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Rutherford backscattering and channelling studies of erbium implanted SIMOX ∗ structures
Nuclear Instruments and Methods in Physics Research B47 (1990) 155-159 North-Holland
RUTHERFORD BACKSCATIZRING SIMOX * STRUCTURES ZHANG
Jingping,
Y.S. TANG,
AND CHANNELLING
P.L.F. HEMMENT
155
STUDIES
OF ERBIUM
IMPLANTED
and B.J. SEALY
Department of Electronic and Electrical Engineering, University of Surrey, Guildford, Surrey, GU2 5XH, UK Received 17 November 1989
The behaviour of 250 keV ‘%r+ implanted into SIMOX structures has been investigated by Rutherford backscattering and charmelhng analysis. The implantation doses were 1.5 x 1014cmm2and 1.5,X 1015cmd2. Both conventional furnace and rapid thermal annealing were carried out in the temperature range 600 o C-1100 o C. Regrowth of the amorphized silicon and redistribution of the erbium were found to be strongly influenced by the status of the damaged layer. Different regrowth processes of the completely damaged silicon overlayer were suggested respectively for conventional furnace and rapid thermal annealing. It is found that the regrowth rate increases rapidly when the temperature is higher than 900 o C in both cases. The redistribution of the erbium atoms was controlled by the regrowth boundary between the damaged and the recrystallized silicon.
1. Introduction Erbium implanted into silicon has been studied by several authors [l-4] due to its potential device applications in optical fibre telecommunication engineering. According to our previous results [3], the preferential lattice sites of implanted erbium in bulk Si are those which have tetrahedral (Td) symmetry (either interstitial or substitutional). It is important to keep the majority of the implanted erbium atoms on these lattice sites in order to increase the luminescence efficiency of the material. Unfortunately, a significant piling up of the implanted erbium atoms occurs at the surface after the recrystallization of the damaged Si layer, and this redistribution has been found to limit the emission efficiency. In the present study we investigated the regrowth of the damaged Si and the redistribution of the erbium implanted into SIMOX (separation by implanted oxygen) structures after both furnace and rapid thermal annealing. The results indicate that the regrowth of totally amorphized top silicon layer depends on the annealing process, and the redistribution of the implanted erbium is controlled by the recrystallization of the top Si layer.
2. Experimental details The 250 keV 166Er+ ions were implanted into a (100) SIMOX structure having a buried SiO* layer of 430 nm
* SIMOX: separation by implanted oxygen. 0168-583X/90/$03.50 (North-Holland)
0 Elsevier Science Publishers B.V.
and a top Si layer of 180 run with doses of 1.5 X 1014 cm-’ and 1.5 X 1015 cm-‘. The implantation was performed by tilting the wafers to 7O from the surface normal in order to avoid channelling. After the implantation, conventional furnace annealing was then carried out at 600, 750, 900 and 1100” C for 30 min; rapid thermal annealing using a graphite strip heater was also utilized at 750, 900 and 1100°C for 10 s. Both annealing processes were performed in a dry nitrogen atmosphere. Rutherford backscattering (RBS) and channelling using 1.5 MeV 4He+ as the analysing beam has been used to determine the amount of damage and the depth profiles of erbium for both “as implanted” and annealed samples. For the RBS measurements, the samples were mounted on a goniometer which permits simultaneous rotation about two orthogonal directions. The scattered particles are detected at an angle of 160 o with a system energy resolution of 13 keV.
3. Results and discussion 3.1. As implanted samples Shown in fig. 1 is the accumulation of damage in the top silicon layer of (100) SIMOX implanted with 250 keV I’6 Er+ ions. The increase of damage as the erbium dose increases from 1.5 x 1014 to 1.5 x 1015 cm-’ can be observed by comparing the random and channeling spectra. The top silicon layer was completely amorphized for the sample with a dose of 1.5 x 1015 cm-* (fig. la) while the amorphous layer thickness was only 140 nm for the sample implanted with 1.5 X 1014 cm-* (fig. lb).
Zhang Jingping et al. / Erbium implanted SIMOX structures
156
(b)
I
150
I
.
