We have studied the formation of buried oxide in MeV oxygen implanted Si. A continuous oxide layer is formed in the samples implanted with 2x10^(18)/cm^2 oxygen and annealed at 1300° C. The microstructures are studied by cross-sectional transmission electron microscopy and high resolution electron microscopy. Chemical information was obtained by electron energy loss spectroscopy. The effects of implantation temperature are studied. Implantation at a low substrate temperature leads to a well-defined buried SiO_2 layer, inhibits the formation of oxide precipitates in the silicon, and reduces silicon inclusions in the SiO_2

Ahn, C. C.

Fultz, B.

Jamieson, D. N.

Nieh, C. W.

Tombrello, T. A.

Vreeland, T., Jr.

Xiong, F.

Zhou, Z.

Caltech Authors - Main

FORMATION OF BURIED OXIDE IN MEV 
OXYGEN IMPLANTED SILICON 
C.W.NIEH, F. XIONG, C. C. AHN, Z. ZHOU*, D. N. JAMIESON, 
T. VREELAND JR., B. FULTZ, and T. A. TOMBRELLO. 
Materials Research Group 
California Institute of Technology, Pasadena, CA 91125 
73 
• Present address: Department of Nuclear Science, Fudan University, Shanghai, China. 
ABSTRACT 
We have studied the formation of buried oxide in MeV oxygen implanted Si. A contin-
uous oxide layer is formed in the samples implanted with 2xl018 /cm2 oxygen and annealed 
at 1300° C. The microstructures are studied by cross-sectional transmission electron mi-
croscopy and high resolution electron microscopy. Chemical information was obtained by 
electron energy loss spectroscopy. The effects of implantation temperature are studied. 
Implantation at a low substrate temperature leads to a well-defined buried Si02 layer, 
inhibits the formation of oxide precipitates in the silicon, and reduces silicon inclusions in 
the Si02. 
INTRODUCTION 
Considerable effort has been made in fabricating Si-on-insulator structures. High 
dosage oxygen implantation is one of the most promising techniques. It has been demon-
strated that with 200 keY oxygen implantation to doses higher than 1.4xl018 jcm2 , a 
stoichiometric Si02 layer can be formed[l]. Buried oxides formed in subcritical dose oxy-
gen implanted silicon have been reported[2,3]. Usually a high temperature (1300° C) post 
implantation anneal is required for oxygen redistribution[4]. An alternat ive way of pro-
viding mobility for oxygen atoms is to implant at a high substrate temperature (500° 
C). Formation of a continuous oxide layer by high temperature implantation has been re-
ported[5,6] and the microstructure of buried Si02 has been extensively studied[7,8]. Most 
of the previous studies have concentrated on keY oxygen implantation although 1 MeV 
implantation has been used[3]. High energy implantation has the advantage of retaining 
a high quality si licon surface layer since the implanted ions create only electronic damage 
near the surface, and the damage can be easily recovered by thermal annealing. The oxy-
gen concentration in the surface region is much lower since most of the oxygen is deposited 
several microns away from the surface. 
In this paper we first report on the microstructure of silicon implanted with MeV 
oxygen ions at various temperatures. We then show how these microstructures, especially 
the buried oxide layer, is changed after a high temperature annealing treatment. The 
microstructure of the samples was studied by cross-sectional transmission electron mi-
croscopy (XTEM). High resolution electron microscopy (HREM) was performed to study 
the structure of Si/Si02 interfaces and crystal defects. E lectron energy loss spectroscopy 
(EELS) was used to study the bonding state of oxygen and silicon as well as to measure 
the oxygen concentration profiles. 
EXPERIMENTAL 
Mat. Res. Soc. Symp. Proc. Vol. ~07 <1988 Materials Research Society 
74 
N-type (100) silicon wafers were implanted with 2 and 5 MeV o+2 at doses of 1x101r; 
to 2x1018 /cm2 . The implantation was carried out on an EN-Tandem accelerator at a 
current density of 3-8p.A/cm2 . The substrate temperatures during the implantation were 
