Patent Description:
<CIT> describes an RF in-line phase change switch with RF terminals in a planar configuration with heater terminals. The heater is adjacent and under the phase change material (PCM) and can switch the conductivity of the PCM via heating pulses. The PCM bridges the RF terminals over the heater. The RF and heater terminals are formed in one lithography step.

<CIT> describes a phase change switch device with a bridging PCM material. The lithography process of patterning the device architectures utilizes an etch stop layer.

<CIT> describes a phase change memory device with a PCM material. The electrodes are trenched in the substrate. The processing of a deposit or bridge filament on top of these trenches utilizes chemical mechanical polishing.

A method of forming a phase change switching device as defined in claim <NUM> is disclosed. The dependent claims define further embodiments.

The method of the present invention comprises providing a substrate, forming first and second RF terminals on the substrate, forming a strip of phase change material on the substrate that is connected between the first and second RF terminals, forming a heating element adjacent to the strip of phase change material such that the heating element is configured to control a conductive state of the strip of phase change material, wherein the first and second RF terminals and the heating element are formed by a lithography process that self-aligns the heating element with the first and second RF terminals.

The method further comprises forming a region of electrically insulating material on the substrate, and forming a first trench and a second trench in the region of electrically insulating material by the lithography process, wherein the first and second RF terminals are formed in the first and second trenches, respectively.

The method further comprises forming a third trench in the region of electrically insulating material by the lithography process, and wherein the heating element is formed in the third trench.

Separately or in combination, the first, second and third trenches are each formed simultaneously by a single masked etching step. Separately or in combination, the first and second RF terminals are formed in the first and second trenches, respectively, before forming the third trench, and wherein forming the third trench comprises using the first and second RF terminals as an etch mask.

Separately or in combination, the heating element has a different metal composition as the first and second RF terminals.

Separately or in combination, the heating element is disposed below the strip of phase change material.

Separately or in combination, the heating element is disposed above the strip of phase change material.

According to another embodiment, the method comprises forming a region of electrically insulating material on the substrate, depositing a first metal layer on the region of electrically insulating material, structuring the first metal layer to form first, second and third laterally isolated sections of the first metal layer; and configuring the first, second and third laterally isolated sections of the first metal layer such that the first and second laterally isolated sections are first and second RF terminals of the phase change switching device, respectively, and such that the third laterally isolated section is a heating element of the phase change switching device.

Structuring the first metal layer comprises forming first, second and third trenches in the region of electrically insulating material, depositing the first metal layer on the region of electrically insulating material to fill the first, second and third trenches; and planarizing an upper surface of the first metal layer so as to form the first, second and third laterally isolated sections of the first metal layer, wherein the first, second and third trenches are formed by a lithography process that self-aligns the third trench with the first and second trenches.

Separately or in combination, forming first, second and third trenches comprises performing a masked etching process that forms the first, second and third trenches simultaneously.

Separately or in combination, the method further comprises providing an etch stop layer within the region of electrically insulating material, and wherein the masked etching process is performed by etching the region of electrically insulating material until each of the first, second and third trenches reach the etch stop layer.

Separately or in combination, the first metal layer comprises any one or more of: tungsten, tantalum, titanium, and platinum. Separately or in combination, the method further comprises forming a strip of phase change material on the substrate, wherein the heating element is formed adjacent to the strip of phase change material such that the heating element is configured to control a conductive state of the strip of phase change material.

According to another embodiment, which is outside the scope of the present invention, the method comprises providing a substrate, forming a region of electrically insulating material on the substrate, depositing a first metal layer on the of electrically insulating material, structuring the first metal layer to form first and second laterally isolated sections of the first metal layer, forming a central trench in the region of electrically insulating material in between the first and second laterally isolated sections of the first metal layer, forming a second metal region in central first trench, configuring the first and second isolated sections of the first metal layer to be first and second RF terminals of the phase change switching device, respectively, and configuring the second metal region to be a heating element of the phase change switching device that is configured to control a conductive connection between the first and second RF terminals, wherein the central trench is formed by a lithography process that self-aligns the central trench with the first and second laterally isolated sections of the first metal layer.

