Ferroelectric [HfO2/ZrO2] Superlattices with Enhanced Polarization, Tailored Coercive Field, and Improved High Temperature Reliability

Modern microelectronic systems and applications demand an every increasing amount of non‐volatile memories that are fast, reliable, and consume little power. Memory concepts based on ferroelectric HfO2 like the ferroelectric field effect transistor (FeFET) and the ferroelectric random access memory (FeRAM) are promising to satisfy these requirements. As a consequence, continuing high attention is given to improve the ferroelectric properties and the reliability characteristics of the ferroelectric HfO2 films – for instance by using different dopant elements, dopant concentrations, and film thicknesses. Superlattices (i.e., a periodic structure of two materials stacked upon each other) are a promising alternative approach. Herein, [HfO2/ZrO2] superlattices of various sublayer thicknesses and a constant total thickness of 10 nm are embedded into metal‐ferroelectric‐metal (MFM) capacitors and then electrically as well as structurally characterized with special focus on remanent polarization, coercive field, endurance, and high temperature reliability. Compared to a 10 nm (Hf,Zr)O2 solid solution reference film, the use of superlattice stacks significantly improves the above mentioned parameters. In addition, most of these parameters depend on the sublayer thickness, which allows, for instance, tailoring the coercive field of the whole device.


Introduction
The demand for data storage increases persistently in modern microelectronic systems.Many applications like for instance sensors for internet of things devices or microcontrollers for automotive solutions, require non-volatile memories that are fast, reliable, and consume little power. [1,2]Memories based on ferroelectric materials like the ferroelectric field effect transistor (FeFET), the ferroelectric random access memory (FeRAM), and the ferroelectric tunnel junction (FTJ) are auspicious to satisfy these requirements. [3]t zero external electric field, ferroelectric materials have two distinguishable remanent polarization states (P r + and P r -) that can be used to store information.Switching between both states is triggered by applying an external electric field that is larger than the coercive field (|E c |) of the ferroelectric.
First memory concepts based on perovskite-type ferroelectrics were suggested already in the late 1950s. [4]However, these conventional ferroelectrics require high layer thicknesses (due to the small coercive and breakdown fields) and their integration into complementary metal-oxide-semiconductor (CMOS) technologies is rather challenging.Consequently, their use is limited to niche applications. [5]n 2011, ferroelectricity was detected in HfO 2 , [6] a material successfully integrated into standard CMOS high-k metal gate technologies just a few years before. [7]The ferroelectric properties of the HfO 2 -based films are attributed to the polar, non-centrosymmetric, orthorhombic (o) phase of space group Pca2 1 , [8] which can be stabilized, for instance, by introducing impurity atoms. [9,10]21] To further optimize important characteristics of the ferroelectric films, superlattices are a promising alternative approach.Superlattices are periodic structures of two (or more) materials with layer thicknesses in the range of nanometers.This concept is not new.[25][26][27][28] A review about the development of that topic is given in reference. [29]he observation of ferroelectricity in [HfO 2 /ZrO 2 ] x superlattices is (at least) unexpected, since the increased surface/interface energy effect (without doping) does not necessarily explain the stabilization of the ferroelectric phase.33] Herein, we report on the ferroelectric characteristics of [HfO 2 /ZrO 2 ] superlattices with various sublayer thicknesses at a constant total thickness of 10 nm.The stacks are characterized with respect to remanent polarization, coercive field, endurance, and high temperature reliability.These parameters clearly depend on the sublayer thickness, which allows tailoring of for instance the coercive field within certain limits.
Metal-ferroelectric-metal (MFM) capacitors are used as simple test vehicle to characterize the ferroelectric stack.They are also an important building block for embedded ferroelectric memories.Placed in the back-end-of-line (BEoL) of microchips, they can be connected either to the gate or drain contact of a standard logic device to realize a 1T1C FeFET [12,13,34,35] and a 1T1C FeRAM, [36] respectively.

