WO2010103452A1

WO2010103452A1 - Low-frequency filter comprising maxwell-wagner stack

Info

Publication number: WO2010103452A1
Application number: PCT/IB2010/050985
Authority: WO
Inventors: Jinesh Balakrishna Pillai Kochupurackal; Willem Frederik Adrianus Besling
Original assignee: Nxp B.V.
Priority date: 2009-03-11
Filing date: 2010-03-08
Publication date: 2010-09-16

Abstract

The present invention relates to an electric low-pass filter, comprising a layer stack formed by a first electrode layer, a first dielectric layer adjacent to the first electrode layer, a second dielectric layer adjacent to the first dielectric layer, and a second electrode layer adjacent to the second dielectric layer. The first dielectric layer is made of a material and has a layer thickness, which in combination allow a tunellingof first charge carriers of a first charge-carrier polarity from the first electrode layer through the first dielectric layer to an interface between the first and second dielectric layers, under application of a first tunneling voltage of a first voltage polarity between the first and second electrode layers, and a tunneling of second charge carriers of an opposite second charge-carrier polarity from the first electrode layer through the first delectric layer to the interface between the first and second dielectric layers under application of a second tunneling voltage of a second voltage polarity between the first and second electrodes, which second voltage polarity is oppsosite to the first voltage polarity. The second charge carriers have a second charge-carrier mobility in the first dielectric material, which is lower than the first charge-carrier mobility of the first charge carriers.

Description

LOW-FREQUENCY FILTER COMPRISING MAXWELL- WAGNER STACK

FIELD OF THE INVENTION

The present invention relates to an electric low-pass filter and to devices comprising an electric low-pass filter.

BACKGROUND OF THE INVENTION In modern electronic devices, different functional units are packed together in multi-chip modules (MCM) or integrated in a system-on-chip (SOC). For instance, WO 2007/132422 Al discloses a capacitive electroacoustic sensor device, which can be used as a microphone. The device comprises a micro-electric mechanical-system (MEMS) sensor and a low-pass filter in the form of two mono-stable pulse generators integrated either in a MCM or in a SOC.

It would be desirable to provide an alternative low-frequency pass filter, which is suitable for integration into MCM or SOC solutions.

SUMMARY OF THE INVENTION According to the present invention, an electric low-pass filter is provided, which comprises a layer stack formed by a first electrode layer, a first dielectric layer adjacent to the first electrode layer, a second dielectric layer adjacent to the first dielectric layer, and a second electrode layer adjacent to the second dielectric layer. The first and second electrode layers are metallically conductive. The second dielectric layer has a lower electrical conductance than the first dielectric layer. The first dielectric layer is made of a material and has a layer thickness, which combination of material choice and layer thickness allows, a tunelling of first charge carriers of a first charge-carrier polarity from the first electrode layer through the first dielectric layer to an interface between the first and second dielectric layers, under application of a tunneling voltage of a first voltage polarity between the first and second electrode layers, and a transport of second charge carriers of an opposite second charge-carrier polarity in the first delectric layer in a direction pointing from the first electrode layer to the interface between the first and second dielectric layers under application of a second voltage of a second voltage polarity between the first and second electrodes, which second voltage polarity is opposite to the first voltage polarity .

In the electric low-pass filter of the present invention, the second charge carriers, with respect to their transport in the first dielectric layer under application of the second voltage, have a second charge-carrier mobility, which is lower than a mobility of the first charge-carriers with respect to the tunneling process at the tunneling voltage. The stack of the first and second dielectric layers is hereinafter referred to in short as "the Maxwell- Wagner stack". Note that the order numbers "first" and "second" are used in the above paragraphs to provide generic names for certain layers and charge carrier types. For instance the first charge carriers may in one embodiment be electrons and thus negatively charged, but in another embodiment, holes or ions, e.g., protons, and thus positively charged. The order numbers "first" and "second" are in this context not used to refer to a priority of some sense like, e.g., in the context of layers, an order of deposition. The electric low-pass filter of the present invention is based on a capacitive structure, which provides excellent low- frequency pass characteristics, for instance in the frequency range of acoustic signals, which is well below 50 kHz and typically covers a range of up to 20 or 30 kHz. The electric low-pass filter has particularly low dissipation characteristics and a high efficiency, by employing the slower dynamics of the second charge carriers in the first dielectric layer in comparison with the mobility that can be assigned to a tunneling process of the first charge carriers from the first electrode to the interface between the dielectric layers. In the following, embodiments of the electric low-pass filter will be described.

