Patent Description:
The technical requirements for radio frequency (RF) applications using high frequencies, such as radar sensing and mobile communication according to the <NUM> standard, are increasing. In particular, switches having improved characteristics compared to state-of-the-art CMOS switches will be required to meet future demands. Phase change switches are considered as promising candidates for switching RF signals. Such phase change switches use a phase change material (PCM) which typically exhibits a higher electric conductivity in a crystalline phase state than in an amorphous phase state. By changing the phase state of the phase change material, a switch device including such a material may be switched on and off.

For example, to change the phase state from amorphous to crystalline, typically a heater is employed heating the phase change material causing crystallization. This switching on by causing crystallization is also referred to as a set operation. In the set operation, the heater is actuated in such a way that the temperature of the phase change material is above its crystallization temperature, typically about <NUM>, but below the melt temperature typically in a range of <NUM> to <NUM>, for example. The length of the heating pulse caused by the heater is chosen such that any amorphous portion present in the PCM can regrow into the crystalline phase state.

When switching off the switch device, also referred to as reset operation, the heater is actuated in such a way that the temperature of the PCM is raised above the melt temperature (for example above about <NUM> to <NUM>) followed by a comparatively rapid cooldown which freezes the phase change material or at least a portion thereof into an amorphous state.

Suitable phase change materials used for such phase change switches include germanium telluride (GeTe) or germanium-antimony-tellurium (GeSbTe, usually referred to as GST), and heaters may be made of a material like polycrystalline silicon or tungsten.

PCM switch devices promise excellent radio frequency performance in comparison to state of the art CMOS RF switches. In particular, the main figure of merit, the product of on-resistance and off capacitance, is reduced significantly from around <NUM> fsec for CMOS RF switches to values below <NUM> fs for PCM switch devices. In particular, a low off capacitance is desirable in applications like antenna tuning, as resonant modes of tuning networks including such switches may adversely influence the antenna properties at a high operating frequency.

For example, when for tuning purposes such a PCM switch is coupled in series to an inductor having an inductance L, the off state capacitance COFF of the switch creates a series resonance at a frequency <MAT>. This resonance frequency must be shifted to a value outside the operating frequency range of the respective system, for example radio frequency antenna, by either minimizing the inductance value L or minimizing Coff. The latter option is preferred, as it offers a higher degree of freedom in choosing the tuning elements, in particular inductances thereof, of a system.

<CIT>discloses a phase change switch device including an array of phase change switches with associated heaters, which heaters are supplied by a single pulse generator. A switch network selects a heater to be heated by connecting it to the pulse generator, while isolating heaters not to be heated from the pulse generator. Diodes are used for preventing a reverse current.

<CIT> discloses p-i-n diodes to provide a variable impedance.

Post-published <CIT> discloses a phase changes switch including switches for isolating a heater to reduce parasitic capacitances.

A phase change material switch device as defined in claim <NUM> and a method as defined in claim <NUM> are provided. The dependent claims define further embodiments.

According to an embodiment, a phase change material switch device is provided, comprising:.

For at least one of the N heaters the impedance device comprises a first impedance element coupled between the first device terminal and a first terminal of the respective heater, and a second impedance element coupled between the first device terminal and a second terminal of the respective heater.

According to another embodiment, a method of operating a phase change material switch device as defined above is provided, the method comprising:.

In the following, various embodiments will be described in detail referring to the attached drawings. The embodiments described hereinafter are to be taken as examples only and are not to be construed as limiting. For example, while in embodiments specific arrangements or components are provided, in other embodiments other configurations may be used.

Implementation details described with respect to one of the embodiments are also applicable to other embodiments.

Features from different embodiments may be combined to form further embodiments.

Variations and modifications described for one of the embodiments may also be applied to other embodiments and will therefore not be described repeatedly.

In the figures like elements are designated with the same reference numerals. Such elements will not be described repeatedly in each Figure to avoid repetitions. Any directional terminology used when referring to the drawings (e.g. up, down, left, right) is merely for indicating elements and directions in the drawings and is not intended to imply a directional orientation of the actually implemented devices.

Besides features (for example components, elements, acts, events or the like) explicitly shown and described, in other embodiments additional features may be provided, for example features used in conventional switch devices using phase change materials. For example, embodiments described herein relate to equalization devices in phase change material (PCM) switch devices, and other components and features, like spatial arrangement of heaters and phase change material, radio frequency (RF) circuitry using the switch device and the like may be implemented in a conventional manner. Such RF circuitry may be integrated with the described switch devices on the same substrate, but may also be provided separately for example, on one or more separate chip dies, which in some implementations then may be combined with a switch device in a common package. Also, manufacturing implementations like providing phase change material on a substrate like a silicon substrate to implement a PCM switch device or in a part thereof like a trench for manufacturing the switch device and the like may be performed in any conventional manner.

