U.S. patent application number 13/762974 was filed with the patent office on 2013-07-04 for soi structures with reduced metal content.
This patent application is currently assigned to MEMC ELECTRONIC MATERIALS, INC.. The applicant listed for this patent is MEMC ELECTRONIC MATERIALS, INC.. Invention is credited to Larry Flannery, Alexis Grabbe.
Application Number | 20130168802 13/762974 |
Document ID | / |
Family ID | 46576658 |
Filed Date | 2013-07-04 |
United States Patent
Application |
20130168802 |
Kind Code |
A1 |
Grabbe; Alexis ; et
al. |
July 4, 2013 |
SOI STRUCTURES WITH REDUCED METAL CONTENT
Abstract
Methods for producing silicon on insulator structures with a
reduced metal content in the device layer thereof are disclosed.
Silicon on insulator structures with a reduced metal content are
also disclosed.
Inventors: |
Grabbe; Alexis; (St.
Charles, MO) ; Flannery; Larry; (Warrenton,
MO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MEMC ELECTRONIC MATERIALS, INC.; |
St. Peters |
MO |
US |
|
|
Assignee: |
MEMC ELECTRONIC MATERIALS,
INC.
St. Peters
MO
|
Family ID: |
46576658 |
Appl. No.: |
13/762974 |
Filed: |
February 8, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13354788 |
Jan 20, 2012 |
|
|
|
13762974 |
|
|
|
|
61437993 |
Jan 31, 2011 |
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Current U.S.
Class: |
257/506 ;
438/479 |
Current CPC
Class: |
H01L 21/7624 20130101;
H01L 21/3226 20130101; H01L 21/76251 20130101; H01L 21/3221
20130101; H01L 29/0642 20130101; H01L 29/06 20130101; H01L 21/02043
20130101 |
Class at
Publication: |
257/506 ;
438/479 |
International
Class: |
H01L 21/762 20060101
H01L021/762; H01L 29/06 20060101 H01L029/06 |
Claims
1. A silicon on insulator structure comprising a handle wafer, a
silicon device layer having a front surface, and a dielectric layer
between the handle wafer and the silicon device layer, the
dielectric layer and silicon device layer forming an interface
between the dielectric layer and silicon device layer, the silicon
on insulator structure having the amount of metal in the device
layer reduced by: forming a sacrificial oxide layer on the front
surface of the silicon device layer, the sacrificial oxide layer
and the silicon device layer forming an interface between the
sacrificial oxide layer and the silicon device layer; heating the
silicon on insulator structure having a sacrificial oxide layer
thereon to a temperature T.sub.1 sufficient to dissolve all metal
precipitates present in the device layer for a time t.sub.1
sufficient to allow metal atoms to evenly disperse throughout the
device layer, wherein the temperature T.sub.1 is less than a
temperature at which the metal atoms cross the sacrificial oxide
layer-silicon device layer interface and enter the sacrificial
oxide layer; cooling the silicon on insulator structure from
T.sub.1 to a temperature T.sub.2 at which the metal atoms are
substantially immobile in silicon at an average cooling rate R to
cause metal atoms to dissolve to the sacrificial oxide
layer-silicon device layer interface and the silicon device
layer-dielectric layer interface, the cooling rate being
sufficiently high enough to cause substantially no metal
precipitation to occur in the silicon device layer during cooling;
and removing the sacrificial oxide layer and a portion of the metal
atoms at the sacrificial oxide layer-silicon device layer interface
from the front surface of the silicon device layer; wherein the
silicon device layer comprises metal of a first type in an amount
at or below the solubility limit of the metal in silicon at
temperature T.sub.1 after the amount of metal in the device layer
has been reduced.
2. The silicon on insulator structure as set forth in claim 1
wherein the metal content in the silicon device layer is reduced by
heating the silicon on insulator structure for a time sufficient to
allow the average lateral diffusion distance of the metal atoms to
exceed the thickness of the device layer.
3. The silicon on insulator structure as set forth in claim 2
wherein the metal content in the silicon device layer is reduced by
heating the silicon on insulator structure for a time sufficient to
allow the lateral diffusion distance of the metal atoms to exceed
the thickness of the device layer by at least about 100 times.
4. The silicon on insulator structure as set forth in claim 2
wherein the metal content in the silicon device layer is reduced by
heating the silicon on insulator structure for a time sufficient to
allow the lateral diffusion distance of the metal atoms to exceed
the thickness of the device layer by at least about 1000 times.
5. The silicon on insulator structure as set forth in claim 1
wherein the temperature at the sacrificial oxide layer-silicon
device layer interface exceeds the temperature at the silicon
device layer-dielectric layer interface during cooling such that
more metal atoms are positioned at the sacrificial oxide
layer-silicon device layer interface than the silicon device
layer-dielectric layer interface.
6. The silicon on insulator structure as set forth in claim 1
wherein the metal is selected from the group consisting of nickel,
copper and cobalt.
7. The silicon on insulator structure as set forth in claim 1
wherein the metal is nickel.
8. The silicon on insulator structure as set forth in claim 7
wherein T.sub.1 is at least about 490.degree. C.
9. The silicon on insulator structure as set forth in claim 1
wherein T.sub.1 is at least the temperature at which the metal
dissolves in the device layer and is no more than about 25.degree.
C. greater than the temperature at which the metal completely
dissolves in the device layer.
10. The silicon on insulator structure as set forth in claim 7
wherein the time t.sub.1 at which the silicon on insulator
structure is heated is at least about 1 minute.
11. The silicon on insulator structure as set forth in claim 7
wherein the cooling rate R is at least about 0.3.degree.
C./min.
12. The silicon on insulator structure as set forth in claim 7
wherein T.sub.2 is about 440.degree. C.
13. The silicon on insulator structure as set forth in claim 7
wherein T.sub.2 is about 440.degree. C. or less.
14. The silicon on insulator structure as set forth in claim 1
wherein the sacrificial oxide layer is formed by exposing the
structure to ambient air to form a native oxide layer.
15. The silicon on insulator structure as set forth in claim 1
wherein the sacrificial oxide layer is formed by heating the wafer
in an oxygen-containing atmosphere.
16. The silicon on insulator structure as set forth in claim 1
wherein the silicon device layer is less than about 200 nm
thick.
17. The silicon on insulator structure as set forth in claim 1
wherein the silicon device layer is less than about 75 nm
thick.
18. The silicon on insulator structure as set forth in claim 1
wherein the initial concentration of nickel in the silicon on
insulator structure is at least about 1.times.10.sup.10
atoms/cm.sup.3.
19. The silicon on insulator structure as set forth in claim 1
wherein the concentration of nickel in the silicon device layer is
at least about 1.times.10.sup.10 atoms/cm.sup.3.
20. The silicon on insulator structure as set forth in claim 1
wherein the sacrificial oxide layer forming step, heating step,
cooling step and sacrificial oxide layer removal step are repeated
for about 2 cycles or more to further reduce the amount of metal in
the silicon device layer.
21. The silicon on insulator structure as set forth in claim 1
wherein the dielectric layer is composed of SiO.sub.2.
