Method for the formation of monomolecular adsorption films or built-up films of monomolecular layers using silane compounds having an acetylene or diacetylene bond

Abstract: This invention relates to a method for the formation of monomolecular adsorption films of a silane compound having a diacetylene bond. The silane compound is chemically adsorbed on a substrate through =SiO- bonds and the resultant film is significantly reduced in number of defects such as pinholes. When the monomolecular film is irradiated with actinic light, the diacetylene bonds are converted into a conjugated diacetylene polymer. Similarly, a built-up film of a desired number of monomolecular layers of a silane compound having an acetylene bond formed on a substrate by chemical adsorption and a corresponding number of monomolecular layers of an aliphatic acid or alcohol having an acetylene bond and alternately deposited on the first-mentioned layer is also provided. These films have wide utility in electronic fields.

Description: This invention relates to a method for forming a polymerised monomolecular adsorption film of a material having diacetylene bonds on a substrate by chemical adsorption and irradiation. These films have wide utility as a conductive or insulating film in the fields of semiconductive industries. Polydiacetylene materials are of interest because they have a very long conjugated polydiacetylene bond in the molecule and exhibit a non-linear optical effect and electric conductivity. These materials are considered to have utility as a functional material. Methods for obtaining these polymers from monomer molecules in the form of crystals have been studied using the Langmuire-Blodgett technique or vacuum deposition technique. In general, in order to impart good optical properties to polydiacetylene material, it is necessary for the polymer to have a very long polydiacetylene bond. When the Langmuir-Blodgett technique is used, the molecular orientation and density of the monomer can be arbitrarily controlled. This is advantageous in that the reactivity of the monomer by irradiation of actinic light can be easily controlled. However, when the monomolecular film of the polydiacetylene is formed by the technique, the diacetylene monomer on the water surface in the Langmuir-Blodgett trough is crystallized to form defects in spite of the surface pressure from outside. This leads to the problem that the conjugation length after the polymerization is limited. On the other hand, the formation of polydiacetylene by the vacuum deposition technique is advantageous in that the film thickness can be appropriately controlled. However, the drawback of the technique is that when heated excessively, the monomer decomposes. In United States Patent No. 4,673,474 chemical adsorption is mentioned but this patent does not particularly set out the type of material to be adsorbed or adsorption conditions for such a material. US-A-4,539,061 discloses a process for producing built up films by stepwise adsorption of individual monolayers. It also disclosed intralayer polymerisation of adsorbed molecules. EP-A-77577 discloses an article comprising a substrate wherein the substrate carries a layer of a photopolymerisable composition which includes a material having acetylenic bonds. The present invention provides a method for the formation of a polymerised monomolecular adsorption film which comprises: providing a substrate having, on the surface, a layer capable of reaction with a silane compound having a diacetylene bond in the molecule; subjecting the silane compound to chemical adsorption with the layer to form a monomolecular adsorption film of the silane compound on the substrate through the layer wherein the chemical adsorption is carried out either a) at a temperature not higher than 20°C whereby the molecules in the adsorption film are oriented in a vertical direction with respect to the substrate and the resulting film is heated to a temperature sufficient for the molecules to adopt an oblique orientation, or b) at a temperature between the glass transition temperature and the melting point of the silane compound whereby the molecules in the adsorption film are oriented in an oblique direction with respect to the substrate; and c) irradiating the film with an energy beam to form a conjugated diacetylene polymer film. The substrate is preferably made of silicon and the layer is an SiO₂ layer. The orientation of the silane molecules can be controlled either in a direction vertical to the substrate or in a direction oblique to the substrate by heating the film to the level as set forth above. The polymerization of the compound through the diacetylene bonds by application of actinic light or an energy beam capable of conversion of the diacetylene bonds into a conjugated diacetylene polymer depends greatly on this orientation. When the monomolecular adsorption film which has been formed at a temperature of not higher than 20°C is further heated to a temperature between the glass transition temperature and the melting point, say, about 50°C in a desired pattern and is subjected to irradiation of an energy beam, the film can be selectively polymerized according to the pattern to form a conjugated diacetylene polymer of the desired pattern. Alternatively, if the monomolecular adsorption film is formed at a temperature between the glass transition temperature and the melting point of silane compound, say 50°C, or is formed at temperatures lower than 20°C and subsequently heated to the temperature between the glass transition temperature and the melting point and is entirely irradiated with an energy beam, the entire film is polymerized. The heating in a desired pattern is preferably by IR rays. Fundamentally, the above embodiment makes use of chemical adsorption of silane compounds on a substrate having a layer capable of reaction with the silane compound wherein the molecules of the silane compound chemically with the