U.S. patent application number 10/459044 was filed with the patent office on 2003-11-06 for anodizing method and apparatus and semiconductor substrate manufacturing method.
Invention is credited to Sakaguchi, Kiyofumi, Sato, Nobuhiko.
Application Number | 20030205480 10/459044 |
Document ID | / |
Family ID | 12715724 |
Filed Date | 2003-11-06 |
United States Patent
Application |
20030205480 |
Kind Code |
A1 |
Sakaguchi, Kiyofumi ; et
al. |
November 6, 2003 |
Anodizing method and apparatus and semiconductor substrate
manufacturing method
Abstract
A porous layer having a multilayered structure is formed. An Si
substrate (102) to be processed is anodized in a first electrolytic
solution (141, 151) while being held between an anode (106) and a
cathode (104) in an anodizing bath (101). The first electrolytic
solution (141, 151) is exchanged with a second electrolytic
solution (142, 152). The Si substrate (102) is anodized again,
thereby forming a porous layer having a multilayered structure on
the Si substrate (102).
Inventors: |
Sakaguchi, Kiyofumi;
(Yokohama-shi, JP) ; Sato, Nobuhiko;
(Sagimihara-shi, JP) |
Correspondence
Address: |
MORGAN & FINNEGAN, L.L.P.
345 Park Avenue
New York
NY
10154-0053
US
|
Family ID: |
12715724 |
Appl. No.: |
10/459044 |
Filed: |
June 11, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10459044 |
Jun 11, 2003 |
|
|
|
09251668 |
Feb 17, 1999 |
|
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Current U.S.
Class: |
205/159 ;
205/171; 205/333; 257/E21.215; 257/E21.228; 257/E21.57 |
Current CPC
Class: |
H01L 21/02052 20130101;
C25D 11/005 20130101; C25D 11/32 20130101; C25D 17/002 20130101;
H01L 21/0203 20130101; H01L 21/76259 20130101 |
Class at
Publication: |
205/159 ;
205/333; 205/171 |
International
Class: |
C25D 005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 26, 1998 |
JP |
10-045309 |
Claims
What is claimed is:
1. An anodizing method of forming a porous layer on a substrate,
comprising the steps of: preparing an anodizing bath used to
anodize a substrate, anodizing said substrate to be processed in a
first electrolytic solution while holding said substrate between an
anode and a cathode in said anodizing bath, exchanging the first
electrolytic solution with a second electrolytic solution, and
anodizing said substrate in the second electrolytic solution,
thereby forming a porous layer having a multilayered structure on
said substrate.
2. The method according to claim 1, wherein a current density of a
current to be flowed across said anode and said cathode is changed
between anodizing using the first electrolytic solution and
anodizing using the second electrolytic solution.
3. The method according to claim 1, wherein a conductive diaphragm
is inserted between said anode and said substrate to be processed
to prevent contamination of said substrate by said anode.
4. The method according to claim 3, wherein said conductive
diaphragm is arranged to flow the whole current from said anode to
said substrate to be processed through said conductive
diaphragm.
5. The method according to claim 3, wherein said conductive
diaphragm is arranged to cover a surface of said anode that opposes
a backside surface of said substrate to be processed.
6. The method according to claim 3, wherein said conductive
diaphragm is arranged to isolate an electrolytic solution in
contact with a surface of said substrate to be processed, that is
on said anode side, from an electrolytic solution in contact with
said anode.
7. The method according to claim 4, wherein at least a surface of
said conductive diaphragm that opposes said substrate to be
processed is formed from a silicon material.
8. The method according to claim 4, further comprising the step of
changing said conductive diaphragm every time an anodizing
condition is changed.
9. The method according to claim 4, further comprising the step of
preparing a conductive diaphragm corresponding to each anodizing
condition, and every time the anodizing condition is changed, using
a conductive diaphragm corresponding to the condition.
10. The method according to claim 1, further comprising the step of
forming a porous layer having a relatively low porosity as a
surface layer of said substrate to be processed, and forming a
porous layer having a relatively high porosity as an underlayer of
said surface layer.
11. The method according to claim 10, further comprising the step
of anodizing to form a porous layer having a porosity of not more
than 30% as said surface layer.
12. The method according to claim 10, further comprising the step
of anodizing to form a porous layer having a porosity of not less
than 30% as said underlayer of said surface layer.
13. The method according to claim 10, further comprising the step
of anodizing to form a porous layer having a thickness of not more
than 5 nm as said underlayer of said surface layer.
14. An anodizing method of forming a porous layer on a substrate,
comprising the step of: preparing at least two anodizing baths used
to anodize a substrate, anodizing said substrate to be processed
while holding said substrate between an anode and a cathode in one
anodizing bath, and anodizing said substrate while holding said
substrate between an anode and a cathode in the next anodizing
bath, thereby forming a porous layer having a multilayered
structure on said substrate.
15. The method according to claim 14, wherein different
electrolytic solutions are used as electrolytic solutions used for
anodizing in all or some of said at least two anodizing baths.
16. The method according to claim 14, wherein a current density of
a current to be flowed across said anode and said cathode is
changed in anodizing in all or some of said at least two anodizing
baths.
17. The method according to claim 14, wherein a conductive
diaphragm is inserted between said anode and said substrate to be
processed to prevent contamination of said substrate by said
anode.
18. The method according to claim 17, wherein said conductive
diaphragm is arranged to flow the whole current from said anode to
said substrate to be processed through said conductive
diaphragm.
19. The method according to claim 17, wherein said conductive
diaphragm is arranged to cover a surface of said anode that opposes
a backside surface of said substrate to be processed.
20. The method according to claim 17, wherein said conductive
diaphragm is arranged to isolate an electrolytic solution in
contact with a surface of said substrate to be processed, that is
on said anode side, from an electrolytic solution in contact with
said anode.
21. The method according to claim 17, wherein at least a surface of
said conductive diaphragm that opposes said substrate to be
processed is formed from a silicon material.
22. The method according to claim 21, further comprising the step
of changing said conductive diaphragm every time an anodizing
condition is changed.
23. The method according to claim 21, further comprising the step
of preparing a conductive diaphragm corresponding to each anodizing
condition, and every time the anodizing condition is changed, using
a conductive diaphragm corresponding to the condition.
24. The method according to claim 14, further comprising the step
of forming a porous layer having a relatively low porosity as a
surface layer of said substrate to be processed, and forming a
porous layer having a relatively high porosity as an underlayer of
said surface layer.
25. The method according to claim 24, further comprising the step
of anodizing to form a porous layer having a porosity of not more
than 30% as said surface layer.
26. The method according to claim 24, further comprising the step
of anodizing to form a porous layer having a porosity of not less
than 30% as said underlayer of said surface layer.
27. The method according to claim 24, further comprising the step
of anodizing to form a porous layer having a thickness of not more
than 5 .mu.m as said underlayer of said surface layer.
28. A semiconductor substrate manufacturing method comprising the
steps of: processing a first substrate using the anodizing method
of claim 1 to form a porous layer having a multilayered structure
on said first substrate; forming at least one non-porous layer on
said porous layer; bonding a second substrate to a surface of said
non-porous layer of said first substrate; separating a bonded
substrate stack to a side of said first substrate and a side of
said second substrate at a portion of said porous layer; and
removing said porous layer left on the side of said separated
second substrate.
29. The method according to claim 28, further comprising the step
of removing said porous layer left on the side of said separated
first substrate to allow reuse of said first substrate.
30. The method according to claim 28, wherein the separation step
comprises separating said bonded substrate stack at a portion
having a high porosity in said porous layer having the multilayered
structure.
31. The method according to claim 28, wherein said non-porous layer
contains a single-crystal silicon layer.
32. The method according to claim 28, wherein said non-porous layer
comprises a single-crystal silicon layer and a silicon oxide layer
which are sequentially formed on said porous layer.
33. The method according to claim 28, wherein said non-porous layer
contains a compound semiconductor layer.
34. The method according to claim 28, wherein said second substrate
comprises a silicon substrate.
35. The method according to claim 28, wherein said second substrate
comprises a substrate having a silicon oxide layer formed on a
surface to be bonded to said first substrate.
36. The method according to claim 28, wherein said second substrate
comprises a transparent substrate.
37. The method according to claim 28, further comprising, after the
step of removing said porous layer, the step of planarizing a
surface layer on the side of said second substrate.
38. The method according to claim 37, wherein the planarization
step comprises annealing in an atmosphere containing hydrogen.
39. The method according to claim 28, wherein the step of removing
said porous layer comprises selectively etching said porous layer
using, as an etchant, any one of a) hydrofluoric acid, b) a mixed
solution prepared by adding at least one of an alcohol and hydrogen
peroxide to hydrofluoric acid, c) buffered hydrofluoric acid, and
d) a mixed solution prepared by adding at least one of an alcohol
and hydrogen peroxide to buffered hydrofluoric acid.
40. The method according to claim 33, wherein the step of removing
said porous layer comprises selectively etching said porous layer
using an etchant whose etching rate for said porous layer is higher
than that for a compound semiconductor.
41. The method according to claim 28, wherein the step of removing
said porous layer comprises selectively polishing said porous layer
using said non-porous layer as a stopper.
42. The method according to claims 28, wherein the bonding step
comprises the step of bringing said first substrate having said
non-porous layer into tight contact with said second substrate.
43. The method according to claim 28, wherein the bonding step
comprises, after the step of bringing said first substrate having
said non-porous layer into tight contact with said second
substrate, the step of performing a process selected from the group
consisting of anode bonding, pressing, heating, and a combination
thereof.
44. A semiconductor substrate capable of being manufactured by the
manufacturing method of claim 28.
45. A semiconductor substrate in a process of executing the
manufacturing method of claims 28.
46. A substrate having a porous layer formed by the anodizing
method of claims 1.
47. An anodizing apparatus for executing the anodizing method of
claim 1.
48. An anodizing apparatus for forming a porous layer on a
substrate, comprising: an anodizing bath having an anode and a
cathode; a plurality of tanks for storing electrolytic solutions to
be supplied to said anodizing bath; a supply mechanism for
selectively supplying the electrolytic solution stored in any one
of said plurality of tanks to said anodizing bath; and a drain
mechanism for draining off the electrolytic solution from said
anodizing bath back into said tank which supplied the electrolytic
solution.
49. The apparatus according to claim 48, wherein said apparatus
further comprises a holding mechanism for holding a conductive
diaphragm between said anode and a substrate to be processed, and
said conductive diaphragm prevents contamination of said substrate
by said anode.
50. The apparatus according to claim 49, wherein said holding
mechanism holds said conductive diaphragm to flow a whole current
from said anode to said substrate to be processed through said
conductive diaphragm.
51. The apparatus according to claim 49, wherein said holding
mechanism holds said conductive diaphragm to cover a surface of
said anode that opposes a backside surface of said substrate to be
processed.
52. The apparatus according to claim 49, wherein said holding
mechanism holds said conductive diaphragm to isolate an
electrolytic solution in contact with a surface of said substrate
to be processed, that is on said anode side, from an electrolytic
solution in contact with said anode.
53. The apparatus according to claim 49, wherein at least a surface
of said conductive diaphragm that opposes said substrate to be
processed is formed from a silicon material.
54. The apparatus according to claim 49, wherein said holding
mechanism detachably holds said conductive diaphragm.
55. An anodizing apparatus for forming a porous layer on a
substrate, comprising: at least two anodizing baths each having an
anode and a cathode; and a conveyor mechanism for conveying a
substrate anodized in one anodizing bath into the next anodizing
bath.
56. The apparatus according to claim 55, wherein said substrate is
anodized in all or some of said at least two anodizing baths under
different conditions.
57. The apparatus according to claim 55, further comprising a
cleaning unit for cleaning said substrate processed in said final
anodizing bath of said at least two anodizing baths, and a drier
unit for drying said substrate cleaned by said cleaning unit.