Separately or in combination to the method, which is outside the scope of the present invention, the lithography process comprises forming a hardmask layer over the first and second laterally isolated sections of the first metal layer, forming an opening in the hardmask layer that exposes inner ends of the first and second laterally isolated sections of the first metal layer; and etching the region of electrically insulating material through the opening to form the first trench.

Separately or in combination to the method, which is outside the scope of the present invention, the method further comprises depositing a dielectric layer after forming the central trench so as to cover the inner ends of the first and second laterally isolated sections with the dielectric layer, depositing a second metal layer in the central trench over the dielectric layer; and planarizing an upper surface of the second metal layer so as to remove sections of the second metal layer that are outside of the central trench, and wherein the second metal region is formed by the second metal layer.

Separately or in combination to the method, which is outside the scope of the present invention, structuring the first metal layer to form first and second laterally isolated sections of the first metal layer comprises forming first and second trenches in the region of electrically insulating material, depositing the first metal layer to fill the first and second trenches, and planarizing an upper surface of the first metal layer so as to form the first laterally isolated section of the first metal layer in the first trench, and form the second laterally isolated section of the first metal layer in the second trench.

Separately or in combination, the method further comprises forming a strip of phase change material on the substrate, wherein the heating element is formed adjacent to the strip of phase change material such that the heating element is configured to control a conductive state of the strip of phase change material.

The features of the various illustrated embodiments can be combined unless they exclude each other. Embodiments are depicted in the drawings and are detailed in the description which follows.

Embodiments of a PCM (phase change material) switching device and corresponding methods of forming the phase change switching device are described herein. The PCM switching device comprises a strip of phase change material connected between first and second RF terminals, and a heating element disposed adjacent to the strip of phase change material. The heating element is configured to control a conductive connection between the first and second RF terminals by heating the strip of phase change material. Advantageously, the heating element is self-aligned to the first and second RF terminals. As a result, the device has less variation in performance parameters attributable to process variation, e.g., capacitance between the heating element and the RF terminals, thermal resistance between the heating element and the surrounding regions of the PCM switching device. Moreover, the self-aligned techniques for forming the heating element and the RF terminals advantageously eliminate costly lithography steps.

Referring to <FIG>, a PCM switching device <NUM> is depicted, according to an embodiment. The PCM switching device <NUM> includes a substrate <NUM>. Generally speaking, the substrate <NUM> may include any material that is compatible with semiconductor processing techniques, e.g., deposition, etching, etc. For example, the substrate <NUM> may include semiconductor materials such as silicon (Si), carbon, silicon carbide (SiC), silicon germanium (SiGe), etc. In another example, the substrate <NUM> includes non-semiconductor material, e.g., sapphire, glass, diamond, etc. In one particular embodiment, the substrate <NUM> is a commercially available bulk semiconductor wafer, e.g., a silicon wafer. In another example, the substrate <NUM> is a so-called SOI (Silicon on Insulator) substrate <NUM>, which includes a buried layer of insulating material. The substrate <NUM> includes a main surface, which may be a substantially planar surface.

The PCM switching device <NUM> includes a region of electrically insulating material <NUM> that is formed on the main surface of the substrate <NUM>. Generally speaking, the region of electrically insulating material <NUM> can comprise any electrically insulating material that can be formed through typical semiconductor processing techniques such as CVD (chemical vapor deposition). Examples of these electrically insulating materials include semiconductor oxides and nitrides, e.g., silicon nitride (SiN), silicon dioxide (SiO<NUM>), silicon oxynitride (SiOxNY), etc. In another embodiment, the region of electrically insulating material <NUM> comprises aluminum nitride (AlN). The region of electrically insulating material <NUM> may be thermally insulating or thermally conductive. The region of electrically insulating material <NUM> may include multiple layers of the same or different material.

The PCM switching device <NUM> further includes first and second RF terminals <NUM>, <NUM>. The first and second RF terminals <NUM>, <NUM> may be formed from an electrically conductive metal, e.g., copper, aluminum, alloys thereof, etc. As shown, the PCM switching device <NUM> may further comprise an upper level metallization <NUM> that is electrically connected to the first and second RF terminals <NUM>, <NUM> by vertical through-via structures <NUM>. The upper level metallization <NUM> can be a structured metallization e.g., copper, aluminum, alloys thereof, etc., and the vertical through-via structures <NUM> comprise a conductive metal such as tungsten, copper, nickel, aluminum, etc. The upper level metallization <NUM> may be connected to or form externally accessible terminals of the PCM switching device <NUM>.

