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Thermal annealing behavior of ion implanted muscovite mica: Implications for its defect structure
Nuclear Instruments and Methods in Physics Research Bl (1984) 402-408 North-Holland, Amsterdam
402
THERMAL ANNEALING BEHAVIOR OF ION IMPLANTED MUSCOVITE MICA: IMPLICATIONS FOR ITS DEFECT STRUCTURE J.-C. DRAN, Y. LANGEVIN,
J-C. PETIT, J. ~~U~ONT
and B. VASSENT
L&oratoireRentBerms,CNRS, Orsuy, France
High doses ( > 1012 ions cmM2) of low energy ( - 1 keV/amu) heavy ions produce, on insulating surfaces, thin layers of heavily damaged material (“planar tracks” - 1000 A) with an increased chemical reactivity. We have attempted to infer indirectly their defect structure in the case of muscovite mica from experiments ~mbining thermal annealing and etching. We show that planar tracks: (i} have a multi-layered structure with respect to etchability, probably reflecting the damage profile; (ii) are constituted of randomly distributed point defects and extended defects for which an “active” diameter - 100 A is inferred.
the implanted and unimplanted materials respectively at
1. Introduction High doses (> 1Or2 ions cmm2) of low energy (- 1 keV/amu) heavy ions produce on insulating surfaces thin layers (5 1000 A) of heavily damaged material with an increased chemical reactivity (“planar tracks”), the defect structure of which is still ill defined. Indeed, the X-ray scattering technique which has been applied [l] to linear tracks formed by ions of high energy to determine their defect structure has unfortunately not yet been applicable to the thin planar tracks. Therefore, we have attempted to infer indirectly their defect structure from experiments combining thermal annealing and etching. We first recall our experimental approach which has been already described [2] and briefly review the main features of the previous studies on the defect structure of high energy (- 1 MeV/amu) ion tracks performed in our laboratory [l]. Then, we will discuss our results obtained on low energy Pb ion planar tracks.
a depth x as well as an amplification factor kf x) = V*(x)/ Moreover, Ah reaches an asymptotic value Ah* which can be expressed as:
fro.
Ah* =im[l
- l/k(x)]dx.
When the etch rate is constant along the whole thickness of the implanted layer, due for instance to a constant density of defects, Y*(x)= V*, K = V*,/V” and then Ah* = (K- 1)/K(X),, where (X), is the etchable damage range that we define as the thickness of implanted material in which the density of defects is sufficient to induce an observable enhancement in etch rate. The variations of AY (or K ) and Ah * after isochronal(2 h) annealing runs under vacuum at temperatures ranging from 200 to 6OO’C have been investigated. This temperature is the upper limit of annealing as muscovite mica begins to suffer phase tranformation at about 625OC.
2. Experimental Grid-covered cleaved flakes of mica were implanted with either: (i) 10 l4 , 1015 and 3 X 1016 helium ions cmF2 at energies between 1 and 100 keV/amu, or (ii) 1Or3 and 10” lead ions crne2 at energies between 1 and 2.75 keVfamu, which are slightly above the critical doses for increased etchability and amorphization [3], respectively. Some samples were also implanted in the latter range of energies and fluences with H, Ne, Ar, I and Xe ions. After etching in HF (4% or 40%) at 30°C, the target is probed with a diamond stylus device (Talystep) and the ion-induced enhanced etchability is measured by the step-height Ah between irradiated and uuirradiated areas. One can define a differential etch rate AV= V*(x) - v”, where V*(x) and V” are the etch rates of 0168-583X/84/$03.00 0 Elsevier Science Publishers (North-Ho~and Physics Publishing Division)
B.V.
3. Recall of previous results on the defect structure of individual tracks of either high or low energy Dartyge et al. [l] have shown by combined scattering and etching experiments that high energy (- 1 MeV/amu) ion tracks are constituted of (fig. 1): (i) point defects of diameter -C 10 8, which are probably Frenkel pairs; (ii) clusters of defects constituted of several hundreds or thousands of point defects. These clusters have a diameter of a few tens of angstroms depending on the atomic number (Z) and the energy (E) of the incident ions. Moreover, their linear density also strongly depends on Z but moderately on E. These clusters, which dominate the etch rate as well as the annealing properties of the damaged material, are lin-
J. -C. Dran ei al, / Thermal annealing behaviour of ion implanted muscovite mica GAP
A,
JJ
AC ,
LATENT TRACK
Fig. 1. Theoretical defect structure of a high-energy ( - 1 MeV/amu) linear track in muscovite mica and formation of a terrace during etching.