I
250 CHANNEL NUMBER
4
450
350
Fig. 1. Damage profilesof two sampleswith differentdoses: (a) 1.5 X 1Ol5cme2 and (b) 1.5 X 1014cmm2.
layer has recovered almost completely after each amrealing process and all the samples have X,, - 5%. Fig. 2 and fig. 3 show the random and channelling spectra of the samples with a dose of 1.5 x lOI5 cme2 following furnace and rapid thermal annealing, respectively. Fig. 2 shows that about 50% of the tiorphized layer has regrown after furnace annealing (FA) at 600, 750 and 900 OC for 30 min, but the whole of the damaged layer was restored with X,, - 168 after annealing at llOO°C for 30 min. In the case of rapid
The experimental projected range of the implanted erbium determined from the peak position of erbium is 90 mn, which agrees with TRIM 89 simulation. 3.2. Regrowth of the top Si layer For the samples with lower dose of 1.5 x 1014 cmP2, a comparison of the random and charmelling spectra after annealing at 600 and 900” C for 30 min and at 1000 o C for 10 s shows that the amorphized top silicon
annealing conditions: D random A 600”C/30min. - 750W30min. 0 900W30min. ’ 1lOO”C/30min.
I
.
. ***:,. +. *.‘*.*).:. .*. e : ::r.::*** * . ..**.......*
surface
’ . . . . 1. . * .
.I
I
240
265 CHANNEL NUMBER
..*
; A . 0I **I .!!‘I .*I
,,
290
Fig. 2. Furnace annealingtemperaturedependenceof the regrowthof the damagedlayer. The implantationdose is 1.5x lOI cmm2.
Zhang An&ping et al. / Erbium implanted HMOX structures
157
anneai~ng conditions: A random a 75O’Cf IOsec. - 900”C/lOsec. D 1lOO”C/10sec.
? ? m ;;; E-
1
.
L. . .* “‘ 0*
.‘A“,‘., m,,::a .I @a’ 0 . .
.
5 8 :i;;t.+
surface
“,.‘A
:. _I, 5. I *
.4*“.‘.//“an, :.. v, .. * . .:,*.. .’ ..’ . ..*...*....**.*.........
: *.
m 7 + t *-.
t
i
240
265 CHANNEL NUMBER
‘**.llr.r.
290
Fig. 3, Regrowth of tbe damaged layer for the samples with a dose of 1.5X10’* cm -’ after rapid thermal annealing at different temperatures. annealing or rapid thermal annealing at all chosen temperatures. The regrowth of the amo~hous phase is siruihu to the solid-phase epitaxy of amorphous silicon on crystalline silicon [.5] due to the existence of crystalline seeds. But for the case of 1.5 X 10”’ cmW2 implant, the top Si layer was completely amorphized. The spectra in fig. 2 indicate that for the higher dose implant only about half of the top silicon layer near the Si-Si02
thermal anneahng (RTA) {see fig. 3) the amount of recovery of the damaged Si increases with increasing annealing temperature from 750 to llOO°C. The regrowth rate increases rapidly in the case of RTA at temperatures between 90 and 1100°C for 10 s, where about 70% of the damaged silicon was recrystallized. For the case of 1.5 X 1014 cm-* implant, the top silicon layer was fully recovered after either furnace
top Si layer
surface ..,.:A* 1)
.
v.1
‘ .
b .’
L
‘V,
II vI
0
!
I
I
I
.‘,-*
a
.
itct.,:
Er distribution
DEPTH (nm) Fig. 4. The redistribution of implanted erbium in the top silicon layer: (a) “as implanted”, (b) RTA at 900°C and (c) RTA at llOtJ°C.
