Method for making thin silicone coatings.

0001The invention relates to a method for producing thin layers based on silicone by photocuring organosiloxanes and to the use of such thin layers.

0002Thin silicone-based layers serve, among other things, as a surface coating for a wide variety of materials. Paper and highly polymeric materials can be coated with silicones and metals can be coated. Silicon layers can also serve as the primary coating for optical fibers (see, for example: "Adhäsion", 29th year (1985), No. 10, pages 28 to 35, and "Plaste und Kautschuk ", 34th year (1987), No. 5, pages 183 to 190).

0003Unsaturated organosiloxanes are usually polymerized and crosslinked thermally with the addition of suitable initiators, such as peroxides, or catalysts, such as platinum compounds. The photochemical polymerization and crosslinking of unsaturated organosiloxanes is also known, ie the irradiation with light, in particular UV light. Without the addition of photoinitiators and / or photosensitizers, however, long exposure times are required for this. The direct photocrosslinking of organosiloxanes, without the addition of photoinitiators or photosensitizers, requires the presence of chromophores in the molecule, which interact with the light. Structural elements of this type are, for example, maleimide groups.

0004Non-functionalized organosiloxanes, such as poly (dimethylsiloxane) and poly (methylphenylsiloxane), are discolored when exposed to UV light; gel formation, ie crosslinking, takes place only in the presence of atmospheric oxygen (see: "Polymer Preprints", Vol. 20 (1979), No. 2, pages 443 to 446). When irradiated with a low-pressure mercury lamp, poly (methylphenylsiloxane) only forms a surface skin, with only partial crosslinking; after an irradiation period of more than 100 h, a change in the average molecular weight of less than 2% is found (see: "J. Polym. Sci. - Part A-1", Vol. 4 (1966), pages 2107 to 2120).

0005However, from the technical and economic point of view, the above-mentioned processes cannot produce high-purity and thermally stable thin films. Contaminants, in particular due to the required addition of photoinitiators and photosensitizers, have a technically disadvantageous effect. Discoloration, insufficient transparency and inadequate thermal stability, for example of thin layers made from silicone acrylates, are also disadvantageous in the case of UV crosslinking. The long exposure time required is economically disadvantageous (see, for example: "J. Organometal. Chem.", Vol. 148 (1978), pages 213 to 223).

0006The object of the invention is to design a method of the type mentioned in the introduction in such a way that thin layers based on silicone with high purity, thermal stability and good dielectric properties can be produced with short irradiation times by photocuring.

0007This is achieved according to the invention in that non-functionalized organosiloxanes with alkyl groups or alkyl and aryl groups are polymerized and / or crosslinked photochemically by means of pulse laser radiation with a wavelength of <400 nm, the pulse duration being 10 ps to 1 ms, the pulse frequency being 1 Hz to 10 kHz and the energy density is at least 1 J / cm² and the irradiation with one or more pulses follows.

0008It is essential in the process according to the invention that no additives, ie no photoinitiators and / or photosensitizers, are used. In addition, non-functionalized organosiloxanes are used in this process, ie compounds which have no functional groups. Rather, these compounds contain only alkyl groups or alkyl and aryl groups. The molar ratio of aryl groups to alkyl groups of the organosiloxanes used or mixtures made therefrom advantageously a maximum of 2.0; preferably the molar ratio is 0.001 to 0.5. The alkyl groups can be linear, ie unbranched, branched or cyclic in nature. The aryl groups can be alkyl substituted, for example to have a tolyl radical, and condensed systems such as naphthyl can also be present.

0009The organosiloxanes can be linear, branched or cyclic. It is essential that the organosiloxanes have a high degree of purity. When using the layers produced according to the invention in microelectronics, for example, both elements which influence the semiconductor function, such as sodium, and .alpha.-radiators, such as uranium and thorium, and elements which trigger corrosion, such as halogens, must be largely avoided.