ked by the point defects. Both defects are randomly distributed along the path of the ions. For muscovite mica, this discontinuous structure is visualized through
403
characteristic terraces in SEM or Nomarski interferential optical micrographs (fig. 2). The observation of terraces can be explained as follows: when the reagent encounters a cluster of defects the damaged material is rapidly dissolved at a rate V, which is - 10’ times higher than the etch rate V,, parallel to the cleavage planes; thus the corresponding section of the track appears as a narrow cylinder. In the gap of mean length A, between two successive clusters of defects the reagent encounters material containing only point defects and progresses at a lower rate Vs - Vt,; then the reagent has time to enlarge the track, thus leading to a terrace (see fig. 1). Such a terrace structure could be formed due to chemical inhomogeneities in the mineral; however, the data of the present paper strongly support the picture given above. Upon heating at increasing temperatures under
Fig. 2. Characteristic terrace structure of high-energy (7 MeV/amu) linear Fe tracks in mica, observed with SEM(a) and (b), optical microscope in transmission mode (c). and optical microscope in reflexion mode with a Nomarski interferential contrast device (d). VI. ION TRACKS
404
J. -C. Dran et al. / Thermal annealing behaviour of ion implanted muscovite mica
vacuum, these two types of defects anneal in two distinct temperature ranges, thus allowing their indirect identification by thermal annealing experiments. For example, in the case of fission fragment tracks in muscovite mica, these specific ranges are 200-3OO’C for point defects and 400-500°C for clusters of defects. This interesting property has been used in the present work. Borg [5] has extended these concepts to the case of low energy (- 1 keV/amu) individual ion tracks and suggested that for ion masses up to xenon low energy ion tracks only contain point defects, the density of which increases with the atomic number. On the contrary, heavier ions would also produce some clusters of defects inducing a terrace structure in etched tracks (1 to 3 clusters in - 15% of - 1 keV/amu lead ion tracks). The main purpose of this paper is to determine if the defect structure of the planar tracks is strikingly different from that of the original individual tracks as a result of recombination of defects, self annealing during implantation etc.
4. Results and discussion 4.1. Helium planar tracks
In order to definitively ascribe the terrace structure of etched tracks to the presence of clusters of defects, we implanted muscovite mica with - 100 keV/amu helium ions at a fluence of - 1014 ions cm-‘. At this energy, helium ions suffer mainly electronic energy loss,
produce only point defects and do not induce etchable individual tracks. When etched in 40% HF at 30°C for 2 h, the He-implanted sample clearly shows oblique and smooth edges in its irradiated areas (fig. 3). This feature provides two important clues: (i) there is no evidence for terraces when etching damaged layers containing only point defects. Moreover, the accumulation of point defects at this fluence does not lead to clusters of defects by aggregation. The correlation between clusters of defects and terraces is therefore strongly supported. This conclusion is reinforced by the terrace structure clearly present in the fission track contained in the implanted area (see arrow). (ii) The angle of the edges of the etched implanted areas allows evaluation of the ratio V$V,, = - 0.04. For this particular ion energy and dose, one can notice that this ratio is much smaller than the value (- 1) observed with individual high energy heavy ions [l]. This is probably due to the smaller concentration of defects in the corresponding planar track. The amplification factor Kg = V$V,,, where V,, is the etch rate of the undamaged material perpendicular to the planes, can also be inferred from the step-height measurement and is equal to - 4. This very small value compared to that of high energy tracks (- 104) probably explains the fact that He-latent tracks cannot be revealed as etched tracks with conventional techniques of observation. After a 2 h annealing run at 250°C, one notes a significant decrease of K down to < 2 and a marked increase in the roughness of the etched implanted area (see arrow of fig. 3B), characterized by a high density of
Fig. 3. Optical micrographs (Nomarski interferential contrast) of mica implanted with - lOI He cmv2 ( - 100 keV/amu) for 4 h in 40% HF at 30°C (see text). (A) before annealing (see text); (B) after 2 h annealing at 250°C.
and etched
J. -C. Dran et al. / Thermal annealing behaoiour of ion implanted muscovite mica
Fig. 4. Optical micrographs (Nomarski interferential contrast) illustrating the annealing properties of the planar track formed in mica by - lOI Pb cmm2 (E - 1 keV/amu), after 2 h etching in 40% HF at 30°C (see text). (A) before annealing; (B) after 2 h annealing at 500°C.
tiny corrosion dots (- 1 pm), which probably correspond to clusters of defects of higher chemical reactivity; such clusters could result from the aggregation of original point defects induced by annealing. On the low energy samples ( - 1 keV/amu), the AV values and their variation with annealing temperatures were very difficult to measure, due to the small range of the helium ions (- 200 A). However, one notes after a 2 h run at 2OO’C that the 1013 target has been almost completely annealed whereas the 10” target still shows a AL’- 100 A min-’ (in 40% HF) but is finally annealed at 500°C. The Ah* value is - 450 A for doses between 10” and 3 X lOI He cm-‘.
increase in Ah up to - 1500 A at a constant rate of - 900 A min-’ (in 4% HF), followed by a much slower increase at a constant rate of only - 6 A min-’ up to an asymptotic value Ah* - 2000 A after 1 h etching. The AV values for the 1 keV/amu targets are comparable in both parts of the curve and the Ah values are - 700 A after 1 min etching and - 1100 A after 1 h. After annealing (fig. 6), the AV for - 1013 Pb cm-* are moderately affected up to - 300 “C; for instance in 4% HF AV- 500 A min-’ (- 2OWC) and - 190 A min-’ (- 3OOT). On the other hand, AV values drastically decrease down to a negligible value -= 10 A min-’ at - 600°C. For the - 10” target, one observes that the initial AV is of the same order as at - 1013 (- 1000 A
4.2. Lead planar tracks We then investigated the etching properties of leadion planar tracks that are known to produce such clusters. Observation of an etched sample with a Nomarski contrast device revealed pm-sized corrosion dots (fig. 4). The kinetics of the the step-height Ah* for the lower fluence (10 l3 Pb cm-*) and the higher energy (2.75 keV/amu) is represented in fig. 5. The two major etching characteristics of planar tracks (AV and Ah*) were also studied after thermal annealing. The specific features of these parameters give interesting clues to the possible defect structure of low energy lead-implanted planar tracks even though the conclusions are still speculative and are given in the following. 4.2.1. Properties of AV Before annealing (fig. 5), one notes
a very sharp
{Ah(A)
Fig. 5. Variation of the step-height Ah with etching time for - 1 keV/amu lead ions. VI. ION TRACKS
406
J. -C. Dran et al, / Thermal annealing behaviour of ion implanted muscouite mica n
1016.cm-*
t l
IOU.cm-*
AV
i .mn-1
HF 4%
30-c
4, \
f; i
1 i i i i i i i i i i i
i
i
Fig. 6. Variation of AV with the annealing temperature for 2 h periods for - 1 keV/amu lead ions.
