Process for joining inorganic substrates in a permanent manner

Details Process for joining inorganic substrates in a permanent manner Process for joining inorganic substrates in a permanent manner

1999-11-05

2001-12-11

Ball, Michael W. (Department: 1733)

Adhesive bonding and miscellaneous chemical manufacture

Methods

Surface bonding and/or assembly therefor

C156S273300, C156S274400, C427S533000, C427S535000, C427S569000

Reexamination Certificate

active

06328841

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of permanently connecting inorganic substrates and, in particular, to a method of connecting a first and a second silicon wafer by means of a polyimide layer.
DESCRIPTION OF BACKGROUND ART
In the field of microelectronics, future applications require not only an increase in the computing performance or in the storage capacity of an IC component (IC=integrated circuit) but also an extension of the functionality thereof. Such an extension of functionality comprises e.g. the possibility of combining sensor technology, data processing and data storage on one chip component, e.g. for a CCD chip (CCD=charge coupled device), which is integrated directly on one logic level for evaluating and reducing large amounts of data. Starting from the planar standard CMOS technology, efforts concerning the development of a vertical connection technique are therefore increasingly made so as to fulfill the future system requirements in the field of microelectronics.
The desired option, viz. that also wafers with sensitive surface structures can vertically be integrated with a second wafer, makes it necessary to possess a connection technique permitting a wafer bottom and a wafer top to be placed one on top of the other, i.e. to be permanently connected. In the field of technology, it is known to connect surfaces of bodies, which consist of plasma oxide, thermal oxide and silicon, with polyimide.
The only material that can be used for a CMOS-compatible electric connection of two mechanically connected wafers is tungsten which is deposited in a CVD process (CVD=chemical vapour deposition) from the gaseous phase (WF
6
) at 400 to 450° C. This deposition of tungsten can only be carried out after the mechanical connection of the two wafers. This means, in turn, that for the mechanical connection of the wafers a method must be found which provides a mechanical connection of said wafers that is capable of resisting temperatures of up to 450° C.
Conventional adhesive materials are not suitable for establishing a connection of the above-mentioned kind, which is capable of resisting temperatures of up to 450° C., since said materials are not sufficiently temperature stable. Furthermore, conventional adhesive materials cannot be applied to the wafers with the necessary homogeneity in the range of from approx. 1 to 3 &mgr;m thickness. The temperature requirement prevents the use of organic adhesives, the homogeneity requirement prevents the use of mineral-matter-filled, inorganic adhesives for establishing a permanent mechanical connection between two wafers.
DESCRIPTION OF PRIOR ART
The article “DIRECT BONDING OF ORGANIC POLYMERIC MATERIALS”, SPIERINGS G.A.C.M. et al, in Philips J. Res. 49 (1995), pp. 139-149, discloses connection methods, e.g. for connecting a silicon layer with a polyimide layer, in the case of which an originally hydrophobic polyimide layer is rendered hydrophilic by means of a surface treatment, whereupon it is connected to a silicon wafer.
DE-A-4404931 discloses a method and devices for directly connecting two bodies. A reactive gas, such as oxygen and hydrogen is used for forming hydroxide groups on at least one surface of two bodies connected, according to the method and devices described. DE-A-4404931 mentions, by way of example, polyimide, SiO2 or silicon as bodies to be connected.
In Den Besten C. et al, “Polymer Bonding of Micro-Machined Silicon Structures”, Proceedings of the Workshop on Micro Electro Mechanical Systems '92, Travemunde, Feb. 4 to 7, 1992, (No. Workshop 5, Feb. 4, 1992, Benecke W., Petzold H.-C., pp. 104-109, XP000344134) different ways of connecting silicon wafers are described. One possibility described is a direct connection between silicon and silicon, which is based on a chemical reaction between OH groups existing on the surface of SiO
2
. Such connection methods are carried out at 200° C. to 300° C. A further connection mode described in the above-mentioned publication is indirect bonding in the case of which an intermediate layer of glass containing sodium or an intermediate layer of boron glass is used. Furthermore, the use of an intermediate layer of metal for carrying out eutectic bonding is described. The above-mentioned publication additionally discloses a method of indirectly connecting two silicon wafers in the case of which a negative photoresist, a polyimide or an epoxide is used for connecting two silicon wafers indirectly.
Starting from the above-mentioned prior art, it is the object of the present invention to provide a method for permanently connecting two silicon wafers, the connection area being capable of resisting temperatures of up to 500° C. and showing the demanded homogeneity.
This object is achieved by methods of connecting a first and a second silicon wafer according to claims
1
and
4
.
The present invention provides a method of permanently connecting silicon wafers making use of a polyimide layer which is subjected to a separate surface modification prior to the connection operation. Such a polyimide layer allows a wafer bonding process (wafer bonding=establishing an atomic connection between solids) permitting at bonding temperatures of less than 450° C. a stable and, in particular, a blister-free wafer connection. The connection area of two wafers, which are connected by means of a surface-modified polyimide layer, shows a greatly increased tensile strength in comparison with the interface of a conventional polyimide connection.
According to a first embodiment of the present invention, a first silicon wafer is first provided with a polyimide layer on a main surface thereof. Subsequently, a plasma-induced reaction is caused to take place between the polyimide layer and water so as to make the surface of the polyimide hydrophilic. In addition, a plasma-induced reaction is also caused to take place between a main surface of a second silicon wafer, which is to be connected to the first silicon wafer, and chlorine so as to make the main surface of the second silicon wafer hydrophilic. The main surface of the second silicon wafer, which is to be connected to the first silicon wafer, is then treated with hydrolyzed triethoxysilylpropanamine. Subsequently, the first and the second silicon wafer are joined together, whereby the main surfaces thereof are connected.
According to a further embodiment, an SiO
2
layer is formed on the main surface of the second silicon wafer which is to be connected to the first silicon wafer. This SiO
2
layer is treated with hydrolyzed triethoxysilylpropanamine. The treated SiO
2
layer is then either connected directly to the surface-modified polyimide layer or subjected to a plasma-induced reaction with chlorine prior to being connected.
Further developments of the present invention are disclosed in the dependent claims.
Preferred embodiments of the present invention will be explained in detail in the following, part of said explanations referring to the drawings enclosed in which:
FIG. 1
shows schematically a set-up for carrying out a tensile test for two interconnected wafers; and
FIG. 2
shows a “tear-off spectrum” as a result of the tensile test which is schematically shown in FIG.
1
.
In the following, the designation “bonding” constituting the basis of the connection method according to the present invention will be explained. The designation “bonding”, which is used in German in the same way, is used for referring to connections established via a chemical reaction resulting in covalent bonds. This connection technique clearly differs from connection techniques in the case of which much weaker bonds are produced by means of adhesive joining. The connection established by “bonding” does normally not comprise an adhesive body, which may be the limiting factor due to its material-dependent stability under temperature and tensile strain, but is a process, . . . .
The present invention provides a method of permanently connecting silicon wafers making use of a polyimide layer which is subjected to a sepa

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