Electricity: electrothermally or thermally actuated switches – Thermally actuated switches – With bimetallic element
Reexamination Certificate
2000-06-06
2004-11-02
Vortman, Anatoly (Department: 2835)
Electricity: electrothermally or thermally actuated switches
Thermally actuated switches
With bimetallic element
C337S370000, C337S078000
Reexamination Certificate
active
06812820
ABSTRACT:
FIELD OF THE INVENTION
The present invention involves a microsystem with an element which can be deformed by the action of a thermal sensor. Such microsystems can be applied to microswitches for opening or closing electric circuits and microvalves for microfluid applications.
These Microsystems include an element in the form of a beam or a membrane which is deformed by heat. Strongly non-linear behaviour is sought in order to obtain a rapid shift between the two states, an open state and a closed state.
It must be possible to design these Microsystems so that they can be compatible with the making of microelectronic components.
STATE OF THE ART
The microsensors used to trigger the deformation of the deformable element of a microsystem can be put in three main categories as a function of the principles used. First, thermal actuators which use thermal dilatation of one or several of their components. There are also electrostatic actuators which use the electrostatic force generated between two elements with different charges.
Lastly there are magnetic actuators which use forces induced by a magnetic field.
There are also actuators which use piezoelectric and magnetostrictive materials.
The thermal actuators appear to be the most useful because they generally allow for larger deformations than electrostatic actuators whereas magnetic actuators, or those which use piezoelectric and magnetostrictive materials, are generally difficult to use with classic micro-machining processes, particularly for manufacturing which requires technological compatibility with microelectronics. In addition, with a thermal actuator, it is easy to generalise the use of a controlled microswitch to a thermal microswitch (change of state as of a critical temperature) or to a micro circuit breaker (change of state as of a certain critical current intensity).
The simplest way to make a thermal actuator is to use a bimetal. This technique involves two layers of materials having different thermal dilatation coefficients so that a variation in temperature of the whole unit causes a deflection of the bimetal. Temperature elevation is obtained by the Joule effect either by directly passing an electrical current into one of the two layers of the bimetal or into the resistors formed on one of these layers and obtained, for example, by implantation if one of the layers is made of silicon.
The deformation of the bimetal depends on the type of attachment to its support.
FIG. 1
shows the deformation due to the effect of a thermal stress on a free bimetal, i.e. at the ends which are not attached but merely supported, composed of a layer
1
and a layer
2
with different thermal dilatation coefficients. The broken line shows the average position of the bimetal in the absence of a thermal stress. The theory shows that in this case the radius of curvature &rgr; is uniform. It is negative if the coefficient of dilatation of layer
2
is greater than that of layer
1
.
If the deformable structure is embedded at its ends, it is preferable, because of the appearance of the deformity, to place the bimetal in the areas where the dilatation effect acts in the direction of the curvature. Depending on the location of the bimetal, an increase in temperature may deflect the structure in one direction or another.
FIG. 2
shows a first bimetal structure of this type. It includes a first layer
3
and a second layer
4
formed of two parts. The broken line indicates the average position of the bimetal in the absence of a thermal stress. As the thermal dilatation coefficient of the layer
4
is greater than that of the layer
3
, the deformation of the bimetal structure due to the effect of dilatation is in the direction indicated in FIG.
2
.
FIG. 3
shows a second bimetal structure embedded at its ends. It has a first layer
5
, which is embedded, and a second layer
6
which is located on the central part of the layer
5
. The broken line indicates the average position of the bimetal in the absence of a thermal stress. As the thermal dilatation coefficient of the layer
6
is greater than that of the layer
5
, the deformation of the bimetal structure due to the effect of dilatation is in the direction indicated in FIG.
3
.
The amplitude f of the deformation is proportional to the temperature and the deformation thus depends on the surrounding temperature. It is possible however to find structural configurations so that the deformation is independent of the surrounding temperature.
Due to the complex mechanisms involved during the opening and closing of an electric circuit however (electric arc, bounce phenomena, etc.), it is preferable to seek systems for which the change in state (the shift from the open state of the circuit to its closed state) is as rapid as possible. The ideal would be designing systems having a critical temperature beyond which the mechanical equilibrium state changes. This cannot be obtained with just a bimetal however.
The patent U.S. Pat. No. 5,463,233 discloses a micro-machined thermal switch which combines a bimetal and an electrostatic sensor. In the absence of deformation of the bimetal, the electrostatic force is weak, the bimetal is in equilibrium between the electrostatic force and the mechanical restoring force of the structure. When the temperature increases, the bimetal effect brings the electrodes of the sensor closer until the electrostatic force becomes sufficiently strong to overcome the mechanical restoring force and to thus trigger the instantaneous shift of the structure.
Another way to generate a displacement by a change in temperature is to heat an embedded beam or membrane.
FIG. 4
shows an embedded membrane
7
in resting position along the broken line and the deformed position by the solid line. The thermal dilatation compresses the structure. The theory of beams or membranes shows that there is a critical compression stress (and thus a temperature) beyond which the structure buckles. The article “Buckled Membranes for Microstructures” by D. S. Popescu et al., which appeared in the IEEE review, pages 188-192 (1994), describes such as structure in compression. In the case of a beam of thickness h, length L, made from a material with a dilatation coefficient &agr;, the critical compression stress is given by the equation:
θ
cr
=
π
2
h
2
3
α
L
2
(
1
)
The theory also shows that the amplitude f of the deformity of the structure is given by the equation:
f
=
±
θ
θ
cr
-
1
(
2
)
In the case of a square membrane, A is 2.298 h. One of the drawbacks of this method is the indeterminate nature of the sign of f. As
FIG. 4
shows, the membrane
7
may be deformed in the opposite direction and take the position indicated by the broken line. Equation (2) also shows that it is difficult to obtain high displacement amplitudes for structures made by surface technologies, i.e. in thin layers.
Another solution derived from the preceding one is to use a naturally buckled membrane. This is obtained by using silicon oxide membranes for example. The system thus has two stable positions
f
=
±
A
S
S
cr
-
1
,
where S is the internal stress and S
cr
is the critical buckling stress. To shift from one position to another an additional mechanical action is needed. In the article mentioned above by D. S. Popescu et al., this additional mechanical action is from a field of pressure on the membrane.
Embedded bimetals were studied in the article “Analysis of Mi-metal Thermostats” by TIMOSHENKO which appeared in the Journal of the Optical Society of America, vol. 11, pages 233-255, 1925. This article gives in particular a theoretical study of the structure shown in FIG.
5
. The deformable structure is a beam
10
composed of a bimetal, the ends of which are held by two fixed supports
11
and
12
. The retention of the ends eliminates the degree of freedom of translation but leaves the freedom of rotation along an axis perpendicular to the plane of the figure. At rest, i.e. at a temperature such tha
Commissariat a l'Energie Atomique
Thelen Reid & Priest LLP
Vortman Anatoly
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