U.S. patent application number 09/132059 was filed with the patent office on 2002-02-21 for treatment process for molecular bonding and unbonding of two structures.
Invention is credited to ASPAR, BERNARD, MALEVILLE, CHRISTOPHE.
Application Number | 20020022337 09/132059 |
Document ID | / |
Family ID | 9510376 |
Filed Date | 2002-02-21 |
United States Patent
Application |
20020022337 |
Kind Code |
A1 |
MALEVILLE, CHRISTOPHE ; et
al. |
February 21, 2002 |
TREATMENT PROCESS FOR MOLECULAR BONDING AND UNBONDING OF TWO
STRUCTURES
Abstract
Treatment process for bonding two structures (200, 220) by
molecular adhesion on a bonding interface (224), and unbonding of
the two structures along the said bonding interface. According to
the invention: bonding is done using at least one structure (200)
containing an element capable of diffusing towards the bonding
interface, unbonding is done using a heat treatment to make the
said element diffuse towards the bonding interface (224) to weaken
it. Application to the manufacture of devices with integrated
circuits.
Inventors: |
MALEVILLE, CHRISTOPHE;
(BERNIN, FR) ; ASPAR, BERNARD; (RIVES,
FR) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Family ID: |
9510376 |
Appl. No.: |
09/132059 |
Filed: |
August 11, 1998 |
Current U.S.
Class: |
438/406 ;
257/E21.519; 257/E21.568 |
Current CPC
Class: |
H01L 2221/68359
20130101; H01L 21/6835 20130101; Y10S 148/135 20130101; H01L
21/76254 20130101; Y10S 148/012 20130101 |
Class at
Publication: |
438/406 |
International
Class: |
H01L 021/76 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 19, 1997 |
FR |
97 10480 |
Claims
1. Treatment process for bonding two structures (100, 120, 200,
220) by molecular adhesion on a bonding interface (124, 224), and
unbonding of the said two structures along the said bonding
interface, characterized in that: bonding is done using at least
one structure (100, 200) containing at least one element capable of
diffusing within the said structure towards the bonding interface,
unbonding is done using a heat treatment with a sufficient thermal
budget to make the said element diffuse towards the bonding
interface (124, 224) to weaken it.
2. Process according to claim 1, in which hydrogen implantation is
done in at least one of the structures (100) before bonding,
hydrogen forming the said element capable of diffusing within the
structure.
3. Process according to claim 2, in which implantation is done in
silicon with a dose of between 10.sup.16 and 5.10.sup.16 and an
energy of between 20 and 500 keV.
4. Process according to claim 1, in which at least one structure
(200) is used comprising a surface oxide layer (206) formed by a
Plasma Enhanced Chemical Vapor Deposition and containing OH
molecules, the said OH molecules forming the element capable of
diffusing.
5. Process according to claim 3, in which the heat treatment is
done at a temperature of between 600 and 1350.degree. C.
6. Process according to claim 1, in which the heat treatment is
done by exposing the bonded structures to radiation from heating
lamps.
7. Process according to claim 1, in which the heat treatment is
done in a furnace.
8. Process according to claim 1, in which at least one of the
structures is a multi-layer structure.
9. Process according to claim 1, in which the surface of at least
one of the structures is prepared before bonding to form a relief
on it.
10. Process according to claim 1, in which external separation
forces (240, 241) are also exerted on the structures at the time of
unbonding.
Description
[0001] Technical Domain
[0002] This invention relates to a treatment process for bonding
two structures by molecular adhesion, and unbonding.
[0003] A structure is any micromechanical or integrated optical
part, or microelectronic part that could be combined with another
part by bonding. For example, this type of structure could be a
substrate or a support board, equipped or not equipped with
electronic, optical or mechanical components.
[0004] Furthermore, bonding by molecular adhesion refers to bonding
that involves an interaction between chemical terminations present
on the surfaces of structures in contact with each other.
[0005] The invention has applications particularly in the
manufacture of devices with integrated circuits. In some
manufacturing processes, semi-conductor boards containing
integrated circuits must be combined with stiffening substrates,
and then separated at the end of the treatment.
[0006] State of Prior Art
[0007] As mentioned above, and particularly in microelectronic
applications concerning the manufacture of power circuits,
semi-conductor wafers comprising integrated electronic circuits are
in the form of large thin boards. For example, wafers with a
diameter of four inches (.congruent.10 cm) and a thickness of less
than 200 .mu.m are used.
