Semiconductor device manufacturing: process – Coating of substrate containing semiconductor region or of... – By reaction with substrate
Reexamination Certificate
2001-05-21
2002-12-17
Eckert, II, George C. (Department: 2814)
Semiconductor device manufacturing: process
Coating of substrate containing semiconductor region or of...
By reaction with substrate
C438S791000
Reexamination Certificate
active
06495477
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for forming a nitridized interface on a substrate, and more particularly to improving uniformity of an ammonia plasma surface treatment on a semiconductor substrate. Specifically, this invention relates to forming a nitridized polysilicon interface using ammonia plasma doped with fluorine.
2. Description of Related Art
Silicon nitride films are commonly employed in the fabrication of circuits for use in modern semiconductor devices, such as the fabrication of metal-oxide semiconductor (“MOS”) devices for high density integrated circuits and submicron designs. For example, silicon nitride films are employed in the manufacture of MOS devices using Local Oxidation of Silicon processes (“LOCOS”), as well as advanced LOCOS methods such as polysilicon buffered LOCOS processes (“PBLOCOS”). Further information on LOCOS-based processing technology may be found in Wolf.
In a LOCOS manufacturing method, a pad of native oxide is typically formed on a semiconductor substrate for purposes of cushioning transition of stress between a silicon substrate and a silicon nitride film, which is deposited to serve as an oxidation mask. Such pad oxides may be thermally-grown or deposited using chemical vapor deposition (“CVD”). In a PBLOCOS method, a thin pad layer of thermally deposited silicon dioxide (SiO
2
) in combination with a polysilicon buffer layer is formed. PBLOCOS methods are typically utilized to enhance suppression of lateral oxidation and to provide a stress buffer layer between the oxide layer and a subsequently deposited silicon nitride layer.
In a typical LOCOS-based process, silicon nitride and polysilicon buffer layers (when present) are removed selectively to expose those areas where field oxide growth is desired, leaving the active areas of the device covered. Field oxide is then grown “locally” in the etched areas between the active areas covered by silicon nitride film to isolate them from each other. It is desirable to minimize the space required for these isolation zones, as they consume valuable semiconductor space.
It is at the time of field oxide growth that encroachment of oxide into the silicon substrate and interface area between the surrounding silicon nitride and polysilicon buffer layer (if present) typically occurs. This oxide encroachment is commonly referred to as a “first bird's beak” when it extends into the substrate, and as a “second bird's beak” when it occurs in the interface of the silicon nitride and polysilicon. Bird's beak formation can change the active area size and potentially cause gate poly bridging. First bird's beak areas consume additional space as they extend beyond the edges of the isolation zones, and work against the achievement of isolation requirements for submicron devices. Second bird's beak areas tend to interfere with removal of polysilicon in a LOCOS-based process, leaving undesirable “stringers” of poly which may cause shorts and/or leakage. Following field oxide growth, the remainder of the silicon nitride layer and any buffer layer present underneath is typically removed in order that the active areas of the semiconductor device may be formed.
Substrate surfaces are typically nitridized using a CVD process such as ammonia-plasma CVD. During surface nitridization in a plasma enhanced CVD reactor, flow of reactant gasses, as well as reacted byproducts, typically results in nonuniform film thickness. Nonuniformity of treatment may be gauged or measured in terms of both film within wafer thickness nonuniformity, and in wafer to wafer nonuniformity. Film within wafer thickness nonuniformity may be expresses in terms of percentage standard deviation. Wafer to wafer nonuniformity typically averages from about 6 Å to about 10 Å.
Increased film nonuniformity typically results in increased second bird's beak formation. Such variations in film treatment tend to decrease wafer yield by increasing the second bird's beak size, leading to poly stringer formation and reduced process capability as measured on a test wafer run with the product. Process capability is defined as the process spec width divided by (6&egr;), and is typically expressed as C
p
or C
pk
.
As an example of problems encountered with conventional LOCOS-based processes,
FIG. 1
illustrates field oxide
18
formed on a semiconductor substrate
10
in isolation area
11
, using a conventional LOCOS-based process known in the art. As shown in
FIG. 1
, isolation area
11
is defined between active device areas
13
. Active device areas
13
are covered by silicon dioxide pad layer
12
, polysilicon buffer layer
14
and silicon nitride layer
16
. As may be seen in
FIG. 1
, field oxide
18
extends into active areas
13
due to encroachment of oxide
18
into the substrate
10
, forming “first bird's beak” areas
17
, and between silicon nitride layer
16
and polysilicon buffer layer
14
, forming “second bird's beak” areas
19
. Upon subsequent removal of layers
16
,
14
and
12
, residual areas of polysilicon buffer layer
14
may remain between the “bird's beak” areas
17
and
19
, which tends to act as a mask against removal of polysilicon. These residual polysilicon areas are referred to as “poly stringers” and are undesirable due to their tendency to cause shorts and leakage.
SUMMARY OF THE INVENTION
Using the disclosed method, a fluorine doped nitride surface treatment may be employed to form a fluorine-doped nitridized substrate interface. Benefits of the disclosed method include, but are not limited to, reduction in nonuniformity of nitridized substrate surfaces and wafer to wafer nonuniformity, as well as the provision of a more stable process. By increasing uniformity of substrate nitridization, wafer yield may be increased over conventional undoped ammonia plasma treatment processes by, for example, substantially eliminating across-wafer nonuniformity and the presence of a “second bird's beak” in the field oxide edge areas formed in a LOCOS-based process. Advantageously, in one embodiment of the disclosed method, process capability (C
p
, C
pk
) is increased because of this reduction in thickness variation, and/or formation of poly stringers is suppressed or substantially prevented.
While not wishing to be bound by theory, it is believed that lateral oxidation of the poly
itride interface during field oxidation is prevented by tying up available Si atoms at the surface of the poly. This is believed to result due to the breaking of stressed Si—O—Si bonds at the polysilicon surface by HF formation and surface reaction, direct F reaction, and/or bombardment. When this occurs, Si—O—Si bonds are believed to be replaced by Si—F, non-bridging Si—O, and/or Si—N bonds. Formation of Si—N bonds is believed to be the predominate reaction, and it is believed that these bonds create a more uniform interface between a polysilicon layer and a subsequent silicon nitride layer. Furthermore, fluorine is also believed to break Si—H bonds and Si—OH bonds which may form in NH
3
plasma, thus tending to form stronger Si—F bonds. It is believed that one or more of the previously described mechanisms retard lateral diffusion and reaction of oxygen during field oxide growth, thus yielding better uniformity.
In one embodiment of the disclosed method, a source of fluorine, typically carbon hexafluorine, C
2
F
6
(Halocarbon-116), is introduced into an ammonia plasma to nitridize a substrate surface (such as oxide or polysilicon) in a LOCOS-based process, for example, a LOCOS or PBLOCOS isolation scheme. LOCOS-based processes are known in the art and are described, for example, in Wolf, Stanley
Silicon Processing for the VLSI Era, Volume
2
—Process Integration
, Lattice Press, Sunset Beach, Calif., pp. 12-41, 1990, which is incorporated herein by reference.
In one embodiment, C
2
F
6
gas (typically employed as an etchant) is introduced into an ammonia plasma during nitride surface treatment of polysilicon. In this embodiment, additi
Jendresky David F.
Taylor Jonathan J.
Akin Gump Strauss Hauer & Feld & LLP
Eckert II George C.
Peralta Ginette
Samsung Austin Semiconductor, LLC
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