Patent Description:
The technical requirements for radio frequency applications (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 switching device including such a material may be switched on or 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 is also referred to as a set operation of the switching device. In this 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 of 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 region present in the PCM can regrow into the crystalline phase state.

When switching off the switching 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 (e.g. above about <NUM> to <NUM>), followed by rapid cooldown which freezes the phase change material into an amorphous state.

Heating the phase change material causes energy losses when also other adjacent components and structures are heated. Furthermore, for the rapid cooldown, a corresponding thermal coupling to a heat sink is required.

<CIT> discloses a memory cell based on a phase change material disposed in a trench. A heater is configured to heat the phase change material. A similar device is known from <CIT>. <CIT> discloses a PCM RF switch including a thermal buffer layer between a planar PCM or heater and a substrate. <CIT> shows a PCM RF switch comprising a U-shaped PCM surrounding a heater formed on a planar substrate.

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

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

According to another embodiment, a method for manufacturing a switch device is provided, comprising:.

The above summary is merely intended to give a brief overview over some aspects of some embodiments and is not to be construed as limiting in any way.

In the following, various embodiments and a comparative example 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 shapes and arrangements of components of a switch device like a phase change material or a heater are shown, in other embodiments other configurations may be used. For example, while in some embodiments a heater is provided above a phase change material, in other embodiments a heater may be provided below a phase change material.

Besides the 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 as used in conventional switch devices using phase change materials. For example, embodiments herein relate to switch devices where a phase change material is arranged at least partially within a trench formed in a substrate. Other components, like control circuitry for controlling a heater, radio frequency (RF) circuitry using the switch device and the like may be implemented in a conventional manner. Such additional components 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 the switch device in a common package.

Functionally similar components in various figures bear the same references numerals, with the first digit corresponding to the number of the figure. Just to give an example, a heater is designated with reference numeral <NUM> in <FIG>, with reference numeral <NUM> in <FIG>, with reference numeral <NUM> in <FIG> etc..

Therefore, these elements will not be described in detail repeatedly, but general explanations will be made when these elements occur for the first time (e.g. referring to <FIG>), and for following figures only differences to the previously described figures will be described in detail. For example, materials for different components like heaters and phase change materials will be explained with reference to <FIG> and apply likewise to corresponding components of other embodiments.

Turning now to the figures, <FIG>. is a cross-sectional view of a switch device according to an embodiment.

Switch device <NUM> of <FIG> comprises a crystalline semiconductor substrate <NUM>. Crystalline semiconductor substrate <NUM> as used herein means that the part of substrate <NUM> where the device shown is provided is made of a crystalline semiconductor material, for example silicon, gallium arsenide or silicon carbide. For example, crystalline semiconductor substrate <NUM> may be a semiconductor wafer like a silicon wafer. In other embodiments, semiconductor substrate <NUM> may include a crystalline semiconductor wafer with one or more crystalline semiconductor layers grown epitaxially thereon, for example a silicon semiconductor wafer with silicon layers epitaxially grown thereon (doped or undoped). In yet other embodiments, crystalline semiconductor substrate <NUM> may be an active layer of a semiconductor wafer having an insulation layer incorporated therein like an active wafer layer of a silicon-on-insulator (SOI) wafer. The active layer of such a wafer is the layer on which, for example, semiconductor devices are formed. Also such a substrate may further include crystalline epitaxial semiconductor layers formed on the layer. While a crystalline semiconductor substrate will be used as an example herein, in other embodiments other substrates, for example non-semiconductor crystalline substrates may be used, for example sapphire substrates. The term substrate, like crystalline (semiconductor) substrate, as used herein, does not include non-crystalline layers formed on a substrate like a semiconductor wafer, like dielectric layers, and also does not include metal layers. Such dielectric layers or metal layers may for example be formed in so-called back end of line (BEOL) processing. Crystalline semiconductor substrate <NUM> will be referred to simply as substrate in the following.

The same applies to substrates used in further embodiments described further below.