[0008] Standard equipment for the manufacture of microelectronic
devices, for example such as photorepeaters, are not suitable for
the treatment of boards this thin. Furthermore, thin semiconductor
boards are fragile, and this is incompatible with handling steps,
and particularly handling using automated treatment equipment.
[0009] A thin board or a surface layer of a substrate with or
without integrated circuits may be bonded on a treatment support
also called a "handling substrate". The handling substrate thus
provides it with sufficient mechanical strength for all required
treatments and manipulations.
[0010] The attached FIGS. 1 to 3 described below illustrate
transfer of a thin layer comprising integrated circuits, as an
example.
[0011] The thin layer, marked in FIG. 1 as reference 10, is
initially fixed to a substrate 12, called the source substrate. It
comprises integrated electronic components and circuits, which are
not shown.
[0012] The source substrate 12 and the thin surface layer 10 are
transferred to a handling substrate 14 by bonding the thin surface
layer on the handling substrate. The structure thus obtained is
shown in FIG. 1.
[0013] The source substrate is then eliminated by a process such as
grinding or cleavage, by etching and/or polishing to obtain the
structure shown in FIG. 2.
[0014] The thin layer 10 comprising integrated circuits is then
bonded upside down on the handling substrate 14. The handling
substrate thus provides this layer with the stiffness necessary for
other manufacturing operations or treatments.
[0015] In a final step shown in FIG. 3, the thin layer 10
containing the electronic circuits is transferred to a target
substrate or a destination substrate 16, onto which it is
permanently fixed.
[0016] After attachment to the destination substrate 16, the thin
layer 10 is separated from the handling substrate 14. Thus the
handling substrate 14 is shown in dashed lines in FIG. 3.
[0017] This type of process is described in more detail in document
(1), for which the reference is given at the end of this
description.
[0018] The thin layer 10 may be bonded on the handling substrate
14, for example cold using an appropriate glue. Bonding is then
reversible and it is possible to separate the thin layer 10 from
the handling substrate. However, the adhesion obtained between the
thin layer 10 and the handling substrate 14 may be insufficient,
particularly for subsequent treatments at high temperature. In
particular, the glue is incapable of resisting high
temperatures.
[0019] Furthermore, the material (glue) added for bonding can cause
metallic or organic contamination of bonded parts during subsequent
treatments.
[0020] These disadvantages are avoided by preferring bonding by
molecular adhesion which does not use any glue or added material.
Bonding two structures by molecular adhesion includes four main
steps, which are described below.
[0021] A first step is surface preparation of the structures to be
brought into contact. A good quality molecular bonding requires
control of important parameters such as surface roughness, which
should preferably be less than 0.5 nm (4.ANG.) as a root mean
square value, the lack of any dust (particles>0.2 .mu.m) on
surfaces, the planeness of the surfaces to be put in contact, and
the chemical state of these surfaces.
[0022] Thus the first step consists mainly of cleaning the surfaces
of structures to be bonded in order to eliminate foreign particles
and to make these surfaces hydrophile.
[0023] FIG. 4 shows a structure for bonding comprising a silicon
substrate 20, one surface 22 of which has been made hydrophile.
Surface 22 comprises a first hydrophile layer 24 composed
essentially of Si--OH chemical groups and one (or several) layers
of water H.sub.2O 26 adsorbed on the hydrophile layer 24.
[0024] A second step consists of putting the hydrophile surfaces of
the two structures to be bonded into contact. Putting them into
contact brings the water layers adsorbed on these structures
sufficiently close together for them to interact with each other.
The attraction exerted between the water molecules is propagated
gradually along the entire surface of each structure. The surfaces
in contact are then bonded together.
[0025] The bonding energy as measured by a blade insertion method
is of the order of 0.15 J/m.sup.2. This value is typically the
value of hydrogen type adhesion between two water layers, on each
structure.
[0026] Document (2), the reference of which is given at the end of
this description, contains an illustration of the blade insertion
method.
[0027] A third step consists of solidification heat treatment of
the adhesion.
[0028] The heat treatment can eliminate water layers between the
assembled structures, up to a temperature of the order of
200.degree. C.
