Study of Retention Characteristics and Program Speed Enhancement Using Plasma Nitridation on Interpoly Dielectric

Abstract

The interpoly dielectric ͑SiO 2 -SiN-SiO 2 ͒ in floating gate NAND flash memory, as it is used to bolster both programming speed and data retention, has been extensively investigated in studies examining plasma nitridation technologies. The bird's beak, as found on an interpoly dielectric edge, likely contributes to a degradation program/erase performance and reliability in cell operation and is abnormally increased above 12 Å during gate sidewall oxidation. This phenomenon can be effectively reduced to less than 1.5 Å through the application of nitrogen plasma nitridation on the floating gate and on the top oxide of the interpoly dielectric. In terms of control gate coupling capability and program voltage, both the physical and electrical properties of the interpoly dielectric can improve by 6% and 1.2 V, respectively. Nevertheless, this study found that data retention problems resulting from plasma nitridation, a process which induces the oxynitride remaining in shallow trench isolation, are the source of electron leakage through the word line. To reduce charging loss and sustain the coupling capabilities of the control gate, selective oxynitride removal with diluted HF is proposed. This interpoly dielectric can enhance memory characteristics and also extends the functional limitation of the NAND flash memory to the 40 nm generation.Nonvolatile memories have been growing rapidly in recent years due to the increasing demands of portable and embedded devices. Among them, floating gate ͑FG͒ NAND flash is a sequential access device used for mass storage applications. It achieves a higher density at a lower cost per bit, making it ideal for MP3 players and digital cameras as well as applications requiring mass storage of data. However, as the devices have become smaller, problems with the process and the fundamental physical limitations of the FG NAND flash memory cells have risen. First, the abnormal increase in gate oxide and interpoly dielectric ͑IPD͒ thickness degrades the intrinsic oxide reliability during thermal oxidation processes. 1,2 To address these problems, nitrogen was introduced on the polydielectric interface to eliminate gate oxide and IPD deterioration. However, a precise nitrogen profile is difficult to control using a conventional rapid thermal process ͑RTP͒, which may induce device failure. 3,4 Second, in the past decade, the IPD film has become a critical process in the dimensions of an aggressive device. To achieve a high control gate coupling ratio ͑GCR͒ without sacrificing IPD capabilities, the critical thickness of the IPD stack films should be further reduced to maintain the GCR during dimension shrinkage. 5,6 However, the current leaking through the IPD film has been found to be the primary factor that limits reliability. 7-9 To reduce the leakage current or direct tunneling current, a small equivalent oxide thickness ͑EOT͒ with a higher dielectric constant value is required to enhance the GCR and programming speed and to address the reliability problems.The introduction of stronger Si-N ...