Patent Description:
It is sometimes desirable to have circuits capable of selectively filtering one frequency or range of frequencies out of a mix of different frequencies in a circuit. A circuit designed to perform this frequency selection is called a filter circuit, or simply a filter. Filters are used in a vast number of practical applications.

For example, a common need for filter circuits is in high-performance stereo systems, where certain ranges of audio frequencies need to be amplified or suppressed for best sound quality and power efficiency. For example, equalizers allow the amplitudes of several frequency ranges to be adjusted to suit the listener's taste and acoustic properties of the listening area. In contrast, crossover networks block certain ranges of frequencies from reaching speakers. Both equalizers and crossover networks are examples of filters, designed to accomplish filtering of certain frequencies.

Another practical application of filter circuits is in the "conditioning" of non-sinusoidal voltage waveforms in power circuits. Some electronic devices are sensitive to the presence of harmonics in the power supply voltage, and so require power conditioning for proper operation. If a distorted sine-wave voltage behaves like a series of harmonic waveforms added to the fundamental frequency, then it should be possible to construct a filter circuit that only allows the fundamental waveform frequency to pass through, blocking all (higher-frequency) harmonics.

Frequency-selective or filter circuits pass to the output only those input signals that are in a desired range of frequencies (called pass band). The amplitude of signals outside this range of frequencies (called stop band) is reduced (ideally reduced to zero). Typically, in these circuits, the input and output currents are kept to a small value and as such, the current transfer function is less important parameter than the voltage transfer function in the frequency domain.

<FIG> shows a conventional <NUM>st order passive low pass filter <NUM> which includes a resistor <NUM> and a capacitor <NUM> connected in series so that they can accept the same current. The input terminal <NUM> is connected across the whole circuit whereas the output terminal <NUM> is connected across the positive capacitor. The filter <NUM> is simple in implementation, but does not provide a gain greater than <NUM> dB and/or a rapid power roll off with a value more than <NUM> dB/decade around and beyond a cutoff frequency.

<FIG> shows an exemplar Gain vs Frequency curve <NUM> for a first order passive low pass filter <NUM>. Here Gain is defined as 20log(H(f)) wherein H(f)=Vout (f)/Vin(f). The value of Gain for a passive filter in the pass band <NUM> is either <NUM> dB or slightly less than that. A cut off frequency <NUM> is defined such that the gain at that point is -3dB. The power roll off <NUM>, i.e., the slope of Gain curve <NUM> in the stop band beyond cutoff frequency, is -<NUM> dB/decade. A <NUM>st order low pass filter cannot provide a gain greater than <NUM> dB and a power roll off with a value more than -<NUM> dB/decade around and beyond a cutoff frequency. In order to, achieve higher power roll off in a passive low pass filter, two such low pass filters must be cascaded to make it a second order low pass filter. Also, in order to have a gain higher than <NUM> dB an active filter is needed with active elements like transistors, operational amplifiers.

<FIG> shows an exemplar Gain vs Frequency curve <NUM> for a <NUM>nd order passive low pass filter formed by cascading two first order passive low pass filters <NUM>. The Gain is defined as 20log(H(f)) wherein H(f)=Vout (f)/Vin(f), and the value of Gain in the pass band <NUM> is always <NUM> dB or slightly less than that. A cut off frequency <NUM> is defined such that the gain at that point is -3dB. The power roll off that is the slope of Gain curve in the stop band beyond cutoff frequency <NUM> is -<NUM> dB/decade. One thing to be noticed is that the power roll off has been improved significantly in the <NUM>nd order filter. However, the gain is still <NUM> dB or less. In addition, the cascading <NUM>nd order passive low pass filter requires duplication of electric components of the first order passive low pass filter.

Another type of filters is RLC filters implemented based on combinations of resistors (R), inductors (L) and capacitors (C). The RLC filters are also known as passive filters, because they do not depend upon an external power supply and/or they do not contain active components such as transistors. The RLC filters can be configured to form a resonant circuit providing a high gain for a particular band of frequencies.

