Creating phase-change memory devices with GeTe thin films
11/01/2011
Julien Vitiello, Altatech Semiconductor S.A. Montbonnot-Saint-Martin, France
With advantages including low cost, low programming voltage and excellent scalability to nanoscale cell sizes, phase-change memory (PCM) is one of the promising candidates for next-generation nonvolatile memory applications [1]. The technology is based on fast and reversible phase-change effects in chalcogenide materials (such as sulfides, selenides and tellurides), causing them to switch between crystalline and amorphous states. But the inherent instability of these materials in their amorphous states reduces the archival life of the memory cell. This presents a critical problem for embedded applications in which working temperatures can be very high and data must be retained over long periods of time.
In a joint development project with STMicroelectronics, CEA-LETI and the CNRS-LTM laboratory in Grenoble, France, equipment supplier Altatech Semiconductor used its pulsed liquid-precursor, metal-organic CVD (MOCVD) technology to fill confined areas in PCM devices ??? including trenches narrower than 50nm with aspect ratios higher than 2:1 ??? with the chalcogenide material GeTe, an alternative to today's commonly used Ge2Sb2Te5. Because GeTe can be changed from its crystal phase to its amorphous phase by applying low voltages, it can be used as a phase-change material in tight spaces [2]. In addition, this compound material has been shown to withstand higher phase-change temperatures than Ge2Sb2Te5 while also demonstrating the durability to last a decade at 105??C without failure [3].
Optimizing film properties
The AltaCVD tool was used to create bilayer GeTe films by applying Ge and Te as liquid precursors. As with most alloys, the stoichiometry of GeTe is a key factor in determining how the resulting thin films behave. In this project, Ge and Te precursors were controlled separately and kept in their liquid states until just prior to vaporization, which is performed very close to the point of use in the CVD reactor. With the CVD tool's pulsed injection system, each amount of the two liquid precursors was tightly and separately controlled, using a wide allowable frequency scale and injection time in the millisecond range. One example is presented in Table 1.
| Table 1. GeTe stoichiometry tuning. |
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The injection time was the same for each precursor, and the frequency was adjusted to balance the Ge/Te ratio. Thin-film characterization was performed using x-ray photoelectron spectroscopy (XPS) measurements in "quasi" in situ mode, meaning there was no vacuum break between the deposition chamber and the XPS chamber. This was done to minimize any chances of film contamination.
Tuning the process
While optimizing the Ge/Te ratio is the foundation for any process development, the ability to protect the precursors from contamination also has a significant impact on MOCVD deposition. Most metal-organic precursors induce carbon contaminants in deposited films, and GeTe is no exception. Using XPS measurements with surface-etching capability, the presence of in-film carbon contamination can easily be seen (Fig. 1). In this case, we found a strong correlation between the amount of trapped carbon and the transition properties of GeTe layers: the higher the carbon content, the higher the transition temperature needed to be. As shown in Fig. 1, the transition state between amorphous and crystalline phase can be determined by measuring optical reflectivity. Therefore, controlling the degree of carbon contamination can be used as a way to tune the final film's properties.
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| Figure 1. a) Carbon contamination measurement (left graph) and b) its effect on GeTe transition states (right graph) |
Because GeTe layers have relatively low crystalline-to-amorphous transition temperatures of <400??C, CVD and plasma-enhanced CVD methods allow films to be deposited without further annealing. Thus, crystalline films can be directly obtained when the deposition temperature is higher than 300??C (Fig. 2, left image). Since film-growing behavior is mostly in the nucleation/growth regime, the surface roughness of such layers has to be considered. Grain growth limits the process' ability to be transferred from full-sheet deposition to patterned structures while maintaining acceptable conformality.
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| Figure 2. MEB images showing surface morphology for a) crystalline CVD deposition (left image) and b) amorphous PECVD (right image) of GeTe layers. | |
At temperatures below 200??C, the CVD deposition is amorphous. Therefore, surface roughness and conformality are no longer concerns. But low-temperature processing induces high levels of trapped contaminants so films exhibit poor quality, particularly regarding their transition behavior. Plasma enhancement helps to overcome this problem with low-temperature processing. Adding energy to the deposition process, via high-frequency plasma, helps to achieve good film properties and smooth layers (Fig. 2, right image).
Ready for prime time
This project showed that GeTe deposition can be a productive process in applications such as PCM devices where the ability to fill small structures is mandatory. By maintaining a precise Ge/Te ratio and a well-controlled vaporization flow, plasma-enhanced MOCVD thin films can achieve performances that parallel those of physical vapor deposition (PVD) films. In addition, the combination of plasma-enhanced deposition and controlled precursor delivery on a wafer's surface results in a larger process window, compared to PVD processing.
While MOCVD deposition of GeTe has demonstrated very useful results in process development for advanced material integration, this technology also can be combined with a standard wafer-handling system for use in a manufacturing environment. In either engineering or production, liquid-precursor deposition provides a beneficial approach to creating GeTe thin films for PCM embedded applications.
Acknowledgments
AltaCVD is a registered trademark of Altatech Semiconductor S.A.
The author would like to thank partners P. Michallon, E. Gourvest, C. Vall??e, D. Jourde, S. Lhostis, B. Pelissier, C. Bourasseau and A.L. Savin, and the French Ministry for their contributions to this project.
References
1. S. J. Hudgens, "The future of phase-change semiconductor memory devices," J. Non-Crys. Solids, 354, 2748 (2008).
2. S. L. Cho et al., "Highly scalable on-axis confined cell structure for high-density PRAM beyond 256 Mb," Symp. VLSI Tech. Dig. (2005), p. 96.
3. L. Perniola et al., "Electrical behavior of phase-change memory cells based on GeTe," IEEE Electron Device Lett., 31, 5 (2010), pp. 488-490.
Julien Vitiello received his MS in materials science and engineering and his PhD in integrated electronic devices from Institut National des Sciences Appliqu??es (INSA) in Lyon, France, and is the NanoDeposition Group Manager at Altatech Semiconductor S.A., 611 rue Aristide Berg??s, Z.A. de Pr?? Millet, 38330 Montbonnot-Saint-Martin, France; ph.: +33 (0)4 56 52 68 00; email [email protected]
Solid State Technology | Volume 54 | Issue 10 | November 2011
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