Selective nitride: oxide anisotropic etch process

Details Selective nitride: oxide anisotropic etch process Selective nitride: oxide anisotropic etch process

1998-01-28

2003-12-02

Sherrer, Curtis E. (Department: 1761)

Etching a substrate: processes

Gas phase etching of substrate

Application of energy to the gaseous etchant or to the...

C216S079000

Reexamination Certificate

active

06656375

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to anisotropic etching of nitride layers. More particularly, the invention relates to an anisotropic etching process in which the etching gas uses three components, namely, CH
3
F, CO
2
and O
2
.
BACKGROUND OF THE INVENTION
Conventional dry etching processes for etching silicon nitride (Si
3
N
4
) films formed on silicon oxide (SiO
2
) films use a variety of etchant gases. The choice of etchant gas is somewhat limited, however, due to the miniturization of semiconductor devices which contain extremely thin nitride layers. Because these nitride layers have thicknesses on the order of 10
2
-10
3
Å, selectivity and control of etch rates are of great importance to maintaining control of etching processes.
In microelectronic fabrication processes, etching is the selective removal of sections or regions of material from either a silicon substrate or from other thin films on the substrate surface. Etching proceeds in all directions at the same rate in an isotropic etching process. Typically, mask layers are used to protect those regions of material which the fabricator wishes to maintain on the substrate or thin film. Generally, however, mask layers cannot exactly form the pattern desired. This inability results because, as the etching proceeds, some removal occurs in undesired places.
FIG. 1
shows such an occurrence.
Referring now to the drawing, wherein like reference numerals refer to like elements throughout,
FIG. 1
illustrates the results of an isotropic etching process in which some material has been removed in directions other than just the z-direction—despite the presence of a mask layer
100
. Mask layer
100
was placed upon an insulator film layer
110
which is on top of substrate
120
. During the etching process, however, mask layer
100
did not prevent some of insulator film layer
110
from being removed in areas
130
other than directly below the mask layer
100
. Similarly, some parts of insulator film layer
110
remained in areas where etching was supposed to remove it.
FIG. 2
illustrates an anisotropic etching process, in which the insulator film layer
110
has been removed in all but exactly where mask layer
100
was placed. Thus, both the mask layer
100
and the insulator film layer
110
have exactly the same dimensions in the x-y plane. In such a case, where etching occurs in only one direction (here, only in the z-direction), the etching process is said to be completely anisotropic. Because the completely anisotropic process as depicted in
FIG. 2
is only an ideal state (completely single dimensional etching is not achievable), anisotropic processes are those which achieve nearly “completely anisotropic” results.
Current typical anisotropic processing techniques for thin nitride layers (e.g., spacer or liner levels of 100-1,000 Å) include the use of chlorinated-type gases (such as HBr/Cl
2
) to provide selectivity to underlying thin oxide layers. Although very high etch rates and high selectivity may be achieved, these processes offer no selectivity to underlying silicon layers (especially doped silicon layers, such as those used in Direct Random Access Memory (DRAM) processing). In particular, in many applications, such silicon or doped silicon underlying layers are rapidly eroded by chlorine in the event of oxide punch-through.
In an attempt to overcome the above problems, technologies requiring a very thin gate oxide (less than 100 Å) have used a fluorine-based chemistry (e.g., CHF
3
and CO
2
) to provide some measure of selectivity to silicon. Fluorine-based processes offer much improved selectivity to underlying silicon. Unfortunately, however, fluorine-based chemistries offer much lower nitride:oxide selectivity, typically 3:1 or less compared with 8:1 or better for the chlorine process. Fluorine-based chemistries are also generally associated with low etch rates.
Because of low uniformity of these slow, selective processes, punch-through of the oxide is a serious concern. An alternative process, using a hydrofluorocarbon (e.g., CH
3
F and O
2
) having a low fluorine:carbon (F:C) ratio, has been used to enhance the selectivity of the fluorinated processes. Although more selective (approximately 6:1 ratio of nitride:oxide) than the fluorinated process, the etch rate of this process is much lower, at approximately 180 Åmin
−1
. Because of its low etch rate, the time to conduct the process will be substantially increased from the chlorinated process (by a factor of about 3).
Therefore, there remains a need for a process which accelerates the etch rate, improves the selectivity of current fluorine-based processes, and offers sufficient tunability/control for optimization for both thick and thin nitride layers. Accordingly, one object of the present invention is to provide a highly selective nitride:oxide anisotropic etch process for etching the nitride layer on top of an oxide layer.
SUMMARY OF THE INVENTION
To meet this and other needs, and in view of its purposes, the present invention provides a highly selective etching process for a nitride:oxide combination upon a substrate. The process comprises the combined use of a hydrogen-rich fluorohydrocarbon (e.g., CH
3
F or CH
2
F
2
), a strong oxidant (O
2
), and a carbon source (CO
2
or CO). It is preferred that the following amounts of each source be used for optimal performance: 7%-35% CH
3
F; 1%-35% O
2
; and 30%-92% CO
2
by volume.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.


REFERENCES:
patent: 4529476 (1985-07-01), Kawamoto et al.
patent: 4654114 (1987-03-01), Kadomura
patent: 4920065 (1990-04-01), Chin et al.
patent: 5399237 (1995-03-01), Keswick et al.
patent: 60-115232 (1985-06-01), None

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