Anisotropic, fluorine-based plasma etching method for silicon

Details Anisotropic, fluorine-based plasma etching method for silicon Anisotropic, fluorine-based plasma etching method for silicon

1999-04-29

2001-10-16

Kunemund, Robert (Department: 1765)

Semiconductor device manufacturing: process

Chemical etching

Vapor phase etching

C438S714000, C438S723000, C438S724000, C438S725000, C438S733000

Reexamination Certificate

active

06303512

ABSTRACT:

FIELD OF THE INVENTION
The invention proceeds from a method for anisotropic plasma etching of laterally defined patterns in a silicon substrate.
BACKGROUND OF THE INVENTION
The configuration of patterns, for example recesses, in a silicon substrate using the plasma etching method is known. It is also known, for example for applications in micromechanics, to use fluorine compounds for anisotropic plasma etching. The fluorine radicals generated in the plasma act isotropically be with respect to silicon, however; i.e. the lateral etching rate corresponds substantially to the vertical one, which leads to correspondingly large mask undercuts and rounded profile shapes. In order to achieve a vertical sidewall with an etching method that uses fluorine compounds, precautions must additionally be taken in order to protect the sidewall selectively from etching attack, and to confine the etching to the pattern front, i.e. the bottom of the recess. Discrimination between the sidewall of the recess and the etching front is attained via a highly oriented vertical incidence of energetic ions, which are produced simultaneously in the plasma alongside the chemically active neutral radicals. The ions strike the surface of the substrate, the etching front being bombarded strongly by ions but the sidewalls of the recess, on the other hand, only relatively weakly. It is known to use polymer-forming gases such as CHF
3
, which are mixed directly with the fluorine-supplying etching gas, as the protective mechanism for the sidewalls. A polymer layer is deposited onto the sidewall from the polymer-forming monomers present in the plasma, while the fluorine radicals produced in the plasma at the same time etch the silicon substrate on the etching front, which is polymer-free because of the incidence of ions. It is disadvantageous that intensive recombination occurs, in the plasma and on the way to the substrate being etched, between unsaturated polymer-forming monomers and the fluorine radicals. To overcome this disadvantage, it is known to prevent the disruptive recombination of unsaturated polymer-forming monomers and the fluorine radicals capable of silicon etching by separating the plasma etching process into etching steps in which exclusively fluorine-supplying gases are used, and deposition steps in which exclusively deposition gases, such as the polymer-forming gases, are used. Because they are used in the plasma on a temporally separated basis, the two varieties of gas used do not encounter one another, so that appreciable recombination also cannot occur.
It is also known to passivate the sidewalls by using in the plasma, alongside the etching fluorine radicals, oxygen radicals or nitrogen radicals which convert the silicon of the sidewall at the surface into silicon oxide or silicon nitride, respectively. Since the dielectric surface is etched particularly strongly by the fluorine radicals with ion assistance and less strongly without ion assistance, etching proceeds essentially on the etching front, while the sidewall remains relatively protected. One substantial disadvantage of this method is that the silicon oxide or nitride layers generated at the surface are of only atomic thickness, i.e. are on the order of 1 nm thick or less. The silicon oxide or nitride layers generated at the surface therefore do not seal very well, and offer only incomplete protection. The result is that process control becomes more difficult, and that the process result is greatly influenced by secondary effects. The profile shapes of the patterns to be formed are never completely vertical, since sidewall attacks and thus also mask edge undercuts always occur. Cryogenic methods are used in order to increase the effectiveness of this passivation: cooling the silicon substrate to temperatures as low as −100° C., in addition to oxygen or nitrogen passivation, “freezes out” the sidewall reaction. This method is described in U.S. Pat. No. 4,943,344. Disadvantages include the highly complex equipment and the costs associated therewith, as well as the comparatively poor reliability of the components.
SUMMARY OF THE INVENTION
The present invention concerns a method for anisotropic plasma etching of laterally defined patterns in a silicon substrate, protective layers made of at least one silicon compound being deposited before and/or during plasma etching onto the sidewalls of the laterally defined patterns. In a particularly advantageous exemplary embodiment, the present invention provides for depositing silicon oxide layers and/or silicon nitride layers onto the sidewalls of the laterally defined structures, in particular beams, trenches, combs, or tongues. The patterns are preferably defined with the aid of an etching mask. The procedure according to the present invention leads, advantageously, to a thick (i.e. from several nm to several tens of nm thick) silicon oxide or silicon nitride layer on the sidewalls of the pattern. Even at room temperature, this protective layer withstands the etching attack of the radicals formed in the plasma, in particular the fluorine radicals that are preferably used, and thus makes possible a particularly reliable etching operation which is resistant to malfunction. Advantageously, the method can also be carried out at lower substrate temperatures; a specific parameter range of the gas composition leading to the production of vertical etching profiles is to be used at each substrate temperature.
The present invention advantageously provides for the etching gas which releases fluorine radicals in the plasma to be sulfur hexafluoride SF
6
or nitrogen trifluoride NF
3
, optionally as a mixture together with argon. In order to make available the components which form the protective layer, oxide formers and/or nitride formers as well as a secondary reactant are added to the etching gas which supplies the fluorine radicals. Oxygen O
2
, dinitrogen oxide N
2
O, a different nitrogen oxide NO, NO
x
, carbon dioxide CO
2
, or nitrogen N
2
are added as oxide formers or nitride formers. The invention advantageously provides, when NF
3
is used as the etching gas, for no separate nitride formers to be used, since the nitrogen released during dissociation of the etching gas NF
3
serves for nitrification. Advantageously, silicon tetrafluoride SiF
4
is used as the secondary reactant, i.e. as the compound which supplies the silicon component of the protective layer. According to the present invention, the reaction product SiO
2
, Si
x
N
y
, or a mixture made up of Si
x
O
y
N
z
, is deposited onto the sidewall of the pattern from the secondary reactant, i.e. preferably SiF
4
, and the reaction partner (oxygen or nitrogen) deriving from the oxide former or nitride former. The secondary reactant SiF
4
does not react with the fluorine radicals deriving from SF
6
dissociation, but only with oxygen or nitrogen, fluorine radicals in fact additionally being released (SiF
4
+O
2
<==>SiO
2
+4F*; SiF
4
+xO*<==>SiO
x
F
4−
+xF*).
It is also possible to separate the plasma etching according to the present invention into etching and deposition steps that are separated from one another, only etching being conducted during the etching step, and the deposition of the silicon compound described above being performed during the deposition step. In particularly preferred fashion, the etching steps are performed alternately with the deposition steps.
The affinity of the oxide formers or nitride formers preferred according to the present invention for the secondary reactant, i.e. preferably SiF
4
, is low enough that in the gas phase, in particular under the low process pressures that are preferred according to the present invention and the process conditions provided according to the present invention, especially the excess of free fluorine radicals in the plasma, no appreciable reaction takes place between the oxide formers or nitride formers and the secondary reactants, even with high-density plasma excitation. The advantageous result is to prevent

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