U.S. patent application number 15/499011 was filed with the patent office on 2017-08-10 for method for permanently bonding wafers.
This patent application is currently assigned to EV GROUP E. THALLNER GmbH. The applicant listed for this patent is EV GROUP E. THALLNER GmbH. Invention is credited to Christoph Flotgen, Kurt Hingerl, Thomas Plach, Markus Wimplinger.
Application Number | 20170229423 15/499011 |
Document ID | / |
Family ID | 44625808 |
Filed Date | 2017-08-10 |
United States Patent
Application |
20170229423 |
Kind Code |
A1 |
Plach; Thomas ; et
al. |
August 10, 2017 |
METHOD FOR PERMANENTLY BONDING WAFERS
Abstract
This invention relates to a method for bonding of a first
contact surface of a first substrate to a second contact surface of
a second substrate with the following steps, especially the
following sequence: forming a first reservoir in a surface layer on
the first contact surface and a second reservoir in a surface layer
on the second contact surface, the surface layers of the first and
second contact surfaces being comprised of respective native oxide
materials of one or more second educts respectively contained in
reaction layers of the first and second substrates, partially
filling the first and second reservoirs with one or more first
educts; and reacting the first educts filled in the first reservoir
with the second educts contained in the reaction layer of the
second substrate to at least partially strengthen a permanent bond
formed between the first and second contact surfaces.
Inventors: |
Plach; Thomas; (Linz,
AT) ; Hingerl; Kurt; (Wolfern, AT) ;
Wimplinger; Markus; (Ried im Innkreis, AT) ; Flotgen;
Christoph; (Pramerdorf, AT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
EV GROUP E. THALLNER GmbH |
St. Florian am Inn |
|
AT |
|
|
Assignee: |
EV GROUP E. THALLNER GmbH
St. Florian am Inn
AT
|
Family ID: |
44625808 |
Appl. No.: |
15/499011 |
Filed: |
April 27, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14007999 |
Sep 27, 2013 |
|
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PCT/EP11/55470 |
Apr 8, 2011 |
|
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15499011 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 2924/10333
20130101; H01L 2924/10335 20130101; H01L 2224/8309 20130101; H01L
21/187 20130101; H01L 24/27 20130101; H01L 2224/83896 20130101;
H01L 2924/1082 20130101; H01L 2924/10328 20130101; H01L 2924/10332
20130101; H01L 2924/10336 20130101; H01L 24/29 20130101; H01L
2924/20106 20130101; H01L 21/3105 20130101; H01L 2924/1032
20130101; H01L 21/2007 20130101; H01L 2924/1033 20130101; H01L
2224/29188 20130101; H01L 2924/10823 20130101; H01L 2224/27444
20130101; H01L 2924/01013 20130101; H01L 21/0223 20130101; H01L
2924/10329 20130101; H01L 2224/278 20130101; H01L 2224/32145
20130101; H01L 2924/10346 20130101; H01L 24/83 20130101; H01L
2924/10331 20130101; H01L 24/32 20130101; H01L 21/76251 20130101;
H01L 2924/10252 20130101; H01L 21/02255 20130101; H01L 2924/10334
20130101; H01L 2924/10821 20130101 |
International
Class: |
H01L 23/00 20060101
H01L023/00 |
Claims
1. A method for bonding of a first contact surface of a first
substrate to a second contact surface of a second substrate
comprising the following steps: forming a first reservoir in a
surface layer on the first contact surface and a second reservoir
in a surface layer on the second contact surface, the surface
layers of the first and second contact surfaces being comprised of
respective native oxide materials of one or more second educts
respectively contained in reaction layers of the first and second
substrates, the second educts being selected from the group
consisting of Ge, Al, GaP, GaAs, InP, InSb, InAs, GaSb, GaN, InN,
Al.sub.xGa.sub.1-xAs, In.sub.xGa.sub.1-xN, InAlP, CuInSe.sub.2,
CuInGaSe.sub.2, CuInGaS.sub.2, and In.sub.2-xSn.sub.2-xO.sub.3-y;
partially filling the first and second reservoirs with one or more
first educts; and reacting the first educts filled in the first
reservoir with the second educts contained in the reaction layer of
the second substrate to at least partially strengthen a permanent
bond formed between the first and second contact surfaces.
2. The method as claimed in claim 1, wherein the reacting takes
place by diffusion of the first educts of the first reservoir into
the reaction layer of the second substrate.
3. The method as claimed in claim 1, wherein the reacting takes
place at a temperature between room temperature and 200.degree. C.,
during a maximum 12 day period.
4. The method as claimed in claim 1, wherein the permanent bond has
a bond strength of greater than 1.5 J/m.sup.2.
5. The method as claimed in claim 1, wherein a reaction product is
formed in the reaction layer of the second substrate during the
reacting, said reaction product having a greater molar volume than
a molar volume of the second educts contained in the reaction layer
of the second substrate.
6. The method as claimed in claim 1, wherein the reservoirs are
formed by plasma activation.
7. The method as claimed in claim 1, wherein the surface layer of
said first contact surface is comprised of an amorphous material
produced by thermal oxidation.
8. The method as claimed in claim 1, wherein a growth layer is
between the second contact surface and the reaction layer of the
second substrate, said growth layer being comprised of the native
oxide material of the second educts contained in the reaction layer
of the second substrate.
9. The method as claimed in claim 8, wherein before the reacting,
the growth layer and/or the surface layer has an average thickness
"A" between 1 angstrom and 10 nm.
10. The method as claimed in claim 1, wherein one of said first
reservoir and said second reservoir is formed in a vacuum.
11. The method as claimed in claim 1, wherein the reservoirs are
filled by one or more of the following steps: exposing the first
contact surface to an atmosphere, exposing the first contact
surface to at least one fluid selected from the group consisting of
deionized H.sub.2O and H.sub.2O.sub.2, and exposing the first
contact surface to at least one gas selected from the group
consisting of N.sub.2, O.sub.2, and Ar with an ion energy in the
range from 0 to 2000 eV.
