Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
2002-04-18
2003-05-27
Elms, Richard (Department: 2824)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
Reexamination Certificate
active
06569776
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, and more particularly, to a method of removing a silicon nitride film formed on the bottom of a contact hole and the like in fabrication of a semiconductor device.
2. Description of the Related Art
With the miniaturization in fabrication processes of semiconductor devices, the miniaturization is also under way in contact holes, via holes and the like formed through interlayer insulating films in semiconductor devices. Since interlayer insulating films cannot be reduced in thickness in concert with the progress of miniaturization in design rules, the aspect ratios of holes such as contact holes necessarily become larger. In addition, due to a requirement for reducing variations in alignment of contact holes with underlying wires and the like, associated with the miniaturization in the fabricating process, a self-aligned contact (SAC) process has drawn more and more attention because this process can eliminate a design margin for the alignment on a photomask.
While there are several SAC processes, a typical one involves forming a gate electrode and a gate wire, an offset oxide film disposed on the top faces of them, and side walls (oxide films) disposed on side faces of the gate electrode and gate wire, conformally forming a thin SiN (silicon nitride) film over the entire surface as an etching stopper film, subsequently forming an interlayer insulating film (oxide film), and selectively removing the interlayer insulating film at and near positions at which contact holes are to be formed through a photolithographic step. In this event, since the final positions of the contact holes are determined by the side walls and offset oxide film, this can be said a sort of self-aligned process. Finally, the SiN film is removed from the bottoms of the contact holes which are then filled with contact plugs.
Further, in the SAC process, an attempt has been made to use an SiN film instead of the offset oxide film formed on the gate electrode and gate wire, and to use an SiN film as well for the side walls.
Depending on a process which follows the formation of contact holes, the interlayer insulating film, which is an oxide film, can be damaged. For protecting the interlayer insulating film from such a damage, a thin SiN film may be formed on a side wall (inner wall) of a formed contact hole, for example, in thickness of 10 to 20 nm. In this case, the SiN film on the bottom of the contact hole must be removed after the SiN film is formed on the side wall of the contact hole. Particularly, the formation of such SiN film is deemed as essential when a relatively “soft” oxide such as BPSG (borophosphosilicate glass) is used as the interlayer insulating film.
FIG. 1
is a schematic cross-sectional view illustrating a contact hole before an SiN film is removed after the contact hole was formed. A pair of wiring patterns
12
or electrode patterns made of WSi (tungsten silicide) are formed on substrate
11
made of silicon or the like, and similarly patterned offset SiN films
13
are formed on wiring patterns
12
. Further, side walls
14
similarly made of SiN are provided on side faces of wiring patterns
12
and offset SiN films
13
. Then, interlayer insulating film
15
made of silicon oxide is formed over the entire surface of substrate
11
including wiring patterns
12
, offset SiN films
13
and side walls
14
. Interlayer insulating film
15
is formed with a contact hole
16
by an SAC process. Contact hole
16
extends through interlayer insulating film
15
to the surface of substrate
11
in a region sandwiched by the pair of wiring patterns
12
.
Thin SiN film
17
is formed on the bottom and inner side face (side wall) of contact hole
16
. Here, the bottom of contact hole
16
refers to a portion of the contact hole which is in contact with the surface of substrate
11
. This SiN film
17
is provided as a film for protecting interlayer insulating film
15
from a wet etching and the like in subsequent processes. While SiN film
17
is also formed on the top face of interlayer insulating film
15
depending on its deposition process, the SiN film overlying interlayer insulating film
15
may be removed as required by a subsequent CMP (chemical mechanical polishing) process or the like. It should be noted that contact hole
16
formed by the SAC process is generally set such that its diameter on the top face of interlayer insulating film
15
is larger than that on the bottom of contact hole
16
. Therefore, the diameter of contact hole
16
on the bottom thereof is determined by side walls
14
in a self-aligned manner, and shoulder
18
is formed within contact hole
16
.
When contact hole
16
is used for interlayer connection, SiN film
17
must be removed from the bottom of contact hole
16
, as described above, before contact hole
16
is filled with a wiring material or a wiring plug. In this event, since the SiN film on the side wall of contact hole
16
must be left, anisotropic etching is used. Such anisotropic etching may be dry etching such as plasma etching.
When the SiN film on the bottom of contact hole
16
is removed by plasma etching, a gas system conventionally used for this purpose is a gas system of CHF
3
/Ar/O
2
, a gas system of CH
2
F
2
/Ar/O
2
, and the like. When the former gas system is used, an etching reaction is expressed by:
Si
3
N
4
+4CHF
3
→3SiF
4
↑+4HCN↑
In this example, a product having a relatively high vapor pressure, such as SiF
4
, HCN or the like is formed to promote the etching reaction. Likewise, with the latter gas system a product having a relatively high vapor pressure is produced such as SiF
4
, HCN or the like.
However, the dry etching which uses the aforementioned conventional gas system has a problem in that the SIN film on the side face of the contact hole, as well as the SIN film on the bottom of the contact hole is inevitably etched to some degree. Since etching is less advanced as the aspect ratio is larger, in other words, since an upper portion of a contact hole, i.e., a region near the entrance of the contact hole is etched in advance, the etching rate at the shoulder
18
within the contact hole is higher than the etching rate on the bottom of the contact hole. In some cases, as illustrated in
FIG. 2
, an SiN film on side wall
14
has been etched away before the SIN film on the bottom of contact hole
16
is completely removed, causing wiring patterns
13
made of WSi to expose to contact hole
16
. If contact hole
16
is embedded with a contact plug metal with exposed wiring patterns
13
, the contact plug will be short-circuited with wiring patterns
13
.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method of removing a silicon nitride (SiN) film which is capable of reliably removing an SiN film on the bottom of a contact hole without removing an SiN film formed on a side wall of the contact hole, even if the contact hole has a large aspect ratio.
The inventor diligently repeated investigations for achieving the above object, and as a result found that a silicon nitride film on the bottom of a hole such as a contact hole alone can be selectively removed by using a process gas which comprises a first fluorine compound including a carbon atom-carbon atom bond, and a second fluorine compound including at least one hydrogen atom and a single carbon atom in one molecule, thereby completing the present invention. Assume in the present invention that a double bond C═C and a triple bond C≡C also fall under the carbon atom-carbon atom bond, in addition to the single bond C—C. In the present invention, preferably used as the first fluorine compound may be, by way of example, octafluorocyclobutane (C
4
F
8
), hexafluorobutadiene (C
4
F
6
), octafluorocyclopentene (C
5
F
8
) and the like. On the other hand, preferably used as the second fluorine compound may be, by way of example, monofluoromethane (CH
3
F), difluoromethane
Elms Richard
NEC Electronics Corporation
Owens Beth E.
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