In such cases the membrane has to withstand the forces exerted by the pressure difference between the gas and the vacuum chambers. In the case of the VUV light source, it is intended that the energy of the electron beam is deposited in the gas, however some amount of the energy is also transferred onto the membrane through collisions with the atoms and electrons of the membrane material, thus heating the membrane.

The energy deposited in the membrane reduces the available amount of energy to be deposited in the gas and in addition limits the operating parameters (e. g. beam intensity or pulse repetition rate), as the heating of the membrane weakens and ultimately destroys the membrane. The heating is generally countered by three cooling mechanisms: convective cooling (through the contact with the gas on one side of the membrane), conductive

cooling (through the contact with the material surrounding and holding the membrane), and radiative cooling (through the emission of electromagnetic radiation).

In the prior art (as in US 6,052, 401 and DE 44 38 407 C2) membranes made of silicon nitride have been used, because they absorb only about 10% of the beam energy when their thickness is reduced to about 300 nm. It is also possible to achieve pressure differences of up to 15 bar with membranes of about 300 nm thickness and about 0.7 mm width. However, membranes made of silicon nitride are hardly cooled by radiative cooling since they only have a small emissivity £ (2, T) as can be seen from the fact that they are perfectly clear to visible light. Since such membranes however are considerably weakened at temperatures above 1600 K, this limits the achievable beam current.

In the prior art, attempts have been made to increase the emissivity E (A, T) by "blackening"the surface of the membrane, e. g. black anodising, application of colloidal gold or copper, silver-sulfate or dendritic carbon structures of high thickness (> 1 ism).

However these materials have a high atomic number Z and, since the specific energy loss of charged particles is proportional to the number of electrons in the material (and therefore to the atomic number Z), the thickness of the black coating would have to be reduced considerably in order to reduce the energy loss of the particles. This however would again lead to a reduction of the emissivity.

Another approach for membranes that can transmit particle beams is the use of thin carbon foils (< 1 jj. m) which are used as electron strippers of ions or atoms having energies larger than 100 keV, known from [G. Dollinger and P. Maier-Komor, "Heavy ion irradiation damage in carbon stripper foils", Nucl. Instr. and Meth. A, 282,223 (1989) ]. Such foils are cooled through electromagnetic radiation. Being used as electron strippers, they are designed to withstand high irradiation damage, however they are not suitable to separate high gas pressures from vacuum.

It is therefore the object of the present invention to provide a membrane that is able to withstand high pressure differences, that can be transmitted by particle beams without causing too much energy loss to the beams, and that can be efficiently cooled by radiative cooling.

SUMMARY OF THE INVENTION This object is solved by the present invention by providing a membrane that is thick enough (but not thicker, to minimize losses in the membrane) to withstand the desired pressure difference, that consists of chemical elements with low atomic number Z to minimize the loss of energy of the particles of the beam and that comprises at least one layer having a high emissivity of electromagnetic radiation.

The emissivity of a material is a measure for the similarity of its surface to the surface of a black body. A black body perfectly emits and absorbs radiation of any wavelength and is thus most efficiently cooled by radiative cooling. The emission of black body radiation is given by Planck's radiation formula.

Real surfaces, on the other hand, emit thermal radiation in a similar fashion but to a lesser degree, reduced by a factor, the emissivity e (#, T) < 1. The emissivity #(#,T) equals the absorbtivity cT ? and can be calculated from the reflectivity p (,, T) and the transmissivity s (%, T) according to #(#,T) = α(#,T) = 1 - #(#,T) -#(#,T) (Kirchhoff's law), i. e. radiation that is neither reflected nor transmitted is absorbed and eventually re-emitted.

The wavelength x, at which the maximum of the spectral emission from a black body of temperature T is obtained and at and around which the emissivity £ (2, T) should thus be particularly high, is given by Wien's law: #max = 2900 µm K/T. For the highest allowable temperature for silicon nitride membranes, T= 1600 K, this yields #max = 1.8 µm.

