U.S. patent application number 10/875233 was filed with the patent office on 2005-10-06 for methods for preparing a bonding surface of a semiconductor wafer.
Invention is credited to Maleville, Christophe, Maunand Tussot, Corinne.
Application Number | 20050218111 10/875233 |
Document ID | / |
Family ID | 34944614 |
Filed Date | 2005-10-06 |
United States Patent
Application |
20050218111 |
Kind Code |
A1 |
Maleville, Christophe ; et
al. |
October 6, 2005 |
Methods for preparing a bonding surface of a semiconductor
wafer
Abstract
A method for preparing an oxidized surface of a first wafer for
bonding with a second wafer. The method includes treating the
oxidized surface using a mix of NH.sub.4OH/H.sub.2O.sub.2 to
increase the bonding energy between the first and second wafers.
The treatment parameters are chosen such that etching occurs that
is sufficient to remove isolated particles from the oxidized
surface, but that is sufficiently weak to smooth the surface
without creating rough patches thereon. Also described is a thin
layer removal process, which may advantageously be used to
fabricate a semiconductor on insulator structure.
Inventors: |
Maleville, Christophe; (La
Terrasse, FR) ; Maunand Tussot, Corinne; (Meylan,
FR) |
Correspondence
Address: |
WINSTON & STRAWN LLP
1700 K STREET, N.W.
WASHINGTON
DC
20006
US
|
Family ID: |
34944614 |
Appl. No.: |
10/875233 |
Filed: |
June 25, 2004 |
Current U.S.
Class: |
216/33 ; 216/83;
257/E21.568; 438/455 |
Current CPC
Class: |
H01L 21/76254
20130101 |
Class at
Publication: |
216/033 ;
216/083; 438/455 |
International
Class: |
B44C 001/22; H01L
021/302 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 30, 2004 |
FR |
0403273 |
Claims
What is claimed is:
1. A method for preparing an oxidized surface of a first wafer for
bonding with a second wafer, which comprises treating the oxidized
surface of the surface with a solution of
NH.sub.4OH/H.sub.2O.sub.2, wherein treatment parameters are chosen
to provide etching that is sufficient to remove isolated particles
from the oxidized surface but that is sufficiently weak to smooth
the surface without creating rough patches thereon to enable an
increased bonding energy between the first and second wafers when
those surfaces are bonded together compared to bonding without
treating the oxidized surface of the fist wafer.
2. The method of claim 1, which further comprises implanting atomic
species through the oxidized surface of the first wafer prior to
the treating.
3. The method of claim 1, wherein the treatment parameters include
at least one of a predetermined dose of chemical elements, a
predetermined temperature, or a predetermined duration for applying
the treatment.
4. The method of claim 1, wherein the treatment parameters are
chosen such that treating removes isolated surface particles having
an average diameter of more than about 0.1 micrometers.
5. The method of claim 1, wherein the treatment parameters are
chosen such that after treatment any rough patches that appear are
less than about 5 .ANG.RMS.
6. The method of claim 1, wherein the treatment parameters are
chosen such that after treatment any rough patches that appear are
less than about 4 .ANG.RMS.
7. The method of claim 1, wherein the etching occurs to a depth of
about 10 angstroms to about 120 angstroms.
8. The method of claim 1, wherein the etching occurs to a depth of
about 10 angstroms to about 60 angstroms.
9. The method of claim 1, wherein the solution provides a dose per
unit mass of NH.sub.4OH/H.sub.2O.sub.2 in the range from about 1/2
to about 1/1.
10. The method of claim 1, wherein the treating occurs at a
temperature in a range of between about 30.degree. C. and about
90.degree. C. and a cleaning duration of between about 1 and 6
minutes.
11. The method of claim 1, wherein the treatment parameters
comprise a dose per unit mass of NH.sub.4OH/H.sub.2O.sub.2 of about
1/2, a temperature of about 50.degree. C., and a cleaning duration
of about 3 minutes.
12. The method of claim 1, wherein the treatment parameters
comprise a dose per unit mass of NH.sub.4OH/H.sub.2O.sub.2 of about
1/2, a temperature of about 70.degree. C., and a cleaning duration
of about 3 minutes.
