U.S. patent application number 12/007747 was filed with the patent office on 2008-05-15 for augmenting semiconductor's devices quality and reliability.
This patent application is currently assigned to OPTIMAL TEST LTD.. Invention is credited to Gil Balog, Nir Erez.
Application Number | 20080114558 12/007747 |
Document ID | / |
Family ID | 37308382 |
Filed Date | 2008-05-15 |
United States Patent
Application |
20080114558 |
Kind Code |
A1 |
Erez; Nir ; et al. |
May 15, 2008 |
Augmenting semiconductor's devices quality and reliability
Abstract
A method for augmenting quality or reliability of semiconductor
units, including providing few populations of semiconductor units
that are subject to quality or reliability testing. The populations
include few quality or reliability fail candidate populations and
other population(s). The method includes the step of associating
test flows to the populations. Each test flow includes stress
testing sequence. The stress testing sequence for the quality or
reliability fail candidate population includes a stress test of
increased duration compared to duration of a stress test in the
test flow of the other population. The stress test sequence for the
other population includes a stress test of increased voltage
compared to corresponding operating voltage specification for a
semiconductor unit. The method further includes the step of
applying, within a sort testing stage, the corresponding test flow
to the populations and identifying any unit which failed the stress
sequence.
Inventors: |
Erez; Nir; (Nes Zionna,
IL) ; Balog; Gil; (Jerusalem, IL) |
Correspondence
Address: |
Gary M. Nath;THE NATH LAW GROUP
112 South West Street
Alexndria
VA
22314
US
|
Assignee: |
OPTIMAL TEST LTD.
|
Family ID: |
37308382 |
Appl. No.: |
12/007747 |
Filed: |
January 15, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11343209 |
Jan 31, 2006 |
7340359 |
|
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12007747 |
Jan 15, 2008 |
|
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60676289 |
May 2, 2005 |
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Current U.S.
Class: |
702/81 |
Current CPC
Class: |
G01R 31/287 20130101;
G11C 29/56 20130101; G01R 31/2894 20130101; G11C 29/56008
20130101 |
Class at
Publication: |
702/081 |
International
Class: |
G06F 19/00 20060101
G06F019/00 |
Claims
1. A method for augmenting quality or reliability of semiconductor
units, comprising: (a) providing at least two populations of
semiconductor units that are subject to quality or reliability
testing; the populations include at least one quality or
reliability fail candidate population and at least one other
population; (b) associating test flows to said populations; each
test flow includes stress testing sequence; the stress testing
sequence for at least one of said quality or reliability fail
candidate populations includes a stress test of increased duration
compared to duration of a stress test in the test flow of a
population of at least one of said other populations; the stress
test sequence for at least one of said other populations includes a
stress test operable under different temperature compared to
corresponding operating temperature specification for a
semiconductor unit of said semiconductor units; (c) applying,
within a sort testing stage, the corresponding test flow to each
population and identifying any unit which failed the stress
sequence.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a system and method for augmenting
semiconductor integrated circuit quality or reliability through
real-time testing.
BACKGROUND OF THE INVENTION
[0002] Testing has emerged as a key constraint in the push for more
advanced, reliable and cost-effective semiconductor based products.
Advances in semiconductor process technology have enabled chip
designers to pack high volume production chips with 100 million
transistors. Experts predict this will increase to more than 1
billion transistors within the next few years.
[0003] Semiconductor process technology is characterized by Moore's
Law, which states that the numbers of transistors in a given
surface area will double every 18 months. Much of this density
increase is driven by smaller and smaller line widths or
geometries. Today 90 nm devices are common and 25 nm designs are in
development (a nanometer is one billionth of a meter or 1/1200th
width of human hair). These advances place a strain on test systems
as more transistors and structures must be tested. This exponential
growth rate also drives a continual increase in the process, design
and manufacturing complexities which, in turn, can increase
systemic problems that affect quality, yield and product
reliability. All of these factors tend to drive the need for more
test time and more comprehensive testing, thus test cost has become
a major factor in the overall manufacturing cost of an integrated
circuit.
[0004] Even with commodity and other products there is an
ever-increasing demand by customers to push quality, yield and
reliability to new levels. These factors have combined to put major
stresses on conventional test capacity and systems. Current
solutions to the problem are not cost-effective and rely on either
adding expensive capital equipment or relaxing quality
standards.
[0005] Quality is a known term in the semiconductor industry. One
known definition of this term is "a product's nature or features
that reflect capacity to satisfy express or implied statements of
needs" (Deming, W. Edwards, Out of the Crisis, Cambridge, Mass.
Aug. 31, 1993).
[0006] Reliability is a known term in the semiconductor industry.
One known definition of this term is: "the conditional probability,
at a given confidence level, that the equipment will perform its
intended functions satisfactorily or without failure, i.e., within
specified performance limits, at a given age, for a specified
length of time, function period, or mission time, when used in the
manner and for the purpose intended while operating under the
specified application and operation environments with their
associated stress levels" (Kececioglu, Dimitri, Reliability
Engineering Handbook, Vol. 1, 2002). Note, incidentally, that these
definitions are provided for clarity only and, accordingly, the use
of the terms "quality" and/or "reliability" in the context of the
invention is by no means bound by these definitions.
[0007] Currently, testing is performed by two basic methods.
Either, devices are tested one at a time, in singular manner
(sequentially); or, several are tested at the same time in
"parallel". Singular tests are more common with complex products
such as CPUs while memory devices are most often tested in
parallel. Note, however, that these examples are not binding.
[0008] As is well known, there are various manufacturing stages
including "sort" which aims at testing the semiconductor devices at
wafer level and a later final stage which aims at testing the
semiconductor devices after having been packaged.
[0009] The main goal of the specified stages is to separate out
potentially good devices from those that fail testing.
[0010] Each of the specified phases is traditionally divided into a
few sockets, such as sort1, sort2 etc. Each test socket is applied
to the same product (e.g. to all manufactured lots of wafers of the
particular product) in a slightly different test flow. For
instance, for a given socket, a set of tests is performed at a
given temperature, whereas for another socket, the same (or
similar) tests are performed at a different temperature. By way of
another example, functional tests are applied to the first socket
and structural tests are applied to the second socket.
[0011] By the same token, the final test phase is divided into
distinct stages, such as final1, final2, etc.
[0012] As is well known, a semiconductor device is always designed
at the single device level. The resulting single prototype is then
replicated again and again in the semiconductor fabrication process
using lithography or other equipment across the surface of a
silicon wafer.
[0013] Design for Testability (DFT) is the process in which the
device designers create various tests such as functional tests that
include the correct patterns and vectors that are used to test the
device according to its expected functionality, or creation of
parametric tests, etc. A test flow may include functional tests,
parametric tests and/or other tests, (whichever the case may be)
that are used in the testing process.
[0014] Test Program (TP) is composed of test flows that are applied
across all the devices on a product in a nearly identical manner.
The TP is product specific.
[0015] Typically, at a given socket (during sort stage) 100% of all
dice are put through the requisite tests and undergo the entire TP
(100% TP). In other words, all dice will be subjected to the same
test flow, such as continuity, opens, shorts, various functional
tests, stuck at scan, at-speed scan, leakage, icc, iddq and/or many
more--depending upon various parameters such as the device
type.
[0016] There are several inherent limitations of the specified
testing methodologies, for instance: final IC product reliability
can be increased by applying additional tests and stresses,
however, since the test program is identically applied over the
entire wafer, such a methodology will affect the whole product and
thus dramatically increase overall test cost.
[0017] Product reliability is a key customer satisfaction issue and
affects the reputation of the manufacturer.
[0018] Bearing this in mind there follows a list of publications
which may be regarded as related art:
[0019] U.S. Pat. No. 6,184,048 discloses: A method for assuring
quality and reliability of semiconductor integrated circuit
devices, fabricated by a series of documented process steps,
comprising first, electrically testing the devices outside their
specified operating voltage range, yet within the capabilities of
the structures produced by the process steps, thereby generating
raw electrical test data; second, comparing the test data to values
expected from the design of the devices, thereby providing
non-electrical characterization of the devices to
verify-compositional and structural features; and third,
correlating the features with the documented process steps to find
deviations therefrom, as well as structural defects, thereby
identifying outlier devices. After eliminating the outlier devices,
the accepted devices do no longer have to undergo the traditional
burn-in process.
[0020] U.S. Pat. No. 6,618,682 discloses: A method and system are
provided that minimize wafer or package level test time without
adversely impacting yields in downstream manufacturing processes or
degrading outgoing quality levels. The method provides optimization
by determining, a priori, the most effective set of tests for a
given lot or wafer. The invention implements a method using a
processor-based system involving the integration of multiple
sources of data that include: historical and real-time, product
specific and lot specific, from wafer fabrication data (i.e.,
process measurements, defect inspections, and parametric testing),
product qualification test results, physical failure analysis
results and manufacturing functional test results. These various
forms of data are used to determine an optimal set of tests to run
using a test application sequence, on a given product to optimize
test time with minimum risk to yield or product quality.
[0021] US 2003/0120457 discloses: A system and method for
determining the early life reliability of an electronic component,
including classifying the electronic component based on an initial
determination of a number of fatal defects, and estimating a
probability of latent defects present in the electronic component
based on that classification with the aim of optimizing test costs
and product quality.
[0022] There is a need in the art for a new system and method for
augmenting semiconductor integrated circuit quality or reliability
through real-time optimal testing. Moreover, there is a need in the
art to provide a system and method for applying selectively
different test flows to different semiconductor units. The
different test flows may apply, for example, to different
populations in a wafer or in accordance with another example, to
different wafers in a lot. The different populations may be for
example different geographies and/or different lithography
exposures. The unit may be a whole die or module(s) within a
semiconductor device (die). The different test flows may be applied
e.g. during sort testing stage and/or during final testing
stage.
SUMMARY OF THE INVENTION
[0023] In accordance with certain embodiments of the invention,
reliability augmentation is a method that utilizes e.g. IC product
characteristics, product fabrication process and test data
knowledge to optimize device test and stress flows resulting in a
more reliable product. Typically, the more reliable the product,
the lower the Defective Parts per Million (DPPM) score.
[0024] In accordance with certain embodiments of the invention,
there is provided a method and system that provide augmentation of
semiconductor units (device or module(s) thereof) quality or
reliability by determining the set of reliability tests applied
e.g. to a specific IC lot, wafer, areas of a wafer, wafer die, dice
within the same lithography exposure, dice with the same location
across lithography exposures, packaged device or (functional)
modules within an IC.
[0025] In accordance with certain embodiments, there is provided a
data manager, station controller, database and software. The
station controller integrates information provided from the data
manager. The latter gathers historic information from various
sources and analyses this for identifying the relevant
population(s) of semiconductor units which are candidate to fail
quality or reliability tests and associating thereto appropriate
test flow(s) including stress tests. The test flow(s) are handled
at the station controller and are applied to the specified
populations through the product's test program and will augment the
product's reliability as compared to the original product expected
quality or reliability.
[0026] There is provided a method for augmenting quality or
reliability of semiconductor units, comprising: [0027] (a)
providing at least two populations of semiconductor units that are
subject to quality or reliability testing; the populations include
at least one quality or reliability fail candidate population and
at least one other population; [0028] (b) associating test flows to
said populations; each test flow includes stress testing sequence;
the stress testing sequence for at least one of said quality or
reliability fail candidate populations includes a stress test of
increased duration compared to duration of a stress test in the
test flow of a population of at least one of said other
populations; the stress test sequence for at least one of said
other populations includes a stress test of increased voltage
compared to corresponding operating voltage specification for a
semiconductor unit of said semiconductor units; [0029] (c)
applying, within a sort testing stage, the corresponding test flow
to each population and identifying any unit which failed the stress
sequence.