22. The silicon on insulator structure as set forth in claim 1
wherein the sacrificial oxide layer and a portion of the metal
atoms at the sacrificial oxide layer-silicon device layer interface
are removed by contacting the silicon on insulator structure with
the sacrificial layer thereon is contacted with an etching
solution.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is a Divisional of U.S. patent
application Ser. No. 13/354,788, filed Jan. 20, 2012, which claims
the benefit of U.S. Provisional Patent Application No. 61/437,993,
filed Jan. 31, 2011, both of which are incorporated herein by
reference in their entirety.
FIELD OF THE DISCLOSURE
[0002] The field of the present disclosure relates to methods for
producing silicon on insulator structures and, in particular,
methods for producing silicon on insulator structures with a
reduced metal content in the device layer thereof. The present
disclosure also relates to silicon on insulator structures produced
by such methods.
BACKGROUND
[0003] Silicon on insulator structures ("SOI structures" which may
also be referred to herein as "SOI wafers" or "SOI substrates")
generally include a handle wafer, a silicon layer (also
characterized as a "device layer"), and a dielectric layer (such as
an oxide layer) between the handle wafer and the silicon layer.
Transistors built within the top silicon layer of SOI structures
switch signals quickly compared to transistors built on bulk
silicon wafers, run at lower voltages and are much less vulnerable
to signal noise from background cosmic ray particles. Each
transistor is isolated from its neighbor by a complete layer of
silicon dioxide. These transistors are generally immune to
"latch-up" problems and can be spaced closer together than
transistors built on bulk silicon wafers. Building circuits on SOI
structures increases productivity by allowing for more compact
circuit designs, yielding more chips per wafer.
[0004] Metal contamination in semiconductor wafers is detrimental
as the contamination can cause yield losses in the resulting
integrated circuits. Metal contamination has increasingly become a
concern in view of the trend toward smaller devices, devices with
faster operational speeds and lower manufacturing costs. Metal
contamination may be introduced into semiconductor wafers in a
number of processing steps including wafer polishing, cleaning,
bond strength enhancing thermal treatment, epitaxy, oxide
stripping, plasma activation, wet chemical etching, gas phase
chemical etching, high temperature annealing, ion implantation,
oxidation and the like. SOI wafers made by layer transfer are
sensitive to metal contamination as the device layer is thin and
three surfaces must be cleaned. Surface contamination can diffuse
into the thin device layer at a higher concentration relatively to
thicker bulk wafers.
[0005] A continuing need exists for semiconductor wafers with a
reduced metal content and, in particular, silicon on insulator
structures with a reduced metal content in the device layer
thereof. A continuing need also exists for methods for producing
such wafers.
SUMMARY
[0006] One aspect of the present disclosure is directed to a method
for reducing the metal content of a silicon on insulator structure.
The silicon on insulator structure includes a handle wafer, a
silicon device layer having a front surface and a dielectric layer
between the handle wafer and the silicon layer. The dielectric
layer and silicon device layer form an interface between the
dielectric layer and silicon device layer. A sacrificial oxide
layer is formed on the front surface of the silicon device layer.
The sacrificial oxide layer and the silicon device layer form an
interface between the sacrificial oxide layer and the silicon
device layer. The silicon on insulator structure having a
sacrificial oxide layer thereon is heated to a temperature T.sub.1
sufficient to dissolve all metal precipitates present in the device
layer for a time t.sub.1 sufficient to allow metal atoms to evenly
disperse throughout the device layer. The temperature T.sub.1 is
less than a temperature at which the metal atoms cross the
sacrificial oxide layer-silicon device layer interface and enter
the sacrificial oxide layer. The silicon on insulator structure is
cooled from T.sub.1 to a temperature T.sub.2 at which the metal
atoms are substantially immobile in silicon at an average cooling
rate of R to cause metal atoms to dissolve to the sacrificial oxide
layer-silicon device layer interface and the silicon device
layer-dielectric layer interface. The cooling rate is sufficiently
high enough to cause substantially no metal precipitation to occur
in the silicon device layer during cooling. The sacrificial oxide
layer and a portion of the metal atoms at the sacrificial oxide
layer-silicon device layer interface are removed from the
silicon-in-insulator structure.
[0007] A further aspect of the present disclosure is directed to a
silicon on insulator structure having a handle wafer, a silicon
device layer, a dielectric layer between the handle wafer and the
silicon device layer, and a sacrificial oxide layer. The silicon
device layer is bonded to the dielectric layer at a first surface
and is bonded to the sacrificial oxide layer at a second surface
which forms a sacrificial oxide-silicon device layer interface. The
device layer includes metal atoms of a first type and at least
about 50% of the metal atoms of the first type are positioned in
the device layer at the interface.
[0008] Another aspect of the present disclosure is directed to a
silicon on insulator structure having a handle wafer, a silicon
device layer having a front surface, and a dielectric layer between
the handle wafer and the silicon device layer. The dielectric layer
and silicon device layer form an interface between the dielectric
layer and silicon device layer. The silicon on insulator structure
has the amount of metal in the device layer reduced by forming a
sacrificial oxide layer on the front surface of the silicon device
layer thereby forming an interface between the sacrificial oxide
layer and the silicon device layer. The silicon on insulator
structure having a sacrificial oxide layer thereon is heated to a
temperature T.sub.1 sufficient to dissolve all metal precipitates
present in the device layer for a time t.sub.1 sufficient to allow
metal atoms to evenly disperse throughout the device layer. The
temperature T.sub.1 is less than a temperature at which the metal
atoms cross the sacrificial oxide layer-silicon device layer
interface and enter the sacrificial oxide layer. The silicon on
insulator structure is cooled from T.sub.1 to a temperature T.sub.2
at which the metal atoms are substantially immobile in silicon at
an average cooling rate of R to cause metal atoms to dissolve to
the sacrificial oxide layer-silicon device layer interface and the
silicon device layer-dielectric layer interface. The cooling rate
is sufficiently high enough to cause substantially no metal
precipitation to occur in the silicon device layer during cooling.
The sacrificial oxide layer and a portion of the metal atoms at the
sacrificial oxide layer-silicon device layer interface are removed
from the silicon-in-insulator structure. The silicon device layer
includes metal of a first type in an amount at or below the
solubility limit of the metal in silicon at temperature T.sub.1
after the amount of metal in the device layer has been reduced.
[0009] Various refinements exist of the features noted in relation
to the above-mentioned aspects of the present disclosure. Further
features may also be incorporated in the above-mentioned aspects of
the present disclosure as well. These refinements and additional
features may exist individually or in any combination. For
instance, various features discussed below in relation to any of
the illustrated embodiments of the present disclosure may be
incorporated into any of the above-described aspects of the present
disclosure, alone or in any combination.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is cross-section view of a donor wafer with a
dielectric layer disposed thereon;
[0011] FIG. 2 is a cross-section view of the donor wafer and
dielectric layer bonded to a handle wafer;
[0012] FIG. 3 is a cross-section view of the SOI structure upon
cleaving the donor wafer at the cleave plane;
[0013] FIG. 4 is a SOI structure with a sacrificial oxide layer
disposed thereon;
[0014] FIG. 5 is a graph depicting the solubility of nickel in
silicon as a function of temperature;
[0015] FIG. 6 is a schematic view of a SOI structure which shows
migration of metal atoms in the silicon device layer; and
[0016] FIG. 7 is a graph showing the temperature profile of the
wafer during processing according to Example 1.
[0017] Corresponding reference characters indicate corresponding
parts throughout the drawings.
