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Direct formation of device worthy thin film SIMOX structures by low energy oxygen implantation
Nucle ,r Instruments and Methods in Physicr. Re-itch B80/81 (1993) 822--826 North- .Holland
Beam Interactions with Materials &Atoms
Direct formation of device worthy thin film SIMOX structures by low --nergy oxygen implantation A. Nejim a, Y. Li and G .R . Booker
n, e
C .D. Marsh
c,
P.L .h. Hemment ", R.J . Chater (", J .A . Kilner "
Departmeru of Electrical & Electronic Engmeeraig, Unh~ily of Surrey, Gtaldford GL'2 5XH, UK n Department of Materials, Imperial College of Science. Technok~-y and .Medicine, Londrm, UK ` Department of Materials. University of Oxford, Oxford 09X! 3PH, UK
Thin elm SOI/SIMOX samples suitable for fully depleted CMOS circuits rave been directly prepared by low energy oxygen ion implantation. In this paper we report the successful optimisation of the processing parameters, including energy, dose and wafer temperature to form low defect density, device worthy substrates with both the silicon overlayer -f the buried oxide layer tnrctcnesses in the r¢,nge of 7(H)-14(x) A. Samples impiauu . .i a, °., . . . . .^, ke.' the r .. .,ge of 3x If) ., to 6 x 1Or' O'/em = have been intesngated. The wafers were preheated and implanted ' at temperawre~ i .^. the range of 50(, up to 68W.'-' using halogen hntps to pro%ide backgrouna heating . The samples were annealed for 6 h in either nitrogen ambient at 1320°C or in argon +0.5% O_ at a temperature of 13(h)°C and then analyzed using XTEM, SIMS and RBS/channelling analysis . Results indicate a l.ontinnous buried layer of SiO,, planar interfaces and a drop of more than three orders of magnitude in the defect density from IO r down to less than 1(1`/cm= in samples implanted under optimum conditions of 70 keV, 3.3x I() r7 O/cm 2 , and implantation temperature (T,) of 680°C.
1 . Introduction
2 . Experimental
Since the demonstration that silicon on insulator (SOD technology producee significant CMOS performance and fabrication advantages over conventional bulk silicon technology, SIMOX (separation by implantation of oxygen) has emerged as one of the leading SO' technologies for large scale IC fabrication. Recent reports [1,21 indicate that high quality ultrathin SIMOX structures, with extremely low defect densities, can be produced :with low dose 180-2°00 keV oxygen ion implantation, however in order to produce the required range of silicon overlayer thickness (700 <_ t 5 1200 .4) a subsequent wet sacrificial oxidation and thinning has been used [11. Using low energies of 70 or 90 keV and doses below the dose threshold (0,') for the formation of a continuous buried axide during implantation represents an alternative route to produce good quality ultrathin SIMOX structures with suitable t,; values . This method involves only a single implantation step which eliminates the need for subsequent sacrificial oxidation. In this paper the sensitivity of the final microstructure upon the implantation conditions, such as dose and implantation temperature, for both 70 and 9() keV have been investigated and optimum conditions have been identified.
Areas of 8 cm= in the centre of ? in . device grade (FZ) bulk silicon (100) wafers were implanted with 140 or 180 keV molecular oxygen ('`O ; ) which dissociate upon impact to give 70 or 90 keV atomic proiectiles . The implantation iclaperaturr was maintained within the range of 500-680°C using a bank of halogen lamps [3] . Wafers were preheated for 1() min to allow the tempc .aturc to stabiiiLe at the chosen preset value . After implantation the wafers were annLaled for 6 h under nitrogen at 1320°C or argon + 0 .5% O, at a temperature of 1300°C. Specimen were then analyzed by TEM (cross section and plan view), SIMS and RBS/channelling. Table I lists the samples and their history . The doses used in this work are the retained doses calculated from RBS data . They arc within 10% of the nominal dose . Further details of the experimental procedures arc found elsewhere [3,41. 3. Results and discussion 3.1. Microstrtrcture of the as-implanted samples RBS/channelling spectra were recorded and using the disorder parameter, X m the effect of dose and
0168-583X/93/$06.00 Oc 1993 - Elsevier Science Publishers B.V. All rights reserved
823
/ For,-non oJ thitt fib. SIMOX st-m-
A. Wit. et al
Table 1 Processing history of samples analysed in this work Sarlples 1,2,3
4, 5, 6
T8,9,
Energy [keV]
Retained dose [x10 17 cm -2 1
Implant i:mp. [°C]
Annealing ambien°.
90
4,242.2 3.5. 4 .8, 5.2
630 68!)