[0030] There is also provided a method for augmenting quality or
reliability of semiconductor units, comprising: [0031] (a)
providing at least two populations of semiconductor units that are
subject to quality or reliability testing; [0032] (b) associating
test flows that include stress test sequences to said populations;
a first flow from among said flows further includes identical first
pre-stress test sequence and first post stress test sequence; a
second flow from among said test flows further includes identical
second pre-stress test sequence and second post stress test
sequence; the first sequence and the second sequence are different;
[0033] (c) applying, within a sort testing stage: [0034] i) the
test flows that includes a pre stress sequence, a stress sequence
and a post stress sequence to the respective population for
identifying any unit which failed the post stress sequence.
[0035] The invention further provides a method for augmenting
quality or reliability of semiconductor units, comprising: [0036]
(a) providing at least two populations of semiconductor units that
are subject to quality or reliability testing; the populations
include at least one quality or reliability fail candidate
population and at least one other population; [0037] (b)
associating test flows to said populations; each test flow includes
stress testing sequence; the stress testing sequence for at least
one of said quality or reliability fail candidate populations
includes a stress test of increased duration compared to duration
of a stress test in the test flow of a population of at least one
of said other populations; the stress testing sequence for at least
one of said other populations includes a stress test of increased
voltage compared to corresponding operating voltage specification
for the semiconductor unit; [0038] (c) applying, within a final
testing stage, the corresponding test flow to each population and
identifying any unit which failed the stress sequence.
[0039] The invention further provides a method a for providing
augmented quality or reliability tests configured to be applied to
semiconductor units, comprising: [0040] (a) analyzing data gathered
from historical information that pertains to semiconductor units
for identifying at least two populations of semiconductor units
that are subject to quality or reliability testing; the populations
include at least one quality or reliability fail candidate
population and at least one other population; at least one of said
quality or reliability fail candidate populations includes a
geographical area in a wafer that includes a cluster of at least
two consecutive devices, such that any device in the identified
cluster has at least one neighboring device which also belongs to
said cluster.
[0041] The invention further provides a method for providing
augmented quality or reliability tests configured to be applied to
semiconductor units, comprising: [0042] (a) analyzing data gathered
from historical information that pertain to semiconductor units for
identifying at least two populations of semiconductor units that
are subject to quality or reliability testing; [0043] (b) the
populations include at least one quality or reliability fail
candidate population and at least one other population; [0044] (c)
at least one of said quality or reliability fail candidate
populations includes devices with the same lithography
exposure.
[0045] The invention further provides a method for providing
augmented quality or reliability tests configured to be applied to
semiconductor units, comprising: [0046] (a) analyzing data gathered
from historical information that pertain to semiconductor units for
identifying at least two populations of semiconductor units that
are subject to quality or reliability testing; [0047] (b) the
populations include at least one quality or reliability fail
candidate population and at least one other population; [0048] (c)
at least one of said quality or reliability fail candidate
populations includes a devices with same location across
lithography exposures.
[0049] The invention further provides a method for providing
augmented quality or reliability tests configured to be applied to
semiconductor units during a manufacturing stage, comprising:
[0050] (a) analyzing data gathered from historical information that
pertains to semiconductor units for identifying at least two
populations of semiconductor units that are subject to quality or
reliability testing; [0051] (b) the populations include at least
one quality or reliability fail candidate population and at least
one other population; [0052] (c) providing a repertoire of test
flows; [0053] (d) for at least one of said quality or reliability
fail candidate population perform the following, including: [0054]
i) providing a reference burn-in "units fail" score in respect of
the quality or reliability fail candidate population of a
semiconductor sample; [0055] ii) selecting a test flow from said
repertoire of test flows; [0056] iii) applying the test flow to the
quality or reliability fail candidate population of substantially
identical semiconductor sample and logging the tested "units fail"
score; [0057] iv) applying a burn-in test to the quality or
reliability fail candidate population of the semiconductor sample
of (ii) and logging the tested burn-in "units fail" score; units
failed in said tested "units fail" score are excludes from the
burn-in "units fail" score; [0058] v) repeating as many times as
desired (ii) to (iv), in time, in respect to different test flow
from said repertoire of test flows; [0059] vi) determining a
prevailing test flow from among the flows used in (ii), (iii) and
(iv), according to a decision criterion; the decision criterion is
dependent at least on the reference burn-in "units fail" score, and
the tested "units fail" score and tested burn-in "units fail" score
of the prevailing test; and associating the prevailing test to said
quality or reliability fail candidate population.
[0060] The invention further provides a method for providing
augmented quality or reliability tests configured to be applied to
semiconductor units during sort stage, comprising: [0061] (a)
analyzing data gathered from historical information that pertains
to semiconductor units for identifying at least two populations of
semiconductor units that are subject to quality or reliability
testing; [0062] (b) associating a respective selected test flow to
each one of said populations according to a decision criterion that
includes a condition that the fail rate of units in a burn-in test
that follows the selected test flow is lower than a fail rate of
units in a burn in test that follows a test flow other than the
selected test flow.
[0063] The invention further provides a system for augmenting
quality or reliability of semiconductor units, comprising: [0064]
(a) means including a processor and storage configured to provide
at least two populations of semiconductor units that are subject to
quality or reliability testing; [0065] (b) the populations include
at least one quality or reliability fail candidate population and
at least one other population; [0066] (c) means including a
processor and storage configured to associate test flows to said
populations; each test flow includes stress testing sequence;
[0067] (d) the stress testing sequence for at least one of said
quality or reliability fail candidate populations includes a stress
test of increased duration compared to duration of a stress test in
the test flow of a population of at least one of said other
populations; [0068] (e) the stress test sequence for at least one
of said other populations includes a stress test of increased
voltage compared to corresponding operating voltage specification
for a semiconductor unit of said semiconductor units; [0069] (f)
means configured to apply, within a testing stage, the
corresponding test flow to each population and identifying any unit
which failed the stress sequence.
[0070] The invention further includes a system for augmenting
quality or reliability of semiconductor units, comprising: [0071]
means including a processor and storage configured to provide at
least two populations of semiconductor units that are subject to
quality or reliability testing; [0072] means including a processor
and storage configured to associate test flows that include stress
test sequences to said populations; a first flow from among said
flows further includes identical first pre-stress test sequence and
first post stress test sequence; a second flow from among said test
flows further includes identical second pre-stress test sequence
and second post stress test sequence; [0073] the first sequence and
the second sequence are different; [0074] means configured to
apply, within a testing stage the test flows that includes a pre
stress sequence, a stress sequence and a post stress sequence to
the respective population for identifying any unit which failed the
post stress sequence.
[0075] The invention further includes a computer program product
that includes a storage storing population data indicative of at
least two populations of semiconductor units that are subject to
quality or reliability testing; the populations include at least
one quality or reliability fail candidate population and at least
one other population; the storage further storing test flow data
indicative of test flows associated to said populations; each test
flow includes stress testing sequence; the stress testing sequence
for at least one of said quality or reliability fail candidate
populations includes a stress test of increased duration compared
to duration of a stress test in the test flow of a population of at
least one of said other populations; the stress test sequence for
at least one of said other populations includes a stress test of
increased voltage compared to corresponding operating voltage
specification for a semiconductor unit of said semiconductor units;
said population data and test flow data is usable by means
configured to apply, within a testing stage, the corresponding test
flow to each population and identifying any unit which failed the
stress sequence.
[0076] The invention further includes a computer program product
that includes a storage storing population data indicative of at
least two populations of semiconductor units that are subject to
quality or reliability testing; the storage further storing test
flow data indicative of test flows that include stress test
sequences to said populations; a first flow from among said flows
further includes identical first pre-stress test sequence and first
post stress test sequence; a second flow from among said test flows
further includes identical second pre-stress test sequence and
second post stress test sequence; the first sequence and the second
sequence are different; said population data and test flow data is
usable by means configured to apply, within a testing stage the
test flows that includes a pre stress sequence, a stress sequence
and a post stress sequence to the respective population for
identifying any unit which failed the post stress sequence.
BRIEF DESCRIPTION OF THE DRAWINGS
[0077] In order to understand the invention and to see how it may
be carried out in practice, a preferred embodiment will now be
described, by way of non-limiting example only, with reference to
the accompanying drawings, in which:
[0078] FIG. 1A is a generalized system architecture, in accordance
with an embodiment of the invention;
[0079] FIG. 1B is a typical test scanning of a wafer, in accordance
with an embodiment of the invention;
[0080] FIG. 2 is a flow diagram showing data-feed forward sequence
in accordance with an embodiment of the invention;
[0081] FIG. 3A is a schematic illustration of semiconductor wafer
showing distinct quality or reliability fail candidate populations,
in accordance with an embodiment of the invention;
[0082] FIG. 3B-3D illustrate schematically a few distinct
geographies that are subject to the augmentation of quality or
reliability technique, in accordance with an embodiment of the
invention;
[0083] FIG. 4 illustrates schematically a semiconductor device
(including few functional modules) that is subjected to the
augmentation of quality or reliability technique, in accordance
with an embodiment of the invention;
[0084] FIG. 5A illustrates schematically a flash memory device,
that is subjected to the augmentation of quality or reliability
technique, in accordance with an embodiment of the invention;
[0085] FIG. 5B illustrate a group of dice that result from a single
exposure through a mask as well as across exposures, in accordance
with a certain embodiment of the invention;
[0086] FIGS. 6A-6C are three graphs showing various types of
detected outlier dice, in accordance with an embodiment of the
invention;
[0087] FIG. 6D illustrates schematically a flow diagram of a feed
backward sequence of operations, in accordance with an embodiment
of the invention;
[0088] FIG. 7A illustrates schematically a flow diagram of a
sequence of operation in accordance with an embodiment of the
invention;
[0089] FIG. 7B illustrates schematically a mechanism for heating a
wafer to different temperatures, in accordance with an embodiment
of the invention;
[0090] FIG. 8 is a flow diagram showing a quality or reliability
test sequence, in accordance with an embodiment of the
invention;
[0091] FIG. 9 is a flow diagram illustrating a sequence of
operations for associating test flows to distinct quality or
reliability fail candidate populations, in accordance with an
embodiment of the invention; and
[0092] FIGS. 10A-10B illustrate two tables for further exemplifying
the flow chart sequence of FIG. 9;
[0093] FIG. 11 is a flow diagram showing conventional quality or
reliability test flow as known in the prior art;
[0094] FIG. 12 is a flow diagram showing a quality or reliability
test flow in accordance with a certain embodiment of another aspect
of the invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0095] The present invention is primarily disclosed as a method and
it will be understood by a person of ordinary skill in the art that
an apparatus such as a conventional data processor incorporated
with a database, software and other appropriate components could be
programmed or otherwise designed to facilitate the practice of the
method of the invention.
