Semiconductor device manufacturing: process – Bonding of plural semiconductor substrates
Reexamination Certificate
1998-08-11
2002-08-06
Chaudhuri, Olik (Department: 2814)
Semiconductor device manufacturing: process
Bonding of plural semiconductor substrates
C438S458000, C148SDIG001, C148SDIG001
Reexamination Certificate
active
06429094
ABSTRACT:
TECHNICAL DOMAIN
This invention relates to a treatment process for bonding two structures by molecular adhesion, and unbending.
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.
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.
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.
STATE OF PRIOR ART
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 (≡10 cm) and a thickness of less than 200 &mgr;m are used.
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.
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.
The attached
FIGS. 1
to
3
described below illustrate transfer of a thin layer comprising integrated circuits, as an example.
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.
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
.
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
.
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.
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.
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
.
This type of process is described in more detail in document (1), for which the reference is given at the end of this description.
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.
Furthermore, the material (glue) added for bonding can cause metallic or organic contamination of bonded parts during subsequent treatments.
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.
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 Å) as a root mean square value, the lack of any dust (particles >0.2 &mgr;m) on surfaces, the planeness of the surfaces to be put in contact, and the chemical state of these surfaces.
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.
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
2
O
26
adsorbed on the hydrophile layer
24
.
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.
The bonding energy as measured by a blade insertion method is of the order of 0.15 J/m
2
. This value is typically the value of hydrogen type adhesion between two water layers, on each structure.
Document (2), the reference of which is given at the end of this description, contains an illustration of the blade insertion method.
A third step consists of solidification heat treatment of the adhesion.
The heat treatment can eliminate water layers between the assembled structures, up to a temperature of the order of 200° C.
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° C. to 900° C.
Finally, there may be a fourth step consisting of heat treatment at more than 900° 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.
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
2
and temperatures are expressed in °C.
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.
Note that at treatment temperatures above 600° C., it becomes impossible to unbond the two assembled structures without causing severe degradation to them.
When the assembled structures are silicon boards, bonding energies greater than 2 J/m
2
may be obtained. These energies are thus of the same order of magnitude as the cohesion energies of the silicon material.
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.
Thus, the thin layer is separated from the handling substrate by eliminating the handling substrate. For example the handlin
Aspar Bernard
Maleville Christophe
Chaudhuri Olik
Commissariat A l'Energie Atomique
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Rao Shrinivas H.
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