Patent Description:
Some magnetic memory or logic devices employ a magnetic tunnel junction (MTJ) stack. The MTJ stack typically comprises a magnetic free layer and a magnetic reference or fixed layer, which sandwich a tunnel barrier. Depending on the magnetization of the magnetic free layer, with respect to the fixed magnetization of the magnetic reference layer, a tunnel magnetoresistance (TMR) between the magnetic free layer and the magnetic reference layer is larger or smaller. This can be used to distinguish between different (magnetic) states of the magnetic device, e.g. to store data.

However, such MTJ stack based magnetic devices - like magnetoresistive random access memory (MRAM) devices or spin transfer torque (STT) MRAM devices - require considerable current to switch the magnetization of the magnetic free layer. Thus, such MTJ stack based magnetic devices are not sufficiently energy-efficient.

Making use of a voltage to assist the switching of the magnetization may lower the required current. Indeed, some examples of magnetic devices use voltage-assisted switching in MTJ stacks. Those examples include, for instance, VCMA in strain-coupled ferromagnetic (FM) metal/ferroelectric bilayers, and voltage control of the exchange field in FM metal/multiferroic bilayers.

Further, most magnetic memory and logic devices employ magnetic anisotropy determining a preferential orientation of the magnetic moment. Decreasing or removing this magnetic anisotropy is considered a potential tool to manipulate the magnetic state in such magnetic devices. In VCMA, for instance, an electric field may be used to modulate the magnetic anisotropy. However, the VCMA effect is so far too limited to be efficiently employed in magnetic devices based on MTJ stack. Consequently, VCMA based magnetic devices are not yet feasible.

<CIT> discloses a magnetization control method involving controlling a magnetization direction of a magnetic layer, and including: forming a structure including (i) the magnetic layer which is an ultrathin film ferromagnetic layer, and (ii) an insulating layer provided on the ultrathin film ferromagnetic layer and working as a potential barrier. <CIT> discloses a magnetic memory device including a reference magnetic pattern having a magnetization direction fixed in one direction, a free magnetic pattern having a changeable magnetization direction, and a tunnel barrier pattern disposed between the free and reference magnetic patterns.

<NPL>, discloses large voltage-controlled magnetic anisotropy in a SrTiO3/Fe/Cu structure.

<NPL>, discloses ultra-low-power orbital-controlled magnetization switching using a ferromagnetic oxide interface.

In view of the above-mentioned challenges, a goal of the present disclosure is to improve the magnetic devices, and their production methods. An objective is in particular to provide a magnetic structure for a magnetic device, which allows obtaining a stronger VCMA effect. Specifically, a complete suppression of the magnetic anisotropy by a strong VCMA effect is desired. The magnetic structure should be suitable for designing an MTJ stack for a magnetic device. Another aim is to provide specific magnetic device designs, which employ a strong VCMA effect. Thereby, an overall goal of the disclosure is to enable ultra-low-power, energy-efficient magnetic memory or logic devices.

The objective is achieved by the embodiments of the invention provided in the enclosed independent claims. Advantageous implementations of the embodiments of the invention are further defined in the dependent claims.

According to a first aspect, the disclosure relates to a method for manufacturing a magnetic structure for a magnetic device, the method comprising: providing a bottom electrode layer, forming a SrTiO<NUM>, STO, stack on the bottom electrode layer by Atomic Layer Deposition (ALD) of at least two different STO nanolaminates, forming a magnetic layer on the STO stack, and forming a Perpendicular Magnetic Anisotropy (PMA) promoting layer on the magnetic layer, the PMA promoting layer being configured to promote PMA in the magnetic layer.

The magnetic structure, which is fabricated by the method of the first aspect, allows obtaining a significantly stronger VCMA effect (e.g., if a voltage is applied to the bottom electrode layer) than so far, in particular at room temperature. This is at least in part due to the specific fabrication technique of the STO stack, i.e., forming the two different STO nanolaminates by the ALD. In particular, a high permittivity of the STO stack is achieved in this way, which leads to the stronger VCMA effect, for instance, in the magnetic device. The VCMA effect may even be sufficiently strong to suppress altogether the PMA in the magnetic layer. The magnetic structure is suitable for building an MTJ stack in the magnetic device. Overall, the method of the first aspect is able to produce a magnetic structure suitable for fabricating an ultralow-power magnetic memory or logic device.

