ST7294 datasheet(58/69 Pages) STMICROELECTRONICS

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3 GENERAL INFORMATION

SGS-THOMSON offers ST7 devices in EPROM

and OTP as well as in the ROM version. This

range of product options provides the ST7 user

with maximum of flexibility for his application

needs.

The OTP rather than the ROM version of the ST7

is recommended when the customer’s quantity re-

quirement is relatively small on a given code - e.g.

less than 10 thousand pieces. But the OTP solu-

tion is also popular even for volumes when the

customer has a need for reduced leadtime, wheth-

er pre-production, an unforecasted increase in de-

mand, or a quick program change. And the OTP is

often preferred by those customers who have sev-

eral codes running concurrently since they need

purchase and stock only one vendor sales type.

However, it must be understood that the ROM and

OTP follow different production flows and that the

difference between these flows, in particular the

method by which final electrical test is performed,

may have an impact on the customer.

The basic production flow is as follows:

– Wafer diffusion

– Electrical test of dice (wafer sort)

– Assembly (encapsulation)

– Final electrical test

For ROM parts, the customer program is included

at a specific mask of the wafer. Therefore, all of

the product’s functionality is present in both the die

and the assembled product and this functionality

can be fully evaluated at both wafer sort and final

electrical test. SGS-THOMSON fully tests the

ROM version’s functionality at both wafer sort and

final test, thus ensuring conformance to the elec-

trical specification and a low reject rate.

But for OTPs there exists an additional feature

that must be tested: programmability.The program

memory of an OTP should be seen as a collection

of fuses that will be blown during programming.

These fuses can be ”recovered” by lightening the

die with UV light. This recovery is no longer possi-

ble once the OTP die has been encapsulated in an

opaque plastic package. The programmability and

data retention can only be fully tested at wafer

sort.

At this step the dice are electrically tested and the

memory is programmed to verify programmability.

Then the wafers are placed in high temperature

bake in order to provoke any possible memory re-

tention defects. They are then retested to check

data retention. After this test, UV light is used to

”recover” the fuses and the good dice are assem-

bled.

Although the programmability of the OTP dice is

verified as fully functional at probe test, the die en-

capsulation process has the potential to affect this

function in the finished product. It is therefore nec-

essary that in addition to electrical final test, a pro-

grammability test be made. However since UV

light cannot be used to erase an OTP’s pro-

grammed byte once the die has been encapsulat-

ed it is then impossible to write a test pattern and

thus to check 100% of the user program area. For

this reason the final test is limited to a reserved

number of bytes which are programmed and then

verified.

As the programmability of the OTP cannot be fully

tested once the die has been encapsulated, unlike

the ROM or EPROM versions, a customer should

find a programming reject rate below 1.0% and a

data retention reject rate below 0.5% when pro-

gramming is performed using qualified program-

ming tools. Apart from the programmability, the

OTP has the same reject as EPROM and ROM

versions.

In order to lower the reject levels for programma-

bility and data retention, SGS-THOMSON is con-

tinually pursuing technology improvements in are-

as such as soft die handling, low stress com-

pounds, and passivation layer enhancements.

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