U.S. patent application number 09/347260 was filed with the patent office on 2002-02-21 for semiconductor substrate and production method thereof.
Invention is credited to SAKAGUCHI, KIYOFUMI, SATO, NOBUHIKO.
Application Number | 20020022348 09/347260 |
Document ID | / |
Family ID | 26501815 |
Filed Date | 2002-02-21 |
United States Patent
Application |
20020022348 |
Kind Code |
A1 |
SAKAGUCHI, KIYOFUMI ; et
al. |
February 21, 2002 |
SEMICONDUCTOR SUBSTRATE AND PRODUCTION METHOD THEREOF
Abstract
A method of producing an SOI substrate is disclosed which
comprises a step of preparing an Si substrate prepared by the
floating zone process (FZ process), a step of implanting oxygen
ions from the principal surface side of the Si substrate thereinto
to form an ion-implanted layer in the Si substrate, and a buried Si
oxide layer forming step of forming an Si oxide layer buried below
the single-crystal Si layer on the principal surface side, by a
heat treatment of the Si substrate.
Inventors: |
SAKAGUCHI, KIYOFUMI;
(YOKOHAMA-SHI, JP) ; SATO, NOBUHIKO;
(SAGAMIHARA-SHI, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Family ID: |
26501815 |
Appl. No.: |
09/347260 |
Filed: |
July 6, 1999 |
Current U.S.
Class: |
438/480 ;
257/E21.563; 438/370; 438/479; 438/526; 438/530 |
Current CPC
Class: |
C30B 31/22 20130101;
C30B 29/06 20130101; H01L 21/76243 20130101 |
Class at
Publication: |
438/480 ;
438/479; 438/370; 438/530; 438/526 |
International
Class: |
H01L 021/20; H01L
021/36; H01L 021/31; H01L 021/469; H01L 021/425; H01L 021/331 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 8, 1998 |
JP |
10-192897 |
Jun 29, 1999 |
JP |
11-183326 |
Claims
What is claimed is:
1. A method of producing a semiconductor substrate comprising: a
step of preparing a single-crystal silicon substrate; an ion
implantation step of implanting ions into the single-crystal
silicon substrate to form an ion-implanted layer; and a step of
heat treating the single-crystal silicon substrate to form a buried
insulating layer inside the single-crystal silicon substrate,
wherein the single-crystal silicon substrate is one produced by the
floating zone process, and wherein a protective layer is formed on
the single-crystal silicon substrate, prior to the ion implantation
step, and then the ions are injected into the silicon substrate
from the side of the protective layer.
2. The method of producing the semiconductor substrate according to
claim 1, wherein the protective layer is a silicon oxide layer
formed by thermal oxidation of the single-crystal silicon
substrate.
3. The method of producing the semiconductor substrate according to
claim 1, wherein the protective layer is a silicon nitride
layer.
4. The method of producing the semiconductor substrate according to
claim 1, wherein the floating zone process is carried out under an
atmosphere of a mixed gas of argon gas and a small amount of
nitrogen gas.
5. The method of producing the semiconductor substrate according to
claim 1, wherein the ions implanted in the ion implantation step
are ions of oxygen or nitrogen.
6. The method of producing the semiconductor substrate according to
claim 1, wherein the temperature of the single-crystal silicon
substrate in the ion implantation step is in a range of 550.degree.
C. to 650.degree. C.
7. The method of producing the semiconductor substrate according to
claim 1, wherein the ion implantation step is carried out by the
plasma immersion ion implantation.
8. The method of producing the semiconductor substrate according to
claim 1, wherein the heat treatment is carried out under an
atmosphere of a mixed gas of argon and oxygen.
9. The method of producing the semiconductor substrate according to
claim 1, wherein the heat treatment is carried out at a temperature
not less than 1000.degree. C. nor more than 1400.degree. C.
10. The method of producing the semiconductor substrate according
to claim 1, wherein after the heat treatment, a second heat
treatment of the single-crystal silicon substrate is carried out
under an oxidizing atmosphere.
11. The method of producing the semiconductor substrate according
to claim 1, comprising a step of cleaning the single-crystal
silicon substrate after the ion implantation step and before the
heat treatment.
12. The method of producing the semiconductor substrate according
to claim 1, wherein a heat treatment of the single-crystal silicon
substrate is carried out under a reducing atmosphere containing
hydrogen, after formation of the buried insulating layer and after
removal of the protective layer on the single-crystal silicon
substrate.
13. The method of producing the semiconductor substrate according
to claim 12, wherein the heat treatment under the reducing
atmosphere containing hydrogen is carried out at a temperature not
less than 800.degree. C. nor more than 1350.degree. C.
14. The method of producing the semiconductor substrate according
to claim 1, wherein chemical mechanical polishing of a surface of
the single-crystal silicon substrate is carried out after formation
of the buried insulating layer and after removal of the protective
layer on the single-crystal silicon substrate.
15. A method of producing a semiconductor substrate comprising: a
step of preparing a single-crystal silicon substrate; a step of
implanting oxygen ions into the single-crystal silicon substrate to
form an ion-implanted layer; and a step of forming a buried silicon
oxide layer inside the single-crystal silicon substrate, wherein
the single-crystal silicon substrate is one produced by the
floating zone process, and wherein the method further comprises a
step of carrying out the internal thermal oxidation process of the
single-crystal silicon substrate under an oxidizing atmosphere,
after formation of the buried silicon oxide layer.
16. The method of producing the semiconductor substrate according
to claim 15, wherein the internal thermal oxidation process is
carried out at a temperature not less than 1300.degree. C. nor more
than the melting point of silicon.
17. A method of producing a semiconductor substrate comprising: a
step of preparing a single-crystal silicon substrate; a step of
implanting oxygen ions into the single-crystal silicon substrate to
form an ion-implanted layer; and a step of forming a buried silicon
oxide layer inside the single-crystal silicon substrate, wherein
the single-crystal silicon substrate is one produced by the
floating zone process, and wherein the method further comprises a
step of carrying out hydrogen annealing of the single-crystal
silicon substrate under a reducing atmosphere containing at least
hydrogen, after formation of the buried silicon oxide layer.
18. The method of producing the semiconductor substrate according
to claim 15 or 17, further comprising a step of forming a thermally
oxidized film on the single-crystal silicon substrate, prior to the
step of forming the ion-implanted layer.
19. A semiconductor substrate produced by the method as set forth
in either one of claims 1 to 17.
20. A semiconductor substrate produced by the method as set forth
in claim 18.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of producing a
semiconductor substrate having a single crystal semiconductor layer
on an insulating layer. More particularly, the invention concerns
an SOI substrate produced by a method called SIMOX (Separation by
IMplanted OXygen).