A switch based on a phase change material (PCM) will be referred to as a phase change switch (PCS) or PCM switch herein. As explained in the introductory portion, such phase change switches may be set to a crystalline phase state or an amorphous phase change, thus changing the resistance of the phase change material and therefore of the switch by several orders of magnitude. In this way, for example an on-resistance of a switch in a range of <NUM> to <NUM>Ω may be achieved, whereas an off-resistance may be several orders of magnitude higher, for example at least in the Kiloohm range.

PCM switch devices discussed herein may be manufactured for example in layer deposition and pattering processes similar to those used in semiconductor device manufacturing, by depositing or modifying layers on a substrate. In some embodiments discussed herein, cross-sectional views and top views are illustrated. A cross-sectional view essentially corresponds to a cross section through the substrate, whereas a top view is a view in a direction towards a surface of the substrate.

While phase change switch devices in the embodiment below are shown with a configuration where a heater is provided below a phase change material, in other embodiments the heater may be provided above the phase change material. Furthermore, currents through the phase change material and through the heater may run in the same direction or in different, for example perpendicular directions. Therefore, the specific configurations shown are not to be construed as limiting in any way.

In the figures, <FIG> shows a claimed embodiment. The remaining figures show examples illustrating features useable in embodiments.

Turning now to the figures, <FIG> illustrates a phase change material switch device (PCM switch device) <NUM>. PCM switch device <NUM> includes a phase change material <NUM>, electrically contacted via terminals 12A, 12B, for example electrodes 12A, 12B. A heater device <NUM> is placed adjacent to phase change material <NUM>, electrically isolated but thermally coupled to phase change material <NUM>. By heating phase change material <NUM> using heater device <NUM>, as in conventional PCM switch devices, phase change material <NUM> may be selectively set to a crystalline, electrically conducting state or to an amorphous, electrically isolating state. It should be noted that in the amorphous state phase change material <NUM> need not become fully amorphous, but some crystalline portions may remain for example in the vicinity of terminals 12A, 12B, as long as the phase change material in the amorphous state provides an electrical isolation between terminals 12A and 12B. Phase change material <NUM> may be any suitable phase change material described above. Therefore, in the crystalline state phase change material <NUM> provides an electrical connection between terminals 12A and 12B, and in the amorphous state, phase change material <NUM> provides an electrical isolation between terminals 12A and 12B, corresponding to a switch function. In some embodiments, terminals 12A, 12B may be radio frequency (RF) terminals, such that the PCM switch device <NUM> serves to switch RF signals.

Heater device <NUM> is supplied with electrical power by a power source <NUM>. When power source <NUM> supplies power to heater device <NUM>, heater device <NUM> heats up and therefore heats phase change material <NUM>. Power source <NUM> may be a pulse power source and may include further components for regulation of the supplied current or voltage. Heater device <NUM> may include one or more individual heaters.

Power source <NUM> is coupled to terminals 16A, 16B of heater device <NUM> by a coupling device <NUM>.

Coupling device <NUM> is configured to selectively provide a first electrical impedance or a second electrical impedance between power source <NUM> and terminals 16A, 16B of heater device <NUM>. The second electrical impedance is higher than the first electrical impedance, for example at least 100times higher. A magnitude of the second electrical impedance for example may be <NUM> kQ or higher, whereas a magnitude of the first electrical impedance may be <NUM>Ω or less. In some embodiments, the second electrical impedance may effectively decouple power source <NUM> from heater device <NUM>. The terms "first electrical impedance" and "second electrical impedance" are to be understood as not necessarily being fixed values, but may also be values in different ranges, as long as the second electrical impedance is higher than the first electrical impedance.

Coupling device <NUM> is configured to provide the first electrical impedance when electrical power is to be supplied from power source <NUM> to heater device <NUM>, i.e. during heating phases of heater device <NUM>. Coupling device <NUM> is further configured to provide the second electrical impedance outside heating phases, i.e. when no electrical power is to be supplied from power source <NUM> to heater device <NUM>. The first electrical impedance and/or the second electrical impedance may include ohmic and/or reactive components.

As will be explained below in more detail, in some embodiments, coupling device <NUM> may be implemented using p-in diodes. In other embodiments, coupling device <NUM> may be implemented using transistor switches.

