COMPUTER CONTROLLED INTEGRATED CIRCUIT TEST ... Robert E. Huston, MSEE

COMPUTER  CONTROLLED  INTEGRATED  CIRCUIT  TEST ... Robert  E.  Huston,  MSEE
COMPUTER CONTROLLED INTEGRATED CIRCUIT TEST SYSTEMS
Robert E. Huston, MSEE
Manager, Array Test Systems
Fairchild Systems Technology
Sunnyvale, California
Testing is a subject which is of major concern to all manufacturers and users of integrated circuits,
whether they are low complexity gates and flip-flops, or more complex. MSI and LSI devices. There
seems to be general agreement in the industry that low complexity devices can be tested adequately
on the test systems, generally computer controlled, that are available from several manufacturers.
However, there is very little agreement about how the more complex MSI and LSI devices should be
tested. Some commentators have even stated that LSI devices cannot be tested adequately at a
reasonable cost.
This paper reviews the requirements that an integrated circuit test system must fulfill and shows
why a computer controlled system usually, but not necessarily always, is the best choice. Various
possible configurations of test systems are discussed and reasons for their applicability to certain
'test situations are suggested. In the latter part of the paper particular attention is given to testing
more complex MSI and LSI devices. Finally, a short description of one manufacturer's approach to
testing complex devices is given.
It is impossible to separate the philosophy of testing from the design and selection 'of test equipment. Some agreement on the philosophy of testing is essential before product of the complexity
of an integrated circuit can become a reality. Testing is basic in the vendor/cl:'stomer relationship. A
satisfactory definition of test methods and test equipment is essential if there is to be a satisfactory
working relationship between the manufacturer and user. This applies as much When the manufacturer and u'ser are separate departments within the same company as when they are completely different companies. The industry has established reasonable solutions to most of the testing problems
related to low complexity integrated circuits. At this time, however, there is very little agreement
about testing the more complex MSI and LSI devices.
a
It is the author's intention that some of the material in this paper will contribute to establishing
some agreement on the basic philosophies of testing complex integrated circuits. In response to
those who suggest that it is impossible to test these devices adequately and economically, it is relevant to point out that there are many similarities between testing LSI and testing complex digital
functions ir1 the form of printed circuit boards and sub-systems. These devices have been tested for
many years with varying degrees of success. Workable, if not ideal, solutions to the' problems of testing complex. digital functions have been developed and us~d. It is reasonable, therefore, to start
considering the subject of testing LSI from the basis that it can be done.
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TEST ENVIRONMENTS
Integrated circuits test systems are used by manufacturers and users and also by independent test
laboratories (Figure 1). Each of these types of organization use test systems for many different purposes and, for these purposes, require a variety of capabilities. For many purposes it is a requirement
that one test system can satisfy all the requirements although, sometimes, it is more convenient to
have separate test systems for the different purposes. This mainly depends on the size of the organization. Whereas a large organization can afford to have many test systems, each designed to satisfy a
certain requirement, the smaller organization cannot afford to do this and must, therefore, have a
versatile test system.
Integrated circuit manufacturers primarily use test systems to test devices at the wafer probe and
final test stages. For them high throughput is a prime requirement. They are primarily interested in
how much it costs to test a device. Initial cost of the equipment is less important than such factors
as test rate, reliability, down-time, (which includes the time that the system is not testing while it is
being reprogrammed as well as the time that it is not operational) and accuracy. These are the
factors that determine the cost of testing and the cost of errors in testing.
At the wafer probe stage it is usually economic to perform a functional test first on each device as
this allows a rapid decision to be made as to whether or not the device is worth further testing. In
the case of complex devices, functional testing becomes much more important. Following functional
testing it is usual to perform a number of static parameter tests (often referred to as dc tests) to
verify that the device meets its specification. These tests are generally performed on a go/no-go
basis. It is advantageous for process control purposes to accumulate parameter distributioninformation at this stage. To satisfy this requirement it is necessary that the test system has the capability
of making measurements at high speed as well as making go/no-go decisions.
Until quite recently, there has been little need to attempt to make dynamic tests or measurements at
the wafer probe stage of manufacture. However, some manufacturers are beginning to make this
type of measurement. There will, no doubt, be a growing tendency to do this,' particularly in the
case of integrated circuits which are to be assembled into hybrid assemblies. When dynamic testing
is performed at the wafer stage it is necessary that a test system is used that can make both static
and dyn~nnic tests at the same test head. This is because a wafer can only be probed once, otherwise unaccf!ptable scratches on the metallization are likely to occur.
At the final test stage the manufacturer makes tests on packaged devices to ensure that they meet
their published specifications and to verify that the lead bonding within the package is good. The
degree of testing that is performed at this stage depends on the application for which the device is
sold. It is usual at least to perform static and functional tests at room temperature. In addition the
static parameters may be tested at the extremes of temperature for which the device is specified.