[0096] Unless specifically stated otherwise, as apparent from the
following discussions, it is appreciated that throughout the
specification discussions, utilizing terms such as, "processing",
"computing", "calculating", "determining", "applying",
"associating", "providing" or the like, refer to the action and/or
processes of a computer or computing system, or processor or
similar electronic computing device, that manipulate and/or
transform data represented as physical, such as electronic,
quantities within the computing system's registers and/or memories
into other data, similarly represented as physical quantities
within the computing system's memories, registers or other such
information storage, transmission or display devices.
[0097] Embodiments of the present invention may use terms such as,
processor, computer, apparatus, system, sub-system, module, unit,
device (in single or plural form) for performing the operations
herein. This may be specially constructed for the desired purposes,
or it may comprise a general purpose computer selectively activated
or reconfigured by a computer program stored in the computer. Such
a computer program may be stored in a computer readable storage
medium, such as, but not limited to, any type of disk including
optical disks, CD-ROMs, magnetic-optical disks, read-only memories
(ROMs), random access memories (RAMs), electrically programmable
read-only memories (EPROMs), electrically erasable and programmable
read only memories (EEPROMs), magnetic or optical cards, any other
type of media suitable for storing electronic instructions that are
capable of being conveyed via a computer system bus.
[0098] The processes/devices (or counterpart terms specified above)
and displays presented herein are not inherently related to any
particular computer or other apparatus, unless specifically stated
otherwise. Various general purpose systems may be used with
programs in accordance with the teachings herein, or it may prove
convenient to construct a more specialized apparatus to perform the
desired method. The desired structure for a variety of these
systems will appear from the description below. In addition,
embodiments of the present invention are not described with
reference to any particular programming language. It will be
appreciated that a variety of programming languages may be used to
implement the teachings of the inventions as described herein.
[0099] Attention is first directed to FIG. 1A, illustrating general
system architecture in accordance with an embodiment of the
invention. As shown, the system includes two main components each
designated to a distinct process. The first process manages
gathering and analyzing of historical information 10 in a data-feed
forward system 110 and makes the processed data available via a
local area network 104 to a second process that controls the test
cell 109. The latter includes a station controller 105, Automatic
Test Equipment (ATE) 106 and handling equipment 108. A test program
107 that accommodates various test flows is used to program and
sequence the ATE for testing the different populations of Devices
Under Test (DUT's) 111, all as will be explained in greater detail
below. Handling equipment 108 refers to a wafer prober adapted to
handle wafer.
[0100] A similar test cell can be used, for instance for the final
stage test, wherein the handling equipment is a device handler
adapted to handle packaged devices.
[0101] Also shown is burn-in oven cell 120 used to test selected
wafers in a known per se manner.
[0102] The data-feed forward system 110 which gathers, stores, and
processes historical information 10 from multiple sources such as
fabrication (Fab) data 100, wafer sort data 101, and final test
data 102 by utilizing a data manager 103 component which consists
of a suitable database and software. The historical information can
be derived from other sources 11 instead or in addition to the
specified sources, all as will be explained in greater detail
below.
[0103] The so gathered and processed data serves in accordance with
certain embodiments to determine distinct populations which are
subjected to different test flows during the sort stage. Data
indicative of the so determined populations of semiconductor units
(e.g. semiconductor devices or distinct module(s) thereof) are
stored (say, in one of the repositories 10) for further use. The
populations of semiconductor units are associated with distinct
test flows stored (say, in one of the repositories 10) as will be
explained in greater detail below. Note that the gathering and
processing of the data can be performed as a preliminary stage not
necessarily in conjunction with the actual sort testing phase as
performed by the control test cell 109. Thus, for example, in
accordance with certain embodiments, the data feed forward system
110 can be disposed at different locations and operated at
different times than the test cell 109. In the latter case, the
data processed by manager 103 is provided to the station controller
105, e.g. through WAN or other means.
[0104] In accordance with certain other embodiments, the control
test cell 109 and data feed forward system 110 are integrated into
a single system and operated serially such that data processed by
manager 103 is fed to the station controller 105, e.g. through LAN
104.
[0105] Also shown in FIG. 1A, is a station-controller 105 which
consists of a conventional data processor that controls the test
cell 109 via a Test Cell Bus (TCB) 112. The station controller 105
integrates information provided by the data-feed forward system 110
and uses the information to make decisions upon the data relevancy
as applied to each DUT 111. More specifically, in accordance with
certain embodiments, the station controller 105 can load population
data (say, from the wafer sort data repository 101) and feed the
data to the Handling equipment 108 (through BUS 112) enabling the
latter (based on the population data) to identify the location of
the Device(s) Under Test (DUT(s)) that are subjected to the test.
The operation of the Data handler 108 is generally known per se and
therefore will not be further expounded upon herein.
[0106] Data indicative of the so associated test flow(s) is loaded
from the database (say, from the wafer sort data repository 101)
and serves for modifying TP 107, e.g. by updating global variables
of the TP (107). The values of the specified variables may vary
from one test flow to the other. The ATE (106) can then apply the
TP to the designated DUT(s) that constitute the population of
devices under test.
[0107] Turning now to FIG. 1B, there is shown a typical test
scanning sequence of a wafer. Thus, by this example wafer 120
includes 3 distinct populations of dice. The first population
includes dice marked `0` (such as 121). The second population
includes dice marked `1` (such as 122) and the third population
includes dice marked `blank` (such as 123). Each population is
associated with a distinct stress test flow. The manner of
determining the various populations and the associating thereto
test flows will be discussed in greater detail below.
[0108] The serpentine manner of scanning the wafer is illustrated
by serpentine line 124. This test scanning pattern is provided for
illustrative purposes only and is by no means binding. As is well
known, in parallel testing, scanning of the dice may be performed
in parallel such that during each touchdown a few dice are tested
simultaneously. In the example of FIG. 1B, this means that instead
of testing one die at a time according to the testing sequence
prescribed by the serpentine line, a group of dice, say 2.times.2
are tested simultaneously, and thereafter the probes are moved (in
a serpentine manner) to the next touchdown, covering another group
of dice to be tested, etc. The number 2.times.2 is provided, of
course, for illustrative purposes only.
[0109] Reverting now to the station controller 105 of FIG. 1A, in
accordance with certain embodiments, the data manager 103 is in
charge of data manipulation and preparation so that algorithms or
rules that reside inside the station controller 105 can utilize the
historic information and knowledge it provides (e.g. list of
populations and associated test flows) in the decision making
process, including instructing the Handling equipment 108 what are
the DUTs that need to be tested and modifying the TP according to
the appropriate test flows associated to the particular population
of devices. In accordance with certain embodiments the algorithms
are basically "What/If/Then" rules that take into account the
population data and test flow data and instruct accordingly the
Handling equipment as well as dynamically updating the global
variables of the test program.
[0110] The invention is not bound by the architecture and manner of
operation of the system architecture of FIG. 1A.
[0111] The specified processes make intelligent use of various data
to control the test cell 109 and optimize test coverage of each DUT
111 across the entire test coverage chain such that
quality/reliability of the final product is increased.
[0112] FIG. 2 shows a data-feed forward sequence in accordance with
an embodiment of the invention, aims at gathering and analyzing
information for identifying distinct populations of semiconductor
units. FIG. 2 is an exemplary drill-down illustration of the feed
forward process of data gathered from the repositories 10 and
analyzed by Data manager 103 (of FIG. 1A).
[0113] The flow diagram is divided into three semiconductor
manufacturing sections namely, wafer fabrication (Fab) 200, wafer
sort 201 and final test 202. Each of the sections may contain
multiple manufacturing or testing steps where measurements are
performed and data is stored. Examples of data (forming example of
historical data) that can be gathered, processed and fed forward
from the wafer Fab 200 include inline electrical parametric tests
on the wafers scribe line, lithography and metrology data, defects
review and analysis data, Advanced Process Control (APC) data and
other wafer manufacturing data. By way of non-limiting example, a
population of devices exhibiting shift in the results of one or
more of the specified test measurements (exceeding a given
threshold) may be subjected to additional stress test sequences,
all as will be explained in greater detail below.
[0114] The processed historical information may serve for
determining population of semiconductor units that will be
subjected to distinct test flows in accordance with various
embodiments of the invention as will be explained in greater detail
below.
[0115] As further shown in FIG. 2, at sort stage 201, finished
wafers are tested for the first time by applying appropriate test
flows to corresponding populations of semiconductor devices (data
of which is incorporated in the test program) and using to this end
the station controller 105 and ATE 106.
[0116] The historical information that affects the determination of
the populations/tests that will be applicable at sort stage can be
derived also from previous sort stages.
[0117] By way of non-limiting example these data may include data
from all test phases such as hot, cold, functional, structural,
Sort 1, Sort 2, Sort 3, optical inspection data from wafer sort
201; stored data from previous tested wafers and lots; test results
from neighborhood dice.
[0118] In accordance with a certain embodiment, historical
information may be gathered also from previous final test 202 stage
where assembled and packaged devices are tested at various testing
steps such as: final1 and final2 to augment product
reliability.
[0119] Note that it is not, of course, necessarily required to
collect historical information from all the manufacturing stages
and accordingly information can be gathered and analyzed from only
selected one or more stages.
[0120] As will be further discussed below, the historical
information may be gathered and analyzed from other sources,
designated generally as 11 in FIG. 1A.
[0121] There follows now a list of additional sources of data which
may serve as historical information for determining distinct
populations.
[0122] Thus, in accordance with certain embodiments, at least one
of the following sources may be used in addition or in lieu of the
data sources discussed above with reference to FIG. 2.
[0123] 1. Shifts or failure signatures in wafer Fab parametric
E-Test may require more stress testing at wafer sort. For example,
a shift in a parameter tested in the scribe line may provide an
indication for a quality or reliability fail candidate population
that embraces few dice (or sub-die module(s)). This may require
more stress test during future sort stage testing.
[0124] 2. Shifts or failure signatures in wafer Fab Particle
signatures may require more stress testing at wafer sort. For
example, a shift in a particle signature may provide an indication
for a quality or reliability fail candidate population that
embraces few dice (or sub-die module(s)). This may require more
stress test during future sort stage testing.
[0125] 3. Shifts or failure signatures in a given socket at a given
stage (say, sort) may require more stress testing at future test.
For example, it may stipulate on testing of the same or different
socket during future sort testing. Thus, by way of non limiting
example, shifts in signatures in sort3 testing of a given lot, may
prescribe that additional stress testing will be applied in sort3
(i.e. same socket) of the next tested lot. Or, by way of another
non limiting example, it may prescribe that additional stress
testing will be applied in sort2 (i.e. socket -1) of the next
tested lot. Or, by way of yet another non limiting example, it may
prescribe that additional stress testing will be applied in sort4
(i.e. socket +1) of the next tested lot. These examples are of
course not binding and others may apply e.g. shifts or failure
signatures in a given socket at a final stage test may require more
stress testing at future final testing.
[0126] 4. Memory IC's or IC's that contain memories (i.e.,
microprocessors) may have redundancy repair features built-in. IC's
with a high repair rate are usually subjected to more reliability
issues then IC's with minimal repairs and therefore may constitute
an exemplary quality or reliability fail candidate population that
requires more stress test during future sort stage testing. In
other words, the more redundancy is activated in a given
population, the more reliability testing is required in the later
tested population.
[0127] 5. Any process window parameter can go out of statistical
control in the Fab. If the control systems in the Fab do not
correct the problem before the lots move on to other manufacturing
or test steps, some of the material may have reliability issues.
