Composition Controlled Atomic Layer Deposition of Phase Change

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Valerio Adinolfi, Lanxia Cheng, Mario Laudato, is published by the Ryan C. Clarke, Vijay K. American Chemical Society. 1155 Sixteenth Narasimhan, Simone Balatti, Street N.W., Washington, Son Hoang, and DCKarl 20036 A. Littau Published by American Chemical by Society. Subscriber access provided Nottingham Copyright © American Trent University Chemical Society. However, no copyright

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Composition Controlled Atomic Layer Deposition of Phase Change Memories and Ovonic Threshold Switches with High Performance Valerio Adinolfi, Lanxia Cheng, Mario Laudato, Ryan C. Clarke, Vijay K. Narasimhan, Simone Balatti, Son Hoang, Karl A. Littau. Intermolecular, 3011 N. First Street, 95135, San Jose, California, United States of America

ABSTRACT Chalcogenide compounds are the main characters in a revolution in electronic memories. These materials are used to produce ultra-fast ovonic threshold switches (OTSs) with good selectivity and moderate leakage current and phase change memories (PCMs) with excellent endurance and short read/write times when compared with state-of-the-art FLASH-NANDs. The combination of these two electrical elements is used to fabricate non-volatile memory arrays with a write/access time orders of magnitude shorter than state of the art flash-NANDs. These devices have a pivotal role for the advancement of fields such as artificial intelligence (AI), machine learning (ML), and big-data. Chalcogenide films, at the moment, are deposited by using physical vapor deposition (PVD) techniques that allow for fine control over the stoichiometry of solid solutions but fail in providing the conformality required for developing largememory-capacity integrated 3D structures. Here we present conformal ALD chalcogenide films with control over the composition of germanium, antimony, and tellurium (GST). By developing a technique to grow elemental Te we demonstrate the ability to deposit conformal, smooth, composition-controlled GST films. We present a thorough physical and chemical characterization of the solids and an in-depth electrical test. We demonstrate the ability to produce both OTS and PCM materials. GeTe4 OTSs exhibit fast switching times ~ 13 ns. Ge2Sb2Te5 ALD PCM exhibit a wide memory window exceeding two-ordersof-magnitude, short write times (~ 100 ns), and a reset current density as low as ~ 107 A/cm2 – performance matching or improving over state of the art PVD PCM devices. KEYWORDS ALD, chalcogenides, phase-change-memories, ovonic-threshold-switches, PCM, OTS

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Chalcogenides play an increasingly important role in the electronic industry.1–3 These materials are today ubiquitous in a large variety of applications such as solar energy conversion,4,5 light emission and detection,6–9 and electronic memories.1,10,11 The advent of artificial intelligence and the need for managing big-data imposes the necessity for faster non-volatile memories (NVMs).12–14 Chalcogenide-based memory arrays combining phase change memory (PCM) elements with Ovonic Threshold Switches15,16 (OTSs) are the object of intense studies and have been demonstrated, in academia and industry, to produce highcapacity, fast-access NVMs. State-of-the-art arrays rely on the cross-point (X-point) architecture17–21 and are fabricated by depositing the active materials using physical vapor deposition (PVD) techniques.22–24 Multiple PVD sources can be easily operated together to produce binary, ternary, and quaternary alloys with finely tuned film compositions. Such control over the semiconductor stoichiometry is essential; the electrical properties of chalcogenide films are dramatically impacted by their composition.25–27 By definition, increased memory density requires the integration of more memory cells in the same wafer area; as 3D Vertical-NAND Flash has shown,28,29 this problem is best addressed by replacing traditional PVD 3D X-point architectures with more sophisticated three-dimensional architectures such as vertical arrays30,31 allowing for a more efficient usage of the space.32–34 It is clear that the success of chalcogenide based memories will depend on the possibility of integrating these materials in high aspect ratio threedimensional structures. Unfortunately, PVD deposition fails in providing the level of conformality that would be required to fabricate chalcogenide based high-density Vertical 3D X-point arrays (figure 1a). Atomic layer deposition (ALD) is an established deposition technique producing highly conformal thin films with atomic control over the thickness. This technique is currently used in industry to deposit many critical layers, e.g., conformal, ultra-thin gate oxides.35,36 Here we present an ALD deposition technique producing compositionally controlled GexSbyTez (GST) films covering a portion of the triangular composition diagram enabling fabrication of both OTS and PCM materials. In this letter we first provide details on the ALD processes and present a thorough analysis of the surface reactions by using in-situ ellipsometry. Then an in-depth material characterization is carried out: conformality for different compositions was probed by imaging the cross section (scanning electron microscope, SEM) of 20:1 high aspect ratio silicon trench structures covered with GST; the surface morphology of the ALD films was characterized by atomic force microscopy (AFM); films composition was analyzed using Rutherford backscattering spectroscopy (RBS), calibrated X-ray fluorescence (XRF), and X-ray photoemission spectroscopy (XPS). Finally the electrical properties of GST films were investigated unveiling excellent device performance. Tellurium rich compositions exhibited OTS behavior, as expected,37 with a very short switching time ~ 13 ns. Ge2Sb2Te5, well known to produce state-of-the-art PVD PCM memories,38 was