Experimental Section
MFM capacitors were fabricated on highly boron doped 300 mm silicon wafers.The 20 nm thick TiN bottom and top electrodes were deposited via reactive magnetron sputtering.The ferroelectric films were deposited by atomic layer deposition (ALD) at 300°C.As precursors, hafnium tetrachloride (HfCl 4 ), and zirconium tetrachloride (ZrCl 4 ) were used together with water (H 2 O) and argon (Ar) as oxidizing reactant and purging gas, respectively.
Five superlattice stacks with different sublayer thicknesses and a constant total thickness of 10 nm (confirmed by spectroscopic ellipsometry) were embedded as ferroelectric into the MFM structures: [0.25 nm HfO 2 /0.In case of the superlattices, the number of deposition cycles necessary to meet the targeted HfO 2 and ZrO 2 sublayer thicknesses were calculated from the growth per cycle (GPC) of the individual material.The (Hf,Zr)O 2 reference film was achieved by periodically depositing one cycle of HfO 2 and one cycle of ZrO 2 .Therefore, it could be considered as the "superlattice" with the smallest sublayer thickness in the range of the GPC (≈ 0.05 nm).The crystallization anneal was done under N 2 atmosphere within a rapid thermal annealing (RTA) tool.The hold temperature, hold time, and ramp up rate were 400°C, 60 s, and 50 K s -1 , respectively.Cool down was done passively.
Discrete Ti/Pt dots were deposited with electron beam evaporation by using a shadow mask.Subsequently, the TiN between the dot contacts was removed by wet etching (SC1) to form individual MFM capacitors.Figure 1 depicts a schematic drawing of all samples.In addition, Figure 2 depicts the MFM stack with the 1 nm sublayer thickness, which was visually analyzed on crosssection specimen using a FEI Tecnai F20 transmission electron microscope (TEM).
The structural characteristics of the crystalline phases were concluded from X-ray diffraction experiments in a grazingincident diffraction geometry (GIXRD) with a thin-film diffractometer (Bruker D8 Discover).The incident angle was set to 0.5°.To avoid the substrate reflection (311), all samples were mounted with a 45°rotation between the <100 > directions of the (100)oriented silicon wafer and the diffraction plane.
The polarization versus electric field (P-E) measurements were carried out with an Aixacct TF 3000 FE analyzer using a triangular waveform at a frequency of 1 kHz.The endurance tests were performed with an electrical cycling frequency of 10 kHz.

