Testing integrated circuits is an activity that does not always receive the attention it deserves when describing the overall process of chip manufacturing. Many persons, including some of those directly involved in this business, think that once a chip is properly designed and manufactured using a stable technology, it should work, period. Well, that is unfortunately not true. Each and every unit must be carefully checked, often multiple times, before it can reach its final user. And this activity prevents many issues, ranging from inconsequential to catastrophic, that may otherwise emerge later, once the unit sits in a system. Testing also compensates for small differences in behavior of different units, making the underneath technology look more uniform than what it is in reality. Those who recognize the importance of testing often turn completely upside
down the common understanding mentioned earlier, stating that the yield is determined by a good testing methodology. Whether this is true or not, a lot of technology and money are involved in this activity, along with a lot of effort from a number of engineers, who often play a hidden role unlike their design or process colleagues. Without any wish to be exhaustive, this chapter aims to give an overview of the instruments and techniques used to test a nonvolatile memory chip. 9.1 IntroductionFlash memory is ubiquitous. Every PC, tablet, and smartphone contains a flash memory integrated circuit (IC). The continuous drop in flash memory cost per bit is transforming the hard disk market by replacing traditional drives with faster and less-power-consuming solid-state storage devices, or solid-state drives (SDDs). At the same time, since flash memory manufacturing is so unique and has such a high cost and more complex tuning compared to other IC types, only a small number of companies worldwide can afford to do it. This means that only a handful of companies produce and test a tremendous quantity of wafers to meet the market’s growing demand, while simultaneously working to achieve advanced levels of technology in order to maintain a competitive advantage over the other players. In recent years NAND has overcome NOR architecture in flash memory products (Fig. 9.1), and this is in in large part due to a more efficient use of the silicon real estate. While the floating gate technology in production today is close to reaching its limits, companies are trying to store more bits in each memory cell (2-3 bits per cell) and simultaneously conducting research on 3D technology solutions. The drawback of storing more bits per cell is poorer data retention and endurance. A strong dependence on an external controller to cope with the cells’ degraded performance has nowadays become the rule. Memory is so different compared to other types of ICs that any company that wasn’t born exclusively as a memory maker (Intel, AMD, ST, Siemens, etc.) at some point has had to spin out new companies focused on memory. This chapter is not focused on flash memory technology or design (a complete introduction to the subject can be
found in Refs. [1, 2]) but specifically on what is needed to test it, both in terms of equipment and in terms of internal resources.