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tested by collecting current controlled DC current-voltage (IV) curves, resistance-voltage (RV) plots showing clear resistivity control by amorphization and crystallization of the GST solid; a 1000 set-reset cycling was performed to confirm repeatable switching of the PCM. The PCM shows impressively short write time (~ 100 ns pulse width, 10 ns fall time) and reset current density comparable or lower than stateof-the-art PVD devices. In addition to the data presented in this paper we invite the reader to access the supporting information available on line that includes additional data, analysis and material characterization.

Figure 1 a) Vertical 3D X-point array architecture for ultra-high capacity b) Ellipsometric analysis performed, in real-time, during deposition of GeTe0.7. The plot is zoomed on the first 10 cycles of the deposition and tracks the injection of BTMS-Te into the chamber (yellow line) and the film thickness as a function of cycles (brown line). c) GeTe0.7 growth as a function of time. We can clearly identify an initial substrate-dominated regime followed by a film-dominated regime. d) Schematic elucidating the deposition process. On the left the pulse scheme is depicted, showing the use of discrete feeding; by combining a number of GeTe, Te, and SbTe layers in a supercycle (to repeat until the desired thickness is reached) we achieve control over the solid stoichiometry. e) Ternary plot showing the range of composition accessible through the proposed process. We cover a large part of the diagram spacing PCM and OTS compositions. A comparison with previous ALD chalcogenide works is provided.


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RESULTS AND DISCUSSION Among the approaches that have been proposed for depositing chalcogenides via ALD,39 the recent use of alkyl-silyl ligands was demonstrated to produce continuous films and controllable processes.40–43 We started our study by investigating the ALD growth of germanium telluride (GeTe). We chose trichlorogermane (HCl3Ge) and bis(trimethilsilyl)tellurium (BTMS-Te) as the ALD precursors based on the method of Gwon et al.40 By considering, a priori, the reactions between the ligands we can expect the resultant film to show a 1:1 Ge:Te stoichiometry; HGeCl3 is hypothesized to decompose into GeCl2 + HCl reacting then with BTMS-Te in the manner GeCl2 + (Me3Si)2Te  GeTe + 2 Me3SiCl.41 We deposited GeTe films on 300 mm bare silicon wafers in an ALD chamber featuring an in-situ ellipsometer for real time analysis of the film growth. The ellipsometer was also used to monitor the injection of BTMS-Te into the chamber by observing its gas phase optical absorption (figure 1b). The realtime plot of the thickness as a function of time, zoomed on the first 10 cycles, reveals an incubation time of 2 cycles followed by film growth (figure 1b). By extending the temporal variable over the entire deposition time we can distinguish two different growth regimes;44 a substrate-dominated regime characterizes the first part of the growth (initial 2 – 3 nm growth); here the growth curve exhibits a clear concavity. This regime is followed by a film-dominated growth where the curve experiences linearity followed by a slight superlinearity toward the end of the deposition, suggesting improved growth rate for GeTe-on-GeTe growth. Such behavior suggests that the deposition surface plays an important role. We calculated an average growth rate of ~ 0.86 A/cycle for the GeTe growth. We measured a GeTe0.7 film stoichiometry (using RBS, RBS calibrated RBS, and XPS) as also observed in previous reports presenting similar chemical approaches;45 this deviation from a Ge1Te1 could result from concurrent reactions such as GeCl2 + GeCl2  Ge + GeCl4.40 The values of the thickness extracted by ellipsometry were confirmed by using X-ray reflectivity (see supplementary information, S2 and S3). The growth of GeTe0.7 is indeed an important building block for the development of GST films but, because of its composition, of minor interest for applications. It is necessary to produce tellurium rich GeTex compounds to realize OTS selectors25,46,47 and to introduce antimony to fabricate high-performing PCM memories. To gain control over the composition of ALD GST films we developed a process for elemental tellurium. BTMS-Te and tellurium ethoxide (Te(OEt)4) were used as precursors to grow metallic tellurium; BTMS-Te and antimony ethoxide (Sb(OEt)3) resulted instead in Sb2Te3.40,43 GeTe, Te, and Sb2Te3 layers were combined in a supercycle fashion to grow films of desired compositions (figure 1d). The film stoichiometry was tuned by adjusting the number of sub-cycles composing each super-cycle. It can been seen that BTMS-Te was