[0029] Adhesion of structures then takes place by bonding of OH
groups between the layers of Si--OH chemical groups in each
structure, respectively. Note that the layer of Si--OH groups is
shown as reference 24 in FIG. 4. This interaction results in a
reduction of the distance between the two structures in contact and
results in the interaction of additional OH groups. The bonding
energy thus increases for treatment temperatures of 200.degree. C.
to 900.degree. C.
[0030] Finally, there may be a fourth step consisting of heat
treatment at more than 900.degree. C. In this step, the interacting
Si--OH groups change towards Si--O--Si type bonds, which are much
stronger. This then gives a very strong increase in the bonding
energy.
[0031] The graph in FIG. 5 shows the bonding energy per unit area
between structures bonded by molecular adhesion as the ordinate, as
a function of the treatment temperature. Bonding energies are
expressed in J/m.sup.2 and temperatures are expressed in .degree.
C.
[0032] Regions 32, 33 and 34 in the graph are related to the
second, third and fourth steps in the bonding process and
correspond to a hydrogen type interaction between water films, a
hydrogen interaction between OH groups (reference 24), and then an
Si--O--Si) type interaction, respectively. A more detailed
description of bonding of silicon wafers may be found in document
(3), the reference of which is given at the end of this
description.
[0033] Note that at treatment temperatures above 600.degree. C., it
becomes impossible to unbond the two assembled structures without
causing severe degradation to them.
[0034] When the assembled structures are silicon boards, bonding
energies greater than 2 J/m.sup.2 may be obtained. These energies
are thus of the same order of magnitude as the cohesion energies of
the silicon material.
[0035] It is immediately clear that if molecular bonding is used in
a transfer process like that shown in FIGS. 1 to 3, it will be
impossible to detach the handling substrate from the thin layer by
applying mechanical forces, without destroying the thin layer or
the handling substrate.
[0036] Thus, the thin layer is separated from the handling
substrate by eliminating the handling substrate. For example the
handling substrate can be eliminated by grinding and/or
mechanical-chemical abrasion.
[0037] In this case, the process for transferring a thin layer
involves the sacrifice of a handling layer for each treated thin
layer. This sacrifice also introduces a large industrial cost.
DESCRIPTION OF THE INVENTION
[0038] The purpose of this invention is to propose a treatment for
bonding of two structures which can firstly give a very strong
molecular bond between the two structures, and will also enable
unbonding of the structures along the bonding interface.
[0039] Another purpose of the invention is to propose a treatment
enabling unbonding that does not damage the assembled
structures.
[0040] More precisely in order to achieve these objectives, the
purpose of the invention is a treatment process for bonding two
structures by molecular adhesion on a bonding interface, and for
separation of the two structures along the said bonding
interface.
[0041] In accordance with the invention,
[0042] bonding is done using at least one structure containing at
least one element capable of diffusing within the said structure to
the bonding interface, and
[0043] a heat treatment is used for unbonding, with a sufficient
heat budget to make the said element diffuse towards the bonding
interface to weaken it.
[0044] An element capable of causing diffusion refers to any
element or compound either intrinsically present in the material or
added to it, deliberately or accidentally, capable of migrating
within the material towards the bonding interface, to react with
it. This element is then capable of modifying this interface during
the heat treatment and will cause separation of the two parts on
each side of the interface. This separation may be assisted by a
gaseous phase which may form at the interface during the heat
treatment.
[0045] Furthermore, heat budget means the sum of heat treatments
carried out and defined by a time/temperature pair applied to the
structure.
[0046] Thus, the heat treatment designed to separate the two parts
(on each side of the bonding interface) may take account of heat
treatments applied to the assembled structures before
unbonding.
[0047] According to one particular embodiment of the process, a
hydrogen implantation may be done before bonding in at least one of
the structures, the hydrogen forming the said element capable of
diffusing in the structure.
[0048] For example, implantation is done in silicon with a dose of
between 10.sup.16 and 5.10.sup.16 (H.sup.+/cm.sup.2) and an energy
of between 20 and 500 keV. Preferably, the dose may be of the order
of 3.10.sup.16 ions/cm.sup.2 and the implantation energy of the
order of 70 keV. The dose depends on the implantation conditions
and particularly the temperature of the structure during the
implantation.
