With the improvement of the integration density and performance of semiconductor integrated circuits and so on, there has been a growing demand for a microfabrication technique for a semiconductor substrate or the like. In particular, for obtaining a predetermined pattern on a substrate or thin film of an insulator, a semiconductor or metal, a photoetching technique which removes an unnecessary part by an optical or chemical method is important and is now attracting attention as the microfabrication technique.
But, when imaging is performed through use of ultraviolet radiation for microfabrication in the order of submicrons, its wavelength is so long that it is impossible to prevent image lines from "defocusing" which is caused by the diffraction or reflection of light in a mask transfer process.
In the recent years, there has been developed, with a view to achieving ultra-microfabrication, an exposure system which utilizes an electron beam or soft X-rays of a shorter wavelength than the ultraviolet rays. The development of such an exposure system and improvement in a dry etching system which produces no appreciable undercut of a resist material together have made possible ultra-microfabrication in the order of submicrons.
The sensitivity of positive resist, which is a sort of resist, increases with an increase in the G-value used as a measure of the solubility of a main chain and an increase in the ratio between the solubility rates of irradiated and unirradiated parts of the resist during development, that is, an increase in the selective solubility of the resist itself.
Now, consider a benzene ring-containing polymer. This kind of resist is highly resistant to dry etching but since its G-value is small, it is difficult to achieve high sensitivity. It is considered, however, that if the abovesaid selective solubility could be enhanced by some method, a positive electron resist could be obtained which is excellent in the resistance to dry etching and high in sensitivity.
A cross-linking method has been known as one of techniques for improving the selective solubility. This method is one that coats straight-chain high molecules on a substrate and then heat treats the coating to create intermolecular cross-links, whereby the positive resist forms a resist film which is insoluble in a solvent. The positive resist of this kind, even after having been irradiated by an electron beam or X-rays, retains the cross-linking structure in the unirradiated part, so that even if a strong developing solvent is used, the unirradiated part will not undergo any swelling or deformation. With the strong developing solvent, since the irradiated part slightly decomposed by a very small amount of irradiation is also dissolved, high sensitivity can be achieved.
The known cross-linking type positive resist can be roughly divided into the following four kinds according to the type of thermal cross-linking reaction (the following reaction formulae being given noting only functionality groups of polymer side chains).
[1] --COOH+--COCl.fwdarw.--COOCO--+HCl (see U.S. Pat. No. 3,981,985) PA0 [2] --COOCH.sub.3 +--COCl.fwdarw.--COOCO--+CH.sub.3 Cl (See U.S. Pat. No. 4,061,832) PA0 [3] --COOR+--COOR.fwdarw.Cross-linking polymer (where R represents --(CH.sub.3).sub.3 or --CH.sub.2 CCl.sub.3. ) (See U.S. Pat. No. 4,264,715) PA0 [4] --COOC.sub.6 H.sub.5 +--COOH.fwdarw.--COOCO--+C.sub.6 H.sub.5 OH (see K. Harada et al., "Poly(Phenyl Methacrylate-Co-Methacrylic Acid) as a Dry-Etching Durable Positive Electron Resist", IEEE Ed-29 (4) p518 (1982))
Usually these thermally cross-linkable functional groups are located in the same polymer molecules through copolymerization or in dissimilar polymer molecules through mixture of dissimilar polymers. However, since the thermal crosslink density varies with the content of such functional groups and the heat treatment temperature, their accurate control is needed for obtaining sensitivity of excellent reproducibility after development.
Especially, in the case where it is necessary, for the purpose of increasing the dry etching resistance of the resist, to mix therein a monomer unit containing the benzene ring, the resulting resist material is a ternary copolymer which contains a third monomer unit in addition to two monomer units necessary for causing the abovesaid thermal cross-linking reaction. Hence it is very difficult to control the composition of the copolymer. Furthermore, in the case of utilizing the reaction mentioned above in [4], a high-temperature heat treatment at about 200.degree. C. is required, so that especially when the substrate to be worked is made of aluminum or the like, hillock grows, resulting in the device characteristics being degraded.