Where the dynamic parameters of the device are critical these will also be tested. Environmental
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I
MANUFACTURER
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TESTING
FINAL
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STATIC (DC)
TESTING
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QUALITY
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DYNAMIC (AC)
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FUNCTIONAL
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USER
INDEPENDENT
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LABORATORY·
fiGURE 1
HIGH
RELIABILITY
TESTING
ENVIRONMENTAL
TESTING
INCOMING
trJSPECTION
DELTA
TESTING
CHARACTERIZATION·
DATA
ANALYSIS
EVALUATION
DATA
RECORDING
INTEGRATED CIR·:UIT TEST SYSTEM REQUIREMENTS
testing and dynamic parameter testing may be performed on a sample of devices or they may be performed on every device.
The test system requirements for final device testing are similar to those for wafer testing. High
throughput rates, good reliability, and accuracy are all important. One difference between wafer
testing and final testing is that at the latter stage it is often possible, and sometimes even desirable,
to perform different types of tests at separate test stations.
Quality control testing is performed on samples of devices at various stages in the manufacturing
process. For this type of testing accuracy is of great importance. During a working day the quality
control department typically performs tests on a small number of each of many different types of
devices. It is important, therefore, that the test system can rapidly change from testing one type of
device to testing another. Also quality control testing often involves recording parameter values. The
ability to data log without significantly reducing the test rate is, therefore, important.
Delta testing is the measurement of parameter change due to stressing. The most common form of
delta testing is to record parameter values before and after operating devices for a defined period,
and comparing the change in parameter values against predetermined limits. The allowable parameter changes are sometimes stated in absolute values, sometimes in percentages, and sometimes
both an absolute and a percentage delta limit are specified. Clearly, for this type of testing it is very
desirable to have the test system connected to an on-line computer so that the delta results can be
automatically calculated as the testing proceeds.
High reliability testing is very similar to quality control testing except that it involves testing all
devices and rec·ording their parameter values. In addition to recording parameter values it may be
necessary to provide statistical summaries of the parameters and to give delta information. When
testing devices for use in high reliability applications very stringent requirements are placed on the
accuracy and reliability of the test system. It can be very expensive if, due toa test system error, a
large batch of devices has to be retested or, even worse, is rejected.
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IS illlpor lalll thallhu test rnethods wiud by the incolllinu inspection depi:trtment should be very simi
1.11 to those used by the integrated circuit manufacturer. If this is not so, many difficulties will arise
due to discrepancies in measurements at the two locations. Just to give one example of how these
difficulties arise, it is well~known that most semiconductor parameters are temperature dependent.
Due to the internal dissipation of a device, its temperature will rise to some extent during testing.
The actual temperature that will occur during testing depends on the time that power is applied
prior to the time the temperature is measured. This can cause differences in test results at two locations to such factors as different test times used by two different test systems or by performing tests
in a different order.
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The two functions of characterization and evaluation are similar, the former being performed by the
integrated circuit manufacturer as he seeks to gain sufficient knowledge about his devices to write a
convincing data sheet and obtain a profitable yield from his production lines, and the latter being
performed by the user as he tries to compare one manufacturer's device with another and gain some
knowledge about the unspecified parameters of the device. In both cases a very versatile test system
is required and one that has the capability to record and analyze test data.
The independent test laboratory performs a very useful function within the industry. It performs
evaluation and incoming inspection functions for smaller users of integrated circuits and also acts as
a back-up for the larger user when his testing requirements cannot be satisfied internally. Some
independent test laboratories are also very active in the field of high reliability testing. The independent test laboratory requires very versatile equipment and places a high value on the ability to
record and analyze test data.
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TEST EQUIPMENT
A review of the preceeding paragraphs indicates that there are a wide range of applications of integrated circuit test equipment, each of which places a different emphasis on the importance of the
various features of the system. It is important, therefore, that the designer of the equipment is completely cognizant of the use to which it will be put before he starts to design it. The potential
purchaser must be equally sure that he understands his own needs before he decides which equipment to buy.
The following paragraphs suggest some of the considerations that should be borne in mind when
defining integrated circuit test systems.
With the risk of appearing too elementary it is relevant to pay some attention to the purpose of
integrated circuit test systems. Clearly, their function is to test integrated circuits. But why test
integrated circuits? The reason, surely, is to ensure that, when they are soldered into an assembly, .
they will operate correctly and reliably. Integrated circuits are used in a very wide variety of equipment. It is reasonable, therefore, to suppose that the testing that must be done depends on the final
use to which the integrated circuit is to be put.
There are four basic types of electrical tests that are commonly performed on integrated circuits,
stress tests, functional tests, static parameter (dc) tests, and dynamic parameter (ac) tests. Much
time and energy has been spent in arguments about which of these tests should and should not be
performed. Most of this time and energy is wasted because it tends to assume that there is one
answer that suits most, if not all, app,lications. All these types of tests are important and useful, but
not all are necessary or desirable for all applications. Sufficient time and energy should be spent,
prior to any attempt to define an integrated circuit test system, in understanding what each of these
types of tests can achieve within the environment that they may be used. Unfortunately this is not
always done.
Stress tests often take the form of applying voltages in excess of what will normally be experienced
in use to the terminals of an integrated circuit. Correctly designed and interpreted these tests can be
used to indicate the potential reliability of a device. They also give assurance that the device is unlikely tufilil due to transient conditions in the equipment in which they are installed. Stress tests are
frequently used to test the oxide integrity of MOS devices.