12. The method as claimed in claim 1, wherein the permanent bond is
additionally strengthened by a reaction of the first educts filled
in the second reservoir with the second educts contained in the
reaction layer of the first substrate.
13. The method as claimed in claim 1, wherein an average distance
(B) between the first reservoir and the reaction layer of the
second substrate immediately before the reacting is between 0.1 nm
and 15 nm.
14. The method as claimed in claim 1, wherein the first reservoir
and the second reservoir are dimensioned to hold the first educts,
wherein portions of the first educts held by the first and second
reservoirs respectively react with the reaction layers of the
second and first substrates, and wherein remaining portions of the
first educts held by the first and second reservoirs do not
respectively react with the reaction layers of the second and first
substrates and remain within the first and second reservoirs to
hinder a formation of bubbles therein.
15. The method as claimed in claim 1, wherein the second educts are
selected from the group consisting of Ge and Al.
16. A method of bonding a first contact surface of a first
substrate to a second contact surface of a second substrate
comprising the following steps: forming a first reservoir in a
surface layer on the first contact surface and a second reservoir
in a surface layer on the second contact surface, the surface
layers of the first and second contact surfaces being comprised of
respective native oxide materials of one or more second educts
respectively contained in reaction layers of the first and second
substrates, the second educts being selected from the group
consisting of Ge, Al, GaP, GaAs, InP, InSb, InAs, GaSb, GaN, InN,
Al.sub.xGa.sub.1-xAs, In.sub.xGa.sub.1-xN, InAlP, CuInSe.sub.2,
CuInGaSe.sub.2, CuInGaS.sub.2, and In.sub.2-xSn.sub.xO.sub.3-y;
partially filling the first and second reservoirs with one or more
first educts; forming a prebond connection between the first and
second contact surfaces by bringing one or more portions of the
first contact surface into contact with one or more portions of the
second contact surface such that gaps are formed between the first
and second contact surfaces at areas located between the respective
contacted portions of the first and second contact surfaces; and
reacting the first educts filled in the first reservoir with the
second educts contained in the reaction layer of the second
substrate to form a reaction product to at least partially
strengthen a permanent bond formed between the first and second
contact surfaces, the reaction product being formed between the
reaction layer of the second substrate and the surface layer of the
second substrate, the reaction serving to bulge the surface layer
of the second substrate toward the first contact surface to close
the gaps and at least partially strengthen the permanent bond, the
reaction serving to deform a portion of the reaction layer of the
second substrate into the reaction product.
17. The method as claimed in claim 16, wherein the permanent bond
has a bond strength which is at least twice a strength of the
prebond connection.
18. The method as claimed in claim 16, wherein the areas in which
the gaps are formed are filled with the bulged surface layer.
19. The method as claimed in claim 16, wherein the permanent bond
is additionally strengthened by a reaction of the first educts
filled in the second reservoir with the second educts contained in
the reaction layer of the first substrate.
20. The method as claimed in claim 16, wherein the second educts
are selected from the group consisting of Ge and Al.
21. A method of bonding a first substrate to a second substrate,
the first and second substrates being respectively comprised of
first and second reaction layers, the method comprising the
following steps: producing first and second surface layers
respectively on the first and second reaction layers by reacting
the first and second reaction layers with a first educt, the first
and second surface layers having molar volumes that are
respectively greater than molar volumes of the first and second
reaction layers, the first and second surface layers respectively
comprising first and second contact surfaces; the first and second
reaction layers being comprised of one or more second educts;
forming first and second reservoirs respectively in the first and
second surface layers; filling the first and second reservoirs with
the first educt; and bringing a plurality of portions of the first
contact surface into contact with a plurality of portions of the
second contact surface to cause the first educt from the first and
second reservoirs to respectively diffuse through the second and
first surface layers and react with the second and first reaction
layers, thereby bringing all of the portions of the first contact
surface into contact with all of the portions of the second contact
surface and permanently bonding the first substrate to the second
substrate.
22. The method as claimed in claim 20, wherein the second educts
are selected from the group consisting of Ge, Al, GaP, GaAs, InP,
InSb, InAs, GaSb, GaN, InN, Al.sub.xGa.sub.1-xAs,
In.sub.xGa.sub.1-xN, InAlP, CuInSe.sub.2, CuInGaSe.sub.2,
CuInGaS.sub.2, and In.sub.2-xSn.sub.xO.sub.3-y.
23. The method as claimed in claim 22, wherein the second educts
are selected from the group consisting of Ge and Al.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 14/007,999, filed Sep. 27, 2013, which is a U.S. National Stage
Application of PCT/EP2011/055470 filed Apr. 8, 2011, said patent
applications hereby fully incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates to a method for bonding of a first
contact surface of a first substrate to a second contact surface of
a second substrate.
BACKGROUND OF THE INVENTION
[0003] The objective in permanent or irreversible bonding of
substrates is to produce an interconnection, which is as strong and
especially as irreversible as possible, i.e., a high bond force,
between the two contact surfaces of the substrates. There are
various approaches and production methods for this purpose in the
prior art.
[0004] The known production methods and the approaches which have
been followed to date often lead to results which cannot be
reproduced or can be poorly reproduced and which can hardly be
applied especially to altered conditions. In particular, production
methods which are used at present often use high temperatures,
especially >400.degree. C., in order to ensure reproducible
results.
[0005] Technical problems such as high energy consumption and a
possible destruction of structures which are present on the
substrates result from the high temperatures, to some extent far
above 300.degree. C., which have been necessary to date for a high
bond force.
[0006] Other demands include the following: [0007]
front-end-of-line compatibility.
[0008] This is defined as the compatibility of the process during
the production of the electrically active components. In this
respect, the bonding process must be designed such that active
components such as transistors, which are already present on the
structure wafers, are neither adversely affected nor damaged during
the processing. Compatibility criteria include mainly the purity of
certain chemical elements (mainly in CMOS structures) and
mechanical loadability, mainly by thermal stresses. [0009] low
contamination. [0010] no application of force. [0011] temperature
as low as possible, especially for materials with different
coefficients of thermal expansion.