0010The organosiloxanes generally have the following structure:<ul id="ul0001" list-style="none"><li>a) Linear connections:<chemistry id="chem0001" num="0001"><img file="EP0353583A2_D0001.tif" /></chemistry> R₁, R₂, R₃ and R₄ denote alkyl or aryl, where the presence of aryl groups means the above-mentioned ratio of aryl to alkyl groups; the following also applies: m, n ≧ 0.</li><li>b) Branch connections:<chemistry id="chem0002" num="0002"><img file="EP0353583A2_D0002.tif" /></chemistry> R₁, R₂, R₃ and R₄ denote alkyl or aryl, where the presence of aryl groups means the above-mentioned ratio of aryl to alkyl groups; the following also applies: m, o ≧ 0 and n ≧ 1.</li><li>c) Cyclic compounds:<chemistry id="chem0003" num="0003"><img file="EP0353583A2_D0003.tif" /></chemistry> The following applies: n 1 = 3 or 4 and n 2 = 3.</li></ul>

0011The organosiloxanes are subjected to radiation with a wavelength &lt;400 nm; the wavelength is preferably in the range between 190 and 300 nm. A poly (dimethylsiloxane) is advantageously used in the process according to the invention, with irradiation at a wavelength of 193 nm, or a poly (methylphenylsiloxane), with a wavelength of 240 to 280 nm is irradiated. The radiation itself is preferably carried out in an oxygen-free atmosphere. To increase crosslinking and / or to improve interface adhesion, the organosiloxanes can advantageously be added in small amounts of siloxanes containing vinyl groups, such as poly (methylvinylsiloxane); the proportion of such siloxanes is preferably 0.5 to 1.0% by mass.

0012Layers with a thickness of up to 100 μm are produced by the process according to the invention. These layers have a high level of purity and are thermally stable up to over 180 ° C, whereby long-term stability is of particular importance. The thin layers produced by the method according to the invention can advantageously serve as passivation and insulating layers for semiconductor components and electronic circuits. Furthermore, these layers can be used as structured passivation and insulating layers or as etching masks in the production of semiconductor components. For this purpose, the mask is irradiated during the production of the thin layers, ie a mask is arranged between the pulse laser and the organosiloxane to be irradiated, so that imagewise irradiation takes place.

0013The invention will be explained in more detail with the aid of exemplary embodiments.

example 1

Irradiation of a poly (methylphenylsiloxane)

0014A poly (methylphenylsiloxane) of the following structure<chemistry id="chem0004" num="0004"><img file="EP0353583A2_D0004.tif" /></chemistry> with a methyl / phenyl ratio of 20: 1 and a viscosity of 2000 mPa.s (at 25 ° C) is applied to a silicon wafer by means of spin coating. The irradiation takes place in a reaction chamber through which a nitrogen stream was passed for 15 minutes. A dye laser pumped with an Nd / YAG laser is used for the radiation, the following radiation parameters being observed: unfocused laser beam, wavelength λ = 266 nm, frequency γ = 10 Hz, pulse duration t = 10 ns, number of pulses n = 1000, energy density E. ≈ 30 J / cm². After irradiation, development is carried out by treatment with dichloromethane, the exposed area remaining on the silicon wafer; the layer thickness is approx. 20 µm.

Example 2

Irradiation of a poly (dimethylsiloxane)

0015A poly (dimethylsiloxane) of the following structure<chemistry id="chem0005" num="0005"><img file="EP0353583A2_D0005.tif" /></chemistry> with a viscosity of 1000 mPa.s (at 25 ° C) is applied to a silicon wafer by means of spin coating. The irradiation takes place in a reaction chamber which is operated at a pressure of 2 for 30 min. 10⁻⁵ mbar was evacuated. An ArF laser is used for the radiation, the following radiation parameters being observed: unfocused laser beam, λ = 193 nm γ = 50 Hz, t = 30 ns, n = 1000, E ≈ 74 J / cm². After irradiation, development is carried out by treatment with dichloromethane, the exposed area remaining on the silicon wafer; the layer thickness is approx. 2.5 µm.

Example 3

Irradiation of a poly (methylphenylsiloxane) with the addition of a siloxane containing vinyl groups

0016A poly (methylphenylsiloxane) according to Example 1, to which 0.5% by mass of a methylvinylsiloxane / dimethylsiloxane copolymer with a vinyl content of 7.5% was added, is applied to a silicon wafer and irradiated according to Example 1, with the following irradiation parameters being observed: unfocused laser beam, λ = 266 nm, γ = 10 Hz, t = 10 ns, n = 300, E ≈ 10 J / cm². After irradiation, development is carried out by treatment with dichloromethane, the exposed area remaining on the silicon wafer; the layer thickness is approx. 20 µm.