Electricity: electrical systems and devices – Electrostatic capacitors – Variable
Reexamination Certificate
2001-02-28
2003-04-29
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Variable
C361S280000, C361S283100, C361S285000, C361S283400, C257S417000, C257S419000, C438S052000, C438S053000
Reexamination Certificate
active
06556418
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a micromechanical component and a process for its fabrication.
With a view to ever faster and smaller circuit configurations, attempts are made to integrate an electronic circuit together with a micromechanical component, for example, a sensor or an actuator, into a single chip.
In T. Scheiter et al., “Full integration of a pressure sensor system into a standard BiCMCOS-Process”, Eurosensors XI, 11th European Conference on Solid State Transducers, Warsaw, Poland (1997) 1595, a pressure sensor is described that is produced in a standard BiCMOS process on a surface of a silicon substrate. To produce the pressure sensor, a doped region, which acts as a first capacitor electrode of a capacitor, is produced at the surface of the substrate. A field oxide 600 nm thick, which serves as a sacrificial layer, is produced over the doped region. A layer of polysilicon 400 nm thick is deposited over the sacrificial layer. Openings are produced in the polysilicon layer. Then, through these openings, part of the sacrificial layer is removed by etching, which produces a cavity under the polysilicon layer. The openings are closed by a further deposited layer. The further layer is then structured such that parts of the polysilicon layer that are disposed above the cavity are exposed. Parts of the polysilicon layer and parts of the further layer, which are disposed respectively above the cavity, form a membrane. In the region of the openings, the membrane has thickened portions, at which it is inflexible. The parts of the polysilicon layer that are parts of the membrane act as second capacitor electrodes of the capacitor. As a result of deflection of the membrane because of a pressure, the distance between the first capacitor electrode and the second capacitor electrode is changed, which changes the capacitance of the capacitor, which is a measure of the pressure. The size of the area of the exposed parts of the polysilicon layer, that is to say, a deformable region of the membrane, determines the stiffness of the membrane. The higher the pressure range to be measured, the smaller the deformable region should be. One disadvantage is that mechanical loading brought about by the deflection is substantially distributed only to deformable parts of the membrane. For pressures that are greater than about 20 bar, such a pressure sensor is not suitable because the mechanical loading of the deformable membrane regions is close to the fracture limit. Moreover, process fluctuations, such as lithography faults, have an unmanageably large influence on the stiffness of the membrane because of the small size of the deformable regions.
In G. Ehrler, “Piezoresistive Silizium-Elementardrucksensoren” [Piezoresistive silicon-element pressure sensors], Sensormagazin 1/92, 10, a pressure sensor is described in which the pressure is measured with the aid of the piezoresistive effect. Four diffusion regions are produced on a surface of a silicon substrate and are connected to form a Wheatstone bridge. A passivation layer is disposed over the diffusion regions. An opening is produced in a rear of the substrate that reaches as far as the diffusion regions. The depression forms a pressure chamber that, for absolute pressure sensors, is under vacuum and is closed from below. A layer of the substrate, in which the diffusion regions are disposed and which is disposed above the pressure chamber, acts as a membrane of the pressure sensor. A pressure that acts on the passivation layer deflects the membrane, which produces stresses in the membrane. Because of the piezoelectric effect, the stresses lead to changes in the electrical conductivity of the layer of the substrate and, therefore, to changes in the magnitudes of the resistances of the diffusion regions, which are a measure of the pressure. Such a sensor is suitable as a high-pressure sensor. However, the process outlay for producing such a sensor is very high, in particular, due to the processing of the silicon substrate both on the front and on the rear.
In H. Dudaicevc et al., “A fully integrated surface micromachined pressure sensor with low temperature dependence”, Transducers '95 Eurosensors IX (1995) 616, a pressure sensor is described in which a cell includes a first electrode of a capacitor, which is implemented at a doped region on a surface of a silicon substrate. An insulating layer of silicon nitride is applied to the surface. A sacrificial layer of oxide is deposited over the insulating layer and is structured such that it fills a cavity to be produced and has a diameter of about 100 &mgr;m. A thin oxide layer is then deposited and structured to make a spur of oxide laterally adjoin the sacrificial layer. Then, a layer of polysilicon is deposited and structured to cover the sacrificial layer and part of the spur. By etching the oxide selectively with respect to polysilicon and silicon nitride, the thin oxide layer and the sacrificial layer are removed, the spur acting as an etching channel. The cavity is formed under the polysilicon layer. The polysilicon layer is supported on the silicon nitride layer. To seal off the cavity, oxide is deposited and closes the etching channel laterally. To completely cover the etching channel, it is important that the thin oxide layer not be too thick. As a result, the etching process is slow and possibly incomplete. In addition, an etching medium used can be flushed out only with difficulty. A number of identical cells are disposed in an x-y grid and connected in parallel with one another.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a micromechanical component and process for its fabrication that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that can be configured as a high-pressure sensor and, as compared with the prior art, can be fabricated with a low process outlay or with a higher process reliability.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a micromechanical component, including at least one cell having a cavity with a given vertical dimension, a membrane acting as an electrode of a capacitor of the at least one cell, the membrane homogeneously disposed with a substantially uniform thickness over the cavity, a counter-electrode of the capacitor disposed under the cavity, at least one etching channel laterally adjoining the cavity, the at least one etching channel having a vertical dimension equal to the given vertical dimension, and at least one closure adjoining the at least one etching channel from above and disposed outside the membrane.
The objectives of the invention are achieved by a micromechanical component that includes at least one cell having a membrane that acts as an electrode of a capacitor of the cell and that is disposed with substantially uniform thickness over a cavity in the cell. Disposed under the cavity is a mating or counter electrode of the capacitor. At least one etching channel laterally adjoins the cavity. The etching channel has a vertical dimension equal to a vertical dimension of the cavity. A closure adjoins the etching channel from above.
With the objects of the invention in view, there is also provided a process for fabricating a micromechanical component, including the steps of producing a sacrificial layer over a counter-electrode of a capacitor of a cell, structuring the sacrificial layer to fill a region of a cavity to be produced in the cell and to fill an etching channel laterally adjacent to the cavity, conformally applying an upper conductive layer over the sacrificial layer, producing an opening into the upper conductive layer through the etching channel reaching as far as the sacrificial layer, etching the sacrificial layer to produce the cavity, a part of the upper conductive layer disposed over the cavity becoming capable of being deflected and acting as a membrane of the cell and as an electrode of the capacitor, and c
Aigner Robert
Kapels Hergen
Oppermann Klaus-Günter
Dinkins Anthony
Greenberg Laurence A.
Ha Nguyen
Infineon - Technologies AG
Locher Ralph E.
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