FLJilctional tests, also referred to as truth table tests, are used to ensure that a digital device performs
the correct logic function. Patterns of '1's and 'D's are applied to the input pins of the device under
test while the output pins are compared with the expected levels. At the wafer test stage functional
tests are used to make a fast decision whether or not the device is basically good before it is committed to· the more time-consuming static parameter tests. Functional tests become particularly
importarlt in the case of complex devices.
Static parameter tests are those in which measurements of voltages and currents are made at the
terminals of the device under test at a speed which is sufficiently slow that the measurements are
essentially independent of time. Typically static parameter tests are made in periods ranging from
one to one hundred milliseconds. The voltages and currents that are measured ensure that the fan-in
and fan-out, the power dissipation, and the threshold levels of the device under test are correct.
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Dynamic parameter tests are those in which times or, sometimes, time dependent voltages, are measured. The most common of these are transition times and propagation delays. Less frequently
instantaneous noise voltages are measured.
In addition to the four basic types of tests the decision also has to be made whether it is sufficient
to measure the performance of integrated circuits at room temperature or whether measurements
should be made over the temperature range at which the devices will be used.
Idea"y testing should ensure that every integrated circuit operates correctly when it is first installed
and that it will continue to operate correctly over the required life of the equipment. No amount of
testing can absolutely guarantee this. So the facts of probability intrude into the test decisions. The
more money that is spent on testing, providing it is wisely spent, the more confidence can there be
that the testing is effective. A comparison has to be made between the cost of testing and the cost
of not testing. A defective integrated circuit costs money to locate and replace whether this is done
during the manufacture of the equipment or in the field. This cost depends on many factors some
of which are the accessibility of the defective device (the cost of replacing a defective component in
an unmanned satellite is the cost of that satellite plus the cost of putting it in orbit), the size and
complexity of the module (eg printed circuit board) of which it is part, and the cost of not having
the equipment operational while the defective part is being located and replaced. Clearly these costs
vary enormously from one application to another. Equally, therefore, the amount that it is economic
to spend on testing and on the test equipment varies greatly according to the application.
The very varied requirements for integrated circuit test equipment leads to a very clear choice. Either
specific equipment must be designed for particular applications or the equipment must be sufficiently modular that it can easily and economically be tailored to suit many specific applications. It
is rarely economic to design equipment specifically to suit a particular need. Even if the usage of the
equipment is large enough to justify a special design, it is likely that the needs that are identified at
anyone time will change periodically making it necessary to redesign the equipment. This approach
is unlikely to result in efficient and reliable test equipment. Usually, therefore, integrated circuit
test systems are designed on a modular basis.
A specific 'example of a modular test system is briefly described at the end of this paper. This shows
how a modular approach to the design of an LSI test system may be accomplished.
I t is necessary to have a deep understanding of the nature of a device before an adequate system to
test it can be designed. This is particularly true in the case of integrated circuits. Two examples of
this as it applies to integrated circuits are given below.
At first it might seem to be a simple matter to measure the leakage current at an input terminal of
an integrated circuit. Suppose that it is required to measure the leakage current of an input to a
gate which is part of a complex integrated circuit (Figure 2). While the input in question is connected to one of the pins of the device all the other inputs are connected to the outputs of flip-flops
which are buried within the device. To measure the leakage current under worst case conditions it
must be done with all the inputs to the gate except the one being measured connected to a low
logic level (DTL or TTL logic is assumed). To ensure that this state is reached it may be necessary to
exercise the inputs of the device through a large number of logic states. It is desirable that this is
done rapidly in order to minimize test time. From this arises the need to perform rapid function
-7-
PIN UNDER
TEST
FIGURE 2
LEAKAGE CURRENT MEASUREMENT
DEPENDS ON INTERNAL STATE
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INFLUENCE OF CAPACITIVE LOAD
-8-
tests which are interrupted by static parameter tests at specific times. The test system, therefore,
must be capable of switching from the high speed function test mode to the static parameter mode
without producing tranients at the terminals of the device under test which might cause it to change
state.
The second example concerns the capacitive loading that the test system places on the output terminals of the device under test. It is generally well known that some integrated circuits are unable to
drive more than a certain capacitance. What is not understood is that, even though the device under
test can drive a certain capacitance, this capacitance may cause significant errors in the tests that are
made and may even make it impossible to test certain kinds of devices. In the drawing (Figure 3) an
integrated circuit is shown driving a function test comparator that places a capacitive load C L on the
output terminal. The resistor RG represents the resistance between the ground connection of the
gates on the die and the external ground of the device package. This resistance is partly due to the
resistance of the aluminum conductors on the surface of the die, partly due to the bonds between
the die and the pins of the package, and partly due to the package leads. Suppose that the output of
the integrated circuit is at a high logic level and that changes at the input terminal cause the output
voltage to change to the low state. While this is happening the charge stored on the capacitor C L
discharges through the integrated circuit and through the resistor RG . This current causes a temporary change in the internal ground level of the die which will not occur when the device is used ina
system, unless the capacitive load happens to be the same. This temporary change of internal ground
voltage can cause a number of problems. If the input voltage change is slow compared with the
switching speed of the gates on the die it can cause multiple triggering of flip-flops. Even if the input
edges are fast enough to avoid this problem there may be other inputs that are held close to the
threshold levels which will pass through the threshold levels as the ground level changes. If this
happen's the device will not make the required changes in state. For these reasons it is important
that the test system places a minimum capacitive load on the output pins of the device under test.