Providing the second, higher impedance between power source <NUM> and heater device <NUM> outside heating phases, in particular when the PCM switch device <NUM> is used for selective signal transmission, for example of RF signals, in some implementations, may improve the RF voltage handling capabilities of the PCM switch device, may improve the off-state capacitance and resistance of the PCM switch device, may improve the linearity of the PCM switch device and/or may improve insertion losses when the PCM switch device is switched on. In other embodiments, other effects may result.

<FIG> is a flowchart illustrating a method. The method of <FIG> may be implemented using the PCM switch device <NUM> of <FIG> or any of the PCM switch devices discussed further below, and corresponds to a claimed embodiment when implemented using the PCM switch device of <FIG>. In order to avoid repetitions, reference will be made to the above explanations for <FIG> when explaining the method of <FIG>.

At <NUM>, the method comprises providing a first electrical impedance between a power source and a heater device of a PCM switch device during heating phases, i.e. when power is supplied to the heating device for heating a phase change material of the PCM switch device to change the state thereof, as explained above.

At <NUM>, the method comprises providing a second, higher, impedance outside the heating phases as explained above, for example to effectively decouple the power source from the heater device.

It should be noted that the providing of the first and second electrical impedances at <NUM> and <NUM> need not be performed in the order shown and/or may be performed repetitively. For example, the providing of the first electrical impedance at <NUM> may be performed every time when the phase change material is to be heated, and the providing of the second impedance may be generally performed outside heating phases, i.e. every time when no power is supplied to the heater device from the power source.

Next, various example implementations of PCM switch devices and in particular of coupling device <NUM> as discussed above will be discussed. First, with reference to <FIG>, including subfigures 3A to 3C, an example configuration of a structure including a phase change material and a heater device will be discussed. While this configuration is taken as an example, as already mentioned above, other configurations may also be used. For example, while in the configuration shown a heater is arranged below a phase change material, in other embodiments, a heater may be provided above a phase change material, or around a phase change material.

In the example of <FIG>, the PCM switch device is provided as a layered structure, for example by depositing and structuring corresponding layers on a substrate like a semiconductor substrate, for example using processes from semiconductor processes.

<FIG> shows a top view, and <FIG> shows a sectional view of the PCM switch device.

The PCM switch device of <FIG> includes a heater device <NUM> including a single heater and a phase change material <NUM>. Heater device <NUM> and phase change material <NUM> are separated by a dielectric layer <NUM>, which is electrically insulating, but couples heater <NUM> thermally to phase change material <NUM>. The dielectric material of layer <NUM> may for example be an oxide or nitride layer, like silicon dioxide or silicon nitride. Terminals 12A and 12B are provided as electrodes on phase change material <NUM>.

Heater <NUM> heats phase change material <NUM> at least in a middle portion thereof. In some embodiments, portions of phase change material <NUM> contacting terminals 12A, 12B may always remain in a crystalline state with low electrical resistance, whereas the middle portion changes between the amorphous state and the crystalline state based on the heating to switch the switch device on and off. The structure shown in <FIG> may for example be surrounded by a dielectric material, which may be the same material or a different material from the material of layer <NUM>. On heater <NUM>, heater terminals 16A, 16B are provided, via which heater <NUM> may be supplied with electrical power for heating.

<FIG> shows a symbol <NUM> which will be used to represent the arrangement of heater <NUM> and phase change material <NUM>, with terminals 12A, 12B, 16A and 16B, in the following figures. This arrangement will also referred to as PCM switch <NUM> in the following. In embodiments, a PCM switch device includes a PCM switch <NUM>, a power source and a coupling device. <FIG> illustrate the effect of coupling device <NUM> of <FIG> and the method of <FIG>. <FIG> illustrates a state where the first impedance is provided, which may correspond to a low-ohmic electric coupling of power source <NUM> to terminals 16A, 16B of the heater device. <FIG> illustrates a state where the second, higher, impedance is provided, which in some embodiments, as shown in <FIG>, may essentially correspond to a decoupling of power source <NUM> from heater terminals 16A, 16B.

<FIG> illustrates further a PCM switch device. In <FIG>, the coupling device includes a first switch device 50A and a second switch device 50B. Each of switch devices 50A, 50B is configured to selectively couple power source <NUM> to heater terminals 16A, 16B, respectively. In other embodiments, other devices than switch devices may be provided to implement a coupling device, as long as the function of providing the first and second impedance is fulfilled. For example, as will be explained below, instead of switch devices diodes may be used, or other coupling networks may be provided.