Inductors block high-frequency signals and conduct low-frequency signals, while capacitors do the reverse. A filter in which the signal passes through an inductor, or in which a capacitor provides a path to ground, presents less attenuation to low-frequency signals than high-frequency signals and is therefore a low-pass filter. If the signal passes through a capacitor, or has a path to ground through an inductor, then the filter presents less attenuation to high-frequency signals than low-frequency signals and therefore is a high-pass filter. Resistors on their own have no frequency-selective properties, but are added to inductors and capacitors to determine the time-constants of the circuit, and therefore the frequencies to which it responds.

The RLC filters can provide better power roll off than the <NUM>st order passive filter. However, inductors are very bulky due to their need to store energy in a form of current. To that end, fabricating/realizing an inductor in an integrated circuit (IC) in very difficult and consumes a lot of die area. In addition, the RLC filters also do not provide gain greater than <NUM> dB.

Document <NPL> studies negative capacitance in ferroelectric materials.

Document "<NPL> describes an electrical circuit consisting of a resistor, an inductor and a capacitor connected in series or in parallel.

Document <CIT>describes an interconnect structure for dynamic random access memory cells.

Document <NPL> studies capacitance dispersion in hydrogen-doped amorphous barium titanate.

Document <NPL> investigates the behavior of the capacitance switching of HfO<NUM> Resistive nonvolatile Memories.

Document <NPL> studies negative capacitances in metal-insulator-semiconductor structures.

Document <NPL> discloses a field effect transistor, in which a standard insulator is replaced with a ferroelectric insulator.

Document <NPL> describes electrically controlled tunable active low-pass filters with adjustable cut-off frequency.

Document <CIT> describes a filter network that exhibits a bandstop response.

Document <NPL> describes an electronically-controllable lossless floating inductance circuit without any matching condition.

Document<NPL> describes an inductance simulator circuit.

Document <NPL> describes a simulation study of the negative capacitance effect incorporating leakage through the ferroelectric negative capacitor.

Document <NPL> discloses concepts for making low-pass and high-pass filters with RC circuits.

There is a pressing need to develop a compact and efficient circuit that can provide Gain in the pass band and higher roll of frequency without using any active elements, such as transistors and operational amplifier.

The embodiments disclose a filter formed by a capacitor connected in series with a resistor and a ferroelectric oxide capacitor operating in negative capacitance zone. According to simulations, the power rolls off a filter with a negative capacitor can be even higher than <NUM> dB/dec and the gain of the filter with a negative capacitor can be positive.

The negative capacitor differs from the positive capacitor in that the charge associated with a positive capacitor increases with the increase of the voltage across the positive capacitor, while the charge associated with a negative capacitor decreases with the increase of the voltage across the negative capacitor. The embodiments are based on recognition that the voltage across the negative capacitor includes a term resembling the voltage across an inductor. After some simulations and/or experimentations, the embodiments confirm that the negative capacitor acts, in part, as an inductor. Hence, the negative capacitor can be potentially used in place of the inductor. Hence, the negative capacitor used in the circuits for the purpose of its inductive purposes is referred herein as virtual inductor.

The embodiments are based on recognition that the negative capacitor is unstable in isolation, but can be stabilized if connected in series with a positive capacitor. The embodiments are based on another realization that the positive capacitor plays its role in the filters to attenuate high-frequency signals. To that end, the same positive capacitor can play the dual role in the filter, i.e., to attenuate high-frequency signals and to stabilize the negative capacitor. In such a manner, the power rolls off a filter with virtual inductor as a negative capacitor can be increased without the need to use the actual inductor.

The embodiments are based on recognition that typically it is impractical to connect two capacitors in series, because the joint capacitance of two positive capacitors connected in series is less than the sum of their individual capacitance. However, the embodiments are based on realization that when a negative capacitor is added in series with a positive capacitor the joint capacitance is enhanced. In such a manner, the gain of a filter with virtual inductor as a negative capacitor can be positive without the need to use any active elements, such as transistors and operational amplifier.

