Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
1999-12-17
2003-05-27
Kunemund, Robert (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C438S710000, C438S711000, C438S713000, C438S723000, C438S724000
Reexamination Certificate
active
06569773
ABSTRACT:
PRIORITY CLAIM
This application is based on and claims the priorities under 35 U.S.C. §119 of German Patent Application 198 60 271.5, filed on Dec. 24, 1998, and German Patent Application 199 55 145.6, filed on Nov. 17, 1999, the entire disclosures of which are incorporated herein by reference.
FIELD OF THE INVENTION
The invention relates to a method for carrying out anisotropic plasma-chemical dry etching of silicon nitride layers using an etching gas mixture particularly containing SF
6
and CHF
3
.
BACKGROUND INFORMATION
Various techniques of plasma-chemical dry etching, such as reactive ion etching (RIE) for example, are well known and are typically used in the fabrication of semiconductor circuit elements. An advantage of such dry etching techniques in comparison to wet etching techniques is that structures having dimensions of less than 1 &mgr;m can be produced using dry etching but not by using wet etching. This is necessary, for example, for fabricating integrated circuit elements with SiGe transistors.
U. S. Pat. No. 5,433,823 (Cain) discloses a method of carrying out so-called pad window etching using a gas mixture of SF
6
and CHF
3
, among other gas mixtures. Especially, the mole ratio of CHF
3
relative to SF
6
is in the range from about 5/1 to about 20/1. The gas flow rates used in the process are a CHF
3
flow rate of 180 sccm (standard cubic centimeters per minute) and an SF
6
gas flow rate of 20 sccm. The disclosed conventional method is used for etching a passivating semiconductor layer sequence or stack including a layer of Si
3
N
4
having a thickness of approximately 1 &mgr;m and an underlying layer of SiO
2
having a thickness of approximately 0.5 &mgr;m. Both the Si
3
N
4
and the SiO
2
are etched relative to a metal layer of TiW arranged lying under the two semiconductor layers, so as to entirely remove both the Si
3
N
4
and the SiO
2
at a specified window location. The structural dimensions to be etched by the conventional method are in a range around 100 &mgr;m.
Table 3 in column 9 of U.S. Pat. No. 5,433,823 shows the etching selectivities of the etching of the silicon nitride layer relative to the resist material, and of the silicon oxide layer relative to the resist material, when using a conventional SF
6
etching gas in one case (the left data column of Table 3) and when using the disclosed mixture of SF
6
and CHF
3
etching gas in another case (the right data column of Table 3). While the patent points out that the etching selectivity of the silicon nitride relative to the resist and of the silicon oxide relative to the resist, respectively, is improved by using the gas mixture of SF
6
and CHF
3
, a significant disadvantage is also apparent from the data shown in Table 3.
Namely, while the SF
6
etching gas achieves a selectivity of 1.6 when considering the silicon nitride relative to the silicon oxide (1.6/1.0=1.6), the disclosed mixture of SF
6
and CHF
3
achieves a selectivity of only 0.83 for etching silicon nitride relative to silicon oxide (3.3/4.0=0.825). Thus, U.S. Pat. No. 5,433,823 suggests that the addition of CHF
3
to SF
6
in an etching gas mixture reduces the etching selectivity of silicon nitride relative to silicon oxide from 1.6 to 0.83. While that may not be significant in the context of the patent, wherein a silicon nitride layer and a silicon oxide layer are both to be etched selectively relative to a resist layer and a metal underlayer, it is a disadvantage in any application in which a silicon nitride layer is to be etched selectively relative to a silicon oxide layer.
In various conventionally known etching methods, CF
4
, CHF
3
or other fluorine-containing gases or gas mixtures are typically used in combination with O
2
as disclosed in International Patent Publication WO 96/16437. Published European Patent Application EP 0,706,206 similarly discloses an etching process in which a mixture of CF
4
+O
2
is utilized. In all dry etching processes using O
2
as a component of the etching gas mixture, there arises the substantial disadvantage that such processes, i.e. such gas mixtures, cannot be used in the reactor chambers of dry etching systems that use oxidizing or oxidizable electrode materials such as silicon or carbon, because such electrode materials would be attacked and corroded due to the effects of the oxygen component of the gas.
Another type of process is disclosed in German Patent 37 14 144, for example. This German Patent suggests to use a fluorine-containing gas together with chlorine or bromine as a gas mixture. Since these gases, or gas components, are corrosive and toxic, they are not suitable for use in all reactors.
German Patent Laying-Open Document 42 32 475 discloses a plasma-chemical dry etching process for selectively etching silicon nitride layers relative to silicon oxide layers. The etching gas or etching gases used in the disclosed process contain compounds in which respectively one fluorine atom and at least one atom selected from the group consisting of chlorine, bromine and iodine are chemically bonded onto a hydrocarbon framework in the molecular structure. Due to the corrosiveness and toxicity of the gas or its components, such an etching gas is also not suitable or acceptable in all applications.
International Patent Publication WO 96/16433 discloses an anisotropic and selective dry etching process for silicon nitride over thin silicon oxide layers, in which only Cl
2
is used as the etching gas. This leads to very low etching rates, which are not practically applicable for etching relatively thick layers, or any layer other than the very thinnest layers.
Published European Patent Application EP 0,516,053 discloses a process in which a mixture of S
2
F
2
, SF
2
, SF
4
or S
2
F
10
with an inert gas is used as the etching gas, especially in order to etch SiO
2
selectively relative to Si
3
N
4
. The reference discloses that the sulfur that is freed or released during the etching process is redeposited as a passivating layer on the Si
3
N
4
surfaces and thereby hinders the etching of the Si
3
N
4
. Through this mechanism, the SiO
2
may be selectively and preferentially etched in comparison to the Si
3
N
4
.
In the context of the above discussed conventional etching processes, and in addition to the above mentioned disadvantages of the prior art when using oxygen and halides especially containing Cl
2
and Br, studies and experiments conducted by the present inventors have shown that all previously known processes are unable to achieve an adequate or satisfactory control of the edge slope angle of the etched edge of the silicon nitride material. Namely, the resulting slope angle of an edge of an opening or etched-away area of the silicon nitride layer, which is exposed by a corresponding opening in a resist layer or mask, is variable and not readily controllable using the prior art processes.
With the exception of the above mentioned pad window etching process disclosed in U.S. Pat. No. 5,433,823, the other prior art processes described in the literature are used for etching Si
3
N
4
layers having a thickness of less than about 150 nm. With such thin layers, it is not necessary or critical to achieve a high degree of control of the edge slope angle. If the layer thickness is greater than 150 nm, however, then the resulting edge slope angle does become important. Thus, if the layer thickness of the Si
3
N
4
material to be etched is greater than 150 nm, or if the edge slope angle serves an important function for the semiconductor component being fabricated, then the prior art processes are not adequate for achieving the required degree of control of the etched edge slope angle.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the invention to provide a method of the above described general type for the plasma-chemical dry etching of silicon nitride layers selectively relative to silicon oxide layers, wherein the edge slope angle of an etched edge of a silicon nitride layer can be precisely set to a required value, while achieving a high etch rat
Gellrich Norbert
Kirchmann Rainer
Brown Charlotte A.
Fasse W. F.
Fasse W. G.
Kunemund Robert
TEMIC Semiconductor GmbH
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