layer to form the monomolecular adsorption film. The substrate used is preferably a silicon substrate which has a naturally occurring oxide layer on the surface although any material may be used provided that it has a surface layer of material capable of chemical adsorption reaction with the starting silane compound. This chemical adsorption is advantageous in that the monomolecular film has small defects. Figs. 1(a) to 1(i) are illustrative views of the respective steps of a method for forming a monomolecular adsorption film according to one embodiment of the invention; Figs. 2(a) to 2(f) are illustrative views of the steps of a method for forming a monomolecular adsorption film according to another embodiment of the invention; Figs. 3 and 4 are, respectively, UV absorption spectra for different irradiation times; The embodiments of the present invention are described with reference to the accompanying drawings. Fig. 1 shows a first embodiment of the invention. In Fig. 1(a), there is shown a substrate 1 which is made of, for example, silicon. For chemical adsorption of a silane compound having a diacetylene bond, it is necessary that the substrate 1 have a layer 1a capable of chemical adsorption with the silane compound. The silicon used as the substrate 1 is advantageous from the above viewpoint since a SiO₂ layer is naturally formed on the substrate in air. In this connection, however, any material may be used as the substrate provided that a layer capable of undergoing chemical adsorption with the silane compound is formed on the substrate 1. For instance, metals which are able to naturally form an oxide surface layer when exposed to air are generally used and include, for example, not only Si, but also Sn, Al, Ni, Fe, Cr and the like. In addition, metal oxides such as Al₂O₃, SnO₂, Fe₂O₃the like are also useful as the substrate material. Of these, Si, Al, Cr and Al₂O₃ are preferred, Si is most preferable. In a subsequent step, a silane compound having a diacetylene bond in the molecule is applied onto the substrate 1 thereby causing-chemical adsorption with the layer 1a to form a monomolecular adsorption film 2 on the substrate surface as shown in Fig. 1(b). In practice, the chemical adsorption is carried out by immersing the substrate in a solution of the silane compound in solvent at a concentration of from 2 x 10⁻³ to 5 x 10⁻² mols/liter. The solvent may be any compound which is able to dissolve the silane compound and includes, for example, hydrocarbons having from 6 to 20 carbon atoms such as n-hexane, n-heptane, n-octane, n-hexadecane and the like, halogenated hydrocarbons such as trichloromethane, carbon tetrachloride, chloroform, trichloroethylene, trichloroethane and the like, and mixtures thereof. The silane compounds having a diacetylene bond are, for example, compounds of the formula (1) CH₃(CH₂) m C≡C-C≡C(CH₂) n -SiCl₃ (1) wherein m and n are, respectively, an integer of from 0 to 25, and compounds of the formulae (2) and (3) CH₃(CH₂) m C≡C-C≡C(CH₂) n -SiHCl₂ (2) CH₃(CH₂) m C≡C-C≡C(CH₂) n -SiH₂Cl (3) wherein m and n have, respectively the same meaning as defined above. Of these, the compound of the formula (1) is preferred because the adsorption density is higher. The preparation of these compounds is described, for example, in United States Patent No. 4,761,316, For instance, the silane compound of the above formula is dissolved in a mixed solvent of 80% of n-hexane, 12% of carbon tetrachloride and 8% of chloroform at a concentration with the above-defined range. When the substrate is immersed in the solution, the bonding of as 3 in Fig. 1(c) is formed on the SiO₂ surface. If the solvent is removed, the monomolecular adsorption layer is formed on the substrate by the chemical adsorption. It will be noted that Fig. 1(c) is enlarged view of A of Fig. 1(b). When the chemical adsorption is carried out at a temperature significantly lower than the glass transition temperature of the silane compound used, the vibration energy of the methylene chains and the end methyl group in the molecule is low. Under these low temperature conditions, the diacetylene bonds 4 are oriented in a direction vertical to the substrate as is particularly shown in Fig. 1(c). For instance, when the silane compound of the formula (1) where m = 11 and n = 8 is used to carry out the chemical adsorption out at a temperature of from 10 to 20°C which is significantly lower than the melting point of about 70°C or the glass transition point of about 40°C of the silane compound, the vertical orientation is ensured. Fig. 1(d) shows selective heating of the monomolecular film 2. When the film 2 is selectively applied with heat (5) such as from an IR source as shown, the molecules in region 6 of Fig. 1(e) applied with the heat 5 are oriented obliquely. Fig. 1(e) is an enlargement of B of Fig. 1(d). As a result, the monomolecular adsorption film has regions 7 where the molecules are oriented vertically with respect to the substrate and a region 8 where the molecules are obliquely oriented. This is shown in Fig. 1(f). It will be noted that the silane compound having a diacetylene bond is not polymerized by application of heat. In general, where diacetylene compounds are polymerized by irradiation of an energy beam, the molecules which are to be polymerized are more reactive when the diacetylene bonds are oriented obliquely. Thus when an energy beam 9 is applied over the entire surface of the film 2 as shown in Fig. 1(g), it is possible that the region 7 where the molecules are vertically oriented remains unreacted and the region 8 where the molecules are oriented obliquely undergoes the reaction of the diacetylene bonds to form a polydiacetylene bond 10 in a very high efficiency as shown in Fig. 1(h). It will be noted that Fig. 1(h) is an enlargement of C in Fig. 1(g). The resultant film 2 is shown in Fig. 1(i) wherein it has a pattern with a region 11 having electric conductivity or non-linear optical properties