Switch devices may for example be implemented using transistor switches. For providing the first impedance, the switch devices are closed, and for providing the second impedance, the switch devices are opened.

<FIG> illustrates a further PCM switch device. Here, the coupling device is provided by p-i-n diode 60A, 60B coupled as shown between power source <NUM> and heater terminals 16A, 16B, respectively. The polarity of diodes 60A, 60B is selected such that when power is to be supplied to the heater, the diodes are forward biased, such that in the example of <FIG>, current flows from power source <NUM> through diode 60B to heater terminal 16B and back via heater terminal 16A in diode 60A to power source <NUM>. Diodes 60A, 60B in embodiments may be p-i-n diodes, i.e. with an intrinsic region between a p-doped region and an n-doped region of the diode. Particular implementation examples of such p-i-n diodes will be explained further below with respect to <FIG>. Due to the intrinsic region, when no power is supplied by power source <NUM>, diodes 60A, 60B are in an off-state exhibiting a high impedance (essentially a capacitance, which is lower than without the intrinsic region, corresponding to a higher impedance than without the intrinsic region) , thus providing the second impedance. In some implementations, the use of p-i-n diodes may provide high RF voltage handling capabilities in an off-state, for example <NUM> V or higher, may provide low non-linear distortion in the off-state also at high RF voltage excitations, i.e. when high RF signals are conducted by the PCM switch device, and may provide a low load capacitance at the heater terminals, which may lead to an advantageous figure of merit as the product of the on-resistance Ron and the off-capacitance Coff.

The RF voltage handling capability of the diodes may be further improved in some embodiments and the off-state capacity may be reduced by applying a reverse bias voltage from a DC terminal (labelled "DC bias in OFF state" in <FIG>) via a high ohmic resistor <NUM>, which may have a resistance in the range of <NUM> kΩ to <NUM> kΩ, as shown in <FIG>. The DC bias voltage supplied in embodiments may be in a range of +/-3V, but may be higher in other embodiments, for example up to +/- 40V or even higher.

In claimed embodiments, additionally impedance networks may be provided between one or both of the terminals 12A, 12B and the heater terminals 16A, 16B. A corresponding device is shown in <FIG>, which is based on of <FIG>. In addition to the components already described with respect to <FIG>, the PCM switch device of <FIG> includes a first impedance network 80A coupled between terminal 12A and heater terminal 16A, and a second impedance network 80B coupled between terminal 12B and heater terminal 16B. Via the impedance networks 80A, 80B, potentials (voltage levels) at terminals 16A, 16B, may be brought closer to the potentials at terminals 12A, 12B outside heating phases, i.e. when the switch is used to conduct (on-state) or block (off-state) signals. In some embodiments, additionally or alternatively impedance networks 80A, 80B may linearly distribute a voltage at terminal 12A over the heater device. In this way, a voltage handling of the PCM switch device may be improved in some embodiments by reducing the voltage drop between the heater device and the phase change material in the PCM switch device.

As illustrated in <FIG>, impedance networks 80A, 80B may include a capacitive coupling element <NUM>, for example a capacitor, a resistive coupling element <NUM>, for example a resistor, or a combination thereof. Generally, impedance networks 80A, 80B may comprise linear circuit elements or a combination thereof.

In case of capacitive element <NUM>, when the capacitance value equals to the off-state capacitance of the respective coupling device, in case of <FIG>, diode 60A, and one terminal of the switch device (for example terminal 12B) is grounded, then the voltage magnitude at the heater terminal will be half of the applied radio frequency voltage at terminal 12A. Such a heater excitation may improve the voltage handling capability of the PCM switch device. Therefore, in embodiments, the capacitor of the capacitive element <NUM> may be equal to the capacitance of the coupling device, for example diode 60A, 60B, where outside the heating phases, i.e. when the second impedance value is provided.

Similar considerations apply in case of a resistive element <NUM>, when the magnitude of the resistance value equals the magnitude of the impedance based on the off-capacitance of the diode 60A, 60B, or in other words, the impedance magnitudes are equal. Also in this case, the voltage magnitude at heater terminal 16A may be approximately half of the applied radio frequency voltage at terminal 12A, when for example terminal 12B is grounded. Also this may improve voltage handling capabilities of the PCM switch device.

<FIG>, including sub-<FIG>, shows various implementation possibilities for p-i-n diodes usable as diodes 60A, 60B in <FIG>. Other possibilities for implementing the diodes may also be used. <FIG> show cross-sectional views of diode structures.