[0003] 2. Related Background Art
[0004] Formation of a single crystal Si semiconductor layer on an
insulator is commonly known as silicon on insulator (SOI)
technology, and many studies about it have been conducted
heretofore, because devices made by making use of the SOI
technology have various advantages which cannot be achieved by bulk
Si substrates for production of ordinary Si integrated circuits.
Specifically, use of the SOI technology presents the advantages
including the following.
[0005] 1. It facilitates dielectric isolation and permits high
integration.
[0006] 2. It is superior in radiation resistance.
[0007] 3. The parasitic capacitance is reduced, so as to permit
increase of operation speed.
[0008] 4. The well forming step can be omitted.
[0009] 5. The latch up can be prevented.
[0010] 6. Fully depleted field effect transistors can be produced,
based on formation of thin film.
[0011] These are discussed in detail, for example, in the following
literature. Special Issue: "Single-crystal silicon on
non-single-crystal insulators"; edited by G. W. Cullen, Journal of
Crystal Growth, volume 63, no. 3, pp. 429-590 (1983).
[0012] Further, there are many reports in these several years about
use of SOI as a substrate to realize faster operation and lower
power consumption of MOSFET (IEEE SOI conference 1994).
[0013] With use of the SOI structure in which the SOI layer is
present through the insulating layer on the support substrate,
because the insulating layer exists at the lower portion of the
element, the element separation process can be simplified more than
in the case of the elements being formed on a bulk Si wafer; as a
result, device process steps are shortened. Namely, in addition to
the higher performance, the SOI structure is expected to be lower
in the total of wafer cost and process cost than the structure of
MOSFET, IC on bulk Si.
[0014] Among others, the fully depleted MOSFETs are expected to
realize faster operation and lower power consumption based on
improvement in driving force. The threshold voltage (Vth) of MOSFET
is normally determined by an impurity concentration in the channel
part, and in the case of the fully depleted (FD; Fully Depleted)
MOSFET using SOI, the thickness of the depletion layer is also
affected by the thickness of SOI film. There were, therefore,
strong desires for uniformity of the thickness of SOI film in order
to produce large-scale integrated circuits at high yields.
[0015] The research about the formation of SOI substrate has been
becoming active since the 1970s. Considerable research has been
pursued on methods of heteroepitaxial growth of single-crystal Si
on a sapphire substrate being an insulator (SOS: Silicon on
Sapphire), methods of formation of the SOI structure by dielectric
isolation based on oxidation of porous Si (FIPOS: Full Isolation by
Porous Oxidized Silicon), bonding, and oxygen ion implantation.
[0016] This oxygen ion implantation is a method called SIMOX first
reported by K. Izumi (K. Izumi, M. Doken, and H. Ariyoshi:
Electron. Lett. 14, p. 593 (1978)). An Si wafer 91, as illustrated
in FIG. 9A, undergoes implantation of oxygen ions in about
10.sup.17 to 10.sup.18/cm.sup.2 (FIG. 9B). After that, it is
annealed at the high temperature of about 1320.degree. C. in an
argon-oxygen atmosphere to convert an ion-implanted layer 94 to a
silicon oxide layer 95 (FIG. 9C). As a consequence, Si atoms are
bonded to oxygen ions thus implanted around the depth corresponding
to the projected range (R.sub.p) of ion implantation to form a
silicon oxide layer 95, thus obtaining the SOI structure (the SOI
substrate prepared by the use of SIMOX will be referred to
hereinafter as "SIMOX wafer").
[0017] It is common practice to use a CZ wafer as a silicon
substrate for production of the SIMOX wafer. The CZ wafer is a
single-crystal silicon substrate made by the Czochralski
process.
[0018] This CZ wafer includes grown-in defects such as COP (Crystal
Originated Particles), FPD (Flow Pattern Defect), and so on, which
are defects specific to the bulk wafer.
[0019] The size of these COP (Hidekazu Yamomoto, "requirements for
large-diameter silicon wafers," the 23th ultraclean technology
college, (Aug. 1996)) and FPD (T. Abe, Extended Abst. Electrochem.
Soc. Spring Meeting vol. 95-1, pp. 596, (May, 1995)) is
approximately 0.1 to 0.2 .mu.m. In the use of the CZ wafer, there
could occur a defect called OSF (oxidation-induced stacking
fault).
[0020] The details of COP, FPD, and OSF will be discussed
hereinafter.
[0021] In the production heretofore of very large scale integrated
circuits using this CZ wafer, devices were produced with sufficient
margins to the size of the aforementioned grown-in defects, so that
the COP etc. rarely affected the device characteristics.
[0022] However, for example, considering the example of DRAM, the
design rules thereof have been changing as 0.5 .mu.m for 16M-DRAM,
0.35 .mu.m for 64M-DRAM, . . . , and, therefore, the influence of
COP etc. is becoming more and more prominent on the device
characteristics and yields.
[0023] Particularly, in the case of IG-DRAM, it is mentioned that
the design rules will be 0.1 to 0.15 .mu.m.
[0024] Namely, the production of the SIMOX wafer with the CZ wafer
would pose the problem that the aforementioned defects cause
decrease of yield in the production of devices. Therefore, there
were desires for a method of producing the SIMOX wafer without the
defects of COP etc. originating in the CZ bulk wafer or with very
fewer defects than the CZ wafer.
SUMMARY OF THE INVENTION
[0025] A first object of the present invention is to provide a
method of producing a semiconductor substrate that can be an SIMOX
wafer with higher quality than the conventional SIMOX wafers.
[0026] A second object of the present invention is to provide a
method of producing an SIMOX wafer having an Si active layer with
very few defects of COP etc. originating in the CZ bulk wafer.
Another object of the invention is to provide an SOI substrate
excellent in the quality of a buried oxide layer.
[0027] A method of producing a semiconductor substrate according to
the present invention comprises:
[0028] a step of preparing a single-crystal silicon substrate
produced by the FZ process;
[0029] a step of implanting ions into the single-crystal silicon
substrate to form an ion-implanted layer; and
[0030] a step of forming a buried insulating layer inside the
single-crystal silicon substrate.
[0031] The present invention is characterized, particularly, by
forming a protective film on the single-crystal silicon substrate
produced by the FZ process and implanting the ions from the
protective film side.
[0032] Another method of producing a semiconductor substrate
according to the present invention comprises:
[0033] a step of preparing a single-crystal silicon substrate
produced by the FZ process;
[0034] a step of implanting ions into the single-crystal silicon
substrate to form an ion-implanted layer;
[0035] a step of forming a buried insulating layer inside the
single-crystal silicon substrate; and
[0036] a step of heat treating a surface of the single-crystal
silicon substrate under a reducing atmosphere containing at least
hydrogen.
[0037] Further, the present invention is characterized by cleaning
the single-crystal silicon substrate, or the single-crystal silicon
substrate having a protective layer, prior to the formation of the
ion-implanted layer.
