Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Physical deformation
Reexamination Certificate
2003-05-22
2004-11-09
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Physical deformation
C257S416000, C257S417000, C257S418000, C257S419000, C257S420000
Reexamination Certificate
active
06815782
ABSTRACT:
TECHNICAL DOMAIN
This invention relates to a miniature electrostatic actuation device capable of generating movements in a determined direction, these movements being obtained through at least one pair of electrodes, one of which can be moved with respect to the other, under the effect of an electrostatic force exerted between the two electrodes subjected to a potential difference.
The invention also relates to an installation comprising such miniature electrostatic actuation devices, the devices being placed adjacent to each other.
For example, the invention relates to a MEMS (Micro Electro Mechanical System) type component in domains such as astrophysics or ophthalmology, particularly in order to actuate optical instruments such as continuously deformable micro-mirrors.
Moreover, the invention is equally applicable to any other micro-system that needs to be actuated in translation along a determined direction.
STATE OF THE PRIOR ART
Several implementations have already been proposed in this technical field.
Firstly, prior art is familiar with conventional miniature electrostatic actuation devices comprising a fixed electrode and a mobile electrode located parallel to the fixed electrode and at a distance from it.
When an electric voltage is applied between the two electrodes, an electrostatic force is generated causing displacement of the mobile electrode with respect to the fixed electrode, along a direction approximately perpendicular to the plane in which the fixed electrode is located. Thus, when such a device is being used, the resulting displacement is capable of generating translation movements along the determined direction, in this case corresponding to the displacement direction of the mobile electrode.
However, this type of conventional actuation device has many disadvantages that strongly disturb its operation. In particular, these disadvantages occur when such a device is being used to continuously actuate a deformable micro-mirror.
These disadvantages include the sudden pull-in phenomenon applied to this type of device. Conventional electrostatic devices normally have a characteristic instability that results in sudden pull-in of the mobile electrode in contact with the fixed electrode when the voltage applied between the two electrodes exceeds a given value. Consequently, the controlled movement distance of the mobile electrode corresponds to a distance that is very much restricted from the initial distance separating the two electrodes to which a potential difference has not yet been applied. The result is that conventional devices are incapable of generating controlled high amplitude movements in the given direction.
Another disadvantage related to this type of device relates to the lack of linearity between the applied voltage and the generated displacement of the mobile electrode. This constraint occurs throughout the usage range and makes it very difficult to control the device, obviously to the detriment of the actuation precision of the micro-system to be controlled.
Finally, it should be noted that the force developed by movement of the mobile electrode remains relatively small, particularly with respect to the average force required to correctly actuate a continuously deformable micro-mirror.
Prior art proposed another miniature electrostatic actuation device described in document EP-A-0 592 469 in order to overcome these various problems.
The document mentioned above describes a miniature actuation device comprising a diaphragm type membrane and a support on which an electrically conducting layer is supported, together with an insulating layer inserted between the diaphragm and the conducting layer. In the inactive state, in other words when no electrical voltage is applied between the electrically conducting diaphragm and the conducting layer, only the ends of the diaphragm are pulled in to come into contact with the conducting layer. The other part of the diaphragm is then at a distance from the conducting layer, so as to define an empty space with it.
When the device is actuated by the application of a voltage between the two electrically conducting elements, the part of the diaphragm at a distance from the conducting layer might progressively get pulled in to come into contact with the conducting layer, symmetrically about a central part of the diaphragm and from its ends. It should be noted the symmetry of the device introduces a translation displacement of the central part of the diaphragm along a direction approximately perpendicular to the conducting layer.
The actuation device described above is advantageous to the extent that the diaphragm used may be gradually pulled into contact with the conducting layer so as to make the volume of the empty space almost zero, without introducing the “pull-in” type phenomenon causing the diaphragm to come into sudden contact with the conducting layer. In this way, the controlled stroke of the central part of the diaphragm that may be moved along a determined direction corresponds approximately to the initial distance between the central part of the diaphragm and the conducting layer. This stroke is thus fully optimised as a function of the design of such a device.
Furthermore, experiments carried out on this device have demonstrated the existence of linearity between displacement of the central part of the diaphragm and the applied voltage between the conducting elements, this linear relation having been observed over a wide usage range of the device.
Furthermore, as mentioned above, the diaphragm may gradually be pulled into contact with the conducting layer from the ends of this diaphragm, the contact surface between the diaphragm and the conducting layer depending on the electrical voltage applied between these two elements. As a result, it can be seen that a portion of the diaphragm that is not yet in contact and that is directly along the extension of a part that is in contact is extremely close to the conducting layer, at all times while the device is being activated. The force developed by displacement of the diaphragm is inversely proportional to the square of the distance separating the diaphragm and the conducting layer, therefore this force is very high within the usage range of the device.
Note that the progressive pull-in phenomenon between two conducting elements, known particularly under the term “zipping”, is also described in the document entitled “A New Electrostatic Actuator providing improved Stroke length and force, MEMS' 92, J. Branebjerg; P. Gravesen”.
Although the miniature device described in document EP-A-0 592 469 solves problems related to conventional electrostatic actuation devices, it does have a major disadvantage.
After an activation phase of such a device that caused pull-in of the diaphragm into contact with the conducting layer, a simple action to reduce the voltage applied between these two elements is not sufficient to put the diaphragm back into its initial position. Thus, in order to overcome this disadvantage, the device also comprises means for injecting gas under pressure inside the empty space formed between the diaphragm and the conducting layer. In this way, when gas is injected under pressure into the empty space, a force is applied over the entire diaphragm so that the diaphragm can resume its initial shape and position.
Nevertheless, this type of arrangement very much complicates the design of this type of device, such that costs related to the manufacture and use of pressurised gas injection means are excessive compared with the total cost of the overall device.
Furthermore, due to the operating mode of the means necessary to put the diaphragm back into its initial position, the diaphragm and/or the insulating layer must necessarily be sealed in order to assure good cooperation with the means of injecting the pressurised gas, thus naturally introducing constraints in the geometry and in the choice for materials that can be used to make the diaphragm and/or the insulating layer of the device.
OBJECTS OF THE INVENTION
Therefore, the initial object of t
Charton Julien
Stadler Eric
Commissariat a l'Energie Atomique
Erdem Fazli
Flynn Nathan J.
Thelen Reid & Priest LLP
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