These lots referred to as excursion lots may require a different
stress flow at wafer test. This data would be fed-forward to from
wafer Fab to wafer sort and the specified population (e.g. these
excursion lots may require additional stress test during the
subsequent sort and/or final test stages).
[0128] Those versed in the art will readily appreciate that the
examples discussed above (with reference to 1-5) and with reference
to FIG. 2 are by no means binding, and accordingly other sources of
historical information may be used in addition or in lieu of the
above.
[0129] It should be further noted that there may be distinct
populations defined in accordance with the corresponding historical
information. For instance there may be a first population of
devices or modules (being subjected to first test flow) that is
defined based on Shifts or failure signatures in wafer Fab
parametric E-Test (see 1 above) and a different population of
devices or modules (being subjected to a second test flow) that is
defined based on Shifts or failure signatures in wafer Fab Particle
signatures (see 2 above).
[0130] It should be further noted that there is not necessarily one
to one correspondence between the historical source of information
and the identified population. For instance two or more of the
specified sources may be combined to define a given population that
is subjected to a given stress test flow. For instance (and for
illustrative purposes only), an elevation on a parameter like gate
leakage in Etest testing may identify a given population. If a
portion or all the specified population is identified in the sort
test as outliers (e.g. devices having test results which deviate
from the norm of neighboring dice) then this may reinforce the need
to apply a more demanding stress test sequence to the specified
population (e.g. in a wafer from the same lot) that will undergo
future sort testing stage.
[0131] It should be further noted that whilst the description above
focused on determination of populations and associated stress test
sequences that will be applied on semiconductor units during sort
stage, in accordance with certain other embodiments the stress
sequences will be applied mutatis mutandis to populations of
semiconductor devices or modules thereof during the final test
stage (where, normally, the devices are already packaged).
[0132] In accordance with certain embodiments, reliability problems
identified at packaged devices at final stage can normally be
traced back to specific X-Y locations on the wafer. This will be
exemplified in the following non-limiting examples. (i) In the case
that certain packaged devices are identified as fail candidate
population during final test stage, then applying a tractability
technique would allow to identify that all those devices originate,
for instance, from X, Y locations of the edge region of a wafer.
Based on this observation, dice that reside in the X, Y locations
of edge region of yet untested wafers can be subjected to stress
test during future sort testing. (ii) In accordance with another
non-limiting example, assuming that during sort stage, edge wafers
are identified as a fail candidate population that should be
subjected to additional stress tests in the following final test
stage. Then, when the final test stage actually occurs, it is
mandatory to apply tractability techniques in order to identify the
packaged devices that originate from the X, Y locations of the
wafer's edge region in order to apply to these devices the desired
additional stress tests.
[0133] Traceability techniques are generally known per se. Some
products/processes use traceability techniques in order to trace
the device ID (Lot, Wafer & X/Y coordinates) after it has been
"sawed" from the wafer. This traceability can be done by inside the
unit itself using fuses or using database trace techniques. Note
that in certain scenarios in final stage testing there is no need
for applying tractability techniques for identifying the "origin"
of the device. This in particular applies to feed backward
techniques, such as identifying outliers (as will be discussed in
greater detail below with reference to FIG. 6).
[0134] Attention is now drawn to FIG. 3A showing a schematic
illustration of semiconductor wafer showing distinct quality or
reliability fail candidate populations.
[0135] FIG. 3A shows semiconductor devices (dice) in areas on a
typical semiconductor wafer 300 that represent distinct populations
of semiconductor devices (that will undergo test during sort stage
according to associated stress test flows).
[0136] Three examples of systemic common failure areas expressed in
geographies over the wafer 300 are depicted by areas with
reliability problems due to process characteristics 301 yield
common failures 302 and customer common failures 303.
[0137] Thus, by analyzing historic information gathered during the
wafer FAB stage (as exemplified in the description above with
reference to FIG. 2 above), a geography that embraces the
semiconductor devices (marked A) 301 is identified as a
quality\reliability fail candidate population. In accordance with
certain embodiments, reliability problems of population 301 due to
process characteristics are caused by wafer fabrication process
variations in different areas or geographies across the wafer. It
is well known that fabrication process variations will cause
devices to either malfunction or have marginal operating
characteristics, resulting in certain geographies over the wafer
exhibiting different yields. For example, edge dice on a wafer
usually have lower yield than dice in the center of the wafer.
Products from such low yielding geographies are generally subject
to more reliability problems. The population that includes the
specified dice is a quality or reliability fail candidate and is
associated with a test flow that includes stress tests which aim at
anticipating those devices which are susceptible to quality or
reliability problems.
[0138] Similarly, by analyzing historical information that pertain
to yield common failure (as exemplified in the description above
with reference to FIG. 2 above), a geography that embraces the
semiconductor devices (marked B) 302 is identified as a
quality\reliability fail candidate population. As is well known, a
yield common failure relates to a population of that exhibits drop
in the yield score and is characterized by a common failure
signature (say, all failed dice have a Bin=8 score). In accordance
with certain embodiments, reliability problems of population 302
due to yield common failures are identified by lots or wafers with
lower than expected yield. It is well known in the semiconductor
industry that semiconductor devices from lots or wafers exhibiting
a significantly lower yield than expected from historical yield
data are subject to increased reliability problems. For example, a
wafer yielding 70% instead of the normal and expected 90% yield for
that particular product will normally include dice that have
increased reliability issues. The population that includes the
specified devices is a quality or reliability fail candidate and is
associated with a test flow that includes stress tests which aim at
anticipating those devices which are susceptible to quality or
reliability problems.
[0139] By the same token, by analyzing historical information that
pertain to customer common failure (e.g. information collected from
customers identifying common failures in the functionally of the
end product), a geography that embraces the semiconductor devices
(marked C) 303 is identified as a quality\reliability fail
candidate population. Identifying the specified population may
require application of tractability techniques, discussed above.
For example, in accordance with certain embodiments, reliability
problems of population 303 due to customer common failures are
specific products rejected by the customer that can be traced back
to specific X-Y locations on the wafer.
[0140] In accordance with certain embodiments, systemic common
failures are malfunctioning devices (303) within the wafer 300 with
common failure characteristics By this embodiment, such systemic
common failures are analyzed through a common process that tries to
identify such areas either through test measurement analysis,
process analysis or other means. Not all common failure areas are a
potential reliability concern while others are known to be highly
correlative to reliability problems.
[0141] The population that includes the specified devices is a
quality or reliability fail candidate and is associated with a test
flow that includes stress tests which aim at anticipating those
devices which are susceptible to quality or reliability
problems.
[0142] Reverting to FIG. 3A, the unmarked areas 304 on wafer 300
represent the normal ("other") population of semiconductor devices
which are not identified as a quality or reliability fail candidate
population and as such are subjected to less stress testing. There
may be one or more of this other populations (in FIG. 3A there is
only one population area designated 304). The stress testing
sequence for at least one of said other populations (304 in the
case of FIG. 3A) includes applying to the devices a test vector(s)
or pattern(s) at increased voltage (e.g. test VDD) compared to
corresponding operating voltage (e.g. reference VDD) as prescribed
by the specification for this product. Thus, for example, if the
operating voltage specification for the manufactured semiconductor
devices of a given product (including, of course, the dice in wafer
300) is X.+-..DELTA. volts, then the voltage applied during stress
test to the devices in area 304 is Y volts, where (Y>X+.DELTA.).
There may be certain scenarios where there are few power supplies
operating distinct modules in a device. For instance, if the device
includes a memory module and an analog module, it may be the case
that the memory module is powered by a Vdd having operating voltage
specification of X.sub.1.+-..DELTA. volts, whereas the operating
voltage specification for the analog module is X.sub.2.+-..DELTA.
volts, where X.sub.2.noteq.X.sub.1.
[0143] In this particular example, the stress test for the other
population (dice in area 304) may prescribe that (i) the stress
test voltage for the memory module would be Y.sub.1 volts where
Y.sub.1>X.sub.1+.DELTA. or (ii) that the stress test voltage for
the analog module would be Y.sub.2 volts where
Y.sub.2>X.sub.2+.DELTA. or (iii) both conditions apply.
[0144] Note that at least one of the other populations may be
subjected to additional stress testing (not necessarily only
increased voltage).
[0145] The invention is not bound by these examples.
[0146] Those versed in the art will readily appreciate that the
specified three examples of quality\reliability fail candidate
populations (designated `A` dice, `B` dice and `C` dice,
respectively) are by no means inclusive of all possible systemic
failures, but are intended to illustrate non limiting examples of
dice that are subjected to stress tests for improving
reliability.
[0147] The description above referred mainly to population(s) of
semiconductor devices (dice) within a wafer, say four distinct
populations accommodated in wafer 300. This however is not
binding.
[0148] Thus, in accordance with certain embodiments, at least one
of the populations includes all devices within a wafer (or few
wafers) (say, all dice in wafer 300 are `A` dice), whereas other
wafer(s) in the lot accommodate different type of population(s).
For example, another wafer includes dice all belonging to type `B`
and the rest of the wafers in the lot accommodate "normal" dice
(like 304), etc.
[0149] In accordance with certain embodiments the quality or
reliability fail candidate population includes all wafers in a
given lot whereas other lot(s) of the same product includes
different population(s).
[0150] In accordance with certain other embodiments, a combination
of the above applies. For instance, all dice in two wafers of a
given lot include dice of type `A` and a third wafer in the same
lot includes dice of different populations, say, similar to wafer
300.
[0151] Those versed in the art will readily appreciate that the
invention is not bound by these particular examples.
[0152] Note that the discussion with reference to FIG. 3A focused
on semiconductor devices. However, the invention is not bound to
semiconductor devices and, accordingly, other semiconductor units
may apply, such as module(s) of semiconductor devices. In
accordance with certain embodiments the modules are functional
modules. A non-limiting example in which the test may focus on
modules is Multi-Signal Devices (MSD) or System On a Chip (SOC)
integrated circuits that contain different functional modules, each
with different geometries within a single packaged IC. For example,
a SOC can contain a logic module, an analog module, a memory module
and others.
[0153] It should be noted that the stress test sequence may focus
on modules (rather than devices) if the analysis identifies
modules. For instance, the process characteristics analysis may
identify shifts in test results of a given module (say an analog
module or memory module) in any of the dice `A` population (see 301
in FIG. 3A). This may suggest that the specified population (e.g.
the analog module in each die in the specified geography) should be
subjected to additional stress tests.
[0154] Bearing this in mind, attention is directed to FIG. 4,
illustrating schematically a semiconductor device 400 that includes
few functional modules, in accordance with an embodiment of the
invention. By this example the die 400 includes the following
functional modules" Micro-processor 401, DSP 402, Memory 403, I/O
404, a customized ASIC 405, Control module 406, Analog module 407
and RF module 408. The invention is of course not bound by this
example.
[0155] The analysis of historical information for identifying
population(s) of semiconductor units may apply also to modules. For
instance, the population encompassing `A` dice may refer to
distinct module or modules within a die. For instance, the test
flow that includes a given stress test is associated to a
population that includes module(s) of `A` dice, e.g. the Analog
module 407 of dice 400. By this specific example, the other modules
of `A` dice may be classified as normal modules (subjected to less
or no stress), or to different quality or reliability populations,
as discussed in detail above.
