There is an increasing requirement for high-grade adhesives especially in electronics and microelectronics, mechanical engineering, automotive engineering, and also aerospace. In many cases the critical factor is that these adhesives must withstand the extremely high thermal and chemical loads without loss of bond strength or of physical properties. It is also important that they absorb very little water, if any, since water absorption may lead at low temperatures to stresses and cracks, and at high temperatures gives rise to blistering.
The application and testing temperatures here may amount, for example, to between −80° C. to +450° C. Particularly in microelectronics (processes at up to 450° C.) and automotive engineering (adhesive bonds directly in the engine or transmission area), very high temperatures occur. Adhesive bonds in the area of microelectronics, chemical plant, and in the engine area necessitate very high stability of the adhesives toward solvents, acids, bases and/or aggressive gases. At the present time there is a lack in particular of adhesives which meet the requirements specified above and which are highly suitable for bonding a variety of materials, such as silicon, metal, glass, stone and/or ceramic.
In the field of microelectronics and the semiconductor industry the stacking of chips (ICs, integrated circuits) is significant owing, for example, to the increase in memory capacity, since through stacking it is possible to raise the memory capacity without increasing the area of the chip. The stacking technique is especially significant for the combination of different chips; for example, of memory chips and logic chips. Thus during the processing of the silicon wafer it is possible to carry out cost-effective production of only one kind of chips, which are later stacked atop one another and electrically contacted.
In accordance with the state of the art, materials, including chips and/or wafers, are adhesively bonded, for example, by using polyimide adhesives (C. Feger, M. M. Khojasteh, J. E. McGrath, Polyimides: Materials, Chemistry and Characterization, Elsevier Science Publishers B. V., Amsterdam, 1989, p. 151 ff.). Although the polyimides exhibit good temperature stability, the presence of the keto groups means that they absorb a relatively large amount of water, leading to the problems referred to above. Moreover, the adhesion of polyimides to many materials used in particular in microelectronics and optoelectronics is poor.
In the field of microelectronics, polyimides have been used, for example, as follows:
A polyimide is applied to the first wafer, dried, and baked in an oven at about 400° C. The surface of the polyimide layer is then activated in a plasma (argon, oxygen). This wafer is then bonded with a second, likewise plasma-activated, wafer, with the activated sides facing one another. The second wafer can, but need not necessarily, have a polyimide layer. The great disadvantage of this process is that the bonding must be performed within about one hour following activation, since otherwise the surfaces become deactivated. Moreover, owing to the presence of the carbonyl groups, polyimide may absorb water, which may lead to blistering later on when the stack is subjected to temperature. This greatly restricts the usefulness of the process. For chip-on-wafer applications it is practically impossible to employ this process, since in general up to three hours may be needed for the bonding of the chips to a wafer, especially in the case of 200 nm and 300 nm wafers.
EP 807 852 B1 discloses compositions which comprise polyhydroxy amides, a diazoquinone compound, and a phenolic compound and/or an organosilicon compound. In the tests which were carried out here, the adhesiveness fell substantially when the phenol compound or organosilicon compound was removed from the composition or was not present in the amounts stated. Polyhydroxy amide compounds have the disadvantage that, when used as adhesives, in the course of the cyclization to polybenzoxazoles that takes place when they are so used, they eliminate water, which makes the adhesive bonding of relatively large areas more difficult.
Japanese laid-open specification JP 11354591 A (abstract) discloses a photosensitive adhesive for the semiconductor industry, which may comprise polybenzoxazoles.
The operation of chip and/or wafer bonding requires a highly temperature-resistant and chemical-resistant adhesive bond, since a stack of this kind and hence the adhesive may come into contact with aggressive solvents and gases. Moreover, the temperatures are frequently up to 450° C., in the case of tungsten CVD depositions, for example. The adhesive used must absorb very little water, if any, since otherwise there will be blistering at high temperatures and it may in some cases not be possible to produce the contacts reliably.