Semiconductor device manufacturing: process – Bonding of plural semiconductor substrates
Reexamination Certificate
2002-01-09
2004-06-29
Jones, Deborah (Department: 1775)
Semiconductor device manufacturing: process
Bonding of plural semiconductor substrates
C438S457000, C438S459000, C438S761000, C438S766000, C428S620000, C428S457000
Reexamination Certificate
active
06756285
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a multilayer structure obtained by adhesion or adherence, in particular molecular adhesion, characterized by controlled internal stresses, and to a method for producing such a structure.
By multilayer structure with controlled stresses, it is understood a structure comprising at least two layers, so-called main layers, having between them tensile or compression stresses. These stresses are determined and controlled depending on the purpose of the structure.
The invention finds applications in the fields of microelectronics, as a substrate or as a stiffener, but also in the fields of micromechanics for manufacturing membrane sensors, for example.
STATE OF THE PRIOR ART
Among the multilayer structure assembled by means of molecular adhesion techniques (wafer bonding), let us mention SOI (silicon on an insulator) structures, as an example. Typically, a SOI multilayer structure includes a thick layer of silicon serving as a support, an insulating layer in silicon oxide and a surface layer of thin silicon, the thickness of which is between a few tens of nanometers to a few tens of micrometers.
The manufacturing of SOI structures generally consists of bringing two silicon wafers into contact by molecular adhesion, one of which at least is covered by a silicon oxide surface layer.
After bringing them into contact, the wafers generally undergo a heat treatment under a controlled atmosphere. The purpose of this heat treatment is to enhance the close contact and therefore the adherence of the wafers.
During the heat treatment, the present materials, in particular silicon in contact with silicon oxide, may impose stresses on each other. These stresses are in particular related to the differences in thermal expansion coefficients, &Dgr;l/1, of the materials in contact. These differences in the expansion coefficients of materials of the surfaces in contact are also the source of stresses when cooling the closely bound structures.
More generally, it is also known that a SiO
2
film on a silicon wafer when it is produced at certain temperatures, has the effect of inducing deformation on the wafer upon its cooling. The relative deformation under the effect of heat, noted &Dgr;l/1, is the order of 2.6.10
−6
/K for silicon, and of the order of 5.10
−7
/K for silicon oxide (SiO
2
), produced by thermal oxidation of silicon.
When the oxide film is formed on one face of the silicon wafer, the deformation due to the stresses may be quantified by measuring the deflection at the center of the wafer. Because of the difference in thermal expansion coefficients, a decrease of temperature generates a compression of the oxide film on the silicon wafer. This compression is expressed by a convexity of the wafer. The convexity is all the more marked because the oxide film is thick and it may cause a change in the surface morphology.
The appended
FIGS. 1-4
are for illustrating the stresses generated in SiO
2
structures produced by conventional methods through molecular adhesion.
FIG. 1
shows a first main layer
10
a
, or support, as a silicon plate having a thin layer of thermal oxide
20
a
on its surface.
It is seen that the set formed by the first main silicon layer
10
a
and the oxide surface layer
20
a
is bent. The surface of the oxide layer
20
a
is convex.
Reference number
10
b
refers to a silicon wafer forming a second main layer the parallel faces of which are planar In the illustrated example, the main layers
10
a
and
10
b
initially have thicknesses of the same order of magnitude.
FIG. 2
shows the structure obtained by assembling main layers
10
a
and
10
b
. These layers are connected through the oxide layer
20
a
. The assembly, as mentioned earlier, comprises the molecular bonding of the second main layer of silicon
10
b
onto the oxide surface layer
20
a
. This bonding is reinforced by a heat treatment.
It is seen that the structure obtained after assembly virtually does not have any deformation. Indeed, from the moment that the thicknesses of the main silicon layers are of the same order of magnitude, stresses generated by the oxide layer on each of the main layers tend to compensate each other.
The silicon surface film with a SOI type structure is generally a thin film, the thickness of which is adapted to the requirements of electrical insulation of the components, for example. The stiffness of the structure is provided by the thick silicon layer.
Thus, in order to obtain a typical SOI structure from the structure of
FIG. 2
, one of the main silicon layers should be thinned. The thinning may be performed by means of one of the thinning techniques known in different methods, BSOI (Bonded Silicon on Insulator), BESOI (Bonded with Etch stop Layer Silicon on Insulator). On this point, reference may be made to document (7), the reference thereof is specified at the end of the present description.
When one of the main silicon layers is thinned, it appears that the generated stresses at the interfaces with the silicon oxide layer are no longer compensated.
FIGS. 3 and 4
show structures obtained by thinning of the main layers
10
b
and
10
a
, respectively. These structures have a deflection and the surface of the thin silicon layer is convex in each of the cases.
It is seen that the thickness of the main layers and also the thickness of the buried silicon oxide layer, i.e., the oxide layer sandwiched between the main layer and the thin surface layer, are part of the parameters which control the deflection of the finally obtained structure.
As an example, for a buried thermal oxide film
20
a
with a thickness of the order of 1 micrometer, deflection values are obtained which may be larger than 50 &mgr;m when the thin surface film of silicon
10
a
has a thickness of 25 &mgr;m and when the main silicon layer has a thickness of the order of 500 &mgr;m. When the thickness of the surface film of silicon is increased to more than 50 &mgr;m, the deflection decreases by about 25 &mgr;m. This shows the importance of the thickness of the silicon film as compared with that of the oxide film.
A conceivable step for reducing the deformations of the structure would consist of producing a second oxide film on the free face, called the rear face, of the thick main layer of the structure. This step would actually enable the deformation of the plates to be reduced before their bringing into contact. However, in a certain number of applications, it is necessary to remove the rear oxide film. Now, after thinning, if the oxide film is removed from the rear face, it is seen that deformation is restored and finally a deformation of the SOI structure mainly related to the thickness of the oxide film, is obtained.
On this point, reference may be made to document (1) the reference of which is specified at the end of the description.
According to another possibility, illustrated by
FIG. 5
, an attempt may be made to reduce the effect of the stresses by bringing into contact two main silicon layers
10
a
,
10
b
each provided with an oxide film
20
a
,
20
b
at the surface, the films being of comparable thickness. However, it is seen that a deformation appears for the structure when thinning one of the layers. Further, as shown in
FIG. 5
, the initial deflection of both main layers increases the difficulty for bringing into contact the surfaces of the oxide surface layers. This may locally generate areas with poor contact and therefore recesses or defects in the final structure.
The deformation phenomenon described above for a structure combining silicon and silicon oxide layers exists for a large number of pairs of materials. However, the generated deformation may vary depending on the materials brought into contact with each other, and notably on the type of stress which occurs, either a tensile or compression stress.
As an example, as shown in
FIG. 6
, when a silicon nitride film
30
is deposited on a silicon wafer
10
, this coating may generate, depending on the conditions of its implementation and after cooling, stresses also leadin
Aspar Bernard
Cartier Anne-Marie
Moriceau Hubert
Rayssac Olivier
Anderson Kill + Olick P.C.
Jones Deborah
Lieberstein Eugene
Meller Michael N.
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