MBIST-based weak bit screening method for embedded MRAM

Magnetoresistive random access memory (MRAM) is an attractive option to replace eFlash. The recent demonstration of a nano-second write speed and a 10e14 endurance are compelling performances even as an embedded MRAM for cache replacement. Both eFlash and cache applications often use large array sizes, which require tight defect control. The unique defects in MRAMs that are not easily detectable with traditional memory test algorithms can potentially cause test escapes. Test escapes will not only delay the manufacturing process but also cause reliability issues, which is fatal for safety-critical applications such as automotive. This paper presents effective ways of screening hard-to-find defects related to oxide surface quality. The devices with minor oxide degradation have properties in the grey zone, which spec out some of the properties, although they pass the functional test. We introduce a new test method to screen those spec out cells using read reference trimming.