METHOD OF MANUFACTURING A THIN, RADIOTRANSPARENT WINDOW

Abstract: A method of manufacturing a radiotransparent window. In order to achieve suitable radiation transmission, notably for long-wave X-rays, the window is made of a material containing exclusively light elements such as carbon and hydrogen. This material is deposited on a metal or semiconductor substrate, for example of silicon, after which the window material is exposed to such a dose of ion bombardment that the material is converted into glassy carbon. This carbon modification can be realised in very thin layers and is chemically very resistant, so that it can serve very well as an etching stop for etching off (a part of) the substrate material.

Description: Method of manufacturing a thin, radiotransparent window. The invention relates to a method of manufacturing a thin, radiotransparent window of a material having a low atomic number in which a layer which consists mainly of carbon and hydrogen is provided on an etchable substrate, the substrate with the layer being subjected to an etching operation during which the substrate material is etched off over at least a part of the surface of said layer. The invention also relates to a radiotransparent window manufactured by means of such a method, and to a radiation-optical analysis apparatus comprising a radiotransparent window manufactured by means of such a method. A method of this kind is known from United States Patent US 5,090,046. The cited document describes the manufacture of a window for an X-ray detector for soft X- rays whose X-ray transparent layer consists of a polymer consisting mainly of carbon and hydrogen, for example polyimide of a thickness of 0.5 μm. The substrate on which this layer is provided is formed by a metal plate consisting of, for example a copper alloy. In the course of the manufacturing process the plate is partly etched off in conformity with a masking pattern, thus forming a supporting grid for the thin X-ray transparent polymer layer. For such an etching process the substrate material, and hence the etching agents, should be carefully chosen so as to obtain a suitable etching effect without attacking the X-ray transparent film. Moreover, the polymer film is not capable of withstanding high temperatures which could be involved in the process for manufacturing the window. It is an object of the invention to provide a method of the kind set forth in which the window is provided with an X-ray transparent layer which is substantially less susceptible to a variety of etching agents and which is capable of withstanding high temperatures. To this end, the method of the invention is characterized in that said layer is exposed to such a dose of particle radiation that of said layer a carbon-like layer in the form of glassy carbon remains. It has been found that such glassy carbon is resistant to practically all etching agents and that it has such a chemical resistance that it disintegrates in air only when heated to a temperature of 400 °C. In an embodiment of the invention, said layer to be irradiated consists of photoresist. This material is very suitable for the formation of thin layers, so that the thickness of the X-ray transparent layer can be accurately controlled by the manufacturing process. In conformity with a further version of the invention, the particle radiation consists of ion radiation. It has been found that notably ion radiation of an energy of the order of magnitude of from tens to hundreds of kilovolts is effective in forming glassy carbon exhibiting the desired properties. For some applications, such as the use of a radiotransparent window for a gas discharge detector for X-rays, it is desirable that the window is electrically conductive to a given degree so as to prevent charging of the window by the discharge pulses in the detector. To this end, the window in conformity with a further embodiment of the invention is characterized in that said layer is exposed to a first dose of ion radiation so that of said layer a carbon-like layer in the form of glassy carbon remains, and to a second dose of ion radiation of a lower energy so that of said carbon layer in the form of glassy carbon a part which faces the radiation is given an electrical surface resistance which is lower than that of the remainder of said carbon-like layer in the form of glassy carbon. Tests performed on the window have shown that due to the second dose of radiation of lower energy, a part of said layer exhibits the desired electrical conductivity. A possible explanation could be that on the surface of this layer small regions of a somewhat graphite-like nature arise, without the entire surface starting to behave as graphite. In accordance with another embodiment of the invention, the substrate consists of a silicon wafer having a < 110> surface. When the pattern to be etched on the surface is suitably chosen, etching perpendicularly to the surface of the silicon wafer can be readily realised by preferential etching, so that the remaining silicon parts have exactly the correct shape as supporting profiles for the thin X-ray transparent layer. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 is a diagrammatic cross-sectional view of a gas-filled X-ray detector comprising an X-ray transparent window in accordance with the invention; Fig. 2 shows a relevant part of an analysis apparatus comprising an X- ray detector with an X-ray transparent window in accordance with the invention; Fig. 3a is a cross-sectional view of a window structure as used in a radiotransparent window in accordance with the invention; Fig. 3b is a general view of a silicon wafer provided with a mask pattern for a supporting profile for a window in accordance with the invention, and Fig. 3c shows a window cut from the silicon wafer shown in Fig. 3b. Fig. 1 shows an X-ray detector in which the X-ray window in accordance with the invention can be used. The detector comprises a housing 4 provided with an entrance window 2. The housing encloses a space 6 which contains a detector gas and in which further detector components are accommodated, for example an anode wire 8 which is insulated from the metal housing 4 by means of insulators 10. Incident X-rays 12 cause ionization of the detector gas 6 so that a charge pulse is intercepted by the anode wire 8; this pulse is further processed by processing equipment (not shown) to be connected to output 14. The entrance window 2 should be as thin as possible so as to minimize X-ray absorption; however, it should be thick enough to provide suitable gastight sealing in changing operating conditions, such as fluctuating temperatures and pressures, so that severe requirements are imposed as regards the strength of the window material and its support. Fig. shows a relevant part of an analysis apparatus in accordance with the invention. An X-ray source 26 generates an X-ray beam 12 which is incident on a specimen 28 to be examined. X-ray fluorescence in the specimen excites X-rays which are subsequently incident on an analysis crystal 20, via a first beam limiter 16 and a first collimator 18. Wavelength selection of the excited X-rays takes place in this crystal. The X- rays of the selected wavelength are ultimately detected by the X-ray detector 4. Before entering the X-ray detector 4 via the window 2, the radiation passes through a second beam limiter 22 and a second collimator 24 which is arranged against the X-ray window 2. This collimator is of the Soller type, i.e. it consists of a stack of mutually parallel plates of an X- ray absorbing material with a given spacing (as diagrammatically shown in the Figure) so as to allow passage to the X-rays which are thus parallelized. Fig. 3a is a sectional view of an X-ray transparent window 2. The window consists of a carbon-like layer 30 of glassy carbon and supporting profiles 32 of silicon. The radiation to be transmitted is incident perpendicularly to the layer 30 and between the supporting profiles 32. For the manufacture of the window use is made of a layer of a substrate material which can be completely or partly removed by etching. A large number of materials can be used in this respect, for example metals, semiconductor materials such as silicon (Si), germanium (Ge), indium phosphide (InP) and gallium arsenide (GaAs) which can all be etched by means of potassium hydroxide (KOH); it is also possible to use other materials, such as titanium carbide (TiC) or aluminium oxide (Al 2 O 3 ) which can be removed by sputter etching, or glass which can be removed by means of hydrogen fluoride (HF). Use is preferably made of a silicon wafer of a thickness of 0.2 mm as used for the manufacture of integrated semiconductor circuits. A wafer 34 of this kind is shown in Fig. 3b. If this wafer is a < 110> silicon wafer, perpendicular preferential etching is possible. Such a (circular) silicon wafer always comprises a straight part 36 at its circumference, being the reference edge or orientation flat {110} serving as a reference for the crystal orientation in the wafer. For preferential etching the surface of the wafer is provided with a mask pattern 38 with elongate supporting profiles 40. When these supporting profiles are provided at an angle of 35.3° (the angle between the 111 and the 110 direction) with respect to the reference edge, etching will take place perpendicularly to the wafer surface because of the preference of the etching pattern. As is generally known in the manufacture of integrated semiconductor circuits, the etching process then exhibits a preference for etching in a direction perpendicular to the wafer surface and in the longitudinal direction of the supporting profiles, whereas only a low etching rate occurs in the direction perpendicular to the supporting profiles. On the silicon wafer 34 there is provided a layer for which a large number of possibilities exist as regards composition but which must consist almost exclusively of light elements. Any hydrocarbon can be used for this purpose, for as long as this material can be deposited on the silicon wafer in the desired thickness. Deposition can be realised by spinning the material, by pulling the plate out of a solution containing the desired material, or by sputtering. Use is preferably made of a photoresist as is common practice in IC technology. This layer can be deposited with the desired thickness by spinning, the thickness preferably being 2 μm. It has been found that approximately 50% of the original layer thickness remains after the ion bombardment, so that in