Results and Discussion
The back-end of line (BEoL) compatible thermal budget used here (60 s at 400°C) is sufficient to crystallize the solid solution (Hf,Zr)O 2 reference film and all tested [HfO 2 /ZrO 2 ] superlattices with a total thicknesses of 10 nm (cf., Figure 3).The XRD analysis can clearly distinguish the non-polar monoclinic (m)-phase from the cubic (c)-, the tetragonal (t)-, and the ferroelectric (noncentrosymmetric) o-phase.It is worth to mention that the lamination has no considerable impact on the formation of the m-phase, since the fraction of this phase is negligible for all films studied in here.For superlattices starting with the HfO 2 sublayer, a similar result was reported. [30]On the contrary, if the first layer directly on the bottom TiN electrode is ZrO 2 , a significant fraction of the m-phase may be found, as partly reported in the literature, for instance in [30].
It is apparent from the GIXRD pattern that the area of the diffraction line at 2 ≈ 35°(corresponding to {200} planes of the o-phase) changes with the sublayer thickness.To investigate this, the area of this diffraction line is normalized to that of the main diffraction line at 2 ≈ 30.5°, which corresponds to {111} planes of the same phase (cf., Figure 4).Superlattices with sublayer thicknesses between 0.25 nm and 1 nm have the largest fraction of the {200} diffraction line.If the sublayer is thicker and / or in case of the (Hf,Zr)O 2 (HZO) reference, this fraction is significantly smaller.In the literature, an increase of the {200}intensity/area is related to a crystallographic texture that enhances the polarization, as studied by transmission Kikuchi diffraction. [37,38]ince the o-phase diffraction line at 2 ≈ 18°, which belongs to the (100) plane, is not apparent in Figure 4, the same is expected to apply for the superlattices studied here.
The Scherrer equation can be used to calculate the grain size within the superlattices and the reference device.Due to the GIXRD geometry, the grain size is estimated at ≈15°off-axis along the vertical direction.By using the main diffraction line at 2 ≈ 30.5°and a shape factor of 0.9, the estimated size is almost equal to the film thickness (cf. Figure 4).Thus, it can be concluded that the crystal growth is not interrupted at the sublayer interfaces and that the grains are most likely of columnar shape.
Since XRD cannot be used to clearly distinguish between the c-, t-, and o-phase (all mentioned phases have diffraction lines at similar 2 angles), polarization versus electric field (P-E) measurements were conducted to verify the presence of the ferroelectric o-phase.As seen in Figure 5, all MFM capacitors are ferroelectric, which proofs the presence of the o-phase.It is worth to mention that devices with the solid solution (Hf,Zr)O 2 reference film depict almost no pinching of the hysteresis loop in the pristine state.In contrast, whenever the superlattice films are integrated into the MFM capacitors, pronounced antiferroelectriclike characteristics are observed before wake-up cycling.We conclude that the laminates either crystallize preferentially in the antiferroelectric-like t-phase [39] and / or stabilize a higher amount of in-plane oriented domains, which can act antiferroelectriclike within the o-phase. [38]However, less than 1000 field cycles are needed to fully wake-up these integrated superlattice devices (i.e., eliminate the antiferroelectric-like characteristics).During wake-up either a field-induced phase transformation from a trich to an o-rich state occurs with subsequent stabilization of the o-phase due to defect migration, [39] and / or destabilization of antiferroelectric-like in-plane domains due to stress relaxation caused by, for example, defect (dislocation) generation might be present. [38]ssuming that crystallization occurs initially at the interface between the first deposited HfO 2 sublayer and the bottom TiN interface, as suggested for instance in [Ref.[30]], we may derive the following model-based-description to explain ferroelectricity of the [HfO 2 /ZrO 2 ] superlattice stacks: The crystallization is triggered from the bottom TiN electrode either during deposition (depending on the temperature of the ALD process and the sublayer thickness) or during a post metallization anneal, as applied in this study.During this process, the first HfO 2 sublayer gets orientation and phase information, which are determined (among other factors) by the stress conditions introduced from the bottom TiN electrode.Thus, a significant fraction of the o-phase is stabilized.This information is then propagated across the entire laminated stack.This model also explains the significantly higher amount of m-phase in the stack, should the superlattice start with ZrO 2 as a first sublayer. [30]For that, the crystallization (triggered again from the bottom TiN electrode) may start from the ZrO 2 sublayer and other crystallization information is propagated.In the case of thick ZrO 2 sublayers, the propagation of the crystallization information across the stack may be impeded, which explains that the fraction of the t-phase and/or of in-plane domains within the o-phase increases and a pinching of the hysteresis loops is observed that becomes more pronounced for an increasing ZrO 2 sublayer thickness.
One important reliability parameter that can be directly estimated qualitatively from the shape of the P-E curve is the leakage-density, since leakage leads to a "cigar-like" broadening of the hysteresis loop.For the measurement conditions used in Figure 5, no significant leakage is observed for the reference and the superlattice MFM capacitors.
The remanent polarization (2P r ) is another important parameter that characterizes ferroelectric films.For instance in the case of FeFETs, too low 2P r -values could result in a reduced memory window (MW).Figure 6 depicts the remanent polarization of the solid solution reference device and the superlattice devices with various sublayer-thickness in the pristine state (a) and after field cycling (b-d).The superlattice itself and the thickness of the sublayer have strong impact on the measured remanent polarization.In the pristine state, the superlattices show smaller remanent polarization than the reference device.Furthermore, the remanent polarization of the superlattices decreases with increasing sublayer-thickness.This can be explained by the antiferroelectric-like characteristics of the superlattices, which become more prominent for thicker sublayers.After wake-up, the dependency between the sublayer thickness and the remanent polarization changes as follows: Starting from the solid solution reference device, the remanent polarization increases with increasing sublayer thickness.However, whenever surpassing the 1 nm sublayer thickness, this trend reverts and the remanent  polarization starts to decrease with further increasing sublayer thickness.This result is in good agreement with the GIXRD data, where the largest fraction of {200} planes were observed for sublayer thicknesses between 0.25 and 1 nm.That a sublayer thickness of 1 nm results in the highest remanent polarization was reported before for similar stacks, for instance in reference. [24,30]n addition, it should be mentioned that it is not limited to [HfO 2 /ZrO 2 ] superlattices that the sublayer thickness influences the remanent polarization.Similar characteristics were observed for [Si-doped HfO 2 /SiO 2 ] stacks with various SiO 2 thicknesses. [40]he coercive field (|E c |) is another important parameter.Integrated into the gate stack of a logic device to build a FeFET, the MW is defined by the coercive field of the ferroelectric material and its film thickness (t F ): MW ≈ 2t F E c . [41]Furthermore, the coercive field directly impacts the program and erase voltage and thus can be used to tune the power consumption during device operation.Also the endurance limitations related to high coercive field of HfO 2 based ferroelectrics is one important challenge for future memory applications.Using superlattices is a rather easy approach to tailor the coercive field, as the following results demonstrate.