[0156] It should be noted that the scope of the population(s) may
vary in a similar manner as described above in respect to
semiconductor devices. Thus, for example, in accordance with
certain embodiments, a given population extends to one or more
module in one or more devices in a wafer, or in accordance with
certain embodiments a given population extends to one or more
modules in all devices of one or more wafers in a lot, or in
accordance with certain embodiments a given population extends to a
given lot of a product, etc. Other variants are applicable
including, but not limited to, combinations of modules and devices,
e.g. a given stress test flow is associated with a population that
extends to certain devices, whereas other stress test flow is
associated with a population that extends to functional module(s)
of identical or different devices.
[0157] In accordance with certain embodiments, the modules are not
necessarily functional modules. For a better understanding of the
foregoing, attention is directed to FIG. 5A illustrating
schematically a memory device (e.g. a flash memory device) 500,
that is subjected to the augmentation of quality or reliability
technique, in accordance with an embodiment of the invention. As
shown, the device includes rows of memory cells 501 and columns of
memory cells 502. In the context of memory device, a module can be
a row of cells or a portion thereof (e.g. one, or few cells which
the case may be) or a column of cells or portion thereof (e.g. one,
or few cells which the case may be) or an block of cells (e.g. a 3
by 3 block 503). The invention is of course not bound to the
specific example of memory device and to the specific modules
discussed with reference to FIG. 5A. Note also that 500 is not
necessarily a memory device, but can be, for example, a memory
module within a device (as described above with reference to FIG.
4).
[0158] In the context of memory devices or modules (referred to as
memory units), it should be noted that in accordance with certain
embodiments, Memory units may have redundancy repair features
built-in. Thus, Memory units with a high repair rate are usually
subjected to more reliability issues then units with minimal
repairs and may require a different stress flow at final test.
[0159] Having described an embodiment that utilizes a memory device
or a module, attention is reverted to FIG. 3A for describing yet
another embodiment of the invention focusing on identification of
population(s) that encompass geographic area and in accordance with
yet certain other embodiments population(s) that encompass dice
within the same lithography exposure, or dice with the same
location across lithography exposures.
[0160] Thus, based on the analysis of the historical information, a
population of devices is defined that encompasses a geographical
area in the wafer 300. In accordance with certain embodiments, the
analysis leads to identification of a geographical area
encompassing a cluster of at least two consecutive dice. The term
cluster of consecutive dice means that any die in the identified
cluster has at least one neighboring die which also belongs to the
specified cluster. Note that insofar as the specific example of
FIG. 3A is concerned, all the dice marked `A` fall in the cluster
`A`, and likewise all the dice marked `A` fall in the cluster `B`,
however, cluster `C` encompasses the four left hand dice but not
die 303' since the latter has no neighboring die that belongs to
the cluster.
[0161] The invention is not bound by a specific geography. Thus,
FIGS. 3B-3D illustrate schematically a few distinct geographies
that are subjected to the augmentation of quality or reliability
technique, in accordance with an embodiment of the invention. As
shown in FIG. 3B, the wafer 310 is divided to distinct rings 311,
312 and 313 each constituting a distinct fail candidate quality or
reliability population. FIG. 3C illustrates a so called "doughnut"
like geography. As shown, the first geography is the internal ring
321 and the external ring 322 of the wafer 320 and the second
geography is the middle ring 323. FIG. 3D illustrates a so called
"3 o'clock" geography where a certain sector 331 of wafer 330
constitutes a geography whereas the other regions of the wafer 332
will constitute another geography. Obviously, any i o'clock option
(not necessarily 3) is valid. Each of the specified geographies (in
FIGS. 3B-3D) can be deducted, e.g. by analyzing for example process
characteristics failures. Each of the specified geographies, say in
FIG. 3B or 3C or 3D, which the case may be, will be subjected to a
distinct test flow. The invention is of course not bound by the
specific examples of FIGS. 3B-3D.
[0162] As well known, in a typical lithography process (during the
FAB stage) patterns are exposed through mask onto the semiconductor
substrate leading eventually (after undergoing various processes
including development and etching) to the manufactured dice.
Typically, the number of dice that result from a single exposure is
smaller than the number of dice in a wafer and, therefore, few
exposures are required in order to cover the whole wafer. These
groups of dice are referred to as dice within the same lithography
exposure, or in accordance with certain other embodiments dice with
the same location across lithography exposures. Since each exposure
is characterized by unique ambient (and possibly other) parameters,
it may well be the case that the quality or reliability of the
resulting dice from this particular exposure may be different than
the quality or reliability of dice resulting from a different
lithography exposure. Thus, in accordance with certain embodiments,
the historical analysis identifies a population that is confined to
a single lithography exposure. There may be of course more than one
population each associated with a respective lithography exposure,
and obviously also other population(s) as discussed in detail
above.
[0163] For a better understanding of the foregoing, attention is
drawn to FIG. 5B illustrating a group of dice that result from a
single exposure through a mask as well as across exposures, in
accordance with a certain embodiment of the invention.
[0164] For example, wafer 5000 includes (for illustrative purposes
only) few dice each marked as a small square (e.g. 5001). As is
well known the wafer include complete dice (such as die 5002) and
incomplete dice that are only partially accommodated in the wafer
5000 (such as die 5003). Obviously, only the complete dice are of
interest.
[0165] As specified above, the dice are manufactured through a
lithography exposure process, such that in each exposure fewer dice
are treated. In the specific example of FIG. 5B, each lithography
exposure embraces 9 dice and is marked as a bounding box (say 5004
or 5005). As shown, the nine dice marked `A` fall within a
lithography exposure 5005 and can constitute a fail candidate
quality or reliability population (if identified as such in the
data analysis process described above). Similarly, the four dice
marked `C` fall within a lithography exposure 5006 and can
constitute a fail candidate quality or reliability population (if
identified as such in the data analysis process described
above).
[0166] In accordance with certain embodiments the so determined
population may embrace dice within the same location in an exposure
across two or more exposures. This population is of course
identified in the data analysis process described above.
[0167] For example, the two dice marked `B` reside in the upper
right corners of lithography exposures 5007 and 5008, respectively
and may share for example a similar process characteristics problem
and as such be subjected to appropriate stress test sequence.
Similarly, the dice marked `C` reside in the corners of lithography
exposures 5006 and 5009 respectively and may share, for example, a
similar process characteristics problem and as such be subjected to
appropriate stress test sequence.
[0168] The invention is of course not bound by this example. Thus
by way of non limiting example: (i) the dice within exposure are
not bound to a single exposure in a wafer and/or to a single wafer;
(ii) the dice across exposures are not necessarily bound to a
single wafer, (iii) the dice within exposure and dice across
exposures are not necessarily bound to the same wafer; (iv) the
population whether within exposure or across exposures can refer to
semiconductor devices or modules, which the case may be. Other
variants are applicable, all as required and appropriate.
[0169] Note that the specified analysis of historical data
(discussed with reference to FIGS. 3-5 above) can be performed
off-line (e.g. by system 110 of FIG. 1A) and the population (and
possibly the associated flow test) data can be stored and used
during a later sort stage (e.g. to system 109 of FIG. 1A). In
accordance with certain embodiments the specified analysis can be
performed during sort stage and as result update the
populations/associated test flows in a dynamic fashion.
[0170] The description above of the various embodiments with
reference to FIGS. 3-5 was confined to the sort stage. In
accordance with certain embodiments the specified embodiments may
be applied mutatis mutandis to the final stage (where the devices
are normally tested in a packaged form). In most cases, it may be
required to trace the origin of the packaged device (namely the
location of the corresponding die in earlier stage wafer (at sort)
and/or at earlier manufacturing stages). This is normally
achievable using known per se tracing techniques, discussed above.
In certain cases there is no need to trace the origin (e.g. X, Y
location of the die in the wafer) of the test device or module
(during final stage) as will be evident in the example below that
refers to outliers.
[0171] The description above referred to various embodiments of
analyzing historic data in a feed forward approach and based on the
analysis identifying populations that are subjected to
corresponding test flows. The invention is not bound to feed
forward analysis. The description below focuses on a feed backward
mechanism, in accordance with certain embodiments of the
invention.
[0172] Thus, in accordance with certain embodiments, the backward
analysis analyzes data gathered and processed at sort stage (and/or
possibly in other stages, such as final stage) to identify a new
population of devices (or modify an existing population) that is
classified as quality or reliability fail candidate. The latter
population(s) is subjected to additional stress flow tests (either
repeating an already existing stress test flow or applying modified
stress test flow), in order to identify those devices that are
susceptible to quality or reliability problems (based on the
additional data that was gathered and analyzed in the sort stage).
In accordance with certain embodiments the analysis leads to
modifying the test flow (or generating a new one) applied
successively to one or more of the existing populations, e.g. in
order to load additional stress on these population(s) which the
analysis, during sort, emphasized their susceptibility to quality
or reliability problems. Detecting outlier devices are a
non-limiting example of applying feed backward technique, as will
be described with reference to FIG. 6, below.
[0173] Thus, FIGS. 6A-6C are three graphs showing various types of
detected outlier dice, in accordance with an embodiment of the
invention. The specified graphs are of course not binding. FIGS.
6A-6C concern the specific case where the analysis of data gathered
at sort stage aims at identifying a new population (or new
populations) that is subjected to additional stress test sequences.
The sought population is outlier dice. As is well known, outliers
are test measurements outside the six-sigma distribution limits but
within the product specification limits. Devices identified with
such outlier test values are usually subject to increased
reliability problems.
[0174] Reverting to FIG. 6A, a graph of an outlier sub-test value
604 shows an outlier test measurement value 605 that is within the
upper and lower control limits but is significantly different to
all the other test values.
[0175] The graph of FIG. 6B (606) shows a group 607 of test
measurements trending toward an out-of-control condition. Once
again the measured values are within the upper and lower control
limits, however, devices identified to have trending test values
toward either the upper or lower control limit have also been shown
to have increased reliability problems.
[0176] The graph of FIG. 6C (608) shows a group of test
measurements that are close to the lower control limit 609.
Although these test values are within the upper and lower control
limits, devices identified to have marginal test values have also
been shown to have increased reliability problems.
[0177] The other dice (not classified as outliers), say 610 of FIG.
6A may potentially be less susceptible to quality or reliability
problems and therefore require less or no stress tests.
[0178] Having exemplified various non-limiting forms of outliers,
there follows a feed backward procedure (with reference to FIG. 6D)
in accordance with an embodiment of the invention. Thus, at the
onset, an outlier population of dice is identified (601) (e.g. by
using known Part Average Testing (PAT) techniques, or for example
the technique disclosed in U.S. Pat. No. 6,184,048 (Ramon). Next,
new (or existing) stress test flow(s) are associated (602) and
re-applied (603) to the specified outlier population(s).
[0179] The determination of the appropriate test flow that will be
applied to the so detected outliers is normally determined during a
preliminary calibration phase (of associating test flows to
distinct populations). Generally speaking, a repertoire of possible
outlier groups characteristics is a priori defined and is tested in
the preliminary calibration phase in order to assign a test flow
per "outlier group". The latter procedure of associating a test
flow to a distinct population will be discussed in greater detail
with reference to FIGS. 9 and 10. In operation, whenever outlier(s)
are detected (say 607 of FIG. 6B), they are "classified" to an
already defined group of outliers (which during calibration phase
was associated with a given stress test flow) and the latter stress
test flow will be applied to the so detected outliers (in this
example 607).