The PCM switching device <NUM> further includes a strip of phase change material <NUM>. The strip of phase change material <NUM> may have an elongated geometry that extends lengthwise parallel to the main surface of the substrate <NUM>. The strip of phase change material <NUM> is formed from a material that can be transitioned between two different phases that each have different electrical conductivity. For example, strip of phase change material <NUM> may comprise a material that changes from an amorphous state to a crystalline state based upon the application of heat to the phase change material, wherein the phase change material is electrically insulating (i.e., blocks a conductive connection) in the amorphous state and is electrically conductive (i.e., provides a low-resistance current path) in the crystalline state. Generally speaking, phase change materials having this property include chalcogenides and chalcogenide alloys. Specifically, these phase change materials include germanium-antimony-tellurium (GST), germanium-tellurium, and germanium-antimony.

The strip of phase change material <NUM> is connected between the first and second RF terminals <NUM>, <NUM>. That is, the strip of phase change material <NUM> is in low-ohmic contact with both the first and second RF terminals <NUM>, <NUM>, either through direct physical contact or by one or more conductive intermediaries that provide a low-resistance electrical connection. In one example, a conductive material such as TiN, W, TiPtAu is provided between the first and second RF terminals <NUM>, <NUM> and the phase change material to improve the electrical connection between the two. When the strip of phase change material <NUM> is in a conductive state, current flows between the first and second RF terminals <NUM>, <NUM> in a current flow direction of the strip of phase change material <NUM>.

The PCM switching device <NUM> further includes at least one heating element <NUM>. The heating element <NUM> is arranged adjacent to the strip of phase change material <NUM>. In the depicted embodiment, the strip of phase change material <NUM> is disposed above each of the first and second RF terminals <NUM>, <NUM> and the heating element <NUM>. The heating element <NUM> is arranged and configured to apply heat to the strip of phase change material <NUM>. For example, the heating element <NUM> may comprise a conductive or semi-conductive material that converts electrical energy into heat through ohmic heating. The heating element <NUM> may be connected between electrically conductive heating terminals, which are not shown in the cross-sectional view of <FIG>. For example, the heating element <NUM> may extend transversely to the current flow direction of the phase change material <NUM> and contact heating terminals that are in locations that are spaced apart from the cross-sectional plane of <FIG>. The heating terminals are electrically conductive structures that can be biased to force a current through the heating element. The heating element <NUM> is separated from the strip of phase change material <NUM> by an insulating liner <NUM>. The insulating liner <NUM> electrically isolates the heating element <NUM> from the strip of phase change material <NUM> while simultaneously permitting substantial heat transfer between the two. To this end, the insulating liner <NUM> may be a relatively thin (e.g., less than <NUM> thick and more typically less than <NUM> thick) layer of dielectric material, e.g., silicon dioxide (SiO<NUM>), silicon nitride (SiN), etc..

The working principle of the PCM switching device <NUM> is as follows. The heating element <NUM> is configured to control a conductive connection between the first and second RF terminals <NUM>, <NUM> by applying heat to the strip of phase change material <NUM>. In an OFF state of the PCM switching device <NUM>, the phase change material of the strip of phase change material <NUM> is in an amorphous state or partially amorphous. As a result, the strip of phase change material <NUM> blocks a voltage applied to the first and second RF terminals <NUM>, <NUM>. In an ON state of the PCM switching device <NUM>, the phase change material of the strip of phase change material <NUM> is in a crystalline state. As a result, the strip of phase change material <NUM> provides a low-resistance electrical connection between the first and second RF terminals <NUM>, <NUM>. The PCM switching device <NUM> performs a switching operation by using the heating element <NUM> to heat the strip of phase change material <NUM>. The phase change material may be transitioned to the amorphous state by applying a short pulses (e.g., pulses in the range of <NUM> - <NUM>,<NUM> nanoseconds) of high intensity heat which causes the phase change material to reach a melting temperature, e.g., in the range of <NUM>° C to <NUM>° C, followed by a rapid cooling of the material.