In order to minimize losses in the membrane it has to be as thin as possible, while still being able to withstand the desired pressure difference. For membranes with a thickness x much smaller than the wavelength of the emitted radiation, £t (jí, T) = 4snsx/R is a good approximation for the emissivity s (R, T) as long as 8* (A, T) < 1 [G. Dollinger and P. Maier- Komor,"Heavy ion irradiation damage in carbon stripper foils", Nucl. Instr. and Meth. A, 282, 223 (1989) ]. Here n and rare the components of the coniplex refractive index n + iK: For A= 1.8 urn (corresponding to a temperature of the membrane of T = 1600 K) and x = 0.1 µm for example, one gets nK # 1. 4. Under this constraint, the transmissivity (n-1)2 + K2<BR> <BR> becomes negligible and 3 (#,T) = 1 - R, where R= being the reflectivity<BR> <BR> <BR> <BR> (n+1)2 + K2 under perpendicular incidence according to Fresnel's equations is used as an approximation for the total reflectivity of the membrane. R is minimise and therewith £ (R, T) maximised for n =, which gives a second constraint on n and m. The optimum regarding these two constraints is at n = 1.4 and y= 1 for =1. 8) J, m and x = 0.1 µm resulting in an emissivity #(#,T) = 0.83. It is therefore the object to find a material that has an emissivity close to this theoretical maximum.

Materials having a high atomic number Z have been found to be unsuitable for a membrane according to the present invention, since the specific energy loss of charged particles is proportional to the number of electrons in the material, and therefore proportional to the atomic number of the material.

Thermal evaporation-condensation of carbon results in membranes made from a graphite like modification of carbon with a thickness of about 100 nm having a measured emissivity of 0.7 over a wide spectral range (ca. 160 nm to 3000 nm). This is very close to the theoretical maximum for a membrane of such thickness. Such membranes are stable up to 2200 K, compared to 1600 K for membranes made from silicon nitride.

Another advantage of the increased emissivity of the membrane is the T4 dependence of the radiative cooling process. This smoothes out temperature gradients created by an inhomogeneous beam profile of the electron beam source or an inhomogeneous convective cooling by the gas much better than the processes of convective and conductive cooling which show a linear T dependence. Thus thermally induced stresses on the membrane are reduced.

On the other hand, an inhomogeneous beam profile or variations of the intensity of the electron beam can be easily observed by the glow of the membrane. This observation of the glow of the membrane can be employed in closed loop control systems for the beam intensity or in measurements of the beam current and the temperature of the membrane.

Membranes according to the present invention can also be used for the transmission of particle beams other than electron beams, e. g. beam of ions. According to the Bethe-Bloch

formula, the specific energy loss of ions in the membrane, which leads to heating of the membrane, is directly proportional to the atomic number Z of the material of the membrane. Hence it has been found that, in accordance with the present invention, materials having a low atomic number Z are preferable.

Furthermore membranes according to the present invention can be used for the transmission of electromagnetic radiation, e. g. X-rays and radio waves. Absorption of the electromagnetic radiation in the membrane, which again leads to heating of the membrane, increases also with increasing atomic number Z.

A preferred embodiment of the present invention is a membrane consisting of a graphite like modification of carbon made by thermal evaporation-condensation of carbon with a thickness of 20 nm to 300 nm, preferably 80 nm.

Other materials with low atomic number Z, i. e. containing mainly boron, carbon, and nitrogen (BCN-materials) are also well suited. Amounts of other light elements (H, Li, Be, O, F) have been found to enhance properties and there might be advantages to add small amounts of heavy elements. Also carbon modifications other than the already described evaporated-condensated carbon are well suited like tetragonal amorphous carbon, crystalline or nanocrystalline diamond, or other carbon or hydrocarbon films with varying content of graphite-like (sp2) and diamond-like (Sp3) chemical bonds.

Other methods for producing membranes with a high emissivity according to the present invention are, for instance, deposition of carbon by laser plasma ablation-deposition or laser induced arc. Membranes obtained from these processes have a lower emissivity than membranes made by thermal evaporation-condensation of carbon. However, thermal treatment of membranes produced by the processes mentioned above further increases the emissivity of the membranes.

Another preferred embodiment of the invention is a membrane as described above deposited onto a silicon nitride or diamond substrate. Such a membrane is able to withstand much higher pressure differences than the membrane as described above alone.

With silicon nitride membranes of about 300 nm thickness pressure differences of up to 15 bar have been achieved.

In yet another preferred embodiment of the invention the layer having high emissivity is separated from the substrate by a thin diamond-like carbon layer of a thickness of 5 nm to 100 nm, preferably 20 nm, applied to the substrate by laser beam evaporation- condensation of graphite. This additional layer improves adhesion of the layer having high emissivity to the substrate.

In still another preferred embodiment of the invention one or more functional layers are applied to one or both sides of the membrane. These functional layers can include, but are not limited to, protective layers, e. g. against scratching or reactions with the gas, for example made from silicon nitride.