In crystalline semiconductor substrate <NUM>, a trench <NUM> is formed. A trench generally refers to a recess formed in the substrate. Such trenches may be formed by etching. In some embodiments, conventional techniques used for forming trenches in semiconductor substrates may be employed, for example techniques used for forming trenches used for shallow trench isolation (STI) or semiconductor devices built in trenches. In some implementations, the trench may have an elongated shape, which means that the trench is longer in a length direction than in a width direction, for example, more than two times longer, more than three times longer, more than <NUM> times longer or even more than <NUM> times longer.

Compared to trenches used in conventional semiconductor devices, a ratio of the width of the trench to the depth of the trench may be comparatively large, also referred to as shallow trench. For example, an aspect ratio of width : depth (assuming that the width is smaller than the length) may be between <NUM>:<NUM> and <NUM>:<NUM>, for example between <NUM>:<NUM> and <NUM>:<NUM>, for example about <NUM>:<NUM>. An aspect ratio of <NUM>:<NUM> may correspond to a cross-section in the width direction similar to a semicircle. Such shallow trenches in some implementations may lead to improved thermal properties compared to other trench geometries. However, in other implementations also deeper trenches with an aspect ratio width : depth smaller than <NUM>:<NUM> may be used. Such deeper trenches may reduce area requirements in some implementations. Generally, depth, length and width of the trench depend on the desired properties of the switch device. For example, for lower depth a larger width and/or length is needed to have a same area of phase change material (for example to have a same area than a specific conventional planar switch).

In trench <NUM>, various elements may then be formed layer by layer. In the embodiment of <FIG>, first, a thermal buffer layer <NUM> is provided, which may, for example, be made of silicon oxide or other dielectrical materials. Following the thermal buffer layer, a phase change material (PCM) <NUM> is provided. An example for a usable phase change material is germanium telluride.

As can be seen in <FIG>, the phase change material <NUM> is at least partially provided within trench <NUM>. In some embodiments, this may enhance cooling of PCM <NUM> in a reset operation due to enhanced thermal coupling to substrate <NUM>, which serves as a heat sink.

In some embodiments, the shape of a side of PCM <NUM> facing trench <NUM> in at least one cross-section (for example the one shown in <FIG>) matches or follows the shape of trench <NUM>, such that PCM <NUM> is provided along the walls and bottom of trench <NUM>, and separated from trench <NUM> by thermal buffer layer <NUM>. This may help to achieve a desired cooling rate, which in some embodiments may be larger than <NUM><NUM> K/s. As can be seen in <FIG>, a side of PCM <NUM> facing trench <NUM> has a curved portion, in this case at a bottom of trench <NUM>, in at least one cross-section. This curved portion may further serve to enhance cooling. Yet, in other embodiments other shapes are possible, for example angular shapes.

Furthermore, a heater <NUM> is provided separated from PCM <NUM> by a barrier layer <NUM>. Barrier layer <NUM> may, for example, be made of silicon nitride. Suitable heater materials for heater <NUM> include polycrystalline silicon or tungsten. Heater <NUM> in the embodiment of <FIG> is also partially provided within trench <NUM>. In the cross-section of <FIG>, furthermore PCM <NUM> partially surrounds heater <NUM> (on three sides in case of <FIG>). This may lead to an increased heat transfer from heater <NUM> to PCM <NUM> and may reduce heat dissipated to the environment, for example to a dielectric material <NUM> which in the example of <FIG> is used on top of substrate <NUM> as a filler material, for example for planarization purposes and/or as dielectric material in back end of line (BEOL) processes. Furthermore, the curved shape of PCM <NUM> already discussed above together with the heater <NUM> surrounded by PCM <NUM> leads to a greater outer surface of PCM <NUM> towards substrate <NUM>, serving as a heat sink, compared to a surface of heater <NUM>. Therefore, a thermal resistance of the heater may be higher than in a planar design, even when a corresponding thermal resistance of PCM layer <NUM> is the same. This will be explained in more detail further below. Additionally, in some embodiments with this design the volume of heater <NUM> may be reduced compared to conventional solutions.