This is referred to as a "reset pulse. " The phase change material may be transitioned to the crystalline state by applying longer duration pulses (e.g., pulses in the range of <NUM> - <NUM> microseconds) of lower intensity heat, which causes the phase change material to reach a temperature at which the material quickly crystallizes and is highly conductive, e.g., in the range of <NUM>° C to <NUM>° C. This is referred to as a "set pulse.

According to an embodiment, the first and second RF terminals <NUM>, <NUM> and the heating element <NUM> are formed by a lithography process that self-aligns the heating element <NUM> with the first and second RF terminals <NUM>, <NUM>. This means that one photomask and one lithography step form the first and second RF terminals <NUM>, <NUM> and the heating element <NUM>, either by directly forming these features or by forming features such as structured mask layers, trenches, etc. which in turn determine the geometry of the first and second RF terminals <NUM>, <NUM> and the heating element <NUM>. A lithography step utilizes a photomask has a pre-defined pattern that selectively blocks light to replicate the pre-defined pattern in a photosensitive material e.g., a photoresist layer, that is formed on a semiconductor substrate <NUM>. This pattern is used to create the first and second RF terminals <NUM>, <NUM> and the heating element <NUM> through a sequence of processing steps, e.g., etching, deposition, polishing, etc. In some of the processes described herein, the heating element <NUM> and the first and second RF terminals <NUM>, <NUM> are formed simultaneously with one etching step. In other processes described herein, the first and second RF terminals <NUM>, <NUM> are formed by an initial etching step, and the heating element <NUM> is subsequently formed by a second etching step that uses the first and second RF terminals <NUM>, <NUM> as an etch mask. In either case, the first and second RF terminals <NUM>, <NUM> and the heating element <NUM> are self-aligned because each feature owes its geometry to one lithography step.

The advantages of forming the first and second RF terminals <NUM>, <NUM> and the heating element <NUM> according to a self-aligned technique include the following. The location of the heating element <NUM> relative to the first and second RF terminals <NUM>, <NUM> may be well-controlled. For example, the heating element <NUM> may be centered between the first and second RF terminals <NUM>, <NUM> to a great degree of precision. Separately or in combination, the spacing between the heating element <NUM> and the first and second RF terminals <NUM>, <NUM> may be well-controlled to a great degree of precision. By contrast, in a device wherein the first and second RF terminals <NUM>, <NUM> and the heating element <NUM> are not self-aligned (i.e., having a geometry defined by two different lithography steps), the location of the heating element <NUM> relative to the first and second RF terminals <NUM>, <NUM> and/or the spacing between the heating element <NUM> and the first and second RF terminals <NUM>, <NUM> is not as well-controlled, due to the possibility of mask misalignment. Even minor misalignment can have significant impact in device performance by altering the capacitive coupling between the heating element <NUM> and the first and second RF terminals <NUM>, <NUM> and/or by altering the thermal resistance of the heating element <NUM> to the ambient environment. The self-aligned technique described herein substantially mitigates this issue by removing a potential source of unreliability in the manufacturing process. Moreover, the self-aligned technique advantageously eliminates costly lithography steps.

Referring to <FIG>, selected process steps for forming the PCM switching device <NUM> of <FIG> are shown.

As shown in <FIG>, a substrate <NUM> is provided and a region of electrically insulating material <NUM> is formed on the main surface of the substrate <NUM>. The region of electrically insulating material <NUM> can be formed by a deposition technique such as CVD (chemical vapor deposition) wherein one or more layers of electrically insulating material, e.g., silicon nitride (SiN), silicon dioxide (SiO<NUM>), silicon oxynitride (SiOxNY), etc., are formed on the substrate <NUM>. A first trench <NUM>, a second trench <NUM>, and a third trench <NUM> are formed in the region of electrically insulating material <NUM>. The first, second and third trenches <NUM>, <NUM>, <NUM> may be formed by a lithography process that self-aligns the third trench <NUM> with the first and second trenches <NUM>, <NUM>. For example, a layer of photoresist material (not shown) may be provided on the region of electrically insulating material <NUM> and lithographically patterned using a photomask (not shown) to form openings in the layer of photoresist material. The patterned photomask can be used directly as an etch mask to form the first, second and third trenches <NUM>, <NUM>, <NUM>. Alternatively, the patterned photomask can be used to form corresponding openings in a hardmask layer (not shown), which in turn is used to etch the first, second and third trenches <NUM>, <NUM>, <NUM>. In either case, an etching process, e.g., chemical etch, reactive ion etching, plasma etching, etc., can be performed to etch the region of electrically insulating material <NUM>. As a result, the first, second and third trenches <NUM>, <NUM>, <NUM> are formed to be self-aligned with one another.