DETAILED DESCRIPTION
[0018] In accordance with the present disclosure, the metal content
in the device layer of a silicon on insulator structure may be
reduced by forming a sacrificial oxide layer on the surface of the
silicon on insulator structure in a first step S.sub.1. As
schematically shown in FIG. 6, the structure is heated to a
temperature at which a particular metal contaminant (e.g., nickel
precipitates) dissolves for a sufficient amount of time to allow
the metal atoms to evenly disperse throughout the device layer in
step S.sub.2. The cooling rate of the SOI structure is controlled
in step S.sub.3 to allow the metal atoms to migrate to and become
"pinned" at the interface between the sacrificial oxide layer and
the device layer. The sacrificial oxide layer and metal atoms at
the interface therewith are removed by etching in step S.sub.4 to
reduce the concentration of contaminants in the device layer. The
process may be repeated in one or more additional cycles to further
reduce the concentration of metal in the device layer.
[0019] Multi-layered structures and, in particular, silicon on
insulator structures and methods for producing silicon on insulator
structures are generally known by those skilled in the art (see,
for example, U.S. Pat. Nos. 5,189,500; 5,436,175 and 6,790,747,
each of which is incorporated herein by reference for all relevant
and consistent purposes). In an exemplary process for making a
multi-layered structure, two separate structures are prepared,
bonded together along a bond interface, and then delaminated (i.e.,
cleaved) along a separation plane that is different from the bond
interface and which has been formed via an implantation technique.
One structure is typically referred to as the "handle" wafer (or
structure) and the other is typically referred to as the "donor"
wafer (or structure).
[0020] A dielectric layer may be deposited on the surface of the
donor wafer, the handle wafer or both, prior to bonding the donor
and handle together. In this regard, the SOI structures and methods
for preparing the SOI structures are described herein as having a
dielectric layer deposited or grown on the donor wafer and as
having the surface of the handle wafer bonded to the surface of the
dielectric layer. However, it should be understood that the
dielectric layer may be grown or deposited on the handle wafer
alternatively or in addition to growing or depositing the
dielectric layer on the donor wafer and that these structures may
be bonded in any of the various arrangements without limitation.
Reference herein to the dielectric layer being disposed on the
handle wafer alone should not be considered in a limiting
sense.
[0021] Typically, at least the donor wafer and more typically both
the donor wafer and the handle wafer are composed of single crystal
silicon wafers, however it should be noted that other starting
structures may be used such as multi-layered and/or heterolayered
structures without departing from the present disclosure.
[0022] The handle wafer may be obtained from any material common in
the art for preparing multi-layered structures, such as silicon,
germanium, gallium arsenide, silicon germanium, gallium nitride,
aluminum nitride, phosphorous, quartz, sapphire and combinations
thereof. Similarly, the donor wafer may contain silicon, germanium,
gallium arsenide, silicon germanium, gallium nitride, aluminum
nitride, phosphorous, quartz, sapphire and combinations thereof.
Typically however, the handle wafers and donor wafers used
according to the methods of the present disclosure are single
crystal silicon wafers and generally are single crystal silicon
wafers which have been sliced from a single crystal ingot grown in
accordance with conventional Czochralski crystal growing methods.
As such, the following discussion frequently refers to a specific
type of multi-layered structure, i.e., SOI structures, for
illustrative purposes. In this regard, it should be noted that the
handle wafer and/or the donor wafer used in accordance with the
present disclosure may be any diameter suitable for use by those of
skill in the art including, for example, 200 mm, 300 mm, greater
than 300 mm or even 450 mm diameter wafers.
[0023] Referring to FIG. 1, a dielectric layer 15 (e.g., a silicon
oxide and/or silicon nitride layer) is deposited on a polished
front surface of a donor wafer 12. The dielectric layer 15 may be
applied according to any known technique in the art, such as
thermal oxidation, wet oxidation, chemical vapor deposition,
thermal nitridation or a combination of these techniques. Generally
speaking, the dielectric layer 15 is grown to a substantially
uniform thickness sufficient to provide the desired insulating
properties in the final structure. Typically, however, the
dielectric layer has a thickness of at least about 1 nm and less
than about 10,000 nm, less than about 5,000 nm, less than about
1,000 nm, less than about 500 nm, less than about 150 nm, less than
about 100 nm or even less than about 50 nm. The dielectric layer 15
may be any electrically insulating material suitable for use in a
SOI structure, such as a material comprising SiO.sub.2,
Si.sub.3N.sub.4, aluminum oxide, or magnesium oxide. In some
embodiments, the dielectric layer 15 is SiO.sub.2 (i.e., the
dielectric layer consists essentially of SiO.sub.2 with impurities
being present therein). However, it is to be noted that in some
instances, it may alternatively be preferable to use a material for
the dielectric layer which has a melting point which is higher than
the melting point of pure SiO.sub.2 (i.e., higher than about
1700.degree. C.). Examples of such materials are silicon nitride
(Si.sub.3N.sub.4), aluminum oxide, and magnesium oxide.
[0024] Ions (e.g., hydrogen atoms, helium atoms or a combination of
hydrogen and helium atoms) are implanted at a substantially uniform
specified depth beneath the front surface of the donor wafer to
define a cleave plane 17. It should be noted, that when
combinations of ions are implanted, they may be implanted
concurrently or sequentially. Ion implantation may be achieved
using means known in the art. For example, this implantation may be
achieved in a manner similar to the process disclosed in U.S. Pat.
No. 6,790,747. Implantation parameters may include, for example,
implantation of ions to a total dose of about 1.times.10.sup.15 to
about 5.times.10.sup.16 ions/cm.sup.2 at a total energy of, for
example, about 20 to about 125 keV (e.g., H.sub.2.sup.+ may be
implanted at an energy of 20 keV and a dose of 2.4.times.10.sup.16
ions/cm.sup.2). When a combination of ions is used, the dose may be
adjusted between the combination of ions accordingly (e.g., He may
be implanted at an energy of 36 keV and a dose of 1.times.10.sup.16
ions/cm.sup.2 followed by H.sub.2.sup.+ implanted at an energy of
48 keV and a dose of 5.times.10.sup.15 ions/cm.sup.2).
[0025] When implantation is performed prior to deposition of the
dielectric layer, the subsequent growth or deposition of the
dielectric layer on the donor wafer is suitably performed at a
temperature low enough to prevent premature separation or cleaving
along plane 17 in the donor layer (i.e., prior to the wafer bonding
process step). The separation or cleaving temperature is a complex
function of the implanted species, implanted dose and implanted
material. However, typically, premature separation or cleaving may
be avoided by maintaining a deposition or growth temperature below
about 500.degree. C.
[0026] Referring now to FIG. 2, the front surface of the dielectric
layer 15 is then bonded to the front surface of a handle wafer 10
to form a bonded wafer 20 through a hydrophilic bonding process.
The dielectric layer 15 and handle wafer 10 may be bonded together
by exposing the surfaces of the wafers to a plasma containing, for
example, oxygen or nitrogen. Exposure to the plasma modifies the
structure of the surfaces in a process often referred to as surface
activation. The wafers are then pressed together and a bond at the
bond interface 18 is formed there between.
[0027] Prior to bonding, the surfaces of the dielectric layer and
handle wafer may optionally undergo cleaning and/or a brief
etching, planarization, or plasma activation to prepare their
surfaces for bonding using techniques known in the art. Without
being held to a particular theory, it is generally believed that
the quality of the silicon surface of the SOI structure is, in
part, a function of the quality of the surface prior to bonding.