The additional features of different embodiments described hereinafter may be combined to obtain further embodiments, unless such combination is explicitly excluded for embodiments, which form alternatives to each other.

In one embodiment the first dielectric layer is made of a material, in which either holes or hydrogen ions are mobile. Preferably the holes or protons form the second charge carriers, while electrons form the first charge carriers. In this embodiment, the transport of the second charge carriers in the first dielectric layer occurs as a migration of the holes or protons from the first electrode to the interface between the first and second dielectric layers. The slow dynamics of the holes or hydrogen ions in the first dielectric layer, which leads to a slow response to an applied AC signal in the phase of the second voltage polarity, allows constructing low-frequency pass filters with a particularly high efficiency and low dissipation. In this embodiment, the first dielectric layer may advantageously be made of silicon dioxide containing mobile holes and/or mobile protons. A tunneling of electrons under the first voltage polarity, and a transport of holes/protrons under the second voltage polarity, can for instance be achieved if the the first dielectric layer, like for instance the silicon dioxide layer, has a thickness between 1 and 10 nanometer. The layer has a thickness between 2 and 8 nanometer in another embodiment.

As a material of the second dielectric layer, one or a combination of known high-k dielectrics may be chosen. Suitable materials are, for instance, lanthanum zirconate, hafnium oxide, hafnium- erbium oxide, zirconium dioxide, aluminum oxide, or a combination of these materials, such as in the form of a stack or mixture. The thickness of the second dielectric layer is typically much larger than that of the first dielectric layer to avoid a tunneling of charge carriers through the dielectric second layer. In order to let the Maxwell- Wagner effect take place the relative conductivities, the difference in conductivity of the two dielectrics are to be taken into consideration. This is also dependent on the electrical field and the permittivity. In typical embodiments, the second dielectric layer has a thickness of between 2 and 50 nanometer, or between 5 and 30 nanometer.

In one embodiment, at least one of the first and second electrodes is metallically made of a metal. The electric low-pass filter is particularly suitable for integration into an interconnect stack of an integrated-circuit device. Here, it preferably takes the form of a metal- insulator-metal (MIM) structure. However, the electric low-pass filter may also be formed as a metal-insulator-semiconductor (MIS) structure, where a highly- doped, e.g., n⁺⁺ semiconducting material like silicon may be used. The MIS structure is suitable for an arrangement of the low-pass filter directly on a silicon wafer. However, other semiconductor materials are similarly suited in this context. The use of a semiconductor- insulator-semiconductor (SIS) structure is possible, but currently not preferred for device application.

The present filter is very suitable for frequencies in a domein from 0.5 kHz - 20 kHz. The electric-low pass filter is very suitable for integration into MEMS application devices. Such A MEMS device comprises a microelectromechanical structure and an electric low-pass filter according to the present invention or one of its embodiments, preferably integrated on a single substrate or in a multichip package. The MEMS device may for instance be an electroacoustical device, such as a MEMS microphone.

Preferred embodiments of the invention are also defined in the dependent claims. Combination of said embodiments are also envisaged.

DETAILED DESCRIPTION OF THE INVENTION The present invention is further elucidated by the following figures and examples, which are not intended to limit the scope of the invention. The person skilled in the art will understand that various embodiments may be combined.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings:

Fig. 1 shows a schematic cross-sectional view of a electric low- frequency filter device according to an embodiment of the invention in a first operational mode;

Fig. 2 shows the electric low-pass filter of Fig. 1 in a second operational mode;

Fig. 3 shows the capacitance- voltage characteristics of the electric low-pass filter of Figs. 1 and 2 for different operational frequencies between 500 Hz and 20 kHz;