[0012] The reduction of the bond force leads to more careful
treatment of the structure wafer and thus to a reduction of the
failure probability by direct mechanical loading.
SUMMARY OF THE INVENTION
[0013] The object of this invention is therefore to devise a method
for careful production of a permanent bond with a bond force which
is as high as possible and, at the same time at a temperature which
is as low as possible.
[0014] This object is achieved with the features of the claims.
Advantageous developments of the invention are given in the
dependent claims. All combinations of at least two of the features
given in the specification, the claims and/or the figures also fall
within the framework of the invention. At the given value ranges,
values within the indicated limits will also be considered to be
disclosed as boundary values and will be claimed in any
combination.
[0015] The present invention provides a reservoir for holding a
first educt on at least one of the substrates, which educt reacts
after making contact, or producing a temporary bond between the
substrates, with a second educt which is present in the other
substrate, and which thus forms an irreversible or permanent bond
between the substrates. Before or after forming the reservoir in
one surface layer on the first contact surface, generally cleaning
of the substrate or substrates occurs, preferably by a flushing
step. This cleaning should generally ensure that there are no
particles on the surfaces which would result in unbonded sites. The
surface layer as claimed in the invention is comprised mostly of a
native material, especially an oxide material, preferably of a
native silicon dioxide. A layer of native material can be made
especially thin so that the reactions provided as claimed in the
invention (first educt or first group with a second educt or a
second group), especially diffusion processes, due to the reduced
distances between the reaction partners, can proceed especially
promptly. On the opposing second contact surface as claimed in the
invention there can be a growth layer in which the deformation as
claimed in the invention takes place and the first educt (or the
first group) reacts with the second educt (or the second group)
present in the reaction layer of the second substrate. To
accelerate the reaction between the first educt (or the first
group) and the second educt (or the second group) it can be
provided as claimed in the invention that the growth layer located
between the reaction layer of the second substrate and the
reservoir be thinned before the substrates make contact since in
this way the distance between the reaction partners is reduced and
at the same time the deformation/formation of the growth layer as
claimed in the invention is promoted. The growth layer is removed
at least partially, especially mostly, preferably completely, by
the thinning. The growth layer grows again in the reaction of the
first educt with the second educt even if it has been completely
removed.
[0016] As claimed in the invention there can be means for
inhibiting the growth of the growth layer before the contact
surfaces make contact, especially by passivation of the reaction
layer of the second substrate, preferably by exposure to N.sub.2,
forming gas or an inert atmosphere or under a vacuum or by
amorphization. In this connection treatment with plasma which
contains forming gas, especially consists largely of forming gas,
has proven especially suitable. Here forming gas is defined as
gases which contain at least 2%, better 4%, ideally 10% to 15%
hydrogen. The remaining portion of the mixture consists of an inert
gas such as for example nitrogen or argon.
[0017] Alternatively or in addition to this measure, it is
advantageous as claimed in the invention to minimize the time
between the thinning and the formation of the reservoir/reservoirs
and the contact-making, especially <2 hours, preferably <30
minutes, even more preferably <15 minutes, ideally <5
minutes.
[0018] The diffusion rate of the educts through the growth layer is
increased by the growth layer (which has been optionally thinned
and thus is very thin at least at the beginning of the formation of
the permanent bond or at the start of the reaction). This leads to
a shorter transport time of the educts at the same temperature.
[0019] The reservoir and the educt contained in the reservoir
create the technical possibility of inducing a reaction which
increases the bonding speed and strengthens the permanent bond
directly on the contact surfaces after producing the temporary or
reversible bond in a controlled manner, especially by deforming at
least one of the contact surfaces by the reaction, preferably the
contact surface opposite the reservoir.
[0020] The formation of the reservoir, especially by plasma
activation, is chosen as claimed in the invention such that bubble
formation is avoided. Preferably for plasma activation, ions of gas
molecules are used which are at the same time suitable for reaction
with the second educt, especially correspond to the first educt.
This results in that possible byproducts which could arise in the
reaction of the first educt with the second educt are avoided.
[0021] The size of the reservoir is set as claimed in the invention
such that pores on the contact surface between the substrates can
be closed as completely as possible by means of the growth of the
growth layer. That is, the reservoir must be large enough to be
able to hold enough of the first educt in order to be able to
produce therewith a relatively thick/voluminous growth layer by
reaction of the first educt with the second educt which is present
in the reaction layer. The size must however be small enough to
accommodate as little excess first educt as possible which cannot
react with the reaction layer. This largely prevents or precludes
bubble formation.
[0022] In-situ processing is advantageously carried out to
intercalate an educt as pure as possible in the reservoir, and
nonreacting species are excluded as much as possible.
[0023] For the prebonding step, for producing a temporary or
reversible bond between the substrates there are various
possibilities with the objective of producing a weak interaction
between the contact surfaces of the substrates. The prebond
strengths are below the permanent bond strengths, at least by a
factor of 2 to 3, especially by a factor of 5, preferably by a
factor of 15, still more preferably by a factor of 25. As guideline
values the prebond strengths of pure, nonactivated, hydrophilized
silicon with roughly 100 mJ/m.sup.2 and of pure, plasma-activated
hydrophilized silicon with roughly 200-300 mJ/m.sup.2 are
mentioned. The prebonds between the molecule-wetted substrates
arise mainly due to the van-der-Waals interactions between the
molecules of the different wafer sides. Accordingly, mainly
molecules with permanent dipole moments are suitable for enabling
prebonds between wafers. The following chemical compounds are
mentioned as interconnect agents by way of example, but not limited
thereto [0024] water [0025] thiols [0026] AP3000 [0027] silanes
and/or [0028] silanols.