58. The apparatus according to claim 55, wherein each of said
anodizing baths comprises a holding mechanism for holding a
conductive diaphragm between said anode and said substrate to be
processed, and said conductive diaphragm prevents contamination of
said substrate by said anode.
59. The apparatus according to claim 58, wherein said holding
mechanism holds said conductive diaphragm to flow a whole current
from said anode to said substrate to be processed through said
conductive diaphragm.
60. The apparatus according to claim 58, wherein said holding
mechanism holds said conductive diaphragm to cover a surface of
said anode that opposes said substrate to be processed.
61. The apparatus according to claim 58, wherein said holding
mechanism holds said conductive diaphragm to isolate an
electrolytic solution in contact with a surface of said substrate
to be processed, that is on said anode side, from an electrolytic
solution in contact with said anode.
62. The apparatus according to claim 59, wherein at least a surface
of said conductive diaphragm that opposes said substrate to be
processed is formed from a silicon material.
63. The apparatus according to claim 55, wherein said holding
mechanism detachably holds said conductive diaphragm.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an anodizing method and
apparatus and a semiconductor substrate manufacturing method and,
more particularly, to an anodizing method and apparatus for forming
a porous layer on a substrate, and a semiconductor substrate
manufacturing method using the anodizing method as part of the
process.
[0003] 2. Description of the Related Art
[0004] A substrate (SOI substrate) having an SOI (Silicon On
Insulator) structure is known as a substrate having a
single-crystal Si layer on an insulating layer. A device using this
SOI substrate has many advantages that cannot be achieved by
ordinary Si substrates. Examples of the advantages are as
follows.
[0005] (1) The integration degree can be increased because
dielectric isolation is easy.
[0006] (2) The radiation resistance can be increased.
[0007] (3) The operating speed of the device can be increased
because the stray capacitance is small. (
[0008] 4) No well step is necessary.
[0009] (5) Latch-up can be prevented.
[0010] (6) A completely depleted field-effect transistor can be
formed by thin film formation.
[0011] Since an SOI structure has the above various advantages,
researches have been made on its formation method for several
decades.
[0012] As one SOI technology, the SOS (Silicon On Sapphire)
technology by which Si is heteroepitaxially grown on a
single-crystal sapphire substrate by CVD (Chemical Vapor
Deposition) has been known for a long time. This SOS technology
once earned a reputation as the most matured SOI technology.
However, the SOS technology has not been put into practical use to
date because, e.g., a large amount of crystal defects are produced
by lattice mismatch in the interface between the Si layer and the
underlying sapphire substrate, aluminum that forms the sapphire
substrate mixes in the Si layer, the substrate is expensive, and it
is difficult to obtain a large area.
[0013] Attempts have recently been made to realize the SOI
structure without using any sapphire substrate. The attempts are
roughly classified into two methods.
[0014] In the first method, the surface of a single-crystal Si
substrate is oxidized, and a window is formed in the oxide film
(SiO.sub.2 layer) to partially expose the Si substrate.
Single-crystal Si is epitaxially grown laterally using the exposed
portion as a seed, thereby forming a single-crystal Si layer on
SiO.sub.2 (in this method, an Si layer is deposited on an SiO.sub.2
layer).
[0015] In the second method, a single-crystal Si substrate itself
is used as an active layer, and an SiO.sub.2 layer is formed on the
lower surface of the substrate (in this method, no Si layer is
deposited) .
[0016] As a means for realizing the first method, a technique of
directly epitaxially growing single-crystal Si in the horizontal
direction from the single-crystal Si layer by CVD (CVD), a
technique of depositing amorphous Si and epitaxially growing
single-crystal Si laterally in the solid phase by annealing (solid
phase epitaxial growth), a technique of irradiating an amorphous
silicon layer or a polysilicon layer with a focused energy beam
such as an electron beam or laser beam to grow a single-crystal Si
layer on an SiO.sub.2 layer by melting recrystallization (beam
annealing), or a method of scanning band-shaped melting regions by
a rod-like heater (zone melting recrystallization) is known.
[0017] All of these methods have both advantages and disadvantages
and many problems of controllability, productivity, uniformity, and
quality, and therefore have not been put into practical use in
terms of industrial applications. For example, CVD requires
sacrifice oxidation to form a flat thin film. Solid phase epitaxial
growth is poor in crystallinity. In beam annealing, the process
time required to scan the focused beam and controllability for beam
superposition or focal point adjustment pose problems. Zone melting
recrystallization is the most matured technique, and relatively
large-scaled integrated circuits have been fabricated on a trial
basis. However, since a number of crystal defects such as a
subboundary undesirably remain, minority carrier devices cannot be
created.
[0018] As the above second method, i.e., as the method without
using the Si substrate as a seed for epitaxial growth, the
following four techniques can be used.
[0019] As the first technique, an oxide film is formed on a
single-crystal Si substrate having a V-shaped groove formed in the
surface by anisotropic etching. A polysilicon layer having nearly
the same thickness as that of the single-crystal Si substrate is
deposited on the oxide film. After this, the single-crystal Si
substrate is polished from the lower surface, thereby forming, on
the thick polysilicon layer, a substrate having a single-crystal Si
region surrounded and dielectrically isolated by the V-shaped
groove. With this technique, a substrate having satisfactory
crystallinity can be formed. However, there are problems of
controllability and productivity in association with the process of
depositing polysilicon as thick as several hundred micron or the
process of polishing the single-crystal Si substrate from the lower
surface to leave the isolated Si active layer.
[0020] The second technique is SIMOX (Separation by Ion Implanted
Oxygen). In this technique, oxygen ions are implanted into a
single-crystal Si substrate to form an SiO.sub.2 layer. In this
technique, to form an SiO.sub.2 layer in a substrate, oxygen ions
must be implanted at a dose of 10.sup.18 (ions/cm.sup.2) or more.
This implantation takes a long time to result in low productivity
and high manufacturing cost. In addition, since a number of crystal
defects are generated, the quality is too low to manufacture
minority carrier devices.
[0021] As the third technique, an SOI structure is formed by
dielectric isolation by oxidizing a porous Si layer. In this
technique, an n-type Si island is formed on the surface of a p-type
single-crystal Si substrate by proton ion implantation (Imai et
al., J. Crystal Growth, vol. 63, 547 (1983)) or epitaxial growth
and patterning. This substrate is anodized in an HF solution to
convert only the p-type Si substrate around the n-type Si island
into a porous structure. After this, the n-type Si island is
dielectrically isolated by accelerated oxidation. In this
technique, since the Si region to be isolated must be determined
before the device process, the degree of freedom in device design
is limited.
[0022] As the fourth technique, an SOI structure is formed by
bonding a single-crystal Si substrate to another thermally oxidized
single-crystal Si substrate by annealing or an adhesive. In this
technique, an active layer for forming a device must be uniformly
thin. More specifically, a single-crystal Si substrate having a
thickness of several hundred micron must be thinned down to the
micron order or less.
[0023] To thin the substrate, polishing or selective etching can be
used.
[0024] A single-crystal Si substrate can hardly be uniformly
thinned by polishing. Especially, thinning to the submicron order,
the variation range is several ten %. As the wafer size becomes
large, this difficulty becomes more pronounced.
[0025] Selective etching is effective to uniformly thin the
substrate. However, the selectivity ratio is as low as about
10.sup.2, the surface planarity after etching is poor, and the
crystallinity of the SOI layer is unsatisfactory.
[0026] A transparent substrate represented by a glass substrate is
important in forming a contact sensor as a light-receiving element
or a projection liquid crystal display device. To realize highly
precise pixels (picture elements) having higher density and
resolution for the sensor or display device, a high-performance
driving element is required. For this purpose, a demand has arisen
for a technique of forming a single-crystal Si layer having
excellent crystallinity on a transparent substrate.
[0027] However, when an Si layer is deposited on a transparent
substrate represented by a glass substrate, only an amorphous Si
layer or a polysilicon layer is obtained. This is because the
transparent substrate has an amorphous crystal structure, and the
Si layer formed on the substrate reflects the disorderliness of the
crystal structure of the transparent substrate.
[0028] The present applicant has disclosed a new SOI technology in
Japanese Patent Laid-Open No. 5-21338. In this technique, a first
substrate obtained by forming a porous layer on a single-crystal Si
substrate and a non-porous single-crystal layer on its surface is
bonded to a second substrate via an insulating layer. After this,
the bonded substrate stack is separated into two substrates at the
porous layer, thereby transferring the non-porous single-crystal
layer to the second substrate. This technique is advantageous
because the film thickness uniformity of the SOI layer is good, the
crystal defect density in the SOI layer can be decreased, the
surface planarity of the SOI layer is good, no expensive
manufacturing apparatus with special specifications is required,
and SOI substrates having about several hundred angstrom to
10-.mu.m thick SOI films can be manufactured by a single
manufacturing apparatus.
[0029] The present applicant has also disclosed, in Japanese Patent
Laid-Open No. 7-302889, a technique of bonding first and second
substrates, separating the first substrate from the second
substrate without destroying the first substrate, smoothing the
surface of the separated first substrate, forming a porous layer
again, and reusing the porous layer. Since the first substrate is
not wasted, this technique is advantageous in largely reducing the
manufacturing cost and simplifying the manufacturing process.
[0030] To separate the bonded substrate stack into two substrates
while destroying neither of the first and second substrates, for
example, the two substrates are pulled in opposite directions while
applying a force in a direction perpendicular to the bonding
interface, shearing stress is applied parallel to the bonding
interface (for example, the two substrates are moved in opposite
directions in a plane parallel to the bonding interface, or the two
substrates are rotated in opposite directions while applying a
force in the circumferential direction), a pressure is applied in a
direction perpendicular to the bonding interface, a wave energy
such as an ultrasonic wave is applied to the separation region, a
peeling member (e.g., a sharp blade such as knife) is inserted into
the separation region parallel to the bonding interface from the
side surface side of the bonded substrate stack, the expansion
energy of a substance filling the pores of the porous layer
functioning as the separation region is used, the porous layer
functioning as the separation region is thermally oxidized from the
side surface of the bonded substrate stack to expand the volume of
the porous layer and separate the substrates, or the porous layer
functioning as the separation region is selectively etched from the
side surface of the bonded substrate stack to separate the
substrates.
[0031] Porous Si was found in 1956 by Uhlir et al. who were
studying electropolishing of semiconductors (A. Uhlir, Bell Syst.
Tech. J., vol. 35, 333 (1956)). Porous Si can be formed by
anodizing an Si substrate in an HF solution.
[0032] Unagami et al. studied the dissolution reaction of Si upon
anodizing and reported that holes were necessary for anodizing
reaction of Si in an HF solution, and the reaction was as follows
(T. Unagami, J. Electrochem. Soc., vol. 127, 476 (1980)).
Si+2HF+(2-n)e.fwdarw.SiF+2H.sup.1ne
SiF+2HF.fwdarw.SiF.sub.4+H.sub.2
SiF.sub.4+2HF.fwdarw.H.sub.2SiF.sub.6
[0033] or
Si+4HF+(4-.lambda.)e.fwdarw.SiF.sub.4+4H.sup.1.lambda.e
SiF.sub.4+2HF.fwdarw.H.sub.2SiF.sub.6
[0034] where e and e each represent a hole and an electron,
respectively, and n and .lambda. are the number of holes necessary
to dissolve one Si atom. According to them, when n>2 or
.lambda.>4, porous Si is formed.
[0035] The above fact suggests that p-type Si having holes is
converted into porous Si while n-type Si is not converted. The
selectivity in this conversion has been reported by Nagano et at.
and Imai (Nagano, Nakajima, Anno, Onaka, and Kajiwara, IEICE
Technical Report, vol. 79, SSD79-9549 (1979)), (K. Imai,
Solid-State Electronics, vol. 24, 159 (1981)).
