AVS 66th International Symposium & Exhibition
    Plasma Science and Technology Division Tuesday Sessions
       Session PS+EM-TuA

Invited Paper PS+EM-TuA7
Challenges in High-aspect-ratio Hole Etching for 3D Flash Memory

Tuesday, October 22, 2019, 4:20 pm, Room B131

Session: Advanced BEOL/Interconnect Etching and Advanced Memory and Patterning
Presenter: Mitsuhiro Omura, Kioxia Corporation, Japan
Authors: M. Omura, Kioxia Corporation, Japan
J. Hashimoto, Kioxia Corporation, Japan
T. Adachi, Kioxia Corporation, Japan
Y. Kondo, Kioxia Corporation, Japan
M. Ishikawa, Kioxia Corporation, Japan
J. Abe, Kioxia Corporation, Japan
I. Sakai, Kioxia Corporation, Japan
H. Hayashi, Kioxia Corporation, Japan
Correspondent: Click to Email
Memory devices with higher bit density are required for effective use of big data in the internet of things era, and 3D memory architecture is required. 3D flash memory encompasses numerous pillars that punch through control gate plates, and cells are arranged along the pillars [1]. We refer to each pillar as a memory hole. Memory holes are fabricated by dry etching of stacked films, which are generally constructed of dozens to several hundred pairs of SiO2/Si or SiO2/Si3N4 films. Therefore, the aspect ratio of a memory hole reaches several tens. Moreover, the critical dimension and profile of a memory hole must be strictly controlled because these features at each control gate plate directly affect the characteristics of memory cell. Therefore, the key technology of 3D flash memory is a high aspect ratio (HAR) hole etching process. However, the dry etching process of HAR holes has a variety of profile issues, including bowing, shape distortion, twisting of the hole profile, and striation.

In this study, sidewall striation formation in a HAR hole was investigated. In spite of the smooth morphology of the mask, sidewall striation was observed on dielectric films. Results from the carbon mask sample treated with several gas plasmas implies that ion irradiation can increase the degree of striation on the carbon mask, and striation tends to be suppressed by deposition of a fluorocarbon film from fluorocarbon radicals. An ion beam experiment with a simulated hole sidewall using blanket films shows that striation tends to form on the fluorocarbon film rather than on SiO2 and Si3N4 films. In connection with this result, the shallower region with striation had thicker fluorocarbon film than the deeper region with smooth sidewall. Therefore, the possible of sidewall striation formation mechanism is as follows. When the etching depth of the HAR holes reaches a certain depth, striation forms on the deposited fluorocarbon film and is transferred to the dielectric films laterally as the hole diameter increases. The region with striation depends on the aspect ratio, defined as the depth divided by the neck width of the carbon mask. Consequently, as etching progresses, the mask thickness decreases and striation forms in a deeper region, depending on the aspect ratio.

References

[1] H. Tanaka et al., Symposium on VLSI Technical Digest, 14 (2007).