[0038] The present invention is also characterized by having a step
of performing a heat treatment in an oxidizing atmosphere, after
the formation of the buried insulating layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 is a flowchart showing an example of the method of
producing the semiconductor substrate according to the present
invention;
[0040] FIGS. 2A, 2B and 2C are schematic, sectional views showing
an example of the method of producing the semiconductor substrate
according to the present invention;
[0041] FIGS. 3A, 3B, 3C, and 3D are schematic, sectional views
showing a first embodiment of the present invention;
[0042] FIG. 4 is a flowchart for explaining a second embodiment of
the present invention;
[0043] FIGS. 5A, 5B, 5C, 5D, 5E, and 5F are schematic, sectional
views showing the second embodiment of the present invention;
[0044] FIGS. 6A, 6B, 6C, 6D, and 6E are schematic, sectional views
showing a third embodiment of the present invention;
[0045] FIGS. 7A, 7B, 7C, 7D, and 7E are schematic, sectional views
showing an example of the method of producing the semiconductor
substrate according to the present invention;
[0046] FIG. 8 is a schematic view showing an example of the
production step of the FZ silicon wafer; and
[0047] FIGS. 9A, 9B and 9C are schematic, sectional views for
explaining the production steps of the conventional SIMOX
wafer.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0048] First, the present invention will be described referring to
the flowchart illustrated in FIG. 1.
[0049] The first step is to prepare a single-crystal silicon
substrate produced by the floating zone process (FZ process) (which
will be referred hereinafter as "FZ silicon wafer") (S1). The
second step is to form an ion-implanted layer by ion implantation
of oxygen ions into the single-crystal silicon substrate (S2).
After that, the single-crystal silicon substrate is heat treated
under desired conditions to form a buried oxide (BOX: Buried OXide)
layer inside the single-crystal silicon substrate (S3). In this
way, an SOI substrate is completed (S4).
[0050] Next, the present invention will be explained in more
detail.
[0051] As illustrated in FIG. 2A, a single-crystal silicon
substrate 21 produced by the floating zone process (FZ process) is
prepared.
[0052] Next, as illustrated in FIG. 2B, oxygen ions are implanted
by ion implantation to form an ion-implanted layer 24. Inside the
single-crystal silicon substrate 21, an upper area of the
ion-implanted layer 24 is denoted by 22, and a lower area by
26.
[0053] After that, a desired heat treatment is carried out to form
a buried oxide (BOX) layer 25 by making use of the ion-implanted
layer 24 (FIG. 2C).
[0054] This yields an SOI substrate 27 having the single-crystal
silicon layer (SOI layer) 22 on the BOX layer 25.
[0055] The single-crystal silicon substrate (FZ silicon wafer)
produced by the FZ process includes very fewer grown-in defects,
such as COP, FPD, etc. specific to the bulk wafers, than the CZ
wafers. Therefore, use of the FZ wafer permits the SIMOX wafer with
high quality to be produced without the defects of COP etc. in the
SOI layer 22, or with very fewer such defects than the ordinary CZ
wafers. It can also prevent occurrence of OSF on or inside the
silicon wafer.
[0056] Here the OSF (oxidation-induced stacking fault) is such a
fault that a small defect becoming a nucleus thereof is introduced
during growth of a crystal wafer and becomes manifest after
oxidation. For example, there are cases where a ring pattern of OSF
is observed after wet oxidation of the wafer surface.
[0057] The COP and FPD, which can be observed without a heat
treatment, are considered to be defects due to the same cause and
there are no precise definitions to define them. It is, however,
generally known that COP is etch pits that can be detected by a
fine particle detector or a contamination inspection device
utilizing light scattering after the wafer is immersed in SC-1
(NH.sub.4OH/H.sub.2O.sub.2) solution, which is one of element
solutions for RCA cleaning, and that the latter FPD is etch pits
that can be observed with an optical microscope after the wafer is
immersed in Secco solution (K.sub.2Cr.sub.2O.sub.7/HF/H.sub.2O) for
about 30 minutes.
[0058] (FZ Silicon Wafer)
[0059] The FZ silicon wafer is a single-crystal silicon wafer
obtained by carrying out a step of heating and melting a silicon
stock bar 82 by induction heating coil 81, as illustrated in FIG.
8, and moving a molten zone 83 from bottom to top of the bar 82
(which will be referred to hereinafter as "FZ step"). Of course,
there are cases where the movement between bottom and top is
carried out two or more times.
[0060] The CZ silicon wafers contain oxygen of about
4.times.10.sup.17 to 2.times.10.sup.18 atoms/cm.sup.3 due to fusion
of a quartz crucible in the production process of the wafers. In
contrast with it, the FZ silicon wafers are wafers containing a
very low concentration of oxygen, approximately 1.times.10.sup.16
atoms/cm.sup.3 or less, though depending upon conditions. For that
reason, the FZ silicon wafers have few crystalline defects even
with a heat treatment. On the other hand, the FZ silicon wafers are
easy to warp or twist with a heat treatment, because the oxygen
concentration thereof is extremely low. For solving it, for example
as described in Japanese Patent Publication No. 3-41437, it is
preferable to raise the mechanical strength of the silicon wafer by
mixing a small amount (the volume percentage of 0.05 to 5%) of
nitrogen gas into an ambient gas (for example, argon gas). It is
also preferable to uniform nitrogen concentrations in a single
silicon crystal by diffusing nitrogen into the silicon polycrystal
bar in the nitrogen atmosphere, as described in Japanese Patent
Application Laid-Open No. 59-137393.
[0061] As described in Japanese Patent Application Laid-Open No.
8-14590, it is also preferable to carry out the FZ step in such a
way that a ring-shaped member is provided on the upper surface side
and/or on the lower surface side of the induction heating coil 81
and that the inside periphery of the ring-shaped member is put in
the molten zone 83 as a melt of the silicon stock bar. The
ring-shaped member receives polycrystalline grains peeled off the
stock bar during the FZ step, whereby the polycrystalline grains
can be prevented from redepositing onto the growing surface. This
allows the single silicon crystal to be produced with high quality
and high production efficiency. This ring-shaped member is
preferably made of quartz, sapphire, or silicon nitride.
[0062] It is also preferable to apply a static magnetic field in
the FZ step. In this case, the static magnetic field applied to the
molten zone can be either a perpendicular magnetic field or a
horizontal magnetic field. Particularly, it is also preferable to
simultaneously apply the perpendicular magnetic field formed in a
direction of the growth axis of the single-crystal silicon bar and
the horizontal magnetic field formed perpendicular to the
perpendicular magnetic field.
[0063] An impurity doping method in the FZ process is spraying of
argon gas containing phosphine (PH.sub.3), B.sub.2H.sub.6, or the
like to the molten zone by use of a nozzle.
[0064] Neutron irradiation (for example, as described in Japanese
Patent Application Laid-Open No. 6-104199) is preferably used for
uniforming the resistivity in the wafer.