N
680
90 90
10
5 .5,5 .3,5 .9,5 .8
11 12, 13, 14, 15
70 70
1,4,5 3 .6, 4.6, 4 .5, 5.3
16, 1 7, 18, !')
7Q
3 .3,3.3,3 .5,3 .4
energy on the crystalline long range order of the near surface Iaycis has been determined . Xmm plots as a function of oxygen do.,c far f-^m both 70 and 90 keV sample series are shown in figs. la and lb. These plot- show an increase in Xmm (and hence a decrease in the crystalline proportion of the subsurface layer [5]) tip to 80% for a dose of 1 X 10 1" O/cm = at 70 keV and a maximum of 66% for a d,,se of 1 .2 x 10 1" 0/'cm= at 90 keV . The difference b , .tween Xmm plots at these energies can be attributed to two factors. Firstly, the difference in the distribution profile of the deposited oxygen atoms which contributes to the formation of* he amorphous oxide phase. Secondly . the displacenteut damage eau"ed by the nuclear energy deposition rate (dE/dx) . SIMS profiles (not irteluded) show that the oxygen profile shifts closer to the surface as the oxygen energy is reduced and that the peak of oxygen distribution is at 1800 A for 70 keV and 2370 Â for 90 keV. TRIM88 shows that at the peak of the displacement damage profile, there are 2.90 atomic displacement events/(ionA) for the 70 keV and 2 .78 displacement events/(ionA) for 90 keV . Both mecha-
70
Ar Ar
650, 600, 550, 500 680
N
Ar Ar
650, 600, 550,500
nis its lead to an increased level of disorder in the near surface region at the lower c nergy. It is important to none that X mi never reaches 100% so contirming that dynamic annealing has retained the long range order of the silicon matrix. This is evident from Xmm va!-es for the annealed samples which show good recovery to 3-10°! after annealing fog 6 h in either 13'0°C nitrogen or 1300'C argon + 0.5% OZ . The cross section micrographs shown in figs . 2 and 3 are taken from sampi,s implanted at either 70 keV (figs. 2a and 2b) or 00 keV (figs. 3a and 3b) with oxygen doses between the two dose thresholds firstly, Oc', defined as the close required to form a continuous oxidc !aver during implantation, and secondly, 0,A, after implantation and annealing . Both samples show the formation of a continuous amorphous layer containing oxide precipitates which, from SIMS and RBS data (not shown), is found to have an average volume concentration below the stoichiometric value for S'OZ . A continuous stoichiometric oxide layer is formed for doses greater than 0.1. SIMS and RBS data show that
100,
keV
Ibl
90
keJ
~~ras-im0laWed w onnealed (N,) ~w~~ onnealed A )r
c É X
1E+017
Oxygen
dose
1E+018
(ot .c t
1
-2)
Fig. 1 . (a) A plot of Xmm as a function of dose for 70 keV oxygen ions in silicon. As-implanted data ( n ), nitrogen annealed (o) and argon annealed (to) . (b) A plot of Xmm as a function of dose for 90 keV oxygen ions in silicon . As-implanted data (n), nitrogen annealed (o) and argon annealed (9). Illd. SEMICONDUCTOR MODIFICATION (d)
,
82
A. Nejirn et al / Formation
of . 'titi film SIMOXstnictures b
a
"a( O O O
O O O N
Fig 2. XTEM of a sample implanted at 70 keV with 4x 10' 7 O/cm2 at 680'C . (a) As-implanted and (b) annealed under 1320'C nitrogen for 6 h .
