FABRICATION OF BLAZED DIFFRACTIVE OPTICS BY THROUGH-MASK OXIDATION

Abstract

A method for manufacturing a low-angle blazed grating on a semiconductor or silicon substrate, includes spin-coating the substrate with resist layer or hydrogen or polysilsesquioxane, being 100-1000 nm or few hundred nanometers thick, applying grayscale irradiation lithography exposure to the resist layer, generating a dose modulated pattern therein, varying in response to absorbed energy density from irradiation lithography exposure. The coated, irradiated substrate is developed in solution, such as TMAH or NaOH, enabling a blazed profile having structures of thickness-dependent diffusion barriers or SiO.sub.2, with 0-1000 nm height to emerge. Thermal oxidation in oxygen atmosphere at elevated temperature with the developed substrate, converts the upper silicon substrate layer into SiO.sub.2 to a depth depending on the thickness of the pattern in the resist layer above. Hydrofluoric acid fluid removes the SiO.sub.2, creating low-angle low-roughness blazed grating structure on silicon substrate.

Claims

1-5. (canceled)

6. A method for manufacturing low-roughness, low-angle blazed profiles or lines of blazed gratings, on a silicon substrate, the method comprising: a) spin-coating the silicon substrate with a layer of resist having a thickness of between 100 nm and 1000 nm; b) applying gray-scale lithography exposure to the spin-coated silicon substrate to generate a dose modulated pattern into the resist layer, the dose varying locally in response to a density of absorbed energy from the irradiation lithography exposure; c) developing the coated and irradiated silicon substrate in a suitable solution, enabling a profile including structures of thickness-dependent diffusion barriers with a height in a range from 0 nm to 1000 nm to emerge; d) performing thermal oxidation in an oxygen atmosphere at elevated temperature with the developed silicon substrate, converting an upper layer of the silicon substrate into silicon dioxide, to a depth depending on a thickness of the pattern in the resist layer above; and e) removing the silicon dioxide in a hydrofluoric acid fluid, creating the low-roughness, low-angle blazed grating structure on the silicon substrate.

7. The method according to claim 6, which further comprises using hydrogen silsesquioxane (HSQ) or poly-silsesquioxane having a thickness of a few hundred nanometers for the layer of resist.

8. The method according to claim 6, which further comprises using TMAH or NaOH as the solution.

9. The method according to claim 6, which further comprises using silicon dioxide (SiO.sub.2) for the diffusion barriers.

10. The method according to claim 6, which further comprises initially using a flat or curved polished silicon substrate as the semiconductor substrate.

11. The method according to claim 6, which further comprises using an e-beam or a direct laser writer in the gray-scale irradiation lithography exposure.

12. The method according to claim 6, which further comprises replacing steps b) and c) with nanoimprint lithography (NIL), and subsequently coating resulting structures with a single layer coating or a multilayer coating, producing devices with optimized reflectivity usable with multi-keV or hard x-rays or with EUV radiation.

13. The method according to claim 6, which further comprises using the method to produce masters for manufacturing blazed gratings by replication techniques or nanoimprint lithography.

Description

[0026] Preferred embodiments of the present invention are described hereinafter in more detail with reference to the attached drawings which depict in:

[0027] FIG. 1 schematically a process flow to generate a shallow-angle blazed grating on a silicon substrate;

[0028] FIG. 2 a) Atomic force microscopy (AFM) images of a demonstrator sample (left) and a blazed grating with 4 μm pitch (right) and b) a cross-section line scan measured on the blazed grating, demonstrating a 14 nm tall saw-tooth profile (upper image) with roughness of 0.35 nm RMS (lower image), fabricated using HSQ as a resist; and

[0029] FIG. 3 a) Atomic force microscopy (AFM) cross-section line scan measured on the blazed grating with 4 μm pitch, demonstrating pattern transfer for two different profiles exposed in poly-silsesquioxane and b) Using poly-silsesquioxane resist results in blazed structures with the roughness of 0.19 nm RMS, which is lower compared to the blazed gratings obtained with HSQ resist mask.

[0030] In the following, a description of the process flow as used with silicon substrate and HSQ e-beam resist is described in detail. This description also works in principle with any other kind of substrates possibly requiring an adaptation of the resist materials and the chemicals used during the developing and/or the oxidizing step. The oxidizing step could be also replaced by a reaction process with Boron in order to yield a borosilicate glass on silicon substrate which can be also removed selectively, for example with hydrofluoric acid.

[0031] As a starting material a polished monocrystalline silicon substrate has been used. A resist layer—in particular electron beam sensitive hydrogen silsesquioxane (HSQ)—was spin-coated on the silicon substrate to a thickness in the range of 200 nm and baked out at a moderate temperature in the range of 150° C. on a hot plate (FIG. 1a).

[0032] In the following, the grayscale electron beam lithography step has been performed. Dose modulated patterns were exposed into the HSQ resist, leading to local variations of the absorbed energy density inside the resist layer (see FIG. 1b).

[0033] After the development in a suitable developer, e.g. TMAH or NaOH containing solution, for a fixed time and at a fixed temperature micro- and nanostructures made of SiO.sub.x (with x almost equal 2) with varying thickness were obtained. A schematic representation of a blazed HSQ resist profile is depicted in the FIG. 1c).

[0034] During the subsequent oxidation step the relatively high and rough saw-tooth profiles of the HSQ resist were transferred into the surface layer of the silicon substrate. Due to the local difference in the thickness of the HSQ mask and due to reduction of the oxidation rate of silicon with increasing oxide layer thickness, the interface profile between the silicon substrate and the SiO.sub.2 becomes shallower and simultaneously smoother with progressing oxidation depth (see FIG. 1d).

[0035] Finally, the HSQ mask as well as the oxide layer were removed with buffered oxide etch (BOE), a hydrofluoric acid containing solution, as depicted in see FIG. 1e). The AFM measurements on resulting blazed grating structures created on the surface of the silicon substrate confirmed that this process enables the blazed profiles angles about few degrees only and have very low roughness, with RMS values well below 0.5 nm (see FIG. 2).

[0036] The described process works stable with silicon substrates, but it is not restricted to silicon only.

[0037] The pattern transfer into silicon was done by initiating the oxidation reaction of the substrate material in Oxygen atmosphere. For different semiconductor substrate materials chemical reactions involving other gaseous or liquid reactants might be more suitable.

[0038] The choice of the resist is not restricted to HSQ only. The poly-silsesquioxanes was successfully used instead of HSQ (for experimental proof see FIG. 3). Other inorganic or organic resists may be suitable, too. In this case a separate step may be required for the removal of the resist material after the oxidation step and before removal of the thermal oxide.

[0039] The diffusion of the reactant, e.g. Oxygen, through the resist mask has to exhibit clear mask thickness dependence, enabling the propagation of the mask surface profile into the upper layer of the substrate as the interface between the initial and the chemically modified substrate material during the through-mask oxidation step.

[0040] The main potential field of applications of this invention is the fabrication of the reflective optics in a broad range of wavelengths (from hard X-rays to IR), for example, diffraction gratings, Fresnel zone plates, beam shapers etc.

[0041] The resulting structures can afterwards be coated by a single layer coating or a multilayer coating to produce devices with optimized reflectivity that can be used with multi-keV or hard x-rays or with EUV radiation.

[0042] The present invention can be used for example to fabricate a master for replication of the blazed optical elements and devices by nano-imprint lithography (NIL) as well as to transfer a resist pattern created from a master by a nano-replication technique.