[0065] It is also preferable to use the wafer whose number of COP
or FPD in the FZ silicon wafer surface is approximately 0 to 10 per
unit wafer.
[0066] The conductivity type of the FZ silicon wafer can be either
the p-type or the n-type. The specific resistance of the wafer can
be one as desired. Particularly, the FZ silicon wafer with high
resistance is suitably applicable as a substrate for production of
high-frequency devices. Especially, it is preferred to suitably
select the specific resistance such that the specific resistance in
the vicinity of the interface between the BOX layer 25 and the
lower area 26 be not less than 50 .OMEGA.cm, preferably not less
than 100 .OMEGA.cm, and more preferably not less than 300
.OMEGA.cm
[0067] When MOS transistors etc. are formed by use of the SOI
substrate, they can be subjected to perfect isolation by the BOX
layer and LOCOS (LOCal Oxidation of Silicon) oxide film, so that
interference of noise can be decreased between a digital part and
an analog part.
[0068] Therefore, the SOI substrate obtained according to the
present invention can also be suitably applied to one-chip
configurations of analog circuit part, digital circuit part, and
high-frequency components.
[0069] (Protective Layer)
[0070] It is also preferable to form a protective layer on the
substrate, prior to the implantation step of oxygen ions into the
FZ silicon substrate 21. When the ions are implanted from the
protective layer side, the surface of the substrate can be
prevented from roughening because of the implantation step.
[0071] The protective layer can be either a silicon oxide layer or
a silicon nitride layer obtained by oxidizing or nitriding the
surface of the FZ silicon substrate 21.
[0072] For forming the silicon oxide layer on the substrate by
thermal oxidation, applicable methods are as follows; dry
oxidation, wet oxidation, steam oxidation, pyrogenic oxidation,
oxygen partial pressure oxidation, oxidation with hydrochloric
acid, and so on.
[0073] Of course, the protective layer may be formed by depositing
a silicon oxide film or a silicon nitride film on the substrate by
thermal CVD, plasma CVD, or the like, or PVD.
[0074] The thickness of the protective layer is preferably several
nm to several .mu.m approximately.
[0075] (Ion-implanted Layer)
[0076] In the oxygen ion implantation step the acceleration voltage
can be in the range of 1 keV to 10 MeV, and, because the thickness
of the ion-implanted layer varies depending upon the acceleration
voltage, it is preferably determined in the range of several 10 keV
to 500 keV approximately so as to satisfy the desired
conditions.
[0077] The implantation dose is in the range of
1.0.times.10.sup.16/cm.sup- .2 to 1.0.times.10.sup.19/cm.sup.2 and
more preferably in the range of 5.0.times.10.sup.16/cm.sup.2 to
5.0.times.10.sup.18/cm.sup.2. When it is desirable to shorten the
time necessary for the implantation, the dose is preferably in the
range of 5.0.times.10.sup.16/cm.sup.2 to
5.times.10.sup.17/cm.sup.2. Further, if necessary, the ITOX process
may be carried out to increase the dielectric strength.
[0078] The temperature during the ion implantation is preferably in
the range of -200.degree. C. to 600.degree. C., more preferably in
the range of 0.degree. C. to 600.degree. C. or below, and further
preferably in the range of room temperature to 600.degree. C. or
below.
[0079] From the viewpoint of maintaining the crystallinity of the
silicon layer and in order to obtain a good buried oxide layer with
fewer current leaks, the substrate temperature during the
implantation is preferably in the range of 550.degree. C. to
650.degree. C.
[0080] The ion implantation of oxygen ions into the silicon
substrate is normally carried out as follows; oxygen ions (O.sup.+)
are selected out of various ions from ion sources by a mass
separation device, the O.sup.+ ions thus selected are accelerated
by the desired acceleration voltage, and thereafter the ions are
implanted into the silicon substrate with an ion beam obtained by
the acceleration. On this occasion, the implantation step into the
silicon substrate is carried out with scanning of the ion beam in
order to implant the ions over the entire surface of the substrate.
It is a matter of course that the implantation method is not
limited to this method.
[0081] On the other hand, it is also preferable to carry out the
ion implantation step of oxygen ions by plasma immersion doping
(Plasma Immersion Ion Implantation) (Jingbao Liu et al., Appl.
phys. Lett., 67, 2361 (1995)).
[0082] Since this method permits full irradiation over a large area
without use of the beam form, it can shorten the time necessary for
the oxygen ion implantation and can also decrease the cost. Of
course, the implantation can also be carried out by low energy ion
implantation, plasma gas immersion laser doping (PGILD), fast gas
immersion doping, and so on.
[0083] If silicon nitride is desired instead of the silicon oxide
layer as an insulating layer in the SOI substrate, nitrogen ions
can also be implanted instead of the implantation of oxygen
ions.
[0084] The implantation can also be carried out in a plurality of
ion implantation steps with stepwise change of implantation dose
and/or implantation energy (acceleration voltage). In the case of
the implantation with change of implantation energy, the
implantation energy of the second step is preferably set to be
smaller than the implantation energy of the first step
[0085] In the case of the implantation including a plurality of ion
implantation steps, the ion species implanted into the silicon
substrate can also be changed between the steps. In the
implantation comprised of two ion implantation steps, it is
preferable to first carry out ion implantation of the first ions
and thereafter carry out ion implantation of the second ions
lighter than the first ions. For example, the first ions are oxygen
ions and the second ions hydrogen ions.
[0086] In cases where the protective layer is not formed on the
upper surface of the silicon substrate prior to the step of the
formation of the ion-implanted layer, it is also preferable to form
the protective layer on the surface of the silicon substrate after
the step of formation of the ion-implanted layer. In this case, the
surface of the substrate can be prevented from roughening because
of the high-temperature heat treatment during the formation of the
BOX layer.
[0087] (Formation of BOX Layer)
[0088] A heat treatment atmosphere during the formation of the BOX
layer 25 being the buried oxide layer is an ambience mainly
containing a gas selected from oxygen, nitrogen, Ar, He, Ne, and Xe
and, more preferably, a gas ambience of oxygen diluted with an
inert gas (for example, a gas ambience of mixture of argon and
oxygen). For example, the atmosphere is a non-oxidizing atmosphere
in which oxygen of 1% or less is mixed with argon gas.
[0089] The BOX layer can also be formed by a heat treatment in a
reducing atmosphere containing hydrogen.
[0090] The temperature of the heat treatment during the formation
of the BOX layer is not less than 600.degree. C. nor more than the
melting point of silicon, preferably not less than 800.degree. C.
nor more than the melting point of silicon, and still more
preferably not less than 1000.degree. C. nor more than 1400.degree.
C.
