Nonvolatile semiconductor memory with a programming...

Details Nonvolatile semiconductor memory with a programming... Nonvolatile semiconductor memory with a programming...

2001-12-12

2003-11-18

Le, Thong (Department: 2818)

Static information storage and retrieval

Floating gate

Particular connection

C365S185050

Reexamination Certificate

active

06650566

ABSTRACT:

This application relies for priority upon Korean Patent Application No. 2000-75641, filed on Dec. 12, 2000 and No. 2000-75642, filed on Dec. 12, 2000, the contents of which are herein incorporated by reference in their entirety.
FIELD OF THE INVENTION
The present invention relates to nonvolatile semiconductor memory devices, and more specifically to a NAND-type flash memory device in which the threshold voltages of parasitic transistors interposed between memory cell transistors belonging to the same rows are controllable for a programming operation.
BACKGROUND OF THE INVENTION
NAND-type flash memory devices are in great demand for their high storage capacity and high integration density, without a need for refreshing, and their straightforward electrical functions of erasing and programming. The facilities of data retention of the NAND flash memory device, even during a power-off, are very useful for portable electronic systems such as mobile computers, digital still cameras, or PDAs.
The NAND flash memory device has memory cells electrically erased and programmed, i.e., EEPROM cells, each being formed of a source and a drain spaced apart from each other in a bulk region (or a semiconductor substrate), a floating gate positioned over a channel region between the source and drain, and a control gate over the floating gate. Insulation films are interposed between the control and floating gates, and between the floating gate and the channel region.
FIG. 1
shows an arrangement of a memory cell array including the EEPROM cells. Memory cell transistors MC
15
~MC
0
connected in series form a cell string CS
0
(or CS
1
). The memory cell transistor MC
15
is connected to bitline BL
0
(or BL
1
) through string selection transistor SST, and the memory cell transistor MC
0
is connected to common source line CSL through ground selection transistor GST. Control gates of the memory cell transistors M
15
arranged in a row are coupled to wordline WL
15
. In the same manner, control gates of other cell transistors in a row are coupled to their corresponding wordlines.
Threshold voltages of the cell transistors are set at about −3V by erasing, and then a programming operation is carried out for a selected memory cell transistor to raise its threshold voltage. This is done by applying a high voltage of 20V to a corresponding wordline coupled to the selected cell transistor. Threshold voltages of the other non-selected cell transistors do not change from their current values. However, typically a programming disturbance occurs whereby memory cell transistors coupled to a wordline coupled to a selected memory cell transistor are undesirably programmed by the high-leveled program voltage even though the cell transistors are not selected in a programming operation.
A program inhibit technique for preventing the non-selected memory cell transistors from being undesirably programmed has been proposed in U.S. Pat. No. 5,677,873 entitled “Method of programming flash EEPROM integrated circuit memory devices to prevent inadvertent programming of nondesignated NAND memory cells therein”, or U.S. Pat. No. 5,991,202 entitled “Method for reducing program disturb during self-boosting in a NAND flash memory”. In a method of program inhibition called self-boosting, a current path towards a ground is cut off by applying 0V to a gate of the ground selection transistor GST. 0V is applied to a bitline (e.g., BL
0
) assigned to a selected memory cell transistor while a non-designated bitline (e.g., BL
1
) sees a program inhibit voltage of 3.3V or 5V of a power supply voltage VCC.
At the same time, the power supply voltage VCC is applied to a gate of the string selection transistor SST so that a source of the string selection transistor is charged up to VCC-Vth (Vth is a threshold voltage of the string selection transistor). The string selection transistor is substantially shut off. Next, a high program voltage Vpgm and a pass voltage Vpass are applied to a selected wordline and non-selected wordlines, respectively, so that channel voltages of the non-selected memory cell transistors are boosted up to levels that prevent programming. The boosted channel voltages prohibit generation of F-N tunneling between the floating gate and channel region, preventing any change of the non-selected memory cell transistors from their primary erased states.
Nevertheless, there is still a problem of programming disturbance, because the non-selected memory cell transistors adjacent to a selected memory cell transistor tend to be programmed due to leakage current flowing through parasitic MOS (meal-oxide-semiconductor) transistors
10
as shown in FIG.
1
. The parasitic transistors
10
are connected between active regions (sources or drains) of cell transistors coupled to the same wordlines.
Referring to
FIG. 2
which shows a sectional diagram taken along the line A-A′ of
FIG. 1
, channel region
2
of program cell transistor MC
14
p
(to be programmed) and channel region
3
of program-inhibit cell transistor MC
14
i
act as a source and a drain, respectively, of the parasitic transistor. And the wordline W
14
acts as a gate of the parasitic transistor. A substrate region under field oxide
14
between the channel regions
2
and
3
is assigned to a channel region of the parasitic transistor. As a result, if the program voltage Vpgm is higher than a threshold voltage of the parasitic transistor, the parasitic transistor will be turned on, inducing a generation of leakage current flowing into the channel region
2
of the program cell transistor MC
14
p
from the channel region
3
of the program-inhibit cell transistor MC
14
i
through the conductive parasitic transistor. Thus, the self-boosted voltage at the channel region
2
of the program-inhibit cell transistor MC
14
i
becomes lower, resulting in an undesirable programming disturbance.
There is a way to overcome the aforementioned problem by increasing the threshold voltage of the parasitic MOS transistor, using an ion implantation into the substrate region acting as the channel region of the parasitic transistor. However, it is preferred to restrict such an ion implantation into the substrate region because of a decrease in breakdown voltage in the drain region, or because of a scaling-down of a topological size of the memory cell array. Increasing the threshold voltage of the parasitic transistor by biasing the substrate
1
with a negative voltage is also not desirable because a longer time for charging the substrate
1
increases overall program time.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a nonvolatile semiconductor memory device capable of preventing program disturbance due to parasitic MOS transistors, and a method thereof.
It is another object of the invention to provide a nonvolatile semiconductor memory device capable of increasing a threshold voltage of a parasitic MOS transistor interposed between adjacent memory cell transistors, without increasing the wordline voltage during a program operation.
In order to achieve the present objects, a nonvolatile semiconductor memory of the invention includes: a memory cell array formed of a plurality of memory cell strings each connected to a plurality of bitlines; a plurality of page buffers each connected to the bitlines; a plurality of transistors connected between the bitlines and the page buffers; and a bitline voltage controller applying a bitline control voltage to gates of the transistors. The bitline control voltage is charged to a first voltage during a first bitline setup period and charged to a second voltage during a second bitline setup voltage, the second voltage being lower than the first voltage.
Another aspect of the invention is a method of programming in a nonvolatile semiconductor memory device, which has a plurality of memory cell strings connected to a plurality of bitlines and constructed of a plurality of memory cell transistors whose gates are coupled to a plurality of wordlines, and a plurality of registers corresponding to the bi

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