Method for defining a default state of a charge trap based memory cell
A method of generating a default state in an embedded Multi-Time-Programmable-Read-Only-Memory for a high-performance logic technology consisting of a plurality of memory cells featuring a charge trap, each having a first and a second NMOS transistor. The first and second NMOS transistors use a different mask having different threshold voltages. The second NMOS threshold voltage is adjusted to a middle point of the threshold voltage of the first NMOS with or without trapping the charge. When the charge is not trapped by the first NMOS, the NMOS threshold is lowered to the second NMOS, thereby generating a default state. When the charge is trapped to the first NMOS, the NMOS threshold is higher than the second NMOS, generating a second state. Moreover, a reference voltage generation can use two arrays, each consisting of memory cells and reference memory cells such that a default state can be generated for a single transistor per memory cell.
1. An Embedded Multi-Time-Read-Only-Memory (eMTPROM) comprising:
an N-type Metal-Oxide-Semiconductor-Field-Effect-Transistor (NMOS) array consisting of a plurality of cells, each of said cells provided with a first NMOS having a first threshold voltage (VT 1 ) and a second NMOS having a second threshold voltage (VT 2 ), wherein said VT 2 is higher than said VT 1 ;
wherein a default state is generated by developing a differential voltage on a first bitline coupled to said first NMOS, and a second bitline coupled to said second NMOS without trapping a charge to any of said NMOS, thereby enabling a differential sense amplifier coupling to said first and second bitlines.
2. The eMTPROM recited in claim 1 , wherein said VT 1 and said VT 2 are determined by two different implant mask sets.
3. The eMTPROM recited in claim 1 , wherein said VT 2 is higher than said VT 1 by a value ranging between 100 mV and 150 mV.
4. The eMTPROM recited in claim 1 , wherein a level of said VT 2 is programmed by trapping a charge to said second NMOS during manufacturing.
5. The eMTPROM recited in claim 1 , wherein said default state changes to a second state by trapping a charge to said first NMOS wherein said VT 1 is higher than said VT 2 .
6. A method of generating a default state comprising forming an array consisting of a plurality of cells, each of said cells having first NMOS having a first threshold voltage VT 1 without trapping a charge and a second NMOS having a second threshold voltage VT 2 , wherein said VT 2 is different from said VT 1 ;
wherein generating said default state further comprises detecting a differential voltage of said VT 1 and said VT 2 , and wherein said VT 2 is higher than said VT 1 by 100 mV to 150 mV; and
wherein said first NMOS features an increase of said threshold voltage to a third threshold voltage (VT 3 ), wherein said VT 3 is higher than said VT 2 .
7. The method in claim 6 , wherein said VT 2 is adjusted to one-half of a sum of said VT 1 and said VT 3 .