[0036] However, it has also been reported that n-type at a high
concentration is converted into porous Si (R. P. Holmstrom and J.
Y. Chi, Appl. Phys. Lett., vol. 42, 386 (1983)). Hence, it is
important to select a substrate which can be converted into a
porous Si substrate independently of p- or n-type.
[0037] To form a porous layer on an Si substrate, a pair of
electrodes are supported in a process bath filled with an HF
solution, the Si substrate is held between the electrodes, and a
current is flowed across the electrodes. In this process, the metal
element of the anode dissolves into the HF solution to contaminate
the Si substrate. The present applicant has disclosed an anodizing
apparatus for solving this problem in Japanese Patent Laid-Open No.
6-275598. In the anodizing apparatus disclosed in Japanese Patent
Laid-Open No. 6-275598, a conductive diaphragm formed from an Si
material is inserted between an Si substrate and an anode, and
contamination of the Si substrate by the metal element of the anode
is prevented by this conductive electrode.
[0038] To facilitate separation of the bonded substrate stack, the
porosity of the porous layer is preferably made large to some
degree. To form a high-quality single-crystal Si layer on a porous
layer, the porosity is preferably made small to some degree.
Therefore, to facilitate separation of the bonded substrate stack
and simultaneously form a high-quality single-crystal Si layer on
the porous layer, a porous layer with a multilayered structure in
which porous layers having different porosities are stacked is
preferably formed.
SUMMARY OF THE INVENTION
[0039] The present invention has been made in consideration of the
above requirement, and has as its object to provide a method and
apparatus suitable to form a porous region with a multilayered
structure, and a semiconductor substrate manufacturing method using
this method.
[0040] According to an aspect of the present invention, there is
provided an anodizing method of forming a porous layer on a
substrate, characterized by comprising preparing an anodizing bath
used to anodize a substrate, anodizing the substrate to be
processed in a first electrolytic solution while holding the
substrate between an anode and a cathode in the anodizing bath,
exchanging the first electrolytic solution with a second
electrolytic solution, and anodizing the substrate in the second
electrolytic solution, thereby forming a porous layer having a
multilayered structure on the substrate.
[0041] In the anodizing method, a current density of a current to
be flowed across the anode and the cathode is preferably changed
between anodizing using the first electrolytic solution and
anodizing using the second electrolytic solution.
[0042] In the anodizing method, a conductive diaphragm is
preferably inserted between the anode and the substrate to be
processed to prevent contamination of the substrate by the
anode.
[0043] In the anodizing method, the conductive diaphragm is
preferably arranged to flow the whole current from the anode to the
substrate to be processed through the conductive diaphragm.
[0044] In the anodizing method, the conductive diaphragm is
preferably arranged to cover a surface of the anode that opposes a
backside surface of the substrate to be processed.
[0045] In the anodizing method, the conductive diaphragm is
preferably arranged to isolate an electrolytic solution in contact
with a surface of the substrate to be processed, that is on the
anode side, from an electrolytic solution in contact with the
anode.
[0046] In the anodizing method, at least a surface of the
conductive diaphragm that opposes the substrate to be processed is
preferably formed from a silicon material.
[0047] The anodizing method preferably further comprises changing
the conductive diaphragm every time an anodizing condition is
changed.
[0048] The anodizing method preferably further comprises preparing
a conductive diaphragm corresponding to each anodizing condition,
and every time the anodizing condition is changed, using a
conductive diaphragm corresponding to the condition.
[0049] The anodizing method preferably further comprises forming a
porous layer having a relatively low porosity as a surface layer of
the substrate to be processed, and forming a porous layer having a
relatively high porosity as an underlayer of the surface layer.
[0050] The anodizing method preferably further comprises anodizing
to form a porous layer having a porosity of not more than 30% as
the surface layer.
[0051] The anodizing method preferably further comprises anodizing
to form a porous layer having a porosity of not less than 30% as
the underlayer of the surface layer.
[0052] The anodizing method preferably further comprises anodizing
to form a porous layer having a thickness of not more than 5 .mu.m
as the underlayer of the surface layer.
[0053] According to another aspect of the present invention, there
is provided an anodizing method of forming a porous layer on a
substrate, characterized by comprising preparing at least two
anodizing baths used to anodize a substrate, anodizing the
substrate to be processed while holding the substrate between an
anode and a cathode in one anodizing bath, and anodizing the
substrate while holding the substrate between an anode and a
cathode in the next anodizing bath, thereby forming a porous layer
having a multilayered structure on the substrate.
[0054] In the other anodizing method, different electrolytic
solutions are preferably used as electrolytic solutions used for
anodizing in all or some of the at least two anodizing baths.
[0055] In the other anodizing method, a current density of a
current to be flowed across the anode and the cathode is preferably
changed in anodizing in all or some of the at least two anodizing
baths.
[0056] In the other anodizing method, a conductive diaphragm is
preferably inserted between the anode and the substrate to be
processed to prevent contamination of the substrate by the
anode.
[0057] In the other anodizing method, the conductive diaphragm is
preferably arranged to flow the whole current from the anode to the
substrate to be processed through the conductive diaphragm.
[0058] In the other anodizing method, the conductive diaphragm is
preferably arranged to cover a surface of the anode that opposes a
backside surface of the substrate to be processed.
[0059] In the other anodizing method, the conductive diaphragm is
preferably arranged to isolate an electrolytic solution in contact
with a surface of the substrate to be processed, that is on the
anode side, from an electrolytic solution in contact with the
anode.
[0060] In the other anodizing method, at least a surface of the
conductive diaphragm that opposes the substrate to be processed is
preferably formed from a silicon material.
[0061] The other anodizing method preferably further comprises
changing the conductive diaphragm every time an anodizing condition
is changed.
[0062] The other anodizing method preferably further comprises
preparing a conductive diaphragm corresponding to each anodizing
condition, and every time the anodizing condition is changed, using
a conductive diaphragm corresponding to the condition.
[0063] The other anodizing method preferably further comprises
forming a porous layer having a relatively low porosity as a
surface layer of the substrate to be processed, and forming a
porous layer having a relatively high porosity as an underlayer of
the surface layer.
[0064] The other anodizing method preferably further comprises
anodizing to form a porous layer having a porosity of not more than
30% as the surface layer.
[0065] The other anodizing method preferably further comprises
anodizing to form a porous layer having a porosity of not less than
30% as the underlayer of the surface layer.
[0066] The other anodizing method preferably further comprises
anodizing to form a porous layer having a thickness of not more
than 5 .mu.m as the underlayer of the surface layer.
[0067] According to still another aspect of the present invention,
there is provided a semiconductor substrate manufacturing method
characterized by comprising the steps of processing a first
substrate using any one of the above anodizing methods to form a
porous layer having a multilayered structure on the first
substrate, forming at least one non-porous layer on the porous
layer, bonding a second substrate to a surface of the non-porous
layer of the first substrate, separating a bonded substrate stack
to a side of the first substrate and a side of the second substrate
at a portion of the porous layer, and removing the porous layer
left on the side of the separated second substrate.
[0068] The semiconductor substrate manufacturing method preferably
further comprises the step of removing the porous layer left on the
side of the separated first substrate to allow reuse of the first
substrate.
[0069] In the semiconductor substrate manufacturing method, the
separation step preferably comprises separating the bonded
substrate stack at a portion having a high porosity in the porous
layer having the multilayered structure.
[0070] In the semiconductor substrate manufacturing method, the
non-porous layer preferably contains a single-crystal silicon
layer.
[0071] In the semiconductor substrate manufacturing method, the
non-porous layer preferably comprises a single-crystal silicon
layer and a silicon oxide layer which are sequentially formed on
the porous layer.
[0072] In the semiconductor substrate manufacturing method, the
non-porous layer preferably contains a compound semiconductor
layer.
[0073] In the semiconductor substrate manufacturing method, the
second substrate preferably comprises a silicon substrate.
[0074] In the semiconductor substrate manufacturing method, the
second substrate preferably comprises a substrate having a silicon
oxide layer formed on a surface to be bonded to the first
substrate.
[0075] In the semiconductor substrate manufacturing method, the
second substrate preferably comprises a transparent substrate.
[0076] The semiconductor substrate manufacturing method preferably
further comprises, after the step of removing the porous layer, the
step of planarizing a surface layer on the side of the second
substrate.
[0077] In the semiconductor substrate manufacturing method, the
planarization step preferably comprises annealing in an atmosphere
containing hydrogen.
[0078] In the semiconductor substrate manufacturing method, the
step of removing the porous layer preferably comprises selectively
etching the porous layer using, as an etchant, any one of
[0079] a) hydrofluoric acid,
[0080] b) a mixed solution prepared by adding at least one of an
alcohol and hydrogen peroxide to hydrofluoric acid,
[0081] c) buffered hydrofluoric acid, and
[0082] d) a mixed solution prepared by adding at least one of an
alcohol and hydrogen peroxide to buffered hydrofluoric acid.
[0083] In the semiconductor substrate manufacturing method, the
step of removing the porous layer preferably comprises selectively
etching the porous layer using an etchant whose etching rate for
the porous layer is higher than that for a compound
semiconductor.
[0084] In the semiconductor substrate manufacturing method, the
step of removing the porous layer preferably comprises selectively
polishing the porous layer using the non-porous layer as a
stopper.
[0085] In the semiconductor substrate manufacturing method, the
bonding step preferably comprises the step of bringing the first
substrate having the non-porous layer into tight contact with the
second substrate.
[0086] In the semiconductor substrate manufacturing method, the
bonding step preferably comprises, after the step of bringing the
first substrate having the non-porous layer into tight contact with
the second substrate, the step of performing a process selected
from the group consisting of anode bonding, pressing, heating, and
a combination thereof.
[0087] According to still another aspect of the present invention,
there is provided an anodizing apparatus for forming a porous layer
on a substrate, characterized by comprising an anodizing bath
having an anode and a cathode, a plurality of tanks for storing
electrolytic solutions to be supplied to the anodizing bath, a
supply mechanism for selectively supplying the electrolytic
solution stored in any one of the plurality of tanks to the
anodizing bath, and a drain mechanism for draining off the
electrolytic solution from the anodizing bath back into the tank
which supplied the electrolytic solution.
[0088] Preferably, the anodizing apparatus further comprises a
holding mechanism for holding a conductive diaphragm between the
anode and a substrate to be processed, and the conductive diaphragm
prevents contamination of the substrate by the anode.
[0089] In the anodizing apparatus, the holding mechanism preferably
holds the conductive diaphragm to flow a whole current from the
anode to the substrate to be processed through the conductive
diaphragm.
[0090] In the anodizing apparatus, the holding mechanism preferably
holds the conductive-diaphragm to cover a surface of the anode that
opposes a backside surface of the substrate to be processed.
[0091] In the anodizing apparatus, the holding mechanism preferably
holds the conductive diaphragm to isolate an electrolytic solution
in contact with a surface of the substrate to be processed, that is
on the anode side, from an electrolytic solution in contact with
the anode.
[0092] In the anodizing apparatus, at least a surface of the
conductive diaphragm that opposes the substrate to be processed is
preferably formed from a silicon material.
[0093] In the anodizing apparatus, the holding mechanism preferably
detachably holds the conductive diaphragm.
[0094] According to still another aspect of the present invention,
there is provided an anodizing apparatus for forming a porous layer
on a substrate, characterized by comprising at least two anodizing
baths each having an anode and a cathode, and a conveyor mechanism
for conveying a substrate anodized in one anodizing bath into the
next anodizing bath.
[0095] In the other anodizing apparatus, the substrate is
preferably anodized in all or some of the at least two anodizing
baths under different conditions.
