Graphical IDDQ Signatures Reduce Defect Level and Yield Loss

We propose a new I_DDQ testing signature, the graphical I_DDQ signature. We discovered that noise, in the entire set of current measurements for a chip, is a vastly superior feature for classifying chips as good or bad, compared to present methods. The measured I_DDQ current as a function of vectors is defined here as the signature. We examine the shape of the waveform defined by the total set of the I_DDQ measurements, to extract the number of bands that all of the current measurements cluster into, the width and separation of the bands and current glitches or noise among all I_DDQ measurements. We examined the I_DDQ signatures of all SEMATECH experiment chips that were classified as good or bad by a combination of functional, delay, and scan voltage tests. A single I_DDQ threshold, whether absolute or differential, cannot separate good/bad chips reliably. Good chip signatures contain discrete levels (or bands) of varying widths and separations. A faulty chip almost always displays noise and glitches in the band structure. The graphical I_DDQ classifier shows very high accuracy for SEMATECH test data with a test escape rate of 5.97%, compared with 7.5% for the single threshold method, 7.6% for current differences and 7.5% for the DeltaI_DDQ method. The graphical I_DDQ method had a 1.2% test overkill, compared with 2.3% for the single threshold method, 6.1% for current differences and 7.0% for DeltaI_DDQ.