In an implementation form of the method according to the first aspect, the magnetic layer comprises a Co layer and/or the PMA promoting layer comprises a Pt layer or a MgO layer.

For instance, a Co/Pt layer combination (bilayer), or a ferromagnet/MgO bilayer - e.g. a Co/MgO bilayer, or a Fe/MgO bilayer, or a Ni/MgO bilayer, or a bilayer of a Co/Fe/Ni alloy and MgO - can optimize the PMA. The ferromagnet in the ferromagnet/MgO bilayer may also have a boron content. Further, the STO/Co interface may promote the PMA and also the VCMA effect. Further, the MgO layer may conveniently be used as a tunnel barrier in the magnetic device, particularly in an MTJ stack of the magnetic device.

In an implementation form of the method according to the first aspect, the STO stack comprises a first Sr-rich STO nanolaminate and a second Ti-rich STO nanolaminate.

This sequence of Sr-rich SrTiO3 and Ti-rich SrTiO3 in the STO stack leads to a significant enhancement of the permittivity of the STO stack, and thus can lead to a stronger VCMA effect.

In an implementation form of the method according to the first aspect, the first Sr-rich STO nanolaminate comprises a (Sr-rich STO/TiO<NUM>)×n nanolaminate, wherein n is in a range of <NUM>-<NUM>, and preferably n is <NUM>.

In an implementation form of the method according to the first aspect, the second Ti-rich STO nanolaminate comprises Sr in a range of <NUM>-<NUM> %, preferably of <NUM>%.

The above implementation forms provide a specific sequence of Sr-rich SrTiO3 and Ti-rich SrTiO3, which can optimize the permittivity of the STO stack.

In an implementation form of the method according to the first aspect, the bottom electrode layer comprises a tensile TiN layer.

Providing the TiN layer may lead to a tensile strain situation, which strongly enhances the permittivity of the STO stack and thus the VCMA effect.

According to a second aspect, the disclosure relates to a magnetic structure for a magnetic device, wherein the magnetic structure comprises: a bottom electrode layer, a SrTiO<NUM>, STO, stack comprising at least two different STO nanolaminates, wherein the STO
stack is provided on the bottom electrode layer, a magnetic layer provided on the STO stack; and a PMA promoting layer provided on the magnetic layer and configured to promote a PMA in the magnetic layer.

The magnetic structure of the second aspect provides the effects and advantages described above. In particular, it may lead to a stronger VCMA effect, due to using the high-permittivity STO stack, e.g. when a voltage is applied to the bottom electrode layer. The VCMA effect can especially be strong at room temperature. The VCMA effect may even be sufficiently strong to remove the PMA (induced) in the magnetic layer altogether.

In an implementation form of the magnetic structure according to the second aspect, the permittivity of the STO stack is larger than <NUM>, in particular is about <NUM>, at room temperature.

In an implementation form of the magnetic structure according to the second aspect, a grain size of the STO of the STO stack is below <NUM>.

The above described aspects of the disclosure and implementation forms of the present invention will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which:.

<FIG> shows a magnetic structure <NUM> according to an embodiment, wherein the magnetic structure <NUM> is suitable for a magnetic device <NUM> according to an embodiment. In particular, the magnetic structure <NUM> can be used to form a MTJ stack in a magnetic memory or logic device <NUM>.

The magnetic structure <NUM> comprises a bottom electrode layer <NUM> (e.g., comprising a tensile TiN layer), a SrTiO<NUM> (STO) stack <NUM> provided on the bottom electrode layer <NUM>, a magnetic layer <NUM> (e.g., comprising a Co layer) provided on the STO stack <NUM>, and a PMA promoting layer <NUM> (e.g. comprising a Pt layer or a MgO layer) provided on the magnetic layer <NUM>. The PMA promoting layer <NUM> may promote/strengthen/enhance a PMA in the magnetic layer <NUM>. That is, the PMA promoting layer may not be solely responsible for the PMA in the magnetic layer. However, the PMA promoting layer <NUM> could also be a PMA inducing layer, which is configured to induce the PMA in the magnetic layer <NUM>.