Additionally, the quality of both surfaces prior to bonding will
have a direct impact on the quality or strength of the resulting
bond interface.
[0028] In some instances, therefore, the dielectric layer and/or
handle wafer may be subjected to one or more of the following
procedures in order to obtain, for example, a low surface roughness
(e.g., a roughness of less than about 0.5 nm root mean square
(RMS)) prior to bonding: (i) planarization by, for example, CMP
and/or (ii) cleaning by, for example, a wet chemical cleaning
procedure, such as a hydrophilic surface preparation process (e.g.,
an RCA SC-1 clean process wherein the surfaces are contacted with a
solution containing ammonium hydroxide, hydrogen peroxide, and
water at a ratio of, for example, 1:2:50 at about 65.degree. C. for
about 20 minutes, followed by a deionized water rinse and drying).
One or both of the surfaces may also optionally be subjected to a
plasma activation after, or instead of, the wet cleaning process to
increase the resulting bond strength. The plasma environment may
include, for example, oxygen, ammonia, argon, nitrogen, diborane or
phosphine.
[0029] Generally speaking, wafer bonding may be achieved using
essentially any technique known in the art, provided the energy
employed to achieve formation of the bond interface is sufficient
to ensure that the integrity of the bond interface is sustained
during subsequent processing (i.e., layer transfer by separation
along the cleave or separation plane 17 in the donor wafer).
Typically, however, wafer bonding is achieved by contacting the
surface of the dielectric layer and the handle wafer at a reduced
pressure (e.g., about 50 mTorr) and at room temperature, followed
by heating at an elevated temperature (e.g., at least about
200.degree. C., at least about 300.degree. C., at least about
400.degree. C., or even at least about 500.degree. C.) for a
sufficient period of time (e.g., at least about 10 seconds, at
least about 1 minute, at least about 15 minutes, at least about 1
hour or even at least about 3 hours). For example, the heating may
take place at about 350.degree. C. for about 1 hour. The resulting
interface may have a bond strength that is greater than about 500
mJ/m.sup.2, greater than about 1000 mJ/m.sup.2, greater than about
1500 mJ/m.sup.2, or even greater than about 2000 mJ/m.sup.2. The
elevated temperatures cause the formation of covalent bonds between
the adjoining surfaces of the donor wafer and the handle wafer,
thus solidifying the bond between the donor wafer and the handle
wafer. Concurrently with the heating or annealing of the bonded
wafer, the ions earlier implanted in the donor wafer weaken the
cleave plane. A portion of the donor wafer is then separated (i.e.,
cleaved) along the cleave plane from the bonded wafer to form the
SOI structure.
[0030] After the bond interface has been formed, the resulting
bonded structure is subjected to conditions sufficient to induce a
fracture along the separation or cleave plane within the donor
wafer (FIG. 3). Generally speaking, this fracture may be achieved
using techniques known in the art, such as thermally and/or
mechanically induced cleaving techniques. Typically, however,
fracturing is achieved by annealing the bonded structure at a
temperature of at least about 200.degree. C., at least about
300.degree. C., at least about 400.degree. C., at least about
500.degree. C., at least about 600.degree. C., at least about
700.degree. C. or even at least about 800.degree. C. (the
temperature being in the range of, for example, about 200.degree.
C. to about 800.degree. C., or from about 250.degree. C. to about
650.degree. C.) for a period of at least about 10 seconds, at least
about 1 minute, at least about 15 minutes, at least about 1 hour or
even at least about 3 hours (with higher temperatures requiring
shorter anneal times, and vice versa), under an inert (e.g., argon
or nitrogen) atmosphere or ambient conditions.
[0031] In this regard it is to be noted that in an alternative
embodiment, this separation may be induced or achieved by means of
mechanical force, either alone or in addition to annealing. For
instance, the bonded wafer may be placed in a fixture in which
mechanical force is applied perpendicular to the opposing sides of
the bonded wafer in order to pull a portion of the donor wafer
apart from the bonded wafer. According to some methods, suction
cups are utilized to apply the mechanical force. The separation of
the portion of the donor wafer is initiated by applying a
mechanical wedge at the edge of the bonded wafer at the cleave
plane in order to initiate propagation of a crack along the cleave
plane. The mechanical force applied by the suction cups then pulls
the portion of the donor wafer from the bonded wafer, thus forming
a SOI structure.
[0032] Referring to FIG. 3, upon separation, two structures 30, 31
are formed. Since the separation of the bonded structure 20 occurs
along the cleave plane 17 in the donor wafer 12 (FIG. 2), a portion
of the donor wafer remains part of both structures (i.e., a portion
of the donor wafer is transferred along with the dielectric layer).
Structure 30 comprises a portion of the donor wafer. Structure 31
is the silicon on insulator structure and includes the handle wafer
16, the dielectric layer 15, and a silicon layer 25.
[0033] The resulting SOI structure 31 includes a thin layer of
silicon 25 (the portion of the donor wafer remaining after
cleaving) disposed atop the dielectric layer 15 and the handle
wafer 10. The silicon device layer 25 is typically less than about
200 nm thick and the trend toward further miniaturization has led
to development of structures with a device layer thickness of less
than about 100 nm, less than about 75 nm, less than about 50 nm or
even less than about 10 nm (e.g., from about 1 nm to about 200 nm,
from about 50 nm to about 200 nm or from about 1 nm to about 10
nm).
[0034] The cleave surface of the SOI structure (i.e., the thin
layer of silicon of the donor wafer) has a rough surface that may
be smoothed by additional processing. The structure 31 may be
subjected to additional processing to produce a silicon layer
surface having desirable features for device fabrication thereon.
Such features include, for example, reduced surface roughness,
and/or a reduced concentration of light point defects. To smooth
and thin the surface layer of silicon (i.e., cleaved surface),
combinations of annealing, chemical-mechanical polishing,
high-temperature gaseous etching (i.e., epitaxial-smoothing
(epi-smoothing)), or the formation of a sacrificial oxide layer on
the cleaved surface may be used. Pre-epitaxial smoothing anneal
(PESA) processes that subject the SOI wafer to an elevated
temperature (about 1000.degree. C. to about 1200.degree. C.) for
several hours may be used. The elevated temperature heals the
cleaved surface of the SOI wafer by permitting the crystal
structure of the silicon to reorient the dislocations present
therein. Alternatively, any of the additionally processing
operations (e.g., PESA and/or epi-smoothing) may be performed after
the metal reduction process described below.
[0035] As described above, the various surfaces of the SOI
structure 31 are often cleaned and/or polished during fabrication
of the SOI structure to assist in bonding of the various surfaces.
