Fabrication of ultrahigh aspect ratio freestanding gratings on silicon-on-insulator wafers

Abstract

Articles you may be interested inFabrication of nanoscale, high throughput, high aspect ratio freestanding gratings J. Vac. Sci. Technol. B 30, 06FF03 (2012); 10.1116/1.4755815 High-efficiency, large-bandwidth silicon-on-insulator grating coupler based on a fully-etched photonic crystal structure Appl. Phys. Lett. 96, 051126 (2010);The authors report a silicon-on-insulator ͑SOI͒ process for the fabrication of ultrahigh aspect ratio freestanding gratings for high efficiency x-ray and extreme ultraviolet spectroscopy. This new grating design will lead to blazed transmission gratings via total external reflection on the grating sidewalls for x rays incident at graze angles below their critical angle ͑about 1°-2°͒. This critical-angle transmission ͑CAT͒ grating combines the alignment and figure insensitivity of transmission gratings with high broadband diffraction efficiency, which traditionally has been the domain of blazed reflection gratings. The required straight and ultrahigh aspect ratio freestanding structures are achieved by anisotropic etching of ͗110͘ SOI wafers in potassium hydroxide ͑KOH͒ solution. To overcome structural weakness, chromium is patterned as a reactive ion etch mask to form a support mesh. The grating with period of 574 nm is written by scanning-beam interference lithography ͑SBIL͒ which is based on the interference of phase-locked laser beams. Freestanding structures are accomplished by etching the handle and device layers in tetramethylammonium hydroxide and KOH solution, respectively, followed by hydrofluoric acid etching of the buried oxide. To prevent collapse of the high aspect ratio structures caused by water surface tension during drying, the authors use a supercritical point dryer after dehydration of the sample in pure ethanol. The authors have successfully fabricated 574 nm period freestanding gratings with support mesh periods of 70, 90, and 120 m in a 10 m thick membrane on ͗110͘ SOI wafers. The size of a single die is 10ϫ 12 mm 2 divided into four 3 ϫ 3.25 mm 2 windows. The aspect ratio of a single grating bar achieved is about 150, as required for the CAT grating configuration.

Densely Packed Arrays of Ultra‐High‐Aspect‐Ratio Silicon Nanowires Fabricated using Block‐Copolymer Lithography and Metal‐Assisted Etching

“…It is commonly observed that high-aspect-ratio structures cluster at their tips, due to van der Waals or electrostatic charges on the newly formed surfaces, [31] facilitated by capillary forces present during drying after liquid immersion. [32] If the attractive forces between wires are greater than the force required to bend the wires, sheaflike structures result as the wires stick together, as shown in Figure 6a. The capillary force present during drying results primarily from surface tension at the solid/liquid interface as the liquid evaporates.…”

Edge-induced flattening in the fabrication of ultrathin freestanding crystalline silicon sheets

“…Fabrication of such membranes has been successful in metallic systems, 6,7 but has proven challenging in semiconductors like silicon. Freestanding silicon membranes have applications in electronic and photonic materials, [8][9][10][11] micromechanical devices, [12][13][14][15][16] x-ray optics, [17][18][19] macromolecular filters, 20 lithographic templates, 17,19 as sensors, 21,22 and as low-absorption supports in transmission electron microscopy. All of these applications benefit from flat crystalline structures with low lateral inhomogeneity.…”

Fabrication and performance of blazed transmission gratings for x-ray astronomy

“…15,20 The process improvements we have employed since these measurements were taken have greatly reduced the frequency of any SEM-detectable defects. Thus we believe that the diffraction efficiencies can still be improved.…”

“…We developed a fabrication process that uses a <110> silicon-on-insulator (SOI) wafer as a starting point with a device layer thickness equal to the desired grating thickness d. 20 The four main process steps are front side patterning, backside patterning and etching, front side etching, and supercritical drying. The resulting front side patterns consist of a chrome mask that defines a coarse support mesh, and a silicon nitride mask for the CAT grating bars, defined by scanning-beam interference lithography (SBIL).…”