Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2000-08-17
2003-10-21
Loke, Steven (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S595000, C257S600000, C438S050000, C438S052000, C438S053000, C200S181000, C200S600000
Reexamination Certificate
active
06635919
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to integrated circuits, and more particularly to Micro Electro-Mechanical System (MEMS) devices.
BACKGROUND OF THE INVENTION
In the telecommunications industry, the demand for lightweight portable devices such as personal computing devices, Personal Digital Assistants (PDA's) and cellular phones has driven designers to reduce the size of existing components. A Q value is a ratio of the power stored in a device to the dissipated power in a device. Due to the need for Q values beyond the capabilities of conventional IC technologies, board-level passive components continue to occupy a substantial portion of the overall area in transceivers of handheld telecommunications equipment, presenting a bottleneck against further miniaturization. For example, discrete components currently occupy approximately 50% of the space in cellular phones.
Recently MEMS devices including resonators, filters, and switches have been developed that offer an alternative set of strategies for transceiver miniaturization and improvement. MEMS devices are high-Q, chip-level, lower power replacements for board-level components that greatly decrease space and area requirements.
One such MEMS device is an RF switch for switching RF signals, shown in a cross-sectional view in FIG.
1
. RF drumhead capacitive MEMS switch
10
, disclosed by Goldsmith et al. in U.S. Pat. No. 5,619,061, comprises an insulator
14
such as SiO
2
deposited over a substrate
12
, which may comprise silicon, for example. A bottom electrode
16
is formed on insulator
14
and a dielectric
18
is formed over bottom electrode
16
. Capacitor dielectric
18
typically comprises Si
3
N
4
, Ta
2
O
5
or other suitable dielectric materials, for example. An active element comprising a thin metallic membrane
22
is suspended away from electrode
16
by an insulating spacer
20
. Membrane
22
which serves as a top electrode is movable through the application of a DC electrostatic field between membrane
22
and bottom electrode
16
. Membrane
22
, dielectric
18
and bottom electrode
16
comprise a metal-dielectric-metal capacitor when the MEMS switch
10
is in the “on” position, shown in FIG.
2
. In the “off” position shown in
FIG. 1
, with no voltage applied to membrane
22
and bottom electrode
16
, the capacitance value is at a minimum. MEMS switches
10
have low insertion loss, good isolation, high power handling, and very low switching and static power requirements.
A MEMS switch
10
may be designed for use as a varactor. A varactor is a discrete electronic component, usually comprising a P-N junction semiconductor, designed for microwave frequencies, in which the capacitance varies with the applied voltage. Varactors are sometimes referred to as tunable capacitors. Varactors are used in frequency up and down conversion in cellular phone communication, for example. Existing varactors are usually p-n diodes specifically designed for operation in the reverse bias regimes where the capacitance(C
J
) of the depletion region is varied to set frequency (&ohgr;
0
) of operation as reflected in Equation 1:
&ohgr;
0
≈1/(
C
J
*R
S
*R
P
)
½
Equation 1:
where resistances R
P
and R
S
are the parallel and series resistances of the diode, respectively. Some primary requirements of a varactor are that it have a high quality factor (Q) for increased stability to thermal variations and noise spikes, and a large linear tuning range (TR). High-performing varactors are usually made of GaAs. Unfortunately, these devices use a different processing technology that is not amenable to integration into standard Si-CMOS process.
MEMS devices offer a means by which high Q large tuning range varactors can be integrated in higher level devices such as voltage controlled oscillators and synthesizers using the current Si-CMOS process. The drumhead capacitive switch
10
shown in
FIG. 1
may be designed to produce a MEMS varactor. The voltage across the electrodes is varied to pull down and up membrane
22
, which varies the distance D
air
between membrane
22
and dielectric
18
, which changes the capacitance of the device
10
accordingly.
A problem in MEMS devices is stiction, which is the unintentional adhesion of MEMS device
10
surfaces. Stiction may arise from the strong interfacial adhesion present between contacting crystalline microstructure surfaces. The term stiction also has evolved to often include sticking problems such as contamination, friction driven adhesion, humidity driven capillary forces on oxide surface, and processing errors. Stiction is particularly a problem in current designs of MEMS varactors, due to the membrane
22
possibly adhering to dielectric
18
, resulting in device
10
failure, either temporarily or permanently. To prevent stiction, material and physical parameters, and voltage signal levels of the varactor are designed to avoid contact of membrane
22
with dielectric
18
. Coatings such as Teflon-like materials that resist stiction are frequently applied over dielectric
18
.
SUMMARY OF THE INVENTION
The present invention achieves technical advantages as a MEMS varactor designed to operate in a stiction mode. The pull-down electrode or top membrane maintains contact with the underlying dielectric covering the bottom electrode during operation of the varactor. As the voltage across the pull-down electrode and the bottom electrode is varied, the area of the pull-down electrode contacting the dielectric is varied, which varies the capacitance.
Disclosed is a MEMS varactor, comprising a bottom electrode formed over a substrate, a dielectric material disposed over the bottom electrode, and a spacer proximate the bottom electrode. A pull-down electrode is disposed over the spacer and the dielectric material, wherein the varactor is adapted to operate in a stiction mode.
Also disclosed is a method of manufacturing a MEMS varactor, comprising depositing an insulator on a substrate, forming a bottom electrode on the insulator, and depositing a dielectric material over the bottom electrode. A spacer is formed over the insulator, and a pull-down electrode is formed over the spacer and the dielectric material, wherein the varactor is adapted to operate in a stiction mode.
Further disclosed is a method of operating a MEMS varactor, comprising applying a voltage across the bottom electrode and the pull-down electrode to produce a predetermined capacitance across the bottom and pull-down electrode, wherein at least a portion of the pull-down electrode is adapted to contact the dielectric material during operation in a stiction mode.
Advantages of the invention include solving the stiction problems of the prior art by providing a varactor adapted to operate in a stiction mode. The present MEMS varactor is a high Q varactor having a large tuning range. The distance between the dielectric and the membrane may be increased in accordance with the present invention, allowing for a larger tuning range and providing more sensitivity to a change in voltage. A wider range of voltages and capacitances is available with the present MEMS varactor design. Furthermore, the use of Teflon-like coatings on dielectric to prevent stiction of membrane is not required, as in some prior art designs. A wider variety of dielectric materials may be used for dielectric than in the prior art because there is no need for concern about stiction of the membrane to the dielectric. The invention provides an extended tuning range that is not possible with only an air gap for the capacitive medium.
REFERENCES:
patent: 5619061 (1997-04-01), Goldsmith et al.
Z. Jamie Yao et al., “Micromachined Loe-Loss Microwave Switches”, IEEE J. of. Microelectromechanical systems, vol. (8), Jun. 2, 1999, pp 129-134.*
N. Scott Barker et al., “Distributed MEMS True-Time Delay Phase Shifters and Wide-Band Switches”, IEEE Tran. on Microwave Theory and Techniques, vol. 46, No. 11, Nov. 1998, pp 1881-1890.*
Goldsmith et al., Micromechanical Membrane Switches for Microwave Applications, IEEE Microwave Theory
Lin Tsen-Hwang
Melendez Jose L.
Williams Byron
Brady W. James
Kang Donghee
Loke Steven
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
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