Fig. 4 shows a frequency response of the electric low-pass filter of Figs. 1 and 2 in the operational mode of Fig. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 and 2 show a schematic cross-sectional view of a electric low-pass filter 100 according to an embodiment of the present invention. The low-pass filter comprises a layer stack, which is integrated into an interconnect stack (not shown) of an integrated circuit. Only the layers which are necessary for building the low-pass filter are shown in these figures for reasons of graphical simplicity. A first or bottom electrode 102 is provided. On top of the bottom electrode 102, a first or lower dielectric layer 104 is deposited. On top of the lower dielectric layer 104, a second or upper dielectric layer 106 is deposited. The lower and upper dielectric layers 104 and 106 together form a Maxwell- Wagner stack of dielectrics. On top of the Maxwell- Wagner stack 104, 106, a second or top electrode 108 is deposited.

The bottom and top electrodes are made of metallically conductive materials. It is suitable to use electrode materials, which are compatible with the process technology of forming the interconnect stack. For instance, titanium nitride TiN can be used for the bottom and top electrodes 102 and 108. However, it is also possible to place the low-pass dielectric filter 100 on a silicon layer, using a highly doped n⁺⁺ substrate section as a metallically conductive bottom electrode 102. For the lower dielectric layer 104, silicon oxide, typically in the form of silicon dioxide SiO₂, is a particularly suitable material. Silicon dioxide has the advantage, that it has a higher electric conductance than several other known dielectric materials, which are suitable candidates for the upper dielectric layer 106. Furthermore, silicon dioxide as fabricated in industrial fabrication processes used in the semiconductor industry, comprises protons (positive hydrogen ions), which provide mobile positive charge carriers in addition to holes. The use of this property will be explained in more detail further below.

The thickness of the first and second dielectric layers is chosen in consideration of the desired achievement of a Maxwell- Wagner stack. In operation of the low-pass filter, such a stack provides that a tunneling of electrons from the bottom electrode 102 to an interface 105 between the lower and upper dielectric layers 104 and 106 through the lower dielectric layer 104 is enabled under application of a suitable tunneling voltage applied between the bottom and top electrodes 102 and 108, which is schematically indicated by the symbols "+" and "-" in Fig. 1. This tunneling is used in a first operational mode indicated schematically in Fig. 1, which is referred to as "bottom injection" mode. During this mode, also referred to as electron injection, electrons are tunneling from the bottom electrode to the interface 105 between the lower and upper dielectric layers 104 and 106. The bottom injection mode is indicated schematically in Fig. 1 by arrows representing the tunneling of electrodes to the interface 105. It should be noted that the tunneling of electrodes takes place with a very fast kinetics. Under application of an AC signal between the top and bottom electrodes, the electron tunneling mechanism, in phases of the polarity as indicated in Fig. 1, occurs without detectable delay in relation to the variation in the applied signal.

On the other hand, under application of a tunneling voltage of an opposite polarity, as indicated in Fig. 2, a hole tunneling mechanism is triggered. This operational mode is referred to as a "top injection" mode. In this mode, holes and/or other mobile positive charge carriers such as protons tunnel from the bottom electrode to the interface 105 between the lower and upper dielectric layers 104 and 106 of the Maxwell- Wagner stack. The kinetics of this process is much slower than that of the electron tunneling in the substrate injection mode. As a consequence, the hole kinetics can not follow AC signals at high frequencies.

The different kinetics of electron and hole/proton tunneling gives rise to the capacitance- voltage (CV-) characteristics shown in Fig. 3. Fig. 3 is a diagram, in which the capacitance of the low-pass filter 100 of Figs. 1 and 2 is plotted in dependence on the applied voltage. The CV characteristic is plotted for different frequencies of the applied signal ranging between 500 Hz and 20 kHz. For the measurements shown in Fig. 4, a MIM capacitor structure with a doped n⁺⁺ bottom electrode 102, a 4.2 nm silicon oxide layer 104, a 5 nm thick lanthanum zirconate (La_yZri__yO_x) upper dielectric layer 106 and TiN top electrode 108 was used. As can be clearly seen, the range of negative voltages between -6 and 0 V exhibits an almost identical CV characteristic for all frequencies. This range is the bottom injection mode represented by Fig. 1, and is governed by the fast kinetics of electron tunneling. The corresponding range in the CV diagram is labeled with the capital letter B. In contrast, for positive voltages above +4 V, the CV characteristic of the low-pass filter depends strongly on the applied frequency. The capacitance reaches the same value as in the bottom injection mode B for frequencies in the range between 500 Hz and roughly 2 kHz.