The above examples are just two of the many practical points that must be borne in mind when
designing or selecting integrated circuit test systems. There are, of course~ many other equally
critical points.
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WHY USE A COMPUTER?
In the earlier parts of this paper considerable attention has been given to what an integrated circuit
test system must do, both in terms of the testing environme~t and in terms of the device to be
tested. Very little has been said about the configuration of a test 'system or how it operates. It is not
the intention of this paper to describe the hardware of a test system in any detail. Reference should
be made to other papers for this information. In the simplest terms, every test system consists of a
number of modules that are capable of applying defined. conditions to specific terminals of the
device under test and others that are capable of measuring the results of applying these conditions.
From test to test the forcing conditions, the measurement conditions, and the connections to the
terminals of the device under test are changed. This implies the necessity of a control function within the test system. A different sequence of test conditions is required for each device to be tested.
Therefore the system controller must be programmable.
In the case of very simple integrated circuit testers a printed circuit board is used to provide control.
Soldered connections on the board determine the connections to the device under test. Resistor networks provide the correct voltage and current forcing levels and also comparison levels. This type of
tester can provide adequate testing under some conditions but is clearly very restricted in its flexibility. Somewhat greater flexibility is provided in more expensive test systems by providing the test
program on punched paper tape. The test conditions are controlled directly from the paper tape as
it passes through a reader. Again, this type of test system can satisfy some needs, but it has a number
of disadvantages including slow speed, inflexibility and inconvenience.
Most automatic test systems have a means of storing test programs within an internal memory
which take's the form of either a core memory or a magnetic disk. In these systems the stored program is converted into instructions for the modules of the system either by special purpose logic or
by a general purpose digital computer. In older systems special purpose logic was used whereas,
today, general purpose digital computers are used almost exclusively.
There are many reasons why it is advantageous to control an integrated circuit test system by means
of a general purpose digital computer. Some of these reasons are explored in detail below. Before
going into these details, however, it may help to clarify the use of a computer in this type of application if some general comments are made about general purpose digital computers.
The word 'computer' can be mis-leading. When computers were first developed they were designed
to perform scientific calculations. The use of computers in this application enabled calculations to
be made which were previously impossible due to the time that would be required by a human being
operating a desk calculator and due to the inevitability of errors that arise whenever a human is
involved in long and tedious operations. Later computers were developed for business applications
where, again, calculation was their basic function. In this case the advantage of an automatic digital
computer was that it could perform a very large number of simple calculations very rapidly. In both
these types of use the main purpose of the computer was to perform computations. It was very
reasonable, therefore, that it should be known as a computer.
More recently computers have come to be used for applications where their fu-nction is primarily
that of control, rather than computation. In these applications the decision making capability of a
computer is of great importance.
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Both as a calculator and as a controller a computer operates on binary numbers. Binary numbers are
stored in its memory, manipulated in its central processing unit, and written back into the memory.
As a calculator these binary numbers are coded representations of decimal numbers. As a controller
the binary numbers are coded representations of control information. In both cases binary numbers
are also used within the computer as coded instructions for the computer itself.
To act as a system controller, where the system may' be an integrated circuit test system for
example, coded instructions for the various parts of the system are stored within the memory. These
instructions are delivered to the appropriate parts of the test system either in the sequence in which
they are stored or in a sequence which is determined by other information such as, for example,
the results of previously made tests.
One of the principal reasons why general purpose digital computers have come to be used to control
integrated circuit test systems is that of economics. Small general purpose computers are available
today from many manufacturers. They are used in many applications both for computational and
for control purposes. Due to the large number of these computers that are built and the competitive
nature of the market they are available at quite low prices. It is possible to buy a small general
purpose digital computer suitable for controlling an integrated circuit test system for considerably
less than the special purpose logic to do the same task could be built. If the computer had no other
advantages over special purpose logic it would be preferred from a cost point of view. However, as
will be shown in the following paragraphs, the computer has a number of other important advantages.
Figure 4 represents a computer controlled integrated circuit test system. Notice that the test system
main frame is a peripheral to the computer. The diagram shows a large number of input/output
peripheral devices. It is unlikely that any particular installation would have all the peripherals shown.
However, to suit particular requirements any combination of them may be desi'rable.
Most computer controlled systems use a teletypewriter, or similar device, as the basic method of
communicating with the system. The system operating modes are selected by typing in instructions
at the keyboard. Test programs may be read into the computer memory either by typing them
directly in at the keyboard or by reading them in from a prepared punched paper tape. Test results
may be typed out by the computer using the teletypewriter or may be punched onto paper tape.
Other input/output devices such as high speed paper tape readers and punches, card readers and
punches, and magnetic tape drives are used to provide more convenient methods of reading information into the memory of the computer and recording test data. The principal advantage of these
devices is high operating speed.
Magnetic tape drives and magnetic disks prbvide a very convenient way of having a large number of
test programs readily available to the computer. Each has advantages and disadvantages. For many
purposes'the disk is preferred. Magnetic disks have a very short access time. It is possible to store a
very large test program, such as might be used for testing an LSI device, on a disk and transfer it in
sections into the computer memory as the testing proceeds. This enables very long test programs to
be performed without the need for a very large and expensive core memory.
