Page 276 - Semiconductor Manufacturing Handbook

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Source: SEMICONDUCTOR MANUFACTURING HANDBOOK

CHAPTER 19

INSPECTION, MEASUREMENT,
AND TEST

Donald W. Blair
Agilent Technologies
San Diego, California

19.1 INTRODUCTION—OVERVIEW ON TESTING EQUIPMENT

19.1.1 The Need for Tests

Why test at all? The simple answer to this question is to increase the confidence of the IC manufac-
turer that the device-under-test (DUT) will meet its specifications listed in the device data sheet and
will perform as designed in its end application. Semiconductor manufacturing processes are not per-
fect, therefore the good parts need to be sorted from the bad and testing assures customer quality.
Test also plays another, equally important role—that of quantifying and identifying the manu-
facturing defects so they can be reduced and the yield increased. Not only do manufacturers want to
ship only good chips, they want to maximize the yield of good chips rather than scrap. By increas-
ing yield, defects are decreased and the probability of a bad part escaping the test is reduced.
Yield is crucial to the profitability of a fab since it determines how much revenue in the form of
good chips, can be obtained for the cost of producing a wafer. Furthermore, yield, along with cycle
time, sets the capacity of the fab. Thus products, manufacturing processes, and wafer fabrication
facilities all go through yield improvement cycles where designs, procedures, and equipment settings
are adjusted to decrease the rate of defects. The rate of this yield learning determines the return on
the investment in equipping the fab and developing the manufacturing process since it determines
the rate at which capacity can be ramped to match the peak demand before the demand inevitably
falls. That is, the rate of yield learning determines a fab’s ability to hit its product’s market window.
Fast yield learning depends on the ready availability of yield and defect information, and semicon-
ductor manufacturing tests are one of the prime sources.
Semiconductor manufacturing has been advancing rapidly for many years as exemplified by the
steady reduction in the minimum feature size with each process generation or node. This advance
also means that the defect density, and yield, of any given node is also improving rapidly. By the time
a node has been obsoleted by newer nodes its yield can be so high that its manufacturing process can
be very well understood and controlled.
Why does the electronics industry keep using newer, smaller dimension processes that conse-
quently are less understood, less in control, and have higher defect densities? The first answer obvi-
ously is performance—smaller devices and denser circuits operate faster. Therefore, manufacturing
tests increase product performance by allowing the product to use a higher performance and lower
quality process while maintaining acceptable customer quality. The second answer is cost. A denser

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