Variable Capacitor for RF Power Applications

Abstract

A radio-frequency (RF) power variable capacitor capable of operating at, at least, 50 watts in the MHz range. The capacitor has a composite HDK-NDK ceramic dielectric. The HDK (high dielectric constant) component comprises an active matrix of barium strontium titanate, for example. Acoustic resonances are reduced or eliminated by the addition of a metal or metalloid oxide such as magnesium borate (NDK—low dielectric constant), which acts as an acoustic resonance reduction agent (ARRA) in the RF power domain. The acoustic resonances which previously occurred under bias voltage 500 V or 1100 V in prior art RF power variable capacitors are eliminated by the addition of the ARRA.

Claims

1. A radio-frequency (RF) power variable capacitor, capable of operating at at least 50 W, comprising at least two electrodes separated by a dielectric, wherein the capacitance of the capacitor is variable by varying a voltage, denoted by applied across the at least two electrodes, wherein: the dielectric comprises a high dielectric constant-low dielectric constant (HDK-NDK) composite ceramic; the HDK component forms at least 80% by weight, of the material of the dielectric and comprises an active matrix wherein the active matrix comprises a HDK ferroelectric ceramic material, chosen from barium titanate, strontium titanate, barium-strontium titanate, calcium titanate and barium zirconate titanate; the NDK component less than 20% by weight, of the dielectric and comprises an acoustic resonance reduction agent (ARRA), wherein the ARRA comprises a metal oxide ceramic which is stable at 1200° C., and wherein the ARRA is distributed throughout the active matrix.

2. The RF power variable capacitor according to claim 1, wherein the active matrix material comprises barium strontium titanate.

3. The RF power variable capacitor according to claim 1, wherein the active matrix material has a mean grain size between 1.5 μm and 5 μm.

4. The RF power variable capacitor according to claim 1, wherein the active matrix has a first mean grain size and the ARRA has a second mean grain size, and wherein the first and second mean grain sizes differ by, at most, a factor of 2.

5. The RF power variable capacitor according to claim 4, wherein the second mean grain size is less than twice the first mean grain size.

6. The RF power variable capacitor according to claim 5, wherein the dielectric is formed as a substantially planar monolithic block or disk having a thickness between 0.6 and 1.2 mm, and having a cross-sectional area in the plane of the block or disk, of at least 100 mm.sup.2.

7. The RF power variable capacitor according to claim 1, wherein the metal oxide ceramic is Mg.sub.3(BO.sub.3).sub.2.

8. The RF power variable capacitor according to claim 1, wherein the active matrix material has a Curie temperature between 25° C. and 40° C.

9. The RF power variable capacitor according to claim 1, wherein the capacitor has a tunability of between 15% and 50% for a bias voltage range of 0 V to 5 kV.

10. A method of manufacturing the RF power variable capacitor according to claim 1, comprising: a first step in which ingredients of the active matrix component are ground and calcinated; a second step in which ingredients of the ARRA are ground and calcinated; a third step in which the calcinated active matrix and AARA components are mixed together in a predetermined ratio and ground; and a fourth step in which the mixed active matrix and ARRA components are dried, pressed, and sintered at a predetermined sintering temperature to form the capacitor dielectric.

11. The method according to claim 10, further comprising a fifth step, in which a plurality of RF electrodes are formed on the dielectric.

12. The method according to claim 10, wherein the sintering temperature is between 1050° C. and 1150° C.

13. The method according to claim 10, further comprising: an iterative minimizing step in which the ratio of ARRA to active matrix component is adapted, and the third and fourth steps are repeated to determine a value of said ratio which produces a capacitor having a Q-factor value, wherein a dependence of the Q-factor value on a magnitude of a bias voltage has a minimum value, at which an amount of acoustic resonance at a predetermined frequency has a minimum value.

14. The method according to claim 13, wherein the adapted ratio determined in the minimizing step is used as the predetermined ratio in the third step.

15. The RF power variable capacitor according to claim 1, wherein the active matrix material comprises a HDK ferroelectric ceramic material is a composite of one or more of barium titanate, strontium titanate, barium-strontium titanate, calcium titanate, and barium zirconate titanate.

16. The method according to claim 10, further comprising: an iterative minimizing step in which the ratio of ARRA to active matrix component is adapted, and the third and fourth steps are repeated to determine a value of said ratio which produces a capacitor having a Q-factor value, wherein a dependence of the Q-factor value on a magnitude of a bias voltage has a minimum value, or at which an amount of acoustic resonance at a predetermined frequency has a minimum value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The disclosure will be described in detail with reference to the attached drawings, in which:

[0014] FIG. 1 depicts a schematic isometric view of an example of a dielectric for a variable RF power capacitor.