13. The method of claim 1, wherein the treatment parameters
comprise a dose per unit mass of NH.sub.4OH/H.sub.2O.sub.2 of about
3/4, a temperature of about 80.degree. C., and a cleaning duration
of about 3 minutes.
14. The method of claim 2, wherein the first wafer is a donor
wafer, the second wafer is a receiving wafer, and the atomic
species are implanted through the oxidized surface of the first
wafer to form a weakened zone at a predetermined depth to define a
thin layer for subsequent transfer; and the method further
comprises bonding the donor wafer to the receiver wafer; and
supplying energy to detach the thin layer from the donor wafer at
the weakened zone.
15. The method of claim 14, wherein the implanted atomic species
comprise at least one of hydrogen and helium ions.
16. The method of claim 14, further comprising conducting a thermal
oxidation step prior to treating the donor wafer.
17. The method of claim 14, wherein the thin layer and donor wafer
comprise a semiconductor-on-insulator structure.
Description
BACKGROUND ART
[0001] The present invention generally relates to the bonding of
two semiconductor wafers suitable for us in micro-electronics,
optics, or optronics applications. In particular, the invention
relates to preparing an oxidized bonding surface of at least one of
the wafers, wherein treatment parameters are chosen to provide
etching that is sufficient to remove isolated particles from the
oxidized surface but that is sufficiently weak to smooth the
surface without creating rough patches thereon.
[0002] Atomic species may be implanted through an oxidized surface
of a wafer to form a weakened area therein at a pre-set depth
beneath the oxidized surface (thus creating a film on the wafer
surface). It is then possible to detach the surface film from the
implanted wafer after it has been bonded to another substrate. An
example of such a detachment process is the SMART-CUT.RTM. process,
which is known to skilled person in the art (see
"Silicon-on-Insulator Technology, Materials to VLSI", 2nd edition,
by Jean-Pierre Colinge, published by Kluwer Academic Publishers,
pages 50 and 51), and which allows a film to be removed from a
wafer for transfer to another wafer. A semiconductor-on-insulator
structure such as an SOI (Silicon On insulator) structure can be
made in this manner by transferring a thin silicon film from a
donor wafer to a receiver wafer.
[0003] With the increase of miniaturization of electronic
components formed in semiconductor layers, manufacturers of
semiconductor-on-insulat- or substrates are increasingly asked to
make semiconductor-on-insulator structures that include thinner and
thinner semiconductor films. Thus,, it is vitally important to
improve the quality of a transferred layer and therefore to improve
removal techniques.
[0004] Consequently, the quality of the bond between the layer to
be transferred and the receiver substrate is essential in order to
ensure good removal, wherein the quality of the bond is mainly
measured by the bonding energy between the two wafers. To ensure
that the contact area of the two wafers to be joined is of good
quality, it is necessary to implement a cleaning operation to clean
at least one of the two bonding surfaces.
[0005] A trend in the prior art is to chemically treat the wafers
in stages prior to bonding. To clean the surface of a wafer of
oxidized or non-oxidized semi-conductor material, the known
technique is to use a treatment called RCA. The RCA treatment
includes a first bath of SC1(Standard Clean 1) solution that
includes ammonium hydroxide (NH.sub.4OH), hydrogen peroxide
(H.sub.2O.sub.2) and deionised water, and then a second bath of
SC2(Standard Clean 2)solution, which contains hydrochloric acid
(HCl), and hydrogen peroxide (H.sub.2O.sub.2), and deionised water.
The first bath is intended mainly for removing isolated particles
on the wafer surface and for removing particles buried in the
vicinity of the surface and to prevent them from resettling. The
SC2 solution mainly removes any metallic contamination that has
settled on the wafer surface that may, in particular, form
chlorides. However, after implementing such chemical treatments the
resulting surfaces have rough patches, which can, in some cases, be
more significant than that existing prior to treatment. Such rough
patches on the surface of the wafers alter the bonding energy of
the wafers all the more because they have a high RMS (Root Mean
Square) value in angstroms. The presence of isolated particles or
contaminants on the surface of the wafers can also be detrimental
to good bonding of the wafers when they are found at its interface.