Fig . 3. XTEM of a sample implanted at 90 keV with 3.5 x I() ~' O/cm 2 at 680'C. (a) As-implanted and (b) annealed under 1320'C nitrogen for 6 h
for 90 keV ions the value of 1P,1 is about 8 X 10 17 O/cm2 and at 70 keV it is about 7 x 10'' 01,c,,,2 . In contrast, for doses below P^ namely, about 3 X 10 17 O/cn, 2 for 70 keV and about 4 x 10 1 ' O/crn'- foi 90 keV, the as-implanted structure is characterised by a good quality single crystal layer near the surface with ne, resolvable defects being evident in the XTEM micrographs ('igs. 2a and 3a region (i)', although it contains oxide precipitates. Below this surface layer there is a highly damaged region (figs. 2a and 3a region (ü)) in which the silicon mat,' ;, contains larger oxide precipitates . These precipitates are present since the oxygen concentration exceeds the solid solubility in silicon . Bell-v tuffs region (iii), many extended defects are
found which extend to a maximum depth of about 5000 A, for samples implanted with 70 keV oxygen ions (fig. 2a) . 3.2. Microstructure of the annealed .sa, ales 3.2.1 . Dose dependence
For doses at or just abcvc 0^, RBS data from samples implanted at either energy show that, after annealing, a continuous oxi(.c layer is formed which contains a low silicon island density . This is contirmed in the XTEM micrographs from sampl,a 'mplanted with 70 keV oxygen ions which shows less than 107 silicon islands/cm 2 (fig . 4b) . Howc ,.er, for larger oxyc
Fig. 4. XTEM of three samples implanted at 70 keV, 680'C and annealed under argon+0,5% O; for 6 h. Doses are (a) I x 1() 17 O/cm'-, (b)3 .6x 10" 0/cm' and (c)5.3x 101 ' O/cm2 . b
C
a a 8N
Fig. 5 . XTEM of three samples implanted a! 90 keV, 680'C and annealed under argon +0 .5% O z for 6 h . Do,es are (a) 2.2x 10 17 O/cm2 , (b) 6.8x 1017 0/cm2 and (c) 1 x 10'" O/cm 2 .
A. Nelim et al. / Fornratton of tl+.a film SIMOX.stnrctures
825
of lengths extending to more char: 2700 f1 in samples implanted with 1 x 101' O/CM2 at 70 keV.
Fig. 6. XTEM of twosamples implanted at 90 keV, 680°Cand annealed under n, ;on+0.5% O, for 6 h. Doses are (a) . 5.5 x 10 1 ' O,'cn. = and(b)5.2 x 10 17 0/cm=
gen dose, the silicon islands are trapped in the continuous buried oxide (fig . 4c). For doses just above O^ the silicon islands occupy the centre of the layer and span more than 75% of its thickness (figs. 2b and 3b). With increasing dose the islands become variable in size and an distributed throughout the oxide layer with the larger diameter islands positioned closer to the lower SiO z /Si interface (figs . 4c and 5b). With further dose increases the islands shrink in size and become confined to the back SiO,/Si interface (fig. 5c). A similar dose dependence
has been reported elsewhere for oxygen implanted at energies of 50 and 200 keV [6,7]. For doses just below 0`` the structure is free of silicon islands but is characterised by silicon pin holes which extend across the oxide layer and effectively form a noncontinuous oxide
region (fig. 5a). This again is in agreement with samples produced by 50 and 200 !(LV oxygen implantations ['I]. At even 1o1cr doses (1 X 10 1 ' O/cm`) a layer of faceted oxide precipitates is found at the depth of the peak of the oxygen distribution in the as-implanted structure (fig . 4a). Some of these precipitates are linked either to each other or to the surface with dislocations
and O/CM2 For the 90 keV samples a dose of 5.5 x 101' was selected (fig 6a). The threading dislocation density in the silicon overlayer as well as the silicon island
density in the buried oxide are of the same order of magnitude throughout this temperature range . t,, and t, ,_ values, also. did not show any substantial change . A slight reduction of 6% in the dose down to 5.2 X 10' O/CM2 (closer to -P,) for the highest temperature
sample (fig . 66) produced a two orders of ml~gnttude reduction in the threading dislocation density from 2 X 10 1/cm= down to less than 5 X 10`'/cm = . There is
also a reduction in the density of the silicon islands which can be seen in XTEM microgra ihs in fig. 6. Samples were implanted over the same temperature range at 70 keV with doses between 3.3 X 1017 -3.6 X
1017 O/cm=, which is closer to -PA than in the case of the 90 keV series. As with the 90 keV series the silicon island density, t and r_, values are found to be relatively insensitive to T, over this temperature range.