[0180] It is noted that in accordance with certain embodiments, a
shift in the number of outliers or in the characteristic of the
outliers may require more stress testing at sort test. For example,
any identified outliers during sort stage may identify an exemplary
quality or reliability fail candidate population that requires more
stress test during future sort stage testing.
[0181] As mentioned previously, hitherto known techniques would
consider IC's with tests containing outliers, test results trending
in one direction or marginal test results as failures and
consequently discard the "outlier" devices, with the inevitable
result of reducing yield score. In contrast, in accordance with the
various embodiments of the invention discussed above, the
population of outliers is only classified as "fail candidate" and
as such is subjected to additional stress tests and only if the
"outlier" device or module fails the additional stress test(s) it
will be classified as "fail". If, on the other hand, it passes the
additional stress test sequences it may proceed to the succeeding
manufacturing stages and eventually delivered to the customer as a
good IC. The net effect would then be that the yield score
improves.
[0182] It should be noted that whilst generally outliers are
characterized by 6 sigma deviation this is by no means binding and
accordingly lower or higher deviation degree may apply, depending
upon the particular application.
[0183] Those versed in the art will readily appreciate that the
description above with reference to detecting outliers is a non
limiting example of dynamic backward analysis technique that is
applicable in accordance with the invention.
[0184] In accordance with certain embodiments, the backward
analysis (the terms backward analysis and feed backward are used
interchangeably) is used exclusively or in accordance with certain
embodiments in addition to the feed forward technique discussed in
detail above.
[0185] In accordance with certain embodiments the feed backward
mechanism is not confined to semiconductor devices, but rather it
may apply to at least one module of a semiconductor device, all as
discussed in detail above with reference to the feed forward
embodiments.
[0186] In accordance with certain embodiments, the outliers may
extend to one or more semiconductor units (device or module(s)) in
a wafer, or in certain embodiments to one or more wafers in a lot
or in accordance with certain embodiments to one lot or more of a
product, or in accordance with certain embodiments, certain
combinations thereof, all as discussed in detail above with
reference to the feed forward mechanism.
[0187] In accordance with certain embodiments the feed backward
mechanism may be applied at the final stage when the devices are
packaged whilst tracing the location of the corresponding die in
the wafer (during sort) or at earlier stages, all as discussed in
detail above. Insofar as final stage is concerned, there is
generally no need for die tracing, seeing that the same identified
outliers (at final stage) will be subjected to the additional
stress testing.
[0188] Note that for realizing the re-test of the so identified
outlier, known per se techniques may be used. For example, in sort
stage a so called re-test or re-probe technique is used for
applying the stress test sequence to the identified outlier
devices. For final stage, considering that the devices are tested
one at a time, whenever a device is classified as an outlier, it is
normally subjected to the additional stress test sequences.
[0189] After applying the stress test, the units
(devices/module(s)) that failed, are discarded and will not proceed
to packaging and final stage testing. The remaining semiconductor
devices that pass the stress test with a sufficient level of
confidence are less susceptible to quality or reliability problems
and therefore will proceed to the next manufacturing stage. The
enhanced selective stress test applied in accordance with the
various embodiments of the invention, obviate or minimize the
costly burn-in stage (marked schematically as 120 in FIG. 1A) of
detecting devices that are susceptible to quality or reliability
problems.
[0190] Having described various embodiments of the invention
concerning identification of distinct populations in feed forward
and or feed backward mechanisms, there follows a description of a
comprehensive sequence of operation in accordance with certain
embodiments of the invention (with reference to FIG. 7A) focusing
also on the procedure of associating appropriate test flow(s) to
the so determined quality or reliability fail populations.
[0191] The first step 700 in the process involves the
identification of quality or reliability fail candidate populations
as discussed in detail above.
[0192] The next step (701), is the calculation and association of a
test flow (that includes a stress test sequence) to each one of the
specified populations. This will be further elaborated below with
reference to FIGS. 9 and 10.
[0193] The stress test sequence applied to the specified fail
candidate populations includes at least one test with increased
duration compared to the duration of a test applied to other
population. Thus, as exemplified with reference to FIG. 3A, the
type `A` dice have a stress test sequence that includes a given
test (say, injection of given test vector(s) or pattern(s)) to
certain or all inputs of a device) of longer duration than
identical or similar test applied (or applicable) to the normal
dice (residing, e.g. in the same die 304 and/or possibly in other
wafer). In other words, the latter example means that the duration
of the vector (e.g. the vector length) would be longer than a
vector that is injected to a device belonging to a normal
population.
[0194] There follows a few non-limiting examples of stress tests of
longer and shorter durations, exemplified for the non-limiting case
of stress test vector(s). Thus, by way of non-limiting example the
shorter vector applied to the other population 304 can be a prefix
of a vector applied to the devices of a first fail candidate
population 301. In accordance with another non-limiting example,
the longer vector applied to the first fail candidate population
301 can be a repetition (twice or more) of a vector applied to the
devices of the other population 304. In accordance with another
non-limiting example, the longer vector applied to the first fail
candidate population 301 is different to the vector applied to the
devices of the other population 304. In accordance with another
non-limiting example, the longer vector applied to the first fail
candidate population 301 can be a concatenation of two or more
sub-tests compared to a test vector applied to the devices of the
other population 304. The invention is not bound by anyone of the
specified examples.
[0195] The test sequence may possibly further include other tests,
e.g. at least one with different voltage and/or different
temperature and/or different frequency. By a non-limiting example,
an increased frequency means that devices that belong to a given
fail candidate population will receive a test sequence that
includes injecting a test vector at a given frequency--higher or
lower (which the case may be) compared to the frequency of the same
test vector that is injected to devices that belong to a normal
population.
[0196] One non-limiting manner of realizing application of higher
temperature to, say distinct dice in a wafer is by placing the
wafer on the "chuck". This chuck is heated to a specific
temperature. A very simple mechanical object can "separate" the
heat according to areas. Consider--rings, pizza slices, etc.
[0197] For a better understanding of the foregoing, attention is
drawn to FIG. 7B illustrates schematically a mechanism for heating
a wafer to different temperatures, in accordance with an embodiment
of the invention. Thus, as shown a chuck 7100 is heated in a
uniform manner and accordingly when a wafer 7101 is placed on the
chuck 7100 it will be heated to a uniform pre-selected temperature.
In accordance with a different scenario it is required to heat
different portions of the wafer to correspondingly different
temperatures. The specified different areas signify distinct fail
candidate populations that are subjected to different stress test
sequences that include applying the test sequences at respective
different temperatures. Thus, for example the chuck 7100 is sliced
to distinct rings 7102, 7103 and 7104. The implementation provision
of heating/cooling mechanism to distinct parts of the chuck is
generally known per se and will not be expounded upon herein. Each
of the specified rings is subjected to a different temperature. For
example, the inner part 7104 may be an ambient temperature applied
to a normal population (i.e. dice placed at the wafer center). A
higher (or lower) temperature may be applied to the outer ring 7103
which will result in applying test sequences in higher/lower
temperature to dice (belonging to a fail candidate population)
placed at corresponding location in the wafer. And finally, a
different (higher or lower) temperature may be applied to the most
outer ring 7102 which will result in applying test sequences in
higher/lower temperature to dice (belonging to yet another fail
candidate population) placed at corresponding most outer location
in the wafer.
[0198] The invention is of course not bound to the specified "ring"
implementation for provision of different temperatures to different
populations.
[0199] Having completed a preliminary calibration phase of
identifying populations and associating thereto test flows, there
follows a sequence of operation 7000 in accordance with an
embodiment of the invention.
[0200] In operation, it is first inquired whether the backward
analysis mode is active 7001 and if in the affirmative, data is
gathered and analyzed 7002 (during the sort operation) all as
discussed in detail with reference to FIG. 6.
[0201] Regardless of whether data is gathered for feed backward
purposes, data indicative of the tested population 7003 (e.g.
population of dice designated `A` in FIG. 3A) is loaded (from a
database, say sort database 101) by the station controller (105 of
FIG. 1A) and the handling equipment 108 is appropriately controlled
for placing the DUT(s) 111 for testing.
[0202] Next, the associated test flow is loaded 7004 (from a
database, say sort database 101) by the station controller (105 of
FIG. 1A) and the test program (107) is updated for activating the
ATE 106 for applying the test flow 7005 (according to the test
program) to the devices of the population under consideration.
[0203] The failing/passing dice are recorded in an appropriate bin.
This procedure is repeated until all populations are treated 7006
(e.g. populations 301, 302, 303 and normal population 304 [the
latter is subjected to reduced stress test or not at all] of FIG.
3A).
[0204] In the case that the feed backward analysis is applicable
7007 and 7008 (e.g. identified outliers), the appropriate test flow
is loaded and applied to the so identified populations 7010 until
all populations are treated 7011, giving rise to the termination of
the stress test sequence.
[0205] Note that the procedure of testing populations is not
necessarily serial, namely testing all the dice of a given
population and then the devices of the other population, etc.
Attention is again drawn to FIG. 1B illustrating the possible
serpentine manner of scanning the wafer (as illustrated by
serpentine line 124). Assuming that the first population (with its
associated test flow) is the `0` dice (e.g. 121) and that the
second population (with its associated test flow) is the `1` dice
(e.g. 122) and the other population (with its associated test flow)
is the `blank` dice (e.g. 123). As readily shown, the scanning
would prescribe the loading and incorporating in the test program
the test flow that corresponds to the `0` dice, all as described in
detail with reference to FIGS. 1A and 1B, above. The dice are
tested until `0` die 130 is encountered. When the scanning proceeds
to die 123 (belonging to the `blank` population), the appropriate
test flow is loaded and the TP is updated for testing dice 123 and
132. Next die 133 is encountered and the TP is updated again with
the test flow that corresponds to the `0` dice. This switching of
test flows according to the tested dice is continued (e.g.
switching between the test flow that corresponds to blank dice (die
134) and then the test flow that corresponds to `0` dice (die 135),
and finally the test flow that corresponds to `1` dice (die 136).
Note also that if outliers are detected (in the course of
scanning), the appropriate test flow is loaded and applied (e.g.,
in a re-probing technique) to so detected outliers. Note that in
the case of touchdown scanning, where a few dice are tested in
parallel, it is possible to load different TPs to different dies if
the latter belong to different populations.
[0206] Those versed in the art will readily appreciate that the
invention is not bound by the specific sequence of operations
described with reference to FIG. 7A and likewise not by the
corresponding system architecture of FIG. 1A
[0207] Obviously, one or more of the various possible embodiments
discussed above may apply, e.g. (including but not limited to):
[0208] (i) In accordance with certain embodiments the testing may
apply to at least one module of a semiconductor device, all as
discussed in detail above.
[0209] (ii) In accordance with certain embodiments, population(s)
may extend to one or more semiconductor unit(s) (device or
module(s)) in a wafer, or in certain embodiments to one or more
wafers in a lot or in accordance with certain embodiments to one
lot or more of a product, all as discussed in detail above.
[0210] (iii) In accordance with certain embodiments the testing
mechanism may be applied at the final stage when the devices are
packaged whilst typically (although not necessarily) tracing the
location of the corresponding die in the wafer (during sort) or at
earlier stages, all as discussed in detail above.
[0211] The invention is not bound by the specified examples (i) to
(iii), discussed above.
[0212] Having described a sequence of operation with reference to
FIG. 7A, it is recalled that each quality or reliability fail
candidate population is associated with a test flow that includes a
test stress sequence. The description below elaborates on various
embodiments of the so used stress sequences.