PCM <NUM> is coupled to contact pads 14A, 14B, which, for easier distinguishing from contact pads coupled to heater <NUM> described later on, are also labeled RF1 and RF2. Contact pads 14A, 14B may be provided in a metal layer of a conventional back end of line (BEOL) processing used in semiconductor manufacture, and connected to PCM <NUM> via vertical interconnects (e.g. VIA, vertical interconnect axis) 18A, 18B, respectively. Contact pads 14A, 14B, interconnects 28A, 28B, and material <NUM> may be provided as in conventional BEOL processing in semiconductor technology. As briefly mentioned above, heater <NUM> may be also contacted to be able to supply electric power to heater <NUM>. In operation of the switch device, by operating heater <NUM>, PCM <NUM> may be brought to a crystalline phase state having a low-ohmic resistance or an amorphous phase state having a high-ohmic resistance, and therefore a low-ohmic connection between contact pads 14A, 14B may be selectively provided. This may, for example, be used as switch device for RF signals applied to contact pad 14A and/or 14B. It should be noted that contact pads 14A, 14B as well as contact pads for a heater (not described for <FIG>, but described for other embodiments) need not be pads for external contacting, but may also be part of metal leads or wires extending to other parts of a circuit.

For better understanding of switch device <NUM> of <FIG>, it will be compared to a reference switch device <NUM> of <FIG> using a conventional planar technology, such as known from aforementioned <CIT>. <FIG> shows a cross-sectional view of switch device <NUM> which corresponds to the cross-sectional view of <FIG>. In contrast to the embodiment of <FIG>, in a substrate <NUM> no trench is provided, but a PCM <NUM> is provided on substrate <NUM> separated by a thermal buffer layer <NUM> in a planar manner. A heater <NUM> is provided on PCM <NUM> separated by a barrier layer <NUM> also in a planar manner. PCM <NUM> is contacted via contact pads 24A, 24B and vertical interconnects 28A, 28B as shown. A dielectric material <NUM> is used for planarization and isolation.

As can be seen in <FIG>, when heating PCM <NUM>, also filler material <NUM> has to be heated. Furthermore continuous thermal losses occur due to thermal losses via filler material <NUM>.

Furthermore, cooling with substrate <NUM> as a heatsink may be comparatively slow. As mentioned initially, cooling with a high cooling rate is required to bring the PCM to an amorphous phase state.

In contrast thereto, in <FIG> the placement of PCM <NUM> in trench <NUM>, and the placement of heater <NUM> such that it is partially surrounded by PCM <NUM>, essentially makes thermal losses to filler material or BEOL layers negligible, or reduces them significantly. It is to be understood that some losses may remain, but these are significantly smaller than in <FIG>. This may reduce the power needed for heating PCM <NUM>.

Furthermore, by the placement of PCM <NUM> in trench <NUM>, thermal contact to the substrate <NUM> may be enhanced. This may contribute to a fast cooling of PCM <NUM> when resetting the switch device, i.e. putting PCM <NUM> to an amorphous phase state.

Various configurations for arranging a phase change material and a heater in a trench are possible. Some of these possible configurations will be explained now referring to <FIG>. Each of <FIG> includes subfigures A, B and C, i.e. 3A, 3B, 3C for <FIG>, <FIG>, <FIG> for <FIG> etc. Subfigures A show schematic top views on the substrate, and subfigures B and C show schematic cross-sectional views.

In <FIG>, <FIG> shows top view of a switch device according to an embodiment. <FIG> shows a schematic cross-sectional view along a dashed line <NUM>, which is in the length direction of a trench <NUM>. This length direction of the trench is also marked by an arrow <NUM>. <FIG> shows a schematic cross-sectional view along a dashed line <NUM> in <FIG>, which runs in a width direction of trench <NUM>. Generally, as explained previously, a length of the trench in the length direction may be larger than a width of the trench in the width direction.