[0091] When the BOX layer is formed in the reducing atmosphere
containing hydrogen, the treatment is carried out particularly
preferably in the temperature range of not less than 800.degree. C.
nor more than 1350.degree. C. However, in order to suppress the
reduction of the area itself which becomes the BOX layer, the
treatment is carried out preferably in the temperature range of not
less than 800.degree. C. nor more than 1000.degree. C.
[0092] The time of the heat treatment during the formation of the
BOX layer is not less than 0.5 hour nor more than 20 hours and
preferably not less than 2 hours nor more than 10 hours. Although
the heat treatment should be preferably carried out within as short
time as possible, it is desirable to define the time of the heat
treatment so as to form a uniform and continuous BOX layer.
[0093] The pressure during the formation of the BOX layer can be
the atmospheric pressure, a low pressure, or a high pressure.
[0094] With small implantation doses of oxygen ions, a leak of
current could occur where the BOX layer is formed.
[0095] In order to avoid this current leak, it is also possible to
increase the amount of oxygen ions implanted, but the time
necessary for the implantation will also increase considerably with
increase of implantation dose.
[0096] For forming the BOX layer in sufficient thickness without
increase of implantation dose, it is thus preferable to employ the
ITOX (Internal Thermal Oxidation) process. An example of
application of this ITOX process to the present invention will be
described referring to FIGS. 7A to 7E.
[0097] First, the FZ silicon wafer 71 is prepared as illustrated in
FIG. 7A. Oxygen ions are implanted into the FZ silicon wafer to
form the ion-implanted layer 74 (FIG. 7B).
[0098] After that, a cleaning step is carried out if necessary and
thereafter the substrate is annealed under a non-oxidizing
atmosphere. In this case, it is desirable to add 1% or less oxygen
to the aforementioned non-oxidizing atmosphere in order to prevent
the surface roughening due to the heat treatment. Of course, the
atmosphere is not necessarily limited to the non-oxidizing
atmosphere so long as a desired BOX layer is obtained.
[0099] This annealing under the non-oxidizing atmosphere implements
recovery from disorder of crystallinity caused by the ion
implantation and forms the buried oxide layer (BOX layer) 75 (FIG.
7C). The substrate is then annealed at high temperature to bring
about thermal oxidation called the ITOX process (FIG. 7D). This
ITOX process permits the buried oxide silicon layer 75 to
additionally grow without depending upon the oxygen ion
implantation step. Numeral 78 designates an oxidized silicon film.
This ITOX process is carried out under an oxidizing atmosphere (for
example, O.sub.2/Ar) and is preferably carried out at the
temperature not less than 1200.degree. C. and more preferably at
the temperature not less than 1300.degree. C. nor more than the
melting point of silicon in terms of the recovery of crystallinity
and the quality of the buried oxide layer.
[0100] Use of this ITOX step can improve the quality of the BOX
layer and form the SOI layer 72 in an extremely small
thickness.
[0101] After that, the surface silicon oxide film 78 is removed as
occasion may demand, thereby completing the SOI substrate (FIG.
7E).
[0102] It is noted that it is also preferable to form the
protective layer prior to the ion implantation step, though not
illustrated in FIG. 7A.
[0103] The SOI substrate is obtained after the formation of the BOX
layer, and where the surface of the SOI layer 22 or 72 is rough, it
is preferable to carry out a planarization step of the surface of
the SOI layer after the removal of the surface oxide film as
occasion may demand.
[0104] Specifically, the planarization is effected by chemical
mechanical polishing (CMP) or hydrogen annealing. An abrasive in
CMP can be selected from abrasives of borosilicate glass, titanium
dioxide, titanium nitride, aluminum oxide, iron nitrate, cerium
oxide, colloidal silica, silicon nitride, silicon carbide,
graphite, diamond, and so on, or abrasive solutions of mixtures of
the aforementioned abrasives with an oxidizer such as
H.sub.2O.sub.2, KIO.sub.3, or the like or with an alkali solution
of NaOH, KOH, or the like.
[0105] When the planarization of the surface of the SOI layer is
carried out by hydrogen annealing, an atmosphere thereof can be
100% hydrogen gas, or a mixed gas of hydrogen with a rare gas (Ar,
Ne, etc.). During the hydrogen annealing, boron or phosphorus
existing in the SOI layer diffuses outwardly, so as to increase the
resistance of the SOI layer to high resistance.
[0106] The temperature during the hydrogen annealing is not less
than 800.degree. C. nor more than 1350.degree. C. and more
preferably not less than 850.degree. C. nor more than 1250.degree.
C.
[0107] The pressure of the reducing atmosphere containing hydrogen
during the hydrogen annealing can be an atmosphere of either the
atmospheric pressure or a low pressure, but the pressure is
preferably the atmospheric pressure or a pressure not less than
1.times.10.sup.4 Pa nor more than the atmospheric pressure
(1.times.10.sup.5 Pa). It is also more preferable to carry out the
hydrogen annealing under a pressure a little, -100 mmH.sub.2O
approximately, lower than the atmospheric pressure.
[0108] A furnace used for the hydrogen annealing can be either a
vertical heat treatment furnace or a horizontal heat treatment
furnace used normally. A heater can be one selected from a
resistance heating unit, a high-frequency heating unit, and so
on.
[0109] Another annealing method is lamp heating utilizing heat
radiation, used in RTA (Rapid Thermal Annealing). In this case a
rapid annealing device can be one selected from an infrared
annealing device using a halogen lamp, an arc lamp, or the like, a
flash lamp annealing device using a xenon flash lamp or the like,
and so on. Particularly, in the case of the lamp heating, the
hydrogen annealing can be completed within a short time.
[0110] The time necessary for the hydrogen annealing is several
seconds to several ten hours and more preferably several seconds to
several hours.
[0111] In cases where the protective layer is formed on the FZ
silicon wafer 21, the protective layer is removed if necessary,
after the formation of the BOX layer 25. This removal of the
protective layer is carried out by polishing, grinding, CMP, dry
etching, or wet etching (an etchant in this case is one of
hydrofluoric-nitric acid base, ethylene diamine base, KOH base, and
hydrazine base etchants, or can be one selected from hydrofluoric
acid, mixtures of hydrofluoric acid with at least one of hydrogen
peroxide and alcohol, and mixtures of buffered hydrofluoric acid
with at least one of hydrogen peroxide and alcohol).
[0112] (Embodiment 1)
[0113] FIGS. 3A to 3D are schematic, sectional views showing steps
of Embodiment 1 of the present invention.
[0114] First prepared is a substrate 31 of a single-crystal Si
wafer produced by the floating zone process (FZ process), as an Si
substrate. An insulating layer 33 may be further formed on the
surface of the substrate if necessary (FIG. 3A).
[0115] Then oxygen ions are implanted from the principal surface
side of the substrate 31 thereinto. In this way, an ion-implanted
layer 34 is formed to define a lower area 36 and an upper area 32
in the substrate 31 (FIG. 3B).