For instance, the top surface of the handle wafer 10 and the top
surface of the dielectric layer 15 are both cleaned prior to
bonding and the surface of the silicon layer 25 is polished and
cleaned after cleavage. Each of these cleaning operations
introduces metal contaminants into the wafer. The concentration of
metal and, in particular, the concentration of nickel which is the
principal metal contaminant in the structure (i.e., the
concentration in the structure as a whole or "bulk" concentration),
may be at least about 1.times.10.sup.10 atoms/cm.sup.3 and in some
applications may be at least about 1.times.10.sup.11 atoms/cm.sup.3
or at least about 1.times.10.sup.12 atoms/cm.sup.3. The localized
concentration in the silicon device layer 25 may be higher than the
bulk concentration and device layer metal concentrations (and in
particular the concentration of nickel) of at least about
1.times.10.sup.10 atoms/cm.sup.3, at least about 1.times.10.sup.12
atoms/cm.sup.3 or at least about 1.times.10.sup.14 atoms/cm.sup.3
(e.g., from about 1.times.10.sup.10 atoms/cm.sup.3 to about
1.times.10.sup.16 atoms/cm.sup.3) are typical. Metal contaminants
other than nickel may also be present such as, for example, copper
and/or cobalt. In this regard, the methods of the present
disclosure are particularly well-suited for removing nickel which
is more problematic when present in the silicon device layer 25
relative to other metals such as copper and cobalt.
[0036] In a first step S.sub.1 of the metal-reducing process and as
shown in FIG. 4, a sacrificial oxide layer 40 is formed on the
front surface of the silicon device layer 25. The sacrificial oxide
layer 40 may be a native oxide layer that forms upon exposing the
structure to ambient air. Such native oxide layers may be from
about 5 .ANG. to about 15 .ANG. thick. For example, a native oxide
layer may be about 10 .ANG. and typically about 8 .ANG., (i.e.,
about 0.8 nm thick). Alternatively, the sacrificial oxide layer 40
may be formed by heating the wafer in an oxygen-containing
atmosphere. In several embodiments, a small amount of oxygen is
added to the atmosphere in which the SOI structure is heated in
step S.sub.2 described below (i.e., steps S.sub.1 and S.sub.2 occur
in the same apparatus (e.g., furnace) and may occur
simultaneously). The thickness of the oxide layer is not critical
and any thickness may be suitable. In this regard, the sacrificial
oxide layer need not be continuous on the surface of the silicon
device layer, however it is preferred that any gaps in sacrificial
oxide layer coverage be less than the device layer thickness.
[0037] The methods of the present disclosure for reducing the metal
content of the SOI structure may be described in relation to
several interfaces within the structure. As shown in FIG. 4, the
sacrificial silicon oxide layer-containing structure has a first
interface 44 formed between the sacrificial oxide layer 40 and the
silicon device layer 25. A second interface 48 is formed between
the device layer 25 and the dielectric layer 15 (e.g., the silicon
oxide layer) and a third interface 52 is formed between the
dielectric layer 15 and the handle wafer 10.
[0038] After the sacrificial oxide layer 40 is formed, in step
S.sub.2 of the metal-reducing process, the silicon on insulator
structure is heated to a temperature T.sub.1. T.sub.1 should be
chosen to be sufficiently high to allow essentially all metal atoms
(e.g., or at least essentially all metal atoms of a particular type
such as nickel atoms) in the device layer to dissolve (i.e., to
dissociate from metal agglomerates within the device layer).
T.sub.1 should also not exceed a temperature at which the dissolved
metal atoms cross the sacrificial oxide layer-silicon device layer
interface 44 and enter the sacrificial oxide layer 40. By
maintaining T.sub.1 below such temperatures, metal atoms (e.g.,
nickel) in the device layer 40 become "pinned" when they contact
the sacrificial oxide layer-device layer interface 44 or contact
the device layer-dielectric layer interface 48. It has been found
that at temperatures above the dissolution temperature of metal in
the device layer, dissolved metal atoms have a chemical potential
at the various device layer-oxide interfaces that is lower than the
chemical potential for atoms within the bulk which causes the metal
atoms to become "pinned" or "trapped" at the respective interface.
However, if the temperature T.sub.1 of the structures exceeds a
critical value, metal atoms may cross the various oxide interfaces.
This may be problematic as metal from the bulk of the SOI wafer may
then cross the dielectric layer and enter the silicon device layer.
Additionally, metal that has diffused into oxides may dissolve into
the oxide but not migrate to the surface which may require hotter,
longer processes with lower throughput. Further, temperatures at
which metal atoms cross the oxide surface may cause an engineered
bulk microdefect density profile (e.g., oxygen precipitates or
nuclei thereof) to become altered. Accordingly, it is preferred
that T.sub.1 is no more than about 25.degree. C. greater than the
temperature at which the metal completely dissolves in the device
layer 25. In other embodiments T.sub.1 is no more than about
20.degree. C. greater, no more than about 15.degree. C. greater, no
more than about 10.degree. C. greater or even no more than about
5.degree. C. greater than the temperature at which the metal
completely dissolves in the device layer 25.
[0039] As stated above, T.sub.1 is chosen such that essentially all
of the particular metal which is desired to be removed from the
structure dissolves in the silicon layer. T.sub.1 may be determined
empirically. For example, a solubility function based on
experimental data for the particular metal of which it is desired
to reduce the concentration thereof may be used to determine
T.sub.1. In the case of nickel in the silicon device layer, the
solubility of nickel and silicon has been reported in INSPEC, EMIS
Data Reviews, No. 4, Properties of Silicon, p. 446 (1988)
(extrapolated to low temperatures) as
[Ni].sub.Si=1.510.sup.24exp(-1.68 eV/k.sub.BT) (1)
where [Ni].sub.Si is expressed in atoms/cm.sup.3, T is in Kelvin
and K.sub.B is Boltzmann's constant (8.617.times.10.sup.-5 eV/K).
This function is shown graphically in FIG. 5. As can be seen from
FIG. 5, nickel appears to be completely soluble at temperatures
above about 760.degree. K (above about 490.degree. C.).
Accordingly, in embodiments of the present disclosure in which it
is desired to remove nickel, T.sub.1 may be at least about
490.degree. C., at least about 500.degree. C. or at least about
510.degree. C.
[0040] The wafer should be held at a temperature T.sub.1 sufficient
to dissolve essentially all of the particular metal but less than a
temperature at which the metal atoms cross the oxide interfaces 44,
48 for a period of time t.sub.1 sufficient to allow metal atoms to
substantially evenly disperse throughout the device layer. By
allowing the metal atoms to evenly disperse, the atoms are less
likely to recombine during cooling and form metal precipitates. In
this regard, for purposes of the present disclosure, the metal
atoms are "evenly dispersed" in the silicon layer when the mean
axial diffusion distance of metal atoms (i.e., the average distance
at which metal atoms travel toward either the sacrificial oxide
layer-silicon device layer interface 44 or the device
layer-dielectric layer interface 48) is many times larger the
device layer thickness (e.g., at least about 100 times). In some
embodiments, the mean axial diffusion distance of metal atoms is at
least about 500 times the silicon device layer thickness or at
least about 1000 times, at least about 2500 times or even at least
about 5000 times the device layer thickness (e.g., from about 500
to about 10,000 times or from about 2500 to about 10,000 times the
device layer thickness). The dispersal of metal atoms in the device
layer may also be observed by using destructive testing that
involves various thermal cycles, precipitation and etching to
delineate precipitates.
[0041] In this regard, the mean axial diffusion distance of metal
atoms may be determined from an experimentally determined
diffusivity function and by the following equation which relates
diffusivity and soak time to the mean axial diffusion distance,
d.
d= {square root over (4Dt)} (2).
For nickel metal in the silicon device layer, the diffusivity D has
been reported in INSPEC, EMIS Data Reviews, No. 4, Properties of
Silicon, p. 424 (1988) (extrapolated to low temperatures) as
D(Ni).sub.Si=210.sup.-3exp(-0.47 eV/k.sub.BT) (3)
where D(Ni).sub.Si is expressed in cm.sup.2/sec, T is in Kelvin and
K.sub.B is Boltzmann's constant (8.617.times.10.sup.-5 eV/K).