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injected, consecutively, multiple times in the chamber; this approach known as discrete feeding guaranteed better film morphology and improved growth rate (see supplementary information, S4). This approach enabled production of GST films with various compositions producing both OTS and PCM devices (figure 1e).

Figure 2: a) Silicon trench structure (20:1 aspect ratio) used for conformality tests b) SEM image of a silicon trench structure covered with a GeTe4 film. c) SEM image of a silicon trench structure covered with a thin film of Ge2Sb2Te5. Both films show excellent conformality and minimal to no-difference in thickness from the top to the bottom of the trench. d) and e) report the AFM image of the surface of GeTe0.7 and GeTe4 respectively. Surface roughness was found to increase with tellurium content. f) RBS analysis of a GeTe6 film. g) XPS signal for a Ge2Sb2Te5 film.

Figures 2b and 2c show the cross-sectional SEM images for silicon trench structures (20:1 aspect ratio, figure 2a) covered with GeTe4 and Ge2Sb2Te5 respectively. We chose a thickness of 21 nm for GeTe4 and 27 nm for Ge2Sb2Te5, relevant for applications (these thicknesses were used for subsequent electrical testing). The analysis reveals excellent conformality for both compositions. The film thicknesses do not experience appreciable changes when measured at the top, middle, and at the bottom of the features. While the Ge2Sb2Te5 surface looks very smooth and compact, GeTe4 presents some degree of surface roughness. In figure 2d we show an AFM image of the surface morphology of GeTe0.7; figure 2e shows the same analysis performed on a sample richer in Te (GeTe4). The root mean square (RMS) value for the surface roughness increases with the Te content from ~ 0.4 nm to ~ 2.6 nm; such behavior can be attributed to the tendency of Te to produce crystallites even at low temperature (additional analyses are included in the supplementary information, S4). Additional investigations revealed that growth of tellurium rich films on tungsten surfaces (a metal often used as bottom contact for OTS and PCM applications) resulted in smoother films and slightly higher growth rates (see supplementary information,


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S4). The composition of the films was thoroughly characterized using different orthogonal metrologies. Rutherford back scattering (RBS, figure 2f), RBS calibrated XRF, and XPS (figure 2g) were used and confirmed the same stoichiometry. More details are attached in the supplementary information, S6. RBS and XPS were also used to quantify the level of contaminants and impurities such as carbon and chlorine that were found to be negligible (≤ 1%) and further decreased after argon sputtering of the film surface (in the case of XPS analysis). RBS data reveal a not negligible presence of oxygen that is ascribable to surface oxidation.

Figure 3 a) Schematic of the test vehicle used to characterize the GST films. A tungsten plug is connected in series to a poly-silicon line resistor. The GST film is deposited on top by ALD. A TiN top contact is deposited by sputtering and patterned by standard photolithography. b) and c) pulsed-IV curves for a GeTe0.7 and GeTe4 device respectively. GeTe0.7 exhibits a purely resistive behavior while GeTe4 presents clear OTS switching. d) Transient analysis of a GeTe4 OTS selector. The brown line is the external excitation while the yellow line is the device response. e) Leakage current measured at 100 mV for multiple devices as a function of the bottom electrode diameter.

