1995 Microchip Technology Inc. DS00601A-page 1
INTRODUCTION
The endurance of an EEPROM-based device will be
quoted by a manufacturer in terms of the minimum
number of erase/write cycles (write cycles) that the
device is capable of sustaining before failure. A write
cycle is generally considered to be the operation that
changes data in a device from one value to the next.
There are several EEPROM-based devices available
on the market. Microchip Technology Incorporated
makes three general types of EEPROM-based
product: Serial EEPROMs, Parallel EEPROMs, and
EEPROM-based Microcontrollers. As a manufacturer
of many EEPROM products, Microchip is concerned
with endurance and continues to try to educate its
customers on the importance of this topic.
There are many differences in the interpretation of
“endurance” that can result in misleading or inaccurate
information being used in design decisions. This paper
hopes to clear up any questions that the customer ma y
have in the subject of endurance, without becoming so
technical that the information given is not helpful.
There is no widely used standard for any type of
endurance test. Each manufacturer will use their own
endurance testing methodology. This report will
describe all the testing options, and which tests
Microchip performs on its EEPROM-based products.
The MIL-STD cycling test (Method 1033) has not been
updated since 1977 and is well out of date as applied
to EEPROM non-v olatile memories. The standard does
not distinguish the difference between block cycling
and byte cycling, and gives a ver y poor failure criteria.
Microchip does not use this standard.
A uthor: David Wilkie
Reliability Engineer
BASIC TERMS
The definition of “endurance” (as applied to EEPROMs)
in the first part of this introduction contains various words
and phrases that require clear definition and under-
standing. As shown in the f ollowing par agraphs, diff erent
manufacturers will use different standards.
“Endurance cycling” is a test performed by all
manufacturers (and some customers) to determine
how man y “write cycles” the product will achie v e bef ore
failing.
The “minimum number of write cycles” is the least
number of times that you can expect to subject the
product to a “write cycle” before it fails.
“Failure” is a somewhat arbitrary definition, since a
device only truly fails when it no longer meets the
customer expectation, and does not operate in his
system. A failure can be defined in this, the loosest of
definitions, or the most stringent of definitions (whereby
a device would fail if it did not meet any of the data
sheet parameters), as well as a wide range in betw een.
For example, if the device did not correctly store data
into a particular address that the customer was not
using, then the device would work correctly for the
customer but would fail a functional test set by the
manufacturer. Likewise, if the device drew more
current than the data sheet specified after some time,
but the customer application could supply the current
needed, the device would work in the customer
application but would fail a parametric test set by the
manufacturer.
Microchip uses the most stringent definition:
A failure
occurs when the device fails to meet any data sheet
condition under any guaranteed operating
condition of temperature and voltage
.
The number of devices that can fail before a particular
endurance criteria is not met is also somewhat flexible.
Even the most quality conscious manufacturer will
occasionally hav e a failure, so a f ailure level is defined.
The industry standard conditions for many types of
reliability tests are set by JEDEC (the Joint Electronic
De vice Engineering Council). JEDEC defines that if 5%
or less of a given sample fails at a given endurance
goal, then that goal has been met. For example, if a
sample of 100 units are endurance cycled to 1 million
cycles and 3 hav e failed at 100,000 cycles and a further
7 hav e failed at 1 million cycles, then the sample would
have an endurance of 100,000 cycles.
AN601
EEPROM Endurance Tutorial
Thi d t t d ith F M k 4 0 4
AN601
DS00601A-page 2
1995 Microchip Technology Inc.
Microchip uses a more stringent criteria for endurance:
no more than 2.5% of devices can have failed for the
given endurance goal to have been met.
A “write cycle” is also a somewhat flexible definition
since almost every customer will write the device in a
different way. For example, if the customer application
uses only the first three bytes of the array to store
variab le data, and the remainder of the array is used as
a lookup table , then a write cycle will be complete when
the three data bytes have been re-written to their new
data state.
A write cycle is often described as an erase/write cycle,
since almost all technologies employ an “auto-erase”
before the data is actually written to the array. This is
also used by Microchip, but we will use the term “write
cycle” since the auto-erase is invisible to, and cannot
be suppressed by, the customer.