158
Bang Jingping et al. / Erbium impranted SIMOX structures
interface was recrystallized after annealing at 600, 750 and 900 o C for 30 min. This is due to the fact that there is no undamaged silicon to act as a seed for regrowth. A higher temperature is required to recover this fully damaged layer than to regrow the one with a damagecrystalline boundary. For the RTA samples, as shown in fig. 3, less than 35% of the top Si layer was recorded after annealing at 750 or 900°C for 10 s. The regrown region was uniformly increased to more than 70% after annealing at 1100 o C for 10 s. By comparing fig. 2 with fig. 3, it appears that the recrystallization processes taking place for furnace annealing and rapid thermal annealing are different, when the layer is entirely damaged. For the furnace annealing case, the regrowth started approximately from the interface of Si-SiO, for the higher dose samples because this region is not as heavily damaged as the region closer to the surface (within the projected range). Then the regrowth front moves towards the surface with increasing annealing temperature or annealing time. Because no crystalline region exists near the Si-SiO, interface, the regrowth of the top Si layer tends to be random for RTA at 750,900 and 1100 o C for 10 s, which could introduce the growth of crystals with different orientations like twins. One can suggest that there are two steps for the solid-phase regrowth here. Firstly, nucleation sites would be produced throughout the top Si layer, and then spreading of the nucleation sites occurs to form a crystalline silicon layer. The difference between FA and RTA is attributed to the different processes of raising temperature and energy transfer under different conditions. For the case of FA there exists a uniform heat distribution profile. There is enough time and energy to make the crystalline regrowth along one orientation. The regrowth is only decided by the nucleation probability in the top Si layer. As mentioned before, the bombardment damage is heavier near the surface than inside, which causes the amorphous/crystalline boundary to move from the inside to the surface as the annealing temperature increases. But for the case of RTA there is not enough energy to get a unique orientation of the nucleation process in the damaged top silicon layer. This will make the recrystallization easier in several orientations at some places than others. To confirm the idea discussed above, we annealed an RTA (1100 o C for 10 s) sample (as used in fig. 3) in a conventional furnace at llOO°C for 30 min. The channelling analysis on this sample shows that X,, - 25% which is different from the sample originally furnace annealed at 1100°C for 30 min. This means the regrowth of RTA samples is in twins rather than a uniform single crystal. From the results of FA and RTA for the higher dose, it seems there is a critical temperature higher than 900 o C for the regrowth of the heavily damaged region, above which the regrowth rate is much more enhanced. Except for those discussed above, one can also find
from fig. 2 and fig. 3 that the damaged region close to the Si-SiO* interface is more difficult to recover, which might be due to the existence of more defects, such as various kinds of interface states, in this region. 3.3. The redistribution of implanted erbium From the RBS measurements, we observed that the erbium implanted into the top silicon layer of the (100) SIMOX structures migrated to the surface after annealing if there exists a thin undamaged crystalline Si layer near the Si-SiO, interface, which is similar to the case of erbium implanted into bulk Si [3]. The erbium atoms implanted into SIMOX accumulated on the surface for the dose of 1.5 X 1014 cmp2 after both FA and RTA and for the dose of 1.5 x 1015 cme2 after annealing at llOO°C, which corresponds to the regrowth of the whole damaged top Si layer. For the case of the higher dose, fig. 4 shows a comparison of the regrowth of the damaged silicon and the redistribution of the erbium in the top silicon layer for the “as implanted” and two RTA samples. It can be found that the erbium atoms remained at the boundary between reordered and damaged regions after RTA at 600 to 900 QC. Similar results were obtained for the FA samples. It seems likely that the erbium atoms were “pushed out” towards the boundary between the damaged and regrown regions. The segregation of the implanted erbium at the moving boundary of the damaged and recrystallized regions caused a significant piling up of the erbium atoms at the Si surface when the top Si layer regrew completely.
4. Conclusion In conclusion, we have studied the characteristics of erbium implanted into a SIMOX structure. The regrowth of the damaged top Si layer depends upon whether it was amorphized or not. Different regrowth processes were observed for FA and RTA respectively when the implanted dose was 1.5 X 1015 cmp2. The redistribution of erbium implants was controlled by the regrowth boundary of the damaged and recrystallized silicon. The authors thank the staff of the D.R. Chick Laboratory of the University of Surrey for their help with the experiments.
References [l] H. Bnnen, J. Schneider, G. Pomrenke and A. Axmann, Appl. Phys. Lett. 43 (1983) 943.
Zhang Jingping. et al. / Erbium implanted SIh4OX structures [2] H. Ennen, G. Pomrenke, A. Axmann, K. Eisele, W. Haydl and J. Schneider, Appi. Phys. Lett. 46 (1985) 381. [3] Y.S. Tang, K.C. Heasmen, W.P. Gillin and B.J. Scaly, Appi. Phys. Lett. 55 (1989) 432.
159
[4) Y.S. Tang, Zhang Jingping, K.C. Heasman and B.J. Sealy, Solid State Commun. 72 (1989) 991. [S] For example: K.H. Heinig and H.D. G&k, Phys. Status Sotidi A92 (1985) 421; Phys. Status Solidi A93 (1986) 99.
