METHOD OF MANUFACTURING A DEVICE:

Abstract: The invention provides a method for the two-layer photo-resist system having an improved composition of the top layer. The formation of a transition layer is suppressed by the choice of binder and solvent.

Description: The invention relates to a method of manufacturing a device, in which method an intermediate layer is formed from a first photosensitive lacquer on a substrate surface and is covered with a top layer formed from a second photosensitive lacquer, the top layer is patternwise exposed to radiation and is then developed so as to form an apertured mask, the intermediate layer then being exposed to radiation using the developed top layer as a mask, the second photosensitive lacquer comprising a binder, a positive acting photosensitizer and a solvent in which the intermediate layer is only slightly soluble. The method according to the invention is used, for example, in the manufacture of a semiconductor device. The invention additionally relates to a device manufactured by means of the method according to the invention and to a photosensitive lacquer. The method mentioned in the opening paragraph is described in the publication "Materials for multilevel resist schemes" by E. Reichmanis, C.W. Wilkins, Jr. and E. Ong, in Polymer Engineering and Science, 23 , December 1983, pp. 1039-1042, in which the intermediate layer is a polymethyl methacrylate and the top layer is a positive acting photosensitive lacquer based on a novolak resin. Novolak is a condensation product of phenol and formaldehyde. The said publication describes how a transition layer is formed at the interface between the first and the second lacquer layers and this causes difficulties when exposing and developing the intermediate layer, inter alia by light absorption and diffraction. Therefore, additional steps are necessary in the method to remove the transition layer of to make it harmless. On the other hand it is possible to choose a solvent for the top layer in which polymethyl methacrylate has a very poor solubility. As a result of this the formation of the transition layer is only partly inhibited. It is the object of the invention to provide a method of manufacturing a device by using a bilayer photosensitive lacquer system in which the formation of the transition layer takes place to a considerably reduced extent. For that purpose, the method described in the opening paragraph is characterized in that the binder is selected from the group of polymers which is formed by polyhydroxystyrenes and halogenated polyhydroxystyrenes. Upon developing, the top layer shows an especially large solubility difference between the exposed and unexposed parts in an embodiment of the method according to the invention which is characterized in that the binder is poly-para-hydroxystyrene. The formation of the transition layer is counteracted to a particularly strong extent if the binder is a brominated poly-para-hydroxystyrene compound having on an average 1 to 2 substituted bromine atoms per phenyl group. A preferred embodiment of the method according to the invention is characterized in that the binder is poly(2,4-dihydroxystyrene). Although the use of polyhydroxystyrene (also termed) "polyvinylphenol") in a multilayer system has been described in the publication "Multilayer resists for fine line optical lithograph" by E. Ong and E.H. Hu in Solid State Technology, June 1984, pp. 155-160, the polyhydroxystyrene in said publication forms a constituent of a negative acting photosensitive lacquer. There, the disadvantage occurs that the transition layer is exposed and thereby strengthened in the pattern-wise exposure of the top layer. As a result of this, the transition layer is difficult to remove entirely at a later stage. Suitable positive acting photosensitizers are multiple esters of trihydroxybenzophenone and 2-diazo-1-naphthoquinone-4-sulphonyl chloride of 2-diazo-1-naphthoquinone-5-sulphonyl chloride. The tri-esters or a mixture of bi-esters and tri-esters are preferably used. Photosensitive lacquers based on polyhydroxystyrene and esters of o-quinone diazide sulphonic acid are described in German Patent Application DE 3309222, but the use in a bilayer system is not described therein. The lacquers according to the said German Patent Application are not suitable for use in the method according to the invention because the solvents used, for example 2-ethoxy-ethyl acetate, enhance the formation of a transition layer. The transmission of light having a wavelength of 200 nm through the transition layer exceeds 80%, which indicates a very thin transition layer, if in the method according to the invention the