As described above in detail, ultra-microfabrication techniques in the order of submicrons are required with the advancement of the integration density and performance of semiconductor integrated circuits; to meet such a requirement, an electron beam or soft X-ray exposure process and an electron resist material have been developed for imaging a predetermined pattern on the substrate. Moreover, a dry etching process such as plasma etching has recently come into wide use as a precision etching process of limited undercut, while at the same time such advances in the dry etching technique have now called for development of an electron resist material of high dry etching resistance.
At present, however, no particular positive resist is available which is markedly effective for dry etching as referred to above; so there is much left to improve.
On the other hand, as described above, the increased integration density and performance of semiconductor integrated circuits inevitably necessitates ultra-microfabrication in the domain of submicrons; to fulfil such a requirement, there have been developed the exposure techniques using an electron beam or soft X-rays and the electron resist material. In the case of forming a resist pattern on a substrate through use of these techniques and materials, it is important to control the cross-sectional configuration of the resist pattern as well as to microminiaturize the resist pattern itself.
A conventional patterning method utilizes a lift-off process for obtaining a desired pattern without involving the etching of the material to be worked, in the manufacture of an LSI (large scale integrated circuit) or the like. With this method, a material sensitive to an electron beam, light, or soft X-rays is exposed thereto and then developed to form a desired undercut cross-sectional configuration. After the material to be worked is deposited by evaporation or some other method using the pattern as a mask, the mask forming material is removed, obtaining a desired pattern. This method necessitates the formation of the undercut cross-sectional configuration for removing the material forming the mask.
On the other hand, it has been proposed to taper the cross-sectional configuration of the resist pattern which is employed as a mask in the case of making a tapered contact hole through etching for the purpose of preventing the breakage of an interconnection material.
Such an undercut or tapered cross-sectional configuration can be obtained by forming a plurality of layers of positive resists of different solubility rates in the same developer.
The following methods have heretofore been utilized for controlling such a cross-sectional configuration as mentioned above.
(1) A first method makes use of excessive irradiation of a single layer of positive resist by an electron beam, light or X-rays, or excessive development of the layer by which the shape of the cross section of a resist pattern 2 formed on a substrate 1 becomes such a so-called undercut shape that a foot width d.sub.2 is larger than an aperture width d.sub.1, as depicted in FIG. 1. This method is simple but is attended with difficulty in controlling the dimensions and cross-sectional configuration of the resist pattern, that is, the aperture width d.sub.1 and the foot width d.sub.2. In particular, a fine aperture width d.sub.1 is difficult to obtain.
(2) A second method employs positive resists of different solubility rates for the same developer and obtains a pattern of such a cross section as shown in FIG. 2 by forming a two-layer structure with the lower layer built up from a resist of the higher solubility rate and the upper layer from a resist of the lower solubility rate. In FIG. 2, an underlying resist pattern 22 of the higher solubility rate and an overlying resist pattern 23 of the lower solubility rate are formed on a substrate 21. With method, however, when the ratio in the solubility rate between the upper and lower layers is small, dissolution of the upper resist pattern 23 proceeds in the course of dissolution of the lower resist pattern 22, introducing difficulties in obtaining a fine aperture width and in controlling the cross-sectional configuration as is the case with the method described above in (1).
With the method (2) it is also possible to laminate layers of resists of different molecular weights through utilization of the phenomenon that the solubility rate of a resist generally depends upon its molecular weight. In this instance, however, the molecular weight ratio that is feasible is limited and the underlying resist layer is readily dissolved during coating of the overlying resist layer, making it difficult to form, as a unitary structure, two layers of largely different solubility rates.
Furthermore, by reversing the relationship between the solubility rates of the underlying resist layer and the overlying resist layer formed on the substrate in the case of FIG. 2, that is, by selecting the solubility rate of the upper resist layer higher than the solubility rate of the lower resist layer, it is possible to obtain an upwardly tapered cross-sectional configuration. Also in this case, it is necessary, for easy control of the cross-sectional configuration, that the ratio between the solubility rates of the overlying and underlying layers.
It is therefore a first object of the present invention to provide a novel high-sensitivity positive resist mixture which obviates such defects of the conventional positive resists as described above and a patterning method which employs the novel positive resist mixture.
A second object of the present invention is to develop a developing method suitable for development of the novel positive resist mixture and a patterning method which employs the developing method.
A third object of the present invention is to provide a patterning method which employs the novel positive resist mixture and permits the formation of a fine pattern and at the same time high-precision control of its cross-sectional configuration.