Sang-seok Kang

Bit line coupling scheme and electrical fuse circuit for reliable operation of high density DRAM

Two design techniques are presented to improve the yield of high density DRAM product. One is bit line coupling (BLC) scheme and the other is electrical fuse (E-Fuse) circuit for reliable field programmable repair scheme. We obtain an improvement of 100 ms for the data retention time (tREF) using the BLC scheme. BLC scheme also improves the low VCC margin by 0.3 V and the RAS to CAS delay time (tRCD) by 1.5 ns. Differential current evaluation for the E-fuse implementation shows polysilicon fuse fail rate <10/sup -12/.

A Prevenient Voltage Stress Test Method for High Density Memory

The most effective acceleration factor of reliability is the high voltage stress. However high electric field generated on thin gate oxide transistors in nanometer technology becomes the uppermost limit. In this paper, an improved voltage stress method for DRAM with the 6F2 structure and the open bit line scheme is proposed to enhance the Early Life Failure Rates (ELFR) and the yield of package test. The proposed method reduces the degradation of transistors caused by a high voltage stress. Experimental results show that the proposed method improves the yield of package test and the characteristic of refresh, and avoids the degradation of transistors using voltage ramp stress (VRS).

A New Wafer Level Latent Defect Screening Methodology for Highly Reliable DRAM Using a Response Surface Method

Screening latent defects in a wafer test process is very important task in both reducing memory manufacturing cost and enhancing the reliability of emerging package products such as SIP, MCP, and WSP. In terms of the package assembly cost, these package products are required to adopt the KGD (known good die) quality level. However, the KGD requires a long burn-in time, added testing time, and high cost equipments. To alleviate these problems, this paper presents a statistical wafer burn-in methodology for the latent defect screen in the wafer test process. The newly proposed methodology consists of a defect-based wafer burn-in (DB-WBI) stress method based on DRAM operation characteristics and a statistical stress optimization method using RSM (response surface method) on the DRAM manufacturing test process. Experimental data shows that package test yields in the immature fabrication process improved by up to 6%. In addition, experimental results show that the proposed methodology can guarantee reliability requirements with a shortened package burn-in time. In conclusion, this methodology realizes a simplified manufacturing test process supporting time to market with high reliability.