Basics Of VLSI Chip Testing - [ Need For VLSI Chip Testing - Why & What ]

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