However, the threading dislocation density in the silicon overlayer showed a consiste-it decrease with in-
creasing T, reaching a minimum value of less thar. 1 X 105/cm= for the sample implanted at the highest (680°C) temperature . From the above results it is noticed that the 70 keV series displays greater sensitivity to the variation it. F, than the 90 keV series . One possible explanation is
4000 1(b)
4000 (a)
V000
3.2.2. T, dependence In order to define the optimum processing conditions the effect of the implantation temperature T, on, the annealed structure has been i., c,iigated for samp1cs implanted at 70 or 90 keV with doses between 0A
70
90
-3000 keV
-
N Q) 2000 C
keV s! si-IRIS SiO,-IRIS
0 2000
U 1000
1~E17 Oxygen
Dose
' 1E,11 (at.cm-z )
0 1E17
Oxygen
Dose
1E18 (at.cm-Z )
Fig. 7. Plots of layer thickness as a function of dose. t,, experimental data (o), simulation data from IRIS programme experimental data ( O) and simulation data from IRIS programme (---). (a) 70 keVand(b)9(1 keV. Illd . SEMICONDUCTOR MODIFICATION (d)
826
A. Nelim et al. / Fonnanott n( thin ftbn SIMOXstructttre,s
that the 70 keV dos-s !ic cla"cr to the critical dose 0 ^ for this energy than the case for 90 k,-V. The dose chosen for the :'0 keV series represents a "window of opportunity" within which the implatlta'tion conditions can be "tuned" to produce good thin film SIMOX material characterised by a low, threading dislocation density, abrupt and planar interfaces (fig. 4b). The window is narrow as it is found that the microstructure . i s highly sensitive to both dose and T, . A slight increase of 6°h in the dose from 5 .2 a 10 17 to 5.5 x 1017 O/ cm2 away from the optimum dose which is thought to be in the: dose range between 3.8 x 10 17 and 4.5 x 10 17 O/cm 2 at 90 keV, leads to an increase in the threading dislocation density by two orders of magnitude, at least. 3.2.3. Layer thickne aes t, and t,,,, Figs. 71) and 7b show the dose dependence of t and t,ioz for samples implanted at 70 or 90 keV . Theoretical cun~es -rained from simulation program IRIS are included . For doses between -P, and 0,' there is a good agreement between the simulation and the experimental data. Tlic sruall discrepancy which t xtsts is due to (i) included silicon islands in the Luried oxide which effectively increases the overall thickness of the oxide, and (ii) oxidation at the silicon surface during the annealing in the argon + 0.5% O, ambient which effectively reduces t,; by about 200 11 when compared with similar anneals under nitrogen ambient. At both energies the optimum dose "window" is close to pA for the particular energy. 4. Conclusions It has been shown that a single step implantation and annealing schedule can be identified to produce good quality thin film SIMOX at energies of either 70 or 90 keV . We find the optimum dose to be 3,3 x 10 7 O/cm 2 at 70 keV and 3 .8-4 .5 x 10 17 O/em 2 for 90 keV using an implantation temperature of 680°!.'. By
selecting the appropriate dose and energy it -s possible to produce thin film SIMOX with a t s , value of 1100 %1 and t,,o, of 750 Â and also t value of 1600 f1 and t_, of i000 A for samples implanted at 70 and 90 keV, respectively. Acknowledgements The authors would like to thank the staff of D .R . Chick Laboratory, Unversity of Surrey, for their assistance during the implantation and RBS analysis and Southampton University Microelectronic Centre for performing some of the annealing. This work is supported in part by the UK Science and Engineering Research Council OED 1777) . References [1] S . Nakashima, Y . Otnttra and K. Imtmi, Proc. 5th Int . Symr. on SOI Technology and Devices, Electmchem . Soc . 92 (13) (1992) p . 3.58 . [21 .1 . Yoshmo, ibid., p . 321 . (3) A.K. Robinnson, C.D. Marsh, U . Bussmann, J .A . Kilner, `t' . Li, J. Vanhellemont, K .J. Reeson, P .L .F. Hemment and G.R . Booker, Nucl . Insu. and Meth. B55 (1991) 555. [4] Y. Li, J.A . Kilner, R .J . Chaser, P .L.F. Hemment, A . NeJim, A.K. Robinson. K.J. Reeson, C .D . Marsh, and G .R. Booker, Proc. 5th Ins. Symp. on SOI Technology and Devices, Flectrochem . Soc. 92 (13) (1992) p. 368; Y. 1 i, J .A . Kilner, A .K. Robinson, P .L .F. Hemment and C.D . M.arsli, J . Appl. Phys. 70 (11)91) 3605; Y . Li, J .A . Kilner, P.L.F . Hemment, A.K . Robinson, J .P . Zb:mg, K .J . Reeson, C.D . Marsh and ( R. Booker, Nucl . Instr . and Meth . B64 (1992) 750 : Y . Li, J.A . Kilner, P .L.F. Hemment, A K . Robinson, J.P. Zhang, K.1 . Reeson, C.D . Marsh and -R. Booker, Appl. Phys . Lett 59 (1991) 3130. [51 N. Hatzopoulos, R .J. Chatet, U. Bussmwnn, P.L.F. Hemment and J.A . Kilner, Mater. Sci . Eng . '312 (1992) 37. [61 J .C . Mikkciscn, Appl . Phys . Lc!°.. 40 (1096) 336 . [7] A.K. Robinson, Y. Li, C.D . Marsh, R .J . Chater, P.L .F. Hemment, J .A . Kilner and G .R. Booker, Mater. Sci . Eng . B12 (1992) 41 .