The physical and morphological characterization confirmed the good quality of the ALD chalcogenide films, being conformal, smooth, and with a desired composition. To test the electrical behavior of the films we deposited the chalcogenides on a test vehicle (figure 3a) and subsequently patterned titanium nitride (TiN) top contacts; the resultant structure sandwiches GST between a tungsten plug (with diameter ranging from 200 to 1000 nm) and a much larger TiN top contact (used to apply electrical bias), connected in series with a line resistor embedded in the test vehicle. We first tested films with a composition GeTe0.7 (figure 3b). The I-V curve reveals an ohmic behavior regardless of the maximum voltage applied. We then


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tested tellurium rich compositions (figure 3c); as expected GeTe4 films exhibit clear ovonic threshold switching.25 We confirmed this behavior on a large number of devices and for different bottom contact areas. By analyzing the transient electrical performance of the OTS (figure 3d) a response time of ~ 13 ns can be extracted, comparable with GeTe6 PVD selectors.37,48 The response time of OTS devices has been reported to be limited by the escape time of carriers from traps and therefore bias dependent.49 Figure 3e shows the leakage current measured at 100 mV for the GeTe4 film. We can see that the leakage current scales with different plugs diameters. The film exhibit clear OTS behavior and, as already shown in previous reports considering PVD GeTex films,48 suffers from high leakage currents.

Figure 4 a) Current driven IV of an ALD Ge2Sb2Te5 PCM. The curve shows the characteristic ‘snap-back’ at a threshold voltage VT = 1.4 V. b) Resistance-Voltage characterization and d) pulse scheme utilized. The plot shows film crystallization happening at ~ 1 V and full amorphization for voltages close to 6 V (brown curve). Amorphous and crystalline phases result in resistance difference exceeding two orders of magnitude. Prior to measuring each point in voltage, the film was amorphized (yellow curve) to establish a consistent initial condition. c) SET – RESET cycling of the ALD GST PCM for 1000 times. The device shows a memory window exceeding two orders of magnitude throughout the entire cycling.

We continued our study by testing Ge2Sb2Te5, a composition repeatedly shown to be an excellent PCM memory.50–53 Figure 4a shows a DC-IV curve for a Ge2Sb2Te5 film connected to a ~ 0  resistor (short circuit). The plot reveals the typical behavior of a PCM: an exponential regime (characteristic of trap


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limited current transport in disordered solids and chalcogenide glasses54–56) corresponding to the amorphous phase of the cell is followed by a sharp ‘snap back’ at VT ~ 1.4 V indicating the fast crystallization of the film. Figure 4b shows the film crystallization and amorphization as a function of the applied voltage combining this information in a so called resistance-voltage (RV characteristic). Figure 4d summarizes the excitation wave forms used to measure the RV. A ‘reset’ pulse (amplitude 5.8 V, pulse width 100 ns, rise/fall time 10 ns) is used to melt and re-solidify the film in the amorphous state; this condition is established prior applying any programming pulse in the voltage space so allows direct comparison between each point. The resistance, after every write operation is ‘read’ by using a 0.1 V voltage pulse (10 s pulse width, 1 s rise/fall time) ensuring that the memory state is not perturbed by the reading pulse. Finally the write pulse has amplitude spanning from 0.5 to 6 V (VWRITE) and duration and rise/fall time 100 ns and 10 ns respectively. The plot shows an abrupt transition to the crystalline (set) state taking place ~ VWRITE = 1 V. For applied voltages above ~ 4 V the film experience a gradual amorphization completing its transition at VWRITE ~ 5.8 V (reset state). We note that the times of write and reset are particularly short making the memory suitable for fast operations.57–59 Reset currents for different bottom contact sizes are shown in figure 5a. To confirm the repeatability of the memory switching we cycled the PCM (set - reset) for 1000 times (figure 4c). The GST film shows an excellent memory window exceeding two orders of magnitude. The presented ALD GST PCM shows a crystallization time one order of magnitude shorter and a memory window more than one order of magnitude wider than previously reported ALD PCMs.39 The excellent performance of the ALD Ge2Sb2Te5 films were additionally confirmed by comparing our device with state of the art PVD GST PCMs (figure 5a and b). Our ALD GST films exhibit a reset current density JRESET ~ 107 A/cm2 comparable or smaller than state of the art PVD GST PCMs (figure 5b). Memory window, reset current, and write times of the ALD GST PCMs are in line with the performance of state of the art PVD devices.60


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Figure 5: Reset current a) and reset current density are plotted against bottom electrode area for a number of state-of-the-art PVD GST PCMs;47,61–63 the ALD GST PCM is included for comparison. The ALD PCM compares well with state-of-the-art devices showing comparable or better reset current density confirming the high quality of the ALD film.