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Where a large amount of data is to be printed two approaches are possible. One is to record the data
on some high speed medium such as magnetic tape or disk and to translate it into typed copy offline using a separate computer. The other approach is to use a line printer operating directly from
the test system computer. The latter method is preferred as it gives the test system operator the
opportunity to monitor the tests as they are being performed. Without the use of one of these
methods the test system is used very inefficiently due to the slow speed of printing on a teletypewriter. The need for printing large amounts of data arises in high reliability testing where a permanent record of the test results is often required.
A cathode-ray-tube display of test results can be useful in testing LSI devices or printed circuit
boards. Particularly at the time test programs are being debugged and when devices are being evaluated the CRT gives the operator the ability to have an immediate display of test results. Used with a
versatile editing software package the CRT display can be an important and useful testing aid.
Some testing facilities have advanced to the stage where each test system is operated by a local
(dedicated) computer and all are connected to a central computer. The central computer provides
overall control of the test complex. Where a number of test systems are to be operated in the same
location the cost of the installation can be minimized by this approach. For example instead of
each test system having a disk file to store all the test programs a single disk file can be part of the
central computer and access provided to each local computer. Similarly if computation on the test
results is required, instead of each test system having a computer with facilities for the required
computations, this task can be performed by the central computer. In some instances, this type of
installation is used where the test systems are separated by many hundreds of miles.
The diagram (Figure 4) also shows a modem as a peripheral to the test system. This allows data to
be sent to and from the test system over telephone lines.
As stated above, it is unlikely that any test system installation will have all the peripherals mentianed.
However to be generally useful the test system needs to have the capability of having a selectian of
most of them. If the test system is designed using a general purpose digital computer it is likely that
the computer has the capability, both from a hardware and a software point of view, of being interfaced to a versatile range of input/output devices. If the test system is designed to be controlled by
special purpose logic the addition of each input/output device makes necessary the design of interface hardware to. suit that device. This can be expensive and tends to pro.duce reliability problems.
It is usually less expensive and leads to. greater reliability to have an established and proven unit.
The use of a digital camputer to. control a test system provides the opportunity to simplify the
programming language. Programming languages tend to be a subjective matter to a certain extent,
what is simple to. one user may be complex to. another. The users' view of the programming language
depends to' a large extent on his background. This is where the computer provides assistance. The
same computer that cantrols the test system can be used to compile a test program written in a
, 'language' that is simple for the user to understand into the code that operates the system. The same
.. computer can also translate a program written in a language designed for one test system into a
language which can be understood by another .
.In some applications af integrated circuit test systems it is very desirable to havea method of reduciing dr an()lyzing the test data. For example, in the case of wafer testing, distribution data based an
test results' is very useful information on whiCh to base control of the fabrication processes. Data
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TO MASTER
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COMPUTER CONTROLLED TEST SYSTEM
-13.";;
TEST
STATtON
reduction can either be performed off-I ine by recording the test results and using a separate computer or by using the computer that controls the test system to process the data while testing takes
place. The latter approach is greatly to be preferred. One reason for this is that it allows the operator
to monitor the test results while they are being produced. Another is that it makes the reduced data
available immediately after a batch of devices have been tested. The computer controlled test system
has the advantage over the non computer controlled system that it can do this.
Another example of where the computing ability of the computer controlled test system has an
advantage is the case where go/no-go decisions are made as a result of calculated results rather than
as a result of direct measurements. In high reliability testing delta measurements are often made.
Devices are tested and all the measurement results are recorded. The devices are then subjected to
some stress, such as being operated at a certain temperature for a defined time. They are then retested~ Go/no-go decisions are made on the basis of the absolute or percentage change in parameter
values between the two measurements. In some cases go/no-go decisions are made on the basis of
limiting values of both absolute and percentage changes. A computer controlled test system can
make this type of test readily. Without a computer it is necessary to store both sets of test results
and to compute the changes off-line in order to pass or fail the devices. This is inconvenient, timeconsuming, and expensive. It is also subject to errors due to the amount of manual handling both of
the devices and the test data that is involved.
Yet another example of where the computing ability of the computer controlled test system is
valuable'is in the case of tests on devices where the specified parameter is obtained by calculation
from two or more test results. This happens in the case of linear devices.
Computer controlled test systems are often faster than those that are not controlled by computers.
There are several reasons for this. Usually test systems that are not controlled by:a computer do not
have a core memory. Instead they use a magnetic disk as the internal storage medium. The access
time ofa magnetic disk is usually in excess of ten milliseconds, whereas the access time of a core
memory is only a few microseconds. The computer controlled system has virtually immediate access
to information from the memory whenever it is required, whereas the disk controlled system often
has to wait for information. Usually the computer controlled system can be programmed to minimize test times by making use of its capability for sophisticated branching. The 'use of subroutines
in test programs can also reduce testing time and simplify programming.
One of the most important features of a computer controlled test system is its self-diagnostic
capability. Diagnostic programs can be prepared which will check that the entire system is working
correctly and, if a fault is found, will localize the fault so that it can be corrected rapidly. This can
be an important contribution to minimizing the system down-time.