Figure 7 shows the coercive field of the solid solution reference device and the superlattices with different sublayer-thicknesses for the pristine state (a) and after field cycling (b-d).In the case of the pristine state, the superlattices have significantly smaller coercive fields than the reference.In addition, the coercive field further decreases with increasing sublayer thickness.Similar to the trend observed in Figure 5a before, this can be explained by the increasing antiferroelectric-like characteristics of the superlattice devices with increasing sublayer thickness.However, in contrast to Figure 5b-d, the same trend is observed in Figure 7bd after cycling, although the antiferroelectric-like characteristics of the hysteresis loops are vanished.While for the sublayers of 1 nm thickness and below, this might be explained by the out-ofplane texture of the <200 > axes, the origin for the reduced coercive field in the case of thicker sublayers is not expected.From the P-E loops, a slightly decreased slope is observed around the coercive field, indicating that small antiferroelectric-like fractions in the domain configuration remain and thus result in a lower effective coercive field.
The superlattice approach opens the possibility to tailor the coercive field (2|E c |) between 3.1 MV cm -1 (reference film after 10 3 field cycles) and 2.1 MV cm -1 (superlattice with 2.5 nm sublayers after 10 3 field cycles), which corresponds to a reduction of about 30%.Alternatively, the coercive field can be tuned for instance by using hybrid ferroelectric/anti-ferroelectric stacks, [42] Hf x Zr 1-x O 2 films of various compositions, [18] or an interface modification of the bottom electrode through ultraviolet-ozone irradiation. [43]Although the last mentioned approaches allow a modification of the coercive field within similar ranges, they suffer from disadvantages like for instance polarization reduction [18] and/or hysteresis loop shape distortion. [42]igure 8 displays the endurance characteristics.Cycling tests can basically be divided into three stages: wake-up, fatigue, and dielectric breakdown. [44,45]During wake-up, the remanent polarization growths with increasing number of field cycles due to the de-pinching of the prior pinched hysteresis loop.This is observed in particular for the superlattice devices (cf., Figure 8bd), which are more pinched in the pristine state.The impact of the sublayer thickness on the number of wake-up cycles is negligible for sublayer thicknesses up to 2.5 nm.However, stacks with 5 nm sublayers need a reduced number of field cycles for wake-up.This stage is attributed to a gradual decrease in the built-in bias field [44] and/or a partial phase transition of the dielectric material from the t-to the o-phase due to a redistribution of defects (oxygen vacancies). [46]Recently also stress relaxation due to defect generation and thus the reduction of in-plane domains was suggested as the underlying mechanism. [38]After the wake-up process is completed, the reference sample and the superlattice devices get straight into the fatigue (or aging) phase.During this stage, the remanent polarization decreases with increasing field cycles due to the generation and field drift of defects like oxygen vacancies that facilitate charge trapping and / or domain pinning.Finally, the main failure mode is a dielectric breakdown, which occurs abrupt for all tested devices.This stage is attributed to the formation of permanent conduction paths that substantially promote enhanced leakage currents.Usually the field drift and accumulation of oxygen vacancies are attributed to this failure mode, similar to filament  The maximum endurance (defined as the number of field cycles before dielectric breakdown) is shown in Figure 9 for the solid solution reference device and the superlattice of dif-ferent sublayer-thicknesses after cycling at different electrical fields.As expected, the endurance decreases with increasing electrical fields applied to the devices and with increasing sublayer-thickness.It was reported elsewhere that devices with HfO 2 /ZrO 2 /HfO 2 trilayers have improved endurance characteristics compared to superlattices and solid solution (Hf,Zr)O 2 reference films, [30] which can, based on the results observed here, be due to the increased single layer thickness of the trilayered devices.
Certain applications, for instance in the automotive sector, require appropriate reliability characteristics at elevated operation temperatures.Figure 10 depicts the polarization versus electric field (top row) and current density versus electric field (bottom row) characteristics of the superlattice with 1 nm sublayer thickness measured at chuck temperatures between 75 and 150°C.For a constant bias stress like for example 3.5 MV cm -1 , the leakage increases with rising chuck temperature.This expected trend can be seen qualitatively from the "cigar-like" shape of the polarization loops and quantitatively from the current versus electric field plots.It is worth to point out that even at the most challenging bias and temperature stress conditions (3.5 MV cm -1 and 150°C) the superlattice with 1 nm sublayers survives 10 3 field cycles, which is in strong contrast to the other superlattices and the reference device that break under the same stress condition after a significantly lower number of cycles.
Figure 11 summarizes the endurance of the solid solution reference device and the superlattices of different sublayerthicknesses for chuck temperatures up to 150°C.The high temperature reliability strongly depends on the structure of the ferroelectric material: Starting from the reference film, the temperature reliability significantly improves with increasing sublayer thickness up to 1 nm, which is the most promising film.In the case of thicker sublayers, the endurance characteristics degrade again and reach at 5 nm almost the same results as observed earlier for the reference device.
This behavior is rather analogue to another class of superlattices that use Al 2 O 3 interlayers between sublayers of Zr-doped HfO 2 . [47]It was reported that Al 2 O 3 interlayers might facilitate the formation of smaller grains.The reliability improvement can thus be explained by the lengthened leakage paths.However, we observe grain sizes in the range of the total superlattice thickness.Another explanation for the improved reliability characteristics of the superlattice devices can be the comb like band-structure originating from the different band-gap values of the HfO 2 sublayer (5.3-5.9 eV [48] ) and the ZrO 2 sublayer (3.8-4.5 eV [49] ).As a consequence, the interfaces between the sublayers act as a barrier that suppresses electron and hole transport.Contrary, in the case of a perfect solid solution, an intermediary band-gab is formed.

