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SELECTING BETWEEN ROM, FASTROM AND OTP FOR A MICROCONTROLLER

5 OTP RELIABILITY

Why do ROM devices have very low failure rates?

For ROM parts, the customer program is included at a specific mask level of the wafer fab-

rication. Therefore, the complete product functionality is present in both the die and the as-

sembled product. This functionality can be fully evaluated at both wafer probing and final

electrical test, thus ensuring a low reject rate at customer manufacturing stage.

Testing Programmability and Data Retention for OTP devices

To ensure optimal OTP quality, STMicroelectronics tests two important device characteristics:

programmability and data retention. The program memory of an OTP should be seen as a

number of cells that will be activated during programming. These cells can be “deactivated” by

lighting the die with UV light. This recovery is no longer possible once the OTP die has been

encapsulated in an opaque plastic package. Therefore programmability and data retention

can only be fully tested at wafer probing.

At this step the dice are electrically tested and the memory is programmed to verify program-

mability. Then the wafers are placed in high temperature bake to accelerate any possible

memory retention defects. The dice are then tested again to check data retention. After this

test, UV light is used to deactivate the cells and the good dice are assembled.

Description of OTP programmability test

Figure 4. Details about the electrical tests performed on OTP and ROM

Although the programmability of the OTP die is verified as fully functional at probe test, the as-

sembly process can affect this parameter in the finished product (for example by damaging

some cells). It is therefore necessary to make a final test to check programmability. But

since UV light cannot be used to restore OTP's programmed bytes once the die has been en-

Full Electrical

Memory Check

Full Electrical

OTP Programming

Memory Check

Inking

Memory Check

BAKE

Programmability

Data Retention

ROM

OTP

WAFER PROBING

FINAL TEST

Tested

Feature

TEST

TEST

Full Electrical

Memory Check

TEST

Full Electrical

TEST

Full Electrical

Memory Check

TEST

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VR02100E