The above problems are solved by the subject-matter according to the independent claims. Accordingly, one embodiment discloses a filter comprising: a circuit including a resistor, a positive capacitor, and a negative capacitor connected in series to accept the same current; an input terminal to accept an input voltage across the circuit; and an output terminal to deliver an output voltage taken across the resistor, across the positive capacitor, or across the positive capacitor and the negative capacitor; wherein the circuit is an integrated circuit formed on a substrate; wherein the circuit comprises: a dielectric oxide layer sandwiched between a first metal layer and a second metal layer, wherein the first metal layer extends beyond the dielectric layer, and wherein the extended portion of the first metal layer is patterned; and a ferroelectric oxide, FEO, layer sandwiched between the second metal layer and a third metal layer; wherein the negative capacitor is formed by the FEO layer sandwiched between the second metal layer and the third metal layer; wherein the positive capacitor is formed by the dielectric layer sandwiched between the first metal layer and the second metal layer; and wherein the resistor is formed by the patterned extended portion of the first metal layer.

Another embodiment discloses a method for manufacturing a filter, the filter comprising a circuit including a resistor, a positive capacitor, and a negative capacitor connected in series to accept the same current; the method comprising: providing a substrate; forming a first metal layer on the substrate; depositing a dielectric layer on the first metal layer; forming a second metal layer on the dielectric layer; depositing a ferroelectric oxide layer on the second metal layer; forming a third metal layer on the ferroelectric oxide layer; etching away a portion of the dielectric layer to form a portion of the first metal layer extending beyond the dielectric layer; and pattering the extended portion of the first metal layer; wherein the negative capacitor is formed by the ferroelectric oxide layer sandwiched between the second metal layer and the third metal layer; wherein the positive capacitor is formed by the dielectric layer sandwiched between the first metal layer and the second metal layer; and wherein the resistor is formed by the patterned extended portion of the first metal layer.

<FIG> shows a block diagram of a filter <NUM> according to embodiments. The filter <NUM> includes a circuitry <NUM> including a resistor <NUM>, a positive capacitor <NUM>, and a negative capacitor <NUM> connected in series to accept the same current. The order of connection of the resistor and the positive and the negative capacitors can vary. The filter <NUM> also includes an input terminal <NUM> to accept an input voltage across the circuit and an output terminal <NUM> to deliver an output voltage taken across the resistor or the positive capacitor. In embodiments, the filter <NUM> includes only passive elements, such as resistors and capacitors. In embodiments, the filter <NUM> includes only a combination of the resistor, the positive capacitor, and the negative capacitor.

<FIG> shows a circuit diagram of a circuit of a filter according to the embodiment. The circuit includes a resistor <NUM>, a capacitor <NUM> and a negative <NUM> capacitor. As shown on this diagram, all these elements <NUM>, <NUM>, and <NUM> are combined is such a way that once connected to a voltage source, the same current <NUM> pass through all the elements; this particular electrical connection is known as series connection and referred herein accordingly.

The power roll off the filter with a negative capacitor can be even higher than <NUM> dB/decade and the gain of the filter with a negative capacitor can be positive. The negative capacitor differs from the positive capacitor in that the charge associated with a positive capacitor increases with the increase of the voltage across the positive capacitor, while the charge associated with a negative capacitor decreases with the increase of the voltage across the negative capacitor.

<FIG> shows a cross section of a negative capacitor <NUM> according to the embodiments. Those embodiments form the negative capacitor using a Ferroelectric Oxide (FEO) layer <NUM> sandwiched between two metal layers <NUM> and <NUM>.

The embodiments are based on recognition that the voltage across the negative capacitor includes a term resembling the voltage across an inductor. After some simulations and/or experimentations, the embodiments confirm that the negative capacitor acts, in part, as an inductor. Hence, the negative capacitor can be potentially used in place of the inductor. Hence, the negative capacitor used in the circuits for the purpose of its inductive purposes is referred herein as virtual inductor.