It has often' been said that an advantage of the computer controlled test system is that it is less likely
to become 'obsolete than a non computer controlled system. The reason for this is that the functioning of the system is controlled by software. System changes can, therefore be made without changing the hardware. There is a great deal of truth in this philosophy. However, then~ is need for some
caution in interpreting it. It is an important and obvious fact that the software ca'n only instruct the
hardware to operate within its capabilities. Software cannot increase the voltage range of the system
or the number of pins that it can service. Software cannot increase the bandwidth of a time measuring device significantly. Also it must be remembered that software can be expensive to produce. It
may be less expensive to implement an improvement in the system capability 'w'ithhardware than
-14-
with software. Understanding that there are limitations on the changes that can be made with software, it is still valid to claim that the computer controlled system has an in-built flexibility that
helps to prevent it becoming obsolete.
No discussion of a computer controlled system can be complete without a reference to the software.
The computer is useless unless it has good and proven software to control it. Software development
for any computer controlled system is a major development task.
In the case of commercially available computer controlled integrated circuit test systems the executive operating software is supplied by the system manufacturer. The test programs that enable the
system to test specific devices are the responsibility of the system user.
"\
-15-
The Sentry 600 provides the versatile
hardware and data analysis capability
required for in-depth LSI and dynamic MOS
device characterization for critical device
design, yield improvement, and pilot production program and quality assurance
requirements.
The Sentry 600 delivers up to 10 MHz data
rate. In addition, multiple timing generators
provide a conservative clock rate of 20 MHz.
Under complete software control, the
device is powered, functional and DC
parametric tests arc performed automatically, then repeated for all variations required by the test specification. All device
data can be recorded for further processing
by the Sentry. 600.
Special attention has been given to optimization of the electrical testing environment
A unique high performance test head of
compC3.ct design provides specialized pin
electronics in close proximity to the device
under test. Each OUT pin has its own
dedicated pin electronics circuit. Capacitanceis minimized. Waveform characteristics are optimized. Testing of dual function
(I/O buss) pins is a standard feature. Each
pin electronics section contains all the
hardware required to provide device bias,
inputdrive pulses, input clock pulses, detect
output ·pulses, and access to the DC measurement unit.
RAMS and other devices with dual function pins are easily tested. Input! output
definitions are defined as part of the test
procedure in local memory. The pin electronics card rapidly switches as necessary
from input mode to output sensor. This is
accomplished at real time rates not practical on other systems. No special wiring
changes or other adjustments are
necessary.
Testing LSI with Sentry 600
Long test sequences are handled with ease
by the Sentry 600. First, the powerful 24-bit
computer word controlled by FACTOR software allows addressing large memories in
short length instruction. Secondly, the
computer is backed up by an 18 million bit
disc memory for fast access to additional
patterns. Thirdly, since most long test
sequences are highly repetitive, only minor
variat.ions are usually required. A unique.
high speed controller can be programmed
to "loop" blocks of instruction. This allows
reuse of instructions without duplication
time or waste of memory. Standard microprogramming and the disc file make the
number of tests per pin theoretically infinite.
-16-
The versatile test center
Sentry 600, a high speed computercontroller system, tests, measures,
characterizes, and analyzes all electrical
parameters of complex large scale digital
circuits-bipolar and MaS, logic and
memories, single chips and assemblies.
Application
Performing the widest range of semiconductor parameter measurements for the
widest range of components, the Sentry 600
manipulates and arithmetically computes
data, datalogs every selected test parameter, and reports statistical distributions
and all types of data analysis. The system
provides for in-depth study before and after
environmental stresses-temperature,
vibration, shock, and electrical.
The Sentry 600 tests at all stages of
development-from wafer to packaged
component
LOJ wafer probing
I J final test
II pilot runs
1-.1 production
[] trend analysis
LI evaluation engineering
I I quality assurance
I J incoming inspection
I I research and development
Special capabilities
High speed testing
The Sentry 600 performs functional tests at
up to 10 MHz data rate, and greater than
20 MHz four phase clock rate. It makes
go-no go limit comparisons at 2,000 per
second; DC parametric measurements for
datalogging and data analysis at 1,000
per second.
Ability to test complex circuits
Accurate MaS testing ismade possible
through independent and precise control
of data and clock input timing and output
comparator strobe.
RAMS and other devices with dual-function
pins are quickly tested because any pin
of the tester can serve as data input driver
or output detector. Pins are switched from
input to output at real time rates under
software control.
Complex logic arrays and memories require
long function patt~rns. In the Sentry 600,
long pre-determined function patterns are
immediately available through direct access
of deep pattern storage. Behind each OUT
pin, the memory module provides a 256
bit pattern depth, expandable to 512 or
1024 bits per pin. Chaining provides a
pattern 4096 bits deep. Additional pattern
depth is available wit~in software and mass
memory storage.
Powerful, easy-to-use software
All testing hardware is under complete
software control. This software is compatible among all Sentry systems. Data
recorded on magnetic tape is compatible
with IBM 360 and other large computers.
English language FACTOR is easy to write,
modify, and understand, not only by programmers, but also by supervisors and other
departments. Software produces true
understandable reports, not just formatted
data.