[0015] FIG. 2 shows an example of a back-to-back connection arrangement of two such capacitors.

[0016] FIG. 3 shows a schematic equivalent circuit diagram of the back-to-back arrangement of FIG. 2.

[0017] FIG. 4 shows a graph illustrating how the Q-factor of a dielectric may vary with RF frequency of the RF signal applied, comparing the behavior of a capacitor according to the disclosure (curves 21-23) with those of a prior art capacitor (curves 24-26).

[0018] FIG. 5 shows a graph illustrating the variation in Q-factor with applied bias voltage, comparing the behavior of a capacitor according to the disclosure (curve 31) with that of a prior art capacitor (32).

[0019] FIG. 6 shows a graph illustrating the variation in capacitance with applied bias voltage, comparing the behavior of a capacitor according to the disclosure (curve 42) with that of a prior art capacitor (41).

[0020] FIG. 7 shows a scanning-electron-microscope image of a sample of a dielectric material suitable for use in a capacitor according to the disclosure.

[0021] FIG. 8 shows a block flow diagram of an example method according to the disclosure.

[0022] It should be noted that the FIGS. are provided merely as an aid to understanding the principles underlying the disclosure, and should not be taken as limiting the scope of protection sought. Where the same reference numbers are used in different FIGS., these are intended to indicate similar or equivalent features. It should not be assumed, however, that the use of different reference numbers is intended to indicate any particular degree of difference between the features to which they refer.

DETAILED DESCRIPTION OF THE INVENTION

[0023] A ferroelectric ceramic material may be used in its paraelectric phase to form an RF power capacitor whose capacitance is electrically adjustable by varying a high voltage (HV) bias electric field applied across the dielectric. When describing the disclosure, the terms “variable”, “adjustable” and “tunable” are used in this description to refer to the changing of the capacitance of a variable capacitor. The term “capacitor” when referring to the disclosure relates to variable capacitors for use in high-power RF applications, in which for example vacuum variable capacitors have hitherto been used, and they are therefore suitable for use in high power delivery systems used, for example, for powering RF plasma etching or coating processes in semiconductor manufacturing industries. Instead of a mechanical adjustment mechanism of the vacuum variable capacitance (whose speed is limited and inherently slow compared to load impedance variations in RF plasma processes), an electric DC bias voltage is used to generate an electric field in the capacitor dielectric.

[0024] In order to control the relative permittivity in such RF power applications, the magnitude of the applied DC bias voltage should advantageously be significantly greater than the amplitude of the RF application voltage (for example a factor of 10 greater), so that the effect of the RF voltage on the relative permittivity of the paraelectric dielectric can be neglected compared with the effect of the DC bias voltage. The relative permittivity can thus be controlled and adjusted by varying the DC voltage. The speed of reaction of the permittivity to the applied voltage is essentially instantaneous, since dipole orientations in materials react in nanoseconds or less.

[0025] The adjustment of the relative permittivity results directly in an adjustment of the capacitance of a device made with a paraelectric dielectric. The dielectric may be formed as a rectangular block or tablet, or as a disc, for example, with parallel planar conducting electrodes of area A on either side of the dielectric. In this simple geometry, the capacitance is given by C=ε.sub.0ε.sub.rA/d, where d is the dielectric thickness (distance between the electrodes), ε.sub.0 is the permittivity of vacuum (a physical constant), and ε.sub.r is the field-dependent (now DC-bias-voltage-dependent) relative permittivity of the dielectric. The term radio frequency (RF) relates to a frequency range which is used in RF power systems, typically between 400 kHz and 200 MHz. A standardized RF power frequency for use in industry is 13.56 MHz, for example, although other standardized RF frequencies are also used for RF power applications, for example 400 kHz, 2 MHz, 6.78 MHz, 13.56 MHz, 27.12 MHz, 40.68 MHz, 60 MHz, 80 MHz, 100 MHz and 162 MHz. References to power RF are intended to refer to applications in which the RF power output may be rated at 50 W or more, such as 100 W or more, or even 1000 W or more, and it is a requirement of RF power capacitors that they be able to handle such power at such frequencies. References to a ceramic material which is “stable at 1200° C.” or other temperature refer to a material which is suitable for sintering at the stated temperature.

