PMOS avalanche programmed floating gate memory cell structure

Details PMOS avalanche programmed floating gate memory cell structure PMOS avalanche programmed floating gate memory cell structure

1999-01-07

2001-04-10

Nelms, David (Department: 2818)

Static information storage and retrieval

Floating gate

Particular connection

C365S185140, C365S185180, C257S314000

Reexamination Certificate

active

06215700

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to nonvolatile memory, and particularly a non-volatile memory structure optimized for particular applications.
2. Description of the Related Art
Non-volatile memory devices of the type commonly referred to in the art as EPROM, EEPROM, or Flash EEPROM serve a variety of purposes, and are hence provided in a variety of architectures and circuit structures.
As with many types of integrated circuit devices, some of the main objectives of non-volatile memory device designers are to increase the performance of devices, while decreasing device dimensions and consequently increasing circuit density. Cell designers strive for designs which are reliable, scalable, cost effective to manufacture and able to operate at lower power, in order for manufacturers to compete in the semiconductor industry. EEPROM devices are one such device that must meet these challenges. In some applications, such as flash memory cards, density is at a premium, while in applications such as programmable logic devices (PLD's), performance and reliability is more important and space is at less of a premium.
EEPROMS (electrically erasable/programmable read-only memories) generally employ Fowler-Nordheim (F-N) tunneling for both programming and erasing. The term “flash”, when used with “EEPROM”, generally refers to a device programmed by hot electron injection. Generally, flash technology employs a floating gate structure with a thin oxide layer between the floating gate and the drain side of the transistor where Fowler-Nordheim tunneling occurs.
As process technology moves toward the so-called 0.18 and 0.13 micron processes, the conventional “stacked gate” EEPROM structure has given way to different cell designs and array architectures, all intended to increase density and reliability in the resulting circuit.
An alternative to the aforementioned FN tunneling-based cell structure is presented in Ranaweera, et al., “Performance Limitations of a Flash EEPROM Cell, Programmed With Zener Induced Hot Electrons,” University of Toronto Department of Electrical Engineering (1997). Discussed therein is a flash EEPROM cell which accomplishes programming and erase by establishing a reverse breakdown condition at the drain/substrate junction, generating hot electrons which are then coupled to the floating gate to program the cell.
FIGS. 1A
,
1
B and
1
C of Ranaweera, et al. are reproduced as
FIGS. 1A
,
1
B and
1
C of the present application.
FIGS. 1B and 1C
show cross-sections of the cell shown in FIG.
1
A. As shown in
FIG. 1C
, a “ZEEPROM” cell comprises a source and drain region, floating gate and control gate, with a P+ pocket implant extending part way across the width of the drain region to generate hot electrons for programming. The flash ZEEPROM cells are fabricated using CMOS compatible process technology, with the addition of a heavily doped boron implant for the P+ region replacing the LDD region. A sidewall spacer is necessary to form the self-aligned N+ source and drain regions and to avoid counter-doping of the P+ pocket.
To program the flash ZEEPROM cell, the P+ N+ junction is reverse-biased to create an electric field of approximately 10
6
volt/cm. and generate energetic hot electrons independent of the channel length. The P+ region adjacent to the drain enhances this generation. A low junction breakdown current can be used for programming by optimizing the P+ N+ junction depth and controlling the applied drain voltage. One disadvantage of this cell is that a low drain voltage (approximately one volt) must be used to read the cell since the P+ region exhibits a low breakdown voltage which can contribute to “soft programming.” Another disadvantage is that the cell provides lower read current compared with conventional flash memory cells. Erasing in the cell is performed by Fowler-Nordheim tunneling of electrons from the floating gate to the source region using a negative gate voltage and supply voltage connected to the source similar to conventional flash EEPROM cells.
Another alternative cell structure using hot election programming generated by a reverse breakdown condition at the drain is described in the context of a method for bulk charging and discharging of an array of flash EEPROM memory cells in U.S. Pat. No. 5,491,657 issued to Haddad, et al., assigned to the assignee of the present invention. In Haddad, et al., a cell structure similar to that shown in cross-section in
FIG. 1B
of the present application may be used, as well as a substrate-biased p-well in n-well embodiment. In the first embodiment, an N+ source region includes an N+ implant region and an N diffusion region, and the erase operation (removing electrons) is accomplished by applying (−)8.5 volts to the control gate for 100 milliseconds, and (+)5 volts to the source for 100 milliseconds, with the drain being allowed to float. In contrast, programming (adding electrons to the gate) is achieved by applying a negative 8.5 volt to the substrate for 5 microseconds, zero volts to the drain and control gate with the source floating. The bulk charging operation can just as easily be done on the source side rather than the drain side in a case where the cell is provided in a P well by applying −8.5 volts to the P well for 5 microseconds, 0 volts to the source and control gate with the drain being allowed to float.
Yet another structure and method for programming a cell is detailed in co-pending U.S. patent application Ser. No. 08/871,589 inventors Hao Fang, et al., filed Jul. 24, 1998 and assigned to the assignee of the present application.
FIGS. 1A and 1B
of the Fang, et al. application are reproduced herein as
FIGS. 2A and 2B
, and
FIGS. 2A and 2B
of the Fang application are reproduced as
FIGS. 3A and 3B
of the present application. The Fang, et al. application uses the programming method disclosed in Haddad, et al. to form a high density, low program/erase voltage and current, and fast byte programming and bulk erase and fast reading speed non-volatile memory structure specifically designed for programmable logic circuit applications.
In Fang, et al. the non-volatile memory cell
10
in
FIGS. 2A
,
2
B is formed of a P substrate
12
having embedded therein an N+ source region
14
, an N-type diffused drain region
16
, a floating gate
18
capacitively coupled to the P substrate
12
through a tunnel oxide
20
, or other gate dielectric such as nitride oxide; and a control gate
22
coupled capacitively to the floating gate
18
through an oxide
itride/oxide, or other type of inter polysilicon dielectric, film
24
,
26
. Diffused region
16
is formed of a shallowly diffused but heavily doped N-type junction, while source region
14
is formed of a deeply diffused but lightly doped N junction. The relatively thin gate dielectric
20
(an oxide of 60 to 150 Å in thickness) is interposed between top surface of substrate
12
and conductor polysilicon floating gate
18
. Control gate
22
is supported above the floating gate by the inter-poly dielectric layer
24
,
26
. Avalanche program and erase bias configurations of the memory cell of the Fang, et al. application are shown in
FIGS. 3A and 3B
, respectively.
Program and erase operations are illustrated in
FIGS. 3A and 3B
. To program the cell, electron injection is effected from the drain side. In this case, programming operation is accomplished by applying +3 volts on the drain and −6 volts on the P substrate so as to shift upwardly the threshold voltage V
t
by 4 volts in approximately 0.002 seconds. To erase, holes are injected from the drain side by applying +6.5 volts on the drain and −3 volts on the P substrate so as to shift down with the voltage threshold V
t
by 4 volts. Utilizing the substrate bias configuration suppresses hot hole injection due to the fact that the location of the high field is away from the oxide interface, the magnitude of the maximum field strength is re

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