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Erasing EPROM based chips.

What is and what is not allowed.

If programmable and UV erasable chips (EPROM based, next used term EPROM) are to bring the maximum benefit, when they are programmed, it is necessary to ensure the right algorithm on the right type of programmer. This is clear, of course.
But what is necessary to know about UV erasers? A mistake or an incorrect use of an eraser may cause the same amount of problems as an incorrect use of a programmer. To provide a clarification of how such an error may occur, first a little bit of theory:

An erased EPROM has all cells in the HIGH state. A change takes place during the programming. EPROM cells that are to be programmed to the LOW state are charged by the lead-in electric charge with defined parameters. The erasing process means an increase in the conductivity of the otherwise not conductive insulation of the gate, well enough to allow the charge of the cell to migrate into the surrounding silicon. Thus the cell gets into the HIGH state.

Now let me describe how errors may occur while an EPROM is being erased, and thus remove a number of myths surrounding that process.
Fiction:
Erasing a chip for 5 minutes and the subsequent check of the erasure are sufficient for finding out whether the chip has been erased correctly.
TRUTH:
For the correct erasure, an EPROM needs a precisely specified dose of UV radiation. The dose is specified by EPROM manufacturer in the form of a total (integrated) dose of radiation per a square unit. The dose is typically 15Ws/cm2. The specified value is the minimum one necessary for the correct erasure, and it may vary by manufacturer and by the type of chip (let’s mention the heroic Soviet EPROMs). For the calculation of the erasing period it is therefore necessary to know the integrated dose (let’s assume it is 15Ws/cm2) and the UV power of the EPROM eraser used (EREPROM-01 provides about 19 mW/cm2). That gives an erasing period of 15 (Ws/cm2)/0.019 (W/cm2) = 790 seconds, i.e. 15 minutes with some reserve..
Let’s introduce a term ‘undererased’ EPROM (or EPROM cell) for further use. It is an EPROM (or an EPROM cell) that has not received the prescribed dose of UV radiation.
Fiction:
An undererased EPROM is shown as not erased in a blank check or in an erasure test before programming.
TRUTH:
An undererased EPROM will pass erasure tests with about a 90 % chance. In the case of most programmers, such tests are done under standard conditions, i.e. VCC = 5 V or VCC = 5 V+10 %. An undererased EPROM, with partially charged cells, is read as erased. During programming, however, the chip is exposed to a VCC of 6 V or higher and to a VPP of 12 V or higher (depending on the type of algorithm), with additional internal heating. Under such conditions, with an increased supply voltage and a growing temperature, the undererased cells that you do not intend to program may be read as already programmed. In the better case it will cause an error during programming, but if such an EPROM passes the programming process and its unprogrammed cells are shown to be erroneous later, your will be wasting your precious time, if your are trying to identify an error in the device..
Things to remember:
  • A chip read as erased may not be actually empty.
  • Always make sure that chips are erased for a sufficiently long time and thus receive at least the minimal prescribed dose of UV radiation
  • Make sure the window of the chip is clean during erasing. Remnants of glue after the removal of the protection sticker may reduce UV radiation by 90 %.
  • If you are not sure about the quality of the UV tube in the eraser, replace it sooner rather than later, e.g. once a year or after every 2,000 hours of operation
  • If you have a production programmer with a possibility of a marginal test (verification at the minimum and maximum allowed supply voltages) after the programming, switch to this option.
  • ? Remove the sticker from the window of the chip only for the period of erasing.
Final advice: Stop looking into the eraser’s lamp before you stop seeing it.



Presov, June 2004
Author: Jan Puobis, Elnec



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