Suitably for nickel atoms in the silicon device layer, at an anneal
temperature ranging from 490.degree. C. to about 515.degree. C.,
anneals of at least about 15 minutes (corresponding to a mean axial
diffusion distance of about 0.79 mm at a temperature of about
500.degree. C. which is at least 4000 times the thickness of
typical device layers) may be used or even anneals as short as
about 10 minutes or more, about 5 minutes or more or even as short
as 1 minute or more may be used.
[0042] Further in this regard, it should be understood that, for
purposes of the present disclosure, T.sub.1 need not be a single
temperature at which the SOI structure is maintained for a period
of time t.sub.1. Rather, the wafer may be maintained within a range
of temperatures which allows essentially all of the particular
metal to be dissolved but is less than a temperature at which the
metal atoms cross the oxide interfaces 44, 48 for a time period
t.sub.1 sufficient to allow metal atoms to substantially evenly
disperse throughout the device layer.
[0043] After time period t.sub.1, the wafer is cooled from T.sub.1
to a temperature T.sub.2 in step S.sub.3, T.sub.2 being a
temperature at which the metal atoms are substantially immobile in
silicon, at an average cooling rate of R. The cooling rate R is
sufficiently low to cause substantially no metal precipitation to
occur in the silicon device layer during cooling. As the wafer
cools, the metal atoms in the device layer seek their lowest
chemical potential which has been found to occur at the oxide
interfaces 44, 48. If the wafer cools too slow, the metals may
instead nucleate and combine into precipitates. Accordingly, the
cooling rate R is controlled to be sufficiently fast to cause
substantially no metal precipitation to occur in the silicon device
layer during cooling. Suitably, the cooling rate of the silicon
wafer may be maintained to be at least about 0.3.degree. C./min or,
as in other embodiments, at least about 1.degree. C./sec, at least
about 5.degree. C./min, at least about 30.degree. C./min, at least
about 60.degree. C./min or at least about 100.degree. C./min (e.g.,
from about 0.3.degree. C./min to about 5000.degree. C./min, from
about 0.3.degree. C./min to about 1000.degree. C./min or from about
1.degree. C./min to about 500.degree. C./min).
[0044] For purposes of the present disclosure, the temperature at
which metal atoms are substantially immobile in silicon is the
temperature at which 80% of the metal of interest remains
undissolved in silicon. As can be seen from FIG. 5, nickel is
substantially immobile in silicon at about 715.degree. K (about
440.degree. C.) (i.e., T.sub.2 is about 440.degree. C. or less). In
other embodiments, T.sub.2 is about 425.degree. C. or less, about
400.degree. C. or less or about 300.degree. C. or less. It should
be noted that steps S.sub.2 and S.sub.3 may be performed in the
same ambient and suitable ambients include gases which are inert
with respect to silicon and silicon oxide atoms. In some
embodiments, the ambient comprises an amount of oxygen to grow a
sacrificial oxide layer on the surface of the SOI structure in step
S.sub.1 as described above.
[0045] In addition to controlling the cooling rate such that
substantially no metal precipitation occurs in the silicon device
layer during cooling, the cooling rate should also be high enough
to cause a temperature gradient across the wafer such that the
temperature at the sacrificial oxide layer-silicon device layer
interface 44 exceeds the temperature at the silicon device
layer-dielectric layer interface 48 during cooling. This gradient
allows more metal atoms to migrate and be pinned at the sacrificial
oxide layer-silicon device layer interface 44 relative to the
silicon device layer-dielectric layer interface 48 as metals at the
silicon device layer-dielectric layer interface 48 are mobile for
an increased amount of time compared to the metals at the
sacrificial oxide layer-silicon device layer interface 44. Suitable
cooling rates include rates of at least about 0.3.degree. C./sec or
even at least about 5.degree. C./sec as described above.
[0046] The movement of metal during steps S.sub.2 and S.sub.3 of
the present disclosure is shown schematically in FIG. 6. FIG. 6
schematically shows a SOI wafer 31 with a sacrificial oxide layer
40 on the surface thereof. The temperature profile of the wafer as
a function of time is shown within the dielectric layer 15. Before
step S.sub.2, the device layer 25 includes a number of metal
precipitates 53 (e.g., nickel precipitates). Upon heating during
step S.sub.2, the precipitates dissolve and metal atoms become
evenly dispersed throughout the silicon device layer 25. During
cooling in step S.sub.3, the metal atoms migrate to their lowest
chemical potential which is at the oxide interfaces 44, 48,
preferably with the greater amount migrating to the interface 44
with the native oxide layer 40 than the interface 48 with the
dielectric layer 15. In this regard, steps S.sub.1, S.sub.2 and
S.sub.3 result in a SOI structure with a sacrificial oxide layer
and a device layer comprising metal atoms of a first type (e.g.,
nickel atoms), wherein at least about 50% of the metal atoms of the
first type are positioned in the device layer at the interface 44
formed between the sacrificial oxide layer 40 and device layer 25.
In some embodiments, at least about 55%, at least about 60% or even
at least about 70% of the metal atoms of the first type are
positioned in the device layer at the interface 44. It should be
noted that, as used herein, metal atoms located "at the interface
44" either (i) make contact with both the device layer 25 and
sacrificial oxide layer 40 or (ii) are within the device layer 25
at a distance from the interface 44, the distance being
sufficiently small such that the atoms are accessible to
conventional cleaning processes (i.e., are capable of being removed
by such processes) after the sacrificial oxide layer is
removed.
[0047] The sacrificial oxide layer 40 is removed as well as the
metal atoms at the interface 44 therewith in step S.sub.4 of the
metal-reducing process. The sacrificial oxide layer 40 and metal
atoms may be removed by contacting the SOI structure with an
etching solution. Any suitable etching solution known to those of
skill in the art for semiconductor cleaning may be used including
SC1 (a NH.sub.4OH and H.sub.2O.sub.2 mixture) and HF cleaning
baths. Generally the solutions are applied under conditions
suitable to remove the sacrificial oxide layer as may be determined
by those of skill in the art. The surface of the SOI structure may
be contacted with the etchant by immersing (or at least partially
immersing) the SOI structure in an etchant bath or by selectively
etching the surface by contacting the surface with an etchant gas
(e.g., HF etchant gas). Batch-immersion (cleaning benches) may be
used for etching and spin etching techniques may also be used
without departing from the scope of the present disclosure.
[0048] In some particular embodiments, the solution is an aqueous
HF solution that may optionally contain an amount of
H.sub.2O.sub.2. The concentration of HF in the solution (by weight)
may be less than about 50% and, in other embodiments, is less than
about 25%, less than about 10%, less than about 5% or less than
about 2% (e.g., from about 0.5% to about 50% or from about 0.5% to
about 10%). The HF solution may contain one or more other
components such as complexing ligands or surfactants; however, in
certain embodiments the etchant consists essentially of water and
HF. The etchant may also be a gas that is contacted with the
surface of the structure. Suitable gases include HF gas that may be
diluted in a carrier gas such as hydrogen, argon or nitrogen.
