Wave transmission lines and networks – Long line elements and components – Switch
Reexamination Certificate
2002-12-30
2004-06-15
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Long line elements and components
Switch
C200S181000
Reexamination Certificate
active
06750742
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a radio frequency device; and, more particularly, to a radio frequency device using a micro-electronic-mechanical system (MEMS) technology.
DESCRIPTION OF RELATED ART
Generally, a micro-electronic-mechanical system (MEMS) technology is called a micromachining, micro-system or ultra-small size precise machine technology. The technology is used to manufacture ultra-small three-dimensional structure by processing a wafer.
The methods for applying the MEMS technology to the radio frequency (RF) area are studied actively, especially in the areas of radio communication and national security. In particular, the low-loss RF switch and low-loss filter draw explosive attention from the radio communication area.
The low-loss RF switch uses an electrostatic attractive force. The switch has two types: one moving the beams of the switch right and left, and the other moving them up and down. The two types of low-loss RF switches are divided again into a direct contact switch (or it is called a resistive switch) and a capacitive switch.
The conventional resistive or capacitive MEMS switch is mounted on a substrate. A top electrode is formed in the form of a cantilever or a membrane, and it works as an actuator, which makes a movement by the electrostatic attractive force with a bottom electrode, which is a signal line. The conventional resistive or capacitive MEMS switch uses the principle of the top electrode and the bottom electrode connected to each other through the electrostatic attractive force to transmit an RF signal.
In case where the resistive MEMS switch is desired to be operated under an operating voltage of 3V in the current mobile communication area, the spring constant k should be as sufficiently small as 1 N/m~3 N/m. To make the spring constant that small, the physical length of the switch should be longer than 500 &mgr;m. After all, this increase in the physical length drops the reliability of the MEMS switch device, and increases the switching time as much as several milliseconds.
Meanwhile, if the physical length of the MEMS switch device is reduced, a problem of increasing operating voltage emerges. Therefore, researchers are studying to develop a switch with short physical length and small spring constant.
In case where a capacitive MEMS switch should be operated at a high speed of several microseconds (&mgr;s), more than 20V of high operating voltage is required. To speed up the switch, various efforts have been attempted, such as making an air hole in the actuator to thereby reduce the mass, or modifying the shape of the actuator to make the spring constant small and thus reduce the operating voltage and improve the switch rate of the switch.
As described above, low operating voltage and rapid switching time are required to apply the switch, which can be operated in the RF range, to the mobile communication terminal.
In case of the capacitive MEMS switch, operating voltage as high as 50V should be supplied to make the switch operate at a high speed of 4-6 &mgr;s. [Z. Jamie Yao, Shea Chen, Susan Eshelman, David Denniston and Chuck “Micromachined Low-Loss Microwave Switches,” IEEE Journal of Micro-electro-mechanical Systems, Vol. 8, pp. 129, 1999]
Meanwhile, when the capacitive switch that operates at a high voltage is embodied to operate at a low temperature, the operation of the switch needs to be optimized according to the shape change of a bridge structure, and the air gap has to be smaller. However, when the air gap is reduced, the isolation of the RF signal is deteriorated. Therefore, the air gap should be maintained around 1-4 &mgr;m. [J. M. Huang, K. M. Liew, C. H. Wong, S. Rajendran, M. J. Tan and A. Q. Liu, “Mechanical Design and Optimization of Capacitive Micromachined Switch,” Sensors and Actuators A 93 pp. 273, 2001]
Particularly, since the switching characteristic of the capacitive MEMS switch is more improved, as the capacitance ratio between on and off is large, a dielectric substance having a higher dielectric rate may be applied. [G. M. Rebeiz and J. B. Muldavin, “RF MEMS Switches and Switch Circuit,” IEEE Microwave Magazine, Vol. 2, pp. 67, 2001; and Wallace W. Martin, Yu-Pei Chen, Byron Williams, Jose Melendez and Darius L. Crenshaw, “Micro-electronic-mechanical Switch with Fixed Metal Electrode Dielectric Interface with a Protective Cap Layer,” U.S. Pat. No. 6,376,787, April, 2002.] However, the capacitive MEMS switch still operates at a high operating voltage over 20V.