[0096] The other anodizing apparatus preferably further comprises a
cleaning unit for cleaning the substrate processed in the final
anodizing bath of the at least two anodizing baths, and a drier
unit for drying the substrate cleaned by the cleaning unit.
[0097] In the other anodizing apparatus, preferably each of the
anodizing baths comprises a holding mechanism for holding a
conductive diaphragm between the anode and the substrate to be
processed, and the conductive diaphragm prevents contamination of
the substrate by the anode.
[0098] In the other anodizing apparatus, the holding mechanism
preferably holds the conductive diaphragm to flow a whole current
from the anode to the substrate to be processed through the
conductive diaphragm.
[0099] In the other anodizing apparatus, the holding mechanism
preferably holds the conductive diaphragm to cover a surface of the
anode that opposes the substrate to be processed.
[0100] In the other anodizing apparatus, the holding mechanism
preferably holds the conductive diaphragm to isolate an
electrolytic solution in contact with a surface of the substrate to
be processed, that is on the anode side, from an electrolytic
solution in contact with the anode.
[0101] In the other anodizing apparatus, at least a surface of the
conductive diaphragm that opposes the substrate to be processed is
preferably formed from a silicon material.
[0102] In the other anodizing apparatus, the holding mechanism
preferably detachably holds the conductive diaphragm.
[0103] Further objects, features and advantages of the present
invention will become apparent from the following detailed
description of embodiments of the present invention with reference
to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0104] FIGS. 1A to 1F are views showing the steps of manufacturing
a semiconductor substrate according to a preferred embodiment of
the present invention;
[0105] FIG. 2 is a view showing the schematic arrangement of an
anodizing apparatus according to the first embodiment;
[0106] FIG. 3 is a view showing the schematic arrangement of an
anodizing apparatus according to the second embodiment;
[0107] FIG. 4 is a view showing the schematic arrangement of an
anodizing apparatus according to the third embodiment;
[0108] FIG. 5 is a view showing a modification of the third
embodiment;
[0109] FIG. 6 is a view showing an improved example of an anodizing
bath shown in FIG. 5 which allows batch process of a number of
substrates;
[0110] FIG. 7 is a view showing the schematic arrangement of an
anodizing apparatus according to the fourth embodiment; and
[0111] FIG. 8 is a view showing the schematic arrangement of an
automatic process line incorporating the anodizing apparatus shown
in FIG. 7.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0112] Preferred embodiments of the present invention will be
described below with reference to the accompanying drawings.
[0113] The steps of manufacturing a semiconductor substrate
according to a preferred embodiment of the present invention will
be described first. FIGS. 1A to 1F are views showing the steps of
manufacturing a semiconductor substrate according to this
embodiment. In the step shown in FIG. 1A, a single-crystal Si
substrate 11 is prepared, and two porous layers 12 and 13 having
different porosities are formed on the upper surface side of the
single-crystal Si substrate 11. Three or more porous layers may be
formed.
[0114] The uppermost porous layer 12 preferably has a low porosity
of, e.g., 30% or less to form a high-quality epitaxial layer
thereon. On the other hand, the second porous layer 13 preferably
has a high porosity of, e.g., 30% or more to facilitate separation.
The thickness of the porous layer 13 is preferably, e.g., 5 .mu.m
or less.
[0115] In the step shown in FIG. 1B, at least one non-porous layer
is formed on the uppermost porous layer 12. The resultant structure
is used as a first substrate. In the example shown in FIG. 1B, two
non-porous layers 14 and 15 are formed. As the lower non-porous
layer 14, for example, a single-crystal Si layer is suitable. This
single-crystal Si layer can be used as an active layer. As the
non-porous layer 15 on the upper surface side, for example, an
SiO.sub.2 layer is suitable. This SiO.sub.2 layer is suitable to
separate the active layer from the bonding interface.
[0116] As the non-porous layer, a polysilicon layer, an amorphous
Si layer, a metal layer, a compound semiconductor layer, or a
superconductor layer is also suitable. A device such as a MOSFET
may be formed on the non-porous layer at this time point.
[0117] In the step shown in FIG. 1C, the first substrate and an
independently prepared second substrate 16 are brought into tight
contact with each other at room temperature while sandwiching the
non-porous layers. After this, the first and second substrates are
bonded by anode bonding, pressing, heating, or a combination
thereof.
[0118] When a single-crystal Si layer is formed as the non-porous
layer 14, the first and second substrates are preferably bonded
after an SiO.sub.2 layer is formed on the surface of the
single-crystal Si layer by, e.g., thermal oxidation, as described
above.
[0119] As the second substrate, an Si substrate having an SiO.sub.2
layer formed thereon, a transparent substrate consisting of silica
glass, quartz or a sapphire wafer is suitably used in addition to
the Si substrate. However, the second substrate is not limited to
this, and any other substrate can be used as far as it has a
sufficiently flat surface to be bonded.
[0120] When the non-porous layer 14 is not formed from Si, or no Si
substrate is used as the second substrate 16, the non-porous layer
15 as an insulating layer need not be formed.
[0121] In bonding, another insulating thin plate may be inserted
between the first and second substrates to form a three-layered
structure.
[0122] In the step shown in FIG. 1D, the bonded substrate stack is
separated into two substrates at the porous layer 13. To separate
the bonded substrate stack, for example, a fluid such as water is
injected between the substrates, a pressure is applied, an external
pressure such as a tensile or shearing force is applied, the porous
Si layer 13 is oxidized to expand from the peripheral portion to
generate an internal pressure in the porous Si layer 13, thermal
stress is applied to the porous layer 13 by heat changing in a
pulse shape, or the porous layer 13 is softened. Any other methods
can also be employed.
[0123] In the step shown in FIG. 1E, the porous layers 12 and 13 on
the second substrate 16 are removed. When the non-porous layer 14
is a single-crystal Si layer, only the porous Si layers 12 and 13
are etched by electroless wet chemical etching using at least one
etchant selected from a normal etchant for etching Si, hydrofluoric
acid as an etchant for selectively etching porous Si, a mixed
solution prepared by adding at least one of an alcohol and hydrogen
peroxide to hydrofluoric acid, buffered hydrofluoric acid, or a
mixed solution prepared by adding at least one of an alcohol and
hydrogen peroxide to buffered hydrofluoric acid, thereby leaving
the non-porous layers 14 and 15 on the second substrate 16. Since
the non-porous Si layer has a large surface area, only porous Si
can be selectively etched even by a normal Si etchant, as described
above.
[0124] Alternatively, the porous Si layers 12 and 13 may be
selectively removed by polishing using the non-porous layer 14 as a
polishing stopper.
[0125] When a compound semiconductor layer is formed as the
non-porous layer 14, only the porous Si layers 12 and 13 can be
selectively chemically etched using an etchant whose etching rate
for Si is higher than that for a compound semiconductor to leave a
thin single-crystal compound semiconductor layer (non-porous layer
14) on the second substrate 16. Alternatively, the porous Si layers
12 and 13 may be selectively removed by polishing using the
single-crystal compound semiconductor layer (non-porous layer 14)
as a polishing stopper.
[0126] FIG. 1E shows the semiconductor substrate manufactured by
the above steps. According to the above steps, a flat non-porous
thin film (e.g., a single-crystal Si thin film) having a uniform
thickness can be formed in the entire region of the second
substrate 16.
[0127] For example, a semiconductor substrate having a
single-crystal Si layer as the surface-side non-porous layer 14 and
an SiO.sub.2 layer as the inner non-porous layer 15 can be used as
an SOI substrate. When an insulating substrate is used as the
second substrate 16, a semiconductor substrate suitable for forming
an electrically insulated electronic element can be
manufactured.
[0128] In the step shown in FIG. 1F, the porous layer 13 left on
the first substrate side, i.e., on the single-crystal Si substrate
11 is removed. If the surface planarity falls outside the allowable
range, the surface of the single-crystal Si substrate 11 is
planarized. With this process, this substrate can be used as a
substrate (single-crystal Si substrate 11) for forming a first
substrate, or the second substrate 16.
[0129] Embodiments of an anodizing apparatus for performing the
step shown in FIG. 1A, i.e., forming a porous layer having a
multilayered structure will be described below.
[0130] [First Embodiment]
[0131] In an anodizing apparatus of this embodiment, every time one
porous layer is formed, the electrolytic solution in an anodizing
bath is exchanged to change the anodizing conditions, thereby
forming a porous layer having a multilayered structure.
[0132] FIG. 2 is a view showing the schematic arrangement of the
anodizing apparatus according to the first embodiment. This
anodizing apparatus comprises an anodizing bath 101, solution tanks
121, 122, 131, and 132, and solution supply and removal
mechanisms.
[0133] The anodizing bath 101 has a cathode (consisting of, e.g.,
platinum) 104, a cathode holder 105 fixing the cathode 104 in the
bath, an anode (consisting of, e.g., platinum) 106, an anode holder
107 fixing the anode 106 in the bath, and a substrate holder 103
for fixing an Si substrate 102 to be processed between the pair of
electrodes 104 and 106. The substrate holder 103 has an opening
portion for bringing the lower surface of the substrate 102 to be
processed into contact with the portion of an electrolytic solution
on the anode 106 side.
[0134] In the anodizing bath 101, the cathode 104 side of the Si
substrate 102 to be processed, i.e., the side on which a porous
layer is formed on the Si substrate 102 is isolated from the anode
106 side of the Si substrate 102 when the Si substrate 102 is set
on the substrate holder 103.
[0135] The cathode side of the anodizing bath 101 is filled with an
electrolytic solution in the tank 121 or 122. The anode side is
filled with an electrolytic solution in the tank 131 or 132.
[0136] The electrolytic solution filling the cathode side of the
anodizing bath 101 and that filling the anode side may be the same
electrolytic solution or different electrolytic solutions. As the
electrolytic solution filling the cathode side of the anodizing
bath 101, an electrolytic solution essential for anodizing, i.e.,
an electrolytic solution containing HF must be used. However, the
electrolytic solution filling the anode side need only have an
appropriate conductivity. To prevent any adverse influence
resulting from mixing of the electrolytic solution on the cathode
side with that on the anode side, the two electrolytic solutions
are preferably of the same type.
[0137] A description will be made below assuming that first
electrolyte solutions 141 and 151 are stored in the first and
second tanks 121 and 131, respectively, and first electrolyte
solutions 142 and 152 different from the first electrolytic
solution are stored in the third and fourth tanks 122 and 132,
respectively. Different electrolytic solutions mean electrolytic
solutions having different mixing ratios of chemical substances or
electrolytic solutions having different compositions.
[0138] In this anodizing apparatus, first, the Si substrate 102 is
anodized using the first electrolyte solutions 141 and 151 to form
a first porous layer. Subsequently, the Si substrate 102 is
anodized using the second electrolyte solutions 142 and 152 to form
a second porous layer.
[0139] The flow of process by the anodizing apparatus will be
described below in detail.
[0140] First, the electrolytic solution in the anodizing bath 101
is drained off, and valves 161 and 163 are closed. In this state,
the Si substrate 102 to be processed is conveyed to the substrate
holder 103 by an automatic conveyor robot or the like and fixed by
a vacuum chuck mechanism (not shown) by chucking.
[0141] A valve 162 is opened to the first tank 121 side. The first
electrolyte solution 141 is pumped up by a pump 172 and supplied to
the cathode side of the anodizing bath 101 through a filter 181.
Simultaneously, a valve 164 is opened to the second tank 131 side.
The first electrolyte solution 151 is pumped up by a pump 174 and
supplied to the anode side of the anodizing bath 101 through a
filter 182.
[0142] When the cathode and anode sides of the anodizing bath 101
are filled with the first electrolyte solutions 141 and 151,
respectively, a current having a first current value is flowed
across the cathode 104 and the anode 106 for a predetermined time
to anodize the Si substrate 102, thereby forming a first porous
layer on the cathode side of the Si substrate 102.