the present numerical example a layer of 1 μm remains. The thickness of the thinnest layer possible is of the order of magnitude of 10 nm; in that case the so-called "e-beam resist" is used for the layer to be irradiated. The largest thickness is of the order of magnitude of 5 μm; in the case of even larger thicknesses, there is a risk that internal stresses occur in the layer which is then liable to rupture. Bombardment can take place by a variety of particles, such as electrons, neutrons or ions. Ion bombardment is preferably used. The nature of the ions is not important; for example, nitrogen (N^, borium (B) or neon (Ne) can be used. The radiation dose at which "glassy carbon" arises can be readily determined experimentally; it has been found that the desired glassy carbon form is obtained in the case of a dose of between 10 M and 10 ions per cm 2 preferably between 10 15 and 10 16 ions per cm 2 . The ion acceleration voltage can also be readily determined experimentally; it also depends on the nature of the ions to be accelerated. As a rule of thumb it may be stated that a layer thickness of 1.3 μm (prior to bombardment) requires an acceleration voltage of 300 kV when bombarded by borium. Roughly speaking, for a layer which is n times thicker, the acceleration voltage must be n times higher, whereas for ions which are n times heavier, the acceleration voltage must be n times higher. For applications requiring an electrically conductive X-ray transparent layer, a second dose of ion radiation is applied with an energy which is substantially lower than that of the first dose. In said example involving borium ions, a second irradiation takes place with an acceleration voltage of 30 kV. The dose can be readily determined by measuring the resistance of the irradiated layer during irradiation. After the ion bombardment, the other side of the silicon wafer is provided with a mask pattern so as to form a supporting grid for the thin X-ray transparent layer. This pattern can be provided by means of a customary photoresist. Between the photoresist and the silicon a layer SiO 2 of a thickness of 50 nm is provided on the silicon by thermal deposition and on this layer a layer of Si 3 N 4 of a thickness of 120 nm is deposited by Low-Pressure Chemical Vapour Deposition (LPCVD). The pattern thus formed causes openings in the resist layer, after which the underlying Si 3 N 4 is removed by dry etching and subsequently the underlying SiO 2 is removed by wet etching. The remaining resist is subsequently removed and the silicon can then be etched off by means of KOH. This is a customary technique in IC technology. Fig. 3c shows a finished radiotransparent window made from the wafer 34 shown in Fig. 3b. The window has been removed from the wafer 34 along the cutting lines 42, together with a silicon part 44 so that it can be handled. In this window the supporting grid is visible which consists of the elongate supporting elements 40 on which the radiotransparent layer is provided.

Claims: CLAIMS: 1. A method of manufacturing a thin radiotransparent window of a material having a low atomic number, * in which a layer which consists mainly of carbon and hydrogen is provided on an etchable substrate, * in which the substrate with the layer is subjected to an etching operation during which the substrate material is etched off over at least a part of the surface of said layer, characterized in that * said layer is exposed to such a dose of particle radiation that of said layer a carbon-like layer in the form of glassy carbon remains. 2. A method as claimed in Claim 1, characterized in that said layer to be irradiated consists of photoresist. 3. A method as claimed in Claim 1, characterized in that the particle radiation consists of ion radiation. 4. A method as claimed in Claim 3, characterized in that said layer is exposed to a first dose of ion radiation so that of said layer a carbon-like layer in the form of glassy carbon remains, and to a second dose of ion radiation of a lower energy so that of said carbon-like layer in the form of glassy carbon a part which faces the radiation is given an electrical surface resistance which is lower than that of the remainder of said carbon-like layer in the form of glassy carbon. 5. A method as claimed in Claim 1, characterized in that the substrate consists of a silicon wafer having a < 110> surface. 6. A method as claimed in Claim 1, characterized in that said layer has a thickness of between 10 nm and 5 μm prior to irradiation. 7. A radiotransparent window provided with a layer of a material of a low atomic number on a substrate, characterized in that said layer of material consists of glassy carbon. 8. A radiotransparent window as claimed in Claim 7, characterized in that the substrate is made from a silicon wafer whose side facing said layer is a < 110> surface. 9. A radiation-optical analysis apparatus comprising a radiotransparent window as claimed in Claim 7 or 8.

Inventors: MAAS HENRICUS GODEFRIDUS RAFAE, POLITIEK JARIG, VERHOEVEN JOHANNES FRANCISCUS, MAAS, HENRICUS, GODEFRIDUS, RAFAEL, POLITIEK, JARIG, VERHOEVEN, JOHANNES, FRANCISCUS, CORNELIS, MARIA
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1995
Main classification: H01J5
IPC codes: G01T7, G21K5, H01J5, H01J47, H01L21