Models for Bridging Defects Test and Diagnosis - Models in Hardware Testing - page 61

Tabl e 2. 5 Identical I DDQ
behaviour of the two
examples in Fig. 2.22

V A V B V C V F V G I DDQ

0 0 0 0 0 Leakage

0 0 1 0 1 igh

0 1 0 0 0 Leakage

0 1 1 0 1 igh

1 0 0 0 0 Leakage

1 0 1 0 1 igh

1 1 0 1 0 igh

1 1 1 1 1 Leakage

b c

a

V B V B V B

V C

V C V C

V A V D V E V A V D V E V A V D V E

R b R b

R b

Fig. 2.23 Network excitations for example in Fig. 2.22 a . ( a ) One pMOS transistor on (NAND
gate), ( b ) both pMOS transistors on ( c ) nMOS network on (NAND gate)

I DDQ threshold method. Indeed, the discrimination between these two faults is pos-
sible provided that the bridged network strengths are considered ( Arumı et al. 2007 )
as shown below.
For the example in Fig. 2.22 a , there are three different network excitations, as
depicted in Fig. 2.23 . Every excitation adds a different equivalent resistance between
power and ground, generating thus different quiescent currents. If a set of patterns
are applied so that all the possible combinations of the bridged networks are excited,
the I DDQ measurements would follow the behaviour of Fig. 2.24 a , where four current
levels are clearly observed. The lowest level corresponds to those patterns which
do not excite the bridge. The three upper levels correspond to the patterns which
activate the bridge. In these cases, apart from the leakage current, extra current is
flowing through the bridged networks. According to Fig. 2.24 a , letters a, b and c
relate the current level with their corresponding excited network in Fig. 2.23 . The
highest current level corresponds to the case where both pMOS transistors of the
NAND gate are in the on state (Fig. 2.23 b ), since the equivalent resistance composed
by the parallel pMOS transistors of the NAND gate, the bridge resistance and the
nMOS transistor of the inverter is lower than in the other two cases.
Regarding the bridge between the outputs of the inverters in Fig. 2.22 b , there are
only two equivalent network excitations. In fact, assuming identical inverters, there
is only one different excitation. For that reason, the I DDQ measurements would only
show two current levels, the lowest one corresponding to the leakage current and the
upper level when the bridge is activated, as depicted in Fig. 2.24 b .