society, Pittsburg, 1988). [=I J. Margail, J. Stoemenos, C. Jaussaud and M. Bruel, Proc. European SOI Workshop, Meyland, France (March 1988). S.J. Pennycook, T.P. Sjoreen and J. [I31 M.K. El-Ghor, Narayan, Mater. Res. Symp. Proc., vol. 74, Beam-Solid Interactions and Transient Processes, eds. S.T. Picraux, M.O. Thompson and S.J. Williams (The Materials Research society, Pittsburg 1987) p. 591. [I41 W.P. Maszara, Mater. Res. Symp. Proc., vol. 93, Materials Modification and Growth Using Ion Beams, eds. U. Gibson, A.E. White and P.P. Pronko (The Materials Research Society, Pittsburg, 1987) p. 143. [I51 K.J. Reeson, Nucl. Instr. and Meth. B19/20 (1987) 269. [161 K.J. Reeson, P.L.F. Hemment, J. Stoemenos, J.R. Davis and G.K. Celler, Inst. Phys. Conf. Ser. 87 (1987) 427. u71 K.J. Reeson, P.L.F. Hemment, J. Stoemenos, J.R. Davis and G.K. Celler, Mater. Res. Sot. Symp. Proc., vol. 107, Silicon-On-Insulator and Buried Metals, eds. C.K. Chen, P.L.F. Hemment, J.C. Sturm and L. Pfeiffer (The Materials Research Society, Pittsburg, 1988). WI A.E. White, K.T. Short, R.C. Dynes, J.P. Garno and J.M. Gibson, Appl. Phys. Lett. 50 (1987) 95. u91 A.E. White, K.T. Short, R.C. Dynes, J.M. Gibson and R. Hull, Mater. Res. Sot. Symp. Proc., vol. 107, Silicon-OnInsulator and Buried Metals, eds. C.K. Chen, P.L.F. Hemment, J.C. Sturm and L. Pfeiffer (The Materials Research Society, Pittsburg, 1988). Mater. Res. Sot. Symp. Proc., vol. 53, WI P.L.F. Hemment, Semiconductor-on-Insulator and Thin Film Transistor Technology, eds. A. Chiang, M.W. Geis and L. Pfeiffer (The Materials Research Society, Pittsburg, 1986) p. 207. [21] S.J. Krause, C.O. Jung, T.S. Ravi, S.R. Wilson and D.E. Burke, Mater. Res. SG. Symp. Proc., vol. 107 Silicon-OnInsulator and Buried Metals, eds. C.K. Chen, P.L.F. Hemment, J.C. Strum and L. Pfeiffer (The Materials Research Society, Pittsburg, 1988). J. Stoemenos, J. Margain, M. Dupuy and C. Jaussaud, Phys. Scripta 35 (1987) 42. A.H. van Ommen, B.H. Koek and M.P.A. Viegers, Appl. Phys. Lett. 49 (1986) 628. A. Bourret, J. Thibault-Desseaux and D.N. Seidman, J. Appl. Phys. 55 (1984) 825. A. Bourret, Inst. Phys. Conf. Ser. 87 (1987) 39.
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[26] A.H. van Ommen and M.P.A. Viegers, Appl. Surf. Sci. 30 (1987) 383. 1271 G. Harbeke, E.F. Steigmeier, P.L.F. Hemment and K.J. Reeson, Semicon. Sci. Technol. 2 (1987) 687. [28] D.J. Olego, H. Baumgart and G.K. Celler, Appl. Phys. Lett. 52 (1988) 483. [29] W.M. Duncan, P.-H. Chang, B.-Y. Mao and C.-E. Chen, Appl. Phys. Lett. 51 (1987) 773. [30] J. Weber, H. Baumgart, J. Petruzello and G.K. Celler, Mater. Res. Sot. Symp. Proc., vol. 107, Silicon-On-Insulator and Buried Metals, eds. C.K. Chen, P.L.F. Hemment, J.C. Sturm and L. Pfeiffer (The Materials Research Society, Pittsburg, 1988). [31] J. Narayan, S.Y. Kim, K. Vedam and R. Manukonda, App. Phys. Lett. 51 (1987) 343. [32] I. Jastnebski and G. Harbeke. IEEE SOS/SOI Technology Workshop. Durango Colorado (1987). 1331 E.F. Steigmeier, G. Harbeke, H. Auderset. P.L.F. Hemment and K.J. Reeson, Proc. European SO1 Workshop, Meylan, France (March 1988).
