Semiconducting or conducting wafers or stacks of wafers are today used for many types of equipment. The most conventional use of wafers is to act as a mechanical support for electronic equipment. In recent years, when also mechanical systems have become miniaturized, wafers are also used for more mechanical purposes, acting as supports and or parts of the actual mechanical systems. MEMS (Micro Electro Mechanical System) technology is today used for producing many kinds of mechanical or combined mechanical and electrical systems.
In most applications, the wafer assemblies are provided with different types of electrical equipment, and there is typically a need for providing electrical contact between the different sides of a wafer, or between different surfaces in a wafer stack. There is a need for electrical connections or vias through or around the wafer assembly. In prior art, electrical connections are typically provided by inserting a metal pin through a hole provided through a wafer. The pin is electrically connected at each side of the wafer. However, when the electrical structures become smaller and smaller, the handling of such electrical connections becomes difficult, due to the small dimensions. Also, electrical connections based on metal pins are difficult to make vacuum tight. One approach is then to manufacture an electrical connection directly based on the wafer. One example of how such an electrical connection can be provided in prior art can be found e.g. in the published U.S. patent application 2003/0022475.
Furthermore, in many applications today, several wafers are provided on top of each other in a wafer stack, possibly bonded together. Electrical connections are often provided between different surfaces of these wafers. During the bonding process, the wafers are heated to high temperatures, which means that it is difficult to provide soldered electrical connections or electrical connections comprising metals of low melting temperature before the bonding process. Furthermore, if metal electrical connections are used the thermal expansion differs between the wafer and the electrical connections. In particular for electrical connections through multiwafer stacks, the difference in thermal expansion can be considerable, which leads to mechanical stress of the connections. This is particularly important, e.g. during bonding processes or other high temperature treatments. In the published U.S. patent application 2007/0020926, electrical connections consisting of low-resistance silicon is manufactured from the wafer itself. The use of low-resistance silicon as electrical connection material solves some of the above mentioned problems. The thermal expansion of the electrical connections becomes identical to the thermal expansion of the wafer, which removes any mechanical strain upon heating the treatment. However, the solution of U.S. 2007/0020926 is not very easily applicable on multiwafer stacks. The solution is also only applicable in cases where the electrical connection can be made in the same material as the main substrate.
One remaining problem with electrical connection solutions according to prior art is that they are incompatible with high-temperature treatments of wafers and wafer stacks. Furthermore, reliable mechanical connections of external mechanical or electrical parts to the electrical connections are difficult to provide. Also, electrical connections can not be provided in systems where the main substrate comprises a material that is not suitable to be utilized as an electrical connection material.