After bonding, these particles which are enclosed at the bonding
interface, may cause surface blisters to form in the structure
obtained after using a SMART-CUT.RTM. detachment technique, and/or
cause surface blisters in areas not transferred between the area at
the level of which the species were implanted and the surface of
the structure. These blisters increase in size and/or grow during
any subsequent heat treatment, for example, a heat treatment used
after bonding to strengthen the bond.
[0006] A known solution for increasing separation of the isolated
particles is to conduct the chemical treatment while applying
megasonic waves. The megasonic waves cause the isolated particles
to vibrate and therefore to separate off. It is preferable,
however, to avoid implementing an additional process when cleaning
the wafers to avoid complicating the cleaning stage. Furthermore,
additional equipment would be required in order to generate the
megasounds.
SUMMARY OF THE INVENTION
[0007] Presented is a method for preparing an oxidized surface of a
first wafer for bonding with a second wafer. The method includes
treating the oxidized surface using a solution of
NH.sub.4OH/H.sub.2O.sub.2 to increase the bonding energy between
the subsequently bonded first and second wafers. The treatment
parameters are advantageously chosen to provide etching that is
sufficient to remove isolated particles from the oxidized surface
but that is sufficiently weak to smooth the surface without
creating rough patches thereon to enable an increased bonding
energy between the first and second wafers when those surfaces are
bonded together compared to bonding without treating the oxidized
surface of the fist wafer.
[0008] Advantageously, the treating is conducted after atomic
species are implanted through the oxidized surface.
[0009] In an embodiment, the treatment parameters of the method
include at least one of a predetermined dose of chemical elements,
a predetermined temperature, or a predetermined duration for
applying the treatment. These treatment parameters are
advantageously chosen such that treating removes isolated surface
particles having an average diameter of more than about 0.1
micrometers. In a beneficial implementation, the treatment
parameters are chosen such that after treatment any rough patches
that appear are less than about 5 .ANG.RMS. In a variation, the
treatment parameters are chosen such that after treatment any rough
patches that appear are less than about 4 .ANG.RMS.
[0010] Advantageously, the method limits the etching that occurs to
a depth of about 10 angstroms to about 120 angstroms, or to a depth
of about 10 angstroms to about 60 angstroms. The dose per unit mass
of NH.sub.4OH/H.sub.2O.sub.2 may beneficially be in the range from
about 1/2 to about 1/1, and treating may occur at a temperature in
a range of between about 30.degree. C. and about 90.degree. C. and
for a cleaning duration of between 1 and 6 minutes. In an
implementation, the treatment parameters comprise a dose per unit
mass of NH.sub.4OH/H.sub.2O.sub.2 of about 1/2, a temperature of
about 50.degree. C., and a cleaning duration of about 3 minutes. In
a variation, the treatment parameters comprise a dose per unit mass
of NH.sub.4OH/H.sub.2O.sub.2 of about 1/2, a temperature of about
70.degree. C., and a cleaning duration of about 3 minutes. In yet
another beneficial implementation, the treatment parameters
comprise a dose per unit mass of NH.sub.4OH/H.sub.2O.sub.2 of about
3/4, a temperature of about 80.degree. C., and a cleaning duration
of about 3 minutes.
[0011] In another aspect of the invention, the first wafer is a
donor wafer and the second wafer is a receiving wafer. The method
includes the step of removing a thin layer from the donor wafer and
transferring it to the receiving wafer. The atomic species are
implanted through the oxidized surface of the donor wafer to form a
weakened zone at a predetermined depth to define the thin layer,
and then the donor wafer surface is treated with the
NH.sub.4OH/H.sub.2O.sub.2 solution. The method also includes
bonding the donor wafer to the receiver wafer, and supplying energy
to detach the thin layer from the donor wafer at the level of the
weakened zone to transfer it to the receiving wafer.