[34] T.K. Kamins and J.-P. Colinge, Electron. Lett 22 (1986) 187. [35] M.P.A. Viegers, B.H. Koek and A.H. van Ommen, Extended abstracts 18th Conf. on Solid State Devices and Materials, Tokyo (1986) p. 557. [36] O.W. Holland, D. Fathy, T.P. Sjoreen, J. Narayan and K. More, Proc. Sot. Photo-opt. Instrum. Eng. 530 (1985) 255. [37] S. Maillet, R. Stuck, J.J. Grab, A. Golanski, R. Pantel and A. Perio, Nucl. Instr. and Meth. B19/20 (1987) 294. [38] L. Nesbit, S. Stiffler, G. Slusser and H. Vinton, J. Electrochem. Sot. 132 (1985) 119. [39] A.H. van Ommen, H.J. Ligthard, J. Politiek and M.P.A. Viegers, Mater. Res. Symp. Proc., vol. 93, Materials Modification and Growth llsing Ion Beams, eds. U. Gibson, A.E. White and P.P. Pronko (The Materials Research Society. Pittsburg, 1987) p.143. 1401 K.J. Reeson, P.L.F. Hemment, CD. Meekison, CD. Marsh, G.R. Booker, R.J. Chater, J.A. Kilner and J. Davis, Nucl. Instr. and Meth. B32 (1988) 427.
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Section III. SIMOX NEW TRENDS
and silicides
IN SIMOX
A.H. VAN OMMEN
Recent developments of the SIMOX technology, in which a silicon-on-insulator structure is synthesized by high-dose implantation of oxygen into Si, have been reviewed. A problem with these structures is that the Si layer generally contains a high density of dislocations (lo”-lo9 dislocations/cm2). However, recently three different methods have been reported in which the density of these defects was reduced to less than 10’ dislocations/cm* by optimization of the implantation conditions. These methods have been discussed and relations have been established between the processing conditions, the steady-state point defect concentrations and the microstructure of the material. The relations may be used as guidelines for further improvement of these structures. The success of the SIMOX technology has also stimulated the research on the synthesis of other compounds in Si. In general the structures formed by the ion beam synthesis technique are not only of good quality, but they frequently exhibit unique features, illustrating the potential of the SIMOX and other ion beam synthesis techniques,
1. Introduction The feasibility of the technology of forming a silicon-on-insulator structure by implanting a high dose of oxygen into Si at elevated temperatures was first demonstrated by Izumi et al. [l] in 1978. He named the technique SIMOX, an acronym for Separation by IMplanted Oxygen. In the last decade the quality of the material, that is formed by this technique, has been steadily improved. Moreover, high current (100 mA) oxygen implanters have become commercially available [2]. Small geometry CMOS devices fabricated in this material exhibit characteristics that are superior to those of similar devices formed in bulk Si [3]. Additionally, some circuits of moderate complexity like a 16K SRAM [4] and a microprocessor [5] have been formed successfully in this material. Consequently, it appears that all boundary conditions are fulfilled for SIMOX to become a mainstream technology. In the SIMOX process oxygen is impl~ted into Si at a high energy so that the maximum in the oxygen distribution, where the oxide layer will start to form, lies well be low the surface. The implantations are performed at elevated temperatures to continuously anneal the damage created by the collision cascades, accompanying the penetration of energetic ions. The second step comprises a high-temperature annealing treatment which is required to remove the precipitated oxygen from the silicon film above the dielectric. Jaussaud et al. [6] demonstrated that temperatures above 1200 o C are necessary to remove the oxide precipitates. Celler et al. [7] developed a special technique in which the implanted wafers can be annealed just below the melting point of silicon. These high-temperature annealing treatments render silicon-on-insulator (SOI) struc0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