mm-‘) although the density of defects is multiplied by a factor probably between 10 and 100 if one takes into account the sputtering phenomenon. Moreover, AV remains nearly unaffected up to - 400~500°C before dramatically decreasing to negligible values at - 600°C. This low sensitivity of the etch rate with the defect concentration at high fluences is predicted by our Monte-Carlo model of etching reported elsewhere (see companion paper, page 557). Indeed, the major conclusion of this model is that the accumulation of damaged islands of increased chemical reactivity characterized by an intrinsic amplification factor ki induces a percolation threshold in the implanted layers. We recall that the average amplification factor K of the implanted material first very slowly increases with $I up to a critical density of such islands (corresponding to - 20% occupancy of the total volume in mica) where it markedly increases and then rapidly saturates to ki. Therefore, the K value at the higher fluence is an evaluation of the intrinsic ki of the defects that do dominate the etch rate of these planar tracks. In 40% HF the AY estimated in the first part of the curve is 17000 A mm-’ which is compatible with the V, value (104) measured for high energy heavy ion linear tracks, thus suggesting that clusters of defects also dominate the dissolution of these planar tracks. Another way to evaluate the possible role of both types of defects is to calculate their theoretical densities for the two fluences and their corresponding degrees of occupancy in the damaged layer. By assuming that one individual lead ion produces - 2000 point defects [4] with an approximate volume of - 10 A3, their density should be - 1.3 X 10” cme3 at 1013 ions cme2 and
- 1.3 x 1O23 at 1015 ions cme2 and the degrees of occupancy u - 0.013 and - 1.3 respectively. As these two fluences are both greater than the experimental critical fluence - 5 X 1012 Pb cmd2 previously reported (refs. 2 and S), the two values of u should be > 0.20. This demonstrates that point defects cannot be responsible for the observations. In addition, if one assumes that each incident lead ion produces on average - 0.25 clusters of defects [5] with a volume - 3 X lo4 K, roughly estimated by the superior limit measured by small angle X-ray scattering for high energy ion tracks, their density should be - 1.7 X 10” cme3 at 1013 ions cme2 and - 1.7 X 1019 cmm3 at 1015 ions cmd2. The degrees of occupancy o are thus - 0.005 and - 0.5 respectively, and for the same reasons as above the accumulation of clusters of defects directly produced by individual ions cannot explain the observations. However, Dartyge et al. [l] have shown that in the case of this second type of defect, the etch rate ruling parame ter is not the apparent size estimated by X-ray scattering but rather an actual “active” diameter X,, corresponding to the total width of the defect distribution in the cluster. For instance in the case of 1 MeV/amu Fe ions, X, - 700 A. For 1 keV/amu lead ions, the critical fluence +c - 5 X 1012 ions cmv2 implies a value X, of - 100 A which is smaller than that for high energy ions and suggests that the apparent diameter which could be inferred from X-ray studies would be significantly smaller than 40 A. This conclusion is reinforced by the relatively small effect of annealing on the Al/-values in the temperature range (25-300%) where point defects are known to vanish. On the contrary AV does markedly decrease in the temperature range (400-600°C) which characterizes annealing of clusters of defect. It must be emphasized that the 1015 targets show a higher resistance to annealing compared to those implanted at lower fluences since a 2OO’C excess annealing temperature is required to significantly reduce AV. This can be explained either by a larger size of the clusters of defects due to the aggregation of primary defects, or by a higher density of clusters. 4.2.2. Properties of Ah* The results concerning Ah* are as shown in fig. 7. The lower fluence samples clearly show a two step annealing behavior: first a marked decrease at a temperature 5 200°C then an almost constant value up to - 400°C followed by a second drastic diminution at higher temperatures. In contrast, the 1015 targets present a one-step annealing behavior. In fact, Ah * remains constant up to - 3OO’C and then linearly decreases but maintains a value - 1000 A at the highest annealing temperature (6OO’C). Before annealing, the A h*-values are identical (1500 A) for the two fluences investigated and, more-
407
J.-C. Dran et al. / Thermal annealing behauiour df ion implanted muscovite mica
Fig. 8. Variation of Ah* with the energy of incident ions. Fig. 7. Variation of Ah* with the annealing temperature (2 h periods) for - 1 keV/amu lead ions etched in 4% HF at 3O’C.