[0029] Suitable substrates as claimed in the invention are those
whose material is able to react as an educt with another supplied
educt to form a product with a higher molar volume, as a result of
which the formation of a growth layer on the substrate is caused.
The following combinations are especially advantageous, to the left
of the arrow the educt being named and to the right of the arrow,
the product/products, without the supplied educt or byproducts
which react with the educt being named in particular: [0030]
Si.fwdarw.SiO.sub.2, Si.sub.3N.sub.4, SiN.sub.xO.sub.y [0031]
Ge.fwdarw.GeO.sub.2, Ge.sub.3N.sub.4 [0032] .alpha.-Sn=SnO.sub.2
[0033] B.fwdarw.B.sub.2O.sub.3, BN [0034] Se.fwdarw.SeO.sub.2
[0035] Te.fwdarw.TeO.sub.2, TeO.sub.3 [0036] Mg.fwdarw.MgO,
Mg.sub.3N.sub.2 [0037] Al.fwdarw.Al.sub.2O.sub.3, AlN [0038]
Ti.fwdarw.TiO.sub.2, TiN [0039] V.fwdarw.V.sub.2O.sub.5 [0040]
Mn.fwdarw.MnO, MnO.sub.2, Mn.sub.2O.sub.3, Mn.sub.2O.sub.7,
Mn.sub.3O.sub.4 [0041] Fe.fwdarw.FeO, Fe.sub.2O.sub.3,
Fe.sub.3O.sub.4 [0042] Co.fwdarw.COO, Co.sub.3O.sub.4, [0043]
Ni.fwdarw.NiO, Ni.sub.2O.sub.3 [0044] Cu.fwdarw.CuO, Cu.sub.2O,
Cu.sub.3N [0045] Zn.fwdarw.ZnO [0046] Cr.fwdarw.CrN,
Cr.sub.23C.sub.6, Cr.sub.3C, Cr.sub.7C.sub.3, Cr.sub.3C.sub.2
[0047] Mo.fwdarw.Mo.sub.3C.sub.2 [0048] Ti.fwdarw.TiC [0049]
Nb.fwdarw.Nb.sub.4C.sub.3 [0050] Ta.fwdarw.Ta.sub.4C.sub.3 [0051]
Zr.fwdarw.ZrC [0052] Hf.fwdarw.HfC [0053] V.fwdarw.V.sub.4C.sub.3,
VC [0054] W.fwdarw.W.sub.2C, WC [0055] Fe.fwdarw.Fe.sub.3C,
Fe.sub.7C.sub.3, Fe.sub.2C
[0056] The following mixed forms of semiconductors are also
conceivable as substrates: [0057] III-V: GaP, GaAs, InP, InSb,
InAs, GaSb, GaN, AlN, InN, Al.sub.xGa.sub.1-xAs,
In.sub.xGa.sub.1-xN [0058] IV-IV: SiC, SiGe, [0059] III-IV: InAlP,
[0060] nonlinear optics: LiNbO.sub.3, LiTaO.sub.3, KDP
(KH.sub.2PO.sub.4) [0061] solar cells: CdS, CdSe, CdTe,
CuInSe.sub.2, CuInGaSe.sub.2, CuInS.sub.2, CuInGaS.sub.2 [0062]
conductive oxides: In.sub.2-xSnxO.sub.3-y
[0063] As claimed in the invention, on at least one of the wafers
and directly on the respective contact surface there is the
reservoir (or reservoirs) in which a certain amount of at least one
of the supplied educts for the volume expansion reaction can be
stored. Educts can therefore be for example O.sub.2, O.sub.3,
H.sub.2O, N.sub.2, NH.sub.3, H.sub.2O.sub.2, etc. Due to the
expansion, especially dictated by oxide growth, based on the
tendency of the reaction partners to reduce system energy, possible
gaps, pores, and cavities between the contact surfaces are
minimized and the bond force is increased accordingly by narrowing
the distances between the substrates in these regions. In the best
possible case the existing gaps, pores and cavities are completely
closed so that the entire bonding area increases and thus the bond
force as claimed in the invention rises accordingly.
[0064] The contact surfaces conventionally show a roughness with a
quadratic roughness (R.sub.q) of 0.2 nm. This corresponds to
peak-to-peak values of the surfaces in the range of 1 nm. These
empirical values were determined with Atomic Force Microscopy
(AFM).
[0065] The reaction as claimed in the invention is suitable for
allowing the growth layer to grow by 0.1 to 0.3 nm for a
conventional wafer surface of a circular wafer with a diameter of
200 to 300 mm with 1 monolayer (ML) of water.
[0066] As claimed in the invention it is therefore provided that at
least 2 ML, preferably at least 5 ML, even more preferably at least
10 ML of fluid, especially water, be stored in the reservoir.
[0067] The formation of the reservoir by exposure to plasma is
especially preferable, since plasma exposure causes smoothing of
the contact surface and hydrophilization as synergy effects. The
surface is smoothed by plasma activation predominantly by a viscous
flow of the material of the surface layer. The increase of the
hydrophilicity takes place especially by the increase of the
silicon hydroxyl compounds, preferably by cracking of Si--O
compounds present on the surface, such as Si--O--Si, especially
according to the following reaction:
Si--O--Si+H.sub.2O.rarw..fwdarw.2SiOH
[0068] Another side effect, especially as a result of the
aforementioned effects, consists in that the prebond strength is
improved especially by a factor of 2 to 3.