tures with atomically sharp SiO,/Si interfaces and a precipitate free Si film. A problem with these structures is that they generally contain a very high density of dislocations. After implantation a dislocation density of 109-10’o dislocations/cm’ is commonly reported, Annealing can reduce this density by about one order of magnitude, so that the remaining defect density is still rather high. However, recently there have been several reports on SIMOX material with a dislocation density of less than lo5 dislocations/cm’. The improvement was achieved in all cases by optimization of the implantation conditions, since these defects are a remnant of the implantation damage. The three different methods of improvement that have been established thusfar are: Implantation at low dose rates into a channeling direction of the Si crystal [S] giving rise to ordering of the precipitates [9], sequential low-dose implantation and annealing [lo-121 and implantation at high dose rates Ieading to the formation of cavities in the silicon overlayer 113,141. We will discuss these new developments and relate the observations to phenomena that occur during the implantation. The success of the SIMOX technology has also been a stimulating factor in the research on the formation of other compounds in Si. The synthesis of buried Si,N, layers by implantation of nitrogen has been studied parallel to the work on oxygen (see e.g. ref. [1.5]). In addition to these insulating layers, buried semiconducting SIC films have been formed by implantation of carbon into Si [16,17]. A very spectacular development is the formation of buried single crystaltine silicide layers by implantation of transition metals. This technique was introduced by White et al. [IS], who named it “mesotaxy” to indicate that in this process the growth takes place in the bulk of the crystal. In the
A.H. Van Ommen / New trends WISIMOX
meantime buried layers of CoSi, [18], Nisi,, TiSi,, C&i,, YSi, and even Co,Ni,_,Si, [19] have been synthesized in Si. We will also discuss some of the exciting results in this area. The developments discussed above indicate that SIMOX is just one example of the vast potential of ion beam synthesis of buried compounds.
2. Microstructure
of SIMOX
structures
The general microstructure of an as-implanted SIMOX structure (2.5 x 10” O/cm*; 250 keV; 7’, = 600°C) is shown in the cross-sectional transmission electron microscope (XTEM) micrograph of fig. 1, together with some high resolution electron microscopy (HREM) images of specific microstructural features. The micrograph shows that a 490 nm thick buried oxide layer has been formed during the implantation below a 350 nm thick layer of Si. The oxygen concentration in the top Si film greatly exceeds the solid-solubility limit and precipitation occurs in the form of oxide precipitates. The left-hand
195
HREM micrograph of the upper Si-SiO, interface (a) shows these spherical precipitates. The size of these precipitates increases with increasing oxygen concentration and is also dependent on the implantation temperature [20,21]. Fig. 1 shows that the upper interface is much sharper than the lower interface. The HREM image of the interface region (A) shows that it is built up from rows of large precipitates embedded in Si. At a somewhat greater depth there is a region (B) in which we find the same spherical precipitates as above the oxide. The size of the precipitates below the oxide decreases rapidly with depth. The roughness of the lower interface is related to the fact that the oxidation stops at this point once a continuous oxide layer forms during implantation. From that moment on, Si interstitials that are generated to compensate the volume effect of the oxidation reaction can no longer reach the surface to annihilate, and the saturation of interstitials prevents further oxidation [15]. Fig. 1 reveals a very high density of dislocations (= 10” dislocations/cm*) in the top Si film. Other defects that have been found in this region after implantation are dislocation loops [22], stacking faults [21],
Fig. 1. Cross-sectional TEM micrograph and high resolution images of the microstructure commonly obtained after high-dose implantatio n of oxygen into Si. Implantation conditions were 2.5 X 10” O/cm*, 200 keV, 600 o C. III. SIMOX AND SILICIDES
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196
microtwins [21] and large interstitial Frank dipoles on (ill)-Si planes [22]. Below the oxide, only a high density of (113) defects is found [23] in region C of fig. 1. initially, we [23] interpreted these defects as the highpressure silica phase coesite [24], but Bourret’s more recent explanation [25] in terms of precipitated excess Si interstitials appears more likely.