The term “data changes” is occasionally used in place
of “write cycle” or “er ase/write cycle.” A data change will
occur when an auto-erase cycle is initiated, and a
second data change will occur upon the write cycle,
therefore, one “erase/write cycle” is equivalent to two
“data changes.” The term “data change” also implies
that a different type of cycling is being used than
“erase/write cycle.” This will be described later.
The term “write cycle” does not define under what
conditions the cycling was done (unless explicitly
stated) nor does it define the type of cycling that was
done. The endurance cycling can be done at any
number of conditions of voltage and temperature (e.g.,
85
°
C and 5.5V, or 25
°
C and 5.0V) that may or may not
meet with a customer’s application. The cycling mode
used in endurance cycling can affect the endurance of
the product. All these effects will be described later.
Microchip uses the most stringent conditions that are
reasonable f or endurance cycling. W e use b yte or page
mode cycling at a temperature of 85
°
C at 5.5V. All data
not explicitly defined at other conditions is taken at
these conditions.
A “read cycle” is completed when any number of bytes
of data have been read from the device. For the
FLOTOX-design EEPROM-based devices made by
Microchip a read cycle does not affect endurance,
since the data in the EEPROM is not changed. Other
technologies, such as Ferroelectric technology, may
have a limited number of read cycles since data is
corrupted during a read.
System Design Considerations
There are a number of design considerations that the
system designer can use to maximize the endurance of
an EEPROM-based device, if endurance is the
application’s limiting factor.
As will be described in more detail later , if the designer
has any control over certain environmental or operation
conditions he should observe the following basic
guidelines:
• Keep the application temperature as low as
possible
• Keep the application voltage (or the V
CC
voltage
on the EEPROM-based device) as low as
possible
• Write as few bytes as possible
• Use page write features wherever possible
• Write data as infrequently as possible
With these basic guidelines applied to the fullest
extent, the endurance of EEPROM-based devices can
be extended well beyond the guaranteed minimum
endurance. Under certain very specific conditions,
Microchip Serial EEPROMs have been shown to last
for well over 100 million cycles.
1995 Microchip Technology Inc. DS00601A-page 3
AN601
WRITE MODES IN EEPROMS
There are three ways that EEPROM-based devices
can have the entire array data contents changed.
These are: byte mode, page mode, and block (or bulk)
mode. Some types of devices support all three modes,
others may only suppor t one or two modes. The mode
that you use to write an EEPROM-based device will
affect the long term endurance of the product.
Byte mode writing is used when the contents of the
array are changed one byte at a time. For many
devices this is the only user-accessible write mode
available. To change the entire contents of a Serial
EEPROM in this way would take up to 10 seconds
(using 10 ms per page on a 64K Serial EEPROM).
Parallel EEPROMs such as a 28C64, which have a
faster byte write time (1 ms rather than 10 ms), but no
page mode would also take up to 10 seconds.
Page mode writing is a popular feature on many new
designs of EEPROM memory products. This feature
allows up to 8 b ytes of data to be written to the memory
in the same time that one byte would normally take. In
this mode, the write time f or a 64K Serial EEPROM can
be cut from eight seconds to one second.
Block cycle is gener ally a test mode used b y EEPR OM
manufacturers to make it easier to test the products.
Some types of EEPROM-based products have these
modes as user options (such as the ERAL and WRAL
mode in 93CXX products, or the Chip Clear mode in
28CXXX products) but generally this mode is not user
accessible. A block write can be done in as little as
1 ms, allowing millions of write cycles to be completed
in a few hours.
A general rule to follow in choosing write modes is that
the larger the number of b ytes being written in a single
instruction, the longer the device will last. For e xample ,
in byte mode a device might star t to fail after 300,000
cycles under a particular set of conditions, but the
device may last 600,000 cycles in page mode under
the same conditions. In block mode the device might
last 1 million cycles, under the same conditions.
The reason for this is related to the internal design of
any FLOTOX EEPROM-based product. In these
devices, an internal “charge pump” takes the applied
V
CC
voltage (typically 2.5 to 5.5V) and increases it to
15 to 25V. This voltage is required to induce the
“Fowler-Nordhiem Tunneling” or “Enhanced Emission”
effects that are used to program and erase
EEPROM-based devices. The specific effect that is
used is manufacturer-dependent. Microchip
EEPROMs program by Fouler-Nordheim Tunneling.
