Multiple bits per-cell flash EEPROM capable of concurrently programming and verifying memory cells and reference cells

An improved programming structure for performing a programming operation and a concurrent verify operation in an array of two bits-per-cell flash EEPROM memory cells on a more effective and efficient basis, said programming structure comprising in combination:

• a memory core array (12) having a plurality of memory core cells arranged in rows of word lines and columns of bit lines intersecting said rows of word lines, each of said memory core cells including a floating gate array transistor having its control gate connected to one of said rows of word lines, its source connected to one of said columns of bit lines, and its drain connected to a common array ground line;
• row decoder means (14) responsive to row address signals and being operatively connected to said memory core array for selecting certain ones of said rows of word lines;
• column decoder means (16,20) responsive to column address signals and being operatively connected to said memory core array for selecting certain ones of said columns of bit lines;
• reference cell array means (22) including a plurality of reference core cells arranged in rows of word lines corresponding to the number of rows in said memory core array and in columns of reference bit lines corresponding to three programmable memory states and which are selected together with the selected memory core cells;
• each of said reference core cells including a reference cell transistor having its control gate connected to the same one of said rows of word lines, its source connected to one of the columns of reference bit lines, and its drain connected to the common array ground line;
• pre-charging means (36) for initially precharging all of said array bit lines and all of said reference bit lines to a predetermined potential prior to said programming operation;
• reference voltage generator means (134) having an output for selectively generating one of three target memory core cell bit line program-verify voltages, each one corresponding to one of the three programmable memory states;
• switching means (36a) for selectively connecting a program current source to the selected certain ones of said columns of bit lines containing the selected memory core cells which are to be programmed;
• sensing logic means (26,27) having a first input connected to the output of said reference voltage generator means and having a second input connected to one of the selected certain ones of said columns of bit lines for continuously comparing a potential on the one of the selected bit lines and one of the three target program-verify voltages generated by said reference voltage generator means;
• said sensing logic means having an output for generating a logic signal which is switched to a low logic level when said potential on said one of the selected bit lines falls below said selected one of the three target program-verify voltages;
• and
• said switching means being responsive to said low logic level for disconnecting said program current source so as to inhibit further programming of said selected memory core cells.

A method for performing a programming operation in an array of multiple bits-per-cell flash EEPROM memory cells on a more effective and efficient basis, said method comprising the steps of:

• providing a memory core array having a plurality of memory core cells arranged in rows of word lines and columns of bit lines intersecting the rows of word lines;
• providing a reference cell array including a plurality of reference core cells arranged in rows of word lines corresponding to the number of rows in the memory core array and in columns of reference bit lines corresponding to the number of programmable memory states and which are selected together with a selected memory core cell;
• precharging initially all of the array bit lines and all of the reference bit lines to a predetermined voltage potential prior to said programming operation;
• selectively generating one of a plurality of target memory core cell bit line program-verify voltages, each one corresponding to one of a plurality of programmable memory states;
• selectively connecting a program current source to the selected certain ones of the columns of array bit lines containing the selected memory core cells which are to be programmed;
• disconnecting the program current source so as to inhibit further programming of the selected memory core cells when a potential on the selected bit line falls below the selected one of the plurality of target program-verify voltages;
• generating a plurality of target reference cell bit line program-verify voltages, each one corresponding to one of a plurality of reference cell levels;
• and
• connecting second program current sources to the reference bit lines at the same time as the memory core cells are being programmed.