[0116] Then the substrate 31 undergoes a heat treatment to form the
buried oxide layer 35, as illustrated in FIG. 3C.
[0117] In this way an SOI substrate 37 is completed with the
single-crystal silicon layer 32 on the oxide layer 35.
[0118] Since the single-crystal Si layer 32 remaining on the Si
oxide layer 35 is one formed by the FZ process, occurrence of FPD
and COP is well suppressed therein.
[0119] Then the surface oxide film 33 is removed to obtain a
semiconductor substrate (SIMOX wafer) according to the present
invention, as illustrated in FIG. 3D. Of course, the surface oxide
film 33 may be left without being removed, until immediately before
the device process in order to avoid contamination of the surface.
The single-crystal Si layer 32 obtained in this way is formed as a
flat and uniform thin layer with intervention of the Si oxide layer
35 across a large area over the entire wafer surface. The
semiconductor substrate thus obtained can be suitably used from the
viewpoint of the production of dielectric-isolated semiconductor
devices, too.
[0120] Further, the substrate may be subjected to a heat treatment
in a reducing atmosphere containing hydrogen (hydrogen annealing)
after the removal of the surface oxide film 33 as occasion may
demand. This heat treatment makes the roughness of the surface
smoother. This smoothing method by the heat treatment permits
smoothing of the surface without use of touch polishing having
stronger mechanical polishing action than chemical etching action,
so as to prevent fine scratches from being introduced into the
surface.
[0121] When the single-crystal layer contains a relatively large
amount of boron, the above hydrogen annealing causes boron to
diffuse outwardly out of the substrate, so that the boron
concentration can be decreased in the single-crystal layer.
[0122] Since the substrate 31 as a starting material used in the
present invention is one produced by the FZ process, the SOI layer
32 can be obtained with very fewer defects of COP etc. than the
SIMOX wafers produced using the CZ silicon wafers.
[0123] The implantation amount (dose) of oxygen ions used in the
present invention is 1.times.10.sup.16 cm.sup.-2 to
1.times.10.sup.19 cm.sup.-2, preferably 5.times.10.sup.16 cm.sup.-2
to 5.times.10.sup.18 cm.sup.-2, and more preferably
4.times.10.sup.17 cm.sup.-2 to 5.times.10.sup.18 cm.sup.-2.
[0124] The atmosphere of the heat treatment in the formation of the
Si oxide layer after the oxygen ion implantation is an atmosphere
mainly containing a gas selected from oxygen, nitrogen, Ar, He, Ne,
and Xe, and more preferably a gas atmosphere of oxygen diluted with
an inert gas (for example, argon).
[0125] The temperature of the heat treatment in the formation of
the buried Si oxide layer is not less than 1250.degree. C. and more
preferably not less than 1300.degree. C. but less than the melting
point of single-crystal Si.
[0126] The time of the heat treatment is not less than 2 hours and
more preferably not less than 4 hours.
[0127] The reason is that too short time of the heat treatment will
result in insufficient formation of the buried oxide layer.
[0128] It is more preferable to carry out the cleaning step, the
ion implantation step, and the heat treatment step at least once
more after the formation of the Si oxide layer by one cycle of ion
implantation and heat treatment.
[0129] This permits formation of a uniform buried Si oxide layer
with very suppressed current leak.
[0130] The reducing atmosphere during the heat treatment for
smoothing used in the present invention is 100% hydrogen, or a
reducing atmosphere consisting of hydrogen and an inert gas. The
inert gas used herein is either of He, Ar, Ne, Xe, N.sub.2, and so
on.
[0131] The time of the heat treatment is not less than 10 minutes
and preferably not less than 1 hour, depending upon the size of the
Si substrate and the thickness of the surface layer from which the
defects are removed by the heat treatment.
[0132] (Embodiment 2)
[0133] The second embodiment of the present invention will be
described first referring to the flowchart of FIG. 4.
[0134] The FZ silicon wafer is prepared (S1). The protective layer
is formed on the FZ silicon wafer and thereafter oxygen ions are
implanted into the FZ silicon wafer from the protective layer side
to form an ion-implanted layer (S2). After that, the single-crystal
silicon substrate undergoes a heat treatment under the desired
conditions to form the BOX layer inside the single-crystal silicon
substrate (S3). The steps so far are similar to those in the
flowchart of FIG. 1. It is noted that the protective layer forming
step can be omitted from the step (S2) as occasion may demand.
[0135] In the present embodiment, after the formation of the BOX
layer, the FZ silicon wafer is cleaned (S4). Then the ion-implanted
layer forming step is carried out again (S5). After that, the heat
treatment is carried out again, as in step (S3), to form the BOX
layer (S6). The SIMOX wafer is completed in this way (S7). If there
exists a particle on the surface of substrate on the occasion of
the ion implantation it can work as a mask and form a region where
the ion-implanted layer is not formed. Since the present embodiment
involves the steps of forming the BOX layer once, thereafter
cleaning the surface of the silicon substrate, and again implanting
oxygen ions, unevenness in implantation of oxygen ions can be
prevented well.
[0136] Although FIG. 4 shows the example of two ion implantation
steps, the number of ion implantation steps can be determined
arbitrarily as occasion may demand. It is also preferable to clean
the silicon substrate after the step (S1) and before the step
(S2).
[0137] The heat treatment (S3, S6) for formation of the BOX layer
may be carried out only once after completion of the final ion
implantation step.
[0138] FIGS. 5A to 5F are schematic, sectional views showing the
steps in Embodiment 2 of the present invention.
[0139] In FIG. 5A, first, the single-crystal silicon wafer 51
produced by the FZ process is prepared as an Si substrate. Further,
it is better to form an Si oxide layer 53 on the surface of the
substrate 51 according to the necessity in order to prevent the
surface roughening during the ion implantation (FIG. 5B).
[0140] Next, oxygen ions are implanted from the principal surface
of the substrate 51 (FIG. 5C). In this figure, numeral 59
represents a particle on the substrate. The ion-implanted layer 54
is formed inside the substrate 51 so as to exist between the lower
area 56 and the upper area 52 of the substrate 51. Of course, where
the protective layer 53 is formed, it is desirable to implant the
ions from the side of the protective layer 53.
[0141] Next, as illustrated in FIG. 5D, the substrate 57 is heat
treated to change the ion-implanted layer 54 into oxidized Si,
thereby forming the buried Si oxide layer (BOX layer) 55.
[0142] (Multi-step Implantation: Multi-I/I)
[0143] After that, the substrate 57 is cleaned and the ion
implantation step (FIG. 5E) and the heat treatment step (FIG. 5F)
are carried out again in similar fashion to the above steps.
[0144] If there remains a particle on the wafer surface during the
previous ion implantation it can work as a mask to form a region
without implanted ions (FIG. 5D). However, such nonuniformity of
the BOX layer can be prevented by again carrying out the ion
implantation after the cleaning step between the implantation
steps.
