Importance of VLSI Chip Testing:
Nowadays testing have
become an important aspect for every chip being fabricated. To analyse it consider
a simple example of car air bag activator control system as shown in the figure
1.1.
Figure 1.1 Car air bag activator control
System
The system is designed
in such a way that whenever a car is subjected to crash, either of the crash
sensor 1 or crash sensor 2 gets enabled, there by an airbag activator signal is
generated to activate the air bag which saves the lives of the people. From
figure 1.1 consider a simple logic implementation of car crash sensor control
system by simple NOR gate and Inverter gate in series.
Table 1.1 Truth table for air bag activator
control system
When either of the
inputs to NOR gate goes high, the output of NOR gate is 0 and the inverted
output will be 1, which activates the air bag. The truth table for air bag
activator is as shown in table 1.1.
Figure 1.2 Transistor and physical layout of
inverter
Assume this simple
airbag control system is being fabricated, say inverter logic in the system
is been fabricated. Normally assumption for inverter physical implementation
would be like as shown in figure 1.2.
As due to some
manufacturing imperfections at different levels of fabrication, from figure 1.3
may be at metal deposition level, an extra metal would be deposited between output
and Vss of inverter or at etching level certain path between output and Vss would
not be etched properly.
Such direct paths
between Vss and output will always lead the output to stuck at 0, termed as a
defect in inverter. If such an inverter is used in logic block of car airbag
activator control system, the air big will never get activated, such that
system is subjected to failure, this is a system failure.
Figure 1.3 Inverter layout due improper
etching.
costs the lives of number
peoples. The truth table for such a failure system is shown in table 1.2. Even
though either of the crash sensors are going high, still air bag activator
remains 0 due to defect at the output. Hence there must be a criteria or
methodology to check such defects after fabrication. The solution is provided
by subjecting chip after fabrication to testing.
FOR TUTORIAL VIDEO OF ABOVE DISCUSSION CLICK ON THE LINK BELOW
https://www.youtube.com/watch?v=ztT3ttaCtuE
https://www.youtube.com/watch?v=ztT3ttaCtuE