-196° C, room temperature and 500° Cas monitored by a thermocouple on the backside of 
the substrates. Due to ion beam heating the surface temperatures might be 200° C higher 
than the thermocouple reading. For the annealing experiments, the samples were sealed 
in quartz tubes. Before sealing the tubes were purged with He gas and evacuated. The 
oxygen partial pressure is estimated to be less than 10-G Torr. The annealing temperatures 
were measured by an optical pyrometer with an accuracy of ± 10° C. 
The conventional TEM work and HREM work employed a Philips EM 430 microscope 
operating at 300 kV. The EELS was performed with a Gatan 607 Electron Energy Loss 
Spectrometer mounted on the same microscope operating at 200 kV. 
RESULTS AND DISCUSSION 
For the 5MeV oxygen implanted samples, the area of heaviest damage is about 3p.m 
below the surface. For low dose (less than 101u / cm2) implantation, point defect clusters 
were formed near the damage peak. For the samples implanted with a dose higher than 
lxl010 / cm2 , an amorphous Si layer was formed at about 4J.Lm from the surface. The 
amorphous layer thickness increased as the ion dose increased. However, an ion dose two 
orders of magnitude higher than lxl010 f cm2 only doubled the amorphous layer thickness. 
Fig.l shows the as-implanted structure of the sample implanted at room temperature with 
1.4xl018 o+2 fcm2 • Heavily damaged crystalline zones were observed on both sides of the 
amorphous layer due to straggling of oxygen ions. The zones were slightly thicker than 
those in specimens implanted with a lower dose. No lattice damage was observed in the 
surface silicon layer. These results indicate that one can increase the amount of oxygen 
(and thus the thickness of oxide) without sacrificing the crystallinity of the surface silicon 
by use of 5 MeV o+2 implantation. High resolution imaging shows there are point defect 
clusters on (111) planes in samples implanted at room temperature. (see Fig.lb)but not 
in the samples implanted at low temperature. 
Fig.l (a)XTEM micrograph of sample as implanted with 1.4xl018 fcm2 oxygen at 
room temperature, (b)HREM micrograph of the same sample as in (a) showing the 
planar defects. 
75 
The maximum oxygen concentration found by EELS was located at about 41-'m from 
the surface, in good correspondence with the microstructure observation of the amorphous 
layer. Fig.2 (a) shows EELS spetra from areas at two different depths from the outer 
interface of the sample as implanted at low temperature. The lower trace shows that little 
oxygen is present immediately adjacent to the outer interface. The upper spectrum is 
taken from an area. 41-'m from the surface and reveals an oxygen K peak with a near edge 
structure identical to that of Si02 . The near edge structure of this oxygen K transition 
gives a good indication of the local environment of oxygenJ9J which is different from that 
of oxygen gasjlOJ. 
2000 
~ 
c: 
:J 
0 
0 1000 
500 600 
2000 
1000 
0 
ev 
~ 
., 
c: 
Cl) 
c 
100 120 140 ev 
Fig.2 (a)EELS spectrum showing the oxygen K edge from the amorphous region of 
the specimen as implanted at low temperature, (b) EELS spectra showing the Ledge 
from Si02 and from the same amorphous region as in (a). 
Nevertheless, even the center of this amorphous region does not consist entirely of 
amorphous Si02. Examination of the Si L23 edge in Fig.2(b) shows that the L23 structure 
seen in the upper spectrum is not identical to that of SiL23 from a purely amorphous 
Si02. Rather, it appears to consist of a sum of contributions from both Si02 as in the 
lower spectrum, and from amorphous Si as in the middle spectrum. The amorphous region 
immediately adjacent to the inner interface appears to consist entirely of amorphous silicon 
as in the region by the outer interface. 
For the sample implanted to the critical dose {1.4xl018 /cm2) and annealed a.t 1050° 
C for 1 h, the amorphous silicon recrystallized by both solid phase epitaxial regrowth 
