S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Materials Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
Si1-xGex crystallization for vertical channel in VNAND
수직 구조 낸드 플래시 메모리 내 수직 채널 적용 목적으로의 Si1-xGex 결정화 연구
- 공과대학 재료공학부
- Issue Date
- 서울대학교 대학원
- Vertical NAND flash memory (VNAND); Solid phase crystallization (SPC); Grain growth; Si1-xGex; In situ transmission electron microscopy (TEM); Transmission electron microscopy (TEM)
- 학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2015. 2. 윤의준.
- Recently, the three-dimensional vertical NAND (VNAND) flash memory structure is developed to overcome the scaling limit and degree of integration issues of conventional two-dimensional planar NAND flash memory. The solid phase crystallization is used as a crystallization technique for vertical channel in VNAND. However, the solid phase crystallized film has fatal limitations related to the electrical properties when used as a channel material due to the high density of grain boundaries and intra-grain defects in the microstructure. Especially, it is expected that the string current degradation as the number of cell layers is increased for higher bit densities in the next-generation VNAND. Therefore, having a high quality poly-crystalline channel that is large-grained and less-defective microstructure is very important for next-generation VNAND as well as devices currently in use.
In this study, the Si1-xGex/Si bi-layer structure is proposed in order to obtain the high quality poly-crystalline Si channel microstructure. This bi-layer structure is simple, easy and directly applicable to the VNAND process. The bi-layer shows Si1-xGex surface nucleation and induces equiaxial grains that leads to large-grained and less-defective microstructure in the Si channel layer. Furthermore, the Si1-xGex lateral growth method is proposed based on the crystallization behavior of Si1-xGex bi-layer in order to obtain even large grains in the Si channel layer. The Ge/Si bi-layer is introduced to maximize the lateral growth effect. It was confirmed that the Ge layers are crystallized through the lateral growth only without any nucleation. After Si growth, the laterally grown grains are shown through the Ge and Si layers. These results mean that the very large Si grains over film thickness can be obtained after the Ge nucleation layer stripping. Accordingly, using a bi-layer as a channel structure is expected to improve the electrical properties of each cells and to minimize the degradation of string current in VNAND.
Additionally, the poly-crystalline Ge single layer is proposed as a vertical channel structure in VNAND. The secondary grain growth which shows the very larger grains over film thicknesses is applied as a crystallization technique. The grains three or four time larger than adjacent grains and over film thickness are detected after annealing near VNAND thermal budget. Besides, the two-step annealing was proposed combining different grain growth kinetics in order to enlarge the grain size in a given thermal budget. The two-step annealed films showed greatly increased hole mobility compare to conventional single-step annealing and expected microstructural evolution was also discussed.