RUTHERFORD BACKSCATIZRING SIMOX * STRUCTURES ZHANG
Jingping,
Y.S. TANG,
AND CHANNELLING
P.L.F. HEMMENT
155
STUDIES
OF ERBIUM
IMPLANTED
and B.J. SEALY
Department of Electronic and Electrical Engineering, University of Surrey, Guildford, Surrey, GU2 5XH, UK Received 17 November 1989
The behaviour of 250 keV ‘%r+ implanted into SIMOX structures has been investigated by Rutherford backscattering and charmelhng analysis. The implantation doses were 1.5 x 1014cmm2and 1.5,X 1015cmd2. Both conventional furnace and rapid thermal annealing were carried out in the temperature range 600 o C-1100 o C. Regrowth of the amorphized silicon and redistribution of the erbium were found to be strongly influenced by the status of the damaged layer. Different regrowth processes of the completely damaged silicon overlayer were suggested respectively for conventional furnace and rapid thermal annealing. It is found that the regrowth rate increases rapidly when the temperature is higher than 900 o C in both cases. The redistribution of the erbium atoms was controlled by the regrowth boundary between the damaged and the recrystallized silicon.
1. Introduction Erbium implanted into silicon has been studied by several authors [l-4] due to its potential device applications in optical fibre telecommunication engineering. According to our previous results [3], the preferential lattice sites of implanted erbium in bulk Si are those which have tetrahedral (Td) symmetry (either interstitial or substitutional). It is important to keep the majority of the implanted erbium atoms on these lattice sites in order to increase the luminescence efficiency of the material. Unfortunately, a significant piling up of the implanted erbium atoms occurs at the surface after the recrystallization of the damaged Si layer, and this redistribution has been found to limit the emission efficiency. In the present study we investigated the regrowth of the damaged Si and the redistribution of the erbium implanted into SIMOX (separation by implanted oxygen) structures after both furnace and rapid thermal annealing. The results indicate that the regrowth of totally amorphized top silicon layer depends on the annealing process, and the redistribution of the implanted erbium is controlled by the recrystallization of the top Si layer.
2. Experimental details The 250 keV 166Er+ ions were implanted into a (100) SIMOX structure having a buried SiO* layer of 430 nm
* SIMOX: separation by implanted oxygen. 0168-583X/90/$03.50 (North-Holland)
0 Elsevier Science Publishers B.V.
and a top Si layer of 180 run with doses of 1.5 X 1014 cm-’ and 1.5 X 1015 cm-‘. The implantation was performed by tilting the wafers to 7O from the surface normal in order to avoid channelling. After the implantation, conventional furnace annealing was then carried out at 600, 750, 900 and 1100” C for 30 min; rapid thermal annealing using a graphite strip heater was also utilized at 750, 900 and 1100°C for 10 s. Both annealing processes were performed in a dry nitrogen atmosphere. Rutherford backscattering (RBS) and channelling using 1.5 MeV 4He+ as the analysing beam has been used to determine the amount of damage and the depth profiles of erbium for both “as implanted” and annealed samples. For the RBS measurements, the samples were mounted on a goniometer which permits simultaneous rotation about two orthogonal directions. The scattered particles are detected at an angle of 160 o with a system energy resolution of 13 keV.
3. Results and discussion 3.1. As implanted samples Shown in fig. 1 is the accumulation of damage in the top silicon layer of (100) SIMOX implanted with 250 keV I’6 Er+ ions. The increase of damage as the erbium dose increases from 1.5 x 1014 to 1.5 x 1015 cm-’ can be observed by comparing the random and channeling spectra. The top silicon layer was completely amorphized for the sample with a dose of 1.5 x 1015 cm-* (fig. la) while the amorphous layer thickness was only 140 nm for the sample implanted with 1.5 X 1014 cm-* (fig. lb).
Zhang Jingping et al. / Erbium implanted SIMOX structures
156
(b)
I
150
I
.
