Models for Bridging Defects Test and Diagnosis - Models in Hardware Testing - page 44

parameter, the critical resistance value R C associated with a given test vector is a
deterministic parameter that depends on the technological and topological transistor
parameters.
So in this very simple example, a faulty logic value appears on the output of the
driven gate if the unpredictable parameter Rsh is smaller than the critical resistance
R C . Consequently even if it is not possible to guarantee the detection of the bridge,
it is demonstrated that the bridge defect is detected by vector #2 if the unpredictable
parameter Rsh falls into the interval [0; R C ]. This interval is called the ' Analogue
Detectability Interval ' (ADI) associated to vector #2.

2.3.2 Logic Detectability Techniques

The analysis performed in the previous section has established that a bridging de-
fect may result in a defective or defect-free effect depending on the value of the
unpredictable bridge resistance. The concept of Analogue Detectability Interval has
then been introduced to represent the value of the bridge resistance creating a de-
fective effect. It is now interesting to study the dependence between ADIs and test
vectors. Indeed in the previous example of Section 2.3.1 , the ADI [0; R C ] associ-
ated to vector #2 is defined by the value of the critical resistance R C . This critical
resistance corresponds to the intersection of the Vn 1 characteristics with the logic
threshold Vth c of the driven gate. Consequently, the value of the critical resistance
depends on both the shape of the Vn 1 vs. Rsh characteristics (defect excitation) and
the location of the logic threshold Vth c (defect propagation).

2.3.2.1 Defect Excitation

The Vn 1 vs. Rsh characteristic depends on the electrical parameters of the transis-
tor(s) driving the bridged node. These driving transistors are fully determined by
the input vector. As an example, the vector #2 in Fig. 2.9 turns 'ON' the two p-
transistors of the driving NAND gate 'a' (with I 1 I 2 D 00), while the vector #6 turns
'ON' only one p-transistor (with I 1 I 2 D 01). As a consequence, excitation with vec-
tors #2 and #6 results in different values of the critical resistance, as illustrated in
Fig. 2.11 . Two different ADIs are therefore defined: [0; R C ] corresponding to vector
#2 and [0, R C ] corresponding to vector #6. This simple example demonstrates that
for a given defect, different ADIs exist depending on the excitation defined by the
input vector.

2.3.2.2 Defect Propagation

The Analogue Detectability Interval also depends on the logic threshold Vth of the
driven gate(s). Here again, the gates that propagate the effect of the defect are fully