3. Relation between conditions
microstructure
and implantation
The two predominant processes that occur during the implantation are [26]: (i) Frenkel defect generation by the collision cascades of the oxygen ions: Si” 8 si’ + v. (ii) Reaction
(I)
of the implanted
2Si”+ 20’ * (SiO,)‘+ 40 Aa
oxygen ions to SiO,:
Si’
(2)
46 A3
in which the superscripts indicate substitutional (s) and interstitial (i) lattice positions. Reaction (1) is the dominant source of point defects, since each oxygen ion typically generates lO’-lo3
New trends in SIMOX
Frenkel pairs. The distribution of these defects is determined by the damage profile, which has its maximum closer to the surface than the oxygen concentration profile. Additionally, the oxidation reaction (2) gives rise to the generation of silicon interstitials, since volume has to be conserved in this internal oxidation process. Although the number of defects generated by this reaction is much smaller, it shows that the reaction rate depends on the point defect concentrations. A supersaturation of Si interstitials will stop the reaction, as is observed at the lower oxide interface. Another important effect of reaction (2) is that it gives rise to internal strain in the Si. It is the combination of the supersaturation of point defects and the presence of this strain that will give rise to the nucleation of extended defects. The preceeding shows that the microstructure will greatly depends on the steady-state point defect concentrations. Point defects are generated by reactions (1) and (2) at a rate proportional to the ion flux. The generated defects will migrate through the crystal with a diffusion coefficient which depends on the implantation temperature, and thus controls the rate at which the defects are annihilated. It should however be noted that, due to the different migration enthalpies of vacancies and interstitials, changing the implantation temperature
f
510
520 Stokes
530
shift (cm-‘)
Fig. 2. Micro-Raman measurements, recorded from a low-angle lapped bevel of the as-implanted structure. Bevel shown in the optical micrograph on the right. Implantation conditions were: 2 X 10” O/cm’, 200 keV, 600 * C. The wavelength of the laser light was 410 nm, limiting the information depth to 125 nm. The locations of the spectra correspond to the positions at which they were recorded. The various depths can be read on the right-hand scale.
A.H. Van Ommen
will not only change the absolute defect concentrations but also the relative concentrations of vacancies and interstitials. Annihilation of excess point defects can occur by various mechanisms, but when the diffusion lengths are large enough the surface will be the predominant sink. In that case the chemical state of the surface (i.e. bare, oxide covered, etc.) will also affect the steady-state point defect concentrations. The influence of sinks on microstructure is clearly demonstrated at the lower oxide interface, where the oxidation stops due to the lack of a sink for Si interstitials. Processing variables that will affect the steady-state point defect equilibria and thereby the microstructure of the SIMOX material are: implantation energy, dose, temperature, ion flux, angle of incidence of the ions and chemical state of the surface. Once of the major goals in the present research on SIMOX is to reduce the dislocation density. Excess point defects and strain play a crucial role in the formation of these defects. The oxidation reaction (2) gives rise to compressive stress in the oxide, as is observed by the frequency shift of the infrared absorption bands [27]. The presence of tensile strain in the top Si film of as-implanted SIMOX structures has been shown by Raman scattering measurements [8,27,28]. Fig. 2 illustrates the depth dependence of this strain, as measured after implantation on a low-angle lapped bevel. Taking the perfect Si spectrum as a reference (dashed line), one can see that the Si below the oxide appears to be under compressive strain (shift of the spectrum to higher wave numbers), whereas the Si above the oxide is clearly under tensile strain. Although these are only preliminary results to demonstrate the feasibility of the technique, it does present a good example of the rapid evolution of optical analysis techniques, most of which are nondestructive, that are being developed to characterize SIMOX structures. In addition to infrared absorption [27] and Raman scattering [8,27,28] photoluminescence [29,30], spectroscopic ellipsometry [31], UV reflection [32], elastic light scattering [33] and optical interference methods [34] have been employed.