[0049] The period of time for which the etchant contacts the
silicon layer may vary according to the corrosiveness of the
etchant, its concentration and the thickness of the oxide that must
be removed before the metal atoms trapped at the device
layer-sacrificial oxide interface are exposed and removed. For
oxide thicknesses less than about 1 nm, time scales typical for
conventionally-used SC1 or HF baths (no more than 18 minutes, and
typically less than 5 minutes) may be used. In general, SC1
processing is followed by rinsing and acid treatment (i.e., a SC2
clean) to remove metal deposited on the surface by the SC1 clean,
as is conventional in the field of the present disclosure.
Processing conditions are well known to those of skill in the art
and any number of combinations of chemical baths and process times
to remove the surface oxide and the metal atoms pinned immediately
underneath may be used without departing from the scope of the
present disclosure.
[0050] Step S.sub.4 of the present disclosure results in removal of
metal atoms at the interface 44 which reduces the total metal
content of the silicon device layer. For instance, the total metal
content of the device layer may be reduced to the solubility limit
of the metal of interest in silicon at temperature T.sub.1 or even
below this solubility limit.
[0051] To further reduce the metal content in the silicon device
layer, steps S.sub.1, S.sub.2, S.sub.3 and S.sub.4 may be repeated
for one or more additional cycles such as, for example, about 2
cycles or more, about 3 cycles or more or even about 5 cycles or
more.
EXAMPLES
Example 1
Reduction of Metal-Related Defects in SOI Wafers that were
Pre-Epitaxial Layer Smooth Annealed (PESA) but not Smoothed by Gas
Etching (Epi-Smoothed)
[0052] Eight SOI wafers with Secco wand haze (i.e., very scattered
visible light caused by small nickel precipitates) and which had a
native oxide layer at the surface thereof were tested. The SOI
wafers had been bonded, cleaved and annealed at 1125.degree. C.
(i.e., pre-epitaxial layer smooth annealing (PESA)) but had not
been subjected to high-temperature gaseous etching (i.e.,
epitaxial-smoothing (epi-smoothing)). Four wafers were used as a
control and the four remaining wafers were heated to 500.degree. C.
in a vertical furnace (A412, ASM (The Netherlands)) for 15 minutes
with a small amount of oxygen insufficient to grow appreciable
oxide (<3 .ANG.). The temperature profile for the wafers is
illustrated in FIG. 7 in which the temperatures for the various
paddle zones of the furnace are shown. The ramp down cooling rate
was about 1.8.degree. C./min.
[0053] Two wafers that were annealed at 500.degree. C. were then
etched by use of a HF solution for 8 minutes and the other two were
etched by use of a HF/H.sub.2O.sub.2 solution for 8 minutes. The
concentration of HF was about 0.5 wt % and, when used, the
concentration of H.sub.2O.sub.2 was about 2.5 wt %. All wafers were
then epi-smoothed (E3000 EPI reactor, ASM (The Netherlands)) in a
two step etching process in which the wafers were heated to
1100.degree. C. for a set period of time and then cooled to
1000.degree. C. while contacting the wafers with about a 0.7 vol %
HCl in hydrogen ambient. Defects were measured by highlighting
metal precipitates using a decorative etch. The decorative etch
included a standard Secco etch to etch through to any metal
precipitates followed by an HF etch to undercut the buried oxide
layer in any areas where the Secco etch created a pathway to the
BOX layer. The resulting pits were detected by a Kodak bright
light. The defects were measured for each wafer in two different
quadrants (tests "a" and "b" below).
[0054] The results of the test are shown in Table 1 below. Results
of the bright light assessment are shown in the "Defects Present?"
column. The column titled "Extension" indicates the distance the
wand haze extended from the edge of the wafer toward the center. A
field of view of the wafer in an area were wand haze was present
was selected to count defects. A random field of view was selected
for wafers where no defects were observed. The microscope objective
was chosen such that the defects could properly be counted. Only
one field of view was used per quadrant. The total number of
defects measured for each field of view is shown in the "Defect
Count" column and the number of defects per cm.sup.2 is shown in
the "Defect Density" column.
TABLE-US-00001 TABLE 1 Defect amounts in SOI wafers heated to
500.degree. C. for 15 minutes and in control wafers processed
according to Example 1. Defect Density Metal Reduction Defects
Extension Defect (defects/ Wafer Treatment Etch Present? (mm)
Objective Count cm.sup.2) 1a 500.degree. C., 15 min HF None #N/A
1-5x 0 0 1b 500.degree. C., 15 min HF None #N/A 1-5x 0 0 2a
500.degree. C., 15 min HF + H.sub.2O.sub.2 None #N/A 1-5x 0 0 2b
500.degree. C., 15 min HF + H.sub.2O.sub.2 None #N/A 1-5x 0 0 3a
500.degree. C., 15 min HF None #N/A 1-5x 0 0 3b 500.degree. C., 15
min HF None #N/A 1-5x 0 0 4a 500.degree. C., 15 min HF +
H.sub.2O.sub.2 None #N/A 1-5x 0 0 4b 500.degree. C., 15 min HF +
H.sub.2O.sub.2 None #N/A 1-5x 0 0 5a None HF Wand 30 2-20x 7 1892
Haze 5b None HF Wand 42 2-20x 6 1622 Haze 6a None HF +
H.sub.2O.sub.2 Wand 36 2-20x 17 4595 Haze 6b None HF +
H.sub.2O.sub.2 Wand 33 2-20x 3 811 Haze 7a None HF + H.sub.2O.sub.2
Wand 33 2-20x 6 1622 Haze 7b None HF + H2O2 Wand 30 2-20x 6 1622
Haze 8a None HF Wand 33 2-20x 4 1081 Haze 8b None HF Wand 30 2-20x
2 541 Haze
[0055] All defects are believed to be attributable to nickel due to
its behavior and the thermal recipe used in the processing recipes.
As can be seen from Table 1, the 500.degree. C. heat treatment
resulted in no defects being detected.
Example 2
Reduction of Metal-Related Defects in End-of Line SOI Wafers
[0056] Four end-of-line SOI wafers (i.e., wafers that were
processed as in Example 1 and then processed through a final
cleaning and subjected to applicable inspections (particles,
flatness and layer thickness inspections)) that had Secco wand haze
were heated according to the process of Example 1 (500.degree. C.
for 15 minutes with 1.8.degree. C./min ramp down). As these wafers
had previously been stripped of the native oxide layer, the
sacrificial oxide layer was formed during annealing as a result of
the partial pressure of oxygen present during heating. Each wafer
was selected from a lot of wafers that were graded into one of four
defect categories depending on the extent to which one wafer in the
lot contained Secco wand haze. A second wafer from each lot was
also used as a control (4 control wafers). After the 500.degree. C.
heat treatment, the heat-treated wafers were combined with the
control wafers and all wafers received a HF clean, a final clean
(SC1 and SC2 clean) and a final inspection on a particle inspection
tool (SP207, KLA-Tencor (Milpitas, Calif.)). All eight wafers were
then subjected to Secco testing in accordance with the process set
forth in Example 1 with the results being shown in Table 2. The
wafer tested in lot 3 had two "hot spots" near the wafer edge.