over, they are markedly greater than the calculated damage range (X) ,, - 400 A of individual lead ions in mica, and even than the sum (X), + (AXz)‘~z (mean width distribution). These two features can be tentatively interpreted as follows: (i) since mica is an extremely sensitive track detector (K - 104), a very small density of defects can induce a measurable enhancement in etch rate; as the etching time for measuring Ah* was very long (2 h) it is expected that we have revealed the entire width of defect distribution therefore exceeding the (X), + (AX2)‘k2. In contrast, in the case of soda-lime glass which is much less sensitive (K < lo), Ah* is very close to the value of the mean width distribution [3]. Moreover, the variations of Ah* with energy between 0.3 and 7.5 keV/amu studied for lead (lot3 and 1015 ions cmm2) and argon (10” ions cmm2) are linear as predicted by the corresponding theoretical damage distributions (fig. 8). This small density of defects at the bottom of the implanted layer could also be due to the knock-on implantation of light constituent elements of mica such as oxygen. Indeed, the experimental variations of Ah* with the Z of the incident ions (fig. 9) do correspond quite well to the theoretical ranges calculated for secondary oxygen ions (mass M,) when using the maximum energy E,_ transferred by the primary ions (Z, M,, E) in the classical formula E mm. = [Q%M,/(M,
ent kind of defect less resistant to annealing. This last assumption would probably favor the role of knock-on implantation in the enhancement of the thickness of the damaged layer at high fluences. In contrast, the first 4 of the damaged layer is much more resistant to thermal annealing. This is illustrated by the fact that, even for the highest temperature, the Ah* of the 10” target is precisely equal to the thickness of this sublayer. For this fluence the discontinuity is therefore revealed at 6OO’C. However, a second discontinuity in the first f sublayer of the 1013 target is reflected by the second annealing step for T 5 400°C. The remaining Ah* - 400 A then corresponds roughly to (X) n. In addition, these observations should be compared to the data on the partial annealing of various high energy heavy-ion tracks. At low annealing temperature, the etch rate of - 7 MeV/amu Fe tracks is markedly reduced while that of Kr tracks is unaffected [l]. This result is indicative of a much lower density of clusters of defects in the Fe tracks than that due to Kr ions. A qualitative description of the defect structure of the implanted layer would thus be the following: at both fluences the first sublayer of thickness (5(X),)
Ah*&
t
+ M212] E.
After annealing, the first characteristic worth noticing is the marked decrease of Ah* in the temperature range 2%200°C for the lower fluence whereas this parameter remains constant up to 300°C for the higher fluence. This clearly indicates for the lOI3 target a discontinuity in the structure of the implanted layer, with the lower j sublayer containing either a smaller density of defects than the first 4 sublayer, or a differ-
0
I
2 * 30
Fig. 9. Variation of Ah* keV/amu incident ions.
*o
66
with the atomic
number
for
-I
VI. ION TRACKS
408
J.-C. Dran et al. / Thermal annealing behaviour of ion implanted muscovite mica
would contain point defects and clusters of defects induced by the incident lead ions. The density of clusters would be high enough to satisfy the percolation threshold at both fluences. In the last f sublayer, where the density of clusters would rapidly diminish, the percolation threshold would only be reached in the 1Ol3 target by a combination of both types of defects whereas it could still be satisfied by clusters in the 10” target. It is also possible that u, is reached in this last sublayer by the accumulation of point defects due to knock-on inplantation of light ions for both fluences. After annealing at low temperature (200°C), point defects would have almost completely vanished. In the case of the 1Ol3 target, the density of defects in this last sublayer would be insufficient to satisfy the percolation threshold and thus this sublayer would be no longer etchable. When increasing the annealing temperature (200-400°C), the remaining point defects would completely disappear without any significant change in the etching characteristics. In the temperature range 400-500°C, the X, parameter would begin to diminish thus affecting the degree of occupancy of the clusters of defects themselves and consequently the percolation threshold would be no longer satisfied, therefore leading to a second annealing step. For the 1015 target, the percolation threshold would always be satisfied up to the range of temperatures (400-500°C) where the clusters of defects would be affected by annealing.
ments combining annealing and etching, although indirect, show that low energy lead-ion planar tracks are also probably made up of point defects and clusters of defects with an “active” diameter - lOOA, which delineate a multi-layered structure. The variations of the asymptotic step-height Ah* with the energy and the atomic number of incident ions have also been shown to be compatible with theoretical predictions derived from models of particle-solid interaction. It is now necessary to further support these preliminary results by more direct experimental evidence. For this purpose, we are currently preparing thin films of amorphous silica (few thousands of A) which will be implanted and observed by means of small angle X-ray scattering. The defect structure of the planar track, which should be directly observable, will then be compared to the one deduced from its etching properties. We acknowledge the very valuable help of F. Lalu in performing the implantations, and thank G. Vidal for the Talystep measurements, and G. Goutiere for his constant interest in this work.
References [l] E. Dartyge, [2]
5. Conclusion We have shown that the terrace structure of etched linear tracks of high energy heavy ions is indeed due to the presence of clusters of defects. Moreover, experi-
[3] [4] [5]
J-P. Duraud, Y. Langevin and M. Maurette, Phys. Rev. 23 (1981) 5213. J-C. Dran, M. Maurette and J-C. Petit, Science 209 (1980) 1518. J-C. Petit, These de doctorat d’Etat et sciences physiques, Universitt de Paris XI (1982). G.H. Kinchin and P.S. Pease, Rep. Prog. Phys. 18 (1955) 1. J. Borg, These de doctorat d’Etat, Universitt de Paris XI (1980).
