BACKGROUND OF THE INVENTION
Field of the Invention
The invention lies in the processing technology field and relates, more specifically, to a process for the anisotropic dry etching of an organic antireflection layer.
Integrated circuits on semiconductor wafers, primarily made of silicon wafers, are produced with the aid of planar technology. The structuring of the semiconductor layers in order to form the individual components is carried out virtually without exception with the aid of lithographic technology. The desired component structures are first of all generated, via a photomask, in a thin, radiation-sensitive film, most often an organic photoresist layer, on the oxidized semiconductor wafer, and are transferred into the layers lying underneath with the aid of specific etching processes. The irradiation of the photoresist layer is carried out with narrow-band light, high-pressure mercury lamps or lasers primarily being used. During the exposure, pronounced interference effects often occur between the incident light waves and those reflected at the semiconductor wafers, which lead to line width fluctuations in the light on the photoresist layer. These line width fluctuations in turn undesirably reduce or enlarge the structure transferred to the photoresist by using the mask.
In order to reduce the interference effects, antireflection layers are applied between the semiconductor substrate and the photoresist layer, absorb the light waves reflected back by the semiconductor substrate into the photoresist layer and/or cancel them out by means of destructive interference. In this case, use is made in particular of antireflection layers consisting of organic polymers which are distinguished by a high light absorption. Organic antireflection layers are additionally also suitable for planarizing edges and steps in the semiconductor layers. Such a leveling action is often required, in particular before the application of a metal plane, wherein conductor tracks for wiring the components of the semiconductor chip are formed. Because of the limited conformity of the metal sputtering coating which is conventionally used, this is because the thickness of the metalization on steep edges on the semiconductor surface can be so low that the conductor track cross sections turn out to be considerably lower than on planar surfaces. In such areas, undesirably high current intensities then occur. The various possibilities of the photoresist technique with an antireflection layer are described, inter alia, by Widmann, Mader, and Friedrich in Technologie hochintegrierter Schaltungen [Technology of Highly Integrated Circuits], 1996, Springer.
One variant of the photoresist technique with an antireflection layer is what is referred to as the trilevel resist technique. Here, a so-called bottom resist layer is sputtered onto the semiconductor substrate. The bottom resist layer is a positive resist or its reswherein can be made highly light-absorbing by the addition of an absorber or by high baking out. Onto this so-called bottom resist layer, a spin-on-glass intermediate layer and then a so-called top resist layer are applied. This topmost top resist layer is the actually photochemically active layer. The bottom resist layer, on the other hand, ensures that virtually no light is reflected back from the semiconductor substrate into this top resist layer. In addition, the bottom resist layer is also made sufficiently thick to level any steep steps which are present on the semiconductor substrate, so that the top resist layer can be sputtered on with a uniform thickness which is not influenced by these surface steps. The top resist layer is then exposed and developed via a mask which contains the desired semiconductor component structure. As a result of the development, the top resist layer is dissolved at the exposed points while the non-irradiated areas remain masked. The spin-on-glass intermediate layer is then etched at the exposed points. The latter is then used as an etching mask during the anisotropic etching of the bottom resist layer which is finally in turn used as a masking layer for etching the semiconductor layer, for example silicon dioxide, which lies underneath and is to be structured.
As an alternative to the trilevel resist construction, a bilevel resist technique can also be used. There, the spin-on-glass intermediate layer is omitted. This is possible when the top resist structure is resistant to the bottom resist etching.
Opening the organic antireflection layer is conventionally carried out with the aid of dry etching techniques, the following requirements being placed on the etching process: the anisotropy factor of the etching should be as close as possible to 1, in order to achieve high profile accuracy, that is to say steep resist edges. In addition, the mask structure is to be transferred as accurately as possible to the semi-conductor layer lying underneath the organic antireflection layer, that is to say the etching is to be CD (critical dimension) accurate. Finally, high selectivity with respect to the mask layer and with respect to the semiconductor layer lying underneath the layer to be etched is required of the etching.
The dry etching process normally uses gaseous media, which are excited by a gas discharge in the high-frequency alternating field. The discharge process takes place in the vacuum area, so that a long free path length for the ions between collisions is achieved. In order to achieve highly fine structures, the chemical/physical dry etching process is primarily used, wherein, in addition to a purely physical removal of material by atoms or molecules being thrown out of the layer to be etched, chemical removal of material is carried out by means of a reactive gas. The most important etching process in the chemical/physical dry etching is reactive ion etching, oxygen primarily being used as the reaction gas for anisotropic etching of an organic antireflection layer, since oxygen forms volatile reaction products with the polymer constituents.
However, since the oxygen exhibits a very isotropic etching behavior and poor selectivity, further gases are generally mixed with the hydrogen in order to improve the etching behavior. In this case, use is primarily made of chlorine or nitrogen although not all the requirements on the etching process can continue to be met in the case of etching gas chemistry of this type.
Although physical/chemical dry etching with an oxygen/chlorine chemistry is distinguished by an accurate transfer of the mask structure in the etching process to the semiconductor structure lying underneath the organic antireflection layer, the profile accuracy is low, on the other hand, and in addition undesired concomitant etching of the semiconductor layer occurs. When an oxygen/nitrogen chemistry is used for the chemical/physical dry etching of an organic antireflection layer, although an improved selectivity with respect to the semiconductor base under the antireflection layer can be achieved, the CD accuracy, on the other hand, is low.