Beam Interactions with Materials &Atoms
Direct formation of device worthy thin film SIMOX structures by low --nergy oxygen implantation A. Nejim a, Y. Li and G .R . Booker
n, e
C .D. Marsh
c,
P.L .h. Hemment ", R.J . Chater (", J .A . Kilner "
Departmeru of Electrical & Electronic Engmeeraig, Unh~ily of Surrey, Gtaldford GL'2 5XH, UK n Department of Materials, Imperial College of Science. Technok~-y and .Medicine, Londrm, UK ` Department of Materials. University of Oxford, Oxford 09X! 3PH, UK
Thin elm SOI/SIMOX samples suitable for fully depleted CMOS circuits rave been directly prepared by low energy oxygen ion implantation. In this paper we report the successful optimisation of the processing parameters, including energy, dose and wafer temperature to form low defect density, device worthy substrates with both the silicon overlayer -f the buried oxide layer tnrctcnesses in the r¢,nge of 7(H)-14(x) A. Samples impiauu . .i a, °., . . . . .^, ke.' the r .. .,ge of 3x If) ., to 6 x 1Or' O'/em = have been intesngated. The wafers were preheated and implanted ' at temperawre~ i .^. the range of 50(, up to 68W.'-' using halogen hntps to pro%ide backgrouna heating . The samples were annealed for 6 h in either nitrogen ambient at 1320°C or in argon +0.5% O_ at a temperature of 13(h)°C and then analyzed using XTEM, SIMS and RBS/channelling analysis . Results indicate a l.ontinnous buried layer of SiO,, planar interfaces and a drop of more than three orders of magnitude in the defect density from IO r down to less than 1(1`/cm= in samples implanted under optimum conditions of 70 keV, 3.3x I() r7 O/cm 2 , and implantation temperature (T,) of 680°C.
1 . Introduction
2 . Experimental
Since the demonstration that silicon on insulator (SOD technology producee significant CMOS performance and fabrication advantages over conventional bulk silicon technology, SIMOX (separation by implantation of oxygen) has emerged as one of the leading SO' technologies for large scale IC fabrication. Recent reports [1,21 indicate that high quality ultrathin SIMOX structures, with extremely low defect densities, can be produced :with low dose 180-2°00 keV oxygen ion implantation, however in order to produce the required range of silicon overlayer thickness (700 <_ t 5 1200 .4) a subsequent wet sacrificial oxidation and thinning has been used [11. Using low energies of 70 or 90 keV and doses below the dose threshold (0,') for the formation of a continuous buried axide during implantation represents an alternative route to produce good quality ultrathin SIMOX structures with suitable t,; values . This method involves only a single implantation step which eliminates the need for subsequent sacrificial oxidation. In this paper the sensitivity of the final microstructure upon the implantation conditions, such as dose and implantation temperature, for both 70 and 9() keV have been investigated and optimum conditions have been identified.
Areas of 8 cm= in the centre of ? in . device grade (FZ) bulk silicon (100) wafers were implanted with 140 or 180 keV molecular oxygen ('`O ; ) which dissociate upon impact to give 70 or 90 keV atomic proiectiles . The implantation iclaperaturr was maintained within the range of 500-680°C using a bank of halogen lamps [3] . Wafers were preheated for 1() min to allow the tempc .aturc to stabiiiLe at the chosen preset value . After implantation the wafers were annLaled for 6 h under nitrogen at 1320°C or argon + 0 .5% O, at a temperature of 1300°C. Specimen were then analyzed by TEM (cross section and plan view), SIMS and RBS/channelling. Table I lists the samples and their history . The doses used in this work are the retained doses calculated from RBS data . They arc within 10% of the nominal dose . Further details of the experimental procedures arc found elsewhere [3,41. 3. Results and discussion 3.1. Microstrtrcture of the as-implanted samples RBS/channelling spectra were recorded and using the disorder parameter, X m the effect of dose and
0168-583X/93/$06.00 Oc 1993 - Elsevier Science Publishers B.V. All rights reserved
823
/ For,-non oJ thitt fib. SIMOX st-m-
A. Wit. et al
Table 1 Processing history of samples analysed in this work Sarlples 1,2,3
4, 5, 6
T8,9,
Energy [keV]
Retained dose [x10 17 cm -2 1
Implant i:mp. [°C]
Annealing ambien°.
90
4,242.2 3.5. 4 .8, 5.2
630 68!)
