Process for making solid optical single and multiple interference layers.

p0001The present invention relates to a method for producing solid optical single and multiple interference layers of metal oxides on a substrate, which contains on the substrate per layer with an immersion solution by turning on and appearance of a liquid film, the reactive compounds from which subsequently, the layers are produced, is applied and the liquid film is converted into the solid layer.

p0002There already exist several methods for producing optical layers on a substrate.

p0003have become known in which, from DE-AS 10 62 901 (Schröder et al.) process for the preparation of sight glasses are applied dissolved in organic solvents, starting materials for the layers on the surface, the resulting film is dried, and thereafter the solvent and any volatile reaction products driven off by heating. As layer materials here are, in particular SiO₂ and TiO₂ and / or ZrO₂ in question.

p0004A review of such dip coatings are H. Dislich, Glastech. Ber.<u>57</u> (1984) 229ff.

p0005A method for producing transparent electrically conductive infrared-reflecting indium oxide layers, and in particular glass panes, is described in DE-OS 33 00 589th It consists in that the glass sheets are immersed in a first solution containing a hydrolyzable silicon compound and hydrolyzable compounds of titanium, zirconium, aluminum, tin or tantalum, that the glass sheets are heated in a moisture-containing atmosphere to a maximum of 450 ° C that the precoated sheets are immersed in a second solution containing hydrolyzable compounds of indium and of tin, that the discs are dried at temperatures below 250 ° C and that the slices are then heated in a reducing atmosphere to a maximum of 500 ° C.

p0006A disadvantage of the methods described above, that the applied liquid films as the precursors of the layers first dried and then must be transferred by an additional heating step in the final layer form.

p0007On the other hand, can be applied from the gas phase to the formation of thin layers on a solid surface, the laser-induced chemical deposition. A representative of the method referred to here is for example the "CVD". In this thermally induced material deposition process from the gas phase (CVD = chemical vapor deposition) are relatively high temperatures (600 to 900 ° C) required. In the plasma CVD, the temperatures can be through the use of electric discharges at 350 ° C to 450 ° C lower.

p0008there are three possibilities for the photochemical deposition of materials from the gas phase by IR or UV laser:

p0009When pyrolytic method, the laser light is not absorbed in general the reaction gas, but the substrate surface locally heated strongly on. Due to the resulting large temperature gradients, the material is deposited in a limited area end, so that the transport of the material from the gas phase can be effected from various directions.

p0010In the second option of the cut is made is released with a laser beam from a target instead of the otherwise mostly used Ionenzerstäubungstechnik; this results in a better targeted and more homogeneous beam of material going to landfills.

p0011In the photolytic process, the third possibility that the cut is made forming compounds are selectively brought into an excited state. The products of Laserphotodissoziation can either condense directly on the surface to be coated or react with other existing in the near-surface layer parts.

p0012These methods have in common that can be used as starting materials for the coating production only slightly-volatile and easily vaporize without decomposition compounds of the cut off material, for example SiCl₄, TiCl₄, Si (OR) ₄ R = alkyl; because of the strong Hydrolysetendenz of these substances, which have to be used for the reaction in a highly pure state but, high demands are made with regard to the impermeability to the devices for performing these methods.

p0013From DD-PS 244 704 another method is known, according to which a uniform coating of, for example, a liquid can be produced by irradiating with UV laser. When the coating is polymerized, transparent or pigmented inks or paints that must contain a photo initiator which is selectively excited by the laser light.

p0014Transparent coatings of this type have a thickness of 2 to 150 microns, so that optical interference layers by this method may not be obtained.

p0015Object of the present invention is to provide a method of the aforementioned type, with easily the liquid film produced in an immersion process can be converted into a layer of optical quality quickly and easily. Underlayer optical quality is a layer of uniform thickness, constant refractive index and of special surface quality, and low dispersion are understood throughout the volume of the layer. By about creating processes interference layers are to be produced; therefore, the requirements mentioned in layers of thickness must be fulfilled in the wavelength of visible light. The to be established method should be applicable to the diverse layers on various substrates.