<FIG> shows a schematic of voltages across a negative capacitor and an inductor. The voltage <NUM> across the negative capacitor is <MAT> where Q is the total charge, tFE is the thickness of ferroelectric oxide forming the negative capacitor, l, ρ, α, β and γ are material constants of FE oxide.

The voltage <NUM> across an inductor is <MAT>.

Some embodiments are based on realization that the first term <NUM> of Eqn. <NUM> is quite similar to the term <NUM> of Eqn. To that end, it is realized that a negative capacitor has a built-in virtual inductance of value ltFE.

The embodiments are based on recognition that the negative capacitor is unstable in isolation, but can be stabilized if connected in series with a positive capacitor. The embodiments are based on another realization that the positive capacitor plays its role in the filters to attenuate high-frequency signals. To that end, the same positive capacitor can play the dual role in the filter, i.e., to attenuate high-frequency signals and to stabilize the negative capacitor. In such a manner, the power roll off a filter with virtual inductor as a negative capacitor can be increased without the need to use the actual inductor.

<FIG> shows a schematic of a negative capacitor <NUM> stabilized with help of a positive capacitor <NUM> according to the embodiments. In this embodiment, the positive capacitor <NUM> is formed using dielectric oxide layer <NUM> sandwiched between two metal layers <NUM> and <NUM>.

<FIG> shows a plot of the charge voltage characteristics of a positive capacitor formed by a layer <NUM> according to the embodiments. This plot demonstrates that the charge associated with a positive capacitor increases with the increase of the voltage across the positive capacitor.

<FIG> shows a plot of the charge voltage characteristics of a negative capacitor formed by the FEO layer <NUM> according to the embodiments. This plot demonstrates that the charge associated with a negative capacitor goes down with the increase of the voltage across the negative capacitor.

<FIG> shows a plot of the Energy vs Charge characteristics of a positive capacitor formed by a layer <NUM> according to the embodiments. This plot has a "V" shaped Energy vs Charge curve (U vs Q). The curvature of U vs Q curve gives the value of capacitance.

<FIG> shows a plot of the Energy vs Charge characteristics of a negative capacitor employed by the embodiments. This plot has an inverted/upside down "V" shaped Energy vs Charge curve (U vs Q). The curvature of U vs Q curve gives the value of capacitance. However, such a negative capacitor is unstable without additional assistance and configuration.

<FIG> shows a plot of the energy landscape curve <NUM> of a ferroelectric oxide material as a function of charge used in FEO layer <NUM> according to the embodiments. The energy landscape curve of a FEO material has "W" shape <NUM>. This curve <NUM> around zero charge value has negative curvature giving rise to negative capacitance, referred herein as a "negative capacitance zone". Normally a FEO material cannot stay in this zone because it has higher energy and end up being either of the two local minima <NUM> and <NUM>. However, adding a capacitor <NUM> in series to have the same charge can make the ferroelectric oxide stable in negative capacitance zone. This is because adding a normal capacitor makes the overall energy of the system lower.

To that end, some embodiments select the thickness of the FEO layer <NUM> as a function of charge of the positive capacitor. For example, one embodiment selects the thickness of the FEO layer to be less than a critical thickness Tc determined based on <MAT> wherein α is a material based parameter the material of the FEO layer, Ccap is the capacitance of the positive capacitor.

<FIG> shows the measured polarization vs electric field characteristics of ferroelectric oxide. The electric labeled in this figure as <NUM> is known as coercive electric field (Ec) and the polarization at zero electric field labeled by <NUM> is known as remnant polarization PR. Once PR and EC are known from the P-E measurement, one can calculate α by using the following equation, <MAT>.

