TOPIC : Introduction to FaultsTOPIC : Introduction to Faults

UNIT 2: Modeling and Simulation

Module 1 : Logical faults due to physical faults

FaultFaultFault is any deficiency in the system either

during design or manufacturing which ultimately results in the deviation in the expected value.

Faults need to be identified and corrected.A faulty sub-part in a chip might ask for the

entire reconstruction of the chip.

Why to model faults?Why to model faults?To verify the interconnections of the circuitReal defects are difficult to analyzeA fault model identifies targets for testingA fault model makes fault analysis easy

Causes of faultsCauses of faultsFaults due to materialsFaults due to masksFaults due to improper order of process

stepsFaults due to error in the design rules

It is difficult to simulate these faults. So there is a need to visualize these faults as logical faults so that the analysis become easier.

Contd …Contd …Material defects

◦ Surface impurities◦ Body defects

Processing defects◦ Missing contact windows◦ Parasitic transistors◦ Oxide breakdown

Packaging defects◦ Contact degradation◦ Seal leaks

Physical faults Physical faults Logical Logical faultsfaultsAny physical fault can be represented by

logical fault.Advantages

◦ Fault analysis becomes logical analysis rather than a physical problem

◦ Many physical faults can be modeled by one logical fault

◦ Logical fault models are technology independent◦ Some physical faults are too complex to be

analyzed

Different fault modelsDifferent fault models

Logical fault model can be explicit or implicit.Explicit fault model – defines a fault universe

in which each fault is individually identified Implicit fault model – defines a fault universe

by collectively identifying the faults of interest

Fault universe – set of all possible faults in the design.

Different types of faultsDifferent types of faultsStructural faults – faults defined in

conjunction with a structural model◦ Modify the interconnections among the components

Functional faults – faults defined in conjunction with a functional model◦ Modify the truth table of the component or the

model in which it is represented

Short and Open Structural Short and Open Structural faultsfaultsAssumes that the faults are due to

interconnects only and the components are fault free.

Typical structural faults◦ Short – by connecting unnecessary nodes◦ Open – by breaking of a connection

For example shorting the inputs of a component or breaking the connection to supply or ground.

Stuck-at-faultsStuck-at-faultsA short between signal and supply or ground

will make the node remain at a fixed voltage.The node is said to be stuck-at-voltage (v) V € {0,1}Represented by s-a-v.

Any line X s-a-v € {0,1} represent following physical faults◦ X open◦ X shorted to ground or supply◦ Any internal fault in the component driving X that

keeps its value at ‘v’.

ExampleExampleSuppose the line ‘W’ is unfortunately shorted

to ground It can be represented as s-a-0One can observe the error at the output.

Bridging faultBridging fault It is a logical fault representing a short

between two signal lines which creates a new logic function.

Depending on the function created, there are two types of bridging faults◦ AND bridging faults◦ OR bridging faults

AND bridging fault OR bridging fault

ExampleExampleAssume OR bridge between the inputs of the

NAND gate

In Original CircuitB = (Y.Z)’ NAND gate

In faulty circuitB = {(Y+Z).(Y+Z)}’ = (Y+Z)’ NOR gate

The output function will become NOR instead of NAND due to bridging fault.

Original circuit Faulty circuit

Transistor FaultsTransistor FaultsMOS transistor can be considered as an ideal

switch.Two types of transistor faults

◦ Stuck-open◦ Stuck-short

Stuck-open: A transistor is permanently stuck in the open state.Stuck-short: A transistor is permanently shorted irrespective of its gate voltage.

Thank YouThank You