High-aspect ratio silicon structures by displacement Talbot lithography and Bosch etching

Abstract

Despite the fact that the resolution of conventional contact/proximity lithography can reach feature sizes down to ~0.5-0.6 micrometers, the accurate control of the linewidth and uniformity becomes already very challenging for gratings with periods in the range of 1-2 µm. This is particularly relevant for the exposure of large areas and wafers thinner than 300 µm. If the wafer or mask surface is not fully flat due to any kind of defects, such as bowing/warpage or remaining topography of the surface in case of overlay exposures, noticeable linewidth variations or complete failure of lithography step will occur.We utilized the newly developed Displacement Talbot lithography to pattern gratings with equal lines and spaces and periods in the range of 1.0 to 2.4 µm. The exposures in this lithography process do not require contact between the mask and the wafer, which makes it essentially insensitive to surface planarity and enables exposures with very high linewidth uniformity on thin and even slightly deformed wafers. We demonstrated pattern transfer of such exposures into Si substrates by reactive ion etching using the Bosch process. An etching depth of 30 m or more for the whole range of periods was achieved, which corresponds to very high aspect ratios up to 60:1. The application of the fabricated gratings in phase contrast x-ray imaging is presented.

Microfabrication of X-ray Optics by Metal Assisted Chemical Etching: A Review

“…Four inch wafer scale MacEtch gratings have been demonstrated with good uniformity and high control of trench profile and etching direction for aspect ratio up to 30:1 [39]. The process itself has no limits in terms of patterning area such as LIGA, photolithographic processes are available for Si based technology up to 12 inch wafer scale, periodic linear gratings with pitch size in the range of 100 nm can be patterned on wafer scale by interference lithography [89], nanoimprinting processes can be implemented to further increase the patterned area with nanoscale resolution [55]. High quality structures with high aspect ratio require an etching regime that is dominated by the diffusion of the reactive species.…”

“…The combination of unconventional processing and the freedom from microelectronics constrains enrich the spectrum of capabilities and give a new life to the "old silicon material", with revolutionizing advancements in nanotechnology [37]. We recently reported about micro-and nano-fabrication processing for X-ray gratings, including lithography [89], dry [37,90,91] and wet etching methods [39], Au electroplating [22], Ir atomic layer deposition [92] and metal casting [19,20]. The use of MacEtch as a microfabrication process for X-ray optical devices was first reported in 2014 for Fresnel zone plate structures [17,18].…”

Towards the Fabrication of High-Aspect-Ratio Silicon Gratings by Deep Reactive Ion Etching

“…Wafers with lower resistances (0.001–0.01 Ω·cm) are preferred to facilitate the following gold electroplating process, for filling the silicon templates with highly absorbing X-ray materials in order to create the absorption gratings [ 26 ]. Either standard photolithography (for gratings with periods larger than 2 μm) or displacement Talbot lithography (DTL) (for gratings with periods smaller than 2 μm) [ 15 ] was used for preparing the pattern of grating lines. MICROPOSIT S1800 series positive photoresists were used for standard photolithography, and SUMIRESIST PFI-88A7 positive photoresist together with AZ Barli-II bottom anti-reflective coating (BARC) were used for DTL.…”

“…For the fabrication of HAR gratings, especially G0 and G2 gratings, researchers have several mature technologies to choose from: LIGA (Lithographie, Galvanik und Abformung) [ 8 , 9 ], anisotropic wet-etching [ 10 ], metal assisted chemical etching (MacEtch) [ 11 , 12 , 13 ] and deep reactive ion etching (DRIE) [ 9 , 11 , 14 , 15 ]. The LIGA process relies highly on the availability of a synchrotron source; wet-etching gives very good sidewall smoothness but has poor etching depth uniformity and is very sensitive to precise crystallographic alignment; MacEtch could achieve an extremely high aspect ratio, but still it has limited reliability [ 13 ].…”

The Micro Solar Flare Apparatus (MiSolFA) instrument concept

“…New etching-based techniques (metal-assisted chemical etching: Romano et al (2016Romano et al ( , 2017, Bosch etching with seedless gold electroplating: Jefimovs et al (2017), Kagias et al (2019)) were attempted but failed to produce prototype gratings of functional thickness. However, success was found with the LIGA (X-ray lithography, electroplating and moulding) method employed by MicroWorks GmbH.…”