Patent Number: 048624900
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIGS. 1 and 2, a vacuum window 10, in accordance with the present invention, includes a support substrate 12, a front membrane 14, and a back membrane 16. Substrate 12 can be made from many different types of materials including silicon, glass, quartz, sapphire, or tungsten, and is provided with a window aperture 18 which, in the illustrated embodiment, is substantially cylindrical. The front membrane 14 has a perimeter portion 20 attached to a front surface of substrate 12, and a window portion 22 aligned with window aperture 18. The window portion 22 has a number of pane openings 24 surrounded by a plurality of ribs 26, and a plurality of pane sections 28 formed within pane openings 24. The back membrane 16 is preferably made from the same material as the front membrane 14, and has a perimeter portion 30 attached to a back surface of substrate 12. Second thick membrane 16 is provided with a cylindrical aperture 32 which is aligned with the window aperture 18. Three materials that have been found to be suitable membrane material are boron nitride, boron carbide, and silicon carbide. All three of these materials have low atomic numbers and permit the formation of the thin pane sections 28 and thick ribs 26. Taking boron nitride as an example for the membrane material, a 37% transmission rate for soft x-rays can be obtained by making front membrane 14 four micrometers thick at the ribs 26, 0.1 micrometers thick at the pane sections 28, and by making the pane sections 68 micrometers square. A method for producing a vacuum window in accordance with the present invention starts with the selection and preparation of a suitable substrate material. As mentioned previously, a clean, polished silicon wafer has been found to be a suitable substrate. A relatively thick boron nitride membrane is grown on both sides of the silicon wafer using low-pressure chemical vapor deposition (LPCVD) techniques that are well known to those skilled in the art of integrated circuit manufacturing. For example, in a preferred embodiment, the boron nitride is deposited on the silicon wafers in a furnace tube at 470.degree. C. at a pressure of 900 m Torr with a flow of 11 standard cubic centimeters per minute (SCCM), of NH.sub.3 mixed with 145 SCCM of 10% diborane (B.sub.2 H.sub.6) in hydrogen dilution gas plus 345 SCCM of hydrogen carrier gas. The silicon wafers are serially arranged relative to axial flow of reactant gases within the furnace tube with the normals to their major face axially aligned with each other and parallel to the axis of revolution of the tube with 2 cm spacing between wafers. The deposition rate is approximately 1 .mu.m per hour and the deposition time is 6 to 8 hours to form a 6 to 8 .mu.m thick layer having a tensile stress of 1.times.10.sup.9 dynes/cm.sup.2. Thereafter, in a preferred embodiment, the wafer is coated by evaporation on both sides with a thin layer, as of 1000 .ANG. of Ni masking material. Next, a photolithographic process is used to pattern the thick boron nitride on the back side of the substrate to make a window aperture mask. The photolithographic process preferably includes the steps of applying a layer of photoresist to the boron nitride, curing the photoresist in a soft-bake cycle, exposing the photoresist through a suitable mask, developing the photoresist. The exposed nickel mask is patterned by etching in standard aluminum etch. Then, the remaining photoresist is removed. The photolithographic process is, once again, well known to those skilled in the art of integrated circuit manufacturing. After the window aperture mask is created on the back surface of the wafer, the relatively thick boron nitride on the front surface of the wafer is patterned to produce the pane opening sections and the ribs. At this point, the pane openings extend through the relatively thick front membrane to the upper surface of the silicon substrate. For example, in a preferred embodiment, a 1 .mu.m thickness of photoresist is spun onto the back side of the nickel-coated wafer. The photoresist is patterned to expose the nickel. Photoresist 1 .mu.m thick is then spun onto the front side of the nickel-coated wafer and patterned with the front side mask to expose the front side nickel through the photoresist. The wafer is then immersed in a wet etch for the nickel, as of conventional wet aluminum etchant commercially available from KTI of Sunnyvale, California, to expose the boron nitride on both sides of the wafer through the patterned