N
680
90 90
10
5 .5,5 .3,5 .9,5 .8
11 12, 13, 14, 15
70 70
1,4,5 3 .6, 4.6, 4 .5, 5.3
16, 1 7, 18, !')
7Q
3 .3,3.3,3 .5,3 .4
energy on the crystalline long range order of the near surface Iaycis has been determined . Xmm plots as a function of oxygen do.,c far f-^m both 70 and 90 keV sample series are shown in figs. la and lb. These plot- show an increase in Xmm (and hence a decrease in the crystalline proportion of the subsurface layer [5]) tip to 80% for a dose of 1 X 10 1" O/cm = at 70 keV and a maximum of 66% for a d,,se of 1 .2 x 10 1" 0/'cm= at 90 keV . The difference b , .tween Xmm plots at these energies can be attributed to two factors. Firstly, the difference in the distribution profile of the deposited oxygen atoms which contributes to the formation of* he amorphous oxide phase. Secondly . the displacenteut damage eau"ed by the nuclear energy deposition rate (dE/dx) . SIMS profiles (not irteluded) show that the oxygen profile shifts closer to the surface as the oxygen energy is reduced and that the peak of oxygen distribution is at 1800 A for 70 keV and 2370 Â for 90 keV. TRIM88 shows that at the peak of the displacement damage profile, there are 2.90 atomic displacement events/(ionA) for the 70 keV and 2 .78 displacement events/(ionA) for 90 keV . Both mecha-
70
Ar Ar
650, 600, 550, 500 680
N
Ar Ar
650, 600, 550,500
nis its lead to an increased level of disorder in the near surface region at the lower c nergy. It is important to none that X mi never reaches 100% so contirming that dynamic annealing has retained the long range order of the silicon matrix. This is evident from Xmm va!-es for the annealed samples which show good recovery to 3-10°! after annealing fog 6 h in either 13'0°C nitrogen or 1300'C argon + 0.5% OZ . The cross section micrographs shown in figs . 2 and 3 are taken from sampi,s implanted at either 70 keV (figs. 2a and 2b) or 00 keV (figs. 3a and 3b) with oxygen doses between the two dose thresholds firstly, Oc', defined as the close required to form a continuous oxidc !aver during implantation, and secondly, 0,A, after implantation and annealing . Both samples show the formation of a continuous amorphous layer containing oxide precipitates which, from SIMS and RBS data (not shown), is found to have an average volume concentration below the stoichiometric value for S'OZ . A continuous stoichiometric oxide layer is formed for doses greater than 0.1. SIMS and RBS data show that
100,
keV
Ibl
90
keJ
~~ras-im0laWed w onnealed (N,) ~w~~ onnealed A )r
c É X
1E+017
Oxygen
dose
1E+018
(ot .c t
1
-2)
Fig. 1 . (a) A plot of Xmm as a function of dose for 70 keV oxygen ions in silicon. As-implanted data ( n ), nitrogen annealed (o) and argon annealed (to) . (b) A plot of Xmm as a function of dose for 90 keV oxygen ions in silicon . As-implanted data (n), nitrogen annealed (o) and argon annealed (9). Illd. SEMICONDUCTOR MODIFICATION (d)
,
82
A. Nejirn et al / Formation
of . 'titi film SIMOXstnictures b
a
"a( O O O
O O O N
Fig 2. XTEM of a sample implanted at 70 keV with 4x 10' 7 O/cm2 at 680'C . (a) As-implanted and (b) annealed under 1320'C nitrogen for 6 h .
Fig . 3. XTEM of a sample implanted at 90 keV with 3.5 x I() ~' O/cm 2 at 680'C. (a) As-implanted and (b) annealed under 1320'C nitrogen for 6 h
for 90 keV ions the value of 1P,1 is about 8 X 10 17 O/cm2 and at 70 keV it is about 7 x 10'' 01,c,,,2 . In contrast, for doses below P^ namely, about 3 X 10 17 O/cn, 2 for 70 keV and about 4 x 10 1 ' O/crn'- foi 90 keV, the as-implanted structure is characterised by a good quality single crystal layer near the surface with ne, resolvable defects being evident in the XTEM micrographs ('igs. 2a and 3a region (i)', although it contains oxide precipitates. Below this surface layer there is a highly damaged region (figs. 2a and 3a region (ü)) in which the silicon mat,' ;, contains larger oxide precipitates . These precipitates are present since the oxygen concentration exceeds the solid solubility in silicon . Bell-v tuffs region (iii), many extended defects are
found which extend to a maximum depth of about 5000 A, for samples implanted with 70 keV oxygen ions (fig. 2a) . 3.2. Microstructure of the annealed .sa, ales 3.2.1 . Dose dependence
For doses at or just abcvc 0^, RBS data from samples implanted at either energy show that, after annealing, a continuous oxi(.c layer is formed which contains a low silicon island density . This is contirmed in the XTEM micrographs from sampl,a 'mplanted with 70 keV oxygen ions which shows less than 107 silicon islands/cm 2 (fig . 4b) . Howc ,.er, for larger oxyc
Fig. 4. XTEM of three samples implanted at 70 keV, 680'C and annealed under argon+0,5% O; for 6 h. Doses are (a) I x 1() 17 O/cm'-, (b)3 .6x 10" 0/cm' and (c)5.3x 101 ' O/cm2 . b
C
a a 8N
Fig. 5 . XTEM of three samples implanted a! 90 keV, 680'C and annealed under argon +0 .5% O z for 6 h . Do,es are (a) 2.2x 10 17 O/cm2 , (b) 6.8x 1017 0/cm2 and (c) 1 x 10'" O/cm 2 .
