Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Physical deformation
Reexamination Certificate
2002-08-29
2004-05-25
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Physical deformation
C257S415000, C257S417000, C257S421000, C257S432000, C257S458000, C257S467000, C257S680000, C200S181000, C361S233000
Reexamination Certificate
active
06740946
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-261999, filed Aug. 30, 2001, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a micromechanical device using surface micromachine technologies.
2. Description of the Related Art
Electrically controlled switching elements used in various electronic devices include semiconductor (solid-state) switches and reed relays. From the standpoint of an ideal relay, they have merits and demerits.
The semiconductor switches have merits of being capable of being downsized and operating at high speed and being high in reliability. They can also be easily integrated as an array of switches. For instance, PIN diodes, HEMTs (High Electron Mobility Transistors) and MOSFETs have been used as switches for switching antennas adapted for microwave, millimeter wave, etc. In comparison with switches which close or break mechanical contacts, however, the semiconductor switches are high in the on impedance and low in the off impedance and have large stray capacitance.
In comparison with the semiconductor switches, on the other hand, the reed relays are high in the on/off impedance ratio and can be designed to minimize insertion loss and ensure signal fidelity. For this reason, the reed relays have been frequently used in semiconductor testers by way of example. However, they are large in size and low in switching speed.
Recently, attention has been paid to micromechanical switches which have the merits of semiconductor switches and reed relays. Among others, micromechanical switches that are formed using surface micromachine technologies and are operated electrostatically can be implemented at low cost because they can be formed through the use of semiconductor thin-film techniques.
FIG. 1A
is a plan view of a conventionally proposed micromechanical switch and
FIG. 1B
is a sectional view taken along line
1
B—
1
B of FIG.
1
A. This switch has a source electrode
51
, a drain electrode
52
, and a gate electrode
53
therebetween, which are all formed on a substrate
50
made of, say, silicon. A conductor beam
54
is formed above the gate electrode
53
with a predetermined gap therebetween. Although the electrodes are named source, drain and gate after those of MOSFETs, the switch is different in structure from the MOSFETs.
The conductor beam
54
has its one end fixed to the source electrode
51
to form an anchor portion
55
. The other end of the beam is made open to form a moving contact (contact chip)
56
. When a voltage is applied to the gate electrode
53
, the conductor beam
54
is deflected downward by resulting electrostatic force, allowing the moving contact
56
to come into contact with the drain electrode
52
. When the gate electrode
53
is deenergized, the conductor beam
54
is restored to its original position.
An analysis of deflection of the conductor beam using a mechanical model has been made by P. M. Zavracky et al. (“Micromechanical Switches Fabricated Using Nickel Surface Micromachining” Journal of Microelectromechanical Systems, Vol. 6, No. 1, March 1997). According to this analysis, when gate voltage is applied, the conductor beam
54
connected to the source electrode
51
is held in a position d(x) above the gate electrode
53
with x as the distance from the source. The gate voltage required to hold the conductor beam
54
in a deflected state increases monotonously with increasing deflection but, after it has been deflected to a certain extent or more, decreases monotonously. The system therefore becomes unstable. At some gate voltage (threshold voltage Vth), the beam bends, closing the switch.
The threshold voltage Vth according to this model is represented by
Vth
=(2/3)×
d
0
×(2
kd
0
/3&egr;
0
A
)
1/2
where d
0
is the initial gap between the conductor beam and the gate electrode, k is the effective spring constant of the conductor beam, A is the area of portions of the conductor beam and the gate electrode which are opposed to each other, and &egr;
0
is the dielectric constant of air.
From this it can be seen that Vth is lowered by increasing A (increasing electrostatic force acting on the beam), reducing k, and decreasing d
0
. However, reducing k results in a reduction in maximum switching speed and decreasing d
0
results in an increase in electrostatic coupling between the gate electrode and the conductor beam. Another method of lowering Vth is to increase the amount of downward projection of the moving contact
56
, i.e., to decrease the gap g between the moving contact
56
and the drain electrode
52
. Thereby, the switch can be closed before the unstable point is reached.
Thus, manufacturing of the gaps d
0
and g with precision is essential in lowering the threshold voltage Vth. The manufacture of the micromechanical switch involves complicated processes. To be specific, the source electrode
51
, the drain electrode
52
and the gate electrode
53
are first formed on the substrate. A sacrificial layer of, say, silicon oxide, is then deposited on these electrodes. The sacrificial layer is subjected to two-step etch processing. In the first step, the sacrificial layer is partly etched to form the contact chip portion
56
. In the second step, in order to form the anchor portion
55
, the sacrificial layer is etched until the source electrode
51
is reached.
Subsequently, a conductive layer is deposited over the sacrificial layer and then patterned. Finally, the sacrificial layer is etched away in order to separate the conductor beam
54
from the substrate.
To manufacture the micromechanical switch as described above, the following four lithographic processes (masking processes) are involved:
(1) Patterning of the source electrode, etc.
(2) Patterning of the contact chip portion in the sacrificial layer
(3) Patterning of the anchor portion in the sacrificial layer
(4) Patterning of the conductive layer
There has also been a proposal for use of a mechanical vibrator manufactured through similar micromachine technologies as a high-frequency filter; in fact, a bandpass filter of the order of 100 MHz has been manufactured (see C, Nguyen, et at., “VHF free-free beam high Q michromechanical resonators.” Technical digest, 12th International IEEE Micro Electro Mechanical Systems Conference, 1999, pp. 453-458). The advantages of mechanical vibrator filters are that the Q value is extremely high in comparison with electrical LC filters and the size can be made extremely small in comparison with dielectric filters and SAW filters.
FIG. 2A
is a plan view showing the unit configuration of such a vibrator filter and
FIG. 2B
is a sectional view taken along line
2
B—
2
B of
FIG. 2A. A
vibrator
61
, an input terminal
62
and an output terminal
63
are formed on a substrate
60
by means of micromachine technologies. The vibrator
61
is formed of polycrystalline silicon integrally with four supporting beams
64
a
to
64
d
. The supporting beams
64
a
to
64
d
have their ends fixed to the anchors
65
a
,
65
b
, and
65
c
, whereby the vibrator
61
is held floating above the substrate.
As with the vibrator
61
, the input terminal
62
is formed from a film of polycrystalline silicon. The underlying metal is extended so that its one end is located just below the vibrator, forming a gate electrode (driving electrode)
66
. The output terminal
63
and the vibrator
61
are formed on a common metal electrode
67
. In practice, a mechanical filter with a given passband is manufactured by connecting a plurality of such unit vibrator filters in parallel with one another.
The vibrator
61
is driven by the driving electrode
66
to vibrate in an up-and-down direction. The resonant frequency f
0
of the vibrator
61
is represented by f
0
=(1/2&pgr;)T(k/m)
1/2
where k is the spring constant of the vibrator and m is the mass of the vibrator. With the structure and dimensions in
Kabushiki Kaisha Toshiba
Nelms David
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Tran Mai-Huong
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