The embodiments are based on recognition that typically it is impractical to connect two capacitors in series, because the joint capacitance of two positive capacitors connected in series is less than the sum of their individual capacitance. However, the embodiments are based on realization that However, some embodiments are based on realization that, when a negative capacitor is added in series with a positive capacitor the joint capacitance is enhanced. The amplification of the input voltage in the invented filter can be understood from Kirchhoff's Voltage Law (KVL), according to this law, <MAT>.

Here, VR is the voltage across the resistor and VFE is the voltage across the negative capacitor. In most applications VR~<NUM> and <MAT>; using these values we get, <MAT>.

Therefore, Voutput > Vinput, since for a positive applied voltage charge is positive.

In such a manner, the gain of a filter with virtual inductor as a negative capacitor can be positive without the need to use any active elements, such as transistors and operational amplifier.

<FIG> shows a plot of Gain vs Frequency characteristics a filter with virtual inductor according to some embodiments. Here Gain is defined as 20log(H(f)) wherein H(f)=Vout(f)Vin(f). As is evident from the figure, the value of Gain for the filter in the pass band <NUM> is +<NUM> dB which is greater than 0dB. And power roll off that is the slope of Gain curve in the stop band beyond cutoff frequency <NUM> is greater than -<NUM> dB/decade.

A frequency of interest can be defined as a critical frequency fc at which the Gain is <NUM> db. The said critical frequency can be thought of a frequency beyond which the negative capacitance effect is nullified. As shown in <FIG> the gain G(ω) is a function almost constant at the low frequency regime and decreases sharply at the higher frequency. An expression for gain is <MAT>.

An expression that relates fc with the circuit parameters and the material based parameter of FEO is <MAT> where <MAT>.

As long as the required the cutoff frequency of the filter is less than that of the critical frequency, the negative capacitance can provide positive Gain.

A second order filter can be implemented by having an inductor in series with the first order filter, however having an inductor in an integrated circuit is expensive as mentioned earlier. In addition to power roll of and gains benefits of the filter with the virtual inductor implemented via negative capacitor, such a filter according to various embodiments is more compact than the filter with actual inductor and can be advantageously implemented as an is an integrated circuit formed on a substrate.

<FIG> shows a <NUM>-D schematic of the filter in semiconductor platform according to the embodiments The filter includes a negative capacitor which is formed by a ferroelectric oxide layer <NUM> sandwiched between two metal layers <NUM>, <NUM>; a positive capacitor formed by a dielectric layer <NUM> sandwiched between two metal layers <NUM> and <NUM>; and a resistor formed by pattering the extended portion labeled <NUM> of the metal <NUM>. Here, the negative capacitor, positive capacitor and the resistor are in series combination. For example, the dielectric layer <NUM> can include one or combination of Al2O3, SixOy, SixNy, SixOyNz, Teflon, HfO2, or any other dielectric with a dielectric constant below <NUM>. For example, the substrate can include one or combination of silicon (Si), silicon carbide (SiC), diamond, and gallium nitride (GaN).

Depending on arrangement of the output terminal, the filter of some embodiments can for a low-pass filter, a high-pass filter, and a band-pass filter that can be also configured to form a resonant circuit.

<FIG> shows a schematic of a circuit <NUM> of a low pass according to one embodiment. In this embodiment, the filter built with a series combination of a resistor <NUM>, a capacitor <NUM> and a negative capacitor <NUM> wherein the input terminal <NUM> is taken across the whole series combination and output terminal <NUM> is taken across the positive capacitor <NUM>. Such combination of input and output terminals would provide low pass filter operation, which presents less attenuation to low-frequency signals than high-frequency signals. Specifically, with the output voltage taken across the positive capacitor, such that the filter is a passive low-pass filter with the output voltage greater than the input voltage in a pass band and a power roll off greater than 20dB per decade.

