Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
2000-10-05
2002-11-05
Powell, William A. (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C156S345240, C156S345260, C156S345480, C216S060000, C216S067000, C216S079000, C438S009000, C438S723000, C438S729000, C438S732000, C438S743000
Reexamination Certificate
active
06475918
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a dry etching system and a dry etching method used for fine fabrication of semiconductor devices and, more in particular, it relates to a dry etching system and a dry etching method for attaining highly accurate dry etching fabrication for insulation films.
In a semiconductor device, for electrically connecting transistors formed on a wafer and metal wirings, as well as connection between metal wirings, contact holes are formed to a insulation film formed on a transistor structure and between wirings (thin film mainly composed of SiO
2
, hereinafter referred to as an oxide film) and electric conductors are filled-in the contact holes. In the dry etching, etching gas is introduced into a vacuum chamber, a high frequency bias or &mgr; wave is applied to the gas to generate plasmas, the oxide films are etched selectively by active species and ions formed in the plasmas to form contact holes. Upon etching, a resist thin film transferred with a hole pattern is formed on the oxide film.
In the contact hole fabrication, the oxide film has to be etched selectively to silicon forming a resist film, a wiring layer below the contact hole, or insulation film different from the etched film.
For the fabrication of the contact hole, etching is conducted by introducing fluoro carbon gas such as CF
4
, CHF
3
, C
4
F
8
, C
4
F
6
or C
5
F
8
and Ar gas into an etching device, conducting high frequency wave plasma discharge under a gas pressure condition of from 0.5 Pa to 10 Pa and under the condition of applying a Vpp voltage from 0.5 to 2.0 kV to the wafer. In a case where the thickness of the oxide film between the wiring layers is large and the aspect ratio of the contact hole (depth/diameter) is high, oxygen gas and CO gas are added for improving the hole penetration and addition of the CO gas has an effect of further enhancing the selectivity to the resist and the nitride film
In recent years, Cu is used as a wiring material for higher speed operation of semiconductor devices, and use of organic insulation film and organic silicon oxide film as the insulation film between the wirings has been considered. Nitrogen-containing gas is used for the organic insulation film and gas substantially identical with that for the oxide film is used for an organic silicon oxide film.
A technique of forming contact holes at high aspect ratio to an oxide film using polycrystal Si as a mask is disclosed in Jpn. J. Appl. Phys. Vol. 36 (1997), pp 2470-2476.
However, in a case of fabricating contact holes at high aspect ratio in the existent etching systems, oxygen gas or CO gas has to be added in excess in order to avoid the problem that etching is interrupted failing to form an opening. Since CO constitutes a source for supplying oxygen radicals, etching has been conducted under oxygen radical rich condition. However, as the oxygen radicals become excessive, deposition films are not formed on the lateral side in the upper portion of the hole, and the hole fabricated shape is enlarged by the incidence of ions scattered by a mask. Extension of the hole occurs at a position somewhat deeper than the hole opening (upper portion) Referring more specifically, a hole diameter is enlarged compared with the hole opening and the hole-diameter is smaller as the hole depth is greater. That is, side etching occurs in the midway of the hole in the fabricated shape. Such a phenomenon that the hole diameter is enlarged is referred to as bowing.
When bowing should occur, gaps are formed when electroconductive materials such as polycrystal silicon or tungsten is filled in the hole to cause failure in semiconductor devices. Since the bowing becomes conspicuous as the hole aspect ratio is higher, it results in a bar for the size-reduction of semiconductor devices. Particularly, in the oxide film etching, bowing appears at an aspect ratio of 6 or more, and the bowing increases as the aspect ratio increases. The size of semiconductors has been reduced and fabrication at an aspect ratio of 10 or more is necessary but size reduction becomes difficult in view of trade off with the bowing shape.
The bowing phenomenon also occurs for the materials other than the oxide film and it becomes conspicuous, particularly, in organic insulation films. Depending on the condition, the bowing is conspicuous in the organic insulation films and organic silicon oxide films at an aspect ratio of about 2.5 or more. Since such thin films are used as insulation films between wirings, the aspect ratio is about from 5 to 10 in fine portions.
In order to prevent bowing, depositing gas is sometimes added but this results in a side effect such as lowering of the etching rate.
SUMMARY OF THE INVENTION
A problem to be solved by this invention is to suppress radicals causing bowing during etching, thereby reducing bowing to attain fine fabrication for insulation films.
In the case of etching for oxide films by C
4
F
8
gas and CO gas, CF
2
, F, O and C are formed mainly due to dissociation in plasmas (the term “radical” is sometimes attached at the head of names of atoms and molecules but this indicates same materials). In addition, while CF
3
, CF, C
2
F
4
, C
3
F
7
are also formed, explanations therefor are to be omitted since they have no influence in the aspect of this invention. The sticking coefficient of CF
2
, F, O and C on the lateral surface of a hole is represented as: SC>SF≈S
O
>S
CF2
(S
C
, S
F
, S
O
, S
CF2
represents respectively sticking coefficients for C, F, O and CF
2
). For the sake of convenience, F or O is also represented by the sticking coefficient and the sticking coefficient corresponds to the etching probability for the deposition film.
FIG. 1
is a schematic view for determining the dependence hole aspect ratio of the relative wall deposition in the mixed gas process of Ar, C
4
F
8
, O
2
, CO using the foregoing relation of sticking coefficients. Depending on the relation of the sticking coefficients, C can be a protective film to a mask but, as shown in a curve
102
, the reaching amount into the hole decreases abruptly as the aspect ratio of the hole becomes higher. On the other hand, as shown in a curve
103
, it scarcely decreases even if the aspect ratio of the hole increases. On the contrary, since F and O react with depositing radicals C and CF
2
in the upper portion of the hole, the reaching amount to the vicinity of the bottom is decreased in a case of a hole at a high aspect ratio as shown by a curve
101
.
For fabricating a hole at a high aspect ratio, it is necessary to supply a great amount of C and F radicals in order to suppress interruption of etching caused by excessive CF
2
. Therefore, O and F radicals become excessive in a shallow portion of the hole and side wall deposition film with C and CF
2
is scarcely formed. Therefore, when ions scattered in the upper portion of the hole are entered to the lateral surface of the hole, SiO
2
film is etched (or sputtered) to bring about bowing. Assuming that the deposition film
2
formed of C and CF
2
is etched by F and O, a dimension shift for the fabricated shape occurs as shown by a curve
201
in FIG.
2
. In this case, negative size shift means bowing. Bowing occurs at an aspect ratio of about 3.5.
In a case where holes are etched under the same condition (hereinafter referred to as uncontrolled etching), it is necessary to correspond the amount of F and O radicals to the hole at the highest aspect ratio. However, since the hole aspect ratio increases along with the progress of etching, an aspect ratio is about 3 for the diameter of 0.2 &mgr;m or less at the initial stage of etching even if the resist film thickness is considered. Since the resist film thickness is reduced as the size reduction of the devices progresses, the aspect ratio at the initial stage does not change so much depending on the generation of the semiconductor devices. As has been described above, since the aspect ratio at the initial stage is small, F and O are excessive at the initial stage of the etching in uncontrolled etching
Izawa Masaru
Momonoi Yoshinori
Negishi Nobuyuki
Tachi Shinichi
Yokogawa Ken'etsu
Mattingly Stanger & Malur, P.C.
Powell William A.
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