402
THERMAL ANNEALING BEHAVIOR OF ION IMPLANTED MUSCOVITE MICA: IMPLICATIONS FOR ITS DEFECT STRUCTURE J.-C. DRAN, Y. LANGEVIN,
J-C. PETIT, J. ~~U~ONT
and B. VASSENT
L&oratoireRentBerms,CNRS, Orsuy, France
High doses ( > 1012 ions cmM2) of low energy ( - 1 keV/amu) heavy ions produce, on insulating surfaces, thin layers of heavily damaged material (“planar tracks” - 1000 A) with an increased chemical reactivity. We have attempted to infer indirectly their defect structure in the case of muscovite mica from experiments ~mbining thermal annealing and etching. We show that planar tracks: (i} have a multi-layered structure with respect to etchability, probably reflecting the damage profile; (ii) are constituted of randomly distributed point defects and extended defects for which an “active” diameter - 100 A is inferred.
the implanted and unimplanted materials respectively at
1. Introduction High doses (> 1Or2 ions cmm2) of low energy (- 1 keV/amu) heavy ions produce on insulating surfaces thin layers (5 1000 A) of heavily damaged material with an increased chemical reactivity (“planar tracks”), the defect structure of which is still ill defined. Indeed, the X-ray scattering technique which has been applied [l] to linear tracks formed by ions of high energy to determine their defect structure has unfortunately not yet been applicable to the thin planar tracks. Therefore, we have attempted to infer indirectly their defect structure from experiments combining thermal annealing and etching. We first recall our experimental approach which has been already described [2] and briefly review the main features of the previous studies on the defect structure of high energy (- 1 MeV/amu) ion tracks performed in our laboratory [l]. Then, we will discuss our results obtained on low energy Pb ion planar tracks.
a depth x as well as an amplification factor kf x) = V*(x)/ Moreover, Ah reaches an asymptotic value Ah* which can be expressed as:
fro.
Ah* =im[l
- l/k(x)]dx.
When the etch rate is constant along the whole thickness of the implanted layer, due for instance to a constant density of defects, Y*(x)= V*, K = V*,/V” and then Ah* = (K- 1)/K(X),, where (X), is the etchable damage range that we define as the thickness of implanted material in which the density of defects is sufficient to induce an observable enhancement in etch rate. The variations of AY (or K ) and Ah * after isochronal(2 h) annealing runs under vacuum at temperatures ranging from 200 to 6OO’C have been investigated. This temperature is the upper limit of annealing as muscovite mica begins to suffer phase tranformation at about 625OC.
2. Experimental Grid-covered cleaved flakes of mica were implanted with either: (i) 10 l4 , 1015 and 3 X 1016 helium ions cmF2 at energies between 1 and 100 keV/amu, or (ii) 1Or3 and 10” lead ions crne2 at energies between 1 and 2.75 keVfamu, which are slightly above the critical doses for increased etchability and amorphization [3], respectively. Some samples were also implanted in the latter range of energies and fluences with H, Ne, Ar, I and Xe ions. After etching in HF (4% or 40%) at 30°C, the target is probed with a diamond stylus device (Talystep) and the ion-induced enhanced etchability is measured by the step-height Ah between irradiated and uuirradiated areas. One can define a differential etch rate AV= V*(x) - v”, where V*(x) and V” are the etch rates of 0168-583X/84/$03.00 0 Elsevier Science Publishers (North-Ho~and Physics Publishing Division)
B.V.
3. Recall of previous results on the defect structure of individual tracks of either high or low energy Dartyge et al. [l] have shown by combined scattering and etching experiments that high energy (- 1 MeV/amu) ion tracks are constituted of (fig. 1): (i) point defects of diameter -C 10 8, which are probably Frenkel pairs; (ii) clusters of defects constituted of several hundreds or thousands of point defects. These clusters have a diameter of a few tens of angstroms depending on the atomic number (Z) and the energy (E) of the incident ions. Moreover, their linear density also strongly depends on Z but moderately on E. These clusters, which dominate the etch rate as well as the annealing properties of the damaged material, are lin-
J. -C. Dran ei al, / Thermal annealing behaviour of ion implanted muscovite mica GAP
A,
JJ
AC ,
LATENT TRACK
Fig. 1. Theoretical defect structure of a high-energy ( - 1 MeV/amu) linear track in muscovite mica and formation of a terrace during etching.