Conclusion
Herein, [HfO 2 /ZrO 2 ] superlattices with sublayer thicknesses ranging from 0.25 nm to 5 nm and an overall thickness of 10 nm were embedded into MFM capacitors.The electrical and structural characterization revealed that, compared to a pure 10 nm (Hf,Zr)O 2 reference film, superlattice-based devices have higher remanent polarization, allow tailoring of the coercive field, and provide better reliability characteristics, in particular at high operation temperatures.Regarding remanent polarization and high temperature reliability, a sublayer thickness of 1 nm seems to be the most promising.The corresponding superlattice is, thus, very interesting for high temperature applications as found for instance in the broad automotive sector.Another important advantage of the superlattices is the possibility to tailor the coercive field by the sublayer thickness.A sublayer thickness of 2.5 nm provides the smallest coercive field without significantly sacrificing the remanent polarization and the reliability characteristics (both are superior compared to the reference device).This, in turn, makes the corresponding superlattice promising for low power applications.

Figure 1 .
Figure 1.Schematic cross section of the metal-ferroelectric-metal (MFM) stack used to characterize the ferroelectric films.Figure a shows the reference device with the single phase (Hf,Zr)O 2 solid solution.Figure b) illustrates the integration of the [HfO 2 /ZrO 2 ] superlattices into the MFM stack.The sublayer thicknesses range from 0.25 to 5 nm, whereas the total thickness of the superlattices is kept constant at 10 nm.