[0143] Subsequently, to exchange the first electrolytic solutions
in the anodizing bath 101 with the second electrolytic solutions,
the valve 161 is opened to the first tank 121 side. The first
electrolyte solution is drained off by a pump 171 from the lower
portion of the anodizing bath 101 on the cathode side into the
first tank 121, and the valve 161 is closed. Simultaneously, the
valve 163 is opened to the second tank 131 side. The first
electrolyte solution is drained off by a pump 173 from the lower
portion of the anodizing bath 101 on the anode side into the second
tank 131, and the valve 163 is closed.
[0144] Next, the valve 162 is opened to the third tank 122 side.
The second electrolyte solution 142 is pumped up by the pump 172
and supplied to the cathode side of the anodizing bath 101 through
the filter 181. Simultaneously, the valve 164 is opened to the
fourth tank 132 side. The second electrolyte solution 152 is pumped
up by the pump 174 and supplied to the anode side of the anodizing
bath 101 through the filter 182. The interior of the anodizing bath
101 is preferably cleaned by a cleaning solution such as water
before the second electrolyte solutions 142 and 152 are supplied to
the anodizing bath 101. Upon completing cleaning, the cleaning
solution is preferably completely drained off from the anodizing
bath 101 to prevent any adverse influence on the electrolytic
solutions to be supplied next.
[0145] When the cathode and anode sides of the anodizing bath 101
are filled with the second electrolyte solutions 142 and 152,
respectively, a current having a second current value is flowed
across the cathode 104 and the anode 106 for a predetermined time
to anodize the Si substrate 102, thereby forming a second porous
layer on the cathode side of the Si substrate 102. The second
porous layer is formed on the lower side of the first porous
layer.
[0146] The valve 161 is opened to the third tank 122 side. The
second electrolyte solution is drained off by the pump 171 from the
lower portion of the anodizing bath 101 on the cathode side into
the third tank 122, and the valve 161 is closed. Meanwhile, the
valve 163 is opened to the fourth tank 132 side. The second
electrolyte solution is drained off by the pump 173 from the lower
portion of the anodizing bath 101 on the anode side into the fourth
tank 132, and the valve 163 is closed. After the second electrolyte
solutions 142 and 152 are drained off, the interior of the
anodizing bath 101 is preferably cleaned by a cleaning solution
such as water. Upon completing cleaning, the cleaning solution is
preferably completely drained off from the anodizing bath 101 to
prevent any adverse influence on the electrolytic solutions to be
supplied next.
[0147] After formation of the second porous layer, a third porous
layer may be formed by filling the anodizing bath 101 with the
first electrolyte solutions 141 and 151 again or another
electrolytic solution. More porous layers may be formed by
repeating this process.
[0148] [Second Embodiment]
[0149] An anodizing apparatus of this embodiment has a plurality of
anodizing baths. With this arrangement, every time one porous layer
is formed, the anodizing bath is exchanged, thereby forming a
porous layer having a multilayered structure.
[0150] FIG. 3 is a view showing the schematic arrangement of the
anodizing apparatus according to the second embodiment. This
anodizing apparatus is used to form a porous layer having a
four-layered structure and comprises four anodizing baths 101a,
101b, 101c, and 101d. Each of the anodizing baths 101a, 101b, 101c,
and 101d substantially has the same arrangement as that of the
anodizing bath 101 shown in FIG. 2 and comprises a pair of
electrodes 104 and 106 and a substrate holder 103.
[0151] The anodizing baths 101a, 101b, 101c, and 101d are filled
with first, second, third, and fourth first electrolyte solutions
201, 202, 203, and 204, respectively. The third electrolyte
solution 203 may be the same as the first electrolyte solution 201.
The fourth electrolyte solution 204 may be the same as the first or
second electrolyte solution 201 or 202.
[0152] According to this anodizing apparatus, after an Si substrate
102 is processed in one anodizing bath, the Si substrate 102 is
transferred to another anodizing bath filled with a different
electrolytic solution to change the anodizing conditions, and the
next process is executed. Hence, the electrolytic solution need not
be exchanged every time one porous layer is formed In addition,
since a free anodizing bath can be used to process the next Si
substrate 102, high throughput can be realized.
[0153] More specifically, first, the Si substrate 102 is set on the
substrate holder 103 of the first anodizing bath 101a filled with
the first electrolyte solution 201 and anodized under the first
conditions. Subsequently, the Si substrate 102 is set on the
substrate holder 103 of the second anodizing bath 101b filled with
the second electrolyte solution 202, and anodized under the second
conditions. Then, the Si substrate 102 is set on the substrate
holder 103 of the third anodizing bath 101c filled with the third
electrolyte solution 203 and anodized under the third conditions.
Finally, the Si substrate 102 is set on the substrate holder 103 of
the fourth anodizing bath 101d filled with the fourth electrolyte
solution 204 and anodized under the fourth conditions.
[0154] [Third Embodiment]
[0155] This embodiment is an improvement of the anodizing apparatus
of the first embodiment. In the anodizing apparatus according to
the third embodiment, an Si substrate 102 to be processed is
prevented from being contaminated by the metal material (e.g.,
platinum) of an anode, which is dissolved into an electrolytic
solution.
[0156] FIG. 4 is a view showing the schematic arrangement of the
anodizing apparatus according to the third embodiment.
[0157] In this anodizing apparatus, a conductive diaphragm 108 for
preventing contamination of the electrolytic solution and the Si
substrate 102 to be processed is inserted between an anode 106a and
the electrolytic solution. This conductive diaphragm 108 is
preferably comprised of an Si substrate and, more specifically, an
Si substrate having almost the same resistivity as that of the Si
substrate 102 to be processed. When the conductive diaphragm 108 is
formed from the same material as that of the Si substrate 102 to be
processed, the Si substrate 102 to be processed can be prevented
from being contaminated.
[0158] The conductive diaphragm 108 is preferably detachable. In
the example shown in FIG. 4, for example, a vacuum chuck mechanism
is preferably arranged on the surface of the anode 106a or in an
anode holder 107a The conductive diaphragm 108 and anode 106a need
be electrically connected. When a gap is formed between the
conductive diaphragm 108 and the anode 106a, the gap must be filled
with a conductive solution or a conductive material.
[0159] When an Si substrate is to be processed using this anodizing
apparatus, the conductive diaphragm 108 is set on the anode holder
107a, and then, an anodizing bath 101 is filled with first
electrolyte solutions 141 and 151 to process the Si substrate 102.
After this, the Si substrate 102 may be processed after exchanging
the first electrolyte solutions 141 and 151 with second electrolyte
solutions 142 and 152 without exchanging the conductive diaphragm
108.
[0160] However, for example, when an Si substrate of the same type
as the Si substrate to be processed is used as the conductive
diaphragm, and an electrolytic solution (e.g., a solution
containing HF) is used as the electrolytic solutions 151 and 152
filling the anode 106a side, the conductive diaphragm is preferably
exchanged with a diaphragm dedicated to each electrolytic solution
every time the electrolytic solution is exchanged. The reason for
this is as follows.
[0161] When the space between the Si substrate 102 to be processed
and the conductive diaphragm 108 is filled with an electrolytic
solution such as a solution containing HF, and anodizing is
performed, a porous layer is formed not only on the cathode side of
the Si substrate 102 to be processed but also on the cathode side
of the conductive diaphragm 108. When the process of forming a
porous layer having a multilayered structure while changing the
anodizing conditions is repeated for the number of Si substrates
102, a thick multilayered structure of a number of porous layers
with different porosities is formed on the conductive diaphragm
108. Finally, the porous layers break to contaminate the interior
of the bath or Si substrate to be processed. The probable causes
for break of the porous layers on the conductive diaphragm 108 are
as follows.
[0162] 1) Since porous layers having different porosities are
stacked, the pore walls cannot withstand stress.
[0163] 2) When a porous layer having a high porosity is formed
under (inside) a porous layer having a low porosity, the porosity
of the porous layer with a high porosity increases in proportion to
the depth from the surface of the conductive diaphragm 108 to the
porous layer (the total thickness of the porous layers). Hence,
when a number of Si substrates are processed, the porosity of the
porous layer formed at the deepest portion of the conductive
diaphragm 108 reaches the critical value to break the pore
walls.
[0164] Break of pore walls hardly occurs when anodizing is repeated
under the same condition. This is because a nearly constant
porosity is maintained in the direction of depth of the conductive
diaphragm 108.
[0165] A suitable process procedure of forming a porous layer using
this anodizing apparatus will be described below. In this case,
assume that a porous layer having a multilayered structure is to be
formed on each of 25 Si substrates.
[0166] First, the electrolytic solution in the anodizing bath 101
is drained off, and valves 161 and 163 are closed. In this state,
the conductive diaphragm 108 for the first electrolyte solution 151
is conveyed and fixed on the anode holder 107a by an automatic
conveyor robot or the like.
[0167] The first Si substrate 102 to be processed is conveyed and
fixed on a substrate holder 103 by an automatic conveyor robot or
the like.
[0168] A valve 162 is opened to the first tank 121 side. The first
electrolyte solution 141 is pumped up by a pump 172 and supplied to
the cathode side of the anodizing bath 101 through a filter 181.
Simultaneously, a valve 164 is opened to the second tank 131 side.
The first electrolyte solution 151 is pumped up by a pump 174 and
supplied to the anode side of the anodizing bath 101 through a
filter 182.
[0169] When the cathode and anode sides of the anodizing bath 101
are filled with the first electrolyte solutions 141 and 151,
respectively, a current having a first current value is flowed
across a cathode 104 and the anode 106a for a predetermined time to
anodize the first Si substrate 102, thereby forming a first porous
layer on the cathode side of the Si substrate 102.
[0170] The valve 161 is opened to the first tank 121 side. The
first electrolyte solution is drained off by a pump 171 from the
lower portion of the anodizing bath 101 on the cathode side into
the first tank 121, and the valve 161 is closed. In the mean time,
the valve 163 is opened to the second tank 131 side. The first
electrolyte solution is drained off by a pump 173 from the lower
portion of the anodizing bath 101 on the anode side into the second
tank 131, and the valve 163 is closed.
[0171] The first Si substrate 102 fixed on the substrate holder 103
is exchanged with the second Si substrate 102.
[0172] The valve 162 is opened to the first tank 121 side. The
first electrolyte solution 141 is pumped up by the pump 172 and
supplied to the cathode side of the anodizing bath 101 through the
filter 181. Also, the valve 164 is opened to the second tank 131
side. The first electrolyte solution 151 is pumped up by the pump
174 and supplied to the anode side of the anodizing bath 101
through the filter 182.
[0173] When the cathode and anode sides of the anodizing bath 101
are filled with the first electrolyte solutions 141 and 151,
respectively, a current having a first current value is flowed
across the cathode 104 and the anode 106a for a predetermined time
to anodize the second Si substrate 102, thereby forming a first
porous layer on the cathode side of the Si substrate 102.
[0174] This process is repeated until the 25th Si substrate
102.
[0175] Subsequently, to exchange the first electrolytic solutions
in the anodizing bath 101 with the second electrolytic solutions,
the valve 161 is opened to the first tank 121 side. The first
electrolyte solution is drained off by the pump 171 from the lower
portion of the anodizing bath 101 on the cathode side into the
first tank 121, and the valve 161 is closed. Simultaneously, the
valve 163 is opened to the second tank 131 side. The first
electrolyte solution is drained off by the pump 173 from the lower
portion of the anodizing bath 101 on the anode side into the second
tank 131, and the valve 163 is closed.
[0176] The conductive diaphragm 108 for the second electrolyte
solution 152 is conveyed and fixed on the anode holder 107a by an
automatic conveyor robot or the like.
[0177] Next, the valve 162 is opened to the third tank 122 side.