[0026] The inventors have discovered that the prior art BST dielectric is liable to develop major unwanted acoustic resonance at particular RF frequencies when the dielectric is subjected to its biasing field. This kind of acoustic (mechanical) resonance may be associated with piezoelectric characteristics of the ceramic material (BST), for example, and can have such a severe detrimental effect that it can render the device unusable due to excessive mechanical stress and/or due to excessive localized heating in the ceramic material. Note that this effect is distinct from the ‘background’ electrical losses which typically occur in a ceramic dielectric even when the dielectric is not operating under a high biasing voltage. It has been found that the BST ceramic material of the prior art may be usable in an RF power variable capacitor at frequencies where acoustic resonance is not significant, but not at those frequencies where damaging resonance occurs.

[0027] The electrical losses of a capacitor are commonly quantified in terms of the Q factor or Quality factor of a capacitor. This represents the efficiency of a given capacitor in terms of energy losses, and is defined as: Q=X.sub.c/R.sub.c=1/(2πf C R.sub.c) where X.sub.c is the reactance of the capacitor, C the capacitance of the capacitor, Rc is the equivalent series resistance (sometimes abbreviated as ESR) of the capacitor, and f is the frequency at which the measurement is taken. The effect of acoustic resonance at a particular RF frequency under bias voltage may be detected by a significant decrease in the Q-factor at that frequency (referred to as a resonance peak), as discussed below with reference to FIG. 4.

[0028] The resonant characteristics of a particular disc-shaped block of ceramic dielectric (hereafter also referred to as a pellet) are principally dependent on the dimensions of the pellet. Such acoustic resonance can be compensated for or cancelled out at individual frequencies by changing the dimensions. The dimensions of the dielectric are critical to the acoustic wave behavior, in particular their resonances or the suppression of their resonances. Changing dimensions typically only shifts a damaging resonance peak to a different frequency. Furthermore, it may not always be an option to adjust dimensions of the pellet in order to eliminate acoustic resonances because the dimensions of the pellet also influence other factors such as the maximum bias and RF voltages which can be applied, and the capacitance values which can be achieved with the capacitor.

[0029] FIG. 1 shows a simple example of an RF power variable capacitor 1 comprising two RF electrodes 5 separated by a ferroelectric ceramic dielectric 7 operating in its paraelectric phase. RF connections 4 and DC bias voltage connections 2, 3 are provided to the electrodes 5. A variable bias voltage source Vc provides the variable bias voltage between the electrodes 5, and thus across the dielectric 7, thereby altering the capacitance value of the capacitor 1. The larger the applied bias voltage Vc, the lower the capacitance value of the capacitor 1. As indicated in FIG. 1, the dielectric can be fabricated as a contiguous monolithic block of the composite HDK-NDK ceramic material described below. Thanks to the inclusion of an acoustic resonance reduction agent (ARRA) in the dielectric material, the dielectric block can be formed without the kind of layered or interposed electrode arrangement proposed in the prior art, and without the need for tuning the dimensions (e.g. the thickness) of the block to particular operational frequencies measures which would otherwise be needed in order to avoid damaging acoustic resonance (although such interposed electrodes, dimension-tuning or other measures may optionally be included in addition to the ARRA).

[0030] FIG. 2 shows how a pair of capacitors 1 and 1′ such as the one depicted in FIG. 1 can be arranged back-to-back so as to provide a dual-capacitor unit 1″ whose outer (RF) electrodes 5 and 5″ are at substantially the same DC potential, while the high voltage bias voltage Vc is connected to the sandwiched inner electrode 5′. This is just one example of how multiple capacitors may be interconnected. Other configurations can be used. The physical back-to-back arrangement of FIG. 2 may instead be achieved electrically by arranging the two capacitors 1 and 1′ physically separate but electrically connected in the same way.

[0031] FIG. 3 shows an electrical schematic of the two-capacitor arrangement of FIG. 2. The RF power is supplied at input contact 4 and the RF output is at output contact 4′. Variable DC bias voltage Vc is connected across capacitor 1 via circuit nodes 2 and 3, and across capacitor 1′ via circuit nodes 3 and 2′. FIGS. 2 and 3 also illustrate how filters 8 may be provided to decouple the RF from the Vc bias voltage. Such an arrangement may be required to avoid the bias voltage affecting other connected components, and to avoid the Vc source being affected by the RF power signal. Each decoupling filter 8 may be implemented with resistive or reactive (e.g. LC) components in the conventional manner, for example.