[0049] According to one variant, at least one structure may also be
used comprising a surface oxide layer formed by plasma enhanced
chemical vapor deposition and containing OH molecules, the said OH
molecules forming the element capable of diffusing.
[0050] For example, the heat treatment for unbonding may be done at
a temperature of between 600 and 1350.degree. C. for silicon. This
temperature would be chosen to be of the order of 200 to
600.degree. C. for gallium arsenide (AsGa). For silicon carbide
(S.sub.iC), the chosen temperature will be between 600.degree. C.
and the melting temperature which exceeds 1350.degree. C.
[0051] For example, the heat treatment may take place under heating
lamps or in a furnace.
[0052] The structures to be assembled may be structures made of a
single solid material, or may be multi-layer structures containing
zones which may or may not have been treated.
[0053] The multi-layer nature of the structures may beneficially
generate internal stresses that facilitate separation of structures
during the unbonding step.
[0054] Similarly, the surface of at least one of the structures to
be assembled may be prepared before bonding to form a relief. This
relief may also facilitate separation of the structures when
unbonding.
[0055] Finally, external separation forces may be exerted on the
structures to further facilitate unbonding. For example, tension or
bending forces, or shear forces, may be exerted on the structures
by inserting a blade at the interface between the structures.
[0056] Other characteristics and advantages of the invention will
become clearer from the following description with reference to the
figures in the attached drawings. This description is given for
illustration only, and is in no way restrictive.
BRIEF DESCRIPTION OF THE FIGURES
[0057] FIG. 1, already described, is a schematic section of a
structure comprising a thin layer on a substrate called the source
substrate, assembled with another substrate called the handling
substrate.
[0058] FIG. 2, already described, is a schematic section of the
handling substrate on which the thin layer is bonded.
[0059] FIG. 3, already described, is a schematic section of the
thin layer transferred onto a substrate called the destination
substrate.
[0060] FIG. 4 is a schematic section of a structure prepared for
molecular bonding.
[0061] FIG. 5 is a graph showing the bonding energy between two
structures during molecular bonding steps.
[0062] FIG. 6 is a schematic section of a structure comprising a
silicon wafer and illustrates the preparation of this structure for
molecular bonding according to the invention.
[0063] FIG. 7 shows a section of the structure in FIG. 6 bonded to
another structure comprising a silicon wafer.
[0064] FIGS. 8 and 9 show sections of the assembled structures in
FIG. 7, and illustrate a treatment step for unbonding in accordance
with the invention.
[0065] FIG. 10 shows a section of another assembly obtained by
molecular bonding of two structures in accordance with the
invention.
[0066] FIG. 11 shows a section of the assembly in FIG. 10 during an
unbonding treatment in accordance with the invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0067] FIG. 6 shows a structure 100 to be bonded by molecular
adhesion according to the invention.
[0068] The structure 100 comprises a silicon board 102 covered by a
surface silicon oxide layer 104.
[0069] An implanted hydrogen layer 106 is formed by implantation of
hydrogen ions in the silicon board 102 through the oxide layer 104.
In this example, the hydrogen layer, implanted at an energy of the
order of 70 keV, has a concentration of 3.10.sup.16 ions/cm.sup.2,
a thickness of 400 nm and is buried at a depth of the order of 300
nm below the upper surface 110 of the structure 100. (The figures
are not to scale, and are not in these proportions).
[0070] Hydrogen refers to gaseous substances formed either in their
atomic form (for example H) or in their molecular form (for example
H.sub.2) or in their ionic form (H.sup.+, H2.sup.+, etc.) or in
their isotopic form (Deuterium) or isotopic and ionic form.
[0071] If necessary, surface 110 of structure 100 is then cleaned
in order to make it hydrophile and to remove all particles. A water
film (not shown in the figure) is formed on the surface 110.
[0072] A small roughness may also be applied to or kept on the
surface 110 of structure 100.
[0073] As shown in FIG. 7, the structure 100 is then assembled with
a second structure 120. The second structure comprises a silicon
board 122, of which the surface in contact with structure 100 has
also beneficially be cleaned to make it hydrophile.
[0074] Reference 124 denotes the interface between the assembled
structures 100 and 120.
[0075] The structures are then firstly annealed at a temperature of
the order of 500.degree. C. which eliminates water layers between
the assembled structures and forms molecular bonds between the
surfaces in contact.