I
250 CHANNEL NUMBER
4
450
350
Fig. 1. Damage profilesof two sampleswith differentdoses: (a) 1.5 X 1Ol5cme2 and (b) 1.5 X 1014cmm2.
layer has recovered almost completely after each amrealing process and all the samples have X,, - 5%. Fig. 2 and fig. 3 show the random and channelling spectra of the samples with a dose of 1.5 x lOI5 cme2 following furnace and rapid thermal annealing, respectively. Fig. 2 shows that about 50% of the tiorphized layer has regrown after furnace annealing (FA) at 600, 750 and 900 OC for 30 min, but the whole of the damaged layer was restored with X,, - 168 after annealing at llOO°C for 30 min. In the case of rapid
The experimental projected range of the implanted erbium determined from the peak position of erbium is 90 mn, which agrees with TRIM 89 simulation. 3.2. Regrowth of the top Si layer For the samples with lower dose of 1.5 x 1014 cmP2, a comparison of the random and charmelling spectra after annealing at 600 and 900” C for 30 min and at 1000 o C for 10 s shows that the amorphized top silicon
annealing conditions: D random A 600”C/30min. - 750W30min. 0 900W30min. ’ 1lOO”C/30min.
I
.
. ***:,. +. *.‘*.*).:. .*. e : ::r.::*** * . ..**.......*
surface
’ . . . . 1. . * .
.I
I
240
265 CHANNEL NUMBER
..*
; A . 0I **I .!!‘I .*I
,,
290
Fig. 2. Furnace annealingtemperaturedependenceof the regrowthof the damagedlayer. The implantationdose is 1.5x lOI cmm2.
Zhang An&ping et al. / Erbium implanted HMOX structures
157
anneai~ng conditions: A random a 75O’Cf IOsec. - 900”C/lOsec. D 1lOO”C/10sec.
? ? m ;;; E-
1
.
L. . .* “‘ 0*
.‘A“,‘., m,,::a .I @a’ 0 . .
.
5 8 :i;;t.+
surface
“,.‘A
:. _I, 5. I *
.4*“.‘.//“an, :.. v, .. * . .:,*.. .’ ..’ . ..*...*....**.*.........
: *.
m 7 + t *-.
t
i
240
265 CHANNEL NUMBER
‘**.llr.r.
290
Fig. 3, Regrowth of tbe damaged layer for the samples with a dose of 1.5X10’* cm -’ after rapid thermal annealing at different temperatures. annealing or rapid thermal annealing at all chosen temperatures. The regrowth of the amo~hous phase is siruihu to the solid-phase epitaxy of amorphous silicon on crystalline silicon [.5] due to the existence of crystalline seeds. But for the case of 1.5 X 10”’ cmW2 implant, the top Si layer was completely amorphized. The spectra in fig. 2 indicate that for the higher dose implant only about half of the top silicon layer near the Si-Si02
thermal anneahng (RTA) {see fig. 3) the amount of recovery of the damaged Si increases with increasing annealing temperature from 750 to llOO°C. The regrowth rate increases rapidly in the case of RTA at temperatures between 90 and 1100°C for 10 s, where about 70% of the damaged silicon was recrystallized. For the case of 1.5 X 1014 cm-* implant, the top silicon layer was fully recovered after either furnace
top Si layer
surface ..,.:A* 1)
.
v.1
‘ .
b .’
L
‘V,
II vI
0
!
I
I
I
.‘,-*
a
.
itct.,:
Er distribution
DEPTH (nm) Fig. 4. The redistribution of implanted erbium in the top silicon layer: (a) “as implanted”, (b) RTA at 900°C and (c) RTA at llOtJ°C.