The STO stack <NUM> comprises at least two different STO nanolaminates. A (STO) nanolaminate may thereby comprise a set of (STO) layers, e.g. nanolayers formed by ALD. A set of layers may comprise one or more layers. The two different STO nanolaminates comprise a first Sr-rich STO nanolaminate and a second Ti-rich STO nanolaminate. The first STO nanolaminate may, accordingly, comprises a first set of Sr-rich layers. The second STO nanolaminate may, accordingly, comprise a second set of Ti-rich layers.

The permittivity (k) of the STO stack <NUM> may be larger than k=<NUM>, in particular the permittivity may be k=<NUM>-<NUM>, or even k≥<NUM>, specifically at room temperature. Further, a grain size of the STO of the STO stack <NUM> may be below <NUM>, or even below <NUM>. The STO stack <NUM> supports an enhanced VCMA effect in the magnetic structure <NUM> (e.g., enhanced compared to a conventional high-k stack), in particular in a magnetic device <NUM> employing the magnetic structure <NUM>.

The strong VCMA effect may be obtained by an exemplary magnetic structure <NUM> comprising a TiN\STO\Co\Pt layer sequence, according to the layer examples described above. The TiN layer <NUM> and the STO dielectric layer <NUM> may be deposited such that the permittivity of the dielectric STO stack <NUM> is optimized and reaches k≥<NUM>. The Co\Pt layer combination (bilayer) can be designed to strengthen/optimize the PMA, and the STO\Co interface may be oxidized to promote the PMA and the VCMA effect. The Pt layer <NUM> may be chosen specifically for the Co layer <NUM>, because it enhances the PMA of Co significantly. However, other possibilities are available. For example, an MgO layer <NUM> deposited on the Co layer <NUM> can also promote a strong PMA. These two examples are, therefore, interchangeable in the present disclosure, as the PMA promoting layer <NUM>.

The stoichiometric STO stack <NUM> may be obtained by combining the first STO nanolaminate, specifically (Sr-rich STO/ TiO2)xn, wherein n is an integer number in the range of n=<NUM>-<NUM>, preferably n=<NUM>, and a second STO nanolaminate (Ti-rich STO). The Sr content may be in the range of <NUM>-<NUM>%, with a preferred value being <NUM>%, and it may be deposited after an optional thermal treatment was applied to the first STO nanolaminate. An optional second thermal treatment may lead to an intermixing of the two STO nanolaminates, with the formation of the STO layer <NUM> comprising STO with a controlled grain size below <NUM>, a dielectric constant of k=<NUM>-<NUM>, and a low leakage current in the order of <NUM>-<NUM> A/cm<NUM>.

The TiN layer <NUM> may act as a template during the crystallization of the STO, thus enabling a good quality interface between the metal and the dielectric STO. The combination of the TiN layer <NUM>, the ALD process producing the two or more nanolaminates, and the Co magnetic layer <NUM>, may lead to a tensile strain situation, which strongly enhances the permittivity and, as such, the VCMA effect.

<FIG> shows a schematic representation of a method <NUM> for manufacturing the magnetic structure <NUM> of <FIG>. The method <NUM> comprises the following steps:.

Notably, in this disclosure, forming a layer "on" another layer may mean growing/depositing these layers one upon the other. Thus, surfaces of these layers are in contact. Forming a layer "above" another layer may mean that this layer is formed after the other layer, but there are formed one or more layers in between.

<FIG> show exemplary designs for magnetic devices <NUM>, which are based on the magnetic structure <NUM> of <FIG>, and employ the strong VCMA effect enabled by the magnetic structure <NUM>. In particular, the magnetic devices <NUM> are MTJ based magnetic devices <NUM>.

In each of <FIG>, a white arrow placed into a layer represents a magnetization of that layer. The orientation of the magnetization may correspond to the orientation of the arrow. The thinner white arrows represent the magnetization of the magnetic layer <NUM>, i.e. the layer where the magnetization can switch. The thicker white arrows represent fixed/pinned magnetization.