Versatile
More test stations can .be added to increase
throughput. Station functions are modular
and can be modified or expanded to accommodate advances in the state-of-the-art.
-17-
System operation
Typical system operation begins with the
translation of a test specification into the
Sentry programming language FACTOR.
FACTOR statements are transferred
to punched cards, and are entered
into the system via the card reader. The
program is digested by the computer and
then stored on the 18-megabit disc file for
use by any test station. The di'sG can carry
an entire library of test programs. It stores
operating software system diagnostics,
device test programs and any temporary
work. Any test station can call up any program, facilitating broad system usage. The
magnetic tape transport adds virtually
infinite bulk storage.
The Video Keyboard Terminal, VKT, is the
primary man/machine interface. It controls
the operating modes of the Sentry 600,
displays test programs, provides for pro-:gram alterations directly from the keyboard,
and displays test data. A simple keyboard
command assigns the desired test program to the appropriate test station.
The operators' involvement is limited to
inserting the device, pressing the test start
button and observing Pass/Faillamp and/or
binning lamps. All testing and decision
making is performed by the Sentry 600. For
testing operations involving wafer probing,
device handlers or temperature chambers,
the inking or binning is accomplished
under system control.
When the test station requests service,
either manually (when the operator presses
the start test button)-or automatically (by
wafer prober or device handler), the
assigned test program is transferred from
disc to computer memory.
Acting under that test program, the computer automatically translates and directs
individual test commands to required hardware elements. Bias voltages and "0"
and "I" reference voltages are generated
in the mainframe and directed through
the test station controller to the device
under test at the test station.
Specialized high speed electronics are
triggered and the' preprogrammed truth
table flows into the device under test. Simultaneously, the function pattern flowing out
of the OUT is compared against the expected pattern. Any error in the OUT output
is automatically detected and recorded for
further processing per test program instructions stored in the computer.
-18-
In addition to the function test described
above, the computer automatically controls
and directs the measurement of all DC
electrical parameters of the device under
test. These results are also detected and
recorded for further processing as
requested by the test program.
During any testing sequence, the measured
and function fail data may be collected in
the computer memory for further processing.
Test results are formatted and, at user
option:
o displayed on the Video Keyboard
Terminal (VKT)
o printed at high speed on a line printer
o recorded on IBM compatible magnetic
tape for later processing by the 600
computer or larger off-line computer
o or recorded on the magnetic disc for
later retrieval.
Software
The FACTOR language, used for all
programming, allows the user to add many
levels of decision-making into the test
program. This allows exhaustive testing of
any device-type of any complexity. A
monitor mode allows the user to modify
and experiment with test conditions at any
test station. He can verify orogrammed
voltage and current levels, modify them
while observing the element being tested,
check out power suppiy levels, run strobe
Signals through an oscilloscope, etc.
Device characterization requires more than
thorough testing. The test results must be
analyzed. Lot data must be analyzed for
parameter distribution, mean value,
percentages within Sigma limits, and Delta
comparisons. The same FACTOR language
employed for test programming is used to
analyze the data in any manner required.
FACTOR power provides the testing and
data analysis tool to:
o characterize the device
o analyze the device
o analyze lot data
o provide device and yield improvement
data
o monitor process control
o monitor vendor shipments
o provide device engineering data and
controls
The entire 600 is focused on the testing and
data analysis requirements of the LSI
device. The system is organized to perform
that task. This description summarizes the
integrated system elements that focus on
the Device Under Test.
System description
COMPUTER CONTROL &
MAINFRAME GROUP
MAINFRAME CPU
Memory extension
I'cripheral
Interface
REF voltanes
Bias voltaqe
System ground
HIGH SPEED TEST
STATION GROUP
TEST STATION
CONTROLLER
Local memory
Control registers
-----PMU
Time control
LINF f'FIINT[H
OPTION
Operator display/
control
Pin electronics
DUT test head
The Sentry 600 includes
two hardware groups
The mainframe provides all central
functions: computer, its control functions,
peripherals, digital to analog converters for
reference voltage and bias supplies, and
multiplex control.
The high speed test station group provides
all specialized hardware necessary to
perform high speed functional and DC
parametric tests of wafers, packaged
devices, or higher level assemblies.
The mainframe group
System Computer
Fairchild's FST -1 process computer
controls the total system. Using FACTOR,
an English test program language, and
powerful 24-bit words, the computer
transmits control data and receives
subsystem status reports, interrupt requests
and test data. The computer uses 16K of
memory to control high speed test stations.
Operating in a direct memory access mode,
the computer minimizes test execution time
by direct loading of local memory.
MI\CNf:IIC lM'rOI'TION
System peripherals
Supporting core memory is an 18-megabit
disc mass memory unit with a fixed head
per track. The disc provides fast access to
any test program, operating software
systems, and system diagnostic and
self-check programs. About 350 test
programs averaging 1,500 words each can
be stored on disc along with the entire
operating systems software.
A high speed card reader provides fast.
convenient, and flexible test program entry
for complex LSI circuits. It also frees Video
Keyboard Terminal (VKT) for user/system
communication. The VKT controls the
system, displays test programs and test
results. It provides rapid access to the
system for quick switchover between
device t'ypes.
