Adhesive bonding and miscellaneous chemical manufacture – Differential fluid etching apparatus – With radio frequency antenna or inductive coil gas...
Reexamination Certificate
2002-02-12
2003-12-09
Alejandro, Luz L. (Department: 1763)
Adhesive bonding and miscellaneous chemical manufacture
Differential fluid etching apparatus
With radio frequency antenna or inductive coil gas...
C118S7230IR, C118S7230AN
Reexamination Certificate
active
06660127
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to a method for providing a stable plasma useful in the etching of films. The method is particularly useful in the etching of silicon-containing films which differ in overall chemical composition, when it is desired to simultaneously etch such films at a substantially equivalent etch rate. For example, adjacent areas of a semiconductor substrate where polysilicon or silicon is doped and undoped can be etched at substantially the same rate. Further, the method provides critical dimension bias control for the feature being etched. The stable plasma provided by the method makes possible a reliable timed etch end point. And, finally, the method offers the advantage that the process chamber in which the etching is carried out remains particularly clean during the etch process, with minimal etch byproduct deposited upon chamber surfaces.
2. Brief Description of the Background Art
K Suzuki et al., in an article entitled “Power transfer efficiency and mode jump in an inductive RF discharge”, Plasma Sources Sci. Technol. 7 (1998) 13-20, describe a plasma density jump observed in a large-diameter (50 cm) high density (>10
11
cm
−3
) plasma, where the plasma is produced in a few mTorr argon by inductive RF discharge using a conventional external antenna or a plasma-immersed internal antenna. A mechanism for the density jump is explained in terms of the density dependence of the power transfer efficiency. The density jump is said to correspond to the mode jump from the capacitive to the inductive discharges. Representative formulae are given for capacitive power absorption,
Pcap
and inductive power absorption P
ia
.
In recent process development work related to a method for etching silicon-containing layers, a method for obtaining simultaneous cleaning of etch byproducts from the etch chamber wall was discovered. This technology is disclosed in pending U.S. patent application, Ser. No. 08/969,122 of Qian et al., Entitled: “Self-Cleaning Etch Process”, filed Nov. 12, 1997, and is hereby incorporated herein by reference. Subsequently, in a related development, a method was developed which permitted simultaneous etching of silicon-containing layers having different dopant concentrations where the etch rate was substantially uniform across the differing materials. This technology is disclosed in pending U.S. patent application, Ser. No. 09/116,621 of Nallan et al., Entitled: “Process For Etching Silicon-Containing Layers On Semiconductor Substrates”, filed Jul. 15, 1998, and is hereby incorporated herein by reference. This second patent application is a continuation-in-part of the Qian et al. application described above, and both of these applications are assigned to the assignee of the present invention.
These patent applications describe high density plasma etching in a dual power processing apparatus. In particular, in the dual power processing apparatus, the shape of the processing chamber may be specially designed and the power applied to the apparatus for generation of the plasma is separately controlled from the power applied to bias the substrate. This has a substantial effect on the etch results obtained, as is evident in the disclosure of the inventions of Qian et al. and Nallan et al. referenced above.
The Qian et al. and Nallan et al. applications disclose the use of a plasma source gas which includes four main components: a bromine-containing gas including one or more of HBr, Br
2
, and CH
3
Br; a chlorine-containing gas including one or more of Cl
2
, and HCl; an inorganic fluorinated gas including one or more of NF
3
, CF
4
, and SF
6
; and an oxygen-comprising gas. Typically, the oxygen-comprising gas is a mixture of an inert gas with oxygen, such as He—O
2
, where the O
2
ranges from about 20% to about 30% by volume of the gas mixture. The volumetric flow ratio of the various plasma source gas components is selected so that the relative etch rates of the different silicon-containing films vary by less than about 10%.
In one embodiment of the disclosed technology a two step etching process is used. In the first step, the plasma source gas includes all of the four components described above. The CF
4
component both tends to equalize the etch rate among the different silicon-containing films (such as doped and undoped silicon) and tends to assist in removal of etch byproducts from the etch process chamber wall. However, with CF
4
present, the selectivity ratio of polysilicon: silicon oxide is lower. This is important, for example, when a silicon film (doped in some areas) overlies a thin silicon-oxide film (functioning as a gate for a transistor). The etch rate for the silicon oxide in the four-component plasma etchant system is sufficiently rapid relative to the polysilicon etch rate that there is a danger of etching through the thin silicon oxide film before the polysilicon etching process can be terminated. To solve this problem, in a second step (which may be part of a continuous process), the CF
4
is removed from the plasma source gas so that a “soft landing” is achieved when the end point of the polysilicon etch process occurs. By removing the CF
4
and increasing the selectivity (etch rate ratio) of polysilicon to silicon oxide, it is possible to etch up to a thin underlying silicon oxide layer without the danger of etching through.
It was desired to use a timed etch end point for the first etch step described above and a process variable monitored etch end point for the second step. However, there was difficulty in using a timed etch end point for the first step, because the etch rate was not sufficiently predictable.
SUMMARY OF THE INVENTION
Subsequent to development of the technology described above, we discovered that under some process conditions an unstable plasma developed. A brightening of the plasma was observed, with an accompanying change in etch rate. This unstable plasma was observed in some instances and not in others. We investigated this unstable plasma in an attempt to obtain a more predictable etch rate, with the goal in mind of enabling the use of a timed etch end point for the first etch step.
In accordance with the present invention, the etch plasma density is controlled to provide plasma stability. We have discovered that it is possible to operate a stable plasma with a portion of the power deposited to the plasma being a capacitive contribution and a portion of the power deposited being an inductive contribution. In particular, a stable plasma may be obtained within two process regions. In the first region, the gradient of the capacitive power to the power applied to the inductively coupled source for plasma generation [∂P
cap
/∂P
RF
] is greater than 0. In the second region, plasma stability is controlled so that [∂P
cap
/∂P
RF
] is less than 0 and so that P
cap
<<P
RF
. Typically, the magnitude of P
cap
is about 10% or less of the magnitude of P
RF
. In these equations, P
cap
is the calculated capacitive power deposited to the plasma, and P
RF
is the actual power applied. The stability of power deposition to the substrate (workpiece) is achieved by choosing both the RF power level applied to the inductively coupled plasma generation source and the operating pressure in the process chamber to provide a stable plasma. It is preferred to operate under conditions where [∂P
cap
/∂P
RF
] is greater than 0, because the etch results are superior, due to a more favored ion to neutral ratio in the plasma.
We have also discovered that, at a given application of power to the plasma generation source, the stability of the plasma may be extended by increasing the pressure in the etch process chamber. This enables operation of the etch process using lower power application for plasma generation. The stable plasma operating regime is overlaid upon the process which permits etching of different silicon-containing layers at substantially the same etch rate, while reducing the need to clean the pro
Holland John
Lill Thorsten
Nallan Padmapani
Todorov Valentin
Alejandro Luz L.
Applied Materials Inc.
Bach Joseph
Church Shirley L.
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