158
Bang Jingping et al. / Erbium impranted SIMOX structures
interface was recrystallized after annealing at 600, 750 and 900 o C for 30 min. This is due to the fact that there is no undamaged silicon to act as a seed for regrowth. A higher temperature is required to recover this fully damaged layer than to regrow the one with a damagecrystalline boundary. For the RTA samples, as shown in fig. 3, less than 35% of the top Si layer was recorded after annealing at 750 or 900°C for 10 s. The regrown region was uniformly increased to more than 70% after annealing at 1100 o C for 10 s. By comparing fig. 2 with fig. 3, it appears that the recrystallization processes taking place for furnace annealing and rapid thermal annealing are different, when the layer is entirely damaged. For the furnace annealing case, the regrowth started approximately from the interface of Si-SiO, for the higher dose samples because this region is not as heavily damaged as the region closer to the surface (within the projected range). Then the regrowth front moves towards the surface with increasing annealing temperature or annealing time. Because no crystalline region exists near the Si-SiO, interface, the regrowth of the top Si layer tends to be random for RTA at 750,900 and 1100 o C for 10 s, which could introduce the growth of crystals with different orientations like twins. One can suggest that there are two steps for the solid-phase regrowth here. Firstly, nucleation sites would be produced throughout the top Si layer, and then spreading of the nucleation sites occurs to form a crystalline silicon layer. The difference between FA and RTA is attributed to the different processes of raising temperature and energy transfer under different conditions. For the case of FA there exists a uniform heat distribution profile. There is enough time and energy to make the crystalline regrowth along one orientation. The regrowth is only decided by the nucleation probability in the top Si layer. As mentioned before, the bombardment damage is heavier near the surface than inside, which causes the amorphous/crystalline boundary to move from the inside to the surface as the annealing temperature increases. But for the case of RTA there is not enough energy to get a unique orientation of the nucleation process in the damaged top silicon layer. This will make the recrystallization easier in several orientations at some places than others. To confirm the idea discussed above, we annealed an RTA (1100 o C for 10 s) sample (as used in fig. 3) in a conventional furnace at llOO°C for 30 min. The channelling analysis on this sample shows that X,, - 25% which is different from the sample originally furnace annealed at 1100°C for 30 min. This means the regrowth of RTA samples is in twins rather than a uniform single crystal. From the results of FA and RTA for the higher dose, it seems there is a critical temperature higher than 900 o C for the regrowth of the heavily damaged region, above which the regrowth rate is much more enhanced. Except for those discussed above, one can also find
from fig. 2 and fig. 3 that the damaged region close to the Si-SiO* interface is more difficult to recover, which might be due to the existence of more defects, such as various kinds of interface states, in this region. 3.3. The redistribution of implanted erbium From the RBS measurements, we observed that the erbium implanted into the top silicon layer of the (100) SIMOX structures migrated to the surface after annealing if there exists a thin undamaged crystalline Si layer near the Si-SiO, interface, which is similar to the case of erbium implanted into bulk Si [3]. The erbium atoms implanted into SIMOX accumulated on the surface for the dose of 1.5 X 1014 cmp2 after both FA and RTA and for the dose of 1.5 x 1015 cme2 after annealing at llOO°C, which corresponds to the regrowth of the whole damaged top Si layer. For the case of the higher dose, fig. 4 shows a comparison of the regrowth of the damaged silicon and the redistribution of the erbium in the top silicon layer for the “as implanted” and two RTA samples. It can be found that the erbium atoms remained at the boundary between reordered and damaged regions after RTA at 600 to 900 QC. Similar results were obtained for the FA samples. It seems likely that the erbium atoms were “pushed out” towards the boundary between the damaged and regrown regions. The segregation of the implanted erbium at the moving boundary of the damaged and recrystallized regions caused a significant piling up of the erbium atoms at the Si surface when the top Si layer regrew completely.
4. Conclusion In conclusion, we have studied the characteristics of erbium implanted into a SIMOX structure. The regrowth of the damaged top Si layer depends upon whether it was amorphized or not. Different regrowth processes were observed for FA and RTA respectively when the implanted dose was 1.5 X 1015 cmp2. The redistribution of erbium implants was controlled by the regrowth boundary of the damaged and recrystallized silicon. The authors thank the staff of the D.R. Chick Laboratory of the University of Surrey for their help with the experiments.
References [l] H. Bnnen, J. Schneider, G. Pomrenke and A. Axmann, Appl. Phys. Lett. 43 (1983) 943.
Zhang Jingping. et al. / Erbium implanted SIh4OX structures [2] H. Ennen, G. Pomrenke, A. Axmann, K. Eisele, W. Haydl and J. Schneider, Appi. Phys. Lett. 46 (1985) 381. [3] Y.S. Tang, K.C. Heasmen, W.P. Gillin and B.J. Scaly, Appi. Phys. Lett. 55 (1989) 432.
159
[4) Y.S. Tang, Zhang Jingping, K.C. Heasman and B.J. Sealy, Solid State Commun. 72 (1989) 991. [S] For example: K.H. Heinig and H.D. G&k, Phys. Status Sotidi A92 (1985) 421; Phys. Status Solidi A93 (1986) 99.