[0032] The arrangements shown in FIGS. 1 to 3 are merely examples illustrating how an RF power variable capacitor may be implemented using two RF electrodes separated by a ceramic dielectric. In the example capacitor illustrated in FIG. 1, the RF electrodes are also used as the V, bias electrodes. Alternatively, one or more of the bias electrodes could be implemented separately from the RF electrodes. It/they may for example be located embedded in the ceramic body of the dielectric 7. The bias electrodes may be electrically insulated from the RF electrodes they may be located such that they are not in electrical contact (and optionally not in physical contact) with the dielectric 7, but nevertheless such that they generate the required electric field in the body of the dielectric 7.

[0033] As mentioned above, it has previously been proposed that the dielectric 7 be made of a ferroelectric material such as barium strontium titanate (BST), behaving as a paraelectric material (i.e. above its Curie temperature) at the operating temperature of the capacitor. By judicious choice of composition of the ceramic, the properties of the material can be adjusted so that its Curie temperature is near room temperature, for example the barium to strontium ratio can be selected to bring the Curie temperature to between 0° C. and 50° C., or preferably to between 25° C. and 40° C. This means that the dielectric will be in its paraelectric phase at an operating temperature of 50° C., for example. It has been found that such a dielectric, when fabricated with the dimensions required for an RF power variable capacitor, exhibits acoustic resonance which can be so severe as to render the device unusable at particular frequencies, or even at all. However, the inventors have discovered that it is possible to eliminate or greatly reduce the acoustic resonances without the necessity for fine-tuning the dimensions of the dielectric. By adding a second component (referred to as the acoustic resonance reduction agent) to the ceramic mix before sintering, it is possible to create a composite of a high permittivity and a low permittivity ceramic material (referred to as a composite HDK-NDK ceramic material) in which, at least for the dielectric dimensions required for RF power operation, acoustic resonances are substantially eliminated. Furthermore, the inventors have identified that there exist such compositions in which the required capacitance variability, the required dielectric properties (e.g. dielectric constant, background Q-factor) for RF power applications and the required reduction/elimination of acoustic resonance are all present.

[0034] Such a composite HDK-NDK ceramic material may comprise at least 60% (preferably more than 80%) by weight of an active matrix (e.g. BST) as the majority HDK component, and less than 40% (preferably less than 20%) of an acoustic resonance reduction agent, abbreviated as ARRA (e.g. a metal oxide ARRA such as magnesium borate) as the minority NDK component, for example. The German abbreviation HDK indicates a high dielectric constant (e.g. ε.sub.r>100) and NDK a low dielectric constant (e.g. ε.sub.r<100). The active matrix material has a mean grain size of between 0.5 μm and 20 μm, or preferably between 1 μm and 8 μm, or more preferably between 1.5 μm and 5 μm. The NDK should preferably be distributed throughout the active matrix: it has been found that very localized and larger aggregates of NDK component are not as effective in reducing acoustic resonances as well-distributed grains of similar size to those of the HDK component. Preferably the difference in mean grain size of the HDK and the NDK is less than a factor five, more preferably less than a factor two.

[0035] FIG. 4 shows (curves 24, 25, 26) how the Q-factor of a prior art ceramic based RF power capacitor varies with applied RF frequency for bias voltages of 0 V, 500 V and 1100 V respectively. As can be seen from curve 24, there is little sign of acoustic resonance effects when the dielectric is unbiased, but curves 25 and 26 show how the acoustic resonance at certain frequencies (e.g. 10.5 MHz, 16 MHz for the particular sample concerned) is reducing the Q-factor to near zero, particularly under higher bias conditions (1100 V for curve 26).

[0036] Curves 21, 22 and 23, by contrast, show how the Q-factor varies with the frequency of the applied RF power signal at different bias voltages (curves 21, 22 and 23 correspond to bias voltages of 1100 V, 500 V and 0 V respectively). The Q-factor is not only significantly greater; it also increases slightly with applied bias voltage. More significant for the purposes of the disclosure, however, is that the acoustic resonance peaks which were present in the prior art capacitor under bias voltages of 500 V and 1100 V are no longer present in the capacitor which uses the composite ceramic dielectric including the ARRA as described above.

[0037] The dielectric is preferably formed as a substantially planar monolithic block or disc having a thickness of between 0.5 mm and 1.5 mm or preferably between 0.6 and 1.2 mm, and having a cross-sectional area in the plane of the block or disk, of at least 20 mm.sup.2 or preferably at least 50 mm.sup.2, or more preferably at least 100 mm.sup.2. This can generate capacitance values of several thousand pF, for example, and is suitable to be biased with voltages up to 6 kV. This high bias voltage allows the permittivity of the dielectric block (in particular a barium strontium titanate dielectric with ARRA) to be adjusted over a wide range, thereby enabling capacitors with tunabilities greater than 15%, or preferably greater than 30%, and up to 40% or up to 50% or more. FIG. 5 shows the variation in Q-factor with applied bias voltage, corresponding to the values for approximately 13 MHz in FIG. 4. Curve 32 shows how the Q-factor of the prior art capacitor decreases with applied voltage, while curve 31 shows how the Q-factor increases with applied bias voltage V.sub.c.