[0213] Thus, as specified above the test sequence includes tests of
longer duration compared to test applied to a population other than
the fail candidate. (e.g. a test applied for 200 msec, to, say,
population `A` 301 compared to 100 msec to, say, regular population
304). If desired, the test flow may further include one or more of
the following tests: different voltage (say +30% compared to the
voltage defined by the device's operational specification.). The
higher voltage can be applied to the device's Vdd and/or to any
other relevant input.
[0214] If further desired the test sequence may be performed at
different temperature conditions compared to the temperature
defined by the device's operational specification. If further
desired the test sequence may include a test of different frequency
compared to that defined by the device's operational
specification.
[0215] In the specific case of memories (whether as a device or
module(s) in a device), the longer duration may include larger
number of test cycles, say larger number of read commands and/or
larger number of write commands, or e.g. in the case of flash
memory larger number of program/erase commands. Note that the
invention is not bound by the specified examples and obviously not
by any specific memory type and or commands. After applying the
sequence of operation as described by way of specific embodiment
with reference to FIG. 7A, certain devices or modules which are
susceptible to quality or reliability problems are classified as
"failed" (whether during the sort or final stage which the case may
be), and obviously will be discarded. One of the benefits of
identifying and utilizing an optimized stress test flow at sort
and/or final test for each IC is that burn-in could possibly be
reduced or eliminated for a given IC product, thus reducing overall
testing cost.
[0216] Bearing this in mind, attention is drawn to FIG. 8 showing a
flow diagram of a quality or reliability test sequence, in
accordance with an embodiment of the invention. As shown, at the
onset, a set of conventional (non-stress) test steps 800 are
executed (during sort stage) consisting of several sub-tests such
as Test A 801 each of which performs a binning decision such as Bin
A 802 if the device fails the test.
[0217] The specified pre stress sub-tests can be e.g. one or more
of the following: continuity, opens, shorts, various functional
tests, stuck at scan, at-speed scan.
[0218] Next, there follows the stress test phase wherein distinct
test flows are applied to distinct populations, as described with
reference to FIG. 7A above. In FIG. 8, three test sequences are
applied 803, 806 and 809 each including a test flow that includes a
stress test sequence. 803 is directed to, say the other population
(e.g. 304 in FIG. 3), whereas 806 and 809 are directed to, say, the
`fail` populations (e.g. 301 and 302, respectively).
[0219] For instance, the test flow 803 includes a stress test
sequence 804 that includes a an increased voltage compared to that
defined by the operating voltage specification of the device (say,
+30% voltage) followed by post stress tests A, B and C The stress
test duration is, say 50 ms). It should be noted already at this
stage that the post stress tests operate under normal conditions,
all as will be explained in greater detail below.
[0220] Reverting to the stress test sequence 804, the binning
decision of failed dice is indicated in 805.
[0221] The second test flow 806 includes a stress test sequence 807
that includes a test of longer duration and increased voltage (e.g.
+40% voltage level for 200 ms) followed by post stress tests A, B
and C that will be discussed in greater detail below. The binning
decision of failed dice is indicated in 808. The population that
corresponds to the second test flow 806 can be composed of dice
with reliability problems due to yield common failures (such as die
designated as 301 in FIG. 3A).
[0222] The third test flow 809 includes a stress test sequence 8000
that includes a test of longer duration and increased voltage (e.g.
+30% voltage level for 100 ms) followed by post stress tests A, B
and C that will be discussed in greater detail below. The binning
decision of failed dice is indicated in 8001. The population that
corresponds to the third test flow 809 can be composed of dice with
reliability problems due to customer common failures (such as die
designated as 302 in FIG. 3A).
[0223] For illustrative purposes only, had the stress test flows
not been used, the current reliability in terms of DPPM for the
product being tested would be for example 540 DPPM, whereas after
using the specified stress test sequences (targeted to the
designated populations) the reliability in terms of DPPM for the
product being tested using the augmented reliability flow is
reduced to 290. This improved result is accomplished due to the
fact that many devices that would otherwise be revealed as "faulty
devices" at (costly and lengthy) burn-in stage, or later at final
test stage or after having been shipped to customers, would now be
classified as "faulty devices" in response to applying the stress
test sequences, and consequently be discarded already at this early
sort stage (and/or final stage which the case may be).
[0224] The invention is of course not bound by these particular
examples which are provided for illustrative purposes only.
[0225] It is therefore noted that as described previously, there
may be reliability concerns on certain semiconductor devices
originating from various factors such as process characteristics,
yield common failures, customer common failures or other issues.
These concerns are not addressed by using a pre-defined set of
tests and stresses. In other words, certain devices may need more
or less testing and stressing to optimize stress flow steps,
thereby optimizing product reliability. Many semiconductor
companies report DPPM rates of 200 to 500 based on their products
and struggle to lower those rates. The method and system in
accordance with certain embodiments of the invention will augment
semiconductor reliability by determining effective stress flow
comprised of an optimized set of quality or reliability stress
sub-tests for at least one member in a group that includes: IC lot,
wafer, areas of a wafer, wafer die, dice within the same
lithography exposure, dice with the same location across
lithography exposures, packaged device and functional modules
within an IC. The net effect is the use of a best fit stress test
flow for each unit resulting in reliability augmentation which
increases product reliability in terms of lower DPPM. The method
and system in accordance with certain embodiments of the invention
of the invention can also be used to reduce or eliminate burn-in on
IC's, thus reducing cost.
[0226] Having described a sequence of operation in accordance with
certain embodiments, attention is reverted to FIG. 7A, and in
particular to step 701 of associating the test flows to the so
identified populations which will now be described in greater
detail with reference also to FIGS. 9 and 10.
[0227] Accordingly, attention is directed to FIG. 9, showing a flow
diagram of a sequence of operation for associating test flows to
distinct quality or reliability fail candidate populations, in
accordance with an embodiment of the invention.
[0228] The flow diagram shows an example of an automated process to
determine the best fit reliability augmentation for a specific
population using a skew lot, in accordance with an embodiment of
the invention. Note that the invention is not bound by the use of
skew lot.
[0229] As is well known, skew lot is generally defined in the
semiconductor industry as a group of wafers or packaged IC's that
collectively exhibit the full window of allowable variation
(typically +/-3 Sigma) of at least one Fab process-parameter for
the Fab process being used to manufacture the wafers. Examples of
Fab process parameters are transistor channel length/width, gate
capacitance, saturation current, poly gate critical dimension and
gate threshold voltage. A skew lot is typically created by
purposely inducing maximum allowable process parameter shifts
during wafer fabrication. A skew lot can also be discovered by
reviewing test results from multiple lots to find a lot that
exhibits the full window of allowable variation. In this
connection, attention is drawn to FIG. 10A, illustrating a table
1000 elaborating 3 skew lots, the first identifying Fast material
(characterized by "poly" target -3 Sigma), the second identifying
Typical material (characterized by "poly" target) and the third
identifying Slow material (characterized by "poly" target +3
Sigma). In the example Fab lot depicted in the Fab skew lot
definition table 1000, the devices within the lot exhibited
variation of a specific parameter, namely a parameter that
correlates to processor speed, and the distribution of the devices
within the lot were classified as follows: fast material, typical
material and slow material.
[0230] Once a skew lot has identified semiconductor units (devices,
or modules(s) which the case may be) with similar characteristics
(i.e. belonging to the same population) all as described in detail
above, these units are split equally between n test flows 900. The
n test flows constitute the repertoire of available stress test
flows which all devices that belong to a given population will
undergo and the results will be compared in order to identify the
prevailing test flow which will be associated to this particular
population.
[0231] The procedure described with reference to FIG. 9 is
performed in respect of each population distinctly. A population
may be defined for instance as dice of `A` type (in FIG. 3A). This
means that the specified population will be subjected to its
associated test flow for the skew of slow material and for the skew
of the typical material and also for the skew of the fast
material.
[0232] Other populations may likewise be defined for `B` type dice,
`C` type dice and normal dice.
[0233] Note that the invention is not bound by the use of skew lots
as a factor for determining a population for testing.
[0234] At the onset, a baseline (reference) test flow 901 is
invoked per selected population (say `A` dice for fast material).
It includes wafer test for the baseline 902 which uses the original
test program flow (without invoking any additional stress test) and
burn-in test for the baseline split 903. The base line flow
provides a reference of "units-fail" score in the burn-in phase.
For instance, the percentage of failed unit detected in the burn-in
stage 903. This procedure is repeated for each one of the specified
populations and the appropriate reference test results are logged,
per population.
[0235] Obviously, it is expected that the logged "units fail" score
after applying the prevailing test flow (with the stress test
sequence) will be improved (compared to the "reference" test) since
some (or hopefully most) of the units which would otherwise be
detected as failed units during the costly and long burn-in process
(or afterwards when shipped to the customers) will now be revealed
at an earlier stage (e.g. sort or final stage, whichever the case
may be), due to the augmented reliability test technique offered in
accordance with the various embodiments of the invention.
[0236] To this end, a 904 selected test flow from the n possible
test flows is applied to a given population 905 (say the `A` dice
for slow material). The "unit fail score" is logged (e.g. the
percentage of failed units) and then the same wafer is subjected to
burn-in stage 906 and likewise the "unit fail" score is logged.
[0237] Then the procedure is repeated in respect of the other test
flows, logging the corresponding results.
[0238] Next, the baseline split test results are compared to the
reliability test splits (1 . . . n) 907 and the prevailing test is
determined according to a decision criterion. A non-limiting manner
of comparison between the results and deciding on the prevailing
flow will be exemplified with reference to FIGS. 10A-10B.
[0239] Finally one (out of n) reliability flow is chosen and
associated to the specified population 908.
[0240] Turning now to FIGS. 10A-10B, there are shown two tables for
further exemplifying the flow chart sequence of FIG. 9.
[0241] FIG. 10 contains two tables in 10A (discussed above) and
10B.
[0242] In this example, assume that processor speed is the
reliability issue of interest. Table 1001 describes an example
stress flow testing experiment for n devices that constitute a
given population using various combinations of stress voltage and
stress duration. The invention is of course not bound by this
example and other variants can apply, e.g. additional stress test
parameter(s) in addition of instead of the above. In accordance
with another example the constraint of modifying only one parameter
at a time does not necessarily apply, etc.
[0243] Turning now to table 1001, first the selected population of
devices (say `A` dice in wafer 300 of FIG. 3) is tested with the
original baseline stress flow (duration 50 msec and an additional
voltage, say Vdd compared to the operating voltage specification of
the device). Next, similar groups of n devices (e.g. `A` dice from
another wafer in the same lot) are tested using various reliability
test flows using combinations of stress voltage, stress duration.
For example, in the first stress flow (second line in table 1001)
longer duration is selected (100 msec) and increased voltage (+40%
compared to the operating voltage specification). Another test flow
in the repertoire (third line in table 1001) stipulates test
sequence with test of longer duration (100 msec) and increased
voltage (+30%). The procedure is repeated for all stress test
sequences of the repertoire of which a few are indicated for
illustrative purposes only in table 10B.
[0244] During each of the specified applications of stress test
sequences, failing (bad) reliability bins are captured and stored
for each device and the percent failures are calculated and logged.
After all the dice in the specified population are stress tested,
those that managed to pass the sort stress test are subjected to
burn-in testing which accelerates future reliability failures. The
failure rates during the subsequent burn-in phase are also
calculated and logged.
