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DIFFERENTIAL SENSE AMPLIFIER REFERENCE CURRENT MODE FOR PRODUCTION SCREENING

IP.com Disclosure Number: IPCOM000009626D
Original Publication Date: 2000-Jan-01
Included in the Prior Art Database: 2002-Sep-05
Document File: 5 page(s) / 222K

Publishing Venue

Motorola

Related People

Jon Choy: AUTHOR [+3]

Abstract

The use of a highly parallelized logic tester EXISTING SENSE AMPLIFIER SCHEME early in production screening called wafer level burn-in has necessitated an embedded flash memory When reading a flash memory array, a sense test mode where the sense amplifier reference cm- amplifier compares the value of a reference current rent can be changed to a non-bitcell based reference. against the current produced from a flash memory bitcell. Initially, we had a sensing scheme which compared a bitcell based reference current against a bitcell in the flash memory array exclusively.

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DIFFERENTIAL SENSE AMPLIFIER REFERENCE CURRENT MODE FOR PRODUCTION SCREENING

by Jon Choy, Dave Chrudimsky and Thomas Jew

  The use of a highly parallelized logic tester EXISTING SENSE AMPLIFIER SCHEME early in production screening called wafer level
burn-in has necessitated an embedded flash memory When reading a flash memory array, a sense test mode where the sense amplifier reference cm- amplifier compares the value of a reference current rent can be changed to a non-bitcell based reference. against the current produced from a flash memory bitcell. Initially, we had a sensing scheme which compared a bitcell based reference current against a bitcell in the flash memory array exclusively.

Fig. 1

Q Momm,a, Inc. m 3 JanuaTy 2m

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0 M MO7VROLA

  This scheme is pretty effective, once the refer- ence is set. However, we have no way of guarantee- ing that the state of a flash memory bitcell reference level is at the correct level coming out of the fabri- cation plant. On the other hand, in order to exercise the flash memory array, a very specific reference current needs to be defined. Even to program or erase a given memory cell, we need to ensure that the bitcell reaches a program or erase state by "sens- ing" it after an erase or program operation. As men- tioned above, the way that the sense amplifier works is that it compares the current of the reference bit-

her

Poor reference level

Program current level

current --------____ Good reference level level

Erase current level

lower

cell to the current of the memory array bitcell. If the reference is greater than the array bitcell, then the sense amplifier may for example output a logic level 1 and if it is less than the array bitcell then it will produce a logic level 0. The idea is for the sense amplifier to give an output which is different for programmed bitcells versus erased bitcells. If the reference is skewed to one side or the other, the result may be that the output is the same for both the erased bitcells and the programmed bitcells (see the figure below). Thus, if the reference is based on a bitcell, it must be trimmed to the correct level.

7 ------------

I

I-----------_

Poor reference level

Fig. 2

0 Motomla, Inc. 2cm January 2ooo

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Technical MOTOROLA @ Developments

REQUIREMENTS FOR SElTlNG done with the flash memory's internal charge pump THE SENSE AMPLIFIER REFERENCE or alternatively by applying high voltage (as high as

                                1OV) through external i/o pins. The next require- To set the reference current bitcell, this requires ment is to directly measure the current of the refer- two steps to occur. One, set the current level of the ence bitcell to verify it is at the desired level. This reference bitcell by altering its threshold voltage is typically done through an external i/o pin as well, (Vt) or turn on voltage by applying the appropriate Figure...