solvent of the second photosensitive lacquer is selected from the group formed by cyclohexanone and 2-ethoxy-ethanol. The top layer can be developed in the conventional manner, for example, by using a metal ion-free basic solution, for example trismethyl ammonium hydroxide in water. In a preferred embodiment of the method according to the invention the intermediate layer comprises a polymer or a copolymer of methyl methacrylate. Very suitable is polymethyl methacrylate, but copolymers with, for example, other alkyl methacrylates, methyl acrylate and methacrylonitrile can also be used successfully. The intermediate layer can be developed in a conventional manner. The intermediate layer may, for example be developed by using a solvent, for example methyl isobutyl ketone by which the remainders of the top layer are also dissolved. In connection with the further treatments to which the device is subjected (for example chemical etching, ion etching or deposition of a metal layer, succeeded by removal of the remainders of the intermediate layer), it may be of importance for the remainders of the top layer to remain on the developed intermediate layer as an extra protection. In such a case it is efficacious for the intermediate layer to be developed with a solvent selected from the group formed by chlorobenzene, xylene and toluene. Embodiments of the method according to the invention and comparative examples will be described in greater detail with reference to the drawing, in which: Figures 1 a - e show diagrammatically a possible application of the method according to the invention, Figures 2 a - c show diagrammatically an alternative application of the method according to the invention, and Figures 3 a - b show an embodiment of the photosensitive component of the top layer. Example 1 of the method according to the invention. Figure 1 a is a diagrammatic sectional view of a silicon substrate 1 on the surface of which steps and bosses 2 may be present. An intermediate layer 3 of polymethyl methacrylate having a thickness of 1 to 4 µm is provided in a conventional manner on the substrate 1, for example, by spinning a solution in chlorobenzene. This solvent is then removed by heating to 150°C for 30 minutes. The intermediate layer 3 has a substantially flat surface. A top layer 4 using a second photosensitive lacquer, is then provided, for example, also by spinning. Said photosensitive lacquer consists of 17% by weight of poly-para-hydroxystyrene as a binder (molecular weight 1500 - 7000 of Messrs. Polyscience Inc.), 78% by weight of cyclohexanone as a solvent and 5% by weight of the tri-ester of 2,3,4-trihydroxybenzophenone and 2-diazo-1-naphthoquinone-5-sulphonyl-chloride as a photosensitive component (Figure 3 a , wherein R is shown in Figure 3 b ). Before providing the lacquer it is filtered through a filter having pore diameters of at most 0.2 µm. The top layer is dried at 80°C for 30 minutes and after drying has a thickness from 0.3 to 1.5 µm. A transition layer 5 is formed at the interface between the intermediate layer 3 and the top layer 4. This layer is very thin and does not cause the problems occurring when other binders and solvents are used in the second photosensitive lacquer. The transmission of light through the transition layer is 90% at a wavelength of 200 nm and 95% at 220 nm. For comparison: a transition layer having a thickness of not more than 30 nm which is formed when a known photosensitive lacquer HPR 204 of Messrs. Hunt (on the basis of novolak) is used causes a transmission of only 30% at 220 nm. The transmission is measured by an absorption determination on a quartz substrate with polymethylmethacrylate as the reference. The substrate 1 with the photosensitive lacquer layers 3 and 4 is exposed in the form of a pattern through a mask (not shown) by means of light in a wavelength range from 350 - 450 nm (see Figure 1 a ). The polymethylmethacrylate in layer 3 is not sensitive to said light, but is sensitive to light having a wavelength shorter than 250 nm. The exposed parts of the top layer 4 are dissolved (see Figure 1 b ) with developer N437 of Messrs. Shipley, diluted 1 : 1 with water, for 1 minute. Said developer is a metal ion-free basic solution. The substrate 1 with the photosensitive layers 3 and 4 is then exposed throughout the surface to parallel UV radiation having a wavelength of 190 - 250 nm (see Figure 1 b ), the mask formed in the top layer 4 ensuring a patterned radiation of the intermediate layer 3. The exposed part of the intermediate layer 3 is then dissolved in toluene, apertures being formed in the intermediate layer having side-walls extending substantially perpendicularly to