DD252846A1 - PROCESS FOR BORCARBIDE CARBON SHEET PRODUCTION BY DEPOSITION FROM THE GAS PHASE - Google Patents

Abstract

The process is used for surface finishing, especially wear reduction, but also for the production of layers with special physical properties. It is Z. B. applicable in mechanical engineering, electronics, electrical engineering. The process is characterized in that carbon and boron-containing components in a molecular gas mixture are simultaneously excited and dissociated by absorption of wavelength-selective pulsed laser radiation using real molecular transitions and chemically react and deposit a layer on a substrate under certain gas and laser intensity conditions.

Description

Field of application of the invention

The process is used for surface refinement, especially for the reduction of wear, but also for the production of layers with further special physical properties.

It can be used, for example, in mechanical engineering, electrical engineering, electronics and scientific instrument manufacturing.

Characteristic of the known state of the art

Boron carbide-containing or boron carbide layers are already known. In this case, gas mixtures of boron trichloride, hydrogen and hydrocarbons (or other gases of suitable composition) are thermally decomposed at very highly heated substrate surfaces by the CVD method (chemical vaper deposition method). In DD-PS 224339, C23 C, 11/08 X-ray amorphous solid solutions of boron carbide and carbon are described as very hard layers.

A disadvantage of these layers is the risk of substrate damage and the application of intermediate layers to avoid chemical reactions with the substrate material itself due to the high heating temperatures.

In DD-PS 226907A1, C23 C, 14/32 a process for the production of carbon film from the gas phase is shown. In this process, carbonaceous molecular gas mixtures (preferably low-atomic hydrocarbons) are dissociated by absorption of wavelength-selective pulsed laser radiation.

Carbon radicals in defined states of energy are provided which cause the formation of a carbon layer of defined structural composition on a substrate. The substrate is not thermally damaged in this method, but in this case the relatively low deposition rates are disadvantageous.

A method has also been proposed in which low-molecular hydrocarbon molecules in a defined volume in the vicinity of a substrate dissociate to carbon radicals or carbon-containing radicals by at least two IR laser pulses of different frequencies acting on them spatially completely overlapping in the region of the volume a carbon layer of defined structure and thickness is deposited on the substrate.

Disadvantages of this method are the relatively low deposition rates.

Object of the invention

The object of the invention is the preparation of boron carbide and boron carbide carbon layers having particular physical properties, e.g. B. amorphous carbon layers miteingebeten.kristallinen boron carbide phases up to amorphous solutions of boron carbide in carbon with high deposition rate and without thermal damage to the substrate to be coated.

Explanation of the essence of the invention

The invention has for its object to provide a method with which takes place without thermal stress of the substrate to be coated, a deposition of boron carbide and carbon-containing layers with selectively adjustable StöchiomQtrie and structure.

According to the invention the object is achieved in that the components of a molecular gas mixture consisting of low-atomic hydrocarbons, boron trichloride and hydrogen are excited and dissociated simultaneously by laser-induced sequential Mehrphotonenabsorption using real molecule transitions, the simultaneous excitation of the carbon and boron-containing reactants triggers a synergistic effect, the leads to a particularly high reaction rate of the solid state separation.

A preferred composition of the molecular gas mixture is BCI ₃ , C ₂ H ₄ and hydrogen.

The frequency of the laser radiation is determined by the bonding ratios of the selected gaseous starting materials and is in the infrared range of the electromagnetic spectrum between λ = 9nm and λ = 11 nm. There are tunable pulse laser with pulse lengths around Ιμ, β used. Depending on the spectral structure of the molecules used, the excitation takes place in the IR range at one wavelength or at two different wavelengths or by UV or a combination of IR and UV laser radiation.

The maximum values of the gas pressure and the laser intensity in the reaction chamber are to be limited so that the development of a laser-induced plasma is suppressed and the formation of a local thermal equilibrium and an increase in the translation temperature only after times that are greater than the reaction time of the reactants. The total pressure in the cuvette depends on the gas composition and is a maximum of 50 Torr.

The layers deposited on the substrate are in their stoichiometry and structure dependent on the composition of the gas mixture, the gas pressure and the structure of the substrate itself. The structure of the layers is also influenced by whether the focused laser beam is guided parallel or perpendicular to the substrate surface.