[0069] The reservoir in the surface layer on the first contact
surface of the first substrate (and optionally of a second
reservoir in the surface layer on the second contact surface of the
second substrate) is formed for example by plasma activation of the
first substrate which has been coated with a native oxide,
especially silicon dioxide. Advantageously the second surface of
the second surface is also activated or an additional reservoir is
formed to which the features described for the first reservoir
apply analogously. The plasma activation is carried out in a vacuum
chamber in order to be able to adjust the conditions necessary for
the plasma. As claimed in the invention, for the plasma discharge,
N.sub.2 gas, O.sub.2 gas or argon gas with ion energies in the
range from 0 to 2000 eV is used, as a result of which a reservoir
is produced with a depth of up to 10 nm, preferably up to 5 nm,
more preferably up to 3 nm, of the treated surface, in this case
the first contact surface. As claimed in the invention, any
particle type, atoms and/or molecules which are suitable for
producing the reservoir can be used. Preferably those atoms and/or
molecules are used that produce the reservoir with the required
properties. The relevant properties are mainly the pore size, pore
distribution and pore density. Alternatively, as claimed in the
invention gas mixtures such as for example air or forming gas
consisting of 95% Ar and 5% H.sub.2 can be used. Depending on the
gas used, in the reservoir during the plasma treatment among others
the following ions are present: N+, N.sub.2+, O+, O.sub.2+, Ar+.
The first educt can be accommodated in the unoccupied free space of
the reservoir/reservoirs. The surface layer and accordingly the
reservoir can extend into the reaction layer.
[0070] Advantageously there are different types of plasma species
which can react with the reaction layer and comprise at least
partially, preferably mostly of the first educt. To the extent the
second educt is Si/Si, an O, plasma species would be
advantageous.
[0071] Advantageously, oxygen ions are mainly used since they can
react with Si to form silicon oxide and therefore do not bond again
into oxygen molecules. The preferred bonding to silicon prevents
oxygen gas from leading to bubble formation after the bond process.
Analogous considerations apply to the other substrate-gas
combinations. Generally therefore, the ion species which can bond
more easily in the system and which has a very low tendency, or
none at all, to pass into the gaseous state is always
preferred.
[0072] The reservoir is formed based on the following
considerations: The pore size is smaller than 10 nm, preferably
smaller than 5 nm, more preferably smaller than 1 nm, even more
preferably smaller than 0.5 nm, most preferably smaller than 0.2
nm.
[0073] The pore density is preferably directly proportional to the
density of the particles which produce the pores by striking
action, most preferably can even be varied by the partial pressure
of the striking species, and depending on the treatment time and
the parameter, especially of the plasma system used.
[0074] The pore distribution preferably has at least one region of
greatest pore concentration under the surface, by variation of the
parameters of several such regions which are superimposed into a
preferably plateau-shaped region (see FIG. 7). The pore
distribution converges toward zero with increasing depth. The
region near the surface during bombardment has a pore density which
is almost identical to the pore density near the surface. After the
end of plasma treatment the pore density on the surface can be
reduced as a result of stress relaxation mechanisms. The pore
distribution in the thickness direction with respect to the surface
has one steep flank and with respect to the bulk a rather flatter,
but continuously decreasing flank (see FIG. 7).
[0075] For the pore size, the pore distribution and pore density,
similar considerations apply to all methods not produced with
plasma.
[0076] The reservoir can be designed by controlled use and
combination of process parameters. FIG. 7 shows a representation of
the concentration of injected nitrogen atoms by plasma as a
function of the penetration depth into a silicon oxide layer. It
was possible to produce two profiles by variation of the physical
parameters. The first profile 11 was produced by more highly
accelerated atoms more deeply in the silicon oxide, conversely the
profile 12 was produced after altering the process parameters in a
lower density. The superposition of the two profiles yields a sum
curve 13 which is characteristic for the reservoir. The
relationship between the concentration of the injected atom and/or
molecule species is evident. Higher concentrations designate
regions with higher defect structure, therefore more space to
accommodate the subsequent educt. A continuous change of the
process parameters which is controlled especially in a dedicated
manner during the plasma activation makes it possible to achieve a
reservoir with a distribution of the added ions over the depth,
which (distribution) is as uniform as possible.
[0077] According to one embodiment of the invention, the filling of
the reservoir can take place especially advantageously at the same
time with the formation of the reservoir by the reservoir being
applied as a coating to the first substrate, the coating already
encompassing the first educt.
[0078] The reservoir is conceivable as a porous layer with a
porosity in the nanometer range or as a layer which has channels
with a channel density smaller than 10 nm, more preferably smaller
than 5 nm, even more preferably smaller than 2 nm, most preferably
smaller than 1 nm, most preferably of all smaller than 0.5 nm.
[0079] For the step of filling of the reservoir with a first educt
or a first group of educts, as claimed in the invention, the
following embodiments, also in combination, are conceivable: [0080]
exposing the reservoir to the ambient atmosphere, [0081] flushing
with especially deionized water, [0082] flushing with a fluid which
contains the educt or which consists of it, especially H.sub.2O,
H.sub.2O.sub.2, NH.sub.4OH, O.sub.x.sup.+ [0083] exposing the
reservoir to any gas atmosphere, especially atomic gas, molecular
gas, gas mixtures, [0084] exposing the reservoir to a water vapor-
or hydrogen peroxide vapor-containing atmosphere.
[0085] The following compounds are possible as educts:
O.sub.x.sup.+, O.sub.2, O.sub.3, N.sub.2, NH.sub.3, H.sub.2O,
H.sub.2O.sub.2, and/or NH.sub.4OH.
[0086] The use of the above cited hydrogen peroxide vapor is
regarded as the preferred version in addition to using water.
Hydrogen peroxide furthermore has the advantage of having a greater
oxygen to hydrogen ratio. Furthermore, hydrogen peroxide
dissociates above certain temperatures and/or via the use of high
frequency fields in the MHz range into hydrogen and oxygen.
[0087] Mainly when using materials with different coefficients of
thermal expansion the use of methods for dissociation of the
aforementioned species which do not cause any noteworthy
temperature increase or at best a local/specific temperature
increase is advantageous. In particular there is microwave
irradiation which at least promotes, preferably causes the
dissociation.
[0088] According to another advantageous embodiment of the
invention it is provided that the formation of the growth layer and
strengthening of the irreversible bond takes place by diffusion of
the first educt into the reaction layer.