In <FIG>, the p-i-n diode is implemented as a polysilicon diode including a highly p-doped portion <NUM>, an intrinsic portion <NUM> and a highly n-doped portion <NUM>. Intrinsic portion, in this respect, may indicate a nominally undoped portion, or a portion with very low defined dopant concentration. The polysilicon diode is implemented on a shallow trench isolation (STI) <NUM> on a substrate <NUM>. STI <NUM> electrically isolates the p-i-n diode from the substrate.

<FIG> shows an implementation of a p-i-n diode including highly p-doped region <NUM>, intrinsic region <NUM> and highly n-doped region <NUM> on a silicon on insulator (SoI) substrate with a buried oxide (BOX) <NUM> on substrate <NUM>. In this case, the diode may be a polysilicon diode or may be a single crystal silicon diode. BOX <NUM> electrically isolates the p-i-n diode from the substrate.

<FIG> show so-called lateral implementations of the diode. <FIG> show vertical implementations. In <FIG>, an intrinsic region <NUM> is provided in a highly p-doped substrate <NUM>. An n-doped region <NUM> is formed in the intrinsic region <NUM>, thus forming a p-i-n diode.

In <FIG>, the implementation is in a so-called triple well structure. Again, a p-doped substrate <NUM> is used. As shown in <FIG>, in p-type substrate <NUM>, an n+ well <NUM> is formed, in which an intrinsic region <NUM> is provided, in which in turn a p-doped region <NUM> is provided. Regions <NUM>, <NUM> in this case serve as n- and p-doped regions of the p-i-n diode, respectively. The triple well structure also provides an electrical isolation from the substrate <NUM>. Contacting of the diodes may be performed in any conventional way known in semiconductor manufacturing, for example by depositing metal electrodes on the respective p- and n-doped portion of the diodes.

Instead of diodes, as already mentioned above, also switch devices (see switch devices 50A, 50B of <FIG>) like transistor switches may be used. <FIG> shows a corresponding example with an implementation example of switch devices.

In <FIG>, power source <NUM> is coupled to terminal 16A via N stacked MOSFETs 1000_1 to 1000_N, and to heater terminal 16B via N stacked MOSFETs <NUM><NUM> to <NUM> N. The number of MOSFETs used may depend on the voltages present. Generally, a higher number N leads to a higher voltage stability in an off-state. In some embodiments, a single MOSFET may be sufficient (N=<NUM>) whereas in other embodiments more than one MOSFET may be required.

MOSFETs 1000_1 to 1000_N are biased with a bias voltage Vbias via a resistive ladder 1002_1 to 1002_N as shown, and MOSFETs <NUM><NUM> to <NUM> N are biased via a resistive ladder <NUM><NUM> to 1003_N with voltage Vbias. The voltage Vbias may be changed to switch the MOSFETs on and off, to provide the first and second impedances, respectively. Although not shown in <FIG>, similar to <FIG>, impedance networks 80A, 80B may be provided, which may be matched to an off-capacitance on MOSFETs 1000_1 to 1000_N, 1001_1 to 1001_N when switched off, as explained with respect to <FIG> for the off-capacitance of diode 60A, 60B.

In devices described previously, a heater device including a single heater is provided heating phase change material <NUM>. In claimed embodiments, a plurality of heaters are provided, heating different portions of a phase change material <NUM> or phase change materials <NUM> coupled in series. Corresponding devices will be described next.

<FIG> shows a device where a heater device includes three heaters 13A, 13B, 13B provided to heat different portions of phase change material <NUM> in a phase change switch <NUM>. The number of three heaters 13A to 13C is merely an example, as indicated by dots, and other numbers of heaters, for example two heaters or more than three heaters, may also be provided.

In <FIG>, between portions heated by heaters 13A, 13B, 13C, phase change material <NUM> always remains crystalline, thus providing an electrical connection between the portions heated by heater. In other embodiments, phase change material <NUM> may be discontinuous, and for example the discontinuous portions of phase change material <NUM> may be connected by metal connections or by other electrically conducting materials. In both cases, effectively the arrangement corresponds to a series connection of a plurality of phase change switches, which may increase a voltage tolerance of the PCM switch device.

In some embodiments, separate power sources may be provided to the individual heaters, each connected to the respective heater as discussed above, i.e. via a coupling device which may be implemented using diodes, switches, stacked MOSFETs or the like. In other embodiments, for example in <FIG>, a single power source <NUM> is provided. Power source <NUM> is then connected to heaters 13A to 13C via a coupling device, which in case of <FIG> includes individual switch devices 50A to 50F.

