Metal Gate Recessed Access Device (RAD) for DRAM Scaling
A functional DRAM with higher data retention characteristics than a planar access device has been demonstrated, using a metal gate recessed access device (RAD). Chemical vapor deposition (CVD) and atomic layer deposition (ALD) were used to deposit titanium nitride (TiN) and tantalum nitride (TaN), respectively. CVD TiN and ALD TaN-CVD TiN laminate gate stacks were integrated with a RAD module. ALD TaN-CVD TiN laminate gates showed enhanced drive current (IDS), higher transconductance (GM), higher mobility (¿EFF) and reduced off current (IOFF) characteristics compared to CVD TiN gates. Device characteristics and reliability data for both the planar devices and RADs are presented. The ALD TaN-CVD TiN laminate metal gate RAD showed much improved data retention characteristics compared to a conventional planar device with a poly silicon gate. The optimum thickness of ALD TaN in the laminate stack is discussed.
Roles of Residual Stress in Dynamic Refresh Failure of a Buried-Recessed-Channel-Array Transistor (B-CAT) in DRAM
We clarify the role of metal gates (e.g., TiN) on the degradation of the state-of-the-art buried-channel-array transistor (B-CAT) in dynamic random access memory (DRAM) chips. Unless the thermal budget during the processing step for integration is well controlled, residual stress caused by grain growth of the metal gate can result in a dynamic refresh failure of B-CAT through the negative shift in threshold voltage (-ΔVth). A hole trapping model is proposed to explain this phenomenon. Uncontrolled grain growth of the metal gate increases the residual stress level on SiO2, and, consequently, it breaks the strained Si-O-Si bonds, which can serve as precursor sites for incoming holes. Residual stress in the three-dimensional transistor architecture, therefore, must be well controlled to improve the reliability of commercial DRAM chips.
Nuclear sphingolipid metabolism.
Nuclear lipid metabolism is implicated in various processes, including transcription, splicing, and DNA repair. Sphingolipids play roles in numerous cellular functions, and an emerging body of literature has identified roles for these lipid mediators in distinct nuclear processes. Different sphingolipid species are localized in various subnuclear domains, including chromatin, the nuclear matrix, and the nuclear envelope, where sphingolipids exert specific regulatory and structural functions. Sphingomyelin, the most abundant nuclear sphingolipid, plays both structural and regulatory roles in chromatin assembly and dynamics in addition to being an integral component of the nuclear matrix. Sphingosine-1-phosphate modulates histone acetylation, sphingosine is a ligand for steroidogenic factor 1, and nuclear accumulation of ceramide has been implicated in apoptosis. Finally, nuclear membrane-associated ganglioside GM1 plays a pivotal role in Ca(2+) homeostasis. This review highlights research on the factors that control nuclear sphingolipid metabolism and summarizes the roles of these lipids in various nuclear processes.