According to an embodiment, an etch stop layer <NUM> is provided within the region of electrically insulating material <NUM>. The etch stop layer <NUM> is less selective to the etchant that is used to form the first, second and third trenches <NUM>, <NUM>, <NUM> than the superjacent region of the electrically insulating material <NUM>. For example, the etch stop layer <NUM> may include a nitride and/or a metal whereas the superjacent material includes an oxide. In this case, the masked etching process is performed by etching the region of electrically insulating material <NUM> until each of the first, second and third trenches <NUM>, <NUM>, <NUM> reach the etch stop layer <NUM>. In this way, the depth of the first, second and third trenches <NUM>, <NUM>, <NUM> and hence the thickness of the functional elements of the PCM switching device <NUM> is well-controlled.

As shown in <FIG>, a first metal layer <NUM> is deposited on the region of electrically insulating material <NUM>. The first metal layer <NUM> is conformably deposited so as to completely fill the first, second and third trenches <NUM>, <NUM>, <NUM>. That is, a thickness of the first metal layer <NUM> is at least equal to the depth of the first, second and third trenches <NUM>, <NUM>, <NUM>. Generally speaking, the first metal layer <NUM> can comprise any metal or metal alloy with sufficient material properties to perform the function of the heating element <NUM> as described above. Examples of these metals include tungsten, tantalum, titanium, platinum, and any alloy or combination thereof.

As shown in <FIG>, an upper surface of the first metal layer <NUM> is planarized. The planarization step can be performed using any technique that successively removes material from the upper surface of the first metal layer <NUM>, e.g., polishing such as CMP (chemical-mechanical polishing). The planarization step removes all portions of the first metal layer <NUM> outside of the first, second and third trenches <NUM>, <NUM>, <NUM>. As a result, first, second, and third sections <NUM>, <NUM>, <NUM> of the first metal layer <NUM> remain within the trenches. The first, second, and third sections <NUM>, <NUM>, <NUM> are laterally isolated from one another, meaning that there is no conductive path between each section.

Subsequent processing may be performed after the step illustrated in <FIG> to complete the PCM switching device <NUM>. The strip of phase change material <NUM> and the insulating liner <NUM> may be formed by blanket deposition and subsequent masked etching step, for example. A further layer or layers of the electrically insulating material may be formed on top of the functional elements of the PCM switching device <NUM> by a deposition technique such as CVD (chemical vapor deposition), for example. The upper level metallization <NUM> and the through via <NUM> may be formed by etching and deposition techniques, for example. In the completed device, the first and second laterally isolated sections <NUM>, <NUM> of the first metal layer <NUM> correspond to the first and second RF terminals <NUM>, <NUM> of the PCM switching device <NUM>, respectively, and the third laterally isolated section <NUM> of the first metal layer <NUM> corresponds to the heating element <NUM> of the PCM switching device <NUM>.

Instead of the process illustrated with respect to <FIG>, alternate metal structuring techniques may be used to create the first, second, and third laterally isolated sections <NUM>, <NUM>, <NUM> of the first metal layer <NUM>. For example, the first metal layer <NUM> may be deposited on a planar surface of electrically insulating material and subsequently structured using direct metal etching techniques, such as wet or dry etching techniques. In another example, the first, second, and third laterally isolated sections <NUM>, <NUM>, <NUM> of the first metal layer <NUM> may be formed by a lift-off technique. According to this technique, a structured layer of lift-off material is provided on a planar surface of the electrically insulating material. The first metal layer <NUM> is conformally deposited on the structured layer so as to fill the openings of the structured layer of lift-off material. The lift-off material is removed, e.g., by chemical dissolution such that the only portions of the first metal layer <NUM> disposed within openings remain. In each case, only one photomask is used to structure the first metal layer <NUM> and the first, second, and third laterally isolated sections <NUM>, <NUM>, <NUM> are self-aligned.

