Process and integrated circuit for a multilevel memory cell with an asymmetric drain

• US 6,525,361 B1
• Filed: 11/19/1999
• Issued: 02/25/2003
• Est. Priority Date: 07/29/1998
• Status: Expired due to Term

• Current Assignee: Macronix International Co., Ltd.
• Original Assignee: Macronix International Co., Ltd.
• Inventors: Lin, Chin Hsi, Wang, Mam Tsung, Lin, Hon Sui, Chen, Chung Ju, Ni, Ful Long, Lu, Tao Cheng
• Primary Examiner(s): Flynn, Nathan J.
• Assistant Examiner(s): MANDALA, VICTOR A
• Application Number:
  US09/443,964
• Time in Patent Office: 1,194 Days
• Field of Search: 257/296, 257/288, 257/390, 257/391, 257/404, 257/403, 257/402, 257/297, 257/300, 257/304, 257/305
• US Class Current: 257/297
• CPC Class Codes: G11C 11/56 using storage elements with more than two stable states represented by steps, e.g. of voltage, current, phase, frequency
• CPC Class Codes: G11C 11/5642 Sensing or reading circuits; Data output circuits
• CPC Class Codes: G11C 11/5692 read-only digital stores using storage elements with more than two stable states
• CPC Class Codes: H10B 20/00 Read-only memory [ROM] devices
• CPC Class Codes: H10B 20/383 Channel doping programmed

Abstract

An asymmetric multilevel memory cell provides an inhibited source read current. The inhibited source read current dramatically reduces the likelihood of a cell type misread error for a memory array comprising multilevel cells. The method for fabricating the asymmetric multilevel memory cell comprises a source only implant, formation of a spacer on the drain side of the gate prior to source/drain implant, and the resultant formation of an offset region disposed between the channel and the drain. The offset region is not controlled by the gate voltage. The drain current at 1.5 volts is more than 3.5 times larger than the source current at 1.5 volts for spacer width of 0.12 micrometers. Asymmetric multilevel memory cells in a memory array, where the cells have a common source configuration, are accurately read in one direction because neighboring cells on the word line have substantially lower source current than the read cell drain current.

Claims

1. An integrated circuit comprising:

an array of memory cells, the memory cells comprising transistors having channels in channel regions of a substrate, selected asymmetric multilevel memory cells in the array storing multiple bits;

a word line and a bit line coupled respectively with rows and columns of memory cells in the array by which to read data stored in the array, the bit line comprising a first patterned layer of the integrated circuit, the word line comprising a second patterned layer of the integrated circuit;

each of the selected asymmetric multilevel memory cells having a source formed in the substrate, a gate, a gate oxide disposed between the substrate and the gate;

each of the selected asymmetric multilevel memory cells further comprising;

an asymmetric drain formed in the substrate, the asymmetric drain having a width;

a channel formed in the substrate, the channel in contact with the gate oxide and aligned with the gate, the channel extending from the source towards the asymmetric drain, the channel separated from the asymmetric drain by an offset, the channel adapted to store multiple bits, the channel adapted to form a depletion layer in the channel proximal to the gate oxide in response to a gate voltage; and

an offset region disposed in the substrate and disposed between the channel and the asymmetric drain, the offset region having an initial conduction state, the offset region adapted to maintain the initial conduction state proximal to the gate oxide in response to the gate voltage;

each of the selected asymmetric multilevel memory cells having a first asymmetric drain read current corresponding to a drain voltage and a source read current corresponding to a source voltage, the source voltage equal to the drain voltage, the drain read current having a different value than the source read current;

the offset of each of the selected memory cell transistors sufficiently large to ensure that the selected memory cell transistors can only be accurately read by the corresponding word lines and corresponding bit lines in one direction, wherein the gate of each of the selected asymmetric multilevel memory cells is addressable by a corresponding word line, and the integrated circuit further comprises;

a sense amplifier having a sense amplifier conductor, the sense amplifier conductor having a first side and a second side;

a ground having a conductor, the ground conductor having a first side and a second side;

the bit line disposed proximal to the ground conductor, the bit line disposed on the first side of the ground conductor, the bit line disposed proximal to the sense amplifier conductor, the bit line disposed on the second side of the sense amplifier conductor;

a first selected memory cell disposed between the bit line and the second side of the sense amplifier conductor, the asymmetric drain of the first selected memory cell in communication with the sense amplifier conductor; and

a second selected memory cell disposed between the bit line and the first side of the ground conductor, the asymmetric drain of the second selected memory cell in communication with the ground conductor;

the first selected memory cell and the second selected memory cell having a common source, the bit line in communication with the common source of the first and second selected memory cells;

wherein, the first memory cell can only be accurately read by the corresponding word line and the corresponding bit line in a first direction, the second memory cell can only be accurately read by the corresponding word line and the corresponding bit line in a second direction, the second direction opposite the first direction.

2. The integrated circuit of claim 1, wherein the offset is in a range from 0.02 micrometers to 0.20 micrometers.

3. The integrated circuit of claim 1, wherein the offset region is adapted to form a depletion layer proximal to the gate oxide in response to the drain voltage;

and the offset region is adapted to maintain the initial conduction state proximal to the gate oxide in response to the source voltage.

4. The integrated circuit of claim 1, further comprising bank select lines coupled with rows of transistors in the array, responsive to applied potentials on the bank select lines adapted to determine which selected asymmetric multilevel memory cell is read by the array, the rows of transistors coupled with the bank select lines comprising a plurality of high threshold voltage transistors, the high threshold being higher than said applied potentiais on the bank select lines, the high threshold voltage transistors disposed in the array to block alternate current paths.

5. The integrated circuit of claim 1, wherein the first direction is from the asymmetric drain of the first selected memory cell to the source of the first selected memory cell, the second direction is from the asymmetric drain of the second selected memory cell to the source of the second selected memory cell.

6. The integrated circuit of claim 1, wherein the first direction is from the asymmetric drain of the first selected memory cell to the source of the first selected memory cell, the second direction is from the asymmetric drain of the second selected memory cell to the source of the second selected memory cell;

a portion of the second direction current flows from the source of the first selected memory cell to the asymmetric drain of the first selected memory cell, the portion of the second direction current in the first selected memory cell is smaller than forty percent of the second direction current of the second selected memory cell;

a portion of the first direction current flows from the source of the second selected memory cell to the asymmetric drain of the second selected memory cell, the portion of the first direction current in the second selected memory cell is smaller than forty percent of the first direction current of the first selected memory cell.