The charge pump voltage is used to program however
many EEPROM-cells are being programmed. For
example, in byte mode, all the cells in a byte (8 to 16)
are biased with the charge pump voltage. In block
mode, all the cells in the array (up to 100,000,
depending on the device) are biased with the charge
pump voltage. The charge pump is like a current
source during conditions of high load, so the voltage
put out by the charge pump will be reduced slightly if
more bytes are being written. If the whole arr ay is being
programmed then the charge pump voltage will be
significantly reduced, but the programming current Ipp
will be very high.
Generally, the lower the charge pump voltage the
better the endurance (there is a limit since the charge
pump voltage needs to be high enough to program the
cell) and so the best endurance is generally achieved
by using b lock mode cycling. Page mode is worse than
block mode, but better than byte mode. Block mode is
generally not a very useful cycling mode to the end
user, since the data contents in the whole array will be
changed to the same value (generally 00 or FF).
When Microchip tests EEPROM-based products we
use byte mode cycling on devices which do not have a
page mode, and page mode cycling for those that do.
We encourage our customers to use page mode writing
on all products which have page mode, to ensure high
endurance.
AN601
DS00601A-page 4
1995 Microchip Technology Inc.
ENDURANCE TESTING
METHODOLOGIES
Different manufacturers use different ways to both
endurance cycle and test EEPROM-based products.
There is no standard for endurance cycling, or testing
devices after cycling.
There are two groups of testing that Microchip
performs on all products: qualification and production.
Qualification testing is done for all new products, and
major changes to a product or manufacturing process.
Production testing is done on all devices shipped to
customers.
Qualification testing at Microchip is used to test the
reliability of new products, and to guarantee that the
device is reliable. A great deal of testing is done,
including endurance testing on all EEPROM-based
products. Endurance cycling is done at the maximum
rated data book value, generally 85
°
C or 125
°
C. After
the rated number of cycles (10,000, 100,000, or
1,000,000) the sample (around 300 from multiple waf er
lots) is tested to a full production test program. After
endurance, the units are subject to “data retention” to
guarantee that the required 40 years of data retention
will be achieved, after the maximum number of cycles
has been completed.
Endurance cycling is done under the conditions
pre viously described, and the data retention test is per-
formed after this. After the data retention stress is
completed (which takes six weeks) the devices are
tested again, to confirm the functionality of the device
to all data sheet parameters.
No more than 2.5% failures are allowed after
endurance, and a f ailure rate higher than 100 FITs after
1000 hours of data retention stress (equivalent to more
than 10 years at 55
°
C) is unacceptable.
Production testing is done by Microchip on all devices
shipped to a customer. Production testing begins
immediately after a wafer lot is finished being
processed, continuing in various stages until the
devices are shipped to a customer.
The first tests that are done on EEPROM-based
products at Microchip are called wafersort. They are
done before the wafer is cut up into dice for assembly.
There is a series of tests which include large numbers
of write cycles (up to 5000) to ensure reliability by
weeding out weak devices so they never get shipped.
After assembly full testing is done which includes
further write cycles across the guaranteed temperature
range, to ensure de vice functionality at the temperature
e xtremes. After the normal production testing a sample
of 128 units is taken from e v ery waf er lot, and cycled to
10,000 cycles (Parallel EEPROMs) or 1,000,000
cycles (Serial EEPROMs and EEPROM
microcontrollers) at 85
°
C using the conditions already
described. After endurance testing the devices are
tested for functionality at 85
°
C.
Any sample of Parallel EEPROMs which shows
significant failures at the end of cycling causes the
entire wafer lot to be pre-cycled prior to production
testing. Serial EEPROMs and EEPROM-based
microcontrollers do not receive the same disposition.
Lots with significant failures come under scrutiny and
full f ailure analysis with corrective actions is done. This
is, however, a rare occurrence.
The testing that Microchip does is unique.
Manufacturers will generally do different testing from
each other. Microchip firmly believes that our testing
ensures excellent quality and reliability.
1995 Microchip Technology Inc. DS00601A-page 5
AN601
THE EFFECT OF TEMPERATURE ON
EEPROM ENDURANCE
The temperature at which cycling is done will aff ect the
number of write cycles that can be executed before the
device fails. The higher the temperature, the worse the
endurance will be. Generally, and approximately, a
de vice which f ails at 10 million cycles at 25
°
C will f ail at
2 million cycles at 85
°
C and 1 million cycles at 125
°
C.
