Method of local oxidation using etchant and oxidizer

Details Method of local oxidation using etchant and oxidizer Method of local oxidation using etchant and oxidizer

1998-10-14

2001-01-02

Fahmy, Wael (Department: 2823)

Semiconductor device manufacturing: process

Chemical etching

Combined with coating step

C438S694000, C438S706000, C438S715000, C438S770000, C438S774000, C438S443000

Utility Patent

active

06169035

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for fabricating a semiconductor device, and more particularly to a local oxidation (LOCOS) method of forming a field oxide (FOX) structure.
2. Description of Related Art
The purpose of an isolation region in an IC device is to prevent a carrier from drifting between two adjacent device elements, such as two adjacent transistors, through a semiconductor substrate to cause leakage. For example, carriers can drift between two adjacent transistors through a common substrate if there is no isolation region between the transistors. Conventionally, isolation regions are formed between field effect transistors (FETs) in an IC device, such as a dynamic random access memory (DRAM) device, to prevent current leakage from occurring. A local oxidation (LOCOS) process is a typical isolation process widely used to form the isolation region. LOCOS technology has been well developed and it can effectively isolate the transistors or other kinds of device elements with good and reliable performance and a low fabrication cost.
FIGS.
1
A-
1
E are cross-sectional views schematically illustrating a conventional fabrication flow of a LOCOS process to form a FOX layer. In
FIG. 1A
, a pad oxide layer
102
is formed over a semiconductor substrate
100
by, for example, thermal oxidation. The substrate
100
typically includes silicon. A mask layer
104
including, for example, silicon nitride is formed over the pad oxide layer
102
by, for example, lowpressure chemical vapor deposition (LPCVD).
In
FIG. 1B
, the mask layer
104
and the pad oxide layer
102
are patterned by, for example, photolithography and etching to form an opening
101
in order to expose the substrate
100
. The region other than the opening
101
is an active region. The purpose of the opening
101
is to form an isolating structure therein.
In
FIG. 1C
, an wet oxidation process with water vapor is performed to oxidize the exposed semiconductor substrate
100
within the opening
101
so that a field oxide (FOX) layer
106
is formed in the opening
101
. Since the mask layer
104
is made of silicon nitride, it effectively resists H
2
O or O
2
molecules in a high temperature environment. A portion of the substrate surface covered by the mask layer
104
is not oxidized but a bird's beak structure
108
forms at the edge of the opening
101
. A silicon nitride spot
110
due to a reaction on the silicon substrate
100
may form around the bird's beak structure
108
. Details of how the spot is formed are described later. In
FIG. 1D
, the mask layer
104
is removed by, for example, heated phosphoric acid or fluorine ion plasma. In
FIG. 1E
, the pad oxide layer
102
is removed by, for example, wet etching with HF acid solution.
However, the conventional LOCOS process still has some problems. In the above description, the formation of the bird's beak structure
108
shown in
FIG. 1C
forms because H
2
O and O
2
molecules can horizontally diffuse at the edges of the opening
101
. The mask layer
104
cannot resist horizontal diffusion. An oxidation along a horizontal inner forward direction continues to occur at the edge even though the oxidation phenomenon gradually decreases. This is why a bird's beak structure is formed. The bird's beak structure induces a tensile stress on the substrate
100
at its contact portion, where dislocation of silicon crystal cells occurs to cause a leakage current. Moreover, the bird's beak structure
108
also consumes the effective active region. The integration of an integrated circuit device is thereby limited.
Furthermore, during thermal oxidation to form the bird's beak structure, H
2
O molecules can react with the silicon nitride to form NH
3
molecules. If the bird's beak structure extends farther, NH
3
molecules have a higher probability of diffusing along the edge of the bird's beak structure and even may diffuse through the oxide layer
102
to react with the silicon substrate
100
. This results in formation of the localized silicon nitride spot
110
on the surface of the substrate
100
. Since silicon nitride is usually used as a mask for growing oxide, the silicon nitride spot
110
behaves as a micro-oxidation masking layer. When a gate oxide layer (not shown) is subsequently formed over the active region, the gate oxide layer below or around the micro-oxidation masking layer becomes thinner. This thinner portion of the gate oxide layer appears as a white ribbon structure under a photo-microscope, so that this phenomenon is called the white ribbon phenomenon. The white ribbon phenomenon reduces the charge trapping capability of the gate oxide layer and the oxide breakdown phenomenon therefore occurs earlier. This seriously undermines the long-term reliability of the gate oxide layer.
SUMMARY OF THE INVENTION
It is therefore an objective of the present invention to provide a local oxidation (LOCOS) method, which can reduce the occurrence of the bird's beak structure on the field oxide (FOX) layer. This allows the effective active region to be increased and the leakage current is effectively avoided.
It is another an objective of the present invention to provide a local oxidation (LOCOS) method, which can reduce the extension of the bird's beak structure so as to reduce the white ribbon phenomenon, thereby ensuring the long-term reliability of the subsequently formed gate oxide layer.
It is still another an objective of the present invention to provide a local oxidation (LOCOS) method, which can simultaneously form a FOX layer and remove a mask layer so that the LOCOS method reduces the procedure by one step.
In accordance with the foregoing and other objectives of the present invention, a LOCOS method uses a reagent mixed of etchant and oxidizer to simultaneously perform the step of forming the FOX layer and the step of removing mask layer of the conventional LOCOS method. The applied temperature is about 950-1150° C. In the LOCOS method of the invention. the length of the bird's beak structure is reduced to avoid all related problems such as the occurrence of the white ribbon phenomenon. The bird's beak structure may even be completely eliminated. The etchant is used to remove the mask layer, and the oxidizer is used to form the FOX layer on a silicon substrate.


REFERENCES:
patent: 4595601 (1986-06-01), Horioka et al.
patent: 5312518 (1994-05-01), Kadomura
patent: 5641383 (1997-06-01), Jun

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