Electricity: circuit makers and breakers – Electrostrictive or electrostatic
Reexamination Certificate
2003-09-15
2004-06-22
Donovan, Lincoln (Department: 2832)
Electricity: circuit makers and breakers
Electrostrictive or electrostatic
C335S078000, C257S414000
Reexamination Certificate
active
06753488
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a microswitch and a method of manufacturing the same, particularly, to a microswitch using a thin film made of a carbon series material and a method of manufacturing the same.
2. Description of the Related Art
In recent years, attentions are paid to a MEMS (Micro Electro Mechanical Systems) technology for manufacturing an integral structure comprising a fine mechanical structure and an electronic circuit by using a semiconductor fine fabrication technology. The MEMS technology is employed in various technical fields including, for example, a fine mechanical switch, hereinafter referred to as “microswitch”. The microswitch exhibits frequency characteristics which are more satisfactory than the semiconductor switch and, thus, is expected to be utilized in the field of telecommunications. Also, the microswitch can be miniaturized and integrated more easily than the conventional relay utilizing the electromagnetic force and, thus, is expected to be utilized in the field of vehicles.
As an example of the microswitch utilizing the particular MEMS technology, a microswitch manufactured by utilizing the Ni plating is disclosed in “Paul M, Zavracky et al., Micromechanical Switches Fabricated Using Niker Surface Micromachining”, which is reported in “Journal of Microelectro Mechanical Systems, (USA), (IEEE/IEE), 1997, Vol. 6, p. 3, FIG. 2”.
The method of manufacturing the microswitch is schematically illustrated in
FIG. 2
of the publication noted above, and the fabrication procedure is described in this publication with reference to FIG.
2
. In the fabrication procedure disclosed in this publication, a silicon oxide film is formed first on a Si substrate, followed by forming on the silicon oxide film a Cr film and a Au film forming a first contact layer. Then, a source electrode, a gate electrode and a drain electrode are formed by the patterning using the photolithography. After formation of these electrodes, a Cu film acting as a sacrificial layer is formed, followed by etching the Cu layer so as to form a hemispherical concave portion and a hole extending to reach the source electrode. Then, a resist layer is patterned so as to form a Au film acting as a second contact layer and subsequently forming a beam by a Ni plating. Finally, the resist layer and the Cu layer acting as the sacrificial layer are removed so as to finish the manufacture of the device.
If a voltage is applied to the gate electrode in the microswitch of the construction described above, the beam is electrostatically deformed toward the substrate. If the voltage exceeds a certain value, the electrostatic force overcomes the elastic force of the beam so as to permit the hemispherical contact formed in the tip of the beam to be brought into contact with the drain electrode, with the result that the source-drain passageway is rendered conductive so as to form an “on” state. If the voltage ceases to be applied to the gate electrode, the beam is brought back to the original state, with the result that the contact is moved away from the drain electrode so as to form an “off” state.
In general, Au is widely used as a material of the contact included in the microswitch. It should be noted in this connection that the on-resistance of the microswitch includes in general a contact resistance and a film resistance. The contact resistance is derived from the situation that the irregularity on the contact surface causes the actual contact area to be markedly smaller than the apparent contact area. On the other hand, the covering resistance is derived from the situation that the contact surface is covered with a thin insulating layer. In order to diminish the former one of the contact resistance, it is necessary to increase the contact force so as to enlarge the contact area, or to use a material that is likely to be deformed. On the other hand, in order to diminish the latter one of the covering resistance, it is necessary to increase the contact force so as to mechanical destroy the insulating layer on the surface, or to use a material on which an insulating layer is unlikely to be formed. However, an electrostatic force is used in general as a driving force in the microswitch. What should be noted is that the electrostatic force is very small. In general, the electrostatic force is capable of generating only about &mgr;N to mN of the contact force. Such being the situation, Au, which can be deformed easily and on the surface of which an insulating film is not formed, is widely used as the material of the contact included in the microswitch.
However, the microswitch thus manufactured leaves room for further improvement in respect of its life. Particularly, Au used for forming the contact layer is known as a contact material that is likely to bring about a so-called “sticking”, which is the phenomenon that the both poles thereof are stuck to each other so as to make it difficult to permit the both poles to be separated from each other. It follows that Au used for forming the contact layer is said to leave a problem in respect of the reliability for a long time.
A microswitch using a diamond thin film is reported as a measure for overcoming the sticking problem noted above in, for example, an article entitled “Surface micromachined diamond microswitch” included in a magazine “Diamond and Related Materials, Vol. 9, p.970, FIG. 2” by S. Ertl, et al, which was published by “Elsevier Science, the Netherlands” in 2000”.
The method of manufacturing a microswitch by using a diamond thin film is illustrated in
FIG. 2
of the publication quoted above. According to the description relating to
FIG. 2
, an i-type diamond thin film forming an insulating layer is formed first on a Si substrate. Then, a p
+
-type diamond thin film having a high dopant concentration is formed, followed by patterning the p
+
-type diamond thin film so as to form a gate electrode and a first contact layer. Further, a SiO
2
layer acting as a sacrificial layer is formed, followed by selectively etching the SiO
2
layer so as to form a convex portion and a hole. After the selective etching step, a p
+
-type diamond thin film is formed, followed by pattering the diamond thin film so as to form a beam. In the beam forming step, a contact is formed at the tip of the beam. Finally, the sacrificial layer is removed so as to form a metal electrode, thereby finishing the manufacture of the device.
Likewise, a diamond microswitch structure is disclosed in, for example, an article entitled “Diamond microwave microswitch” by M. Adamschik, et al, which is included in a magazine “Diamond and Related Materials, Vol. 11, p.672“published by “Elsevier Science, the Netherlands” in 2002”. Disclosed in this prior art is the structure that the current passageway starting from the metal electrode to reach again the metal electrode through the contact made of the p
+
-type diamond and the first contact layer is made as short as possible. It is reported that the particular structure makes it possible to decrease the resistance component derived from the bulk resistance of the diamond thin film.
Graphite, which is made of carbon like diamond, is known to be a material that does not bring about the sticking problem and, thus, was widely used in the past as a material of the sliding contact. It is reported by the research group referred to above that the sticking is not brought about in diamond, too, as reported in, for example, an article entitled “Surface micromachined diamond microswitch” included in a magazine “Diamond and Related Materials, Vol. 9, p.970, FIG. 2” by S. Ertl, et al, which was published by “Elsevier Science, the Netherlands” in 2000”.
Diamond exhibits satisfactory mechanical characteristics, the thermal conductivity, and the corrosion resistance and, thus, makes it possible to achieve a microswitch capable of stably switching a large current over a long period of time.
The microswitch using a diamond thin film referred to above exhibits satisfactory mechanical ch
Ono Tomio
Sakai Tadashi
Sakuma Naoshi
Suzuki Mariko
Donovan Lincoln
Kabushiki Kaisha Toshiba
Lee K.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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