Nonvolatile semiconductor memory

Details Nonvolatile semiconductor memory Nonvolatile semiconductor memory

2003-08-12

2004-10-05

Phung, Anh (Department: 2824)

Static information storage and retrieval

Floating gate

Particular biasing

C365S185140

Reexamination Certificate

active

06801458

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a nonvolatile semiconductor memory capable of changing memory cell data in units of bytes.
EEPROMs are conventionally known as nonvolatile semiconductor memories for changing memory cell data in units of bytes.
Reference 1 (W. Johnson et al., “A 16 Kb Electrically Erasable Nonvolatile Memory,” ISSCC Digest of Technical Papers, PP. 152-153, February 1982) has proposed an EEPROM which changes memory cell data in units of bytes using FLOTOX (Floating Gate Tunnel Oxide) cells.
FIG. 65
is a plan view showing an example of a memory cell section of an EEPROM capable of byte erase.
FIG. 66
is a sectional view taken along a line LXVI—LXVI in FIG.
65
.
This EEPROM uses FLOTOX cells in the memory cell section. As a characteristic feature of a FLOTOX cell, an about 10-nm tunnel oxide film
22
a
is formed between an N
+
drain
20
a
and a floating gate
21
a
, and charges are transferred between the N
+
drain
20
a
and the floating gate
21
a
by applying an electric field to the tunnel oxide film
22
a.
A current flowing to the tunnel oxide film
22
a
is an F-N (Fowler-Nordheim) tunneling current generated by the F-N tunneling phenomenon.
FIG. 67
is a view showing the energy band of a MOS capacitor section.
When an electric field is applied to the MOS capacitor (N
+
drain—tunnel oxide film—floating gate), an F-N tunneling current flows to the tunnel oxide film (SiO
2
) on the basis of equation (1):
I=S·&agr;·E
2
exp(−&bgr;/
E
)  (1)
S
: area,
E
: electric field
&agr;=
q
3
/8
&pgr;h&PHgr;B
=6.94×10
−7
[A/V
2
]
&bgr;=−4(2
m
)
0.5
&PHgr;B
1.5
/3
hq
=2.54×10
8
[V/cm]
As is apparent from equation (1), the electric field with which the F-N tunneling current starts flowing is about 10 MV/cm. This electric field theoretically corresponds to a case wherein a voltage of 10V is applied to a tunnel oxide film of 10 nm.
Referring to
FIGS. 65 and 66
, assume that when a voltage is applied between the N
+
drain
20
a
and a control gate
23
a
, the capacitance ratio (coupling ratio) between the control gate
23
a
and the floating gate
21
a
is 0.5.
In this case, to apply a voltage of 10V to the tunnel oxide film
22
a
between the N
+
drain
20
a
and the floating gate
21
a
, a voltage as high as 20V must be applied between the N
+
drain
20
a
and the control gate
23
a.
For example, in the erase mode, the N
+
drain
20
a
is set at 0V and the control gate
23
a
at 20V to move electrons from the N
+
drain
20
a
to the floating gate
21
a
. In the “1” program mode, the N
+
drain
20
a
is set at 20V and the control gate
23
a
at 0V to move electrons from the floating gate
21
a
to the N
+
drain
20
a.
The disadvantage of the EEPROM using FLOTOX cells is that two elements, a memory cell and a select transistor, are required to store 1-bit data, as shown in
FIGS. 65 and 66
.
FIG. 68
shows another example of the memory cell section of the EEPROM capable of byte erase.
As characteristic features of this EEPROM, FLOTOX cells are used in the memory cell section, and a byte control transistor Tr is prepared in correspondence with memory cells of 8 bits (1 byte).
Table 1 shows bias conditions in each mode of this EEPROM.
TABLE 1
Unselected Byte
Unselected Byte
Connected to the
Connected to
Same Word Line
the Same Bit
Selected
as That of
Line as That of
Mode
Byte
Selected Byte
Selected Byte
Erase
Word Line
High
High
Low
(“0” pro-
Byte
High
Low
High
gram-
Control
ming)
Bit Line
Low
Low
High
“1” Pro-
Word Line
High
High
Low
gramming
Byte
Low
Low
Low
Control
Bit Line
High or
Low
High or Low*
2
Low*
1
*
1
= Data Dependent
*
2
= Don't Care
When such a memory cell section is used, various operation errors (disturbances) can be avoided. However, since 2+(1/8) transistors are required to store 1-bit data, the cell area increases to result in an increase in cost.