For a mixture of C ₂ H ₄ : BCl 3: H ₂ = 2Torr: 16Torr: 20Torr, a deposition rate of about 4 nm per laser pulse is achieved parallel to the focus from a substrate of NaCl with a gold layer. Information about the possible strength of deposition rates in the case of perpendicular laser irradiation is given by the layers deposited on the cuvette windows in the case of various molecular gas mixtures. For example, with a mixture of BCI ₃ IC ₃ H ₄ = 15Torr: 15Torr, they are still more than ₄ nm per laser pulse.

embodiment

The invention will be explained below by an embodiment:

In a brass cuvette with NaCI windows, previously evacuated to a pressure of less than 10 ^-3 Torr and purified by deliberate use of the gettering action of boron trichloride, BCI ₃ , H ₂ O, and oxygen, there is a mixture of purified gases of BCI ₃ and C ₂ H ₄ or BCI ₃ , C ₂ H ₄ or BCI ₃ , C ₂ H ₄ and H ₂ . In this gas mixture is focused with a NaCI lens, focal length: 10cm, the radiation of a TEA-CO ₂ laser. The pulse length of this laser is about 1.5 ^ s, with about 50% of the energy of the pulse on the start of the pulse with a length of about 100 ns omitted. Energy densities up to 200 J / cm ^{2 are achieved} for the used wavelength λ = 10.59 / nm (10 P [20] lens, the CO ₂ laser.) Parallel to the focus is in a few mm distance in the known manner open or closed reaction chamber within the cuvette the substrate of NaCI with gold layer

arranged. ·

Depending on the composition of the gas mixture, a layer on the substrate and the cuvette window forms more or less rapidly when laser bombarded. In a mixture C ₂ H ₄ : BCI ₃ : H ₂ = 2Torr: 16Torr: 20Torr z. B. achieved in parallel irradiation to the substrate with a gold layer deposition rates of about 4 nm per laser pulse. Thermal damage to the substrate is avoided by the parallel irradiation.

In the case of molecular gas mixtures in which layers on the cuvette windows also form on the cuvette window in parallel irradiation, for example with a mixture of BCI ₃ : C ₃ H ₄ = 15Toprr: 15Torr, the deposition rates are still more than 4 nm per laser pulse.

Analysis of the layers for the layer on the cuvette windows reveals that they are layers of amorphous carbon with embedded crystalline BC phases, while amorphous carbon layers form on the NaCl gold layer substrate.

Claims (9)

1. A process for boron carbide-carbon layer production by deposition from the gas phase by means of laser, characterized in that carbon and boron-containing components are excited and dissociated simultaneously in a molecular gas mixture by absorption of wavelength-selective, pulsed laser radiation using real molecule transitions and chemically. reacting and depositing a layer of defined stoichiometry and structure on a substrate located in a known manner open or closed reaction chamber, at a gas pressure and a laser intensity whose maximum values prevent the development of a laser-induced plasma and the formation of a local thermal equilibrium an increase in the translation temperature can occur only after times that are greater than the reaction time of the reactants. 2. The method according to claim 1, characterized in that the molecular gas mixture of boron trichloride, low-atomic hydrocarbons and hydrogen. 3. The method according to claim!, Characterized in that on an arranged parallel to the focus substrate at a mixture of C ₂ H ₄ = BCI ₃ IH ₂ = 2Torr: 16Torr: 20Torr an amorphous Borkohlenstoffschicht of about 4 nm per laser pulse is deposited. 4. The method according to claim 1, characterized in that the pressure of the molecular gas mixture in the cuvette is less than 50 Torr. 5. The method according to claim 1, characterized in that the excitation and dissociation is effected by IR radiation of a wavelength. 6. The method according to claim 1, characterized in that the excitation and dissociation by IR laser radiation takes place at two different wavelengths 7. The method according to claim 1, characterized in that the excitation and dissociation is effected by UV laser radiation. 8. The method according to claim!, Characterized in that the excitation and dissociation takes place in a combined field of IR and UV laser radiation. 9. The method according to claim 1 and one of claims 5 to 8, characterized in that the laser pulse duration is smaller than the translation relaxation times of the molecules.