p0016This object is achieved with a method in which the liquid film by exposure to high intensity radiation, suitable for very quick, direct, and limited to the thickness of the liquid film itself substantially heating to high temperature, which is necessary for conversion of the respective liquid film in the respective layer energy is supplied.

p0017The inventive method combines the advantages of dip coating - quick and easy manufacture of thin homogeneous films on large substrate areas, a large vorrichtungsmäßer effort is not necessary - with the advantages of laser treatment - very fast, direct and limited to the thickness of the liquid film itself essentially heating at high temperature -. In contrast to the known dip coating process with subsequent film forming pre-drying of the liquid film prior to conversion by heating the substrate including coating is eliminated.

p0018A preferred embodiment of the inventive method is carried out so that the conversion of the liquid film is already made during the emergence of the coated substrate. This embodiment is both time- and space-saving. It falls no "intermediate", since the conversion of the film has already been completed for a short time after the complete appearance. There is thus obtained almost immediately after the appearance of the end product.

p0019The advantage of the process of the invention is that in contrast to the known laser-assisted CVD process is not the reaction gas or the substrate to be coated is heated by absorption of the laser beam, but that the liquid film is directly absorbs the laser beam and is heated thereby. This happens very quickly (milliseconds), so that the directly heated thickness range of the film and the underlying substrate is generally at about 10 microns.

p0020The absorption rate of the CO₂ laser radiation as glass is approximately 3 x 10 cm. It follows that at a penetration depth of 1 micron is about 26%, at 5 microns about 78% at 10 microns and about 95% at 15 microns about 99% of the radiation be absorbed.

p0021Approximately applies for heating T an irradiated surface depending on the radiation intensity (= intensity) B in W / cm² and radiation duration t in sec: T_t = T_O + 7.17 √t. B T o = initial temperature.

p0022The usual for oxide layers immersion education temperatures, eg TiO₂ or SiO₂, are at about 450 ° C. the required temperature is, for a given irradiance of 2000 W / cm² reached in about 1/1000 sec. In comparison, the required times are the conventional heating in the range of several minutes.

p0023The required amount of energy that is required to achieve the required temperatures for the film formation is low for these reasons. In the method for forming dip layers previously used substrate layer and had to be brought together to the required temperature. Since this is no longer necessary, resulting in a significant energy saving.

p0024In contrast to the deposition from the gas phase as in the CVD method or the vacuum, the dipping method is characterized in that all of the material is applied in a single coating operation. All the more surprising, that can be obtained in the very short time available for stratifying available, at least equivalent, in the majority of cases even better layers qualities, as posted in the refractive index. For example, by the inventive process TiO₂ layers produced a substantially higher refractive index than the previously known TiO₂ submersible layers.

p0025The constancy of the refractive index in the entire layer volume and the layer thickness constancy, which is particularly suitable for the production of interference filters are important, outstanding.

p0026It was particularly surprising that even freshly prepared liquid films without any further purchase or pre-drying can be converted to the corresponding optical layer directly by irradiation with a laser, whereby the quality of the coatings was at least as good as that of layers produced by conventional methods.

p0027With the inventive method, layers can be applied on a variety of substrates. Particularly preferred are glass substrates in sheet or tubular form.

p0028In order to supply the necessary energy for converting commercially available lasers can be used with sufficient energy. Due to their high efficiency to CO₂ laser (wavelength of emitted monochromatic radiation 10.3 microns) can be used particularly advantageously. For liquid films which absorb strongly in the UV or in the visible, UV laser or lasers operating in the wavelength range of the visible, be used alone or in combination.

p0029As a further laser XeCl, Kr⁺-, ArF, Nd-glass and ruby ​​laser may be used.

p0030The intensity of coherent monochromatic radiation, which is supplied to the respective liquid film for the conversion, is from 300 to 5000 watts / cm², preferably 600 to 1000 Watt / cm².

p0031As already indicated, can by a suitable choice of the irradiation intensity (= intensity) depending on the duration of irradiation, the temperature reached by the irradiated liquid-film controlled and thus the specific for the respective liquid film forming temperature for the conversion to be set.

