Phase-change properties of GeSbTe thin films deposited by plasma-enchanced atomic layer depositon
© Song et al.; licensee Springer. 2015
Received: 16 November 2014
Accepted: 12 February 2015
Published: 28 February 2015
Phase-change access memory (PCM) appears to be the strongest candidate for next-generation high-density nonvolatile memory. The fabrication of ultrahigh-density PCM depends heavily on the thin-film growth technique for the phase-changing chalcogenide material. In this study, Ge2Sb2Te5 (GST) and GeSb8Te thin films were deposited by plasma-enhanced atomic layer deposition (ALD) method using Ge [(CH3)2 N]4, Sb [(CH3)2 N]3, Te(C4H9)2 as precursors and plasma-activated H2 gas as reducing agent of the metallorganic precursors. Compared with GST-based device, GeSb8Te-based device exhibits a faster switching speed and reduced reset voltage, which is attributed to the growth-dominated crystallization mechanism of the Sb-rich GeSb8Te films. These results show that ALD is an attractive method for preparation of phase-change materials.
KeywordsPhase-change memory Atomic layer deposition Microstructure Electric properties
Phase-change memory (PCM) has been regarded as one of the most promising nonvolatile memories for the next generation because of the advantages of high speed, low power, good endurance, high scalability, and fabrication compatibility with complementary metal-oxide semiconductor (CMOS) process [1-4]. PCM uses the reversible phase change between the crystalline and amorphous states of chalcogenide materials brought about by Joule heating. Ternary Ge2Sb2Te5 (GST) compounds are widely regarded as the most commercially viable and practical phase-change family of materials for this application. These materials are currently being used commercially, and processes which deposit GST films by RF sputtering are being implemented into production lines .
In PCM cells, the high level of reset current, which is required for switching the GST material from the crystalline to the amorphous state in a planar cell structure, has been the major obstacle to the further scaling of PCM because of the limited on-current drive capability of the cell transistor . It has been reported that a confined cell structure where the phase-change material is formed inside a contact via is expected to be essential for the next-generation PCM device because it requires lower switching power [7,8]. This structure requires more complex deposition of the active chalcogenide into a very small cell pore. However, it can be easily anticipated that the fabrication of this confined structure is not possible using the conventional sputtering process for the GST film deposition due to its inherent poor step coverage. Therefore, it is necessary to deposit the GST film using a process that offers good conformality in terms of its thickness as well as its chemical composition, such as atomic layer deposition (ALD) or chemical vapor deposition (CVD).
Recently, a number of studies have been carried out to investigate the deposition of phase-change materials by ALD and CVD techniques [9-15]. It has been reported that the precursor and substrates have an important influence on the microstructure and composition of Ge-Sb-Te films. Mikko Ritala et al. reported Sb2Te3, GeTe, and GST films were deposited by ALD at remarkably low temperature of 90°C using (Et3Si)2Te, SbCl3, and GeCl2 · C4H8O2 as precursors . Byung Joon Choi et al. reported the different nucleation and growth behaviors of the GST films deposited by the combined plasma-enhanced CVD/ALD on various types of substrates. The nucleation of the GST films on the SiO2, Si3N4, and ZrO2 substrates was seriously retarded compared to those on the TiN and TiO2 substrates . Adulfas Abrutis et al. reported the deposition of smooth GST films by using a hot-wire CVD technique . Most of these works focus on the microstructure properties and the deposition process. However, the electric properties and switching properties of ALD/CVD-deposited phase-change materials (especially Sb-rich GeSbTe films) have rarely been reported in literature. In this study, ALD of Sb2Te3, GST, and GeSb8Te thin films were attempted with Ge [(CH3)2 N]4, Sb [(CH3)2 N]3, Te(C4H9)2 as Ge, Sb, and Te precursors, respectively, and plasma-activated H2 gas as reducing agent of the metallorganic precursors. The microstructural and electric properties of these materials have been studied.
The Sb2Te3 and Ge-Sb-Te films were deposited on Si3N4/Si substrates using a plasma-enhanced ALD reactor (Beneq TFS 500 ALD system) at wafer temperatures ranging from 190 to 250°C. The Si3N4 films were prepared by inductively coupled plasma CVD on Si substrate at temperature of 130°C. The SiH4 and NH3 were used as reactive gases. Ge [(CH3)2 N]4, Sb [(CH3)2 N]3, and Te(C4H9)2 were used as the Ge, Sb, and Te precursors, respectively. Each precursor deposition cycle for the Ge, Sb, and Te process was composed of four consecutive pulses: (i) a pulse of precursor vapor, (ii) a purge pulse, (iii) a pulse for exposure to H2 plasma, and (iv) another purge pulse. The sequence of precursor pulses was Sb-Te-Ge. To control the cation composition ratio in Ge-Sb-Te films, the pulse ratio of precursor cycles was changed. During the H2 plasma pulse period, a radio frequency (rf) plasma was applied (rf power of 150 W, rf frequency of 13.56 MHz) to the reaction chamber. The growth rates of GST and Sb2Te3 films are about 0.2 and 0.1 nm/cycle, respectively. The uniformity of Sb2Te3 films was very good, but the uniformity of GST was very sensitive to the deposition process. Similar results have been found by other researchers . The stoichiometry of the deposited films was confirmed by electron dispersive spectroscopy (EDS). The thickness and microstructure of the films were determined by field-emission scanning electron microscopy (FESEM). T-shaped PCM cells are fabricated using 0.18 μm CMOS technology to verify the electrical switching behaviors of the alloys. The bottom W electrode with diameter of 260 nm is covered by phase-change films of approximately 100 nm thickness, while TiN (20 nm) and Al (300 nm) are deposited sequentially as top electrodes. For comparison, physical vapor deposition (PVD) GST film was also fabricated with the same structure. The current–voltage (I-V) and resistance-voltage (R-V) characteristics of the PCM cells are monitored employing an arbitrary waveform generator (Tektronix AWG5002B) and a Keithley-2400 meter.
Results and discussion
In this study, GST and GeSb8Te thin films were deposited by plasma-enhanced ALD method using Ge [(CH3)2 N]4, Sb [(CH3)2 N]3, Te(C4H9)2 as precursors and plasma-activated H2 gas as reducing agent of the metallorganic precursors. The measured set speed of the ALD-deposited GST films is slower than that of PVD-deposited GST films, which is supposedly due to the N, C impurities. Compared with GST-based device, GeSb8Te-based device exhibits a faster switching speed and reduced reset voltage, which is attributed to the growth-dominated crystallization mechanism of the Sb-rich GeSb8Te films. These results show that ALD is an attractive method for preparation of phase-change materials.
This work was supported by the ‘Strategic Priority Research Program’ of the Chinese Academy of Sciences (XDA09020402), National Key Basic Research Program of China (2013CBA01900, 2011CBA00607, 2011CB932804), National Integrate Circuit Research Program of China (2009ZX02023-003), National Natural Science Foundation of China (61261160500, 61376006, 51201178), and Science and Technology Council of Shanghai (13DZ2295700, 13ZR1447200, 14ZR1447500, 14DZ2294900).
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