Nonvolatile memory test structure and nonvolatile memory reliability

Abstract

A test structure is formed by an array of memory cells connected in parallel and including each a memory transistor and a select transistor connected in series. The gate terminals of the select transistors of all memory cells are biased to a value next to the threshold voltage of the select transistors. Therefore, in each memory cell, the drain current is limited by the memory transistor for control gate voltages below the threshold voltage of the memory transistor, and by the select transistor at higher voltages; for high control gate voltages, the drain current is clamped to a constant maximum value. Since the clamping effect of the select transistors acts on each memory cell, the total maximum current of the test structure may be held below a value causing a limitation in the current generated by the entire array because of the resistance in series to the output of the test structure. Thus also the right side of the threshold distribution may be evaluated and the presence of defective cells causing injection of electrons in the floating gate of the memory transistors may be detected.