[0145] It is also preferable to carry out the third ion
implantation step and the third heat treatment step after further
cleaning.
[0146] The buried oxide layer 55 can be obtained with good quality
by repeatedly carrying out several cycles of the cleaning, ion
implantation, and heat treatment steps in this way.
[0147] Further, a smooth surface having the surface roughness not
more than several nm can be obtained by removing the surface oxide
film 53 and thereafter carrying out a heat treatment in the
reducing atmosphere containing hydrogen.
[0148] (Embodiment 3)
[0149] FIGS. 6A to 6E are schematic, sectional views showing the
steps in Embodiment 3 of the present invention.
[0150] In FIG. 6A, first, the substrate 61 of the single-crystal Si
wafer produced by the FZ process is prepared as an Si substrate.
The protective layer 63 of an Si oxide film or the like may be
formed on the surface of the substrate 61 according to the
necessity to prevent the surface roughening during the ion
implantation.
[0151] Next, the oxygen ions are implanted from the principal
surface of the substrate 61, as illustrated in FIG. 6B, to form the
ion-implanted layer 64. Where the protective layer 63 is formed,
the ions are implanted from the side of the protective layer
63.
[0152] Next, as illustrated in FIG. 6C, the substrate is heat
treated to form the buried Si oxide layer 65. It is a matter of
course that the desired cleaning step may be carried out prior to
the heat treatment. The protective layer 63 may be removed after
the ion implantation as occasion may demand.
[0153] (ITOX Step)
[0154] After that, the substrate 67 undergoes a heat treatment in
the oxidizing atmosphere (FIG. 6D).
[0155] This heat treatment in the oxidizing atmosphere again forms
a surface oxide layer in the surface of the single-crystal Si layer
62 and also increases the thickness of the buried oxide layer 65
inside, thereby enhancing the reliability of the buried Si oxide
layer 65. Therefore, the thickness of the single-crystal Si layer
becomes thinner, so as to obtain the single-crystal Si thin layer
62 (FIG. 6E).
[0156] It is desirable to make the oxidizing atmosphere of oxygen
and an inert gas. For restraining forming rates of the oxide film
in the surface and promoting increase in the thickness of the
internal oxide layer, it is desirable to decrease the oxygen
concentration in the atmosphere and increase the temperature of the
heat treatment.
[0157] Then the surface oxide film 63 is removed to obtain the
semiconductor substrate having the single-crystal Si thin layer 62
with few defects on the buried Si oxide layer 65. Since the SIMOX
wafer thus produced is made based on the FZ substrate, the SOI
layer can be obtained with very few defects of COP and the
like.
[0158] Further, after the removal of the surface oxide film 63, the
substrate may be subjected to the heat treatment in the reducing
atmosphere containing hydrogen as described previously. This heat
treatment can smooth the surface down to the surface roughness not
more than several nm.
[0159] FIG. 8 is a view showing the principle of the method of
producing the single-crystal Si substrate according to the FZ
process.
[0160] The ring-shaped heater 81 is placed around the cylindrical
crystal silicon ingot 82 having the diameter of 100 mm to 300 mm,
and this heater 81 is moved vertically. Si 83 melted in the belt
form by the heater 81 is retained by surface tension to be
crystallized into a single crystal of a circular cylinder shape.
This single-crystal Si ingot is sliced in a disk shape and polished
to obtain FZ Si wafers. The coil of the heater is, for example, a
melting coil to which high-frequency power of 1 to 3 MHz is
supplied from a high-frequency oscillator.
[0161] When the single-crystal silicon is produced by the FZ
process as described above, it is formed without contact with other
materials such as the quartz crucible in the CZ process, and the
substrate itself is thus unlikely to be contaminated by oxygen,
boron, aluminum, and so on. Since the CZ waqfer has COP inside of
the wafer, there is a case where oxygen implantred
EXAMPLE 1
[0162] As silicon substrates, there were prepared an 8-inch
single-crystal silicon substrate (the FZ silicon wafer, p-Si (100))
produced by the FZ process and a CZ silicon wafer for
comparison.
[0163] The ion implantation of oxygen ions was carried out up to
1.5.times.10.sup.18 cm.sup.-2 by the acceleration energy of 180
keV. The temperature of the substrate during the implantation was
550.degree. C.
[0164] After that, each of the substrates was heat treated at
1350.degree. C. in an O.sub.2 (10%)/Ar (90%) atmosphere for four
hours.
[0165] The SIMOX wafers (SOI wafers) were obtained each in the
structure of single-crystal Si semiconductor layer (SOI layer) 179
nm/buried Si oxide layer (BOX layer) 400 nm.
[0166] After that, for detecting COP in the surface of the SOI
layer, the SOI wafers were treated with the SC-1 cleaning solution
(a mixture of NH.sub.4OH 1.0 wt %, H.sub.2O.sub.2 6.0 wt %, and
water) for ten minutes. Then the number of COP (approximately 0.1
to 0.2 .mu.m) in the surface of the SOI wafers was counted using a
surface particle inspector (SP-1 available from KLA-Tencor
Inc.).
[0167] The number of COP per unit wafer was 200 in the surface of
the SOI layer formed based on the CZ silicon wafers. The "per unit
wafer" in the present example means "per area of wafer"; for
example, in the case of an 8-inch wafer, the number of COP per unit
wafer is equivalent to the number of COP per about 324
cm.sup.2.
[0168] On the other hand, no COP was counted per unit wafer in the
case of the SOI substrate being produced using the FZ silicon
wafer. In this way, the SOI wafer was obtained substantially
without COP which was the defects originating in the CZ-Si
substrate.
[0169] If the surface of the Si substrate is oxidized before the
oxygen ion implantation step to form the protective layer, the
surface roughening due to the ion implantation can be prevented
effectively.
[0170] If the substrate is further subjected to hydrogen annealing
after the formation of the BOX layer, still more decrease can be
achieved in the defects of COP and the like.
[0171] It is also preferable to thermally oxidize the surface of
the silicon substrate to form a silicon oxide layer as a protective
layer, prior to the formation of the ion-implanted layer. When the
surface of the ordinary CZ wafers is thermally oxidized to form a
silicon oxide layer, OSF (oxidation-induced stacking fault) can
appear in the wafers. If the fault exists in an area to become the
SOI layer, it will affect the quality of the SOI layer. However,
since the surface of the FZ silicon wafer is oxidized in the
present invention, introduction of OSF can be prevented. A
conceivable reason is that the oxygen concentration in the surface
of substrate is lower in the FZ wafer than in the CZ wafer.
[0172] Since the CZ wafer contains COP inside of the wafer too,
there is a case where the oxygen implanted by the heat treatment
during formation of the BOX layer is concentrated also at the
periphery of the COP. This may result in formation of larger COP
(defect). The presence of the larger COP in the SOI layer may
degrade the device characteristics.