The second electrolyte solution 142 is pumped up by the pump 172
and supplied to the cathode side of the anodizing bath 101 through
the filter 181. Simultaneously, the valve 164 is opened to the
fourth tank 132 side. The second electrolyte solution 152 is pumped
up by the pump 174 and supplied to the anode side of the anodizing
bath 101 through the filter 182. The interior of the anodizing bath
101 is preferably cleaned by a cleaning solution such as water
before the second electrolyte solutions 142 and 152 are supplied to
the anodizing bath 101. Upon completing cleaning, the cleaning
solution is preferably completely drained off from the anodizing
bath 101 to prevent any adverse influence on the electrolytic
solutions to be supplied next.
[0178] When the cathode and anode sides of the anodizing bath 101
are filled with the second electrolyte solutions 142 and 152,
respectively, a current having a second current value is flowed
across the cathode 104 and the anode 106a for a predetermined time
to anodize the Si substrate 102, thereby forming a second porous
layer on the cathode side of the Si substrate 102. The second
porous layer is formed on the lower side of the first porous
layer.
[0179] The valve 161 is opened to the third tank 122 side. The
second electrolyte solution is drained off by the pump 171 from the
lower portion of the anodizing bath 101 on the cathode side into
the third tank 122, and the valve 161 is closed. At the same time,
the valve 163 is opened to the fourth tank 132 side. The second
electrolyte solution is drained off by the pump 173 from the lower
portion of the anodizing bath 101 on the anode side into the fourth
tank 132, and the valve 163 is closed.
[0180] The first Si substrate 102 fixed on the substrate holder 103
is exchanged with the second Si substrate 102.
[0181] Next, the valve 162 is opened to the third tank 122 side.
The second electrolyte solution 142 is pumped up by the pump 172
and supplied to the cathode side of the anodizing bath 101 through
the filter 181. Simultaneously, the valve 164 is opened to the
fourth tank 132 side. The second electrolyte solution 152 is pumped
up by the pump 174 and supplied to the anode side of the anodizing
bath 101 through the filter 182.
[0182] When the cathode and anode sides of the anodizing bath 101
are filled with the second electrolyte solutions 142 and 152,
respectively, a current having a second current value is flowed
across the cathode 104 and the anode 106a for a predetermined time
to anodize the Si substrate 102, thereby forming a second porous
layer on the cathode side of the Si substrate 102.
[0183] This process is repeated until the 25th Si substrate
102.
[0184] Finally, the valve 161 is opened to the third tank 122 side.
The second electrolyte solution is drained off by the pump 171 from
the lower portion of the anodizing bath 101 on the cathode side
into the third tank 122, and the valve 161 is closed. Also, the
valve 163 is opened to the fourth tank 132 side. The second
electrolyte solution is drained off by the pump 173 from the lower
portion of the anodizing bath 101 on the anode side into the fourth
tank 132, and the valve 163 is closed. After the second electrolyte
solutions 142 and 152 are drained off, the interior of the
anodizing bath 101 is preferably cleaned by a cleaning solution
such as water. Upon completing cleaning, the cleaning solution is
preferably completely drained off from the anodizing bath 101 to
prevent any adverse influence on the electrolytic solutions to be
supplied next.
[0185] In the above series of processes, every time the Si
substrate 102 is exchanged, the electrolytic solutions are
temporarily drained off from the anodizing bath 101. However, when
electrolytic solutions of the same type are supplied to both the
cathode and anode sides of the anodizing bath 101 and, especially,
when corrosion of the automatic conveyor robot for conveying the Si
substrate 102 need not be taken into consideration, the Si
substrate 102 can be exchanged while keeping the anodizing bath 101
filled with electrolytic solutions.
[0186] After formation of the second porous layer, a third porous
layer may be formed by filling the anodizing bath 101 with the
first electrolyte solutions 141 and 151 again or another
electrolytic solution. More porous layers may be formed by
repeating this process.
[0187] FIG. 5 is a view showing a modification of the third
embodiment. An anodizing bath 301 shown in FIG. 5 has a conductive
diaphragm holder 103a exclusively used to hold the conductive
diaphragm 108 in place. The anodizing bath 301 is used while
filling the gap between the conductive diaphragm 108 and the anode
106 with a conductive solution. This conductive solution is used to
simply electrically connect the conductive diaphragm 108 and anode
106 and therefore need not be exchanged every time a porous layer
having a multilayered structure is formed on the Si substrate 102
to be processed. The anodizing bath 301 is replaced with, e.g., the
anodizing bath 101 shown in FIG. 4.
[0188] FIG. 6 is a view showing an improvement example of the
anodizing bath shown in FIG. 5 which allows batch process of a
number of substrates. An anodizing bath 401 has a plurality of
substrate holders 103.
[0189] In this embodiment, the conductive diaphragm and Si
substrate to be processed are electrically connected by filling the
gap therebetween with a conductive solution However, the conductive
diaphragm and Si substrate to be processed may be brought into
direct contact with each other.
[0190] [Fourth Embodiment]
[0191] This embodiment is an improvement of the anodizing apparatus
of the second embodiment. The anodizing apparatus according to the
fourth embodiment has a means for preventing an Si substrate 102 to
be processed from being contaminated by the metal material (e.g.,
platinum) of an anode 106a, which is dissolved into an electrolytic
solution.
[0192] FIG. 7 is a view showing the schematic arrangement of the
anodizing apparatus according to the fourth embodiment.
[0193] In this anodizing apparatus, a conductive diaphragm 108 for
preventing contamination of the electrolytic solution and the Si
substrate 102 to be processed is inserted between the anode 106a
and the electrolytic solution. This conductive diaphragm 108 is
preferably comprised of an Si substrate and, more specifically, an
Si substrate having almost the same resistivity as that of the Si
substrate 102 to be processed. When the conductive diaphragm 108 is
formed from the same material as that of the Si substrate 102 to be
processed, the Si substrate 102 to be processed can be prevented
from being contaminated.
[0194] The conductive diaphragm 108 is preferably detachable. In
the example shown in FIG. 7, for example, a vacuum chuck mechanism
is preferably disposed on the surface of the anode 106a or in an
anode holder 107a. The conductive diaphragm 108 and anode 106a need
be electrically connected. When a gap is formed between the
conductive diaphragm 108 and the anode 106a, the gap must be filled
with a conductive solution or a conductive material.
[0195] The anodizing apparatus of this embodiment has a plurality
of anodizing baths. With this arrangement, every time one porous
layer is formed, the anodizing bath is exchanged, thereby forming a
porous layer having a multilayered structure. Since anodizing can
be executed under the same conditions in each anodizing bath, the
problem described in the third embodiment, i.e., the problem of
break of the conductive diaphragm 108 need not be taken into
consideration. Since the conductive diaphragm 108 need not be
frequently exchanged, this anodizing apparatus is excellent in
throughput.
[0196] The process procedure by this anodizing apparatus is the
same as in the second embodiment.
[0197] Each of first anodizing baths 101a, 101b, 101c, and 101d of
the anodizing apparatus of this embodiment may be replaced with the
anodizing bath 301 shown in FIG. 5 or anodizing bath 401 shown in
FIG. 6.
[0198] In this embodiment, the conductive diaphragm and Si
substrate to be processed are electrically connected by filling the
gap therebetween with a conductive solution. However, the
conductive diaphragm and Si substrate to be processed may be
brought into direct contact with each other.
[0199] FIG. 8 is a view showing the schematic arrangement of an
automatic process line incorporating the anodizing apparatus shown
in FIG. 7. This automatic manufacturing line has two anodizing
baths 101a and 101b for forming a porous layer having a two layered
structure. Three or more anodizing baths may also be used, as a
matter of course.
[0200] The process procedure by this automatic manufacture line
will be described below.
[0201] A wafer carrier 702 storing Si substrates 102 to be
processed is mounted on a loader 701. The start of the process is
instructed by operating a control panel (not shown).
[0202] In response to this, a first conveyor robot 721 extracts one
Si substrate 102 from the wafer carrier 702 by chucking it from the
lower surface and dips it into the first anodizing bath 101a on the
cathode 104 side. A second conveyor robot 722 receives the Si
substrate 102 by chucking it from the lower surface and moves the
Si substrate 102 to a position where the Si substrate 102 comes
into contact with the chuck surface of a substrate holder 103. In
this state, the vacuum chuck mechanism of the substrate holder 103
is activated to chuck the Si substrate 102 on the chuck
surface.
[0203] A predetermined current is flowed across the electrodes 104
and 106a of the first anodizing bath 101a, thereby forming a first
porous layer on the surface of the Si substrate 102.
[0204] After the second conveyor robot 722 chucks the Si substrate
102 in the first anodizing bath 101a by chucking it from the lower
surface, and vacuum chuck by the substrate holder 103 is canceled,
the Si substrate 102 is separated from the substrate holder 103.
The Si substrate 102 is transferred from the second conveyor robot
722 to the first conveyor robot 721.
[0205] The Si substrate 102 is conveyed to the second anodizing
bath 101b by the first conveyor robot 721, transferred to the
second conveyor robot 722, and set on the substrate holder 103 of
the second anodizing bath 101b.
[0206] A predetermined current is flowed across the electrodes 104
and 106a of the second anodizing bath 101b, thereby forming a
second porous layer under the first porous layer on the Si
substrate 102.
[0207] After the second conveyor robot 722 chucks the Si substrate
102 in the second anodizing bath 101b by chucking it from the lower
surface, and vacuum chuck by the substrate holder 103 is canceled,
the Si substrate 102 is separated from the substrate holder 103.
The Si substrate 102 is transferred from the second conveyor robot
722 to the first conveyor robot 721.
[0208] The Si substrate 102 is stored by the first conveyor robot
721 in the wafer carrier 702 which has been dipped into a washing
tank 703 in advance.
[0209] The above process is continuously executed to process all
the Si substrates 102 in the wafer carrier 702 on the loader 701
and store them in the wafer carrier 702 in the washing tank 703.
After this, the Si substrates 102 are cleaned.
[0210] Finally, a third conveyor robot 731 extracts the Si
substrates 102 in the washing bath 703 while keeping them stored in
the wafer carrier 702 and conveys them to a spin drier 704. After
the Si substrates 102 are dried by the spin drier 704, the third
conveyor robot 731 conveys the Si substrates 102 onto an unloader
705 while keeping them stored in the wafer carrier 702.
[0211] Examples of anodizing by the above anodizing apparatuses
will be described next.
EXAMPLE 1
[0212] A single-crystal Si substrate was set in each of the
anodizing apparatuses of the above embodiments, and a porous layer
having a two-layered structure was formed. The first and second
anodizing conditions were as follows.
1 <First Anodizing Conditions> Current density: 7 (mA
.multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:1:1 Process time: 11 (min)
Thickness of porous Si layer (target): 12 (.mu.m) Porosity
(target): to 24% <Second Anodizing Conditions> Current
density: 10 (mA .multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:2:2 Process time: 3 (min)
Thickness of porous Si layer (target): 3 (.mu.m) Porosity (target):
to 35%
[0213] In each of the anodizing apparatuses of the first to fourth
embodiments, a substrate having a porous layer with a two-layered
structure could be manufactured. In each of the apparatuses of the
second and fourth anodizing apparatuses, two anodizing baths were
used.
[0214] In the anodizing apparatus of the third embodiment, after a
first porous layer was formed on each of 25 Si substrates, the
electrolytic solutions and conductive diaphragm were exchanged, and
a second porous layer was formed on each of the 25 Si substrates.
Porous layers sequentially formed on each Si substrate used as the
conductive diaphragm did not break.
[0215] In the anodizing apparatus of the fourth embodiment, first
and second porous layers were sequentially formed on each of 25 Si
substrates. Porous layers sequentially formed on each Si substrate
used as the conductive diaphragm did not break.
