Systems and methods for components for manufacturing photonic devices

Abstract

The present disclosure provides a method for fabricating high aspect ratio patterns as elements of a photonic device in a semiconductor substrate by using a continuous metal mesh and etching in the presence of air and an etchant in a continuous flow. In one method, a stable catalyst that forms a stable metal-semiconductor alloy allows etching of the substrate in the vertical direction even under conditions of very low oxidant concentration (e.g., the oxidant species present in air) without any external bias or magnetic field, thereby achieving very high aspect ratio structures in the semiconductor substrate. The metal layer on the semiconductor substrate reacts with the oxidant contained in the air and catalyzes semiconductor etching through the etchant. In one method, the etchant is supplied by evaporating a water-diluted HF solution. The continuous flow of air near the metal layer allows a constant oxidant concentration to be maintained near the metal layer. This facilitates the mass transfer of reactant species and etching by-products, and thus the process can continue for a long time to form very high aspect ratio structures. Once the etched semiconductor structure is formed, the continuous air flow supports the diffusion of reactant species through the etched semiconductor structure, thereby maintaining a uniform etching rate for the high aspect ratio structure. The continuous air flow supports the diffusion of reaction by-products, thus avoiding inhibition of the etching reaction. Structures with an aspect ratio of about 10,000:1 can be obtained by this method. The method has excellent pattern transfer ability at the nanoscale. Since the oxidant can be provided by ordinary air, the system has particular implementation advantages because it does not require any handling of hazardous and flammable gases (such as O2 gas) or unstable chemicals (such as H2O2).

Description

[0001] The present invention relates to a method for fabricating high aspect ratio patterns of elements that can be used as photonic devices in a semiconductor substrate.

[0002] Generally, photonic devices are components for generating, manipulating, or detecting light. This can include laser diodes, light emitting diodes, solar cells and photovoltaic cells, displays and optical amplifiers, diffraction patterns, periodic refractive and diffractive structures, gratings, and lenses.

[0003] In this context, metal-assisted chemical etching (MacEtch) is a technique capable of fabricating 3D nano- and microstructures of various shapes and applications, such as nanoporous layers, nanowires, 3D objects, MEMS, microfluidic channels, vias, X-ray optics, and sensor devices in a few semiconductors (Si, Ge, polycrystalline-Si (poly-Si), GaAs, SiC), and uses different catalysts (Ag, Au, Pt, Pd, Cu, Ni, Rh). In typical MacEtch, when a metal-patterned semiconductor substrate is immersed in a solution (electrolyte) containing an etchant (e.g., HF) and an oxidizer (e.g., H 2 O 2 ), local electrochemical etching occurs. The metal serves as a catalyst for H 2 O 2 reduction, and then holes are deeply injected into the valence band of the semiconductor. In the region around the metal catalyst, the hole concentration becomes higher, where the semiconductor is easily oxidized and removed by HF, forming reaction by-products such as silicon fluoride compounds. As the catalyst is pulled into the substrate, the reaction continues.

[0004] When the electrolyte evaporates and condenses on the surface of the metal-patterned silicon, the same reaction can occur. It has been demonstrated that the MacEtch reaction occurs at room temperature in the presence of aerated HF in a manner similar to the air corrosion of metals. Oxygen diffusion through the condensed HF / water layer limits the etching rate and the maximum etching depth, thus the maximum depth of 6 μm is etched within 3 hours. According to other sources, the etchant can evaporate from a liquid solution containing HF and H 2 O 2 and adsorb on a heated substrate (35 °C to 60 °C), where a condensed thin layer is formed and the MacEtch reaction occurs. MacEtch in the liquid phase demonstrates the ability to etch nanostructures (e.g., nanowires) with very high aspect ratios, but as a wet etching technique, it undergoes bending and coalescence during drying due to van der Waals forces and capillary forces between adjacent surfaces at the interface between the liquid and air. Nanostructure coalescence is highly undesirable and is considered a limiting factor in all applications where the surface is directly related to device efficiency (such as solar cells or sensor devices).

[0005] In fact, large bundles prevent conformal coating, deteriorate the optical properties of the nanostructure array, and may cause higher series resistance. A drying step after etching is required to minimize nanostructure coalescence. For example, critical point drying based on CO 2 shows excellent results, but it still requires additional processing steps as well as the use of high pressure and careful handling of the samples.

[0006] Patterning nanostructures requires high-precision pattern transfer and high lateral resolution during growth or etching. In the case of MacEtch in liquid, this corresponds to conditions of very high HF concentration in the etching solution. During MacEtch under high HF concentration conditions, poor adhesion of the Au catalyst to the silicon substrate and adverse pattern peeling have been reported. On the other hand, for nanoporous Au catalysts under conditions of low HF concentration and high oxidant (e.g., H 2 O 2 ) concentration, uniform high aspect ratios have been reported. Under these conditions, the etching is more isotropic, and the top of the trench appears wider relative to the bottom, compromising the fidelity of pattern transfer in the lateral dimension. Therefore, this process is not suitable for high aspect ratio nanostructures with high-precision pattern transfer.

[0007] It is necessary to perform MacEtch under conditions of low oxidant concentration and very high HF concentration. In addition, an effective catalyst must be selected to maximize the etching rate. Pt has a faster reported etching rate for MacEtch due to its excellent catalytic activity. The use of Pt as a MacEtch catalyst has been mainly studied in the form of nanoparticles or added as a top layer of a thick Au film. Pt has the advantage of forming stable silicides (PtSi and Pt 2 Si) on the Si surface at relatively low temperatures. The formation of Pt silicides has been widely reported in the literature for annealing temperatures in the range of 400 °C to 600 °C. The silicon oxide layer at the metal-substrate interface is usually a barrier layer for metal silicide formation, but it has been reported that Pt silicides also form in the presence of a native oxide layer. In the case of annealing in an oxidizing environment, an SiO 2 top layer may form.

[0008] It has been demonstrated that the use of an interconnected metal pattern effectively reduces off-vertical catalyst movement during MacEtch. The thermal anti-wetting of a thin platinum film provides a simple and low-cost method for producing an etching mask for fabricating semiconductor nanowires with large area dimensions. Anti-wetting occurs when a thin film on a solid substrate is heated, causing the film to aggregate. The film structure morphology (feature size, roughness, pore distribution) and pattern evolution largely depend on the film / substrate parameters (film material, film thickness, substrate material, defects) and experimental conditions (deposition rate, annealing temperature, annealing environment, etc.).

[0009] Some research fields such as X-ray optics, optical devices, microfluidics and bioengineering, thermoelectric materials, battery anodes, black silicon, solar cells, sensors, and MEMS technology can utilize the use of MacEtch as a nanofabrication technology and a microfabrication technology. In particular, MacEtch can have applications for manufacturing X-ray optical elements, such as gratings for grating-based X-ray interferometry, zone plates, speckles for speckle-based X-ray phase contrast imaging, and other optical diffraction structures that can be used as components of photon devices.