A. Nelim et al. / Fornratton of tl+.a film SIMOX.stnrctures
825
of lengths extending to more char: 2700 f1 in samples implanted with 1 x 101' O/CM2 at 70 keV.
Fig. 6. XTEM of twosamples implanted at 90 keV, 680°Cand annealed under n, ;on+0.5% O, for 6 h. Doses are (a) . 5.5 x 10 1 ' O,'cn. = and(b)5.2 x 10 17 0/cm=
gen dose, the silicon islands are trapped in the continuous buried oxide (fig . 4c). For doses just above O^ the silicon islands occupy the centre of the layer and span more than 75% of its thickness (figs. 2b and 3b). With increasing dose the islands become variable in size and an distributed throughout the oxide layer with the larger diameter islands positioned closer to the lower SiO z /Si interface (figs . 4c and 5b). With further dose increases the islands shrink in size and become confined to the back SiO,/Si interface (fig. 5c). A similar dose dependence
has been reported elsewhere for oxygen implanted at energies of 50 and 200 keV [6,7]. For doses just below 0`` the structure is free of silicon islands but is characterised by silicon pin holes which extend across the oxide layer and effectively form a noncontinuous oxide
region (fig. 5a). This again is in agreement with samples produced by 50 and 200 !(LV oxygen implantations ['I]. At even 1o1cr doses (1 X 10 1 ' O/cm`) a layer of faceted oxide precipitates is found at the depth of the peak of the oxygen distribution in the as-implanted structure (fig . 4a). Some of these precipitates are linked either to each other or to the surface with dislocations
and O/CM2 For the 90 keV samples a dose of 5.5 x 101' was selected (fig 6a). The threading dislocation density in the silicon overlayer as well as the silicon island
density in the buried oxide are of the same order of magnitude throughout this temperature range . t,, and t, ,_ values, also. did not show any substantial change . A slight reduction of 6% in the dose down to 5.2 X 10' O/CM2 (closer to -P,) for the highest temperature
sample (fig . 66) produced a two orders of ml~gnttude reduction in the threading dislocation density from 2 X 10 1/cm= down to less than 5 X 10`'/cm = . There is
also a reduction in the density of the silicon islands which can be seen in XTEM microgra ihs in fig. 6. Samples were implanted over the same temperature range at 70 keV with doses between 3.3 X 1017 -3.6 X
1017 O/cm=, which is closer to -PA than in the case of the 90 keV series. As with the 90 keV series the silicon island density, t and r_, values are found to be relatively insensitive to T, over this temperature range.