In the embodiment of <FIG>, the cross-sectional view of <FIG> in this case essentially corresponds to switch device <NUM> discussed with reference to <FIG>.

The switch device of <FIG> includes a crystalline semiconductor substrate <NUM>, trench <NUM> formed in substrate <NUM>, a thermal buffer layer <NUM>, a phase change material <NUM>, a barrier layer <NUM>, and a heater <NUM>.

In the embodiment of <FIG>, the phase change material is provided in the width direction of trench <NUM>, and the heater is provided in the length direction of trench <NUM>. This means, as seen in <FIG> and <FIG>, that contact pads 54A, 54B for phase change material <NUM> are provided such that the signal flow through phase change material <NUM> is in the width direction. Contact pads 54A, 54B are provided in filler material <NUM> and coupled to phase change material <NUM> via vertical interconnects 58A, 58B.

For providing current to heater <NUM>, contact pads 510A, 510B also labelled H1 and H2 are provided, which are electrically coupled to heater <NUM> by vertical contacts 514A, 514B, which, as seen in <FIG>, extend into heater <NUM> to provide a more uniform current distribution. Current through heater 5A, when present, therefore flows essentially in the length direction of the trench.

<FIG> illustrates a further example configuration. <FIG> illustrates a top view of a switch device according to a further embodiment, <FIG> shows a schematic cross-sectional view along a dashed line <NUM>, which runs in a length direction of a trench <NUM> as indicated by an arrow <NUM>, and <FIG> shows a schematic cross-sectional view along a line <NUM> running in a width direction of trench <NUM>.

In the embodiment of <FIG>, a depth of the trench may be about half a length of the trench.

In the embodiment of <FIG>, phase change material <NUM> is arranged in the length direction, i.e. signal flow is in the length direction, and heater <NUM> is arranged in the width direction, i.e. current flow through heater <NUM>, is in the width direction.

As can be seen in <FIG> and <FIG>, contact pads 64A, 64B for contacting phase change material <NUM> are provided such that current flows in the length direction. Contact pads 64A, 64B are coupled to phase change material <NUM> by vertical interconnects 68A, 68B which extend into the trench for better current distribution. Contact pads 610A, 610B for providing current through heater <NUM> are arranged in the width direction, as seen in <FIG> and <FIG>.

As seen in <FIG>, filler material <NUM> extends into the trench to provide a U shape of heater <NUM> in the cross-sectional view of <FIG>, to ensure that current flows through the heater essentially "along" phase shape material <NUM>. Without this filler, material, i.e. if heater <NUM> had a filled-out U shape in <FIG>, current would mainly flow directly from contact pad 610A to contact pad 610B above substrate <NUM>, such that the lower part of heater <NUM> would hardly be heated.

<FIG> illustrates a switch device according to a further embodiment. <FIG> shows a top view, <FIG> shows a schematic cross-sectional view along a dashed line <NUM> running in a length direction of a trench <NUM> as indicated by an arrow <NUM>, and <FIG> shows a schematic cross-sectional view along a dashed line <NUM> running in a width direction of trench <NUM>.

In the embodiment of <FIG>, both phase change material <NUM> and heater <NUM> are provided in a length direction, i.e. both signal flow through phase change material <NUM> and current flow through heater <NUM> are in the length direction. As best seen in the cross-sectional view of <FIG>, contact pads 75A, 75B for contacting phase change material <NUM> and contact pads 710A, 710B for contacting heater <NUM> may be arranged in different metal layers. In other embodiments, an arrangement in a same metal layer may be used. As can be seen in the cross-sectional view of <FIG>, vertical interconnects 78A, 78B extend from contact pads 75A, 75B into phase change material <NUM>, and vertical interconnects 714A, 714B extend from contact pads 710A, 710B into the heater <NUM>, to ensure an appropriate current distribution.

<FIG> shows a switch device according to a further embodiment. <FIG> shows a top view, <FIG> shows s schematic cross-sectional view along a dashed line <NUM> running in a length direction of a trench <NUM> as indicated by an arrow <NUM>, and <FIG> shows a schematic cross-sectional view along a dashed line <NUM> running in a width direction of trench <NUM>.