[0010] With the rise of X-ray grating interferometry, for conventional X-ray sources, phase contrast and scattering contrast have been obtained, thus promoting the potential for medical and industrial applications. This is achieved by using a grating with a micron-sized period that modulates the phase or intensity of X-rays. The key challenge currently faced is to fabricate such a grating to provide a high-contrast image over an extended area (at least a 4-inch wafer). Considering that the phase or intensity modulation ability of the grating is directly related to its height / depth, a high aspect ratio is required for applications operating in the X-ray energy range for medical or industrial use.

[0011] Therefore, the object of the present invention is to provide a method for fabricating a high aspect ratio pattern of a component (such as a diffraction grating) of a photon device in a semiconductor substrate. A photon device is a component for generating, manipulating, or detecting light. This can include laser diodes, light-emitting diodes, solar cells and photovoltaic cells, displays and optical amplifiers, diffraction patterns, periodic refraction and diffraction structures, gratings, and lenses.

[0012] According to the present invention, this object is achieved by a method for fabricating a component of a photon device using metal-assisted chemical etching in the gas phase, the method comprising the following steps:

[0013] (a) providing a semiconductor substrate and a patterned metal layer thereon;

[0014] (b) exposing the semiconductor substrate and the patterned metal layer thereon to a gaseous reactant, wherein the reactant comprises an oxidizing gas and an etchant gas, wherein the oxidizing gas comprises air and wherein the etchant gas comprises hydrofluoric acid, and wherein the reactant is supplied to the semiconductor substrate and the patterned metal layer thereon in a continuous flow or a pulsed flow, wherein the concentration of oxygen in the oxidizing gas is locally increased by decomposing H 2 O 2 on a platinum surface, the platinum surface being a solid piece containing platinum immersed in a liquid solution containing H 2 O 2 wherein H in the liquid phase 2 O 2 decomposes on the platinum surface to produce gaseous O 2 , wherein the liquid solution is placed in a container and the liquid does not contact the semiconductor substrate and the patterned metal layer thereon.

[0015] Furthermore, according to the present invention, this object is achieved by a method for fabricating an element of a photonic device using metal-assisted chemical etching with a reactant in a liquid phase or a gaseous phase, comprising a semiconductor substrate and a patterned metal layer thereon, wherein the semiconductor substrate and the patterned metal layer thereon comprise the following steps:

[0016] (a) forming a semiconductor oxide on the semiconductor substrate;

[0017] (b) forming a plurality of different metal layers in the patterned metal layer, wherein a first metal layer contacts the semiconductor oxide of the substrate and the last metal layer contacts the etching reactant, wherein the first metal layer comprises a metal that forms a stable metal-semiconductor alloy, and wherein the metal-semiconductor alloy comprises a compound selected from the group consisting of silicides and germanides of one or more metals selected from the group consisting of Pt, Pd, Cu, Ni, and Rh;

[0018] (c) heating the substrate and the metal layer thereon to simultaneously achieve the formation of the metal-semiconductor alloy and metal dewetting, wherein metal dewetting comprises forming an interconnected metal pattern having features, wherein the features of the interconnected metal pattern include holes in the metal layer, and wherein the characteristic size of the holes is at least 1 nm.

[0019] Accordingly, the present disclosure provides a method for fabricating high aspect ratio patterns of elements (such as diffraction gratings) as photonic devices in a semiconductor substrate by using a continuous metal mesh with a stable catalyst involved in forming a stable metal-semiconductor alloy and etching in the presence of air and an etchant in a continuous flow. The presence of the stable catalyst allows etching of the substrate in the vertical direction even under conditions of very low oxidant concentration (e.g., oxidant species present in air) without the need for any external bias or magnetic field, thereby achieving very high aspect ratio structures in the semiconductor substrate. Photonic devices are components for generating, manipulating, or detecting light. This can include laser diodes, light emitting diodes, solar cells and photovoltaic cells, displays and optical amplifiers, diffraction patterns, periodic refraction and diffraction structures, gratings, and lenses.

[0020] In a preferred embodiment of the present invention, the patterned metal layer may include an underlying layer of a metal-semiconductor alloy, wherein the metal-semiconductor alloy contains a compound selected from silicides and germanides of Pt, Pd, Cu, Ni, Rh.

[0021] Preferably, the oxidant gas may include air.

[0022] In a preferred embodiment of the present invention, the etchant may contain vapor-phase HF evaporated from a liquid solution containing HF diluted with water.

[0023] In a preferred embodiment of the present invention, the etchant may contain a solution of HF diluted with water in a liquid phase.

[0024] In a preferred embodiment of the present invention, the semiconductor substrate may include a semiconductor selected from Si, Ge, or group III to V semiconductors, and wherein the metal may include a metal selected from Au, Ag, Pt, Pd, Cu, Ni, Rh as a top catalyst.

[0025] In a preferred embodiment of the present invention, the semiconductor substrate and the patterned metal layer thereon may be heated to a temperature in the range of 30 °C to 90 °C during exposure to the oxidant gas and the etchant.

[0026] In a preferred embodiment of the present invention, the oxidant gas may be generated by decomposition on a platinum surface, and the platinum surface is a solid piece containing platinum immersed in a liquid solution containing H 2 O 2 diluted with water. 2 O 2

[0027] In a preferred embodiment of the present invention, the method may be carried out in the presence of an inert gas selected from nitrogen, argon, and helium.

[0028] In a preferred embodiment of the present invention, the method can be carried out in the presence of an alcohol selected from isopropanol, methanol, and ethanol.

[0029] In a preferred embodiment of the present invention, the oxidant gas and the etchant gas can be connected to a closed etching chamber in separate gas lines.

[0030] In a preferred embodiment of the present invention, the patterned metal layer can include a continuous reticular pattern, and wherein the etched semiconductor structure can include a nanowire array with an aspect ratio of at least 10:1.

[0031] In a preferred embodiment of the present invention, the patterned metal layer can include an X-ray diffraction grating pattern with periodic features, and wherein the etched semiconductor structure can include an X-ray diffraction grating with periodic features.

[0032] The preferred embodiments of the present invention are described in more detail hereinafter with reference to the accompanying drawings depicting the following:

[0033] Figure 1 Schematically shows a semiconductor substrate covered with a multi-layer metal catalyst. The semiconductor substrate can have a thin oxide layer. Metal n.l forms a stable metal-semiconductor alloy with the semiconductor substrate. The attached metal can consist of several metal layers.