However, the threading dislocation density in the silicon overlayer showed a consiste-it decrease with in-
creasing T, reaching a minimum value of less thar. 1 X 105/cm= for the sample implanted at the highest (680°C) temperature . From the above results it is noticed that the 70 keV series displays greater sensitivity to the variation it. F, than the 90 keV series . One possible explanation is
4000 1(b)
4000 (a)
V000
3.2.2. T, dependence In order to define the optimum processing conditions the effect of the implantation temperature T, on, the annealed structure has been i., c,iigated for samp1cs implanted at 70 or 90 keV with doses between 0A
70
90
-3000 keV
-
N Q) 2000 C
keV s! si-IRIS SiO,-IRIS
0 2000
U 1000
1~E17 Oxygen
Dose
' 1E,11 (at.cm-z )
0 1E17
Oxygen
Dose
1E18 (at.cm-Z )
Fig. 7. Plots of layer thickness as a function of dose. t,, experimental data (o), simulation data from IRIS programme experimental data ( O) and simulation data from IRIS programme (---). (a) 70 keVand(b)9(1 keV. Illd . SEMICONDUCTOR MODIFICATION (d)
826
A. Nelim et al. / Fonnanott n( thin ftbn SIMOXstructttre,s
that the 70 keV dos-s !ic cla"cr to the critical dose 0 ^ for this energy than the case for 90 k,-V. The dose chosen for the :'0 keV series represents a "window of opportunity" within which the implatlta'tion conditions can be "tuned" to produce good thin film SIMOX material characterised by a low, threading dislocation density, abrupt and planar interfaces (fig. 4b). The window is narrow as it is found that the microstructure . i s highly sensitive to both dose and T, . A slight increase of 6°h in the dose from 5 .2 a 10 17 to 5.5 x 1017 O/ cm2 away from the optimum dose which is thought to be in the: dose range between 3.8 x 10 17 and 4.5 x 10 17 O/cm 2 at 90 keV, leads to an increase in the threading dislocation density by two orders of magnitude, at least. 3.2.3. Layer thickne aes t, and t,,,, Figs. 71) and 7b show the dose dependence of t and t,ioz for samples implanted at 70 or 90 keV . Theoretical cun~es -rained from simulation program IRIS are included . For doses between -P, and 0,' there is a good agreement between the simulation and the experimental data. Tlic sruall discrepancy which t xtsts is due to (i) included silicon islands in the Luried oxide which effectively increases the overall thickness of the oxide, and (ii) oxidation at the silicon surface during the annealing in the argon + 0.5% O, ambient which effectively reduces t,; by about 200 11 when compared with similar anneals under nitrogen ambient. At both energies the optimum dose "window" is close to pA for the particular energy. 4. Conclusions It has been shown that a single step implantation and annealing schedule can be identified to produce good quality thin film SIMOX at energies of either 70 or 90 keV . We find the optimum dose to be 3,3 x 10 7 O/cm 2 at 70 keV and 3 .8-4 .5 x 10 17 O/em 2 for 90 keV using an implantation temperature of 680°!.'. By
selecting the appropriate dose and energy it -s possible to produce thin film SIMOX with a t s , value of 1100 %1 and t,,o, of 750 Â and also t value of 1600 f1 and t_, of i000 A for samples implanted at 70 and 90 keV, respectively. Acknowledgements The authors would like to thank the staff of D .R . Chick Laboratory, Unversity of Surrey, for their assistance during the implantation and RBS analysis and Southampton University Microelectronic Centre for performing some of the annealing. This work is supported in part by the UK Science and Engineering Research Council OED 1777) . References [1] S . Nakashima, Y . Otnttra and K. Imtmi, Proc. 5th Int . Symr. on SOI Technology and Devices, Electmchem . Soc . 92 (13) (1992) p . 3.58 . [21 .1 . Yoshmo, ibid., p . 321 . (3) A.K. Robinnson, C.D. Marsh, U . Bussmann, J .A . Kilner, `t' . Li, J. Vanhellemont, K .J. Reeson, P .L .F. Hemment and G.R . Booker, Nucl . Insu. and Meth. B55 (1991) 555. [4] Y. Li, J.A . Kilner, R .J . Chaser, P .L.F. Hemment, A . NeJim, A.K. Robinson. K.J. Reeson, C .D . Marsh, and G .R. Booker, Proc. 5th Ins. Symp. on SOI Technology and Devices, Flectrochem . Soc. 92 (13) (1992) p. 368; Y. 1 i, J .A . Kilner, A .K. Robinson, P .L .F. Hemment and C.D . M.arsli, J . Appl. Phys. 70 (11)91) 3605; Y . Li, J .A . Kilner, P.L.F . Hemment, A.K . Robinson, J .P . Zb:mg, K .J . Reeson, C.D . Marsh and ( R. Booker, Nucl . Instr . and Meth . B64 (1992) 750 : Y . Li, J.A . Kilner, P .L.F. Hemment, A K . Robinson, J.P. Zhang, K.1 . Reeson, C.D . Marsh and -R. Booker, Appl. Phys . Lett 59 (1991) 3130. [51 N. Hatzopoulos, R .J. Chatet, U. Bussmwnn, P.L.F. Hemment and J.A . Kilner, Mater. Sci . Eng . '312 (1992) 37. [61 J .C . Mikkciscn, Appl . Phys . Lc!°.. 40 (1096) 336 . [7] A.K. Robinson, Y. Li, C.D . Marsh, R .J . Chater, P.L .F. Hemment, J .A . Kilner and G .R. Booker, Mater. Sci . Eng . B12 (1992) 41 .