and a region 12 exhibiting no electric conductivity or optical properties. This monomolecular film may have utility in the fields of molecular elements or optics. The energy beam used in the present invention is, for example, a far UV ray having a wavelength of 250±10 nm, at which the diacetylene bonds undergo polymerization, an X-ray or an electron beam. Reference is now made to Fig. 2 wherein a second embodiment of the invention is illustrated. In the figure, the reference numerals indicate the same members or parts as in Fig. 1. In Fig. 2(a), the substrate 1 having the SiO₂ layer 1a is similar to that of Fig. 1(a). The chemical adsorption is carried out in the same manner as in Fig. 1 except that the silane compound is applied at a temperature between the glass transition temperature and the melting point, for example 50°C, thereby forming the monomolecular adsorption film 2 as in Fig. 2(b). Fig. 2(c) is an enlargement of D in Fig. 2(b), from which it will be seen that the diacetylene bonds 13 of the film 2 are oriented obliquely with respect to the substrate since the application temperature is high. More particularly, the adsorption temperature of 50° is between the glass transition temperature (about 40°C) and the melting point (about 70°C) of the afore-indicated silane compound. These temperature characteristics, of course, depend on the type of silane compound. Accordingly, adsorbed molecules can move when they have heat energy. This results in the oblique orientation of the diacetylene bonds. Similar results may be obtained with respect to the orientation when the silane compound is applied at lower temperatures and the resultant film is subsequently heated to a higher temperature as used in the above procedure. When the monomolecular adsorption film 2 is applied or irradiated with the energy beam 9 such as a far UV ray over the entire surface of the film as shown in Fig. 2(d). The results are shown in Fig. 2(e) which is an enlarged view of E in Fig. 2(d). More particularly, all the molecules in the film 2 are oriented at such an angle as to allow ready reaction of the diacetylene bonds with one another, so that a very long polydiacetylene bond 10 is formed as shown. Since the monomolecular adsorption film 2 is formed along the substrate 1 through the =SiO₂ bonds, a monomolecular adsorption film 11 of the polydiacetylene having a very long polydiacetylene bond is formed in this step, which is substantially free of any defect such as a pinhole or void as is shown in Fig. 2(f). In Fig. 3, there is shown a UV absorption spectrum of a monomolecular absorption film of the silane compound of the formula, CH₃(CH₂)₁₁C≡C-C≡C(CH₂)₈-SiCl₃ in the region 7 of the first embodiment shown in Fig. 1(h). In the region 7, the film does not undergo any polymerization when a far UV ray with an illuminance of 0.24 mWatt when determined at 365 nm is used for spectrum measurement for different times of 0, 10, 15 and 90 minutes corresponding to the spectra a to d in the figure, respectively. Likewise, Fig. 4 shows a UV absorption spectrum of the region 8 in Fig. 1(h) where the silane compound is polymerized through the diacetylene bonds under the same conditions as indicated above. The irradiation times for the spectra a to d in Fig. 4 are the same as those in Fig. 3. From the results of Fig. 3, it will be seen that no absorption of polydiacetylene takes place. Fig. 4 reveals that the absorption of the polydiacetylene having λ max at 650 is confirmed and becomes greater for a long irradiation time.

Claims: A method for the formation of a polymerised monomolecular adsorption film which comprises: providing a substrate having, on the surface, a layer capable of reaction with a silane compound having a diacetylene bond in the molecule; subjecting the silane compound to chemical adsorption with the layer to form a monomolecular adsorption film of the silane compound on the substrate through the layer wherein the chemical adsorption is carried out either a) at a temperature not higher than 20°C whereby the molecules in the adsorption film are oriented in a vertical direction with respect to the substrate and the resulting film is heated to a temperature sufficient for the molecules to adopt an oblique orientation, or b) at a temperature between the glass transition temperature and the melting point of the silane compound whereby the molecules in the adsorption film are oriented in an oblique direction with respect to the substrate; and c) irradiating the film with an energy beam to form a conjugated diacetylene polymer film. A method according to claim 1, wherein said substrate is made of silicon and said layer is an SiO₂ layer. A method according to claim 1 or 2, wherein the chemical adsorption is carried out at a temperature not higher than 20°C, and the adsorption film is heated in a desired pattern to a temperature sufficient to cause the molecules of the silane compound in the heated portions to be oriented obliquely. A method according to claim 1, 2 or 3 wherein the chemical adsorption is effected at a temperature between the glass transition temperature and the melting point, and the adsorption film is irradiated with an energy beam capable of conversion of the diacetylene bonds into a conjugated diacetylene polymer, so that a conjugated diacetylene polymer is formed entirely in the film. A method according to any one of the preceding claims wherein said silane compound is of the formula CH₃(CH₂) m C≡C-C≡C(CH₂) n -SiCl₃ wherein m and n are, respectively, an integer of from 0 to 25. A method according to any one of the preceding claims wherein the chemical adsorption is effected by immersing the substrate in a solution of said silane compound in a solvent at a concentration of from 2 x 10⁻³ to 5 x 10⁻² mols/liter.

Inventors: OGAWA KAZUFUMI, TAMURA HIDEHARU, OGAWA, KAZUFUMI, TAMURA, HIDEHARU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1989
Main classification: B05D1
IPC codes: B05D1, H01L21