4. Low-dislocation
density
SIMOX
Now that we have gained some insight into the phenomena that occur during the implantation we will discuss the three methods that have been found to reduce the dislocation density in the top Si film of SIMOX structures.
4.1. Channeling implants at low dose-rates The use of low dose-rates will result in lower steadystate defect concentrations, since there will effectively be more time to anneal the defects. A second way to
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/ New trends in SIMOX
Fig. 3. XTEM micrograph of a SIMOX structure implanted at a low dose rate into a channeling direction. The laminar contrast is due to precipitate ordering.
reduce the densities of these point defects is by implanting into a channeling direction of the Si crystal. This will significantly reduce the number of displacive collisions, specifically in the top Si film [8]. The microstructure that is obtained after channeling implantation of 2.5 x 10’s O/cm2 at an energy of 300 keV a temperature of 570°C and a flux of 1.5 uA/cm’, is shown in the XTEM micrograph of fig. 3. Three regions (I-III) can be distinguished in the top Si film. Region I consists of perfect Si and we have shown [8] that this region is denuded of oxygen by outdiffusion during implantation. Region II exhibits a laminar contrast which is due to ordering of the oxide precipitates [35] into a simple cubic superlattice which has a periodicity of about 5 nm and is aligned with the (100) directions of the Si lattice. The size of the precipitates (2 nm) is fairly constant of this region. A similar microstructure has been observed by a number of other groups working on oxygen implanted Si [9,22,36,37], but also for nitrogen [38] and carbon [17] implanted Si. The superlattice is not very well resolved in the (110) cross-section of fig. 3. Rotation of this structure through 45” to the (010) direction of Si, renders the micrographs in fig. 4 in which the superlattice can be clearly distinguished. The inset shows that it also gives rise to additional spots in the electron diffraction pattern. The thickness of region II (fig. 3) is limited by the maximum concentration of oxygen that can be accomodated in the superlattice structure (1.6 x 102’ O/cm’) [8]. When this concentration is exceeded the microstructure of region III results in which the precipitates are much larger in size (10 nm) and which contains a very high density (= 10’ dislocations/cm2) III. SIMOX
AND
SILICIDES
A.H. Van Ommen / New
trendsin SIMOX
Fig. 5. XTEM micrograph annealing
Fig. 4. XTEM micrograph similar to that of fig. 3 but rotated from the (110) to the (010) direction of Si to show the super-
of the structure in fig. 3 after
at 1300 o C for 1 h.
considerably less strain [El. This is probably related to precipitate ordering, in which the precipitates align with the (100) directions of Si along which strain is most easily accommodated. In general we find that when Raman measurements reveal little strain in the as-implanted state, low-dislocation SO1 is obtained after annealing.
lattice. The inset shows the electron diffraction pattern revealing extra first order diffraction spots in (100) directions, due to the ordered precipitate structure.
