Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2001-09-05
2002-09-24
Loke, Steven (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S315000, C257S316000, C257S324000, C257S326000
Reexamination Certificate
active
06455890
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a structure for forming a semiconductor device, and more particularly to a structure of fabricating high gate performance for NROM technology.
2. Description of the Prior Art
Semiconductor devices typically include multiple individual components formed on or within a substrate. As illustrated in prior art
FIG. 1A
, a memory device such as a flash memory
100
comprises one or more memory arrays
102
and peripheral regions
104
on a substrate
106
. The memory array
102
typically consists of at least one MXN array of individually addressable, substantially identical floating-gate type memory cells and the peripheral region
104
typically includes input/output (I/O) circuitry and circuitry for selectively addressing the individual cells (such as decoders for connecting the source, gate and drain of selected cells to predetermined voltages or impedances to effect designated operations of the cell such as programming, reading or erasing).
The memory cells within the array
102
are coupled together in an NAND-type circuit configuration, such as, for example, the configuration illustrated in prior art FIG.
1
B. Each memory cell
108
has a drain
108
a
, a source
108
b
and a stacked gate
108
c
. A plurality of memory cells
108
connected together in series with a drain select transistor at one end and a source select transistor at the other end to form a NAND string as illustrated in prior art FIG.
1
B. Each stacked gate
108
c
is coupled to a word line (WL
0
, WL
1
, . . . , WLn) while each drain of the drain select transistors are coupled to a bit line (BL
0
, BL
1
, . . . , BLn). Lastly, each source of the source select transistors are coupled to a common source line Vss. Using peripheral decoder and control circuitry, each memory cell
108
can be addressed for programming, reading or erasing functions.
Prior art
FIG. 1C
represents a fragmentary cross section diagram of a typical memory cell
108
in the array
102
of prior art
FIGS. 1A and 1B
. Such a cell
108
typically includes the source
108
b
, the drain
108
a
and a channel
110
in a substrate or P-well
112
; and the stacked gate structure
108
c
overlying the channel
110
. The stacked gate
108
c
further includes a thin gate dielectric layer
114
a
(commonly referred to as the tunnel oxide) formed on the surface of the P-well
112
. The stacked gate
108
c
also includes a polysilicon floating gate
114
b
which overlies the tunnel oxide
114
a
and an interpoly dielectric layer
114
c
overlies the floating gate
114
b
. The interpoly dielectric layer
114
c
is often a multilayer insulator such as an oxide-nitride-oxide (ONO) layer having two oxide layers sandwiching a nitride layer. Lastly, a polysilicon control gate
114
d
overlies the interpoly dielectric layer
114
c
. The control gates
114
d
of the respective cells
108
that are formed in a lateral row (see, for example, prior art FIG.
1
B). In addition, the drain regions
108
a
of the respective cells in a vertical column are connected together by a conductive bit line (BL). The channel
110
of the cell
108
conducts current between the source
108
b
and the drain
108
a
in accordance with an electric field developed in the channel
110
by the stacked gate structure
108
c.
The process for making such NAND type flash memory devices includes numerous individual processing steps. There are numerous concerns associated with making flash memory devices that provide consistent performance and reliability. For example, the thicker the floating gate, the more likely undesirable cracking occurs in the tungsten silicide layer and the more likely etch problems occur due to high aspect ratios and high topographies. However, the thicker the floating gate, higher the stress released on the tunnel oxide layer, improved tunnel oxide reliability, improved conductivity and better circuit performance result. The thinner the floating gate, the more likely undesirable punch through during etch occurs, especially the Poly 1 contact etch for the select gate, as well as an undesirable increase in pinhole defects. Further, when the thickness of the Poly 1 is too thin, an HF dip cleaning step (prior to forming the ONO multilayer dielectric film) may degrade the Poly 1 and attack the tunnel oxide.
If the doping level is too low in the control gate, wordline resistance and contact resistance become undesirably high, and specifically resistivity for the select gate becomes undesirably high. However, low doping in floating gate results in a smooth tunnel oxide-floating gate interface. Low doping also results in fewer charge gain (loss) problems. If the doping level is too high in the floating gate, undesirable dopant segregation to the tunnel oxide occurs, undermining the tunnel oxide integrity. Undesirably high doping levels lead to severe surface roughness between the floating gate and the tunnel oxide, resulting in high local electric fields, lower oxide dielectric strength, and program/erase endurance cycling problems.
The prior art defect of the poly gate has depletion effect, and less dopant activation, weak reliability interface between polysilicon/oxide or poly-SiGe/SiO
2
, and low gate oxide integrity.
For the forgoing reasons, there is a necessary structure of fabricating high gate performance for NROM technology.
SUMMARY OF THE INVENTION
The present invention is directed toward a structure of fabricating high gate performance for NROM. The key aspect of the present invention is that the Poly-SiGe layer is to replace conventional polysilicon layers for the floating gate and control gate in the array and peripheral regions. The amorphous silicon layer is deposited on the silicon oxide layer, and the Poly-SiGe layer is deposited on the amorphous silicon layer, wherein the amorphous silicon layer is formed with doped germanium in the polysilicon layer which is the Poly-SiGe layer formed ways. After activation annealing (S/D anneal), germanium diffuses into the polysilicon layer and amorphous silicon layer to avoid germanium diffusion into silicon oxide layer. Therefore amorphous silicon layer became polysilicon layer by annealing. This thin amorphous silicon layer improves the gate oxide quality such as breakdown voltage.
In accordance with the present invention, a structure of fabricating high gate performance for NROM technology that substantially can reduce poly depletion effect and equivalent oxide thickness in conventional process.
One object of the present invention is to provide a structure of fabricating high gate performance for NROM technology to improve gate oxide integrity in peripheral regions.
Another object of the present invention is to provide a structure of fabricating high gate performance for NROM technology to reduce poly depletion effect and equivalent oxide thickness.
One another object of the present invention is to provide a structure of fabricating high gate performance for NROM technology to improve active dopant concentration and yield.
In order to achieve the above object, the present invention provides a structure of fabricating high gate performance for NROM technology. The method at least includes the following steps. First of all, an tunnel oxide layer on the silicon substrate. The tunnel oxide layer has a thickness from about 120 angstroms to about 180 angstroms. Then, a amorphous silicon layer on the tunnel oxide layer, and a poly-SiGe layer (a polysilicon layer with doped germanium) on the amorphous silicon layer. The amorphous silicon layer is formed by using a low pressure chemical vapor deposition method. The amorphous silicon layer has a thickness from about 30 angstroms to about 60 angstroms. Next, an interpoly dielectric layer on the poly-SiGe layer. The interpoly dielectric layer comprises a first oxide layer, a nitride layer over the first oxide layer, and a second oxide layer over the nitride layer. Finally, a polysilicon layer on the interpoly dielectric layer.
REFERENCES:
patent: 6153470 (2000-11-01), H
Chang Kent Kuohua
Chiu Jen-Yuan
Hu Shouxiang
Loke Steven
Macronix International Co. Ltd.
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