The reasons f or this are not conclusiv e (although there
is much technical literature supporting one theory or
another) but it is apparent that the failure mode of
EEPROM cells (generally considered to be electron
trapping in the tunnel dielectric causing shielding and
dielectric breakdown) is strongly dependent on
temperature.
Data taken by Microchip suggests that if the typical
failure of an EEPROM-based device is 10 million
cycles at 25
°
C, the mean failure will occur at other
temperatures according to the following table:
TABLE 1: TEMPERATURE MEAN FAILURE
This data was taken on Microchip FLOTOX
Fowler-Nordhiem Tunneling EEPROMs and formed a
part of the data set used to create the
Total Endurance
disk. Other technologies (such as
FLOTOX Enhanced Emission or Ferroelectric
technologies) may have different characteristics.
As is clearly seen, any cycling done at 25
°
C can be
misleading in the extreme if the application requires a
de vice that can be cycled 10 million times at, say, 55
°
C.
Write Cycle
Temperature Mean Failure (Cycles)
-40
°
C 37.1 million
0
°
C 16.7 million
25
°
C 10.0 million
40
°
C 7.4 million
55
°
C 5.4 million
70
°
C 4.0 million
85
°
C 2.9 million
100
°
C 2.2 million
125
°
C 1.3 million
THE EFFECT OF VOLTAGE ON
EEPROM ENDURANCE
The voltage at which a device is written can also aff ect
the endurance. This is simply because the charge
pump (used to program and erase EEPROM cells) is
more powerful at higher voltages. As has already been
described, a higher programming voltage will reduce
the endurance of an EEPROM cell, and a stronger
charge pump will produce a higher voltage.
Data taken by Microchip suggests that if typical failure
of an EEPROM-based device is 1 million cycles when
endurance cycling is done at 5.5V, mean f ailure occurs
at other temperatures according to the following table:
TABLE 2: VOLTAGE MEAN FAILURE
This data was taken on Microchip FLOTOX
Fowler-Nordhiem Tunneling EEPROMs and formed a
part of the data set used to create the Total Endurance
disk. Other technologies (such as FLOTOX Enhanced
Emission or Ferroelectric technologies) may have
different characteristics.
Endurance Cycling
Voltage Mean Failure (Cycles)
5.5V 1.0 Million
5.0V 1.2 Million
4.5V 1.4 Million
4.0V 1.7 Million
3.5V 2.0 Million
3.0V 2.4 Million
2.5V 2.8 Million
2.0V 3.3 Million
AN601
DS00601A-page 6
1995 Microchip Technology Inc.
THE EFFECT OF WRITE MODE ON
EEPROM ENDURANCE
As has been discussed there are three basic ways of
writing data to an EEPROM-based device:
• Byte mode
• Page mode
• Block mode
This is related to the strength of the charge pump in
applying the required programming voltages to the
EEPROM cells.
Data taken by Microchip suggests that if the typical
failure of an EEPROM-based device is 1 million cycles
when the endurance cycling is done in byte mode, the
mean f ailure will occur in other modes according to the
following table:
TABLE 3: ENDURANCE MEAN FAILURE
This data was taken on Microchip FLOTOX
Fowler-Nordhiem Tunneling EEPROMs and formed a
part of the data set used to create the Total Endurance
disk. Other technologies (such as FLOTOX Enhanced
Emission or Ferroelectric technologies) may have
different characteristics. This data was taken from a
Microchip 24LC16.
As you can see, the use of the block cycle data to
guarantee endurance can be misleading.
Endurance Cycling
Mode Mean Failure (Cycles)
Byte 1.0 Million
Page 4.6 Million
Block 13.2 Million
THE T OTAL ENDURANCE
PREDICTIVE SOFTWARE
Microchip has a Windows
-based software model
called Total Endurance. This program, based on all of
the customers endurance parameters, will predict the
failure level at the expected end of application life. This
tool is invaluable for system designers who would like
to fine-tune their application in favor of endurance. It is
available now from your local Microchip distributor.
1995 Microchip Technology Inc. DS00601A-page 7
AN601
NOTES:
DS00601A-page 8
1995 Microchip Technology Inc.
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