HITEQUEST
Apr 2004
 Introduction to SCAN and IDDQ

 
 

Scan is a design technique used in IC manufacturing to increase the overall testability of a circuit. Scan tests are generated by ATPG tools - Automatic Test Pattern Generation. Unlike the functional tests that check chip functionality, scan tests cover stuck-at-faults, caused by manufacturing problems.

Physical manufacturing defects, such as

  • silicon defects
  • photolithography defects
  • mask contamination
  • process variation
  • defective oxide
cause electrical defects, such as
  • shorts (bridging faults)
  • opens
  • transistor stuck on open
  • changes in threshold voltage
that in their turn cause logical defects, such as
  • logic stuck at 1/0
  • slower transition (delay fault)
  • AND-bridging,OR-bridging
Here is an example of stuck-at-false. You can see how a contamination on a wafer is causing a resistive "short" of a transistor, resulting in a stuck-at-zero faulse of the gate.




How does the scan work?
Basically all FFs in the design are replaced with scan type FFs. It makes all FFs in the design controllable and observable by chaining them together and shifting test data in and out.



Scan type FF contains a MUX to select either a normal mode data D or a scan data SI. SE is a control input. The first step is to put a circuit into a scan mode. The whole chip is splited into parts (chaines),serial data is applied to the inputs and clocking/shifting to the outputs. Every scan chain Output is strobed for the proper data coming out.



All FF replacements ATPG tool does automatically based on design_setup file prepared by designer. Scan insertion adds 15%-30% of area per FF.

IDDQ - is another test technique used in IC manufacturing. It covers manufacturing defects,such as bridging faults. The idea of this test is to measure drain current through the chip, while it is in the static state. Normally a CMOS logic consumes very little current in a static state. If there are any internal shorts then high Icc current should indicate this. This test takes relatively long time since many static states must be covered. IDDQ test does not cover timing problems.


References:
1.John F. Wakerly Digital design,principles and practices,Second Edition,Prentice Hall,1994
2.Alfred Crouch Design-for-Test for digital ICs and embedded core systems,Prentice Hall,1999
3.Abramovici, Digital system testing and testable design,Computer scince press,1990


HITEQUEST
Apr 2004