Referring to <FIG>, a PCM switching device <NUM> is depicted, according to an embodiment. The PCM switching device <NUM> may be substantially identical to the PCM switching device <NUM> described with reference to <FIG>, except that the strip of phase change material <NUM> is disposed below each of the first and second RF terminals <NUM>, <NUM> and the heating element <NUM>.

As shown in <FIG>, a substrate <NUM> is provided and a region of electrically insulating material <NUM> is formed on the main surface of the substrate <NUM>. Subsequently, the strip of phase change material <NUM> and the insulating liner <NUM> are formed. This may be done by depositing a blanket layer of phase change material on the region of electrically insulating material <NUM> and subsequently structuring this blanket layer in a similar manner as previously described. The insulating liner <NUM> may be formed as a blanket layer and structured at the same time as the phase change material. Alternatively, the insulating liner <NUM> may be formed by a separate sequence of deposition and etching.

As shown in <FIG>, electrically insulating material is further deposited to grow the region of electrically insulating material <NUM>. As a result, the strip of phase change material <NUM> and the insulating liner <NUM> are embedded within the region of electrically insulating material <NUM>. A planarization step, e.g., polishing such as CMP (chemical-mechanical polishing) may be performed after the deposition of the electrically insulating material so as to planarize the upper surface of the electrically insulating material <NUM>, thereby preparing this surface for the masked etching step described below.

As shown in <FIG>, first, second and third trenches <NUM>, <NUM>, <NUM> are formed in region of the electrically insulating material. The first, second and third trenches <NUM>, <NUM>, <NUM> may be formed by a self-aligned masked etching technique, e.g., in the same manner as described with reference to <FIG>. The third trench <NUM> is formed to expose the insulating liner <NUM> and the first and second trenches <NUM>, <NUM> is formed to expose the outer ends of the strip of phase change material <NUM>.

As shown in <FIG>, first, second, and third sections <NUM>, <NUM>, <NUM> of the first metal layer <NUM> are formed within the first, second and third trenches <NUM>, <NUM>, <NUM>, respectively. This may be done by depositing a first metal layer <NUM> and subsequently planarizing the first metal layer <NUM> in a similar manner as described with reference to <FIG>. As a result, the functional elements of the PCM switching device <NUM> are formed. Subsequent processing may be performed after the step illustrated in <FIG> to complete the PCM switching device <NUM> in a similar manner as previously described.

Referring to <FIG>, a PCM switching device <NUM> is depicted, according to an embodiment. The PCM switching device <NUM> differs from the previously described embodiments in the following way. In the previously described embodiments, each of the first and second RF terminals <NUM>, <NUM> and the heating element <NUM> are formed by the first metal layer <NUM>, and hence have the same metal composition. By contrast, in the PCM switching device <NUM> of <FIG>, the heating element <NUM> has a different metal composition as the first and second RF terminals <NUM>, <NUM>. For example, the first and second RF terminals <NUM>, <NUM> may be formed from a first metal or metal alloy with preferable electrically conductive characteristics, e.g., copper, aluminum, alloys thereof. The heating element <NUM> may be formed from a second metal or metal alloy that is different from the first metal or metal alloy and has preferable heating characteristics, e.g., tantalum, tungsten, nickel, etc. and alloys thereof. In this way, there is no tradeoff between the preferable characteristics for the heating element <NUM> and the preferable characteristics for the first and second RF terminals <NUM>, <NUM>. Additionally, the PCM switching device <NUM> comprises a dielectric layer <NUM> that separates the first and second RF terminals <NUM>, <NUM> and the heating element <NUM>. The thickness of the dielectric layer <NUM> can be well-controlled according to the deposition technique described below such that advantageous control over the lateral positioning of the heating element <NUM> is maintained.