[0034] Figure 2 Schematically shows a Si semiconductor substrate (A) covered with a Pt metal catalyst layer, undergoing metal dewetting to form a reticular pattern and forming Pt silicide to stabilize the catalyst (B); the etching mechanism in the presence of air and HF as an etchant in the vapor phase (C); the formation of an etched structure in the Si substrate (D).

[0035] Figure 3 Schematically shows an example of a device for manufacturing elements of a high aspect ratio photonic device using air as an oxidant and vapor HF as an etchant, and a side view (A) and a top view (B) of an open chamber. The figure is not drawn to scale.

[0036] Figure 4 Schematically shows in side view (A) and top view (B) an example of a device for manufacturing elements of a high aspect ratio photonic device using vapor HF and O 2 gas, the vapor HF and O 2 gas being supplied by the reaction of H 2 O 2 in a liquid solution containing HF, water, and H 2 O 2 with a solid Pt piece. The figure is not drawn to scale.

[0037] Figure 5 An example of an apparatus for fabricating elements for high aspect ratio photonic devices is schematically shown, having an enclosed etch chamber connected at least to an oxidant gas line and an etchant gas line and finally to an inert gas line. The figures are not drawn to scale.

[0038] Figure 6 A plan view scanning electron microscope (SEM) image of a Pt (bright contrast region) film on a Si (dark contrast region) substrate that has undergone metal anti-wetting by heat treatment at different temperatures is shown.

[0039] Figure 7 A plan view SEM (A) of a continuous reticulated patterned Pt layer on Si and cross-sectional SEMs of the etched samples after exposure to air and after HF generated by evaporation of a liquid solution containing HF diluted with water for 10 minutes (B) and 1 hour (C). High magnification SEMs of the cross-sections of the formed nanowire blanket (D) and the nanowire bottom (E).

[0040] Figure 8 The etch rate is shown as a function of substrate temperature (A) and HF concentration in the liquid solution (B), with the etchant evaporating from the liquid solution and the oxidant being air. Cross-sectional SEM (C) of etched nanowires with a length of 107 μm for 4 hours.

[0041] Figure 9 The etch rate is shown as a function of different alcohols in the liquid solution (A) and at different temperatures (B). The etchant evaporates from a liquid solution containing HF diluted with water and an alcohol, and the oxidant is air. Examples of nanowires obtained in the presence of isopropyl alcohol at 40 °C (C) and 55 °C (D). SEM (E) of the nanowires showing the etch depth difference at the metal pattern boundary and the decrease in nanowire length difference as a function of temperature and the presence of alcohol (F).

[0042] Figure 10 The processing steps for achieving high aspect ratio patterns in a semiconductor substrate are schematically shown: a resist layer covers the semiconductor substrate (A); the pattern is exposed and developed by lithography (B); a thin metal layer is deposited (D) and the resist is stripped (E); a continuous reticulated pattern with a stable metal-semiconductor alloy is formed by heat treatment (F); the metal patterned layer is exposed to an oxidant and an etchant (G), forming an etched structure with finally remaining nanowires (H).

[0043] Figure 11 Some examples of high aspect ratio X-ray diffraction optical elements obtained by the present disclosure are shown.

[0044] Figure 12, SEM images show details of etched silicon by MacEtch with air as the oxidant (A, B) and with HF and H 2 O 2 (C, D) in the liquid phase. The typical mesoporous structure of liquid-phase MacEtch of low-resistivity silicon is highlighted by the characteristic inverted V-shape (C, D).

[0045] A photon device is a component for generating, manipulating, or detecting light. This can include laser diodes, light-emitting diodes, solar cells and photovoltaic cells, displays and optical amplifiers, diffraction patterns, periodic refraction and diffraction structures, gratings, and lenses. The present disclosure provides a method for fabricating high aspect ratio patterns as elements of photon devices (e.g., diffraction gratings) in a semiconductor substrate by using a continuous metal mesh having a stable catalyst involving the formation of a stable metal-semiconductor alloy and etching in the presence of air and an etchant in a continuous flow. The presence of the stable catalyst allows etching of the substrate in the vertical direction even at very low oxidant concentrations (e.g., oxidant species present in air) without the need for any external bias or magnetic field, thereby achieving very high aspect ratio structures in the semiconductor substrate. The metal layer on the semiconductor substrate reacts with oxygen contained in the air and catalyzes semiconductor etching through the etchant. The continuous flow of air near the metal layer allows a constant oxidant concentration to be maintained near the metal layer. The etchant can be an HF solution diluted with water, or it can be provided by evaporating hydrofluoric acid from a solution containing HF diluted with water. The continuous air flow supports the diffusion of reactant species (e.g., oxygen and the etchant) through the etched semiconductor, thereby maintaining a uniform etching rate for the high aspect ratio structure. The continuous air flow supports the diffusion of reaction by-products to avoid inhibiting the etching reaction. Since the oxidant gas is provided by ordinary air, the system has particular implementation advantages because it does not require any handling of hazardous and flammable gases such as O 2 gas or unstable chemicals such as H 2 O 2 .

[0046] The method includes providing a semiconductor substrate and a metal pattern thereon. In certain embodiments, the semiconductor substrate can include an oxygen-terminated layer or a thin semiconductor oxide layer at the interface between the semiconductor bulk material and the metal layer. In certain embodiments, the metal pattern can be composed of a plurality of different metal layers. Figure 1 An example of the above multi-layer structure is reported in

[0047] The first metal layer is in contact with the oxygen-terminated surface of the substrate, and the last metal layer is in contact with the etchant reactant. The metal of the first layer is selected from a list of metals that form a stable metal-semiconductor alloy with the substrate. The metal of the last layer is selected from the following list of MacEtch catalysts: Ag, Au, Pt, Pd, Cu, Ni, Rh. In some embodiments, a single metal layer is selected, and the metal is selected from the following list: Pt, Pd, Cu, Ni, Rh. The metals in the above list can be used as catalysts for MacEtch and form stable metal-semiconductor alloys called silicides and germanides with Si and Ge as the substrate, respectively. Some examples of stable silicides that can be formed by thin-film reactions are: PtSi, Pt 2 Si, PdSi, Pd 2 Si, Pd 3 Si, Pd 4 Si, Pd 5 Si, Cu 3 Si, NiSi, Ni 2 Si, Ni 3 Si, Ni 5 Si 2 , Ni 3 Si 2 , Rh 3 Si. Some examples of stable germanides that can be formed by thin-film reactions are: PtGe, PtGe 2 , PdGe, Pd 2 Ge, Cu 3 Ge, Cu 5 Ge 2 , NiGe, Ni 5 Ge, RhGe, Rh 2 Ge, Rh 3 Ge, Rh 5 Ge 3 , Rh 3 Ge 4 .