The present invention relates also to sources for vacuum-ultraviolet (VUV) light utilizing any of the membranes described above. Such light sources usually comprise an evacuated chamber in which an electron source produces an electron beam of low energy (5 keV-40 keV). The electron beam is transmitted through a membrane into a chamber filled with pressurized gas (usually noble gases). The atoms of the gas are excited by the electron beam and form excimers (excited dimers) which emit light in the VUV spectral range when decaying. To keep the electron source as small and as cheap as possible, the energy loss of the electrons when passing the membrane must be minimized. The energy still deposited in the membrane leads to heating of the membrane, so efficient cooling must be applied to allow as high a current as possible to be transmitted through the membrane.

A further application for membranes according to the present invention is in devices where an electron beam is used to induce chemical reactions in gases. Such a device is similar to the VUV light source as described above, with an evacuated chamber comprising an electron source and a second chamber filled with the gas or gases in which the chemical reactions are to take place connected to the evacuated chamber via a membrane according to the present invention. The difference is that the main aspect of such a device is not the generation of light, but the inducing of chemical reactions.

The electron source can be operated either continuously or in a pulsed mode. In the continuous mode the energy of the beam is limited by the heating of the membrane.

Although in the pulsed mode the energy of the beam is limited by the heat capacity and the

specific heat of the membrane, the improved cooling of a membrane provided by the present invention will enable greatly increased repetition rates.

In the following a detailed description of some of the preferred embodiments of the present invention will be given in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The drawings show in FIG. 1 a diagrammatic sectional view of a first preferred embodiment of the present invention; FIG. 2 a diagrammatic sectional view of a second preferred embodiment of the present invention; FIG. 3 a diagrammatic sectional view of a third preferred embodiment of the present invention; FIG. 4 a diagrammatic sectional view of a fourth preferred embodiment of the present invention; and FIG. 5 a diagrammatic sectional view of a fifth preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a diagrammatic sectional view of the membrane (10) connecting an evacuated chamber (1) and a chamber (2) containing a pressurized gas. The membrane (10) is mounted on a frame (3), made for example from silicon, which leaves an opening (4) big enough for the electron beam (5) to be transmitted from the evacuated chamber (1) to the gas filled chamber (2). The membrane (10) consists of an entry film (16), made for example from silicon nitride, having a low emissivity of electromagnetic radiation and a coating (17), made for example from a graphite like carbon-modification, having a high emissivity of electromagnetic radiation, especially in the spectral range corresponding to the operating temperature of the membrane (10). The coating (17) is on the side facing the evacuated chamber (1) and is thus safe from chemical reactions with the pressurized gas in the second chamber (2).

FIG. 2 shows a diagrammatic sectional view of the membrane (20) connecting an evacuated chamber (1) and a chamber (2) containing a pressurized gas. The membrane (20) is mounted on a frame (3) similar to the frame shown in FIG. 1. The membrane (20) consists of an entry film (26), made for example from silicon nitride, having a low emissivity of electromagnetic radiation and a coating (27), made for example from a graphite like carbon-modification, having a high emissivity of electromagnetic radiation, especially in the spectral range corresponding to the operating temperature of the membrane (20). The coating (27) is on the side facing the gas filled chamber (2).

FIG. 3 shows a diagrammatic sectional view of the membrane (30) connecting an evacuated chamber (1) and a chamber (2) containing a pressurized gas. The membrane (30) consists of three layers: an entry film (36), made for example from silicon nitride, having a low emissivity of electromagnetic radiation, a coating layer (37), made for example from a graphite like carbon-modification, having a high emissivity of electromagnetic radiation, especially in the spectral range corresponding to the operating temperature of the membrane (30), and a further layer (38), made for example from silicon nitride, having a low emissivity of electromagnetic radiation. Since the coating (37) is covered on both sides, it is protected from physical damages and chemical reactions with the gas in the second chamber (2).

FIG. 4 shows a diagrammatic sectional view of the membrane (40) connecting an evacuated chamber (1) and a chamber (2) containing a pressurized gas. The membrane (40) consists of an entry film, made for example from a graphite like carbon-modification, having a high emissivity of electromagnetic radiation, especially in the spectral range corresponding to the operating temperature of the membrane (40). Due to the greater thickness of the membrane (40) its emissivity and thus the cooling is increased, as compared to FIGs. 1 to 3.

FIG. 5 shows a diagrammatic sectional view of the membrane (50) connecting an evacuated chamber (1) and a chamber (2) containing a pressurized gas. The membrane (50) consists of an entry film (56), made for example from a graphite like carbon- modification, having a high emissivity of electromagnetic radiation, especially in the spectral range corresponding to the operating temperature of the membrane (50), and one or more functional coatings (57). One of the functional coatings (57) can be made for example from silicon nitride. This embodiment combines the advantages of high emissivity of the entry film (56) with protection from chemical reactions with the gas in the second chamber (2).