<FIG> shows a schematic a circuit <NUM> of a high pass filter according to one embodiment. In this embodiment, the filter built with a series combination of a resistor <NUM>, a capacitor <NUM>, and a negative capacitor <NUM>, where the input terminal <NUM> is across the whole circuit and the output terminal <NUM> is across the resistor. Specifically, with the output voltage is taken across the resistor, such that the filter is a passive high-pass filter with the output voltage greater than the input voltage in a pass band and a power roll off greater than 20dB per decade. In some implementations, this circuit behaves like a high pass filter and is capable of providing gain in the pass band and would yield more than -<NUM> dB/decade power roll off.

<FIG> shows a schematic of a circuit <NUM> of a resonant circuit according to one embodiment. In this embodiment, the filter built with a series combination of a resistor <NUM>, a capacitor <NUM>, and a negative capacitor <NUM>, where the input terminal <NUM> is across the whole circuit and the output terminal <NUM> is across the capacitor and the negative capacitor. When the circuit operates at the resonant frequency the reactance due to the virtual inductor coming from the negative capacitor cancels out the reactance of the positive capacitor and a high current flows through the circuit, in this way a series resonance is achieved.

<FIG> shows the block diagram of a method for fabricating a semiconductor device according to the embodiments. The method includes providing substrate <NUM>, here the substrate includes but not limited to silicon (Si), silicon carbide (SiC), diamond, gallium nitride (GaN) and so on. Further the method includes the formation <NUM> of a first metal layer for the resistor and positive capacitor. The formation of this metal layer can be done by Lithography → Metal Deposition → Lift-off and/or Metal deposition → Lithography → Etching. Here the lithography could be performed using, including but not limited to photolithography, electron-beam lithography. Metal deposition can be done using one or combination of an ebeam deposition, a joule evaporation, a chemical vapor deposition and a sputtering process.

The method also includes <NUM>, deposition of dielectric layer on the first metal layer to form positive capacitor. Then etch away the dielectric layer from the extended region of the first metal layer so that an electrical contact can be made while doing measurements. The method further includes deposition <NUM> of a second metal layer on the dielectric layer and deposition <NUM> of ferroelectric oxide layer on the second metal layer and deposition <NUM> of the third metal layer on the ferroelectric oxide layer to form negative capacitor. The extended portion of the first metal layer and/or the third metal layer can be pattered to form a resistor. Also, in some implementations, the first and third metal serves as the input terminal and second and third mental serves as the output terminal.

Claim 1:
A filter (<NUM>), comprising:
a circuit (<NUM>, <NUM>, <NUM>, <NUM>) including a resistor (<NUM>, <NUM>), a positive capacitor (<NUM>, <NUM>), and a negative capacitor (<NUM>, <NUM>) connected in series to accept the same current (<NUM>);
an input terminal (<NUM>, <NUM>) to accept an input voltage across the circuit (<NUM>, <NUM>, <NUM>, <NUM>); and
an output terminal (<NUM>, <NUM>, <NUM>, <NUM>) to deliver an output voltage taken across the resistor (<NUM>, <NUM>), across the positive capacitor (<NUM>, <NUM>), or across the positive capacitor (<NUM>, <NUM>) and the negative capacitor (<NUM>, <NUM>);
wherein the circuit is an integrated circuit formed on a substrate;
wherein the circuit comprises:
a dielectric oxide layer (<NUM>) sandwiched between a first metal layer (<NUM>) and a second metal layer (<NUM>), wherein the first metal layer extends beyond the dielectric layer, and wherein the extended portion of the first metal layer is patterned; and
a ferroelectric oxide, FEO, layer (<NUM>) sandwiched between the second metal layer (<NUM>) and a third metal layer (<NUM>);
wherein the negative capacitor (<NUM>, <NUM>) is formed by the FEO layer (<NUM>) sandwiched between the second metal layer (<NUM>) and the third metal layer (<NUM>);
wherein the positive capacitor (<NUM>, <NUM>) is formed by the dielectric layer (<NUM>) sandwiched between the first metal layer (<NUM>) and the second metal layer (<NUM>); and
wherein the resistor (<NUM>, <NUM>) is formed by the patterned extended portion (<NUM>) of the first metal layer (<NUM>).