[0048] Figure 1 An example of the metal layer structure is reported. The stable metal-semiconductor alloys formed with Si or Ge can be detected by XPS, TEM or RBS analysis.

[0049] In some embodiments, the semiconductor substrate with a metal pattern thereon is heated. During the heating, the semiconductor substrate with a metal pattern thereon is exposed to an oxidizing gas containing O 2 (e.g., air) in a continuous flow and an acidic gas containing HF, such as vapor generated by the evaporation of a liquid solution containing water-diluted HF. The reactant gas species (containing O 2Gases such as HF diffuse through the patterned metal layer and etch the metal-covered regions of the semiconductor substrate, thereby forming an etched semiconductor structure.

[0050] Once the etched semiconductor structure is formed, a continuous gas flow supports the diffusion of gas species through the etched semiconductor structure. This facilitates the mass transfer of reactant species and etch by-products, enabling the process to continue for a long time to form very high aspect ratio structures.

[0051] The presence of a stable catalyst that involves forming a stable metal-semiconductor alloy allows for uniform etching of the substrate in the vertical direction even under conditions of very low oxidant concentration and very dense patterns (such as X-ray diffraction gratings).

[0052] This method allows for very high etching rates in the range of 20 μm / hour to 24 μm / hour, comparable to the values of liquid-phase MacEtch. Referring to the previous report by Hu et al., where a maximum depth of 6 μm was achieved due to a series of 6 wet / dry cycles and an etching rate of 2 μm / hour, in the case of certain embodiments of the present disclosure, the etching rate is increased by at least 10 times. Relative to the previous report by Hu et al., where the length of the nanowires was limited to a maximum of 6 μm due to the limited diffusion of oxygen through the liquid etchant layer, this method allows for etching nanowires with a length at least 17 times longer.

[0053] The method of the present disclosure uses a very low oxidant concentration, which limits the injection of excess charge carriers from the metal catalyst into the semiconductor, which is the main cause of undesirable porosity in the etched structure. Therefore, the method of the present disclosure produces almost negligible porosity without the need for any external bias. Additionally, the process is very stable for any pattern size and feature without the need for any external bias or magnetic field. Relative to the previous report by Hildreth et al., the presence of a stable catalyst involving the formation of a stable metal-semiconductor alloy and a continuous reticular pattern allows for uniform etching of the substrate, where the depth and shape of the etched structure are uniform in the vertical direction.

[0054] As a MacEtch reaction, this method is a promising low-cost technology for producing high aspect ratio nanostructures over a large area by overcoming the limitations of other nanoscale gas-phase etching techniques (such as reactive ion etching). As a gas-solid reaction, it can be used for applications sensitive to viscosity without the need for an additional post-etch drying process. Relative to previous disclosures, the innovation of this method lies in using ordinary air as the oxidant gas instead of H 2 O 2 evaporated from a liquid solution containing water-diluted HF and H 2 O 2 vapor. Since H 2 O2 is a less volatile substance in the liquid solution. Therefore, relative to MacEtch in the liquid phase (e.g., 1%), it is necessary to significantly increase the volume of H in the solution 2 O 2 (e.g., 30%). The volume of H in the liquid solution 2 O 2 limits the amount of HF concentration in the etchant vapor. Therefore, the presence of H in the liquid solution 2 O 2 significantly reduces the concentration of HF in the gas phase. The method of the present disclosure maximizes the HF concentration in the etchant gas, having the advantages of extremely high pattern transfer accuracy and a very high etching rate in the range of 20 μm / hour. The advantage of this method is that it is carried out with materials sensitive to exposure to 30% H 2 O 2 , such as: copper, brass, carbon steel, cast iron, tungsten carbide, styrene-butadiene rubber, polysulfide polymer, thermoplastic elastomer, thermoplastic polyurethane, nitrile, neoprene, polyester elastomer, and polyamide.

[0055] In addition, the method has the advantage of avoiding handling high concentrations of H 2 O 2 while ordinary air is present everywhere and free. In addition, once a very high aspect ratio structure is formed, the presence of a continuous air flow helps to diffuse the reactive substances through the etched substrate. The continuous air flow through the etched substrate promotes the supply of reactive substances to the metal catalyst, thus allowing etching to continue for several hours. The continuous air flow along the surface of the etched substrate promotes the release and dispersion of reaction by-products such as water (which is not conducive to the viscous-sensitive nanostructures). In certain embodiments of the present disclosure, the etching is a "dry" process, which can be used for viscous-sensitive applications without the need for an additional post-etch drying process.

[0056] Referring to Figure 2 described is a method for fabricating high aspect ratio patterns (such as diffraction gratings and other diffraction periodic structures in a semiconductor substrate) in a semiconductor substrate by using metal-assisted chemical etching utilizing a continuous air and hydrofluoric acid flow. Then, Figure 3 , Figure 4 and Figure 5 describe some examples of systems for implementing elements for fabricating photonic devices using the method of the present disclosure.

[0057] First referring to the flowchart of Figure 2 , the method requires depositing a metal layer on the semiconductor substrate ( Figure 2A). The metal may include platinum (but not limited to platinum). The semiconductor substrate may include silicon (but not limited to silicon). The semiconductor substrate may include an oxide layer (such as a native Si oxide layer, but not limited to native silicon oxide). The method requires forming a continuous network pattern of the metal layer ( Figure 2 B). In certain embodiments, platinum is used as the metal layer and silicon with native silicon oxide is used as the semiconductor substrate. The continuous network pattern of the metal layer is formed by thin film dewetting. For a Pt film thickness of 10 nm, the dewetting temperature is about 250 °C.

[0058] The method requires forming a stable metal-semiconductor alloy, which serves as a stable layer for the metal catalyst between the metal layer and the semiconductor substrate. In certain embodiments, platinum is used as the metal layer and silicon with native silicon oxide is used as the semiconductor substrate. The stable metal-semiconductor alloy (e.g., Pt silicide, PtSi, Pt 2 Si) is formed by annealing at a temperature in the range of 250 °C to 600 °C. Pt silicide ensures that the metal adheres firmly to the Si substrate during MacEtch under conditions of high HF concentration. The method requires an oxidant and an etchant. In certain embodiments, the oxidant is air and the etchant is HF. In certain embodiments, the oxidant is air and the etchant is HF evaporated from a water-diluted HF solution.

[0059] The method requires exposing the semiconductor substrate and the patterned metal layer thereon to air and the etchant during heating, and the air and the etchant diffuse on the patterned metal layer ( Figure 2 C). In one example, silicon with native silicon oxide is used as the semiconductor substrate, and the remaining silicon oxide layer is etched away during exposure to HF ( Figure 2 C). The metal layer acts as a catalyst. The oxidant present in the air selectively oxidizes the region of the semiconductor substrate under the patterned metal layer, and the etchant selectively removes the oxidized region ( Figure 2 D). Thus, the metal-covered region of the semiconductor substrate is etched, causing the patterned metal layer to sink into the semiconductor substrate ( Figure 2 D).