For frequencies higher than that, i.e. 4, 8, 10 and 20 kHz, the capacitance drops strongly, and does not exhibit much of an increase with increasing voltage for a frequency of 20 kHz. This range of voltages is governed by the slow kinetics of the top injection mode referred to in Fig. 2. The lower capacitance of the layer structure 102 to 108 for high frequencies in the positive voltage range accounts for the low-pass filter characteristics of the low-pass filter 100.

As can be seen from Fig. 3, the low-pass filter can be used for letting signal frequencies in the range below 20 kHz pass. Thus, the slower hole kinetics alone is responsible for the asymmetric CV characteristics of the low-pass filter 100. Fig. 4 is a diagram showing a dependence of the capacitance of the low-pass filter 100 on the frequency between 0 and 20 kHz. As can be seen, the capacitance drops by about 25 % from ca. 0.125 nF to ca 0.09 nF in the covered frequency range. This frequency range is covered by the slow hole-conduction kinetics. Above 20 kHz, electron kinetics would start playing a role, thus changing the capacitance in the top-electrode injection mode as well.

In summary, by providing a MIM capacitor structure with a Maxwell- Wagner stack, in which the dielectric layer with a higher electric conductance provides a mechanism for transporting positive charges like holes or protons, an efficient low-pass filter device can be fabricated. The low-pass filter device is useful for low-frequency filtering, especially in the range of acoustic frequencies. The low-pass filter device can be integrated on silicon next to electro acoustic MEMS structures such as microphones.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. An electric low-pass filter, comprising: a layer stack formed by a first electrode layer, a first dielectric layer adjacent to the first electrode layer, a second dielectric layer adjacent to the first dielectric layer, and a second electrode layer adjacent to the second dielectric layer; wherein the second dielectric layer has a lower electrical conductance than the first dielectric layer; the first dielectric layer is made of a material and has a layer thickness, which combination of material choice and layer thickness allows - a tunelling of first charge carriers of a first charge-carrier polarity from the first electrode layer through the first dielectric layer to an interface between the first and second dielectric layers, under application of a tunneling voltage of a first voltage polarity between the first and second electrode layers, and allows a transport of second charge carriers of an opposite second charge-carrier polarity in the first delectric layer in a direction pointing from the first electrode layer to the interface between the first and second dielectric layers under application of a second voltage of a second voltage polarity between the first and second electrodes, which second voltage polarity is opposite to the first voltage polarity; wherein further the second charge carriers, with respect to their transport in the first dielectric layer under application of the second voltage, have a second charge-carrier mobility, which is lower than a mobility of the first charge-carriers with respect to the tunneling process at the tunneling voltage.

2. The electric low-pass filter of claim 1, wherein the first dielectric layer is made of a material, in which either holes or hydrogen ions are mobile.

3. The electric low-pass filter of claim 2, wherein the first dielectric layer is made of silicon dioxide containing mobile protons.

4. The electric low-pass filter of claim 2 or 3, wherein the first dielectric layer has a thickness of between 1 and 10 nanometer.

5. The electric low-pass filter of claim 3, wherein the second dielectric layer is made of a lanthanum zirconate, hafnium oxide, hafnium- erbium oxide, zirconium dioxide, aluminum oxide, or of a combination of these materials.

6. The electric low-pass filter of claim 5, wherein the second dielectric layer has a thickness between 2 and 50 nanometer.

7. The electric low-pass filter of claim 1, wherein at least one of the first and second electrodes is made of a metal.

8. The electric low-pass filter of claim 1, which is integrated into an interconnect stack of an integrated-circuit device.

9. A MEMS device, comprising a microelectromechanical structure and an electric low-pass filter according to any of claims 1-8 integrated on a single substrate or in a single multichip package.

10. An electroacoustical device, comprising the MEMS device of claim 9.