EXAMPLE 2
[0216] A single-crystal Si substrate was set in each of the
anodizing apparatuses of the above embodiments, and a porous layer
having a three-layered structure was formed. The first to third
anodizing conditions were as follows.
2 <First Anodizing Conditions> Current density: 7 (mA
.multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:1:1 Process time: 11 (min)
Thickness of porous Si layer (target): 12 (.mu.m) Porosity
(target): to 24% <Second Anodizing Conditions> Current
density: 10 (mA .multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:2:2 Process time: 3 (min)
Thickness of porous Si layer (target): 3 (.mu.m) Porosity (target):
to 35% <Third Anodizing Conditions> Current density: 7 (mA
.multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:1:1 Process time: 1 (min)
Thickness of porous Si layer (target): 1.1 (.mu.m) Porosity
(target): to 25%
[0217] In each of the anodizing apparatuses of the first to fourth
embodiments, a substrate having a porous layer with a three-layered
structure could be manufactured. In each of the anodizing
apparatuses of the first and third embodiments, the anodizing
solution for the first and third anodizing processes was supplied
from the first and second tanks, and the anodizing solution for the
second anodizing process was supplied from the third and fourth
tanks. In each of the anodizing apparatuses of the second and
fourth embodiments, three anodizing baths were used.
[0218] In the anodizing apparatus of the third embodiment, a first
conductive diaphragm was set, and a first porous layer was formed
on each of 25 Si substrates under the first anodizing conditions.
Subsequently, a second conductive diaphragm was set, and a second
porous layer was formed on each of the 25 Si substrates under the
second anodizing conditions. Then, a third conductive diaphragm was
set, and a third porous layer was formed on each of the 25 Si
substrates under the third anodizing conditions. Porous layers
sequentially formed on each Si substrate used as the conductive
diaphragm did not break.
[0219] In the anodizing apparatus of the fourth embodiment, first
to third porous layers were sequentially formed on each of 25 Si
substrates. Porous layers sequentially formed on each Si substrate
used as the conductive diaphragm did not break.
EXAMPLE 3
[0220] A single-crystal Si substrate was set in each of the
anodizing apparatuses of the above embodiments, and a porous layer
having a two-layered structure was formed. The first and second
anodizing conditions were as follows.
3 <First Anodizing Conditions> Current density: 7 (mA
.multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:1:1 Process time: 5 (min)
Thickness of porous Si layer (target): 6 (.mu.m) Porosity (target):
to 24% <Second Anodizing Conditions> Current density: 30 (mA
.multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:1:1 Process time: 110 (sec)
Thickness of porous Si layer (target): 3 (.mu.m) Porosity (target):
to 40%
[0221] In each of the anodizing apparatuses of the first to fourth
embodiments, a substrate having a porous layer with a two-layered
structure could be manufactured. In each of the apparatuses of the
second and fourth anodizing apparatuses, two anodizing baths were
used.
[0222] In the anodizing apparatus of the third embodiment, after a
first porous layer was formed on each of 25 Si substrates, the
electrolytic solutions and conductive diaphragm were exchanged, and
a second porous layer was formed on each of the 25 Si substrates.
Porous layers sequentially formed on each Si substrate used as the
conductive diaphragm did not break.
[0223] In the anodizing apparatus of the fourth embodiment, first
and second porous layers were sequentially formed on each of 25 Si
substrates. Porous layers sequentially formed on each Si substrate
used as the conductive diaphragm did not break.
EXAMPLE 4
[0224] A single-crystal Si substrate was set in each of the
anodizing apparatuses of the above embodiments, and a porous layer
having a two-layered structure was formed. The first and second
anodizing conditions were as follows.
4 <First Anodizing Conditions> Current density: 7 (mA
.multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:1:1 Process time: 11 (min)
Thickness of porous Si layer (target): 12 (.mu.m) Porosity
(target): to 24% <Second Anodizing Conditions> Current
density: 10 (mA .multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:2:2 Process time: 3 (min)
Thickness of porous Si layer (target): 3 (.mu.m) Porosity (target):
to 35%
[0225] In each of the anodizing apparatuses of the first to fourth
embodiments, a substrate having a porous layer with a two-layered
structure could be manufactured. In each of the apparatuses of the
second and fourth anodizing apparatuses, two anodizing baths were
used.
[0226] In the anodizing apparatus of the third embodiment, after a
first porous layer was formed on each of 25 Si substrates, the
electrolytic solutions and conductive diaphragm were exchanged, and
a second porous layer was formed on each of the 25 Si substrates.
Porous layers sequentially formed on each Si substrate used as the
conductive diaphragm did not break.
[0227] In the anodizing apparatus of the fourth embodiment, first
and second porous layers were sequentially formed on each of 25 Si
substrates. Porous layers sequentially formed on each Si substrate
used as the conductive diaphragm did not break.
[0228] The substrate having a porous layer was oxidized in an
oxygen atmosphere at 400.degree. C. for 1 hr. Upon this oxidation,
the inner wall of each pore in the porous Si layer was covered by a
thermal oxide film.
[0229] Next, a 0.3-.mu.m thick single-crystal Si layer was
epitaxially grown on the porous Si layer by CVD (Chemical Vapor
Deposition). The growth conditions were as follows. Since the
surface of the porous Si layer is exposed to H.sub.2 in the initial
stage of epitaxial growth, the surface pores are filled to
planarize the surface.
5 <Epitaxial Growth Conditions> Source gas:
SiH.sub.2Cl.sub.2/H.sub.2 Gas flow rate: 0.5/100 (liter/min) Gas
pressure: 80 (Torr) Temperature: 950 (.degree. C.) Growth rate: 0.3
(.mu.m/min)
[0230] Subsequently, a 200-nm thick SiO.sub.2 layer was formed on
the surface of the epitaxially grown single-crystal Si layer by
thermal oxidation (completion of the first substrate).
[0231] The surface of this SiO.sub.2 layer was brought into tight
contact with the surface of an independently prepared Si substrate
(second substrate), and these substrates were bonded by annealing
at 1,000.degree. C. for one hr.
[0232] A water jet with a diameter of 0.2 mm was injected to the
beveling gap of the bonded substrate stack to separate the bonded
substrate stack into two substrates at the second (lower) porous Si
layer.
[0233] The porous Si layer left on the second substrate side was
etched using a mixed solution of 49% hydrofluoric acid, 30%
hydrogen peroxide, and water. At this time, the single-crystal Si
layer functioned as an etching stopper, so the porous Si layer was
selectively etched.
[0234] The etching rate of non-porous single-crystal Si by the
etchant is very low, and the ratio of the etching rate of porous
single crystal Si to non-porous single-crystal Si reaches 10.sup.5
or more. For this reason, the etching amount (about several ten
angstrom) of the non porous layer can be neglected for practical
use.
[0235] With the above process, an SOI substrate having a 0.2-.mu.m
thick single-crystal Si layer on the Si oxide film was obtained.
The thickness of the single-crystal Si layer of this SOI substrate
was measured at 100 points on the entire surface. The film
thickness uniformity was 201 nm.+-.4 nm.
[0236] The resultant structure was annealed in hydrogen at
1,100.degree. C. for 1 hr, and the surface roughness was evaluated
with an atomic force microscope. The root mean square roughness in
a 50-.mu.m square area was approximately 0.2 nm. This nearly equals
that of a commercially available Si wafer.
[0237] Cross section observation with a transmission electron
microscope showed that no new crystal defects were formed in the Si
layer, and satisfactory crystallinity was maintained.
[0238] The same result as described above was obtained even when no
oxide film was formed on the surface of the epitaxially grown
single-crystal Si layer.
[0239] Further, the same result as described above was obtained
even when oxide film was formed on the surface of the second
substrate or both of the surfaces on the first and second
substrates.
[0240] The porous Si layer left on the first substrate side was
also selectively etched using a mixed solution of 49% hydrofluoric
acid, 30% hydrogen peroxide, and water. At this time, the
single-crystal Si layer functioned as an etching stopper, so the
porous Si layer was selectively etched. This substrate can be used
again as a substrate for forming a first substrate in anodizing or
as a second substrate in bonding.
[0241] Before the substrate was reused to form a first substrate,
the substrate may be annealed in hydrogen at 1,100.degree. C. for 1
hr to restore the surface roughness (microroughness) due to
micropores back to normal. However, planarization of the
microroughness need not always be performed because when the
substrate is to be reused to form a first substrate, surface
planarization is performed simultaneously with sealing of pores on
the surface of the porous Si layer during prebaking in hydrogen
before epitaxial growth.
[0242] The microroughness due to micropores may be planarized not
by annealing in hydrogen but by surface touch polishing.
EXAMPLE 5
[0243] Example 5 is a modification of Example 4. More specifically,
the first and second anodizing conditions were changed as
follows.
6 <First Anodizing Conditions> Current density: 7 (mA
.multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:1:1 Process time: 5 (min)
Thickness of porous Si layer (target): 6 (.mu.m) Porosity (target):
to 24% <Second Anodizing Conditions> Current density: 30 (mA
.multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:1:1 Process time: 110 (sec)
Thickness of porous Si layer (target): 3 (.mu.m) Porosity (target):
to 40%
EXAMPLE 6
[0244] Example 6 is a modification of Example 4. More specifically,
the manufacturing conditions of Example 4 were changed as
follows.
[0245] 1) Thickness of epitaxial Si layer: 2 .mu.m
[0246] 2) Thickness of thermal oxide film on epitaxial Si layer:
0.1 .mu.m
[0247] 3) Second substrate: Si substrate having a 1.9-.mu.m thick
SiO.sub.2 layer
[0248] 4) Bonding: after the surfaces of first and second
substrates were exposed to a nitrogen plasma, the substrates were
brought into tight contact with each other and annealed at
400.degree. C. for 10 hrs.
EXAMPLE 7
[0249] Example 7 is a modification of Example 4. More specifically,
the manufacturing conditions of Example 4 were changed as
follows.
[0250] 1) Second substrate: silica or quartz substrate
[0251] 2) Bonding: after the surfaces of first and second
substrates were exposed to a nitrogen plasma or after the surfaces
of first and second substrates were rinsed with water, the
substrates were brought into tight contact with each other and
annealed at 200.degree. C. for 24 hrs.
[0252] 3) Annealing in hydrogen: the resultant structure was
annealed in hydrogen at 970.degree. C. for 2 hrs, and the surface
roughness was evaluated with an atomic force microscope. The root
mean square roughness in a 50-.mu.m square area was approximately
0.2 nm. This nearly equals that of a commercially available Si
wafer.
[0253] 4) Reuse: the first substrate side after separation was used
in anodizing as a substrate for forming a first substrate.
EXAMPLE 8
[0254] A single-crystal Si substrate was set in each of the
anodizing apparatuses of the above embodiments, and a porous layer
having a two-layered structure was formed. The first and second
anodizing conditions were as follows.
7 <First Anodizing Conditions> Current density: 7 (mA
.multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:1:1 Process time: 5 (min)
Thickness of porous Si layer (target): 6 (.mu.m) Porosity (target):
to 24% <Second Anodizing Conditions> Current density: 30 (mA
.multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:1:1 Process time: 110 (sec)
Thickness of porous Si layer (target): 3 (.mu.m) Porosity (target):
to 40%
[0255] In each of the anodizing apparatuses of the first to fourth
embodiments, a substrate having a porous layer with a two-layered
structure could be manufactured. In each of the apparatuses of the
second and fourth anodizing apparatuses, two anodizing baths were
used.
[0256] In the anodizing apparatus of the third embodiment, after a
first porous layer was formed on each of 25 Si substrates, the
electrolytic solutions and conductive diaphragm were exchanged, and
a second porous layer was formed on each of the 25 Si substrates.