[0060] Thus, an etched semiconductor structure is formed. The etching mechanism is reported in Figure 2 D and is described in detail below. The O 2 species present in the air diffuse on the patterned metal layer, and the metal acts as a catalyst for the following cathodic reaction:

[0061] O 2 + 4H + + 4e - → 2H2 O (1)

[0062] Therefore, hole charge carriers are deeply injected into the valence band of the semiconductor. The concentration of holes becomes higher in the region around the metal catalyst. Just below the metal layer, the current density of holes reaches its maximum value and becomes high enough to dissolve Si there (anodic reaction). According to the literature, Si can be dissolved using two different reactions, namely direct dissolution (Equation 2):

[0063] Si + 4h + + 4HF → SiF 4 + 4H + (2)

[0064] or by oxidation of Si (Equation 3),

[0065] Si + 2H 2 O + 4h + → SiO 2 + 4H + (3)

[0066] Then there is the dissolution of the oxide (Equation 4):

[0067]

[0068] As the catalyst is pulled into the substrate, the reaction continues. Etching in the gas-phase reaction occurs through a slow gas-solid reaction. H 2 O is formed as a by-product of the cathodic reaction (Equation 1) and can ultimately catalyze the anodic reaction of Si oxidation (Equation 3).

[0069] Figure 3 An example of a system for fabricating elements of a photonic device, such as a diffraction grating, is shown. The system includes:

[0070] 1) causing air to flow as an oxidant gas;

[0071] 2) evaporating HF from a liquid solution containing HF diluted with water;

[0072] 3) placing a semiconductor substrate having a metal pattern thereon on a heating holder;

[0073] 4) placing the semiconductor substrate having a metal pattern thereon close to the liquid solution, i.e., within a few centimeters;

[0074] 5) heating the semiconductor substrate having a metal pattern thereon and causing etching to occur through a gas-solid reaction in which no liquid condensation forms on the sample;

[0075] 6) Support the retainer on four spacers on the container of the liquid HF solution to form a reaction chamber with an open channel for air to flow in;

[0076] 7) Place the system on a workbench under an air laminar flow.

[0077] In this example, a sample including a patterned catalyst layer on a semiconductor substrate is supported on a hot plate or other heating system and maintained within a few centimeters above a liquid solution containing HF diluted with water. The system is implemented by modifying a simple commercially available vapor HF tool, keeping the liquid solution at room temperature, and holding the sample approximately 2 cm above the liquid solution by using an HF-compatible chuck with a resistive heating system and substrate temperature control.

[0078] The sample holder is placed on a set of four spacers made of Teflon, and the set of four spacers is placed on the boundary of the container of the liquid solution. This makes the etching chamber open and air can flow in easily. The system is placed on a workbench in an aerated environment under a laminar flow providing clean air. The innovative implementation of a conventional vapor HF tool lies in achieving an open etching chamber through a set of four spacers between the retainer and the liquid solution container. The air flow is achieved by placing the system in the air under a laminar flow, while conventional vapor HF tools are usually located in a fume hood with air suction.

[0079] In an etching system with an open chamber, air can flow in and diffuse over the patterned metal layer and through the etching structure. Referring to the previous report by Hu et al., where only air was used for drying and a long series of wet / dry cycles were used to achieve MacEtch of a silicon substrate, the innovation of this method lies in exposing the sample to air during the entire etching process, having the advantage of a continuous etching process. In addition, in this method, the sample is heated during MacEcth so that the MacEtch reaction occurs through a slow gas / solid reaction rather than a liquid / solid reaction as in the previous report by Hu et al. Once the etching structure is formed in the semiconductor substrate, the air flow present on the etching structure also helps to diffuse the reactant substances inside the etching structure and remove the reaction by-products.

[0080] The presence of the air flow is related to etching very deep semiconductor structures (such as trenches deeper than 10 μm) with a very high aspect ratio (e.g., an aspect ratio higher than 10:1). The sample holder has an HF-compatible chuck with substrate temperature control, and the sample is heated to a temperature within the range of 35 °C to 60 °C. The heating temperature has a relevant effect on avoiding water condensation and nanostructure adhesion. In addition, it is reported that the etching rate of wet MacEtch increases with temperature, so it is expected that the efficiency of the disclosed method increases with the increase of the reaction temperature.

[0081] Figure 4 Another example of a system for fabricating elements (e.g., diffraction gratings) of a photonic device using the method of the present disclosure is shown. Referring to Figure 4 , O 2 O 2 gas is generated in a liquid solution containing water-diluted HF and water-diluted H 2 and a solid platinum piece. Liquid H 2 O 2 decomposes on the surface of the solid platinum piece immersed in the liquid solution and O 2 gas is produced as a byproduct. O 2 gas forms bubbles in the liquid, and then the bubbles burst and release O 2 gas. O 2 gas can diffuse and reach the catalyst layer on top of the sample to be etched.

[0082] The O 2 O 2 gas obtained from the decomposition of H 2 on the platinum surface increases the O 2 concentration in the air to support MacEtch. The amount of O 2 gas released from the liquid solution can be varied by selecting the specific volume of water-diluted H 2 O 2 present in the liquid solution containing water-diluted HF and water-diluted H 2 O 2 . The amount of O 2 gas released from the liquid solution can be varied by selecting the specific surface area of the solid platinum piece to be immersed in the liquid solution containing water-diluted HF and water-diluted H 2 O 2 . The uniformity of the O 2 gas released from the liquid solution can be varied by selecting the specific shape (e.g., platinum wire mesh) of the solid platinum piece to be immersed in the liquid solution containing water-diluted HF and water-diluted H 2 O 2 . This embodiment of the method allows the concentration of O 2 gas in the air to be supplied by keeping the etching chamber closed. Relative to the previous reports of Hildreth et al., the method of the present disclosure allows a higher etching rate to be obtained because the concentration of the oxidant is increased relative to the concentration of evaporated H 2 O 2 .

[0083] Figure 5Another example of a system for fabricating elements of a photonic device using the method of the present disclosure is shown. This example includes at least two separate and independent gas lines: at least one gas line for an oxidant gas and at least one gas line for an etchant gas, each gas line being fluidly connected to an etching chamber. Additional gas lines may provide a non-reactive gas for purging (e.g., nitrogen or argon). A semiconductor substrate and a metal pattern having a stable metal-semiconductor alloy thereon are placed on a sample holder, which is placed in an enclosed etching chamber that can ultimately be evacuated. The sample holder can ultimately provide sample heating. The gas flow in each gas line fluidly connected to the etching chamber can be adjusted independently. The sample can be exposed to the oxidant and etchant gases by flowing both gases simultaneously or by flowing one gas at a time in the case of a final step of purging gas and a final step of chamber evacuation. The oxidant and etchant gases can flow and diffuse over the metal pattern, thereby forming an etched semiconductor structure. The sample can ultimately be heated during exposure to the oxidant and etchant gases.