of defects. The most important feature in fig. 3 is that these defects are now confined to a region close to the buried oxide interface, whereas in fig. 1 they we present throughout the entire top Si film. The advantage of the confinement of the dislocations immediately becomes clear after annealing at 1300°C for 1 h (fig. 5). At this high temperature all of the small precipitates in the former region II have been dissolved resulting in the growth of the larger precipitates in region III. These growing precipitates pin the dislocations and prevent them from extending to the surface [39]. Prolonged annealing (8 h) at 1300 ’ C leads to the elimination of both the precipitates and the dislocations, resulting in the structure shown in fig. 6. The Si inclusions near the lower SiO,/Si interface are a remnant of the roughness of the interface in the as-implanted state. Plan-view TEM revealed that the dislocation density in the top Si film is less than 10’ disl~ations/cm2 1391. The reduction of the dislocation density is only partially due to the reduction in the steady-state point defect concentrations. Raman measurements have shown that the as-implanted structures also contain
A number of groups [lo-121 have recently reported on low-dislocation SIMOX (< lo5 dislocations/cm’) obtained by sequential implantation and annealing. In the first reports six implants of 3 x lOI O/cm* at 150
micrograph of the structure in fig. 3 after at 1300 o C for 8 h. Part of the Si film was lost by oxidation during the anneal.
Fig. 6. XTEM annealing
A.H.
Az476.4
“m
Van Ommen
5~10170/Ul12 200
fn
510
520 Stokes
keV
530
Shift (cm-‘)
Fig. 7. Stokes Raman spectra recorded form SIMOX structures implanted with a relatively low dose at two different implantation temperatures. The spectrum of virgin Si (dashed) is given as a reference.
keV (475O C) and six annealing treatments of 16 h at 1275 “C were used. Margail et al. [12] performed a sequence of two implantations of 8 X 10” O/cm2 at 200 keV (600 o C) and two annealing steps of 6 hours at 1300 o C. In addition to a dislocation density of less than lo5 dislocations/cm2 these authors also found that the final structure did not contain the silicon inclusions which are commonly observed near the lower interface (fig. 6). In this method the point defect concentrations are reduced by interrupting the implantation and eliminating them by annealing. However, this procedure still requires optimization of the implantation conditions. In fig. 7 we show Raman spectra in the as-implated state for implantations of 5 X 10” O/cm’ at 200 keV and at temperatures of 550” C implant reveals a significant amount of strain in the Si, whereas for the 6OO’C
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/ New trends WISIMOX
implant hardly any strain is present. Plan-view TEM micrographs (fig. 8) of material annealed for 4 h at 1350” C reveal that the strain introduced by the low-temperature implantation leads to a high dislocation density of 3 x 10’ dislocations/cm2, whereas the specimen implanted at 600” C exhibits a dislocation density of less than lo5 dislocations/cm2. (The dark areas in these figures are due to silicon inclusions and discontinuities in the oxide film.) Evidently, when using optimized implantation conditions, repetition of this implantation and annealing treatments can result in low-dislocation SO1 structures containing thicker buried oxide layers. 4.3. High dose-rate implantations The use of high dose rates (34 pA/cm2) was recently found to lead to the formation of cavities in the upper part of the top Si film. The cavities are believed to be filled with oxygen gas [13]. Maszara [14] applied even higher instantaneous current densities (= 1 mA/cm’) and found that these spherical cavities build up a regular structure consisting of parallel columns of voids. A surprising observation was that the direction of these columns aligned with the ion beam, which was unrelated with any major crystallographic axis of Si. Maszara [14] has tentatively attributed this to overlapping ionization thermal spikes. Voids were only found to occur when the implantation was carried out into bare Si and were absent when the implanation was carried out through a protective layer of SiO,. The XTEM micrograph of Maszara in fig. 9, shows the columns of voids on the left for implantations into bare Si, and the absence of these features on the right, where part of the
Fig. 8. Plan-view TEM micrographs after high-temperature annealing of the specimens in fig. 7. (Conditions: 5 x 10” O/cm’; 200 keV; 4 h at 1350 o C.) The strain after implantation at 550 o C gives rise to a dislocation density of 3 X 10’ cme2 after annealing, whereas for the 600° C implantation it is less than lo5 cmm2. III. SIMOX
AND
SILICIDES
A.H. Van Ommen / New trends in SIMOX
200
Fig. 9. XTEM micrograph from Maszara [14], illustrating the columnar structure of voids for high dose-rate implantations into bare Si (left) and the absence of these features below a protective oxide (right).