[0089] According to another advantageous embodiment of the
invention it is provided that the formation of the irreversible
bond takes place at a temperature of typically less than
300.degree. C., advantageously less than 200.degree. C., more
preferably less than 150.degree. C., even more preferably less than
100.degree. C., most preferably at room temperature, especially
during a maximum 12 days, more preferably a maximum 1 day, even
more preferably a maximum 1 hour, most preferably a maximum 15
minutes. Another advantageous heat treatment method is dielectric
heating by microwaves.
[0090] Here it is especially advantageous if the irreversible bond
has a bond strength of greater than 1.5 J/m.sup.2, especially
greater than 2 J/m.sup.2, preferably greater than 2.5
J/m.sup.2.
[0091] The bond strength can be increased especially advantageously
in that during the reaction, as claimed in the invention, a product
with a greater molar volume than the molar volume of the second
educt is formed in the reaction layer. In this way growth on the
second substrate is effected, as a result of which gaps between the
contact surfaces can be closed by the chemical reaction as claimed
in the invention. As a result, the distance between the contact
surfaces, therefore the average distance, is reduced, and dead
spaces are minimized.
[0092] To the extent the formation of the reservoir takes place by
plasma activation, especially with an activation frequency between
10 and 600 kHz and/or a power density between 0.075 and 0.2
watt/cm.sup.2 and/or with pressurization with a pressure between
0.1 and 0.6 mbar, additional effects such as smoothing of the
contact surface and also a clearly increased hydrophilicity of the
contact surface are effected.
[0093] According to another advantageous embodiment of the
invention it is provided that the reaction layer consists of an
oxidizable material, especially predominantly, preferably
essentially completely, of Si, Ge, InP, GaP or GaN or one of the
other materials mentioned alternatively in the above list. An
especially stable reaction which especially effectively closes the
existing gaps is enabled by oxidation.
[0094] Here it is especially advantageous as claimed in the
invention if between the second contact surface and the reaction
layer there is a growth layer, especially predominantly of native
oxide material, preferably silicon dioxide. The growth layer is
subject to growth caused by the reaction as claimed in the
invention. The growth takes place proceeding from the transition
Si--SiO.sub.2 by re-formation of amorphous SiO.sub.2 and the
deformation caused thereby, especially bulging, of the growth
layer, especially on the interface to the reaction layer, and
especially in regions of gaps between the first and the second
contact surface. This causes a reduction of the distance or a
reduction of the dead space between the two contact surfaces, as a
result of which the bond strength between the two substrates is
increased. A temperature between 200 and 400.degree. C., preferably
roughly 200.degree. C. and 150.degree. C., more preferably a
temperature between 150.degree. C. and 100.degree. C., most
preferably a temperature between 100.degree. C. and room
temperature, is especially advantageous. The growth layer can be
divided into several growth regions. The growth layer can at the
same time be a reservoir formation layer of the second substrate in
which another reservoir which accelerates the reaction is
formed.
[0095] It is especially advantageous if the growth layer and/or the
surface layer has an average thickness "A" between 0.1 nm and 5 nm
prior to formation of the irreversible bond. The thinner the growth
layer and/or the surface layer, the more quickly and easily the
reaction takes place between the first and the second educt through
the growth layer and/or the surface layer, especially by diffusion
of the first educt through the growth layer and/or the surface
layer to the reaction layer. Furthermore the activation of the
surface can promote the diffusion by the generation of point
defects. The diffusion rate of the educts through the growth layer
is increased by the growth layer (which has optionally been thinned
and thus is very thin at least at the beginning of the formation of
the permanent bond or at the start of the reaction). This leads to
a lower transport time of the educts at the same temperature.
[0096] Here thinning can play a decisive part since the reaction
can be further accelerated hereby and/or the temperature further
reduced. Thinning can be done especially by etching, preferably in
a moist atmosphere, still more preferably in-situ. Alternatively
etching takes place especially by dry etching, preferably in-situ.
As used herein, "in-situ" means performance in the same chamber in
which at least one previous and/or one following step is/are done.
Wet etching takes place with chemicals in the vapor phase, while
dry etching takes place with chemicals in the liquid state. To the
extent the growth layer consists of silicon dioxide, etching with
hydrofluoric acid or diluted hydrofluoric acid can be done. To the
extent the growth layer consists of pure Si, etching can be done
with KOH.
[0097] According to one embodiment of the invention it is
advantageously provided that the formation of the reservoir is
carried out in a vacuum. Thus contamination of the reservoir with
unwanted materials or compounds can be avoided.
[0098] In another embodiment of the invention, it is advantageously
provided that filling of the reservoir takes place by one or more
of the steps cited below: [0099] exposing the first contact surface
to the atmosphere, for filling the reservoir with atmospheric
humidity and/or oxygen contained in the air, [0100] exposing the
first contact surface to a fluid consisting especially
predominantly, preferably almost completely, of especially
deionized H.sub.2O and/or H.sub.2O.sub.2, [0101] exposing the first
contact surface to N.sub.2 gas and/or O.sub.2 gas and/or Ar gas
and/or forming gas, especially consisting of 95% Ar and 5% H.sub.2,
especially with an ion energy in the range from 0 to 2000 eV,
[0102] vapor deposition for filling the reservoir with any already
named educt.
[0103] According to another advantageous embodiment of the
invention it is provided that the formation and filling of a
reservoir take place additionally on the second contact surface,
especially in the growth layer, and the formation of the permanent
bond is additionally strengthened by reaction of the first educt
with a second educt which is contained in a reaction layer of the
first substrate (1).
[0104] It is especially effective for the process sequence if the
reservoir is formed preferably in a thickness R between 0.1 nm and
25 nm, more preferably between 0.1 nm and 15 nm, even more
preferably between 0.1 nm and 10 nm, most preferably between 0.1 nm
and 5 nm. Furthermore, according to one embodiment of the invention
it is advantageous if the average distance B between the reservoir
and the reaction layer immediately before formation of the
irreversible bond is between 0.1 nm and 15 nm, especially between
0.5 nm and 5 nm, preferably between 0.5 nm and 3 nm. The distance B
can be influenced or produced as claimed in the invention by the
thinning.
