Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2001-06-20
2003-05-13
Lee, Eddie (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S296000, C257S303000, C257S306000, C257S310000
Reexamination Certificate
active
06563153
ABSTRACT:
BACKGROUND
The present invention relates to the field of electrically tunable devices e.g. for microwave (radio frequency) circuits. Particularly it relates to a thin film ferroelectric varactor device, use of such a device in microwave (or millimeter wave) circuits and to a method of producing such a device.
Various tunable devices for use in microwave and millimeter wave devices have been proposed in the past. A varactor is a variable capacitance device in which the capacitance depends on a voltage applied thereto. Varactors are known to be used in RF tuning applications among others due to the fact that the capacitance variations of the varactor caused by an applied voltage has corresponding effects on frequency tuning.
Varactors based on semiconductors are known. However, such devices are disadvantageous in many aspects. They suffer e.g. from a low tunability (low dynamic range) at microwave frequencies, i.e. above 10-20 GHz, and the microwave losses are also high. Due to the inherent properties of semiconductor materials such varactors are susceptible to overheating and burnout if forward biased or reverse biased with an excessive applied voltage. Semicondutor PN junction devices have a depletion region which is subjected to high electric field stress, and as a consequence thereof, such devices may break down as the applied voltage is varied. Still further, semiconductor materials have dielectric constants between about 10-15, i.e. low dielectric constants which limits the capacitance and this is very disadvantageous for a plurality of applications.
Microelectromechanical varactors are also known. As opposed to semiconductor varactors they have a high dynamic range, or a high tunability and low microwave losses but the tuning speed is limited to tens of microseconds. In addition thereto they are sensitive to mechanical vibrations, they have a short lifetime and they are also not reliable.
Varactors based on ferroelectric materials or non-linear dielectrics are also known, e.g. from U.S. Pat. No. 5,472,935. The main disadvantage of the varactors disclosed in the above mentioned document, as well as other tunable microwave devices based on (bulk) ferroelectrics, is that the parameters are extremely temperature dependent which is related to the inherent temperature dependence of the ferroelectric materials. This is illustrated in
FIG. 1A
,
FIG. 1B
which show the extreme temperature dependence close to the maximum of the dielectric constant of typcial ferroelectric materials, Barium Titanate (BaTiO
3
, BTO) and Strontium Titanate (SrTiO
3
, STO). The dependence of the dielectric constant on an applied electrical DC field (the tunability) is also stronger close to the maximum of the dielectric constant whereas away from the maximum of the dielectric constant, the tunability is low. STO for example is not tunable at room temperature at a reasonably low applied electric field (E<100 kV/cm). This means that capacitors based on STO are not tunable at about room temperature (i.e. they are actually no varactors). This means that a high temperature stability only can be achieved in combination with a low tunability.
In addition thereto a transition layer is formed in the surface of the ferroelectric material at the interface between the electrodes of metal, e.g. gold, and the ferroelectric material in the varactor. The internal electric field of this layer will reduce the dielectric constant of the ferroelectric material and as a consequence thereof it also reduces the sensitivity to the applied external DC fields. In other words, the tunability of the varactor is reduced.
Ferroelectric varactors based on bulk material suffer among others from the drawback that the thickness of such devices limits the total capacitive effect.
It has been found advantageous to use thin ferroelectric films for the production of tunable capacitors since the dielectric constant of the ferroelectric films is tunable by variation of a voltage applied to the film. At high frequencies such films intrinsically show comparatively low losses.
U.S. Pat, No. 5,640,042 shows a simple ferroelectric varactor comprising a plurality of thin film layers. A carrier substrate layer is provided on which a metallic conductive layer is deposited. The thin film ferroelectric is in turn deposited on the metallic conductive layer and a plurality of longitudinally spaced metallic conductive means are disposed on the thin film ferroelectric. The carrier substrate layer, the metallic conductive layer and the thin film ferroelectric layer may have matching lattices to form a matched crystal structure. However, even if higher capacitance values than for example in semiconductor varactors can be obtained resulting in a higher tunability, such devices do not work satisfactorily for a plurality of implementations, e.g. because the temperature stability is not good enough and the extent to which such a device can be tuned is not sufficient.
SUMMARY
What is needed is therefore an improved varactor device. More particularly a varactor device is needed which has a high dynamic range (a large range of tunability) and at the same time shows a high temperature independence, i.e. which shows a high degree of temperature stability. Particularly a ferroelectric varactor device is needed which is reliable, has a long lifetime and which do not suffer from mechanical stresses or vibrations or similar. Still further a varactor device is needed for which the tuning speed is high. The tuning speed can be defined as dC/dt, i.e. the time (t) derivative of the capacitance (C) and shows how fast the capacitance can be tuned. Further a ferroelectric varactor device is needed which is easy to fabricate and which moreover is not expensive to fabricate. Further yet a varactor device is needed which is suitable for a large number of applications, particularly for microwave or millimeter wave applications or even more particularly for microwave radio frequency applications. Particularly a varactor device is needed which has a high tunability (high dynamic range) and which is temperature independent in a given temperature interval.
A method of producing such a varactor device, fulfilling one or more of the above mentioned objects, is also needed which method particularly is easy to implement. A method of operating a tunable ferroelectric varactor device as referred to above is also needed.
Therefore a thin film ferroelectric varactor device comprising a substrate layer, a ferroelectric layer structure and an electrode structure is provided wherein the ferroelectric layer structure comprises a number of ferroelectric layers and a number of intermediate buffer layers arranged in an alternating manner. At least a first and a second of said ferroelectric layers, between which an intermediate buffer layer, which may be dielectric, is arranged, have different Curie temperatures. The Curie temperature is specifically defined as a temperature characterizing the temperature dependence of the dielectric constant. Specifically it is the temperature for which the dielectric constant has a maximum. According to the invention, different Curie temperatures for the respective ferroelectric layers is provided through giving ferroelectric layers a different chemical composition, or by chemically isolating the ferroelectric layers from one another, such that different Curie temperatures are provided. The content of at least one element of the elemental composition of the respective layer is different in the at least two layers. (Specially the content of an element may be zero in one of the layers.)
Advantageously at least some of the layers of the varactor device have lattice matched crystal structures. Even more particularly all layers, i.e. the layers of the ferroelectric layer structure, the electrode structure and the substrate layer, have lattice matched crystal structures.
In a preferred implementation the layers, particularly the intermediate buffer layers and the ferroelectric layers and the substrate layer comprise single crystalline films (epitaxial films).
In a
Gevorgian Spartak
Ivanov Zdravko
Petrov Peter
Wikborg Erland
Fenty Jesse A.
Lee Eddie
Telefonaktiebolaget LM Ericsson (publ)
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