When the resistive MEMS switch is embodied to be operated under 3V, which is the operating voltage in the current mobile communication area, the spring constant k should be as sufficiently small as 1~3 N/m. Accordingly, the physical length of the switch becomes as long as more than 500 &mgr;m, thus causing a problem in the device reliability and switching rate. [Robert Y. Loo, Adele Schmitz, Julia Brown, Jonathan Lynch, Debabani Cohoudhury, James Foshaar, Daniel J. Hyman, Juan Lam, Tsung-Yuan Hsu, Jae Lee, Mehran Mehregany “Design and Fabrication of Broadband Surface-Micromachined micro-electro-mechanical Switches for Microwave and Millimeter Wave Applications,” U.S. Pat. No. 6,046,659, April, 2000; and L. R. Sloan, C. T. Sullivan, C. P. Tigges, C. E. Sandowal, D. W. Palmer, s. Hietala, T. R. Christenson, C. W. Dyck, T. A. Plut, and G. R. Schuster “RF Micro-mechanical Switches That Can Be Post Processes on Commercial MMIC,” Electric Component and Technology Conference 2001.]
Meanwhile, when the physical length of the switch device is shortened, there is a problem that the operating voltage is raised. So, researchers are studying to find a MEMS switch of a new structure using an electrostatic attractive force, and a new material. When a new material is to be found, the area of the membrane should be large and the mass should be small to make the switch operate at a low voltage, and these conditions are contrary to each other.
Hereinfrom, the conventional resistive and capacitive MEMS switches are described with embodiments.
FIG. 1
is a plane figure showing a conventional membrane-type capacitive switch, and
FIG. 2
is a cross-sectional view illustrating the capacitive switch of
FIG. 1
, cut along the line a-a′.
Referring to
FIGS. 1 and 2
, a conventional capacitive switch
18
, which is fabricated in the micro-fabrication process technique, such as photolithography, etching, deposition and lifting-off, is provided to a substrate
10
having such a characteristic as insulation, semi-insulation or semiconduction, and polymerization.
The capacitive switch
18
largely has two parts: a part fixed on the substrate
10
(to be referred to as a fixed part, herefrom), and the other part that makes a mechanical movement, that is, actuating part (to be referred to as an actuator, herefrom).
The part fixed on the substrate
10
includes an insulation layer
11
, a bottom electrode
12
, a capacitive dielectric layer
13
, and a grounding surface
17
, and the actuator includes a top electrode
15
.
To be more concretely, the insulation layer
11
is formed on the substrate
10
, and a plurality of grounding surfaces
17
, which are connected with an active zone (not shown) formed inside the substrate
10
or the conduction layer, are embodied and arranged through metal wires. Between the grounding surfaces
17
, there is the bottom electrodes laid, and on the bottom electrode
12
, the dielectric layer
13
covering the bottom electrode
12
is positioned. On top of the dielectric layer
13
, there is the top electrode
15
supported by the supporting material
14
positioned at both ends of the insulation layer
11
. Therefore, the top electrode
15
forms a membrane structure having a regular space (d) with the dielectric layer
13
under the top electrode
15
by the cavity formed in the lower part of the top electrode
15
.
The top electrode
15
is an actuator. S, when an electric voltage is supplied to the top electrode
15
, the top electrode
15
is drawn to the bottom electrode
12
Jung Sung Hae
Kang Sung Weon
Kim Yun Tae
Yang Woo Seok
Blakely & Sokoloff, Taylor & Zafman
Electronics and Telecommunications Research Institute
Takaoka Dean
LandOfFree
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