[0105] A device for executing the method is formed, as claimed in
the invention, with a chamber for forming the reservoir, a chamber
provided especially separately for filling the reservoir, and an
especially separately provided chamber for forming the prebond, all
of which chambers are connected directly to one another via a
vacuum system.
[0106] In another embodiment the filling of the reservoir can also
take place directly via the atmosphere, therefore either in a
chamber which can be opened to the atmosphere or simply on a
structure which does not have jacketing but can handle the wafer
semi-automatically and/or completely automatically.
[0107] Other advantages, features and details of the invention will
become apparent from the following description of preferred
exemplary embodiments and using the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0108] FIG. 1a shows a first step of the method as claimed in the
invention immediately after the first substrate makes contact with
the second substrate,
[0109] FIG. 1b shows an alternative first step of the method as
claimed in the invention immediately after the first substrate
makes contact with the second substrate,
[0110] FIGS. 2a and 2b show other steps of the method as claimed in
the invention for forming a higher bond strength,
[0111] FIG. 3 shows another step of the method as claimed in the
invention which follows the steps according to FIG. 1, FIG. 2a and
FIG. 2b, with substrate contact surfaces which are in contact,
[0112] FIG. 4 shows a step as claimed in the invention for
formation of an irreversible/permanent bond between the
substrates,
[0113] FIG. 5 shows an enlargement of the chemical/physical
processes which proceed on the two contact surfaces during the
steps according to FIG. 3 and FIG. 4,
[0114] FIG. 6 shows a further enlargement of the chemical/physical
processes which proceed on the interface between the two contact
surfaces during the steps according to FIG. 3 and FIG. 4 and
[0115] FIG. 7 shows a diagram of the production of the reservoir as
claimed in the invention.
[0116] The same or equivalent features are identified with the same
reference numbers in the figures.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0117] In the situation which is shown in FIG. 1a only one extract
of the chemical reactions which proceed during or immediately after
the prebond step between a first contact surface 3 of a first
substrate 1 and a second contact surface 4 of a second substrate 2
is shown. Surface layers 6, 6' adjoin the contact surfaces 3, 4
respectively and are formed from oxidizable, native silicon dioxide
and are very thin. The surfaces are terminated with polar OH groups
and are accordingly hydrophilic. The first substrate 1 and the
second substrate 2 are held by the force of attraction of the
hydrogen bridges between the OH groups present on the surface and
the H.sub.2O molecules and also between the H.sub.2O molecules
alone. The hydrophilicity of at least the first contact surface 3
is increased by plasma treatment of the first contact surface 3 in
a preceding step.
[0118] It is especially advantageous to additionally subject the
second contact surface 4 to plasma treatment, especially at the
same time with the plasma treatment of the first contact surface 3
according to the alternative embodiment.
[0119] A reservoir 5 in the surface layer 6 consisting of native
silicon dioxide, as well as in the alternative embodiment according
to FIG. 1b a second opposing reservoir 5' in the surface layer 6',
are formed as claimed in the invention by plasma treatment. Plasma
treatment with O.sub.2 ions with ion energies in the range between
0 and 2000 eV yields an average thickness R of the reservoir 5 of
roughly 10 nm, the ions forming channels or pores in the surface
layer 6 (and optionally the surface layer 6').
[0120] Between the reservoir formation layer 6 and the reaction
layer 7 there is a growth layer 8 on the second substrate 2 which
can be at the same time at least partially the reservoir formation
layer 6'. Accordingly there can additionally be another growth
layer between the reservoir formation layer 6' and the reaction
layer 7'.
[0121] Likewise the reservoir 5 (and optionally the reservoir 5')
is filled at least largely with H.sub.2O as the first educt prior
to the step shown in FIG. 1 and after plasma treatment. Reduced
species of the ions present in the plasma process can also be
located in the reservoir, especially O.sub.2, N.sub.2, H.sub.2,
Ar.
[0122] The contact surfaces 3, 4 still have a relatively wide gap,
especially dictated by the water present between the contact
surfaces 3, 4, after making contact in the stage shown in FIGS. 1a
and 1b. Accordingly the existing bond strength is relatively low
and is roughly between 100 mJ/cm.sup.2 and 300 mJ/cm.sup.2,
especially more than 200 mJ/cm.sup.2. In this connection the prior
plasma activation plays a decisive part, especially due to the
increased hydrophilicity of the plasma-activated first contact
surface 3 and a smoothing effect which is caused by the plasma
activation.
[0123] The process which is shown in FIG. 1 and which is called
prebond can preferably proceed at ambient temperature or a maximum
50.degree. C. FIGS. 2a and 2b show a hydrophilic bond, the
Si--O--Si bridge arising with splitting of water by --OH terminated
surfaces. The processes in FIGS. 2a and 2b last roughly 300 h at
room temperature. At 50.degree. C. roughly 60 h. The state in FIG.
2b occurs at the indicated temperatures without producing the
reservoir 5 (or reservoirs 5, 5').
[0124] Between the contact surfaces 3, 4, H.sub.2O molecules are
formed and provide at least partially for further filling in the
reservoir 5 to the extent there is still free space. The other
H.sub.2O molecules are removed. In the step according to FIG. 1
roughly 3 to 5 individual layers of OH groups or H.sub.2O are
present and 1 to 3 monolayers of H.sub.2O are removed or
accommodated in the reservoir 5 from the step according to FIG. 1
to the step according to FIG. 2a.
[0125] In the step shown in FIG. 2a the hydrogen bridge bonds are
now formed directly between siloxane groups, as a result of which a
greater bond force arises. This draws the contact surfaces 3, 4
more strongly to one another and reduces the distance between the
contact surfaces 3, 4. Accordingly there are only 1 to 2 individual
layers of OH groups between the contact surfaces 1, 2.