As shown in <FIG>, a substrate <NUM> is provided and a region of electrically insulating material <NUM> is formed on the main surface of the substrate <NUM>. First and second laterally isolated sections <NUM>, <NUM> of a first metal layer <NUM> are formed in the region of electrically insulating material <NUM>, e.g., using the same technique described with reference to <FIG>. The first metal layer <NUM> used to form the first and second laterally isolated sections <NUM>, <NUM> may comprise a first metal or metal alloy with preferable conductive characteristics, e.g., copper, aluminum, alloys thereof.

As shown in <FIG>, a hardmask layer <NUM> is formed over the first and second laterally isolated sections of the first metal layer <NUM>. The thickness and material composition of the hardmask layer <NUM> are such that the hardmask layer <NUM> prevents the first and second laterally isolated sections <NUM>, <NUM> from being etched during the subsequent etching process to be described below. The hardmask layer <NUM> is structured, e.g., using a lithographic patterning technique, to form an opening that exposes inner ends of first and second laterally isolated sections <NUM>, <NUM>. Subsequently, an etching process (e.g., wet chemical etch, reactive ion etching, plasma etching, etc.) is performed to remove the electrically insulating material through the opening in the hardmask layer <NUM>. During this etching step, the first and second laterally isolated sections <NUM>, <NUM>, which correspond to the first and second RF terminals <NUM>, <NUM> of the PCM switching device <NUM>, are used as an etch mask to form a central trench <NUM> (shown in <FIG>) in between the first and second laterally isolated sections <NUM>, <NUM>. The central trench <NUM> is thus self-aligned to the first and second laterally isolated sections <NUM>, <NUM> of the first metal layer <NUM>, as the geometry of the central trench <NUM> is directly defined by the first and second RF terminals <NUM>, <NUM>, and the geometry of each structures is attributable to a single photomask.

As shown in <FIG>, a dielectric layer <NUM> is deposited within the central trench <NUM> such that the dielectric layer <NUM> covers the inner ends of the first and second laterally isolated sections <NUM>, <NUM>. The dielectric layer <NUM> may be a relatively thin (e.g., less than <NUM> thick and more typically less than <NUM> thick) layer of dielectric material, e.g., silicon dioxide (SiOz), silicon nitride (SiN), etc..

As shown in <FIG>, a second metal layer <NUM> is deposited in the central trench <NUM> over the dielectric layer <NUM>. The second metal layer <NUM> may be conformally deposited with sufficient thickness to completely fill the central trench <NUM>. The second metal layer <NUM> may comprise a second metal or metal alloy with preferable heating characteristics, e.g., tantalum, tungsten, nickel, etc. and alloys thereof.

As shown in <FIG>, an upper surface of the second metal layer <NUM> is planarized so as to remove sections of the second metal layer <NUM> that are outside of the central trench <NUM>. The planarization step can be performed using any technique that successively removes material from the upper surface of the second metal layer <NUM>, e.g., polishing such as CMP (chemical-mechanical polishing). As a result, a second metal region <NUM> is formed in the central trench <NUM>, wherein the second metal region <NUM> has a different material composition as the first and second laterally isolated sections <NUM>, <NUM>.

After performing the above-described steps, the second metal region <NUM> can be configured as the heating element <NUM> of the PCM switching device <NUM> described with reference to <FIG>, and the first and second laterally isolated sections <NUM>, <NUM> of the first metal layer <NUM> can be configured as the first and second RF terminals <NUM>, <NUM> of the PCM switching device <NUM> according to previously describe techniques. For example, the insulating liner <NUM> and the strip of phase change material <NUM> may be formed on top of the second metal region <NUM> by masked etching techniques. As illustrated in <FIG>, a via <NUM> may be formed that extends through the dielectric layer <NUM> so as to complete the electrical connection between the strip of phase change material <NUM> and the first and second RF terminals <NUM>, <NUM>.

Referring to <FIG>, a PCM switching device <NUM> is depicted, according to an embodiment. The PCM switching device <NUM> may be substantially identical to the PCM switching device <NUM> described with reference to <FIG>, except that the strip of phase change material <NUM> is disposed below each of the first and second RF terminals <NUM>, <NUM> and the heating element <NUM>. In this case, the dielectric layer <NUM> provides the electrical isolation between the strip of phase change material <NUM> and the heating element <NUM> in a similar manner as the previously described insulating liner <NUM>.