[0084] The proposed etching tool is different from the etching tool of Hu et al. because the present method does not pass oxygen through a liquid HF solution. The innovative feature disclosed herein is the presence of separate gas lines for the oxidant and the etchant. In particular, in the present invention, the etchant gas can be anhydrous HF, and the semiconductor substrate having a metal pattern thereon is heated during exposure to the etchant atmosphere to minimize the presence of water because water condensation is not conducive to the production of high aspect ratio nanostructures.

[0085] Using a catalyst that has a highly efficient reaction with an oxidant (e.g., platinum), the method of the present disclosure can etch a semiconductor substrate in a gas atmosphere containing a very small amount of oxidant and a high concentration of etchant for several hours, thereby producing very deep trenches (e.g., 100 μm), large aspect ratio structures (in the range of 1000 to 10000 to 1), and very sharp features on the order of 1 nm to 100 nm. In certain embodiments, a self-assembled platinum metal pattern on top of a silicon substrate is used to produce a blanket of high aspect ratio silicon nanowires. In certain embodiments, heat treatment is used to cause dewetting of the platinum film, followed by the formation of a nanostructured metal pattern. Dewetting occurs for Pt deposition on an oxygen-terminated Si surface, while no dewetting is observed under the same experimental conditions when the native oxide is removed by immediately dipping the substrate into HF prior to Pt deposition.

[0086] Referring to Figure 6Describes an example of adjusting the size distribution of pores generated by the dewetting of a thin Pt film on a Si substrate with a native silicon oxide layer. In this example, the Si substrate with native silicon oxide is cleaned by oxygen plasma and then a Pt film is deposited by electron beam evaporation at a deposition rate of 0.5 nm / minute, and the Pt film thickness is in the range of 5 nm to 20 nm. The substrate with the metal film thereon is annealed in air at a temperature in the range of 250 °C to 600 °C to cause dewetting of the metal film. Figure 6 A through 6I show SEM images of the Pt film morphology at different dewetting temperatures. Referring to Figure 6 A through 6I, the metal has a bright contrast, while the pores show the silicon substrate in a darker gray. Thus, the metal layer is patterned into self-assembled nanostructures, and the metal pores have a size distribution in the range of a few nanometers to a few hundredths of a nanometer.

[0087] Thus, Figure 6 A through 6I of the perforated Pt film is an example of a self-assembled metal mask for nanowire realization by MacEtch. Dewetting (250 °C to 350 °C) occurs with a gradual increase in the film fracture density, and finally pore formation (400 °C to 500 °C) occurs, followed by a coalescence process of pore expansion (550 °C to 600 °C). Once the film thickness and deposition conditions are determined, the dewetting temperature can be used as an adjustment parameter for the characteristic size of the Pt pattern, and the average pore size increases from a few nanometers (<400 °C) to dozens of nanometers (450 °C to 550 °C) and hundreds of nanometers (>550 °C). For annealing temperatures in the range of 400 °C to 600 °C, Pt silicide formation has been widely reported in the literature.

[0088] The silicon oxide layer at the metal-substrate interface is usually a barrier layer for metal silicide formation, but it has been reported that Pt silicide is also formed in the presence of a native oxide layer. In the case of annealing in an oxidizing environment, SiO 2 top layer may be formed. In all Figure 6 asymmetries in pore growth during dewetting are observed, and it indicates silicide formation.

[0089] Referring to Figure 7 Describes the realization of nanowires by the method of the present disclosure using a Pt self-assembled metal mask through dewetting. Nanowires can be used as diffractive optical elements in spot-based X-ray phase contrast imaging. Compared with sandpaper or other films with feature sizes in the micron range, it is expected that nanowires can improve the sensitivity of spot-based X-ray imaging by generating spots with smaller sizes and better uniformity.

[0090] A thin Pt film is deposited on a Si substrate with a native silicon oxide layer, and the substrate with a metal film thereon is annealed in air at 550 °C to produce metal film dewetting. Figure 7 Scanning electron microscope (SEM) micrographs in plan view are reported in A. The substrate with a metal pattern thereon is heated at 55 °C for 10 minutes and then exposed to a vapor etchant. The vapor etchant is produced by using Figure 3 a system. The oxidant is provided by flowing air. The etchant evaporates from a liquid solution containing HF diluted with water, and the HF concentration in the liquid is in the range of 1 mol / l to 20 mol / l. The substrate with the metal pattern is held 2 cm above the liquid surface. The vapor etchant diffuses through the metal pattern, the silicon substrate behind the metal is etched, and the metal pattern sinks into the substrate. Thus, a silicon etch structure is formed. After 10 minutes of etchant exposure, the substrate is clearly etched, and the silicon etch structure looks like columns, as Figure 7 shown in the SEM micrograph in the cross-section of B.

[0091] Figure 7 The metal mask of A and Figure 7 the silicon pillars of B show a good match of the structural features, indicating that the method has excellent pattern transfer ability at the nanoscale.

[0092] Figure 7 C shows the silicon structure after one hour of exposure to the etchant. The silicon pillars are now 6 μm long and can be called nanowires. A uniform silicon nanowire blanket ( Figure 7 C) is formed by the method disclosed in the present invention. Figure 7 D shows a magnified image of the silicon nanowires. The top nanowires are well separated and have reduced agglomeration compared to the nanowires produced by the wet etching method. Figure 7 E shows a magnified image of the bottom of the silicon nanowires. The cross-section of the nanowires is measured by SEM and is in the range of 10 nm to 100 nm. The aspect ratio of the nanowires is calculated by the ratio of the average cross-section diameter (e.g., 10 nm to 100 nm) and the nanowire length (e.g., 6 μm). Thus, the aspect ratio is in the range of 60 to 1 to 600 to 1.