[0012] In an advantageous embodiment, the implanted atomic species
comprise at least one of hydrogen and helium ions. The process also
beneficially includes conducting a thermal oxidation step prior to
treating the donor wafer. The structure that includes the thin
layer and donor wafer resulting from use of the process according
to the invention is advantageously a semiconductor-on-insulator
structure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Other aspects, purposes and advantages of the invention will
become clear after reading the following detailed description with
reference to the attached drawings, in which:
[0014] FIGS. 1a to 1d show the different stages in a SMART-CUT.RTM.
removal process.
[0015] FIG. 2 is a graph showing a plot of measurements of the
depths of etch in angstroms to the values of rough patches in RMS
angstroms on wafers after different cleaning operations.
[0016] FIG. 3 is a graph showing a plot of the same measurements as
those shown in FIG. 2, but used here to predict the resultant rough
patches due to more substantial cleaning operations.
[0017] FIG. 4 is a graph showing a plot of measurements of the
effectiveness of surface particle removal from a wafer as a
function of the depths of etch caused by the cleaning.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] The wafer cleaning process according to the invention may be
used with the thin layer removal method according to the
SMART-CUT.RTM. process. Referring to FIG. 1a, a first stage
includes oxidizing a semi-conductor wafer to create a donor wafer
10 having an oxide layer 11 on its surface. This oxidation process
may be native, or may be conducted under a heat treatment (i.e.
thermal oxidation), or by deposit of aggregates of SiO.sub.2.
[0019] With reference to FIG. 1b, the oxidized donor wafer 10 is
subjected to an implantation of atomic species through one of the
oxidized surfaces. The atomic species may be hydrogen and/or helium
ions. The atomic species used during implantation are dosed and are
implanted with a predetermined energy to form a weakened zone 15 at
a pre-set depth under the surface of the donor wafer 10. The
weakened zone 15 has a particular weakness relative to the rest of
the donor wafer 10. A film 16 is thus formed that is delimited by
the weakened zone 15 and the oxidized surface 12.
[0020] Referring to FIG. 1c, a receiver wafer 20 is brought into
contact with the oxidized surface 12 through which implantation has
taken place. Bonding by molecular adhesion takes place between the
surfaces that are brought into contact. An annealing stage may be
applied to reinforce the bonding interface. Next, sufficient
energy, such as heat and/or mechanical energy, is supplied to break
the weak bonds of the weakened zone 15. This causes detachment of
the thin film 16 from the donor wafer 10, thus forming the
semiconductor-on-insulator structure 30 shown in FIG. 1d. The thin
film 16 removed from the donor wafer 10 forms the semiconductor
part, and the subjacent oxide layer 17 forms the electrically
insulating part of the structure 30.
[0021] A finishing stage, using for example mechanical chemical
polishing, may then be implemented to minimize any defects and
rough patches that appeared when detaching the thin film. The final
structure may then be used in applications for micro-electronics,
optics or optronics. For example, it would be possible to form
components in the detached layer.
[0022] It is thus possible to make semiconductor-on-insulator
structures such as SOI, SGOI (Silicon Germanium on Insulator), SOQ
(Silicon on Quartz), GeOI (Germanium On Insulator) structures, an
alloy made of components belonging to the Group Ill-V on insulator
family; each having an insulating layer including the cleaned oxide
layer according to the invention introduced between the detached
layer and another wafer.
[0023] As shown above, the SMART-CUT.RTM. process may be used to
bond the donor wafer 10 to the receiver wafer 20, and the present
invention improves upon the overall process. One goal is to improve
the bonding between the two wafers 10 and 20, which can be achieved
by satisfying the following three objectives. First, remove
isolated particles from the bonding surface of at least one of the
wafers to reduce the appearance of post-bonding defects. Second;
reduce the size and the number of the rough patches on the wafer
surface to increase the contact areas of the bonding surfaces which
results in improving the bonding energy. Third, make the surfaces
hydrophilic. The three objectives can be achieved by utilizing a
simple, fast and cost-effective technique according to the
invention. Another goal is to create a semiconductor-on-insulator
structure 30 by using the SMART-CUT.RTM. process and incorporating
a stage according to the invention.
