1. Field of the Invention
The invention relates to the production of microsystems with microelectromechanical structures, a vertical electrical throughplating of cover disks being used for the microelectromechanical structure.
2. Description of Related Art
In the silicon-based microsystem technology it is necessary in the production of microelectromechanical systems (MEMS) to protect the miniaturized, freely movable mechanical structures which are thus very sensitive to mechanical damage, humidity and corrosion and which are in contact with electrical components by means of a cover (typically silicon or glass). Mostly, its application already takes place in the wafer composite as an effective parallel joining process (wafer bonding), which, in addition, does not only offer a protection in the use during the service life, but also for the further processing (separation, assembly and connection technologies). As a rule, a defined atmosphere with a specific pressure is enclosed in the case of a hermetically sealed attachment of the cover, which is of importance for the function of the component. However, it must be ensured when covering the microelectromechanical structure that electric contact lines for the energy supply to the microelectromechanical system and the reading out of signals (e.g. sensor output signals) are guided into the hermetically sealed area and/or guided out of the same. As a rule, this is done by means of a horizontal leadthrough of conductor strips in the area of the connection point of system wafer and cover chip (and/or wafer). Here, the leadthroughs of conductor strips must be buried in an expensive fashion during direct and anodic bonding in order to obtain a plane sealing connection surface. During bonding with soft intermediate layers (glass solder bonding, adhesive bonding) metallic standard strip conductors can be embedded into the connection material in a hermetically sealed fashion. In both cases connecting faces (bond pads) of cover and system chip (and/or wafer) for the later wire bonding must be outside the cover in the connection plane.
This arrangement entails the following disadvantages:
1. The cover wafers must have large openings through which the connecting faces (in most cases a great number) are accessible. These openings can only be effectively made in the wafer composite prior to and after bonding, which, however, is always connected with a high expenditure (e.g. deep silicon etching, glass structuring).
However, the mechanical strength of the cover wafers is greatly reduced due to the openings so that they become susceptible to fractures. Consequently, the openings for the connecting faces cannot be dimensioned and arranged in an optional large size.
2. The connecting surfaces must imperatively be located next to the active microelectromechanical structures, i.e. additional space on the chip is required, whereby the integration density is limited and high costs are incurred. In extreme cases the connecting surfaces and the active microelectromechanical structures may have the same space requirements.3. The connecting surfaces are not located on the chip surface as this is customary in the case of standard integrated circuits, but are lowered by the thickness of the cover. This renders the use of standard methods of the assembly and connection technologies (wire bonding) more difficult and/or makes the use of special technologies such as the flip-chip bonding very expensive or even impossible.
To overcome the disadvantages vertical throughplatings can be used in order guide an electric contact from the front side to the rear side through an opening in the cover wafer so that covered structures may be electrically connected. Various processes are known for the production of such throughplatings. Thus, the through holes can e.g. be completely filled with metals in chemical and/or electrochemical processes. Here, the special difficulty consists in ensuring a gas-tight filling. A method is known from literature, cf. Wiemer, Technologieentwicklung für Beschleunigungssensoren and Drehratensensoren unter Nutzung von Wafer-Bondverfahren, Dissertation April 1999, Chemnitz, in which the holes are only metallized on the side walls and their hermetically sealed closure is implemented by means of the bonded microcover.
All processes known so far are very expensive and, due to their lengthy individual processes (metal filling of the holes), they are very expensive and error-prone.