[0076] The bonding energy of the molecular bonding achieved at
500.degree. C. is of the order of 0.5 J/m.sup.2. For example, this
bonding energy will be sufficient to bond a silicon board
containing integrated circuits to a handling substrate; in
particular, it is sufficient for all treatments envisaged for the
wafer in equipment normally used for manufacturing microelectronic
devices.
[0077] When the heat treatment is prolonged, or when another heat
treatment is carried out at a temperature of the order of
800.degree. C. or more, unbonding areas (indicated as reference 130
in FIGS. 8 and 9) appear at the interface 124 between the assembled
structures 100 and 120.
[0078] The formation of unbonding areas is controlled by the
thermal budget applied to structures. Heat treatment forces
hydrogen to diffuse from layer 106 implanted in structure 100
towards the bonding interface 124 (through the oxide if it is
present). The hydrogen that diffuses is trapped at the interface
124, accumulates on the interface and may move along it in gaseous
form. Thus, the accumulation of hydrogen at interface 124 can at
least partly overcome the bonding forces. Arrows 132 in FIGS. 8 and
9 show the diffusion of hydrogen towards the bonding interface
124.
[0079] Depending on the magnitude of the thermal budget used, the
unbonding areas 130 may be local (FIG. 8) or may extend over the
entire surface of the interface (FIG. 9).
[0080] The thermal budget necessary to obtain complete unbonding
depends on the quantity of the element capable of migrating present
in the material from at least one of the structures. Consequently,
in this example the thermal budget is related to the hydrogen
implantation dose. For example, the budget for a dose of 3.16.sup.6
cm.sup.3 may be 900.degree. C.-30 min.
[0081] FIG. 10 indicates another example embodiment of the
invention. It shows a sectional view of an assembly of a first
structure 200 and a second structure 220 which are bonded by
molecular bonding as described above. Reference 224 denotes the
bonding interface between the surfaces of assembled structures.
[0082] The first structure comprises essentially a silicon wafer
202 at the surface of which an oxide layer 206 has been formed. The
oxide in this layer 206 is deposited by a Plasma Enhanced Chemical
Vapor Deposition technique. This type of oxide has the specific
feature that it contains OH molecules capable of diffusing.
[0083] A first heat treatment carried out at a temperature of less
than or equal to about 500.degree. C. increases the molecular
bonding energy.
[0084] When the heat treatment is continued or resumed at
temperatures exceeding about 500.degree. C., the OH groups
contained in the oxide layer 206 diffuse and migrate, particularly
towards the interface 224 where they are trapped. Diffusion of the
OH groups towards the interface is indicated with arrows 232.
[0085] At interface 224, the OH molecules evolve in gaseous form
and concentrate at the bonding interface. This phenomenon causes
the formation of bubbles and weakens the bonding interface.
[0086] Under the effect of pressure generated by the gas, an
unbonded area 230 shown in FIG. 11 propagates until the two
structures 200 and 220 are completely unbonded. Separation of the
structures may be facilitated by applying external mechanical
separation forces. These forces are shown in FIG. 11 by arrows 240,
241. The forces are tension forces 240 and/or shear forces 241.
[0087] It is thus clear that the process according to the invention
may be used particularly for molecular bonding of integrated
circuit wafers, without any added material, while enabling
reversible bonding.
[0088] Furthermore, the invention is applicable to any type of
structure as defined above. Note that the invention relates not
only to structures containing silicon, but also other
semiconducting structures (Si, SiC, AsGa, etc....), insulating
structures (glass, quartz, etc.) and even conducting structures
(metal alloys, etc.).
REFERENCED DOCUMENTS
[0089] (1) FR-A-2 744 285
[0090] (2) "Bonding of silicon wafers for silicon-on-insulator" W.
P. Maszara, G. Goetz, A. Caviglia and J. B. McKitterick Aerospace
Technology Center, Allied Signal Aerospace Company, Columbia, Md.
21045(Received Apr. 12, 1988, accepted for publication Jul. 28,
1988).
[0091] (3) "A model for the silicon wafer bonding process" R.
Stengl, T. Tan and U. Gosele School of Engineering, Duke
University, Durham, N.C. 27706, USA. (Received May 8, 1989,
accepted for publication Jul. 15, 1989). Japanese Journal of
Applied Physics, vol. 28, No. 10, October 1989, pp. 1735-1741.
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