Etching the organic antireflection layer with chlorine as a reactive constituent is, moreover, generally not compatible with the subsequent chemistry for the semiconductor etching, in particular when a silicon dioxide layer is to be etched. This is because the reaction products of the oxygen with the polymer constituents of the organic antireflection layer can be removed from the etching chamber only with great difficulty and then combined with the etching chemistry of the subsequent semiconductor etching process which changes the etching parameters and therefore the etching structure in an undesired way.
Jpn. J. Appl. Phys. Part 1, Vol 32 (1993), pages 747-52 discloses a process for removing photoresist (stripping), wherein the etching gas consists of O2-CF4 and contains an addition of N2-H2. The process is carried out in an MRIE apparatus. In addition, the etching of organic ARC layers with O2 or O2-H2-Ar is disclosed by Japanese document JP 11-150 115 A and U.S. Pat. No. 5,910,453.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a method of processing organic antireflection layers which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which provides a process for the anisotropic dry etching of an organic antireflection layer which is distinguished by high selectivity, improved structure accuracy, and good compatibility with subsequent etching processes.
With the above and other objects in view there is provided, in accordance with the invention, a process for anisotropically dry etching of an organic antireflection layer, which comprises etching the organic antireflection layer with an etching gas composition primarily containing hydrogen and nitrogen.
In a preferred mode of the invention, the organic antireflection layer is etched with an etching gas composition consisting essentially of hydrogen and nitrogen.
In the process according to the invention for the anisotropic dry etching of the organic antireflection layer, the etching gases used are substantially hydrogen and nitrogen. Using this etching chemistry, physical/chemical dry etching is possible wherein incipient etching of the semiconductor layer lying underneath the organic antireflection layer is largely prevented. In this way, using the etching chemistry which, as the reactive etching gas according to a preferred embodiment, contains at least 80% nitrogen and hydrogen, a selectivity of more than 1:50 of the organic antireflection layer etching in relation to etching the semiconductor layer lying underneath can be achieved. In addition, the reaction gas composition of nitrogen and hydrogen ensures accurate transfer of the structure of the etching mask and high profile accuracy of the resist edges. Finally, using a hydrogen/nitrogen mixture as reaction gases for etching the organic antireflection layer, good compatibility with the subsequent semiconductor etching, in particular the silicon dioxide etching, is achieved, so that the etching operations can be carried out in the same reaction chamber.
In accordance with an added feature of the invention, hydrogen and nitrogen are adjusted to a ratio of 1:1, that is, the reaction gas is composed of equal proportions of hydrogen and nitrogen. This reaction mixture permits highly accurate anisotropic etching of the organic antireflection layer even in the case of very high layer thicknesses, such as are provided in particular if the organic antireflection layer is additionally used to planarize steps and edges in the semiconductor layer underneath. The organic antireflection layer can be opened in a structurally accurate manner with such a reaction gas composition even in the case of severe overetching, without the semiconductor base being attacked. In this case, even structures below 0.2 μm, in particular, can be achieved reliably.
In accordance with an additional feature of the invention, the etching gas composition contains at least 80% hydrogen and nitrogen as reactive etching gases. Preferably, the etching gas composition contains, as reactive etching gases, only hydrogen and nitrogen.
In accordance with another feature of the invention, the etching gas composition contains additives for improving etching gas properties in the respective dry etching process that is utilized.
In accordance with a further feature of the invention, a photoresist layer is used as an etching mask for the organic antireflection layer, and the etching gas composition is adjusted such that a vertical removal of the photoresist corresponds at most to an etching rate of the organic antireflection layer. In this embodiment, when a photoresist layer is used as the etching mask for the organic antireflection layer, the etching gas composition with hydrogen and nitrogen is set such that the vertical removal corresponds at most to the etching rate of the organic antireflection layer. This achieves the situation where only slight faceting of the photoresist layer used as the etching mask occurs during the etching process, and the organic antireflection layer lying underneath continues to have steep edges after the etching process.
In accordance with again a further feature of the invention, the physical/chemical dry etching with a reaction gas mixture of hydrogen and nitrogen is carried out using reactive ion etching technique in a pressure range of 2.67 to 26.67 Pa (20 to 200 mTorr) and with a gas flow of 0.17 to 1.67 10−6m3sec−1 (10 to 100 sccm). During such an etching process, the etching behavior can be controlled particularly well with regard to homogeneity, etching rate, etching profile and selectivity and, in addition, high reproducibility can be achieved. It is also preferred to carry out the reactive ion etching with the assistance of a magnetic field, in a magnet field of up to 120 Gauss.
In accordance with a preferred mode of the invention, therefore, the layer is exposed to a magnetic field strength from 0 to 120 Gauss and processed with magnetic field-assisted reactive ion etching.
In accordance with a concomitant feature of the invention, the organic antireflection layer is etched with a plasma from an electron cyclone resonance plasma source, with an inductively coupled plasma, or a Helicon source.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a process for organic antireflection layers, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.