and by homogeneous nucleation (see Fig.3). A high density of microtwins was formed 
in the epitaxial regions. The microtwins are typical defects formed during the epitaxial 
growth of amorphous silicon that contains a high concentration of oxygen. Homogeneous 
nucleation of polycrystals was observed between the two epitaxial regions with grain sizes 
of 200A near the original oxygen peak position and 400A near the epitaxial regions (see 
Fig.3). The d ifference in oxygen concentration might cause this grain size difference. 
Oxide precipitates might act as nucleation sites for poly-silicon and can also inhibit the 
grain growth. The thicknesses of the epitaxial regions at the outer and inner interfaces 
of the buried layer are 200nm and 50nm, respectively. This difference in epitaxial layer 
thickness also arises from the difference in oxygen concentration near the two interfaces 
since the growth rate is strongly dependent on oxygen concentrationjllj. Oxide precipitates 
were formed near the original crystalline damaged zone. Near the outer interface a finer 
76 
dispersion of oxide precipitates was found. After annealing the oxygen concentration profile 
was largely unchanged. 
Fig.3 XTEM micrograph of the sample 
implanted at room temperature with 
a doseage of 1.4x1018 /cm2 after 
annealing at 1050° C. 
For the samples implanted with 2x1018 fcm2 oxygen and annealed at 1300° C for 1 
h , the microstructure strongly depends on the implantation temperature. For the sample 
implanted at room temperature (Fig.4a), the interfaces between the buried oxide and 
silicon are not flat and have a roughness of about 50nm. There are two bands of silicon 
inclusions inside the Si02 layer. The inclusions are between 10 to 60nm in s ize and are 
basically in the same orientation as the substrate which suggests that the inclusions are 
the original silicon substrate that was left behind as the oxide/silicon interface advanced 
into the silicon region. The position of the silicon inclusion bands corresponds to the large 
grain poly-silicon regions observed in the 1050° C annealed sample. There is also a high 
density of oxide precipitates in the silicon regions. The oxide precipitates are distributed 
within 1J.im from the Si/Si02 interfaces. A high density of dislocations was observed 
in these layers, the dislocations appeared to be pinned by precipitates. The size of oxide 
precipitates near the outer and inner interfaces were 200nm and lOOnm, respectively. Both 
the oxide precipitates and silicon inclusions are in the shape of a truncated octahedron 
bounded by (111) and {100) planes. Similar precipitate shape was reported in 200 keY 
oxygen implanted silicon but with a smaller size[2]. This shape is the result of the fact that 
the (111) planes have a higher oxidation rate. Precipitates and inclusions tend to develop 
into an octahedral shape which expose the most (111) planes, however, formation of (100) 
planes at octahedron tips reduces the curvature at the tip and further reduces the surface 
energy. 
For the sample implanted at low temperature, a continuous oxide layer was formed 
after the 1300° C anneal (Fig.4b). A band of si licon inclusions is seE'n inside the oxi<.le 
layer about 50nm from the inner interface. The silicon inclusions are smaller than observed 
in Fig.4(a), but their density is higher. The Si/Si02 interfaces are fiat and no oxide 
precipitates were observed in the silicon regions. Two bands of epitaxial silicon with a high 
density of microtwins were formed. A polysilicon layer is seen between the Si02 and outer 
epitaxy region. The thickness of the Si02 layer is 900nm. This is the maximum thickness 
that we can expect with this oxygen dose because it accounts for all the implanted oxygen 
atoms. Including all Si02 precipitates and the uniform Si02 layer, the total volume of Si02 
in Figs.4 (a) and (b) is nearly equal. Threading dislocations with a density of 107 / cm2 were 