ked by the point defects. Both defects are randomly distributed along the path of the ions. For muscovite mica, this discontinuous structure is visualized through
403
characteristic terraces in SEM or Nomarski interferential optical micrographs (fig. 2). The observation of terraces can be explained as follows: when the reagent encounters a cluster of defects the damaged material is rapidly dissolved at a rate V, which is - 10’ times higher than the etch rate V,, parallel to the cleavage planes; thus the corresponding section of the track appears as a narrow cylinder. In the gap of mean length A, between two successive clusters of defects the reagent encounters material containing only point defects and progresses at a lower rate Vs - Vt,; then the reagent has time to enlarge the track, thus leading to a terrace (see fig. 1). Such a terrace structure could be formed due to chemical inhomogeneities in the mineral; however, the data of the present paper strongly support the picture given above. Upon heating at increasing temperatures under
Fig. 2. Characteristic terrace structure of high-energy (7 MeV/amu) linear Fe tracks in mica, observed with SEM(a) and (b), optical microscope in transmission mode (c). and optical microscope in reflexion mode with a Nomarski interferential contrast device (d). VI. ION TRACKS
404
J. -C. Dran et al. / Thermal annealing behaviour of ion implanted muscovite mica
vacuum, these two types of defects anneal in two distinct temperature ranges, thus allowing their indirect identification by thermal annealing experiments. For example, in the case of fission fragment tracks in muscovite mica, these specific ranges are 200-3OO’C for point defects and 400-500°C for clusters of defects. This interesting property has been used in the present work. Borg [5] has extended these concepts to the case of low energy (- 1 keV/amu) individual ion tracks and suggested that for ion masses up to xenon low energy ion tracks only contain point defects, the density of which increases with the atomic number. On the contrary, heavier ions would also produce some clusters of defects inducing a terrace structure in etched tracks (1 to 3 clusters in - 15% of - 1 keV/amu lead ion tracks). The main purpose of this paper is to determine if the defect structure of the planar tracks is strikingly different from that of the original individual tracks as a result of recombination of defects, self annealing during implantation etc.
4. Results and discussion 4.1. Helium planar tracks
In order to definitively ascribe the terrace structure of etched tracks to the presence of clusters of defects, we implanted muscovite mica with - 100 keV/amu helium ions at a fluence of - 1014 ions cm-‘. At this energy, helium ions suffer mainly electronic energy loss,
produce only point defects and do not induce etchable individual tracks. When etched in 40% HF at 30°C for 2 h, the He-implanted sample clearly shows oblique and smooth edges in its irradiated areas (fig. 3). This feature provides two important clues: (i) there is no evidence for terraces when etching damaged layers containing only point defects. Moreover, the accumulation of point defects at this fluence does not lead to clusters of defects by aggregation. The correlation between clusters of defects and terraces is therefore strongly supported. This conclusion is reinforced by the terrace structure clearly present in the fission track contained in the implanted area (see arrow). (ii) The angle of the edges of the etched implanted areas allows evaluation of the ratio V$V,, = - 0.04. For this particular ion energy and dose, one can notice that this ratio is much smaller than the value (- 1) observed with individual high energy heavy ions [l]. This is probably due to the smaller concentration of defects in the corresponding planar track. The amplification factor Kg = V$V,,, where V,, is the etch rate of the undamaged material perpendicular to the planes, can also be inferred from the step-height measurement and is equal to - 4. This very small value compared to that of high energy tracks (- 104) probably explains the fact that He-latent tracks cannot be revealed as etched tracks with conventional techniques of observation. After a 2 h annealing run at 250°C, one notes a significant decrease of K down to < 2 and a marked increase in the roughness of the etched implanted area (see arrow of fig. 3B), characterized by a high density of
Fig. 3. Optical micrographs (Nomarski interferential contrast) of mica implanted with - lOI He cmv2 ( - 100 keV/amu) for 4 h in 40% HF at 30°C (see text). (A) before annealing (see text); (B) after 2 h annealing at 250°C.
and etched
J. -C. Dran et al. / Thermal annealing behaoiour of ion implanted muscovite mica
Fig. 4. Optical micrographs (Nomarski interferential contrast) illustrating the annealing properties of the planar track formed in mica by - lOI Pb cmm2 (E - 1 keV/amu), after 2 h etching in 40% HF at 30°C (see text). (A) before annealing; (B) after 2 h annealing at 500°C.
tiny corrosion dots (- 1 pm), which probably correspond to clusters of defects of higher chemical reactivity; such clusters could result from the aggregation of original point defects induced by annealing. On the low energy samples ( - 1 keV/amu), the AV values and their variation with annealing temperatures were very difficult to measure, due to the small range of the helium ions (- 200 A). However, one notes after a 2 h run at 2OO’C that the 1013 target has been almost completely annealed whereas the 10” target still shows a AL’- 100 A min-’ (in 40% HF) but is finally annealed at 500°C. The Ah* value is - 450 A for doses between 10” and 3 X lOI He cm-‘.
increase in Ah up to - 1500 A at a constant rate of - 900 A min-’ (in 4% HF), followed by a much slower increase at a constant rate of only - 6 A min-’ up to an asymptotic value Ah* - 2000 A after 1 h etching. The AV values for the 1 keV/amu targets are comparable in both parts of the curve and the Ah values are - 700 A after 1 min etching and - 1100 A after 1 h. After annealing (fig. 6), the AV for - 1013 Pb cm-* are moderately affected up to - 300 “C; for instance in 4% HF AV- 500 A min-’ (- 2OWC) and - 190 A min-’ (- 3OOT). On the other hand, AV values drastically decrease down to a negligible value -= 10 A min-’ at - 600°C. For the - 10” target, one observes that the initial AV is of the same order as at - 1013 (- 1000 A
4.2. Lead planar tracks We then investigated the etching properties of leadion planar tracks that are known to produce such clusters. Observation of an etched sample with a Nomarski contrast device revealed pm-sized corrosion dots (fig. 4). The kinetics of the the step-height Ah* for the lower fluence (10 l3 Pb cm-*) and the higher energy (2.75 keV/amu) is represented in fig. 5. The two major etching characteristics of planar tracks (AV and Ah*) were also studied after thermal annealing. The specific features of these parameters give interesting clues to the possible defect structure of low energy lead-implanted planar tracks even though the conclusions are still speculative and are given in the following. 4.2.1. Properties of AV Before annealing (fig. 5), one notes
a very sharp
{Ah(A)
Fig. 5. Variation of the step-height Ah with etching time for - 1 keV/amu lead ions. VI. ION TRACKS
406
J. -C. Dran et al, / Thermal annealing behaviour of ion implanted muscouite mica n
1016.cm-*
t l
IOU.cm-*
AV
i .mn-1
HF 4%
30-c
4, \
f; i
1 i i i i i i i i i i i
i
i
Fig. 6. Variation of AV with the annealing temperature for 2 h periods for - 1 keV/amu lead ions.