Two optional peripherals are IBM
compatible read/write magnetic tape
transport and high speed line printer. Tape
adds virtually infinite program storage and
extra datalogging capability. The high speed
printer offers rapid print-outs:
o computer aided test programs for
immediate use by the originator
o device data in report form for immediate
use by supervisors and management.
Villi ()
f,I-Y(l()Alli )
I ',,;play
C')lllr,>I
-19-
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t head on wn(er prober lowers your
vcstnwnt mql1lrcments.
Test heaci used as wAfer prober
Wnft'{ p{ohl' /Jsoej for rnnnlJnllc:stinq
Controlled by the computer, the mainframe
acts upon the assigned device test program.
The mainframe multiplex allows several
test stations to be controlled from one
center.
The mainframe contains the D to A
converters. These units provide
programmable voltages and currents. They
also provide all voltage references for the
zero and one logic levels used for function
testing at the individual test stations.
High speed test station group
This group includes the high speed test
station controller console and up to four
high speed test stations. Under control of
main system, this sub-system provides all
specialized hardware necessary for high
speed functional and DC parametric tests.
The controller console contains high speed
special purpose controller, timing
generators, and digital pin electronics,
reference voltage buffers and associated
pin select logic, the precision measurement
unit, and test station multiplexer.
Each test station contains test head
assembly with test head pin electronics. The
pin electronics is the final link between the
600 and the Device Under Test (DUT). Pin
electronics provide pin drivers and detector
functions to each pin of the DUT and access
to DC testing and performance board.
The compact test head is designed for
optimal device testing. Each pin electronics
card is located in a "carrousel"
configuration that provides minimum yet
uniform lead length to each DUT pin. The
assembly will accommodate up to 30 pin
cards to service up to 60 DUT pins. Each
pin electronics card can function as data
input driver, clock driver, device power
supply, and data output comparator. Inputs
can be switched to outputs within one data
period. No pin swap boards are needed.
Test heads function with manual test, wafer
p'rober, or automatic device handler. This
provides maximum device throughput with
optimum data correlation between wafer
test and final test.
-20-
High speed special purpose controller
The heart of the high speed test station
controller console unit is the high speed
special purpose controller, high speed
registers, and timing generators that control
function testing. The function test
patterns are stored in the local memory of
the special purpose controller.
The specialized controller consists of a
bipolar random access local memory,
control registers, and instruction set. One
local memory channel is assigned to each
DUT pin. The depth of the memory channels
is expandable from 256 to 1024 bits. The
channels can be "chained" under software
program control in order to double or
redouble the available function test pattern
depth.
In addition, the local memory architecture
and instruction set provides the following
capability under complete software program
control:
o Load function patterns into the local
memory from the FST-1 in a Direct
Memory Access (DMA) mode
o Store device input patterns and expected
output patterns for up to 60 DUT pins to
a depth of 1024 bits per pin
o Transfer the function patterns to the OUT
at full rated speed
o Compartmentalize the local memory into
major and minor "loops" with separate
start andstop addresses
o Alter any portion of the local memory
while testing continues at full rated speed
o Initialize the DUT (if required) then
proceed to testing without interruption
of the programmed test rate
o Branch to a new function pattern based
on test results
o Repeat the stored pattern at various
worst case voltage and timing conditions
o Remain in continuous recycle mode for
program debug or other manual test
req ui rements.
I/J(l/I I)('r/offllnnr:o pin oInctronics
High speed registers dedicated to each OUT
pin provide adqitional function test
capability under software program control
on a pin-by-pin basis:
.
o Selection of the input data format:
Return-to-Zero (RZ) or
Non-return-to-Zero (NRZ)
o Set up of the OUT pin as an input only,
output only or input/output (1/0 Buss)
pin
o Selection of one of two separate I/O
definition registers
o Change the set up of I/O Buss pins from
input to output on the fly at full rated
speed
o Masking 'of the output comparator during
any "don't care" condition of the output
pin
[1 Selection of one of two separate mask
registers so that function testing can
proceed from OUT initialization (a don't
care condition) to OUT testing (a care
condition) without interruption of the
testing rate
Timing generators
Up to eight timing generators provide
function test synchronization and timing of
input data and clock wave forms and
comparator strobes for clocked and
multiphase circuits. Any generator can be
assigned to any pin or pins under software
control. Each generator is completely and
independently programmable for both
delay and width. This independent control
provides the capability to accurately
program a narrow width pulse at the end
of a long delay.
-21-
Precision measurement unit
Precision measurement unit (PMU) performs
complete, full range OC parametric
measurements on any OUT pin under
software program control. This multi mode
unit may be used for stress, breakdown,
leakage, and loading measurements for
either golno go testing, or subsequent
datalogging/ data analysis. It is also the
primary instrument for system self check
tests. It makes go-no go limit comparisons
at 2,000 per second; OC parametric
measurements for datalogging and data
analysis at 1,000 per second.
The unit contains a digitally programmed
voltage supply with programmable current
limit, a digitally programmed current supply
with programmable voltage clamp, and a
programmable precision current or voltage
detector for high speed go-no go testing.
Either voltage-force/ current-sense or
current-force/voltage-sense measurements
may be made. The results are digitized for
formatting and analysis by the FST-1
computer or for transmittal to the magnetic
tape option for off-line storage, and
subsequent data analysis.
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