Electricity: circuit makers and breakers – Electrostrictive or electrostatic
Reexamination Certificate
2001-06-29
2003-11-11
Scott, J. R. (Department: 2832)
Electricity: circuit makers and breakers
Electrostrictive or electrostatic
C333S262000
Reexamination Certificate
active
06646215
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to microelectromechanical devices, and more particularly, to a microelectromechanical device in which a cantilever is electrostatically pulled away from a conductive pad.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Microelectromechanical devices, or devices made using microelectromechanical systems (MEMS) technology, are of interest in part because of their potential for allowing integration of high-quality devices with circuits formed using integrated circuit (IC) technology. For example, MEMS switches may exhibit lower losses and a higher ratio of off-impedance to on-impedance as compared to transistor switches formed from conventional IC technology. However, a persistent problem with implementation of MEMS switches has been the high voltage required (often about 40V or higher) to actuate the switches, as compared to typical IC operating voltages (about 5V or lower).
These relatively high actuation voltages of MEMS switches are caused at least in part by a tradeoff between the closing and opening effectiveness of a given switch design. In the case of a cantilever switch, for example, approaches to lowering the actuation voltage of the switch include reducing the stiffness of the cantilever beam and reducing the gap between the beam and the underlying conductive pad. Unfortunately, these design changes typically have the effect of making opening of the switch more difficult. MEMS cantilever switch designs generally use an applied voltage to close the switch, and rely on the spring force in the beam to open the switch when the applied voltage is removed. In opening the switch, the spring force, or restoring force, of the beam must typically counteract what is often called “stiction”. Stiction refers to various forces tending to make two surfaces stick together, such as van der Waals forces, surface tension caused by moisture between the surfaces, and/or bonding between the surfaces (e.g., through metallic diffusion). In general, modifications to a switch which act to lower the closing voltage also tend to make the switch harder to open, such that efforts to form a switch with a lowered closing voltage can result in a switch which may not open reliably (or at all).
Another problem with MEMS devices is that they tend to deform due to residual stresses contained within the devices. More specifically, the residual stresses within a MEMS switch may cause a beam within the device to curl either away from its underlying contact structures or toward the contact structures. In the event that the beam curls down and closes a contact prematurely, the switch may become inoperable because significant electrostatic repulsion between the gate and the beam cannot be established. In this manner, the switch may not be opened by removing an applied voltage as described above. Residual stresses typically arise when a MEMS device has layers of differing properties. For example, the device may include layers of differing materials. Alternatively or in addition, the properties of the layers may change if deposition conditions change as the layers are formed. As such, the variation of materials within conventional MEMS devices may be limited. In addition, fabrication steps may be tightly controlled such that changes in layer properties do not occur.
It would therefore be desirable to develop a MEMS device which relaxes the constraints imposed by the above-described tradeoff between opening and closing effectiveness and the presence of residual stresses within the device.
SUMMARY OF THE INVENTION
The problems outlined above may be in large part addressed by a device adapted to electrostatically pull a cantilever away from a conductive pad and a method for fabricating such a device. In particular, a microelectromechanical device is provided which includes a fulcrum contact structure interposed between two electrodes spaced under a cantilever. The device further includes a conductive pad arranged under the distal end of the cantilever and laterally adjacent to one of the electrodes. Such a device may be adapted to initially bring the cantilever in contact with the conductive pad by an application of a closing voltage to one of the electrodes. The device may be further adapted to deflect the cantilever away from the conductive pad upon an application of voltage to the other of the electrodes such that the cantilever contacts the fulcrum contact structure. In another embodiment, the device may be adapted to deflect the cantilever away from the conductive pad upon a release of the closing voltage after the application of the voltage to the other electrode. In yet another embodiment, the device may be adapted to deflect the cantilever away from the conductive pad upon an increase of the voltage applied to the other electrode after a release of the closing voltage.
In some embodiments, the cantilever may include residual forces with which to bring the cantilever in contact with the conductive pad. In such an embodiment, the application of voltage to one or both of the electrodes may pull the cantilever in contact with the fulcrum contact structure. In this manner, the device may serve as a functional switch since contact at the fulcrum structure may be made and/or released by actuating either one or both of the gate structures. In addition, the application of a voltage to an electrode interposed between the fulcrum contact structure and a support structure of the cantilever, in such an embodiment, may be sufficient to pull the cantilever apart from the conductive pad. In an alternative embodiment, the residual stresses within the cantilever may cause the beam to curl away from the conductive pad. In such an embodiment, the device may be adapted to pull the cantilever away from the fulcrum contact structure. In particular, the application of a voltage to an electrode arranged laterally adjacent to the contact pad, in such an embodiment, may be sufficient to pull the cantilever apart from the fulcrum contact structure. In addition, the device may be adapted to bring the cantilever in contact with both the conductive pad and the fulcrum contact structure.
In an embodiment, a microelectromechanical device as described above may include first and second electrodes spaced under a cantilever. In addition, the device may include a fulcrum contact structure interposed between the first and second electrodes and a conductive pad arranged under a distal end of the cantilever and laterally adjacent to the second electrode. In some embodiments, the conductive pad may be interposed between the fulcrum contact structure and the second electrode. Alternatively, the second electrode may be interposed between the fulcrum contact structure and the conductive pad. In addition, the conductive pad and/or fulcrum contact structure may include multiple sections spaced apart from each other.
The cantilever may be supported by a support structure at the end opposite the distal end of the cantilever. In some embodiments, the support structure may include an electrical terminal. The cantilever may further include an insulating element interposed between the supported end and the distal end of the cantilever. In addition or alternatively, the cantilever may have a dimpled portion above at least one of the fulcrum contact structure and conductive pad. On the other hand, the cantilever may be substantially uniform. In addition or alternatively, at least one of the fulcrum contact structure and the conductive pad may include a raised section arranged upon its respective surface. Regardless of the configuration, the spacing between the fulcrum contact structure and its overlying respective portion of the cantilever is preferably smaller than the spacing between the first and second electrodes and their overlying respective portion of the cantilever when the cantilever is not in contact with the conductive pad. As such, the cantilever may include a dimpl
Conley & Rose, P.C.
Daffer Kevin L.
Scott J. R.
Teravicin Technologies, Inc.
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