mm-‘) although the density of defects is multiplied by a factor probably between 10 and 100 if one takes into account the sputtering phenomenon. Moreover, AV remains nearly unaffected up to - 400~500°C before dramatically decreasing to negligible values at - 600°C. This low sensitivity of the etch rate with the defect concentration at high fluences is predicted by our Monte-Carlo model of etching reported elsewhere (see companion paper, page 557). Indeed, the major conclusion of this model is that the accumulation of damaged islands of increased chemical reactivity characterized by an intrinsic amplification factor ki induces a percolation threshold in the implanted layers. We recall that the average amplification factor K of the implanted material first very slowly increases with $I up to a critical density of such islands (corresponding to - 20% occupancy of the total volume in mica) where it markedly increases and then rapidly saturates to ki. Therefore, the K value at the higher fluence is an evaluation of the intrinsic ki of the defects that do dominate the etch rate of these planar tracks. In 40% HF the AY estimated in the first part of the curve is 17000 A mm-’ which is compatible with the V, value (104) measured for high energy heavy ion linear tracks, thus suggesting that clusters of defects also dominate the dissolution of these planar tracks. Another way to evaluate the possible role of both types of defects is to calculate their theoretical densities for the two fluences and their corresponding degrees of occupancy in the damaged layer. By assuming that one individual lead ion produces - 2000 point defects [4] with an approximate volume of - 10 A3, their density should be - 1.3 X 10” cme3 at 1013 ions cme2 and
- 1.3 x 1O23 at 1015 ions cme2 and the degrees of occupancy u - 0.013 and - 1.3 respectively. As these two fluences are both greater than the experimental critical fluence - 5 X 1012 Pb cmd2 previously reported (refs. 2 and S), the two values of u should be > 0.20. This demonstrates that point defects cannot be responsible for the observations. In addition, if one assumes that each incident lead ion produces on average - 0.25 clusters of defects [5] with a volume - 3 X lo4 K, roughly estimated by the superior limit measured by small angle X-ray scattering for high energy ion tracks, their density should be - 1.7 X 10” cme3 at 1013 ions cme2 and - 1.7 X 1019 cmm3 at 1015 ions cmd2. The degrees of occupancy o are thus - 0.005 and - 0.5 respectively, and for the same reasons as above the accumulation of clusters of defects directly produced by individual ions cannot explain the observations. However, Dartyge et al. [l] have shown that in the case of this second type of defect, the etch rate ruling parame ter is not the apparent size estimated by X-ray scattering but rather an actual “active” diameter X,, corresponding to the total width of the defect distribution in the cluster. For instance in the case of 1 MeV/amu Fe ions, X, - 700 A. For 1 keV/amu lead ions, the critical fluence +c - 5 X 1012 ions cmv2 implies a value X, of - 100 A which is smaller than that for high energy ions and suggests that the apparent diameter which could be inferred from X-ray studies would be significantly smaller than 40 A. This conclusion is reinforced by the relatively small effect of annealing on the Al/-values in the temperature range (25-300%) where point defects are known to vanish. On the contrary AV does markedly decrease in the temperature range (400-600°C) which characterizes annealing of clusters of defect. It must be emphasized that the 1015 targets show a higher resistance to annealing compared to those implanted at lower fluences since a 2OO’C excess annealing temperature is required to significantly reduce AV. This can be explained either by a larger size of the clusters of defects due to the aggregation of primary defects, or by a higher density of clusters. 4.2.2. Properties of Ah* The results concerning Ah* are as shown in fig. 7. The lower fluence samples clearly show a two step annealing behavior: first a marked decrease at a temperature 5 200°C then an almost constant value up to - 400°C followed by a second drastic diminution at higher temperatures. In contrast, the 1015 targets present a one-step annealing behavior. In fact, Ah * remains constant up to - 3OO’C and then linearly decreases but maintains a value - 1000 A at the highest annealing temperature (6OO’C). Before annealing, the A h*-values are identical (1500 A) for the two fluences investigated and, more-
407
J.-C. Dran et al. / Thermal annealing behauiour df ion implanted muscovite mica
Fig. 8. Variation of Ah* with the energy of incident ions. Fig. 7. Variation of Ah* with the annealing temperature (2 h periods) for - 1 keV/amu lead ions etched in 4% HF at 3O’C.