p0032The inventive method can be an extraordinary variety of metal oxide produced. Advantageously, the inventive method can be used in the preparation of layers of oxides of the following metals or mixtures thereof: Mg, La, Ba, Sr, Al, Ga, In, Si, Ge, Sn, Pb, Sb, Be, Te, Zr, Hf, Nb, Ta, Cr, Mo, W, Mn, Fe, Ru, Co, Rh, Ni, Pd, Zn, Cd, Ca and the rare earths.

p0033Another extraordinary advantage of the claimed method is that it is possible to produce semiconducting layer structures, such as antimony or fluorine-doped tin oxide layers and tin-doped indium oxide layers. The production of these films or film structures can be made directly in a forming gas or only in an oxidizing atmosphere, then but with the necessary training to the semiconductor properties step can subsequently be carried out by simple heat treatment in a forming gas.

p0034Which option is chosen, depends solely on the not going to explain here the possible procedural advantages of one or another way.

p0035In this way, a variety of other semiconducting layers to this simple type can be produced. should be mentioned: p-Cu₂O, n-TiO₂, CdO, Cd₂SnO₄, CdSnO₃, n-Fe₂O₃.

p0036Especially advantageous is a reducing treatment in a forming doped tin oxide or indium oxide layers with the inventive method indium, tin-indium and cadmium are formed with a UV laser, with the cadmium-doped layers depends on the concentration of cadmium not necessary to be needed.

p0037The invention will be explained by way of example to the production of a TiO₂ layer:

p0038An alcoholic solution of acetylacetone stabilized tetraethoxy (15-30 g TiO₂ / l) is applied in an air conditioned room to a glass substrate by the glass substrate carefully immersed in the solution and at a rate of 0.3 to 0.5 cm / sec is withdrawn. The coated substrate is 1 to 2 minutes left hanging and then bombarded with a CO₂ laser, said at a power density of 850 watts / cm², the time to form the TiO₂ layer about 10⁻² sec lasts. a second layer of for example SiO₂ can be applied to a thus produced TiO₂ layer. Here, an alcoholic solution of a defined anhydrolysierten silica methyl ester is used. The substrate provided with the TiO₂ layer is immersed in this solution and in turn pulled out depending on the desired layer thickness with a matched to the layer thickness rate, dried briefly at room temperature and also irradiated with a CO₂ laser. The training periods and the power density used can be selected as the TiO₂ layer.

p0039Advantageously, the application of the liquid films also be effected by spin coating, the layers can also be immediately thereafter formed with a CO₂ laser.

p0040For irradiating the liquid film with laser light can thus be taken that the coated substrate mounted on an xy "stage" and at a constant speed of in the range, for example, from 16.6 to 20 cm / sec at a CO₂-laser, the radiation of a emits intensity of 58 watts / cm², is passed. A TiO₂ layer thus produced has a refractive index of n_d = 2.40 and a SiO₂ layer correspondingly produced a refractive index of n _d = 1.46.

p0041In the process described defined interference layer systems can be produced in high quality, the produced at least equivalent to the usual method, are clearly superior in many cases. Especially in multi-layer systems, the time advantage over the conventional dipping process makes noticeably advantageous because the drying of the liquid film can be dispensed with in a separate operation in each case prior to conversion to the finished film.

p0042to be mentioned in particular the much higher refractive index thus produced TiO₂ layers (n_D = 2.40 to 2.60) in comparison to the previously described TiO₂- dipping layers (n_D = 2.0 to 2.3). SiO₂ layers produced by the novel process also have a slightly higher refractive index, which is close to the theoretically expected of quartz (1.46). The usual dip-coating method, the layers have to go through a defined and time-consuming annealing process. An advantage of the method described herein is that the substrate is heated only in the uppermost boundary layer and therefore cools very rapidly again after the conversion and the next layer can be applied by dipping, so that is why successive layers in multilayer interference Systemenen at shorter intervals in succession applied and the entire system can be made possible much faster than before.

p0043Using two-axis positioning with stepper motor controller directly simple structures can be produced, as can be generally are not acted upon by the laser places very easily removed by a suitable solvent. In particular, it is also possible by means of suitable masks microstructures generate up in the micron range.