Wave transmission lines and networks – Coupling networks – Frequency domain filters utilizing only lumped parameters
Reexamination Certificate
2001-02-14
2004-06-01
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Frequency domain filters utilizing only lumped parameters
C333S174000, C333S262000
Reexamination Certificate
active
06744335
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The invention relates to a micromechanical tunable capacitor and an integrated tunable resonator. In particular the invention relates to an RF resonator realised with a micromechanical tunable capacitor with improved tuning range and high Q-(Quality factor) value.
BACKGROUND OF THE INVENTION
Integrated LC tank circuits are basic building blocks for IC integrated filters, oscillators and matching circuits. Prior art RF resonators typically employ various LC designs where an inductor (L) and a capacitor (C) are connected in series or in parallel. Integrated tunable RF resonators in accordance with the prior art usually comprise an integrated inductor and a micromechanical or a varactor based tunable capacitor.
Prior art fabrication technologies have been optimized for low frequency (<1 MHz) applications and used mainly for inertial and pressure sensors. The design of micromechanical RF components for 1 to 5 GHz applications used in mobile terminals sets demands on micromachined structures. These demands are partly different from the problems in the low frequency Micro Electromechanical Systems (MEMS) applications. In order to create an integrated high Q value LC tank circuit the series resistance and the substrate losses in the inductor-capacitor system must be minimized. Tunability of the LC resonator furthermore requires that the instability of the electromechanical system is taken into account in the structure design and that the parasitic capacitance is minimized in the overall structure. The MEMS RF components must therefore be optimized with respect to following constraints:
the tuning range should give more than 15% resonance frequency change when the capacitance value changes over 50% which can be obtained by simultaneously
1) minimizing the parasitic capacitance to less than 1 pF, and
2) eliminating the electromechanical instabilities,
the series resistance must be minimized to a value smaller than 1 ohm,
the temperature dependencies must be minimized,
the vibration and acceleration sensitivity must be minimized,
The major limitation for the tuning range of the micromechanical capacitors is the instability of the electromechanical system.
FIG. 1
demonstrates the deflection of the flexible capacitor plate as a function of the voltage U across the capacitor plates. The flexible capacitor plate deflects towards the fixed electrode until the electrostatic force due to its non-linear dependence on the plate distance exceeds the maximum possible mechanical spring counter force, and the capacitor plates collapse together at this particular critical voltage value U
pull-in
, called pull-in voltage.
FIG. 2
illustrates a simple piston structure with a spring &kgr; and a mass m, and parallel capacitor plates C
a
and C
b
, wherein the pull-in happens independent of the dimensions when the displacement x of the capacitor plate is one third (33%) of the original distance x
0
between the capacitor plates. For deflecting beam or diaphragm, the deflection can be slightly larger as seen in FIG.
1
. Furthermore, after the pull-in has happened the capacitor plates can be separated only by decreasing the voltage significantly below the release voltage as shown in FIG.
1
. The pull-in effect limits the maximum relative change in the capacitance below 50%.
Prior art micromechanical capacitors and integrated RF resonators based on them have therefore disadvantages related to those requirements. The achieved tuning range of prior art micromechanical capacitors is inadequate to many applications. Series resistance and parasitic capacitance are also high in prior art RF resonators based on tunable micromechanical capacitor and integrated planar inductor. In addition, prior art RF circuits suffer from temperature dependence, due to the mismatch of thermal expansion coefficients of the micromechanical structure and the substrate. These factors may severely limit the tuning range and lead to high losses, thermal unstability and unreliability of the micromechanical capacitors and RF resonators.
SUMMARY OF THE INVENTION
The purpose of the invention is to achieve improvements related to the aforementioned disadvantages. The arrangement for micromechanical tunable capacitor and an integrated RF resonator based on it in accordance with the invention presents a micromechanical tunable capacitor and integrated resonator that facilitate a significant increase in the tuning range and minimizing the series resistance, parasitic capacitance and temperature dependence. Hence, the invention presents a substantial improvement to the tuning range, quality factor, stability and reliability of the micromechanical tunable capacitor and the RF resonator based on it.
The aforementioned advantages of the invention are preferably implemented with a micromechanical tunable capacitor with a movable electrode and two-piece electrode structure where the active electrode and the tuning electrode are separate. The two different electrodes may also have different gap heights, meaning that the gap height between the tuning electrodes and the electrode beam is different from the gap height between the active electrode and the beam. With the two-piece electrode structure it is possible to achieve a large relative shift between the active electrodes without the pull-in effect. The electrode forming the circuit capacitance is here called the active electrode.
A resonator according to the invention is preferably implemented with such a micromechanical tunable capacitor integrated on the same substrate with an integrated inductor coil. An integrated coil is preferably a planar coil; however, also integrated solenoid or toroid coils are possible.
The inventive concept of a micromechanical tunable capacitor can advantageously be realised with a movable electrode and a two-piece electrode structure, where the active electrode and the tuning electrodes are separate, using one or several of the following details:
divided electrode structure with variable gap height to improve the tuning range of the variable capacitor. The two different electrodes may have different gap heights;
use of metal thin films to reduce the serial resistance in the capacitor structure down to the 0.1 ohm level;
reduction of the parasitic capacitance by using the diaphragm or the doubly-supported beam as a ground electrode, i.e., the diaphragm or the beam is connected to the same electrical potential as the substrate. The fixed (anchor) parts of the micromechanical structure are thus not creating any parasitic capacitance, and the tuning range can be improved;
reduction of the parasitic capacitance by removing the substrate under that part of the capacitor structure that is forming actual variable capacitance. It is advantageous to remove the substrate only under the capacitor electrode, not under the film anchors and the tuning electrodes, to have mechanically a more robust structure. The substrate removal can be done by either isotropic or anisotropic etching; and the etching can be done from either front or back side of the wafer;
corrugating the diaphragm or the doubly-supported beam to reduce the effect of the stress induced by the mismatch of the thermal expansion coefficients of the micromechanical structure and the substrate;
metal film beam or diaphragm is preferably corrugated by two or more folds so that the depth of the fold is more than the thickness of the metal film. Preferably the depth of the corrugation is over 10 times the thickness of the film; and
vertical sides of the folds of the corrugated film are preferably thinner than the lateral portions of the film. The vertical sides of the folds are also preferably weakened by etching holes through the vertical portions of the film.
The inventive concept of a RF resonator that is based on a integrated inductor and a micromechanical tunable capacitor can advantageously be realised with a capacitor with a movable electrode and a two-piece electrode structure, where the active electrode and the tuning electrode are separate, using one or several of the followi
Ermolov Vladimir
Lind Mikael
Ryhänen Tapani
Silanto Samuli
Nokia Mobile Phones Ltd.
Pascal Robert
Perman & Green LLP
Takaoka Dean
LandOfFree
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