Measuring and testing – Fluid pressure gauge – Electrical
Reexamination Certificate
2001-03-22
2002-08-20
Oen, William (Department: 2855)
Measuring and testing
Fluid pressure gauge
Electrical
Reexamination Certificate
active
06435033
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a microsystem with a flexible membrane for a pressure sensor and its manufacturing process.
A microsystem with a flexible membrane refers to a pressure sensor cell, for example of a capacitive measurement type, or a switch cell triggered by pressure.
The invention has applications in the manufacture of microsensors, microswitches, variable microcapacitors, and more generally micro-components that can be integrated with an associated electronic circuit. One particular application of the invention is the manufacture of a fingerprints sensor.
2. Background Art
French patent publication 2 700 003 published Jul. 1, 1994, describes the manufacture of pressure micro-sensors with a deformable silicon membrane.
Due to their manufacturing process, compatibility difficulties occur when making this type of microsensor jointly with CMOS type integrated circuits. Furthermore, these sensors include a generally conducting membrane, which may cause electrical insulation problems when components are miniaturized.
Insulating membrane sensors made on glass substrates are also known. For example, this type of sensor is described in Canadian patent publication CA 2,130,505, published Aug. 19, 1994.
The reference entitled “A Surface Micromachined Miniature Switch for Telecommunication Applications with Signal Frequencies from DC up to 4 GH
z
” from J. Jason and F. Chang, International Conference on Solid State, Sensors and Actuators, Stockholm, Sweden, Jun. 25-29, 1995, describes the manufacture of sensors associated with electronic circuits and adjacent to each other on the same substrate. This type of design is particularly attractive for production manufacturing.
However, it has a number of limitations concerning interconnection problems between sensors and the corresponding electronic circuits. The electrical connections between the sensors and integrated circuits placed on the same substrate generate harmful parasite capacitance effects. Furthermore, the electrical connections are cumbersome and form an obstacle to increased miniaturization of the devices.
Finally, separate manufacturing of sensors and associated electronic circuits result in high manufacturing costs.
SUMMARY OF THE INVENTION
The purpose of this invention is to propose a process for manufacturing Microsystems with a flexible membrane that do not have the limitations mentioned above. In particular, one purpose is to propose microsystems with a flexible membrane made integrally on a substrate according to a process compatible with the manufacture of MOS integrated circuits in the same substrate.
Another purpose is to propose pressure sensors or integrated microswitches that can be laid out in the form of a large number of adjacent cells, for example in the form of a matrix.
Another purpose is to propose a fingerprints detector using this type of sensor.
Finally, another purpose is to reduce the manufacturing cost of Microsystems with flexible membranes.
In order to achieve these objectives, the purpose of the invention is more precisely a process for manufacturing a microsystem for a pressure sensor comprising the following steps:
a) deposit and forming of at least a first conducting layer on a support, the conducting layer forming at least a first electrode,
b) deposit and forming of at least one layer of sacrificial material covering the first conducting layer,
c) deposit and forming of a second conducting layer on the layer of sacrificial material in a region located above the first conducting layer, the second conducting layer forming a second electrode,
d) formation of a first membrane layer covering and surrounding the layer of sacrificial material and the second conducting layer,
e) elimination of the layer of sacrificial material,
f) forming the first membrane layer.
The steps in the process are preferably carried out in the order mentioned above. However the order of steps e) and f) can be reversed. Furthermore, in one special embodiment, the layers mentioned in steps b) and c) may be formed at the same time.
In particular, when major cutting or thinning operations are necessary on the first membrane layer, it is useful to not release it by eliminating the sacrificial layer, until it has been completely formed.
The layer of sacrificial material is formed so as to define the shape and size of a closed chamber, in which one wall is formed by the first membrane layer.
The process according to the invention can be used to make simple and inexpensive Microsystems, and particularly cells for pressure sensors suitable for common integration with CMOS or BICMOS type circuits on the same substrate. This aspect will be described in more detail in the rest of the description.
Beneficially, with the process according to the invention, the first and second conducting layers forming the electrodes are not separated by any material layer. After the layer of sacrificial material has been eliminated, the second conducting layer is kept separated from the first conducting layer by means of the flexible membrane layer, when there is no pressure exerted on this membrane layer.
A particularly sensitive sensor cell can be made by adjusting the thickness of the layer of sacrificial material.
Furthermore, the first and second layers of conducting material may be separated by a very small distance only, for example of the order of about 0.1 &mgr;m to 5 &mgr;m. This characteristic is also useful for making sensitive sensors. For example, the first and second layers could form the armatures of a capacitor in which the capacitance varies as a function of a deformation of the membrane layer.
In other applications, the microsystem obtained by the process according to the invention could also form a microswtich. The first and second conducting layers may in this case form the terminals of the switch. For example, this type of switch is open when the deformation of the membrane layer is insufficient for the first and second conducting layers to come into contact with each other, and is closed when the conducting layers are pressed in contact with each other.
According to one alternative embodiment of the microsystem for use as a switch, a groove is formed in the first layer of conducting material to separate and delimit two electrodes, and the second conducting layer is then formed on the layer of sacrificial material in a region above at least one portion of the groove.
In a microsystem made according to this alternative, when the membrane layer is deformed, the second conducting layer electrically connects the two electrodes in the first conducting layer which thus form the terminals of the switch.
According to one specific embodiment of the process according to the invention, elimination of the layer of sacrificial material may include opening of at least one etching channel through the first membrane layer, etching of the layer of sacrificial material and the formation of a second membrane layer covering the first membrane layer and closing the etching channel, the second membrane layer also being formed during step f) in the process.
For example, step f) may include partial elimination of the second membrane layer and thinning of the first membrane layer in a region surrounding the second electrode.
Thinning of the first membrane can precisely adjust its suppleness or flexibility and therefore the sensitivity of the microsystem.
Preferably, thinning takes place in a region surrounding the electrode formed in the second conducting layer but not above this electrode. This characteristic increases the stiffness of the membrane in the region of the second electrode and thus prevents excessive deformation of this electrode when pressure is applied to the membrane.
Preferably, formation of the first membrane layer may include the successive deposit of three sublayers of material, in which at least one sublayer forms an etching stop sublayer. The first membrane layer is then thinned by etching, stopping the etching on the etching stop sublayer.
Thus the final thickness of th
Anderson Kill & Olick
Commissariat a l'Energie Atomique
EugeneLieberstein
Meller Michael N.
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