[0024] Another purpose is to control the preparation of an oxidized
surface 12 that has been subject to implantation for subsequent
bonding. It has been observed that such a surface is about 5 times
more sensitive to such preparation than if it had not been subject
to implantation. Consequently, it is important to accurately
calibrate and to correctly set preparation parameters.
[0025] The wafer to be cleaned may be made of any type of
semiconductor material. However, with regard to the following
discussions, the wafer material is silicon, which material has been
studied as described below. In an implementation, a wafer was
oxidized naturally (or has a native oxide) or artificially (for
example, the case of a thermally formed oxide). The invention
proposes a process for preparing a surface of the wafer for bonding
with another wafer, implementing at least one chemical treatment
stage that employs ammoniated chemical species mixed with molecules
of H.sub.2O.sub.2. In a preferred embodiment, such chemical species
are supplied in a moist medium. The chemical species are, for
example, diluted in de-ionized water. An ammoniated solution of
this kind is also called an SC1 solution.
[0026] Cleaning by means of an SC1 solution results in the
following effects (obtained by the chemical action of this
solution). The surface is etched, making it possible to dig under
the particles and thus to "strip" them (otherwise known as a
"lift-off" effect). An opposite electric potential between the
surface and the particles is created, linked directly to the high
pH of the solution which causes detachment of isolated particles.
The opposite electrical potential prevents migration of particles
from the bath to the plate. This cleaning is therefore linked
particularly to the high pH of the ammoniated solution, including
as a result a significant concentration of OH- ions in solution.
During the etching of the oxide by the ammonia, these ions react
with the pendant bonds generated on the surface and saturate them
in SiOH termination. This layer of SiOH formed on the surface then
creates the repelling opposite potential, which detaches particles
bonded to the surface (in other words the isolated particles) and
prevents them from resettling. These surface SiOH bonds will also
be the point of insertion of water molecules on the surface of the
wafer, thus causing a hydrophilic condition. This hydrophilic
condition improves the bonding with another wafer.
[0027] With reference to FIG. 2, the results are shown of a study
conducted to find the relationship between the thickness of etched
materials (etch depth) by different SC1 solutions, to the rough
patches that are present and measured on the wafer surface. The
etch depths were measured by reflectometry, and the rough patches
were measured using an AFM (Atomic Force Microscope), on oxidized
silicon wafers that are and have been subject to ion
implantation.
[0028] The level of particle removal is determined by measurements
taken prior to and following each SC1 treatment. Measurements were
taken by reflectometry, typically by using a laser adjusted to a
pre-set light spectrum, to about 0.13 microns. This value is
constitutes the average diameter of the smallest particles
detectable by reflectometry. The x-coordinate of the graph in FIG.
2 shows the etch depths obtained with different SC1 solutions,
expressed in angstroms. The y-coordinates of the graph in FIG. 2
show the values of rough patches measured on the wafer for the
different etches carried out on the wafers, and these rough patch
values are expressed in RMS angstroms. The rough patches are
presented as a function of the etches implemented on the wafer
surface, and are shown on the graph by black dots.
[0029] A first result of the measurement is that the average
roughness increases with the etch depth. A second result is that a
roughly linear relationship was obtained between the etch depth and
the roughness values.
[0030] Referring to FIG. 3, a linear extension of the curve 1 of
FIG. 2 is shown that takes the substantially linear relationship
between the etch depth and the roughness values noted above into
account. The result is the curve 2 shown in FIG. 3. Using this
linear extension of the curve 1, and knowing that a maximum pre-set
roughness value beyond which the bonding energy becomes
insufficient exists, it is then possible to deduce and predict the
maximum depth of etch that is associated with it, beyond which the
bonding energy becomes insufficient. In an implementation, the
maximum roughness value was set at about 5 RMS angstroms, in
compliance, for example, with the results of measurements disclosed
in "Detailed characterization of wafer bonding mechanisms", C.
