Microchip 11AA02E64 Manual de Usario

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AN1019

BASIC TERMS

The definition of “endurance” (as applied to EEPROMs)

contains various words and phrases that require clear

definition and understanding. As shown in the following

paragraphs, different manufacturers use different

standards. “Endurance cycling” is a test performed by

all manufacturers (and some customers) to determine

how many “write cycles” the product will achieve before

failing.

Microchip defines “endurance” as the minimum

number of write cycles the product can be subjected to

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 between.

For example, if the device does not correctly store data

into a particular address that is not used, then the

device would work correctly for the customer but would

fail a functional test by the manufacturer. Likewise, if

the device draws more current than the data sheet

specifies 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 specified operating

condition of a 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 have a failure, so a failure level is defined.

There are several industry standard conditions for

many types of reliability tests. For example, IEEE-Std-

1005-1998 defines a maximum cumulative failure rate

of 1%. JEDEC (The Joint Electronic Device Engineer-

ing Council), JESD47, defines that if three lots of 77

units each have no fails (equivalent to an LTPD of 1%)

at a given endurance goal, then that goal has been

met.

Microchip uses a more stringent criteria for endurance:

No fails out of a sample of 256 units per product, and

no fails out of 3 lots of 256 units each per technology

for the given endurance goal to have been met. That is

equivalent to 0/768 or less than 0.3% LTPD.

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

variable data, and the remainder of the array is used as

a look-up 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 EEPROM 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 “erase/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, an “erase/write cycle” is equivalent to two

“data changes.” The term “data change” may also imply

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 for endurance cycling: Byte or Page mode

cycling at a temperature or 85°C at 5.5V. All data not

explicitly defined at other conditions is taken at these

conditions.

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.

Author: David Wilkie

Microchip Technology Inc.

EEPROM Endurance Tutorial

AN1019

As will be described in more detail later, if the designer

has any control over certain environmental or operating

conditions, he should observe the following basic

guidelines:

• Keep the application temperature as low as

possible.

• Keep the application voltage (or the VCC voltage

on the EEPROM) as low as possible.

• Write as few bytes as possible.

• Use page write feature whenever 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 specified minimum

endurance. Under certain very specific conditions,

Microchip Serial EEPROMs have been shown to last

for over 100 million cycles, and 10-million cycle

applications are common.

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 support one or two modes. The mode

that is used to write an EEPROM-based device may

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 some 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 6 minutes (using 5 ms per byte on

a 512K Serial EEPROM). Page mode writing is a

popular feature on many designs of EEPROM memory

products. Again using 512K serial EEPROM as an

example, this feature allows up to 128 bytes of data to

be written to the memory in the same time that one byte

would normally take. In this mode, the write time for a

512K Serial EEPROM can be cut from 6 minutes to 3

seconds. Block cycle is generally a test mode used by

EEPROM 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 93LXXX 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 bytes being written in a single

instruction, the longer the device will last. For example,

in Byte mode, a device might start 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 EEPROM-based product. In these devices, an

internal “charge-pump” takes the applied VCC voltage

(typically 1.8V to 5.5V) and increases it to 15V to 20V.

This voltage is required to induce “Fowler-Nordhiem

Tunneling” that is used to program and erase

EEPROM-based devices.

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 or 16)

are biased with the charge pump voltage. In Block

mode, all the cells in the array (up to 1M 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 array is being programmed

then the charge pump voltage will be significantly

reduced.

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 Block 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 that have Page mode in order to get the

highest endurance.

ENDURANCE TESTING

METHODOLOGIES

Different manufacturers use different ways to both

cycle and test EEPROM-based products. There is no

standard for endurance cycling or for testing devices

after cycling.

There are two groups of testing that Microchip performs

on all products: qualification and production. Qualifica-

tion 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 our

customers.

Qualification testing at Microchip is used to ensure that

the device is reliable. A great deal of testing is done,

including endurance testing on all EEPROM-based

products. Endurance cycling is generally done at 85°C.

After the rated number of cycles, the sample is tested

to a full production test program. After endurance, the

units are subject to “data retention” to ensure that the

required 200 years of data retention will be achieved

after the maximum number of cycles has been

completed.

AN1019

Endurance cycling is done under the conditions

previously described and the data retention test is

performed after this. After the data retention stress is

completed (which takes up to six weeks), the devices

are tested again to confirm the functionality of the

device to all data sheet parameters. No failures are

allowed after the endurance or data retention stress

(equivalent to more than 200 years at 55°C).

Production testing is done 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 conducted on EEPROM-based products

at Microchip are called wafer sort. They are completed

before the wafer is cut up into dice for assembly. There

is a series of tests which include Block cycle (up to

5000) to ensure reliability by weeding out weak devices

so they never get shipped. After assembly, full testing

is again done to ensure device functionality at the

temperature extremes.

The testing that Microchip does is unique. Microchip

firmly believes that our testing ensures the highest

quality and reliability.

THE EFFECT OF TEMPERATURE ON

EEPROM ENDURANCE

The temperature at which cycling is done will affect 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

device which fails at 10 million cycles at 25°C will fail at

2 million cycles at 85°C and 1 million cycles at 125°C.

The reasons for this are not conclusive (although there

is much technical literature supporting one theory or

another), but it is apparent that the failure mode of

EEPROM cells (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:

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™

model. Other technologies may have different charac-

teristics. As is clearly seen, any cycling done at 25°C

can be misleading in the extreme if the application

requires a device that can be cycled 10 million times at,

say, 55°C.

THE EFFECT OF VOLTAGE ON

EEPROM ENDURANCE

The voltage at which a device is written can also affect

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.

More recently designed Serial EEPROM products at

Microchip have minimized this effect in order to provide

a more consistent endurance across VDD voltage.

Data taken by Microchip suggests that if the typical

failure of an EEPROM-based device is 1 million cycles

when endurance cycling is done at 5.5V, mean failure

occurs at other temperatures according to the following

table:

TABLE 1: TEMPERATURE MEAN

FAILURE

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

Especificaciones del producto

Marca:	Microchip
Categoría:	No categorizado
Modelo:	11AA02E64

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