CONCLUSION In this letter we presented a method to grow GST chalcogenides by ALD with control over the film stoichiometry. We demonstrated control over the composition spanning an area of the triangular plot enabling fabrication of both OTS and PCM devices. We carried out a thorough physical characterization of the films confirming excellent conformality, film roughness, and low impurity content. We presented a GeTe4 OTS selector exhibiting a short response time of ~ 13 ns. We presented an in-depth electrical characterization of an ALD Ge2Sb2Te5 PCM showing excellent device performance comparable with state of the art PVD memories previously reported. We note that the use of a smaller bottom electrode would greatly benefit the device operation decreasing operating currents, electrical power, and thermal stress. This work poses the foundations for the future development of ultra-high capacity 3D chalcogenide memory arrays. More elements could be easily added to the ALD GST system (e.g. selenium and arsenic) following similar approaches to further improve device performance, especially for ALD OTS selectors.20,64– 66


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METHODS ALD deposition: all ALD films were deposited in a 300 mm wafers compatible custom chamber at 60⁰C and at constant pressure of 500 mTorr. GeTe0.7 was deposited by using BTMS-Te and HGeCl3 as ALD precursors. A needle valve was used to limit the flow of the high-vapor-pressure chlorinated precursor that was kept at 3⁰C by using an external chiller. Elemental tellurium was deposited using BTMS-Te and Te(OEt)4 as the chemical precursors. Three pulses of BTMS-Te (5 s long) divided by a 10 s argon purge and a 9 s Te(OEt)4 injection followed by a 10 s Ar purge were used. The BTMS-Te canister was kept at a controlled temperature of 30⁰C while Te(OEt)4 was maintained at 60⁰C. Antimony was introduced by growing Sb2Te3 by using BTMS-Te (3 pulses 5 s long divided by 10 s long Ar purge) and Sb(OEt)3 (a single 9 s long injection pulse followed by a 10 s Ar purge) as chemical precursors. BTMS-Te and Te(OEt)4 were delivered by using 100 sccm of Ar as a carrier gas. Given their high volatility, HGeCl3 and Sb(OEt)3 were introduced into the chamber by using a vapor draw approach. The recipes here described were used for depositing films on both planar and structured substrates. HGeCl3 yielded ~ 23 nm of film per gram; BTMSTe and Te(OEt)4 had a yield superior than HGeCl3 but was not quantified. Ellipsometry: ellipsometry was carried out using a J.A. Woollam M-200 ellipsometer. Measurements were performed in a 200 -1000 nm wavelength range. Data were analyzed using the complete easy software using a Tauc-Lorentz model. SEM images: SEM images were taken by using a FEI NANOSEM 630 SEM system. XPS analysis: XPS measurements were performed in a K-Alpha Photoelectron Spectrometer System at a 20 eV pass energy XRF measurements: XRF measurements were collected using a Panalytical 2830 ZT wafer analyzer AFM: all AFM measurements were carried out in a Bruker dimension icon AFM system in tapping mode. Device fabrication: the test-vehicle surface was cleaned using forming gas annealing (5 min at 450⁰C) followed by a 30 s 10:1 H2O:HF wet cleaning. The GST film (~30 nm and ~20 nm for the GeTe4 and Ge2Sb2Te5 respectively) was deposited by ALD. The TiN was deposited by reactive sputtering of Ti and N at 25⁰C and 250⁰C pedestal temperature for the OTS and the PCM device respectively. Top contacts were patterned using standard photolithography. TiN was then etched using H2O2 at room temperature for ~


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35 min. The GST film outside the top contact was also removed by using an Ar/CHF3 (70%/30%) plasma etching. Electrical test: all electrical data were collected using a Cascade micro-probe system. DC curves were taken by using a Keysight B1500 Semiconductor Device Parameter Analyzer. During pulsed measurements the time dependent excitation was provided by a B1525A HV-SPGU pulse module while the output signal was collected by using a Keysight DSO6104A digital oscilloscope after been amplified by using a Femto DHPCA-100 high-speed current amplifier large bandwidth operational amplifier.

ACKNOWLEDGEMENT The authors thank Dr. S. Weeks for the useful conversations and his support. Thanks to D. Livingstone and the EQE team at IMI for the technical support. Thanks to R. Huertas, R. Meck, S. Jewhurst, and Y. Stone for their help with metal depositions.

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