For each heater individually, in this case during heating phases, a first impedance as discussed above may be provided, and outside heating phases a second, higher, impedance is provided, as explained above.

Also in the case of a plurality of heaters, impedance networks are provided. <FIG> illustrates a corresponding embodiment, where impedance networks 80A to 80F couple terminal 12A to the terminals of heaters 13A to 13V as shown. Impedance networks 80A to 80F may be implemented as explained previously with respect to <FIG>.

As can be seen, in <FIG> one impedance network is coupled to first terminal 12A, while another impedance network is coupled to terminal 12B, while in <FIG> all impedance networks are coupled to terminal 12A. For all embodiments including further impedance networks discussed herein, both alternatives are possible, as long as a first and a second impedance element are coupled between the first device terminal and a first and a second terminal of at least one heater. In yet other examples, one or more impedance networks may only be coupled between first terminal 12A and one of the heater terminals, not both.

The alternative used may depend on the implementation of the heater and use of the PCM switch device.

For example, <FIG> shows an implementation of bootstrapping the heater, which also works bidirectionally. In implementations where one side, for example first terminal 12A can be identified as 'hot' (e.g. have an RF signal applied to it) while the other side, e.g. terminal 12B, is grounded, network 80B in <FIG> can be omitted to minimize the penalty on switch off-capacitance Coff and the voltage on the heater is given by the voltage division between impedance network 80A and diodes 60A/B.

<FIG> is such an embodiment where second terminal 12B is typically connected to ground, so no impedance networks need to be connected to second terminal 12B.

In yet other examples, if the heater resistance is low ohmic (as in the case of tungsten-based heater) only one of the two heater terminals (e.g. one of heater terminals 16A, 16B) needs to be connected to first terminal 12A.

In <FIG> and <FIG>, switch devices 50A to 50F are coupled in parallel. In other devices, series connections may be used. <FIG> shows such device, where switch devices 50A, 50C and 50E are coupled in series, and switch devices 50B, 50D and 50F are coupled in series. Heaters 13A to 13C are coupled to the series connection as shown in <FIG>, to nodes between switch devices or, in case of heater 13A, to switch devices 50E and 50F. Instead of switch devices, other coupling devices like the diodes previously discussed may be used.

Also with such a series connection of switches, impedance networks may be used. <FIG> illustrates a corresponding device, where impedance networks 80A, 80B are coupled between terminal 12A and nodes between switch devices 50E, 50F, respectively, and heater 13A as shown. Impedance networks 80A, 80B may be dimensioned and implemented as explained with reference to <FIG>, matching the off-capacitance of switch devices 50A to 50F when they are at an off-state.

PCM switch devices as mentioned above may be used in a variety of applications, including antenna-tuning applications. <FIG> illustrates an example antenna-tuning application using switch devices as discussed above.

Claim 1:
A phase change material switch device (<NUM>), comprising:
a phase change material (<NUM>),
a heater device (<NUM>; 13A-13C) thermally coupled to the phase change material (<NUM>), wherein the heater device (<NUM>; 13A-13C) comprises N heaters, wherein N is greater than <NUM>,
a power source (<NUM>; <NUM>),
a coupling device (<NUM>) electrically coupled between the power source (<NUM>; <NUM>) and the heater device (<NUM>; 13A-13C),
a first device terminal (12A) coupled to the phase change material (<NUM>),
a second device terminal (12B) coupled to the phase change material (<NUM>), and
an impedance device (80A-80F) coupled between the first device terminal (12A) and the heater device (<NUM>; 13A-13C),
wherein the coupling device (<NUM>) is configured to:
provide a first electrical impedance between the power source (<NUM>) and the heater device (<NUM>; 13A-13C) in a first state where current supplied to the heater device (<NUM>; 13A-13C) from the power source (<NUM>; <NUM>) for heating the phase change material (<NUM>), and
provide a second electrical impedance between the power source (<NUM>; <NUM>) and the heater device (<NUM>; 13A-13C) higher than the first electrical impedance in a second state outside heating phases of the heater device (<NUM>; 13A-13C),
characterized in that
for at least one of the N heaters the impedance device (80A-80F) comprises a first impedance element (80A, 80C, 80E) coupled between the first device terminal (12A) and a first terminal of the respective heater, and a second impedance element (80B, 80D, 80F) coupled between the first device terminal (12A) and a second terminal of the respective heater.