Figure 2 .
Figure 2. Transmission electron microscopy (TEM) image and schematic drawing of a MFM stack showing the integrated [1 nm HfO 2 /1 nm ZrO 2 ] 5 superlattice before RTA treatment.Although the superlattice stack has a certain roughness, the individual HfO 2 and ZrO 2 sublayers are clearly distinguishable (i.e., not intermixed).

Figure 3 .
Figure 3. Grazing-incident X-ray diffraction (GIXRD) pattern of the (Hf,Zr)O 2 reference film and the [HfO 2 /ZrO 2 ] superlattice stacks embedded in MFM structures.At the bottom of the figure, the positions of the diffraction lines of individual HfO 2 phases are displayed.In addition, the positions of the TiN diffraction lines are added by dashed vertical lines.

Figure 4 .
Figure 4. Area of the diffraction line at 2 ≈ 35°, corresponding to {200} planes, normalized to the area of the diffraction line at 2 ≈ 30.5°that corresponds to the {111} planes (red circles) as well as the grain size calculated with the Scherrer equation (black squares) displayed for both, the (Hf,Zr)O 2 (HZO) reference film and the [HfO 2 /ZrO 2 ] superlattices with different sublayer thicknesses.

Figure 5 .
Figure 5. Polarization versus electric field (P-E) measurements of the (Hf,Zr)O 2 reference film and the [HfO 2 /ZrO 2 ] superlattices embedded in MFM structures for the pristine state a-f) and after 10 3 wake-up cycles g-l).For statistical propose, each figure consists of at least five P-E loops.The sublayer thickness is given as legend of the respective hysteresis loop color.

Figure 6 .
Figure 6.Remanent polarization of the (Hf,Zr)O 2 (HZO) reference film and the [HfO 2 /ZrO 2 ] superlattices for a) the pristine state and after b) 10 3 , c) 10 4 , and d) 10 5 field cycles.The sublayer thickness is given at the bottom of the individual figures.Each box plot consists of at least 5 measurements.The maximum field of the P-E measurement is 3.5 MV cm -1 .

Figure 7 .
Figure 7. Coercive field (2|E c |) of the (Hf,Zr)O 2 (HZO) reference film and the [HfO 2 /ZrO 2 ] superlattices for a) the pristine state and after b) 10 3 , c) 10 4 , and d) 10 5 field cycles.The sublayer thickness of the respective superlattice is given at the bottom of each individual figure.Each box plot consists of at least 5 measurements.The maximum field of the P-E measurement is 3.5 MV cm -1 .

Figure 8 .
Figure 8. Remanent polarization as a function of field cycles for the (Hf,Zr)O 2 reference film and the [HfO 2 /ZrO 2 ] superlattices stressed at electrical fields between 2.75 and 3.5 MV cm -1 .The sublayer thickness of the respective superlattice is given in each individual figure.

Figure 9 .
Figure 9. Room temperature endurance characteristics of the (Hf,Zr)O 2 reference and the [HfO 2 /ZrO 2 ] superlattices for three different electrical fields: a) 2.75, b), 3.0, and c) 3.5 MV cm -1 .In the case of the superlattices, the respective sublayer thickness is given at the bottom of each individual figure.

Figure 10 .
Figure 10.Polarization versus electric field loops (top row) and current density versus electric field characteristics (bottom row) measured after 10 3 wake-up cycles on the [HfO 2 /ZrO 2 ] superlattice with 1 nm sublayer thickness embedded in MFM structures.In the horizontal direction, the chuck temperatures increases from 75 to 150°C.

Figure 11 .
Figure 11.High temperature endurance characteristics of the (Hf,Zr)O 2 reference and the [HfO 2 /ZrO 2 ] superlattices at 3.5 MV cm -1 .The chucktemperature during the measurements as well as the thickness of the sublayers are displayed at the bottom of each individual figure.