[0173] On the other hand, since the FZ wafer contains little COP
inside of the wafer, there is no possibility that COP may be
increased by implanted oxygen, so that SOI substrates of high
quality can be provided.
[0174] EXAMPLE 2
[0175] Single-crystal Si substrates produced by the FZ process were
prepared as Si substrates.
[0176] Further, the surface of these Si substrates was thermally
oxidized to form a surface Si oxide film in the thickness of 50 nm.
This oxide film was formed for the purpose of preventing the
surface roughening during the ion implantation.
[0177] The ion implantation of O.sup.+ was carried out up to
2.times.10.sup.18 cm.sup.-2 by 180 keV through the Si oxide film of
the surface. The temperature during the implantation was
550.degree. C. This formed the ion-implanted layer having a
concentration peak near 400 nm from the surface of the
substrate.
[0178] After this, the substrates were heat treated at 1350.degree.
C. in the O.sub.2 (10%)/Ar (90%) atmosphere for four hours. Then
the surface oxide film was removed to obtain the SOI wafers in the
structure of single-crystal Si semiconductor layer (SOI layer) 150
nm/buried Si oxide layer 400 nm.
[0179] Since this SOI layer is part of the single-crystal Si layer
with few defects of the FZ wafers produced by the FZ process, there
were substantially no defects of COP, FPD, etc. originating in the
CZ-Si substrate.
[0180] The SOI substrates thus completed were immersed in 49% HF
solution for 15 minutes and then observed with an optical
microscope. If there were COP inside the SOI layer the Si oxide
layer would be etched through the COP part with HF to observe HF
Voids from which the Si oxide layer was dissolved out in a circular
pattern. In the case of the present example, HF Voids were observed
in the density of 0.05 void/cm.sup.2.
EXAMPLE 3
[0181] The FZ-silicon wafer was prepared as in Example 1.
[0182] The ion implantation of O.sup.+ was carried out up to
2.times.10.sup.17 cm.sup.-2 by 180 keV through the Si oxide layer
of the surface. The temperature during the implantation was
550.degree. C.
[0183] After this, the substrate was heat treated at 1350.degree.
C. in the O.sub.2 (10%)/Ar (90%) atmosphere for four hours. The
buried Si oxide layer thus obtained had the thickness of about 100
nm.
[0184] This wafer was cleaned and thereafter the ion implantation
of O.sup.+ was carried out again up to 5.times.10.sup.17 cm.sup.-2
by 180 keV. Then the heat treatment similar to above was carried
out. This cycle of cleaning, implantation, and heat treatment was
repeatedly carried out before the total implantation amount of
oxygen became 2.times.10.sup.18 cm.sup.-2.
[0185] Then the surface oxide film was removed to obtain the SOI
wafer in the structure of SOI layer 150 nm/buried Si oxide layer
400 nm.
[0186] Since this SOI layer was originally part of the FZ wafer
layer prepared by the FZ process, there were substantially no
defects of COP, FPD, etc. originating in the CZ-Si substrate.
EXAMPLE 4
[0187] The FZ-Si wafer was prepared as in Example 1.
[0188] Further, an Si oxide film 50 nm thick was formed by thermal
oxidation in the surface of the surface single-crystal Si layer
(SOI layer).
[0189] The ion implantation of O.sup.+ was carried out up to
4.times.10.sup.17 cm.sup.-2 by 180 keV through the Si oxide film of
the surface. The temperature during the implantation was
550.degree. C.
[0190] After this, the substrate was heat treated at 1350.degree.
C. in the O.sub.2 (10%)/Ar (90%) atmosphere for four hours to
change the ion-implanted layer into the buried Si oxide layer. In
this way the SOI wafer was obtained in the structure of SOI layer
300 nm/buried Si oxide layer 90 nm.
[0191] After this, a further heat treatment was carried out at
1350.degree. C. in an O.sub.2 (70%)/Ar (30%) atmosphere for four
hours. Then the surface oxide layer on the SOI layer was removed to
obtain the SOI wafer in the structure of SOI layer 200 nm/buried Si
oxide layer 120 nm.
[0192] Since this SOI layer was originally part of the
single-crystal Si layer of the FZ wafer, there were substantially
no defects of COP, FPD, etc. originating in the CZ-Si
substrate.
EXAMPLE 5
[0193] The Si wafer produced by the FZ process was prepared.
[0194] Further, the surface of this substrate was thermally
oxidized to form a silicon oxide film in the thickness of 50
nm.
[0195] The ion implantation of O.sup.+ was carried out up to
4.times.10.sup.17 cm.sup.-2 by 180 keV through the silicon oxide
film in the wafer surface. The temperature during the implantation
was 550.degree. C.
[0196] After this, this wafer was heat treated at 1350.degree. C.
in the O.sub.2 (10%)/Ar (90%) atmosphere for four hours. The SOI
wafer was obtained in the structure of SOI layer 300 nm/buried
oxide layer 90 nm.
[0197] After this, a further heat treatment was carried out at
1350.degree. C. in the O.sub.2 (70%)/Ar (30%) atmosphere for four
hours. Then the oxide film in the surface of wafer was removed to
obtain the SOI wafer in the structure of SOI layer 200 nm/buried
oxide layer 120 nm.
[0198] This SOI layer had no defects of COP, FPD, etc. originating
in the CZ-Si substrate.
EXAMPLE 6
[0199] Prepared was a p.sup.+ type FZ-Si wafer having the specific
resistance of 0.01 .OMEGA..multidot.cm.
[0200] Further, the surface of the FZ wafer was thermally oxidized
to form the SiO.sub.2 layer 50 nm thick.
[0201] The ion implantation of O.sup.+ was carried out up to
2.times.10.sup.18 cm.sup.-2 by 180 keV through the Si oxide film of
the surface. The temperature during the implantation was
550.degree. C.
[0202] After this, the substrate was heat treated at 1350.degree.
C. in the O.sub.2 (10%)/Ar (90%) atmosphere for four hours. Then
the surface Si oxide film was removed to obtain the SOI wafer in
the structure of SOI layer 150 nm/buried oxide layer 400 nm.
[0203] Since this SOI layer was originally part of the FZ wafer,
there were substantially no defects of COP, FPD, etc. originating
in the CZ-Si substrate.
[0204] After this, the SOI wafer was subjected to a heat treatment
in an atmosphere of 100% high-purity hydrogen purified by a
hydrogen purifier using a palladium alloy (1100.degree. C., 4 h).
After this, the surface roughness of the SOI wafer was measured and
it was found that the surface roughness was improved from Rrms =0.5
nm before the heat treatment to 0.3 nm. Further, the boron
concentration of the SOI wafer, which was
2.times.10.sup.18/cm.sup.3 in the SOI layer before the heat
treatment, was also reduced to 5.times.10.sup.15/cm.sup.3 or less
after the heat treatment.
* * * * *