Flash EEPROMs aim at eliminating this problem. A conventional EEPROM is very convenient because data can be erased or programmed in units of 1-bit data.
However, when a computer hard disk requiring a large memory capacity is to be formed from an EEPROM, the EEPROM need not have a function of erasing or programming data in units of 1-bit data. In most hard disks, data is often erased or programmed in units of sectors (or in units of blocks).
It is more advantageous to attain a large memory capacity by cell area reduction and reduce the cost of products while omitting the function of changing data in units of 1-bit data. On the basis of such an idea, flash EEPROMs have been developed.
Details of a flash EEPROM are described in, e.g., reference 2 (F. Masuoka et al., “A new Flash EEPROM cell using triple polysilicon technology,” IEDM Technical Digest, pp. 464-467 December 1984).
FIG. 69
shows the structure of a memory cell of a flash EEPROM.
The memory cell of the flash EEPROM has a control gate and floating gate, like a memory cell of a UV erase EPROM. In the flash EEPROM, data is programmed by injecting hot electrons to the floating gate, as in the UV erase EPROM. Data is erased by removing electrons from the floating gate using the F-N tunneling phenomenon, like a byte EEPROM.
In the flash EEPROM, the erase operation for the individual memory cells is the same as in the byte EEPROM. However, the operation for the entire memory cell array is completely different from that in the byte EEPROM. More specifically, the byte EEPROM erases data in units of bytes while the flash EEPROM erases all bit data at once. Employing such an operation method, the flash EEPROM realizes a memory cell section with one transistor per bit and achieves a large memory capacity.
In the flash EEPROM, data can be programmed in units of bits, like the UV erase EPROM. More specifically, the flash EEPROM is the same as the UV erase EPROM in that all bit data are erased at once, and data can be programmed in units of bits.
To realize a memory chip with a large memory capacity, a NAND flash EEPROM has been proposed on the basis of the above-described flash EEPROM.
Reference 3 (F. Masuoka et al., “New ultra high density EPROM and Flash EEPROM with NAND structured cell,” IEDM Technical Digest, pp. 552-555 December 1987) discloses a NAND flash EEPROM.
The memory cell array portion of a NAND EEPROM has a NAND unit in which a plurality of (e.g., 16) memory cells are serially connected to form a NAND series with select transistors connected to its two ends, respectively, as shown in
FIGS. 70 and 71
.
In the NAND EEPROM, a bit line contact section and source line need be formed not for each memory cell but for one NAND unit. Two adjacent memory cells of the plurality of memory cells forming the NAND series share one diffusion layer. For this reason, the memory cell size per bit can be largely reduced, and a memory chip having a large memory capacity can be realized.
FIG. 72
shows a NOR flash EEPROM. In the NOR flash EEPROM, a 1-bit (one) memory cell is formed between a bit line and a source line.
In terms of cost, the above-described NAND flash EEPROM has a characteristic feature suitable to a large-capacity file memory: the cost per bit is low because the cell size can be reduced as compared to the NOR flash EEPROM. In terms of function, the NAND flash EEPROM has a higher data change rate and lower power consumption than those of the NOR flash EEPROM.
In terms of function, the NAND flash EEPROM is characterized in the scheme for changing data. More specifically, the NAND flash EEPROM achieves program and erase by charge transfer between the silicon substrate (channel) and the floating gate.
To transfer charges, the F-N tunneling phenomenon is used. A current necessary for programming is an F-N tunneling current flowing from the silicon substrate (channel) to the floating gate. Unlike the NOR flash EEPROM that uses hot electrons for programming, the NAND flash EEPROM has very small current consumption.
In a 64-Mbit NAND flash EEPROM, data

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