Further, in each of <FIG>, dashed black arrows represent stray magnetic field outside of the magnetic device <NUM>. The stray field may, for example, originate from the field bias layer <NUM>, and may tilt the magnetization of the magnetic layer <NUM>. The stray field supports controllable precessional switching.

<FIG> shows a schematic representation of an exemplary three-terminal magnetic device <NUM> according to an embodiment. The magnetic device <NUM> employs/includes the magnetic structure <NUM> shown in <FIG>. Same elements in <FIG> and <FIG> are labelled with the same reference signs. The magnetic device <NUM> shown in <FIG> accordingly comprises the bottom electrode layer <NUM>, the STO stack <NUM> provided on the bottom electrode layer <NUM>, the magnetic layer <NUM> provided on the STO stack <NUM>, and the PMA promoting layer <NUM> on the magnetic layer <NUM>.

Specifically, in the magnetic device of <FIG>, the magnetic layer <NUM> comprises or functions as a free magnetic layer (e.g., comprising Co). Further, the PMA promoting layer <NUM> functions as a tunnel barrier <NUM> (e.g., comprising an MgO layer). That is, in the magnetic device <NUM>, the tunnel barrier <NUM> promotes PMA in the free magnetic layer <NUM>. Further, a magnetic reference layer <NUM> is provided on the tunnel barrier <NUM>, thus forming an MTJ stack together with the magnetic layer <NUM> and the tunnel barrier <NUM>. In addition, a top electrode layer <NUM> is provided above the tunnel barrier <NUM>. A field bias layer <NUM> is provided between the magnetic reference layer <NUM> and the top electrode layer <NUM>.

The magnetic device <NUM> is, in this exemplary embodiment, a three-terminal T1, T2, T3 VCMA device. The magnetic device <NUM> comprises two or more pillars (two are illustrated), wherein both pillars are structured based on (e.g., patterned and/or etched from) the magnetic reference layer <NUM>, the field bias layer <NUM>, and the top electrode layer <NUM>, respectively. That is, these layers <NUM>, <NUM>, and <NUM>, are patterned such that they form two separate pillars. Each pillar consequently comprises a part of each of these layers <NUM>, <NUM>, and <NUM>. Further, the first terminal T1 is connected to the top electrode layer <NUM> of one of the pillars (left side pillar in <FIG>), and the second terminal T2 is connected to the top electrode layer <NUM> of the other one of the pillars (right side pillar in <FIG>). The third terminal T3 is connected to the bottom electrode layer <NUM>.

<FIG> shows a schematic representation of an exemplary three-terminal magnetic device <NUM> according to another embodiment, which is similar to the embodiment of <FIG>. The magnetic device <NUM> of <FIG> employs/includes the magnetic structure <NUM> shown in <FIG>. Same elements in <FIG> and <FIG> are labelled with the same reference signs, may be implemented likewise, and may function likewise.

In particular, as shown in <FIG>, compared to the magnetic device <NUM> of <FIG>, the field bias layer <NUM> is positioned higher up in the stack. In particular, in this exemplary embodiment, the top electrode layer <NUM> is provided on the magnetic reference layer <NUM>, and the field bias layer <NUM> is embedded into the top electrode layer <NUM>. The first terminal T1 is again connected to the top electrode layer <NUM> of one of the pillars (left side pillar in <FIG>), and the second terminal T2 is again connected to the top electrode layer <NUM> of the other one of the pillars (right side pillar in <FIG>). The third terminal T3 is connected to the bottom electrode layer <NUM>.

<FIG> shows a schematic representation of an exemplary three-terminal magnetic device <NUM> according to another embodiment, which is similar to the embodiments shown in <FIG> and <FIG>, respectively. The magnetic device <NUM> of <FIG> employs/includes the magnetic structure <NUM> shown in <FIG>. Same elements in <FIG> or <FIG> and in <FIG> are labelled with the same reference signs, may be implemented likewise, and may function likewise.