openings in the nickel and photoresist masks. The boron nitride layers 16 and 14 are then plasma etched to expose the silicon through the boron nitride, Ni and photoresist masks. A suitable plasma etch is 96% CF.sub.4 and 4% O.sub.2 at 75 watts and 200 m Torr. The front side etch is stopped immediately upon etching through the boron nitride to the silicon so as not to pit or significantly etch the polished silicon surface. A residual gas analyzer is employed for analyzing the gaseous reaction products of the plasma etching process to determine when the silicon starts to be etched. Etching is terminated when these products are detected. The resist and nickel masks are then stripped, and the wafer is cleaned in boiling sulfuric peroxide, to assure particle-free pane openings. Next, a thin layer of boron nitride is deposited over the front layer of boron nitride to form thin layers or pane sections against the front surface of the wafer at the bottom of the pane openings. The pane sections are very uniform in nature, and are free of such defects as particles, pinholes and fractures because they were formed by deposition rather than by some other, less controllable process such as being etched down from a thicker deposition. For example, in a preferred embodiment, the thin layer of boron nitride, which forms the pane portions 28 of the x-ray window, is deposited in essentially the same manner as the aforedescribed thick membranes 14 and 16, except that the flow conditions are varied slightly to reduce the tensile stress of the deposited layer to about 2.times.10.sup.8 dynes/cm.sup.2. Suitable flow conditions into the furnace tube are 15 SCCM of NH.sub.3, 100 SCCM 10% diborane and hydrogen and 385 SCCM hydrogen. The deposition rate is about 1 .mu.m per hour and the deposition time is chosen to deposit between 1000 and 2500 .ANG. boron nitride onto the front surface covering the ribs 26 and exposed silicon at the bottom of the recesses defined between intersecting ribs 26. Thereafter, the wafer is diced, as by sawing to separate individual x-ray windows 10 from the wafer. The silicon substrate portion remaining under the pane portion 28 supports the pane 28 during the sawing operation and prevents fracture thereof by the sawing slurry and shock and vibration associated with sawing. Next, a silicon etching acid mixture is used to etch a window aperture through the wafer as defined by the window aperture pattern mask of the back layer of thick boron nitride. Finally, the vacuum window is cleaned and mounted in a suitable holder. For example, in a preferred embodiment, the individual window die are placed in a holder and immersed in a wet silicon etchant which will not etch the boron nitride. A suitable room temperature silicon etchant is the conventional isotropic silicon etchant consisting of 1 part nitric acid, 1 part hydrofluoric acid, and 2 parts of acetic acid, all by volume and of industry standard concentration. A preferred etchant is the same as above, except without the acetic acid constituent. The industry standard concentration of nitric, HF and acetic are 69-71%, 48-51% and 99.7%, respectively. In a preferred method for mounting the x-ray window in a suitable holder, the wafer, before dicing, is coated, as by evaporation, on its back side, overlaying the boron nitride layer 16, through a suitable mask with 300-500 .ANG. of either Cr, Ti or Ni, followed by 5000 .ANG. of aluminum. This back side metallization is confined by the mask to the periphery of the window frame portion. After dicing, individual die are anodically, i.e., thermoelectrically, or electric field assisted, bonded to a Pyrex glass holder having an opening aligned with the back side recess 18 of the x-ray window 10. Typical anodic bonding conditions are 3000 V negative applied to the glass relative to the potential of the silicon substrate 12 for 10 to 20 minutes at 250.degree. to 300.degree. C. While this invention has been described with reference to a single preferred embodiment, it is contemplated that various alterations and permutations of the invention will become apparent to those skilled in the art upon reading of the preceding descriptions and a study of the drawing. For example, another suitable membrane material for membranes 14 and 16 and panes 28 is silicon nitride. The etchants employed for the boron nitride examples above are also suitable for etching boron carbide, silicon carbide and silicon nitride. The silicon etchants above are also suitable for use with membranes of boron carbide, silicon carbide and silicon nitride.