observed in the sample implanted at room temperature, whereas the threading dislocation 
density in the sample implanted at low temperature is less than 10° fcm2 • HREM shows 
77 
that all the Si02 interfaces in Fig4. (a) and (b) are atomically sharp. 
Fig.4 XTEM micrographs of samples implanted with 2x10 18 /cm2 oxygen at (a)room 
temperature, and (b)low temperature, after annealing at 1300° C. 
For the high tempecature implantation studies, 2 MeV o+2 was used with a dose of 
1.4x1018 / cm2 . The substrate was kept at 500° C during implantation. The as-implanted 
structure is shown in Fig.5 (a). A band of alternating rows of two kinds of plates was 
formed. HREM reveals that one kind of plate is amorphous and the other kind is a network 
of silicon single crystals embedded in an amorphous material, see Fig.5 (b). Between this 
band and the silicon are highly defective crystalline zones. A high density of defects with 
dislocations, stacking faults and oxide precipitates was formed in a 1J.Lm band at a depth 
1J,Lm from the surface. 
Fig.5 (a)XTEM micrograph of specimen implanted at 500° C with 2 MeV, 1.4xl018 / cm2 
oxygen, (b) HREM micrograph of same specimen showing the plate structure. 
During ir.:tplantalion and annealing, the oxygen atoms lend to migrate to the nearest 
stable oxide precipitate. There is a critical radius for oxide precipitates, below which the 
oxide is not stable and tends to dissolve. This critical radius increases with temperature. 
From Fig.2 and 4(a), the smallest precipitate sizes observed at 1050° C and 1300° C are 
5nm and 50nm, respectively. To form a uniform buried Si02 layer with a sharp Si/Si02 
interface, it is important to avoid the formation of any Si02 precipitates which exceed 
78 
the critical radius. Implantation at low temperature might inhibit the early nucleation of 
oxide and thus eliminate the formation of oxide precipitates in the silicon regions. 
SUMMARY and CONCLUSIONS 
Silicon on insulator structure was formed by 5 MeV oxygen ions implantated at 
2x10 18 fc m 2 after a 1300° C anneal. The structure contains a 3Jlm surface of high quality 
single crystal silicon and a 900nm buried Si02 layer. The detailed structures for samples 
implanted at different s ubstrate temperatures are studied. Implantation at low temper-
ature significantly improves the buried oxide structure. Low temperature implantation 
e liminates the oxide precipitates inside the crystalline si licon and reduces the silicon inclu-
sions inside the Si02 buried layer. 
ACKNOWLEDGEMENT 
This work was supported in part by the NSF grant [DMR-8421119[. 
REFERENCES 
1. G. K. Geller, Solid State Technology/ March, 93 {1987) 
2. J. Stoemenos, J. Margail, C. Jausscud, M. Dupuy, and M. Bruel, Appl. Phys. Lett. 
48, 1470 {1986) 
3. A. E. White, K. T. Short, J. L. Batstone, D. C. Jacobson, J. M. Poate, and K. W. 
West, Appl. Phys. Lett. 50, 19 {1986) 
4. G. K. Geller, P. L. F. Hemment, W. K. West, and J. M. Gibson, Appl. Phys. Lett. 
48, 532 ( 1986) 
5. C. G. Tuppen, M. R. Taylor, P. L. F. Hemment, and R . D. Arrowsmith, Appl. Phys. 
Lett. 45, 57 (1984) 
6 . A. E. White, K. T. Short, L. N. Pfeiffer, K. W. West, and J. L. Batstone, Mat. Res. 
Soc. Symp. Proc. Vol.74 , 585 (1987) 
7. M. K. El-ghor, S. J. Pennycook, T. P. Sjoreen, and J. Narayan, Mat. Res. Soc. Symp. 
Proc. Vol.74, 591, (1987) 
8. J. L. Batstone, A. E. White, K. T. Short, J. M. Gibson, and D. C. Jacobson, Mat. 
Res. Soc. Symp. Proc. Vol.74, 597, 1987. 
9. Tafto, J. and Zhu, J., Ultramicroscopy, 9, 349-354 (1982) 
10. C. C. Ahn, and 0. L. Krivarek, EELS Atlas, Center for Solid State Science, Arizona 
State University, Tempe AZ 85287 (1983) 
11. E. F. Kennedy, L. Csepregi, J. M. Mayer, and T . W . Sigmon, J. Appl. Phys. 48, 4241 
(1977) 


Formation of Buried Oxide in MeV Oxygen Implanted Silicon

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Formation of Buried Oxide in MeV Oxygen Implanted Silicon

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