over, they are markedly greater than the calculated damage range (X) ,, - 400 A of individual lead ions in mica, and even than the sum (X), + (AXz)‘~z (mean width distribution). These two features can be tentatively interpreted as follows: (i) since mica is an extremely sensitive track detector (K - 104), a very small density of defects can induce a measurable enhancement in etch rate; as the etching time for measuring Ah* was very long (2 h) it is expected that we have revealed the entire width of defect distribution therefore exceeding the (X), + (AX2)‘k2. In contrast, in the case of soda-lime glass which is much less sensitive (K < lo), Ah* is very close to the value of the mean width distribution [3]. Moreover, the variations of Ah* with energy between 0.3 and 7.5 keV/amu studied for lead (lot3 and 1015 ions cmm2) and argon (10” ions cmm2) are linear as predicted by the corresponding theoretical damage distributions (fig. 8). This small density of defects at the bottom of the implanted layer could also be due to the knock-on implantation of light constituent elements of mica such as oxygen. Indeed, the experimental variations of Ah* with the Z of the incident ions (fig. 9) do correspond quite well to the theoretical ranges calculated for secondary oxygen ions (mass M,) when using the maximum energy E,_ transferred by the primary ions (Z, M,, E) in the classical formula E mm. = [Q%M,/(M,
ent kind of defect less resistant to annealing. This last assumption would probably favor the role of knock-on implantation in the enhancement of the thickness of the damaged layer at high fluences. In contrast, the first 4 of the damaged layer is much more resistant to thermal annealing. This is illustrated by the fact that, even for the highest temperature, the Ah* of the 10” target is precisely equal to the thickness of this sublayer. For this fluence the discontinuity is therefore revealed at 6OO’C. However, a second discontinuity in the first f sublayer of the 1013 target is reflected by the second annealing step for T 5 400°C. The remaining Ah* - 400 A then corresponds roughly to (X) n. In addition, these observations should be compared to the data on the partial annealing of various high energy heavy-ion tracks. At low annealing temperature, the etch rate of - 7 MeV/amu Fe tracks is markedly reduced while that of Kr tracks is unaffected [l]. This result is indicative of a much lower density of clusters of defects in the Fe tracks than that due to Kr ions. A qualitative description of the defect structure of the implanted layer would thus be the following: at both fluences the first sublayer of thickness (5(X),)
Ah*&
t
+ M212] E.
After annealing, the first characteristic worth noticing is the marked decrease of Ah* in the temperature range 2%200°C for the lower fluence whereas this parameter remains constant up to 300°C for the higher fluence. This clearly indicates for the lOI3 target a discontinuity in the structure of the implanted layer, with the lower j sublayer containing either a smaller density of defects than the first 4 sublayer, or a differ-
0
I
2 * 30
Fig. 9. Variation of Ah* keV/amu incident ions.
*o
66
with the atomic
number
for
-I
VI. ION TRACKS
408
J.-C. Dran et al. / Thermal annealing behaviour of ion implanted muscovite mica
would contain point defects and clusters of defects induced by the incident lead ions. The density of clusters would be high enough to satisfy the percolation threshold at both fluences. In the last f sublayer, where the density of clusters would rapidly diminish, the percolation threshold would only be reached in the 1Ol3 target by a combination of both types of defects whereas it could still be satisfied by clusters in the 10” target. It is also possible that u, is reached in this last sublayer by the accumulation of point defects due to knock-on inplantation of light ions for both fluences. After annealing at low temperature (200°C), point defects would have almost completely vanished. In the case of the 1Ol3 target, the density of defects in this last sublayer would be insufficient to satisfy the percolation threshold and thus this sublayer would be no longer etchable. When increasing the annealing temperature (200-400°C), the remaining point defects would completely disappear without any significant change in the etching characteristics. In the temperature range 400-500°C, the X, parameter would begin to diminish thus affecting the degree of occupancy of the clusters of defects themselves and consequently the percolation threshold would be no longer satisfied, therefore leading to a second annealing step. For the 1015 target, the percolation threshold would always be satisfied up to the range of temperatures (400-500°C) where the clusters of defects would be affected by annealing.
ments combining annealing and etching, although indirect, show that low energy lead-ion planar tracks are also probably made up of point defects and clusters of defects with an “active” diameter - lOOA, which delineate a multi-layered structure. The variations of the asymptotic step-height Ah* with the energy and the atomic number of incident ions have also been shown to be compatible with theoretical predictions derived from models of particle-solid interaction. It is now necessary to further support these preliminary results by more direct experimental evidence. For this purpose, we are currently preparing thin films of amorphous silica (few thousands of A) which will be implanted and observed by means of small angle X-ray scattering. The defect structure of the planar track, which should be directly observable, will then be compared to the one deduced from its etching properties. We acknowledge the very valuable help of F. Lalu in performing the implantations, and thank G. Vidal for the Talystep measurements, and G. Goutiere for his constant interest in this work.
References [l] E. Dartyge, [2]
5. Conclusion We have shown that the terrace structure of etched linear tracks of high energy heavy ions is indeed due to the presence of clusters of defects. Moreover, experi-
[3] [4] [5]
J-P. Duraud, Y. Langevin and M. Maurette, Phys. Rev. 23 (1981) 5213. J-C. Dran, M. Maurette and J-C. Petit, Science 209 (1980) 1518. J-C. Petit, These de doctorat d’Etat et sciences physiques, Universitt de Paris XI (1982). G.H. Kinchin and P.S. Pease, Rep. Prog. Phys. 18 (1955) 1. J. Borg, These de doctorat d’Etat, Universitt de Paris XI (1980).