SAFC Hitech Advances on Phase Change Memory Front
Staff -- Semiconductor International, 9/4/2008 10:19:00 AM
SAFC Hitech (St. Louis) said it has made “significant progress” in developing germanium antimony telluride (GST) precursors for use in high-volume manufacturing of phase change memories (PCMs).
SAFC Hitech researchers have been developing GST precursors for deposition with conventional metal-organic chemical vapor deposition (MOCVD) techniques. SAFC Hitech Chief Technology Officer Ravi Kanjolia said, “Until now, PCM materials have generally been deposited by sputtering or other physical vapor deposition (PVD) techniques, which are line-of-sight methods with inherent weaknesses relating to uniformity of deposition.”
Vapor phase deposition techniques, such as MOCVD, offer several advantages for GST precursors, including better step coverage for deposition on patterned substrates, industrial scaling, and high compositional control. “We have achieved advances in precursor chemistries that allow similar layers to be deposited using conventional MOCVD, without the need for an activation process,” he said.
Since 2005, SAFC Hitech has been participating in the European Commission-supported CHEMAPH project, a consortium set up to look at deposition methods for GST films. Researchers at the company’s Bromborough, U.K. facility 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,” Kanjolia said. After synthesizing a number of different chemicals and characterizing their physical properties, a combination of sources was found with improved thermal stability to allow decomposition to the same degree when simultaneously introduced to the deposition reactor chamber.
The actual chemicals of choice 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 equipment. Recent growth trials have resulted in “successful deposition of device-quality GST,” using nitrogen as a carrier gas, the company said.
The materials are now available to customers, following collaborations with partners to test the different combinations.
Kanjolia said, “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.”
Going forward, the researchers will seek to control the growth, with 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 said.