[0245] In accordance with certain embodiments, identification of
the best fit reliability stress flow involves comparing the failing
reliability bins and percent failures from the stress flow testing
experiment to the failing reliability bins and percent failures of
the burn-in process. In this example the baseline reliability
stress flow at wafer test had yield fallout of 3%. In other words
the original baseline stress flow at wafer test detected 3% of the
failures directly related to the reliability issue of interest and
burn-in detected an additional 5% of those failures.
[0246] The goal of the experiment is to identify prevailing test
flow. A possible non-limiting decision criterion can be detecting
the highest number of failures at sort test and the lowest number
of failures at burn-in. In this example the stress flow that
appears to qualify is test No. 4: Additional stress voltage
+duration which at wafer test detected 6% (tested "unit fail"
score) of the failures directly related to the reliability issue of
interest and burn-in detected 1% (tested burn-in "unit fail"
score). In this case the stress flow with additional stress voltage
and duration as prescribed Test No. 4 is the optimal reliability
stress flow and therefore this selected test flow is associated to
the specified population (701 in FIG. 7A). As specified, one of the
benefits of identifying and utilizing an optimized stress test flow
at wafer test or package test for each IC is that burn-in could
possibly be reduced or eliminated for a given IC product, thus
reducing overall testing cost.
[0247] Table 10B illustrates a specific repertoire using only
duration and voltage. This is of course not binding and accordingly
other parameters can be used, e.g. duration, and one or more of the
following: voltage, frequency and temperature.
[0248] Regardless of the embodiment under consideration, whenever
desired, at least one of the test flows further includes a
post-stress sequence that is applicable to the corresponding
quality or reliability fail candidate population. A typical post
stress test may include applying one or more of the following
exemplary list of tests: various functional tests, stuck at scan,
at-speed scan, leakage, icc, iddq.
[0249] For example with reference to FIG. 3, the `A` dice may be
subjected to a test flow that includes their designated stress test
and a subsequence post stress test sequence. After applying the
stress test and logging the units (devices/module(s)) that failed,
there follows a subsequent post test sequence logging also the
devices which passed the stress test but failed the post test
sequence. The specified devices which failed the post test sequence
will be discarded, thereby packaging and shipping (the remaining)
semiconductor devices that are less susceptible to quality or
reliability problems whilst obviating or minimizing the costly
burn-in phase of detecting devices that are susceptible to quality
or reliability problems.
[0250] In accordance with certain embodiments, the population is
not necessarily bound to geographic area. Reverting to the example
of FIG. 10, as may be recalled a first population is composed of
dice `A`, the second is composed of dice `B` etc. As may be
recalled, the test flow associated with the dice `A population was
applied for both skew slow material, typical material and fast
material.
[0251] In accordance with certain embodiments the population is not
defined only by geographic consideration (such as dice in area 301,
etc.), but rather on additional one or more parameters. By way of
non-limiting example, the population may be defined in accordance
with the geographic consideration and the skew parameter.
Accordingly by this non-limiting example a first population will be
`A` dice for skew of slow material (associated with a first test
flow). The second population would be `A` dice for skew of typical
material (associated with a second test flow), a third population
would be `B` dice for skew of slow material (associated with a
third test flow), etc.
[0252] In accordance with a second aspect of the invention, the
invention concerns utilization of pre-stress and post-stress
testing.
[0253] Attention is first directed to FIG. 11 showing a flow
diagram of a conventional quality or reliability test flow, in
accordance with prior art.
[0254] As is well known, device quality or reliability is usually
tested through a set of sub-tests called a stress flow. First, a
set of normal (pre-stress) test steps 1100 are executed followed by
a set of stress flow test steps 1107. The idea is to test the
device under normal operating conditions and then stress the device
with stress test e.g. 1103, say higher voltage levels (e.g. +30%).
The conventional test steps 1100 may consist of several sub-tests
such as Test A 1101, each of which performs a binning decision,
such as Bin A 1102 if the device fails the test. Typical, yet not
exclusive, examples of a normal test sequence is at least one of
the following tests: continuity, opens, shorts, various functional
tests, stuck at scan, at-speed scan.
[0255] During the succeeding stages 1107, stress flow test sequence
1103 is activated which makes a quality bin decision 1104 if the
device fails the test. Following the stress sub-test 1103, it is
customary to subject the device to several post stress tests such
as post stress Test D 1105, which makes a binning decision 1106 if
the device fails the test. The set of post stress sub-tests may be
the same or different from the set of normal pre-stress test steps
1100. It is important to note that in a typical quality and
reliability test flow all devices receive the identical number and
type of post stress tests. Namely, all the devices of a given
product will receive the same pre-stress sequence (say: A, B and C)
as well as the same post stress sequence, say D, E and F. A typical
post stress test may include at least one of the following: various
functional tests, stuck at scan, at-speed scan, leakage, icc,
iddq.
[0256] One of the shortcomings of this approach is that all devices
are tested with the identical stress flow which utilizes
pre-defined deterministic decisions, therefore the opportunity to
increase product reliability by customizing the pre-stress and post
stress tests per population is not met.
[0257] Bearing this in mind, attention is drawn to FIG. 12 showing
a flow diagram of a quality or reliability test flow in accordance
with certain embodiments of another aspect of the invention.
[0258] First, a set of normal pre-stress test steps 1200 are
executed consisting of several sub-tests such as Test A 1201 (and
subsequent tests B-E), each of which performs a binning decision
such as Bin A 1202 if the device(s) fails the test. The normal
pre-stress tests may include at least one of the following
non-exclusive list: continuity, opens, shorts, various functional
tests, stuck at scan, at-speed scan.
[0259] Next, a set of quality/reliability test flow is performed
1203 which, as is well known, aims at identifying in-advance
devices which are susceptible to failure. In the example of FIG.
12, there is shown a test 1203 which performs a binning decision
1204 if the device(s) fail(s) the quality or reliability tests.
Note that in accordance with certain embodiments of this aspect of
the invention, the quality or reliability tests can be performed
according to prior art (e.g. performed uniformly on all devices),
or in accordance with certain other embodiments, the quality or
reliability tests are performed in accordance with any of the
teachings of the first aspect of the invention.
[0260] Next, a set of normal post-stress test steps 1205 are
executed consisting of several sub-tests such as Test A 1206, each
of which performs a binning decision such as Bin A 1207 if the
device(s) fails the test. The normal post-stress tests may include
at least one of the following non-exclusive list: various
functional tests, stuck at scan, at-speed scan, leakage, icc,
iddq.
[0261] In accordance with certain embodiments of the second aspect
of the invention, there are determined at least two distinct
populations of semiconductor units that are subjected to the
pre-stress and post-stress sequence. The determination of the
specified populations can be, e.g. in accordance with any of the
teachings described with reference to the first aspect of the
invention described above.
[0262] Having determined at least two populations, each of the
populations is subjected to a test flow that includes pre-stress,
stress, and post-stress sequences, of the kind described with
reference to FIG. 12. Note that the test flow for the first
population includes identical pre-stress test sequence and post
stress test sequence. This is readily shown in FIG. 12, where the
same sequence (e.g. pre-stress Tests A,B,C,D, and E 1200) is
provided also as a post stress test (e.g. Tests A,B,C,D, and E
1205).
[0263] The second population will also be subjected to identical
pre-stress sequence and post-stress sequence, however, obviously
different than that of the first population. Thus, for example, if
the pre-stress and post-stress test sequence for a first population
is A,B,C,D,E, then for a second population the sequence used as a
pre-stress and post-stress sequence would be, e.g. B,C,E,F.
[0264] The advantage of using a unique pre- and post stress
sequence per population is by adapting the most suitable sequence
of pre- and post stress tests per population. The manner of
associating a test flow per population can be realized, e.g. in
accordance with the teachings of the various embodiments described
with reference to FIGS. 9 and 10.
[0265] It is also recalled that the unique pre-stress and
post-stress sequence per population can be used with conventional
quality or reliability stress test techniques or in accordance with
any of the embodiments discussed above with reference to a first
aspect of the invention.
[0266] Reverting now to a distinct population, as specified above,
it is subjected to an identical pre-test sequence and post-stress
sequence. If a given device (in the population) passes a pre test
sequence (including, say test A 1201) and then passes the quality
or reliability test 1203 then, on the face of it, the device is
less susceptible to quality or reliability problems when shipped to
the customer (after undergoing the subsequent packaging and final
test). At this stage, the device should pass the post stress test
(including test A 1206) since it has already passed the same
sequence as a pre-stress test. Indeed, those devices which are
fault-free are likely to pass the identical post-stress test
sequence, however, those that are vulnerable to reliability
failures (after having been "pressurized" by the stress test
sequence) may now fail (namely have a reliability fail
signature--say, reliability fail bin 1207) if being subjected to
the same sequence that they already passed before having been
"pressurized" through stress-test sequence.
[0267] The net effect would be that additional fail candidate
devices can be traced during sort stage (using the approach of the
second aspect of the invention), improving the DPPM and reducing or
obviating the need to utilize the costly and lengthy burn-in
procedure for detecting faulty devices.
[0268] Note that whereas the description above referred to
identical pre-stress test sequence and post stress test sequence
(e.g. A,B,C,D and E in the example above), this is not necessarily
binding. Thus, at least one of the pre-stress tests and the
post-stress tests may be identical. For instance, for the pre
stress test sequence A,B,D,E and the post stress sequence B,C,F,S
the test B serves as the identical test for the pre-stress and post
stress sequence. Thus, if a device that already passed test B as a
pre-stress test, fails the same test at post stress, it indicates
that this device is susceptible to a reliability problem.
[0269] In accordance with certain embodiments, at least one of the
test flows further includes additional post-stress sequence that is
applicable to a corresponding population. The additional
post-stress test need not be part of the pre-test sequence. Thus,
as shown in FIG. 12, additional post-stress tests 1208 are applied
to a given population, such as tests F (1209) with binning decision
1210, G and H. As shown this F,G,H post-stress test sequence was
not used during the pre-stress sequence.
[0270] In accordance with certain embodiments, the system
architecture described, for example, with reference to FIG. 1, is
configured to carry out the pre-stress test, stress test and
post-stress test sequences in accordance with the various
embodiments of the second aspect of the invention.
[0271] Those versed in the art will readily appreciate that the
gathering and analyzing of data for identifying distinct
populations may be carried out in accordance with the various
embodiments described with reference to the first aspect of the
invention, described above. This includes and is not limited to
applying feed forwards and/or feed backward processing as described
in detail above.
[0272] Those versed in the art will readily appreciate that the
various embodiments described with reference to semiconductor
devices, may apply if desired also (or in lieu) to semiconductor
module(s), all as described in detail with reference to the first
aspect of the invention.
[0273] Those versed in the art will readily appreciate that the
various embodiments described with reference to sort stage may be
applicable also (or in lieu) to final stage, all as described in
detail with reference to the first aspect of the invention,
above.
[0274] It will also be understood that the system according to the
invention may be a suitably programmed computer. Likewise, the
invention contemplates a computer program being readable by a
computer for executing the method of the invention. The invention
further contemplates a machine-readable memory tangibly embodying a
program of instructions executable by the machine for executing the
method of the invention.
[0275] The present invention has been described with a certain
degree of particularity, but those versed in the art will readily
appreciate that various alterations and modifications may be
carried out without departing from the scope of the following
Claims:
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