Cast: An electrical stress test to monitor single bit failures in flash-EEPROM structures

To increase yield and reliability of flash-EEPROM devices, great effort has been devoted to improve the way of monitoring the tunnel oxide quality, both as regards the electrical measurements and the related test structures. The most popular test is an electrical stress to evaluate the charge or the electric field at breakdown on large area or edge intensive capacitors. Although the capacitor test is a fundamental means to evaluate the oxide quality, it can not detect the subtle defects which are responsible for the most likely flash-EEPROM failure mechanisms such as the single bit over-erasure or failure after an electrical stress. To overcome this difficulty the cell array stress test (CAST, patent pending) has been conceived: by means of a suitable test structure it is possible to detect defective cells in a flash-EEPROM array. Correlation with actual flash EEPROM yield is demonstrated. This test can be used either as a short-loop monitor for process control and improvement or as an end-of-process wafer level screening. Copyright (C) 1996 Elsevier Science Ltd