TABLE-US-00002 TABLE 2 Defect amounts in SOI wafers heated to
500.degree. C. for 15 minutes and in control wafers processed
according to Example 2. Metal Reduction Quadrant Defect Extension
Defect Density Wafer Lot Treatment Etch Defect Grading Tested Class
(mm) Objective Count (defects/cm.sup.2) 1a None HF Edge Band 1 None
N/A 1-5x 0 0 1b None HF Edge Band 4 None N/A 1-5x 0 0 2a None HF
Light Wand Haze 1 Wand Haze 30 1-5x 147 2450 2b None HF Light Wand
Haze 4 Wand Haze 30 1-5x 16 267 2c None HF Light Wand Haze 4 Hot
Spot 4 1-5x 246 4100 3a None HF Heavy Wand Haze, 1 Wand Haze 3 1-5x
189 3150 small extension 3b None HF Heavy Wand Haze, 4 Wand Haze 3
1-5x 677 11284 small extension 3c None HF Heavy Wand Haze, 4 Hot
Spot 3 1-5x 143 2383 small extension 3d None HF Heavy Wand Haze, 4
Hot Spot 3 2-20x 36 9731 small extension 4a None HF Heavy Wand Haze
1 Wand Haze 4 1-5x 297 4950 4b None HF Heavy Wand Haze 4 Wand Haze
33 1-5x 313 5217 4c None HF Heavy Wand Haze 4 Hot Spot 4 1-5x 306
5100 5a 500.degree. C., 15 min HF Edge Band 1 None N/A 1-5x 0 0 5b
500.degree. C., 15 min HF Edge Band 4 None N/A 1-5x 0 0 6a
500.degree. C., 15 min HF Light Wand Haze 1 None N/A 1-5x 0 0 6b
500.degree. C., 15 min HF Light Wand Haze 4 None N/A 1-5x 0 0 7a
500.degree. C., 15 min HF Heavy Wand Haze, 1 None N/A 1-5x 0 0
small extension 7b 500.degree. C., 15 min HF Heavy Wand Haze, 4
None N/A 1-5x 0 0 small extension 8a 500.degree. C., 15 min HF
Heavy Wand Haze 1 None N/A 1-5x 0 0 8b 500.degree. C., 15 min HF
Heavy Wand Haze 4 None N/A 1-5x 0 0
[0057] As can be seen from Table 2, the 500.degree. C. heat
treatment resulted in no defects being detected.
Example 3
Comparison of Defect Density Before and after 500.degree. C. Heat
Treatment
[0058] Sixteen wafers from sixteen different lots of wafers that
were previously stripped of oxide were subjected to pre-epitaxial
layer smooth annealing but were not subjected to epi-smoothing. The
wafers were subjected to the 500.degree. C. heat treatment of
Example 1, were stripped of oxide and subjected to epi-smoothing
and other end-of-line processing (e.g., a final cleaning and
subjected to applicable inspections). The wafers were then analyzed
for defects. Several wafers in the same lot were also analyzed
(pre-epi smoothing) for defects before the 500.degree. C. heat
treatment for comparison purposes.
TABLE-US-00003 TABLE 3 Defect amounts in SOI wafers before and
after 500.degree. C. heat treatment processed according to Example
3. Before 500.degree. C. Heat Treatment After 500.degree. C. Heat
Treatment Defect Defect Density Density Wafer Extension Defect
(defects/ Extension Defect (defects/ Lot (mm) Objective Count
cm.sup.2) (mm) Objective Count cm.sup.2) 1a 27 1-5x 6 100 NA 1-5x 0
0 1b 24 2-20x 3 811 NA 1-5x 0 0 2a 39 2-20x 96 25949 3 2-20x 41
11082 2b 42 2-20x 103 27841 6 2-20x 98 26490 3a NA 1-5x 0 0 3b NA
1-5x 0 0 4a 4 1-5x 375 6250 4b 9 1-5x 769 12818 5a 30 2-20x 4 1081
NA 1-5x 0 0 5b 36 2-20x 2 541 NA 1-5x 0 0 6a 27 2-20x 5 1352 NA
1-5x 0 0 6b 33 2-20x 3 811 NA 1-5x 0 0 7a 39 2-20x 60 16218 2 1-5x
18 300 7b 27 2-20x 74 20002 2 1-5x 19 317 8a 27 2-20x 28 7569 3
1-5x 13 217 8b 30 2-20x 91 24598 4 1-5x 317 5284 9a 24 2-20x 91
24598 60 1-5x 2404 40069 9b 24 2-20x 159 42978 2 1-5x 23 383 10a 30
2-20x 12 3244 NA 1-5x 0 0 10b 33 2-20x 42 11353 NA 1-5x 0 0 11a 24
2-20x 2 541 NA 1-5x 0 0 11b 21 2-20x 7 1892 NA 1-5x 0 0 12a 18
2-20x 2 541 NA 1-5x 0 0 12b 12 1-5x 5 83 NA 1-5x 0 0 13a NA 1-5x 0
0 13b 9 2-20x 50 13515 14a NA 1-5x 0 0 14b 3 2-20x 8 2162 15a NA
1-5x 0 0 15b 8 2-20x 13 3514 16a 9 2-20x 77 20813 16b 15 2-20x 272
73523
[0059] As can be seen from Table 3, the 500.degree. C. heat
treatment resulted in significant reduction of nickel-related
defects.
Example 4
Analysis of Multi-Cycle Metal Reduction Processes
[0060] Four SOI wafers were selected from four lots and subjected
to a 500.degree. C., 15 minute heat treatment followed by removal
of the sacrificial oxide layer as in Example 1. The heat treatment
and oxide removal were then repeated in a second cycle. Defects
densities were determined before the first heat treatment, after
the first heat treatment and after the second heat treatment.
Defect densities are shown in Table 4 below.
TABLE-US-00004 TABLE 4 Defect amounts in SOI wafers before and
after a 500.degree. C. heat treatment and after a second
500.degree. C. heat treatment. Before 500.degree. C. Heat
Treatments Defect After First 500.degree. C. Heat Treatment After
Second 500.degree. C. Heat Treatment Density Defect Defect
Extension Defect (defects/ Extension Defect Density Extension
Defect Density Wafers (mm) Objective Count cm.sup.2) (mm) Objective
Count (defects/cm.sup.2) (mm) Objective Count (defects/cm.sup.2) 1a
39 2-20x 96 25949 3 2-20x 41 11082 5 2-20x 223 60278 1b 42 2-20x
103 27841 6 2-20x 98 26490 #N/A 1-5x 0 0 2a 4 1-5x 375 6250 #N/A
1-5x 0 0 2b 9 1-5x 769 12818 6 2-20x 58 15678 3a 27 2-20x 28 7569 3
1-5x 13 217 7 1-5x 10 167 3b 30 2-20x 91 24598 4 1-5x 317 5284 #N/A
1-5x 0 0 4a 9 2-20x 77 20813 4 1-5x 88 1467 4b 15 2-20x 272 73523 6
1-5x 112 1867
[0061] As can be seen from Table 4, the second outdiffusion results
in further reduction of defects for several wafers.
[0062] When introducing elements of the present disclosure or the
preferred embodiments(s) thereof, the articles "a", "an", "the" and
"said" are intended to mean that there are one or more of the
elements. The terms "comprising", "including" and "having" are
intended to be inclusive and mean that there may be additional
elements other than the listed elements.
[0063] As various changes could be made in the above apparatus and
methods without departing from the scope of the disclosure, it is
intended that all matter contained in the above description and
shown in the accompanying figures shall be interpreted as
illustrative and not in a limiting sense.
* * * * *