[0038] FIG. 6 shows how the capacitance of the same devices varies with applied bias voltage. Curve 41 shows how the capacitance of the prior art device decreases more markedly than the capacitance of the device (shown by curve 42) with a dielectric according to the disclosure. The magnitude of the capacitance of the inventive capacitor including the ARRA component is significantly reduced, and the gradient of the curve 42 is significantly shallower than the prior art device. However, the absolute values of the capacitance and the variability of the capacitance are nevertheless suitable for use in an RF power variable capacitor. For a bias voltage range of 0 V to 5 kV, a tunability of 70% or more may be achievable with a BST dielectric known in the prior art (curve 41), whereas the tunability achievable with the dielectric including the ARRA is significantly lower (curve 42), and lies in the range 15% to 50%, for example, for a bias voltage range of 0 V to 5 kV.

[0039] FIG. 7 shows a scanning electron microscope image of a section through a sample of a dielectric according to the disclosure. In this example, the ARRA 51 is present in the form of grains of a similar size to those of the active matrix 52. The ARRA component grains may, as in this example, have a different morphology from that of the grains of the active matrix (e.g. BST). In this case, part of the ARRA, e.g. in the upper right corner of FIG. 7, is more amorphous (glass-like) than the more crystalline active matrix. In both cases (for the active matrix and the ARRA), the mean grain sizes are preferably similar (e.g. differing by a factor of less than 5, or preferably less than 2).

[0040] FIG. 8 shows a block diagram of an example of a method for manufacturing an RF power variable capacitor as described above. In steps 62 and 63, raw components 60 of the active matrix HDK component of the composite ceramic (such as barium carbonate, strontium carbonate and titanium dioxide) and dopants 61 (such as 0.5% or 1% iron or manganese, for example) are ground together 62 and calcinated 63 to form the base active matrix material. The ARRA material 65, a metal or metalloid oxide, preferably magnesium borate, for example, is ground in step 64, optionally together with other additives or dopants 66, and calcinated in step 67 to form the NDK component of the HDK-NDK composite ceramic. The calcinated active matrix and ARRA components are then re-ground and mixed, in predetermined proportions, in step 68, with additional materials 69, e.g. additives for facilitating the subsequent compression and sintering steps (some may be optional depending on which precise steps are intended). The ground mixture is then dried in step 70, for example by spray drying, pressed into a mold in step 71 and sintered in step 72 to form the ceramic dielectric block, disc, pellet or other shape as required for the capacitor. In step 73, RF electrodes are formed (for example by metallization) on the surfaces of the dielectric, and the bias electrodes are formed if separate bias electrodes are used. In step 74, RF and Vc bias connections are provided to the RF electrodes and bias electrodes respectively. Filter components 8 may be added as described in relation to FIG. 3. Multiple dielectrics 7 may be assembled and connected as described in relation to FIG. 2. In step 75, the capacitor may be provided with insulation, packaging or other finishing features for providing the finished component. Step 76 indicates an iteration which may optionally be performed when customising or prototyping the capacitor. In this step, one or more iterations may be performed with different ratios of ARRA to active matrix, to determine an optimal proportion of ARRA, i.e. the proportion which minimizes the variation in Q-factor with bias voltage, for example, or to address particular acoustic resonances at particular frequencies. In addition, other characteristics of the dielectric can be maximized, such as the Q-factor.

[0041] A composite with the following formula has been found to be effective in reducing or eliminating acoustic resonances for a wide range of dielectric geometries and RF frequencies:

Ba.sub.ASr.sub.BMn.sub.cTi.sub.D0.sub.3  Active matrix:

[0042] where Fe or other metallic elements may be substituted for Mn, A=0.6-0.8, B=0.2-0.4, C=0-0.015, and D=0.985-1

[0043] ARRA: magnesium borate Mg.sub.3 (B0.sub.3).sub.2, for example. Other metal or metalloid oxides may be usable instead.

[0044] The ratio of ARRA to active matrix may be between 40:60 and 5:95, or preferably between 20:80 and 8:92, or more preferably between 10:90 and 5:95 by volume in the sintered state.