[0093] Figure 8 A reports Figure 3 the etch rate of the system shown in Figure 3 as a function of the heating temperature of the silicon substrate and the metal pattern thereon and the molar concentration of HF in the liquid solution containing HF diluted with water. The etch rate is calculated by measuring the length of the nanowires produced in 2 hours in the 2The square, silicon substrate is an N-type <100> single crystal with a resistivity in the range of 0.001 Ωcm to 0.01 Ωcm. The liquid solution has been obtained by adding deionized water to a commercially available 50% water-diluted HF solution. By increasing the temperature to the range of 35 °C to 40 °C, the etching rate increases, which is consistent with previous studies on MacEtch kinetics in liquids. For a high HF concentration (18 mol / l), the etching rate has a distinct maximum at 40 °C and then decreases as a function of temperature, indicating that the reaction rate is limited by HF desorption. For a low HF concentration (12 mol / l), the etching rate increases slightly with temperature, but over the entire temperature range (35 °C to 55 °C), the change in the etching rate is very small (15%). In fact, this shows a significant stable processing window where the decrease in HF concentration over time can have a negligible effect on the etching rate.

[0094] Figure 8 B shows Figure 3 the etching rate of the system shown in at 55 °C as a function of the molar concentration of HF in a liquid solution containing water-diluted HF. The liquid solution is obtained by adding deionized water to a commercially available 50% water-diluted HF solution. For very high HF concentrations, very high etching rates of 20 μm / hour to 24 μm / hour are reported, and these values are comparable to MacEtch in the liquid phase. Referring to a previous report by Hu et al., where a maximum depth of 6 μm was achieved due to a series of six wet / dry cycles and an etching rate of 2 μm / hour, using the method of the present disclosure, the etching rate is increased by at least an order of magnitude.

[0095] Figure 8 C shows a cross-section SEM of silicon nanowires with a length of at least 107 μm obtained by heating silicon with a platinum mask on it at 55 °C for 4 hours during exposure to air and evaporation of HF from a liquid solution with an HF molar concentration in the range of 20 mol / l to 29 mol / l. As a nanowire portion in the range of 10 nm to 100 nm, Figure 8 the aspect ratio of the nanostructures in B is in the range of 1000 to 1 to 10000 to 1. The nanowires have low agglomeration, indicating that the reaction occurs in the gas phase throughout the time. Relative to a previous report by Hu et al., where the nanowire length was limited to a maximum of 6 μm due to the limited diffusion of oxygen through the liquid etchant layer, the method of the present disclosure allows for etching nanowires with a length at least 17 times longer. At Figure 8 the bottom of the SEM image of C, the Pt catalyst layer is still visible and appears flat, indicating that the Pt catalyst layer remains stable even after long-term etching and exposure to a high concentration of HF gas.

[0096] The stability of the catalyst indicates that vapor MacEtch can continue and produce even longer nanowires. Thus, Figure 8 shows the ability of the present invention to etch extremely deep trenches with a huge aspect ratio (10,000 to 1) in silicon with very high precision.

[0097] In another example, an etchant is obtained by evaporating a liquid solution containing water-diluted HF and an alcohol as an additive. An alcohol with a low vapor pressure and low surface tension is used instead of water vapor as a catalyst to minimize the capillary force at the gas-liquid interface. The alcohol helps the vapor etching to proceed with less water condensation because it is highly volatile and tends to evaporate easily with water.

[0098] Figure 9 Results were reported using Figure 3 setups and methanol, isopropyl alcohol, and ethanol as additives in a liquid solution containing water-diluted HF. The samples were 1×1 cm Figure 6 squares cut from a silicon substrate with a platinum self-assembled mask, and the silicon substrate was an N-type <100> single crystal with a resistivity in the range of 0.001 Ωcm to 0.01 Ωcm. The HF molar concentration of the liquid solution was in the range of 1 mol / l to 20 mol / l, and the alcohol volume was in the range of 10% to 20% of the entire liquid solution. 2 A shows the etching rate calculated by measuring the length of the nanowires produced in 2 hours at 40 °C using the

[0099] Figure 9 system. As reported in Figure 3 A, the etching rate decreases in the presence of alcohol. In this particular example, isopropyl alcohol has the highest etching rate value. Figure 9 A, the etching rate decreases in the presence of alcohol. In this particular example, isopropyl alcohol has the highest etching rate value. Figure 9 B reports SEM images of nanowires produced by adding isopropyl alcohol to a liquid solution containing water-diluted HF and heating a substrate with a metal pattern on it at 40 °C. Liquid condensation causes the nanowires to form large bundles. The alcohol catalyzes the HF reaction by producing water as a byproduct, so the thickness of the condensation layer increases with the alcohol content in the vapor.

[0100] As Figure 9 reported in B, the etching rate decreases as a function of the substrate temperature. By raising the temperature to 55 °C, no nanowire bundles were detected.

[0101] Figure 9 D reports SEM images of nanowires produced by adding isopropyl alcohol to a liquid solution containing water-diluted HF and heating a substrate with a metal pattern on it at 55 °C. Figure 9 The nanowires in D have the same as Figure 9The nanowires in C have the same length, but they appear to be well separated. Thus, in the method of the present disclosure, the heating temperature is a relevant parameter to avoid water condensation and nanostructure adhesion (stiction). The etching is carried out at a higher etching rate at the boundaries of the pattern, and an example of this effect is visible in Figure 9 E.

[0102] Figure 9 F shows the relative change (ΔL) in the length of the nanowires between the center and the boundary of the sample as a function of the substrate temperature. The higher the temperature, the smaller the ΔL. In the presence of alcohol, the decrease in ΔL is even more relevant. Thus, the etching uniformity can be improved by increasing the heating temperature and in the presence of alcohol.

[0103] Figure 10 A flowchart showing the elements (e.g., diffraction gratings) for manufacturing a photonic device using the method of the present disclosure is shown. In certain embodiments, a metal pattern is prepared by a photolithography method using a positive resist on a silicon substrate. A suitable resist is coated on the semiconductor substrate ( Figure 10 A), and the resist type and thickness depend on the desired pattern.

[0104] In one example, the positive photoresist MICROPOSITTM S1805 is used for photolithography according to a process reported elsewhere. In another example, PMMA, which is a positive resist, is used for electron beam lithography. The resist is exposed to UV or electron beam lithography ( Figure 10 B) and then developed ( Figure 10 C). A short plasma clean (10 seconds to 60 seconds in a standard oxygen RF plasma etch) is used to clean the resist residues, and the time is adjusted to avoid excessive thinning of the resist. In certain embodiments, Pt is used as the metal catalyst, and Pt is deposited using an electron beam evaporator at a deposition rate of 0.5 nm / minute. The Pt thickness ranges from 5 nm to 20 nm ( Figure 10 D). Then, lift-off is carried out, for example, by immersing the sample in acetone ( Figure 10 E). The sample is cleaned in a cleaning solvent, then cleaned in isopropyl alcohol and dried by nitrogen purging. The Pt film dewetting and Pt-silicide are obtained by annealing on a hot plate in air in the temperature range of 250 °C to 600 °C ( Figure 10 F) to stabilize the catalyst layer. The dewetting step is related to ensuring uniform etching of the metal pattern with a feature size greater than 500 nm in one direction.