the substrate 1, the remainders of the top layer 4 not being dissolved (Figure 1 c ). Other solvents, for example chlorobenzene or xylene, may be used instead of toluene. If on the contrary a more polar solvent, for example, methyl isobutyl ketone, is used, the remainders of the top layer 4 also dissolve. However, if the pattern formed in the intermediate layer 3 is used to serve as a mask in an ion etching treatment of the substrate 1, it is efficacious to use the remainders of the layer 4 as a protection of the non-exposed parts of the intermediate layer 3. The pattern formed in the intermediate layer 3 may be used in various conventional manners in the further manufacture of the device. The pattern may serve as a mask in a chemical or plasma etching treatment or ion implantation. It is also possible (Figure 1 d ) to provide a metal layer 6, for example, by vapour deposition of aluminium. When the remainders of the layers 3 and 4 are then dissolved, a part of said metal layer 6 may be removed (so-called "lift-off" process) with the result that the substrate 1 has a pattern of metal tracks (Figure 1 e ). Example 2 of the method according to the invention. Figure 2 a shows a silicon substrate 11 on the surface of which steps and bosses 12 may be present. The substrate 11 is covered with a conductor layer 16, for example of polysilicon, and with the intermediate and top layers 13 and 14 in which a pattern is provided in the manner described in example 1. This pattern is used as a mask during etching of the conductive layer 16. Etching may be carried out in any known manner, for example, by chemical etching or by etching with reactive ions. A pattern (Figure 2 b ) is formed in the conductive layer 16 after which the remainders of the intermediate and top layers 13 and 14 may be removed, for example by a treatment with an oxygen plasma. The width of the conductor tracks, for example 17 and 18, is determined by the mask used in the first exposure and by the nature of the further processes, but hardly by the presence of steps or bosses 12 on the substrate 11, as a result of which track width variations at the crossings of such steps occur only to a small extent. Alternative example of the method according to the invention. In the method according to the invention other solvents may also be used in the second photosensitive lacquer, provided the solvent does not attack the intermediate layer 3. Particularly suitable is, for example, 2-ethoxy-ethanol the use of which even results in the measured transmission through the transition layer 5 being larger than when cyclohexanone is used, which indicates a still thinner transition layer 5. The overall concentration of binder and photosensitizer in the lacquer may be adapted at will. In order to obtain a good spinnability this overall concentration is, for example, 10 to 40% by weight, preferably approximately 25% by weight. The ratio of binder to photosensitizer in the composition of the second photosensitive lacquer can freely be chosen within certain limits. If the ratio of binder to photosensitizer is greater than 8 : 1, however, the rate of dissolution of the unexposed part of the top layer becomes too large, for example more than 1 µm/min, as a result of which the top layer 4 shows little selectivity. On the other hand it is not advisable to choose a ratio smaller than 2 : 1 because there is no further improvement of the result. Due to the comparatively high cost of the photosensitizer a ratio of approximately 3 : 1 is to be preferred. Poly-para-hydroxystyrene is used as a binder in the above-described example. Poly-ortho-hydroxystyrene and poly(2,4-dihydroxystyrene) may also be used in the lacquer layer 4 in the method according to the invention. When brominated poly-para-hydroxystyrene compounds with on an average 1.5 bromine atoms per phenyl ring are used, the transmission of light of a wavelength of 220 nm through the transition layer 5 is even more than 98%. When the brominated compounds are used it is necessary to further dilute the developer, for example, in a ratio of 1 : 4 to 1 : 8, with water, the rate of developing decreasing. Because the problems with the transition layer are considerably reduced in the method according to the invention, there is more freedom in the choice of the molecular weight and the glass transition temperature of the binder and in the drying temperature than, for example, when novolak is used. Poly-para-hydroxystyrene having a molecular weight between 1,500 and 30,000 has proved to be useful, the molecular weight preferably being smaller than 7,000 (All molecular weights stated are number-averaged molecular weights of the polymer