In the embodiment of <FIG>, both phase change material <NUM> and heater <NUM> are arranged in the width direction, i.e. signal flow through phase change material <NUM> and current flow through heater <NUM> is in the width direction. As can be best seen in <FIG> and <FIG>, similar to the embodiment of <FIG>, contact pads 84A, 84B which contact phase change material <NUM>, via vertical interconnects 88A, 88B and contacts pads 810A, 810B contacting heater <NUM> via vertical interconnects 514A, 514B may be arranged in different metal layers. In other embodiments, they may be arranged in the same metal layer.

Furthermore, similar to <FIG>, filler material <NUM> extends into the trench to provide a U form of heater <NUM> in the cross-section of <FIG>, similar to what was explained with reference to <FIG>.

As can be seen from <FIG>, various configurations of providing phase change material and heater in the trench and contacting them are possible. Therefore, it is evident that this disclosure is not limited to any specific configuration.

<FIG> is a flowchart illustrating a method of manufacturing a switch device according to some embodiments. The method of <FIG> may be used to manufacture any of the switch devices discussed previously, and in order to avoid repetitions, will be described referring to the previous explanations. However, it is to be understood that the method of <FIG> may also be used to manufacture other switch devices than the ones explicitly described previously.

At <NUM>, the method comprises providing a crystalline semiconductor substrate, like substrates <NUM>, <NUM>, <NUM>, <NUM> or <NUM> described previously.

At <NUM>, the method comprises forming a trench in the substrate, for example by etching, like trench <NUM>, <NUM>, <NUM>, <NUM> or <NUM> described previously.

At <NUM>, the method comprises providing a phase change material in the trench, for example phase change material <NUM>, <NUM>, <NUM>, <NUM> or <NUM> described previously. The phase change material may be provided partially in the trench, as also explained. For example at least <NUM> %, at least <NUM> %, at least <NUM> % or at least <NUM> % of the phase change material <NUM> may be provided in the trench.

At <NUM>, the method comprises providing a heater, which also may be provided fully or partially within the trench, for example heater <NUM>, <NUM>, <NUM>, <NUM> or <NUM> described previously. Further process parts not explicitly shown and described in <FIG> may include contacting the phase change material or the heater, for example as shown and described with reference to <FIG>, providing a barrier material between phase change material and the heater (e.g. barrier layer <NUM>) between <NUM> and <NUM>, providing a thermal buffer between the phase change material and the substrate, or providing filler material, as also shown and described with reference to <FIG>.

<FIG> illustrates a method of operating a switch device, wherein the switch device is implemented using a trench in a crystalline semiconductor substrate as describe previously. Acts or events described with respect to <FIG> may be performed in different order and may be performed repeatedly to turn the switch on and off.

At <NUM>, the method comprises providing power to the heater to turn the switch on. In this case, the power is provided in a manner to perform the already described set operation, i.e. heat the phase change material to a temperature above the crystallization temperature and below the melt temperature.

At <NUM>, the method comprises providing power to the heater to turn the switch off, i.e. here the power is provided in a manner as described for the reset operation above, that the temperature of the phase change material is raised above the melt temperature (for example above about <NUM>) followed by a fast cooling (for example larger than <NUM><NUM>K/s) to turn the phase change material into an amorphous phase state.

Pulse length of power provided to the heater at <NUM> or <NUM> may be of the order of <NUM> or below.

Claim 1:
A switch device (<NUM>), comprising:
a substrate (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>),
a trench (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>) formed in the substrate (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>),
a phase change material (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>) having a U-shape in a cross-sectional view along a direction (<NUM>; <NUM>; <NUM>; <NUM>) and disposed at least partially in the trench (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>), and
a heater (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>) configured to heat the phase change material (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>), wherein the phase-change material surrounds the heater on at least two sides of the heater in the cross-sectional view along the direction.