[0105] During MacEtch, metal dewetting produces nanowires. The influence of the etched nanowires on the final pattern can be minimized by adjusting the metal film thickness and annealing temperature so that the cross-sectional size of the nanowires is much smaller than the pattern feature size, for example, in Figure 6In some embodiments, MacEtch is performed by exposing the Si substrate and the Pt patterned layer with a stable Pt silicide layer thereon to air and HF during heating ( Figure 10 G). The metal layer acts as a catalyst. The oxidant selectively oxidizes the semiconductor substrate region under the patterned metal layer, and HF selectively removes the oxidized region ( Figure 10 H).

[0106] Thus, the metal-covered regions of the semiconductor substrate are etched, causing the patterned metal layer to sink into the semiconductor substrate ( Figure 10 H). Thereby, an etched semiconductor structure is formed.

[0107] Figure 11 Some examples of grating structures obtained by the process shown in Figure 10 are shown. For the example in Figure 11 A, the metal layer is patterned by UV lithography, and for the examples in Figure 11 B to 11D, the metal layer is patterned by electron beam lithography. Figure 11 A shows a linear grating with a pitch size of 4.8 μm. The nanowires generated by Pt dewetting are visible in the SEM image, but it has minimal X-ray absorption. Figure 11 B shows a circular grating with a pitch size of 1 μm. Residues of nanowires are visible in the Si trenches due to catalyst dewetting. A depth of 29 μm is achieved by heating the sample at 55 °C and exposing it in the system described in Figure 3 for 4 hours. The resulting aspect ratio is approximately 80:1. The smoothness of the etched Si lines is visible in the high-resolution images of the grating from the top view ( Figure 11 C) and the bottom view ( Figure 11 D). The etching is very uniform over the entire patterned area, as shown by the uniform moiré pattern visible in the SEM image ( Figure 11 B).

[0108] Figure 12 An example of a linear grating with a pitch size of 1 μm and a Si width of 300 nm is shown, where the metal pattern is generated by electron beam exposure of PMMA resist and Pt deposition. Figure 12 A and 12B (B is a high-magnification detail of A) are cross-sectional SEMs of the bottom of the etched structure by the method of the present disclosure as described in Figure 3 . Etching is achieved at 55 °C with air and an etchant generated by evaporating a liquid solution containing water-diluted HF with a molar concentration in the range of 1 mol / l to 20 mol / l.

[0109] Figure 12C and 12D (D is a highly magnified detail of C) are cross-sectional SEMs of the bottom of the etched structure by liquid-phase MacEtch. The liquid solution contains HF diluted with water with a molar concentration in the range of 1 mol / l to 5 mol / l and H 2 O 2 . Figure 12 It is intended to show the Si porosity of the structure achieved by MacEtch in the gas phase compared to the liquid phase for Si N-type <100> single crystals with a resistivity in the range of 0.001 Ωcm to 0.01 Ωcm. In the gas phase ( Figure 12 A to 12B), the etched Si structure has the same contrast as the bulk Si (under the catalyst), and fewer nanowires are visible on the catalyst layer. Multiple small holes are visible in the SEM of the structure produced by liquid-phase MacEtch ( Figure 12 C to 12D). In addition, a characteristic distribution of holes is observed at the bottom of the etched structure ( Figure 12 C to 12D), and there are so many holes that it looks like a complete mesoporous structure, and the unetched bulk Si is clearly visible at the bottom of the image. The mesoporous Si has a poorer contrast in the SEM compared to the unetched bulk Si region. The mesoporous structure starts at the interface with the unetched bulk Si substrate and has a characteristic inverted V-shape near the Pt catalyst layer.

[0110] Related prior art

[0111] Y.Hu, K.-Q.Peng, Z.Qiao, X.Huang, F.-Q.Zhang, R.-N.Sun, X.-M.Meng & S.-T.Lee, Metal-Catalyzed Electroless Etching of Silicon in Aerated HF / H2O Vapor for Facile Fabrication of Silicon Nanostructures, Nano Letters 14(2014)4212-4219.

[0112] O.J.Hildreth & D.R.Schmidt, Vapor Phase Metal-Assisted Chemical Etching of silicon, Advanced Functional Materials 24(2014)3827-3833.

[0113] Catalyst-assisted chemical etching using a vapor-phase etchant according to US 2018 / 0090336 A1.

Claims

1. A method of fabricating an element of a photonic device by metal-assisted chemical etching in a gas phase, comprising the steps of: (a) providing a semiconductor substrate and a patterned metal layer thereon; (b) Expose the semiconductor substrate and the patterned metal layer thereon to a gaseous reactant, wherein the reactant comprises an oxidizing gas and an etchant gas, wherein the oxidizing gas comprises air and wherein the etchant gas comprises hydrofluoric acid, and wherein the reactant is supplied to the semiconductor substrate and the patterned metal layer thereon in a continuous flow or a pulsed flow, wherein the concentration of oxygen in the oxidizing gas is locally increased by the decomposition of H 2 O 2 on the platinum surface, the platinum surface being a solid piece containing platinum immersed in a liquid solution containing H 2 O 2 , wherein the decomposition of H 2 O 2 on the platinum surface generates gaseous O 2 , wherein the liquid solution is placed in a container and the liquid does not contact the semiconductor substrate and the patterned metal layer thereon.

2. The method according to claim 1, wherein the etchant comprises vapor-phase HF evaporated from a liquid solution containing HF diluted with water.

3. The method according to claim 1 or 2, wherein the semiconductor substrate comprises a semiconductor selected from Si, Ge, or an alloy containing elements from groups III and V of the periodic table, and wherein the patterned metal layer comprises a metal selected from Au, Ag, Pt, Pd, Cu, Ni, Rh.

4. The method according to claim 1 or 2, wherein during exposure to the oxidant gas and the etchant, the semiconductor substrate and the patterned metal layer thereon are heated to a temperature in the range of 30 °C to 90 °C.

5. The method according to claim 1 or 2, which is carried out in the presence of an inert gas selected from nitrogen, argon, and helium.

6. The method according to claim 1 or 2, which is carried out in the presence of an alcohol selected from isopropyl alcohol, methanol, and ethanol.

7. The method according to claim 1 or 2, wherein the etchant gas is supplied to a closed etching chamber through a dedicated gas line.

8. The method according to claim 1 or 2, wherein the patterned metal layer comprises a continuous mesh pattern, and wherein the etched semiconductor structure comprises a nanowire array with an aspect ratio of at least 10:

1.

9. The method according to claim 1 or 2, wherein the patterned metal layer comprises an X-ray diffraction grating pattern with periodic features, and wherein the etched semiconductor structure comprises an X-ray diffraction grating with periodic features.

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