Malleville et al., published by Electromechanical Society
Proceedings, volume 97-36, page 50, section 3. It has been shown
that for a roughness value above about 5 RMS angstroms the bonding
energy may be drastically reduced. Consequently, with reference to
FIG. 3, it may be deduced that the maximum etch depth is around 120
angstroms.
[0031] Wafer bonding, when applied to making an SOI structure by
using the SMART-CUT.RTM. technique, requires a bond strength that
is sufficient, and in particular much greater, than the
implantation force of the buried (having been implanted) atomic
species. This is achieved experimentally with regard to rough
patches of less than about 4 RMS angstroms, thus reducing (again
with reference to FIG. 3) the maximum depth etch to around 60 to 70
angstroms. These measurements underscore the need to restrict as
far as possible the etching action on the wafer surface, with a
maximum limit on the etch depth that must not be exceeded.
[0032] FIG. 4 depicts another study undertaken to find
relationships between the efficiency of surface particle removal
from the wafer, and the etch depth of the wafer when using
different SC1 solutions. When these measurements were taken, the
wafers were deliberately contaminated by depositing a pre-set
number of isolated particles, which represented the particles to be
removed. The efficiency of removal of these particles was measured
by taking LPD (Light Point Defect) measurements on the surfaces of
different wafers that had been deliberately contaminated in a
similar manner. An LPD is a defect that is detectable by laser
light scattering optical measurements. An LPD defect is also known
as a "highlight".
[0033] An LPD measurement is made by illuminating the wafer surface
using an incident optical wave emitted by the laser source. The
light scattered by the LPD defects present on the surface is
detected by means of an optical detector. The light scattering on
the wafer surface can be correlated with the number of residual
particles on the wafer surface, and thus light scattering
measurements provide information on the number of residual
particles. Other residual particle measurement techniques may be
implemented, alone or in combination with the LPD measurements.
[0034] Etch depth is typically measured by using reflectometry, in
substantially the same way as that used to measure the rough
patches as explained above with reference to FIG. 2. The
x-coordinate of FIG. 4, in the same way as that for FIGS. 2 and 3,
shows the different etch depths effected by means of different SC1
solutions, expressed in angstroms. The y-coordinate shows isolated
particle removal efficiencies, expressed as a percentage relative
to the estimated total number of present isolated particles on the
wafer surface. Particle removal efficiency measurements as a
function of the etch depths are shown on the graph as black
dots.
[0035] FIG. 4 illustrates that beyond an etch depth of about 10
angstroms, particle removal efficiency is close to 100%. In
contrast, below the value of about 10 angstroms, particle removal
is much less impressive, having an efficiency of around 50% to 60%.
Thus, for etches of less than about 10 angstroms, particle removal
is insufficient for allowing good bonding conditions. This means
that if the etched thickness is too small, the particles are no
longer separated from the surface and their removal efficiency
falls very quickly.
[0036] Optionally, it is possible to simultaneous use an SC1 bath
and to apply megasounds to help separate the particles from the
surface.
[0037] It is noted again, that an oxidized surface that has been
subject to implantation is particularly sensitive to chemical
treatments. This sensitivity is about 5 times greater than that of
the same type of surface that has not been subject to implantation.
Thus, the implementation and the calibration of the chemical
treatment must be carefully conducted.
[0038] The measurements discussed above with reference to FIGS. 2
and 4 make it possible to evaluate the desired etch depth when the
wafer to be cleaned will be brought into the presence of an SC1
solution. The etch depth is bound to be located in the range
between about 10 angstroms and about 120 angstroms, or between
about 10 angstroms and about 60 angstroms in an embodiment using an
SOI structure formed by using the SMART-CUT.RTM. technique. Within
this authorized range of etch depths, a considerable number of
experiments were conducted to attempt to optimize etch conditions
using SC1 solutions, with a view to further increasing the
post-cleaning bonding energy. These etch results typically employed
a dosing per unit mass of NH.sub.4OH/H.sub.2O.sub.2 in the range
from about 1/2 to about 4/4 or 1/1, temperatures in a range of from
about 30.degree. C. to about 80.degree. C., and etch durations of
from about a few seconds to several hours. Generally, the
parameters are chosen so that the cleaning duration is relatively
short, on the order of between about 1 and 6 minutes.