Porous layers sequentially formed on each Si substrate used as the
conductive diaphragm did not break.
[0257] In the anodizing apparatus of the fourth embodiment, first
and second porous layers were sequentially formed on each of 25 Si
substrates. Porous layers sequentially formed on each Si substrate
used as the conductive diaphragm did not break.
[0258] The substrate having a porous layer was oxidized in an
oxygen atmosphere at 400.degree. C. for 1 hr. Upon this oxidation,
the inner wall of each pore in the porous Si layer was covered by a
thermal oxide film.
[0259] Next, a 1-.mu.m thick single-crystal GaAs layer was
epitaxially grown on the porous Si layer by MOCVD (Metal Organic
Chemical Vapor Deposition). The growth conditions were as
follows.
8 <Epitaxial Growth Conditions> Source gas:
TMG/AsH.sub.3/H.sub.2 Gas pressure: 80 (Torr) Temperature: 700
(.degree. C.)
[0260] The surface of the GaAs layer was brought into tight contact
with the surface of an independently prepared Si substrate (second
substrate).
[0261] A water jet with a diameter of 0.2 mm was injected to the
beveling gap of the bonded substrate stack to separate the bonded
substrate stack into two substrates at the second (lower) porous Si
layer.
[0262] The porous Si layer left on the second substrate side was
etched at 110.degree. C. using a mixed solution (etchant) of
ethylenediamine/pyrocatechol/water (at a ratio of 17 ml:3 g:8 ml).
At this time, the single-crystal GaAs layer functioned as an
etching stopper, so the porous Si layer was selectively etched.
[0263] The etching rate of single-crystal GaAs by the etchant is
very low, and the etching amount (about several ten angstrom) of
the single-crystal GaAs can be neglected for practical use.
[0264] With the above process, a substrate having a 1-.mu.m thick
single crystal GaAs layer on the single-crystal Si layer was
obtained. The thickness of the single-crystal GaAs layer of this
substrate was measured at 100 points on the entire surface. The
film thickness uniformity was 1 .mu.m.+-.29.8 nm.
[0265] Cross section observation with a transmission electron
microscope revealed that no new crystal defects were formed in the
GaAs layer, and satisfactory crystallinity was maintained.
[0266] When an Si substrate having an oxide film was used as a
support substrate, a substrate having a GaAs layer on an insulating
film could be formed.
[0267] The porous Si layer left on the first substrate side was
also selectively etched using a mixed solution of 49% hydrofluoric
acid and 30% hydrogen peroxide under stirring. At this time, the
single-crystal Si layer functioned as an etching stopper, so the
porous Si layer was selectively etched. This substrate can be used
again as a substrate for forming a first substrate in anodizing or
as a second substrate in bonding.
[0268] Before the substrate was reused to form a first substrate,
the substrate may be annealed in hydrogen at 1,100.degree. C. for 1
hr to restore the surface roughness (microroughness) due to
micropores back to normal. However, planarization of the
microroughness need not always be performed because when the
substrate is to be reused to form a first substrate, surface
planarization is performed simultaneously with sealing of pores on
the surface of the porous Si layer during prebaking in hydrogen
before epitaxial growth.
[0269] The microroughness due to micropores may be planarized not
by annealing in hydrogen but by surface touch polishing.
EXAMPLE 9
[0270] A single-crystal Si substrate was set in each of the
anodizing apparatuses of the above embodiments, and a porous layer
having a two-layered structure was formed. The first and second
anodizing conditions were as follows.
9 <First Anodizing Conditions> Current density: 7 (mA
.multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:1:1 Process time: 11 (min)
Thickness of porous Si layer (target): 12 (.mu.m) Porosity
(target): to 24% <Second Anodizing Conditions> Current
density: 10 (mA .multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:2:2 Process time: 3 (min)
Thickness of porous Si layer (target): 3 (.mu.m) Porosity (target):
to 35%
[0271] In each of the anodizing apparatuses of the first to fourth
embodiments, a substrate having a porous layer with a two-layered
structure could be manufactured. In each of the apparatuses of the
second and fourth anodizing apparatuses, two anodizing baths were
used.
[0272] In the anodizing apparatus of the third embodiment, after a
first porous layer was formed on each of 25 Si substrates, the
electrolytic solutions and conductive diaphragm were exchanged, and
a second porous layer was formed on each of the 25 Si substrates.
Porous layers sequentially formed on each Si substrate used as the
conductive diaphragm did not break.
[0273] In the anodizing apparatus of the fourth embodiment, first
and second porous layers were sequentially formed on each of 25 Si
substrates. Porous layers sequentially formed on each Si substrate
used as the conductive diaphragm did not break.
[0274] The substrate having a porous layer was oxidized in an
oxygen atmosphere at 400.degree. C. for 1 hr. Upon this oxidation,
the inner wall of each pore in the porous Si layer was covered by a
thermal oxide film.
[0275] A 1-.mu.m thick single-crystal InP layer was epitaxially
grown on the porous Si layer by MOCVD (Metal Organic Chemical Vapor
Deposition).
[0276] After the surfaces of the InP layer and an independently
prepared silica or quartz substrate (second substrate) were exposed
to a nitrogen plasma, these surfaces were brought into tight
contact with each other and annealed at 200.degree. C. for 10
hrs.
[0277] A water jet with a diameter of 0.2 mm was injected to the
beveling gap of the bonded substrate stack to separate the bonded
substrate stack into two substrates at the second (lower) porous Si
layer.
[0278] The porous Si layer left on the second substrate side was
selectively etched using a mixed solution of 49% hydrofluoric acid
and 30% hydrogen peroxide under stirring. At this time, the
single-crystal InP layer functioned as an etching stopper, so the
porous Si layer was selectively etched.
[0279] The etching rate of single-crystal InP by the etchant is
very low, and the etching amount (about several ten angstrom) of
the single-crystal InP can be neglected for practical use.
[0280] With the above process, a substrate having a 1-.mu.m thick
single-crystal InP layer on the silica or quartz substrate was
obtained. The thickness of the single-crystal InP layer of this
substrate was measured at 100 points on the entire surface. The
film thickness uniformity was 1 .mu.m.+-.29.0 nm.
[0281] Cross section observation with a transmission electron
microscope indicated that no new crystal defects were formed in the
InP layer, and satisfactory crystallinity was maintained.
[0282] The porous Si layer left on the first substrate side was
also selectively etched using a mixed solution of 49% hydrofluoric
acid and 30% hydrogen peroxide under stirring. At this time, the
single-crystal Si layer functioned as an etching stopper, so the
porous Si layer was selectively etched. This substrate can be used
again as a substrate for forming a first substrate in
anodizing.
[0283] Before the substrate was reused to form a first substrate,
the substrate may be annealed in hydrogen at 1,100.degree. C. for 1
hr to restore the surface roughness (microroughness) due to
micropores back to normal. However, planarization of the
microroughness need not always be performed because when the
substrate is to be reused to form a first substrate, surface
planarization is performed simultaneously with sealing of pores on
the surface of the porous Si layer during prebaking in hydrogen
before epitaxial growth.
[0284] The microroughness due to micropores may be planarized not
by annealing in hydrogen but by surface touch polishing.
EXAMPLE 10
[0285] In Example 10, the bonded substrate stack separation method
in Examples 4 to 9 was changed. More specifically, in Example 10,
instead of using the water jet method, a thin resin wedge was
inserted into the beveling gap of the bonded substrate stack to
separate the bonded substrate stack into two substrates at the
second (lower) porous Si layer.
EXAMPLE 11
[0286] In Example 11, the processes described in Examples 4 to 9
were performed for both surfaces of a single-crystal Si substrate
for forming a first substrate.
EXAMPLE 129
[0287] A single-crystal Si substrate was set in each of the
anodizing apparatuses of the above embodiments, and a porous layer
having a four-layered structure was formed. The first to fourth
anodizing conditions were as follows.
10 <First Anodizing Conditions> Current density: 7 (mA
.multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:1:1 Process time: 5 (min)
Thickness of porous Si layer (target): 6 (.mu.m) Porosity (target):
to 24% <Second Anodizing Conditions> Current density: 10 (mA
.multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:2:2 Process time: 3 (min)
Thickness of porous Si layer (target): 3 (.mu.m) Porosity (target):
to 35% <Third Anodizing Conditions> Current density: 7 (mA
.multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:1:1 Process time: 5 (min)
Thickness of porous Si layer (target): 6 (.mu.m) Porosity (target):
to 25% <Fourth Anodizing Conditions> Current density: 20 (mA
.multidot. cm.sup.-2) Anodizing solution:
HF:H.sub.2O:C.sub.2H.sub.5OH = 1:1:1 Process time: 80 (sec)
Thickness of porous Si layer (target): 1 (.mu.m) Porosity (target):
to 45%
[0288] In each of the anodizing apparatuses of the first to fourth
embodiments, a substrate having a porous layer with a four-layered
structure could be manufactured. In each of the anodizing
apparatuses of the first and third embodiments, the anodizing
solution for the first, third, and fourth anodizing processes was
supplied from the first and second tanks, and the anodizing
solution for the second anodizing process was supplied from the
third and fourth tanks. In each of the anodizing apparatuses of the
second and fourth embodiments, four anodizing baths were used.
[0289] In the anodizing apparatus of the third embodiment, a first
conductive diaphragm was set, and a first porous layer was formed
on each of 25 Si substrates under the first anodizing conditions.
Subsequently, a second conductive diaphragm was set, and a second
porous layer was formed on each of the 25 Si substrates under the
second anodizing conditions. Then, a third conductive diaphragm was
set, and a third porous layer was formed on each of the 25 Si
substrates under the third anodizing conditions. Finally, a fourth
conductive diaphragm was set, and a fourth porous layer was formed
on each of the 25 Si substrates under the fourth anodizing
conditions. Porous layers sequentially formed on each Si substrate
used as the conductive diaphragm did not break.
[0290] In the anodizing apparatus of the fourth embodiment, first
to fourth porous layers were sequentially formed on each of 25 Si
substrates. Porous layers sequentially formed on each Si substrate
used as the conductive diaphragm did not break.
[0291] [Others]
[0292] In the above examples, to epitaxially grow a non-porous
layer such as a single-crystal Si layer on the porous Si layer, not
only CVD but also MBE, sputtering, or liquid phase epitaxial growth
can be employed.
[0293] The etchant for selectively etching the porous Si layer is
not limited to the mixed solution of 49% hydrofluoric acid and 30%
hydrogen peroxide. For example,
[0294] 1) a mixed solution of hydrofluoric acid and water
[0295] 2) a mixed solution prepared by adding at least one of an
alcohol and hydrogen peroxide to a mixed solution of hydrofluoric
acid and water
[0296] 3) buffered hydrofluoric acid
[0297] 4) a mixed solution prepared by adding at least one of an
alcohol and hydrogen peroxide to buffered hydrofluoric acid, or
[0298] 5) a mixed solution of hydrofluoric acid, nitric acid, and
acetic acid can be used. The porous Si layer has a large surface
area and therefore can be selectively etched using various
etchants.
[0299] To separate the bonded substrate stack, various methods can
be employed in addition to the separation method using a fluid as
an application of the water jet method.
[0300] The remaining processes can be performed not only by the
methods of the above examples but also by various methods.
[0301] As described above, by exchanging the electrolytic solution
for anodizing every time one porous layer is formed, the porosity
of the resultant porous layer can be precisely controlled.
[0302] In addition, when one conductive diaphragm is used under the
same anodizing conditions, break of the conductive diaphragm can be
suppressed.
[0303] According to the present invention, a porous layer having a
multilayered structure can be formed.
[0304] The present invention is not limited to the above
embodiments and various changes and modifications can be made
within the spirit and scope of the present invention. Therefore, to
apprise the public of the scope of the present invention, the
following claims are made.
* * * * *