As shown in <FIG>, a substrate <NUM> is provided and a region of electrically insulating material <NUM> is formed on the main surface of the substrate <NUM>. A strip of phase change material <NUM> is formed to be embedded within the region of electrically insulating material <NUM>, e.g., in a similar manner as previously described with reference to <FIG>. A planarization step, e.g., polishing such as CMP (chemical-mechanical polishing) may be performed after the deposition of the electrically insulating material so as to form a planar upper surface in the region of electrically insulating material <NUM>, thereby preparing this surface for the masked etching step described below.

As shown <FIG>, first and second laterally isolated sections <NUM>, <NUM> of a first metal layer <NUM> are formed in the region of electrically insulating material <NUM>. The first and second laterally isolated sections <NUM>, <NUM> may be formed by forming first and second trenches <NUM>, <NUM> in the region of electrically insulating material <NUM>, depositing the first metal layer <NUM> to fill the first and second trenches <NUM>, <NUM>, and planarizing an upper surface of the first metal layer <NUM>, e.g., in a similar manner as previously described with reference to <FIG>.

As shown in <FIG>, a hardmask layer <NUM> is formed over the first and second laterally isolated sections <NUM>, <NUM> of the first metal layer <NUM>, an opening is formed in the hardmask layer <NUM> that exposes inner ends of the first and second laterally isolated sections <NUM>, <NUM> of the first metal layer <NUM>, and the region of electrically insulating material <NUM> is etched through the opening to form the central trench <NUM>, e.g., in a similar manner as previously described with reference to <FIG>.

As shown in <FIG>, a dielectric layer <NUM> is deposited within the central trench <NUM> such that the dielectric layer <NUM> covers the inner ends of the first and second laterally isolated sections <NUM>, <NUM>, e.g., in a similar manner as previously described with reference to <FIG>.

As shown in <FIG>, a second metal region <NUM> is formed in the central trench <NUM> over the dielectric layer <NUM>, e.g., in a similar manner as previously described with reference to <FIG>.

The methods and structures disclosed herein with reference to specific figures are equally applicable to all other embodiments to the extent consistent with these other embodiments. For instance, particular techniques, materials, steps and so-forth describing a method of forming a device represented by one figure may be applied to any other method represented by other figures, to the extent consistent with these other methods. Likewise, particular device features, structures or arrangements disclosed in connection with a device represented by one figure may be incorporated into a device represented any other figures, to the extent consistent with these other devices.

The term "electrically connected," "directly electrically connected" and the like as used herein describes a permanent low-impedance connection between electrically connected elements, for example a direct contact between the relevant elements or a low-impedance connection via a metal and/or a highly doped semiconductor.

As used herein, the terms "having," "containing," "including," "comprising" and the like are open-ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles "a," "an" and "the" are intended to include the plural as well as the singular, unless the context clearly indicates otherwise.

It is to be understood that the features of the various embodiments described herein may be combined with each other, unless specifically noted otherwise.

Claim 1:
A method of forming a phase change switching device (<NUM>), the method comprising:
providing a substrate (<NUM>);
forming first (<NUM>) and second (<NUM>) RF terminals on the substrate;
forming a strip of phase change material (<NUM>) on the substrate (<NUM>) that is connected between the first (<NUM>) and second (<NUM>) RF terminals;
forming a heating element (<NUM>) adjacent to the strip of phase change material (<NUM>) such that the heating element (<NUM>) is configured to control a conductive state of the strip of phase change material (<NUM>),
wherein the first (<NUM>) and second (<NUM>) RF terminals and the heating element (<NUM>) are formed by a lithography process that self-aligns the heating element (<NUM>) with the first (<NUM>) and second (<NUM>) RF terminals;
wherein the lithography process is characterized in that it comprises the following steps:
forming a region of electrically insulating material (<NUM>) on the substrate (<NUM>) ;
forming a first trench (<NUM>) and a second trench (<NUM>) in the region of electrically insulating material (<NUM>);
forming the first (<NUM>) and second (<NUM>) RF terminals in the first (<NUM>) and second (<NUM>) trenches, respectively;
forming a third trench (<NUM>) in the region of electrically insulating material (<NUM>); and
forming the heating element (<NUM>) in the third trench (<NUM>).