Pressure sensor and process for producing the pressure sensor

Measuring and testing – Fluid pressure gauge – Electrical

Reexamination Certificate

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Details

C073S754000, C073S756000

Reexamination Certificate

active

06732590

ABSTRACT:

BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a pressure sensor which is subjected to an external pressure, having a semiconductor chip which has a pressure-sensitive area, and a process for producing such a pressure sensor.
Such pressure sensors constructed on the basis of semiconductor materials and using what is known as the MEM technique or microelectromechanical technique are used for automotive applications, such as motor vehicle tires, and exhibit a high failure rate with increased requirements in relation to increased attack of aggressive media and increased accelerative loadings.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a pressure sensor which withstands the increased requirements in relation to environmental influences, to accelerative and external pressure loadings and attacks of aggressive media.
According to the invention, the pressure sensor has a semiconductor chip having a pressure-sensitive area which is subjected to an external pressure, and contact areas which are arranged on a pressure-insensitive area of the semiconductor chip. The semiconductor chip is arranged in a hollow housing which has a housing base on which the semiconductor chip is adhesively bonded. The hollow housing additionally has a housing wall which surrounds the semiconductor chip and through which flat conductors project with an internal section into a housing interior. External sections of the flat conductors project out of the housing wall. Arranged between the contact areas of the semiconductor chip and the internal sections of the flat conductors are connecting elements which connect predetermined contact areas electrically to corresponding internal sections.
The connecting elements, the internal sections and the contact areas within the surrounding housing wall are covered by a first plastic compound. The pressure-sensitive area of the semiconductor chip is at least partly covered by a second plastic compound. Because of its material characteristics, under the same external pressure loading, the first plastic compound is subjected to lower deformations than the second plastic compound.
This pressure sensor according to the invention has the advantage that, as a result of extensive covering of the. pressure-insensitive areas within the housing of a first plastic compound with a negligible deformation, the deformation loadings at increased external pressure are reduced, in particular for deformation-sensitive components such as the connecting elements. As a result of limiting the highly deforming second plastic compound to a pressure-sensitive area of the sensor, the remaining areas and components within the housing are protected against distortions and displacements as a result of tensile, compressive and shear stresses during deformation of the pressure-sensitive area.
The pressure sensor according to the invention is able to satisfy the increased requirements on temperature-cycle resistance and pressure resistance and erosion resistance with respect to aggressive media without failing. A pressure sensor protected in this way by two different plastic components can advantageously be used for continuous operational monitoring of the tire pressure in rotating vehicle tires up to a tire pressure of 100 MPa without it being possible to determine large temperature hysteresis values in the operating temperature range between −50° C. and +150° C. The scatter in the temperature response is likewise reduced as compared with pressure sensors merely having a silicone gel covering on all sides.
The first plastic compound preferably has a thermosetting plastic made of an epoxy resin or a silicone resin. These resins, with appropriate fillers, can exhibit a coefficient of thermal expansion which is matched to the coefficient of expansion of the semiconductor material and/or the material of the hollow housing. The hollow housing has either a ceramic substance or a plastic material. In the ceramic substance or the plastic material, a transition layer of a flat conductor is embedded in such a way that an internal section of the flat conductor projects into the interior of the hollow housing and an outer section of the flat conductor projects outward from the housing wall.
The flat conductor is anchored in the hollow housing by the transition section. In order to support the internal section, the hollow housing can have a ledge on the housing inner wall, to which the internal section of the flat conductor is fitted. This ensures secure bonding of a bonding wire between a contact connection area of the internal section of the flat conductor and a bonding wire which is intended to connect the flat conductor to contact areas on the pressure-insensitive areas of the semiconductor chip.
In the event of thermal loading, in particular the flat connections between the bonding wire and the contact connecting area and the bonding wire and the contact area are endangered if, in these intrinsically pressure-insensitive areas of the pressure sensor, highly deformable plastic protective layers made of a resilient elastomer are applied. It is therefore advantageous to protect these areas not serving as sensors against thermal stresses and severe deformations of a covering plastic compound such as the second plastic compound. Furthermore, the first plastic compound adheres both to the pressure-insensitive areas of the semiconductor chip and to the inner walls of the hollow housing, so that the interfaces between the first plastic compound and semiconductor chip and also between the first plastic compound and the hollow housing are protected against aggressive media.
The second plastic compound preferably has a plastic gel of a resilient elastomer based on silicone. In this case, the high resilience permits protection of a membrane of semiconductor material arranged underneath in the pressure-sensitive area of the semiconductor chip without hysteresis effects building up. Such resilient elastomers are based on dimethyl polysiloxane or phenyl polysiloxane and can be used for operating temperatures in the range from −55° C. to +200° C. or −120° C. to +200° C., depending on the base material. A further preferred resilient elastomer is based on fluorosiloxane and can be used at operating temperatures between −55° C. and +175° C. Fluorosiloxanes of this type can be used in particular for the vehicle sector, since they are resistant with respect to fuels and solvents.
For resilient elastomers of this type, based on silicone, the energy loss factor at a predefined pressure cycle frequency, of the order of magnitude of minus four powers of ten, is extremely low, so that such a second plastic compound follows the deformations of the pressure-sensitive area of the semiconductor chip with negligible energy loss. Furthermore, elastomers based on silicone have the advantage that they can form intensive adhesion to silicone resins. The risk of microcracks in the interface between the first plastic compound and the second plastic compound can therefore be reduced if a silicone resin is used as the first plastic compound and an elastomer based on silicone is used as the second plastic compound.
The hollow housing can have a housing cover with an opening which leaves the pressure-sensitive area and the second plastic compound free. A housing cover of this type can advantageously be matched to the inner dimensions of the housing wall, by its external:dimensions permitting a clearance fit with respect to the inner dimensions of the housing wall of the hollow housing. Following the application of the first plastic compound and still before the crosslinking of the resin, this housing cover is pressed onto said resin, at the same time any joints with respect to the housing wall being sealed off. A second plastic compound can be introduced into the opening that leaves the pressure-sensitive area of the semiconductor chip free, before or else after the fitting of the housing cover.
In order to construct the semiconductor chip as a pressure

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