In this embodiment of <FIG>, the magnetic device <NUM> comprises a first pillar (left side pillar in <FIG>), which is structured based on the tunnel barrier <NUM>, the magnetic reference layer <NUM>, the field bias layer <NUM>, and the top electrode layer <NUM>, respectively. That is, compared to <FIG>, also the tunnel barrier <NUM> is patterned. Further, the magnetic device <NUM> comprises a second pillar (right side pillar in <FIG>), which is structured from a further electrode layer <NUM> provided on the magnetic layer <NUM>. That is, this pillar is different than the right-side pillar of <FIG>. Otherwise the magnetic devices <NUM> are similar.

In the magnetic device <NUM> of <FIG>, the first terminal T1 is connected to the top electrode layer <NUM> of the first pillar, the second terminal T2 is connected to the further electrode layer <NUM> of the second pillar, and the third terminal T3 is connected to the bottom electrode layer <NUM>. The second terminal T2 is accordingly arranged in direct contact with the magnetic layer <NUM> (that functions as the free magnetic layer), so that the resistance of the MTJ stack and, therefore, the readout energy consumption, can be decreased.

The magnetic devices <NUM> shown in <FIG>, <FIG> and <FIG>, respectively, are VCMA devices <NUM>. In these magnetic devices <NUM>, a precessional switching of the magnetization of the free magnetic layer <NUM> may be achieved by employing the VCMA effect to suppress the PMA, which is induced/present in the free magnetic layer <NUM> by means of the tunnel barrier <NUM>. In particular, the magnetization of the magnetic layer <NUM> is tilted by the field bias layer <NUM> (stray field, see above). This enables efficient precessional switching of the magnetization of the magnetic layer <NUM>, by applying a giant voltage (VCMA) pulse to the third terminal T3 that removes the PMA. The magnetization of the magnetic layer <NUM> may be switched by controlling, for instance, the pulse duration of the voltage pulse applied to the third terminal T3.

Further, the state of the magnetization of the magnetic layer <NUM> can be read out by applying a TMR (current) pulse between the first terminal T1 and the second terminal T2. In this case, a TMR current flows from the first terminal T1 to the second terminal T2. The current differs depending on the magnetization state of the magnetic layer <NUM>, due to corresponding low-resistance or high-resistance (TMR) states.

In order to make the magnetic devices <NUM> function better than conventional MTJ based devices, a strong VCMA effect is employed. Here, in the magnetic devices <NUM>, the STO stack <NUM> and the magnetic layer <NUM> provided on the STO stack <NUM>, enable a giant VCMA effect, which is sufficiently strong to improve the magnetic devices <NUM> over conventional MTJ based devices.

The strong VCMA effect (already at room temperature) even allows removing completely the anisotropy energy barrier between the two magnetization states of the magnetic layer <NUM>, when applying the large voltage pulse to the third terminal T3. This enables the efficient precessional switching (by controlling, e.g., the duration of the voltage pulse) of the magnetization of the magnetic layer <NUM>. Thus, a magnetization switching current is drastically reduced compared to a conventional MTJ stack based device. Further, since the VCMA effect is strong in the magnetic device <NUM>, the anisotropy energy barrier can be designed generally larger, which leads to an increased data retention in the magnetic device <NUM>.

A top-pinned MTJ stack is preferred for use in the magnetic device <NUM> according to an embodiment. This is because the magnetic layer <NUM>, tunnel barrier <NUM> and magnetic reference layer <NUM> (MTJ stack) can then be deposited on top of the high-k dielectric STO stack <NUM> (i.e., can be formed after the STO stack <NUM>). The ALD grown STO stack <NUM> reaches its highest permittivity after a high temperature anneal, in this case, e.g., at <NUM>. The MTJ stack could deteriorate above a thermal budget of, e.g., <NUM> and, therefore, is preferably not exposed to the crystallization anneal of the STO stack <NUM>. Thus, the STO stack <NUM> is preferably formed before the MTJ stack. In addition, the STO crystallinity is promoted by the bottom electrode layer <NUM>. The magnetic layer <NUM>, which is deposited on top of the STO stack <NUM>, contributes, but to a lesser extent.