protective oxide can still be seen to be present. There are indications that this material also has potential to render low-dislocation SO1 material after annealing [13]. A low dislocation density has been reported for annealing treatments upto 1250” C. However, at these temperatures large facetted cavities are still found to be present in the top Si layer. After elimination of these cavities at 1300 o C dislocations are found to be present. Hence in this case further optimization of the annealing procedure is required. In these experiments the steady-state point defect concentrations were affected by the high dose rate and by the chemical state of the surface. A bare Si surface is an effective sink for excess interstitials, and the difference in mobility of the defects leads to a supersaturation of vacancies. This again shows that relations exist between the processing conditions, the steady-state point defect concentrations and the microstructure, which can be used as guidelines for further improvement of SIMOX material.
to the formation of buried dielectric layers, Reeson et al. [16,17] have recently also studied the synthesis of the wide band-gap semiconductor SIC by implanting C into Si. They found that after subsequent high temperature annealing a buried /I-Sic film is formed, which has an epitaxial relationship with the (100)~Si substrate. The microstructure of C-implanted Si turns out to be very complex [17] containing interesting microstructural features, like voids and precipitate ordering, that have also been found in oxygen implanted Si.
5. Synthesis of other buried compounds There have been a number of interesting developments in addition to the “traditional” work on buried 50, and Si,N, layers. To combine the strengths of these two materials, Reeson et al. [40] have studied the formation of buried SiO,N, layers by implanting combinations of O+ and N+ ions or NO+ ions. The resulting structure was found to depend strongly on the sequence in which the ions were implanted. In addition
Fig. 10. XTEM micrograph of a buried single-crystalline Co%, layer in (100)-a, formed by implantation of 2 X 10” Co/cm2 and annealing. The insets are HREM images of the interfaces, illustrating the coherence of the two lattices.
A.H. Van Ommen
Finally, also buried metallic films can be synthesized within the Si crystal. White et al. [18] have shown that a buried layer of CoSi, can be formed within Si by Co implantation and annealing. CoSi, exhibits the cubic CaF, structure and has a lattice mismatch of only 1.2% with Si. Epitaxial growth of CoSi, has been observed on (111)-a, although in a twinned orientation, but attempts to grow monocrystalline films on (lOO)-Si have been unsuccessful as yet. In fig. 10 we show a XTEM micrograph of a single heteroepitaxial Si/CoSi,/Si structure in (100) oriented Si obtained by implantation of 2 x 10” Co/cm2 at 170 keV (450 o C), followed by an annealing treatment of 30 min at 1000°C. The HREM images of the interfaces show that the buried monocrystalline CoSi, film is coherent with the Si lattice. This example demonstrates that ion beam synthesis can provide unique structures of very high quality.
6. Conclusions Recent developments have shown that the quality of SIMOX material can still be significantly improved by optimization of the implantation conditions the microstructure after implantation has been shown to depend strongly on the steady-state point defect concentrations and on internal strain. Reduction of these two quantities by optimization of the processing parameters will lead to better quality material. The success of SIMOX has also stimulated research into the synthesis of other buried compounds in Si. Semiconducting and metallic buried layers of excellent quality have been synthesized, indicating the great potential of ion beam synthesis techniques like SIMOX. The authors thanks W.P. Maszara for the permission to publish fig. 9 and K.J. Reeson and P.L.F. Hemment for communicating many of their new results prior to publication. He is pleased to acknowledge the valuable contributions from J.J.M. Ottenheim, J. Politiek, H.J. Ligthart, B.H. Koek, M.P.A. Viegers, A.H. Reader, R. Hokke, C.W.T. Bulle-Lieuwma and H. van den Boom.
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