Models for Bridging Defects Test and Diagnosis - Models in Hardware Testing - page 53

a b

500

500

400

400

300

300

200

200

100

100

0

0

0 500 1000 1500 2000 2500

0 500 1000 1500 2000 2500

Pattern number Number of patterns

Fig. 2.17 I DDQ test for a real 0:18 m defective device. ( a ) Non-ordered and ( b ) current signature

I DDQ ' ( Thibeault 1997 ; Miller 1999 ; Kruseman et al. 2001 ) . Instead of observing the
absolute value of the power supply current, 'Delta I DDQ ' considers the difference of
the power supply current among successive test vectors. This difference is treated
probabilistically to determine if the circuit is defective or not.
Another extension of the I DDQ testing technique is based on the use of current
signatures, which was proposed by Gattiker and Maly ( 1996 ). The measured I DDQ
data is not compared to a single threshold value, but the current for the whole test set
is measured ( Gattiker and Maly 1996 ; Nigh and Gattiker 2004 ) . A current signature
is generated by ordering all the obtained measures from the smallest to the highest
value. This technique looks for sharp changes (or steps) in the current signature,
which indicates some kind of defect in the device. In case of bridges, the number of
steps may give information about the number of network excitations that have been
activated. Figure 2.17 illustrates the I DDQ data for a real CMOS 0:18 m defective
device. On one hand, Fig. 2.17 a showstheI DDQ values in the same order as in the
test procedure. On the other hand, the values are ordered in Fig. 2.17 b . Notice that
different steps are observed for the current signature of the defective device. Current
signatures avoid the problem of I DDQ and Delta I DDQ testing when deciding the
current threshold limit.
The current ratios technique ( Maxwell et al. 1999 ) is based on the same idea as
current signatures, but tolerating parameter variations. The basic idea relies on the
fact that the slopes of the rank-ordered current signatures for dies having differences
in the absolute I DDQ values are quite similar. Therefore, it is possible to set a test
limit based on the ratio of the maximum to minimum I DDQ value. This value is more
or less constant and independent of the mean of the I DDQ measurements for each die.
This ratio is determined by means of an iterative process. Once obtained the ratio,
the vector which typically gives the minimum current is identified. The current for
that vector is measured. Subsequently, the maximum current is computed due to the
ratio previously obtained. Outliers are then identified.
Other solutions have been proposed in order to overcome the leakage problem
( Keshavarzi et al. 1997 ; Sachdev 1997 ; Figueras and Ferre 1998 ; Meijer et al. 2004 ) ,