molecules). The glass transition temperature of polyhydroxystyrene is approximately 200°C, that of novolak, dependent on the type, 100 - 130°C. The result of this is that the mobility of the polyhydroxystyrene molecules remains low and little interdiffusion takes place between the top layer and the intermediate layer, as a result of which the formation of a transition layer is suppressed. Comparative examples. The transmission values recorded in the Table are measured at a wavelength of 200 nm through the transition layer which is formed on polymethyl methacrylate when various combinations of binders and solvents in the second photosensitive lacquer are used to form the top layer. The novolak PN 430 of Messrs. Hoechst comprises a diazoquinone as a photosensitive component, the poly-hydroxystyrenes of Messrs. Polyscience Inc. comprise the above-described tri-ester (Figures 3 a , b ).MW denotes the average molecular weight. It will be obvious from the Table that the compositions according to the invention show a particularly great transmission and hence a thin transition layer. It furthermore appears that 2-ethoxy-ethanol and cyclohexanone are very suitable solvents for use in the method according to the invention. TABLE It has proved possible by means of the method according to the invention to make well-defined patterns with tracks having a width which is smaller than 1 µm, in which the width does not vary substantially if such a track crosses a step on the surface of the substrate. It will be obvious that the above-described method permits many modifications without departing from the scope of this invention. The method may also be applied to other substrate materials and to multilayer substrates. Other acrylate or methacrylate-containing polymers which are sensitive to radiation of short-wave UV-light may also be used for the intermediate layer.

Claims: 1. A method of manufacturing a device, in which method an intermediate layer is formed from a first photosensitive lacquer on a substrate surface and is covered with a top layer formed from a second photosensitive lacquer, the top layer is patternwise exposed to radiation and is then developed so as to form an apertured mask, the intermediate layer then being exposed to radiation using the developed top layer as a mask, the second photosensitive lacquer comprising a binder, a positive acting photosensitizer and a solvent in which the intermediate layer is only slightly soluble, characterized in that the binder is selected from the group of polymers which is formed by polyhydroxystyrenes and halogenated polyhydroxystyrenes. 2. A method as claimed in Claim 1, characterized in that the binder is poly-para-hydroxystyrene. 3. A method as claimed in Claim 1, characterized in that the binder is a brominated poly-para-hydroxystyrene compound having on an average 1 to 2 substituted bromine atoms per phenyl group. 4. A method as claimed in Claim 1, characterized in that the binder is poly(2,4-dihydroxystyrene). 5. A method as claimed in any of the Claims 1 to 4, characterized in that the photosensitizer is a multiple ester of trihydroxybenzophenone and 2-diazo-1-naphthoquinone-4-sulphonyl chloride or 2-diazo-1-naphthoquinone-5-sulphonyl chloride. 6. A method as claimed in any of the Claims 1 to 5, characterized in that the solvent of the second photosensitive lacquer is selected from the group formed by cyclohexanone and 2-ethoxy-ethanol. 7. A method as claimed in any of the Claims 1 to 6, characterized in that the intermediate layer comprises a polymer or copolymer of methyl methacrylate. 8. A method as claimed in Claim 7, characterized in that the intermediate layer is developed by means of a solvent selected from the group formed by chlorobenzene, xylene and toluene. 9. A device manufactured by means of the method as claimed in any of the Claims 1 to 8. 10. A photosensitive lacquer comprising a binder, a positive acting photosensitizer and a solvent, characterized in that the binder is selected from the group of polymers which is formed by polyhydroxystyrenes and halogenated polyhydroxystyrenes. 11. A photosensitive lacquer as claimed in Claim 10, characterized in that the photosensitizer is a multiple ester of trihydroxybenzophenone and 2-diazo-1-naphthoquinone-5-sulphonyl chloride or 2-diazo-1-naphthoquinone-5-sulphonyl chloride. 12. A photosensitive lacquer as claimed in Claim 10 or 11, characterized in that the solvent is selected from the group formed by cyclohexanone and 2-ethoxy-ethanol.

Inventors: GEOMINI, MARCELLINUS JOSEPH HENDRIKUS JACOBUS, WIJDENES, JACOB
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1986
Main classification: G03F7
IPC codes: G03C1, G03F7