[0039] The following Table 1 lists some conditions wherein cleaning
by using SC1 proved to be particularly impressive:
1 TABLE 1 % per unit mass SC1 NH.sub.4OH/H.sub.2O.sub.2 T(C)
Cleaning time 1/2 50 3 min {fraction (2/4)} (or 1/2) 70 3 min 3/4
80 3 min
[0040] In particular, if a % per unit mass
NH.sub.4OH/H.sub.2O.sub.2 equal to approximately 1/2 is used at a
temperature of about 70.degree. C., and with a cleaning time of
about 3 minutes, then an etch of about 20 angstroms was obtained.
This resulted in a roughness value of about 3 RMS angstroms, and a
level of particle removal of more than about 90%, thus attaining an
optimum bonding energy.
[0041] Optionally, one or more cleaning stages may precede or
follow the previous cleaning stage. In this manner, an SC2
treatment is advantageously implemented subsequent to the SC1
treatment. The SC2 treatment may be conducted with a solution
comprising a mix of HCl and of H.sub.2O.sub.2. This treatment is
typically applied at temperatures of between about 70.degree. C.
and about 80.degree. C. The action of the SC2 solution makes it
possible to remove mainly metal contaminants from the wafer
surface.
[0042] After cleaning at least one of the two oxidized bonding
surfaces of the two wafers that are to be bonded, the wafers are
brought into close contact with each other. Oxidized wafer cleaning
thus makes it possible to restrict a sizeable number of large-size
particles and to avoid defects that would result in a downgrade of
the wafers. Wafers are downgraded when the bonding energy is not
sufficient to obtain non-defective final structures. The two wafers
10 and 20 (see FIG. 1c) are advantageously brought into contact
just after cleaning, without any intermediate treatment stage. The
two wafers can be bonded by adhesion of the molecules present on
their bonding surfaces. This adhesive property is explained mainly
by the hydrophilic properties present on the wafer surface. In
particular, water molecules are present on the wafer surface which
give rise to Si-OH bonds and to water diffusion in the vicinity of
the wafer surfaces. The Si-OH bonds of a wafer bonding surface are
linked via hydrogen bonds to the surface of the other wafer, thus
forming a bond strength between the two wafers 10 and 20 that is
sufficiently significant to create a sufficient binding adhesion.
It is then advantageous to apply a heat treatment to increase the
bonds between the two wafers 10 and 20. This heat treatment may be
applied at one or more pre-set temperatures and for a pre-set
period of time to optimize the bonding efficiency and to avoid
creating structural defects on the wafer surface. The heat
treatment causes the disappearance of a large part of the Si-OH
bonds to the benefit of the covalent Si-O-Si (stronger) bonds.
[0043] With reference to the FIGS. 1c and 1d, after bonding of two
wafers, a thin film 16 is detached at the level of a weakened zone
15 to form the structure 30. The detachment step may be imperfect
if, for example, non-transferred areas appear that result from the
presence of intervening particles at the bonding interface that
were imprisoned during bonding. These apparent defects may be
accentuated or created during a subsequent heat treatment such as a
heat treatment to solidify the bonding interface. Such defects are
reduced as much as possible by the cleaning stage according to the
invention, which is implemented prior to the bonding step. The SC1
chemical treatment is carried out under conditions and in
accordance with treatment parameters chosen to maximally reduce the
number of isolated particles at the bonding interface, while
reducing interfacial rough patches as much as possible. The SC1
treatment also takes into account the particular etching
sensitivity of an oxidized surface that has been subject to
implantation.
[0044] The present invention relates to preparing the surface of
oxidized wafers of any kind of material relating to the field of
semi-conductors. Thus, any material belonging to atomic Group IV
family such as silicon or a Silicon-Germanium alloy, and extending
also to other types of alloys of the Group IV-IV, Group III-V or
Group II-VI family. It should also be understood that these alloys
may be binary, ternary, quaternary or of higher degree.
* * * * *