Effect of abrasive material properties on polishing rate selectivity of nitrogen-doped Ge 2 Sb 2 Te 5 to SiO 2 film in c
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Ungyu Paikb) Department of Ceramic Engineering, Hanyang University, Seoul 133-791, Korea (Received 20 June 2008; accepted 15 August 2008)
We investigated the polishing rate and selectivity of nitrogen-doped Ge2Sb2Te5 (NGST) to SiO2 film for different abrasive materials (colloidal silica, fumed silica, and ceria abrasives). They both were strongly dependant on abrasive material properties. The polishing rate of nitrogen-doped NGST decreased in the order ceria, fumed silica, and colloidal silica abrasives, which was determined by abrasive material properties, such as abrasive hardness, crystal structure, and primary and secondary abrasive sizes. In addition, the polishing rate slope of NGST film was not significantly different for different abrasive materials, indicating that the polishing of NGST film is mechanical dominant polishing. In contrast, the polishing rate slope of SiO2 film decreased in the order ceria, fumed silica, and colloidal silica abrasives, indicating that the polishing of SiO2 film is chemical dominant polishing. Furthermore, the difference in polishing rate slopes between NGST and SiO2 film gave a polishing rate selectivity of NGST to SiO2 film higher than 100:1 with colloidal silica abrasive. I. INTRODUCTION
Phase-change random-access memory (PRAM) is one candidate for next-generation nonvolatile memory because of its large sensing signal, moderately fast writing time, good endurance for repetitive writing, and wellmatched fabrication process with Si technologies.1,2 Ge2Sb2Te5 (GST) chalcogenide material has been used for its capability as a phase change memory material.3–6 In particular, nitrogen-doped Ge 2 Sb 2 Te 5 (NGST) memory elements have been demonstrated to have smaller reset current and enhanced endurance because the incorporated nitrogen suppresses grain growth.7–10 This suppression of crystalline grain growth can reduce the reamorphization current and enhance the endurance properties of PRAM devices.11 To reduce the contact volume between GST and the electrode by minimizing the phase changing volume of GST, the PRAM memory cell structure was changed from a planar (i.e., T-shaped) structure fabricated by physical vapor deposition (PVD) to a confined structure fabricated by chemical vapor deposition (CVD). The advantages of the confined memory cell structure include a simple integration
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2008.0397 J. Mater. Res., Vol. 23, No. 12, Dec 2008
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process, a lower reset current, and lower thermal crosstalk.12,13 The use of a confined memory cell structure makes the chemical mechanical polishing (CMP) of GST film an essential process for avoiding etching damage and facilitating easy mass production of PRAM devices. CMP is a major process for achieving local and global planarity of integrated devices in the manufacture of integrated circuits (ICs) with a feature size of 0.25 m or less. The technique has been cont
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