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12 September 2008


SAFC Hitech demos device-quality GST precursors for phase-change memory

SAFC Hitech of St Louis, MO, USA (a unit of SAFC within Sigma-Aldrich Group) says that it has made significant progress in developing germanium antimony telluride (GeSbTe, or GST) precursors for high-volume manufacturing of phase-change memory (PCM). Development work has been conducted with both the precursors and with conventional MOCVD techniques, resulting in device-quality GST.

The firm claims that the advances represent a major step toward achieving a commercially viable solution to address the aggressive memory device scaling issues faced by the semiconductor industry to keep pace with Moore’s Law.
PCM, a non-volatile computer memory, takes advantage of the unique behavioral properties of chalcogenide compounds to enable scaling of ultimate feature sizes further than possible with conventional Flash memories, boosting storage capacity and performance. Chalcogenide compounds such as GST are attractive materials for PCM, and have already been used as the basis for optical storage media and prototype PCM devices.

“Until now, PCM materials have generally been deposited by sputtering or other physical vapor deposition (PVD) techniques, which are line-of-sight methods and have inherent weaknesses relating to uniformity of deposition,” says SAFC Hitech’s chief technology officer Ravi Kanjolia. “Vapor phase deposition techniques such as MOCVD offer several advantages in relation to GST precursors, in particular a better step coverage for deposition on patterned substrates, industrial scaling, and high compositional control,” he adds. “Furthermore, we have achieved advances in precursor chemistries that allow similar layers to be deposited using conventional MOCVD, without the need for an activation process.”

Since 2005, SAFC Hitech has been a participant in the European Commission-supported CHEMAPH project, a consortium set up to look at deposition methods for GST films. Researchers at the firm’s Bromborough facility in the UK have been investigating a variety of GST sources suitable for MOCVD, and have matched the physical properties of each metal precursor to enhance efficiencies at the desired growth parameters.

“Variations in cracking efficiencies were one major hurdle that we had to overcome,” explains Kanjolia. “After extensive work to synthesise a number of different chemicals and characterize their physical properties, a combination of sources was found with a much improved match of thermal stability to allow decomposition to the same degree when simultaneously introduced to the deposition reactor chamber.”

The preferred chemicals were found to be Ge(NMe2)4, Sb(NMe2)3 and iPr2Te. SAFC Hitech then developed synthesis protocols to allow the isolation of high-purity product in both small and large laboratory-scale equipments. These materials are now commercially available, with quality guaranteed by in-house analysis. Samples have been shipped to various centers, and collaborations with partners have tested the different combinations. Recent growth trials have resulted in the deposition of device-quality GST using nitrogen as a carrier gas, says SAFC Hitech.  

“While a full process to make MOCVD devices remains to be demonstrated on anything other than very small research structures, the quality of the films on flat substrates is improving, and the precursor chemistry is ideally suited,” concludes Kanjolia. “The next challenge is to get the correct parameters in place to control the growth and lay down the correct layers in the correct structure. The temperature window with this process remains critical, and highlights both the difficulties associated with this system and the need for advanced precursors to move forward with integration into future phase-change memory applications,” he adds. “We are confident that our sources will allow the development of next-generation devices to maintain the speed of performance enhancement required to meet market targets.”

See related item:

SAFC Hitech launches vapor-phase MO distribution system and large-scale bubbler

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