<FIG> shows a schematic representation of an exemplary two-terminal magnetic device <NUM> according to an embodiment. In particular, in this exemplary embodiment, the magnetic device <NUM> is a VCMA device. The magnetic device <NUM> of <FIG> employs/includes the magnetic structure <NUM> shown in <FIG>. Same elements in <FIG>, <FIG> or <FIG>, and in <FIG>, are labelled with the same reference signs, may be implemented likewise, and may function likewise.

The magnetic device <NUM> of <FIG> comprises the bottom electrode layer <NUM>, the STO stack <NUM> on the bottom electrode layer <NUM>, the magnetic (free) layer <NUM> on the STO stack <NUM>, the (PMA promoting) tunnel barrier <NUM> on the magnetic layer <NUM>, the magnetic reference layer <NUM> (could be a synthetic antiferromagnet) on the tunnel barrier <NUM>, the field bias layer <NUM> on the magnetic reference layer <NUM>, and the top electrode layer <NUM> on the field bias layer <NUM>.

The first terminal T1 is connected to the top electrode layer <NUM>. There is no second terminal T2. The third terminal T3 is connected to the bottom electrode layer <NUM>. In this magnetic device <NUM>, a precessional switching of the magnetization of the magnetic layer can be achieved through the VCMA effect, when a voltage pulse is applied to the third terminal T3. The tunnel barrier <NUM> (e.g., comprising MgO) and the high-k properties of the STO stack <NUM> are preferably tuned carefully, such that the MgO and high-k bands lead to two different resistance states.

<FIG> shows a schematic representation of an exemplary magnetic device <NUM> according to an embodiment. In particular, in this embodiment, the magnetic device <NUM> is a three-terminal STT device. The magnetic device <NUM> of <FIG> employs/includes the magnetic structure <NUM> shown in <FIG>. Same elements in <FIG>, <FIG>, <FIG> or <FIG>, and in <FIG>, are labelled with the same reference signs, maybe implemented likewise, and may function likewise.

The magnetic device <NUM> comprises the bottom electrode layer <NUM>, the STO stack <NUM> on the bottom electrode layer <NUM>, and the magnetic layer <NUM> on the STO stack <NUM>. Here the magnetic layer <NUM> comprises an antiferromagnetic (AFM) bias layer <NUM> on the STO stack <NUM>, an AFM coupling layer <NUM> on the AFM bias layer <NUM>, and a magnetic free layer <NUM> on AFM coupling layer <NUM>. Further, the magnetic device <NUM> comprises the (PMA promoting) tunnel barrier <NUM> on the magnetic layer <NUM>, the magnetic reference layer <NUM> (could be a synthetic antiferromagnet) on the tunnel barrier <NUM>, and the top electrode layer <NUM> on the magnetic reference layer <NUM>.

In this embodiment, the first terminal T1 is connected to the top electrode layer <NUM>, the second terminal T2 is connected to the magnetic layer <NUM>, and the third terminal T3 is connected to the bottom electrode layer <NUM>. A voltage pulse to the third terminal T3 may remove the anisotropy from the high-anisotropy antiferromagnetic (AFM) bilayer of the magnetic layer <NUM> (i.e., the bilayer of the AFM bias layer <NUM> and AFM coupling layer <NUM>). The low anisotropy magnetic free layer <NUM> of the magnetic layer <NUM> can be tilted, and can be switched with a small STT current through the tunnel barrier (current pulse flowing from first terminal T1 to second terminal T2 through the tunnel barrier <NUM>).

Claim 1:
A method (<NUM>) for manufacturing a magnetic structure (<NUM>) for a magnetic device (<NUM>), the method (<NUM>) comprising:
providing (<NUM>) a bottom electrode layer (<NUM>);
forming (<NUM>) a SrTiO<NUM>, STO, stack (<NUM>) on the bottom electrode layer (<NUM>) by Atomic Layer Deposition, ALD, of at least two different STO nanolaminates;
forming (<NUM>) a magnetic layer (<NUM>) on the STO stack (<NUM>); and
forming (<NUM>) a Perpendicular Magnetic Anisotropy, PMA, promoting layer (<NUM>) on the magnetic layer (<NUM>), the PMA promoting layer (<NUM>) being configured to promote PMA in the magnetic layer (<NUM>).