[0126] In the step shown in FIG. 2b, in turn with separation of
H.sub.2O molecules according to the reaction which has been
inserted below, covalent compounds in the form of silanol groups
are now formed between the contact surfaces 3, 4 which lead to a
much stronger bond force and require less space so that the
distance between the contact surfaces 3, 4 is further reduced until
finally the minimum distance shown in FIG. 3 is reached based on
the contact surfaces 3, 4 directly meeting one another:
Si--OH+HO--Si.rarw.Si--O--Si+H.sub.2O
[0127] Up to stage 3, especially due to the formation of the
reservoir 5 (and optionally of the additional reservoir 5'), it is
not necessary to unduly increase the temperature, rather to allow
it to proceed even at room temperature. In this way an especially
careful progression of the process steps according to FIG. 1 to
FIG. 3 is possible.
[0128] In the process step shown in FIG. 4, the temperature is
preferably increased to a maximum 500.degree. C., more preferably
to a maximum 300.degree. C., even more preferably to a maximum
200.degree. C., most preferably to a maximum 100.degree. C., most
preferably of all not above room temperature in order to form an
irreversible or permanent bond between the first and the second
contact surface. These temperatures which are relatively low, in
contrast to the prior art, are only possible because the reservoir
5 (and optionally in addition the reservoir 5') encompasses the
first educt for the reaction shown in FIGS. 5 and 6:
Si+2H.sub.2O.fwdarw.SiO.sub.2+2H.sub.2
[0129] Between the second contact surface 4 and the reaction layer
7 there is a growth layer 8 which can be identical to the surface
layer 6'. To the extent a reservoir 5' has been formed according to
the second embodiment, between the first contact surface 3 and
another reaction layer 7' which corresponds to the reaction layer 7
there is also another growth layer 8', the reactions proceeding
essentially reciprocally. By increasing the molar volume and
diffusion of the H.sub.2O molecules, especially on the interface
between the surface layer 6' and the reaction layer 7 (and
optionally in addition on the interface between the surface layer 6
and the reaction layer 7'), volume in the form of a growth layer 8
increases, due to the objective of minimizing the free Gibb's
enthalpy enhanced growth taking place in regions in which gaps 9
are present between the contact surfaces 3, 4. The gaps 9 are
closed by the increase in the volume of the growth layer 8. More
exactly:
[0130] At the aforementioned slightly increased temperatures,
H.sub.2O molecules diffuse as the first educt from the reservoir 5
to the reaction layer 7 (and optionally from the reservoir 5' to
the reaction layer 7'). This diffusion can take place either via a
direct contact of the surface layer 6 and growth layer 8 which are
formed as native oxide layers (or via a gap 9 or from a gap 9 which
is present between the oxide layers). There, silicon dioxide,
therefore a chemical compound with a greater molar volume than pure
silicon, is formed as a reaction product 10 of the aforementioned
reaction from the reaction layer 7. The silicon dioxide grows on
the interface of the reaction layer 7 with the growth layer 8 (or
the interface of the reaction layer 7' with the growth layer 8')
and thus deforms the layer of the growth layer 8 formed as native
oxide in the direction of the gaps 9. Here H.sub.2O molecules from
the reservoir are also required.
[0131] Due to the existence of the gaps which are in the nanometer
range, there is the possibility of bulging of the native oxide
layer (growth layer 8 and optionally growth layer 8'), as a result
of which stresses on the contact surfaces 3, 4 can be reduced. In
this way the distance between the contact surfaces 3, 4 is reduced,
as a result of which the active contact surface and thus the bond
strength are further increased. The weld connection which has
arisen in this way, which closes all pores, and which forms over
the entire wafer, in contrast to the products in the prior art
which are partially not welded, fundamentally contributes to
increasing the bond force. The type of bond between the two native
silicon oxide surfaces which are welded to one another is a mixed
form of covalent and ionic portion.
[0132] The aforementioned reaction of the first educt (H.sub.2O)
with the second educt (Si) takes place in the reaction layer 7
especially quickly or at temperatures as low as possible to the
extent an average distance B between the first contact surface 3
and the reaction layer 7 is as small as possible.
[0133] Therefore the choice of the first substrate 1 and the
selection/pretreatment of the second substrate 2 which consists of
a reaction layer 7 (and optionally 7') of silicon and a native
oxide layer as thin as possible as a growth layer 8 (and optionally
8') are decisive. A native oxide layer as thin as possible is
provided as claimed in the invention for two reasons. The growth
layer 8 is very thin, especially due to additional thinning, so
that it can bulge through the newly formed reaction product 10 on
the reaction layer 7 toward the surface layer 6 of the opposite
substrate 1, which surface layer is likewise made as a native oxide
layer, predominantly in regions of the nanogaps 9. Furthermore,
diffusion paths as short as possible are desired in order to
achieve the desired effect as quickly as possible and at a
temperature as low as possible. The first substrate 1 likewise
consists of a silicon layer and a native oxide layer as thin as
possible present on it as a surface layer 6 in which the reservoir
5 is formed at least partially or completely.
[0134] The reservoir 5 (and optionally the reservoir 5') as claimed
in the invention is filled at least accordingly with the amount of
the first educt which is necessary to close the nanogaps 9 so that
an optimum growth of the growth layer 8 (and optionally 8') can
take place to close the nanogaps 9 in a time as short as possible
and/or at a temperature as low as possible.
REFERENCE NUMBER LIST
[0135] 1 first substrate [0136] 2 second substrate [0137] 3 first
contact surface [0138] 4 second contact surface [0139] 5, 5'
reservoir [0140] 6, 6' surface layer [0141] 7, 7' reaction layer
[0142] 8, 8' growth layer [0143] 9 nanogaps [0144] 10 reaction
product [0145] 11 first profile [0146] 12 second profile [0147] 13
sum curve [0148] A average thickness [0149] B average distance
[0150] R average thickness
* * * * *